• [Introduction]
• [Radio direction-finding and radio-navigation terminology]
• [The Telefunken stepwise-rotating beacon: 1906-1918]
• [The constant-speed rotating beam beacons at Orford Ness and Farnborough]
• [The equisignal beacon system of Otto Scheller: 1907 - today]
• [Leader-cable course guidance]
• [The fixed-multi-beam "X-Method" E/T beacon system of Johannes Plendl]
• [The "Knickebein" - Luftwaffe fixed-single-beam E/T beacon]
• [The "Y-system" - Luftwaffe fixed single-beam + transponder]
• [The "Hermes" Luftwaffe rotating talking beacon]
• ["Elektra", "Sonne", "Mond", Truhe" - Luftwaffe multi-beam beacons]
• [Hyperbolic radio navigation systems]
• [Radar systems: 1904 - 1945]
• [Other Luftwaffe radio navigation systems]
• [Patents]
• [References]

Note: this page contains a large number of photos and diagrams. Current overall page download size is 26 MB. Depending on your internet access, the page may take a while to completely download. Please be patient!

• ["Bernhard/Bernhardine" Luftwaffe radio-navigation system]
• [FuG120 "Bernhardine" airborne Hellschreiber printer system]
• [FuSAn 724/725 "Bernhard" ground station]
• ["Bernhard" station locations]

©2004-2021 F. Dörenberg, unless stated otherwise. All rights reserved worldwide. No part of this publication may be used without permission from the author.

Latest page update: 13 January 2021 (complete overhaul & expansion of this page is in progress since April 2020).

INTRODUCTION

Clearly, the Luftwaffe Bernhard/Bernhardine radio navigation and command upload system that is described in great detail on this website, is a "rotating radio beam system" for fighter aircraft navigation and guidance. But where does this system fit within the domain of "Radio Direction Finding" (D/F), Radio Location, Radio Guidance and Navigation"? This page provides an overview of the history of this domain, through the end of World War 2. The purpose is not to add yet another survey or taxonomy without verifiable and publicly accessible references, and rife with errors or omissions based on lack of language skills, or the usual lazy copy-and-paste from other bad sources. Instead, to present an historic overview with documented origins, undistorted by persisting propagandistic or nationalistic versions of history. With extremely few exceptions, history school books reflect the either the "winner's" point of vie, and WW2 is no exception whatsoever. I did take the liberty to place some accents based on my personal curiosity and interests, including as a pilot and user of such systems. And, like Humpty Dumpty put it so well (in Lewis Caroll's "Through the looking glass"): "When I use a word, it means just what I choose it to mean — neither more nor less."

Fig. 173: The Luftwaffe rotating beacon ground station "Bernhard" Be-10 at Hundborg/Denmark

(source: www.gyges.dk, used with permission; US gov't = no ©)

TERMINOLOGY, CONCEPTS, PRIMARY PATENTS

First, we need to introduce some basic terminology regarding air navigation (which, of course, is very similar to nautical navigation). Rather than describing this textually, I will let the figure below speak for itself:

Fig. 41: Some basic terminology of air navigation - very similar to nautical navigation

Directional radio-frequency (RF) waves were discovered during the late 1880s. Starting in the early 1900s, concepts, techniques, and devices were invented and developed, to apply RF to direction-finding and navigation. So, we have to introduce some more terminology:

• Radio direction finding (RDF). As the name suggests, this is a technique for determing the direction to or from a radio transmitter. I.e., determining Angle of Arrival (AoA) or Angle of Depature (AoD), respectively. The transmitter station can be fixed-base (stationary) or mobile, and cooperative or not ( = "enemy"). The direction is measured and expressed relative to a reference direction at the D/F station or at that transmitter station. The reference direction is typically True North, Magnetic North, or the longitudinal axis of the vehicle (ship, aircraft, land vehicle, surfaced submarine). A distinction is sometimes made between:
• RDF ("Fremdpeilung"): RDF-ing of a mobile transmitter station by a receiver station with known position.
• Reverse-RDF ("Eigenpeilung"): RDF-ing by a mobile receiver station of a fixed transmitter station with known position.
• Radio location, a.k.a. radio positioning: determing the position (own or of a target) by radio means. This is also known as taking a "position fix", or "fix" for short. The position can be the absolute 2D position within some coordinate system on the face of the earth, or a relative 2D position ( = D/F direction + distance/range), or the 3D position of an aircraft ( = 2D position + altitude).
• Radio navigation, in particular in-flight. One of the mantras that I remember well from my own pilot training, is about the top priorities of every pilot/navigator: always "aviate" ( = fly the airplane), "navigate" ( = figure out where you are, where you're going, and how to get there in 4D), "communicate" (with ATC) - in that specific order of priority. Radio navigation is pre-dated by the following other forms of navigation, even though those were/are also used by airplane navigators, in particular on trans-oceanic and trans-polar routes, out of range of radio nav stations:
• Pilotage: visual navigation by reference to landmarks and man-made objects on the ground (or in the water).
• Celestial navigation: based assessing the angle between one or more celestial bodies (stars, sun, moon, planets) and the horizon. This method has been used by mariners since ancient times. Most of these methods require the knowledge of time (e.g, local noon). In aircraft, navigators used periscopic sextants.
• A related technique is Dead Reckoning (DR). It estimates the current position, based on a previous position fix (or known position), and an estimated position-change. The latter is based on estimated speed, direction, drift and elapsed time since that previous fix. The same approach can be used to estimate position at some time in the future, based on current position (known or estimated) and conditions.

Clearly, the above radio-based activities are of strategic and tactical importance during times of armed conflict, and preparation therefore. Which also explains why corresponding RF-based countermeasures (interference, jamming, locate-and-destroy, etc.) have a similar level of importance.

Fig. 3: The alternative to radio navigation....

(source: 1987, unknown)

Note that "determining" position or direction is actually "estimating". Estimations always have an "accuracy" and a "precision". These terms are often confused, and even used interchangeably - which they are not! Simply put, "accuracy" expresses how close estimates are to the true value. "Precision" expresses how close multiple estimates of the same true value are to each other, i.e., "repeatability".

Note that with a single transmitter/DF-station pair, only a direction ( = bearing angle) can be determined - not position. The result of D/F-ing is basically a continuous straight (or great-circle) line of possible positions, emanating from the position of the D/F station, through the position of the transmitter, and beyond. This is a linear Line of Position (LoP, a.k.a., position line; D: "Standlinie"). See the left-hand panel of Figure 42 below.

Important: without further information, the position of the target on an LoP is not known! For a given position of the D/F or transmitter station, the LoP can be drawn on a map ("chart" in navigation parlance). Note that the bearing from a ground station to the aircraft (or vice versa) should not be confused with the aircraft's heading ( = the way the nose is pointing), nor with the aircraft's course ( = direction of the ground track, which is affected by wind).

Fig. 42: Linear, circular, and hyperbolic Lines of Position

Some other RF methods do not determine direction, but rather the distance ( = range) between an observer/anchor station and the "target". All points with the same distance to the anchor station now lie on a circle that is centered on that station. That is, all these points combined form a circular LoP. See Figure 42. Again, without further information, the target's position on an LoP is not known. Through air and space, radio waves propagate at the speed of light: close to 300000 km/sec, or close to 30 cm ( = 1 foot) per nanosecond. Now you know how long a nanosecond is! This finite-speed property makes it possible to use radio waves for determing distance: speed x time = distance traveled. The standard methods are as follows:

• An fixed or mobile anchor station transmits a radio-wave pulse. That pulse is scattered from, and reflected by, the surface of the target. The target may be a cooperative/friendly, or non-cooperative/enemy aircraft, ship, land vehicle, or surfaced submarine. A reflected pulse is received back at the anchor station. The pulse has made a round-trip. The total time-of-flight (ToF) to and back from the target covers twice the distance between station and target. By measuring the time-difference (delay) between transmitting a pulse and receiving the echo, that distance (a.k.a. "range") is known. This is the "ranging" ("Enfernungsmeßung") part of what is called "Radio Detection and Ranging" (acronym: "radar") since late-WW2. The range from a radar station on ground to an airborne aircraft is actually "slant range", which is not the same as "down range". The latter is distance-over-ground, measured along the earth's surface, to the point on that surface, directly below the aircraft.
• Rather than bouncing a radio pulse of a target, an "interrogator" station can transmit a pulse (or coded sequence thereof), and a compatible mobile "transponder" station replies with another pulse (or sequence thereof) on the same or (usually) different radio frequency. Transponders typically apply a pre-defined reply-delay. Again, half the round-trip time (minus any reply-delay) is equivalent to slant range. Modern transponders are required to apply a 3.0 ± 0.5 μsec reply delay.
• The interrogator and transponder roles can be reversed: an airborne interrogator and a ground-based transponder. The latter is often co-located with a radio navigation beacon. The common post-war implementation of this is called Distance Measuring Equipment (DME).
• In the world's first transponder-based ranging system (in 1927 patent nr. 632304 by Koulikoff & Chilkowsky), there were two interrogator/transponder stations. One initiated a pulse, and from thereon, the two stations ping-ponged the pulse. The resulting beat-tone was a measure of the distance between them.
• Instead of transmitting and replying with a pulse signal, it is also possible to transmit a tone-modulated continuous wave (CW) signal, have the transponder send this back (typ. with a reply-delay) on a different frequency, and measure the phase-difference between the two signals. This round-trip difference represents a time-shift, hence distance. To avoid distance ambiguity, the wavelength of the transmitted tone has to be longer than the round-trip distance. E.g., a 10 kHz modulation tone has a wavelength of about 33 km (≈20 statute miles).

OK, just one more type of LoP to discuss! Above, we covered using the "time-difference = 2x round-trip distance" approach. It resulted in circular LoP's. This can be expanded to a system with not one but two anchor stations with known position. Of this station pair, one is referred to as the "master", the other a "slave". The master transmits an omnidirectional pulse. Upon receipt, the slave also transmits a pulse, on the same frequency. I.e., the slave is synchronized to the master. Both pulses are received by the target, where they arrive at slightly different times. Again, we have a time-difference (Time-Difference-of Arrival, TDoA). However, now this time-difference is equivalent not to a distance, but to a distance-difference! It is the difference between 1) the distance between the target and the master station, and 2) the distance between the target and the slave station. All points that have the same absolute distance difference, lie on two open curves. They are two branches of one hyperbola. The two anchor stations are the "foci" of the hyperbola. The two mirror-image curves of each hyperbola pass symmetrically between these foci. In fact, as with the linear and circular LoPs, there is an infinite family of LoP's. In this case, covering all possible distance-differences. Depending on the +/- sign of the distance difference, the target lies on one curve or on the other. So, we now have hyperbolic LoP's. See Figure 42.

In analogy with the various circular LoP methods, the same hyperbolic LoP's can also be created by pair of master-slave anchor stations that transmit a continuous-wave carier signal instead of pulses. Again, the slave is synchronized to the master. But now the LoP represents a fixed phase difference between their (continuous) transmissions, instead of a fixed time difference between pulse-pair receptions. But a phase difference between two sinewaves (audio or RF) of the same frequency is equivalent to a time difference.

As repeatedly mentioned above, without further information, the position of the target on an LoP is not known! It could be anywhere on the LoP - within the reception coverage area of the radio aids. How can this be resolved? By combining two or more independent lines of position into a point of position (PoP). For this approach to work, we need LoP's that intersect. I.e., LoPs that cross, or at least touch each other. This can be achieved with linear, circular, and hyperbolic LoP's, as illustrated in Figure 43:

Fig. 43: Combining Lines of Position to determine position

It should be intuitively obvious from this figure, that creating an accurate, clear and concise equivalent textual description is a rather tall order, and not necessarily more comprehensible or instructive. So, I will not attempt to do so. That said, a couple of words anyway...

The case of two "crossing linear LoP's" is standard classical triangulation ("Kreuzpeilung") - the simplest form of multi-lateration. This has been used since many centuries, if not millenia. Note that it works both ways: two (or more) fixed D/F stations can determine the position of a mobile transmitter. This was also done in the early days of radio D/F, and the position estimated by the D/F stations was reported to the transmitter station (ship, airship) via radio. Conversely, a mobile D/F station can determine its own position by using two (or more) beacon stations with known position. Accuracy ( = uncertainty) of the position estimate depends primarily on the angle between intersection LoPs. I.e., distance between the beacon pair or D/F station pair, as wel as the distance from the target to the baseline between the beacon or D/F station pair.

We can also combine a linear LoP (bearing) and a circular LoP (range). These two LoP's can be obtained with two spatially separated anchor stations: one D/F station and one range-finding station. Once can also combine a linear LoP and a circular LoP into a single system, at a single anchor station. Radar is a prime example of this. Two independent and spatially separated range-finding stations generate two overlapping circular LoP's. Conversely, a mobile interrogator station may determine the range to two spatially separated transponder stations with known position. Generally, two overlapping circles intersect at two points, not one - unless the target is located exactly on the straight baseline between the two anchor stations. I.e., there generally is ambiguity as to which of these two points is the actual position of the target. Overlapping hyperbolic LoP's can also have two intersect points.

Of course, the concept of positioning by means of intersecting LoP's also applies to hyperbolic LoP's. This is done with a chain of (at least) three anchor stations, one of which is the master to which the remaining stations of the chain are synchronized. Early such systems used maps (charts) with a lattice of hyperbolic LoP's, like the pink and light blue lines in the right-hand panel of Fig. 43 above. Each line of the lattice was marked with the associated time-difference.

Note: all necessary basic methods for, and concepts of, radio D/F and radio location/positioning were patented by 1935! See the time-line diagram below:

Fig. 44: Time-line of primary radio direction-finding/location/navigation patents through 1935

(source: patent table-3)

Note: basically nothing fundamentally new has been added by subsequent radio D/F, positioning, and navigation methods, including those that are based on satellites, WiFi networks, mobile/cellular telephone networks, etc!

THE TELEFUNKEN "COMPASS" STEPWISE-ROTATING BEACON: 1906 - 1918

Directional radio beacons can be thought of as the radio equivalent of optical rotating beacons: nautical lighthouses. The Prussian Building Authority ("preußische Bauverwaltung") were part of the Royal Prussian Ministry of Public Works ("Königlich-Preußisches Ministerium der öffentlichen Arbeiten"). They considered coastal fog signalization to be of prime importance. Air-acoustics warning devices, such as fog horns, sirens, whistles, guns, and bells, do no allow determination of direction and distance. Around 1905, underwater (submarine) acoustic systems were introduced, to mark obstacles and lightships, and to avoid ship-to-ship collisions during fog. Sounds were generated with underwater bells, and received with hydrophones (one on each side of the ship's hull). While at sea, this enabled reasonable determination of the direction to the signal source. In coastal areas, their use was limited to lightships, as the sound waves must basically be received head-on. It was very expensive to install acoustic underwater systems fixed to the sea bed. Also, the high cost of on-board equipment and their maintenance was only affordable for large ships. Therefore, in 1906, the Building Authority started to investigate directional radio signals for fog signaling, under the direction of Privy Councillor ("Geheimrat") Walter Körte. Ref. 187A1, 187A6. The Government Secretary ("Regierungsbausekretär") of the Building Authority proposed that, at times of fog, lighthouses should transmit radio signals that could be received by small ship-board receivers, and an automatically rotating parabolic antenna that would stop turning and point in the direction of the transmitter. Körte contacted the Telefunken company mid-1906, who informed him that they had already been experimenting with directional radio waves - without satisfying results.

During 1906-1908, the Building Authority performed radio direction-finding tests. These initial lab and field tests were partly done with support from the physics department of the scientific institute Urania-Berlin and equipment from the Telefunken company. Field tests took place July-August 1906 near the then-Prussian Baltic port city of Swinemünde (about 160 km northeast of Berlin; since October 1945: Świnoujście, on the Polish side of the border). Ref. 187A5-187A7. Test signals from Swinemünde were received by a steamer at 16 nautical miles (≈30 km). The ship-board receiver used a directional parabolic antenna system (ref. 187A7). Possibly a wire antenna configured, e.g., per Fig. 2 in ref. 229E2 was used. During the 1920s, the British also experimented with rotating parabolic reflectors, but at VHF frequencies (≈50 MHz, wavelength ≈6 m), ref. 228A. Such parabolic systems were heavy, large, cumbersome, expensive, and therefore considered unsuitable for boats and small ships. To be effective, the focal length of the parabolic antenna had to be larger than a quarter wavelength! Ship-board direction-finding systems also required ship-specific calibration due to the metal hull and structures. At the same time, similar ship-board DF experiments by their French counterparts came to the same conclusions. In parallel, the Italians Artom, Bellini, and Tosi also pursued radio direction-finding (ref. 184E), but with a different antenna system arrangement. The latter approach was evaluated by the German Imperial Postal Administration ("Reichspostverwaltung"), and was deemed too large and complicated to be promising. These conclusions are not surprising for the large radio wavelengths that were practicable at the time. Also, receivers were still without electronic tube (valve) amplifiers.

Meanwhile, private industry had continued to improve the spark-gap transmitter ("Knallfunkensender", "Knatterfunkensender"). It is based on using high voltage pulses to generate an electrical spark, like the spark plugs of an automobile combustion engine. The final major improvement to the spark gap transmitter was proposed by Max Wien in 1906 (ref. 186C): the so-called “tonal” or "singing" quenched-spark transmitter ("Löschfunkensender", "Tonfunkensender", "transmetteur à étincelle musicale", "transmisores de chispa sonora"). He subsequently joined the Telefunken company, where his idea was developed and commercialized. See further below. In parallel, the C. Lorenz Company (ref. 263A) had bet on Valdemar Poulsen's "spark-less" light-arc transmitter ("Lichtbogensender") technology. Telefunken had unsucessfully attempted to bypass Poulsen's patent during 1906-1908, then rejected the light-arc transmitter as inferior (ref. 187B).

Fig. 45: Simplified time-line of radio transmitter technology up to 1930

(source: ref. 186A-186P)

In 1909, the Prussian Building Authority considered the quenched-spark transmitter to be sufficiently mature, and abandoned mobile radio direction-finding receivers. Instead, it decided to pursue a directional radio beacon on land, and a relatively simple standard receiver on-board. Mid-1909, it was proposed to build two transmitter stations on Müggelsee Lake (about 20 km southeast of the city center of Berlin), ref.187A5:

• A 3-4 mast Bellini-Tosi system on the site of the Royal Inland Fisheries Institute ("königliches Institut für Binnenfischerei") at Friedrichshagen.
• A directional transmitting station to the east of there, on higher terrain outside the nearby village of Rahnsdorf. The antenna system was to comprise 32 masts that supported 16 dipole antennas in a star configuration. The dipoles were to be aligned with compass directions. The dipoles were to be energized in sequence, to obtain a (stepwise) rotating radio beam - the world's first rotating radio navigation beacon! Per ref. 2, the switching mechanism was motorized. A separate Morse-code-like sequence of "dots" and "dashes" was to be transmitted by each dipole. Most likely, this was exactly the same scheme as used during follow-on tests around 1912 at Cape Arkona (see further below). The receiver station determined the direction from which the transmitted signals arrived ( = Angle of Arrival, AoA), based on identifying the dipole from which the smallest signal was received.

Fig. 46: Map of Müggelsee (ca. 4.3x2.6 km / 2.7x1.6 miles) with Friedrichshagen and Rahnsdorf

(source: "Pharus-Plan Berlin-Oberspree 1919" tourist map)

During 1912-1913, the offices and labs of the Maritime Navigation Markers Test Site ("Seezeichenversuchsfeld") of the Maritime Navigation Markers Office ("Seezeichenausschuss") of the Ministry of Public Works were built next to the Fisheries Institute (ref. 187A2-187A3, also right-hand image in Fig. 46 above), and their facilities moved there from Berlin-Tiergarten.

The mention of a Bellini-Tosi (B-T) system for transmitting is strange. Such systems (two crossing loop antennas with radio goniometer coupling) were still only used as DF receiving stations, not transmitter stations. Also, a 3-mast B-T would have been non-standard. Per ref. 187A7, only one small 16-dipole transmitting station was built on Müggelsee Lake. Per ref. 187C1-187C3 (Telefunken, 1912), there were actually two transmitter stations, one of which had a rather large 16-dipole antenna system: a diameter of 200 m (≈660 ft). Telegraphy engineer Franz Kiebitz (at that time working at the Imperial Postal Administration) participated in the receiving tests from a boat on the lake.

Per ref. 187A5, two medium-wave frequencies were proposed to be used: around 2.4 and 3 MHz (wavelength of 125 and 100 m, respectively). This coulds explain the simultaneous proposal for two transmitter stations. These frequencies were roughly the demonstrated practical upper limit of spark transmitters at the time. However, these two frequencies are actually too close together to warrant two separate installations, or even two test series. Per ref. 187A7, the actual antenna system was small. However, per ref. 187C1-187C3 (Telefunken, 1912), the actual circular antenna installation had a diameter of about 200 m (≈660 ft). That would have been more appropriate for a standard ½ wavelength wire dipole, resonant on an operating frequency around 300/(2x200) ≈ 0.715 MHz = 715 kHz. Undoubtedly, small standard commercial quenched spark gap transmitters from Telefunken were used. They had an operating frequency of ca. 500-1000 kHz. However, there is no law of physics that states or implies that an antenna will not radiate efficiently, unless it is dimensioned so as to be resonant at the operating frequency. Shorter dipoles could have been used (with loss of directivity), with appropriate impedance adaptation between the transmitter output and the antenna (which causes loss of power). Then again, the stated qualification "small" is relative, and no reference is provided, such as the electrical wavelength or the physical size of a human. Note that either way, even a diameter of 200 m was small compared to the wavelength of several km (!) of early transmitter systems.

You may wonder "16 does not divide nicely into the 360° of a compass. So, why 16 dipoles and not 18?" Astronomers already divided the circle into 360 degrees in antiquity. However, the magnetic compass came into use only about 1000 years ago. Initially, sailors referred to eight principal wind directions: north, northeast, east, etc. I.e., the four cardinal directions and the four intercardinal (= ordinal) directions. As nautical compasses improved, the compass rose ("scale") was further divided into eight "half winds", to form the 16-wind compass rose. A further split added 16 "quarter winds", for a total of 32 "compass points" ("nautische Strich", "Kompassstrich"). Each point of the 32-wind rose represents a compass sector of 360°/32=11¼°.

The Minister of Public Works explicitly stated in a letter of November of 1909, that the Prussian Building Authority went public with their system concept in order to make it "unpatentable" (ref. 187A1, 187A5). Per § 1 and §2 of the German patent law of 1891 (applicable in 1909): "Patents are only awarded for new inventions that have a commercial application" and "An invention is not considered new, if at the time of filing, the invention has already been published within the last 100 years, or has evidently been used within the country such that an other expert could use it." Therefore, the publication indeed effectively blocked such a patent.

The transmitter antenna system comprised dipole antennas, because 1) they have a clear directional radiation pattern, and 2) their construction is very simple. Note that in some old literature and patents, the term "dipole" is used for only a "dipole half" (a.k.a. "leg"), each "half" is considered and counted as a separate antenna. Likewise, "transmitter" ("Sender") is used for a radiating/transmitting antenna, not for the electronic device that energizes the antenna with radio-frequency signals. The next figure shows the doughnut (torus) shaped radiation pattern of a dipole antenna:

Fig. 47: Radiation pattern of a dipole in "free space"

Fig. 49: Radiation pattern of a dipole close(r) to ground

Clearly, the radiation pattern is symmetrical. It has a "null" or minimum direction in two opposite directions. Likewise, it has a maximum in two opposite directions. These bi-directional min and max directions are at right angles (90°, orthogonal). These symmetries pose a problem for direction-finding methods that are based on detection of the min or max direction. There are always two such opposite directions. I.e., there is always a 180° ambiguity! Note that this direction-finding ambiguity problem is exactly the same when using a rotating or rotable symmetrical pattern on the transmitter side, or on the receiver side. Such direction-finding systems provide a Linear Line of Position, as discussed in the previous section. The ambiguity can only be resolved by additional information: triangulation with another station, or being able to exclude one of the two LoP directions by other methods (e.g., one LoP direction is over land, whereas the receiver is at sea).

Due to delays and "adverse cirumstances", the Building Authority performed no further tests during 1909-1910. However, the tests at Müggelsee Lake had been successful enough to warrant construction in 1911 of a new antenna system, this time at the Fog Signal Test Station (“Funknebelsignalversuchsstation”) at Cape Arkona. Arkona (misspelled Arcona in some literature) is located 250 km due north of the center of Berlin, on the tip of the Baltic isle of Rügen, Germany´s largest island. This site was chosen to test under realistic operational conditions, and also such that in case of successful completion of the tests program, the system could be used to secure shipping traffic of the state rail ferry between Sassnitz (Saßnitz) on Rügen, and Trelleborg on the southern tip of Sweden (≈100 km).

Contrary to the installation at Friedrichshagen, this time it was a large antenna system. It comprised 32 short masts (2.6 m, ≈8.5 ft), evely spaced on a circle with a diameter of 115 m (≈377 ft), with a tall mast (2.6+43.8=46.4 m, ≈152 ft) at the center. Sixteen dipoles were strung from the central mast. Ref. 187C1-187C3. The dipoles were arranged radially, in a star configuration, i.e., one for every 11¼ degrees. The center of this star, the feedpoint of the dipoles, was raised more than the tips of the dipoles. Such dipoles are also called "inverted-V" dipoles. The span of the dipoles was about equal to the diameter of the circle. For standard half-wavelength dipoles, this implies a medium-wave operating frequency of about 300/(2x115) = 1.3 MHz = 1300 kHz. Due to the size of the installation, it was decided to involve the Telefunken company. To reduce cost and expedite construction, Telefunken recommended an antenna configuration that was different from the one used at Müggelsee Lake. The Müggelsee configuration turned out to be more suitable. So, the Building Authority decided to continue without Telefunken. Maybe this is why the 1912 Telefunken publications on this topic (ref. 187C1-187C3) mention Friedrichshagen but not Arkona!

Fig. 50: 1911 site plan for the Fog Signal test site at Arkona, with encoding of 16 dipole signals

(source: adapted from de.wikipedia.org, retrieved 15 April 2020)

During the summer of 1912, the Building Authority conducted a second series of tests, this time with an antenna arrangement "similar to that used at Müggelsee Lake, with minor modifications". Construction of this station was completed by October of 1912. It was a small system: eight 20 m tall wooden masts (≈66 ft), evenly spaced around a circle with a diameter of 40 m (≈130 ft), and a transmitter equipment shed near the center. See the photo below. There appears to be a support mast with a ball on top, just to the left of the shed. It is only about half as tall as the other masts, and is not at the center of the circle.

Fig. 51: The eight wooden antenna masts and small equipment shed at the Arkona test site - 1912

(source: 187K)

These antennas were installed within the earthen walls of the 12^th century slavic Jarosmargburg hillfort. Remnants of the eight tripod masts and the associated tie down stakes were recorded in an archeologic site survey of 1921:

Fig. 52A: A 1921 archeological survey of the medieval Jarosmarburg site on Cape Arkona

(source: adapted from de.wikipedia.org; original 1921 survey drawn by Robert Koldewey; tripod masts marked blue, stakes marked red)

Fig. 52B: The tip of Cape Arkona in modern times - approximate location of the antenna ring is marked with a yellow circle

(source original unedited photo: unknown)

A much larger antenna circle would not have fit within these earthen walls. The 1921 survey also did not record remnants of a larger circle of masts. So the large antenna system must have been installed somewhere else on the Cape, outside the walls. Note that the entire terrain inside these walls slopes upward towards the cliffs on the eastern side, with a rather steep 6% grade ( ≈3 m rise across the 40 m circle).

Tests were conducted through the end of 1912, using ship-board receivers on a passenger steamer and on one of the rail ferries. These were basically still simple, passive crystal sets ("Detektorempfänger"). Direction finding was possible out to 32 nautical miles (≈60 km), and signals could be distinginguished all the way to Trelleborg (≈100 km). Due to lack of funding, tests could not be repeated under various sorts of bad weather conditions and day/night variations in radio propagation. Possible improvements were considered in 1913 and planned to be implemented and tested at Arkona in 1914, with equipment of the Building Authority, Telefunken, and of the company “Dr. Huth-Berlin”. However, World War I interfered with this plan. War can indeed be quite a nuisance and a kill-joy...

Per ref. 187A7, tests with this small system showed that an accuracy of half a compass point (i.e., ≈6°) was feasible, based on averaging. But doesn't eight masts imply only four dipole antennas?! So, how was this demonstrated directivity obtained? Well, even in Germany, there was no law that dipoles had to be straight! Four dipoles in a star configuration gives you eight dipole halves, evenly spaced by 360°/8=45°. These halves can also be connected as horizontal-V dipoles (bent dipoles), instead of straight dipoles. This way, the "null beam" can be steered in twice as many fixed paired directions, thereby doubling the achievable bearing accuracy without changing the numebr of dipoles. See the figure below (c.f. the 3-dipole example on p. 421 in ref. 186A). The aformentioned Franz Kiebitz actually conceived the required "half angle" antenna switching arrangment:

Fig. 53: Sixteen "null" (or "maximum") directions obtainable with 8 dipoles

(the four possible straight dipoles are marked in green; source antenna switching arrangement: Fig. 1067 in ref. 187R)

Down to a V-angle of 45°, the general shape of the far-field dipole radiation pattern is not very sensitive to that angle. It is also not very sensitive to the sag of the dipole wires towards their feed-point at the center of the circle. But even so, there would still only be 16 paired "null/minimum" (or "maximum") directions spread out over 360°... Note that with 16 dipoles, the same technique would result in 16x2=32 paired "null" directions. Of course, this would have made the antenna switching system significantly more complicated! But that's not all! The system concept requires that the receiver station identify the dipole with the strongest or weakest signal in the direction of the receiver. For this to work, all dipoles must generate the same field strength. This was basically done by ensuring that all dipoles had close to the same antenna current when connected to the transmitter ("it is antenna current that radiates"). For identical dipoles and a homogeneous environment, this is relatively straightforward. This only required a single adjustment of the coupled coils at the output of the transmitter. However, when the dipole configuration is changed from "straight" to "shallow V" or "sharp V", the feed point impedance of the dipole changes significantly. This would then have required switcheable adjustment of the coupling coils, or switching between at least two sets of such coils. This was definitely a disadvantage with respect to simply doubling the number of dipoles. Note that the available literature does not mention how fast any of the above systems rotated. Of course, for proof-of-concept testing and application to navigation of ships, this was not of prime importance.

As mentioned above, the Prussian Building Authority first contacted the Telefunken company in 1906, at which time the company was already investigating directional radio transmissions - without much success. The Authority first engaged Telefunken in 1911 for the activities at Arkona. In 1907, Austrian-born Alexander Meißner (also spelled Meissner) joined Telefunken in Berlin, where he worked on radio direction-finding, radio location, directional radio transmission, and associated antenna technology. Ref. 187G, 187L. Meißner (also) pioneered transmitters using radio tubes/valves, and conceived a vacuum-tube feedback oscillator around the same time (1911-1913) as F. Lowenstein, L. de Forest, E. Reisz, C.S. Franklin, H.J. Round, E.H. Armstrong, I. Langmuir, O. Nussbaumer, S. Strauss, A. Sinding-Larsen, and others. Due to near-insuppressible worldwide nationalism (at least regarding inventions), rather lax patent criteria notably in the USA, general ignorance regarding "invention" vs. "patent", and limited understanding of foreign languages, typically only one of these names is recognized and credited in any individual country.

But anyway, in 1911, Meißner developed what became known as the "Telefunken Kompass Sender". Ref. 184AE, 187C-187H. This literally translates to "compass transmitting station". Note that this is the opposite of the term "radio compass" in the English language, which is a radio direction finding receiving installation. As is clear from his 1912 US patent 1135604, Meißner's system makes several important practical improvements to the rotating dipole-beam system of the Prussian Building Authority:

• Sequentially transmitting an identical signal (e.g., a short pulse) via the dipoles, instead of a dipole-specific pulse-sequence or tone.
• Using a timing signal to announce the start of the rotation sequence. This could be a single pulse or a station identifcation code, distinct from the signal transmitted via the dipoles. Of course, this signal now had to be transmitted in a non-directional/omni-directional manner, as it had to be received throughout the entire coverage area of the beacon. A natural choice would be to start the dipole sequence with the dipole that has its null in the north-south direction. The the timing signal would then be equivalent to a North-signal.
• Patent text page 1 (numbered lines 94-96) proposes to create the required non-directional pattern by simultaneously connecting all dipoles in parallel. From a switching point of view, this not as easy as it sounds. Possibly, this was the configuration tried by Telefunken at Arkona and deemed problematic by the Building Authority... However, the patent also proposes a separate omni-directional "umbrella" antenna (lines 99-101 on text page 1, lines 16, 19, 20 on page 2, and in patent figure 2 shown in Fig. 54 below). Consistent with this, the commutator/distributor in the patent figure connects to the dipoles as well as to an omni-antenna. The latter is accommodated just like an additional dipole would have been, by simply adding two contacts to the commutator.
• Note that the same transmitter is used for the non-directional transmission as for the directional transmissions. During non-directional transmssion, the radio energy is spread out evenly in all 360° directions, instead of being concentrated in the figure-of-8 beam of single dipole. Hence, the received omni-signal is always weaker than the strongest dipole signal.
• As shown in patent figure 2, the omni and dipole antennas would share a central mast. The dipole legs are no longer straight: each leg is bent into an inverted-V, and the dipole feed point is near ground level. This is not to be confused with an inverted-V dipole: it has straight legs, and the feed point is higher than the tips of the dipole.
• The dipoles switching part of the Building Authority system was motorized. A rotary contact arrangement would have been the logical choice. That system also had secondary motorized switching mechanism for creating the dipole-specific "dot" and "dash" pulse sequences. In the patent diagram, this is implemented as an interupter disk in series with the signal from the transmitter. Unfortunately, there are no details available of the Building Authority system that allow us to determine if, or to what extent, Meißner's commutator was different.
• Significantly simplifying the telegraphist's job at the receiver station. It was no longer needed to interpret the various dot/dash dipole signals. Instead, he simply had to measure the time between the north/time-signal and passage of the weakest dipole signal. The patent proposes a special stopwatch for this purpose. Its needle rotates at the same speed as the antenna commutator.

Interestingly, Meißner's patent was filed in August of 1912, after Telefunken 1912 publications such as ref. 187C1-187C3, and after Telefunken demonstrated a working scale model at the April 1912 Allgemeine Luftfahrzeug-Ausstellung (ALA) aviation exhibition in Berlin, ref. 187M! The patent obviously lists Meissner as inventor. For some reason, it does not list any assignee, even though he was working at Telefunken. There is no equivalent German patent, and elsewhere, there only appears to be one in The Netherlands (octrooi nr. 981, filed July 1912). The same switched multi-dipole star-configuration, but now for radio direction finding receiving purposes, appears in the 1919 German patent 423014 about inductive leader-cable systems for ship navigation.

Fig. 54: Cross-section of the antenna system, the transmitter/commutator, and the Telefunken stopwatch

(source: diagram adapted from Meißner's 1912 US patent 1135604; also Fig. 3 in ref. 187C1-C3; stopwatch adapted from Fig. 2 in ref. 187D)

The face of the Telefunken stopwatch shows a circle with 34 tick marks, see Fig. 54. The ticks at the 12 and 6 o'clock position are marked "z" (probably for "zurück" = reset). There is one tick mark pair for each dipole antenna. I.e., 32 ticks in total. They are counted from 0 to 31, where "North" = 0 and "South" = 16. The ticks for the cardinal and intercardinal points of the compass are marked with the standard corresponding German letters N, O, S, W, and NO, SO, SW, NW, respectively. Note that the stopwatch does not have a conventional hand (needle), but a double one - across the entire watch face. It represents the two null-directions of a dipole antenna, and the associated 180° ambiguity.

According to ref. 187C1-187C3, the Telefunken Compass transmitted the north/timing signal after each half revolution of the commutator. This makes perfect sense. During a full commutator revolution, each of the 16 dipoles is energized twice, and the receiver station observes two null-passages. If the north/time signal were only sent once, then the second of these null passages would normally be wasted, and double the time between measurements. Note that with a single north-time-signal, using the second null instead of the first, does not change the measured bearing angle, with its 180° ambiguity. The antenna commutator turned at 2 rpm, i.e., one full revolution every 30 sec (though ref. 187F2 states 1 rpm). Hence, half a revolution took 15 sec.

The principle of the Telefunken Compass system is illustrated in the animation below. In this animation, the audio pulse tone is 1000 Hz, the standard for Telefunken quenched spark transmitters, see further below. The tone is not sinusoidal, but a "spark" spike with a decaying tail (see top left-hand corner of Fig. 45 above). This replicates the rough sound of a quenched spark tone. Some background static noise was also included, for added realism.

Reports that the system achieved an accuracy of about 3-4° (e.g., ref. 187C1-187C3, 1912), may refer to accuracy of reading the angle on the stopwatch (ref. 2) or after averaging multiple "null" passages. The average human reaction time to aural stimuli is about 0.25 sec, which would correspond to 6°/sec x 0.25 sec ≈ 1.5°. A significantly improvement the system accuracy would have required an impractical number of dipoles (ref. 184L, 1921).

A prototype of Meißner's Kompaßstation was built and tested at a Telefunken test site in Berlin-Gartenfeld (spelled Gartenfelde in old Telefunken publications). This is an 800 m (≈½ mi) wide triangular area, surrounded by canals. It is located between Berlin-Spandau and what had been canon and rifle shooting ranges of the Imperial Artillery ("kaiserliche Artillerie") and home of the 1^st Prussian Airship Battalion ("1. preußische Luftschiffer-Batalion") at Reinickendorf. The ranges were closed after near-misses with boats on the adjacent lake and a granade hitting a residential house in 1908. The former shooting range and airship area became Berlin-Tegel airport in 1948. Gartenfeld was acquired by the Siemens company late 1910 and was appended to Siemensstadt ("Siemens Town"), adjacent to the south. The Siemens Gartenfeld Cable Works ("Kabelwerk Gartenfeld", part of the Pirelli company since 1998) closed in 2002.

Fig. 55: Section of a 1912 map of greater Berlin - the Gartenfeld island is marked by the blue triangle

(source: adapted from wikipedia.org)

Per ref. 187C1-187C3, the central mast of the prototype antenna system was about 20 m tall. This is consistent with the roof ridge of the equipment shed in the photo below being about 2.5 m above ground.

Fig. 56: The Telefunken Compass test site at Gartenfeld near Berlin-Spandau

(source: ref. 187C2)

The figure below shows three photos of Meißner's antenna commutator system. The photo at the center was taken inside the equipment shed in Fig. 56 above. The photo on the right was taken at Telefunken's equipment exhibition at the June 1912 International Radiotelegraph Conference in London (ref. 187D). It was decided at this same conference to relegate transmissions of telegraphy signals used exclusively for direction-finding and location of ships to wavelengths up to 150 m (i.e., frequencies above 2 MHz).

Let's first look at the left-hand photo. It corresponds to Meissner's 1912 US patent 1135604. At the top is a ring with porcelain insulators: one pair for each of the 16 connected dipole antennas and one omni-antenna), each with a ball-shaped contact at the bottom. It appears that these balls could "swing" to and from the central support. The antenna feed-line wires were connected just above the ball contacts. Below this ring are the two rotating arms of the commutator. There is a porcelain insulators at the end of each arm, again with a ball-shaped contact, now pointing upward. Just below the rotating arms are two slip rings that rotate with the arms. Each of the associated sliding contacts is mounted on a vertical rod, with a porcelain insulator at the base. Just above these insulators, the rods are connected to a quenched spark transmitter.

Fig. 57: The "Meißner" motorized transmitter distributor / antenna commutator

(source: (left) ref. 187C2 (also in 187C1 & C3); (center) ref. 187C2; (right) ref. 187D)

Also note the vertical disk between the motor and one of these two insulators. The motor is down-geared, and drives the disk and the rotating arms in synchrony. The disk has a series of cams/notches that actuate a normally-closed switch contact. One of the two connections to the transmitter is passed through this contact. This allowed a short Morse-code station identification to be transmitted instead of the start/North pulse signal, while the commutator was connected to an omni-directional antenna. Note that there is no such disk near the commutator-motor in the center and right-hand photos. In the center photo, it appears to be mounted on a small shelf against the outside wall, below a set of eight porcelain insulator disks. This disk has its own motor, so it was not synchronized to the commutator.

In the center and right-hand photo above, the transmitter is mounted directly below the base plate of the commutator. See the next figure. It is a Telefunken model "0,5 T.K." quenched-spark gap transmitter. As the model designator suggests, it had an output power of 0.5 kW. It dissipated over 1 kW. There is a large spiral inductor on the front side of the transmitter. It is made of copper tape. Behind this coil, there are three "Leyden jar" capacitors, connected in parallel. Combined, the inductor and capacitors determine the operating frequency of the transmitter. The frequency was adjustable by selecting taps on the front of the coil. Standard frequency range of the "0,5 TK" was ca. 500-1000 kHz, equivalent to a "medium wave" wavelength of 300-600 m. This could be customized down to 330 kHz (900 m, longwave). Next to the antenna-current meter, there is a frame in which a stack of porcelain insulators and copper electrode disks (with 0.2-0.3 mm mica insulating spacer rings) is compressed. This is a 10-section series spark gap "Funkenlöschstrecke": the actual quenched spark gap device! The copper disks were partially silver plated. They had a large diameter, to increase cooling and thereby improve "quenching" of the sparks. In the center and right-hand photos above, three additional copper-tape coils are visible below the transmitter. They are part of an adjustable antenna tuning circuit.

Fig. 58: Telefunken quenched-spark transmitter model "0,5 T.K." used in the early model "Kompass Sender"

(source: (left) ref. 187C; color photos adapted from ref. 186D)

The transmitter was powered by an AC motor-generator. It converted standard 220 V / 50 Hz AC power to 220 V / 500 Hz. A step-up transformer increased the latter to the required high voltage: ca. 8000 volt. The number of spark gaps depended on the desired transmitter power, e.g., 60 gaps for 35 kW. The rpm of the motor-generator was adjustable. This allowed ±30% variation of the nominal tone of the transmitted amplitude-modulated radio signal: 500 Hz AC power generated a 1000 Hz tone, as the transmitter "fires" each half-wave of the AC power (i.e., twice per 500 Hz cycle).

The Telefunken "0,5 TK" sales brochure mentions a range of 100-150 km over land (200 km at night), and 200-300 km over open sea (400 km at night), when using 20-30 m tall T-antennas at both the transmitter and receiver. The 1.5 kW model "1,5 TK" also operated on 500-1000 kHz and had an advertised range of 700 km. Ref. 186D, 186J1.

The Telefunken "Kompass Sender" was used for long-range navigation of dirigibles built by the Zeppelin, Parseval, Schütte-Lanz, and Basenach companies (ref. 1, 2, 185G, 185H, 235L). In 1912, a complete network of 33 of such beacons was proposed, spaced 50 - 100 km along the entire "political border" of Germany (ref. 187C1-187C3, 187H):

Fig. 59: Map of Telefunken Compass beacons along the border of the German Reich, as proposed in 1912

(source: ref. 187H)

The beacon stations were to have used Telefunken ½ kW transmitters, powered by the local electricity grid, and be fully automated: no need for operating staff/personnel.

In 1914, the US Navy began to evaluate radio direction finding systems on Fire Island, a narrow barrier island along the south side of Long Island, New York. The Navy Bureau of Equipment purchased a Bellini-Tosi directional receiver system and a Telefunken Compass transmitter system. Construction drawings of the foundations of the "Telefunken Aerial" and the "Telefunken Compass Building" are dated July 1914 - June 1915 (ref. 187P). Actual testing probably took place in 1916 (ref. 187E1). Per ref. 187N, the central mast was about 30 m tall (100 ft) and the dipole antennas spaced 20°. This spacing implies eight dipoles, so the spacing was actually two compass points, i.e., 2 x 11¼° = 22½°. As shown in Fig. 53 above, it would have been possible to transmit with one compass point spacing. That same reference states that the commutator shorted out all dipoles, except the one being used to transmit. It also states that a different code letter was transmitted via each dipole. I.e., Telefunken had "borrowed" the scheme from the original system of the Prussian Building Authority! Demonstrated bearing accuracy was about 5 - 10°, which was deemed acceptable for rough navigation of commercial vessels, but not for war-time military vessels.

In the end, only two "Telefunken Compass" stations were built in Germany. They were operational through the end of WW1 (November 1918). One station was built at the village of Bedburg-Hau, on the southeast side of Cleve in Germany, close to where the river Rhine crosses into The Netherlands. These days, the "Funkturmstraße" (lit. "Radio Tower Street") in Hau is a reminder of exactly where the station was located. Cleve is spelled Kleve since a German spelling reform in July of 1935, and is spelled Cleves in English. In 1939, a large "Knickebein" fixed-beam station was built in Materborn, on the west side of Cleve. The second Compass station was built at Tönder (ca. 43 km northwest of Flensburg; Tønder/Denmark before 1864 and after World War I), with its airship base of the German Imperial Navy ("kaiserliche Marine"). Obviously, the beacons were deactivated at the end of WW1. The station at Cleve was dismantled in 1926, on orders of the Belgian occupational forces in that area (ref. 187J).

According to ref. 187R, the main wooden antenna mast stood 75 m tall (≈245 ft). Per ref. 187J, the trellis mast was made of American pitch pine, a dense high-strength hard wood. Thirty (!) dipoles were suspended from the top of the main mast, down to sixty support masts (standard "Telegraphenstangen", i.e., telegraph/telephone poles) that were spaced evenly on a circle with a diameter of 2x125=250 m (≈820 ft). Note: ref. 187R mentions both 125 and 130 m as diameter. The support masts were 12 m (≈40 ft) tall. There were five evenly spaced "crow's nest" platforms on the main mast, see Fig. 60 below. The dipole wires were strung from each support mast to an insulator on the outside the upper platform (i.e., several meters below the top of the mast), and from there all the way straight down, attached to an isolator at each platform. This implies that there was no separate omni-directional antenna, and that omni-directional transmission was done via all dipoles simultaneously, as confirmed by the schematic in Fig. 61 below.

Fig. 60: The wooden central mast of the Telefunken "Kompass Sender" near Cleve (left) and Tönder (center)

(source: ref. 187J (left), Fig. 1090 and Fig. 1085 in ref. 187R)

Per ref. 187R, there was no Meißner-commutator between the transmitter and the dipoles. Instead, there was a very large toroidal coil ("Trommelspule", "Ringspule") with a diameter of 2 m (6.6 ft)! This coil comprised 720 wire-turns with 60 evenly spaced taps for the 30 dipole antennas. Inside the coil, similar to the Meißner-commutator, an arm rotated. There was a sliding brush at both ends of the arm that connected via slip rings to a 4 kW Telefunken quenched spark transmitter (possibly a model "4 TV", ref. 186J1). Compared to the standard radio goniometer, this toroidal coil had a more homogeneous coil field and rotation did not cause undesirable capacitance variation (ref. 184B).

This final implementation of the Telefunken Compass was in fact not longer a step-wise rotating beam system, but the very first operational continously rotating directional beacon!

Fig. 61: The transmission control system of the Telefunken Kompass stations at Kleve and Tönder

(source: adapted from Fig. 1091 in ref. 187R)

A large number of cam disks was used to transmit the various Morse-code identifier and timing signals, and to switch between the directional and the omni-directional antenna configuration. At the center of the diagram above, there is a cam disk marked "Flimmerzeichen" , i.e., blinker. Its purpose is to transmit rapid short pulses instead of a constant signal, to improve accurate detection by the receiver station of the signal null/minimum.

The shafts of the cam disks were driven by a system made by the company Carl Zeiss (renowned for its lenses and scientific instruments) in the city of Jena, about 225 km southwest of Berlin. The drive system was regulated by an accurate clock (±0.2 sec/day), made by the Riefler precision-clock company in Munich. For omni-directional transmission, all dipole wires were connected together by bridging the two sliding brushes, and one side of the transmitter was connected to ground/earth.

Per ref. 187R, the Telefunken Kompass stations at Cleve and Tönder used the following transmision sequence, see Fig. 63:

• First, the station at Cleve ("Station C") repeatedly transmitted its identifier ("Stationszeichen"), the Morse-code letters "CCC", 3x "─ • ─ •", during 42 sec. This was an omni-directional transmission.
• This was followed by the "start" sequence ("Loszeichen"): the Mors-code punctuation character "=" (dash-dot-dot-dot-dash, "─ • • • ─"), followed by the letters "O" (dash-dash-dash, "─ ─ ─") and "S" (dot-dot-dot, "• • •"). The last "dot" of this sequence was the actual start signal. This sequence was also transmitted omni-directionally.
• Then a constant tone signal ("Peilungsstrich") was transmitted with rotating directivity for 80 sec, at two rpm. So, receiving stations would observe a signal null/minimum every 15 sec.
• Finally, a termination signal ("Schlußzeichen") was sent, again omni-directionally: the Morse-code procedure sign (prosign) AR (for the Morse procedure word "new page", dot-dash-dot-dash-dot, "• ─ • ─ •"), followed by the punctuation character ":" (dash-dash-dash-dot-dot-dot , "─ ─ ─ • • •"). The final "dot" coincided with "north" passage of the null-direction, exactly 90 sec after the last "dot" of the "start" sequence.
• Ten second laters, the station at Tönder ("Station B") would start its 90 sec sequence, but used the Morse-code letters "BBB" (3x "─ • • • ") as station identifier.
• Both stations transmitted on the same radio frequency, around 165 kHz (1800 m, long-wave).

Fig. 62: Relative timing of the Cleve and Tönder Telefunken Kompass stations

(source: Fig. 1088 in ref. 187R)

During 1918, civilian radio listeners in The Netherlands (a neutral country during WW1) became aware of regular transmissions by "mysterious" directional radio stations in Germany, with cyclically varying signal strength. Their observations, conjectures, and conclusions were discussed in several issues of the monthly magazine of The Netherlands' Telegraphy Association, ref. 187Q1-187Q9. These private observations were supplemented with measurements by several direction-finding and listening stations of the Navy, Army, and schools. For example, for the "c" station, the following was observed:

• Transmission sequences started at 4 minutes past the full and half hour.
• Each sequence started with omni-directional transmission that consisted of:
• five groups of three letters "C" (5x3x "─ • ─ •"),
• then the Morse-code prosign AS (used for both "wait" and for ampersand, i.e., the character "&", "• ─ • • •"),
• and finally the group "EEE", i.e., three "dots" ("•"). The last "dot" would have signified the "start stopwatch" signal.
• On other occasions, the "C" station sent its sequences every 15 minutes:
• the C-series,
• followed by the "double dash" punctuation character "=" (dash-dot-dot-dot-dash, "─ • • • ─ ")
• and finally the punctuation character ":". The latter character is "dash-dash-dash-dot-dot-dot" ("─ ─ ─ • • •). Again, the last "dot" signified the "start stopwatch" signal.
• Some observations only mention gradually increasing and decreasing signal strength, i.e., not stepwise. Others reported that the directional transmission sequence comprised long dashes of about three seconds each (ref. 187Q1).
• Note that for an antenna system with 16 dipoles, this implies one revolution in 60 seconds, with a 0.75 sec pause between dashes. I.e., as if there was a commutator switches between consecutive dipoles.
• Some observations mention passage of 5 or 6 minimums during a 90 sec cycle. I.e., 15 sec between minimums. This is consistent with the 2 rpm rotational speed.
• Based on the received sound, it had to be a "fluitvonkstation", literally a whistling spark station, i.e., a quenched spark transmitter.

By triangulation, listeners determined that the "C" station was the Telefunken Radio Compass at Cleve, and "B" the station at Tönder. All participating receiving stations were within 100-200 km (≈ 60-125 mi) from beacon "C". Signals received from that beacon never disappeared completely during the minimums. I.e., the minimums of the radiation pattern were not true "nulls". The receivers were 175-500 km (≈ 110-310 mi) from beacon "B". During the long (wide) minimum, no signal was received. Only one listener ever claimed to have heard the "é" ("dot-dot-dash-dot-dot", "• • ─ • •") station.

The 1919 Reichs patent nr. 328279 of Leo Pungs and Hans Harbich (both of Telefunken's competitor C. Lorenz AG) proposes to replace the toroidal coil with an arrangement of two concentric cylindrical coils, the outer one being a stationary non-contact drum armature ("Trommelspule") with a special coil winding scheme, the inner one rotating and coupled to the transmitter (see Fig. 1083 in ref. 187R). Mr. Pungs (author of ref. 187R) also patented a special stop-clock for use with a Radio Compass (RP 328274, May 1917), to accurately measure the time between two successive null/minimum signal passages, instead of between the North/Start-signal and the first null/minium passage.

Of course, the original Telefunken "Kompass Uhr" stopwatch could still be used, with its nautical "compass points" scale (see Fig. 54). However, a special high-resolution direction-finding stop-clock "Peiluhr" was developed, see Fig. 62 below. Its single-hand made a step every 0.1 sec, making its rotation seem continuous. This, combined with the high number of dipoles (30 vs. 16) and the goniometer instead of a commutator, the obtainable accuracy was improved to about 1° (ref. 187R).

Fig. 62: "Peiluhr" direction-finding clock for use with the Telefunken Kompass

(source: Fig. 1089 in ref. 187R)

As Meissner mentions in his 1928 German patent 529891, the accuracy that could be obtained with the Telefunken Compass system depended on the stopwatch operator and the relatively low rotational speed of the beacon. This required averaging of several consecutive bearing measurements, a rather time-consuming process that was only acceptable for slowly moving boats and ships. His patent proposes to replace the stopwatch with an optical indicator that "somehow" rotates synchronously with the beam of a beacon that rotates at very high speed (e.g., 20 rps = 1200 rpm!). The pulses received from sweep-by of the rotating beam would be converted to two almost-stationary light spots (spaced 180°) at the corresponding positions on a compass scale.

Clearly, the concept of the "rotating beam" radio compass is highly independent of transmitter technology. Spark, arc, and machine transmitters were doomed by the advent of vacuum tube transmitters around 1910 (see Fig. 45 above). The demise of spark transmissions was also caused not just by their inefficiency, but primarily by their very large occupied bandwidth. This often caused interference, and severely limited the number of operating frequencies ("tunes") that could be used simultaneously. Therefore, the International Radiotelegraph Conference of 1927 (ref. 186H) decided to immediately forbid new spark transmitter installations on land, and per 1 January 1930 in ships and aircraft (except low power). Not only installation of new stations, but also the use of undamped wave "spark" transmissions was phased out: first forbidden below 375 kHz as of 1 January 1930, then forbidden from land-based stations per 1 January 1935, and completely by 1 January of 1940 (except with less than 300 W power supply consumption, i.e., no more than about 150 watt transmitted power). §2 of article 4 of the adopted regulations also implied an immediate ban on all amateur radio use of spark transmitters. But by that time, simple vacuum tube telegraphy transmitters had already become relatively inexpensive and very efficient compared to sparkers.

This was basically the end of the line for this first generation of rotating-beam radio navigation beacons - until the latter half of the 1920s...

THE CONSTANT-SPEED ROTATING BEAM BEACONS AT ORFORD NESS AND FARNBOROUGH

During first decades of the 20^th century, there was a lot of experimentation going on in the field of directional antenna systems (ref. 185M), the use of directional radio reception and transmission, in particular for maritime navigation. Most of it contributed to knowledge and experience, but did not lead to long lived applications. An interesting British example of this is the large rotating beam system constructed in 1920 on the small isle of Inchkeith in the Firth of Forth, some 10 km northeast of downtown Edinburg. Ref. 228V6. It was a cooperation of the Marconi company (designed and developed by Charles Samuel Franklin) and Trinity House, in its role as General Lighthouse Authority. The beacon was operated for about a year, and tested with the steam ship "Royal Scot" of the London & Edinburgh Steamship Co. A second such beacon was erected at South Foreland near Dover/England. The antenna system was about 10 m tall (32 ft, ref. 228U), with arms of similar length (ref. 185F, p. 34). It comprised two back-to-back parabolic reflector "curtain" screens, each made up of about two dozen parallel vertical wires, spaced about one foot. Ref. 228V7. Also see the 1919 US Marconi/Franklin "reflectors" patent nr. 1301473. A vertical monopole transmitter antenna was placed on the axis of symmetry of each parabola, typically about ¼ wavelength in front of it. This configuration generated two sharp radio beams in opposite directions, sweeping at a constant speed of ½ rpm (one revolution in 2 minutes = one beam passage per minute), very much like an optical lighthouse. Ship-board, only a simple receiver installation was required (ref. 228V8). A bearing accuracy of a quarter compass point was obtained (≈2.8°). Ref. 228V2, 228V3. The spark transmitter operated on a wavelength around 4 m (ref. 228V2; but ≈ 6 m per ref. 228A), i.e., a frequency around 70 MHz (VHF). A distinct Morse signal was sent every half compass point = every 5.625°. I.e., the same approach as pioneered by the stepwise rotating beam system of the Prussian Building Authority ten years earlier!

Fig. 64: The  antenna of the Marconi Rotating Beam system - under construction on Inchkeith

(source: (left image) Fig. 5 in ref. 228V6, (right image) ref. 228V1)

Fig. 65: Receiving antennas for the Marconi Rotating Beam system on the "S.S. Royal Scot"

(source: adapted from ref. 228V1)

The British government established the Department of Scientific and Industrial Research (DSIR) in 1916. The Department formed the Radio Research Board (RRB) in January of 1920. In 1925, (Sub-)Committee "C" (Directional Wireless) of the Board initiated preliminary tests of a radio beacon system with a loop antenna that rotated through 360°. Loop antennas have radiation pattern similar to dipole antennas, i.e., a figure-of-eight shape, with two null/minimum directions that are spaced by 180°, and two flat maximum directions. See Fig.48 above and Fig. 64 below. Using rotating loops for direction-finding reception was common practice, and based on reciprocity, the same directional pattern applies to transmission. This was not a novelty at that time (ref. 228A). These tests were performed by the Royal Aircraft Establishment (RAE) based at Farnborough airfield, located about 50 km (≈33 miles) southwest of downtown London. The test installation was set up at Gosport's WW1 RAF airfield (at nearby Ft. Monckton per ref. 228B, 228D, 228S2), located on the Channel coast near Portsmouth, about 60 km southwest of Farnborough. This beacon operated on a wavelength of 707 m (424 kHz) and the loop antenna was a 5x5 ft (≈1.5x1.5 m) square with six turns of wire (ref. 228U). To reduce the wire's loss resistance (important, as the antenna was extremely small compared to the wavelength), the wire comprised 1458 insulated strands of SWG 40 wire ( = 0.1118 mm diam). Ref. 228R, 228S1. Experiments showed that for open-sea ranges up to 50-60 miles, all observed bearings agreed to within 5° of bearing estimated with other methods (themselves typ. with 1-2° accuracy), and about 70% of all cases agreed within 2°. During subsequent experiments with ships anchored at 90-100 miles, the bearings agreed within 4°. At distances beyond 60 miles, noticeable night-effects were observed, as with other DF methods. These effects were more serious beyond 90 miles. Ref. 228C, 228D.

Early 1928, it was concluded that these experiments were promising enough to warrant full-scale trials with a more permanent beacon. The decision to proceed was taken shortly thereafter. The costs and the work were to be shared by the Air Ministry and Trinity House. Ref. 228K. In November of 1928, the cost was estimated to amount to GB£5k, ref. 228J. Based simply on the inflation rate data for general goods and services, this would be equivalent to about £316 thousand in 2019 (≈€360k). The site selected for these trials was the Orford Ness, for its coastline location and for financial reasons. The ness had been acquired by the British War Department in 1913/14, and a military airfield became operational there in 1915. As the name suggests, Orford Ness is a headland, or "ness". There are no records of monstrous nessy beings ever having been sighted here - at least not of the aquatic kind and not more than in the general human population. This narrow peninsula is some 15 km (≈9 miles) long, and is separated from the Suffolk/England mainland by the Alde-Ore estuary. The widest part of the ness is off the village of Orford, about 130 km (≈80 miles) northeast of downtown London. The ness became the site of the first Royal Flying Corps (RFC) air research station in 1913. The associated military airfield became operational in 1915. The Ness remained a restricted military site through 1982, including WW2 radar development, atomic and conventional weapons research, and over-the-horizon radar trials.

Formal purpose of the trials with the Orfordness (one word!) Beacon was to test practical utility for various classes of ships and for aviation, investigate transmitter power requirements vs. operating frequency [for some desired operating range], investigate limitations regarding siting, and estimate operating and maintenance cost for full-scale operation. However, per communications from the Air Ministry to the Treasury, the Ministry's actual motivation was the importance to RAF aerial navigation, in particular at night, and for direction-finding simply with standard on-board radio equipment. Ref. 228F2, 228K. Construction of the building for supporting the rotating-loop antenna and to install associated radio equipment started in January of 1929. The beacon building is located 475 m west of the 1792 Orford Lighthouse, also located on the ness. The beacon officially entered into service on 20 June 1929, after some initial test flights by the RAF.

Fig. 66: The Orfordness Beacon with barracks to the left and electrical "power house" to the right

(source: adapted from Orford_Ness_23, © 2008 Simon James, Creative Commons Attribution-Share Alike 2.0 Generic license)

The ground level of the beacon building is an octagonal concrete box with an external buttress at each corner. This concrete base is close to 3 m heigh (≈10 ft), ref. 228Q. Applying this dimension as a reference to available photos, the pointed roof of the beacon building is about 5 m across and the tip of the roof at about 10.5 m (≈35 ft) above ground. The upper structure housed the transmitter and motor drive for the antenna. It is timber framed and covered with tarred wooden clapboard (weatherboard) siding. This is why the National Trust (who acquired the Ness from the UK Ministry of Fefence in 1993) named it the "black beacon". Entry to the upper levels is via external stairs. The delapidated beacon building was restored in 1994. The original timber central drive shaft of the loop antenna is still inside the building. The beacon was powered via cables by large (30 kW) WW1-vintage generators at the airfield site on the ness. The brick powerhouse in the photo above (about 6x8 m in size) housed a much smaller generator. It was built early 1933, to reduce operating costs and also to serve a new bombing range that was under construction at the time.

The loop antenna was a small multi-turn "frame coil" loop of about 3x3 m square (10x10 ft, ref. 228M3). The loop was vertically oriented and mounted on a vertical wooden shaft that poked through the roof of the beacon. No other technical details are available regarding this antenna. It rotated with a speed of 1 rpm, i.e., 6° per sec. To ensure accuracy of the system, this speed had to be constant and precise. A "phonic motor" was used to achieve this (ref. 228K). Its concept was invented by Poul la Cour in Denmark in 1885 and patented by him in Britain in 1887. Ref. 228P. It was originally used to synchronize telegraphy and teleprinter systems, as well as J.L. Baird's television system. In essence, it uses a stable electric oscillator to drive a synchronous motor. Since the 1920s, this was implented with a simple electronic audio tone generator. Its signal drives an electromagnet that is coupled to a mechanical tuning fork and continuously excites the fork. Tuning forks can only oscillate at a specific audible frequency (hence "phonic"). The resulting precise, constant fork vibration is captured via capacitive coupling. This signal is then amplified to the required power level for the synchronous AC motor. The fork's frequency was based on the rpm set-point and the number of poles per phase of the AC motor. Deriving an audio frequency from a highly stable and precise source, like a high frequency quarz crystal oscillator, was neither practicable, nor were such oscillators and the necessary frequency dividers available at the time. Per ref. 228L, this drive system maintained the beacon's 1 rpm to within 0.01 sec per rev. (note: this implies a rather amazing 0.02%!). "North" was aligned with True North, not Magnetic North (ref. 228N). In modern aviation, beacons such as VOR (VHF Omi-directional Range) are referenced to Magnetic North, not True North, as local Magnetic North is the only reference direction that can still be determined when all of the aircraft's electrical and vacuum systems have failed. I.e., when only the magnetic "whisk(e)y" compass remains.

Fig. 67: The Orfordness Beacon - with what the antenna might have looked like, and the loop radiation pattern

(source: adapted from Orford_Ness_23, © 2008 Simon James, Creative Commons Attribution-Share Alike 2.0 Generic license)

The beacon transmitted on a long-wave frequency of 288.5 kHz, i.e., a wavelength of about 1040 m. There are no clear references regarding the output power of the transmitter, of which only a small fraction was actually radiated, given the very small size of the loop compared to the wavelength. When operational (ref. 228H1-228H11), the beacon repeated a fixed transmission sequence, starting at the full hour. During the first minute of each sequence, the station call sign (GFP) was repeatedly sent in slow Morse code. Then a continuous tone-modulated carrier signal was sent for five minutes. This was followed by five minutes of silence. Ref. 228H1-228H11, 228K, 228N.

In April of 1930, it was decided to build a second such beacon, at the Royal Aircraft Establishment (RAE) site at Cove near Farnborough/Hampshire (ref. 228H4, 228K). The RAE had a Radio & Navigation Department at Cove. The stated objective was "to test the general utility of this system of direction finding and to ascertain in particular, whether by obtaining bearings from the two beacons [ = triangulation], aircraft can fix their position with sufficient accuracy for practical purposes". The beacon became operational in November of 1930. The Cove/Farnborough beacon transmitted during the five-minute intervals during which the Orfordness beacon was silent.

Fig. 68: Particulars of the two primary beacons and beacon locations

(source: based on ref. 228H4)

An omni-directional "North" signal was transmitted while the "nulls" of the antenna radiation pattern swept through the North and South direction. This signal was a single Morse-code character (see Fig. 68). As soon as the constant tone started after this Morse character, the receiving station started the stopwatch (ref. 228N). The stopwatch was stopped upon subsequent passage of the beam's "null". This is exactly the same method as patented by Meißner in 1912 and implemented with the Telefunken "Compass". See the section above. This is referred to as an "ingenious technique" in ref. 262F (p. 4, pdf p. 18; 1948). In the Telefunken Compass, the rotation was stopped during transmission of the North signal, and that signal was transmitted via a separate, omni-directional antenna. Here, only a directional antenna is used, and it rotates non-stop. So, if the receiver was located close to due north or south of the beacon, the North signal could not be received. Therefore, the beacon also transmitted an "East" signal that could be used instead of the "North" signal. Of course, 90° had to be added to the bearing measurement). The "East" signal was not (and could not) be used to resolve the 180° ambiguity that is caused by the antenna radiation pattern having two diametrically opposed null-directions.

A "sufficiently accurate" stopwatch or other chronograph had to be used. Such a stopwatch could have a special dial "somewhat similar in type to that proposed for use with the Telefunken Compass in 1912" (ref. 228L), see Fig. 68 below. Instead of a stopwatch, a sort of strip-chart or other ondulator recorder could also be used to measure the time between "North" or "East" signal and subsequent "null" passage. Ref. 228M1-228M3.

Fig. 69: Stopwatch dials for use with the rotating beacon

(source: adapted from ref. 228L)

All civil pilots and merchant marine radio operators were invited to use the beacons and report accuracy to the Air Ministry (ref. 228H4). During the initial nine months of operation (ref. 228D), the general conclusion from reports submitted by ships was that accurate bearings were obtainable with "ordinary" receivers at a range of 50-100 miles (presumably nautical miles), and up to 250 miles with "more elaborate" receivers. During the subsequent nine months (ref. 228E, 228F2), about 160 ships submitted reception reports. Participating ships, anchored within 45 miles, recorded an accuracy no worse than 1° compared to true bearings accurately determined by other DF means(themselves typically with 1-2° accuracy). Overall, with "normal modern" receivers (i.e., 1- or 2-tubes/valves), a reliable range on the order of 100 miles was obtained, day and night, with an accuracy no worse than 2° in about 80% of the cases. With slightly degraded-but-workable accuracy, a range of 250 miles was obtained, and up to 500 miles with more sensitive receivers. A few ships reported accurate bearings at ranges from 400 - 900 miles. Ref. 228F2. The German Küstenfunkstelle (coastal radio station) at Cuxhafen and at Norddeich, located at ca. 425 and 525 km northeast of Orford, also provided signal reports. Ref. 228K. As to be expected for a land-based long- or medium-wave beacon, ships also observed a 1-2° shoreline effect (a.k.a. "coastal deviation", "coast refraction") around certain directions. I.e., beam bending towards the shore line. Ref. 228F2, 228N. The RAF performed tests with three bombers in May of 1931, but results were inconclusive. Ref. 228K.

In October of 1934, it was decided to shut down both the Orford and Cove/Tangmere beacon (ref. 228K). From a military point of view, such beacons were considered to have a fatal flaw: they could be hijacked by enemy transmitters. The British revisited this perceived vulnerability when they attemped to bend and "spoof" the German WW2 "Knickebein" beam in 1939/1940. However, it appears that the 1934 decision was rescinded at some point: per Air Ministry Notice to Airmen No. 32 of 1938 (ref. 228H11), the Orfordness beacon was (still or again) active in 1938. Per ref. 228N (1939), the beacons at Orford and Tangmere (though with the Farnborough call sign GFT) were still active in August of 1939. Also, in 1939, there was an experimental rotating loop beacon at the mouth of the China Bakir River in Myanmar (frmr. Burma, under British rule until January of 1948), about 50 km south of Rangoon. It transmitted on 285.7 kHz with the callsign XZP. Other than the call sign and the North signal, "the remaining signals are as for Orfordness Beacon" (ref. 228N).

At the beginning of WW2, the only radio navigation aids at the disposal of the RAF were medium frequency (MF) and high frequency (HF) radio direction finding ground stations, and the Lorenz Standard Beam Approach (SBA) "bad weather" landing-beacon system (ref. 230V, 1945)...

THE EQUISIGNAL BEACON SYSTEM OF OTTO SCHELLER: 1907 - TODAY

Otto Scheller obtained well over 70 patents, primarily while working at the C. Lorenz company in Berlin. Two of his patents have been absolutely fundamental and groundbreaking for radio navigation. They have found widespread application in aircraft radio navigation, from the late 1920s to the present time (!) - both en route and for approach and landing.

Fig. 70: The 1907 and 1916 Scheller patents for course-line radio beacons

Scheller's first patent, German Imperial Patent nr. 201496, dates back to 1907! Keep in mind, both “radio” and "aviation" were still in their early infancy! This patent proposes the following (ref. 229C1):

• Using directional radio means to create a sharply defined line in space. This line is easy to locate, even under poor weather conditions, incl. reduced visibility. This could be used by mobile receiving stations as a position marker or course line for marking shipping lanes.
• Note: in those days, aviation did not yet need the means to navigate other than by simple visual reference to landmarks and man-made objects (a.k.a., "pilotage").
• Such course lines to be generated by two co-located antenna systems, each with a "figure-of-8" directional radiation pattern and operating on the same radio frequency.
• Scheller proposes two pairs of vertical antennas, see Figure 60 below. The paired vertical arrangement is covered by Scheller's patent nr. 192524, also from 1907. It uses a single transmitter source of "undamped" (continuous) waves, inductively coupled to both vertical radiators of the pair.
• This 2-pair antenna configuration was "borrowed" over a decade later by Frank Adcock, as part of his 1919 Direction Finding patent (GB 130,490). Adcock also proposed a configuration with elevated vertical dipoles (not practical for LF/MF/HF frequencies) instead of monopoles, and added compensation/elimination of common-mode signals by adding 180° phase shift between the two antennas of each pair, resulting in a "2 crossing H's" configuration. Likewise, around 1933, the U.S. National Bureau of Standards abandoned pairs of loop antennas (also with a "figure-of-8" directional radiation pattern) in favor of the Scheller arrangement of two pairs of vertical antennas. See further below.
• The two antenna systems to be angled with respect to each other, such that the lobes of their radiation patterns partially overlap, and a narrow common line of same-strength signals is created. This is referred to as the equi-signal beam / zone / course line.
• As the patent drawing above illustrates, this creates not one, but four equi-signal course lines that emanate from the beacon. One pair of these can be narrower than the other.
• The two antenna systems to be energized in a distinct alternating on/off manner. E.g., one transmits “dots” ("•", the letter "E" in Morse code), the other “dashes” ("─", the letter "T"), such that one is always transmitting. I.e., it is an "interlocked" system: there are no pauses between successive
• "E/T" is the simplest combination of distinct interlocking pulses: "E" = "dot ", "•", "T" = "dash", "─". The most widely used complementary Morse code letter combination was A/N ("A" = "dot dash", "• ─", "N" = "dash dot", "─ •").
• Other combinations that were used are F/L ("F" = "dot dot dash dot", "• • ─ •", "L" = "dot dash dot dot", "• ─ • •"); D/U ("D" = "dash dot dot", "─ • •", "U" = "dot dot dash", "• • ─"), K/I ("K" = "dash dot dash", "─ • ─, "I" = "dot dot", "• •"); X/S ("X" = dash dot dot dash", "─ • • ─", "S" = "dot dot dot", "• • •"); O/I ("O" = "dash dash dash", "─ ─ ─", "I" = "dot dot", "• •"); A/I ("A" = "dot dash", "• ─", "I" = "dot dot", "• •"). Ref. 229D, 229R18, 235X4, 254.
• Note: Scheller's patent does not consider sending two different tones instead of two distinct on/off signals - “wireless telephony” transmission with variable tone modulation was still in the very early experimental stage at that time.
• On the equi-signal line, a mobile receiver station will hear a constant sound. When moving away from the equi-signal line, one of the two audio signals will become predominant. This allows detection of course deviation as such, as well as determination of the direction of this deviation (i.e., to the "E" side or to the "T" side of the equi-signal line).
• The direction of the four equi-signal course lines can be changed with respect to each other, by modifying the relative transmission power of the two antenna systems.
• This is expanded by Scheller’s 1916 patent (nr. 299753), in which he proposes to use a radio-goniometer to rotate the entire transmitted 4-course radiation pattern. I.e., without the need to physically rotate the entire antenna system, or move the relative positions of the transmitting antennas. This 1916 patent also mentions the use of a radio-goniometer to make the equi-signal beams wider or narrower.
• Multiple, sharply defined equi-signal lines can be created by changing the relative transmission power of the two antenna systems in a cyclic manner.
• This was later done in the 12-Course Radio Range system in the US, and German World War II multi-beam beacon systems such as “Erika”, “Sonne”, etc.
• If only a single course line is desired, two uni-directional antenna systems should be used, instead of bi-directional ones.
• This concept was later used in all VHF/UHF Instrument Landing Systems (ILS).
• Use a stationary radio receiver station located on the equi-signal line, for monitoring the transmissions.
• Constant monitoring rapidly became standard practice for all radio navigation beacons.

The following plots show the horizontal radiation pattern of two pairs of vertical antennas, placed at the corners of a square, as in Scheller's patent. The polar plot on the left corresponds to the lines A₁1-A₂ and B₁-B₂ crossing at right angles (90°) in the figure above. The plot on the right is for crossing at 45°/135°: two of the overlapping zones are now much narrower, two are much wider.

Fig. 71: Horizontal radiation pattern of the Scheller antenna configuration - for 90°/90° and 45°/135° crossing angles

(the two pairs of associated NEC files of my 4NEC2 antenna simulation model are here, here, here, and here)

The next figure shows the 3D radiation patterns for the same two pairs of vertical antennas, for the 90° crossing-angle case:

Fig. 72: 3D radiation pattern of the Scheller antenna configuration - separately for each antenna pair

(the two associated NEC files of my 4NEC2 model are here and here)

That was 1907. Then… nothing much happened with Scheller's invention for about ten years. In 1917, Franz Kiebitz, while serving in the German kaiserliche Marine (Imperial Navy) during World War I, was the first to build and test Scheller's patented concept with its 2+2 vertical antennas arrangement. Ref. 184L, 229A, 229B, 229G. Rather than using the interlocking Morse characters "E" and "T" as proposed by Scheller, he used the likewise complementary characters "A" and "N". So, it was Kiebitz who made the world's first A/N equi-signal beam system. He tested this system with both ships and aircraft. This confirmed the ability of the equi-signal beam to mark a narrow course line, and allow detection of deviation from this line. With a 150 W tube transmitter and a 32° crossing-angle between the antenna pairs, he obtained an equi-signal beam width of 3° and a reliable range of 130 km (≈80 mi). With two beacons, each with three vertical antennas in an equilateral triangle configuration, he even obtained circular equi-signal lines (ref. 229B). He also observed that errors were introduced by the directional characteristics of a receiving antenna-wire trailing behind the aircraft. Due to the operating wavelength (350-500 m or ≈545-860 kHz medium wave band, ref 229B), such antennas were physically long (e.g., 60 m = 200 ft, ref. 185T). This undesirable effect was confirmed several years later (1923), in the USA (ref. 229E). Kiebitz also observed shore-line effect (beam bending due to land-sea transition) of up to 5°.

During the aftermath of World War I, there was no immediate urge in Germany to continue Kiebitz's activities. Also, the rather French-biased "Peace" Treaty of Versailles, signed 28 June 1919, imposed severe restrictions and reparations. Note that Germany made the final World War I reparation payment to France on 3 October 2010 (!). Part V Section III of the treaty covers "Air Clauses". In particular, Articles 198, 201, and 202 state that the armed forces of Germany must not include any military or naval air forces, and that no dirigibles shall be kept. Furthermore, all military and naval aeronautical material, incl. all aircraft [FD: i.e., dirigibles, airplanes, and seaplanes], whether complete, or being manufactured, repaired, or assembled, are to be handed over to the Governments of the Principal Allied and Associated Powers. Also, for a period of six months, the manufacture and importation of aircraft, parts of aircraft, engines for aircraft, and parts of engines for aircraft, shall be forbidden in all German territory. The implementation of the articles was under control of the Inter-Allied Commissions of Control. It was this CoC (i.e., not the Treaty as such) that subsequently imposed additional orders and rulings during the aforementioned six months period. In particular, the ban on military aircraft production was progressively extended until 5 May of 1922. Performance and other restrictions imposed on civil aircraft and engines actually stimulated the German aviation industry to rapidly become a world leader in aerodynamics, as well as in low-weight engines and constructions (incl. self-supporting structures, such as "stressed skin").

Especially in the USA, long-distance aviation developed at a high pace, both trans-continental airmail service (ref. 235D) and passenger air transport. Hence, the need arose for navigation aids for the growing network of routes between airports - in addition to using landmarks and prominent man-made structures such as railroad lines ("steel-beam navigation"). In particular at night, as the mail service basically worked 24/7. This started in 1919 with bonfires, scattered along the air routes (ref. 229D3, 229F, 229J). Early 1923, the US Post Office Department began to construct a transcontinental airway system with optical beacons (enhanced nautical lighthouses). In 1926, this activity was transferred to the brand new Aeronautics Branch of the US Department of Commerce. The first airway light beacon of the Aeronautics Branch entered service in December of 1927. By 1933, about 1500 optical beacons were in place. A standard beacon station comprised a steel lattice tower (standard sizes from 51 up to 152 ft tall, ≈15-46 m), with a powerful 24 or 36 inch diameter rotating-mirror light (500 W or 1 kW), two 18 inch stationary pencil-beam course lights, and an illuminated windsock. Ref. 229R3-No.15. The color of the rotating and stationary light beams indicated whether the beacon served a landing field, a waypoint between landing fields, or an obstruction. Next to the tower was a shack, marked with airway designators on its roof. At remote sites, it housed two gasoline generators (one on standby), activated by a timer or a photocell. A large concrete course-arrow (ca. 70 ft long, ref. 229R3-No.15) next to the tower also pointed in the direction of the airway. The FAA officially decommisioned the last US federal airway beacon in 1973, near Palm Beach, CA.

Fig. 73: Airway Light Beacons and a 5¢ "Beacon on Rocky Mountains" stamp from July 1928 (airmail field near Sherman/WY)

(source left image: ref. 247; center image: Cibola County Historical Society - Aviation Heritage Museum)

The remainder of this section is currently (January 2021) in the process of being overhauled and significantly expanded on a near-daily basis.

As useful as this "light line" system was, it still required the pilot to have visual contact. During times of reduced visibility (clouds at or below the aircraft altitude, fog, precipitation), no such contact could be established or maintained, or only at close range to a beacon.

Light systems elsewhere, e.g., France and Germany (federal states, Luft Hansa's own network), incl. landing zone projectors, lanterns, smoke pots, etc.

By 1920, the US National Bureau of Standards (NBS) was seriously involved in R&D regarding "electron" tubes (vacuum tubes, thermionic valves) and radio. The NBS was an agency of the US Department of Commerce, and renamed National Institute of Standards & Technology (NIST) in 1988. This work included cooperation with the Bureau of Lighthouses for a radio-based fog signal system. I.e., just like their German counterpart over 10 years before, see the "Radio Compass" section above. Furthermore, the NBS developed automatic radio transmitter sets for lighthouses, radio compasses, radio direction finders, and the renowned broadcast radio station WWV. This station started in May of 1920 and changed in 1923 to transmission of accurate reference time signals on standard longwave and shortwave frequencies. It is active to this day. Ref 229D12-229D15. Around that time, Percival D. Lowell and Francis W. Dunmore of the NBS worked on loop antennas, designed vacuum tube amplifiers and ship-ship and ship-shore radio communication systems. Ref. 229D14. They also appear to have been co-owners of the Radio Instrument Co., to whom the NBS outsourced work. In 1920, Lowell arrived at the same idea as patented by Scheller in 1907: use overlapping radio beams to create pairs of fixed-direction equi-signal beams. His idea was not pursued at the NBS until another two years later.

Fig. 7X: "Some degree" of similarity.......

(source: German patent 201496 and US patent 2172365)

By the latter half of the 1920s, it became clear that.... Ref. 229D1-229D15, 229E1, 229F, 229X.

• In US, contrary to Europe, RadNav R&D almost exclusively by various branches of the federal government, instead of private industry.
• US developments reinvented Schellers system, but US Army Air was fully aware of his patents when they took over ca 1926.
• Four-Course Range system, Low-frequency Radio Range (LFR) ; a characteristic of the low-frequency range was the Cone of Silence immediately above the station (cf. 3D radiation pattern in Fig. 62 above).
• Ref. 185F: 22 A/N "signals" [ = characters] per minute [TBC: 22x A + 22x N or 11+11]; interrupted every 15 min for station identification by voice from the omni-directional co-located radiotelephone station.
• Here, the word "range" is used in the sense of area of open land, e.g., a site for testing equipment. I.e., not in the sense of "distance" (as in the acronym "radar"). Radio ranges do not provide proper distance information, though strength of the received signals (relative field strength) provides some indication of relative distance.
• "A" / "N quadrant (clean A/N); "A"/"N" twilight zones = bi-signal zone; on-course / equisignal zone; "cone of silence" [add illustration, w/wo "fill in" beacon] over the beacon range ca 100 miles, 1:60 rule of thumb (180/pi=57.xxx) 1° triangle --> 1 mile lateral for every 60 miles distance, so a 3°-wide equisignal zone is roughly 3x1=3 miles wide at 60 miles from the beacon, or about 5 miles (4+ NM) at the 100 miles range limit, more difficult to accurately track to course line
• Tests by/at Bureau of Air Commerce, Army:
• 1923, at National Bureau of Standards Washington/DC: two 1-turn loops, 43.75x15.25m/150x50ft, 36.5°/143.5° crossing angle, 2 kW quenched spark transmitter, A/N, loops tuned to 300 kHz; ref. 229E1.
• McCook field @ Dayton , ref. 229N.
• Wilbur Wright field (Wright Patterson), ref. 229N.
• Typically, multiple airways that lead to/from the same city/airfield/intersection do not cross at right ( = 90°) angles. Also, airway courses are typically also not aligned with north/south and east/west. Therefore, the courses of a Radio Range are typically rotated simultaneously, and bent or shifted (ref. 229Q, pp. 36-43):
• "course rotation": changing the all courses of a Radio Range by the same amount and in the same direction, i.e., without changing the angles between the course-pairs. (as proposed in Ottos Scheller's patent - most elegantly done with a radio goniometer).
• "course bending": changing the angle between the two opposite courses of a course-pair, from their normal 180° angle. Typ. by no more than 30°, i.e., to 150-210°.
• "course shifting": changing the angle between the two course-pairs of a radio range from the standard 90°+90°. Typ. by no more than 30°, i.e., to 60°+120°.
• The Aeronautics Branch standardized a type of 4-Course Radio Range during the course of 1928.
• Loop type ranges vs tower (simultaneous voice) type ranges, incl. (dis)advantages: p. 13 ref 229W5.
• Visual-Aural Range system (VAR), 4-course beacon; a 4-course range, comprising a 2-course Aural Range and 2-course Visual Range; "visual", as it provided on-beam/deviation to the pilot via an indicator instrument, rather than via sound on his headphones.
• Per ref 185F pp. 37ff: constant tones of diff audio modulation freq, instead of A/N keying. 65 & 86.7Hz (originally 60 & 85 Hz, but abandoned due to....) 86⅔, per ref. 229R4-No.6; originally ) and 75 & 100 Hz. Marker beacons transmitting ID code sigs , primarily to ID intersections of courses from adjacent ranges, "double frequency" marker beacons, alternating between the two. Single -freq beacons used to mark emergency landing fields, abrupt terrain elevation changes, dangerous landmarks; 5 miles max range. Marker beacon freq same as associated Radio Range freq. Marker stations also low power radiophone station, for emergency communications or emergency WX broadcast. Air nav facilities operated on freqs 237-285 kHz (LW) and 315-350 kHz (MW), with 6 kHz channel spacing.
• one Visual Range system with two course lines (150 Hz and 90 Hz tones, visual indicator in the cockpit, full meter needle deflection for ≥ 10° off-beam deviation)
• one Aural Range system with 2 course lines (1020 Hz A/N system); equi-signal beam appr. 1½-2° wide.
• First demonstrated in 1937 by the Bureau of Air Commerce (VHF, 63 MHz), operational in the US from 1944 - 1960 (VHF, 112-118 MHz).. Also used in Australia, operational 1947 to at least 1980
• Lorenz' 1938 Swiss patent 206464: Motorized rotating antenna arrangement of 2 pairs of vertical antennas (grounded monopoles or dipoles) at corners of a square, Adcock arrangement, simultaneously fed by transmitter via , central vertical monopole, fed simultaneously by same transmitter; creates rotating equi-signal beams; using shortwave to obtain long range.
• Lorenz A/N AFF, "blind landing system"
• adopted in Britain via Lorenz/ITT as the Standard Beam Approach System (SBAS) - indeed, it was.
• Using a separate glide path/slope equisignal beacon was patented by Ernst Kramar (Lorenz) in 1937 (German patent 734130, and equivalent 1939 Kramar/Hahnemann (Lorenz) US patent 2210664). A variation with a separate Lorenz beacon abeam the touchdown zone was patented in 1940 by Andrew Alford (IT&T, parent company of Lorenz, US patent 2294882).
• ILS (Localiser & Glideslope), SCR51: by ITT, parent company of Lorenz.

"Such equisignal course lines are established by the radiation of directional fields, each of which is identified by a characteristic tone modulation (AM) or signal (keying rythm), and which fields overlap or intersect in space to produce zones of equal signal intensity, which provides the beacon course [or courses]"

Fig. XX: Rotable square loops, hinged on telegraph poles at "compass house" near the Radio Building of the Bureau of Standards

(sources: Fig. 4 in ref. 229E1 (image left), p. 151 in ref. 229D15 (image right); note rotable RDF loop antenna on the roof)

Next Figure: the experimental Aural Radio Beacon station at College Park/MD/USA.College Park is located about 13 km (8 miles) eastnortheast of the Bureau of Standards in Washington/DC, and about the same distance northeast of the White House. Triangular antennas in two crossing vertical planes [crossing vertical Delta-loops], replacing the square loop "coil" antennas. Goniometer was housed at the base of the 70-foot wooden tower. From 1918-1934, the National Bureau of Standards (NBS), the U.S. Navy, the Post Office, and other government and private organizations regularly used College Park Airport to design and test "blind flight" systems. By 1930, a very similar station was installed at Mitchel Field, on Long Island/NY, just outside Hampstead and Garden City (not to be confused with Mitchell Field); ref. 235U3 (p. 24).

Fig. XX: The experimental "Directive Aural Radio Beacon" station of the Bureau of Standards at College Park/MD/USA - ca. 1930

(source image: adapted from Fig. 6 in ref. 229V and Fig. 2 in ref. 229D3; also see Fig. 19 in ref. 229L8 & p. 154 in ref. 229D15)

The wooden tower of the beacon at College Park was 70 ft tall (≈21 m) and painted dark yellow. At the base it was 10 ft wide. The radio room measured 10x14 ft (≈3.3x4.3 m). The two orthogonal single-turn triangular loop antennas spanned ca. 2x150=300 ft (≈90 m). Total wire length per loop was 628 ft (≈190 m). The horizontal wires that ran back to the radio room, were suspended 8 ft above ground by three posts on each side of the mast. Ref. 229Y.

Fig. XX: The inside of the radio room of the beacon at College Park/MD/USA - official inauguration on 10 May 1927

(source: adapted from ref. 229V3; the person in the photo is George K. Burgess, Director of the Bureau of Standards)

Fig. 74: The 2-loop "polydirectional" 4-Course Range at Wayne County Airport near Detroit/Michigan/USA

(source: Fig. 11 in ref. 185F; also Fig. 30 in ref. 229L14; loop dimensions per ref. 229L14; loops at right angles)

Fig. xx: Four-course patterns of the Scheller-array for various parameter settings

(source: adapted from ref. 229M)

Fig.XX: Aerial view of the Four-Course Radio Range at Elizabeth City NC/USA

(source: Fig. 3 in ref. 229W3 (1940) & 229W4)

"Radio range" beacon : a directional radio beacon that transmits in such a way as to mark out a fixed straight line / provide radio-marked courses (as for directing the course of airplanes to or from a landing field). Here, the word "range" has nothing to do with "distance".

Types RA, RL, MRA, MRL, ML (ref. 229R10).

The following audio clip is the realistic simulation (incl. keyclick suppression) of the receiver sound that would be heard when being on the "A" side of the equibeam of a Four-Course Radio Range AN-beacon. The "N" signal is also heard, but much weaker. As it is complementary to the keying of the "A" signal, it sounds like a continuous background signal. In the middle of the clip, there is a 3-letter Morse code identification (here: ABC), transmitted sequentially on the "A" and "N" beam. Tone frequency is 1020 Hz.

Audio simulation for the A side of an LF Four-Course Radio Range, while also receiving the (weak) N-beam signal

(source: "LF Range Navigation Sound 70% "A"" © Bob Denny, accessed 27 March 2020)

Simulated sound of crossing an A/N beam back & forth and approach beacons - TBD!!!

(source: © ipse)

Fig. XX: Growth of the Radio Range network in the Contiguous USA - 1932, 1935, 1940, 1944

(sources: Fig. 1 in ref. 235P5, Fig. 31 in ref. 229W1, Fig. 46 in ref. 229W2, Fig. 72 in ref. 229W3, Fig. 147 in ref. 229W4)

"Visual-type": vibrating tuned reeds, placed side-by-side, and tipped with a white metal strip about 3/32 x 5/32 inch (≈ 2.4x4 mm). Ref. 229D1, 229D9, 229D10, 229D18, 229D26, 229L13. NBS tested a "vibrating reed" visual radio beacon indicator in 1927.

Fig. 75A: The tuned-reed course-deviation indicator - construction details

(sources: left image: ref. 229V4, 1928; center: ref. 229R2-No4; right: adapted from ref. 229R1-No22)

"When [the] receiving two tones, the reeds vibrate and move the tips in rapid vertical vibrations, forming what appears as two "ribbons" and varying in amplitude according to the strength of the received signals. When the aircraft is "on course", both ribbons or "reeds" are of equal amplitude. If the aircraft moves "off course" , - "longest reed shows side off course" - head A/C towards in direction of the shorter reed to get back on "on course" " [iff flying TO!!!]. relative length diff is measure for cross-track error, ref. Instrument has a TO/FROM "reversing switch", to ensure deflection of the pointer is in the same direction as deviation of the aircraft from the course. Ref. 229R4-No.06

Fig. 75B: The tuned-reed course-deviation indicator for the 2-Course Visual Range

(source: adapted from ref. 229V; note: reed indication is independent of airplane heading / nose pointing direction!)

The 4-course system can be expanded to a 12-course system, not by adding a third loop antenna with the figure-of-eight radiation pattern (or a thrird vertical antenna pair) instead of two, but by coupling a third transmitter chain to the existing loop pair. ref. 229R1-No.4, 229D1, 229D15 (p. 157): antenna system still comprises two crossing loops, as with the 4-course system, but goniometer with 3 stator coils spaced 120°, one connected to each PA of the TX (now 3 PA's and modulators), 2-coil rotor as before. Tested at College Park/MD. Shifting courses per 4-course beacon method, or - less complicated in the 12-crs system - displacing the stator windings from their normal 120° positions.

Rather than Visual Range with its two tones (65 Hz & 86⅔ Hz), now three different constant modulation tones were used: added 108⅓ Hz. Clearly, the deviation indicator and associated electronics also had to be adapted, ref. 229D1, 229D9, 229L7. E.g., reed instrument with 3 reeds (since 3 audio freqs), but 4 tabs, such that any pair of freq could be selected by sliding shutter/mask. So, always selecting one of 3 4-courses (red/black, green/amber, blue/brown). Actual airway beacon(s) actually ever implemented? No!

Fig. 75C: The 12-course radio range pattern and prototype TO/FROM rotable reed instrument

(source left image: adapted from Fig. 3 in ref. 229D9; right image: adapted from 229V4, 1928)

Fig. 75D: The 3-reed cockpit instrument for the 12-course radio range

(source: adapted from Fig. 1 & 2 in ref. 229D9 and Fig. 25 in 229D3)

Fig. XX: Blind Landing System - instrument panel of test airplane of the National Bureau of Standards, 1930

(source: "Blind Landing of Aircraft collection" of National Institute of Standards and Technology (NIST) Digital Archives)

WW2: also US mobile (truck mounted 4+1 "TL" antenna config) military VHF (100-156 MHz) 2-CRS A/N aural radio range system for ldg field or waypoint marking, w periodic sector ident via D/U keying, w simultaneous voice capability (e.g., AN/MRN-2, p. 38 in ref. 230U2), with

Fig. XX: Experimental VHF Glide Path antennas - Bureau of Standards (left) and, "inspired" by it, of the German DVL (right)

(source left image: ref. 235Y4 (1930, 100 MHz, dipole + reflector + 6 directors); right image: ref. 2 (1931, 63-64 MHz, 5 directors))

LW/MW "Marconi" range in Australia.

Rotating beam system - stationary antennas. Ca. 1928, the C. Lorenz company in Berlin began to use the 1907+1916 "Scheller" patents, which they owned. Interesting aspect: alternatingly connecting the transmitter to the two input coils of the motorized radio goniometer was done with two switches, iron-powder toroidal transformer cores ("Pungs Drossel), each with a DC-powered control winding, driving the core into saturation, causing a high series impedance to the transmitter signal ("Tastdrosselverfahren". lit. "choke-coil keying method"). "Magnetic-bias keying", ref. 229N. A/N sequence. 4-course beacon. Rotable, not rotating. Before NBS in USA. Lorenz test site at Versuchsfunkstelle Eberswalde (on the Finow canal, about 48 km, 30 miles, northwest of down-town Berlin). Freq: 385 kHz, long-wave 780 m wavelength, 800 W transmitter power. In 1931/32, goniometer motorized, to get a rotating beacons. "Umlaufende Richtfunkbake Eberswalde". Range ca. 350 km. But two crossing loops antenna system, instead of Scheller's 2 pairs of vertical antennas (copied by Adcock), hence, limited use due to sky wave night-effect. Ref. 2. Note: same approach with "DC transformer" / "saturating transformer" was standard high-power light dimmer device for (movie) theaters etc. for many decades.

In 1913/1914 Leo Pungs and Felix Gerth (both at C. Lorenz AG) developed the first practical and satisfactory method for amplitude modulating the RF antenna current of high power transmitters with voice and music. It used a choking coil on a closed laminated iron core. The series-impedance of that coil was controlled by varying the magnetic saturation level of the core via the DC current through a secondary coil on the same core. This was referred to as a "telephony choke" or "Pungs choke" ("Steuerdrossel", "Pungs-Drossel"). The concept was originally proposed by Reginald Fessenden around 1902, who never got it to work properly. In 1913 Ludwig Kühn of the Dr. E.F. Huth company in Berlin revived the method (cf. 1923 US patent nr. 1653859). By hard-switching between zero and full saturation, this type of choke coil could also be used as an on/off telegraphy keying-choke ("Tastdrossel"), i.e., as an RF switch, instead of an AM modulating choke.

Fig. 76: The experimental Lorenz long-wave rotable/rotating four-course A/N beacon at Eberswalde/Germany

(source: adapted from ref. 2)

Fig. 76: The experimental Lorenz long-wave rotable/rotating four-course A/N beacon at Eberswalde

(source: ref. 2)

In 1930, de C. Lorenz AG company was acquired by Standard Elektrizitätsgesellschaft, a subsidiary of the US American International Telephone and Telegraph Corporation (ITT, also IT&T), from the Dutch firm N.V. Philips' Gloeilampenfabrieken, who owned 98% of Lorenz shares by 1929. Ref. 263A-263C. ITT was created by the Puerto Rico Telephone Company (Ricotelco) in 1920. From 1922 through 1925, ITT acquired all overseas subsidiaries of Western Electric, and a number of European telephone companies through its subsidiary C. Lorenz AG. This included Standard Telephones & Cables Ltd (STC) in Britain, Standard Elektrik Lorenz (SEL) in Germany, Bell Telephone Manufacturing (BTM) in Belgium, and Compagnie Générale de Constructions Téléphoniques (CGCT) in France. The A.E.G. Telefunken company also had affiliations with a major US American conglomerate: International General Electric (IGE). Their facilities were not bombed during WW2, other than accidentally. They were actually on American "do-not-bomb" lists, as were e.g., the Ford Motor Co. facilities. Note that Siemens (as was Brown Boveri) had no close ties with US companies. Their production sites where the specific target of Allied bombing raids. Ref. 8.

During 1932/33, Ernst Kramar of the Lorenz company applied the concept of the Lorenz-Scheller A/N-system to a "blind landing system" for aircraft. Ref. 28, 188, 235C2, 235C3. Note that "blind landing" [or, more generally, "flying" = "solely by reference to instruments"] is somewhat of a misnomer, as the system did not provide precision vertical guidance down to the actual touch-down of the landing and subsequent roll-out. Hence, it is only an approach-beacon (D: "Ansteuerungsfunkfeuer", AFF). These days, we would refer to this beacon as a non-precision "localizer" approach system: the horizontal ( = lateral) component of an Instrument Landing System (ILS).

As discussed above and shown in Fig. 60/61, the ground-station of the Scheller system had a radiation pattern with four main lobes, in fixed orthogonal directions. See Figure 41A. Two of the lobes transmitted the Morse code letter "A", the other two the letter "N". Where lobes overlap and are of equal strength, the combination of "A" and "N" results in a constant tone signal, the so-called "equi-signal". This signal had a beam width of about 1-5°. This was the first "A/N" system, later used in several other Lorenz radio-navigation systems. Subsequent variations of this scheme used narrow "A" and "N" beams, with a much narrower overlap, allowing more accurate determination of the course line of the equi-signal. In the 1907 Scheller patent, the directional radiation patterns are obtained with four equidistant vertical antennas.

A-zone (≈10-15°), bi-signal / "twilight zone" (≈2x15°, in one half A dominates, N in the other), equi-signal/on-course zone (≈1-5°), N-zone (≈10-15°). For visual-type radio range beacons, the 65 Hz tone beam corresponds to "A" and 86⅔ Hz to "N". Ref. 229R7-No. 6.

The antenna system is very simple: a vertical exciter dipole of standard length (½ λ), with a vertical reflector to the left and to the right. See Figure 77. This was patented in 1932 by Ernst Kramar of the C. Lorenz AG company in Berlin (Reichspatent 577350, British patent 405727). The dipole is excited continously by the transmitter. The reflectors are completely passive: they are never connected to a transmitter. Each reflector can be "opened" at its mid-point with a relay. This reconfigures the reflector into two unconnected half-length rods - much too short to affect the radiation pattern of the active dipole. The two relais are energized in a interlocked fashion: when the contact of the Relay 1 is open, the contact of Relay 2 is closed, and vice versa. This makes it very easy to implement complementary keying (E/T, A/N, etc.).

Ref. 185H, p. 12 ff. 1932 flight tests at Berlin-Tempelhof.

Ref. 235Y4, The constant-intensity glide path proposed in 1929, BoS/Diamond & Dunmore.

Lorenz' Swiss patent 206463  : standard Lorenz landing beam arrangement with only a single switched refelector.

Fig. 77: The antenna arrangement of the Lorenz-Scheller E/T localizer beam ("Lorenz Beam")

(source: ref. 31)

The patents covers a distance of 0.2 - 0.5 λ between dipole and reflectors, which primarily affecting sharpness of the beam. The patent also considers reflector length shorter/same as/longer than the dipol. This primarily affects side lobes. For a parallel rod to work effectively as a reflector, its electrical length must typically be within 5-10% of length of the dipole.

Fig. 78: Radiation pattern of a vertical dipole with one reflector to the left of it

(cases similar to those covered by Ernst Kramar's patents RP577350 & GB405727; note: radiation patterns are for "free space" case = without ground)

Fig. 79: The beam pattern of the Lorenz beam - simulated vs British patent 405727

(source: ref. 31)

The signal transmitted by the dipole induces current into the parallel reflector. In turn, this induced current causes the reflector to (re)radiate. This radiation combines with that of the dipole. Depending on the distance ( = phase) between dipole and reflector, the strength of the dipole radiation is decreased in directions behind the reflector, and increased on the opposite side of the dipole. I.e., the radiation pattern of the dipole is no longer omni-directional. Basically "vertical 2-element beam" antenna. How the antenna works. The radio waves from each element are emitted with a phase delay [physical distance + inductive-lag=long=reflector/capacitive-lead=short=director], so that the individual waves emitted in the forward direction are in phase, while the waves in the reverse direction are out of phase. Therefore, the forward waves add together, (constructive interference) enhancing the power in that direction (constructive interference / EM wave combination)), while the backward waves partially cancel each other (destructive interference), thereby reducing the power emitted in that direction. At other angles, the power emitted is intermediate between the two extremes.

Two overlapping "Scheller" beams with equisignal zone:

1. Complementary keying with same tone frequency + aural assessment of audio signals and equisignal. No viusal indicator.
2. Same, with additional visual indication with a galvanometer instrument with needle that "kicks" to left or right in the rhythm of pos & neg induction pulses that are derived from the leading / trailing edges of the keyed tone pulses, with the inductance of a transformer; hence, impossible to make accurate reading of needle deflection and also requires simple dot/dash keying patterns.
3. Both beams transmitting continously, each modulated with a different tone frequency + visual indication of the relative signal strength of the received tones; no aural assessment possible (e.g., when pilot performing other tasks). Indicator can be tuned reeds, or galvanometer needle-instrument, with summed rectified demodulated tones, one of which with inverted sign.
4. Combination of 1 & 3: complementary keying with two tones + aural of tone pulses + visual of the relative strength of those pulses. Aural & visual indications cannot be guaranteed to be consistent.
5. TBC: like 2., but with with galvanometer needle-instrument instead of kicking meter, TBC conversion of tone or inductive pulses. Patent?

In the laboratories at Tohoku Imperial University, beginning in 1924, Professor Hidetsugu Yagi and his assistant, Shintaro Uda, designed and constructed a sensitive and highly-directional antenna system using closely-coupled parasitic elements. The antenna system, using a driven element with closely coupled parasitics (usually a reflector and one or more directors) for short-wave work, was first described by S. Uda, a professor at Tohuku University in Japan, in 1926, in the IEEJ (Japan). Associated patents filed in Japan [69115], the USA [1860123], and Germany [475293] that same year - only bear Yagi's name.... -------- VS Marconi et al parabolic reflector screen 1919 [US patent nr. 1301473]

Kramar's 1937 patents expand this scheme with a complementary-keyed (e.g., E/T) beam system for vertical guidance. This [the latter?] was simply re-patented in 1940 in the USA by others (e.g.,...ITT?)

front course, back course - revert L/R mentally or switch instrument of switch beam keying when active crs changed.

Insrtument localizer: front/forward course beam that enables approaching aircraft to establish lateral alignment with the runway / runway centerline.

In 1934/35, Telefunken developed their version of the Lorenz AFF/VEZ/HEZ "Landeleitstrahlanlage" system (ref. 2, 235A), to Lorenz specifications.

Fig. 79: Telefunken and Lorenz localizer-beacon ground stations

(sources: ref. 2 & 235A (left, Telefunken), ref. 31 (center), ref. 137B / 225C2 / 235P26 (Lorenz, at Berlin-Tempelhof airport))

Fig. 80: 1937 Lorenz beacons - left & center: at Zürich-Dübendorf/Switzerland airport, right: at Heston/Middlesex/UK aerodrome

(sources: ref. 235B (left), ref. 137B (center), ref. 235P6 (right))

Fig. XX: Typical dimensions of a Lorenz beam ground station

Fig. 81: Lorenz VHF marker beacons - horizontal dipole above a "chicken wire" ground screen and a transmitter "dog house"

(sources: ref. 254 (left image; also ref. 235Q), Fig. 7 in ref. 235E)

Fig. 82: Lorenz and AEG/Telefunken VHF marker beacon

(source: ref. 235F (left), ref. 2 (Telefunken))

The two reflector dipoles were activated alternatingly, to deform the dipole beam slightly to the right and to the left. This effectively created a directional beacon ("Richtfunkfeuer") with a twin-beam radiation pattern. At the centerline of the beams (aligned with the centerline of the runway), the "E" and "T" beams would merge into an 1150 Hz equi-signal zone that had an aperture of about 5 degrees. The antenna system was located at the far end ( = departure end) of the runway, so as to provide left/right guidance throughout the entire approach, landing, and roll-out. During approach to landing, the arriving aircraft would intercept and track the equi-signal beam. The beam-system operated at frequencies in the 30 - 36.2 MHz range (λ ≈ 10 m). The pilot would hear the E/T audio signals, and also have a Left/Right course deviation indicator. At two fixed distances from the runway, a marker-beacon ("Einflugzeichenbake", EFZ-Bake) was installed. An Outer Marker ("Vor-EFZ") at 3 km, and a Main Marker ("Haupt-EFZ") at 300 m, ref. 32. These beacons transmitted on 38 MHz, with a narrow upwardly pointing fan-beam, extending across the approach course and at right angles to it. This allowed the pilot to determine when to initiate descent to the runway from a standard altitude and with a standard descent rate (3 degrees flight path). Ref. 26B, 235L1-235L5. This "Lorenz beam" system entered service in 1934 with the German national carrier, Deutsche Luft Hansa (a 1926 merger of Deutsche Aero Lloyd and Junkers Luftverkehr, "Luft Hansa" became "Lufthansa" at its post-WW2 re-start in 1953). It was then commercialized worldwide.

Fig. XX: Schematic depiction of the Lorenz "bad weather" landing procedure

(source: adapted from ref. 31)

"Funknavigationsanzeiger" of „Lorenz-Blindlandungs-Empfangsanlage für Flugzeuge“

Fig. XX: Ca. 1936 beam approach indicators made by C. Lorenz in Berlin and a WW2 Type 3 Mod. S-47 by Sangamo Weston Ltd.

(sources - left to right: ref. 235C3 (also 235P4/P26); ref. 235Y4 (also 229L20, 235Q, 235P7/P8/P18, 254); aeronautique.com (accessed August 2020))

Fig. XX above: in January of 1938, Sangamo Weston Ltd. received a contract from the British government to manufacture a copy of the Lorenz Beam Approach Indicator, ref. 235Z, 235P17.

To have same interpretation of left/right meter deflection on both of the two opposite-direction equi-signal course lines, Lorenz' 1935 patent 180996 proposes to install outer & inner marker beacons on both front- and back-course, but inversed the keying of the reflectors, based on which of these two courses is in use.

Fig. XX: Overview of the intertwined history of the Lorenz, ITT, and STC companies in volved with ILS

(note: this overview is quite simplified, e.g., 1960 to 1977, ITT acquired more than 350 companies)

Note the A/N Radio Ranges in the USA were "aural" only.

1937: Approximately 35 Lorenz ground equipments have been installed in various parts of the world, 14 of which in Germany, and about 200 receivers installed in planes engaged in air transportation in various countries. Ref. 229L20. By 1938, some 38 of these beacons were installed at airports throughout the German Reich. The above beacon provides lateral ( = horizontal, left/right) guidance. In 1937, Lorenz/Kramar created a separate system for providing vertical approach-to-landing guidance, by turning the antenna system 90 degrees and placing it next to the runway, abeam the touch-down point. The combined system with lateral- and vertical-guidance beams is called Instrument Landing System (ILS). It is used to this day. For a general treatise of such beam systems by Ernst Kramar himself, see ref. 254 (1938).

In 1936/37, Lorenz installed its beam systems at three aerodromes around London: Croydon, Heston, and Gatwick. Sept 1936: operational at Heston, decided to proceed with installation at Croydon, order placed for installation at Gatwick; all installed by STC. Ref. 235B2, 235P6:

In response to the need for long-range radio navigation, the Lorenz company investigated and developed a long-range course beacon, based on its standard VHF landing beacon system. Throughout the 1930s, propagation of VHF radio waves (frequencies 30-300 MHz) beyond visual range had already been extensively investigated and reported in publications in the USA and Germany (e.g. by the "DVL Deutsche Versuchsanstalt für Luftfahrt" (DVL, German Aeronautics Research Institute), ref. 229L21 and publications referenced therein). 1937 Lorenz tests at Essendon (installed by its ITT sister-company STC Pty. Ltd.), standard Lorenz beam antenna config (i.e., dipole+2reflectors), 30 m wooden tower, with 500 W transmiter, 9 m wavelength [European standard 33.3 MHz beacon freq]. Results so favorable that decided to plan intro in AUS of a network of VHF beacons instead of long-wave beacons, potentially with an additional reflector pair to create VHF 4-course beacons.

Lorenz UKW Bake installed at Essendon Airport (Melbourne/Australia) by Lorenz (p. 96, 97 in ref. 2), 1937; Kastrup/Denmark, 1937, Malmi-Helsinki 1937 (see advert-TFK-AEG-ILS-Finland-Aero-Vol17-193709.jpg).

Fig. XX: Lorenz Radio Range beacons in Australia (left to right) - Essendon ca. 1938, Nhill Aeradio Station ca. 1939, Seymour 1944

(source: Civil Aviation Historical Society & Airways Museum/Australia; left-to-right: Essendon (EN), Nhill (NHL), Seymour (SYR))

Fig. XX: Advertising for Lorenz beacon equipment by its ITT sister company STC Pty. Ltd. in Australia, ca. 1939

(source: ref. 229P2)

Mobile Luftwaffe version, using same transmitters as the civil version. However, the antennas of the main and the marker beacons are different. The main beacon antenna (the standard "vertical dipole + two switched reflectors" configuration) is scaled down by about 25%, i.e., resonant around 43 MHz instead of 31 MHz. Three variometers are used to adapt them to the 30-33.3 MHz transmitter frequency. For the equisignal beacon, either the 120 watt "AS 2" transmitter was used, or the 500 watt "AS 4".

Fig. XX: "UKW-Landefunkfeuer 120 Watt und 500 Watt" - mobile Lorenz landing beam beacon system of the Luftwaffe

(sources: adapted from ref. 39B-39G)

Introduced into Luftwaffe in 1933? LFF vs Jagd JFFF, see ref. 6F.

British Royal Air Force - Standard Beam Approach ground Installations (ref. 235V1):

• Fixed Ground Radio Installation type 5069 (F.G.R.I.5069):
• Transportable Ground Radio Installation type 5041 (T.G.R.I.5041):

The F.G.R.I.5069 comprises a main beacon transmitter of type T.1345, with the actual radio transmitter equipment installed in a hut. The T.G.R.I.5041 comprised a main beacon transmitter type T.1254, with the equipment installed in a "radio vehicle" trailer. The installation generated 500 watt "aerial power", which presumably refers to radiated power, not transmitter output power. The crystal-controlled operating frequency was in the range of 30.5-40.5 MHz, with standard Lorenz 1150 Hz modulation. The installation consumed 4-5 kW of supply power (220/250 volt, 50 Hz). Power was supplied via a burried cable from the airfield main supply, or by a diesel-electric generator set. A "Type 2" automatic keying device was used with both types of G.R.I. (ref. 235V2). This device comprises two motorized cam discs. The "interval cam disc" interrupted the normal beacon keying every 1, 2½, or 5 minutes for a period of 5 sec max. During this interval, the "code cam disc" keyed the 2-letter Morse code beacon identification, with 1/6 sec dot-length. During ID transmission, both reflectors of the antenna system were disabled, so as to obtain an omni-directional radiation pattern. There was a possible option to operate the main beacon with either E/T keying (dot/dash, Lorenz standard 1/8 sec and 7/8 sec respectively), or A/N keying. The T.1345 and T.1254 superseded the type T.1122. All transmitter types include the associated antennas, cables, etc. A remote control unit and a monitoring arrangement was located in the airfield's control room. It provided switching devices, transmitter status indication, and control of obstruction lights. Associated aircraft receivers are the R.1124A and R.1125A (ref. 235V3, 235V4).

Fig. XX: The "T.R.G.I. 5041" - SBA main beacon trailer and antennas

(source: adapted from ref. 235V1)

Both the fixed and the transportable G.R.I. include two marker beacon transmitters of type T.1295, basically a type T.1123 with an improved radiation pattern. The transmitter generated 5 watt "aerial power" on 38 MHz, with standard Lorenz keyed-tone modulation of 700 Hz (Outer Marker) or 1700 Hz (Inner Marker). The transmitter consumed 150 watt of supply power (220/250 volt, 50Hz). Power was supplied via a burried cable from the airfield main supply, or by a petrol (US: gasoline) generator set. This transmitter system includes two pairs of identical horizontal ½λ dipole antennas. The dipoles of each pair are spaced by ½λ, and fed in-phase. For the Inner Marker, they are installed in parallel, on either side of the extended approach path / runway center line. For the Outer Marker, they are installed end-to-end in-line (a.k.a. collinear) and on the approach path center line.

AP1186 §104: "The aeroplane should be flown at a constant air speed directly across the beam at a distance of 20 miles from the transmitter and at a height of about 3000 feet."

Fig. XX: "T.1295" - SBA outer marker beacon (left) and inner marker beacon transmitter installations

(source: adapted from ref. 235V1)

The antenna installation of any Lorenz main beacon system transmits a pair of overlapping beams in two opposite directions: the normal approach (a.k.a. "front-course") and the reciprocal approach (a.k.a. "back-course"). The standard pair of marker beacons is placed on the front-course. Inherent to the particular antenna system, the left/right keying on the back-course is reversed compared to the front-course. To be able to use the back-course as front-course, there was a provision to be able to swap the keying of the overlapping beams. In these cases, the F.G.R.I.5069 or T.G.R.I.5041 included a second T.1295 pair of marker beacons, placed on the normal back-course line.

"T.R.G.I. 5041" vs "T.U. 3" main beacon installation + "M.U. 3" marker beacon installation (ref. 235P14)

Set, Complete, System (not "Signal Corps System"), model no. 51 ( = SCS-51): TBC designed by ITT Federal Telephone & Radio Corp./Labs, manufactured by ITT Federal Mfg. & Eng. Co.?? ITT being parent company of the Lorenz AG. comprising the following system components [add family tree!]:

• Localizer:
• AN/MRN-1 (introduced mid 1942): VHF (108.3-110.3 MHz, 25 W), Basic Component BC-751 localizer transmitter (w BC-752 90/150 Hz modulators & RF bridge) + monitors (BC-753 (fixed) or BC-754 (portable) course detector + BC-755 filed intensity meter + BC-777 indicator alarm; range 40/70/100 miles at 2500/6000/10000 ft altitude; 5 Alford loop antennas (LP-24, DF Loop for RC-107 & RC-109 U/W MC-528 & MP-79-A) in same horizontal plane; replaces SCR-241;  installed in a K-53 truck.
• AN/CRN-3 (same, but without truck = fixed), transportable, radio equipment in a tent.
• AN/CRN-10 (same, with radio equipment on small "V6" (?) trailer and antenna array on simple support structure.#
• Vs. version with 5/6/7 antennas; with/without anti back-course reflector MC-528, antenna reflector (comprising 2 reflector screens: Z-2004, Z-2005), [optional] part of antenna equipment RC-109 (Loop LP-24-A, Mast MA-5-A, mounting base MP-79-A, various cables)).
• Aircraft receiver: RC-103. Cross-pointer indicator: I-101.
• Glide path:
• AN/MRN-2: "portable" UHF (5 MHz segment within the 100-156 MHz range), 2-course aural Radio Range (100 W carrier + 50 W side-band; 25 W "cone-of-silence" fill-in), with station ID and periodic quadrant ID, simultaneous phone transmission.
• AN/CRN-2: Glide Path transmitter, double beam MCW. UHF (335 MHz, 25 W CW). Preceeded SCR-592 in Feb 1944.
• Aircraft receiver set: AN/ARN-5. Similar to AN/CRN-5 set.
• Marker beacons and Compass Locator Marker
• AN/MRN-3: mobile (jeep mounted transmitter RC-115 / BC-902-B) marker beacon with 1 horizontal dipole, vertical fan-shaped pattern. VHF (75 MHz, 1 W, CW/MCW); replaces BC-302.
• Three sets: Outer/Middle/Inner marker, located inside airfield boundary / ca. 1 mile & 4.5 miles from RWY approach end, respectively.
• Aircraft radio equipment: RC-39, RC-43, RC-193, RC193Z, AN/ARN-8, AN/ARN-12. TBC.
• Compass Locator: ?
• Local ground comm.
• SCR-610, 20 W, FM, 27-38.9 MHz (2 xtal-controlled channels out of 120 possible Xtals), battery powered, range 5 miles.

AN = Joint Army-Navy nomenclature system (a.k.a. JETDS), MRN = Mobile (Ground) Radio Navigational aid, CRN = Air transportable Radio Navigational aid.

Aircraft equipment: RC-103 ILS receiver set (BC-733 receiver + I-101 indicator + control box + antenna + dynamotor); RC-39 (incl. 12 volt BC-341 marker beacon RX, 67-80 MHz), RC-43 (incl. 24 volt BC-357 marker beacon RX), RC-193 (same, post-WW2 designator). TBD/TBC.

AN/CRN-10 is also mobile LOC installation (ref. 230U2); AN/MRN-1 cabin + antenna system could also be dismounted from the truck.

Fig. XX: AN/MRN-1 - truck version with antennas in operational position; MC-528 reflector is used to suppress LOC back-course

(source: adapted from ref. 230U2)

Fig. XX: Alford loop antenna of the AN/MRN-1 and radiation pattern of the RC-109 5-loop localizer antenna array

(source: adapted from ref. 235W1; note the very narrow overlap of the yellow & blue beams)

Fig. XX: AN/CRN-10 - frame-mounted version with V-shaped folded dipoles instead of Alford loop antennas

(source: adapted from ref. 230U2)

Fig. XX: AN/CRN-2 - air transportable Glide Path transmitter for operation from trailer, with 30 ft antenna mast

(source: adapted from ref. 230U2, 235W1, 235W3)

Fig. XX: AN/CRN-2 -

(source: adapted from ref. XXXXXXXXXXXXXX)

Fig. XX: AN/CRN-2 - vertical radiation pattern of the upper & lower antenna system

(source: adapted from ref. XXXXXXXXXXXXXX)

Fig. XX: AN/MRN-3 - marker beacon set, for operation from "jeep" truck

(source: adapted from ref. 230U2, TM11-997, and TM11-277)

Of course, by the late 1920s, flying "in the soup", as it is sometimes called, was nothing new or extraordinary. Even "blind" landings under so-called "zero-zero" conditions were successfully made, years before the advent of radio landing beacons and somewhat accurate altimeters! Here, "zero-zero" refers to zero vertical visibility and zero horizontal visibility (e.g., Runway Visual Range, RVR) outside the cockpit. For instance, one recorded true zero-zero landing took place at Croydon Airport near London in 1925, the scheduled destination of a regular passenger flight of Imperial Airways, with captain G.P. Olley at a the helm. Ref. 235M. Regarding "blind" flights using radio navigation aids from take-off to landing, there are three notable "first" events. Regrettably, most publications only credit the first one, even though the third one is the most impressive:

• On 24 September 1929, Lt. James "Jimmy" Doolittle of the U.S. Army Air Corps, performed a series of flights and landings, including several in heavy fog. He was piloting from the rear cockpit of a Consolidated Aircraft Corp. model  NY-2 "Husky" trainer biplane, with safety pilot Benjamin S. Kelsey in the front cockpit. Doolittle's cockpit was completely covered with a hood that completely blocked his view outside the cockpit. The cockpit of the safety pilot had no such vision restrictions. They used a large grassy air field (Mitchel Field) with an obstacle-free approach path. Doolittle used then-standard cockpit instruments and several additional, newer ones, including an artificial horizon indicator, a directional gyroscope, and an altimeter that could be corrected for changes in barometric pressure, based on two-way radio communication with the ground. They used radio navigation aids developed by the National Bureau of Standards: a Radio Range and a Marker Beacon. One of the conclusions was the lack of stable, sufficiently accurate indication of true height above terrain. Doolittle made the first “blind” takeoff, 15-minute flight around the airfield, and landing — all by reference to instruments alone, and with safety pilot. Ref. 235H3, 235N, 235U3.
• Flying "under the hood" originated in France, where Lucien Rougerie introduced the foldable cloth dome at the first school for flying without visibility ("École de pilotage sans visibilité (PSV)"), part of the flying school that Henri Farman established in 1911 at the Toussus-le-Noble aerodrome, some 20 km southwest of the center of Paris. Rougerie also developed a simplistic fixed-base (i.e., non-motion) instrument flight simulator: a "ground training bench for pilotage without external visibility" (French 1928 patent 655874, US 1929 patent 1797794). Hans A. Roeder patented a full dynamic-response training simulator for aircraft (both airplanes and airships) and submarines in 1929 (German patent 568731). During the late 1920s, Edwin A. Link also developed a flight trainer, which he patented in 1930 as a "combination training device for student aviators and entertainment apparatus” (US patent 1825462). He upgraded his initial commercial model in 1933 for practicing "blind" flying, and expanded its features in the 1936 model C-3 with a dynamically responding altimeter and compass, and a ground-track plotter. Ref. 235P20, 229W6. Link's trainers are commonly known as Link Pilot Maker, Link Trainer, Pilot Trainer, and "the blue box" - for their standard paint scheme. A large variety of "synthetic training devices" evolved rapidly during WW2.
• On 5 September 1931, Marshall S. Boggs, a U.S. Department of Commerce pilot, made the first “completely blind” landing in the history of aviation using only radio signals for guidance. He too flew with a safety pilot: James L. Kinney. This historic flight took place at College Park, Maryland, using a narrow hard-surface runway (100x2000 ft, ≈30x600 m), with a small obstacle below the approach path, and a VHF (90.8 MHz) landing beam. Ref. 235N, 235Y1.
• On 9 May 1932, Albert Francis Hegenberger of the US Army Air Corps is credited with the first complete solo blind flight, from take-off to landing, i.e., without a safety pilot. He used a runway localizer course beacon and a marker beacon at McCook/Wright Field. He received the 1934 Collier Trophy for his achievement. Ref. 235H1, 235H2.

Fig. XX - left-to-right: Rougerie's 1928 trainer, a WW2 RAF Link Trainer poster, and my own 1971 "flight" in a Link Trainer

(1971 photo taken at Nationaal Luchtvaart Museum "Aviodome", then still at Amsterdam Schiphol Airport; note my very special aviator sandals!)

INTRO

W.I.T. officials created the Air-Track Corporation of College Park to sell the landing equipment to commercial airports. Pittsburgh, PA, officials bought and installed the system leading to the first blind landing [using only radio signals for guidance ??] of a passenger-carrying flight on January 28, 1938. Ref. 235P2, 235P10. In US or worldwide? Unfortunately, pilots never trusted the system enough to encourage airlines and airports to invest in this system.

"Diamond-Dunmore-originated at BoS" (p. 465 in ref. 229D23, 1933: "The coordination of the two sets of course indications into a single reading is of utmost importance to the pilot, relieving him of the need for considerablemental effort.") cross-pointer / crossed-needle / combined instrument / combined indicator: ergonomically much better than two stacked or side-by-side instruments, and standard to this day. Below: Bureau of Standards experimental combined instrument (1930) and , for LF Visual (2-tone) Course Beacon + VHF (90 MHz) visual glide path beacon. The two [colored] zones at the bottom ofthe instrument face were colored green and red, left to right, respectively. These we relater changed to yellow and blue because green and red could not [cannot] not be read under red night-time cockpit lighting. Blue/yellow (orig. red/green) as per approach chart symbology Blue/yellow: corresponds to colors of sectors used on Approch Charts and Visual Range symbology; found to be of little value.

Some ILS indicators have needles that are hinged and move like wipers, others have needles that move rectilinearly. Round instruments measures 3.25 inches diameter, 8.25 cm. Vertical/pendulum pointer corresponds to deviation wrt VAR or LOC 150 Hz/blue 90Hz yellow

MIT Ground Controlled Aproach (GCA) - dead end?

Ref. 164B pp. 20-25: Lorenz ILS, autopilot coupled vs ca 1938 in US 1st coupled landing?

Fig. XX: 1930 evolutions of Bureau of Standards separate field strenght / glide path meter and course-beam deviation indicator

(sources: ref. 235Y4, 229D23, 229V4 (also in 235C4, 229D24, 229D15, 235C4))

Fig. XX: Crossed-pointer instruments - Bureau of Standards 1930 prototype model and inside of a mature model

(source - left & center image: adapted from ref. 229V4 (same in ref. 235C4); right image: adapted from ref. 235P25)

Fig. XX: Bureau of Standard 1933 final model, WW2 US model I-101-C, Weston Electrical Instrument Corp. model 888-3Y2

(adapted from sources (left-to-right): ref. 229D23; aeronautique.com (accessed Aug.2020); eBay article 312913997455)

In the right-hand image of the figure above, both pointers are marked with a red "off" flag. Such flags drop into view to indicate that the affected pointer is not valid, due to no valid signal being received, or detected equipment failure, or equipment being in a powered down state.

The intersection of the two needles (pointers) represents the relative position of the aircraft with respect to the landing course beam and the glide path. The latter is represented by the small circle at the center of the instrument. Same instrument but reversed L/R Abv/Blo input signals or instrument installed upside-down. Purpose of instrument is to provide guidance ("fly left" "fly right" "descend" "maintain" "climb" for intercepting and tracking the lateral and vertical landing beams. Two philosophies: 1) the center of the instrument represents the crossing equisignal planes of the Runway Localizer and Glide Path beams and the intersection of the two pointers/needles indicates the relative position of the aircraft with respect to center of the ILS beams, vs. 2) the opposite, i.e., the center of the instrument represents the aircraft, and the needles the equisignal planes of the Runway Localizer and Glide Path beams. Per ref. 229R12-No.3, the BoS/CAA and the Army Air Forces used opposite sensing at least until 1940 (basically the same instruments, but wired in reverse). This was harmonized by the RTCA in favor of the AAF standard: the center of the instrument represents the aircraft. This became the world-wide ICAO standard in 1946. Clearly, arguments can be made both ways. However, the needles physically move with respect to the center of the instrument, so they move with respect to the aircraft in which the instrument is installed. Hence, it makes more sense that the center of the instrument should represent the aircraft.

Fig. XX: The two oppposite interpretations of Left/Right & Above/Below of pointer deflections

Lateral: intercept and track the inbound course to the runway, i.e., localizes that course line --> Localizer (LOC). Vertical: curved Glide Path (GP), when straight path (i.e., an angled/sloped flat plane) became standard: Glide Slope (GS).

Equipment also sold by Lorenz to airlines and RAF, where it was know as Standard Approach Beam (SBA).

Between the two World Wars, a divergence evolved regarding aviation in the USA and Europe. In the USA, the postal, freight, and passenger aviation industry required a single consistent continent-wide system of official airways that were marked by radio navigation beacons. The European nations never arrived at recognizing the need for, or attempt to create, such a standardized system. The USA also transitioned well before Europe from aerodromes that were merely large grassy fields (hence, "airfield") with no specific takeoff and landing directions, to aerodromes with one or more hard surface runways aligned with the prevailing wind(s). Not suitable for inherently inaccurate curved glide path descents to landing, also requiring a narrower localizer beam than standard in Europe (3° vs 6°).

• Lorenz claimed the following system advantages for its landing beam system: simple antenna system, and a single transmitter as the course beam could also used for curved glide path to landing. "Disadvantage of a head-start"
• However, Lorenz failed to recognize that these advantages had become moot, or even a disadvantage - in particular, the "curved glide path to landing".

Advantages initially generally quoted for the curved "constant field strength" landing path (ref. 229R2-No.4):

• The landing path may be so directed as to clear all obstructions.
• The landing path may be adjusted to suit different landing fields, esp. important for getting into small fields.
• The landing path automatically levels off, facilitating a normal landing.
• The landing glide may be begun at any desired altitude, within a rather wide range (say, 500 to 5000 ft).
• Easy to use landing-beam indications - no tuning, no adjustment of receiver audio volume, as line of constant field intensity is followed.

However, extensive tests (ref. ????) showed straight glide path with level off close to ground was much better. E.g., ref. 235J. Curved --> steep descent at beginning, continuous adjustment of aircraft attitude and engine power setting - unacceptable, even more so upon intro of faster, aerodynamically more efficient aircraft with high wing-loading (weight divided by the surface area of its wings), and long flat float (eg ref 254, p. 11). Basically: power glide, descending at about 400 ft/min until contact with ground is made (i.e., no flare / round-out!), then cut off engine power completely.

Blind landing: OK, at that time, without accurate ILS, is was possible to do so successfully. As the pilots' saying goes, "any landing you can walk away from, is a good landing"! In a small and slow airplane, with a forgiving landing gear designed for rough, unpaved (grass), ondulating runways (e.g., Junkers Ju-52 transport airplane, with an approach speed of ≈150 km/h, ≈80kts; landing speed of a 100 km/h, ≈54 kts). This is actually akin to the procedure for landing on absolutely flat calm water, so-called glassy water, without as much as a ripple. On approach to the "landing", such flat water looks like a mirror and it is impossible for the pilot to get a sense of depth and judge height above the watery runway. Not recommended (or even allowed!) at night. I enjoyed practicing this for my pilot rating for seaplanes (both floatplanes and flying-boats)!

Die Lorenz-Funkbake, welche für die Zentralstelle für Flugsicherung in Berlin-Tempelhof Auf-stellung fand, hatte folgende Charakteristik: Die Antenne bestand aus einem Gestell von 9 Me-ter Höhe mit einem, vom Sender – 70 Watt moduliert – erregten Dipol und zwei Dipolreflektoren, in denen die Tastung durch Unterbruch mittelst Relais erfolgte (in einem ein Ruhestrom-relais, im andern ein Arbeitsstromrelais, was die reziproke Tastung ergab).

Da bis dahin sieben deutsche und ein österreichischer Flugplatz für 7,89 m eingerichtet waren, setzte die 4. Conférence européenne des experts radiotelegraphistes de l’aéronautique, welche im September 1934 in Warschau tagte, an Stelle der 50 cm-Welle für Signale von Blindlandeanlagen diejenige von 7,89 m.

In the UK, it became the "Standard Beam Approach System" (SBAS) system, where "Standard" refers to the "Standard Telephones & Cables Ltd." the British part of ITT's International Western Electric Co. that was aquired by ITT form ATT in 1925. Lorenz, with all its IP, was acquired by ITT in 1930 (see Fig. Lorenz/ITT/ATT org chart). YEAR??? Copied?? via Lorenz UK?? Former German Lorenz system used at civil airports and Royal Air Force airfields. Evolution?? Difference w.r.t. BABS - Beam Approach Beacon System, widely used approach system at Royal Air Force airfields? ref. 235P14 Pt 2 p50: SBA regularly interrupted beam keying (i.e, reflectors both deactivated, omni transmission by dipole) and 2-lettter beacon Morsee-ID was transmitted for a few seconds.

S.C.S. (Signal Corps System) 51, S.C.S. 51 is the forerunner of the future I.C.A.O. approved runway approach system, known as I.L.S. (Instrument Landing System).

The Low-Frequency Radio Range (LFR), also known as the Four-Course Radio Range, the A-N Radio Range or the Adcock Radio Range, was developed in the late 1920. This 1937 Westinghouse transmitter is identified as "simultaneous" because, unlike earlier versions, it was capable of transmitting the range navigation signals (A and N) and voice transmissions at the same time.

Flight into "instrument meteorological conditions" by non-qualified pilots typically ends catastrophically in a matter of a few minutes.

Fig. XX: Indication of passage of Outer, Middle, and Inner Marker on standard modern indicator or display

(associated audio signals: OM 400 Hz 2 dots/sec, MM 1300 Hz 2 dashes or 2 dots/sec, IM 3000 Hz 6 dots/sec)

Simulated sound of over-flying the Outer, Middle, and Inner Marker of a modern ILS

THE FIXED-MULTI-BEAM "X-METHOD" E/T BEACON SYSTEM OF JOHANNES PLENDL

First, we have to get some terminology straight, as there is some general and persistent confusion, particularly in non-German publications (except 230R3):

• X-Verfahren / X-Method (process, system)
• X-Bake / X-Beacon , X-Station (transmitter station; German code name "Wotan I" (but not for original version?), Brit. code name "Ruffian")
• X-Gerät / X-Equipment (apparatus): equipment set in the aircraft: X-Empfänger (TFK (dev) /Siemens (prod) FuG 22 Leitstrahl Doppelempfänger. "Anna" ), = EBL2 with additional amplifier/differentiator stage for the AFN2 ?

"Bombenabwurfverfahren", "Bomben-Ziel-Abwurf-Verfahren", "gezielter Blind-Bomben-Wurf" says it all (ref 183).

The "Lorenz Beam" was designed for flying a specific course-line towards a short-range beacon that had relatively wide beam-aperture (5º). In 1932, Dr. Johannes (Hans) Plendl of the Deutsche Versuchsanstalt für Luftfahrt (DVL, German Aviation Test Establishment) already identified the need for a directional beam system, to guide bombers to a target along a course-line away from the beacon, at night and in poor weather (visibility) conditions. Plendl was the national commissary for RF research ("Bevollmächtigte für Hochfrequenzforschung") from November 1942 until December 1943, and also headed up the national agency for RF research "Reichsstelle für Hochfrequenzforschung" (RHF) that was established mid-1943.

Make distinction between "pre-Knickebein" and "Post-Knickebein".

Proof-of-concept was done with "Lorenz Beam" systems. However, these commercial systems had neither the required range, nor the required accurate and narrow equi-beam (±0.1º aperture). Therefore, the operating frequency was increased from 30-36.2 MHz (wavelength λ ≈ 9 m ± 9%) to the 66-77 MHz range (wavelength λ ≈ 4 m ± 8%), and the beacon was equipped with more powerful transmitters. The German code name for the "X"-station ("X-Station", "X-Bodenstelle", "X-Peiler", "X-Bake") was "Wotan I". The basic antenna system comprised two vertical dipoles. The antenna system as such was rotable to the desired [primary] beam direction, but not (continuously) rotating. One dipole was energized continuously with an AM carrier that was modulated with a 2000 Hz tone. The second dipole was placed at a distance of 3½ λ, and was energized via a motorized capacitive phase-shifter. The phase was changed stepwise, every 0.5 sec [??how many steps in 360? rpm?]. The resulting radiation pattern had 14 or 18 E/T-beams of about equal strength [lobe size, see Fig. 84], and an equi-signal zone with a width of less than ±0.1º. The large number of major lobes was very awkward: the aircraft had to fly across the lobes, and count the passages of the "T" (dash) zones to find the intended guide beam ("Marschleitstrahl", the 7^th of 14 (as in Figure 84 below), or the 9^th of 18 in the larger system).

Ref. 185H: in 1933 Hans Plendel (DVL) proposed a VHF precision nav system for bombing without visual contact with the target; development contract for this "X-System" (a.k.a. "Wotan 1") was awarded to him in 1934. "The “X-System” worked on the principal that a guiding beam was directed over the target and served as a course beam. Two other beams on different frequencies intersected the course beam at right angles prior to the target. The [crossing] beams allowed the pilot to determine his speed over the ground with a special "X-Clock" to determine the correct release time for his bombs." After extensive testing (@ Köthen), the "Wotan I" VHF navigation system (66-77 MHz) was operational in 1937. 14-beam antenna system: 2 vertical arrays, 1 transmitted a continuous signal, 1 an intermittent 120 Hz signal (?); 2000 Hz E/T tone pulse modulation, +/- 0.5° equisignal beam width. A.k.a. "multi-beam Rechlin system". A larger 18-beam system was subsequently developed, with "2 reflectors". Eight built throughout Germany in 1938, late 1939 most moved to the western front, later to the French channel coast. X-receiver: Siemens FuG22 "Anna", bascially an E Bl 2 with additional amplifier/pulse-differentiator stage. AFN2 indicator used [per 230R3 not initially], but connected such that 1 needle showed LOC guide beam, the other the transverse/crossing beam [????]. The system was used as follows: The frequency of the UHF-beam (A) was tuned into the first receiver which was tracked by the pilot. See the figure on the next page. The flight engineer manned the second receiver and it was his job to determine the time that the aircraft crossed the transverse beam signals (B) and (C) which transmitted at different frequencies. Usually these beams were detected 18 and 6 km before the target and used to determine the ground speed of the aircraft. A “X-Clock” (fabricated by Hartmann & Braun, later by Baeurle& Sons) was used to calculate the time to the target. Wind drift, altitude, aircraft speed was measured and accounted for. The clock would then show the time for the aircraft to drop its bomb. X system used September 1939 during invasion of Poland, and from France August-1940 to June-1941 in bombing raids at targets in England.

Ref. 6G, §29: The first operations of the war with mobile X-Stations were on two bombing missions against a munitions factory in Poland. §39: Kampfgruppe 100 later flew in Russia with mobile "X" stations, which were set up with great speed.

Ref. 6G: After several years Plendl completed his apparatus, and experiments were conducted on the X-System by Versuchs Regiment Köthen (Ln Abt. 100. Luftwaffe Signal Corps Detachment 100) with the bomber Gruppe that later [FD: late 1941] became K.G. 100 [FD: 1. Kampfgruppe 100, 1.K.Gr.100; Köthen, ca. 135 km / 84 miles southwest of downtown Berlin: small civil airfield since 1928, airfield + industrial site acquired by Luftwaffe mid-1936, 360 hectares (≈900 acres) total, Nov 1937 home of the Luftnachrichten Lehr- und Versuchsregiment, source: ref. 230R2, 230R3). Antenna system: 2 vertical arrays (vs single antenna), one transmitting omni-directional signal (2000 Hz tone modulation), the other transmitted 120 pulses per minute = intermittent (on/off). E/T rythm. Radiation pattern: 14 beams (in order to obtain high beam "resolution") with ~0.05° ???? width. Later an 18-beam version was developed. Large number of beams not user-friendly whatsoever, as hard to identify which beam to use (or for enemy: determine which "target" beam was in use). Range later increased by adding reflector behind each antenna. Crossing beams 18 and 6 km before the target.

Q: difference between transportable, mobile, and fixed-base antenna systems?

Fixed-base Wotan I station locations?

3-beam system: Marschleitstrahl + Vorsignal (Querleitstrahl) + Hauptsignal (Querleitstrahl 5 Km vor Ziel); vs 1+3= 4 beams or more?

FD: The need for (near) right angles is illustrated in Fig. XX; this also applies to all other crosssing-beam systems, such as "Knickebein".

Ref. 183. Plendl's concept used two directional beacons, sufficiently spaced apart. The centerline of their beams crossed each other at the target. Plotted on a chart, the two beam lines form a big "X". Hence, this concept was referred to as the "X-Method" ("X-Verfahren", "Bomben-Ziel-Abwurf-Verfahren", "gezielter Blind-Bomben-Wurf" - like the Y-Verfahren). The aircraft would intercept and track the E/T equi-signal of a director-beam ("Marschleitstrahl") to the target. A second beacon would transmit (at least) three E/T beams that would cross the director-beam at certain distances just before reaching the target. Upon reaching the first cross-beam, the aircraft had to well established on the equi-signal course-line. Reaching the centerline of the second and third cross-beam was used to determine when to release the bombs. This was done with a special stopwatch, the "X-Clock" ("X-Uhr"), see Figure 45B. This mechanical calculator computed the "bomb release" time, based on actual groundspeed towards the target (derived from the times between the cross-beams), release altitude, and type of bomb. In a simplified version, only a single cross-beam was used.

Ref. 185C.

Fig. XX: The importance of (near) perpendicular crossing beams - it minimizes inaccuracy in the position estimate

(note: distance from Station A & B to the LoP-intersection is the same in both cases)

The (lead) aircraft required an "X-Gerät" ("X-Equipment") installation. This comprised (see p. 106 in ref. 2, ref. 229R, 230R3):

• Two dedicated "X-receivers", FuG 22 "Anna" receivers for 66-77 MHz (20x RV12P2000 tube/valve), each with a ¼λ vertical rod (dipole?) antenna; vs. FuG-17-X receivers?
• Two AFN2 ("Anzeigegerät Flug-Navigation") course-deviation indicators (right-hand instrument in Figure 82A).
• The vertical down-pointing needle indicates left/right deviation from the course line. The needle pulses to the left or right, in the rythm of the dominating "E" or "T" signals ("Zuckanzeige", "kicking meter"). When receiving the beacon and the position is "on course", the needle is centered.
• The horizontal needle (pointing at the vertical scale on the left) is a signal-strength indicator, and acts as a simplistic near / far indication of distance to the beacon.
• The indicator lamp in the center is illuminated when receiving a marker beacon during approach to landing. It is a line-replacable neon lamp. On the AFN1, it is located at the top.
• Two AVP units ("Anzeigeverstärker Plendl", "Plendl-method Indicator Amplifier"), one for each receiver/AFN2 pair.
• A power converter unit for the required anode & grid DC-voltages, and a power-distribution unit,
• A "Prüfuhr" (a.k.a. "X-Uhr"): "Bombenabwurfautomat" (automatic bomb-release timer/computer; multi-stopwatch bomb-release timer (see Figure 83 below).

Dual RX/AVP/AFN, to be able to simultaneous monitor the main guide beams and the crossing beam.

Fig. 82A: Anzeigegerät für Funk-Navigation (Radio-Navigation Indicator) - model AFN1 and AFN2

(vertical needle/pointer indicates Left/Right course deviation, horizontal needle shows signal strength = appr. distance, "nahe" = "near")

Note that these two instruments do not have the same size. The AFN1 is a somewhat larger: its bezel has an outer diameter of 83 mm (≈3.3 in), whereas the cylindrical housing has a diameter of 79.4 mm (≈3.1 in) and a length of 72 mm (excl. protruding connectors; ≈2.8 in). For the AFN2, these dimensions are 66.7 (≈2.6 in), 57 mm (≈1.3 in), and 57 mm, respectively. See these diagrams.

AFN1 and AFN2: at least as early as 1939. AFN2 desigend and manufactured by Siemens SAM (LGW-Hakenfelde/Berlin). AFN2 was also used during aproach to landing flight-phase, and for D/F purposes with sets such as "Bordfunkgerät FuG 16: Empfänger E16Z + Zielflugvorsatzgerät ZVG 16", "Zielflfuggerät Peil G IV" (1941), "Funk-Peil-Gerät FuG141" (receiver E141) and "Bordpeilgerät Peil G 6", though no use for the neon lamp.

AFN1 and AFN2 are the Luftwaffe version of the Lorenz "1936 beam approach indicator" instruments, see Fig. xx. The AFN2 has a 40 μA moving-coil microammeter for the left/right course deviation needle, and an 80 μA meter for the signal strength needle.

Converter circuitry (eg AVP) was integrated into the EBl 2 receiver.

Figure 83A: X-Uhr automatic bomb-release timer/computer - model Pr.U.1

(source: ref. 230A but with its explanatory legends corrected!)

In the photo above, the pointers are at the position that would be typical for the moment approaching bomb release: the red pointer is about to overlap the black pointer, and the black pointer is set back from the green pointer.

Figure 83B: Front and top view of X-Uhr model Pr.U.28

(source left image: ”Pr.U 28, 1939” page of deutscheluftwaffe.de, used with permission)

There were two manufacturers of the Prüfuhr. The Hartmann & Braun A.-G. company in Frankfurt/Main was a maker of electrical test and measurement instruments (incl. strip chart recorders, pressure gauges, temperature gauges, fuel gauges) and control equipment. They made the initial Prüf-Uhr model 1, the Pr.U.1. The Tobias Bäuerle & Söhne company was a maker of clocks and mechanical calculators in St. Georgen in the Black Forest - a traditional clock making region, and not just cuckoo clocks with intricate wood carvings. They built the smaller X-clock model Pr.U.2., as well as the later version model Pr.U.28. On the Pr.U.28, the setting knob for the black pointer is lower than on the Pr.U.2. Per ref. 3, Hartmann & Braun received an order for 100 Pr.U.1 clocks, and Tobias Bäuerle & Söhne for 13000 (!!!) model Pr.U.2 clocks.

The equipment label in the Figure above shows that the clock was developed at "E.Re.F". This is the "Erprobungsstelle Rechlin, Abteilung Funkforschung". That is: the radio research department of the Luftwaffe test site at Rechlin, about 100 km north-northeast of downtown Berlin. This department covered "Hochfrequenzforschung und Leitstrahlverfahren": research of high frequency radio and flight guidance beams. The clock was developed there by Dr. W. Hepper.

The winding key can be removed from the winding mechanism and stuck into a small cylindrical holder to the right of it. The clock is actually wound through the back of the instrument, via external right-angle gearings to the top of the housing, see the center image in Fig. 83B above. The housing of the Pr.U.2 and Pr.U.28 clocks has a diameter of 17.5 cm (≈7 inch), and the clock weighs 3.65 kg (≈8 lbs), ref. 212B. Note that the Pr.U.28 X-Uhr in Fig. 83B has a third small scale that is marked "0 - 4 hours", which the Pr.U.1 does not have. It indicates to what extent the clock spring is wound. Model Pr.U.1 does not have this feature.

Ref. 185H: cross-beams crossed main around 18 and 6 km prior to reaching the target. FD: not fixed distances, hence adjustable ratio on X-clock.

The X-Uhr calculations the bomb-release moment based on a fixed, standard approach altitude and the actual speed over ground. This speed is based on a fixed distance (≈ 10 km) between the points where the two secondary guide beams cross the main guide beam that is pointed at the target location. The time that elapses between these crossings mechanically derived from the two moments when the operating lever of the clock is pressed. As the next Figure shows, the calculator mechanism is quite complex. It uses two variable-ratio bevel gearing to calculate "time of flight" and drift of the bombs. The 2-prong electrical connector is connected to a switch contact for automatic bomb release.

Figure 83B: Inside of a Pr.U.2 X-clock (made by the Tobias Bäuerle & Söhne company) and its dimensions

(source left image: "Ln.28901 Prüfuhr PR.U 28, 1939" page of deutscheluftwaffe.de, used with permission; right: adapted from ref. 212B)

X-clock operation: ref. 230F vs.230A/Fig.83A, vs. 230E ("The second intersecting beam was at 20 km distance, and told the navigator to start a special clock with two independent hands, one of which started rotating at this point. The third beam intersected at 5 km from the target, and was used to start the second hand on the clock. When the two hands of the clock aligned, an automated bomb release was triggered electrically"). FD: no fixed distances, as these distances from teh target depended on the distance from the X-beacon to the target, the fixed angles of the X-beacon's radiation pattern, and the line from the main beam to the target. Hence the need for adjustability of the X-Clock, to account for these !

Fig. 84: Radiation pattern of the Wotan I beacon

(source: adapted from ref. 2)

Fig. xxA: 3D radiation pattern of the Wotan I beacon (in free space) - for 3½ λ antenna spacing

(the NEC file of my 4NEC2 model is here)

Fig. xxB: 3D radiation pattern of the Wotan I beacon (in free space) - for 4½ λ antenna spacing

(the NEC file of my 4NEC2 model is here)

Fig. XX: Creation of E/T equisignal beams with 3½ λ antenna spacing

E/T equisignal beams created by stepwise toggling phase shift between the two antennas in rhythm of the E/T keying. Note that when phase shift causes rotation of the radiation pattern by more than 2°, additional (and undesirable) equisignal beams emerge. This characteristic limits the achievable minimum narrowness/sharpness of the equisignal beams.

Ref. 230R3: fixed ground station/installation had "fan" pattern with 14 lobes, the mobile station had 3 lobes. Various antenna configurations: 2 vertical dipoles each with a reflector (also active dipole?), vs. 6 horizontal + 6 vertical dipoles vs. 10 dipoles. All rotable (not rotating). Dot/dash keying. Prototypes completed late 1934.

Vs: transportable station with 6+6 vertical dipoles (2 side-by-side broadside arrays) for single equisignal beam (have photo).

Vs.: single-beam fixed-base "cross-beam" station with 2 vertical dipoles, each dipoel with 2 reflectors (or reflector + director???; have photo)

The complexity of the method required extensive pilot training. Only a rather limited number of aircraft was equipped with the X-Gerät. To cope with jamming by the British, the system was modified to use additional frequency channels, and a tone modulation well above the standard audio bandwidth of regular receivers. This provided some temporary relief from the jamming. The "Battle of the Beams" between Germany and Britain took place from late-1939 through mid 1941. Ref. 6D, 28, 38A, 230C, 230D, 230E, 230F, 230G. During this period, the British developed countermeasures to German radio-navigation systems and to radio-telephony communication of fighter/bomber control systems, to which the Germans responded with modifying those systems and introducing new systems.

By May of 1941, the Lorenz X-System was abandoned in favor of the "Y-System", see further below. A secondary reason for abandoning the X-system was the absence of a system for formation flying "in the clouds". This limitation had been recognized, and implied that a simpler system with crossing beams would be just as effective.

"KNICKEBEIN" - LUFTWAFFE ROTABLE SINGLE-BEAM E/T COURSE GUIDANCE BEACON

The Telefunken company had already been tasked early 1939 to develop a simple beam system that was compatible with the Lorenz Funklande Empfangsanlage Fu Bl 1 ("blind approach & landing receiver system") that was standard equipment in Luftwaffe aircraft (ref. 32). The receivers had much higher sensitivity than required for operation with a landing beacon, to enable long-range navigation. This new system was also an E/T beam system, and also used two crossing marker-beam beacons. Telefunken's extremely rapid development was headed by Adalbert Lohmann, who was later in charge of the development of the Bernhard/Bernhardine system. The new system operated in the 30-33.3 MHz band, i.e., a wavelength of 9 - 10 m. Obtaining a sufficiently narrow equi-signal beam at these frequencies required an antenna system with two large dipole arrays. The ground stations of this directional-beam system ("Richtfunkfeuer") were Telefunken Funk-Sende-Anlage ("radio transmitter installation") FuSAn 721. Their German code name was "Knickebein". The interlocked Morse code "E" (= "dot ", "•",) and "T" ( = "dash", "─") pulses were modulated with a 1500 Hz tone. The "E" dots were 1/8 sec wide, the "T" dashes 7/8 sec - the same as the standard Lorenz blind landing system. This way, the off-the-shelf transmitters (with integrated E/T keying) of those landing systems could be used.

Simplified "X-Verfahren", could be considered as two long-range, high-accuracy, Lorenz landing beam systems (see below).

By the end of 1939, two Knickebein installations were operational along the western border of Germany: station Knickebein-2 (K2) at Stollberg/Bredstedt on the North Sea coast in the far north (later the location of Bernhard station Be-9), and K4 at Kleve (the German town that is closest to London and the Midlands). K12 was constructed during the winter of 1939/1940 at Maulburg (near Lörrach, in the far southwest of Germany, near the German/Swiss/French border. All three stations in Germany were the large Knickebein version ("Großanlage", "große Bauform").

Fig. 7X: Table with the location of all Knickebein stations

(the exact map coordinates of all Knickebein stations are provided in ref. 230Q1)

Fig. 71: Map showing the location of all Knickebein stations (modern-day political borders)

(see ref. 230Q1 for exact coordinates and associated aerial photos & satellite images; an interactive Google-map version of this map is here)

To point the guide-beam at a bombing target in Britain, the system could be rotated on a circular track (diameter 96 m per ref. 5B, 94 m per ref. 230Q3). This large circular track was not a railway rail, but a single flat steel band on a concrete ring (a curved I-beam, embedded in the concrete; see Fig. 75B below). The outer vertical trusses of the frame were mounted onto a small platform, with a single line of multiple small support wheels underneath. The track diameter was somewhat smaller than the span of the truss frame. Likewise, the center vertical truss was supported by small wheels on a small circular track. See Fig. 71A. I have no details about the electrical motorized rotation drive system.

Fig. 71A: Large Knickebein K2 at Bredstedt/Germany - under construction (replaced by Bernhard Be-9 in 1944)

(source: Fig. 36 in ref. 181; red circle shows the size of a man)

Fig. 71B: The fenced-in Large Knickebein K2 at Bredstedt/Germany - construction completed

(source: p. 9 in ref. 261L)

The "E" and "T" beams were offset by 7.5° to the left and to the right, respectively, of the direction perpendicular to the plane of the large truss frame. This created a narrow zone where these sub-beams overlapped in such a way, that they had the same field strenght. This equi-signal zone was about 0.3º wide. Looking at the antenna system from above, it had a slight V-shape ("crooked leg", "dog leg") of 180° - 15° = 165°. Note that the large steel truss box frame is completely straight! One of the two 7.5° angles is easy to see at the top left-hand corner of the dipole array in the photo above. The same angle is easier to see in photos of the Small Knickebein (Fig. 75 below).

Fig. 71C: Large Knickebein K12 at Maulburg near Lörrach/Germany - under construction

(source: ref. 230Q2)

After the invasion of their neighbor countries, the Germans installed another nine Knickebein stations along the coasts of southern Norway (1x), The Netherlands (2x), and France (6x, from the Channel coast down to Brittany). Construction of station K13 on the isle of Sicily/Italy was advanced, but never completed or operational. These were Small Knickebein ("kleine Bauform", "Kleinanlage") systems. The rotable installation had a width of about 45 m, and a track diameter of 31 m. and had a single row of 4 dipoles plus "reflectors" per sub-beam, instead of 2x8. I.e., only one quarter the overall size of the Large Knickebein. Hence, the width of the equi-beam was larger (≈0.6º) and the side-lobes were stronger. These small stations were installed closer to their targets than the large K2, K4, and K12 stations in Germany. So, over the target location, the width of the equi-beam was still acceptable.

Small stations usable both as main guide beam and crossing beam.

Fig. 75A: Small Knickebein station (left: under construction in France, August 1941, possibly K6 (location: see ref. 229Z))

(sources: Bundesarchiv Bild 101I-228-0322-04/Friedrich Springorum/CC-BY-SA 3.0 (left) and Fig. 37 in ref. 181)

The left-hand photo in Fig. 75A above clearly shows that the "reflectors" are indeed driven dipoles - see the split between the upper & lower dipole halves. So, they are "active", i.e., powered by the transmitter, and not passive ("parasitic")reflector rods.

Circular track: spoked wheel; a curved steel I-beam, either embedded in a concrete ring, or attached with regular rail-chairs onto wooden cross-ties (UK: "sleepers") on a track bed of crushed rock or gravel:

Fig. 75B: Circular tracks of the Small Knickebein K10 at Sortosville/France (left) and K13 at Noto/Italy (summer 1943)

(source left image: normandy1944.org.uk, accessed 31 July 2020; right image: ref. 90-TBD!!!!)

The Large Knickebein installations K2, K4, and K12 had an enormous rectangular antenna system, see Figure 71. The rectangular steel truss frame measured ca. 95 x 30 m (WxH, ≈310 x 100 ft). The center vertical truss divided the antenna system into two sides - one to generate the "E" sub-beam and one for the "T" sub-beam:

• Each sub-beam was created with a large array ("Gruppenantenne") of vertical dipoles. All dipoles had the same 1λ length. For the operating frequency of 30-33.3 MHz, the wavelength λ is about 9.5 m.
• Each array comprised two rows of eight vertical dipoles, one row right above the other. I.e., a "stacked array". These parallel dipoles were spaced horizontally by a standard ½λ.
• Each of these dipoles had another dipole right behind, at a distance of ≈¼λ. These were not passive reflector rods, but active dipoles, driven by the single transmitter. This makes for more effective side-lobe reduction (see, e.g., p. 71 in ref. 137A).
• The dipoles and reflectors were made of large-diameter metal tubes instead of wires, see Fig. 71B, 75. A larger radiator diameter makes an antenna more broadband ( = usable over a wider frequency range, without the need for "re-tuning" the system).

The Small Knickebein installations (K1, K3, K5-K11, and K13) also had a symmetrical antenna system with a 165°/15° angle, but a lot fewer dipoles:

• Again, each sub-beam was created with an array of parallel vertical 1λ dipoles, with ½λ horizontal spacing.
• But now, there were only four dipoles per side, not eight!
• Also, each array comprised only single row of dipoles, not two vertically stacked rows.
• Here too, each dipole had another dipole right behind it, again at a distance of ≈¼λ.
• As a result, the frontal area of this antenna system was only one quarter the size of that of the Large Knickebein.

The diagram below shows the configurations of the Large and Small Knickebein antenna systems:

Fig. 7x: Dipole array configurations of the Large and Small Knickebein antenna systems

There are several Knickebein-beam radiation pattern diagrams floating around in literature, without reference to any reputable source. As always: trust, but verify! This is why I decided to create a simple model of the large Knickebein antenna array myself. I always use the fabulous 4NEC2 antenna modeling freeware tool. The complete antenna system comprises two independent identical arrays side-by-side (one for the dash-beam, one for the dot-beam), but only one beam transmits at a time. So, I only modeled one sub-beam. The results are shown above and below. Note that, as is standard for radiation pattern diagrams, a logarithmic scale (decibel, dB) is used for the signal-strength (see far left side of the two figures below).

Fig. 73: Top, oblique, and side view of the radiation pattern of a Large Knickebein sub-beam ("E" or "T") - in free space

(the NEC file of my 4NEC2 model is here - it is not optimized)

The top views above are actually quite similar to the generic patterns shown in publications. However, they are only valid for an antenna in so-called "free space". I.e., without any objects anywhere near, or ground below, the antenna. This is unrealistic, in particular for an antenna system close to the ground (in terms of the number of wavelengths λ of the transmitted signals), as is the case with Knickebein (with λ ≈ 9.5 m). The figure below shows the impact of ground reflections (assuming conductivity and dielectric constant of standard "real ground"), clearly beyond the modeling capabilities of the era. My model does not include the steel trusses around the Knickebein arrays, which could cause some pattern distortions. In practice, this was "not disturbing" (pdf p. 4 in ref. 184F1).

Fig. 74: Top, oblique, and side view of the radiation pattern of a Large Knickebein sub-beam ("E" or "T") - over ground

(the NEC file of my 4NEC2 model is here)

As illustrated above, the antenne array of the Large Knickebein comprises two stacked rows of vertical dipole antennas. The second row does not significantly change the radiation pattern. However, the overall transmitter power is divided 50/50 between the two rows. I.e., each row only gets half the transmitter power. This is not the case with the Small Knickebein: the full transmitter power goes to a single row of dipoles. That row comprises only half the number of dipoles of its Large counterpart, which concentrates less radiated power into the forward lobe of the pattern: less directivity. This makes the resulting sub-beam and equi-signal beam about twice as wide as those of the Large Knickebein. For the installation locations of the Small Knickebein stations, this reduced performance was still sufficient, at much reduced cost, size, and construction time.

Fig. 7X: Comparison of a Large & Small Knickebein sub-beam ("E" or "T") - in free-space & over ground

Fig. 72: The alternating "E" (dot) and "T" (dash) beams of the Knickebein beacon

Simulated sound of crossing the "Knickebein" E-beam, Equi-Signal, and T-beam back & forth

(source: ©2020 Frank Dörenberg)

On 21 June 1940, an aircraft of the RAF Blind Approach Training & Development Unit (BATDU, replaced several months later by the Wireless Intelligence Development Units, WIDU) intercepted Knickebein signals with a Hallicrafters model S-27 VHF superhet receiver (27.8 - 143 MHz), and determined the direction of their source. Ref. 5, 230D. During the winter of 1940/41, the Knickebein system became increasingly unreliable and unusable over Britain, due to jamming by the British. Reports on "spoofing" and "beam bending" by the British being undetected and effective are contradictory (e.g., ref. 5 vs. §27-28 in ref. 6G). The jamming tone pulses sounded different from the true Knickebein pulses (§28 in ref. 6A), possibly due to better keyclick suppression in the jammer. Also, the British jamming systems were incapable of synchronizing to the Knickebein pulses, hence, their jamming signals created easily detectable "wailing" beat tones. The system continued to be used in the lead aircraft ("Pfadfinder") for navigation towards the target, but those now relied on the X-System (described further below) to locate and mark the actual target. In September of 1941, the Luftwaffe aircraft receivers were upgraded from FuBl 1 to FuBl 2, which supported a large increase in the number of available frequency channels in the same band, and a range of 600 km at 6000 m altitude (20 thousand feet).

Questions/Notes:

• Since standard E/T system, based on Lo AFF, used standard 500 W landing beacon transmitter ("Landebakensender") = Lo AS4, ...?
• Human ear, fly not on the center of the beam but right on the edge, where it is easier to detect deviation (ref. ADIK???); 0.2-0.3 dB - but only in absence of background noise. UKW Fernfunkfeuer - VHF Long-Range Beacon
• Zyklop (Cyclop): §28-30 in ref. 6D: "This was the latest form of the well-known Knickebein, working on 30 - 33,3 mc/s and received by E.B.L.3 in the aircraft. It was a mobile station which could be fully erected into operation within a week. 120 watt transportable beacon station, derived from Knickebein (late 1943, per ref 6G), operated on Knickebein frequencies, but range of 300-350 km vs Knickebein 450 km.
• A still more mobile unit known as the Bock-Zyklop had been introduced, operating on FuG16 frequencies ( = ???? MHz, VHF). Ref. 6D §29. This could be set up in three days and could be adapted for use on the FuGe 16 frequency although as yet, according to documents, no visual indicator for the FuGe 16 had been developed. The 120 W ground transmitter was called the ???? which gave a beam 0.5° wide and a range of 300 km. at a height of 5,000 meters. The Zyklop systems had been made use of on the Russian [also near Borisow/Belorus?] front up to the end of the hostilities. Ref. 6G, §41: It was developed by Dr. Künhold at Köthen [see above in Xsystem section; add bookmark link]. Ref. 21A: 1940/41, "Zyklopfeuer" FuSAn 722 ground station + FuBl 1 / FuBl 2 airborne radio set.
• There are some references (e.g., AIR 14/2343, AIR 14/3246) to Knickebein stations (e.g., K3) being reactivated starting December 1943 - not for guidance of Luftwaffe bombers, but of fighter aircraft for intercepting Allied bombers arriving from Britain. British codename: "Ottokar". This is unconfirmed by original German documents. E.g., there is no mention such a procedure in the Kriegstagebuch (KTB, War Diary) of the I. Jagdkorps (1^st Fighter Corps, responsible fighter units within the Reich). Technically, it would have been possible to point a Small Knickebein beacon at a bomber stream, scramble fighters on a vector to intercept the beam and inform the fighters of the direction of the beam.

THE "Y-SYSTEM" - LUFTWAFFE FIXED-SINGLE-BEAM + TRANSPONDER

Already in 1939, an other successor to the "X-System" was conceived. It retained the director-beam ("Marschleitstrahl") concept of the "X-System". But rather than using a cross-beam to mark the position of the target along that beam, it used a transponder system (?? "Emil" Kontroll-E-Messung???) that allowed the ground controller to determine the aircraft's distance from the beacon (of course "slant range", not distance over ground). After a fixed delay time, the transponder in the aircraft would retransmit the received signal at a different frequency (1.9 MHz lower). The interrogator ground station would derive the range from the total round-trip signal delay, minus the fixed delay. The ground controllers would command release of the bombs ("Abwurfkommando", based on the range, via VHF R/T. This was called the "Y-System" ("Y-Verfahren", ref. 6G, 244D), US/UK Allied code name "Benito" (system or beacon?). It became operational in September of 1940. As the procedure involved a ground controller, the number of aircraft that could use the system simultaneously, was limited.????

Why "Y"? Successor to "X"?

Ref. 6G, §29, 30: Like X-system beam nav, Y-system also conceived by Plendl but developed at the Technisches Amt instead of at Köthen.

Ref. 185C.

As with the "X-Verfahren" above, ,we have to get some terminology straight, as there is some general and persistent confusion, particularly in non-German publications.

Y-Verfahren ("Y-Procedure"), Y-Gerät ("Y-Equipment"); the "Y" beam-guidance-to-bombing target method is not to be confused with "Y-Verfahren" procedure for guiding fighter aircraft to intercepting enemy aircraft (ref. 244R), with its Y-stations/Y-sites (ref. 6E) incl. Y-Peiler ("Y-D/F system"), nor the British Y-Service! The "Y-Bake" ground station was called "Wotan II" (FuSAn 733). Not to be confused with "Y-Bodenstelle", "Y-Peiler", ref. 6E "Equipment of a Y-Site", ref. 244R ??? "Y-Procedure for fighers", which useda variety of transmitters was used (Bertha I, Bertha II), and a number of co-located transponder transmitters (Y-Stations in Germany: Sender 16 Boden / S16B, Y-statins in France: "Société Anonyme des Industries Radioélectriques" Sadir 80/100; Elsewhere any?).

The (lead) aircraft required a "Y-Appraratus" ("Y-Gerät") installation (ref. 2, 6E, 229P, 230R1, 230R3). This comprised:

• One dedicated beacon receiver: the "UKW Leitstrahlempfangsgerät" FuG28a, comprising:
• A slightly modified E17 receiver of the modular Lorenz FuG17 VHF radio-telephony transceiver "UKW Sprechgerät" (42.15 - 47.75 MHz, 10 Watt, ref. 40C). Manufacturers of the E17 include "Dr. Georg Seibt A.G." in Berlin; plus (dedicated or shared?) ca. 1 m long rod antenna ("Stabantenne"). Seven stage, tubes: 9x RV12P2000. Weight: 5.3 kg (incl. tubes), 138x210x205 m (WxHxD)
• An AW28 "Auswertegerät" / "Bakenwandler" - signal processing / interpretation unit (e.g., for converting received audio signals from course-guidance beams (e.g., E/T pulses from Lorenz Landing beam, Knickebein, X-System, Y-System) left/right course-deviation signal to drive a lateral auto-pilot. An electro-mechanical (?) equivalent of the X-system's AVP unit ?
• AW28 manufacturer, based on Fertigungskennzeichen mfr code "ded" on equipment label (see Fig. XX): "Heliowatt Werke, Elektrizitäts-Aktiengesellschaft" factory in the town of Schweidnitz (Świdnica) in Silesia (post-WW2 south-central Poland). Heliowatt was originally established by Hermann Aron in 1897 as a subsidiary of "H. Aron Elektrizitätszähler-Fabrik GmbH" in Berlin-Charlottenburg, renamed "Aron Elektrizitäts GmbH" in 1912. The "Aron" name was Germanized in 1933 to "Heliowatt". Already in 1923, Aron had begun to produce broadcast radio receivers under the ethnically-neutral brand name "Nora" ("Aron" in reverse). In 1935, the company was aryanized (i.e., expropriated for ethnic reasons) and sold off to Siemens.
• An LKZG ("Leitstrahl-Kurssteuerungs-Zwischengerät") to interface the FuG28a to the lateral-mode autopilot ("Kursregler"), for automatically tracking an equi-beam,
• One course-deviation indicator (CDI): "IJ 28" (Bauart: Siemens Apparate und Maschinen GmbH, Mfr: Philips in Berlin, see below).
• Alternatively ?: AFN2 ("Anzeigegerät Flug-Navigation"), see Fig. 45A.
• Unclear how IJ 28 was connected to the FuG28(a), as a converter unit would be necessary. Per L-Dv-702-1-Heft-205 (Anlage 4), FuG17 included a "Nav-Gerät", which could connected to the E17 "Tel." headphones output.
• One FuG16ZE or FuG16ZY transceiver (38.5-42.3 MHz and 38.4 - 42.4 MHz, respectively; ref. 40A, 40B; other differences?), 1.9 MHz transponder shift; used by Y-station for D/F.
• Associated power converter units, to provide the receivers and transponders with the required DC supply voltages.
• Various control panels.

Fig. 50A: Front of the FuG28a "UKW Leitstrahlempfangsgerät" (VHF guide-beam receiver)

(source FuG 17 image: deutscheluftwaffe.de)

Fig. 50: View into the top of the FuG28a

(source: www.cdvandt.org)

As the suffix "28" implies, the IJ 28 was specific to the FuG 28.

Fig. 50B: A 1942 course-deviation indicator model IJ 28 of the FuG 28 system and label on the housing of the unit

(source left & center image: eBay article nr. 383651899889, 2020)

The equipment label states that the IJ 28 was designed by Siemens Apparate & Maschinen GmbH (SAM) in Berlin (most likely the SAM Luftfahrtgerätewerk Hakenfelde (LGW) in Berlin-Spandau). The label lists Philips in Berlin as the manufacturer. The Dutch company Philips Gloeilampenfabrieken N.V. established a German national subsidiary Deutsche Philips GmbH in Berlin-Mitte in 1926. Late 1935, the company created a special department: "Kathograph". It developed and manufactured measurement instruments (e.g., cathode-ray oscillographs, "Kathodenstrahl Oszillograph") and TV's at the central Philips-Haus offices at Kurfürstenstraße 126. In March of 1937, this department was incorporated as Philips-Electro-Special G.m.b.H. Berlin (PESB). After WW2, it was reestablished in Hamburg as "Elektro Spezial GmbH" (1949).

The bottom part of the glass of this instrument (and other serial numbers) has a large black crescent painted on the inside. Unclear what it covers up. The unit's rear connector has two pins for the moving-coil meter.

THE "HERMES" - LUFTWAFFE ROTATING TALKING BEACON

An interesting beacon system is the Hermes/Hermine "Sprechdrehbake" system ("Talking rotating Beacon"). The system was originally developed in response to a tactical requirement formulated during the second part of 1942, as a navigational aid for the purpose of giving an approximate bearing to single-engine night fighters engaged in "Wilde Sau" [lit. "Wild Boar"] air-defence operations. The pilot could determine the bearing from the beacon, without having to look at an instrument. The beacon stations (FuSAn 726) transmitted real-time voice-announcements of the beam azimuth, every 10º. I.e., the numbers 1 - 35 (multiples of 10º), and the "station call-sign" at 360º = 0º = True North passage. Each digit was pronounced separately: e.g., "12" = "1-2" (as is proper /standard practice in worldwide aviation radio comunication to ensure intelligiblity, except in France), not "twelve". The voice stream was pre-recorded as an optical track on an endless/continous film strip ("Tonfilm") + photoelectric cell; rotated with the shaft of the antenna system (--> "optical disk"?). The voice signal was transmitted (FM modulated) with an omni-directional antenna. At the same time, and on the same frequency, a strong constant audible 1150 Hz tone was transmitted. However, it was transmitted with a rotating cardioid antenna pattern. The null of this pattern ( = no 1150 Hz interference signal) coincided with the direction as announced by the voice announcement at that very moment. So, the voice could only be heard (briefly) in that particular momentary direction. Due to the width of the null (effectively about 15° (ref. 8), equivalent to (360°/15°)x60=2.5 sec), the immediately preceding and following announcement was only partially audible, at much reduced volume.

The airborne counterpart, FuG125 "Hermine-Bord", comprised the standard EBl 3 receiver, its FBG2 control panel, and a small audio-amplifier (model V3a or ZV3), (separate?) FM demodulator.

Notes:

• The system was developed in 1943/44 by Ernst Kramar et al of the Lorenz company.
• Ref. 164B (pp. 13-15): antenna system atop a 16 m tall steel tower, accuracy ~2°, headphone in a/c, transmitter 150 W type AS3 (Lorenz landing beacon TX), 30-33.3 MHz; antenna system had gain of single dipole; configuration: 2 pairs of vertical dipoles at 4 corners + 1 central omni for voice announce; antenna current ratio dipole pair-1 : pair-2: omni = 1.0 : 1.8 : adjusted to min (?); bandwidth +/- 500 kHz without changing dipole length (30-33.3. MHz when changing length); antenna system mechanically rotated at 1 rpm; feedlines through hollow shaft --> slip rings; noise transmission turned of during station ID announcement (1100 Hz, not 1150?); single/same transmitter for tone and voice? ; monitoring RX at 200-300 m, with audio via telephone cable to control/equipment room. Hermine/FuG125 had to be minor modded to increase audio bandwidth (with add'l on/off ctl).
• Ref. 229Q, p. 92: cardioid pattern.
• Ref. 185H: Yet another receiver was developed by the Lorenz Company (Dr. Kramar) which was designated FuG 125, code named “Hermine.” The receiver was made up of the EBl 3 F receiver, the FBG 2 remote control, a V3a amplifier for volume control. A FuG 16 ZY antenna was connected in parallel. This unit was designed for Instrument Flight Rule weather. On the 14th of September 1944 an order of 18,000 units was given to the Strassfurter-Rundfunk-GmbH. In the last part of the war only a few dozen units were delivered and used in the Me 109, Fw 190, Ta 152, Do 335 and 15 Me 262
• Ref. 6D §58-64: The Hermine system was originally developed, in response to a tactical requirement formulated during the second part of 1942, as a navigational aid for the purpose of giving an approximate bearing to single-engine night fighters engaged on “Wilde Sau” operations. By the time the initial difficulties in development had been overcome Wilde Sau night fighting had almost ceased; it was found however that Hermine could be used to advantage by day fighters, and it came into operational use. 60. An accuracy of ±5° was assumed, but it was found in practice that this could be improved upon to ±3° by experienced pilots. Thirteen or fourteen ground stations were in operation by Easter 1945 which, P/W claimed, gave complete coverage of the Reich. It was intended to fit two Schlechtwetter (bad weather) Fighter Geschwader with the necessary airborne equipment, and this program had been one-third completed by May 1945. One P/W had heard that ten to fifteen Me.262's of K.G.51 were amongst the aircraft so equipped. The following may be added in modification of the description of the Hermine system given in A.D.I.(K) 125/1945 [ = ref. 6C] , paras.59 to 62. The Hermine rotating beacon transmits a continuous tone on which is superimposed a speaking clock which counts from 1 to 35, each figure representing tens of degree. Over an angle of about 15° the continuous tone falls to a minimum and rises again. During this period the voice appears to become more audible and the pilot can estimate where the minimum of continuous tone occurs, and so obtain his bearing from the beacon. The beacon recognition is given by means of a self-evident code name for example, "Berolina” for Berlin – which is spoken by the voice in place of 000°. The airborne equipment is the FuGe 125 consisting of the E.B.L.3 with the Tzg (Telephoniezusatzgerät) which enables the 30.0 - 33.3 mc/s transmission picked up on the E.B.L.3 receiver to be heard in the pilot's headphones. Though the Hermine beacons were fully operational there was a scarcity of FuGe 125 sets, as a result of which practical experience of this system was too limited to judge of its efficiency or to lead to further improved tactical requirements been formulated.
• Ref. 2: Range 200-250 km at 15000 ft. Accuracy 3-5°. Antenna system: 4 vertical dipoles with "Drehefeldspeisung" at 4 corners of a square with 1/8 λ sides, plus a central radiator (vertical dipole?). Antenna system rotating at 1 rpm. Purpose: enable fighters to home to airfield. Five beacons operational in January of 1944, 13-14 by Easter of 1945, spread out over the Reichsgebiet. Receiver: Lorenz FuG 125 "Hermine" = E Bl 3 F [F = Fernbedienung = remote control] + Zwischenverstärker ZF 3 amplier + Anzeigezusatz[gerät] ZuG 125 + one AFN2 [see Fig + text above, add #]. Shared antenna with the VHF R/T transceiver FuG 16 ZY.

Fig. 51: Principle of the "Hermes/Hermine" talking beacon system

(source photo: deutschesatlantikwallarchiv.de)

Fig. 53: Radiation pattern of the rotating "null" beam

Simulated sound of a "Hermine" beacon (60 sec rotation)

(source: ? © ? used with permission)

"ELEKTRA", "SONNE"/"CONSOL", MOND", "TRUHE" - LUFTWAFFE MULTI-BEAM BEACONS

Sound clip of a Sonne/Consol sequence (42 sec)

(source: de.wikipedia.org, retrieved March 2020)

An other sound clip of a Sonne/Consol sequence (3 min)

(source: www.geocaching.com, retrieved March 2020)

HYPERBOLIC NAVIGATION SYSTEMS

Fig. YY: The hyperbolic radio navigation system for aviation

(source: National Air & Space Museum, Smithsonian Institution, "Time & Navigation - Navigating in the Air", 2012, artist: Bruce Morser)

Sound clip of Loran/Loran-A pulses (33 sec)

(source: A. Cordwell, retrieved February 2020)

An EM wave is called "vertically polarized", if the E (= electrical) field is vertically polarized. The H (magnetic) field is always perpendicular to the E field.

Fig. XX: Linearly polarized EM wave (left) and E-field of such a wave inducing current in an aligned dipole antenna

(image source: (left) wikimedia.org CC license; (right) wikimedia.org, CC license)

DISTANCE MEASURING EQUIPMENT

Circular LoP.

Radio-transponder based ranging (here: range in sense of distance) invented and patented in France FR632304 & GB GB288233 in 1927 by Alexandre Koulikoff & Constantin Chilkowsky.

WW2 Germany + UK/GB.

Derived from "Identification Friend or Foe" (IFF), ground-based interrogator + airborne responder or transponder. Reverse roles: airborne interrogator + ground based transponder.

1944 Convention on International Civil Aviation (also known as Chicago Convention), looked at developing a form of distance measurement, based on the Rebecca-Eureka ‘Identification Friend or Foe’ (IFF) secondary radar system, and operating in the 200 MHz band. Adapted to DME(A) in Australia in 1946.

1945 CERCA London meeting

"Up to that time whilst continuous azimuth guidance on airways was provided by various types of radio beacons, progress along track could only be measured by station passage of a beacon - in other words, by flying over it."

1000 MHz DME(I) international, 200 MHz DME(A) Australian (discontinued December 1995)

• interrogator + responder/transponder (not the same!).
• Slant range.
• Ground-based/fixed-base/stationary interrogator + mobile/airborne responder/transponder: WW2: IFF, E-Meßung. Modern: secondary radar, transponder.
• Mobile/airborne interrogator + Ground-basedfixed-base/stationary responder/transponder: modern: DME, often colocated with VOR = VOR-DME or mil TACAN = VORTAC; being phased out slowly in favor of GPS?
• Example: UK/RAF/WW2: Rebecca (airborne interrogator; name supposedly derived from "Recognition of Beacons") + Eureka (gnd based xpdr, since freq shift; fixed DeltaT --> min range of 2 milles; coded pulsed reply; A coding unit which is part of the Eureka beacon periodically causes the width of the beacon response pulses to vary at Morse code intervals for identification. This function may also be manually controlled for transmission of simple messages.), developed by TRE (frmr AMES research ctr). Mk-I 1941, operational by 1943. Became integrated in BABS (E/T keyed VHF course beacon + xpdr; homing approach beam), PRF 300, 4-5 usec pulses, 170-234 MHz Mark X: 1 GHz), slant-range determined from round-trip time-of-flight with radar display pulse scope (time-base synced to TX). Many equipment variations built, US-built versions: AN/PPN-1 (fixed), AN/PPN-2 (portable), AN/TPN-1 (xprtable) xpdr. AN/APN-2 (interrgtr) aka SCR-279. Rebecca-H (for use with 2 xpdrs) used in combo with Gee-H.
• Ref. 265A.
• Radio/radar altimeter: developed during 1930s in parallel in Germany (Siemens-Halske) and USA (esp. Bell Labs). Refs: 268A-268K.
• Patents?
• Nodal (e.g., E.F.W. Alexanderson/GE Co.: indicated position between nodes produced by transmitted and reflected radio waves.) vs FM (e.g., W.L. Everitt: FM ("carrier wave varied by rotation of air condenser"). Beat note of transmitted and reflected waves has pitch that is a direct function of the altitude) vs pulse
• Height above terrain (vs barometric altitude, based on pressure difference wrt a reference pressure level [fixed, eg above 18 kft, or wrt local/regional altimeter setting = baro pressure at ground level (typ. airport/aerodrome)].
• Difference Radio Alt (RA) vs Radar Alt (RadAlt).
• Two modulation types: Frequency Modulation Continuous Wave (FMCW, triangular/sawtooth waveform "chirp" with fixed period and df/dt (rate), the second utilizes pulsed modulation. Frequencies: high enough to get usable echo from ground/terrain, i.e., ??? Both based on round-trip time-of-flight of transmitted signal, either as Δf=ΔT x df/dt (RA) or ΔT directly (RadAlt)
• FMCW: Separate TX and RX antennas (aka bistatic?). As with radars, several techniques to avoid interference between such altimeters. Modern: ca. 50 Hz, 150 Hz, 12-1600 Hz waveform repitition freq, TX power < 1 W.
• Pulsed: single antenna as with radar, modern: 6000-20000 pps, TX power 5-100 W.
• ICAO standard performance: accuracy 3 ft / 0.9 m or better
• Challenges include: bank angle, "jagged" terrain
• 1930s/WW2: USA: RCA ABY-1, RC-24; based on R&D since 1929/30 at Bell Labs etc. USA, UK, D, F,...
• WW2: Germany, Luftwaffe
• <= 1940: Funkhöhenmesser FuG-101/101 A (FM), Siemens/LWG, λ = 75-89 cm, 337-400 MHz, 1.5 W; ref. 268A, 268G. 268H. Subsequently/1944 same technology subsequently used in "Marabu" proximity fuses of ground-to-air/surface-to-air missiles.
• FuG102, FuG103, FuG104.
• Siemens FuG101, FM (ref. 7A (section III c on p. 358-359)
• Modern: stdrd ICAO frq range 4.2-4.4 GHz (RadAlt), height above terrain: upto 3000 or 4000 ft?
• precision approach & landing aid (esp. auto-pilot coupled and in low-vis conditions), ground/terrain proximity awareness/clearance/collision avoidance (later GPWS/TAWS, highly significant contribution to flight safety)
• Sonic & capacitive altimeters:
• Capacitive: ref. 7A (section III b on p. 358): Siemens, 1935-1938; ref. 235P35.
• Sonic: ref. 268J, 268K

RADAR SYSTEMS

This section only covers radar applied to navigation / guidance (fixed/ground-based: OBOE, airborne: H2S/X ground mapping - nav + ground target ID). Primarily: British/Allied?  I.e., not air-air intercept, not early warning, not FLAK, not gun-laying,...

Finally, after decades of shameful denial, the world-renowned IEEE finally redeemed itself in 2019. It formally recognizing that, contrary to popular belief, perpetuated Allied WW2 propaganda and general ignorance, radar was not only invented and patented in 1904 in Germany by Christian Hülsmeyer (patents 165546, 13170, 25608), but he also publicly demonstrated his "telemobiloscope"/"Telemobilskop"in Cologne/Germany (Köln), and in Rotterdam/The Netherlands that same year. Ref. 261A, 261B.

3 general categories: Earyl warning, Ground control, intercept, ..

The term "radar", for Radio Detection and Ranging, was actually introduced in 1940. "Funkmeß" = lit. radio measurement

Types of Radars:
• Pulse Radar: time of flight-> range; transmit short pulses, measure echo delay; ref. 261S.
• Doppler (CW) Radar: frequency shift -> velocity; transmit constant frequency, measure frequency shift due to (relative) motion
• Chirp (FMCW) Radar: frequency difference -> range; transmit varying frequency, measure how much frequency changed during echo delay

OTHER LUFTWAFFE RADIO NAVIGATION SYSTEMS

In addition to the beacon systems described above, there was a number of other German beacon systems (ref. 1, 2, 8, 26B, 230A-230C, pp. 7-19 in ref. 164B), such as:

• "Dezimeterwelle-Richstrahl-Drehfunkfeuer" a.k.a. "Drehbake M" (UHF rotating-beam beacon). Ref. 3; ref. 2A, p. 87ff: DVL/Plendl, 261-268 MHz carrier frequency [≈1.1 m, so not truely "decimeter"], variable azimuth-segment dependent modulation (5.3-6.0 kHz in the 270°-0°-90° semi-circle direction, 8.0-10.0 kHz in the 90°-180°-270° semi-circle) , 100 W telefunken magnetron transmitter. Transportable, mobile (truck), and fixed-base installations. Antenne system: broadside planar array (4 m wide) of 18 ½λ-dipoles (2 stacked rows of 9?) + as many reflectors behind them, motorized rotation 1 rps (60 rpm!). 3 test sites: Rechlin, Schneeberg (≈ 90 km northeast of Nürnberg (Nuremberg), ≈ 30 km from the Czech border; with 1050 m the highest peak in the Bavarian Fichtelgebirge mountain range), the Wendelstein (1836 m, peak in the Bavarian alps, ≈ 60 km southeast of München/Munich, close to the Austrian border). Associated on-board receiver system: "Dezimeterwelle-Funkfeuer-Empfangsgerät AF2/3" a.k.a. "M-Gerät", comprising Telefunken BFO receiver "Lina", a Siemens "Zusatzgerät", a Siemens "Anzeige-Meßbrücke" with "Peilwertanzeige", and a "Rollkarte mit Lichtzeiger" (per DVL/Hepper) moving map/chart with light-pointer. Alternatively: standard AFN1 kick-meter ("Zuckanzeige") indicator, as for Lorenz ILS and Knickebein. Planned: course setpoint selection and auto-pilot coupling. Achieved RDF and course-tracking accuracy: +/1 1° and range up to 500 km. Development halted spring 1939 in favor of "Erika", due to complexity (temperature & auto volume/gain control), some issues with side-lobes of the radiation pattern, and reflections from terrain (e.g., cliffs & montains).
• Baldur, VHF system range measuring system (basically DME), comparable to British "G-H". ref. 6D. Airborne set: FuGe 126 and FuGe 126k "klein". Wavelenght: ca. 2-4m (75-150 MHz). Locations: possiblyonly 2 experimental stations in Lower Silesia (check Ln maps/charts) . Further developments (never operational):
• Baldur-Truhe (combination of Baldur and Truhe)
• Baldur-Bernhardine (combination of Baldur and Bernhard, for simultaneously obtaining bearing and range; with a "Bernhardine" Hellschreiber printer for bearing & range indication; ref. 6D: The range indication was to be obtained by the pilot pressing a knob when the range would appear in kilometres on a dial. This system was suggested for use by both day fighters and bombers.).
• Elektra: long wave beam system (initially ca. 480 kHz, later 270-330 kHz), range over land 800-1200 km (500 and 300 kHz respectively), range over sea 1700-200 km (500 and 300 kHz respectively). Three antennas per beacon station. Transmitter power: 1.5 kW. Ref. 230A, 230B, 164B. Ref. 164B, p. 7: antenna height only 50 m and goniometer to swing the beam to the desired direction.
• Stations (p. 6, 7 in ref. 164B): Huisen/The Netherlands (1940, 460 kHz, 10 λ spacing), Bayeux/France (Nov 1940, 300 kHz, 5.7 λ spacing, (temporarily?) dismantled late 1943); Stavanger (March 1941), Morlaix (?), one near Warsaw.
• Elektra kurz (480 kHz, λ = 625 m; 1939-1941), Elektra lang (300 kHz, λ = 1000 m). Ref. 230K.
• Dreh-Elektra? = Sonne?
• Sonne ["Sun"]: long wave (several LF frequencies between 270 and 330 kHz vs 300 kHz nominal and 250-350 kHz), long-range system of the Kriegsmarine (navy). Lorenz. It was based on "Elektra", but rather than physically rotating a loop antenna, Sonne used three stationary antennas spaced about 1 km (about 3.86 wavelengths) and a single transmitter + goniometer, to electronically sweep the direction of the beams. Range over land 1200 km. Range over sea 2000 km. Transmitter power: 1.5 kW. Ref. 230A-230C, 230K1, 230K2, 230K3, 164B (p. 8, 9)). Collapsed hyperbolic system (extreme case of hyperbolic). Became operational June 1942.
• Goldsonne. Ref. 164B, pp. 15-17: proposed by Dr Goldmann to overcome disadvantages of Sonne and Komet.
• Goldwever/weber: a "Sonne" derivative that never became operational. Ref. 164B, p. 12: substitute for Komet. Ref. 38B.
• Mond ["Moon"]: experimental system, intended to improve range and accuracy of "Sonne", while operating on higher frequencies (3 MHz, 6 MHz; 30-30 MHz per ref. 230A), at night.
• Stern ["Star"]: an experimental Sonne derivative, operating at VHF frequencies, hence range basically limited to line of sight. Not developed to completion.
• Esseker: Sonne derivative, with quicker indication of position.
• Elektra-Sonne: beacon could be operated alternately as "Elektra" and "Sonne", to combine advantages of both. Range was intended to be increased by raising transmitter power to 60 kW. Three stations were built during 1944-1945 but were never operational. Ref. 230A.
• Erich: "UKW-Phasendrehfunkfeuer"., i.e., a VHF rotating-phase navigation system.
• Fernmeldetechnisches Entwicklungslaboratorium Dr. Ing. H. Kimmel ("Development lab for telecom equipment") in Munich, later Münchener Apparatebau für Elektronische Geräte Kimmel GmbH & Co. KG. Their 3-letter military manufacturer's code was "bes". Kimmel also made the "NF Phasenuhr" (audio frequency phase-indicator, with 360º scale) that was part of the on-board equipment of the 1943 Lorenz VHF rotating-phase navigation system "Erich" (very similar to the VOR system developed in parallel in the USA). Like "Bernhard/Bernhardine", the "Erich" system also used the EBl 3 radio receiver.
• Ref. 2/2A: R&D at Lorenz started in 1943, on the familiar/standard 30-33.3 MHz freq, based on a 1940 patent (nr ???). Antenna system: 4 vertical dipoles at 4 corners of a square with 1/8 λ sides, plus a central radiator (vertical dipole?). The 4 dipoles were excited with a (standard) 500 W landing beam beacon transmitter, unmodulated carrier only, via motorized rotating radio goniometer [antennas stationary!]. Creates a cardoid, rotating at 50 rpm, resulting in a 50 Hz amplitude variation at the receiver, similar to 50 Hz AM modulation. Central (omni-directional ) antenna powered by a separate 10 W FM-transmitter with constant 50 Hz modulation, referenced to "North" passage of the rotating cardioid. Phase difference between the two received 50 Hz signals [after AM and FM demodulation, respectively] is [linearly] dependent on the bearing from the beacon to the receiver. The [demodulated signals where compared] with an "NF-Phasenmesser" / "NF-Phasenuhr" ["AF phase clock"] with two electric motors driving an indicator pointer w.r.t. a 360° scale, via a differential gear. See blockdiagram in ref. This system is very similar to the simultaneously and independently VHF Omni-directional Range (VOR) beacon [here too: "Range" is not "distance" but "directional beacon"]. For the given cardioid radiation pattern, the achievable accuracy was "only" 1°, and the fact that demodulators and cockpit indicator were needed,[and the industrial stage of the war], the project was cancelled in favor of the "Hermes/Hermine" "talking beacon", [with a slowly rotating "4 vertical dipoles + central omni antenna" antenna system] which only required the existing standard receiver.
• Erika: a hyperbolic beacon system [TBC], comprising a line of six antenna arrays (spaced ca. 46, 18, 27, 27???, and 124 m) each with a transmitter. Similar to the British Gee system. Developed in 1942 (per ref. 164B p9: mid-1941 by Dr Goldmann / Lorenz), briefly operational, replaced by Bernhard. Ref. 8. Developed by P. von Handel, Pfister (who proposed the 6 antenna solution) @ DVL,et al.
• Ref. 230K1, 164B.
• Ref. 20: accuracy obtained ≈0.015°, avoided HF phase measurements by converting HF phase to NF phase via "umlaufende Interferenzlinien" ("circular interference lines" ???). Transmitter station comprised three dipole pairs. Dipole pair #1: excited with f₁ and f₁ + Δf, respectively. The combined waves fronts create interference lines that are modulated with Δf, such that at all points in the coverage area, a signal with the carrier frequency f₁ that is wobbled (sweeping FM modulated) with the beat-frequency Δf is received. Transmitter 2: dipole pair #2: similar to pair #1, but excited with f₂ (close to f₁) and f₂ + Δf. Lines of constant phase difference are hyperbola. At the receiver station, 2 receivers are required: tuned to f₁ and f₂, respectively. The audio output of each receiver drives a small synchronous motor. The 2 motors drive a differential gear, the output of which moves a pointer that indicates the momentary phase difference between the beat-frequency wobbles about f₁ and f₂. To be able to determining position (by intersecting hyperbolic LoP's), f₃ and f₃ + Δf were transmitted via a third dipole pair. On-board equipment comprised 4 receivers and 2 Phase-clocks; or: 2 receivers (with the tuning of the second one switched between f₂ and f₃) and 1 phase clock. At a distance of 500 km, accuracy obtained on frequencies of ≈43 MHz (λ ≈ 7 m) was 100-200 m.
• Stations (p. 9-11 in ref. 164B). Transmitters built by LMT in France (with 50 W (?) and 5 kW PA stages built by SADIR in France). Station in France at Boulogne and Cherbourg (#2) cancelled ? Training stations to be comleted by end of 1943 near Vienna and Ischemberg/Munich.
• Dora: ca. 1940, shortwave, 2 pairs of vertical dipoles in crossed configuration, with a vertical antenna at the center, rotating dual cardioid pattern; not satisfactory. 1.5 kW beacon; tool, only used for calibrating Erika.
• Komet (FuSAn 712) / Komet-Bord (FuG 124); RDF system (with acccuracy +/- 1.5° ?)
• Antenna system comprised concentric rings of (vertical) antennas for multiple operational HF frequencies (short wave; 3-25 MHz); angular spacing of 3° --> 120 antennas per ring, used pair-wise (diametrically opposed) at a time, in combination with omni-directional antenna at center of the rings. Development / evaluation / experimental 1941- EOW.
• Proved impossible/impractical to properly adjust/calibrate. Development and evaluation was done from 1941 through the end of the war (vs. abandoned per ref. 6D). Large HF ground station with an antenna array with 127 (120?) masts and 19 control huts (boxes?), with "Kometschreiber" bearing recorder/indicator in the aircraft (FuG124). Ref. 8. Experimental stage only (Bordeaux/France, Kølby/Denmark vs. Wullenweber HFDF at Kølby, ref. 230S1).
• Ref. 20: Experimental installation with single antenna pair at Ahlimbsmühle (ca 50 km north of downtown Berlin). A complete system was also installed 1944 in the Ismaninger Moos bog area, ca. 15 km northeast of downtown Munich, but never operational. Circle diameter 2λ. Primary frequencies 5470 kHz (λ ≈ 55 m), 8345 (λ ≈ 36 m), 11455 kHz (λ ≈ 26 m).
• Truhe, VHF hyperbolic pulse system, compatible with the British "Gee" system (where "Gee" stands for the letter "G" in "Grid"), which was referred to as "Hyperbel" by the Germans.
• ref. 6D: was ultimately to cover the 20-100 MHz band and employed various types of ground transmitters including Feuerhilfe, Feuerstein, Feuerzange and Feuerland. All these transmitters could also be used to the "Gee" system. Truhe I: 46-50 MHz, Truhe II: 30-60 MHz.
• Locations per ref. 6: A chain of Truhe stations was built around Berlin, primarily for training purposes and there were in addition groups of ground stations in the Schwarzwald and in Pomerania. The last named was intended for operations against Russia and it is not known if the stations were destroyed before their capture.
• Airborne sets: FuGe 122 (46-50 MHz), and FuGe 123 (25-75 MHz). Ref. 6D, 230Q, 261AC1.
• Schwanboje: ref. 6D, §22-25.
• Diskus: ref. 6D, §75.
• Wullenwever: p. 12 in ref. 164B.

PATENTS

Below is a listing of patents related to radio direction finding, radio location, radio navigation (generally covering the early 1900s through WW2).Patent source: DEPATISnet. Patent office abbreviations: KP = Kaiserliches Patentamt (German Imperial Patent Office), RP = Reichspatentamt (Patent Office of the German Reich), DP = deutsches Patentamt (German Federal Patent Office), US = United States Patent Office, GB = The (British) Patent Office, F = Office National de la Propriété Industrielle (French patent office), AU = Dept. of Patents of the Commonwealth of Australia, NL = Nederlandsch Bureau voor den Industrieelen Eigendom (patent office of The Netherlands).

Note: in the USA and other countries, a company or business cannot apply for a patent. In such cases, the employee-inventor (i.e., the invention was made as part of the employment) has to apply for the patent (or the patent is applied for in the inventor's name), and then transfer (assign) the patent rights and ownership the employer/company. This assignment transfer is typically done during the application process. An inventor who is not obliged to assign the patent to an employer, may assign his patent (transfer of rights, not of invention) to any other party.

REFERENCES

• Ref. 1: "Bernhard and Bernhardine", p. 24 in "Some historical and technical aspects of radio navigation, in Germany, over the period 1907 to 1945", Arthur O. Bauer, 28 pp. Source: www.cdvandt.org.
• Ref. 2: "Die deutschen Funkführungsverfahren bis 1945", Fritz Trenkle, Alfred Hüthig Verlag, 1987, ISBN 3778516477, 215 pp.
• Ref. 2A: pp. 76-110, 224
• Ref. 3: pp. 62, 94-102 in "Die deutschen Funk-Navigations- und Funk-Führungsverfahren bis 1945", Fritz Trenkle, Motorbuch Verlag, 1995, 208 pp., ISBN-10: 3879436150.
• Ref. 5: "Instruments of Darkness: The History of Electronic Warfare, 1939-1945", new ed., Alfred Price, Greenhill Books, 2005, 272 pp., ISBN-10: 1853676160; original edition: William Kimber and Co., Ltd, 1967. See note 1
• Ref. 5A: pp. 236-237; Same as pp. 274-275 in the excellent German translation: "Herrschaft über die Nacht: Spionen jagen Radar", Alfred Price, publ.: Bertelsmann Sachbuchverlag Reinhard Mohn, 1968, 304 pp., ASIN B0000BT35X.
• Ref. 5B: p. 82 - second of sixteen photo pages.
• Ref. 6: Transcribed reports from the British Air Ministry, Assistant Director of Intelligence (Prisoner Interrogation), A.D.I. (K) (a.k.a. "Felkin Reports", after their author, Samuel Denys Felkin, who was Chief Interrogator at Bletchley). Source: The National Archives of the UK, ref. AIR40/2875 and 2876. Retrieved from www.cdvandt.org.
• Ref. 6A: §19-23 in "Some further notes on G.A.F. Pathfinder procedure" [Egon procedure, Freya control, Bernhard/Bernhardine, Erika, Y-Gerät, X-Clock, Knickebein jamming], Samuel Denys Felkin, A.D.I.(K) Report No. 187/1944, 25 April 1944, 5 pp.
• Ref. 6B: §57-59 in "G.A.F. night fighters - R.A.F. Bomber Command countermeasures and their influence on German night fighter tactics" [Luftwaffe intercept methods (Free-lance/Ungeführte Zahme Sau, R/T procedures, SN2, running/Gruppe W/T commentaries, beacon commentaries), Window, jamming, Naxos, Mosquitos], Samuel Denys Felkin, A.D.I.(K) Report No. 599/1944, 2 November 1944, 16 pp.
• Ref. 6C: G.A.F. Night Fighters - Recent Developments in German Night Fighting" [incl. Bernhard/Bernhardine, IFF, Hermine, SN2, SN3, Naxos, H2S/Berlin-gerät], Samuel Denys Felkin, A.D.I. (K) Report No. 125/1945, January 1945, 18 pp.
• Ref. 6D: "Radio and Radar Equipment in the Luftwaffe - II, Navigational Aids" [incl. D/F (PGe6, FuGe141, FuGe142, FuGe145), homing beacons (Schwanboje, Biene), beam systems (Zyklop, Sonne, Mond, Stern, Dora, Komet, Erika, Bernhard, Hermine), pulse systems (Ingolstadt, Truhe, Baldur, Baldur-Truhe, Baldur-Bernhardine), ground control systems (Benito/Y, Egon, Nachtlicht, Nachtfee, Barbara, Barbarossa, Rübezahl)], Samuel Denys Felkin, A.D.I. (K) Report No. 357/1945, 1945, 18 pp.
• Ref. 6E: "Equipment of a Y-Site" [incl. D/F (Y-stations, Heinrich I, IIM, IIU, III), transmitters (S16B, SADIR 80/100); range measurement (Rechlin unit; Siemens), FuGe16ZE, FuGe16-ZY], Samuel Denys Felkin, A.D.I.(K) Report No. 527B/1944, 25 Sept 1944, 13 pp.
• Ref. 6F: "Radio and Radar Equipment in the Luftwaffe- I. Blind Landing and Airborne Communications Equipment" [report of interrogations of General Martini and other PoW's; topics: LFF, JLFF, Fu Bl 2, AWG 1 (autopliot coupler for E/T course deviation), FuG 10 P, FuGe 16, FuGe 17, FuGe 18, FuGe 15, FuGe 24, FuGe 29], Samuel Denys Felkin, A.D.I.(K) Report No. 343/1945, 1 July 1945, 6 pp.
• Ref. 6G: "The GAF (German Air Force) Signals Organisation in the War" [incl. X-System, X-beam, Knickebein, Y-System (Benito), Zyklop/Cyclop, Erika, Sonne beacons in Spain, radar, FuG25/25A, IFF, Panorama, Centimetre Wave R&D, Egon, Gee, H2S, Berlin-Gerät, Kammhuber night fighting, Window, Wilde Sau, HS293], Samuel Denys Felkin, A.D.I.(K) Report No. 334/1945, 1 July 1945, 27 pp.
• Ref. 7: "Beiträge der Firma Siemens zur Flugsicherungstechnik und Luftfahrt-Elektronik in den Jahren 1930 bis 1945 (Teil 1 & 2)", H.J. Zetzmann, in "Frequenz - Zeitschrift für Schwingungs- und Schwachstromtechnik"
• Ref. 7A: Part 1: Vol. 9, Nr. 10, 1955, pp. 351-360.
• Ref. 7B: Part 2 (pp. 387, 388, 392): Vol. 9, Nr. 11, 1955, pp. 386-395.
• Ref. 8: "General Electric funds Hitler", Chapter 3 in "Wall Street and the Rise of Hitler", Antony C. Hutton, G S G & Associates Publ., June 1976 (reprint), 162 pp., ISBN 0945001533.
• Ref. 13: p. 405 and 4.09 in "Japanese Electronics", OPNAV-16-VP101, Photographic Intelligence - Report 1, U.S. Navy Dept., Office of the Chief of Naval Ops., Air Intelligence Group, Div. of Naval Intelligence, Naval Photographic Intelligence Center, January 1945, 166 pp. [33 MB]
• Ref. 14: summary item 27 in "The German Wartime Electricity Supply - Conditions, Developments, Trends", British Intelligence Objectives Sub-comittee (BIOS), Final Report 342, Item No. 33, 28 selected pages. Source: www.cdvandt.org.
• Ref. 15: "Beschreibung und Betriebsvorschrift für Funk-Navigationsanlage FuG 120" [Description and Operating Manual for Radio-Navigation System FuG 120 "Bernhardine", with 2-channel Hellschreiber radio-navigation printer], Telefunken G.m.b.H., document FN-T-GB Nr. 1932, December 1944, 43 pp. [File size: 66 MB - a good-but-lower resoluton file is here 26 MB]
• Ref. 19: p. 122 in "Die Erprobungstelle Rechlin", Christoph Regel, pp. 60-149 in "Flugerprobungsstellen bis 1945: Johannisthal, Lipezk, Rechlin, Travemünde, Tarnewitz, Peenemünde–West", Heinrich Beauvais, Max Mayer, Bernard & Graefe Verl., 1998, 364 pp., ISBN: 3763761179; Vol. 27 of "Die deutsche Luftfahrt : Buchreihe über die Entwicklungsgeschichte der deutschen Luftfahrttechnik", Theodor Benecke, Deutsches Museum
• Ref. 20: pp. 59-63 of "Richt- und Drehfunkfeuer" ["Directional and rotating radio beacons"; 4-Course, Telefunken Compass, Bernhard, Erika, Komet, ILS/LOC/GS], Chapter 3 of “Leitfaden der Funkortung: Eine systematische Zusammenstellung der Verfahren und Anlagen der Funkortung“ [Guide to radio location: a systematic survey of radio location methods and installations], Vol. 1 of "Lehrbücherei der Funkortung", Walter Stanner, 4^th ed., Deutsche RADAR-Verlagsgesellschaft m.b.H., 1957, 160 pp.
• Ref. 21: "Bordfunkgeräte - vom Funkensender zum Bordradar", Fritz Trenkle, Bernard und Graefe Verlag (publ.), 1986, 283 pp., ISBN 3-7637-5289-7
• Ref. 21A: p. 61-63, "Kommandoübertragungszusätze".
• Ref. 21B: pp. 97-103, "Leitstrahl-Verfahren" (beam methods).
• Ref. 21C: photo "Große Knickebein Anlage bei Kleve", p. 108 [note: incorrectly identified by Trenkle as station K4 at Kleve, instead of K2 at Bredstedt]
• Ref. 21D: "Drehfunkverfahren", pp. 119-130
• Ref. 25: "Verzeichnis deutscher Bordfunkgeräte aller Art (einschl. der mit Fu G-Nr. belegten akust. und UR-Geräte)". "Kenntnis der Allierten von deutschen Bordfunkgeräten (Winter 1944/45, nach engl. amerik. Anweisungen zur Auswertung deutscher Beutegeräte, Stand Februar 1945" [list of German on-board radio equipment, incl. acoustic and infrared; list of equipment types/models captured by Allied forces]. Manuscript notes of Fritz Trenkle, incl. for "Funkgeräte Katalog Deutschland 1908-1918 und 1919-1945", 44 pp. Source: Archives of Deutsches Technik Museum Berlin, retrieved from www.deutscheluftwaffe.com on 19 February 2020. [pdf]
• Ref. 28: "Ernst L. Kramar Pioneer Award 1964", in "IEEE Trans. on Aerospace and Electronic Systems", Vol. 11, Issue 2, AES-2, No. 4, June 1964, pp. 81-85 [pdf]
• Ref. 31: "Das Funk-Blindlandegerät" [Fu Bl I, EBl 1, EBl 2, Fu Bl 2], Werner Thote, in "Radiobote", Vol. 2, Nr. 9, May-June 2007, pp. 20-25. Source: radiobote.at. [pdf]
• Ref. 32: "Beschreibung und Betriebsvorschrift für Funklande-Empfangsanlage Fu Bl 1 Ex" [Description and operating instructions for landing receiver system Fu Bl 1 Ex"], DTA 140, C. Lorenz AG, 1940, 59 pp. Source: www.cockpitinstrumente.de
• Ref. 33: pp. xxxxx in "Confound and Destroy: 100 Group and the bomber support campaign", Martin Streetly, Jane's Publishing, Inc. (publ.), 1978, 379 pp. [pdf, file size 35 MB]. See note 1
• Ref. 35: pp. 8-11, 33 in "The Hut Six Story - Breaking the Enigma Codes", Gordon Welchman, M & M Baldwin Publ., 6^th ed., 2011, 263 pp.
• Ref. 38: documents of the Sub-Committee for the Investigation of German Electronic and Scientific Organisation (SIGESO). Source: www.cdvandt.org.
• Ref. 38A: "Navigational Aids for Bombers" [Knickebein, X, Y, Benito, Erika, Sonne, Zyklop], sheet 3-6 in Section 0.1 of A.L. No. 46 of SIGESO, 12/12/1945, Report Vol. 1, Part 2. 
• Ref. 38B: "The Goldwever System of Navigational Aid for U-Boats", sheet 30 in Section 2.2, A.L. No. 61 of SIGESO,12/2/46.
• Ref. 39: Luftwaffe manuals for ground and on-board landing beam systems element. Source: www.cdvandt.org. Retrieved: January 2018, September 2020.
• Ref. 39A: "Funk-Landegerät Fu Bl I - Geräte-Handbuch, Beschreibung und Betriebsvorschrift", Luftwaffe Druckschrift D. (Luft) T.4065, April 1942, 50 pp.
• Ref. 39B: "UKW-Landefunkfeuer 120 Watt und 500 Watt, Aufbau-Anweisung", Luftwaffe Druckschrift D. (Luft) T.4458, June 1943, 8 pp.
• Ref. 39C: "AS 4 Anflugführungssender 4 - Geräte-Handbuch", Luftwaffe Druckschrift D. (Luft) T.4456, June 1943, 36 pp.
• Ref. 39D: "Ansteuerungs-Sender AS 2 - Geräte-Handbuch, Beschreibung und Wirkungsweise sowie Bedienung und Wartung", Luftwaffe Druckschrift D. (Luft) T.4452, May 1941, 38 pp.
• Ref. 39E: "Antenne AFFA 2- Geräte-Handbuch, Beschreibung und Wirkungsweise sowie Bedienung", Luftwaffe Druckschrift D. (Luft) T.4454, September 1941, 17 pp.
• Ref. 39F: "Beschreibung und Betriebsvorschrift für 5 Watt-Ultrakurzwellen-Einflugzeichensender EZS 2", Luftwaffe Druckschrift D. (Luft) T.4451, February 1941, 30 pp.
• Ref. 39G: "Beschreibung und Betriebsvorschrift für Funkfeuer-Kontrolempfänger EBKS", Luftwaffe Druckschrift D. (Luft) T.4450, February 1941, 17 pp.
• Ref. 40: documents about FuG 16 and FuG17
• Ref. 40A: "Die Bordfunkgeräte FuG 16 und FuG 17", pp. 28-49 in "Berühmte Bordfunkgeräte - ein Beitrag zur Geschichte der Elektrotechnik" [FuG10, FuG16, FuG17, FuG25a, FuG101a, ...], H. Sarkowski, Expert Verlag, 1983, 80 pp.
• Ref. 40B: "Bordfunkgerät FuG 16 ZY mit Aufbauvorschrift für Antenne des Zielflug-Senders", Luftwaffe Druckschrift D.(Luft)T.4069, 5 August 1944, 114 pp. Source: cdvandt.org.
• Ref. 40C: "Das Bordfunkgerät FuG17", Heft 205 of "Teil 1 - Gerätebeschreibungen" of Luftwaffe Druckvorschrift L.Dv.702/1, April 1941, 28 pp. Source: cdvandt.org, accessed 23 September 2020.
• Ref. 41: to be allocated
• Ref. 77: "History of the German night fighter force, 1917-1945", Gebhard Aders, Jane's Publishing Company, 1^st ed., 1979, 284 pp. [file size 62 MB].  See note 1
• This is the translated and edited version (by A. Vanags-Baginskis and B. Gallagher) of the original German version "Geschichte der deutschen Nachtjagd: 1917-1945", Gebhard Aders, Motorbuch Verlag (publ.),1977, 391 pp.
• Ref. 77A: pp. 96, 182.
• Ref. 77B: pp. 194, 195.
• Ref. 77C: pp. 246, 237.
• Ref. 90:
• Ref. 90A: private message to me in March of 2015 in the Axis History Forum
• Ref. 90B: pp. 21, 57, 58 in "Stations radar et radio-navigation sur le Mur de l'Atlantique - Spécial Normandie d'Antifer à Granville", Alain Chazette, Bernard Paich, Alain Destouches, Jacques Tomine, Jörg Poweleit, Michaël Svejgaard, Histoire & Fortifications, 2015, 96 pp
• supplement: "Stations radar et de radio-navigation sur le Mur de l'Atlantique (complement photos)", 32 pp.
• Ref. 90C: p. 161 in "Stations radar et radio-navigation sur le Mur de l'Atlantique - Volume 2 -Spécial Belgique - Nord - Pas-de-Calais - Picardie - Haute-Normandie", Alain Chazette, Bernard Paich, Pierre Nowak, Alain Destouches, Jacques Tomine, Ingrid Paindavoine, Histoire & Fortifications, 2016, 160 pp.
• Ref. 90D: "Drawing of a Funk Sende Anlage Bernard 724/725 bunker", 14-March-2015 thread in Axis History Forum; used with permission
• Ref. 90E: "German Military Symbols", U.S. War Dept., General Staff, Military Intelligence Service, January 1943, 152 pp. (public domain, no ©)
• Ref. 137: "Ausstrahlung, Ausbreitung und Aufnahme Elektromagnetischer Wellen", Ludwig Bergmann, Hans Lassen, Vol. 2 of "Lehrbuch der drahtlosen Nachrichtentechnik" ["Textbook of wireless communication"], Nicolai von Korshenewsky (ed.), Wilhelm T. Runge (ed.), Springer -Verlag (publ.), 1940, 286 pp. Source: libarch.nmu.org, retrieved 18 May 2020. [file size: 22 MB]
• Ref. 137A: "Die Dipolreihe, Dipolgruppe, und Dipolebene" ["The dipole row, dipole array, dipole plane"], pp. 62-73.
• Ref. 137B: "Richtantennen für Leitstrahlanordungen (Funkbaken)" ["Directional antennas for guide-beams (radio beacons)"], p. 97-98 in Chapter II.
• Ref. 151: p. 16, 17, 22 in "Jagdschloß A (Lehrunterlagen) Teil I", 2^nd ed., Lehrschule für Fernmeldetechnik, Detmold, November 1944, 115 pp. Source: www.cdvandt.org
• Ref. 164: documents of the Combined Intelligence Objectives Sub-Committee (CIOS). Source: cdvandt.org.
• Ref. 164A: "Institutes of the Bevollmaechtigter fuer Hochfrequenz-Forschung", CIOS, Item No. 1 & 7, File No. XXXI-37, May 1945, 215 pp.; source: cdvandt.org.
• Ref. 164B: "The I.T.T., Siemens and Robert Bosch Organizations" [incl. Lorenz: Elektra, Sonne, Mond, Knickebein, Erika, Goldwever, Hermine, 6-mast Lorenz Adcock D/F based on British patent, Lorenz Blind Approach/Landing, IFF, radar], CIOS, Item No. 1, 7 & 9 (Radar, Signal Communications, Physical & Optial Instruments and Devices), File No. XXXI-38. Source: cdvandt.org. Retrieved 22 September 2019.
• Ref. 164C: "Report on C. Lorenz A.G." [history, factories, relationship with Philips & Telefunken, Stuttgart UHF radio link (FuG0s, FuG03a), Feuerzauber & Feuermolch Oboe-jammer, FuG200, FuG226/Neuling], CIOS, Item No. 1, File No. XXV-12, 13 pp. Source: cdvandt.org. Retrieved 7-Feb-2020.
• Ref. 172: copy of item in file AIR 29/284 "Central Interpretation Unit (CIU) Medmenham; Interpretation reports and aerial photos (1943)". Item is in the collection of The National Archives; material with UK Crown Copyright, used in accordance with the Open Government License [pdf].
• Summary of the contents of ref. 172A.
• Ref. 172A: "German “Windjammer” R.D.F. Stations", part of "Monthly interpretation review for July 1943", 7 pp.
• Ref. 173: copy of items in file AIR 14/3577 "Signals investigation on 27 to 35 Mc/s "Windjammer" (1943/1944)". Items are in the collection of The National Archives; material with UK Crown Copyright, used in accordance with the Open Government License [pdf].
• Summary of the contents of ref. 173A-173E.
• Ref. 173A: "'Windjammer" observation", by R.A. Fareday (Noise Investigation Bureau [Electronic Intelligence], N.I.B., London), dated 20^th June 1944, 1 page.
• Ref. 173B: "Possible "Windjammer" transmissions", report by Flight Lieutenant Douglas of 192 Sq., dated 16^th December 1943, 1 page.
• Ref. 173C: "192 Squadron Flight report No. 215/43" by F/Lt Robinson to Squadron Leader Burtler, dated 15^th November 1943 (actual report by P/O G.F. Evans of 13^th November 1943), 6 pages.
• Ref. 173D: "Windjammer – Arcachon", letter from Air Ministry A.I.4. [intelligence branch section supervising RAF Y Service] to Commanding Officer of 192 Squadron, dated 16^th July 1943, 1 page + 1 aerial photo.
• Ref. 173E: "The Windjammer and Dreh-Elektra", by 192 Squadron Leader J. Whitehead, dated 18^th June 1943, 1 page.
• Ref. 174: copy of items in file AIR 14/3594 "Windjammer" [ ="Bernhard"] station: photographs and interpretation reports. Includes vertical and low oblique aerial photographs of "Windjammer radar" sites in Germany and France (1943/1944)". Items are in the collection of The National Archives; material with UK Crown Copyright, used in accordance with the Open Government License [pdf].
• Summary of the contents of ref. 174A-174J.
• Ref. 174A: Letter entitled "W/T Bergen/Belvedere" by Squadron Leader C.W. Swanell on behalf of the Group Captain commanding R.A.F. Station Medmenham to R.V. Jones (A.D.I. Science), dated 9^th April 1943, 1 page + 1 photo
• Ref. 174B: Aerial photo of station "Bergen/Belvedere" [The Netherlands], photo No. 3022, taken 22^rd March 1943 by 541 Squadron
• Ref. 174C: Letter entitled "W/T – Bergen/Belvedere" by Group Captain commanding R.A.F. Station Medmenham to R.V. Jones (A.D.I. Science), dated 9^th April 1943, 1 page. (note: photos referenced in letter not on file)
• Ref. 174D: Letter entitled "W/T Desvres/Le Bois Julien" on behalf of Group Captain commanding R.A.F. Station Medmenham to R.V. Jones (A.D.I. Science), dated 15^th November 1942, 1 page + 2 photos.
• Ref. 174E: Letter entitled "W/T: Desvres/Le Bois Julien" on behalf of Group Captain commanding R.A.F. Station Medmenham to R.V. Jones (A.D.I. Science), dated 29^th March 1943, 1 page + 1 photo.
• Ref. 174F: "Interpretation Report No. G. 308" dated 28^th June 1942, of aerial photos taken over Desvres/Le-Bois-Julien at altitude of 20k ft during Sortie A/945 on 20^th June 1942, 1 page + 1 photo.
• Ref. 174G: "Interpretation report No. G.590" dated 6^th October 1942, of aerial photo taken over locality Morlaix, W/T station Mt. St. Michel, at altitude of 12k ft during Sortie Q/21 on 24^th September 1942, 2 pages + 1 photo.
• Ref. 174H: Letter entitled "W/T: Pouzauges/St.Michel-Mont-Mercure" on behalf of Group Captain commanding R.A.F. Station Medmenham to Squadron Leader Whitehead (A.I.4), dated 29^th March 1943, 1 page + 3 photos.
• Ref. 174J: Photos No. 4065 and 4066 of station at St. Vaast / La Pernelle, taken 31^st March 1943 from off shore. [station is fully, though vaguely, visible on horizon]
• Ref. 181: "Drehfunkfeuer System Telefunken - Teil 1: Verfahrensbeschreibung EC1-4262" [Telefunken rotating radio beacon, part 1: description of the method], Adalbert Lohmann, Berlin, October 1942, 129 pp., copy nr. 29, personal copy of Albrecht Leyn [note: this document was never printed, other than a very limited number of personal copies, individually approved by Dept. LC-4 (Technisches Amt) of the RLM; ref. 183]; source: corporate archives of DTM Berlin, part of file nr. I.2.060C-06172  [file size: 62 MB]
• Ref. 183: "Das Drehfunkfeuer-Verfahren Bernhard und Bernhardine, System Telefunken" ["Verfahrensbeschreibung Bernhard, Bernhardine", description of the Bernhard-Bernhardine method], Adalbert Lohmann, Telefunken Gesellschaft für drahtlose Telegraphie m.b.H., Berlin-Zehlendorf, Telefunken document EC 1 4310, July 1943, 28 pp., copy nr. 11; source: corporate archives of DTM Berlin, file nr. I.2.060C-04403.
• Ref. 184: radio direction finding
• Ref. 184A: articles on Watson-Watt direction finding method
• Ref. 184A1: "Basics of the Watson-Watt Radio Direction Finding Technique", RDF Products Web Note WN-002, December 1998, 12 pp. Source: www.rdfproducts.com, accessed 3 March 2020. [pdf]
• Ref. 184A2: "Busting Watson Watt DF ambiguity - maths edition", David A Moschella, cyntony.com blog post, 13 July 2017. Accessed 16 March 2020. [pdf]
• Ref. 184A3: "Adcock/Watson-Watt Radio Direction Finding", Ismael Pellejero (EA4FSI) Technical Articles, 15 August 2012. Accessed 3 March 2020. [pdf]
• Ref. 184A4: "Über den Rahmeneffekt eines aus vertikalen Linearantennen bestehenden Adcock-Peilers: Der Zusammenbruch eines Dogmas" ["Loop effect of an Adcock D/F - the collapse of a dogma"; Adcock Watson-Watt configuration does not always eliminate horizontally polarized signals], Gottfried Eckart, pp. 151-178 in "Sitzungsberichte", Verlag der Bayerischen Akademie der Wissenschaften (publ.), 1 January 1972, 28 pp.  [pdf]
• Ref. 184B: "Het radio-peilen" ["Radio D/F"; in Dutch language; DF, multi-path, Telefunken Compass toroidal coil coupling, radio goniometer, rotating loop, fixed-course crossing loop pair, night effect, shoreline effect], Anthonet Hugo de Voogt, pp. 74-94 in "Tijdschrift van het Nederlandsch Radiogenootschap", 23 April 1921. Source: kivi.nl (Deel 01), accessed 18 March 2020.
• Ref. 184C: "Antennas for radio direction finding (RDF)", Chapter 23 (pp. 439-456) in "Practical Antenna handbook", 4^th ed., Joseph J. Carr, McGraw-Hill, 2001, 625 pp.
• Ref. 184D: "Summary Technical Report", E.C. Jordan et al, Technical Report No. 4, The Radio Direction Finding Research Laboratory, Dept. of Electrical Engineering, University of Illinois, Urbana/IL/USA, 15 April 1948, 70 pp. Source: Defense Technical Information Center (DTIC). Accessed 12 March 2020. [pdf] [Summary]
• Ref. 184E: "1957 Pioneer Awards in Aeronautical and Navigational Electronics: Alessandro Artom" [invention of the radio goniometer by Alessandro Artom, i.e., before Bellini & Tosi], Robert I. Colin, pp. 44-47 in "IRE Trans. on Aeronautical and Navigational Electronics", Vol. ANE-4, Issue 2, June 1957 [pdf]
• Ref. 184F: "Sammlung der Vorträge anlässlich der Arbeitstagung "Navigation", Arbeitskreis "Navigation", Bevollmächtigte der Hochfrequenzforschung ["Collection of presentations made at the workshop of the "Navigation" working group of the Commissioner for RF Research"], at Ferdinand Braun Institute, Landsberg am Lech, Germany, 23/24 March 1944. Source: cdvandt.org. Accessed 14 March 2020.
• Ref. 184F1: "Über grundsätzliche Fragen der Richtungs- und Entfernungsmessung" ["About fundamental questions regarding radio direction and distance measurement"], Paul von Handel, pp. 9-26 [pdf]
• Ref. 184F2: "Funknavigation mittels Laufzeitverfahren auf Kurzwelle" ["Short-wave radio navigation by means of time-of-flight method"], W. Dieminger, pp. 54-67. [pdf]
• Ref. 184F3: "Empfangs- und Peilanlagen mit gebündelter Charakteristik (Sektorpeilanlagen)" ["Directional reception and DF installations (sector-DF)"; RDF "Guben", "Wullenwever, "Brommy"], Hermann Janssen, pp. 98-120 in  [pdf]
• Ref. 184G: "Reduction of Night Error in Radio Direction-Finding Equipment for Aerodromes", H. Busignies, in "Electrical Communication - A Journal of Progress in the Telephone, Telegraph and Radio Art", Int'l Standard Electric Corp., Vol. 16, No. 3, January 1938, pp. 213-232. Source: www.worldradiohistory.com, retrieved 22 May 2020. [pdf]
• Ref. 184H: "Funkortung", Wilhelm Tolmé Runge, pp. 7, 8 in "Telefunken Hausmitteilungen", Telefunken, Vol. 20, Nr. 82, December 1939.
• Ref. 184J: "Air Navigation Systems: Chapter 3. The Beginnings of Directional Radio Techniques for Air Navigation, 1910–1940", Brian Kendal, in "The Journal of Navigation", Volume 43, Issue 3, September 1990, pp. 313-330.
• Ref. 184K: "Introduction into Theory of Direction Finding", pp. 62-85 in "Rohde & Schwarz Radiomonitoring & Radiolocation Catalog 2016", 24 pp. Retrieved 28 February 2020.
• Closely related: "An Introduction to Radio Direction Finding Methodologies", Paul Denowski, Rohde & Schwarz, 99 pp. Accessed 3 March 2020. [pdf]
• Ref. 184L: "Drahtloses Peilen" [wireless direction finding], A. Esau, pp. 3-12 in "Telefunken-Zeitung", Vol. IV, Nr. 22, March 1921. [Summary]. Source: radiomuseum.org.
• Ref. 184M: "War secrets in the ether", Wilhelm F. Flicke, translation by Ray W. Pettengill of Flicke's original German manuscript "Kriegsgeheimnisse im Aether", source: US National Security Agency (NSA), retrieved 8 March 2020.
• Part I & II (the period from the inception of the German Radio Intercept Service to the End of WW I, and the period between the two World Wars), NSA Document ID A59421, 288 pp. [pdf]
• Part III (aspects of the large scale espionage and the counterespionage (crypto, radio agents,..)), NSA Document ID A59332, 428 pp. [pdf]
• Ref. 184N: "Geschichte der Funkpeiltechnik" [History of radio direction finding], Rudolf Grabau, in "Funkgeschichte" (Mitteilungen der Gesellschaft der Freunde der Geschichte des Funkwesens (GFGF) e.V). [Keywords]
• Ref. 184N1: "(1) - Entwicklung der Funkpeilung bis 1945" [Development up to 1945], Vol. 28, 2005, Nr. 164, pp. 268-276. Source: www.radiomuseum.org. Accessed 28 March 2020. [pdf]
• Ref. 184N2: "(2) - Entwicklung der Funkpeilung ab 1945" [Development from 1945 on], Vol. 29, 2006, Nr. 165, pp. 24-30. Source: www.radiomuseum.org. Accessed 28 March 2020. [pdf]
• Ref. 184P: "Radio Direction Finding", US Army Field Manual No. 30-476, 8 April 1977, 221 pp. [file size 25 MB]
• Ref. 184Q: "Direction and Position Finding by Wireless" [directional reception & transmission, incl. loop, cardioid, propagation] , Ronald Keen, The Wireless Press (publ.), 1922, 411 pp. Source: archive.org. Accessed: 6 May 2020.
• Ref. 184R: "Radio direction-finding and navigational aids, some reports on German work issued in 1944-45", Scientific and Industrial Research (D.S.I.R.) Radio Research Board, Special Report No. 21, 1951, 96 pp.
• 1-page review of the above document: "Radio Direction-Finding and Navigational Aids", in "Nature", Vol. 169, Nr. 4308, 24 May 1952, p. 785. Source: nature.com, retrieved 14 October 2020.
• Ref. 184S: "Beschreibung für Großpeil- und Empfangsanlage Wullenwever Type HF 2076" [Description of large RDF and receiver installation "Wullenwever"], 11 March 1946, 27 pp. Source: cdvandt.org. Retrieved 3 February 2020.
• Ref. 184T: "The determination of the direction of arrival of short radio waves", H.T. Friss, C.B. Feldman, W.M. Sharpless, in "Proceedings of the Institute of Radio Engineers", Vol. 22, Nr. 1, January 1934, pp. 47- 78. Source: worldradiohistory.com, retrieved 21 July 2020.
• Ref. 184U: articles in "Electrical Communication - A Journal of Progress in the Telephone, Telegraph and Radio Art", published by "International Standard Electric Corp". Source: worldradiohistory.com, accessed 17 August 2020.
• Ref. 184U1: "The automatic radio compass and its application to aerial navigation", H. Busignies, in Vol. 15, No. 2, October 1936, pp. 157-172.
• Ref. 184U2: "Mountain effects and the Use of Radio Compasses and Radio Beacons for Piloting Aircraft", H. Busignies, in Vol. 19, No. 3, 1941, pp. 44-70.
• Ref. 184U3: "Evaluation of Night Errors in Aircraft Direction Finding, 150-1500 Kilocycles", H. Busignies, in Vol. 23, No. 1, March 1946, pp. 42-62.
• Ref. 184V: "Funkpeilung als alliierte Waffe gegen deutsche U-Boote 1939 - 1945: Wie Schwächen und Versäumnisse bei der Funkführung der U-Boote zum Ausgang der "Schlacht im Atlantik" beigetragen haben" [file size 74 MB], Arthur O. Bauer, Ralph Erskine, Klaus Herold, Liebich Funk GmbH (publ.), 1997, 323 pp., ISBN 3-00-002142-6. Source: cdvandt.org, retrieved 28 September 2020.
• An excerpt in English is: "HF/DF An Allied Weapon against German U-Boats 1939-1945", Arthur O. Bauer, 1998, 23 pp. Source: cdvandt.org, retrieved 28 September 2020.
• Ref. 184W: "Radio Direction Finding, 10 kc/s to 550 kc/s", Walter C. Weaver, MsEE Thesis, U.S. Naval Postgraduate School, 1957, 107 pp. Source: archive.org, retrieved 14 October 2020.
• Ref. 184X: "Direction finder and antenna research" [file size 38 MB; full resolution file is here, 242 MB], Office of Scientific Research and Development, National Defense Research Committee, Division 13 (Electrical Communication), Summary Technical Report, 1946 (declassified 1960), 292 pp. Source: loc.gov, retrieved 19 February 2021.
• Ref. 185: radio location & navigation, general articles, articles covering multiple systems
• Ref. 185A: "Survey of Radio Navigational Aids" [DF, Shoran, Oboe, DME, transponders, radar, Loran, GEE, Decca, Raydist, Micro-H, Consol, Sonne, Navaglobe, A-N ranges, CAA Omnirange, ILS landing beam, radio altimeters, rotating beacons, Orfordness range, Navar, Teleran, Navascope], Robert I. Colin, in "Electrical Communication" (Technical Journal of the International Telephone & Telegraph Corporation and Associate Companies), Vol. 24, No. 2, June 1947, pp. 219-261. Source: www.worldradiohistory.com; accessed 27 March 2020. [pdf]
• Ref. 185B: "The Geography of Radionavigation and the Politics of Intangible Artifacts", William Rankin, in "Technology and Culture", Volume 55, Number 3, July 2014, pp. 622-674 [pdf]
• Ref. 185C: "Electronic Navigation Systems", Summary Technical Report of Division 13 (Electrical Communication) of the National Defense Research Committee (NDRC) - Vol. 2B, 1946, 374 pp. [file size 211 MB; a lower-but-good resolution version is here, 49 MB]. Source: www.loc.gov. Retrieved 8 August 2008. [Beacons, Oboe, Decca, A-N, Radio Range, Sonne/Consol, Gee, POPI, ground-based radar, airborne radar, Loran, Shoran, Micro-H, Four-Course Aural Range, Federal Long-Range Navigation System, ADF, Elektra, Benito, Knickebein, Ruffian, Hermine, Rübezahl/Egon, Bernhard/Bernhardine, Hyperbol, Truhe, Zyklop/cyclop, Dora, Erika, Diskus, Schwanboje, Nachtfee]
• Ref. 185D: "Air Navigation", U.S. Navy Hydrographic Office H.O. Publication No. 216, corrected print 1963, 717 pp.
• Ref. 185D1: "Lines of Position, Bearings, and Fixes", Chapter IX, pp. 188-202
• Ref. 185D2: "Low Frequency Radio Range", pp. 288-300 in Chapter XI "Radio" (pp. 261-300).
• Ref. 185D3: "Hyperbolic Navigation Systems" [Loran, Decca, Lorac, Sofar, Consol, Sonne, Consolan, GEE], Chapter XIII, pp. 345-365.
• Ref. 185E: "On the origins of RF-based location", Hans Gregory Schantz, in "Proc. 2011 IEEE Radio & Wireless Symposium", Phoenix/AZ/USA 16-20 Jan., 2011. Source: researchgate.net, retrieved 16 Jan 2020.
• Ref. 185F: "Radiobeacons and radiobeacon navigation", George R. Putnam, U.S. Dept. of Commerce, Lighthouse Service, 1 July 1931, 44 pp. [pdf]
• Ref. 185G: p. 260 in "A survey of continuous-wave short-distance navigation and landing aids for aircraft", Caradoc Williams, in "Journal of the Institution of Electrical Engineers - Part IIIA: Radiocommunication", Volume 94, Issue 11, March-April 1947, pp. 255 - 266.
• Ref. 185H: "History of radio flight navigation systems - Memoirs of Dr. E. Kramar", M. Hollmann & P. Aichner (translation, ed.), 15 pp. Source: radarworld.org. [Scheller A/N, Lorenz E/T, Telefunken Knickebein, Hermine, X-System / Wotan I, Four-Course Range, ILS, Elektra, Consol, Erika, Komet, Hohentwiel]
• Ref. 185J: "Navigation und Luftsicherung" ["Navigation and air traffic control"], Leo Brandt, pp. 25-80 in "Arbeitsgemeinschaft für Forschung des Landes Nordrhein-Westfalen", Vol. 13, Springer Fachmedien, 1952, 98 pp. Accessed 3 March 2020. [pdf]
• Ref. 185K: "Die Funknavigation der Luftfahrt", August Leib, pp. 9-68 in "Telefunken Hausmitteilungen", Telefunken, Vol. 20, Nr. 82, December 1939.
• Ref. 185L: "A Brief Description of the Major Second World War Navigational Aids", Brian Kendal, in "Journal of Navigation", Vol. 45, No 1, Januay 1992, pp. 70 - 79.
• Ref. 185M: "Highlights of antenna history", Jack Ramsay, in "IEEE Communications Magazine", Vol. 19, Iss. 5, September 1981, pp. 4-16. Accessed 6 May 2020. [pdf]
• Ref. 185N: "Flugsicherung durch Richtfunkbaken" ["Radio-beacon aids to aerial navigation"], H. Rahskopff, in "Zeitschr. des Vereins deutscher Ingenieure (V.D.I.)", Vol. 75, 4 January 1931, pp. 116-117.
• Ref. 185P: "Radio navigation in the 1920s", C. Powell, in "Journal of the Institution of Electronic and Radio Engineers (IERE)", Vol. 56, Nr. 8-9, August/September 1986, pp. 293-297.
• Ref. 185Q: "La Radionavigation" [in French], J. Piergo, pp. 159-167 in "Science et Vie", No. 349, October 1946.
• Ref. 185R: "The Aeronautical Navigational Radio Service" [Consol, Decca, Loran, GEE, SBA, BABS, MF Range], Chapter VII (pp. 101-109) in "The Civil Aviation Communications Handbook", Vol. 5 of CAP series (M.C.A.P.5), Great Britain Ministry of Civil Aviation, 2^nd ed., 1949, 286 pp. Source: atchistory.files.wp.com. Accessed 19 May 2020. [pdf, file size 26 MB]
• Ref. 185S: "RF Positioning - Fundamentals, Applications, and Tools" [incl. RDF, TFK Kompass, Orefordness, Sonne, Gee, Oboe, Gee-H, Loran], Rafael Saraiva Campos, Lisandro Lovisolo, Artech House (publ.), 29 pp. Accessed: 31 January 2020. [pdf]
• Ref. 185T: "Mail Planes Radio Equipment", in "Radio Topics", March 1924, pp. 15-16. Source: worldradiohistory.org, accessed 14 April 2020.
• Ref. 185U: "Airlines and Air Mail - The Post Office and the birth of the commercial aviation industry", F. Robert van der Linden, The University Press of Kentucky, 2002, 367 pp. Source: core.ac.uk, retrieved 12 February 2021 [file size: 25 MB]
• Ref. 186: Transmitter technology - spark gap, arc converter, machine generator, vacuum tube
• Ref. 186A: pp. 420-423 in "Handbuch der drahtlosen Telegraphie und Telephonie - Ein Lehr- und Nachschlagebuch der drahtlosen Nachrichtenübermittlung" ["Handbook of wireless telegraphy and telephony - a textbook and reference book of wireless communication"] , Vol. 1, Eugen Nesper, Julius Springer (publ.), 1921, 708 pp. Source: archive.org, accessed 30 March 2020. [pdf; file size: 65 MB]
• Ref. 186B: "Die »Tönenden Funken« - Geschichte eines frühen, drahtlosen Kommunikationssystems, 1905-1914" ["Tonal quenched spark gap transmission - history of an early wireless communication system"], Michael Friedewald, Vol. 2 of "Aachener Beiträge zur Wissenschafts- und Technikgeschichte des 20. Jahrhunderts", GNT-Verlag (publ), 1999, 185 pp., ISBN 978-3-928186-38-4. [Table of Contents]
• Ref. 186C: "Über die Intensität der beiden Schwingungen eines gekoppelten Senders" ["About the intensity of both oscillations of a coupled transmitter"; quenched-spark transmitter], Max Wien, in "Physikalische zeitschrift", Vol. 7, Nr. 23, 15 November 1906, pp. 871-872.
• Ref. 186D: "Beschreibung der kommerziellen Land-Schiff-Station Telefunken Type 0,5 T.K. 0,5 KW Antennen-Schwingungsenergie bei 1,5 KW Primärenergie" ["Description of the Telefunken commercial land/ship-board transmitter model 0,5 T.K. 0.5 kW output power for 1.5 kW dissipation"], 1916 Telefunken brochure (transcribed and adapted by Heinrich Busch, 2016). Accessed 3 April 2020. [pdf]
• Ref. 186E: "The Spark Transmitter", "Section "A", 27 pp, in "Wireless telegraphy Theory", Vol. II of "Admiralty Handbook of Wireless Telegraphy", H.M. Signal School, 1938, 1943 revised ed. , B.R.230, 530 pp. Accessed 15 April 2019. [pdf]
• Ref. 186F: "Die technische Entwicklung der Verschiedenen F.T.-Systeme" ["The technical development of various spark transmitter systems"; Poulsen, Wien, HF machine-generators], pp. 3-6 in "Funkentelegraphie für Flugzeuge" [Spark gap telegraphy for aircraft], Erich Niemann, Vol. IX of "Handbuch der Flugzeugkunde", Richard Carl Schmidt & Co. (publ.), 1921, 434 pp. Accessed 15 April 2020. [pdf]
• Ref. 186G: "Wireless telegraphy", Jonathan Zenneck, McGraw-Hill Book Co. (publ.), 5^th ed., 1918, 443 pp. [pdf; file size: 350 MB]; translation of the German original "Lehrbuch der drahtlosen Telegraphie", Jonathan Zenneck, F. Enke (publ.), 1913, 521 pp.
• Ref. 186G1: Chapter VII: "Transmitters of damped oscillation" [Marconi, Braun, Wien, stationary & rotating electrodes, quenched], pp. 173-212. [file size: 35 MB]
• Ref. 186G2: Chapter VIII: "High frequency machines for undamped oscillations" [Fessenden, Alexanderson, C. Lorenz Co., Goldschmidt, Eberswalde], pp. 213-219.
• Ref. 186G3: Chapter IX: "Undamped oscillations by the arc method" [Poulsen], pp. 220-245. [file size: 21 MB]
• Ref. 186H: §8 of Article 5 (pp. 36-37), §1 & §3 of Article 16 (p. 74-76) in International Telegraph Convention 1927 - Treaties, Washington/DC/USA, 1927, 171 pp. Source: International Telecommunication Union, accessed 29 April 2020.
• Ref. 186J: "Zusammenstellung der modernsten tönenden und ungedämpften Radio-Stationen und -Geräte" ["Overview of the most modern quenched spark and undamped (continous wave) radio sets and equipment"], Telefunken Gesell. für drahtlose Telegraphie m.b.H., product catalog, November 2020 (product status of early 1919), 244 pp. [file size: 32 MB]. Source: www.cdvandt.org. Accessed 1 May 2020.
• Ref. 186J1: Chapter 2, Group II: "Gruppe II. Land- und Schiffs-Stationen" ["Group II, Land and shipstations"], 30 pp. [singing spark transmitter models 0,2 TK; 0,5 TK; Debeg-Schrank-Station; 1,5 TK; 2,5 TK; 5 TK; 7,5 TK; 10 TK; 15 TK; 25 TK; 35 TK; 0,4 TV Behelfs-Station; 0,5 TV; 1 TV; 2 TV; 2,5 TV; 4 TV; 10 TV; all of these spark transmitter models appear to have alreday been marketed by mid-1913; continuous wave vacuum tube transmitter models 0,5 kW; 1 kW, 5 kW].
• Ref. 186J2: Chapter 2, Group III: "Gruppe III. Flugzeug- und Luftschiffstationen" ["Group III, Aeroplane and airship stations"], 16 pp. [AFS 35; Sender-Empfänger A, D4, G, N, R, ALS 49; Röhren-Sender-Empfänger ARS 80a; Empänger E; Dynamo Maschine B, C, D16, D17, G, R, RS, TL 3,5/20]
• Ref. 186K: "Funkentelegraphie und -telephonie mit ungedämpften Schwingungen" ["Spark gap telegraphy and telephony with undamped waves"], Rudolf Grabau, in "Funkgeschichte", Vol. 29, Nr. 168, 2006, pp. 177-185. Accessed 19 April 2020. [pdf]
• Ref. 186L: "Elektromagnetische Schwingungen und Drahtlose Telegraphie" ["Electromagnetic oscillations and wireless telegraphy"], Jonathan Zenneck, Ferdinand Enke (publ.), 1905, 1056 pp. Source: us.archive.org, accessed 19 April 2020.
• Ref. 186M: "Sendeprinzipien" ["Principles of radio transmitters"], pp. 332-335 in "Radios von gestern: Das Sachbuch für Sammler und Radio-Amateure", Ernst Erb, 3^rd ed., 1998, 456 pp. Accessed 19 April 2020 [pdf]
• Ref. 186N: "Historical remarks to the history of electrical oscillators", Wolfgang Mathis, in "Proc. of Mathematical Theory of Networks and Systems Conference", Padua/Italy, July 1998", 5 pp. Source: www.researchgate.net. Accessed 1 May 2020.
• Ref. 186P: "Die Technik der Funkentelegraphie mit gedämpften Schwingungen", ["Spark gap telegraphy and telephony with damped waves"], Rudolf Grabau, in "Funkgeschichte", Vol. 29, Nr. 167, 2006, pp. 136-147. Accessed 19 April 2020. [pdf]
• Ref. 187: Telefunken Compass
• Ref. 187A: articles about the German Fog Signal Service and the "Seezeichenversuchsfeld" (Maritime Navigation Markers Test Site)
• Ref. 187A1: "Elektrische Wellen im Nebelsignaldienst" ["Electrical waves in the Fog Signal Service"], Walter Körte, pp. 570-571 in "Zentralblatt der Bauverwaltung", Nr. 87, 30 October 1909. Source: digital.zlb.de. Retrieved February 2020.
• Ref. 187A2: "Die Entwicklung des deutschen Seezeichen-Versuchswesens" ["The development of the German maritime signalling research institute"], Breuer, in "Zentralblatt der Bauverwaltung", Vol. 50, Nr. 1, 8 January 1930, pp. 44–46. Source: digital.zlb.de. Retrieved 17 April 2020.
• Ref. 187A3: "Das Seezeichen-Versuchsfeld des Reichsverkerhsministeriums in Berlin-Friedrichshagen" ["The maritime signalling test site of the Ministry of Transport in Berlin-Friedrichshafen], Breuer, in "Zentralblatt der Bauverwaltung", Vol. 50, Nr. 25, 25 June 1930, pp. 452-457, and Nr. 26, 2 July 1930, pp. 467-471. Source: digital.zlb.de. Retrieved 17 April 2020.
• Ref. 187A4: "Nautischer Verein zu Bremen" ["The Nautical Association in Bremen"], in "HANSA, Deutsche Nautische Zeitschrift", Vol. 50, No. 49, 6 December 1913, p. 1034. Source: digishelf.de. Retrieved 16 April 2020.
• Ref. 187A5: "Funknavigation (Seefunkfeuer) - Arbeiten der preußischen und deutschen Seezeichen Verwaltung und des Seezeichenversuchsfeld - Eine Zusammenfassung der Entwicklungsgeschichte der funktechnischen Seezeichen aus Akten von 1905 - 1939; Teil 1 (1905 - 1910)", Johannes Braun, 1962. Source: web.archive.org (part 2 not available). Retrieved 17 April 2020.
• Ref. 187A6: "Geheimer Oberbaurat Körte" ["Privvy Counsel Körte"], Roloff, in "Zentralblatt der Bauverwaltung", Vol. 34, Nr. 39, 16 May 1914, pp. 297-298. Source: digital.zlb.de. Retrieved 17 April 2020.
• Ref. 187A7: "Die Versuche der deutschen Verwaltung mit elektrischen Wellen im Nebelsignaldienst (Funkfeuer / Seefunkfeuer)" ["Experiments by the Authorities with electrical waves in the Fog Signal Service (radio beacons / maritime radio beacons)"], Johannes Braun, Fachstelle der WSV für Verkehrstechnik (publ.), Koblenz 1962. [Summary]. Retrieved in part from web.archive.org on 25 February 2020
• Ref. 187B: p. 3 in "Telefunken und der deutsche Schiffsfunk, 1903 - 1914" ["Telefunken and German marine radio"], Michael Friedewald, in "Zeitschrift für Unternehmensgeschichte", Vol. 46, Issue 1, April 2001, pp. 27-57. Retrieved 19 April 2020. [pdf]
• Ref. 187C: Telefunken-Kompass; 3 near-identical Telefunken articles. [Summary]
• Ref. 187C1: "Telefunken-Kompass", in "Dinglers Polytechnisches Journal", Year 93, Vol. 327, No. 34, 24 August 1912, pp. 538–541. Source: DFG Digitalisierung des Polytechnischen Journals (Creative Commons 3.0 license).
• Ref. 187C2: "Telefunken-Kompass", pp. 77-84 in "Telefunken Zeitung", Vol. 1, Nr. 5, April 1912. Source: radiomuseum.org. [pdf]
• Ref. 187C3: "Telefunken Kompass", in "Jahrbuch der drahtlosen Telegraphie und Telephonie sowie des Gesamtgebietes der elektromagnetischen Schwingungen", Nr. 1, July 1912, pp. 85-92, Nr, 2, September 1912, p. 198.
• Ref. 187D: "Telefunken-Kompaß", pp. 15 & 17 in "Telefunken Zeitung", Vol. 2, Nr. 7, August 1912. Source: radiomuseum.org. [pdf]
• Ref. 187E: articles from the "Popular Science Monthly" magazine.
• Ref. 187E1: "Safeguarding Vessels by Radio" [Bellini-Tosi directional receiver, Telefunken Compass], Annis Salsbury, in "Popular Science Monthly", Vol. 88 [file size: 92 MB], No. 3, March 1916, pp. 451-453. Retrieved 15 April 2020.
• Ref. 187E2: "How the Zeppelin Raiders Are Guided by Radio Signals", in "Popular Science Monthly, Vol. 92" [file size: 92 MB], No. 4, April 1918, pp. 632-634. Retrieved 27 February 2020.
• Ref. 187F: "50 Jahre Telefunken - Festschrift zum 50 jährigen Jubileum der Telefunken Gesellschaft für drahtlose Telegraphy m.b.h. - Gleichzeitig als 100. Ausgabe der Telefunken Zeitung", Vol. 26, Nr. 100, May 1953, 164 pp. Retrieved 19 February 2020.
• Ref. 187F1: "Die Zeit des Funkensenders" [The era of the spark gap transmitters], J. Zenneck, pp. 153-158.
• Ref. 187F2: "Fernsehen, Richtstrecken und Funkortung" [Television, directional microwave links, and radio D/F], W.T. Runge, pp. 181-190.
• Ref. 187G: "Meißner, Alexander (Meissner)", pp. 92-93 in "Biografien österreichischer Physiker - Eine Auswahl", D. Angetter, M. Martischnig, Österreichisches Staatarchiv, 2005, 175 pp.
• Ref. 187H: "Orientierung von Luftschiffen" ["Airship navigation"], pp. 110, 111, 114 in "Telefunken Zeitung", Vol. 2, Nr. 11, April 1913. Source: radiomuseum.org. [pdf]
• Ref. 187J: "Der Peilsender Kleve der Kaiserlichen Marine in Hau" ["The directional beacon (Telefunken Compass station) of the Imperial Navy at Kleve/Hau", Bernd-Rüdiger Ahlbrecht, pp. 20-24 in "Geschichtsbrief Bedburg-Hau", Nr. 14, 2019, Geschichtsverein Bedburg-Hau e.V. (publ.),72 pp. Accessed 15 April 2019.
• Ref. 187K: p. 71 in "Die Antenne - Zeitschrift für drahtlose Nachrichtenübermittlung und verwandte Gebiete", Dr. Erich F. Huth G.m.b.H. – Gesellschaft für Funkentelegraphie (publ.), Vol. 2, Nr. 4, July 1913. Source: de.wikipedia.org.
• The same image is also used on p. 408 of "Das deutsche Seezeichenwesen – 1850–1990 zwischen Segel- und Container-Schiffsverkehr", Gerhard Wiedemann (ed.), Johannes Braun, Hans Joachim Haase, DSV Verlag (publ.), Hamburg 1998, 640 pp.
• Ref. 187L: "Meißner, Alexander, Funkingenieur" [radio engineer], 3-page bio, pp. 695-697 in "Maly - Melanchton", Vol. 16 of "Neue deutsche Biographie", Otto zu Stolberg-Wernigerode (ed.), Duncker & Humblot (publ.), 1990, 785 pp. Accessed 16 April 2020.
• Ref. 187M: "Telefunken auf der Allgemeinen Luftschiff-Ausstellung (Ala)", pp. 53-56 in "Telefunken Zeitung", Vol. 1, Nr. 5, April 1912. Source: radiomuseum.org. [pdf].
• Ref. 187N: "The Radio Direction Finder", Chapter XXII (pp. 261-265) in "History of Communications-Electronics in the United States Navy", Linwood S. Howeth, Bureau of Ships and Office of Naval History, 1963, 698 pp. Source: hathitrust.org, retrieved May 2020.
• Ref. 187P: Appendix A3 (pp. 33-43) in "Fire Island Lighthouse and Keeper's Dwelling", Vol. 3 of "Historic Structure Report", 470 pp. Source: US National Archives and Records Service. [file size: 37 MB]. Retrieved 2 May 2020.
• Ref. 187Q: articles [in Dutch] in "Radio-Nieuws - Maandblad van de Nederlandsche Vereeniging voor Radiotelegrafie". Source: nvhrbiblio.nl. Accessed: 13 April 2020.
• Ref. 187Q1: "Een geheimzinnig station" ["A mysterious station"], Vol. 1, No. 3, 1 March 1918, p. 40.
• Ref. 187Q2: "Geheimzinnige stations" ["Mysterious stations"], Vol. 1, No. 4, 1 April 1918, pp. 82-84.
• Ref. 187Q3: "De A. B. C. stations" ["The A.B.C. stations"], Vol. 1, No. 5, 1 May 1918, pp. 89-90.
• Ref. 187Q4: "De B.C.-stations" ["The B.C.-stations"], Vol. 1, No. 6, 1 June 1918, pp. 125-128.
• Ref. 187Q5: "Nog eens het CCC-station" ["Once more, the CCC-station], Vol. 2, No. 2, 1 February 1919, pp. 33-35.
• Ref. 187Q6: "Is het toch Kleef geweest?" ["Was it Cleves after all?"], Vol. 2, No. 3, 1 March 1919, pp. 70-71.
• Ref. 187Q7: "Nog eens het CCC-station" [Once more, the CCC-station"], Vol. 2, No. 3, 1 March 1919, pp. 71-73.
• Ref. 187Q8: "Weer het CCC-station" ["Again, the CCC-station"], Vol. 2, No. 4, 1 April 1919, pp. 115.
• Ref. 187Q9: "Het b-station" ["The b-station"], Vol. 2, No. 12, 1 December 1919, p. 405-409.
• Ref. 187R: "Die Richtungssendeanlagen Cleve und Tondern" ["The directional transmitting stations Clev and Tondern], pp. 961-964 in "Die Funkpeilung", Leo Pungs, F. Banneitz, Section 3 of Chapter VI of Part 5 in "Taschenbuch der drahtlosen Telegraphie und Telephonie", Vol. 1, F. Banneitz (ed.), Springer-Verlag (publ.), 1927, 1254 pp.
• Ref. 188: List of 1929-1940 patents of the Conz company (and its employees) regarding frequency conversion and motor speed control; source: DEPATISnet (search-engine of the German patent & trademark office, DPMA).
• Ref. 212: "Flugzeug-Ausrüstungsgeräte - Teil 9, Mappe 637", RLM, Jan/Sept 1944; source: www.DeutscheLuftwaffe.de [file size 41 MB]
• Ref. 212A: pdf pp. 17, 18, 330-350, 388-392
• Ref. 212B: pdf p. 299 "Prüfuhr Ln 28901, PrU 28, 124-1416 A, T. Bäuerle u. Söhne, 1941/42"
• Ref. 228: Rotating loop beacons
• Ref. 228A: "Directional Wireless as an Aid to Navigation" [direction-finding, night effect, course-ranges, directional transmission, beam, VHF reflector-antenna, rotating loops, fixed-course loops], R.L. Smith-Rose, in "Nature", Volume 120, No. 3030, 26 November 1927, p. 774-776. Accessed 11 April 2020. [pdf]
• Ref. 228B: "Some experiments on the applications of the rotating-beacon transmitter to marine navigation", R.L. Smith-Rose, S.R. Chapman, in "Journal of the Institution of Electrical Engineers", Vol. 66 , Iss. 375, March 1928, pp. 256-269. Abstract.
• Ref. 228C: "Directional Wireless and Marine Navigation: the Rotating-Loop Beacon", R.L. Smith-Rose, in "Nature", Volume 121, 12 May 1928, p. 745.  [pdf]
• This is an update to ref. 228A.
• Ref. 228D: "Radio Direction-Finding by Transmission and Reception", R.L. Smith-Rose, in "Nature", Vol. 125, No. 3154, 12 April 1930, pp. 568-569. [pdf]
• Ref. 228E: "Some Observations on the Orfordness Rotating Beacon", R.L. Smith-Rose, in "Journal of the I.E.E.", Vol. 69, Iss. 412, April 1931, pp. 523-532. [Abstract]
• Also published under the same title in "Proceedings of the Wireless Section of the Institution of Electrical Engineers", Vol. 6, Iss. 17/18 , June-September 1931, pp. 137-146. [Abstract].
• Ref. 228F: Reports of the UK Department of Scientific and Industrial Research, Radio Research Board (DSIR 11):
• Ref. 228F1: "An investigation of a Rotating Radio Beacon" [Gosport], R.L. Smith-Rose, S.R. Chapman, Research Report No. 6, 1928.
• Ref. 228F2: "The Orfordness Rotating Beacon and Marine Navigation" [principle of the beacon, bearing taking with stopwatch and automatic recorder, bearing accuracy], R.L. Smith-Rose, Special Report No. 10, 1931, 14 pp. [Abstract / report review].
• Ref. 228G: pp. 200-204 in "Rotating beacons", Section 4.12 of "Radio Aids to Civil Navigation", Reginald Frederick Hansford (ed.), Heywood & Co. Ltd. (publ.), 1960, 623 pp.
• Ref. 228H: British Air Ministry Notices/Pamphlets
• Ref. 228H1: "Orfordness Rotating Beacon : Instructions for taking bearings. (Provisional) : direction and position finding by means of directional wireless transmission", Pamplet No. 38, year unknown.
• Ref. 228H2: "Orfordness Rotating Radio Beacon", Notice No. 56 of 1929, in "Air Ministry Notices to Airmen, p. 1045 in "Flight", Vol. XXI, no. 38, 20 September 1929, 56 pp. Source: archive.org. Accessed 3 May 2020.
• Ref. 228H3: "Orfordness rotating radio beacon", General Notice No. 19 of 1930.
• Ref. 228H4: "Farnborough Rotating Wireless Beacon", General Notice No. 31 of 1930, in "Air Ministry Notices to Aircraft Owners and Ground Engineers", p. 1388 in "Flight", Vol. XXII, No. 48, 28 November 1930. Source: archive.org. Accessed 3 May 2020.
• Ref. 228H5: "Farnborough rotating wireless beacon" - General Notice No. 33 of 1930.
• Ref. 228H6: "Farnborough and Orfordness rotating wireless beacons", Notice No. 43 of 1931 Series A, p. 823 in "Flight", Vol. XXIII, No. 33, 14 August 1931, 26 pp. Source: archive.org. Accessed 4 May 2020.
• Ref. 228H7: "Orfordness rotating radio beacon", Notice No. 21 of 1932 Series A, p. 499 in "Flight", Vol. XXIV, No. 23, 3 June 1932, 22 pp. Source: archive.org. Accessed 3 May 2020.
• Ref. 228H8: "Orfordness rotating radio beacon", Notice No. 58 of 1932 Series A.
• Ref. 228H9: "Orfordness and Tangmere rotating radio beacons", Notice No. 30 of 1933 Series A.
• Ref. 228H10: "Orfordness rotating radio beacon", Notice No. 63 of 1933 Series A.
• Ref. 228H11: "Orfordness radiobeacon", Notice No. 32 of 1938. Excerpt.
• Ref. 228J: "Direction-Finding System (Beacon)", UK Parliament, Commons Chamber, Record of Oral Answers To Questions, Vol. 222, nr. 29, 21 November 1928. Source: hansard.parliament. Accessed 3 May 2020.
• Ref. 228K: "First of the Beams", 9 pp. in "Inter-War Years", Chapter 5 in "Most Secret: The Hidden History of Orford Ness", Paddy Heazell, The History Press (publ.), 2011, 256 pp. Accessed 4 May 2020. [pdf]
• Ref. 228L: "Note on a special dial for time-pieces to be used with rotating wireless or other beacons", R.L. Smith-Rose, in "Journal of Scientific Instruments", Vol. 5, No. 3, 1 March 1928 [file size: 49 MB], pp. 93-96. Accessed 4 May 2020.
• Ref. 228M:
• Ref. 228M1: "An automatic recorder of signals from a rotating beacon transmitter", R.L. Smith-Rose, H.A. Thomas, in "Journal of Scientific Instruments", Vol. 8, No. 3, 1 March 1931, pp. 81-88. Abstract.
• Ref. 228M2: "An automatic recorder of signals from a rotating beacon transmitter", R.L. Smith-Rose, H.A. Thomas, in "The Nautical Magazine", Vol. 127, January 1932.
• Ref. 228M3: 2-page extract of ref. 228M2, "An automatic recorder of signals from a rotating beacon transmitter" in "International Hydrographic Review", Vol. IX, No. 2, 1932. [pdf].
• Ref. 228N: pp. 35, 36, 79, 87, 154 in "1939 Radio Aids to Navigation - Including Details of Direction-finder Stations, Radiobeacons, Navigational Warnings, Time Signals, Etc.", United States Hydrographic Office (Navy Dept., Bureau of Navigation), publication H.O. No. 205, 1939, 315 pp.
• Ref. 228P: "The invention of synchronous rotations by means of Paul la Cour's Phonic Wheel as used in Telegraphy", P. Chr. Dresing, in "The Telegraphic Journal and Electrical Review", Vol. XX, No. 476, 7 January 1887, pp. 31, 32.
• Ref. 228Q: "Orford Ness: the Black Beacon and associated power house", National Heritage List for England (NHLE). Accessed 8 May 2020. [pdf]
• Ref. 228R: pp. 454-455 in "Wireless - A treatise on the theory and practice of high-frequency electric signalling" [Farnborough rotating beacon], L.B. Turner, Cambridge University Press (publ.), 1931, 531 pp. Source: archive.org [file size: 32 MB]. Accessed 10 May 2020.
• Ref. 228S: articles in "Journal of the Institution of Electrical Engineers", Vol. 66, Iss. 375, March 1928.
• Ref. 228S1: "Rotating-loop radio transmitters, and their application to direction-finding and navigation", T.H. Gill, N.F.S. Hecht, pp. 241-255. Abstract.
• Ref. 228S2: "Some experiments on the applications of the rotating-beacon transmitter to marine navigation", R.L. Smith-Rose, S.R. Chapman, pp. 256 -269. Abstract.
• Ref. 228S3: "A theoretical discussion of various possible aerial arrangements for rotating-beacon transmitters", R.L. Smith-Rose, pp. 270-279. Abstract.
• Ref. 228S4: "Discussion on the papers “Rotating-loop radio transmitters, and their application to direction-finding and navigation”, by Messrs. Gill and Hecht, “Some experiments on the applications of the rotating-beacon transmitter to marine navigation”, by Messrs. Smith-Rose and Chapman, and “A theoretical discussion of various possible aerial arrangements for rotating-beacon transmitters”, by Dr. Smith-Rose, respectively, before the Wireless Section, 4th January, 1928", p. 274-278.
• Ref. 228S5: "The authors' replies to the discussion on “Rotating-loop radio transmitters, and their application to direction-finding and navigation”, “Some experiments on the applications of the rotating-beacon transmitter to marine navigation” and “A theoretical discussion of various possible aerial arrangements for rotating-beacon transmitters”, T.H. Gill, N.F.S. Hecht, R.L. Smith-Rose, S.R. Chapman, pp. 278-279. Accessed 10 May 2020. [pdf]
• Ref. 228T: documentation about the Orfordness ebacon, available at The National Archives, Kew/England
• Ref. 228T1: "COMMUNICATIONS. Wireless: Orfordness; erection of an experimental wireless rotating beacon", archive file Catalog reference T 161/576/1, file covers period 24 May 1928 - 4 May 1933.
• Ref. 228T2: "Signals (Code 133): Rotating wireless beacon - Orfordness. Key number(s): 5688, 4496....", archive file Catalog reference MT 9/1951, file covers period 1928 - 1930, file contains 14 items.
• Ref. 228U: pp. 453-455 in "Wireless" [Farnborough rotating loop], L. B. Turner, Cambridge University Press, 1931, 578 pp. Source: archive.org, retrieved 20 October 2020.
• Ref. 228V: articles about Marconi's parabolic reflector antenna beacons
• Ref. 228V1: "The "Radio Lighthouse" - An Amazing New Use for Marconi Beams", p. 77 in "Popular Science Monthly", Vol. 105, No. 6, December 1924. Accessed 10 May 2020.
• Ref. 228V2: "Short-wave Directional Wireless Telegraph" [reflector antennas, Inchkeith beacon], C.S. Franklin, in "Nature", Vol. 110, No. 2754, 12 August 1922. Accessed 10 May 2020. [pdf]
• Ref. 228V3: "The Marconi wireless beam reflector on Inchkeith", N. Wells, in "Engineering", Vol. 119, 13 March 1925, p. 309-311.
• Ref. 228V4: pp. 10-13 in "Short wave wireless communication", Alan Wilfrid Ladner, Charles Robert Stoner, John Wiley & Sons, Inc. (publ.), 1933, 348 pp. Source: hathitrust.org, retrieved 20 October 2020.
• Ref. 228V5: pp. 442-444, 455 in "Wireless", L. B. Turner, Cambridge University Press, 1931, 578 pp. Source: archive.org, retrieved 20 October 2020.
• Ref. 228V6: "Short-wave directional wireless telegraphy", C.S. Franklin, in "The Wireless World and Radio Review",Vol. X, No. 8, 20 May 1922, p. 219-225. Source: worldradiohistory.com, retrieved 13 January 2021.
• Ref. 228V7: "Directional short - wave wireless telephony", §6 in "Electrons, electric waves and wireless telephony - XIX", J.A. Fleming, in "The Wireless World and Radio Review", No. 182 (Vol. XI, No. 19), 10 February 1923, p. 626-629. Source: archive.org, retrieved 13 January 2021.
• Ref. 228V8: "Directional wireless as an aid to navigation", in "The Wireless World and Radio Review", No. 189 (Vol. XI, No. 26), 31 March 1923, pp. 859-860. Source: archive.org, retrieved 13January 2021.
• Ref. 229: civil equi-signal beam systems (Fixed-course & Four-Course ranges, E/T & A/N beacons, Visual Ranges, Visual Aural Ranges)
• Ref. 229A: "Über die Wirkung von Schellers drahtlosen Kursweiser auf das Flugzeug" ["About the impact of Scheller's wireless direction pointer on aircraft", Eberhard Buchwald, in "Zeitschrift für drahtlose Telegraphie und Telephonie sowie des Gesamtbereichs der elektromagnetischen Schwingungen", Vol. 15, Nr. 2, February 1920, pp. 114-122.
• Ref. 229B: "Neue Versuche über funkentelegraphische Richtsender" ["New trials with directional spark transmitter stations"; A/N system], Franz Kiebitz, in "Zeitschrift für drahtlose Telegraphie und Telephonie sowie des Gesamtbereichs der elektromagnetischen Schwingungen", Vol. 15, Nr. 4, April 1920, pp. 299-310.
• Ref. 229C: articles in "Electrical Communication - Technical Journal of the International Telephone and Telegraph Corporation and Associate Companies". Source: worldradiohistory.com. Accessed January-August 2020.
• Ref. 229C1:"Otto Scheller and the Invention and Applications of the Radio-Range Principle" [incl. ITT Landing System, SCS-51 (mobile)], R.I. Colin, in "ITT Electrical Communication", Vol. 40, Nr. 3, 1965, pp. 359-368, retrieved 23 January 2020. [Summary]. Reprinted as "Otto Scheller: The Radio Range Principle", R.I. Colin, in "IEEE Trans. on Aerospace and Electronic Systems", Vol. AES-2, No. 4, July 1966, pp. 481-487 [pdf].
• Ref. 229C2: "Federal Telephone and Radio Corporation - A historical review: 1909-1946" [incl. spark transmitter, arc transmitter, Duddell, Poulsen, Federal Telegraph Company, Mackay System, ITT], F.J. Mann, in "ITT Electrical Communication", Vol. 23, No. 4, December 1946, pp. 377-406.
• Ref. 229C3: "An Ultra-High Frequency Radio Range with Sector Identification and Simultaneous Voice", Andrew Alford, Armig G. Kandoian, Frank J. Lundburg, Chester B. Watts, in "Electrical Communication" Journal of the Int'l Telephone & Telegraph Corp. and Associated companies", Vol. 23, No. 2, 1946, pp. 179-189.
• Ref. 229D: Papers published in the "Bureau of Standards Journal of Research". Source: US National Institute of Standards and Technology (NIST). Accessed March-April 2020. Also articles about activities of the Bureau of Stds.
• Ref. 229D1: "A 12-Course Radio Range for Guiding Aircraft with Tuned Reed Visual Indicator", H. Diamond, F.G. Kear, Research Paper 154 (RP154), in "Journal of Research", Vol. 4, Issue 3, March 1930, pp. 351-369. [pdf] [Abstract]
• Ref. 229D2: "Applying the Radio Range to the Airways", F.G. Kear, W.E. Jackson, Research Paper 155 (RP155), in "Journal of Research", Vol. 4, Issue 3, March 1930, pp. 371-381. [pdf] [Abstract]. NOTE: this article was also published in the Proc. of the IRE, see ref. 229L8.
• Ref. 229D3: "Development of the Visual Type Airway Radio-Beacon System", J.H. Dellinger, H. Diamond, F.W. Dunmore, Research Paper RP159, in "Journal of Research", Vol. 4, Issue 3, March 1930, pp. 425-459.  [pdf]   [Abstract, Summary]
• Ref. 229D4: "The cause and elimination of night effects in radio range-beacon reception", H. Diamond, Research Paper 513 (RP513), in "Journal of Research", Vol. 10, Issue 1, January 1933, pp. 7-34. [pdf]
• Ref. 229D5: "A radiobeacon and receiving system for blind landing of aircraft", H. Diamond, F.W. Dunmore, Research Paper 238 (RP238), in "Journal of Research", Vol. 5, Issue 4, October 1930, pp. 897-931. [pdf]
• Ref. 229D6: "Unidirectional radiobeacon for aircraft", E.Z. Stowell, Research Paper 35 (RP35), in "Journal of Research", Vol. 1, Issue 6, December 1928, pp. 1011-1022. [pdf]   [Abstract]
• Ref. 229D7: "Applying the visual double-modulation type radio range to the airways", Research Paper 148 (RP148), H. Diamond, in "Journal of Research", Vol. 4, Issue 3, March 1930, pp. 265-289. [pdf]   [Abstract]
• Ref. 229D8: "A course-shift indicator for the double-modulation type radiobeacon", H. Diamond, F.W. Dunmore, Research Paper 77 (RP77), in "Journal of Research", Vol. 3, Issue 1, 1 July 1929 (paper dates from 20 February 1929), 10 pp. [pdf]
• Ref. 229D9: "A tuned-reed course indicator for the 4 and 12 course aircraft radio range", F.W. Dunmore, Research Paper RP160, in "Journal of Research", Vol. 4, Issue 4, April 1930, pp. 461-474. [pdf]   [Abstract]
• Ref. 229D10: "Design of tuned reed course indicators for aircraft radiobeacon", F.W. Dunmore, Research Paper 28 (RP28), in "Journal of Research", Vol. 1, Issue 5, November 1928, pp. 751-769. [pdf, file size: 26 MB]
• Ref. 229D11: "A method of providing course and quadrant identification with the radio range-beacon system" [Adding a vertical antenna to the cross-loops], F.W. Dunmore, Research Paper 593 (RP593), in "Journal of Research", Vol. 11, Issue 3, September 1933, pp. 309-325. [pdf] [Abstract]
• Ref. 229D12: "The Radio Work of the Dept. of Commerce", J.H. Dellinger, in "QST", June 1921, pp. 18-21. Accessed 14 April 2020. [pdf]
• Ref. 229D13: "A Century of WWV", Glenn K. Nelson, in "Journal of Research of the National Institute of Standards and Technology", Vol. 124, Article 124025, 2019. Accessed 14 April 2020. [pdf]
• Ref. 229D14: "Radio publications of the Bureau of Standards", NBS DoC, Letter Circular LC 40, 25 November 1922, 22 pp. Accessed 14 April 2020. [pdf]
• Ref. 229D15: "Radio - The aviator's guiding hand", pp. 147-170 in "Antennas, instruments, and systems in development" Chapter VI in "Achievement in Radio - Seventy Years of Radio Science, Technology, Standards, and Measurements at the National Bureau of Standards" [file size: 72 MB; history of the NBS radio section], Wilbert F. Snyder, Charles L. Bragaw, National Bureau of Standards, NIST Special Publication 55 (SP555), October 1986, 884 pp. Accessed 14 April 2020. [pdf, file size: 72 MB]
• Ref. 229D16: "Automatic volume control for aircraft radio receivers", W.S. Hinman, Research Paper 330 (RP330), in "Journal of Research", Vol. 7, Issue 1, July 1931, pp. 37-46. Accessed 15 July 2020. [pdf]
• Ref. 229D17: "A Course Indicator of Pointer Type for the Visual Radio Range-Beacon System", F.W. Dunmore, Research Paper 336 (RP 336), in "Journal of Research", Vol. 7, Issue 1, July 1931, pp. 147-170. Accessed 5 June 2020. [pdf] [Abstract]
• Ref. 229D18: "Theory of design and calibration of vibrating-reed indicators for Radio Range beacons", G.L. Davies, in "Journal of Research", Vol. 7, Issue 1, July 1931, pp. 195-213. Accessed 10 June 2020. [pdf] [Abstract] NOTE: this article was also published in the Proc. of the IRE, see ref. 229L13.
• Ref. 229D19: "A simultaneous radiotelephone and Visual Range beacon for the airways", F.G. Kear, G.H. Wintermute, Research Paper 341 (RP341) in "Journal of Research", Vol. 7, Issue 2, August 1931, pp. 261-287. Accessed 10 June 2020. [pdf] [Abstract] NOTE: this article was also published in the Proc. of the IRE, see ref. 229L7.
• Ref. 229D20: "Phase Synchronization in Directive Antenna Arrays with Particular Application to the Radio Range Beacon", F.G. Kear, Research Paper 581 (RP581), in "Journal of Research", Vol. 11, No. 1, July 1933, pp. 123-140. [pdf] [Abstract]
• Ref. 229D21: "A course-shift indicator for the double-modulation type radiobeacon", H. Diamond, F.W. Dunmore, in "Journal of Research", Vol. 3, Issue 1, July 1929, p. 1. [pdf]
• Ref. 229D22: "Graphical determination of polar patterns of directional antenna systems", G.L. Davies, W.H. Orton, Research Paper 435 (RP435), in "Journal of Research", Vol. 8, Issue 5, May 1932, pp. 555-569. [pdf]
• Ref. 229D23: "Performance tests of radio system of landing aids", H. Diamond, Research Paper 602 (RP602), in "Journal of Research", Vol. 11, Issue 4, October 1933, pp. 463-490. [pdf] [Abstract]
• Ref. 229D24: "A radio direction finder for use on aircraft", Wilbur S. Hinman, Research Paper 621 (RP621), in "Journal of Research", Vol. 11, Issue 6, December 1933, pp. 733-741. [pdf] [Abstract]
• Ref. 229D25: "Experiments with underground ultra-high-frequency antenna for airplane landing beam", Harry Diamond, Francis W. Dunmore, Research Paper 1006 (RP1006), in "Journal of Research", Vol. 19, Nr.1, July 1937, 19 pp. [pdf] [Abstract]. NOTE: this article was also published in the Proc. of the IRE, see ref. 229L17.
• Ref. 229D26: "Consolidated Instrument Company" & "The Radio-beacon Tuned Reed Indicator", pp. 44 & 74 in "US Air Services - Feature Aeronautical Magazine, Commercial and Military", November 1929. [contract to manufacture the U.S. Bureau of Standards Visual Radio beacon Vibrating Reed Indicator awarded to Julien P. Friez & Sons, Inc., Baltimore/MD, subsidiary of Consolidated Instrument Co. of America, Inc.].
• Ref. 229D27: "The comparative accuracy of various existing and proposed radio navigation systems" [Radio Range, azimuthal-type, hyperbolic-type, composite range/azimuth type], William Q. Chrichlow, National Bureau of Standards, Central Radio Propagation Laboratory [ = Department of Commerce, Boulder/CO, Laboratories Building 1], Report No. CRPL-4-1, 30 December 1946, 44 pp. Source: nist.gov, retrieved 20 October 2020.
• Ref. 229D28: "Receiving sets for aircraft beacon and telephony", Haraden Pratt, Harry Diamond, in "J. of Research", Vol. 1, Issue 4, October 1928, pp. 543-563, Research Paper 19 (RP19), 19 June 1928, 23 pp. Source: nist.gov, retrieved 23 February 2021.
• Ref. 229D29: "Characteristics of airplane antennas for radio range-beacon reception" [vertical, T-, L-, V-antennas] H. Diamond, G.L. Davies, in "J. of Research", Vol. 6, Issue 5, May 1931, pp. 901-916, Research Paper 313 (RP313), 24 January 1931, 16 pp. Source: nist.gov, retrieved 23 February 2021.
• Ref. 229D30: "Theory of design and calibration of vibrating reed indicators for radio range beacons", G.L. Davies, in "J. of Research", Vol. 7, Issue 1, July 1931, pp. 195-213, Research Paper 338 (RP338), 25 March 1931, 19 pp. Source: nist.gov, retrieved 23 February 2021.
• Ref. 229E: Papers published in "Scientific Papers of the Bureau of Standards". Source: US National Institute of Standards and Technology (NIST). Accessed April 2020.
• Ref. 229E1: "A Directive Type of Radio Beacon and its Application to Navigation" [test/eval of multi-turn crossed-loop antennas, A/N equi-beam, effect of aircraft trailing-wire receive antenna], F.H. Engel, F.W. Dunmore, in "Scientific Papers of the Bureau of Standards", Vol. 19, No. 480 (S480), 8 September 1923, pp. 281-295.  [pdf] [Summary]
• Ref. 229E2: "Directive radio transmission on a wave length of 10 meters" [successful experiments of making the radiation pattern of a vertical dipole directional, with various configs of a reflector screen made of tuned & un-tuned vertical wires; shortwave, instead of then-standard long & medium wave], Francis W. Dunmore, Francis H. Engel, in "Scientific Papers of the Bureau of Standards", Vol. 19, Scientific Paper No. 469 (S469), 9 January 1923, pp. 1-16. [pdf]
• Ref. 229E3: "Principles of radio transmission and reception with antenna and coil aerials", J.H. Dellinger, Scientific Paper No. 354 (S354), Scientific Papers of the Bureau of Standards, Vol. 15, June 1919, pp. 435-495. [pdf]
• Ref. 229E4: "The radio direction finder and its application to navigation", Frederick A. Kolster, Francis W. Dunmore, Scientific Paper 428 (S428), July 1921), Scientific Papers of the Bureau of Standards, Vol. 17, January 1922, pp. 529-566. [pdf]
• Ref. 229E5: "The field radiated from two horizontal coils", G. Breit, Scientific Paper No. 431 (S431), Scientific Papers of the Bureau of Standards, Vol. 17, Issue 3, 1922, pp. 589-606. [pdf]
• Ref. 229F: "The Evolution of Airway Lights and Electronic Navigation Aids", Roger Mola, U.S. Centennial of Flight Commission, 2003. [pdf]
• Ref. 229G: "The Equi-Signal Zone Radio Beacon and Air Navigation", R.L. Smith-Rose, in "Nature", Vol. 126, No. 3168, 19 July 1930, pp. 98-100. Accessed 1 March 2020. [pdf]
• Ref. 229H: "Projector of the Sharpest Beam of Electric Waves", Hidetsugu Yagi, Shintaro Uda, in "Proceedings of the Imperial Academy" (Japan), Vol. 2, Iss. 2, 1926, pp. 49-52. [pdf]
• Ref. 229J: "Lighting the Airways" and "The Emergence of Radio", in Chapter II "The Republican Era, 1926-1932", in "Bonfires to beacons: Federal civil aviation policy under the Air Commerce Act, 1926-1938" [file size: 29 MB], Nick A. Komons, U.S. Department of Transportation / Federal Aviation Administration (publ.), 1978, 472 pp. Source: hathitrust.org, accessed 21 May 2020. 
• Ref. 229K: "Flying the beam: navigating the early US airmail airways, 1917-1941", Henry R. Lehrer, Purdue University Press (publ.), 2014, 234 pp.
• Ref. 229L: articles in Proceedings of the Institute of Radio Engineers. Source: worldradiohistory.com, retrieved June 2014 - June 2020.
• Ref. 229L1: "Apparatus used on British and European Airways", Edward H. Furnival, in "Proc. of the I.R.E", Vol. 17, Nr. 12, December 1929, pp. 2123-2136.
• Ref. 229L2: "The civil airways and their radio facilities", H.J. Walls, in "Proc. of the I.R.E.", Vol. 17, Nr. 12, December 1929, pp. 2141-2157.
• Ref. 229L3: "Applying the visual double-modulation type Radio Range to the airways", H. Diamond, in "Proc. of the I.R.E", Vol. 17, Nr. 12, December 1929, pp. 2158-2184. [Abstract]
• Ref. 229L4: "Radio in aeronautics - Its technical status and the organization for its application in Germany", F. Eisner, H. Fassbender, in "Proc. of the I.R.E.", Vol. 17, Nr. 12, December 1929, pp. 2185-2229.
• Ref. 229L5: "Applying the Radio Range to the airways", F.G. Kear, W.E. Jackson, in "Proc. of the I.R.E.", Vol. 17, Nr. 12, December 1929, pp. 2268-2282.   [Abstract]. NOTE: this article was also published in the Bureau of Standards Journal of Research, see ref. 229D2.
• Ref. 229L6: "Radio beacons for transpacific flights", Clayton C. Shangraw, in "Proc. of the I.R.E.", Vol. 16, Nr. 9, September 1928, pp. 1203-1235. [Abstract].
• Ref. 229L7: "A simultaneous radiotelephone and Visual Range beacon for the airways", F.G. Kear, G.H. Wintermute, in "Proc. of the I.R.E.", Vol. 20, Nr. 3, March 1932, pp. 478-515. NOTE: this article was also published in the Bureau of Standards Journal of Research, see ref. 229D18. [Abstract].
• Ref. 229L8: "Development of radio aids to air navigation", J.H. Dellinger, Haraden Pratt, in "Proc. of the I.R.E.", Vol. 16, No. 7, July 1928, pp. 890-920. [Abstract].
• Ref. 229L9: "Apparent Night Variations with Crossed-coil Radio Beacons", Haraden Pratt, in "Proc. of the I.R.E.", Vol. 16, Nr. 5, May 1928, p. 652-657. [Abstract].
• Ref. 229L10: "Simultaneous Radio Range and Telephone Transmission", W.E. Jackson, D.M. Stuart, in "Proc. of the I.R.E.", Vol. 25, Nr. 3, March 1937, pp. 314-326.  [Abstract].
• Ref. 229L11: "Radio Guidance" [two rotating radio beacons (simultaneously on the same frequency), triangulation on a display (physical map in aircraft instrument + projecting 2 intersecting light beams)], J. Edward Miller, in "Proc. of the I.R.E.", Vol. 20, Nr. 11, November 1932, pp. 1752-1762. [Abstract].
• Ref. 229L12: "Loop antennas for aircraft" [D/F], George F. Levy, in "Proc. of the I.R.E.", Vol. 31, Nr. 2, February 1943, pp. 56-66. [Abstract].
• Ref. 229L13: "Theory of design and calibration of vibrating-reed indicators for Radio Range beacons", G.L. Davies,  in "Proc. of the I.R.E.", Vol. 20, Nr. 1, January 1932, pp. 161-181. [Abstract]. NOTE: this article was also published in the Bureau of Standards Journal of Research, see ref. 229D17.
• Ref. 229L14: "The Development of a Visual Type of Radio Range Transmitter Having a Universal Application to the Airways" [very comprehensive!!! 4-course, 12-course visual range, transmitters, frequency tripler, loops, goniometer], W.E. Jackson, S.L. Bailey, in "Proc. of the IRE", Vol. 18, Nr. 12, December 1930, pp. 2059-2101. [Abstract]
• Ref. 229L15: "A Radio Range beacon free from night effects", Howard Allan Chinn, in "Proc. of the I.R.E.", Vol. 21, Nr. 6, June 1933, pp. 802-807.
• Ref. 229L16: "On the solution of the problem of Night Effects with the Radio Range beacon system", H. Diamond, in "Proc. of the I.R.E.", Vol. 21, Nr. 6, June 1933, pp. 808-832. [Abstract]
• Ref. 229L17: "Rectangular short-wave frame aerials for reception and transmission", L.S. Palmer, D. Taylor, in "Proc. of the I.R.E.", Vol. 22, Nr. 1, January 1934, pp. 93-114
• Ref. 229L18: "Experiments with underground ultra-high-frequency antenna for airplane landing beam", Harry Diamond, Francis W. Dunmore, in "Proc. of the I.R.E.", Vol. 25, Nr. 12, December 1937, pp. 1542-1560. [Abstract] Note: this article was also published in the Bureau of Standards Journal of Research, see ref. 229D25.
• Ref. 229L19: "The status of instrument landing systems" [overview BoS activities, test at College Park/MD & Newark, Lorenz 1933 40 MHz and kick-meter, US Army 1932/33 at Wright Field, Lorenz/ITT tests at Indianapolis issues, Bendix/UAL tests 1936 with yagi antennas], William Elvin Jackson, in "Proc. of the I.R.E.", Vol. 26, Nr. 6, June 1938, pp. 681-699. [Abstract]. Note: same article was also published in 1937 as CAA Technical Development Report No. 1 (ref. 229R20).
• Ref. 229L20: "The Ultra-Short-Wave Guide-Ray Beacon and Its Application", E. Kramar, W. Hahnemann, in "Proc. of the I.R.E.", Vol. 26, No.1, January 1938, pp. 17-44.
• Ref. 229L21: "Ultra-Short-Wave Propagation", Paul von Handel, Wolfgang Pfister, in "Proc. of the IRE", Vol. 25, Nr. 3, March 1937, pp. 346-363.
• Ref. 229L22: "Field intensity characteristics of double modulation type of directive radio beacon", Haraden Pratt, in "Proc. IRE", vol. 17, Nr. 5, May 1929, pp. 873-878. Source: worldradiohistory.com, retrieved 23 February 20121.
• Ref. 229M: "Adcock Antennas" [4-Course A/N systems], section 1.13.3 in "Radio Antenna Engineering", Edmund A. Laport, McGraw-Hill Book Co. (publ.), 1952, 574 pp.
• Ref. 229N: "The stationary and rotating equisignal beacon" [Bellini-Tosi, goniometer, Telefunken Compass, A/N fixed-course, tests at McCook Field / Dayton, and Wilbur Wright Field, loop crossing-angle effects], W. H. Murphy, L. M. Wolfe, in "SAE Transactions", Vol. 21, Part II, 1926, pp. 979-1015.
• Ref. 229P: articles about Australian "Marconi" Radio Range, Lorenz/AWA beams & ILS
• Ref. 229P1: "First radio beacon in Australia opened by the Minister for Defence", p. 3 in "The Radiogram - The Staff Journal of the Amalgamated Wireless (Australasia) Ltd.", Vol. III, no. XIII, July 1936. Source: worldradiohistory.com, retrieved 15 June 2020.
• Ref. 229P2: p. 128, 148, 149 in "Air Crash: the story of how Australia's airways were made safe, Vol. 1 - 1929-1939", Macarthur Job, Aerospace Publications Australia (publ.), 1991, 169 pp.
• Ref. 229Q: "Principles of Aeronautical Radio Engineering", P.C. Sandretto, McGraw-Hill Book Co. (publ.), 1^st ed., 1942, 424 pp. Source: us.archive.org [file size 20 MB], retrieved 15 June 2020.
• Chapters II-VIII: "The Radio Range", "The Ultra-High-Frequency Radio Range", "Aircraft Direction Finders", "Markers", "Instrument Landing", "Absolute Altimeters", "Direction Finding from Ground Stations".
• Key words: radio ranges; derivation of loop antenna field-strength pattern; goniometer; apparent/phantom loop vs. physical/true loop; course-shifting & -bending; TL range antenna system; "night effect" elimination; Simultaneous Radio Range (beacon + voice); Visual Radio Range; reed instruments; 12-course (never in service); UHF radio range; shore-effect (dielectric constant & soil conductivity); multiple/split courses (altitude dependent); TL tower; Alford Loop; Visual 2-Course Range (90/150 Hz modulation + 1020 Hz A/N quadrant ID & station passage); RCA VHF Omni-directional range (VOR) being developed (1942), preceded by LW omni with rotating goniometer + north signal; cardioid pattern;
• Ref. 229R: US Dept. of Commerce, bi-weekly Air Commerce Bulletins (ACB) 1929-1939, Aeronautics Bulletin (AB) 1936, monthly Civil Aeronautics Journals (CAJ) 1944-1950, Technical Development Reports (TDR), Technical Manuals (TM), Miscellaneous Publications (MP). Source ACB: hathitrust-org, source AB: hathitrust.org; source CAJ: hathitrust.org (1) and  hathitrust.org (2);  retrieved June-September 2020. Source TM: hathitrust.org, retrieved February 2021; [abstracts/summaries of all ACB & CAAJ articles below]
• Ref. 229R1: ACM, Vol. 1 (July-December 1929, January-June 1930) [abstracts / summaries] ‒
  No. 2 (July-'29), pp. 19-20, No. 3 (Aug-'29), p. 13, No. 4 (Aug-'29), pp. 5-11, 21-23, No. 5 (Sept-'29), pp. 10-11, No. 8 (Oct-'29), pp. 15-17, No. 10 (Nov-'29), pp. 1-3,  No. 11 (Dec-'29), pp. 11-12, 29, No. 12 (Dec-'29), p13, No. 13 (Jan-'30, pp. 19-20, No. 14 (Jan-'30), p. 18, No. 15 (Feb-'30), p. 18,  No. 16 (Feb-'30), pp. 14,  No. 18 (Mar-'30), p. 16,  No. 19 (Apr-'30), p. 15, No. 22 (May-'30), pp. 3-8, No. 23 (June-'30), pp. 5-6, No. 24 (June-'30), p. 9.
• Ref. 229R2: ACM, Vol. 2 (July-December 1930, January-June 1931) [abstracts / key words] ‒
  No. 4 (Aug-'30), pp. 79-87, No. 5 (Sept-'30), pp. 120-121, No. 8 (Oct-'30), pp. 201-203, No. 10 (Nov-'30), pp. 272-275, No. 14 (Jan-'31), pp. 366-367, 370-371, No. 15 (Feb-'31), pp. 393, No. 17 (Mar-'31), pp. 437-439, No. 20 (Apr-'31), pp. 526-529.
• Ref. 229R3: ACM, Vol. 3 (July-December 1931, January-June 1932) [abstracts / summaries] ‒
  No. 3 (Aug-'31), pp. 55-58, No. 4 (Aug-'31), p. 102, No. 8 (Oct-'31), p. 109, No. 15 (Feb'-32), pp. 361-365, No. 18 (Mar-'32), pp. 433-438, 440, 441.
• Ref. 229R4: ACM, Vol. 4 (July-December 1932, January-June 1933) [abstracts / summaries] ‒
  No. 2 (July-'32), pp. 33-45, No. 5 (Sept-'32), pp. 121-126,  No. 6 (Sept-'32), pp. 135-150, No. 11 (Dec-'32), pp. 260-264, No. 12 (Dec-'32), pp. 293-294, No. 17 (Mar-'33), pp. 424-426, No. 18 (Mar-'33), pp. 441-427, No. 19 (Apr-'33), pp. 467-469, 470-472, No. 21 (May-'33), pp. 525-529, No. 22 (May-'33), pp. 555-569.
• Ref. 229R5: ACM, Vol. 5 (July-December 1933, January-June 1934) [abstracts / summaries] ‒
  No. 1 (July-'33), pp. 3-7, 16, No. 5 (Nov-'33), pp. 127-133, No. 7 (Jan-'34), pp. 165-168, No. 8 (Feb-'34), pp. 202, No. 9 (Mar-'34), pp. 223-225, No. 11 (May-'34), pp. 265-271.
• Ref. 229R6: ACM, Vol. 6 (July-December 1934) [abstracts / summaries] ‒
  No. 3 (Sept-'34), pp. 55-59, No. 5 (Nov-'34), pp. 108, 109.
• Ref. 229R7: ACM, Vol. 8 (July-December 1936, January-June 1937) [abstracts / summaries] ‒
  No. 3 (Sept-'36), pp. 65-70, No. 4 (Oct-'36), pp. 83-93, No. 5 (Nov-'36, pp. 127-129, No. 8 (Feb-'37), pp. 169-175.
• Ref. 229R8: ACM, Vol. 9 (July-December 1937, January-June 1938) [abstracts / summaries] ‒
  No. 4 (Oct-'37, pp. 77-85, No. 5 (Nov-'37) , p. 119, No. 6 (Dec-'37), pp. 141-143, No. 8 (Feb-'38), pp. 189-191, No. 12 (June-'38), pp. 304-306.
• Ref. 229R9: ACM, Vol. 10 (July-December 1938, January-June 1939) [abstracts / summaries] ‒
  No. 4 (Oct-'38), p. 116, No. 12 (June-'39), p. 301.
• Ref. 229R10: AB, No. 24 (1936), "The federal airways system" [incl. airway operations, intermediate landing fields, optical airway beacons, radio range beacons (types RA, RL, MRA, MRL, ML), radio communication stations, weather service], U.S. Dept. of Commerce, Bureau of Air Commerce, 1 July 1936, 25 pp.
• Ref. 229R11: ACM, Vol. 11 (July-December 1939) [abstracts / summaries] ‒
  No. 5 (Nov-'39), pp. 121, 124, No. 6 (Dec-'39), pp. 155-157.
• Ref. 229R12: CAJ, Vol. 1 (1940)  [abstracts / summaries] ‒
  No. 3 (Feb-'40, pp. 37, 38, 52), No. 7 (Apr-'40, pp. 109-111, 156.
• Ref. 229R13: CAJ, Vol. 5 (1944) [abstracts / summaries] ‒
  No. 3 (Mar-'44, pp. 29, 39, No. 12 (Dec-'44), p. 140.
• Ref. 229R14: CAJ, Vol. 6 (1945) [abstracts / summaries] ‒
  No. 2 (Feb-'45), pp. 13, 17.
• Ref. 229R15: CAJ, Vol. 7 (1946) [abstracts / summaries] ‒
  No. 6 (June-'46), pp. 72, 78, No. 7 (July-'46), pp. 85, 95, No. 11 (Nov-'46), pp. 137, 142, 148, No. 12 (Dec-'46), p. 150.
• Ref. 229R16: CAJ, Vol. 8 (1947) [abstracts / summaries] ‒
  No. 3 (Mar-'47), pp. 30, 31, 33, No. 4 (Apr-'47, pp. 37, 44.
• Ref. 229R17: "Circuit design for low-frequency radio ranges" [general description, field intensity distributions, coupling system, antenna currents, phase between antenna pairs vs. course shift, equipment], Donald M. Stuart, "CAA Technical Development Report No. 23 (formerly CAA Technical Development Div. Report No. 8)", November 1939, 23 pp. Source; hathitrust.org, retrieved 26 June 2020.
• Ref. 229R18: "Visual-Aural Ranges and Omniranges", Bulletin No. 3 of "CAA Airways Operations Training Series", March 1949, 24 pp. Source: hathitrust.org, retrieved 26 June 2020.
• Ref. 229R19: "Flying the Omnirange", Department of Commerce,  Civil Aeronautics Administration, Aviation Information, June 1950. 24 pp. Source: hathitrust.org, retrieved 26 June 2020. Revised version was issued by the CAA as "Basic omnirange flying", February 1951, 9 pp.
• Ref. 229R20: "The status of instrument landing systems" [NBS, Lorenz system, Army Air Force, Washington Institute of Technology, Indianapolis, Transcontinental / Western Air / United Airlines, Bendix], William Elvin Jackson, Department of Commerce, Bureau of Air Commerce, Civil Aeronautics Authority, Safety and Planning Division, Technical Development Report No. 1 (formerly CAA Technical Development Report No. 1 (TDR 1), October 1937, 15 pp. Source: hathitrust.org, retrieved 1 July 2020. [Abstract]. Note: this article was also published in 1938 the Proc. of the I.R.E. (ref. 229L19).
• Ref. 229R21: "Preliminary report on a four course ultra-high-frequency radio range", J.C. Hromada, Dept. of Commerce, Bureau of Air Commerce, Safety and Planning Division, Technical Development Report No. 3 (TDR 3, formerly Report No. 3), January 1938, 7 pp., CAA reprint, 1940. Source: hathitrust.org, retrieved 1 July 2020.
• Ref. 229R22: "Geographical separation of Radio Range stations operating on the same or adjacent frequencies in the 200-400 kilocycle band", A.E. Harrison, U.S. Department of Commerce, Civil Aeronautics Authority, Technical Development Report No. 4, January 1938, 6 pp. Source: hathitrust.org, 16 January 2021.
• Ref. 229R23: "The Ultra-High-Frequency Aural Radio Range", J.C. Hromada, P.B. King, Part I of "Development of the Ultra-High-Frequency Radio Range", U.S. Dept. of Commerce, Civil Aeronautics Administration, Radio Development Section, Technical Development Division, Technical Development Report No. 42, June 1944, 15 pp.
• Ref. 229R24: "Testing of the UHF Radio Ranges and Towers", P.B. King, T.A. Kouchnerkavich, Part II of "Development of the Ultra-High-Frequency Radio Range", U.S. Dept. of Commerce, Civil Aeronautics Administration, Radio Development Section, Technical Development Division, Technical Development Report No. 43, July 1944, 40 pp. [file size: 34 MB]
• Ref. 229R25: "A Visual-Aural Ultra-High-Frequency Radio Range with Simultaneous Voice", Part III of "Development of the Ultra-High-Frequency Radio Range", U.S. Dept. of Commerce, Civil Aeronautics Administration, Radio Development Section, Technical Development Division, Technical Development Report No. 49, June 1945, 116 pp. [file size: 40 MB]
• Ref. 229R26: "Pilots' Radio Handbook" [incl. Visual Radio Range, Aural Radio Range, Omni Range / VOR, ILS], U.S. Dept. of Commerce, Civil Aeronautics Administration (C.A.A.), Technical Manual No. 102, September 1953, revised March 1954, 127 pp. [file size: 31 MB]
• Ref. 229R27: pp. 295-298 in "The tide of commerce and industry (1920-30)", Chapter V (pp. 221-298) in "Measures for Progress - A history of the National Bureau of Standards", Rexmond C. Cochrane, James. R. Newman (ed.), U.S. Dept. of Commerce, Miscellaneous Publication 275 (MP 275), 1966, 683 pp. Source: nist.gov, retrieved 19-Feb-2021.
• Ref. 229R28: "Airplane radio" [1921 NBS support to Army Air Service experiments with loops crossing at 45° for generating equisignal course], p. 68 in "Radio Communication", in "Annual Report of the Director Bureau of Standards to the Secretary of Commerce for the Fiscal Year Ended June 30, 1921", Dept. of Commerce, Miscellaneous Publication No. 47 (MP 47), 1921, 276 pp. Source: digital.library.unt.edu, retrieved 22 February 2021.
• Ref. 229S: "American Aviation Heritage - Identifying and Evaluating Nationally Significant Properties in U.S. Aviation History, A National Historic Landmarks Theme Study", U.S. Dept. of the Interior, National Park Service, Rev. March 2011, 320 pp. Source: npshistory.com, retrieved 20 June 2020. [pdf]
• Incl.: College Park Airfield/MD (pp. 84-87, p. 261), Army's McCook Field in Dayton/OH (p. 54, 67, 71, 125, 134, 267; all facilities moved to Wright Field in 1927 and was then closed down), Wright Field (also near Dayton/OH, later Wright-Patterson AFB; pp. 3-5, 125, 134-136), Bellefonte/PA Air Mail Field (p. 84, 85, 271; NBS 4-course range test site), Mitchel Field near Garden City/NJ (p. 168)].
• Ref. 229T: articles/books about radio beacons developed and/or installed in France
• Ref. 229T1: "Navigation radioélectrique (principe des appareils)" ["Radio Navigation (principles of the equipment)"; D/F, rotating loop, rotating beacons with radio goniometer, course-beacons, Lorenz & L.M.T. landing systems, ], Ministère des Travaux Public et des Transports, Secrétariat Général à l'Aviation Civile et Commerciale, Section des Instructions Aéronautique, preliminary edition, 1944, 44 pp. Source: calameo.com, retrieved 3 July 2020.
• Ref. 229T2: "Les phares aéronautiques - France, Allemagne, États-Unis, Europe du Nord, 1910-1960" ["Aeronautical beacons - France, Germany, USA, northern Europe"], Collection Mémoire de l'Aviation Civile, Direction Générale de l'Aviation Civile (DGAC), 2018, 99 pp. Source: Ministère de la Transition écologique et solidaire / Mission Mémoire de l'Aviation Civile, retrieved July 2020.
• Ref. 229T3: "Chronique de la navigation aérienne" ["Chronicle of radio air navigation", file size: 30 MB], Jean Hubert, École Nationale de l'Aviation Civile (ENAC, National Civil Aviation School, Toulouse/France, publ.), 1987, 349 pp. Source: Ministère de la Transition écologique et solidaire / Mission Mémoire de l'Aviation Civile, retrieved December 2019. [Notes]
• Ref. 229T4: "Navigation radioélectrique (principe des appareils)" ["Radio navigation - principle of the systems"; radio-goniometer & error sources, automatic goniometer/DF, radio compass, omni-directional radio beacons, fixed & rotating course-beacons, radio altimeter, bad-weather landing methods & beacons, Lorenz], Ministère des Travaux Public et des Transports, Secrétariat Général à l'Aviation Civile et Commerciale, Section des Instructions Aéronautique, preliminary edition, 1944, 44 pp. Source: calameo.com, retrieved 3 July 2020.
• Ref. 229T5: "Phares lumineux ou radiophares?" ["Optical or radio beacons?"], P. Franck, in "L'Aéronautique - Revue mensuelle illustrée", Vol. 9, No. 100, September 1927, pp. 271-274. Source: Bibliothèque National de France (BNF) online library, retrieved 11 July 2020.
• Ref. 229T6: "Un important problème de la navigation áerienne : Le guidage des avions 1-2-3" ["An important problem in aerial navigation: the guidance of aircraft", Parts 1-3], M. Volkringer, in "Revue Aéronautique de France", Vol. 20, No. 3, March-April 1930, pp. 4-6; No. 4, April-May 1930, p. 7; No. 7, July-August 1930, pp. 5-6. Source: Bibliothèque National de France (BNF) online library, retrieved 11 July 2020.
• Ref. 229T7: "L'atterissage sans visibilité des avions par l'emploi des ondes ultra-courtes - 1" ["Aircraft landing without visibility by using ultra-short waves" - Part 1 of 3], Vol. 29, No. 7-8, July-August 1939, pp. 10-12. Source: Bibliothèque National de France (BNF) online library (the library does not hold issues of this magazine with the remaining part(s) of the article), retrieved 7 September 2020.
• Ref. 229T8: "Les radiophares d'atterrissage" ["Radio beacons for landing"], Paul Larivière, in "L'Aéronautique", Vol. 18, No. 207, August 1936, pp. 155-163. Source: Bibliothèque National de France (BNF) online library, retrieved 11 July 2020.
• Ref. 229T9: "Note sur les aériens et les diagrammes de rayonnement des radiophares de guidage à enchevêtrement / Notice concerning the aerials and the diagrams of radiation of the radio range beacons at interlocking signals" [article in both French & English], Yves Rocard, in "Bulletin de la Société Française Radio-Électrique (S.F.R.)", Vol. 11, No. 2, 2^nd Quarter 1937, pp. 33-60. Source: Bibliothèque National de France (BNF) online library, retrieved 11 July 2020.
• Ref. 229T10: "Les radiophares interférentiels S.A.D.O.D.-Aicardi" ["Directional radio beacons of the company Société des Ondes Dirigées (SADOD), system Aicardi], Paul Larivière, in "L'Aéronautique", Vol. 19, No. 219, August 1937, pp. 198-202. Source: Bibliothèque National de France (BNF) online library, retrieved 11 July 2020.
• Ref. 229T11: "Effets de diffraction affectant la signalisation des radioalignements" ["Effects of refraction (shore effect) on the signalling of radio course beacons"], Yves Rocard, in "Revue Scientifique", Vol. 78, No. 5-6, May-June 1940, pp. 267-272. Source: Bibliothèque National de France (BNF) online library, retrieved 11 July 2020.
• Ref. 229T12: "Un dispositif français d'atterrissage sans visibilité" ["A French device for blind landing"], in "Aviation Française", No. 138, 22 October 1947, p. 8. Source: Bibliothèque National de France (BNF) online library, retrieved 11 July 2020.
• Ref. 229T13: "L'atterrissage dans la brume" ["Landing in fog"], Henry Porra, in "Science et Vie", No. 356, May 1947, pp. 223-233. [ZZ method, Toulouse, SCS 51, GCA, CSF].
• Ref. 229T14: "Le temps des ingénieurs de la navigation aérienne - Mémoires techniques 1945-1985" ["The era of radio navigation engineers"; file size: 34 MB], Groupement des Ingénieurs de L'aviation Civile Rétraités (Bordeaux), 2013, 243 pp. Source: calameo.com, retrieved 4 October 2020.
• Ref. 229U: "Report on electronic systems of air navigation - "Technical and economic characteristics of LF/MF non-directional beacons, Standard Loran, Consol [Sonne-Consolan], Navarho, Decca, "GEE" system, LF/MF four-course radio range, VHF omni-directional range, Distance Measuring Equipment", Air Coordinating Committee, Air Traffic Control & Navigation Panel, Special Working Group No. 9; U.S. Dept. of Commerce, Office of Technical Services, March 1954, pp. Source: hathitrust.org, retrieved 1 July 2020.
• Ref. 229V: articles, books, and other material about the history of the U.S. DoC National Bureau of Standards and its beacons
• Ref. 229V1: "History and development of the Bureau of Standards radio beacon experiment station at College Park, Maryland", Robert W. Beckham, 18 December 1936, 40 pp. Records of the Phi Mu Fraternity, Special Collections, University of Maryland Libraries, University of Maryland, College Park. Source: archive.org, retrieved 3 July 2020. [Introduction]
• Ref. 229V2: pp. 293-298, 407 in "Measures for progress: A history of the National Bureau of Standards" [file size: 32 MB], Rexmond C. Cochrane, U.S. Dept. of Commerce, National Bureau of Standards, MP 275, 2^nd ed., 1974, 718 pp. Source: US National Institute of Standards and Technology (NIST),  accessed 3 September 2020. [pdf]
• Ref. 229V3: "The first radio beacon is inaugurated", photo of the interior of the Bureau of Standards beacon radio hut at College Park/MD/USA, with its director, George K. Burgess, 30 May 1927. Retrieved 19 February 2021.
• Ref. 229V4: Historic images in the Digital Archives of the National Institute of Standards and Technology (NIST, Gaithersburg, MD 20899). Records: Dials for blind landing aircraft (Aug. 1930), Blind landing system - 1 (Dec. 1930), Blind landing system - 2 (Dec. 1930), Interior of blind landing system (Dec. 1930), Landing beacon indicator and control panel (April 1930), Aids for blind landing of aircraft (July 1930), College Park field station (1926), Aircraft course indicators using two metal reeds, 1928. Accessed: June-August 2020.
• Ref. 229W: U.S. Dept. of Commerce and US Army Air Forces documents covering air navigation (incl. US radio range maps)
• Ref. 229W1: "Practical air navigation and the use of the aeronautical charts of the Department of Commerce", Thoburn C. Lyon, Special Publication No. 197, U.S. Dept. of Commerce, Coast and Geodetic Survey, 1935, 63 pp. [file size: 23 MB] [p. 36 - Radio Range chart] Source: hathitrust.org.
• Ref. 229W2: "Practical air navigation and the use of the aeronautical charts of the U.S. Coast and Geodetic Survey", Thoburn C. Lyon, Special Publication No. 197, U.S. Dept. of Commerce, Coast & Geodetic Survey, 2^nd ed. (1938), 204 pp. [p. 53 - Radio Range chart] [file size: 52 MB] Source: noaa.gov, accessed 21 July 2020.
• Ref. 229W3: "Practical air navigation", Thoburn C. Lyon, Civil Aeronautics Bulletin No. 24, U.S. Dept. of Commerce, Civil Aeronautics Administration (CAA), September 1940, 263 pp. [p. 97 - Radio Range chart] [file size: 49 MB]. Source: hathitrust.org, accessed 22 July 2020.
• Ref. 229W4: "Practical air navigation", Thoburn C. Lyon, 1945, 359 pp. [p. 200 - Radio Range chart] [file size: 112 MB] Source: hathitrust.org, accessed 23 July 2020.
• Ref. 229W5: "Instrument Flying: Advanced (with Radio Aids)" [incl. A/N Radio Ranges, Marker Beacons], U.S. Army Air Forces, Technical Order No. 30-100B-1, 15 January 1944, 80 pp. Source: aafcollection.info, accessed 23 September 2020.
• Ref. 229W6: "The Evolution of Instrument Flying in the U.S. Army", David M. McIntosh, Air Command and Staff College, Air University, Maxwell AFB, Student Report Nr. 88-1760, April 1988, 65 pp. Source: US Defence Technical Information Center, retrieved 7 October 2020.
• Ref. 229X: "Neuere Arbeiten auf dem Funkbaken-Gebiete" ["Recent work in the field of radio beacons"; Lorenz, E/T keying, magnetic-bias keying, VHF beacon], E. Kramar, "Hochfrequenztechnik und Elektroakustik", Vol./Nr. 40, September 1932, pp. 88-92. [Abstract].
• Ref. 229Y: articles in (popular) radio and aviation magazines, primarily 1920s
• Ref. 229Y1: "Radio beacons to aid air mail flyers", R.S. Winters, in "Radio Age", Vol. 7, Nr. 1, September 1927, pp. 21-24, 48. Source: worldradiohistory.com,, retrieved 23 February 2021.
• Ref. 229Y2: "Guiding airplanes by radio beacons", S.R. Winters, in "Popular Aviation", Vol. 1, Nr. 4, November 1927, pp. 23-26.
• Ref. 229Y3: "Landing of aircraft in fog by radio", H. Diamond, in "Electronics", Vol. 6, Nr. 6, June 1933, pp. 158-161. Source: worldradiohistory.com, retrieved 23 February 2021.
• Ref. 229Y4: "Airports and Airways" [radio beacon at Hadley Field], in "Aviation" (pre AvWeek), Vol. 23, No. 1, 29 August 1927, p. 486, 488. Source: us.archive.org, retrieved 24 February 2021.
• Ref. 229Z: articles and other material about the Henry Ford A/N radio beacons
• Ref. 229Z1: photos from the digital collection expert set of The Henry Ford, accessed 15 February 2021 - "Radio Beacon Transmitting Station and Antennas at Ford Airport, 10 September 1926" (image THF123653); "Radio Station "WFO" License for Operating the Ford Aircraft Radio Beacon, 26 October 1926" (image THF255154); "Detail of Goniometer Used inside the Radio Beacon Transmitting Station at Ford Airport, 10 September 1926" - (image THF99457), "Equipment Inside the Radio Beacon Station Building at Ford Airport, March 16, 1927" (image THF255074; goniometer motor drive & scale).
• Ref. 229Z2: "Radio beacon tests at Ford Airport", in "The New York Times", February 27, 1927, Section XX, p. 18.
• Ref. 229Z3: "Radio beacon used by commercial airplane", William F. Gilmore, in "U. S. Air Services", Vol. XII, Nr. 3, March 1927, p. 15.
• Ref. 229Z4: "The new Ford transport", A.M. Jacobs, p. 68 in "Air Corps News Letter", Vol. XI, No. 3, 10 March 1927. Source: media.defense.gov, retrieved 19 February 2021.
• Ref. 229Z4: "Guiding aerial flight by radio beacon", in "Ford News", Vol. 6, 8 April 1927, pp. 4-5.
• Ref. 229Z5: "The radio beam", in "Ford News", Vol. 20, November 1940, p. 247.
• Ref. 229Z6: "Radio beacon", James Barbier, in "Herald", Vol. 13, 15 June 1940, pp. 291, 297
• Ref. 229Z7: "1^st with a radio beam for planes!", 1-page Ford Co. advertising in "Boy's Life - For All Boys" magazine, May 1945, p. 19; on p. 12 in "Life", 5 February 1945; on p. 33 in "Popular Science", January 1945.
• Ref. 229Z8: pp. 204-207 in "Henry Ford: His life - His work - His genius", William Adams Simonds, Floyd Clymer (publ.), 1946 (revised ed.), 365 pp. Source: us.archive.org, retrieved 19 February 2021.
• Ref. 229Z9: "Aircraft Radio Beacon" chapter 10 (pp. 97-100) in "Beyond the Model T - The other adventures of Henry Ford", Ford R. Bryan, Wayne State University Press (publ.), revised edition, 1997, ISBN 0-8143-2682-X, 232 pp.
• Ref. 229Z10: "Ford Airport Hangar", National Register of Historic Inventory - Nomination Form, U.S. Dept. of the Interior, National Park Services, 10 April 1985, 25 pp. Source: catalog.archives.gov, retrieved 15 February 2021. [file size 25 MB] 
• Ref. 229Z11: website "Flying the beams: The history of the Low Frequency Radio Range, aviation's first radio navigation station", accessed 15 February 2021.
• Ref. 229Z12: "How the Ford radio beacon station changed aviation", YouTube video, accessed 15 February 2021.
• Ref. 229Z13: "Lansing Municipal Airport", 4 pp. in "Forgotten Chicago Airfields", Nicolas C. Selig, The History Press (publ.), 2014, 128 pp.
• Ref. 229Z14: "Ford Motor Company" [renewal of license for experimental radio stations W8XC, W9XH, W8XE], pp. 150-153 in "Decisions, reports, and orders of the Federal Communications Commission of the United States - July 1934 - July 1935 - Volume 1", Federal Communications Commission Reports, Federal Communications Commission, 1936, 338 pp.
• Ref. 230: German & British WW2 RDF, radio-navigation systems, and associated jamming systems / countermeasures
• Ref. 230A: p. 42 in "The Secret War", Brian Johnson, Pen and Sword (publ.), 2004, 352 pp. [pdf, file size 50 MB] See note 1
• Ref. 230B: Table 1 in "Verfahren und Anlagen der Funkortung" ["Radio-navigation methods and installations"], W. Stanner, in "Elektrotechnische Zeitung (ETZ)", Ausgabe A, Vol. 75, Nr. 13, 1 July 1954, pp. 438-442. [circular LoP, hyperbolic LoP, Consol, Consolor, Decca, Loran, range of various systems incl. "Erika", "Erich", "Hermine", and "Mond"]
• Ref. 230C: "Pulling the crooked-leg", R.V. Jones, in "New Scientist", 23 February 1978, pp. 493-496.
• Ref. 230D: "Most Secret War: Britisch Scientific Intelligence 1939-1945", R.V. Jones, Hamish Hamilton (publ.), 1978, 576 pp. See note 1
• Ref. 230E: "Milestones - Battle of the Beams", Carlo Kopp, in "Defence Today", January/February 2007, pp. 76, 77.
• Ref. 230F: "The Battle of the Beams - Part 1-3", D.V. Pritchard (G4GVO), in "Practical Wireless", Vol. 64, No. 1, Issue 970, January 1989, pp. 43-47; No. 2, Issue 971, February 1988, pp. 46-49; No. 3, Issue 972, March 1988, pp. 30-34. Source: worldradiohistory.com, accessed 1 September 2020. Also published as "The Battle of the Beams - Part 1-3", D.V. Pritchard (G4GVO), in "Ham Radio Magazine", June 1989, pp. 29-38; August 1989, pp. 20-29; October 1989, pp. 53-61. Source: worldradiohistory.com, accessed January 2014.
• Ref. 230G: "Chapter II and III in "R. V. Jones and the Birth of Scientific Intelligence", James Martinson Goodchild, PhD thesis, University of Exeter, March 2013, 640 pp. [pdf] Accessed 12 May 2019.
• Ref. 230H: "Electronic Warfare and the Night Bomber Offensive", Rob O'Dell, pp. 97 - 117 in "Air Power Review", Royal Air Force, Volume 10, Number 1, Spring 2007.
• Ref. 230J: "Radio Navigation Systems for Aviation and Maritime Use — A Comparative Study" [RDF, Consol/Consolan, Navaglobe, VOR, VORTAC, VOR/DMET, Navarho/Navarho-H/-HH/-Rho, Decca, Standard Loran (Loran-A), Loran-C, Radio Mesh Systen], W. Bauss (tech ed.), Advisory Group for Aeronautical Research and Development (AGARD), North Atlantic Treaty Organization (NATO), AGARDOgraph 63, Pergamon Press, 1963, 232 pp. Translation of the German publication "Funkortungssysteme für Luft- und Seefahrt - Eine vergleichende Gegenüberstellung", Verkehrs- und Wirtschaftsverl. Dr. Borgmann (publ.). [pdf, file size 58 MB]. The following are articles taken from this book.
• Ref. 230J1: "Radio Direction-Finding on Board Aircraft and Ships", W.T. Runge, pp. 19-28.
• Ref. 230J2: "Consol and Consolan", Ernst Kramar, pp. 29-39.
• Ref. 230J3: "VOR-System", K. Bärner, pp. 43-57.
• Ref. 230J4: "Decca". H. Lueg, pp. 81-101.
• Ref. 230J5: "Standard-Loran", Ernst Kramar, pp. 113-118.
• Ref. 230K: articles about Elektra, Sonne, Consol/Consolan, Elektra-Sonne, Mond
• Ref. 230K1: "The Navigational Beam System "Elektra-Sonne" [Elektra, Sonne, Elektra-Sonne, Mond; complete German description, short translated summary in English], Otto von Heil, FIAT Final Report No. 1105, Field Information Agency Technical (FIAT), US Office of Military Government for Germany, 17 June 1947, 177 pp. Source: www.cdvandt.org. Accessed: March 2019.
• Ref. 230K2: "Funknavigation, Elektra, Sonne, Mond, Stern, Erika", J. Goldmann (Lorenz), Vorträge vor Fernmelde-Ingenieuren der Luftwaffe - Luftnachrichtenschule Halle (Saale) [Luftwaffe Signals School], February 1944, 22 pp. [file size: 25 MB]. Source: BArch file nr. (Signatur) RL 2-V/48, used in accordance with "Erstinformation für Ihren Besuch im Bundesarchiv in Freiburg, Stand Juni 2016".
• Ref. 230K3: "Sonne Planungen" ["Planning of Sonne sites"; mentions Sonne station Liebau, Sonne stations 12 (Warsaw) & 23, Großsonne station 32 (Danzig), and new Sonne site near Oppeln], Luftnachrichten telegram, dated 29 July 1944, signed by Capt. Franz, 1 p. Source: BArch file nr. (Signatur) RL 2-V/6, used in accordance with "Erstinformation für Ihren Besuch im Bundesarchiv in Freiburg, Stand Juni 2016".
• Ref. 230K4: "Consol - Beacon Direction Finding system of high accuracy" [Consol, Sonne, SBA], John E. Clegg, in "Wireless World", Vol. 52, No. 7, July 1946, pp. 233-235. Source: worldradiohistory.com, retrieved 6 January 2021.
• Ref. 230L: "Radio vs. U-boat" - An account of the part played by radio in the Battle of the Atlantic", G.M. Bennet, in "Wireless World", Vol. 52, No. 12, December 1946, pp. 408-411. Source: worldradiohistory.com, retrieved 6 January 2021.
• Ref. 230M: "Demonstrations of Radio Aids to Civil Aviation: Technical Descriptions of Radio Aids Demonstrated in the United Kingdom on Behalf of P.I.C.A.O during September 1946" [Consol, Post Office Position Indicator (POPI), Automatic DF (Radio Compass), weather radar, Gee with L & θ indicator, Decca, Rebecca Mk II & IV + Orbit Computer, VHF Omni-directional Radio Beacon(ORB), VHF Rotating Beacon, VHF Cathode Ray DF (CRDF), Airfield Control radar (ACR), SCS-51, Airfield Surface Movement Indicator (ASMI), BABS Mk II, radio altimeter slot antenna, GEE/BABS/Rebecca trainers, H2S/ASV trainers], Great Britain, Ministry of Civil Aviation, Provisional International Civil Aviation Organisation, H.M. Stationery Office, 1946, 84 pp. Source: chingchic.com, retrieved 5 January 2021. [file size: 25 MB]
• Ref. 230N: articles about VHF rotating-phase navigation systems VOR, Erich
• Ref. 230N1: "An Omnidirectional Radio-Range System - Part I & II" [VOR], David G.C. Luck, in "RCA Review", Vol. VI, Nr. 1, July 1941, pp. 55-81, Vol. VI, Nr. 3, January 1942, pp. 344-369. Source: worldradiohistory.com, retrieved 25 October 2020.
• Ref. 230N2: "Omnirange = Air Safety", Samuel Freedman, in "Radio Electronics", February 1951, pp. 24-26. Source: worldradiohistory.com, retrieved 27 January 2021.
• Ref. 230P: original correspondence of the Director General of Luftwaffe Signals Corps (General Nachrichtenführer). Source: Bundesarchiv file nr. (Signatur) RL 2-V/5., used in accordance with "Erstinformation für Ihren Besuch im Bundesarchiv in Freiburg, Stand Juni 2016".
• Ref. 230P1: "Anruf Gen.Martini wegen Erstellung Boden-Truhe West" [telephone call with General Martini regarding construction of Boden-Truhe West]. Letter/telegram from Gen. Nachrichtenführer (1.Abt.II), addressed to Chef. der Ln.Inspektion. Letter ref. OKL Gen.Nafü. Nr. 10 955/44 g.Kdos. (1. Abt.II). Letter is dated 18 June 1944. Letter states that due to current situation, the forward-looking Boden-Truhe West station will not be constructed and construction of the rearward-looking Boden-Truhe West station will be accelerated, using transmitters of the prior. The planned transmitters will be transported from France, but with trucks/lorries (wood gas) instead of by rail.
• Ref. 230P2: "Fernmündliche Rücksprache Major Kluge - Hptm. Gottschalk am 30.6.44" [telephone conversation Major Kluge - Captain Gottschalk]. Letter from Gen.Nafü (1.Abt.), addressed to Lfl.Kdo. 3 - Höh.Nafü, General der Navigation, and General der Kampfflieger. Letter ref. OKL Gen.Nafü. Nr. 11 092/44 g.Kdos. (1. Abt.II). Letter dated 8 July 1944. Letter states that, due to operational and test reasons, construction of radio navigation stations "Komet 2 (Laharie)", "Komet 3 (Labouheyre), and "Dora 2 (Morlaix) cannot be finished. Due to failure of "Erika 2 (Cherbourg)", the "Erika" system can no longer be used in the West. Therefore, "Erika 1 (Boulogne)" can be dismantled and parts (transmitter etc.) be secured.
• Ref. 230P3: "Sender for Bodentruhe" [transmitters for Truhe ground station]. Letter from Gen.Nafü (1.Abt.), addressed to Gen.Nafü/Ln.Insp (5.Abt/6.Abt). Letter is dated 8 July 1944. Letter describes allocation of 3 "Feuerstein" transmitters instead of "Feuerzange" transmitters to "Bodentruhe West", also mentions 3 "Merkur" transmitters are to be modified for "Bodentruhe West", schedule for delivery of additional "Merkur" and "Feuerstein" transmitters to be provided.
• Ref. 230P4: "Abschalten von Rundfunksendern." [Shutdown of (public) radio broadcast transmitters]. Letter from OKL.Gen.Nachrichtenführer. Letter ref. 11 987/44 geh. (1.Abt.II). Letter is dated 2 July 1944. Letter states that, as agreed with OKW and RPromin [Reichs Propaganda Ministerium], the request for shutting down radio broadcast transmitters during jamming/interference of radio beacons for day & night fighters, is denied. Reasons given by RPromin: 1) the current regulations already imply large scale shutdowns, further reduction is unacceptable for the propaganda, 2) the population interprets shutdowns as sign of imminent air raids. Shutdown for other reasons would cause unrest, and 3) Broadcast transmitter frequencies are fixed. If broadcast transmitters interfere with radio beacons or other services, Lfl.Kdo. must make sure that those services use other frequencies. As in other Luftflotten regions, restriction of radio beacons improves spread of utilized frequencies.
• Ref. 230Q: articles about Knickebein
• Ref. 230Q1: "Location of Knickebein station K1-K13" [map coordinates, satellite images, aerial photos], Frank Dörenberg, 12 August 2020, 8 pp.
• Ref. 230Q2: pp. 245-247 in "Gleichgeschaltet: Maulburg im Nationalsozialismus und die Rolle von Hermann Burte im Dritten Reich", Hansjörg Noe, Verlag Waldemar Lutz (publ.), 448 pp., ISBN 978-3-922107-09-5. [Extract]
• Ref. 230Q3: p. 16 of "Knickebein" thread in the forum of geschichtsspuren.de, post by ChristianCH on 30-Mar-2014. [Extract]
• Ref. 230Q4: p. 2, 3 in "Krigsminner 1940-45 i Klepp kommune" ["War memories 1940-45 in the Klepp municipality"], F. Ravndal, T. Ødemotland, A. Jakobsen, T. Erga, O. Håland, B. Bore, A. Hatteland, J. Sørbø, T. Reve, Laget (publ.), 1990, 8 pp. Source: Norwegian National Library, accessed 31 July 2020. [pdf]
• Ref. 230R: articles about X-Verfahren ("X-Procedure), X-Gerät ("X-Equipment"), Y-Verfahren ("Y-Procedure"), Y-Gerät ("Y-Equipment"); the Y-guidance method is not to be confused with Y-Peiler ("Y-D/F system"), nor the British Y-Service!
• Ref. 230R1: pp. 48-49 (X-Gerät, Y-Gerät) in "German Radio Communication Equipment", US War Department Technical Manual, TME 11-227, June 1944, 61 pp.
• Ref. 230R2: "Fliegerhorst Köthen" ["Köthen airfield"], source: Militärhistorisches Museum Anhalt, accessed 11 August 2020. [pdf]
• Ref. 230R3: pp. 14-27 in "The First Pathfinders - The Operational History of Kampfgruppe 100, 1939-1941", Kenneth Wakefield, William Kimber (publ.), 1981, 265 pp.
• Ref. 230S: articles about "Komet" ("Comet")
• Ref. 230S1: "Tysk Retningsantenneanlæge i Kølby Vest for Nibe" ["German Directional Antenna Installation in Kølby, west of Nibe"; rotating-beam system "Komet" ????], Commission for the Inspection of German Radio Stations Constructed in Denmark, report by Capt. Bahnsen and Prof. Jørgen Rybner of site visit on 12 Dec 1945, 3 pp. Courtesy M. Svejgaard, used with permission. English translation by me is here.
• Ref. 230T: "Scientific Intelligence", R.V. Jones, 12 February 1947 lecture, "CIA Studies in Intelligence" Vol. 6, No. 3 (Summer 1962), pp. 55-76. Source: cia.org, accessed 12 August 2020. This is a slighly edited version of the original 12 February 1947 lecture, as first published in "Journal of the Royal United Services Institution", nr. 42, August 1947, pp. 352-360.
• Ref. 230U: "Graphic survey of radio and radar equipment used by the Army Air Force", U.S. Army Air Forces, Air Technical Service Command
• Ref. 230U1: Section one, "Countermeasures equipment", 1 February 1945, 89 pp.
• Ref. 230U2: Section three, "Radio navigation equipment", 1 May 1945, 68 pp. [file size: 21 MB]
• Ref. 230U3: Section four, "Radar navigation equipment", 1 July 1945, 138 pp.
• Ref. 230V: "Navigational Aids" and "Radar and Radar Approach Aids" [MF & HF D/F, SBA, Radio Ranges, Gee, SCR 277, B/T, BABS, GCA], pp. 24-32 in "The signals war: A brief history of no. 26 Group", AIR 14/3562, UK Air Ministry, Bomber Command, December 1945.
• Ref. 230W: "Aerial Navigation and Traffic Control with Navaglobe, Navar, Navaglide, and Navascreen", H. Busignies, Paul R. Adams, Robert I. Colin, in "Electrical Communication - A Journal of Progress in the Telephone, Telegraph and Radio Art", published by "International Standard Electric Corp.", Vol. 23, No. 2, June 1946, pp. 113-143. Source: worldradiohistory.com, retrieved 17 August 2020.
• Ref. 234: general articles about aviation in Germany through WW2
• Ref. 234A: "Die Flugsicherung in Deutschland vor 1945 - Ein Rückblick" ["Air traffic control and air navigation in Germany before 1945 - a review"], part 1 of 3 of "Entwicklung der Flugsicherung in Deutschland" Frank W. Fischer, International Advisory-Group Air Navigation Services (ANSA, publ.), 2016, 464 pp. [key words incl. blind landing with infrared light, accoustic signal, and the Lorenz system]
• Ref. 234B: "German Commercial Air Transport until 1945", Liudger Dienel, Martin Schiefelbusch, in "Revue belge de philologie et d'histoire – Belgisch Tijdschrift voor Filologie en Geschiedenis", vol. 78, nr. 3-4, 2000, pp. 945-967. [pdf]
• Ref. 235: "blind" landing systems, instrument landing systems, Ground Controlled Approach (GCA)
• Ref. 235A: "Schlechtwetterlandeanlagen" ["Bad-weather landing systems"], Telefunken commercial brochure W.B.160D (2000), 1936 (?), 4 pp. Source: www.cdvandt.org.
• Ref. 235B: installations at various airports
• Ref. 235B1: "20 Jahre Funkstation Dübendorf 1919–1939" ["20 years radio station Dübendorf], Max Unterfinger, Josef Baumgartner, unpublished typescript, 1939, 472 pp. The referenced photos were published in "Ein Beitrag zur Flugsicherungs Geschichte" ["A contribution about air traffic control history"], Hans H. Jucker, July 2014, 61 pp. Source: www.wrd.ch. Retrieved 18 May 2020.
• Ref. 235B2: pp. 40, 43, 45, 48, 54 in "Gatwick: The Evolution of an Airport" [file size: 21 MB], John King, Issue No. 16 of "Sussex Industrial History - Journal of the Sussex Industrial Archeology Society", 1986, 74 pp. Source: docplayer.net, accessed 24 September 2020.
• Ref. 235C: articles of the Institute of Radio Engineers (IRE) -  "Proceedings of the IRE" and "IRE Transactions on Aeronautical and Navigational Electronics" (ANE). Source "Proc. of the IRE": worldradiohistory.com, retrieved June 2014 - May 2020.
• Ref. 235C1: "Report on Experiments with Electric Waves of about 3 Meters: Their Propagation and Use", Abraham Esau, Walter M. Hahnemann, in "Proc. of the I.R.E.", Vol. 18, Issue 3, March 1930, pp. 471-489. [Abstract].
• Ref. 235C2: "A new field application for ultra-short waves", Ernst Kramar, in "Proc. of the IRE", Vol. 21, Nr. 11, November 1933, pp. 1591-1531.
• Ref. 235C3: "The present state in the art of blind landing of airplanes using ultra-short waves in Europe" [1934/35, 33 MHz LOC, 38 MHz markers], Ernst Kramar, in "Proc. of the I.R.E.", Vol. 23, Nr. 10, October 1935, pp. 1171-1182. [Abstract]
• Ref. 235C4: "A Radio Beacon and Receiving System for Blind Landing of Aircraft", H. Diamond, F.W. Dunmore, in "Proc. of the I.R.E.", Vol. 19, Nr. 14, April 1931, pp. 585-626. [Abstract]. NOTE: this is an expanded version of ref. 235Y4 below.
• Ref. 235C5: "A new system for blind landing of aircraft" [VHF (345 MHz) CW transmitter horizontal dipole antenanna [FD: bad!] in aircraft, directional receiving system on ground (2 crossing loop antennas with "equi" direction aligned with runway center line). Long-wave (350 kHz) AM transmitter on ground transmits Localizer deviation (modulation depth) to aircraft], K. Baumann, A. Ettinger, in "Proc. of the I.R.E.", Vol. 24, Nr. 5, May 1936, pp. 751-754.
• Ref. 235C6: "Coupled Networks in Radio-Frequency Circuits" [transmission-line bridge with coupled TL-section, as used in AAC LOC system], Andrew Alford, in "Proc. of the I.R.E.", Vol. 29, Nr. 2, February 1941, pp. 55-70.
• Ref. 235C7: "Instrument Landing at the National Bureau of Standards", Frank G. Kears, in "IRE Trans. on ANE", Vol. 6, Issue 2, June 1959, pp. 61-67. [Abstract]
• Ref. 235C8: "Operational Flight Testing of Early Instrument Landing Systems", E.A. Cutrell, in "IRE Trans. on ANE", Vol. 6, Issue 2, June 1959, pp. 67-70. [Abstract]
• Ref. 235C9: "Ground-Controlled Approach---Its Development and Early Operational Use", Chester Porterfield, in "IRE Trans. on ANE", Vol. 6, Issue 2, June 1959, pp. 71-75. [Abstract]
• Ref. 235C10: "All-Weather Landing", J.L. Anast, in "IRE Trans. on ANE", Vol. 1, Issue 2, June 1959, pp. 75-77. [Abstract]
• Ref. 235C11: "A Survey of Instrument Approach Systems in the United States", Henry L. Metz, in "IRE Trans. on ANE", Vol. 6, Issue 2, June 1959, pp. 78-84. [Abstract]
• Ref. 235C12: "Improvements on the Instrument Landing System", in "IRE Trans. on ANE", Vol. 6, Issue 2, June 1959, pp.85-94. [Abstract]
• Ref. 235C13: "Electronic Landing Aids for Carrier Aircraft", A. Brodzinsky, in "IRE Trans. on ANE", Vol. 6, Issue 2, June 1959, pp. 95-99. [Abstract]
• Ref. 235C14: "Glide-Slope Antenna Arrays for Use under Adverse Siting Conditions", in "IRE Trans. on ANE", Vol. 6, Issue 2, June 1959, pp. 100-111. [Abstract]
• Ref. 235C15: "The FAA Philosophy and Program of Instrument Approach and Landing System Development", L.C. Wright, D.J. Sheftel, in "IRE Trans. on ANE", Vol. 6, Issue 2, June 1959, pp. 112-117. [Abstract]
• Ref. 235C16: "A Look at the Future of Automatic Landing Systems", G.B. Litchford, A. Tatz, F.H. Battle, in "IRE Trans. on ANE", Vol. 1, Issue 2, June 1959, pp. 118-128. [Abstract]
• Ref. 235C17: "An Automatic Landing System", F.D. Powell, in "IRE Trans. on ANE", Vol. 6, Issue 2, June 1959, pp. 135-142. [Abstract]
• Ref. 235C18: "The AN/MSN-3: An Automatic Ground-Controlled Approach System", H. Goldstein, B. Cutler, in "IRE Trans. on ANE", Vol. 6, Issue 2, June 1959, pp. 142-148. [Abstract]
• Ref. 235D: "Airmail: A Brief History", United States Postal Service, March 2018, 7 pp. Source: usps.com, retrieved 21 October 2020.
• Ref. 235E: "Operation and Maintenance of Lorenz Equipment" and "Ultra-Short-Wave Blind-Approach System", pp. 55-64 and pp. 114-128 in "Aeroplane Radio Equipment - Dealing with Marconi, Standard, and North American radio equipment, with special notes on direction finding equipment, Lorenz equipment, and bonding and screening", Edward Molloy (ed.), Ernest Walter Knott (ed.), Vol. 17 of "Aeroplane Maintenance and Operation Series", Chemical Publishing Company, Inc. (publ.), 1941, 133 pp. Source: hathitrust.org. Retrieved 22 May 2020.
• Ref. 235F: "Schaltung und Aufbau der Sender" ["Circuit and construction of the transmitters"], Part II of "Die Sender und Sendeanlagen der Reichsflugsicherung" ["Circuits & construction of ground-station transmitters"], Vol. 3 of "Flugsicherungstechnische Lehrbücher" ["Air traffic control textbooks"], Hans-Joachim Zetzmann, Georg Siemens Verlagsbuchhandlung (publ.), 1938, 106 pp.
• Ref. 235G: "Die Ultrakurzwelle-Funkbake" ["The VHF radio beacon"; general description and method of operation of an A/N approach guide-beam beacon], E. Kramar, in "Elektrische Nachrichten-Technik (E.N.T.)", Vol. 9, Nr. 12, December 1932, pp. 469-473. Source: cdvandt.org.
• Ref. 235H: articles in "Air Corps News Letter" (source: Air Force Historical Support Division), "Air Force", and "Air Force Magazine" (source: airforcemag.com):
• Ref. 235H1: "First solo blind flight a success" [Hegenberger, localizer + marker beacon], Joseph S Edgerton, in "Air Corps News Letter", Vol. XVI, No. 6, 28 May 1932, pp. 4-5, retrieved 6 September 2020.
• Ref. 235H2: "Collier Trophy presented to Captain Hegenberger", in "Air Corps News Letter", Vol. XVIII, No. 14, 1 August 1935, pp. 4-5.
• Ref. 235H3: "Flying Blind" [Doolittle 1929 flight], C.V. Glines, in "Air Force Magazine", September 1989, pp. 140-141, retrieved August 2020.
• Ref. 235H4: "Shooting landings by radio", F.L. Moseley, in "Air Force: the official journal of the U. S. Army Air Forces", Vol. 27, No. 10, October 1944, pp. 41-44.
• Ref. 235J: articles by Eric M. Conway
• Ref. 235J1: "Blind Landings - Low-Visibility Operations in American Aviation 1918-1958" [general history, why runways?, curved & straight glide path guidance, leader-cable system, SCS-51], Erik M. Conway, The Johns Hopkins University Press (publ.), 2006, 235 pp. Retrieved 2 February 2020 [pdf]
• Ref. 235J2: "The Politics of Blind Landing", Erik M. Conway, in "Technology and Culture", Vol. 42, No. 1, January 2001, pp. 81-106. Source: jstor.com, accessed 23 August 2020. [pdf]
• Ref. 235K: "Blind flying on the beam, aeronautical communication, navigation and surveillance: its origins and the politics of technology". Source: NASA Technical Reports Server (NTRS), retrieved 13 January 2020.
• Ref. 235K1: "Part one: Form and Function", Randy Johnson, in "Journal of Air Transportation", Vol. 8, No. 1, 2003, pp. 37-68
• Ref. 235K2: "Part II: "Political Oversight and Promotion", Randy Johnson, in "Journal of Air Transportation", Vol. 8, No. 2, 2003, pp. 57-78
• Ref. 235K3: "Part III: Emerging Technologies, The Radio Range - The Radio Beacon and Visual Indicator", Randy Johnson, in "Journal of Air Transportation", Vol. 8, No. 2, 2003, pp. 79-104
• Ref. 235L: articles in "Journal of the IEE, Part IIIA: Radiocommunication"
• Ref. 235L1: "The development of c.w. radio navigation aids, with particular reference to long-range operation”, R. V. Whelpton, P. G. Redgment, in Vol. 94, Issue: 11, March-April 1947, p. 244-254. [Abstract]
• Ref. 235L2: "C.W. Radio Aids to Homing and Blind Approach of Naval Aircraft", D. Quinn, R.D. Holland, in "J. of the IEE, Part IIIA: Radiocommunication", in Vol. 94, Issue 16, March-April 1947, pp. 953-960.
• Ref. 235L3: "C.W. Radio Aids to Approach and Landing", M. Birchall, in Vol. 94, Issue 16, March-April 1947pp. 943-952
• Ref. 235L4: "Discussion on "C.W. Navigational Aids" at the Radiocommunication Convention, 2^nd April 1947", in Vol. 94, Issue 16, March-April 1947pp. 1022-1028.
• Ref. 235L5: "Discussion on "C.W. Navigational Aids" - The Author's Replies to the Above Discussion", in Vol. 94, Issue 16, March-April 1947pp. 1029-1030.
• Ref. 235M: pp. 43-45 in "Croydon - Britain's Liverpool of the Air", chapter 2, pp. 29-45 in "European skyways; the story of a tour of Europe by airplane", Lowell Thomas, Boston Houghton Mifflin Co. (publ.), 1927, 524 pp. Source: hathitrust.org, retrieved 9 October 2020.
• Ref. 235N: "The Search for an Instrument Landing System, 1918-1948", William M. Leary, Chapter 3, pp. 80-99 in "Innovation and the Development of Flight", Roger D. Launius (ed.), Texas A&M University Press (publ.), 1^st ed., 1999, 335 pp. [pdf]
• Ref. 235P: articles in (popular) radio and aviation magazines, primarily 1930s
• Ref. 235P1: "Flying the Radio Beam", Henry W. Roberts, in "Short Wave Craft", February 1936, pp. 582, 583, 624, 625. Source: worldradiohistory.com, retrieved 28 February 2020.
• Ref. 235P2: "The "Air-Track" System of "Blind Landing"", Charles E. Planck, in "Radio-Craft", Vol. IX, Nr. 4, April 1937, pp. 202, 248, 255. Source: worldradiohistory.com, retrieved 1-July-2020.
• Ref. 235P3: "The Lorenz Blind Landing System", Roderick Denman, in "The Wireless World - The Practical Radio Journal", Vol. XXXVI, Nr. 14, Nr. 814, 5 April 1935, pp. 332-335. Source: worldradiohistory.com. Accessed 21 July 2020.
• Ref. 235P4: "Landing aircraft by sound - a demonstration of the Lorenz Blind Landing System" [Lorenz installation at Heston airport, demo flight with British Continental Airways airplane], in "The Wireless World - The Practical Radio Journal", Vol. XXXVIII, Nr. 26, Nr. 878, 26 June 1936, p. 627. Source: worldradiohistory.com. Accessed 21 July 2020.
• Ref. 235P5: "Radio Aloft", John B. Brennan, in "Radio News", Vol. XIV, Nr. 3, September 1932, pp. 140-141. Source: worldradiohistory.com, accessed 22 July 2020.
• Ref. 235P6: "Air traffic control", in "Wonders of World Aviation", Vol. 1, Part 7, 19 April 1938. Source: wondersofworldaviation.com, accessed 18 August 2020.
• Ref. 235P7: "Down the beam - an amateur tries Heston's Blind Approach System : Technique which requires only practice", H.A. Taylor, pp. 648-649 in "Flight" ["Flight International" since 1962], 18 June 1936.
• Ref. 235P8: "Instrument Landing System for Aircraft - Part I", Henry W. Roberts, in "Aero Digest including Aviation Engineering", Vol. 29, Nr. 10, October 1936, pp. 43-46. Source: library.upenn.edu, retrieved May/July 2020.
• Ref. 235P9: "Instrument Landing System for Aircraft - Part II" [Lorenz blind landing system, burried cable systems (Loth, Simon)], Henry W. Roberts, in "Aero Digest including Aviation Engineering", Vol. 29, Nr, 11, November 1936, pp. 32-36. Source: library.upenn.edu, retrieved May/July 2020.
• Ref. 235P10: "The W.I.T. "Air-Track" System of Instrument Landing", Henry W. Roberts, "Aero Digest including Aviation Engineering", Vol. 30, Nr. 4, April 1937, pp. 60, 64. Source: library.upenn.edu, retrieved May/July 2020.
• Ref. 235P11: "Lorenz B-L System" [B-L = Blind Landing], in "Aero Digest including Aviation Engineering", Vol. 30, Nr. 6, June 1937, p. 66. Source: library.upenn.edu, retrieved May/July 2020.
• Ref. 235P12: "Lorenz Blind Approach Receivers", advertising by Smith's Aircraft Instruments Ltd., in "Flight", November 1936.
• Ref. 235P13: "Lorenz Blind Approach Receivers", advertising by Smith's Aircraft Instruments Ltd., in "The Aeroplane", 13 January 1937.
• Ref. 235P14: "On the Beam: an explanation of "Standard" Beam Approach - a radio aid for landing aircraft in conditions of poor visibility" (Part 1 & 2) [SBA, Lorenz MCW system, T.U. 3 main LOC beacon, M.U. 3 marker beacons, receiver set, RAF], Frank Preston, in "Practical Wireless", Vol. 22, No. 474, December 1945, pp. 4-9 and Vol. 22, No. 475, January 1946, pp. 50-51. Source: worldradiohistory.com. Accessed 25 August 2020.
• Ref. 235P15: "Radio Landing Systems" [MIT GCA, Lorenz, Lorenz/ITT SBA, SCS-51], P.R. Darrington, in "Wireless World", April 1978, pp. 38-43, 56. Source: worldradiohistory.com, retrieved 20 August 2020.
• Ref. 235P16: "Radio method of blind landing" [Hegenberger, US Army Air Corps, localizer + marker beacon, sonic altimeter], in "Popular Aviation", Vol. 14, No. 3, March 1934, pp. 155-156, 192-193.
• Ref. 235P17: "Weston Aircraft Instruments", Blind Approach Indicator advertising by Sangamo Weston Ltd, 1945-1946.
• Ref. 235P18: "Lorenz Blind Approach Receivers", advertising by Smith's Aircraft Instruments Ltd, as distributor of Standard Radio equipment, 1937.
• Ref. 235P19: "The Problems of Blind Landing", H. C. Pritchard, in "The Aeronautical Journal", Vol. 50, Issue 432, December 1946, pp. 935-958, with "Discussion" on pp. 958-973. [Summary]
• Ref. 235P20: "Link Trainer: The infamous "blue box" - history's most prolific flight simulator", pp. 70-71 in "Aeroplane", Issue No. 567, Vol. 48, No. 7, July 2020, pp. 70-71.
• Ref. 235P21: "Fluglandetechnik" [instrument landing], Karl Durst, pp. 102-107 in "Radio-Rundschau - Technisch-wirtschaftliche Zeitschrift", Vol. 1, Nr. 6, September 1946. Source: archive.org, retrieved 15 May 2020.
• Ref. 235P22: "Radiolandingsbakens, Part I - XVI" ["Radio landing beacons", in Dutch language, file size: 21 MB], in "Radio Bulletin", Vol. 18, No. 7-12 (July 1949, pp. 221-224, 246; August 1949, pp. 267-270; September 1949, pp. 304-307, 309; October 1949, pp. 339-342, 355; November 1949, pp. 380-384, 398; December 1949, pp. 440-443, 453), Vol. 19, No. 1-8, 10, 11 (January 1950, pp. 26-29; February 1950, pp. 66-68; March 1950, pp. 105-108; April 1950, pp. 135-137; May 1950, pp. 183-187; June 1950, pp. 209-211, 229; July 1950, pp. 245-248; August 1950, pp. 279-281; October 1950, pp. 351-354, 365; November 1950, pp. 339-402). Source: nvhrbiblio.nl, retrieved 30 September 2020.
• Ref. 235P23: "Latest Type AAF Blind Landing Equipment" [ITT Federal, Indianapolis system, LOC AN/MRN-1, MC-528, GS pattern], M.E. Montgomery, in "Electronic Industries", January 1945, pp. 100-103, 190, 192, 194, 196, 198. Source: worldradiohistory.com, retrieved 4 October 2020.
• Ref. 235P24: "New UHF landing system at Laguardia", in "Electronic Industries", January 1943, pp. 62-64, 107, 108.
• Ref. 235P25: ""Happy Landings in Fog", Julian Legget, in "Popular Mechanics", Vol. 59, No. 6, June 1933, pp. 973-975.
• Ref. 235P26: "Lorenz-ultra-korte golf-landingsbaken voor vliegtuigen" ["Lorenz VHF landing beacon for airplanes", in Dutch], H. Veenstra, in "Radio-Nieuws", Vol. 18, No. 3, 15 May 1935, pp. 49-61. Source: nvhrbiblio.nl, retrieved 14 October 2020.
• Ref. 235P27: "IL at ID - The latest demonstration of instrument landing (IL) equipment, the Lorenz system at Indianapolis (ID)", Donald G. Fink, in "Aviation", July 1937, pp. 20, 21, 72, 75. Source: archive.org, retrieved 14 October 2020.
• Ref. 235P28: "La T.S.F." [Télégraphie sans Fil, i.e., "Radio" application to aviation; incl. Lorenz landing beacon & associated cockpit instrument], L. Portier, in "Potentiel Aérien Mondial", special 1936 issue for the 15^th "Salon de l'Aéronautique à Paris" of "L'Aérophile", pp. 63-69. Source: Bibliothèque nationale de France; public domain.
• Ref. 235P29: "Landing on a radio beam", Greg Grant, in "Electronics World + Wireless World", February 1991, pp. 121-124. Source: worldradiohistory.com, retrieved 19 October 2020.
• Ref. 235P30: "The CAA Instrument Landing System - Part I & II", Peter Caporale, in "Electronics", Vol. 18, February 1945, pp. 116-124, March 1945, pp. 128-135. Source: worldradiohistory.com, retrieved 25 October 2020.
• Ref. 235P31: "Landing of aircraft in fog by radio - installation at Newark Airport", H. Diamond, in "Electronics", Vol. 4, No. 6, June 1933, pp.158-161. Source: worldradiohistory.com, retrieved 25 October 2020.
• Ref. 235P32: "40-cm waves for aviation" [MIT 700 MHz localizer beam system], in "Electronics", Vol. 9, No. 11, November 1939, pp. 12-15. Source: worldradiohistory.com, retrieved 25 October 2020.
• Ref. 235P33: "Cathode-Ray Tube used as aircraft instrument indicator" [first integrated CRT-based electronic display: ILS LO/GS deviations or altimeter, artificial horizon, directional gyro, airspeed indicator], in "Electronics", Vol. 10, Nr. 3, March 1940, pp. 36-38. Source: worldradiohistory.com, retrieved 25 October 2020.
• Ref. 235P34: "All-weather flying" [CAA landing system, Omni Ranges, DME, radar, automatic landing], in "Electronics", Vol. 16, Nr. 9, September 1946, pp. 84-87. Source: worldradiohistory.com, retrieved 25 October 2020.
• Ref. 235P35: "Air navigation by wireless - The work of the radio beacons" [Mitchel Field/NY, leader cable, capacity altimeter], in "Wireless World", No. 522, Vol. XXV, No. 9, 28 August 1929, pp. 196-198. Source: nvhrbiblio.nl, retrieved 14 January 2021.
• Ref. 235P36: "Ground-Controlled Approach for aircraft" [GCA], C. W. Watson, in "Electronics", Vol. 15, Nr. 11, November 1945, pp. 112-115. Source: worldradiohistory.com, retrieved 25 October 2020.
• Ref. 235P37: "GCA for Control of Commercial Aviation" [post-WW2 civil version by Gilfillan Bros. Co. of military GCA], in "Electronics", Vol. 16, Nr. 5, pp. 160-161. Source: worldradiohistory.com, retrieved 25 October 2020.
• Ref. 235P38: "Status of VHF facilities for aviation" [VHF communication, VHF Range (Localizer), ILS glide-path, VOR], in "Electronics", Vol. 17, Nr. 10, November 1947, pp. 90-95. Source: worldradiohistory.com, retrieved 25 October 2020.
• Ref. 235P39: "Air navigation: survey of radio aids to civil navigation", M.G. Scroggie, in "Wireless World", Vol. 52, No. 11, November 1946, pp. 352-356.Source: worldradiohistory.com, retrieved 3 January 2021.
• Ref. 235P40: "Aerial navigation - radio aids to sky pilotage" [DF, B-T, goniometer, TFK Compass, neon lights, leader cable], Douglas G. Jeffrey, in "Aeronautics" [predecessor of "Popular Aviation" and "Flying Magazine"], Vol. 5, Nr. 6, December 1929, pp. 39-40, 66, 68, 70.
• Ref. 235Q: "C. Lorenz Aktiengesellschaft, Berlin-Tempelhof", pp. 172-182 in "Reichsverband der deutschen Luftfahrt-Industrie auf der Luftfahrt-Ausstellung, Stockholm, 1936" [catalog by the German aviation industry/trade association of German companies and their products exhibited at the International Aerospace Exhibition at Stockholm/Sweden, May/June 1936; ILIS 1936], 232 pp. Source: justus.ownit.nu, retrieved 31 May 2020. Catalog cover page and table of contents is here, courtesy B. Justusson.
• Ref. 235R: "The development of the Civil Aeronautics Authority instrument landing system at Indianapolis", W. E. Jackson, A. Alford, P.F. Byrne, H.B. Fischer in IEEE "Electrical Engineering", Vol. 59, Nr. 12, December 1940, pp. 849-858. Originally published as AIEE Tech. Paper 40-43, December 1939. [Abstract]
• Ref. 235S: "The history and development of the Washington Institute of Technology" [Air-Track, ILS, College Park], Joseph M. Marzolf, 18 November 1938, 23. Source: archive.org, retrieved 3 July 2020. [Summary & introduction].
• Ref. 235T: "The RAE contribution to all-weather landing" [SCS-51, approach lighting, ILS, BLEU], John Charnley, in "Journal of Aeronautical History", Vol. 1, Paper No. 2011/1, 21 pp; Accessed 18 July 2020. [pdf]
• Ref. 235U: reports of the Daniel Guggenheim Fund for the Promotion of Aeronautics
• Ref. 235U1: "The Full-Flight Laboratory", Chapter VII, pp. 29-32 in "The second report of the Daniel Guggenheim Fund for the Promotion of Aeronautics - 1928", January 1929, 48 pp. Source: hathitrust.org, retrieved 25 February 2021.
• Ref. 235U2: "Solving the problem of fog flying; a record of the activities of the fund's full flight laboratory to date", Daniel Guggenheim Fund for the Promotion of Aeronautics, 1929, 52 pp. Source: hathitrust.org, retrieved 21 February 2021.
• Ref. 235U3: "Equipment used in experiments to solve the problem of fog flying - A record of the instruments and experience of the Fund's Full flight Laboratory" [Doolittle's first instrument flight, aircraft equipment], The Daniel Guggenheim Fund for the Promotion of Aeronautics, Inc., March 1930, 57 pp. Source: hathitrust.org, retrieved 18 July 2020.
• Ref. 235V: articles/documents about the RAF military version of the Lorenz landing beam system (RAF FGRI-5069/TGRI-5041)
• Ref. 235V1: "Standard Beam Approach Ground Installations (F.G.R.I.5069 and T.G.R.I.5041)" [Fixed Ground Radio Installation 5069, Transportable Ground Radio Installation 5041, Transmitter types T.1122, T.1123, T.5041, T.1245], Chapter 4 in "Ground Station Equipment", Section 1 of "Radio Aids to Navigation", Part IV of "Royal Air Force Signal Manual", in Vol. I of "Air Publication 1168" (A.P. 1186, revised ed., June 1940, and AP 1186B, December 1942 / June 1943). Courtesy J. Harris, 2020.
• Ref. 235V2: "Automatic Keying device, Type 2" Chapter 6 in "Ground Station Equipment", in same Section 1 as above, 8 pp. Courtesy J. Harris, 2020.
• Ref. 235V3: "Blind Approach Receivers, R.1124A and R.1125A" [file size: 38 MB], Chapter 7 in "Receivers", Section 3 in "Radio Aids to Navigation", in Part IV of "Royal Air Force Signal Manual" in Vol. I of "Air Publication 1168" (AP 1186), revised ed., June 1940, 55 pp. Courtesy J. Harris, 2020.
• Ref. 235V4: "Beam Approach Aircraft Equipment", chapter 18 of "Standard Notes for Wireless Mechanics", "Air Publication 1938" (A.P. 1938), Air Ministry, reprint of May 1944, 10 pp. Source: blunham.com, retrieved September 2020.
• Ref. 235W: articles/documents about the USAAF military version of the Lorenz landing beam system (USAAF SCS-51)
• Ref. 235W1: "Army Air Forces' Portable Instrument Landing System", Sidney Pickles, in in "Electrical Communication - A Journal of Progress in the Telephone, Telegraph and Radio Art" [broadband "Alford" loop omni-antennas], published by "International Standard Electric Corp", Vol. 22, No. 4, 1945, pp. 262-294. Source: worldradiohistory.com, accessed 17 August 2020.
• Ref. 235W2: "Instrument Flying: Army Air Forces Instrument Approach System", U.S. Army Air Forces, Technical Order No. 30-100F-1, 10 November 1943, 16 pp. Source: aafcollection.info, accessed 1 September 2020.
• Ref. 235W3: "First-Hand: Development of the Instrument Landing System Glide Path" [SCS-51, CAA, Signal Corps, ITT Federal Laboratories, ITT Standard Telephone and Cable, Standard Elektrik Lorenz, 330 MHz, hyperbolic path, straight path, constant rate of descent], Leon Himmel, source: etwh.org, retrieved 25 June 2020.
• Ref. 235W4: "Air service: radio range and SCS 51 equipment", Exchange of notes between the US Ambassador to the UK and the UK Minister of Civil Aviation, London May 8 and July 31, 1946, regarding transfer of certain WW2 air navigational and operational facilities in the UK, to the UK government. pp. 795-799 in "Treaties and Other International Agreements of the USA, 1766-1949", Vol. 12 "United Kingdom - Zanzibar". Source: US Library of Congress, retrieved 20 October 2020.
• Ref. 235W5: "Synthesis of multifunctional tactical landing system" [incl. SCS-51], G.B. Litchford, Air Force Flight Dynamics Laboratory, Air Force Systems Command, Wright-Patterson Air Force Base, Technical Report AFFDL-TR-67-188, January 1968, 201 pp. Source: dtic-mil, retrieved 29 November 2020.
• Ref. 235X: articles in "Electrical Communication - A Journal of Progress in the Telephone, Telegraph and Radio Art", published by "International Standard Electric Corp". Source: worldradiohistory.com, accessed 17 August 2020.
• Ref. 235X1: "Ultra-Short Wave Radio Landing Beam - The C. Lorenz A.G. Radio Beacon Guide Beam System", R. Elsner, E. Kramar, in Vol. 15, No. 3, January 1937, pp. 195-206.
• Ref. 235X2: "Aviation Radio" section (pp. 218-220) in "Electrical Communication in 1938", in Vol. 17, No. 3, January 1939, pp. 205-229. [Busignies' HF-DF model R.C.5; LMT transportable Adcock; NDB network in Europe, most major airports in Europe now equipped with Lorenz-system VHF ILS; similar development in USA; Australia using similar AN beacons for point-to-point navigation; Lorenz ILS type marker beacons now used along routes in US; 1936/37 experiments with by LMT resulted in improved Lorenz-localizer track and demonstrated in Feb 1938 in France and selected by AF; Western Electric Co. radio altimeter (FM, UHF)]
• Ref. 235X3: "Ultra-High Frequency Loop Antennae", A. Alford, A.G. Kandoian, in Vol. 18, No. 4, April 1940, pp. 255-265.
• Ref. 235X4: "Development of the C.A.A. Instrument Landing System at Indianapolis", Vol. 18, No. 4, April 1940, pp. 285-302.
• Ref. 235X5: "Aviation" [ITT receives order from CAA for "Indianapolis System" ILS at 6 major US cities, to be come operational mid-1941], p. 5 in "Electrical Communication in 1940", Vol. 19, No. 3, 1941, pp. 3-10.
• Ref. 235X6: "Instrument Landing System" and "Ultra High Frequency Two Course Radio Range with Sector Identification", p. 80  in "Western Hemisphere I. T. & T. System Communication: Contributions of 1942", in Vol. 21, No. 2, 1943, pp. 75-84.
• Ref. 235X7: "1938 - First installation of instrument landing equipment at three major London Airports", p. 217 in "Standard Telephones and Cables, Limited, London - 60^th Anniversary", C.W. Eve, in Vol. 21, Nr. 4, 1944, pp. 213-217.
• Ref. 235X8: "Development of Aircraft Instrument Landing Systems", H. H. Buttner, A. G. Kandoian, in Vol. 22, No. 3, 1945, pp. 179-192
• Ref. 235X9: "1937 - Awarded contract for supply of radio instrument landing equipment for the Defence Department, Australian Commonwealth", p. 324 in "Standard Telephones & Cables Pty. Ltd., Australia - 50th Anniversary", J. Clarke, in Vol. 22, No. 4, 1945, pp. 322-325.
• Ref. 235X10: "Standard Beam Approach" p. 6, 11 in "Electrical Communication: 1940-1945, War Years Review - Part I", in Vol. 23, No. 1, 1946, pp. 3-13.
• Ref. 235X11: "Landing aircraft with ground radar" [AN/MPN-1C, Luis Alvarez, Ground Controlled Approach (GCA), Precision Approach Radar (PAR)], J.S. Engel, Vol. 24, No. 1, March 1947, pp. 72-81.
• Ref. 235Y: articles and documents from the US Department of Commerce (incl. Civil Aeronautics Authority / Administration), US National Academy of Sciences, and National Advisory Commission on Aeronautics (NACA)
• Ref. 235Y1: "Diamond-Dunmore", pp. 16-20 in "Federal Science Progress", U.S. Dept. of Commerce, Vol. 1, No. 3, April 1947.
• Ref. 235Y2: "Description and theory of Instrument Landing System", Federal Airways Manual of Operation IV-B-1-4, U.S. Dept. of Commerce, Civil Aeronautics Administration, 1st ed., 15 October 1949, 43 pp.
• Ref. 235Y3: "The CAA Radio Instrument Landing System with Simultaneous Voice", U.S. Department of Commerce, Civil Aeronautics Administration, Office of Federal Airways, Technical Development Services, 1946, 31 pp.
• Ref. 235Y4: "A radio system for blind landing of aircraft in fog", H. Diamond, F.W. Dunmore, in "Proceedings of the National Academy of Sciences [PNAS] of the Unite States of America", Vol. 16, Nr. 11, 15 November 1930, pp. 678-685. [pdf]
• Ref. 235Y5: "History of Instrument Landing System glide paths", Chapter 1 (pp. 1-20) in "An Airport Glide-Path System Using Flush-Mount, Travelling-Wave Runway Antennas, Vol. II", Richard H. McFarland, Ohio State University Research Foundation, Report 891-2, for the FAA Aviation R&D Service (frmr. Bureau of R&D), 31 March 1961, 177 pp. Source: dtic.mil, retrieved 25 October 2020.
• Ref. 235Y6: "The development of a straight-line glide path", J.M. Lee, H.I. Metz, Civil Aeronautics Administration, U.S. Department of Commerce, Technical Development Report No. 55, June 1947, 34 pp. Source: hathitrust.org, retrieved 16 January 2021.
• Ref. 235Y7: "The CAA-RTCA instrument landing system - Part I: Development and installation" [low quality scan], Henry I. Metz, Civil Aeronautics Administration, U.S. Department of Commerce, Technical Development Report No. 35 (TDR 35), October 1943, 83 pp. Source: planeandtrainwrecks.com, retrieved 16 January 2021.
• Ref. 235Y8: "The CAA-RTCA instrument landing system - Part II: Tests and modifications" [low quality scan],, Henry I. Metz, Civil Aeronautics Administration, U.S. Department of Commerce, Technical Development Report No. 36 (TDR 36), October 1943, 29 pp. Source: planeandtrainwrecks.com, retrieved 16 January 2021.
• Ref. 235Y9: "Development of a VHF directional localizer - Part I (preliminary tests) & II (the monitor)", Chester B. Watts, Samuel E. Taggart, Kennard E. Voyles, CAA Technical Development & Evaluation Center, Technical Development Report No. 183 (CAA TDR 183), May 1954, 43 pp. Source: hathitrust.org, retrieved 25 October 2020.
• Ref. 235Y10: "Methods for facilitating the blind landing of airplanes", M. Heinrich Gloeckner, National Advisory Commission on Aeronautics (NACA), Technical Memorandum No. 687 (NACA-TM-687), 1 October 1932, 28 pp. Source: ntrs.nasa.gov, retrieved 19 February 2021. Translated from "Verfahren zur Erleichterung von Blindlandungen", pp. 347-355 in "Zeitschrift für Flugtechnik und Motorluftschiffahrt" (ZFM), Vol. 23, No. 12, 24 June 1932.
• Ref. 235Z: pp. 114-115 in "Sangamo in Peace and War", Part 2 of "Sangamo, a history of fifty years", Robert Carr Lanphier, Benjamin Platt Thomas, 1949, 145 pp. Source: hathitrust.org, accessed 10 September 2020.
• Ref. 241: "Erprobungsstellen der Luftwaffe" [Luftwaffe test sites, incl. Rechlin, Süd/Foggia, Tarnewitz, Travemünde, Udetfeld, Werneuchen]. Bestandsbeschreibung [file description] of Bundesarchiv file nr. (Signatur) RL36. Retrieved 28 August 2019.
• Ref. 244: Luftwaffe fighter intercept & control methods ("Jagdverfahren"), maps, and related topics
• Ref. 244A: "Nachtjagd" [Night fighting; intro, descriptions, and evaluations], Luftwaffe document, date unknown, 18 pp. Source: German Russian Project for digitization of archives in the Russian Federation. Retrieved 29 August 2019.
• Ref. 244B: "Bestimmungen über Nachtjagd" [Descriptions of methods, and instructions to Flak organisation regarding night fighting], 1^st Flakdivision, Berlin, 19 November 1943, 12 pp. Source: German Russian Project for digitization of archives in the Russian Federation. Retrieved 29 August 2019.
• Ref. 244C: "Mosquito Nachtjagd" [Specific fighter and flak tactics against incoming British "Mosquito" fighter-bombers], 1^st Flakdivision, Berlin, 7 March 1944, 5 pp. Source: German Russian Project for digitization of archives in the Russian Federation. Retrieved 29 August 2019.
• Ref. 244D: "Das Y-Verfahren für Tag- und Nachtjagd" [The Y-system for day and night fighting], document without reference number, without date, without place, without author, 113 pp. [file size: 115 MB; good-but-lower-reslution file is here 36 MB]
• Complete description of the "Y" procedure for day & night fighter control, from ground plotting stations with short wave equipment.
• Source: BArch file nr. (Signatur) RL 2-V/38, used in accordance with "Erstinformation für Ihren Besuch im Bundesarchiv in Freiburg, Stand Juni 2016".
• Ref. 244E: "Anlage 1 zu Luftflottenführungsabteilung Ia op2 Nr. 500/41 geh." [file size: 28 MB; good-but-lower-resolution file is here, 4 MB]. Map is not dated.
• Map covers area of Belgium, The Netherlands, Denmark, northern Germany incl. Berlin. Map is marked with locations of militärische Sperrgebiete, Nachtsperrgebiete, Nachtjagdgebiete, Dunkle Nachtjagdsräume.
• Map size: 4x2 (WxH) A4-sheets. Map is low-quality blueprint copy.
• Source: BArch file nr. (Signatur) RL 20/186/K, used in accordance with "Erstinformation für Ihren Besuch im Bundesarchiv in Freiburg, Stand Juni 2016".
• Ref. 244F: "Einsatzbeispiel für einen Feindeinflug" [example of response to intruding enemy aircraft]. Map is not dated. [file size: 84 MB; good-but-lower-resolution file is here, 10 MB].
• Map shows ground track of a night-time enemy bomber stream arriving from Britain with target Frankfurt, return corridor, intercepting fighters, timing, etc. Map includes large & detailed table of the entire nightfighter intercept process, from long-range radar detection to "kill", with step-by-step status/activity/communication at the level of FlugmeldekopF, Nachtjagdraum, Zentralgefechtsstand, Fühlungshalter, Jagdgruppe, Flugmeldungszentrale, Fluko, and Flak; the entire sequence covers 1 hr 40 min.
• Map size: 4x5 (WxH) A4-sheets.
• Source: BArch file nr. (Signatur) RL 36/443, used in accordance with "Erstinformation für Ihren Besuch im Bundesarchiv in Freiburg, Stand Juni 2016".
• Ref. 244G: "Luft-Navigationskarte in Merkatorprojektion - Erweitertest Blatt Deutschland mit Jägernetz", Bodenorganisation Großraum-Nachtjagd Luftflotte Reich, July 1944. Source: Bestand/File 500, Findbuch/Index 12452, Akte/record 286 of the German-Russian Project for the Digitization of German Documents in Archives of the Russian Federation. Retrieved 28 August 2019.
• Ref. 244H: "Nederland en de Duitse Nachtjacht - Van jager tot prooi" (The Netherlands and German night fighting - from hunter to prey), W.H. Lutgert, R. de Winter, pp. 536- 545 in "Militaire Spectator", vol. 183, nr. 12, December 1994. Accessed September 2019. [pdf]
• Ref. 244J: "Nederland en de Duitse Nachtjacht - Van jager tot prooi (Deel 2)" (The Netherlands and German night fighting - from hunter to prey - part 2), W.H. Lutgert, R. de Winter, pp. 5-17 in "Militaire Spectator", vol. 184, nr. 1, January 1995. Accessed September 2019. [pdf]
• Ref. 244K: "De Luftwaffe en Nederland - Balans van een oorlogserfenis" (The Luftwaffe and The Netherlands - legacy of a war), W.H. Lutgert, R. de Winter, pp. 450- 459 in "Militaire Spectator", vol. 184, nr. 10, October 1995. Accessed September 2019. [pdf]
• Ref. 244L: "Nachtjagdnavigationskarte - herausgegeben von NJG.3.-NO" [night-fighter navigation chart issued by the Nachrichten-Offizier of the no. 3 night fighter-wing, Nachtjagdgeschwader 3]. [file size: 25 MB]
• Map is not dated. Bernhard-stations Be-0, Be-6, Be-8 through Be-12 are also marked on this map. Source: collection R. Grywatz.
• Ref. 244M: "Tag- und Nachtjagd, 3. Jagddivision (als beispiel)". Map is dated 29 July 1944. [file size: 24 MB; good-but-lower-resolution file is here, 9 MB].
• Map covers area of The Netherlands to Heilbronn/Germany. Map is marked with location of Tagjagdstellungen and Fu.M.G. sites (1., 2., and 3. Ordnung; dunajafähig vs. nicht dunajafähig).
• Map size ca 3x2 (WxH) sheets of size A4. Map scale 1:1.000.000
• Source: BArch file nr. (Signatur) RL-3-1527, used in accordance with "Erstinformation für Ihren Besuch im Bundesarchiv in Freiburg, Stand Juni 2016".
• Ref. 244N: "Jagd-Einsatz im November 1943", Anlage 3 (appendix 3) to "Lfl.Kdo3, Führ.Abt(1)/1c No. 15684/43 Kdos". [file size: 19 MB]
• Large bar graph for each day of November 1943, with total number of fighter sorties for each day, split into "day" and "night" fighter. Also: table with statistics for each day: number (with type(s) of aircraft) for each type of sortie/mission: "Alarmstart" / "Überwachung und Sperre" / "Geleitschutz" / "Begleitschutz u. Aufnahme"; / "Nachtjagd" (night fighter) / "Fernjagd" (long range intercept); also: total number of sorties/missions, number of enemy kills (incl. type of enemy aircraft), own losses.
• Size: size: ca. 3x3½ (WxH) A4-sheets.
• Source: BArch file nr. (Signatur) RL-3-1527, used in accordance with "Erstinformation für Ihren Besuch im Bundesarchiv in Freiburg, Stand Juni 2016".
• Ref. 244P: "Abschüsse der Nachtjagd im Bereich Luftwaffenbefehlshaber Mitte", Anlage 2 (appendix 2).
• Graph with three lines: "hell" (helle Nachtjagd), "dunkel" (dunkle Nachtjagd), "kombiniert" (kombinierte Nachtjagd). Graph covers monthly statistics regarding enemy kills for the period April 1941 - March 1942.
• Source: BArch file nr. (Signatur) RL-3-1527, used in accordance with "Erstinformation für Ihren Besuch im Bundesarchiv in Freiburg, Stand Juni 2016".
• Ref. 244Q: "Anzahl der voll u. bedingt (ab Juliu 1941) bzw. eingeschränkt (ab Mai 1943) einsatzfähigern Besatzungen" and "Anzahl der einsatzbereiten Kampf-Flugzeuge" .  [file size: 17 MB]
• Graph with monthly statistics for the period mid-1939 - mid-1939: number of available flight crews and number of operational fighter planes; separate curves for "Tagjäger" (day fighters) and "Nachtjäger" (night fighters; from late 1940 onward).
• Also: interesting large table with 53 key dates of the WW2, from 1 September 1939 (German invasion of Poland) through 15 August 1944 (anglo-american landing in southern France).
• Size: ca. 1½ x3 (WxH) A4-sheets.
• Source: BArch file nr. (Signatur) RL-3-1527, used in accordance with "Erstinformation für Ihren Besuch im Bundesarchiv in Freiburg, Stand Juni 2016".
• Ref. 244R: "Fighter defence of Germany - Control of fighters by the "Y" Procedure", Samuel Denys Felkin (Chief Interrogator at Bletchley), transcribed report from the British Air Ministry, Assistant Director of Intelligence (Prisoner Interrogation), A.D.I. (K) Report No. 525/1944, 14 pp. Source: The National Archives of the UK, ref. AIR40/2875 and 2876. Retrieved from www.cdvandt.org.
• Ref. 247: "Air mail beacon Farmerville, Pa., Lighted by Kohler Electric Plant", cover panel of "Light for the Night Air Mail", 3 page folded sales brochure of "Kohler of Kohler - Automatic Electric Plants" of the Kohler Co. in Kohler, Wisconsin/USA. Date unknown. Source: atchistory.org. Accessed 21 July 2020.
• Ref. 254: "Richtfunkfeuer und Drehfunkfeuer" [directional and rotating radio beacons], E. Kramar, Berlin, 18 pp., in "Ringbuch der Luftfahrttechnik", Vol. 15, VC3 (V. Ausrüstung, C. Funkpeilung, 3), Berlin-Adlershof, Zentrale f. wiss. Berichtswesen b. d. Deutschen Versuchsanst. f. Luftfahrt, 1938. [file size: 28 MB]
• Ref. 259: pp. 75-84 in "Zwischen Möhrenfeld und Panzerkute - verblassende Erinnerungen im märkischen Sand. Die märkischen Dörfer Groß- und Kleinbeuthen und ihre Umgebung in schwerer Zeit um 1945", Bernd Saalfeld, self published book (available via Heimatverein Beuthen e.V.), 2018, 156 pp.
• Ref. 261: German and Allied radar systems used in WW2 European war theatre
• Ref. 261A: radar-related articles from "IEEE Aerospace and Electronic Systems Magazine"
• Ref. 261A1: "IEEE HISTORIC MILESTONE - Christian Hülsmeyer: Invention and First Demonstration of Radar, 1904", Hugh Griffiths, Peter Knott, Wolfgang Koch, in "IEEE AES Magazine", Vol. 34, Issue 9 , September 2019, pp. 56-60. A preliminary version of this article is here, 9 pp. Retrieved 22 January 2020.  [pdf]
• Ref. 261A2: "The long prelude (1873-1922): Phase I of the invention of radar", J.B. McKinney, in "IEEE AES Magazine", Vol. 21, Iss. 8, September 2006, pp. 17-25.
• Ref. 261A3: "The rise of radio (1922-1930): Phase II of the invention of radar", J.B. McKinney,in "IEEE AES Magazine", Vol. 21, Iss. 8, September 2006, pp. 27-39.
• Ref. 261A4: "The arrival of radar (1930-1935): Phase III of the invention of radar", J.B. McKinney,in "IEEE AES Magazine", Vol. 21, Iss. 8, September 2006, pp. 41-54.
• Ref. 261A5: "The race with destiny (1935-1939): Phase IV of the invention of radar", J.B. McKinney, in "IEEE AES Magazine", Vol. 21, Iss. 8, September 2006, pp. 55-73.
• Ref. 261A6: "Radar becomes operational (1939-1941): Phase V of the invention of radar", in "IEEE AES Magazine", Vol. 21, Iss. 8, September 2006, pp. 75-78.
• Ref. 261B: "Hülsmeyer and the early days of radar inventions, sense and nonsense, a survey" and "Part II", Arthur O. Bauer, 2004/2005, 74 pp. (Part I) and 18 pp. (Part II). Source: www.cdvandt.org. [pdf Part I] [pdf Part II]
• Ref. 261C: "Some Aspects of German Airborne Radar Technology, 1942 to 1945", Arthur O. Bauer, 2 December 2006, 32 pp. Source: www.cdvandt.org, retrieved May 2019.
• Ref. 261D: "CH - The First operational Radar" [Chain-Home], pp. 73-83 in "GEC Journal of Research - Incorporating the Marconi Review", Vol. 3, No. 2, 1985. Retrieved 27 January 2020. [pdf]
• Ref. 261E: "Bomber's Radar - General Survey of the Three Primary Systems Used by Bomber Command", C.B. Baily-Watson, pp. 252-254 in "Flight", 6 September 1945.
• Ref. 261F: "Einsatz "Berlin"-Gerät" [Operation of "Berlin"-device = German version of the British "H2S" ground-mapping radar].
• Letter from Gen.Nafü (1.Abt.), addressed to gen.Nafü/Ln.Insp. (6.Abt.). Letter ref. Nr. 12493/44 geh. (1.Abt.II). Letter is dated 17 Juli 1944. Letter states that after referral with Generals Martini & Peltz, the IX.Fl.Korps will, until further notice, not use the "Berlin"-Gerät", as 1) current test/evaluation status is not yet sufficient for operational service, and 2) this navigation aid is not needed in the "Landekopfraum" [Allied landing/invasion area = Normandy]. The "Berlin"-Gerät" is not allowed to be used without permission from Lw.Fü.Stab.
• Source: BArch file nr. (Signatur) RL-2-V/5, used in accordance with "Erstinformation für Ihren Besuch im Bundesarchiv in Freiburg, Stand Juni 2016".
• Ref. 261G: Untitled letter/telegram about "Lichtenstein SN 2" and "Lichtenstein B/C"
• Letter/telegram from OKL Fü.Stab/Gen.Nafü.(Robinson), addressed to Luftfl.Kdo.1,4, 6. and Lw.Kdo. Südost. Letter ref. Nr. 10910/44. Letter is from 1944 (exact date not marked). Letter states that loss [to the enemy] of night fighter radar search equipment "Lichtenstein SN 2" must absolutely be avoided, to make it harder for the enemy to develop interference means. Therefore, night fighters missions with "Lichtenstein SN 2" over enemy territory and bandit territory [ = occupied territories with active resistance groups] is forbidden. Night fighters with missions over bandit territory in the East will only be equipped with "Lichtenstein B/C" until further notice.
• Source: BArch file nr. (Signatur) RL-2-V/5, used in accordance with "Erstinformation für Ihren Besuch im Bundesarchiv in Freiburg, Stand Juni 2016".
• Ref. 261H: "Deflating British radar myths of World War II", Maj. G.C. Clark, Air Command and Staff College Research Dept., AU/ACSC/0609F/97-3.
• Ref. 261J: "Radar Origins Worldwide: History of Its Evolution in 13 Nations Through World War II", R.C. Watson, Trafford Publishing, 2009, 420 pp.
• Ref. 261K: "Geschichte der Funkortung - Funktionsmodell des Funkmessgerätes "Würzburg" FuMG 62 (D)" [History of radar, funtional model of the "Würzburg" radar], Hans-Peter Opitz, pp. 12-16 in "Funkgeschichte - Mitteilungen der Gesellschaft der Freunde der Geschichte des Funkwesens (GFGF)", Vol. 34, Nr. 195, February/March 2011. See note 1
• Ref. 261L: p. 9 in "The Century of Radar - from Christian Hülsemeyer to Shuttle Radar Topography Mission", Wolfgang Holpp, based on his presentation "The Century of Radar" at the German Radar Symposium, Bonn/Germany, 2002 with 2004 update, 27 pp. [pdf]
• German version: "Das Jahrhundert des Radars - von Christian Hülsemeyer zur Shuttle Radar Topography Mission"  [pdf].
• Ref. 261M: "Deckname „Würzburg“ - Ein Beitrag zur Erhellung der Geschichte des geheimnisumwitterten deutschen Radargeräts 1937-1945" ["The technical history of the Würzburg radar system"], Arthur O. Bauer, Verlag Historischer Technikliteratur (publ.), 103 pp. Source: www.cdvandt.org. Accessed 28 March 2020. [pdf]
• Ref. 261N: "The History of Radar Technology in Germany - Reference to its Application to Radio Location" [Würzburg-A/C/Riese, Freya, Philips, GEMA, Lorenz 39L/40L, Elefant, Mammut, Seeburg, Lichtenstein SN2] , H. Diehl. Special Felkin report.
• Ref. 261P: "The State of German Centimetre Wave Technology at the End of the Second War", L. Brandt, special Felkin report?
• Ref. 261Q: "Eureka-H Radar Beacons in World War II" [VHF DME], F.R. Hunt, 12 pp., Chapter 14 of "Canadians on Radar - Royal Canadian Air Force 1940-1945", George K. Grande, Sheila M. Linden, Horace R. Macaulay, 2003; retrieved 2 May 2020.
• Ref. 261R: "Radar development in Canada" [WW2], Frederick H. Sanders, in "Proceedings of the Institute of Radio Engineers (I.R.E.)", Vol. 35, Nr. 2, February 1947, pp. 195-200. Source: worldradiohistory.com, retrieved 28 June 2020.
• Ref. 261S: "A Historical Survey of the Application of the Doppler Principle for Radio Navigation", Ernst Kramar, in "IEEE Trans. on Aerospace and Electronic Systems", Vol. AES-8, Iss. 3, May 1972, pp. 258-263.
• Ref. 261T: "Radar Handbook", Merril I. Skolnik (ed. in chief), McGraw-Hill, 2^nd ed., 1990, 846 pp. Source: University of Zürich, Dept. of Geography, accessed 14 July 2020 [file size: 41 MB]. [pdf]
• Ref. 261U: "History of Doppler Radar Navigation", Walter R. Fried, in "Navigation - Journal of the Institute of Navigation", Vol. 40, Issue 2, Summer 1993, pp. 121-136. [Abstract]
• Ref. 261V: "The radar war, 1930-1945", Gerhard Hepcke (mediocre translation into English by H. Liebmann), 53 pp. Source: radarworld.org, accessed 5 September 2020. [brief overview/timeline keywords description of: pip-squeak, ILS, hyperbolic, Ingolstadt, Elektra, Sonne, Seetakt ship radar, Freya ground radar, Stichling IFF for Freya, Würzburg ground radar, Erstling IFF for Würzburg, Chain Home (CH) coastal ground radar, Type 79Y ship radar, Anti Surface Vessels (AVS) airborne radar, Air Intercept (AI) radar, Breslau I & II jamming stations, R3000 IFF, Masking Beacons (Meacon), Knickebein, X Procedure, Y Procedure, Postkutsche Procedure, Caruso jamming station, FuMB 1 Gee, Rebecca-Eureka, Heinrich, Metox R600 warning receiver, Olga warning receiver, Lichtenstein night fighter airborne radar, Mannheim FLAK radar, Moonshine, Wasserman groundradar, Karl jamming station, Wespe IFF, Leigh-light procedure, Oboe I Bumerang procedure, Window Chaff Tinsel, Würzlaus, K-Laus, H2S Rotterdam Gerät, Naxos & Naxos-W & Naxburg & Korfu airborne warnign receivers, Roderick jamming transmitter, Tuba ground jamming transmitter, SS Loran, Shoran, Hohentwiel U submarine radar, Oboe navigation procedure, Mandrel, Taxanble, Glimmer, Titanik, GH Discus, Feuerstein, Rotterheim, Berlin A & N1 & N2 airborne radar, Roland & Feuerball, Jagdschloß & Forsthaus ground panorama radar, Pauke gun laying radar] [pdf]
• Ref. 261W: radar articles from Wireless World magazine, source: worldradiohistory.com, retrieved Sept-2020 - Jan-2021
• Ref. 261W1: "Radiolocation - Part 1", R.L. Smith-Rose, in "Wirelesss World", Vol. LI, Nr. 2, February 1945, pp. 34-37, source: worldradiohistory.com, accessed 17 September 2020.
• Ref. 261W2: "Radiolocation - Part 2: History of its development", R.L. Smith-Rose, in "Wirelesss World", Vol. LI, Nr. 3, March 1945, pp. 66-70. Part 1 & 2 were reprinted as "Radar" [RDF, radiolocation, radar], R.L. Smith-Rose, in "Electrical Communication - A Journal of Progress in the Telephone, Telegraph and Radio Art", published by "International Standard Electric Corp.", Vol. 22, No. 3, 1945, pp. 171-178.
• Ref. 261W3: "Achievements of Radar - Its part in the war", in "Wireless World", Vol. 51, No. 9, September 1945, pp. 269-270.
• Ref. 261W4: "Fundamentals of Radar 1. - Ground stations: The development of Pulse Technique" [Chain Home (CH, CHL)], in "Wireless World", Vol. 51, No. 10, October 1945, pp. 299-303.
• Ref. 261W5: "Fundamentals of Radar 2. - Night fighter equipment : relation between power, beam width and range", in "Wireless World", Vol. 51, No. 11, November 1945, pp. 326-329.
• Ref. 261W6: "Fundamentals of Radar 3. - Radar as a weapon of offense : ASV and H2S", in "Wireless World", Vol. 51, No. 12, December 1945, pp. 363-365.
• Ref. 261W7: "Fundamentals of Radar 4. - Pulse methods applied to navigation" [Oboe, Gee], in "Wireless World", Vol. 52, No. 1, January 1946, pp. 23-26.
• Ref. 261W8: "Fundamentals of Radar 5. - Beacons employing pulse technique" [ASV, IFF, H2S, Eureka, BABS], in "Wireless World", Vol. 52, No. 2, February 1946, pp. 55-56.
• Ref. 261X: "A Brief History of the Development of Radar in Great Britain up to 1945", Richard M. Trim, in "Measurement and Control", Vol. 35, December 2002, pp. 299-301. Source: journals.sagepub.com, accessed 18 September 2020.
• Ref. 261Y: "Technical History of the Beginnings of Radar" [basic principles of radar and historical aspects of their evolution; limited to the pre-cavity magnetron era of radar], Sean S. Swords, IET History of Technology Series 6. Source: b-ok.cc, retrieved 22 October 2020, file size 22 MB. [pdf - See note 1]
• Ref. 261Z: "Radar for Blind Bombing - Part I & II" [H2X airborne radar], J.V. Holdam, S. McGrath, A.D. Cole, in "Electronics", Vol. 16, Nr. 5, May 1946, pp. 138-143, Vol. 16, Nr. 6, June 1946, pp. 142-149. Source: worldradiohistory.com, retrieved 25 October 2020.
• Ref. 261AA: "Directory of German Radar Equipment" [airborne and ground-based], War Department Technical Manual TM E11-219, Apri l1945, 74 pp.
• Ref. 261AB: "Die deutschen Funkmeßverfahren bis 1945" ["German radar through 1945"], Fritz Trenkle, Motorbuch Verlag Stuttgart, 1979, 209 pp.
• Ref. 261AC: articles about Oboe
• Ref. 261AC1: "Bumerangstörung im Ruhrgebiet" [Jamming of "Bumerang" (German codename for Oboe-guided British Mosquitoes) in the Ruhr area), Nr. 82 514/44 g.Kdos. (3.Abt.III). Source: BArch file nr. (Signatur) RL 2-V/6, used in accordance with "Erstinformation für Ihren Besuch im Bundesarchiv in Freiburg, Stand Juni 2016".
• Ref. 262: articles about hyperbolic radio navigation systems
• Ref. 262A: "GEE and LORAN - Radar Navigational Systems World War II", W.P. Campbell, 16 pp., Chapter 16 of "Canadians on Radar - Royal Canadian Air Force 1940-1945", George K. Grande, Sheila M. Linden, Horace R. Macaulay, 2003; retrieved 2 May 2020.
• Ref. 262B: "An Introduction to Loran", John Alvin Pierce, in "Proc. of the Institute of Radio Engineers (IRE) and Waves and Electronics", Vol. 34, Nr. 5, May 1946, pp. 216-234 . Source: worldradiohistory.com, retrieved 27 June 2020.
• Also reprinted as "An Introduction to Loran", John Alvin Pierce, in "IEEE Aerospace and Electronic Systems Magazine", Vol. 5, No. 10, October 1990, pp. 16-33. [pdf]
• Ref. 262C: "Robert J. Dippy: The Hyperbolic Radio Navigation System", Robert I. Colin, in "IEEE Trans. on Aerospace and Electronic Systems", July 1966, pp. 476-481.
• Ref. 262D: "GEE and LORAN - Radar Navigational Systems World War II", W.P. Campbell, 16 pp. [pdf]
• Ref. 262E: "1969 IEEE Pioneer Award - William Joseph O'Brien & Harvey Fischer Schwarz" [Award for invention, development, implementation of DECCA], in "IEEE Trans. on Aerospace & Electronic Systems", November 1969, pp. 1013-1020. [pdf]
• Ref. 262F: "LORAN Long Range Navigation", J.A. Pierce, A.A. McKenzie, R.H. Woodward, Vol. 4 of Massachusetts Institute of Technology (MIT) Radiation Laboratory Series, McGraw-Hill Book Co. Inc. (publ.), 1948, 490 pp. [file size: 25 MB].
• Ref. 262G: "LORAN (Long Range Aid to Navigation)", U.S. Dept. of Commerce, Civil Aviation Authorities (CAA), Airways Operations Training Series, Bulletin No. 7, June 1949, 16 pp., source: hathitrust.org, retrieved 6 June 2020.
• Ref. 262H: "The Loran system - Part I, Part II (Loran Receiver-Indicator), Part III (Loran transmitting-station)", in "Electronics", Vol. 15, Nr. 11, November 1945, pp. 94-99, Vol. 15, Nr. 12, December 1945, pp. 110-115, Vol. 16, Nr. 3, March 1946, pp. 109-115.
• Ref. 262J: "Decca Navigator - Continuous-wave navigation system", in "Wireless World", Vol. 52, No. 3, March 1946, pp. 93-95. Source: worldradiohistory.com, accessed 17 September 2020.
• Ref. 262K: "Decca Navigator stations - details of the transmitter chain", M.G. Scroggie, in "Wireless World", Vol. 52, No. 8, August 1946, pp. 260-262. Source: worldradiohistory.com, accessed 17 September 2020.
• Ref. 263: articles about the C. Lorenz A.G. company
• Ref. 263A: "Das Auge am Marktgeschehen – Die Lorenz-Röhre und der Sieg nach 1945" ["Watching the market - the Lorenz valve and victory after 1945"], Jakob Tschandl, pp. 45-53 in "Fliegen und Funktechnik - Die Flugzeugfabrik der Luftwaffe Berline-Tempelhof 1933-1945" ["Aviation and radio - The Luftwaffe aircraft factory at Berlin-Tempelhof 1933-1945"], Marcus Popplow (ed.), Beate Winzer (ed.), Universitätsverlag der TU Berlin (publ.), 60 pp. Creative Commons License CC BY 4.0. Retrieved 22 May 2020.
• Ref. 263B: "Telephone and telegraph industry: Manufacture of equipment", pp. 42-50 in "American Industry in Europe", Frank Allan Southard, Vol. 6 of "The Evolution of International Business 1800-1945", Houghton Mifflin (publ.) 1931, 264 pp. Source: hathitrust.org. Accessed 22 May 2020.
• Ref. 263C: "ITT: The Management of Opportunity", Robert Sobel, Beard Books (publ.), 2000, 421 pp.
• Ref. 263C1: pp. 72-73 [Lorenz]
• Ref. 263C2: pp. 102-111, 158-161 [ITT, Federal Telephone & Radio Corp., Federal Labs]
• Also see ref. 164C.
• Ref. 265: articles about IFF, interrogator-transponder/responder, Distance Measuring Equipment (DME)
• Ref. 265A: "The Eureka-Rebecca compromises: another look at Special Operations security during World War II", Chris Burton, in "Air Power History", Vol. 52, No. 4, Winter 2005, pp. 24-37. Source: afhistory.org, retrieved 19 June 2020.
• Ref. 266: articles about radio course guidance via radiating & magnetic leader cables (C. Stevenson (1893), Owens (1901-1903), F.A. Kolster (1918/19), W.A. Loth, Blancard, E.J. Simon, A.H. Marriott, et al.)
• Ref. 266A: "Le pilotage par câble électrique, système Loth, des navires et des aéronefs" ["Pilotage of ships and aircraft by means of the "Loth System" electric cable"], P. Letheule, in "Le Génie Civil : revue générale des industries françaises et étrangères", Vol. LXXIX, no. 24, no. 2052, 10 December 1921, pp. 505-510. Source: Bibliothèque nationale de France; public domain.
• Ref. 266B: "The Loth guide cable - an interesting French aid to air navigation", in "Flight International Magazine", Vol. XIV, No. 11, 16 March 1922, pp. 163-164. Source: archive.org, retrieved 21 July 2020.
• Ref. 266C: "The Loth guide cable for flying in fog. French invention for guiding aircraft through fog described. System functions in preliminary trial." in "Aviation" (predecessor of "Aviation Week"), Vol. 12, No. 15, 10 April 1922, pp. 422-423. Source: hathitrust.org, accessed 21 July 2020.
• Ref. 266D: NACA/NASA Reports & Memos. Source: NASA Technical Reports Server (NTRS).
• Ref. 266D1: "On the problem of guiding aircraft in a fog or by night when there is no visibility", William Loth, National Advisory Committee for Aeronautics (NACA), Technical Memo 57 (NACA-TM-57), January 1932, 5 pp. (translated from "Comptes redues des séances de l'académie des sciences", nr. 23, 5 December 1921). Source: NTRS, retrieved 21 July 2020. [Abstract]
• Ref. 266D2: "Automated landing, rollout, and turnoff using MLS and magnetic cable sensors", S. Pines, S.F. Schmidt, F.I. Mann, NASA Contractor Report 2907 (NASA-CR-2907), 1 October 1977, 152 pp. Source: NTRS, retrieved 21 July 2020.
• Ref. 266D4: "Results from tests, with van-mounted sensor, of magnetic leader cable for aircraft guidance during roll-out and turnoff" [B-737], James C. Young, W. Thomas Bundick, Stewart H. irwin, NASA Technical Paper 2092, January 1983. 40 pp. Source: NTRS, retrieved 24 August 2020.
• Ref. 266D4: "Results of aircraft open-loop tests of an experimental magnetic leader cable system for guidance during roll-out and turn-off", W. Thomas Bundick, David B. Middleton, NASA Technical Memorandum 4135 (NASA-TM-4135), NASA, Scientific & Technical Information Div., 1990, 39 pp. Source: NTRS, retrieved 21 July 2020.
• Ref. 266D5: "Effects of cable geometry and aircraft attitude of a magnetic leader cable system for aircraft guidance during rollout and turnoff", W. Thomas Bundick, NASA Technical Paper 1978, April 1982, 31 pp.
• Ref. 266D6: "Results of aircraft open-loop tests of an experimental magnetic leader cable system for guidance during roll-out and turn-off", W. Thomas Bundick, David B. Middleton, NASA Technical Memorandum 4135, NASA, Scientific & Technical Information Div., 1990, 39 pp. Source: NASA Technical Reports Server, retrieved 21 July 2020.
• Ref. 266E: "The Loth leader cable system for electrical steering of aeroplanes", John. S. Gray, in "Minutes of proceedings of the Institution of Aeronautical Engineers", Nr. 9, 1923, pp. 7-30.
• Ref. 266F: articles in "L'Aérotechnique", "L'Aérophile", "L'Aéro", and "Les Ailes" magazines - 1921-1935
• Ref. 266F1: "Utilisation des procédés Loth pour le guidage des avions par ondes hertziennes -  Partie 1: "Étude critique des procédés Loth pour la navigation aérienne" A. Verdurand, Partie 2 "Point de vue et réfutations de la S.I.P.L."" ["Use of the Loth method for guiding aircraft with radio waves - Part 1 "Critical review of the Loth method of air navigation", Part 2 "Opinion of, and rebuttal by, the Société Industrielle des Procédés Loth (SIPL) company"], A. Verdurand, J. Blancard, in "L'Aérotechnique", Vol. 8, Nr. 137, October 1930, pp. 363-376. Source: Bibliothèque nationale de France; public domain.
• Ref. 266F2: "Le guidage des avions par câbles électriques" ["Aircraft guidance by means of electric cables"], P. Franck, A. Volmerange, in "L'Aérotechnique", Vol. 4, nr. 33, February 1922, pp. 39-47. Source: Bibliothèque nationale de France; public domain.
• Ref. 266F3: "Le câble de guidage - son emploi pour atterissage sans visibilité" ["Guidance cables - their use for landing without visibility"], Paul Larivière, in "L'Aérotechnique", Vol. 17, Nr. 190, March 1935, pp. 33-39.  Source: Bibliothèque nationale de France; public domain.
• Ref. 266F4: "La Navigation aérienne par temps de brume" ["Aerial navigation during fog"], A. de Gramont de Guiche, in "L'Aérophile", Vol. 29, Nr. 1-2, 1-15 February 1921, pp. 42-45. Source: Bibliothèque nationale de France; public domain.
• Ref. 266F5: "Les câbles-guides de M. Loth - La direction des aéronefs sans aucune visibilité" ["The guide cables of Mr Loth - aircraft guidance without any visibility"], P. James, in "L'Aérophile", Vol. 30, Nr. 1-2, 1-15 January 1922, pp. 18-21. Source: Bibliothèque nationale de France; public domain.
• Ref. 266F6: "L'infrastructure aérienne" ["Aviation infrastructure"], in "L'Aéro - Organe hebdomadaire de la locomotion aérienne", Vol. 25, No. 87, No. 1301, 5 May 1933, p. 5. Source: Bibliothèque nationale de France; public domain.
• Ref. 266F7: "La navigation dans le brouillard - où en est la pose du cable Loth" ["Navigation in the fog - the status on installation of the Loth cable"], in "Les Ailes - Journal hebdomadaire de la locomotion aérienne", Vol. 4, No. 180, 27 November 1924, pp. 2-3. Source: Bibliothèque nationale de France; public domain.
• Ref. 266G: "Guidage magnétique des aéronefs et aérodromes de securité" ["The Magnetic Guidance of Aircraft; Safety Aerodromes"], William Loth, in "Comptes rendus hebdomadaires des séances de l'académie des sciences" ["Weekly minutes of the sessions of the academy of sciences"], Vol. 189 (July-December 1929), 14 October 1929, pp. 572-573. Source: Bibliothèque nationale de France; public domain. [Summary
• Ref. 266H: "The Loth navigation system: a twin rotating beacon method of emitting wireless signals that can be picked up on an ordinary receiver", R.J. de Marolles, in "Aircraft Engineering and Aerospace Technology", Vol. 2 No. 5, 1 May 1930, pp. 107-108.
• Ref. 266J: "Making a technology fit: evolution of the leader cable system" [complete general history, 1893 - mid-1930s], pp. 46-56 in ref. 235J, .
• Ref. 266K: "Field localizers", p. 899 in ref. 229D5
• Ref. 266L: "Simon system" [concentric burried cables for approach to landing from any direction; unsuitable for airfield with runway(s)], pp. 35-36 in ref. 235P9.
• Ref. 266M: articles in "Nature" magazine, 1921
• Ref. 266M1: "The Leader Cable System", in "Nature", Vol. 106, No. 2767, 10 February 1921, pp. 760-762. Source: nature.com, accessed 20 August 2020. [pdf]
• Ref. 266M2: ""Leader" cables for aircraft", in "Nature", Vol. 108, No. 2721, 22 December 1921, pp. 539. Source: nature.com, accessed 20 August 2020. [pdf]
• Ref. 266P: "New fog-landing system installed at Materiel Division" [US Army Air Corps, "Loth" leader cable system (with U/I keying) for landing in fog, tested at Patterson Field, Dayton/OH/USA ], in "Air Corps News Letter", Vol. XVI, No. 1. 25 January 1932, pp. 8-9. Source: Air Force Historical Support Division, retrieved 6 September 2020.
• Ref. 266Q: "An Instrument Landing System", Edward Dingley, in "Communications" (merger of "Radio Engineering", "Communication & Broadcast Engineering", & "The Broadcast Engineer"), Vol. 18, Nr. 6, June 1938, pp. 7-9, 30-31. Source: worldradiohistory.com, retrieved 11 September 2020.
• Ref. 266R: "Piloting vessels by electrically energized cables" and "Discussion", A. Crossley in "Proc. of the Institute of Radio Engineers" (IRE), Vol. 9, Nr. 4, August 1921, pp. 273-299. Source: worldradiohistory.com, retrieved 19 February 2021.
• Ref. 266S: "Blind Landing Experimental Unit" [BLEU]. Source: wikiwand.com, retrieved 28 November 2020.
• Ref. 266T: "The Portsmouth "Leader" Cable", in "The Electrical Review", Vol. 86, No. 2209, 26 March 1920, p. 392 and No. 2211, 9 April 1920, p. 458.
• Ref. 266U: in ref. 235Y10 - pp. 9-16, 18 + Fig. 8-17 (leader cable at Ford Airport / Chicago-Lansing/IL/USA, Coock principle & patent, Loth system, Cromwell-Johannson method)
• Ref. 266V: "Finding aircraft landing stations by means of audio frequency receivers", Earl C. Hanson, in "Aerial Age Weekly", Vol. 9, No. 10, 19 May 1919, pp. 489-490. Source: hathitrust.org, retrieved 20 February 2021.
• Ref. 266W: "Underwater pilot cable", p. 335 in Chapter XXVIII in in "History of Communications-Electronics in the United States Navy", Linwood S. Howeth, Bureau of Ships and Office of Naval History, 1963, 698 pp. Source: hathitrust.org, retrieved 20 February 2021.
• Ref. 268: articles about acoustic (sonic) / electro-acoustic / capacitive / radio / radar altimeters
• Ref. 268A: pp. 26-42 [FuG101, FuG101A, AN/APN-1, FuG102, FuG103, SCR-718, AN/APS-13] in "Ein Beitrag zur Flugsicherungs Geschichte" ["A contribution about air traffic control history"], Hans H. Jucker, July 2014, 61 pp. Source: www.wrd.ch. Retrieved 18 May 2020.
• Ref. 268B: "Demonstration of clearance indicator for airplanes", pp. 84 in "Bell Laboratories Record", Vol. XVII, No. III, November 1948. Source: worldradiohistory.com, accessed 17 August 2020.
• Ref. 268C: "Historic firsts: radio altimeter", pp. 18-19 in "Bell Laboratories Record", Vol. 26, No. 1, January 1948. Source: worldradiohistory.com, accessed 17 August 2020.
• Ref. 268D: "Altitude by Radio", Cass H. Maxwell, in "Popular Aviation", Vol. XXVI, Nr. 2, February 1940, pp. 44-46, 94.
• Ref. 268E: "A Terrain Clearance Indicator", Lloyd Espenschied, R.C. Newhouse, in "The Bell System Technical Journal", Vol. XVIII, No. 1, January 1939, pp. 222-234. Source: worldradiohistory.com. Accessed 22 August 2020.
• Ref. 268F: "1967 Pioneer Award - Lloyd Espenschied, Russel Conwell Newhouse" [radio altimeter], Robert I. Colin, in "IEEE Transactions on Aerospace and Electronic Systems", July 1967, pp. 736-742. [pdf]
• Ref. 268G: "Funkhöhenmesser FuG 101a mit Frequenzmodulation" ["Radio altimeter FuG 1010 with frequency modulation"], Hans Jucker, 6 pp. Source: cdvandt.org, accessed 22 August 2020. Also see pp. 26-32 in ref. 268A.
• Ref. 268H: "Elektrischer Höhenmesser FuNG 101 - Kurzbeschreibung und Betriebsvorschrift E 93001" ["Electrical altimeter FuG 101 - description and operating instructions"], Luftfahrtgerätewerk Hakenfelde GmbH, 21 pp. Source: cdvandt.org, accessed 22 August 2020.
• Ref. 268J: "The sonic altimeter for aircraft" [history and status of the sonic altimeter], C.S. Draper, National Advisory Committee for Aeronautics, Technical Notes No. 611, August 1937, 127 pp. [file size: 74 MB; Abstract]. Source: NASA Technical Reports Server (NTRS), accessed 22 August 2020.
• Ref. 268K: "Pursuiters test sonic altimeter", in "Air Corps News Letter", Vol. XVI, No. 1. 25 January 1932, p. 14. Source: afhistory.af.mil, accessed 6 September 2020
• Ref. 268L: see ref. 235Y10 - pp. 16, 17 (acoustic, electro-acoustic, Behm System), p. 17 (radio), pp. 17-18 & Fig. 19 (capacitive)
• 271, etc: to be allocated.

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