red-blue line


The "Bernhard/Bernhardine" system is a radio-navigation system that was used by the Luftwaffe to assist fighter aircraft with the intercept of enemy bombers. The system went into operational service late 1941. Ref. 1, 2, 3, 4, 5, 6, 7A, 8. As shown in Figure 1, the system comprises a beacon station and a Hellschreiber printer-system in the aircraft:

As it operated in the 30-33.1 MHz frequency range, it is by definition a VHF system (30-300 MHz). However, from a radio propagation point of view it behaves more like an HF system (3-30 MHz). The British and US military and intelligence also referred to this system as "Windjammer" (ref. 9, 10, 11, 12, p. 4.09 in ref. 13).

Bernhard Bernhardine system diagram

Fig. 1: "Bernhard/Bernhardine"  = "Bernhard" ground-station + "Bernhardine" airborne Hellschreiber printer system

(click here for a full size image)

The original concept of this radio navigation system is explained in the 1936 Telefunken patent 767354 (see the patent table near the bottom of this page). Its purpose is a highly accurate rotating-beam system that overcomes the stated weaknesses of other such systems:

Obviously, the pre-war patent does not mention that this system was significantly less susceptible to jamming (and spoofing) by the enemy, which had rendered radio-navigation systems such as "Knickebein" and the "X-Verfahren" either unusable or inaccurate. Moreover, the command-uplink capability of the Bernhard/Bernhardine system also freed up capacity of fighter-control "Reportage" messages, that were normally broadcast via Morse code telegraphy and radio telephony (which was also subject to jamming). Note that by the time that the first VHF Bernhard/Bernhardine system became operational, the focus on the western front (Britain) had already shifted to (night-) fighter control (i.e., defensive), rather than for guiding bombing missions over enemy territory (i.e., offensive). The latter had moved to the eastern front (Russia).

Bernhard beam concept Rotating beacon cartoon

Fig. 2: Concept of the Bernhard/Bernhardine radio-navigation system

The "Bernhardine" Hellschreiber-printer in the aircraft prints two parallel tracks on a paper tape, see Figure 3:

":Bernhardine" hell printout

Fig. 3: Signal strength bar graph, azimuth data, and station identifier - printed with a 2-channel Hellschreiber

(The strip indicates that the receiver is on a bearing of 239 degrees from ground station "R"; source: ref. 3)

The paper tape only moves when a sufficiently strong signal is received. So, the print-out does not need to be interpreted immediately.


The original trial systems were developed by Telefunken mid-1935 through 1938, see the time line in Figure 4 below. Note that Telefunken was one of the pioneers of radio navigation beacons: 1907-1918 they had already developed and operated the Telefunken-Kompass-Sender, primarily for long-distance navigation by Zeppelin dirigibles (ref. 1, 2, 16, 17A, 18).

The original Bernhard system operated at a UHF frequency around 300 MHz (λ = 1 m) and the transmitters had an output power of 20 W. Tests were carried out by Telefunken at several locations (ref. 3):

Several versions of the UHF system were evaluated: ground-test only, a system with two receivers without a Hellschreiber printer, a dual system with four receivers (for triangulation) and a 2+2-channel Hellschreiber, and a simplex system with two receivers and a 2-channel Hellschreiber. Initially, test signals were recorded on a standard wax-paper strip-chart recorder.

Late 1940, development and introduction of the system was halted. The primary reason being that the UHF operating frequency required an additional receiver and antenna in the aircraft, since standard equipment was VHF. Therefore, the system was converted from low-UHF (300 MHz) to low-VHF (30 MHz). This, of course, made the ground station a factor 10 larger. However, now the already standard-equipped VHF receivers in the aircraft could be used (as used for Knickebein and X-Verfahren), and only the printer system needed to be added.

Bernhard development time-line

Fig. 4: Time-line of the development and deployment of the Bernhard/Bernhardine system

(based on ref. 3)


Basic characteristics of the Bernhard/Bernhardine system are:

Bernhard system range vs. altitude graph

Fig. 5: Operational range of the Bernhard/Bernhardine system vs. aircraft altitude

(source: based on data in ref. 15)

The above graph is based on a table in the official "Bernhardine" manual (p. 22 in ref. 15). The beacon is assumed to be at zero altitude. Note: as FuSAn 725 never entered service, the claimed range was obtained with 500 W transitters. This range makes "Bernhard/Bernhardine" a medium-range system, like other Luftwaffe radio-navigation systems such as "Erika", "Erich", "Hermine", and "Mond" (ref. 22).

The original concept (patent 767354) comprises a primary and a secondary beacon signal. The primary signal is a constant carrier that is transmitted via a rotating directional antenna with a narrow beam ("Richtstrahl-Drehfunkfeuer"). The beam pattern of the primary antenna has two lobes that are slightly offset (20º in the figure below), such that there is a deep, steep null between them. The secondary signal marks passage of the directional beam through the reference direction (true north or magnetic north). This signal is transmitted via an antenna with an omni-directional pattern. The bearing from the ground station is determined from the time ( = distance on the print-outs) that elapses between the reference "tick", and the null of the rotating beacon signal. Accuracy requires constant rotational speed of the rotating beam, and accurate timing at the receiver. A first improvement to this concept was to print both the signal strength and reference pulse on a single (but wider) paper strip. One major disadvantage remains: the operator must analyze and interpret the printed information from the two printed traces. Obviously this is not practical when being bounced around in a dimly lit and maneuvering aircraft.

In a subsequent update to this concept (patent 767512), the omni-directional reference signal is replaced with a second, co-rotating directional signal. Its beam pattern is at maximum where the primary beam has the null between its two main lobes. Ref. 23.

Bernhard antenne pattern

Fig. 5: Antenna radiation patterns of the antennas for the primary signal (curves D) and the secondary signal (curves K)

(Left: Fig. 2 in patent 767354, right: Fig. 1 in patent 767523)

Bernhard antenne pattern

Fig. 6: Radiation pattern of the "Bernhard" antenna system

(source: Fig. 4.28 in ref. 24)

The associated receiver continuously records (prints) the signal strength level of the primary rotating beam. This is done with a simple Hellschreiber printer. The antenna beam pattern makes it easy to accurately detect the passage of the beam at the azimuth of the receiver, as shown in the diagram below.

Berhard signal strength plot

Fig. 7: signal strength received from the rotating constant-carrier directional beam vs. time

(source: based on Fig. 4 in patent 767354)

A Hellschreiber printer comprises an inked spindle that is placed across, and slightly above, a moving paper tape. Below the paper tape is an electromagnet with a hammer blade. When the magnet is energized, the hammer pushes the paper tape against the continuously turning spindle. This causes a line segment to be printed across the paper tape. The length of the printed line depends on the amount of time that the electromagnet is energized. See the "How it works" page. The signal strength is determined by rectifying the received tone. This signal level is then converted to the width of an energization pulse that is fed to the printer's electromagnet. The clever way how this was done, is described here on the "Bernhardine"page. The result is a bar graph of that signal strength.

Berhard signal strength plot

Fig. 8: signal strength bars, as printed with the Bernhardine-Hellschreiber

(source: ref. 3)

In Hellschreiber-format, any graphical information that is to be transmitted, is decomposed into a number of consecutive pixel columns. A 360-degree compass card comprises a band of tick marks and numbers. This band can be converted to Hell-format by decomposing it into columns of pixels as shown in Figure 9B below. Consecutive columns are transmitted top-to-bottom.

Patin remote compass

Fig. 9A: Remote compass, as used e.g. in Fw190

(made by A. Patin & Co. G.m.b.H. of Berlin)

Bernhard compass card

Fig. 9B: Radiation pattern of the "Bernhard" antenna system

(sources: patent 767524 (top), ref. 15 (bottom))

The original 1936 Telefunken patent (767354) mentions transmission of the azimuth data via the principles of fax or TV ("Bildfunk oder Fernsehprinzip"). Other early patents are explicitly based on transmission of compass-rose information via video (Nipkow-disk image scanning): patent 562307 (1929, J. Robinson) and 620828 (1933, Marconi Co.).

Patent 767524 and the manual of the "Bernhardine" Hellschreiber printer (ref. 15) show the following pixel format (see Figure 9B above):

The antenna of the Bernhard system made one full revolution every 30 sec. For the above Hell-format, this implies 25920 / 30 = 864 pixels per sec. Hence, a pixel duration of 1000 / 864 = 1.15 msec per pixel. Applying the "2-pixel rule" of Hellschreiber implies a minimum pulse duration of 2 x 1.15 = 2.3 msec (ref. 15). The shortest pulse cycle (1 black pulse + 1 white pulse) is 2 x 2.3 = 4.6 msec. This is equivalent to a maximum pixel-rate of 1000 / 4.6 = 217 Hz, and a telegraphy speed of 217 Bd. Note that this is 10% less than the speed of Presse Hell (225 Bd), and less than twice the speed of Feld-Hell (122.5 Bd). Siemens-Halske recommended limiting the transmitter bandwidth to 1.6 times the pixel rate - here: 1.6 x 217 ≈ 350 Hz. This is consistent with the 400 Hz wide filter of the actual Bernhard-transmitter (ref. 15).

The pixel sequence of the entire compass card is captured as an optically encoded track at the circumference of a disk ("Kennzeichenscheibe"). The disk is mounted on the central shaft of the rotating antenna system. The pixel track passed between a light source and a photocell. The output of the photocell was used to key the Telefunken transmitter.

The disk was made of glass (p. 62 in ref. 20). Patent 767524 proposes to implement the pixels as lines, engraved into a disk with a blackened surface. However, the same patent suggests a "negative" master disk, from which a copy can be made for each beacon station. This suggests a photo-chemical process, rather than engraving. Engraving (as suggested in ref. 20, and p. 124 of ref. 21) would have been an extremely (and unnecessarily) laborious process, given the number of pixels.

I have no information on the size of the disk. Let's suppose that the disk measured about 30 cm (12") in diameter - i.e., the size of an old 78 rpm phonograph record. The circumference would be about 94 cm for 25920 pixels. The resulting 2-pixel width is a mere 0.14 mm. With a very large disk, say 1 meter diameter (≈40"), a single pixel would measure less than 0.25 mm, so 0.5 mm for the minimum 2-pixel width.

Optical disk

Fig. 10: Optically encoded disk (only part of the pixel track shown)

(source: patent 767524)

Telefunken patents 767524 and 767354 also considers other implementations of the disk: a pixel track implemented as interconnected metal patches that are scanned with a slip contact (similar to the character drum of the Feld-Hellschreiber), or an optical pixel track with holes through the disk.

The Bernhard navigation system comprised a chain of ground stations. So the station had to be identified by its transmission. This was done by adding a station-identifier letters (callsign) between the 10-degree numbers of the transmitted compass rose. The associated pixels could be implemented in several ways (patents 767524 and 767528):

Patent 767937 proposes an optical disk with multiple (e.g., 8) concentric tracks. This would enable the quick change of the station's identifier letter, and/or expansion to a 2-letter callsign, to be able to distinguish more than 26 beacons.

Patent 767528 includes the cross-section diagram of what a 2-disk attachment might look like. One disk with the azimuth tick marks and degree numbers ("Gradeinteilungen" and "Gradzahlen"), and a second disk for the beacon's station-identifier letter ("Funkfeuer kennzeichnender Buchstabe").

Optical disk mount

Fig.11: two stacked optical disks with, mounted on the shaft of the rotating antenna system

(source: Figure 4 of patent 767528)

Patent 767354 proposes that the light source be a gas discharge lamp, powered with an AC signal. This also makes the output signal of the photocells AC, which is easy to amplify. Patent 767529 discusses shaping the output signal of the photocell by amplification and clipping, to obtain square keying pulses for the transmitter.

Patent 767528 states that the limiting factors for the upper limit of the antenna's rotational speed, are the printing speed of the Hellschreiber and the required pixel resolution of the printed information. Given the large size and weight of the antenna system, there are obviously also mechanical considerations for the upper speed limit. The patent proposes to resolve this, by quadrupling the number of antenna beams, spaced at 90º intervals. Each optical disk would simply have four "light source plus photocell" pairs (two pairs shown in the diagram above), that could be adjusted to account for angular offsets between the beam centerlines.

On the printed paper strip, the sharp dip in the bar graph now points directly at the aircraft's current azimuth (bearing from the station), as printed on the trace right below it. See figure 12 below. The format of the azimuth track in this print-out appears to be similar to that of patent 767524.

This bar-graph pattern is insensitive to variations in the rotating speed of the antenna, and the speed of the printer spindle and paper transport. The achievable accuracy of the system is primarily limited by the pointing accuracy of the twin-lobe beam, and the visual alignment of the printed Hellschreiber traces. Later on, accuracy was improved to 0.5 deg, by adding 0.5 deg tick marks on the azimuth scale (ref. 3). The figure shows a total width of the twin-lobe beam of about 2x20 = 40 deg - at the particular distance at which the recording was made.

":Bernhardine" hell printout

Fig. 12: Signal strength bar graph, azimuth data, and station identifier as printed with the "Bernhardine" Hellschreiber

(The strip indicates that the receiver is at on bearing of 239 degrees from ground station "R"; source: ref. 3)

":Bernhardine" compass card

Fig. 13: Re-created signal strength bar graph, azimuth data, and station identifier "M"

I have used FontStruct™ to capture the above signal-level track and compass-rose segment as Feld-Hell character of a "TrueType" font that can be used in regular Windows® and MacOS® programs (e.g., Word®, PowerPoint®). Click here for the two-character font (capital letters A and B) for the signal level. Click here for the 18-character font (capital A-R) for the 360º compass-rose.

With a single Bernhard ground station, only relative bearing to/from that particular station can be determined. I.e., a position line ("Standlinie"), and neither distance (range) from the station nor a position point. Position determination is done by combining the bearing from at least two ground stations with know location. I.e., by means of conventional triangulation. Of course, the second ground station need not be another Bernhard station, but could be another type of beacon system or even a public broadcast station. Note that the bearing from a ground station to the aircraft should not be confused with the aircraft's heading (the way the nose is pointing), nor with the aircraft's course (ground track).


Fig. 14: Triangulation with two ground stations

(D = beacon ("Drehfunkfeuer"), α = azimuth = bearing from station = angle measured clockwise from north)

Patents 767937 and 767538 propose to facilitate triangulation by simultaneously printing the signals (strength + azimuth) of two ground station on double-wide paper strip. This does require a double printer (2+2 tracks), as well as a second receiver.


Fig. 15: Triangulation made easy with a Bernhardine Registrierungschrieb (print-out) from two ground stations on a single, extra wide paper strip during test with the UHF (300 MHz) development system

(source: ref. 3; also see patent 767537)

Patent 767512 refers to the option of transmitting an extra heavy tick mark in the azimuth trace, see Figure 16. This could be used for tactical purposes, e.g., to indicate a target azimuth (the radial from the ground station) that is to be intercepted. This could be simply implemented with a notch (cam) on the same shaft as that of the optical disk. The position of the notch could be adjusted to the desired target azimuth. The notch would actuate a switch that is simply connected in parallel with the output of the photocell of the optical disk. However, this target-azimuth marker method would not have been very practical: it could only be received by aircraft that are flying on nearly the same bearing from the beacon station as that target.

Notched optical disk

Fig. 16A: Notched optical disk

(source: Figure 2 in patent 767512)

Tarfet azimuth bar

Fig. 16B: Target azimuth bar superimposed on azimuth data

(source: hand-drawn Figure 3 in patent 767512)

The above target-azimuth bar is the simplest form of a (unidirectional) data-link for sending tactical data to an aircraft, for guidance or command purposes ("Kommando-Übertragung"). A more sophisticated data link was implemented by transmitting text messages in Hellschreiber-format on the azimuth data channel. When the messages had to be sent, the output of the photocell of the optical azimuth disk was simply disconnected, and the pixel stream of the text messages was used instead.

These were short messages, consisting of simple coded groups of letters and numbers. The message could consist of information about an enemy bomber formation, e.g.:

This is the same format as used for "Running Commentary" ("Laufende Reportage") broadcasts on HF and VHF via via radio telephony and Morse beacons. This was used in German fighter control systems such as "Zahme Sau" ("Tame Boar") and "Wilde Sau" ("Wild Boar"). See §22-25 in ref. 6, ref. 25.

Bernhard reportage track

Fig. 17: "Bernhardine" print-out with Reportage track at the bottom

(source: ref . 6, 1945)

Note that these command messages were transmitted instead of the azimuth data, not in addition to, or in parallel with the azimuth data. There was no additional, third printer-track for these messages - contrary to what Figure 17 above may suggest! Figure 17 also incorrectly suggests that the printed message characters had a height of about half the azimuth track. And it suggests that the station-identifier (here "X") is repeated every 20 degrees. In Figures 15 and 16 it is clearly 10 degrees. Unfortunately, I have not seen any pictures of original Bernhardine print-outs with a data-link message.

Obviously, this message broadcast system had the same robustness against jamming as the azimuth part. An other advantage of this messaging system, is that it provided relief for the already busy frequencies for voice communication.

Note that the Bernhard/Bernhardine system was the first and only operational ground-to-air data-link system of the second World War that had freely formattable messages! Since about the year 2000, the same concept has been introduced to "modern" civil aviation: Controller-Pilot Data Link Communications (CPDLC). This is for up-linking routine air traffic control instructions and clearances to aircraft via digital radio. However, contrary to the Bernhard system, the pilot can now respond to messages, request clearances and information, and declare an emergency. Apparently, late 1943 / early 1944, the Lorenz company also experimented with an expanded Hellschreiber-based command data-link system, referred to as "Sägezahn" ("sawtooth", ref. 26A).

The Bernhardine printer only receives and prints the signals from the Bernhard station during 3-5 sec of the 30 sec revolution of the antenna system. The entire message must be received during this short beam-passage period, independent of the bearing from the station on which the aircraft is flying within the operating range of the system. This limits the number of text characters that can be put in a single message. Let's assume that transmission of the message is continuously repeated. To always guarantee that a complete message is received, two back-to-back copies of the message should fit within a single beam passage. This limits the length of a complete message (including delimiter symbol) to about 10 characters. The actual message is then delimited by two "+" symbols. 

Bernhard reportage track

Fig. 18: re-created Bernhardine print-out with reportage track

In some literature, allied bomber formations are generically referred to as "bomber streams". However, the term only refers to a specific form of sequencing (night) bombers, used by the RAF from the end of May 1942 until the end of the war. The purpose of this tactic was to create a string of bombers (with designated altitude bands and time slots), that would pass through the narrow German (night) air defense system via a minimum number of "boxes". This defense system, established in 1940 by then-colonel Josef Kammhuber, comprised a chain of rectangular airspace zones ("boxes"). The chain eventually reached from Denmark to the north of France, and was referred to by the British as the "Kammhuber Line". Ref. 27. The zones had search and tracking radars (first Freya and Würzburg radar systems, later also Würzburg Riese) and groups of search lights (some radar controlled). Funneling all bombers through one or a few boxes, quickly overloaded the defense capability of the boxes (two night fighters per box, an estimated 6 intercepts per hour).

As stated above, the "Reportage" messages were sent instead of the azimuth data from the optical disk. So, somewhere, these messages were converted from text to Hellschreiber pixel streams. This could have been done with a keyboard and tape-puncher, combined with a "punch tape to Hellschreiber pulse-sequence converter". This was the normal way with the "Presse Hellschreiber" system. The tape could be looped through the reader to repeat the message. Of course, speed and text font may have had to be adapted.

The conversion from text string to Hellschreiber pixel streams could have been done at the Bernhard station, based on telephone or teleprinter messages from the regional fighter command & control center. However, this communication step would have introduced unnecessary delays in the time-critical "Reportage" broadcasts.

Note that the Kriegsmarine used a 1 megawatt (!) VLF transmitter to broadcast Morse and Hellschreiber messages to submarines around the world, even submerged. See the "Goliath transmitter" section of the “Military radios used with Feld-Hell” page. That transmitter was located 135 km northwest of Berlin. It was used remotely from the "Koralle" Kriegsmarine headquarters, 135 km from the transmitter. Tuning of the transmitter was done locally, but the Morse and Hellschreiber keying of the transmitter was done "live" from “"Koralle", via the Wehrmacht telecom network. This implies sending Hellschreiber tone pulses via phone or teleprinter lines.

So, most likely, the text messages were sent in Hellschreiber format from the fighter command & control center directly to the Bernhard transmitter. But how were the Hellschreiber pixel streams generated? The conventional way would have been with a keyboard, punch tape reader / character-generator. However, p. 87 in ref. 2 and p. 392 in ref. 7B suggest that a different method was used to program the text character sequence: inserting jumpers ("Stöpsel") into some sort of patch-board. Assume that the message consisted of 10 characters, and that the first character was a fixed message delimiter symbol "+". This leaves 9 characters to be selected. A character could be the figure 0 - 9, or the letter A - Z. The patch-board could have had 10 jacks for the figures and 26 jacks for the letters. With 9 patch cords, one for each character position, the 9-character string could be selected. This is similar to the method used in the Hellschreiber T empf 44 in the 1960s. It has a character-drum with seven notched disks to generate the characters A - G, ten notched disks for the figures 0 - 9, and one disk for the character "-". The machine would send a strings of eight characters, based on a discrete code at its inputs.

The azimuth Hellschreiber printer of the "Bernhardine" system uses the degree tick-marks of the received compass rose segment to synchronize the printer spindle to the pixel stream. The "Bernhardine" manual (ref. 15) suggests that it took 1-2 minutes for the synchronization circuitry to settle (i.e., the aircraft being illuminated by the beam 2-4 times). It is unclear how this synchronization was done when printing the Reportage messages instead of the compass rose. Unless the characters/font of those messages also included tick marks at the top (which would not have been too distracting)… Unfortunately there are extremely few photos of such print-outs (some hand-drawn images), and the ones I have are from the UHF prototypes of the system, where the printer speed could have been adjusted manually.

Also still unclear is whether the switch-over from transmitting azimuth data to transmitting reportage messages was done by crew at the Bernhard station (upon a phone call or teleprinter message), or also by remotely, by the fighter command & control center. Either way involves switching a relay or switch. That could have easily been done remotely. “Presse Hellschreiber” printers used by news agencies worldwide ca. 1934-1980 via radio, included relatively simple relay-circuitry for remote on/off. This used the same tone frequency as for the Hell-format pixel stream (pulses of several millisecond duration), but much longer pulses (8 sec to turn on, 0.5 sec to turn off).

Reportage - Kontrollpult

Fig. 19: Reportage control console ("Kontrollpult") of the Luftwaffe's regional control center (Großraum-Gefechtsstand) at Grove airbase in Denmark

(source:, used with permission)


Clearly, Bernhard/Bernhardine is a "beam system" for aircraft guidance. The origins of such systems date back at least to the early 1900s, when Otto Scheller of the Lorenz company invented the ""Wireless course indicator and telegraph" system (patents from 1907 and 1916 ). At that time (before the widespread advent of aviation), the system was intended for guiding ships. The ground-station of the Scheller system had a radiation pattern with four main lobes in fixed orthogonal directions. See Figure 20. Two of the lobes transmitted the Morse letter "N" (dash dot), the other two the letter "A" (das dot"). Where lobes overlap, the combination of "A" and "N" results in a constant tone signal ("Dauerton"): the "equi-signal". This was the first "A/N" system, later used in several other Lorenz radio-navigation systems. Later 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.

Lorenz-Schiller A/N system

Fig. 20: The Lorenz-Scheller A/N system

In 1908, Telefunken developed a rotating beacon system, the "Kompass Sender". It was used for long-range navigation of Zeppelins. The antenna system comprised 18 dipoles (i.e., every 10 degrees) of 2 x 60 m, and a motorized antenna switching system. See Figure 21. Individual dipoles would be successively connected to the transmitter via a motorized distributor (later replaced with a goniometer without contacts). This created a rotating dipole radiation pattern with a constant signal. Once per revolution, at the "north" position, the station identifier would be sent in Morse code via all dipoles simultaneously (omni-directional). Bearing from the beacon is determined by measuring the time between the "north" marker and passage of maximum signal (actually much harder to do than determining the minimum). As the radiation pattern of a dipole is a figure "8", it has two identical maxima. Hence, a 180 degree ambiguity. The ambiguity is resolved by determining the bearing from another beacon.

Telefunken Kompass-Sender

The array of 18 dipoles

Telefunken Kompass-Sender

The motorized distributor at the center of the array

Fig. 21: the 1908 Telefunken "Kompass Sender"

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, 29, 30. Note that "blind landing" is misleading, as the system did not provide precision vertical guidance down to the actual landing. Hence it is only an approach-beacon "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).

Rather than using the Morse letters "A" and "N", this system used "E" (= "dot") and "T" ( = "dash"), which has the same effect but is simpler to implement. The antenna system was simple: a vertical dipole, with a vertical reflector to the left and to the right, at a distance of ¼ λ. See Figure 22. This was patented in 1932 (Reichspatent 577350). Kramar's 1937 patents expand this with a complementary-keyed (e.g., E/T) beam system for vertical guidance. This was re-patented in 1940 in the USA by others.

Lorenz Beam Lorenz Beam

Fig. 22: Antenna arrangement of the "Lorenz Beam" system

(photo source: ref. 31)

The reflectors were activated alternately, 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 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. During approach for 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 an Inner Marker ("Haupt-EFZ") at 300 m, ref. 32. These beacons transmitted upward at 38 MHz, with a fan-beam. This allowed the pilot to determine when to initiate decent to the runway from a standard altitude and with a standard descent rate (3 degrees flight path). Ref. 17A/B/C, 26B. This "Lorenz beam" system entered service with the German national carrier, the Lufthansa, in 1934 and was then commercialized worldwide. In the UK it became the "Standard Beam Approach" system.

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.

Plendl's concept used two directional beacons, sufficiently spaced apart, crossing 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"). The aircraft would intercept and track the E/T equi-signal of a director-beam ("Leitstrahl") to the target. A second beacon would transmit 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 23B. 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.

The (lead) aircraft required an "X-Equipment" ("X-Gerät") installation. This comprised (cf. p. 106 in ref. 2):

AFN2 indicator

Fig. 23A: AFN2 Anzeigegerät für Funk-Navigation - Left/right course deviation indicator

(the horizontal needle is a signal-strength indicator, as a simplistic near/far distance indication; the indicator lamp in the center is illuminated when receiving a marker beacon during approach)


Figure 23B: an X-Uhr "Bombenabwurfautomat" (automatic bomb-release timer/computer)

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 to the 66 - 77 MHz range, and equipped with more powerful transmitters. The German codename for the "X" ground-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 beam direction, but not (continuously) rotating. One dipole was energized permanently with a 2000 Hz modulation. The second dipole, placed at a distance of 3.5 λ, was energized via a motorized capacitive phase shifter. The phase was changed stepwise, every 0.5 sec. The resulting radiation pattern had 14 or 18 E/T-beams of about equal strength and an equi-signal zone with a width of less than ±0.1º. The large number of major lobes was rather awkward: the aircraft had to fly across the lobes, and count the passages of the "T" zones to find the intended guide beam ("Marschleitstrahl", the 7th of 14 (as in Figure 24 below), or the 9th of 18).

Wotan I beacon pattern

Fig. 24: Radiation pattern of the Wotan I beacon

(source: adapted from ref. 2)

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 more 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. 8, 28, 34, 35, 36, 37, 38. During this period, the British developed countermeasures to German radio-navigation systems and radio-telephony communication of fighter/bomber control systems, to which the Germans responded with modified or new systems.

By May of 1941, the 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 as effective. Hence, the Telefunken company had already been tasked very early on, to develop a simple beam system that was compatible with the Lorenz Funklande Empfangsanlage Fu Bl 1 ("blind approach & landing 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 beacons. Its development was headed by Adalbert Lohmann, who later developed the Bernhard/Bernhardine system. It operated in the 30-33.3 MHz band, i.e., a wavelength of 9 - 10 m. Obtaining a sufficiently narrow equisignal-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 codename was "Knickebein".

By the end of 1939, three Knickebein installations were operational along the western border of Germany: Stollberg/Bredstedt on the North Sea coast in the far north (later the location of a Bernhard station), Kleve (Cleves, where the Rhine river crosses into the Netherlands), and Lörrach/Maulburg (in the far south, near the Swiss/French border). These were installations with an enormous rectangular antenna system, see Figure 25A. Its truss frame measured ca. 90 x 30 m (WxH). Suspended in the frame were two dipole arrays, one for the "E" beam and one for the "T" beam. Each of these sub-arrays comprised 8 vertical wire-dipoles. Each wire-dipole had a dipole-reflector. The system could be rotated on a circular track, to point the beam at the target (in Britain). To obtain a narrow equi-beam (±0.3º), the "E" and "T" beams were offset 7.5 degrees to the left and to the right of the desired equi-beam. This was done by angling the left and right hand half of the antenna system by 15 degrees from each other. Looking at the antenna system from above, it had a slight V-shape ("crooked leg") of 165 degrees.

large Knickebein ground-station

Fig. 25A: Large Knickebein ground-station

Small Knickebein ground station

Fig. 25B: Small Knickebein ground station

After the invasion of their neighbor countries, the Germans installed another nine Knickebein stations along the coasts of Norway (1x), The Netherlands (2x), and France (6x, from the Channel coast down to Brittany). Construction of a tenth station in Italy was never completed. However, these stations had a smaller antenna system. It had half the width of the large system (i.e., 45 m, and a track diameter of 31 m), and 2x4 dipoles plus reflectors per beam, instead of 2x8. Hence, the width of the equi-beam was larger (±0.6º). On the other hand, they were installed closer to the targets in Britain than the large stations in Germany. In September of 1941, the aircraft receivers were upgraded from Fu Bl 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 6000m altitude (20 thousand feet). During the fall of 1940, the Knickebein system became increasingly unreliable and unusable over Britain, due to jamming and spoofing by the British. It continued to be used for navigation towards the target, but relied on the X-System (described further below) in lead aircraft ("Pfadfinder") to locate and mark the actual target.

AFN1 indicator

Fig. 26: AFN1 Anzeigegerät für Funk-Navigation - Left/right course deviation indicator of the FuBl 1 system

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 using a cross-beam to mark the position of the target along that beam, it used a transponder system 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 aircaft would retransmit the received signal at a different frequency (1.9 MHz lower). The ground station would derive the range from the total round-trip signal delay. The ground controllers would command release of the bombs, based on the range. This was called the "Y-System" ("Y-Verfahren", US/UK allied code name "Benito"). 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.

The (lead) aircraft required a "Y-Equipment" ("Y-Gerät") installation (ref. 2, 39). This comprised:

FuG17 + AG28a

FuG17 (left) + AG28a (right)

FuG17 + AG28a

Bottom of the unit

Fig. 27: FuG28a "UKW Leitstrahlempfangsgerät" (VHF guide-beam receiver)


The "Y-Station" ground station ("Y-Bodenstelle", "Y-Peiler", "Y-Bake") was called "Wotan II" (FuSAn 733). A variety of antenna configurations, and beam transmitters was used (Bertha I, Bertha II), and a number of co-located transponder transmitters (S16B, Sadir 80/100).

An interesting beacon system is the Hermes/Hermine "Sprechdrehbake" system ("Talking 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" air-defence operations. The pilot could determine the bearing from the beacon, without having to look at an instrument. The beacons (FuSAn 726) transmitted recorded voice-announcements of the beam azimuth in real-time, every 10º. I.e., the numbers 1 - 35 (multiples of 10º), and the "station call-sign" for 360º = 0º = north. The airborne FuG125 "Hermine" comprised an EBl 3 receiver with increased audio bandwidth, an FBG2 control panel, and a small audio-amplifier (model V3a or ZV3). The system was developed in 1943/44 by Ernst Kramar of the Lorenz company.

In addition to the beacon systems described above, there was a number of other German beacon systems (ref. 1, 2, 26B, 33), such as:

In general, the British developed countermeasures (ground-based and airborne) to all of the operational systems, with varying degrees of effectiveness.


Below is a listing of patents related to Bernhard/Bernhardine.

Patent number Patent office Year Inventor(s) Patent owner(s) Title (original) Title (translated)
577350 RP 1932 E. Kramar C. Lorenz A.G. Sendeanordnung zur Erzielung von Kurslinien Transmitter arrangement for producing course lines
662457 RP 1935 W. Runge
K. Röhrich
Telefunken GmbH Antenneanordung zur Aussendung von zwei oder mehreren einseitig gerichteten Strahlen Antenna arrangement for transmission of two or more uni-directional beams
737102 RP 1935 W. Runge Telefunken GmbH Anordnung zur ständigen Kontrolle und zur Ein- bzw. Nachregulierung der geometrischen Lage eines Leitstrahls während des Leitvorganges Arrangement for monitoring and adjustment of the location of a directional beam
767354  RP 1936 - Telefunken G. für drahtlose Telegraphie m.b.H. Verfahren zur Richtungsbestimmung Method for direction-finding [this is the primary "Bernhard" patent]
767512 RP 1938 - Telefunken GmbH Verfahren zur Richtungsbestimmung mittels rotierender Richtstrahlung Method for direction finding by means of a rotating directional beam
767513 RP 1939 A. Lohman Telefunken GmbH Empfangsseitige Schreibvorrichtung zur Durchführung eines Verfahrens zur Richtungsbestimmung [Wachsschreiber] Receiver-side printer for the implementation of a method for direction-finding [wax printer, infinite loop, erasable tape]
767515 RP 1940 A. Lohman Telefunken GmbH Anwendung des Registrierverfahrens nach Patent 767354 für ein Verfahren zur Führung eines Luftfahrzeuges während des Landungsvorganges Application of the printing method per Patent 767354 for a method for aircraft guidance during landing
767523  RP 1938 A. Lohman
A. Bittighofer
Telefunken GmbH Empfangseinrichtung zur Durchführung des Verfahrens zur Richtungsbestimmung Receiver-side device for the implementation of the method for direction-finding
767524  RP 1938 A. Lohman Telefunken GmbH Verfahren zur Richtungsbestimmung mittels rotierender Richtstrahlung Method for direction-finding with a rotating directional beam
767525 RP 1938 A. Lohman Telefunken GmbH Einrichtung zur Speisung eines rotierenden Richtantennensystems Device for capacitive coupling of a transmitter to a rotating directional antenna system
767526  RP 1938 A. Lohman Telefunken GmbH Verfahren zur Richtungsbestimmung Method for direction finding
767527 RP 1938 A. Lohman Telefunken GmbH Einrichtung zur periodischen Ein- bzw. Ausschaltung einer Registriervorrichtung Device for switching on and off of a printer
767528 RP 1936 A. Lohman Telefunken GmbH Verfahren zur Richtungsbestimmung Method for direction finding [optical disks, quadruple antenna]
767529 RP 1938 A. Lohman
A. Bittighofer
Telefunken GmbH Einrichtung zur Erzeugung angenähert rechteckiger, zur Modulation des Kennzeichensenders dienender Abtastimpulse bei einem Verfahren zur Richtungsbestimmung mittels Drehfunkfeuer Device for the generation of an approximately square pulse envelopes, for the direction finding method by means of a rotating beacon
767530 RP 1938 A. Lohman Telefunken GmbH Verfahren zur Richtungsbestimmung Method for direction-finding [frequency shift for beacon tone frequencies]
767531 RP 1939 A. Lohman Telefunken GmbH Verfahren zur Richtungsbestimmung Method for direction-finding [dipole antenna array arrangement with side-lobe suppression]
767532 RP 1939 A. Lohman Telefunken GmbH Sendeanordnung zur Durchführung eines Verfahrens zur Richtungsbestimmung Antenna arrangement for the implementation of a method for direction finding
767534 RP 1940 A. Lohman Telefunken GmbH Verfahren zur Richtungsbestimmung Method for direction-finding
767536 RP 1940 A. Lohman Telefunken GmbH Empfangsseitige Schreibvorrichtung zur Durchführung eines Verfahrens zur Richtungsbestimmung Receiver-side printer for the implementation of a method for direction-finding
767537 RP 1938 A. Lohman Telefunken GmbH Anwendung des Peilverfahrens nach Patent 767354 für die Standortbestimmung Application of the direction finding method of patent 767354, for position finding [printing of two beacons for triangulation]
767538 RP 1939 A. Lohman Telefunken GmbH Anwendung des Verfahrens nach Patent 767354 für die Standortbestimmung Application of the method of patent 767354, for position finding [receiver/printer arrangement for triangulation with two beacons]
767919 RP 1940 H. Muth Telefunken GmbH Verfahren zur Richtungsbestimmung unter Verwendung eines rotierenden Funkfeuers Method for direction-finding with a rotating beacon [using only twin-lobe beam]
767937 RP 1939 A. Lohman Telefunken GmbH Einrichtung zur Durchführung eines Verfahrens zur Richtungsbestimmung Device for implementation of a process for direction finding [multi-track optical disk for quick change of identifier]
730635 RP 1937 R. Hell Dr.-Ing. Rudolf Hell Verfahren zur Registrierung des Verlaufes veränderlicher Stromkurven Method for printing the trace of varying signals [Hell printer for signal-level track of Bernhardine]

Here are some ancillary patents:

Patent number Patent office Year Inventor(s) Patent owner(s) Title (original) Title (translated)
562307 RP 1929 J. Robinson J. Robinson Funkpeilverfahren Method for direction finding [transmission of coursepointer, or compass rose info via Nipkow-video]
620828 RP 1933 - C. Lorenz AG Funkpeilverfahren Method for direction finding [transmission of compass rose info via Nipkow-video]

Patent office abbreviation: RP = Reichspatentamt (Patent Office of the Reich), DP = deutsches Patentamt (German Patent Office)
Patent source: DEPATISnet


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:
Ref. 2: pp. 76-110, 224 in "Die deutschen Funkführungsverfahren bis 1945", Fritz Trenkle, Alfred Hüthig Verlag, 1987, ISBN 3778516477, 215 pp.
Ref. 3: pp. 94-102 in "Die deutschen Funk-Navigation und Funk-Führungsverfahren bis 1945", Fritz Trenkle, Motorbuch Verlag, 1995, 208 pp., ISBN-10: 3879436150.
Ref. 4: p. 260 in "A survey of continuous-wave short-distance navigation and landing aids for aircraft", C. Williams, Journal of the Institution of Electrical Engineers - Part IIIA: Radiocommunication, Volume 94, Issue 11, March-April 1947, pp. 255 - 266.
Ref. 5: pp. 236-237 in "Instruments of Darkness: The History of Electronic Warfare, 1939-1945", new ed., Alfred Price, Greenhill Books, 2005, 272 pp., ISBN-10: 1853676160.
Ref. 6: "G.A.F. Night Fighters - Recent Developments in German Night Fighting" [transcript], Air Ministry, Air Ministry, Assistant Director of Intelligence - Prisoner Interrogation, Wing Cmdr. S.D. Felkin, A.D.I. (K) Report No. 125/1945, January 1945, 18 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: "Radio and Radar Equipment in the Luftwaffe - II, Navigational Aids" [transcript], Air Ministry, Assistant Director of Intelligence - Prisoner Interrogation, Wing Cmdr. S.D. Felkin, A.D.I. (K) Report No. 357/1945, 1945, 18 pp. Source:
Ref. 9: AIR 29/284 "Central Interpretation Unit (CIU) Medmenham; Appendices 3299-3313. Interpretation reports covering Andaman and Nicobar Islands, France, Germany, Greece, Italy and others. Contains maps, plans and aerial photographs. Includes reports on the German "Windjammer" RDF stations and the German aircraft carrier 'Graf Zeppelin'", Air Ministry, Bomber Command.
Ref. 10: AIR 14/3594 "Windjammer" stations: photographs and interpretation reports. Includes 17 photographs depicting: Vertical and low oblique aerial photographs of `Windjammer' radar sites in Germany and France (places named). Dated 1943-1944", March 1943 - October 1944, Air Ministry, Bomber Command.
Ref. 11: AIR 14/3592 "Radio Direction Finding Stations", September 1942 - March 1945, Air Ministry, Bomber Command.
Ref. 12: AIR 14/3577 "Signals investigation on 27 to 35 Mc/s "Windjammer", June 1943 - September 1944, Air Ministry, Bomber Command
Ref. 13: p. 405 and 4.09 in "Japanese Electronics", OPNAV-16-VP101, Photographic Intelligence - Report 1", U.S. 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:
Ref. 15: "Beschreibung und Betriebsvorschrift für Funk-Navigationsanlage FuG 120" [Description and Operating Manual for Radio-Navigation System FuG 120], Telefunken G.m.b.H., document FN-T-GB Nr. 1932, December 1944, 43 pp.  [30 MB]
Ref. 16: "C.W. Radio Aids to Homing and Blind Approach of Naval Aircraft", D. Quinn, R.D. Holland, J. of the IEE, Part IIIA: Radiocommunication, Vol. 94, Issue 16, March-April 1947, pp. 953-960.
Ref. 17A: "C.W. Radio Aids to Approach and Landing", M. Birchall, J. of the IEE, Part IIIA: Radiocommunication, Vol. 94, Issue 16, March-April 1947, pp. 943-952
Ref. 17B: "Discussion on "C.W. Navigational Aids" at the Radiocommunication Convention, 2nd April 1947", J. of the IEE, Part IIIA: Radiocommunication, Vol. 94, Issue 16, March-April 1947, pp. 1022-1028.
Ref. 17C: "Discussion on "C.W. Navigational Aids" - The Author's Replies to the Above Discussion", J. of the IEE, Part IIIA: Radiocommunication, Vol. 94, Issue 16, March-April 1947, pp. 1029-1030.
Ref. 18: "History of radio flight navigation systems", translated into English by M. Hollmann, P. Aichner, 15 pp. Source:
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", Chapter 3 of “Leitfaden der Funkortung: Eine systematische Zusammenstellung der Verfahren und Anlagen der Funkortung“ ("Lehrbücherei der Funkortung: Band 1"), Walter Stanner, 4th ed., Deutsche RADAR-Verlagsgesellschaft m.b.H., 1957, 160 pp.
Ref. 21: "Drehfunkverfahren", pp. 119-130 in “Bordfunkgeräte - vom Funkensender zum Bordradar“, Fritz Trenkle, Bernard und Graefe Verlag, 1986, 283 pp., ISBN 3-7637-5289-7
Ref. 22: table 1 in "Verfahren und Anlagen der Funkortung", W. Stanner, Elektrotechnische Zeitung (ETZ), Ausgabe A, Vol. 75, Nr. 13, 1 July 1954, pp. 438-442.
Ref. 23: "Drehfunkfeuer – System Telefunken; Teil 1: Verfahrensbeschreibung", A. Lohmann, October 1942.
Ref. 24: p. 200-204 in "Rotating beacons", Section 4.12 of "Radio Aids to Civil Navigation", Reginald Frederick Hansford (ed.), Heywood & Co. Ltd., 1960, 623 pp.
Ref. 25: §57, 58 in "G.A.F. night fighters - R.A.F. Bomber Command countermeasures and their influence on German night fighter tactics", A.D.I.(K) Report No. 599/1944, Air Ministry, Assistant Director of Intelligence - Prisoner Interrogation, Wing Cmdr. S.D. Felkin, 2 Nov 1944, 16 pp. Source:
Ref. 26: “Bordfunkgeräte - vom Funkensender zum Bordradar“, Fritz Trenkle, Bernard und Graefe Verlag, 1986, 283 pp., ISBN 3-7637-5289-7
Ref. 26A: p. 61-63, "Kommandoübertragungszusätze".
Ref. 26B: pp. 97-103, "Leitstrahl-Verfahren" (beam methods).
Ref. 27: "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. 28: "Ernst L. Kramar - Pioneer Award 1964", IEEE Trans. on Aerospace and Navigational Electronics, Vol. 11, Issue 2, June 1964, pp. 81-85.
Ref. 29: "A new field application for ultra-short waves", Ernst Kramar, Proc. of the IRE, Vol.21, Nr. 11, November 1933, pp. 1591-1531.
Ref. 30: "The present state in the art of blind landing of airplanes using ultra-short waves in Europe", Ernst Kramar, Proc. of the IRE, Vol.23, Nr. 10, October 1935, pp. 1171-1182.
Ref. 31: "Das Funk-Blindlandegerät" [Fu Bl I, EBl 1, EBl 2, Fu Bl 2, Werner Thote, pp. 20-25 in "Radiobote", Jg. 2, Heft 9, May-June 2007   See note 1
Ref. 32: "Beschreibung und Betriebsvorschrift für Funklande-Empfangsanlage Fu Bl 1 Ex", DTA 140, C. Lorenz AG, 1940, 59 pp. Source:
Ref. 33: "Die deutschen Funkführungsverfahren bis 1945". F. Trenkle, Dr. Alfred Hüthig Verlag, Heidelberg 1987, ISBN 3-7785-1647-7.
Ref. 34: "Most Secret War", R.V. Jones, Hamish Hamilton, 1978, 576 pp. See note 1 
Ref. 35: "Milestones - Battle of the Beams", Carlo Kopp, Defence Today, January/February 2007, pp. 76, 77.
Ref. 36: "The Battle of the Beams - Part 1-3", D.V. Pritchard (G4GVO), Ham Radio Magazine, June 1989 pp. 29-38, August 1989 pp. 20-29, October 1989 pp. 53-61.
Ref. 37: "Pulling the crooked-leg", R.V. Jones, New Scientist, 23 February 1978, pp. 493-496.
Ref. 38: "Navigational Aids for Bombers", sheet 3-6 in Section 0.1 of A.L. No. 46 of Sub-Committee for the Investigation of German Electronic and Scientific Organisation (SIGESO), 12/12/1945, Report Vol. 1, Part 2. Source:
Ref. 39: "Equipment of a Y-Site", A.D.I.(K) Report No. 527B/1944, Air Ministry, Assistant Director of Intelligence - Prisoner Interrogation, Wing Cmdr. S.D. Felkin, 25 Sept 1944, 13 pp. Source:
Ref. 40: "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. 41: "Bordfunkgerät FuG 16 ZY mit Aufbauvorschrift für Antenne des Zielflug-Senders", D.(Luft)T.4069, 5 August 1944, 114 pp. Source:

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