- [Antenna system]
- [Concrete ring & circular rail track]
- [Locomotive system]
- [Central-support building]
- [Monitoring antenna mast & receiver]
- [Electrical power]
- [Unknown/unclear aspects]
Last page update: 29 January 2017
- ["Bernhard/Bernhardine" Luftwaffe radio-navigation system]
- [FuG120 "Bernhardine" airborne Hellschreiber printer system]
- ["Bernhard" station locations]
"Bernhard" is the German codename for the ground-station ("Stellung", "Anlage") of the "Bernhard/Bernhardine" radio-navigation system that was used by the Luftwaffe during part of WW2. The beacon ground-station is a complete radio transmitter installation - including the antennas. A complete installation is a "Funk Sende-Anlage", abbreviated "FuSAn". The FuSAn developed for the "Bernhard" system was FuSAn 724. Its two transmitters had an output power of 500 W. The available literature often refers to "Bernhard" as FuSAn 724/725. The 725-version was intended to have more powerful transmitters: 5000 W. However, there is no evidence that these transmitters were ever developed and entered into service. Ref. 20 and 21 state that they were planned only. The exact reasons for the power increase is unknown, but would typically be extended range and improved immunity against interference and jamming. The FuG 120 "Bernhardine" is the airborne counterpart of the "Bernhard" beacon. It comprises a Hellschreiber-printer and control electronics. It printed the bearing data transmitted by the selected "Bernhard" beacon. Note that over a dozen different fixed and mobile FuSAn types were developed for various radar and radio-navigation systems of the Luftwaffe, primarily by Telefunken, Lorenz, and the DVL (Deutsche Versuchsanstalt für Luftfahrt), ref. 141.
The "Bernhard" FuSAn comprised a large rotating antenna system (ca. 25 x 35 meter). The main sub-systems of this rotating navigation beacon are (see Figure 1):
- The upper structure, consisting of:
- a large antenna system that comprises three antenna arrays,
- a cabin with the transmitters,
- a small square "shed" near each of the four corners of the cabin.
- A large concrete ring, with
- a circular rail track, and
- four locomotives for rotating the upper structure.
- A small round central-support building in the middle of the ring.
- A remote antenna mast, for monitoring the signals transmitted by the beacon.
- Sources of electrical power.
Fig. 1: The "Bernhard" ground-station of the "Bernhard/Bernhardine" radio-navigation system
(click here to get full size)
Basic characteristics of the "Bernhard/Bernhardine" system are:
- Frequency: 30 - 33.1 MHz.
- Transmitter power: 2 × 500 watt (FuSAn 724) or 2 x 5000 watt (FuSAn 725). There is no evidence that the 5 kW transmitters were ever developed and entered into service; ref. 20 and 21 state that they were planned only.
- Antenna system dimensions: ≈28 x 35 m (HxW, 92x115 ft).
- Antenna system track diameter: 22.6 m (74 ft).
- Antenna system weight: 120 tons (265000 lbs,
ref. 21); some literature states the weight as 102 tons (ca. 256000 lbs), or 100 tons (ref.
- I have tried to do a "sanity check" on this number. With many assumptions, I arrived at at least 50-60 tons (see this spreadsheet). However, if extreme precision is required (ref. 20 suggests an unrealistic 1 mm !), an extremely stiff construction would have been necessary. This implies a much heavier construction. One of these days, I will update my analysis...
- Antenna rotational speed: 12 degrees per second (2 revolutions per minute). This means that the small locomotives that turned this enormous antenna installation, moved at a respectable speed of about 8.3 km per hour (5.2 mph).
- Accuracy: initially ±1°, then improved to ±0.5°, finally reduced to ±4° by using a single-trace ( = simpler) printer system and a single transmitter (though unclear if this was ever operational).
- Operational range: 150-500 km (80-270 nm), depending on aircraft altitude with respect to the "Bernhard" antenna (p. 22 in ref. 15).
Below is a 1945 US Intelligence synopsis of the Bernhard system. It appears to confuse the "Bernhard" system ("Windjammer") with the "Y-System" ("Benito"). Contrary to the Bernhard system, the Y-System could provide "slant range", i.e., line-of-sight distance (not distance-over-ground) between the aircraft and the ground-station. The referenced frequency also belongs to the "Y-System" (42-48 MHz), not the Bernhard system (30-33.1 MHz).
Fig. 2: Flawed 1945 U.S. description of the "Bernhard" system
(source: ref. 13)
The "Bernhard" ground station is the rotating radio-navigation beacon of the "Bernhard/Bernhardine" system. This means that its transmission is not simultaneous in all directions (i.e., omni-directional), but it sweeps the horizon with its directional radio beam. Secondly, it is a 2-channel transmission: one channel is used to transmit the azimuth value of the beam-direction in Hellschreiber-format, the second channel is used to transmit a signal that disappears when the associated beam is pointing straight at the antenna of the navigation-radio receiver in the aircraft. So, how is this all done? Clearly, we need two antenna sub-systems: one for each transmission channel. The antenna sub-system for the azimuth-data must create a single-beam radiation pattern. The antenna sub-system for the pointer-channel must create a twin-beam radiation pattern, with a sharp null between the two beam. That is, two narrow beams, that point slightly to the left and right of the centerline of the single-beam. To get the sweeping effect, the two antenna systems must be aligned (pointing in the same direction), and be continuously rotated together.
Let's look at the actual antenna systems to see how all of this was implemented. Figure 3 below shows three development stages of the antenna system of the initial UHF Bernhard system. It operated at a frequency of 300 MHz, which is equivalent to a wavelength of 1 meter. The fourth development form (1940) used a parabolic dish antenna with an aperture of 6 λ (ref. 3), which translates to almost 10°. I have no photos of this installation.
Fig. 3: Three antenna systems developed for the UHF-version of "Bernhard"
(left: 1935 (twin-beam only), center: 1936 , right: 1940; source: ref. 3)
The antenna system in the center photo of Figure 3 most clearly shows the arrangement of a large number of vertically-oriented antenna elements:
Fig. 4: The three antenna sub-systems and the resulting radiation pattern (top view)
(dipole arrays diagram: adapted from Reichspatent 767532)
The antenna sub-system at the top (in the green box) shows a group of five identical, parallel antennas that are interconnected. These antennas are simple dipoles. As the name suggest, a dipole has two identical "poles", typically straight metal rods or wires. The two halves of the dipole are connected to the transmitter via a 2-wire feedline. The next figure illustrates the radiation pattern of a single, vertically oriented dipole antenna in free-space (i.e., sufficiently far from other objects and ground). It is a doughnut-shaped omni-directional pattern - which is not what we want!
Fig. 5: The omni-directional radiation pattern of a single vertical dipole antenna in free-space
(left: 3D pattern of the strength of the radiated signal; center: vertical cross-section of the pattern, right: top view)
So how do we get a directional beam-pattern? This requires that we concentrate the transmitted energy in the desired direction(s), and radiate less energy in all other directions. This can be done in (at least) three ways (ref. 139K-a):
- By placing a reflector surface behind the (dipole) radiator.
- By using a "longwire" antenna (such as a Rhombus antenna), with a radiator length of several wavelengths. This is not very practical for a rotary antenna system that operates at a wavelength of ca. 10 m.
- By combining the radiation pattern of multiple dipoles.
or a combination thereof.
Let's keep it simple and use several identical dipoles that are placed in parallel, and arranged on a straight line ( = linear). As all dipoles lie in the same plane, this is called a planar array. Let's use the same spacing ( = equi-distant) between adjacent dipoles: a linear array. We feed all dipoles of this array with the same radio-frequency current (= same amplitude and same phase). I.e., a uniform distribution of the dipole excitation. The in-phase (co-phase) feeding makes the array concentrate the radiated energy in the direction that is perpendicular ( = broadside) to the plane that is made up by the parallel dipoles. We now have a uniform linear broadside array. The actual pattern depends on the distance between the dipoles. Figures 6 and 7 below show this dependency for an array of 3 and of 4 parallel dipoles, respectively. This is beginning to look like what we want!
Fig. 6: Top view of radiation patterns of a uniform 3-dipole broad-side array (dipoles spaced by 0.2 - 1 λ)
Fig. 7: Top-view radiation patterns of a uniform 4-dipole broad-side array (dipoles spaced by 0.1 - 2 λ)
We can make several general observations:
- The direction with maximum radiation intensity, is independent of the number of parallel dipoles and their spacing.
- For spacing larger than 0.1 λ, all patterns have one or more clear side-lobes, in addition to the "forward" radiation of the main beam. This is not desirable.
- For spacing equal to, or larger than, half a wavelength (0.5 λ), some side-lobes become very significant and approach the strength of the main beam. These are called grating lobes. This is also not desirable. Note: maximum gain for a given number of dipoles is typically obtained for a spacing of 0.5 to 0.7 λ.
- The main beam becomes narrower ( = directivity) and stronger ( = gain), as the number of dipoles and/or spacing increases. This is desirable. However, each additional dipole adds less gain than the previous addition ("law of diminishing returns"). The latter is illustrated in Figure 8A below. Also, if the number of dipoles is increased without increasing the overall size (span) of the array, then the directivity is reduced (main beam becomes wider), ref. 139J.
- As, for a given number n of dipoles, the spacing increases, the number of (equal strength) maxima increases. Between these maxima, there are n - 2 smaller lobes.ref. 139J.
Fig. 8A: The radiation pattern of uniform broadside arrays of 1-8 parallel dipoles (spacing < 0.5 λ)
(source: Figure 3.36 in ref. 139A)
Fig. 8B: Radiation pattern of uniform broadside array of 3 parallel dipoles (spacing = 0.5 λ) - without reflector screen or dipoles
(left: array configuration; center: 3D radiation pattern; right: top view of the radiation pattern; my 4NEC2 model is here)
How get can we reduce, or even eliminate, the side-lobes and rear-lobe? There are several techniques, with varying degrees of complexity. The basic parameters that we can adjust, are amplitude and phase of the dipole current, spacing between the dipoles, and the use of reflectors (ref. 139A-139H):
- One way is to not feed all dipoles with the same current amplitude. That is, non-uniform excitation instead of uniform. Instead, use amplitudes that taper off towards the dipoles at both ends of the array. This is called current "grading" or "tapering". The amplitude coefficients should be equal to those of a binomial series. For dipole spacing less than 0.5 λ, this eliminates all side-lobes. Current-grading coefficients may also take the form of a Fourier-series distribution, Power-of-Cosine distribution, Taylor-series distribution, etc. However, this approach very significantly complicates the system for feeding the dipoles. Also, the uniform distribution produces the highest directivity (see p. 518 in ref. 139H), though the first side-lobe is at best only about 13 dB down from the main-lobe.
- Side-lobes can be reduced by putting a reflector surface behind (and parallel to) the plane of the dipoles. This is what we see in the three "Bernhard" antenna systems of Figure 3 above. For UHF frequencies (λ ≤ 1 m), constructing such a conductive "mirror" surface. Likewise for small VHF arrays (2-3 dipoles). Reflectors are typically placed at a distance of 0.25 λ from the dipoles (1939 Telefunken/Lohmann patent 767531, lines 114-116; also p. 18 in ref. 138).
- For true broadband operation (i.e., a bandwidth of at least +/- 10% about the center frequency, whereas "Bernhard" is "only" +/- 5%), a reflector distance of 0.34 λ actually provides much less variation in feedpoint impedance, with that impedance closer to being purely resistive (ref. 139K-b).
- Instead of a solid metal surface, as wire mesh may be used, as long as the size of the openings is less than 0.1 λ. Instead of a mesh, it is also possible to use a sufficiently dense "curtain" of un-tuned wires that are parallel to the dipoles.
- To get the desired effect, the planar surface or mesh may have to be extended around the array. I.e., the array is placed inside a shallow metal box or wire basket, that is open in the broadside / forward direction of the array.
- Instead of a reflector surface or mesh, a reflector-dipole can be placed behind (and parallel with) each primary dipole, e.g., at a distance of 0.25 λ (e.g., p. 18 in ref. 138). The reflector-dipoles may be passive ("parasitic"). In this case, they are not connected to the transmitter, but receive radiation from the other dipoles and re-radiate it. The reflector-dipoles may also be actively powered by the transmitter. The result of the latter method is illustrated in Figure 9 below.
Fig. 9: The radiation pattern of uniform broadside arrays with identical array placed 0.25 λ behind it, fed with 90° phase difference
(source: Figure 3.42 in ref. 139A)
Figure 10 shows my simulation of the effect of placing a reflector mesh-screen (0.1 λ mesh openings) at 0.25 λ behind the 3-dipole array: a significant reduction (14.4 dB) of the side-lobes (compared to Fig. 8B above).
Fig. 10: Radiation pattern of 3-dipole broadside array with a reflector screen (not a cage)
(left: array configuration; center: 3D radiation pattern; right: top view of the radiation pattern; my 4NEC2 model is here)
Radiation pattern of the 3-dipole array with reflector screen - rotating at 2 rpm
(simulation in free-space, i.e., effect of the earth's surface not taken into account)
So, now we basically know how to create a single-beam pattern with acceptably small side-lobes. But how do we get the required twin-beam pattern? How about using two separate broadside uniform arrays that are placed side-by-side? Then we "just" have to create a small angular difference between the two main beams. This can be done both mechanically and electrically. The mechanical approach was used in the small version of the "Knickebein" beacon system. The left-hand and right-hand dipole arrays are clearly placed at an angle, and the two beams cross-over in front of the antenna system. See Figure 11. Looking down onto the antenna system, the two sides form a shallow "V". Hence the name "Knickebein" ("crooked leg" = "dog leg").
Fig. 11: Antenna system of the small "Knickebein" beacon
The electrical method for pointing the main beam of a single broadside-array in a direction other than perpendicular, is called "beam slewing" or "electronic beam steering" (see Section 3.14 / pp. 271 in ref. 139A). Here, the dipole-current in the dipoles of one half of the array, is given a phase-angle difference with respect to the dipoles in the other half of the array. The size of the phase-offset determines the amount of beam-slew or beam-tilt. To get a twin-beam pattern, one could use two such split-arrays and put them side-by-side.
However, this is unnecessarily complicated for what we are trying to achieve! All we need to do, is put two uniform linear broadside arrays side-by-side, and simply feed them in an anti-phase manner (180° phase difference between the two arrays). See p. 72 in ref. 137. The resulting radiation pattern has two main-beams, symmetrically with respect to the normal (perpendicular) direction, with a small angle and a sharp, deep null between the two beam directions. See Figure 12 for a 4+4 dipole array configuration. Other than the rear-lobes, this is what we want!
Fig. 12: Radiation pattern of two side-by-side arrays that are fed anti-phase (without reflector screen or dipoles)
(left: array configuration; center: 3D radiation pattern; right: top view of the radiation pattern; my 4NEC2 model is here)
Before discussing the VHF-version of "Bernhard", there are two aspects of the UHF antenna system that need to be mentioned:
- The configuration of the twin-beam dipole array.
- Feeding a rotating antenna system from stationary transmitters.
The twin-beam antenna system comprised two side-by-side arrays (see the magenta boxes in Figure 4 above). The photo shows that each array actually comprised two sub-arrays of five dipoles each. These sub-arrays are placed one above the other (a so-called "stacked" array). This was either done as part of side-lobe suppression (see Section 3.16 (p. 276) in ref. 139A), or to increase gain. However, the latter is less likely. The system used two identical transmitters (i.e., same output power). The power of one of these transmitters was split in two, for the two halves of the twin-beam array. Increasing the gain of the twin-beams would increase the operational range of the beacon, but it would not make sense for the twin-beam system to have a significantly larger (or smaller) range than the single-beam system above it.
In the UHF "Bernhard" installations, the transmitters did not rotate with the antenna system: they were located in the small stationary building below the antenna system. This arrangement requires a rotary coupler between the antenna system and the transmitters. This may have been implemented as a set of slip-rings on the shaft of the rotating antenna system. Note that a slip-ring approach ("HF-Schleifringkopplung") was actually used in several German systems, for instance the FuMG 404 "Jagdschloß" radar (designed by GEMA, built by Siemens). Ref. 151. The largest version had a 24 m wide array antenna system (4 x 16 horizontal dipoles) that weighed 25-30 metric tons. It rotated at 10 rpm, with the central shaft driven by a 75 kW 3-phase motor. It was a UHF system (120-240 MHz, depending on the version). It transmitted 1- 2 μsec pulses of 8-20 kW, with a pulse repetition frequency (PRF) of 500 / 3000 Hz. However, there is also a 1936 Telefunken patent (nr. 767525), by Adalbert Lohmann. He was Telefunken's expert on rotary navigation beacons, including the Bernhard system. This patent proposes a method for a contact-free rotary coupler: no contact resistance, no arcing at brushes! The coupler comprises two sets of stator and rotor disks that form capacitors. See Figure 13. The disks are installed coaxially: the rotor plates are fixed to a rigid shaft, the stator plates to the housing of the coupler. The transmitters are wired to the edge of the respective stator plate. Note that in the VHF (30 MHz) Bernhard system, the transmitters were located on the same rotating platform as the antenna system. Hence, no rotary couplers were required for between the transmitters and the antennas. Of course, now electrical power for the transmitters must be provided via slip-rings.
Fig. 13: Contact-free rotary coupler for RF signals
(source: the 1936 Telefunken/Lohmann patent 767525)
The VHF "Bernhard" system operated at frequencies in the 30-33.1 MHz band instead of 300 MHz. That is, a nominal wavelength of 9.5 m instead of 1 m. This basically means that the antenna system of the VHF "Bernhard" is about ten times as big as its UHF predecessor. Figure 14 below shows the large antenna system. It measured about 35x25 m (WxH, ≈82x115 ft). We recognize the upper dipole array (green box) for the single-beam transmission, above the two side-by-side arrays (magenta boxes) for the twin-beam transmission.
Fig. 14: The two antenna sub-systems of the Be-10 "Bernhard" station at Thisted/Denmark
Note that the dipole-array configurations are not the same as those of the UHF "Bernhard" system:
- The single-beam array now comprises three parallel dipoles instead of five. Clearly, having used arrays of five dipoles would have made the entire antenna system much larger.
- There is no solid reflector surface behind the top-array. Instead, there is a wire-mesh reflector behind and around the array. Obviously the mesh is much lighter, and also has less wind resistance. It is placed at a distance of 0.25 λ from the dipoles.
- The twin-beam array now comprises two side-by-side sub-arrays of four dipoles each, instead of five. These two sub-arrays are fed in an anti-phase manner (180° phase difference between the two arrays).
- There is no reflector surface behind the bottom-array. Instead, there are reflector-dipoles. They are placed at 0.25 λ behind the primary dipoles. The reflector-dipoles are "active", i.e., powered by the transmitter. Active reflectors are more effective for side-lobe reduction than passive/parasitic reflector dipoles or rods (see p. 71 in ref. 137). The phase angle of the excitation of the reflector-dipoles leads that of the main dipoles in front of them by 90°.
- There are two different configurations of reflector-dipoles, see Figure 15A/B:
- a reflector-dipole behind each primary dipole (Figure 15A).
- a reflector-dipole only behind the two primary dipoles of each sub-array that are closest to the centerline of the antenna system (Figure 14 and 15B).
Fig. 15A: Top view of the 2x(4+4) array configuration of the twin-beam antenna
Fig. 15B: Top view of the 2x(4+2) array configuration of the twin-beam antenna
Fig. 16: Radiation pattern of two side-by-side arrays that are fed anti-phase (with active reflector dipoles at 0.25 λ)
Radiation pattern of the 2x(4+4) arrays - rotating at ≈2 rpm
(simulation in free-space, i.e., effect of the earth's surface not taken into account)
Below are illustrations of the radiation pattern of the "Bernhard" antenna system. The left and center image are from pre-WW2 Telefunken/Lohmann patents, and therefore relate to the UHF-version of "Bernhard". However, they are equally applicable to the later VHF-version of "Bernhard" (see p. 95 in ref. 3). Note that the pattern has significant side-lobes and rear-lobes. This is confirmed by the official manual of the FuG 120 "Bernhardine" bearing-printer system that was used in the aircraft (p. 6 in ref. 15). Note the consistency between the twin-beam radiation patterns in Figure 16 and 17.
Fig. 17: Single-beam and twin-beam radiation patterns of the "Bernhard" antenna system
(left: Figure 2 in the 1936 patent 767354; center: Figure 1 in 1938 patent 767523; right: Figure 4.28 in ref. 24)
Based on available photos, the 2x(4+4) configuration was used at the "Bernhard" installation of Trebbin (Be-0), Mt.-St.-Michel-de-Brasparts (Be-2), and La Pernelle (Be-7). The 2x(4+2) configuration was used at the installation of Bredsted (Be-9), Thisted (Be-10), and Arcachon (Be-14). This suggests that the transition between the two configurations was made around station Be-8 in Schoorl. Why remove the outermost reflector-dipoles? Figure 18 shows the difference in the 3D radiation pattern of the two configurations. Without the outermost reflector dipoles, there are more (and slightly stronger) rear-lobes. However, this may not necessarily have had a negative impacted on the performance of the "Bernhard/Bernhardine" system, based on the time-constants of the automatic receiver-gain control of the SV 120 printer-amplifier in the aircraft.
Fig. 18: Radiation pattern of the 2x(4+4) array configuration (left) and of the 2x(4+2) configuration
Transmitter power output (TPO) is the radio-frequency (RF) power that the transmitter produces at its output. This is not the same as the effective radiated power (ERP) of the antenna system. ERP is basically the product of transmitter output power, losses in the feedline system (and radiation by that system) including splitters, and gain of the antenna system in the direction of maximum gain. ERP is referenced to the same TPO being input to a single dipole. The "Bernhard" dipole arrays concentrate the RF energy into the main lobes of the radiation pattern. This provides a gain with respect to a omni-directional reference antenna. Based on my simulations of the antenna arrays, the main lobes of the twin-beam pattern had a gain of 13.81 dBi. That is: 13.81 dB compared to an isotropic radiator (i.e., 13.81 dBi). ERP is referenced to a dipole, which has a gain of 2.15 dBi. Hence, the twin-beams had an estimated gain of 13.81 - 2.15 = 11.66 dBd. With a 500 watt "Bernhard" FuSAn 724 transmitter, this would have resulted in an ERP of 7.3 kW - far from the unrealistic 5 MW ( = 40 dB) that is sometimes suggested in literature.
Figure 19 below illustrates the length, thickness, and spacing of the dipoles, as well as the spacing between the dipoles and the reflector-cage. The size of the two men in the photo can be used as a reference. Note that the VHF "Bernhard" operated at a frequency of 31.55 MHz ±1.55 MHz. Hence, the nominal wavelength λ is 9.5 m.
Fig. 19: Dimensions and spacing of the dipoles and the wire-mesh reflector cage
The table below shows the dipole and spacing dimensions for three different assumptions regarding the height of the men in the photo above:
Fig. 20: Estimated dipole and spacing dimensions, based on photogrammetric analysis
I have run antenna simulations for about 30 different combinations of parameters in Figure 20, for the array of 2x(4+4) dipoles. A table with the results is here. General conclusions are consistent with antenna theory:
- When the dipole length is decreased, gain of the main beams decreases (not desirable), the front-to-back ratio decreases (not desirable), and the strength of the first left & right side-lobe increases relative to the two main beams (not desirable)
- The gain of the two main beams is relatively insensitive to spacing between the front dipoles and the reflector dipoles.
- The strength of the first side-lobes left & right of the two main beams, is relatively insensitive to the variation of the parameters. The relative strength is -9 to -12 dB.
- The strength of the other side- and rear-lobes is sensitive to spacing between the front dipoles and the reflector dipoles, and to the dipole diameter.
- The width of the two main beams is relatively insensitive to array dimensions, other than the number of dipoles. The beam-width is 10°-12°.
- The angle between the two main beams is relatively insensitive to array dimensions, other than the number of dipoles. The angle is 20°-24°.
- The total number of pattern-lobes increases when lateral spacing between the dipoles is increased. For the evaluated configurations, this even number varied from 10 to 16.
As in the UHF "Bernhard" system, the dipoles are "full-wavelength": they have a nominal length of 1 λ. Note that this is contrary to the original 1936 Telefunken/Lohmann Reichspatent nr. 767531 and 767532, in which all dipoles are "half-wave" (½ λ). Why use dipoles 1 λ dipoles instead of ½ λ dipoles?
- The radiation pattern of a 1 λ dipole has slightly more gain and slightly narrower beams than that of a ½ λ dipole.
- 1 λ dipoles are more suitable for broad(er)band operation than antenna systems with ½ λ dipoles
- A 1 λ dipole may be harder to properly match to a modern solid-state transmitter via a 50 Ω coax cable than ½ λ dipole (at resonance), but that is not a real argument against 1 λ dipoles when using a transmitter with tube amplifiers in combination with an open-wire feedline.
- The mid-point of each "leg" of a 1 λ dipole in principle has (near) zero voltage. This makes it a convenient point for attaching the dipole to a support structure. This is discussed further below. The neutral point of a ½ λ dipole is the feedpoint, which is not convenient for attachment.
- Implementing a uniform array is easier with 1 λ dipoles. They have high feedpoint impedance. Therefore, they are voltage-fed instead of current-fed. A feed-system for supplying all dipoles of an array with same amplitude and phase, is easier with voltage than with current. The dipoles need to be fed in-phase ( = co-phase). That is, the phase difference between all dipoles within a sub-array must be 0° = n x 360°, where n is an integer number. Half of the 360° is simply obtained by having a feedline length of ½ λ = 180° between adjacent dipoles. Obviously, with a 2-wire feedline, this is easy to do: just space the dipoles ½ λ (with a small adjustment for the velocity factor of the wire). A tuned ½ λ section of feedline also has the advantage that it does not act as an impedance transformer: the impedance at one end, is transferred 1:1 to the other end. The second half of the required 360° is simply obtained by connecting the top element of the even dipoles and the bottom element of the odd dipoles to the same feedline wire, and vice versa. I.e., switching polarity at each dipole. The latter switch-over can be implemented two ways, see Figure 21:
- By crossing the feedline wires between dipoles. This (standard) approach is illustrated in the 1936 Telefunken/Lohmann Reichspatent nr. 767531 and 767532, However, with this method, the feedline wires are not perfectly parallel. This may disturb the characteristic impedance Z0 of the feedline.
- Without crossing the feedline wires between the dipoles. Instead, the cross-over is made at the feedpoint of every other dipole (see p. 72 in ref. 137).
Fig. 21: In-phase voltage feeding of a uniform array of 1λ dipoles
Available documentation does not mention the spacing between the feedline wires and the diameter of those wires. It can also not be determined accurately enough from the photos. Hence, the characteristic impedance of the feedline cannot be calculated. However, it was probably at least several 100 Ω. As shown in Figure 21 above, the feedline from the transmitter was attached to the end of each 4-dipole sub-array. The 3-dipole top array was fed at the center dipole. This provided a sufficiently broadband feed system for the "Bernhard" operating frequency range of 31.5 MHz +/- 5%. See Fig. 2 in ref. 139K-c. Note that open-wire feedlines do have some disadvantages (ref. 139K-d):
- the wires always radiate to some extent, which may affect the radiation pattern of the antenna system
- snow and rime accretions affects the impedance characteristics
- depending on their placement, capacitance between insulators (see Fig. 23 below) affects the impedance characteristics.
Thick dipole radiators have a large cross-section. At the dipole feedpoint these large cross-sections would be facing each other and form a significant capacitance. This is undesirable. Furthermore, the large cross-section of the radiators must be connected to the feedline wires. The feedline wires have a much smaller diameter, typically no more than several mm. A large step-transition in conductor diameter is also undesirable in antenna systems. A standard solution is to give the feedpoint-end of the dipole radiators a pointed shape, like the tip of a sharp pencil. See p. 70/71 in ref. 135. See Figure 22. The pointed tip can also easily be adapted to implement a connection to the feedline wires without having to cross those over between adjacent dipoles.
Fig. 22: Conical feedpoint-end of the dipole radiators
Fig. 23: Pointed radiator-feedpoint tips of several "Bernhard" installations and of a small "Knickebein"
The dipole radiators were made of large diameter tubing ("tube" is specified by outside diameter, "pipe" by inside diameter). Ref. 13. A solid rod would have been much heavier, cost a lot more material, and not perform better as an antenna: only the "skin" radiates,. It is unknown what material they were made of: steel, copper, brass,... Copper would have had less losses, but steel pipe would have been much more easily available, stronger, and could easily be welded to an attachment arm.
Figure 24 below shows the standard textbook diagram of the sinusoidal distribution of current and voltage along the radiators of a full-wave dipole. The curves are only valid for "vanishingly thin" radiators.
Fig. 24: current & voltage distribution of a full-wave dipole in free-space
(note: only valid in free space (far enough away from ground and objects), and for very thin radiators)
When the radiators have a diameter that is not infinitesimally small, the current distribution is no longer purely sinusoidal - see Figure 25 below. The current at the feedpoint becomes significant. This explains the reduction in feedpoint resistance when the diameter of the radiators is increased (i.e., the λ/d ratio is decreased).
Fig. 25: Current distribution of a thick full-wave dipole in free-space
(dashed line: Fig. 4.9 in ref. 135; solid line: my 4NEC2 model)
The feedpoint resistance and resonance length of a dipole depend on the ratio of the wavelength λ, and the diameter of the dipole radiators. The diagram below shows this dependence for a single dipole. For the same relative thickness, a full-wavelength dipole has bandwidth that is about 1.3x larger than that of a half-wave dipole (see Section 4.3 in ref. 135). Note that each "Bernhard" system operated at a fixed frequency in the range of 31.5 MHz ±5%. It was obviously highly desirable to be able to use the same dipoles at all "Bernhard" installations. This required a relatively broadband antenna system, which was facilitated by using full-wave dipoles.
Fig. 26: Feedpoint resistance and resonance length as function of the diameter of the dipole radiator
(source: figure 4.7 in ref. 135)
Photogrammetric analysis of the available photos suggests that the "Bernhard" dipole radiators were quite thick: a diameter of about 9 cm (3½ inch) for a wavelength of about 9.5 m. I.e., a ratio λ/d ≈ 100. Based on this λ/d ratio and the graph above, the dipoles should have a length of ≈0.87 λ, not 1 λ. This is consistent with the photogrammetric estimate of the actual dipole length. The bandwidth of a dipole not only depends on its length, but also on its diameter. A dipole has a feedpoint impedance that consists of resistance and reactance. The resistance is relatively insensitive to the dipole diameter, but the reactance (capacitive or inductive) is not! The thinner the dipole, the more the reactance changes for a given frequency change away from resonance. In other words: the smaller the bandwidth. So: thicker is better!
The voltage distribution in Figure 24 above shows that the voltage is zero at a distance of ¼ λ from the feedpoint. That is: at the mid-point of each dipole radiator "leg". This means that this "neutral" point can be connected to ground, without affecting the performance of the antenna. This makes it a convenient point for attaching a dipole leg to the structure of the antenna system, without needing some form of insulation. However, this applies only to a single dipole in free-space. In a real dipole, the voltage and current distribution are distorted due to coupling with nearby objects (in particular other dipoles in the antenna system and the support structure) and ground (esp. with vertically oriented dipoles). Assuming that the same dipole length was used at all "Bernhard" stations, the position of the neutral point would also depend on the operating frequency of the particular station (one of 32 channel-frequencies in the 30-33.1 MHz range). From the available photos of "Bernhard" installations, it cannot be determined whether or not an insulated attachment was used. There is another advantage of attaching the dipole legs at mid-point: the high-voltage parts of the antenna (the feedpoint and the tips) can be kept away from the antenna support structure. Hence, the effect of stray capacitance ( = loss and pattern distortion) between the dipoles and the structure is minimized.
Fig. 27: Attachment of dipole neutral-points to the support structure of the antenna system
The same neutral-point attachment method was used in several dipole-array antenna systems, such as that of the FuG200 "Hohentwiel" ship-detection radar (ref. 136, multiple arrays of four 1λ dipoles with passive reflector dipoles, operating at around 550 MHz, i.e., λ = 55 cm), the DMG-3 microwave link (ref. 138), and the "Knickebein" beacons.
Fig. 28: Junkers Ju 188 with 4 arrays of 4 driven + 4 passive dipoles of the FuG 200 "Hohentwiel" radar
(right-hand image clearly shows crossed-over feedline wires between the dipoles )
Fig. 29: Antenna sub-systems of a small "Knickebein" beacon - 2x(4+4) driven full-wave dipoles
(source left-hand photo: ref. 33; right-hand: France 1941, Bundesarchiv Bild 101I-228-0322-04)
The antenna systems were built by Hein, Lehman & Co., Eisenkonstruktionen, Brücken- und Signalbau of Berlin-Reinickendorf. This company was incorporated in 1888, and was active in sheet metal, steel constructions, bridges, railway signals, hangars for "Zeppelin" dirigibles, etc. Ref. 85H, 140.
The company had a department ("Abt. Funkbau") that constructed and installed (very) large antenna masts and towers ("Funkmaste, Funktürme") for Telefunken (incl. the "Funknachrichten und Navigation" dept.). Examples are the famous Funkturm (radio tower) in Berlin-Charlottenburg (1926, still standing tall to this date (2016)), and antennas for the Langwellensender around the world, notably at Lahti/Finland (1928), Nauen/Germany, Kootwijk/The Netherlands, and Sidney/Australia. Also the huge antennas of the gigantic 1 megawatt Goliath VLF transmitter station of the Kriegsmarine. It was used for worldwide communication to (submerged) submarines - including broadcasts in Hellschreiber format.
The antenna structure of the "Bernhard" station Be-7 at La Pernelle/France is reported as having grey-black camouflage colors (ref. 128).
Fig. 30: British intelligence compiled by RAF Photographic Reconnaissance Units (P.R.U.) and "agents" (e.g., members of the resistance movement in various countries, such as Yves Rocard in France, ref. 128)
(source: plate 15 in ref. 34, reproduced in ref. 91)
Not surprisingly, given its size and shape, the Bernhard ground station was easily mistaken for a German radar installation of the era:
Fig. 31: The "Bernhard" antenna system was easily (and often) mistaken for a German radar installation
(source: Figure 26b in ref. 34, p. I-30 in ref. 13)
THE TWO "BERNHARD" TRANSMITTERS
The photo below show the two 20 watt transmitters of the the 1936 UHF "Bernhard". The drum-shaped object between the transmitters may have housed the optical disk with the 360° compass-rose in Hellschreiber format.
Fig. 32: The two 20 watt transmitters of the 1936 UHF "Bernhard" system
(source: p. 95 in ref. 3)
The VHF "Bernhard" beacon FuSAn 724 comprised two identical transmitters, each with an output power of 500 watt:
- One connected to the 3-dipole upper antenna array (to the center dipole, from where it was distributed to the outer two dipoles).
- One connected to the 2x(4+4) or 2x(4+2) dipole-arrays of the lower antenna. The transmitter output was split once, and equally divided between the reflector sub-arrays and the front sub-arrays. The latter had a 90° phase delay. This was probably implemented by simply making the feedline to the front-array ½ λ ( = 90°) longer than to the reflector-array.
I have no photos of the FuSAn 724 transmitters. Figure 33 shows other Luftwaffe beacon transmitters of approximately the same era and output power. The AS4 on the left had an output power of 500 W, measured 120x120x70 cm (WxHxD, without the feet; 4x4x2.3 ft) and weighed 402 kg (900 lbs). Ref. 143. The S 341 N had an output power of 700 W and operated on 33.3 MHz. It measured 175x175x70 cm (WxHxD, 5.8x5.7x2.3 ft) and weighed 1250 kg (2765 lbs). Ref. 2, 144. Both were crystal-controlled and modulated with a 1150 Hz tone.
Fig. 33: Two landing-beacon transmitters - Lorenz AS4 (left) and Telefunken S341N (right)
The planned 5000 watt transmitters of the never realized FuSAn 725 may have consisted of simply adding a final-amplifier stage and associated power-supply to each of the FuSAn 724 transmitters. I.e., using the latter as powerful modulators. This was not an uncommon approach. E.g., the 5 kW Lorenz Lo5000 "Ehrenmahl" transmitter used an 800 W "Ehrenmal" transmitter with its 3 kV power-supply, and added a 5 kW final-amplifier with a 7.5 kV power-supply. See Figure 34.
Fig. 34: Left-to-right - 3 kV power supply, 800 W transmitter (2 columns), 5 kW amplifier and 7.5 kV power supply of the Lo5000
(source: J.Bieschke, used with permission of seefunknetz.de)
The frequencies of the two FuSAn 724 transmitters were crystal-controlled and spaced by 10 kHz (ref. 24). The frequencies were chosen such that the two carrier frequencies were -5 KHz and +5 KHz with respect to the center of the (large!) bandwidth of the EBl 3 receiver used by the FuG120 "Bernhardine" printer system in the aircraft. This receiver had 34 fixed channels in the 30-33.3 MHz band, spaced 100 kHz. The lowest 32 channels were allocated to the Bernhard/Bernhardine system.
The EBl 3 is an AM receiver, so both transmitters were amplitude-modulated. The transmitter for the twin-lobe beam was modulated with a constant 1800 Hz tone (emission designator A2N). Ref. 15. Hence, the spectrum of the transmitted signal consisted of the carrier frequency and a sideband at 1800 Hz distance on both sides of this carrier. See Figure 35.
Fig. 35: RF-spectrum of an AM transmitter modulated with a constant 1800 Hz tone
The carrier for the mono-beam was modulated with a 2600 Hz tone. Hence, the spectrum of the transmitted signal consisted of the carrier (10 kHz away from the other carrier), and a sideband at 2600 Hz on both sides of the carrier. This is similar to the spectrum of the carrier modulated with the constant 1800 Hz tone. However, the 2600 Hz signal was not a constant tone! It was keyed on/off with the pixel-pulses in Hellschreiber-format of the compass-rose information that was to be transmitted. I.e., Modulated Continuous Wave (MCW), Amplitude Shift Keying (AFSK), emission designator A2A. The spectrum of a tone with frequency f0 that is keyed on & off with a square wave with period 2T, consists of a spectral line at f0, and an infinite Fourier-series of sideband lines at odd multiples N of 1/(2T) on both sides of f0. The height of the sideband lines decreases with N, via a (1/Nπ)2 relationship. See the left-hand image in Figure 36. Keying a tone on/off with a square wave signal is not particularly useful for conveying information. If the tone is keyed with a (quasi) random sequence of on & off pulses with duration T, then the sideband spectrum lines are "smeared" into half sine-wave envelopes. See the right-hand image in Figure 36.
Fig. 36: spectrum of a constant tone frequency that is modulated with fixed-length on & off pulses
(left: keying with a square wave with on & off pulses with duration T; right: keying with random sequence of such pulses)
The shortest Hellschreiber-pulses of the "Bernhard" system (one black or white pixel) had a duration T = 2.3 msec (ref. 15). This implies a shortest pixel cycle ( = 1 black pixel, followed by 1 white pixel) of 2 x 2.3 = 4.6 msec. A stream of such pixel cycles is a square wave with a period 2T = 4.6 msec. Hence, the spectral lines were spaced by 1000 / 4.6 = 217 Hz. Obviously, the stream of Hellschreiber pixels did not consists of alternating black and white pixels, all with the same 2.3 msec duration. The black pixel pulses were 1 to 10 pixels long, the white pulses 1 to 14. The tone-pulse transmitter limited the bandwidth of the sidebands to 400 Hz (ref. 15). This is consistent with the 1935 recommendation from Hellschreiber manufacturer Siemens-Halske, the Cable & Wireless company in the UK, and the Reichspostzentralamt (central office of the German national postal authority), to limit the transmitted bandwidth to 1.6 times the pixel-rate (here: 217 Hz, resulting in 1.6 x 217 = 350 Hz). Ref. 142. See the "Hellschreiber bandwidth" page. This is sufficient to ensure proper printing. Figure 37 shows what the combined RF-spectrum of the two transmitters looked like. I have not been able to determine if the frequency of carrier-2 was 10 kHz above carrier-1, or vice versa.
Fig. 37: RF spectrum of the "Bernhard" transmitter outputs
The audio output of the EBL3 shortwave AM navigation receiver consisted of the constant 1800 Hz tone from the "Bernhard" beacon's twin-beam antenna, and the 2600 Hz Hellschreiber tone pulses that represent the azimuth symbology. See Figure 38.
Fig. 38: Audio spectrum of the EBl 3 receiver output
To the best of my knowledge, no original "Bernhard" audio recordings exist today. So I have simulated the sound of a transmission. The recording below comprises two beam-passages: once without the azimuth information in Hellschreiber-format (i.e., only the constant 1800 Hz tone), and once the constant tone plus the constant stream of 2600 Hz Hellschreiber-pulses. The simulated beam-passage duration is about 3½ seconds. Actual beam-passage duration was 3-5 sec, depending on distance from the beacon.
Simulated sound of two "Bernhard" beam-passages - without & with Hellschreiber tone pulses
The photo below is the only one that I have of "Bernhard"-equipment related to the transmitters. The equipment stack on the right has (at least) two units related to modulation of the two transmitters.
Fig. 39: Equipment inside the rotating cabin of the Bernhard installation at Hundborg
(source: Figure 30 in ref. 93)
As stated before, one of the two "Bernhard" transmitters is modulated with a constant 2600 Hz tone. This tone is keyed on/off with the pixels of the compass-rose information (antenna bearing) and station identifier letter. These pixels were captured as a track on an optically encoded disk ("Kennzeichenscheibe"). The disk was 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 2600 Hz tone on/off.
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 mechanical 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.
Fig. 40: Optically encoded disk (only part of the pixel track shown)
(source: patent 767524)
Telefunken patents 767524 and 767354 also consider 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):
- on the same disk as the compass card data, integrated with the azimuth data track.
- on the same disk as the compass card data, but as a separate track.
- as a dedicated track on a separate disk, mechanically aligned and synchronized with the compass card disk. This has the advantage that the same compass card disk could be replicated for all ground stations, and the station identifier be put on a "personalized" disk. In case of a separate track (on the same disk or on a separate disk), the output of the associated photocell is simply combined (logical "OR") with the output of the photocell of the azimuth track.
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 (see Figure 41). 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"). The manual for the FuG 120 "Berhardine" printer system (see §14 in ref. 15) is consistent with that patent, in that it states that the light source was mounted above the disk and the photo cell below it.
Fig. 41: 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 rectangular keying pulses for the transmitter.
Below the antenna installation, and rotating with it, was a long cabin. The cabin was made of wooden planks. It was mounted on a large rectangular frame made of heavy I-beams. This frame was suspended from the large lattice truss-joist of the support structure of the antenna system. The cabin had full-height sliding panels on the outside, to protect the windows. At some "Bernhard" sites, these sliding covers are on the outside of the framework that suspends the cabin from the truss-joist. At other sites, these panels slide between the cabin and that framework.
Fig. 42A: The cabin at Hundborg/Denmark
Ref. 13 (p. 4.09) suggests that the cabin was divided into three sections. The section on the right (looking at the front of the antenna system) contained the transmitter equipment. The center section housed the controls of the four electric locomotives. The section on the left, and closest to the door, was a workspace. A plaque at La Pernelle states that it had one or more beds in it. However, the following photo shows antenna cables emanating from the bottom of a hexagonal box that is mounted to the right of the top of the door, and a ventilation screen below it, at floor level. This suggests that the transmitters were located in the section near the door...
Fig. 42B: The entrance end of the cabin at Bredsted
(source: Australian War Memorial photo SUK14634; also part of photo on page 5 in ref. 5)
There are no photos available of the cabin that were taken from the rear side of the antenna system.
Fig. 43: The wooden cabin below the antenna system
Photometric analysis of available photos suggests that the outside of the cabin measured approximately 4½x2½x19 m (WxHxL; ≈15x8x62 ft). The length of the cabin fits inside the concrete ring of the installation. The photo below is the only one that I have of the inside of the cabin. The height of the ceiling is consistent with the estimated cabin height.
Fig. 44: German engineer describing controls of the "Bernhard" system at Bredstedt to a member of the RAF-ADW
(source: Australian War Memorial photo SUK14636, public domain; ca. August 1945)
THE FOUR CORNER-SHEDS
At each of the four corners of the cabin, there is a "closet" of about 1½x1½x2 m (WxDxH; ≈5x5x6½ft). These corner sheds are mounted on a cantilever, away form the main cabin and above the rear bogie of the locomotive underneath. There are no openings for ventilation in the walls or the roof. No conduits appear to emanate from the bottom of the sheds. From the available photos it is clear that there are no windows, and it appears that there is no door. Whatever was in there, never required access! So it was not electrical or mechanical equipment, nor a container with break sand for the locomotives. Hence, the purpose of these sheds remains entirely unclear. Possibly they contained dead weight (stone, lead) or were actually a block of concrete, to get more weight on the traction wheels of the locomotives - even though the entire structure carried by the locomotives was already quite heavy by itself. No such blocks have been found at "Bernhard" sites were there still are visible remains...
Fig. 45: One of the cantilevered corner sheds at La Pernelle
(source: 1946 film clip Cinémathèque de Normandie)
At some sites, these sheds have a roof that is pointed with four sides (e.g., Be-7 at La Pernelle, Be-14 at Arcachon). At others, the roof is flat (e.g., Be-9 at Bredstedt, Be-10 at Hundborg).
Figure 46A: Corner-sheds with a pointed roof (Be-7 at La Pernelle/France)
Figure 46B: Corner-sheds with a flat roof (Be-10 at Hundborg/Denmark)
Unlike the main cabin, the walls are not made of wooden planks: there are no seams. In available post-war photos of "Bernhard" installations (La Pernelle, Arcachon), the main cabin is completely stripped of its materials - but not the four corner-sheds! Either the material was hard to remove and carry away, not valuable enough, or not usable for some other reason.
Fig. 47A: Post-war photo of Arcachon - installation dismantled and stripped, except for the corner sheds
Fig. 47B: Post-war photo of La Pernelle - installation dismantled and stripped, except for the corner sheds
THE CONCRETE RING AND CIRCULAR RAIL TRACK
The base of the entire rotating "Bernhard" superstructure is a large concrete ring. Standard width is 1.5 m (5 ft) and an outer diameter of almost 23 m (75 ft). The ring has a circular rail track on top of it, for the four locomotives that actually rotated the system. The superstructure (equipment cabin + antenna systems) is basically a large turntable.
Fig. 48: Concrete ring with the circular rail track
Fig. 49: Satellite image of the "Bernhard" site at Arcachon/France - overhead view (ca. 2013)
A rail track comprises two main elements:
- the track structure:
- steel rails
- fastening system. This typically takes the form of ties (crossties; UK: sleeper, D: Querschwelle), and fasteners to fix the rails to the ties. Their purpose is to maintain the correct distance between the rails ( = gauge), hold the rails upright, and transfer the loads from the rails to the underlying track bed and ground. Fasteners take the form of rail anchors, tie-plates, base-plates, sole-plates, rail-chairs (D: "Schienenstuhl"), bolts and nuts.
- the track bed or foundation: typically layers of ballast, sub-ballast, and subgrade (layers of crushed rock, gravel, and sand) on top of the natural ground. For applications with very high loading ("weight on wheels") the track bed may be "ballastless": a continuous slab of reinforced concrete on top of a subgrade. The latter is the case of the "Bernhard" track. This has a major advantage compared to the traditional track structure: no need for regular heavy maintenance (e.g., tamping the ballast and re-aligning the rails) to restore the desired track geometry and smoothness.
The standard rail profile of the Reichsbahn (and the Bundesbahn until 1963) was S 49 (weight 49 kg / m). It was, and still is, also used for narrow gauge tracks, tramway and subway tracks. It is unknown what rail profile was used for the "Bernhard" track, but it would have made sense to use the readily available national standard profile.
Fig. 50: Standard Reichsbahn rail profile S 49
The "Bernhard" track is "narrow gauge": the distance between the rails is about 1 m. I have not (yet) been able to determine the exact gauge (possibly available in ref. 85E). The documented "Bernhard" installations have at least six different fastening systems. In all cases, the rails are fixed in place either with 120 ties, or with 120 sets of anchors in the concrete.
- A: the concrete ring has 120 pairs of rectangular vertical holes, with a square dimple half way between them. Example: Be-15 Szymbark/Bytów. Possibly pre-fab ties were used that had a rectangular vertical post at both ends, and those ends were inserted into the vertical holes. The purpose of the dimples is unknown.
- B: the concrete ring has 120 pairs of rectangular vertical holes, but there is only a dimple for every third pair of those holes. Example: Aidlingen/Venusberg. There, the holes measure about 11x22 cm and are about 50 cm deep; the dimples are 17x17 cm. A pair of steel rods is anchored in the bottom of each rectangular hole. The part of the rods that sticks out above the ring is threaded (M20). The holes are not placed very accurately. However, as the upper part of the steel rods can be moved around, this is not an issue when installing the rail fasteners.
- C: the concrete ring has 120 pairs of rectangular vertical holes, but there are no dimples. There are no steel rods anchored in the holes. Examples: Be-12 Nevid/Plzň, Be-16 Sonnenberg/Hornstein.
Fig. 51A: The various forms of rail fastening and associated features in the concrete ring
- D: the concrete ring has no holes, but there are 120 sets of 2+2 threaded rods sticking out where there are vertical holes in versions A-C. This requires more accurate placement of the rods than in cases A-C. Examples: Be-7 La Pernelle, Be-11 Trzebnica/Trebnitz.
- E: the ties are sections of I-beam that are embedded into the top of the concrete ring; 2+2 vertical bolts are welded onto each end of the I-beams.
- concrete ring with a flat top. Examples: Be-10 Hundborg, Be-4 Le-Bois-Julien, Be-6 Marlemont, Be-2 Mt.-St.-Michel-de-Brasparts, Be-14 Arcachon. At Archachon, the I-beams are 15½ cm wide and 17 cm tall ( = standard I-beam per DIN 1025), and are embedded in a concrete layer that was poured separately.
- concrete ring with a rounded top. Example: Be-8 Schoorl/Bergen, where the ties are 1.3 meter long and the bolts are placed at 13 and 27 cm from each end of the ties. Rounding the concrete top must have required additional effort. It is unclear why it was done.
- F: two sets of 120 ties, one set is narrower than the ties at all other sites. Example: Be-0 Trebbin which was also a "Bernhard" test site. The narrow ties have a width of 7 cm, and may have been from an initial version of the track. However, both sets of ties are embedded into the same layer of concrete, i.e., both sets were already installed when the concrete of the top layer was poured. The bolts on the narrow ties appear to have been removed with a grinding tool, suggesting that they predate the wide ties.
Fig. 51B: The various forms of rail fastening and associated features in the concrete ring
Fig. 51C: Pairs of threaded rods
I visited the remains of the "Bernhard" ring of Buke in April of 2015. This ring was destroyed by the British after the war, exposing the cross-section of the concrete. It suggests that the concrete was cast with as many as six distinct parts, see the photo below. The top layer is about 7 cm (3 inch) thick.
Fig. 51D: Cross-section of the ring at Buke
A number of the "Bernhard" rings have rail-ties (sections of I-beam) that are embedded into a separate layer of concrete on top of the ring:
Fig. 51E: Top layer of concrete, with embedded I-beam rail-tie at Arcachon
The inside rail of the track has a radius of less than 10 meters. This is very small for a rail track, and causes a large (5%) difference in the speed between the inside and outside wheels of the locomotive bogies. This "slippage" causes problems with normal bogies that have rigid axles with wheels that cannot turn independently. It also causes problems with traction, and wear of the wheels and the rails. One way to solve this, is to use wheels with a smaller diameter (here: 5%) on the inside rail of the track. In turn, this requires that the inside rail be raised slightly (several cm). See Figure 52.
Fig. 52A: Raising the inside rail with a block on the ties
Fig. 52B: A block on the inside end of all ties of Be-6 at Marlemont
Fig. 52C: A block on the mounting plate on the inside end of all ties of Be-10 at Hundborg
(source: Hundborg Lokalhistoriske Arkiv; used with permission)
Generally, the aim is for trains to run without any contact between the flange of the wheels and the rail head. Such contact causes stress on the frame of the bogie, and wear on the wheels and rails. This is why wheel treads normally have a conical shape that widens towards the flange of the wheel. On a circular track with a very small radius, it may be necessary to align the bogie axels radially with the curvature of the track. I.e., the axles always point at the center of the circular track, perpendicular to the rails. No information is available about the bogie design of the Bernhard locomotives. So it is unknown if this approach was actually used.
Fig. 53: Bogie axles angled towards the center of the circular track
Obviously the circular rail track was not transported to the "Bernhard" sites in one piece. Most likely, they were delivered as sections of curved rail. It is unknown which type of rail joints were used
- Conventional joints with a small expansion gap (D: "Stoßlücke"). The rail sections are connected with so-called "fishplate" joint-bars ("Schienenlasche") and bolts through the rail. The "fishing" is the vertical web between rail head and rail base. The gaps cause the familiar clickety-clack noise when the train wheels bump over them. There is a tie underneath both of the joining rail ends. None of the "Bernhard" tracks have pairs of ties that are placed right next to each other. So it is unlikely that this type of joint was used.
- Expansion joints (a.k.a. "breather switch", "adjustment switch", D: "Dehnungsfuge", "Schienenauszug"). The ends of the mating rail sections are tapered diagonally, and are not bolted together with fishplates. This type of joint allows for smoother transitions (reduced noise and vibration) than conventional joints.
- Fusion-welded seamless joints . The welding is done on-site with the "Thermit" process that is described below. The result is a continuous welded rail (CWR, D: "durchgehen geschweißtes Gleis", "lückenloses Gleis"). Such rails need very solid anchoring, in order to to avoid warping of the tracks due to thermal contraction/expansion (e.g., "sun kink").
- The "Bernhard track has a length of π x track diameter = π x 22 ≈ 69 m. The expansion of steel rails is ca. 12 mm per °C per km. Assuming a very moderate summer-winter difference in rail temperature of 50 °C, the expansion would have been ca. 4 cm (≈2½ inch).
- For CWR, the Deutsche Bahn installs a heavy concrete rail-tie every 60 cm. To minimize expansion/contraction forces, the rails are installed when the temperature is around 20 °C. The ties of the "Bernhard" track are spaced by... 60 cm!
Fig. 54: Three standard types of rail joints
(left-to-right: conventional joint, expansion joint, seamless joint)
The "Thermit" process was discovered by the German chemist Hans Goldschmidt around 1890. He was a student of the German chemist Prof. Robert Bunsen (yes, of the famous "Bunsen burner" - actually the "Bunsen & Desaga" burner). Goldschmidt patented the process (originally intended for purifying metals) in Germany 1895, and named it "Thermit". It is a simple but intense exothermal reaction: a mixture of powdered iron oxide (commonly known as "rust") and aluminum is converted into iron and aluminum oxide, plus enough heat - over 2400 °C (4300 °F) - to fully melt the mixture in a matter of seconds! The reaction is typically started by igniting a magnesium "sparkler" stick or ribbon that is put into the mixture. Unlike blast furnace smelters, basically no external heat needs to be applied. The aluminum slugs will float on top of the melted iron. The mix normally contains additives, to obtain the desired steel alloy. The process can be used for welding large steel parts, such as shafts, cables, pipes, and rails. It can also be used for casting of parts, and for under water welding. The first commercial welding application were tramway rail projects in Essen/Germany in 1899 and Berlin in 1901. The deutsche Reichsbahn followed in 1928. The process was patented in the USA in 1928, by the Metal & Thermit Company (named Goldschmidt Detinning Company until the first world war). Ever since the 1920s, it is the standard process world-wide for welding rail tracks. The intense pyrotechnic process has also found military applications (grenades, incendiary devices, etc.). The aluminothermic Thermit rail-welding process is as follows:
- The ends of two rail sections that are to be welded, are brought together with 2-3 cm spacing (1 inch) and are aligned.
- The gap is clamped with a two-part ceramic shell that has the same shape as the cross-section of the rail. This is needed to avoid the molten steel from running off. The sides of the clamp are sealed with special molding sand.
- The ends of the two rail sections are pre-heated to ca. 1000 °C with a blow torch.
- A ceramic "funnel" pot is placed on top of the molding clamp, and is filled with Thermit-mixture.
- The mixture is ignited with magnesium ribbon, and then boils. The entire process only takes about 25 sec.
- After cooling off, the pot, clamp, and slugs are removed.
- The weld is ground, to get a smooth rail head.
Fig. 55: Thermit-welding of tramway tracks in the city center of Bremen/Germany around 1900
Rails that are dirty (grease, decomposing leaves, rain and ice) significantly reduce traction of the locomotives. Traction heavily depends on the adhesion coefficient μ between the steel wheels and the steel rails (ref. 158):
μ ≈ 30% for clean, dry rails.
μ ≈ 20% for rails that are clean but wet or icy, and for greasy rails.
μ ≈ 5-10% for rails covered with (decomposing) plant leaves.
This is why a number of "Bernhard" stations had a track-cover that moved with the rotating installation. Based on available photos, such a cover was installed at least at the "Bernhard" stations Be-7 at La Pernelle, Be-8 at Schoorl/Bergen, Be-9 at Bredstedt, and Be-10 Hundborg. The "Bernhard" station Be-0 at Trebbin did not have such a cover.
Fig. 56: A steel track-cover moves with the rotating platform
Fig. 57: Cross-section of the track cover - made of sheet metal and support braces
Fig. 58A: The cover has a "box" around each pair of support wheels
Fig. 58B: Side-view of the track cover - "box" around each pair of support wheels
Fig. 58C: Cross-section of the track cover - "box" around each pair of support wheels
Ther are two stairs. One is attached to the rotating platform, near the entry door of the cabin. The second one is attached to the rotating rail cover.
Fig. 59A: Two stairs: one is attached to the rotating platform (left), the other to the track cover
Fig. 59B: The destroyed "Berhard" Be-8 at Bergen/Schoorl - upside-down section of track cover
(source: ref. 127)
Fig. 59C: Staircase section of the track cover
(photo - turned right side up: ref. 127)
THE LOCOMOTIVE SYSTEM FOR ROTATING THE ANTENNAS AND CABIN
The superstructure (antenna system + cabin) of the "Bernhard" was rotated by four electrically powered locomotives on the circular rail track. Each locomotive has two bogies (US: trucks). The photo below shows that each bogie has two axle-boxes, and leaf-spring suspension. Such axle-boxes typically have greased sliding bearings (a.k.a., journal bearings, not ball bearings). There are 4x (2x (2+2)) = 32 wheels in total. So the weight per wheel is a little under 4 metric tons. This is well below the standard railway limit of about 16 tons/wheel, for the load at which the rail head and (full size) wheels are damaged.
Fig. 60: Each locomotive has two bogies (Be-10 at Hundborg)
(source: www.gyges.dk, used with permission)
Fig. 61: Close-up of a locomotive (left) and external gearing of a locomotive (right) of Be-7 at La Pernelle
(source: 1946 film clip Cinémathèque de Normandie)
The photos above show that the locomotives supported the weight of the superstructure at the point halfway between the bogies - which makes sense. The motor was placed at the front of the locomotive (on the left in Figure 60). There was an additional down-gearing between the motor and the forward bogie, on the outside of the locomotive (on the inside of the ring). The down-gearing ratio is small: about 1.6:1. The gearing is covered, so it is not known if it was a belt or a chain. It appears that only the forward bogie was driven by the motor. It is not known if both axles of that bogie were driven, or only one. Dividing motor power evenly between multiple axles optimizes the use of the available traction, but complicates the construction.
The motors of the electro-locomotives of the Bernhard station at Buke were built by Ziehl-Abegg Elektrizitätsgesellschaft m.b.H., of Berlin-Weißensee (ref. 99). Their power was 10 kW (13.6 metric horsepower, 13.4 US hp), with a large down-gearing ratio. The motors at (some) other Bernhard stations may have been built by Siemens (e.g., ref. 103). This was probably Siemens-Schuckert.
Ziehl-Abegg is a company specialized in electric motors. It was founded in 1910 by Emil Ziehl and the Swedish investor Eduard Abegg (who dropped out of the partnership the same year, as he could not come up with the required funds). Ref. 154. In 1897, Emil Ziehl invented the external rotor motor (stator inside the rotor - very compact and excellent weight balancing). In 1904, he invented electrically powered gyroscopes with gimballed suspension. Prior to 1910, Emil Ziehl had developed electric motors and tested generators at AEG, and developed gyro-compasses at Berliner Maschinenbau AG (BEMAG, frmr. Eisengießerei und Maschinen-Fabrik von L. Schwartzkopff). BEMAG was a manufacturer of locomotives powered by steam, compressed air, and electricity. Ziehl-Abegg made DC-DC converters (DC-motor + generator) for Zeppelin airships and airplanes. Telefunken was a major customer. They also made electro-mechanical transverters ("Drehstrom-Gleichstrom-Umformer", i.e., AC-motor + DC-generator, as in Ward-Leonard Drive Systems) for elevators and generation of anode voltage of large transmitters, transformers for directional-gyros (e.g., SAM-LKu4), motor-generators such as the U 4a, and the motor-generator-alternator of the U 120 of the "Bernhardine" system. After the war, the production facilities were carried off to the Soviet Union. In 1947, the company restarted, this time in the south of Germany. These days, Ziehl-Abegg AG is a manufacturer of electric motors for elevators, ventilation and air-conditioning systems.
Fig. 62: 1912 advertizing poster, listing in 1943 Berlin phonebook, wall plaque on building of Fig. 63
(source of poster: ref. 155)
Fig. 63: Company buildings of Ziehl-Abegg Elektrizitätsgesellschaft m.b.H in Berlin-Weißensee
How powerful did the locomotives have to be? Let's do a simplistic reasonableness check, using the definition of "horsepower". On a level (horizontal) track, the required locomotive horsepower HPloc is (ref. 156, 157):
HPloc = W x T x S / 375
W = total gross train weight in tons (1000 lbs)
T = total Train Resistance per ton = 8 lbs/ton. Note: this is a standard value used in the railway industry.
S = speed in mph
Converting this to metric units, we get:
HPloc = W x T x S / 271
W = total gross train weight in metric tons (1000 kg)
T = total Train Resistance per metric ton = 8 kg/ton
S = speed in km/h = mph / 1.609
Total weight of the rotating system (antenna system, cabin, locomotives) is 120 metric tons. The track diameter (between the inside & outside track) is about 22 m. I.e., a circumference of π x 22 ≈ 69 m. As the installation rotates at 2 rpm, the speed is 2 x 69 m/min ≈ 8.3 km/h. Hence, the required total locomotive horse power is 120 x 8 x 8.3 / 271 = 29.4 HP at the traction wheels. There is down-gearing between the motor and the driven wheels. Let's assume a reasonable transmission efficiency of 85%. For the required total motor horsepower we now get:
HPmotor-total = HPloc / 0.85 = 29.4 / 0.85 = 34.6 HP
The "Bernhard" system used four locomotives. So, the required motor horsepower per locomotive is:
HPmotor = HPmotor-total / 4 = 34.6 / 4 ≈ 8.7 HP
The actual motors supposedly produced 13.6 HP each. This would have left margin for acceleration and wind load.
Note that rolling resistance of a railway vehicle (which is a science all by itself) is the sum of all forces acting through the wheels and the axles, that oppose motion of that vehicle. There are many sources of rolling resistance. Some vary only with weight (e.g., journal bearing resistance, rolling friction, track resistance), some are linearly proportional to speed (e.g., wheel-flange contact, wheel-rail interface, lateral and vertical movement), some depend on the square of the speed (e.g., aerodynamic), some on the fourth time-derivative of displacement. Examples:
- Bearing friction
- Elastic deformation of the wheel-tread and of the rail, in the wheel-rail contact area. Note that the contact surface between a wheel and the rail is a very small elliptical area, called the "contact patch", that is typically only about 15 mm (0.6 inch) across! The weight of the wheel and the load that it carries, makes a "dent" in the surface of the rail head. This dent moves with the wheel: trains actually always go uphill, even if the track is horizontal!
- Losses due to wheel creep (during accelerations and decelerations, the elastic deformations cause the actual wheel displacement to be be different from its rolling distance)
- Losses due to grinding of wheel flanges against the rail head, and "hunting" (horizontal back-and-forth waving movement of the bogies on the track).
- Wheel noise (vibration and resonances in the wheels)
- Suspension "jounce" (the fourth time-derivative of displacement), due to impacts on rail joints (if any), and associated rebounds. Bumps and bounces convert horizontal momentum into vertical momentum; the associated energy is dissipated in the suspension.
- Track deformation (Rayleigh waves)
- Aerodynamic drag that acts on exposed wheels and on the body of the locomotive. At low speed, this is normally quite small, if not negligible. However, in the case of "Bernhard", there is also drag of the large antenna system due to system movement, but also due to wind load. The antenna system is symmetrical with respect to the vertical axis of rotation. So there is a "push & pull" effect, depending on whether the movement is upwind or downwind.
- Curve resistance, due to the radius of the curvature of the track. Note that regular "1 meter" narrow gauge track (i.e., about the gauge of the "Bernhard" track) typically has a minimum curve radius of 45 - 60 meters, about 4 - 6 times that of the circular "Bernhard" track! Without special measures, the curve resistance would have been quite high.
In normal rail applications, total resistance at low speed (less than about 15 km/h, 10 mph) is dominated by friction of the axle bearings (ref. 159).
Electrically powered rail vehicles (electric and diesel-electric train locomotives, streetcars/tramways, subways) traditionally used DC traction motors. This remained the case until well after the advent of solid-state power electronics, in particular gate turn-off (GTO) thyristors, in the early 1960s. The DC motors where primarily of the series-wound brushed type (i.e., with a commutator), and the field windings in series with the motor's armature windings. Note that brushless DC-motors only date back to the late 1950s, ref. 160. Series DC-motors can produce their highest torque at low speed: as much as 3-8 times the full-load torque at nominal speed. This is ideal for traction applications. For a given field flux, DC motor speed is determined by the armature voltage, whereas the delivered torque is driven by the armature current.
Fig. 64: Basic types of wound Direct Current (DC) motors - classified by placement of the field winding
Fig. 65: Basic characteristics of series, shunt, and compound DC motors
The signals transmitted by the "Bernhard" beacon were printed aboard aircraft with the "Bernhardine" Hellschreiber-printer. One of the two channels of this printer was synchronized to pulses sent by the beacon. The timing of those pulses depended on the rotational speed of the beacon. This only allowed for a small speed differences between the beacon and the printer motor: no more than 1.5%. This required quite accurate control and regulation of the speed of the locomotives! Here "control" relates to the ease and accuracy of achieving a desired speed (a fixed or variable set-point), whereas "regulation" refers to the sensitivity of the motor drive system to variations in the mechanical load.
As stated above, the speed of a DC motor depends on the voltage across the motor's armature and the field flux. Standard methods to vary the armature voltage of a series motor are:
- An adjustable resistance placed in series with the motor's armature
- Ward-Leonard drive system
- Rectified adjustable AC-voltage (ref. 161)
There are many other flavors of motor speed control. They are generally beyond the scope of this discussion.
The "adjustable series resistance" (rheostat) method has a major disadvantage: poor speed regulation ( = speed is highly load-dependent). There are also very high losses ( = heat dissipation) in the series resistance at low speed. That is, during speed-up from, and slow-down to stand-still. Some speed-control methods for series DC-motors are illustrated in Figure 66A/B.
Fig. 66A: Speed control of a series DC-motor via field or armature diverter, tapped field-winding, variable series resistance
Fig. 66B: Speed control of a series DC-motor via series-parallel reconfiguration of split field-winding and multiple motors
The Ward-Leonard Drive System (D: "Leonardsatz", "Ward-Leonard-Umformer", "Leonard Doppel-Umformer") is basically an electro-mechanical way of generating a variable DC-voltage to control the speed of a DC-motor. Ref. 162A-162G. It was invented by Harry Ward Leonard in 1892. For about 75 years, there were few practical alternatives to this system - until the advent of solid-state power-electronics such as thyristors in the 1960s. Worldwide, this was the normal way to provide smooth, step-less control of the speed of high-power DC-motors, from zero to full speed. It has been - and still is - used in many applications, such as cannon/gun- and turret-aiming, elevators, rolling mills, cranes, hoists, mining (colliery) winders, diesel-electric propulsion of locomotives and of special ships, strip-mining shovels, and heavy radar antennas. German WW2 radar antenna systems with a Ward-Leonard drive include the "Wassermann S" (FuMG 42; see figure 2 in ref. 162G; 36-60 m tall / 4 m diameter column, weight up to 60 tons) and the AEG-Telefunken "Würzburg Riese" (FuSE 65) with its large dish antenna (7.5 m diameter, 9.5 tons; ref. 162F). Allied radar system also used Ward-Leonard drives. E.g., the 20 ton antenna of the British "Marconi Type 7" was rotated with a 15 HP DC-motor, controlled with a Ward-Leonard set comprising a 24 HP 3-phase motor, a main DC-generator, and a small DC exciter generator. Ref. 162H.
The Ward Leonard Drive System consists of a Ward Leonard Drive Unit and a shunt-wound DC motor. The Drive Unit consists of a motor-generator. The motor (referred to as the "prime mover") has a near-constant speed. This can be a 3-phase or single-phase synchronous AC motor, or a combustion engine (diesel, gasoline/petrol) with a speed governor. The output shaft of the motor is coupled (direct-drive) to the input shaft of a DC-generator. The output voltage of this DC-generator is connected to the armature of the DC-motor that drives the load. The DC-motor need not be located near the motor-generator. The shunt-field of the DC-motor is connected to a constant voltage source; hence, the motor's excitation field (flux per motor-pole) is constant, and the torque only depends on the armature current - independent of the motor speed. The shunt-field of the DC-generator is connected to that same constant-voltage source, though via a rheostat (large variable resistor).
Fig. 67: Ward-Leonard Drive System
The generator's output voltage is varied by changing the generator's field current with the rheostat. In turn, this changes the DC-motor's armature voltage, and hence its speed. The constant voltage source may be a rectified AC voltage (if the prime mover is an AC-motor). It can also be generated with a small DC exciter-generator ("selbsterregter Erregergenerator"), that is also driven by the prime mover, and has its shunt-field connected to its own armature (hence, "self-excited").
The Ward-Leonard drive unit is an electro-mechanical multi-kilowatt amplifier: a small change in the input current (generator field) results in a large change at the output (generator armature voltage and current). It is an open-loop control system. Note that both the AC-motor (or the engine) and the DC-generator must be dimensioned for the full and peak power of the load-driving DC-motor(s) and system inefficiencies. System efficiency is driven by the product of the efficiency of the three machines (AC-motor, DC-generator. DC-motor), and typically lower than that of rheostat control and field control methods. A single Ward Leonard drive unit can control multiple load-sharing DC motors in parallel ("group control"). Variations of the Ward-Leonard drive system are electro-mechanical amplifiers such as the Metadyne (1930s) and the Amplidyne (1940s).
As elegant and effective as the Ward-Leonard drive system may be, it was not what was used in the "Bernhard" to provide a variable DC-voltage to the locomotive drive system! In May of 2015, I obtained the black & white photo shown below. It shows a large Mercury Arc Rectifier (MAR, a.k.a. Mercury Vapor Rectifier; D: "Quecksilberdampfgleichrichter"). Ref. 163A-163E. It was installed in the Be-10 "Bernhard" at Hundborg/Denmark. The only reason to have such a rectifier in a "Bernhard" beacon, is as part of the speed control system for one or more large DC-motors!
Fig. 68: "Rectifier for Bernhard equipment - Hundborg /Denmark" (left) and a demonstration cubicle (right)
(source: left-hand photo - ref. 93; right-hand photo: collection of Technical University Freiberg, used with permission)
A 6-anode MAR with a height of 90 cm (3 ft, about the size of the "Bernhard"-MAR) can typically handle as much as 350 amps at 650 volts. The photo on the right shows a small MAR, rated for only 220 volt / 100 amps (22 kW). It was manufactured in the 1950s by by Elektro-Apparate-Werke J.W. Stalin in Berlin-Treptow. This was the post-war continuation in the Soviet-occupied part of Germany, of the AEG company Apparate-Werke Berlin-Treptow (AT) that was founded in 1928. The MARs used in the "Bernhard" system may have been manufactured at the latter factory, by Siemens-Schuckert Werke, or one of the Brown-Boveri & Cie. (BBC, now ABB) MAR-plants in Germany (such as the Gleichrichter Gesellschaft m.b.H company in Berlin, that was acquired by BBC).
The discovery of the unidirectional current-flow of an arc between a mercury pool and a carbon electrode, goes back at least to the early 1880s. Mercury arc rectifiers were invented around 1900, and patented by Peter Cooper-Hewitt in 1902. They are a form of cold-cathode gas discharge tube. The rectifier consists of a glass or stainless steel vessel. The vessel is evacuated, or filled with inert gas. There is a pool of liquid mercury at the bottom of the vessel. This is the cathode. The vessel has one or more upward arms with a graphite anode. Like a fluorescent lamp, a MAR must be started. Conduction is initiated by dipping the starting (igniting) electrode into the mercury pool, passing a high current, and retracting the electrode. This locally heats up the mercury (the "cathode spot" or "emission spot") and vaporizes it. This starts abundant emission of electrons by the mercury cathode. The mercury vapor is ionized by the stream of electrons that flows to the anode, and causes plasma discharge (arc) between the anode and the cathode. The mercury ions emit both visible blue-violet light, and a large amount of ultra-violet radiation. The light may have another color when the vessel is filled with an inert gas. Evaporated mercury condenses on the cool wall of the vessel (hence the large bulbous form), and returns to the mercury pool at the bottom of the device. The plasma discharge stops as soon as the anode voltage drops below a certain level, or anode current is interrupted. Hence, for rectification of an AC voltage, ignition must be synchronized with that voltage. Alternatively, excitation electrodes may be used to maintain the plasma. The anode material does not emit electrons, so electrons can only flow from the cathode to the anode. I.e., current can only flow from the anode to the cathode. The ripple on the DC output current is smoothed with a series-inductance ("choke coil"). The above photo of the MAR-cubicle of the Hundborg "Bernhard" shows no collocated choke-coil. Some DC-motors do not require current smoothing. Also, the coil(s) may have been collocated with the speed control circuitry, or even the motors themselves.
MARs can be constructed for hundreds of kilovolts and tens of thousands of amps. They have been used, and sometimes still are, as rectifiers for locomotives, radio transmitters, control of industrial motors, welding equipment, aluminum smelters, high-voltage DC power transmission, etc. The heat of the mercury vapor must dissipate through the glass envelope in order to condense. To help keep the glass envelope cool enough, an electric fan is typically installed below the MAR
(as clearly visible in the left-hand photo of Fig. 68 above).
temperatures below 10 °C (50 °F), special measures must be taken to protect the MAR against damage from instable operation, and the attached transformers against current surges. This may be done with surge diverters and
cathode heating. Ref. 161. Mercury freezes around -39 °C (-39 °F). Glass-bulb MAR designs are typically limited to 250 kW (500 volt , 500 amps). For higher power levels, a steel-tank version was developed around 1908. Siemens-Schuckert developed a
compact double-wall water-cooled tank rectifier around 1920. MARs technology was succeeded
by ignitrons and thyratrons (ref. 163F), and then GTOs (e.g., thyristors).
Full-wave rectification of a single-phase AC voltage requires two anodes. See Figure 69. For rectifying 3-phase AC power, the MAR must have at a multiple of three anodes.
Fig. 69: left: Principle diagram with a 2-anode MAR (left) and an active 3-anode MAR
The "Bernhard" MAR in Figure 68 has six anodes. This reduces the ripple in the rectified voltage. It also requires a more complicated main-transformer connection to AC power, e.g., a delta/double-star or star/double-star configuration. See pp. 18-24 in ref. 163B. Six-anodes is typically sufficient. More anodes does reduce the output ripple (with a lowest harmonic of six times the 50 or 60 Hz main power), but the reduction when going from 6-phase to 12-phase is less than half the reduction when going from 3-phase to 6-phase. Also, cost increases rapidly without increasing rectifier output, and the already low power factor (undesirably large phase angle between AC supply voltage & current) is further reduced.
Fig. 70: A 6-anode Mercury Arc Rectifier with delta/double-star main-transformer connection to 3-phase AC power
(source: Figure 7.41 in ref. 163E)
Here is a 36 sec video clip of a 6-anode MAR in action (you may want to turn the audio volume down a bit):
A six-anode Mercury Arc Rectifier in actionSource: YouTube
Controlling the DC output voltage of a MAR means varying the AC input voltage. Two of the standard methods for this are:
- Grid-controlled MAR, see pp. 175-200 in ref. 163B, §6.4 in ref. 161. This requires that the MAR has a control-grid at each anode, and analog phase-shift circuitry to control the firing of the grids. However, the anodes in the "Bernhard" MAR of Figure 68 above do not appear to have control-grid electrodes! So this method does not appear to have been used in the "Bernhard" system. The grid-control method varies the fraction of the time that the anodes are "on", out of each half cycle (180° phase) of the AC power, which varies the DC-output voltage. This is basically the same method a used in household dimmers for incandescent light bulbs. Control can be open-loop (manual) or closed-loop (automatic, based on the speed feedback-signal from a tachometer).The required phase-shift ("firing angle") between anode and control grid can be provided in two basic forms:
- A grid voltage that is a fixed-amplitude AC voltage, that is delayed ( = phase shifted) with phase shift with respect to the anode voltage. Due to the smooth sinusoidal shape of the control voltage, this method is not very precise. The phase shift can be varied , e.g., with a "induction regulator" rotary variable transformer.
- A grid voltage that is a narrow pulse, the relative position ( = phase) of which is varied.
- Free-firing MAR - the MAR has no control-grids, and the firing of the anodes is driven by the AC input voltage.
- Transformer tappings. The standard way to adjust an AC-voltage for traction motor control has traditionally been to use an auto-transformer with tapped secondary windings (Tapped Auto-Transformer, TAT). The output voltage of the auto-transformer depends on the selected output tap. This basically provides a transformation ratio that can be changed in a number of discrete steps. Having selectable transformer output-taps means that we need a tap-selector. For the control of traction motors, the current should not be interrupted when changing taps. I.e., changing taps should be done with the motor-load connected (via the MAR). This is called "on load". Also, consecutive taps should not be short-circuited when changing taps, as this can damage the tap-selector and the transformer. The standard tap-selector that meets both requirements is a so-called On-Load Tap Changer (OLTC). OLTCs date back to the 1920s and are still used today.
- The auto-transformer provides "up" transformation of the input voltage, hence "down" transformation of the current. This reduces the size of the transformer, and makes it easier to add taps, as the secondary winding of the transformer has has many turns. The selected output voltage of the auto-transformer is then reduced with a step-down transformer, to a level that is suitable for the DC-motor. Multiple load-sharing motors may be connected in parallel.
- This has been a long-time standard in trains and streetcars/tramways, where the tap changes are often quite noticeable to the passengers. However, those applications are only powered by single-phase AC line voltage. When using 3-phase AC line voltage, the transformer and the OLTC switching elements have to be triplicated. A 3-phase transformer can be constructed either by connecting three single-phase transformers, or by using one 3-phase transformer that consists of three pairs of single-phase windings mounted onto one single laminated core.
- "Sliding-contact variable transformer". This regulator is an extreme form of a TAT: the entire secondary winding of the transformer is exposed, and a sliding-contact can be moved along the length of that winding. This is equivalent to a tap at every secondary turn. This mechanism is similar to a rheostat variable wire-wound resistor. This approach is common when the motor is driving a heavy load, and it takes a long time to accelerate to nominal speed.
The diagram below shows a typical OLTC, for an arbitrarily chosen number of taps. There are two concentric contract rings (the blue and red circles in the diagram). Outside each contact ring is a ring of contact notches. The even-numbered transformer taps are connected to one set of notches (here the red ones), whereas the odd-numbered taps are connected to the other set of notches (the blue ones in the diagram). There are two tap selector arms. They are not interconnected electrically. One arm can be rotated to interconnect any blue notch to the blue contact ring. The other can interconnect any red notch to the red contact ring. The two contact rings are connected to two transition switches (TS1 and TS3). A transition resistor bridges TS3 to a third transition switch (TS2). The position of the selector arms and the diverter switches cannot be changed arbitrarily: the arms can never be more than one notch apart. They are actuated by a camshaft mechanism that can be controlled remotely (solenoids, air pressure).
Fig. 71: Principle of DC-motor speed control with an On-Load Tap Changer and full-wave mercury arc rectifier
(shown for single-phase AC voltage; rectifier igniter control-circuitry not shown)
In the diagram above, switch TS1 is closed, and blue notch number 1 is selected with the selector arm. Hence, transformer tap number 1 is selected. Red notch 2 is (pre-)selected on the red contact ring, but switch TS3 is open (as is TS2). To select tap 2, first TS2 is closed - the resistor limits the short-circuit current between tap 1 and tap 2. Then TS1 is opened, to disconnect tap 1. This is followed by closing of TS3. Finally, TS2 is opened. Now tap 2 is selected. The blue selector is moved to notch 3, to pre-select tap 3. A similar sequence of "make-before-break" selector arm movements and opening/closing of the three switches is used for all tap changes. It is not possible to skip a notch, e.g., to jump from notch 4 to notch 11.
When the motor voltage is reduced to slow down the motors (e.g., to brake to standstill), the large inertia of the "Bernhard" turntable will reverse the load torque of the motors. However, current can only flow through the MAR in one direction - it is a rectifier/diode. So, regenerative braking (using the motor as a generator and feeding the generated electricity back tio a power grid) is not an option, and the motor cannot exert a braking torque. As a result, the armature voltage will increase to undesirably high levels. This is typically handled by switching-in a large dummy-load resistors across the armature, and dissipating the generated power as heat.
I do not know exactly which AC voltage control method was used with the "Bernhard" MAR. Figure 44 above shows a large control panel inside the cabin of Be-9 in Bredstedt. It was either associated with speed control of the locomotive motors, or with the transmitters.
THE ROUND CENTRAL-SUPPORT BUILDING
The antenna system and the cabin are mounted on a frame that is made of large I-beams. The corners of this frame are supported by the four locomotives. The frame spans the large diameter of the concrete ring. This is why there is a support structure in the middle of the ring. It is a round building with a flat concrete roof that has a diameter of about 4 m (13 ft). This is the main pivot (kingpin, D: "Königstuhl") of the rotating platform.
Fig. 72: Concrete ring with central- support building
Fig. 72A: Left-to-right - the round building of Be-14 at Arcachon, Aidlingen/Venusberg, and Be-4 at Le-Bois-Julien
(photo Le-Bois-Julien: ©2006 T. Oliviers, used with permission)
Fig. 72B: The round building of Be-8 at Bergen/Schoorl (left), and Be-12 at Nevid/Plzeň
(sources: photo Be-8: ref. 127; Be-12: © Jacek Durych, used with permission)
The walls of the building are made of brick. There are four windows of 1.2x1.2 m (4x4 ft), and a door. Whatever equipment was installed inside this building, must have fitted through the door or a window. Floor-to-ceiling height inside the building is about 3 m (10 ft), so the floor is well below the base of the concrete ring. This is why there is a small trench and steps that lead down to the door.
The three diagrams below show the cross-section (with measured dimensions) of the concrete ring and round building of the "Bernhard" installation at Aidlingen/Venusberg, Arcachon, and Hundborg:
Fig. 73A: Cross-section of the installation at Aidlingen/Venusberg
(based on the measurements that I took in June of 2012)
Fig. 73B: Cross-section of the installation at Arcachon
(based on the measurements that I took in July of 2012)
Fig. 73C: Cross-section of the Bernhard ring on Gåsbjerg hill at Hundborg
(based on ref. 115)
The concrete roof of the round building is supported by four columns, made of massive steel I-beams (H-beams, D: "Doppel T Träger"): the flanges are 30 cm (1 ft) wide and 24 mm (1") thick! The web of the beams is 32 cm (12½ inch) wide and 15 mm (0.6 inch) thick. So these columns have a cross-section of 30x37 cm. The columns are spaced evenly in the round wall. The roof-joists are made of the same heavy I-beams. The following diagrams show more details of the steel structure:
- 4 large steel I-beam columns, with end-plates. The flange of these vertical I-beams is 30 cm (1 ft) wide, and the height of the beams is 37 cm (1 ft 3")
- 4 large steel I-beam joist, with a joist-to-column brace and a triangular filler plate. The brace is a heavy steel plate (3 cm thick), as wide as the flanges of the columns and the joists. The joist and brace are mounted to the column with 8 bolts. The brace prevents the structure from racking ( = sideways swaying of the tops of the columns). The brace is mounted to the column at a 50° angle.
- 4 steel doubler-plates, to better transfer vertical forces on the joist to the column, and to distribute any bending force to both flanges of the columns. The doubler-plate is mounted onto the end-plate of the column with 8 bolts, and to the joist with 8 bolts.
- 2 identical steel octagonal plates, to interconnect the four joists. Each joist is mounted to the two octagonal plates with 6 bolts. The plates have hole at the center.
- 4 small steel plates that form the sides of a box that is placed between the octagonal plates. The corners of the box are butted up against the web of the joists, but do not appear to be welded to them. The function of the box is unclear - possibly to prevent the center of the top octagonal plate from being pushed down, possibly they where used to pre-assemble the two octagonal plates.
- 4 small steel I-beams, connecting the joists above the braces. This makes the structure torsionally stiff. I found no sign that the columns are interconnected at the bottom.
- numerous steel concrete reinforcement rods/bars ("rebar"), placed radially inside the concrete of the roof. The ends of the rods are curled back.
Fig. 74: The major elements of the steel support structure of the round building
(based on my measurements of the "Bernhard" at Aidlingen/Venusberg)
Fig. 75: Dimensions of the steel "skeleton" of the round building
(based on my measurements of the "Bernhard" at Aidlingen/Venusberg)
A doubler-plate reinforces the joint of each joist and the associated column:
Fig. 76: Dimensions of the column-to-joist doubler plate
(based on my measurements of the "Bernhard" at Aidlingen/Venusberg)
Fig. 77: Details of the steel support structure of the "Bernhard" at Aidlingen/Venusberg
Two heavy octagonal plates interconnect the four large I-beams joists at the center of the roof. One plate on top, one from below. There are two brackets on the bottom plate, for suspending equipment. The space between the brackets is about 55 cm (22 inch).
Fig. 78: The octagonal mounting plate against the ceiling - with mounting brackets to suspend equipment
Fig 79: Dimensions of the octagonal mounting plates and associated brackets
(based on my measurements of the "Bernhard" at Aidlingen/Venusberg)
Four smaller I-beams make the top of the structure torsionally stiffer:
Fig. 80: Top view of the steel support structure
(based on my measurements of the "Bernhard" at Aidlingen/Venusberg)
The next photo shows the remains of the steel structure (upside down), after removing the walls and collapsing the roof:
Fig. 81A: The remains of the steel structure at Nevid, after destruction of the building in 2015
(source: © jdvlavicka)
Fig. 81B: The remains of the steel structure at Nevid, after destruction of the building in 2015
(source: © jdvlavicka)
A large ball bearing was installed in the middle of the roof:
Fig. 82: The round building with the raceway of a large ball bearing in the middle of the roof
(Be-12 at Nevid/Plzeň; source: © Jacek Durych, used with permission)
The ball bearing held a cylindrical tube, that rotated with the antenna system and the cabin. The bottom of this tube has a flange, for connecting to equipment that rotated with the tube. The tube had a diameter of about 14 cm (5½ inch).
Fig. 83: Tubular shaft descending through the roof of Be-6 at Marlemont and Be-10 at Hundborg
(source Hundborg photo: www.gyges.dk, used with permission; note the original wiring)
A small turntable is mounted on top of the tube:
Fig. 84: small turntable on top of the shaft
(Be-6 at Marlemont; note the large mounting bolts inserted into the turntable from below)
One major question is: what kind of equipment was suspended from the ceiling of this round building, and connected to the vertical shaft through the ceiling? This is basically the rotating entry point to the cabin for:
- Electrical power. The four electric locomotives and the transmitters in the rotating cabin needed electrical power. The power (probably 3-phase AC) came from a motor-generator in a nearby bunker or building, or from the local public power grid.
- Interfaces with the optical disk. One of the two transmitters transmitted the momentary azimuth of the antenna ( = pointing direction) in the form of a stream of pulses that corresponded to the pixels of a compass-rose. See the transmitter section above. These pixels were captured on an optical disk. This disk had to rotate with the antenna system. The pixels on the disk were detected with a light source above the disk, and a photo cell below the disk. Obviously, the light source and photo cell did not turn with the disk.
- Audio signals from a radio receiver at the monitoring-antenna mast nearby the "Bernhard" station. The signals were printed with a Hellschreiber-printer inside the cabin.
- A telephone line. The personnel inside the cabin and in the nearby command/control bunker needed to be able to communicate (including when the system was rotating).
So it is safe to assume that the suspended equipment comprised a stack of slip-rings for electrical power, a phone line, and the interfaces with the light source and photo cell of the optical disk, an also a sub-assembly with that disk.
No photos are available of the slip-rings and optical disk unit. As an example, the photo below shows the slip-ring assembly of German "Panther" and "Tiger" tanks of the same era. The slip-rings have a diameter of 12 cm (≈5 inch). There are four brass rings for electrical power (12 & 24 volt, 50 amps) and seven rings for communication and lighting.
Fig. 85: The slip-ring stack of Panzer V "Panther" and VI "Tiger" tanks
(source: ref. 145)
REMOTE MONITORING-ANTENNA MAST & RECEIVER
The signals transmitted by the "Bernhard" beacon were monitored via an antenna that was located at a distance of 0.4 - 1 km (400 m per ref. 13). The signals were printed with a "Bernhardine" Hellschreiber-printer in the cabin (see Figure 39 above). So far, I have been able to determine the location of the monitoring antenna of the following Bernhard stations:
- Le Bois-Julien (Be-4): ca. 500 m northwest of the concrete ring
- Bredstedt (Be-9): ca. 550 m southeast
- Hundborg (Be-10): ca. 1 km southwest
- Trzebnica/Trebnitz (Be-11): ca. 445 m south
- Aidlingen/Venusberg: ca. 900 m due west. Ref. 103.
- Buke: ca. 550 m northwest.
The photo below shows the mast of Be-11 at Trebnica/Trebnitz. It is the only "Bernhard" mast that has survived to date (2014). It is being used for antennas of a local FM radio station. The officially registered height of this "object" is 22 m (ref. 129). According to ref. 13, the steel lattice masts (truss mast, cage mast; D: "Gittermast") by itself stood an estimated 30 m (≈100 ft) tall, and the vertical antenna on top of it about 2.4 m (8 ft). Photometric analysis of the photo below shows that the antenna radiator (on top of the box at the top of the mast) measures 1/8 the height of the mast (without the box). The mast is built on a standard 3x3 m foundation. Based on the latter, the mast is about 28½ m tall, and the antenna about 3½ m (4½ m including the box). Assuming the mast is 30 m tall, then the antenna radiator would measure 30 / 8 = 3.8 m. Assuming the mast is 22 m tall, the antenna radiator would be about 2.8 m. The antenna has a pointed tip, just like the feedpoint of the dipoles of the Bernhard's antenna arrays. So it was probably one leg of such a dipole. A ladder has been integrated into the mast. It consists simply of horizontal sections of L-bracket, mounted between one of the mast legs, and the braces to one of the adjacent mast legs. The box on which the antenna radiator is mounted, is about 50 cm wide. The tall mast was delivered pre-assembled to the "Bernhard" site - at least at Aidlingen/Venusberg (ref. 103).
Fig. 86 The monitoring antenna mast of Be-11 at Trzebnica/Trebnitz
(photo (1980s): courtesy C. Piotrowski, used with permission)
The next photo shows a man just below the top of the mast. Assuming he is 1.7 m tall, the mast measured about 23 m.
Fig. 87: Monitoring mast - note the equipment and cable installed at the bottom of the mast
(source: ref. 13; probably Be-7 at La Pernelle, based on the other photos in ref. 13)
Fig 88: Looking up inside the monitoring antenna mast, and the base of the mast at Be-11 Trzebnica/Trebnitz
(©2014 C. Piotrowski, used with permission)
Fig. 89: The dimensions of the standard concrete foundation of the monitoring masts
(data source: Czarek Piotrowski, used with permission)
Each mast was placed on a 3x3 meter concrete slab foundation. The concrete slab is at least 75 cm thick (2½ ft). The one shown below on the left, has two rectangular dimples (see arrows); their purpose is unknown. Possibly they are vertical holes in the concrete, for inserting steel posts for mounting equipment, such as shown in Fig. 86 above.
Fig. 90: Concrete foundations of the monitoring mast of Be-9 at Bredstedt (left) and Be-10 at Hundborg
(sources: R. Grzywatz (Be-9); Hundborg Lokalhistoriske Arkiv (Be-10); both used with permission)
As stated before, the signals transmitted by the "Bernhard" beacon were monitored via a remote antenna, by printing them printed with a "Bernhardine" Hellschreiber-printer in the cabin (see Figure 39 above). There was a cable between the remote antenna and the the "Bernhard ring" (ref. 13).
- Un-amplified signal from the remote antenna to a radio receiver in the cabin.
- The Wehrmacht had special cable("konzentrisches Hochfrequenz-Spezialkabel") with an attenuation of only 2.6 dB/km, comparable to high-quality modern coax cable. Note: cheap modern coax cable (e.g., RG-58/U) has an attenuation of 82 dB/km !
- This may have been an option, but unnecessarily expensive.
- RF-amplifier at the antenna or at the base of the mast, and the amplified signal via a cable to a radio receiver in the cabin.
- This option requires a remote amplifier, and either batteries at the amplifier, or a cable with power from the cabin to the remote mast.
- As with the un-amplified antenna signal, an expensive cable would be needed.
- Radio receiver at the remote mast and its audio output via cable to cabin
- This option also requires either batteries at the mast, or a cable with power from the cabin to the remote mast.
- The audio output signals could be sent to the cabin via regular field telephone cable.
- "Crystal radio" receiver at the antenna or at the base of the mast, its audio output via cable to cabin
- Ref. 99 refers to the antenna mast as a "Diodenmast". This German term suggest that a "Diodenempfänger" was used (a.k.a. "Detektorempfänger", "Kristalldetektorempfänger"). This is known in English as a "crystal radio" or "crystal set". They were popular in the early days of radio, and got their name from a small piece of crystal that was used in the signal detector. In the 1930s, the inconvenient "crystal detector" was replaced with a diode. Such diode-receivers are not only simple, they are also passive. No separate source of electric power, such as a battery, is required: the simple circuit is powered by the received radio signals.
- The audio output signals could be sent to the cabin via regular field telephone cable.
Would the audio output of a diode-receiver have been strong enough? This depends on three parameters:
- The signal level required at the input of the Hellschreiber printer-amplifier in the cabin. A Wehrmacht Hell Feldfernschreiber had a specified nominal output signal amplitude of 2.5 volt (900 Hz tone pulses). To ensure proper printing at a receiving Feld-Hell machine, the maximum allowed cable damping was 5 Neper, which is about 43 dB, or a voltage attenuation factor of about 140x. That is, the printer amplifier required an audio input signal with a minimum amplitude of 2500 / 140 ≈ 18 mV.
- The signal attenuation (damping) of the audio signals over field telephone cable with a length of 1 km (the maximum distance between the mast and the "Bernhard" ring). According to a 1945 manual of the Hell Feldfernschreiber (ref. 146), its range over standard field telephone cable of type DL500 was 36 km (22 km when wet), 60 km over regular pupin-cables (cable with a loading coil/inductance at regular intervals, typ. 250 m for German field cable; "bespultes Kabel", "Pupin-Kabel"), and 160 km over special pupin-cable of type FL250. That is, worst-case 22 km for 43 dB damping, or no more than 2 dB for 1 km. The required minimum 18 mV plus 2 dB is about 23 mV. So that would have been the required minimum output signal of the diode-receiver.
- The RF field-strength induced at the remote antenna by the "Bernhard" transmitters and antenna system. Per the definition of the ITU (ITU-R BS.561-2), the field strength of a ½λ-dipole with an effective radiated power (ERP) of 1 kW is 222 mV per meter, at a distance of 1 km. Also see ref. 150. The "Bernhard" installation had an ERP of at least 7 kW (my estimate). I..e, at least 1000 mV per meter at the monitoring antenna. That would have been more than enough to generate 23 mV at the receiver output!
Field telephone line installations often had lightning protection at both ends: fuses ("Blitzschutzpatronen") in a junction box ("Anschlußkasten", AK). Figure 85 above shows two equipment boxes that are installed near the bottom of the mast. This may have been a crystal radio and a fuse box.
The schematic below shows a crystal radio that can power a small loudspeaker (ref. 147), similar to a small 1-transistor radio. It has a double-tuned circuit, a full-wave diode rectifier, followed by a voltage-doubler. The output voltage across the capacitors can be connected directly to standard high-impedance headphones (4000 Ω). It can also be connected to a low-impedance load, such as a loudspeaker or a standard "600 Ω" phone line. To match that impedance, a simple output- transformer (about 1:6 to 1:10) must be used. Solid-state diodes were readily available at the time, such as "Sirutor" diodes that were used in various Hellschreiber models. These were quite suitable for crystal radios (ref. 148, 149), as they have a forward voltage (a.k.a. "knee" or "turn-on" voltage) of only 0.2-0.3 volt (Sirutor type 1b).
Fig. 91: Schematic of a "crystal radio" receiver, capable of driving a small loudspeaker
(source: adapted from ref. 147)
Fig. 92: A "crystal radio" receiver built per the schematic above
(source: ref. 147)
SUPPLY OF ELECTRICAL POWER
The "Bernhard" installation had several consumers of electrical power:
- Four electric locomotives, reportedly 10 HP each ≈ 13.6 kW, hence about 55 kW in total.
- Two transmitters, each with an output power of 500 watt, or 1 kW total. The table below shows the specified power supply for a number of transmitters of the era, including other beacon transmitters. The power supplies are dimensioned for at least four times the transmitter output power. Let's assume five times, i.e., 5 kW.
- Miscellaneous (e.g., heating & lighting in the cabin) - let's assume 1 kW.
Adding up the three types of electrical loads, we arrive at an estimated total load of 55 + 5 + 1 = 61 kW. To translate this to the required power from an AC source, we have to assume a worst-case phase angle between the generated voltage and current. For a reasonable cos(φ) = 0.8, the AC-power source would have to be dimensioned for at least 61 / 0.8 = 76 kVA (excluding power required for barracks near the "Bernhard" ring).
Fig. 93: Specified power supply rating for a number of Wehrmacht transmitters
(based on data sheets)
To be able to operate independently from the local public power grid, the "Bernhard" installation had its own local power generator, driven by a combustion engine (diesel or gasoline/petrol). The "Bernhard" power supplies must have been able to adapt to this. Based on the local situation (access to the local public power grid), the "Bernhard" installation often included a high-voltage bunker ("Hochspannungsbunker"), e.g., ref. 99. It contained one or more transformers, to connect to the multi-phase regional Hochspannungsnetz (high voltage public power grid, 110-220 kV in Germany), or to the local Mittelspannungsnetz (6-60 kV, typically 10 or 20 kV). The local Niederspannungsnetz (several km) carried less than 1 kV. Incidentally, towards the end of the war, the minimum frequency of the 50 Hz power grid was reduced to 43.3 Hz in the Central German block, and 41 Hz in the Western German block. Ref. 14.
Fig. 94: The two sources of electrical power of the "Bernhard" installation
The next table lists the horse power (HP) of the engine for a number of Wehrmacht electrical power generators ("Maschinensätze") for transmitters, various models of Flak-Scheinwerfer (search-lights for Anti Aircraft gun installations; ref. 152A, 152B) and even a field bakery. This ranges roughly from 1.5 - 3 HP/kVA. This implies an engine with 115 - 240 HP for the "Bernhard" generator. The specification of the actual "Bernhard" generator and associated engine is unknown.
Fig. 95: Engine power for a number of Wehrmacht generators
Transmitters with more than 200 watt output power typically had a separate power supply ("Netzanschlußgerät"), to convert 3-phase 220/380 volt AC-power to the required high and intermediate DC voltages for the anode of the transmitter's amplifier tubes (valves), and high-current AC or DC power for the heater filaments of the tubes. The power supplies had metal rectifiers or an AC-motor with DC-generator, voltage regulators, filtering, and overload protection.
Ref. 103 suggests that the generator(s) of the "Bernhard" station at Aidlingen/Venusberg were powered by two engines that came from high-speed patrol boats of the French navy. However, the French navy's "vedette rapide" boats were standard British-built "Fairmile" models. The smallest model had two Hall-Scott Defender V12 gasoline (UK: petrol) engines of 650 BHP. The larger "A" model had three 600 HP engines. Clearly more power than needed for a Bernhard installation, even if power was also generated for the local FLAK unit and its search-light(s). Perhaps the second engine was used for a back-up power generator.... On the other hand, this particular "Bernhard" station was built towards the end of the war, when materials other than rocks and stone were in increasingly short supply. A strong engine that cost nothing, is better than a correctly sized engine that is not available... There are two stone buildings near the ring at Aidlingen/Venusberg. In the middle of one of them,, there is a rectangular concrete slab that measures 1.4 x 3.25 meters (4.6x10.7 ft). The slab has 6 pairs of shallow round dimples, 8 and 10 cm in diameter. The purpose of the slab in unknown. Possibly the dimples corresponded to mounting feet of a motor-generator.
Fig. 96: A concrete slab in the middle of an ancillary building at Aidlingen/Venusberg
The original label of the photo below is "Power supply and generator of the Hundborg installation". It was installed in a bunker or barrack near the ring. Given the vertical arrangement of the "generator", it does not appear to have been driven by a combustion engine.
Fig. 97: Power supply and generator of the Hundborg installation
(photo courtesy Mike Dean, US National Archives & Records Adm. (NARA) image nr. 111 SC 269041; US gov't = no ©)
Below one of the windows of the round building of the "Bernhard" at Arcachon, there is an old 4-prong junction box with triangular shape. Near this box, several old cables enter the building through the wall, just above floor level. This appears to be where 3-phase AC-power entered from the generator in a nearby building.
Fig. 98: Large 4-prong (3-phase) junction box on inside wall of the round building of Be-14 at Arcachon
Below is a listing of patents related to the "Bernhard" system.
|Patent number||Patent office||Year||Inventor(s)||Patent owner(s)||Title (original)||Title (translated)|
|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]|
|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. Lohmann||Telefunken GmbH||Verfahren zur Richtungsbestimmung mittels rotierender Richtstrahlung||Method for direction-finding with a rotating directional beam|
|767525||RP||1939||A. Lohmann||Telefunken GmbH||Einrichtung zur Speisung eines rotierenden Richtantennensystems||Device for capacitive coupling of a transmitter to a rotating directional antenna system|
|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|
|767531||RP||1939||A. Lohmann||Telefunken GmbH||Verfahren zur Richtungsbestimmung||Method for direction-finding [dipole antenna array arrangement with side-lobe suppression]|
|767532||RP||1939||A. Lohmann||Telefunken GmbH||Sendeanordnung zur Durchführung eines Verfahrens zur Richtungsbestimmung||Antenna arrangement for the implementation of a method for direction finding|
Patent office abbreviation: RP = Reichspatentamt (Patent Office of the Reich), DP = deutsches Patentamt (German Patent Office)
Patent source: DEPATISnet
UNKNOWN / UNCONFIRMED / UNCLEAR ASPECTS
- Transmitter types
- The purpose of FuSAn725
- From the available photos of "Bernhard" installations, it cannot be determined whether the neutral point of the dipoles was mounted to the truss of the structure with an insulated attachment, or conductive
- Feedline between transmitters and array feedpoints: 2-wire, 2-shielded-wire, ...
- Type of feedline cable between transmitters and each antenna array
- Open-wire feedline between the dipoles: characteristic impedance (spacing between the wires, wire diameter)
- Was the carrier frequency for the constant tone 10 kHz than the carrier for the Hellschreiber azimuth-signal, or vice versa.
- Gauge of the rail track, rail profile, continuous welded rail or jointed rails
- Size (diameter, thickness) of the optical disk with the azimuth data in Hell-format
- Implementation of the station-identifier: separate track on the optical disk, integrated with azimuth data, separate disk?
- Power rating and type of generator-motor/engine and the of the generator
- Speed control & regulation of the electric locomotives, incl. if it was closed-loop speed control or manual
- Accuracy of speed regulation vs. Hellschreiber-printer synchronization margin
- Purpose/content of the four corner-sheds
- Number of powered axes per locomotive
- Purpose of dimples in the concrete ring, between rail ties/shoes
- Remote receiver at the monitoring mast, telephone cable?
- Slip-rings inside round building (type, size, number of rings)
- 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. 5: pp. "Instruments of Darkness: The History of Electronic Warfare, 1939-1945", new ed., Alfred Price, Greenhill Books publ., 2005, 272 pp., ISBN-10: 1853676160; original edition: William Kimber and Co., Ltd, 1967.
- excellent German translation: "Herrschaft über die Nacht: Spionen jagen Radar", Alfred Price, publ.: Bertelsmann Sachbuchverlag Reinhard Mohn, 1968, 304 pp., ASIN B0000BT35X
- 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: www.cdvandt.org
- 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. 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. 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. 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. 85: Files available in the company archive of the Deutsches Technikmuseum, Berlin/Germany:
- Ref. 85C: "Materialsammlung zur Geschichte der Ortung, insbesondere Leitstrahl und Drehfunkfeuer "Bernhard" und "Bernhardine" (FuG 120)" [Material regarding the history of navigation, in particular beams, rotary beacon "Bernhard", and "Bernhardine" (FuG 120); technical description of beam-systems "Wotan I", "Wotan II" (X-System), "Knickebein" (Y-System); correspondence & documents about beam-systems and "Bernhard"/"Bernhardine", drawings of the "Bernhard" installation at Trebbin/Glau, technical description of the Telefunken rotary beacon (part 1) and system description ED1-4262 by A. Lohmann]; Archive file I.2.060C (Fa. AEG-Telefunken), I-Num. 07823, 1942-1967
- Ref. 85E: "Stücklisten und Schaltbilder für Drehfunkfeuer-Anlage "Bernhard"" [Parts lists and schematics of the rotary "Bernhard" beacon; cable lists; adjustment & test specifications for the circular rail track]; Archive file I.2.060C (Fa. AEG-Telefunken), I-Num. 07753, 1941-1947
- Ref. 85F: "Stücklisten und Schaltbilder für Drehfunkfeuer-Anlage "Bernhard" für die Flugzeugortung" [Parts lists and schematics of the rotary "Bernhard" aircraft navigation beacon; installation instructions]; Archive file I.2.060C (Fa. AEG-Telefunken), I-Num. 07755, 1941-1946
- Ref. 85G: "Beschreibung zur technischen Änderung des Lüfters für die Skalensenderanlage "Bernhard"" [Description of the modification of the ventilation of the "Bernhard" compass-rose transmitter]; Archive file I.2.060C (Fa. AEG-Telefunken), I-Num. 04404, 1942
- Ref. 85H: "Forderungen u.a. von Hein, Lehmann & Co KG, Berlin (Antennenanlagen)" [Invoices, e.g., from Hein, Lehmann & Co]; Archive file I.2.060C (Fa. AEG-Telefunken), I-Num. 00541, 1944-1957
- Ref. 91: pp. 60-69 in "Mémoires sans concessions", Yves Rocard, Grasset, 1988, 302 pp., ISBN-10: 2246411211
- Ref. 93: pp. 116, 117 in "The High-Frequency War - A Survey of German Electronic Development", E.S. Henning, HQ Air Materiel Command (AFMC), Wright Field, Dayton/OH, Summary report No. F-SU-1109-ND, 4 June 1946 [pdf of full document See note 1
- Ref. 99 "Völlig in Vergessenheit geraten: Funkanlage bei Buke", Hans-Walter Wichert, in "Die Warte - Heimatzeitschrift für die Kreise Paderborn und Höxter", Nr. 24, December 1979, pp. 14-15, ISSN 0939-8686 (courtesy Mr. R. Gellhaus and H.-W. Wichert)
- Ref. 103: "Aidlingen Venusberg", web-page of Forschungsgruppe Untertage e.V. [pdf] Also part of "Pressebericht Tageswanderung zum Venusberg", Reiner Schopf, p. 28 in "Stadt Holzerlingen Nachrichtenblatt", 1 August 2014.
- Ref. 115: Drawing of the cross-section of the Bernhard at Hundborg/Gåsbjerg hill, Jens Salmonson, 1982, source: Hundborg Lokalhistoriske Arkiv, used with permission.
- Ref. 127: "Vliegveld Bergen NH 1938-1940", J.H. Schuurman, A.W. de Poel (ed.), Uitgeverij De Coogh, 2001, 336 pp., ISBN 9075440049
- Ref. 128: personal correspondence with Y. Rose, December 2013 - April 2014
- Ref. 129: "Trzebnica *Raszów*", source: RadioPolska website
- Ref. 135: Section 4.2 and 4.3 in "Antennbuch", Karl Rothammel (DM2ABK), Deutscher Militär Verlag, 7th edition, 1969, 593 pp.
- Ref. 136: "Die Antennen", Section I in "Fibel FuG 200 für den Funkwart und Bordfunkmechaniker" 2nd ed., April 1944, 53 pp.
- Ref. 137: "Ausstrahlung, Ausbreitung und Aufnahme elektromagnetischer Wellen", L. Bergmann, H. Lassen (eds.), Julius Springer Verlag, 1940, 280 pp., Vol. II of "Lehrbuch der drahtlosen Nachrichtentechnik", N. v. Korshenewsky, W.T. Runge (eds.) [pdf]
- Ref. 138: p. 15-20 in "Richtverbindungsgeräte", Vol. 2 of "Richtverbindungen", Riv-2, Luftnachrichtenschule Halle (Saale), 1st ed., September 1942
- Ref. 139: array antennas
- Ref. 139A: "High-frequency Antennas" in "Radio Antenna Engineering", Edmund A. Laport, McGraw-Hill Book Co., 1952, 560 pp., [pdf 30 MB] © Creative Commons License; also as free online e-Book here.
- Ref. 139B: "The Linear Broadside Array", Chapter 4 in "Array and Phased Array Antenna Basics", Hubregt J. Visser, John Wiley & Sons, 2005, 378 pp.
- Ref. 139B: "Reflective Array Antenna", Wikipedia [pdf]
- Ref. 139C: "Phased Array Handbook", Robert J. Mailloux, 2nd ed., Artech House, 2005, 515 pp.
- Ref. 139D: "Wire Array Antennas", Chapter 8 in "Antenna Toolkit", Joseph J. Carr (K4IPV), 2nd ed., Newnes, 2001, 263 pp.
- Ref. 139E: "Multi-Element Arrays", Chapter 8 in "The ARRL Antenna Handbook", 21st ed., 2007, 976 pp.
- Ref. 139F: "Antenna arrays", Mississippi State University ECE4990 course notes nr. 6, Patrick Donohoe, 2014 [pdf]
- Ref. 139G: "Array Antennas", Chapter 20 in "Electromagnetic Waves and Antennas", Sophocles J. Orfanidis, 2014 [pdf]
- Ref. 139H: "Arrays of Dipoles and of Apertures", Chapter 11 in "Antennas", John D. Kraus, 2nd ed., McGraw-Hill Book Co., 1988, 924 pp.
- Ref. 139J: p. 170 in "Über die Richtcharakteristik von in einer Ebene angeordneten Strahlern" [About the directivity of planar arrays], H. Stenzel, in "Elektrische Nachrichten-Technik" (ENT), Vol. 6, Nr. 5, May 1929, pp. 165-181; source: cdvandt.org
- Ref. 139K: "Ausgewählte Fragen über Theorie und technik von Antennen - Zusammengestellt aus Vorträgen anläßlich der Arbeitsbesprechung "Antennen" veranstalltet vom Generalbevollmächtigten für technische Nachrichtenmittel in Verbindung mit dem Vierjahresplan-Institut für Schwingungsforschung", 24-26 March 1943, Vol. 2, Zentrale für wissenschaftliches Berichtewesen der Luftfahrtforschung des Generalluftzeugmeisters (ZWB), Berlin-Adlershof; source: cdvandt.org
- Ref. 139K-a: pp. 134-144, "Grundsätzliches über Richtantennen" [basics of directional antennas], W. Moser
- Ref. 139K-b: pp. 45-63, "Der Einfluss der Reflektorwand auf die Ortskurve von Breitband - Rohrdipolen" [Effect of a reflector surface on the impedance of thick broadband dipoles], G. Dieckmann
- Ref. 139K-c: pp. 5-29, "Grundlagen der Breitbandantennenanlagen" [basics of broad-band antenna systems], O. Zinke
- Ref. 139K-d: pp. 223-231, "Neue Erfahrungen der Deutschen Reichspost in der Speisung von Antennen für lange und kurze Wellen" [new experiences of the with feeding long-wave & shortwave antennas], Ellrodt
- Ref. 140: "Zum 75 jährigen Bestehen der Firma Hein, Lehmann & Co., A.-G., Düsseldorf. 1888 - 1963", Hein, Lehmann & Co., 1963, 123 pp.
- Ref. 141: "FuSAn, Peil- und Funkanlagen", Harry Lippmann, 2007, Deutsches Atlantikwall-Archiv [pdf]
- Ref. 142: "Bandbreitenfragen bei Anwendung der Siemens-Hell-Fernschreibtechnik" [signal bandwidth issues with A2 and A3 modulation], Rudolf Zimmermann, 7 pp., Technische Mitteilungen des Fernmeldewerks, Siemens & Halske A.G., Wernerwerk, Abteilung für Telegrafengerät, Berlin-Siemensstadt, May 1940, SH 7998, 1. 8. 40. T T1.
- Ref. 143: "AS 4 Anflugführungssender Geräte-Handbuch", D. (Luft) T. 4456, June 1943, 36 pp.; source: www.cdvandt.org
- Ref. 144: "Schlechtwetterlandeanlagen", Telefunken brochure W.B.160D (2000), 1936 (?), 4 pp.; source: www.cdvandt.org
- Ref. 145: "Schleifringübertrager allgemein", pp. 166-169 in "Funk- und Bordsprechanlagen in Panzerfahrzeugen", Vol. 3 of "Die deutschen Funknachrichtenanlagen bis 1945", H.-J. Ellissen, Marketing & Technik Verlag, 1991, 240 pp., ISBN 3-928388-01-0
- Ref. 146: p. 3 in "Instruktion för Hellskriv-apparat" [in Swedish], Kungl. Arméförvaltningens Tygavdelning, SiB/040:3045, 13 July 1945, 40 pp. (courtesy Willi Reppel, SM6OMH)
- Ref. 147: "High-Power Crystal Set", Walter B. Ford, in "Popular Electronics", August 1960, pp. 63-65
- Ref. 148: "Empfangsversuche mit dem Sirutor", F. Nitturra, in "Funkschau", Vol. 13, Nr. 2, February 1940, p. 29
- Ref. 149: "Anregungen zum Detektorempfang", H.-A. Dennig, in "Funkschau", Vol. 16, Heft 8/9, August/September 1943, p. 80
- Ref. 150: "Crystal Set Analysis", Berthold Bosch (DK6YY), March 2002, 15 pp. [pdf]
- Ref. 151: p. 16, 17, 22 in "Jagdschloß A (Lehrunterlagen) Teil I", 2nd ed., Lehrschule für Fernmeldetechnik, Detmold, November 1944, 115 pp. Source: www.cdvandt.org
- Ref. 152: Luftwaffe search-lights
- Ref. 152A: "Scheinwerfer der deutschen Luftwaffe", C. Mattner, in "Zeitschrift des Vereines deutscher Ingenieure", Bd. 18, Nr. 33, 14 August 1937, pp. 953-955
- Ref. 152B: "pp. 136-139 in "Der Flakscheinwerferkanonier", Ernst Schlachtmann, "Handbücher der Luftwaffe - Der Dienstunterricht in der Flakartillerie - Ausgabe für den Flakscheinwerferkanonier", 3rd ed., Verlag E.S. Mittler & Sohn, 1939, 155 pp.
- Ref. 154: "Das Mädchen, die Katze und der Amboss - Auszüge aus '100 Jahre Ziehl-Abegg'", source: industrieanzeiger.de
- Ref. 155: p. 16 in "PLAKAT - Gedanke und Kunst: Einiges aus der Plakat-Praxis", Propaganda Stuttgart, 1912, 63 pp.
- Ref. 156: "The Science of Locomotion - Why Rail Has 20X Energy Saving Advantage Over Rubber Tire Road Vehicles". Source: Brooklyn Historic Railway Association.
- Ref. 157: "Railway Engineering", Karim H. Al Helo, Department of Building Construction Engineering, University of Technology, Iraq, 64 pp.
- Ref. 158: "Adhesion in the wheel-rail contact under contaminated conditions", Yi Zhu, Licentiate thesis at the Royal Institute of Technology, Stockholm/Sweden, 2011, 100 pp. [pdf]
- Ref. 159: slide 24 in "Railroad Transportation Energy Efficiency", C. Barkan, University of Illinois at Urbana-Champaign, 2007, 61 slides [pdf]
- Ref. 160: "Some graphical solutions to electric railway problems", A.M. Buck, in "University of Illinois Bulletin", Vol. XIII, No. 47, 1916, 40 pp.
- Ref. 161: "The characteristics and control of rectifier-motor variable speed drives" [mercury-arc rectifiers], P. Bingley, in "Proc. of the IEE Part II: Power Engineering", Vol. 99, Issue 69, June 1052, pp. 189-202 ( Author's reply to discussion: pp. 205-206)
- Ref. 162: Ward-Leonard drive systems
- Ref. 162A: "Ward Leonard drives 75 years of development", K.A. Yeomans, IEEE Electronics and Power, Vol. 14, Nr. 4, pp. 144-148
- Ref. 162B: "Electrical Transmission of Power", H. Ward Leonard, United States Patent Office, Patent No. 463,802 [pdf]
- Ref. 162C: "A new system of electric propulsion", Harry Ward Leonard, in "Transactions of the American Institute of Electrical Engineers (AIEE)", Vol. IX, Issue 1, January 1892, pp. 566-577
- Ref. 162D: "Volts vs. Ohms; Speed Regulation of Electric Motors", Harry Ward Leonard, in "Transactions of the American Institute of Electrical Engineers (AIEE)", Vol. XIII, January 1986, pp. 373-386
- Ref. 162E: "Ward Leonard System", pp. 445-446 in "Synchros and Servomechanisms", Chapter 21 in "Basic Electricity", United States. Dept. of the Army, Naval Education and Training Program Development Center, FM 55-506-1, 22 April 1977, 490 pp.
- Ref. 162F: "Leonard Doppel-Umformer U65a (neuere Form)", F 054a, Ln 20 347-1, 2 pp. (3 kW generator, 9 kW motor). Source: www.cdvandt.org
- Ref. 162G: "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", Part 1: Vol. 9, Nr. 10, 1955, pp. 351-360, Part 2 (pp. 387, 388, 392): Vol. 9, Nr. 11, 1955, pp. 386-395
- Ref. 162H: "Type 7", RAF Ventnor Radar website [pdf]
- Ref. 163: Mercury Arc / Vapor Rectifiers
- Ref. 163A: "Mercury cathode tubes: Ignitron, Excitron, Sendytron", Chapter 6, pp. 210-255, of "Gas-discharge tubes", H.L. van der Horst, Philips Technical Library series, 1964, 318 pp.
- Ref. 163B: "Mercury Arc Rectifier Practice", F.C. Orchard, Pittsburg Instruments Publ. Co., 1936, 255 pp. [pdf 19 MB]
- Ref. 163C: "Mercury Arc Power Rectifiers - Theory and Practice", O.K. Marti, H. Winograd, 1st ed., McGraw-Hill Book Co. Inc., 1930, 478 pp. [pdf 38 MB]
- Ref. 163D: "Der Quecksilberdampf-Gleichrichter (Stromrichter) der Starkstromtechnik", Weissbach, in "Funk-Technik - Zeitschrift für das gesamte Elektro- Radio- und Musikwarenfach", pp. 15, 18, 19 in Nr. 1, 1948, and pp. 63, 66, 67 in Nr. 3, February 1948
- Ref. 163E: pp. 681-688 in "Electrical machines" R.K. Rajput, 4th ed., Laxmi Publications, 2006, 881 pp.
- Ref. 163E: "Converters, the mercury arc rectifier and supply to electric railways", Chapter 10 in "Electric Railways 1880-1990", Michael C. Duffy, IET, 2003, 452 pp. [pdf]
- Ref. 163F: "Some notes on ignitrons and thyratrons", F. Dörenberg, 2014, 3 pp.
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