The "Bernhard" antenna radiation pattern
"Bernhard" was a navigation system developed during the early part of World War II to replace the ailing "Knickebein" system, which had been effectively put out of business by Allied jamming. Another problem with the Knickebein system, which consisted of a series of semi-fixed intersecting radio beams, was that the British were able to work out, by searching for the beams, where the next raids were to be expected, so the defending aircraft could be waiting.
Given the constraints of existing airborne equipment, especially receivers, "Bernhard" was developed to operate at the same frequency range (30 - 33.3 MHz), and with the same receivers, as the already available Lorentz landing aid system and the "Knickebein". So right from the start, the developers knew that robust performance in the presence of jamming was important, as was some method of hiding the intended destination of the aircraft.
An essential property of these earlier navigation systems, and many others since, was the radiation pattern of the transmitting antenna - two fixed narrow lobes spaced a few degrees apart, so a null was formed between them. In the earlier systems, a single tone modulated transmitter fed the two antennas, the signal being switched between the antennas by the Morse letter "A", so when one beam was in use the other was not. Thus (work it out), while one beam sent "A", the other beam transmitted the letter "N" in the gaps. An aircraft flying exactly in the null between the beams heard a steady tone, to one side an "A", to the other an "N".
This essential property was retained in the Telefunken FuSAn 724/725 "Bernhard" system, but some special ingredients were added:
The upper antenna transmitted the bearing information in Hell, the lower pair transmitted the double lobe pattern with a null centered on the bearing lobe.
"Bernhard" was originally intended for short range airfield use, and experimental systems operated at 300 MHz. Sensitive receivers for this frequency were a problem, as was generating sufficient RF power to provide the necessary range, so the system was adapted to 30 MHz to suit existing equipment.
The Transmitters
Two transmitters were used at each ground station. They were each capable of
5kW output, switchable to 500W on low power. The two transmitters were operated
close enough in frequency that both signals were received at the same time
by the relatively wide receiver. The transmitters were housed in a large
rotating building about 25m long and 6m wide, mounted underneath the antenna.
One transmitter was unmodulated, but switched between antennas at an audio rate as described above, while the other was AM modulated with a Hellschreiber keyed tone. The bearing information was derived from a rotating glass disc, engraved with the Hellschreiber bearings in numerals, every 10 degrees, and with a mark every five degrees. The disc rotated in synchronism with the antenna and the Hell signal was read off optically as the glass disc rotated.
It was also possible for the ground station to be used to send short coded letter or number groups to the aircraft for guidance or command purposes. This was done using a conventional Hell transmitter, in place of the bearing readout keyer.
Antennae
The Hell modulated transmitter fed the upper antenna of the array, a
six dipole vertical collinear array over 10m high, with a massive screen reflector, used to ensure
very low side and rear lobes. The other transmitter fed two
identical antennas mounted side by side, each an eight dipole
vertical collinear array with rod reflectors, each over 10m high and some 15m wide.
These antennas were angled in azimuth relative
to the upper antenna a few degrees either side, to provide a null
that corresponded to the Hell signal peak.
The huge antenna array, 28m high, 35m wide and weighing about 120 tonne, was electrically rotated, by driving it around a circular track 22m in diameter. The antenna rotated twice every minute, so the beam passed by an aircraft every 30 seconds, and was received for only 3-5 seconds.
The receiver antenna was probably a vertical whip.
Receiver
The standard aircraft navigation receiver was used, and the audio
output taken to the radionavigation printer, a specially designed and
compact "Bernhardine" Hellschreiber receiver. This receiver contained an audio
amplifier, special audio filters to separate the null tone and the Hell
signal, and a reasonably conventional rotating helix
Hellschreiber printing mechanism.
The FuG 120a receiver operated in a Start-Stop mode, possibly controlled by reception of the tone, so the paper strip moved and the current position was indicated only as the beam went past. It is not known what method was used to correct the phase of the Hell transmissions, although apparently the text was only printed once, as in a post-war GL-Hell system.
An additional indication was printed on the paper, which displayed the current strength of the bearing null tone from the two lower antennas. This of course provided a sharp null at the exact heading, and provided a useful and precise "pointer" for the reading of the heading from the Hellschreiber scale below. It is not known how this indication, a series of vertical lines, was converted from audio tone to printed mark.
At the end of the war a simplified Hell receiver, probably the "Hermine", was developed. This lacked the bulky audio filters, and provided poorer accuracy (it probably lacked the null tone scale pointer). This receiver, the FuG 120k, was small enough to be fitted to single seat aircraft such as the Fw190. The lower ±4° resolution was considered adequate for this application.
Construction History
The 30 MHz "Bernhardine" system was first tested at Tebbin in 1941, and by May 1944
seven stations were completed or under construction. By the end of the war,
about 2500 aircraft had been fitted with the receivers, and perhaps 12,000
sets awaited installation. Most of the installations were in Ju88 and Do335
aircraft. Do any of these FuG 120a receivers survive?
Performance
The range was 400 km maximum, depending on aircraft altitude.
An accuracy of ±1° azimuth was achieved, and later this
improved to ±0.5°.
The interference immunity of the system was proven to be extremely good, not being seriously affected by jamming, and the Hell bearing display of course never gave false positions. In addition, because the beam rotated continuously, no clue to the course or destination of the aircraft using the system was revealed.