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THE TRANSMISSION GALLERY
ALEXANDRA PALACE AT WAR
By Peter Bigwood
On 26- Feb-1935 an experiment using the 6MHz BBC Empire Service transmitter at Daventry detected a Heyford bomber flying at ten thousand feet. So began the British development of early warning radar that led to the building of the wartime coastal 'Chain Home' radar stations in time for the Battle of Britain. Meanwhile, Germany was using their radio technology to devise bomber navigation systems based on the principal of the Lorenz beam.
At the start of the war, the Germans deployed two similar systems, Knickebein and X-Gerät (Wotan I). These consisted of Lorenz type beams at around 30 and 70 MHz respectively that intersected over the target. Simple jamming of these beams should have been relatively easy but the British monitoring equipment was not accurate enough to define the exact frequency of the beams for every raid or the frequency of the tone modulation. Accurate frequency information decoded by Bletchley Park often did not arrive in time. It was believed at the time that the devastating raid on Coventry on the night of 14th November 1940 was a result of inaccurate detection of the beam frequencies.
By the end of 1940, Britain was having more success in jamming these beams and the Germans began deploying Y-Gerät or Wotan II. This system was unique in that it used only one Lorenz type beam along which the bombers would fly. A second transmission around 45MHz was aimed at a transponder in the aircraft, which re-transmitted on an offset frequency. The home station was able to deduce the range of the aircraft, and hence distance from the target, by calculating the return delay. The home station would order the release of bombs or markers as the aircraft reached the target. The system was potentially very accurate.
The British immediately realised that the powerful Alexandra Palace TV transmitter was capable of transmitting on the transponder frequencies and instigated 'Operation Domino'. Using the receiving station at Swains Lane, Highgate, the return signal from the aircraft's transponder was retransmitted back to the aircraft on its receiving frequency by the Alexandra Palace TV transmitter and hence back to the aircraft's home station. This extra loop producing a false distance reading.
The Swains Lane receiver station was connected by Post Office landline to the Alexandra Palace transmitter. By using a low-voltage motor, this line controlled any drifting in the lock-on carrier beam, thus eliminating any give-away heterodyning beat-notes. It took only ten minutes to retune all the stages of RF circuits in the transmitter to the exact transponder frequency; however, the Luftwaffe only needed thirty minutes from take-off before bombing over London. The operators at Alexandra Palace were equipped to listen to the Luftwaffe pilots receiving confused messages from their home stations in Northern France. For its trouble, Alexandra Palace received a stick of six bombs parallel to and 100 yards from the famous south wing of the station.
The 'Domino' counter measures from Alexandra Palace were a complete success and by the spring of 1941 the Germans had abandoned all the beam systems against Britain. They were used later, notably in the Baedeker raids, but it was the British who, in the end, developed the most accurate and successful radio bombing system which became known as 'Oboe'.
DOMINO versus Y-GERAT - A first-hand account
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