page_title: Battle of the Beams
section_title: Countermeasure
hierarchical_section_title: Battle of the Beams / German systems / Y-Gerät / Countermeasure
caption_attribution_description: English: Alexandra Palace, Muswell Hill, London. The image was taken with a Panasonic Lumix DMC-TZ1.
context_page_description: The Battle of the Beams was a period early in the Second World War when bombers of the German Air Force used a number of increasingly accurate systems of radio navigation for night bombing in the United Kingdom. British scientific intelligence at the Air Ministry fought back with a variety of their own increasingly effective means, involving jamming and distortion of the radio waves. The period ended when the Wehrmacht moved their forces to the East in May 1941, in preparation for the attack on the Soviet Union.
context_section_description: The British were ready for this system even before it was used. By chance, the Germans had chosen the operating frequency of the Wotan system very badly; it operated on 45 MHz, which just happened to be the frequency of the powerful-but-dormant BBC television transmitter at Alexandra Palace. All Jones had to do was arrange for the return signal to be received from the aircraft and then sent to Alexandra Palace for re-transmission. The combination of the two signals modified the phase shift – and thus the apparent transit delay. Initially, the signal was re-transmitted at low power, not powerful enough for the Germans to realise what was happening, but enough to spoil the accuracy of the system. Over subsequent nights, the transmitter power was gradually increased.
  As Y-Gerät's use went on, the aircrew accused the ground station of sending bad signals and the ground station alleged the aircraft had loose connections. The whole scheme appealed to Jones as he was a natural practical joker, and remarked that he was able to play one of the largest practical jokes with virtually any national resource that he required. The gradually increasing power conditioned the Germans such they did not realise that anyone was interfering with the system, but believed that it suffered several inherent defects. Eventually, as the power was increased enough, the whole Y-Gerät system started to ring with all the feedback.
  The Luftwaffe, finally realising that the British had been deploying countermeasures from the very first day that the system was used operationally, completely lost faith in electronic navigation aids (as the British had predicted) and did not deploy any further system against Great Britain, although by this time Hitler's attention was turning toward Eastern Europe.
caption: Alexandra Palace
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