Y-Gerät

The Battle of the Beams was a period early in the Second World War when bombers of the German Air Force (Luftwaffe) 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.

Prior to the war, Lufthansa and the German aircraft industry invested heavily in the development of commercial aviation and various systems and methodologies that would improve its safety and reliability. Among these was a considerable amount of research and development of blind landing aids which allowed aircraft to approach an airport at night or in bad weather. The primary system developed for this role was the Lorenz system, developed by Johannes Plendl, which was in the process of being widely deployed on large civilian and military aircraft.

The Lorenz system worked by feeding a special three-element antenna system with a modulated radio signal. The signal was fed to the centre dipole, which had a slightly longer reflector element on either side set slightly back. A switch rapidly alternated the opened midpoint connection of each reflector in turn, sending the beam slightly to the left and then slightly to the right of the centreline of the runway. The beams widened as they spread from the antennas, so there was an area directly off the runway approach where the two signals overlapped. The switch was timed so it spent longer on the right side of the antenna than the left.

An aircraft approaching the airport would tune one of their radios to the Lorenz frequency. If the crew found they were on the left side of the centreline they would hear a series of short tones followed by longer pauses - the pauses being the time the signal was being sent out the other side of the antenna. Hearing the "dots", they would know they had to turn to the right in order to be flying down the centreline. If they started on the right side, they would instead hear a series of longer tones followed by short pauses, while the signal was on the "dot" side of the antenna. Hearing the "dashes", they would turn to the left to capture the centreline. In the centre, the radio would receive both signals, which sounded like a continual signal, the so-called "equisignal". Flying in the known direction of the runway and keeping the equisignal on the radio, the Lorenz could guide an aircraft down a straight line with a relatively high degree of accuracy, so much so that the aircraft could then find the runway visually in all but the worst conditions.

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Wikipedia