Serrate radar detector

Serrate was an Allied radar detection and homing device used by night fighters to track Luftwaffe night fighters equipped with the earlier UHF-band BC and C-1 versions of the Lichtenstein radar during World War II. It allowed RAF night fighters to attack their German counterparts, disrupting their attempts to attack the RAF's bomber force.

The first Serrate systems were developed from the AI Mk. IV radar systems, which just happened to have antennas almost perfectly suited to receiving the Lichtenstein signals. Serrate simply disconnected the display from their own transmitter and receiver and connected it to one tuned to the Lichtenstein. This produced a display indicating the direction to the German aircraft, but not the range. When the operator felt the range to be suitable, the display was reconnected to the Mk. IV's own electronics to provide both ranging and directional information during the last moments of the attack.

Serrate operations began by No. 141 Squadron RAF on the night of 7 September 1943. 179 operational sorties yielded 14 claimed fighters shot down, for 3 losses. After that point, the Luftwaffe realized what was happening and quickly introduced new versions of their radars working on different frequencies. Unlike the earlier sets, which just happened to operate on frequencies very close to the British radars, the new sets required entirely new detectors, which took some time to develop. New versions of Serrate were introduced, and moved from the Bristol Beaufighter to the faster de Havilland Mosquito, but later operations were never as successful as the original.

The AI Mk. IV radar was the first operational airborne interception radar, first used experimentally in April 1940, and entering widespread service in early 1941. These systems used a set of four receiver antennas that were arranged so they were most sensitive in different directions; two were sensitive above or below the aircraft, and the other two to the left and right. The output from these antennas was rapidly switched back and forth on the display, producing two blips for any given target, with the length of the blip indicating the strength of the signal in that direction. By comparing the length of the blips from, say, the upper and lower antennas, the operator could determine if the target was above or below their fighter.

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