Development
The PPI effort started fairly early in the history of radar; Hans Hollmann filed a patent for the basic concept in 1936. At that time development of GEMA's other radars, notably the Freya, took priority, and work on the system did not start until 1939. By this time, radar development had progressed to the point were a prototype could be constructed by re-using systems from various production radars.
Just such a system was built south of Berlin, known as the Tremmen Radar Tower. It mounted a large antenna consisting of two rows of four half-wave dipoles aligned horizontally, rotating on a shaft located at the top of the tower. It was found that at least five pulses needed to be returned in order for the target to become visible on the scope, so the rotation rate of the antenna was adjusted to synchronize with the pulse repetition frequency of the radar. The radio equipment was taken from the Wassermann and Freya units, and operated on a basic wavelength of 2.4 m (~125 MHz).
Read more about this topic: Jagdschloss Radar
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