Klein Heidelberg Parasit
Description of the radar set, tactical-technical characteristics
Figure 1: Klein Heidelberg Parasit, back-to-back on a Wassermann antenna
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Klein Heidelberg Parasit
Klein Heidelberg Parasit, also shortened to Klein Heidelberg or Heidelberg-Gerät, was a passive, bistatic radar that was tuned to the active emissions of the transmission towers of the British Chain Home in the shortwave range. Because of its passive mode of operation, it could not be detected and thus could not be jammed. One of the sets was located at Coastal Base No. 24, about 2.5 km from Oostvoorne, 8 km southwest of Hoek van Holland.
The method was developed by Dipl.-Ing. Fritz Wächter of the Telefunken company, evaluated the reconnaissance results of radio technicians of the Reichspost for this purpose. Research on this project began around 1942 in response to the Allied jamming of active radar frequencies. Although the Chain Home radio towers all transmitted on the same frequency, they did so in a fixed time rhythm, named “Running Rabbit” after the fairy tale “The Rabbit and the Hedgehog.” The Klein Heidelberg Parasit was able to synchronize itself to this time rhythm so that only one specific transmitter tower whose exact position was known could ever be selected for measurement.
A total of six receiver positions were set up on the west coast of France, Belgium, and the Netherlands. Initially, the receiving antennas were additionally mounted on the back of existing Wassermann antennas. They consisted of a group of six rows with three half-wave dipoles each. Later, separate, somewhat wider antennas based on a Wassermann turntable and mast were used. A smaller antenna, set up at a distance of about 60 m, was permanently aligned on the selected transmitter tower and provided the reference signal.
The evaluation device was called the “Waechter-Geraet.” It contained two J-scopes. The directly received reference signal was displayed on the left J-scope. The reflected signal is on the display on the right. On this display unit, the start of deflection relative to that of the reference signal was delayed by a handwheel and a precise potentiometer until both display units showed the same image. Using a scale switch, this adjustment could be made with high precision. The mechanical display on the handwheel then indicated the exact delay time. The conversion to position information was done in a graphical diagram by selecting an ellipse matching the delay time. A message about the target then only named the number of the ellipse and the measured bearing angle.