Abstract: When the transmitter is operated in the keyed mode, the HF properties of the Gunn diode change because the diode warms up during the pulses and cools again in the pulse intervals. With a transmission frequency of 30.5 GHz a frequency change of 10 MHz was found during the duration of a pulse, this frequency change being highest at the beginning of the pulses and lowest at the end of the pulses. For this reason the evaluation circuit is constructed so that it evaluates only during a very short period of the pulse duration. Then, namely, the doppler signal can be made "visible" to a sufficient extent.