Abstract: An apparatus for testing the operation of a CW radar is disclosed. A test antenna is placed within the field of radiation of the radar's transmit and receive antenna. The sample of millimeter wave energy collected by the test antenna is coupled to a switch that routes the sample to either a dissipative load or to a reflective short. When the sample is coupled to the load the energy is absorbed. When the test antenna is coupled to the short, the sample is reflected back to the test antenna and re-radiated. A control signal produced by a square wave generator is coupled to the switch and determines whether the sample is absorbed or reflected. The result is the re-radiation of an amplitude modulated sample of the energy transmitted by the radar to form a test signal. Means are provided to adjust the frequency of operation of the square wave generator.