Can you buy a working radar ... sensor, and it’s definitely a solid lesson in design. Still, if your tastes run more toward using this module on the 1.25-cm ham band, have a look at this HB100 ...

HB100 miniature microwave motion sensor is an X-band, Bi-static doppler transceiver module. This module is ideal for alarm, motion detection, vehicle speed measurement, automatic lighting, and door ...