Gunnplexers are becoming scarce in the surplus market, but X-band motion detection is very much alive. X-band detection modules are available for OEM applications, and fortunatelly, Parallax has a unit based on this module available for $34.99.
Parallax’s X-Band Motion Detector operates at 10.525 GHz and indicates detected movements with oscillations in its high/low output. All you need to make it transmit is to supply 5VDC at 8 mA and connect its “ENABLE” pin to +5V. The Parallax operates the GH100 microwave sensor module in pulsed mode, but I intend to modify the module (and write instructions on how to do it) to run CW.
The antenna end is shown in the picture below. Its microwave output is linearly polarized