Someone, a few months ago said there was something on ebay for old intrution alarms. I was wondering were in the world I could get a gun diode.

I'm not familiar with any other modern way to do doppler thn with gun diodes. Except, once I read something about handheld radars being demonstrated in the laboratory. I think it might of been made with new transistors rather than gun diodes. But I don't know if it was doppler.

Have you considered ultrasound? Bats seem to do just fine with it.