The HB100 costs about $3 on eBay and is intended for use as a Doppler radar for a garage door opener.    It consists of a mixer, local oscillator, transmit and receive antennas  on a little PCB and metal can. 

It can be repurposed as either a transmitter or receiver at 10 GHz ( 3 cm).   As a transmitter the audio is applied to the mixer (IF) and the audio level controls the modulation index.  The received signal comes out as a FM modulated signal riding just above the the FM Broadcast band.  Using a HackRF helped locate the output signal.

The downside is these devices are very unstable and drift just sitting.  This is magnified by transmitter and receiver LO drifting  differently.  A slight tap will cause the frequency to jump and I assume that they are sensitive to temperature also.   The signal is received with SDR# and is shown here.  I can block the transmit path to attenuate the signal.

I put the HB100s in little boxes to help orient the antennas.

More to follow with better production values.