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.