Gunn diode microwave oscillator with a ferrite rod with a magnet bias to provide a circulator. This is all located in a small X band waveguide cavity with a receiver detector diode. Typically supplied with a small horn antenna.
The'plexer' is short for duplexer, used to partially separate the Tx path from the Rx path. In the gunnplexer it dribbles some of the Tx local oscillator back to the Rx detector diode to make a simple Rx mixer. Tx power is about 20-30 mW depending how much DC voltage supplied to gunn diode. Was made by MA/Com and very low cost. Used in old police speed radars. The Tx signal bounced of the car with a doppler frequency shift based on speed and was pickup up and mixed down to audio frequency range which ran into a frequency counter that read out the equivalent speed of the car based on doppler shift.