Resonant cavity mixers

Hi all,

I was reading about "The thing" which uses a resonant cavity for modulating the input signal to increase its frequency three times. Could somebody please explain what happens in the cavity that it acts like a mixer?

The article is this one Passive Resonant Cavity & "Spycatcher" Technical Surveillance Devices


Thanks in advance

The spy device was no "resonant mixer"! It was a microwave resonator with one wall used as an acoustical membrane. If someone talked in the room, the voice modulated resonant frequency.
The spies across the street installed a microwave transceiver sending a CW signal; the returned signal was mixed with it, and the IF output from the mixer was amplified into a headphone. The operator could listen this way to conversations in the room across the street.
The principle is similar to a "solar phone" used a century ago; a mirror was set to reflect solar light across a street where it hit a simple resistive light detector connected to a headphone. Talking to the mirror modulated the light amplitude.

Passive Spycatcher was most-like an RFID system used today, but without using a detector and a converter at the tag side.
I've seen a very well technically documented movie about this story and seems that is almost impossible to replicate it in normal conditions and getting stable and consistent results.