Resonant cavity mixers

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MobiNaz

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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.
 

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