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The Thing [2] is also worth reading. Soviet Union was well-aware that RF/LO leakage was being constantly monitored by the NSA, so they made a passive eavesdropping device in 1945, that didn't use a power supply. Instead, when eavesdropping was needed, an agent near the field would transmit an unmodulated carrier wave at 330 MHz, which was received by The Thing with an antenna and tuned circuit, thus activates the device. The recording is then rebroadcasted at a higher harmonic frequency. When the device is inactive, it's almost impossible to detect.

[2] https://en.wikipedia.org/wiki/The_Thing_(listening_device)

I remember when i first heard of this, i was so amazed at the cleverness of the whole thing.
The Thing is also known as "The Great Seal Bug".
Banned so publicly that it was regularly on the evening news, and so was a roaring success.