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by unnouinceput
2563 days ago
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There is nothing unknown in the article. You can do this at home as well. Is very well explained how it works. For start I would suggest you get technical of how Yagi antennas work, it's the same principle coupled with cavity resonator + membrane movement used in a microphone. In the end you get your high frequency beamed back modulated by the sound. And is not AM modulation, is FM modulation. But what article indeed lacks is the power required to get the device working properly. In my experience I would guess at least 1KW would've been beamed directly at the embassy walls when Soviets were doing their surveillance using this device. |
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In order to get FM modulation from a passive resonator, the decay time of that resonator has to be much longer than the modulation; i.e. the resonator Q has to be very high, much narrower bandwidth than the modulation.
A back of the envelope calculation, for 3 kHz audio bandwidth, and a 300 MHz resonator, that sets the unloaded Q=100E3, which you can't get even close with a passive resonator, short of a superconductor. Not to mention the loaded Q from the monopole radiation is going to lower it even more.
You can get FM modulation out of an oscillator because the feedback is generating a negative resistance to compensate for the resonator resistance; the active circuit is a Q multiplier, getting you that massive Q.
The interrogator would have to set up not only a standing wave, but provide coherent feedback. Sort of a stand-off oscillator. Even if you had low-level FM modulation, it's still going to have the spectrum of DSB AM as the Bessel components are pretty far down.
Maybe it is doing that stand-off oscillator, but my hunch is the receiver is a coherent AM demodulator. You have the carrier available , so might as well use it.