[madengr]

Maybe someday the Soviet side of things will be declassified, and we can read the technical account from that end. Would be interesting.

There seem to be some technical unknowns in the article. I don’t think you can get FM back from the passive cavity, just AM, unless you can pump the cavity with feedback. Same goes for re-radiating at a harmonic. Maybe if the Q of the cavity were super high. Again, would be interesting to see the technical details of the receiving equipment.

[unnouinceput]

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.

[madengr]

Ha ha, I know how a Yagi works; 25 years of RF/Microwave circuit and antenna design. I don't see how you are going to get FM modulation out of this thing. This is akin to passing a CW tone through a filter at it's cutoff, then modulating the filter response. You get AM sidebands as the passband varies with modulation. The frequency does not change.

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.

[TheOtherHobbes]

I'm finding it hard to imagine no one noticed an intermittent 1kW at 330MHz from somewhere close.

[unnouinceput]

Well, the article does say a radio-amateur stumbled upon it and alerted the embassy. So in the end someone did. Since Soviets did the surveillance on key moments, and not continuous, that's what enabled for device to remain undetected for so long

[elvecinodeabajo]

I think it stills classified because it stills being in use.

[ascagnel_]

This wouldn't surprise me -- some stuff, even if it's older, is still in use because it works well. Numbers stations[0] are still a thing because they're (relatively) cheap to set up, reliable, and virtually impossible to trace the source.

[elvecinodeabajo]

The great old engineering mantra: 'If it works, don't change it'.

[ambicapter]

You forgot to link your citation.

[Crespyl]

I'm not ascagnel_, but the Wikipedia page covers the topic pretty well: https://en.wikipedia.org/wiki/Numbers_station

[chadcmulligan]

da comrade, waiting for the great soviet empire to rise up again

[VLM]

My guess is the excitation power is broadband, which explains the unusually high power required.

Smooth white noise with a spectrum 50 KHz wide centered on the theoretical center of the resonator, then the 3rd harmonic can rebroacast a resonant tiny high Q slice of up to 150 KHz deviation FM as the microphone diaphragm wiggles with sound.

Imagine a transmitter design where you make a very powerful white noise signal, then filter out a little bit of some tuned frequency and pass that narrow sliver out the antenna. A historical example of this transmitter topology would be old fashioned spark gap transmitter generating noise from DC to daylight as they used to say, then couple it to a resonant antenna. "The Thing" is merely a much more elaborate and refined variation on the idea.

[madengr]

See my response above about the FM modulation; I don't think it will work like that; it would be AM. Assuming the resonator diaphragm is linear and small signal (WRT to the sound pressure), you won't get a radiated carrier 2nd harmonic. A cavity resonator (2nd order) would produce a 2nd harmonic in AM audio as the carrier rides that resonator response.

If the diaphragm goes non-linear (say from high sound pressure), then perhaps you'd get harmonics and maybe an f/2 response. At this point it's acting like a parametric amplifier, with your carrier frequency and pump frequency. Parametric amps need a strongly non-linear reactance, hence pumping a varactor diode with a strong signal.

[georgeNT]

The receiving equipment is a standard CW illuminator, see compatible model here:

https://www.schneier.com/blog/archives/2014/01/ctx4000_nsa_e...