The book spycatcher https://en.wikipedia.org/wiki/Spycatcher is an engaging read that has many interesting bits about bugging capabilities. Its author, Peter Wright played a significant role in figuring out how the device worked.
Wright determined that the bugging device, dubbed The
Thing, was actually a tiny capacitive membrane (a
condenser microphone) that became active only when 330
MHz microwaves were beamed to it from a remote
transmitter. A remote receiver could then have been used
to decode the modulated microwave signal and permit
sounds picked up by the microphone to be overheard. The
device was eventually attributed to Soviet inventor,
Léon Theremin." -- https://en.wikipedia.org/wiki/Peter_Wright
Those who are more informed about these things than the layman don't like the book as much.
It's best known for its otherworldly sounds. But the control mechanism is relevant here: two antennas are positioned to create a 2D field, and the motion of the musician's hands within that field controls tone and amplitude. Clearly, the guy was a talented radio researcher.
He invented it in 1920 in Russia, emigrated to the US in the late 20s, and ended up back in Russia in the late 30s. Sounds like a fascinating guy. Does anyone have any recommendations for biographies of him?
My favorite factoid about him is that late in life, Theremin became convinced that the secret to immortality lay in "young blood". Certain billionaires seem to be testing his theory today.
Thanks to your explanation I now imagine the 'Thing' to be a bit like Theremin player Dorit Chrysler in this live performance by Trentemøller: Silver Surfer, Ghost Rider Go!!!:
It seems basically a tuned circuit connected to an aerial where the frequency of the tuned circuit varied as the diaphragm moved. I guess in the same way that if you shout at a tuning fork it will vibrate at its usual frequency, the circuit would oscillate at a frequency corresponding to the diaphragm position. I wonder if this could be used for an IoT device.
A friend of mine told me that when he was a baby, his parents were US diplomats stationed in East Berlin. Knowing that the East Germans would be assigning someone to go through their garbage, they made sure to put bits of paper in the kids' diapers.
I'm pretty sure the signal cut out because the Soviets feeding it energy realized we were on to it, the guys weren't hammer the seal with the device when it cut out. Unlikely we'd not finish the job in discovering it, but that was the least worst thing they could do at that point.
I first learned about this in the excellent and thought provoking book "Code Warriors" which covers all the ways that US intelligence efforts relied on code breaking after the end of WWII, but were often trumped by the Soviet's superior spy-craft.
I just saw this on Wednesday along with many other historical spy devices at the International Spy Museum in Washington, DC, USA. Definitely worth a visit - figure on spending at least 2-3 hours. There's so much to see.
"The device he came up with consisted of an antenna and a cylinder with a thin membrane that acted as a microphone. Soviet agents stationed across the street from Spaso House would turn the device “on” by focusing a radio signal on it, which then bounced back to their radio receiver. When the ambassador or anyone else in the study spoke, the sound waves caused the membrane to resonate and alter the signal that returned to the Soviets, allowing them to hear the conversation."
The wikipedia link in a sibling comment has even more information.
There's a lot of info out there if you search at all. It acted a lot like an RFID chip it was passively powered by the receiver transmitting on given frequency. The mic was capacitive and when powered changed the signal that was being returned by the bug.
> Soviet agents stationed across the street from Spaso House would turn the device “on” by focusing a radio signal on it, which then bounced back to their radio receiver. When the ambassador or anyone else in the study spoke, the sound waves caused the membrane to resonate and alter the signal that returned to the Soviets, allowing them to hear the conversation.
It could contain more detail but sounds like it was altering a radio signal that was completely external to the device and therefore had no battery at all.
just to clarify the wording of your essentially correct supposition, the radio signal emanated from across the street "completely external to the device", but when this signal encountered the device, the device resonated (like an opera singer breaking a glass across a room, or even like your eardrum vibrating while listening, except EMF waves not sound) and the listeners across the street could detect the signal from this modulated resonance... so ultimately, not "completely external" to the device.
this type or degree of "vibrating resonance" is passive "signal electronics" rather than the active power electronics inside RFID, which uses a stronger signal resonance to generate power to power up an active broadcast and computing circuit.
it's not entirely unlike bouncing a laser off the outside of a window to detect the conversation inside, though that higher tech and frequencies means the system does not need to be tuned to a resonant frequency.
I think this works on the basic wave mixing principle. You mix two frequencies together and the products include the sum and the difference. My theory is this gadget was able to reflect the RF wave sent to it mixed with the sound waves from the room.
