Theremin's Bug

How the Soviet Union Spied on the US Embassy for 7 Years

The Soviets would sit outside the embassy, either in another building or in a van. From this remote location they would aim a radio transmitter at the great seal. The bug inside would receive this signal and transmit voices in the room on a second, higher frequency. It did all of this with no standard internal components. No resistors, no tubes, no traditional capacitors, or the like. There were capacitive properties to the mechanism. For instance, a capacitor is formed between the diaphragm and the tuning peg of the device.

Receive tuning (if it can be called such) was achieved by the precisely cut antenna. The RF carrier transmitted by the Russians would be received at the antenna and travel into the body of the device which was a resonant cavity. That resonant chamber was capacatively coupled to the thin conductive diaphragm which formed the microphone.

Sound waves would cause the diaphragm to move, which would vary the capacitance between the body and diaphragm, forming a condenser microphone. It is important to note that the bug didn't transmit and receive on the same frequency. According to Peter Wright, the excitation frequency used by the Russians was actually 800 MHz. The cavity would resonate at a multiple of this base frequency, producing the 1.6 GHz output seen by Bezjian.

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13 Responses:

  1. Thomas Lord says:

    How many accidental bugs are there in the built environment, without forcing a Great Seal on someone?

    • Rena says:

      A couple times I've had random old electronics/scrap metal pick up local radio stations. You'd actually hear the music coming out of the metal. No doubt things like this bug could also easily exist by accident.

      • Thomas Lord says:

        Well, of course a bug needs the special additional property of (when excited) converting adjacent acoustical signal into modulation of the reflected carrier but I agree your anecdote says that's probably not rare.

        Bonus question: Suppose as a nightmare that spying this way is way more common that you'd wish. What's a good counter-measure to catch the listeners? They can use very narrow beams, even lasers, to pick up audio ... what's a cheap easy way to catch them out?

        • James says:

          Talk about a dead drop in a remote area and see if anyone shows up to look at it.

        • Nate says:

          You look for the carrier wave being transmitted in if you want to catch them. But that's not the most effective countermeasure.

          If you look at SCIF design, they isolate the room from vibration and make the walls transparent so that bugs in its surface are visible. Then, they add an active countermeasure of a vibrator on each wall to overpower any vibrations from conversation.

          Supposedly the US embassy let Russian contractors pour the foundation, and they dumped various passive components into the concrete so that active scanning for bugs would result in a lot of false positives.

      • joe says:

        heck i've heard that grass can transduce RF from meteors into sound...

      • robert_ says:

        I gather it's quite common in houses that are right next to major radio transmitters for metal devices, such as saucepans, to 'play' audio.

  2. Rena says:

    So is the 9-inch antenna rod connected to that T-shaped part? In the diagram it looks like it's not, but is that just indicating some of its length isn't shown here?

    • Asm says:

      Yes, it is; the "break" used here is the standard method that was used to signify "this has been shortened so it does not look ridiculous".

    • Ben Coburn says:

      That diagram definitely shows the antenna rod being connected to the T-shaped part. That particular style of break is the standard way to show "this part is longer than is shown" in engineering drawings.

  3. The article doesn't mention it, but I could swear I saw that diagram as a kid in a back issue of Scientific American's old "Amateur Scientist" column.

  4. Nikola Tesla says:

    For some reason I keep reading the title of this entry as "Theremin's Blog", and every time it makes me smile.

    "Got some more moldy bread, today. Made some electronics. Better than being worked to death in Siberia tho, amirite? LOL #zeklife #sovietfederation #moldybread"

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