Adam Fabio, hackaday.com; via JFS on Facebook

The man leaned over his creation, carefully assembling the tiny pieces. This was the hardest part, placing a thin silver plated diaphragm over the internal chamber. The diaphragm had to be strong enough to support itself, yet flexible enough to be affected by the slightest sound. One false move, and the device would be ruined. To fail meant a return to the road work detail, quite possibly a death sentence. Finally, the job was done. The man leaned back to admire his work.
The man in this semi-fictional vignette was Lev Sergeyevich Termen, better known in the western world as Léon Theremin. You know Theremin for the musical instrument which bears his name. In the spy business though, he is known as the creator of one of the most successful clandestine listening devices ever used against the American government.

In 1920, while working on his dielectric measurement device, Theremin noticed that an audio oscillator changed frequency when he moved his hand near the circuit. The Theremin was born. In November of 1920 Léon gave his first public concert with the instrument. He began touring with it in the late 1920’s and in 1928, he brought the Theremin to the United States. He set up a lab in New York and worked with RCA to produce the instrument.
Theremin’s personal life during this period was less successful than his professional endeavors. His wife, Katia, had come to America with him and studied medicine at a school about 35 miles from the City. For much of this time, Léon and Katia lived apart, seeing each other only a couple of times a week. While at school, Katia became associated with a fascist organization. The Russian Consulate caught wind of this and summarily divorced Léon from Katia. They couldn’t risk their rising star being associated with the Nazis.
Theremin eventually remarried, this time to Lavinia Williams, a ballerina. Lavinia was African-American and the couple faced ridicule in American social circles due to their mixed race. However, the Soviet Consulate did not have a problem with their relationship. In 1938, with the Nazi threat growing stronger, Theremin returned to Russia. He expected to send for his wife a few weeks after his arrival. Unfortunately, that wasn’t to be the case. Léon and Lavinia never saw each other again.
Upon arrival in Leningrad, Theremin was imprisoned, suspected of crimes against the state. He found himself working in a laboratory for the state department [JB ????]. This was not an unusual situation. Aircraft designer Andrei Tupolev and missile designer Sergei Korolyov were two of many others who faced a similar fate.
It was during this time as a prisoner that Theremin designed his listening device.
It was during this time as a prisoner that Theremin designed his listening device.
Placing the bug

Discovered!

Theory of Operation

Regardless of who figured out the device, the method of operation is devilishly simple. 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.

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.
While bugs of this type have fallen out of favor, the idea of “illuminating” a device with an external transmitter lives on. Check out [Elliot’s] description of the RageMaster bug from the ANT catalog here. Resonant cavities have found common use as well. Every microwave oven or radar system with a magnetron uses one.
A Political Pawn

A replica of the great seal is on display at the NSA National Cryptologic Museum.
Afterward

Theremin finally visited the United States in 1992, reuniting with old friends.He performed in a concert at Stanford and was interviewed by Robert Moog, who considered him to be a hero of the electronic music world. After filling in many of the blanks of his story, Theremin asked Moog and co-interviewer Olivia Mattis to be responsible when writing up their story. “But if you write that I have said something; against the Soviet government and that I have said that it is better to work elsewhere, then I shall have difficulties back home [ironic laughter]”. Even then at the twilight of his life, with the fall of the Soviet Union underway, Theremin was still looking over his shoulder, worried about what the government might do if he offended them.
Theremin passed away in 1993. The unlikely master of this spy-gadget was 97 years old.
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