Viewer Comment: Theremin Story

By The History Detectives Team
8 July 2010
Category: Viewer Mailbag

Viewer Patrick MacKinnon shared this comment about the story

Watched your interesting video on the theremin and learned much I didn’t know. However there is something I do know that was not mentioned in your presentation.

The device concealed in the Great Seal of the US in the Moscow embassy was very ingenious. The usual means of detecting bugs is by means of a broadband receiver used to pick up any emitted signal. As you do mention this device did not require any batteries but you did not state the reason for this. The reason was because the signal it provided was not generated in the device itself. In a nearby building the Soviets placed a UHF radar transmitter which was directed at the location of the great seal. Concealed in the seal was a short tuned metal rod with a resonant chamber at one end. One end of this chamber was a diaphram which picked up and vibrated in response to any sound waves in the vicinity. This difference in effective length modulated the reflected UHF signal. These echo modulations were stripped from the reflected signal and amplified separately to provide readable sound waves.

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