This is the first part of articles about the Great Seal Bug (also called “The Thing”). These articles will cover: history, theory of operation , a practical reconstruction of this device and a working demo.
This device, even 75 years later, is still often mentioned at security publications and conferences.
On 4 August 1945, at the end of WW II, the Pioneer scouts (children from the Pioneer Organization of the Soviet Union) gave Averell Harriman, the USA ambassador, a beautiful wooden replica of the Great Seal of the United States. This was a “gesture of friendship” to USA, USSR’s war ally.
This replica was hung in the conference room in the residence of the US Ambassador in Moscow.
According to different accounts, the US learned around 1951-1952 that a listening device has been planted at the ambassador’s residence. There are two main versions circulating: in 1951 the British altered the Americans that a listening device is active, in 1952 the Americans found the bug themselves. Espionage and counterespionage are difficult subjects, and many other versions also could be possible, including the possibility that the bug was left on purpose to feed misinformation to the eavesdroppers.
It was found that the seal contained a novel passive microphone. The microphone was mounted in the inside of the wooden carving, with small holes under the eagle’s beak to pass the sound.
The bug consisted of an antenna coupled to a microwave distributed element resonator, part of which was a flexible microphone membrane. This is irradiated by a carrier at the resonant frequency of the resonator, which is picked up by the antenna, and then re-radiated. The resonant frequency of the resonator is very dependent on the membrane position, so when the membrane moves it changes the amount of re-radiated carrier signal, resulting in AM and PM modulation of the re-radiated signal. The working frequency was about 1.7-1.8GHz.
On May 26, 1960 the bug was presented at the United Nations Security Council to show that the US was constantly being targeted by eavesdropping by the USSR.
A detailed drawing of “The Thing” was published in the March 1968 edition of Scientific American:
The bug was designed by Leonid Theremin , inventor of the theremin instrument . 
In 2019 the FBI unclassified a detailed report about “The Thing” from 1952. This report contains detailed drawings of this device, with dimensions . It is avaliable at this link: https://www.cryptomuseum.com/covert/bugs/thing/files/GREAT_SEAL_BUG.pdf
A lot of information about this device and it’s history, including information not published elsewhere, is published by Murray Associates in 
Author: Jacek Lipkowski SQ5BPF
 https://counterespionage.com/great-seal-bug-part-1/ https://counterespionage.com/great-seal-bug-part-2/ https://counterespionage.com/great-seal-bug-part-3/ https://counterespionage.com/great-seal-bug-part-4/ https://counterespionage.com/great-seal-bug-part-5/