Soviet young pioneers gave a carving of the Great Seal of the United States to U.S. Ambassador Averell Harriman. It hung in the ambassador’s Moscow residential office until 1952 when the State Department discovered that it was ‘bugged’. The Soviets were able to eavesdrop on the U.S. ambassador’s conversations for six years. A discovered bug was puzzle, since it had not batteries, no electric circuits and it was named The Great Seal Bug or The Thing.
Operating Principles
The Great Seal Bug was one of the first covert listening devices to use passive techniques to transmit an audio signal. It is considered a predecessor of current RFID technology.
The Thing was designed by Leon Theremin. It consisted of a tiny capacitive membrane connected to a small quarter-wavelength antenna; it had no power supply or active electronic components. The device, a passive cavity resonator, became active only when a radio signal of the correct frequency was sent to the device from an external transmitter. Sound waves caused the membrane to vibrate, which varied the capacitance “seen” by the antenna, which in turn modulated the radio waves that struck and were retransmitted by The Thing. A receiver demodulated the signal.
Theremin’s design made the listening device very difficult to detect, because it was very small, had no power supply or active components, and did not radiate any signal unless it was actively being irradiated remotely. These same design features, along with the overall simplicity of the device, made it very reliable and gave it a potentially unlimited operational life.
For his work, developer of the system, the prisoner Theremin, earned a personal freedom.
MORE: THE GREAT SEAL BUG STORY by Kevin D. Murray; Passive Resonant Cavity & "Spycatcher" Technical Surveillance Devices from archive.is
