Microwave eavesdropping has a long and clandestine history. And there’s an odd connection between music and microwave eavesdropping. Mention the name Leon Theremin and many people will immediately think of the musical instrument bearing his name. Born Lev Sergeyevich Termen, his contributions to the former Soviet Union spanned the history of the Soviet Union from the original Bolshevik Revolution to the collapse of the USSR.

Termen was a mathematics prodigy who was drawn into the emerging field of electronics. Termen came across the basic principle for one of the first electronic musical instruments while working on television. The theremin (Termen called it an “etherophone”) produced music with modulated static. Players used two antennae, one for pitch and one for volume, producing musical tones by waving their hands around the antennae. Skilled musicians reportedly could recreate the sounds of many existing instruments.

Termen traveled to the US under the pretense of demonstrating the wonders of the theremin. The real purpose of his trip was to collect information about German and US technology and manufacturing, reporting them to the USSR. His return to the Soviet Union came at an inopportune time in history. Because Termen had spent years in America, he became a suspect of counterrevolutionary thoughts and was sent to the prison camp/gold mine of Kolyma. Most non-Russians believed that Termen had disappeared in the middle of the night, kidnapped by Soviet agents.

Ultimately the pogrom against anti-Stalinists left so few experts available for espionage work that Termen was pressed into service. Among his inventions was a passive cavity audio bug.

While there are a great many myths about when and where the bug was most famously deployed, the NSA provides a definitive history.

From the NSA web site:

“On August 4, 1945, Soviet school children 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 microphone hidden inside was passive and only activated when the Soviets wanted it to be. They shot radio waves from a van parked outside into the ambassador's office and could then detect the changes of the microphone's diaphragm inside the resonant cavity. When Soviets turned off the radio waves it was virtually impossible to detect the hidden 'bug.' The Soviets were able to eavesdrop on the U.S. ambassador's conversations for six years.”



Figure 1 Carved Great Seal of the United States hid resonant cavity bug.



Figure 2 Hiding place for resonant cavity bug.

The existence of the bug was accidentally discovered by a British Radio Operator when he heard conversations on an open radio channel.

In 'Spycatcher' Peter Wright, former assistant director of MI5, detailed how he uncovered the workings of the microwave bug. He had been working with resonance cavities trying to create practical spy equipment. The existence of the Russian device proved the concept possible, but he was stymied by how the device worked. Nothing that he did to it caused it to broadcast any audio. It was about eight inches long, with an antenna on top which fed into a cavity. Inside the cavity was a metal mushroom with a flat top which could be adjusted to give a variable capacity. Behind the mushroom was a thin diaphragm, to receive speech. They had assumed that the metal plate needed to be opened out to increase resonance, but in actual operation the closer the plate was to the mushroom the great the sensitivity. Inspiration caused Wright to try a new experiment that activated the bug.

The resonant cavity transmitter is simple technical device called a passive radiator. A layer of thin metalized material is stretched across a closed metal tube. The size of the tube determined its resonant frequency. An antenna, is attached to the base of the cavity. The cavity is irradiated with a beam of radio frequency energy from an external source. The size of the cavity and the length of its antenna are designed so that a harmonic of the inbound radio frequency energy is rebroadcast. The metalized diaphragm acts as a transducer, and the audio range energy modulates the returned radio frequency signal that, in turn, is picked up by a receiver in a nearby listening post. It is important to note that the microwave signal that “powers up” the device is not the same frequency as the outbound signal.



Figure 3 Resonant cavity bug structure.



Figure 4 How the bug worked.

The microwave eavesdropping equipment discussed in “Eavesdropping using microwaves” uses no resonant cavities. It is very useful because there is no need to plant a device in the room. US Patent 4359683 filed in 1980 gives a preview of the new invention. In the patent, the inventor John W.H. Chivers of Rolls Royce, details a microwave interferometer for measuring distances, but fails to disclose any idea that the equipment might be useful for intercepting sounds. In US Patent 4280055 inventor Reinhold Gerharz takes a different view of microwaves to describe an imaging device. Neither of these two patents tell us how to use microwaves to directly eavesdrop on sounds without a resonant cavity.

A commercial product gets us closer to the eavesdropping solution. 2010/2010A – Doppler Stethoscopes are used to detect motion. The 2010 is a stand-alone probe designed for use with a high gain amplifier. The units generate a small amount of radio energy which penetrates any soft sided package such as an attache case, suitcase, wooden or cardboard box, and returns a signal which indicates movement within the package. This includes many types of watch mechanisms, mechanical timers, radio control escapements and tape recorder motors. Still, the stethoscope does not directly extract speech.

In US Patent Application 2005/0220310 William R. McGrath details how microwaves can be reflected to intercept sounds. The important part of this patent is the detailing of how microwaves can be reflected by an object and signal processed to recover audio information.

The continuing development of audio intercept technology has important legitimate uses. But the more technically sophisticated approaches that can be built simply carry with them an increased risk of abuse.