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It used a straight rod, led through an insulating bushing into a cavity, where it was terminated with a round disc that formed one plate of a capacitor. Given the radio technology of the time, the frequency of 330 MHz is most likely). The device consisted of a 9-inch-long (23 cm) monopole antenna (quarter-wave for 330 megahertz frequencies, but it was also able to act as half-wave or full-wave the accounts differ. These same design features, along with the overall simplicity of the device, made it very reliable and gave it a potentially unlimited operational life. Theremin's design made the listening device very difficult to detect, because it was very small, had no power supply or active electronic components, and did not radiate any signal unless it was actively being irradiated remotely. A receiver demodulated the signal so that sound picked up by the microphone could be heard, just as an ordinary radio receiver demodulates radio signals and outputs sound. The movement of the membrane varied the capacitance "seen" by the antenna, which in turn modulated the radio waves that struck and were re-transmitted by the Thing. Sound waves (from voices inside the ambassador's office) passed through the thin wood case, striking the membrane and causing it to vibrate. This is referred to in NSA parlance as "illuminating" a passive device. 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.

The Thing consisted of a tiny capacitive membrane connected to a small quarter-wavelength antenna it had no power supply or active electronic components. The seal opened exposing the Soviet bugging device, on display at the NSA's National Cryptologic Museum.