The Thing is a so-called resonant cavity microphone, consisting of a resonant cavity, combined with a condenser microphone. The diagram below shows an 'educated guess' of the construction of The Thing, based on various reports and publications. The device consists of a copper cylinder with a highly polished silver-plated interior, that acts as a high-Q resonant cavity of the reentrant type. At the center is an adjustable mushroom-shaped disc with a flat surface, acting as a capacitor in combination with a very thin 75µm membrane that closes the open end. An antenna enters the cavity through an insulated hole in the side of the cylinder and is capacitively coupled.





The cavity has a diameter of 19.7 mm and is 17.5 mm long. The antenna is ~22.8 cm long (9"). The membrane, or diafragm, at the front of the cylinder is just 75 micrometers thick (3 mil). The tuning post can be adjusted to increase or decrease the capacity of the mushroom. The flat face of the mushroom has machined grooves to reduce the pneumatic damping 1 of the diafragm. According to one report [24], the distance between the mushroom and the diafragm was 230µm.





The dimensions of the cavity are carefully chosen so that it is resonant at a very high frequency (e.g. 1320 MHz). It is then illuminated, or exited, by a strong signal from the outside, as shown in the illustration below. Any sound in the room (speech) causes the membrane to vibrate, which decreases and increases the space inside the cavity and also the capacity between the membrane and the mushroom. As a result, the bug produces a combination of Amplitude Modulation (AM) and Frequency Modulation (FM). In practice, only the AM component was used by the Russians.





It is currently unclear what the illumination frequency was. In the original investigation reports, it is suggested that the illumination frequency was the same as the resonance frequency (i.e. the output). Although this poses technical restrictions, such as overloading of the receiver, it is the most likely scenario. Receiver overloading can be solved by using directional antennas (e.g. helical antennas) and by mixing part of the transmitted signal with the input of the receiver, in order to cancel-out the excess signal. The block diagrams below give some useful suggestions.





The antenna problem can be solved effectively, by adding a 3-port circulator between the transmitter and the receiver, as illustrated in the diagram below. The circulator ensures that all transmission power is passed to the antenna and that returned energy is passed to the receiver.





In theory, it would also be possible to use a sub-harmonic of the resonance frequency as the illumination signal. Such sub-harmonics are often easier to generate at high power, and cause far less interference between transmitter and receiver. Take for example 440 MHz, which is 1/3 of the resonance frequency of our example above. 2 In that case, the cavity is used as a tripler.



However, in order for the cavity to generate the 3rd harmonic of the exitation frequency, it must have non­linear properties, such as thin oxidized layers between contacts, similar to a semi­conductor (diode), or loose contacts, in which case the cavity acts as a contact generator. In the given situation, this effect is arbitrary however, and its behaviour would be difficult to predict and to reproduce. It is therefore unlikely that the Russian resonant cavity was used as a multiplier.