The Thing

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 device consisted of a 9-inch (23 cm) long monopole antenna (quarter-wave for 330 megahertz (MHz) frequencies, but able to also act as half-wave [at 660 MHz] or full-wave [at 1320 MHz], the accounts differ – given the radio technology of the time, the frequency of 330 MHz is most likely) — 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. [5]

The original device was located with the can under the beak of the eagle on the Great Seal presented to W. Averell Harriman (see below); accounts differ on whether holes were drilled into the beak to allow sound waves to reach the membrane.

Source: en.wikipedia.org