The Cone of Silence may have been a CONTROL-issue bit of kit, but the term predates the television show. The first system of practical radio navigation used by American aviators beginning in the 1930s was known as the Adcock Range or Low Frequency Radio Range (LFR) system. This system, which predated the Victor Airways linking VOR beacons, was composed of a network of LFR ground stations ("Adcock ranges") which produced distinct audio signals depending on the relative position of the aircraft to the beacon. Pilots would listen to the tones produced by the beacons, adjusting course until two distinct signals merged into a continuous tone, to maintain a plotted course. This was called 'riding the beam.'
The LFR beacons produced distinct signal pairs in each of four quadrants around the station, and along the four intersections of the quadrants (the "beam") they produced a continuous tone. Those four beams were used as navigation pathways. However, as the airplane approached the station, there was a region directly above the beacon where the signal was not audible due to the directional antennas not transmitting directly upwards. This region was roughly cone shaped with the apex at the surface, increasing in radius with altitude, and was called the Cone of Silence. When your aircraft entered the Cone of Silence, you knew you were directly over the beacon (or nearly so) and might need to make a course change to intercept the desired beam leaving the station.
(IN5 11/30)