The plans for this CONELRAD monitor appeared in the January 1960 issue of Radio Electronics magazine. As part of the nation’s civil defense structure, the CONELRAD system was designed to alert Americans to an incoming attack, but also make sure that broadcast signals did not serve as beacons for incoming bombers.
To prevent this from happening, all radio stations ceased broadcasting. Selected stations then resumed broadcasts, but only on two frequencies, 640 and 1240 kHz, in order to confuse the navigators of those incoming bombers.
When the station first left the air, this would serve as the first warning to the public. And this device sounded a bell when the monitored station left the air. In some more remote areas, a more sensitive and selective receiver might be required. But in most areas, the ubiquitous “All American Five” receiver could be used. This alarm tapped into the receiver’s AVC circuit. If the incoming carrier disappeared, the bell would sound.