CONELRAD may sound like the name of a fictional android, but it is actually an acronym for control of electronic radiation. This was a system put in place by the United States at the height of the cold war (from 1951 to 1963) with two purposes: One was to disseminate civil defense information to the population and, also, to eliminate radio signals as homing beacons for enemy pilots.
Here’s how it worked: In case of an attack, certain key stations were notified. They would use a very simple sequence to indicate there was an alert. All FCC-licensed stations had to cease transmission once the alert sounded. This wasn’t a bad idea. In World War II, bombers used radio stations to find nearby targets.
However, it did leave the government without a way to communicate with the people. Through advertising, the US let people know that in an emergency they should tune to 640 kHz or 1240 kHz. Certain commercial radio stations would move to those frequencies and take turns transmitting the same information. One station would transmit for a few minutes before another took over. This way there wasn’t a lengthy transmission for enemy bombers to home in on.