In the event of an emergency, all United States television and FM radio stations were required to stop broadcasting. Upon alert, most AM medium-wave stations shut down. The stations that stayed on the air would transmit on either 640 or 1240 kHz. They would transmit for several minutes and then go off the air, and another station would take over on the same frequency in a "round robin" chain. This was to confuse enemy aircraft who might be navigating using radio direction finding. By law, radio sets manufactured between 1953 and 1963 had these two frequencies marked by the triangle-in-circle ("CD Mark") symbol of Civil Defense.
In the event of an emergency, all United States television and FM radio stat:ions were required to stop broadcasting. Upon alert, most AM medium-wave stations shut down. The stations that stayed on the air would transmit on either 640 or 1240 kHz. They would transmit for several minutes and then go off the air, and another station would take over on the same frequency in a "round robin" chain. This was to confuse enemy aircraf!t who might be navigating using radio direction finding. By law, radio sets manufactured between 1953 and 1963 had these two frequencies marked by the triangle-in-circle ("CD Mark") symbol of Civil Defense.