The CONELRAD concept was originally known as the Key Station System. According to an FCC document created during the "Informal Government - Industry Technical Conference" on March 26, 1951:
"The primary plan for alerting broadcast stations that is currently being considered by the FCC Study Group is known as the Key Station System. The arrangement requires certain telephone circuits (private wire or direct line to Toll Board) between the Air Defense Control Centers (A.D.C.C.) and specified radio stations to be known as "Basic Key Stations".
Additional telephone circuits (direct line to Toll Board) will be required in certain cases, between "Basic Key Stations" and other stations to be known as "Relay Key Stations". Each "Basic Key Station" receiving an alert or warning signal from the A.D.C.C. shall, if so directed, proceed to broadcast a predetermined message and also relay the message by telephone to all "Relay Key Stations" under his control as specified." CONELRAD was officially introduced on December 10, 1951.
CONELRAD had a simple system for alerting the public and other "downstream" stations, consisting of a sequence of shutting the station off for five seconds, returning to the air for five seconds, again shutting down for five seconds, and then transmitting a tone for 15 seconds. Key stations would be alerted directly. All other broadcast stations would monitor a designated station in their area.
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 frequencies marked by the triangle-in-circle ("CD Mark") symbol of Civil Defense.
Although the system by which the CONELRAD process was initiated (switching the transmitter on and off) was simple, it was prone to numerous false alarms, especially during lightning storms.
Transmitters could also be damaged by the quick cycling. The switching later became known informally as the "EBS Stress Test" (due to many transmitters failing during tests) and was eventually discontinued when broadcast technology advanced enough to make it unnecessary.
Beginning in 1957, operating U.S. amateur radio stations were required to verify at least once every 10 minutes that a normal broadcast station was on the air. If not, the amateurs were required to stop transmitting. Several companies marketed special receivers that would sound an alarm and automatically deactivate the amateur's transmitter when the monitored broadcast station went off the air.
There was also a hardcore punk-rock band in Pittsburgh, Pennsylvania, named Conelrad.
There is also a teleplay for "The Twilight Zone" called "The Shelter" involving the CONELRAD station as well as an episode of Quantum Leap called "Nuclear Family" that plays part of a CONELRAD broadcast.
Bob Dylan references the system in the song "Talkin' World War III Blues".
In the 1963 comedy album Jose Jimenez - Our Secret Weapon, Bill Dana played a Civil Defense volunteer. When asked what he would listen to during the event of an atomic attack, he said he would listen to Comrade. When corrected and asked if he meant CONELRAD, he explained "No, Comrade will know about it first!"