It was the early fifties and the threat from the Soviets, our WWII allies, was increasing daily.  The entire military establishment was gearing up for the defense of the North American continent, and there was serious concern over the ability of the population to survive a  nuclear attack by Soviet bombers. 

The belief was that, if the United States were to be attacked, our many commercial broadcasting stations would serve as valuable navigational aids for enemy aircraft and that those stations would have to be silenced if an attack came.  At the same time, it was recognized that it was vital to keep the people informed and that the only viable means of disseminating timely information was via radio.  

It was with those considerations that CONELRAD was born, in 1951.  An acronym for "Control of Electromagnetic Radiation," the idea was to deny the Soviets the ability to home in on specific identifiable broadcast stations, while still providing the American public with an information medium. 

Under the plan, all regular broadcasting would cease when the alarm was given, and all further broadcasts would be carried out by designated regional stations on the frequencies of 640 and 1240 KHz.   (KHz, or kilohertz, was known as Kc. or kilocycles in the US in those days.)   It was hoped that, by concentrating all broadcasting on two frequencies, at lower than usual power, it would be difficult or impossible for the enemy to single out individual stations for navigation.

All radio and TV broadcast stations, except for the designated CONELRAD stations, were required to monitor a designated station and to broadcast the CONELRAD announcement in the event of an alert, then cease transmitting.

It didn't stop with broadcast stations, though.  Even Amateur Radio (ham) stations had to monitor a broadcast station at all times, and to stop transmitting if there was a CONELRAD alert.   Beginning in 1953, all new AM radios sold (FM wasn't that common in those days) were required to be marked with a "Civil Defense" triangle at 640 and 1240 on the dial.

Times changed and, in the early sixties, an attack by Soviet bombers seemed more remote; the threat now was from Intercontinental Ballistic Missiles (ICBMs,) which had sophisticated guidance systems, that didn't rely on radio navigation.  Now obsolete, CONELRAD evolved into the Emergency Broadcast System (EBS,) in 1963.

The EBS was similar to CONELRAD, in that most stations left the air, but the designated EBS stations continued to broadcast on their own frequencies.  At first, the EBS used the old CONELRAD alerting system, which consisted of the monitored station leaving the air, or "dropping carrier."  This was found to be unreliable and the familiar dual-tone system was adopted in the 70s.  The rules were also relaxed over the years, to permit segments of the EBS to be used for state and local emergencies. 

Due to criticism that it was unreliable, less than responsive to local requirements and because it relied on a "daisy-chain" of stations, in addition to numerous other technical reasons, the Emergency Broadcast System began to be replaced, in 1997, by the Emergency Alerting System (EAS.)  This digital system ends, for the first time, the need to rely on intermediate stations, and can be activated as required by the proper authorities, and the station returned to normal operations, without the need for intervention by station personnel. 

Control of Electromagnetic Radiation

Emergency Alert System (EAS). The EAS is a national public warning system that requires broadcasters, cable television systems, wireless cable systems, satellite digital audio radio service providers, and direct broadcast satellite (DBS) providers to provide the communications capability to the President to address the American public during a national emergency. 

The system can also be used by state and local authorities to deliver important emergency information, such as AMBER alerts and weather information targeted to specific areas. All services are required to maintain an Emergency Alert System folder that contains the current authenticator word list.

The lists were mailed annually to all services  who in the event of an actual emergency would use the authenticator list to verify that a true emergency had be initiated.   The EAS replaced CONELRAD, which was the original notification system developed for broadcasters.