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 TV and FM stations were required to stop broadcasting. Upon alert, most AM 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.
False Alert - 1970s.
Emergency Broadcast System (E.B.S.)
Anyone who worked in broadcasting in the 70s or 80s
will recall these envelopes which were kept securely with the station's control room. Upon receipt of an emergency message from the federal government the duty operator was supposed to compare the authenticator words with the list of test words on the front of the envelope. If the words did not match they were supposed to open the envelope and verify the message using the word list inside.
Opened only under penalty of severe government sanctions the above image is one an actual authenticator list.