[AMRadio] OT. Retuning for Conelrad?

[Robert Nickels]

This recording of the 1961 Operation Alert CONELRAD test maybe of interest:

https://youtu.be/8N5l_Zx1kvg

Note the comment that stations on 640 or 1240 may be operating on 
reduced power, thus may be weaker than normal.     Per this article that 
appeared during the early planning phase of the Key Station System which 
turned into CONELRAD, the cost of implementing 640 or 1240 kHz 
transmitting capability fell to each individual station owner.

https://www.radiomuseum.org/forumdata/upload/conelrad_sys_def.pdf

The alternative was to leave the air (unless they were "drafted" by the 
FCC).    I've seen references to setting up a back-up transmitter with 
CONELRAD capability, and since non-directional transmission was required 
on 640 and 1240, some stations used separate auxiliary antennas when in 
CONELRAD mode.   Thanks to the excellent search capability of the 
WorldRadioHistory website I've read a number of references describing 
how crystal switching and re-tuning components were relay-switched in 
transmitters including the RCA BTA-1R1, using either a front-panel 
switch or the stations regular remote control equipment.

Here is a brief outline of how CONELRAD alerting network operated, per 
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 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."

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The above carrier drop sequence was detected by CONELRAD receivers in 
radio stations and by other licensed radio services, including hams.

73, Bob W9RAN