Retrotechtacular: Radio To Listen To When You Duck And Cover

CONELRAD may sound like the name of a fictional android, but it is actually an acronym for control of electronic radiation. This was a system put in place by the United States at the height of the cold war (from 1951 to 1963) with two purposes: One was to disseminate civil defense information to the population and, also, to eliminate radio signals as homing beacons for enemy pilots.


Here’s how it worked: In case of an attack, certain key stations were notified. They would use a very simple sequence to indicate there was an alert. All FCC-licensed stations had to cease transmission once the alert sounded. This wasn’t a bad idea. In World War II, bombers used radio stations to find nearby targets.

However, it did leave the government without a way to communicate with the people. Through advertising, the US let people know that in an emergency they should tune to 640 kHz or 1240 kHz. Certain commercial radio stations would move to those frequencies and take turns transmitting the same information. One station would transmit for a few minutes before another took over. This way there wasn’t a lengthy transmission for enemy bombers to home in on.

Tuning During an Air Raid

Special receivers that could pick up the CONELRAD signal were available. In addition, all AM radios were required to have markings at the CONELRAD frequencies (see triangle on the radio dial below).

Detecting the alert was simple but error prone. The key stations would stop transmitting for five seconds, returning to the air for five seconds, and then shut down for another five seconds. The station would then return to the air and transmit a 1 kHz tone for fifteen seconds.

It wasn’t unusual to get false alarms. Also, some transmitters would fail because of the rapid on/off cycling. This led to the replacement of the system by 1963 with the Emergency Broadcasting System. Most monitors would simply look for a signal on one of the two CONELRAD frequencies. Others would look for the tone along with an ordinary station dropping out.

What If?

Luckily, the CONELRAD system was never used for a real event. Unlike later systems, CONELRAD was not used for severe weather alerts. You have to wonder about its success had it been activated. There were still tube sets and the transmitters of the day were probably all tube-based. But electromagentic pulse effects would have certainly taken out the transistor devices that did exist. On top of that, bomb air burst would have played havoc with radio communications anyway.

34 thoughts on “Retrotechtacular: Radio To Listen To When You Duck And Cover

  1. London’s TV transmitter – at Alexandra Palace – was cut off in 1939 to stop the Luftwaffe using it as a homing beacon. Orders must have come in pretty suddenly, as the broadcast was famously terminated in the middle of a Mickey Mouse cartoon. When permission came in to start up the transmitter again in 1945 the BBC considerately dug up the same cartoon so that anyone who had survived, and still had a house to watch it from, could find out how it ended.

  2. It was obviously intended to be used *while* the Soviet bombers were flying towards the USA. After some bombs have been dropped, it a rather questionable whether the radio propagation conditions (and listeners) would be in good condition, as Al Williams said.

    And later, ICBMs and SLBMs made attack much faster, with even less possibility for survival.

    Regarding the EMP, US testing showed that it is device design, not the active element (tube/transistor) that determines damage to radio equipment. Military radios were then transistorized and protected against EMP.

      1. Some radios have 2 antiparallel diodes connected at the antenna terminals to protect against lightning etc. That would probably work.

        Vacuum tubes had a limit to how high voltage can be applied btw cathode and a heater for example. It was less than 100 V on many types.

        At any rate, a proper metal case would help.

        1. I don’t know, how much energy an EMP would dump in this case. At least against lightning damage a multi stage approach is necessary: crude spark gaps, gas discharge surge arrester tubes and semiconductor devices like varistors or diodes.

      1. somehow that makes me think of an old book that was named something like -The Nuking-Zapping of America.
        There was a section about TV & Radio station techs who purportedly looked into the RF path of equipment, so as to check alignment.
        So the “line of sight” and FM kind of are a bit co-mingled in my fuzzy brain cells.

      1. Also, not sure when it was mapped/understood but late 40s at least, the jetstream was a mysterious force that swept up planes and dragged them miles away from where they wanted to go…. which also due to low cloud cover or being over featureless expanses of ocean, ice, desert, would have not been realised as a ground speed a couple of hundred mph wrong.

    1. Radio navigation from whatever source was used in low visibility flying weather which was and is common in the UK, especially bad for night flying. Also, wartime blackouts made using illuminated landmarks such as roads and cities difficult to impossible to use at night.

    2. Non-Directional Beacon (NDB) radio navigation was standard for flying before VOR/TACAN and is still in use where those facilities don’t exist and works near the AM band. Though most pilots won’t admit it, the best use these days is to listen to ball games while en route. Or so I hear. It’s still useful to find a city if everything else goes dark, but these days the battery-powered GPS in your phone is much better.

      All of this predates LORAN/GPS, and inertial navigation was (and is) pretty finicky stuff. Maps, compasses and stopwatches (ded reckoning – from “deduced reckoning”) can still be (and has been) used but it’s not much good for dropping loud things on people’s heads.

  3. “This is only a test! This radio station is conducting a test of the Emergency Broadcast System. The radio stations in your area, in cooperation with the FCC blah blah blah…Had this been an actual emergency, you would be instructed on where to tune. Again, this is only a test!” TOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO!

  4. And if any of you, like me, grew up in the Westchester CO area of Croton, they would realize that the local police offices was also the local CD offices. For a while I’d drop by and get involved in their activities. I ended up learning a little too much regarding what to do if the impossible happens.

    Fortunately it never did, but now? Who knows.

    1. You know, those North Korean missiles that can reach the US mainland by the nature of things can also reach Europe.
      And if there is a conflict NATO might get pulled in, and then what?
      Also the Russian navy recently did an exercise in the Baltic sea together with the Chinese navy I read. The Baltic sea which is a stonethrow away from Britain, with the Chinese navy, wut.

      1. I don’t know much about missiles. Guam is a US territory in the western part of the pacific ocean, i.e. practically Asia. It is not part of the US mainland. Can you provide a reference for US mainland in reach of NK missiles? It’s also not entirely clear to me how any missile which can reach the US mainland can necessarily also reach Europe…

        Obviously conflict is undesirable…

        1. My reasoning is US to NK distance being the same as EU to NK distance, although so far NK is pointing all its anger at SK and Japan and the US.
          As for the missile capabilities of NK, I think you can find info on every damn news-site about their latest test and the growing capabilities, as well as the appropriate wikipedia entries I would think.

  5. Its funny how technology reverses itself over time. In WW1 Radio Triangulation was used by German Zeppelin commanders to help them navigate through the always foul weather over the English Channel but reception was poor the further they got from Germany and the Brits soon learned how to jam their signals. Zeppelin commanders considered it good enough if they happened to be over the right cities when they dropped their bombs. 20 years later the Germans used British broadcast signals to home in on England requiring radio blackouts to mitigate, and 20 years after that the threat of Russia doing the same to the US with nuclear bombers created the Conelrad and EBS systems. Nowadays you again don’t have to aim the bombs – just getting them in the right time zone will be good enough.

  6. In my archives I have an old RDF with a rotatable antenna. You pick a station, rotate the antenna for the best signal, or highest reading on the S meter,, get the call letters, look it up in Whites Radio log, and mark the angle on the RDF through the transmitters location you got from Whites Radio Log. Than you pick another station and do it again. You are at the place on the map where the lines intersect, give or take. The more stations you can get, the more intersecting lines and the better the fix. Neat piece of old electronics.

  7. In i961 I built a “CONELRAD Monitor” as a high school science project. It was my first transistor project, up to then had been vacuum tube. Used a CK722 and 2N107. A relay would tend to drop out on vibration, I reclassified the bug as an “earthquake detector feature.” Yes, I am that old.

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