Conelrad: Radio meets the Civil Defense

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Conelrad: Radio meets the Civil Defense 
12.Nov.11 10:47

Emilio Ciardiello (I)
Articles: 533
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Emilio Ciardiello

In the early fifties, in an increasing climate of cold war, United States defined plans to control radio emissions in case of enemy attacks. The primary objective was to shut down broadcast stations that could have been used as guidance reference by ADF equipment of enemy bombers. At the same time there was the need to alert and keep people informed. In 1951 a shut down of any broadcast and ham emission, leaving a couple of AM channels available for Civil Defense, was planned.

The system evolved through the use of dedicated FM an telephone networks, as did few months before, the Washington automatic C-D warning system, with 70W six-foot horn loudspeaker clusters to broadcast information and instruction to population.

Automatic C-D warning system, from Electronics, May 1951.

The idea was to use existing AM stations to inform people all over the States, while preventing their use by radio direction finders aboard of enemy bombers. Conelrad was based upon a network of AM transmitting stations all operating at common synchronous frequencies. This because direction finders could not give reliable readings when several sources from different directions emit at the same frequency.

Definition of the system, from Electronics, August 1951.

Alarm was triggered via telephone and other dedicated lines by the Air Defense Control Centers to some key stations, that started the sequence. Power of the key stations was pulsed down for a minute or two. The missing signal was decoded by other relay stations and propagated to the entire broadcast network. All the associated broadcasters, key and relay stations, in case of alarm had to limit the antenna power to 5 KW. Emission of each station had to be pulsed down for approximately one minute alternating with other station of the same group on a random basis. The pulsed sources, all similar as per frequency and power, randomly appearing and disappearing from several points, contributed to deceive enemy ADF sets.

In case of alarm, 640 and 1240 KHz were the only allowed frequencies. Broadcasters who aimed to join the Conerlad network had to upgrade their sets, in order to switch to one of the said frequencies, also adding capabilities to modulate the antenna power down to 5KW and to pulse it to zero.

All other emissions, ham, TV, AM or FM, had to cease at all. Hams and broadcasters were asked to monitor the issue of Conelrad alarm to shut down their equipment. Special monitors were built, capable of triggering a relay, usually upon a significative drop in the AVC voltage in receivers tuned to the nearest Conerlad station. We find several solutions, ranging from custom built, fixed frequency receivers, to modified general coverage radios, down to simple adapters intended to be connected to the AVC of any existing radio.

A crystal-controlled single channel Conelrad monitor, Electronics, October 1952.

A fine example of custom built Conelrad monitor is this Hallicrafters 5R10A from the collection of Bob F.Maccubbin, Springville, NY USA. It derives from a standard 5R10A receiver with the addition of a large pilot light and of a rotary switch on the front panel.

One of the most popular adapter was the Heathkit CA-1. Its input was connected to the AVC of any receiver. A 2D21 thyratron fired upon the detection of the alarm condition and a manual reset was required to release the relay.

Schematic diagram of Heathkit CA-1 Conelrad adapter.

But even all the commercial radio sets were asked to comply with Conelrad specs. By law in the late fifties through the early sixties all US AM radios had two C-D marks (or even just two stylized triangles) on the tuning dial, at 640 and 1240 KHz emergency frequencies, so that people, when all other stations were down, could readily tune one of them and listen for information and for instructions by Civil-Defense.



Here the picture of the tuning dial of a typical AM radio, kindly supplied by Konrad Birkner. Note the two marks, two red triangles in this case, indicating the exact position of the two C-D frequencies.

In 1963, since the menace of enemy bombers was no longer actual, Conelrad was replaced by the updated EBS, Emergency Broadcast System,


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Looking for more info 
13.Nov.11 16:39
159 from 33451

Emilio Ciardiello (I)
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Emilio Ciardiello

The Conelrad System involved the whole radio market in US for several years. Radio receivers, transmitters and alarm monitors built in US since the early fifties were all compliant with its rules. Probably nobody cares to see two C-D marks on the tuning dial of his vintage radio, since all sets carried these marks. But there were many special equipment, as the monitors installed in every broadcast or ham transmitting station, and the same high-power broadcast transmitters that are not documented here.

Even a lot of radio sets imported in US from, say, Japan or Germany or elsewhere are not fully documented. Sets built overseas for US almost certainly should differ from domestic versions.

The Telefunken Hymnus Stereo 5014 Wk is one of the few examples in my collection of German radios of Conelrad compliant AM tuning dial. Here is the picture of its tuning dial, with highlighted C-D marks along the upper AM scale.


More or less the same difference must exist for many other models. It would be nice if these details could be evidenced even in the model pages.


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14.Nov.11 23:59
313 from 33451

Paul Reid (USA)
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I sure remember Conelrad. I forget which was the first radio we had with the CD triangles; for a while all our radios did. I sure do remember the annoying BEEEEEEEP when they tested the system. I was young, but I knew (and the test announcement reminded us) that in case of a "real" emergency we would tune to one of the triangles and listen for information.

My brother's 1941 Plymouth Philco C-1808 radio does not have the CD triangles, which agrees with your history. I am sure the 1966 and 1967 Fords did; I do not remember Mom's 1957 Plymouth that well.

We still have an emergency alert but the trigger signal is just a few seconds of complex tone, not a full minute(?) of 100% two-tone.

I did not know about the sequencing to foil direction-finders. Interesting, but moot after long-range rockets made aircraft attack less likely.

I am also of the generation which practiced getting under our desks in school.

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CD dial markings 
16.Nov.11 20:55
393 from 33451

Martin Bösch (CH)
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Martin Bösch

Dear Emilio, thank You for Your in-depth description of the conelrad system. I found this markings on American sets and did a internet search a while ago but did not find so well prepared information.

I got out my Hallicrafters S53A small all wave radio (built in 1950 - 1958/9)

yes - there were there, the CD triangles.

But I have another S53A in my cellar, the pictures of that one are slighly better quality as newer

and with this set - no CD markers on the dial.
So this set might be slightly older, we have production years 1950 - 1958/9, maybe the set above was produced after 1951.
There are slight differences of the markings on the frontpanel, when doing a search in the internet, You find images of both types.

Kind regards Martin

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The actual starting time of CONELRAD 
16.Nov.11 22:14
409 from 33451

Ernst Erb (CH)
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Ernst Erb

As far as I know at the moment, only in 1953 the manufacturers were asked to have the necessary scale - but to our knowledge never were obliged. Some started already before, like Zenith with the model J615 where you can find some models with and some without CONELRAD CD signs. Zenith has used the prefix for the model year (H = 1951, J = 1951/52, K = 1952, L = 1953) and the H615 models have (at least here) no marks - and we show quite many different sets in different colors. It is to say that those prefixes are not a lways a sure thing. We can only prove when having worked through this period for Zenith with original material like dated fliers, advertisements etc.

We should not guesswork and I might have to correct this statement, but we are researching on the models and Konrad Birkner has already marked some models. But some changes are necessary there.

We want to separate some models which are made in both versions by creating a copy as a variant and adding a variant to the original - but to do this only where we detect two different scales:

The model without CD marks gets in the field variant: no conlerad
The model with CD marks gets in the field variant: CONELRAD
We normally don't use capital letters but in this case the distinction is much better.

Second: For those models we enter a sentence in the text with a link to this article and containing Conelrad. This enables us to find all such marked models in the SEARCH. But also to distinguish in the "simple model search" between "no conelrad" and "conelrad".

Again: This is only done if both variants exist for a model, but that link in the notes can be done for any conelrad marked radios/equipment.

After a certain amount of models is marked, we can get results. And we will not only link to here but link here to some models to demonstrate what was done really. Naturally we will also do some more researches about articles - but so far in the different forums we have not really found proofs and sometimes quite contradictory posts. I would delete such posts here.

We also will try to research the situation at the end time of CD marks in this way.

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Some dates 
22.Nov.11 16:26
572 from 33451

Emilio Ciardiello (I)
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Emilio Ciardiello

A certain degree of uncertainty appears about the transition dates to C-D compliant tuning dials, even because of the existence of both dials, old and new, on some models. In order to better date the diffusion of radio sets with C-D marks on the tuning dial, we can look at the early tests of the Conelrad system. We should consider the system start as a growing project, going ahead from the definition of its specs and gradually involving all the subjects over a reasonable span: first military and civil defense, then broadcasters, radio manufacturers and at last people.

In the above attached article from Electronics, September 1951, we can read the complete description of the Conelrad System. Nevertheless no reference to ‘Conelrad’ acronym can be found there. To know more about the starting dates of the Conelrad System, I gave a look at the on-line archives of New York Times. Here are the relevant dates I found; abstracts of the articles are evidenced in blue.

The very early announcement of a secret military system under the name “Conelrad” was given by the North American Newspaper Alliance on March 14, 1952. Here the abstract:

- WASHINGTON, March 13 -- The Government has devised a secret system to prevent enemy planes from riding in on a radio beam in the event of a sudden air attack. The project, set up by the Air Force in conjunction with the Federal Communications Commission, goes by the code name "Conelrad."

We have to wait more than one year, until May 1953, to read again about the system. The article announced that the alert plan to control all broadcasts was going to a start.

- May 15, 1953 - A nation-wide emergency plan for unifying the broadcasts of all standard radio stations in case of an air alert will go into effect today. It was set up through the joint efforts of the Federal Communications Commission, civil defense authorities and broadcasters.

Upon May 31, New York Times gave the announcement of a test in New York City to be run in September. Note that just a vague indication of possible variations in radio channels was given here. Yet no details about the two C-D channels.

- May 31, 1953 - Plans for New York's biggest test of atom bomb defenses, a September exercise that will send school children to shelters, halt automobiles and some subways, ground airplanes and possibly change the channels of radio stations, were announced last week.

Announcements of the nation-wide test, scheduled for September 16, appeared in the days before. People were warned of possible missing radio and TV signals.

- September 13, 1953 - RADIO TEST SLATED TO FOIL BOMBERS; System of 'Planned Confusion' on Broadcast Beams Gets Workout on Wednesday

WASHINGTON, Sept. 12 (AP) -- Conelrad, the radio operation designed to thwart any effort to use broadcasting signals to lead bombers or guided missiles to American cities, will have its first national test on Wednesday.


- September 15, 1953 – AIR DEFENSE TEST SLATED; Trial Will Close Down Standard Programs Early Tomorrow

WASHINGTON, Sept. 14 (UP) -- Don't be alarmed if you can't tune in the radio or television programs early Wednesday morning. It will be just a test of the "conelrad" civil defense radio set.

On September 17 the success of the test was synthesized by an article reporting how the ADF set of a C-47 Skytrain observation aircraft returned useless bearing indications, the pointer jumping all around the dial.

- September 17, 1953 - By PETER KIHSS  - EMERGENCY RADIO A JUMBLED SUCCESS; City Joins Nation in Test of CONELRAD, Defense System That Is Puzzling to Foe
A shining needle darted every few seconds from one point on a cockpit dial to another point, sometimes a quarter circle or more away. In the blackness of 2:45 A. M. yesterday, flying 170 miles an hour 4,000 feet above Mastic Beach, L. I., Capt. John W. Wilkin and his Eastern Air Defense Force crew in a C-47 observation plane called it completely confusing -- and therefore good.

Again no definite reference to C-D channels can be found at the date. We can guess that the network of synchronous transmitters was not yet complete and ready to reach population everywhere. Probably in marginal areas, even in New York City, instructions to people were still given through speakers.

We must wait until the exercise of November 17, 1954 to read that the civil defense information reached people through eleven stations joining the Conelrad System.


Eleven radio stations in the metropolitan area participated early today in a nation-wide test of CONELRAD, a system for communicating civil defense information without providing guidance for enemy aircraft. The exercise was conducted from 1:30 to 4:30 A. M.

Indeed, form the ‘Honolulu Record, Volume 9 No. 29, Thursday, February 14, 1957 p. 2’, we learn that the Conelrad network coverage had been extended to Hawaii only two years later, in 1956. The above dates give us the span required to reach full coverage and full operation all over US.

We can then expect that radio sets, with tuning dial modified to facilitate tuning of the two distress channels, gradually appeared on the market before and around these dates. Of course even old dials could do the job, just asking for a slightly more difficult aural tuning. Anyway we must assume that already built non-compliant sets, still on the shelves of manufacturers and distributors, sooner or later reached the market, maybe just the Hawaiian market.

In conclusion, we could find either Conelrad compliant and non compliant radios sold through the years from 1952 to 1956. A good practice to identify variants of the base model is to assign different suffixes, in order to better handle spare parts. But in this case the variant just consisted in the addition of two extra marks on the tuning dial. To service an old radio set with a broken glass (or cabinet in case of small sets), a dial with C-D marks was even better that the old one, since the radio was automatically updated after the replacement. We can then assume that probably some manufacturers did not assign any variants to their sets with updated dials. Probably for these models a ‘Conelrad’ variant could be handled, upon its evidence, in the same page of the base model itself, more or less as today happens for vacuum tubes.

Of course more information as well as direct witness are always welcome.

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Some CONELRAD and EBS history  
08.Dec.11 21:10
891 from 33451

David Josephson (USA)
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America is a young country, with many of its citizens having recently come from other countries whose governments they found repressive. Since 1774 we have been very reluctant to give the government too much power. So, it’s not surprising that most of the work of the American civil defense system was accomplished by public encouragement rather than actual law. You can find President Kennedy's 1961 speech on YouTube, seach for "JFK Endorses CONELRAD." Participation in the CONELRAD system, except for some critical details, was voluntary. I can find no requirement for marking of radio receivers with the CD symbol, and FCC had no jurisdiction over receivers at that time. Many have assumed that there was some government requirement that radios be marked with the CONELRAD frequencies, but in fact it seems to have been entirely voluntary.  Dr. Harvey Smith, who served as a science and technology adviser in the Office of Emergency Preparedness in the White House starting in 1967 (and later an expert on terrorism, before returning to academia to teach math) wrote me:

In those (pre-Viet Nam) days, industry was eager to cooperate with national defense
efforts.  Most, if  not all, of the radio manufacturers were defense contractors,
eager to stay on the good side of DoD. Since the cost would be minimal, I think
that simply telling the manufacturers that putting the CD  markings on the dials
would be a useful thing to do would have sufficed, without making it a formal 

Development of CONELRAD-marked radios

Research credit to Eric Huffstutler who found this clipping from "Tele-Tech and Electronic Industries" April 1953. This is the magazine of the Radio-Television Manufacturers Association, the trade organization responsible (through several name changes) for many consumer electronic standards in the 1930s through the late 2000s.

Similar articles appeared in other publications about the same time. The government actually obtained


a design patent on the CD logo, and promptly released it for anyone to use on radio receivers as shown in this excerpt from the Federal Register, 18 Fed Reg 3276 (1953).

Broadcasters' role and the FCC

There is a comprehensive treatment of the subject in the National Association of Broadcasters Engineering Handbook, 5th edition (1960), pages 8-123 through 8-156. NAB is concerned about the broadcasters' side of the problem, not the radio manufacturers' side, and along with the other FCC rules  on their operation, they were required to go off the air unless they were participating in CONELRAD. It's clear that through the 1950s there was a lot of experimentation going on, as it was up to individual stations to decide whether and how to participate (only half of the cost of their equipment was paid by the federal government). There was, as usual with government projects, a lot of bureaucracy involved and individual stations' efforts had to be coordinated through their local and then state-level civil defense managers. But the engineers concerned were free to experiment, and the NAB Handbook (and articles in broadcasting magazines of the time) describe many of their attempts.

The only law about CONELRAD was the original establishment of the idea, which was President Truman’s Executive Order 10312, which you can find online. It doesn’t say anything about radio receivers. FCC took this as a mandate to make their regulations governing the requirements for radio broadcasters (which in the US were and are entirely privately owned.)

End of CONELRAD, beginning EBS

Dr. Smith also had some interesting comments about the jumbled development and abandonment of CONELRAD:

My general impression is that CONELRAD was primarily  intended to deny attacking 
manned bombers the ability to use our radio stations as navigation aids  while 
still allowing the authorities to communicate with the public via AM radio.  As 
we moved into  the 1960s, this became of less importance because the expected 
modes of attack changed and the  capabilities of navigation systems improved.  
I believe that the Cuban missile crisis in October 1962  led to a re-examination 
of exactly what we were doing and the changes you mention. I have a vague 
recollection of one of the old hands at OEP telling me that there had been an 
attempt  to exercise the CONELRAD system during the Cuban missile crisis and 
that it was a fiasco, so the  system was just abandoned and the EBS set up 
instead. I don't think the bureaucracy would have  wanted to announce that their 
system wasn't practical and needed to be abandoned, so I suspect that  they 
just quietly walked away from it and hoped nobody would ask embarrassing 
questions. In that  case they would try not to create the paper trail you are 

Of course the system became impractical for several reasons, the first of which was that by that time, an attack on the mainland of the US was very unlikely to come from manned bombers using 1940-era radio gear to find their target. Rather than scrap the entire project, only the "confuse the bombers" aspect was canceled. CONELRAD was modified in 1963 to become the Emergency Broadcast System (EBS) which used the same methods for alerting listeners to critical information, but allowed stations to participate on their normal radio frequency. EBS was used this way from 1963 to 1976. In 1976 the system of switching the carrier on and off, followed by a 15 second 1 kHz tone, was replaced with a two-tone attention signal and no carrier interruption; this lasted until 1997 when it was replaced with the Emergency Alert System (EAS) which uses internet and weather radio channels as well as broadcast.

I don't think actual synchronous operation was ever attempted. The article in Emilio's post mentions 20 Hz frequency tolerance, which was and is the standard required of standard broadcast stations anyway. The NAB handbook reports that such a system design had been completed on paper for stations on the east coast of the US, but only three stations in the New York area ever did this, achieving tolerance of ± 3 to 4 Hz. Subsequently the plan was changed, so that instead of having stations on the air simultaneously, they would be organized into clusters, switching more or less randomly between stations. As of 1960, NAB reported that over 200 clusters of stations were operational, and 500 more single stations in the "on-off" category. One cluster, in Washington, DC is described using three stations, with a switcher made to "vary the interval that each station will be on the air from 6 to 40 seconds" and run for 1 hour and 40 minutes before repeating the sequence.

Alerting Receivers

Some radio receivers were provided not only with CD marks at 640 and 1240, but alert relays to sound an alarm if the monitored station's carrier dropped. The list provided in the NAB Handbook includes Ameco, Paul Revere (Dumont), Conalert II (Kaar), Air Alert I and II (Miratel), Moradco (Morrow Radio Mfg.), Motorola and RCA. These same receivers were often used in broadcast stations after CONELRAD was replaced by EBS. Some of my earliest tasks as a broadcast engineer were to log other stations' EBS tests and turn our own transmitters on and off as we were initially required to do. In the station where I started, it was a common nuisance that something would knock the primary station off the air for a few seconds and bells would ring throughout all the stations monitoring, to wake up the engineers on duty...

Stations that were not directly participating in CONELRAD had the requirement only to monitor the "key" or primary stations, which in CONELRAD times were the ones that would switch to 640 or 1240. Monitoring the primary station was the responsibility of all stations. When CONELRAD was replaced by EBS, these same key stations remained in their primary roles but did not shift frequency.

Amateur radio stations were required to monitor their local key stations and go off the air on hearing an alert, but this was only in effect from 1957 (per the ARRL handbook of 1958). Emilio has already added a model for the Heathkit CA-1 alarm that many hams used to perform this function. The Heathkit actually had provisions to remove power from a ham's transmitter on receiving an alert signal.

Sirens, speakers, lights

The suggestion that loudspeakers were used in America for public alerting is not correct, as far as I know. While there is a history of “wired radio” in Europe and Asia, it was never common in the US. Air raid sirens having clearly defined patterns were used and many towns still have them (typically not in use except for volunteer fire departments). The telephone company (until 1984, a monopoly called the Bell System that operated with very close coordination with the federal government) offered a “Bell and Lights” system that allowed levels of alert to be communicated. Subscribers to “Bell and Lights” were typically police departments, schools and big factories. I have the Bell System Practice on this system if anyone is interested.

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