Category Archives: Conelrad

July 20, 1956 CONELRAD Test

Milwaukee Civil Defense Director Don E. Carleton and Col. Anthony F. Levno assess damage after simulated attack on Milwaukee. Milwaukee Journal, Jul. 20, 1956.

Milwaukee Civil Defense Director Don E. Carleton and Col. Anthony F. Levno assess damage after simulated attack on Milwaukee. Milwaukee Journal, Jul. 20, 1956.

Today marks the 60th anniversary of what was, as far as I’ve been able to determine, the only nationwide test of CONELRAD, the system designed to give Americans emergency information about a nuclear attack.

As I wrote in a previous post, all radio and TV broadcast stations in the U.S. left the air at 3:10 PM Eastern Time.  Designated stations came on the air on 640 or 1240 kHz, alternating between transmitters to confuse enemy bombers.  In some cities, such as Milwaukee, local exercises were conducted in conjunction with the CONELRAD test.  In the photo shown here, civil defense planners are examining the hypothetical ruins of Milwaukee.

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1954 High Gain Low Drain Portable


The plans for this “high gain, low drain portable radio” appeared in the March 1954 issue of Radio Electronics magazine.  It appears to be a good performer, but it was designed both for performance and low battery drain, and the author explained why:

When anybody anybody mentions “portable radio,” most of us think immediately of the entertainment it can provide.  The Set can go with us on picnics, vacation trips, and boat rides. We can enjoy ball games, national events and all our favorite programs while we work, play, and travel. But there is a serious side to this matter. In these days of H-bombs and supersonic jets, a portable radio might mean the difference between life and death. If that terrible day should ever come when air-raid sirens wail for real, the Conelrad system will go into effect. If worst comes to worst, if power goes out and wires down, civil defense messages and other essential communications will continue. In such an emergency, a portable radio can become a very important item.

With that in mind, the key design factor was low battery drain. A few years later, transistors would make that goal easy. But the current transistors on the market, while suitable for low-powered audio, weren’t yet ready for use with RF. Therefore, the design of this set was a hybrid–it contained two tubes, a 1E8 serving as oscillator and mixer, with a 1AD5 serving as IF amplifier. Then, solid state took over, with a 1N34 diode as the detector, and two CK722 transistors providing enough audio amplification to drive a speaker. For more distant stations, a headphone jack was provided.

Because of the hybrid design, the set required three batteries, but they were all set up to minimize current draw. The transistors were powered by 4.5 volts, provided by three dry cells. The filaments were powered by another 1.5 volt dry cell, but the author noted that these could be run on a battery so low that it was no longer useful for a flashlight. In addition, there was a potentiometer in series with the filaments. This would serve, to a certain extent, as a volume control. But more importantly, it would allow the filament current to be set to the lowest possible position.

The B+ for the tubes was provided by a 45 volt battery. Here, another battery saving trick was employed. The set contained a switch for local/long-distance. For strong local stations, a 56k resistor was switched in series with the B battery. This reduced the B+ current to only 50 microamps, just barely enough to keep the RF section running. For more distant stations, this resistor was bypassed, and the tubes ran on the full 45 volts, drawing about 1.6 mA.

The author reported that the set worked well with local stations, even in a skyscraper, or even in a subway or tunnel. He reported pulling in stations as far as 500 miles away with the telescoping antenna, normally designed for use as an automobile antenna.


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1955 CONELRAD Monitor at WRFD

1955WRFDConelradFrom 1951-1963, U.S. broadcast stations were required to participate in CONELRAD, a system designed to alert the public to enemy attack, but also deprive enemy bombers of radio signals which could be used for navigation purposes.  Under a CONELRAD alert, all stations would cease broadcasting on their normal frequencies. Designated stations would switch to broadcasting on either 640 or 1240 kHz.  The result would be that listeners would be able to hear alerts, but enemy bombers would hear only a confusing jumble of signals on those frequencies.

For the system to work, each station needed an alarm.  For smaller stations, this would consist of an alarm tuned to another station in the area.  If the primary station went off the air, then the smaller station would be alerted.  If it turned out to be an actual alert, they would need to leave the air or switch to their designated frequency.  Alarms were available commercially for broadcast stations, and simpler models were also available for hams, who were later required to participate in CONELRAD.  Many hams built their own, and there were many plans published over the years.  Sixty years ago this month, the October 1955 issue of Radio Electronics magazine carried the plans for the unit shown here, which was in use at WRFD, a Worthington, Ohio, 5000 watt daytime only station affiliated with Fram Bureau Insurance, with a format aimed at the agricultural market.

The author of the construction article was Harold Schaaf, the station’s chief engineer, who noted a critical defect in many of the existing CONELRAD alarms.  Most of them depended on a normally open relay which would close in case of an alert.  If the alarm circuit failed for some reason, there would be no relay action.

Schaaf noted that “such a system cannot be considered reliable, since it can go out of order without the operator knowing it.” Schaaf’s circuit instead included a relay that remained energized during normal operation. In the case of a circuit fault such as a failure of one of the tubes, the relay would de-energize, which would cause the alarm to sound, requiring the station operator to investigate. The result was what the title of the article described as a “failure-proof CONELRAD alarm.”

Like most other CONELRAD alarms, this one was hooked to the AVC circuit of a superheterodyne receiver tuned to the station being monitored. As long as there was an AVC voltage present, the alarm would remain silent. If the monitored station went off the air, the AVC voltage would disappear, which would trigger the alarm, which could consist of an external bell.

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More CONELRAD Crystal Sets

1956 Boys' Life Conelrad Receiver.

1956 Boys’ Life Conelrad Receiver.

We’ve had a number of posts about the use of crystal sets for reception of CONELRAD signals.   For example, a 1956 Boys’ Life article contained instructions for building a one-transistor set for use in receiving CONELRAD during an attack.  A later 1965 article pared down the set to a simple crystal set.

Boy Scout and Civil Defense leaders in Spokane apparently thought it was a good idea, as reported in the November 24, 1958 issue of the Spokane Daily Chronicle.

Chester L. Brown, the civil defense communications chief, prepared a special bulletin on Conelrad crystal radio receivers, which were distributed by Boy Scouts of the Inland Empire Council.

According to the article, the idea was suggested by a scout mother who had been active in civil defense. She had read a newspaper article in which an Atomic Energy Commission official had proposed that all households should be equipped with a receiver capable of operating without commercial power.

The CD pamphlet contained a diagram for a set, and noted that the parts could be purchased in kit form for as little as $1.25.


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1965 Emergency Broadcast System Simulated Script

Terrorism Section Content Nuclear Blast
A January 1965 report prepared for the Office of Civil Defense reported on several simulated fallout shelter exercises conducted in the Pittsburg area to investigate various shelter management factors. For example, in one study, the designated shelter manager arrived late. In another study, the effects of total darkness upon shelter operation were studied.

In three of the experiments, the stress level was increased by periodically playing simulated emergency broadcast system messages. The script of those broadcasts is particularly interesting, since it gives an insight into what civil defense planners thought might be a plausible scenario for a nuclear attack on the United States, and how news would be communicated to the public.

Here is the text of the nineteen radio broadcasts that might have been heard during a nuclear war:

EBS #1, Friday, 7:00 PM

ATTENTION. ATTENTION. This is the Emergency Broadcast System. Take shelter immediately. Take shelter immediately. This is not a drill.
Repeat: This is not a drill. An enemy attack is being launched against
the United States. Take shelter immediately and stay tuned to this
frequency for further instructions.


EBS #2, Friday, 7:10 PM

ATTENTION. ATTENTION. This is the Emergency Broadcast System. We have just been informed that the city is now on the emergency power system. Please inform the control center if your shelter is without light-., Repeat: The city is now on the emergency power system. Please inform the control center if your shelter is without lights. We also have ….. we also have word here that there has been no confirmed report of a missile strike in this area.  There has been no confirmed report of a missile strike in this area.

EBS #3, Friday, 7:15

(Phone is heard ringing in background.)

ATTENTION. ATTENTION. This Is the Emergency Broadcast System. A missile attack has been launched against the United States. Reports about the attack are fragmentary and unconfirmed. The strategic missile bases west of the Mississippi appear to have borne the brunt of the attack. As of this moment there has been no official report of a nuclear detonation in our immediate vicinity. Fallout has begun to descend on the western portions of our city and is expected in other areas imminently. Do not communicate with the emergency operations center unless absolutely necessary.

EBS #4, Friday, 8:00 PM

ATTENTION. ATTENTION. This is the Emergency Broadcast System. Stay tuned for an important message. (DISTANT VOICE: Okay, stand by now. We’ve got a remote from Washington.) Static ——- Noise. Another voice: This is a report from the emergency national command post in Washington. The President and his key civilian and military aides have been safely evacuated to the emergency seat of government. This evening at 6:35 PM the enemy launched an attack against the strategic retaliatory forces of the United States and its NATO allies. An intelligence warning allowed us to launch a portion of our land-based missile force against the enemy’s remaining strategic forces.
Polaris missiles have also been launched. In addition, our airborne alert and a portion of our ground alert aircraft forces have been sent against the enemy’s non-missile strategic forces. Our damage assessment reports indicate that many of our SAC bases have been destroyed or severely damaged. A number of communities near SAC bases have also suffered great damage. The fallout monitoring network reports that radiation is heavy in the western portion of our country and is increasing in the midwest and eastern portions of our nation. Although there have been several nuclear detonations in the east, it appears as if these have been the result of errant missiles, rather than a planned attack against population centers.  The President, whom, I repeat is alive and well, will address the nation as soon as his command duties permit. This is the end of the Priority One report. Local EBS stations may resume Priority Two broadcasting.

EBS #5, Friday, 8:30 PM

ATTENTION. ATTENTION. This is the Emergency Broadcast System. Short wave monitoring has disclosed that our air strike forces are currently launching attacks on the enemy homeland. These forces are utilizing a new….. what?  What do you mean it’s not for release? (Another voice: Priority One. Now.. for heaven’s sake! Announcer: Well what the hell …… 1)


EBS #6, Friday, 8: 50 PM

Has this one been cleared?
ATTENTION. ATTENTION. This is the Emergency Broadcast System. We have Just received word that the President has been evacuated to sea in the floating Whitehouse. The location of this ship is unknown. The floating Whitehouse is a battle cruiser, fully equipped for command and control functions. Our government has survived the attack. I repeat, our government has survived the attack.

EBS #7, Friday, 9:30 PM

ATTENION. ATTENTION. This is the Emergency Broadcast System, We have just been informed that a message is to be delivered from the governor’s office in Harrisburg. Please stand by,

This is a report from the governor’s office in Harrisburg. The state of
conditions in Pennsylvania is serious, but not critical. Erie has been
severely damaged by what is believed to have been a stray missile. No other cities have reported being hit, but the fallout level is rapidly increasing, particularly in western Pennsylvania. Apparently neighboring states have borne the brunt of the attack, particularly those in the western portions of the country. All citizens should seek shelter immediately. Do not attempt to evacuate your area until you are instructed to do so. Local law enforcement personnel should remain in their respective areas. State police have been assigned to more critical areas, and additional state aid will become available and be assigned when fallout levels permit.

EBS #8, Friday, 10:15 PM

ATTENTION. ATTENTION. This is the Emergency Broadcast System. Fallout began to descend on the Pittsburgh area several hours ago and radiological monitoring reports indicate that radiation levels are dangerously high in many parts of our city. No one should attempt to leave shelters. Repeat: No one should attempt to leave shelters. Youngstown, Ohio and Erie, Pennsylvania have suffered severe damage as a result of nuclear detonations.
As of the moment there have been no nuclear blasts in our immediate area.  The municipal power has been temporarily disrupted in some parts of the city. Power should be restored shortly. No further official reports on our retaliatory attacks on the enemy homeland are available. Unofficially, the absence of any significant second wave of enemy attack, plus the size of our surviving strategic force, allows cautious optimism that we will suffer no further major damage from any attack. Until further word is transmitted by this station, everyone must remain in shelters.

EBS #9, Friday, 10:30 PM

ATTENTION. ATIENTION. This is the Emergency Broadcast System. In order to evaluate the damage to Pittsburgh, the emergency operations center requests every shelter to gather the following information and to report it to the local emergency operations center. Is this a fallout or a blast shelter? How many persons are in the shelter? How many of these persons are injured? How many persons are suffering from radiation sickness? What is the condition, of your equipment? Is your shelter structure damaged? Do you have adequate electricity?
Do you have adequate ventilation? What is the state of your food supplies? What is the state of your water supply? Do you have any illness other than radiation sickness? As soon as we have received reports from district control centers we will relay such information on to you. When emergency missions are possible, disaster teams will be sent to those shelters which need medical supplies, food and water. Attempts will also be made to report specific areas of damage in our city. Please stay tuned for additional announcements.

EBS #10, 11:30 PM

ATTENTION. ATTENTION. This is the Emergency Broadcast System. Wee have hundreds of people in the area who do not have shelter with an adequate protection factor. They must be moved to other shelters in order to survive.
Please advise the emergency operations center as to the number of additional people you can take into your shelter. This is imperative. Please inform the emergency operations center as to the number of additional people you can take into your shelter.

EBS #11, Saturday. 1:30 AM

ATTENTION. ATTENTION. This is the Emergency 3roadcast System. Radiological monitoring teams report that the radiation levels in the Pittsburgh area are still high. However, there is no additional accumulation of radioactive dust. The fallout on the ground is beginning to decay. It is simply a matter of waiting out this decay time before we can undertake further civil defense measures.  Everyone is to remain inside until further notice. Please do not leave your shelters.

EBS #12, Saturday, 2:15 AM

ACCIDENTALLY OVERHEAR A SHORT WAVE BROADCAST. “Hello Tower…. to checkpoint two…. ” Static and short wave noise.

EBS #13, Saturday, 3:00 AM


ATTENTION. ATTENTION. This is the Emergency Broadcast System.

EBS #14, Saturday, 3:45 AM

ATTENTION. ATTENTION. This is the Emergency Broadcast System. Reports have been received that there are bands of looters wandering about the city.  Attempts have been made to loot shelters in this area. Be alert to this situation and act accordingly. Security police will begin patrolling the area as soon as the radiation level permits.

EBS #15, Saturday, 4:15 AM

“Angels 46 — Same heading — Roger, Angels 52 — Fuel 30 … ”
Much static.

EBS #16, Saturday, 6:00 AM

ATTENTION. ATTENTION. This is the Emergency Broadcast Svytem. Stay tuned for an important message. Okay, stand by to switch.
MUCH STATIC ———– “Please stand by.”
This is a Priority One report from the emergency national command post in Washington. It appears that the enemy attack is over. There have been no further reports of missile strikes since early last evening. Radio monitoring indicates no further enemy air activity. Damage assessment reports indicate that the brunt of this attack was borne by western states. Many of our SAC bases have been destroyed or severely damaged. Communities near SAC bases have also been severely damaged. The central and eastern portions of the country have escaped extensive damage although stray missiles have struck some of the smaller population centers. Fallout is moving across the country in an easterly direction, carried on westerly winds. All citizens should remain in shelters until instructed otherwise by local civil defense commands. The President and key members of his cabinet are still aboard the U. S. S. Northampton. The President will address the American people as soon as his command duties permit.

This is the end of the priority ….. this is the end of the Priority One report. Local EBS stations may resume priority two broadcasting.

EBS #17, Saturday, 7:30 AM

ATTENTION. ATTENTION. This is the Emergency Broadcast System. Emergency teams have been established and have begun to operate in various sections of Pittsburgh. There is a shortage of able-bodied men to serve on work details in Shadyside, East Liberty, Bloomnfield, and Morningside. Will all shelters submit to the emergency operations center the names of able-bodied volunteers who may be asked to leave shelters before radiation levels are completely safe for permanent exit. Phone the names into the emergency operations center. Further information will be provided as to when and where the rescue volunteers will report.

EBS #18, Saturday. 10:00 AM

ATTENTION. ATTENTION. This is the Emergency Broadcast System. Weather monitoring teams report that there is a severe storm approaching the Pittsburgh area. What’s that? It appears that this storm is bearing with it a radioactive dust cloud and we expect the levels of radiation to increase severely. Some shelters do not have adequate protection facilities against this cloud. There is a possibility that some shelters will have to mobilize and be moved. (PAUSE) We will contact these shelters by phone within the next few minutes. Please do not call the emergency operations center. If your shelter is one of these that has to be mobilized and be moved, we will contact you. Please stand by.

EBS #19, Saturday 3:00 PM

ATTENTION. ATTENTION. This is the Emergency Broadcast System. Radiological calculations of fallout levels in Pittsburgh indicate that permanent exit from some shelters will be possible in the near future. At the present time recovery teams are surveying the city to locate and to prepare facilities for post-shelter operations. It is imperative that you do not attempt to leave your shelter without prior notice from the emergency operations center. There are still many dangerous radiological “hot spots” in the city. Therefore, regardless of the radiological readings in your immediate vicinity, wait for official notification. from your government in the emergency operations center.


For more insight into 1960’s civil defense, my website contains a scan of the 1962 St. Paul, MN, civil defense operational plan, which includes the manual that shelter managers would use in the operation of a fallout shelter.

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1958 Conelrad Monitor


An article in the February 1958 issue of Popular Electronics admonishes that “all of us should use a receiver that warns when the Conelrad alert begins,” and the magazine offered a simple circuit, shown here. The concept was about as simple as possible: The little self-contained receiver monitored continuously. If the carrier of the local station disappeared, then a buzzer would sound.  Since all stations (other than designated stations on 640 and 1240) were required to leave the air during a CONELRAD alert, this would warn the owner of an impending attack.

It would seem that this particular design would be prone to false alarms. Obviously, it would work only with a 24-hour station. Otherwise, when the station signed off each night, the buzzer would sound until morning. And even most 24-hour stations signed off occasionally for transmitter maintenance. These periods often took place early Monday mornings, so someone relying on this receiver would probably get some rude awakenings if the receiver was left on.

And even though the receiver drew minimal current, it would probably go through the three penlight cells quite quickly.

But since hams were then required to monitor Conelrad, the receiver might have served a useful purpose, even though it probably wouldn’t work too well in continuous service.

1958PEconelradSchematicAs can be seen from the circuit here, the CK722 served as detector, and the 2N170 was hooked up to a relay, which would turn off the buzzer when the station left the air. The author, I.C. Chapel, notes that the relay is set to trip at 6 mA. At the conclusion of the article, there is an editor’s note stating that this figure “amazed” them, and concluded that the author must be very close to the broadcast station to get these results with his 10-foot antenna. One editor constructed the unit and tried it about 25 miles from New York City, and none of the stations there was strong enough to make the circuit work. With a more sensitive relay and a 150 foot antenna, though, they were able to get it to work with a 250-watt station 10 miles away.

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1955 CONELRAD Receiver

1955ConelradRXUnder the headline, “Are You Ready For CONELRAD?”, the April 1955 issue of Radio News carried the plans for this small CONELRAD receiver which could be easily carried in a pocket or glove compartment. The circuit is very similar to the Conelrad receiver carried by Boys’ Life the following year.  This receiver also consists of a crystal set followed by a one-transistor audio amplifier.

The article describes CONELRAD, and notes that “the responsibility of the individual citizen and particularly of the electronic technician and experimenter would be to provide themselves and their families well ahead of time with some means for receiving Conelrad broadcasts under any or all emergency conditions.” The article noted that receivers should be exactly calibrated, to avoid losing valuable seconds fumbling around trying to find 640 and 1240 on the dial. The article suggests using a signal generator to calibrate and mark the dial before an emergency.

1955ConelradSchematicIt then offers the circuit shown here, since it would be “highly advisable to have available at all times a portable battery-operated radio. The average portable radio is fairly cumbersome to carry about over any appreciable distance and its power requirements are high enough to make it impossible to keep in operation continuously.

The author notes that his original plan was to have one transistor serve as the detector, but had some difficulty in designing the circuit, due to the low input impedance of most transistors. Therefore, like the author of the Boys’ Life design, he settled on a diode detector, with one stage of audio amplification.

Unlike the Boys’ Life model, which was mounted in a cigar box, this one is mounted snugly in a small plastic box. The author notes that “as most old crystal set men will recall, a fairly long antenna and a good ground are required for best results.” Therefore, he recommended taping 25-50 feet of fine wire to the case for use as an emergency antenna.

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1957 CONELRAD The Easy Way

1957ConelradStarting in 1957, U.S. Amateur Radio operators were required to participate in CONELRAD. (If you’re unfamiliar with CONELRAD, I explain it in other posts, including this one.) Under the regulations that took effect that year, hams were required to monitor an AM broadcast station whenever transmitting. If that station went off the air, the ham was required to check to see if the absence from the air was due to a CONELRAD alert. If so, he was required to leave the air.

The regulations could be satisfied by keeping an AM radio on low volume in the background, but the preferred method was to have an automated alarm that would sound if an AM station left the air. One popular receiver for that purpose was the Heathkit CA-1 CONELRAD alarm, which was an external monitor that would be hooked to a receiver. Other dedicated receivers wwere available, such as the Kaar Engineering Conalert II, although a unit such as that would be out of the price range of most hams, and probably used mostly by broadcast stations.

The April, 1957, issue of Radio News carries an article entitled “Conelrad the Easy Way,” with a simple method of converting a five-tube broadcast receiver into a CONELRAD monitor. As shown in the schematic above, it required only three parts, and allowed the radio to be used for normal listening. The additions to the circuit are the three parts inside the dotted lines.

This circuit ties in to the AVC voltage of the first audio amplifier. As long as there is an AVC voltage present, the added resistor biases the first tube to silence the radio. But if the AVC voltage disappears (because there is no signal present), then the output of the final audio amplifier gets fed back to the first audio amplifier, causing the two stages to break into oscillation to emit a loud squeal.

It’s a pretty ingenious and easy modification, and the author reports that many hams were using it and that he thought “it is the answer to the Conelrad needs of most hams.” He even notes that the circuit “is so simple that many broadcast listeners may want to install it on their receivers, just in case.”

The author, by the way, is John T. Frye, W9EGV. If that name rings a bell, it is because Frye was a prolific writer in many electronics and radio magazines. He was most famous as the author of the “Carl and Jerry” stories that appeared in Popular Electronics from 1954-1964.

Another Crystal Set for CONELRAD Reception


60 years ago, CONELRAD was the system planned for keeping the American public informed in the event of a nuclear attack. As I’ve explained previously, the idea was for designated broadcast stations to operate on 640 or 1240 kHz. Stations would not transmit station identification, transmissions from individual stations would be short, and enemy bombers would be presented with a cacophony of signals useless for navigation purposes.

But power might be out. Battery-operated sets were rare, and most of those that existed sucked through expensive batteries quickly, since they had to power the filaments of the tubes. Undaunted, radio enthusiasts realized that a crystal set could be put to use. As I previously reported, Boys’ Life magaine touted a crystal set that could be put to use in an emergency.
Another Boys’ Life article included a CONELRAD receiver with one transistor that could run on two penlight batteries. And in a pinch, that set could be used without a battery, operating as a simple crystal set. And during the 1956 CONELRAD test, a Heathkit crystal set performed surprisingly well at receiving the emergency broadcasts, even outperforming commercial tube and transistor radios.

EmergencyXtalSetRadioTVExperimenter1955SchematicAnother example of crystal sets for emergency use is shown here, in the 1955 edition of Radio-TV Experimenter.  Author George P. Pearce (probably shown in the illustration above) describes the need:

If flood, tornado or air raids cause power failures, could you get emergency directions from the Conelrad stations the government has at 640 and 1240 on the dial? Even battery-powered sets couldn’t operate over an extended period of weeks, so why not build a crystal set that needs no power except the broadcast signal.

The author describes this set, which uses two 1N35 diodes along with two .001 uF capacitors in a voltage-doubler circuit. It uses basket-wound high-Q coils to pull in weak signals. It recommends a 100 foot antenna and good ground. He also suggests the use of the house wiring as an antenna, using a lamp cord, capacitor, and plug going in to the 120 volt house wiring. This ought to work, but if the power is on, you would be putting a lot of faith in that capacitor not being leaky as you put the headphones hooked to that antenna onto your head, just like they place the electrode of an electric chair.

The author notes that there’s nothing to wear out, and his set has operated for over three years.

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CONELRAD Test, July 20, 1956

Milwaukee Civil Defense Director Don E. Carleton and Col. Anthony F. Levno assess damage after simulated attack on Milwaukee.  Milwaukee Journal, Jul. 20, 1956.

Milwaukee Civil Defense Director Don E. Carleton and Col. Anthony F. Levno assess damage after simulated attack on Milwaukee. Milwaukee Journal, Jul. 20, 1956.

At 3:10 PM Eastern Standard Time on July 20, 1956, CONELRAD conducted its first (and as far as I can tell, only) nationwide test. At that time, all radio and television stations left the air for 15 minutes, and the only broadcast signals coming from the United States were those of the CONELRAD system on 640 and 1240 kHz.

CONELRAD  was obsolete almost as soon as it was put into effect, but the idea was that during an enemy attack, attacking bombers must be deprived of the ability to use American broadcast stations for direction finding and navigation. Aviation routinely made use of AM stations for navigation, and the locations of broadcast stations and their frequencies are still printed on aviation charts. It was a reasonable concern, but it became much less critical when the bomber was replaced by the ICBM as the main component of both Soviet and American strategic war planning.

CONELRAD was created by President Truman in 1951, and hung on until 1963, when it was replaced by the Emergency Broadcast Sytem, in which participating stations continued to broadcast on their normal frequency.

Under CONELRAD, all stations in the U.S. would operate on the same two frequencies.  The navigator of an enemy bomber tuning to either of those frequencies would be confronted with hundreds of stations on the same frequency, rendering them useless for navigation.  And each station would transmit for only a few seconds or minutes.  In smaller markets with only one station, the station would quickly give instructions, and then sign off for a few minutes.  In larger cities, the stations would be linked together by telephone lines.  A continuous program could be sent, but it would switch quickly from one transmitter to another, hopelessly confusing enemy bombers.

But for those 12 years, CONELRAD was the method by which Americans would be warned of war, and in 1956, it was put to a test.  At 3:10 PM Eastern Time, each participating station was to transmit the program which had been delivered by record.  The introduction to this broadcast can be heard at the following YouTube video:

In many cities, such as Chicago, the CONELRAD test was conducted in conjunction with other civil defense exercises.  The Chicago Tribune for July 20, 1956, details some of the preparations being made there.  The next day’s paper  reports 325,000 simulated deaths in the Land of Lincoln.

There was surprisingly little reporting on how well the test went: Namely, whether the public was actually able to hear the broadcasts. One of the few actual tests was carried out by Radio News magazine, and reported in the October 1956 issue.

The magazine arranged receiving sites at four locations around the New York area. They were in a steel building in Brooklyn, a steel building in Manhattan, a home about 25 miles from the city, and a home about 50 miles from the city. At each location, writers for the magazine had multiple receivers ready for the test. They then rated the percentage of the broadcast they were able to receive intelligibly.

An outdoor antenna proved to be the greatest asset. At the home 25 miles from the city, the editor reported a 100% satisfactory signal using a Hallicrafters S-40 hooked to an outside TV antenna, and also with a Heathkit crystal set with a 100 foot outdoor antenna. At the same location, a Westinghouse battery portable without external antenna gave only 75% satisfactory reception.

In Brooklyn, the best performer turned out to be the Regency TR-1 transistor portable,
which gave a 100% reliable signal, but with continual retuning as the signal shifted from one transmitter to another. At the same location, the Knight tube portable was only 75% satisfactory, with the remaining signal too weak without reorienting and retuning the radio.

In Manhattan, the transistor portable, a Zenith Royal 500, outperformed the tube portable, with 85% satisfactory reception compared to 65%.

50 miles from the city, a Grundig tube portable gave 85% satisfactory reception, outdoing the Zenith portable, which had only 60% reliability. At this more distant location, the main problem came from interference from stations in other cities’ CONELRAD networks.

It is somewhat surprising that so much “retuning” was necessary. Presumably, the stations all had a crystal for their assigned frequency (the article didn’t state whether New York was using 640 or 1240), so it’s unlikely that the individual transmitters were drifting. More likely, some of the individual transmitters were slightly off frequency, resulting in the need to retune when the signal switched from one to another.

The article did stress the importance of having nondirectional antennas, something that was lacking in most AM portables. Most of the receivers, other than those using outdoor antennas, had to be reoriented when the signal switched transmitter locations. The article noted that extreme sensitivity wasn’t necessarily a good thing, because of the danger of interference from adjacent networks. These two factors explain why the crystal set had such good results in the test.

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