Radio-Electronics Monthly Review
March 1946 Radio-Craft

March 1946 Radio-Craft [Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Craft, published 1929 - 1953. All copyrights are hereby acknowledged.

EME (earth-moon-earth) amateur radio enthusiasts will be interested in reading this report of the first known instance of bouncing a radio signal off the surface of the moon and then receiving it. The event was reported in the April 1946 issue of Radio Craft magazine. A highly modified "SCR-271" radar set was used, and operated at the standard frequency of 111.6 MHz. Aside from customizing the pulse width and repetition rate, an adjustment to the receiver was necessary to account for the Doppler shift of the return frequency due the the moon's motion relative to the earth. Handling Doppler frequency shifts was necessary in the course of normal operation, but the moon, depending on its position in the sky, could be moving as fast as 750 miles per hour - faster than any airplanes of the era. Also presented this month was the first commercially operating RF food heater, built by the Automatic Canteen Company (1953 lawsuit). Working with General Electric, their engineers experimented with more than a thousand frequencies, power levels, and mechanical configurations to arrive at a solution which properly heated hot dogs and buns together.

Radio-Electronics Monthly Review

Electronic Hot Dogs, long distributed to spectators at demonstrations of high-frequency heating, will be available to the public at the drop of a dime into an ordinary vending machine, General Electric engineers announced last month.

Electronic hot dogs have long been cooked by high-frequency engineers to show what dielectric heating could do - one of the first set-ups was pictured on the cover of the Gernsback magazine Short Wave Craft in November, 1933. So far only a chosen few have been privileged to taste them. Now they will be as available as such other mechanically sold products as Coca-Cola and cigarettes.

The electronic canteen, which will serve the hot dogs, hamburgers, or grilled cheese sandwiches at the drop of a dime and the push of a button, is slightly larger than the usual soft drink or cigarette machine. It plugs into the regular 117-volt outlet, has a decorative front door with appropriate mirror; push-button selectors for choice of food; a glass window in front of the electronic unit and coil so the customer can see his food getting the heat - by radio; and below a glass door and compartment into which the hot dog or sandwich drops when ready to eat.

The oscillator used to heat the canteen items is operated by two specially developed high-frequency power oscillator tubes. Over a thousand different types of oscillators were built and tried before the problem of heating rolls and meat uniformly, without burning was overcome, according to G.E. transmitter division experts who worked out the mechanical and electrical designs with Automatic Canteen Co. engineers. Some frequencies would heat the roll but not the frankfurter. Other frequencies would heat the frankfurter but burn the roll. Then when it looked as though both bun and meat would heat uniformly, one end of the bun would burn. Finally the right frequency was found and tubes and a special coil developed which would deliver the right amount of heat uniformly over the hot dog.

It should not be construed from this development that the electronic stove is just around the corner, electronic engineers hasten to explain. The canteen grill and the electronic stove present two different kinds of problems and the accomplishments in the development of the former should not be interpreted as solving the problems yet to be overcome in the field of electronic cooking.

Peace-Time Radar is already an accomplished fact, stated Raytheon Manufacturing Co., in a report last month. Approximately 75 percent of the transports returning soldiers to this country were equipped with radar at the time the report was prepared. It was expected that by the time of the report's release, the number would have been increased to 100 percent.

Radar eliminates the delays caused by bad weather or poor visibility. A pencil-sharp beam constantly searches the area all around the ship, giving a map-like presentation on the radar indicator of anything that falls within its range. Other ships, icebergs, buoys ... even driftwood, are spotted with an accurate indication of their bearing and distance. It is estimated that the return of troops has already been speeded up by the use of radar. It has, to a great extent, eliminated the necessity for reducing speed during periods of poor visibility and for waiting outside of harbors for fog to lift.

At least one disaster was averted by the efficient use of radar during and following storms in British waters this winter. No troop-transport accident has ever occurred on ships equipped with SO-1 or SO-8 radar.

Radio Signals from the moon were reported January 10 by a group of Signal Corps scientists working under Lieutenant-Colonel John H. DeWitt. The radio signals received did not originate on the moon, but were return pulses of radar transmissions beamed at that body and picked up on the return trip by special transmitting and receiving apparatus designed by the Signal Corps.

The transmitter was very much like a standard war-time radar outfit. Although many components of a regular "SCR-271" set were used, and operated at the standard frequency of 111.6 megacycles, the long range of the "target" made certain deviations necessary. A much longer pulse repetition rate was used, somewhere between 3 and 5 seconds, whereas the usual pulse rate is of the order of thousands of times per second. Also the "pulse width" which in radar parlance means the length of time each separate pulse of energy exists, varied from 1/10th to 1/2 second, an enormously long interval compared to the usual radar pulse width for war purposes, measured in millionths of a second. The electronic transmit-receive (TR) switch was dispensed with for these low repetition rates, ordinary electric relays being used.

Many other problems had to be solved in this investigation. For example, the question of the relative speed of the moon to the earth as it travels across the heavens from rising to setting. Such speed varies from 750 miles an hour faster than the earth's rotation at Belmar, New Jersey, to zero when the moon is at its zenith, and then again to 750 miles an hour slower than the speed of the earth's surface at that point in New Jersey when the moon is setting.

Such variations were important because of the very special receiver designed to pick up the weak signals from the moon. It was a quadruple superheterodyne with 4 i.f.'s. The lowest and last of these operated at 200 cycles per second. Band pass of the receiver was only 60 cycles. Sensitivity was in the order of 0.01 microvolt absolute.

The highly selective receiver made the Doppler effect (lengthening or shortening of waves when transmitter and receiver are moving rapidly away from or toward each other) important. The receiver had to be tuned at times to a frequency differing as much as 200 cycles from the transmitted frequency. Calculations as to the relative speeds of the earth and moon had to be made before each attempt at contact, so that the change in frequency could be allowed for.

A double antenna was used, containing 64 dipoles instead of the usual 32. As the antenna could not be rotated vertically, it was necessary to transmit near moonrise or moonset. Peak transmitter power was three kilowatts.

Lieutenant-Colonel DeWitt, who was an amateur radioist and broadcast engineer before the war, admits having made attempts to radio the moon as an amateur experimenter in 1940. A true ham, he is still not entirely satisfied with the greatest DX ever attained by one of the fraternity, and is already looking forward to working the moon on 'phone! Army scientists, he said, hope to increase their transmitter's power so that it could be modulated by voice. We should like to be able to say 'Hello' and hear the moon say 'Hello' back... I hope the moon doesn't answer, 'Goodby'," he declared.

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