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Radar Reaches the Moon
April 1946 Radio News Article

April 1946 Radio News


"These pulses speed toward the moon at the fantastic speed of light… through the ionosphere and on into the unknown void surrounding the earth's atmosphere [emphasis added]." Hard as it might be to imagine nowadays, in 1946 there was no empirical data regarding the Earth's upper atmosphere other than the few instrumented sounding rockets that had been launched for studies. Orbiting man-made communications satellites were still a decade away when engineers at the Evans Signal Corps Engineering Laboratory in New Jersey made the first Earth-Moon-Earth (EME, aka 'moon bounce') signal bounce using a massive radar and antenna that blasted 10 MW EIRP pulse at the lunar surface. It was a big deal then; it's no big deal today. Amateur radio hobbyists routinely conduct EME communications from the comfort of their home-based Ham shacks, using equipment vastly superior to and less expensive than the 1946 setup. This article is yet another example of reminding us how far we've come, and who the pioneers were who got us here.

Radar Reaches the Moon

By Tom Gootée

A new era of scientific exploration begins with development of the first lunar radar.

Radar Reaches the Moon, April 1946 Radio News - RF Cafe

Special radar antenna array of 64 dipoles used for the transmission of pulses and the reception of radar echoes from the moon.

Pulses of r.f. energy shoot heavenward from the massive radar set-up and out into the darkness of unknown space. It might be like other nights during the five long years of war - when other radar sets swept other skies in search of enemy planes. But this is different.

This is lunar radar.

The radar antenna - one of the largest ever constructed-points toward no military target. Its dipoles concentrate the r.f. pulses toward a great ball of whiteness, the moon, just rising above the New Jersey horizon.

In a tiny shack near the base of the antenna tower, components of the radar set generate the sharp pulses of r.f. energy. The pace is slow, compared to military radar sets. The transmitter functions only once every five seconds. Like the heavy, labored pulsing of a giant heartbeat.

Then, concentrated into a narrow beam by the antenna array, these pulses speed toward the moon at the fantastic speed of light - more than 186,000 miles per second - through the ionosphere and on into the unknown void surrounding the earth's atmosphere. The pulses probe where man has never been before, where man has never even dared explore before - with radio waves.

But the men who guide these pulses across distant space have never left their prosaic, tiny shack near Belmar, New Jersey. They wait quietly for results of their interplanetary effort, they wait for echoes of the radar pulses to return to earth.

Seconds seem like eternities, as the base line crawls across the face of a single 9-inch oscilloscope. Even the scope is geared to cosmic thinking: its calibrated scale is not in miles, but in hundreds of thousands of miles!

Suddenly, through the "grass" of noise a wide pip appears along the base line of the scope. And simultaneously, a 180-cycle tone is heard from a speaker on the receiver console. Both last for almost half a second, then fade out. The time base finishes its sweep, there is a microseconds pause, and the entire procedure is repeated.

Output stage of the radar transmitter - RF Cafe

Output stage of the radar transmitter. Two W1-530 tubes (inside large copper shields) supply 50 kilowatts of pulsed power to the antenna. Blowers and filter equipment are in the lower compartment.

An echo pip lasts for almost half a second, and for every radar pulse transmitted: a received pip - appearing at the same place along the base line, indicating a reflecting surface about 238,000 miles distant! An almost stationary image appears on the base line that represents - the moon!

Radar echoes from the moon!

No scientific dream this, no wild tale of phantasy.

Almost every night and day for the past few months, radar engineers and scientists at the Evans Signal Corps Engineering Laboratory in New Jersey have repeated this astounding feat. And the results have been proven beyond a doubt, by leading scientists.

Radar echoes from the moon!

This is the outstanding scientific achievement since the revelation of the atomic bomb. Radar, itself, was a miracle of science - bent to the defensive and offensive requirements of modern warfare: to detect and locate air and surface vessels.

But this extension of the use of radar - to measure vast distances that heretofore could only be computed in theory - becomes a singular and major step forward in the field of science.

Planned Strategy

Contact with the moon was no mere accident.

Within a few hours after V-J Day, work was begun on the equipment - under the personal direction of Lt. Col. John H. DeWitt and his four chief associates. Some degree of secrecy was deemed necessary - at least until results were obtained, and proven certain and definite.

The new project was referred to only as the "Diana Project." And the men went to work designing, building, rebuilding, and adapting suitable radar equipment to do the job.

All preparations were completed for a test on January 10th.

On that day the moon rose at 11 :48 a.m. At about that time the first radar pulses were transmitted, and the first echoes appeared on the oscilloscope - indicating success.

Accurate timing of each pulse and its reflected echo indicated that it took 2.5 seconds for the echo to return.

Since radio waves travel at a fixed rate of speed - about 186,000 miles per second - it wasn't difficult to compute the distance from the radar set to the reflecting surface: about 238,000 miles. Col. DeWitt and his radar engineers were convinced they had contacted the moon, because there was nothing else in space at that distance from the earth.

But additional tests were made on following days and nights - each time the moon rose and set.

Said Col. DeWitt, "We knew our months of thinking, planning, calculations, and design were on the right track, but to make doubly positive and sure, as our Army Laboratories must be, we aimed our radar beam at the rising and setting satellite time and time again, so that we knew without question of a doubt that our pulses were striking the moon and echoes were rebounding back to earth."

Block diagram of the radar  - RF Cafe

Block diagram of the radar set that was used in the original detection of the moon echoes.

Finally, a group of distinguished but unidentified scientists visited Belmar and verified all of the findings and conclusions of Col. DeWitt and his group.

Only then, weeks after the initial contact with the moon, did the War Department announce details of the Diana Project. The first earth-to-moon, interplanetary circuit had been definitely established. All records of long-distance radio transmission had been broken.

And repercussions from the announcement were heard around the world - speculating on both the war-time and peacetime applications of the new long-range radar equipment.

For there are many possible, future applications of such radar equipment - some almost beyond the immediate comprehension of mankind.

A new and far more accurate study of the solar system will be entirely feasible, as soon as adequate and more powerful equipment can be built.

One war possibility is radar-beam control of long-range rockets and jet-propelled missiles. Man has gained control of outer space.

But the primary significance of the Signal Corps' achievement is that this is the first time scientists have known with certainty that a very high frequency radio wave sent out from the earth can actually penetrate the electrically charged ionosphere which encircles the earth and stratosphere.

And this proves to be a curious parallel with history.

Link with the Past

Herbert Kauffman, radio engineer who worked on the "Diana" project - RF Cafe

Herbert Kauffman, radio engineer who worked on the "Diana" project, adjusts one of the many stages of frequency multiplication.

More than two decades before contact with the moon was recorded in New Jersey, radio waves were first used for a very similar purpose: to determine the distance to the reflecting surface of the ionosphere.

In 1924, a particular portion of the upper atmosphere, called the Heaviside layer, was believed responsible for the transmission (by reflection) of low-frequency radio signals around the earth. In that year, experiments were begun in England by Dr. Edward Appleton and M. A. F. Barnett. Using frequency-modulated transmissions of a large broadcasting station, they were able to prove that the received signal varied in intensity with frequency - because it consisted of a direct wave and a reflected component. And a measure of signal intensity caused by a known change in wavelength resulted in a measure of the height of the reflecting layer.

In the same year in the United States, Dr. Gregory Breit and Dr. Merle A. Tuve, of the Carnegie Institute of Washington, used pulses of continuous waves for measuring the distance to the reflecting surface of the ionosphere. Their technique consisted of sending skyward a train of very short pulses - a small fraction of a second in length - and measuring the time it took the reflected pulse to return to earth. Fairly low-frequency radio waves were employed. And, after completion of the experiments, pulse ranging soon became the accepted method of ionospheric investigation.

The advent of short-wave radio transmission and the success of these early experiments led scientists in many countries to speculate on the possibility of using such energy to detect the presence of man-made reflectors; such as ships and airplanes. When powerful sources of high-frequency energy, highly sensitive receivers, and refinements in radio technique became available, these possibilities of detection were converted into working devices. Then, with the coming of war, pulse ranging-or radar - was developed under great impetus. And the story of radar's part in winning the war is now well known.

But even before the war, there were a few radio engineers and scientists who saw in the pulse ranging method a means for measuring phenomenal distances: to the moon, other planets, even, perhaps some day, the sun.

One of these men was John H. DeWitt - then chief engineer of station WSM in Nashville. He was also a "ham", and an amateur astronomer. Using his then meager knowledge of pulse ranging, in 1940 he built his own equipment and attempted to contact the moon. His efforts were wholly unsuccessful, but he was undaunted. He looked forward to the day when he might experiment on a really grandiose scale. A year later the country was plunged into war, and DeWitt en­tered the Armed Forces.

It was not until after the defeat of Japan that his thoughts returned to contacting the moon. Then a Lieutenant Colonel in the Army Signal Corps, he had participated directly in much of the radar development activity of the Army - particularly as Director of the Evans Signal Laboratory near Belmar, New Jersey, with its wartime personnel of over 6000.

Signal Corps scientists responsible for contacting the moon by radar - RF Cafe

The five Signal Corps scientists responsible for contacting the moon by radar. (Left to right) Jacob Mofenson. Dr. Harold Webb. Lt. Col. J. H. DeWitt. E. King Stodola. and Herbert Kauffman.

Seizing the opportunity, Col. DeWitt immediately started work on equipment suitable for measuring great distances. Four key civilian radar engineers joined his group: E. K. Stodola. Dr. Harold Webb, Herbert Kauffman, and Jacob Mofenson. They had all worked at Evans during the war developing military radar equipment.

First problem facing the group of men was a new philosophy of thought.

They no longer could think of radar ranges in terms of a few hundred miles. The distance to the moon is about 238,567 miles. But this figure varies from day to day, as the moon revolves and moves in an elliptical orbit around the earth. And both the earth and the moon move around the sun.

A staff of mathematicians and physicists spent weeks computing the trend in relationship between the earth and moon, before assembly of the equipment began. It was necessary to determine accurately the speed of the moon relative to the movement of the earth. And this speed varies - with respect to the earth's rotation - from 750 miles faster to 750 miles slower, at Belmar, New Jersey.

Variations in speed and positions of the earth and its satellite must be taken into consideration each time the moon is contacted. Because the net effect of two variables of movement causes a Doppler effect - a shift in the frequency of radio waves. Often this shift is greater than the receiver bandwidth. Thus the relative speeds of earth and moon must be calculated each day with the radar receiver tuned and adjusted to take advantage of the Doppler effect.

Only in this way can a positive check be made on the direct range measurements of the oscilloscope. These calculations are the most reliable verification that the moon is actually being contacted.

General Characteristics

Equipment used on the Diana Project comprised extensive adaptations to the standard wartime long-range radar known as the type SCR-271 - originally designed in 1937 and used widely during the war.

Principal components are; transmitter, receiver, antenna system, and indicator. The timer or keyer is part of the indicator unit.

The radar transmitter sends out bursts of radio energy, known as pulses. During intervals between pulses the transmitter is turned off, but the radar receiver functions - and picks up any echo reflections which may be received from distant objects or surfaces. These echoes are amplified and then displayed on the time base of a calibrated oscilloscope. The elapsed time between the transmission of a pulse and the reception of its echo is a measure of the distance from the radar set to the reflecting surface - because radio energy travels through space at the speed of light: about 186,000 miles per second.

radar signals reflected by moon - RF Cafe

Photographic record of reception of radar signals reflected by moon on night of Jan. 22, 1946. Heavy pulse at 0 represents initial transmission of energy toward the moon. Jagged lines indicate general noise reception. Distinct echo at about 238,000 miles represents reception of radar echo 2.4 seconds later, after echo had actually traveled a round trip distance of over 477,000 miles between earth and moon. Actual mean distance from earth to moon is 238,857 miles.

Because of the distance and nature of the target, this radar set had to have a number of special features.

A very slow pulse rate was necessary - since the radio signal must travel a round trip distance of more than 477,714 miles. Time must be allowed for an echo to be received, before another pulse is transmitted to the moon.

And each radar pulse must be of appreciable duration-from 1/4 to 1/2 second - to insure a strong signal at the receiver after reflection by the moon.

A three-kilowatt radar transmitter was available for the experiments, and this was modified to supply an output of fifty kilowatts. Through the use of a high-gain antenna, effective radiation was raised to about 10 megawatts, or 10 million watts.

Strength of the received echo reflection has been estimated to be only a few tenths of a watt. Thus the most difficult step in contacting the moon was not so much in the transmission, but in the design and construction of an extremely sensitive receiver.

This receiver-using 34 tubes, and four different intermediate frequencies - has a sensitivity of about 0.01 microvolts.

A good idea of the overall equivalent sensitivity is that the radar set could pick up an airplane at a distance of more than 1900 miles - assuming, of course, that the target was within the set's line of sight.

The complete radar set incorporates a number of new design techniques, thus a detailed analysis of the Diana Project is worthy of further study.

Details of Components

The radar transmitter consists of a series of frequency multiplying stages which raise the frequency of a 516.20 kc. crystal to 111.6 megacycles (the carrier frequency). A pair of type WL-530 tubes are used in the output. These are driven by a pair of type 450-TH tubes (triplers) which, in turn, are driven by a pair of type 257-B tubes (doublers) which, in turn, are driven by an 807 which, in turn, is driven by tubes in the radar receiver.

The same crystal controls both the transmitter frequency and the heterodyne voltage for the receiver.

A pulse of variable width is supplied the transmitter by the electronic keyer or timer-a physical part of the indicator unit. A pulse duration of from 1/4 to 1/2 second can be used.

Servicing and aligning intricate circuits of the sensitive radar receiver - RF Cafe

Servicing and aligning intricate circuits of the sensitive radar receiver. Col. DeWitt discusses procedure with Herbert Kauffman and Dr. Harold Webb.

Pulse recurrence frequency is also variable. The electronic keyer or timer can supply a pulse once every three to five seconds. This is equivalent to p.r.f. of 1/3 to 1/5 cycles per sec.

The peak output of the radar transmitter during pulses is fifty kilowatts.

The transmitter feeds through a mechanical, low-loss T/R switch to the antenna system .The T/R switch consists of specially constructed relays to obtain positive low-loss action on the long and relatively low peak power pulse used.

The antenna consists of a broadside array of 64 half-wave dipoles. The array is movable and mounted 100 feet above ground.

The antenna system has a forward power gain of about 200. It has a beam width of 15 degrees at half power points - in both the vertical and horizontal planes.

Received echoes are applied to the radar receiver - the real secret of the set's ability to pick up reflections, from such distant targets. The receiver is a 4-mixer superheterodyne, with all but one of the mixer injection frequencies directly controlled by the transmitter crystal - to provide locking with the transmitter frequency. Fourth mixer is provided with an adjustable frequency crystal; this sets exactly the final intermediate frequency and depends upon the actual radio frequency being received. The received frequency of the radar signal differs from the transmitted frequency by an amount depending upon the Doppler effect which, in turn, is caused by the moon's relative velocity.

Operating noise factor of the receiver is about 8 db. The receiver bandwidth is about 50 cycles.

A loudspeaker is coupled to the out­put of the last i.f. amplifier to provide audible indications of echoes.

A long-persistence oscilloscope is used to display the echo output from the detector. The scope uses a type "A" time base with a three to five second sweep, depending upon the desired pulse recurrence frequency. Direct coupling is used for both sweep and deflection circuits.

New Equipment

German radar successfully jammed by the Allies - RF Cafe

One of the German radars successfully jammed by the Allies. Used for ground control of fighters and. later on in the war, for direction of anti-aircraft fire.

Work has already begun on the design of new and more compact permanent equipment to replace the composite gear used on the first experiments.

The multi-dipole antenna array will probably be replaced by a parabolic reflector - forty or fifty feet in diameter - capable of movement in three dimensions. The base will be comparable in design to bases used for large telescopes at astronomical observatories.

It is also fairly certain that the operating frequency will be considerably increased, when the present radar transmitter is replaced with a more powerful one - possibly using magnetrons.

Other improvements will be incorporated in other components in an attempt to increase both the output power and the sensitivity of the lunar radar.

The Signal Corps intends to continue experiments in this fascinating new realm of exploration and discovery. The War Department has already embarked upon a long-range research program to develop more reliable and informative techniques for radar study of the moon and the ionosphere.

Study of the effect of radio waves in traveling through the atmosphere is of utmost importance. This includes bending and refraction of radio waves, and more complete data concerning the Doppler effect on radio signals passing beyond the earth's atmosphere.

Another valuable application of lunar radar will be the provision of new meteorological and astronomical information. Cosmic dust in space can be detected and located. And not only may it be possible to construct topographical maps of distant planets with the aid of radar data, but scientists may be able to determine the composition and atmospheric characteristics of other celestial bodies by means of long-range radar.





Posted March 10, 2015

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