"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
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. 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.
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
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 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, 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 en-tered 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.
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.
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.
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
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. 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 out-put 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.
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
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
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
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
Posted December 2, 2020
(updated from original post on 3/10/2015)