"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
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.
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 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."
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
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.
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
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
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 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
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
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
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
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