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Title (Dublin Core)
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U.S. Army employ FM Radio
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Article Title and/or Image Caption (Dublin Core)
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Title: FM Radio Joins the Army
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extracted text (Extract Text)
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JUST in time for America's supreme war
effort comes a new system which jumps
many of the hurdles in radio communication
for military and emergency defense work.
It is called FM, in commercial broadcast-
ing already famous as a static eliminator.
A few years ago, communication with a
mechanized army in the field had to be ei-
ther two-way communication by the old
telegraphic code signals requiring trained
operators, or one-way communication by
voice. Radiotelephony broadcast to mobile
units such as command cars, trucks, recon-
naissance patrols, tanks, etc. required a
powerful transmitter to overcome noise and
interference. Such a transmitter was too
large, heavy, and cumbersome to be in-
stalled in a mobile unit to provide talking
back. For tanks which, in themselves, set
up violent disturbances, even one-way voice
communication was unreliable.
Today, the operator of a tank, speaking
in an ordinary tone, can talk quietly with
other tanks or with command cars. He can
receive orders from command posts or send
back news of his position or of the action in
which he is engaged. Only the voice is used.
The tank operator's voice is not communi-
cated through an ordinary microphone but
through two disks pressed against his
throat. He hears through headphones built
into his helmet. Thus his hands are left en-
tirely free. Tuning is done automatically by
the use of push buttons and requires no skill.
Ordinary communication from a tank is
short-range—about one mile. Power can,
however, be increased by the operator if
necessary either to increase the range or to
blank out another signal—a new FM tech-
hique which will be described
later.
A tank platoon has been put through rap-
id and complex operations entirely by radio
with no visual signals. This simple two-way
system has made possible a novel method of
reconnaissance called the Combat Zone
Warning Service, especially useful in anti-
tank maneuvers. CZWS has, in effect, mech-
anized the old scouts which used to move out
on all sides of an army unit to give warning
of the enemy's approach or other doings.
At headquarters is a 250-watt FM trans.
‘miter mounted on a truck. From it orders
are sent out through relay stations to small
reconnaissance units. These have receivers
to pick up messages, and transmitters by
which they can talk back. Reconnaissance
units are small vehicles such as “jeeps”
which may be concealed in woods, bushes,
or other cover. Any of them which detects
enemy activity can send a message to the
relay station which may either relay it back
to headquarters or directly warn a combat
unit.
"All this has become possible through FM
—or rather through the particular frequen-
y-modulation system invented and perfect-
ed by Major Edwin H. Armstrong. As every-
one who has followed radio development.
knows, this system was invented primarily
to do away with static and man-made dis-
turbances which produce noise in the re.
ceiver. In the words of Major Armstrong,
this Is done by transmitting “a wave which
is different in character from the static”
and by using a “receiver which responds
only to this new type of wave and is deaf to
the ‘amplitude modulations’ of the noise
currents and substantially deaf also to the
small frequency swings occurring in the
noise by using a wide frequency band.” Ac-
tually, this “different” wave is a wave of
constant amplitude whose frequencv 1s
widely varied by the sound pattern im; >sed
upon It—an exact reversal of previous ac-
cepted practice. FM is further insured
against noise by operating in the ultra-high
frequencies. The informed general public
has come, in the last few years, to under-
stand this.
But what the public is only beginning to
learn is the immense advantage gained
from other incidental ef-
fects of FM which adapt it
s0 beautifully to military
and police use. Apart from
noise reduction, these are,
briefly: low-power opera-
tion, low-cost operation,
short-range transmission,
easy adjustment of range,
compactness of units, fa-
cility of operation by un-
skilled operators, complete
absence of distortion or
“squeal” caused by the in-
terference between signals.
and the opening up of a multitude of new
communication channels. Most of these ef-
fects are, of course, derived from the basic
properties of FM which make possible the
elimination of static and noise.
With the amplitude-modulation system,
called AM, the high power necessary to
overcome both noise and interference in-
creases operation cost and size of ap-
paratus. Furthermore, interfering signals
on the same frequency are difficult to drown
out.
FM does not drown out an interfering sig-
nal, it “blanks” it out. Only one signal can
come through at a time in an FM receiver.
The familiar jumble of sounds coming from
two stations using the same frequency—
which everyone has heard in his AM set—
is impossible with FM. The more powerful
signal always come through, clear and un-
distorted, even if its amplitude is only a
little greater. So the operator of an FM
transmitter can blank out other signals at
will by only slightly increasing his power.
Actually a ratio of 2 to 1 will blank out a
weaker signal on the same frequency
whereas a ratio of 20 to 1 is necessary in
AM to overcome such interference.
At the same time, this blanking out can-
not be done successfully at great distances,
for FM is essentially a short-range system.
Commercial broadcasting over distances
greater than 50 miles requires high antennas
on high ground. Therefore the enemy, even
if he possesses FM field equipment, cannot
blank out or “jam” signals unless he is very
near.
The Army learned of FM from the police.
About a year ago, a number of Signal Corps
officers and engineers from Fort Monmouth
were invited to Hartford, Conn, to see a
demonstration by the pioneer police users of
the Armstrong system—the Connecticut
State Police. The story of what they
saw will explain, better than any theo-
retical description, why the Army has
gone in for FM.
Connecticut, an area of 4,965 square
miles, is divided into ten barracks areas.
The barracks are not in the centers of
the areas they represent, and there are
no high antenna poles rising from them.
Yet in each barrack, a dispatcher sits
at a desk talking by radio with police
cars and with other barracks.
The dispatcher is handling the trans-
mitter by remote two-wire control, be-
cause a barrack and a transmitter re-
quire two different sets of conditions.
A barrack must be on a main highway,
easily accessible to motorists and handy
for sending out cars in a hurry. An
FM transmitter using ultra-high fre-
quencies should be on the highest
ground available and should be in the
center of the area to which it transmits.
So the 200-foot steel masts carrying the
antennas are put on hilltops, often in
the middle of woods or fields. In a
welded-steel shack below each mast is
housed the transmitter and receiving
equipment. From this shack—called
the “station” —wires run to the area's
barrack.
‘The dispatcher, sitting at his desk at
the barracks, is able to hear both the other
stations and the cars in his own area. He
may, if he wishes, ground the station-to-
station receiver by remote control so that
he will hear only a car which is calling him.
Police cars can also talk directly with other
cars.
Because of the factors of distance and
power, the local station is able to control
its local cars and blank out all other signals.
One of the Army visitors at the police
demonstration asked if the remote-control
wires were not vulnerable to saboteurs. The
answer to this question by Sydney E. War-
ner, Radio Supervisor of the Connecticut
State Police, reveals another advantage of
the FM system as it is laid out for police
work:
“That is a serious objection where an en-
tire system is dependent upon a single
transmitter. However, in this particular
system, there is ample overlap in the range
of each transmitter so that if, for any rea-
son, one fails, its traffic can be handled
through other barracks.”
Tests have shown that, in emergency,
even a car can be sent to high ground and
used us a dispatching point. “Thus,” says
Mr. Warner, “we also know that even if all
our ten fixed stations were put out of serv-
ice, strategic placing of cars around the
state would still enable the State Police
radio system to function.”
The Signal Corps men saw many tests, in-
cluding some in cities where ignition dis-
turbance was very high. They were able
to listen to both FM and AM receivers in
these traffic-congested areas. It was proved
conclusively that in nine or ten New York
City blocks (about half a mile) AM signals
were eliminated by noise, whereas FM sig-
nals were perfectly clear over a five-mile
distance.
tis easy to see why, after these demon- |
strations, the Army adapted the Armstrong
FM system to its needs. Quickly they vis-
ualized the small apparatus made for the
police, which could be stowed away in the
baggage compartment of a coupe, being
efficiently packed in a command car or a
small truck, or built into a tank. They im-
agined the quick automatic tuning being
handled, if necessary, by a “rookie.” Espe-
clally, they saw how signals could be kept
from the enemy by limiting the range. They
saw that signals could never be confused.
They saw, in short, speed, clarity, foolproof |
operation, and invulnerability.
Another wartime application of FM radio |
communication has come in the extremely
ticklish handling of freight in the railroad
yards of Government arsenals where the
cars carry immense loads of explosives and
where even a slight accident might have
terrific repercussions. To see how FM has
come to fit here, we should take a brief
look at the background of ordinary freight-
yard management.
Most Americans would be shocked to see
how old-fashioned and primitive this is. Not
even common block signaling Is used in
most railroad freight yards. Old visual sig-
nals such as the waving of flags and swing-
ing of lanterns are still in use when they
are not stopped by fog. A switching engi-
neer, after he has fulfilled one order, must
return to the dispatcher for the next. Ac-'
cidents in freight yards occur oftener than
railroad men like to tell. |
When the imzese new Government ar-
senals like Kingsbury and Elwood were
projected, the difficulties of a quick, efficient,
and safe handling of ordnance freight by
common methods was quickly foreseen. At
least, thought the traffic engineers, a block-
signal system should be installed. But as
this was investigated it turned out to be
not only extremely costly but its installation
would require much valuable time. So the
engineers turned to radio for a solution of
their problems.
Common AM communication had been
tried with long-run passenger traffic. When
ordinary steam locomotives were used it
worked weis enough. But it did not take
much imagination to see that any AM re-
ceiver installed in a Diesel-electric locomo-
tive such as were planned for the arsenal
yards would be stopped by noise. It was
then that the FM system came to the
notice of the men who were planning the
arsenal-yard communications. Today half
a dozen Government ordnance plants are
being equipped with it.
An example is the Elwood arsenal at
Joliet, Til, whose yard occupies 22 square
miles. There are 80 miles of track, 212
switches and 300 of the arsenal’s own
freight cars. Here nine locomotives will
handle trainloads of TNT, artillery shells,
aircraft bombs, and antitank mines 24
hours a day.
A 50-watt FM transmitter is controlled
(remotely) by the train dispatcher. A simi-
lar station is controlled by the yardmaster.
From these stations orders are sent out to
the locomotives. Each locomotive has a
transmitter (25 watts) by which he can
talk back to the stations, as well as a re-
ceiver.
With this Kind of communication, any
locomotive may be instantly located and
controlled. At Elwood there are also gaso-
line-operated maintenance cars and guard
cars. These may be summoned in case of
breakdown or sabotage, to any part of the
yard.
As the FM radio installation for large
freight yards costs less than one tenth of
the block-signal layout, it has been suggest-
ed that it should be adopted in commercial
yards throughout the country. Communi-
cations engineers to whom I have talked
estimate a 300 to 400-percent speed-up in
the handling of freight. With such an in-
crease in efficiency applied to transportation
of war material, the impending shortage of
freight cars due to steel restrictions could
easily be met.
It is always heartening to hear of “new
weapons.” FM has no direct destructive
power. But as a means of multiplying the
speed of armies and civilian defense, it is a
new aid to all weapons.
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Contributor (Dublin Core)
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Roger Burlingame (Article Writer)
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Language (Dublin Core)
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eng
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Date Issued (Dublin Core)
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1942-04
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pages (Bibliographic Ontology)
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82-86
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Rights (Dublin Core)
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Public Domain (Google Digitized)
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Archived by (Dublin Core)
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Roberto Meneghetti
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Marco Bortolami (editor)
Popular Science Monthly, v. 140, n. 4, 1942
