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SCR-299 mobile communications unit
by Ted Wise
The Signal Corps' SCR-299 is the best example of Yankee ingenuity before U.S. entry into World War II. The 299 was the civilian and military communications experts' effort to give a long-range communication advantage to the U.S. Army and its allies.
Military observers were unanimous that the success of modern, fast-moving "blitz" tactics was directly linked to how efficient armies' communications systems were. Up to 1940, communications by messenger, signal flags, telephone and other signal devices had been adequate to meet the challenge of the slower-moving military machinery.
However, Germany's initial successes in a highly mechanized form of warfare were attributed to their efficient and highly reliable communications system the lightning-like movements of their panzer divisions could be coordinated and timed for split-second action. These tactics produced an ever-changing battle line ranging over distances of many hundreds of miles.
Communications officers could see that if the United States became involved in the war, our army also would have to be equipped with modern communications to coordinate our combined ground, air, sea and armored forces none could function efficiently without the others' aid.
To meet these demands, a high-powered radio transmitter was required capable of infallible voice communications over 100 miles; self-powered; sturdy enough to work in all conditions (cold North, hot South, jungle humidity or dry desert heat); flexible enough to cover a wide range of frequencies; and able to operate in motion or at fixed locations. It had to be entirely independent in its mobility-containing repair and replacement parts to ensure its continued operation on detached missions.
The problem to find, procure, test and field such a unit under all conditions and produce it in time to give our army what it needed was the concern of Col. Roger Colton, chief of engineering and technical services at Fort Monmouth, N.J.
"Olmstead's Baby," as the project was called (after the Chief Signal Officer at the time), was to find commercial and military parts already produced and make them work together. Colton's program gathered commercially built transmitters with the necessary requirements for Signal Corps adaptation. Out of the various sets sent from U.S. vendors, and after considerable experimentation, Hallicrafters' Standard HT-4 transmitter was chosen as the desired radio's basis.
Production began in early spring 1942. The HT-4 designed for amateur use and commercially available for several years was compact and stable. It could deliver 325 watts of power on voice and 450 watts on code. It was crystal-controlled but provided optimal use of the master-oscillator power amplifier and was able to work over a wide range of frequencies.
The Signal Corps' requested changes were achieved by Hallicrafters' engineers working with Army technicians at Fort Monmouth. Adapting the transmitter to military use required incorporating minor changes, augmenting the basic unit with more electronic devices permitting the transmitter to handle a wider range of frequencies and standardizing control equipment.
Toughening the equipment
The engineers and techs strengthened the steel cabinets and shock-proofed individual units so they could withstand the terrific pounding they'd encounter in military vehicles. Cables, connectors and plugs were designed specially to permit the extreme flexibility of operations in motion.
Also, some component parts had to be treated specially to prevent the corrosion likely in some climates.
Other minor changes included adding several relays to permit automatic changeover of military- operations circuits. The engineers designed an overload relay system that proved to be completely chatter- proof. An ordinary relay would kick out when traveling over rough terrain; reset controls were placed on the unit's front panel and made readily accessible to operators.
A master oscillator was also designed which provided continuous coverage without a maze of crystals ground to the many frequencies required for military communications. A switch on the tuning unit controlled the changeover from crystal to master oscillator.
The antenna problem was met after much experimentation by designing a vertical whip some 35 feet long. To match the antenna to the transmitter's wide frequency ranges, a special antenna coupler was designed to allow accurate loading at any frequency the transmitter covered. Commercial engineers devised a continuously variable network which, when connected to the vertical, provided the necessary matching required.
The SCR-299's variety of replacement parts were packed uniformly and standardized for easy location.
After mechanical considerations were met and a layout adopted for the various units, the Signal Corps techs turned their attention to designing a suitable truck. A 1 1/2-ton capacity truck would be required, with a dual operation system for the two operators on duty at all times. Either of the operators would have complete control of receiving and transmitting equipment.
Also, another unit (trailer) was required to transport the gasoline generator that powered the transmitter, receivers and communications-truck lighting. The two units had to be separated so mechanical vibration as well as the generator's noise wouldn't interfere with communications.
Field telephones with plenty of cable were carried in each unit. They were used either as interphones or to control and modulate the transmitter from a remote point.
The HT-4 transmitter's new version became known as the BC-610 transmitter. The receivers finally supplied were the BC-312 and BC-342, plus the BC-614 (speech amplifier), BC-729 (tuning unit) and BC-211 (frequency meter), along with the PE-95 (power unit). All these became part of the truck-and- trailer unit called the SCR-299 later better known as the "mobile communications unit."
The SCR-299 was part of the first equipment to land on the African shores and did yeoman duty during those hectic days when the fate of the United Nations' African campaign was in the balance. For long periods it was the only means of communication linking Oran and England, Oran and Casablanca, Gibraltar, Algiers and Accra.
By this time our allies had heard of this famous equipment that gave such phenomenal results and, through Lend-Lease, gained many complete units. British generals Montgomery and Alexander used the SCR-299 to coordinate their successful efforts against the Germans in North Africa.
Gen. Dwight Eisenhower credited the SCR-299 in his successful reorganization of the American forces and final defeat of the Nazis at Kasserine Pass. In the invasion of Sicily and later Italy, the SCR-299 was used with telling results.
Though the original Signal Corps requirements were for communication points up to 100 miles, under favorable conditions these transmitters made and maintained contact over 2,300 miles of land and sea.
Without adequate communications, the Army's numerous divisions couldn't have been used to their fullest. World War II proved communications and split-second timing were crucial in overpowering Germany's panzer units. The SCR-299 provided the necessary answer to the Blitz.
Mr. Wise, curator of the Signal Corps' museum at Fort Gordon, Ga., has a bachelor's degree in graphic art/art history from Cameron University and a graduate degree in museum studies from University of South Carolina.
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