SCR-270 Radar - Building of The Radar

Building of The Radar

The Signal Corps had been experimenting with some radar concepts as early at the late 1920s, under the direction of Colonel William R. Blair, director of the Signal Corps Laboratories at Fort Monmouth, New Jersey. Although the Army focused primarily on infra-red detection systems (a popular idea at the time), in 1935 work turned to radar again when one of Blair's recent arrivals, Roger B. Colton, convinced him to send another engineer to investigate the US Navy's CXAM radar project. William D. Hershberger went to see what they had, and returned a positive report. Gaining the support of James B. Allison, the Chief Signal Officer, they managed to gather a small amount of funding and diverted some from other projects. A research team was organized under the direction of civilian engineer Paul E. Watson.

By December 1936 Watson's group had a working prototype, which they continued to improve. By May 1937 they were able to demonstrate the set, detecting a bomber at night. This demonstration turned out to be particularly convincing by mistake; the Martin B-10 bomber had originally been instructed to fly to a known point for the radar to find it, but could not be located at the agreed upon time. The radar operators then searched for the bomber and located it about ten miles (16 km) from its intended position. It was later learned that winds had blown the bomber off course, so what was to be a simple demonstration turned into an example of real-world radar location and tracking. Development of this system continued as the SCR-268, which eventually evolved into an excellent short-to-medium range gun laying system.

In April 1937 a LtC. Davis, an officer in an Army Air Corps Pursuit Squadron in the Panama Canal Zone (CZ), sent a request for a "Means of Radio Detection of Aircraft" to the US Army's Chief Signal Officer (CSig.), bypassing normal channels of command. The SCR-268 was not really suited to this need, and after its demonstration in May they again received a request for a long-range unit, this time from "Hap" Arnold who wrote to them June 3, 1937.

Shortly thereafter the Signal Corps became alarmed that their radar work was being observed by German spies, and moved development to Sandy Hook at Fort Hancock, the coast artillery defense site for Lower New York Bay. After the move, work immediately started on the Air Corps request for what was to become known (in 1940) as the "Radio Set SCR-270". Parts of the SCR-268 were diverted to this new project, delaying the completion of the -268.

Key to the -270's operation was the primary water-cooled 8 kW continuous/100 kW pulsed transmitting tube. Early examples were hand-built, but a contract was let to Westinghouse in October 1938 to provide production versions under the Westinghouse designation "WL-530" and the Signal Corps type number "VT-122". A pair of these arrived in January 1939, and were incorporated into the first SCR-270 in time to be used in the Army's maneuvers that summer. Several improved components followed as the Army offered additional contracts for eventual production.

The original -270 consisted of a four-vehicle package including a K-30 operations van for the radio equipment and oscilloscope, a K-31 gasoline-fueled power-generating truck, a K-22B flatbed trailer, and a K-32 prime mover. The antenna folding mount was derived from a well-drilling derrick, and was mounted on the trailer for movement. When opened it was 55 feet (17 m) tall, mounted on an 8-foot (2.4 m) wide base containing motors for rotating the antenna. The antenna itself consisted of a series of 36 half wave dipoles backed with reflectors, arranged in three bays, each bay with twelve dipoles arranged in a three-high four-wide stack. (Later production versions of the SCR-270 used 32 dipoles and reflectors, either eight wide by four high (fixed) or four wide by eight high (mobile)).

In use, the antenna was swung by command from the operations van, the angle being read by reading numbers painted on the antenna mount. The radar operated at 106 MHz, using a pulse width from 10 to 25 microseconds, and a pulse repetition frequency of 621 Hz. With a wavelength of about nine feet, the SRC-270 was comparable to the contemporary Chain Home system being developed in England, but not to the more advanced microwave systems in Germany. This frequency did turn out to be useful, as it is roughly the size of an airplane's propeller, and provided strong returns from them depending on the angle. Generally it had an operational range of about 150 miles (240 km), and consistently picked up aircraft at that range. A nine-man field operating crew consisted of a shift chief, two oscilloscope operators, two plotters, two technicians, and two electricians.

The declassified US military document "U.S. Radar -- Operational Characteristics of Available Equipment Classified by Tactical Application" gives performance statistics for the SCR-270-D, namely "maximum range on a single bomber flying at indicated heights, when set is on a flat sea level site":