page_title: SCR-270
section_title: Technical description
hierarchical_section_title: SCR-270 / Technical description
caption_attribution_description: English: Original Caption: Parts of an early SCR-270 installed in a K-30 truck. This is the same equipment used by an SCR-270 unit located at Opana point that detected the Pearl Harbor attacking planes 55 minutes prior to the attack. alteration- this version has a border cropped which credits the photo to Lieutenant Zahl. Original at the above location. This photo is credited to First Lieutenant Harold Zahl, who pioneered Radar use by the Army air corps. The photo appears in a paper by him from the period. these photos would not have been saved were it not for Dr. Howard W. Andrews, who had the foresight to preserve them along with Lieutenant (Dr.) Zahl's papers for posterity. Credit and thanks to H.W. Andrews for preseving these photos.
context_page_description: The SCR-270 was one of the first operational early-warning radars. It was the U.S. Army's primary long-distance radar throughout World War II and was deployed around the world. Its also known as the Pearl Harbor Radar, since it was an SCR-270 set that detected the incoming raid about 45 minutes before the December 7, 1941, attack on Pearl Harbor commenced.
  Two versions were produced, the mobile SCR-270, and the fixed SCR-271 which used the same electronics but used an antenna with somewhat greater resolution. An upgraded version, the SCR-289, was also produced, but saw little use. The -270 versions were eventually replaced by newer microwave units based on cavity magnetron that was introduced to the US during the Tizard Mission. The only early warning system of the sort to see action in World War II was the AN/CPS-1, which was available in mid-1944, in time for D-day.
context_section_description: Key to the SCR-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 (rotated) by command from the operations van, the azimuth angle being read by observing with binoculars the numbers painted on the antenna turntable. The maximum rotation rate was one revolution per minute. 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 3 meters (nine feet), the SRC-270 was comparable to the contemporary Chain Home system being developed in England, but not to the more advanced UHF Würzburg radars being developed in Germany. This wavelength 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":
caption: SCR-270 operations van components
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