The Navy’s First Radar Turns 75! | XAF Flying Bedspring

The Navy’s First Radar Turns 75! | Armed with Science

XAF Antenna on USS New York in Dec 1938

XAF Antenna Today @ NRL

 December 2013 marks the 75th anniversary of one of the Naval Research Laboratory‘s most important contributions to technology development and national defense: the Fleet’s first shipboard radar.

Weapons & Sensors -- Radar -- U.S. Navy XAF Radar
 The "XAF", an experimental radar that resulted from several years' technical progress by the Naval Research Laboratory (NRL), was constructed in 1938, following a late February decision to install a radar set on a major warship. Operating at 200 megacycles (1.5 meter wavelength) at a power of 15 kilowatts, the XAF featured a "bedspring"-like antenna about 17 feet square. This was mounted in a rotating yoke that allowed it to scan around the horizon, and to elevate for what was hoped would be improved aircraft detection. This large antenna and yoke had to be strong enough for sea service, while remaining as light as possible to avoid excessive topside weight. Accordingly, the Brewster Aeronautical Corporation (then also building the Navy's first monoplane shipboard fighter, the F2A "Buffalo"), was given the job of fabricating a suitable duralumin structure. The XAF's transmitter, receiver and other equipment were made by NRL.

The unit was installed on BB34, the USS New York, a 27,000 ton battleship built in 1914, atop the pilot house. At the conclusion of these tests, New York's Commanding Officer recommended installation of radar in all aircraft carriers (whose vulnerability to surprise air attack was very well-understood), while the Commander of the Atlantic Squadron commented "The XAF equipment is one of the most important military developments since the advent of radio ...". Later in 1939, the XAF was reengineered and placed in production by the Radio Corporation of America. Designated CXAM, six of these production models were delivered in 1940 and installed on an aircraft carrier, a battleship and four cruisers.

The CXAM was operationally quite different from the British Chain Home. Instead of using separate, fixed, broad-beam transmitting and receiving arrays, it employed a common  antenna for transmitting and receiving. To produce as narrow as beam as possible, it operated at the then ultrahigh frequency of 195 Mc/see, and employed a “mattress” array of dipoles with reflectors, giving a gain of 40 and a beam 14° wide in azimuth by about 70° in elevation. The antenna could be rotated in azimuth at a speed of 5 rpm, or manually trained to follow a particular target. The peak pulse power was 15 kw, the pulsewidth 3 µsec, and the repetition rate 1640 pps. Range against bombers was about 70 miles, against fighters about 50 miles.

The display was an A-scope in which the trace was lengthened by causing the sweep to take place from left to right across the tube, then drop down and return from right to lem Range was estimated with the help of marks on the face of the tube; bearing was determined as the direction of antenna-pointing which yielded maximum signal. The height of targets could be estimated with the help of nulls in the vertical antenna pattern.

An improved version, CXAM-1, with a simplified antenna, was produced in greater numbers. By the time the United States entered World War II in December 1941, the use of radar in the U.S. Navy was rapidly expanding. Despite its early design and its lack of adequate coverage against low flying aircraft, this simple, rugged equipment proved highly satisfactory in service use. It was the direct forerunner of the later shipborne long range air-search radar equipments (SA, SC and its various redesigns, SK and its redesigns) used on large ships until the end of the war. 


The XAF radar's antenna survived World War II as a historic artifact. For several decades in the middle and later Twentieth Century it was exhibited in Willard Park, close to the Washington Navy Yard's waterfront. However, the outdoor environment contributed to serious deterioration in the antenna's condition, and it was placed in storage in the mid-1990s. In May 2008 it was loaned to the Historical Electronics Museum, located in Linthicum, Maryland, where it is to be restored and again placed on public exhibit.

New York went on to be of great service. With the coming of war to Europe, New York participated in Neutrality Patrol operations, and, as the U.S. drew closer to the conflict in 1941, helped in the occupation of Iceland and in escorting convoys. Her convoy activities continued after the United States became a combatant in December 1941. In November 1942, New York also took part the North African invasion, providing gunfire support for landings at Safi, Morocco. She spent 1943 and most of 1944 on escort and training duties, steaming to the Pacific war zone in early 1945. In February, New York's big guns were active bombarding Iwo Jima before and during the Marines' assault on that island. She was similarly employed off Okinawa from late March until June, and was lightly damaged by a suicide plane on 14 April 1945.

Following the Japanese capitulation in August 1945, New York moved back to the Atlantic and was at New York City for the Navy Day fleet review in late October. Her last active service was as a target during the atomic bomb tests at Bikini, Marshall Islands, in July 1946. Too radioactive and far too old for further use, she decommissioned a month later. In July 1948, USS New York was towed out to sea off Pearl Harbor and sunk as a target for Navy aircraft and ships.