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| SJ radar | |
|---|---|
| Name | SJ |
| Country | United States |
| Manufacturer | General Electric |
| Introduced | 1944 |
| Type | Surface-search radar |
| Frequency | S band (approx. 3 GHz) |
| Range | up to 40+ nmi (ship- and model-dependent) |
| Azimuth | 360° (rotating antenna) |
| Elevation | fixed or limited |
SJ radar
The SJ radar was a United States World War II–era surface-search radar system deployed aboard United States Navy warships to detect surface ships and submarine periscopes at short to medium ranges. Designed for use on destroyers, cruisers, and auxiliary vessels, the system complemented air-search sets such as CXAM and SK and navigation aids like SG. It played a role in major Pacific Theatre actions including the Battle of Leyte Gulf and supported convoy operations with Allied navies including the Royal Navy and the Royal Canadian Navy.
SJ entered service during the latter half of World War II to provide improved surface detection over earlier metric-band sets. Its adoption reflected lessons from engagements such as the Battle of the Atlantic and night surface actions around Solomon Islands campaign, where radar-directed gunfire and maneuver became decisive. The radar was developed to meet requirements from the Bureau of Ships and to integrate with fire-control systems like the Mark 37 Gun Fire Control System and navigation suites aboard capital ships and escorts.
Development began as part of wartime radar expansion programs managed by the United States Navy Bureau of Ships and contractors including General Electric and Westinghouse Electric Corporation. Engineers who had worked on earlier systems such as the CXAM and SG adapted S-band magnetron technology refined by researchers at institutions like the Radiation Laboratory (MIT). The SJ design emphasized a compact antenna, ruggedized chassis for shipboard vibration, and pulse characteristics optimized to detect small targets in high sea states. The set interfaced with bridge displays and integration efforts with systems like the Mark 1 Fire Control Computer and radio direction-finding gear were trialed during acceptance trials at naval yards including Philadelphia Naval Shipyard.
SJ operated in the S band near 3 GHz, using a pulsed magnetron transmitter and a duplexer to allow a single antenna for transmit and receive. Typical pulse repetition frequency and pulse width choices balanced range resolution and maximum detection distance, yielding effective surface-search ranges often in excess of 20–40 nautical miles depending on antenna height, sea conditions, and target radar cross-section. Antenna patterns were engineered to provide a narrow horizontal beam for azimuth resolution and limited elevation discrimination; units produced echo displays on A-scope or plan-position indicators in subsequent modifications to aid navigation and gunnery. Power supplies, cooling arrangements, and shock mounting conformed to Naval Shipboard Standards of the period, and maintenance manuals referenced components common to other wartime radars like the SG and SX.
SJ-equipped ships participated in late-war operations across the Pacific and Atlantic theatres. Destroyer escorts, destroyers, and cruisers used SJ for convoy escort, night action, and amphibious-support roles during operations such as the Invasion of Okinawa and Leyte Gulf. The radar's ability to reveal small craft and periscopes proved valuable against Japanese Special Attack Units and in anti-submarine warfare paired with sonar systems like the QC series. After World War II, many SJ sets remained in service through the Korean War era, aboard pre-war conversions and newly commissioned ships involved in United Nations maritime operations and patrol duties.
Several production variants and field modifications addressed size, power, and display technologies. Later marks incorporated plan-position indicator (PPI) displays and improvements to the transmitter, following wartime experience and lessons documented by blind-spot and performance studies conducted by the Bureau of Ships and research organizations such as the Office of Scientific Research and Development. Some Navy yards retrofitted SJ hardware to interface with emerging fire-control radars and gyro-stabilized platforms used by classes like the Gearing-class destroyer and Fletcher-class destroyer escorts. Export and lend-lease transfers saw modified SJ types employed by Allied fleets, sometimes adapted under local engineering programs in United Kingdom dockyards.
SJ installations appeared on a wide spectrum of U.S. and Allied vessels: destroyers, destroyer escorts, light cruisers, escort carriers, and amphibious support ships. Tactical doctrine placed SJ as the primary short-range surface-search and navigation radar, cooperating with air-search sets like SK and fire-control radars such as the Mark 12 fire-control radar family. Crews trained at fleet radar schools and on-the-job at naval bases including Pearl Harbor, Norfolk Navy Yard, and Subic Bay to operate and maintain SJ under combat conditions. Postwar refits sometimes replaced SJ with newer centimeter-band radars as technology evolved during the early Cold War.
A number of SJ units and antennas have been preserved in museums and aboard museum ships. Surviving examples are displayed on restored vessels such as museum destroyers and cruisers moored at sites like USS Kidd and other naval heritage collections, where volunteers and curators document wartime electronics alongside artifacts like fire-control directors and communications gear. Naval museums and institutions maintain technical manuals, schematics, and oral histories from radar technicians, contributing to preservation efforts and public interpretation of mid-20th-century maritime radar technology.
Category:Naval radars Category:World War II military equipment of the United States