Left Jab was an airborne radio direction finding system used by the US Army. The system was mounted on modified U-21 Ute utility aircraft, designated JU-21A.
Development of the Left Jab system started in 1966 as part of a need to supplement existing direction finding systems then deployed in support of operations in Vietnam. Left Jab was a limited system using 3 JU-21A aircraft. The most notable modification was the addition of a large oval antenna that was extended in flight well below the bottom of the aircraft. The Left Jab system had 2 operators, which was an improvement over existing single operator systems, but had only an intercept and direction finding capability.
The aircraft were deployed to Hue Phu Bai, Vietnam, in 1971 where they were assigned to the 138th Aviation Company (Radio Research). During operations a crew and aircraft were lost to a hostile surface-to-air missile. A Left Jab JU-21A from the 138th Aviation Company flew what was reported to be the last direction finding mission in South Vietnam on 16 February 1973.
The remaining 2 Left Jab aircraft remained with the 138th Army Security Agency Company, also known as the 138th Aviation Company (Electronic Warfare), US Army Reserve in Orlando, Florida until the mid-1980s when the aircraft were removed from service.
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