J-pole antenna
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The J-pole antenna is an end-fed omnidirectional dipole that is matched to the feedline by a quarter wave stub. Since this is a half-wave antenna, it will exhibit some gain over a quarter-wave ground-plane antenna. The SWR of a J-pole is relatively insensitive to nearby objects. This is probably due to the quarter wave matching stub.
A well known variation of the J-pole is the "Slim Jim". The name may have been derived from its slim construction and the J type matching stub (J Integrated Matching). The antenna should ideally be fed with balanced line, however a coax feed line may be used if a balun is added.
It is more than likely however that the short quarter wave stub part of the antenna simply isolates the half-wavelength upper part of the longer element, essentially relieving it of any of the influence of the mounting. When the antenna is cut correctly to the working frequency, matching to the feed-line is easily archived by sliding the connection of the feedline back and forth along the stub until a 1:1 match is obtained..
No part of the antenna should be grounded; it should be fully insulated from its mount points and have at least a quarter wavelength of free space around itself.
The invention of the J-pole antenna is attributed to Fred Judd.
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