J-pole antenna
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The J-pole antenna, also called the Zepp' antenna (short for Zeppelin), was first invented by the Germans for use in their lighter than air balloons. Trailed behind the airship it consisted of a quarter wave one half wave. This was later modified into the J-Pole configuration popular with amateur radio hobbyists.
The J-pole antenna is an end-fed omnidirectional dipole antenna that is matched to the feedline by a quarter wave transmission line stub. Matching to the feed-line is achieved by sliding the connection of the feedline back and forth along the stub until a VSWR as close as possible to 1:1 is obtained. Since this is a half-wave antenna, it will exhibit gain over a quarter-wave ground-plane antenna. The J-pole is somewhat sensitive to surrounding metal objects, and should have at least a quarter wavelength of free space around it.
A well known variation of the J-pole is the "Slim Jim", invented by Fred C. Judd (G2BCX). The name was derived from its slim construction and the J type matching stub (J Integrated Matching).
Both antennas should ideally be fed with balanced line, however a coax feed line may be used if a balun is added. Commonly, a choke balun is used, or an air transformer, using about five turns of coax. Typical construction materials include copper pipe, ladder line, or twin-lead. Coax can be used to match the J-Pole as somewhere between the closed circuit and open circuit of the stub an exact 50 ohm impedance match exists.
The J-pole antenna is popular with amateur radio operators, as it is effective and fairly simple to build.
