The J-pole antenna is an end-fed omnidirectional half-wave 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. Because this is a half-wave antenna, it provides a small gain over a quarter-wave ground-plane antenna. Like all antennas, the J-pole is sensitive to surrounding electrically conductive objects and should maintain sufficient separation to minimize near field interactions. The J-Pole is very sensitive to conductive support structures and will achieve best performance with no electrical bonding between antenna conductors and the mounting structure.
The J-pole antenna and its variations should ideally be fed with balanced line, however a coax feed line may be used if it includes a means to suppress feed-line RF currents. A choke balun is often used, with about five turns of coax, or an air transformer. Typical construction materials include copper pipe, ladder line, or twin-lead. The feed-point of the J-pole is somewhere between the closed low-impedance bottom and open high-impedance top of the J stub. Between these two extremes a match to any impedance between the low to high impedance points is available.
The J-pole design functions well when fed with a balanced feed (via balun, transformer or choke) and no electrical connection exists between its conductors and surrounding supports. A common approach extends the conductor below the bottom of the J-pole resulting in additional and undesirable RF currents flowing over every part of the mounting structure. This modifies the far field antenna pattern typically, but not always, raising the primary lobes above the horizon reducing antenna effectiveness for terrestrial service. J-pole antennas with electrical connection to their supports often fare no better, and often much worse, than the simpler Monopole antenna.
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