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The J-pole antenna, also called the Zepp' antenna (short for Zeppelin), and more properly known as the J antenna, was first invented by the Germans for use in their lighter-than-air balloons. Trailed behind the airship, it consisted of a single element, one half wavelength long radiator with a quarter wave parallel feedline tuning stub. This was modified into the J-pole configuration by at least 1936 with continuing refinements by 1943 and became popular with amateur radio operators because it is effective and relatively simple to build.
The J-pole antenna is an end-fed omnidirectional half-wave antenna that is matched to the feedline by a quarter wave parallel transmission line stub. Matching to the feed-line is achieved by sliding the connection of the feedline back and forth along the stub until an impedance match is obtained. Being 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 electrically conductive objects in its induction fields (aka reactive near-field region) and should maintain sufficient separation to minimize near field interactions. The quarter wave parallel transmission line stub must be kept free of moisture, snow, ice and should be kept away from other conductors including downspouts, metal window frames, flashing, etc. by a distance of two to three times the spacing between the parallel stub conductors. 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.
Feed and mounting
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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. Historical documentation of the J antenna suggests the lower end of the matching stub is at zero potential with respect to earth and can connect to a grounding wire or mast with no effect on the antenna's operation. Later research confirms the tendency of the mast or grounding wire to draw current from the antenna potentially spoiling the antenna pattern. 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. A mast decoupling stub reduces mast currents.
Slim Jim antenna
A variation of the J-pole is the Slim Jim antenna, also known as 2BCX Slim Jim, that is related to the J-pole the way a folded dipole is related to a dipole. The Slim Jim is one of many ways to form a J-Pole. Invented by Fred Judd (G2BCX), the name was derived from its slim construction and the J type matching stub (J Integrated Matching).
The Slim Jim variation of the J-pole antenna has characteristics and performance similar to a simple or folded Half-wave antenna and identical to the traditional J-pole construction. Judd found the Slim Jim produces a lower takeoff angle and better electrical performance than a 5/8 wavelength ground plane antenna. Slim Jim antennas made from ladder transmission line use the existing parallel conductor for the folded dipole element.. In the copper pipe variation, the Slim Jim uses more materials for no performance benefit. Slim Jim antennas have no performance advantage over the traditional J-pole antenna.
The Super-J variation of the J-pole antenna adds an additional collinear half-wave radiator above the traditional J and connects the two with a phase stub to ensure both vertical half-wave sections radiate in current phase. The phasing stub between the two half-wave sections is often of the Franklin style.
The Super-J antenna compresses the vertical beamwidth and has more gain than the traditional J-pole design. Both radiating sections have insufficient separation to realize the maximum benefits of collinear arrays resulting in less than the optimal 3 dB over a traditional J-pole or halfwave antenna.
Collinear J-pole antenna
The collinear J-pole antenna improves the Super-J by separating the two radiating half-wave sections to optimize gain using a phasing coil. The resulting gain is closer to the optimum 3 dB than the Super-J.
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