J-pole antenna

E-plane gain measurements of J antenna with respect to reference dipole.

Primarily a dipole, the J-pole antenna exhibits a mostly circular pattern in the H plane with an average free-space gain near 2.2 dBi (0.1 dBd).[6] Measurements and simulation confirm the quarter-wave stub modifies the circular H-plane pattern shape increasing the gain slightly on the side of the J stub element and reducing the gain slightly on the side opposite the J stub element.[6][7] At right angles to the J-stub, the gain is closer to the overall average: about 2.2 dBi (0.1 dBd).[6] The slight increase over a dipole's 2.15 dBi (0 dBd) gain represents the small contribution to the pattern made by the current imbalance on the matching section.[6] The pattern in the E plane reveals a slight elevation of the pattern in the direction of the J element while the pattern opposite the J element is mostly broadside.[7] The net effect of the perturbation caused by quarter-wave stub is an H-plane approximate gain from 1.5 to 2.6 dBi (-0.6 dBd to 0.5 dBd).[7]

Like all antennas, the J-pole is sensitive to electrically conductive objects in its induction fields[8] (aka reactive near-field region [9]) and should maintain sufficient separation to minimize these near field interactions as part of typical system installation considerations.[10] The quarter wave parallel transmission line stub has an external electromagnetic field with strength and size proportional to the spacing between the parallel conductors.[11] The parallel conductors 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.[4] 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.[12][13]

Typical construction materials include metal tubing,[1] ladder line, or twin-lead.[14]

The J-pole antenna and its variations may be fed with balanced line.[1] A coax feed line may be used if it includes a means to suppress feed-line RF currents.[12][15] The feed-point of the J-pole is somewhere between the closed low-impedance bottom and open high-impedance top of the J stub.[1][3] Between these two extremes a match to any impedance between the low to high impedance points is available.[1][3]

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.[12][13] 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.[1] Later research confirms the tendency of the mast or grounding wire to draw current from the antenna potentially spoiling the antenna pattern.[16] 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.[12] This modifies the far field antenna pattern[16] typically, but not always, raising the primary lobes above the horizon reducing antenna effectiveness for terrestrial service.[13] J-pole antennas with electrical connection to their supports often fare no better, and often much worse, than the simpler monopole antenna.[12] A mast decoupling stub reduces mast currents.[16][17][18][19]

A variation of the J-pole is the Slim Jim antenna, also known as 2BCX Slim Jim,[20] that is related to the J-pole the way a folded dipole is related to a dipole.[21] The Slim Jim is one of many ways to form a J-Pole.[21] Introduced by Fred Judd (G2BCX) in 1978, the name was derived from its slim construction and the J type matching stub (J Integrated Matching).[20]

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 conventional J-pole construction.[21] Judd found the Slim Jim produces a lower takeoff angle and better electrical performance than a ​⁵⁄₈ wavelength ground plane antenna.[20] Slim Jim antennas made from ladder transmission line use the existing parallel conductor for the folded dipole element.[7] In the copper pipe variation, the Slim Jim uses more materials for no performance benefit.[7] Slim Jim antennas have no performance advantage over the conventional J-pole antenna.[7][21]

The approximate gain in the H-plane of the Slim Jim is from 1.5 to 2.6 dBi (-0.6 dBd to 0.5 dBd).[7]

The Super-J variation of the J-pole antenna adds another collinear half-wave radiator above the conventional J and connects the two with a phase stub to ensure both vertical half-wave sections radiate in current phase.[22] The phasing stub between the two half-wave sections is often of the Franklin style.[22][23][24]

The Super-J antenna compresses the vertical beamwidth and has more gain than the conventional J-pole design.[25] Both radiating sections have insufficient separation to realize the maximum benefits of collinear arrays, resulting in slightly less than the optimal 3 dB over a conventional J-pole or halfwave antenna.[25][26]

The approximate gain in the H-plane of the Super-J antenna is from 4.6 to 5.2 dBi (2.4 dBd to 3.1 dBd).[26]

The collinear J antenna improves the Super-J by separating the two radiating half-wave sections to optimize gain using a phasing coil.[26] The resulting gain is closer to the optimum 3 dB over a conventional J-pole or halfwave antenna.[26]

The approximate gain in the H-plane of the Collinear J antenna is from 4.6 to 5.2 dBi (2.4 dBd to 3.1 dBd).[26]

The graph compares the E-plane gain of the above three variations to the conventional J antenna.

The conventional J antenna and SlimJIM variation are nearly identical in gain and pattern. The Super-J reveals the benefit of properly phasing and orienting a second radiator above the first. The Collinear J shows slightly higher performance over the Super-J.

The basic J antenna resonates on the third harmonic of its lowest design frequency.[27] Operating a 3/2 wavelengths this way produces an antenna pattern unfavorable for terrestrial operation.[28]

To address the pattern change a variety of techniques exist to allegedly constrain a J antenna operating at or near the third harmonic so only one half-wave is active in the radiator above the stub. All involve the use of a high impedance choke at the first voltage loop.[28] These methods fall short of the goal as choking a high impedance point with a high impedance allows energy to pass the choke.[28][29]