J-pole antenna

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J-Pole Antenna.

The J-pole antenna, more properly known as the J antenna,[1] was first invented by Hans Beggerow in 1909 for use in Zeppelin airships.[2] Trailed behind the airship, it consisted of a single element, one half wavelength long radiator with a quarter wave parallel feedline tuning stub. This concept evolved to the J configuration by 1936[3] attaining the name J Antenna by 1943.[1]

Characteristics

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.[1] 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.[1] Being a half-wave antenna, it provides a small gain over a quarter-wave ground-plane antenna.[4]

Gain and radiation pattern

Primarily a dipole, the J-pole antenna exhibits a mostly circular pattern in the H plane with free-space gain near 2.5 dBi (0.4 dBd).[5] 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.[5][6] At right angles to the J-stub, the gain is closer to the overall average: about 2.3 dBi (0.2 dBd).[5] 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.[5] 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.[6] 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).[6]

Environment

Like all antennas, the J-pole is sensitive to electrically conductive objects in its induction fields[7] (aka reactive near-field region [8]) and should maintain sufficient separation to minimize these near field interactions as part of typical system installation considerations.[9] The quarter wave parallel transmission line stub has an external electromagnetic field with strength and size proportional to the spacing between the parallel conductors.[10] 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.[11] 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]

Feed and mounting

Construction

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

Feed

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] A folded-balun, sleeve balun or common-mode choke will suppress feed-line RF currents.[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]

Mounting

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]

Variations

Slim Jim antenna

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J-pole Antenna and variations of same.
E-plane gain plots of J antenna variations

A variation of the J-pole is the Slim Jim antenna, also known as 2BCX Slim Jim,[18] that is related to the J-pole the way a folded dipole is related to a dipole.[19] The Slim Jim is one of many ways to form a J-Pole.[19] Invented by Fred Judd (G2BCX), the name was derived from its slim construction and the J type matching stub (J Integrated Matching).[18]

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.[19] Judd found the Slim Jim produces a lower takeoff angle and better electrical performance than a 5/8 wavelength ground plane antenna.[18] Slim Jim antennas made from ladder transmission line use the existing parallel conductor for the folded dipole element.[6] In the copper pipe variation, the Slim Jim uses more materials for no performance benefit.[6] Slim Jim antennas have no performance advantage over the traditional J-pole antenna.[6][19]

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).[6]

Super-J 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.[20] The phasing stub between the two half-wave sections is often of the Franklin style.[20][21][22]

The Super-J antenna compresses the vertical beamwidth and has more gain than the traditional J-pole design.[23] 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 traditional J-pole or halfwave antenna.[23][24]

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).[24]

Collinear J antenna

The collinear J antenna improves the Super-J by separating the two radiating half-wave sections to optimize gain using a phasing coil.[24] The resulting gain is closer to the optimum 3 dB over a traditional J-pole or halfwave antenna.[24]

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).[24]

Antenna patterns of the variations

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

The traditional 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.

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 Lua error in package.lua at line 80: module 'strict' not found.
  2. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  3. ↑ 3.0 3.1 3.2 US patent 2124424, Laurance McConnell Leeds, "Antenna System", published 1938-07-19 
  4. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  5. ↑ 5.0 5.1 5.2 5.3 Lua error in package.lua at line 80: module 'strict' not found.
  6. ↑ 6.0 6.1 6.2 6.3 6.4 6.5 6.6 Lua error in package.lua at line 80: module 'strict' not found.
  7. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  8. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  9. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  10. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  11. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  12. ↑ 12.0 12.1 12.2 12.3 12.4 Lua error in package.lua at line 80: module 'strict' not found.
  13. ↑ 13.0 13.1 13.2 Lua error in package.lua at line 80: module 'strict' not found.
  14. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  15. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  16. ↑ 16.0 16.1 16.2 Lua error in package.lua at line 80: module 'strict' not found.
  17. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  18. ↑ 18.0 18.1 18.2 Lua error in package.lua at line 80: module 'strict' not found.
  19. ↑ 19.0 19.1 19.2 19.3 Lua error in package.lua at line 80: module 'strict' not found.
  20. ↑ 20.0 20.1 Lua error in package.lua at line 80: module 'strict' not found.
  21. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  22. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  23. ↑ 23.0 23.1 Lua error in package.lua at line 80: module 'strict' not found.
  24. ↑ 24.0 24.1 24.2 24.3 24.4 Lua error in package.lua at line 80: module 'strict' not found.

External links

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