J-pole antenna

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

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).[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 2.2 dBi (0.1 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

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]

E-plane gain 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 2 3 4 5 6 7 8 9 "Very-High-Frequency Antennas". Antennas and Antenna Systems (TM 11-314) (PDF). U.S. War Department. 1943-11-30. pp. 163–164. Retrieved 6 May 2016.
  2. ↑ Beggerow, Hans (1909). "Zeppelin Antenna" (PDF). Retrieved 28 January 2016.
  3. 1 2 3 US patent 2124424, Laurance McConnell Leeds, "Antenna System", published 1938-07-19
  4. ↑ Huggins, John S. "1/4 Wave Monopole vs. 1/2 Wave J-Pole EZNEC Shootout". Retrieved 30 January 2012.
  5. 1 2 3 4 Cebik, L. B. "Some J-Poles That I Have Known Part 1: Part 1: Why I Finally Got Interested in J-Poles and Some Cautions in Modeling Them". Cebik.Com. Archived from the original on April 22, 2014. Retrieved 1 October 2015.
  6. 1 2 3 4 5 6 7 Huggins, John S. "Slim Jim vs. Traditional J-pole Antenna". Retrieved 28 August 2015.
  7. ↑ Griffith, B. Whitfield (1962). Radio-Electronic Transmission Fundamentals. New York, NY: McGraw Hill Book Company, Inc. pp. 322–323.
  8. ↑ Balanis, Constantine (1982). Antenna Theory. Harper & Row, Publishers, Inc. pp. 116–118. ISBN 0-06-040458-2.
  9. ↑ Collins, Brian (1984). "VHF and UHF Communication Antennas". In Johnson, Richard. Antenna Engineering Handbook (2nd ed.). New York, NY: McGraw-Hill. pp. 27.21–27.22. ISBN 0-07-032291-0.
  10. ↑ Griffith, B. Whitfield (1962). Radio-Electronic Transmission Fundamentals. New York, NY: McGraw Hill Book Company, Inc. pp. 243–244.
  11. ↑ Hall, Gerald (1988). The ARRL Antenna Book (15th ed.). American Radio Relay League. p. 24.25. ISBN 0-87259-206-5.
  12. 1 2 3 4 5 Huggins, John S. "J-Pole Antenna – Should I ground it?". Retrieved 30 January 2012.
  13. 1 2 3 Richardson, Dan (March 1998). "The J-Pole Revisited" (PDF). CQ Magazine: 34–41. Retrieved 30 January 2012.
  14. ↑ Fong, Edison (March 2007). "The DBJ-2: A Portable VHF-UHF Roll-Up J-pole Antenna for Public Service". QST. Newington, CT: ARRL, Inc.
  15. ↑ Straw, Dean (2007). "26 - Coupling the Line to the Antenna". The ARRL Antenna Book. Newington, CT: The ARRL, Inc. ISBN 0-87259-987-6.
  16. 1 2 3 Huggins, John S. "Have your J-Pole and ground it too". Retrieved 2015-03-04.
  17. ↑ Huggins, John S. "Mast Mountable J-Pole Antenna". Retrieved 2015-06-17.
  18. 1 2 3 Judd, Fred. "Slim Jim - 2 Metre Aerial". Practical Wireless - Out of Thin Air: 37–39. Retrieved 24 April 2014.
  19. 1 2 3 4 Cebik, L. B. "What is a Slim Jim?". Cebik.Com. Archived from the original on April 24, 2014. Retrieved 30 January 2012.
  20. 1 2 Steve Cerwin (2007). "Mobile and Maritime Antennas - The Super-J Maritime Antenna". In Straw, Dean. ARRL Antenna Book (21st ed.). The ARRL, Inc. pp. 16.23–16.26. ISBN 0-87259-987-6.
  21. ↑ Franklin, Charles (1924). "Franklin Antenna" (PDF). Retrieved 28 January 2016.
  22. ↑ Collins, Brian (1984). "VHF and UHF Communication Antennas - Base-Station Antennas". In Johnson, Richard; Henry Jasik. Antenna Engineering Handbook (2nd ed.). New York: McGraw-Hill. p. 27_14. ISBN 0-07-032291-0.
  23. 1 2 Cebik, L. B. "Some J-Poles That I Have Known Part 4: Some Things We Can and Cannot Do With a J-Pole". Cebik.Com. Archived from the original on April 22, 2014. Retrieved 21 April 2014.
  24. 1 2 3 4 5 Huggins, John S. "Improving the Super J-Pole Antenna". Retrieved 21 April 2014.

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