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]
CONSTRUCTION OF DIPOLE ANTENNAS A dipole antenna is a symmetrical antenna, consisting of two quarter wave elements, connected to each of the two conductors (inner and outer) of a coaxial cable. You will need a center insulator for connection of the two quarter wave elements and the coaxial cable. Sznde durmayan, kendisine gvenilmeyen, itin teki. A bag of nerves. A bed of roses. Gllk glistanlk durum. A bee in one's bonnet. Fikri sabit, saplant.
- 1Characteristics
- 2Feed and mounting
- 3Variations
Characteristics[edit]
The J-pole antenna is an end-fed omnidirectional half-wave antenna that is matched to the feedline by a quarter wave parallel transmission linestub[1] of Lecher system form.[2] 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[edit]
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[edit]
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[edit]
Construction[edit]
Typical construction materials include metal tubing,[1]ladder line, or twin-lead.[14]
Feed[edit]
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]
Mounting[edit]
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]
Variations[edit]
E-plane gain plots of J antenna variations
Slim Jim antenna[edit]
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 (JIntegrated 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 traditional 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.[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][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).[6]
Super-J antenna[edit]
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.[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 traditional 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 traditional 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]
Collinear J antenna[edit]
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 traditional 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]
E-plane gain patterns of the variations[edit]
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.
Dual-band operation near 3rd harmonic[edit]
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]
References[edit]
- ^ abcdefghi'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.
- ^ abBeggerow, Hans (1909). 'Zeppelin Antenna'(PDF). Retrieved 28 January 2016.
- ^ abcUS patent 2124424, Laurance McConnell Leeds, 'Antenna System', published 1938-07-19
- ^Huggins, John S. '1/4 Wave Monopole vs. 1/2 Wave J-Pole EZNEC Shootout'. Retrieved 30 January 2012.
- ^ abcdCebik, 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.
- ^ abcdefgHuggins, John S. 'Slim Jim vs. Traditional J-pole Antenna'. Retrieved 28 August 2015.
- ^Griffith, B. Whitfield (1962). Radio-Electronic Transmission Fundamentals. New York, NY: McGraw Hill Book Company, Inc. pp. 322–323.
- ^Balanis, Constantine (1982). Antenna Theory. Harper & Row, Publishers, Inc. pp. 116–118. ISBN0-06-040458-2.
- ^Collins, Brian (1984). 'VHF and UHF Communication Antennas'. In Johnson, Richard (ed.). Antenna Engineering Handbook (2nd ed.). New York, NY: McGraw-Hill. pp. 27.21–27.22. ISBN0-07-032291-0.
- ^Griffith, B. Whitfield (1962). Radio-Electronic Transmission Fundamentals. New York, NY: McGraw Hill Book Company, Inc. pp. 243–244.
- ^Hall, Gerald (1988). The ARRL Antenna Book (15th ed.). American Radio Relay League. p. 24.25. ISBN0-87259-206-5.
- ^ abcdeHuggins, John S. 'J-Pole Antenna – Should I ground it?'. Retrieved 30 January 2012.
- ^ abcRichardson, Dan (March 1998). 'The J-Pole Revisited'(PDF). CQ Magazine: 34–41. Retrieved 30 January 2012.
- ^Fong, Edison (March 2007). 'The DBJ-2: A Portable VHF-UHF Roll-Up J-pole Antenna for Public Service'. QST. Newington, CT: ARRL, Inc.
- ^A folded-balun, sleeve balun, or common-mode choke will suppress feed-line RF currents. See: Straw, Dean (2007). '26 - Coupling the Line to the Antenna'. The ARRL Antenna Book. Newington, CT: The ARRL, Inc. ISBN0-87259-987-6.
- ^ abcHuggins, John S. 'Have your J-Pole and ground it too'. Retrieved 2015-03-04.
- ^Huggins, John S. 'Mast Mountable J-Pole Antenna'. Retrieved 2015-06-17.
- ^Huggins, John. 'Mast Mountable Antenna'. USPTO via Google. US Government. Retrieved 22 July 2017.
- ^US patent D798847, 'Antenna', issued 2017-10-03
- ^ abcJudd, Fred (1978). 'Slim Jim - 2 Metre Aerial'. Practical Wireless - Out of Thin Air: 37–39. Retrieved 24 April 2014.
- ^ abcdCebik, L. B. 'What is a Slim Jim?'. Cebik.Com. Archived from the original on April 24, 2014. Retrieved 30 January 2012.
- ^ abSteve Cerwin (2007). 'Mobile and Maritime Antennas - The Super-J Maritime Antenna'. In Straw, Dean (ed.). ARRL Antenna Book (21st ed.). The ARRL, Inc. pp. 16.23–16.26. ISBN0-87259-987-6.
- ^Franklin, Charles (1924). 'Franklin Antenna'(PDF). Retrieved 28 January 2016.
- ^Collins, Brian (1984). 'VHF and UHF Communication Antennas - Base-Station Antennas'. In Johnson, Richard; Henry Jasik (eds.). Antenna Engineering Handbook (2nd ed.). New York: McGraw-Hill. p. 27_14. ISBN0-07-032291-0.
- ^ abCebik, 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.
- ^ abcdeHuggins, John S. 'Improving the Super J-Pole Antenna'. Retrieved 21 April 2014.
- ^Huggins, John. 'Can a 2m J-Pole be used at 440?'. Hamradio.me. Retrieved 12 June 2019.
- ^ abcHuggins, John. 'Antenna radiator decoupling stub flub?'. Hamradio.me. Retrieved 12 June 2019.
- ^Huggins, John. 'Where quarter-wave radiator decoupling stubs work… and don't work'. Hamradio.me. Retrieved 12 June 2019.
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