J-pole antenna

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J-Pole Antenna.

The J-pole antenna, also called the Zepp' antenna (short for Zeppelin), was first invented by the Germans for use in their lighter-than-air balloons.[1] 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 1943[2] and became popular with amateur radio operators because it is effective and relatively simple to build.

Characteristics[edit]

The J-pole antenna is an end-fed omnidirectional dipole antenna that is matched to the feedline by a quarter wave transmission line stub. Matching to the feed-line is achieved by sliding the connection of the feedline back and forth along the stub until a VSWR as close as possible to 1:1 is obtained. Because this is a half-wave antenna, it provides a small gain over a quarter-wave ground-plane antenna.[3]

Like all antennas, the J-pole is sensitive to surrounding electrically conductive objects and should maintain sufficient separation to minimize near field interactions. 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.[4][5]

Feed and mounting[edit]

Feed[edit]

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. Commonly, a choke balun is used, or an air transformer, using about five turns of coax. 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.

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.[4][5] 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.[4] This modifies the far field antenna pattern typically, but not always, raising the primary lobes above the horizon reducing antenna effectiveness for terrestrial service.[5] J-pole antennas with electrical connection to their supports often fare no better, and often much worse, than the simpler Monopole antenna.[4]

Variations[edit]

Slim Jim antenna[edit]

J-pole Antenna and variations of same.

A variation of the J-pole is the Slim Jim antenna, also known as 2BCX Slim Jim,[6] that is related to the J-pole the way a folded dipole is related to a dipole.[7] The Slim Jim is one of many ways to form a J-Pole.[7] Invented by Fred Judd (G2BCX), the name was derived from its slim construction and the J type matching stub (J Integrated Matching).[6]

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.[7] Judd found the Slim Jim produces a lower takeoff angle and better electrical performance than a 5/8 wavelength ground plane antenna.[6] The Slim Jim does not significantly improve the electrical performance of the traditional J-pole antenna.[7]

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.[8] The phasing stub between the two half-wave sections is often of the Franklin style.[8][9]

The Super-J antenna compresses the vertical beamwidth and has more gain than the traditional J-pole design.[10] 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.[10][11]

Collinear J-pole antenna[edit]

The collinear J-pole antenna improves the Super-J by separating the two radiating half-wave sections to optimize gain using a phasing coil.[11] The resulting gain is closer to the optimum 3 dB than the Super-J.[11]

References[edit]

  1. ^ Rogers, G. E. "J-Poles Handbook, 4th Edition". Retrieved 30 January 2012. 
  2. ^ Antennas and Antenna Systems (TM 110314). U.S. War Department. 1943. 
  3. ^ Huggins, John S. "1/4 Wave Monopole vs. 1/2 Wave J-Pole EZNEC Shootout". Retrieved 30 January 2012. 
  4. ^ a b c d Huggins, John S. "J-Pole Antenna – Should I ground it?". Retrieved 30 January 2012. 
  5. ^ a b c Richardson, Dan (March 1998). "The J-Pole Revisited". CQ Magazine: 34–41. Retrieved 30 January 2012. 
  6. ^ a b c Judd, Fred. "Slim Jim - 2 Metre Aerial". Practical Wireless - Out of Thin Air: 37–39. Retrieved 24 April 2014. 
  7. ^ a b c d Cebik, L. B. "What is a Slim Jim?". Cebik.Com. Retrieved 30 January 2012. 
  8. ^ a b 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. 
  9. ^ 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. 
  10. ^ a b 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. Retrieved 21 April 2014. 
  11. ^ a b c Huggins, John S. "Improving the Super J-Pole Antenna". Retrieved 21 April 2014. 

External links[edit]