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. 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 and became popular with amateur radio operators because it is effective and relatively simple to build.
The J-pole antenna is an end-fed omnidirectional half-wave 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.
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.
Feed and mounting
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. A choke balun is often used, with about five turns of coax, or an air transformer. 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.
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. 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. This modifies the far field antenna pattern typically, but not always, raising the primary lobes above the horizon reducing antenna effectiveness for terrestrial service. J-pole antennas with electrical connection to their supports often fare no better, and often much worse, than the simpler Monopole antenna.
Slim Jim antenna
A variation of the J-pole is the Slim Jim antenna, also known as 2BCX Slim Jim, that is related to the J-pole the way a folded dipole is related to a dipole. The Slim Jim is one of many ways to form a J-Pole. Invented by Fred Judd (G2BCX), the name was derived from its slim construction and the J type matching stub (J Integrated Matching).
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. Judd found the Slim Jim produces a lower takeoff angle and better electrical performance than a 5/8 wavelength ground plane antenna. The Slim Jim does not significantly improve the electrical performance of the traditional J-pole 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. The phasing stub between the two half-wave sections is often of the Franklin style.
The Super-J antenna compresses the vertical beamwidth and has more gain than the traditional J-pole design. 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.
Collinear J-pole antenna
The collinear J-pole antenna improves the Super-J by separating the two radiating half-wave sections to optimize gain using a phasing coil. The resulting gain is closer to the optimum 3 dB than the Super-J.
- Rogers, G. E. "J-Poles Handbook, 4th Edition". Retrieved 30 January 2012.
- Antennas and Antenna Systems (TM 110314). U.S. War Department. 1943.
- Huggins, John S. "1/4 Wave Monopole vs. 1/2 Wave J-Pole EZNEC Shootout". Retrieved 30 January 2012.
- Huggins, John S. "J-Pole Antenna – Should I ground it?". Retrieved 30 January 2012.
- Richardson, Dan (March 1998). "The J-Pole Revisited" (PDF). CQ Magazine: 34–41. Retrieved 30 January 2012.
- Judd, Fred. "Slim Jim - 2 Metre Aerial". Practical Wireless - Out of Thin Air: 37–39. Retrieved 24 April 2014.
- Cebik, L. B. "What is a Slim Jim?". Cebik.Com. Retrieved 30 January 2012.
- 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.
- 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.
- 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.
- Huggins, John S. "Improving the Super J-Pole Antenna". Retrieved 21 April 2014.
- J-Pole Double Header
- 2 Meter half wave J-pole made from 450 ohm ladder line
- 144/440 Dual band Open Stub J-Pole Antenna