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. This was later modified into the J-pole configuration, which became popular with amateur radio operators because it is effective and relatively simple to build.
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 will exhibit gain over a quarter-wave ground-plane antenna.[2] The J-pole is somewhat sensitive to surrounding metal objects, and should have at least a quarter wavelength of free space around it. 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.[3][4]
A well known variation of the J-pole is the Slim Jim antenna, which 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.[5] Invented by Fred Judd (G2BCX), the name was derived from its slim construction and the J type matching stub (J Integrated Matching).
Both antennas should ideally be fed with balanced line, however a coax feed line may be used if a balun is added. 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. Coax can be used to match the J-pole as somewhere between the closed circuit and open circuit of the stub an exact 50 ohm impedance match exists.
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.[3][4] 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.[3] This modifies the far field antenna pattern typically, but not always, raising the primary lobes above the horizon reducing antenna effectiveness for terrestrial service.[4] J-pole antennas with electrical connection to their supports often fare no better, and often much worse, than the simpler Monopole antenna.
[edit] References
- ^ Rogers, G. E. "J-Poles Handbook, 4th Edition". http://www.buxcomm.com/jpoles4ever.htm. Retrieved 30 January 2012.
- ^ Huggins, John S. "1/4 Wave Monopole vs. 1/2 Wave J-Pole EZNEC Shootout". http://www.hamradio.me/nec-shootouts/monopole-vs-jpol-eznec-shootout.html. Retrieved 30 January 2012.
- ^ a b c Huggins, John S. "J-Pole Antenna – Should I ground it?". http://www.hamradio.me/antennas/j-pole-antenna-should-i-ground-it.html. Retrieved 30 January 2012.
- ^ a b c Richardson, Dan (March 1998). "The J-Pole Revisited". CQ Magazine: 34-41. http://techdoc.kvindesland.no/radio/vhf_antennas/20061030163006789.pdf. Retrieved 30 January 2012.
- ^ Cebik, L. B. "What is a Slim Jim?". Cebik.Com. http://www.cebik.com/content/a10/vhf/sj.html. Retrieved 30 January 2012.
[edit] External links
- 100 J-pole antenna plans
- Build a 440 mHz J pole antenna
- How to build a 2 meter J pole antenna
- J-Pole Double Header
- 2 meter copper J-Pole construction plans
- 2 Meter half wave J-pole made from 450 ohm ladder line
- Slim Jim antenna project
- Slim Jim antenna calculator & info
- Cebik: What is a Slim Jim? (Free registration required)
- Cebik: Some J-Poles That I Have Known (Free registration required)
