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The J-Pole is a end-fed halfwave dipole. Sometimes referred to as a folded zepp. Zepps are 1/2 wavelength end-fed antennas, named after the German Zeppelin dirigibles where they were used.

- the J-Pole needs no radials
- the J-Pole has low angle radiation
- the J-POLE has greater bandwidth.
- the J-Pole has greater immunity to terrestrial noise
- the J-Pole is great for local nets or distant repeaters
- Its length means that the J pole antenna also provides
  some gain over a normal quarter wavelength vertical
- the J-Pole is more durable than most Ground Planes
- the J-Pole meets most "stealth" antenna restriction
  agreements
- the J-Pole has less static-charge noise,
  and static-charge build-up.

The J pole antenna uses the stub to provide a good match to 50 Ohm cable. The feed point is moved up or down the stub to provide the best match, and adjustment can be made once the antenna is in position if required.
The main disadvantage is that it can be a little more difficult to adjust than some other forms. The reason for this is that impedance matching has to be accomplished by altering the trimming length of the stub.

The following designs are for 2 meters, but it will listen well on 440.
You can use the calculator listed below for other bands.

Copper Cactus   Copper Cactus
Source: "The Copper Cactus J-Pole for 2m and 70 cm", by John Post KE7AX, 73 Amateur Radio Today, Feb. 1992.

J Pole Handbook Fig 1
According to the J pole Handbook, A BUX VBALUN ($20) should be installed at the antenna feed point, or where the coax or feed-line attaches to the J-Pole antenna. BUX BALUNs are used to connect balanced antennas to unbalanced transmission lines, such as coax cable. Their primary purpose is to prevent antenna (RF) currents from flowing down the outside of the cable. Another function of the BUX BALUN41 is to match the impedance of an unbalanced coax to the balanced feed point of a balanced input antenna(s). BUX BALUNS may also be used as "line isolators" anywhere along the cable to prevent the destructive influence of induced RF currents (VSWR). BUX 1:1 BALUNs are current BALUNs. They consist of several large, number 73, ferrite type 44 cores.
See: buxcomm.com/catalog/
Some guidelines in the J-Pole Tutorial from Gary E. O'Neil, (N3GO). Some are contrary to popular opinion.
  • Adjusting the feedpoint "tap" is not the proper way to tune a J-pole.
  • 300 ohms is the optimal transmission line impedance to use in the construction of J-poles.
  • 300 ohm TV twin-lead is minimally susceptible to RF current induced in the feedline coaxial cable, enabling the J-pole to perform well without a balun.
  • The "stub" portion of the J-pole is electrically longer than a quarter wavelength.
  • The J-pole "shorting bar" can be connected to an earth ground reference if and only if a balun is employed at the antenna feedpoint.
  • The velocity factor is the most critical transmission line parameter to consider when designing J-poles.
  • J-poles are easy to build, and tune (even by the inexperienced). As a consequence, they are easy to reproduce.
Links:
J pole Handbook Glynn E. "Buck" Rogers Sr., (K4ABT) at PacketRadio.com
"The Copper Cactus J-Pole for 2m and 70 cm" (In spanish at qslbrasil.com), by John Post KE7AX, 73 Amateur Radio Today, Feb. 1992.
Build a 2 Meter, 5/4 Wave Antenna using twin lead and PVC pipe.
J-Pole Tutorial from Gary E. O'Neil, (N3GO)
J Pole Antenna at Radio_electronics.com
Copper J-Pole Antennas by dxzone.com
J-Pole dimension calculator for different frequencies at mJw.com
J-Pole Dimensions at N6mrx
Dual band - Open stub by Chuck W4CLL
J-Pole with 5/8 wave section above 1/2 wave
Roll-uP J-Pole Antenna at the Conejo Valley Amateur Radio Club
Another Roll-up
Yet another roll-up

See Also:
Electromagnetic Spectrum
Electromagnetic Principles

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last updated 22 Mar 2008