J-Pole Antennas WB5CXC

J-Pole Antenna J-Pole antennas are shorted end feed antennas. They consist of two elements, that are shorted together at the bottom. The shorter element is ¼ λ and the longer is ¾ λ. It is feed with 50 ohm coax, a little off the bottom of the shorted portion. The most common J-Pole is known as the ‘Copper Cactus’ because it is made from copper water pipe (and looks like a cactus).

Copper Cactus Copper Cactus can be made for 10M, 6M, 2M, 220, and 440. The 10M J-Pole is very large and is probably impractical. They are very popular on 6M and above.

70cm (440) Copper Cactus

Adding Adjustment Bolts Some of the articles add bolts to the J-Poles. These provide adjustments to the lengths of the elements. I used the standard calculation and did a model of a 2M Copper Cactus. The SWR was probably within acceptable limits. I then added some bolts and added some length (antenna too short). The first attempt proved the bolts are a good addition.

SWR – used adjustment bolts SWR – built to formula SWR – used adjustment bolts (first try)

Wire J-Poles You can also make J-Poles out of wire. Most are made from twinlead (flat TV cable) or ladder line. Common Ladder Line comes in 300 or 450 Ohm. You can use your standard formula for ladder line, but you have to use the velocity factor to get the final calculations.

300 ohm flat twinlead J-Pole