The J antenna is a simple, omni-directional, vertically polarized, coax-fed antenna, with some gain, doesn’t have radials to get in the way, and is easy to build…BUT. The trouble with J poles is the
hams building them (and most of the instructions, e.g., see QST’s copper tubing J poles) don’t know how they work. As a result, they are not omni-directional, and they do not have any gain over a ground plane antenna. They certainly work, but not nearly as good as they should.
The J uses a vertical half-wave antenna, which has all the above-mentioned advantages, except it is not easy to feed with coax. The J uses end feed to the dipole, which is a high-impedance feed, about 2400 Ohms, and you cannot feed coax to it. Therefore a J pole uses a quarter wavelength of open line as a matching transformer between the coax and the bottom of the half-wave vertical. The quarter wave matching section provides and impedance transformation of 50 Ohms to 2400 Ohms…IF…it is built correctly. If it is not built correctly, it is a poor matching section and becomes a part of the antenna.
This is because open transmission line matching sections will act like an antenna if the line is spaced over 0.02 wave lengths (see ARRL Handbook, 1941, p.122 and in every issue since). A separation of 0.02 wavelength on 2-meters is 1 – 1.5 inch. That means the separation between the two wires or tubes at the bottom of your J should be under 1.5 inch; If it is more, the bottom matching section will radiate RF just like the antenna. Three bad things happen when the matching section radiates. First, the two sections, the half-wave and the quarter-wave, radiate RF energy out of phase with each other and a lot of the RF cancels out, making your signal weaker. Second, a matching section does not radiate an omni-directional signal, since the two wires or tubes are displaced from each other. Therefore your J is not omni-directional. Third, a radiating matching section does not provide a good match between the coax and the bottom of the half-wave antenna.
Results are poor in some directions, and the coax must be moved up from the bottom of the
matching section to get a low SWR. So, if you have these problems, take a look at the spacing on the matching section. I’ll bet it’s too wide.
Chuck Teeters – W4MEW
Published in the December 2005 edition of The Splatter