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Antenna article
On Sunday, February 16, 2014 11:12:51 AM UTC-6, Jeff Liebermann wrote:
On Sun, 16 Feb 2014 06:32:38 -0800 (PST), W5DXP wrote: On Saturday, February 15, 2014 9:58:10 PM UTC-6, Jeff Liebermann wrote: However, a J-pole (or Zepp) is not a 1/2 wave antenna. The driven element is a 1/4 wavelength long, and therefore DOES require a ground plane. Sorry, that is not correct. Agreed. See my followup to my posting where I noticed that I've been building J-pole antennas with the coax feed connected to the wrong element. Judging by some of the photos I've found, I'm not alone. I never built J-poles myself. Never was that fond of the design for some reason. I always preferred the "ringo" method of feeding a base fed half wave. IE: a flat single turn tapped coil, and coax capacitor. I used them on 10m mostly. They work decently well in most cases with no decoupling section. But they work even better with decoupling. I used a 1/4 wave length of coax to a union which was attached to the mast, and had a set of three radials. But... The 5/8 ground planes were always better than the 1/2 waves on distant 10m local stations. Even the decoupled version. And being low angle space wave stuff, it's a pretty good test. Both antennas were at about 36 ft up at the base. According to a modeling exercise I did once, the best way to run a 5/8 GP is with 5/8 radials. I've also used 3/4 wave radials, which seemed to work well. But according to the modeling, the 5/8 radials will give more gain. Starts to resemble a dual 5/8 collinear. And naturally, you would want to use a decoupling section for the best performance. This is pretty old, but compares the different lengths for 10m use. http://home.comcast.net/~nm5k/acompari.htm |
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