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Slim Jim v J Pole
Baron wrote:
The difference is negligible. They both suffer from unbalanced feed currents and thus feeder radiation. There is or was a commercial design that attempted to reduce that. 73's Modeling of J-poles and ground plane verticals shows considerable mutual coupling to the feedline, and the Slim Jim almost certainly behaves the same in this regard. The amount of common mode current this induces and consequently the amount of pattern distortion it causes depends on the feedline length and its path to ground. When the current is significant, you'll probably need two current baluns (common mode chokes), one at the feed point and the other about a quarter wavelength down, to reduce it to a low value. I theorize that the dependence of common mode current on feed line length is a possible cause for the widely varying reports on how well a J-pole radiates -- some people get lucky, some don't. Roy Lewallen, W7EL |
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