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Slim Jim v J Pole
I've seen a couple of postings so far recommending a single balun at the
feedpoint. While this takes care of conducted common mode current, it doesn't eliminate current induced on the feedline from mutual coupling. All the single balun does is to insure that the current is near zero at the single point where the balun is inserted. Current is still induced on the feedline, and you end up with a current maximum a quarter wavelength down the line -- the distribution looks just like it does on an antenna (which the feedline has actually become), with the balun location being the end of the "antenna". The balun can even make the induced current worse if the effective common mode open circuit at the balun insertion point results in a more nearly resonant feedline length. The only way to really effectively reduce the induced current to a low level is to break up the resonance of the feedline by inserting a second balun about a quarter wavelength below the feedpoint as I recommended earlier. Roy Lewallen, W7EL |
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