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Good evening, Gentlemen.
A thought experiment: Start with a regular 1/4-wave monopole ground plane. The literature says it looks like half the value of a dipole, about 35 Ohms, when resonant. It would be nice to have the resistance at the terminals be a bit higher, and I very much value a grounded element anyway, so let's let it evolve into a folded monopole. The literature says it should now have about 4 times the terminal resistance of the original 1/4-wave we started with (about 140 Ohms). Huh. Now it's a bit high. They tell me that shortening the antenna below resonance will lower the resistance and introduce capacitance. But I think I have also seen in the literature that the antenna can be viewed as a transmission line. A shorted portion of parallel conductor transmission line (the folded monopole) less than 1/4-wave long looks inductive. But wait! Which will win? Will the shortness of the antenna look capacitive or will the transmission line dominate and the antenna will look inductive? Even better, is there some choice of the folded section wire diameters and spacing that will give an inductance that will exactly offset the capacitance due to shortness? So, then, is there a folded monopole of such dimensions that the resistance is 50 Ohms (due to being shorter than 1/4 wave) with no terminal reactance (due to the inductive design of the "transmission line" cancelled by the shortness of the antenna's capacitance)? Brain hurts. John, KD5YI |
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