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Antenna Reactance Question
In my dipole/vertical modeling and analyzer measurements, as frequency
is increased past quarter-wave resonance, I've noticed with interest that both reactance and resistance peak at different times, as they increase with frequency toward the half-wave point. Resistance peaks at approximately the half-wave point as expected, but inductive reactance always peaks a little earlier (lower frequency). This is also indicated in the ARRL Antenna Book in Figures 3 through 5 on pages 2-3 and 2-4 (it's the bulging on the right side of each of the curves). For a given antenna of particular length, adding inductive or capacitive reactance changes the magnitude of the reactance peak, but not the frequency at which it occurs. Changing the thickness of the radiating elements changes (lowers) the frequency at which the reactance peak occurs, but it also changes (lowers) the frequency at which resistance peaks, and the difference in these two freqencies stays approximately the same. Why does the reactance peak occur slightly earlier than half-wavelength? Can it be mathmatically predicted/explained? Any help would be appreciated. Al, WA4GKQ |
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