On 6/1/16 2:06 PM, Steve Hunt wrote:
It's clearly not as simple as a "capacitance to ground" effect.
The cyclic nature of the resonant frequency vs height, plus the fact
that it is more pronounced over better-conducting ground, suggests to me
that is related to the "interference" effect outlined in the ARRL
Antenna Book in the section "The effects of Ground in the Reactive Near
Field".
There's that, but I think of that as more a detuning, loading thing for
the ground in "near" proximity to the radiating element (<1/10th
wavelength).
When you're in that "fraction of a wavelength" area, you're not so much
in the reactive near field (well, technically you are) but you're in the
region where you can think about the ground as a mirror with an image.
Actually, it's all the same math in the details (e.g. it all basically
is a solution of Maxwell's equations), but there are good approximation
models in various regimes.
Ultimately, it's all about different conceptual and math models for the
same underlying phenomenon. You can consider a transmission line with
forward and reflected waves, or, you can consider it as a 2 port network
which does impedance transformations, or, etc.
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