On 3/28/17 8:21 PM, Hans Hammarquist via TowerTalk wrote:
Jim,
I can't give you "right" on all your points here:
## What method would you suggest to explain that the resonance frequency is
reduce when you put the dipole closer to real (or ideal) ground?
Explaining it? In the sense of a simple conceptual model, or in terms of
how to calculate it. For the latter, I use FEM codes and don't worry
about analytical approximations.
For the former, it's because the antenna is no longer in free space -
the medium has an relative permittivity that's not 1. But I don't
bother trying compute the effective permittivity or the change in
resonant frequency by analytical means. I just assume that the
resonance will be different, and generally lower. How much lower? That's
what the modeling code is for.
One could use the "dielectric loading" explanation (just like why a
dipole with insulated wire has a lower resonant frequency than bare),
but I suspect that's not fully correct. That is, if you compute the
effective epsilon from the insulation and the surrounding free space,
that doesn't account for ALL of the change in resonant frequency,
particularly for higher order resonances.
---
I just don't like the idea of "end effect" - it's just too facile and I
don't think it actually represents what's going on with the actual EM
field. An infinitely thin dipole has no real "end" with some sort of
capacitance - it's not like it's a flat plate or hemisphere at the end
of a rod where you can calculate a capacitance to free space.
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