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Re: [TowerTalk] Fwd: Fwd: Fwd: OCFD

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Fwd: Fwd: Fwd: OCFD
From: jimlux <jimlux@earthlink.net>
Date: Tue, 28 Mar 2017 22:43:14 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
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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