[TowerTalk] Fwd: Change in Frequency As Antenna Height Rises

[jimlux]

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.