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

[Jim Brown]

On Wed,6/1/2016 1:45 PM, Hans Hammarquist via TowerTalk wrote:
> Anybody with a "simple" explanation?

It can be tough to provide a simple explanation of a complicated issue, 
but I tried about a week ago in this thread. :)  It is FAR more than 
capacitance to ground, although that's a component.

The complex part of the interaction of a horizontal dipole with the 
earth below it is that the earth is somewhat conductive, current flows 
in it, and it couples back into the dipole like any other passive 
element. The amplitude and phase of the reflection depend on height and 
on the current in the ground, which in turn depends on soil 
conductivity. At some heights, the reflection adds more nearly in phase, 
at others more nearly 180 out of phase, and at a different phase 
relationship at every point in between. THAT'S why the feedpoint Z 
changes in the complex way it does.

I've seen plots of this in one of the ARRL antenna publications, 
probably the ON4UN book. For an antenna that's "high" as a fraction of a 
wavelength, Z varies around 70 ohms, for a low one, around 50 ohms.

73, Jim K9YC