The reflection that you care about for your high horizontal
antennas isn't dependent on where the local water table is
located. The ground conductivity in the region immediately below
the antenna is nearly completely irrelevant if the antenna is
a half wavelength or more above it.
What follows is a grossly oversimplified discussion. But in
practical terms it is valid for the values of the various
constants that you will encounter in the real world near earth.
Note that I am not attempting to discuss anything relating to
The important reflection region is located far away from your
antenna site and is not strongly affected by the electrical
conductivity of the ground in its locality. The reflection is
supported at the air / ground interface and is mostly due to the
difference between the speed of light in air versus the speed of
light in the region near and in the earth. This is primarily due
to the difference in the relative dielectric constant of the two
media. For air e = (very close to) 1 and for earth e is
generally taken to be 13 approximately.
In a heavily vegetated region, the reflection plane (for HF) is
actually above ground by approximately the amount of the height
of the vegetation. You should figure the "effective height" of
your horizontal antennas as the distance above the plane where
the effective dielectric constant begins to differ significantly
from 1.0. In the horizontal plane, the important part of this
region begins at about one wavelength from your tower and extends
out to 20 (or more depending on antenna height) wavelengths
For an antenna at a height of a half wavelength, the "ground" in
the region 1 wavelength out from the tower is affecting the
takeoff angles near 30 degrees. For a takeoff angle of 2
degrees, the reflection zone of importance is out approximately
14 wavelengths. The high angle reflections are more dependent on
ground conductivities. By the time you get below takeoff angles
of 20 degrees or so, the conductivity is much less important but
the reflection is strongly dielectric constant gradient
I may be reading your post incorrectly, but if you counted the
water table distance the way I think you meant to, you would be
tempted to install the antennas below the optimum height.
73, Eric N7CL
>Date: Sat, 07 Aug 1999 08:04:47 -0400
>From: Dave Jordan <email@example.com>
>Reading N4KG's response to Steve (TH11 and Shorty 40 , Revisited)
>got me wondering about my own antenna plans for the farm...I have
>a couple of 100ft towers that I'd like to put up down south in
>the Northern Neck of Virginia. The land is sandy loam on the
>edge of a very large creek of tidal water from the Chesapeake.
>The water is salty (brakish). We've done some excavating on the
>property and the land is pretty dry and about 25ft above the
>creek water level. I presume the surface water is somewhere below
>the 25ft level. So, I wonder are my towers sitting on a big
>insulator made of sand -- if I should be adding the depth "to"
>the water in the creek to the effective height of my beams when
>considering placement on the tower? If the beams are installed
>120ft above the sand is it really more like 145ft+???
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Administrative requests: towertalk-REQUEST@contesting.com