Eric: Your reply re-kindled a question that I have always had about my
installation. I have a TH-7 at 95' in a small clearing surrounded by
heavy vegetation (trees) averaging 80' in height. Using your logic
below, I would surmise that my antenna is 15' above the effective
reflecting surface, which I know is not the case, based on the
performance of the antenna.
My suspicion is that the trees attenuateand/or scatter the reflection
path, so that the reflected signal has less strength, and therefore
contributes less to the combined signal. Another way of looking at this
is that the transition from air to trees is not abrupt, but gradual and
variable. Probably has more of a scattering/attenuating characteristic
than a ground surface. Not to mention the complications that come with
wet vs. dry vegetation. I figure this is good and bad - my effective
angle is probably lower (if there was 100% attenuation it would be 0
degrees?), but on the other hand I loose the gain factor in the main
lobe from the contribution of the reflected signal. Is this close to
reality?
Another point to ponder. I have stuck with an antenna on top of the
tower. If I were to stack another antenna below the treeline, would the
contribution be significant enough to make the investment worth while?
How does one evaluate an antenna in this type of location?
Thanks for your effort to help us all better understand antennas.
73, Wayne N1WR
Mchael D. Ihry wrote:
>
> Hi Dave,
>
> 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
> vertical antennas.
>
> 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
> radius.
>
> 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
> dependent.
>
> 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
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