[TowerTalk] Terrain Effects

[Jim Lux]

To my recollection, HFTA does geometric theory of diffraction, in a 2D
plane. So there is a big difference between a flat 3 degree slope and a 1000
ft high ridge 4 miles away. The key here is the size of the features in
terms of wavelengths: knife edges vs rounded mountain tops. It's also 2D, so
a conical needle sticking up would model the same as a circular ridge would
model the same as a straight ridge, even though in real life the propagation
would be different.

I don't think it works as far out as 30 miles, either (although, perhaps the
limit is on the number of points defining the terrain).  I don't know if it
takes into account the curvature of the earth (which would be important at
30 mi kinds of distances).


----- Original Message -----
From: "Michael Tope" <W4EF at dellroy.com>
To: "TowerTalk" <towertalk at contesting.com>
Sent: Thursday, December 02, 2004 11:37 PM
Subject: [TowerTalk] Terrain Effects


> Using tools like TA has anyone compared the effects of
> very close-in terrain to distant horizon clutter. Specifically
> which would be more detrimental to take-off angle, an HF
> antenna mounted directly over a gentle 3 degree upward
> slope for a mile or two, or that same antenna mounted over
> flat terrain which looks at a range of +3 degree high mountains
> say 30 miles off in the distance? Simplistically, it seems
> that these two cases would be about equal in terms of their
> impact on TOA, but the high mountains might provide
> some interesting diffraction effects, whereas the flat local
> slope would probably just skew the antenna elevation
> pattern up +3 degrees.
>
> I was looking at a site (rural setting with a very low noise
> floor) that I was considering for portable operation that
> exhibits a gentle upward slope toward the SW with a
> nearby visual horizon (+0.5 miles away) that is approximately
> concident with the mountains that are ~30 miles away. Both
> the close-in and far distance visual horizons sit at about 2.5
> degrees elevation (e.g. you can see the mountain ridges
> peaking just slightly above the close-in visual horizon). In
> other directions the terrain is very favorable. Any thoughts
> on this?
>
> Thanks,
>
> Mike, W4EF..........................................
>
>
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