At 07:45 PM 2/3/02 -0500, Tom Rauch wrote:
>Do you really think NEC models can predict the effective height of
>and antenna over earth within five feet? I don't.
>I don't believe it is possible because none of us have homogeneous
>earth below the antenna, as NEC models assume it is, let alone
>have areas free of things like wiring, fences, and so on.
>Now granted it is the best thing going for us, but predicting null
>patterns at four wavelengths height or null angles within a few
>degrees elevation? I wouldn't bet on it. Not unless you live in the
>middle of many wavelengths of empty flat metal sheeting.
Tom, I think you're getting the horizontal and vertical polarization cases
mixed, with maybe also a little too much focus on the 160m case. In his
book, Dave Leeson observes that ground characteristics have relatively
little to do with vertical pattern formation from horizontally polarized
antennas, because "the reflection is from the conductive or dielectric
discontinuity between the air and the surface of the ground(giving effect
to skin depth)..." As for smoothness, he observes that "for radio waves in
the HF band this shows that ground nearly always behaves as a perfectly
smoth surface, particularly for low angles of incidence."
Thre most precise tool available to amateurs for modeling the vertical
radiation pattern of horizontally-polarized antennas over ground appears to
be K6STI's TA, because it can accept OpenPF antenna pattern files that
allow it effectively to model any such antenna over irregular ground. To
get the data needed for those OpenPF files, however, you need first to use
NEC to model the antenna over a perfectly flat foreground.
To sum up what I believe to be the truth of the matter, you really need
both tools, and even then, there will be cases where they will not be
reliable. Most of these cases are well-known, and in some of them remedies
are known (such as aligning segments of closely-separated parallel
conductors). As long as you stay within the limitations of the software,
and use good technique to build and evaluate the models, the saving in time
and effort over cut and try experimentation is immense.
73, Pete N4ZR
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just a tower
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