I won't retrace the ground that K1TTT has already covered, but just want to
add a few points.
The computer codes that are used for modeling (currently NEC-2 and NEC-4 in
amateur circles) have been fairly well verified aganst range data,
including the famous helicopter tests by WA3FET and others.
All of these modeling methods have specific limitations -- circumstances
they don't model well, or special precautions that need to be observed in
particular situations. I think it is equally important to be sure that a
particular model has converged -- that is, breaking up the antenna into
shorter segments does not produce major changes in the gain or pattern.
I've been bitten by THAT one a few times.
For real-world terrain, you definitely need to combine antenna modeling
with terrain modeling (YT or TA). When it comes to interactions with other
antennas, I have found it useful to maintain a model of my entire station
(including tower and guy wires as well as antennas). I'm convinced this
has saved me a few times from making changes that would have had undesired
side-effects on other antennas in the mix. The only problem with EZNEC in
this application is that its 500-segment limit is easily exceeded on the
higher frequencies.
And finally, it's worth mentioning that ARRL offered to accept gain and
pattern claims in antenna advertising a couple of years ago if
manufacturers would submit NEC-4 models of their antennas for verification.
73, Pete N4ZR
Sometimes a tower is
just a tower
________________________________________________________________________
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Order online at http://store.eham.net.
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