>From: km1h@juno.com (km1h @ juno.com)
To: <topband@contesting.com>
>Date: Wed, 18 Mar 1998 10:25:31 EST
>
Snip...
>Re..your other post and the April '98 QST comments by
>N6BV... Does not the military use NEC-4xx to model their field
>installed and stealth antennas? The question remains; are those
>models accurate to within < 1dB?
>
In a lot of cases they can be. Just as other modeling based on
previous versions of the NEC code can be. In this specific case
of correctly accounting for all of the system losses of a
vertical with a radial field of varying density under it, at
varying distances in close proximity to earth, apparently not. I
say this because they report results that do not agree with
measurements taken at various times spanning many years in
various locations by various researchers.
>If they are accurate, then how do we justify the 5dB or greater
>claims as mentioned by others?
>
If they were accurate, you couldn't. The real question is: "If
the models don't report the losses actually measured for a
particular case, why cling to the modeling results?"
By the way... I personally use several of the modeling programs
both for my hobby and at work. I find them to be extremely
useful. I find them to be even more useful if you understand
their limitations (which I don't pretend to do completely), and
can devise model adjustments and/or compensations to mitigate the
effects of the limitations. But from MININEC right on up, I have
found that in general, unless you have a pathological model
construction, they get the pattern SHAPE very nearly exactly
correct when fed the correct constants for conductor and ground
losses.
I'm not running down the modeling programs at all. I just
recognize that for this particular case, in my experience, they
do not completely agree with reality. They still permit me to
gain vastly more insight into the antenna systems than I could
without the models to play with.
73, Eric N7CL
--
FAQ on WWW: http://www.contesting.com/topband.html
Submissions: topband@contesting.com
Administrative requests: topband-REQUEST@contesting.com
Problems: owner-topband@contesting.com
|