There is a great difference with a model that allows to design an
antenna, a model for predicting propagation or a model that tries to
show as an antenna will perform over a media.
When is sure that one can define a parameter, i.e. a certain height
from ground or a certain number and lenght of radials, then it's
perfectly possible to trust results, knowing that those results are
true in that specifical condition.
In other words, I can design with optimal results and precision a
complex antenna in free space or an horizontal array like a yagi over a
real ground if this ground is far enough to allow me to neglect its
effect within inherent antenna performances in term of elements
detuning.
On the other end I must be aware that when parameters are simplyfied
and assumed instead of beeing real and over full control, ground
reflections and morfology, what I get is a result that is valid within
the assumption I made but not necessarily with the real world, if this
differs from the model.
73,
Mauri I4JMY
> >The REASON we have models is the actual system is too complex to
> >consider. While the models are certainly very good in some aspects,
> >they always miss some things.
>
>
> This is certainly true, but there is another good reason, and that is
that
> the sheer magnitude of the task required to accurately measure a pheno
menon
> is impractical.
>
> I remember that back in the 60's and 70's VOA used to employ people wh
ose
> job involved monitoring VOA transmitters' signals at various locations
> around the globe. Expensive, if you're going to compile a large-
enough
> data set to be more than anecdotal. They adopted VOACAP as their stan
dard
> for propagation analysis and transmitter siting in the 80's, presumabl
y
> after satisfying themselves that its results were sufficiently consist
ent
> with the empirical data.
>
> I've always wondered, though, how they made their decisions on their b
ig MW
> installations, since these would be near or below the threshhold of
> accuracy of VOACAP.
>
> One final comment, on the utility to giving NEC model results to two
> decimal places. For those of us who are still learning how to use the
se
> programs effectively, it can be useful to have these numbers, even tho
ugh
> we know the experimental accuracy may be in the +/- 1 dB range. For
> example, one important source of error in these models is not using en
ough
> segments to get convergence. If I look at Bill's results, and they ar
en't
> the same as mine, then I want to know why, and in the process I can le
arn
> something.
>
>
> 73, Pete N4ZR
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