On 5/13/2011 7:32 AM, Diane & Edward Swynar wrote:
> IMHO, ALL of the antenna articles in QST---and everywhere else, for that
> matter---rely far too much upon modeling these days.
I'll take exactly the opposite view -- if you can't make it work in the
model, it isn't likely to work when you build it either. Modeling
software, like NEC, simply does the math for a design that YOU must
conceive. That math is nothing more (or less) than applying the
equations that have been developed over the years by careful study of
the laws of physics.
There are several major problems with modeling. First, properly plugging
the antenna into the modeling software. Second, knowing and
understanding the limitations of the modeling software (usually the
underlying equations and computation method) that the software uses. I
don't pretend to be an expert on NEC, but it's my understanding that a
primary weakness is in accurately modeling the effect of elements of the
antenna in close proximity to lossy earth. This can cause some
inaccuracy of the result in terms of EFFICIENCY. Third, NEC breaks an
antenna down into small pieces, computes the contribution of each piece,
and combines them. NEC has problems with elements that are very close
together, especially if the pieces are not equal.
For nearly 25 years, I worked as a consultant designing sound systems
for large public spaces, and I used modeling extensively. No question
that it works if carefully applied. Since I moved to CA and 8 acres of
trees, I've been doing a lot of modeling and then building antennas
based on what I learn. So far, I have yet to find an antenna whose
actual on-the-air performance does not come very close to what the model
predicts. The only differences for those 160M slopers I described
yesterday were 1) the result of interaction of the radials with the
earth (a known limitation of NEC), and 2) interaction of the wavefront
with the very dense redwood forest surrounding the antennas (how ya
gonna model that?) . Both of these effects are likely to increase loss a
bit, and that's the only difference I've seen.
Many years ago, I came to the conclusion that if you think there's a
difference between "theory" and the real world, you don't know enough
about one or the other. Nothing I've learned since has caused me to
change my thinking about that.
73, Jim K9YC
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