On 8 Dec, 18:17, "Mike Kaliski" wrote:
"Roy Lewallen" wrote in message

...





Mike Kaliski wrote:
. . .
NEC based programs follow well proven principles, but are not the holy
grail in being able to predict the performance of all antenna types or
configurations. They can accurately predict 'established' antenna design
performance in most circumstances and give a useful insight into what
properties a new and previously untried design might have. . .

They are used daily by a very large number of people to predict, with
excellent accuracy, how new and untried antenna designs will work.

The programs are, of necessity, constrained by the accuracy of the
physical measurements and formulae used in writing the original program.

That's not quite correct. The "formulae" used are very fundamental
electromagnetic equations. The "physical measurements" used are
fundamental constants such as the speed of light and permittivity and
permeability of free space, all of which are known to much greater
accuracy than antenna performance measurements are capable of delivering.
So neither constitutes any real constraint on program accuracy. The main
constraints are the inability to model all physical structures. For
example, NEC has no provision for including a slab of dielectric, making
microstrip antennas impossible to model accurately (except the rare ones
with air dielectric). There are many such constraints, of which the user
has to be acutely aware. The programs are very accurate if, and only if,
the model is indeed representative of the physical antenna.

My point entirely - It is impossible to take into account absolutely every
factor that someone might want to model. It is either so esoteric as to be a
'one off', or someone wishes to work outside of the constraints of accepted
design principles.





I do not consider the challenge to Newton's Laws to be valid. Just
because there is no true vacuum anywhere in the universe, as far as can
be established, then all objects will eventually come to rest due to
friction. This does not invalidate the premis that in the absence of
friction or any other external influence, an object would continue in
motion along a straight path forever.

This would be funny if it weren't sad. This newsgroup is one of the few
places I can think of where the silly statement I posted about moving
bodies and friction would be taken seriously. But it's really no surprise,
since it's much less unreasonable than the imaginative alternative
theories which are seriously presented, and just as seriously argued, here
daily.

My postulate about objects in motion was a parody of Cecil's rejection of
theoretical cases on the basis that they can't exist in practice, my
intent being to show how such a rejection leads to incorrect results. But
I see it's drawing the same serious response as Cecil's and Art's
postings. All that's missing is one of Richard's quotes from Terman and
support from Derek.

Well I felt I had to comment in case someone was taking your remark
seriously. As you rightly say there have been plenty of arguments over many
less obvious tongue in cheek remarks.





Ultimately whatever predictions are made by computer modelling programs
or theory, the only measure of success is by physical measurement in the
real world.

That's almost true. It's important also to know that even moderately
accurate antenna measurements are extremely difficult and exacting to
make. Consequently, there are often cases where model results are a better
indicator of an antenna's performance than measured ones. Antennas and
models follow the same physical laws. So to the extent that the model
imitates reality, the results are the same.

This philosophy isn't restricted to antenna modeling. Circuit designers
use programs to model circuits which can't be measured. I've designed
circuitry which operates from a few GHz to several tens of GHz, which is
still in production. It's simply impossible to measure voltages and
currents in such circuits, so modeling is heavily used. Often, every PC
board or hybrid circuit pad and trace has to be included as a transmission
line model; I've even had to build a transmission line model of a
capacitor which was 80 mils long. That the overall performance matched the
models' predictions is evidence that the models matched the physical
circuits. IC designers rely entirely on models to predict the performance
of circuits which can be measured only after huge financial investments to
commit the model results to physical reality. The principles are exactly
the same for antenna modeling as for circuit modeling.

Roy Lewallen, W7EL

I think we are in agreement Roy that the doubts cast on computer modelling
are generally raised by those who have an incomplete understanding of the
problem they are trying to model. While it may be fun to chuck in some
random figures to see what happens, the results are unlikely to be of use
without a thorough understanding of the basics. That only comes through a
lot of hard work and experience.

Regards

Mike G0ULI- Hide quoted text -

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Mike let me put things into perspective. I use the programs a lot
and trust them. At the same time there is an article in a compendium
that relates the difficulties they have had with calculations meeting
empirical solutions.They then added a condition which infers that
the time varient used is that of the current.But when I used the
computor
by following Gauss and the terms of equilibrium the result is a tank
circuit which contradicts the added condition. So in a sense the
program corroberates my work despite the imposed condition which
the program itself describes as erronious.
When I read of the difficulties associated with antenna programs
and how they were adjusted especially with an erronious condition
which is not allowable in mathematics then the corroberation of
my work is seriously strained to what appears as the correct results
for the wrong reasons. A specific case is where using magnetic and
electrical standards one can use vectors in a known direction
the summation of which is not parallel to the axis of the radiator.
The paradox is that the computor results are in line with Maxwells
laws
when one is confined to a situation of equilibrium demanded by Gauss.
Thus the question is how can this be if the program itself has
imposed
a condition around the laws of Maxwell whereas the program itself
overrules the validity of that same condition.
This obviously leaves the computor analysis in a precarious position.
For myself I would like to know how the programmers implemented this
condition they imposed and how it affected derived results.If this
inconsistency is not resolved to my satisfaction then the thesis
is tenacious at best and subject to serious question.
Best Regards
Art Unwin KB9MZ....xg (uk)