On May 11, 1:38*pm, Jim Lux wrote:
Ralph Mowery wrote:
"tom" wrote in message
et...
On 5/10/2010 3:12 PM, wrote:
As Clint said in the wonderful old movie, "A man's gotta know his limits".
For antenna modelers it should read, "A man's gotta know the program's
limits".

Of course, Art thinks things have changed and the computer modelers have a
better grasp upon reality than the ones even he calls "the masters". He is
an example of the blind man leading himself.

tom
K0TAR

The computer program should know its limits.

yes and no. *For EM modeling codes originally intended for use by
sophisticated users with a knowledge of the limitations of numerical
analysis, they might assume the user knows enough to formulate models
that are "well conditioned", or how to experiment to determine this.
NEC is the leading example here. It doesn't do much checking of the
inputs, and assumes *you know what you are doing.

There were modeling articles in ARRL pubs 20 years ago that described
one way to do this at a simple level: changing the number of segments in
the model and seeing if the results change. *The "average gain test" is
another way.

In many cases, the constraints on the model are not simply representable
(a lot of "it depends"), so that raises an issue for a "design rule
checker" that is reasonably robust. *Some products that use NEC as the
backend put a checker on the front (4nec2, for instance, warns you about
length/diameter ratios, almost intersections, and the like)

It's sort of like power tools vs hand tools. *The assumption is that the
user of the power tool knows how to use it.

* Anytine a program allows the

data entered to be too large or small for the calculations, it should be
flagged as being out of range. *Also many computer programs will use
simplified formulars that can mast the true outcome. *Usually it is not very
much, but as all errors start to add up the end results may be way off.

There's whole books written on this for NEC. *Part I of the NEC
documents, in particular, discusses this. *There's also a huge
professional literature on various FEM computational techniques and
their limitations. *NEC, like most numerical codes (for mechanics,
thermal, as well as EM), is very much a chainsaw without safety guards.
* It's up to the user to wear gloves and goggles and not cut their leg off.

Jim Lux of NASA no less!
All of the programs clearly state that they are based on Maxwells
equations. The bottom line of that equation is that for accountability
for all forces involved are required and where the summation of all
equals zero. This is nothing new and has been followed thru for
centuries. The equations requires first and formost equilibrium and
what the program supplies is easily checked that it meets these
requirements. It is very simple. Showing that the solution is that
inside an arbitrary boundary all within as with the whole must be
resonant and in equilibrium.It requires no more than that to show if
the program has achieved its object. I understand your preachings but
you presented no point that can be discussed.
Now you will respond that I must do such and such to back the
statement above despite that those requirements are the basis of
physics. So to you I will supply the same that I have supplied to
others which they reject, no one has stated why.
A arbitrary gaussian border containing static particles
( not waves as many summize. Gauss was very clear about the presence
of static particles) in equilibrium may be made dynamic by the
addition of a time varying field such that Maxell's equations can be
applied to solve.I have stated the over checks that can be applied to
provide correctness of this procedure. You may, of course, join the
poll that swells on behalf of NASA in opposition to the above but it
would provide me a great deal of delight if you provided more than to
just say "I am wrong". Nobody as yet provided one mathematical reason
that disputes the above, so in the absence of such you will not be
alone, only your credibility suffers but you will remain in the
majority of the poll in the eyes of the ham radio World.
Regards
Art Unwin