Art Unwin wrote:
On May 11, 1:38 pm, Jim Lux wrote:

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.

Jim Lux of NASA no less!
Speaking, however, as Jim Lux, engineer, not necessarily on NASA's behalf.

All of the programs clearly state that they are based on Maxwells
equations.
snip
I understand your preachings but
you presented no point that can be discussed.


While NEC and its ilk are clearly based on Maxwell's equations, one
should realize that they do not provide an analytical closed form
solution, but, rather, are numerical approximations, and are subject to
all the limitations inherent in that. They solve for the currents by
the method of moments, which is but one way to find a solution, and one
that happens to work quite well with things made of wires.

Within the limits of computational precision, for simple cases, where
analytical solutions are known to exist, the results of NEC and the
analytical solution are identical. That's what validation of the code
is all about.

Further, where there is no analytical solution available, measured data
on an actual antenna matches that predicted by the model, within
experimental uncertainty.

In both of the above situations, the validation has been done many
times, by many people, other than the original authors of the software,
so NEC fits in the category of "high quality validated modeling tools".

This does not mean, however, that just because NEC is based on Maxwell's
equations that you can take anything that is solvable with Maxwell and
it will be equally solvable in NEC.

I suspect that one could take the NEC algorithms, and implement a
modeling code for, say, a dipole, using an arbitrary precision math
package and get results that are accurate to any desired degree. This
would be a lot of work.

It's unclear that this would be useful, except perhaps as an
extraordinary proof for an extraordinary claim (e.g. a magic antenna
that "can't be modeled in NEC"). However, once you've done all that
software development, you'd need independent verification that you
correctly implemented it.

This is where a lot of the newer modeling codes come from (e.g. FDTD):
they are designed to model things that a method of moments code can't do
effectively.