Ralph Mowery wrote:
"tom" wrote in message
t...
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.