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#8
May 11th 10, 07:38 PM
posted to rec.radio.amateur.antenna
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Computer model experiment
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. |
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