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Computer model experiment
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. |
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