NEC-LIST: SM Patches

The patch discussion has elicited the standard cautions about using patches.
However, there are some models floating about on the web that use patches
(SM commands) to simulate flat surfaces (without forming a significant
volume). I recall a horn model using this technique. Since the surfaces do
not require direct excitation, the question raised by such models is whether
their results are useful.

I did some comparative modeling using wire-grid structures and SM structures
for both planar and corner reflectors. If the SM structure is correctly
oriented for the driver ahead of it, it will yield ballpark results
comparable but not identical to a wire-grid structure. However, one must
use care when joining SM-based structures, as in a corner reflector, or the
results will be meaningless, since the vectors will not simulate reflection
from the desired surfaces. As well, the SM structures do not show the
effects to the rear from edge diffraction as well as do the wire-grid
structures. The apparent advantage of the SM structures is speed of the
run. One has to know that the structure is right if one uses something like
the SM to approximate a correct design before committing to the wire-grid
reflector and its longer run time. For such work, I personally find it less
troublesome to start with a collection of wire-grid reflectors in Green's
files, and then investigate various drivers by choosing reflector results
serially.

L. B. Cebik

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