Re: NEC-LIST: Which nec variant, what are the limits?

At 09:50 AM 8/21/2005, Eric Fort wrote:
>After doing some looking at the various incantations of nec2 available
>there seems to be various limits upon model size and complexity as well as
>single vs. Double precision. What differences do these things make in
>practical use?

If you're using NEC2, don't even consider using the single precision
versions. That the two versions exist is more a manifestation of days gone
by, when DP ran 4 times slower than SP. Such is not really the case any
more, and besides, modern PCs are so fast, even if it WAS slower, you'd
never notice.

The model size thing has to do with the size of the arrays that are
dimensioned in the source code (FORTRAN doesn't (didn't) have dynamic
allocation). If you have limited RAM, you'd limit the size of the model to
avoid running "out of core" (i.e. swapping to disk) which is a LOT
slower. Again, with modern PCs having half a gig of RAM, it's less of an
issue.

>how complex do models really need to be (number of segments. Etc) to get
>decent results? I'd like to be able to model things like an array of 16,
>24 element yagis with decent results.

Experiment. Try it with different numbers and see how much the results
change. For half wave dipoles, I use 21 segments for run of the mill
models, unless there's something close to it.

>What practical differences will I see in result accuracy and speed of
>execution between single and double precision code?

Run double.

James Lux, P.E.
Spacecraft Radio Frequency Subsystems Group
Flight Communications Systems Section
Jet Propulsion Laboratory, Mail Stop 161-213
4800 Oak Grove Drive
Pasadena CA 91109
tel: (818)354-2075
fax: (818)393-6875

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