Re: NEC-LIST: Significance of ?/10 segments in dipole design

At 09:25 AM 7/12/2004 -0700, wadavis_at_vt.edu wrote:
>Actually, you need to be careful in the assumption about the
>leveling off of the impedance. As I recall, NEC tends to use the
>thin-wire approximation. If you continue to increase the number of
>unknown, it will level off but never converge. The problem has to
>do with the source modeling as the number of unknowns is increased.
> This was documented in an APS paper back in the 70s. An easy way
>to see this phenomena is to plot the resistance or reactance as a
>function of (1/N) as the number of segments N is increased.
>
>However, in general your comment is correct. Bill
>
>
> > Yes lambda/10 would be the upper limit. You can explore this by
> > modeling the dipole with an increasing number of segments. At some
> > point the impedance and resonant impedance will level off.
> >

There's also the issue when segment length starts getting close to the wire
radius.

Now that computers are so fast, and there are a lot of front ends for NEC
that allow symbolic substitution, it's easy to run a bunch of models with
different segmentation to look at where the segments start getting too long
or short. This is the true value of this kind of modeling, because you can
do it quickly enough to get insight and understanding rather than just
printing out a bunch of numbers.

James Lux, P.E.
Spacecraft Telecommunications 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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