Re: NEC-LIST: Currents

The segment current in NEC is the current in that segment, with the
excitation of the antenna being whatever you specified on the EX card. If
you look at the segment that corresponds to the feedpoint, you'll see that
the current in that segment is the same as the feedpoint current in the
"feedpoint" section of the output. The apparent Z and Y are just calculated
by current and voltage (Z=E/I, Y=I/E).

As for finding the answer in the source code... I'd have looked in the
program documentation, particularly Part 1 and Part 2, which describe in
quite some detail how NEC works, what it computes, etc. While well
documented code is a "good thing", one shouldn't expect the entire theory of
operation, nor the "how to use it" to be in the comments. I've found the
comments in the NEC code fairly complete, assuming one knows what the
program is trying to do (i.e. solve the general method of moments problem)
and how it does it. Also, don't forget that NEC was written in the early
80's, in a development environment that differs quite substantially from
what is "usual" today. To have the theory separately documented from the
code would be (and still is) standard practice, if only because the theory
and program design would have to go through design reviews before
implementation.

There is also a certain expection of sophistication and knowledge on the
part of the creators of NEC2. That is, they probably assumed that the
original intended users of NEC2 would be knowledgable about
method-of-moments, antenna simulation theory, etc. I doubt that the original
authors ever expected that thousands of people would be running dozens of
NEC2 models on PC's at home. In 1981, running a 1000 segment model would
have required some serious (and very expensive) computer resources: I sure
wouldn't have been running it on my CP/M Z80 machine with 48K of RAM. The
"mass market" use of MoM codes has been greatly facilitated by later
products like EZNEC, 4nec2, etc.etc.etc. which, to a certain extent, reduce
the need to understand EM modeling theory. This is, as always at the risk
that you'll "get bit" by some inherent limitation... There have been some
good articles over the years in the amateur radio literature about
understanding the limitations of MoM codes, etc.

I think that the endurance of the basic NEC2 code is a testament to the
design process which created it in the first place. There aren't many other
software packages that are 20+ years old that are being run by casual users
in a form substantially unchanged from the original.

----- Original Message -----
From: "Rob" <rob_at_pythonemproject.com>
To: "Pat Foster" <prf_at_maasdesign.co.uk>; <nec-list_at_gweep.ca>
Sent: Saturday, December 07, 2002 8:33 AM
Subject: Re: NEC-LIST: Currents

> ASAP will do this, and give you a nice list. You'd have to look at the
> code. I believe the current from a 1V rms source into Zin at the
> antenna terminals is the default source, which then gives/prints the
> mag/phase current on each segment. The current is what the matrix
> routine solves for. Thats a big plus for readable source code.
>
> I think NEC does the same, but its not a symmetrical matrix like ASAP,
> so LU decomposition is used to solve it. Eznec gives a nice relative
> plot of all of the currents, so thats what I use when I want a pretty
> picture of antenna currents.
>
> Rob.
>
> Pat Foster wrote:
> >
> > It is possible to output the currents on all segments of a structure in
NEC.
> > But exactly what current is being output? I have hunted through the code
but
> > have not had much joy.
> >
> > Regards
> >
> > Pat Foster
> >
> > ***********************************
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> > Microwave and Antenna Systems
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> > Malvern
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> > email prf_at_maasdesign.co.uk
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