NEC-LIST: Negative VSWR (!) -- some comments.

Dear folks,

Recently we was working on a wide band design - consisting of stack of
dipoles at different bands inclined at 45degrees to ground - and noticed
we were getting negative values of VSWR for many frequencies.

To cut a long story short, the reason was that the parallel feed lines
were barely 1.5*radius apart and NEC resultantly was giving some numerical
error. This caused a huge overshoot in VSWR graph, and apparently when it
overshot the maximum signed integer that can be stored by the computer,
the computer rolled back ,and stored the number as negative.

Now the NEC manual available at http://www.nec2.org/, in its WIRE MODELING
section, says that, and I quote

"Although extensive tests have not been conducted, it is safe to specify
that wires should be several radii apart."

Well, one may indeed quantify the "several radii" aspect, because
numerical unstability is severe for wires so close.

Thus, the purpose of this mail is to highlight following two points:

1]. If you do get negative VSWR's, a most probable explanation is roll
back of number storage by the computer,

2]. The problem of parallel wires being within a few radii is quite
serious, and one should attempt to quantify the minimum separation for
which NEC is stable.

Hope this information is of help to you all,

Thank you,

Regards,
Nikhil Mehta
Visiting Student
Raman Research Institute
Sadashivnagar Bangalore 560 080
Ph.: 09886229248

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