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Richard Clark · September 27th 06, 08:19 AM posted to rec.radio.amateur.antenna

On 27 Sep 2006 06:52:46 GMT, (John E. Davis)
wrote:

Please look at the ftp://space.mit.edu/pub/davis/misc/nec/swr.png
for a plot of the SWR using a spacing of 0.01 MHz. I suspect that the
noise that is showing up may be due to truncation error. I believe
that spacings of higher values, e.g., 0.2 MHz result in a different
sampling of the noise.

Yow! That is a lot of trash.

The version I am using (see
http://packages.debian.org/stable/hamradio/nec) contains this warning:

This version contains code which hasn't been extensively tested for
errors, which was input by hand from a report -- use with care. The
numerics are currently only SINGLE PRECISION.

Hi John,

Yes, this confirms the shift to double precision in EZNEC lowering
artifacts in the fine data.

However, I think it goes beyond simple matters of single or dual
precision math. When I was designing Fourier Analysis packages while
I was on contract to HP, I discovered there was a world of variability
in math library's transcendental functions.

Microsoft's product was abysmal, whereas Borland's was superlative. A
telling example is that for the transform of a sine wave into the
frequency domain under Microsoft math libraries, the noise floor was
at 60 to 80 dB below the fundamental peak with harmonics. When I
switched to Borland math libraries, there was a single bin response
and the noise floor plunged to 200dB down!

For others following this description, they may wonder at the
terminology of noise floor for a simple sine wave transform. Fourier
Analysis is done by parts through FFTs and this departs from classical
Fourier which is continuous. Because FFTs are discrete (bound by an
arbitrary start and stop), this injects spurious responses into the
transform. On top of that, rounding errors attributable to series
expansions of the transcendentals would give lower accuracy -
statistical (largely uncorrelated) noise. The sine wave transform was
a method of self-validation of the library used and Microsoft failed
big time (especially considering the 10:1 cost differential for the
more expensive M$).

If EZNEC were available for linux, I would look into it. Also, can it
be driven in "batch" mode without a GUI?

There have been various reports of success and failure when Windows
emulators have been used. I cannot report any personal experience
because my Linux machine is largely confined to Server development
(XAMMP/Wiki/MySQL/RubyOnRails).

73's
Richard Clark, KB7QHC