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Tom Donaly · May 10th 09, 10:52 PM posted to rec.radio.amateur.antenna

wrote:
Tom,

You posted earlier "Why would anyone use MoM if there were a set of
symbolic equations that would work just as well?"

We now have a situation where MoM (EZNEC) and a set of symbolic
equations (Corum method) are predicting very similar SRFs.

"Verbal sparring" about measurement methods aside, I think it's
important to try to understand what's causing the differences between
the EZNEC/Corum value and the measured value. Of course it's possible
that EZNEC is in error, but it seems odd to me that the Corum value
(derived by a completely unrelated technique) would also produce the
same erroneous value.

My EZNEC model had the coil 12" above Real Mininec ground, and
connected to ground via a 12" vertical wire with the source in it. I
simply looked for the frequency at which the source impedance was
purely resistive. The SRF was relatively insensitive to the height of
the coil above ground, and changing the ground type made no
difference.

I wonder if that model is anything like your own test set-up, and if
not whether the differences could explain the different SRFs we're
seeing.

73,
Steve G3TXQ

The presence of anything at all near the coil should lower its resonant
frequency. Even the measuring apparatus should have an effect. I think
it would require very careful planning and implementation to find an
exact resonant frequency. You'd have to ask Richard Clark how to do it
if you wanted high accuracy. I'm unwilling to find fault with either
Corum or EZNEC at this point. Making accurate models can be just
as hard as making valid experiments, and I wish you luck working with
your models. I do urge you to make a coil to test your results against,
though. It isn't difficult, and with a little help from some of your
fellow experimenters, you should get results that are very close to
being meaningful.
73,
Tom Donaly, KA6RUH

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