Tom,

I thought I'd quoted some numbers in the related "Dish reflector"
thread - apologies if I did not. Here are some mo

* I modelled a coil as a spiral in EZNEC (40T, diameter=6",
length=12", #14 copper wire)
* I added a 6ft "stinger" and found the frequency where the
combination was resonant: 3.79 MHz
* I checked the feedpoint impedance without the coil present: 0.46-
j2439
* That tells me the "lumped circuit equivalent" reactance of the coil
at 3.79 MHz is +j2439 ohms
* I found the frequency where the coil was resonant with no "stinger":
6.2 MHz

Now I look at what ON4AA's "Corum method" inductance calculator tells
me:

* "Lumped circuit equivalent" reactance at 3.79 MHz: +j2449
* Self-resonant frequency: 6.3 MHz

Unless I'm missing an option, if I want to predict the RF
characteristics of a "bugcatcher" it seems I have 3 choices:

* Use Wheeler's formula
* Build a helical model in EZNEC
* Use the Corum method

Wheeler's formula is inappropriate at frequencies close to a coil's
SRF.

EZNEC and the Corum method give very close results. The Corum formulas
are not difficult to use; even if they were, there is an on-line
calculator which removes the need for any maths. So it seems to me the
Corum formulas would be the more convenient tool to use, at least for
a "first look".

73,
Steve G3TXQ


On May 9, 7:06*am, "Tom Donaly" wrote:

You know, you haven't shown that the Corum model accurately measures
the bugcatcher coil. You have stated - and I have no reason to
disbelieve you - that the Corum model agrees with EZNEC. If that's the
case, it's just as easy to use EZNEC, right or wrong. MoM is a method of
obtaining numerical solutions to integral equations. The only reason to
do that is if symbolic solutions are either too difficult or impossible
to puzzle out of those same integral equations. In other words, some
very deep thinkers decided that MoM would give results superior to
algebraic approximations and hand waving, so they applied it to antenna
analysis. I don't think it's perfect. It's certainly useful. If you
think Corum is good enough for your purposes, though, go for it.