wrote:
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



Did you make such a coil and measure its self-resonant frequency? The
reason I ask is that I put the dimensions of an old coil I had
(D=155mm,length=140mm,wire diameter=1.3mm,N=27 turns) into ON4AA's
calculator and got a self resonant number of 7.4137 Mhz. When I measured
it, though, it was 8.93 Mhz. Where did I go wrong? Maybe I entered the
numbers incorrectly.
73,
Tom Donaly, KA6RUH

Tom,

I just modelled that coil in EZNEC and it was self-resonant at 7.5MHz
- very close to the Corum model prediction of 7.4137MHz.

Interesting that both EZNEC and the Corum are wrong and by the same
amount!

73,
Steve G3TXQ



On May 9, 10:06*pm, "Tom Donaly" wrote:

Did you make such a coil and measure its self-resonant frequency? The
reason I ask is that I put the dimensions of an old coil I had
(D=155mm,length=140mm,wire diameter=1.3mm,N=27 turns) into ON4AA's
calculator and got a self resonant number of 7.4137 Mhz. When I measured
it, though, it was 8.93 Mhz. Where did I go wrong? Maybe I entered the
numbers incorrectly.
73,
Tom Donaly, KA6RUH

wrote:
Tom,

I just modelled that coil in EZNEC and it was self-resonant at 7.5MHz
- very close to the Corum model prediction of 7.4137MHz.

Interesting that both EZNEC and the Corum are wrong and by the same
amount!

Did you guys agree on the same ground plane? The self resonant
frequency of my 75m Texas Bugcatcher coil is very different
on my insulated wooden work-bench vs two inches above my GMC
pickup.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

Cecil,

I tried mine above Perfect, Mininec Average, Mininec Salt Water. Self
resonant frequency was the same in each case.

Steve G3TXQ



On May 9, 10:49*pm, Cecil Moore wrote:

Did you guys agree on the same ground plane? The self resonant
frequency of my 75m Texas Bugcatcher coil is very different
on my insulated wooden work-bench vs two inches above my GMC
pickup.
--
73, Cecil, IEEE, OOTC, *http://www.w5dxp.com

Cecil Moore wrote:
wrote:
Tom,

I just modelled that coil in EZNEC and it was self-resonant at 7.5MHz
- very close to the Corum model prediction of 7.4137MHz.

Interesting that both EZNEC and the Corum are wrong and by the same
amount!

Did you guys agree on the same ground plane? The self resonant
frequency of my 75m Texas Bugcatcher coil is very different
on my insulated wooden work-bench vs two inches above my GMC
pickup.

I followed Reg's advice and hung mine from the ceiling with a thin
thread.
73,
Tom Donaly, KA6RUH

Tom Donaly wrote:
I followed Reg's advice and hung mine from the ceiling with a thin
thread.

That would certainly make it act differently
than over a good ground plane.

Where was your ground/counterpoise? Which self-
resonant frequency did you see? 1/4WL? 1/2WL?
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

wrote:
Tom,

I just modelled that coil in EZNEC and it was self-resonant at 7.5MHz
- very close to the Corum model prediction of 7.4137MHz.

Interesting that both EZNEC and the Corum are wrong and by the same
amount!

73,
Steve G3TXQ



On May 9, 10:06 pm, "Tom Donaly" wrote:
Did you make such a coil and measure its self-resonant frequency? The
reason I ask is that I put the dimensions of an old coil I had
(D=155mm,length=140mm,wire diameter=1.3mm,N=27 turns) into ON4AA's
calculator and got a self resonant number of 7.4137 Mhz. When I measured
it, though, it was 8.93 Mhz. Where did I go wrong? Maybe I entered the
numbers incorrectly.
73,
Tom Donaly, KA6RUH


I think it's interesting, too. Maybe someone better acquainted with
testing procedures than I am can resolve the difference.
73,
Tom Donaly, KA6RUH

Tom Donaly wrote:
I put the dimensions ... into ON4AA's
calculator and got a self resonant number of 7.4137 Mhz. When I measured
it, though, it was 8.93 Mhz. Where did I go wrong?

The self resonant frequency of a coil varies wildly
with the ground provided. What ground did you provide?
Perfect? Average? None?
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

Cecil Moore wrote:
Tom Donaly wrote:
I put the dimensions ... into ON4AA's
calculator and got a self resonant number of 7.4137 Mhz. When I measured
it, though, it was 8.93 Mhz. Where did I go wrong?

The self resonant frequency of a coil varies wildly
with the ground provided. What ground did you provide?
Perfect? Average? None?

If that's true, the Corum brothers should have included that in
their formulas.
73,
Tom Donaly, KA6RUH

Tom Donaly wrote:
If that's true, the Corum brothers should have included that in
their formulas.

Their formulas assume a near-perfect ground plane.
See Figure 2.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com