Richard Harrison wrote:
"A wire antenna is a circuit with distributed constants; hence the
current distribution in a wire antenna that results from the application
of a localized voltage follows the principles discussed in Chap. 4
(Transmission Lines), and depends upon the antenna length, measured in
wavelengths; the terminations at the ends of the antenna wire; and the
losses in the system."

In other words, an antenna acts a lot like a lossy
transmission line where the loss from the system
is radiation.

Here's a transmission line example using resistance
wire in a transmission line to emulate the losses
normally due to radiation in an antenna. Note that
the feedpoint impedance is around 35 ohms.

http://www.w5dxp.com/stub_dip.EZ
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

Tom,

OK, I tried what you suggested. I put my loading coil midway up a 20ft
vertical wire in the EZNEC model. I reduced the number of turns to
lift the resonant frequency to 5.6MHz. EZNEC predicted that the
magnitude of the current at the top of the coil would be 77% of the
magnitude at the bottom.

Then I removed the coil in the model, replaced it with a straight wire
containing an EZNEC lumped load, and adjusted that load for antenna
resonance at 5.6MHz again. I needed +j1630.

Given the dimensions of the coil, the Corum calculator predicted a
lumped circuit equivalent reactance of +j1573, and it predicted a
current fall-off across the coil of 78%.

73,
Steve G3TXQ


On May 9, 5:35*pm, K7ITM wrote:

Steve, this is fine for a base loading coil, but I'd suggest you try
your experiment with a loading coil well up the antenna, where the
coil is significantly larger diameter than the straight conductor in
which it's placed. *The same size coil you described (though
presumably a different number of turns), placed at least half way up
something like a 15 or 20 foot long thin wire, should illustrate the
point. *Is the EZNEC model then in such good agreement with placing a
reactive load at that point in the antenna, where the reactance is
from ON4AA's online calculator?

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

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

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

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

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