RadioBanter - Dual-Z0 Stubs

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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

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

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.

Please describe your test setup. Where was ground?
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

Cecil Moore wrote:
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.

Please describe your test setup. Where was ground?

I'll tell you what, Cecil. If you'll test your coil, and
describe how you did it, I'll tell you what I did. Fair?
I know you're champing at the bit to claim my test setup was all
wrong and that I used standing wave current when I should have
been using traveling wave current, and all that, but you'll just
have to wait. I reported this in order to get you keyboard theorists
off your fundaments and start dealing with the real world. If you don't,
you'll forever be plagued by the nagging thought that I might be right.
Horrors!
73,
Tom Donaly, KA6RUH

(P.S. If you don't know how to test for a coil's self-resonant
frequency, you should go back to Texas A&M and ask for your money
back.)

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

Tom Donaly wrote:
I know you're champing at the bit to claim my test setup was all
wrong and that I used standing wave current when I should have
been using traveling wave current, and all that, but you'll just
have to wait.

That's not true at all, Tom. It's a trivial procedure
to use standing waves to determine the approximate 1/4WL
self-resonant frequency. But as with 1/4WL monopoles,
there is *NO 1/4WL self-resonant frequency* without a
ground plane or counterpoise and the quality of the
ground-plane/counterpoise has an effect on the self-
resonant frequency. Sorry, I don't have anything
approaching a perfect ground plane for 4 MHz.

I suppose one could use two identical coils and turn it
into a dipole but I don't have another 75m Texas Bugcatcher
loading coil.

Let me say once again: A 1/4WL monopole, all by itself
in free space is *NOT resonant*. After all, making guy wire
segments 1/4WL long is one way of breaking up their resonance.

We are not looking for super high accuracy/precision/resolution
here. Almost everything is an approximation because we don't
share exactly the same test environments. All I am after is
the technical truth - there's nothing personal involved.

Neither is a 1/4WL coil self-resonant all by itself in free
space. I don't know what Reg was thinking if he advised hanging
the coil from the ceiling without a counterpoise.

A traveling wave can be used to determine 1/4WL self-resonance
but it is a little more complicated than using an MFJ-259B
with standing waves. A load resistor minimizes reflections
while current probes are used to measure the phase shift through
the coil. When the phase shift is 90 degrees, that's the 1/4WL
self-resonant frequency.

I'll tell you what, Cecil. If you'll test your coil, and
describe how you did it, I'll tell you what I did. Fair?

1. I connected my 75m Texas Bugcatcher coil through about
a foot of wire to the bumper mount on my GMC pickup and
connected my MFJ-259B to the coax connector under the
bumper. I tuned for lowest impedance above 4 MHz. The
1/4WL self-resonant frequency was ~6.6 MHz. These
measurements were done and reported in March, 2006.

2. I used a tri-mag mount sitting on the hood of my GMC
pickup fed through ~6' of coax. The 1/4WL self-resonant
frequency was ~6.9 MHz.

3. I used a traveling wave on top of a wooden bench and
found the frequency at which the phase shift through the
coil was ~90 degrees. That frequency was ~7.2 MHz.

4. I modeled the coil with EZNEC and got some segmentation
length warnings. EZNEC reported the 1/4WL self-resonant
frequency to be 7.96 MHz.

The spread in the above frequency figures is about +/-8%.

Again Tom, the only thing I am after is the technical
truth. If that is also what you are after, we should
have no personal conflicts.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

Cecil Moore wrote:
4. I modeled the coil with EZNEC and got some segmentation
length warnings. EZNEC reported the 1/4WL self-resonant
frequency to be 7.96 MHz.

The above modeling was done using a traveling wave,
i.e. with the coil terminated in its Z0.

5. Using standing waves, i.e. no termniation, EZNEC
said the 1/4WL self-resonant frequency was 7.724 MHz.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

Tom Donaly wrote:
The presence of anything at all near the coil should lower its resonant
frequency.

That's my experience. Conversely, the farther away from the
coil the ground plane is located, the higher the self-resonant
frequency. The location of the ground plane has a significant
effect on the self-resonant frequency of the coil.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com