The performance of a coaxial choke inserted at the antenna end of the
feedline cannot be properly studied without knowledge of the
feedline's length in wavelengths and how the feedline is terminated at
the transmitter end. No two systems are the same.

Choke performance depends on the condition of the system before the
choke is inserted. The choking effect depends on the impedance seen
by the choke looking into the outer conductor of the coaxial feedline.

The input impedance of the coaxial outer conductor depends on Zo, 400
to 550 ohms, of the single-wire line, its length in wavelengths, and
on the very uncertain resistance of the ground connection at the
transmitter end.

If the ground connection at the transmitter end is very low and the
line is an odd number of 1/4-wavelengths there will be a very high
impedance at the antenna end and longitudinal current will negligible.
Insertion of the choke will have no effect.

If the line is a whole number of 1/2-wavelengths, with a low
resistance ground the input impedance of the coaxial outer conductor
will also be very low and longitudinal current in the line will be at
a maximum. Insertion of the choke will be very effective.

With the usual random line length and on different bands, choke
performance will always be completely random and unpredictable. Which
explains the different opinions and arguments about whether a choke is
necessary or not.

Program SELFRES3 models choke behaviour.
----
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. ..........

On Tue, 23 Aug 2005 18:17:23 -0500, Tom Ring
wrote:

Walter Maxwell wrote:

snip
Walt, W2DU

Thank you for that, Walter. This one gets printed and goes in the binder.

tom
K0TAR

I'm pleased that you find my data useful, Tom,

Walt

Reg Edwards wrote:
If the line is a whole number of 1/2-wavelengths, with a low
resistance ground the input impedance of the coaxial outer conductor
will also be very low and longitudinal current in the line will be at
a maximum. Insertion of the choke will be very effective.

To complicate things a little more, the common-mode current may
consist of standing waves. If the choke is installed at a current
minimum point, it may have little effect.
--
73, Cecil http://www.qsl.net/w5dxp

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"Cecil Moore" wrote in message
...
Reg Edwards wrote:
If the line is a whole number of 1/2-wavelengths, with a low
resistance ground the input impedance of the coaxial outer
conductor
will also be very low and longitudinal current in the line will be
at
a maximum. Insertion of the choke will be very effective.

To complicate things a little more, the common-mode current may
consist of standing waves. If the choke is installed at a current
minimum point, it may have little effect.
--
73, Cecil http://www.qsl.net/w5dxp
====================================

Dear Cec, It doesn't complicate things. It merely repeats what I have
already said. When the line is an odd number of 1/4-waves long and
there is an (albeit indeterminate) ground at the transmitter end, then
there will be a high impedance at the choke (DUE TO STANDING WAVES)
and the choke will then be wasting its time.

If you havn't, moons ago, already downloaded the program then do so
and read the program notes. There are so many unknowns in it the
results are only approximate but nevertheless sufficiently accurate
for the intended purpose - which is just to demonstrate choke
behaviour.

The only accurate result is the calculated-from-first-principles
self-capacitance of the coil on which the impedance/frequency response
depends.

I had checked calculations, years back, by winding many coils of
various sizes and shapes, (one 7 feet long), with various numbers of
turns and measuring the coils' self-resonant frequency over the HF
band using hand-held instruments with coils suspended in free space
from strings. I always knew the results would be useful at some time
or other, never even dreaming about choke baluns.

Did I ever tell you that for several years I was Head of a
Measurements Standards Laboratory which I personally set up from
scratch and which was in the 2nd echelon from the British National
Physical Laboratory. The NPL.

There were certain electrical parameters for which the laboratory was
capable of making measurements more accurate than the NPL but for
obvious political reasons it was not possible for the laboratory to
claim such performance on calibration certificates.

Some time after I had left the position on promotion, the wicked Mrs
Thatcher had the laboratory dismantled and sold it off in bits because
it did not produce any immediate, short-term profits. She was a
chemist who went into politics. Often I still wonder what happened to
the oven-enclosed battery of 12 standard Western cells.

One day, if I can force myself to find the time, I may write my
memoirs. I could tell you some amusing stories.
----
Yours, Reg, G4FGQ

Reg Edwards wrote:
When the line is an odd number of 1/4-waves long and
there is an (albeit indeterminate) ground at the transmitter end, then
there will be a high impedance at the choke (DUE TO STANDING WAVES)
and the choke will then be wasting its time.

And what I am saying is that exactly the same thing can happen NO
MATTER WHAT THE LENGTH OF THE FEEDLINE if the choke is located at
a current minimum point. I'm not arguing with you - just expanding
upon what you said.
--
73, Cecil http://www.qsl.net/w5dxp

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Speaking of useful data, what is the status of reflections III Sir?

"Walter Maxwell" wrote in message
...
On Tue, 23 Aug 2005 18:17:23 -0500, Tom Ring
I'm pleased that you find my data useful, Tom,

Walt

On Wed, 24 Aug 2005 16:01:06 +0000 (UTC), "Reg Edwards"
wrote:


One day, if I can force myself to find the time, I may write my
memoirs. I could tell you some amusing stories.
----
Yours, Reg, G4FGQ


Please force yourself, Reg, we'd all like to hear those stories. And
they should be published for all to read.

Walt

On Wed, 24 Aug 2005 13:49:21 -0400, "Fred W4JLE"
wrote:

Speaking of useful data, what is the status of reflections III Sir?

"Walter Maxwell" wrote in message
.. .
On Tue, 23 Aug 2005 18:17:23 -0500, Tom Ring
I'm pleased that you find my data useful, Tom,

Walt

Hi Fred,

Reflections 3 is at the publisher. I don't know the current status,
but thanks for asking. I'll keep the rraa up to date at it progresses.
It'll also be announced on my web page at w2du.com.

Walt