I have some professional Fischer current probes (clamp-on current
transformers).

For a simple dipole, balun, 50 ohm coax feed system...
It seems to be that I can simply clamp one on the rig side of the
balun, and see how much current is flowing and compare it to antenna
current. I can clamp it on either, or each antenna leg near the feed
point to measure antenna current.

It'd also be interesting to probe several points along the feed line.

Anyone see anythhing wrong with this reasoning?

73, Steve
PLEASE also respond via email
K9DCI @ arrl dot net

On Mon, 16 Aug 2010 19:10:07 -0700 (PDT), Noskosteve
wrote:

It'd also be interesting to probe several points along the feed line.

Anyone see anythhing wrong with this reasoning?

Hi Steve,

You will have the length of the line leading to the probe in the
radiation field and it will disturb the transmission line coupling to
that same field, changing the current characteristics. At some point,
it won't matter, at other points it may matter considerably - the
trick is knowing one point from the other. What will be happening is
that you will force a common mode into a balanced circuit, or will be
further unbalancing an already unbalanced load with that long length
of line.

In other words, it will be a joker in the deck unless you choke that
line too.

The question then comes to this: Why are you doing this? Is this
some form of classroom experiment with what was called Lecher lines?
Measuring SWR? If so, you might find that you are pushing the current
node along the line (like a bead on a string) as you slide the probe.
At some point the node will pop back to its original position and if
you are trying to plot current points, they will exhibit a curiously
distorted shape. This can occur through overcoupling the probe to the
line.

73's
Richard Clark, KB7QHC

Noskosteve wrote in news:3aaecad9-d181-4981-a45a-
:

....
It'd also be interesting to probe several points along the feed line.

You realise of course that the common mode current is almost certainly a
standing wave, and meaasurement at a single point is not a reliable way
to characterise a standing wave.

I know that this is not consistent with the widely held view that a
common mode choke creates a demarcation point where common mode current
can flow on one side and not the other.

This is the pitch for the very popular Carolina Windom which advertisers
and promoters claim leverages the radiation from the vertical feedline
section on one side of the "isolator" but there is no undesirable common
mode current on the tx side of the "isolator".


Anyone see anythhing wrong with this reasoning?

To assess effectiveness, don't you first need to take inventory of what
it is supposed to do, then assess its performance against those criteria?
Have you a list of expected benefits?

There are a lot of flawed common mode current measurment articles around,
and even some commercial instruments that seem not cognizant of the fact
the line currents are complex quantities (eg at least one MFJ
'instrument'). Most reports I have seen of "I have common mode current
20dB down" speak more of the reporters lack of knowledge of the problem
and measurment technology than 'clean' status.

If for instance, one of your benefits was reduction of receive noise
level due to proximity of common mode conductors with household noise
sources, then your current test is not likely to be a good measure of
that outcome.

If the objective is EMC improvement on tx, then isn't the best test
observation of interference to yours and your neighbors TV, HiFi,
computer, telephone etc? These effects usually incorporate a threshold
effect, and if field strengths are below threshold, your home free.

I am not familiar with the equipment you have, so ask the question, can
you measure the phenomena without significantly disturbing it? If I was
designing a common mode current probe, I would think of a small self
contained thing with a panel DMM readout, battery powered, and that could
be clamped to a conductor and read remotely with a telescope. Anything
with substantial length of conductors is likely to be a problem.


Owen

On Aug 17, 4:58*am, Owen Duffy wrote:
I know that this is not consistent with the widely held view that a
common mode choke creates a demarcation point where common mode current
can flow on one side and not the other.

This is the pitch for the very popular Carolina Windom which advertisers
and promoters claim leverages the radiation from the vertical feedline
section on one side of the "isolator" but there is no undesirable common
mode current on the tx side of the "isolator".

Ideally, the high-impedance "isolator" causes a reflection of the
forward common-mode wave back toward the antenna. This ideally results
in a current node (minimum) at the "isolator" and ideally confines the
common-mode standing wave to the feedline between the "isolator" and
the antenna. That's the theory behind the Carolina Windom. How it
performs in reality is another question.

It should be easy to measure the common-mode current at a few points
between the isolator and the antenna and at a few points between the
isolator and the shack to determine just how effective the Carolina
Windom "isolator" really is. It should also be possible to model the
situation with EZNEC - no, not a perfect model, just close enough to
observe the effect of the "Isolator".

Does anyone know the choking impedance for the Carolina Windom
"isolator" over the spec'ed frequency range?
--
73, Cecil, w5dxp.com

Owen Duffy wrote:

I am not familiar with the equipment you have, so ask the question, can
you measure the phenomena without significantly disturbing it? If I was
designing a common mode current probe, I would think of a small self
contained thing with a panel DMM readout, battery powered, and that could
be clamped to a conductor and read remotely with a telescope. Anything
with substantial length of conductors is likely to be a problem.


A strategy that is used in antenna research with bolometers is to use
wires with an impedance of 377 ohms/square so they "look" like free
space and don't perturb the field. I haven't seen these as a
commercial item, so I suspect that they are fabricated by the
researcher: perhaps by cutting from sheets of so called space cloth or
paper (which has the required sheet resistance)

Check out the work by Bolomey, et al., with measuring fields with
"modulated scattering probes" (put a small dipole in the field with a
diode in it that you can turn on and off with a DC bias, supplied by
resistive lines)

On 8/16/2010 7:10 PM, Noskosteve wrote:


I have some professional Fischer current probes (clamp-on current
transformers).

For a simple dipole, balun, 50 ohm coax feed system...
It seems to be that I can simply clamp one on the rig side of the
balun, and see how much current is flowing and compare it to antenna
current. I can clamp it on either, or each antenna leg near the feed
point to measure antenna current.

It'd also be interesting to probe several points along the feed line.

Anyone see anythhing wrong with this reasoning?

73, Steve
PLEASE also respond via email
K9DCI @ arrl dot net

You could just pass the coax though a suitable toroid. This toroid
having a few turns of wire, rectification (diode) and some type of
readout. This could easily be slid along the length of the coax and
"relative measurements" taken. You could probably add some calculations
and with the proper choice of components, actually get "real" readings.
A simple turn of non-shorted coax around the inside circumference of
the toroid, plus windings, would provide a measure of capacitance
decoupling. I am sure, someone, somewhere has already done this and
would have some meaningful data what is possible or what might be
expected ... if nothing else, a "poor mans' indicator."

Regards,
JS

On 8/24/2010 1:20 PM, John Smith wrote:
A simple turn of non-shorted coax around the inside circumference of the
toroid,

should have read, "A simple turn of non-shorted copper foil around the
inside circumference of the toroid,"

I know, do error checking before posting ... yeah, get right on that ...

Regards,
JS

On 8/16/2010 7:10 PM, Noskosteve wrote:

...
It'd also be interesting to probe several points along the feed line.

Anyone see anythhing wrong with this reasoning?

73, Steve
PLEASE also respond via email
K9DCI @ arrl dot net

I am sure you are already familiar with this, but never hurts to review
when working with baluns/ununs:

"The maximum attenuation of a ferrite core is not where the inductance
is maximum, but instead is where the magnitude of the impedance is
maximum. This impedance peak may even be where the impedance has no
inductive component nor capacitive component i.e. at resonance.

The frequencies that you are trying to attenuate determine the core that
you should use."

One mans' experience and experiments he
http://www.eham.net/articles/16319

Regards,
JS

From: John Smith
Date: Tue, 24 Aug 2010 Time: 13:20:44

You could just pass the coax though a suitable toroid.

Like this:

http://www.ifwtech.co.uk/g3sek/clamp-on/clamp-on.htm

--
73
Ian, G3NRW

On 8/27/2010 2:48 AM, Ian Wade G3NRW wrote:
From: John Smith
Date: Tue, 24 Aug 2010 Time: 13:20:44

You could just pass the coax though a suitable toroid.

Like this:

http://www.ifwtech.co.uk/g3sek/clamp-on/clamp-on.htm

Just goes to show you, if you can think of it, most likely, someone else
has already done it.

I would use a core with no air gap (but, if it was all I had, I'd use
one such as the author of the article did.) If you have/like ferrite
material, a FT114-43 (FT114-61, FT114-77, etc.) should work. If you
like iron a T130-2, T130-6, etc. should work. I simply mention these
because the ID would pass over a pl259 with a few turns of thin wire,
larger ID toroids should be fine. I would think you can use whatever
you have which will fit over the connectors you are using on your coax.
Simply wrap the turns on the toroid, pull the connector off the
tuner/rig, slip coax through, reconnect coax.

I just happened to have a T200-6 in a drawer here. So, I wrapped a few
turns on, slipped it on the coax, ran the secondary through the 50ohm
shut, diode/cap, and to a cheap VOM. With ~100w and the coax into a
50ohm dummy load, the reading I get is just reflecting the loss in the
coax from the 95% braid I am using, most likely. With the coax from the
ant, it appears it would be usable from 40 - 2m (bands limited to my
antennas.) This should easily cover 160m to well below 2m. For SHF and
EHF you may wish to choose a torid of differing material.

Thank you for finding and posting that article. It was a fun read!

Regards,
JS