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Owen Duffy · August 17th 10, 11:58 AM posted to rec.radio.amateur.antenna

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