I find after some interesting conversations that readers of posts
often read as if written to them. So I write my posts as to the
general audience. If an expressed situation is so specific as to be
exceptional to general use, I reply in private.
K4XS had such a situation, as does **any case** where the feedline has a
long distance to earth. The general application is a choke with reactance
pretty much always aids the system when the feedine to ground path is short
in terms of wavelength.
A reactance caused by spooling cable into a "choke" might not be nearly as
effective, and can actually hurt the system, if the lead is long.
A 1/4 wave to ground on the cable forms a very effective isolation, and even
1/8th wave is not bad, so in this case (or any other case without a cable
shield ground close to the antenna) we have to be careful.
This applies to verticals and dipoles EQUALLY, because the interaction we
worry about is identical. There is no distinction between the two.
*Of course* there will be a few situations where this general purpose
rule won't add any value. The question is whether a specific owner
has the equipment and technical knowledge to identify those
situations, which are highly specific and subject to all kinds of
local influences, and whether one can post a textbook to explain the
choices.
I agree. Simple answers are both easy and popular, but air wound chokes are
not good baluns or CM chokes.
They are cheap and easy, but do not fit into "always do this" applications.
Here is a general rule that always works:
When the path along the braid from CM source to ground point is very short
in terms of wavelength, a coil of coax can work fine. It might add
significant impedance to the path, because the path is probably a low
impedance and inductive.
When the path is longer, adding a coil of coax often will not do anything,
or can even make matters worse. With a long distance to shield grounding, we
are better off to plan the system.
Despite simplicity and almost universal popular acceptance, coils of coax
are NOT good universal CM chokes or baluns. They can even make things worse.
All of us should be worried about their use when we see the system has 1/8th
wave or longer to ground along the feedline path. Sometimes a simple
additional ground can be much more effective.
Long cable lengths to ground are tricky, and it doesn't do us any good to
pretend they are not tricky. Air core chokes or added reactive impedances
should be used carefully, because they don't always increase common mode
impedance.
73 Tom
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