I will need to make common mode choke for 160m "T" shape vertical for
my club station. I always used to think I should aim at 3000+ Ohms
impedance, but lately I found article at
http://rudys.typepad.com/files/160m-common-mode-choke.pdf which says
thousands of Ohms is not necessary here. Several hundred Ohms is adequate
and air core chokes are OK for 160m. Now I'm not sure what target to
set for myself.
What do you think about that?
Unless you have a truncated or very small counterpoise, or a unique feed
system, Rudy is right on target.
Very small counterpoises and off-tune "floating" counterpoises create large
potential differences from the counterpoise common to earth because they
have intense electric fields near the feedpoint and at other places along
the counterpoise.
There will be cases where even several thousand ohms common mode will not be
enough. There are other cases where almost no impedance is required. For
nearly all resonant counterpoises, or even moderately truncated
counterpoises, several hundred ohms will be enough.
Whether an air coil is OK or not depends entirely on the reactance of the
shield's common mode path from counterpoise to "earth". There are many cases
where adding an air coil of coax increases problems, and there are many
where an air coil works fine.
The problem we have is no one has defined the system and goals properly and
in detail. It would take a good sized book to go through all the different
situations, so all we really have are generalized cases that have been
offered as generalized rules.
What ground system, feedline routing, and antenna do you have? Most of the
problems, and the acceptable choice of decoupling, would depend feedline
routing and the particular counterpoise or ground system.
73 Tom
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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