Topband: Air core chokes for 160m

[Tom W8JI]

> 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