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Re: [Antennaware] modeling feedlines- coils - traps

To: "Jim Miller" <JimMiller@STL-OnLine.Net>, "AntennaWare Reflector" <>
Subject: Re: [Antennaware] modeling feedlines- coils - traps
From: "K9AY" <>
Date: Fri, 18 Apr 2008 16:29:59 -0500
List-post: <">>
> If this doesn't seem like enough information, you have a sense of how 
> goofy it
> can be to model this stuff.
> 73, Guy.

I'd like to confirm Guy's notes and add my 2 cents on one specific topic.

Numerous references purporting to explain common mode currents in feedlines 
claim that there is a Kirchoff-style split of currents where the shield 
meets the antenna, which creates the current on the outside of the shield. 
This is only part (maybe the smallest part) of the story. The real culprit 
for common mode is coupling by radiation (mutual coupling)  from the antenna 
to the feedline, the same as it is for other unwanted objects. This is why 
the problem is never solved simply by using "balanced" open-wire feeders.

As Guy makes clear, multiple chokes are often required. My own practice is 
to first pay attention to the dress of the feedline, maintaining symmetry, 
running the feedline at right angles to the antenna instead of toward one 
end or the other, and getting the coax to ground level (or buried) in the 
shortest distance. With problems minimized by layout, a choke has "less 
work" to do. Modeling should confirm the value of this practice -- at least, 
that's how I've interpreted things... ;-)

73, Gary

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