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Re: [TowerTalk] Choking on chokes

To: <towertalk@contesting.com>
Subject: Re: [TowerTalk] Choking on chokes
From: "Earl Morse" <kz8e@wt.net>
Reply-to: kz8e@wt.net
Date: Wed, 3 Dec 2014 15:00:10 -0800
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
Yeah, matched was a bad choice of words.  I wasn't speaking totally of 
impedance.  Hooking up a balanced dipole to an unbalanced coaxial transmission 
line isn't really matched and begs the dividing of the shield coaxial current 
into the current on the inside and outside of the coax.  Adding the CM choke 
inserts the impedance on the coaxial shield that reduces that Icm component on 
the outside of the coaxial shield.  Pretty much means that you'll have to do 
something if you are going to feed a balanced antenna with an unbalanced 
feedline in order to reduce the CM current on the outer coaxial shield.

Choking on chokes was a good title for this topic.

----------------------------------------------------------
The level of CM current on a transmission line is *not* directly related 
to how well it is matched. You can have a badly mismatched line with no 
CM current, and you can have a perfectly-matched line with lots of CM 
current.

If what you say is true, you would not need a CM choke when feeding a 50 
Ohm balanced antenna with 50 Ohm coax.

Steve G3TXQ


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