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Re: [TowerTalk] Balun Recommendation

To: "'Frank'" <>, "'Joe Subich, W4TV'" <>
Subject: Re: [TowerTalk] Balun Recommendation
Date: Sat, 21 Apr 2012 16:54:58 +0000
List-post: <">>
>I have a basic question regarding CM chokes.  My understanding is that you
make a choke by windng the feedline 
>around a form to create an inductance.  If the inductance is sufficient it
bucks the CM current trying to sneak back 
>into the shack.  

It is trying to stop current on the outside of the shield, be it at the
shack or antenna end.  If its at the antenna end then it is usually stopping
current from getting to the antenna feed point.

>All the conductors in the feedline are coiled.   Even the center conductor
is coiled and probably has 
>inductance as well.  Won't that buck the signal to the antenna?  On receive
as well as transmit.

No, the center conductor is shielded by the shield so it does not create an
inductance for the signal on the inside of the coax.

>Could it be that preceived noise reduction is nothing more than a signal

No, the noise reduction is because of signals picked up on the outside of
the coax getting up to the antenna feedpoint and then down into the
receiver.  The desired signal picked up by the antenna is not affected by a
coil or ferrite choke.

>I remember seeing decoupling sleeves used at VHF.  These were supposed to
cure CM current on the feedline.  They were 1/4 wave tubes installed at the 
>antenna feedpoint, open at the antenna end and electrically connected at
the other end to the coax shield.  Seems that such an approach would be 
>preferable to coiling up the feedline.  At least on frequencies where this
would be practical.  Obviously a single band solution.

Its just another way of choking the current on the outside of the shield, as
you note it is a single band one.

>Perhaps someone could develop a system where a choke could be inserted only
in series with the shield instead of turning the 
>entire feedline into an RF choke.

As noted above, the center conductor is not choked.

David Robbins K1TTT
AR-Cluster node: 145.69MHz or telnet://


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