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Re: [TowerTalk] Current imbalance on 450 ohm Ladder Line

To: Garry <>,
Subject: Re: [TowerTalk] Current imbalance on 450 ohm Ladder Line
From: "Dan Zimmerman N3OX" <>
Date: Thu, 15 Mar 2007 17:03:24 -0400
List-post: <>
>. Next I reversed the leads from the tuner to the
> MFJ-835. Now the reading on the ammeter reversed with high current going to
> opposite side of the meter.

My guess is that the balun on the tuner has insufficient impedance.

Even a "balanced" tuner relies on the balun to force current
balance... and if it's not an a balun with extremely high common mode
impedance, there will be some bands on which current can flow on the
outside of the coax shield and radio... the common mode currents on
the transmission line form a standing wave pattern and if your input
balun is at a current node where the current would be low anyway, it
won't do anything unless it is of exceptionally high impedance.

A good high power, high impedance common mode choke on the output of
the tuner might help.  You'd need a well designed one; a high power
current balun modified to have standoff insulators on the input
instead of an SO-239 would work... you might actually want to dispense
with the case entirely.


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