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

Subject: Re: [TowerTalk] Current imbalance on 450 ohm Ladder Line
From: "Dan Zimmerman N3OX" <>
Date: Fri, 16 Mar 2007 09:20:03 -0400
List-post: <>
>but it is also
>possible to have common mode currents plus have matched differential
>mode currents, although I can't think of a good example of this with

Thanks for keeping me honest ;-)

It's easy to have nonzero common mode and equal and opposite
differential mode currents with coax and two seperate sources...  just
make current flow on the shield with one source while feeding power to
a dummy load or balanced antenna through the coax with the other.

I think it's impossible with open wire, seperate sources or no, just
because the vector addition of a perfect differential-mode flow with a
common mode one reduces the current in one conductor and increases it
in the other.  If they're two different frequencies (no coherent phase
relationship), and you had a frequency-sensitive current meter, you
could still read perfect balance on the line at the frequency of the
differential mode current, and 100% common mode at the frequency of
the common mode current, otherwise you'd read an imbalance.

You could, in principle, feed 17m to the open wire and the doublet
while using the whole thing as a tee antenna for 160m.  I'll leave the
design of the appropriate diplexer to the brave ;-)  Maybe a good(?)
idea for the low-profile SO2R crowd?


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