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[TowerTalk] RFI suppression

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Subject: [TowerTalk] RFI suppression
From: (John Petrich)
Date: Thu, 22 Mar 2001 05:35:06 -0800

    It sounds like you have been given some very sound advise about your RFI
problems.  Your theory about the common mode RF on your feedline is the same
theory that is behind the proposed solutions that you have been offered.
Your problem sounds solvable, but it will take some trial and error to find
the correct combination.  You have major common mode RF on the chassis of
your rig and accessories.

    My suggestion is to try the common mode choke approach as the initial
step.  Inserting a common mode choke is quick and easy to do, and you'll
have an immediate answer as to whether you have made an improvement or not.
These chokes can be constructed by either making an air coil with your
feedline coax,  by making a coax feedline coil through a number of large
ferrite toroids of the proper mix, or by inserting a commercial 1:1 choke
balun (one used for antennas) in the feedline at the rig end.  Inserting a
1:1 choke balun, of the broad band variety, in the feedline at the rig is
often the quickest and easiest way to see if the common mode choke approach
is effective.  The choke that I am referring to are the same ones used to
feed antennas and can be easily purchased or borrowed from a friend.  Broad
bandedness is probably important in your installation, due to RFI from 80
through 10 meters.  The coiled coax approach can be effective, but is
somewhat frequency selective.  The ferrite based baluns, coiled on toroids
or bead style are more broad band  in their operation.

    Some people supplement their feedline decoupling by placing some of
those clip on Radio Shack type ferrite chokes on the mouse and keyboard
cables to their computers to eliminate the last wiggle on the screen.

John Petrich, W7FU

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