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[TowerTalk] common mode chokes

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Subject: [TowerTalk] common mode chokes
From: (Rob Atkinson, K5UJ)
Date: Fri Jun 27 14:15:19 2003

Common mode chokes, a.k.a. line isolators are 1:1 ununs.  If you are using 
50 beads or baluns at your antennas you probably don't need line isolators 
(at those locations).   line isolators are usually used to control errant RF 
so it stays out of the shack and out of the audio freq. circuitry when you 
transmit (the dreaded RF in your transmit audio).   It's usually a problem 
if you have your antenna close to your transmitter (for example if you are 
on a small lot) or due to the way your gear is grounded, you have rf 
travelling on loops between for example, your exciter and amp (via the coax 
shield between the two), and back from amp to exciter via your common ground 
bus.  Most hams put feedline isolators between exciters, amps and 
transmatches to break up these loops.  They may also put them out in the 
line near the antenna to attenuate RF coming back from that point on the 
external surface of the feedline shield.  if you don't have problems with rf 
in the shack (distorted audio, stuff such as your mic biting you when you 
grab it & transmit) then you are probably okay without them.

Rob Atkinson


Question in regard to Common-mode chokes.

I use Force 12 B-1 baluns on 10-40M. On 80 and 160M I use 50 of the small
beads on RG141.
All the Buryflex and hardline is tied together and buried a few inches in
sand for about 125'. It all ends at Ice 303 lightning arrestors (302
arrestor for 2M) on a grounded aluminum panel in my basement. In addition
there is an ICE coax switch, low pass filter for HF and a 2M bandpass filter
on the panel. My amp is tied into this panel. I have one coax line for HF
and one coax line for 2M coming to the room right above it to my rig.
Should I be using Common-mode chokes anywhere? What benefit would they buy
me? Where would I put them?
Tnx for any feedback

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