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Re: [TowerTalk] Fair rite materials for choke baluns

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Fair rite materials for choke baluns
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Tue, 5 Jul 2016 23:01:03 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
On Tue,7/5/2016 5:02 PM, Jim Thomson wrote:
##  I never saw one of those systems ever use a CMC at the feedpoint, and they 
all
had 3-4 db gain..and typ  20 db  FB.   The theory was...a  half wave sloping 
dipole
is unbalanced to begin with..so deep 6 the CMC at the feedpoint.   On those 
setups,
at most you might require a CMC  on the main feed line coming up the tower.

##  I never saw anybody using CMC at the feedpoint of a 1/4  wave sloper either.
But on 1/4 wave slopers, I always ran the coax down the inside of the tower, 
since it
was easy to do, since the tower I used at the time was 33 inches  across. ( I 
climbed
up the inside of it).
Somehow, you seem to miss the point, which is NOT gain, but rather keeping RF picked up on the feedline from coupling to the antenna, and from there via the feedline in differential mode to the receiver.

##  the yagis at the time all used gamma or omega matches... an no CMCs  used.

Just because you never saw it doesn't mean that it's not good practice. It just means you never saw an installation where someone was smart enough to do it. :) Good practice is to do whatever the mfr of the antenna designed for matching, and then ADD a common mode choke.

When I first published my work on using ferrite chokes at the feedpoint of antennas, guys in our contest club started doing it and found that their antennas were quieter. Guys in multi-transmitter stations found less interaction between stations. This includes everything from dipoles, to monoband and multiband arrays of aluminum, to SteppIRs, and is independent of how they are fed. Even verticals benefit from a coax choke at the feedpoint.

73, Jim K9YC




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