[TowerTalk] Fair rite materials for choke baluns

[Jim Brown]

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