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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