Date: Wed, 20 Sep 2017 16:42:42 -0500
From: Stan Stockton <wa5rtg@gmail.com>
To: Dave Sublette <k4to@arrl.net>
Cc: Dan Maguire via TowerTalk <towertalk@contesting.com>
<A question regarding chokes for K9YC.
<Let's assume a coax going to an antenna is buried to the tower, the shield is
grounded with a short length at the base, the coax is inside the tower and the
shield is electrically bonded to the top of the tower and anything else that
would be considered good <practice for a good RF and lightning ground.
<How is common mode noise making its way to the feed point and back to the
receiver if you have something less than multiple cores at the feed point of
the antenna?
<Stan, K5GO
### CM goes up the tower faces to the top of the tower, then to the
ant...then back down the center conductor.
## Easiest way to see if the CM choke is doing the job, or to compare no
choke, lousy choke, good choke,
is to use a clamp on RF ammeter, like the deluxe version that MFJ sells.
MFJ-854. Measure it at the base of the tower,
then in the shack. Then replace your CM choke with a good one, different
one, 2 in series, nose to tail etc, etc.
The limitation on the MFJ-854 is... its only good up to 213-U OD. If hardline
used, you could temp insert a real
short length of 58 / 213 at base of tower.
## Pretty easy to get 7500-11,000 ohms of RS these days.... using just 4 x
cores + teflon RG-142U..and thats
from 160-10m. http://myantennas.com/wp/product/cmc-230-5k/ He can put a
balanced output on the box...
with whatever coax connector you want on the input side. Put 2 of these in
series for an eye opener.
## Then watch the RX noise drop.....a bunch. Also no more rfi, tvi, etc.
Folks have seen between
1-4 S units of noise redux on RX..with 3-5 k ohms of RS. . Even a few db is
well worth it.
Jim VE7RF
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