[TowerTalk] 40M rotary dipole

[Jim Thomson]

Date: Wed, 20 Sep 2017 16:42:42 -0500
From: Stan Stockton <wa5rtg at gmail.com>
To: Dave Sublette <k4to at arrl.net>
Cc: Dan Maguire via TowerTalk <towertalk at 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