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Topband: Testing for common mode noise

Subject: Topband: Testing for common mode noise
From: Don Kirk <>
Date: Thu, 18 Aug 2011 11:22:03 -0400
List-post: <">>
A few weeks ago I installed 3 half size point fed pennants (each aimed 
approximately 120° apart), and have been wondering if I need to install any 
chokes and/or ground rod on the feedline near the feedpoint of this antenna 
system to negate any common mode noise.

I then did a lot of research and see that testing for common mode currents is 
difficult (W8JI said it's a PITA), but I hate to add extra components without 
verifying their need (also want to make sure I don't actually make things 
worse).  This got me wondering what simple test can I do to see if common mode 
currents are really an issue.  I'm most concerned about man made noise, etc. 
(very happy with my front to back ratio), so here is my idea of a simple test, 
and I would like to see what others think of my proposal.

Proposed Test
Step 1) With HF rig in the shack connected to the antenna via the feedline, 
measure the noise level (S meter reading just listening to noise).

Step 2) Take HF rig right out to the antenna and connect it to the antenna 
using the shortest length of feedline possible (just a few feet) and power the 
HF rig with a battery (12 volt SLA battery) so the system is totally isolated 
from commercial electrical wiring, ground, etc. and basically no feedline, and 
then measure the noise level again.

If the noise level with the radio located in the shack is not higher than the 
noise level with the radio located at the antenna then common mode noise is 
insignificant (and I can forget about chokes and ground rod).  (Note : this 
test assumes feedline loss is minimal, but probably a good test to also perform 
would be a signal to noise ratio test with the radio at both locations to 
compensate for feedline losses).

Note : I guess I could also do an A vs. B test (in place of my above proposed 
test) in which I install a ground rod and make a choke, and then I place these 
devices in and out of the system to compare noise levels with the rig in the 

I wonder what the topband community has to say about my "Proposed Test".

Don (wd8dsb) 

UR RST IS ... ... ..9 QSB QSB - hw? BK
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