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Re: [Amps] RF in the Audio

To: <>, <>
Subject: Re: [Amps] RF in the Audio
From: "Peter Voelpel" <>
Date: Sun, 25 Sep 2011 02:20:20 +0200
List-post: <">>
Hi Jim,

That is not something new to me ;-))

But the question concerned that perfectly! balanced open feeder which
certainly carries no common mode current. 


-----Original Message-----
From: [] On
Behalf Of Jim Brown
Sent: Sonntag, 25. September 2011 01:56
Subject: Re: [Amps] RF in the Audio

On 9/24/2011 3:47 PM, Peter Voelpel wrote:
> How could common mode current appear on a near or perfectly balanced
> parallel wire feedline?

Common mode current results from imbalance at either end of the 
feedline. Most ham antennas are unbalanced to at least some extent by 
their surroundings -- trees, buildings, sloping earth, sloping antennas, 
off-center-fed antennas are all common causes of unbalanced antenna 
current in a transmission line. The DIFFERENCE between the current in 
one conductor and the current in the other is called common mode 
current. That common mode current radiates from the line, AND that 
imbalance will cause the feedline to act as a receiving antenna.

There are at least three important PRACTICAL reasons to kill common mode 
current. First, to prevent RX from coupling into the antenna (like from 
your neighbor's computer, battery chargers, etc.). Second to kill 
conduction of RF into your shack, where it can excite Pin One problems. 
Third, to prevent radiation from the feedline, which is often closer to 
your neighbor's stereo rig than your antenna.

The RX noise issue is one important reason for having a really good 
choke on the line -- a mediocre choke may be good enough to eliminate 
pattern distortion, RF in the shack and interference to your stereo for 
100W, but if you're trying to work the weak ones, every few dB can make 
a difference. How much difference a better choke will make depends on 
your antenna, proximity to noise sources, etc. 4-6dB reduction is not  

73, Jim K9YC
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