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[TowerTalk] Common-mode current on feedline

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Subject: [TowerTalk] Common-mode current on feedline
From: (Chuck Counselman)
Date: Wed Jun 25 22:43:27 2003
At 1:32 AM +0000 6/26/03, Howard Klein K2HK wrote:
>This is a very interesting question.

It's nice to hear that someone else thinks it's interesting.  :-)

>I imagine that we haven't heard more about it is because the average 
>ham does not have the equipment to make the measurements.

I agree.  I probably would not have done it myself, had I not been 
able to borrow the instruments.

>Some good data from a large sampling might lead to some interesting 
>and useful conclusions.

As you've probably guessed, I suspect that most hams have very little 
(in other words, very poor) common-mode isolation.  Here's one 
example of why I think so:

Recently I visited a ham who is Amateur Extra, has two degrees in EE 
from MIT, and is an active and accomplished HF DX contester.  He 
lives in a more rural neighborhood than mine; his neighbors' houses 
are relatively far from his; and I saw no electric power line above 
ground near him.  Yet, his received noise level on the lower HF 
bands, at mid-day when very little noise arrives via skywave, was 
nearly 50 dB greater than mine!  He also had an RFI problem in his 
house, despite transmitting only 100 watts.

IMO, much power-line noise was being coupled from his house wiring 
into his antenna, and that much of his transmitted power was being 
coupled into his house wiring, via common-mode current on his 
transmission line -- even though he was using a popular brand of 
common-mode choke, or "balun."  [I don't claim ('cuz I don't know) 
that this is the whole story; I have not yet had an opportunity to 
return to his QTH with instrumentation to investigate.]

>I would be quite interested in seeing where this goes. One question. 
>How do you have your tuner 70 ft away from your shack? Is it 
>remotely tuned?

To minimize loss, I wanted to run open parallel-wire line all the way 
from the feedpoint of my doublet to my shack; but I calculated that 
there'd be too much coupling between such a line (even a four-wire 
line) and the various other cables that it would have to pass near. 
So I ran parallel-wire line as far as I could; put a balun and an 
antenna coupler there; and ran coax the rest of the way.  I put the 
antenna coupler there to minimize loss in the coax, by reducing the 
SWR.  This coupler was indeed remotely tuned.

Then I was able to find one _fixed_ setting of the tuner for which 
the SWR on the coax (and therefore the loss) was tolerably low across 
_all_ HF bands 80 through 10 meters, except 30 meters where I don't 
transmit much power anyway.  Since then I've left the tuner fixed. 
Now QSYing is much faster.  I intend some day to write up and publish 
my method for finding such a magic setting.  I've found them for 
various line lengths, different tuners, different baluns, etc.; but 
have not yet found a general (never-fail) solution.  However, I 
believe that there's hope.

73 de Chuck, W1HIS

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