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[TowerTalk] Common-mode current on feedline

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Subject: [TowerTalk] Common-mode current on feedline
From: (Scott Townley)
Date: Thu Jun 26 00:01:32 2003
Good stuff Chuck.
One very important thing to remember is that the common-mode current "sees" 
a transmission line of unknown impedance.  One conductor is the outside 
surface of the coaxial line, and the "other" surface (could be multiple) is 
whatever conductor is nearby...side of a house, fence, earth surface, etc.
So you can get all sorts of interesting impedance transformation 
issues.  For example, the "common" common-mode choke consisting of ferrite 
beads surrounding the outer conductor of a coaxial line would "like" to see 
a short circuit "behind" that you have a large-ish series impedance 
followed by a shunt short circuit...maximum rejection of signal.  If the 
coax is grounded/bonded 1/2 wavelength from the choke position, that will 
do the that frequency (and all multiples).  But at a frequency 
where the grounded coax shield point is 1/4 wavelength away from the choke 
point...the choke is much less useful.  I have personally measured this 
effect using a setup similar to yours.
Back in my DoD days when doing wideband HF engineering/testing, we would 
always bond the coaxial shields to ground (where we could) at least every 
1/10wl (at the highest frequency of interest) between exciter and 
antenna.  This kept the common-mode impedance low and greatly aided the 
effectiveness of common-mode chokes used.
Unfortunately I have never done a CM/DM measurement like you did; actually 
30dB sounds pretty reasonable to me at HF, but I have no basis for that 

At 19:42 2003-06-25, Chuck Counselman wrote:
>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 
>>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
>See:  for "Self Supporting Towers", "Wireless 
>Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with 
>any questions and ask for Sherman, W2FLA.
>TowerTalk mailing list

Scott Townley NX7U
Gilbert, AZ  DM43di

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