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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 18:05:47 2003
Has anyone here measured the ratio between the (undesired) 
common-mode current on the feedline from his transmitter to his 
antenna, and the (desired) difference-mode, in other words the 
transmission-line current?

Can anyone refer us to any such measurement that has been published, 
preferably on the web?

In view of how much we hear about baluns, quantitative information 
about their effectiveness should be abundant -- right?  Yet, although 
I've read several editions of the ARRL Handbook, several editions of 
the ARRL Antenna Book, about six volumes of ARRL Antenna Compendia, 
hundreds of issues of QST including practically every antenna article 
and antenna tuner/balun review, J. Sevick's excellent book 
_Transmission_Line_Transformers_, and countless web pages relating to 
antennas, transmission lines, tuners, common-mode chokes, and baluns, 
I can't recall _ever_ seeing any _quantitative_ data on common-mode 
isolation!  Is it just my bad memory?

I just finished measuring the ratio of the RMS common- and 
difference-mode currents on the coax at the output of my TX, in my 
shack.  I used an AILTech model 94106-4 current probe with an HP 
3586C Selective Level Meter, and I measured the currents while 
transmitting CW at each of five HF ham-band frequencies.  The 
results, expressed in dB (where an RMS-current ratio of 1/10 
corresponds to -20 dB) and rounded to the nearest 1 dB, were:

(MHz)| (dB)
  3.5 | -32
   7  | -22
  14  | -25
  21  | -20
  28  | -33

For context and to facilitate comparisons with data from measurements 
at other stations, my antenna is a horizontal doublet about 33 feet 
above ground and 100 feet away horizontally from my 2nd-floor shack. 
Starting at the 50-ohm coaxial output of my TX, there are about five 
feet of coax, a low-pass filter, and a coaxial common-mode choke; 
then 70 feet of 50-ohm coax; then another coaxial common-mode choke, 
an antenna tuner, and a 50-ohm unbalanced to 200-ohm balanced "balun" 
transformer; then 45 feet of ladder line to the center of the 
doublet, which is about 10 degrees from being perpendicular to the 
ladder line.  The coax-shield side of the unbalanced side of the 
balun transformer is connected to a sheet-copper counterpoise of area 
equal to about square meters.  Should anyone care, I could provide 
further details.

I was surprised to see how _poor_ my common-mode isolation is, 
because I thought that I had done a better job of common-mode choking 
and balancing than most hams have done.  However, as mentioned, I've 
never seen a measurement of the common-mode isolation at any other 
ham station.

73 -Chuck, W1HIS
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