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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: Thu Jun 26 11:01:26 2003
At 7:27 AM -0400 6/26/03, Pete Smith wrote:
>This is intriguing.  I note that in his book (page 7-19) ON4UN 
>advocates using two 100-bead chokes located at least 5 meters from a 
>Beverage feedpoint, with the shield of the coax grounded between the 
>two chokes.  He asserts that with 1500 ohms of impedance in each 
>choke (at 1.8 MHz) he gets minimum 70 dB suppression of common mode 
>currents on the outside of the feedline.  Since this methods doesn't 
>seem to be terribly dependent on the point on the feedline where it 
>is installed, I wonder if you think it would provide good 
>common-mode suppression on the higher bands as well, for use on a 
>dual-purpose coax feedline?

The effectiveness of this method depends critically on the impedance 
of the "ground" connection between the chokes.  A low-impedance 
ground is difficult to get at higher frequencies.

For a quantitative answer I'd have to use NEC-4 to model the entire 
geometry, including the cable (shield only), the ferrite-bead 
loading, the conductor(s) to the ground rod(s), the ground rod(s), 
and the soil as a semi-infinite medium having finite conductivity and 
dielectric permittivity.  I could do this easily enough but I'm not 
presently inclined to spend the time because this method is 
infeasible at my QTH.

If someone else is interested in investigating the method, two 
variations worth considering for higher frequencies are: (1) breaking 
the string of beads into more than two sections, with "grounds" 
between all sections; and (2) using counterpoises rather than driven 
or buried grounds.

73 de Chuck, W1HIS
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