[TowerTalk] Common-mode current on feedline

[Pete Smith]

At 01:03 AM 6/26/03 -0400, you wrote:
>I agree.  Because the impedance of a common-mode choke approaches zero as 
>the frequency approaches zero, it tends to be difficult to achieve 
>effective common-mode choking at low frequencies.  Because the 
>lowest-frequency band that I use is 80 meters, I placed two common-mode 
>chokes 70 feet apart -- about one-quarter wavelength for this band -- on 
>my coaxial line.  In effect, the current node at one choke is transformed 
>(potentially) to a current maximum at the other choke, which maximizes the 
>effect of the other choke.  This arrangement is probably responsible for 
>the superior, rather than inferior as one might expect, common-mode 
>isolation that I measured on 80 meters.  On the even-harmonically-related 
>upper bands, the spacing of these two chokes is the worst-possible, of 
>course.  But another trick, described below, is effective at higher 
>frequencies. (Did you notice that my best common-mode isolation occurs at 
>the highest and lowest frequencies?)


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?


73, Pete N4ZR
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