[TowerTalk] Common-mode current on feedline

[Chuck Counselman]

At 9:01 PM -0700 6/25/03, Scott Townley wrote:
>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" it...so 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 trick...at 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 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?)


>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.

I wish I could do this, but my feedline is at least twenty feet above 
ground all the way from my antenna to my upstairs shack.

So I inserted a common-mode, L-C-L, T-network, low-pass filter at the 
interface between the open-wire line and the 70-ft. coaxial line, as 
follows:

The 4:1 balun transformer between the open-wire line and coax is a 
"current" balun, i.e., a common-mode choke.  It is the first "L" of 
the common-mode, L-C-L, T-network.  The shield of the short coax 
jumper between this balun and the antenna tuner is connected to a 
two-square-meter sheet of copper on the floor of my attic.  This 
sheet has capacitance (C) to "ground," or infinity.  The second "L" 
of the common-mode "T" network is provided by the coaxial common-mode 
choke on the other side of the antenna tuner.

In effect, the C to ground provides the low source-impedance that is 
required for a choke to be effective.

A NEC-4 simulation of this L-C-L T-network scheme showed that it 
provided substantially more common-mode isolation than a single choke 
(L), or the two L's in series, would provide.  In this simulation I 
modeled my whole three-dimensional universe, including the antenna, 
the open-wire line, the metal boxes containing the balun and the 
tuner, the copper-sheet counterpoise, the coax, the c-m choke in the 
shack, metal boxes for my transmitter etc., continuing with the power 
wiring (and the c-m chokes in it, which I haven't mentioned 
previously) to the basement entrance panel, its connection to the 
buried metal water pipe to the street, the buried power cable to the 
street, and the water and power mains under the street.

This simulation showed that more RF current would flow into my shack 
via the power line from the street, where the buried mains parallel 
my antenna, than via the coaxial line from the antenna.  So I added 
power-line choking.

73  -Chuck, W1HIS