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## [TowerTalk] Common-mode current on feedline

 To: [TowerTalk] Common-mode current on feedline ccc@space.mit.edu (Chuck Counselman) Thu Jun 26 01:03:48 2003
 ```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 ```
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