[TowerTalk] Common-mode current on feedline

[Chuck Counselman]

Pete N4ZR wrote:
>...I've always been of the impression that open-wire line at HF, if 
>properly configured, would radiate very little because of 
>cancellation between the two conductors, which are spaced quite 
>closely together relative to wavelength.  Wouldn't it follow that 
>because of that cancellation, such a line would also be pretty good 
>about NOT picking up power line noise, etc.?  I know that the 
>"properly configured" part is important, but does the rest compute, 
>or have I been operating on a misimpression?


Good question; thanks for asking.  I'll answer in two parts because 
there are two distinct issues, relating respectively to the common 
mode and to the difference mode of transmission on an open-wire line:

1. Re: the difference, or ordinary transmission-line, mode -- Your 
impression regarding radiation is quite correct; but the _near_ (in 
other words induction rather than radiation) field is a problem when 
you're looking only, say, three feet from an open-wire line having 
five-inch spacing.  For a two- or four-wire line, the near field is 
that of a dipole or a quadrupole, respectively, so it's easy to 
calculate; but it's even easier to let NEC do the work, as I did. 
Using NEC-4 I modeled helically twisted lines, too.

It's been a few years and I no longer remember numbers; I just 
remember concluding, reluctantly, that an open-wire line running the 
63-ft. length of my attic (as my layout requires) would couple too 
much to the copious 60-Hz-power, alarm-system, computer-network, 
TV-antenna, HVAC-control and telephone cables, and to the 
air-conditioning ducts that connect to AC-power-line-connected 
machinery, that are already up there.  There isn't enough room to 
keep an open-wire line far enough away from all these other 
conductors, many of which run parallel to the transmission-line for 
tens of feet.

2. The more difficult problem is that of common-mode current.  It's 
more difficult to build an effective common-mode choke for an 
open-wire line than for a coaxial cable.

The fact that a transmission line is balanced parallel-wire, rather 
than unbalanced coaxial, doesn't mean that it won't carry common-mode 
current.  I have measured the common-mode and the difference-mode 
current components on my 600-ohm open-wire line where it connects to 
my 4:1 balun.  This is an unusually accurate (home-made rather than 
commercial) Guanella _current_ balun, so I had expected the ratio of 
RMS common-mode to difference-mode current to be very small.  It was 
not.  On 80 meters it was 1/5, in other words -14 dB.

The common-mode is excited in at least three ways: (1) asymmetry of 
the antenna; (2) non-perpendicularity of the antenna and its 
feedline; and (3) unbalance of the balun transformer.  It's difficult 
to make a balun that's very well balanced throughout the HF range, 
and that also handles high power with a high SWR; and it's difficult 
to avoid having a high SWR on some band(s) with a multiband antenna 
of practical size -- unless you don't care about efficiency.  :-)

73  -Chuck, W1HIS