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
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