[Antennas] Balanced line optimum conductor spacing?

Fri, 4 Jan 2002 15:58:59 -0800

All the discussion of window line (which I've always preferred) and ladder
line, got me to wondering if anyone has seen any definitive tests or
analysis of the effects of the spacing between the two balanced conductors?
For a given frequency there should be a tradeoff between wider spacing that
yields lower loss and higher impedance versus poorer balancing of the
complementary fields surrounding the two conductors. I seem to remember
reading somewhere that 0.1% of the wavelength was a good compromise, but I'm
not certain. That would suggest that for the 10m band you should use 10mm
wire spacing (~0.4"), and for the 160m band you should use 160mm wire
spacing (~6.3"). It seems that most home-built ladder line I've seen is
built with something on the order of five to six inches spacing, while
window line is around 1 inch or a little less, so this calculation seems
like it's in the ballpark. Since we hams have a nasty habit of wanting to
operate on multiple bands, I'm just curious how much balance you lose with
real wide conductors when operating from a higher frequency band, and how
much extra loss you get when you operate narrow spaced lines on a lower
frequency band.
Probably not too significant, but just something that got me wondering.
As I recall, the NEC2 models don't yield much useful information about
balanced lines.
Maybe one of the transmission line calculators could give the results of
this analysis. Of course there are other factors, like conductor quality and
diameter, dielectric quality, etc.
- Tom Scott, Field Applications Engineering
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