[Antennas] Balanced line optimum conductor spacing?

[Dave Shrader]

The answer is really founded in Electromagnetic Field Physics. Each side
of the balanced line radiates. One line is radiating from a positive
current, for example, while the complimentary line is radiating based on
a negative current. At a 'far field' broadside pattern the vector sum of
the two fields cancel for a net of zero radiation. [Note: it's been too
many years from my academic days, 43 years, to do the math, but the
principle is well understood]. For broadside configurations the spacing
is secondary, but for collinear configurations the spacing in electrical
degrees becomes more critical. There will ALWAYS be some radiation in
the collinear configuration, but if it is small ... ???

In reality, good engineering practice and compromise provides a workable
solution. A spacing of 10 degrees at the highest frequency of interest
will begin to produce noticeable changes in the antenna pattern. This is
a spacing of about 10 inches for 10 meters. The 10 inches is
approximately 720 ohms versus 4 inches at 600 ohms. A design of 3.6
degrees spacing at the highest frequency produces a 1% imbalance in the
antenna pattern.

So, Radio Shack ladder line at approximately 1 inch spacing should be
usable up to 2.5 meters with negligible impact on antenna pattern, and
up to 2 meters with minor impact. While a 4 inch line has a 'max'
frequency of about 10 meters.

Here, science [Physics] and engineering [Electromagnetic Design] will
never agree as to the degree of precision; but, engineers use the tools
of Physics to provide workable but not perfect solutions.

With that caveat stated: I am a retired engineer, not a retired
Physicist!

W1MCE

Tom Scott wrote:
> 
> 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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