I suspect the popularity of 600 ohms is because that is the standard Z of an
open wire voice/telegraphy pair from the 19th century. And *that* is probably
determined by the physical size of insulators, with the wire size chosen for
the mechanical properties to accommodate the pole to pole span.
All those 600 ohm transformers in telephones and such: even though they’re
actually fed by twisted pair, which is almost certainly in the 100-150 ohm
range.
There might be a paper from the late 1800s that discusses the selection of open
wire Z for telephony and telegraphy.
On Sun, 31 Dec 2023 05:22:57 -0600, Rob Atkinson <ranchorobbo@gmail.com> wrote:
I think we should back up and get more information from the fellow
asking for help. It's understandable to assume the question pertains
to a pair of wires but open wire line is a term encompassing different
designs. For example, the line could be an unbalanced line with a
coaxial geometry with a large center conductor and four or more
smaller diameter conductors spaced around it equidistant from the
center and each other. Or, it could be a balanced line with an even
number of conductors greater than two. In that case, two pairs of
different diameter may or may not work (I'm not sure offhand). So, to
the ham asking for help, please tell us what design of open wire line
you are attempting to assemble. If in fact it is two wire balanced
line, there's no good reason I can think of for one wire diameter to
be different from the other, and I'd like to know why you are trying
this other than wire availability. As N6RK said, different diameters
will lead to different resistances, different currents, and line loss
via radiation. Therefore, formulas are irrelevant because you
shouldn't do it. BTW from my experience, AWG 16 copper stranded
works fine for US ham power levels. For a reasonable impedance for
multiband use (600 ohms), going wider diameter will mean spacing
insulator length of 5 or more inches. 600 ohms line Z is sort of the
target if you are going to use a center fed half wave dipole as a load
across multiple bands with half wave length on the lowest frequency
because 600 ohms is roughly in the middle of the load Z extremes.
73
Rob
K5UJ
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