On Oct 25, 3:31*pm, Owen Duffy wrote:
Ian Wade G3NRW wrote in news:F3UuvlXZYcxMFwB2
@ntlworld.com:

We all know that for an open-wire feedline, Zo = 276 log 2S/d, where S=
the center-to-center distance between the two conductors, and d= the
outer diameter of each conductor.

Well, that is an approximation that falls down at closer spacings.



As far as I know, this formula applies to single-core wires. Is there

It applies to circular conductors.

any significant difference to Zo where multi-strand wire is used
instead? I'm thinking of how the RF skin effect might affect things. Or
is it simply a question of treating the overall diameter of all the
strands as one solid conductor?

I have seen some proposed corrections for stranded bare wires, but the
factors are small in the scheme of things for seven strand and smaller
for 19 strand.



Further, if the multi-strand wire is plastic coated, what effect does
that have on Zo?

Assuming you mean bare wires twisted and then served with insulation...

Dielectric with rel permitivitty 1 will reduce Zo.

Have a play with TWLLC athttp://www.vk1od.net/calc/tl/twllc.htm.

An effective way to estimate the effective permittivity is to measure the
velocity factor of a sample of line, and use permittivity=vf^-2.

Owen



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Just wondering how a few mils of PVC insulation would effect the
permittivity of a couple of 12 guage wires spaced a couple of inches
apart.

Jimmie