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.

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

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

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73
Ian, G3NRW