My experience is that the velocity factor of foam dielectric cable
like
RG8X varies considerably from batch to batch and manufacturer to
manufacturer. Apparently they have poor control over the foam
density in
the manufacturing process. For any application where it really
matters,
you really have to measure it for the particular piece of cable
you'll
be using.

Roy Lewallen, W7EL
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And the same applies to impedance Zo and attenuation.
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Reg.