If you know the ID and OD of the dielectric
section of the coax, it is easy to calculate
what the velocity factor needs to be in order to get
a characteristic impedance of 50 ohms
(or 75 ohms as the case may be).
If this isn't the velocity factor you are
measuring, then the characteristic impedance
will be off. Some manufacturers can't control
VF very well, and therefore you shouldn't expect
Zo to be very well controlled either. This
means that if you are building phasing lines,
you can't simply cut the lines to whatever
corrected length achieves the correct electrical
length to fix VF errors, because that doesn't fix
the Zo errors. I often see references to the
effect that VF doesn't matter if you custom cut
the cables to length.
Rick N6RK
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