Actually, velocity factor is usually pretty accurate for the value at
VHF, where VF has reached its asymptotic value. More to the point --
any of these numbers are going to be within a few percent, and if you're
trying to estimate how much coax is on a spool or in a big coil, that's
plenty close enough. :)
A capacitance meter assumes a lumped capacitance -- that is, one that
is physically small enough that there are no wavelength effects. They
begin to show up at roughly 1/20 wavelength, so If the cable you're
trying to measure is longer than about 1/10 of a wavelength at the
frequency of the measurement, the error will be quite significant
because you're measuring a transmission line, not a capacitor.
73, Jim K9YC
On Sat,9/10/2016 2:16 PM, Zivney, Terry wrote:
TDR and/or stub measurement assumes you know velocity factor. The manufacturer does not
specify velocity factor to the same precision that they use for capacitance. Hence, a
low frequency measurement of capacitance will probably be as good as "assuming"
a velocity factor.
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