[TowerTalk] AVA7-50 cable length

[Jim Brown]

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.