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5.2.3. Velocity factor measurement
For a known resonant frequency and physical length of a cable, the actual value of the
velocity factor can be easily measured:
1.
Locate a resonant frequency as described above.
Example:
10.09 meters (33.10 feet) of open-circuited cable.
Resonant frequency is 4835 kHz at the 1/4-wave point.
2.
Calculate the speed of electromagnetic wave in this cable. Divide the length by
1/4, 1/2, 3/4, etc. (depending on the location of the resonant frequency), then
multiply by the resonant frequency (in Hz).
Example:
10.09 / (1/4) · 4,835,000 = 195,140,600 meters per second
- or –
33.10 / (1/4) · 4,835,000 = 640,154,000 feet per second
3.
Finally, find the velocity factor. Just divide the above speed by the
electromagnetic constant.
Example:
195,140,600 / 299,792,458 = 0.65
- or –
640,154,000 / 983,571,056 = 0.65
5.2.4. Cable fault location
To locate the position of the probable fault in the cable, just use the same method as
when measuring its length. Watch the behavior of the reactive component (X) near the
zero frequency:
•
If the value of X is moving from –
∞
to 0, the cable is open-circuited.
•
If the value of X is moving from 0 to +
∞
, the cable is short-circuited.
