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Each type of cable has different velocity factor: for instance, for RG-58 it is 0.66.
Notice that this parameter may vary depending on the manufacturing process and
materials the cable is made of.
To measure the physical length of a cable,
1. Locate a resonant frequency by using single-point measurement mode or R,X
graph.
Example:
The 1/4-wave resonant frequency of
the piece of open-circuited RG-58 cable
is 4835 kHz
2. Knowing the electromagnetic constant and the velocity factor of the particular
type of cable, find the speed of electromagnetic wave in this cable.
Example:
299,792,458 · 0.66 = 197,863,022 meters per second
- or -
983,571,056 · 0.66 = 649,156,897 feet per second
3. Calculate the physical length of the cable by dividing the above speed by the
resonant frequency (in Hz) and multiplying the result by the number which
corresponds to the location of this resonant frequency (1/4, 1/2, 3/4, 1, 5/4, etc.)
Example:
197,863,022 / 4,835,000 · (1/4) = 10.23 meters
- or -
649,156,897 / 4,835,000 · (1/4) = 33.56 feet
(The actual length of this cable is 10.09 meters or 33.1 feet, which is
about 1% off the calculated result.)
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