31
5.2.2. Cable length measurement
Resonant frequencies of a cable depend on its length as well as on the velocity factor.
A velocity factor is a parameter which characterizes the slowdown of the speed of the
wave in the cable compared to vacuum. The speed of wave (or light) in vacuum is
known as the electromagnetic constant: c=299,792,458 meters per second or
983,571,056 feet per second.
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 9400 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
To Next Page
Loading...
