RigExpert AA-1500 ZOOM
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
(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 the
R,X chart
.
Example:
The 1/4-wave resonant frequency of a
piece of open-circuited RG-58 cable is
4100 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.
299,792,458
×
0.66 =
197,863,022 meters per second
- or -
983,571,056
×
0.66 =
649,156,897 feet per second
15
