18 Chapter 1
Introduction and Measurement Theory
Cable Impedance and Structural Return Loss Measurement Theory
SRL and Periodic Cable Faults
SRL is the measure of the reflection of incident energy that is caused by
imperfections or disturbances (bumps) in the cable which are distributed throughout
the cable length. These bumps may take the form of a small dent, or a change in
diameter of the cable. These bumps are caused by periodic effects on the cable while
in the manufacturing process. For example, consider a turn-around wheel with a
rough spot on a bearing. The rough spot can cause a slight tug for each rotation of
the wheel. As the cable is passed around the wheel, a small imperfection can be
created periodically corresponding to the tug from the bad bearing.
Each of these small variations within the cable causes a small amount of energy to
reflect back to the source due to the non-uniformity of the cable diameter. Each
bump reflects so little energy that it is too small to observe with fault location
techniques. However, reflections from the individual bumps can sum up and reflect
enough energy to be detected as SRL. As the bumps get larger and larger, or as more
of them are present, the SRL values will also increase. The energy reflected by these
bumps can appear in the return loss measurement as a reflection spike at the
frequency that corresponds to the spacing of the bumps. The spacing between the
bumps is one half the wavelength of the reflection spike and is described by
equations 7 and 8.
Equation 7
Equation 8
The wavelength/2 spacing corresponds to the frequency at which down and back
reflections will add coherently (in-phase). The reflections produce a very narrow
response on the analyzer display that is directly related to the spacing of the bumps.
The amount of reflected energy is observed as return loss. When this return loss
measurement is normalized to the cable impedance, the return loss becomes
structural return loss.
wavelength
c
f
--
≈
c
speed of light
= f frequency=
wavelength
2
------------------------------
spacing between the bumps
=
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