5. TDR Measurement Theory
Figure 5.2.
A short cable fault shows a downward step edge at the location of the fault.
long, shallow reflected rise caused by cable attenuation of high frequency components of the step
pulse.
Figure 5.3.
An open cable fault at 824 ft.
Faults with reactive components such as capacitance and inductance appear as either dips below
or bumps above the characteristic impedance of the cable, depending on whether they are in
series with the conductor or represent a fault to ground.
Normal connectors will show up as an impedance discontinuity simulating a small fault.
Depending on the type and quality of the connectors, the expected impedance variation will
differ, as shown in Figure 5.4 and Figure 5.5 for SMA and BNC type connections. Connector
damage and corrosion can change the impedance profile of a connector over time, typically
increasing the excess impedance of the connector. Periodic surveillance with TDR can be used
to confirm connector performance.
Soft/partial faults can appear anywhere along the cable’s length. Any fault has the potential to
reduce the incident pulse strength for subsequent cable faults and may change the apparent
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