A TDR (time domain reflectometer) uses the radar principle to identify faults on cables. A pulse is “fired” down the cable. Any changes in the impedance of the cable will result in reflections being sent back down the table.

TDRs require 2 conductors running in parallel to operate – they identify the change in impedance. Any connection, change of cable type, break in the cable, or fault will cause a change of impedance. Each type of change has a different effect on the display of the TDR; a positive reflection shows a higher impedance, a lower reflection shows a lower impedance.

Velocity Factor (VF)

VF = Velocity of Propagation / Speed of light (300 m/μs)