Locating water ingress in telephone cables using frequency domain reflectometry

It is important to locate water ingress in telephone cables because water severely attenuates DSL signals used for Internet access. The paper initially reviews the frequency domain reflectometry method and how it measures the distance, magnitude and phase angle of reflections caused by transmission line discontinuities. When water enters a cable it lowers the magnitude and angle of the characteristic line impedance, causing reflections at the points where the water starts and ends. The dry and wet characteristic impedances of a 300-meter section of telephone line were experimentally measured and the reflection coefficient was estimated to have a phase angle of 20deg. Measurements of a properly terminated telephone line with 300 meters of the line immersed in water showed a first reflection with a phase angle of 162deg (180deg-18deg). A second reflection, from the end of the wet section, was measured with a phase angle of 25deg. These reflection angles differ from the 0deg or 180deg typical of bridge taps, open circuits or short circuits and this allows a technician to specifically identify water ingress