Signals Passing Through Asymmetric Faults in Transmission Lines

Unexpected results have been seen in multiple papers involving spread spectrum time domain reflectometry (SSTDR) measurements on twin lead cables with one line containing a fault. A small portion of the signal is able to transmit past the fault, reflect off the end of the cable, return back through the fault, and be recorded by the SSTDR. This paper explains the physics of why this can happen, and the effect of nearby ground planes. The transmission through the fault involves electric and magnetic fields coupling to nearby conductive objects. This coupling allows for a small amount of signal to move beyond the fault. We use multi-conductor transmission line theory to explain this effect. A finite difference time domain simulation of the multi-conductor system is compared to measured results to demonstrate why this effect occurs, and the effect of the nearby ground.