Cable Shielding Performance and CW Response

We show that the reference for the measurement of the shielding performance of a cable sheath should be the sheath current. However, unless the phase length of the cable is short, the response at the ends of the line will be sensitive to frequency even if the sheath current is uniform, and the coaxial line formed by the core wires and the sheath is matched at both ends. This is because the driving field in the coaxial line and the waves that it generates have different phase speeds and directions. We present supporting experimental data. Because of this effect it is not meaningful simply to state that a cable provides a specifled attenuation, except for a cable that is short at the highest frequency of interest. We discuss the effects of connectors and of cracks or holes in the sheath. Because the total leakage is proportional to the sum of leakages through the sheath (the connectors and the cracks) and not to their product, it is necessary to assign these contributions with some arbitrariness in order to compare them in decibels. We give an example that shows the importance of RF-tight connectors.