Effect of Pigtails on Coupling to Shielded Wires

An investigation of the effect of pigtails (exposed sections of shielded wires) on the coupling to braided-shield cables is presented. It is found that even though the lengths of these exposed sections of a shielded wire may constitute only a very small portion of the total line length, they may, depending on the terminal impedances of the shielded line, constitute the dominant coupling mechanism for the braided-shield cable. For situations in which pigtail coupling is dominant, the shield simply serves to reduce the exposed section of the interior wire from what it would be if no shield were present. Thus, the shield provides some reduction in coupling, but the effectiveness of the shield is shown to be as much as 30 dB less than it would be if the pigtail lengths were minimized. Experimental results are given to support these conclusions.