Effect of Pigtails on Crosstalk to Braided-Shield Cables

An investigation of the effect of pigtail sections (terminal segments of braided-shield cables in which the shield braid has been stripped back exposing the interior wire) on the electromagnetic coupling to braided-shield cables from adjacent wires is presented. Experimental and computed data indicate that, even though the lengths of these pigtail sections may be only a very small portion of the total cable length, they may, over certain frequency ranges, 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 in reducing crosstalk is shown to be as much as 30 dB (over certain frequency ranges) less than it would be if the pigtail sections were eliminated. A low-frequency model which explains this phenomenon is given. The effect of various shield grounding configurations is also investigated.