Propagation characteristics of coaxial cable with a helically wound ground shield

Many coaxial cables depart from the 'ideal' geometry of a coaxial cable due to manufacturing constraints. In the case considered here, the role of the ground return is fulfilled by a drain wire. The ground shield is made of a plastic backed metal strip that is wound around the center conductor, dielectric core, and drain wire. This differs from the ideal case in which the ground return is entirely fulfilled by the ground shield, which is continuous. In the case studied here, the drain wire and the ground shield only make contact periodically, so there are two distinct current paths on the ground side of the circuit. Because of the plastic backing on the ground shield, the shield is not electrically continuous where it overlaps, and this results in a helical break in the ground shield. The combination of the the helical break in the ground shield and the existence of multiple signal paths results in signal dispersion. The helical break also makes the resulting electromagnetic fields non-TEM and thus precludes the use of two-dimensional solution methods normally used to characterize the coaxial problem. The characteristics of the electromagnetic waves propagating in this structure are analyzed with the use of a finite-difference time-domain solver under development at the Ultrahigh Speed Digital Electronics Laboratory at the University of Illinois. This solver uses a time-split Lax-Wendroff finite-difference time-domain scheme in conjunction with Maxwell's equations expressed in generalized coordinates in order to support the use of non-orthogonal meshing. The use of non-orthogonal meshing is required for a reasonable treatment of the problem under consideration. The results of this analysis are used to develop a circuit model of the cable behavior. This model is used to estimate the frequency ranges over which dispersion takes place.