Evaluation of pigtail termination of STP cable using modal equivalent circuit of four-conductor transmission systems

For investigating mode conversion in four-conductor transmission systems we have developed a modal-equivalent-circuit expressed by an imbalance factor of transmission line, a current division factor, in the same way as we developed the one in three-conductor transmission systems. The modal equivalent circuit consists of three modal equivalent circuits of normal mode, primary-common mode, and secondary-common mode. In this paper, the influence of mode conversion by the pigtail termination of a shielded-twisted-pair (STP) cable was analyzed using the modal equivalent circuit and mixed-mode S parameters with respect to normal mode and primary-common mode. As a result, it was found that mode conversion depends on the imbalance factor and that the modal equivalent circuit helps to explain mode conversion that occurs in terminating STP cables with pigtail.