Field-and-Circuit Co-Analysis of Mode Conversion Caused by Continuous Imbalance Factor Changes in Three-Conductor System

The direct connection of a microstrip line (MSL) to a parallel-line cable causes mode conversion that is proportional to the difference in the imbalance factor of the transmission lines, which can be represented by the modalequivalent-circuit model. In this paper, we first demonstrate that mode conversion occurs where the distance between the parallel-line cable and the system ground changes continuously in the immunity evaluation system. The mode conversion occurs when the parallel-line cable is placed perpendicular to the system ground, whereas no mode conversion occurs when the parallel-line cable is placed parallel to the system ground. Next, we clarify that the mode conversion can be represented by using a staircase approximation to represent the continuously changing imbalance factor as the distance between the parallelline cable and the system ground changes. It was found that the differential-mode (DM) voltage caused by mode conversion was successfully analyzed using the modal equivalent-circuit model that utilizes field and circuit co-analysis in the commercial simulator HFSS. Furthermore, it was found that increasing the number of steps in the staircase approximation approaches the spectrum obtained in the full-wave calculations.