<p>When a shielded-twisted-pair (STP) cable is connected with a printed circuit board (PCB) via an Ethernet (RJ45) connector, mode conversion between primary- and secondary-common modes occurs at the connector section due to their structural difference that causes the difference in the imbalance factor of the transmission line. In this paper, we investigate the suppression of the mode conversion at the connector section by using a modal-equivalent circuit model based on imbalance matching. We focus on improving the PCB pattern below the shielded Ethernet connector by placing a copper layer on the PCB surface, and the inadequate shielding at the connector section by soldering and wrapping with copper tape. The application of this improvement based on imbalance matching at the connector section makes the imbalance factor of the connector section closer to that of the cable section and results in the suppression of the mode conversion. Based on the concept of imbalance matching, we confirmed the effect of the shield-improved connector with an improved PCB pattern on the suppression of the mode conversion at the connector section through circuit simulation and measurement, and it was validated that the circuit simulation results obtained from the modal-equivalent circuit model agree well with the measurement results.</p>