Generation and Propagation of Common-Mode Currents in a Balanced Two-Conductor Line

Based on the transmission line theory, modal transformation is applied to a balanced two-conductor line connected to a signal source and a load, which yields an equivalent circuit expressed in differential and common modes. This newly developed equivalent circuit clearly shows that the common-mode currents on a balance two-conductor line are generated by (1) direct injection of the common-mode current by a connected RF source; (2) multiple mode conversions by the source and load; and (3) multiple reflections of the modal currents at the source and load. The effects of branch lines connected to the feeder line are also investigated, focusing on generation of the common-mode current. It is concluded that the common-mode currents are also generated by (4) series connection of a branch line. Particularly, in the case of a switching branch line, the connection of a feeder line makes the branch line imbalanced, resulting in the common-mode current in the branch line. The aforementioned theoretical findings are compared with computer simulation using the method of moments with the numerical electromagnetic code (NEC2).