The correlation between common mode currents and radiated emissions

Common mode currents are usually suspected of causing the majority of emission problems in equipment under test (EUT). The meaning of “common mode” current in three dimensional (3D) structures in unclear. This term is really only defined for two dimensional (2D) transmission line structures. Other authors use the term “asymmetric mode”. Again the word “mode” implies a transmission line structure. Perhaps a better word for these currents would be “antenna currents” or “asymmetric currents”. In this paper we clearly define what we mean by antenna or asymmetric currents and how one can measure it. We also show that the magnitude of these so called antenna currents are not necessarily directly correlated with radiated emissions. This is verified by using a full wave 3D simulator. It is easy to construct an example where antenna currents increase but overall emissions decrease