Image and Return Current Modeling of PCB Traces for Radiated Emissions

This paper establishes that the image current spread induced on a ground plane by a balanced PCB trace is several times wider than the return current spread of an unbalanced PCB trace. The magnitude of the image current, unlike that of the return current, is not only very small but it also has a nonuniform flow pattern similar to eddy current in a transformer. Therefore, the combined radiated emission levels of a balanced trace and ground plane current is always higher than that of an unbalanced trace and sometimes higher than that of an isolated balanced trace. Radiated emission measurements for an isolated balanced trace loop without a ground plane, a balanced trace loop over a ground plane, and an unbalanced trace over a ground plane were made. The resulting radiated emissions data are presented as Cartesian and polar plots. The variation of current spread levels with respect to spread width is also investigated.