Radiated emission, susceptibility and crosstalk control on ground plane printed circuit boards

In the case of conductors of finite width and with dielectrics of finite thickness it is shown that the closeness of the inductive coupling between conductor and ground plane is important in minimising the equivalent loop area at high frequencies. Alternating currents tend to follow a path which encloses the least magnetic flux, but in the case of a ground plane return it may be difficult to believe. An acceptance of this phenomenon is essential if certain advantages of ground planes are to be understood, therefore the results of a simple experiment which effectively demonstrates this are presented. As the current density under a conductor using the ground plane as a return has a direct influence on crosstalk, this effect can contribute a significant amount to crosstalk between conductors near ground plane boundaries. A method of plotting the mean current density over straight paths using simple probes which couple inductively into the plane was used to investigate this effect in a square board