Effect of an image plane on printed circuit board radiation

It is shown that the predominant radiation from a PCB (printed circuit board) with or without an attached cable is the result of common-mode current. For the PCB investigated here, the common-mode current was caused primarily by a source proportional to the signal voltage driving the traces and the cable as an asymmetric dipole antenna. Moreover, the PCB was an effective common-mode antenna even without the cable. The concept of partial inductance was used to calculate the inductance of the ground-return trace. The effect of placing an image plane beneath and close to a PCB was shown. The image plane dramatically reduced the radiated emissions from a PCB even though the plane was not electrically connected to the PCB. The image plane also dramatically reduced the emissions from a PCB with an attached cable when the image plane was correctly connected to the PCB