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Traces on a printed circuit board (PCB) couple to the cavity consisting of the PCB ground plane and a metallic enclosure cover. We introduce this common-mode coupling of a PCB trace to the cavity field by an analytical model that is verified with 3-D simulations using HFSS. The cavity field causes radiated emission from the slots of the cavity. For an accurate calculation of the cavity field inside the enclosure and the radiated emission, we consider the radiation loss by a multiport approach. Comparisons of the analytical results for the radiated field to measurements show good agreement. Radiated emission can be calculated for arbitrary geometric enclosure shapes as a function of frequency, position of the trace on the PCB, and trace load/driver impedances.