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A closed-form expression is derived for determining the maximum possible radiated emissions from a heatsink over a printed circuit board or chassis plane as a function of the maximum voltage between the heatsink and plane. Two modes of radiation are considered: a patch-antenna mode and a monopole-antenna mode. The maximum emissions are the sum of the emissions due to each of these modes. Circuit board dimensions are assumed to be large compared to the heatsink dimensions. The most relevant parameters affecting the emissions are the dimensions of the heatsink. The closed-form expression is validated by comparing its results to full-wave simulation results for a variety of heatsink sizes and shapes.