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This paper discusses modal decomposition and its application to wideband antenna measurements. Applying network analysis, a simple finite Fourier series decomposes measured far-field patterns into characteristic modes to determine the power in each mode. The technique is useful for identifying measurement inaccuracies and errors due to antenna design or manufacturing problems. The analysis can be applied to a general class of antennas that radiate a set of characteristic modes; however, this paper demonstrates the concept with wideband, multi-mode spiral antenna measurements.