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Mutual coupling has negative impact on the radiation properties of compact antenna arrays when the spacing of individual radiators is considerably closer than half a wavelength. We have previously put forward a description of the radiation efficiency of an arbitrary antenna array in terms of orthogonal radiating eigenmodes. The present publication elaborates on this concept and presents a descriptive equivalent circuit model for a coupled antenna array. We show that eigenmodes undergo independent Rayleigh fading under appropriate conditions. Also, we demonstrate how practical closed-form expressions in terms of the array's scattering matrix can be derived, which characterize the diversity performance and therefore are convenient measures for the radiation quality of a compact antenna array.