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The equivalence between physical optics (PO) and aperture integration (AI) is extended to evaluate the coupling between two open-ended waveguides of arbitrary cross-section. Furthermore, a Kirchhoff-type coupling coefficient is derived in terms of a double line integration of incremental coupling coefficients. A useful tool when the mutual impedance of two modal distributions has to be calculated in the framework of a method of moment procedure is provided which is formulated in terms of mode-shaped basis functions.
