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A numerical method is presented for determining all of the coefficients of coupled-mode equations for natural, single-phase, unidirectional SAW transducers (NSP-UDTs). Substituting numerical results computed by the hybrid finite element method for infinite NSPUDTs with shorted and open electric ports into several relations derived from the coupled-mode theory, we can determine all of the coefficients. Specifically, the edge frequencies of a stop-band and the static capacitances yield the self-coupling coefficients and the amplitudes of mutual-coupling and transduction coefficients, and the electric potential standing wave on the substrate surface, which can be derived from the projection of the standing wave distributions of particle displacements and electric potential in the whole substrate onto the set of those predicted by the coupled-mode theory, yields the phases of mutual-coupling and transduction coefficients. NSPUDTs on ST-25/spl deg/X quartz, Y-51.25/spl deg/Z LiTaO/sub 3/, and 50/spl deg/Y-25/spl deg/XLa/sub 3/Ga/sub 5/SiO/sub 14/ substrates are investigated. Our results agree well with the earlier experimental ones.