An equivalent network theory and a coupled mode theory are described for the analysis of excitation characteristics of interdigital transducers (IDT) for surface acoustic waves. Circuit parameters of equivalent circuits and coupling and transduction coefficients of coupled mode equations can be theoretically determined by applying the finite-element method to an infinite array corresponding to an electrically shorted or open IDT. Examples are computed for two types of IDTs fabricated on a 128° Y-X LiNbO3 substrate. One is a uniform type of IDT and the other is a single-phase unidirectional transducer utilizing internal floating electrodes.