`Patch' improvements to COM model for leaky waves

Coupling-of-modes (COM) model is known to be very accurate and easy to use for the design of narrow band SAW filters and resonators. The use of the model is so successful that there is a temptation to expand the application of the model to other types of SAW,like leaky SAW, and to use it for the design of relatively wide band devices. Thus it is desirable to modify the COM model so that it gives acceptable results for wide band devices based on the use of leaky waves, while keeping the skeleton of the COM model unchanged. When applied directly to leaky wave, 3 to 5% bandwidth resonator filters, standard COM model has the following drawbacks: The dispersion, (i.e. the frequency dependence of the reflectivity, velocity changes and other COM parameters) must be taken into account; It doesn't take into account the increase of attenuation at frequencies higher than the stopband due to the scattering into the bulk waves; The generation of bulk waves, which occurs also at frequencies slightly higher than the stopband, results in an additional loss mechanism. In the present work we propose simple analytical expressions for the three above mentioned effects, as terms to be added to the COM model to improve the agreement between theory and the measured curves. Comparisons between this “patched” COM model and wideband devices on LiNbO₃ show improved agreement