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The properties of longitudinal leaky surface waves (LLSW) under a periodic SiO/sub 2//Al structure on Li/sub 2/B/sub 4/O/sub 7/ (LBO) substrate, were investigated theoretically and experimentally, in order to improve the high propagation losses of LLSWs under a periodic Al grating with the normalized thickness over 2%. In the theoretical analysis, the previously presented method based on the boundary integral equations for the periodic metal grating structure on the substrate was extended to include the dielectric layer. In the experiments, devices with Al electrodes recessed into a SiO/sub 2/ groove on LBO were fabricated, and the propagation losses of them were estimated. As a result, it was shown that, when the surface of the structure was flattened, the propagation losses were sufficiently low and the first Bragg stopband width decreased.