Return-loss investigation of the equivalent width of substrate-integrated waveguide circuits

Five different models to determine the equivalent width of substrate-integrated waveguide (SIW) circuits are investigated. The reflection coefficients between all-dielectric waveguides of equivalent width and SIW circuits are analyzed by full-wave techniques. It is found that one of the models yields consistently inferior results while the others depend on the ratio of the via-hole diameter and the center-to-center spacing of the via holes. Moreover, the influence of the substrate's permittivity with respect to the via-hole diameter and spacing is demonstrated. Recommendations are derived as to the use of respective models for different via diameters and spacings.