A precise equivalent circuit for TM mode dielectric resonators in circular waveguide below cutoff

An equivalent circuit for characterizing the transition between circular homogeneous waveguide and dielectric partially loaded waveguide, both operating on transverse magnetic modes, is presented. It allows the accurate representation of dielectric resonators in circular waveguide below cutoff, which are widely used for realizing bandpass filters. Although the model represents only the TMO modes in the two guides (homogeneous and dielectric-loaded) in order to make possible synthesis applications, its parameters are computed by means of a multimodal approach (the mode matching technique) which allows a high degree of accuracy. The proposed equivalent circuit makes easy to design bandpass filters following the well known direct coupled resonators approach.