A DUAL-MODE DIELECTRIC WAVE-GUIDE RESONATOR AND ITS APPLICATION TO BANDPASS-FILTERS

The fundamental TE(10) mode in a rectangular waveguide of a square cro ss section is degenerate with TE(01) mode. A quarter wavelength resona tor made of a dielectric square waveguide is, therefore, applied for a small-sized bandpass filter,just like dual mode filters for base stat ions in the mobile communication. In this paper, the methods to couple the two modes are first studied, including cutting a corner of the re sonator and adding some metal electrodes on its end face. Both methods help to flow the rf current of the odd mode at the corner, resulting in decrease of the series inductance and thus increase of the resonant frequency. The coupling constant, that is proportional to the differe nce of the odd and even-mode's resonant frequency, can be controlled b y the perturbations mentioned above. The coupling to the external circ uit is adjusted by an electrode fabricated also on the end face. It is connected to a microstrip line and capacitively couples to the resona nt modes. The coupling strength increases with the dimension of the el ectrode. The adjustment of the resonant frequency is carried out by th e similar electrode on the end face and connected to the center of the side of the square cross section. The frequency decreases with the le ngth of the electrode. The unloaded Q is measured to be of around 500 for 5 x 5 x 10 mm resonator of epsilon(r)=93. The optimum aspect ratio for the resonator is found in terms of the Q value. The simplest band pass filter, i.e., a two-stage bandpass filter is designed and fabrica ted using 5 x 5 x 10 mm resonator. It is mounted in a square hole made in a printed circuit board and excited by a microstrip line. The freq uency characteristics are in good agreement with the expected values.