A review of centrally loaded multimode microstrip resonators for bandpass filter design

The increasing demand for good performance filters in the field of radio frequency and wireless communication leads to the advancement in the design and development of compact microstrip resonator filters. This paper reviews the development of multimode microstrip resonators in recent decade. Half-wavelength open loop resonator is a well-known design approach for a single-mode resonator design, the half-wavelength line can be centrally loaded with an open-circuited stub, a short-circuited stub or a grounded via to form a dual-mode resonator. For symmetrical resonator structure, the resonant frequencies can be analysed using odd- and even-mode analysis. Triple-mode, quadruple-mode, quintuple-mode resonators are considered for wide bandwidth filters. The multimode resonators, in general, can be designed into filter using three different design methods. The first method uses cross-coupling resonators topology. The second method considers the distribution of the fundamental resonant frequencies of the resonator according to the Chebyshev's insertion loss function. The third approach is to design all fundamental resonant frequencies of the resonator to be located within the 3-dB frequencies. All these three design approaches will be reviewed and discussed in this paper. In addition, a design example using the last approach for a fourth-order bandpass filter using a quadruple-mode resonator is given.