Multi-mode resonators with quad-/penta-/sext-mode resonant characteristics and their applications to bandpass filters

In this article, a new class of dual-/tri-band and ultra-wideband (UWB) bandpass filters (BPFs) using novel multi-mode resonators are proposed. The classical even-/odd-mode method is applied to analyze the resonant characteristics of the proposed resonators, which exhibit controllable resonant modes with different dimension parameters under the same configuration. According to the analysis, three resonators with quad-/penta-/sext-mode resonant characteristics are obtained by choosing the specific dimension parameters. Then, the quad-mode resonator is used to design a dual-wideband BPF centred at 2.39/5.14 GHz with 3-dB fractional bandwidths (FBWs) of 36.9%/18.9%, and the penta-mode resonator is utilized to design an UWB BPF with 3-dB FBW of 102.2%, whereas the sext-mode resonator is applied to design a tri-band BPF with centre frequencies of 2.09/3.52/5.46 GHz and 3-dB FBWs of 11.3%/20%/12.1%. All these three filters are fabricated and measured, and the measured results are in good agreement with the simulated ones.