Highly selective microstrip bandpass filters using dual-mode patch resonator loaded with short via

In this paper, highly selective dual-mode microstrip bandpass filters (BPFs) are presented utilizing novel square patch resonator loaded with short via and periodic metallic vias array. Characteristics of the patch resonator are studied, and a new nondegenerate dual-mode patch resonator operating with the second and third higher-order modes is proposed. A two-pole BPF based on one dual-mode patch cavity is analyzed, and two transmission zeros (TZs) are produced. To realize higher-order filtering responses, four-pole BPFs with two cascaded dual-mode patch resonators are proposed, and different cascaded methods of two dual-mode patch resonators by a lambda g/4 microstrip connecting line (MCL) are analyzed and compared to maintain high selectivity and small size. For the demonstration, two four-pole filters with the center frequency at 3.3 GHz and five transmission zeros is designed, fabricated and measured.