A Compact and Wideband Bandpass Filter with Mixed Coupling via Multimode Patch Resonators

This work presents a compact and wideband bandpass filter (BPF) based on multimode patch resonators. Compared with conventional mixed coupling topologies using multiple resonators, the proposed design achieves miniaturization with only two resonators. Firstly, shorting pins are introduced to excite TM10mode of the circular patch, leading to an electric-field null region. Owing to the positions of the shorting pins coincide with the electric-field nulls of TM11mode, the resonant frequency of reshaped TM10(rTM10) mode gradually approaches that of TM11mode as the pin spacing increases, while TM11mode remains almost unaffected. Subsequently, when two identical resonators are vertically coupled, mixed coupling primarily governed by electric coupling (Ec) is established between rTM10modes of Resonator 1 (R1) and Resonator 2 (R2), thereby generating a transmission zero 1 (TZ1) in the lower band. In contrast, a mixed coupling dominated by magnetic coupling (Mc) is formed between their TM11modes, resulting in another TZ2in the upper band. Finally, through structural optimization and full-wave simulations, a wideband quad-mode patch BPF with two TZs on both sides is successfully implemented, thereby verifying the effectiveness and feasibility of the proposed structure.