Synthesis Design of Microstrip Triple-Passband Dual-Stopband Filter Based on $\lambda /4$ Uniform-Impedance Resonators

A triple-passband, dual-stopband filter based on $\lambda /4$ uniform-impedance resonators (UIRs) is proposed in this letter. The triple-passband and dual-stopband frequency response is obtained by placing four transmission zeros inside a virtual wide passband. Presented with the equivalent circuit of the proposed filter, the analytical formulas of the structure for initial design are derived according to the synthesized coupling matrix without time-consuming tuning and optimization. Meanwhile, the low-insertion losses are achieved by the direct line connection of all resonators. Moreover, the fabricated filter features good return loss, wide bandwidth, and operating single frequency.