Simple quad-band bandpass filter implemented on a 50-Ω microstrip line

Abstract

In this study, we implement a compact quad-band bandpass filter (BPF) on a 50-Ω microstrip line. The proposed structure comprises an input/output stage with one end terminated at a 50-Ω C-shaped open-loop resonator. When the gap between the input stage and the output stage is located at the center and the filter structure is symmetric, it acts as a dual-band BPF, but when the gap moves and the filter structure becomes asymmetric, it acts as a quad-band BPF. The proposed structure is very small in size because it is implemented on a 50-Ω line without any additional space. Even though it supports four frequency bands, the design complexity is minimized by reducing the number of design parameters. In addition, analysis of the current distribution and parameter study suggest a method to control only one or two specific resonant frequencies among four resonant frequencies. The fabricated BPF demonstrates four passbands located at 1.5, 3.6, 5.4, and 7.2 GHz. Within the four passbands, the minimum insertion losses are 0.2, 0.6, 1.8, and 1.7 dB, respectively.