Compact Three-Stage SIW Bandpass Filter Based on Circular CSRRs for 4.7 GHz Wireless Systems

Abstract

This paper presents the design of a compact bandpass filter in SIW (Substrate Integrated Waveguide) technology based on the integration of circular CSRR (Complementary Split Ring Resonator) resonators. The study deals successively with three configurations: a single-stage topology, followed by a two-stage version and finally a threestage architecture. The operating principle is based on the utilization of evanescent modes below the cut-off frequency of the waveguide, which allows the creation of a controlled bandwidth. The filter performance is optimized by adjusting the geometry and arrangement of the CSRR resonators, which reduces unwanted modes, improves selectivity, and increases out-of-band rejection. The design and simulation were carried out using CST Microwave Studio software. The results show that the three-stage configuration achieves a bandwidth of 630 MHz (at -3 dB), extending from 4.3 GHz to 4.93 GHz, which corresponds to a fractional bandwidth of 13.4%. At the center frequency of 4.7 GHz, the return loss reaches -20 dB, while the insertion loss remains below $-\mathbf{0. 3 8 ~ d B}$. Outside the passband, the out-of-band rejection exceeds -55 dB. Furthermore, the proposed filter exhibits a 22.95% frequency miniaturization compared to a reference SIW filter operating at 6.1 GHz, highlighting its efficiency in achieving compactness without compromising performance. These features make this filter ideal for various applications, such as wireless communication systems and short-range radars.