Design of Evanescent-Mode Rectangular Waveguide Inline Bandpass Filters With Transmission Zeros Using Split Ring Resonators

This article presents a novel design method for inline bandpass filters exhibiting prescribed transmission zeros (TZs) above the passband. The basic structure consists of evanescent-mode rectangular waveguide (EMRW) sections loaded with split ring resonators (SRRs) that are arranged on the cross section. For the first time, the position of the TZs is controlled above the bandwidth by turning the SRR. The proposed design method uses a cascaded triplet topology to simplify the control over the cross couplings. A prototype is fabricated and measured in the $X$ -band to validate the practical viability of the presented filters, revealing good agreement with simulated results and high-performance characteristics for a moderate bandwidth: insertion loss (IL) = 0.85 dB, return loss (RL) = 19 dB, and fractional bandwidth (FBW) = 5.69%. In addition to achieving control over the position of the prescribed TZs in the filter frequency response compared with other SRR-based filters reported in the literature, significant size reduction has also been obtained with respect to other triplet implementations. This design method is extended by adding TZs below the bandwidth. To do this, the sign of one of the main couplings in the triplet is reversed by arranging the SRR along the $E$ -plane of the evanescent RW. A fifth-order bandpass filter with TZs at both sides of the passband is designed to demonstrate the effectiveness of the proposed method.

I. Introduction

Over the last decades, microwave filters are undergoing a constant process of transformation in order to satisfy the increasing demands of the industry on several aspects, particularly the improvement of out-of-band rejection (to prevent channel interference) and miniaturization [1].