Design of a compact dual-band microstrip bandpass filter using square open-loop, stub-loaded, and spiral resonators

This study introduces a dual-band bandpass filter (BPF) architecture that utilizes two λ g / 2 open-loop resonators to define the initial passband. The second frequency response is achieved by embedding a pair of stub-loaded resonators between the primary open-loop structures, resulting in a compact dual-band performance. This paper's principal contribution is the novel incorporation of RSRs into the input and output feed lines instead of the resonating structure. This method adds more transmission zeros, improves frequency selectivity, and keeps the filter structure small without adding to the complexity of the design. For experimental validation, the filter is fabricated on a Rogers RO4003C substrate ( ε r = 3.55, h = 1.524 mm, tan δ = 0.0027) with a copper metallization thickness of t = 0.0175 mm. The proposed filter exhibits excellent dual-band performance, with measured return losses of 15.93 dB and 18.53 dB, and low insertion losses of 0.34 dB and 1.27 dB for the first and second passbands, respectively. The integration of RSRs into the input and output feed lines introduces two transmission zeros (TZs) at 3.00 GHz and 3.41 GHz, enhancing stopband rejection and frequency selectivity. Furthermore, the simulated and measured results show good agreement, confirming the effectiveness of the proposed RSR-integrated dual-band filtering approach.