A low-loss dual-band bandpass filter using open-loop stepped-impedance resonators and spur-lines for sub-6 GHz 5G mobile communications

This article presents the design of a low-loss microstrip dual-band bandpass filter with improved inter-band isolation and selectivity for 5G sub-6 GHz mobile communications. The proposed filter utilizes the two first resonance mode frequencies provided by the stepped-impedance resonator to generate its two passbands at 3.6 GHz and 3.5 GHz, and spur-lines located before the input/output ports to improve the isolation and selectivity between passbands. The filter is designed using an RT/Duroid 5870 substrate with a relative permittivity of 2.33 and a thickness of 0.79 mm, manufactured and tested to validate the proposed design. The experimental results show that the filter operates at 3.61 GHz and 5.51 GHz with a 3-dB fractional bandwidth of 12.74% and 16.7%, respectively. Insertion losses at the two passbands center frequencies are 0.6 dB and 0.9 dB. In addition, the proposed filter has the advantage of covering the licensed and unlicensed 5G bands and provides a simple structure without using vias or DGS structures. © 2023 the Author(s)