Miniaturized Single- and Dual-Band Bandpass Filters With Low Loss Based on Folded Stepped-Impedance Resonators

In this article, miniaturized single- and dual-band bandpass filters (BPFs) with low loss are proposed for WI-FI application. A novel structure of a half-wavelength (λ/2) and quarter-wavelength (λ/4) vertically folded stepped-impedance resonators (FSIRs) is introduced, which significantly reduce the overall size of the filters. Additionally, the multi-zero characteristics of the λ/2 FSIR and the introduction of flexible mixed-coupling enhance the stopband performance of the single-band BPF. Meanwhile, a coplanar waveguide feeding structure is chosen, allowing both single- and dual-band BPFs to be implemented on a low-cost single-layer dielectric substrate. Small-sized single-band BPF with a low loss of 0.22 dB and dual-band BPF with a low loss of 0.77 dB/0.93 dB are implemented and fabricated. The measured results agree well with the full-wave simulations.