Compact Fully Uniplanar Bandstop Filter Based on Slow-Wave Multimodal CPW Resonators

In this letter, a compact, fully uniplanar bandstop filter is presented. It is based on multimodal coplanar waveguide resonators implemented using a slow-wave periodic structure that greatly reduces the filter's horizontal dimension. A multimodal circuit model for the filter is proposed and experimentally validated. A second-order filter prototype at 1.9 GHz, with a compact size of 0.31 λ_g × 0.19 λ_g, was designed and fabricated. The filter features measured fractional bandwidths of 9.3% and 3.7% for -20- and -30-dB rejection levels, respectively, and a 1.3-dB insertion loss in the passband.