A Miniature Wideband VHF Superconducting Filter Using Double-Surface Quasi-CPW Spiral Structures

Double-surface quasi-coplanar-waveguide spiral structures to attain strong couplings in low-frequency wideband filter designs are presented. Such structures yield coupling coefficients of up to 0.86 at the VHF band. A compact four-pole high-temperature superconducting filter centered at 112 MHz with a 38% fractional bandwidth, 0.03 dB insertion loss, and 17.5 dB return loss is demonstrated. The simulated and measured results show excellent agreement.