Design of an Ultrahigh Bandwidth Ratio Dual-Band HTS Filter With High Isolation Based on a Parallel Structure

A design methodology for a dual-band high-temperature superconducting (HTS) filter with high isolation, high out-of-band suppression, and low insertion loss is proposed by paralleling two transmission-path units with an ultrahigh bandwidth ratio. Two types of HTS resonators—hairpin-line and open-ring structures—are designed based on electromagnetic analysis of their coupling and resonant characteristics. These resonators are tailored for ultranarrowband and wideband responses, respectively, and are well-suited for implementing the CQ coupling topology. The methodologies and architectural configurations applied in the implementation of parallel design are proposed based on the impedance matching of the shunt branches, enabling the realization of a highly isolated dual-band response. The measured results show that the center band of the two passbands is at 2705 and 3365 MHz, with insertion losses (ILs) of 0.30 and 0.20 dB and return losses (RLs) better than 17 and 18 dB, respectively. The corresponding band-edge roll-off factors are 15,199.46 and 837.76. The figures of merit (FOMs) for the passbands are 413,189 and 22,239, with relative bandwidths of 1.1% and 21.7%, respectively. The relative bandwidth ratio between the two bands is 19.8, which is the highest reported in this frequency range for dual-band filters.