Improvement of Filter Properties of Independently Tunable Superconducting Dual-Band Bandpass Filter

We previously reported an independently tunable high-temperature superconducting (HTS) dual-band bandpass filter (DBPF) using dielectric rods. The DBPF is designed with stub-loaded hair-pin resonators. The dielectric rods are used to tune the center frequencies of both passbands. We successfully tuned both of the center frequencies independently. However, when the second band center frequency was tuned to the lower frequency, the bandwidth became smaller and stop-band rejection became worse. In this study, we investigated the reason why the second band frequency responses of the tunable DBPF got worsened and proposed a modified tuning rod to overcome these problems. Our new tuning rods were applied to the previously reported four-pole DBPF. For the second band tuning, stopband rejection was improved and bandwidth reduction was suppressed.