Compact ultra-narrowband superconducting filter using N-spiral resonator with open-loop secondary coupling structure

A novel N-spiral resonator with open-loop secondary coupling structure (OLSCS) is proposed to realize a compact ultra-narrowband high temperature superconducting (HTS) filter. The coupling strength and polarity between the resonators can be significantly reduced and changed by introducing OLSCS, thus the required weak coupling can be achieved in a very compact size. A six-pole superconducting filter at 1701 MHz with a fractional bandwidth of 0.19% is designed to validate this method. The filter is fabricated on MgO substrate with a compact size of 15 mm x 10 mm. The measured insertion loss is 0.79 dB, and the return loss is better than 17.4 dB. The experimental results show a good agreement with the simulations.