UHF Band Ultra-Narrowband Superconducting Filter With Double U-Type Secondary Coupling Structure

A double U-type secondary coupling structure (DUSCS) is used for a compact ultra-narrowband filter in the ultra-high frequency (UHF) band. It can effectively reduce the coupling and thus spacing between coupled twin-spiral resonators. A six-pole superconducting filter at 600 MHz with a fractional bandwidth of 0.083% is designed and fabricated to validate this technique. The filter size is reduced by 15% compared with the filter designed without DUSCSs.