Characterization of Nb-based superconducting microstrip lines around the gap frequency

Submillimter-wave SIS (superconductor-insulator-superconductor mixers usual ly adopt SIS junctions associated with an integrated tuning circuit, which tunes out the junction's geometric capacitance and is typically an inductiv e thin-film superconducting microstrip line. This paper mainly investigates the characteristic of Nb-based superconducting microstrip lines around the junction's gap frequency, at which the surface resistance of Nb films beco mes considerable, and its effect on the performance of SIS mixers.