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.