EVALUATION OF NIOBIUM TRANSMISSION-LINES UP TO THE SUPERCONDUCTING GAP FREQUENCY

The frequency dependence of the coupling of niobium superconducting tr ansmission lines is measured for frequencies up to and above the super conducting gap frequency. For this purpose superconducting microstrips are integrated in log-periodic planar antennas and the spectral respo nse is measured by means of a Fourier transform spectrometer with 1 mu m2 Nb/Al2O3/Nb SIS junctions as detectors. Resonances of microstrips a re observed up to 650 GHz, the superconducting gap frequency of niobiu m, but above that frequency no resonances have been detected. The meas ured resonance frequencies are in good agreement with a dispersive mod el for superconducting transmission lines based on the Mattis-Bardeen theory. The radiation coupling is lower than calculated, indicating si gnificant dielectric losses.