LINE IMPEDANCE AND PROPAGATION COEFFICIENT OF NARROW SUPERCONDUCTING COPLANAR LINES MADE OF YBACUO

The microwave properties of coplanar waveguides with line widths from 1 mu m to 40 mu m made of superconducting YBaCuO films with a thicknes s t = 180 nm on LaAlO3 are investigated. The line impedance Zr and the normalized propagation coefficient beta/beta(0) of these waveguides a re measured between 45 MHz and 26.5 GHz at temperatures between 77.4 K and 92 K. The ratio of the line width w to the distance of the ground layers d is constant with w/d = 0.2. Therefore, Z(L) and beta/beta(0) are independent of w for perfectly conducting waveguides. For superco nducting waveguides it is found that Z(L) and beta/beta(0) differ from the values of perfectly conducting waveguides. They increase for smal ler line widths at a constant temperature. At w = 1 mu m and T = 80 K, Z(L) and beta/beta(0) are nearly twice as high as calculated for perf ect conductors, Furthermore, Z(L) and beta/beta(0) increase with the t emperature. It is shown that these effects are attributed to an increa se of the inductance per unit length L' due to the superconducting mat erial, whereas the capacitance per unit length C' behaves like C' of p erfectly conducting waveguides. Using these results, the dimensions of the superconducting waveguides, which are necessary to obtain a desir ed Z(L) at a given line width w, are calculated.