Improved HTSC surface loss measurements evaluating a multipath transmission signal

Today´s best high temperature superconductive films (HTSC) have residual losses almost as small as those of the classical superconductor niobium at 4.2 K. A cavity measurement system at W-band (75-110 GHz) has been described which allows the determination of these very low surface resistances using a copper cavity. It has been shown that a single-mode cavity model is no longer adequate for the present task, but additional signal paths must be included in order to measure unloaded quality factors with sufficient accuracy. An ultimate sensitivity of /spl plusmn/0.3 m/spl Omega/ has been achieved. At the cost of a smaller dynamic range, the sensitivity could be improved to /spl plusmn/0.1 m/spl Omega/ by optimising the cavity geometry. Three samples of YBCO on LaAlO/sub 3/ have been measured at 76 GHz over temperature. The lowest value observed was 3.0 m/spl Omega/ at 15 K. Wafers of any size can be measured including now available 4-in. (101.6 mm) wafers. As a cooling apparatus any low cost cooling system in combination with a temperature controller is applicable. Expensive cooling down to liquid helium temperature is avoided completely.