In this dissertation measurements on the surface impedance of superconducting niobium, tantalum, and a 30 at% niobium-70 at% tantalum (Nb30-Ta70) alloy are reported. The measurements were made on X-band TM010 mode cavities at a nominal frequency of 8.6 GHz. The surface resistance and the change of surface reactance for the three materials were measured from 1.3 K up to their respective transition temperatures. These data were compared to computer calculations based on the Mattis Bardeen expressions for the surface resistance and reactance of super-conductors.
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