Cryogenic Microwave Wideband Measurements of Superconducting Thin Films

We discuss calibration issues in wideband (2-20 GHz) surface impedance measurements in a cryogenic and high-magnetic-field environment. We show that, in the case of measurements taken on superconducting thin films, the substantial impossibility of a full, three-standard calibration can be partially overcome by exploiting appropriate approximations and the basic properties of superconductors. We discuss a modification of the so-called "short-only" calibration, in order to increase the accuracy of the measurements. We check experimentally the applicability of the approximations at the ground of the method, and we report sample measurements of the surface impedance of a MgB2 superconducting film.