Wideband Surface Impedance Measurements in Superconducting 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 relate the material properties of a sample under study to the measured surface impedance, which is affected by the geometry of the measurement, and we discuss the effects on the proposed approximate calibration procedure. We experimentally check the applicability of the approximations at the ground of the method, and we thoroughly discuss the uncertainties related to the approximations employed. Finally, we report the sample measurements of the surface impedance of two MgB2 superconducting films.