Spatially resolved measurements of HTS microwave surface impedance

We describe further development of a novel technique for the characterization of microwave properties of HTS films which allows the spatial variation of this important physical parameter to be measured. The method employs a dielectric puck system that can be moved over the surface of a large HTS wafer, sampling the surface impedance at a number of discrete frequencies between 5 and 15 GHz. The surface impedance can also be rapidly measured as a function of microwave magnetic field strength. Spatial resolution for the prototype system is as small as 1-2 mm. The surface resistance and the shift in surface reactance can be measured by using a loop oscillator which can be interrupted by a fast microwave switch. The decay of microwave power in the resonator is then monitored as a function of time to determine the power dependent surface impedance parameters. This process is extremely fast and straightforward and the loop oscillator configuration permits only relatively inexpensive components to be used. We describe measurements made at 11.5 GHz of the spatial variation of the non-linear surface impedance of a number of HTS films at 77 K.