We present a new technique to measure the complex surface impedance of the mixed state of superconducting thin films over the broad frequency range 45 MHz–20 GHz. The surface impedance is extracted from measurements of the complex reflection coefficient made on the film using a vector network analyzer. The technique takes advantage of a special geometry in which the self‐fields from currents flowing in the film are everywhere parallel to the film surface, making it an ideal configuration in which to study vortex dynamics in superconductors. The broadband nature of the measurement system allows us to explore a region of magnetic field–temperature–frequency parameter space of superconductors previously inaccessible with other measurement techniques. The power of the technique is illustrated by measurements on thin films of the high temperature superconductor YBa2Cu3O7−δ.

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