A broadband electron spin resonance spectrometer is described which operates at frequencies between 4 and 40 GHz and can be used in superconducting magnets. A tunable cylindrical cavity is connected to a vector network analyzer via coaxial cables, and the radiation is fed into the cavity by a coupling loop. No field modulation is employed. Resonance frequencies below 14 GHz are obtained by inserting dielectrics with different permittivities into the cavity. The setup allows for measurements with the microwave magnetic field either parallel or perpendicular to the external field. 2010 American Institute of Physics.