The paper describes an experimental technique for measuring microwave magnetic diffraction fields. It establishes the capabilities of the modulated scattering method when a small loop containing two subminiature diodes in parallel is used as the scatterer. Extensive data for amplitude and phase distributions of both the electric and the magnetic fields over the entire region of a circular aperture irradiated by a plane wave have been obtained. Distributions of diffraction fields both in front of and behind the aperture are presented. Discussions on cross-polarization components and the effect of screen thickness are also included.