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A new antenna diagnostics technique has been developed for the DTU-ESA Spherical Near-Field Antenna Test Facility, which is operated by the Technical University of Denmark (DTU) for the European Space Agency (ESA). The technique is based on the transformation of the spherical-wave expansion (SWE) of the radiated field, obtained from a spherical near-field measurement, to the plane-wave expansion (PWE). It allows an accurate reconstruction of the field in the extreme near-field region of the antenna under test (AUT), including the aperture field. The fundamental properties of the SWE-to-PWE transformation, as well as the influence of finite measurement accuracy, have been reported previously. We validate here the new antenna diagnostics technique through an experimental investigation of a commercially available offset reflector antenna, where a tilt of the feed horn and surface distortions were intentionally introduced. The effects of these errors were detected inthe antenna's far-field pattern, and the accuracy and ability of the diagnostic technique to subsequently identify them were investigated. Real measurement data were employed for each test case.