A robust method for antenna diagnostics that can provide the reconstruction of the aperture field from spherical near- or far-field measurements, is presented. This method is based on the equivalence principle which consists in the rewriting of the spherical wave expansion of the radiated field in terms of infinitesimal electric and magnetic dipoles distributed over the antenna main surface. This method presents the advantage of being very stable and extremely robust beside the measurement noise. Synthesized data (simulated antenna) used with different level of noise will show the viability of this technique for antenna diagnostics purpose.