Una soluzione alternativa al problema della ricostruzione olografica della superficie delle antenne a riflettore da dati in far-field o in near-field

Abstract

The retrieval of surface profile of a reflector antenna is an important task, mainly for radio astronomical application. Up to now the microwave holographic reconstruction technique has been applied to far field data either directly measured or extrapolated from near field measurements by using the Fourier transform inversion. The main step of this technique is the computation of the induced currents on the reflector surface. In this thesis the relation between the far field (or near field) and the current on the parabolic reflector surface is formulated as a discrete data inverse problem. Since this problem is strongly ill conditioned, an alternative technique is proposed to find a solutionin the least squares sense and to regularize it by using a Singular Value Decomposition. This technique supplies a flexible regularization scheme, able to take into account also the noise level of the data. Moreover it allows to reconstruct a surface distortions map using either far field data or the direct use of Fresnel zone field data, i.e. these data are not used to extrapolate the far field pattern, but directly used by the procedure. A comparison with the standard technique based on the FFT inversion algorithm has been made on a number of different cases, with noise free and noisy far field data generated by a commercial software. The results show that this alternative procedure is able to delect, with a very good accuracy, both the position and the amount of the distortions of the reflector surface starting from both far field and near field data.