Antenna pattern determination by MST from square complex field data

Near field techniques for antenna testing deal sometimes with incomplete data. In this paper we consider the Modulated Scattering Technique (MST), which, in the monostatic measurement configuration, probes directly the complex square of the near field. In this case, at each measurement point, the correct phase determination must be associated with the available signal in order to compute the radiation pattern. Such a non linear problem is formulated as the inversion of a quadratic operator connecting the unknown near field distribution over a plane to the available square field data. The solution is then defined as the global minimum of an appropriately defined fourth order functional. A second set of square field data, collected over a different planar surface, plays an important role in order to overcome the problem of the local minima occurrence in the minimization procedure. The similarity of the present problem with the so-called phase retrieval problem for the same scanning configuration is also emphasized. The numerical results for a planar geometry confirm the reliability of this approach when realistic data are used.