On the performances of two algorithms in phaseless antenna measurements

Antenna diagnostics techniques from phaseless measurements of the near field are receiving more and more attention, because they are the only viable methods at millimeter and submillimeter frequencies for the testing of antennas, with a diameter of some thousands of wavelengths and with low sidelobe levels. Two techniques for predicting the radiation pattern of the antenna from intensity measurements of the near field are currently under consideration. The first one is the Gabor holography which employs the interference intensity distribution between the antenna under test and a known reference source. However, such a technique presents some drawbacks concerning the measurement procedure. The second technique is phase retrieval, which requires a knowledge of the amplitude information of the near field over two scanning surfaces. Since the reference wave is not required, such a method is not affected by stability problems and the achievable accuracy is limited only by the low intensity drift. However, the phase retrieval method requires a knowledge of the intensity distributions over two scanning surfaces; this can be overcome when enhanced holographic techniques, which require three measurements of intensity distributions over a single surface, are employed. In any case, for the phase retrieval method the problem of the determination of a complex function, i.e. the near field, from amplitude data arises. The performance of the two approaches are compared with respect to the local minima problem by investigating the different mathematical properties