Implementation of phase retrieval techniques for improving the results of phased array antennas near-field measurements

Abstract

Abstracf - To enhance the performance of existing near field antenna test facilities the idea of far field pattern determination using both the amplitude and phase measurements and the phaseless measurements techniques is proposed. In this way some difficulties related to precise phase measurements are overcome. The feasibility and the performance of the introduced techniques are shown through numerical examples. Both simulation and measurement results will be presented to demonstrate the utility of such approach. Comparison will be made behveen the antenna patterns calculated using traditional near field - far field (NF/FF) transformations, the phaseless measurements and the ones obtained by applying both the amplitude and phase measurements. The most widespread techniques for alignment and testing large phased array antennas (PAA) with electrically scanned beams are based on near field - far field (NF/FF) transformations. They requires the precise measurements of both the amplitude and the phase of the radiated near field on a prescribed surface. Whereas amplitude measurements are relatively easy to perform with sufficient accuracy, for phase measurements many problems arise. For obtaining precise phase information sophisticated equipment is required, which is more and more expensive as the frequency becomes higher and higher, considerably increasing the overall cost of the testing facility. In order to reduce these drawbacks of available NFiFF techniques and improve the performances of less expensive measurement set-ups, thus extending the usable frequency range, the idea of reconstructing the far field from only amplitude near field data has been proposed and successively developed for the planar scanning geometry [I4 This approach requires the knowledge of the near field amplitude data over two different surfaces and the solution is searched for in two steps. First, a near field complying with the available data is sought for. Next the transformation to the corresponding far field is performed by a standard technique. Recently, the use of phase retrieval techniques has been proposed for antenna diagnostics. Different kinds of developed approaches are essentially based on the retrieval of the phase distribution from the amplitude or from the squared amplitude of the measured field [5-9]. The main drawback of the amplitude only approach is related to the fact that it is essentially based on the retrieval of the phase of the measured field and thus involves the solution of a non-linear inverse problem. As a consequence, like the phase retrieval problem, the far-field determination from the amplitude only near-field data (especially corrupted) is an ill-conditioned problem, where solution is to be found by looking for a generalized solution which minimizes a proper functional. However, because the objective functional casually exhibits many local minima, the solution algorithm may be trapped in a false solution, thus making the corresponding near-field far-field transformation technique unreliable.