RADIATION-PATTERN EVALUATION FROM NEAR-FIELD INTENSITIES ON PLANES

This paper describes, analyzes, and implements an approach to far-fiel d determination from phaseless measurements over two planar surfaces, A proper formulation of the problem is considered as a quadratic inver se one whose data is the square amplitude of the near field, A solutio n is introduced as the global minimum of an appropriate functional. Ne xt, to perform such a minimization procedure, a finite dimensional rep resentation of the field radiated by sources whose plane-wave spectrum becomes negligible outside a fixed angular domain is used, A detailed investigation of the properties of the mapping connecting the unknown s to the data makes it possible to analyze how to escape from the loca l minima possibly met in the course of the minimization procedure, To this end, the crucial role of the availability of phaseless data over two different surfaces and of appropriate weights in the functional de finition is emphasized, and a reliable iterative procedure converging on the solution, regardless of the starting point, is thus obtained. T his property is confirmed by experimental results concerning near-zone data from a shaped reflector at 9 Ghz. It can be readily appreciated that when only the field intensity is detected the complexity and the cost of the equipment required for near-field techniques in antenna te sting and diagnostics can be reduced to a very large extent.