Radiation pattern evaluation from near-field intensities on planes

TLDR

The problem is formulated as a quadratic inverse problem where the data are the squared amplitudes of the near‑field, and analysis of the mapping properties shows how to avoid local minima during minimization. The study proposes and implements a method to determine far‑field radiation patterns from phaseless measurements taken on two planar surfaces. The approach seeks the global minimum of a weighted functional, using a finite‑dimensional representation of the source field limited to a fixed angular domain, and employs an iterative algorithm that converges to the solution regardless of the initial guess. Experimental results at 9 GHz on a shaped reflector confirm the method’s accuracy and demonstrate that relying solely on field intensity measurements can dramatically reduce the complexity and cost of near‑field antenna testing.

Abstract

This paper describes, analyzes, and implements an approach to far-field determination from phaseless measurements over two planar surfaces. A proper formulation of the problem is considered as a quadratic inverse one whose data is the square amplitude of the near field. A solution is introduced as the global minimum of an appropriate functional. Next, to perform such a minimization procedure, a finite dimensional representation 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 unknowns to the data makes it possible to analyze how to escape from the local 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 sarfa,ces and of appropriate weights in the functional definition is emphasized, and a reliable iterative procedure converging on the solution, regardless of the starting point, is thus obtained. This 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 testing and diagnostics can be reduced to a very large extent.