Planar NF-FF with direct optimization-source reconstruction using amplitude only data

The study of near field to far field (NF-FF) techniques based on the knowledge of only amplitude data is mainly encouraged by the complexity of antenna measurements at millimeter frequencies and the economical advantage of amplitude measurement equipment. We present a direct optimization procedure using the known phase-less electric field data for the reconstruction of an equivalent magnetic current density that represents the radiating structure or antenna under test. The technique of using an equivalent magnetic current (EMC) distribution and a matrix method to reconstruct the sources and perform NF-FF transformation from amplitude and phase information over arbitrary scanning has been previously addressed by Petre and Sarkar (1992) and by Las-Heras (see IEE Electronic Letters, vol.35, no.3, p.211-12, 1999). In the case of amplitude only data, a cost function regarding the difference between the known amplitude data and the field radiated by the EMC distribution has now been defined. The conjugate gradient method has been used to minimize the cost function and obtain the EMC distribution. Once the equivalent magnetic current density is determined, the electric field at any point can be calculated. This approach is applied to the near field to far field transformation problem as well as to diagnostic tasks. Numerical and measurement results from one plane and two planes are presented.