Far-field radiation estimation from near-field measurements and image theory

This paper proposed a near-field to far-field transformation method for the radiation sources located on a large ground plane, based on the Huygens's principle and image theory. This method uses the tangential electromagnetic fields on a small near-field plane and the vertical electric fields, one tangential component (parallel to the ground plane) of magnetic fields around the near-field plane to extract the equivalent current sources. The far-field radiations are calculated from these equivalent sources and their images. The application of this method in several simulation models indicates that it has very good performance on both simple printed circuit boards and antenna radiation estimation. The method proposed in this paper can decrease the total scanning area in real measurements.

I. Introduction

The near-field to far-field (NF-FF) transformation technique was introduced for the antenna near-field measurements since 1950's. Initial efforts measured the complete electromagnetic fields configurations surrounding an antenna. The fields were utilized to form the equivalent source to determine the far-field pattern by the Huygens-Fresnel principle [1]. Since it is very difficult to obtain the measured near-field electromagnetic fields over a closed surface, planar, cylindrical or spherical wave functions are used to do the model expansion to estimate the far-field radiation [2]. However, the shortcoming of the model expansion method is the fields outside the measurement area are truncated as zero. Thus, only the particular angular sector of the far fields could be well predicted. On the other hand, paper [3] presents the far-field estimation method of equivalent sources determined from near-field measurements. This method employs procedure of integral equation and method of moment to solve the equivalent sources.