Evaluating near-field radiation patterns of commercial antennas

A source reconstruction technique from the measured near fields is proposed to obtain a set of equivalent currents that will characterize the forward and backward radiation patterns of an antenna. Once the equivalent sources are determined, the electromagnetic field at any aspect angle and distance from the antenna can be calculated. In this paper, the method is applied to the evaluation of the radiation from commercial antennas at any observation point. The electric field patterns of a DCS base station antenna at 1800 MHz and a horn antenna at 2500 MHz have been calculated and plotted at several distances from the antenna. This method can be used in characterizing the "reference volumes" or exclusion zones for transmitting antennas dealing with the maximum levels of electromagnetic radiation safe for human exposure, as stated in many national and international regulations

I. Introduction

For most commercial antennas, only the far field pattern along the principal planes is specified. Although this can be useful for most cases, this does not provide information about the pattern at other aspect angles out of the main planes. Sec-ondly, no phase information is available. Thirdly, one cannot derive how the antenna radiates at near distances except for the one provided. This makes it difficult to know what is the safe distance for human exposure for a given input power level to the antenna.