TABLE OF CONTENTS Somewhere, within almost every treatise on antenna theory and design can be found a passage, or more typically several passages, that describe techniques for determining the far-field properties of a given radiator using Maxwell's equations, and utilizing a detailed knowledge of the mechanical structure of the antenna and the enforced current distribution. Although effective, such methodologies are overly restrictive for antenna measurements as such detailed information is not typically available to the antenna metrologist. Thus, recourse to more generic treatments becomes unavoidable. The approach usually adopted within the field of near-field antenna measurements is to sample the, potentially very complicated, electric field produced by an antenna over a convenient enclosing surface and then to try and determine the properties of that field elsewhere in space.
In general, if the properties of an arbitrary, potentially complex, field distribution are known in one region of space it is difficult to determine the properties of that field elsewhere. However, this is not the case for certain types of elementary field, that is, mode. If for example, the amplitude, phase and direction of propagation of a plane wave are known, then the properties of this field can instantly be determined everywhere in space. Thus, if the complex field distribution associated with some radiator can be decomposed into a summation of plane waves propagating in diverse directions then similarly, the properties of the complex field distribution can be determined throughout space from a summation of the properties of the respective plane waves. Furthermore,...
