A Theory of Antenna Electromagnetic Near Field—Part I

We present in this paper a comprehensive theory of antenna near fields in two parts, highlighting in particular the engineering perspective. Part I starts by providing a general conceptual framework for the more detailed spectral theory to be developed in Part II. This paper proceeds by proposing a general spatial description for the electromagnetic field in the antenna exterior region based on an asymptotic interpretation of the Wilcox expansion. This description is then extended by constructing the fields in the entire exterior domain by a direct computation starting from the far-field radiation pattern. This we achieve by deriving the Wilcox expansion from the multipole expansion, which allows us to analyze the energy exchange processes between various regions in the antenna surrounding domain, spelling out the effect and contribution of each mode in an analytical fashion. The results are used subsequently to evaluate the reactive energy of arbitrary antennas in a complete form written in terms of the TE and TM modes. Finally, the concept of reactive energy is reexamined in depth to illustrate the inherent ambiguity of the circuit total electric and magnetic reactive energies. We conclude that the traditional reactive field concept is inadequate to the characterization of the antenna near field in general.

I. Introduction

Antenna practice has been dominated since its inception in the research of Hertz by pragmatic considerations, such as how to generate and receive electromagnetic waves with the best possible efficiency, how to design and build large and complex systems, including arrays, circuits to feed these arrays, and the natural extension toward a more sophisticated signal processing done on site. However, we believe that the other aspects of the field, such as the purely theoretical, nonpragmatic study of antennas for the sake of knowledge for itself is in a state altogether different. We believe that to date the available literature on antennas still appears in need of a sustained, comprehensive, and rigorous treatment for the topic of near fields, a treatment that takes into account the peculiar nature of the electromagnetic behavior at this zone.