Interpretation of Antenna Scattering Phenomena with the Aid of Characteristic Mode Theory

This article is a study of electromagnetic (EM) antenna scattering phenomena from the Characteristic Mode (CM) standpoint and aims to shed some light on scattering from antennas. Using CM theory, the modes can be classified to yield two different sets of modes. One set is orthogonal with the localized antenna's source field in the vicinity of the feed point and this set of modes are referred to as the short circuit “Scattered Modes”. The other set includes the modes that can be excited by the antenna's source at the feed point and this set forms the so-called re-radiated fields or “Antenna Modes”. These modes are studied to determine the effect of each set on the overall scattering by an antenna within these scattering/re-radiation environments. This analysis provides additional physical insight as well as a new and different approach that can be used in various antenna applications.

I. Introduction

Characteristic Mode (CM) Theory is an ideal tool to analyze an antenna for both radiation and/or scattering cases. In antenna scattering, the incident field is generated by a source away from the antenna. The scattered field by an antenna contains a component, that is usually referred to as the “short-circuit” scattered field, and a second component that is proportional to the radiated field. The radiated field is defined as the field that can be generated by a source located at the feed point of the antenna. Using CM theory, the CM modes can also be classified based on these two components to yield two different sets of modes. One set is orthogonal to the localized antenna's source field at the feed point and thus is referred to as the short-circuited scattered field or “Scattering Modes” because it cannot be excited by the antenna source located at the feed point. The other scattered set includes modes that can interact with the load attached to the antenna (internal impedance of antenna source). Because these modes can induce a non-zero voltage at the feed point, they are referred to as the “Antenna Modes”. These latter modes are proportional to the radiation pattern of the antenna. In this article, a study of this phenomenon from the CM standpoint will be performed aiming to shed some light on antenna scattering. The CM modes are studied to determine the effect of each set on the overall antenna within these scattering/radiation environments.