On the natural frequencies of an annular ring above a conducting half space

Abstract

The natural resonant frequencies of an object embedded in a stratified conducting medium are considered. The special case of an annular ring is considered in detail. Extensive numerical results are examined for the case of a ring in free space above a conducting half space. This simple problem provides insight into the behavior of the natural frequencies of airborne radar targets above the earth, ice, or sea surface. Results reveal an intricate pattern of pole trajectories in the complex plane. These trajectories are studied for various combinations of height, ring geometry, constitutive parameters, and modal index.