Semi-transparent Spherical Shells as Omnidirectional Retroreflectors

Abstract

The use of microwave sensors (radars) in the areas of assisted and autonomous driving (cars) and flying (UAVs) suggests the need for radarreflectors which could operate over a wide range of spatial angles, ideally fully independent of the mutual position of the radar and the vehicle. It is also desirable that the reflector be passive such that no external energy supply would be necessary. The functionality of the conventional passive radarreflectors, however, is limited in space, e.g. to a close vicinity of the normal direction (metal plates) or a 90° spherical sector (corner reflectors) or a hemisphere (Luneburg lenses). A metal sphere is perfectly omnidirectional but its backscattered signal is weak. This paper shows that spherical shells with semi-transparent walls, dielectric or resistive, may serve the need by providing fully omnidirectional response, much stronger than equally large metal spheres.