Artificial Dielectrics and Fresnel Lenses for High Power Microwave Applications

"Lens antennas with large apertures could be useful in a variety of High-Power Microwave (HPM) and Ultra-Wideband (UWB) applications. Such lenses are needed in the frequency range of 0.2-2 GHz, and they must have low loss and dispersion. Existing lensesmade of solid dielectric materials, such as polyethylene, are too dense and weigh too much to be practical for large apertures. To build a lightweight lens, one would consider an artificial dielectric (AD) material. Commercially available artificialdielectrics are typically composed of conductively coated glass micro-spheres embedded in epoxy. While they have one-sixth the density of polyethylene, they could be made even lighter with an improved design. A lighter artificial dielectric can bemanufactured in a variety of forms, the most promising of which appears to be a uniform array of discs embedded into a lightweight foam. The density of such a material is roughly proportional to the bandwidth of the material. If we limit the high end ofthe material performance to 2 GHz, then our calculations show that metallic discs embedded in foam will have a density one-third that of commercially available ADs, and one-eighteenth that of polyethylene. To achieve even lighter lenses, we propose the useof Fresnel lenses for the narrowband HPM case. Fresnel lenses use less material than conventional lenses because they allow for 360-degree phase shifts in the aperture fields as they pass through the