Review of Soft and Hard Horn Antennas, Including Metamaterial-Based Hybrid-Mode Horns

This paper is divided into three parts. Part one gives an overview of the early history of the design and implementation of hard horn antennas, and of the concept formulation of soft and hard electromagnetic surfaces. Part two presents a review of all known classes of soft and hard hybrid-mode horns. Part three presents a new class of hybrid-mode horns, based on the use of a low-index metamaterial liner on the wall. Moment-Method analysis of both soft and hard metamaterial horns indicates that it may be feasible to realize these horns with very large bandwidth. This is because the desired (analyzed) metamaterial dispersion is similar to the Drude dispersion curve (monotonically increasing permittivity as a function of frequency), which represents typical electromagnetic dispersion in dense media. A successful implementation of these horns depends on whether low-index metamaterials can be implemented with polarization-independent boundary impedance, favorable dispersion characteristics, and reasonable loss, mass, and production cost.