Artificially soft and hard surfaces in electromagnetics and their application to antenna design

This paper give an introduction to the concept of artificially soft and hard surfaces in electromagnetics. The soft and hard surfaces are defined from the boundary conditions of the E-field by an analogy with soft and hard surfaces in acoustics. The electromagnetic soft and hard surfaces have polarisation-independent boundary conditions (at the centre frequency), in contrast to a metallic conductor that has different boundary conditions for the tangential and normal components of the E-field. This makes the soft and hard surfaces desirable in many practical antennas, in particular when dual or circular polarisation is required. The soft and hard surfaces are realised by providing a metal conductor by an anisotropic surface impedance. This can be obtained by providing it with corrugations, or with a strip-loaded dielectric layer. The most known application of the soft surface is in hybrid-mode horns, more commonly known as corrugated horns. The corrugated "soft" surface is also often called "chokes" and used to reduce coupling and sidelobes in particular directions (e.g. choke-horn). The hard surface has only been known a few years. Two of the most important applications of the hard surface are described in the present paper. The first is to design hard horns which have very high aperture efficiency for application in cluster feeds and arrays. The second is to reduce scattering from masts or struts that block the aperture or radiation fleld of an antenna. The paper also describes how known calculation methods now are extended to analyse these types of loaded surfaces.