• AMC
• Artificial Impedance Surfaces
• Artificial Magnetic Conductor
• EBG
• Electromagnetic Absorbers
• Electromagnetic Bandgap
• High-impedance Surfaces
• HIS
• Metamaterial
• Metasurfaces
• Radar Absorbing Material
• RAM

High-impedance surfaces (HIS) are resonant cavities synthesized by printing a periodic frequency selective surface (FSS) on the top of a grounded dielectric slab. The use of such metasurfaces allows to achieve remarkable improvements in the performance of several microwave devices.
The first part of the thesis is dedicated to the modeling of high-impedance surfaces and frequency selective surfaces which constitute the key element of the HIS structures. A model for analyzing frequency selective surfaces is first introduced and then employed also in the analysis of high-impedance surfaces. The circuital approach is also employed to derive a new definition of the bandwidth of the metasurfaces.
High-impedance surfaces are employed in the design of low-profile antennas and ultra-thin electromagnetic absorbers. In the latter case thin high-impedance surfaces are used to replace the typical quarter-wavelength grounded dielectric slab making the absorbing structure much thinner. Most part of this thesis is dedicated to this issue and in particular the physical mechanisms leading to the absorption of electromagnetic waves are studied by the aforementioned transmission line model. Different designs based on resistive frequency selective surfaces for synthesize thin, wideband and lightweight absorbers with enhanced performances with respect to the available solution are proposed and analyzed. Other techniques as the use of vias as instrument to enlarge the operating bandwidth for oblique incidence and the use of the HIS boundary in place of the electrical one in classical commercial absorbers are proposed.
High-impedance surfaces are also employed as ground plane for make an horizontal dipole to radiate efficiently. In this configuration, the interaction between the wire antenna and the high-impedance surface excites complex waves and generates different resonant phenomena that require a rigorous analysis. The problem is thoroughly analyzed by breaking up the problem in two different contributions depending on the surface waves that are allowed to propagate in the HIS structure and important design rules are derived. The validity of the homogeneization models for approximating the finite antenna structure is also addressed. The last part of the thesis deals with the so-called Fabry-Perot antenna in which high-impedance surfaces are used to strongly reduce the height of this antenna which typically is equal to a half wavelength. In particular the thesis describes an active structure which is able to work as a tunable high-gain antenna over a wide band.
The presented theoretical work is supported by several experimental demonstrations.