Enhancement of directivity by using metamaterial substrates

An interesting aspect of using metamaterials in radiating systems is the possibility to create highly directive beams from simple sources placed inside planarly layered structures. Assuming for simplicity an isotropic model for the metamaterial, such enhancement of the radiative performance may be obtained through the realization of materials with either very low or very high positive values of the permeability and/or the permittivity. Conventional designs of planar directive antennas are based on resonant structures, in which a simple feed is placed inside a partially open one-dimensional cavity (a sort of Fabry–Pérot Cavity, FPC). In this chapter the operation of such FPC-based antennas is reviewed and compared with that of metamaterial-based designs, emphasizing the common role played by leaky waves in establishing the main radiation properties of the antennas. A particular focus will be given to the case of a grounded slab made of a metamaterial with a very low positive value of the permittivity (epsilon near zero, ENZ). This particular structure allows us to illustrate clearly the mechanism of radiation and also to describe the interesting performance of a possible practical implementation of the ENZ medium by means of a periodic arrangement of parallel metal wires (wire medium).