A study is presented on the directivity of grounded low-permittivity metamaterial slab structures that achieve highly directive broadside radiation. Two-dimensional (2D) configurations excited by electric line sources are considered, adopting a scalar plasma-like dispersive permittivity for the metamaterial medium that suitably models a wire medium in the presence of a 2D electromagnetic field, with the electric field directed along the wire axes. The role of leaky waves in producing the high directivity attainable with such structures is illustrated by comparing it with a simple ray-optic model for the radiation mechanism. We show how for increasing substrate heights, the leakywave effect (dominant for "optimal" thicknesses, and responsible for the very high directivity) evolves into a lensing effect purely explained with ray-optics, when losses are present. © 2006 Wiley Periodicals, Inc.

A grounded wire-medium slab has recently been shown to support leaky modes with azimuthally independent propagation wavenumbers capable of radiating directive omnidirectional beams. In this paper, the analysis is generalized to wire-medium slabs in air, extending the omnidirectionality properties to even modes, performing a parametric analysis of leaky modes by varying the geometrical parameters of the wire-medium lattice, and showing that, in the long-wavelength regime, surface modes cannot be excited at the interface between the air and wire medium. The electric field excited at the air/wire-medium interface by a horizontal electric dipole parallel to the wires is also studied by deriving the relevant Green's function for the homogenized slab model. When the near field is dominated by a leaky mode, it is found to be azimuthally independent and almost perfectly linearly polarized. This result, which has not been previously observed in any other leaky-wave structure for a single leaky mode, is validated through full-wave moment-method simulations of an actual wire-medium slab with a finite size. © 2006 IEEE.

Radiation features are studied for a grounded wire-medium slab excited by a simple canonical source, i.e., a horizontal electric dipole. For the first time, an approximate analysis based on a homogenized model for the wire medium as well as a rigorous full-wave analysis of the actual periodic structure are presented. The homogeneous model takes into account both anisotropy and spatial dispersion of the metamaterial medium in the long-wavelength regime. One rather surprising result is that this structure allows for directive pencil beams at broadside that are azimuthally symmetric (in spite of the directionality of the wires). The structure also allows for conical beams that point at a chosen scan angle, where the beam angle and beamwidth are azimuthally independent, and the beam peak in the elevation planes remains approximately constant during the scanning process, in contrast with other types of planar leaky-wave antennas. These remarkable features are explained in terms of the azimuthal independence of the wavenumber for the leaky mode that is responsible for the beam. © 2008 IEEE.

In this paper, an analytical method is proposed to estimate the 3dB power bandwidth of a Fabry-Pérot Cavity antenna covered by a thin frequency selective surface. The 3dB power bandwidth of the antenna is found using the broadside radiation power density of the antenna calculated based on the reciprocity theorem along with a transmission line model of the antenna and the circuit model of the FSS layer. The accuracy of the proposed method is verified by numerical simulations. © 2011 IEEE.

In this paper an investigation is presented of metamaterial structures excited by a line source aimed at producing narrow directive beams. The structure under consideration is a grounded slab made of a homogeneous metamaterial medium with a plasma-like dispersive permittivity; for low values of the slab permittivity an extremely directive beam pointing at broadside can be obtained. Conditions for the maximization of radiation at broadside are given and the narrow-beam effect is shown to be related to the excitation of a leaky mode supported by the slab, with radiation maximization corresponding to small and equal values of the phase and attenuation constants. The frequency bandwidth and directivity are expressed in a simple closed form in terms of the attenuation constant of the leaky mode. By increasing the slab height for a fixed frequency, the leaky mode is analytically shown to give rise to a beam that is scanned from broadside to the critical angle for plane-wave refraction, thus being confined to a narrow angular region around broadside. Numerical results are given that illustrate these features, and full-wave simulations of a metamaterial structure made of an array of metallic cylinders are presented that confirm the results of the analytical study. The case of a line source inside a semi-infinite metamaterial region is also considered and its radiation characteristics compared with those of the metamaterial slab. © 2006 IEEE.

Radiation features of an elemental electric dipole source embedded inside a grounded wire-medium slab are investigated. A homogenous model is adopted which takes into account both anisotropy and spatial dispersion of the metamaterial medium in the long wavelength limit. An equivalent transverse network is derived for plane waves TE and TM with respect to the wires' axis, coupled by a four-port network associated with the air-slab interface, and the far-field pattern is then derived via the reciprocity theorem. Numerical results are provided which show the possibility of achieving a narrow azimuthally symmetric pencil beam at broadside by properly choosing the physical and geometrical parameters of the structure. MoM results for the wire medium, not shown in this summary, confirm the phenomenology observed with the homogenous model. ©2006 IEEE.

Leaky modes supported by a grounded wiremedium slab are studied here in detail for the first time. Such leaky modes are responsible for highly-directive radiation when the structure is excited by a dipole source. The analysis is carried out by means of a rigorous full-wave numerical approach based on the method-of-moments and also by using an approximate homogenized model. The use of a homogenized model that properly accounts for anisotropy and spatial dispersion leads to a remarkable and surprising result: the wavenumber of the leaky mode is independent of the azimuth angle of propagation on the surface. This result, which has not previously been observed in any other leaky-wave structure, allows for the creation of narrow conical beams that are azimuthally symmetric, as well as symmetric broadside beams. This conclusion is confirmed by the full-wave method-of-moments analysis. © 2007 IEEE.

This review paper summarizes various aspects of directive beaming and explains these aspects in terms of leaky waves. Directive beaming occurs in antenna design where a narrow beam is obtainable by using fairly simple planar structures excited by a single source. These structures include Fabry-Pérot cavity structures as well as metamaterial structures made from artificial low-permittivity media. Directive beaming also occurs in the optical area where it has been observed that highly directive beams can be produced from small apertures in a metal film when an appropriate periodic patterning is placed on the film. One aspect that these phenomena all have in common is that they are due to the excitation of one or more weakly attenuated leaky waves, the radiation from which forms the directive beam. This is established in each case by examining the role of the leaky waves in determining the near-field on the aperture of the structure and the far-field radiation pattern of the structure. © 2011 IEEE.