13-10-2010, 05:07 PM
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A COMPARATIVE STUDY OF CONVENTIONAL AND METAMATERIAL-BASED
PLANAR LEAKY-WAVE ANTENNAS FOR DIRECTIVE RADIATION AT BROADSIDE
(2) “La Sapienza” University of Rome, Electronic Engineering Dept. - Via Eudossiana 18, 00184 Roma, Italy.
(3) University of Siena, Information Engineering Dept. – Via Roma 56, 53100 Siena, Italy.
(4) University of Houston, Electronic and Computer Engineering Dept. - Houston, Texas, 77204-4005 USA.
Abstract:
In this work, one-dimensional and two-dimensional leaky-wave antennas based on planar structures are considered, aimed at producing narrow directive beams pointing at broadside. A comparative analysis is presented of different classes of radiators, namely, conventional grounded slabs covered with metallic or dielectric partially-reflecting surfaces, and metamaterial grounded slabs with very low values of permittivity or permeability. The antenna performance is expressed in terms of enhancement of broadside power density with respect to free space, of broadside directivity, and of the relevant frequency bandwidth, as a function of the attenuation constant of the involved leaky modes. The latter is in turn related to the antenna structural parameters, in order to ascertain advantages and drawbacks of the various structures. Numerical full-wave results for antennas excited by printed dipoles are provided.
INTRODUCTION
Leaky-wave antennas (LWAs) based on planar-layered structures have been investigated in recent years due to their advantages in terms of low cost and simplicity. By properly exciting leaky waves on such structures with a simple feed, narrow-beam radiation patterns may be obtained, which are scannable in elevation by varying frequency. The planar LWA typically consists of a grounded dielectric slab with a ‘partially-reflecting surface’ (PRS) on top, which may assume various practical forms. For example, it may consist of a high-permittivity dielectric layer, or a stack of alternating high- and low-permittivity layers [1] (see Fig. 1(a)), or a conducting plate with a periodic array of slots, or a periodic array of metallic patches (see Fig. 1(b)) [2]. In any case, the PRS creates a leaky parallel-plate waveguide region between the PRS and the bottom ground plane, in which the leaky mode propagates. A simple source, such as an infinitesimal dipole, inside the substrate can be used to launch the leaky wave. The advent of metamaterials has opened up new possibilities for the creation of planar LWAs. Metamaterials are artificial materials (typically constructed from a periodic structure) that can be described in certain frequency ranges as homogeneous media with frequency-dependent constitutive parameters that can assume very small and even negative values. Progress has already been made in studying both surface and leaky modes supported by planar structures that include metamaterial layers, and in using such materials for antenna applications [3]. An interesting example of this type of antenna consists of a metamaterial layer with a small and positive permittivity placed on a ground plane with a source (such as an elemental dipole or line source) inside (see Fig. 1©). Such an antenna is capable of producing a narrow beam of radiation at broadside, with the beam becoming more directive as the permittivity of the layer decreases. Although the fundamental operating principle was originally explained in terms of a lensing effect (ray refraction), recent work has demonstrated that this antenna is fundamentally a leaky-wave antenna [4]. Furthermore, a careful analysis of this structure has allowed for the development of simple approximate formulas to predict the main antenna characteristics such as gain and pattern bandwidth. In this work, the goal is to make a comparative study of two types of LWAs: those using a grounded substrate and a PRS, and those using a grounded metamaterial slab. The comparison will be made for the practical quantities of interest, such as physical size, directivity, bandwidth, broadside power density enhancement, efficiency, etc. Numerical results will also be presented for the input impedance of both ordinary and metamaterial LWAs excited by printed dipoles.