A Critical Look at the Performance Enhancement of Small Antennas using Metamaterials

Currently, there is a great deal of interest in enhancing the performance of small antennas using metamaterials. Of course, the interest itself is not new, since the search has always been actively pursued for ways to increase the directivity, reduce the sidelobe and backlobe levels, improve the bandwidth, etc., of small antennas whose dimensions are limited by the available real estate. What is novel on the scene, then, is the advent of metamaterials, which have been touted as the magic bullet that would lead in to the holy grail of small antenna design, namely, a size that is small comparable to the wavelength, but one that has a high directivity (and even gain), wide impedance and pattern bandwidths, etc., but with little or no limitations that compromise the performance of the antenna. The purpose of this paper is twofold. First, we compile the various definitions of metamaterials that have appeared in the literature with a view to examining them critically and then coming up with one that not only appears to have a consensus, but also one that makes practical sense. We separate the metamaterials that are artificial dielectrics - typically periodic in nature into two principal categories: (i) those with unit cell sizes that are small comparable to the wavelength (aplambda/10) and display SNG or DNG characteristics; (ii) the periodic structures whose unit cell dimensions are on the order of lambda - typically in the range of lambda/2 - and which are also referred to as EBG materials and frequency selective screens (FSSs). Some examples are shown. The paper then presents examples of both antenna/metamaterial and antenna/EBG composites, used as substrates, superstrates as well as shell type of covers enclosing small antennas. Following this, the paper examines some theoretical limits on the directivity, gain-bandwidth product, etc., to see if the use of SNG or DNG metamaterials enables us to breach these limits. Finally, the paper also poses the question whether the use of DNG offers any advantage over the conventional use of EBG substrates and superstrates, and attempts to answer it via some examples