Challenges in Antenna Designs and Some Novel Techniques for Meeting Them

In this paper we identify some challenges in antenna designs-both small and large types-that we have recently encountered in the process of designing antennas for personal communication, GPS, UWB, Millimeter waves, BANs (body area networks), sensors, RFID, and other similar applications. The primary challenges in designing small antennas arise from the specifications that are often very demanding in terms of performance expectations, e.g., gain, bandwidth, pattern characteristics (including polarization), decreased sensitivity to the environment, etc., even as the above specs concurrently place severe restrictions on the size of the available real estate in which the antenna is to be placed. We should mention that even though the antenna itself may be small, the degrees of freedom (DOFs) associated with the problem description can still be large, especially when dealing with antenna/metamaterial composites, which often pose a considerable challenge when one attempts to simulate them. Needless to say that the topic of metamaterial has attracted considerable attention in recent years, as evidenced by the number of researcher contributing to the subject, who have been very prolific indeed. It would be presumptive to attempt to fully cover and renew these developments in this presentation-given the time limitation-because it would be impossible to do justice to the diverse away of facets that have all been grouped under the general umbrella of "Metamaterials." In view of this, we narrow our focus down to the topic of "Performance Enhancement of Small Antennas using Metamaterials," because such enhancements are certain to have a very large impact on communication antenna design, which, we believe to be a centerpiece topic of this conference. The first impression that comes to one's mind when one scans through and takes a cursory look at the vast array of publications that discuss antenna-material composites is that the metamaterials provide the proverbial "Midas touch," a veritable magic bullet for curing everything that ails the small antennas. If we were to compile a wish-list for the desired features of small antennas, we would most probably include: wide bandwidth in terms of both impedance match (return loss) and radiation pattern, high directivity, and conformal nature-to name just a few of the desirable attributes of the antennas that we might be designing for communication, RFID, GPS, LAN or BAN applications. Some of the small antenna/metamaterial composite type of configurations to be discussed in the presentation are shown in Figs. 1 through 5. For large antennas, the design goals are often very different, and the emphasis here may be to realize ultra-low sidelobes, reduce EMI, achieve superior EMC characteristics, etc., just to name a few. Here, the challenge often is how to model these large structures in an accurate and efficient manner so that their performance can be predicted reliably and confidently, before the antenna is actually fabricated-the motto being "Correct by Design," or CBD. Commercial softwares are often quite limited in their ability to handle the structures which require a problem description with a very large number of DOFs that could reach-or exceed- 10
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unknowns, because of their complexity, multi-scale characteristics and inhomogeneous nature. With the advent of powerful computers such as the IBM/BlueGene (see Fig.6), it is now possible to solve problems of this magnitude accurately and efficiently. Illustrative examples of antenna configurations that fall in this category are: Array feeds for the Square Kilometer Array (SKA); coupling between two Phased arrays mounted on a large platform, e.g., the topside of a ship; and, the design of large doubly-curved subreflectors, comprised of multilayer FSS screens, for frequency reuse applications. A few of these representative large antenna problems are shown in Figs. 7 through 10, and would be discussed in the presentation.