IMPROVING THE PERFORMANCE OF ANTENNAS WITH METAMATERIAL CONSTRUCTS

Abstract

Metamaterials (MTMs) are artificial materials that can be designed to have exotic properties. Because their unit cells are much smaller than a wavelength, homog-enization leads to effective, macroscopic permittivity and permeability values that can be used to determine the MTM behavior for applications. There are four possible combinations of the signs of permittivity and permeability values. The desired choice of sign depends on the particular application. Inspired by these MTM concepts, several MTM-inspired structures are adopted in this dissertation to improve various performance characteristics of several different classes of antennas. Three different metamaterial-inspired engineering approaches are introduced to achieve enhanced antenna designs. First,the transmission-line (TL) type of MTM is used to modify the dispersion characteristics of a log-periodic dipole array (LPDA) antenna. When LPDA antennas are used for wideband pulse applications, they suffer from severe frequency dispersionbecause the phase center location of each element is frequency dependent. By incorporating MTM-based phase shifters, the LPDA frequency dispersion properties are improved significantly. Both eight and ten element MTM-modified LPDA antennasare designed to enhance the fidelity of the resulting output pulses. Second, epsilon-negative unit cells are used to design several types of electrically small, resonant parasitic elements which, when placed in the very near field of a driven element, leadto nearly complete matching (i.e., reactance and resistance) of the resulting electrically small antenna system to the source and to an enhanced radiation efficiency. However, despite these MTM-inspired electrically small antennas being very efficient radiators, their bandwidth remains very narrow, being constrained by physicallimitations. Third, we introduce an active parasitic element to enhance the band-width performance of the MTM-inspired antennas. The required active parasitic element is derived and an implementation methodology is developed. Electricallysmall active Z, stub, and canopy antennas are designed. It is demonstrated that an electrically small antenna with ka around 0.046 and over a 10% bandwidth can be realized, in principle.