IMPROVING THE PERFORMANCE OF ANTENNAS BY USING METAMATERIAL-INSPIRED CAPACITIVELY-LOADED LOOP (CLL) NEAR-FIELD RESONANT PARASITIC (NFRP) ELEMENTS

Abstract

While double negative (DNG) metamaterials (MTMs) were proposed over forty years ago, they have been experimentally demonstrated only in the last decade. The adaptation of a variety of epsilon-negative (ENG), mu-negative (MNG), and double negative (DNG) metamaterials, as well as single ENG, MNG,and DNG MTM unit cells, to achieve antenna systems exhibiting enhanced performance characteristics has received considerable research attention. These include, for example, electrically small antennas, multi-functional antennas, leaky-wave antenna arrays, and higher directivity antennas. Inspired by these metamaterial concepts, several metamaterial-engineered antennas have been investigated in this dissertation to achieve additional enhanced performance characteristics. First, several fabricated and tested variations of the three dimensional (3D) magnetic EZ (easy) antenna at 300 MHz and 100 MHz were examined. The 3D magnetic EZ antenna is composed of an electrically small loop antenna that is coaxially-fed through a finite ground plane and that is integrated with an extruded capacitively loaded loop (CLL) element. This 3D CLL structure is designed to be a near-field resonant parasitic (NFRP) element. With the proper placement of the NFRP element in the very near field of the driven element, it was demonstrated that the overall antenna system achieved complete matching to the source without any external matching network, along with an enhancement of the overall radiation efficiency. Additionally, multi-functional 3D magnetic EZ antennas were designed for wireless communication applications. For instance, by incorporating multiple NFRP elements, several dual-band versions were realized. Second, by properly combining and phasing their effective magnetic dipoles, electrically small multi-band, circular polarized (CP), metamaterial-inspired wire antennas were perfected that are nearly completely matched to a 50 Ω source and have high radiation efficiencies. Again, they were accomplished by incorporating multiple NFRP elements into their designs. Finally, two tri-band-notched UWB antennas were developed and tested successfully. The notched filters were achieved by introducing printed, electrically small CLL resonators into the UWB antenna design. Each CLL element has a high-Q characteristic and a compact size, which made it a very suitable candidate for a band-stop filter function. Like the split ring resonator (SRR) element, the CLL element is self-resonant and has a resonance frequency that is determined primarily by its loop inductance and the capacitances resulting from the cuts which opened the loop. In contrast, the CLL element has a much simpler, more compact design. It was demonstrated that by placing one, two or three CLL elements near the feedline of the UWB antenna and by tuning their sizes, one can control the band-notched frequencies of the radiator, while minimizing their space requirements, to achieve single, dual, and tri-band notched-filter UWB antennas.