A "metamaterial" by its widely accepted definition is an artificially engineered structure that gains its material properties from its structure as opposed to its intrinsic material composition. The field of metamaterials has gained much attention within the scientific community over the past decade. With continuing advances and discoveries leading the way to practical applications, metamaterials have earned the attention of technology based corporations and defense agencies interested in their use for next generation devices. With the fundamental physics developed and well understood in some areas, current research efforts are driven by the demand for practical applications, with a famous example being the well-known microwave "invisibility cloak." Gaining exotic electromagnetic properties from their structure as opposed to their intrinsic material composition, metamaterials can be engineered to achieve tailored responses not available using natural materials. With typical designs incorporating resonant and dispersive elements much smaller than the operating wavelength, a homogenization scheme is possible, which leads to the meaningful interpretation of effective refractive index, and hence electric permittivity and magnetic permeability. Most metamaterials consist of scattering element arrays embedded in a host matrix. The scattering elements are typically identical, and the electromagnetic properties of the medium can be inferred from the properties of the unit cell. This convenience allows the designer to engineer the effective electromagnetic parameters of the medium by modifying the size, shape, and composition of the unit cell. An important rule when designing a metamaterial is to keep the size and periodicity of the scattering elements significantly smaller than the operating wavelength (lambda0=10 or smaller), as this allows for meaningful interpretation of the material's bulk properties based on the behavior of the unit cell.;This dissertation summarizes several key projects related to my research efforts in metamaterials. The main focus of this dissertation is to develop practical approaches to frequency tunable and reconfigurable metamaterials. Chapter one serves as a background and introduction to the field of metamaterials. The purpose of chapters two, three and four is to develop different methods to realize tunable meta-materials -- a broad class of controllable artificially engineered metamaterials. The second chapter develops an approach to characterizing metamaterials loaded with RF MEMS switches. The third chapter examines the effects of loading metamaterial elements with varactor diodes and tunable ferroelectric thin film capacitors (BST) for external tuning of the effective medium parameters, and chapter four develops a more advanced method to control the response of metamaterials using a digitally addressable control network. The content of these chapters leads up to an interesting application featured in chapter five -- a reconfigurable frequency selective surface utilizing tunable and digitally addressable tunable metamaterials. The sixth and final chapter summarizes the dissertation and offers suggestions for future work in tunable and reconfigurable metamaterials. It is my hope that this dissertation will provide the foundation and motivation for new researchers in the field of metmaterials. I am confident that the reader will gain encouragement from this work with the understanding that very interesting and novel practical devices can be created using metamaterials. May this work be of aid and motivation to their research pursuits.
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