This dissertation shows a detailed investigation on reconfigurable low profile antennas using tunable high impedance surfaces (HIS). The specific class of HIS used in this dissertation is called a frequency selective surface (FSS). This type of periodic structure is fabricated to create artificial magnetic conductors (AMCs) that exhibit properties similar to perfect magnetic conductors (PMCs). The antennas are intended for radiometric sensing applications in the biomedical field. For the particular sensing application of interest in this dissertation, the performance of the antenna sub-system is the most critical aspect of the radiometer design where characteristics such as small size, light weight, conformability, simple integration, adjustment in response to adverse environmental loading, and the ability to block external radio frequency interference to maximize the detection sensitivity are desirable. The antenna designs in this dissertation are based on broadband dipole antennas over a tunable FSS to extend the usable frequency range. The main features of these antennas are the use of an FSS that does not include via connections to ground, their low profile and potentially conformal nature, high front-to-back radiation pattern ratio, and the ability to dynamically adjust the center frequency. The reduction of interlayer wiring on the tunable FSS minimizes the fabrication complexity and facilitates the use of flexible substrates. This dissertation aims to advance the state of the art in low profile tunable planar antennas. It shows a qualitative comparison between antennas backed with different unit cell geometries. It demonstrates the feasibility to use either semiconductor or ferroelectric thin film varactor-based tunable FSS to allow adjustment in the antenna frequency in response to environment loading in the near-field. Additionally, it illustrates how the coupling between antenna and HIS, and the impact of the varactor losses affect the antenna performance and it shows solutions to compensate these adverse effects. Novel hybrid manufacturing approaches to achieve flexibility on electrically thick antennas that could be transitioned to thin-film microelectronics are also presented. The semiconductor and ferroelectric varactor-based tunable low profile antennas demonstrated tunability from 2.2 GHz to 2.65 GHz with instantaneous bandwidths greater than 50 MHz within the tuning range. The antennas had maximum thicknesses of λ/45 at the central frequency and front to back-lobe radiation ratios of approximately 15dB. They also showed impedance match improvement in the presence of a Human Core Model (HCM) phantom at close proximity distances of the order of 10-20 mm. In addition, the use of thin film ferroelectric Barium Strontium Titanate (BST) varactors in the FSS layer enabled an antenna that had smaller size, lower cost and less weight compared to the commercially available options. The challenging problems of fabricating robust flexible antennas are also addressed and novel solutions are proposed. Two different types of flexible antennas were designed and built. A series of flexible microstrip antennas with slotted grounds which demonstrated to be robust and have 42% less mass than typically used technologies (e.g., microstrip antennas fabricated on Rogers® RT6010, RT/duroid® 5880, etc.); and flexible ferroelectric based tunable low profile antennas that showed tunability from 2.42 GHz to 2.66 GHz using overlapping metallic plates instead of a continuous ground plane. The bending test results demonstrated that, by placing cuts on the ground plane or using overlapping metallic layers that resemble fish scales, it was possible to create highly conductive surfaces that were extremely flexible even when attached to other solid materials. These new approaches were used to overcome limitations commonly encountered in the design of antennas that are intended for use on non-flat surfaces. The material presented in this dissertation represents the first investigation of reconfigurable low profile antennas using tunable high impedance surfaces where the desired electromagnetic performance as well as additional relevant features such as robustness, low weight, low cost and low complexity were demonstrated.

abstract: Flexibility, reconfigurability and wearability technologies for antenna designs are presented, investigated and merged in this work. Prior to the design of these radiating elements, a study is conducted on several flexible substrates and how to fabricate flexible devices. Furthermore, the integration of active devices into the flexible substrates is also investigated. A new approach of designing inkjet-printed flexible reconfigurable antennas, based on the concept of printed slot elements, is proposed. An alternate technique to reconfigure the folded slot antenna is also reported. The proposed radiator works for both Wireless Local Area Network (WLAN) and Worldwide Interoperability for Microwave Access (WiMAX) applications. The flexible reconfigurable antenna is also redesigned to resonate at both (2.4/5.2 GHz) for WLAN devices and its Multiple-Input Multiple-Output (MIMO) configuration is reported. Two orthogonal elements are used to form the MIMO antenna system for better isolation. The wearability of the proposed flexible reconfigurable radiator is also discussed. Since wearable antennas operate close to the human body, which is considered as a lossy tissue, an isolation between the radiating elements and human body is required to improve the radiation characteristics and to reduce the Specific Absorption Rate (SAR). The proposed antenna is redesigned on an Artificial Magnetic Conductor (AMC) surface that also functions as a ground plane to isolate the radiator from the human body. To examine its performance as a body-worn device, it is measured at different positions on the human body. Furthermore, simulations show that the SAR level is reduced when using the AMC surface. The proposed wearable antenna works for both Wireless Body Area Network (WBAN) and WiMAX body-worn wireless devices. Electromagnetic bandgap (EBG) structures are used to suppress surface wave propagation in printed antennas. However, due to the presence of vias, not all of them can be utilized in flexible radiators. Thus, a Perforated High Impedance Surface (PHIS) is proposed which suppresses the surface waves without the need of vias, and it also serves as a ground plane for flexible antennas. The surface wave suppression and the antenna applications of the proposed PHIS surface are discussed. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2017

This thesis describes the possibilities of wearable antennas and the basic properties description. Numeric model for ISM 2,4 GHz band is created by CST Microwave Studio®. The thesis also deals with the human body proximity effect by two models of human tissue. The effect of flexibility to antenna parameters is evaluated. Several samples of the antenna has been created and measured. Obtained results has been compared with numeric model and with the literature.

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Made available in DSpace on 2014-12-17T14:55:19Z (GMT). No. of bitstreams: 1 GustavoAC_TESE.pdf: 3178455 bytes, checksum: bdea1ce583a318f3a35fb4f3221877a8 (MD5) Previous issue date: 2014-04-28 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / The use of flexible materials for the development of planar circuits is one of the most desired and studied characteristics, lately, by researchers. This happens because the flexibility of the substrate can provide previously impracticable applications, due to the rigidity of the substrates normally used that makes it difficult to fit into the circuits in irregular surfaces. The constant interest in recent years for more lighter devices, increasingly more compacts, flexible and with low cost, led to a new line of research of great interest from both academic and technological views, that is the study and development of textile substrates that can be applied in the development of planar circuits, for applications in the areas of security, biomedical and telecommunications. This paper proposes the development of planar circuits, such as antennas , frequency selective surfaces (FSS) and planar filters, using textile (cotton ticking, jeans and brim santista) as the dielectric substrate and the Pure Copper Polyester Taffeta Fabric, a textile of pure copper, highly conductive, lightweight and flexible, commercially sold as a conductive material. The electrical characteristics of textiles (electric permittivity and loss tangent) were characterized using the transmission line method (rectangular waveguide) and compared with those found in the literature. The structures were analyzed using commercial software Ansoft Designer and Ansoft HFSS, both from the company Ansys and for comparison we used the Iterative Method of Waves (WCIP). For the purpose of validation were built and measured several prototypes of antennas, planar filters and FSS, being possible to confirm an excellent agreement between simulated and measured results / A utiliza??o de materiais flex?veis para o desenvolvimento de circuitos planares ? uma das caracter?sticas mais desejadas e estudadas, ultimamente, pelos pesquisadores, pois essa maleabilidade do substrato proporciona aplica??es antes imposs?veis, devido ? rigidez dos substratos normalmente utilizados o que dificultava a adequa??o dos circuitos em superf?cies irregulares. O constante interesse nos ?ltimos anos por dispositivos mais leves, cada vez mais compactos, flex?veis e com custo reduzido, levou a uma nova linha de pesquisa de grande interesse tanto do ponto de vista acad?mico quanto tecnol?gico que ? o estudo e desenvolvimento de substratos t?xteis que possam ser aplicados no desenvolvimento de circuitos planares, para aplica??es nas ?reas de seguran?a, biom?dica e telecomunica??es. Este trabalho prop?e o desenvolvimento de circuitos planares, tais como antenas, superf?cies seletivas de frequ?ncia (FSS) e filtros planares, utilizando tecidos (lona, jeans e brim santista) como substrato diel?trico e o tecido Pure Copper Polyester Taffeta Fabric, um tecido de cobre puro, altamente condutivo, leve e flex?vel, comercialmente vendido como material condutivo. As caracter?sticas el?tricas dos tecidos (permissividade el?trica e tangente de perda) foram determinadas utilizando o m?todo de linha de transmiss?o e comparadas com os encontrados na literatura. As estruturas foram analisadas utilizando os softwares comerciais Ansoft Designer, Ansoft HFSS ambos da empresa Ansys e para efeito de compara??o foi utilizado o M?todo Iterativo das Ondas (WCIP). Para efeito de valida??o foram constru?dos e medidos v?rios prot?tipos de antenas, FSS e filtros planares sendo poss?vel constatar uma excelente concord?ncia entre os resultados simulados e medidos