Development of Textile Antennas for Energy Harvesting

The current socio-economic developments and lifestyle trends indicate an increasing consumption of technological products and processes, powered by emergent concepts, such as Internet of Things (IoT) and smart environments, where everything is connected in a single network. For this reason, wearable technology has been addressed to make the person, mainly through his clothes, able to communicate with and be part of this technological network. Wireless communication systems are made up of several electronic components, which over the years have been miniaturized and made more flexible, such as batteries, sensors, actuators, data processing units, interconnectors and antennas. Turning these systems into wearable systems is a demanding research subject. Specifically, the development of wearable antennas has been challenging, because they are conventionally built on rigid substrates, hindering their integration into the garment. That is why, considering the flexibility and the dielectric properties of textile materials, making antennas in textile materials will allow expanding the interaction of the user with some electronic devices, by interacting through the clothes. The electronic devices may thus become less invasive and more discrete. Textile antennas combine the traditional textile materials with new technologies. They emerge as a potential interface of the human-technology-environment relationship. They are becoming an active part in the wireless communication systems, aiming applications such as tracking and navigation, mobile computing, health monitoring and others. Moreover, wearable antennas have to be thin, lightweight, of easy maintenance, robust, and of low cost for mass production and commercialization. In this way, planar antennas, the microstrip patch type, have been proposed for garment applications, because this type of antenna presents all these characteristics, and are also adaptable to any surface. Such antennas are usually formed by assembling conductive (patch and ground plane) and dielectric (substrate) layers. Furthermore, the microstrip patch antennas, radiate perpendicularly to a ground plane, which shields the antenna radiation, ensuring that the human body is exposed only to a very small fraction of the radiation. To develop this type of antenna, the knowledge of the properties of textile materials is crucial as well as the knowledge of the manufacturing techniques for connecting the layers with glue, seam, adhesive sheets and others. Several properties of the materials influence the behaviour of the antenna. For instance, the bandwidth and the efficiency of a planar antenna are mainly determined by the permittivity and the thickness of the substrate. The use of textiles in wearable antennas requires thus the characterization of their properties. Specific electrical conductive textiles are available on the market and have been successfully used. Ordinary textile fabrics have been used as substrates. In general, textiles present a very low dielectric constant, εr, that reduces the surface wave losses and increases the impedance bandwidth of the antenna. However, textile materials are constantly exchanging water molecules with the surroundings, which affects their electromagnetic properties. In addition, textile fabrics are porous, anisotropic and compressible materials whose thickness and density might change with low pressures. Therefore, it is important to know how these characteristics influence the behaviour of the antenna in order to minimize unwanted effects. To explain some influences of the textile material on the performance of the wearable antennas, this PhD Thesis starts presenting a survey of the key points for the design and development of textile antennas, from the choice of the textile materials to the framing of the antenna. An analysis of the textile materials that have been used is also presented. Further, manufacturing techniques of the textile antennas are described. The accurate characterization of textile materials to use as a dielectric substrate in wearable systems is fundamental. However, little information can be found on the electromagnetic properties of the regular textiles. Woven, knits and nonwovens are inhomogeneous, highly porous, compressible and easily influenced by the environmental hygrometric conditions, making their electromagnetic characterization difficult. Despite there are no standard methods, several authors have been adapting techniques for the dielectric characterization of textiles. This PhD Thesis focuses on the dielectric characterization of the textile materials, surveying the resonant and non-resonant methods that have been proposed to characterize the textile and leather materials. Also, this PhD Thesis summarizes the characterization of textile materials made through these methods, which were validated by testing antennas that performed well. Further a Resonant-Based Experimental Technique is presented. This new method is based on the theory of resonance-perturbation, extracting the permittivity and loss tangent values based on the shifts caused by the introduction of a superstrate on the patch of a microstrip antenna. The results obtained using this method have shown that when positioning the roughest face of the material under test (MUT) in contact with the resonator board, the extracted dielectric constant value is lower than the one extracted with this face positioned upside-down. Based on this observation, superficial properties of textiles were investigated and their influence on the performance of antennas was analysed. Thus, this PhD Thesis relates the results of the dielectric characterization to some structural parameters of textiles, such as surface roughness, superficial and bulk porosities. The results show that both roughness and superficial porosity of the samples influence the measurements, through the positioning of the probes. Further, the influence of the positioning of the dielectric material on the performance of textile microstrip antennas was analysed. For this, twelve prototypes of microstrip patch antennas were developed and tested. The results show that, despite the differences obtained on the characterization when placing the face or reverse-sides of the MUT in contact with the resonator board, the obtained average result of εr is well suited to design antennas ensuring a good performance. According to the European Commission Report in 2009, “Internet of Things — An action plan for Europe”, in the next years, the IoT will be able to improve the quality of life, especially in the health monitoring field. In the Wireless Body Sensor Network (WBSN) context, the integration of textile antennas for energy harvesting into smart clothing is a particularly interesting solution for a continuous wirelessly feed of the devices. Indeed, in the context of wearable devices the replacement of batteries is not easy to practice. A specific goal of this PhD Thesis is thus to describe the concept of the energy harvesting and then presents a survey of textile antennas for RF energy harvesting. Further, a dual-band printed monopole textile antenna for electromagnetic energy harvesting, operating at GSM 900 and DCS 1800 bands, is also proposed. The antenna aims to harvest energy to feed sensor nodes of a wearable health monitoring system. The gains of the antenna are around 1.8 dBi and 2.06 dBi allied with a radiation efficiency of 82% and 77.6% for the lowest and highest frequency bands, respectively. To understand and improve the performance of the proposed printed monopole textile antenna, several manufacturing techniques are tested through preliminary tests, to identify promising techniques and to discard inefficient ones, such as the gluing technique. Then, the influence of several parameters of the manufacturing techniques on the performance of the antenna are analysed, such as the use of steam during lamination, the type of adhesive sheet, the orientation of the conductive elements and others. For this, seven prototypes of the printed monopole textile antenna were manufactured by laminating and embroidering techniques. The measurement of the electrical surface resistance, Rs, has shown that the presence of the adhesive sheet used on the laminating process may reduce the conductivity of the conductive materials. Despite that, when measuring the return loss of printed monopole antennas produced by lamination, the results show the antennas have a good performance. The results also show that the orientation of the conductive fabric does not influence the performance of the antennas. However, when testing embroidered antennas, the results show that the direction and number of the stitches in the embroidery may influence the performance of the antenna and should thus be considered during manufacturing. The textile antennas perform well and their results support and give rise to the new concept of a continuous substrate to improve the integration of textile antennas into clothing, in a more comfortable and pleasure way. A demonstrating prototype, the E-Caption: Smart and Sustainable Coat, is thus presented. In this prototype of smart coat, the printed antenna is fully integrated, as its dielectric is the textile material composing the coat itself. The E-Caption illustrates the innovative concept of textile antennas that can be manipulated as simple emblems. The results obtained testing the antenna before and after its integration into cloth, show that the integration does not affect the behaviour of the antenna. Even on the presence of the human body the antenna is able to cover the proposed resonance frequencies (GSM 900 and DCS 1800 bands) with the radiation pattern still being omnidirectional. At last, the exponential growth in the wearable market boost the industrialization process of manufacturing textile antennas. As this research shows, the patch of the antennas can be easily and efficiently cut, embroidered or screen printed by industrial machines. However, the conception of a good industrial substrate that meets all the mechanical and electromagnetic requirements of textile antennas is still a challenge. Following the continuous substrate concept presented and demonstrated through the E-Caption, a new concept is proposed: the continuous Substrate Integrating the Ground Plane (SIGP). The SIGP is a novel textile material that integrates the dielectric substrate and the conductive ground plane in a single material, eliminating one laminating process. Three SIGP, that are weft knitted spacer fabrics having one conductive face, were developed in partnership with the Borgstena Textile Portugal Lda, creating synergy between research in the academy and industry. The results of testing the performance of the SIGP materials show that the integration of the ground plane on the substrate changes the dielectric constant of the material, as a consequence of varying the thickness. Despite this, after the accurate dielectric and electrical characterization, the SIGP I material has shown a good performance as dielectric substrate of a microstrip patch antenna for RF energy harvesting. This result is very promising for boosting the industrial fabrication of microstrip patch textile antennas and their mass production and dissemination into the IoT network, guiding future developments of smart clothing and wearables.Os atuais desenvolvimentos socioeconómicos e tendências de estilo de vida apontam para um crescimento do consumo de produtos e processos tecnológicos, impulsionado por conceitos emergentes como a Internet das Coisas, onde tudo tudo está conectado em uma única rede. Por esta razão, as tecnologias usáveis (wearable) estão a afirmar-se propondo soluções que tornam o utilizador possivelmente através das suas roupas, capaz de comunicar com e fazer parte desta rede. Os sistemas de comunicações sem fios são constituídos por diversos componentes eletrónicos, que com o passar dos anos foram sendo miniaturizados e fabricados em materiais flexíveis, tais como as baterias, os sensores, as unidades de processamento de dados, as interconexões e as antenas. Tornar os sistemas de comunicações sem fios em sistemas usáveis requer trabalho de investigação exigente. Nomeadamente, o desenvolvimento de antenas usáveis tem sido um desafio, devido às antenas serem tradicionalmente desenvolvidas em substratos rígidos, que dificultam a sua integração no vestuário. Dessa forma, considerando a flexibilidade e as propriedades dielétricas dos materiais têxteis, as antenas têxteis trazem a promessa de permitir a interacção dos utilizadores com os dispositivos eletrónicos através da roupa, tornando os dispositivos menos invasivos e mais discretos. As antenas têxteis combinam os materiais têxteis tradicionais com novas tecnologias e emergem assim como uma potencial interface de fronteira entre seres humanos-tecnologias-ambientes. Expandindo assim a interação entre o utilizador e os dispositivos eletrónicos ao recurso do vestuário. Assim, através das antenas têxteis, o vestuário torna-se uma parte ativa nos sistemas de comunicação sem fios, visando aplicações como rastreamento e navegação, computação móvel, monitorização de saúde, entre outros. Para isto, as antenas para vestir devem ser finas, leves, de fácil manutenção, robustas e de baixo custo para produção em massa e comercialização. Desta forma, as antenas planares do tipo patch microstrip têm sido propostas para aplicações em vestuário, pois apresentam todas estas características e também são adaptáveis a qualquer superfície. Estas antenas são geralmente formadas pela sobreposição de camadas condutoras (elemento radiante e plano de massa) e dielétricas (substrato). Além disso, as antenas patch microstrip irradiam perpendicularmente ao plano de massa, que bloqueia a radiação da antena, garantindo que o corpo humano é exposto apenas a uma fração muito pequena da radiação. Para desenvolver este tipo de antena, é crucial conhecer as propriedades dos materiais têxteis, bem como as técnicas de fabricação para conectar as camadas, com cola, costuras, folhas adesivas, entre outros. Diversas propriedades dos materiais influenciam o comportamento da antena. Por exemplo, a permitividade e a espessura do substrato determinam a largura de banda e a eficiência de uma antena planar. O uso de têxteis em antenas usáveis requer assim uma caracterização precisa das suas propriedades. Os têxteis condutores elétricos são materiais específicos que estão disponíveis comercialmente em diversas formas e têm sido utilizados com sucesso para fabricar o elemento radiante e o plano de massa das antenas. Para fabricar o substrato dielétrico têm sido utilizados materiais têxteis convencionais. Geralmente, os materiais têxteis apresentam uma constante dielétrica (εr) muito baixa, o que reduz as perdas de ondas superficiais e aumenta a largura de banda da antena. No entanto, os materiais têxteis estão constantemente a trocar moléculas de água com o ambiente em que estão inseridos, o que afeta as suas propriedades eletromagnéticas. Além disso, os tecidos e os outros materiais têxteis planares são materiais porosos, anisotrópicos e compressíveis, cuja espessura e densidade variam sob muito baixas pressões. Portanto, é importante saber como estas grandezas e características estruturais influenciam o comportamento da antena, de forma a minimizar os efeitos indesejáveis. Para explicar algumas das influências do material têxtil no desempenho das antenas usáveis, esta Tese de Doutoramento começa por fazer o estado da arte sobre os pontos-chave para o desenvolvimento de antenas têxteis, desde a escolha dos materiais têxteis até ao processo de fabrico da antena. Além disso, a tese identifica e apresenta uma análise dos materiais têxteis e técnicas de fabricação que têm sido utilizados e referidos na literatura. A caracterização rigorosa dos materiais têxteis para usar como substrato dielétrico em sistemas usáveis é fundamental. No entanto, pouca informação existe sobre a caracterização das propriedades eletromagnéticas dos têxteis vulgares. Como já referido, os tecidos, malhas e não-tecidos são materiais heterogéneos, altamente porosos, compressíveis e facilmente influenciados pelas condições higrométricas ambientais, dificultando a sua caracterização eletromagnética. Não havendo nenhum método padrão, vários autores têm vindo a adaptar algumas técnicas para a caracterização dielétrica dos materiais têxteis. Esta Tese de Doutoramento foca a caracterização dielétrica dos materiais têxteis, revendo os métodos ressonantes e não ressonantes que foram propostos para caracterizar os materiais têxteis e o couro. Além disso, esta Tese de Doutoramento resume a caracterização de dieléctricos têxteis feita através dos métodos revistos e que foi validada testando antenas que apresentaram um bom desempenho. No seguimento da revisão, apresenta-se uma Técnica Experimental Baseada em Ressonância. Esta nova técnica baseia-se na teoria da perturbação de ressonância, sendo a permitividade e tangente de perda extraídas com base nas mudanças de frequência causadas pela introdução de um superstrato no elemento radiante de uma antena patch microstrip. Os resultados de caracterização obtidos através deste método revelam que, ao posicionar a face mais rugosa do material em teste em contato com a placa de ressonância, o valor da constante dielétrica extraída é inferior ao valor extraído quando esta face é colocada ao contrário. Com base nesta observação, as propriedades estruturais da superfície dos materiais têxteis foram investigadas e a sua influência no desempenho das antenas foi analisada. Assim, esta Tese de Doutoramento relaciona os resultados da caracterização dielétrica com alguns parâmetros estruturais dos materiais, como rugosidade da superfície, porosidades superficial e total. Os resultados mostram que tanto a rugosidade como a porosidade superficial das amostras influenciam os resultados, que dependem assim do posicionamento do material que está a ser testado. Também foi analisada a influência do posicionamento do material dielétrico na performance das antenas têxteis tipo patch microstrip. Para isso, foram desenvolvidos e testados doze protótipos de antenas patch microstrip. Os resultados mostram que, apesar das diferenças observadas durante o processo de caracterização, o valor médio da permitividade é adequado para a modelação das antenas, garantindo um bom desempenho. De acordo com o relatório da Comissão Europeia, “Internet das Coisas - Um plano de ação para a Europa”, emitido em 2009, nos próximos anos a Internet das Coisas poderá melhorar a qualidade de vida das pessoas, nomeadamente pela monitorização da saúde. No contexto das Redes de Sensores Sem Fios do Corpo Humano, a integração de antenas têxteis para recolha de energia em roupas inteligentes é uma solução particularmente interessante, pois permite uma alimentação sem fios e contínua dos dispositivos. De fato, nos dispositivos usáveis a substituição de baterias não é fácil de praticar. Um dos objetivos específicos desta Tese de Doutoramento é, portanto, descrever o conceito de recolha de energia e apresentar o estado da arte sobre antenas têxteis para recolha de energia proveniente da Rádio Frequência (RF). Nesta tese, é também proposta uma antena impressa do tipo monopolo de dupla banda, fabricada em substrato têxtil, para recolha de energia eletromagnética, operando nas bandas GSM 900 e DCS 1800. A antena visa recolher energia para alimentar os nós de sensores de um sistema usável para monitorização da saúde. Os ganhos da antena apresentada foram cerca de 1.8 dBi e 2.06 dBi, aliados a uma eficiência de radiação de 82% e 77.6% para as faixas de frequência mais baixa e alta, respetivamente. Para entender e melhorar o desempenho da antena impressa tipo monopolo de dupla banda em substrato têxtil, várias técnicas de fabrico foram testadas através de testes preliminares, de forma a identificar as técnicas promissoras e a descartar as ineficientes, como é o caso da técnica de colagem. De seguida, analisou-se a influência de vários parâmetros das técnicas de fabrico sobre o desempenho da antena, como o uso de vapor durante a laminação, o tipo de folha adesiva, a orientação dos elementos irradiantes e outros. Para isto, sete protótipos da antena têxtil monopolar impressa foram fabricados por técnicas de laminação e bordado. As medições da resistência elétrica superficial, Rs, mostrou que a presença da folha adesiva usada no processo de laminagem pode reduzir a condutividade dos materiais condutores. Apesar disso, ao medir o S11 das antenas impressas tipo monopolo produzidas por laminagem, os resultados mostram que as antenas têm uma boa adaptação da impedância. Os resultados também mostram que a orientação do tecido condutor, neste caso um tafetá, não influencia o desempenho das antenas. No entanto, ao testar antenas bordadas, os resultados mostram que a direção e o número de pontos no bordado podem influenciar o desempenho da antena e, portanto, estas são características que devem ser consideradas durante a fabricação. De um modo geral, as antenas têxteis funcionam bem e seus resultados suportam e dão origem ao um novo conceito de substrato contínuo para melhorar a integração de antenas têxteis no vestuário, de maneira mais confortável e elegante. A tese apresenta um protótipo demonstrador deste conceito, o E-Caption: A Smart and Sustainable Coat. Neste protótipo de casaco inteligente, a antena impressa está totalmente integrada, pois o seu substrato dielétrico é o próprio mat

A Study of Microfluidic Reconfiguration Mechanisms Enabled by Functionalized Dispersions of Colloidal Material for Radio Frequency Applications

Communication and reconnaissance systems are requiring increasing flexibility concerning functionality and efficiency for multiband and broadband frequency applications. Circuit-based reconfiguration mechanisms continue to promote radio frequency (RF) application flexibility; however, increasing limitations have resulted in hindering performance. Therefore, the implementation of a "wireless" reconfiguration mechanism provides the required agility and amicability for microwave circuits and antennas without local overhead. The wireless reconfiguration mechanism in this thesis integrates dynamic, fluidic-based material systems to achieve electromagnetic agility and reduce the need for "wired" reconfiguration technologies. The dynamic material system component has become known as electromagnetically functionalized colloidal dispersions (EFCDs). In a microfluidic reconfiguration system, they provide electromagnetic agility by altering the colloidal volume fraction of EFCDs - their name highlights the special considerations we give to material systems in applied electromagnetics towards lowering loss and reducing system complexity. Utilizing EFCDs at the RF device-level produced the first circuit-type integration of this reconfiguration system; this is identified as the coaxial stub microfluidic impedance transformer (COSMIX). The COSMIX is a small hollowed segment of transmission line with results showing a full reactive loop (capacitive to inductive tuning) around the Smith chart over a 1.2 GHz bandwidth. A second microfluidic application demonstrates a novel antenna reconfiguration mechanism for a 3 GHz microstrip patch antenna. Results showed a 300 MHz downward frequency shift by dielectric colloidal dispersions. Magnetic material produced a 40 MHz frequency shift. The final application demonstrates the dynamically altering microfluidic system for a 3 GHz 1x2 array of linearly polarized microstrip patch antennas. The parallel microfluidic capillaries were imbedded in polydimethylsiloxane (PDMS). Both E- and H-plane designs showed a 250 MHz frequency shift by dielectric colloidal dispersions. Results showed a strong correlation between decreasing electrical length of the elements and an increase of the volume fraction, causing frequency to decrease and mutual coupling to increase. Measured, modeled, and analytical results for impedance, voltage standing wave ratio (VSWR), and radiation behavior (where applicable) are provided

Artificial materials for microwave applications

This thesis has focussed on the properties and manufacturing techniques of artificial RF materials. These artificial materials can be divided into two types depending on the whether their individual unit cell is resonant or non-resonant. Both these types have been discussed. It has been shown that the efficiency and bandwidth of a patch antenna using a flexible 3D printed substrate can be improved by using composite materials as heterogeneous substrates. Composite materials with a large range of relative permittivity values were manufactured by combing 3D printing with commercial laminates. An equation to design such composite materials has been presented. The engineering tolerance and repeatability of 3D printing as a manufacturing process to fabricate ‘on demand’ dielectrics has been presented. [Continues.

Parasitic Element Based Frequency Reconfigurable Antenna with Dual Wideband Characteristics for Wireless Applications

A Microstrip Frequency Reconfigurable circular patch slot antenna for switchable Bluetooth, WiMAX, WLAN, and satellite communication applications is analyzed and presented in this work. The optimized overall size of 47 mm x40 mmx1.6 mm is utilized in the design, and which can cover wide range of frequencies below 10 GHz. In the initial phase, different monopole antennas are designed with various shapes of same size and later parasitic patch elements has been added to those monopole antennas. The circular monopole driven element and parasitic element are connected with a PIN diode, and which reinforced in achieving frequency reconfigurability. The proposed antenna is resonating at various frequencies of 2.4 GHz, 4 GHz, and 8.4 GHz when the diode in ON condition and resonating at 3 GHz, 5.4 GHz, and 8.4 GHz when the diode is in OFF condition. The performance of the designed antenna prototype is scaled and differentiated with the results of simulation and found good matching with respect to performance characteristics

Design and analysis of a novel tri-band flower-shaped planar antenna for GPS and WiMAX applications

This paper presents the design of a tri-band flower-shaped planar monopole antenna operating at three frequencies i.e. 1.576 GHz (GPS), 2.668 GHz and 3.636 GHz (Mobile WiMAX). The radiating element of the antenna is backed by a 1.6 mm thicker FR-4 substrate having a dielectric constant of 4.3. The substrate is backed by a truncated ground plane. The antenna is fed through a 50 Ω microstrip line. The flower shape of the radiating element is derived from the basic circular shape by introducing in it rounded slots of various radii. The upper part of the antenna is flower-shaped while the lower part comprises a microstrip feed line and two branches, each having two ‘leaves’ at the end. The leaves and branches contribute in the impedance matching of the lower (1.576 GHz) and middle (2.668 GHz) frequency bands. The antenna gives an acceptable simulated efficiency >70% in the three frequency bands. Suitable gains of 1.63, 2.59 and 3.23dB are obtained at 1.576 GHz, 2.668 GHz and 3.636 GHz, respectively. The antenna matched with a VSWR<1.2 in the three frequency bands. The prototype of the antenna is fabricated and tested in the laboratory, and good agreement in simulated and measured results is achieved. The proposed design is a visually appealing and may find uses as an external antenna in GPS and WiMAX applications

Impedance Transformers

Non

Metamaterial

In-depth analysis of the theory, properties and description of the most potential technological applications of metamaterials for the realization of novel devices such as subwavelength lenses, invisibility cloaks, dipole and reflector antennas, high frequency telecommunications, new designs of bandpass filters, absorbers and concentrators of EM waves etc. In order to create a new devices it is necessary to know the main electrodynamical characteristics of metamaterial structures on the basis of which the device is supposed to be created. The electromagnetic wave scattering surfaces built with metamaterials are primarily based on the ability of metamaterials to control the surrounded electromagnetic fields by varying their permeability and permittivity characteristics. The book covers some solutions for microwave wavelength scales as well as exploitation of nanoscale EM wavelength such as visible specter using recent advances of nanotechnology, for instance in the field of nanowires, nanopolymers, carbon nanotubes and graphene. Metamaterial is suitable for scholars from extremely large scientific domain and therefore given to engineers, scientists, graduates and other interested professionals from photonics to nanoscience and from material science to antenna engineering as a comprehensive reference on this artificial materials of tomorrow