Small antennas have attracted significant attention due to their prolific use in consumer electronics. Such antennas are highly desirable in the healthcare industry for imaging and implants. However, most small antennas are not highly directive and are detuned when in the presence of a dielectric. The human body can be compared to a series of lossy dielectric media. A novel antenna design, the orthogonal coil, is proposed to counter both of these shortcomings. As loop antennas radiate primarily in the magnetic field, their far field pattern is less influenced by nearby lossy dielectrics. By exciting two orthogonal coil antennas in quadrature, their beams in the H-plane constructively add in one direction and cancel in the other. The result is a small, yet directive antenna, when placed near a dielectric interface. In addition to present a review of the current literature relating to small antennas and dipoles near lossy interfaces, the far field of the orthogonal coil antenna is derived. The directivity is then plotted for various conditions to observe the effect of changing dielectric constants, separation from the interface, etc. Numeric simulations were performed using both Finite Difference Time Domain (FDTD) in MATLAB and Finite Element Method (FEM) in Ansys HFSS using a anatomically accurate high-fidelity head mesh that was generated from the Visible Human Project® data. The following problem has been addressed: find the best radio-frequency path through the brain for a given receiver position - on the top of the sinus cavity. Two parameters: transmitter position and radiating frequency should be optimized simultaneously such that (i) the propagation path through the brain is the longest; and (ii) the received power is maximized. To solve this problem, we have performed a systematic and comprehensive study of the electromagnetic fields excited in the head by the aforementioned orthogonal dipoles. Similar analyses were performed using pulses to detect Alzheimer’s disease, and on the femur to detect osteoporosis.

There is a demand for broadband electrically small antennas that cover large frequency bands without any requirement of reconfiguration techniques. This is particularly true at low frequencies (VHF/UHF), where wavelengths are long and antennas are physically large. The fundamental gain-bandwidth limitation was related to the electrical size of passive electrically small antennas by Wheeler and Chu; their result implied that an electrically small antenna exhibits high quality factor which limits the bandwidth. Additionally, the gain-bandwidth limitation was related to impedance matching conditions by the Bode-Fano criteria, which restricts available bandwidth using conventional reactive elements. A non-Foster circuit approach has been presented which delivers a broadband input impedance match and also overcomes the aforementioned fundamental limits. These non-Foster impedance circuits can be realised by negative impedance converters (negative inductance and/or capacitance). The thesis also explores the advantages and challenges of antenna impedance matching using negative impedance circuits based on two topologies: (1) conventional transistorbased circuits, and (2) a novel resonant tunnelling diode approach. The advantages of non-Foster circuits in the implementation of broadband small antennas include wideband performance around one-tenth of the self-resonant frequency and overcoming of the fundamental limits associated with passive antennas. Diode-based circuits are more compact, easily configurable, less sensitive to stability, have low power consumption and are less complex as compared to the transistor based designs. These features makes it a potential candidate for array and meta-material applications. However, there are few challenges for non-Foster circuit integration with an antenna due to high noise figure, which affects the system channel capacity and receiver performance in a communication system. A detailed design procedure has been developed to mitigate the effects of noise and instability and also, the system performance and measurement of the non-Foster circuit integrated antennas have been discussed.

The millimetre-wave (mm-wave) spectrum offers considerable advantages in terms of antenna form factor and spectrum availability. However, use of this region often requires reconfigurable antennas and systems. Initially, a review of the various applications which are taking hold in the lower regions of the mm-wave spectrum (30 to 100 GHz) is undertaken. Specifically, reconfigurable reflectarray technologies are selected for further research, and critical analysis of the reconfiguration techniques for including these in antennas is considered. Silicon as an optically activated semiconductor is chosen as the reconfiguration mechanism due to its low cost and the scope for improvement in this area. A new form of illumination is used, replacing traditional infra-red (IR) lasers with high power IR-LEDs enclosed in a cavity, increasing the efficiency of the silicon illumination. However, to make use of this novel illumination source, and subsequently integrate it into an antenna, the silicon response has to be characterised within Ka-band. This is done through measurements in a waveguide-based characterisation test cell, from which the complex electromagnetic properties of silicon under IR-LED illumination are retrieved with the aid of full-wave simulations. Using the measured conductivity properties of the illuminated silicon, reflectarrays with non-uniform amplitude distributions can be designed. Through variation of illumination intensities of IR-LEDs throughout the array, it is shown through measurements and full-wave simulations that unit cell reflections can be modified while phases are kept relatively constant. This theoretically allows switching between, for example a low side-lobe pattern binomial array, or a narrow beamwidth pattern Chebyshev array. To implement this, a novel multilayer unit-cell is designed, integrating the IR-LED. This is then used in a full reflectarray design which is measured. The key contributions of this work include the novel illumination mechanism and its integration into a reflectarray antenna, and the use of reconfigurable photoconductive materials to provide a mechanism for beam shaping and pattern synthesis at Ka-band.

L'évolution rapide dans les systèmes de communication sans fil nécessite plus de miniaturisation de divers composants électroniques en plus de l'élément majeur de la technologie sans fil : l'antenne. Dans ce cas, une antenne occupant un espace limité devrait être miniaturisée pour fonctionner aux bandes de communication souhaitées. Cependant, à mesure que la taille électrique de l'antenne diminue, ses performances se dégradent considérablement et sa bande passante, son efficacité et sa directivité sont limitées. Les techniques classiques de réduction de la taille avec chargement de matériau et mise en forme géométrique de l'antenne souffrent d'une bande passante étroite et d'une faible efficacité de rayonnement. D'autre part, les tentatives d'augmenter la directivité des petites antennes en utilisant des réseaux superdirectifs sont également associées à une faible efficacité de rayonnement bande passante très étroite. Pour pallier ces inconvénients, nous proposons de booster les performances des antennes compactes en utilisant des charges réactives embarquées. En plaçant correctement les charges (actives ou passives) à l'intérieur de l'antenne, il est possible de contrôler les courants pour améliorer de manière significative les performances de l'antenne en termes de bande passante et de directivité. Cependant, pour un succès des critères de chargement, il est obligatoire d'analyser les modes naturellement supportés par l'antenne étudiée. On les appelle les modes caractéristiques, qui fournissent des aperçus physiques profonds sur le comportement de l'antenne et ses modes de rayonnement. En combinant cette théorie avec l'algorithme d'optimisation, il devient possible de manipuler de manière optimale les courants à l'intérieur de l'antenne en utilisant des charges réactives pour obtenir des conceptions large bande, superdirectives et efficaces. / The rapid evolution in the wireless communication systems requires more miniaturization of various electronic components in addition to the major element of the wireless technology: the antenna. In this case, an antenna occupying a limited space should be miniaturized in order to operate at the desired communication bands. However, as the electrical size of the antenna decreases, its performance degrades dramatically and it becomes limited in bandwidth, efficiency, and directivity. Classical size reduction techniques with material loading and geometry shaping of the antenna suffer from narrow bandwidth and low radiation efficiency. On the other hand, attempts to increase the directivity of small antennas using superdirective arrays are also associated with low radiation efficiency and very narrow bandwidth. To overcome these drawbacks, we propose boosting the performance of compact antennas using embedded reactive loads. By properly placing loads (active or passive) inside the antenna, it is possible to control the currents to significantly enhance the antenna performance in terms of bandwidth and directivity. Yet, for a successful loading criteria, it is mandatory to analyze the modes that are naturally supported by the antenna under study. These are called the characteristic modes, which provide deep physical insights about the behaviour of the antenna and its radiating modes. By combining this theory with and optimization algorithm, it becomes possible to optimally manipulate the currents inside the antenna using reactive loads to achieve wideband, superdirective and efficient designs.

Mutual Coupling (MC) is the exchange of energy between multiple antennas when placed on the same PCB, it being one of the critical parameters and a significant issue to be considered when designing MIMO antennas. It appears significantly where multiple antennas are placed very close to each other, with a high coupling affecting the performance of the array, in terms radiation patterns, the reflection coefficient, and influencing the input impedance. Moreover; it degrades the designed efficiency and gain since part of the power that could have been radiated becomes absorbed by other adjacent antennas' elements. The coupling mechanism between multiple antenna elements is identified as being mainly through three different paths or channels: surface wave propagation, space (direct) radiation and reactive near-field coupling. In this thesis, various coupling reduction approaches that are commonly employed in the literature are categorised based on these mechanisms. Furthermore, a new comparative study involving four different array types (PIFA, patch, monopole, and slot), is explained in detail. This thesis primarily focuses on three interconnected research topics for mutual coupling reduction based on new isolation approaches for different wireless applications (i.e. Narrowband, Ultra-wide-band and Multi-band). First, a new Fractal based Electromagnetic Band Gap (FEBG) decoupling structure between PIFAs is proposed and investigated for a narrowband application. Excellent isolation of more than 27 dB (Z-X plane) and 40 dB (Z-Y plane) is obtained without much degradation of the radiation characteristics. It is found that the fractal structures can provide a band-stop effect, because of their self-similarity features for a particular frequency band. Second, new UWB-MIMO antennas are presented with high isolation characteristics. Wideband isolation (≥ 31 dB) is achieved through the entire UWB band (3.1-10.6 GHz) by etching a novel compact planar decoupling structure inserted between these multiple UWB antennas. Finally, new planar MIMO antennas are presented for multi-band (quad bands) applications. A significant isolation improvement over the reference (≥ 17 dB) is achieved in each band by etching a hybrid solution. All the designs reported in this thesis have been fabricated and measured, with the simulated and measured results agreeing well in most cases.

Submitted by Cátia Araújo (catia.araujo@unipampa.edu.br) on 2017-01-24T13:19:49Z No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Estudo de antenas de microfita com dupla-banda e dupla-polarização para aplicação em redes retro-diretivas.pdf: 5490330 bytes, checksum: db372388acb59d1f9f94845c1ffc044e (MD5) / Approved for entry into archive by Cátia Araújo (catia.araujo@unipampa.edu.br) on 2017-01-24T13:22:19Z (GMT) No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Estudo de antenas de microfita com dupla-banda e dupla-polarização para aplicação em redes retro-diretivas.pdf: 5490330 bytes, checksum: db372388acb59d1f9f94845c1ffc044e (MD5) / Made available in DSpace on 2017-01-24T13:22:19Z (GMT). No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Estudo de antenas de microfita com dupla-banda e dupla-polarização para aplicação em redes retro-diretivas.pdf: 5490330 bytes, checksum: db372388acb59d1f9f94845c1ffc044e (MD5) Previous issue date: 2015-11-13 / O presente trabalho apresenta o desenvolvimento de uma antena dupla-faixa e com dupla-polarização para instalação em plataformas de alta altitude (do inglês, HAPs – High-Altitude Platforms). Como principal contribuição científica, o objetivo foi a obtenção de um irradiador com as características acima citadas e com duas portas, uma para cada banda de operação, altamente isoladas. A antena foi desenvolvida em tecnologia de microfita e pode ser utilizada para a composição de uma rede de antenas retro-diretiva. Dada a alta isolação obtida entre as portas, pode-se utilizar o irradiador desenvolvido simultaneamente nos modos de recepção (em 5,8 GHz) e transmissão (em 7,0 GHz). Esta antena caracteriza-se como contribuição original desta dissertação. Diferentes técnicas que possibilitam a obtenção de característica dupla-faixa e dupla-polarização para antenas construídas em tecnologia de microfita são estudadas, especificamente voltadas para aplicações em redes retro-diretivas. As faixas de interesse encontram-se no intervalo de 5,75 GHz a 5,85 GHz para a banda inferior e 6,95 GHz a 7,05 GHz para a banda superior. Além de atender às especificações técnicas nas faixas de frequência especificadas, os requisitos de bom coeficiente de reflexão, polarização circular, ganho e impedância de entrada devem ser devidamente satisfeitos. Primeiramente, visando a obter uma geometria para compor uma rede de antenas retro-diretivas, quatro estruturas com característica de dupla-banda e dupla-polarização são estudadas. Os princípios de funcionamento e particularidades de cada geometria estão detalhadamente descritos ao longo do trabalho. Os resultados preliminares de duas das quatro geometrias de antenas são apresentados através de simulação com os pacotes Ansoft Designer® e ANSYS HFSSTM. Foram levantados e estudados os parâmetros elétricos de cada irradiador em função da variação de suas dimensões físicas. Finalmente, dentre as antenas analisadas foi realizada a construção de duas geometrias com a finalidade de validar experimentalmente suas características de irradiação. Os resultados numéricos e experimentais obtidos permitem afirmar que uma das geometrias construídas é capaz de operar em duas bandas e também com polarização circular à direita e à esquerda nas bandas inferior e superior, respectivamente. / This work presents the development of a dual-band and dual-polarized antenna for installation in High-Altitude Platform Station (HAPs). As the main scientific contribution, the objective was to obtain a radiator with the above mentioned features and with two ports with high isolation between then, whereby one port should be used for one operating band. The antenna is designed in microstrip technology and can be used to compose a retrodirective antenna array. With the high isolation achieved between the two ports, the developed radiator can be used simultaneously in the receiving (at 5.8 GHz) and transmitting modes (at 7 GHz). This antenna is an original contribution of this dissertation. Different techniques that allow obtaining the dual-band and dual-polarization characteristics for antennas implemented in microstrip technology are studied, especially those suitable for application in retrodirective arrays. The bands of interest are in the range of 5.75 GHz to 5.85 GHz for the lower band and 6.95 GHz to 7.05 GHz for the upper band. In addition to meeting the technical specifications in this frequency bands, the requirements of good reflection coefficient, circular polarization, gain and input impedance must be satisfied. Firstly, in order to obtain a geometry to compose a retrodirective antenna array, four structures with dual-band and dual-polarization characteristics are studied. The operating principles and the main features of each geometry are described in detail along the work. Preliminary results for two of them are presented through simulation packages Ansoft Designer® e ANSYS HFSSTM. The electrical parameters of each radiator have been studied by means of parametric simulations of its physical dimensions. Finally, among the analyzed antennas, two geometries were prototyped in order to validate experimentally their radiation properties. The numerical and experimental results validate that one of built geometries can be operate in the two bands and with RHCP and LHCP in the lower and higher bands, respectively.

Lam, Fuk Ming. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2008. / Includes bibliographical references. / Abstracts in English and Chinese. / Abstract --- p.I / Acknowledgements --- p.V / Table of Contents --- p.VI / List of Figures --- p.IX / List of Tables --- p.XVI / Chapter Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Background --- p.1 / Chapter 1.2 --- Overview of the Work --- p.5 / Chapter 1.3 --- Original contribution of this thesis --- p.6 / Chapter 1.4 --- Organization of this thesis --- p.7 / Chapter 1.5 --- Remarks on frequency dependent parameters in this thesis --- p.8 / Reference --- p.9 / Chapter Chapter 2 --- Small Antennas for mobile phone applications --- p.10 / Chapter 2.1 --- Introduction --- p.10 / Chapter 2.2 --- Definitions --- p.11 / Chapter 2.2.1 --- Quality factor --- p.11 / Chapter 2.2.2 --- Efficiency --- p.12 / Chapter 2.2.3 --- Return Loss and impedance bandwidth --- p.12 / Chapter 2.2.4 --- Antenna gain and radiation pattern --- p.13 / Chapter 2.3 --- Fundamental limitations of small antenna --- p.14 / Chapter 2.4 --- Low-profile and Dual-band techniques --- p.16 / Chapter 2.4.1 --- Inverted-L/F and Planar Inverted-F Antenna --- p.16 / Chapter 2.4.2 --- Dual-band PIFA --- p.21 / Chapter 2.4.3 --- Discussion on miniaturization of mobile phone antenna --- p.23 / Chapter 2.5 --- Ground plane effect of mobile phone antenna --- p.26 / Chapter 2.5.1 --- Optimal location to excite antenna over a finite ground plane --- p.26 / Chapter 2.5.2 --- Dependence of resonant frequency and impedance bandwidth on ground plane length --- p.27 / Chapter 2.5.3 --- Dual-resonator model for mobile phone antenna --- p.32 / Chapter 2.6 --- Summary --- p.38 / Reference --- p.38 / Chapter Chapter 3 --- A Multilayeredly Rolled Inverted-F Antenna for Dual-band Mobile Phones --- p.42 / Chapter 3.1 --- Introduction --- p.42 / Chapter 3.2 --- Literature review on rolled antennas --- p.43 / Chapter 3.3 --- Proposed MRIFA --- p.47 / Chapter 3.3.1 --- Antenna configuration --- p.47 / Chapter 3.3.2 --- Simulation studies --- p.51 / Chapter 3.3.3 --- Prototype and Experimental results --- p.59 / Chapter 3.3.4 --- Comparison with a reference PIFA --- p.66 / Chapter 3.4 --- Mobile phone installed with the MRIFA --- p.70 / Chapter 3.5 --- Summary --- p.80 / Reference --- p.81 / Chapter Chapter 4 --- A fast method to evaluate Total Isotropic Sensitivity (TIS) in mobile phone active measurement --- p.82 / Chapter 4.1 --- Introduction --- p.82 / Chapter 4.2 --- Proposed fast method for TIS evaluation --- p.85 / Chapter 4.2.1 --- Observed relationship between total Effective Isotropic Radiated Power (EIRP) and total Effective Isotropic Sensitivity (EIS) --- p.85 / Chapter 4.2.2 --- EIS's estimation from EIRPs for TIS evaluation --- p.89 / Chapter 4.3 --- Summary --- p.92 / Reference --- p.93 / Chapter Chapter 5 --- Conclusion --- p.94 / List of Publications --- p.96 / Appendix A - Details of sample mobile phones investigated in this thesis research --- p.97 / Appendix B - Active measurement of mobile phone's transmit power and receiver sensitivity --- p.107 / Appendix C - MRIFA realization procedure --- p.118

This thesis work builds on the concept of reconfiguring the antenna properties (frequency, polarization, radiation pattern) using Radio Frequency (RF) Micro Electro Mechanical Systems (MEMS). This is a part of the overall research performed at the RF Micro/Nano Electro Mechanical Systems (uNeMS) Laboratory at Utah State University, which includes design, microfabrication, test, and characterization of uNeMS integrated cognitive wireless communication systems (Appendix A). In the first step, a compact and broadband Planar Inverted F Antenna (PIFA) is designed with a goal to accommodate reconfigurability at a later stage. Then, a Frequency Reconfigurable Antenna (FRA) is designed using MEMS switches to switch between the Public Safety (PS) bands, 152-162 MHz and 406-512 MHz, while maintaining the integrity of radiation pattern for each band. Finally, robust mechanical designs of the RF MEMS switches accompanied by different analyses have been performed. These analyses are instrumental in obtaining high yield, reliable, robust microfabrication processes including thin film metal deposition and patterning.

Thesis (Ph. D.)--University of Hong Kong, 2007. / Title proper from title frame. Also available in printed format.

Thesis (Ph. D.)--Ohio State University, 2007. / Title from first page of PDF file. Includes bibliographical references (p. 216-221).