Metasurfaces for Antennas, Energy Harvesting and Imaging

Metamaterials are materials with artificial structures, engineered to produce electromagnetic properties not readily available in nature. Metamaterials have generated broad interest and utilization in various applications because of their engineer-able permittivity and permeability. Metasurfaces form the most-used class of metamaterials in all electromagnetic applications, from microwave to optical, because of their simplicity compared to bulky 3D structures. Metasurfaces are created using an ensemble of electrically small resonators. Previously, the metasurface concept was used to redirect or focus light, with the surface profile being tailored to control the phase and magnitude of the current at each cell. In the first part of this dissertation, a metasurface is used to create a new antenna concept by tailoring the feed for each resonator to create optimal radiation behaviour. The resonators are placed on a flat surface and connected to one feed point using different feed mechanisms to achieve desired current phase at each resonator. Unlike conventional array antennas, in which the distance between adjacent antennas is maintained at approximately half the wavelength to reduce mutual coupling between adjacent antennas, here the distance between the radiating elements is electrically very small. This effects good impedance matching of each resonator to its feed. The metasurface antenna has strong potential for a variety of traditional and non-traditional applications. Its flexible design (high degree of optimization freedom) facilitates its use on a variety of non-Cartesian and platforms. A prototype was fabricated and tested, showing positive agreement between numerical simulations and experimental results of the metasurface antenna. In this part, a concept is presented to enable a systematic design of low-profile conformal antennas. The concept is based on using closely spaced electrically-small radiators. An ensemble of the radiators is placed in a periodic arrangement and the phase of the feed for each element is set to create a phase front orthogonal to the direction where maximum radiation is desired. The phase front is created based on the assumption that each electrically-small radiator is essentially a Huygens source radiating in the open space. A novel method is proposed in the second part of the thesis that emerge metasurface for energy harvesting and wireless power transfer. Unlike earlier designs of metamaterial harvesters where each small resonator was connected to a load, in this design, the power received by the resonators is channeled collectively into one load, thus maximizing the power density per load. Another contribution of the metasurface harvester of this work, based on the concept of perfect absorbance and channeling to one load, is the design of a metasurface medium with near unity electromagnetic energy harvesting. Two different feed networks with different impedances matching techniques are proposed to deliver the maximum power collected by all cells to just one load. Prototypes were fabricated and tested; the numerical simulation and the experimental measurements showed that the proposed metasurface harvester was sufficient to collect microwave energy and deliver it to one load through vias using one feed network. The third part presents a new paradigm of imaging objects using metasurfaces. In this part, an extensive study has been done to examine the metasurface panels for imaging. In conventional imaging methods, a raster scan is used to sense any differences or changes in the object, whereas here, objects are imaged without any scan. The mothed leveraging the voltage from each cell and by using a simple Matlab code these voltages will build the image of the object. This method showed promising results through the numerical simulation of imaging for both metal and dielectric materials

Sparse Array Architectures for Wireless Communication and Radar Applications

This thesis focuses on sparse array architectures for the next generation of wireless communication, known as fifth-generation (5G), and automotive radar direction-of-arrival (DOA) estimation. For both applications, array spatial resolution plays a critical role to better distinguish multiple users/sources. Two novel base station antenna (BSA) configurations and a new sparse MIMO radar, which both outperform their conventional counterparts, are proposed.\ua0We first develop a multi-user (MU) multiple-input multiple-output (MIMO) simulation platform which incorporates both antenna and channel effects based on standard network theory. The combined transmitter-channel-receiver is modeled by cascading Z-matrices to interrelate the port voltages/currents to one another in the linear network model. The herein formulated channel matrix includes physical antenna and channel effects and thus enables us to compute the actual port powers. This is in contrast with the assumptions of isotropic radiators without mutual coupling effects which are commonly being used in the Wireless Community.\ua0Since it is observed in our model that the sum-rate of a MU-MIMO system can be adversely affected by antenna gain pattern variations, a novel BSA configuration is proposed by combining field-of-view (FOV) sectorization, array panelization and array sparsification. A multi-panel BSA, equipped with sparse arrays in each panel, is presented with the aim of reducing the implementation complexities and maintaining or even improving the sum-rate.\ua0We also propose a capacity-driven array synthesis in the presence of mutual coupling for a MU-MIMO system. We show that the appearance of\ua0grating lobes is degrading the system capacity and cannot be disregarded in a MU communication, where space division\ua0multiple access (SDMA) is applied. With the aid of sparsity and aperiodicity, the adverse effects of grating lobes and mutual coupling\ua0are suppressed and capacity is enhanced. This is performed by proposing a two-phase optimization. In Phase I, the problem\ua0is relaxed to a convex optimization by ignoring the mutual coupling and weakening the constraints. The solution of Phase I\ua0is used as the initial guess for the genetic algorithm (GA) in phase II, where the mutual coupling is taken into account. The\ua0proposed hybrid algorithm outperforms the conventional GA with random initialization.\ua0A novel sparse MIMO radar is presented for high-resolution single snapshot DOA estimation. Both transmit and receive arrays are divided into two uniform arrays with increased inter-element spacings to generate two uniform sparse virtual arrays. Since virtual arrays are uniform, conventional spatial smoothing can be applied for temporal correlation suppression among sources. Afterwards, the spatially smoothed virtual arrays satisfy the co-primality concept to avoid DOA ambiguities. Physical antenna effects are incorporated in the received signal model and their effects on the DOA estimation performance are investigated

Spin Coated Plasmonic Nanoparticle Interfaces for Photocurrent Enhancement in Thin Film Si Solar Cells

Nanoparticle (NP) arrays of noble metals strongly absorb light in the visible to infrared wavelengths through resonant interactions between the incident electromagnetic field and the metal's free electron plasma. Such plasmonic interfaces enhance light absorption and photocurrent in solar cells. We report a cost effective and scalable room temperature/pressure spin-coating route to fabricate broadband plasmonic interfaces consisting of silver NPs. The NP interface yields photocurrent enhancement (PE) in thin film silicon devices by up to 200% which is significantly greater than previously reported values. For coatings produced from Ag nanoink containing particles with average diameter of 40 nm, an optimal NP surface coverage of 7% was observed. Scanning electron microscopy of interface morphologies revealed that for low surface coverage, particles are well-separated, resulting in broadband PE. At higher surface coverage, formation of particle strings and clusters caused red-shifting of the PE peak and a narrower spectral response.Comment: 25 pages, 7 figure

Superdirectivity from arrays of strongly coupled meta-atoms

This is the final version of the article. Available from AIP Publishing via the DOI in this record.We explore the possibility of achieving superdirectivity in metamaterial-inspired endfire antenna arrays relying on the good services of magnetoinductive waves. These are short-wavelength slow waves propagating by virtue of coupling between resonant meta-atoms. Magnetoinductive waves are capable of providing a rapidly varying current distribution on the scale of the free space wavelength. Using dimers and trimers of magnetically coupled split ring resonators with only one element driven by an external source, we introduce an analytical condition for realising superdirective current distributions. Although those current distributions have been known theoretically for a good 60 years, this is the first time that a recipe is given to realise them in practice. Our key parameters are the size of the array, the resonant frequency and quality factor of the elements, and their coupling constant. We compare our analytical results for coupled magnetic dipoles with numerical results from CST simulations for meta-atoms of various shapes. The calculated bandwidth of 5 MHz for a dimer operating at 150 MHz indicates that, contrary to popular belief, superdirective antennas exist not only in theory but may have practical applications.Financial support by the John Fell Fund (University of Oxford) and by the EPSRC UK (SYMETA, EP/N010493/1) is gratefully acknowledged

DEVELOPMENT OF A WEB-LIKE HEXAGON SHAPED ELECTROMAGNETIC (EM) TRANSMITTER

Direct detection of hydrocarbon using powerful electromagnetic (EM) source in seabed logging (SBL) application has remained a challenge because the size of recent transmitter is too huge. Thus, a new web like hexagon shaped transmitter is proposed as a new invention, this is due to its abilities in concentrating the EM field on its centre and minimizing the size of transmitter. Hexagon shape is the best design in terms of radiation power with 579.86% compared to the dipole transmitter by using Applied Wave Research (AWR) software. From Computer Simulation Technology (CST) software, web like hexagon shape with 20 turns shows the 215.85% increasing of electric field compared to 5 turns. Combination of EM transmitter with Ni0.8Zn0.2Fe2O4 toroid could enhance the EM field at the transmitter. Hence, web like hexagon shaped transmitter was constructed by using copper wire in diameter of 0.08 cm with 20 turns and comparison of magnetic field with and without magnetic feeder were conducted. Ni0.8Zn0.2Fe2O4 nanoparticles were successfully synthesized using self combustion technique. The samples were then sintered at 800 0C, 900 0C and 1200 0C for 4 hours respectively. All sintered samples exhibited single phase Ni0.8Zn0.2Fe2O4 that was confirmed by the [311] plane as the major plane as revealed by XRD results. The calculated average crystallites sizes for these three samples were in the range of 20 – 50 nm determined using Scherer equations. Agilent LCR Impedance Analyzer Meter was used to determine the magnetic properties of Ni0.8Zn0.2Fe2O4 toroids which involved the measurements of initial permeability Qfactor and relative loss factor (RLF). Finally, magnetic field of web like hexagon shaped transmitter had increased by 1228.21 % at further offset by placing Ni0.8Zn0.2Fe2O4 as a magnetic feeder for 20 turns

Numerical Investigation on Flat-Top Beam Arrays for Wireless Power Transfer in the Millimeter-Wave Range

This thesis aims to analyze a far-field WPT link at mm-Wave proposing a transmitter i.e. 10x10 planar dipole antenna array operating at 31 GHz that radiates a ‘flat-top beam’ for uniform power distribution on the receiver side. The proposed link's reception system is made up of a number of single patch antennas that are arranged in space to completely cover the area invested by the flat radiation. To investigate the behavior of the flat-top beam, two different excitation configurations (in terms of amplitude and phase) were applied. The two configurations were named “Configuration 1” (in which the excitation was applied in a 1-dimensional way) and “Configuration 2D” (which had the excitation applied in a 2-dimensional manner) and the flatness was optimized through amplitude and phase variation. The receiving system can be viewed as a DC combiner because each patch antenna has a separate rectifier circuit that can collect RF power and convert it to DC. All the DC contributions were summed to compare the RF to DC efficiency of the entire mm-Wave for the two flat-top configurations. After comparing the results at different distances between the transmitting array and the receiving system for the two configurations, it was found that configuration 1 produced better results in terms of RF-DC efficiency per patch element whereas, configuration 2D produced better overall results in terms of total rectified power by the receiving system because a large number of receiving antennas could be accumulated inside the area covered by the beam. The average RF-DC efficiency was almost constant for every receiving patch which was a consequence of constant received power which was achieved through the flat-top bea

Characteristics of different focusing antennas in the near field region

Focusing antennas are of interest in many application including microwave wireless power transmission, remote (non-contact) sensing, and medical applications. Different kinds of antennas such as array antennas, reflector antennas and Fresnel zone plate (FZP) antennas have been used for these applications. Here, first, a new scheme in designing focused array antennas with desired sidelobe levels (SLLs) in the near field region is presented. The performance of the large focused array antennas is predicted based on the knowledge of the mutual admittances of a smaller array. The effects of various focal distances on the near field pattern of these antennas are investigated. Then, electric field pattern characteristics of the focused Fresnel zone plate lens antennas in the near-field region are presented. The FZP antenna fed by a circular horn is implemented and the effects of various focal lengths on the near field pattern of this antenna are examined. It is shown that the maximum field intensity occurs closer to the antenna aperture than to the focal point and this displacement increases as the focal point moves away from the antenna aperture. The focusing properties of ultra-wideband (UWB) array antennas are also presented. Large current radiator (LCR) antennas are modeled by replacing the antenna with a set of infinitesimal dipoles producing the same near field of the antenna. LCR antenna arrays are used to provide high concentration of microwave power into a small region. It is shown that the defocusing effect occurs in pulse radiating antennas as well. Invasive weed optimization (IWO), a new optimization algorithm, is also employed to optimize the pulsed array antenna. In the attempt of optimizing the focused arrays, a new scenario for designing thinned array antennas using this optimization method is introduced. It is shown that by using this method, the number of elements in the array can be optimized, which yields a more efficient pattern with less number of elements. By applying this new optimization method to UWB arrays, the peak power delivered to a localized region can be increased