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

1-D broadside-radiating leaky-wave antenna based on a numerically synthesized impedance surface

A newly-developed deterministic numerical technique for the automated design of metasurface antennas is applied here for the first time to the design of a 1-D printed Leaky-Wave Antenna (LWA) for broadside radiation. The surface impedance synthesis process does not require any a priori knowledge on the impedance pattern, and starts from a mask constraint on the desired far-field and practical bounds on the unit cell impedance values. The designed reactance surface for broadside radiation exhibits a non conventional patterning; this highlights the merit of using an automated design process for a design well known to be challenging for analytical methods. The antenna is physically implemented with an array of metal strips with varying gap widths and simulation results show very good agreement with the predicted performance

Beam scanning by liquid-crystal biasing in a modified SIW structure

A fixed-frequency beam-scanning 1D antenna based on Liquid Crystals (LCs) is designed for application in 2D scanning with lateral alignment. The 2D array environment imposes full decoupling of adjacent 1D antennas, which often conflicts with the LC requirement of DC biasing: the proposed design accommodates both. The LC medium is placed inside a Substrate Integrated Waveguide (SIW) modified to work as a Groove Gap Waveguide, with radiating slots etched on the upper broad wall, that radiates as a Leaky-Wave Antenna (LWA). This allows effective application of the DC bias voltage needed for tuning the LCs. At the same time, the RF field remains laterally confined, enabling the possibility to lay several antennas in parallel and achieve 2D beam scanning. The design is validated by simulation employing the actual properties of a commercial LC medium

Phase Shaping In The Infrared By Planar Quasi-periodic Surfaces Comprised Of Sub-wavelength Elements

Reflectarrays are passive quasi-periodic sub-wavelength antenna arrays designed for discrete reflected phase manipulation at each individual antenna element making up the array. By spatially varying the phase response of the antenna array, reflectarrays allow a planar surface to impress a non-planar phasefront upon re-radiation. Such devices have become commonplace at radio frequencies. In this dissertation, they are demonstrated in the infrared for the first time--at frequencies as high as 194 THz. Relevant aspects of computational electromagnetic modeling are explored, to yield design procedures optimized for these high frequencies. Modeling is also utilized to demonstrate the phase response of a generalized metallic patch resonator in terms of its dependence on element dimensions, surrounding materials, angle of incidence, and frequency. The impact of realistic dispersion of the real and imaginary parts of the metallic permittivity on the magnitude and bandwidth of the resonance behavior is thoroughly investigated. Several single-phase reflectarrays are fabricated and measurement techniques are developed for evaluating these surfaces. In all of these cases, there is excellent agreement between the computational model results and the measured device characteristics. With accurate modeling and measurement, it is possible to proceed to explore some specific device architectures appropriate for focusing reflectarrays, including binary-phase and phase-incremental approaches. Image quality aspects of these focusing reflectarrays are considered from geometrical and chromatic-aberration perspectives. The dissertation concludes by briefly considering two additional analogous devices--the transmitarray for tailoring transmissive phase response, and the emitarray for angular control of thermally emitted radiation

Electromagnetic Energy Transduction Using Metamaterials and Antennas

The advent of rectenna systems almost half a century ago has enabled numerous applications in a number of areas, with the main goal of recycling the ambient microwave energy. In previously presented rectennas, microstrip antennas were the main energy source used to capture and convert microwaves to AC power. However, the conversion efficiency of antennas have never been examined in terms of their capability of absorbing microwave energy, and hence any enhancements to the overall efficiency of rectenna systems were mainly attributed to the rectification circuitry instead of the antenna. In the first part of this dissertation, a novel electromagnetic energy collector is presented, consisting of an array of Split Ring Resonators (SRRs), used for the first time as the main electromagnetic source of energy in a rectenna system. The SRR array is compared to an array of patch antennas to determine the radiation to AC efficiency when both arrays are placed on the same footprint. Numerical simulations and experimental tests show that the SRRs achieve higher efficiency and wider bandwidth than microstrip antennas. The idea of electromagnetic energy harvesting using metamaterials is further explored by designing a metamaterial slab based on the full absorption concept. The metasurface material parameters are tuned to achieve a surface that is matched to the free space impedance at a certain band of frequencies to minimize any reflections and ensure full absorption within the metasurface. The absorbed energy is then channeled to a resistive load placed within each element of the metasurface. Different from previous metasurface absorber designs, here the power absorbed is mostly dissipated across the load resistance instead of the substrate material. A case study is considered where the metamaterial slab is designed to operate at 3 GHz. The simulation and experimental results show radiation to AC efficiencies of 97% and 93%, respectively. A novel method is proposed in the second part of the thesis that significantly increases the conversion efficiency of electromagnetic energy harvesting systems. The method is based on utilizing the available vertical volume above a 2-D flat panel by vertically stacking panels while maintaining the same 2-D footprint. The concept is applied to SRRs and folded dipole antennas. In both cases, four vertically stacked arrays are compared to a single array panel, both occupying the same flat 2-D footprint in terms of power efficiency. The numerical and experimental results for both the SRRs and the antennas show that the stacking concept can increase the conversion efficiency by up to five times when compared to a single 2-D flat panel. The third part presents the design of a near unity electromagnetic energy harvester that uses a Tightly Coupled Antenna array. Compared to the unit cell of metamaterial surfaces, the dimension of a TCA unit cell is about five times larger, thus providing simplified channeling networks and cost-effective solutions. The TCA surface contains an array of Vivaldi shape unit cells with a diode at each cell to convert the harvested electromagnetic energy to dc power. The dc power from each unit cell is channeled to one single load via series inductors. A sample 4 X 4 TCA array, when simulated, fabricated and tested shows solid agreement between the simulated and measured results. The thesis then discusses the idea and design of a dually polarized metasurface for electromagnetic energy harvesting. A 4 X 4 super cell with alternating vias between adjacent cells is designed to allow for capturing the energy from various incident angles at an operating frequency of 2.4 GHz. The collected energy is then channeled to a feeding network that collects the AC power and feeds it to rectification circuitry. The simulation results yield a radiation to AC, and AC to DC conversion efficiencies of around 90% and 80%, respectively. As a proof of concept, an array consisting of nine super cells is fabricated and measured. The experimental results show that the proposed energy harvester is capable of capturing up to 70% of the energy from a plane wave with various incident angles and then converting it to usable DC power. As future work, the last part introduces the concept of metasurface energy harvesting in the infrared regime. The metasurface unit cells consist of an H-shaped resonator with the load placed across the gap of the resonator. Different from infrared meta-material absorber designs, the resonator is capable of not only full absorption but also maximum energy channeling across the load resistance. The numerical simulation demonstrates that 96% of the absorbed energy is dissipated across the load resistance. In addition, a cross-polarized H-resonator design is presented that is capable of harvesting infrared energy using dual polarization within three frequency bands

Wide Band Embedded Slot Antennas for Biomedical, Harsh Environment, and Rescue Applications

For many designers, embedded antenna design is a very challenging task when designing embedded systems. Designing Antennas to given set of specifications is typically tailored to efficiently radiate the energy to free space with a certain radiation pattern and operating frequency range, but its design becomes even harder when embedded in multi-layer environment, being conformal to a surface, or matched to a wide range of loads (environments). In an effort to clarify the design process, we took a closer look at the key considerations for designing an embedded antenna. The design could be geared towards wireless/mobile platforms, wearable antennas, or body area network. Our group at UT has been involved in developing portable and embedded systems for multi-band operation for cell phones or laptops. The design of these antennas addressed single band/narrowband to multiband/wideband operation and provided over 7 bands within the cellular bands (850 MHz to 2 GHz). Typically the challenge is: many applications require ultra wide band operation, or operate at low frequency. Low frequency operation is very challenging if size is a constraint, and there is a need for demonstrating positive antenna gain

Programmable Beam-Steering Capabilities Based on Graphene Plasmonic THz MIMO Antenna via Reconfigurable Intelligent Surfaces (RIS) for IoT Applications

The approaching sixth-generation (6G) communication network will modernize applications and satisfy user demands through implementing a smart and reconfigurable system with a higher data rate and wider bandwidth. The controllable THz waves are highly recommended for the instantaneous development the new technology in wireless communication systems. Recently, reconfigurable intelligent surfaces (RIS), also called codded/tunable programmable metasurfaces, have enabled a conspicuous functionality for THz devices and components for influencing electromagnetic waves (EM) such as beam steering, multi-beam-scanning applications, polarization variation, and beam focusing applications. In this article, we proposed a graphene plasmonic two-port MIMO microstrip patch antenna structure that operates at a 1.9 THz resonance frequency. An E-shape MTM unit cell is introduced to enhance the isolation of the antenna from −35 dB to −54 dB. An implementation of controllable and reconfigurable surfaces based on graphene meta-atoms (G-RIS) placed above the radiating patches with a suitable separated distance to control the radiated beam to steer in different directions (±60°). The reconfigurable process is carried out via changing the (ON/OFF) meta-atoms states to get a specific code with a certain beam direction. The gain enhancement of the antenna can be implemented through an artificial magnetic conductor (AMC) based on graphene material. The G-AMC layer is located underneath the (MIMO antenna, G-RIS layer) to improve the gain from 4.5 dBi to 10 dBi. The suggested antenna structure results are validated with different techniques CST microwave studio and ADS equivalent circuit model. The results have asymptotic values. So, the proposed design of the MIMO antenna that is sandwiched between G-RIS and G-AMC is suitable for IoT applications

Antennas and Propagation

This Special Issue gathers topics of utmost interest in the field of antennas and propagation, such as: new directions and challenges in antenna design and propagation; innovative antenna technologies for space applications; metamaterial, metasurface and other periodic structures; antennas for 5G; electromagnetic field measurements and remote sensing applications

Wideband Bi-Static and Monostatic Star Antenna Systems

The thesis presents the design, and development of novel wideband Simultaneous Transmit And Receive (STAR) antenna systems. A STAR or in-band full-duplex system has the potential to double the throughput of a communication channel, which is highly important for the next-generation wireless networks. Similarly, these systems could increase the effectiveness of Electronic Warfare (EW) and Support (S) operation, by facilitating spectrum/channel sensing while jamming. A self-interference (SI) phenomenon, where the transmitter disrupts its own receiver is a major challenge in the practical realization of any radio frequency (RF) system. High isolation (>130 dB) is often needed to overcome this SI. A common approach to achieve this high isolation is some combination of cancellation levels such as antenna, analog, digital and signal processing layers. The thesis focuses on maximizing the isolation at the antenna layer, which is crucial for the system implementation. This is attained by researching bi-static, monostatic, and quasi-monostatic architectures that do not rely on polarization multiplexing.Bi-static configurations use separate TX and RX antennas. Hence, the SI can be minimized by increasing the separation between apertures, embedding high impedance surfaces (HISs), or by recessing the RX antenna inside the absorber, as demonstrated in this thesis. The advantages and limitations of each of these techniques are analyzed through full-wave simulations and measurements. High power capable, wideband, metallic quad ridge horn (QRH) antennas are first developed and bi-static, dual polarized STAR system is realized with them. Measured isolation >60 dB is demonstrated between the TX and RX apertures operating over 6-19 GHz and separated by 4λ at the turn on frequency. Isolation >70 dB is obtained in the 18-45 GHz bi-static dual polarized in-band duplex antenna system. Further, the influence of scatterers on system isolation is discussed.Bi-static configurations are robust, and system isolation is less sensitive to the asymmetries in the geometry. However, they require significant area particularly when highly directive apertures are needed. When a bi-static approach is applied to reflector-based systems, the overall size of the system is often prohibitively large. Hence, a monostatic configuration is highly desired for a high gain system. In this thesis, a monostatic STAR configuration, operating from 4-8 GHz, is developed by feeding the designed circularly polarized (CP) reflector antenna with all-analog beamforming network (BFN) consisting of two 90° and 180° hybrids and two circulators. The BFN is arranged to cancel the coupled/leaked signal from the antenna and circulators, by creating 180° phase difference between the TX and RX reflected signals. Theoretically, with ideal devices this approach can provide infinite isolation. In practice, the isolation is limited by the electrical and geometrical imbalances. Nonetheless, using COTS components with noticeable imbalances, average isolation >30 dB is achieved with the fabricated system, which is on average 15 dB higher than the isolation obtained with a conventional circulator approach.Finally, a quasi-monostatic STAR approach is proposed to address the limitations of bi-static and monostatic configurations. The demonstrated configuration can achieve 30 dB (on average) higher isolation than the monostatic reflector architecture with the same BFN components. The quasi-monostatic STAR antenna system consists of a parabolic reflector antenna for transmission, and a receiving antenna mounted back-to-back with the reflector feed. To increase the system isolation both the TX feed and the RX antenna are CP. Further, to achieve the same TX and RX polarization the TX feed is LHCP, and the RX antenna is RHCP. The LHCP fields from the TX feed undergo polarization reversal after bouncing back from the reflector, thereby, the TX and RX operate in the same polarization. T