Candidate
Thamer Almoneef
Title
Electromagnetic Energy Transduction Using Metamaterials and Antennas
Supervisor
Omar Ramahi
Abstract
The advent of rectenna systems almost half a century ago has enabled numerous applications in a number of areas since their main goal is to recycle the ambient microwave energy. In previously presented rectennas, microstrip antennas were the main energy sources used to capture and convert microwaves to AC power. However, the conversion e?ciency of antennas have never been examined in terms of their capability of absorbing microwave energy and hence the enhancements on the overall e?ciency 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) which is used for the first time as the main electromagnetic source of energy in a rectenna system. The SRR array was then compared to an array of patch antennas in terms of radiation to AC e?ciency where both arrays were placed on the same footprint area. Numerical simulations and experimental tests show that the SRRs were able to achieve higher e?ciency and wider bandwidth relative to microstrip antennas. The idea of electromagnetic energy harvesting using metamaterials was further explored by designing a metamaterial slab based on the full absorption concept. The metasurface material parameters were tuned to achieve a surface that is matched to the free space impedance at a certain band of frequencies to minimize any reflections and insure 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 efficiency of 97% and 93%, respectively.
A novel method is proposed in the second part of the thesis to significantly increase 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, 4 vertically stacked arrays were compared to a single 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 of up to 5 times when compared to a single 2-D flat panel.
The third part presents the design of a near unity electromagnetic energy harvester using 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 channelling 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 of a 4 × 4 TCA array was simulated, fabricated and tested showing great agreement between the simulated and measured results.
The following part of the thesis discusses the idea and design of a dual polarized metasurface for electromagnetic energy harvesting. A 4 × 4 super cell with alternating vias between adjacent cells was 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 a rectification circuitry. The simulation results yielded radiation to AC and an AC to DC conversion efficiencies of around 90% and 80%, respectively. As a proof of concept, an array consisting of 9 super cells was fabricated and measured. The experimental results show that the proposed energy harvester is capable of capturing up to 70% of the energy from a planewave having various incident angles and converting it to usable DC power.
As a 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 metamaterial absorber designs, the resonator is capable of not only full absorption but also maximum energy channeling across the load resistance. Numerical simulation demonstrated that 96% of the absorbed energy is dissipated across the load resistance. In addition, cross-polarized H-resonators design is presented that is capable of harvesting infrared energy using dual polarization within three frequency bands.