title: Broadband Rectifying-Antennas for Ambient RF Energy Harvesting and Wireless Power Transfer
Creators: name: Song, C
Publisher: name: University of Liverpool Repository
Year: 2017

description: Ambient wireless energy harvesting (AWEH), by means of capturing RF (radio frequency) and microwave energy from ambient and converting it to electric energy, has become an emerging technology and attracted an upsurge of research interests during the past five years. It could become a good addition to complement the existing energy harvesting technologies for such as vibration, solar and wind energies. One of the ultimate objectives of using these energy harvesting technologies is to create self-sustainable, truly standalone wireless sensor platforms and low power devices for smart home, smart cities and IoT applications, which will have a significant impact on our life in the future. However, the optimal design of a rectifying-antenna, or rectenna (as one of the vital devices for AWEH systems), is still very challenging. A number of key issues and research problems have been identified for broadband rectenna designs, such as the low conversion efficiency and strong nonlinearity under the ambient power conditions. The purpose of this thesis is to present a comprehensive study into broadband rectennas, aiming at overcoming the most challenging research problems of this topic. This thesis is comprised of three main research areas. The first area under investigation focuses on how to improve the overall power conversion efficiency of a broadband rectenna at ambient low power levels. In AWEH applications, a precise knowledge of ambient electromagnetic fields is essential. Therefore, a citywide electromagnetic field measurement campaign was conducted at Liverpool to identify the suitable frequency bands as well as the average power density at different ambient environments for AWEH. A novel broadband rectenna was designed and optimized at the ambient power levels obtained from the measurement campaign. Several novel techniques have been introduced to improve the overall conversion efficiency of the rectenna. Experimental results show that the harvested power and overall efficiency of this broadband rectenna are much higher than that of previously reported designs. This design confirms the feasibility of capturing RF energy from a typical indoor office environment for low power applications. The second area concentrates on how to reduce the nonlinear effect of broadband rectennas, since the performance of most existing rectennas can be significantly affected by the input power and load impedance variations due to this effect. Two designs are presented in this part. Page| xxi The first design is a novel six-band CP (circular-polarisation) rectenna using an improved impedance matching technique. A novel rectifier is introduced with a special matching network section to reduce the impedance mismatch caused by the load impedance variations. A miniaturized ultra-wideband CP receiving antenna is presented as well. Experimental results demonstrate that this design covers a wide frequency band (from 550 MHz to 2.5 GHz) and has constant conversion efficiency over a wide load impedance range (from 10 to 75 kΩ). The second design is an ultra-wide band rectenna using a hybrid resistance compression technique. The broadband matching network of this design is mixed with a resistance compression network to reduce the impedance variation of the rectifier. This design also demonstrates constantly high conversion efficiency and good impedance matching performance over a wide frequency band and load impedance range. The last area under investigation focuses on how to reduce the complexity of broadband rectennas. There are two designs presented in this part. In the first design, a cutting-edge technology to eliminate the need of an impedance matching network is introduced. A special broadband high impedance antenna is designed to conjugately match with the rectifier impedance directly. This design shows a very simple structure and design process. Experimental results demonstrate that this design without matching networks still achieves an excellent conversion efficiency, a good impedance matching performance and a reduced nonlinear effect. The second design is an adaptive rectenna with a wide band frequency-tunable feature. This design is also achieved without the need of impedance matching networks. The rectenna is configured with multiple output ports connected to a number of adaptive rectifier. Experiment results show that this design works well for a variety of frequencies, input powers and load impedance. Both designs show a much-simplified structure and reduced cost compared with other broadband rectenna designs. This thesis has successfully demonstrated a number of novel design methods for broadband rectennas. The most challenging issues such as the nonlinear effect and low conversion efficiency have been significantly overcome by using these presented technologies. The research and knowledge in this thesis should be of great significance to the future development of rectennas and have definitely increased the boundary of this topic to a new level.