Research On Microwave Time Reversal Systems And Related Key Techniques

Time reversal(TR) technique was widely used in the acoustics field years ago. It was introduced into the electromagnetics field recently. Due to the simultaneous temporal and spatial focusing characteristic of time-reversed electromagnetic wave, which can be potentially utilized in the applications of microwave communications or radar systems, TR technique has been paid much more attention on by electromagnetics researchers. This dissertation focuses on the research of microwave TR systems and related key techniques, including microwave time reversal mirror(TRM) system based on analog signal processing(ASP) technique, multiple-antenna array system based on microwave TR focusing, source imaging system based on TR technique, and the physical mechanisms and key techniques of these systems. The contents of this dissertation mainly contain three parts displayed as follows.In the first part, the system of microwave TRM based on ASP technique is studied. Firstly, a scheme to achieve TR for microwave signals by using chirp transform(CT) theory is analyzed, simulated and experimented. The operating bandwidths and time durations of each component in the system are analyzed numerically. In addition, the key modules and components of the system are discussed. Two microwave wideband chirped delay lines(CDL), which work at reflection modes, are designed and fabricated. At last, the TRM system is set up and tested. Through this system, the time-reversed signals of two triangle waves, each with a 2 ns time duration, are obtained respectively. Secondly, another scheme to achieve TR for microwave signals by means of time lens(TL) theory is analyzed theoretically and simulated numerically. Two kinds of conditions for TR by this system are deduced and validated through simulations. The simulated results agree with the theoretical ones. The time-reversed signal of a triangle wave with a 6 ns time duration can be obtained through this TRM system in the simulation. The simulated results are also compared to those in the former system based on CT theory.In the second part, the system of multiple-antenna array based on microwave TR focusing is researched. Firstly, two novel wideband subwavelength arrays with compact structures based on TR technique are designed. Each array is composed by four elements, which are traditional planar monopoles etched with some microstructured slots both in the monopole parts and the ground parts. The radiation properties of the etched element are studied by simulations. Moreover, the focusing property of the subwavelength array is analyzed theoretically according to signal processing. It is verified by experimental results that the proposed two array systems can both exhibit the far-field TR super-resolution property. Secondly, a simplified pulse-shaping network(PSN) using dispersion feeding approach is proposed for TR focusing. The CDL with two approximatively independent channels for two ports is adopted as the dispersion feed line for pre-distortion. By using the novel PSN with a single CDL, two microwave TR systems with different receiving parts are constructed, analyzed, and tested, respectively. The first system contains a 1′2 subwavelength array as the receiving array and the second system has only one receiving antenna. It has been proved experimentally that the time-reversed signals can focus well at the expected output port in both systems while the TR focusing with far-field super-resolution can be realized in the first system.In the third part, the system for source imaging based on TR technique is studied. Aiming at the potential applications of microwave TR technique, two system models with respect to TR source imaging are founded. For the first system, when a TRM is laid outside a slotted rectangle metal cavity, the focusing characteristics of time-reversed electromagnetic wave in the cavity are studied by theoretical analysis and simulations. The channel theory is utilized to analyze the reason for the TR focusing and mirror focusing. By means of embedding scatterers and adjusting the slot position, two effective ways to eliminate the phenomenon of mirror focusing are proposed. For the second system, the focusing properties of time-reversed electromagnetic wave in the arrays with minute metal wires are studied by simulations. A single-antenna model and a double-antenna model are set up respectively. Via rotating the whole metal wires array, the focusing results of time-reversed signals at the source antenna are analyzed and discussed. Furthermore, the single-antenna model is used to study the affections on the final results of TR focusing when properly changing the voltage amplitude and the time length of the time-reversed signals respectively.