Researches On Propagation Characteristics Of Timereversed Electromagnetic Wave In Complex Media And Its Application

With the property of super-resolution focusing,time reveral technique has gained increasing attention among researchers due to its broad and potential applications in ultrawideband communications,radar detection and imaging systems.In order to meet the increasing multhiple function requirements of wireless communication systems in the aspects such as large capacity,high rates,ministration,it is significant to achieve multiple antenna system with independent channel response in limited space.The difficulty in constructing super-resolution transmission technique of electromagnetic wave is the design of special medium.This dissertation focuses on the research of the propagation characteristics of time-reversed electromagnetic wave in dielectric medium and metallic medium.Several sub-wavelength arrays in 2-D and 3-D are constructed by cylinderic scatterers with different materials,the influences of the sub-wavelength arrays on the scattered characteristics of the source are studied.Based on the analyzed results,three sub-wavelength antenna arrays are proposed to achieve the super-resolution transmission of time-reversed electromagnetic wave.It is aimed to have great guiding significance for the design of multiple-antenna system based on the time reversal technique.The main research contents of the dissertation are summarized as follows:In chapter 1,the research background of this dissertation is presented.The research background of time reversal technique is introduced.The development of the theory about time reversal focusing in recent years are described,the research status of super-resolution imaging and sub-wavelength antenna arrays are reviewed.In chapter 2,the propagation characteristics of time-reversed eletramagnetic waves in complex medium is investigated.With the Green function of the cylindrical scatterers,the forward and time-reversed field distributions in the designed medium with the excitation of TM polarization are solved.The resolution value of time-reversed waves in different media are theoretically analyzed and numerically verified.In addition,in a low dielectric constant environment,1/5-wavelength resolution can be achieved for the original source imaging when combined with time-reversal technique,no matter where the source is located.The work greatly enriches the theory of time-reversal imaging based on dieletric medium and have great guiding significance for the design of the medium used to high-resolution imaging.In chapter 3,the propagation characteristics of time-reversed eletramagnetic waves in metal medium is investgated.Firstly,the idea that single-frequency electromagnetically excited signal can possess the property of super-resolution focusing under time reversal technique is proposed.The feasibility of the idea is investigated from the channel theory.A micro-structured model with inter-element spacing of 1/4 wavelength is proposed based on the traditional coaxial probe antenna.Simulated results verify that in combination with time reversal technique,the model shows consistent super-resolution focusing characteristics under single-frequency and broadband input signal excitation.The theoretical analysis and simulated results have great guiding significance for the superresolution research of the single-frequency and multi-band antennas.Secondly,from the view of the intrinsic modes,it is verified that the super-resolution transimission of singlefrequency excitation attributes to the detection of excited egienmode in the far field.As a result,when a wide signal excitation including several excited intrinsic mode frequencies is used,1/35-wavelength resolution is presented at the source antenna.The relationship between the propagation of intrinsic modes and super-resolution transmission performance of the sub-wavelength antenna element is established.Finally,based on the parametric analysis,including the parameters of inter-element spacing among resonant elements,the length of the resonant element and the distribution of the resonant structure,it can be confirmed that the length of resonant element proves a decisive factor to the frequency band with super-resolution performance,which provides extremely reference value for the quantifiable design of sub-wavelength antenna arrays.In chapter 4,the application study of sub-wavelength antenna arrays based on the time-reversed electromagnetic waves are introduced in the aspects of frequency diversity and space diversity.Firstly,based on the decisive role of the resonant element length in the frequency band with super-resolution performance,a basic combination constructed by two metal-wire resonators with different lengths is proposed and its resonant characteristics is analyzed using the method of moments.Based on the time reversal technique,this kind of combination can be applied to a sub-wavelength two-element antenna array with a 1/40-wavelength interval to make the elements work simultaneously with little interference in the frequency band of 1.0-1.5 GHz and 1.5-2.0 GHz,respectively.The simulations and experiments show that analysis of the method of moments and the application of the resonators can be used to design multi-frequency subwavelength antenna arrays efficiently.This general design method is convenient and can be used for frequency diversity to weaken jamming effectiveness in communication systems and sub-wavelength imaging in a broad frequency band.In addition,two subwavelength antenna arrays for space diversity are proposed,asymmetric resonant structure with three antenna elemenets and symmetric one with four antenna elements.The numerical and measured results verify that the signal transmission with a 1/40-wavelength resolution can be achieved at each antenna element when combined with time-reversal technique.Compared with the sub-wavelength antenna array without resonant structure,physical properties of the resonant element contributing to the radiation patterns make the channel responses different.The results have great guiding significance for the pratical applications of sub-wavelenth antenna arrays to the multipleinput multiple-output communication systems.Finally,in chapter 5,the research of this dissertation is summarized and future research about the applications of time reversal technique and sub-wavelength antenna arrays is proposed.