Researches On Propagation Property Of Time Reversal Electromagntic Wave In Complex Media

Since Time reversal(TR) technique was introduced to the engineering in 1990 s, it arouses people great interest and concern, because time revesal waves have adaptive time-spatial focusing property and super-resolution property under certain conditions. The time reversal adaptive focusing property means the time reversal waves can adaptively converge back to its initial source where the initial source sends out a pulse and the signals are recorded, flipped and re-transmitted by time reversal mirrors(TRM). The time reversal super-resolution focusing property means the time reversal waves have sub-wavelength super-resoltion property when the time reversal adaptive focusing occurs.This dissertation, on the one hand, investigates the time reversal adaptive time-spatial property and the property of the time reversal super-resolution in different environments using different methods; on the other hand, proposes two applications of time reversal technique for the time reversal communication and the time reversal detection measurement, based on the theoretical models. The main contents are as follows:1. Global difference operator and its time reversal propertyFirst, based on the Helmholtz equation with source, the global difference operator including the boundary information is founded to be equivalent to the Helmholtz equation with source. In contrast with the ordinary difference operator, the global difference operator can not only avoid the calculation of eigenvector or solving the distribution of electromagnetic fields by iterative methods, but also solve the divergence problem of the difference scheme of Helmholtz equation with source when operating frequency is high; Furthermore, this method also simplifies the time reversal process because of the introduction of virtual sources.Next, the conditions in which the global time reversal operators satisfy the electromagnetic reciprocity and the time reversal symmetry are deduced. Furthermore, in order to obtain the perfect time reversal focusing, the requirement for global time reversal operator is given.Finally, the time reversal difference operators are investigated in a metal cavity, where the effects on the focusing of time reversal electromagnetic waves are considered by changing locations of sources, positions and numbers of time reversal mirrors, and adding metal scatterers in the cavity.2. Adaptive focusing of time reversal electromagnetic wave and the sub-wavelength super-resolutionFirst, in order to explain the time reversal super-resolution of time reversal electromagnetc wave in the sub-wavelength dipoles array with random metal wires, the random metal wire array is simplied to a sub-wavelength periodic metal wire array, and further a dielectric model is equivalent to the periodic metal wire array. Using transmission line method, the time reversal super-resolution of time reversal electromagnetc wave is obtained in the periodic metal wire array. At the same time, in order to understand how the parameters of the metal wire array acts on the time reversal super-resolution, the influence of the parameters of the metal wire array on the time-spatial focusing property of time reversal electromagnetic wave is discussed.Next, instead of the gaps among the metal wires, the periodic slots are used as boundaries to investigate the focusing property of time reversal electromagnetic waves in two dimensions. Based on Fresnel- Kirchhoff diffraction theory, the propagation property of time reversal electromagnetic waves is investigated in near fields and in far fields, where the effects on the focusing of time reversal electromagnetic waves are considered by changing positions and numbers of time reversal mirrors, the number of slots, and the distance between the slots.Finally, based on the results of the two above-investigated time reversal models, in order to simplify the time reversal communication wireless terminal antenna, a time reversal communication subwavelength antenna array is designed by abandoning the random wires which surround the dipoles. The elements of the array, which have independent channels, are etched with microstructures. Compared with the subwavelength antenna array which is composed of other microstructure elements, the array, whose radiation property of individual antennae is similar as others, shows better far-field time reversal super-resolution focusing characteristic in time reversal focusing experiments.3. Detection based on time reversal focusing peaksFirst, a new iterative time reversal approach is proposed to determine the relative permittivity of periodic stratified medium, using the peak value of iterative time reversal wave. By the relationship between the peak value of iterative time reversal wave and the relative permittivities, once the peak value of iterative time reversal wave is obtained, the relative permittivities of periodic stratified medium is gotten. The results also show that the iterative time reversal method is not needed if the medium only has one layer, and in that case the measurement of the relative permittivity can be done only by utilizing the peak value of time reversal wave.Next, the above-mentioned time reversal method is proposed to detect the state of bolt loosening. Using the relationship between the state of bolt loosening and the peak value of time reversal wave, the state of bolt loosening can be effectively measured. Both the experiments and the theoretical model are carried out to show the feasibility of the proposed method. Furthermore, the experimental results also demonstrate the characteristics of anti-noise and the advantage of detecting in low signal-to-noise-ratio signals with this method.