Researches On Propagation Characteristics Of Time Reversal Electromagnetic Wave

Based on the space-time focusing characteristics of the time-reversal electromagnetic waves, the time-reversal technique has received widespread attention in wireless communication, nondestructive test, medical imaging, oncotherapy, energy transfer, etc. Compared with the booming development of time reversal application,researches on the propagation properties of the time-reversal electromagnetic waves are relatively little. However, with an understanding of the propagation property, a recognition of the physical mechanism of the space-time focusing characteristics, and an investigation of the mechanism of the sub-wavelength super resolution phenomenon of the time-reversal electromagnetic waves, it is beneficial to deepen the knowledge of the electromagnetic theory and there are remarkable referent values to the applications of the time-reversal electromagnetic waves.This dissertation mainly researches on the propagation characteristics of the time-reversal electromagnetic waves and is organized as follows:Chapter 1 introduces the relevant backgrounds and the development status of time-reversal.Chapter 2 researches the propagation characteristics of the time-reversal electromagnetic waves in waveguides. Firstly, according to the theory of time-reversal cavity, the analytical expressions of the time-reversal field in the rectangular waveguide are derived. The analytical expressions show that, the Green’s function or the normal derivatives of Green’s function on the metal boundaries is zero due to the metal boundary conditions, thus, the favorable focusing effect could be obtained only by a small number of time-reversal mirrors in the metal cavities. Then, the results of the research on the temporal focusing property of the time-reversal field in waveguide manifest that only at the focusing time point can the maximum signal amplitude be received at the position of the source. Furthermore, time-reversal field in discontinuous parallel plate waveguides has been analyzed by using mode-matching technique, the results show that, time-reversal field at the location of the source is restored to its initial phase and achieves the reconstruction of the original source through compensating the phases of the propagation patterns based on time-reversal technique. Finally, this dissertation achieves focal spots as small as one-fifth wavelength in a parallel-platewaveguide structure with two sets of scatters in it. With the mode-matching technique,the results of the time-reversal field in the second parallel-plate waveguide indicate that far-field sub-wavelength focusing can be achieved only by effectively exciting the evanescent modes.Chapter 3 analyses the impact of the antenna performance on the effects of time-reversal focusing. Firstly, the influence brought by antenna gain pattern, flatness within the bandwidth, polarization and the operating bandwidth has been theoretically researched. The results demonstrate that due to the different multipath attenuation loss in different directions, the time-reversal focusing signal of the directional antenna is different from that of the omnidirectional antenna. Since the frequency domain distortion varies in different directions of the antenna, different superimposed signal at the time-reversal focusing peak happens and more serious signal distortion emerges as a consequence. The mismatched polarization between the transmitting and receiving antennas as well as the inconsistency polarization of different directions of the antennas will change the energy of the received time-reversal signal. A better performance for time-reversal focusing would be achieved with a wider bandwidth and a lower correlation between the channels. Nextly, specific effects of antenna parameters on time-reversal focusing have been verified by the experimental results. Comparing the effect of the time-reversal focusing brought by the omnidirectional monopole antenna with the directional Vivaldi antenna, the results show that if the directional antennas are chosen as the time reversal mirror, the multipath shown in the received signals are less,whereas, the energy density is higher than that of omnidirectional antennas. Since the gain flatness of the log-periodic antenna within a specified band is relatively poor, and the time-reversal technique does not compensate the attenuation distortion, serious distortion occurs to the received final focusing signal. Lastly, a novel reconfigurable antenna pattern has been designed, and a time-reversal communication mechanism of improved user selectivity has been constructed based on the proposed reconfigurable antenna. The effectiveness of the proposed time-reversal communication mechanism has been verified by experiment.Chapter 4 discusses the propagation of time-reversal electromagnetic waves in anisotropic media such as wire array and split ring plane array. First of all, the periodically arranged metal array has been theoretically analyzed, furthermore, it has also been verified by simulation that the metal array could help transfer the energy ofthe evanescent waves into propagate waves in the near-field. Secondly, the experimental results verifie that the resolution of time reversal focusing has been improved through loading split ring plate array between the antennas. Thereafter, the reasons why the split ring plate array can improve the time-reversal focusing resolution have been researched both in frequency and time domain. Finally, impact of the introduced split ring plate array on the antenna performance has also been described.Chapter 5 summarizes the works and the conclusions of the researches, and then introduces the following researches to be done. Prospects on the development of TR technology are also presented.