Time reversal techniques in electromagnetic wave propagation

The time reversal method is a novel scheme utilizing the scattering components in a highly cluttered environment to achieve super-resolution focusing beyond Rayleigh criteria. In acoustics, time reversal effects are comprehensively analyzed and utilized in underwater target detection and communication. Successful demonstrations of the time reversal method using low frequency waveform in acoustics have generated wide interest in utilizing time reversal method by radio frequency electromagnetic waves. However, applications of the time reversal method in electromagnetics are considered to be emerging research topics and lack extensive analyses and studies.; In this thesis, we present a systematic study in which a series of novel time reversal techniques have been developed for target detection and imaging in highly cluttered environments where higher order scattering is substantial. This thesis also contributes to insightful understanding of basic time reversal properties in electromagnetic (EM) wave propagation in such environment. EM time reversal focusing and nulling effects using both single and multiple antennas are first demonstrated by FDTD simulations. Based on these properties, single antenna time reversal detection indicates significant enhancement in detection capability over traditional change detection scheme. A frequency selection scheme utilizing the frequencies with strong constructive interference between the target and background environment is developed to further improve the performance of the time reversal detector. Moreover, a novel time reversal adaptive interference cancellation (TRAIC) detection scheme developed based on TR properties can obtain null of the background through the time reversal nulling effect and achieve automatic focusing on the target through the time reversal focusing effect. Therefore, the detection ability, dynamic range and signal to noise ratio of a radar system can be significantly enhanced by the time reversal method.; In highly cluttered environment, conventional radar imaging methods, such as synthetic aperture radar (SAR) suffer from higher order scattering, which result in random ghost patterns in the radar images. On the other hand the ghost patterns in the difference SAR imaging with and without targets also provide information about the scattering between the targets and the environment even when the targets are not directly "visible". We have developed a new imaging scheme based on the exploitation of the ghost patterns which is substantially enhanced by adding a time reversal procedure in the difference SAR scheme, referred to as TR-SAR. By matching the obtained ghost image with forward FDTD simulations using the scatters obtained from SAR imaging, a complete image of the point/extended target can be reconstructed with relatively high confidence. With a highly cluttered environment that generates sufficient secondary scattering, ghost images are distinctively different for distinctively different point/extended targets, yielding a high distinction correlation in the forward matching process.; The thesis contains both numerical simulation studies and experimental measurements. Finite-difference-time-domain (FDTD) technique has been employed for all the numerical simulations.