Research On The Key Technologies For The Realization Of The High Efficient Time Reversal Electromagnetic System

A complete time reversal electromagnetic system consists of two parts: timereversal mirror and receiving terminal. The main functions of the time reversal mirrorare making time reversal transformation on channel detecting signals andre-transmitting the time-reversed signals. The main functions of the receiving terminalare receiving the time-reversed signals and making further processing to achieveoptimum focusing effect. Both time reversal mirror and the receiving terminal willaffect the focusing performance of time-reversed electromagnetic wave. Therefore,study of the key technologies for the realization of time reversal mirror and thereceiving terminal will help improve the performance of the complete time reversalsystem.Firstly, the background and the development trends of time reversalelectormagnetics are introduced in the dissertation. The approach to obtaintime-reversed electromagnetic signals and the method to design sub-wavelength arraybased on time reversal technique are summarized and two critical issues for therealization of the time reversal electromagnetic system are pointed out.Then, the implementations of time reversal transformation of high frequencyelectromagnetic waves based on analog signal processing technique are thoroughlystudied. The analog signal processing technology is based on the physical properties ofthe devices to complete the signal processing. Compared with digital signal processingtechnique, because of requiring no discretizing processing, the analog signal techniquewill be unlimited by the sampling rates of AD/DA converters any more and theprocessing speed of the analog signal processing is much superior to the digital signalprocessing technique. The dissertation investigates two approaches of waveformtransformation based on analog signal processing technique. Firstly, the waveformtransformation approach based on temporal imaging technique is studied. Theconditions to achieve time-reversed waveform of microwave signals in thetemporal-imaging system are derived by comparing with the spatial-imaging system.Design methods of the chirped delay lines serving as the key components in the temporal imaging system are discussed. By employing the fabricated chirped delay lines,the approach to obtain time-reversed electromagnetic signal based on time lens theory isfurther validated in system simulation. The time-reversed waveform of a singletriangular signal with a1.6ns time duration is obtained in the hardware in loopsimulation. On that basis, a new approach to obtain time-reversed electromagneticsignals based on dispersion compensation principle is proposed. The conditions toachieve time-reversed waveform of microwave signals based on dispersioncompensation technique are derived and the new approach is validated by bothsimulation and experiment. Compared with the temporal-imaging technique, the newapproach requires less dispersion delay lines and reduces the system losses. Thesestudies on time reversal transformation of electromagnetic signals based on analogsignal processing technique will provide new ideas for the realization of high efficienttime reversal mirror.Further more, the approaches to design the sub-wavelength antenna array by usinganalog signal processing technique are proposed. The sub-wavelength antenna arrayplays an important role in the receiving terminal of the time reversal electromagneticsystem. The method to design sub-wavelength antenna array based onevanescent-to-propagating spectrum conversion mechanism has a low success rate, so itis unsuitable to replication. By analyzing the cross-coupling between the elements of thesub-wavelength array, the conditions to achieve time reversal super-resolutioncharacteristic for sub-wavelength array are determined. On that basis, two new schemesare proposed to design the sub-wavelength antenna array. In the first scheme, thechirped delay lines with different dispersion properties are embedded into the feedingnetworks of the array elements. Because the embedded chirped delay lines break thestrong correlations of the channel responses from the elements to the time reversalmirror, when time reversal technique is used, the focusing signal with maximum peakvoltage will be received only by the target element of the sub-wavelength array. In thesecond scheme, in order to reduce the correlations of the channel responses from theelements to the time reversal mirror, the pulse shaping circuits with different transferfunctions are embedded into the feeding networks of the elements. Combining with timereversal technique, the sub-wavelength array can also achieve time reversalsuper-resolution property. Compared with the first scheme, due to requiring no directional coupling devices, the the radiation efficiency of the element antennas arefurther improved by using the second scheme. Compared with the approach based onevanescent-to-propagating spectrum conversion mechanism, the efficiency and thedecorrelation level are much easier to control and predict by using the new approaches.At last, the impact factors of the focusing effect of time-reversed electromagneticwaves are discussed. The influences caused by the special metamaterials on the focusingeffect of time reversal electromagnetic waves are studied by using simulations andexperiments. In addition, the influences caused by antennas in the time reversal mirrorare also discussed and some preliminary solutions are proposed.