Research On Time-modulated Array Antennas

The time-modulated array(TMA)antenna,based on the time-modulation technique,is a new type of array antenna,which introduces "time" as the fourth-dimensional degree of freedom in three-dimensional,conventional array antenna design,making the radiation characteristics to be controlled.In this dissertation,the theory and the analysis method of time modulated array antennas have been studied first.Then,from the view point of array architecture simplification,cost reduction,and advanced performance,the existing per-formance deficiency of time-modulated array antennas for single beam scanning has been addressed,the applications and benefits of the time-modulation technique in retrodirective array antennas have been exploited,and the relevant theory and potential applications of time-modulated frequency diverse array antennas(i.e.,generalized time-modulated array antennas)have been explored.The main innovations in this dissertation are as follows:1.The instantaneous and average behaviors of time-modulated array antennas in time and frequency domains:an analytical analysis method based on the in-array mu-tual impedance formula has been proposed to accurately analyze and simulate time-modulated array antennas and also fully obtain the antenna behaviors in the presence of mutual coupling effects,which greatly improves the full-wave analysis efficiency for TMAs optimization design.Based on the proposed analytical method,the instantaneous and average behaviors of a time-modulated array antenna in time and frequency domains have been analyzed,and the full-wave time-domain and frequency-domain expressions for calculating the power pattern and directivity have been given.To validate the accuracy and efficiency of the proposed method,low sidelobe pattern synthesis and constant instantane-ous directiviy systhesis in a uniformly-excited,uniformly-spaced TMA and nonuni-fromly-spaced TMA have been carried out and the numerical results have been given in comparison with the simulated results.2.Single-sideband time-modulated phased array(TMPA)antenna:In the case of single-beam scanning,to address the issues of quantization errors,complex controlled circuits,and high cost of conventional array and to address the extremely low radiation ef-ficiency issue when using a TMA because of the unwanted radiations at the center and sideband frequencies,a single sideband time-modulated phase-only weighting technique has been proposed which is realized by controlling the pulse time-sequences,resulting in a phase array function without using the phase shifters.Based on the rectuangular pulses(i.e.,considering ideal RF switches)and trapezoidal pulses(i.e.,considering the rise/fall time of realistic RF swithces),the single beam scanning behaviors of single sideband TMPA have been investigated,and the comparison on the performances(sideband level and available power efficiency)of TMPA between using the two types of pulse time-sequences has been made.3.Time-modulation-based reconfigurable coaperture array antennas:Based on the time-modulation and retrodirecitvity techniques,two types of TM-based reconfigurable coaperture array antennas have been proposed,i.e.,time-modulated retrodirective and self-null-steering array(TM-RDNA)and time-modulated/retrodirective/time-modulated retrodirective array(TM-RDA).The theories of both TM-RDNA and TM-RDA have been given.Compared with conventional RDNA antenna system,the proposed TM-RDNA has the advantanges of architecture simplification,low cost and complexity,and space saving.The proposed TM-RDA achieves multiple functionalities and the functionality reconfigu-ration is realized by controlling the time sequences of the SP3T switches,resulting in the advantanges of being simple reconfiguration,high accuracy,and low cost.To verify the feasibility and validity of the proposed schemes,a four-element TM-RDNA and a four-element TM-RDA,by using printed dipole antenna element,have been designed and fabricated as well as measured.The measured results have been given with the corre-sponding simulated results included for comparison.4.Time-modulated frequency diverse array antennas for long-range focusing:To realize four dimensional spatial-time(i.e,azimuth agnle ?,pitch angle ?,range r,time t)focusing for a long-range target,a frequency diverse array based on time-modulated fre-quency offset has been proposed.Two types of time-modulated nonlinear frequency offsets,i.e.,time-modulated logarithmic frequency offset(TMLFO)and time-modulated optimized frequency offset(TMOFO),have been given.The former is analytical solution while the latter is optimized solution based on the pattern optimization synthesis.Through the com-parative analysis on the spatial-time performance between using the TMLFO and TMOFO in the case of both ideal and non-ideal frequency offsets,it is verified that the frequency diverse array using TMOFO results in a better and more robust spatial-time focusing per-formance.Moreover,in view of the application of TMOFO schemes for a long-range target,a time-modulated frequency diverse array based on angle-independent TMOFO for multi-ple targets in long-range has been proposed.The proposed array,based on the combination of angle-independent TMOFO(which depends only on the distances of the targets and in-dependent of the angles of targets)and the multiple sidebands radation feature of a TMA,results in the simultaneous spatial-time focusing capability for multiple long-range targets with common distance.5.Time-modulated frequency diverse array antennas for short-range focusing:To address the issue of four dimensional spatial-time focusing error caused by the time-inavariant phase error,which is due to the the large interelement frequency offsets when using the TMOFO scheme for short-range focusing,a compensated TMOFO scheme,i.e.,a frequency diverse array using the compensanted TMOFO,has been proposed.The compensated TMOFO,which compensates the phase contribution of time-variant quadratic phase term by modifying the frequency offsets across the array elements,results in the ar-ray with accurate spatial-time focusing for a short-range target.To verify the validity and reliability of the proposed scheme,the comparative analysis on the spatial-time focusing performance between using compensated TMOFO and TMOFO have been made.In addi-tion,a frequency diverse array based on time-window compensated TMOFO for multiple targets in short-range has been proposed.In the proposed scheme,the pulse duration is di-vided into multiple time windows and the spatial-time focusing for a single short-range target is performed at a time window,and thus enabling the spatial-time focusing at multi-ple target locations in short-range.