| As an important part of the modern electromagnetic research field， the optimaldesign in electromagnetic research is being paid more and more attention by researchers.The cost functions in the optimization of electromagnetic problems are always highlynonlinear, nondifferentiable and have multiple peaks, which bring many challenges tothe classical optimization methods in solving the practical problems. As a simple andefficient global search algorithm, differential evolution algorithm (DE) is very suitablefor dealing with the complex optimization problems in the electromagnetic field. Evenso, because of the complexity in the practical electromagnetic applications, thecomputation of the object functions needs noticeable amount of time. How to acceleratethe convergence speed of the algorithm and reduce the total time consumption of theoptimization process is a key problem requiring to be solved at present. As the greatachievements in microwave integrated circuit and the growing demands for the spacetechnology, the conformal array antennas which fully conform to the surfaces of thecarriers have attracted more and more attentions. Since the structure of a conformalarray makes it difficult to synthesize the pattern of the array, the traditional synthesismethods and the local optimization techniques are not applicable anymore, but the highefficient DE algorithm becomes a very effective method for the synthesis of conformalarrays. The development of Wireless Local Area Networks (WLAN) and WorldwideInteroperability for Microwave Access (WiMAX) makes the antennas which satisfyseveral kinds of communication standards with high performance and compact sizegreatly demanded. The trial-and-error procedure in the traditional antenna designprocess is very dependent on the enumeration of possible parameter combinations bythe experience of designers, which makes the design process very complex and timeconsuming. As previously unimaginable computational resources become commonplace,the optimization of antenna design by the combination of DE and computationalelectromagnetism method is imminently desired by the antenna designers.Based on the DE algorithm, this dissertation is mainly concerned with theimprovements of DE and its applications in synthesis of conformal arrays and optimaldesign of multiband antennas. The author’s major contributions can be summarized asfollows:1. The study of DE algorithm. A modified DE (MDE) algorithm is proposed by introducing the strategies of random best mutation and random local search in thestandard DE algorithm. The proposed algorithm can make a relative balance betweenthe convergence speed and the diversity of the population as well as enhance the localsearch ability, and then improves the overall performance. After that, seven standardbenchmark functions are optimized and a linear array with wide null and low sidelobelevel pattern is synthesized, the optimization results are compared with several otherevolution algorithms, which demonstrate the superiority of the proposed MDEalgorithm.2. The study of optimal design and synthesis of conformal arrays. Firstly, thepolarization problem in conformal arrays is analyzed and studied, and the selectionmethod of active region is given. Secondly, a cylindrical conformal array with8×12elements is designed, and its patterns with low sidelobe levels when the main beamscans to three different directions are synthesized by the MDE. During this process, theapplication of active-element-pattern can take the influence of mutual coupling betweenthe elements and carrier to the array pattern into consideration. The modified Bernsteinpolynomial is employed to obtain the excitation amplitudes of the cylindrical conformalarray, which can overcome the drawbacks of high oscillation of amplitude weightsacross the phased array and significantly reduce the optimization variables in thesynthesis process. Finally, a hemispherically covered sphere conformal array is designed.The dual polarized array elements are adjusted to improve the polarization of the arrayafter the selection of the active region, and then the proposed MDE is applied tosynthesize the array patterns for low sidelobe levels, which can scan over a wide scope.3. The study of multi-objective DE and array synthesis. Firstly, an improvedmulti-objective differential evolution algorithm (IGDE) is proposed by introducingnovel mutation strategies and individual preservation mechanism in the GeneralizedDifferential Evolution (GDE3). The optimal results of five standard benchmarkfunctions demonstrate that the obtained Pareto solutions by the proposed algorithm arebetter than the original algorithm both in convergence and diversity. Then, the proposedIGDE is applied in two different synthesis problems, which effectively overcome thedefects in single objective evolution algorithm, the Pareto front of the beamwidths andsidelobe levels are obtained, and all the possible optimal results satisfied the constraintsare optimized at once.4. The study of optimal design in planar printed multiband antennas. Threeprinted multiband antennas with different work mechanisms are studied and designedby the trial-and-error procedure. Firstly, by extending strips from the rectangular ring, a rectangular ring monopole antenna with multiple resonation strips is designed, thisdesign is simple and easy to tune. Secondly, a wide slot is excited by a fork-shaped strip,then a multiband antenna for WLAN/WiMAX applications is designed withoutchanging the shape of slot or introducing coupling strips. Thirdly, a compact multibandantenna is designed by introducing several notches in a wide frequency band, the noveletched slots and the asymmetric coplanar strip (ACS) fed structure used in this designcan reduce the overall antenna size to only one half of the original design. Finally, theMDE and the commercial electromagnetic simulation software are combined to design adual band printed antenna for WLAN/WiMAX applications. |
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