Analysis And Design Of Conformal Antenna Based On Characteristic Mode Theory

Conformal antennas have been widely used in military and civilian applications.In military,for all kinds of aerocraft(such as missiles,aircrafts,etc.),conformal antennas do not affect the aerodynamic performance of the carrier and do not increase the radar cross section area(RCS),which improves the battlefield survivability of the weapon.In the daily life of the society,all kinds of mobile interconnected information devices(such as mobile phones,smart watches,etc.)often need conformal antennas in order to be portable and beautiful in appearance.Therefore,the analysis and design of conformal antennas is not only an important practical application requirement,but also an academic problem to be solved urgently in the field of antenna theory and design.The theory of characteristic modes(TCM)is widely used in solving various electromagnetic problems,and the TCM has two outstanding features that other numerical methods do not have.The first feature is that its modal parameters have clear physical meaning,which helps to understand the antenna working principle and optimize antenna feed.The second feature is that the characteristic modes are the inherent property of the object and are independent of the feed.It makes it unnecessary to consider the feed in the design of the potential antenna structure,thus it has a greater degree of freedom and helps to design a conformal antenna.Focusing on how to apply the TCM to analyze and design the conformal antenna,this dissertation discusses and studies the related problems.The main contents and contributions of the dissertation are as follows:1.Taking the characteristic mode feature of a real antenna as an example,the physical meanings of the characteristic mode parameters are explained.This dissertation firstly introduces classical characteristic mode theory,and the physical meanings of its modal parameters are illustrated by the results of characteristic mode analysis(CMA)of a real conformal antenna.2.A double modal parameter tracking method to characteristic mode analysis is proposedHow to perform mode tracking accurately and efficiently is the prerequisite of the application of the theory of characteristic modes.In view of the problems of low accuracy of single modal parameter tracking methods,this dissertation proposes a double modal parameter tracking method(DMPTM),which improve the accuracy of mode tracking with very little added computational burden.An embedded miniaturized omnidirectional conformal antenna is successfully designed by using the DMPTM,which verifies the reliability and effectiveness of the DMPTM.In addition,in order to verify the correctness of tracking results,the dissertation also explores the method of mode tracking result judgment based on the characteristic current distribution.3.An embedded miniaturized omnidirectional conformal antenna is proposed.The design of omnidirectional conformal antenna is a difficult work in the design of conformal antenna under the precondition that the antenna size is strictly limited.As an application,DMPTM is used to analyze several metal cavities with inner rings and short-circuit pins.According to the mode tracking results,the "combined inverted L ring mode" is recognized to be suitable for the vertical polarized omnidirectional antenna.With this information,a vertical polarized omnidirectional conformal antenna is proposed which can mount on the horizontal position of the carrier platform.4.A method for optimizing the feed design of conformal antennas based on TCM is proposed.In the absence of antenna feed,the CMA can reveal the current distribution of the specified structure.This feature provides a theoretical basis for the optimal design of the conformal antenna feeds.After deeply studying the optimal design of the conformal antenna feeds based on the TCM,this dissertation proposes a method for optimizing the antenna feed.Firstly,the underdetermined equations concerning modal weighting coefficients and the feed vector are established.Then the Li norm minimization solver is used to solve the equations to obtain an initial feed varying with frequencies.In order to obtain an antenna-feed independent of frequency in a specified bandwidth,an iteration method is used which include averaging and thresholding.In order to verify the effectiveness of the proposed method,this method is used to effectively determine the feed locations of multiple patch antennas.5.A synthesis method of conformal antenna structure based on localization of characteristic current distribution is proposed.Among the characteristic current distributions of the platform,there are some special currents whose characteristic current distributions are concentrated in the local area while other areas are weak or absent.Considering this situation,the dissertation proposes a synthesis method for the conformal antenna structure.The synthesis method firstly establishes the method of using modal parameters to quickly evaluate the performance of a conformal antenna,then uses the "0-1" binary codes to correspond to the antenna meshes,finally uses the genetic algorithm(GA)and the direct matrix manipulation(DMM)to find out the best structure satisfying the requirements.In the method for rapidly evaluating the performance of the conformal antenna,the inversion of the self-impedance matrix of the platform is performed just once,which greatly improves the computational efficiency of the whole synthesis method.To verify the effectiveness of the synthesis method,the dissertation uses it to design a miniaturized dual-frequency conformal antenna for a smart wristband.