Research On Design Of Reconfigurable Antenna And Filtering Antenna Synthesis Based On Characteristic Mode Theory

In the field of antenna design,the characteristic mode analysis can be performed to the radiation structure of the antenna before the feeding location is specified.Then,an appropriate feedpoint is determined so that the desired modes can be excited for best performances of the antenna.The characteristic mode analysis method can guide the antenna design based on the physical insignt they bring into the radiating phenomena taking place in the antenna and thus this method has received widespread attentions.In this thesis,the reconfigurable antenna and the filtering antenna shape synthesis have been studied based on the characteristic mode theory.This dissertation presents a pattern-reconfigurable antenna based on the theory of characteristic mode for body area networks communication.The radiation pattern of the antenna is able to switch between the omnidirectional and broadside directional patterns,which can satisfy the on-body and off-body communication links,simultaneously.Firstly,for the needs of the miniaturization,an array of 3×2 rectangular plates with the off-body communication mode at the WIFI band is used as the main radiator.It is worth mentioning that the proposed radiation patch can effectively reduce the back-lobe of the off-body mode.Then four copper posts between the radiator and the ground are introduced to generate the on-body mode.Finally,the mode excitation method is used to selectively excite two modes to achieve the pattern-reconfigurable antenna.The footprint of the proposed antenna is0.44?₀×0.44?₀×0.05?₀.For demonstration,the proposed antenna has been fabricated and measured,and the measured results agree well with the simulated ones.The proposed antenna has a simple topology,pattern diversity features and satisfy the different requirements of the on-body and off-body communication links,which make it a good candidate for wearable application.In this dissertation,a method to design a pattern-reconfigurable two-port multiple-input multiple-output(MIMO)antenna system is first proposed by resorting to the orthogonality of the characteristic mode and symmetrical excitation method.Firstly,the characteristic mode analysis is applied to a rectangular metal patch at 2.45GHz,and three desirable characteristic modes i.e.,mode 1,mode 6,and mode 8,are selected.Then four capacitively coupling elements(CCEs)are located around the radiation structure to selectively excite the desired modes.The CCE1 and the CCE2 as a pair of the excitation structures are used to selectively excite the modes 1 and 8,while the CCE3 and the CCE4 are used as the other pair of the excitation structures to selectively excite the modes 1 and 6.The selective excitation is achieved by giving each pair of the CCEs the excitation signals with the same magnitude and phase,or the same magnitude and a phase difference of 180°.Finally,two feeding networks related to two ports have been designed for providing the desirable feeding signals to the CCEs.Compared with traditional design methods,the pattern-reconfigurable2MIMO antenna system with a simple structure in this thesis is realized by using only one radiating structure,and the antenna system has a good isolation in each state due to the orthogonality of radiation patterns between the characteristic modes.The symmetrical excitation method is used to realize the pattern-reconfigurable characteristics of each port,and then three operating states can be obtained for the proposed antenna design,which is suitable for the dynamic communication system.This dissertation has developed a polarization-reconfigurable antenna based on the theory of characteristic mode(CM),which can electronically alter its polarization states between linear polarization(LP),left-hand circular polarization(LHCP),and right-hand circular polarization(RHCP).Firstly,the influence of the modal significance(MS)and the characteristic angle on the polarization of the antenna are discussed.Based on the discussion,a square patch with an I-shaped slot is proposed as the main radiator,which has two orthogonal CMs with the same MS and 90°characteristic angle difference at 2.4 GHz.Then,a simple feeding network is designed to excite the different combinations of the two modes.By properly choosing the different states of the excitations,the LP,LHCP,and RHCP waves can be generated.Finally,a 1×4 antenna array is designed for the demonstration of the array application.The experiments are carried out to validate the proposed design.The proposed antenna has a simpler structure and the smaller size with the good radiation performance,making it more suitable for antenna array applications.In this dissertation,a filtering antenna shape synthesis method is proposed based on the characteristic mode theory.The shape of the radiation structure is automatically determined by the optimization algorithm through minimizing the objective function.The objective function is established using model parameters of the radiation structure.The location of an appropriate feedpoint is selected after shape optimization.Firstly,a multi-objective optimization algorithm,i.e.,NSGA-II,is used to develop an antenna shape synthesis method based on the characteristic mode theory,and the corresponding relationship between the filtering antenna performance and the mode parameters is proposed.Then,in order to achieve radiation null,the characteristic electric field and the characteristic electric current are,respectively,proposed as the objective functions.Finally,based on the proposed optimization method,the filtering antennas are developed.Two radiation nulls are realized at both band edges of the antenna realized gain curve,leading to good filtering radiation performances,which verifies the accuracy and the efficiency of the proposed filtering antenna shape synthesis method.