| In recent years,for the potential applications in wireless communication,radar detection,imaging and so on,the vortical electromagnetic wave carrying orbital angular momentum has been paid more and more attention by researchers.As a basic property of electromagnetic wave,orbital angular momentum was not first used in the optical band until the end of the 20 th century,while the related research of orbital angular momentum electromagnetic wave in the microwave band is in the stage of vigorous development.In fact,orbital angular momentum multiplexing can greatly broaden the bandwidth of wireless communication and improve the spectral efficiency.In addition,breakthrough in the azimuth resolution can be achieved by using the orbital angular momentum carrying electromagnetic wave for radar imaging.Therefore,it is of great significance to study the generation,propagation and scattering characteristics of vortical electromagnetic wave for the development of wireless communication and other related fields.Based on the basic theory of vortical electromagnetic wave and spectrum analysis theory,the generation method of vortical electromagnetic wave,the radiation characteristics such as the purity of phase distribution,the property of amplitude distribution and scattering characteristics when irradiating simple targets are discussed by means of simulation and experimental test.Following aspects are mainly included in this paper:Firstly,study on the generation of vortical electromagnetic wave.The principle and method of helical antenna used to generate vortical electromagnetic wave are introduced systematically,including spiral antenna,cylindrical helical antenna and conical helical antenna.Based on this,a novel multi-arm helical orbital angular momentum antenna with simple and compact structure,high modal purity and easy control is proposed.Secondly,study on the propagation characteristics of vortical electromagnetic wave.Based on the phase gradient method of vortex detection,a variance criterion for the modal purity of vortical electromagnetic wave is proposed,and the influencing factors of the modal purity of vortical electromagnetic wave are analyzed by using the criterion.Besides,the influencing factors and control methods for the spatial extent of the null region of the radiated beam are explored for two kinds of vortex wave generation methods: uniform circular array and helical antenna.Thirdly,Study on the scattering characteristics of vortical electromagnetic wave.On the premise of establishing the scattering model of the vortex wave,the case of the simple targets irradiated by the vortex wave are analyzed theoretically.Then based on the spectrum analysis theory,referenced to the concept of radar cross section,the parameter "echo dominant-mode ratio" is defined,which is used to judge the ability of the scattering target to intercept a certain mode of vortical electromagnetic wave.The rationality and stability of this parameter are demonstrated simultaneously.The echo dominant-mode ratio of simple targets such as spheres,cylinders and dihedral angles of different sizes and complex targets composed of simple targets are analyzed,and some simple rules of vortex wave scattering are obtained through the analysis. |
