Research On Object Scattering Characteristics Of Vortex Electromagnetic Wave

Vortex electromagnetic wave is popularized from vortex beam.It has a broad application prospect in the field of communication such as information modulation and channel expansion due to carrying orbital angular momentum(OAM).Compared with the traditional plane wave in radar imaging,object detection and recognition,it has strong advantages.Therefore,it is particularly necessary to study the object scattering characteristics of vortex electromagnetic waves.The object characteristics of OAM wave in optical frequency band mainly focus on the analytical solution of standard objects.However,the OAM object scattering in the electromagnetic wave band is mainly acquired by experiment,and the far field RCS(Radar Cross Section)is obtained by the physical optics(PO)integration of the simple body surface.It is difficult to further reveal the complex object scattering and its vector characteristics.In this thesis,the object scattering mechanism of high order Bessel vortex electromagnetic waves is studied.The plane wave angular spectrum expansion method is used to characterize the Bessel beam.Combined with the high frequency approximate PO method,the scattering field is obtained by PO integration of the decomposed plane wave on a facet element of object partition.The PO solution of all the decomposed plane waves is integrated and summed to obtain the Bessel beam scattering field of a certain facet element.The vector superposition of the Bessel beam scattered field of all facet elements is used to obtain the object characteristics of the Bessel beam.The details are as follows:1.Several typical vortex beams,such as the high-order Bessel beam,high-order Laguerre-Gaussian beam and Airy vortex beam,are derived by the special solution of the wave equation.Based on the original analytic expression,the transverse amplitude and phase distributions perpendicular to the transmission direction in Cartesian coordinate system are given respectively.The Gaussian beam and Bessel beam are expanded by vector angular spectrum,and the vector propagation characteristics of Bessel beam after expansion are revealed.The influence of the approximate order of different phase factors on the intensity and phase distribution of Bessel vortex beam at different positions is studied.2.According to the Fresnel reflection law,the interaction mechanism between Bessel vortex beam and the interface of infinite conductor and medium is demonstrated.The reflected field of vortex wave under the action of boundary surface is characterized based on angular spectrum expansion.The incident field and reflected field are characterized as the superposition of vertical and parallel polarization components.The reflected field is obtained by numerical integration of the product of Fresnel reflection coefficient and reflected wave angular spectrum.Based on the case of oblique incidence interface,the distribution characteristics of reflected field under the different irradiation distance and the different reflected propagation distance are studied in depth.3.The plane element subdivision of flat plate and electrically large sphere object are carried out based on 3D software.The Bessel beam scattering field on each illuminated facet element is obtained by PO integration,and the vortex scattered field of the whole object is obtained by vector summation scattering of all facet elements.The numerical results are verified by FEKO simulation.The results show that the scattered field still has significant orbital angular momentum characteristics when the linearly/circularly polarized incident Bessel beam is vertically incident on the conductor/medium plate and when the axial incident conductor/medium coated sphere.The circular polarization purity of the scattered vortex wave decreases at oblique incidence.4.A near-field measurement system is built to measure the scattering of simple objects irradiated by Bessel vortex waves.The fabricated holographic metasurface antenna generates quasi Bessel vortex electromagnetic wave,and the conductor/medium plate and the conductor sphere are irradiated by the antenna far field.The "system" composed of the antenna and the object can obtain the near field scattered data through the plane near field scanning.The far-field RCS is obtained by converting the near-field scattered data to the far field.The scattered results of PO numerical calculation are compared by the measurement and the relative error analysis is conducted,which verifies the correctness of the proposed PO scattering algorithm of Bessel vortex wave.5.The reliability and scientific rationality of this method are demonstrated by the simple object scattering calculation,FEKO simulation and experimental measurement.The scattering characteristics of projectile and aircraft complex objects by Bessel beam are calculated,and the scattering characteristics of Bessel vortex wave to common convex object is obtained.The PO numerical method discussed in this thesis has certain practical application value and research significance,which provides an important theoretical basis and scientific basis for the application of vortex electromagnetic waves in object radar detection and identification.