Near-field Scattering Of VLF Waves In The Anisotropic Ionosphere And Radiation Field In The Layered Medium

The theoretical analysis of electromagnetic(EM)wave propagating and scattering in the anisotropic plasma and multi-layered media has been widely used in remote sensing,wireless communication,aerospace,and other fields.With the increasing importance of the very low frequency(VLF)wave in the communication system,it is likely to become an essential means for ionospheric and deep space exploration in the future.This paper mainly investigates the near-field scattering of a finite-length cylinder by VLF waves in the anisotropic ionosphere and the radiation field in the multilayered media.Since the ionosphere exhibits anisotropy in the VLF band,the VLF wave will split into ordinary waves(O-waves)and extra-ordinary waves(E-waves).Due to the different attenuation rates of the two modes,the scattering mechanism are different.In the near zone,O-and E-waves exist simultaneously.In the far-field,only E-waves exist due to the slow attenuation.In fact,the scatterer can be regarded as a secondary antenna that is excited by the incident wave,and its radiation field is the scattering field.It is noted that the near-field determines the current distribution on the antenna,so the O-and E-waves should be both considered in the near-field scattering.In addition to the detection of targets in the ionosphere,the lunar exploration has also been an attention by scientists in recent years.Moon detection can generally be idealized as the radiation field excited by a dipole in the multi-layered media.The main research work and results are summarized as follows:1.The model of scattering by a finite-length conducting cylinder under the oblique incidence for VLF waves in an anisotropic plasma is established.Analytical expressions of the near-field scattering and the surface current distribution coefficients are derived by the integral equation method and method of moment(Mo M),respectively.Furthermore,the radar cross-section(RCS)is analyzed in detail.Numerical computations are used to analyze the influence of the size of the cylinder and the incident angle on the current distribution,RCS,and scattering characteristics,respectively.Finally,to verify the feasibility of the proposed method in this thesis,the expressions are reduced to an infinite-length cylinder in an isotropic medium and compared with the results obtained by the direct method,which shows in good agreement.2.Considering the cross-polarization caused by the oblique incidence,the scattering matrix is used to calcalate the scattering coefficient and coupling scattering coefficient so that the proportion of the coupling scattering field in the total field can be analyzed.3.Since the boundary conditions of the dielectric and conducting cylinder are different,the surface equivalent theory is presented.Furthermore,the integral equation method and Mo M are used to solve the near-field scattering of VLF waves obliquely incident on a finite-length dielectric cylinder.Taking as an example,the influence of the size of the cylinder and the incident angle on the surface current distribution is analyzed in detail.In addition,the differences between the RCS of the dielectric cylinder and conducting cylinder are analyzed.The conclusion that the dielectric coating on the conductor can reduce the RCS is verified.Finally,to verify the feasibility of the method,the expressions are reduced to an infinite-length cylinder in an isotropic medium and compared with the results obtained by the direct method,which shows in good agreement.4.The radiation field excited by the vertical magnetic dipole(VMD)inside the moon is studied.The multiplication iteration method and the asymptotic method are proposed to solve the problem that the series expressions converge slowly when the source off the surface of a large-size sphere.Meanwhile,analytical expressions of the radiation field inside the sphere are obtained in which rays can describe the propagation path.Further,the influence of propagation distance,propagation angle,operating frequency,and height of the source on the EM field is discussed.Finally,relations between EM field of the planar-,transition-,and the spherical region are analyzed.5.The radiation field generated by a dipole in the planar multi-layered media is investigated.The pole equations are obtained more accurately through the piecewise root finding method.Secondly,the asymptotic method is used to obtain the radiation field in the spherical coordinate system.Based on the numerical calculation,the propagation characteristics of the total field,direct wave,reflect wave,lateral wave(DRL),and the trapped surface wave excited by the dipole in the four-layered medium are analyzed.In addition,contributions of the trapped surface wave to the total field are investigated.