| With the human being to explore the Earth’s outer atmosphere,the advantages of near-space are gradually being discovered.At the same time,various types of hypersonic vehicles have been developed.During such aircraft fly at high speeds in near space,the huge kinetic energy is converted into heat energy,air around the aircraft ionized,and forming a plasma flow field which contains free electrons,ions and uncharged neutral particles covering around the vehicle.In addition,the thermal protection material on the surface of the aircraft also abates and produce ablation particles due to high temperature.These ablated particles will be mixed together with the plasma flow field to form plasma sheath.Plasma sheath are complex dusty plasmas,which will cause the transmission signal attenuation,distortion,and even causes ’communication blackout’.How to overcome the ’communication blackout’ has been studied most extensively in recent years,containing the propagation,scattering and plasma parameter diagnosis of electromagnetic(EM)waves in plasmas.The essential reason for the "communication blackout" is that the existence of the plasma sheath,which the emission,propagation,and reception of radio signals.This dissertation based on the propagation theory of EM waves in plasma,on the one hand,we investigate the effects of dust plasma and moving plasma on the polarization properties of EM waves and the polarization mismatch loss.On the other hand,according to the influence of the plasma on the characteristic parameters of the microband antenna and combined with the Bayesian regularization neural network,the inhomogeneous plasma electron density was obtained.Main contents and conclusions of this dissertation are as follows:1.Polarization properties of EM waves in the magnetized weakly ionized dusty plasma are studied.Based on the theory of interaction between the dust particles and the plasmas,and combined with dynamic theory,the permittivity of weakly ionized dusty plasma with an external magnetic field is obtained.The transmission model of the EM waves obliquely incident on weakly ionized dusty plasma is established,and the transmission coefficients for different polarization states of the incident waves are obtained by using the equivalent impedance method.At the same time,combined with the polarization theory of EM waves,the axial ratio,the phase difference and the mismatch loss are analyzed.Analysis indicates that EM waves with different polarization states have different attenuation.The external magnetic field aggravated the depolarization and caused more polarization mismatch loss;The polarization mismatch loss can be alleviated by regulating and controlling the radii and density of dust particles in plasma sheath.2.Polarization properties of EM waves in the moving plasma are studied.Based on the Lorentz transformation and phase invariance principle,the tranmission line method in moving plasma is derivated.Then the relationship between the axis ratio,phase difference and polarization mismatch loss and normalized velocity of moving plasma with different directions is obtained.Results indicates that the polarization mismatch loss and the axial ratio have the similar change trends.They are inversely proportional to the electron density,collision frequency and the incidence angle,and proportional to the incident wave frequency.The effects of the moving plasma on the polarization characteristics of the EM waves are closely related to whether the plasma is close to or away from the incident wave.The moving speed can be adjusted according to the change of the incidence angle of the EM waves to reduce the polarization mismatch loss.3.The non-uniforn plasma electron density is inverted based on Bayesian regularized neural network.The interaction model of microband antenna and cylindrical plasma is established by CST,and then the electric field distribution under different electron density is obtained.Using Bayesian regularization neural network and the cubic spline algorithm,the cylindrical plasma radial two-dimensional electron density distribution is obtained based on electron field intensity at fewer spatial position.Results indicate that the electron density distribution of the cylindrical plasma can be obtained through the spatial electric field distribution at fewer spatial position.The relative error is related to the change range of the electron density.The larger the change span,the greater the relative error.4.Based on the performance parameters of microstrip antennas and Bayesian regularized neural networks,the axial electron density of plasma is inverted.The complex nonlinear relationship between the electron density and the microband antenna performance parameters is fitted by the Bayesian regularization neural network.The inversion for uniform electron density is realized according to the voltage-standing wave ratio(VSWR)and the resonant point.In addition,a database of the antenna VSWR and S11 parameter under different electron density is established,and the electron density distribution is estimated according to the S11 parameter and VSWR at different positions of the cylindrical plasma axis.Results indicate that high accuracy electron density can be obtained according to VSWR and the antenna resonance point.By Establishing a database of different electron density and their corresponding antenna characteristic parameters,the distribution of electron density along the plasma axial direction can be obtained by comparing the antenna characteristic parameter data at different positions along the axial direction with the database data. |
