Researches On The Influence Of Plasma Parameters On The Radiation Characteristics Of Dipole Antennas

As a special form of electrically small antennas,the dipole antenna is characterized by the low radiation resistance and high reactance,leading to low radiation efficiency.The loading of metamaterial is capable to enhance the radiation of electrically small antennas effectively.The plasma can be seen as a kind of artificial dielectric media.And when the plasma frequency is higher than the radiation electromagnetic wave frequency,the plasma is of negative permittivity.Compared with the negative permittivity medium obtained by structure design,the parameters of plasma system is easier to control,thus facilitating the modulation of dipole antenna radiation characteristics.In this thesis,in order to reveal the improvement of the radiation efficiency of dipole antennas modulated by such plasmas,a variety of mathematical tools are used to establish the radiation model of dipole antenna covered by plasma layer.And the variations of the relative radiated power with plasma parameters are investigated by the numerical simulation,the equivalent circuit analysis and experimental comparison,respectively.The content mainly includes four aspects as follow:Based on the radiation properties of dipole antennas,the characteristics of near-field and far-field,as well as the directional properties of the antenna are derived.And the constitutive parameters of the plasma are derived by applying the Drude model.Furthermore,the radiation model of the dipole antenna covered by plasma is established,and the model structure,control equations,boundary conditions and the numerical calculation method are discussed.In order to explore the effect of plasma parameters on the radiation characteristics,the variations of relative radiation power with the plasma shape,collision frequency and density distribution are simulated,and the mechanism of radiation enhancement is analyzed from the aspects of near-field and far-field characteristics.The results have demonstrated that the plasma layer with less thickness near the feed,less collision and decreased radial distribution is beneficial for the antenna radiation intensification.To acquire a simple method to evaluate the effect of plasma parameters on the antenna radiation,an equivalent circuit model of the dipole antenna loaded by plasma layer is established.The circuit element parameters are given qualitatively.Then,the influences of the plasma density,collision frequency and thickness on the antenna radiation are studied from the point of view of inductance capacitance resonance.However,the circuit model needs further improvement,since it cannot reflect the distribution characteristics of the plasma layer shape and density.Based on the achievements in principle understanding,the effective parameters of plasma layer applied to enhance the dipole antenna radiation are obtained.And the simulating results are consistent with the experimental measurements qualitatively,which demonstrates the feasibility of the numerical model.However,because of the complexity of the experimental conditions,there exit some quantitative differences between the simulation results and the experimental measurements.In addition,the 2D numerical model is upgraded to 3D model,and the effect of plasma density on the antenna radiation is investigated based on 3D model.