VLF Antenna/Antenna Array Theory Electromagnetic Environment Modeling In The Ionosphere

Very Low Frequency(VLF:3-30 kHz),which has high penetrating ability and long propagation distance,has a particularly important role in national defense and military fields.However,its long wavelength leads to low radiation efficiency and high construction costs,while the ionosphere has a large refractive index and can significantly reduce the wavelength of electromagnetic waves.Therefore,space-borne antennas have been used in contemporary communication technologies with the VLF transmission.With this background,this paper investigates the theoretical analysis and calculations of kinds of satellite-borne VLF antennas.1.Using the equivalent network theory,the current distribution,kernel,as well as the current integral equation of the multi-feed antenna are all derived.The approximate current distribution is then further calibrated with the method of moments to obtain the final current distribution and input impedance.Computations show that,compared to single-feed antenna,multi-feed antenna can provide more uniformly distributed current moment and is therefore more advantageous in improving the antenna radiation efficiency.Also,it is found that the node input impedance of a multi-feed antenna can be optimized by setting a larger feed voltage in the middle of the antenna,or by deploying the feed locations along the antenna more averagely.2.In order to study the mutual coupling effect of linear antenna arrays,the N simultaneous integral equations are established and solved so as to determine the current distribution on each element,and the evaluation of mutual impedance is realized through the induced electromotive force method.The driving voltage applied on the center of each element is set in the way that the current distributions maintained on the elements have identical amplitudes but a progressive phase difference.By revealing the variation trend of the mutual impedance with different parameters,this approach may provide theoretical support to the prediction of mutual impedance in practical engineering applications and may help determine the optimal array parameters that lead to less mutual coupling.3.By considering the V-type antenna as the superposition of the z-oriented dipole and the arbitrarily oriented dipole,this thesis further investigated the current integral equation,the current distribution,as well as the input impedance of the V-type antenna immersed in the ionosphere.The influences of the electrical length and the radius of the antenna on the current distribution and impedance characteristics are analyzed by numerical calculations.The study shows that of the current distribution of two arms of antenna are different.The input impedance increases sharply with the increase of the angle of the antenna and is very sensitive to variations of the environment parameters.4.In this thesis,the radiation field of the near-field and far-field of a VLF linear antenna oriented to the geomagnetic field in anisotropic cold plasma is investigated for the first time.The kernels for both the exordinary wave and ordinary wave are derived by calculating the field due to an electric dipole,and the current distribution of the antenna is determined.By reasonable approximation,the radiation field of the VLF linear antenna can be expressed as a product of the field excited by a dipole and the correction factor.Computations show that the radiation patterns for the exordinary wave and ordinary wave are disparate,but they will affect the total field in the near zone collectively.It is also found that the magnitude of the correction factor will increase with the antenna length as a whole,but there also exist minima at some particular points.Under far-zone conditions,the energy of the radiated field basically comes from the E-wave,but unlike isotropic medium,the radiation pattern in the anisotropic plasma always concentrates on the direction of the geomagnetic field,with the shape of the pattern not influenced by the antenna length any more.5.To facilitate the analysis of the effects of the ionospheric electrostatic field,this thesis model the global distribution of the Earth’s external electrostatic field base on calibration of the electric field of the DEMETER satellite.The formulas of ionospheric electrostatic field are given reasonably.The distribution models of three components of electric field are established respectively.The coefficients with the best optimization are listed and the figures of the distribution of the electric field which are in good agreement with the satellite original data are also plotted.