Research On VLF Near-Field And Antenna Theory In An Anisotropic Ionosphere

As the U.S.and Europe began to develop the very low frequency(VLF:3-30 kHz)space-borne transmission and propagation experiments during the past thirty years,space-borne transmitting antennas have been playing a more and more important role in contemporary VLF communication and navigation systems,which are very likely to become an important approach for communication and navigation for underwater objects or those on the sea surface.Taking the VLF space-borne transmission and propagation experiments as research background,this thesis investigated the VLF near-field as well as the antenna theory in a homogeneous and anisotropic ionosphere by combining both analytical and numerical methods.Due to the remarkable anisotropy of the ionosphere at the VLF range,there will exist two modes in the ionosphere simultaneously for VLF wave propagation,i.e.,the ordinary wave(O-wave)and the extraordinary wave(E-wave).When a VLF space-borne antenna is located at the height of a low earth orbit(LEO)satellite,the O-wave excited by the antenna is an evanescent wave with large attenuation factor,while the E-wave is the propagable mode with relatively small attenuation rate.Hence,in previous works,the influence of the O-wave is usually neglected when computing the VLF far-field in the ionosphere.Considering that the current distribution and input impedance of an antenna are largely dependent on its near-field,both the effects of the O-wave and E-wave should be taken into account when studying the VLF near-field and antenna problem in an anisotropic ionosphere.The research achievements of this thesis are summarized as follows:1.Firstly,this thesis proposed a semi-analytical method for calculating the VLF near-field in an anisotropic ionosphere.By applying the complex function theory and a speed-up convergence algorithm,this method transformed the original oscillating integral for the near-field into two fast convergent integrals plus an analytical solution.2.By regarding the arbitrarily oriented electric dipole as a superposition of two dipoles which are parallel or perpendicular to the geomagnetic field,and by adding the factor of the geomagnetic inclination angle,this thesis further investigated the computational approach for the near-field of a VLF electric dipole which is at an arbitrary angle to the geomagnetic field in an anisotropic ionosphere.3.Based on the current distribution formula for a linear antenna in an isotropic medium,this thesis proposed a possible current distribution form on a linear antenna when it is placed in an anisotropic ionosphere,and derived the computational method for the current distribution and input impedance of a VLF space-borne linear antenna.This method can provide theoretical basis for practical space-borne applications by determining the optimal antenna parameters.4.Considering that the angle between a space-borne antenna and the geomagnetic field will change with the satellite moving along the orbit,this thesis further analyzed the effect of the geo-magnetic inclination angle on the current distribution and input impedance of a VLF space-borne linear antenna.The solution for the kernel function of an arbitrarily oriented linear antenna in an anisotropic ionosphere was also given by introducing the fast Hankel transform(FHT).5.Finally,by considering the currents aggregating on the surface of the antenna,this thesis also examined the VLF tubular antenna in an anisotropic ionosphere.And the computational methods for the kernel function,the current distribution,as well as the input impedance of a VLF tubular antenna are derived through the Gauss-Legendre quadrature(GLQ).Computations show that in the near zone the O-wave still has comparable amplitudes with the E-wave and will to some extent affect the total field,while the phase mutations of the O-wave will exactly coincide the minima of its field strength.It is also found that there exists a pronounced"cohesion effect" in the radiation pattern of the near-field for the E-wave,i.e.,the field strength in the direction of the geomagnetic field is evidently larger than that in other propagation directions.For an electric dipole of arbitrary orientation,the dipole perpendicular to the geomagnetic field will obviously contribute more to the synthesized field in the near region than the dipole parallel to the geomagnetic field.In addition,this thesis found that both the current distribution and input impedance of a VLF space-borne linear antenna will change with the antenna parameters such as the electrical length,the operating frequency,etc,and the antenna has a smallest input impedance when it is oriented along the geomagnetic field.It is seen that for a VLF linear antenna in an anisotropic ionosphere,the current proportion corresponding to the O-wave is far greater than that corresponding to the E-wave,while the coefficients for the O-wave and E-wave in the current distribution of a VLF tubular antenna are similar to each other in magnitude.Moreover,it is found that the input impedance of a VLF tubular antenna will decrease with the radius of the antenna.In practical engineering applications,the orientation of a VLF space-borne linear antenna should be as parallel as possible to the direction of the geomagnetic field in order to achieve maximum radiation efficiency.And the overall design of a space-borne antenna should be further optimized by determining the best antenna parameters through theoretical analyses.