Study On VLF Antenna Theory In The Ionosphere And ELF Wave Propagation In Layered Media

Over the past 30 years,space-borne antennas have played an important role in contemporary communication technologies with the very low frequency(VLF: 3–30 k Hz)space-borne transmission and propagation experiments conducted by the United States,Russia,and several other countries.This bodes well for the future of submarine communications and potential navigation applications.Due to its long operating wavelength and high penetration capability,the super low frequency(SLF: 30–300 Hz)and extremely low frequency(ELF: 3–30 Hz)electromagnetic waves are also widely used in geological explorations and underwater communications.In this thesis,the theory of an arbitrarily oriented VLF tubular antenna in an anisotropic ionosphere is investigated.The expressions for the integral equation,current distribution as well as the input impedance are derived via both analytical and numerical approaches.Subsequently,an extension to the case where the insulated antenna placed perpendicular to the geomagnetic field is conducted.Beyond that,the quasi-static solutions for the field excited by an ELF vertical magnetic dipole near the surface of a lossy half-space are also discussed.Finally,the locus of the Poynting vector of both the electrical type(TM)and magnetic(TE)type waves in three-layer seafloor media are further evaluated.Based on the above studies,the following conclusions can be drawn from the detailed analyses and simulations:1.Computations show that the current distribution and input impedance under anisotropic conditions are very sensitive to variations of the environment parameters.In general,the input impedance of the VLF space-borne tubular antenna does not vary monotonously,but will decrease with significant fluctuations with the increase of antenna radius or electrical length,and will increase with the geomagnetic inclination angle.Nevertheless,pronounced minima can still be observed at certain points.It is likely to find the optimal antenna parameters in a specific range.In contrast,due to the thin insulation layer,the antenna characteristics and sensitivity to the surroundings are greatly reduced.Since different types of antennas lead to quite different electromagnetic properties,the requirements for the electromagnetic properties of insulated antennas are also different.A suitable antenna model can be obtained depending on the actual requirements in practical applications.By determining the optimal antenna parameters in a variable ionospheric environment,the proposed method may provide heuristic advice to the design of a practical VLF space-borne tubular antenna.2.Comparisons with the numerical solutions show that the amplitudes and phases of the field components are almost the same,and it is possible to keep the results always in agreement with the numerical solutions as the propagation distance gradually increases.Therefore,to ensure the high accuracy of the final results,the propagation distance k?(k: wave number,?:propagation distance)should not be greater than 0.6.As a special case,we simultaneously compare the results with the exact solutions when both the source and receiver are located on the boundary to further verify the reliability of the proposed method.Rigorous analyses of the position of the Poynting vector in the rock layer reveal the distribution and orientation of the electromagnetic wave energy flow.It illustrates that the electromagnetic waves always tend to propagate through a medium with less loss.As the propagation distance increases,the lateral waves can reach a greater skin depth,but the measured electric and magnetic field strengths are also subject to greater attenuation at the receiving point.An effective way to avoid this phenomenon is to determine the required maximum skin depth before deciding the location of the receiving point.Simulations also show that the field components of ELF waves in stratified media will be affected by the operating frequency,the location of the dipole source,and the location of the receiving point.In addition,the locus of the energy flow is closely related to the propagation distance,the thicknesses of the sediment and the seafloor,and the conductivity of the rock layer.In summary,these analyses can provide theoretical guidance for actual seawater/seabed explorations.