| Nowadays,the information transmission across sea-air media,especially in a way of electromagnetic wave,is of great significance to build an integrated information network of sea,air and space.Very low frequency(VLF)electromagnetic waves are receiving increasing attention to their low propagation attenuation in seawater and the progress of miniaturized transceiver antenna technology.In this paper,a two-dimensional electromagnetic wave propagation model across seawaterair media is established based on the heterogeneous parallel numerical computing technology of Finite-Difference Time-Domain(FDTD)and Graphic Processing Unit(GPU).The variations of the radiated electromagnetic field by electric and magnetic dipole sources with different depths and tils in the VLF frequency band in different seawater-air environments are analyzed,and the performance of the dipole antenna in the seawater medium are further simulated.The specific research content is as follows:The propagation path of electromagnetic waves from air to seawater and from seawater to air is firstly studied based on the law of refraction,and it is shown that the electromagnetic waves always propagate along paths with relatively small attenuation.In order to further quantitatively study the propagation law of electromagnetic waves across sea and air media,a two-dimensional numerical simulation model of VLF electromagnetic wave propagation across sea-air media is established based on FDTD and Uniaxial Perfectly Matched Layer(UPML)technology.According to the propagation parameters of VLF electromagnetic waves in seawater and air as well as the numerical stability requirements of the FDTD technique,the sizes of the propagation model and the discrete grid are determined,and the optimal combination of the parameters in the boundary loss layer is analyzed and determined.In order to improve the calculating speed,the GPU-based parallel numerical calculation technique is implemented,which is nearly 250 times faster than the CPU-based serial calculation.Based on the established numerical model,the propagation law of VLF electromagnetic wave across sea-air media is simulated and analyzed.Firstly,the electric field attenuation around the sea-air interface region produced by electric dipole and magnetic dipole current sources with different depths underwater,altitudes in the air,and inclination angles is analyzed,respectively,assuming that the conductivity of seawater is uniformly distributed.The results show that the radiation performance of magnetic dipoles is better than that of electric dipoles,horizontal electric or magnetic dipoles are generally better than vertical electric or magnetic dipoles,and the performances of dipole antennas with other tilt angles are lied between those of horizontal and vertical dipole antennas.In addition,when the transmitting and receiving points are located around the sea-air interface region,the attenuation is found to reduce greatly compared to that in infinite seawater.Furthermore,the characteristics of electric wave propagation around the sea-air interface region in the shallow sea area with different sea depths,in which case the submarine land is included in the model,are discussed.The study shows that when the sea depth is greater than 10 m,the excitation source is located 5 m underwater,the influence of the seabed on the wave propagation around the sea-air interface region is almost negligible.Finally,the influence of the seawater with different conductivities along the longitudinal direction on the propagation of radio waves across the sea-air interface region is discussed.In order to further carry out the measurement of electric wave propagation in an indoor pool in the future,the performance parameters of horizontal electric and magnetic dipole antennas of different sizes in seawater are simulated and studied as a function of seawater conductivity.Studies show that the inductance of the antenna changes with the size of the antenna and the conductivity of the seawater,and the matching network needs to be continuously adjusted to obtain the specified resonant frequency. |
PDF Full Text Request