Calculating HF Electromagnetic Wave Transmission Characteristics With FDTD Method In Complex Environment

The transmission characteristics of the HF electromagnetic wave are affected by the complex propagation environments, such as the earth's curvature effects, above which exists an inhomogeneous atmosphere, the inhomogeneous profile of the parameters of the earth's surface, the rough sea surface and the land-sea mixed paths when the wave travels over the sea. The change of the transmission characteristics will bring problems, such as the instable signal and the discontinuous signal in communicating at long bowls and the HF radar. So studying the speciality of wave concretely and practically has a very significant meaning. The traditional analytic method to calculate the field strength has difficulty in dealing with the problems of propagating through multi-path and mixed path, and the conventional numerical method always bases on 2-D method instead of 3-D method. These methods will bring errors.In this thesis,3-D moving window finite difference time domain (MWFDTD) method is used to calculate the transmission characteristics of the HF electromagnetic wave instead of the conventional 2-D method. This method can reduced the errors which are brought by traditional methods. In order to obtain the information of the wide-band signal just by calculating once, The excitation has a pulse character in time. Because the 3-D method has the shortcomings of more memory space and low computation speed, we use the method of compensating the numerical dispersion to enlarge the size of the grid, and this method can expedite the computation greatly. The surface impedance boundary condition is used to reduce the memory space when the wave propagates over lossy medium.In this paper, the method above is used to calculate the transmission characteristics of the HF electromagnetic wave when the wave meets the islands on its trip over the sea. Firstly, how the island's size, stuff and shape affect the transmission characteristics of the HF electromagnetic wave on calm sea is studied. Then the transmission characteristics of the HF electromagnetic wave is calculated when the wave meets the islands and also when it leaves the islands several kilometers. The second scatterance will occur when the wave meets the island. The scatter wave brings the HF electromagnetic wave additional loss, and the additional loss is affected by the size, the stuff and the shape of the island. The additional loss will reduce when the wave leaves the island, and the additional loss will be approaching to zero when the wave is far from the island. The results of the modeling consistent with the theory. This proved the efficiency of the method this thesis gived.