Study On Electromagnetic Wave Propagation Across Seawater To Air Interface

Data exchange using electromagnetic(EM) wave as the carrier between seawater and air is still a scientific and technical problem to break through. Study on this problem is of great significance for underwater wireless sensor networks and submarine communication. This dissertation studies the characteristics of EM wave propagation across sea–to–air interface, and reaches some conclusions in underwater dipoles’ performance when EM wave is used for data transmission between sea and air. Physical mechanism of EM wave propagation across interface can be explained by a current sheet model built in this dissertation. Theoretical and experimental studies are combined together to investigate the problem. The main contributions are as follows:1. EM fields in air radiated by undersea dipole antennas are studied. The problem of radiation field in air produced by underwater dipole antennas is considered. Fast Fourier transform is used for numerical calculation. The performance of four types of dipole antennas are analyzed and compared, and it presents that horizontal dipoles are superior to vertical ones, and magnetic dipoles are superior to electric ones under the same conditions. Considering environment factors and underwater dipoles’ parameters, the effects of seawater depth, frequency and transmitting antenna depth to EM wave in air are studied. This dissertation systematically studies EM field in air produced by four types of underwater dipoles for the first time. The results show that EM wave can be used for undersea information transmission across the interface in shallow or deep water, and horizontal magnetic dipoles have better performance.2. The current sheet model is built on the basis of current induced on the sea surface. The physical process of EM wave transmission across sea–to–air interface is studied. A current sheet, induced by an underwater dipole antenna, is considered at sea–to–air interface. This current behaves like an antenna located on the surface and creates a wave that propagates in the atmosphere. The wave in the atmosphere is considered as the field that is radiated by underwater dipole antenna. The calculation results of current sheet model agree well with three layered model, and the process is more simpler. Software FEKO is used to study the continuity of field near the interfaces of sea–to–air and sea–to–seabed. The new model can reasonably describe the physical mechanism in EM wave propagation across interface.3. The effects of variation of underwater antenna’s attitude to EM propagation across sea–to–air interface are studied. Marine environment can affect underwater antenna’s attitudes, so the fields in air produced by underwater antennas with inclined and random variation attitudes are considered. The results show that field intensity in air changes due to variations of underwater antenna’s attitudes, especially for random variations. It is concluded that horizontal dipoles, compared with vertical ones, are more suitable to use in practical marine environment. The studies in this part consider the varying ocean environment for EM wave propagation across sea surface. The model is more practical for underwater data transmission to air using EM wave in actual marine environment.4. The experimental studies about EM wave propagation across interface are conducted. The experimental system bout EM wave propagation across sea–to–air interface was exploited. The experiment on sea was conducted. It was presented that long distance of EM wave transmission across sea–to–air interface is mainly depended on the path in air. Moreover, the experimental results are respectively compared with theoretical calculations of previous models. The agreements of the related data demonstrate that the experimental system is reliable, theoretical models are reasonable, and these techniques are feasible. Furthermore, it is presented that EM waves which have transmitted across sea surface can propagate up to several kilometers when they overcome intense attenuation in near-field. Then if radiation power of the transmitting system in this dissertation can increase by 10 d B, distance may increase to tens of kilometers. It shows the possibility of long distance transmission. The studies in this paper provide reliable theoretical and experimental evidences for long distance of data transmission from seawater to air using EM waves.