| Shortwave Radio System is widely used in the fields of electromagnetic environment monitor, sea-state remote sensing, broadcasting communication, radar detecting and so on for its long detection distance, strong invulnerability,relay omittability and other advantages. However, antennas, who play the role as an important transformation component of electromagnetic power in shortwave radio systems, usually get the disadvantage of large size, which makes it uneasy for erection, transportation and hiding; or they get so low gain and efficiency that could not meet the increasingly improved need of the communication, military and commercial use when they are small enough. Most of the research in our country in this field still stays on theory stage, the results need to be completed, so its urgent to develop research on miniaturization of shortwave antennas.This paper mainly discussed the performance indexes and analysis methods of electrically small antennas; the miniaturization technology of antennas is researched and simulated; a method for miniaturization of shortwave antennas is proposed, antennas who used this kind of method may get the advantage of small size, simple structure, high efficiency and so on.In this dissertation, methods of miniaturization of short wave antenna are researched systematically based on the basic principles of antennas. Firstly, the research on miniaturization of shortwave antennas in the world is analysised in this paper, the features and dificiency of the research institutions are points out, which provided the focus point of this dissertation. Based on the detail discuss on the performance indexes of the electrically small antennas, efficiency, gain, working frequency band and directivity are ensured as the most important performace indexes of the miniaturized shortwave antennas in this paper. At the same time, the common electrically small antennas and theoratical methods that are usually used to analysis them are introduced. The miniaturization effects of the miniature methods such as top loading and dielectric loading are simulated by the commercial software Microwave Studio(?) of CST Corporation, the disadvantages and advantages, the application scope of these methods are analysised. A miniaturized shortwave antenna which works at the band of 3~15MHz, based on dipole antenna, with top-lumped components compound loading structure is proposed based on the comparasion of the performaces of the monopole and dipole antenna. Then three methods to improve the working efficiency of the miniaturized shortwave antenna are proposed, including the umbelliform prolong of the loading metal bar method, the dielectric loading method and the method of using helical structure to replace the metal bar. The helical type antenna, with its 75(?) feeding impedance is determined as the final method of miniaturization of shortwave antennas lastly based on the comprehensive analysis of the advantages and disadvantages of every method and their applicable situation. The antenna, whose structure is combined multi-heical structure, has got a size scope of 2m×2m×2m,its biggest size is only 1/50 of its lower frequency wavelength, and 1/10 of its higher frequency wavelength. The total efficiency, gain of the antenna are both above -20dB in the whole 3~15MHz band. This antenna has got the advantages of small size, simple structure and high efficiency and so on, it is applicable in the fields of Broadcasting, Environment monitoring, Radar detecting and so on. |
