The Research Of Multi-band And Broadband Antenna Based On Fractal Structure

Since 1975 the concept of fractal proposed by French mathematician Mandelbrot,developing an important subject today.Its unique space-filing and self-similarity characteristics can be applied to all kinds of fields,including applications to the theory of electromagnetics and antenna design.Simultaneously,the rapid development of multimedia wireless communications make immediate needs of miniaturization,multi-band,broadband and integrated antennas.Through the study of fractal antenna show that fractal geometry combination with antenna design can proposed better antenna size and frequency index.Therefore,fractal antenna technology has become one of the focus research topics of antenna engineering.This thesis reviews the fractal geometry and the cognition,applications of fractal antennas.There are varies of fractal forms in nature,as well as the applications of designed fractal antennas,for instance,log-periodic antenna,helical antenna are obvious fractal antennas.Moreover,each fractal antenna has different characteristics and is widely applied,as it can be used in different application fields for different characteristics.In this paper,the multi-frequency and broadband characteristics of fractal antennas are mainly studied.Firstly,the study on multi-band antenna based on fractal structure,two types of multi-frequency antennas are analyzed and discussed.In addition,a multi-frequency semi-circularized antenna is designed for wireless communication.The antenna can work in four bands at the same time,and the radiation characteristics of the antenna in each band are similar.Secondly,it is the research and design of broadband antenna based on fractal structure.A broadband circular polarization antenna with fractal boundary is designed,and the purpose of broadening the circular polarization bandwidth is realized.Also a UBW antenna is designed with fractal boundary,and the antenna is largely miniaturized and expanded the band.Finally is for dual-band fractal antenna research.On the one hand,a new dual-band broadband antenna based on tree fractal structure is designed,and the two frequencies are realized by using the similar loading branch of tree fractal structure,the antenna can be used simultaneously in GSM band and WLAN band and has broadband characteristics,the relative bandwidth of the low-band GSM band is 55.5 %,and the high-band WLAN(5G band)is 14 %.On the other hand,a new type of Sierpinski fractal dual-band circular polarization antenna is designed,the dual-band circular polarization is realized by the method of cascade,and the complex current path of fractal structure realizes the broadband characteristic of the antenna.The circular polarization bandwidth of the antenna is(3.3-3.5GHz),(6.5-7.5GHz),and the relative bandwidth is 5.8% and 14.2% respectively.