Research On Widebeam And Broadband Circularly Polarized Antenna For BDS

At present, Chinese BeiDou Satellite Navigation System (BDS) is in rapid constructionand has gradually come into service. Due to polarized antenna is very applicable to satellitecommunication system, the research on circularly polarized antenna has very vitalsignificance. Widebeam antenna with radiation pattern covering the upper hemisphere spacecould receive satellite signals from more directions, so as to improve the precision ofnavigation and positioning. Satellite navigation system often has several working frequencybands, while a broadband antenna can cover these bands simultaneously, so as to reduce thenumber of antennas needed in the system and realize the miniaturization of equipment. Planecircularly polarized antenna and solid quadrifilar helix antenna (QHA) are commonBDS antennas. Input impedance is an important circuit parameter of an antenna andstudy on the input impedance of QHA has important guiding significance to thedesign of this kind of antenna.This paper first summarizes the research status of widebeam and broadbandcircularly polarized antennas. The realization of wide beam is mainly through theintroduction of parasitic unit around the antenna. Besides some antennas itself hasbroadband circularly polarized performance, methods for broadening axial ratiobandwidth mainly include using multimode to produce multiple axis ratio minimumpoints and improvement of the feed network.Then in the second chapter, the relationship between the input impedance ofQHA and its structure parameters is studied. A series of fitting formulas for inputimpedance of QHA are presented and it enriches the theory of this kind of antenna. Inthe third chapter, using the methods of loading QHA with cross dipoles parasitic unitand cage-shaped parasitic unit respectively, two widebeam circularly polarizedantennas for BDS first generation are designed, and their3dB beam can cover thewhole upper hemisphere space, and has the characteristics of high gain at lowelevation angle. Finally in the fourth chapter, a broadband circularly polarized slotantenna is proposed. By introducing T-shaped metal strip and L-shaped slot, twocircularly polarized modes are created and by changing the frequency of the minimum axial ratio points, axial ratio bandwidth of the antenna can be easily controlled.Impedance matching of the antenna within its axial ratio bandwidth is realized byloading the feed line with a metal strip. A broadband circularly polarized antenna forBDS second generation is designed using this presented structure with axial ratiobandwidth of30%, covering all of frequency bands of the BDS second generation andcan be compatible with GPS L1and L2frequencies at the same time. A double cellantenna array is designed followed. The problem of the asymmetry of the radiationpattern at high frequency is overcome and the axial ratio bandwidth is about64.2%.