Research On Broadband And Wide Beam Dual/Circularly Polarized Antennas

With the development of wireless communication system,the higher performance for antennas is proposed in wireless communication system.For example,the antennas used in the satellite communication system are expected to have the characteristic of wide beam and the characteristic of high gain at low elevation angle.However,the operating bandwidth of wide beam antennas can’t meet the requirements of specific applications in the published literature at present.And when the wide beam antennas are installed on a large metal platform,the wide beam radiation pattern of the antenna will be distorted due to the coupling effect of the metal installation platform.Therefore,how to make the antennas achieve wide beam coverage in broadband,and how to decrease the impact of metal platforms on wide beam antennas to be compatible with installation platforms are urgent problems to be solved in engineering applications at present.The problems that we mentioned are studied in this thesis.Firstly,the background and significance of the topic selection are discussed,then the basic theory related to this thesis is given,and then the research work of this thesis is described.The research contents of this thesis are summarized as follows:1.Design of the compact and wide beam dual-polarized antennaTo solve the problem of narrow bandwidth for wide beam coverage of wide beam dual-polarized antenna,a compact and wide beam dual-polarized antenna is studied.Firstly,orthogonal trapezoidal dipoles are adopted as the main radiation structure to realize the dual-polarized of the antenna.Secondly,the impedance matching of the antenna at high frequency band is improved by adding parasitic elements.The impedance matching of the antenna at low frequency band is improved by the special-shaped back cavity structure.And the beam width of the antenna in the whole operating frequency band is also widened by the special-shaped back cavity structure.The results show that the relative impedance bandwidth of the antenna is 46.4%(|S₁₁|≤-10 d B,3.29 GHz-5.28 GHz).In the frequency band of 3.3 GHz-5.0 GHz,the stable half-power beamwidths（HPBWs）of the H-plane and E-plane is achieved greater than 110°and 102°,respectively.The compact size of the antenna is 0.43×0.43×0.16λ₀³（λ₀is the wavelength at the center frequency of 3.29 GHz-5.28 GHz）.2.Design of the broadband and wide beam dual-polarized orthogonal dipoles antennaAiming at the problem of poor front-to-back ratio of the compact wide beam dual-polarized antenna designed in Chapter 3 and how to further improve the impedance bandwidth of the antenna,a broadband and wide beam dual-polarized orthogonal dipoles antenna is designed.In order to further improve the impedance bandwidth,the orthogonal trapezoidal dipoles in the antenna designed in Chapter 3 are further evolved into orthogonal deformed fan-shaped dipoles,and four deformed fan-shaped parasitic elements are used to further improve the impedance bandwidth of the antenna.The beam width of the antenna in the whole operating frequency band is broadened by the metal cavity with slotted side wall.The current path on the side wall of the metal cavity is extended by the rectangular grooves,and the impedance bandwidth at low frequency band is improved.Then,a special-shaped metal reflector is added under the metal cavity to improve the front-to-back ratio of the antenna.The HPBWs of E-plane and H-plane of the antenna are greater than 90°in 1.7 GHz-3.22 GHz（61.8%,VSWR<2.0）,and the gain is greater than0d Bi in the range of±60°pitch angle.The front-to-back ratio is greater than 17d B,and the isolation of the two ports is greater than 29d B.3.Design of the wide beam quadrifilar helix antenna（QHA）based on multi-structure loadingAccording to the requirements of the research project,to solve the problems of how to use a pair of antenna to realize two different polarization modes in different angular areas of space,how to make the antenna achieve±90°wide beam coverage and how to weaken the adverse impact of large metal installation platform on the radiation pattern,a wide beam quadrifilar helix antenna（QHA）based on multi-structure loading is studied.Firstly,the traditional monopole is bent into"leaf-shaped",and the four"leaf-shaped"monopoles are fed by a double-layer Wilkinson power divider with a phase difference of 90°in turn.The circular polarization of the antenna is realized in the medium and high elevation area,and the vertical polarization is realized in the low elevation area by adjusting the layout spacing of the four"leaf-shaped"monopoles.The influence of the metal platform on the wide beam radiation pattern of the antenna is decreased by using the laminated cavity,and the gain of the antenna is further improved by loading the parasitic elements.The measured results show that when the antenna is placed on a metal platform with a diameter of 2λ₀,the vertical polarization gain is greater than-3.8d Bi in the low elevation area with an elevation of 0°to 20°and the circular polarization gain is greater than-2.8d Bic in the medium and high elevation area with an elevation of more than 20°in the operating frequency band off₀±130MHz.And the antenna can achieve wide beam coverage in the space angle area of±90°.Finally,the main research achievements of this thesis and the problems that need further research are discussed in the conclusion.