| With the rapid development of wireless communication technology,more and more high performance antennas are demanded for modern wireless communication system.It is crucial to design an antenna with the performance of broadband and high-efficiency communication system.Therefore,the design of the broadband antenna and circularly polarized antenna has become a research hotspot in recent years.Compared with traditional antenna design methods,Characteristic Mode Theory(CMT)reveals the physical characteristics of the antenna and can clearly provide the physical meaning of the antenna resonance and radiation mechanism,which is conductive to designing high-performance complex antennas.In this thesis,combining with the background of modern wireless communication application,broadband metallic loop antenna loaded with distributed inductances,multi-resonant broadband dipole antenna,and dual-circularly-polarized metasurface antenna are carried out.The specific research contents are as follows:1.Based on CMT,the multimode resonance of the antenna is analyzed,and a metallic loop antenna loaded with distributed inductances is designed.According to the inherent mode significance and characteristic current distribution of the metallic loop,two characteristic modes with resonance characteristic are selected,and distributed inductances are loaded at appropriate positions to ensure their resonant frequencies are close to each other.Then,the feeding structure is placed at the strongest position of the current distribution of the two characteristic modes.Therefore,the two characteristic modes can be excited simultaneously,achieving a broadband antenna with a good impedance bandwidth of 51.1%(2.46GHz-4.15GHz).2.Based on CMT,the multiple resonators are analyzed,and a multi-resonant broadband dipole antenna is designed.Then,the principle of broadband radiation is clearly explained.First of all,the original inverted T-shaped metallic conductor is analyzed by the use of characteristic mode.According to the characteristic current distributions of different modes,parasitic dipoles are loaded at the right position to introduce a new characteristic mode.Then,the position of common strongest current distribution of different modes is determined,where is in the center of the inverted T-shaped structure.And the feeding structure is added at this position to excite three different characteristic modes simultaneously.Finally,the multi-resonant broadband dipole antenna achieves a good impedance bandwidth of 33.4%(1.82GHz-2.55GHz).3.A dual-circularly-polarized metasurface antenna is proposed,and the design process of modifying the metasurface structure to achieve circular polarization performance is provided.Firstly,the physical properties of the original metasurface antenna are analyzed through characteristic mode,and the metasurface structure is changed according to characteristic current distribution and mode significance.Therefore,two degenerate characteristic modes are obtained in the operating frequency band.Subsequently,cross-vertical microstrip feeding lines are placed at selected position to excite these two modes by means of cross-slot coupling.The two degenerate modes have a phase difference of 90° and the same excitation amplitude,thus achieving dual-circularly-polarized radiation. |
