| The orbital angular momentum(OAM)beam is a kind of special electromagnetic wave with spiral phase front and "doughnut" shaped intensity distribution.The existence of phase singularity results in a sunken zone around the main optical axis.The orthogonality between each mode and theoretical-infinite modes break the limitation of currently-scanty spectrum resources,satisfying the requirements of communication capacity and providing potential solutions for new communication degree of future communication system.In this thesis,some novel OAM antenna designs,based on conventional antenna design methods,new materials and structures,are proposed.A series of performance tests are carried out after samples fabrication to verify the properties of the proposed OAM antennas.The main research contents and innovations in this dissertation are listed as follows:1.Employing F-shaped radiation patch and circular slot structure,the bandwidth enhance is realized in microwave and terahertz band.The research results demonstrate that the F-shaped structure,acting as LC oscillator circuits,can make antenna acquire broadband effect.Etching a circular slot on the ground plate brings a new resonator for the proposed antenna,doubling the bandwidth while keeping the antenna size invariant.Then,these broadband antennas are applied in OAM antenna array for OAM beams generation.OAM beams at any frequency within bandwidth can be generated by adjusting the feeding signals.2.A new method,using circularly polarized antenna for OAM beams generation,is presented.This method studies the relationship between the phase and electric field vector.It is found that the rotation of antenna can realize the same effect as changing the phase of feeding signals.Based on this characteristic,the phase requirement of generating OAM beams is satis fied by directly rotating antennas,which avoids the utilization of phase shifters and simplifies the OAM generation system.Moreover,the omnidirectional radiation of circularly polarized antenna is beneficial for standard waveform and improves electromagnetic performance of the OAM beam.3.Combined with the design idea of metamaterials,a novel circularly polarized metasurface antenna is proposed.To simplify the design process and enhance antenna properties,a V-shaped metasurface structure is etched in radiation layer.This asymmetric is a common structure applied in conventional circularly polarized antenna design,and the low axial ratio(AR)is easily obtained by employing the periodic structure.The simulated results illustrate the AR can be adjusted by the position of feed point while keeping the resonant frequency invariant.This feature come up with great convenience for circularly polarized antenna design.4.A metasurface OAM antenna is designed,fabricated and tested.In our design,a circular arrangement and phase gradient are used to simplify the structure of the metasurface and obtain the small divergence angle,respectively.As we know,phase gradient has capacity of modulating electromagnetic wave vector.Using reasonable calculation and design,it is possible to focus the OAM beam.The circular arrangement,in consistence with the contour of OAM beam,can help to generate standard waveform.According to simulated results,only one unit cell distributed in radial direction also generates OAM beam.Therefore,a circular ring is proposed for OAM antenna design,which is favorable to miniaturization of radio-frequency OAM antenna. |
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