| In recent years,the mobile internet has been in a period of rapid development since the number of mobile terminals increases dramatically.To satisfy the growing demand for mobile data services,there is an urgent need for the next generation of wireless communication technologies that can provide faster speeds,wider bandwidth and higher efficiency.Orbital angular momentum(OAM)multiplexing,which allows multiple signals to be transmitted on the same frequency by exploiting the orthogonality between different modes of OAM,is an effective approach to enhancing wireless channel capacity.However,the energy null in the center of the vortex waves poses a significant challenge for the transmission and signal detection of vortex waves in the RF field.Moreover,the size of the energy null increases with propagation distance.Metasurface offers great advantages such as ease of fabrication,simple feeding structure,low profile,and strong regulation ability,which has been widely employed for the generation of vortex waves.Therefore,this paper investigates the manipulation of vortex waves based on the metasurface and proposes a novel OAM antenna.The main contributions of this paper can be summarized as follows:1.This paper presents a hybrid phasing meta-atom that combines a variable size approach and meta-atom rotation approach.The meta-atoms consisting of a windmill ring patch and a circular ring patch are etched on a single layer substrate.The rotation of the windmill ring patch not only changes the orientation of the windmill blades but also the patch dimensions of the entire meta-atoms.The simulated results show that a linear phase shift curve which covers 473.6°phase range is achieved by optimizing the parameters.Moreover,the phase shift curves are nearly parallel in the range from 27.5 GHz to 42.5 GHz,it indicates the proposed element features broadband property.2.According to the principle of spatial phase compensation of vortex waves,the spherical wave excited by the feed needs to be converted to a planar wave at first.Hence,the paper presents a reflectarray antenna that can produce a high-gain pencil wave based on the above hybrid phasing meta-atom,which can convert the spherical wave into a plane wave.The measured peak gain is 27.86 d Bi with the aperture efficiency of 51.7%.Moreover,the measured first sidelobe is lower than-17.93 d B and 3-d B gain bandwidth of 35.71% is also achieved.3.The above planar wave is converted into a vortex wave.A broadband OAM antenna that generates vortex waves has been designed using the same feed configuration as the above antenna.The proposed OAM antenna can generate vortex waves with mode l=-1 over the range of 26.5GHz-40 GHz and the 3-d B gain bandwidth is 32.85%.Moreover,the proposed antenna features low crosstalk and high mode purity.More importantly,the traditional vortex waves are used as a case to provide ideas for the generation of new types of vortex waves.4.We have analyzed the formation principles of traditional vortex waves and explained the reason for the energy null at the center of the wave.In order to eliminate the energy null,a theoretical hypothesis is presented with the corresponding verification scheme designed.The Simulated and measured results demonstrate that the new vortex wave maintains a spiral phase distribution during the propagation without an energy null,which provides an approach for generating the new vortex waves. |
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