Research On Antennas Generating Electromagnetic Vortex Beam

With the rapid development of science and technology,large-bandwidth and high-speed transmission has become a big trend for wireless communication.However,the finite radio frequency spectrum resource is very precious,so it is important to develop some new techniques to utilize the electromagnetic spectrum better,and the method of increasing the spectral efficiency and the communication capacity has become one of the recent research hotspots.Orbital angular momentum(OAM)has many eigenstates,and each of the OAM eigenstates with integer topological charges is orthogonal and independent to the others,all these attributes make OAM a promising candidate for the breakthrough in radio communications and radar applications.The existing antennas in practical use can generate only the single-mode or dual-mode OAM beams,and their design and architecture are very complicated;the concept of phased array antenna for generating multi-mode OAM beams has remained up to now mainly at simulation level.To address these problems,we studied and designed a few antennas for generating OAM radio beams,including horn-lens antennas for OAM modes ?= +1 or ?=-1,perforated dielectric reflectarray antennas for generating single-mode or mixed-mode OAM beams,higher-order mode patch antenna for generating dual circularly-polarized dual-mode OAM beams,four-element array for generating dual circularly-polarized dual-mode OAM beams,Rotman lens-based circular array for generating five-mode OAM radio beams and circular horn array for generating multi-mode OAM radio waves with three polarizations.All these antennas are with very simple architecture and high OAM mode purity.The research train of thoughts are from single-mode OAM to multi-mode OAM,and from single-polarization to multi-polarization.The major contributions of this thesis are summarized as follows:1.Based on the methodology of area filling factor,research on the characterisations of perforated dielectrics has been conducted,and the horn-lens antennas for generating OAM modes ?= +1 or ?=-1 are provided in the millimeter frequency band.By covering the aperture of a conventional horn antenna with a perforated planar dielectric lens inducing an azimuthal phase correction,the overall size is reduced and good OAM-carrying radio beams for modes ?= +1 or ?=-1 can be generated,and the intensity distributions and phase maps of both OAM states are also presented and analyzed in detail.Based on the Floquet-port method in HFSS,research on the reflectance characteristics of perforated dielectrics has been conducted for oblique incidence to the two-dimensional periodic structure,and the discrete dielectric antenna reflectarrays for OAM generation of single-mode ?=-1 or mixed-mode ?=-1&-2 are provided in the millimeter frequency band.2.In order to overcome the complicated structures of the multi-mode OAM antennas,a succinct dual circularly-polarized(CP)dual-mode OAM patch antenna and a dual-CP dual-mode OAM array are both presented.For the patch antenna,the dual-CP21 TM mode excited by using the hybrid coupler generates the electromagnetic vortices with OAM modes ?= ±1,and the antenna is designed with very simple architecture and a higher purity of the OAM modes in a very wide inclination angle range.For the dual-CP dual-mode OAM array,the dual-mode operation for OAM is achieved through the opposite phase differences generated for LHCP and RHCP,when the dual-CP elements are sequentially rotated in the clockwise direction.Since the four elements are fed with the same phase,the feed networks of the array are much simpler and ease of implementation.The measured results coincide well with the simulated ones,which verified the effectiveness of the proposed design and confirm the generation of the LHCP wave with OAM mode ? =+1 and the RHCP wave with mode ?=-1.3.To achieve multi-mode OAM multiplexing communications,a two-layer Rotman lens-fed antenna array is introduced to generate five-mode OAM radio beams.Through the integrated implementation of the multilayer lens-fed antenna array,the feeding network for the circular array is greatly simplified,the overall dimension of the antenna is reduced,and the separation of the antenna array from the lens body is also helpful to prevent spurious radiation.Through the optimization of the array structure and elements placement,the mutual coupling between elements and the complexity of the feed network are both reduced.The measured results coincide well with the simulated ones,which verified the effectiveness of the proposed design for generating five-mode OAM beams.When comparing with the phased array antenna system,the antenna is with very simple architecture and is fairly easy to realize.All these features make the proposed antenna a promising candidate for OAM multiplexing communications and radar applications.4.Based on the demand for the higher communication channel capacity,an effective solution of generating multi-mode OAM-carrying radio beams with three polarizations is provided by using phased circular horn antenna array.Through the reasonable configuration of the antenna array using elements with three polarizations,the OAM radio waves with all three polarizations for different states can be generated.When combining both the polarization states and OAM states in multiplexing,communication systems would have tremendously increases in capacity and spectral efficiency.The radiation patterns,electric fields as well as phase distributions of all three polarizations for different OAM states are analyzed in detail,and the superpositions of two coaxial OAM states are also carried out.