| With the rapid development of wireless communication systems,it is difficult to improve spectrum efficiency and expand system capacity due to the limited spectrum.Orbital angular momentum（OAM）has attracted the attentions among worldwide scholars because of its unique helical phase wavefront.The wave carrying OAM is characterized by a phase distribution of exp（jlφ）,where l is an integer known as the mode index andφis the azimuth angle.Theoretically,the OAM contains unlimited range of orthogonal eigenstates,which provide a promising method to enlarge the channel capacity.This thesis focuses on the OAM antenna research.The main work of this thesis includes as follows:1.Research on design method of OAM water antenna.For the first time,liquid material is used in the electromagnetic wave for generating the OAM wave.The antenna is of moderate size,and its operating mode can be easily adjusted.The antenna is transparent as a whole.Based on the radiation principle of the annular metal patch,a water antenna carrying the frequency reconfigurable OAM is designed firstly.The antenna can radiate the OAM vortex electromagnetic waves with the different modes at different frequencies.When the antenna works at 2.2 GHz and 2.4 GHz and 2.6 GHz and 2.9 GHz,the annular metal patch operates in theTM2 1,TM3 1,TM4 1,TM5 1 modes,respectively.Therefore,the OAM beams with the modes of l=1,l=2,l=3,l=4 can be generated.On the basis of this principle,a water antenna carrying the mode reconfigurable OAM is further designed.The vortex electromagnetic waves with the modes l=+1 and l=-2 are generated by feeding the inner and outer rings of the water antenna,respectively.When the inner and outer rings of the water antenna are fed at the same time,the vortex electromagnetic waves with the mixed mode（l=+1,l=-2）are radiated.In order to facilitate the feeding,a feeding network is designed.The feed network consists of a T-type power divider,a Wilkinson power divider,a 90o phase shifter and two RF PIN diodes.By controlling the ON/OFF state of the PIN diodes in the feed network,the inner loop and the outer loop of the water antenna can be excited,so that the OAM waves with reconfigurable modes can be achieved.The prototype is fabricated.Good agreement between the simulation results and the experiment results are observed to verify the proposed design.2.A study of the Archimedean spiral for the OAM with multiple modes.Based on the radiation principle of traditional Archimedean spiral antenna,a two-arm complementary,symmetrical Archimedean spiral antenna is designed.For the first time,a single antenna is used to radiate the OAM wave with the 15 modes.From single mode to four-mixed mode,different OAM waves can be radiated flexibly,which provides a feasible method for communication based on OAM waves.The OAM waves with the modes of l=+1,l=+2,and l=（+1,+2）can be generated when the antenna is fed from the inside of the Archimedean spiral antenna.The OAM waves with the modes of l=-1,l=-2,and l=（-1,-2）can be radiated when the antenna is fed from the outside of the Archimedean spiral antenna.When the antenna is fed from the inside and outside of the Archimedean spiral antenna at the same time,the vortex electromagnetic waves of other mixed modes can be generated.The OAM modes radiated by the antenna contain four kinds of the single modes,i.e.,l=+1,l=-1,l=+2,l=-2,six kinds of the mixed dual modes,i.e.,（+1,-1）,（+1,-2）,（+1,+2）,（-1,-2）,（-1,+2）,（-2,+2）,four kinds of the mixed three modes,i.e.,（+1,+2,-1）,（+1,+2,-2）,（-1,-2,+1）,（-1,-2,+2）,and a kind of the mixed four modes,i.e.,（+1,-1,+2,-2）.For the convenience of feeding the antenna,a phase shifter network is designed.The phase shifter network consists of three Wilkinson power dividers and four digital phase shifters.The output phase can be controlled by applying the DC voltages to the input pins of the phase shifter,thus resulting in the output phase differences including 0o,90o and 180o.The prototype is fabricated.The measured results are given to demonstrate that the proposed antenna can radiate the OAM waves from the single mode to the mixed four modes. |
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