Design And Research Of Orbital Angular Momentum Spiral Antenna

With the rapid development of modern wireless communication technology,5G communication technology as the protagonist of future communication has integrated the multiplexing technology of orbital angular momentum(OAM)electromagnetic vortex technology into it,so as to improve the efficiency of spectrum utilization and the capacity of communication system,and meanwhile solve the contradiction between the limited spectrum resources and its huge demand.At present,most of the OAM antennas proposed are narrow-band,which can not guarantee the purity of the OAM beam in broadband,and thus can not meet the needs of high data transmission rate.So it is urgent to study the wideband OAM antenna.The spiral antenna proposed in this thesis has the advantages including good frequency indepdendent property,mature theory and low cost.Based on the equivalent principle and superposition principle,the far-field radiation formula of the helix antenna is deduced,and the OAM beam mode is determined by the feeding phase of the helix antenna.At the same time,the feed network can be designed reasonably to realize multiple modes and mode reconfiguration of the OAM wave.The OAM based spiral antennas have a certain research value in the field of wireless communication.In this thesis,the far-field formula of the electric field of the spiral antenna is deduced firstly,and the relevant formulas of the phase factor and the topological charge are derived.The feasibility of the OAM beam generation and the influencing factors of the mode conversion are determined.Then,the antenna is modeled and simulated by using the electromagnetic simulation software.By reasonably adjusting the internal and external diameters of the spiral antenna and the curvature of its spiral arm,the appropriate bandwidth and matching of the antenna can be ensured.At the same time,the reasonable number of antenna arms is selected to ensure that the angular momentum waves with six modes including l=± 1,± 2 and ± 3 are generated.The simulation bandwidths of the spiral antenna with center frequency at 2.75 GHz are 1GHz,0.58 GHz and 0.73 GHz respectively,corresponding to the modes of l=1,2 and 3.The resultant fractional bandwidths are 36.36%,21.09% and 26.55% respectively,all of which are broadband,meeting the design requirements.The conversion of the angular momentum wave beams between modes l=1,2 and 3 are realized by using reconfigurable microstrip feed network including multiple groups of switching diodes.Finally,the fabrication of the spiral antenna is completed.The bandwidths of the fabricated antenna are 1GHz,0.57 GHz and 0.73 GHz for the modes l=1,2 and 3 respectively,and the fractional bandwidths are 36.36%,20.72% and 26.55% respectively.The measurent results are the same as those of the simulation,with a slight frequency offset.The OAM spectrum analysis is carried out by the Fourier transform.The results show that the beam purities of the simulated angular momentum waves with the modes l=1,2 and 3 are 97.77%,98.83% and 92.64%,respectively.The beam purities of the measured angular momentum wave are 95.47%,90.2% and 91.48%,respectively,for the mode l= 1,2 and 3.All beam purities are above 90%,which meets the design requirements.Note that the orbital angular momentum beam mode is equal to the measured number of modes minus one,which means the angular momentum in one mode is the orbital angular momentum in zero mode.The phase structure of the first mock exam is caused by the combined action of spin angular momentum and orbital angular momentum.In addition,the purity of the OAM wave radiated by the antenna decreases to some extent,which is caused by the misalignment of the antenna and the machining error in the measurement.Because the gain of the antenna decreases with the increase of the OAM mode,the tapered antenna and its array have been designed to improve itunder the condition of wide band.The conical equiangular spiral(CLS)antenna is of simple structure,small size and unidirectional radiation characteristics.The gain of the 4-element array composed of the CLS antenna is high.By controlling the phases of the currents fed into the different elements,the OAM beams of different modes can be generated.The gain of the CLS antenna is up to 10 d B in mode 0 and 6.6 d B in mode 1.The gain of each mode is higher than that of the plane spiral antenna.However,the OAM beam purity of CLS antenna is low,which needs further improvement and optimization.