Research On Wideband Large Depth-of-field Non-diffractive Beam Antenna

Diffraction is the common phenomenon of the electromagnetic wave propagating in space,which limits the ability of electromagnetic wave to propagate in a localized way.The non-diffractive beam can avoid the diffraction in a certain area to realize the directional and localized propagation of the electromagnetic wave.It is widely used in many fields,such as wireless energy transmission,terahertz space waveguide,near-field detection,microwave medical treatment,near-field ultra-high-speed communication and others.However,the study of non-diffractive beam antenna is still in an initial stage,with some drawbacks like non-uniform evaluation criterias,low frequency,narrow bandwidth,short non-diffractive distance,and fixed beam direction.There are still many problems that deserve to be studied in millimeter-wave non-diffractive beam antennas.Based on the theory of non-diffractive beam,this dissertation mainly studies the generation,steering and scanning of the non-diffractive beam with large depth-of-field in millimeterwave band.The main work of this dissertation is summarized as follows.Since the bandwidth definition of the quasi-non-diffractive beam is not specific,two kinds of evaluation criteria of the bandwidth of quasi-non-diffractive beam antenna are proposed,namely depth-of-field bandwidth and focal spot bandwidth.Aiming at the shortcomings of low frequency band,narrow bandwidth and limited depth-of-field of most present non-diffractive beam antennas,a new kind of phase shifting surface(PSS)element with wideband dispersion compensation in millimeter-wave band is proposed,which can compensate the dispersion on aperture in the entire band.The model of the wideband large depth-of-field quasi-non-diffractive beam antenna is established,and the prototype antenna is fabricated and measured.For the first time,a relative bandwidth of 14% and the farthest normalized depth-of-field is achieved in millimeter-wave band.Considering the beam steering requirement of the quasi-non-diffractive wave,the off-axis quasi-non-diffractive beam antenna is studied.The aperture field distribution of the quasi-non-diffractive beam with wideband and large depth-of-field is analyzed.The beam forming method and its mathematical model are proposed,and the accuracy of this model is verified by the near-field numerical simulation.To achieve broadband beam steering and scanning with large scan angles,a novel broadband PSS is proposed.Its honeycomb densely distributed topology can effectively increase the beam steering range,and its hexagonal multi-resonance unit can effectively achieve the dispersion compensation in the entire band.The model and design criteria of the off-axis quasi-nondiffractive beam antenna with wideband and large depth-of-field are established,and the prototype antenna was fabricated and measured.For the first time,the off-axis quasi-nondiffractive beam is generated with accurate depth-of-field and steerable scan angle in the range of 0°~60°.In order to realize the two-dimensional(2D)beam steering,the mechanical rotating quasi-non-diffractive beam steering antenna is studied.The mathematical model and design processes of the near-field Risley prism antenna which can generate and steer the quasi-non-diffractive beam in two dimensions are proposed.The accuracy and effectiveness of the model are proved by the near-field numerical simulation.The phase error of the model is analyzed and its impact on non-diffractive beams is evaluated.A two-layer wideband dispersion compensation PSS works as the wavefront transforming surface to realize the 2D scanning of the quasi-non-diffractive beam.The experiments show that the beam can be steered in two dimensions for the first time within the front half of the antenna,that is,within the pitch angle of 0°~60° and the azimuth angle of 0°~360°.To overcome some shortcomings of mechanical rotating quasi-non-diffractive beam steering antennas,such as slow scanning speed,unstable depth-of-field and focal spot,inflexible beam shaping,high power consumption and low reliability,a novel wideband wide-angle high-speed programmable digital electromagnetic surface is proposed.Its feeding,controlling and real-time coding modules are integrated to realize the high-speed electronically steering of the quasi-non-diffractive beam.An optimization method for the discretization of the phase distribution on the aperture is proposed,which can effectively improve the gain or depth-of-field of the beam.A prototype programmable digital electromagnetic surface with 400 elements was fabricated by using standard four-layer PCB process.Its overall power consumption is less than 10 W.Its thickness is less than 5mm,and its beam steering speed can reach 40,000 times per second.The digital electromagnetic surface adopts online programming technology,so the scan angle,depth of field and focal spot can be dynamically steered in real time.Experiments show that the relative bandwidth of the antenna reaches 33%,which basically covers the entire X-band.Two-dimensional scanning of the beam is realized within the pitch angle of 0°~60° and the azimuth angle of 0°~360°,and the switching time of any two beams is only 25?s.