High-resolution Radar Imaging Based On Electromagnetic Vortex

Nowadays,with the increasing complexity of electromagnetic environment and the increasing volume and calculation of radar,,the existing radars can hardly meet the demand.There is an urgent need to explore new methods of information acquisition.Radio-frequency vortex(RF-OAM)beams carrying orbital angular momentum can achieve the differential scaling of different targets under the same vortex beam irradiation due to their unique spiral phase front,which can effectively improve the azimuthal resolution and is a good choice for a new generation of radar signals.This paper will focus on the unique advantages of RF-OAM beams in radar systems,and conduct in-depth research on the RF-OAM beam generation mechanism based on imaging and the radar imaging model design,aiming to fully combine the characteristics of RF-OAM beams with radar systems,laying the foundation for the future.The main work of this paper is listed as follows:(1)By discussing the evolution of RF-OAM radar technology in detail,two key issues to be solved in this paper are clarified:how to efficiently generate reliable RFOAM beams and how to effectively extract target’s information from echo signals.In response to these two problems,this paper will be refined into two parts:how to ensure that the RF-OAM beams does not diverge and how to effectively extract the location information from the echo signal.Then,according to the radiation characteristics of the RF-OAM beam,the basic principle of the RF-OAM imaging is deduced,and the influence of the mode range on the imaging effect is analyzed from the three directions:azimuth resolution,phase blur and radar cross section.(2)An optimization scheme is proposed to reduce the divergence angle of RFOAM beams based on the radial phase plate.The generalized refraction theorem is used to adjust the divergence angle of RF-OAM beams,which can effectively weaken the divergence degree of RF-OAM beam and unify the directions of different mode.Since the intensity distribution of the RF-OAM beam is related to the Bessel function,it generally presents a "doughnut" shape,and the hollow area of the distribution has a certain inclination angle(divergence angle).As the beam propagates,the hollow area will continue to become larger,resulting in the inconsistency of the energy irradiated to the target.In addition,RF-OAM beams of different mode have different divergence angles,so that the energy irradiated to the target by each mode is different,which will also affect the final imaging result.In the simulation experiment,the divergence angle of the RF-OAM beam is corrected and set to zero by the phase plate,and the divergence angle is effectively weakened.Compared with the multi-ring circular array and the phased array scheme,the advantages of this scheme are that the structure is relatively simple,and the purity of RF-OAM beam is high.This scheme will provide some reference for the transmitter design of the RF-OAM radar system.(3)An azimuth-elevation high-resolution 2D RF-OAM radar imaging scheme based on rotating antenna is proposed to pave the way for 3D target imaging with RFOAM beams.The ability to extract the pitch information has always been the missing part of the current RF-OAM radar method.One of the main reasons is that the pitch angle information is located in the intensity term,which is hard to extract.In this paper,through mathematical analysis,the RF-OAM beam is regarded as the superposition of multiple plane waves,so that the pitch angle information is separated from the intensity term,which is more convenient for extraction.These plane waves are sequentially generated by the rotating antenna and processed separately at the receiving end,and finally the pitch-azimuth two-dimensional image is obtained through the compressed sensing technology.Simulations show that the scheme can obtain high-resolution images with a few modes and has strong robustness.