Research On Radar Imaging Using Electromagnetic Vortex Wave

The orbital angular momentum(OAM)of electromagnetic waves is an important and underutilized fundamental physical quantity,which provides a new degree of freedom for information modulation in the physical layer.It greatly enhances the ability of information modulation,and has received more and more concerns in recent years.The electromag-netic waves carrying OAM are called vortical electromagnetic waves or electromagnetic vortex.Theoretically,the OAM can span an infinite state space,which are orthogonal to each other and propagate independently in space.Therefore,it allows information-rich ra-dio experiments and paves the way for novel wireless communication and radar detection concepts.At present,the applications of vortical electromagnetic waves mainly focus on the field of communications,and there is little research in radar application.The unique wavefront phase distribution of vortex electromagnetic waves exhibits angular diversity characteristics,which is expected to provide a new way for radar imaging.In this thesis,the investigation and study of vortical electromagnetic wave in radar imaging are explored in an attempt to establish a set of electromagnetic vortex imaging theory and method.We hope to obtain some preliminary research results in terms of the characteristics of vorti-cal electromagnetic wave,the principle of electromagnetic vortex imaging,the imaging model and imaging methods and the experimental verification.Compared with the plane wave,the phase front of the vortical electromagnetic waves is no longer a plane,but has a twisted helical structure in space,which enhances the information acquisition ability of the electromagnetic waves.Firstly,the principle of the azimuth resolution of electromagnetic vortex imaging is analyzed from the perspective of wave-front modulation,compared with the plane-wave and random irradiation.Then,the imaging process is given followed by the formulation of the mathematical models of vortex electromagnetic field and electromagnetic vortex imaging.According to the principle of generating the vortical electromagnetic wave,a new method of generating vortical electromagnetic waves by spiral array is proposed,which changes the OAM mode by frequency control.Finally,a set of experimental system of electromagnetic vortex imaging based on uniform circular array is designed.The experimental data are used for the analysis of vortex electromagnetic wave characteristic and the verification of imaging methods.Before the imaging application,the characteristics of the vortex electromagnetic ra-diation related to the imaging process must be investigated at first.Therefore,the third chapter studies the radiation intensity and phase distribution characteristics of the vor-tex electromagnetic field,including the main-lobe direction of the radiation pattern,the width of the main lobe and the side-lobe performance,and gives two methods for side lobe suppression.The mode characteristic of vertical electromagnetic waves produced by the uniform circular array is studies thoroughly.In order to facilitate the study of the non-pure vortex waves,two kinds of indexes are defined to quantitatively represent the modal purity of the vortex electromagnetic waves.The size of the pure region and the condition of the mainlobe in the pure region are obtained given the array parameters and the threshold of the pure mode,which provides guidance for the design of the imaging system parameters.Finally,the influence of the array errors on the modal purity of vor-tex electromagnetic wave is analyzed.The above theoretical results are verified by the analysis of the experimental data.According to the two-dimensional echo received by the array,the target images can be reconstructed by the two-dimensional Fourier transform method.Due to the influence of the echo amplitude modulation,we study the situations of large and small elevation an-gles.When the target elevation angle is large,the imaging equation can be approximated,and there exist artifacts in the imaging results.The large elevation angle is not suitable for practical application due to the weak radiation energy and the mode superposition.Then,the feasibility of imaging under small elevation angle is studied by means of the point spread function,and the minimum elevation angle available for imaging is obtained.Finally,the comparisons with other array imaging methods in terms of the imaging reso-lution,imaging resources and other aspects are conducted.The main-lobe direction of the vortex electromagnetic wave changes with the OAM mode.Therefore,the target cannot be illuminated by the main lobes of all modes at the same time.In this thesis,two methods are proposed to adjust the main-lobe direction of the vortex beams with different OAM modes to identity.The first method uses the uniform concentric circular arrays,each of which produces one or a few modal vortex waves.According to the law of the main lobe of the vortex electromagnetic wave,the main lobe is set to the desired direction by designing the radius of each circular array reasonably.The second method is to use the phased array technique to apply a steering phase to each of the elements to guide the beam axis to a specified direction while satisfying the phase configuration to produce the designed mode.Using this method for beam steering,the regularization method is used to solve the imaging equation.At the end of this thesis,the imaging results using darkroom measurement data of electromagnetic vortex imaging experiment system are given.As the imaging system uses a single antenna to receive the target echo,the imaging model using a single receiver is studied first.An additional phase should be compensated for OAM dimensional echo before imaging.Aiming at the issue that the spatial resolution of the Fourier transform method is too low,the parametric spectral estimation and regularization method are used to reconstruct the image and improve the imaging resolution.Then,the influence factors of image quality are analyzed.These two methods can obtain the imaging resolution beyond the Rayleigh limit and reduce the azimuthal side lobe,but are more sensitive to noise and model errors.The experimental results verify the effectiveness of the electromagnetic vortex imaging principle and the proposed imaging method.