Design method of broadband low RCS metasurface based on SAM-OAM coupling

Abstract

The present invention belongs to the technical field of electronic materials, and in particular, relates to a broadband low RCS metasurface design method based on SAM-OAM coupling. The present invention realizes broadband coverage of M basic units of 2π phase based on geometric phase, changes the wavefront phase of electromagnetic waves, and forms a vortex metasurface; at the same time, the incident circularly polarized plane wave is reflected by the vortex metasurface, and the SAM carried by the circularly polarized incident wave is coupled and superimposed with the OAM generated by the vortex metasurface, so as to realize the spiral distribution characteristics and phase singularity of the reflected electromagnetic wave wavefront phase exp(ilφ). The vortex wave reflected by the metasurface of the present invention has the characteristic of "central void", which is a typical feature caused by the phase singularity of the vortex wave, so that the electric field amplitude at the center of the reflected wave is zero, and the broadband design of the basic unit finally realizes a metasurface structure with low RCS characteristics in a broadband range, which has broad development prospects in the microwave field.

Description

Technical Field

[0001] The invention belongs to the technical field of electronic materials, and in particular relates to a broadband low RCS metasurface design method based on SAM-OAM coupling. Background Art

[0002] Radar cross section (RCS) is a physical quantity that characterizes the scattering characteristics of a target. It refers to the equivalent scattering area of ​​a target under the illumination of an incident wave. In the military field, by reducing the scattering of radar waves by the target, the detection capability of the enemy radar is reduced and the survivability of one's own equipment on the battlefield is improved. RCS reduction technology mainly includes shape design, stealth material loading, passive stealth and active stealth. When the shape of the target has been determined, stealth material loading has become the most effective means of RCS reduction, and therefore has been widely studied.

[0003] For traditional stealth materials, there are certain limitations in practical applications due to the limitations of their intrinsic properties. With the rapid development of metamaterials / metasurfaces, their unique electromagnetic properties and controllable design have brought new development space for material design and greatly expanded the research of stealth materials. Metasurfaces such as artificial magnetic conductors, frequency selective surfaces, electromagnetic bandgap structures, polarization rotating reflective surfaces, etc. have been proven to have outstanding advantages in reducing RCS and have been widely studied. In many such schemes, the reduction of RCS is achieved by manipulating the amplitude, phase and polarization of electromagnetic waves.

[0004] Angular momentum is an intrinsic property of electromagnetic waves. It can be decomposed into spin angular momentum (SAM) and orbital angular momentum (OAM). In 1909, Poynting pointed out that circularly polarized waves related to polarization carry spin angular momentum SAM; while orbital angular momentum OAM is independent of polarization and belongs to another degree of freedom of electromagnetic waves. Orbital angular momentum is related to the spatial distribution of electromagnetic waves, which can be obtained by introducing a spiral phase factor in the field distribution of plane waves. In the phase factor, l represents the number of OAM modes, which is an arbitrary integer. The azimuth angle of the cross section perpendicular to the propagation direction, and the electromagnetic wave carrying orbital angular momentum is called vortex electromagnetic wave. In recent years, vortex electromagnetic wave has become one of the research hotspots in the field of microwave and radio frequency.

[0005] The main methods for generating vortex beams include spiral phase plates, shaped parabolic antennas, holographic plates, uniform annular arrays, and metasurfaces. The annular array generates a phase difference of 2π for the incident wave through different antenna units, thereby forming a vortex beam. However, this method usually requires a complex feeding network, which increases the design difficulty and cost. The design idea of ​​the spiral phase plate is that when the beam passes through the phase plate surface at different heights, different phase factors are added to the reflected wave, thereby forming an electromagnetic vortex wave. Due to the limitation of the manufacturing accuracy of the spiral gradient, it is difficult to obtain a high-quality OAM beam. The metasurface technology is optimized and designed according to the specific application requirements of the unit structure to meet the requirements of phase and bandwidth, mainly for the unit phase response covering 0 to 2π in a broadband range and the reflection amplitude close to 1. The beam with orbital angular momentum has the characteristic of zero radiation ("hollow") in its radiation direction. This typical feature is caused by the amplitude zero point brought by the phase singularity of the vortex wave. The zero radiation characteristic of the vortex wave phase singularity has the potential for extremely low scattering applications (CN202210866571.8, CN202210870683.0). However, with the changes in science and technology, higher technical requirements are put forward for scattering control, requiring both ultra-wideband and extremely low RCS indicators. Summary of the invention

[0006] In view of the above-mentioned problems or shortcomings, in order to solve the difficult problem of scattering characteristics with broadband low RCS, the present invention provides a broadband low RCS metasurface design method based on SAM-OAM coupling, which realizes the broadband extremely low RCS characteristics of the target body based on the unique characteristics of the vortex wave metasurface.

[0007] A broadband low RCS metasurface design method based on SAM-OAM coupling. The incident circularly polarized plane wave is reflected by the vortex metasurface, and the SAM carried by the circularly polarized incident wave is coupled and superimposed with the OAM generated by the vortex metasurface to realize the wavefront phase of the reflected electromagnetic wave. The spiral distribution characteristics and phase singularity of the . Specifically include the following steps:

[0008] Step 1: Design a basic unit with a reflection coefficient greater than 0.95 in the 9-22.5 GHz frequency band; and introduce a geometric phase to obtain a reflection phase covering 0 to 2π for the basic unit.

[0009] The planar shape of the basic unit is a square with a side length of p, and includes, from bottom to top, a stacked bottom metal layer, an intermediate dielectric layer and a top metal pattern layer; the bottom metal layer and the intermediate dielectric layer of the basic unit have the same physical size, and the top metal pattern layer is a polarized fractal patch stacked centrally on the intermediate dielectric layer; the top metal pattern layer satisfies that the co-polarization reflection coefficient of the basic unit at 9-22.5GHz is greater than 0.95, which is one of the conditions for ensuring the generation of vortex waves in a broadband range.

[0010] By changing the rotation angle of the top metal pattern layer in the basic unit, the corresponding geometric phase is obtained. The geometric phase is related to the spin-orbital angular momentum interaction, which can establish a direct connection between SAM and OAM. times, and then we get M basic units that satisfy the adjacent phase difference of According to the geometric phase principle, the reflection phase obtained by the M basic units can achieve 0°~360°, so that the basic unit covers the reflection phase of 0~2π, which is another important condition to ensure the generation of perfect vortex waves.

[0011]

[0012]

[0013]

[0014]

[0015] where r ll 、r rr 、r xx 、r yy are the co-polarization reflection coefficients under normal incidence of x-polarization, y-polarization, left-hand circular polarization, and right-hand circular polarization, respectively. lr 、r rl 、r xy 、r xy are the corresponding cross-polarization coefficients, is the rotation angle of the basic unit.

[0016] The metals of the bottom metal layer and the top metal pattern layer are both made of metal materials with a reflectivity of ≥90%, and the middle dielectric layer is made of lossy materials with a dielectric constant of 2.2-2.7.

[0017] Step 2: According to the M basic units for realizing vortex waves determined in step 1, the center point of the entire metasurface is taken as the origin and divided into M regions (to ensure that the phase gradient between two adjacent basic units satisfies the relationship of 360° / M or -360° / M); and the planar structure of the entire metasurface is divided into m×n basic units arranged in a matrix, m≥15, n≥15.

[0018] Step 3: According to the compensation phase formula, program design is performed in MATLAB to establish the relationship between the phase of the designed M basic units and the compensation phase, and finally the arrangement position of the m×n basic units of the entire metasurface is obtained, and then all the basic units are arranged to obtain a broadband low RCS metasurface based on SAM-OAM coupling.

[0019] The present invention realizes M basic units with broadband coverage of 2π phase based on geometric phase, changes the wavefront phase of electromagnetic wave, and forms a vortex metasurface; at the same time, the incident circularly polarized plane wave is reflected by the vortex metasurface, and the SAM carried by the circularly polarized incident wave is coupled and superimposed with the OAM generated by the vortex metasurface to realize the wavefront phase of the reflected electromagnetic wave. The spiral distribution characteristics and phase singularity of the metasurface. The vortex wave reflected by the metasurface of the present invention has the characteristic of a central void (zero radiation). This typical feature is caused by the phase singularity of the vortex wave, so that the electric field amplitude at the center of the reflected wave is zero. Based on the broadband design of the basic unit, a metasurface structure with low RCS characteristics in a broadband range is finally realized, which has broad development prospects in the microwave field. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a structural schematic diagram, a top view and a coordinate transformation diagram of a basic unit of an embodiment;

[0021] Figure 2 1 is the reflection amplitude phase curve of 12 basic unit structures under vertical incidence of the embodiment;

[0022] Figure 3 Schematic diagram of reflection phase distribution of all unit structures of vortex metasurface in the embodiment;

[0023] Figure 4 Schematic diagram of the vortex super surface structure of the embodiment;

[0024] Figure 5 is the far-field radiation pattern of the theoretical l=2 vortex metasurface at an incident frequency of 9.8 GHz;

[0025] Figure 6 is the far-field radiation pattern of the theoretical l=2 vortex metasurface at an incident frequency of 13 GHz;

[0026] Figure 7 is the far-field radiation pattern of the theoretical l=2 vortex metasurface at an incident frequency of 20.8 GHz;

[0027] Figure 8 The broadband single-station RCS simulation results of the embodiment when the circularly polarized wave is vertically incident. DETAILED DESCRIPTION

[0028] The present invention is further described in detail below with reference to the accompanying drawings and embodiments.

[0029] In this embodiment, M=12. The length and width of the bottom metal layer and the middle dielectric layer are both 6 mm, the thickness of the dielectric layer is 2.6 mm, and the bottom metal layer and the top metal layer are both made of a conductivity of 5.8×10 7The metasurface is composed of metallic copper with a dielectric constant of 2.65 and a loss of 0.002. The size of the metasurface is 300mm×300mm, m=50, and n=35.

[0030] A broadband low RCS metasurface design method based on SAM-OAM coupling, the specific steps are as follows:

[0031] Step 1: First, design an initial unit with broadband reflection characteristics and perform structural optimization in the 3D electromagnetic simulation software CST.

[0032] The basic unit structure includes a bottom metal layer, an intermediate dielectric layer, and a top metal pattern layer. The bottom metal layer and the intermediate dielectric layer have the same periodic size, the top pattern layer is a dumbbell-shaped polarized fractal metal patch (a circular patch is symmetrically cut out of a rectangle of the same size on both sides), the length and width of the bottom metal layer and the intermediate dielectric layer are both 6mm, the thickness of the dielectric layer is 2.6mm, and the top metal pattern layer is optimized with the geometric parameters r and w of the patch unit as variables, where r is the radius of the polarized fractal circular patch, and w is the length of the cut rectangular patch (such as Figure 1 structure shown). Figure 1 (a) is a schematic diagram of the basic unit structure of the present invention, Figure 1 (b) is a top view of the basic unit and a coordinate transformation diagram of the present invention.

[0033] Step 2: According to the amplitude and phase requirements for achieving OAM, the geometric parameters r and w of the dumbbell-shaped metal patch are optimized in CST to obtain amplitudes close to 1; further based on the geometric phase, the patch unit is rotated counterclockwise 12 times around the center to achieve 360° or -360° full coverage, thereby obtaining 12 basic units with a phase difference of 30° and full coverage of 360° required to generate OAM.

[0034] The basic unit parameters obtained by optimization according to this principle are: p = 6mm, w = 2.7mm, h = 2.6mm, r = 3mm. Based on the principle of geometric phase, the units are rotated 15° in sequence to obtain a phase difference of 30° between adjacent units, and the amplitudes of the 12 basic units are all greater than 0.95. Among them, m = 50 and n = 35.

[0035] Step 3: According to the basic principle of generating OAM vortex waves, the compensation phase required by the metasurface is obtained by subtracting the output phase of the vortex wave from the input phase of the plane wave. Based on this relationship, a program is designed in MATLAB to establish the relationship between the compensation phase of the metasurface and the phases of the 12 basic unit structures designed, and the required distribution form of the entire metasurface structure is obtained; Figure 4 This is a schematic diagram of the supersurface structure finally determined in this embodiment.

[0036] All the basic unit structure parameters determined in this embodiment are modeled and simulated in CST to form a 300mm×300mm metasurface. Circularly polarized plane waves are selected as the field source for simulation calculation. Figure 5-Figure 7 The far-field radiation patterns observed by the CST field monitor at 9.8 GHz, 13 GHz, and 20.8 GHz are given, which can intuitively show that the metasurface structure based on the square patch unit produces perfect vortex waves at three extreme reduction points. It can be seen that there is a "central void" at the center of the beam. This typical feature is formed by the central phase singularity of the vortex wave, thus causing the amplitude zero point. It shows that the simulation results of the designed artificial electromagnetic metasurface have the typical characteristics of the orbital angular momentum vortex wave amplitude being hollow. Based on this feature, the RCS of the metasurface can be greatly reduced. According to the attached Figure 8 It can be shown that the design of the present invention can achieve a -10dB reduction effect in the entire frequency band of 9-22.5GHz.

Claims

1. A broadband low RCS metasurface design method based on SAM-OAM coupling, characterized in that: The following steps are involved: Step 1, design a basic unit with a reflection coefficient greater than 0.95 in the 9-22.5 GHz frequency band; At the same time, the geometric phase is introduced to obtain the reflection phase of the basic unit covering 0~2π; The planar shape of the basic unit is a square with a side length of p, and includes, from bottom to top, a stacked bottom metal layer, an intermediate dielectric layer, and a top metal pattern layer; the bottom metal layer and the intermediate dielectric layer of the basic unit have the same physical size, and the top metal pattern layer is a polarized fractal patch stacked centrally on the intermediate dielectric layer; The top metal pattern layer satisfies the requirement that the co-polarization reflection coefficient of the basic unit at 9-22.5 GHz is greater than 0.95; Rotate the polarized fractal patch times, and then we get M basic units that satisfy the adjacent phase difference of Based on the geometric phase principle, the reflection phase obtained by the M basic units can achieve 0° to 360°, so that the basic unit covers the reflection phase of 0 to 2π; where r ll 、r rr 、r xx 、r yy are the co-polarization reflection coefficients under normal incidence of x-polarization, y-polarization, left-hand circular polarization, and right-hand circular polarization, respectively. lr 、r rl 、r xy 、r xy are the corresponding cross-polarization coefficients, is the rotation angle of the basic unit; The metals of the bottom metal layer and the top metal pattern layer are both made of metal materials with a reflectivity of ≥90%, and the intermediate dielectric layer is made of a lossy material with a dielectric constant of 2.2-2.7; Step 2: According to the M basic units for realizing vortex waves determined in step 1, the center point of the entire metasurface is taken as the origin and divided into M regions to ensure that the phase gradient between two adjacent basic units satisfies the relationship of 360° / M or -360° / M; The entire metasurface planar structure is divided into m×n basic units arranged in a matrix, m≥15, n≥15; Step 3: According to the compensation phase formula, program design is performed in MATLAB to establish the relationship between the phase of the designed M basic units and the compensation phase, and finally the arrangement position of the m×n basic units of the entire metasurface is obtained, and then all the basic units are arranged to obtain a broadband low RCS metasurface based on SAM-OAM coupling.

2. The broadband low RCS metasurface design method based on SAM-OAM coupling as claimed in claim 1, characterized in that: The M=12, p=6 mm, the bottom metal layer and the top metal pattern layer are both made of 5.8×10 7 The dielectric layer is composed of metal copper with a dielectric constant of 2.65 and a loss of 0.002, and the thickness is 2.6mm.

Images