CN107465000B - Broadband, polarization-insensitive helical encoding RCS reduction metasurface and its design method - Google Patents

Abstract

The invention belongs to the technical field of radar stealth, in particular to an ultra-wideband, polarization-insensitive spiral coding RCS reduction metasurface and a design method thereof. The metasurface is a two-dimensional finite-size structure, composed of 8×8 8 linear superunits with different gradient directions arranged in a helical sequence; the gradient directions of the 8 linear superunits are 0 ^o , 45 ^o , and 90 o , respectively. ^o , 135 ^o , 180 ^o , 225 ^o , 270 ^o , and 315 ^o , each linear superunit is a two-dimensional finite-size structure, which is composed of 6 × 6 artificial electromagnetic structural units with different sizes according to linear gradients. It completely covers 360 ^° , and the phase gradient is 60 ^° ; the helical sequence is a single-helix cyclic arrangement from outside to inside, and the artificial electromagnetic structural unit is a rotationally symmetrical reflective structure, which consists of an upper inner closed loop and an outer Jerusalem metal It consists of a structure, a middle layer dielectric plate and a bottom metal copper plate. The RCS reduction metasurface of the invention has excellent characteristics such as thin thickness, good robustness, ultra-wideband operation, easy processing and the like.

Description

Broadband, polarization-insensitive helical encoding RCS reduction metasurface and its design method

technical field

The invention belongs to the technical field of radar stealth, and in particular relates to an ultra-wideband, polarization-insensitive RCS reduction metasurface based on a single-spiral cyclic coding arrangement and a linear phase gradient and a design method thereof.

Background technique

With the rapid development of radar detection technology, stealth characteristics have gradually become an important indicator to measure the performance of flying targets. The detection of the target by radar mainly depends on the radar cross section (RCS) of the detected object. Therefore, reducing the RCS becomes the most important means to achieve stealth. The traditional stealth technology mainly realizes the reduction of RCS by changing the shape of the flying target and coating the radar absorbing material (RAM). 4 layers of RAM to maximize RCS reduction, similar to the F-22 Raptor for radar stealth. However, due to the limitation of aerodynamic factors of fast-flying targets, the reduction of RCS through shape design will cause a sharp drop in target maneuverability. At the same time, multi-layer coating of RAM will increase the weight and thickness of the target. In addition, it can be applied to multi-band high absorption rate RAM comparison. Expensive, making it expensive in targeted stealth applications. Relevant information shows that the B-2 and F-22 need to replace the RAM every time they fly, and each replacement takes 35 hours, which is expensive. Given the many deficiencies in form factor and RAM stealth, researchers began to explore more cost-effective ways to achieve target stealth.

Metasurface is a new type of artificial composite structure composed of subwavelength artificial electromagnetic structures arranged in a certain way and has strong electromagnetic wave regulation ability. As an emerging technology, it is expected to solve the above-mentioned defects in stealth schemes. It is expected to be conformal to some high-speed flying targets such as aircraft and missiles, and has broad application prospects in military, aerospace, and communication systems. Due to the excellent properties of metasurfaces and strong electromagnetic control capabilities, their potential applications in target electromagnetic stealth have been gradually discovered, and staged progress has been made. In 2007, a checkerboard-structured artificial magnetic conductor (AMC) metasurface was first applied in the field of RCS reduction, although its narrow-band properties, polarization sensitivity, and region-specific (backward) RCS reduction limit its practical application, the method It has greatly stimulated the research upsurge of broadband, polarization insensitivity, large-angle incidence and dual-station RCS reduction. It is hoped that this technology can be used in practice one day and become a new stealth material to replace traditional RAM.

The invention discloses an ultra-wideband RCS reduced metasurface based on a single helical cyclic coding arrangement and a linear phase gradient and a design method thereof. The method is simple and efficient, and solves the time-consuming caused by the need for a large number of optimizations in the existing RCS reduced metasurface design. At the same time, it has excellent performances such as large angle incidence and polarization insensitivity, and has important potential application prospects in the field of radar electromagnetic stealth.

SUMMARY OF THE INVENTION

The purpose of the present invention is to propose an ultra-broadband RCS reduction metasurface with polarization insensitivity and large angle incidence and a design method thereof.

The polarization-insensitive, large-angle incident ultra-wideband RCS reduction metasurface proposed by the present invention integrates helical coding technology, linear phase gradient metasurface and multi-mode cascading into a whole new stealth device. As shown in Fig. 1, the ultra-wideband, polarization-insensitive helical encoding RCS reduced metasurface is a two-dimensional finite-size structure composed of 8 × 8 linear superunits (modules) with different gradient directions arranged in a helical sequence ; The gradient directions of the 8 kinds of linear superunits are respectively 0°, 45°, 90°, 135°, 180°, 225°, 270° and 315°, and are sequentially numbered as 1, 2, 3, 4, 5, 6, 7 and 8, each linear superunit is a two-dimensional finite-size structure, which is composed of 6×6 6 artificial electromagnetic structural units with different sizes according to a linear gradient, all of which completely cover 360°, and the phase gradient is 60°; The helical sequence is a single-helix cyclic arrangement from the outside to the inside, namely 1234567812345678..., the number number represents the linear superunit corresponding to the gradient direction; the artificial electromagnetic structural unit is a rotationally symmetrical reflection structure, which is closed by the upper interior The ring (resonant ring) consists of the outer Jerusalem metal structure, the middle dielectric plate and the bottom metal copper plate (see Figure 2).

Denote the size of the artificial electromagnetic structure unit as a, in the upper metal structure, the width of the metal strips of the inner resonant ring and the outer Jerusalem structure is d, the size of the resonant ring is a/2, the thickness of the dielectric plate is b, and the bottom metal The copper plate thickness is c; the unit period is p.

The design method and steps of the polarization-insensitive helical encoding RCS reduction metasurface of the present invention are specifically given below, and the design flow mainly includes the following three steps.

Step 1: Broadband, polarization-insensitive, subwavelength metasurface element design:

Broadband, polarization-insensitive RCS reduction metasurface design, the primary problem is to design a metasurface element with broadband operation and polarization-insensitive characteristics. Polarization insensitive properties. According to the knowledge in the field of polarization transformation, in order to obtain polarization insensitivity, the element must have the characteristic of rotational symmetry around it. At the same time, in order to obtain broadband characteristics, the unit reflection amplitude must be close to 1, and the phase response has excellent linearity and low quality factor in a wide frequency range.

Based on the above analysis, we invented a multi-mode reflective unit structure composed of an inner rectangular resonator ring and an outer Jerusalem cross metal structure, as shown in Fig. 2. The whole unit is composed of an upper metal structure, a middle dielectric plate and a bottom metal floor. Multiple magnetic resonance modes resonating at different frequencies are provided through the coupling of the upper metal structure and the floor, and the phase of the unit at the edge frequency is opened by using the idea of multi-mode cascade. , to improve the linearity of the unit phase and the phase variation range with the structural parameters (covering more than 360°), so as to finally achieve the purpose of expanding the working bandwidth of the unit. At the same time, due to the rotational symmetry of the unit, similar scattering spectra will be generated when electromagnetic waves of different polarizations irradiate the metasurface, and the electromagnetic scattering response of the unit is polarization-insensitive to x and y linearly polarized waves.

In order to maximize the phase characteristics of the unit, the commercial simulation software CST is used to optimize the structural parameters of the unit, so that each resonance mode is reasonably cascaded. The final unit period is p=6mm, the dielectric plate is an epoxy glass cloth plate with a thickness of 2mm, a relative permittivity of 4.3, and an electrical loss tangent of 0.001. The width of the resonant ring and the Jerusalem structure is d=0.2mm. The thickness of the underlying metal copper plate may be 0.036mm. The required phase gradient of each unit can be obtained by changing the size a of the entire metal structure while keeping other parameters unchanged. The final superunit is composed of 6 units of different sizes a, the phase gradient is designed at f ₀ =15GHz, the phase gradient is 60°, and the phases corresponding to the 6 units are 0°, 60°, 120°, 180°, 240°, respectively. ° and 300°, reaching 0° to 360° phase coverage. By reasonably selecting the size a of the six units, the phase characteristic curve of each unit has perfect parallelism in a wide bandwidth range near _f0 , as shown in Figure 3. The final structural dimensions a of the six different units are 4.62mm, 3.95mm, 3.36mm, 3.04mm, 2.78mm and 2.3mm, respectively.

The second step: 8 kinds of supercell designs based on the generalized Snell reflection law:

由下式决定：The generalized Snell reflection law when θ _i = 0° is shown in Fig. 4. According to the generalized Snell reflection law, when the incident wave is incident on the metasurface at angle θ _i , the pitch angle θ _r and azimuth angle of the reflected main beam are

It is determined by the following formula:

是入射电磁波在真空中的波失，

分别为二维平面上x、y方向的相位梯度，

分别为x、y方向上相邻单元之间的相位差，p为单元周期。上式表明通过合理设计x、y方向上的相位梯度，可以任意操控超表面的梯度波矢k_a。而异常反射波矢k_r是镜反射波矢k_r0与梯度波矢k_a的合成，因此通过操控k_a可以进一步任意操控反射波束的偏折方向(波束指向)。in,

is the wave loss of the incident electromagnetic wave in vacuum,

are the phase gradients in the x and y directions on the two-dimensional plane, respectively,

are the phase differences between adjacent cells in the x and y directions, respectively, and p is the cell period. The above formula shows that by reasonably designing the phase gradients in the x and y directions, the gradient wave vector _ka of the metasurface can be manipulated arbitrarily. The abnormal reflection wave vector k _r is the synthesis of the mirror reflection wave vector _{k r0} and the gradient wave vector ka, so the deflection direction (beam pointing) of the reflected beam can be further arbitrarily controlled by manipulating _ka .

为60°，y方向

为0°。此时梯度方向为x方向；二维梯度超单元中，相位梯度同时存在于x、y方向，且

和

均为60°。此时梯度方向为

方向。如图6所示，通过对上述一维、二维超单元分别进行90°、180°和270°旋转，可以生成另外6种梯度方向不同的超单元，最终获得的8种超单元梯度方向分别为

45°，90°，135°，180°，225°，270°和315°，依次编号为1、2、3、4、5、6、7和8，达到了梯度方向在二维平面内的360°覆盖。According to the above theoretical analysis, a one-dimensional gradient supercell and a two-dimensional gradient supercell are designed respectively. As shown in Figure 5, in a one-dimensional gradient supercell, the phase gradient exists only in the x direction and

is 60°, y direction

is 0°. At this time, the gradient direction is the x direction; in the two-dimensional gradient superunit, the phase gradient exists in both the x and y directions, and

and

Both are 60°. At this time, the gradient direction is

direction. As shown in Figure 6, by rotating the above one-dimensional and two-dimensional superunits by 90°, 180°, and 270°, respectively, six other superunits with different gradient directions can be generated, and the final eight superunit gradient directions are obtained respectively. for

45°, 90°, 135°, 180°, 225°, 270° and 315°, numbered 1, 2, 3, 4, 5, 6, 7, and 8 in turn, to achieve the gradient direction in the two-dimensional plane. 360° coverage.

Step 3: Modeling and design of broadband, polarization-insensitive helical encoding RCS reduction metasurfaces:

Based on the metasurface element design in the first step and the 8 types of superelement designs in the second step, the third step is how to reasonably utilize the 8 types of superelements to construct the RCS reduced metasurface.

First, the size of the metasurface, that is, the number of superunits, needs to be determined. Here, for the convenience of design, the metasurface adopts a square layout, that is, the number of supercells L and M in the x and y directions are the same. The effects of computing time, sample fabrication cost and the finite size of the metasurface on the reduction characteristics of RCS are comprehensively measured. Here, the number of superunits in the metasurface is L×M=8×8, and the size is 288×288mm ² ;

Secondly, determine the coding sequence, that is, the arrangement of the 8 superunits. In order to disperse the incident electromagnetic waves to the greatest extent, reduce the scattering intensity of the target at each given angle, and thus reduce the radar detection probability under the double-station RCS detection, the gradient directions of any adjacent supercells in the metasurface are different. Based on this consideration, the present invention designs a single helix cyclic arrangement from the outside to the inside: 1234567812345678..., as shown in Figure 7, namely: 12345678-1234567-8123456-781234-567812-34567-81234-5678 -1234-567-812-34-56-7-8. Among them, "-" represents the turning point of the helix, and the numbers represent different superunits corresponding to the gradient directions, corresponding to the number sequence in Figure 6. The gradient direction distribution of the final spiral-encoded RCS reduced metasurface is shown in Figure 8. The arrows represent the gradient directions of the superunits. The metasurface designed by this arrangement ensures that the gradient directions of any adjacent superunits are different, so that the maximum possible Destroy the iso-phase surface of the uniform scattering of the metasurface to achieve the purpose of dispersing electromagnetic waves to the maximum extent.

Finally, according to the cyclic arrangement of superunits and single helixes of superunits and through the root-seeking algorithm, the helical-encoded metasurface structure is established by using VBA macros in CST.

Different from the previous RCS reduction methods based on singular beam deflection and surface wave conversion, the RCS reduction metasurface of the present invention proposes for the first time a helical coding based on multiple linear phase gradient superunits to solve the electromagnetic stealth problem under double-station RCS detection, making the incident After the electromagnetic wave is incident on the metasurface, the electromagnetic scattering energy can be uniformly dispersed in all directions in all directions; at the same time, the polarization insensitivity problem of the RCS reduction metasurface is realized by using the rotational symmetry element, so that the RCS reduction characteristic does not change with the polarization of the incident wave. The change can be linearly polarized waves with different polarization angles; finally, the invention realizes the ultra-broadband operation of the RCS reduced metasurface through multi-mode cascading. The ultra-broadband and polarization-insensitive RCS reduction characteristics of the invention do not need to be optimized, and have excellent characteristics such as good robustness, easy processing and thin thickness.

Description of drawings

Figure 1 is a structural diagram of a single-helix loop encoding metasurface.

Figure 2 is a structural diagram of a multi-mode resonant cascaded metasurface unit.

Figure 3 is the reflection phase spectrum of the metasurface unit with different sizes a.

Figure 4 is a schematic diagram of the generalized Snell reflection law when electromagnetic waves are vertically incident.

Figure 5 shows (a) a one-dimensional linear phase gradient superunit and (b) a two-dimensional linear phase gradient superunit composed of dual resonant units.

Figure 6 shows 8 different gradient orientation superunits generated by rotation of 2 linear phase gradient superunits.

Figure 7 is a sequence coding diagram of a single helix loop coding metasurface.

Figure 8 is a graph of the gradient patterns of each superunit in a single helical loop encoding metasurface.

FIG. 9 is a gradient pattern of each supercell in a linear phase gradient metasurface.

Figure 10 is a graph of the RCS spectrum of a single helical cyclic encoding metasurface irradiated by x- and y-polarized electromagnetic waves.

Figure 11 shows the three-dimensional scattering patterns of the metal plate, linear phase gradient and single helical cyclic encoding metasurface at normal incidence at 15.2, 18.2 and 22 GHz.

Figure 12 shows (a, b) near-field distributions and (c, d) surface current distributions of (b, d) single-spiral loop-encoding metasurfaces and (a, c) linear phase gradient metasurfaces under normal incidence.

Fig. 13 is the simulated and post-test RCS reduction spectrograms of the helical encoding metasurface under oblique incidence at different angles.

面内金属平板、线性相位梯度和单螺旋循环编码超表面的双站RCS分布图。Figure 14 is

Two-station RCS distributions of in-plane metallic slabs, linear phase gradients, and single-helix loop-encoding metasurfaces.

面内金属平板、线性相位梯度和单螺旋循环编码超表面的双站RCS分布图。Figure 15 is

Two-station RCS distributions of in-plane metallic slabs, linear phase gradients, and single-helix loop-encoding metasurfaces.

Detailed ways

According to the previously established single-helix loop-encoded metasurface design method, we can rapidly and automatically design helical-encoded metasurfaces with arbitrary dimensions. In the following, the design process, design results, and analysis of the results are introduced by taking a metasurface composed of 8 × 8 superunits through single helical cyclic coding as an example. In order to reveal the superiority of the method of the present invention, the results are compared with the scattering properties of conventional linear phase gradient metasurfaces without helical encoding. The gradient directions of each superunit in the linear phase gradient metasurface are shown in Fig. For a fair comparison, the metasurface dimensions, dielectric plate specifications, and experimental conditions are identical in both cases.

In the design and production, the dielectric board adopts epoxy glass cloth board, its dielectric constant ε _r = 4.3, the electric tangent loss tanσ = 0.001, the thickness of metal copper foil is 0.036mm, the thickness of the dielectric board is h = 2mm, and the unit period is p = 6mm , the geometric parameters of the six units constituting the linear gradient superunit are: a=4.62mm, 3.95mm, 3.36mm, 3.04mm, 2.78mm, 2.30mm.

The working principle of the metasurface unit: when the electromagnetic wave is incident vertically, induced current will be generated on the thin metal patch parallel to the polarization direction under the action of the y and x polarized electric fields, and the coupling between the metal backplane and the superstructure makes the The metal backplane generates a reverse current, and a displacement current is generated in the dielectric plate. The induced current and the displacement current eventually form a closed loop and generate magnetic resonance. The multiple modes of the upper metal structure will generate different localized magnetic resonance circuits at different frequencies. Due to the rotational symmetry of the unit, the y and x polarizations have exactly the same electromagnetic response.

The principle of metasurface diffuse reflection: On the one hand, the deflection directions of the eight superunits are different. When a plane wave is vertically irradiated on the metasurface composed of these phase gradient superunits, according to the generalized Snell reflection law, the reflected wave will be reflected by these different Dispersion of superunits in the deflection direction: On the other hand, the helical coding makes the deflection directions of adjacent superunits different. This aperiodic arrangement can further disperse the reflected waves, thereby achieving scattering characteristics and RCS reduction. function, while helical coding makes the arrangement of superunits rule-based, deterministic, and does not require optimization.

By optimizing and adjusting the metal structure parameters of the unit, the spectral position of each resonance mode under y and x polarization can be controlled to tune the reflection phase of the entire unit, and obtain 360° phase coverage, optimal linearity and broadband phase response.

In order to verify the broadband scattering characteristics of the single helical cyclic coding metasurface of the present invention, the commercial simulation software CSTMicrowave Studio was used to conduct electromagnetic simulation on the scattering spectrum of the metasurface, wherein the six boundaries along the x, y, and z directions were all open boundary conditions, plane wave Vertically incident along the z-direction, and vertically irradiated with linearly polarized waves polarized in x and y, respectively.

Example: Ultra-wideband RCS reduction results for a single helical loop-encoded metasurface of size 288 × 288 ^mm2

In the following, the reduction results of the metasurface RCS designed by the above formula are explained and verified. The metasurface was irradiated with x- and y-polarized waves, respectively, and the results are shown in Figure 10. It can be seen that, whether excited by x-polarized wave or y-polarized wave, the single-spiral cyclic coding metasurface can reduce the backward RCS well, and has almost the same scattering spectral response, which verifies the polarities of the metasurface of the present invention. Insensitive characteristics. At the same time, the RCS reduction spectrum results show that the single-spiral cyclic encoding metasurface has excellent RCS reduction characteristics in the range of 13-23GHz, and the RCS reduction values are all over 10dB.

In order to verify the double-station RCS reduction characteristics of the metasurface, the far-region scattering field was simulated to obtain the 3D scattering pattern of the metasurface. Figure 11 presents the far-region 3D scattering patterns at 15.2, 18.2, and 22 GHz for an equal-sized ideal metal plate, a linear phase gradient metasurface, and a single-helix loop-encoding metasurface, respectively. It can be seen that, compared with the strong mirror scattering of the metal plate, the single-spiral cyclic encoding metasurface can perfectly uniformly disperse the reflected electromagnetic waves in all directions of space, and the scattered energy is maximally smooth and uniform in all angles. Good uniform diffuse reflection characteristics are achieved, and the scattered waves of the linear phase gradient metasurface are mainly distributed in three directions, corresponding to three scattering modes of n=-1, n=0 and n=+1, and three strong scattering modes The existence of , makes the linear phase gradient metasurface very easy to be detected by the enemy radar in these three directions, and has no stealth characteristics.

In order to explain the uniform diffuse reflection mechanism of the single helical cyclic encoding metasurface of the present invention, Fig. 12 shows the surface current distribution and electric field distribution of the single helical cyclic encoding metasurface and the linear phase gradient metasurface at 15.2G. It can be seen that the single-spiral cyclic encoding metasurface has more fragmentation, disordered current and near-field distribution than the linear phase gradient metasurface, indicating that the uniform scattering iso-phase surface of the former is completely destroyed, which explains and further verifies the metasurface of the present invention. It has the ability to significantly disperse scattered electromagnetic waves and homogenize the spatial scattering field, while the linear phase gradient metasurface has a relatively obvious isophase surface, explaining the strong scattering in a specific direction in space.

In order to further verify the ability of the method of the present invention to disperse electromagnetic waves and realize ultra-broadband RCS reduction, a single-spiral loop-encoded metasurface sample with an area of 288×288 mm ² was fabricated and its scattering intensity was measured in a microwave anechoic chamber. Due to the wide frequency band to be measured, two groups of linearly polarized speakers in different frequency bands were used to test the scattering intensity of the sample. The first group of linear polarization transceiver speakers work at 10-18GHz, and the second group of linear polarization transceiver speakers work at 18-26GHz. During the test, the transmitter and receiver speakers are placed at the same height as the sample to be tested. In addition, in order to measure the robustness of the RCS reduction of the metasurface under large-angle incidence, the scattered waves of the sample under oblique incidence at different angles were also tested. The test results are shown in Fig. 13. It can be seen that the simulation results are in good agreement with the test results under the 0°, 30° and 60° incident cases. In all cases, the metasurface can well achieve broadband RCS reduction. When the incident angle is 0°, the 10dB RCS reduction simulation bandwidth of the helical encoding metasurface is 13-23GHz, and the test bandwidth is 12.2-23.4GHz; when the incident angle is 30°, the 10dB simulation bandwidth is 13-23.2GHz, and the test bandwidth is 11-23GHz; when the incident angle is 60°, the RCS reduction performance decreases, but both meet 8.7dB and the simulation bandwidth reaches 13.2-23.2GHz, and the test bandwidth reaches 12-23GHz. In summary, the helical encoding metasurface still has good RCS reduction properties under wide-angle (up to 60°) incidence.

和

平面内的双站RCS仿真曲线图。可以看出，

面内线性相位梯度超表面的最高副瓣值出现在-31.5°，为6.49dB，而螺旋编码超表面的最高副瓣值出现在+34°，为4.89dB，峰值缩减1.6dB；

面内线性相位梯度超表面的最高副瓣值出现在+36.5°，为6.16dB，而螺旋编码超表面的最高副瓣出现在+34°，为3.35dB，峰值缩减为2.81dB。Figures 14 and 15 show that the electromagnetic waves are incident vertically

and

Two-station RCS simulation plot in plane. As can be seen,

The highest sidelobe value of the in-plane linear phase gradient metasurface appears at -31.5°, which is 6.49dB, while the highest sidelobe value of the helical encoding metasurface appears at +34°, which is 4.89dB, and the peak value is reduced by 1.6dB;

The highest sidelobe value of the in-plane linear phase gradient metasurface appears at +36.5° with 6.16dB, while the highest sidelobe value of the helical-encoded metasurface appears at +34° with 3.35dB, with a peak reduction of 2.81dB.

To sum up, the near-field distribution, far-field scattering pattern and RCS reduced spectrum all show the ultra-wideband, polarization-insensitive RCS reduction characteristics and the ability to uniformly disperse electromagnetic waves of the single helical cyclic coding metasurface. It still maintains very well in the case of large incident angles. Compared with the time-consuming optimization design of random encoding metasurfaces, the single-spiral cyclic encoding metasurfaces are simple in design, have obvious effects, have good robustness to polarization and incident angle, do not require optimization, and have the inherent ability to disperse electromagnetic waves. It has important potential application value in the field of electromagnetic stealth in the future.

Claims (3)

1. An ultra-wideband, polarization-insensitive helical encoding RCS reduction metasurface, characterized in that it is a two-dimensional finite-size structure, formed by 8 × 8 8 linear superunits with different gradient directions arranged in a helical sequence ; The gradient directions of the 8 linear super-units are 0°, 45°, 90°, 135°, 180°, 225°, 270° and 315°, respectively, and the corresponding super-units are sequentially numbered 1, 2, 3 , 4, 5, 6, 7, and 8; each linear superunit is a two-dimensional finite-size structure, composed of 6×6 6 artificial electromagnetic structural units with different sizes according to a linear gradient, all of which completely cover 360°, and the phase The gradient is 60°; the helical sequence is a single helix cyclic arrangement from outside to inside, namely 1234567812345678..., the number number represents the linear superunit corresponding to the gradient direction; the artificial electromagnetic structural unit is a rotationally symmetrical reflection structure , which consists of the upper inner closed ring, namely the resonant ring, the outer Jerusalem metal structure, the middle layer dielectric plate and the bottom metal copper plate; Denote the size of the artificial electromagnetic structure unit as a, in the upper metal structure, the width of the metal strips of the inner resonant ring and the outer Jerusalem structure is d, the size of the resonant ring is a/2, the thickness of the dielectric plate is b, and the bottom metal The copper plate thickness is c; the unit period is p. 2. The ultra-wideband, polarization-insensitive helical encoding RCS reduction metasurface according to claim 1, characterized in that, the unit period p=6mm, the thickness of the dielectric plate b=2mm, the relative permittivity is 4.3, the resonant ring and The metal strip width of the Jerusalem structure is d=0.2mm; the thickness of the underlying metal copper plate is 0.036mm; the supercell is composed of 6 units of different sizes a, the phase gradient is at f ₀ =15GHz, the phase gradient is 60°, and the 6 types of units correspond to The phases are 0°, 60°, 120°, 180°, 240° and 300°, respectively, reaching 0° to 360° phase coverage; the structural dimensions a of the 6 different units are 4.62mm, 3.95mm, and 3.36mm, respectively. , 3.04mm, 2.78mm, 2.3mm. 3. the design method of ultra-wideband as claimed in claim 1, polarization-insensitive helical coding RCS reduction metasurface, is characterized in that, concrete steps are as follows: Step 1: Broadband, polarization-insensitive, subwavelength metasurface element design: The basic theoretical basis of the design is that the cluster response of the metasurface composed of a series of units inherits the broadband and polarization-insensitive characteristics of the unit; in order to obtain the polarization-insensitive characteristic, the unit must have the characteristic of rotational symmetry around it; at the same time, in order to obtain the broadband characteristic, the unit The reflection amplitude must be close to 1, and the phase response has good linearity and low quality factor over a wide frequency range; According to this, a multi-mode reflective unit structure consisting of an inner rectangular resonant ring and an outer Jerusalem cross metal structure is first designed. The whole unit consists of an upper metal structure, a middle dielectric plate and a bottom metal floor, and the resonance is provided through the coupling of the upper metal structure and the floor. For multiple magnetic resonance modes of different frequencies, the multi-mode cascade idea is used to open the phase of the unit at the edge frequency, so as to improve the linearity of the unit phase and the phase variation range with the structural parameters, so as to finally achieve the purpose of expanding the working bandwidth of the unit; At the same time, due to the rotational symmetry of the unit, the electromagnetic response of the unit also has the characteristic of polarization insensitivity; In order to maximize the phase characteristics of the unit, the commercial simulation software CST is used to optimize the unit structure parameters, so that each resonance mode is reasonably cascaded; the final unit period is p=6mm, the thickness of the dielectric plate is 2mm, the relative permittivity is 4.3, The epoxy glass cloth plate with electrical loss tangent of 0.001, the width of the resonant ring and the Jerusalem structure is d=0.2mm; the required phase gradient of each unit is obtained by changing the size a of the entire metal structure and keeping other parameters unchanged; the final superunit is composed of It consists of 6 units of different sizes a, the phase gradient is designed at f ₀ =15GHz, the phase gradient is 60°, and the corresponding phases of the 6 units are 0°, 60°, 120°, 180°, 240° and 300°, respectively. , the phase coverage from 0° to 360° is achieved; the size a of 6 kinds of units is selected, so that the phase characteristic curves of each unit have perfect parallelism at f ₀ ; 4.62mm, 3.95mm, 3.36mm, 3.04mm, 2.78mm and 2.3mm; The second step: 8 kinds of supercell designs based on the generalized Snell reflection law: According to the generalized Snell reflection law, when the incident wave is incident on the _metasurface at angle _θi , the pitch angle θr and azimuth angle of the reflected main beam are

It is determined by the following formula:

in

is the wave loss of the incident electromagnetic wave in vacuum,

are the phase gradients in the x and y directions on the two-dimensional plane, respectively,

are the phase differences between adjacent units in the x and y directions, respectively, and p is the unit period; the above formula shows that by reasonably designing the phase gradients in the x and y directions, the gradient wave vector _ka of the metasurface can be manipulated arbitrarily. _ka can further arbitrarily control the deflection direction of the reflected beam, that is, the beam pointing; Accordingly, a 1D gradient superunit and a 2D gradient superunit are designed respectively: in a 1D gradient superunit, the phase gradient only exists in the x direction and

is 60°, y direction

is 0°, and the gradient direction is the x direction; in the two-dimensional gradient superunit, the phase gradient exists in the x and y directions at the same time, and

and

Both are 60°, and the gradient direction is

Orientation; by rotating the above one-dimensional and two-dimensional superunits by 90°, 180°, and 270° respectively, another 6 superunits with different gradient directions are generated, and finally 8 superunits are obtained, whose gradient directions are

They are 0°, 45°, 90°, 135°, 180°, 225°, 270° and 315°, respectively, and the corresponding super-units are numbered 1, 2, 3, 4, 5, 6, 7, and 8. Achieve 360° coverage of the gradient direction in a two-dimensional plane; Step 3: Modeling and design of broadband, polarization-insensitive helical encoding RCS reduction metasurfaces: Based on the metasurface element design in the first step and the 8 kinds of superelement designs in the second step, the third step is how to rationally use the 8 kinds of superelements to construct the RCS reduced metasurface; First, determine the size of the metasurface, that is, the number of superunits; here, the metasurface adopts a square layout, that is, the number of superunits L and M in the x and y directions are the same; comprehensively measure the calculation time, sample production cost and the finite size of the metasurface. Influenced by the reduction characteristics of RCS, the number of superunits in the designed metasurface is L×M=8×8, and the size is 288×288mm ² ; Secondly, determine the coding sequence, that is, the arrangement of the 8 super-units: in order to disperse the incident electromagnetic waves to the greatest extent, reduce the scattering intensity of the target at each given angle, and thus reduce the radar detection probability under the double-station RCS detection, the super-surface in the The gradient directions of any adjacent superunits are different; accordingly, a single helical cyclic arrangement from the outside to the inside is designed: 1234567812345678… 567-812-34-56-7-8; in which, "-" represents the turning point of the spiral, and the numbers represent different superunits corresponding to the gradient directions. The metasurface designed in this arrangement ensures that the gradient directions of any adjacent superunits are different. Therefore, the iso-phase surface of the uniform scattering of the metasurface can be destroyed to the greatest extent, and the purpose of dispersing electromagnetic waves to the greatest extent can be achieved; Finally, according to the cyclic arrangement of superunits and single helixes of superunits and through the root-finding algorithm, a helical-encoded metasurface structure was established in CST using VBA macros.

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