An Oblique Incidence Stable Ultra-Wideband Polarization Insensitive Frequency Selective Surface Absorber

Abstract

The present invention discloses an oblique-incidence stable ultra-wideband polarization-insensitive frequency-selective surface absorber, comprising a second lossy layer, a first lossy layer, and a ground layer, arranged sequentially from top to bottom; the first lossy layer and the second lossy layer both comprise a substrate made of an FR-4 board, the graphene coating structure on the upper surface of the first lossy layer comprises a first pattern, and the graphene coating structure on the upper surface of the second lossy layer comprises a second pattern. The symmetrical graphene coating structure enables the present invention to achieve better angular stability and expand the absorption bandwidth; the substrate of the ground layer is an FR-4 board, the upper surface of the ground layer is covered with a layer of PEC material, the PEC material completely covers the upper surface of the ground layer, and the ground layer and the first lossy layer, as well as the first lossy layer and the second lossy layer, are fixedly connected by insulating studs. The present invention further analyzes the function of the proposed structure based on the corresponding equivalent circuit model, verifying the polarization insensitivity and wide-angle absorption stability of the present invention.

Description

Technical Field

[0001] The invention belongs to the field of spatial filters, and in particular relates to a frequency selective surface absorber. Background Art

[0002] Frequency-selective surfaces (FSS), also known as artificial electromagnetic metasurfaces, are periodic electromagnetic modulation structures. Due to their excellent electromagnetic wave modulation capabilities, they have attracted considerable attention in recent years. Compared to traditional materials, FSS designs often offer significant advantages, such as thinner materials, wider bandwidth, and higher efficiency. Consequently, FSS are becoming increasingly competitive in aerospace, antennas, microwaves, optical instruments, and other fields. FSS absorbers play a crucial role in stealth technology. In the stealth field, the traditional term for quantifying radar detectability is radar cross section (RCS), defined as the equivalent area of a target as seen by the radar. Reducing a target's RCS is the most effective method for enhancing stealth. To better achieve stealth, various RCS reduction techniques have been proposed, including the use of radar-absorbing materials, geometric shapes, and polarization-converting metasurfaces. Frequency-selective absorbers, due to their unique characteristics and advantages, have been extensively studied in the stealth field in recent years.

[0003] In existing technologies, most FSS absorbers consist of multilayer structures, with lossy modules typically consisting of resistors, resistive inks, or resistor sheets. These absorbers can be categorized as single-band or multi-band, depending on the resonant properties of the structure. The absorption bandwidth can be increased by employing multiple resonant structures, multilayers, resistors, and fractal modes. Researchers have proposed an ultra-wideband FSS absorber with excellent angular stability, using a metal-graphene hybrid structure. To date, many lumped-element FSS absorbers have been proposed as circuit absorbers, but angular stability remains a key issue that researchers need to improve. Summary of the Invention

[0004] The purpose of the present invention is to provide an ultra-wideband polarization-insensitive frequency selective surface absorber with stable oblique incidence. By designing a method for optimal impedance matching at oblique incidence angles, calculation simulation and optimization of geometric design parameters are used to solve the problem that the absorption performance of traditional absorbers deteriorates as the incident angle increases.

[0005] In order to achieve the above object, the solution of the present invention is:

[0006] An oblique-incidence stable ultra-wideband polarization-insensitive frequency selective surface absorber comprises a second lossy layer, a first lossy layer and a ground layer arranged in sequence from top to bottom; the first lossy layer and the second lossy layer both comprise substrates made of FR-4 boards, the upper surfaces of the two substrates are divided into a plurality of square units of the same size and the square units of the two substrates correspond to each other up and down, each square unit of the first lossy layer is printed with a first pattern using graphene ink, the first pattern being a square with a hollow circle, the center of the hollow circle coinciding with the center of the square unit in which it is located, and the side length of the square of the first pattern being equal to the side length of the square unit. The same; each square unit of the second lossy layer is printed with a second pattern with graphene ink, and the second pattern includes a circular ring, the center of the circular ring coincides with the center of the square unit in which it is located, and the outer periphery of the circular ring extends to the four sides of the square unit in which it is located to form a rectangle, the long side of the rectangle is parallel to the direction in which the center of the circular ring points to the midpoint of the four sides of the square unit, and there is a gap between the outermost short side of the rectangle and the edge of the square unit; the substrate of the grounding layer is an FR-4 board, and the upper surface of the FR-4 board is covered with a layer of PEC material, and the PEC material completely covers the upper surface of the grounding layer.

[0007] The thickness of the FR-4 board is t=0.25 mm, and the dielectric constant is 4.3.

[0008] There are gaps between the ground layer and the first lossy layer, and between the first lossy layer and the second lossy layer. The distance h1 from the lower surface of the ground layer to the lower surface of the first lossy layer is 8 mm, and the distance h2 from the lower surface of the ground layer to the lower surface of the second lossy layer is 12 mm.

[0009] The graphene ink is prepared by the following method: a conductive material, carbon black, a solvent, a thickener, a dispersant, a defoamer, a crosslinker and an adhesive are mixed in a planetary mixer or a deaerator, and then ultrasonically dispersed in an ultrasonic cleaning machine to obtain the graphene ink.

[0010] The conductive material includes carbon black and graphene flakes; the solvent includes deionized water and propylene glycol; the thickener is sodium carboxymethyl cellulose (CMCC); the dispersant is sodium polyacrylate (ACUMER 9300); the defoaming agent is Silok 4600; the crosslinking agent is epoxy silane XR500; and the adhesive is acrylic resin AR.

[0011] The method for printing the graphene ink on the first lossy layer / the second lossy layer to form the first pattern / the second pattern is to print the graphene ink on the upper surface of the substrate of the first lossy layer / the second lossy layer and perform high-temperature curing to form the first pattern / the second pattern.

[0012] The ground layer and the first lossy layer are fixedly connected via insulating studs, and the first lossy layer and the second lossy layer are fixedly connected via insulating studs.

[0013] The square unit size of the first lossy layer and the second lossy layer is a*a. The first lossy layer and the second lossy layer are both square, and the side length is n*a, where n is the number of square units divided in each row / column.

[0014] The value of a mentioned above is 13mm, and n≥1.

[0015] The radius of the hollow circle in the first pattern is 2 mm, the width of the ring in the second pattern is 3.5 mm, and the size of the rectangle is 1.2 mm*1.5 mm.

[0016] The present invention further analyzes the function of the proposed structure based on the corresponding equivalent circuit model. The incident wave causes the generation of surface currents. Due to these surface currents on the metal design, inductive and capacitive effects are induced, which are modeled by corresponding inductance and capacitance, respectively; the dielectric and air layers are replaced by separate transmission lines; the thickness of lossy layer I (first lossy layer) and lossy layer II (second lossy layer) are represented by characteristic impedance, and are characterized by a short transmission line model with a length equivalent to the substrate thickness.

[0017] The absorption mechanism of the absorber is the impedance matching principle. For the proposed structure, the input impedance of the absorber is close to 377Ω within a wide bandwidth, and the imaginary part is close to zero at oblique incidence, which matches the conjugate wave impedance of air. At this time, the electromagnetic energy radiated into the absorber is almost not reflected, and most of it enters the absorber. Using graphene as a loss material, the electromagnetic energy entering the absorber is quickly dissipated, achieving perfect absorption.

[0018] The present invention provides an oblique-incidence stable ultra-wideband polarization-insensitive frequency selective surface absorber, which has the following beneficial effects:

[0019] (1) Without affecting the absorption performance at normal incidence, the absorption stability at large angles of incidence is achieved by designing the best impedance matching effect at oblique incidence angles, performing calculations and simulations, and optimizing the geometric design parameters.

[0020] (2) Graphene ink is used to replace traditional resistors, resistor inks or resistor sheets, achieving better angular stability;

[0021] (3) The multi-layer structure is used to expand the absorption bandwidth; the complex structure based on the ring is used to stimulate multiple mode resonances to expand the bandwidth;

[0022] (4) The upper surfaces of lossy layer I and lossy layer II are both printed with a symmetrical graphene coating structure to meet the requirements of polarization insensitivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a comparison chart of the differences between the traditional structure and the proposed structure;

[0024] Among them, (a) is the best absorption incident angle and good absorption range diagram of the traditional absorber; (b) is the best absorption incident angle and good absorption range diagram of the present invention.

[0025] Figure 2 It is a unit structure diagram of the present invention.

[0026] Figure 3 It is a side view of the unit structure diagram of the present invention and a top view of each layer structure;

[0027] Among them, (a) is a side view of the unit structure of the present invention; (b) is a top view of the metal bottom layer of the present invention; (c) is a top view of the lossy layer I of the present invention; and (d) is a top view of the lossy layer II of the present invention.

[0028] Figure 4 It is an equivalent circuit model diagram of the present invention.

[0029] Figure 5 It is a graph of the ECM results of ADS and the full-wave results of CST.

[0030] Figure 6 This is a curve diagram of impedance variation at 0° and 45° of the present invention;

[0031] Among them, (a) is the curve diagram of the change of the real part of the impedance; (b) is the curve diagram of the change of the imaginary part of the impedance.

[0032] Figure 7 is the reflection coefficient curve of ECM and CST at 0° and 45°.

[0033] Figure 8 is a graph of reflection coefficient at different incident angles;

[0034] Among them, (a) is the curve diagram of TM polarization; (b) is the curve diagram of TE polarization.

[0035] Figure 9 It is the absorption-absorption rate result curve graph of the present invention.

[0036] Figure 10 It is a reflection performance curve diagram of the present invention at different polarization angles. DETAILED DESCRIPTION

[0037] The technical solutions and beneficial effects of the present invention will be described in detail below with reference to the accompanying drawings.

[0038] like Figure 1As shown in the figure, the traditional absorber has the best absorption performance at normal incident angles, and the absorption performance deteriorates when the incident angle increases. To solve this problem, the present invention proposes an oblique-incident stable ultra-wideband polarization-insensitive frequency selective surface absorber, which has the best absorption performance at oblique incidence. Compared with traditional absorbers, the absorber proposed in the present invention has a larger good absorption range.

[0039] like Figure 2 and Figure 3 As shown in FIG, a lossy FSS based on graphene ink is printed on a FR-4 substrate with lossy layer I and lossy layer II to form a symmetrical graphene coating structure. The thickness of the substrate is 0.25 mm and the dielectric constant is 4.3. The purpose of using a two-layer lossy structure is to obtain a larger bandwidth. Both lossy layer I and lossy layer II have an absorption band. Combining the two lossy layers results in an ultra-wide absorption band. The symmetrical graphene coating structure makes the absorber insensitive to different incident polarizations. To ensure zero transmission, the upper surface of the ground layer is covered with a PEC material, as shown in FIG. Figure 3 As shown in (b) of Figure 1, the PEC material completely covers the upper surface of the ground layer. After computational simulation, the optimized geometric design parameters are shown in Table 1, where h1 represents the distance from the bottom of the ground layer to the lower surface of lossy layer I, and h2 represents the distance from the bottom of the base plate to the lower surface of lossy layer II.

[0040] Table 1

[0041] parameter a <![CDATA[h1]]> <![CDATA[h2]]> t <![CDATA[r1]]> <![CDATA[r2]]> <![CDATA[r3]]> b c Value (mm) 13 8 12 0.25 2 1 4.5 1.2 12

[0042] The graphene coating structures formed on the lossy layer I and the lossy layer II are different. The graphene coating structure on the lossy layer I is composed of n 2 The same first patterns are arranged in the form of n rows and n columns, such as Figure 3 As shown in (c), the first pattern is a square with a hollow circle, the center of the hollow circle coincides with the center of the square unit where it is located, and the side length of the square of the first pattern is the same as the side length of the square unit; the graphene coating structure on the lossy layer II is composed of n 2 The same second patterns are arranged in the form of n rows and n columns, such as Figure 3 As shown in (d) in the figure, the second pattern includes a circular ring, the center of the circular ring coincides with the center of the square unit in which it is located, and the outer periphery of the circular ring extends to the four sides of the square unit in which it is located to form a rectangle, the long sides of the rectangle are parallel to the direction from the center of the circular ring to the midpoints of the four sides of the square unit, and there is a gap between the outermost short side of the rectangle and the edge of the square unit.

[0043] The graphene ink used in the present invention is a prior art and is composed of a conductive material, carbon black, a solvent, a thickener, a dispersant, a defoaming agent, a crosslinking agent and an adhesive. The conductive material includes carbon black and graphene flakes; the solvent includes deionized water and propylene glycol; the thickener is sodium carboxymethyl cellulose (CMCC); the dispersant is sodium polyacrylate (ACUMER 9300); the defoaming agent is Silok 4600; the crosslinking agent is epoxy silane XR500; and the adhesive is acrylic resin AR. The specific values of the materials in this embodiment are shown in Table 2:

[0044] Table 2

[0045]

[0046] To ensure uniform dispersion of carbon black and graphene flakes, they were mixed in a planetary mixer or a deaerator (the machine model used in this embodiment was a MAZERUSTAR KK300SSE manufactured by KURABO, Osaka, Japan), and then ultrasonically dispersed in an ultrasonic cleaning machine to obtain graphene ink. The graphene ink was then printed on an FR-4 board with lossy layer I and lossy layer II, and cured at high temperature to obtain a graphene coating structure. The composite dielectric function of the graphene coating structure was evaluated using its electrical conductivity, which depends on its chemical potential. Its surface conductivity is obtained by Kubo's formula:

[0047]

[0048] Where e is the electron charge, ω is the angular frequency, k B is the Boltzmann constant, To simplify the Planck constant, T is fixed at 300K, μ c is the Fermi level, τ is the scattering rate, and j is the sign of the imaginary number.

[0049] like Figure 4 As shown in Figure 2, the function of the proposed structure is further analyzed based on the corresponding equivalent circuit model (ECM). The incident wave leads to the generation of surface flow. Due to these surface currents on the metal design, inductive and capacitive effects are induced and simulated by corresponding inductors and capacitors respectively; the dielectric layer and the air layer can be replaced by separate transmission lines; the thickness of the lossy layer I and lossy layer II can be represented by the characteristic impedance of The short transmission line model is characterized by the length being equivalent to the substrate thickness, where Z0 = 377Ω represents the free space wave impedance, and ε t is the relative permittivity of the dielectric material.

[0050] In order to better predict the frequency response of the proposed FSS through ECM, Figure 4The capacitors (C1, C2, and C3) and inductors (L1, L2, L3, and L4) in the circuit are approximated by the following equations:

[0051]

[0052]

[0053] Where μ0 and ε0 are the vacuum permeability and vacuum permittivity, respectively. t and ε t represents the magnetic permeability and dielectric constant of the dielectric material, a is the period of the unit structure, l and d are the length and width of the graphene structure, respectively.

[0054] After fitting optimization, the final circuit parameter values are shown in Table 3.

[0055] Table 3

[0056]

[0057] The present invention designs the best impedance matching effect when the incident angle is 45°. It can be seen from the calculation formulas of L and C that when the incident angle increases along the +x axis, the increased incident angle mainly affects the value of d and the period along the x axis. Theoretically, when the incident angle is 45°, a and d in the original formula change to and So by and Substituting the L and C calculation formulas into the equations allows us to estimate the changes in L and C, resulting in more accurate values at 45°. The final values of capacitance and inductance vary within a very small range, requiring further minor adjustments. Finally, increasing the angle of incidence primarily affects the capacitance, with changes to C1 = 0.171pF, C2 = 0.0318pF, and C3 = 1.5pF.

[0058] The present invention uses Keysight Advanced Design System (ADS) software and CST Studio Suite (CST) software to simulate the ECM and unit geometry of the FSS absorber, and the simulated reflection coefficient of the lossy layer is as follows: Figure 5 As shown in Figure 2, at normal angles of incidence, the reflection band below -10 dB is approximately 3 GHz to 19 GHz. Both circuit simulation and full-wave simulation show very similar results. Furthermore, the proposed FSS absorber geometry is axially symmetric, thus exhibiting polarization insensitivity.

[0059] The present invention also extracts the input impedance of the absorber through the simulated parameters and quantitatively explains the working mechanism of the absorber. The real and imaginary parts of the 0° and 45° impedances are as follows: Figure 6As shown in Figure 2, it is proved that the impedance of 45° is indeed better than that of 0°. Among them, (a) shows that in the range of about 4GHz to 26GHz, the real part of the 45° absorber impedance is closer to 377Ω than the real part of the 0° absorber impedance. At around 6GHz and 15GHz for 0° and around 22GHz for 45°, the real part is very close to 377Ω and the imaginary part is close to 0. Therefore, the impedance matching is better, which is consistent with the Figure 6 The frequency point where the imaginary part is close to zero and the frequency point where the real part is close to 377Ω are both moved to the high frequency band, which means that the absorption bandwidth is larger, which is consistent with Figure 7 The results correspond to Figure 7 It is clearly shown that the absorption band extends from 18 GHz to 26 GHz.

[0060] The present invention also studies the design performance at different oblique incident angles to determine whether the absorber is suitable for applications requiring stable performance at large oblique angles. Figure 8 Figure 2 shows the simulation results of the TE reflection coefficient of the absorber under oblique incidence, where (a) clearly demonstrates that the -10 dB reflection band expands from approximately 18 GHz to 26 GHz with increasing incident angle. (b) shows that when the incident angle increases to 40°, no significant degradation of the TM polarization reflection coefficient is observed near the operating frequency. Overall, the absorption bandwidth changes little at low frequencies and is significantly optimized at high frequencies. Therefore, the absorber exhibits good absorption stability around 50°.

[0061] In order to more clearly show the absorption performance of the structure, the absorption-absorption rate results are shown in Figure 9 As shown in the figure, it can be seen that when the incident angle increases to 50°, the absorbance in the working band can be maintained above 90%, indicating that the structure has good absorption stability.

[0062] Figure 10 The reflection coefficient at different polarization angles is shown, proving that the structure is polarization insensitive, with the polarization angle rotating from 0° to 90°. The results show that the absorber has polarization insensitive performance.

[0063] In summary, the present invention provides an oblique-incidence stable ultra-wideband polarization-insensitive frequency-selective surface absorber, comprising a second lossy layer, a first lossy layer, and a ground layer, arranged sequentially from top to bottom; the first lossy layer and the second lossy layer each comprise a substrate made of an FR-4 board, the upper surfaces of both substrates being printed with a symmetrical graphene coating structure, the graphene coating structure on the upper surface of the first lossy layer comprising a first pattern, and the graphene coating structure on the upper surface of the second lossy layer comprising a second pattern. The symmetrical graphene coating structure enables the present invention to achieve better angular stability and expand the absorption bandwidth; the substrate of the ground layer is an FR-4 board, the upper surface of the ground layer being covered with a layer of PEC material, the PEC material completely covering the upper surface of the ground layer, and the ground layer and the first lossy layer, as well as the first lossy layer and the second lossy layer, are fixedly connected by insulating studs. The present invention further analyzes the function of the proposed structure based on the corresponding equivalent circuit model, verifying the polarization insensitivity and wide-angle absorption stability of the present invention.

[0064] The above embodiments are only for illustrating the technical idea of the present invention and cannot be used to limit the protection scope of the present invention. Any changes made on the basis of the technical solution in accordance with the technical idea proposed by the present invention shall fall within the protection scope of the present invention.

Claims

1. An oblique-incidence stable ultra-wideband polarization-insensitive frequency selective surface absorber, characterized by: The invention comprises a second lossy layer, a first lossy layer and a ground layer, which are arranged in sequence from top to bottom; the first lossy layer and the second lossy layer both comprise a substrate made of an FR-4 board, the upper surfaces of the two substrates are divided into a plurality of square units of the same size, and the square units of the two substrates correspond to each other from top to bottom; each square unit of the first lossy layer is printed with a first pattern using graphene ink, wherein the first pattern is a square with a hollow circle, the center of the hollow circle coincides with the center of the square unit in which it is located, and the side length of the square of the first pattern is the same as the side length of the square unit; each square unit of the second lossy layer is printed with a second pattern using graphene ink, wherein the second pattern comprises a circular ring, the center of the circular ring coincides with the center of the square unit in which it is located, and the outer periphery of the circular ring extends to the four sides of the square unit in which it is located to form a rectangle, the long side of the rectangle is parallel to the direction in which the center of the circular ring points to the midpoints of the four sides of the square unit, and a gap is formed between the outermost short side of the rectangle and the edge of the square unit; the substrate of the ground layer is an FR-4 board, and the upper surface of the FR-4 board is covered with a layer of PEC material.

2. The frequency selective surface absorber according to claim 1, wherein: The thickness of the FR-4 board is t=0.25 mm, and the dielectric constant is 4.

3.

3. The frequency selective surface absorber according to claim 1, wherein: There are gaps between the ground layer and the first lossy layer, and between the first lossy layer and the second lossy layer. The distance h1 from the lower surface of the ground layer to the lower surface of the first lossy layer is 8 mm, and the distance h2 from the lower surface of the ground layer to the lower surface of the second lossy layer is 12 mm.

4. The frequency selective surface absorber according to claim 1, wherein: The graphene ink is prepared by the following method: a conductive material, carbon black, a solvent, a thickener, a dispersant, a defoamer, a crosslinker and an adhesive are mixed in a planetary mixer or a deaerator, and then ultrasonically dispersed in an ultrasonic cleaning machine to obtain the graphene ink.

5. The frequency selective surface absorber according to claim 4, wherein: The conductive material includes carbon black and graphene flakes; the solvent includes deionized water and propylene glycol; the thickener is sodium carboxymethyl cellulose (CMCC); the dispersant is sodium polyacrylate (ACUMER 9300); the defoaming agent is Silok 4600; the cross-linking agent is epoxy silane XR500; and the adhesive is acrylic resin AR.

6. The frequency selective surface absorber according to claim 1, wherein: The method of printing the graphene ink on the first lossy layer / the second lossy layer to form the first pattern / the second pattern is to print the graphene ink on the upper surface of the substrate of the first lossy layer / the second lossy layer and perform high-temperature curing to form the first pattern / the second pattern.

7. The frequency selective surface absorber according to claim 1, wherein: The ground layer is fixedly connected to the first lossy layer via an insulating stud, and the first lossy layer is fixedly connected to the second lossy layer via an insulating stud.

8. The frequency selective surface absorber according to claim 1, wherein: The square unit size of the first lossy layer and the second lossy layer is a*a. The first lossy layer and the second lossy layer are both square, and the side length is n*a, where n is the number of square units divided in each row / column.

9. The frequency selective surface absorber according to claim 8, wherein: The value of a is 13mm, and n≥1.

10. The frequency selective surface absorber according to claim 9, wherein: The radius of the hollow circle in the first pattern is 2 mm, the width of the ring in the second pattern is 3.5 mm, and the size of the rectangle is 1.2 mm*1.5 mm.

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