An ultra-thin low-frequency absorber based on electromagnetic metamaterials

Abstract

The present invention discloses an ultrathin low-frequency absorber based on electromagnetic metamaterials, comprising a periodic array of unit structures. The unit structures comprise, from top to bottom, a metal resonant layer, a first dielectric layer, a metal patch layer, a second dielectric layer, and a metal ground layer. The metal resonant layer and metal patch layer are etched on either side of the first dielectric layer, and the metal ground layer is adhered to the underside of the second dielectric layer. The two components are stacked to form the present invention. The absorber of the present invention achieves over 90% absorption at 0.3 GHz, has a thickness of 1 / 1072 of the incident wavelength, and exhibits polarization insensitivity and wide-angle stability. This low-frequency absorption characteristic stems from the strong capacitive effect between the metal resonant layer and the metal patch layer. Absorption can be achieved in a wider range of frequency bands by varying the narrow slit amplitude and patch size. The absorber of the present invention achieves low-frequency absorption with an ultrathin structure, is simple in design, and is easy to process. It has high practical value in fields such as military stealth, long-distance communications, and radar shielding.

Description

Technical Field

[0001] The present invention relates to a microwave absorber, and in particular to an ultra-thin low-frequency absorber based on electromagnetic metamaterials, which can be used in the fields of military stealth, long-distance communication, radar shielding, etc. Background Art

[0002] Electromagnetic absorption technology not only improves the stealth and electromagnetic compatibility of specific targets but also suppresses electromagnetic radiation pollution, finding widespread application in both military and civilian fields. Metamaterial microwave absorbers, which enable efficient absorption of electromagnetic waves, are currently a frontier and hot topic in absorber design.

[0003] There are four main indicators for judging the quality of microwave absorbers: "thin, light, wide, and strong." "Thin" refers to the absorber's small relative thickness, where τ is the absorber's thickness and τ is the wavelength of the electromagnetic wave; "light" refers to the absorber's low mass; "strong" refers to the microwave absorber's strong reflection loss of incident electromagnetic waves. Generally, we believe that if it can absorb 90% of the incident wave energy, that is, the reflection loss reaches τ, its absorption strength is relatively excellent; "wide" refers to the wide frequency range over which the material's reflection loss reaches τ. Thinning the absorber can significantly reduce the weight of the entire system, which is especially important for applications with strict weight requirements such as aerospace, satellites, drones, and military equipment. Especially in high-tech fields such as aerospace, military, and electronic communications, thin and light microwave absorbers will become an important factor in improving equipment performance and reliability.

[0004] Low-frequency microwave absorbers have long been a focus of research. They typically operate in the lower microwave frequency range, typically between a few hundred megahertz (MHz) and several gigahertz (GHz). These absorbers are primarily used to absorb relatively low-frequency microwave signals. Due to the longer wavelengths of low-frequency microwaves, absorbers typically require larger dimensions and greater thickness to effectively absorb incident electromagnetic waves. Low-frequency electromagnetic waves have strong penetrating power, and reducing or shielding unwanted low-frequency waves is a core concern in the microwave industry. Therefore, low-frequency microwave absorbers play a vital role in radar systems, communications systems, and electromagnetic interference protection.

[0005] Lawrence of the University of Exeter was one of the first researchers to focus on the concept of ultrathin structure absorbers. In 2009, he reported a polarization-independent thin-structure microwave absorber with a thickness of only 1 / 130 of the operating wavelength and a minimum absorption frequency of 20 GHz. Although this absorber achieved multi-band performance, its absorption effect was poor. In 2019, Professor Huang Weiqing of Hunan University reported a multi-band ultrathin ideal metamaterial absorber based on a split-ring resonator-like structure, which obtained three discrete absorption peaks at 1.8 GHz, 3 GHz, and 4.1 GHz. The ratios of the dielectric sheet thickness to the wavelengths of the three absorption peaks were 1 / 100, 1 / 59, and 1 / 43, respectively, all smaller than the three-band absorber devices at the time. In 2020, Jin Yi's team at Zhejiang University achieved a new breakthrough in ultrathin structure absorbers, reporting a metamaterial absorber with a thickness of only 1 / 474 of the operating wavelength and an absorption frequency of 1.5 GHz. However, the only drawback was that this absorber did not achieve polarization independence. In 2022, Kim achieved a new breakthrough in absorber thickness, reporting absorption at 1.5 GHz with a relative thickness of only 1 / 1250 of the operating wavelength. Although the above reports have made breakthroughs in thickness, low-frequency absorption below 1 GHz remains a challenge. Bui Xuan Khuyen of Hanyang University, concerned about the difficulties of low-frequency absorption, reported an ultra-low-frequency multi-band absorber in 2017. This absorber achieved absorption at 0.3 GHz and 0.36 GHz, but the thickness was relatively large, only 1 / 378 and 1 / 320 of the operating wavelength, and did not achieve an ultra-thin design.

[0006] In summary, ultrathin absorbers and ultra-low-frequency absorbers have attracted considerable research attention in recent years. However, reports on high-performance absorbers that combine low frequency and ultra-thin structure are rare, especially for ultra-thin absorbers below 1 GHz. Because low-frequency electromagnetic waves have longer wavelengths, absorbers must be thick enough to effectively absorb these long-wavelength electromagnetic waves. Low-frequency waves are more easily reflected, and a sufficiently thick absorption layer can eliminate reflections, increase energy absorption, and minimize the effects of the transmission layer. Developing an absorber that achieves both low-frequency absorption and an ultra-thin structure is both a hot topic and a challenge in current absorber research. Summary of the Invention

[0007] Purpose of the invention: In order to break through the traditional thickness of metamaterial absorbers and overcome the difficulty of microwave absorbers in absorbing microwaves at low frequencies, the present invention provides a design of an ultra-thin low-frequency absorber based on electromagnetic metamaterials. This absorber has the advantages of ultra-thin structure, low-frequency absorption, polarization insensitivity, and stability at large angles of incidence. In addition, it has a simple design, small size, and easy processing. It has high practical value in the fields of military stealth, long-distance communication, and radar shielding.

[0008] To achieve the above purpose, the technical solution adopted by the present invention is:

[0009] An ultra-thin structure low-frequency absorber based on electromagnetic metamaterials is characterized by comprising: more than one absorber unit structure arranged in a spatial periodic square, wherein the absorber module comprises, from top to bottom, a metal resonance layer (1), a first dielectric layer (2), a metal patch layer (3), a second dielectric layer (4), and a metal grounding layer (5), and the geometric centers of the layers coincide vertically; the metal resonance layer is a metal surface formed by engraving two orthogonal curved narrow slits, the curved narrow slits extending from the center of the unit structure to the midpoints of the four sides of the unit structure, the curved narrow slits between adjacent unit structures are interconnected, and the slits have rotational symmetry; the metal patch layer is a true circular metal surface, and the center of the circle coincides vertically with the intersection of the two orthogonal curved narrow slits; the metal resonance layer and the metal patch layer are respectively etched on both sides of the first dielectric layer; the metal grounding layer is attached to one side of the second dielectric layer, and the first dielectric layer with the etched structure on both sides and the second dielectric layer with the metal surface attached on one side together constitute an ultra-thin structure low-frequency absorber based on electromagnetic metamaterials.

[0010] Preferably, the metal resonance layer, metal patch layer, and metal ground layer are made of copper, with a conductivity of 5.8×10 7 S / m, thickness h=0.01mm.

[0011] Preferably, the unit structure period p=8.5 mm.

[0012] Preferably: the narrow slit is a sine function Description, where A is the amplitude, is the initial phase, ω = 2π / p, and the two narrow slits are orthogonal to each other.

[0013] Preferably, the narrow gaps between the adjacent unit structures are interconnected.

[0014] Preferably, the first dielectric layer is made of a polyimide film with a relative dielectric constant ε=4, a loss tangent tan(δ)=0.005, and a thickness d1=0.002 mm.

[0015] Preferably, the metal patch layer is a true circular metal surface with a circular radius r=4.1 mm.

[0016] Preferably, the second dielectric layer is made of polytetrafluoroethylene plate with a relative dielectric constant ε=2.2, a loss tangent tan(δ)=0.001, and a thickness d2=0.9 mm.

[0017] Preferably, the metal resonant pattern is processed by a chemical etching process on the upper side of the first dielectric layer, and the metal patch pattern is processed by a chemical etching process on the lower side.

[0018] Preferably, a metal copper film is applied to the lower side of the second dielectric layer as a metal grounding layer.

[0019] The present invention provides an ultra-thin low-frequency absorber based on electromagnetic metamaterials, which has the following advantages over the prior art:

[0020] 1. The ultra-thin low-frequency absorber based on electromagnetic metamaterials of the present invention achieves perfect low-frequency absorption at 0.3 GHz with an absorption rate of more than 90%.

[0021] 2. The ultra-thin low-frequency absorber based on electromagnetic metamaterials of the present invention has ultra-thin characteristics and a total structural thickness of only 0.932 mm, which is only 1 / 1072 of the wavelength of the absorption frequency. The relative thickness of the absorber structure reaches the leading level of currently reported work.

[0022] 3. The ultra-thin low-frequency absorber based on electromagnetic metamaterials of the present invention has excellent properties such as low-frequency absorption, polarization insensitivity, polarization stability, and stability at large angles of incidence. It plays an important role in military stealth, long-distance communications, and radar shielding.

[0023] 3. The ultra-thin low-frequency absorber based on electromagnetic metamaterials of the present invention uses polytetrafluoroethylene plates and polyimide films as dielectric layers. The metamaterial absorber can be bent and folded to a certain extent, and has better adaptability to the covering environment. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a schematic diagram of the wave absorbing principle of an ultra-thin low-frequency absorber based on electromagnetic metamaterials according to the present invention;

[0025] Figure 2 This is a schematic diagram of the layered structure of an ultra-thin low-frequency absorber based on electromagnetic metamaterials according to the present invention;

[0026] Figure 3 This is a schematic diagram of the metal resonant layer structure of an ultra-thin low-frequency absorber based on electromagnetic metamaterials according to the present invention;

[0027] Figure 4 This is a schematic diagram of the metal patch layer structure of an ultra-thin low-frequency absorber based on electromagnetic metamaterials according to the present invention;

[0028] Figure 5 This is a side view of an ultra-thin low-frequency absorber based on electromagnetic metamaterials according to the present invention;

[0029] Figure 6 This is a 3D schematic diagram of an ultra-thin low-frequency absorber based on electromagnetic metamaterials according to the present invention;

[0030] Figure 7This is an absorption spectrum diagram of an ultra-thin structure low-frequency absorber based on electromagnetic metamaterials according to the present invention;

[0031] Figure 8 Absorption spectra at different polarization angles of an ultra-thin low-frequency absorber based on electromagnetic metamaterials according to the present invention;

[0032] Figure 9 This is the absorption spectrum of the ultra-thin low-frequency absorber based on electromagnetic metamaterials under TE mode at different incident angles according to the present invention;

[0033] Figure 10 This is the absorption spectrum of the ultra-thin structure low-frequency absorber based on electromagnetic metamaterials at different incident angles in the TM mode described in the present invention. DETAILED DESCRIPTION

[0034] The present invention is further illustrated below with reference to the accompanying drawings and specific embodiments. It should be understood that these examples are only used to illustrate the present invention and are not used to limit the scope of the present invention. After reading the present invention, modifications of various equivalent forms of the present invention made by those skilled in the art all fall within the scope defined by the claims attached to this application.

[0035] like Figure 1 As shown, the present invention provides an ultra-thin structure low-frequency absorber based on electromagnetic metamaterials, comprising more than one absorber unit structure arranged in a square with a spatial period, the absorber module, such as Figure 2 As shown, from top to bottom are the metal resonance layer (1), the first dielectric layer (2), the metal patch layer (3), the second dielectric layer (4), and the metal ground layer (5), and the geometric centers of each layer coincide vertically; Figure 3 As shown, the metal resonance layer is a metal surface formed by two orthogonal curved narrow slits, the curved narrow slits extending from the center of the unit structure to the midpoints of the four sides of the unit structure, the curved narrow slits between adjacent unit structures are interconnected, and the narrow slits have rotational symmetry; Figure 4 As shown, the metal patch layer is a perfect circular metal surface, and the center of the circle coincides with the intersection of the two orthogonal curved narrow slits in the vertical direction; the metal resonance layer and the metal patch layer are etched on both sides of the first dielectric layer respectively; the metal ground layer is attached to one side of the second dielectric layer, as shown in FIG. Figure 5 As shown, the first dielectric layer with etched structures on both sides and the second dielectric layer with a metal surface on one side together constitute an ultra-thin structure low-frequency absorber based on electromagnetic metamaterials. Figure 6 This is a 3D schematic diagram of the absorber.

[0036] The metal resonance layer, metal patch layer, and metal ground layer are made of copper, with a conductivity of 5.8×10 7 S / m, thickness h=0.01mm.

[0037] The unit structure period p=8.5 mm.

[0038] The narrow slit is a sine function Description, where A is the amplitude, is the initial phase, ω = 2π / p, and the two narrow slits are orthogonal to each other.

[0039] The narrow gaps between the adjacent unit structures are interconnected.

[0040] The first dielectric layer is made of a polyimide film with a relative dielectric constant ε=4, a loss tangent tan(δ)=0.005, and a thickness d1=0.002 mm.

[0041] The metal patch layer is a perfect circular metal surface with a circular radius r=4.1 mm.

[0042] The second dielectric layer is made of polytetrafluoroethylene plate with a relative dielectric constant ε=2.2, a loss tangent tan(δ)=0.001, and a thickness d2=0.9 mm.

[0043] The metal resonant pattern is processed by a chemical etching process on the upper side of the first dielectric layer, and the metal patch pattern is processed by a chemical etching process on the lower side.

[0044] The lower side of the second dielectric layer is covered with a metal copper film as a metal grounding layer.

[0045] Because metamaterial absorbers utilize subwavelength structures, the larger the unit structure, the lower the absorption frequency. Therefore, different processing methods are required for metamaterial absorbers within different absorption frequency ranges. Microwave metamaterial absorber unit structures are typically manufactured using printed circuit board technology, as the unit structure dimensions are measured in millimeters. In this invention, polytetrafluoroethylene (PTFE) sheets and polyimide films are used as dielectric layers, and copper is used as the metal layer. A metal resonant structure and a metal patch structure are obtained by chemically etching both sides of a double-sided copper-clad polyimide film. A metal film is then applied to one side of the PTFE dielectric layer to serve as a metal grounding layer.

[0046] The preparation process of the ultra-thin structure low-frequency absorber based on electromagnetic metamaterials described in this embodiment is as follows:

[0047] (1) Ingredients: Through ingredient preparation, polyimide film and polytetrafluoroethylene plate with electromagnetic parameters close to those of the materials used in simulation are obtained.

[0048] (2) Impregnation: The prepared polyimide film and polytetrafluoroethylene plate are placed in an organic impregnation solution with a vacuum pressure of 0.05 MPa, taken out, and kept in a preheated infrared drying tunnel for 40 to 60 minutes.

[0049] (3) Assembly: Use a cardboard cutting machine and a copper foil cutting machine to cut the polyimide film, polytetrafluoroethylene plate and copper foil into squares of sufficient size.

[0050] (4) Hot pressing: Place the copper plate and glass fiber plate into a hot press for pressing, set the time to 35 minutes and the temperature to 240°C.

[0051] (5) Polishing: To make the copper-clad dielectric board smoother, use fine water sandpaper to polish it and clean it.

[0052] (6) Rubbing: Rubbing the PCB sketch exported from CAD onto the cleaned metal surface.

[0053] (7) Pattern: Use tape to cover the metal surface and remove the tape except for the pattern on the copper foil surface.

[0054] (8) Corrosion: The corrosion liquid is a ferric chloride aqueous solution with a concentration range of 30% to 40%. Select an appropriate temperature (below 50°C) and gently brush it with a brush or clamp the circuit board with a wooden stick and shake it slightly to speed up the corrosion. After the corrosion is completed, rinse with clean water.

[0055] (9) Peeling off the film: Peel off the tape attached to the metal surface.

[0056] (10) Cleaning: Use a dilute acetone solution as the cleaning liquid, dip a cotton ball into it and gently wipe off the protective paint on the surface to obtain the corresponding product.

[0057] The following is a brief introduction to the electromagnetic simulation calculation results of the present invention:

[0058] The present invention uses electromagnetic simulation software for calculations, adopts finite integration technology to divide the structure into grids, and applies Maxwell's equations and boundary conditions to each grid for electromagnetic simulation calculations. A plane electromagnetic wave is used as the incident wave source for this simulation, with the incident wave vector k, incident along the positive z-axis; E is the electric field vector and is parallel to the x-direction, and H is the magnetic field vector and is parallel to the y-direction. In the x and y directions, the boundary is set to a unit cell period, and in the z direction, the boundary is set to an open boundary Open (add space). In this way, the entire boundary appears as a Floquet periodic boundary, which can simulate the absorption effect of the absorbing unit after it is expanded into a periodic array.

[0059] The absorption performance of the ultra-thin low-frequency absorber based on electromagnetic metamaterials described in the present invention can be expressed by the absorptivity A(ω), which can be derived from the reflectivity R(ω) and the transmittance T(ω). The derivation process is as follows: the sum of the absorptivity, reflectivity, and transmittance is 1, that is, A(ω)+R(ω)+T(ω)=1, from which A(ω)=1-R(ω)-T(ω) can be obtained, and R(ω) and T(ω) can be expressed by scattering parameters (S parameters), R(ω)=|S 11 | 2 , T(ω)=|S 21 | 2 Since the thickness of the metal layer covering the bottom is greater than the skin depth of the copper film for the incident electromagnetic wave, the electromagnetic wave cannot be transmitted outward through the metal bottom plate, so T(ω) can be considered to be 0. Therefore, the absorption rate A(ω) can be simplified as follows: A(ω)=1-R(ω)=1-|S 11 | 2 , which is the expression for the absorptivity.

[0060] The absorption spectrum of the ultra-thin structure low-frequency absorber based on electromagnetic metamaterials of the present invention is as follows: Figure 7 As shown, the absorber can have an absorption peak with high efficiency absorption in the 0.3 GHz band, with an absorption rate of more than 90%. The total thickness of the absorber is 0.932 mm, and the working wavelength at the absorption frequency is 1000 mm. It can be concluded that the overall thickness of the absorber is 1 / 1072 of the incident wavelength.

[0061] In practical applications, electromagnetic waves often hit the surface of electromagnetic metamaterial absorbers at a certain angle. Therefore, it is necessary to study the absorption of electromagnetic waves at different incident angles by the absorber. is the angle between the x-axis and the xoy plane, and the incident angle θ is defined as the angle between the position vector of the incident point source and the z-axis.

[0062] First, we explored the polarization angle The electromagnetic absorber of the present invention has C4 symmetry, so it is only necessary to explore the angle change within the range of 1 / 4 period. While keeping the above settings unchanged, set θ to 0° and The variation range is set to 0°~90°. Figure 8 As shown, when a plane wave is vertically incident on the absorber, the polarization angle varies in the range of 0° to 90°, and the absorption spectrum shows good polarization angle stability at the absorption frequency. It can be concluded that the absorber of the present invention has the property of polarization insensitivity.

[0063] Secondly, we explored the corresponding absorption spectra when the electromagnetic wave is incident obliquely on the absorber surface in TE mode and TM mode. The so-called TE mode here refers to the electric field E of the incident wave always remaining parallel to the absorber surface; while the TM mode refers to the magnetic field H of the incident wave always remaining parallel to the absorber surface. In the electromagnetic simulation software, the mode excitation was set to TE mode and TM mode respectively, and two numerical simulations were performed respectively. During the simulation, the polarization angle was set to 0. Set it to 0°, and let the incident angle θ vary between 0° and 80°. The corresponding absorption rates are as follows: Figure 9 、 Figure 10 As shown in the figure: In the TE mode, the absorber can maintain high-efficiency absorption at an incident angle of 0° to 45°, and the absorption rate decreases gently as the incident angle increases further; in the TM mode, the absorber always maintains high-efficiency absorption at an incident angle of 0° to 80°, but the absorption peak shifts to a certain extent as the angle increases. This is because the spacing between adjacent absorption units is too close, the coupling between units is extremely strong, and it is sensitive to changes in the angle of oblique incidence, resulting in poor stability of the absorbed electromagnetic waves and thus a shift.

[0064] The ultrathin low-frequency absorber based on electromagnetic metamaterials described in this invention overcomes the traditional thickness limitations of microwave absorbers, achieving a leading relative thickness. This overcomes the difficulty of microwave absorbers absorbing microwaves at low frequencies, resulting in a highly efficient absorber that combines ultrathin structure with low-frequency absorption. Furthermore, the invention offers advantages such as polarization insensitivity and wide-angle stability. Its simple design, compact size, and ease of fabrication make it highly practical in applications such as military stealth, long-distance communications, and radar shielding.

[0065] The above specifically describes the preferred embodiments of the present invention. Of course, the present invention can also adopt forms different from the above embodiments. Equivalent transformations or corresponding modifications made by technicians familiar with the field without violating the spirit of the present invention should fall within the scope of protection of the present invention.

Claims

1. An ultra-thin low-frequency absorber based on electromagnetic metamaterials, characterized by: The absorber unit structure is arranged in a spatial periodic square, wherein the absorber unit structure is sequentially composed of a metal resonance layer (1), a first dielectric layer (2), a metal patch layer (3), a second dielectric layer (4), and a metal ground layer (5) from top to bottom, and the geometric centers of the layers coincide in the vertical direction; the metal resonance layer is a metal surface formed by engraving two orthogonal curved narrow slits, and the narrow slits are formed by a sine function. Description, where A is the amplitude, is the initial phase, ω=2π / p, and p is the unit structure period; the curved narrow slit extends from the center of the unit structure to the midpoints of the four sides of the unit structure, the curved narrow slits between adjacent unit structures are interconnected, and the narrow slit has rotational symmetry; the metal patch layer is a perfect circular metal surface, and the center of the circle coincides with the intersection of the two orthogonal curved narrow slits in the vertical direction; the metal resonant layer and the metal patch layer are respectively etched on both sides of the first dielectric layer; the metal grounding layer is attached to one side of the second dielectric layer, and the first dielectric layer with the etched structure on both sides and the second dielectric layer with the metal surface attached on one side together constitute an ultra-thin structure low-frequency absorber based on electromagnetic metamaterials.

2. The ultra-thin low-frequency absorber based on electromagnetic metamaterial according to claim 1, characterized in that: The metal resonance layer, metal patch layer, and metal ground layer are made of copper, with a conductivity of 5.8×10 7 S / m, thickness h=0.01mm.

3. The ultra-thin low-frequency absorber based on electromagnetic metamaterial according to claim 1, characterized in that: The unit structure period p=8.5 mm.

4. The ultra-thin low-frequency absorber based on electromagnetic metamaterial according to claim 1, characterized in that: The first dielectric layer is made of a polyimide film with a relative dielectric constant ε=4, a loss tangent tan(δ)=0.005, and a thickness d1=0.002 mm.

5. The ultra-thin low-frequency absorber based on electromagnetic metamaterial according to claim 1, characterized in that: The metal patch layer is a perfect circular metal surface with a circular radius r=4.1 mm.

6. The ultra-thin low-frequency absorber based on electromagnetic metamaterial according to claim 1, characterized in that: The second dielectric layer is made of polytetrafluoroethylene plate with a relative dielectric constant ε=2.2, a loss tangent tan(δ)=0.001, and a thickness d2=0.9 mm.

7. The ultra-thin low-frequency absorber based on electromagnetic metamaterial according to claim 1, characterized in that: The metal resonant pattern is processed on the upper side of the first dielectric layer by using a chemical etching process, and the metal patch pattern is processed on the lower side by using a chemical etching process.

8. The ultra-thin low-frequency absorber based on electromagnetic metamaterial according to claim 1, characterized in that: The lower side of the second dielectric layer is covered with a metal copper film as a metal grounding layer.

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