A broadband low-profile frequency selective metasurface and design method

Abstract

The application discloses a broadband low-profile frequency selective surface and a design method, comprising a plurality of periodically arranged frequency selective surface units, the frequency selective surface units sequentially comprise an upper layer metal resonant structure, a first dielectric layer, an intermediate metal resonant structure, a second dielectric layer and a lower layer metal resonant structure; the upper layer metal resonant structure and the lower layer metal resonant structure constitute a double-mode coupled resonator, the intermediate metal resonant structure is used for providing an additional resonant frequency and adjusting electromagnetic coupling of the upper layer metal resonant structure and the lower layer metal resonant structure, so that the frequency selective surface forms a plurality of adjacent resonant frequencies in a target frequency band and realizes a broadband bandpass response. The application realizes a broadband bandpass response through cooperation of a plurality of resonant frequencies, and improves a control freedom degree of structure parameters on resonant frequencies and coupling strength.

Description

Technical Field

[0001] This invention relates to the technical fields of mobile communication base station antennas, radomes, electromagnetic metasurfaces, and frequency selective surfaces, specifically to a broadband low-profile frequency selective metasurface and its design method. Background Technology

[0002] Frequency selective surfaces (FSS) are artificial electromagnetic structures composed of periodic metallic or dielectric units that exhibit frequency-selective transmission or reflection characteristics to electromagnetic waves propagating in space. Therefore, they are commonly used as space filters, electromagnetic windows, radomes, or radar domes. With the development of advanced radar and communication systems, engineering applications have placed demands on FSSs for low profile, wide passband, dual-polarization stability, steep roll-off, and multi-resonance coordination.

[0003] Traditional low-order bandpass frequency selective surfaces are typically thin and easy to fabricate, but their passbands are usually narrow, making it difficult to meet the application requirements of broadband communication or broadband electromagnetic windows. To extend the passband bandwidth, existing technologies often achieve higher-order bandpass responses by cascading multiple resonators, employing three-dimensional frequency selective structures, introducing substrate-integrated waveguide structures, or stacking multi-layer antenna-filter-antenna structures. However, these approaches usually significantly increase the overall profile height, fabrication complexity, and assembly difficulty, which is not conducive to low-profile system integration.

[0004] Two-mode coupled resonators can provide more resonant frequencies at the same profile height, making them an effective way to achieve high-order frequency selective surfaces with low profiles. However, existing two-mode coupled structures can usually only primarily adjust one of the electrical or magnetic coupling, limiting the degree of freedom in structural control. The relative positions between multiple resonant frequencies are difficult to control flexibly, making it still difficult to achieve a broadband bandpass response under low profile conditions.

[0005] In base station antenna systems, radomes or front-end electromagnetic windows typically need to maintain low insertion loss and stable transmission within the operating communication frequency band to avoid affecting the gain, efficiency, and radiation pattern of the array antenna. Meanwhile, base station antennas often employ dual-polarized arrays and may face obliquely incident electromagnetic waves within a certain angle range. Therefore, the frequency selection structure must maintain a stable frequency response under both dual-polarization and oblique incidence conditions. Furthermore, base station antenna systems have high requirements for structural profile, weight, and manufacturing complexity. Traditional high-order broadband frequency selection structures, due to their high profile or numerous layers, are difficult to directly integrate into base station radomes or array front-ends.

[0006] Therefore, it is necessary to propose a new low-profile broadband frequency-selective surface structure and its design method, which can achieve broadband bandpass response by multiple resonant frequencies working together without significantly increasing the profile height, and improve the degree of freedom of structural parameters in controlling resonant frequency and coupling strength. Summary of the Invention

[0007] In order to overcome the above-mentioned shortcomings and deficiencies of the prior art, the purpose of this invention is to provide a broadband low-profile frequency selective metasurface and its design method that can be used in the electromagnetic window or radome structure of the front end of a base station antenna.

[0008] This invention addresses the problem that existing base station radomes or array front-end frequency selective structures struggle to simultaneously achieve broadband low-loss transmission, low-profile integration, dual-polarization stability, and oblique incidence stability. It also solves the problems of existing high-order frequency selective surfaces typically having high profiles, complex structures, and insufficient degrees of freedom in coupling and control.

[0009] The objective of this invention is achieved through the following technical solution:

[0010] A broadband low-profile frequency-selective metasurface includes multiple periodically arranged frequency-selective metasurface units. Each frequency-selective metasurface unit sequentially includes an upper metal resonant structure, a first dielectric layer, an intermediate metal resonant structure, a second dielectric layer, and a lower metal resonant structure. The upper and lower metal resonant structures constitute a two-mode coupled resonator. The intermediate metal resonant structure provides an additional resonant frequency and adjusts the electromagnetic coupling between the upper and lower metal resonant structures, enabling the frequency-selective surface to form multiple adjacent resonant frequencies within the target frequency band and achieve a broadband bandpass response.

[0011] Furthermore, the upper and lower metal resonant structures are perpendicular to the thickness direction of the frequency selective metasurface unit and are arranged correspondingly to each other. Each of the upper and lower metal resonant structures includes four sub-units, and the four sub-units are arranged in a fourfold rotational symmetry around the center of the frequency selective metasurface unit.

[0012] Furthermore, each of the sub-units includes a fractal metal patch and a groove. A truncated structure is provided at the top of two sub-units disposed opposite each other on one of the diagonals of the first dielectric layer. The groove is disposed inside the fractal metal patch near the center of the frequency-selective metasurface unit, or disposed in the gap between adjacent sub-units.

[0013] Furthermore, the fractal metal patch is a third-order fractal metal patch.

[0014] Furthermore, the truncated structure is specifically a right triangle, used to distinguish the two orthogonal resonance modes of the dual-mode coupled resonator.

[0015] Furthermore, the intermediate metal resonant structure includes a rectangular metal patch with a cross-shaped slit at its center and notches at the centers of its four sides, the notches being connected by narrow strip-shaped metal connectors.

[0016] Furthermore, the cross-shaped gap is used to introduce magnetic coupling between the upper metal resonant structure and the lower metal resonant structure, and the gap between adjacent sub-units is used to introduce electrical coupling, so that the frequency-selective metasurface unit has both electrical coupling control degree of freedom and magnetic coupling control degree of freedom.

[0017] Furthermore, the upper and lower metal resonant structures are coupled to form two sets of dual-mode resonant modes, thereby generating four resonant frequencies; the middle metal resonant structure generates an additional resonant frequency, and the five resonant frequencies together constitute a fifth-order bandpass response.

[0018] A design method for the aforementioned broadband low-profile frequency-selective metasurface includes:

[0019] Determine the center frequency, passband range, and profile height constraints of the target broadband passband;

[0020] Design an upper metal resonant structure and a lower metal resonant structure to form a dual-mode coupled resonator and generate multiple resonant frequencies;

[0021] Design an intermediate metal resonant structure to provide an additional resonant frequency and simultaneously modulate the electrical and magnetic coupling between the dual-mode coupled resonators;

[0022] An equivalent circuit model is established, and the correspondence between the resonant frequency, coupling parameters, and physical structure parameters is determined based on the equivalent circuit model. The structural parameters are adjusted according to the correspondence to make multiple resonant frequencies cooperatively distributed within the target passband, thereby obtaining a broadband bandpass frequency selective surface.

[0023] Furthermore, the equivalent circuit model includes a resonant branch for characterizing the intermediate metal resonant structure, a resonant branch for characterizing the two orthogonal modes of the dual-mode coupled resonator, and an admittance converter for characterizing the coupling relationship between free space and each resonant branch.

[0024] The sources of each resonant frequency are determined by the odd-mode and even-mode current distributions, and the control regions of electric and magnetic coupling are determined by the electric and magnetic field distributions, thereby determining the direction of influence of each physical structural parameter on the target resonant frequency.

[0025] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0026] First, this invention does not simply stack multiple resonators to obtain broadband, but achieves a fifth-order bandpass response in a low-profile three-metal-layer structure. The upper and lower metal resonant structures constitute a two-mode coupled resonator, generating four resonant frequencies; the middle metal resonant structure generates an additional resonant frequency, and the five resonant frequencies together constitute a fifth-order broadband bandpass response, which is beneficial for significantly extending the passband bandwidth of the frequency-selective surface.

[0027] Second, the intermediate metal resonant structure not only provides an additional resonant frequency but also simultaneously modulates electrical and magnetic coupling. Electrical coupling is introduced through the gaps between adjacent sub-units, while magnetic coupling is introduced through the cross-shaped gaps. This gives the structure the ability to control hybrid electromagnetic coupling, rather than relying solely on a single electrical or magnetic coupling, thus increasing the design freedom of the resonant frequency and passband shape.

[0028] Third, this invention presents a design method based on odd and even modes, electromagnetic field distribution, and equivalent circuit models. This method determines the source of each resonant frequency and establishes the correspondence between the resonant frequency, coupling parameters, and physical structure parameters, thereby guiding the adjustment of structural parameters to achieve broadband or multi-bandpass response.

[0029] Fourth, the present invention obtains a stable dual-polarization response through a quadruple rotationally symmetric unit structure, so that the frequency-selective surface has consistent or nearly consistent bandpass characteristics for two orthogonally linearly polarized incident waves.

[0030] Fifth, the structure of the present invention only includes multiple planar metal patterns and dielectric layers, with a low overall profile and simple processing method, making it suitable for planar printed circuit technology and engineering integration applications. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the overall layered structure of the low-profile broadband frequency selective surface unit in Embodiment 1 of the present invention.

[0032] Figure 2 This is a schematic diagram of the upper and lower dual-mode coupled resonator metal structure of Embodiment 1 of the present invention.

[0033] Figure 3 This is a schematic diagram of the intermediate metal resonant structure of Embodiment 1 of the present invention.

[0034] Figure 4 This is a cross-sectional schematic diagram of Embodiment 1 of the present invention.

[0035] Figure 5 This is a schematic diagram of the broadband bandpass response and the distribution of five resonant frequencies in Embodiment 1 of the present invention.

[0036] Figure 6 This is a schematic diagram of the surface current, electric field, and magnetic field distribution at different resonant frequencies in Embodiment 2 of the present invention.

[0037] Figure 7 This is a schematic diagram of the equivalent circuit model of Embodiment 2 of the present invention.

[0038] Figure 8 This is a schematic diagram comparing the simulation and equivalent circuit model results of Embodiment 2 of the present invention.

[0039] Figure 9 Figure 3 shows the test and oblique incidence stability verification results of Embodiment 3 of the present invention. Figure (a) shows the comparison between the simulated and measured transmission response, and Figure (b) shows the transmission response under different incident angles and the oblique incidence test. Detailed Implementation

[0040] The present invention is further described below through specific embodiments, but the scope of protection of the present invention is not limited thereto.

[0041] Example

[0042] like Figure 1 As shown, this embodiment provides a broadband low-profile frequency-selective metasurface, comprising multiple frequency-selective metasurface units arranged periodically along a two-dimensional plane, such as... Figures 2-4 As shown, the frequency-selective metasurface unit comprises, from top to bottom, an upper metal resonant structure 1, a first dielectric layer 2, an intermediate metal resonant structure 3, a second dielectric layer 4, and a lower metal resonant structure 5; the metasurface has an axisymmetric structure, and both the first and second dielectric layers are rectangular, which can be microwave dielectric substrates with a relative permittivity of approximately 3.5, and the thickness of a single dielectric layer can be 1.524 mm; in other embodiments, the dielectric material and thickness can be scaled or replaced according to the target operating frequency band.

[0043] like Figure 2 As shown, the upper metal resonant structure 1 and the lower metal resonant structure 5 use the same metal patch pattern and have the same structure. The upper and lower metal resonant structures are arranged correspondingly to each other perpendicular to the thickness direction of the frequency selective metasurface unit, and the thickness direction is from top to bottom. In one embodiment, the upper and lower metal resonant structures are mirror images of each other about the plane where the intermediate metal resonant structure is located. Both the upper and lower metal resonant structures include four sub-units, which are arranged in a fourfold rotational symmetry around the center of the frequency selective metasurface unit. The four sub-units are arranged in pairs, symmetrically arranged on the two diagonals of the first dielectric layer.

[0044] Each subunit includes a fractal metal patch and a groove line, wherein the fractal metal patch includes a central metal patch and multi-level fractal branches extending outward from the central metal patch; in one embodiment, the fractal metal patch is a third-order fractal metal patch.

[0045] In one embodiment, two of the four sub-units are positioned opposite each other along a diagonal of the frequency-selective metasurface unit. These two sub-units have truncated corner structures at their outer corners, away from the center of the frequency-selective metasurface unit. The other two sub-units may not have truncated corner structures, or their truncated corner dimensions may be zero. The truncated corner structure can be a right-angled triangle, formed by cutting off the outer corner of the sub-unit along two mutually perpendicular edges. The truncated corner dimension is denoted as c. This truncated corner structure is used to change the equivalent electrical length of the two orthogonal modes, thereby distinguishing the two orthogonal resonant modes of the two-mode coupled resonator.

[0046] The groove is located on the side of the fractal metal patch near the center of the frequency-selective metasurface unit, or at the edge of the gap between adjacent fractal metal patches. The groove length s0 and groove width s1 are used to extend the surface current path and reduce the corresponding resonant frequency, thereby increasing the number of tunable resonant frequencies under low profile conditions.

[0047] like Figure 3 As shown, the intermediate metal resonant structure 3 includes a rectangular metal patch, a cross-shaped slot in the center region of the rectangular metal patch, and a narrow strip-shaped metal connector. The narrow strip-shaped metal connector and the rectangular metal patch together form an intermediate layer resonator, which is used to provide an additional resonant frequency independent of the dual-mode coupled resonator; the cross-shaped slot is located in the region with a strong magnetic field between the upper and lower metal resonant structures, and is used to adjust the magnetic coupling between the upper and lower layers.

[0048] The rectangular metal patch has notches at the center of its four sides, and narrow strip-shaped metal connectors connect the two ends of the notches. These narrow strip-shaped metal connectors are used to connect adjacent metal regions or adjust the equivalent inductance of the intermediate metal resonant structure. The width of the slender microstrip connecting line is denoted as inc_w, and its length-related parameters are denoted as inc_l.

[0049] Further explanation: the cross-shaped slits include a first slit and a second slit that intersect each other, with their intersection center coinciding with the center of the frequency-selective metasurface unit; in one embodiment, the extension directions of the first slit and the second slit are respectively aligned with the two diagonal directions of the frequency-selective metasurface unit. In this embodiment, the two slits have different dimensions: the first slit has a length l1 and a width w1, and the second slit has a length l2 and a width w2.

[0050] The gaps between adjacent sub-units are located in regions with strong electric fields, which can introduce electrical coupling; the cross-shaped gaps in the intermediate metal layer are located in regions with strong magnetic fields, which can introduce magnetic coupling. Therefore, the frequency-selective metasurface in this embodiment does not rely solely on electrical or magnetic coupling, but utilizes hybrid electromagnetic coupling to achieve flexible adjustment of multiple resonant frequencies.

[0051] like Figure 5 As shown, in this embodiment, the upper and lower metal resonant structures constitute a two-mode coupled resonator, generating four resonant frequencies; the middle metal resonant structure generates an additional resonant frequency. The five resonant frequencies are adjacent and coupled within the target passband, forming a fifth-order broadband bandpass response. Taking a specific design as an example, the overall thickness of the frequency-selective surface is approximately 0.05 times the free-space wavelength corresponding to the center frequency of the target passband. Simulation results show that its passband can cover 4.12 GHz to 5.95 GHz, with a relative bandwidth of approximately 37%.

[0052] like Figure 5 As shown, in this embodiment, by adjusting the relative positions of the multiple resonant frequencies within the target passband, adjacent resonant frequencies are coupled to each other and together form a continuous broadband passband.

[0053] Because the unit structure in this embodiment has fourfold rotational symmetry, the frequency-selective metasurface exhibits essentially identical bandpass responses to x-polarized and y-polarized incident electromagnetic waves, enabling stable dual-polarization operation. Furthermore, since multiple resonant frequencies are achieved jointly by the planar metallic structure and the dielectric layer, the frequency-selective metasurface maintains a low profile while possessing a wide passband, making it suitable for low-profile electromagnetic windows and broadband spatial filters.

[0054] In this embodiment, the geometric parameters of the frequency-selective metasurface unit can adopt the following preferred values, and the parameters can be scaled proportionally or adaptively adjusted according to the target operating frequency band, dielectric material and unit period.

[0055] p=16 mm, h0=1.524 mm, r1=3.649 mm, c=3.792 mm, r2=1.824 mm,

[0056] L=13.29 mm, r3=0.912 mm, inc_l=2.2 mm, l1=6.0 mm, inc_w=1.2 mm,

[0057] l2=3.0 mm, s0=1.0 mm, w1=0.6 mm, s1=0.4 mm, w2=0.9mm, w3=13.8 mm

[0058] q=0.5, k=3.

[0059] In this embodiment, the frequency-selective metasurface may further include a protective layer, adhesive layer, support layer, or encapsulation layer disposed on the outside of the upper metal resonant structure and / or the outside of the lower metal resonant structure. The protective layer, adhesive layer, support layer, or encapsulation layer does not change the main resonance mechanism formed by the upper metal resonant structure, the intermediate metal resonant structure, and the lower metal resonant structure.

[0060] Example 2

[0061] This embodiment provides a design method for designing the aforementioned low-profile broadband frequency selective surface, including:

[0062] First, the center frequency, lower boundary, upper boundary, allowable insertion loss, polarization requirements, and maximum profile height of the target passband are determined based on system requirements. Then, the cell period, dielectric material, and metal patch size are preliminarily determined according to the target center frequency.

[0063] Secondly, the upper and lower metal resonant structures are designed to generate four resonant frequencies related to the target passband. By adjusting the side length, truncated corner size, slot length, and slot width of the fractal metal patch, the equivalent electrical lengths of the two orthogonal modes of the dual-mode coupled resonator can be changed, thereby altering the corresponding resonant frequencies.

[0064] Next, the intermediate metal resonant structure is designed to provide an additional resonant frequency within the target passband. By adjusting the size of the rectangular patch and the width and length of the narrow strip metal connector, the equivalent inductance and equivalent capacitance of the intermediate layer resonator can be adjusted, thereby shifting the position of this additional resonant frequency.

[0065] Finally, the main control regions for electric and magnetic coupling are determined based on the distribution of electric and magnetic fields. The gaps between adjacent sub-units primarily affect the electric coupling strength, while the length, width, and position of the cross-shaped slots primarily affect the magnetic coupling strength. The dielectric layer thickness simultaneously affects both the interlayer coupling strength and the resonant frequency. Through the coordinated adjustment of these parameters, the five resonant frequencies are distributed within the target broadband range, and the transmission responses between adjacent resonant points are connected into a continuous passband.

[0066] Furthermore, such as Figure 6 As shown, the distribution of surface current, electric field, and magnetic field at different resonant frequencies can be used to determine the source of each resonant frequency and the main areas of electric and magnetic coupling. Figure 7 As shown, an equivalent circuit model is established. This model may include parallel LC resonant branches representing the intermediate layer resonator, parallel LC resonant branches representing the two orthogonal modes of the two-mode coupled resonator, and admittance converters representing the coupling relationship between free space and each resonant branch. Through odd-mode and even-mode decomposition, the resonance source corresponding to each resonant frequency can be determined, and the correspondence between resonant frequency, coupling parameters, and specific physical structure parameters can be established. For example... Figure 8 As shown, the calculation results of the equivalent circuit model and the full-wave simulation results have the same frequency response trend in the target broadband passband, which can be used to guide the subsequent adjustment of structural parameters.

[0067] The structural parameters include at least one of the following: the gap size between sub-units, the length and width of the cross-shaped slot, the truncated angle size of the fractal metal patch, the length and width of the groove, the width of the narrow strip metal connector, the side length of the rectangular metal patch, and the thickness of the dielectric layer. In one embodiment, the extension directions of the first and second slots are respectively aligned with the two diagonal directions of the frequency selective surface unit.

[0068] Finally, the physical structure parameters are iteratively optimized based on the adjustment direction given by the equivalent circuit model, and the transmission coefficient and reflection coefficient are verified by full-wave electromagnetic simulation. When multiple resonant frequencies together form a continuous passband within the target passband with satisfactory insertion loss, the final structural parameters of the low-profile broadband frequency selective surface are obtained.

[0069] Example 3

[0070] This embodiment provides a broadband low-profile frequency selective metasurface, including 10×10 periodically arranged frequency selective metasurface units. The first dielectric layer and the second dielectric layer can be made of microwave dielectric material with a dielectric constant of about 3.5, and a multilayer planar structure is formed by bonding film or lamination process.

[0071] like Figure 9 As shown in (a), the transmission coefficient of the frequency-selective metasurface can be measured using the free-space method during testing. The simulation results are basically consistent with the measured results within the wide passband, indicating that the frequency-selective metasurface can maintain bandpass transmission characteristics over a wide frequency band. In one embodiment, the test passband is approximately 3.95 GHz to 5.85 GHz, and the minimum insertion loss within the passband is approximately 0.3 dB. Figure 9 As shown in (b) above, the frequency-selective metasurface can still maintain a relatively stable broadband transmission response under different incident angles, indicating that the structure has good oblique incident stability.

[0072] Those skilled in the art will readily understand that the above description is merely an embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A broadband low-profile frequency-selective metasurface, characterized in that, The device comprises multiple periodically arranged frequency-selective metasurface units, each of which sequentially includes an upper metal resonant structure, a first dielectric layer, an intermediate metal resonant structure, a second dielectric layer, and a lower metal resonant structure. The upper and lower metal resonant structures constitute a two-mode coupled resonator. The intermediate metal resonant structure provides an additional resonant frequency and adjusts the electromagnetic coupling between the upper and lower metal resonant structures, enabling the frequency-selective surface to form multiple adjacent resonant frequencies within the target frequency band and achieve a broadband bandpass response.

2. The broadband low-profile frequency-selective metasurface according to claim 1, characterized in that, The upper and lower metal resonant structures are perpendicular to the thickness direction of the frequency selective metasurface unit and are arranged correspondingly to each other. Each upper and lower metal resonant structure includes four sub-units, which are arranged in a fourfold rotational symmetry around the center of the frequency selective metasurface unit.

3. The broadband low-profile frequency-selective metasurface according to claim 2, characterized in that, Each of the sub-units includes a fractal metal patch and a groove. The tops of two sub-units disposed opposite each other on one of the diagonals of the first dielectric layer are provided with a truncated structure. The groove is disposed inside the fractal metal patch near the center of the frequency-selective metasurface unit, or disposed in the gap between adjacent sub-units.

4. The broadband low-profile frequency-selective metasurface according to claim 3, characterized in that, The fractal metal patch is a third-order fractal metal patch.

5. The broadband low-profile frequency-selective metasurface according to claim 3, characterized in that, The truncated structure is specifically a right triangle, used to distinguish the two orthogonal resonance modes of the dual-mode coupled resonator.

6. The broadband low-profile frequency-selective metasurface according to claim 1, characterized in that, The intermediate metal resonant structure includes a rectangular metal patch with a cross-shaped slit at its center and notches at the centers of its four sides. The notches are connected by narrow strip-shaped metal connectors.

7. The broadband low-profile frequency-selective metasurface according to claim 6, characterized in that, The cross-shaped gap is used to introduce magnetic coupling between the upper metal resonant structure and the lower metal resonant structure, and the gap between adjacent sub-units is used to introduce electrical coupling, so that the frequency selective metasurface unit has both electrical coupling control degree of freedom and magnetic coupling control degree of freedom.

8. The broadband low-profile frequency-selective metasurface according to claim 1, characterized in that, The upper and lower metal resonant structures are coupled to form two sets of dual-mode resonant modes, thereby generating four resonant frequencies; the middle metal resonant structure generates an additional resonant frequency, and the five resonant frequencies together constitute a fifth-order bandpass response.

9. A design method for a broadband low-profile frequency-selective metasurface based on any one of claims 1-8, characterized in that, include: Determine the center frequency, passband range, and profile height constraints of the target broadband passband; Design an upper metal resonant structure and a lower metal resonant structure to form a dual-mode coupled resonator and generate multiple resonant frequencies; Design an intermediate metal resonant structure to provide an additional resonant frequency and simultaneously modulate the electrical and magnetic coupling between the dual-mode coupled resonators; An equivalent circuit model is established, and the correspondence between the resonant frequency, coupling parameters, and physical structure parameters is determined based on the equivalent circuit model. The structural parameters are adjusted according to the correspondence to make multiple resonant frequencies cooperatively distributed within the target passband, thereby obtaining a broadband bandpass frequency selective surface.

10. The design method according to claim 9, characterized in that, The equivalent circuit model includes a resonant branch for characterizing the intermediate metal resonant structure, a resonant branch for characterizing the two orthogonal modes of the dual-mode coupled resonator, and an admittance converter for characterizing the coupling relationship between free space and each resonant branch. The sources of each resonant frequency are determined by the odd-mode and even-mode current distributions, and the control regions of electric and magnetic coupling are determined by the electric and magnetic field distributions, thereby determining the direction of influence of each physical structural parameter on the target resonant frequency.

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