Dual-passband low-frequency passband independent tunable frequency selective surface

Abstract

According to the dual-passband low-frequency passband independent tunable frequency selective surface, a metal column penetrates through a cross-shaped structure metal layer to be connected with an upper dual-passband metal layer and a lower dual-passband metal layer, so that a multi-layer cascade structure is formed, the bandwidth is broadened, and the characteristics in passbands are adjusted; the upper dual-passband metal layer and the lower dual-passband metal layer comprise an outer metal frame and four branches in the outer metal frame, each branch is provided with a variable capacitance diode, and the side edge of each branch extends towards the outer metal frame and / or the outer metal frame extends towards the side edge of each branch to form a first gap; when direct current feed is carried out through the metal column, the variable capacitance diode provides a first capacitor, the first gap provides a second capacitor, the first capacitor is larger than the second capacitor, the first capacitor is used for controlling a low-frequency passband, and the second capacitor is used for controlling a high-frequency passband. When different direct-current voltages are applied to the two ends of the variable capacitance diode, the low-frequency passband moves, the high-frequency passband is kept unchanged, and independent tuning of the low-frequency passband is achieved.

Description

Technical Field

[0001] The present invention belongs to the field of communication technology, and further relates to a dual-passband low-frequency passband independently tunable frequency selective surface in the field of electromagnetic field and microwave technology, which can be used in dual-frequency, multi-frequency systems and stealth radar antenna covers. Background Art

[0002] As the demand for multi-frequency continues to increase, frequency selective surfaces with multi-band characteristics are gaining more and more attention. Traditional frequency selective surfaces are difficult to meet the corresponding broadband, ultra-wideband, multi-frequency and variable frequency requirements.

[0003] On September 12, 2024, the applicant filed a patent application for a dual-passband low-frequency passband independently switchable frequency selective surface (application number CN 202411278680.3), which adopts a three-layer structure design, in which the upper and lower layers are dual-passband metal layers, the middle layer is a cross metal structure, and 4 pin diodes are placed in the middle gap of each layer of the upper and lower layers, and each pin diode is fed through a metal column. On the one hand, this patent can realize independent control of the dual passbands, and on the other hand, by applying a forward voltage to the pin diode, the low-frequency passband can be independently switched, and the high-frequency passband can be stably maintained. At the same time, it has excellent in-band and out-of-band characteristics, wide passband and high-frequency ratio.

[0004] During further exploration, the applicant discovered that, for the frequency selective surface, in addition to the independent control and independent switching of the dual passbands mentioned above, whether it can achieve independent tunability is also a problem that needs to be solved. Summary of the invention

[0005] Based on patent CN 202411278680.3, the present invention provides a dual-passband low-frequency passband independently tunable frequency selective surface to improve the tuning function of the frequency selective surface.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is:

[0007] A dual-passband low-frequency passband independently tunable frequency selective surface comprises an upper dual-passband metal layer, a cross-structure metal layer and a lower dual-passband metal layer from top to bottom, wherein the upper dual-passband metal layer has the same structure as the lower dual-passband metal layer and both have dual-polarization characteristics; a metal column passes through the cross-structure metal layer to connect the upper dual-passband metal layer and the lower dual-passband metal layer, forming a multi-layer cascade structure to broaden the bandwidth and adjust the characteristics within the passband, wherein:

[0008] The upper dual passband metal layer and the lower dual passband metal layer include an outer metal frame and four branches in the outer metal frame, each branch is provided with a varactor diode, the side of the branch extends toward the outer metal frame, and / or the outer metal frame extends toward the side of the branch to form a first gap;

[0009] When direct current feeding is performed through the metal column, the varactor diode is used to provide a first capacitance, the first gap is used to provide a second capacitance, and the first capacitance is greater than the second capacitance, the first capacitance is used to control a low-frequency passband, and the second capacitance is used to control a high-frequency passband;

[0010] When different DC voltages are applied across the varactor diode, the low-frequency passband moves and the high-frequency passband remains unchanged, thus achieving independent tuning of the low-frequency passband.

[0011] In one embodiment, the upper dual passband metal layer is printed on the upper surface of the upper dielectric substrate, the lower dual passband metal layer is printed on the lower surface of the lower dielectric substrate, the cross structure metal layer is printed on the lower surface of the upper dielectric substrate or the upper surface of the lower dielectric substrate, and the metal column runs through the upper dielectric substrate and the lower dielectric substrate, connecting the upper dual passband metal layer, the cross structure metal layer and the lower dual passband metal layer.

[0012] In one embodiment, the upper dual passband metal layer and the lower dual passband metal layer are both composed of an inner cross metal structure, an outer metal frame, a varactor diode and a circular aperture; the outer metal frame is a square, the inner cross metal structure is located at the center of the outer metal frame, and its four branches are perpendicular or parallel to the edges of the outer metal frame, cross-intersecting and converging at a central disk, the circular aperture is located at the center of the central disk for the metal column to pass through, and the varactor diode is connected between a branch of the inner cross metal structure and an edge of the outer metal frame.

[0013] In one embodiment, a square metal block is respectively disposed at the four inner corners of the outer metal frame, the midpoints of the four sides are respectively connected to an extended metal block in the direction of a branch, and the varactor is connected at the second gap formed between the extended metal block and the corresponding branch.

[0014] In one embodiment, the positive electrode of a varactor is connected to the extended metal block, and the negative electrode is connected to a branch of the inner cross metal structure. When the outer metal frame is connected to the positive electrode of the power supply and the cross structure metal layer is connected to the negative electrode of the power supply, the varactor is fed with power.

[0015] In one embodiment, the extended metal block includes a first metal strip, a wing-shaped metal block, and a second metal strip connected in sequence, one end of the first metal strip is connected to the midpoint of an edge of the outer metal frame, one end of the second metal strip is connected to a varactor diode, and a first gap is provided between the wing-shaped metal block and the square metal block.

[0016] In one embodiment, the cross-structure metal layer is composed of a cross metal structure, a metal disc and a circular hole; the branches of the cross metal structure are parallel or perpendicular to the branches of the inner cross metal structure, and the crosses converge on the metal disc. The circular hole is located at the center of the metal disc for the metal column to pass through.

[0017] In one embodiment, the varactor diode is of model MA46H120.

[0018] In one embodiment, when the voltage of the DC power supply is changed, the junction capacitance of the varactor changes, thereby changing the low-frequency characteristic impedance of the frequency selective surface and moving the low-frequency passband. The capacitance variation range of the varactor is 0.15pF-0.6pF. When the DC voltage changes between 2-20V, the movement range of the low-frequency passband is 3.31-6.81GHz, that is, the tuning range of the low-frequency passband center frequency is 3.31-6.81GHz.

[0019] In one embodiment, the independent tuning of the low frequency passband is implemented as follows:

[0020] The positive pole of the power supply is connected to the outer metal frame of the upper and lower structures, and the negative pole of the power supply is connected to the cross structure metal layer. When the applied power supply is 2V, the capacitance of the varactor diode is 0.6pF, and the center frequency of the low-frequency passband is 3.31GHz; when the applied power supply is 4V, the capacitance of the varactor diode is 0.4pF, and the center frequency of the low-frequency passband is 4.22GHz; when the applied power supply is 6V, the capacitance of the varactor diode is 0.3pF, and the center frequency of the low-frequency passband is 4.8GHz; when the applied power supply is 20V, the capacitance of the varactor diode is 0.16pF, and the center frequency of the low-frequency passband is 6.81GHz.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] First, since the dual-passband metal layer in the present invention adopts a multi-resonance structure to realize the dual-passband characteristics on a single-layer structure, and in order to realize the independent control of the dual-passband, the filtering characteristics of the capacitor at different frequencies in the circuit principle are adopted, that is, a small capacitor can be equivalent to an open circuit in the low-frequency range, and a large capacitor can be equivalent to a short circuit in the high-frequency range. The varactor diode is placed in the middle gap of the dual-passband layer to provide a large capacitor, and the gaps between the four corners of the outer metal frame and the internal strips provide a small capacitor. Therefore, the dual-passband layer can be equivalent to two circuit structures in the low-frequency and high-frequency parts, thereby realizing that the low-frequency passband is only controlled by a large capacitor, and the high-frequency passband is only controlled by a small capacitor. The problem that the traditional frequency selective surface of the prior art is difficult to realize multi-frequency independent control is overcome, so that the present invention has the advantages of independent control of dual passbands and small thickness.

[0023] Second, since the overall structure of the present invention adopts a multilayer structure of an upper dual passband metal layer, a middle cross metal layer and a lower dual passband metal layer, setting the dual passband layer as a double-layer structure is conducive to expanding the bandwidth, and at the same time, the cascade coupling effect between the middle layer and the dual passband layer improves the passband characteristics, making the dual passband flatter. This makes the present invention have the advantages of wide passband and small insertion loss on the basis of independent controllable dual passbands.

[0024] Third, since the varactor diode feeding method in the present invention adopts the frequency selective surface self-feeding method, the metal column is regarded as a feeder structure connecting the upper and lower layers, and a DC voltage is applied to the middle layer and the dual passband layer to realize active regulation of the varactor diode. This makes the present invention have the advantages of simple structure and easy feeding. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is the electrical schematic diagram of the present invention.

[0026] Figure 2 It is an overall structural decomposition diagram of the dual-passband low-frequency passband independently tunable frequency selective surface of the present invention.

[0027] Figure 3 It is a schematic diagram of the structure of the upper and lower double-passband metal layers of the present invention.

[0028] Figure 4 It is a schematic diagram of the intermediate metal layer structure and metal pillars of the present invention.

[0029] Figure 5 It is a schematic diagram of the feeding method of the present invention.

[0030] Figure 6 It is a transmission coefficient curve diagram of the varactor diode when different forward voltages are applied in the simulation experiment according to the embodiment of the present invention. DETAILED DESCRIPTION

[0031] The embodiments of the present invention are described in detail below with reference to the accompanying drawings and examples.

[0032] Patent CN 202411278680.3 realizes the independent switchable design of the frequency selective surface. On this basis, the present invention further realizes the independent tunable function of the frequency selective surface by adjusting the diode function and the power-on signal.

[0033] refer to Figure 1 , Figure 2 , Figure 3 , Figure 4 and Figure 5The overall structure of the electromagnetic surface of the present invention mainly includes three cascaded metal layers, namely, an upper dual passband metal layer 12, a cross-structured metal layer 17, and a lower dual passband metal layer 15. The upper dual passband metal layer 12 and the lower dual passband metal layer 15 both have dual polarization characteristics, and the two can adopt exactly the same structure. The metal column 16 passes through the cross-structured metal layer 17 to connect the upper dual passband metal layer 12 and the lower dual passband metal layer 15, forming a multi-layer cascade structure to widen the bandwidth and adjust the characteristics within the passband. The present invention adopts a multi-layer cascade structure to generate a second-order resonance, thereby widening the passband bandwidth, and the passband stopband effect presented is more intuitive.

[0034] The upper dual passband metal layer 12 and the lower dual passband metal layer 15 include an outer metal frame 111 and four branches in the outer metal frame 111. The present invention sets a variable capacitance diode 11 on each branch to replace the pin diode in patent CN 202411278680.3. The side of the branch extends toward the outer metal frame 111 or the outer metal frame 111 extends toward the side of the branch, or extends toward each other, thereby forming eight first gaps 112 between the side of the branch and the outer metal frame 111. By utilizing the property that the junction capacitance of the varactor diode 11 changes with the DC voltage, according to its filtering characteristics at different frequencies, that is, a small capacitor can be equivalent to an open circuit in the low-frequency range, and a large capacitor can be equivalent to a short circuit in the high-frequency range. The varactor diode 11 provides a large capacitor, namely the first capacitor C1, and the first gap 112 provides a small capacitor, namely the second capacitor C2. Therefore, the dual passband layer can be equivalent to two circuit structures in the low-frequency and high-frequency parts, thereby achieving the low-frequency passband being controlled only by the large capacitor and the high-frequency passband being controlled only by the small capacitor, thereby meeting the tunable purpose of the present invention.

[0035] By arranging a varactor diode 11 on each branch, the present invention realizes dual passband characteristics on a single-layer structure, and can realize independent control of the dual passbands. For the tuning function, it is also necessary that the positive electrode of the varactor diode 11 on the upper dual passband metal layer 12 is electrically connected to the upper dual passband metal layer 12, the positive electrode of the varactor diode 11 on the lower dual passband metal layer 15 is electrically connected to the lower dual passband metal layer 15, and the negative electrode of each varactor diode 11 is electrically connected to the cross structure metal layer 17, such as Figure 5 shown.

[0036] At this time, the metal column 16 serves as a feeder structure for regulating the varactor diode 11. The DC feeding of each varactor diode 11 is simultaneously realized through the metal column 16. When different DC voltages are applied across the varactor diode 11, the low-frequency passband moves and the high-frequency passband remains unchanged, thereby realizing independent tuning of the low-frequency passband.

[0037] Specifically, according to the above structure, when the voltage of the DC power supply is changed, the junction capacitance of the varactor diode 11 changes, thereby changing the low-frequency characteristic impedance of the frequency selective surface and shifting the low-frequency passband. It is found in experiments that when the DC voltage changes from 2 to 20 V, the capacitance of the varactor diode 11 changes from 0.15 pF to 0.6 pF, and the low-frequency passband moves from 3.31 to 6.81 GHz.

[0038] In a further embodiment of the present invention, reference Figure 2 In actual manufacturing, the present invention includes a double-layer dielectric substrate, namely an upper dielectric substrate 13 and a lower dielectric substrate 14. Among them, the upper dual-passband metal layer 12 is printed on the upper surface of the upper dielectric substrate 13 and connected to the metal column 16 penetrating the upper dielectric substrate 13, the lower dual-passband metal layer 15 is printed on the lower surface of the lower dielectric substrate 14 and connected to the metal column 16 penetrating the lower dielectric substrate 14, and the cross-structured metal layer 17 can be printed on the lower surface of the upper dielectric substrate 13 or on the upper surface of the lower dielectric substrate 14.

[0039] In a further embodiment of the present invention, reference Figure 3 In (a), for the upper dual passband metal layer 12 and the lower dual passband metal layer 15 having bipolar characteristics, they must each have four branches. Figure 3 (a) shows a structure of the upper dual passband metal layer 12 and the lower dual passband metal layer 15, which are mainly composed of an inner cross metal structure 114, an outer metal frame 111, a varactor diode 11 and a circular aperture 115. The outer metal frame 111 is a square, and the inner cross metal structure 114 is located at the center of the outer metal frame 111. It is a combination of a cross structure and a disk structure, that is, a cross structure is radially diverged from a disk structure, and the four branches of the cross structure are perpendicular or parallel to the edge of the outer metal frame 111, and cross and converge at the center disk. The circular aperture 115 is located at the center of the center disk for the metal column 16 to pass through. The varactor diode 11 is connected between a branch of the inner cross metal structure 114 and an edge of the outer metal frame 111.

[0040] The characteristic of the above structure is that two parallel resonances can be generated. Figure 1 The circuit schematic diagram corresponds to that of the embodiment to achieve the purpose of independent tunability of dual passbands.

[0041] Furthermore, a square structure is extended from the four corners of the outer metal frame 111, and a portion of the strip structure is extended from the midpoints of the four sides, that is, a square metal block 1111 is respectively provided at the four inner corners. An extended metal block 1112 is connected to the midpoints of the four sides in the direction of a branch, and four second gaps 113 are formed between the four extended metal blocks 1112 and the four branches, and four varactor diodes 11 are respectively connected between the four second gaps 113. At this time, the extended metal block 1112 and the outer metal frame 111 are a whole, and the inner cross metal structure 114 is connected to the cross structure metal layer 17 through the metal column 16. More specifically, the positive electrode of a varactor diode 11 is connected to an extended metal block 1112, and the negative electrode is connected to a branch of the inner cross metal structure 114. When the outer metal frame 111 is connected to the positive electrode of the power supply and the cross structure metal layer 17 is connected to the negative electrode of the power supply, the feeding of the varactor diode is realized, such as Figure 5 shown.

[0042] In order to form the gap 112 described in the present invention, the side of the branch can be extended to the outer metal frame 111 or the outer metal frame 111 can be extended to the side of the branch, but both methods will cause interlacing in the outer metal frame 111, and multi-layer technology is required. Therefore, the present invention preferably extends the side of the branch and the outer metal frame 111 toward each other at the same time. For example, for the upper dual-passband metal layer 12 or the lower dual-passband metal layer 15, the strip structure extending from the midpoint of the four sides of the outer metal frame 111 is expanded into a wing-shaped structure in the middle part, thereby generating 8 first gaps 112 with the square metal blocks 1111 at the four corners. Its function is to provide the first gap 112 to load the varactor diode 11 and participate in the resonance to produce a dual passband. More specifically, the extended metal block 1112 includes a first metal strip 11121, a wing-shaped metal block 11123 and a second metal strip 11122 connected in sequence, wherein one end of the first metal strip 11121 is connected to the midpoint of one side of the outer metal frame 111, and the other end is connected to the midpoint of the long side of the wing-shaped metal block 11123; one end of the second metal strip 11122 is connected to the varactor diode 11, and the other end is connected to the midpoint of the short side of the wing-shaped metal block 11123; a first gap 112 is formed between the side of the wing-shaped metal block 11123 and the square metal block 1111.

[0043] Figure 1 The principle of the present invention is shown. Z0 in the figure is the wave impedance in free space, which is 377Ω. In the low-frequency state, the extended metal block 1112 is equivalent to the first inductor L1, and the four sides of the outer metal frame 111 are equivalent to the third inductor L3. C1, L1, and L3 produce parallel resonance to produce a low-frequency passband; in the high-frequency state, the first metal strip 11121 is equivalent to the second inductor L2, and the four sides of the outer metal frame 111 are equivalent to the fourth inductor L4. C2, L2, and L4 produce parallel resonance to produce a high-frequency passband.

[0044] Reference Figure 3 As shown in (b), the dimensions of each part of the outer metal frame 111 of the dual passband metal layer are: d2 = 1mm, d3 = 3mm, d4 = 1.7mm, L2 = 0.5mm, L3 = 2mm, L4 = 0.5mm, L5 = 0.8mm, L6 = 2.4mm, D = 0.9mm, S = 10mm, and the dimensions of each part of the inner cross metal structure 114 are: d1 = 0.7mm, L1 = 0.66mm, R1 = 1.1mm, A = 0.3mm, and R2 = 0.25mm, which is slightly larger than the radius of the metal column 16. The upper dielectric substrate 13 and the lower dielectric substrate 14 are made of a thickness of 2.2mm and a dielectric constant of ε r =2.2, dielectric substrate with loss tangent tanδ=0.0009.

[0045] Further, refer to Figure 4 The cross-structure metal layer 17 of the present invention is mainly composed of a cross metal structure 116, a metal disk 117 and a circular aperture 118; the metal disk 117 is located at the center of the cross metal structure 116, and the branches of the cross metal structure 116 are parallel or perpendicular to the branches of the inner cross metal structure 114, and cross and converge at the metal disk 117. The size of the metal disk 117 is: W = 0.38mm, L = 4.5mm, and the circular aperture 118 is located at the center of the metal disk 117, and its size is R4 = 0.5mm. The metal column 16 passes through the circular aperture 118 to connect the upper dual-passband metal layer 12 and the lower dual-passband metal layer 15, and its size is: P = 4.4mm, R3 = 0.2mm. The cross metal structure 116, on the one hand, acts as a feed line to feed the varactor diode 11, and on the other hand, interacts with the upper and lower layers to improve the performance of the dual passband.

[0046] refer to Figure 5 , the DC feed of the varactor diode 11 is fed by the frequency selective surface itself, the positive and negative electrodes of the power supply are connected to the cross structure metal layer 17 and the upper dual passband metal layer 12 and the lower dual passband metal layer 15 to feed the varactor diode 11, and the metal column 16 is used as a feeder structure to connect the dual passband layer and the cross structure metal layer 17. The varactor diode 11 used in this embodiment is model MA45H120, and when the DC voltage is 2V, 4V, 6V, and 20V, the capacitance of the varactor diode is 0.6pF, 0.4pF, 0.3pF, and 0.16pF, respectively. The independent tuning of the low-frequency passband in this embodiment is implemented as follows:

[0047] The positive pole of the power supply is connected to the outer metal frame 111 of the upper and lower structures, and the negative pole of the power supply is connected to the cross-structure metal layer 17. When the applied power supply is 2V, the capacitance of the varactor diode is 0.6pF, and the center frequency of the low-frequency passband is 3.31GHz; when the applied power supply is 4V, the capacitance of the varactor diode is 0.4pF, and the center frequency of the low-frequency passband is 4.22GHz; when the applied power supply is 6V, the capacitance of the varactor diode is 0.3pF, and the center frequency of the low-frequency passband is 4.8GHz; when the applied power supply is 20V, the capacitance of the varactor diode is 0.16pF, and the center frequency of the low-frequency passband is 6.81GHz.

[0048] The following is a further description of the technical effects of the present invention in combination with simulation experiments:

[0049] The S parameter curve obtained by modeling and simulating the embodiment of the present invention using the commercial simulation software Ansoft HFSS19.0 is as follows: Figure 6 shown. Figure 6 The horizontal axis is the frequency value in GHz, and the vertical axis is the transmission coefficient S 21 , in dB. Figure 6 The four solid lines in the figure show that the embodiment of the present invention can achieve the function of low-frequency passband tunability and high-frequency passband retention. The low-frequency passband tunable range is 3.31Ghz-6.87Ghz, the high-frequency passband is basically maintained at 14.9Ghz-18.5Ghz, and the insertion loss is less than -1dB. The structure has the characteristics of high frequency ratio, wide passband, and low-frequency passband independent tunability.

[0050] In summary, the three-layer structure of the present invention can realize independent control of dual passbands. By applying different forward voltages to the varactor diode, the low-frequency passband can be independently tuned, and the high-frequency passband can be stably maintained. The structure has good in-band and out-of-band characteristics, wide passband and high-frequency ratio. The present invention is suitable for dual-frequency and multi-frequency systems, adapting to increasingly complex electromagnetic environments, and can be used in applications such as radar stealth.

[0051] Although the specific implementation of the present invention is described in detail in conjunction with the drawings, it should not be understood as limiting the scope of protection of this patent. Within the scope described in the claims, various modifications and variations that can be made by those skilled in the art without creative work still fall within the scope of protection of this patent.

[0052] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Any simple modification or equivalent change made to the above embodiment based on the technical essence of the present invention shall fall within the protection scope of the present invention.

Claims

1. A dual-passband low-frequency passband independently tunable frequency selective surface, comprising from top to bottom an upper dual-passband metal layer (12), a cross-structured metal layer (17) and a lower dual-passband metal layer (15), wherein the upper dual-passband metal layer (12) and the lower dual-passband metal layer (15) have the same structure and both have dual-polarization characteristics; a metal column (16) passes through the cross-structured metal layer (17) to connect the upper dual-passband metal layer (12) and the lower dual-passband metal layer (15), forming a multi-layer cascade structure to broaden the bandwidth and adjust the characteristics within the passband, characterized in that: The upper dual passband metal layer (12) and the lower dual passband metal layer (15) include an outer metal frame (111) and four branches in the outer metal frame (111), each branch being provided with a variable capacitance diode (11), the side edges of the branches extending toward the outer metal frame (111), and / or the outer metal frame (111) extending toward the side edges of the branches to form a first gap (112); When direct current feeding is performed through the metal pillar (16), the varactor diode (11) is used to provide a first capacitance, the first gap (112) is used to provide a second capacitance, and the first capacitance is greater than the second capacitance, the first capacitance is used to control a low-frequency passband, and the second capacitance is used to control a high-frequency passband; When different DC voltages are applied to both ends of the variable capacitance diode (11), the low-frequency passband moves and the high-frequency passband remains unchanged, thereby achieving independent tuning of the low-frequency passband.

2. The dual-passband low-frequency passband independently tunable frequency selective surface according to claim 1, characterized in that: The upper dual-passband metal layer (12) is printed on the upper surface of the upper dielectric substrate (13), the lower dual-passband metal layer (15) is printed on the lower surface of the lower dielectric substrate (14), the cross-structure metal layer (17) is printed on the lower surface of the upper dielectric substrate (13) or the upper surface of the lower dielectric substrate (14), and the metal column (16) penetrates the upper dielectric substrate (13) and the lower dielectric substrate (14), connecting the upper dual-passband metal layer (12), the cross-structure metal layer (17) and the lower dual-passband metal layer (15).

3. The dual-passband low-frequency passband independently tunable frequency selective surface according to claim 1, characterized in that: The upper dual passband metal layer (12) and the lower dual passband metal layer (15) are both composed of an inner cross metal structure (114), an outer metal frame (111), a varactor (11) and a circular aperture (115); the outer metal frame (111) is square, the inner cross metal structure (114) is located at the center of the outer metal frame (111), and its four branches are perpendicular or parallel to the edges of the outer metal frame (111) and cross and converge at a central disk; the circular aperture (115) is located at the center of the central disk for the metal column (16) to pass through; and the varactor (11) is connected between a branch of the inner cross metal structure (114) and an edge of the outer metal frame (111).

4. The dual-passband low-frequency passband independently tunable frequency selective surface according to claim 3, characterized in that: A square metal block (1111) is respectively arranged at the four inner corners of the outer metal frame (111); the midpoints of the four sides are respectively connected to an extended metal block (1112) in the direction of a branch; and the variable capacitance diode (11) is connected at a second gap (113) formed between the extended metal block (1112) and the corresponding branch.

5. The dual-passband low-frequency passband independently tunable frequency selective surface according to claim 4, characterized in that: The positive electrode of a varactor diode (11) is connected to the extended metal block (1112), and the negative electrode is connected to a branch of the inner cross metal structure (114). When the outer metal frame (111) is connected to the positive electrode of the power supply and the cross structure metal layer (17) is connected to the negative electrode of the power supply, the varactor diode is fed with power.

6. The dual-passband low-frequency passband independently tunable frequency selective surface according to claim 4, characterized in that: The extended metal block (1112) comprises a first metal strip (11121), a wing-shaped metal block (11123) and a second metal strip (11122) which are connected in sequence, one end of the first metal strip (11121) is connected to the midpoint of an edge of the outer metal frame (111), one end of the second metal strip (11122) is connected to the varactor diode (11), and a first gap (112) is provided between the wing-shaped metal block (11123) and the square metal block (1111).

7. The dual-passband low-frequency passband independently tunable frequency selective surface according to claim 4, characterized in that: The cross-structured metal layer (17) is composed of a cross-metal structure (116), a metal disc (117) and a circular hole (118); each branch of the cross-metal structure (116) is parallel or perpendicular to each branch of the inner cross-metal structure (114), cross-intersecting and converging at the metal disc (117); the circular hole (118) is located at the center of the metal disc (117) for the metal column (16) to pass through.

8. The dual-passband low-frequency passband independently tunable frequency selective surface according to claim 1, characterized in that: The type of the varactor diode (11) is MA46H120.

9. The dual-passband low-frequency passband independently tunable frequency selective surface according to claim 8, characterized in that: When the voltage of the DC power supply is changed, the junction capacitance of the varactor diode (11) changes, thereby changing the low-frequency characteristic impedance of the frequency selective surface and shifting the low-frequency passband. The capacitance variation range of the varactor diode (11) is 0.15 pF-0.6 pF. When the DC voltage changes between 2 and 20 V, the low-frequency passband moves within a range of 3.31-6.81 GHz.

10. The dual-passband low-frequency passband independently tunable frequency selective surface according to claim 1, characterized in that: The independent tuning of the low frequency passband is achieved as follows: The positive electrode of the power supply is connected to the outer metal frame (111) of the upper and lower structures, and the negative electrode of the power supply is connected to the cross structure metal layer (17). When the power supply is 2V, the capacitance of the varactor diode is 0.6pF, and the center frequency of the low-frequency passband is 3.31GHz; when the power supply is 4V, the capacitance of the varactor diode is 0.4pF, and the center frequency of the low-frequency passband is 4.22GHz; when the power supply is 6V, the capacitance of the varactor diode is 0.3pF, and the center frequency of the low-frequency passband is 4.8GHz; when the power supply is 20V, the capacitance of the varactor diode is 0.16pF, and the center frequency of the low-frequency passband is 6.81GHz.

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