Low RCS frequency selective surface unit structure with continuously adjustable wave-transparent window and array

Abstract

The invention discloses a low-RCS frequency selective surface unit structure with a continuously adjustable wave-transparent window and an array. The low-RCS frequency selective surface unit structure comprises a top-layer polarization conversion surface and a bottom-layer metal resonance structure, the top-layer polarization conversion surface comprises patch units which are distributed in a diagonal symmetry manner; the patch units are connected through welded first lumped resistors; the patch units of two adjacent top layer polarization conversion surfaces are connected through a welded second lumped resistor; the bottom layer metal resonance structure comprises a square metal ring with a bent branch knot and a cross-shaped metal patch; the variable capacitance diode is welded between the bent branch knot and the cross-shaped metal patch; and the metal through holes are communicated with the cross-shaped metal patches at the centers of the upper surface and the lower surface of the dielectric substrate in the vertical direction, so that the upper surface and the lower surface serve as feeder lines to provide reverse bias voltage for the variable capacitance diodes at the bottom layer, and the resonant frequency of the wave-transparent window is regulated and controlled. The direct current feed mode adopted by the invention does not need an additional feed line, and has the advantages of low RCS out of band and continuous and adjustable passband.

Description

Technical Field

[0001] This invention belongs to the field of electromagnetic field and microwave technology, specifically relating to a low RCS frequency selective surface unit structure and array with continuously adjustable wave-transmitting windows. Background Technology

[0002] Frequency-selective surfaces (FSSs) have been widely used in numerous fields due to their unique ability to customize transmission or reflection performance on demand. In particular, bandpass FSSs are widely used as stealth radar radomes, reducing out-of-band back reflections without affecting in-band antenna radiation. However, with the rapid development of advanced radar technology, this simple out-of-band reflection characteristic cannot meet the growing demand for low observability. Therefore, in the past few decades, various methods for reducing radar cross-section (RCS) have emerged, such as absorptive frequency-selective structures, phase cancellation techniques, or combinations of both. Among these, arranging polarization-conversion surfaces in a checkerboard pattern can achieve broadband RCS reduction based on diffuse reflection mechanisms and has been extensively studied due to its simple and effective structure.

[0003] To simultaneously achieve in-band transmission and out-of-band RCS reduction, some research has attempted to combine the FSS and polarization conversion surface by modifying the top layer shape of a multilayer bandpass FSS. However, their broadband transmission characteristics weaken in-band stealth capabilities to some extent, and since they are all passive structures, they are difficult to adapt to complex and variable electromagnetic environments. Although traditional active FSSs can solve this problem, the additional feed lines introduced by loading active components severely limit their integration with RCS reduction structures.

[0004] In Y. Ma, W. Wu, Y. Yan, S. Wang, W. Yuan, J. Huang, and N. Yuan, “Broadband RCS reduction metasurface with a reconfigurable high-selectivity transmission band,” IEEE Trans. Microw. Theory Techn., vol. 72, no. 2, pp. 878–891, Feb. 2024, a multi-layer bandpass FSS structure with low out-of-band RCS and switchable passband was proposed. Although it can maintain an RCS reduction capability of more than 10 dB in the 3.26–9.7 GHz frequency band and a relative bandwidth of about 99.4%, this unit structure adopts a multi-layer design and the different layers are separated by air layers, which makes it difficult to achieve miniaturization.

[0005] To date, there has been little research on single-dielectric-layer FSS structures that can simultaneously achieve in-band reconfigurability and out-of-band RCS reduction.

[0006] In L.-M. Zhang, X. Ding, and W. Shao, “A multifunctional active frequency-selective surface with scattering diffusivity and transmission passbandtunability,” IEEE Antennas Wireless Propag. Lett., vol. 23, no. 1, pp. 229–233, Jan. 2024, a dual-dielectric-layer tunable FSS structure based on a randomly coded metasurface was proposed. Although no air layer is needed between the dielectric layers, and the overall profile height is significantly reduced, it still requires an additional feeder, which increases the manufacturing cost and process complexity to some extent. Moreover, this structure only achieves a 12 dB RCS reduction at a single frequency of 9.1 GHz, and the RCS reduction bandwidth needs to be further improved.

[0007] To meet the practical application requirements of engineering fields such as advanced radar radomes, further research is needed on how to achieve in-band reconfigurability and out-of-band broadband RCS reduction capabilities while minimizing design complexity and cost. Summary of the Invention

[0008] In order to overcome the shortcomings of the existing technology, the purpose of this invention is to propose a low RCS frequency selective surface unit structure and array with continuously adjustable transmission window, so as to achieve adjustable in-band transmission window and reduced out-of-band RCS, while greatly reducing the unit size without the need for additional feed lines.

[0009] In order to achieve the above object, the present invention provides the following technical solutions:

[0010] This invention first provides a low RCS frequency selective surface unit structure with continuously adjustable transmission window, comprising a dielectric substrate and frequency selective surface units distributed on the dielectric substrate; the frequency selective surface unit includes a top polarization conversion surface on the upper surface of the dielectric substrate, a first lumped resistor and a second lumped resistor loaded on the upper surface of the dielectric substrate, a bottom metal resonant structure on the lower surface of the dielectric substrate, a varactor diode loaded on the lower surface of the dielectric substrate, and a metal via connecting the upper and lower surfaces, wherein:

[0011] The top polarization conversion surface includes patch units distributed diagonally symmetrically; the patch units are connected by a first lumped resistor welded together; and the patch units of two adjacent top polarization conversion surfaces are connected by a second lumped resistor welded together.

[0012] The underlying metal resonant structure is symmetrical about the X and Y axes and includes: a square metal ring with bent branches and a cross-shaped metal patch; the cross-shaped metal patch is connected to a rectangular patch with an arc edge on one side of the upper surface of the dielectric substrate through a metal through-hole; the square metal ring with bent branches and the cross-shaped metal patch are connected by a varactor diode; the positive terminal of the varactor diode is connected to the cross-shaped metal patch, and the negative terminal of the varactor diode is connected to the square metal ring with bent branches;

[0013] The positive terminal of the DC power supply is connected to the square metal ring with bent branches in the bottom metal resonant structure; the negative terminal of the DC power supply is connected to the top polarization conversion surface, and a reverse bias voltage is provided to the cross-shaped metal patch at the center of the bottom metal resonant structure through the metal via; by changing the reverse bias voltage value, the capacitance value of the varactor diode is controlled, thereby regulating the transmission frequency of the frequency selection surface and realizing the continuously adjustable function of the transmission window.

[0014] The patch unit includes an L-shaped stub, a rectangular pad, and a rectangular patch with a curved edge on one side; the rectangular pad is located at the end of the L-shaped stub, and the rectangular patch with a curved edge on one side is located at the center connection of the L-shaped stub; the gap between the end of the L-shaped stub and the rectangular pad is connected by a soldered second lumped resistor; adjacent rectangular patches with a curved edge on one side are connected by a soldered first lumped resistor.

[0015] The rectangular patch with one curved edge is a structure composed of a semi-circular patch and a rectangular patch connected together. The center of the semi-circular patch is located at the center of the wide side of the rectangular patch, and the diameter of the semi-circular patch is equal to the width of the rectangular patch.

[0016] The two ends of the rectangular pad are connected to two adjacent frequency selective surface units; a second lumped resistor is welded to the gap in the middle of the rectangular pad to electrically connect all adjacent frequency selective surface units.

[0017] The bent stubs on the square metal ring include a first bent stub and a second bent stub, and the second bent stub has a longer bending length and a wider line width than the first bent stub, which facilitates the welding of the varactor diode between the second bent stub and the cross-shaped metal patch.

[0018] The second bent branch is located at the center of the side length of the square metal ring, and the first bent branch is located between the vertex and the center point of the side length of the square metal ring.

[0019] The top polarization conversion surface is symmetrical about the diagonal; the bottom metal resonant structure is symmetrical about the X and Y axes, that is, about the center.

[0020] The first and second lumped resistors are both 0.6mm × 0.3mm in size and have a resistance of 40.2kΩ, providing isolation between radio frequency and DC signals.

[0021] The varactor diode measures 1.6mm × 0.8mm and is capable of providing a variable capacitance between 0.35 and 3.2pF under a reverse bias voltage between 20 and 0V.

[0022] The period of the frequency selective surface unit structure is 7.5 mm; the dielectric substrate is made of Rogers 4350B material with a thickness of 2.032 mm, a dielectric constant of 3.66, and a loss tangent of 0.004; the diameter of the metal via is 0.28 mm.

[0023] The present invention also provides a low RCS frequency selective surface array with continuously adjustable transmission window, wherein the low RCS frequency selective surface unit structure with continuously adjustable transmission window provided above is used as array unit for arrangement.

[0024] The array units are arranged in a 2×2 checkerboard array, which includes a first subarray and a second subarray. The subarrays along the diagonal are identical, and each subarray consists of 8×8 frequency selective surface units.

[0025] The frequency-selective surface elements used in the first subarray and the second subarray have a phase difference of 180°.

[0026] By rotating the frequency selective surface unit structure 90° around the Z-axis, a 180° phase change of the frequency selective surface unit can be achieved.

[0027] Compared with the prior art, the low RCS frequency selection structure with continuously adjustable transmission window disclosed in this invention has the following advantages:

[0028] 1. This invention integrates the polarization conversion surface with the tunable frequency selection surface, enabling the simultaneous achievement of both continuously adjustable in-band transmission window and out-of-band broadband RCS reduction using only a single-layer dielectric substrate.

[0029] 2. This invention achieves continuous tuning characteristics in the transmission passband within the 1.4-4 GHz range by controlling the reverse bias voltage of the varactor diode, with an insertion loss of less than 4.67 dB. Simultaneously, it achieves broadband RCS reduction characteristics within the 7.8-18.4 GHz range, with a reduction value exceeding 8 dB.

[0030] 3. The present invention employs a bent structure and compact array to miniaturize the adjustable frequency selective surface, maintaining a stable transmission window and broadband RCS reduction capability when the TE and TM incident waves change from 0° to 30°.

[0031] 4. This invention only needs to provide bias voltage to the varactor diode through its own structure, without the need for additional feed lines, which reduces design complexity and manufacturing difficulty, and saves costs. Attached Figure Description

[0032] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0033] Figure 1 This is a schematic diagram of the overall structure of the adjustable frequency selective surface unit provided in an embodiment of the present invention.

[0034] Figure 2 This is a schematic diagram of the top polarization conversion surface of the adjustable frequency selective surface unit provided in an embodiment of the present invention.

[0035] Figure 3 This is a schematic diagram of the bottom metal resonant structure of the adjustable frequency selective surface unit provided in an embodiment of the present invention.

[0036] Figure 4 This is a schematic diagram of the overall structure of the adjustable frequency selective surface array provided in an embodiment of the present invention.

[0037] Figure 5(a) is a schematic diagram of the electric field decomposition of the top polarization conversion surface for incident electromagnetic waves in the u and v directions provided in the embodiment of the present invention.

[0038] Figure 5(b) is a schematic diagram showing the comparison of reflection amplitude and phase before and after loading a metal via on the adjustable frequency selective surface unit provided in the embodiment of the present invention.

[0039] Figure 6(a) is a schematic diagram of the simulation results of the adjustable frequency selective surface structure provided in the embodiment of the present invention when the varactor diode is at different capacitance values.

[0040] Figure 6(b) is a schematic diagram of the simulation results of the polarization conversion rate of the adjustable frequency selective surface structure provided in the embodiment of the present invention when the varactor diode is at different capacitance values.

[0041] Figure 7(a) is a schematic diagram of the oblique incidence simulation results of the adjustable frequency selective surface structure provided in the embodiment of the present invention when TE polarized electromagnetic waves are incident.

[0042] Figure 7(b) is a schematic diagram of the oblique incidence simulation results of the tunable frequency selectable surface structure provided in the embodiment of the present invention when TM polarized electromagnetic waves are incident.

[0043] Figure 8(a) is a schematic diagram of the oblique incidence simulation results of the adjustable frequency selective surface structure provided in the embodiment of the present invention with RCS reduction when TE polarized electromagnetic wave is incident.

[0044] Figure 8(b) is a schematic diagram of the oblique incidence simulation results of the adjustable frequency selective surface structure provided in the embodiment of the present invention with RCS reduction when TM polarized electromagnetic waves are incident.

[0045] The following are the labels in the diagram: 1. Dielectric substrate; 2. Top polarization conversion surface; 21. L-shaped stub; 22. Rectangular pad; 23. Rectangular patch with one side arc edge; 3. Bottom metal resonant structure; 31. Square metal ring with bent stub; 311. First bent stub; 312. Second bent stub; 32. Cross-shaped metal patch; 4. First lumped resistor; 5. Second lumped resistor; 6. Varactor diode; 7. Metal via; 8. Checkerboard array; 81. First subarray; 82. Second subarray. Detailed Implementation

[0046] To enable those skilled in the art to better understand the technical solutions of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. Examples of the embodiments are shown in the accompanying drawings. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0047] This invention discloses a low RCS frequency selective surface unit structure and array with continuously adjustable transmission window, such as... Figure 1 As shown in Figure -8.

[0048] The present invention provides a low RCS frequency selective surface unit structure with continuously adjustable wave transmission window, comprising: a dielectric substrate 1, and frequency selective surface units distributed on the dielectric substrate 1.

[0049] Each frequency-selective surface unit includes a top polarization conversion surface 2 on the upper surface of the dielectric substrate 1, a first lumped resistor 4 and a second lumped resistor 5 loaded on the upper surface of the dielectric substrate 1, a bottom metal resonant structure 3 on the lower surface of the dielectric substrate 1, a varactor diode 6 loaded on the lower surface of the dielectric substrate 1, and a metal via 7 connecting the upper and lower surfaces.

[0050] In one embodiment, such as Figure 1As shown, the structure involved in this invention is formed by printing a top polarization conversion surface 2 and a bottom metal resonant structure 3 on both sides of a 2.032mm thick Rogers 4350B dielectric substrate 1 (dielectric constant is 3.66, loss tangent is 0.004); the period of the entire structural unit is 7.5mm, and the overall thickness is 2.067mm (metal thickness is 0.017mm).

[0051] The top polarization conversion surface 2 includes: an L-shaped stub 21, a rectangular pad 22, and a rectangular patch 23 with an arc edge on one side; the rectangular pads 22 are distributed at the ends of the L-shaped stub 21, and the gaps between the rectangular pads 22 are connected by a soldered second lumped resistor 5; the rectangular patches 23 with an arc edge on one side are distributed at the center connection of the L-shaped stub 21, and adjacent rectangular patches 23 with an arc edge on one side are connected by a soldered first lumped resistor 4.

[0052] The rectangular patch 23 with a curved edge on one side is a structure composed of a semi-circular patch and a rectangular patch connected together. The center of the semi-circular patch is located at the center of the wide side of the rectangular patch, and the diameter of the semi-circular patch is equal to the width of the rectangular patch.

[0053] The two ends of the rectangular pad 22 are connected to two adjacent frequency selective surface units; a second lumped resistor 5 is soldered in the gap in the middle of the rectangular pad 22 to electrically connect all adjacent frequency selective surface units.

[0054] In a specific embodiment, see Figure 2 The unit period of the top polarization conversion surface 2 is p = 7.5 mm; the length of the L-shaped branch 21 is l1 = 3.45 mm and the width is w1 = 0.15 mm; the length of the rectangular pad 22 is l2 = 0.6 mm and the width is w2 = 0.15 mm, with a gap s1 = 0.3 mm in the middle; the sum of the lengths of the two symmetrical rectangular patches 23 with one side arc edge (including the gap of 0.3 mm) is l3 = 0.924 mm and the width is w3 = 0.5 mm; the diameter of the metal via 7 is d1 = 0.28 mm.

[0055] Specifically, the first lumped resistor 4 and the second lumped resistor 5 loaded on the top polarization conversion surface 2 are both 0.6mm × 0.3mm in size and have a resistance value of 40.2kΩ, which can provide isolation between radio frequency and DC signals.

[0056] Specifically, the top polarization conversion surface 2 is symmetrical about its diagonal. The unit metal pattern resembles a drone, with two L-shaped branches and four rectangular pads symmetrical about the diagonal resembling the drone's wings, while the two rectangular patches in the center with one side of an arc-shaped edge resemble the drone's head. This metal pattern enables better polarization conversion functionality of the overall structure.

[0057] The bottom metal resonant structure 3 includes: a square metal ring 31 with bent branches and a cross-shaped metal patch 32; the cross-shaped metal patch 32 is connected to a rectangular patch 23 with an arc edge on one side of the upper surface of the dielectric substrate 1 through a metal through-hole 7; the square metal ring 31 with bent branches and the cross-shaped metal patch 32 are connected by a varactor diode 6; the positive terminal of the varactor diode 6 is connected to the cross-shaped metal patch 32, and the negative terminal is connected to the square metal ring 31 with bent branches.

[0058] The bent stub (31) on the square metal ring includes a first bent stub (311) and a second bent stub (312), and the second bent stub (312) has a longer bending length and a wider line width than the first bent stub (311), which facilitates the welding of the varactor diode (6) between the second bent stub (312) and the cross-shaped metal patch (32);

[0059] The second bent branch 312 is located at the center of the side length of the square metal ring, and the first bent branch 311 is located between the vertex and the center point of the side length of the square metal ring.

[0060] In a specific embodiment, see Figure 3 In the bottom-layer metal resonant structure 3, the distance from the vertex of the square metal ring to the first bent spur is l4 = 0.95 mm, the width of the first bent spur is w4 = 0.25 mm, the vertical bending length (perpendicular to the side length of the square metal ring) of the first bent spur is h1 = 0.85 mm, and the horizontal bending length (parallel to the side length of the square metal ring) is l5 = 1.1 mm. The second bent spur is located at the center of the side length of the square metal ring, with a vertical bending length of h2 = 1.3 mm and a vertical bending width of... The bend length is w4 = 0.25 mm, the horizontal bend length is l6 = 1.6 mm, and the horizontal bend width is w6 = 0.6 mm. The central cross-shaped metal patch can be seen as a square patch with a small square cut off at each of the four vertices, with a corresponding side length of l7 = 2.9 mm and a side length of s2 = 0.65 mm for the cut-off small square. A varactor diode is arranged in the gap between the second bend and the cross-shaped metal patch, with a gap length of l8 = 1 mm and a varactor diode width of w8 = 0.8 mm.

[0061] Specifically, the varactor diode 6 loaded on the underlying metal resonant structure 3 is model SMV2020-079LF, SC-79 package, with a size of 1.6mm × 0.8mm, and can provide a variable capacitance between 0.35-3.2pF under a reverse bias voltage between 20-0V.

[0062] Specifically, the bottom-layer metal resonant structure 3 is symmetrical about the X and Y axes, i.e., symmetrical about the center. The unit metal pattern is a parallel LC resonant structure with bandpass frequency selectivity. The structural prototype consists of a square metal ring and a central square patch. Based on this, the square metal ring is bent twice, and a small square is cut off from each of the four corners of the central square patch. Through these two methods, the final bottom-layer metal resonant structure 3 has lower insertion loss and wider passband bandwidth.

[0063] The present invention provides a low RCS frequency selective surface array with continuously adjustable transmission window, which arranges the frequency selective surface unit structure in a 2×2 checkerboard array to achieve the function of low out-of-band RCS.

[0064] The 2×2 checkerboard array 8 includes a first subarray 81 and a second subarray 82. The subarrays along the diagonal are identical, and each subarray consists of 8×8 frequency-selective surface units.

[0065] The frequency-selective surface elements used in the first subarray 81 and the second subarray 82 have a phase difference of 180°.

[0066] By rotating the frequency selective surface unit structure 90° around the Z-axis, a 180° phase change of the frequency selective surface unit can be achieved.

[0067] In a specific embodiment, see Figure 4 The two frequency-selective surface units with a 180° phase difference have identical structures, differing only in that they are rotated 90° around the Z-axis, making implementation simple. The overall structure of the frequency-selective surface array 8 consists of 16×16 unit structures, with an overall size of 120mm×120mm. Each subarray comprises 8×8 units, resulting in a total of 2×2 subarrays. With the X and Y axes as boundaries, the first subarray 81 is located in the second and fourth quadrants, while the second subarray 82 is located in the first and third quadrants.

[0068] Please see again. Figure 1 , Figure 2 and Figure 3The positive terminal of the DC power supply is connected to the second bent branch 312 of the square metal ring in the bottom metal resonant structure 3; the negative terminal of the DC power supply is connected to the L-shaped branch 21 in the top polarization conversion surface 2; further, the top polarization conversion surface 2 is connected to the cross-shaped metal patch 32 at the center of the bottom metal resonant structure 3 through the metal through hole 7; the top polarization conversion surface 2 is connected to the adjacent unit through the first lumped resistor 4 and the second lumped resistor 5, and the square metal ring 31 with bent branches on the periphery of the bottom metal resonant structure 3 is connected to the adjacent unit.

[0069] Specifically, for the frequency selective surface array 8 structure, the entire top-layer polarization conversion surface 2 structure is completely connected to form a whole, providing a negative voltage for the varactor diode 6; all the square metal rings 31 with bent branches in the bottom-layer metal resonant structure 3 are completely connected to form a whole, providing a positive voltage for the varactor diode 6. Therefore, for the proposed frequency selective surface structure, without additional feed lines, the reverse bias voltage value can be changed through the structure itself to control the change in the capacitance value of the varactor diode 6, thereby regulating the transmission frequency of the frequency selective surface structure and realizing the continuously adjustable function of the transmission window.

[0070] The technical effects of this invention will be further explained below with reference to simulation experiments:

[0071] To better illustrate the polarization conversion characteristics of the top polarization conversion surface 2, in this embodiment of the invention, the electric field components of the incident and reflected electromagnetic waves in the u and v directions of the top polarization conversion surface 2 are decomposed and simulated, as shown in Figure 5.

[0072] Specifically, referring to Figure 5(a), the incident electric field E i Along the positive X-axis, its components in the u and v directions are E, respectively. iu and E iv Reflected electric field E r Along the positive Y-axis, its components in the u and v directions are E, respectively. ru and E rv When the reflected electric field component E ru and E rv If the amplitudes are equal and the phase difference is 180°, it means that a good polarization conversion effect has been achieved within a specific frequency band.

[0073] Furthermore, in order to explore the influence of the introduction of the metal via 7 on the polarization conversion characteristics of the frequency-selective surface structure, the reflection amplitude and phase of the top polarization conversion surface 2 with and without the metal via 7 were simulated along the u and v directions. The results are shown in Figure 5(b).

[0074] Specifically, referring to Figures 5(a) and 5(b), it can be observed that within the 8-18 GHz frequency band, the reflection amplitudes of the top polarization conversion surface 2 are basically equal in the u and v directions, with a reflection phase difference of around 180°. Moreover, the introduction of the metal via 7 generates two zero-reflection points f1 and f2 in the u direction, located on both sides of the polarization conversion frequency band, but has no effect in the v direction, thus significantly improving the selectivity on both sides of the polarization conversion frequency band.

[0075] To verify the continuously adjustable transmission window and out-of-band polarization conversion characteristics of the proposed adjustable frequency selective surface unit structure, the transmission amplitude and polarization conversion rate controlled by the varactor diode in the embodiment of the present invention were simulated, and the results are shown in Figure 6.

[0076] Specifically, referring to Figure 6(a), it can be observed that as the capacitance of varactor diode 6 changes from 0.35pF to 3.2pF, the passband resonant frequency can be continuously tuned within the 1.4-4GHz frequency range, covering the entire S-band and part of the L-band, with a relative bandwidth of 96.3%, and the insertion loss is consistently less than 4.67dB. Furthermore, for an insertion loss of 3dB, the passband resonant frequency can be continuously tuned within the 1.8-4GHz frequency range, with a relative bandwidth of 75.9%.

[0077] Specifically, referring to Figure 6(b), it can be seen that as the capacitance value of the varactor diode 6 changes from 0.35pF to 3.2pF, the polarization conversion frequency band will shift to lower frequencies and the bandwidth will become narrower, but it can still basically cover the X and Ku bands.

[0078] To verify that the proposed adjustable frequency selection unit structure has angular stability characteristics in the transmission passband, the transmission amplitude of the embodiment of the present invention was simulated when TE and TM polarized electromagnetic waves were obliquely incident at different angles. The results are shown in Figure 7.

[0079] Specifically, refer to Figure 7(a), which shows the transmission amplitude curves of TE-polarized electromagnetic waves at different incident angles. It can be seen that as the angle of oblique incident electromagnetic waves increases to 45°, the bandwidth of the passband will decrease slightly and the insertion loss will increase slightly. However, the resonant frequency of the passband is basically the same as that when the wave is perpendicularly incident, and the insertion loss in the passband is always less than 1dB, showing good angular stability.

[0080] Specifically, refer to Figure 7(b), which shows the transmission amplitude curves of TM polarized electromagnetic waves at different incident angles. It can be seen that as the angle of oblique incident electromagnetic waves increases to 45°, the bandwidth of the passband increases slightly, the insertion loss remains basically unchanged, and the resonant frequency of the passband is basically the same as that of vertical incident waves. The insertion loss in the passband is always less than 1dB, showing good angular stability.

[0081] To verify that the proposed adjustable frequency selective surface array has angular stability in its RCS reduction capability, the RCS reduction values ​​of the embodiments of the present invention were simulated when TE and TM polarized electromagnetic waves were obliquely incident at different angles. The reference object was a metal plate with the same size as the proposed adjustable frequency selective surface array (120mm×120mm), and the results are shown in Figure 8.

[0082] Specifically, refer to Figure 8(a), which shows the RCS reduction curves of TE-polarized electromagnetic waves at different incident angles. It can be seen that as the angle of oblique incident electromagnetic waves increases to 45°, in the 7.8-18.4 GHz frequency band, compared with a metal plate of the same size, the RCS is reduced to always above 8 dB, showing good angular stability.

[0083] Specifically, refer to Figure 8(b), which shows the RCS reduction curves of TM polarized electromagnetic waves at different incident angles. It can be seen that as the angle of oblique incident electromagnetic waves increases to 30°, in the 7.8-18.4 GHz frequency band, compared with a metal plate of the same size, the RCS is reduced to always remain above 8 dB, showing good angular stability.

[0084] In summary, the low RCS frequency selective surface unit structure and array with continuously adjustable transmission windows provided by the embodiments of the present invention achieves continuous adjustment of the transmission windows in the L and S bands while maintaining efficient RCS reduction capabilities in the X and Ku bands. The structure of the embodiments of the present invention is simple, miniaturized through a bending structure based on a single-layer dielectric substrate; and it requires no additional feed lines, only needing to provide bias voltage for the varactor diode through its own structure, saving costs. Through the control of the varactor diode, the embodiments of the present invention ultimately achieve continuously adjustable transmission window characteristics with a relative bandwidth of 96.3% in the 1.8-4 GHz frequency band, and achieve RCS reduction characteristics under TE and TM polarized incident waves in the 7.8-18.4 GHz frequency band, with reduction values ​​exceeding 8 dB. Simultaneously, due to the miniaturized design of the units, the overall structure can maintain 30° angular stability.

[0085] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A low RCS frequency selective surface unit structure with continuously adjustable wave-transmitting window, comprising a dielectric substrate (1) and frequency selective surface units distributed on the dielectric substrate (1); characterized in that, The frequency selective surface unit includes a top polarization conversion surface (2) on the upper surface of the dielectric substrate (1), a first lumped resistor (4) and a second lumped resistor (5) loaded on the upper surface of the dielectric substrate (1), a bottom metal resonant structure (3) on the lower surface of the dielectric substrate (1), a varactor diode (6) loaded on the lower surface of the dielectric substrate (1), and a metal via (7) connecting the upper and lower surfaces, wherein: The top polarization conversion surface (2) includes patch units distributed diagonally symmetrically; the patch units are connected by a first lumped resistor (4) soldered together; the patch units of two adjacent top polarization conversion surfaces (2) are connected by a second lumped resistor (5) soldered together. The underlying metal resonant structure (3) is symmetrical about the X-axis and Y-axis, including: a square metal ring (31) with bent branches and a cross-shaped metal patch (32); the cross-shaped metal patch (32) is connected to a rectangular patch (23) with an arc edge on one side on the upper surface of the dielectric substrate (1) through a metal through hole (7); the square metal ring (31) with bent branches and the cross-shaped metal patch (32) are connected by a varactor diode (6); the positive terminal of the varactor diode (6) is connected to the cross-shaped metal patch (32), and the negative terminal of the varactor diode (6) is connected to the square metal ring (31) with bent branches; The positive terminal of the DC power supply is connected to the square metal ring (31) with bent branches in the bottom metal resonant structure (3); the negative terminal of the DC power supply is connected to the top polarization conversion surface (2), and a reverse bias voltage is provided to the cross-shaped metal patch (32) in the center of the bottom metal resonant structure (3) through the metal through hole (7); the capacitance value of the varactor diode (6) is controlled by changing the reverse bias voltage value, thereby regulating the transmission frequency of the frequency selection surface and realizing the continuous adjustable function of the transmission window.

2. The low RCS frequency selective surface unit structure with continuously adjustable transmission window according to claim 1, characterized in that, The patch unit includes an L-shaped stub (21), a rectangular pad (22), and a rectangular patch (23) with a curved edge on one side; the rectangular pad (22) is located at the end of the L-shaped stub (21), and the rectangular patch (23) with a curved edge on one side is located at the center connection of the L-shaped stub (21); the gap between the end of the L-shaped stub (21) and the rectangular pad (22) is connected by a soldered second lumped resistor (5); adjacent rectangular patches (23) with a curved edge on one side are connected by a soldered first lumped resistor (4).

3. The low RCS frequency selective surface unit structure with continuously adjustable transmission window according to claim 2, characterized in that, The patch unit has two rectangular patches (23) with one side arc edge and four L-shaped branches (21), with two L-shaped branches (21) connected to each rectangular patch (23) with one side arc edge.

4. The low RCS frequency selective surface unit structure with continuously adjustable transmission window according to claim 2, characterized in that, The rectangular patch (23) with a curved edge on one side is a structure composed of a semi-circular patch and a rectangular patch connected together. The center of the semi-circular patch is located at the center of the wide side of the rectangular patch, and the diameter of the semi-circular patch is equal to the width of the rectangular patch.

5. The low RCS frequency selective surface unit structure with continuously adjustable transmission window according to claim 2, characterized in that, The two ends of the rectangular pad (22) are connected to two adjacent frequency selective surface units; a second lumped resistor (5) is welded to the gap in the middle of the rectangular pad (22) to make all two adjacent frequency selective surface units electrically connected.

6. The low RCS frequency selective surface unit structure with continuously adjustable transmission window according to claim 1, characterized in that, The bent stub (31) on the square metal ring includes a first bent stub (311) and a second bent stub (312), and the second bent stub (312) has a longer bending length and a wider line width than the first bent stub (311), which facilitates the welding of the varactor diode (6) between the second bent stub (312) and the cross-shaped metal patch (32); The second bent branch (312) is located at the center of the side length of the square metal ring, and the first bent branch (311) is located between the vertex and the center point of the side length of the square metal ring.

7. The low RCS frequency selective surface unit structure with continuously adjustable transmission window according to claim 1, characterized in that, The first lumped resistor (4) and the second lumped resistor (5) are both 0.6 mm × 0.3 mm in size and have a resistance of 40.2 kΩ, which can provide isolation between radio frequency and DC signals; The varactor diode (6) has a size of 1.6 mm × 0.8 mm and is capable of providing a variable capacitance between 0.35 and 3.2 pF under a reverse bias voltage between 20 and 0 V.

8. The low RCS frequency selective surface unit structure with continuously adjustable transmission window according to claim 1, characterized in that, The period of the frequency selective surface unit structure is 7.5 mm; the dielectric substrate (1) is made of Rogers 4350B material with a thickness of 2.032 mm, a dielectric constant of 3.66, and a loss tangent of 0.004; the diameter of the metal via (7) is 0.28 mm.

9. A low RCS frequency selective surface array with continuously adjustable transmission window, characterized in that, The array is arranged using a low RCS frequency selective surface unit structure with continuously adjustable transmission windows as described in any one of claims 1-9.

10. The low RCS frequency selective surface array with continuously adjustable transmission window according to claim 9, characterized in that, The array units are arranged in a 2×2 checkerboard array (8) to achieve the function of low out-of-band RCS; the 2×2 checkerboard array (8) includes a first subarray (81) and a second subarray (82), the subarrays along the diagonal direction are completely identical, and each subarray is composed of 8×8 frequency selective surface units; The frequency-selective surface unit used in the first subarray (81) and the second subarray (82) has a phase difference of 180°; By rotating the low RCS frequency selective surface unit structure with continuously adjustable transmission window as described in any of claims 1-9 by 90° about the Z-axis, a 180° change in the phase of the frequency selective surface unit can be achieved.

Images