Active frequency selective surface with band-edge steepness and polarization stability

Abstract

This invention discloses an active frequency selective surface with a steep passband edge drop and polarization stability, belonging to the field of microwave device technology. The structure comprises periodically arranged unit cells, each employing a three-layer symmetrical configuration, including a top and bottom layer of identical square annular aperture metal patch, a middle layer of cross-shaped mesh metal patch, and an interlayer dielectric substrate. Multiple varactor diodes are symmetrically integrated within the square annular aperture. By continuously adjusting their reverse bias voltage, the junction capacitance is changed, thereby achieving continuous and smooth tuning of the center resonant frequency within the range of 7.75 GHz to 11.2 GHz. This invention achieves a rapid passband edge drop over a wide tuning range through the coordinated resonance of a multi-layered symmetrical structure. The transition band width from -3 dB to -10 dB does not exceed 0.6 GHz, while also exhibiting excellent polarization independence and angular stability, making it suitable for high-performance applications requiring dynamic frequency selection, such as microwave communication and radar detection.

Description

Technical Field

[0001] This invention relates to the field of microwave device technology, specifically to an active frequency selective surface with a steep passband edge and polarization stability. Background Technology

[0002] A Frequency Selective Surface (FSS) is a functional electromagnetic device based on a periodically arranged unit structure that can selectively transmit or reflect the frequency of incident electromagnetic waves. It has wide applications in microwave communications, radar systems, electromagnetic shielding, and antenna design. The filtering characteristics (such as resonant frequency, bandwidth, passband / stopband response) of traditional passive FSSs are fixed by the physical dimensions of their unit structure, substrate material, and thickness, and cannot be dynamically adjusted once fabricated. This presents significant limitations when facing the increasing demands of modern wireless communication and radar systems for flexible operating frequency bands, environmental adaptability, and dynamic reconfiguration capabilities. For example, when the system's operating frequency band needs to be switched or broadened, the FSS device must be replaced or redesigned, resulting in high costs and slow response.

[0003] To overcome the aforementioned drawbacks, Active Frequency Selective Surfaces (AFSS) have emerged. By integrating adjustable components such as PIN diodes, MEMS switches, or varactor diodes into the active unit and using external control signals to change their electrical states, dynamic reconstruction of the FSS filtering characteristics can be achieved. Among these, varactor diodes, due to their junction capacitance changing continuously and smoothly with the applied reverse bias voltage, have become one of the ideal devices for achieving continuous frequency tuning of AFSS.

[0004] However, existing AFSS designs based on varactor diodes still face several key technical bottlenecks that limit their practical application in high-performance systems: First, the tuning range and continuity are insufficient. Most existing designs have a narrow effective tuning range, typically only 1–2 GHz, which is insufficient to cover wide-band requirements such as the X-band and adjacent bands (e.g., 7.75 GHz–11.2 GHz). Furthermore, during tuning, problems such as nonlinear resonant point shift and significantly increased insertion loss can easily occur, affecting the consistency of filtering performance across the entire frequency band.

[0005] Secondly, the frequency selectivity (passband edge roll-off) is poor. Existing AFSSs have a wide transition band between the passband and stopband, with a slow edge roll-off, typically below 20 dB / GHz. This makes adjacent channel signals prone to crosstalk in dense spectrum environments, resulting in insufficient out-of-band suppression and reduced system signal-to-noise ratio and anti-interference capability.

[0006] Furthermore, the polarization and angular stability are poor. In pursuit of a wider tuning range, some designs employ asymmetric unit structures, making them sensitive to the electromagnetic wave polarization direction. The resonant frequencies under horizontal and vertical polarization may shift significantly. In addition, as the incident angle of the electromagnetic wave increases, the resonant frequency and filtering response are prone to drift or deterioration, limiting their applicability in wide-angle incident scenarios such as airborne and vehicle-mounted applications.

[0007] Finally, tuning efficiency and circuit design complexity are issues. The loading position and electromagnetic coupling design of the varactor diode in the structure are often not fully optimized, resulting in low sensitivity of capacitance changes to frequency tuning. The range of capacitance changes required to achieve a certain frequency offset is too large, increasing the design difficulty of the bias circuit and potentially introducing additional impedance mismatch and performance degradation.

[0008] Therefore, there is an urgent need in the existing technology for an active frequency selective surface that can achieve continuous and stable tuning over a wide frequency band, while possessing high frequency selectivity, excellent polarization and angular stability, in order to meet the urgent needs of modern RF systems for high-performance reconfigurable filters. Summary of the Invention

[0009] The purpose of this invention is to provide an active frequency selective surface and its manufacturing method with passband edge steep drop characteristics and polarization stability, so as to solve the performance shortcomings of existing varactor diode-loaded AFSSs mentioned in the background art in terms of tuning range, filtering steepness, polarization adaptability and angle stability.

[0010] To achieve the above objectives, the present invention provides the following technical solution: Active frequency selective surfaces with passband edge steep drop characteristics and polarization stability include periodically arranged unit cell structures; The unit structure includes at least two metal patches and a dielectric layer disposed between adjacent metal patches; At least one layer of the metal patch has an opening or a slot, and a plurality of varactor diodes are loaded at the opening or slot; By changing the reverse bias voltage applied to the varactor diode, its junction capacitance is continuously adjusted, thereby making the resonant frequency of the active frequency selective surface continuously adjustable in the range of 7.75 GHz to 11.2 GHz.

[0011] Furthermore, the unit structure has three layers, including a first metal patch layer, a second metal patch layer, and a third metal patch layer stacked sequentially from top to bottom. The first and third metal patch layers have the same pattern, both being patches with square annular apertures. The second metal patch layer has a cross-shaped grid with a central square patch. The varactor diodes are symmetrically loaded within the square annular apertures.

[0012] Furthermore, four varactor diodes are symmetrically loaded within each square annular aperture, and the unit structure as a whole simultaneously satisfies both central symmetry and axial symmetry.

[0013] Furthermore, the period P of the unit structure is 5mm, the total thickness H of the dielectric layer is 5mm, the metal linewidth W of the square annular aperture is 0.2mm, the side length L3 of the central square patch is 1.8mm, the outer side length L1 of the square annular aperture is 4mm, and the inner side length L2 of the square annular aperture is 3.5mm.

[0014] Furthermore, the junction capacitance of the varactor diode is continuously adjustable in the range of 0.4pF to 2.0pF, corresponding to a reverse bias voltage adjustment range of 20V to 4V.

[0015] Furthermore, the model number of the varactor diode is GC15008-00.

[0016] Furthermore, by adjusting the capacitance of the varactor diode, the bandwidth required for the transmission coefficient at the passband edge of the active frequency selective surface to decrease from -3dB to -10dB in any tuning state is less than or equal to 0.6GHz.

[0017] A method for manufacturing an active frequency selective surface includes the following steps: S1: The dielectric layer is printed layer by layer using photopolymerization 3D printing technology, and a groove for accommodating the varactor diode is formed at a preset position; S2: On the surface of the formed dielectric layer, selective laser melting printing technology is used to print the metal patch layer by layer using spherical copper powder as raw material; S3: Install the varactor diode into the reserved groove on the metal patch and fix it using laser welding process.

[0018] Furthermore, in step S1, a resin medium layer is printed using Digital Light Processing (DLP) photocuring technology, with a single layer printing thickness of 50 μm, and then UV cured; the total printing thickness of the medium layer is 5 mm; in step S2, the particle size range of the spherical copper powder used is 15 μm to 53 μm.

[0019] Furthermore, after step S3, step S4 is also included: performing vapor deposition encapsulation on the welded varactor diode using a parylene coating; and the method sequentially performs the following steps: printing a first dielectric layer; printing a first metal patch on the first dielectric layer and integrating the varactor diode; printing a second dielectric layer; printing a second metal patch on the second dielectric layer; printing a third dielectric layer; printing a third metal patch on the third dielectric layer and integrating the varactor diode.

[0020] Compared with the prior art, the beneficial effects of the present invention are: (1) Achieve wideband continuous tuning: By adjusting the reverse bias voltage of the varactor diode, its junction capacitance is continuously varied in the range of 0.4pF to 2.0pF, thereby driving the center resonant frequency of the active frequency selective surface to continuously and smoothly shift in the wide range of 3.45GHz from 7.75GHz to 11.2GHz; (2) Obtaining a steep passband edge: Based on the synergistic resonance effect generated by the multi-layer cascaded (three-layer) symmetrical structure, the passband edge of the structure exhibits a fast roll-off characteristic throughout the entire wideband tuning range. The transition band width required for the transmission coefficient to drop from -3dB to -10dB does not exceed 0.6GHz, and it has good frequency selectivity. (3) Maintaining excellent polarization stability: The unit structure satisfies both centrosymmetry and axisymmetry. The top and bottom layers use the same square annular aperture pattern, and the middle layer is a cross-shaped grid. This symmetrical design makes its response to TE and TM waves basically the same, and the resonant frequency does not change with the polarization direction of the incident electromagnetic wave; (4) Good angular stability: The symmetrical multilayer structure and the uniform dielectric substrate work together to reduce the influence of electromagnetic waves when incident at an oblique angle. When the incident angle reaches 30°, the resonant frequency shift is small (typical value ≤0.3GHz), and the passband characteristics remain stable, making it suitable for large-angle incident scenarios; (5) Optimized tuning efficiency and reliability: The varactor diodes are symmetrically loaded within the square annular aperture, which improves the sensitivity of capacitance changes to frequency tuning. The adopted "photopolymerization 3D printing medium substrate + selective laser melting printing metal patch" combination process helps to achieve precision manufacturing of multilayer structures and high-reliability integration of active devices. Attached Figure Description

[0021] Figure 1 This is a flowchart illustrating the manufacturing method of the active frequency selective surface with passband edge steep drop characteristics and polarization stability according to the present invention. Figure 2 This is the equivalent circuit diagram of a varactor diode; Figure 3A schematic diagram of a tuned AFSS unit with a loaded varactor diode; Figure 4 The transfer characteristics of a tuned AFSS at different varactor diode capacitance values; Figure 5 The tuning state is 1-TE polarization; Figure 6 For tuning state 1-TM polarization; Figure 7 The tuning state is 2-TE polarization; Figure 8 For the tuned state 2-TM polarization; Figure 9 3-TE polarization is in the tuning state; Figure 10 The state is 3-TM polarization. Detailed Implementation

[0022] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] This invention provides an active frequency selective surface with a steep passband edge and polarization stability. For example... Figure 3 As shown, its core lies in a periodically arranged unit structure. The unit structure includes at least two metal patches and a dielectric layer disposed between adjacent metal patches; at least one of the metal patches has an opening or gap, and multiple varactor diodes are loaded at the opening or gap; by changing the reverse bias voltage applied to the varactor diodes, their junction capacitance is continuously adjusted, thereby making the resonant frequency of the active frequency selective surface continuously tunable in the range of 7.75 GHz to 11.2 GHz. This AFSS, by introducing active devices and combining a specific multilayer symmetrical configuration, achieves continuous and smooth tuning of the resonant frequency over a wide frequency band, while also possessing excellent performance in terms of steep passband edge drop, polarization stability, and angle stability.

[0024] Specifically, the unit structure is a three-layer symmetrical configuration. See also Figure 3 The structure, from top to bottom, includes: a first metal patch layer (top layer), a first dielectric substrate, a second metal patch layer (middle layer), a second dielectric substrate, and a third metal patch layer (bottom layer).

[0025] Top and bottom metal patches: The first metal patch layer 1 and the third metal patch layer 5 have identical patterns, both being metal patches with square annular apertures. These patterns can be periodically arranged along a two-dimensional direction to form the planar periodic structure of the FSS.

[0026] Intermediate metal patch: The pattern of the second metal patch layer 3 consists of a cross-shaped grid and a square patch located in the center of the grid.

[0027] Dielectric substrates: The first dielectric substrate 2 and the second dielectric substrate 4 are preferably made of low-loss resin, which is used to support and isolate the metal layers.

[0028] Active device loading: To achieve tunable characteristics, multiple varactor diodes are symmetrically loaded within the gaps of the square annular aperture. In one specific design, four varactor diodes are loaded symmetrically within each square ring. The entire unit structure is designed to simultaneously satisfy centrosymmetry and axial symmetry, which is crucial for ensuring its polarization insensitivity.

[0029] The unit structure has a period P of 5mm, a total dielectric layer thickness H of 5mm, a metal linewidth W of 0.2mm for the square annular aperture, a side length L3 of 1.8mm for the central square patch, an outer side length L1 of 4mm for the square annular aperture, and an inner side length L2 of 3.5mm for the square annular aperture. The junction capacitance of the varactor diode is continuously adjustable from 0.4pF to 2.0pF, corresponding to a reverse bias voltage adjustment range of 20V to 4V. The varactor diode is model GC15008-00. By adjusting the capacitance of the varactor diode, the bandwidth required for the transmission coefficient at the passband edge of the active frequency selective surface to decrease from -3dB to -10dB in any tuning state is less than or equal to 0.6GHz.

[0030] To achieve optimal performance in the 7.75GHz-11.2GHz frequency band (covering parts of the C-band and X-band), this embodiment provides a set of optimized design parameters as follows: Unit period (P): 5mm Total thickness of dielectric substrate (H): 5mm Square annular aperture metal linewidth (W): 0.2mm The side length (L3) of the central square patch in the intermediate layer is 1.8mm. The outer side length (L1) of the square annular aperture is 4mm. Inner side length (L2) of the square annular aperture: 3.5mm The core active device for achieving electrical tuning is the varactor diode. Its equivalent circuit is as follows: Figure 2 As shown, its junction capacitance C j With the applied reverse bias voltage V RThe voltage changes continuously. In this embodiment, a varactor diode of model GC15008-00 is preferred. When the reverse bias voltage V... R When the voltage changes from 20V to 4V, its junction capacitance C j It is continuously adjustable within the range of 0.4pF to 2.0pF. This capacitance variation range works in conjunction with the aforementioned three-layer symmetrical structure to efficiently convert capacitance changes into resonant frequency shifts. The working principle of this AFSS is as follows: When an electromagnetic wave is incident perpendicularly, its multi-layered symmetrical structure (especially the square annular apertures of the top / bottom layers and the cross-shaped mesh of the middle layer) will generate coordinated resonance, forming a passband response. The varactor diode loaded in the annular gap has a junction capacitance C... j It directly participates in the equivalent LC resonant circuit of the structure. The reverse bias voltage V applied to the varactor diode is changed by an external control circuit. R C can be continuously and precisely adjusted j This allows for continuous changes in the equivalent capacitance of the structure and the overall resonance conditions, thereby shifting the resonant frequency.

[0032] Figure 4 The simulated transmission characteristics (S21 parameters) of the AFSS are shown when the varactor diode has different capacitance values ​​(0.4pF, 1.2pF, 2.0pF). The solid and dashed lines in the figure represent TE (transverse electric) and TM (transverse magnetic) polarized incident waves, respectively. The following key characteristics can be observed: Wideband continuous tuning: with C j Increasing the voltage from 0.4pF to 2.0pF, the center resonant frequency of the passband continuously shifts from approximately 11.2GHz to approximately 7.75GHz, fully covering the design target frequency band, with a tuning range exceeding 3GHz.

[0033] Sharp roll-off at the passband edge: In each tuning state, the passband edge exhibits a rapid roll-off characteristic. For example, when C... j At C = 0.4pF, the transition band width required for the lower passband to drop from -3dB to -10dB is only about 0.6GHz; when C j At 2.0 pF, this bandwidth is approximately 0.5 GHz. This achieves frequency selectivity similar to that of a high-order filter.

[0034] Excellent polarization stability: for every fixed C j The transmission curves of the TE and TM waves almost completely overlap. This proves that the centrally symmetric and axisymmetric unit structure effectively eliminates the dependence on the polarization direction of the incident wave and has excellent polarization stability.

[0035] Angular stability verification

[0036] To evaluate the angular stability of this AFSS, the filtering characteristics were calculated at the incident angle. (Figure 5 to...) Figure 10 The diagram shows the transmission characteristics of this tuned AFSS under TE and TM polarization in tuning states 1 (varactor diode capacitance of 0.4 pF), 2 (varactor diode capacitance of 1.2 pF), and 3 (varactor diode capacitance of 2.0 pF). In tuning state 1, the AFSS exhibits a stable passband frequency response within an incident angle range of 0° to 40°. When the incident angle θ > 40°, the resonant frequency shifts by 0.2 GHz and 0.3 GHz, respectively. In tuning state 2, the resonant frequency shifts slightly to a lower frequency when the TE wave is obliquely incident compared to when it is perpendicularly incident. The AFSS performance is worse when the TM wave is perpendicularly incident compared to when it is obliquely incident. In tuning state 3, the resonant frequency remains essentially unchanged, and the transmission characteristics do not change significantly. Overall, the designed AFSS exhibits good angular stability.

[0037] The AFSS of this invention can achieve high-precision integrated manufacturing using advanced additive manufacturing (3D printing) technology. The manufacturing process can be referred to [reference needed]. Figure 1 The specific steps are as follows: Dielectric substrate molding: DLP photopolymerization 3D printing technology is used to print the dielectric substrate layer by layer using resin as the material. The printing layer thickness can be precisely controlled to 50μm, and each layer is cured with ultraviolet light. When printing the top and bottom dielectric layers, precision grooves for placing varactor diodes need to be pre-reserved at the gap positions of the corresponding square annular apertures of the metal patch, according to the design. The total thickness of the dielectric substrate is controlled to 2.5mm.

[0038] Metal pattern fabrication: Selective laser melting (SLM) technology is used to directly print metal patch patterns on the surface of each solidified dielectric substrate. Spherical copper powder with a particle size range of 15-53μm is used as raw material and is selectively melted by a high-energy laser beam to precisely form a top-layer square ring patch, a middle-layer cross-shaped grid patch, and a bottom-layer square ring patch in sequence.

[0039] Active device integration: Precisely position the GC15008-00 varactor diode into the groove reserved in step 1. Use laser micro-welding technology to reliably connect the diode's electrodes to the surrounding metal patches. Laser welding has a small heat-affected zone, making it suitable for integrating such microstructures.

[0040] Protective Encapsulation (Optional): To improve the long-term reliability of the device in complex environments, a thin film of parylene can be coated onto the overall structure surface using a vapor deposition process after all soldering is completed. This coating provides uniform, dense, and pinhole-free insulating protection, serving as a moisture-proof and corrosion-resistant layer.

[0041] The above-mentioned combination process of photopolymer 3D printing media and SLM printing metal avoids the alignment and lamination problems in traditional multilayer board processes, and can realize the integrated, high-precision and high-reliability manufacturing of complex three-dimensional structures and active devices. It is particularly suitable for multilayer AFSS containing embedded active devices, as described in this invention.

[0042] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. An active frequency selective surface with a steep passband edge and polarization stability, characterized in that, Including periodically arranged unit structures; The unit structure includes at least two metal patches and a dielectric layer disposed between adjacent metal patches; At least one layer of the metal patch has an opening or a slot, and a plurality of varactor diodes are loaded at the opening or slot; By changing the reverse bias voltage applied to the varactor diode, its junction capacitance is continuously adjusted, thereby making the resonant frequency of the active frequency selective surface continuously adjustable in the range of 7.75 GHz to 11.2 GHz.

2. The active frequency selective surface according to claim 1, characterized in that, The unit structure has three layers, including a first metal patch layer, a second metal patch layer, and a third metal patch layer stacked from top to bottom. The first and third metal patch layers have the same pattern, both being patches with square annular apertures. The second metal patch layer has a cross-shaped grid with a central square patch. The varactor diodes are symmetrically loaded inside the square annular apertures.

3. The active frequency selective surface according to claim 2, characterized in that, Each of the square annular apertures contains four varactor diodes symmetrically loaded, and the unit structure as a whole simultaneously satisfies both central symmetry and axial symmetry.

4. The active frequency selective surface according to claim 2, characterized in that, The unit structure has a period P of 5mm, a total dielectric layer thickness H of 5mm, a metal linewidth W of 0.2mm for the square annular aperture, a side length L3 of 1.8mm for the central square patch, an outer side length L1 of 4mm for the square annular aperture, and an inner side length L2 of 3.5mm for the square annular aperture.

5. The active frequency selective surface according to claim 1 or 2, characterized in that, The junction capacitance of the varactor diode is continuously adjustable from 0.4pF to 2.0pF, and the corresponding reverse bias voltage adjustment range is from 20V to 4V.

6. The active frequency selective surface according to claim 5, characterized in that, The model number of the varactor diode is GC15008-00.

7. The active frequency selective surface according to claim 1, characterized in that, By adjusting the capacitance of the varactor diode, the bandwidth required for the transmission coefficient at the passband edge of the active frequency selective surface to decrease from -3dB to -10dB in any tuning state is less than or equal to 0.6GHz.

8. A method for manufacturing an active frequency selective surface as described in any one of claims 1-7, characterized in that, Includes the following steps: S1: The dielectric layer is printed layer by layer using photopolymerization 3D printing technology, and a groove for accommodating the varactor diode is formed at a preset position; S2: On the surface of the formed dielectric layer, selective laser melting printing technology is used to print the metal patch layer by layer using spherical copper powder as raw material; S3: Install the varactor diode into the reserved groove on the metal patch and fix it using laser welding process.

9. The method according to claim 8, characterized in that, In step S1, DLP photopolymerization technology is used to print the resin medium layer, with a single layer printing thickness of 50μm, and then UV curing is performed; the total printing thickness of the medium layer is 2.5mm; in step S2, the particle size range of the spherical copper powder used is 15μm to 53μm.

10. The method according to claim 8, characterized in that, Following step S3, the method further includes step S4: performing vapor deposition encapsulation on the welded varactor diode using a parylene coating; and the method sequentially performs the following steps: printing the first dielectric layer; A first metal patch is printed on the first dielectric layer, and a varactor diode is integrated on this layer; Print the second dielectric layer; print the second metal patch on the second dielectric layer; print the third dielectric layer; A third metal patch is printed on the third dielectric layer, and a varactor diode is integrated on this layer.

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