A liquid crystal tunable frequency selective surface with dual polarization characteristics

Abstract

A liquid crystal tunable frequency selective surface with dual polarization characteristics. The present invention belongs to the technical field of microwave device engineering. The frequency selective surface of the present invention is a single-layer or double-layer liquid crystal frequency selective surface, and the double-layer liquid crystal frequency selective surface is formed by supporting two groups of single-layer liquid crystal frequency selective surfaces through an interlayer support material. The single-layer frequency selective surface is formed by arranging structural units in a periodic manner, and the structural unit is composed of a top dielectric plate, an upper metal structure layer, a liquid crystal layer, a lower metal structure, and a bottom dielectric plate stacked in sequence from top to bottom. The upper metal structure layer is a rectangular metal patch with two intersecting Z-shaped slots, one of the Z-shaped slots is rotated 90° by the other Z-shaped slot, and the two ends of one of the Z-shaped slots are not connected to the two bent portions of the other Z-shaped slot. The outer contour of the two intersecting Z-shaped slots is an unclosed rectangle, and the lower metal structure layer is formed by flipping the upper metal structure layer.

Description

Technical Field

[0001] The present invention belongs to the technical field of microwave device engineering, and particularly relates to a liquid crystal tunable frequency selective surface with dual polarization characteristics. Background Art

[0002] A frequency selective surface is a two-dimensional periodic array with band-pass or band-stop characteristics, and is widely used in fields such as antennas, radars, and wireless communications. Tunable functions can be realized by loading varactor diodes, ferroelectrics, graphene, or liquid crystals into the structural cells of the frequency selective surface. The advantages of varactor diodes are large tuning range and simple design method, but affected by the inductance of packaging leads, they can only operate in the low-frequency microwave band (below the X band). Although ferroelectrics and graphene can operate above the Ku band, limited by the preparation process, the electromagnetic consistency of both ferroelectric thin films and graphene thin films cannot be guaranteed.

[0003] Liquid crystal is a uniaxial crystal, and its molecular long axis has a fixed orientation. Under the action of an external electric field or magnetic field, the orientation of the long axis of the liquid crystal molecules will deflect, thereby causing a change in the macroscopic dielectric constant of the liquid crystal. Such a characteristic enables the frequency selective surface loaded with liquid crystal to have an electro-tuning ability. Compared with varactor diodes, ferroelectrics, and graphene, the liquid crystal tunable frequency selective surface has many advantages such as a wide operating frequency range (from low frequency to terahertz), low loss, and low cost. However, the existing liquid crystal tunable frequency selective surfaces have problems such as low tuning rate, large structural unit size (resulting in poor angular stability), lack of dual polarization and square-wave filtering characteristics, and their application scenarios are greatly limited. Summary of the Invention

[0004] The present invention is to solve the technical problems of the current liquid crystal tunable frequency selective surface having a small tuning rate, a large structural unit size, and lack of dual polarization and square-wave filtering characteristics, and provides a liquid crystal tunable frequency selective surface with dual polarization characteristics.

[0005] A liquid crystal tunable frequency selective surface with dual polarization characteristics of the present invention is formed by periodically arranging structural units. The structural unit is composed of a top dielectric plate, an upper metal structure layer, a liquid crystal layer, a lower metal structure, and a bottom dielectric plate stacked in sequence from top to bottom. The upper metal structure layer is a rectangular metal patch with two intersecting Z-shaped slits, one Z-shaped is formed by rotating the other Z-shaped by 90°, and the two ends of one Z-shaped are not connected to the two bent parts of the other Z-shaped. The outer contour of the two intersecting Z-shaped slits is an unclosed rectangle, and the lower metal structure layer is formed by flipping the upper metal structure layer.

[0006] Further defined, the top dielectric plate and the bottom dielectric plate have the same shape, size, and material.

[0007] Further limitation: the shape of the top dielectric plate is rectangular, and the side length of the rectangle is 0.07 - 0.15 dielectric wavelengths.

[0008] Further limitation: the shape of the top dielectric plate is a rectangle with adjacent sides equal.

[0009] Further limitation: the thickness of the top dielectric plate is 100μm - 1500μm.

[0010] Further limitation: the side length of the rectangle of the upper metal structure layer is 0.07 - 0.15 dielectric wavelengths.

[0011] Further limitation: the distance between the two parallel sides of the two intersecting Z-shaped slits and the edge of the upper metal structure layer is 0.1mm - 0.3mm.

[0012] Further limitation: the slit width of the two intersecting Z-shaped slits is 100μm - 300μm.

[0013] Further limitation: the inner contour of the two intersecting Z-shaped slits is rectangular, and the side length of the rectangle is 0.02 - 0.09 dielectric wavelengths.

[0014] Further limitation: the shape of the liquid crystal layer is the same as that of the top dielectric plate, the side length of its rectangle is 0.07 - 0.15 dielectric wavelengths, and the thickness of the liquid crystal layer < 500μm.

[0015] A liquid crystal tunable frequency selective surface with dual polarization characteristics according to the present invention is a double-layer structure, supported by a low-loss support material between the layers. Each layer in the double-layer structure is composed of periodically arranged structural units. The structural unit is composed of a top dielectric plate, an upper metal structure layer, a liquid crystal layer, a lower metal structure, and a bottom dielectric plate stacked in sequence from top to bottom. The upper metal structure layer is a rectangular metal patch with two intersecting Z-shaped slits, one Z-shaped is rotated 90° from the other Z-shaped, and the two ends of one Z-shaped are not connected to the two bent parts of the other Z-shaped. The outer contour of the two intersecting Z-shaped slits is an unclosed rectangle, and the lower metal structure layer is formed by flipping the upper metal structure layer.

[0016] Further limitation: the thickness of the low-loss support material is 0.05 - 0.34 dielectric wavelengths, and the low loss means that the dielectric loss tangent tanδ < 0.04.

[0017] In the present invention, the dielectric wavelength is determined according to the operating frequency of the absorber and the material parameters of the top and bottom dielectric plates.

[0018] The advantages of the present invention compared with the prior art are as follows:

[0019] The present invention designs two kinds of liquid crystal tunable frequency selective surfaces. One is a single-layer liquid crystal tunable frequency selective surface, which has miniaturization, a large tuning rate and dual polarization characteristics. The other is a double-layer liquid crystal tunable frequency selective surface, which has miniaturization, a large tuning rate, dual polarization characteristics and square-wave filtering characteristics. Compared with the existing liquid crystal tunable frequency selective surfaces, the outstanding advantages of the present liquid crystal tunable frequency selective surface are mainly reflected in the following three points:

[0020] 1) The present invention designs a quasi-three-dimensional spiral structure, which has obvious effects on improving the tuning rate of the liquid crystal tunable frequency selective surface and reducing the size of the structural unit of the liquid crystal tunable frequency selective surface;

[0021] 2) The structure is rotationally symmetric by 90°, so it is insensitive to polarization and can thus work in dual polarization;

[0022] 3) Through the structural design of the double-layer liquid crystal tunable frequency selective surface, the filtering characteristic of the electromagnetic passband being square-wave is realized. Description of the Drawings

[0023] Figure 1 Schematic diagram of the structure of the liquid crystal tunable frequency selective surface (single layer) with dual polarization characteristics in Embodiment 1; where 1 - top dielectric plate, 2 - upper metal structure layer, 3 - liquid crystal layer, 4 - lower metal structure layer, 5 - bottom dielectric plate;

[0024] Figure 2 Overall structural schematic diagram of the liquid crystal tunable frequency selective surface (single layer) with dual polarization characteristics in Embodiment 1; Figure 2 a - overall three-dimensional structural schematic diagram, Figure 2 b - structural unit structural schematic diagram, Figure 2 c - upper metal structure layer structural schematic diagram; where 1 - top dielectric plate, 2 - upper metal structure layer, 2-1 - two crossed Z-shaped slits, 3 - liquid crystal layer, 4 - lower metal structure layer, 5 - bottom dielectric plate;

[0025] Figure 3 Simulated current distribution diagram of the liquid crystal tunable frequency selective surface (single layer) with dual polarization characteristics in Embodiment 1 when the liquid crystal molecules are at an angle of 90°; where Figure 3 (a) - upper metal structure layer, Figure 3 (b) - lower metal structure layer;

[0026] Figure 4 Transmission characteristic curve diagram of the single-layer liquid crystal frequency selective surface in Embodiment 1 at different liquid crystal molecule deflection angles; where Figure 4 a - TE polarization, Figure 4 b - TE polarization;

[0027] Figure 5Schematic diagram of the structure of the liquid crystal tunable frequency selective surface (double layer) with dual polarization characteristics in Embodiment 2; the arrow direction is the incident direction and the outgoing direction of the electromagnetic wave;

[0028] Figure 6 Transmission characteristic curve of the double-layer liquid crystal frequency selective surface in Embodiment 2 at different deflection angles of liquid crystal molecules; where Figure 6 a - TE polarization, Figure 6 b - TE polarization. Detailed implementation manner

[0029] Embodiment 1 (in combination with Figure 1-2 ): A liquid crystal tunable frequency selective surface with dual polarization characteristics in this embodiment is formed by periodically arranging structural units. The structural units are composed of a top dielectric plate 1, an upper metal structure layer 2, a liquid crystal layer 3, a lower metal structure layer 4, and a bottom dielectric plate 5 stacked in sequence from top to bottom. The upper metal structure layer 2 is a rectangular metal patch with two intersecting Z-shaped slits 2-1, where one Z-shaped is rotated 90° from the other Z-shaped, and the two ends of one Z-shaped are not connected to the two bent parts of the other Z-shaped. The outer contour of the two intersecting Z-shaped slits 2-1 is an unclosed rectangle, and the lower metal structure layer 4 is formed by flipping the upper metal structure layer 2;

[0030] The shape of the top dielectric plate 1 is rectangular, and the side length P 1 = P 2 = 2.5 mm, the thickness h = 508 μm, and the material is Rogers 5880;

[0031] The material of the upper metal structure layer 2 is copper. The rectangular side length of the upper metal structure layer 2 = 2.5 mm. The distance w between the two parallel sides of the two intersecting Z-shaped slits 2-1 and the edge of the upper metal structure layer 2 = 0.18 mm. The slit width g of the two intersecting Z-shaped slits 2-1 = 0.1 mm. The inner contour of the two intersecting Z-shaped slits 2-1 is rectangular, and the side lengths a = b = 1.8 mm;

[0032] The lower metal structure layer 4 has the same shape, size, and material as the upper metal structure layer 2;

[0033] The shape of the liquid crystal layer 3 is the same as that of the top dielectric plate 1, its rectangular side length = 2.5 mm, and the thickness of the liquid crystal layer 3 = 250 μm;

[0034] The bottom dielectric plate 5 has the same shape, size, and material as the top dielectric plate 1.

[0035] In the frequency selective surface of this embodiment, the upper metal structure layer and the lower metal structure layer are rotationally symmetric by 180°, thus forming a quasi-three-dimensional spiral inductor structure, enabling the liquid crystal tunable frequency selective surface structural unit to obtain a sufficiently large inductance within a very small circuit area. At the same time, this structure can also guide the current through the parallel triangular or rectangular metal patch parts in the upper and lower metal structure layers, thereby providing a relatively large equivalent capacitance for the liquid crystal tunable frequency selective surface structural unit, making the liquid crystal tunable frequency selective surface structural unit further miniaturized and enhancing the regulation ability of the liquid crystal on the frequency selective surface. In addition, the upper and lower metal structure layers are symmetric, so they are insensitive to the polarization of electromagnetic waves, thus obtaining the dual-polarization working characteristic.

[0036] The sizes of the rectangular sides a and b of the inner contour of the metal patch etched with two intersecting Z-shaped slits 2-1 mainly affect the equivalent capacitance value. The thickness of the liquid crystal layer is less than 500 microns, and this parameter has a significant impact on the equivalent capacitance value of the liquid crystal tunable frequency selective surface structural unit. The smaller the thickness of the liquid crystal layer, the larger the equivalent capacitance value, the better the miniaturization effect of the liquid crystal tunable frequency selective surface structural unit, and the stronger the regulation ability of the liquid crystal on the frequency selective surface. Therefore, within the range allowed by the process, the smaller the thickness of the liquid crystal layer, the better.

[0037] The simulation results of the transmission characteristics of the single-layer liquid crystal tunable frequency selective surface operating in the Ku band of Embodiment 1 at different liquid crystal molecule deflection angles are as Figure 3-4 shown. From Figure 4 it can be seen that when the deflection angle of the liquid crystal molecules changes from 0° to 90°, the center frequency of the single-layer liquid crystal tunable frequency selective surface decreases from 14.68 GHz to 13.32 GHz, and the tuning rate reaches 9.7%. In addition, referring to the free space wavelength λ 0 corresponding to the lowest operating frequency of 13.32 GHz, the unit size of this frequency selective surface is only 0.11λ 0 ×0.11λ 0 .

[0038] Embodiment 2 (in combination with Figure 5):A liquid crystal tunable frequency selective surface with dual polarization characteristics in this embodiment is a double-layer structure, and a low-loss support material is used to support between the layers. Each layer in the double-layer structure is formed by periodically arranging structural units (that is, each layer in the double-layer structure is the single-layer liquid crystal tunable frequency selective surface of Embodiment 1). The structural unit is composed of a top dielectric plate 1, an upper metal structure layer 2, a liquid crystal layer 3, a lower metal structure layer 4, and a bottom dielectric plate 5 stacked in sequence from top to bottom. The upper metal structure layer 2 is a rectangular metal patch with two intersecting Z-shaped slots 2-1, where one Z-shaped is rotated 90° from the other Z-shaped, and the two ends of one Z-shaped are not connected to the two bent parts of the other Z-shaped. The outer contour of the two intersecting Z-shaped slots 2-1 is an unclosed rectangle. The lower metal structure layer 4 is formed by flipping the upper metal structure layer 2;

[0039] The shape of the top dielectric plate 1 is rectangular, and the side length P of the rectangle 1 =P 2 =2.5 mm, the thickness h = 508 μm, and the material is Rogers 5880;

[0040] The material of the upper metal structure layer 2 is copper. The rectangular side length of the upper metal structure layer 2 = 2.5 mm. The distance w between the two parallel sides of the two intersecting Z-shaped slots 2-1 and the edge of the upper metal structure layer 2 = 0.18 mm. The slot width g of the two intersecting Z-shaped slots 2-1 = 0.1 mm. The inner contour of the two intersecting Z-shaped slots 2-1 is rectangular, and the side lengths a = b = 1.8 mm;

[0041] The shape, size, and material of the lower metal structure layer 4 are the same as those of the upper metal structure layer 2;

[0042] The shape of the liquid crystal layer 3 is the same as that of the top dielectric plate 1, and its rectangular side length = 2.5 mm. The thickness of the liquid crystal layer 3 = 250 μm;

[0043] The shape, size, and material of the bottom dielectric plate 5 are the same as those of the top dielectric plate 1;

[0044] The thickness of the low-loss support material = 2.4 mm. The low loss means that the dielectric loss tangent tanδ = 0.

[0045] The double-layer liquid crystal tunable frequency selective surface of this embodiment is formed by stacking two groups of Figure 1 the single-layer liquid crystal frequency selective surfaces shown. Compared with the single-layer liquid crystal tunable frequency selective surface, the double-layer liquid crystal tunable frequency selective surface has better square-wave filtering characteristics, and at the same time can maintain the miniaturization, large tuning rate, and dual polarization characteristics of the single-layer liquid crystal tunable frequency selective surface.

[0046] The numerical simulation results of the transmission characteristics of the dual-layer liquid crystal tunable frequency selective surface in Example 2 at different liquid crystal molecule deflection angles are as Figure 6 shown. It can be seen from Figure 6 that when the deflection angle of the liquid crystal molecules changes from 0° to 90°, the resonant frequency of the liquid crystal frequency selective surface decreases from 14.76 GHz to 13.31 GHz, the tuning rate reaches 10.1%, and the flat-top passband bandwidth of the frequency selective surface in any liquid crystal state reaches 860 MHz, and the 15 dB rectangularity coefficient reaches 2.33.

Claims

1. A liquid crystal tunable frequency selective surface with dual polarization characteristics, Characterized in that, The frequency selective surface is formed by periodically arranging structural units. The structural units are composed of a top dielectric plate (1), an upper metal structure layer (2), a liquid crystal layer (3), a lower metal structure layer (4), and a bottom dielectric plate (5) stacked in sequence from top to bottom. The upper metal structure layer (2) is a rectangular metal patch with two intersecting Z-shaped slots (2-1). One Z-shaped is rotated 90° from the other Z-shaped, and the two ends of one Z-shaped are not connected to the two bent parts of the other Z-shaped. The outer contour of the two intersecting Z-shaped slots (2-1) is an unclosed rectangle, and the lower metal structure layer (4) is formed by flipping the upper metal structure layer (2).

2. The liquid crystal tunable frequency selective surface with dual polarization characteristics according to claim 1, Characterized in that, The top dielectric plate (1) and the bottom dielectric plate (5) have the same shape, size, and material. The shape of the top dielectric plate (1) is rectangular, the side length of the rectangle is 0.07 to 0.15 dielectric wavelengths, and the thickness of the top dielectric plate (1) is 100 μm to 1500 μm.

3. The liquid crystal tunable frequency selective surface with dual polarization characteristics according to claim 2, Characterized in that, The shape of the top dielectric plate (1) is a rectangle with equal adjacent sides.

4. The liquid crystal tunable frequency selective surface with dual polarization characteristics according to claim 1, Characterized in that, The side length of the rectangle of the upper metal structure layer (2) is 0.07 to 0.15 dielectric wavelengths.

5. The liquid crystal tunable frequency selective surface with dual polarization characteristics according to claim 1, Characterized in that, The distance between the two parallel sides of the two intersecting Z-shaped slots (2-1) and the edge of the upper metal structure layer (2) is 0.1 mm to 0.3 mm, and the slot width of the two intersecting Z-shaped slots (2-1) is 100 μm to 300 μm.

6. The liquid crystal tunable frequency selective surface with dual polarization characteristics according to claim 1, Characterized in that, The inner contour of the two intersecting Z-shaped slots (2-1) is rectangular, and the side length of the rectangle is 0.02 to 0.09 dielectric wavelengths.

7. The liquid crystal tunable frequency selective surface with dual polarization characteristics according to claim 1, Characterized in that, The shape of the liquid crystal layer (3) is the same as that of the top dielectric plate (1), and the side length of its rectangle is 0.07 to 0.15 dielectric wavelengths.

8. The liquid crystal tunable frequency selective surface with dual polarization characteristics according to claim 1, Characterized in that, The thickness of the liquid crystal layer (3) < 500 μm.

9. A liquid crystal tunable frequency selective surface with dual polarization characteristics, Characterized in that, The frequency selective surface is a double-layer structure, and a low-loss support material is used to support between the layers. Each layer in the double-layer structure is formed by periodically arranging structural units. The structural unit is composed of a top dielectric plate (1), an upper metal structure layer (2), a liquid crystal layer (3), a lower metal structure layer (4), and a bottom dielectric plate (5) stacked in sequence from top to bottom. The upper metal structure layer (2) is a rectangular metal patch with two intersecting Z-shaped slots (2-1), where one Z-shaped is rotated 90° from the other Z-shaped, and the two ends of one Z-shaped are not connected to the two bent parts of the other Z-shaped. The outer contour of the two intersecting Z-shaped slots (2-1) is an unclosed rectangle. The lower metal structure layer (4) is formed by flipping the upper metal structure layer (2).

10. A liquid crystal tunable frequency selective surface with dual polarization characteristics according to claim 9, characterized in that, the thickness of the low-loss support material is 0.05 to 0.34 dielectric wavelengths, and the low loss means that the dielectric loss tangent tanδ < 0.04.

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