Infrared and microwave emissivity synergistically regulated stealth metasurface

Abstract

The application belongs to the technical field of metamaterials, and particularly relates to a stealth super surface with synergistic regulation of infrared and microwave emissivity, comprising a plurality of periodically arrayed basic units, wherein the basic unit comprises an infrared emissivity regulation layer and a microwave emissivity regulation layer which are sequentially stacked from top to bottom, and the microwave emissivity regulation layer comprises a second ITO layer, a third ITO layer and a fourth ITO layer which are sequentially stacked from top to bottom; the application can simultaneously realize low emissivity in the infrared frequency band and high emissivity in the microwave frequency band, thereby reducing the visibility of the target in the infrared thermal imager and the microwave radiometer.

Description

Technical Field

[0001] This invention belongs to the field of metamaterials technology, specifically relating to a stealth metasurface that combines synergistic modulation of infrared and microwave emissivity. Background Technology

[0002] With the diversification of target detection methods, infrared thermal imagers and microwave radiometers are increasingly widely used in modern military reconnaissance, target identification, and environmental monitoring. Infrared thermal imagers reveal the difference in infrared radiation between the target and the background by detecting the infrared radiation characteristics of the target surface, offering significant advantages in low-light or harsh weather conditions. Microwave radiometers, on the other hand, detect ground targets by measuring the microwave radiation characteristics of the target and utilizing the brightness-temperature difference between the target and the background, offering all-weather, all-day operation. With the combined effect of multiple detection methods, the infrared and microwave radiation characteristics of a target become a key factor in its ability to achieve effective camouflage. Therefore, how to synergistically control the infrared and microwave radiation characteristics of a target to reduce its visibility in infrared thermal imagers and microwave radiometers has become a crucial challenge in improving target camouflage.

[0003] Metamaterials are artificial composite materials with extraordinary physical properties not found in naturally occurring materials. Their electromagnetic properties are achieved through the design and arrangement of special subwavelength structural units, and they have broad application prospects in fields such as stealth technology and perfect lenses. Metamaterials can achieve anomalous reflection, refraction, and absorption of electromagnetic waves. By precisely controlling the propagation path or reflection behavior of incident waves, the electromagnetic response characteristics of a target can be effectively controlled. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a stealth metasurface that combines the synergistic regulation of infrared and microwave emissivity, thereby achieving low emissivity in the infrared band and high emissivity in the microwave band, thereby reducing the visibility of the target in infrared thermal imagers and microwave radiometers.

[0005] This invention provides a stealth metasurface with synergistic control of infrared and microwave emissivity, comprising multiple periodically arrayed basic units. Each basic unit includes an infrared emissivity control layer and a microwave emissivity control layer stacked from top to bottom. The microwave emissivity control layer includes a second ITO layer, a third ITO layer, and a fourth ITO layer stacked from top to bottom.

[0006] The second ITO layer includes a circular patch with four symmetrically distributed grid slots, and a cross-shaped patch is disposed in each grid slot.

[0007] The third ITO layer includes a strip that forms a rectangle, with micro rectangular patches at the four corners of the rectangle, an inner rectangular patch inside the rectangle, rectangular slots at the four corners of the inner rectangular patch, a circular slot inside the inner rectangular patch, and a gap slot between the rectangular strip and the inner rectangular patch.

[0008] Preferably, the infrared emissivity modulation layer includes a first ITO layer, which includes a plurality of periodically arranged first rectangular patches.

[0009] Preferably, the base unit and the first rectangular patch are squares, the side length of the first rectangular patch is 3-8% of the side length of the base unit, and the distance between the first rectangular patches is 10-15% of the side length of the first rectangular patch.

[0010] Preferably, the infrared emissivity modulation layer includes a first ITO layer, and a dielectric layer is disposed between the first ITO layer, the second ITO layer, the third ITO layer and the fourth ITO layer.

[0011] Preferably, the first ITO layer, the second ITO layer, the third ITO layer and the fourth ITO layer are all disposed on the substrate.

[0012] Preferably, the dielectric layer is a PVC board and the substrate is a PET board.

[0013] Preferably, the sheet resistance of the second ITO layer is greater than that of the third ITO layer, and the sheet resistance of the third ITO layer is greater than that of the first ITO layer and the fourth ITO layer.

[0014] Preferably, the mesh groove is rectangular in shape, and the cross-shaped patch divides the mesh groove into four identical rectangles.

[0015] Preferably, the micro-rectangular patch is square, and the width of the spacing groove is smaller than the side length of the micro-rectangular patch.

[0016] Preferably, the fourth ITO layer is a non-patterned ITO film.

[0017] The beneficial effect of this invention is that it can achieve low emissivity in the infrared band while simultaneously achieving high emissivity in the microwave band. Simulation results show that in the infrared band, this invention can achieve a low infrared emissivity of 0.28; in the microwave band, when TE and TM polarized electromagnetic waves are incident perpendicularly, this invention can achieve a high microwave emissivity of over 90% in the range of 4.57–26.38 GHz, with a relative bandwidth of up to 140.94%. When electromagnetic waves with different polarization angles are incident, this invention can achieve results consistent with those of TE and TM polarized electromagnetic waves incident perpendicularly within a polarization angle range of 0° to 90°, indicating that this invention has polarization insensitivity characteristics.

[0018] The metasurface designed in this invention, which combines synergistic modulation of infrared and microwave emissivity, possesses characteristics of low infrared emissivity, high microwave emissivity, broadband high microwave emissivity, polarization insensitivity, and optical transparency. It also offers advantages such as low profile, lightweight design, ease of fabrication, and low cost. This invention can effectively reduce the visibility of targets in infrared thermal imagers and microwave radiometers, and reduce the infrared / microwave contrast between the target and the background, thereby improving the target's survivability in various detection environments and making it suitable for scenarios such as target camouflage. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the overall structure of the present invention.

[0020] Figure 2 This is a side view of the overall structure of the present invention.

[0021] Figure 3 This is a structural diagram of the infrared emissivity modulation layer (first ITO layer) of the present invention.

[0022] Figure 4 This is a structural diagram of the first layer (i.e., the second ITO layer) of the microwave emissivity modulation of the present invention.

[0023] Figure 5 This is a structural diagram of the second layer (i.e., the third ITO layer) of the microwave emissivity modulation of the present invention.

[0024] Figure 6 This is a structural diagram of the third layer (i.e., the fourth ITO layer) of the microwave emissivity modulation of the present invention.

[0025] Figure 7 This is the patch size performance control curve of the infrared emissivity control layer (i.e., the first ITO layer) of the present invention.

[0026] Figure 8 This is the microwave transmittance curve of the infrared emissivity modulation layer (i.e., the first ITO layer) of the present invention.

[0027] Figure 9 This is the microwave emissivity curve of the overall structure of the present invention.

[0028] Figure 10 These are microwave emissivity curves for different polarization angles incident according to the present invention.

[0029] Figure 11 This is the microwave emissivity curve of Comparative Example 1 of the present invention.

[0030] Figure 12 This is the microwave emissivity curve of Comparative Example 2 of the present invention.

[0031] Figure 13 This is the microwave emissivity curve of Comparative Example 3 of the present invention.

[0032] Figure 14 This is the microwave emissivity curve of Comparative Example 4 of the present invention.

[0033] In the figure, 1 is the first ITO layer, 101 is the first rectangular patch, 2 is the first PET layer, 3 is the first dielectric layer, 4 is the second ITO layer, 5 is the second PET layer, 6 is the second dielectric layer, 7 is the third ITO layer, 8 is the third PET layer, 9 is the third dielectric layer, 10 is the fourth ITO layer, and 11 is the fourth PET layer.

[0034] 41 Circular patch, 42 Grid groove, 43 Cross-shaped patch;

[0035] 71. Strip, 72. Micro rectangular patch, 73. Inner rectangular patch, 74. Rectangular groove, 75. Circular groove, 76. Spacer groove. Detailed Implementation

[0036] 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.

[0037] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments, but this is not intended to limit the present invention.

[0038] Example 1

[0039] like Figure 1-3 As shown, a stealth metasurface with synergistic modulation of infrared and microwave emissivity comprises multiple periodically arrayed basic units, each basic unit being square with a side length of [missing information]. p It is 10mm.

[0040] The basic unit includes an infrared emissivity modulation layer and a microwave emissivity modulation layer stacked sequentially from top to bottom. The infrared emissivity modulation layer includes a first ITO layer 1; the first ITO layer 1 is disposed on a first PET layer 2.

[0041] The microwave emissivity modulation layer includes a second ITO layer 4, a third ITO layer 7, and a fourth ITO layer 10 stacked from top to bottom. The second ITO layer 4 is disposed on the second PET layer 5, the third ITO layer 7 is disposed on the third PET layer 8, and the fourth ITO layer 10 is disposed on the fourth PET layer 11.

[0042] A dielectric layer is disposed between the first ITO layer 1, the second ITO layer 4, the third ITO layer 7 and the fourth ITO layer 10. Therefore, the basic unit includes the first ITO layer 1, the first PET layer 2, the first dielectric layer 3, the second ITO layer 4, the second PET layer 5, the second dielectric layer 6, the third ITO layer 7, the third PET layer 8, the third dielectric layer 9, the fourth ITO layer 10 and the fourth PET layer 11 arranged in sequence from top to bottom.

[0043] The first dielectric layer 3, the second dielectric layer 6, and the third dielectric layer 9 are made of PVC material with a relative permittivity of 2.4 and a loss tangent of 0.06. The thickness h1 of the first dielectric layer 3 and the third dielectric layer 9 is 2 mm, and the thickness h2 of the second dielectric layer 6 is 3 mm.

[0044] The first ITO layer 1 and the fourth ITO layer 10 are ITO conductive films with a sheet resistance of 6Ω / sq, the second ITO layer 4 is an ITO conductive film with a sheet resistance of 150Ω / sq, and the third ITO layer 7 is an ITO conductive film with a sheet resistance of 60Ω / sq.

[0045] The first PET layer 2, the second PET layer 5, the third PET layer 8, and the fourth PET layer 11 are all made of PET material, with a relative permittivity of 3.2, a loss tangent of 0.003, and a thickness of [missing information]. t It is 0.125mm.

[0046] like Figure 3 As shown, the first ITO layer 1 has multiple periodically arranged first rectangular patches 101. The first rectangular patch 101 is a square, and its side length is much smaller than the side length of the base unit. The first rectangular patch 101 is an ITO conductive film. The side length m of the first rectangular patch 101 is 0.44 mm. The interval between adjacent first rectangular patches 101 is 0.06 mm. The total number is 20×20.

[0047] like Figure 4 As shown, the second ITO layer 4 includes a circular patch 41. The circular patch 41 has four symmetrically distributed grid slots 42 (the grid slots 42 are rotationally symmetrical about the center of the circular patch 41). A cross-shaped patch 43 is disposed within each grid slot 42, dividing the grid slot 42 into four equal parts. This means the grid slot 42 is composed of four identical squares spaced apart. The grid slot 42 is rectangular in shape, and the cross-shaped patch 43 divides the grid slot 42 into four identical rectangles (preferably squares). The four identical rectangles are rotationally symmetrical about the center of the cross-shaped patch 43.

[0048] The circular patch 41 and the cross-shaped patch 43 are made of ITO conductive film, while the grid groove 42 itself is made of non-ITO conductive film.

[0049] The radius r of the circular patch 41 is 4.5 mm, the interval t between adjacent grid slots is 1 mm, the side length d of the square divided by the cross-shaped patch 43 in the grid slot 42 is 1.05 mm, and the interval g between adjacent squares is 0.4 mm.

[0050] like Figure 5 As shown, the third ITO layer 7 includes a strip 71 that forms a rectangle. Micro-rectangular patches 72 are positioned at the four corners of the rectangle. An inner rectangular patch 73 is positioned inside the rectangle. Rectangular grooves 74 (forming cross-shaped patches) are formed at the four corners of the inner rectangular patch 73. A circular groove 75 is formed inside the inner rectangular patch 73. A spacing groove 76 is provided between the rectangular strip and the inner rectangular patch. The micro-rectangular patches 72 are square, and the width of the spacing groove 76 (0.8 mm) is less than the side length of the micro-rectangular patch 72 (1 mm).

[0051] The strip 71 that forms a rectangle is a hollow rectangle (side length 71). l 2 is 9.4mm, and the width of the strip w2 is 0.4mm. The material is ITO film. The materials of micro rectangular patch 72 and inner rectangular patch 73 are both ITO film. The rectangular groove 74, circular groove 75 and spacer groove 76 are all non-ITO film.

[0052] The micro rectangular patch 72 is a square with a side length of... l 3 is 1mm, the inner rectangular patch 73 is a square, the side length is... l 1 is 7mm, and the rectangular groove 74 is a square with a side length of ( ). l 1- w 1) / 2 = 1 mm, where w 1 is 5mm. The radius of the circular groove is 75. r 1 is 1.5mm.

[0053] like Figure 6 As shown, the fourth ITO layer 10 adopts a non-patterned design and completely covers the fourth PET layer 11. Its size is consistent with the substrate size.

[0054] Example 2

[0055] like Figure 7 The figure shows the relationship between the size of the first rectangular patch 101 in the first ITO layer 1 and the duty cycle and infrared emissivity. The infrared emissivity can be adjusted according to the surface infrared emissivity formula, i.e. ,in Infrared emissivity, The infrared emissivity of ITO This represents the area ratio of ITO on the substrate surface. Let m be the infrared emissivity of the PET substrate. Based on the formula, parameter analysis was performed on the length m of the first rectangular patch 101, scanning the parameter m from 0.4 to 0.49. The figure shows that the duty cycle of the ITO is positively correlated with the size of the first rectangular patch 101, while the corresponding infrared emissivity is negatively correlated.

[0056] like Figure 8 The figure shows the microwave transmittance results of the first rectangular patch 101 in the first ITO layer 1 at different sizes. The first ITO layer 1 needs to achieve low emissivity in the infrared band while ensuring high transmittance in the microwave band; therefore, the microwave transmittance of the first rectangular patch 101 at different sizes was analyzed. As shown in the figure, the microwave transmittance gradually decreases as the size of the first rectangular patch 101 increases. Combined with... Figure 7 and Figure 8 Based on the analysis results, in order to achieve low infrared emissivity while ensuring high microwave transmittance, the length m of the first rectangular patch 101 is selected as 0.44 mm as the optimal size.

[0057] like Figure 9 The image shows the use of Figure 1 The basic unit structure of the metasurface shown is illustrated using microwave emissivity results from simulations conducted with the commercial electromagnetic simulation software CSTStudio Suite, spanning 1–30 GHz. The horizontal axis represents frequency, and the vertical axis represents microwave emissivity. Figure 9 It can be seen that the proposed metasurface can achieve a broadband high emissivity characteristic of 90% in the range of 4.57–26.38 GHz under the incident of TE and TM polarized waves.

[0058] like Figure 10 The figure shows the emissivity performance of the present invention at different polarization angles. (From...) Figure 10 As can be seen, with the increase of the incident polarization angle, the present invention can maintain microwave emissivity results that are essentially identical to those when TE and TM polarized waves are incident perpendicularly. This result is attributed to the rotationally symmetric structure of the designed metamaterial, which exhibits consistent results for arbitrary polarization wave incidentness. Therefore, the present invention possesses polarization insensitivity characteristics.

[0059] In summary, this invention proposes a metasurface with synergistic modulation of infrared and microwave emissivity. In the infrared band, the invention exhibits a low infrared emissivity of 0.28, while the infrared emissivity modulation layer possesses a high microwave transmittance of greater than 85% in the microwave band. In the microwave band, the microwave emissivity modulation layer achieves a broadband microwave high emissivity characteristic of greater than 90% within the range of 4.57–26.38 GHz. Furthermore, due to the rotational symmetry of the invention's structure, it exhibits essentially consistent microwave emissivity results for electromagnetic waves incident at different polarization angles, indicating that the invention possesses polarization insensitivity. These characteristics demonstrate that the invention can effectively reduce the visibility of targets in infrared thermal imagers and microwave radiometers, improve the survivability of targets in various detection environments, and is suitable for scenarios such as target camouflage.

[0060] Comparative Example 1

[0061] Comparative Example 1 is based on Example 1 above, but the structure of the second ITO layer 4 is replaced with a circular patch with a radius of 4.5 mm. That is, the circular patch does not have a grid groove 42. Everything else is the same as in Example 1.

[0062] The microwave emissivity characteristics were analyzed under TE and TM polarization, respectively. Figure 11 As shown, when electromagnetic waves are incident with TE and TM polarization, a microwave emissivity trough appears in the range of 7.32–11.07 GHz, especially at 8.99 GHz where the microwave emissivity is only 87%, making it difficult to form a broadband microwave emissivity characteristic greater than 90%. Due to the axisymmetric characteristics of the circular patch, its results under TE and TM polarization are consistent.

[0063] Therefore, it can be concluded that the structure of Comparative Example 1 (i.e., the circular patch) exhibits a certain impedance mismatch in the 7.32–11.07 GHz range under TE-polarized and TM-polarized electromagnetic wave incidence, making it difficult to achieve broadband high microwave emissivity. Thus, by comparing it with the second ITO layer 4 structure designed in this invention, it can be seen that this comparative structure has not yet achieved broadband high microwave emissivity, limiting its stable performance over a wide frequency band.

[0064] Comparative Example 2

[0065] Comparative Example 2 is based on Example 1 above, but the structure of the second ITO layer 4 is replaced with a circular patch with a radius of 4.5 mm and a square groove with a side length of 6 mm in the middle. Everything else is the same as in Example 1.

[0066] The microwave emissivity characteristics were analyzed under TE and TM polarization, respectively. Figure 12As shown, when electromagnetic waves are incident with TE and TM polarization, the microwave emissivity is lower than that of Example 1, and its microwave emissivity decreases significantly in the low-frequency band. In particular, a microwave emissivity trough appears in the range of 9.04–12.39 GHz, with an emissivity of less than 90%, making it difficult to form broadband high emissivity characteristics. Since the structure of Comparative Example 2 has axisymmetric characteristics, its results under TE and TM polarization are consistent.

[0067] Therefore, it can be seen that the structure of Comparative Example 2 (i.e., a square slot on a circular patch) exhibits a significant trough in microwave emissivity between 9.04 and 12.39 GHz under TE-polarized and TM-polarized electromagnetic wave incidence, making it difficult to achieve broadband high microwave emissivity characteristics. Thus, by comparing it with the second ITO layer 4 structure designed in this invention, it can be seen that the structure of Comparative Example 2 has not yet achieved broadband high microwave emissivity characteristics, limiting its stable performance over a wide frequency band.

[0068] Comparative Example 3

[0069] Comparative Example 3, based on the above embodiment 1, replaces the structure of the third ITO layer 7 with only a square ring and four micro rectangles that are rotationally symmetrical on their inner sides. That is, the third ITO layer 7 includes a strip 71 that forms a rectangle as a whole, and micro rectangular patches 72 are provided at the four corners of the rectangle. Everything else is the same as in embodiment 1.

[0070] The microwave emissivity characteristics were analyzed under TE and TM polarization, respectively. Figure 13 As shown, when electromagnetic waves are incident with TE and TM polarization, the structure forms three peaks with microwave emissivity greater than 90% at 6.19 GHz, 17.50 GHz, and 28.14 GHz, respectively. However, two microwave emissivity troughs appear at 12.43 GHz and 21.94 GHz, with microwave emissivity of only 75% and 69%, respectively. Due to the axisymmetric nature of this comparative structure, its results under TE and TM polarization are consistent.

[0071] Therefore, it can be concluded that the Comparative Example 3 structure (i.e., the square ring and its four rotationally symmetrical inner microrectangles) only achieved three high microwave emissivity peaks greater than 90% under TE-polarized and TM-polarized electromagnetic wave incidence, making it difficult to form broadband high microwave emissivity characteristics. Thus, by comparing it with the third ITO layer 7 structure designed in this invention, it can be seen that the microwave emissivity achievable by the Comparative Example 3 structure is limited and cannot meet the requirements for broadband high microwave emissivity.

[0072] Comparative Example 4

[0073] Comparative Example 4 is based on Example 1 above, except that the third ITO layer 7 structure is replaced with a circular groove with a radius of 1.5 mm on a rectangle with a side length of 7 mm. Everything else is the same as in Example 1.

[0074] The microwave emissivity characteristics were analyzed under TE and TM polarization, respectively. Figure 14 As shown, when electromagnetic waves are incident with TE and TM polarization, the structure exhibits a microwave emissivity trough in the range of 8.96–11.63 GHz, with the microwave emissivity falling below 90%. Furthermore, the microwave emissivity begins to decrease significantly at 22.34 GHz, making it difficult to achieve broadband high microwave emissivity characteristics. Due to the axisymmetric nature of this comparative structure, its results under TE and TM polarization are consistent.

[0075] Therefore, it can be concluded that the microwave emissivity of Comparative Example 4 (i.e., a rectangular structure with a circular slot) is below 90% in the range of 8.96–11.63 GHz under TE-polarized and TM-polarized electromagnetic wave incidence, and the microwave emissivity begins to decrease significantly at 22.34 GHz. Thus, by comparing it with the third ITO layer 7 structure designed in this invention, it can be seen that the microwave emissivity achievable by Comparative Example 4 is limited and cannot meet the requirements for broadband high microwave emissivity.

[0076] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of protection of this application is limited to these examples; within the framework of this application, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of different aspects of one or more embodiments of this application as described above, which are not provided in detail for the sake of brevity.

[0077] One or more embodiments in this application are intended to cover all such substitutions, modifications, and variations that fall within the broad scope of this application. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of one or more embodiments in this application should be included within the protection scope of this application.

Claims

1. An invisibility cloak metasurface with synergistic regulation of infrared and microwave emissivity, characterized in that, The base unit comprises an infrared emissivity control layer and a microwave emissivity control layer arranged in sequence from top to bottom, and the microwave emissivity control layer comprises a second ITO layer (4), a third ITO layer (7) and a fourth ITO layer (10) arranged in sequence from top to bottom; The second ITO layer (4) comprises a circular patch (41) provided with four symmetrically distributed grid slots (42), and the grid slots (42) are provided with cross-shaped patches (43). The third ITO layer (7) comprises a strip (71) constituting a rectangle as a whole, four micro-rectangular patches (72) are arranged on the four corners in the rectangle, an inner rectangular patch (73) is arranged in the rectangle, rectangular slots (74) are arranged on the four corners of the inner rectangular patch (73), circular slots (75) are arranged in the inner rectangular patch (73), and a spacing slot (76) is arranged between the strip of the rectangle and the inner rectangular patch. The infrared emissivity control layer comprises a first ITO layer (1), and the first ITO layer (1) comprises a plurality of periodically arranged first rectangular patches (101). The base unit and the first rectangular patch (101) are square, the side length of the first rectangular patch (101) is 3-8% of the side length of the base unit, and the distance between the first rectangular patches (101) is 10-15% of the side length of the first rectangular patch (101). The fourth ITO layer (10) is an unpatterned ITO film.

2. The cloaking metasurface of claim 1, wherein, The infrared emissivity control layer comprises a first ITO layer (1), and a dielectric layer is arranged between the first ITO layer (1), the second ITO layer (4), the third ITO layer (7) and the fourth ITO layer (10).

3. The cloaking metasurface of claim 2, wherein, The first ITO layer (1), the second ITO layer (4), the third ITO layer (7) and the fourth ITO layer (10) are all arranged on a substrate.

4. The cloaking metasurface of claim 3, wherein, The dielectric layer is a PVC plate, and the substrate is a PET plate.

5. The cloaking metasurface of claim 2, wherein, The sheet resistance of the second ITO layer (4) is greater than that of the third ITO layer (7), and the sheet resistance of the third ITO layer (7) is greater than that of the first ITO layer (1) and the fourth ITO layer (10).

6. The cloaking metasurface of claim 1, wherein, The grid slot (42) is rectangular in shape, and the cross-shaped patch (43) divides the grid slot (42) into four identical rectangles.

7. The cloaking metasurface of claim 1, wherein, The micro-rectangular patch (72) is square, and the width of the spacing slot (76) is less than the side length of the micro-rectangular patch (72).

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