Radio wave absorbing sheet

Abstract

To provide a radio wave absorption sheet which has high possibility in which the radio wave absorption sheet is visually recognized, and has an adjustable appearance.SOLUTION: A radio wave absorption sheet 10 includes: a base material 20 including a first surface 21 and a second surface 22 located on the opposite side of the first surface; a plurality of first conductor layers 31 including a third surface 32 facing the first surface, a fourth surface 33 located on the opposite side of the third surface, and a first side surface 34 located between the third surface and the fourth surface; a first cover layer 60 covering the fourth surface and the first side surface; and a second conductor layer 41 facing the second surface. The first side surface expands so as to be displaced outward as it goes from the fourth surface to the third surface. The first cover layer includes a first colored layer 61 containing a colorant.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] Embodiments of this disclosure relate to radio wave absorbing sheets. [Background technology]

[0002] To suppress the effects of radio waves on electronic devices, radio wave absorbing sheets are used, for example, as disclosed in Patent Document 1. For example, Patent Document 1 proposes a radio wave absorbing sheet that includes a dielectric layer, a conductive layer, and a reflective layer, has a thickness of 1005 μm to 1300 μm, and has a peak frequency of 6.4 GHz. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Patent No. 6063631 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] With the advancement of wireless technology, the demand for radio wave absorbing sheets is increasing. For example, in wireless network technologies such as local 5G, it is necessary to suppress the leakage of radio waves outside the service area. For this reason, it is expected that radio wave absorbing sheets will be installed in various locations within the service area.

[0005] As the number of locations where radio wave absorbing sheets are installed increases, the likelihood of them being visible also increases. Therefore, it is preferable that the appearance of the radio wave absorbing sheets be adjustable.

[0006] The embodiments of this disclosure have been made with these points in mind, and aim to provide a radio wave absorbing sheet with an adjustable appearance. [Means for solving the problem]

[0007] Embodiments of this disclosure relate to the following [1] to

[24] . [1] A substrate including a first surface and a second surface located opposite the first surface, A plurality of first conductive layers, including a third surface facing the first surface, a fourth surface located on the opposite side of the third surface, and a first surface located between the third surface and the fourth surface, A first cover layer covering the fourth surface and the first surface, The material comprises a second conductive layer facing the second surface, The first side includes a first end connected to the third side and a second end connected to the fourth side, The first end is located further out than the second end in a plan view. The first cover layer is a radio wave absorbing sheet comprising a first colored layer containing a coloring agent.

[0008] [2] In the radio wave absorbing sheet described in [1], the first side surface may include an inward curved surface located inside a hypothetical straight line passing through the first end and the second end in the cross-sectional view.

[0009] [3] In the radio wave absorbing sheet described in [2], the first side surface may form a first angle with respect to the third surface at the first end, and the first side surface may form a second angle with respect to the fourth surface at the second end, and the sum of the first angle and the second angle may be greater than 90° and less than 175°.

[0010] In the radio wave absorbing sheet described in [4] [3], the second angle may be 135° or less.

[0011] [5] In the radio wave absorbing sheet described in [1], the first side surface may include an outward curved surface located outside the imaginary straight line passing through the first end and the second end in the cross-sectional view.

[0012] [6] In the radio wave absorption sheet according to [5], the first side surface forms a first angle with the third surface at the first end, the first side surface forms a second angle with the fourth surface at the second end, and the sum of the first angle and the second angle may be more than 185° and less than 270°.

[0013] [7] In the radio wave absorption sheet according to [1], the first side surface may include a flat surface.

[0014] [8] In the radio wave absorption sheet according to [7], the first side surface forms a first angle with the third surface at the first end, the first side surface forms a second angle with the fourth surface at the second end, and the sum of the first angle and the second angle may be 175° or more and 185° or less.

[0015] [9] In the radio wave absorption sheet according to [1], the first side surface may include an eleventh side surface and a twelfth side surface that is connected to the eleventh side surface at a first connection portion and is located between the first connection portion and the fourth surface.

[0016]

[10] In the radio wave absorption sheet according to [9], the eleventh side surface or the twelfth side surface may include an inner curved surface that is located inside a virtual straight line passing through the first end and the second end in a cross-sectional view.

[0017]

[11] In the radio wave absorption sheet according to [9] or

[10] , the eleventh side surface or the twelfth side surface may include an outer curved surface that is located outside a virtual straight line passing through the first end and the second end in a cross-sectional view.

[0018]

[12] In the radio wave absorption sheet according to any one of [9] to

[11] , the eleventh side surface or the twelfth side surface may include a flat surface.

[0019]

[13] In the radio wave absorption sheet according to any one of [1] to

[12] , the first cover layer may include an upper surface parallel to the first surface.

[0020] In the radio wave absorbing sheet described in

[14]

[13] , the first colored layer may include a lower surface parallel to the first surface.

[0021] In the radio wave absorbing sheet described in

[15]

[13] , the first cover layer may include a layer that is in contact with the first side surface and includes an upper surface that, in a plan view, has a step that overlaps with the first side surface.

[0022]

[16] In the radio wave absorbing sheet described in any one of [1] to

[12] , the first cover layer may include an upper surface in which a step appears that overlaps the first side surface in a plan view.

[0023]

[17] In the radio wave absorbing sheet described in any one of [1] to

[16] , the first colored layer may be in contact with the fourth surface and the first side surface.

[0024]

[18] In the radio wave absorbing sheet described in any one of [1] to

[17] , the first cover layer may include a first adhesive layer located between the first conductive layer and the first colored layer and in contact with the fourth surface and the first side surface.

[0025]

[19] In the radio wave absorbing sheet described in any one of [1] to

[18] , the first cover layer may include a second coloring layer containing a coloring agent.

[0026] In the radio wave absorbing sheet described in

[20]

[19] , the first cover layer may include a second adhesive layer located between the first colored layer and the second colored layer.

[0027] In the radio wave absorbing sheet described in

[21]

[19] , the first cover layer may include a first transparent layer located between the first colored layer and the second colored layer.

[0028]

[22] In the radio wave absorbing sheet described in any one of [1] to

[21] , the plurality of first conductive layers may include a plurality of first shaped layers and a plurality of second shaped layers having a shape different from the first shaped layers in a plan view.

[0029]

[23] One embodiment of the present disclosure is A structure having a surface, A wireless network system comprising a radio wave absorbing sheet described in any one of [1] to

[22] attached to the aforementioned surface.

[0030]

[24] In the wireless network system described in

[23] , the surface of the structure may include a curved surface, and the radio wave absorbing sheet may be attached to the curved surface. [Effects of the Invention]

[0031] According to embodiments of this disclosure, a radio wave absorbing sheet with adjustable appearance can be provided. [Brief explanation of the drawing]

[0032] [Figure 1] This figure shows an example of a wireless network. [Figure 2] This figure shows one example of the use of a radio wave absorbing sheet. [Figure 3] This is a perspective view showing an example of a radio wave absorbing sheet. [Figure 4] This is a plan view showing the radio wave absorbing sheet in Figure 3 with the first cover layer removed. [Figure 5] Figure 3 is a cross-sectional view of the radio wave absorbing sheet as seen from the VV direction. [Figure 6] This is an enlarged cross-sectional view showing the first side surface of the first conductive layer in Figure 5. [Figure 7] This diagram shows the unit cells that make up the radio wave absorbing sheet. [Figure 8] This diagram shows the equivalent circuit of a radio wave absorbing sheet. [Figure 9]This figure shows an example of a method for manufacturing a radio wave absorbing sheet. [Figure 10] This figure shows an example of a method for manufacturing a radio wave absorbing sheet. [Figure 11] This figure shows an example of a method for processing the first conductive layer. [Figure 12] This figure shows an example of a method for processing the first conductive layer. [Figure 13] This is a cross-sectional view showing a modified example of the first side surface of the first conductive layer. [Figure 14] This is a cross-sectional view showing a modified example of the first side surface of the first conductive layer. [Figure 15] This is a cross-sectional view showing a modified example of the first side surface of the first conductive layer. [Figure 16] This is a cross-sectional view showing a modified example of the first side surface of the first conductive layer. [Figure 17] This is a cross-sectional view showing a modified example of the first side surface of the first conductive layer. [Figure 18] This is a cross-sectional view showing a modified example of the first side surface of the first conductive layer. [Figure 19] This is a cross-sectional view showing a modified example of the first side surface of the first conductive layer. [Figure 20] This is a cross-sectional view showing a modified example of the first cover layer. [Figure 21] This is a cross-sectional view showing a modified example of the first cover layer. [Figure 22A] This is a cross-sectional view showing a modified example of the first cover layer. [Figure 22B] This is a cross-sectional view showing a modified example of the first cover layer. [Figure 22C] This is a cross-sectional view showing a modified example of the first cover layer. [Figure 23] This is a cross-sectional view showing a modified example of the first cover layer. [Figure 24] This is a cross-sectional view showing a modified example of the first cover layer. [Figure 25] This is a cross-sectional view showing a modified example of the first cover layer. [Figure 26] This is a cross-sectional view showing a modified example of the first cover layer. [Figure 27]This is a cross-sectional view showing a modified example of a radio wave absorbing sheet. [Figure 28] This is a cross-sectional view showing an example of a surface layer. [Figure 29] This is a cross-sectional view showing a modified example of a radio wave absorbing sheet. [Figure 30] This is a cross-sectional view showing a modified example of a radio wave absorbing sheet. [Figure 31] This figure shows an example of a method for manufacturing a radio wave absorbing sheet. [Figure 32] This figure shows an example of a method for manufacturing a radio wave absorbing sheet. [Figure 33] This figure shows an example of a method for manufacturing a radio wave absorbing sheet. [Figure 34] This figure shows an example of a method for manufacturing a radio wave absorbing sheet. [Figure 35] This figure shows an example of a method for manufacturing a radio wave absorbing sheet. [Figure 36] This figure shows an example of a method for manufacturing a radio wave absorbing sheet. [Figure 37] This figure shows an example of a method for manufacturing a radio wave absorbing sheet. [Figure 38] This is a plan view showing one embodiment of a radio wave absorbing sheet. [Figure 39] This figure shows the unit cells that make up the radio wave absorbing sheet shown in Figure 38. [Figure 40] This is a plan view showing one embodiment of a radio wave absorbing sheet. [Figure 41] This figure shows the unit cells that make up the radio wave absorbing sheet shown in Figure 40. [Figure 42] This is a plan view showing one embodiment of a radio wave absorbing sheet. [Modes for carrying out the invention]

[0033] The configuration of the radio wave absorbing sheet 10 according to one embodiment of this disclosure will be described in detail with reference to the drawings. Note that the embodiments shown below are examples of embodiments of this disclosure, and this disclosure is not construed to be limited to these embodiments. Furthermore, in this specification, terms such as "substrate," "base material," "sheet," and "film" are not distinguished from each other solely on the basis of differences in designation. For example, "substrate" and "base material" are concepts that include components that may be called sheets or films. Moreover, terms used in this specification to specify shape, geometric conditions, and their degree, such as "parallel" and "orthogonal," as well as values ​​of length and angle, are interpreted not in strict terms, but to include a range that allows for the expectation of similar functionality.

[0034] In this specification, if multiple candidate upper limits and multiple candidate lower limits are given for a certain parameter, the numerical range of that parameter may be constructed by combining any one candidate upper limit and any one candidate lower limit. For example, consider the case where it is stated that "Parameter B is, for example, A1 or greater, and may be A2 or greater, and may be A3 or greater. Parameter B is, for example, A4 or less, and may be A5 or less, and may be A6 or less." In this case, the numerical range of parameter B may be A1 or greater and A4 or less, A1 or greater and A5 or less, A1 or greater and A6 or less, A2 or greater and A4 or less, A2 or greater and A5 or less, A2 or greater and A6 or less, A3 or greater and A4 or less, A3 or greater and A5 or less, and A3 or greater and A6 or less.

[0035] In the drawings referenced in this embodiment, identical or similar reference numerals are used to denote identical parts or parts with similar functions, and repeated descriptions may be omitted. Furthermore, the dimensional ratios in the drawings may differ from the actual ratios for illustrative purposes, and some components may be omitted from the drawings.

[0036] The embodiments of this disclosure will be described below.

[0037] In recent years, radio waves with high frequencies, such as microwaves, millimeter waves, and sub-millimeter waves, have begun to be used in various fields. Microwaves are radio waves in the frequency band between approximately 0.3 GHz and 300 GHz. Millimeter waves are radio waves in the frequency band between approximately 30 GHz and 300 GHz. Sub-millimeter waves are radio waves in the frequency band between approximately 10 GHz and 30 GHz. Examples of fields include local 5G, fifth-generation mobile communication systems and mobile devices, communication systems for automobiles, radar for collision avoidance systems, and biosensing in medicine. As the use of radio waves increases, there are concerns that malfunctions such as malfunctions of electronic devices and deterioration of communication environments may occur.

[0038] One product known to resolve these problems is radio wave absorbing sheets. Radio wave absorbing sheets can absorb noise and unwanted radio waves emitted from electronic devices. By using radio wave absorbing sheets, it is expected that effects such as preventing malfunctions in electronic devices and improving the communication environment can be achieved.

[0039] Radio wave absorbing sheets are also used to suppress the leakage of radio waves outside a specific area. Figure 1 shows an example of a wireless network system 100. The wireless network system 100, for example, constructs a local area network, so-called local 5G, using 5G wireless technology. The wireless network system 100 includes a service area 125. In the service area 125, a base station 120 communicates wirelessly with terminal devices 130. Terminal devices 130 include personal computers and smartphones.

[0040] Figure 2 shows an example of the use of the radio wave absorbing sheet 10. In the example shown in Figure 2, reference numeral 121 denotes a structure that indicates the boundary of the service area 125. The structure 121 is, for example, a building wall. As shown in Figure 2, multiple radio wave absorbing sheets 10 may be attached to the structure 121. The multiple radio wave absorbing sheets 10 may be arranged in a specific direction. The radio wave absorbing sheets 10 may be located around an opening 122 formed in the structure 121. Doors, windows, etc., may be installed in the opening 122.

[0041] The radio wave absorbing sheet 10 may be installed in a location visible to people. For example, the radio wave absorbing sheet 10 may be attached to the surface of the structure 121. The radio wave absorbing sheet 10 may be located inside the building or outside the building. For example, the radio wave absorbing sheet 10 may be attached to the inner surface of the building's wall or to the outer surface of the wall. Although not shown in the figures, the radio wave absorbing sheet 10 may be located inside the structure 121. That is, the radio wave absorbing sheet 10 may be installed in a location not visible to people.

[0042] As the number of locations where radio wave absorbing sheets 10 are installed increases, the likelihood of the radio wave absorbing sheets 10 being visible also increases. In other words, the likelihood of the radio wave absorbing sheets 10 being installed on structures 121 that are easily visible to people increases.

[0043] When the radio wave absorbing sheet 10 is installed in a location visible to people, it is preferable that the appearance of the radio wave absorbing sheet 10 be adjustable. For example, it is preferable that the radio wave absorbing sheet 10 has an appearance that harmonizes with the surrounding environment. In this embodiment, it is proposed to adjust the appearance of the radio wave absorbing sheet 10 using a first cover layer 60. Specifically, as shown in Figure 3, the radio wave absorbing sheet 10 comprises a first conductive layer 31 and a first cover layer 60 that covers the first conductive layer 31. Figure 3 is a diagram showing an example of the radio wave absorbing sheet 10. The first cover layer 60 has a pattern. For example, the first cover layer 60 has a pattern similar to that of the surface of the structure 121. This makes it possible to suppress the appearance of the structure 121 from being impaired by the radio wave absorbing sheet 10.

[0044] Figure 4 shows the state after removing the first cover layer 60 from the radio wave absorbing sheet 10 shown in Figure 3. Figure 5 is a cross-sectional view of the radio wave absorbing sheet 10 shown in Figure 3, viewed from the VV direction.

[0045] The radio wave absorbing sheet 10 comprises a base material 20, a first conductive layer 31, a second conductive layer 41, and a first cover layer 60. The base material 20 includes a first surface 21, a second surface 22, and a side surface 23. The second surface 22 is located on the opposite side of the first surface 21. The side surface 23 extends from the first surface 21 to the second surface 22. The first conductive layer 31 is located on the side of the first surface 21. The first cover layer 60 covers the first conductive layer 31 on the side of the first surface 21. Although not shown, the first cover layer 60 may also cover the side surface 23 of the base material 20. The first cover layer 60 may cover a part of the side surface 23 or all of the side surface 23. The second conductive layer 41 is located on the side of the second surface 22. The base material 20 and the first cover layer 60 are insulating. The first conductive layer 31 and the second conductive layer 41 are conductive. The substrate 20, the first conductive layer 31, and the second conductive layer 41 may constitute a resonant radio wave absorbing sheet. The first cover layer 60 may affect the absorption characteristics of the radio wave absorbing sheet 10. The absorption characteristics of the radio wave absorbing sheet 10 include the position of the absorption peak, the height of the absorption peak, the -3dB bandwidth, the -15dB bandwidth, etc.

[0046] The frequency of radio waves targeted by the radio wave absorbing sheet 10 is not particularly limited. The frequency of radio waves targeted by the radio wave absorbing sheet 10 may be, for example, 500 MHz or higher, but may also be 1 GHz or higher, 3 GHz or higher, 6 GHz or higher, 10 GHz or higher, or 20 GHz or higher. The frequency of radio waves targeted by the radio wave absorbing sheet 10 may also be 110 GHz or lower, but may also be 80 GHz or lower, 30 GHz or lower, 20 GHz or lower, 10 GHz or lower, 6 GHz or lower, 3 GHz or lower, or 1 GHz or lower. When the radio wave absorbing sheet 10 is used in local 5G, fifth-generation mobile communication systems, or radar equipment for automobiles, a value of 3 GHz or higher may be adopted from the above-mentioned upper and lower limits regarding frequency.

[0047] As shown in Figure 5, the radio wave absorbing sheet 10 may include a first adhesive layer 36 located between the substrate 20 and the first conductive layer 31. Alternatively, the radio wave absorbing sheet 10 may include a second adhesive layer 46 located between the substrate 20 and the second conductive layer 41.

[0048] The components of the radio wave absorbing sheet 10 will be described below.

[0049] (base material) The base material 20 may be flexible. In this case, the radio wave absorbing sheet 10 can be attached to a structure 121 that includes a curved surface. For example, the radio wave absorbing sheet 10 may be attached to a curved surface that constitutes the surface of the structure 121.

[0050] As shown in Figures 3 and 4, the substrate 20 may have a rectangular outer edge in plan view. For example, the substrate 20 may include an outer edge 20Y extending in a first direction D1 and a second direction D2 perpendicular to the first direction D1. Plan view means viewing the radio wave absorbing sheet 10 along the normal direction of the first surface 21.

[0051] The base material 20 includes an insulating material. The insulating material may be an organic material, an inorganic material, or a combination of both.

[0052] Inorganic materials that can be used include silicon oxide (SiO2), silicon nitride (Si3N4), silicon oxynitride (SiON), aluminum oxide (Al2O3), aluminum nitride (AlN), silicon carbide (SiC), silicon carbide nitride (SiCN), carbon-doped silicon oxide (SiCO), borosilicate glass, and quartz glass.

[0053] As organic materials, polyimide, epoxy resin, benzocyclobutene resin, polyamide, phenolic resin, fluororesin, liquid crystal polymer, polyamide-imide, polybenzoxazole, cyanate resin, aramid resin, polyolefin, polyester, BT resin, polyacetal, polybutylene terephthalate, syndiotactic polystyrene, polyphenylene sulfide, polyetheretherketone, polyethernitrile, polycarbonate, polyphenylene etherpolysulfone, polyethersulfone, polyarylate, polyetherimide, etc. can be used. Furthermore, fibers, fillers, etc. may be embedded within the layers of these organic materials. The materials of the fibers and fillers may be insulating materials such as glass, talc, mica, silicon dioxide, aluminum oxide, titanium dioxide, etc., or conductive materials such as carbon, metal, etc.

[0054] Relative permittivity ε of base material 20 r The relative permittivity ε of the substrate 20 can be less than 20.0, 15.0 or less, 10.0 or less, or 5.0 or less. r Even when the relative permittivity ε of the base material 20 is lower than that of conventional radio wave absorbers, it can still absorb radio waves. r It may be 2.0 or higher, 3.0 or higher, or 5.0 or higher.

[0055] The dielectric loss tangent tanδ of the substrate 20 may be 0.20 or less, 0.15 or less, 0.10 or less, or 0.05 or less. The dielectric loss tangent tanδ may be, for example, 0.0001 or more, 0.005 or more, or 0.01 or more.

[0056] As the resin material for the base material 20, fluororesin, liquid crystal polymer (LCP), polypropylene (PP), modified polypropylene (modified PP), polyimide (PI), etc., can be used. Examples of fluororesin include fully fluorinated resins such as polytetrafluoroethylene (PTFE), partially fluorinated resins such as polychlorotrifluoroethylene (PCTFE), and fluorinated resin copolymers such as ethylene-tetrafluoroethylene copolymer (ETFE). Fluororesin has a relative permittivity of, for example, 2.0 to 3.0. Liquid crystal polymer has a relative permittivity of, for example, 2.9 to 3.7. Polypropylene has a relative permittivity of, for example, 2.2 to 2.6. Polyimide has a relative permittivity of about 3.5. The relative permittivity values ​​in this application are for an ambient temperature of 20°C and a radio wave frequency of 10 GHz.

[0057] The base material 20 may consist of a single layer or multiple layers.

[0058] The thickness T0 of the base material 20 is, for example, 10 mm or less, but may also be 5 mm or less, 2 mm or less, 1 mm or less, 900 μm or less, 700 μm or less, 500 μm or less, 300 μm or less, 200 μm or less, or 100 μm or less. The thickness T0 of the base material 20 is, for example, 10 μm or more, but may also be 20 μm or more, 50 μm or more, or 100 μm or more.

[0059] (First conductive layer) The shape and arrangement of the first conductive layer 31 will now be described. As shown in Figures 3 and 4, the radio wave absorbing sheet 10 may have a plurality of first conductive layers 31 located on the first surface 21. The plurality of first conductive layers 31 may be arranged with gaps between them. That is, each of the plurality of first conductive layers 31 may be independent of the others. In plan view, the first conductive layer 31 may be a circle with radius r.

[0060] Multiple first conductive layers 31 may be arranged in the in-plane direction of the substrate 20. For example, multiple first conductive layers 31 may be arranged in the first direction D1 in the first period P1. Multiple first conductive layers 31 may be arranged in the second direction D2 in the second period P2. The second direction D2 is a different direction from the first direction D1, for example, a direction perpendicular to the first direction D1. The first period P1 is the distance between the center points C0 of two adjacent first conductive layers 31 in the first direction D1. The second period P2 is the distance between the center points C0 of two adjacent first conductive layers 31 in the second direction D2. The first period P1 and the second period P2 may be the same or different. The periods P1 and P2 may be, for example, greater than 0.1 mm, 1.0 mm or more, 10 mm or more, 30 mm or more, or 50 mm or more.

[0061] The configuration of the multiple first conductive layers 31 may be determined so that resonance occurs at the frequency of the target radio wave. For example, the radius r, period P1, P2, etc. of the first conductive layer 31 are determined so that the occupancy rate of the first conductive layer 31 on the first surface 21 is 0.85 or less. The occupancy rate may be 0.75 or less, 0.60 or less, 0.50 or less, or 0.30 or less. The occupancy rate may be 0.05 or more, or 0.10 or more.

[0062] The occupancy rate may be calculated by dividing the total area of ​​the multiple first conductive layers 31 by the area of ​​the region where the first conductive layers 31 are distributed. The area of ​​the region where the first conductive layers 31 are distributed may be, for example, the area of ​​the rectangular region 35 surrounding the multiple first conductive layers 31, as indicated by reference numeral 35 in Figure 4.

[0063] When multiple first conductive layers 31 are arranged periodically, the occupancy rate may be calculated by dividing the area of ​​one first conductive layer 31 by the area of ​​the unit cell 12. Figure 7 is a perspective view showing a unit cell 12. The unit cell 12 includes one first conductive layer 31, a substrate 20, and a second conductive layer 41, and has an area corresponding to one first conductive layer 31. For example, in plan view, the unit cell 12 is a quadrilateral including a pair of first sides extending in a first direction D1 and a pair of second sides extending in a second direction D2. The length of the first side is equal to the first period P1, and the length of the second side is equal to the second period P2. In the example shown in Figure 8, the area of ​​the unit cell 12 is P1 × P2. The area of ​​the first conductive layer 31 is πr 2 Therefore, the occupancy rate is πr 2 It is (P1 × P2).

[0064] In Figure 4, the symbol S1 represents the dimension of the first conductive layer 31 in the first direction D1. Dimension S1 is, for example, less than 50 mm, and may be 30 mm or less, 10 mm or less, 5 mm or less, 1 mm or less, or 0.5 mm or less. The symbol S2 represents the dimension of the first conductive layer 31 in the second direction D2. Dimension S2 is, for example, less than 50 mm, and may be 30 mm or less, 10 mm or less, 5 mm or less, 1 mm or less, or 0.5 mm or less. In the example shown in Figure 4, dimensions S1 and S2 are the diameters of the first conductive layer 31. The conditions regarding the range of these dimensions may be satisfied by both dimension S1 and dimension S2, or by only one of them. For example, dimension S1 may be less than 5.0 mm, while dimension S2 may be 5.0 mm or greater. Similarly, the conditions regarding the range of periods P1 and P2 described above may be satisfied by both periods P1 and P2, or by only one of them.

[0065] Next, the cross-sectional structure of the first conductive layer 31 will be described. As shown in Figure 5, the first conductive layer 31 includes a third surface 32, a fourth surface 33, and a first side surface 34. The third surface 32 faces the first surface 21 of the substrate 20. The fourth surface 33 is located on the opposite side of the third surface 32. The first side surface 34 is located between the third surface 32 and the fourth surface 33.

[0066] The first conductive layer 31 contains a conductive material. For example, the first conductive layer 31 contains metallic materials such as copper (Cu), gold (Au), silver (Ag), and aluminum (Al), or alloys using these materials. The first conductive layer 31 may also contain transparent conductive materials such as indium tin oxide (ITO) and indium zinc oxide (IZO). The first conductive layer 31 may also contain carbon-based conductive materials such as graphite, carbon nanotubes, graphene, and fullerene.

[0067] The method for forming the first conductive layer 31 is not particularly limited. For example, the first conductive layer 31 may be formed by plating, printing, sputtering, vapor deposition, etc. The foil constituting the first conductive layer 31 may be attached to the substrate 20 via the first adhesive layer 36. The foil may be manufactured by electrodeposition, rolling, etc.

[0068] The thickness T1 of the first conductive layer 31 may be 0.01 μm or more, 0.1 μm or more, 1 μm or more, or 5 μm or more. The thickness of the first conductive layer 31 may be 50 μm or less, 35 μm or less, or 20 μm or less.

[0069] The arithmetic mean roughness (Ra) of the third surface 32 of the first conductive layer 31 may be 3.0 μm or less, 2.0 μm or less, or 1.0 μm or less.

[0070] Furthermore, if the arithmetic mean roughness (Ra) and maximum height roughness (Rz) of the third surface 32 are small, the adhesion of the third surface 32 to the first surface 21 of the substrate 20 may decrease. In this case, it is preferable to provide a first adhesive layer 36 between the first surface 21 and the third surface 32. This makes it possible to suppress the peeling of the first conductive layer 31 even when the arithmetic mean roughness (Ra) and maximum height roughness (Rz) of the third surface 32 are small.

[0071] The arithmetic mean roughness (Ra) and maximum height roughness (Rz) are defined in accordance with JIS B 0601:2013.

[0072] Figure 6 is an enlarged cross-sectional view showing the first side surface 34 of the first conductive layer 31 in Figure 5. The first side surface 34 widens outward as it moves from the fourth surface 33 toward the third surface 32. Therefore, in a plan view, the first side surface 34 is not hidden by the fourth surface 33. This makes it easier for the first cover layer 60 to come into contact with the first side surface 34 when forming the first cover layer 60 to cover the fourth surface 33 and the first side surface 34. Therefore, it is possible to suppress the formation of a gap between the first side surface 34 and the first cover layer 60. "Outer side" means the side moving from the center of the radio wave absorbing sheet 10 toward the outer edge 20Y in a plan view. "Inner side," which will be described later, means the side moving from the outer edge 20Y toward the center of the radio wave absorbing sheet 10 in a plan view.

[0073] If a gap exists between the first side surface 34 and the first cover layer 60, the gap also affects the properties of the radio wave absorbing sheet 10. For example, the gap affects the properties of the radio wave absorbing sheet 10 based on the dielectric constant of air. The shape and volume of the gap change depending on the ambient temperature, the temperature of the radio wave absorbing sheet 10, etc. For example, as the ambient temperature rises, the volume of the gap increases in accordance with the expansion of air. When the volume of the gap changes, the properties of the radio wave absorbing sheet 10 also change. For this reason, if a gap exists between the first side surface 34 and the first cover layer 60, the properties of the radio wave absorbing sheet 10 may become unstable.

[0074] According to this embodiment, by devising the shape of the first side surface 34, it is possible to suppress the formation of a gap between the first side surface 34 and the first cover layer 60. This stabilizes the characteristics of the radio wave absorbing sheet 10. For example, it is possible to suppress changes in the resonant frequency of the radio wave absorbing sheet 10 in response to changes in ambient temperature.

[0075] The first side surface 34 includes a first end 341 connected to the third surface 32 and a second end 342 connected to the fourth surface 33. In a plan view, the first end 341 is located outside the second end 342. As shown in Figure 6, the first side surface may include a curved surface that is convex inward. "Convex inward" means that in a cross-sectional view, the first side surface 34 is located inside a hypothetical straight line K3 passing through the first end 341 and the second end 342. A curved surface that is convex inward is also called an inward curved surface. The first side surface 34 may be composed of an inward curved surface extending from the first end 341 to the second end 342.

[0076] In Figure 6, the symbol θ1 represents the angle (also referred to as the first angle) that the first side surface 34 makes with respect to the third surface 32 at the first end 341. The first angle θ1 is the angle between the tangent K1 of the first side surface 34 at the first end 341 and the third surface 32. The first angle θ1 is less than 90°. If the first side surface 34 is composed of an inwardly curved surface, the first angle θ1 is, for example, 45° or less, may be 40° or less, or may be 35° or less. The first angle θ1 is, for example, 5° or more, may be 10° or more, or may be 15° or more.

[0077] In Figure 6, the symbol θ2 represents the angle (also called the second angle) that the first side surface 34 makes with respect to the fourth surface 33 at the second end 342. The second angle θ2 is the angle between the tangent K2 of the first side surface 34 at the second end 342 and the fourth surface 33. The second angle θ2 is greater than 90°. If the first side surface 34 is composed of an inwardly curved surface, the second angle θ2 is, for example, 95° or more, may be 100° or more, or 105° or more. The second angle θ2 is, for example, 135° or less, may be 130° or less, or 125° or less.

[0078] When the first side surface 34 is composed of an inwardly curved surface, the sum of the first angle θ1 and the second angle θ2 is greater than 90° and less than 175°. The sum of the first angle θ1 and the second angle θ2 is, for example, 95° or more, may be 100° or more, or 105° or more. The sum of the first angle θ1 and the second angle θ2 is, for example, 145° or less, may be 140° or less, or 135° or less.

[0079] (1st adhesive layer) The first adhesive layer 36 is located between the first surface 21 of the substrate 20 and the third surface 32 of the first conductive layer 31. The first adhesive layer 36 adheres the first surface 21 and the third surface 32. As shown in Figure 5, the first adhesive layer 36 may extend beyond the first side surface 34 of the first conductive layer 31. That is, the first adhesive layer 36 may include a region that does not overlap the first conductive layer 31 in a plan view.

[0080] The first adhesive layer 36 is selected according to the environment in which the radio wave absorbing sheet is used, as well as the first cover layer 60 and the substrate 20. The first adhesive layer 36 may contain a fluorine-based adhesive including a fluororesin. For example, the first adhesive layer 36 may contain a carboxyl group-containing styrene elastomer, epoxy resin, acrylic resin, etc.

[0081] The thickness of the first adhesive layer 36 is, for example, 30 μm or less, may be 25 μm or less, or 20 μm or less. The thickness of the first adhesive layer 36 is, for example, 2 μm or more, may be 5 μm or more, or 10 μm or more. The thickness of the first adhesive layer 36 may be even greater, for example, 50 μm or more, or 100 μm. The substrate 20 may be composed of an adhesive such as a fluorine-based adhesive. For example, a first conductive layer 31 and a second conductive layer 41 may be provided on the first surface 21 and the second surface 22 of the substrate 20 obtained by solidifying the adhesive.

[0082] (First Cover Layer) As shown in Figures 5 and 6, the first cover layer 60 covers the fourth surface 33 and the first side surface 34 of the first conductive layer 31. Preferably, the first cover layer 60 is in contact with the fourth surface 33 and the first side surface 34 without any gaps. The first cover layer 60 may be in contact with the entire area of ​​the fourth surface 33 and the first side surface 34.

[0083] The first cover layer 60 is configured to exhibit some color. That is, the first cover layer 60 is not a colorless and transparent layer. By providing the first cover layer 60, the visibility of the first conductive layer 31 can be suppressed. Therefore, the appearance of the structure 121 can be suppressed from being impaired by the first conductive layer 31. Colorless and transparent means that it has no color and the transmittance of light rays with wavelengths of 150 nm to 400 nm is 95% or more.

[0084] The first cover layer 60 includes at least a first colored layer 61. In this embodiment, the first colored layer 61 is in contact with the fourth surface 33 and the first side surface 34 of the first conductive layer 31. The first colored layer 61 contains a coloring agent and a binder resin. Therefore, the color exhibited by the first colored layer 61 is visible as part of the appearance of the radio wave absorbing sheet 10. The binder resin may be a UV-curable resin or an EB-curable resin. The first colored layer 61 may be in contact with the entire area of ​​the fourth surface 33 and the first side surface 34. Although not shown, a layer to enhance the color development of the first colored layer 61 may be provided between the first colored layer 61 and the first conductive layer 31.

[0085] The first colored layer 61 may be configured to exhibit the same color throughout its entire surface. For example, the coloring agent may be uniformly distributed throughout the entire surface of the first colored layer 61. The first colored layer 61 may be configured to represent some kind of pattern. For example, the coloring agent may be distributed in some kind of pattern across the first colored layer 61. The first colored layer 61 may be configured to exhibit one color, or it may be configured to exhibit two or more colors. The first colored layer 61 may include transparent portions.

[0086] The colorant may contain organic ink or inorganic ink. Organic ink is a pigment mainly composed of organic compounds. Inorganic ink is a natural mineral pigment or synthetic inorganic pigment mainly composed of inorganic compounds. As shown in Figure 6, when the thickness of the portion of the first colored layer 61 that overlaps the first conductive layer 31 in a plan view is smaller than the thickness of the portion of the first colored layer 61 that does not overlap the first conductive layer 31 in a plan view, the transmittance of the first colored layer 61 that overlaps the first conductive layer 31 becomes relatively larger. In this case, it is preferable that the colorant contains inorganic ink. This makes it possible to improve the light-shielding properties of the first colored layer 61. Therefore, even when the thickness of the first colored layer 61 that overlaps the first conductive layer 31 is small, the absolute value of its transmittance can be made sufficiently small. This makes it possible to suppress the difference between the transmittance of the first colored layer 61 that overlaps the first conductive layer 31 and the transmittance of the first colored layer 61 that does not overlap the first conductive layer 31. This makes it possible to suppress the visibility of the pattern of the first conductive layer 31. As will be described later in Figure 22A, when the first colored layer 61 includes a bottom surface parallel to the first surface 21 of the substrate 20, differences in transmittance due to differences in thickness are less likely to occur. In this case, even if the coloring agent contains organic ink, it is possible to suppress the visibility of the pattern of the first conductive layer 31.

[0087] The thickness T4 of the first cover layer 60 is, for example, 5 μm or more, may be 10 μm or more, 20 μm or more, 50 μm or more, or 100 μm or more. The thickness T4 of the first cover layer 60 is, for example, 2 mm or less, may be 1 mm or less, 500 μm or less, or 200 μm or less.

[0088] The thickness T41 of the first colored layer 61 is, for example, 5 μm or more, may be 10 μm or more, may be 20 μm or more, or may be 50 μm or more. The thickness T41 of the first colored layer 61 is, for example, 500 μm or less, may be 300 μm or less, or may be 100 μm or less.

[0089] As shown in Figure 6, the upper surface of the first cover layer 60 may be parallel to the first surface 21 of the substrate 20. For example, the upper surface of the first colored layer 61 may be parallel to the first surface 21 of the substrate 20. In other words, the influence of the first side surface 34 does not have to be visible on the upper surface of the first cover layer 60 that is in contact with the first side surface 34. This suppresses the visibility of the pattern of the first conductive layer 31. When the radio wave absorbing sheet 10 is installed indoors, such as in a commercial facility, it is preferable that the upper surface of the first cover layer 60 is parallel to the first surface 21.

[0090] With respect to the first cover layer 60 and the layers constituting the first cover layer 60, "upper surface" means the surface located on the opposite side of the "lower surface." "Lower surface" means the surface of the first cover layer 60 and the layers constituting the first cover layer 60 that faces the first surface 21 of the base material 20.

[0091] "The upper surface of the first cover layer 60 is parallel to the first surface 21 of the substrate 20" means that the difference between the first distance M1 and the second distance M2 is 20 μm or less. The first distance M1 is the distance between the upper surface of the first cover layer 60 that overlaps the first conductive layer 31 in a plan view and the first surface 21 of the substrate 20. The second distance M2 is the distance between the upper surface of the first cover layer 60 that does not overlap the first conductive layer 31 in a plan view and the first surface 21 of the substrate 20. The difference between the first distance M1 and the second distance M2 may be 15 μm or less, 10 μm or less, 5.0 μm or less, 3.0 μm or less, or 1.0 μm or less.

[0092] The upper surface of the first cover layer 60 preferably has a low surface roughness. This helps to suppress the visibility of the radio wave absorbing sheet 10. For example, the maximum height roughness (Rz) on the upper surface of the first cover layer 60 may be 3.0 μm or less, 2.0 μm or less, or 1.0 μm or less. For example, the arithmetic mean roughness (Ra) on the upper surface of the first cover layer 60 may be 3.0 μm or less, 2.0 μm or less, or 1.0 μm or less.

[0093] (Second conductive layer) As shown in Figure 5, the second conductive layer 41 includes a fifth surface 42 facing the second surface 22 of the substrate 20, and a sixth surface 43 located on the opposite side of the fifth surface 42. In a plan view, the second conductive layer 41 may extend so as to overlap with a plurality of first conductive layers 31. As shown in Figure 3, the second conductive layer 41 includes a second outer edge 41Y. The second outer edge 41Y may coincide with the outer edge 20Y of the substrate 20.

[0094] The second conductive layer 41 contains a conductive material. The material for the second conductive layer 41 can be one of the materials exemplified for the first conductive layer 31. The material for the second conductive layer 41 may be the same as or different from the material for the first conductive layer 31.

[0095] The thickness T2 of the second conductive layer 41 may be 0.01 μm or more, 0.1 μm or more, 1 μm or more, or 5 μm or more. The thickness T2 of the second conductive layer 41 may be 50 μm or less, 35 μm or less, 20 μm or less, or 10 μm or less.

[0096] The second conductive layer 41 faces the first conductive layer 31 in the thickness direction of the substrate 20. This allows an electric field to be generated between the first conductive layer 31 and the second conductive layer 41.

[0097] (Second adhesive layer) The second adhesive layer 46 is located between the second surface 22 of the substrate 20 and the fifth surface 42 of the second conductive layer 41. The second adhesive layer 46 adheres the second surface 22 and the fifth surface 42.

[0098] The material for the second adhesive layer 46 can be the same as the material exemplified for the first adhesive layer 36. The material for the second adhesive layer 46 may be the same as or different from the material for the first adhesive layer 36.

[0099] The thickness of the second adhesive layer 46 is within the range of the thicknesses exemplified for the first adhesive layer 36. The thickness of the second adhesive layer 46 may be the same as or different from the thickness of the first adhesive layer 36.

[0100] (base layer) The component located between the third surface 32 of the first conductive layer 31 and the fifth surface 42 of the second conductive layer 41 is also referred to as the base layer 50. In the example shown in Figure 5, the base layer 50 includes the base material 20, the first adhesive layer 36, and the second adhesive layer 46. Although not shown, the base layer does not necessarily include adhesive layers such as the first adhesive layer 36 and the second adhesive layer 46.

[0101] The relative permittivity of the base layer 50 may be 4.0 or less, 3.5 or less, 3.0 or less, 2.5 or less, or 2.0 or less. The relative permittivity of the base layer 50 may be 1.0 or more, or 1.5 or more.

[0102] The dielectric loss tangent tanδ of the underlayer 50 may be 0.01 or less, 0.005 or less, 0.001 or less, or 0.0005 or less.

[0103] The thickness T3 of the base layer 50 is, for example, 2 mm or less, and may be 800 μm or less, 500 μm or less, or 200 μm or less. The thickness T3 of the base layer 50 is, for example, 10 μm or more, and may be 20 μm or more, 50 μm or more, or 100 μm or more.

[0104] The open-resonator method can be used to measure the relative permittivity and dielectric loss tangent of each component. A measurement system that performs the open-resonator method includes a network analyzer, a millimeter-wave multiplier, a millimeter-wave detector, and a Fabry-Perot resonator.

[0105] The thickness of each component is calculated based on a cross-sectional image of a sample of the radio wave absorbing sheet 10. The image is acquired by a scanning electron microscope.

[0106] The overall thickness of the radio wave absorbing sheet 10 is, for example, 3 mm or less, and may be 800 μm or less, 600 μm or less, 500 μm or less, 300 μm or less, or 200 μm or less. The overall thickness of the radio wave absorbing sheet 10 is, for example, 10 μm or more, and may be 20 μm or more, 50 μm or more, or 100 μm or more.

[0107] By reducing the overall thickness of the radio wave absorbing sheet 10, the constraints on the installation location of the radio wave absorbing sheet 10 can be reduced. In other words, the degree of freedom in the layout of the radio wave absorbing sheet 10 is increased. Furthermore, the thermal resistance of the radio wave absorbing sheet 10 in the thickness direction can be reduced. Therefore, when the radio wave absorbing sheet 10 is installed in electronic equipment, the temperature rise of electronic components caused by the radio wave absorbing sheet 10 can be suppressed. As a result, the radio wave absorbing sheet 10 can be installed in electronic equipment that has components whose temperature rises to about 150°C, for example.

[0108] Next, the operation of the radio wave absorbing sheet 10 will be explained. Figure 8 shows the equivalent circuit of the radio wave absorbing sheet 10. The equivalent circuit includes a primary circuit 14, a secondary circuit 15, and a transformer 16 that connects the primary circuit 14 and the secondary circuit 15.

[0109] The primary circuit 14 represents radio waves propagating through the atmosphere. The primary circuit 14 has a characteristic impedance based on the dielectric constant of the atmosphere. The secondary circuit 15 represents a resonant circuit composed of a substrate 20, a first conductive layer 31, and a second conductive layer 41. The resonant circuit includes, for example, resistors, capacitors, and coils connected in parallel. The transformer 16 is composed of multiple first conductive layers 31.

[0110] The resonant circuit will now be explained. As described above, the first conductive layer 31 is circular with radius r in a plan view. In this case, the resonant frequency fr of the resonant circuit is calculated based on the following equations (A1) and (A2).

number

[0111] The frequency range of the radio waves targeted by the radio wave absorbing sheet 10 is, for example, 500 MHz to 300 GHz. In this case, the radio wave absorbing sheet 10 is configured such that the resonant frequency fr is between 500 MHz and 300 GHz. For example, when the radio wave absorbing sheet 10 is used in local 5G, fifth-generation mobile communication systems, or automotive radar equipment, the radio wave absorbing sheet 10 is configured such that the resonant frequency fr is 3 GHz or higher. For example, the radio wave absorbing sheet 10 is configured such that the resonant frequency fr is between 20 GHz and 300 GHz. For example, if the frequency range of the radio waves targeted by the radio wave absorbing sheet 10 is 79 GHz, the thickness T0 and relative permittivity ε of the base material 20 are set so that the resonant frequency fr is the same as or approximately the same as 79 GHz. rThe radius r of the first conductor layer 31 is determined. When the thickness T0 and relative permittivity ε of the base material 20 are predetermined, the radius r of the first conductor layer 31 is determined so that the resonance frequency fr becomes a desired value. Thereby, the target radio wave can be absorbed using resonance. r When they are predetermined, the radius r of the first conductor layer 31 is determined so that the resonance frequency fr becomes a desired value. Thereby, the target radio wave can be absorbed using resonance.

[0112] Next, the transformer 16 will be described. In order for the radio wave absorption sheet 10 to effectively absorb radio waves, it is preferable that the characteristic impedance of the secondary circuit 15 is the same as or approximate to the characteristic impedance of the primary circuit 14. When the resonance circuit of the secondary circuit 15 satisfies the resonance condition, the characteristic impedance of the secondary circuit 15 is N 2 R. N is the turns ratio of the transformer 16. R is the resonance resistance of the secondary circuit 15.

[0113] N 2 and R are calculated based on the following formulas (A3) and (A4).

Equation

[0114] Assuming that the characteristic impedance in the atmosphere is the same as the characteristic impedance in vacuum, the impedance of the primary circuit 14 is 120π. The radio wave absorption sheet 10 is preferably configured such that N 2 R becomes the same as or approximates 120π. For example, the dielectric loss tangent tanδ of the base material 20, the radius r of the first conductor layer 31, and the area P of the unit cell 12 are determined so that the ratio of N 2 R to 120π is 0.8 or more and 1.2 or less. When the dielectric loss tangent tanδ of the base material 20 and the radius r of the first conductor layer 31 have already been determined, the ratio of N 2The area P of the unit cell 12 is adjusted so that the ratio of R is a desired value. For example, the first period P1 and the second period P2 of the first conductive layer 31 are adjusted. This allows the radio wave absorbing sheet 10 to effectively absorb radio waves. N relative to 120π 2 The ratio of R may be between 0.9 and 1.1, or between 0.95 and 1.05.

[0115] Next, an example of a method for manufacturing the radio wave absorbing sheet 10 will be described.

[0116] As shown in Figure 9, a laminate is prepared comprising a first conductive layer 31, a substrate 20, and a second conductive layer 41 in that order. The laminate may also include a first adhesive layer 36 located between the first conductive layer 31 and the substrate 20. The laminate may also include a second adhesive layer 46 located between the second conductive layer 41 and the substrate 20. Subsequently, as shown in Figure 9, a resist layer 150 is formed on the fourth surface 33 of the first conductive layer 31.

[0117] Next, as shown in Figure 10, the first conductive layer 31 is processed by wet etching using the resist layer 150 as a mask. This yields multiple first conductive layers 31 arranged at intervals. After that, the resist layer 150 is removed.

[0118] Next, a first colored layer 61 is formed on the fourth surface 33 and the first side surface 34 of the first conductive layer 31. For example, a solution containing a coloring agent and a binder resin is applied to the first conductive layer 31 by a printing method such as an inkjet method. The solution may also be applied to the first surface 21 of the substrate 20 or to the first adhesive layer 36. After that, the solution is solidified. For example, the solution is dried. This gives rise to the first colored layer 61. In this way, an electromagnetic wave absorbing sheet 10 is manufactured, which includes a first cover layer 60 that covers the fourth surface 33 and the first side surface 34 of the first conductive layer 31. When the first colored layer 61 is formed using a printing method such as inkjet printing, the first colored layer 61 offers a high degree of design freedom. Therefore, it is easy to apply colors and patterns to the first colored layer 61 that harmonize with the surrounding environment. When forming the first colored layer 61 using a printing method such as an inkjet method, a step may be taken to flatten the upper surface of the first colored layer 61 before it hardens. For example, a roller may be brought into contact with the upper surface of the first colored layer 61 before it hardens. This can reduce the surface roughness of the upper surface of the first colored layer 61.

[0119] The process for processing the first conductive layer 31 will be described in detail with reference to Figures 11 and 12. Figures 11 and 12 show the etching of the first conductive layer 31 by the etching solution 155.

[0120] Etching by the etching solution 155 proceeds not only in the thickness direction of the first conductive layer 31 but also in the planar direction of the first conductive layer 31. For example, as shown in Figure 11, near the edge 151 of the resist layer 150, the portion of the first conductive layer 31 in contact with the resist layer 150 is etched in the planar direction.

[0121] Etching of the first conductive layer 31 in the in-plane direction begins at the fourth surface 33. Therefore, etching in the plane direction on the fourth surface 33 progresses more rapidly than etching in the plane direction on the third surface 32. As a result, as shown in Figure 12, the first side surface 34 is formed, which expands outward as it moves from the fourth surface 33 toward the third surface 32.

[0122] The function of the radio wave absorbing sheet 10 will be explained.

[0123] The radio wave absorbing sheet 10 includes a first cover layer 60 that covers the first conductive layer 31. The first cover layer 60 includes a first colored layer 61 containing a coloring agent. This suppresses the visibility of the first conductive layer 31. Consequently, for example, it is possible to suppress the deterioration of the appearance of the structure 121 due to the first conductive layer 31.

[0124] The first side surface 34 of the first conductive layer 31, which is covered by the first cover layer 60, expands outward as it moves from the fourth surface 33 towards the third surface 32. As a result, the first side surface 34 is not hidden by the fourth surface 33 in a plan view. This makes it easier for the first cover layer 60 to come into contact with the first side surface 34 when forming the first cover layer 60 to cover the fourth surface 33 and the first side surface 34. This suppresses the formation of gaps between the first side surface 34 and the first cover layer 60. This stabilizes the characteristics of the radio wave absorbing sheet 10. For example, it suppresses changes in the resonant frequency of the radio wave absorbing sheet 10 in response to changes in ambient temperature.

[0125] The first cover layer 60 can also enhance the weather resistance of the radio wave absorbing sheet 10. Weather resistance refers to the property of preventing deterioration of the radio wave absorbing sheet 10 caused by the surrounding environment. For example, if the radio wave absorbing sheet 10 is located outside a building, it will be affected by various environmental factors such as sunlight, rain, and snow. By providing the first cover layer 60, which has resistance to environmental changes, to the radio wave absorbing sheet 10, the weather resistance of the radio wave absorbing sheet 10 can be enhanced.

[0126] It is possible to make various modifications to the embodiments described above. The following descriptions of modifications will be made with reference to the drawings as needed. In the following descriptions and the drawings used therein, parts that can be configured similarly to the embodiments described above will be given the same reference numerals as those used for the corresponding parts in the first embodiment, and redundant explanations will be omitted. Furthermore, if it is clear that the effects and advantages obtained in the embodiments described above can also be obtained in the modifications, the explanation may be omitted.

[0127] (First variation) Figure 13 is a cross-sectional view showing the first conductive layer 31 of the radio wave absorbing sheet 10 according to the first modified example. As shown in Figure 13, the first side surface 34 of the first conductive layer 31 may include a flat surface. The first side surface 34 may be composed of a flat surface extending from the first end 341 to the second end 342. In this case, the sum of the first angle θ1 and the second angle θ2 is approximately 180°. For example, the sum of the first angle θ1 and the second angle θ2 is 175° or more and 185° or less. The first side surface 34 including the flat surface can be obtained, for example, by processing the first conductive layer 31 by dry etching. Dry etching can be, for example, plasma etching or sandblasting.

[0128] A "flat surface" is a surface that extends in a certain direction. For example, the difference between the direction in which the flat surface extends at the upper end and the direction in which the flat surface extends at the lower end is 5° or less. The "upper end" is the end of the flat surface on the side furthest from the base material 20, and the "lower end" is the end of the flat surface on the side closer to the base material 20. In the example shown in Figure 13, the second end 342 is the upper end and the first end 341 is the lower end. Therefore, the angle between the tangent to the first side surface 34 at the second end 342 and the tangent to the first side surface 34 at the first end 341 is 5° or less.

[0129] When the first side surface 34 is composed of a flat surface, the first angle θ1 is, for example, 30° or more, may be 35° or more, or may be 40° or more. The first angle θ1 is, for example, 60° or less, may be 55° or less, or may be 50° or less.

[0130] If the first side surface 34 is formed by a flat surface, the second angle θ2 is, for example, 120° or more, may be 125° or more, or 130° or more. The second angle θ2 is, for example, 150° or less, may be 145° or less, or 140° or less.

[0131] (Second variation) Figure 14 is a cross-sectional view showing the first conductive layer 31 of the radio wave absorbing sheet 10 according to a second modified example. As shown in Figure 14, the first side surface 34 of the first conductive layer 31 may include a curved surface that is convex outward. "Convex outward" means that in the cross-sectional view, the first side surface 34 is located outside of a hypothetical straight line K3 passing through the first end 341 and the second end 342. The first side surface 34 may be composed of an outward curved surface extending from the first end 341 to the second end 342.

[0132] When the first side surface 34 is composed of an outward curved surface, the first angle θ1 is, for example, 45° or more, may be 50° or more, or may be 55° or more. The first angle θ1 is, for example, 85° or less, may be 80° or less, or may be 75° or less.

[0133] When the first side surface 34 is formed by an outward curved surface, the second angle θ2 is, for example, 175° or less, may be 170° or less, or 165° or less. The second angle θ2 is, for example, 135° or more, may be 140° or more, or 145° or more.

[0134] When the first side surface 34 is composed of an outward curved surface, the sum of the first angle θ1 and the second angle θ2 is greater than 185° and less than 270°. The sum of the first angle θ1 and the second angle θ2 is, for example, 265° or less, may be 260° or less, or 255° or less. The sum of the first angle θ1 and the second angle θ2 is, for example, 215° or more, may be 220° or more, or 225° or more.

[0135] (Third variation) Figure 15 is a cross-sectional view showing the first conductive layer 31 of the radio wave absorbing sheet 10 according to the third modified example. As shown in Figure 15, the first side surface 34 may include an eleventh side surface 34a and a twelfth side surface 34b that are connected at the first connection portion 343. The eleventh side surface 34a is located between the third surface 32 and the first connection portion 343. The eleventh side surface 34a may extend from the first end 341 to the first connection portion 343. The twelfth side surface 34b is located between the fourth surface 33 and the first connection portion 343. The twelfth side surface 34b may extend from the fourth surface 33 to the first connection portion 343.

[0136] The 11th side surface 34a and the 12th side surface 34b may be flat surfaces. The surface direction of the 11th side surface 34a is different from the surface direction of the 12th side surface 34b. Therefore, in the first connection portion 343, the surface direction of the contact surface of the first side surface 34 changes discontinuously.

[0137] The 11th side surface 34a and the 12th side surface 34b are formed, for example, by processing the first conductive layer 31 by dry etching under different conditions.

[0138] The 11th side surface 34a and the 12th side surface 34b may be connected such that the first connecting portion 343 is convex outward. The first connecting portion 343 may be located outside the imaginary straight line K3 passing through the first end 341 and the second end 342.

[0139] (Fourth variation) Figure 16 is a cross-sectional view showing the first conductive layer 31 of the radio wave absorbing sheet 10 according to the fourth modified example. As shown in Figure 16, the 11th side surface 34a and the 12th side surface 34b may be connected such that the first connecting portion 343 is convex inward. The first connecting portion 343 may be located inside a hypothetical straight line K3 passing through the first end 341 and the second end 342. The 11th side surface 34a and the 12th side surface 34b may be flat surfaces. At the first connecting portion 343, the surface direction of the contact surface of the first side surface 34 changes discontinuously.

[0140] (Fifth variation) Figure 17 is a cross-sectional view showing the first conductive layer 31 of the radio wave absorbing sheet 10 according to the fifth modified example. As shown in Figure 17, the 11th side surface 34a and the 12th side surface 34b may be inwardly curved surfaces that are convex inward. In the first connection portion 343, the surface direction of the contact surface of the first side surface 34 changes discontinuously.

[0141] (Sixth variation) Figure 18 is a cross-sectional view showing the first conductive layer 31 of the radio wave absorbing sheet 10 according to the sixth modified example. As shown in Figure 18, the 11th side surface 34a may be a flat surface and the 12th side surface 34b may be an inwardly curved surface. Although not shown, the 11th side surface 34a may be an inwardly curved surface and the 12th side surface 34b may be a flat surface. In the first connection portion 343, the surface direction of the contact surface of the first side surface 34 changes discontinuously.

[0142] (Seventh variation) Figure 19 is a cross-sectional view showing the first conductive layer 31 of the radio wave absorbing sheet 10 according to the seventh modified example. As shown in Figure 19, the 11th side surface 34a may be an inwardly curved surface and the 12th side surface 34b may be an outwardly curved surface. Although not shown, the 11th side surface 34a may be an outwardly curved surface and the 12th side surface 34b may be an inwardly curved surface.

[0143] The first side surface 34 may be formed by a combination of an eleventh side surface 34a and a twelfth side surface 34b, which are not shown. For example, the eleventh side surface 34a may be one of an inward curved surface, an outward curved surface, and a flat surface, and the twelfth side surface 34b may be one of an inward curved surface, an outward curved surface, and a flat surface.

[0144] (Variation 8) In the above-described embodiment, an example was shown in which the upper surface of the first cover layer 60 in contact with the first side surface 34 is parallel to the first surface 21 of the substrate 20. For example, an example was shown in which the upper surface of the first colored layer 61 in contact with the first side surface 34 is parallel to the first surface 21 of the substrate 20. In other words, an example was shown in which the influence of the first side surface 34 does not appear on the upper surface of the first cover layer 60 in contact with the first side surface 34.

[0145] In this modified example, we will describe an example in which the influence of the first side surface 34 appears on the upper surface of the first cover layer 60 that is in contact with the first side surface 34. Figure 20 is a cross-sectional view showing the radio wave absorbing sheet 10 according to the eighth modified example. As shown in Figure 20, a step corresponding to the shape of the first side surface 34 may appear on the upper surface of the first cover layer 60 that is in contact with the first side surface 34. For example, a step 611 corresponding to the shape of the first side surface 34 may appear on the upper surface of the first colored layer 61 that is in contact with the first side surface 34.

[0146] When the first colored layer 61 has a step 611, the difference in thickness between the portion of the first colored layer 61 that overlaps the first conductive layer 31 in a plan view and the portion of the first colored layer 61 that does not overlap the first conductive layer 31 in a plan view becomes smaller. Therefore, for example, the difference between the magnitude of shrinkage that occurs in the portion of the first colored layer 61 that overlaps the first conductive layer 31 in a plan view and the magnitude of shrinkage that occurs in the portion of the first colored layer 61 that does not overlap the first conductive layer 31 in a plan view becomes smaller. This makes it possible to suppress the occurrence of defects such as cracks in the first colored layer 61. Shrinkage of the first colored layer 61 occurs, for example, in the process of heating the first colored layer 61 to harden it.

[0147] If the first colored layer 61 has a step 611, the radio wave absorbing sheet 10 may be installed outdoors. This makes it possible to suppress the visibility of the pattern of the first conductive layer 31 corresponding to the step 611.

[0148] The height T5 of the step 611 may be approximately equal to the thickness T1 of the first conductive layer 31. That is, the shape of the step 611 of the first cover layer 60 in contact with the first side surface 34 may be similar to the shape of the first side surface 34. The height T5 is, for example, 0.7 times or more the thickness T1, and may be 0.8 times or more, or 0.9 times or more.

[0149] (9th variation) Figure 21 is a cross-sectional view showing the radio wave absorbing sheet 10 according to the ninth modified example. As shown in Figure 21, the shape of the step 611 of the first cover layer 60 in contact with the first side surface 34 does not have to be similar to the shape of the first side surface 34. The height T5 is, for example, less than 0.7 times the thickness T1, may be 0.6 times or less, or 0.5 times or less. The height T5 may be, for example, 0.1 times or more the thickness T1, may be 0.2 times or more, or 0.3 times or more.

[0150] (10th variation) Figure 22A is a cross-sectional view showing a radio wave absorbing sheet 10 according to the 10th modified example. As shown in Figure 22A, the first cover layer 60 may include a first adhesive layer 64 located between the first conductive layer 31 and the first colored layer 61. The first adhesive layer 64 may be in contact with the fourth surface 33 and the first side surface 34 of the first conductive layer 31. The first colored layer 61 may be in contact with the first adhesive layer 64.

[0151] The first adhesive layer 64 is a layer for attaching the first colored layer 61 to the first conductive layer 31 and the base layer 50. The adhesive force of the first adhesive layer 64 to the first conductive layer 31 is higher than the adhesive force of the first colored layer 61 to the first conductive layer 31. By providing the first adhesive layer 64, it is possible to suppress the formation of a gap between the first side surface 34 and the first cover layer 60. In this application, the "adhesive layer" may be a layer that can be peeled off the object. Alternatively, the "adhesive layer" may be a layer that is difficult to peel off the object, or a layer that is not intended to be peeled off the object. The first adhesive layer 64 may be transparent. The material of the first adhesive layer 64 may be, for example, OCA (optical adhesive sheet) or a polyester urethane-based adhesive.

[0152] The thickness T44 of the first adhesive layer 64 may be greater than the thickness T1 of the first conductive layer 31. The thickness T44 of the first adhesive layer 64 may be 2 μm or more, or 5 μm or more. The thickness T44 of the first adhesive layer 64 may be 50 μm or less, or 30 μm or less.

[0153] The radio wave absorbing sheet 10 in Figure 22A is manufactured, for example, by attaching a decorative sheet containing a first colored layer 61 and a first adhesive layer 64 to the first conductive layer 31 and the base layer 50. By using a decorative sheet, the first cover layer 60 can be formed efficiently.

[0154] As shown in Figure 22A, the first colored layer 61 may include a lower surface parallel to the first surface 21 of the substrate 20. In this case, the thickness of the portion of the first colored layer 61 that overlaps the first conductive layer 31 in a plan view is approximately equal to the thickness of the portion of the first colored layer 61 that does not overlap the first conductive layer 31 in a plan view. This makes it possible to suppress the visibility of the first conductive layer 31.

[0155] "The lower surface of the first colored layer 61 is parallel to the first surface 21 of the substrate 20" means that the difference between the third distance M3 and the fourth distance M4 is 20 μm or less. The third distance M3 is the distance between the lower surface of the first colored layer 61 that overlaps with the first conductive layer 31 in a plan view and the first surface 21 of the substrate 20. The fourth distance M4 is the distance between the lower surface of the first colored layer 61 that does not overlap with the first conductive layer 31 in a plan view and the first surface 21 of the substrate 20. The difference between the third distance M3 and the fourth distance M4 may be 15 μm or less, 10 μm or less, 5.0 μm or less, 3.0 μm or less, or 1.0 μm or less.

[0156] Figure 22B is a cross-sectional view showing an example of a radio wave absorbing sheet 10 according to the 10th modified example. As shown in Figure 22B, steps corresponding to the shape of the first side surface 34 may appear in both the layer in contact with the first side surface 34 and the layer constituting the upper surface of the first cover layer 60.

[0157] The symbol T5' represents the height of the step in the layer in contact with the first side surface 34. The symbol T5 represents the step appearing in the layer that constitutes the upper surface of the first cover layer 60. The height of step T5' and the height of step T5 may be 0.7 times or more, 0.8 times or more, or 0.9 times or more of the thickness T1, as in the case of the eighth modification. The height of step T5' and the height of step T5 may be less than 0.7 times the thickness T1, 0.6 times or less, 0.5 times or less, 0.1 times or more, 0.2 times or more, or 0.3 times or more, as in the case of the ninth modification.

[0158] In the example shown in Figure 22B, the upper surface of the first adhesive layer 64 in contact with the first side surface 34 includes a step 641 corresponding to the shape of the first side surface 34. The step 641 has the height T5' described above. In the example shown in Figure 22B, the upper surface of the first colored layer 61 constituting the upper surface of the first cover layer 60 includes a step 611 corresponding to the shape of the first side surface 34. The step 611 has the height T5 described above.

[0159] The first cover layer 60 shown in Figure 22B is manufactured, for example, by attaching a decorative sheet that is thin enough to conform to the shape of the first side surface 34 to the first conductive layer 31 and the base layer 50.

[0160] Figure 22C is a cross-sectional view showing an example of a radio wave absorbing sheet 10 according to the 10th modified example. The upper surface of the layer in contact with the first side surface 34 includes a step corresponding to the shape of the first side surface 34, and the upper surface of the layer constituting the upper surface of the first cover layer 60 may be parallel to the first surface 21 of the base material 20.

[0161] In the example shown in Figure 22C, the upper surface of the first adhesive layer 64 in contact with the first side surface 34 includes a step 641 corresponding to the shape of the first side surface 34. In the example shown in Figure 22C, the upper surface of the first colored layer 61 constituting the upper surface of the first cover layer 60 is parallel to the first surface 21 of the substrate 20.

[0162] (11th variation) Figure 23 is a cross-sectional view showing the radio wave absorbing sheet 10 according to the 11th modified example. As shown in Figure 23, the first cover layer 60 may include a second colored layer 62 located on the first colored layer 61. The second colored layer 62 contains a coloring agent. The second colored layer 62 may be in contact with the first colored layer 61.

[0163] As the coloring agent for the second colored layer 62, the coloring agents exemplified for the first colored layer 61 can be used. The coloring agent for the second colored layer 62 may be the same as or different from the coloring agent for the first colored layer 61.

[0164] The thickness T42 of the second colored layer 62 may be greater than, less than, or the same as the thickness T41 of the first colored layer 61.

[0165] As shown in Figure 23, when the first cover layer 60 includes multiple layers, the method of forming the first cover layer 60 is arbitrary. For example, multiple layers may be sequentially laminated onto the substrate 20 by a printing method or the like. For example, a sheet comprising multiple layers may be attached to the substrate 20.

[0166] (12th variation) Figure 24 is a cross-sectional view showing a radio wave absorbing sheet 10 according to the twelfth modified example. As shown in Figure 24, the first cover layer 60 may include a second adhesive layer 65 located between the first colored layer 61 and the second colored layer 62.

[0167] The second adhesive layer 65 is a layer for attaching the second colored layer 62 to the first colored layer 61. The adhesive strength of the second adhesive layer 65 to the first colored layer 61 is higher than the adhesive strength of the second colored layer 62 to the first colored layer 61.

[0168] The material for the second adhesive layer 65 can be the same as the material for the first adhesive layer 64. The material for the second adhesive layer 65 may be the same as or different from the material for the first adhesive layer 64.

[0169] The thickness T45 of the second adhesive layer 65 may be greater than, less than, or the same as the thickness T44 of the first adhesive layer 64.

[0170] (13th variation) Figure 25 is a cross-sectional view showing a radio wave absorbing sheet 10 according to the 13th modified example. As shown in Figure 25, the first cover layer 60 may include a first colored layer 61 in contact with the fourth surface 33 and the first side surface 34, and a second colored layer 62. The second colored layer 62 may be in contact with the first colored layer 61. A layer such as an adhesive layer may be provided between the first colored layer 61 and the second colored layer 62.

[0171] In the example shown in Figure 25, the first colored layer 61 may have adhesive properties to the first conductive layer 31. For example, the adhesive strength of the first colored layer 61 to the first conductive layer 31 may be higher than the adhesive strength of the second colored layer 62 to the first conductive layer 31.

[0172] (14th variation) Figure 26 is a cross-sectional view showing a radio wave absorbing sheet 10 according to the 14th modified example. As shown in Figure 26, the first cover layer 60 may include a first transparent layer 67 located between the first colored layer 61 and the second colored layer 62. The first transparent layer 67 is made of a transparent material. "Transparent" means that the color exhibited by the light emitted from the first transparent layer 67 is influenced by the layer located between the first transparent layer 67 and the substrate 20. For example, in the example in Figure 26, the color exhibited by the light emitted from the first transparent layer 67 is influenced by the first colored layer 61. The material of the first transparent layer 67 is, for example, a transparent olefin film, a transparent polypropylene film, etc.

[0173] The thickness T47 of the first transparent layer 67 is, for example, 2 μm or more, and may be 5 μm or more. The thickness T47 of the first transparent layer 67 is, for example, 50 μm or less, and may be 30 μm or less.

[0174] (15th variation) Figure 27 is a cross-sectional view showing a radio wave absorbing sheet 10 according to the 15th modified example. As shown in Figure 27, the first cover layer 60 may include a surface layer 90 that constitutes the upper surface of the first cover layer 60. The surface layer 90 may have weather resistance. The material of the surface layer 90 is, for example, a UV-curable resin or an EB-curable resin.

[0175] By providing a weather-resistant surface layer 90 on the radio wave absorbing sheet 10, the weather resistance of the radio wave absorbing sheet 10 can be improved.

[0176] The thickness T6 of the surface layer 90 is, for example, 2 μm or more, and may be 5 μm or more. The thickness T6 of the surface layer 90 is, for example, 50 μm or less, and may be 30 μm or less.

[0177] Figure 28 is an enlarged cross-sectional view of the surface layer 90. The surface layer 90 may include recesses 91 and protrusions 92. The recesses 91 and protrusions 92 may be configured to give the surface of the radio wave absorbing sheet 10 a pattern or texture, to prevent reflection, or to prevent the adhesion of dirt, bacteria, etc. Although not shown, such recesses and protrusions may be formed on the upper surface of the first colored layer 61.

[0178] The depth T7 of the recess 91 is, for example, 100 nm or more, and may be 200 nm or more. The depth T7 of the recess 91 is, for example, 28000 nm or less, and may be 4800 nm or less. The depth T7 of the recess 91 may be constant regardless of position, or may vary depending on position.

[0179] (16th variation) Figure 29 is a cross-sectional view showing a radio wave absorbing sheet 10 according to the 16th modified example. As shown in Figure 29, the radio wave absorbing sheet 10 may include a second cover layer 70 that covers the sixth surface 43 of the second conductive layer 41. This can improve at least one of the aesthetic appeal or weather resistance of the radio wave absorbing sheet 10. For example, if the structure 121 is made of transparent glass, the second cover layer 70 will be visible. The aesthetic appeal of the radio wave absorbing sheet 10 can be enhanced by having the second cover layer 70 have an appropriate color, pattern, etc.

[0180] The layer structure and materials of the second cover layer 70 can be those exemplified for the first cover layer 60. For example, the second cover layer 70 may contain a coloring agent. The layer structure and materials of the second cover layer 70 may be the same as or different from those of the first cover layer 60. The second cover layer 70 may consist of a single layer or multiple layers.

[0181] The thickness T8 of the second cover layer 70 may be within the range of the thickness T4 exemplified for the first cover layer 60. The thickness T8 of the second cover layer 70 may be within the sum of the range of the thickness T4 exemplified for the first cover layer 60 and the range of the thickness T6 exemplified for the surface layer 90. The thickness T8 of the second cover layer 70 may be the same as or different from the thickness T4 of the first cover layer 60.

[0182] (17th variation) Figure 30 is a cross-sectional view showing a radio wave absorbing sheet 10 according to the 17th modified example. As shown in Figure 30, the radio wave absorbing sheet 10 may include a third conductive layer 38 facing the fourth surface 33 of the first conductive layer 31. The third conductive layer 38 may be located on the first cover layer 60.

[0183] The third conductive layer 38 contains a conductive material. The material for the third conductive layer 38 can be one of the materials exemplified for the first conductive layer 31. The material for the third conductive layer 38 may be the same as, or different from, the material for the first conductive layer 31.

[0184] The thickness T9 of the third conductive layer 38 may be 0.01 μm or more, 0.1 μm or more, 1 μm or more, or 5 μm or more. The thickness T9 of the third conductive layer 38 may be 50 μm or less, 35 μm or less, 20 μm or less, or 10 μm or less.

[0185] Although not shown in the diagram, the radio wave absorbing sheet 10 may have multiple third conductive layers 38 that overlap one first conductive layer 31 in a plan view. When multiple third conductive layers 38 are provided, a layer having adhesive properties, colorability, transparency, etc., may exist between two adjacent third conductive layers 38 in a plan view.

[0186] The third conductive layer 38 may be a layer that affects the properties of the radio wave absorbing sheet 10. The third conductive layer 38 may also be a layer provided for the aesthetic purposes of the radio wave absorbing sheet 10.

[0187] As shown in Figure 30, the radio wave absorbing sheet 10 may include a third cover layer 80 that covers the third conductive layer 38. The layer structure and materials of the third cover layer 80 can be the same as those exemplified for the first cover layer 60. For example, the third cover layer 80 may contain a coloring agent. The layer structure and materials of the third cover layer 80 may be the same as or different from those of the first cover layer 60. The third cover layer 80 may consist of a single layer or multiple layers.

[0188] The thickness T9 of the third cover layer 80 is within the range of the thickness T6 exemplified for the first cover layer 60. The thickness T9 of the third cover layer 80 may be the same as or different from the thickness T6 of the first cover layer 60.

[0189] If the third conductive layer 38 is a layer that affects the characteristics of the radio wave absorbing sheet 10, the third side surface 39 of the third conductive layer 38 may spread outward as it approaches the first cover layer 60, similar to the first side surface 34 of the first conductive layer 31. In this case, the third side surface 39 is not hidden in a plan view. This makes it easier for the third cover layer 80 to come into contact with the third side surface 39 when forming the third cover layer 80. Therefore, it is possible to suppress the formation of a gap between the third side surface 39 and the third cover layer 80.

[0190] (18th variation) In this modified example, an example in which the first conductive layer 31 is fabricated by transfer is described. Figures 31 to 34 show the method for manufacturing the radio wave absorbing sheet 10 according to this modified example.

[0191] As shown in Figure 31, a first conductive layer 31 is formed on the substrate 160. The first conductive layer 31 includes a third surface 32 facing the substrate 160 and a fourth surface 33 located on the opposite side of the third surface 32. The method for forming the first conductive layer 31 is arbitrary. For example, the first conductive layer 31 may be formed by wet etching, as in the embodiment described above.

[0192] Next, as shown in Figure 32, a first cover layer 60 is formed on the substrate 160 so as to cover the first conductive layer 31. The first cover layer 60 may be formed by printing. The first cover layer 60 may also be formed by attaching a decorative sheet or the like to the substrate 160.

[0193] Next, as shown in Figure 33, the first cover layer 60 is peeled off from the substrate 160. The adhesive force of the first cover layer 60 to the first conductive layer 31 is higher than the adhesive force of the substrate 160 to the first conductive layer 31. Therefore, the first conductive layer 31 is peeled off from the substrate 160 together with the first cover layer 60. In other words, the first conductive layer 31 is transferred to the first cover layer 60.

[0194] Next, as shown in Figure 34, the first cover layer 60 and the first conductive layer 31 are attached to the base layer 50. In this way, a radio wave absorbing sheet 10 comprising the base layer 50, the first conductive layer 31, and the first cover layer 60 is obtained.

[0195] (19th variation) In this modified example, we will also describe an example in which the first conductive layer 31 is fabricated by transfer. Figures 35 to 37 show the method for manufacturing the radio wave absorbing sheet 10 according to this modified example.

[0196] As shown in Figure 35, a first conductive layer 31 is formed on the substrate 170. The first conductive layer 31 includes a fourth surface 33 facing the substrate 170 and a third surface 32 located on the opposite side of the fourth surface 33. The method for forming the first conductive layer 31 is arbitrary.

[0197] Next, as shown in Figure 36, the first conductive layer 31 on the substrate 170 is brought into contact with the base layer 50. Then, as shown in Figure 37, the substrate 170 is separated from the base layer 50. The adhesive force of the base layer 50 to the first conductive layer 31 is higher than the adhesive force of the substrate 170 to the first conductive layer 31. Therefore, the first conductive layer 31 remains on the base layer 50. In other words, the first conductive layer 31 is transferred to the base layer 50.

[0198] Next, a first cover layer 60 is formed on the base layer 50 so as to cover the first conductive layer 31. In this way, a radio wave absorbing sheet 10 comprising the base layer 50, the first conductive layer 31, and the first cover layer 60 is obtained.

[0199] (20th variation) Figure 38 is a plan view showing one embodiment of the radio wave absorbing sheet 10. As shown in Figure 38, the shapes of the multiple first conductive layers 31 may be different. For example, the multiple first conductive layers 31 may include a plurality of first shaped layers 31A and a plurality of second shaped layers 31B having a different shape from the first shaped layers 31A in a plan view. In the example shown in Figure 38, both the first shaped layer 31A and the second shaped layer 31B are circular. That is, the shape of the second shaped layer 31B is similar to the shape of the first shaped layer 31A.

[0200] As shown in Figure 38, multiple first shaped layers 31A may be arranged in the first direction D1 in the first period P1. Multiple second shaped layers 31B may also be arranged in the first direction D1 in the first period P1. As shown in Figure 38, in the second direction D2, the first shaped layers 31A and the second shaped layers 31B may be arranged alternately in the second period P2.

[0201] Although not shown in the diagram, the period in which multiple second shape layers 31B are arranged may be different from the first period P1 in which multiple first shape layers 31A are arranged.

[0202] The first shaped layer 31A has a radius r1. The second shaped layer 31B has a radius r2 that is smaller than radius r1. Therefore, the area Sb of the second shaped layer 31B is smaller than the area Sa of the first shaped layer 31A.

[0203] The spectrum showing the absorption characteristics of the radio wave absorbing sheet 10, as shown in Figure 38, includes a first peak appearing at a first frequency fr1 corresponding to the first shape layer 31A, and a second peak appearing at a second frequency fr2 corresponding to the second shape layer 31B. The second frequency fr2 is higher than the first frequency fr1. The first and second peaks may partially overlap. In this case, the radio wave absorbing sheet 10 can absorb radio waves in the bandwidth between the first frequency fr1 and the second frequency fr2. For example, the amount of reflection of the radio wave absorbing sheet 10 in the bandwidth between the first frequency fr1 and the second frequency fr2 is -3 dB or less. Therefore, the -3 dB bandwidth BW of the spectrum showing the absorption characteristics of the radio wave absorbing sheet 10 can be expanded. For example, the -3 dB bandwidth BW of the spectrum showing the absorption characteristics of the radio wave absorbing sheet 10 can be made larger than 2 × fr(ave) × tanδ. fr(ave) is the average of the first frequency fr1 and the second frequency fr2, and is calculated by the following formula. fr(ave) = (fr1 + fr2) / 2

[0204] The first frequency fr1 is the same as, or nearly the same as, the resonant frequency fr calculated by substituting radius r1 for r in equations (A1) and (A2) above. The second frequency fr2 is the same as, or nearly the same as, the resonant frequency fr calculated by substituting radius r2 for r in equations (A1) and (A2) above.

[0205] In this modified example, the occupancy rate of the first conductive layer 31 is calculated by dividing the total area of ​​the multiple first conductive layers 31 by the area of ​​the region 35 in which the first conductive layers 31 are distributed. The occupancy rate may also be calculated by dividing the area of ​​the group containing the multiple first conductive layers 31 by the area of ​​the unit cell 12 corresponding to the group. Figure 39 is a perspective view showing the unit cell 12. The unit cell 12 has an area corresponding to two first conductive layers 31 aligned in the first direction D1 and two first conductive layers 31 aligned in the second direction D2. For example, in a plan view, the unit cell 12 is a quadrilateral including a pair of first sides extending in the first direction D1 and a pair of second sides extending in the second direction D2. The length of the first side is equal to twice the first period P1, and the length of the second side is equal to twice the second period P2. In the example shown in Figure 39, the unit cell 12 includes two first shaped layers 31A aligned in the first direction D1, two second shaped layers 31B aligned in the first direction D1, a substrate 20, and a second conductive layer 41. The area of ​​the unit cell 12 is 4 × P1 × P2. The total area of ​​the first conductive layer 31 is 2 × πr1. 2 +2 × πr² 2 Therefore, the occupancy rate is (2 × πr¹ 2 +2 × πr² 2 ) / (4×P1×P2).

[0206] Incidentally, if the difference between the first frequency fr1 of the first peak and the second frequency fr2 of the second peak becomes too large, there is a possibility that the amount of reflection will exceed -3dB in the bandwidth between the first frequency fr1 and the second frequency fr2. The difference between the first frequency fr1 and the second frequency fr2 increases as the ratio of the area Sa of the first shape layer 31A to the area Sb of the second shape layer 31B deviates from 1. Taking this into consideration, it is preferable that Sa / Sb be (1+8×tanδ) or less. This makes it possible to achieve a amount of reflection of -3dB or less in the bandwidth between the first frequency fr1 and the second frequency fr2. Sa / Sb may also be (1+7×tanδ) or (1+6×tanδ) or less.

[0207] On the other hand, if the difference between the first frequency fr1 of the first peak and the second frequency fr2 of the second peak becomes too small, the -3dB bandwidth BW of the spectrum may narrow. Taking this into consideration, it is preferable that Sa / Sb be (1+1×tanδ) or greater. Sa / Sb may also be (1+2×tanδ) or greater, or (1+3×tanδ) or greater.

[0208] (21st variation) Figure 40 is a plan view showing the radio wave absorbing sheet 10 according to the 21st modified example. As shown in Figure 40, the first conductive layer 31 may be a rectangle having length L and width W in a plan view. The width W may be the same as the length L, or it may be smaller than the length L.

[0209] In the example shown in Figure 40, the edges of the first conductive layer 31 having length L extend in the first direction D1, and the edges of the first conductive layer 31 having width W extend in the second direction D2. Although not shown, the edges of the first conductive layer 31 having length L may extend in a direction different from the first direction D1.

[0210] Figure 41 is a perspective view showing the unit cell 12 in this modified example. The unit cell 12 includes one rectangular first conductive layer 31. As in the embodiments described above, the characteristics of the resonant circuit realized by the radio wave absorbing sheet 10 are determined based on the configuration of the unit cell 12.

[0211] In this modified example, the resonant frequency fr is calculated based on the following equations (B1), (B2), (B3), and (B4).

number

[0212] In this modified example, the frequency of the radio waves targeted by the radio wave absorbing sheet 10 is, for example, 500 MHz to 110 GHz. In this case, the radio wave absorbing sheet 10 is configured such that the resonant frequency fr is 500 MHz to 110 GHz. For example, when the radio wave absorbing sheet 10 is used in local 5G, fifth-generation mobile communication systems, or automotive radar equipment, the radio wave absorbing sheet 10 is configured such that the resonant frequency fr is 3 GHz or higher. For example, the radio wave absorbing sheet 10 is configured such that the resonant frequency fr is 20 GHz to 110 GHz. For example, if the frequency of the radio waves targeted by the radio wave absorbing sheet 10 is 79 GHz, the thickness T0 and relative permittivity ε of the base material 20 are set so that the resonant frequency fr is the same as or approximately the same as 79 GHz. r The length L and width W of the first conductive layer 31 are also determined. The thickness T0 and relative permittivity ε of the substrate 20 are also determined. r If the resonant frequency fr is predetermined, the length L and width W of the first conductive layer 31 are determined so that the resonant frequency fr is a desired value. This allows the target radio waves to be absorbed by utilizing resonance.

[0213] (22nd variation) Figure 42 is a plan view showing a radio wave absorbing sheet 10 according to the first modified example. As shown in Figure 42, both the first shaped layer 31A and the second shaped layer 31B may have a rectangular shape in plan view. For example, the shape of the first shaped layer 31A may be a rectangle having length L1 and width W1. The shape of the second shaped layer 31B may be a rectangle having length L2 and width W2. The area Sb of the second shaped layer 31B is smaller than the area Sa of the first shaped layer 31A. For example, the length L2 and width W2 may be smaller than the length L1 and width W1.

[0214] (23rd variation) In the above-described embodiment, an example was shown where the structure 121 to which the radio wave absorbing sheet 10 is attached indicates the boundary of the service area 125, but it is not particularly limited. For example, the structure 121 may be the inner or outer surface of a wall, ceiling, beam, column, or other building component. For example, the structure 121 may be the inner or outer surface of a utility pole, traffic light, tunnel wall, sidewalk step, or other building structure. For example, the structure 121 may be the ground surface, a tree, or other natural structure. The structure 121 may be part of an automobile. For example, the structure 121 may be a pillar of an automobile.

[0215] The above-described modifications may be combined as appropriate and applied to the above-described embodiment. [Explanation of Symbols]

[0216] 10 Radio wave absorbing sheets 20 Base material 21 Page 1 22 Side 2 31. First conductive layer 32 Page 3 33 Page 4 34 First aspect 341 1st end 342 2nd end 343 First connection section 34a 11th aspect 34b 12th aspect 36 1st adhesive layer 38 Third conductive layer 39 Third aspect 41 Second conductive layer 42 Page 5 43 Page 6 46 Second adhesive layer 50 Base layer 60. First Cover Layer 61 1st colored layer 62 2nd colored layer 64 1st adhesive layer 65 Second adhesive layer 67 1st transparent layer 70 Second Cover Layer 80 Third Cover Layer 90 Surface layer 100 Wireless Network Systems 120 base station 125 Service Areas (Smart factories, factories, buildings, public facilities, power plants, 130 Terminal devices

Claims

1. A substrate including a first surface and a second surface located opposite the first surface, A plurality of first conductive layers, including a third surface facing the first surface, a fourth surface located on the opposite side of the third surface, and a first surface located between the third surface and the fourth surface, A first cover layer covering the fourth surface and the first side surface, The material comprises a second conductive layer facing the second surface, The first side surface includes a first end connected to the third surface and a second end connected to the fourth surface, The first end is located further out than the second end in a plan view. The first cover layer is a radio wave absorbing sheet comprising a first colored layer containing a coloring agent.

2. The radio wave absorbing sheet according to claim 1, wherein the first side surface includes an inward curved surface located inside a hypothetical straight line passing through the first end and the second end in a cross-sectional view.

3. The first side surface forms a first angle with respect to the third surface at the first end, The first side surface forms a second angle with respect to the fourth surface at the second end, The radio wave absorbing sheet according to claim 2, wherein the sum of the first angle and the second angle is greater than 90° and less than 175°.

4. The radio wave absorbing sheet according to claim 3, wherein the second angle is 135° or less.

5. The radio wave absorbing sheet according to claim 1, wherein the first side surface includes an outer curved surface located outside a hypothetical straight line passing through the first end and the second end in a cross-sectional view.

6. The first side surface forms a first angle with respect to the third surface at the first end, The first side surface forms a second angle with respect to the fourth surface at the second end, The radio wave absorbing sheet according to claim 5, wherein the sum of the first angle and the second angle is greater than 185° and less than 270°.

7. The radio wave absorbing sheet according to claim 1, wherein the first side surface includes a flat surface.

8. The first side surface forms a first angle with respect to the third surface at the first end, The first side surface forms a second angle with respect to the fourth surface at the second end, The radio wave absorbing sheet according to claim 7, wherein the sum of the first angle and the second angle is 175° or more and 185° or less.

9. The radio wave absorbing sheet according to claim 1, wherein the first side surface includes an eleventh side surface and a twelfth side surface connected to the eleventh side surface at the first connection portion and located between the first connection portion and the fourth side surface.

10. The radio wave absorbing sheet according to claim 9, wherein the 11th or 12th side surface includes an inward curved surface located inside a hypothetical straight line passing through the first end and the second end in a cross-sectional view.

11. The radio wave absorbing sheet according to claim 9, wherein the 11th or 12th side surface includes an outer curved surface located outside a hypothetical straight line passing through the first end and the second end in a cross-sectional view.

12. The radio wave absorbing sheet according to claim 9, wherein the 11th side surface or the 12th side surface includes a flat surface.

13. The radio wave absorbing sheet according to any one of claims 1 to 12, wherein the first cover layer includes an upper surface parallel to the first surface.

14. The radio wave absorbing sheet according to claim 13, wherein the first colored layer includes a lower surface parallel to the first surface.

15. The radio wave absorbing sheet according to claim 13, wherein the first cover layer includes a layer that is in contact with the first side surface and has an upper surface that overlaps the first side surface in a plan view.

16. The radio wave absorbing sheet according to any one of claims 1 to 12, wherein the first cover layer includes an upper surface in which a step appears that overlaps the first side surface in a plan view.

17. The radio wave absorbing sheet according to any one of claims 1 to 12, wherein the first colored layer is in contact with the fourth surface and the first side surface.

18. The radio wave absorbing sheet according to any one of claims 1 to 12, wherein the first cover layer is located between the first conductive layer and the first colored layer and includes a first adhesive layer in contact with the fourth surface and the first side surface.

19. The radio wave absorbing sheet according to any one of claims 1 to 12, wherein the first cover layer comprises a second colored layer containing a coloring agent.

20. The radio wave absorbing sheet according to claim 19, wherein the first cover layer comprises a second adhesive layer located between the first colored layer and the second colored layer.

21. The radio wave absorbing sheet according to claim 19, wherein the first cover layer comprises a first transparent layer located between the first colored layer and the second colored layer.

22. The radio wave absorbing sheet according to any one of claims 1 to 12, wherein the plurality of first conductive layers include a plurality of first shaped layers and a plurality of second shaped layers having a shape different from the first shaped layers in a plan view.

23. The equivalent circuit of the radio wave absorbing sheet includes a primary circuit, a secondary circuit, and a transformer that connects the primary circuit and the secondary circuit, The secondary circuit is a resonant circuit composed of the substrate, the first conductive layer, and the second conductive layer. The radio wave absorbing sheet according to any one of claims 1 to 12, wherein the ratio of the characteristic impedance of the secondary circuit to the characteristic impedance of the primary circuit is 0.8 or more and 1.2 or less.

24. The radio wave absorbing sheet according to claim 23, wherein the characteristic impedance of the primary circuit is 120π.

25. A structure having a surface, A wireless network system comprising a radio wave absorbing sheet according to any one of claims 1 to 12, which is attached to the aforementioned surface.

26. The surface of the structure includes a curved surface, The wireless network system according to claim 25, wherein the radio wave absorbing sheet is attached to a curved surface.

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