Radio wave absorption body

Abstract

Provided is a highly durable radio wave absorption body.　A radio wave absorption body according to the present invention includes: a first dielectric layer that is formed from a thermoplastic adhesive and has a first surface and a second surface, the first surface being on the side at which radio waves are incident; a plurality of conductive films that are positioned on the first surface and have a rectangular shape as seen in plan view; and a radio wave reflection layer that is positioned on the second surface.

Description

Radio wave absorber

[0001] The present disclosure relates to a radio wave absorber.

[0002] Conventionally, there has been a semi-transparent wave absorber in which a first transparent dielectric layer, a first resistive film, a second transparent dielectric layer, a second resistive film, and a third transparent dielectric layer are bonded together with double-sided adhesive tape, and the thickness is 5 mm or more and 25 mm or less (see, for example, Patent Document 1).

[0003] Japanese Patent Application Laid-Open No. 2022-120897

[0004] Incidentally, in a conventional semi-transparent wave absorber (wave absorber), the first transparent dielectric layer, the first resistive film, the second transparent dielectric layer, the second resistive film, and the third transparent dielectric layer are bonded together with double-sided adhesive tape. Since double-sided adhesive tape is highly hygroscopic and has low durability, the durability of conventional wave absorbers is low.

[0005] Therefore, an object of the present invention is to provide a radio wave absorber with high durability.

[0006] The radio wave absorber of an embodiment of the present disclosure has a first surface located on the radio wave incident side and a second surface, and includes a first dielectric layer formed of a thermoplastic adhesive, a plurality of conductive films located on the first surface and having a rectangular shape in a planar view, and a radio wave reflecting layer located on the second surface.

[0007] A highly durable radio wave absorber can be provided.

[0008] 1 is a diagram showing an example of an application of the radio wave absorber of the embodiment. FIG. 2 is a diagram showing an example of an application of the radio wave absorber of the embodiment. FIG. 3 is a cross-sectional view showing an example of the configuration of the radio wave absorber of the embodiment. FIG. 4 is a diagram showing an example of the configuration of a radio wave absorber of the embodiment. FIG. 5 is a diagram showing an example of the configuration of a radio wave absorber of a modified embodiment of the embodiment. FIG. 6 is a diagram showing an example of the frequency characteristics of the radio wave absorption rate in the radio wave absorber of the embodiment. FIG. 7 is a diagram showing an example of the frequency characteristics of the radio wave absorption rate in the radio wave absorber of the embodiment. FIG. 8 is a diagram showing an example of the frequency characteristics of the radio wave absorption rate in the radio wave absorber of the modified embodiment. FIG. 9 is a diagram showing an example of the result of a simulation (Part 1). FIG. 10 is a diagram showing an example of the result of a simulation (Part 1). FIG. 11 is a diagram showing an example of the result of a simulation (Part 1). FIG. 12 is a diagram showing an example of the result of a simulation (Part 1). FIG. 13 is a diagram showing an example of the aspect ratio of the conductive film 120A used in the simulation (Part 1). FIG. 14 is a diagram showing an example of the result of a simulation (Part 2). FIG. 1 is a diagram showing an example of a result of a simulation (part 2). FIG. 2 is a diagram showing an example of a result of a simulation (part 2). FIG. 3 is a diagram showing an example of a result of a simulation (part 2). FIG. 4 is a diagram showing an example of a result of a simulation (part 2). FIG. 5 is a diagram showing an example of a result of a simulation (part 3). FIG. 6 is a diagram showing an example of a result of a simulation (part 3). FIG. 7 is a diagram showing an example of a result of a simulation (part 3). FIG. 8 is a diagram showing an example of a result of a simulation (part 3). FIG. 9 is a diagram showing an example of a simulation model used in a simulation (part 4). FIG. 10 is a diagram showing an example of a result of a simulation (part 4). FIG. 11 is a diagram showing an example of a result of a simulation (part 4).FIG. 10 is a diagram showing an example of the results of a simulation (part 4).

[0009] Hereinafter, embodiments to which the radio wave absorber of the present disclosure is applied will be described. In the following, the same elements will be denoted by the same reference numerals, and duplicated explanations may be omitted.

[0010] In the following description, an XYZ coordinate system is defined. The direction parallel to the X axis (X direction), the direction parallel to the Y axis (Y direction), and the direction parallel to the Z axis (Z direction) are perpendicular to each other. The X axis is an example of a first axis, the Y axis is an example of a second axis, and the Z axis is an example of a third axis. Furthermore, a planar view refers to a view from the XY plane. In the following description, the length, width, thickness, etc. of each part may be exaggerated to make the configuration easier to understand. Furthermore, terms such as parallel, right angle, orthogonal, horizontal, vertical, up and down, etc., are intended to allow for deviations to the extent that they do not impair the effects of the embodiments.

[0011] In the following description, "radio waves" refers to a type of electromagnetic wave, and generally, electromagnetic waves below 3 THz are called radio waves. Hereinafter, electromagnetic waves emitted from outdoor base stations or relay stations will be referred to as "radio waves," and electromagnetic waves in general will be referred to as "electromagnetic waves." In addition, in the following, "millimeter waves" or "millimeter wave band" will include not only the frequency band of 30 GHz to 300 GHz, but also the quasi-millimeter wave band of 24 GHz to 30 GHz.

[0012] The radio waves absorbed by the radio wave absorber of the embodiment are preferably radio waves in the millimeter wave band of the fifth generation mobile communication system (5G) or the like, or in the frequency band of 1 GHz to 30 GHz, including the Sub-6 band. The radio waves absorbed by the radio wave absorber of the embodiment may be LTE (Long Term Evolution), LTE-A (LTE-Advanced), or UMB (Ultra Mobile Broadband). The radio waves absorbed by the radio wave absorber of the embodiment may be IEEE802.11 (Wi-Fi (registered trademark)), IEEE802.16 (WiMAX (registered trademark)), IEEE802.20, UWB (Ultra-Wideband), Bluetooth (registered trademark), LPWA (Low Power Wide Area), or the like. Hereinafter, unless otherwise specified, radio waves in the range of 2 GHz to 10 GHz will be used as an example for explanation.

[0013] 1A and 1B are diagrams showing an example of an application of a radio wave absorber 100 according to an embodiment. Base stations BS1 and BS2 are shown in 1A and 1B. The base stations BS1 and BS2 are capable of communication at a predetermined frequency. As an example, the predetermined frequency is 4.8 GHz.

[0014] 1A, when radio waves are transmitted from base station BS1 to base station BS2, in addition to radio waves D that reach base station BS2 directly from base station BS1, radio waves R that are reflected by a wall W also reach base station BS2, and radio waves D and R are in opposite phases. Because radio waves D and R cancel each other out and are attenuated, the radio wave reception conditions at base station BS2 are very poor, and radio waves D and R are hardly received at all.

[0015] 1B, if a radio wave absorber 100 is placed on the wall W, part of the radio waves R will be absorbed by the radio wave absorber 100, and the strength of the radio waves R reaching the base station BS2 will decrease. As a result, the base station BS2 will be able to receive the radio waves D, and the radio wave reception conditions at the base station BS2 will improve.

[0016] In reality, it is unlikely that the radio waves D and R that reach base station BS2 will be completely out of phase, but if there is a large phase difference between radio waves D and R, radio waves D and R will cancel each other out, causing attenuation and worsening the reception conditions at base station BS2. In such a case, if a radio wave absorber 100 is provided as shown in Figure 1B, the reception conditions at base station BS2 will improve and radio waves D will be able to be received. The radio wave absorber 100 may also be provided between base station BS1 and wall W.

[0017] <Configuration of radio wave absorber 100> Fig. 2A is a cross-sectional view showing an example of the configuration of the radio wave absorber 100. Fig. 2B is a diagram showing an example of the configuration of a plurality of conductive films 120A of an FSS (Frequency Selective Shield) layer 120. The plurality of conductive films 120A are an example of a plurality of conductive films.

[0018] The radio wave absorber 100 includes adhesive layers 110A and 110B, an FSS layer 120, a radio wave reflecting layer 130, and glass plates 140A and 140B. The adhesive layer 110A is an example of a first dielectric layer. The glass plate 140A is an example of a first protective layer. The adhesive layer 110B and the glass plate 140B are an example of a second protective layer. The adhesive layer 110B is an example of a second dielectric layer, and the glass plate 140B is an example of a substrate.

[0019] 2A, the incident surface of the radio wave absorber 100 is a main surface 141A on the +Z direction side of a glass plate 140A. The radio wave absorber 100 absorbs radio waves of a predetermined frequency (4.8 GHz, for example) incident from the main surface 141A.

[0020] In this embodiment, as an example, a form will be described in which the radio wave absorber 100 is used with the main surface 141A standing perpendicular to a horizontal plane.

[0021] The radio waves incident on the incident surface of such a radio wave absorber 100 include TE (Transverse Electric field) waves and TM (Transverse Magnetic field) waves. TM waves are an example of first polarization, and TE waves are an example of second polarization. TE waves vibrate in the Y direction, while TM waves vibrate in the X direction.

[0022] In this case, the length X1 of the conductive film 120A in the X direction is set to a length corresponding to the angle of incidence of the TM wave whose electric field oscillates in the X direction. The length X1 of the conductive film 120A in the X direction is an example of a first length, and the length Y1 of the conductive film 120A in the Y direction is an example of a second length.

[0023] The horizontal plane is the XZ plane. The incident surface of the radio wave absorber 100 is parallel to the XY plane, and radio waves arrive from the +Z direction. That is, radio waves arrive from a direction horizontal to the incident surface (principal surface 141A) of the radio wave absorber 100.

[0024] The incident angle of the radio wave on the incident surface is expressed as an angle in the XZ plane, with the +Z direction being the 0 degree direction. The incident angle of the radio wave on the incident surface is expressed as the angle it forms with the +Z direction in the XZ plane, and as an example, the incident angle on the +X side of the +Z direction is expressed as a positive angle (0 to 90 degrees), and the incident angle on the -X side of the +Z direction is expressed as a negative angle (0 to -90 degrees). However, due to symmetry with respect to the Z axis in the XZ plane, the following explanation will be given for incident angles in the range of 0 to 90 degrees.

[0025] The usage of the radio wave absorber 100 is not limited to the above-described configuration in which the main surface 141A is set perpendicular to a horizontal plane. The length X1 in the X direction of the conductive film 120A may be set to a length corresponding to the angle of incidence of the TM wave component of the radio waves incident on the radio wave absorber 100, and the length Y1 in the Y direction of the conductive film 120A may be set to a length corresponding to the angle of incidence of the TE wave component of the radio waves incident on the radio wave absorber 100.

[0026] The radio wave absorber 100 preferably has a visible light transmittance of 50% or more and a haze value of 5% or less. If the visible light transmittance is 50% or more, a user can visually recognize a commodity or a scene through the radio wave absorber 100. The visible light transmittance is preferably 53% or more, and more preferably 55% or more. The visible light transmittance is measured in accordance with Japanese Industrial Standard JIS R 3106:1998, and can be calculated by a calculation formula when a standard D65 light source is used. The haze value is determined in accordance with Japanese Industrial Standard JIS K7136:2000.

[0027] The adhesive layers 110A and 110B, the FSS layer 120, the radio wave reflecting layer 130, and the glass plates 140A and 140B are arranged in the following order from the +Z direction side to the -Z direction side: glass plate 140A, FSS layer 120, adhesive layer 110A, radio wave reflecting layer 130, adhesive layer 110B, and glass plate 140B.

[0028] As an example, the radio wave absorber 100 is produced by sandwiching the FSS layer 120, the adhesive layer 110A, the radio wave reflecting layer 130, and the adhesive layer 110B between glass plates 140A and 140B and then thermocompression bonding them together. The radio wave absorber 100 has a configuration in which the FSS layer 120 and the radio wave reflecting layer 130 are sandwiched between glass plates 140A and 140B of laminated glass.

[0029] <Adhesive Layers 110A and 110B> The adhesive layer 110A has a surface 111A located on the radio wave incident side (+Z direction side) and a surface 112A on the opposite side (-Z direction side) from surface 111A. Surface 111A is an example of a first surface, and surface 112A is an example of a second surface. The adhesive layer 110A is located between a glass plate 140A having an FSS layer 120 formed on the surface on the -Z direction side and the radio wave reflecting layer 130, and bonds the glass plate 140A and the radio wave reflecting layer 130 together.

[0030] The adhesive layer 110B is located on the opposite side of the radio wave reflecting layer 130 from the adhesive layer 110A. The adhesive layer 110B is located between the radio wave reflecting layer 130 and the glass plate 140B, and bonds the radio wave reflecting layer 130 and the glass plate 140B together. The adhesive layer 110B has a surface 111B located on the +Z direction side and a surface 112B on the opposite side (−Z direction side) from the surface 111B.

[0031] The adhesive layers 110A and 110B are adhesive layers formed of a thermoplastic adhesive. The adhesive layers 110A and 110B formed of a thermoplastic adhesive have high moisture resistance and durability. The adhesive layers 110A and 110B are, for example, polyvinyl butyral (PVB) resin, ethylene vinyl acetate (EVA) resin, cycloolefin polymer (COP) resin, or thermoplastic polyurethane (TPU) resin. The water absorption of the adhesive layers 110A and 110B is, for example, 3% by mass or less. The water absorption of the adhesive layers 110A and 110B is measured in accordance with Japanese Industrial Standard JIS K 7209:2000. If the water absorption of the adhesive layers 110A and 110B is 3% by mass or less, they are less likely to deteriorate even in high-temperature, high-humidity environments. The PVB resin, EVA resin, COP resin, and TPU resin may all have a water absorption of 3% by mass or less. The adhesive layers 110A and 110B preferably have a water absorption rate of 1% by mass or less, and 0.01% by mass or more.

[0032] Furthermore, the thickness of the adhesive layer 110A is equal to or less than 1 / (10√εr) of the wavelength at the frequency at which the maximum radio wave absorption rate is obtained, where εr is the relative dielectric constant of the adhesive layer 110A. This may be the same for the thickness of the adhesive layer 110B. The maximum radio wave absorption rate is the maximum value of the radio wave absorption rate.

[0033] Here, a configuration will be described in which the radio wave absorber 100 includes glass plates 140A and 140B. In addition, if the radio wave absorber 100 does not include the glass plate 140B, a glass plate may be used instead of the adhesive layer 110B. The radio wave absorber 100 having such a configuration will be described later with reference to FIG. 2D .

[0034] <FSS Layer 120> The FSS layer 120 is located between the glass plate 140A and the adhesive layer 110A. As an example, the FSS layer 120 is formed on the main surface 142A on the −Z direction side of the glass plate 140A. Here, the FSS layer 120 will be described using FIGS. 2B and 2C in addition to FIG. 2A. FIG. 2B is a diagram showing an example of the configuration of the FSS layer 120 in a plan view. FIG. 2B shows only the FSS layer 120, with other components omitted. FIG. 2C is a diagram explaining the reflection of radio waves in the FSS layer 120. FIG. 2C shows an incident wave (dashed line) passing through the FSS layer 120, the incident wave reflected by the FSS layer 120, the phase of the incident wave, and the phase of the reflected wave.

[0035] As shown in FIG. 2B , the FSS layer 120 includes a plurality of conductive films 120A. The conductive films 120A are arranged at intervals along the first axis (X-axis) and second axis (Y-axis) of the surface 111A, for example. The FSS layer 120 is configured to have an overlapping band between a frequency band in which the absorption rate of TM waves incident on the conductive films 120A at a predetermined angle of incidence is maximized and a frequency band in which the absorption rate of TE waves incident on the conductive films 120A at a predetermined angle of incidence is maximized. Details of this will be described later with reference to FIGS. 3A to 5 . For example, the predetermined angle of incidence is between 0 and 60 degrees.

[0036] The plurality of conductive films 120A are arranged at equal intervals in the X direction and at equal intervals in the Y direction. For example, the interval ΔX between the plurality of conductive films 120A in the X direction and the interval ΔY between the plurality of conductive films 120A in the Y direction are equal.

[0037] The conductive films 120A are all equal in size. The size of the conductive films 120A is represented by a length X1 in the X direction and a length Y1 in the Y direction. The length X1 in the X direction is an example of a first length, and the length Y1 in the Y direction is an example of a second length. The ratio (X1 / Y1) of the length X1 in the X direction to the length Y1 in the Y direction of each of the conductive films 120A is 1.00 or more and 2.00 or less, more preferably 1.03 or more and 1.30 or less, and even more preferably 1.08 or more and 1.18 or less.

[0038] Furthermore, the interval between the plurality of conductive films 120A in the X direction and the interval between the plurality of conductive films 120A in the Y direction are preferably 30 μm or more and 500 μm or less.

[0039] Furthermore, it is preferable that the difference between the spacing between the plurality of conductive films 120A in the X direction and the spacing between the plurality of conductive films 120A in the Y direction be 30% or less of the spacing between the plurality of conductive films 120A in the X direction.

[0040] The plurality of conductive films 120A are insulated from one another and are formed, for example, of a transparent conductive film such as an ITO (indium tin oxide) film or a Low-E (low emissivity) film. For example, the plurality of conductive films 120A can be produced by dividing a transparent conductive film such as an ITO film or a Low-E film formed on the main surface 142A of the glass plate 140A by laser processing.

[0041] The sheet resistance of the plurality of conductive films 120A is preferably, for example, 1 Ω / sq. or more and 30 Ω / sq. or less.

[0042] 2C, the FSS layer 120 transmits some of the radio waves (dashed lines) and reflects other parts of the radio waves (solid lines). In addition, the FSS layer 120 absorbs some of the radio waves.

[0043] When radio waves reach the conductive film 120A, free electrons in the conductive film 120A move in the direction opposite to the electric field direction of the radio waves, causing a current to flow through the conductive film 120A. At the same time, energy is periodically accumulated and released in the gaps between the multiple conductive films 120A due to the electric field generated. As a result, a propagation delay occurs in the radio waves passing through the conductive film 120A. A delay time occurs between the time the radio waves enter the conductive film 120A and the time they are re-radiated. In other words, the phase of the radio waves passing through the FSS layer 120 and the phase of the radio waves reflected by the FSS layer 120 both change relative to the phase of the radio waves entering the FSS layer 120. The radio waves that pass through the FSS layer 120 are reflected in the +Z direction by the radio wave reflecting layer 130. The conductive film 120A and the radio wave reflecting layer 130 form a λ / 4 type radio wave absorber.

[0044] Therefore, the distance in the Z direction between the conductive film 120A and the radio wave reflecting layer 130 can be made smaller than λ / 4, thereby reducing the thickness of the radio wave absorber 100. λ is the wavelength in free space of the radio wave incident on the conductive film 120A.

[0045] The distance in the Z direction between the conductive film 120A and the radio wave reflecting layer 130 is preferably 0.3 mm to 6 mm, more preferably 0.4 mm to 2 mm, and even more preferably 0.5 mm to 1 mm. The thickness of the conductive film 120A is not particularly limited, but is, for example, 50 nm to 300 nm when the conductive film 120A is a Low-E film.

[0046] Details of setting the length X1 of the conductive film 120A in the X direction to a length according to the incident angle of the TM wave will be described later with reference to FIGS. 3A to 5. FIG.

[0047] <Radio Wave Reflecting Layer 130> The radio wave reflecting layer 130 is located on the surface 112A of the adhesive layer 110A. In other words, the radio wave reflecting layer 130 is located between the adhesive layers 110A and 110B.

[0048] The radio wave reflecting layer 130 reflects radio waves that have passed through the plurality of conductive films 120A. The radio wave reflecting layer 130 is conductive. The sheet resistance of the radio wave reflecting layer 130 is, for example, less than 3 Ω / sq., preferably less than 1 Ω / sq., and more preferably 0.1 Ω / sq. or less.

[0049] The radio wave reflecting layer 130 is made of a metal mesh, for example. The metal mesh has openings, and therefore can achieve a higher visible light transmittance than a metal film. From the viewpoint of suppressing reflection of visible light, the metal mesh is preferably one that has been subjected to an oxidation treatment and a blackening treatment. Stainless steel is generally used as the metal mesh. If the mesh wire diameter is 0.01 mm or more, the radio wave absorber 100 is easy to handle when manufactured. If the mesh wire diameter is 0.1 mm or less, the radio wave absorption properties are good. From the viewpoint of ensuring both the transparency and radio wave absorption properties of the radio wave absorber 100, the mesh wire diameter is particularly preferably 0.01 mm to 0.08 mm, and more preferably 0.02 mm to 0.06 mm.

[0050] The radio wave reflecting layer 130 may also be made of a transparent conductive film. Examples of transparent conductive films include an ITO film and a Low-E film. There are no particular limitations on the thickness of the radio wave reflecting layer 130, but when a transparent conductive film such as a Low-E film is used for the radio wave reflecting layer 130, the thickness is, for example, 100 nm to 500 nm.

[0051] The radio waves that have passed through the FSS layer 120 are reflected by the radio wave reflecting layer 130. The radio waves reflected by the plurality of conductive films 120A of the FSS layer 120 and the radio waves reflected by the radio wave reflecting layer 130 interfere with and cancel each other out. As a result, the radio waves are absorbed by the radio wave absorber 100.

[0052] The radio wave reflecting layer 130 may have an insulating substrate that supports the transparent conductive film. The insulating substrate is, for example, a resin film. For example, the transparent conductive film may be formed on the surface of the insulating substrate by a vapor deposition method, a sputtering method, or the like. The transparent conductive film may also be formed by printing using a conductive ink.

[0053] <Glass Plate 140A> The glass plate 140A has principal surfaces 141A and 142A. The principal surface 141A is located on the radio wave incident side. The principal surface 141A is the radio wave incident surface of the radio wave absorber 100. The principal surface 142A is located on the opposite side to the principal surface 141A. A plurality of conductive films 120A of the FSS layer 120 are formed on the principal surface 141A.

[0054] The glass plate 140A protects the FSS layer 120. The first protective layer can be made of glass, ceramics, resin, or the like. From the viewpoint of weight reduction, resin is preferable. Specific examples of resin include polyethylene terephthalate (PET) resin, polycarbonate (PC) resin, and acrylic resin. On the other hand, from the viewpoint of scratch resistance, glass or ceramics are preferable.

[0055] When the first protective layer is a glass substrate or a resin substrate, the thickness is, for example, 2 mm to 6 mm, preferably 2 mm to 4 mm. When the first protective layer is a ceramic substrate, the thickness is, for example, 0.2 mm to 4.0 mm, preferably 0.3 mm to 2.5 mm, more preferably 0.3 mm to 1.0 mm.

[0056] A resin film may be provided on the surface of the glass plate 140A on the +Z direction side to prevent cracks.

[0057] <Glass Plate 140B> The glass plate 140B has main surfaces 141B and 142B. The main surface 141B is located on the radio wave reflecting layer 130 side. The main surface 142B is located on the opposite side to the main surface 141B. An adhesive layer 110B is adhered to the main surface 141B.

[0058] The glass plate 140B protects the radio wave reflecting layer 130. Ceramics, resin, or the like may be used for the glass plate 140B instead of glass. The material of the glass plate 140B can be selected from the same materials as those described above for the first protective layer. The thickness of the glass plate 140B can be selected within the thickness range described above for the first protective layer. The material and thickness of the glass plate 140B may be the same as or different from the material and thickness of the first protective layer.

[0059] <Configuration of Radio Wave Absorber 100 According to Modification of the Embodiment> FIG. 2D is a cross-sectional view showing an example of the configuration of a radio wave absorber 100A according to a modification of the embodiment.

[0060] The radio wave absorber 100A of the modified example has a configuration in which the adhesive layer 110B is omitted from the radio wave absorber 100 shown in Fig. 2A. The radio wave absorber 100A of the modified example includes the adhesive layer 110A, an FSS layer 120, a radio wave reflecting layer 130, and glass plates 140A and 140B. The adhesive layer 110A is an example of a first dielectric layer. The glass plate 140A is an example of a first protective layer. The glass plate 140B is an example of a second protective layer.

[0061] The radio wave absorber 100A of the modified example differs from the radio wave absorber 100 shown in FIG. 2A in that it does not include the adhesive layer 110B (see FIG. 2A ), but the other configurations are similar. In the radio wave absorber 100A shown in FIG. 2D , the radio wave reflecting layer 130 is formed on a main surface 141B on the +Z direction side of a glass plate 140. As an example, the radio wave absorber 100A shown in FIG. 2D is produced by thermocompression bonding in a state in which the FSS layer 120 and the adhesive layer 110A are sandwiched between a glass plate 140A and a glass plate 140B having the radio wave reflecting layer 130 formed on its main surface 141B. The radio wave absorber 100A shown in FIG. 2D has a configuration in which the FSS layer 120 and the radio wave reflecting layer 130 are sandwiched between glass plates 140A and 140B of laminated glass.

[0062] <Length X1 of Conductive Film 120A in X-Direction> FIGS. 3A and 3B are diagrams showing an example of frequency characteristics of the radio wave absorption rate in the radio wave absorber 100. FIG.

[0063] The radio wave absorption rate is the ratio of the power of the reflected wave to the power of the incident wave, for radio waves that are incident from a certain incident direction on main surface 141A of glass plate 140A of radio wave absorber 100 and are reflected by radio wave absorber 100 at a reflection angle corresponding to the incident angle.

[0064] The frequency band is a predetermined range including the center frequency. The predetermined range is preferably a band of ±5% of the center frequency, more preferably a band of ±3% of the center frequency, and even more preferably a band of ±1% of the center frequency. In this embodiment, the predetermined frequency is 4.8 GHz, for example. Therefore, for example, if the 4.8 GHz band has a band of ±5%, the predetermined frequency is 4.8 GHz ±0.24 GHz; if the 4.8 GHz band has a band of approximately ±3%, the predetermined frequency is 4.8 GHz ±0.15 GHz; and if the 4.8 GHz band has a band of approximately ±1%, the predetermined frequency is 4.8 GHz ±0.05 GHz.

[0065] Here, the length Y1 in the Y direction of the conductive film 120A optimized so that the frequency characteristics of the absorption rate of TE waves arriving from a direction with an incident angle of 0 degrees in the horizontal plane reach a maximum value at 4.8 GHz is, for example, 11.6 mm. Furthermore, the length X1 in the X direction of the conductive film 120A optimized so that the frequency characteristics of the absorption rate of TM waves arriving from a direction with an incident angle of 0 degrees in the horizontal plane reach a maximum value at 4.8 GHz is, for example, 11.6 mm. Here, as an example, the results of actual measurements using the lengths X1 and Y1 of 11.6 mm in the X and Y directions of the conductive film 120A optimized in this way will be described.

[0066] 3A shows an example of the measured radio wave absorption rate when the conductive film 120A has an X-direction length X1 of 11.6 mm and a Y-direction length Y1 of 11.6 mm, and TE radio waves (TE waves) and TM radio waves (TM waves) are incident at angles of incidence of 0 degrees and 45 degrees. The X-direction and Y-direction spacings ΔX and ΔY of the conductive film 120A are set to 0.1 mm, and the sheet resistance of the conductive film 120A is set to 1.8 Ω / sq. The Z-direction thicknesses of the glass plate 140A, adhesive layer 110A, adhesive layer 110B, and glass plate 140B are set to 2.73 mm, 0.485 mm, 0.485 mm, and 2.73 mm, respectively.

[0067] 3A, the characteristics of the TE wave with an incident angle of 0 degrees overlap with the characteristics of the TM wave with an incident angle of 0 degrees. As shown in Fig. 3A, the TE wave with an incident angle of 0 degrees, the TM wave with an incident angle of 0 degrees, and the TE wave with an incident angle of 45 degrees all had absorption rates of approximately 85% or more in the 4.8 GHz band, but the frequency at which the absorption amount of the TM wave with an incident angle of 45 degrees was maximum was approximately 5.4 GHz, and the absorption rate was approximately 95%.

[0068] The visible light transmittance of the wave absorber 100 measured using a spectrophotometer (Colour Cure i colorimeter manufactured by Suga Test Instruments Co., Ltd.) was 59.67%. The haze value of the wave absorber 100 measured using a haze meter (HGM-2 direct reading haze computer manufactured by Suga Test Instruments Co., Ltd.) was 2.3%.

[0069] 3B shows an example of the measured radio wave absorption rate when the X-direction length X1 of the conductive film 120A was set to 13.7 mm, the Y-direction length Y1 was set to 11.6 mm, and TE and TM radio waves were incident at angles of incidence of 0 and 45 degrees. The X-direction and Y-direction spacings ΔX and ΔY of the conductive film 120A were set to 0.1 mm, and the sheet resistance of the conductive film 120A was set to 1.8 Ω / sq. The Z-direction thicknesses of the glass plate 140A, adhesive layer 110A, adhesive layer 110B, and glass plate 140B were set to 2.73 mm, 0.485 mm, 0.485 mm, and 2.73 mm, respectively.

[0070] As shown in FIG. 3B , the TE wave radio waves with incident angles of 0 degrees and 45 degrees and the TM wave radio waves with an incident angle of 45 degrees had an absorption rate of 85% or more in the 4.8 GHz band, but the frequency at which the absorption rate of the TM wave radio waves with an incident angle of 45 degrees reached its maximum was approximately 4.3 GHz, and the absorption amount was approximately 90%.

[0071] From the results of FIGS. 3A and 3B , for TE waves, when the length X1 in the X direction of the conductive film 120A was both 11.6 mm and 13.7 mm, the absorption amount was 85% or more in the 4.8 GHz band at both the incident angles of 0 degrees and 45 degrees.

[0072] Furthermore, for TM waves, when the length X1 in the X direction of the conductive film 120A was changed from 11.6 mm to 13.7 mm, the absorption amount in the 4.8 GHz band became 85% or more when the incident angle was 45 degrees, but when the incident angle was 0 degrees, the frequency at which the absorption amount was maximum shifted from the 4.8 GHz band and decreased to approximately 4.3 GHz.

[0073] From this, it can be considered that for radio waves whose electric field oscillates in the X direction, such as TM waves, it may be possible to accommodate the angle of incidence by adjusting the length X1 in the X direction of the conductive film 120A.

[0074] 3C shows an example of measured values ​​of the radio wave absorption rate when the predetermined frequency is set to 6 GHz, the X-direction length X1 of the conductive film 120A is set to 9.7 mm, the Y-direction length Y1 is set to 9.7 mm, and TE and TM radio waves are incident at an incident angle of 0 degrees in the radio wave absorber 100A of the modified example. The spacings ΔX and ΔY of the conductive film 120A in the X and Y directions are set to 0.1 mm, and the sheet resistance of the conductive film 120A is set to 6.1 Ω / sq. The Z-direction thicknesses of the glass plate 140A, the adhesive layer 110A, and the glass plate 140B are set to 4.73 mm, 0.76 mm, and 2.73 mm, respectively.

[0075] As shown in Figure 3C, the frequency characteristics of the radio wave absorption rates of TE and TM waves at an incident angle of 0 degrees almost overlap, and the absorption rate in the 6 GHz band is 95% or more. This confirms that even when the predetermined frequency is set to 6 GHz, results similar to those of the frequency characteristics of the radio wave absorption rates of TE and TM waves at an incident angle of 0 degrees in Figure 3A when the predetermined frequency is set to 4.8 GHz can be obtained.

[0076] 3D shows an example of measured values ​​of the radio wave absorption rate when the predetermined frequency was set to 3.5 GHz, the X-direction length X1 of the conductive film 120A in the radio wave absorber 100 was set to 12 mm, the Y-direction length Y1 was set to 12 mm, and TE radio waves and TM radio waves were incident at an incident angle of 0 degrees. The X-direction and Y-direction spacings ΔX and ΔY of the conductive film 120A were set to 0.1 mm, and the sheet resistance of the conductive film 120A was set to 1.8 Ω / sq. The Z-direction thicknesses of the glass plate 140A, the adhesive layer 110A, the adhesive layer 110B, and the glass plate 140B were set to 2.73 mm, 0.97 mm, 0.485 mm, and 2.73 mm, respectively.

[0077] As shown in Fig. 3D, the frequency characteristics of the radio wave absorption rates of TE and TM waves at an incident angle of 0 degrees almost overlap, and the absorption rate in the 3.5 GHz band is 95% or more. This confirms that even when the predetermined frequency is set to 3.5 GHz, results similar to those of the frequency characteristics of the radio wave absorption rates of TE and TM waves at an incident angle of 0 degrees in Fig. 3A when the predetermined frequency is set to 4.8 GHz can be obtained.

[0078] From the above measurement results, it appears that for radio waves whose electric field oscillates in the X direction, such as TM waves, it may be possible to accommodate the angle of incidence by adjusting the length X1 in the X direction of the conductive film 120A, and therefore a simulation was performed to confirm this tendency.

[0079] 4A to 4F are diagrams showing examples of the results of the simulation (part 1). The simulation was performed in a state where the radio wave absorber 100 was placed with the main surface 141A standing perpendicular to a horizontal plane.

[0080] FIG. 4A shows an example of a simulation result in which the absorption rates of TE and TM waves are calculated while the length Y1 of the conductive film 120A in the Y direction is fixed at 11.6 mm and the length X1 of the conductive film 120A in the X direction and the incident angle are changed.

[0081] The length X1 in the X direction and the length Y1 in the Y direction of the conductive film 120A were set, and a simulation was performed so that the amount of absorption of TE and TM waves in the 4.8 GHz band would be maximized when the incident angle was 0 degrees, 15 degrees, 30 degrees, 45 degrees, and 60 degrees.

[0082] The length X1 of the conductive film 120A in the X direction was set to 11.6 mm when the incident angle was 0 degrees, 11.9 mm when the incident angle was 15 degrees, 12.5 mm when the incident angle was 30 degrees, 13.7 mm when the incident angle was 45 degrees, and 15.1 mm when the incident angle was 60 degrees.

[0083] 4B to 4F show the simulation results when the incident angle is 0 degrees (X1 = 11.6 mm), when the incident angle is 15 degrees (X1 = 11.9 mm), when the incident angle is 30 degrees (X1 = 12.5 mm), when the incident angle is 45 degrees (X1 = 13.7 mm), and when the incident angle is 60 degrees (X1 = 15.1 mm), respectively.

[0084] 4A to 4F, the distances ΔX and ΔY in the X and Y directions of the conductive film 120A were set to 0.1 mm, and the sheet resistance of the conductive film 120A was set to 1.8 Ω / sq. The thicknesses in the Z direction of the glass plate 140A, the adhesive layer 110A, the adhesive layer 110B, and the glass plate 140B were set to 2.73 mm, 0.485 mm, 0.485 mm, and 2.73 mm, respectively.

[0085] As described above, the conductive film 120A has been optimized so that the frequency characteristics of the absorption rates of the TE and TE waves arriving from a direction with an incident angle of 0 degrees in the horizontal plane both reach a maximum value at 4.8 GHz. The lengths X1 and Y1 of the conductive film 120A in the X and Y directions are both 11.6 mm.

[0086] In other words, setting the lengths X1 and Y1 of the conductive film 120A in the X and Y directions to 11.6 mm means setting the lengths X1 and Y1 to be optimized so that the absorption rates of the TM and TE waves arriving from a direction with an incident angle of 0 degrees in the horizontal plane both reach their maximum values ​​at 4.8 GHz.

[0087] The simulation (Part 1) was performed based on the condition that the lengths X1 and Y1 of the conductive film 120A in the X and Y directions were set to lengths optimized so that the absorption rates of both the TM wave and the TE wave arriving from a direction with an incident angle of 0 degrees in the horizontal plane would reach their maximum values ​​at 4.8 GHz.

[0088] As shown in Figure 4A, the absorption rate of TE and TM waves in the 4.8 GHz band was 70% or higher for all incident angles of 0, 15, 30, 45, and 60 degrees. As can be seen from Figures 4B to 4F, the frequency band in which the TE and TM waves had their maximum values ​​was 4.8 GHz for all incident angles of 0, 15, 30, 45, and 60 degrees. Furthermore, there was a tendency for the absorption rate to decrease slightly as the incident angle increased, and in particular, the decrease in absorption of TE waves was greater than that of TM waves when the incident angle was 30 degrees or greater.

[0089] As a result, there is an overlapping band between the frequency band in which the absorption rate of TM wave radio waves incident on the plurality of conductive films 120A at a predetermined incident angle is at its maximum and the frequency band in which the absorption rate of TE wave radio waves incident on the plurality of conductive films 120A at a predetermined incident angle is at its maximum.

[0090] Furthermore, the maximum radio wave absorption rate in the overlapping band between the frequency band in which the radio wave absorption rate of TM waves incident on the plurality of conductive films 120A at a predetermined incident angle is maximum and the frequency band in which the radio wave absorption rate of TE waves incident on the plurality of conductive films 120A at a predetermined incident angle is maximum was 70% or more.

[0091] From the above simulation results, it has been confirmed that, in a state where the radio wave absorber 100 is installed with the main surface 141A standing perpendicular to the horizontal plane, for radio waves whose electric field oscillates in the X direction, such as TM waves, the incident angle can be accommodated by adjusting the length X1 in the X direction of the conductive film 120A.

[0092] Furthermore, it was confirmed that by installing the radio wave absorber 100 with the main surface 141A standing perpendicular to the horizontal plane and setting the length X1 in the X direction of the conductive film 120A according to the angle of incidence of the TE wave and TM wave radio waves in the horizontal plane, it is possible to make the frequency bands in which the absorption rates of the TE wave and the TM wave radio waves reach their maximum values ​​the same.

[0093] For radio waves whose electric field oscillates in the X direction, such as TM waves, the effective length of the conductive film 120A in the X direction projected onto the incident direction can be considered to determine the frequency at which the amount of radio wave absorption is maximum.

[0094] 5 is a diagram showing an example of the aspect ratio of the conductive film 120A used in the simulation (Part 1). The aspect ratio is the ratio (X1 / Y1) of the length X1 in the X direction of the conductive film 120A to the length Y1 in the Y direction.

[0095] As shown in Fig. 5, the aspect ratio was 1.00 to 1.30 when the incident angle was 0 to 60 degrees. Although not shown in Figs. 4A to 4F and 5, it has been confirmed that the aspect ratio of the conductive film 120A is preferably 1.00 or more and 2.00 or less.

[0096] As shown in FIG. 5, the aspect ratio of the conductive film 120A is more preferably 1.03 or more and 1.30 or less, and even more preferably 1.08 or more and 1.18 or less.

[0097] 6A to 6F are diagrams showing examples of the results of the simulation (part 2). The simulation was performed in a state where the radio wave absorber 100 was placed with the main surface 141A standing perpendicular to a horizontal plane.

[0098] FIG. 6A shows an example of a simulation result in which the absorption rates of TE and TM waves are calculated while the length Y1 of the conductive film 120A in the Y direction is fixed at 6.9 mm and the length X1 of the conductive film 120A in the X direction and the incident angle are changed.

[0099] The length X1 in the X direction and the length Y1 in the Y direction of the conductive film 120A were set, and a simulation was performed so that the amount of absorption of TE and TM waves in the 4.8 GHz band would be maximized when the incident angle was 0 degrees, 15 degrees, 30 degrees, 45 degrees, and 60 degrees.

[0100] The length X1 of the conductive film 120A in the X direction was set to 6.9 mm when the incident angle was 0 degrees, 7.1 mm when the incident angle was 15 degrees, 7.6 mm when the incident angle was 30 degrees, 8.7 mm when the incident angle was 45 degrees, and 10.1 mm when the incident angle was 60 degrees.

[0101] 6B to 6F show the simulation results when the incident angle is 0 degrees (X1 = 6.9 mm), when the incident angle is 15 degrees (X1 = 7.1 mm), when the incident angle is 30 degrees (X1 = 7.6 mm), when the incident angle is 45 degrees (X1 = 8.7 mm), and when the incident angle is 60 degrees (X1 = 10.1 mm), respectively.

[0102] 6A to 6F, the distances ΔX and ΔY in the X and Y directions of the conductive film 120A were set to 0.1 mm, and the sheet resistance of the conductive film 120A was set to 6.1 Ω / sq. The thicknesses in the Z direction of the glass plate 140A, the adhesive layer 110A, the adhesive layer 110B, and the glass plate 140B were set to 2.73 mm, 0.97 mm, 0.485 mm, and 2.73 mm, respectively.

[0103] As described above, the conductive film 120A has been optimized so that the frequency characteristics of the absorption rates of the TE and TE waves arriving from a direction with an incident angle of 0 degrees in the horizontal plane both reach a maximum value at 4.8 GHz. The lengths X1 and Y1 of the conductive film 120A in the X and Y directions are both 6.9 mm.

[0104] In other words, setting the lengths X1 and Y1 of the conductive film 120A in the X and Y directions to 6.9 mm means setting the lengths X1 and Y1 to be optimized so that the absorption rates of the TM and TE waves arriving from a direction with an incident angle of 0 degrees in the horizontal plane both reach their maximum values ​​at 4.8 GHz.

[0105] The simulation (Part 2) was performed based on the condition that the lengths X1 and Y1 of the conductive film 120A in the X and Y directions were set to lengths optimized so that the absorption rates of both the TM wave and the TE wave arriving from a direction with an incident angle of 0 degrees in the horizontal plane would reach their maximum values ​​at 4.8 GHz.

[0106] As shown in Figure 6A, the absorption rate of TE and TM waves in the 4.8 GHz band was 80% or higher for all incident angles of 0, 15, 30, 45, and 60 degrees. As can be seen from Figures 6B to 6F, the frequency band in which the TE and TM waves exhibited the maximum values ​​was the 4.8 GHz band for all incident angles of 0, 15, 30, 45, and 60 degrees. Furthermore, there was a tendency for the absorption rate to decrease slightly as the incident angle increased, and in particular, the decrease in absorption of TE waves was greater than that of TM waves when the incident angle was 30 degrees or greater.

[0107] As a result, there is an overlapping band between the frequency band in which the absorption rate of TM wave radio waves incident on the plurality of conductive films 120A at a predetermined incident angle is at its maximum and the frequency band in which the absorption rate of TE wave radio waves incident on the plurality of conductive films 120A at a predetermined incident angle is at its maximum.

[0108] Furthermore, the maximum radio wave absorption rate in the overlapping band between the frequency band in which the radio wave absorption rate of TM waves incident on the plurality of conductive films 120A at a predetermined incident angle is maximum and the frequency band in which the radio wave absorption rate of TE waves incident on the plurality of conductive films 120A at a predetermined incident angle is maximum was 80% or more.

[0109] The above simulation results confirmed that, when the wave absorber 100 is installed with the main surface 141A perpendicular to the horizontal plane, the incident angle of radio waves whose electric field oscillates in the X direction, such as TM waves, can be accommodated by adjusting the X-direction length X1 of the conductive film 120A. Furthermore, although the X-direction and Y-direction lengths X1 and Y1 of the conductive film 120A are different from those in Part 1, the sheet resistance of the conductive film 120A and the Z-direction thicknesses of the glass plate 140A, the adhesive layer 110A, the adhesive layer 110B, and the glass plate 140B can be optimized. Under these optimized conditions, using a conductive film 120A smaller than that in Part 1, it was confirmed that, in the 4.8 GHz band, the frequency band in which the absorption rate of TM waves incident on the conductive film 120A at a predetermined incident angle is maximized can overlap with the frequency band in which the absorption rate of TE waves incident on multiple conductive films 120A at a predetermined incident angle is maximized.

[0110] Furthermore, it was confirmed that by installing the radio wave absorber 100 with the main surface 141A standing perpendicular to the horizontal plane and setting the length X1 in the X direction of the conductive film 120A according to the angle of incidence of the TE wave and TM wave radio waves in the horizontal plane, it is possible to make the frequency bands in which the absorption rates of the TE wave and the TM wave radio waves reach their maximum values ​​the same.

[0111] For radio waves whose electric field oscillates in the X direction, such as TM waves, the effective length of the conductive film 120A in the X direction projected onto the incident direction can be considered to determine the frequency at which the amount of radio wave absorption is maximum.

[0112] 7A to 7F are diagrams showing examples of the results of the simulation (part 3). The simulation was performed in a state where the radio wave absorber 100 was placed with the main surface 141A standing perpendicular to a horizontal plane.

[0113] FIG. 7A shows an example of a simulation result in which the absorption rates of TE and TM waves are calculated while the length Y1 of the conductive film 120A in the Y direction is fixed at 12.4 mm and the length X1 of the conductive film 120A in the X direction and the incident angle are changed.

[0114] The length X1 in the X direction and the length Y1 in the Y direction of the conductive film 120A were set, and a simulation was performed so that the amount of absorption of TE and TM waves in the 3.5 GHz band would be maximized when the incident angle was 0 degrees, 15 degrees, 30 degrees, 45 degrees, and 60 degrees.

[0115] The length X1 of the conductive film 120A in the X direction was set to 12.4 mm when the incident angle was 0 degrees, 12.6 mm when the incident angle was 15 degrees, 13.5 mm when the incident angle was 30 degrees, 15.1 mm when the incident angle was 45 degrees, and 17.1 mm when the incident angle was 60 degrees.

[0116] 7B to 7F show the simulation results when the incident angle is 0 degrees (X1 = 12.4 mm), when the incident angle is 15 degrees (X1 = 12.6 mm), when the incident angle is 30 degrees (X1 = 13.5 mm), when the incident angle is 45 degrees (X1 = 15.1 mm), and when the incident angle is 60 degrees (X1 = 17.1 mm), respectively.

[0117] 7A to 7F, the distances ΔX and ΔY in the X and Y directions of the conductive film 120A were set to 0.1 mm, and the sheet resistance of the conductive film 120A was set to 1.8 Ω / sq. The thicknesses in the Z direction of the glass plate 140A, the adhesive layer 110A, the adhesive layer 110B, and the glass plate 140B were set to 2.73 mm, 0.97 mm, 0.485 mm, and 2.73 mm, respectively.

[0118] As described above, the conductive film 120A has been optimized so that the frequency characteristics of the absorption rates of the TE and TE waves arriving from a direction with an incident angle of 0 degrees in the horizontal plane both reach a maximum value at 3.5 GHz. The lengths X1 and Y1 of the conductive film 120A in the X and Y directions are both 12.4 mm.

[0119] In other words, setting the lengths X1 and Y1 of the conductive film 120A in the X and Y directions to 12.4 mm means setting the lengths X1 and Y1 to be optimized so that the absorption rates of the TM and TE waves arriving from a direction with an incident angle of 0 degrees in the horizontal plane both reach their maximum values ​​at 3.5 GHz.

[0120] The simulation (Part 3) was performed based on the condition that the lengths X1 and Y1 of the conductive film 120A in the X and Y directions were set to lengths optimized so that the absorption rates of both the TM wave and the TE wave arriving from a direction with an incident angle of 0 degrees in the horizontal plane would reach their maximum values ​​at 3.5 GHz.

[0121] As shown in Figure 7A, the absorption rate of TE and TM waves in the 3.5 GHz band was 80% or higher for all incident angles of 0, 15, 30, 45, and 60 degrees. As can be seen from Figures 7B to 7F, the frequency band in which the TE and TM waves exhibited the maximum values ​​was the 3.5 GHz band for all incident angles of 0, 15, 30, 45, and 60 degrees. Furthermore, there was a tendency for the absorption rate to decrease slightly as the incident angle increased, and in particular, the decrease in absorption of TE waves was greater than that of TM waves when the incident angle was 30 degrees or greater.

[0122] As a result, there is an overlapping band between the frequency band in which the absorption rate of TM wave radio waves incident on the plurality of conductive films 120A at a predetermined incident angle is at its maximum and the frequency band in which the absorption rate of TE wave radio waves incident on the plurality of conductive films 120A at a predetermined incident angle is at its maximum.

[0123] Furthermore, the maximum radio wave absorption rate in the overlapping band between the frequency band in which the radio wave absorption rate of TM waves incident on the plurality of conductive films 120A at a predetermined incident angle is maximum and the frequency band in which the radio wave absorption rate of TE waves incident on the plurality of conductive films 120A at a predetermined incident angle is maximum was 80% or more.

[0124] The above simulation results confirmed that, when the wave absorber 100 is installed with the main surface 141A perpendicular to a horizontal plane, the incidence angle of radio waves whose electric field oscillates in the X direction, such as TM waves, can be accommodated by adjusting the X-direction length X1 of the conductive film 120A. Furthermore, although the X-direction and Y-direction lengths X1 and Y1 of the conductive film 120A are different from those in Part 1, the sheet resistance of the conductive film 120A and the Z-direction thicknesses of the glass plate 140A, the adhesive layer 110A, the adhesive layer 110B, and the glass plate 140B, etc., were optimized. Under these optimized conditions, by using the conductive film 120A that is enlarged by lowering the predetermined frequency (3.5 GHz) compared to Part 1, it was confirmed that, in the 3.5 GHz band, the frequency band in which the absorption rate of TM waves incident on the conductive film 120A at a predetermined incidence angle reaches a maximum can overlap with the frequency band in which the absorption rate of TE waves incident on the plurality of conductive films 120A at a predetermined incidence angle reaches a maximum.

[0125] Furthermore, it was confirmed that by installing the radio wave absorber 100 with the main surface 141A standing perpendicular to the horizontal plane and setting the length X1 in the X direction of the conductive film 120A according to the angle of incidence of the TE wave and TM wave radio waves in the horizontal plane, it is possible to make the frequency bands in which the absorption rates of the TE wave and the TM wave radio waves reach their maximum values ​​the same.

[0126] For radio waves whose electric field oscillates in the X direction, such as TM waves, the effective length of the conductive film 120A in the X direction projected onto the incident direction can be considered to determine the frequency at which the amount of radio wave absorption is maximum.

[0127] In the simulation results of Parts 1 to 3 above, the maximum radio wave absorption rate in the overlapping band between the frequency band in which the radio wave absorption rate of TM waves incident on the plurality of conductive films 120A at a predetermined angle of incidence is maximum and the frequency band in which the radio wave absorption rate of TE waves incident on the plurality of conductive films 120A at a predetermined angle of incidence is maximum was 70% or more. Various simulation results and the like have shown that it is desirable for the maximum radio wave absorption rate in such an overlapping band to be 50% or more.

[0128] <Simulation (Part 4)> FIG. 8 is a diagram showing an example of a simulation model used in the simulation (Part 4).

[0129] The simulation model shown in Figure 8 has a room and a passageway. Here, the explanation will be given using xyz coordinates indicated by lowercase letters. The xy plane is a horizontal plane, and the +z direction is the vertically upward direction. There is an entrance / exit on the +y side of the room, which is connected to the passageway.

[0130] A radio wave transmission source Tx was placed at the corner of the +x and +y sides of such a room, and a simulation was performed on the strength distribution of radio waves within the room. In the simulation, the length of the room in the x direction was set to 4 m, the length in the y direction to 5 m, the length of the corridor in the x direction to 6 m, the width of the corridor in the y direction to 2 m, and the height of the room and corridor in the z direction to 3 m, and conditions were set such that a ceiling was present.

[0131] 9A to 9D are diagrams showing an example of the results of the simulation (part 4).

[0132] 9A shows the radio wave intensity distribution inside a room and a passageway when the side walls inside the room are concrete walls. The radio wave intensity distribution inside the room is not uniform, with areas of high intensity and areas of low intensity coexisting. Radio waves leak into the passageway, with the radio wave intensity being higher near the entrance / exit. When the side walls inside the room are concrete walls, it was confirmed that the radio waves are not absorbed but reflected by the side walls of the room, resulting in an unsatisfactory radio wave intensity distribution inside the room and passageway.

[0133] Figure 9B shows the radio wave intensity distribution inside a room and a corridor when an ideal radio wave absorber is installed on the side wall inside the room. An ideal radio wave absorber has a radio wave absorption rate of 100%, and is completely non-reflective and completely non-transparent. Compared to Figure 9A, the radio wave intensity distribution inside the room was more uniform, and the intensity itself was also higher. In the corridor, the radio wave intensity was high in the area indicated by the solid ellipse (the left corner of the corridor), but this is thought to be due to waves arriving directly from the transmitting source Tx.

[0134] Fig. 9C shows the intensity distribution of radio waves inside a room and a passageway when the radio wave absorber 100 is installed on a side wall inside the room. Compared to Fig. 9A, the uniformity of the intensity distribution of radio waves inside the room was higher and the intensity itself was also higher, but compared to Fig. 9B, the uniformity of the intensity distribution of radio waves inside the room was slightly lower and the radio wave intensity in the passageway was slightly higher.

[0135] Figure 9D is a diagram in which the path of the radio waves from the transmission source Tx is added to Figure 9C. When the radio wave absorber 100 is installed on the side wall of the room, as shown in Figure 9D, the reflected waves that are not absorbed by the radio wave absorber 100 propagate within the room, and it is thought that the uniformity of the intensity distribution of the radio waves within the room is slightly lower than that of an ideal radio wave absorber (Figure 9B). However, it was confirmed that the radio wave absorber 100 can sufficiently absorb the reflected waves, and a good radio wave environment can be obtained within the room.

[0136] The above description concerns a configuration in which the multiple conductive films 120A are configured such that the frequency band in which the absorption rate of TM wave radio waves incident on the multiple conductive films 120A at a predetermined incident angle is maximized overlaps with the frequency band in which the absorption rate of TE wave radio waves incident on the multiple conductive films 120A at a predetermined incident angle is maximized. However, the multiple conductive films 120A may be designed with a relationship to only TM wave radio waves. In this case, the X-direction length X1 of each of the multiple conductive films 120A may be set to a length corresponding to the incident angle of the TM wave radio waves on the multiple conductive films 120A in the horizontal plane so that the absorption rate of TM wave radio waves in a predetermined frequency band is maximized in terms of the relationship between the frequency of the TM wave radio waves and the frequency of the TM wave radio waves whose electric field oscillates in the X direction.

[0137] <Effects> The radio wave absorber 100 has a surface 111A (first surface) located on the radio wave incident side and a surface 112A (second surface), and includes a first dielectric layer (adhesive layer 110A) formed of a thermoplastic adhesive, a plurality of conductive films 120A located on the surface 111A (first surface) and having a rectangular shape in a plan view, and a radio wave reflecting layer 130 located on the surface 112A (second surface). The adhesive layers 110A and 110B formed of a thermoplastic adhesive have high moisture resistance and durability.

[0138] Therefore, it is possible to provide a radio wave absorber 100 with high durability.

[0139] Furthermore, the plurality of conductive films 120A are arranged at intervals along the first axis (X-axis) direction and the second axis (Y-axis) direction of the surface 111A, and there is an overlapping band between the frequency band in which the absorption rate of the first polarized radio wave incident on the plurality of conductive films 120A at a predetermined incident angle is maximized and the frequency band in which the absorption rate of the second polarized radio wave incident on the plurality of conductive films 120A at a predetermined incident angle is maximized. Therefore, the radio wave absorber 100 can absorb the first polarized radio wave and the second polarized radio wave that are incident on the conductive film 120A at a predetermined incident angle and that are in the same frequency band.

[0140] Therefore, it is possible to provide a radio wave absorber 100 that is highly durable and capable of absorbing radio waves of the first polarized wave and the second polarized wave that are in the same frequency band at various angles of incidence.

[0141] Furthermore, the plurality of conductive films 120A may be arranged at equal intervals in the first axis (X-axis) direction and at equal intervals in the second axis (Y-axis) direction. By arranging the plurality of conductive films 120A in this manner, it is possible to provide a radio wave absorber 100 with better radio wave absorption characteristics.

[0142] Furthermore, the intervals between the plurality of conductive films 120A in the first axis (X-axis) direction and the intervals between the plurality of conductive films 120A in the second axis (Y-axis) direction may be equal. By arranging the plurality of conductive films 120A in this manner, it is possible to provide a radio wave absorber 100 with even better radio wave absorption characteristics.

[0143] Furthermore, the plurality of conductive films 120A may have the same size. By making the plurality of conductive films 120A have the same size, it is possible to provide a radio wave absorber 100 with even better radio wave absorption characteristics.

[0144] Furthermore, each of the plurality of conductive films 120A has a first side extending in the first axis (X-axis) direction and a second side extending in the second axis (Y-axis) direction, and the ratio of the length of the first side to the length of the second side of each of the plurality of conductive films 120A may be not less than 1.00 and not more than 2.00. By setting the ratio of the length of the first side to the length of the second side (aspect ratio), it is possible to provide a radio wave absorber 100 that can more reliably absorb first polarized and second polarized radio waves at a predetermined incident angle.

[0145] Furthermore, the difference between the spacing between the plurality of conductive films 120A in the first axis (X-axis) direction and the spacing between the plurality of conductive films 120A in the second axis (Y-axis) direction may be 30% or less of the spacing in the first axis (X-axis) direction. By making the difference between the spacing in the first axis (X-axis) direction and the spacing in the second axis (Y-axis) direction small, it is possible to provide a radio wave absorber 100 with good radio wave absorption characteristics.

[0146] Furthermore, the interval between the plurality of conductive films 120A in the first axis (X-axis) direction and the interval between the plurality of conductive films 120A in the second axis (Y-axis) direction may be 30 μm or more and 500 μm or less. By setting the interval between the first axis (X-axis) direction and the interval between the second axis (Y-axis) direction within a range such as k, it is possible to provide a radio wave absorber 100 with good radio wave absorption characteristics.

[0147] Furthermore, the first axis (X-axis) direction is the horizontal direction, and the second axis (Y-axis) direction is the vertical direction, and the first polarized wave may be a TM wave whose electric field oscillates in the first axis (X-axis) direction, and the second polarized wave may be a TE wave whose electric field oscillates in the second axis (Y-axis) direction. Therefore, the radio wave absorber 100 can absorb radio waves of TM waves and TE waves which are incident on the conductive film 120A at a predetermined incident angle and which are in the same frequency band, and it is possible to provide the radio wave absorber 100 which can absorb radio waves of TM waves and TE waves which are in the same frequency band at various incident angles.

[0148] Furthermore, the first axis (X-axis) direction is horizontal, and the second axis (Y-axis) direction is vertical, the first polarization is TM waves whose electric field oscillates in the first axis (X-axis) direction, and the second polarization is TE waves whose electric field oscillates in the second axis (Y-axis) direction. Each of the plurality of conductive films 120A has a first side extending in the first axis (X-axis) direction and a second side extending in the second axis (Y-axis) direction. The length X1 of the first side of each of the plurality of conductive films 120A may be set to a length corresponding to the angle of incidence of the TM wave radio waves on the plurality of conductive films 120A in a plane (horizontal plane) including the first axis (X-axis) and the third axis (Z-axis) so that the absorption rate of the TM wave radio waves in a predetermined frequency band is maximized in terms of the relationship between the TM wave radio wave frequency and the absorption rate of the TM wave radio waves. Therefore, by setting the length X1 of the first side of each of the plurality of conductive films 120A to a length according to the angle of incidence, it is possible to absorb TM and TE radio waves of a predetermined frequency in the radio wave absorber 100. Therefore, it is possible to provide the radio wave absorber 100 that can absorb TM and TE radio waves of the same frequency band at various angles of incidence.

[0149] Furthermore, the maximum radio wave absorption rate in the overlapping band between the frequency band in which the absorption rate of the first polarized radio wave incident on the plurality of conductive films 120A at a predetermined angle of incidence is maximum and the frequency band in which the absorption rate of the second polarized radio wave incident on the plurality of conductive films 120A at a predetermined angle of incidence is maximum may be 50% or more. By having the maximum radio wave absorption rate in the overlapping band of 50% or more, it is possible to provide a radio wave absorber 100 with good radio wave absorption characteristics.

[0150] Furthermore, the length X1 of the first side may be set to be longer as the incident angle increases. By making the effective length projected in the incident direction a length corresponding to the predetermined frequency, it is possible to provide the radio wave absorber 100 that can absorb radio waves of TM waves and TE waves in the same frequency band at various incident angles.

[0151] Furthermore, the length Y1 of the second side of each of the plurality of conductive films 120A may be constant regardless of the angle of incidence in the horizontal plane. Even if the angle of incidence in the horizontal plane changes, the length Y1 of the second side in the vibration direction of the TE wave does not change, and therefore, by making the length Y1 of the second side constant, it is possible to provide a radio wave absorber 100 that can stably absorb TE wave of a predetermined frequency.

[0152] The radio wave reflective layer 130 may be formed of the transparent conductive film 120A or a metal mesh. It is possible to provide a radio wave absorber 100 that has good visibility by suppressing unevenness due to differences in visible light transmittance while ensuring the function of reflecting radio waves.

[0153] The substrate may further include a first protective layer (glass plate 140A) located on the opposite side of the adhesive layer 110A from the plurality of conductive films 120A. The plurality of conductive films 120A can be protected by the first protective layer (glass plate 140A). The plurality of conductive films 120A can be formed on the first protective layer (glass plate 140A).

[0154] The radio wave absorber 100 may further include a second protective layer located on the opposite side of the radio wave reflecting layer 130 from the adhesive layer 110A. It is possible to provide a radio wave absorber 100 in which the second protective layer is provided on the −Z direction side of the radio wave reflecting layer 130.

[0155] Furthermore, the first protective layer (glass plate 140A) may be a glass or resin substrate (glass plate 140A). When the first protective layer is a glass or resin substrate (glass plate 140A), the plurality of conductive films 120A can be protected by the glass or resin substrate (glass plate 140A). Furthermore, the plurality of conductive films 120A can be formed on the glass or resin substrate (glass plate 140A).

[0156] The second protective layer may also include a second dielectric layer (adhesive layer 110B) formed of a thermoplastic adhesive and located on the opposite side of the radio wave reflecting layer 130 from the first dielectric layer (adhesive layer 110A), and a glass or resin substrate (glass plate 140B) located on the opposite side of the second dielectric layer (adhesive layer 110B) from the radio wave reflecting layer 130. It is possible to provide a radio wave absorber 100 that can protect the plurality of conductive films 120A with the second dielectric layer (adhesive layer 110B) and the substrate (glass plate 140B) provided on the −Z direction side of the radio wave reflecting layer 130.

[0157] The second protective layer may be a glass or resin substrate (glass plate 140B). When the second protective layer is a glass or resin substrate (glass plate 140B), the second protective layer can protect the plurality of conductive films 120A.

[0158] Furthermore, the sheet resistance of the plurality of conductive films 120A may be equal to or greater than 1 Ω / sq. and equal to or less than 30 Ω / sq. By setting the sheet resistance of the plurality of conductive films 120A to an appropriate value, it is possible to provide a radio wave absorber 100 having good radio wave absorption characteristics.

[0159] The thickness of the first protective layer (glass plate 140A) may be 2 mm or more and 6 mm or less. By setting the thickness of the first protective layer (glass plate 140A) to an appropriate value, it is possible to provide a radio wave absorber 100 with good radio wave absorption characteristics.

[0160] The sheet resistance of the radio wave reflecting layer 130 may be 3 Ω / sq. or less. By setting the sheet resistance of the radio wave reflecting layer 130 to an appropriate value, it is possible to provide a radio wave absorber 100 with good radio wave absorption characteristics.

[0161] The thickness of the first dielectric layer (adhesive layer 110A) may be equal to or less than 1 / (10√εr) of the wavelength at the frequency at which the maximum radio wave absorption rate is obtained, where εr is the relative dielectric constant of the first dielectric layer (adhesive layer 110A). By setting the thickness of the first dielectric layer (adhesive layer 110A) to an appropriate value, it is possible to provide a radio wave absorber 100 with good radio wave absorption characteristics.

[0162] Furthermore, the visible light transmittance may be 50% or more and the haze value may be 5% or less, thereby making it possible to provide a radio wave absorber 100 with good visibility.

[0163] The radio wave absorber 100 includes an adhesive layer 110A having a surface 111A and a surface 112A located on the radio wave incident side, a plurality of conductive films 120A located on the surface 111A and having a rectangular shape in a plan view, and a radio wave reflecting layer 130 located on the surface 112A, the plurality of conductive films 120A being arranged at intervals along a first axis (X-axis) direction and a second axis (Y-axis) direction of the surface 111A, and the length of each of the plurality of conductive films 120A in the first axis (X-axis) direction is set to a length that corresponds to the angle of incidence of the first polarized radio waves to the plurality of conductive films 120A in a plane including the first axis (X-axis) and a third axis (Z-axis) so that the absorption rate of the first polarized radio waves in a predetermined frequency band takes on a maximum value in terms of the relationship between the frequency of the first polarized radio waves whose electric field oscillates in the first axis (X-axis) direction and the absorption rate of the first polarized radio waves. Therefore, by setting the length X1 of the first side of each of the multiple conductive films 120A to a length corresponding to a predetermined incident angle, the radio wave absorber 100 can absorb TM waves at a predetermined frequency and a predetermined incident angle.

[0164] Therefore, it is possible to provide a radio wave absorber 100 that is highly durable and capable of absorbing TM waves at a predetermined frequency and a predetermined angle of incidence.

[0165] Furthermore, the length of the conductive film 120A in the first axis (X-axis) direction may be set to be longer as the incident angle increases. By setting the length of the conductive film 120A in the first axis (X-axis) direction to be longer as the incident angle increases, it is possible to provide a radio wave absorber 100 that can absorb TM waves of a predetermined frequency at various incident angles.

[0166] Although exemplary wave absorbers according to the present disclosure have been described above, the present disclosure is not limited to the specifically disclosed embodiments, and various modifications and changes are possible without departing from the scope of the claims.

[0167] The following supplementary notes are further disclosed with respect to the above embodiments. (Supplementary Note 1) A radio wave absorber comprising: a first dielectric layer having a first surface and a second surface located on a radio wave incident side and formed of a thermoplastic adhesive; a plurality of conductive films located on the first surface and having a rectangular shape in a plan view; and a radio wave reflecting layer located on the second surface. (Supplementary Note 2) The plurality of conductive films are arranged at intervals along a first axis direction and a second axis direction of the first surface, and the radio wave absorber according to Supplementary Note 1, wherein a frequency band in which the absorption rate of a first polarized radio wave incident on the plurality of conductive films at a predetermined incident angle is maximized overlaps with a frequency band in which the absorption rate of a second polarized radio wave incident on the plurality of conductive films at the predetermined incident angle is maximized. (Supplementary Note 3) The radio wave absorber according to Supplementary Note 2, wherein the plurality of conductive films are arranged at equal intervals in the first axis direction and at equal intervals in the second axis direction. (Supplementary Note 4) The radio wave absorber according to Supplementary Note 2 or 3, wherein the plurality of conductive films are equal in size. (Supplementary Note 5) The radio wave absorber according to Supplementary Note 4, wherein each of the plurality of conductive films has a first side extending in the first axis direction and a second side extending in the second axis direction, and a ratio of the length of the first side to the length of the second side of each of the plurality of conductive films is 1.00 or more and 2.00 or less. (Supplementary Note 6) The radio wave absorber according to any one of Supplementary Notes 3 to 5, wherein a difference between a spacing between the plurality of conductive films in the first axis direction and a spacing between the plurality of conductive films in the second axis direction is 30% or less of the spacing between the plurality of conductive films in the first axis direction. (Supplementary Note 7) The radio wave absorber according to any one of Supplementary Notes 3 to 5, wherein a spacing between the plurality of conductive films in the first axis direction and a spacing between the plurality of conductive films in the second axis direction are 30 μm or more and 500 μm or less. (Supplementary Note 8) The radio wave absorber according to Supplementary Note 5, wherein the first axis direction is a horizontal direction and the second axis direction is a vertical direction, the first polarized wave is a TM wave whose electric field oscillates in the first axis direction, and the second polarized wave is a TE wave whose electric field oscillates in the second axis direction.(Supplementary Note 9) The radio wave absorber according to any one of Supplementary Notes 2 to 8, wherein the first axis direction is a horizontal direction and the second axis direction is a vertical direction, the first polarization is a TM wave whose electric field oscillates in the first axis direction, and the second polarization is a TE wave whose electric field oscillates in the second axis direction, each of the plurality of conductive films has a first side extending in the first axis direction and a second side extending in the second axis direction, and the length of the first side of each of the plurality of conductive films is set to a length corresponding to an incident angle of the TM wave radio waves to the plurality of conductive films in a plane including the first axis and a third axis so that the absorption rate of the TM wave radio waves in a predetermined frequency band is maximized in a relationship between the frequency of the TM wave radio waves and the absorption rate of the TM wave radio waves. (Supplementary Note 10) The radio wave absorber according to any one of Supplementary Notes 2 to 9, wherein a maximum radio wave absorption rate in the overlapping band of a frequency band in which the absorption rate of a first polarized radio wave incident on the plurality of conductive films at a predetermined incident angle is maximum and a frequency band in which the absorption rate of a second polarized radio wave incident on the plurality of conductive films at the predetermined incident angle is maximum is 50% or more. (Supplementary Note 11) The radio wave absorber according to Supplementary Note 9, wherein the length of the first side is set to be longer as the incident angle is larger. (Supplementary Note 12) The radio wave absorber according to Supplementary Note 9, wherein the length of the second side of each of the plurality of conductive films is constant regardless of the incident angle in a horizontal plane. (Supplementary Note 13) The radio wave absorber according to any one of Supplementary Notes 2 to 12, wherein the radio wave reflective layer is formed of a transparent conductive film or a metal mesh. (Supplementary Note 14) The radio wave absorber according to any one of Supplementary Notes 1 to 13, further comprising a first protective layer located on the opposite side of the plurality of conductive films from the first dielectric layer. (Supplementary Note 15) The radio wave absorber according to any one of Supplementary Notes 1 to 14, further comprising a second protective layer located on the opposite side of the radio wave reflecting layer from the first dielectric layer. (Supplementary Note 16) The radio wave absorber according to Supplementary Note 14, wherein the first protective layer is a substrate made of glass or resin.(Supplementary Note 17) The radio wave absorber according to Supplementary Note 15, wherein the second protective layer comprises: a second dielectric layer located on the opposite side of the radio wave reflecting layer to the first dielectric layer and formed of a thermoplastic adhesive; and a glass or resin substrate located on the opposite side of the second dielectric layer to the radio wave reflecting layer. (Supplementary Note 18) The radio wave absorber according to Supplementary Note 15, wherein the second protective layer is a glass or resin substrate. (Supplementary Note 19) The radio wave absorber according to any one of Supplements 1 to 18, wherein the sheet resistance of the plurality of conductive films is 1 Ω / sq. or more and 30 Ω / sq. or less. (Supplementary Note 20) The radio wave absorber according to Supplementary Note 14, wherein the thickness of the first protective layer is 2 mm or more and 6 mm or less. (Supplementary Note 21) The radio wave absorber according to any one of Supplementary Notes 1 to 20, wherein the sheet resistance of the radio wave reflecting layer is 3 Ω / sq. or less. (Supplementary Note 22) The radio wave absorber according to any one of Supplementary Notes 1 to 21, wherein the thickness of the first dielectric layer is 1 / (10√εr) or less of the wavelength at a frequency at which a maximum radio wave absorption rate is obtained, where εr is the relative permittivity of the first dielectric layer. (Supplementary Note 23) The radio wave absorber according to any one of Supplementary Notes 1 to 22, wherein the visible light transmittance is 50% or more and the haze value is 5% or less. (Supplementary Note 24) The radio wave absorber according to Supplementary Note 2, wherein the length in the first axis direction of each of the plurality of conductive films is set to a length corresponding to an angle of incidence of the first polarized radio wave onto the plurality of conductive films in a plane including the first axis and a third axis, so that the absorption rate of the first polarized radio wave in a predetermined frequency band is maximized in terms of a relationship between the frequency of the first polarized radio wave and the frequency of the first polarized radio wave, the electric field of which oscillates in the first axis direction. (Supplementary Note 25) The radio wave absorber according to Supplementary Note 24, wherein the length in the first axis direction of the conductive film is set to be longer as the angle of incidence is larger.

[0168] The entire contents of the specification, claims, drawings and abstract of Japanese Patent Application No. 2023-121465, filed on July 26, 2023, are incorporated herein by reference as part of the disclosure of the present invention.

[0169] 100, 100A Radio wave absorber 110A Adhesive layer (an example of a first dielectric layer) 111A Surface (an example of a first surface) 112A Surface (an example of a second surface) 110B Adhesive layer (an example of a second dielectric layer) 111B Surface 112B Surface 120 FSS layer 120A Multiple conductive films 130 Radio wave reflecting layer 140A Glass plate (an example of a first protective layer) 141A, 142A Main surface 140B Glass plate (an example of a second protective layer) 141B, 142B Main surface

Claims

1. A radio wave absorber comprising: a first dielectric layer having a first surface located on the radio wave incident side and a second surface, the first dielectric layer being formed from a thermoplastic adhesive; a plurality of conductive films located on the first surface and having a rectangular shape in a plan view; and a radio wave reflecting layer located on the second surface.

2. The radio wave absorber according to claim 1, wherein the plurality of conductive films are arranged at intervals along the first axial direction and the second axial direction of the first surface, and a frequency band in which the absorption rate of a first polarized radio wave incident on the plurality of conductive films at a predetermined incident angle is maximized overlaps with a frequency band in which the absorption rate of a second polarized radio wave incident on the plurality of conductive films at the predetermined incident angle is maximized.

3. The radio wave absorber according to claim 2, wherein the plurality of conductive films are arranged at equal intervals in the first axis direction and at equal intervals in the second axis direction.

4. The radio wave absorber according to claim 2, wherein the plurality of conductive films are equal in size.

5. The radio wave absorber according to claim 4, wherein each of the plurality of conductive films has a first side extending in the first axis direction and a second side extending in the second axis direction, and the ratio of the length of the first side to the length of the second side of each of the plurality of conductive films is 1.00 or more and 2.00 or less.

6. The radio wave absorber according to claim 3, wherein the difference between the spacing between the plurality of conductive films in the first axial direction and the spacing between the plurality of conductive films in the second axial direction is 30% or less of the spacing in the first axial direction.

7. The radio wave absorber according to claim 3, wherein the spacing between the plurality of conductive films in the first axial direction and the spacing between the plurality of conductive films in the second axial direction are 30 μm or more and 500 μm or less.

8. A radio wave absorber as described in claim 5, wherein the first axis direction is a horizontal direction and the second axis direction is a vertical direction, the first polarized wave is a TM wave whose electric field oscillates in the first axis direction, and the second polarized wave is a TE wave whose electric field oscillates in the second axis direction.

9. A radio wave absorber as claimed in any one of claims 2 to 5, wherein the first axis direction is a horizontal direction and the second axis direction is a vertical direction, the first polarization is a TM wave whose electric field oscillates in the first axis direction and the second polarization is a TE wave whose electric field oscillates in the second axis direction, each of the plurality of conductive films has a first side extending in the first axis direction and a second side extending in the second axis direction, and the length of the first side of each of the plurality of conductive films is set to a length according to an incident angle of the TM wave radio waves to the plurality of conductive films in a plane including the first axis and a third axis so that the absorption rate of the TM wave radio waves in a predetermined frequency band is maximized in a relationship between the frequency of the TM wave radio waves and the absorption rate of the TM wave radio waves.

10. The radio wave absorber according to claim 2, wherein the maximum radio wave absorption rate in the overlapping band of a frequency band in which the absorption rate of a first polarized radio wave incident on said plurality of conductive films at a predetermined angle of incidence is maximum, and a frequency band in which the absorption rate of a second polarized radio wave incident on said plurality of conductive films at said predetermined angle of incidence is maximum, is 50% or more.

11. The radio wave absorber according to claim 9, wherein the length of said first side is set to be longer as said incident angle is larger.

12. The radio wave absorber according to claim 9, wherein the length of the second side of each of the plurality of conductive films is constant regardless of the incident angle in the horizontal plane.

13. The radio wave absorber according to claim 2, wherein the radio wave reflecting layer is formed of a transparent conductive film or a metal mesh.

14. The radio wave absorber according to claim 1, further comprising a first protective layer located on the opposite side of said plurality of conductive films from said first dielectric layer.

15. The radio wave absorber according to claim 1, further comprising a second protective layer located on the opposite side of said radio wave reflecting layer from said first dielectric layer.

16. The radio wave absorber according to claim 14, wherein the first protective layer is a substrate made of glass or resin.

17. The radio wave absorber according to claim 15, wherein the second protective layer comprises: a second dielectric layer located on the opposite side of the radio wave reflecting layer to the first dielectric layer and formed from a thermoplastic adhesive; and a glass or resin substrate located on the opposite side of the second dielectric layer to the radio wave reflecting layer.

18. The radio wave absorber according to claim 15, wherein the second protective layer is a substrate made of glass or resin.

19. The radio wave absorber according to claim 1, wherein the sheet resistance of said plurality of conductive films is 1 Ω / sq. or more and 30 Ω / sq. or less.

20. The radio wave absorber according to claim 14, wherein the thickness of the first protective layer is 2 mm or more and 6 mm or less.

21. The radio wave absorber according to claim 1, wherein the sheet resistance of said radio wave reflecting layer is 3 Ω / sq. or less.

22. The radio wave absorber according to claim 1, wherein the thickness of said first dielectric layer is equal to or less than 1 / (10√εr) of the wavelength at the frequency at which the maximum radio wave absorption rate is obtained, where εr is the relative dielectric constant of said first dielectric layer.

23. The radio wave absorber according to claim 1, having a visible light transmittance of 50% or more and a haze value of 5% or less.

24. A radio wave absorber as described in claim 2, wherein the length of each of the plurality of conductive films in the first axis direction is set to a length corresponding to the angle of incidence of the first polarized radio waves onto the plurality of conductive films in a plane including the first axis and the third axis, so that the absorption rate of the first polarized radio waves in a predetermined frequency band reaches a maximum value in terms of the relationship between the frequency of the first polarized radio waves whose electric field oscillates in the first axis direction and the absorption rate of the first polarized radio waves.

25. The radio wave absorber according to claim 24, wherein the length of said conductive film in said first axial direction is set to be longer as said incident angle is larger.

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