Radio wave absorber

Abstract

This radio wave absorber comprises a substrate (110) having a first surface (111) and a second surface (112), a resistance film (120) provided on the first surface (111) of the substrate (110), and a fixing part that fixes the substrate (110) to a glass plate (12) that also has a first surface (111) and a second surface (112) and that has a metal layer (13) provided to the first surface (111), the fixing being achieved so as to provide a face-to-face arrangement between the second main surface (112) of the glass plate (12) and the first surface (111) on which the resistance film (120) is provided. The resistance film (120) has an impedance such that the absorbed amount of radio waves of a predetermined frequency is equal to or greater than a predetermined value.

Description

Radio wave absorber

[0001] This disclosure relates to a radio wave absorber. This application claims priority based on Japanese Patent Application No. 2024-227616 filed in Japan on December 24, 2024, and incorporates its content herein by reference.

[0002] Conventionally, there is a radio wave absorber having a laminated structure, wherein the radio wave absorber has a visible light transmittance of 50% or more, the radio wave absorber absorbs radio waves in a specific frequency band, the radio wave absorber has a first main surface which is the surface on which the radio waves are incident and a second main surface which is the surface opposite to the first main surface, and from the first main surface toward the second main surface, it includes a first dielectric layer, a conductive layer, a second dielectric layer, a radio wave reflection layer, and a third dielectric layer in this order. The conductive layer consists of a plurality of conductors and gaps that dispose the plurality of conductors apart from each other, and the plurality of conductors are insulated from each other. There is a radio wave absorber (for example, see Patent Document 1).

[0003] International Publication No. 2022 / 044958 (A)

[0004] By the way, in the conventional radio wave absorber, the relationship between the distance between the conductive layer and the second dielectric layer and the thickness of the second dielectric layer has not been specified, and there is room for improvement.

[0005] Therefore, an object of the present disclosure is to provide a radio wave absorber with good radio wave absorption characteristics.

[0006] The radio wave absorber of an embodiment of the present disclosure includes a substrate having a first surface and a second surface, a resistive film provided on the first surface of the substrate, a glass plate having a first main surface and a second main surface with a metal layer provided on the first main surface, and a fixing portion that fixes the substrate to the glass plate such that the second main surface of the glass plate faces the first surface on which the resistive film is provided. The resistive film has an impedance such that the absorption amount of radio waves of a predetermined frequency becomes a predetermined value or more.

[0007] According to the present disclosure, a radio wave absorber with good radio wave absorption characteristics can be provided.

[0008] This is a cross-sectional view showing an example of the configuration of the radio wave absorber 100. This is a diagram showing an example of the configuration of the resistive film pattern layer 120. This is a diagram showing an example of the configuration of the resistive film pattern layer 120 in a modified example of the embodiment. This is a diagram showing an example of the configuration of the resistive film pattern layer 120 in a modified example of the embodiment. This is a diagram showing an example of the configuration of the resistive film pattern layer 120 in a modified example of the embodiment. This is a diagram showing an example of the configuration of the resistive film pattern layer 120 in a modified example of the embodiment. This is a diagram showing an example of the characteristics of the amount of absorption of radio waves with respect to the frequency of the radio waves. This is a diagram showing an example of the characteristics of the amount of absorption of radio waves with respect to the frequency of the radio waves in a comparative radio wave absorber. This is a diagram showing an example of the characteristics of the amount of absorption of radio waves with respect to the frequency of the radio waves in a comparative radio wave absorber. This is a characteristic diagram summarizing the results of Figures 3A, 3B, 4A, and 4B in relation to the thickness dw of the glass plate 12 and the amount of absorption of radio waves. This is a characteristic diagram summarizing the results of Figures 3A, 3B, 4A, and 4B in relation to the thickness dw of the glass plate 12 and the thickness da of the adhesive layer 130. This is a diagram showing an example of the results of calculating the amount of radio wave absorption in the radio wave absorber 100 using an electromagnetic field simulator. This is a diagram showing an example of the characteristics of the amount of radio wave absorption in the radio wave absorber 100 with respect to the frequency of the radio wave. This is a diagram showing an example of the characteristics of the amount of radio wave absorption with respect to the frequency of the radio wave. This is a diagram showing an example of the characteristics of the amount of radio wave absorption with respect to the frequency of the radio wave in a comparative radio wave absorber. This is a diagram showing an example of the characteristics of the amount of radio wave absorption with respect to the frequency of the radio wave in a comparative radio wave absorber. This is a characteristic diagram summarizing the results of Figures 7A, 7B, 8A, and 8B in relation to the thickness dw of the glass plate 12 and the amount of radio wave absorption. This is a characteristic diagram summarizing the results of Figures 7A, 7B, 8A, and 8B in relation to the thickness dw of the glass plate 12 and the thickness da of the adhesive layer 130. This figure shows an example of the characteristics of radio wave absorption with respect to radio wave frequency. This figure shows an example of the characteristics of radio wave absorption with respect to radio wave frequency in a comparative radio wave absorber. This figure shows an example of the characteristics of radio wave absorption with respect to radio wave frequency in a comparative radio wave absorber.This is a characteristic diagram summarizing the results in Figures 10A, 10B, 11A, and 11B in relation to the thickness dw of the glass plate 12 and the amount of radio wave absorption. This is a characteristic diagram summarizing the results in Figures 10A, 10B, 11A, and 11B in relation to the thickness dw of the glass plate 12 and the thickness da of the adhesive layer 130.

[0009] Embodiments to which the radio wave absorber of this disclosure is applied will be described below. In the following, the same reference numerals may be used for the same elements, and redundant explanations may be omitted.

[0010] The following describes the XYZ coordinate system. The directions parallel to the X-axis (X direction), the directions parallel to the Y-axis (Y direction), and the directions parallel to the Z-axis (Z direction) are mutually orthogonal. Plane view refers to viewing from the XY plane. In addition, in the following, the length, width, thickness, etc. of each part may be exaggerated to make the structure easier to understand. Furthermore, the terms parallel, right angle, orthogonal, horizontal, vertical, up and down, etc., should be used with a degree of deviation that does not impair the effect of the embodiment.

[0011] Furthermore, in the following explanation, "radio waves" refer to a type of electromagnetic wave, and generally, electromagnetic waves below 3 THz are called radio waves. Below, electromagnetic waves radiated from outdoor base stations or relay stations will be referred to as "radio waves," and when referring to electromagnetic waves in general, the term "electromagnetic wave" will be used. Also, below, when referring to "millimeter waves" or "millimeter wave band," it will include the quasi-millimeter wave band of 24 GHz to 30 GHz in addition to the frequency band of 30 GHz to 300 GHz.

[0012] The radio waves absorbed by the radio wave absorber of the embodiment are preferably in the millimeter wave band, such as fifth-generation mobile communication systems (5G), or in the frequency band of 1 GHz to 30 GHz, including Sub-6. Alternatively, 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). Furthermore, the radio waves absorbed by the radio wave absorber of the embodiment may be IEEE 802.11 (Wi-Fi®), IEEE 802.16 (WiMAX®), IEEE 802.20, UWB (Ultra-Wideband), Bluetooth®, or LPWA (Low Power Wide Area), etc. As the frequency of radio waves increases, propagation loss due to reflection and diffraction increases, making dead zones more likely to occur. Therefore, the radio wave absorber of the embodiment is more suitable for communications handling relatively high frequencies. In the following explanation, unless otherwise specified, we will use radio waves in the 2.4 GHz band and the 5.7 GHz band as examples.

[0013] Furthermore, the radio wave absorber of this embodiment can be used to absorb radio waves in situations where it is necessary to suppress radio wave leakage to the outside of the area where wireless communication is performed, such as in a spatial transmission type wireless power transfer system (WPT). Below, as an example, we will describe a configuration in which the radio wave absorber of this embodiment is attached to double-glazed windows of a building in order to suppress radio wave leakage to the outside of the building, where there is an area inside the building where WPT wireless communication is performed. However, the radio wave absorber of this embodiment can be attached to window glass other than double-glazed windows of a building, and as an example, it can be attached to single-pane glass in building windows, single-pane glass and laminated glass in vehicle windows.

[0014] <Embodiment> <Configuration of Window Glass 10> Figure 1 is a cross-sectional view showing an example of the configuration of the radio wave absorber 100. The radio wave absorber 100 is attached to a window glass 10 installed in the wall 1 of a building. Therefore, before describing the configuration of the radio wave absorber 100, the configuration of the window glass 10 will be described.

[0015] <Window Glass 10> The window glass 10 is a double-glazed glass having glass plates 11 and 12 and a Low-E film 13. The Low-E film 13 is an example of a metal layer and an example of a heat-reflective film. Glass plate 11 is provided on the -Z direction side (outdoor side), and glass plate 12 is provided on the +Z direction side (indoor side). As an example, the Low-E film 13 is formed on the main surface 12A of glass plate 12.

[0016] <Glass plates 11 and 12> Glass plates 11 and 12 are held in place by a window frame fixed to the building wall 1, with spacers and desiccant provided along their outer edges. A hollow layer exists between glass plates 11 and 12. The hollow layer is an air layer between glass plates 11 and 12. The air layer may be a vacuum layer, or it may be a layer of noble gas such as Ar or Kr.

[0017] Glass plates 11 and 12 are transparent, plate-shaped glass plates. "Transparent" means that the visible light transmittance is at least 40%, preferably 60%, more preferably 70%, and even more preferably 80%. The visible light transmittance is measured in accordance with the Japanese Industrial Standard JIS R 3106:1998 and can be calculated using the formula for a standard D65 light source.

[0018] Glass plates 11 and 12 can be made of commonly available glass, such as soda-lime glass, alkali-free glass, borosilicate glass, or quartz glass. Furthermore, glass plates 11 and 12 are not limited to glass plates and may be made of resin such as polycarbonate or acrylic.

[0019] Glass plate 11 has an outdoor main surface 11A and an indoor main surface 11B. Glass plate 12 has an outdoor main surface 12A and an indoor main surface 12B. Main surface 12A is an example of a first main surface, and the indoor main surface 12B is an example of a second main surface. The radio wave absorber 100 is provided on the main surface 12B.

[0020] In the following description, we will explain the state in which the radio wave absorber 100 is attached to a windowpane 10 installed in the wall 1 of a building. Therefore, viewing the windowpane 10 from the front corresponds to viewing the radio wave absorber 100 in plan view (XY plan view). In the following description, there will be cases where we use a plan view and cases where we use a front view of the windowpane 10.

[0021] <Low-E film 13> The Low-E film 13 is a film that reflects far-infrared rays and is provided to improve the heat insulation or heat shielding performance of the window glass 10. The Low-E film 13 is also called a low-emissivity film. The Low-E film 13 includes, for example, a heat-reflective layer. The heat-reflective layer is made of silver (Ag) or tin oxide (SnO ２ ) is included. The Low-E film 13 may include a plurality of layers, and may include a heat-reflective layer and a dielectric layer. The dielectric layer is composed of an oxide, nitride or oxynitride. The Low-E film 13 is formed on the main surface 12A of the glass plate 12 by chemical vapor deposition or sputtering, for example. Although not particularly limited, the Low-E film 13 (metal layer) may be directly provided on the main surface 12A (first main surface) of the glass plate 12. "Directly provided" means, for example, that the Low-E film 13 and the main surface 12A of the glass plate 12 are in contact without an adhesive layer or the like in between.

[0022] The radio wave absorber 100 utilizes the Low-E film 13 as a radio wave reflective layer. Therefore, the configuration of the Low-E film 13 as a radio wave reflective layer will be described later.

[0023] <Configuration of the radio wave absorber 100> The radio wave absorber 100 includes a substrate 110, a resistive film pattern layer 120, and an adhesive layer 130. The resistive film pattern layer 120 is an example of a resistive film.

[0024] This section describes a configuration in which the radio wave absorber 100 is attached to a window pane 10 by bonding a substrate 110, on which a resistive film pattern layer 120 is formed on its surface 111, to the main surface 12B of the glass plate 12 of the window pane 10 via an adhesive layer 130. In other words, this describes a configuration in which the radio wave absorber 100 is attached to an existing window pane 10.

[0025] However, the radio wave absorber 100 may be attached to the window glass 10 and then attached to the building wall 1 together with the window glass 10. In other words, when attaching the window glass 10 to the building wall 1, the window glass 10 with the radio wave absorber 100 attached may be attached to the building wall 1.

[0026] In Figure 1, the incident surface of radio waves on the radio wave absorber 100 is the surface 112 of the substrate 110 on the +Z direction side. The radio wave absorber 100 absorbs radio waves of a predetermined frequency (for example, the 2.4 GHz band) incident from the surface 112.

[0027] As an example, the XZ plane is a horizontal plane. In this embodiment, as an example, a configuration in which the radio wave absorber 100 is used with its surface 112 perpendicular to the horizontal plane will be described. The incident surface of the radio wave absorber 100 is parallel to the XY plane, and the radio waves arrive from the +Z direction. That is, with respect to the incident surface (surface 112) of the radio wave absorber 100, the radio waves arrive from the horizontal direction.

[0028] The radio wave absorber 100 preferably has a visible light transmittance of 50% or more. If the visible light transmittance is 50% or more, users can easily see the product or scenery 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 can be measured in accordance with the Japanese Industrial Standard JIS R 3106:1998 and calculated using the formula for a standard D65 light source.

[0029] Furthermore, it is preferable that the radio wave absorber 100 has a haze value of 5% or less. If the haze value is 5% or less, users can easily see the product or scenery through the radio wave absorber 100. The haze value is determined in accordance with the Japanese Industrial Standard JIS K7136:2000.

[0030] The substrate 110, the resistive film pattern layer 120, the adhesive layer 130, the glass plate 12, the Low-E film 13, the air layer, and the glass plate 11 are arranged in this order from the +Z direction to the -Z direction. The air layer is the air layer between the glass plate 11 and the Low-E film 13 of the double-glazed window glass 10. If the window glass 10 is laminated glass, an interlayer is provided to bond the glass plates 11 and 12 instead of the air layer.

[0031] The thickness of the substrate 110 on which the resistive film pattern layer 120 is provided (the total thickness of the substrate 110 and the resistive film pattern layer 120) is ds, the thickness of the adhesive layer 130 is da, and the thickness of the glass plate 12 is dw. The thickness da of the adhesive layer 130 corresponds to the distance between the resistive film pattern layer 120 and the main surface 12B of the glass plate 12.

[0032] Furthermore, the following description will explain a configuration in which the radio wave absorber 100 absorbs radio waves inside a building, but the reverse is also possible. That is, the radio wave absorber 100 may be configured to absorb radio waves from the outside by attaching a substrate 110, on which a resistive film pattern layer 120 is formed, to the main surface 11A of a glass plate 11 with an adhesive layer 130.

[0033] <Substrate 110> The substrate 110 is a dielectric substrate having a surface 111 on the -Z direction side and a surface 112 located on the opposite side from surface 111. Surface 112 is the surface on the incident side (+Z direction side) of the radio waves. Surface 111 is an example of a first surface, and surface 112 is an example of a second surface.

[0034] A resistive film pattern layer 120 is formed on the surface 111, and the glass plate 12 is bonded to the surface 111 on which the resistive film pattern layer 120 is formed via an adhesive layer 130. The radio wave absorber 100 is attached to the window glass 10 by bonding the surface 111 on which the resistive film pattern layer 120 is formed to the main surface 12B of the glass plate 12 via the adhesive layer 130.

[0035] The substrate 110 is made of any material that is transparent to radio waves emitted from a base station or a user's smartphone or other terminal, and that can support the resistive film pattern layer 120. Transparent to radio waves means, for example, that the transmission loss is 10 dB or less. The substrate 110 being transparent to radio waves means that the transmission loss of the substrate 110 is 10 dB or less, preferably 6 dB or less, more preferably 3 dB or less, and even more preferably 1 dB or less.

[0036] Furthermore, the substrate 110 may be transparent to visible light. "Transparent" to visible light means that the visible light transmittance is at least 50%, preferably 60%, more preferably 70%, and even more preferably 80%.

[0037] As an example, a glass plate is used for the substrate 110. In this case, the substrate 110 is, for example, a glass plate on which a Low-E film or ITO film is provided as a resistive film pattern layer 120. Alternatively, a resin substrate may be used for the substrate 110. As a resin material that satisfies the above conditions, acrylic resins such as polymethyl methacrylate, cycloolefin resins, polycarbonate resins, polyethylene terephthalate (PET), etc. can be used. The thickness of the substrate 110 is preferably 0.025 mm to 10 mm, more preferably 0.1 mm to 6 mm, and even more preferably 1 mm to 3 mm.

[0038] <Resistive Pattern Layer 120> Figure 2A shows an example of the configuration of the resistive pattern layer 120. The resistive pattern layer 120 has a plurality of resistive pattern elements 120A. As an example, Figure 2A shows four resistive pattern elements 120A. The resistive pattern elements 120A are transparent conductive films that are square in shape when viewed from above, and the shape and size of the four resistive pattern elements 120A are the same.

[0039] The four resistive film pattern elements 120A are each arranged within four unit regions divided by solid lines. Each resistive film pattern element 120A is located in the center of each unit region. The pitches of the four resistive film pattern elements 120A are equal in the X and Y directions. The resistive film pattern layer 120, which has multiple resistive film pattern elements 120A composed of a transparent conductive film that is square in shape when viewed from above, has a capacitive impedance.

[0040] The resistive film pattern element 120A is formed, for example, by laser cutting a transparent conductive film provided on the surface 111 of the substrate 110. The resistive film pattern layer 120 may have multiple resistive film pattern elements 120A, and is not limited to four.

[0041] The resistive film pattern element 120A is composed of a transparent conductive film, such as an ITO (indium tin oxide) film or a Low-E (low-emissivity) film, as an example. The term "transparent" in "transparent conductive film" means that it is transparent to visible light, and "transparent" to visible light means that the visible light transmittance is at least 40%, preferably 60%, more preferably 70%, and even more preferably 80%.

[0042] The sheet resistance of the resistive film pattern layer 120 is, for example, 1 Ω / sq. to 377 Ω / sq. The thickness of the resistive film pattern layer 120 is not particularly limited, but for example it is 50 nm to 300 nm.

[0043] When radio waves reach the resistive film pattern element 120A, free electrons in the resistive film pattern element 120A move in the direction opposite to the electric field direction of the radio waves, and a current flows through the resistive film pattern element 120A. Also, at this time, in the gap between the resistive film pattern elements 120A, energy is periodically accumulated and released by the generated electric field. As a result, a propagation delay occurs in the radio waves transmitted through the resistive film pattern element 120A. A delay time occurs from when the radio waves are incident on the resistive film pattern element 120A until they are re-radiated. That is, both the phase of the radio waves transmitted through the resistive film pattern layer 120 and the phase of the radio waves reflected by the resistive film pattern layer 120 change with respect to the phase of the radio waves incident on the resistive film pattern layer 120. The radio waves transmitted through the resistive film pattern layer 120 are reflected in the +Z direction by the Low-E film 13.

[0044] Here, when the relationship between the thickness da of the adhesive layer 130, the thickness dw of the glass plate 12, and the relative permittivity εr of the glass plate 12 is set to a predetermined relationship, radio waves of a predetermined frequency incident on the radio wave absorber 100 from the +Z direction side are absorbed by the resistive film pattern layer 120. The reason why the resistive film pattern layer 120 can absorb radio waves of a predetermined frequency in this way is that the input impedance of the resistive film pattern layer 120 as seen from the surface 112 matches the characteristic impedance in free space. Details of this will be described later.

[0045] Also, the plurality of resistive film pattern elements 120A may have a configuration as shown in FIGS. 2B to 2F. The plurality of resistive film pattern elements 120A shown in FIGS. 2B to 2F are formed, for example, by cutting a transparent conductive film provided on the surface 111 of the substrate 110 by laser processing. Also, FIGS. 2B to 2F show four resistive film pattern elements 120A each, but the number of resistive film pattern elements 120A included in the resistive film pattern layer 120 may be plural and is not limited to four.

[0046] <Figure 2B> The resistive film pattern element 120A shown in Figure 2B is a rectangular annular transparent conductive film in plan view, and the shapes and sizes of the plurality of resistive film pattern elements 120A are equal. The resistive film pattern layer 120 having the plurality of resistive film pattern elements 120A formed of a rectangular annular transparent conductive film in plan view has an impedance corresponding to a series circuit of a coil (L) and a capacitor (C).

[0047] <Figure 2C> The resistive film pattern element 120A shown in Figure 2C is a cross-shaped transparent conductive film in plan view, and the shapes and sizes of the plurality of resistive film pattern elements 120A are equal. The resistive film pattern layer 120 having the plurality of resistive film pattern elements 120A formed of a cross-shaped transparent conductive film in plan view has an impedance corresponding to a series circuit of a coil (L) and a capacitor (C).

[0048] <Figure 2D> The resistive film pattern element 120A shown in Figure 2D has a configuration in which the portion leaving the transparent conductive film and the portion removing the transparent conductive film by laser processing are reversed as compared with the resistive film pattern element 120A shown in Figure 2A. In each unit region, the resistive film pattern element 120A has a rectangular annular transparent conductive film with a rectangular central portion removed and left along the outer edge of the unit region. The rectangular annular transparent conductive film has a rectangular opening at the center. The four rectangular annular transparent conductive films of the four resistive film pattern elements 120A are connected to each other. The shapes and sizes of the four portions where the transparent conductive film is removed in a rectangular shape are equal.

[0049] Therefore, the resistive film pattern layer 120 shown in Figure 2D has a grid-like configuration in which four rectangular slots are arranged in the transparent conductive film. The resistive film pattern layer 120 having the plurality of grid-like resistive film pattern elements 120A having rectangular slots in plan view has an inductive impedance. Let the distance between adjacent rectangular openings in the X direction and the Y direction be GW.

[0050] <Figure 2E> The resistive film pattern element 120A shown in Figure 2E has a configuration in which the portion where the transparent conductive film is left and the portion where the transparent conductive film is removed by laser processing are reversed compared to the resistive film pattern element 120A shown in Figure 2B. In each unit region, the central part of the resistive film pattern element 120A is removed in a rectangular ring shape. The transparent conductive films of the four resistive film pattern elements 120A are connected to each other. The shape and size of the four portions where the transparent conductive film has been removed in a rectangular ring shape are the same.

[0051] Therefore, the resistive film pattern element 120A shown in Figure 2E is composed of rectangular annular slots obtained by removing the transparent conductive film in a rectangular annular shape. The resistive film pattern layer 120 having a plurality of resistive film pattern elements 120A composed of rectangular annular slots in a plan view has an impedance equivalent to a parallel circuit of a coil (L) and a capacitor (C).

[0052] <Figure 2F> The resistive film pattern element 120A shown in Figure 2F has a configuration in which the portion where the transparent conductive film is left and the portion where the transparent conductive film is removed by laser processing are reversed compared to the resistive film pattern element 120A shown in Figure 2C. In each unit region, the central part of the resistive film pattern element 120A is removed in a cross shape. The transparent conductive films of the four resistive film pattern elements 120A are connected to each other. The shape and size of the four portions where the transparent conductive film has been removed in a cross shape are the same.

[0053] Therefore, the resistive film pattern element 120A shown in Figure 2F is composed of cross-shaped slots obtained by removing the transparent conductive film in a cross shape. In a plan view, the resistive film pattern layer 120 having a plurality of resistive film pattern elements 120A composed of cross-shaped slots has an impedance equivalent to a parallel circuit of a coil (L) and a capacitor (C).

[0054] <Adhesive Layer 130> The adhesive layer 130 is a dielectric material that is transparent or translucent and interposed between the substrate 110 and the glass plate 12. The substrate 110 and the glass plate 12 are joined by the adhesive layer 130. Here, as an example, a resistive film pattern layer 120 is formed on the surface 111 of the substrate 110, so more specifically, the adhesive layer 130 joins the resistive film pattern layer 120 and the main surface 12B of the glass plate 12. Also, as an example, the resistive film pattern layer 120 is not formed in the part of the surface 111 of the substrate 110 that is close to the outer edge (outer edge portion), so at the outer edge portion of the substrate 110, the adhesive layer 130 joins the substrate 110 and the glass plate 12.

[0055] Alternatively, instead of joining the substrate 110 and the glass plate 12 with the adhesive layer 130, the substrate 110 and the glass plate 12 may be held in a facing position using a fixing part such as a frame that holds the substrate 110 and the glass plate 12. In this case, an air layer may be interposed between the substrate 110 and the glass plate 12.

[0056] The adhesive layer 130 is an adhesive layer formed from a thermoplastic adhesive. Adhesive layers formed from thermoplastic adhesives have high moisture resistance and high durability. The adhesive layer 130 is, for example, one of the following: polyvinyl butyral (PVB) resin, ethylene vinyl acetate (EVA) resin, cycloolefin polymer (COP) resin, and thermoplastic polyurethane (TPU) resin.

[0057] The water absorption rate of the adhesive layer 130 is, for example, 3% by mass or less. The water absorption rate of the adhesive layer 130 is measured in accordance with the Japanese Industrial Standard JIS K 7209:2000. If the water absorption rate of the adhesive layer 130 is 3% by mass or less, it is less likely to deteriorate even in high temperature and high humidity usage environments. PVB resin, EVA resin, COP resin, and TPU resin may all have a water absorption rate of 3% by mass or less. The water absorption rate of the adhesive layer 130 is preferably 1% by mass or less. Also, the water absorption rate of the adhesive layer 130 is 0.01% by mass or more.

[0058] <Low-E film 13 as radio wave reflective layer> The Low-E film 13 reflects radio waves that have passed through the resistive film pattern layer 120. The radio wave absorber 100 utilizes the Low-E film 13 as a radio wave reflective layer.

[0059] The Low-E film 13 is conductive. The sheet resistance of the Low-E film 13 is, for example, 3 Ω / sq. or less, preferably 1 Ω / sq. or less, and more preferably 0.1 Ω / sq. or less. The thickness of the Low-E film 13 is not particularly limited, but is, for example, 100 nm to 500 nm.

[0060] Radio waves that pass through the resistive pattern layer 120 are reflected by the Low-E film 13. The radio waves reflected by the resistive pattern elements 120A of the resistive pattern layer 120 and the radio waves reflected by the Low-E film 13 interfere with each other and, as a result of their combination, are absorbed by the radio wave absorber 100.

[0061] Alternatively, a transparent conductive film such as an ITO film or a metal mesh may be used instead of the Low-E film 13.

[0062] <Calculation Results (Part 1)> When the amount of radio waves absorbed at a predetermined frequency was calculated in the radio wave absorber 100 while setting the thickness da of the adhesive layer 130 and the thickness dw of the glass plate 12 to various values, it was found that there is a correlation between the amount of radio wave absorbed and the thickness da and thickness dw. The predetermined frequency is, for example, a frequency included in the 2.4 GHz band and the 5.7 GHz band in WPT.

[0063] Furthermore, the amount of radio wave absorption is a physical quantity expressed in dB, which is obtained by subtracting the signal level of the radio waves reflected in the +Z direction inside the radio wave absorber 100 and emitted from the substrate 110 in the +Z direction, and the signal level of the radio waves that pass through the radio wave absorber 100 and are emitted in the -Z direction, from the signal level of the radio waves incident on the substrate 110 from the +Z direction.

[0064] Figures 3A and 3B show examples of the characteristics of radio wave absorption with respect to radio wave frequency. Figures 3A and 3B show examples of calculation results obtained by changing the thickness da of the adhesive layer 130 and the thickness dw of the glass plate 12. Figure 3A shows an example of calculation results when a glass substrate is used as the substrate 110, and Figure 3B shows an example of calculation results when an acrylic resin substrate is used as the substrate 110.

[0065] The relative permittivity εr of the glass plate 12 was set to 6.9. This relative permittivity εr (6.9) corresponds to the relative permittivity of a glass plate used as window glass 10 in a typical building. In addition, the calculation was performed assuming the presence of an air layer instead of the adhesive layer 130.

[0066] The amount of radio wave absorption was calculated for samples of glass plates 12 with thicknesses dw from 3 mm to 12 mm, by varying the frequency from 2 GHz to 7 GHz, including band A (2.41–2.49 GHz) and band B (5.73–5.77 GHz). When setting the thickness dw of the glass plate 12 to each value from 3 mm to 12 mm, the thickness da of the adhesive layer 130 was also set to a value corresponding to bands A and B.

[0067] As a result, as shown in Figures 3A and 3B, the amount of radio wave absorption reached a maximum value in bands A and B. Furthermore, no significant difference was observed between the case where a glass substrate was used as the substrate 110 (Figure 3A) and the case where an acrylic resin substrate was used as the substrate 110 (Figure 3B).

[0068] Next, for comparison, the same calculation was performed on a radio wave absorber in which the position of the Low-E film 13 was changed to the main surface 11B of the glass plate 11. Figures 4A and 4B show examples of the characteristics of the amount of radio wave absorption with respect to the frequency of the radio waves in the comparative radio wave absorber. Figure 4A shows an example of the calculation results when a glass substrate is used as the substrate 110, and Figure 4B shows an example of the calculation results when an acrylic resin substrate is used as the substrate 110. The relative permittivity εr of the glass plate 12 was set to 6.9. In addition, the calculation was performed assuming the presence of an air layer instead of the adhesive layer 130.

[0069] The amount of radio wave absorption was calculated for samples of glass plate 12 with thicknesses dw from 3 mm to 12 mm, by varying the frequency from 2 GHz to 7 GHz, including band A (2.41–2.49 GHz) and band B (5.73–5.77 GHz). When setting the thickness dw of the glass plate 12 to each value from 3 mm to 12 mm, the thickness da of the adhesive layer 130 was also set to values ​​corresponding to bands A and B. The combination of thickness dw and thickness da was set to be the same as the combination of thickness dw and thickness da in the radio wave absorber 100 from which the results in Figures 3A and 3B were obtained.

[0070] As a result, as shown in Figures 4A and 4B, the amount of radio wave absorption reached a maximum value at some frequencies within band A. However, compared to Figures 3A and 3B, the number of samples in band B that yielded a maximum value was smaller, resulting in an overall lower absorption amount. No significant difference was observed between the case where a glass substrate was used as substrate 110 (Figure 4A) and the case where an acrylic resin substrate was used as substrate 110 (Figure 4B).

[0071] Figure 5A is a characteristic diagram summarizing the results from Figures 3A, 3B, 4A, and 4B in relation to the thickness dw of the glass plate 12 and the amount of radio wave absorption. In Figure 5A, the horizontal axis represents the thickness dw of the glass plate 12, and the vertical axis represents the amount of radio wave absorption (dB). Figure 5A shows the results of samples obtained within bands A and B from the results of Figures 3A, 3B, 4A, and 4B.

[0072] As shown in Figure 5A, it was confirmed that the amount of radio waves absorbed by the radio wave absorber 100 of the embodiment was significantly higher than that of the comparative radio wave absorber, regardless of whether the substrate 110 was a glass substrate or an acrylic resin substrate.

[0073] Figure 5B is a characteristic diagram summarizing the results from Figures 3A, 3B, 4A, and 4B in relation to the thickness dw of the glass plate 12 and the thickness da of the adhesive layer 130. In Figure 5B, the horizontal axis represents the thickness dw (mm) of the glass plate 12, and the vertical axis represents the thickness da (mm) of the adhesive layer 130. Figure 5B shows the results of samples obtained within bands A and B from the results of Figures 3A, 3B, 4A, and 4B.

[0074] As shown in Figure 5B, the results for the radio wave absorber 100 of the embodiment (Figures 3A and 3B), and the results for the comparative radio wave absorber (Figures 4A and 4B), were found to fall within a range defined by a linear equation that includes the thickness dw, the thickness da, and the relative permittivity εr of the glass plate 12 as variables.

[0075] More specifically, it was found that among the combinations of thickness dw and thickness da of the radio wave absorber 100 of the embodiment, the preferred range of thickness dw and thickness da that results in a radio wave absorption amount of 5 dB or more is within the range expressed by the following equations (1) and (2). n is an integer of 0 or more. Equation (1) represents the lower limit of the preferred range, and equation (2) represents the upper limit of the preferred range.

[0076] da ≥ -2.7dw - 2εr + 74n + 8.1 (1) and da ≤ -2.7dw - 2εr + 54(n+1) + 8.1 (2)

[0077] In other words, by using combinations of thickness dw and thickness da that fall within the ranges expressed by the following equations (1) and (2), the amount of radio wave absorption in the radio wave absorber 100 of the embodiment will be 5 dB or more.

[0078] Here, equation (A) shown in Figure 5B is the equation obtained by setting εr = 6.9 and n = 1 in equation (2). Equation (D) shown in Figure 5B is the equation obtained by setting εr = 6.9 and n = 1 in equation (1). Equation (E) shown in Figure 5B is the equation obtained by setting εr = 6.9 and n = 0 in equation (2).

[0079] Equation (A) represents the upper limit of the preferred range when n=1, equation (D) represents the lower limit of the preferred range when n=1, and equation (E) represents the upper limit of the preferred range when n=0.

[0080] Furthermore, it was found that among the combinations of thickness dw and thickness da of the radio wave absorber 100 of the embodiment, the combination of thickness dw and thickness da that results in a radio wave absorption amount of 10 dB or more is within the range expressed by the following equations (3) and (4). n is an integer of 0 or more. Equation (3) represents the lower limit of the preferred range, and equation (4) represents the upper limit of the preferred range.

[0081] (3) da ≥ -2.7dw - 2εr + 86n + 8.1 and da ≤ -2.7dw - 2εr + 52(n+1) + 8.1 (4)

[0082] In other words, by using combinations of thickness dw and thickness da that fall within the ranges expressed by the following equations (3) and (4), the amount of radio wave absorption in the radio wave absorber 100 of the embodiment will be 10 dB or more.

[0083] Here, equation (B) shown in Figure 5B is the equation obtained by setting εr = 6.9 and n = 1 in equation (4). Equation (C) shown in Figure 9B is the equation obtained by setting εr = 6.9 and n = 1 in equation (3). Equation (F) shown in Figure 9B is the equation obtained by setting εr = 6.9 and n = 0 in equation (4).

[0084] Equation (B) represents the upper limit of the more preferable range when n=1, equation (C) represents the lower limit of the more preferable range when n=1, and equation (F) represents the upper limit of the more preferable range when n=0.

[0085] By selecting combinations of thickness dw and thickness da as shown in equations (1) to (4), the resistive film pattern layer 120 can absorb radio waves of a predetermined frequency because the input impedance of the resistive film pattern layer 120 as seen from the surface 112 matches the characteristic impedance in free space.

[0086] When n is 2 or greater, the preferred range in which the amount of radio wave absorption by the radio wave absorber 100 of the embodiment is 5 dB or more, and the more preferred range in which the amount of radio wave absorption by the radio wave absorber 100 of the embodiment is 10 dB or more, exist periodically further upward in the characteristics of Figure 5B, but as an example, the range up to n=3 is practically usable.

[0087] <Simulation Results> Figure 6A shows an example of the results of calculating the amount of radio waves absorbed by the radio wave absorber 100 using an electromagnetic field simulator. The amount of radio waves absorbed by the radio wave absorber 100 was calculated when the resistive film pattern layer 120 has four rectangular resistive film pattern elements 120A as shown in Figure 2A.

[0088] In the simulation, the lengths of the resistive film pattern elements 120A in the X and Y directions were set to 24.9 mm, and the lengths of the unit region in the X and Y directions were set to 25 mm. The lengths of the unit region in the X and Y directions correspond to the pitch between adjacent resistive film pattern elements 120A in the X and Y directions. In addition, the thickness ds of the substrate 110 was set to 1.5 mm, the thickness da of the adhesive layer 130 was set to 10.0 mm, and the thickness dw of the glass plate 12 was set to 7.0 mm.

[0089] As shown in Figure 6A, it was confirmed that the amount of radio wave absorption in band A (2.41–2.49 GHz) was 23 dB or more, and the amount of radio wave absorption in band B (5.73–5.77 GHz) was 30 dB or more.

[0090] Figure 6B shows an example of the results of calculating the amount of radio wave absorption in the radio wave absorber 100 using an electromagnetic field simulator. The resistive film pattern layer 120 is grid-shaped as shown in Figure 2D and has resistive film pattern elements 120A composed of four rectangular annular transparent conductive films. The four rectangular annular resistive film pattern elements 120A are connected. The amount of radio wave absorption by the radio wave absorber 100 with such resistive film pattern elements 120A was calculated.

[0091] In the simulation, the length of the unit region in the X and Y directions was set to 9.0 mm, and the distance GW (see Figure 2D) between adjacent rectangular openings in the X and Y directions was set to 3.9 mm. In addition, the thickness ds of the substrate 110 was set to 1.0 mm, the thickness da of the adhesive layer 130 was set to 20.0 mm, and the thickness dw of the glass plate 12 was set to 5.0 mm.

[0092] As shown in Figure 6B, it was confirmed that the amount of radio wave absorption in band A (2.41–2.49 GHz) was 30 dB or more, and the amount of radio wave absorption in band B (5.73–5.77 GHz) was 25 dB or more.

[0093] <Calculation Results (Part 2)> In Calculation Results (Part 2), the same calculation as in Calculation Results (Part 1) was performed under conditions where the relative permittivity of the glass plate 12 was changed. All conditions other than the relative permittivity of the glass plate 12 were the same as in Calculation Results (Part 1).

[0094] Figures 7A and 7B show examples of the characteristics of radio wave absorption with respect to radio wave frequency. Figures 7A and 7B show examples of calculation results obtained by changing the thickness da of the adhesive layer 130 and the thickness dw of the glass plate 12. Figure 7A shows an example of calculation results when a glass substrate is used as the substrate 110, and Figure 7B shows an example of calculation results when an acrylic resin substrate is used as the substrate 110.

[0095] The relative permittivity εr of the glass plate 12 was set to 6.0. This relative permittivity εr (6.0) corresponds to the relative permittivity of glass plates commonly used as laminated glass for vehicles. Furthermore, the calculation was performed assuming the presence of an air layer instead of the adhesive layer 130.

[0096] The amount of radio wave absorption was calculated for samples of glass plates 12 with thicknesses dw from 3 mm to 12 mm, by varying the frequency from 2 GHz to 7 GHz, including band A (2.41–2.49 GHz) and band B (5.73–5.77 GHz). When setting the thickness dw of the glass plate 12 to each value from 3 mm to 12 mm, the thickness da was also set to a value that matched bands A and B.

[0097] As a result, as shown in Figures 7A and 7B, the amount of radio wave absorption reached a maximum value in bands A and B. Furthermore, no significant difference was observed between the case where a glass substrate was used as the substrate 110 (Figure 7A) and the case where an acrylic resin substrate was used as the substrate 110 (Figure 7B).

[0098] Next, for comparison, the same calculations were performed on a comparative radio wave absorber in which the position of the Low-E film 13 was changed to the main surface 11B of the glass plate 11. Figures 8A and 8B show examples of the characteristics of the amount of radio wave absorption with respect to the frequency of the radio waves in the comparative radio wave absorber. Figure 8A shows an example of the calculation results when a glass substrate is used as the substrate 110, and Figure 8B shows an example of the calculation results when an acrylic resin substrate is used as the substrate 110. The relative permittivity εr of the glass plate 12 was set to 6.0. In addition, the calculations were performed assuming the presence of an air layer instead of the adhesive layer 130.

[0099] The amount of radio wave absorption was calculated for samples of glass plate 12 with thicknesses dw from 3 mm to 12 mm, by varying the frequency from 2 GHz to 7 GHz, including band A (2.41–2.49 GHz) and band B (5.73–5.77 GHz). When setting the thickness dw of the glass plate 12 to each value from 3 mm to 12 mm, the thickness da was also set to a value corresponding to bands A and B. The combination of thickness dw and thickness da was set to be the same as the combination of thickness dw and thickness da in the radio wave absorber 100 for which the results in Figures 7A and 7B were obtained.

[0100] As a result, as shown in Figures 8A and 8B, the amount of radio wave absorption reached a maximum value at some frequencies within band A. However, compared to Figures 7A and 7B, the number of samples in band B that yielded a maximum value was smaller, resulting in an overall lower absorption amount. Furthermore, no maximum values ​​for radio wave absorption exceeding 5 dB were obtained in band B. No significant difference was observed between the case where a glass substrate was used as substrate 110 (Figure 8A) and the case where an acrylic resin substrate was used as substrate 110 (Figure 8B).

[0101] Figure 9A is a characteristic diagram summarizing the results from Figures 7A, 7B, 8A, and 8B in relation to the thickness dw of the glass plate 12 and the amount of radio wave absorption. In Figure 9A, the horizontal axis represents the thickness dw of the glass plate 12, and the vertical axis represents the amount of radio wave absorption (dB). Figure 9A shows the results of samples obtained within bands A and B from the results of Figures 7A, 7B, 8A, and 8B.

[0102] As shown in Figure 9A, it was confirmed that the amount of radio waves absorbed by the radio wave absorber 100 of the embodiment was significantly higher than that of the comparative radio wave absorber, regardless of whether the substrate 110 was a glass substrate or an acrylic resin substrate.

[0103] Figure 9B is a characteristic diagram summarizing the results from Figures 7A, 7B, 8A, and 8B in relation to the thickness dw of the glass plate 12 and the thickness da of the adhesive layer 130. In Figure 9B, the horizontal axis represents the thickness dw (mm) of the glass plate 12, and the vertical axis represents the thickness da (mm) of the adhesive layer 130. Figure 9B shows the results for samples obtained within bands A and B from the results of Figures 7A, 7B, 8A, and 8B.

[0104] As shown in Figure 9B, the results for the radio wave absorber 100 of the embodiment (Figures 7A and 7B), and the results for the comparative radio wave absorber (Figures 8A and 8B), were found to fall within a range defined by a linear equation that includes the thickness dw, the thickness da, and the relative permittivity εr of the glass plate 12 as variables.

[0105] More specifically, it was found that among the combinations of thickness dw and thickness da of the radio wave absorber 100 of the embodiment, the preferred range of thickness dw and thickness da that results in a radio wave absorption amount of 5 dB or more falls within the range expressed by equations (1) and (2) used in the calculation results (Part 1). Equations (1) and (2) are shown again here.

[0106] da ≥ -2.7dw - 2εr + 74n + 8.1 (1) and da ≤ -2.7dw - 2εr + 54(n+1) + 8.1 (2)

[0107] In the calculation results (Part 2), using combinations of thickness dw and thickness da that fall within the range expressed by the following equations (1) and (2), the amount of radio wave absorption in the radio wave absorber 100 of the embodiment was 5 dB or more.

[0108] Here, equation (A) shown in Figure 9B is the equation obtained by setting εr = 6.0 and n = 1 in equation (2). Equation (D) shown in Figure 9B is the equation obtained by setting εr = 6.0 and n = 1 in equation (1). Equation (E) shown in Figure 9B is the equation obtained by setting εr = 6.0 and n = 0 in equation (2).

[0109] Equation (A) represents the upper limit of the preferred range when n=1, equation (D) represents the lower limit of the preferred range when n=1, and equation (E) represents the upper limit of the preferred range when n=0.

[0110] Furthermore, it was found that among the combinations of thickness dw and thickness da of the radio wave absorber 100 in the embodiment, the more preferable range of thickness dw and thickness da combinations that result in a radio wave absorption amount of 10 dB or more falls within the range expressed by equations (3) and (4) used in the calculation results (Part 1). Equations (1) and (2) are shown again here.

[0111] (3) da ≥ -2.7dw - 2εr + 86n + 8.1 and da ≤ -2.7dw - 2εr + 52(n+1) + 8.1 (4)

[0112] In the calculation results (Part 2), using combinations of thickness dw and thickness da that fall within the range expressed by the following equations (3) and (4), the amount of radio wave absorption in the radio wave absorber 100 of the embodiment was 10 dB or more.

[0113] Here, equation (B) shown in Figure 9B is the equation obtained by setting εr = 6.0 and n = 1 in equation (4). Equation (C) shown in Figure 9B is the equation obtained by setting εr = 6.0 and n = 1 in equation (3). Equation (F) shown in Figure 9B is the equation obtained by setting εr = 6.0 and n = 0 in equation (4).

[0114] Equation (B) represents the upper limit of the more preferable range when n=1, equation (C) represents the lower limit of the more preferable range when n=1, and equation (F) represents the upper limit of the more preferable range when n=0.

[0115] By selecting combinations of thickness dw and thickness da as shown in equations (1) to (4), the resistive film pattern layer 120 can absorb radio waves of a predetermined frequency because the input impedance of the resistive film pattern layer 120 as seen from the surface 112 matches the characteristic impedance in free space.

[0116] When n is 2 or greater, the preferred range in which the amount of radio wave absorption by the radio wave absorber 100 of the embodiment is 5 dB or more, and the more preferred range in which the amount of radio wave absorption by the radio wave absorber 100 of the embodiment is 10 dB or more, exist periodically further upward in the characteristics of Figure 9B, but as an example, up to about n = 3 is a range that can be practically used.

[0117] <Calculation Results (Part 3)> In Calculation Results (Part 3), the same calculations as in Calculation Results (Part 1) and Calculation Results (Part 2) were performed under conditions where the relative permittivity of the glass plate 12 was changed. All conditions other than the relative permittivity of the glass plate 12 were the same as in Calculation Results (Part 1) and Calculation Results (Part 2).

[0118] Figures 10A and 10B show examples of the characteristics of radio wave absorption with respect to radio wave frequency. Figures 10A and 10B show examples of calculation results obtained by changing the thickness da of the adhesive layer 130 and the thickness dw of the glass plate 12. Figure 10A shows an example of calculation results when a glass substrate is used as the substrate 110, and Figure 10B shows an example of calculation results when an acrylic resin substrate is used as the substrate 110.

[0119] The relative permittivity εr of the glass plate 12 was set to 4.5. Furthermore, the calculation was performed assuming the presence of an air layer instead of the adhesive layer 130.

[0120] The amount of radio wave absorption was calculated for samples of glass plates 12 with thicknesses dw from 3 mm to 12 mm, by varying the frequency from 2 GHz to 7 GHz, including band A (2.41–2.49 GHz) and band B (5.73–5.77 GHz). When setting the thickness dw of the glass plate 12 to each value from 3 mm to 12 mm, the thickness da was also set to a value that matched bands A and B.

[0121] As a result, as shown in Figures 10A and 10B, the amount of radio wave absorption reached a maximum value in bands A and B. Furthermore, no significant difference was observed between the case where a glass substrate was used as the substrate 110 (Figure 10A) and the case where an acrylic resin substrate was used as the substrate 110 (Figure 10B).

[0122] Next, for comparison, the same calculations were performed on a comparative radio wave absorber in which the position of the Low-E film 13 was changed to the main surface 11B of the glass plate 11. Figures 11A and 11B show examples of the characteristics of the amount of radio wave absorption with respect to the frequency of the radio waves in the comparative radio wave absorber. Figure 11A shows an example of the calculation results when a glass substrate is used as the substrate 110, and Figure 11B shows an example of the calculation results when an acrylic resin substrate is used as the substrate 110. The relative permittivity εr of the glass plate 12 was set to 4.5. In addition, the calculations were performed assuming the presence of an air layer instead of the adhesive layer 130.

[0123] The amount of radio wave absorption was calculated for samples of glass plate 12 with thicknesses dw from 3 mm to 12 mm, by varying the frequency from 2 GHz to 7 GHz, including band A (2.41–2.49 GHz) and band B (5.73–5.77 GHz). When setting the thickness dw of the glass plate 12 to each value from 3 mm to 12 mm, the thickness da was also set to a value corresponding to bands A and B. The combination of thickness dw and thickness da was set to be the same as the combination of thickness dw and thickness da in the radio wave absorber 100 from which the results in Figures 10A and 10B were obtained.

[0124] As a result, as shown in Figures 11A and 11B, the amount of radio wave absorption reached a maximum value at some frequencies within band A. However, compared to Figures 10A and 10B, the number of samples in band A that yielded a maximum value was smaller, resulting in a lower overall absorption amount. Furthermore, in band B, no maximum values ​​for radio wave absorption exceeding 5 dB were obtained. No significant difference was observed between the case where a glass substrate was used as substrate 110 (Figure 11A) and the case where an acrylic resin substrate was used as substrate 110 (Figure 11B).

[0125] Figure 12A is a characteristic diagram summarizing the results from Figures 10A, 10B, 11A, and 11B in relation to the thickness dw of the glass plate 12 and the amount of radio wave absorption. In Figure 12A, the horizontal axis represents the thickness dw of the glass plate 12, and the vertical axis represents the amount of radio wave absorption (dB). Figure 12A shows the results of samples obtained within bands A and B from the results of Figures 10A, 10B, 11A, and 11B.

[0126] As shown in Figure 12A, it was confirmed that the amount of radio waves absorbed by the radio wave absorber 100 of the embodiment was significantly higher than that of the comparative radio wave absorber, regardless of whether the substrate 110 was a glass substrate or an acrylic resin substrate.

[0127] Figure 12B is a characteristic diagram summarizing the results from Figures 10A, 10B, 11A, and 11B in relation to the thickness dw of the glass plate 12 and the thickness da of the adhesive layer 130. In Figure 12B, the horizontal axis represents the thickness dw (mm) of the glass plate 12, and the vertical axis represents the thickness da (mm) of the adhesive layer 130. Figure 12B shows the results of samples obtained within bands A and B from the results of Figures 10A, 10B, 11A, and 11B.

[0128] As shown in Figure 12B, the results for the radio wave absorber 100 of the embodiment (Figures 10A and 10B), and the results for the comparative radio wave absorber (Figures 11A and 11B) were found to fall within a range defined by a linear equation that includes the thickness dw, the thickness da, and the relative permittivity εr of the glass plate 12 as variables.

[0129] More specifically, it was found that among the combinations of thickness dw and thickness da of the radio wave absorber 100 of the embodiment, the preferred range of thickness dw and thickness da that results in a radio wave absorption amount of 5 dB or more falls within the range expressed by equations (1) and (2) used in the calculation results (Part 1). Equations (1) and (2) are shown again here.

[0130] da ≥ -2.7dw - 2εr + 74n + 8.1 (1) and da ≤ -2.7dw - 2εr + 54(n+1) + 8.1 (2)

[0131] In the calculation results (Part 3), using combinations of thickness dw and thickness da that fall within the range expressed by the following equations (1) and (2), the amount of radio wave absorption in the radio wave absorber 100 of the embodiment was 5 dB or more.

[0132] Here, equation (A) shown in Figure 12B is the equation in equation (2) where εr = 4.5 and n = 1. Equation (D) shown in Figure 12B is the equation in equation (1) where εr = 4.5 and n = 1. Equation (E) shown in Figure 12B is the equation in equation (2) where εr = 4.5 and n = 0.

[0133] Equation (A) represents the upper limit of the preferred range when n=1, equation (D) represents the lower limit of the preferred range when n=1, and equation (E) represents the upper limit of the preferred range when n=0.

[0134] Furthermore, it was found that among the combinations of thickness dw and thickness da of the radio wave absorber 100 in the embodiment, the more preferable range of thickness dw and thickness da combinations that result in a radio wave absorption amount of 10 dB or more falls within the range expressed by equations (3) and (4) used in the calculation results (Part 1). Equations (1) and (2) are shown again here.

[0135] (3) da ≥ -2.7dw - 2εr + 86n + 8.1 and da ≤ -2.7dw - 2εr + 52(n+1) + 8.1 (4)

[0136] In the calculation results (Part 3), using combinations of thickness dw and thickness da that fall within the range expressed by the following equations (3) and (4), the amount of radio wave absorption in the radio wave absorber 100 of the embodiment was 10 dB or more.

[0137] Here, equation (B) shown in Figure 12B is the equation in equation (4) where εr = 4.5 and n = 1. Equation (C) shown in Figure 12B is the equation in equation (3) where εr = 4.5 and n = 1. Equation (F) shown in Figure 12B is the equation in equation (4) where εr = 4.5 and n = 0.

[0138] Equation (B) represents the upper limit of the more preferable range when n=1, equation (C) represents the lower limit of the more preferable range when n=1, and equation (F) represents the upper limit of the more preferable range when n=0.

[0139] By selecting combinations of thickness dw and thickness da as shown in equations (1) to (4), the resistive film pattern layer 120 can absorb radio waves of a predetermined frequency because the input impedance of the resistive film pattern layer 120 as seen from the surface 112 matches the characteristic impedance in free space.

[0140] When n is 2 or greater, the preferred range in which the amount of radio wave absorption by the radio wave absorber 100 of the embodiment is 5 dB or more, and the more preferred range in which the amount of radio wave absorption by the radio wave absorber 100 of the embodiment is 10 dB or more, exist periodically further upward in the characteristics of Figure 12B, but as an example, up to about n=3 is a range that can be practically used.

[0141] <Summary of Calculation Results (Parts 1-3)> In all of the calculation results (Parts 1-3), both when a glass substrate was used as the substrate 110 and when an acrylic resin substrate was used as the substrate 110, the radio wave absorber 100 of the embodiment achieved sufficient radio wave absorption in both band A (2.4 GHz band) and band B (5.7 GHz band).

[0142] On the other hand, the amount of radio waves absorbed by the comparative radio wave absorber was significantly lower than that of the radio wave absorber 100 in the embodiment. This is thought to be related to the distance between the resistive film pattern layer 120 and the Low-E film 13.

[0143] In the comparative radio wave absorber, the distance between the resistive film pattern layer 120 and the Low-E film 13 is greater than that between the glass plates 11 and 12, compared to the radio wave absorber 100 of the embodiment. Therefore, it is considered that the impedance matching between the resistive film pattern layer 120 and the Low-E film 13 was insufficient.

[0144] <Effects> The radio wave absorber 100 of this disclosure includes a substrate 110 having a surface 111 and a surface 112, a resistive film pattern layer 120 provided on the surface 111 of the substrate 110, and a fixing part for fixing the substrate 110 to the glass plate 12 such that the main surface 12B of the glass plate 12, which has a main surface 12A and a main surface 12B, on which a Low-E film 13 is provided, faces the surface 111 on which the resistive film pattern layer 120 is provided. The resistive film pattern layer 120 has an impedance such that the amount of radio wave absorption of a predetermined frequency is greater than or equal to a predetermined value. Therefore, it is possible to absorb radio waves of a predetermined frequency.

[0145] Therefore, a radio wave absorber 100 with good radio wave absorption characteristics can be provided.

[0146] Furthermore, the fixing portion may be an adhesive layer that bonds the surface 111 on which the resistive film pattern layer 120 is provided with the main surface 12B. This allows for secure fixing of the surface 111 on which the resistive film pattern layer 120 is provided with the main surface 12B, and provides a radio wave absorber 100 with good radio wave absorption characteristics.

[0147] Furthermore, the predetermined frequency is a frequency included in the 2.4 GHz band or the 5.7 GHz band, and the predetermined value may be 5 dB. This makes it possible to provide a radio wave absorber 100 with good absorption characteristics for radio waves in the 2.4 GHz band or the 5.7 GHz band.

[0148] Furthermore, if the distance between the resistive film pattern layer 120 and the main surface 12B is da, the thickness of the glass plate 12 is dw, and the relative permittivity of the glass plate 12 is εr, then the distance da may satisfy the following relationship: da ≥ -2.7dw - 2εr + 74n + 8.1 and da ≤ -2.7dw - 2εr + 54(n+1) + 8.1

[0149] By selecting a combination of distance da and thickness dw that satisfies the above relationship, a radio wave absorber 100 with better radio wave absorption characteristics can be provided.

[0150] Furthermore, the distance da may satisfy the following relationships: da ≥ -2.7dw - 2εr + 86n + 8.1 and da ≤ -2.7dw - 2εr + 52(n+1) + 8.1

[0151] By selecting a combination of distance da and thickness dw that satisfies the above relationship, a radio wave absorber 100 with even better radio wave absorption characteristics can be provided.

[0152] Furthermore, the resistive film pattern layer 120 may have a plurality of resistive film pattern (Frequency Selective Surface) elements 120A provided on the surface 111. By utilizing the resistive film pattern elements 120A, a radio wave absorber 100 with good radio wave absorption characteristics can be provided.

[0153] Furthermore, the sheet resistance of the resistive film pattern layer 120 may be between 1 Ω / sq. and 377 Ω / sq. By setting the sheet resistance of the resistive film pattern layer 120 to an appropriate value, a radio wave absorber 100 with good radio wave absorption characteristics can be provided.

[0154] Furthermore, the Low-E film 13 may be a radio wave reflective layer. By utilizing the reflection of radio waves by the Low-E film 13, a radio wave absorber 100 with good radio wave absorption characteristics can be provided.

[0155] Furthermore, the thickness of the substrate 110 may be between 0.025 mm and 10 mm. By setting the thickness of the substrate 110 to an appropriate value, a radio wave absorber 100 with good radio wave reflection characteristics can be provided.

[0156] Furthermore, the visible light transmittance may be 50% or more and the haze value may be 5% or less. This provides a radio wave absorber 100 with good visibility.

[0157] Another embodiment of the radio wave absorber 100 of the present disclosure includes a glass plate 12 having main surfaces 12A and 12B, a Low-E film 13 provided on the main surface 12A, a substrate 110 having surfaces 111 and 112, a resistive film pattern layer 120 provided on surface 111 of the substrate 110, and a fixing portion for fixing the substrate 110 to the glass plate 12 such that the surface 111 on which the resistive film pattern layer 120 is provided and the main surface 12B face each other, wherein the resistive film pattern layer 120 has an impedance such that the amount of radio wave absorption of a predetermined frequency is greater than or equal to a predetermined value. Therefore, it is possible to absorb radio waves of a predetermined frequency.

[0158] Therefore, a radio wave absorber 100 with good radio wave absorption characteristics can be provided.

[0159] While exemplary radio wave absorbers of this disclosure have been described above, this disclosure is not limited to the specifically disclosed embodiments, and various modifications and changes are possible without departing from the scope of the claims.

[0160] The following additional notes are disclosed with respect to the above embodiments. (Note 1) A radio wave absorber comprising: a substrate having a first surface and a second surface; a resistive film provided on the first surface of the substrate; and a fixing portion for fixing the substrate to the glass plate such that the second main surface of the glass plate having a first main surface and a second main surface, the first main surface of which a metal layer is provided, faces the first surface on which the resistive film is provided, wherein the resistive film has an impedance such that the amount of radio waves of a predetermined frequency is equal to or greater than a predetermined value. (Note 2) The radio wave absorber according to Note 1, wherein the fixing portion is an adhesive layer that bonds the first surface on which the resistive film is provided to the second main surface. (Note 3) The radio wave absorber according to Note 1 or 2, wherein the predetermined frequency is a frequency included in the 2.4 GHz band or the 5.7 GHz band, and the predetermined value is 5 dB. (Note 4) The radio wave absorber described in Note 3, wherein the distance between the resistive film and the second main surface is da, the thickness of the glass plate is dw, and the relative permittivity of the glass plate is εr, such that the distance da satisfies the following relationship.

[0161] da ≥ -2.7dw - 2εr + 74n + 8.1 and da ≤ -2.7dw - 2εr + 54(n+1) + 8.1 (Note 5) The distance da is the radio wave absorber described in Note 4 that satisfies the following relationship.

[0162] da ≥ -2.7dw - 2εr + 86n + 8.1 and da ≤ -2.7dw - 2εr + 52(n+1) + 8.1 (Note 6) The radio wave absorber according to any one of Notes 1 to 5, wherein the resistive film has a plurality of resistive film pattern elements provided on the first surface. (Note 7) The radio wave absorber according to any one of Notes 1 to 6, wherein the sheet resistance of the resistive film is 1 Ω / sq. to 377 Ω / sq. (Note 8) The radio wave absorber according to any one of Notes 1 to 7, wherein the metal layer is a radio wave reflecting layer. (Note 9) The radio wave absorber according to any one of Notes 1 to 8, wherein the thickness of the substrate is 0.025 mm to 10 mm. (Note 10) The radio wave absorber according to any one of Notes 1 to 9, wherein the visible light transmittance is 50% or more and the haze value is 5% or less. (Note 11) The radio wave absorber according to any one of Notes 1 to 10, wherein the metal layer is directly provided on the first main surface of the glass plate. (Note 12) The radio wave absorber according to any one of Notes 1 to 11, wherein the substrate is transparent to radio waves. (Note 13) The radio wave absorber according to any one of Notes 1 to 12, wherein the second surface of the substrate is the incident surface for the radio waves. (Note 14) The radio wave absorber according to any one of Notes 1 to 14, wherein the fixing portion has an air layer interposed between the substrate and the glass plate. (Note 15) A radio wave absorber comprising: a glass plate having a first main surface and a second main surface; a metal layer provided on the first main surface; a substrate having a first surface and a second surface; a resistive film provided on the first surface of the substrate; and a fixing part for fixing the substrate to the glass plate such that the first surface on which the resistive film is provided and the second main surface face each other, wherein the resistive film has an impedance such that the amount of radio waves absorbed at a predetermined frequency is greater than or equal to a predetermined value. (Note 16) The radio wave absorber according to Note 15, wherein the predetermined frequency is a frequency included in the 2.4 GHz band or the 5.7 GHz band, and the predetermined value is 5 dB. (Note 17) The radio wave absorber according to Note 15 or 16, wherein the distance between the resistive film and the second main surface is da, the thickness of the glass plate is dw, and the relative permittivity of the glass plate is εr, and the distance da satisfies the following relationship.da ≥ -2.7dw - 2εr + 74n + 8.1 and da ≤ -2.7dw - 2εr + 54(n+1) + 8.1.

[0163] According to the present invention, it is possible to provide a radio wave absorber with good radio wave absorption characteristics.

[0164] 10 Window glass 11 Glass plate 11A Main surface 11B Main surface 12 Glass plate 12A Main surface (Example of the first main surface) 12B Main surface (Example of the second main surface) 13 Low-E film (Example of a metal layer) 100 Radio wave absorber 110 Substrate 111 Surface (Example of the first surface) 112 Surface (Example of the second surface) 120 Resistive film pattern layer (Example of a resistive film) 120A Resistive film pattern element 130 Adhesive layer (Example of a fixing part)

Claims

1. A radio wave absorber comprising: a substrate having a first surface and a second surface; a resistive film provided on the first surface of the substrate; and a fixing portion for fixing the substrate to the glass plate such that the second main surface of the glass plate having a first main surface and a second main surface, the first main surface of which a metal layer is provided on the first main surface, faces the first surface on which the resistive film is provided, wherein the resistive film has an impedance such that the amount of radio wave absorption of a predetermined frequency is greater than or equal to a predetermined value.

2. The radio wave absorber according to claim 1, wherein the fixing portion is an adhesive layer that adheres the first surface on which the resistive film is provided to the second main surface.

3. The radio wave absorber according to claim 1 or 2, wherein the predetermined frequency is a frequency included in the 2.4 GHz band or the 5.7 GHz band, and the predetermined value is 5 dB.

4. The radio wave absorber according to claim 3, wherein the distance between the resistive film and the second main surface is da, the thickness of the glass plate is dw, and the relative permittivity of the glass plate is εr, such that the distance da satisfies the following relationship: da ≥ -2.7dw - 2εr + 74n + 8.1 and da ≤ -2.7dw - 2εr + 54(n+1) + 8.1 5. The radio wave absorber according to claim 4, wherein the distance da satisfies the following relationship: da ≥ -2.7dw - 2εr + 86n + 8.1 and da ≤ -2.7dw - 2εr + 52(n+1) + 8.1 6. The radio wave absorber according to claim 1, wherein the resistive film has a plurality of resistive film pattern elements provided on the first surface.

7. The radio wave absorber according to claim 1, wherein the sheet resistance of the resistive film is 1 Ω / sq. to 377 Ω / sq.

8. The radio wave absorber according to claim 1, wherein the metal layer is a radio wave reflecting layer.

9. The radio wave absorber according to claim 1, wherein the thickness of the substrate is 0.025 mm to 10 mm.

10. The radio wave absorber according to claim 1, wherein the visible light transmittance is 50% or more and the haze value is 5% or less.

11. The radio wave absorber according to claim 1, wherein the metal layer is provided directly on the first main surface of the glass plate.

12. The radio wave absorber according to claim 1, wherein the substrate is transparent to radio waves.

13. The radio wave absorber according to claim 1, wherein the second surface of the substrate is the incident surface for the radio waves.

14. The radio wave absorber according to claim 1, wherein the fixing portion has an air layer interposed between the substrate and the glass plate.

15. A radio wave absorber comprising: a glass plate having a first main surface and a second main surface; a metal layer provided on the first main surface; a substrate having a first surface and a second surface; a resistive film provided on the first surface of the substrate; and a fixing portion for fixing the substrate to the glass plate such that the first surface on which the resistive film is provided and the second main surface face each other, wherein the resistive film has an impedance such that the amount of radio wave absorption of a predetermined frequency is greater than or equal to a predetermined value.

16. The radio wave absorber according to claim 15, wherein the predetermined frequency is a frequency included in the 2.4 GHz band or the 5.7 GHz band, and the predetermined value is 5 dB.

17. The radio wave absorber according to claim 15, wherein the distance between the resistive film and the second main surface is da, the thickness of the glass plate is dw, and the relative permittivity of the glass plate is εr, and the distance da satisfies the following relationship. da ≥ -2.7dw - 2εr + 74n + 8.1 and da ≤ -2.7dw - 2εr + 54(n+1) + 8.1

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