Radio wave absorber

Abstract

The purpose of the present invention is to provide a radio wave absorber that is highly convenient.　Provided is a radio wave absorber having a laminate. The laminate has a first reflection layer, a first adhesion layer, a first dielectric layer, a resistance layer, a second dielectric layer, a second adhesion layer, and a second reflection layer. These layers are arranged in this order in the lamination direction of the laminate. The first reflection layer is adhered to the first dielectric layer via the first adhesion layer so as to be peelable from the first dielectric layer. The second reflection layer is adhered to the second dielectric layer via the second adhesion layer so as to be peelable from the second dielectric layer.

Description

Radio wave absorber

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

[0002] For example, radio wave absorbers that absorb radio waves are used to avoid the effects of leaked radio waves emitted to the outside from electric circuits, etc., and undesirably reflected radio waves. In recent years, research has been progressing on technologies that utilize centimeter waves with a frequency band of several gigahertz (GHz), millimeter waves with frequencies from 30 gigahertz to 100 gigahertz, and radio waves with frequencies of 100 GHz or higher as radio waves in high frequency bands that exceed the millimeter wave band, in mobile communications such as mobile phones, wireless LANs, and electronic toll collection systems (ETC).

[0003] Here, Patent Document 1 proposes a radio wave absorber that absorbs unwanted radio waves by suppressing their reflection. In the radio wave absorber of Patent Document 1, a resistive layer is provided on the surface of the dielectric layer on which the radio waves are incident, and a reflective layer that reflects the radio waves is provided on the back surface on the opposite side. This radio wave absorber is a radio wave interference type that absorbs radio waves by causing radio waves reflected by the resistive layer and radio waves reflected by the reflective layer to interfere with each other and cancel each other out.

[0004] Japanese Patent Application Laid-Open No. 2023-133310

[0005] Radio wave absorbers do not necessarily have high absorption characteristics over the entire range of a particular frequency, and it may be necessary to use different types depending on the frequency band. In such cases, it may be necessary to prepare different radio wave absorbers, which may reduce convenience.

[0006] An object of the present invention is to provide a radio wave absorber that is highly convenient.

[0007] According to the present invention, there is provided a radio wave absorber having a laminate, the laminate having a first reflective layer, a first adhesive layer, a first dielectric layer, a resistive layer, a second dielectric layer, a second adhesive layer, and a second reflective layer, which are arranged in this order in the stacking direction of the laminate, the first reflective layer being adhered to the first dielectric layer via the first adhesive layer so as to be peelable from the first dielectric layer, and the second reflective layer being adhered to the second dielectric layer via the second adhesive layer so as to be peelable from the second dielectric layer.

[0008] According to the present invention, both the first reflective layer and the second reflective layer can be peeled off, and when the layer structure of the dielectric layer or the resistive layer is devised to give different absorption characteristics to both sides, it is possible to selectively peel off the first reflective layer or peel off the second reflective layer depending on the situation, which is highly convenient.

[0009] FIG. 1A is a cross-sectional view schematically illustrating a laminate 10 of a radio wave absorber 100 according to an embodiment. FIG. 1B (a) illustrates an example of a planar structure of a first resistive layer, and FIG. 1B (b) illustrates an example of a planar structure of a first resistive layer different from that of FIG. 1A. FIG. 2A schematically illustrates a case in which the first reflective layer Rf1 of the radio wave absorber 100 illustrated in FIG. 1A is peeled off for use, and FIG. 2B schematically illustrates a case in which the second reflective layer Rf2 of the radio wave absorber 100 illustrated in FIG. 1A is peeled off for use. FIGS. 3A and 3B illustrate absorption characteristics when radio waves are incident from the first reflective layer side. FIG. 3A is a graph illustrating radio wave absorption characteristics when the resistance value of the first resistive layer is changed from 20 Ω / sq to 60 Ω / sq. FIG. 3B is a graph illustrating radio wave absorption characteristics when the resistance value of the first resistive layer is changed from 70 Ω / sq to 110 Ω / sq. Figures 4A and 4B show the absorption characteristics when radio waves are incident from the first reflective layer side. Figure 4A is a graph showing the absorption characteristics when the resistance value of the first resistive layer is changed from 120 Ω / sq to 160 Ω / sq. Figure 4B is a graph showing the absorption characteristics when the resistance value of the first resistive layer is changed from 170 Ω / sq to 200 Ω / sq. Figures 5A and 5B show the absorption characteristics when radio waves are incident from the first reflective layer side. Figure 5A is a graph showing the absorption characteristics when the resistance value of the second resistive layer is changed from 20 Ω / sq to 60 Ω / sq. Figure 5B is a graph showing the absorption characteristics when the resistance value of the second resistive layer is changed from 70 Ω / sq to 110 Ω / sq. Figures 6A and 6B show the absorption characteristics when radio waves are incident from the first reflective layer side. FIG. 6A is a graph showing the radio wave absorption characteristics when the resistance value of the second resistance layer is changed from 120 Ω / sq to 160 Ω / sq. FIG. 6B is a graph showing the radio wave absorption characteristics when the resistance value of the second resistance layer is changed from 170 Ω / sq to 200 Ω / sq. FIGs. 7A and 7B show the absorption characteristics when radio waves are incident from the second reflective layer side. FIG. 7A is a graph showing the radio wave absorption characteristics when the resistance value of the first resistance layer is changed from 20 Ω / sq to 60 Ω / sq. FIG. 7B is a graph showing the radio wave absorption characteristics when the resistance value of the first resistance layer is changed from 70 Ω / sq to 110 Ω / sq. FIGs. 8A and 8B show the absorption characteristics when radio waves are incident from the second reflective layer side.FIG. 8A is a graph showing the radio wave absorption characteristics when the resistance value of the first resistance layer is changed from 120 Ω / sq to 160 Ω / sq. FIG. 8B is a graph showing the radio wave absorption characteristics when the resistance value of the first resistance layer is changed from 170 Ω / sq to 200 Ω / sq. FIGS. 9A and 9B show the absorption characteristics when radio waves are incident from the second reflective layer side. FIG. 9A is a graph showing the radio wave absorption characteristics when the resistance value of the second resistance layer is changed from 20 Ω / sq to 60 Ω / sq. FIG. 9B is a graph showing the radio wave absorption characteristics when the resistance value of the second resistance layer is changed from 70 Ω / sq to 110 Ω / sq. FIGS. 10A and 10B show the absorption characteristics when radio waves are incident from the second reflective layer side. FIG. 10A is a graph showing the radio wave absorption characteristics when the resistance value of the second resistance layer is changed from 120 Ω / sq to 160 Ω / sq. Fig. 10B is a graph showing the radio wave absorption characteristics when the resistance value of the second resistive layer is changed from 170 Ω / sq to 200 Ω / sq. Fig. 11 shows the radio wave absorption characteristics when radio waves are incident from the first reflective layer Rf1 side, and shows the radio wave absorption characteristics based on the combinations of the resistance values ​​(Ω / sq) of the first resistive layer R1 and the second resistive layer R2. Fig. 12 shows the radio wave absorption characteristics when radio waves are incident from the second reflective layer Rf2 side, and shows the radio wave absorption characteristics based on the combinations of the resistance values ​​(Ω / sq) of the first resistive layer R1 and the second resistive layer R2.

[0010] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. Various features shown in the following embodiments can be combined with each other. Furthermore, each feature can be an invention independently.

[0011] 1. Description of the Configuration of the Embodiment The configuration of a radio wave absorber 100 according to the embodiment will be described with reference to Fig. 1A and Fig. 1B. Note that Fig. 1 is a diagram provided to facilitate understanding of the configuration of the radio wave absorber 100, and the sizes of the members shown in the diagram, particularly the thickness of each layer, are not necessarily expressed in accordance with reality.

[0012] The radio wave absorber 100 is a sheet-like member configured to absorb radio waves. The radio wave absorber 100 has a laminate 10. The radio wave absorber 100 may have a configuration including only the laminate 10, or the radio wave absorber 100 may have, in addition to the laminate 10, a layer member different from the laminate 10 (for example, a member for reinforcing the radio wave absorber 100). The laminate 10 is formed in a flexible sheet shape as a whole.

[0013] The thickness (mm) of the laminate 10 is specifically, for example, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, or 2.0 mm, and may be within a range between any two of the values ​​exemplified here.

[0014] The radio wave absorber 100 can be configured, for example, by a radio wave interference type (also called a λ / 4 type or a reflection type) radio wave absorption sheet. Based on the principle of radio wave absorption, a radio wave interference type absorption sheet can be configured such that the thickness d of a dielectric layer having a dielectric constant ε is d=λ / 4=πc / (2ω√ε) according to the wavelength λ (=1 / frequency) of the radio wave to be absorbed when propagating within the dielectric.

[0015] 1A, the laminate 10 has a first reflective layer Rf1, a first adhesive layer, a first dielectric layer D1, a first resistive layer R1, a third dielectric layer D3, a second resistive layer R2, a second dielectric layer D2, a second adhesive layer, and a second reflective layer Rf2, which are arranged in this order in the stacking direction of the laminate 10. As will be described later, the first resistive layer R1 has a configuration in which a so-called frequency selective surface is formed, and the frequency of radio waves that pass through it is a predetermined frequency.

[0016] Although the first adhesive layer and the second adhesive layer are not shown, the first adhesive layer is interposed between the first reflective layer Rf1 and the first dielectric layer D1, and the second adhesive layer is interposed between the second reflective layer Rf2 and the second dielectric layer D2. The first reflective layer Rf1 is adhered to the first dielectric layer D1 via the first adhesive layer so as to be peelable from the first dielectric layer D1, and the second reflective layer Rf2 is adhered to the second dielectric layer D2 via the second adhesive layer so as to be peelable from the second dielectric layer D2.

[0017] The radio wave absorber 100 according to the embodiment is configured so that, when the layer configurations of the dielectric layers and the resistive layers are devised to have different absorption characteristics on both sides, it can be used by peeling off the first reflective layer Rf1 or by peeling off the second reflective layer Rf2, depending on the situation. The configurations of the dielectric layers and the resistive layers can be selected as appropriate, but in the embodiment, as described above, a configuration is adopted in which the first dielectric layer D1, the first resistive layer R1, the third dielectric layer D3, the second resistive layer R2, and the second dielectric layer D2 are sequentially stacked. Here, the radio wave absorption characteristics of the radio wave absorber 100 when the first reflective layer Rf1 is peeled off from the first dielectric layer D1 so that the first dielectric layer D1 becomes the radio wave incident side are defined as first characteristics, and the radio wave absorption characteristics of the radio wave absorber 100 when the second reflective layer Rf2 is peeled off from the second dielectric layer D2 so that the second dielectric layer D2 becomes the radio wave incident side are defined as second characteristics. In the embodiment, as described above, a configuration is adopted in which the first dielectric layer D1, the first resistive layer R1, the third dielectric layer D3, the second resistive layer R2, and the second dielectric layer D2 are sequentially stacked, but the first characteristic and the second characteristic are different. Note that the first characteristic and the second characteristic are characteristics that represent the relationship between the frequency of radio waves and the radio wave absorption rate (radio wave return loss).

[0018] [When the first reflective layer Rf1 is removed for use (when the first characteristic is exhibited)] As shown in Figure 2A, of the radio waves incident on the first dielectric layer D1, which is the surface dielectric layer, those of a predetermined frequency pass through the first resistive layer R1, while the rest are reflected by the first resistive layer R1. The radio waves reflected by the first resistive layer R1 are referred to as the first radio waves. A portion of the radio waves that pass through the first resistive layer R1 are reflected by the second resistive layer R2. The radio waves reflected by the second resistive layer R2 are referred to as the second radio waves. The radio waves that pass through the first resistive layer R1 pass through the second resistive layer R2 and are reflected by the second reflective layer Rf2. The radio waves reflected by the second reflective layer Rf2 are referred to as the third radio waves.

[0019] In the radio wave absorber 100 according to the embodiment, the incident radio waves interfere with the first radio wave Rw1, the second radio wave Rw2, and the third radio wave Rw3 described above, and the radio waves are absorbed by the radio wave absorber 100. In other words, since the phases of the radio waves are different, the radio waves are attenuated and appear to be absorbed by the radio wave absorber 100.

[0020] The first characteristic of the radio wave absorber 100 is a characteristic in which the frequency range in which the radio wave absorption rate is 80% or more is 60% or more in the frequency range of 50 (GHz) to 300 (GHz). For example, if the radio wave absorption rate of the radio wave absorber 100 is 60% or more in the range of 50 GHz to 280 GHz, this first characteristic is satisfied. Preferably, the first characteristic is a characteristic in which the range in which the radio wave absorption rate is 80% or more is 65% or more, 70% or more, 75% or more, 80% or more, 85% or more, 90% or more, 95% or more, or 100% in the frequency range of 50 (GHz) to 300 (GHz). Preferably, the first characteristic is a characteristic in which the range in which the radio wave absorption rate is 90% or more is 65% or more, 70% or more, 75% or more, 80% or more, 85% or more, 90% or more, 95% or more, or 100% in the frequency range of 50 (GHz) to 300 (GHz).

[0021] The first characteristic of the radio wave absorber 100 is that it has an absorption peak in the frequency range of 50 (GHz) to 300 (GHz). The number of absorption peaks is specifically, for example, 1, 2, 3, 4, or 5, and may be within a range between any two of the numbers exemplified here. Preferably, the absorption peak is a peak with a radio wave absorption rate of 90% or more.

[0022] The above-mentioned radio wave absorption rate corresponds to the ratio of the amount of radio waves absorbed to the amount of radio waves incident on the radio wave absorber 100. The amount of radio waves can be expressed, for example, by converting the radio waves into power. Furthermore, an 80% radio wave absorption rate is approximately 7 dB when converted into decibels (dB), and a 90% radio wave absorption rate is 10 dB.

[0023] [When the second reflective layer Rf2 is removed (when the second characteristic is exhibited)] As shown in Figure 2B, a portion of the radio waves incident on the second dielectric layer D2, which is the surface dielectric layer, is reflected by the second resistive layer R2. The radio waves reflected by the second resistive layer R2 are referred to as the fourth radio waves. Of the radio waves that pass through the second resistive layer R2, those with a predetermined frequency pass through the first resistive layer R1, and the rest are reflected by the first resistive layer R1. The radio waves reflected by the first resistive layer R1 are referred to as the fifth radio waves. The radio waves that pass through the first resistive layer R1 are reflected by the first reflective layer Rf1. The radio waves reflected by the first reflective layer Rf1 are referred to as the sixth radio waves.

[0024] In the radio wave absorber 100 according to the embodiment, the incident radio waves interfere with the fourth radio wave Rw4, the fifth radio wave Rw5, and the sixth radio wave Rw6 described above, and the radio waves are absorbed by the radio wave absorber 100. In other words, since the phases of the radio waves are different, the radio waves are attenuated and appear to be absorbed by the radio wave absorber 100.

[0025] The second characteristic of the radio wave absorber 100 is such that the frequency range in which the radio wave absorption rate is 80% or more is 50% or more in the frequency range of 50 (GHz) to 300 (GHz). Preferably, the second characteristic is such that the range in which the radio wave absorption rate is 80% or more is 55% or more, 60% or more, or 65% or more in the frequency range of 50 (GHz) to 300 (GHz). Preferably, the second characteristic is such that the range in which the radio wave absorption rate is 90% or more is 55% or more, 60% or more, or 65% or more in the frequency range of 50 (GHz) to 300 (GHz).

[0026] The second characteristic of the radio wave absorber 100 is a characteristic in which the radio wave absorption rate has an absorption peak in a frequency range of 50 (GHz) to 300 (GHz). The number of absorption peaks is specifically, for example, 1, 2, 3, 4, or 5, and may be within a range between any two of the numbers exemplified here. It is preferable that this absorption peak is a peak with a radio wave absorption rate of 90% or more. It is also preferable that the frequency of the absorption peak of the first characteristic is different from the frequency of the absorption peak of the second characteristic.

[0027] In the embodiment, the radio wave absorber 100 is described assuming utilization of the above-mentioned frequencies (GHz frequency band), but is not limited thereto, and the radio wave absorber 100 having the layer structure of the embodiment can also be applied to absorption of radio waves in the MHz band or below.

[0028] 1-1. Dielectric Layers Various dielectric materials can be used for the dielectric layers (first dielectric layer D1, second dielectric layer D2, and third dielectric layer D3). The dielectric layers can be configured to include a polymer material. Examples of the polymer material include synthetic resins (including thermoplastic elastomers) such as polyvinyl chloride, polyvinylidene fluoride, acrylic resin, ethylene-vinyl acetate copolymer, polyurethane, acrylic urethane resin, ionomer, polyolefin, polypropylene, polyethylene, silicone resin, polyester, polystyrene, polyimide, polyamide, polysulfone, polyethersulfone, and epoxy resin, or synthetic rubbers such as polyisoprene rubber, polystyrene-butadiene rubber, polybutadiene rubber, chloroprene rubber, acrylonitrile butadiene rubber, butyl rubber, acrylic rubber, ethylene-propylene rubber, and silicone rubber. These materials can be used alone or in combination to form the polymer material. The dielectric layers (first dielectric layer D1, second dielectric layer D2, and third dielectric layer D3) may be made of materials such as glass, titanium oxide, alumina, and barium titanate.

[0029] The first dielectric layer D1, the second dielectric layer D2, and the third dielectric layer D3 may be made of the same dielectric material or different dielectric materials. Each dielectric layer (the first dielectric layer D1, the second dielectric layer D2, and the third dielectric layer D3) may be made by stacking multiple dielectric layers. The thicknesses of the dielectric layers (the first dielectric layer D1, the second dielectric layer D2, and the third dielectric layer D3) may be the same or different. The first dielectric layer D1 and the second dielectric layer D2 may have adhesive layers (first adhesive layer and second adhesive layer) formed on their surfaces facing the radio wave. In other words, the first dielectric layer D1 and the second dielectric layer D2 may include adhesive layers, and the dielectric layers and adhesive layers do not necessarily have to be considered as separate components.

[0030] The relative dielectric constants of the dielectric layers (first dielectric layer D1, second dielectric layer D2, and third dielectric layer D3) can be set appropriately, and specifically, for example, are 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0, 9.5, and 10, and may be within a range between any two of the numerical values ​​exemplified here. The relative dielectric constants of the dielectric layers (first dielectric layer D1, second dielectric layer D2, and third dielectric layer D3) may be the same or different.

[0031] The thickness (μm) of the dielectric layers (first dielectric layer D1, second dielectric layer D2, and third dielectric layer D3) can be, for example, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 800, 850, 900, or 950, and may be within a range between any two of the numerical values ​​exemplified here.

[0032] The thickness of the first dielectric layer D1 is preferably greater than the thickness of the second dielectric layer D2. Specifically, the thickness of the first dielectric layer D1 is, for example, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, or 6.0 times the thickness of the second dielectric layer D2, or may be within a range between any two of the values ​​exemplified here. The thickness of the first dielectric layer D1 is also preferably greater than the thickness of the third dielectric layer D3. Specifically, the thickness of the first dielectric layer D1 is, for example, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, or 6.0 times the thickness of the third dielectric layer D3, and may be within a range between any two of the numerical values ​​exemplified here.

[0033] 1-2. Resistive Layers The resistive layers (first resistive layer R1 and second resistive layer R2) have functions related to the reflection and passage of radio waves. The first resistive layer R1 also has a function related to the selection of the frequency of the radio waves that pass through. The radio wave absorber 100 has at least two resistive layers in this way, which makes radio wave interference more complex and enables it to absorb radio waves in a wide frequency band (for example, 50 GHz to 300 GHz). Furthermore, the radio wave absorber 100 has a configuration in which the radio wave transmission path differs depending on whether it is used with the first reflective layer Rf1 peeled off or the second reflective layer Rf2 peeled off, and therefore the first characteristic and the second characteristic can be made different.

[0034] 1B, the first resistive layer R1 has a periodic structure R1S, which is formed to have periodicity within the plane of the first resistive layer R1 so as to pass a predetermined range of frequencies. In other words, the first resistive layer R1 has a structure (a patch-type resonator structure) having a frequency-selective surface.

[0035] Here, the predetermined range of frequencies that the first resistance layer R1 passes through specifically includes, for example, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, 250, 260, 270, 280, 290, 300, 310, 320, 330, 340, 350, 400, and 500, and may also include a range between any two of the numerical values ​​exemplified here. Note that the predetermined range of frequencies that the first resistance layer R1 passes through may be divided into multiple frequency ranges. For example, when specified using the numerical values ​​listed above, there are frequencies of 10 GHz or more and 40 GHz or less, and frequencies of 80 GHz or more and 120 GHz or less.

[0036] The shape of the periodic structure R1S is not particularly limited. For example, as shown in FIG. 1B(a), the periodic structure R1S can be composed of a plurality of resistor element portions R1a arranged to have a periodicity within the surface of the first resistor layer R1. Note that in the example of FIG. 1B(a), the resistor element portions R1a are shown as having a circular ring shape, but are not limited thereto and may have other shapes, such as a rectangular shape.

[0037] 1B(a), when the periodic structure R1S is composed of a plurality of resistor element portions R1a arranged to have a periodicity within the plane of the first resistor layer R1, the size of the resistor element portion R1a in plan view is preferably such that the vertical width W1 and horizontal width W2 are 0.4 mm or more and 1.0 mm or less. That is, the vertical width W1 (mm) and horizontal width W2 (mm) are specifically, for example, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, or 1.0 mm, and may be within a range between any two of the values ​​exemplified here. The vertical width W1 (mm) and horizontal width W2 (mm) may be the same or different.

[0038] 1B(a), when the first resistance layer R1 is composed of a plurality of resistance element portions R1a arranged periodically within the plane of the first resistance layer R1, the distance between a pair of adjacent resistance element portions R1a is preferably 0.15 mm or more and 0.45 mm or less. That is, the distance d1 (mm) and the distance d2 (mm) are, for example, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, or 0.45, and may be within a range between any two of the values ​​exemplified here. The distances d1 (mm) and d2 (mm) may be the same or different.

[0039] 1B(b), the periodic structure R1S may be configured as a network structure (mesh structure) having periodicity and extending over the entire area of ​​the first resistance layer R1. In this way, the shape of the periodic structure R1S is not particularly limited.

[0040] Furthermore, the periodicity of the periodic structure R1S may exist in one direction (for example, the vertical or horizontal direction in the figure) within the plane of the first resistance layer R1, or may exist in two directions (for example, the vertical and horizontal directions in the figure).

[0041] The resistance value (Ω / sq) of the first resistance layer R1 is, for example, 20, 30, 40, 50, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, or 200, or may be within a range between any two of the values ​​exemplified here. For example, the resistance value (Ω / sq) of the first resistance layer R1 is, for example, 20 or more and 200 or less. The thickness (μm) of the first resistance layer R1 is specifically, for example, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 25, 30, 35, or 40, and may be within a range between any two of the numerical values ​​exemplified here.

[0042] The entire formation area of ​​the first resistive layer R1 should be inside the outer edge of the second resistive layer R2 when the radio wave absorber 100 is viewed from a direction parallel to the thickness direction of the radio wave absorber 100. In other words, the entire first resistive layer R1 should overlap with a part of the second resistive layer R2 when the radio wave absorber 100 is viewed from a direction parallel to the thickness direction of the radio wave absorber 100.

[0043] 1-2-2. Structure of the Second Resistive Layer R2 The second resistive layer R2 is disposed between the first dielectric layer D1 and the second dielectric layer D2, and functions to reflect a portion of the radio waves that pass through the first resistive layer R1 and allow the remainder to pass. The second resistive layer R2 does not have the function of selecting the frequency of the radio waves to be passed, and in this embodiment, the second resistive layer R2 is a layer formed as a so-called solid layer. In other words, the second resistive layer R2 has a sheet-like structure in which the entire area inside the outer edge of the second resistive layer R2 is filled with the constituent material of the second resistive layer R2.

[0044] The resistance value (Ω / sq) of the second resistive layer R2 is, for example, 20, 30, 40, 50, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, or 200, or may be within a range between any two of the values ​​exemplified here. For example, the resistance value (Ω / sq) of the second resistive layer R2 is, for example, 20 or more and 200 or less. The thickness (μm) of the second resistive layer R2 is specifically, for example, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 25, 30, 35, or 40, and may be within a range between any two of the values ​​exemplified here.

[0045] 1-2-3. Materials for Constituting the First Resistive Layer R1 and the Second Resistive Layer R2 The materials for the resistive layers (first resistive layer R1 and second resistive layer R2) may be, for example, conductive organic polymer films, sputtered films, vapor-deposited films, etc. Furthermore, the resistance value of the above-mentioned conductive organic polymer films, sputtered films, and vapor-deposited films can be controlled by the film thickness and formation density, making it easy to form resistive layers with desired resistance values. The materials for the first resistive layer R1 and the second resistive layer R2 may be the same or different.

[0046] The conductive organic polymer used as the resistive layers (first resistive layer R1 and second resistive layer R2) is a conjugated conductive organic polymer, and it is preferable to use polythiophene or its derivatives, or polypyrrole or its derivatives.

[0047] Furthermore, as the resistive layers (first resistive layer R1 and second resistive layer R2), an organic polymer whose main chain is composed of a π-conjugated system can be used, such as a polyacetylene-based conductive polymer, a polyphenylene-based conductive polymer, a polyphenylene vinylene-based conductive polymer, a polyaniline-based conductive polymer, a polyacene-based conductive polymer, a polythiophene vinylene-based conductive polymer, or a copolymer thereof.

[0048] The conductive organic polymer used in the resistance layers (first resistance layer R1 and second resistance layer R2) can use a polyanion as a counter anion. There are no particular limitations on the polyanion, but it is possible to use a conjugated conductive organic polymer containing an anionic group that can generate chemical oxidation doping. Examples of such an anionic group include those represented by the general formula -O-SO ３ X, -O-PO (OX) ２ , -COOX, -SO ３ X (in each formula, X represents a hydrogen atom or an alkali metal atom), and among them, —SO ３ X and -O-SO ３ A group represented by X can be employed.

[0049] The conductive organic polymers may be used alone or in combination of two or more. Among the materials exemplified above, a polymer consisting of one or two selected from polypyrrole, poly(3-methoxythiophene), poly(3,4-ethylenedioxythiophene), poly(2-anilinesulfonic acid), and poly(3-anilinesulfonic acid) is preferred because it has higher transparency and conductivity.

[0050] In particular, it is preferable to use poly(3,4-ethylenedioxythiophene: PEDOT) and polystyrene sulfonic acid (PSS) as a combination of a conjugated conductive organic polymer and a polyanion.

[0051] In addition, in the resistive layers (first resistive layer R1 and second resistive layer R2) of the radio wave absorber 100 according to this embodiment, a dopant can be used in combination to control the electrical conductivity of the conductive organic polymer and obtain a predetermined resistance value. Examples of the dopant include halogens such as iodine and chlorine, and BF ３　 , P.F. ５　 Lewis acids such as these, protonic acids such as nitric acid and sulfuric acid, transition metals, alkali metals, amino acids, nucleic acids, surfactants, dyes, chloranil, tetracyanoethylene, TCNQ, and the like can be used.

[0052] The content of the conductive organic polymer in the resistive layers (the first resistive layer R1 and the second resistive layer R2) is preferably 10% by mass or more and 35% by mass or less, based on the total mass of the solid content contained in the resistive layer composition. If the content is less than 10% by mass, the conductivity of the resistive layer tends to decrease. Therefore, if the surface electrical resistance value of the resistive layer is set within a predetermined range to achieve impedance matching, the film thickness of the resistive layer increases, which tends to thicken the entire radio wave absorber 100 (laminate 10) or, if the resistive layer is translucent, to deteriorate its optical properties. On the other hand, if the content exceeds 35% by mass, the structure of the conductive organic polymer reduces the applicability of the resistive layer when coating it, making it difficult to form a good resistive layer. If the resistive layer is translucent, the haze of the resistive layer increases, which also tends to deteriorate its optical properties.

[0053] Furthermore, the resistive layers (first resistive layer R1 and second resistive layer R2) may be configured to include a carbon material such as carbon microcoils, carbon nanotubes, or graphene.

[0054] Carbon microcoils are a type of vapor-grown carbon fiber obtained primarily by catalytically activated pyrolysis of acetylene, and are materials with a 3D helical / spiral structure with coil diameters on the order of microns. The coil diameter is preferably 1 μm to 10 μm, the carbon fiber forming the coil has a diameter of 0.1 μm to 1 μm, and the coil length is preferably 1 mm to 10 mm.

[0055] Specifically, the carbon nanotubes can be obtained by various methods, such as vapor phase growth methods including arc discharge, laser evaporation, and pyrolysis. The carbon nanotubes used as the resistive layers (first resistive layer R1 and second resistive layer R2) of the radio wave absorber 100 may be either single-walled or multi-walled.

[0056] Graphene can be obtained by, for example, a peeling and transfer method, a SiC pyrolysis method, a chemical vapor deposition method, a method of cutting carbon nanotubes, etc. As graphene to be used as the resistive layer of the wave absorber 100, flake-shaped powder graphene can be used from the viewpoint of easily obtaining a desired aspect ratio and of orientation in the wave absorber 100. As the resin in which the above-mentioned carbon material is dispersed, a water-soluble polyester resin can be used.

[0057] The resistance layers (first resistance layer R1 and second resistance layer R2) can be formed by applying a coating composition serving as a resistance layer-forming paint to a resin substrate and drying it. Examples of methods for applying the resistance layer-forming paint to the substrate include bar coating, reverse coating, gravure coating, microgravure coating, die coating, dipping, spin coating, slit coating, and spray coating. Drying after application is preferably performed at 100 to 150°C for 5 to 60 minutes under conditions that evaporate the solvent components of the resistance layer-forming paint. If necessary, the resistance layer may be formed by irradiating the coating film with UV light (ultraviolet rays) or EB (electron beam) to cure the coating film. While the substrate used to form the resistance layer is not particularly limited, a transparent substrate having transparency is preferred. Examples of materials that can be used for such transparent substrates include various resins such as PET, rubber, glass, and ceramics.

[0058] 1-3. Reflective Layer The reflective layers (first reflective layer Rf1 and second reflective layer Rf2) are configured to reflect radio waves that have passed through the resistive layers (first resistive layer R1 and second resistive layer R2). In other words, the reflective layers are layers that reflect radio waves that have passed through all of the dielectric layers and resistive layers.

[0059] Unlike resistive layers, the reflective layers (first reflective layer Rf1 and second reflective layer Rf2) do not need to pass radio waves, and therefore preferably have as low a resistance value as possible. Metal foil or metal plate can be used as the reflective layer. To ensure flexibility in the radio wave absorber 100, metal foil is more preferable as the material for the reflective layer, and various metal foils such as silver foil, copper foil, aluminum foil, and gold foil can be used. Considering cost and the effects of oxidation in air, aluminum foil can be used as the reflective layer. Metal foils such as aluminum foil for forming the reflective layer can be easily produced by rolling a metal material. Furthermore, when forming the reflective layer using a vapor-deposited film in which a metal is vapor-deposited on the surface of a non-metallic material, it is preferable to appropriately select a vapor deposition method conventionally used to form various vapor-deposited films, taking into account the heat resistance temperature of the metal material to be vapor-deposited and the non-metallic material, such as a resin, that serves as the base material.

[0060] The thickness (μm) of the reflective layer (each of the first reflective layer Rf1 and the second reflective layer Rf2) can be, for example, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 30, 40, 50, 60, 70, 80, 90, 100, 150, 200, 250, 300, 350, 400, or 500, or may be within a range between any two of the numerical values ​​exemplified here. The thicknesses of the first reflective layer Rf1 and the second reflective layer Rf2 may be different.

[0061] 1-4. Adhesive Layers For the adhesive layers (first adhesive layer and second adhesive layer), for example, known materials used as adhesive layers of adhesive tapes, etc., such as acrylic adhesives, rubber adhesives, and silicone adhesives, can be used. Tackifiers and crosslinking agents can be used to adjust adhesive strength and reduce adhesive residue. The adhesive strength can be, for example, 5 N / 10 mm to 12 N / 10 mm. Specific examples of the thickness (μm) of the adhesive layers (first adhesive layer and second adhesive layer) include: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 110, 120, 130, 140, 150, 16 0, 170, 180, 190, 200, 210, 220, 230, 240, 250, 260, 270, 280, 290, 300, 310, 320, 330, 340, 350, 360, 370, 380, 390, 400, 410, 420, 430, 450, and may be within a range between any two of the values ​​exemplified here.

[0062] 2. Description of Effects of the Embodiments In the radio wave absorber 100 according to the embodiment, both the first reflective layer Rf1 and the second reflective layer Rf2 are peelable. When the layer configurations of the dielectric layer and the resistive layer are devised to provide different absorption characteristics on both sides, the radio wave absorber 100 can be used by peeling off the first reflective layer Rf1 or the second reflective layer Rf2, depending on the situation, providing excellent convenience. In the embodiment, the first resistive layer R1 and the second resistive layer R2 have different configurations, and the transmission path of the radio wave is different when the radio wave is incident from the first dielectric layer D1 and when the radio wave is incident from the second dielectric layer D2. In other words, when the radio wave is incident from the first dielectric layer D1, the radio wave first reaches the first resistive layer R1, whereas when the radio wave is incident from the second dielectric layer D2, the radio wave first reaches the second resistive layer R2. As a result, the radio wave absorption characteristics of the radio wave absorber 100 differ depending on the surface onto which the radio wave is incident. Furthermore, the radio wave absorber 100 according to the embodiment can be used by peeling off both reflective layers (the first reflective layer Rf1 and the second reflective layer Rf2). For example, when the radio wave absorber 100 is attached to a metal layer that functions as a reflective layer, both reflective layers can be peeled off to expose both adhesive layers, and one of the adhesive layers can be attached to the metal layer to which the radio wave absorber 100 is attached.

[0063] When the radio wave absorber 100 according to the embodiment is used with the first reflective layer Rf1 peeled away, the first radio wave Rw1, the second radio wave Rw2, and the third radio wave Rw3 act to absorb radio waves in a wide frequency band in the radio wave absorber 100 (first characteristic). Similarly, when the radio wave absorber 100 according to the embodiment is used with the second reflective layer Rf2 peeled away, the fourth radio wave Rw4, the fifth radio wave Rw5, and the sixth radio wave Rw6 act to absorb radio waves in a wide frequency band in the radio wave absorber 100 (second characteristic). In other words, radio waves that have entered the radio wave absorber 100 according to the embodiment pass through, are reflected by, and are selected for passing frequencies by the multiple resistive layers, and the radio wave absorption in the first dielectric layer D1, the radio wave absorption in the second dielectric layer D2, and the radio wave absorption in the third dielectric layer D3 are combined, thereby achieving high absorption characteristics (return attenuation) over a wide frequency band. In particular, the embodiment is effective in absorbing radio waves in the high frequency band (50 GHz to 300 GHz).

[0064] The radio wave absorber 100 according to the embodiment has the first resistive layer R1 having the periodic structure R1S formed thereon, and therefore has effectively improved impedance matching (like a tapered structure), and has good absorption characteristics in a wide frequency band such as 50 GHz to 300 GHz.

[0065] Various embodiments are exemplified below. The embodiments shown below can be combined with each other. [Supplementary Note 1] A radio wave absorber having a laminate, wherein the laminate has a first reflective layer, a first adhesive layer, a first dielectric layer, a resistive layer, a second dielectric layer, a second adhesive layer, and a second reflective layer, which are arranged in this order in a stacking direction of the laminate, wherein the first reflective layer is adhered to the first dielectric layer via the first adhesive layer so as to be peelable from the first dielectric layer, and the second reflective layer is adhered to the second dielectric layer via the second adhesive layer so as to be peelable from the second dielectric layer. [Supplementary Note 2] The radio wave absorber according to Supplementary Note 1, wherein when the radio wave absorption characteristic of the radio wave absorber when the first reflective layer is peeled off from the first dielectric layer and the first dielectric layer becomes the radio wave incident side is defined as a first characteristic, and the radio wave absorption characteristic of the radio wave absorber when the second reflective layer is peeled off from the second dielectric layer and the second dielectric layer becomes the radio wave incident side is defined as a second characteristic, the first characteristic and the second characteristic are characteristics that represent the relationship between the frequency of radio waves and the absorption rate, and the first characteristic and the second characteristic are different. [Supplementary Note 3] The radio wave absorber according to Supplementary Note 2, wherein the first characteristic and the second characteristic are characteristics that the radio wave absorption rate has an absorption peak in a frequency range of 50 (GHz) to 300 (GHz). [Supplementary Note 4] The radio wave absorber according to Supplementary Note 3, wherein the absorption peak is a peak where the absorption rate is 90% or more. [Supplementary Note 5] The radio wave absorber according to Supplementary Note 3 or Supplementary Note 4, wherein the frequency of the absorption peak of the first characteristic is different from the frequency of the absorption peak of the second characteristic. [Supplementary Note 6] The radio wave absorber according to any one of Supplementary Notes 1 to 5, wherein the laminate further has a third dielectric layer, the resistive layer has a first resistive layer and a second resistive layer, and the laminate has the first reflective layer, the first adhesive layer, the first dielectric layer, the first resistive layer, the third dielectric layer, the second resistive layer, the second dielectric layer, the second adhesive layer, and the second reflective layer arranged in this order in the stacking direction.[Supplementary Note 7] The radio wave absorber according to Supplementary Note 6, wherein the first resistive layer has a periodic structure, and the periodic structure is formed to have periodicity within a plane of the first resistive layer so that radio waves of a frequency in a predetermined range pass through. [Supplementary Note 8] The radio wave absorber according to any one of Supplementary Notes 1 to 7, wherein the laminate is formed in a flexible sheet shape as a whole.

[0066] 3. Examples The inventors confirmed the radio wave absorption characteristics of the characteristic structure of the laminate 10 according to the embodiment. The graphs shown in FIGS. 3A to 10B show the results of the absorption characteristics of an object having the configuration of the radio wave absorber 100 described in the embodiment. The vertical axis corresponds to the radio wave absorption rate (return loss) in decibels (dB). The smaller the value on the vertical axis, the better the absorption rate (return loss). The horizontal axis corresponds to the radio wave frequency in GHz.

[0067] The conditions are as follows. The vertical direction here is described with the first reflective layer Rf1 side as the top and the second reflective layer Rf2 side as the bottom. The first dielectric layer D1 is an acrylic OCA layer (relative dielectric constant: 3, dielectric loss 0.05, the same applies below) with a thickness of 425 μm. OCA is an optically transparent adhesive sheet and stands for Optical Clear Adhesive. The first resistive layer R1 has a lower base material (thickness 100 μm) that is a PET layer (relative dielectric constant 3.2, dielectric loss 0.01, the same applies below), and an upper layer that is a resistive element portion, the resistance value of which is 20 to 200 Ω / sq. The second dielectric layer D2 is an acrylic OCA layer with a thickness of 100 μm. The second resistive layer R2 has a lower base material (50 μm thick) made of PET and an upper resistive layer with a resistance value of 20 to 200 Ω / sq. The third dielectric layer D3 is an acrylic OCA layer with a thickness of 150 μm. The first reflective layer Rf1 and the second reflective layer Rf2 are made of aluminum foil with a sheet resistance value of 10 Ω / sq.

[0068] As shown in Figures 3A to 6B, the first characteristic has an absorption peak in the frequency range of 50 GHz to 300 GHz. This absorption peak is a peak where the radio wave absorption rate is 90% or more (-10 dB or less in the graph). Furthermore, as shown in Figures 3A to 6B, the first characteristic has a frequency range where the radio wave absorption rate is 80% or more (approximately -7 dB or less in the graph) that is 90% or more in the frequency range of 50 GHz to 300 GHz, demonstrating favorable absorption characteristics over a wide frequency range.

[0069] In most of the graphs shown in Figures 7A to 10B, the second characteristic has an absorption peak in the frequency range of 50 (GHz) to 300 (GHz). This absorption peak is a peak where the radio wave absorption rate is 90% or more (-10 dB or less in the graphs). Furthermore, in most of the graphs shown in Figures 7A to 10B, the second characteristic has a frequency range in which the radio wave absorption rate is 80% or more (approximately -7 dB or less in the graphs) that is 50% or more in the frequency range of 50 (GHz) to 300 (GHz), demonstrating favorable absorption characteristics over a wide frequency range. It can be seen that the absorption peak of the first characteristic and the absorption peak of the second characteristic are different, demonstrating that the two characteristics are different.

[0070] Fig. 11 shows the radio wave absorption characteristics when radio waves are incident from the first reflective layer Rf1 side, and shows the radio wave absorption characteristics based on the combinations of the resistance values ​​(Ω / sq) of the first resistive layer R1 and the second resistive layer R2. Fig. 12 shows the radio wave absorption characteristics when radio waves are incident from the second reflective layer Rf2 side, and shows the radio wave absorption characteristics based on the combinations of the resistance values ​​(Ω / sq) of the first resistive layer R1 and the second resistive layer R2. Note that the results shown in Figs. 11 and 12 simply expand the verification range of the first resistive layer R1 and the second resistive layer R2, and the conditions other than the resistance values ​​(e.g., the thickness of the dielectric layer) are the same as those for the results shown in Figs. 3A to 11B.

[0071] 11 and 12 show not only the results shown in the above-mentioned Figures 3A to 10B but also other results. The above-mentioned Figures 3A to 10B show results in which the first resistance layer R1 is fixed at 70 (Ω / sq) and the second resistance layer R2 is changed from 20 to 200 (Ω / sq) in increments of 10 (Ω / sq), and results in which the second resistance layer R2 is fixed at 150 (Ω / sq) and the first resistance layer R1 is changed from 20 to 200 (Ω / sq) in increments of 10 (Ω / sq). 11 and 12, in addition to this, the results are shown for the first resistance layer R1 in the range of 20 to 200 (Ω / sq) and for the second resistance layer R2 in the range of 20 to 200 (Ω / sq) in increments of 20 (Ω / sq), and the resistance setting values ​​of the first resistance layer R1 and the second resistance layer R2 were varied over a wider range to verify the characteristics of the radio wave absorber 100. Although graphs are omitted, it was confirmed that the radio wave absorption characteristics on both sides were different.

[0072] Here, N0 to N4 shown in Figures 11 and 12 are as follows: N0 indicates that the frequency range in which the radio wave absorption rate (return loss) is 80% or more (approximately -7 dB or less in the graph) is 90% or more in the frequency range of 50 (GHz) to 300 (GHz). N1 indicates that the frequency range in which the radio wave absorption rate (return loss) is 80% or more (approximately -7 dB or less in the graph) is 60% or more and less than 90% in the frequency range of 50 (GHz) to 300 (GHz). N2 indicates that the frequency range in which the radio wave absorption rate (return loss) is 80% or more (approximately -7 dB or less in the graph) is 40% or more and less than 60% in the frequency range of 50 (GHz) to 300 (GHz). N3 indicates that the frequency range in which the radio wave absorption rate (return loss) is 80% or more (approximately -7 dB or less in the graph) is 20% or more and less than 40% in the frequency range of 50 (GHz) to 300 (GHz). N4 indicates that the frequency range in which the radio wave absorption rate (return loss) is 80% or more (approximately -7 dB or less in the graph) is less than 20% in the frequency range of 50 (GHz) to 300 (GHz).

[0073] In terms of the wide range of frequencies in which absorption is favorable, the order of N0, N1, N2, N3, and N4 is superior. Note that this refers to the wide range of frequencies in which absorption is favorable, and does not mean that case N4 is ineffective. Through the tests shown in Figures 11 and 12, the inventors confirmed that the first resistive layer R1 and the second resistive layer R2 are capable of absorbing radio waves in a predetermined frequency range at least in the range of 20 to 200 (Ω / sq) and that the radio wave absorption characteristics on both sides are different, and confirmed that the effects described in Section "2. Description of Effects of the Embodiments" can be expected.

[0074] 4. Modifications In the embodiment, a configuration having two resistive layers has been described as an example, but this is not limiting. The laminate 10 may also have three or more resistive layers. In this case, the additional resistive layers are arranged with a dielectric layer sandwiched between them. The resistive layer does not have to have two layers, and may have only one layer. In other words, the third dielectric layer D3 is not included. In other words, the laminate 10 may have a first reflective layer Rf1, a first adhesive layer, a first dielectric layer D1, only one resistive layer, a second dielectric layer D2, a second adhesive layer, and a second reflective layer Rf2, arranged in this order in the stacking direction of the laminate 10. In this case, it is preferable to make the thicknesses and relative dielectric constants of the first dielectric layer D1 and the second dielectric layer D2 different. Furthermore, the first resistive layer R1 does not have to have a periodic structure R1S, and like the second resistive layer R2, the first resistive layer R1 may be a so-called solid layer. The adhesive layer may be replaced by a connecting structure (e.g., fine irregularities) formed on the dielectric layer. In other words, the adhesive layer may be configured without using a chemical material as described in the embodiment. In this case, the portion of the dielectric layer where the connecting structure is formed corresponds to the adhesive layer.

[0075] 100: Radio wave absorber 10: Laminate D1: First dielectric layer D2: Second dielectric layer D3: Third dielectric layer R1: First resistive layer R1S: Periodic structure R1a: Resistive element portion R2: Second resistive layer Rf1: First reflective layer Rf2: Second reflective layer Rw1: First radio wave Rw2: Second radio wave Rw3: Third radio wave Rw4: Fourth radio wave Rw5: Fifth radio wave Rw6: Sixth radio wave

Claims

1. A radio wave absorber having a laminate, wherein the laminate has a first reflective layer, a first adhesive layer, a first dielectric layer, a resistive layer, a second dielectric layer, a second adhesive layer, and a second reflective layer, which are arranged in this order in the stacking direction of the laminate, wherein the first reflective layer is adhered to the first dielectric layer via the first adhesive layer so as to be peelable from the first dielectric layer, and the second reflective layer is adhered to the second dielectric layer via the second adhesive layer so as to be peelable from the second dielectric layer.

2. A radio wave absorber as claimed in claim 1, wherein the radio wave absorption characteristics of said radio wave absorber when said first reflective layer is peeled off from said first dielectric layer and said first dielectric layer becomes the radio wave incident side are defined as first characteristics, and the radio wave absorption characteristics of said radio wave absorber when said second reflective layer is peeled off from said second dielectric layer and said second dielectric layer becomes the radio wave incident side are defined as second characteristics, wherein said first characteristic and said second characteristic are characteristics that represent the relationship between the frequency of radio waves and absorption rate, and said first characteristic and said second characteristic are different.

3. A radio wave absorber according to claim 2, wherein the first characteristic and the second characteristic are characteristics in which the radio wave absorption rate has an absorption peak in the frequency range of 50 (GHz) to 300 (GHz).

4. A radio wave absorber according to claim 3, wherein the absorption peak is a peak where the absorption rate is 90% or more.

5. A radio wave absorber according to claim 3, wherein the frequency of the absorption peak of the first characteristic is different from the frequency of the absorption peak of the second characteristic.

6. A radio wave absorber according to any one of claims 1 to 5, wherein the laminate further has a third dielectric layer, the resistive layer has a first resistive layer and a second resistive layer, and the laminate has the first reflective layer, the first adhesive layer, the first dielectric layer, the first resistive layer, the third dielectric layer, the second resistive layer, the second dielectric layer, the second adhesive layer, and the second reflective layer arranged in this order in the lamination direction.

7. A radio wave absorber according to claim 6, wherein the first resistive layer has a periodic structure, and the periodic structure is formed to have periodicity within the plane of the first resistive layer so as to allow radio waves of a predetermined frequency range to pass through.

8. A radio wave absorber according to any one of claims 1 to 5, wherein the laminate is formed in the shape of a flexible sheet as a whole.

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