Radio wave transmission member, method for manufacturing radio wave transmission member, automobile component, emblem, and object detection structure

Abstract

This radio wave transmission member comprises an outer layer, an intermediate layer, and an inner layer in this order, and has a region in which the total thickness X1 of the outer layer and the inner layer in the transmission direction of radio waves satisfies formulas (1) and (3) below, and the thickness X2 of the intermediate layer in the transmission direction of radio waves satisfies formulas (2) and (3) below.　Formula (1): Z-0.6 mm ≤ X1 ≤ Z+0.6 mm. Formula (2): Z-0.3 mm ≤ X2 ≤ Z+0.3 mm. Formula (3): z=c÷√εr×0.5×Y. In the formulas, λ is the wavelength of the radio waves in a vacuum, εr is the relative permittivity of each layer at the frequency of the radio waves, and Y is an integer of 1 or greater.

Description

Radio wave transmitting member, manufacturing method of radio wave transmitting member, automobile part, emblem and object detection structure

[0001] The present disclosure relates to a radio wave transparent member, a method for manufacturing a radio wave transparent member, an automobile part, an emblem, and an object detection structure.

[0002] Recent advances in automobile safety devices have been remarkable, and automatic collision avoidance systems, for example, have become commonplace. Automatic collision avoidance systems automatically apply the brakes using image data from an onboard camera and relative distance information from a radio wave transmitter and receiver, such as millimeter waves.

[0003] It is desirable to place a radio wave transmitter / receiver constituting an automatic collision avoidance system at the front center of a vehicle. The vehicle's emblem is generally located at the front center of a vehicle. Therefore, it is desirable to place the radio wave transmitter / receiver behind the vehicle emblem. Vehicle emblems generally have a structure in which a metal layer for imparting metallic luster is formed on a substrate such as a resin. For example, Patent Documents 1 and 2 disclose structures in which a metal layer is formed on a substrate by silver mirror reaction as an emblem that has metallic luster and is capable of transmitting radio waves.

[0004] Patent Document 1: JP 2003-019765 A Patent Document 2: JP 2019-177311 A

[0005] From the viewpoint of ensuring sufficient performance of an automatic collision avoidance system, it is desirable to suppress attenuation of radio waves that pass through an emblem as much as possible. In view of the above circumstances, the present disclosure aims to provide a radio wave-transmitting member that suppresses attenuation of radio waves passing through the emblem, a method for manufacturing the radio wave-transmitting member, an automotive part, an emblem, and an object detection structure.

[0006] Specific means for achieving the above object are as follows: <1> A radio wave transparent member comprising an outer layer, an intermediate layer, and an inner layer in this order, wherein the total thickness X1 of the outer layer and the inner layer in the radio wave transmission direction satisfies the following formulas (1) and (3), and the thickness X2 of the intermediate layer in the radio wave transmission direction satisfies the following formulas (2) and (3): Formula (1): Z - 0.6 mm ≦ X1 ≦ Z + 0.6 mm Formula (2): Z - 0.3 mm ≦ X2 ≦ Z + 0.3 mm Formula (3): Z = λ ÷ √ε ｒ × 0.5 × Y where λ is the wavelength of the radio wave in a vacuum, and ε ｒ is the relative dielectric constant of each layer at the frequency of the radio wave, and Y is an integer of 1 or more. <2> The radio wave transparent member according to <1>, wherein the intermediate layer is an air layer. <3> The radio wave transparent member according to <1> or <2>, wherein the outer layer and the inner layer are each a layer containing a resin. <4> The radio wave transparent member according to any one of <1> to <3>, further comprising a metal layer that can transmit the radio wave. <5> The radio wave transparent member according to <4>, wherein the metal layer is provided between the outer layer and the intermediate layer or between the inner layer and the intermediate layer. <6> The radio wave transparent member according to any one of <1> to <5>, for transmitting radio waves having a frequency of 20 GHz to 300 GHz. <7> A method for manufacturing a radio wave transparent member having an outer layer, an intermediate layer, and an inner layer in this order, the method comprising determining thicknesses of the outer layer, the intermediate layer, and the inner layer so that a total thickness X1 of the outer layer and the inner layer in the radio wave transmission direction satisfies the following formulas (1) and (3), and a thickness X2 of the intermediate layer in the radio wave transmission direction has a region that satisfies the following formulas (2) and (3): Formula (1): Z - 0.6 mm ≦ X1 ≦ Z + 0.6 mm Formula (2): Z - 0.3 mm ≦ X2 ≦ Z + 0.3 mm Formula (3): Z = λ ÷ √ε ｒ × 0.5 × Y where λ is the wavelength of the radio wave in a vacuum, and ε ｒis the relative dielectric constant of each layer at the frequency of the radio wave, and Y is an integer of 1 or more. <8> An automobile part including the radio wave transparent member according to any one of <1> to <7>. <9> An emblem including the radio wave transparent member according to any one of <1> to <7>. <10> An object detection structure including the radio wave transparent member according to any one of <1> to <7> and a device that irradiates radio waves toward the radio wave transparent member.

[0007] According to the present disclosure, there are provided a radio wave transparent member that suppresses transmission attenuation of radio waves, a method for manufacturing a radio wave transparent member, an automobile part, an emblem, and an object detection structure.

[0008] It is a schematic cross-sectional view showing an example of the configuration of a radio wave transparent member. It is a schematic view showing an example of an automobile part including a radio wave transparent member. It is a schematic view showing an example of an automobile part including a radio wave transparent member. It is a schematic view showing an example of an automobile part including a radio wave transparent member.

[0009] Hereinafter, embodiments for carrying out the present disclosure will be described in detail. However, the present disclosure is not limited to the following embodiments. In the following embodiments, components (including element steps, etc.) are not essential unless otherwise specified. The same applies to numerical values ​​and their ranges, and do not limit the present disclosure.

[0010] In the present disclosure, when a numerical range is indicated using "to", the numerical values ​​before and after "to" are included as the minimum and maximum values, respectively. In the present disclosure, when a numerical range is indicated in a stepped manner, the upper or lower limit value of one numerical range may be replaced with the upper or lower limit value of another stepped numerical range. Furthermore, in the present disclosure, when a numerical range is indicated, the upper or lower limit value of that numerical range may be replaced with a value shown in the examples. In the present disclosure, each component may contain multiple corresponding substances. When multiple substances corresponding to each component are present in a composition, the content or amount of each component refers to the total content or amount of the multiple substances present in the composition, unless otherwise specified. In the present disclosure, the particles corresponding to each component may contain multiple types of particles. In the present disclosure, the terms "layer" and "film" include cases where the layer or film is formed over the entire region when the region where the layer or film is present is observed, as well as cases where the layer or film is formed only in a portion of the region. In the present disclosure, "(meth)acrylic" is a term used to encompass both acrylic and methacrylic.

[0011] <Radio wave transparent member> The radio wave transparent member of the present disclosure is a radio wave transparent member comprising an outer layer, an intermediate layer, and an inner layer in this order, wherein the total thickness X1 of the outer layer and the inner layer in the radio wave transmission direction satisfies the following formulas (1) and (3), and the thickness X2 of the intermediate layer in the radio wave transmission direction satisfies the following formulas (2) and (3): Formula (1): Z - 0.6 mm ≦ X1 ≦ Z + 0.6 mm Formula (2): Z - 0.3 mm ≦ X2 ≦ Z + 0.3 mm Formula (3): Z = λ ÷ √ε ｒ × 0.5 × Y where λ is the wavelength of the radio wave in a vacuum, and ε ｒ is the relative dielectric constant of each layer at the frequency of the radio wave, and Y is an integer of 1 or more.

[0012] The radio wave transparent member of the present disclosure has a structure in which an outer layer, an intermediate layer, and an inner layer are laminated in this order for reasons of ensuring strength, molding process, etc. Therefore, if the layers are made of different materials, radio waves may be reflected at the boundaries between the layers, increasing the amount of attenuation of radio waves passing through the radio wave transparent member.

[0013] In the radio wave transparent member of the present disclosure, the attenuation of radio waves passing through the radio wave transparent member is effectively suppressed by designing the total thickness X1 of the outer layer and the inner layer and the thickness X2 of the intermediate layer to have a region that satisfies formulas (1) to (3). Furthermore, by designing the total thickness X1 of the outer layer and the inner layer to satisfy formulas (1) and (3), radio wave attenuation can be effectively suppressed even if the thickness X of each of the outer layer and the inner layer is not designed to satisfy formulas (1) and (3). Therefore, for example, the transmittance of radio waves can be further improved by reducing the thickness of each of the outer layer and the inner layer.

[0014] Fig. 1 is a schematic cross-sectional view showing an example of the configuration of a radio wave transparent member. The radio wave transparent member 10 shown in Fig. 1 includes an outer layer 1, an intermediate layer 2, and an inner layer 3, in this order, and transmits radio waves irradiated from a device (not shown) that irradiates radio waves, as well as radio waves reflected from an object. The outer layer 1 is disposed on the opposite side of the device that irradiates radio waves to the radio wave transparent member 10. The inner layer 3 is disposed on the side facing the device that irradiates radio waves to the radio wave transparent member.

[0015] The frequency of the radio waves that pass through the radio wave transparent member is not particularly limited. For example, the wavelength in a vacuum may be in the range of 20 GHz to 300 GHz (so-called millimeter waves). When the radio wave transparent member is used in an automatic collision avoidance system for automobiles, radio waves commonly used in automatic collision avoidance systems have frequencies of 24 GHz to 79 GHz, such as 24 GHz, 77 GHz, and 79 GHz. The wavelengths in a vacuum of these radio waves are 12.49135 mm (24 GHz), 3.893409 mm (77 GHz), and 3.794841 mm (79 GHz).

[0016] Y in the formula is not particularly limited as long as it is an integer of 1 or more, and can be set depending on the desired shape, strength, etc. of the radio wave transparent member. For example, Y may be in the range of 1 to 13.

[0017] From the viewpoint of more effectively suppressing the amount of attenuation of radio waves transmitted through the radio wave transparent member, it is preferable that the difference between Z and the total thickness X1 of the outer layer and the inner layer in the radio wave transmission direction is as small as possible. That is, the total thickness X1 of the outer layer and the inner layer in the radio wave transmission direction preferably satisfies the following formula (1'), and more preferably the following formula (1''). Formula (1'): Z - 0.4 mm ≦ X1 ≦ Z + 0.4 mm Formula (1''): Z - 0.3 mm ≦ X1 ≦ Z + 0.3 mm

[0018] From the viewpoint of more effectively suppressing the amount of attenuation of radio waves transmitted through the radio wave transparent member, it is preferable that the difference between the thickness X2 of the intermediate layer in the radio wave transmission direction and Z is as small as possible. That is, it is preferable that the thickness X2 of the intermediate layer in the radio wave transmission direction satisfies the following formula (2'), and more preferably the following formula (2''). Formula (2'): Z - 0.2 mm ≦ X2 ≦ Z + 0.2 mm Formula (2''): Z - 0.1 mm ≦ X2 ≦ Z + 0.1 mm

[0019] When the frequency of radio waves passing through the radio wave transparent member is 77 GHz and the wavelength in a vacuum is 3.893409 mm, the Z value (Y = 1 to 13, unit: mm) when the relative dielectric constant at 77 GHz is 2.574 and the Z value (Y = 1 to 13, unit: mm) when the relative dielectric constant at 77 GHz is 1.0 (i.e., air) are shown in Table 1. The values ​​in the table are rounded to two decimal places.

[0020]

[0021] From the viewpoint of obtaining a sufficient effect of suppressing radio wave attenuation, the area of ​​the region where the total thickness X1 of the outer layer and the inner layer in the radio wave transmission direction and the thickness X2 of the intermediate layer in the radio wave transmission direction satisfy the formulas (1) to (3) preferably accounts for 50% or more, more preferably 60% or more, and even more preferably 70% or more of the total area of ​​the radio wave transparent member. The above-mentioned percentages are values ​​based on the area of ​​the radio wave transparent member when observed from the outer layer side.

[0022] From the viewpoint of the balance between strength and radio wave transmittance, it is preferable that the outer layer and the inner layer each be a layer containing a resin, and from the viewpoint of radio wave transmittance, it is preferable that the intermediate layer be an air layer or a layer containing a resin, and it is more preferable that it be an air layer.

[0023] When the outer layer, inner layer or intermediate layer is a layer containing a resin, the relative dielectric constant thereof may be independently 2.0 to 3.3, 2.3 to 2.9, or 2.5 to 2.8.

[0024] Resins that can be contained in each layer that constitutes the radio wave transparent member include thermosetting resins, thermoplastic resins, and synthetic rubbers.

[0025] Examples of thermoplastic resins include polyethylene (PE), polypropylene (PP), polycarbonate (PC), polystyrene, polyvinyl chloride, vinyl polymers, polyester, polyamide, acrylonitrile-butadiene-styrene copolymer resin (ABS resin), (meth)acrylic resin, acrylonitrile-ethylene-propylene-diene-styrene copolymer resin (AES resin), and thermoplastic elastomer.

[0026] Examples of the thermosetting resin include silicone resin, urethane resin, melamine resin, epoxy resin, phenol resin, and urea resin.

[0027] Examples of synthetic rubber include ethylene-propylene-diene rubber (EPDM), acrylonitrile-butadiene rubber (NBR), isoprene rubber (IR), butadiene rubber (BR), styrene-butadiene rubber (SBR), chloroprene rubber (CR), silicone rubber, and urethane rubber.

[0028] Among the above-mentioned resins, PC, PP, ABS resin, (meth)acrylic resin and AES resin are preferred, and PC, PP and ABS resin are more preferred.

[0029] When the outer layer, intermediate layer, or inner layer is a layer containing a resin, the outer layer, intermediate layer, or inner layer may contain only resin, or may contain resin and a component other than resin. Examples of components other than resin include inorganic particles, colorants, antistatic agents, and the light-diffusing particles described below. When the outer layer, intermediate layer, or inner layer contains resin and a component other than resin, the content of the resin contained in the outer layer, intermediate layer, or inner layer is preferably 60% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, and particularly preferably 90% by mass or more. The upper limit of the resin content is not particularly limited as long as it is less than 100% by mass.

[0030] The outer layer, intermediate layer, or inner layer may have a light diffusion function. When the outer layer, intermediate layer, or inner layer has a light diffusion function, light can be irradiated onto the radio wave transparent member to cause the radio wave transparent member to emit light. This enables various expressions using light.

[0031] Methods for imparting a light diffusing function to the outer layer, intermediate layer, or inner layer include a method of incorporating light diffusing particles, a method of forming irregularities on the surface, etc. The outer layer, intermediate layer, or inner layer having a light diffusing function may have the light diffusing function entirely or partially.

[0032] Among the above methods, the method of incorporating light-diffusing particles is preferred from the viewpoint of productivity. Examples of light-diffusing particles include acrylic resin particles, silicone resin particles, titanium oxide particles, silica particles, zirconia particles, zinc oxide particles, and alumina particles. The light-diffusing particles may be used alone or in combination of two or more types.

[0033] When the outer layer, intermediate layer, or inner layer contains light-diffusing particles, the content of the light-diffusing particles is not particularly limited. For example, the content of the light-diffusing particles may be 0.3% by mass to 10% by mass of the entire outer layer, intermediate layer, or inner layer.

[0034] The thicknesses of the outer layer, intermediate layer, and inner layer are not particularly limited as long as the total thickness of the outer layer and the inner layer and the thickness of the intermediate layer satisfy the conditions of formulas (1) to (3), and can be set depending on the application of the radio wave transparent member, etc. The thickness of the outer layer may be 1.0 mm to 15.0 mm, 1.5 mm to 12.0 mm, or 2.0 mm to 10.0 mm. The thickness of the intermediate layer may be 0.1 mm to 6.0 mm, 0.3 mm to 4.0 mm, or 0.5 mm to 2.0 mm. The thickness of the inner layer may be 1.0 mm to 15.0 mm, 1.5 mm to 12.0 mm, or 2.0 mm to 10.0 mm. The above thicknesses are those in the range where the thickness X of the outer layer or the inner layer satisfies the conditions of formulas (1) to (3), respectively.

[0035] In the present disclosure, the thickness of the radio wave transparent member and each layer constituting the radio wave transparent member can be measured by a known method. For example, the thickness may be measured by observing a cut surface of the radio wave transparent member or may be measured using a microtome or the like.

[0036] The radio wave transparent member may include a metal layer that is capable of transmitting radio waves. By including a metal layer that is capable of transmitting radio waves in the radio wave transparent member, it is possible to impart a metallic luster to the radio wave transparent member.

[0037] When the radio wave transmitting member includes a metal layer that is capable of transmitting radio waves, the metal layer that is capable of transmitting radio waves is preferably provided between the outer layer and the intermediate layer, or between the inner layer and the intermediate layer.

[0038] An example of a metal layer that is permeable to radio waves is a film containing metal particles. When the metal particles are contained in the film, radio waves can pass through the gaps between the metal particles.

[0039] The metal layer may be a metal layer containing silver particles. A metal layer containing silver particles can be formed, for example, by a silver mirror reaction. A method for forming a metal layer by the silver mirror reaction includes contacting an aqueous ammoniacal silver nitrate solution with an aqueous reducing agent solution on a substrate. This causes an oxidation-reduction reaction, resulting in the precipitation of silver particles, and a metal layer containing silver particles is formed on the substrate. When a metal layer is provided between an outer layer and an intermediate layer by the above method, the member that will become the outer layer may be used as the substrate, and when a metal layer is provided between an inner layer and an intermediate layer, the member that will become the inner layer may be used as the substrate.

[0040] From the viewpoint of radio wave transmission, the thickness of the metal layer is preferably 1000 nm or less, more preferably 500 nm or less, and even more preferably 100 nm or less. From the viewpoint of imparting sufficient metallic luster to the radio wave transmission member, the thickness of the metal layer is preferably 10 nm or more.

[0041] When a metal layer is provided between the outer layer and the intermediate layer or between the inner layer and the intermediate layer, an undercoat layer may be provided between the outer layer or the inner layer and the metal layer to improve adhesion between the outer layer or the inner layer and the metal layer. Alternatively, a protective layer may be provided on the metal layer to protect the surface of the metal layer.

[0042] The undercoat layer or protective layer may be a layer containing a resin, and examples of the resin include fluororesin, polyester resin, epoxy resin, melamine resin, silicone resin, acrylic silicone resin, and acrylic urethane resin.

[0043] The thickness of the undercoat layer or the protective layer is preferably 1 μm to 50 μm. When the thickness of the undercoat layer or the protective layer is 1 μm to 50 μm, the effect of suppressing radio wave attenuation by controlling the thickness of the outer layer, intermediate layer, and inner layer can be sufficiently obtained.

[0044] The type of radio wave that can be transmitted through the radio wave transparent member is not particularly limited, and may be, for example, a millimeter wave. In the present disclosure, "millimeter wave" refers to a radio wave having a frequency of 20 GHz to 300 GHz. The radio wave transparent member of the present disclosure is particularly useful for reducing the transmission attenuation of millimeter waves.

[0045] <Method for manufacturing radio wave transparent member> The method for manufacturing a radio wave transparent member of the present disclosure is a method for manufacturing a radio wave transparent member having an outer layer, an intermediate layer, and an inner layer in this order, and includes determining thicknesses of the outer layer, the intermediate layer, and the inner layer such that a total thickness X1 of the outer layer and the inner layer in the radio wave transmission direction satisfies the following formulas (1) and (3), and a thickness X2 of the intermediate layer in the radio wave transmission direction satisfies the following formulas (2) and (3).

[0046] Formula (1): Z-0.6mm≦X1≦Z+0.6mm Formula (2): Z-0.3mm≦X2≦Z+0.3mm Formula (3): Z=λ÷√ε ｒ × 0.5 × Y where λ is the wavelength of the radio wave in a vacuum, and ε ｒ is the relative dielectric constant of each layer at the frequency of the radio wave, and Y is an integer of 1 or more.

[0047] According to the method of the present disclosure, it is possible to manufacture a radio wave transparent member in which attenuation of transmitted radio waves is suppressed. Details and preferred aspects of the radio wave transparent member manufactured by the method of the present disclosure are the same as the details and preferred aspects of the radio wave transparent member of the present disclosure described above.

[0048] <Automotive Parts> The automotive parts of the present disclosure are automotive parts including the radio wave transparent member described above. The type of automotive part is not particularly limited and may be any automotive interior or exterior part. Examples of automotive parts include parts in the front portion of the automobile as illustrated in FIG. 2 , parts in the side and rear portions of the automobile as illustrated in FIG. 3 , and parts in the interior of the automobile as illustrated in FIG. 4 . Specific examples of automotive parts include the emblem 102, lamp 104, garnish 106, bumper 108, and grille 110 shown in FIG. 2 ; the lamp 202, tailgate 204, garnish 206, outer mirror 208, door outer handle 210, and bumper 212 shown in FIG. 3 ; and the room lamp 302, roof garnish 304, room mirror 306, instrument panel garnish 308, and door trim 310 shown in FIG. 4 . The automotive parts may have sensing functions such as obstacle sensors, human presence sensors, and vital signs sensors.

[0049] <Emblem> The emblem of the present disclosure is an emblem including the radio wave-transmitting member described above. The emblem of the present disclosure suppresses attenuation of radio waves that pass through it. Therefore, even if the emblem is attached to the front of a vehicle equipped with a radio wave transmitter / receiver, the radio wave transmission and reception function is maintained satisfactorily.

[0050] The emblem of the present disclosure is attached to the body of an automobile with the outer layer side of the radio wave transparent member facing outward. The outer layer side of the emblem may have an uneven shape for expressing letters, designs, etc.

[0051] The object detection structure of the present disclosure includes the radio wave transparent member described above and a device that irradiates radio waves toward the radio wave transparent member. The object detection structure of the present disclosure exhibits excellent object detection performance because radio wave attenuation by the radio wave transparent member is suppressed.

[0052] The object detection structure of the present disclosure can be suitably used, for example, in an automatic collision avoidance system for automobiles (preferably, an automatic collision avoidance system that uses millimeter waves).

[0053] The present disclosure will be described in more detail below with reference to examples, but the present disclosure is not limited to the following examples.

[0054] (Preparation of Samples) A ​​sample of a radio wave transparent member was prepared using polycarbonate plates with a relative dielectric constant of 2.574 at 77 GHz as the outer layer or inner layer, and a space (air) provided between the polycarbonate plates as the intermediate layer. The thickness of each layer was adjusted to the values ​​shown in Table 2.

[0055] (Measurement of radio wave attenuation) Radio waves having a frequency of 77 GHz and a wavelength in vacuum of 3.893409 mm were irradiated from the inner layer side perpendicular to the thickness direction of the prepared sample, and the attenuation of the radio waves was measured using the free space method. The results are shown in Table 2.

[0056] (Measurement of Absorption and Reflectance) The prepared sample was irradiated perpendicularly to the thickness direction from the inner layer side with radio waves having a frequency of 77 GHz and a wavelength in vacuum of 3.893409 mm, and the radio wave absorption and reflectance were measured using the free space method. The results are shown in Table 2. The smaller the total value of the radio wave absorption and reflectance, the greater the radio wave transmittance.

[0057]

[0058] In the "X1" column in the table, "OK" means that the total thickness X1 of the outer layer and the inner layer satisfies formula (1) and formula (3), and "NG" means that the total thickness X1 of the outer layer and the inner layer does not satisfy formula (1) and formula (3). In the "X2" column in the table, "OK" means that the thickness X2 of the intermediate layer satisfies formula (2) and formula (3), and "NG" means that the thickness X2 of the intermediate layer does not satisfy formula (2) and formula (3). In the "X" column in the table, "OK" means that the thickness X of the outer layer or the inner layer satisfies formula (2) and formula (3) (with X2 replaced with X in formula (2)), and "NG" means that the thickness X of the outer layer or the inner layer does not satisfy formula (2) and formula (3) (with X2 replaced with X in formula (2)).

[0059] As shown in Table 2, Samples 1 to 7, in which the total thickness X1 of the outer layer and the inner layer satisfies formulas (1) and (3) and the thickness X2 of the intermediate layer satisfies formulas (2) and (3), have reduced radio wave attenuation compared to Samples 8 to 14, which do not satisfy these conditions. Among Samples 1 to 7, in which the total thickness X1 of the outer layer and the inner layer satisfies formulas (1) and (3), Sample 1, even though the outer layer and the inner layer do not individually satisfy formulas (1) and (3), has lower radio wave attenuation than Samples 5, 6, and 7, in which the outer layer and the inner layer individually satisfy formulas (1) and (3). This is thought to be because Sample 1 has a smaller overall thickness than Samples 5, 6, and 7, resulting in a higher radio wave transmittance.

Claims

1. A radio wave transparent member comprising an outer layer, an intermediate layer, and an inner layer in this order, wherein the total thickness X1 of the outer layer and the inner layer in the radio wave transmission direction satisfies the following formulas (1) and (3), and the thickness X2 of the intermediate layer in the radio wave transmission direction has a region that satisfies the following formulas (2) and (3): Formula (1): Z - 0.6 mm ≦ X1 ≦ Z + 0.6 mm Formula (2): Z - 0.3 mm ≦ X2 ≦ Z + 0.3 mm Formula (3): Z = λ ÷ √ε ｒ × 0.5 × Y, where λ is the wavelength of the radio wave in a vacuum, and ε ｒ is the relative dielectric constant of each layer at the frequency of the radio wave, and Y is an integer of 1 or more.

2. The radio wave transparent member according to claim 1, wherein the intermediate layer is an air layer.

3. The radio wave transparent member according to claim 1, wherein the outer layer and the inner layer are each a layer containing a resin.

4. The radio wave transparent member according to claim 1, further comprising a metal layer capable of transmitting said radio waves.

5. The radio wave transparent member according to claim 4, wherein the metal layer is provided between the outer layer and the intermediate layer or between the inner layer and the intermediate layer.

6. The radio wave transparent member according to claim 1, for transmitting radio waves having a frequency of 20 GHz to 300 GHz.

7. A method for manufacturing a radio wave transparent member having an outer layer, an intermediate layer, and an inner layer in this order, comprising determining thicknesses of the outer layer, the intermediate layer, and the inner layer such that a total thickness X1 of the outer layer and the inner layer in the radio wave transmission direction satisfies the following formulas (1) and (3), and a thickness X2 of the intermediate layer in the radio wave transmission direction satisfies the following formulas (2) and (3): Formula (1): Z - 0.6 mm ≦ X1 ≦ Z + 0.6 mm Formula (2): Z - 0.3 mm ≦ X2 ≦ Z + 0.3 mm Formula (3): Z = λ ÷ √ε ｒ × 0.5 × Y, where λ is the wavelength of the radio wave in a vacuum, and ε ｒ is the relative dielectric constant of each layer at the frequency of the radio wave, and Y is an integer of 1 or more.

8. An automobile part comprising the radio wave transparent member according to any one of claims 1 to 7.

9. An emblem comprising the radio wave transparent material according to any one of claims 1 to 7.

10. An object detection structure comprising: a radio wave transparent member according to any one of claims 1 to 7; and a device for irradiating radio waves toward the radio wave transparent member.

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