Radio wave absorber

Abstract

Provided is a technique for reducing the thickness of a radio wave absorber.　The present invention is a radio wave absorber comprising a layered structure, wherein: the radio wave absorber has visible light transmittance of at least 50%; the radio wave absorber absorbs radio waves in a given specified frequency band; the radio wave absorber has a first main surface that is a surface on which the radio waves are incident, and a second main surface that is the surface opposite the first main surface; the radio wave absorber includes, from the first main surface to the second main surface, in the stated order, a first dielectric layer, an electroconductive layer, a second dielectric layer, a radio wave reflecting layer, and a third dielectric layer; the electroconductive layer comprises a plurality of conductors and gaps positioned separating the plurality of conductors from each other; and the plurality of conductors are insulated from each other.

Description

[Technical field]

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

[0002] The radio wave absorber described in Patent Document 1 is a laminate that absorbs radio waves for wireless LAN and has a light transmittance of 50% or more. This radio wave absorber is a λ / 4 type, and includes, from the radio wave incident side, (A) a glass layer, (B) a PET film, (C) a resistive film, (D) a dielectric layer, (E) a reflective film, (F) a PET film, and (G) a glass layer, in this order. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2006-86446 A Summary of the Invention [Problem to be solved by the invention]

[0004] In general, a λ / 4 type radio wave absorber has a distance of approximately λ / 4 between the resistive film and the reflective film, where λ is the wavelength of the radio wave. The resistive film reflects part of the radio wave, absorbs another part, and transmits the remainder. The radio waves reflected by the resistive film and the reflective film interfere with each other and cancel each other out. As a result, the radio waves are absorbed.

[0005] As described above, in a λ / 4 type radio wave absorber, the distance between the resistive film and the reflective film is approximately λ / 4. Therefore, the longer the wavelength λ of the radio wave, that is, the smaller the frequency of the radio wave, the thicker the λ / 4 type radio wave absorber becomes.

[0006] One aspect of the present disclosure provides a technique for reducing the thickness of a radio wave absorber. [Means for solving the problem]

[0007] A radio wave absorber according to one embodiment of the present disclosure is a radio wave absorber having a layered structure, the radio wave absorber has a visible light transmittance of 50% or more, the radio wave absorber absorbs radio waves in a certain frequency band, the radio wave absorber has a first principal surface on which the radio waves are incident and a second principal surface opposite to the first principal surface, and includes, from the first principal surface to the second principal surface, a first dielectric layer, a conductive layer, a second dielectric layer, a radio wave reflecting layer, and a third dielectric layer in this order, the conductive layer includes a plurality of conductors and gaps that separate the plurality of conductors from one another, and the plurality of conductors are insulated from one another. At least one of the first dielectric layer, the second dielectric layer and the third dielectric layer is made of glass, and the glass has a relative dielectric constant of 3.5 or more. Effect of the Invention

[0008] According to one aspect of the present disclosure, the thickness of the radio wave absorber can be reduced. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a diagram showing applications of a radio wave absorber according to an embodiment. [Diagram 2] FIG. 2 is a cross-sectional view showing a first example of a radio wave absorber. [Diagram 3] FIG. 3 is a cross-sectional view showing a second example of the radio wave absorber. [Figure 4] FIG. 4 is a cross-sectional view showing a third example of the radio wave absorber. [Diagram 5] FIG. 5 is a plan view showing an example of a slit pattern of the conductive layer. [Figure 6] FIG. 6 is a plan view showing an example of the line width of the first slit and the line width of the second slit. [Figure 7] FIG. 7 is a diagram showing an example of the relationship between the thickness of the second dielectric and the sheet resistance of the conductor of the conductive layer. [Figure 8] FIG. 8 is a cross-sectional view showing an example of a method for measuring the amount of attenuation of a radio wave absorber. [Figure 9] FIG. 9 is a graph showing the attenuation of the radio wave absorber produced in Example 1. [Figure 10]FIG. 10 is a graph showing the ratio of the attenuation of the radio wave absorber produced in Example 1 to the attenuation of a SUS plate. [Figure 11] FIG. 11 is a graph showing the attenuation of the radio wave absorber produced in Example 2. [Figure 12] FIG. 12 is a graph showing the ratio of the attenuation of the radio wave absorber produced in Example 2 to the attenuation of a SUS plate. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In each drawing, the same or corresponding components are denoted by the same reference numerals, and the description may be omitted. In the specification, the symbol "~" indicating a range of values ​​means that the values ​​before and after the symbol are included as the lower and upper limits.

[0011] First, an example of the use of the radio wave absorber 1 will be described with reference to Fig. 1. The radio wave absorber 1 absorbs unnecessary radio waves when radio waves are transmitted and received between, for example, RF (Radio Frequency) tags 3A, 3B, and 3C attached to products 2A, 2B, and 2C and a reader 4. The reflection of unnecessary radio waves can be suppressed, and for example, mistakes in counting the number of products 2A, 2B, and 2C can be prevented.

[0012] The RF tags 3A, 3B, and 3C include a storage medium that stores information for identifying the products 2A, 2B, and 2C. The RF tags 3A, 3B, and 3C are, for example, passive type, and use radio waves emitted from a reader 4 as a power source to transmit radio waves of information previously stored in the storage medium to the reader 4. The frequency of the radio waves emitted from the RF tags 3A, 3B, and 3C is, for example, 913 MHz.

[0013] Although the RF tags 3A, 3B, and 3C are of the passive type in this embodiment, they may be of the active type and may have a built-in battery as a power source.

[0014] The reader 4 receives radio waves transmitted from the RF tags 3A, 3B, and 3C, reads the information contained in the radio waves, and identifies the products 2A, 2B, and 2C. The maximum communication distance of the reader 4 is not particularly limited, but is, for example, about 6 m. The total price of the products 2A, 2B, and 2C identified by the reader 4 is displayed as an image on the display device.

[0015] The radio wave absorber 1 has, for example, a box shape and houses a reading device 4 inside. A purchaser, for example, puts products 2A, 2B, and 2C from the outside to the inside of the radio wave absorber 1. In this case, the radio wave absorber 1 may be required to be transparent so that the purchaser can feel at ease.

[0016] The radio wave absorber 1 may be large enough to accommodate a purchaser. The purchaser enters the inside of the radio wave absorber 1 together with the products 2A, 2B, and 2C. In this case, the radio wave absorber 1 may be required to be transparent so that the purchaser does not feel a sense of obstruction.

[0017] That is, there are cases where the radio wave absorber 1 is required to be transparent so that a purchaser can view a commodity or a scene through the radio wave absorber 1 regardless of the purchaser's position. If the radio wave absorber 1 is transparent, it can be used as a self-checkout or a warehouse where the inside and outside of the radio wave absorber can be viewed and the commodity inside the radio wave absorber 1 can be automatically recognized.

[0018] Next, a first example of the radio wave absorber 1 will be described with reference to Fig. 2. In Fig. 2, the incident direction of radio waves RF is indicated by an arrow.

[0019] The radio wave absorber 1 has a laminated structure, absorbs radio waves of a certain frequency, and has a visible light transmittance of 50% or more. If the visible light transmittance is 50% or more, a purchaser can visually recognize a product or a scene through the radio wave absorber 1. The visible light transmittance is preferably 53% or more, and more preferably 55% or more. In addition, the visible light transmittance is preferably 90% or less, and more preferably 80% or less. The visible light transmittance is measured in accordance with Japanese Industrial Standard JIS R 3106:1998, and calculated by a calculation formula when a standard D65 light source is used.

[0020] The radio wave absorber 1 includes a first principal surface 11 on which radio waves are incident, and a second principal surface 12 which is the surface opposite to the first principal surface 11. The radio wave absorber 1 includes, from the first principal surface 11 to the second principal surface 12, a first dielectric layer 13, a conductive layer 14, a second dielectric layer 15, a radio wave reflecting layer 16, and a third dielectric layer 17, in this order.

[0021] Moreover, the radio wave absorber 1 may further include an adhesive layer 18 between the first principal surface 11 and the second principal surface 12. The adhesive layer 18 is disposed in at least one of between the first dielectric layer 13 and the conductive layer 14, between the conductive layer 14 and the second dielectric layer 15, between the second dielectric layer 15 and the radio wave reflecting layer 16, and between the radio wave reflecting layer 16 and the third dielectric layer 17. The arrangement of the adhesive layer 18 is not particularly limited, as will be described in detail later.

[0022] The first dielectric layer 13 protects the conductive layer 14. The first dielectric layer 13 is made of glass, ceramics, resin, or the like. From the viewpoint of weight reduction, resin is preferable. Specific examples of resin include polyethylene terephthalate (PET) resin, polycarbonate (PC) resin, and acrylic resin. On the other hand, from the viewpoint of scratch resistance, glass or ceramics is preferable.

[0023] When the first dielectric layer 13 is a glass substrate or a ceramic substrate, its thickness is, for example, 0.2 mm to 4.0 mm, preferably 0.3 mm to 2.5 mm, and more preferably 0.3 mm to 1.0 mm. On the other hand, when the first dielectric layer 13 is a resin substrate, its thickness is, for example, 0.4 mm to 5.0 mm, preferably 0.5 mm to 3.5 mm, and more preferably 0.5 mm to 1.5 mm.

[0024] When the first dielectric layer 13 is a glass substrate, the tan δ (also called the dielectric tangent) of the glass substrate is preferably 0.025 or less, more preferably 0.020 or less, and even more preferably 0.010 or less. The lower limit of the tan δ of the glass substrate is not particularly limited, but may be 0.0001 or more. The relative dielectric constant of the glass substrate is preferably 8 or less, and more preferably 6 or less. The lower limit of the relative dielectric constant of the glass substrate is not particularly limited, but may be 3.5 or more.

[0025] When the first dielectric layer 13 is a resin substrate, the tan δ of the resin substrate is preferably 0.2 or less, more preferably 0.1 or less. The lower limit of the tan δ of the resin substrate is not particularly limited, but may be 0.0005 or more. The relative dielectric constant of the resin substrate is preferably 5 or less, more preferably 4 or less. The lower limit of the relative dielectric constant of the resin substrate is not particularly limited, but may be 2 or more.

[0026] Here, tan δ is a value expressed as ε2 / ε1 using the complex dielectric constant, where ε1 is the relative dielectric constant and ε2 is the dielectric loss. The smaller the value of tan δ, the smaller the absorption of radio waves in that frequency band. Tan δ and relative dielectric constant are values ​​measured at a measurement frequency of 1 GHz according to the method specified in IEC (International Electrotechnical Commission) 61189-2-721 (2015).

[0027] The ratio (Ta / Tc) of the thickness (Ta) of the first dielectric layer 13 to the thickness (Tc) of the second dielectric layer 15 is, for example, 0.05 to 0.75. The first dielectric layer 13 is intended to protect the conductive layer 14, and may be thinner than the second dielectric layer 15.

[0028] The conductive layer 14 includes a plurality of conductors 14a that are spaced apart from one another and insulated from one another. The conductors 14a are, for example, transparent conductive films such as an ITO (indium tin oxide) film or a Low-E (low emissivity) film. The conductive layer 14 may be a so-called FSS (Frequency Selective Shield).

[0029] The conductive layer 14 reflects part of the radio wave, absorbs the other part, and transmits the remainder. The radio wave that transmits through the conductive layer 14 is reflected by the radio wave reflecting layer 16. The radio wave reflected by the conductive layer 14 and the radio wave reflected by the radio wave reflecting layer 16 interfere with each other and cancel each other out. As a result, the radio wave is absorbed.

[0030] When the radio wave reaches the conductive layer 14, free electrons in the conductor 14a move in the opposite direction to the electric field direction of the radio wave, and a current flows in the conductor 14a. At that time, energy is periodically stored and released in the gap between the conductors 14a due to the electric field generated. As a result, a propagation delay occurs in the radio wave passing through the conductive layer 14. That is, a delay time occurs from when the radio wave enters the conductive layer 14 until it is re-radiated. Therefore, the distance between the conductive layer 14 (resistive film) and the radio wave reflecting layer 16 (reflecting film) can be made smaller than λ / 4 to reduce the thickness of the radio wave absorber 1. The distance between the conductive layer 14 and the radio wave reflecting layer 16 is preferably 1.5 mm to 20 mm, more preferably 2 mm to 10 mm, and even more preferably 2 mm to 5 mm.

[0031] The thickness of the conductive layer 14 is not particularly limited, but when the conductive layer 14 is a Low-E (low emissivity) film, for example, the thickness is 50 nm to 300 nm. The conductive layer 14 may further include an insulating substrate 14b supporting the plurality of conductors 14a. The insulating substrate 14b is, for example, a resin film. The plurality of conductors 14a are formed on the surface of the insulating substrate 14b. For example, a transparent conductive film is formed on the surface of the insulating substrate 14b by a deposition method, a sputtering method, or the like, and the formed transparent conductive film is patterned by laser processing or the like. The plurality of conductors 14a may be formed by printing using a conductive ink. The thickness of the insulating base material 14b is not particularly limited, but is, for example, 50 μm to 200 μm.

[0032] When the conductive layer 14 includes the insulating base material 14b, the adhesive layer 18 is disposed both between the conductive layer 14 and the first dielectric layer 13 and between the conductive layer 14 and the second dielectric layer 15. On the other hand, when the conductive layer 14 does not include the insulating base material 14b, the adhesive layer 18 is disposed only either between the conductive layer 14 and the first dielectric layer 13 or between the conductive layer 14 and the second dielectric layer 15.

[0033] In the latter case, the conductive layer 14 is adjacent to either the first dielectric layer 13 or the second dielectric layer 15 and is formed directly on either one of them. For example, the conductive layer 14 is adjacent to the second dielectric layer 15 and is formed directly on the second dielectric layer 15 as shown in Fig. 3. Alternatively, the conductive layer 14 is adjacent to the first dielectric layer 13 and is formed directly on the first dielectric layer 13 as shown in Fig. 4. Since the insulating base material 14b is not necessary, the radio wave absorber 1 can be made thinner.

[0034] Next, an example of the slit pattern of the conductive layer 14 will be described with reference to Fig. 5. The conductive layer 14 is composed of a plurality of conductors 14a and gaps that space the plurality of conductors 14a from one another, and the plurality of conductors 14a are insulated from one another by the gaps. The gaps include a plurality of first slits 14c and a plurality of second slits 14d that are perpendicular to one another. The plurality of first slits 14c and the plurality of second slits 14d form a tetragonal lattice, and the conductor 14a is formed in a rectangular shape in a plan view. The rectangle includes a square.

[0035] The interval between adjacent first slits 14c is alternately set to a first interval L1 and a second interval L2 smaller than the first interval L1. Similarly, the interval between adjacent second slits 14d is alternately set to the first interval L1 and the second interval L2. As a result, four types of conductors 14a-1, 14a-2, 14a-3, and 14a-4 are formed in a plan view.

[0036] Conductor 14a-1 is a square with four sides each having a length of L1. Conductor 14a-2 is a square with four sides each having a length of L2. The remaining two conductors 14a-3 and 14a-4 have two opposing sides each having a length of L1 and the remaining two sides each having a length of L2.

[0037] The average outer perimeter of the four types of conductors 14a-1, 14a-2, 14a-3, and 14a-4 is expressed by the formula 2 × (L1 + L2). The average outer perimeter is, for example, 60 mm to 180 mm, and preferably 80 mm to 150 mm.

[0038] Although L1 and L2 are different in this embodiment, they may be the same. In this case, the average perimeter is also expressed by the formula 2×(L1+L2).

[0039] Next, with reference to FIG. 6, the relationship between the line width G1 of the first slit 14c and the line width G2 of the second slit 14d will be described.

[0040] The first slit 14c and the second slit 14d are formed by, for example, laser processing, as described above. The laser beam forms a spot SP on the surface of the conductive layer 14. The first slit 14c is formed by scanning the spot SP in a first direction (X-axis direction). The longitudinal direction of the first slit 14c is the X-axis direction. Moreover, the second slit 14d is formed by scanning the spot SP in a second direction (Y-axis direction) perpendicular to the first direction. The longitudinal direction of the second slit 14d is the Y-axis direction.

[0041] As shown in Fig. 6, the line width G1 of the first slit 14c coincides with the Y-axis dimension of the spot SP, and the line width G2 of the second slit 14d coincides with the X-axis dimension of the spot SP. When the spot SP is elliptical, G1 and G2 are different. When the spot SP is circular, G1 and G2 coincide. When G1 and G2 coincide, there is no need to distinguish between G1 and G2, so they are also simply referred to as G (see Fig. 5).

[0042] Table 1 shows the change in appearance of the radio wave absorber 1 between Example A in which the values ​​of the line widths G1 and G2 are 100 μm and Example B in which they are 4000 μm. The appearance was evaluated as ◯ and determined as pass when the first slits 14c and the second slits 14d of the conductive layer 14 and the conductor 14a could not be distinguished when the radio wave absorber 1 was visually observed from the first principal surface 11 side and from the second principal surface 12 side. On the other hand, the appearance was evaluated as × and determined as fail when the first slits 14c and the second slits 14d of the conductive layer 14 and the conductor 14a could be distinguished at least when the radio wave absorber 1 was visually observed from the first principal surface 11 side or from the second principal surface 12 side. [Table 1]

[0043] In Example A of Table 1, the radio wave absorber 1 was a five-layer laminate having a first dielectric layer 13, a conductive layer 14, a second dielectric layer 15, a radio wave reflecting layer 16, and a third dielectric layer 17 in this order. This laminate was arranged with the first main surface 11 standing vertically, the longitudinal direction of the first slit 14c was horizontal, and the longitudinal direction of the second slit 14d was vertical. The first dielectric layer 13 and the third dielectric layer 17 were both PC resin substrates (relative dielectric constant: 2.96, tan δ: 0.011) with a thickness of 2.0 mm. The second dielectric layer 15 was a glass substrate (relative dielectric constant: 7.0, tan δ: 0.02) with a thickness of 4.0 mm. The conductive layer 14 had L1 of 35 mm, L2 of 21 mm, G1 of 100 μm, G2 of 100 μm, and sheet resistance of 1.8 Ω / sq. The radio wave reflecting layer 16 was a perfect magnetic conductor (PEC).

[0044] In Example B of Table 1, the radio wave absorber 1 (manufactured by Shin-Nihon Radio Absorber Co., Ltd., product number "MS-009T") was a five-layer laminate having, in this order, a conductive layer 14 made of an insulating base material 14b and a conductor 14a, a second dielectric layer 15, and a radio wave reflection layer 16 made of a tile-like layer and a solid film-like layer. This laminate was arranged with the first main surface 11 standing vertically, the longitudinal direction of the first slit 14c was the horizontal direction, and the longitudinal direction of the second slit 14d was the vertical direction. The second dielectric layer 15 was an acrylic resin (relative dielectric constant: 2.7, tan δ: 0.04) with a thickness of 5.0 mm. The conductive layer 14 had L1 of 86 mm, L2 of 86 mm, G1 of 4000 μm, G2 of 4000 μm, and sheet resistance of 5.38 Ω / sq. In the radio wave reflecting layer 16, the sheet resistance of the tile-like layer was 3.21 Ω / sq., the sheet resistance of the solid film-like layer was 2.64 Ω / sq., and the sheet resistance of the entire radio wave reflecting layer 16 was 1.57 Ω / sq.

[0045] G1 and G2 are each independently preferably 30 μm to 500 μm, more preferably 50 μm to 200 μm, and further preferably 50 μm to 150 μm. When G1 and G2 are 30 μm or more, processing is easy. As can be seen from Table 1, when G1 and G2 are 500 μm or less, the distance between adjacent conductive layers 14 (14a-1, 14a-2, 14a-3, 14a-4) is suppressed, so that the slits are hardly visible, and therefore the radio wave absorber 1 having good transparency can be obtained. When G1 and G2 are 150 μm or less, the electric field generated between the conductors 14a is large, and the delay time is long. Therefore, the distance between the conductive layer 14 and the radio wave reflecting layer 16 can be reduced, and the radio wave absorber 1 can be made thin.

[0046] Table 2 shows the variation of frequency at which the amount of radio wave attenuation is minimum when G1 is fixed at 100 μm and G2 is varied. The amount of attenuation (unit: dB) is expressed as a negative value of the intensity of the reflected wave relative to the intensity of the incident wave. The smaller the amount of attenuation, the smaller the intensity of the reflected wave relative to the intensity of the incident wave, and the greater the absorption of the radio wave. For example, an attenuation of -10 dB means that the intensity of the reflected wave is 1 / 10 of the intensity of the incident wave. Also, an attenuation of -20 dB means that the intensity of the reflected wave is 1 / 100 of the intensity of the incident wave. The amount of radio wave attenuation was obtained by electromagnetic field simulation. Microwave Studio (registered trademark) by CST was used as the analysis software. The attenuation in Tables 3 to 8 was also obtained in the same manner.

[0047] [Table 2] In Table 2, the radio wave absorber 1 was a five-layer laminate having a first dielectric layer 13, a conductive layer 14, a second dielectric layer 15, a radio wave reflecting layer 16, and a third dielectric layer 17 in this order. This laminate was arranged with the first main surface 11 standing vertically, the longitudinal direction of the first slit 14c was horizontal, and the longitudinal direction of the second slit 14d was vertical. The first dielectric layer 13 and the third dielectric layer 17 were both PC resin substrates (relative dielectric constant: 2.96, tan δ: 0.011) with a thickness of 2.0 mm. The second dielectric layer 15 was a glass substrate (relative dielectric constant: 7.0, tan δ: 0.02) with a thickness of 4.0 mm. The conductive layer 14 had an L1 of 35 mm, an L2 of 21 mm, and a sheet resistance of 1.8 Ω / sq. The radio wave reflecting layer 16 was a perfect magnetic conductor (PEC: Perfect Magnetic Conductor).

[0048] As is clear from Table 2, if |G1-G2| / G1 is 30% or less, the frequency shift amount at which the attenuation amount is minimum can be suppressed to 5% or less. The frequency shift amount is expressed as a percentage from the frequency at which the attenuation amount is minimum when G1 and G2 are equal (910MHz in Table 2). |G1-G2| / G1 is the ratio of the absolute value |G1-G2| of the difference between the average line width G1 of the first slit 14c and the average line width G2 of the second slit 14d to the average line width G1 of the first slit 14c. |G1-G2| / G1 is preferably 20% or less. In addition, |G1-G2| / G1 is 0% or more.

[0049] The second dielectric layer 15 lengthens the delay time, further shortens the distance between the conductive layer 14 and the radio wave reflecting layer 16, and further thins the radio wave absorber 1. The thickness of the second dielectric layer 15 is, for example, 1.5 mm to 20 mm. If the thickness of the second dielectric layer 15 is within the above range, the distance between the conductive layer 14 and the radio wave reflecting layer 16 becomes an appropriate distance.

[0050] Different materials of the second dielectric layer 15 have different dielectric constants, and therefore different preferred thickness ranges. When the second dielectric layer 15 is a resin substrate, the thickness of the second dielectric layer 15 is, for example, 2.0 mm to 25 mm, preferably 3.0 mm to 20 mm, and more preferably 3.0 mm to 7.0 mm. On the other hand, when the second dielectric layer 15 is a glass substrate or a ceramic substrate, the thickness of the second dielectric layer 15 is, for example, 1.0 mm to 15 mm, preferably 1.5 mm to 10.0 mm, and more preferably 1.5 mm to 5.0 mm.

[0051] The material of the second dielectric layer 15 is glass, ceramics, or resin. From the viewpoint of weight reduction, resin is preferable. Specific examples of resin include PC resin and acrylic resin. On the other hand, from the viewpoint of water absorption, glass or ceramics are preferable. Furthermore, as will be described in detail later, from the viewpoint of the tolerance of the thickness of the second dielectric layer 15, resin is preferable. The tolerance is an allowable difference. The larger the tolerance, the more preferable.

[0052] When the second dielectric layer 15 is a glass substrate, the tan δ of the glass substrate is preferably 0.025 or less, more preferably 0.020 or less, and even more preferably 0.010 or less. The lower limit of the tan δ of the glass substrate is not particularly limited, but may be 0.0001 or more. The relative dielectric constant of the glass substrate is preferably 8 or less, and more preferably 6 or less. The lower limit of the relative dielectric constant of the glass substrate is not particularly limited, but may be 3.5 or more.

[0053] When the second dielectric layer 15 is a resin substrate, the tan δ of the resin substrate is preferably 0.2 or less, more preferably 0.1 or less. The lower limit of the tan δ of the resin substrate is not particularly limited, but may be 0.0005 or more. The relative dielectric constant of the resin substrate is preferably 5 or less, more preferably 4 or less. The lower limit of the relative dielectric constant of the resin substrate is not particularly limited, but may be 2 or more.

[0054] Table 3 shows the variation in the amount of attenuation of radio waves when the thickness of the PC resin substrate, which is the second dielectric layer 15, is varied. Here, the amount of attenuation of radio waves was obtained under the same conditions except for the thickness of the PC resin substrate, which is the second dielectric layer 15.

[0055] [Table 3] In Table 3, the radio wave absorber 1 was a five-layer laminate having a first dielectric layer 13, a conductive layer 14, a second dielectric layer 15, a radio wave reflecting layer 16, and a third dielectric layer 17 in this order. This laminate was arranged with the first main surface 11 standing vertically, the longitudinal direction of the first slit 14c was horizontal, and the longitudinal direction of the second slit 14d was vertical. Both the first dielectric layer 13 and the third dielectric layer 17 were PC resin substrates with a thickness of 2.0 mm. The conductive layer 14 had G of 100 μm, L1 of 34 mm, L2 of 26 mm, and sheet resistance of 10 Ω / sq. The radio wave reflecting layer 16 was a perfect magnetic conductor.

[0056] In Table 3, the case where the attenuation of the radio wave of 915MHz frequency is -20dB or less and the attenuation of the radio waves of the adjacent frequencies of 902MHz and 928MHz is -10dB or less is evaluated as pass. The other cases are evaluated as fail. As is clear from Table 3, when the second dielectric layer 15 is a PC resin board, the tolerance of the thickness of the second dielectric layer 15 for passing the evaluation is about 0.4mm.

[0057] Table 4 shows the variation in the amount of attenuation of radio waves when the thickness of the glass substrate, which is the second dielectric layer 15, is varied. Here, the amount of attenuation of radio waves was obtained under the same conditions except for the thickness of the glass substrate, which is the second dielectric layer 15.

[0058] [Table 4] In Table 4, the radio wave absorber 1 was a five-layer laminate having a first dielectric layer 13, a conductive layer 14, a second dielectric layer 15, a radio wave reflecting layer 16, and a third dielectric layer 17 in this order. This laminate was arranged with the first main surface 11 standing vertically, the longitudinal direction of the first slit 14c was horizontal, and the longitudinal direction of the second slit 14d was vertical. Both the first dielectric layer 13 and the third dielectric layer 17 were glass substrates with a thickness of 2.7 mm. The conductive layer 14 had G of 120 μm, L1 of 30 mm, L2 of 20 mm, and sheet resistance of 1.8 Ω / sq. The radio wave reflecting layer 16 was a perfect magnetic conductor.

[0059] In Table 4, similarly to Table 3, the case where the attenuation of the 915MHz frequency radio wave is -20dB or less and the attenuation of the adjacent frequencies of 902MHz and 928MHz is -10dB or less is evaluated as pass. The other cases are evaluated as fail. As is clear from Table 3, when the second dielectric layer 15 is a glass substrate, the tolerance of the thickness of the second dielectric layer 15 for the evaluation to pass is about 0.05mm.

[0060] From the viewpoint of the tolerance of the thickness of the second dielectric layer 15, from the viewpoint of the tolerance of the thickness of the second dielectric layer 15, it is preferable that the material of the second dielectric layer 15 is resin rather than glass. If the second dielectric layer 15 is made of resin, the tolerance of the thickness of the second dielectric layer 15 is large, and even if the thickness of the second dielectric layer 15 varies between production lots of the second dielectric layer 15, radio waves are sufficiently attenuated.

[0061] Next, an example of the relationship between the thickness of the second dielectric layer 15 and the sheet resistance of the conductor 14a of the conductive layer 14 will be described with reference to Fig. 7. In Fig. 7, the second dielectric layer 15 is a PC resin (relative dielectric constant: 2.96, tan δ: 0.011). When the thickness of the second dielectric layer 15 is x (mm) and the sheet resistance of the conductor 14a of the conductive layer 14 is y (Ω / sq.), it is preferable that the following formula (1) holds.

[0062]

number

[0063]

number

[0064] Table 5 shows the variation in the bandwidth where the attenuation is -10db or less when the sheet resistance of the radio wave reflective layer 16 is varied. Here, the attenuation of radio waves was obtained under the same conditions except for the sheet resistance of the radio wave reflective layer 16.

[0065] [Table 5] In Table 5, the radio wave absorber 1 was a laminate with an eight-layer structure having a first dielectric layer 13, an adhesive layer 18, a conductive layer 14, a second dielectric layer 15, an adhesive layer 18, a radio wave reflecting layer 16, an adhesive layer 18, and a third dielectric layer 17 in this order. This laminate was arranged with the first main surface 11 standing vertically, the longitudinal direction of the first slit 14c was the horizontal direction, and the longitudinal direction of the second slit 14d was the vertical direction. The first dielectric layer 13 and the third dielectric layer 17 were both glass substrates with a thickness of 2.75 mm. The second dielectric layer 15 was a glass substrate with a thickness of 3.7 mm. The conductive layer 14 had G of 120 μm, L1 of 30 mm, L2 of 20 mm, and sheet resistance of 1.8 Ω / sq. Each adhesive layer 18 was a COP film (relative dielectric constant: 2.33, tan δ: 0.00086) with a thickness of 0.5 mm.

[0066] As is clear from Table 5, in order to ensure a bandwidth of 26 MHz or more with an attenuation of -10 dB or less, the sheet resistance of the radio wave reflective layer 16 needs to be less than 3 Ω / sq. It is most preferable that the sheet resistance of the radio wave reflective layer 16 is about 1 Ω / sq.

[0067] The radio wave reflecting layer 16 includes, for example, a metal mesh. Since the metal mesh has openings, it can realize a higher visible light transmittance than a metal film. From the viewpoint of suppressing the reflection of visible light, the metal mesh is preferably one that has been subjected to an oxidation treatment and a blackening treatment. As the metal mesh, one made of stainless steel is generally used. If the mesh wire diameter is 0.01 mm or more, the handling during the production of the radio wave absorber 1 is easy. If the mesh wire diameter is 0.1 mm or less, the radio wave absorption properties are good. From the viewpoint of ensuring both the transparency of the radio wave absorber and the radio wave absorption properties, the mesh wire diameter is particularly preferably 0.01 mm to 0.08 mm, and further preferably 0.02 mm to 0.06 mm.

[0068] The radio wave reflecting layer 16 may also include a transparent conductive film. Examples of the transparent conductive film include an ITO film and a Low-E film (low emissivity film). The thickness of the radio wave reflecting layer is not particularly limited, but when a metal film such as a Low-E film is used for the radio wave reflecting layer 16, the thickness is, for example, 100 nm to 500 nm.

[0069] The radio wave reflecting layer 16 may further include an insulating substrate that supports the transparent conductive film. The insulating substrate is, for example, a resin film. For example, the transparent conductive film is formed on the surface of the insulating substrate by a deposition method or a sputtering method. The transparent conductive film may be formed by printing using a conductive ink.

[0070] When the radio wave reflecting layer 16 includes an insulating base material, an adhesive layer 18 is disposed both between the radio wave reflecting layer 16 and the third dielectric layer 17 and between the radio wave reflecting layer 16 and the second dielectric layer 15, as shown in Figures 2 and 3. On the other hand, when the radio wave reflecting layer 16 does not include an insulating base material, an adhesive layer 18 is disposed only either between the radio wave reflecting layer 16 and the third dielectric layer 17 or between the radio wave reflecting layer 16 and the second dielectric layer 15.

[0071] In the latter case, the radio wave reflecting layer 16 is adjacent to either the third dielectric layer 17 or the second dielectric layer 15 and is formed directly on either one of them. For example, as shown in Fig. 4, the radio wave reflecting layer 16 is adjacent to the third dielectric layer 17 and is formed directly on the third dielectric layer 17. Alternatively, although not shown, the radio wave reflecting layer 16 is adjacent to the second dielectric layer 15 and is formed directly on the second dielectric layer 15. Since an insulating substrate is not required, the radio wave absorber 1 can be made thinner.

[0072] Incidentally, there are Low-E films that include a tin oxide layer or an indium oxide layer, and those that include a silver (Ag) layer.

[0073] Although the Low-E film containing a tin oxide layer or an indium oxide layer has the advantage of being less susceptible to deterioration due to heat, its inherent emissivity is high (normal emissivity εn is about 0.15 or more), so little improvement in heat insulation and heat shielding properties cannot be expected, and heat shielding properties during fires cannot be expected either. In other words, a significant improvement in fire protection performance cannot be expected.

[0074] On the other hand, Low-E films that contain a silver layer have low emissivity (normal emissivity εn is approximately 0.1 or less) and excellent thermal insulation and heat shielding properties, so as long as the emissivity is maintained even in the event of a fire, they can effectively prevent damage to the glass substrate.

[0075] Therefore, it is preferable to use a low-E film containing a silver layer as a low-E film used as a heat-resistant low-E film. Since silver is easily oxidized by heating and its performance is easily deteriorated, a structure that can withstand heat, that is, a structure in which silver is not easily deteriorated by heat, is preferable. For example, the following structures (A) and (B) can be mentioned.

[0076] (A) The Low-E film includes an oxide dielectric layer, a barrier layer, a silver layer, a barrier layer, and an oxide dielectric layer in this order, and is formed directly on a glass substrate in this order. Each barrier layer is made of a metal, and the thickness of each barrier layer is 2 nm or more. By interposing a barrier layer between the silver layer and the oxide dielectric layer, the oxidation of silver can be suppressed, and the deterioration of heat ray reflection performance can be suppressed. In addition, if each barrier layer is made of a metal, and the thickness of each barrier layer is 2 nm or more, the heat resistance is good. The metal constituting the barrier layer is, for example, a zinc alloy (Zn alloy) or titanium (Ti).

[0077] When a metal barrier layer is provided using a normal Low-E film, the thickness of the barrier layer made of a Zn alloy is about 0.7 nm, and the thickness of the barrier layer made of Ti is about 1.5 nm.

[0078] The thicker the barrier layer (A), the better the heat resistance, but the lower the permeability. The thickness of the barrier layer (A) is determined taking into consideration the heat resistance and permeability.

[0079] (B) The Low-E film includes a non-oxide dielectric layer, a barrier layer, a silver layer, a barrier layer, and a non-oxide dielectric layer in this order, and is formed directly on the glass substrate. By sandwiching the silver layer between the non-oxide dielectric layers, it is possible to suppress oxidation of silver in a high-temperature environment, and it is possible to ensure high heat resistance while maintaining high heat ray reflection performance.

[0080] The barrier layer (B) is made of a Zn alloy or Ti, as with the barrier layer (A). This can further improve heat resistance. The barrier layer (B) may be made of the same material as the non-oxide dielectric layer.

[0081] The low-E film may include multiple silver layers. By forming multiple silver layers, the emissivity can be further reduced and εn can be significantly smaller than 0.1. For example, the low-E film includes an oxide dielectric layer, a barrier layer, a silver layer, a barrier layer, an oxide dielectric layer, a barrier layer, a silver layer, a barrier layer, and an oxide dielectric layer in this order, and is formed directly on the glass substrate in this order.

[0082] The third dielectric layer 17 protects the radio wave reflecting layer 16. The third dielectric layer 17 is made of glass, ceramics, resin, or the like. From the viewpoint of weight reduction, resin is preferable. Specific examples of resin include PET resin, PC resin, and acrylic resin. On the other hand, from the viewpoint of scratch resistance, glass or ceramics is preferable.

[0083] When the third dielectric layer 17 is a glass substrate or a ceramic substrate, its thickness is, for example, 0.2 mm to 4.0 mm, preferably 0.3 mm to 2.5 mm, and more preferably 0.3 mm to 1.0 mm. On the other hand, when the third dielectric layer 17 is a resin substrate, its thickness is, for example, 0.4 mm to 5.0 mm, preferably 0.5 mm to 3.5 mm, and more preferably 0.5 mm to 1.5 mm.

[0084] When the third dielectric layer 17 is a glass substrate, the tan δ of the glass substrate is preferably 0.025 or less, more preferably 0.020 or less, and even more preferably 0.010 or less. The lower limit of the tan δ of the glass substrate is not particularly limited, but may be 0.0001 or more. The relative dielectric constant of the glass substrate is preferably 8 or less, and more preferably 6 or less. The lower limit of the relative dielectric constant of the glass substrate is not particularly limited, but may be 3.5 or more.

[0085] When the third dielectric layer 17 is a resin substrate, the tan δ of the resin substrate is preferably 0.2 or less, more preferably 0.1 or less. The lower limit of the tan δ of the resin substrate is not particularly limited, but may be 0.0005 or more. The relative dielectric constant of the resin substrate is preferably 5 or less, more preferably 4 or less. The lower limit of the relative dielectric constant of the resin substrate is not particularly limited, but may be 2 or more.

[0086] The ratio (Te / Tc) of the thickness (Te) of the third dielectric layer 17 to the thickness (Tc) of the second dielectric layer 15 is, for example, 0.05 to 0.75. The third dielectric layer 17 is intended to protect the radio wave reflecting layer 16, and may be thinner than the second dielectric layer 15. The total thickness of the radio wave absorber 1 is, for example, 1.0 mm to 30 mm, more preferably 2.0 mm to 20 mm, further preferably 2.5 mm to 15 mm, and particularly preferably 2.5 mm to 12 mm. When the thickness of the radio wave absorbing layer 1 is within the above range, each of the first dielectric layer 13, the second dielectric layer 15, and the third dielectric layer 17 can have an appropriate thickness, and the radio wave absorber 1 can be made lightweight.

[0087] Adjacent layers are bonded to each other through the adhesive layer 18. The adhesive layer 18 is, for example, any one of a transparent optical adhesive (OCA), a polyvinyl butyral (PVB) resin, an ethylene vinyl acetate (EVA) resin, a cycloolefin polymer (COP) resin, and a thermoplastic polyurethane (TPU) resin.

[0088] The water absorption rate of the adhesive layer 18 is, for example, 3% by mass or less. The water absorption rate of the adhesive layer 18 is measured in accordance with Japanese Industrial Standard JIS K 7209:2000. If the water absorption rate of the adhesive layer 18 is 3% by mass or less, it is less likely to deteriorate even in a hot and humid usage environment. The water absorption rate of the PVB resin, EVA resin, COP resin, and TPU resin may all be 3% by mass or less. The water absorption rate of the adhesive layer 18 is preferably 1% by mass or less. In addition, the water absorption rate of the adhesive layer 18 is 0.01% by mass or more.

[0089] Increasing the dielectric constant of the adhesive layer 18 by dispersing fine particles made of a mineral with a high dielectric constant in the adhesive layer 18 is also effective in reducing the thickness of the entire wave absorber 1. In this case, it is preferable that the difference in dielectric constant between the adhesive layer 18 and each of the first dielectric layer 13, the second dielectric layer 15, and the third dielectric layer 17 is small in terms of suppressing the influence of reflection of radio waves between each member.

[0090] Furthermore, for example, when there is a large difference in the dielectric constant between the first dielectric layer 13 and the second dielectric layer 15, it is preferable that the adhesive layer 18 disposed between the first dielectric layer 13 and the second dielectric layer 15 has a dielectric constant intermediate between these two. Similarly, when there is a large difference in the dielectric constant between the second dielectric layer 15 and the third dielectric layer 17, it is preferable that the adhesive layer 18 disposed between the second dielectric layer 15 and the third dielectric layer 17 has a dielectric constant intermediate between these two.

[0091] Finally, Tables 6 to 8 show examples of the radio wave absorber 1 in which the attenuation of radio waves at a frequency of 915 MHz is −20 dB or less and the bandwidth in which the attenuation is −10 dB or less is 26 MHz or more.

[0092] [Table 6] In Table 6, the radio wave absorber 1 was a five-layer laminate having a first dielectric layer 13, a conductive layer 14, a second dielectric layer 15, a radio wave reflecting layer 16, and a third dielectric layer 17 in this order. This laminate was arranged with the first main surface 11 standing vertically, the longitudinal direction of the first slits 14c was the horizontal direction, and the longitudinal direction of the second slits 14d was the vertical direction. The first dielectric layer 13 and the third dielectric layer 17 were both PC resin substrates with a thickness of 2.0 mm. The radio wave reflecting layer 16 was a perfect magnetic conductor.

[0093] [Table 7] In Table 7, the radio wave absorber 1 was an eight-layer laminate having a first dielectric layer 13, a conductive layer 14, an adhesive layer 18, a second dielectric layer 15, an adhesive layer 18, a radio wave reflecting layer 16, an adhesive layer 18, and a third dielectric layer 17 in this order. This laminate was arranged with the first main surface 11 standing vertically, the longitudinal direction of the first slit 14c was the horizontal direction, and the longitudinal direction of the second slit 14d was the vertical direction. The first dielectric layer 13 and the third dielectric layer 17 were both glass substrates with a thickness of 2.75 mm. The radio wave reflecting layer 16 was a perfect magnetic conductor. Each adhesive layer 18 was a COP film with a thickness of 0.5 mm.

[0094] [Table 8] In Table 8, the radio wave absorber 1 was an eight-layer laminate having a first dielectric layer 13, an adhesive layer 18, a conductive layer 14, a second dielectric layer 15, an adhesive layer 18, a radio wave reflecting layer 16, an adhesive layer 18, and a third dielectric layer 17 in this order. This laminate was arranged with the first main surface 11 standing vertically, the longitudinal direction of the first slit 14c was the horizontal direction, and the longitudinal direction of the second slit 14d was the vertical direction. The first dielectric layer 13 was a glass substrate having a thickness of 2.75 mm, and the third dielectric layer 17 was a PC resin substrate having a thickness of 2.75 mm. The radio wave reflecting layer 16 was a perfect magnetic conductor. Each adhesive layer 18 was a COP film having a thickness of 0.5 mm. EXAMPLES

[0095] The experimental data will be described below. Examples 1 and 2 are both examples of the present invention.

[0096] [Example 1] In Example 1, an eight-layer radio wave absorber was produced, including a first dielectric layer, an adhesive layer, a conductive layer, a second dielectric layer, an adhesive layer, a radio wave reflecting layer, an adhesive layer, and a third dielectric layer, in that order, as shown in Table 9.

[0097] [Table 9] The first and third dielectric layers were soda lime glass with a thickness of 2.75 mm. The second dielectric layer was soda lime glass with a thickness of 3.7 mm. The conductive layer was directly formed on one side of the second dielectric layer. The conductive layer was a laminated film containing silver oxide with a sheet resistance of 1.8 Ω / sq. The conductive layer was patterned using laser processing. The conductive layer had L1 of 34 mm, L2 of 24 mm, and G1 and G2 of 110 μm. The radio wave reflection layer was a metal mesh (sheet resistance: 0.01 Ω / sq.) woven with stainless steel fibers with a line width of 30 μm. The adhesive layer was a COP film with a relative dielectric constant of 2.33 and a thickness of 1000 μm. The adhesion using the COP film was performed at a temperature of 130°C, a pressurized pressure of 1 MPa, and a pressurized time of 90 minutes. The radio wave absorber had a first main surface in the shape of a rectangle measuring 400 mm in length and 900 mm in width, a total thickness of 12.2 mm, and a transmittance of 62%.

[0098] The amount of attenuation of radio waves by the radio wave absorber produced in Example 1 was measured as follows. As shown in FIG. 8, radio waves are incident on the first main surface 11 of the radio wave absorber 1 from the transmission antenna 21 at an incident angle θ i = 30°. And the reflection angle θ rThe radio waves reflected at an angle of 0° to 30° were received by the receiving antenna 22. The transmitting antenna 21 and the receiving antenna 22 were arranged symmetrically with respect to the normal to the first main surface 11 of the radio wave absorber 1, and were installed at a position 1000 mm away from the first main surface 11. The first main surface 11 of the radio wave absorber 1 was set up vertically. The attenuation was calculated as the attenuation of the intensity of the received radio waves relative to the intensity of the transmitted radio waves. The radio wave frequencies were 800 MHz to 1000 MHz. The attenuation was measured in an anechoic chamber to reduce the influence of unnecessary radio waves other than those from the transmitting antenna 21.

[0099] FIG. 9 shows the measurement results of the amount of radio wave attenuation by the radio wave absorber produced in Example 1. For comparison, FIG. 9 also shows the measurement results of the amount of radio wave attenuation by a stainless steel plate (SUS plate), which is a perfect reflector. In FIG. 9, the attenuation by the SUS plate is negative due to the effects of radio wave diffraction and leakage. FIG. 10 shows the ratio of the amount of radio wave attenuation by the radio wave absorber produced in Example 1 to the amount of radio wave attenuation by the SUS plate. As is clear from FIGS. 9 and 10, it was confirmed that the radio wave absorber produced in Example 1 has high absorption properties at frequencies around 880 MHz. In addition, the distance between the conductive layer and the radio wave reflecting layer can be made smaller than λ / 4, which is preferable in terms of weight reduction, etc.

[0100] [Example 2] In Example 2, an eight-layer radio wave absorber was produced, including a first dielectric layer, a conductive layer, an adhesive layer, a second dielectric layer, an adhesive layer, a radio wave reflecting layer, an adhesive layer, and a third dielectric layer, in that order, as shown in Table 10.

[0101] [Table 10] The first and third dielectric layers were soda lime glass with a thickness of 2.75 mm. The second dielectric layer was a PC (polycarbonate) substrate with a thickness of 3.0 mm. The conductive layer was directly formed on one side of the first dielectric layer. The conductive layer was a laminated film containing silver oxide with a sheet resistance value of 1.8 Ω / sq. The conductive layer was patterned using laser processing. The conductive layer had L1 of 40 mm, L2 of 28 mm, and G1 and G2 of 100 μm. The radio wave reflection layer was a metal mesh (sheet resistance value: 0.01 Ω / sq.) woven with stainless steel fibers with a line width of 30 μm. The adhesive layer was a COP film with a relative dielectric constant of 2.33 and a thickness of 380 μm. The adhesion using the COP film was performed at a temperature of 130°C, a pressurized pressure of 1 MPa, and a pressurized time of 90 minutes. The radio wave absorber had a first main surface in a square shape measuring 300 mm in length and 300 mm in width, a total thickness of 9.7 mm, and a transmittance of 64%.

[0102] The attenuation of radio waves by the radio wave absorber produced in Example 2 was measured in the same manner as the attenuation of radio waves by the radio wave absorber produced in Example 1. The measurement results are shown in FIG. 11. For comparison, FIG. 11 also shows the measurement results of the attenuation of radio waves by a SUS plate, which is a perfect reflector. In FIG. 11, the attenuation of the SUS plate is negative due to the influence of diffraction and leakage of radio waves. FIG. 12 shows the ratio of the attenuation of radio waves by the radio wave absorber produced in Example 2 to the attenuation of radio waves by the SUS plate. As is clear from FIG. 11 and FIG. 12, it was confirmed that the radio wave absorber produced in Example 2 has high absorption properties around a frequency of 924 MHz. In addition, the distance between the conductive layer and the radio wave reflecting layer can be made smaller than λ / 4, which is preferable in terms of weight reduction and the like.

[0103] Although the radio wave absorber according to the present disclosure has been described above, the present disclosure is not limited to the above-mentioned embodiments. Various changes, modifications, substitutions, additions, deletions, and combinations are possible within the scope of the claims. These naturally fall within the technical scope of the present disclosure.

[0104] For example, the frequency of radio waves absorbed by the radio wave absorber 1 is not limited to 913 MHz for RFID (Radio Frequency IDentification). The frequency of radio waves absorbed by the radio wave absorber 1 is, for example, 0.5 GHz to 2 GHz, and preferably 0.8 GHz to 1.2 GHz. For example, the frequency of radio waves from a mobile phone is 1 GHz. [Explanation of symbols]

[0105] 1. Radio wave absorber 11 First main surface 12 Second main surface 13 First dielectric layer 14 Conductive layer 14a Conductor 15 Second dielectric layer 16 Radio wave reflective layer 17 Third dielectric layer

[0106] In addition, the entire contents of the specification, claims, drawings and abstract of Japanese Patent Application No. 2020-140958 filed on August 24, 2020 are hereby incorporated by reference as the disclosure of the specification of the present invention.

Claims

1. A radio wave absorber having a laminated structure, 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 wave absorber has a first principal surface onto which the wave is incident and a second principal surface opposite to the first principal surface, the insulating substrate includes, in this order from the first principal surface to the second principal surface, a first dielectric layer, a conductive layer, a second dielectric layer, a radio wave reflecting layer, and a third dielectric layer; the conductive layer includes a plurality of conductors and gaps that space the plurality of conductors from one another, and the plurality of conductors are insulated from one another; at least one of the first dielectric layer, the second dielectric layer, and the third dielectric layer is glass; The radio wave absorber, wherein the glass has a relative dielectric constant of 3.5 or more.

2. The radio wave absorber according to claim 1 , further comprising an adhesive layer between the first principal surface and the second principal surface.

3. 3. The radio wave absorber according to claim 2, wherein the adhesive layer is any one of a transparent optical adhesive (OCA), a polyvinyl butyral (PVB) resin, an ethylene vinyl acetate (EVA) resin, a cycloolefin polymer (COP) resin, and a thermoplastic polyurethane (TPU) resin.

4. 4. The radio wave absorber according to claim 2, wherein the adhesive layer has a water absorption rate of 3 mass % or less.

5. The radio wave absorber according to any one of claims 1 to 4, wherein the conductive layer is adjacent to either the first dielectric layer or the second dielectric layer and is formed directly on either the first dielectric layer or the second dielectric layer.

6. The radio wave absorber according to any one of claims 1 to 5, wherein the radio wave reflecting layer is adjacent to either the second dielectric layer or the third dielectric layer and is formed directly on either the second dielectric layer or the third dielectric layer.

7. The radio wave absorber according to any one of claims 1 to 6, wherein the radio wave reflective layer has electrical conductivity.

8. The radio wave absorber according to claim 7 , wherein the radio wave reflective layer includes a transparent conductive film.

9. The radio wave absorber according to claim 7 , wherein the radio wave reflective layer includes a metal mesh.

10. An electromagnetic wave absorber as described in claim 9, wherein the wire diameter of the metal mesh is 0.01 mm or more and 0.1 mm or less.

11. The radio wave absorber according to any one of claims 7 to 10, wherein the sheet resistance of the radio wave reflective layer is smaller than 3 Ω / sq.

12. An electromagnetic wave absorber described in any one of claims 1 to 11, wherein the second dielectric layer is glass, ceramic or resin.

13. The radio wave absorber according to claim 12 , wherein the second dielectric layer is made of polycarbonate (PC) resin or acrylic resin.

14. 14. The radio wave absorber according to claim 1, wherein the line width of the gap separating the plurality of conductors in the conductive layer is 30 μm to 500 μm.

15. the conductive layer includes a plurality of first slits and a plurality of second slits that are perpendicular to each other as the gaps that separate the plurality of conductors; A longitudinal direction of the first slit is a horizontal direction, and a longitudinal direction of the second slit is a vertical direction, 15. The radio wave absorber according to claim 1, wherein a ratio of an absolute value of a difference between an average value of the line widths of the first slits and an average value of the line widths of the second slits to an average value of the line widths of the first slits is 30% or less.

16. 16. The radio wave absorber according to claim 1, wherein a ratio (Ta / Tc) of a thickness (Ta) of the first dielectric layer to a thickness (Tc) of the second dielectric layer is 0.05 to 0.

75.

17. 17. The radio wave absorber according to claim 1, wherein the frequency of the radio waves is 0.5 GHz to 2 GHz.

18. The radio wave absorber according to any one of claims 1 to 17, wherein the second dielectric layer has a thickness of 1.5 mm to 20 mm.

19. When the thickness of the second dielectric layer is x (mm) and the sheet resistance of the conductor of the conductive layer is y (Ω / sq.), 19. The radio wave absorber according to claim 1, wherein the following formula (1) is satisfied: [0010]

20. The radio wave absorber according to any one of claims 1 to 19, wherein the conductive layer further comprises an insulating substrate supporting the plurality of conductors.

21. An electromagnetic wave absorber according to any one of claims 1 to 20, wherein the bandwidth in which the attenuation is -10 db or less is 26 MHz or more.

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