JP2022047398A - Radio wave absorption sheet and communication device - Google Patents

Abstract

To provide a radio wave absorption sheet that limits an absorption band by the dielectric loss tangent of a dielectric layer, and makes it easy to expand the absorption band.SOLUTION: A radio wave absorption sheet 10 includes a first dielectric layer 20 including a first surface 21 and a second surface 22 located on the opposite side of the first surface, a plurality of first conductor layers 30 located on the first surface and arranged in the first direction, and a second conductor layer 40 located on the second surface, and the plurality of first conductor layers include a plurality of first shape layers 31, and a plurality of second shape layers 32 having different shapes from the first shape layer in a plan view.SELECTED DRAWING: Figure 2

Description

The embodiments of the present disclosure relate to a radio wave absorbing sheet and a communication device.

In order to suppress the influence of radio waves on electronic devices, for example, as disclosed in Patent Document 1, a radio wave absorbing sheet that absorbs radio waves is used. For example, Patent Document 1 proposes a radio wave absorbing sheet that includes a dielectric layer, a conductive film layer, and a reflective layer, has a thickness of 1005 μm to 1300 μm, and has a peak frequency of 6.4 GHz.

Japanese Patent No. 6063631

In the conventional radio wave absorption sheet, the absorption band may be limited by the dielectric loss tangent of the dielectric layer.

The embodiment of the present disclosure has been made in consideration of such a point, and an object of the present invention is to provide a radio wave absorption sheet in which an absorption band can be easily expanded.

One embodiment of the present disclosure is a radio wave absorbing sheet.
A first dielectric layer including a first surface and a second surface located on the opposite side of the first surface,
A plurality of first conductor layers located on the first surface and arranged in the first direction,
A second conductor layer located on the second surface is provided.
The plurality of first conductor layers are a radio wave absorbing sheet including a plurality of first shape layers and a plurality of second shape layers having a shape different from that of the first shape layer in a plan view.

In the radio wave absorbing sheet according to the embodiment of the present disclosure, the shape of the second shape layer may be similar to the shape of the first shape layer.

In the radio wave absorbing sheet according to the embodiment of the present disclosure, the area of the second shape layer may be smaller than the area of the first shape layer.

In the radio wave absorbing sheet according to the embodiment of the present disclosure, the following equation is established between the area Sa of the first shape layer and the area Sb of the second shape layer.
Sa / Sb ≦ 1 + 8 × tanδ
tan δ may be a dielectric loss tangent of the first dielectric layer.

In the radio wave absorbing sheet according to the embodiment of the present disclosure, the following equation is established between the area Sa of the first shape layer and the area Sb of the second shape layer.
Sa / Sb ≧ 1 + 1 × tanδ
tan δ may be a dielectric loss tangent of the first dielectric layer.

In the radio wave absorbing sheet according to the embodiment of the present disclosure, the spectra showing the absorption characteristics of the radio wave absorbing sheet are the first peak appearing at the first frequency fr1 corresponding to the first shape layer and the second shape layer. Correspondingly, the second peak appearing at the second frequency fr2 and the second peak are included.
The -3 dB band BW of the spectrum satisfies the following equation.
BW> 2 x fr (ave) x tanδ
fr (ave) = (fr1 + fr2) / 2
tan δ may be a dielectric loss tangent of the first dielectric layer.

The radio wave absorbing sheet according to the embodiment of the present disclosure may be located on the first surface side and may include an overcoat layer that covers the first conductor layer.

The radio wave absorbing sheet according to the embodiment of the present disclosure may include an adhesive layer located on the second conductor layer.

In the radio wave absorbing sheet according to the embodiment of the present disclosure, the occupancy rate of the first conductor layer on the first surface may be 0.85 or less.

In the radio wave absorbing sheet according to the embodiment of the present disclosure, the dimension of the first conductor layer in the first direction may be less than 5.0 mm.

In the radio wave absorbing sheet according to the embodiment of the present disclosure, the thickness of the first dielectric layer may be 300 μm or less.

In the radio wave absorbing sheet according to the embodiment of the present disclosure, the relative permittivity of the first dielectric layer may be less than 20.

In the radio wave absorbing sheet according to the embodiment of the present disclosure, the dielectric loss tangent of the first dielectric layer may be 0.2 or less.

In the radio wave absorbing sheet according to the embodiment of the present disclosure, the thickness of the radio wave absorbing sheet may be 350 μm or less.

In the radio wave absorbing sheet according to the embodiment of the present disclosure, the spectra showing the absorption characteristics of the radio wave absorbing sheet are the first peak appearing at the first frequency fr1 corresponding to the first shape layer and the second shape layer. Correspondingly, the second peak appearing at the second frequency fr2 and the second peak are included.
Both the first frequency fr1 and the second frequency fr2 may be 20 GHz or more and 110 GHz or less.

One embodiment of the present disclosure is
A communication mechanism that transmits or receives radio waves,
A communication device including the above-mentioned radio wave absorbing sheet.

According to the embodiment of the present disclosure, it is possible to provide a radio wave absorption sheet that can easily expand the absorption band.

It is a top view which shows one Embodiment of a radio wave absorption sheet. It is sectional drawing which shows the case where the radio wave absorption sheet of FIG. 1 is seen from the direction of II-II. It is a top view which shows the radio wave absorption sheet by the form of 1st comparison. It is a figure which shows an example of the spectrum which shows the absorption characteristic of the radio wave absorption sheet of FIG. It is a figure which shows an example of the spectrum which shows the absorption characteristic of the radio wave absorption sheet of FIG. It is a figure which shows the unit cell which constitutes the radio wave absorption sheet of FIG. It is a figure which shows the equivalent circuit of a radio wave absorption sheet. It is sectional drawing which shows an example of the manufacturing method of a radio wave absorption sheet. It is sectional drawing which shows an example of the manufacturing method of a radio wave absorption sheet. It is a figure which shows an example of the cross-sectional shape of the 1st conductor layer of FIG. 9A. It is a figure which shows an example of the cross-sectional shape of the 1st conductor layer of FIG. 9A. It is sectional drawing which shows an example of the communication apparatus provided with the electric wave absorption sheet. It is a top view which shows one Embodiment of a radio wave absorption sheet. It is a top view which shows the radio wave absorption sheet by the 2nd comparative form. It is a top view which shows one Embodiment of a radio wave absorption sheet. It is a top view which shows one Embodiment of a radio wave absorption sheet. It is a figure which shows an example of the spectrum which shows the absorption characteristic of the radio wave absorption sheet of FIG. It is sectional drawing which shows one Embodiment of a radio wave absorption sheet. It is sectional drawing which shows one Embodiment of a radio wave absorption sheet. It is sectional drawing which shows one Embodiment of a radio wave absorption sheet. It is a figure which shows the absorption characteristic of the radio wave absorption sheet of Example 1 and Example 2. FIG. It is a figure which shows the absorption characteristic of the radio wave absorption sheet of Example 3 and Example 4. FIG. It is a figure which shows the absorption characteristic of the radio wave absorption sheet of Example 5 and Example 6. It is a figure which shows the absorption characteristic of the radio wave absorption sheet of Example 7 and Example 8. It is a figure which shows the absorption characteristic of the radio wave absorption sheet of Example 9.

Hereinafter, the configuration of the radio wave absorbing sheet 10 according to the embodiment of the present disclosure will be described in detail with reference to the drawings. It should be noted that the embodiments shown below are examples of the embodiments of the present disclosure, and the present disclosure is not construed as being limited to these embodiments. Further, in the present specification, terms such as "board", "base material", "sheet" and "film" are not distinguished from each other based only on the difference in names. For example, "base material" and "base material" are concepts including members that can be called sheets or films. Furthermore, the terms used herein, such as "parallel" and "orthogonal", and the values of length and angle, which specify the shape and geometric conditions and their degrees, are bound by a strict meaning. Interpret without including the range in which similar functions can be expected.

In the present specification, when a plurality of candidates for an upper limit value and a plurality of candidates for a lower limit value are listed for a certain parameter, the numerical range of the parameter is any one candidate for the upper limit value and any one lower limit value. It may be configured by combining with the candidates of. For example, "Parameter B may be, for example, A1 or more, A2 or more, or A3 or more. Parameter B may be, for example, A4 or less, A5 or less, or A6 or less. It may be. " In this case, the numerical range of the parameter B may be A1 or more and A4 or less, A1 or more and A5 or less, A1 or more and A6 or less, or A2 or more and A4 or less. It may be A2 or more and A5 or less, A2 or more and A6 or less, A3 or more and A4 or less, A3 or more and A5 or less, or A3 or more and A6 or less.

In the drawings referred to in the present embodiment, the same parts or parts having similar functions are designated by the same reference numerals or similar reference numerals, and the repeated description thereof may be omitted. Further, the dimensional ratio of the drawing may differ from the actual ratio for convenience of explanation, or a part of the configuration may be omitted from the drawing.

Hereinafter, embodiments of the present disclosure will be described. First, the background will be described.

In recent years, radio waves in a frequency band of about 30 GHz or more and 300 GHz or less, which are called millimeter waves, have begun to be used in various fields. Examples in the field are 5th generation mobile communication systems, mobiles, radars for collision prevention systems for automobiles, medical biosensing, and the like. On the other hand, as the use of radio waves increases, there is concern that problems such as malfunction of electronic devices and deterioration of the communication environment will occur.

A radio wave absorbing sheet is known as one of the products for solving these problems.
The radio wave absorption sheet can absorb noise and unnecessary radio waves emitted from electronic devices. By using the radio wave absorbing sheet, effects such as prevention of malfunction of electronic devices and improvement of communication environment can be expected.

On the other hand, in the conventional radio wave absorption sheet, the absorption band may be limited by the dielectric loss tangent of the dielectric layer. For example, in a radio wave absorption sheet of a type that absorbs radio waves by utilizing resonance, the -3 dB band of the spectrum showing absorption characteristics is about 2 × fr × tan δ. fr is the resonance frequency of the radio wave absorbing sheet. tan δ is the dielectric loss tangent of the dielectric layer. In the present application, the -3 dB band is the width of the frequency band in which the amount of radio waves reflected from the radio wave absorption sheet is -3 dB or less.

In consideration of such a background, in the present embodiment, we propose a radio wave absorption sheet that can easily expand the absorption band and can correspond to the frequency band of the millimeter wave band.

FIG. 1 is a plan view showing an embodiment of the radio wave absorbing sheet 10. FIG. 2 is a cross-sectional view showing a case where the radio wave absorbing sheet 10 of FIG. 1 is viewed from the II-II direction. The radio wave absorbing sheet 10 includes a first dielectric layer 20 including a second surface 22 located on the opposite side of the first surface 21 and the first surface 21, and a plurality of first conductor layers 30 located on the first surface 21. And a second conductor layer 40 located on the second surface 22.

The radio wave absorbing sheet 10 absorbs radio waves incident on the radio wave absorbing sheet 10 from the first surface 21 side by utilizing resonance. By utilizing resonance, the thickness of the radio wave absorbing sheet 10 can be made smaller than that of other types of radio wave absorbing sheets.

The frequency of the radio wave targeted by the radio wave absorbing sheet 10 is, for example, 20 GHz or more. The frequency of the radio wave targeted by the radio wave absorbing sheet 10 is, for example, 110 GHz or less, and may be 80 GHz or less. Such radio waves are used in 5th generation mobile communication systems, radar devices for automobiles, and the like.

Each component of the radio wave absorption sheet 10 will be described.

The first dielectric layer 20 contains a material having an insulating property. The material having an insulating property may be an organic material, an inorganic material, or a combination of both.

Examples of the inorganic material include silicon oxide (SiO ₂ ), silicon nitride (Si _{3N 4 )} , silicon oxynitride (SiON), aluminum oxide (Al ₂ O ₃ ), aluminum nitride (AlN), silicon carbide (SiC), and the like. Silicon nitride carbide (SiCN), carbon-added silicon oxide (SiCO), borosilicate glass, quartz glass and the like can be used.

Organic materials include polyimide, epoxy resin, benzocyclobutene resin, polyamide, phenol resin, fluororesin, liquid crystal polymer, polyamideimide, polybenzoxazole, cyanate resin, aramid resin, polyolefin, polyester, BT resin, polyacetal, and polybutylene. Telephthalate, syndiotactic polystyrene, polyphenylene sulfide, polyether ether ketone, polyether nitrile, polycarbonate, polyphenylene ether polysulfone, polyether sulfone, polyarylate, polyetherimide and the like can be used. Further, fibers, fillers and the like may be included in the layer of these organic materials. The material of the fiber and the filler may be an insulating material such as glass, talc, mica, silicon oxide, aluminum oxide, titanium oxide, or a conductive material such as carbon or metal.

As the material of the first dielectric layer 20, a compound or a mixture containing both an organic material and an inorganic material may be used. Examples of materials are silicone resin, glass epoxy and the like.

The first dielectric layer 20 may have a heat resistance of about 150 ° C. For example, the glass transition point or the deflection temperature under load of the material of the first dielectric layer 20 may be 150 ° C. or higher. As a result, when the radio wave absorbing sheet 10 is installed in an electronic device including a component whose temperature rises to about 150 ° C., the radio wave absorbing sheet 10 can function appropriately.

The relative permittivity ε _r of the first dielectric layer 20 may be less than 20.0, may be 15.0 or less, may be 10.0 or less, and may be 5.0 or less. You may. By utilizing resonance, radio waves can be absorbed even when the relative permittivity _εr of the first dielectric layer 20 is lower than that of the conventional radio wave absorber. The relative permittivity ε _r of the first dielectric layer 20 may be 2.0 or more, 3.0 or more, or 5.0 or more.

The dielectric loss tangent tan δ of the first dielectric layer 20 may be 0.20 or less, 0.15 or less, 0.10 or less, or 0.05 or less. good.

In general, the relative permittivity and dielectric loss tangent of a dielectric layer are increased by the dielectric layer containing a conductive filler at a high density. On the other hand, when the density of the filler is high, there may be a problem that the dielectric layer becomes brittle. Further, when the density of the filler is increased, the distribution of the filler becomes non-uniform, so that voids may be generated in the dielectric layer or the characteristics of the dielectric layer may be varied.

In the present embodiment, since resonance is used, good radio wave absorption performance can be obtained even when the relative permittivity _εr and the dielectric loss tangent tan δ of the first dielectric layer 20 are limited as described above. Can be expressed. Therefore, it is possible to avoid the problem caused by the high density of the filler.

The thickness h may be, for example, 300 μm or less, 250 μm or less, 200 μm or less, 150 μm or less, or 100 μm or less. By utilizing resonance, radio waves can be absorbed even when the thickness h of the first dielectric layer 20 is smaller than that of the conventional radio wave absorber.

The first dielectric layer 20 may be composed of a single layer or may be composed of a plurality of layers.

As a method for measuring the relative permittivity ε _r and the dielectric loss tangent tan δ of the first dielectric layer 20, an open resonator method can be used. Measurement systems that perform the open resonator method include a network analyzer, a millimeter wave doubler, a millimeter wave detector, and a Fabry-Perot resonator. As the Fabry-Perot resonator, FPR-40, FPR-50, FPR-60, FPR-75, PFR-90, FPR-110 and the like of Keycom Co., Ltd. can be used depending on the frequency. As a measuring instrument for measuring the thickness of the first dielectric layer 20, a length measuring machine can be used, and for example, a Digimicro manufactured by Nikon Corporation can be used. If the thickness cannot be measured by the length measuring machine, the thickness of the first dielectric layer 20 may be calculated based on the image of the cross section of the sample of the radio wave absorbing sheet 10. A scanning electron microscope can be used as a measuring instrument for measuring an image.

Although not shown, an insulating layer such as an adhesive layer may be present between the first surface 21 of the first dielectric layer 20 and the first conductor layer 30. Similarly, an insulating layer such as an adhesive layer may exist between the second surface 22 of the first dielectric layer 20 and the second conductor layer 40.

Although not shown, the interface between the first surface 21 of the first dielectric layer 20 and the first conductor layer 30 may be roughened. For example, the first surface 21 may include irregularities. This makes it possible to improve the adhesion between the first surface 21 and the first conductor layer 30. Rough surface treatment includes wet etching, blasting, polishing, plating and the like. Similarly, the interface between the second surface 22 of the first dielectric layer 20 and the second conductor layer 40 may be roughened. For example, the second surface 22 may include irregularities.

The first conductor layer 30 will be described. The first conductor layer 30 contains a material having conductivity. For example, the first conductor layer 30 includes copper (Cu), gold (Au), silver (Ag), platinum (Pt), palladium (Pd), rhodium (Rh), tin (Sn), aluminum (Al), and the like. It includes metals such as nickel (Ni), chromium (Cr), titanium (Ti), molybdenum (Mo), tungsten (W), and tantalum (Ta), or alloys using these. The first conductor layer 30 may contain an organic conductor such as a conductive polymer, a conductive fiber such as a carbon nanotube (CNT), or the like.

The thickness T1 of the first conductor layer 30 may be 0.1 μm or more, 1 μm or more, or 5 μm or more. The thickness of the first conductor layer 30 may be 50 μm or less, or 20 μm or less.

As shown in FIG. 1, the plurality of first conductor layers 30 may be arranged in the in-plane direction of the first dielectric layer 20. For example, the plurality of first conductor layers 30 may be arranged in the first direction D1 in the first period P1. The plurality of first conductor layers 30 may be arranged in the second direction D2 in the second period P2. The second direction D2 is a direction different from the first direction D1, for example, a direction orthogonal to the first direction D1. The first period P1 is the distance between the center points C0 of two adjacent first conductor layers 30 in the first direction D1. The second period P2 is the distance between the center points C0 of the two adjacent first conductor layers 30 in the second direction D2. The first period P1 and the second period P2 may be the same or different. The cycles P1 and P2 are, for example, larger than 1.5 mm and may be 2.0 mm or more, or may be 2.5 mm or more.

The first conductor layer 30 has a plurality of shapes. For example, the plurality of first conductor layers 30 include a plurality of first shape layers 31 and a plurality of second shape layers 32 having a shape different from that of the first shape layer 31 in a plan view. In the example shown in FIG. 1, both the first shape layer 31 and the second shape layer 32 have a circular shape. That is, the shape of the second shape layer 32 is similar to the shape of the first shape layer 31. "Plane view" means to see the radio wave absorbing sheet 10 along the direction orthogonal to the in-plane direction of the first dielectric layer 20.

As shown in FIG. 1, the plurality of first shape layers 31 may be arranged in the first direction D1 in the first period P1. The plurality of second shape layers 32 may also be arranged in the first direction D1 in the first period P1. As shown in FIG. 1, in the second direction D2, the first shape layer 31 and the second shape layer 32 may be alternately arranged in the second cycle P2.

The first shape layer 31 has a radius r1. The second shape layer 32 has a radius r2 smaller than the radius r1. Therefore, the area Sb of the second shape layer 32 is smaller than the area Sa of the first shape layer 31.

The advantage that the first conductor layer 30 has a plurality of shapes will be described based on the comparison with the first comparative form. FIG. 3 is a plan view showing the radio wave absorbing sheet 210 according to the first comparative embodiment. The radio wave absorbing sheet 210 includes a plurality of first conductor layers 230 having a radius r. The plurality of first conductor layers 230 are arranged in the first direction D1 in the first period P1 and in the second direction D2 in the second period P2.

ｃは、光速である。ｒ_ｅｆｆは、第１導電体層２３０の端部におけるフリンジング効果を考慮した場合の、第１導電体層２３０の実効半径である。 FIG. 4 is a diagram showing an example of a spectrum showing the absorption characteristics of the radio wave absorption sheet 210 of FIG. In the spectrum of the present application, the vertical axis d represents a value obtained by multiplying the common logarithm of the ratio of the power E2 of the radio wave reflected by the radio wave absorbing sheet to the power E1 of the radio wave incident on the radio wave absorbing sheet. That is, d = 10 × log (E2 / E1). Assuming that all the radio waves incident on the radio wave absorption sheet are reflected, the spectrum represented by the vertical axis d corresponds to the absorption spectrum showing the absorption characteristics of the radio wave absorption sheet. Further, the peak of the spectrum represented by the vertical axis d corresponds to the absorption peak.
When the radio wave is absorbed by using the resonance, the frequency of the peak of the spectrum matches the resonance frequency fr of the radio wave absorbing sheet 210. The -3dB band BW of the spectrum is about 2 × fr × tan δ as described above. tan δ is the dielectric loss tangent of the first dielectric layer 20. The resonance frequency fr is calculated based on the following equations (A1) and (A2).

c is the speed of light. r _eff is the effective radius of the first conductor layer 230 when the fringe effect at the end of the first conductor layer 230 is taken into consideration.

As shown in FIG. 4, in the first comparative form, only one peak appears in the spectrum and the -3 dB band BW is narrow.

According to the present embodiment, the -3 dB band BW can be expanded as compared with the case of the first comparative embodiment. FIG. 5 is a diagram showing an example of a spectrum showing the absorption characteristics of the radio wave absorption sheet 10 of FIG. The spectrum includes a first peak corresponding to the first shape layer 31 and appearing at the first frequency fr1 and a second peak corresponding to the second shape layer 32 and appearing at the second frequency fr2. The second frequency fr2 is higher than the first frequency fr1. As shown in FIG. 5, the first peak and the second peak partially overlap each other. Therefore, the radio wave absorption sheet 10 can absorb radio waves in the band between the first frequency fr1 and the second frequency fr2. For example, the reflection amount of the radio wave absorbing sheet 10 in the band between the first frequency fr1 and the second frequency fr2 is -3 dB or less. Therefore, the spectrum showing the absorption characteristics of the radio wave absorption sheet 10 has a -3 dB band BW larger than that in the case of the first comparative form. For example, the -3 dB band BW of the spectrum showing the absorption characteristics of the radio wave absorption sheet 10 can be made larger than 2 × fr (ave) × tan δ. fr (ave) is the average of the first frequency fr1 and the second frequency fr2, and is calculated by the following formula.
fr (ave) = (fr1 + fr2) / 2

The first frequency fr1 is the same as or substantially the same as the resonance frequency fr calculated by substituting the radius r1 into r in the above equations (A1) and (A2). The second frequency fr2 is the same as or substantially the same as the resonance frequency fr calculated by substituting the radius r2 into r in the above equations (A1) and (A2).

The specific configuration of the first conductor layer 30 will be described.

The configuration of the plurality of first conductor layers 30 is determined so that resonance occurs at the frequency of the radio wave of interest. For example, the periods P1 and P2, the radius r1 of the first shape layer 31, the radius r2 of the second shape layer 32, and the like are determined so that the occupancy rate of the first conductor layer 30 on the first surface 21 is 0.85 or less. Has been done. The occupancy rate may be 0.75 or less, 0.60 or less, 0.50 or less, or 0.30 or less. The occupancy rate may be 0.05 or more, or 0.10 or more.

The occupancy rate may be calculated by dividing the total area of the plurality of first conductor layers 30 by the area of the region where the first conductor layers 30 are distributed. The area of the region where the first conductor layer 30 is distributed may be the area of a quadrangle 35 surrounding the plurality of first conductor layers 30, as shown by reference numeral 35 in FIG. 1, for example.

When the groups including the plurality of first conductor layers 30 are arranged periodically, the occupancy rate divides the area of the group including the plurality of first conductor layers 30 by the area of the unit cell 12 corresponding to the group. It may be calculated by. FIG. 6 is a perspective view showing the unit cell 12. The unit cell 12 has an area corresponding to two first conductor layers 30 arranged in the first direction D1 and two first conductor layers 30 arranged in the second direction D2. For example, the unit cell 12 is a quadrangle including a pair of first sides extending in the first direction D1 and a pair of second sides extending in the second direction D2 in a plan view. The length of the first side is equal to twice the length of the first period P1, and the length of the second side is equal to twice the length of the second period P2. In the example shown in FIG. 6, the unit cell 12 includes two first shape layers 31 arranged in the first direction D1, two second shape layers 32 arranged in the first direction D1, and a first dielectric layer 20. Includes a second conductor layer 40. The area of the unit cell 12 is 4 × P1 × P2. The total area of the first conductor layer 30 is 2 × πr1 ² + 2 × πr2 ² . Therefore, the occupancy rate is (2 × πr1 ² + 2 × πr2 ² ) / (4 × P1 × P2).

In FIG. 1, reference numeral S1 represents the maximum value of the dimensions of the first conductor layer 30 in the first direction D1. The dimension S1 is, for example, less than 5.0 mm, may be 4.0 mm or less, may be 3.0 mm or less, may be 2.0 mm or less, or may be 1.5 mm or less. It may be 1.0 mm or less. Reference numeral S2 represents the maximum value of the dimensions of the first conductor layer 30 in the second direction D2. The dimension S2 is, for example, less than 5.0 mm, may be 4.0 mm or less, may be 3.0 mm or less, may be 2.0 mm or less, or may be 1.5 mm or less. It may be 1.0 mm or less. In the example shown in FIG. 1, the dimensions S1 and S2 are the diameters of the first shape layer 31.
The conditions regarding the range of these dimensions may be satisfied by both dimensions S1 and S2, or may be satisfied by only one of them. For example, the dimension S1 may be less than 5.0 mm, but the dimension S2 may be 5.0 mm or more. Similarly, the above-mentioned conditions regarding the range of the period P1 and the period P2 may be satisfied by both the period P1 and the period P2, or may be satisfied by only one of them.

By the way, when the difference between the first frequency fr1 of the first peak and the second frequency fr2 of the second peak becomes too large, the reflection amount becomes -3 dB in the band between the first frequency fr1 and the second frequency fr2. Exceeding range may occur. The difference between the first frequency fr1 and the second frequency fr2 increases as the ratio of the area Sa of the first shape layer 31 to the area Sb of the second shape layer 32 increases from 1. In consideration of this point, it is preferable that Sa / Sb is (1 + 8 × tan δ) or less. Thereby, as supported by the embodiment described later, it is possible to realize a reflection amount of -3 dB or less in the band between the first frequency fr1 and the second frequency fr2. Sa / Sb may be (1 + 7 × tanδ) or less, or may be (1 + 6 × tanδ) or less.

On the other hand, if the difference between the first frequency fr1 of the first peak and the second frequency fr2 of the second peak becomes too small, the -3 dB band BW of the spectrum may become narrow and the advantage over the first comparative form may be lost. There is sex. In consideration of this point, it is preferable that Sa / Sb is (1 + 1 × tan δ) or more. Sa / Sb may be (1 + 2 × tanδ) or more, or may be (1 + 3 × tanδ) or more.

The surface of the first conductor layer 30 may include a layer that has been blackened. For example, the surface of the first conductor layer 30 may include an oxide film obtained by oxidizing the metal constituting the first conductor layer 30.

Next, the second conductor layer 40 will be described. The second conductor layer 40 contains a material having conductivity. As the material of the second conductor layer 40, the material exemplified by the first conductor layer 30 can be used. The material of the second conductor layer 40 may be the same as or different from the material of the first conductor layer 30.

The thickness T2 of the second conductor layer 40 may be 0.1 μm or more, 1 μm or more, or 5 μm or more. The thickness of the second conductor layer 40 may be 50 μm or less, or 20 μm or less.

The second conductor layer 40 faces the first conductor layer 30 in the thickness direction of the first dielectric layer 20. As a result, an electric field can be generated between the first conductor layer 30 and the second conductor layer 40.

The second conductor layer 40 may be spread so as to overlap the plurality of first conductor layers 30 in a plan view. For example, the second conductor layer 40 may be located over the entire second surface 22 of the first dielectric layer 20. Although not shown, the second conductor layer 40 may have openings, slits, or the like.

The surface of the second conductor layer 40 may include a layer that has been blackened. For example, the surface of the second conductor layer 40 may include an oxide film obtained by oxidizing the metal constituting the second conductor layer 40.

The thickness T0 of the entire radio wave absorbing sheet 10 may be, for example, 350 μm or less, 300 μm or less, 250 μm or less, or 200 μm or less. Since the thickness h of the first dielectric layer 20 is small, the overall thickness T0 of the radio wave absorbing sheet 10 is also small. The thickness T0 of the entire radio wave absorbing sheet 10 may be 20 μm or more, or may be 50 μm or more.

By reducing the thickness T0 of the entire radio wave absorbing sheet 10, restrictions on the installation location of the radio wave absorbing sheet 10 can be reduced. That is, the degree of freedom in the layout of the radio wave absorbing sheet 10 is increased. Further, the thermal resistance of the radio wave absorbing sheet 10 in the thickness direction can be reduced. Therefore, when the radio wave absorbing sheet 10 is installed in the electronic device, it is possible to suppress the temperature rise of the electronic component caused by the radio wave absorbing sheet 10. Thereby, for example, the radio wave absorbing sheet 10 can be installed in an electronic device including a component whose temperature rises to about 150 ° C.

Next, the operation of the radio wave absorbing sheet 10 will be described. FIG. 7 is a diagram showing an equivalent circuit of the radio wave absorption sheet 10. The equivalent circuit includes a primary circuit 14, a secondary circuit 15, and a transformer 16 that connects the primary circuit 14 and the secondary circuit 15.

The primary circuit 14 represents a radio wave propagating in the atmosphere. The primary circuit 14 has a characteristic impedance based on the permittivity of the atmosphere. The secondary circuit 15 represents a resonance circuit composed of a first dielectric layer 20, a first conductor layer 30, and a second conductor layer 40. Resonant circuits include, for example, resistors, capacitors and coils connected in parallel. The transformer 16 is composed of a plurality of first conductor layers 30.

In order for the radio wave absorbing sheet 10 to effectively absorb radio waves, it is preferable that the characteristic impedance of the secondary circuit 15 is the same as or close to the characteristic impedance of the primary circuit 14. When the resonance circuit of the secondary circuit 15 satisfies the resonance condition, the characteristic impedance of the secondary circuit 15 is N ² R. N is the turns ratio of the metamorphic device 16. R is the resonance resistance of the secondary circuit 15.

Ｐは、第１周期Ｐ１と第２周期Ｐ２の積である。Ｋは定数である。式（Ａ３）のπｒ^２／Ｐは、第１面２１における第１導電体層２３０の占有率を表す。式（Ａ３）は、第１導電体層２３０の占有率とＮ^２が比例関係にあることを意味する。 As shown in FIG. 3, N ² and R when the first conductor layer 230 is circular having a radius r are shown in the following formulas (A3) and (A4) for reference.

P is the product of the first period P1 and the second period P2. K is a constant. Πr ² / P in the formula (A3) represents the occupancy rate of the first conductor layer 230 on the first surface 21. The formula (A3) means that the occupancy rate of the first conductor layer 230 and N ² are in a proportional relationship.

Assuming that the characteristic impedance in the atmosphere is the same as the characteristic impedance in vacuum, the impedance of the primary circuit 14 is 120π. The radio wave absorbing sheet 10 is preferably configured so that ^N2 R is the same as or close to 120π. For example, the dielectric loss tang tan δ of the first dielectric layer 20, the radius r1 of the first shape layer 31, and the radius r2 of the second shape layer 32 so that the ratio of N ² R to 120 π is 0.8 or more and 1.2 or less. , The first cycle P1 and the second cycle P2 are determined. As a result, the radio wave absorbing sheet 10 can effectively absorb radio waves. The ratio of N ² R to 120 π may be 0.9 or more and 1.1 or less, or 0.95 or more and 1.05 or less.

Next, a method of manufacturing the radio wave absorbing sheet 10 will be described. 8 to 9B are views showing an example of a manufacturing process of the radio wave absorbing sheet 10.

First, the first dielectric layer 20 is prepared. The first dielectric layer 20 may be a flexible member or a rigid member. Subsequently, the first conductor layer 30 is formed on the first surface 21 of the first dielectric layer 20. Further, the second conductor layer 40 is formed on the second surface 22 of the first dielectric layer 20. The method for forming the first conductor layer 30 and the second conductor layer 40 is not particularly limited. For example, the first conductor layer 30 and the second conductor layer 40 may be formed by a plating method, a vapor deposition method, a sputtering method, or the like. The first conductor layer 30 and the second conductor layer 40 may be formed by attaching the metal foil to the first dielectric layer 20. The first conductor layer 30 and the second conductor layer 40 may be formed by printing the conductive paste on the first dielectric layer 20. The method for forming the first conductor layer 30 and the method for forming the second conductor layer 40 may be the same or different.

Subsequently, a patterning step is carried out in which the first conductor layer 30 is processed and divided into the plurality of first shape layers 31 and the plurality of second shape layers 32 described above. For example, as shown in FIG. 8, a plurality of resist layers 37 are formed on the first conductor layer 30. The positions of the plurality of resist layers 37 correspond to the positions of the plurality of first shape layers 31 and the plurality of second shape layers 32 shown in FIGS. 1 and 2, respectively. Subsequently, the first conductor layer 30 is processed by wet etching using the resist layer 37 as a mask. As a result, as shown in FIG. 9A, a plurality of first shape layers 31 and a plurality of second shape layers 32 arranged in the in-plane direction of the first surface 21 can be obtained.

9B and 9C are examples of cross-sectional shapes of the first shape layer 31, the second shape layer 32, and the resist layer 37 of FIG. 9A, respectively. As shown in FIG. 9B, the first shape layer 31 and the second shape layer 32 may include a portion whose dimensions decrease toward the first surface 21. As shown in FIG. 9C, the first shape layer 31 and the second shape layer 32 may include a portion whose dimensions increase toward the first surface 21. Although not shown, the dimensions of the first shape layer 31 and the second shape layer 32 may be constant regardless of the distance from the first surface 21. In any case, the dimensions of the first shape layer 31 and the second shape layer 32 at the position closest to the first surface 21 affect the absorption characteristics of the radio wave absorbing sheet 10. Therefore, the above-mentioned dimension S1 is measured at the position closest to the first surface 21.

After that, the resist layer 37 is removed. In this way, the radio wave absorbing sheet 10 including the first dielectric layer 20, the plurality of first shape layers 31, the plurality of second shape layers 32, and the second conductor layer 40 can be manufactured.

According to the present embodiment, the spectrum showing the absorption characteristics of the radio wave absorption sheet 10 has two peaks. Therefore, the radio wave absorption sheet 10 can absorb radio waves in the band between the first frequency fr1 and the second frequency fr2. As a result, the absorption band of the radio wave absorption sheet 10 can be expanded.

Further, according to the present embodiment, since the radio wave is absorbed by utilizing the resonance, the thickness of the first dielectric layer 20 can be made smaller than that of the conventional radio wave absorber. Therefore, the thickness of the entire radio wave absorbing sheet 10 can be reduced. As a result, the radio wave absorbing sheet 10 becomes lighter.

FIG. 10 is a diagram showing an example of a communication device 100 including a radio wave absorbing sheet 10. The communication device 100 includes a housing 110, a communication mechanism 120 located inside the housing 110 to transmit or receive radio waves, and a control mechanism 130 located inside the housing 110 to control the communication mechanism 120. To prepare for. The communication mechanism 120 transmits or receives the radio wave E having the above-mentioned frequency targeted by the radio wave absorption sheet 10.

In FIG. 10, dotted lines with reference numerals 10A to 10F show an example of arrangement of the radio wave absorbing sheet 10, respectively. For example, as indicated by reference numeral 10A, the radio wave absorbing sheet 10 may be arranged between the communication mechanism 120 and the control mechanism 130. As shown by reference numeral 10B, the radio wave absorbing sheet 10 may be arranged between the front surface 111 of the housing 110 and the communication mechanism 120. As shown by reference numeral 10C, the radio wave absorbing sheet 10 may be attached to the front surface 111 of the housing 110. As shown by reference numeral 10D, the radio wave absorbing sheet 10 may be arranged between the rear surface 112 of the housing 110 and the communication mechanism 120. As indicated by reference numeral 10E, the radio wave absorbing sheet 10 may be attached to the rear surface 112 of the housing 110. As shown by reference numeral 10F, the radio wave absorbing sheet 10 may be attached to the side surface 113 of the housing 110.

According to the present embodiment, since the thickness of the radio wave absorbing sheet 10 is small, the degree of freedom in arranging the radio wave absorbing sheet 10 inside the housing 110 is high.

It is possible to make various changes to the above-described embodiment. Hereinafter, modification examples will be described with reference to the drawings as necessary. In the following description and the drawings used in the following description, the same reference numerals as those used for the corresponding portions in the first embodiment will be used for the portions that can be configured in the same manner as in the above-described embodiment. , Omit duplicate explanations. Further, when it is clear that the action and effect obtained in the above-described embodiment can be obtained in the modified example, the description thereof may be omitted.

(First modification)
FIG. 11 is a plan view showing the radio wave absorbing sheet 10 according to the first modification. As shown in FIG. 11, both the first shape layer 31 and the second shape layer 32 may have a rectangular shape in a plan view. For example, the shape of the first shape layer 31 may be a rectangle having a length L1 and a width W1. The shape of the second shape layer 32 may be a rectangle having a length L2 and a width W2. The area Sb of the second shape layer 32 is smaller than the area Sa of the first shape layer 31. For example, the length L2 and the width W2 may be smaller than the length L1 and the width W1.

The advantage of the radio wave absorbing sheet 10 of FIG. 11 will be described based on the comparison with the second comparative form. FIG. 12 is a plan view showing the radio wave absorbing sheet 210 according to the second comparative form. The radio wave absorbing sheet 210 includes a plurality of first conductor layers 230 having a rectangular shape having a length L and a width W. The plurality of first conductor layers 230 are arranged in the first direction D1 in the first period P1 and in the second direction D2 in the second period P2.

式（Ｂ２）のε_re（Ｌ）は、式（Ｂ３）のｘにＬを代入することによって算出される。式（Ｂ２）のε_re（Ｗ）は、式（Ｂ３）のｘにＷを代入することによって算出される。Ｌ_ｅは、第１導電体層２３０の端部におけるフリンジング効果を考慮した場合の、第１導電体層２３０の実効長さである。 The spectrum showing the absorption characteristics of the radio wave absorption sheet 210 of FIG. 12 has a single peak as in the case of the above-mentioned first comparative form shown in FIG. The resonance frequency fr of the radio wave absorption sheet 210 of FIG. 12 is calculated based on the following equations (B1), (B2), (B3), and (B4).

The ε _re (L) of the equation (B2) is calculated by substituting L for x of the equation (B3). The ε _re (W) of the equation (B2) is calculated by substituting W for x of the equation (B3). _Le is the effective length of the first conductor layer 230 when the fringe effect at the end of the first conductor layer 230 is taken into consideration.

On the other hand, according to the radio wave absorption sheet 10 shown in FIG. 11, the spectrum has the first peak appearing at the first frequency fr1 and the second frequency fr2 as in the case of the above-described embodiment shown in FIG. Includes the second peak that appears in. Therefore, the radio wave absorption sheet 10 can absorb radio waves in the band between the first frequency fr1 and the second frequency fr2. For example, the -3 dB band BW of the spectrum of the radio wave absorption sheet 10 can be made larger than 2 × fr (ave) × tan δ. As a result, the absorption band of the radio wave absorption sheet 10 can be expanded.

The first frequency fr1 is the same as the resonance frequency fr calculated by substituting the length L1 and the width W1 of the first shape layer 31 into L and W of the above equations (B1) to (B4), or It is almost the same. The second frequency fr2 is the same as the resonance frequency fr calculated by substituting the length L2 and the width W2 of the second shape layer 32 into L and W of the above equations (B1) to (B4), or It is almost the same.

Ｋは定数である。式（Ｂ５）のＬＷ／Ｐは、第１面２１における第１導電体層２３０の占有率を表す。式（Ｂ５）は、第１導電体層２３０の占有率とＮ^２が比例関係にあることを意味する。 As shown in FIG. 12, N ² and R when the first conductor layer 230 is a rectangle having a length L and a width W are shown in the following formulas (B5) and (B6) for reference.

K is a constant. The LW / P of the formula (B5) represents the occupancy rate of the first conductor layer 230 on the first surface 21. The formula (B5) means that the occupancy rate of the first conductor layer 230 and N ² are in a proportional relationship.

The radio wave absorbing sheet 10 of this modification is also preferably configured so that ^N2 R is the same as or close to 120π. For example, the dielectric loss tang tan δ of the first dielectric layer 20, the length L1 and the width W1 of the first shape layer 31, and the second shape layer so that the ratio of N ² R to 120 π is 0.8 or more and 1.2 or less. The length L2 and width W2 of 32, the first cycle P1 and the second cycle P2 are determined. As a result, the radio wave absorbing sheet 10 can effectively absorb radio waves. The ratio of N ² R to 120 π may be 0.9 or more and 1.1 or less, or 0.95 or more and 1.05 or less.

In the example shown in FIG. 11, the side of the first shape layer 31 having the length L1 extends in the first direction D1, and the side of the first shape layer 31 having the width W1 extends in the second direction D2. There is. Although not shown, the sides of the first shape layer 31 having a length L1 may extend in a direction different from that of the first direction D1.

In the example shown in FIG. 11, the side of the second shape layer 32 having the length L2 extends parallel to the side of the first shape layer 31 having the length L1. Although not shown, the side of the second shape layer 32 having the length L2 may extend in a direction different from the side of the first shape layer 31 having the length L1.

(Second modification)
FIG. 13 is a plan view showing the radio wave absorbing sheet 10 according to the second modification. As shown in FIG. 13, in the first direction D1, the first shape layer 31 and the second shape layer 32 are alternately arranged in the first period P1, and in the second direction D2, the first shape layer 31 and the second shape layer 31 and the second shape. The layers 32 may be alternately arranged in the second period P2. According to this modification, it is possible to suppress that the absorption characteristic of the radio wave absorbing sheet 10 depends on the in-plane direction of the first surface 21.

(Third modification example)
FIG. 14 is a plan view showing the radio wave absorbing sheet 10 according to the third modification. As shown in FIG. 14, the plurality of first conductor layers 30 include the plurality of first shape layers 31 and the plurality of second shape layers 32, as well as the first shape layer 31 and the second shape layer 32 in a plan view. It may contain a plurality of third shape layers 33 having different shapes from the above.

The plurality of third shape layers 33 may be arranged in the first direction D1 in the first period P1 in the same manner as the plurality of first shape layers 31 and the plurality of second shape layers 32. As shown in FIG. 14, in the second direction D2, the first shape layer 31, the second shape layer 32, and the third shape layer 33 may be arranged in order in the second cycle P2. Although not shown, the first shape layer 31, the second shape layer 32, and the third shape layer 33 may be arranged in order in the second cycle P2 in the first direction D1 as well.

The third shape layer 33 has a radius r1 smaller than the radius r1 of the first shape layer 31 and a radius r2 smaller than the radius r2 of the second shape layer 32. Therefore, the area Sc of the third shape layer 33 is smaller than the area Sa of the first shape layer 31 and the area Sb of the second shape layer 32.

FIG. 15 is a diagram showing an example of a spectrum showing the absorption characteristics of the radio wave absorption sheet 10 of FIG. The spectrum corresponds to the first peak appearing at the first frequency fr1 corresponding to the first shape layer 31, the second peak appearing at the second frequency fr2 corresponding to the second shape layer 32, and the third shape layer 33. The third peak appearing at the third frequency fr3 is included. The third frequency fr3 is higher than the second frequency fr2. As shown in FIG. 15, the second peak and the third peak partially overlap each other. Therefore, the radio wave absorption sheet 10 can absorb radio waves in the band between the second frequency fr2 and the third frequency fr3. For example, the reflection amount of the radio wave absorbing sheet 10 in the band between the second frequency fr2 and the third frequency fr3 is -3 dB or less. Therefore, the spectrum showing the absorption characteristics of the radio wave absorption sheet 10 can have a -3 dB band BW larger than that of the embodiment shown in FIG.

When the difference between the second frequency fr2 of the second peak and the third frequency fr3 of the third peak becomes too large, the reflection amount exceeds -3 dB in the band between the second frequency fr2 and the third frequency fr3. May occur. In consideration of this point, it is preferable that Sb / Sc is (1 + 8 × tanδ) or less. Thereby, as supported by the embodiment described later, it is possible to realize a reflection amount of -3 dB or less in the band between the second frequency fr2 and the third frequency fr3. Sb / Sc may be (1 + 7 × tanδ) or less, or may be (1 + 6 × tanδ) or less.

On the other hand, if the difference between the second frequency fr2 of the second peak and the third frequency fr3 of the third peak becomes too small, the -3 dB band BW of the spectrum becomes narrow, and the advantage for the embodiment shown in FIG. 1 is lost. there is a possibility. In consideration of this point, it is preferable that Sb / Sc is (1 + 1 × tan δ) or more. Sb / Sc may be (1 + 2 × tanδ) or more, or may be (1 + 3 × tanδ) or more.

When the relationship of Sa> Sb> Sc is established, the above-mentioned relationship may not be established between Sa and Sc. For example, Sa / Sc may be larger than (1 + 8 × tanδ).

Although not shown, the area Sc of the third shape layer 33 may be larger than the area Sa of the first shape layer 31 and the area Sb of the second shape layer 32. That is, the relationship Sc> Sa> Sb may be established. In this case, Sc / Sa is preferably (1 + 8 × tan δ) or less. In this case, Sc / Sb may be larger than (1 + 8 × tanδ). Sc / Sa may be (1 + 7 × tanδ) or less, or may be (1 + 6 × tanδ) or less. Sc / Sa may be (1 + 1 × tanδ) or more, (1 + 2 × tanδ) or more, or (1 + 3 × tanδ) or more.

Although not shown, the plurality of first conductor layers 30 may include other shape layers having shapes different from those of the first shape layer 31, the second shape layer 32, and the third shape layer 33 in a plan view. good.

(Fourth modification)
FIG. 16 is a plan view showing the radio wave absorbing sheet 10 according to the fourth modification. The radio wave absorbing sheet 10 may include an overcoat layer 50 located on the first surface 21 of the first dielectric layer 20. The overcoat layer 50 has an insulating property. The overcoat layer 50 covers a plurality of first conductor layers 30. By providing the overcoat layer 50, the insulating property of the surface of the radio wave absorbing sheet 10 can be improved. In addition, it is possible to suppress the occurrence of alteration such as oxidation in the first conductor layer 30. In addition, the heat resistance of the radio wave absorbing sheet 10 can be improved.

The overcoat layer 50 may include a resin layer 51. The resin layer 51 may contain a resin such as polyester, epoxy, phenol, or polyimide. From the viewpoint of heat resistance, it is preferable that the resin layer 51 contains polyimide. The resin layer 51 may contain a curing agent. The resin layer 51 may contain a catalyst for accelerating curing. For example, the resin layer 51 may contain a resist material having photocurability or thermosetting property. The resin layer 51 may be formed by printing a solution containing a resin on the first surface 21.

The thickness of the resin layer 51 is, for example, 1 μm or more, may be 5 μm or more, or may be 10 μm or more. The thickness of the resin layer 51 is, for example, 100 μm or less, 50 μm or less, or 30 μm or less.

When the radio wave absorbing sheet 10 includes the overcoat layer 50, the thickness T0 of the entire radio wave absorbing sheet 10 is, for example, 500 μm or less, 400 μm or less, 300 μm or less, or 200 μm or less. It is also good. The thickness T0 of the entire radio wave absorbing sheet 10 may be 20 μm or more, or may be 50 μm or more.

The relative permittivity of the resin layer 51 may be less than 20.0, may be 15.0 or less, may be 10.0 or less, or may be 5.0 or less. The relative permittivity of the resin layer 51 may be 2.0 or more, 3.0 or more, or 5.0 or more.

The dielectric loss tangent of the resin layer 51 may be 0.20 or less. The dielectric loss tangent of the resin layer 51 may be 0.001 or more.

By the way, when the occupancy rate of the first conductor layer 30 on the first surface 21 is smaller than the occupancy rate of the second conductor layer 40 on the second surface 22, the first dielectric layer 20 is caused by the difference in the occupancy rates. May warp. For example, when the second conductor layer 40 is formed on the entire surface of the second surface 22, the occupancy rate of the second conductor layer 40 on the second surface 22 is 1.00. On the other hand, the occupancy rate of the first conductor layer 30 on the first surface 21 is smaller than 1.00, for example, 0.85 or less. Therefore, when the materials and thicknesses of the first conductor layer 30 and the second conductor layer 40 are the same, the stress generated in the first dielectric layer 20 due to the second conductor layer 40 is the first conductivity. It is larger than the stress generated in the first dielectric layer 20 due to the body layer 30. The larger the difference in stress, the greater the warpage that occurs in the first dielectric layer 20. The overcoat layer 50 may be configured to suppress such warpage. For example, the linear expansion coefficient of the overcoat layer 50 may be adjusted to prevent the first dielectric layer 20 from being warped. For example, the magnitude relationship between the linear expansion coefficient of the first conductor layer 30 and the linear expansion coefficient of the overcoat layer 50 may be matched with respect to the linear expansion coefficient of the first dielectric layer 20. Specifically, when the linear expansion coefficient of the first conductor layer 30 is larger than the linear expansion coefficient of the first dielectric layer 20, the linear expansion coefficient of the overcoat layer 50 is set to the linear expansion coefficient of the first dielectric layer 20. May be larger than. On the contrary, when the linear expansion coefficient of the first conductor layer 30 is smaller than the linear expansion coefficient of the first dielectric layer 20, the linear expansion coefficient of the overcoat layer 50 is smaller than the linear expansion coefficient of the first dielectric layer 20. It may be made smaller. As a result, the stress generated on the first surface 21 of the first dielectric layer 20 due to the first conductor layer 30 and the overcoat layer 50, and the first dielectric layer 20 due to the second conductor layer 40. The difference between the stress generated on the second surface 22 and the stress generated in the second surface 22 can be reduced.

(Fifth modification)
FIG. 17 is a plan view showing the radio wave absorbing sheet 10 according to the fifth modification. The overcoat layer 50 may include a resin layer 51 and a cover film 52 located on the resin layer 51. The overcoat layer 50 shown in FIG. 17 may be formed by attaching a laminate including the resin layer 51 and the cover film 52 to the first surface 21 by hot pressing. Such an overcoat layer 50 is also referred to as a coverlay.

As the material of the resin layer 51, the material mentioned in the fourth modification may be used. The resin layer 51 may contain an adhesive having thermoplasticity, thermosetting property, UV curability and the like. The cover film 52 may be a release paper.

The thickness of the resin layer 51 is, for example, 1 μm or more, may be 5 μm or more, or may be 10 μm or more. The thickness of the resin layer 51 is, for example, 60 μm or less, 30 μm or less, or 15 μm or less.

The thickness of the cover film 52 is, for example, 10 μm or more, may be 15 μm or more, or may be 20 μm or more. The thickness of the cover film 52 is, for example, 40 μm or less, 30 μm or less, or 20 μm or less.

(6th modification)
FIG. 18 is a plan view showing the radio wave absorbing sheet 10 according to the sixth modification. The radio wave absorbing sheet 10 may include an adhesive layer 60 located on the second conductor layer 40. The adhesive layer 60 may be used to attach the radio wave absorbing sheet 10 to the object. For example, the adhesive layer 60 may be used to attach the radio wave absorbing sheet 10 to the front surface 111 of the housing 110. By providing the adhesive layer 60, the radio wave absorbing sheet 10 can easily follow the shape of the object. In addition, the heat resistance of the radio wave absorbing sheet 10 can be improved.

The radio wave absorbing sheet 10 may include a release film 65 located on the adhesive layer 60. By providing the release film 65, it is possible to prevent the radio wave absorbing sheet 10 from sticking to an object other than the object. The release film 6 is removed when the radio wave absorbing sheet 10 is attached to the object.

The adhesive layer 60 may be, for example, a double-sided tape. The double-sided tape may include a core material and adhesive layers located on both sides of the core material. The adhesive layer may contain a resin such as polyester, acrylic, epoxy, phenol, or polyimide. The adhesive layer may contain a filler. This makes it possible to improve the heat dissipation and heat resistance of the adhesive layer. The double-sided tape may be produced by applying a resin to both sides of the substrate.

The adhesive layer 60 may contain a non-woven fabric and a resin impregnated in the non-woven fabric. The resin may be polyester, acrylic, epoxy, phenol, polyimide or the like. The resin may contain a filler. This makes it possible to improve the heat dissipation and heat resistance of the adhesive layer.

The thickness of the adhesive layer 60 is, for example, 1 μm or more, and may be 5 μm or more. The thickness of the adhesive layer 60 is, for example, 200 μm or less, and may be 100 μm or less.

When the radio wave absorbing sheet 10 includes the adhesive layer 60, the thickness T0 of the entire radio wave absorbing sheet 10 is, for example, 500 μm or less, 400 μm or less, 300 μm or less, or 200 μm or less. good. The thickness T0 of the entire radio wave absorbing sheet 10 may be 20 μm or more, or may be 50 μm or more.

The peeling adhesive strength of the adhesive layer 60 is, for example, 0.1 N / 20 mm or more, 1.0 N / 20 mm or more, 5.0 N / 20 mm or more, or 10 N / 20 mm or more. You may. The peeling adhesive strength is measured by the 180 ° peeling test specified in JIS Z 0237: 2009.

(Other variants)
In the above-described embodiment, an example is shown in which the shape of the second shape layer 32 is similar to the shape of the first shape layer 31, and the area of the second shape layer 32 is smaller than the area of the first shape layer 31. .. However, the combination of the shape of the first shape layer 31 and the shape of the second shape layer 32 is not particularly limited as long as the absorption band of the radio wave absorption sheet 10 can be expanded.

For example, the shape of the second shape layer 32 may be rotationally symmetric with the shape of the first shape layer 31. In this case, the first shape layer 31 and the first shape layer 31 have an isotropic shape such as an ellipse or a polygon, not an isotropic shape such as a circle. For example, the shape of the first shape layer 31 may be a square, and the shape of the second shape layer 32 may be a shape obtained by rotating the first shape layer 31 by 45 °.

Further, both the first shape layer 31 and the second shape layer 32 are rectangular, and the ratio of the width to the length of the rectangle of the second shape layer 32 is the ratio of the width to the length of the rectangle of the first shape layer 31. May be different from.

Further, both the first shape layer 31 and the second shape layer 32 are polygons, and the number of corners in the polygon of the second shape layer 32 is different from the number of corners in the polygon of the first shape layer 31. You may.

The plurality of modifications described above may be appropriately combined and applied to the above-described embodiment. For example, the radio wave absorbing sheet 10 may include an overcoat layer 50 of the fourth modification or the fifth modification, and an adhesive layer 60 of the sixth modification.

Next, the embodiment of the present disclosure will be described more specifically by way of examples, but the embodiment of the present disclosure is not limited to the description of the following examples as long as the gist of the present disclosure is not exceeded.

(Example 1)
The absorption characteristics of the radio wave absorption sheet 10 shown in FIG. 1 were evaluated based on a simulation. The plurality of first conductor layers 30 of the radio wave absorbing sheet 10 include a plurality of first shape layers 31 having a radius r1 and a plurality of second shape layers 32 having a radius r2. Both the first shape layer 31 and the second shape layer 32 are arranged in the first direction D1 with a period P1. In the second direction D2, the first shape layer 31 and the second shape layer 32 are alternately arranged in the period P2. The parameters of the components of the radio wave absorbing sheet 10 are as follows.
-Thickness of the first conductor layer 30: 12 μm
The shape of the first shape layer 31: a circle having an area Sa of 7.277 mm ²・ The shape of the second shape layer 32: a circle having an area Sb of 6.353 mm ²・ First period P1: 4.650 mm
・ Second cycle P2: 4.650 mm
-Thickness of the second conductor layer 40: 12 μm
-Thickness of the first dielectric layer 20: 200 μm
Relative permittivity of the first dielectric layer 20 ε _r : 4.2
The dielectric loss tangent tan δ of the first dielectric layer 20: 0.02
Sa / Sb was 1.145.

FIG. 19 shows a spectrum showing the absorption characteristics of the radio wave absorption sheet 10 calculated by simulation. The vertical axis d of the spectrum of the embodiment is the ratio of the radio wave power E2 reflected by the radio wave absorption sheet to the radio wave power E1 incident on the radio wave absorption sheet, as in the case of the spectra shown in FIGS. Represents the value obtained by multiplying the common logarithm of.
The first frequency fr1 of the spectrum was about 27.1 GHz, the second frequency fr2 was about 28.8 GHz, and fr (ave) was about 28.0 GHz. The spectrum had a reflection amount of -3 dB or less in the band between the first frequency fr1 and the second frequency fr2. The -3 dB band BW_1 of the spectrum was about 3.0 GHz. The value obtained by dividing the -3 dB band BW_1 by fr (ave) × tanδ was about 5.4.

(Example 2)
Under the conditions that the area Sa of the first shape layer 31 and the area Sb of the second shape layer 32 of Example 1 were changed as follows, the absorption characteristics of the radio wave absorption sheet 10 were evaluated based on the simulation.
The shape of the first shape layer 31: a circle having an area Sa of 7.031 mm ² , and the shape of the second shape layer 32: a circle having an area Sb of 6.587 mm ² was 1.067.

FIG. 19 shows a spectrum showing the absorption characteristics of the radio wave absorption sheet 10 calculated by simulation. The first frequency fr1 of the spectrum was about 27.6 GHz, the second frequency fr2 was about 28.3 GHz, and fr (ave) was about 28.0 GHz. The spectrum had a reflection amount of −10 dB or less in the band between the first frequency fr1 and the second frequency fr2. The -3 dB band BW_2 of the spectrum was about 2.2 GHz. The value obtained by dividing the -3 dB band BW_2 by fr (ave) × tanδ was about 3.9.

(Example 3)
Simulation under the conditions that the area Sa of the first shape layer 31 of Example 1, the area Sb of the second shape layer 32, the first period P1, the second period P2, and the thickness of the first dielectric layer 20 are changed as follows. The absorption characteristics of the radio wave absorbing sheet 10 were evaluated based on the above.
The shape of the first shape layer 31: a circle having an area Sa of 1.557 mm ²・ The shape of the second shape layer 32: a circle having an area Sb of 1.360 mm ²・ First period P1: 2.200 mm
・ Second cycle P2: 2.200 mm
-Thickness of the first dielectric layer 20: 110 μm
Sa / Sb was 1.145.

FIG. 20 shows a spectrum showing the absorption characteristics of the radio wave absorption sheet 10 calculated by simulation. The first frequency fr1 of the spectrum was about 58.2 GHz, the second frequency fr2 was about 61.6 GHz, and fr (ave) was about 59.9 GHz. The spectrum had a reflection amount of -3 dB or less in the band between the first frequency fr1 and the second frequency fr2. The -3 dB band BW_3 of the spectrum was about 6.2 GHz. The value obtained by dividing the -3 dB band BW_3 by fr (ave) × tan δ was about 5.2.

(Example 4)
Under the conditions that the area Sa of the first shape layer 31 and the area Sb of the second shape layer 32 of Example 3 were changed as follows, the absorption characteristics of the radio wave absorption sheet 10 were evaluated based on the simulation.
The shape of the first shape layer 31: a circle having an area Sa of 1.513 mm ²・ The shape of the second shape layer 32: a circle Sa / Sb having an area Sb of 1.402 mm ² was 1.079.

FIG. 20 shows a spectrum showing the absorption characteristics of the radio wave absorption sheet 10 calculated by simulation. The first frequency fr1 of the spectrum was about 59.0 GHz, the second frequency fr2 was about 60.7 GHz, and fr (ave) was about 59.8 GHz. The spectrum had a reflection amount of −10 dB or less in the band between the first frequency fr1 and the second frequency fr2. The -3 dB band BW_4 of the spectrum was about 4.6 GHz. The value obtained by dividing the -3 dB band BW_4 by fr (ave) × tan δ was about 3.8.

(Example 5)
Simulation under the conditions that the area Sa of the first shape layer 31 of Example 1, the area Sb of the second shape layer 32, the first period P1, the second period P2, and the thickness of the first dielectric layer 20 are changed as follows. The absorption characteristics of the radio wave absorbing sheet 10 were evaluated based on the above.
The shape of the first shape layer 31: a circle having an area Sa of 0.869 mm ²・ The shape of the second shape layer 32: a circle having an area Sb of 0.760 mm ²・ First period P1: 2.173 mm
・ Second cycle P2: 2.173 mm
-Thickness of the first dielectric layer 20: 100 μm
Sa / Sb was 1.143.

FIG. 21 shows a spectrum showing the absorption characteristics of the radio wave absorption sheet 10 calculated by simulation. The first frequency fr1 of the spectrum was about 77.2 GHz, the second frequency fr2 was about 81.2 GHz, and fr (ave) was about 79.2 GHz. The spectrum had a reflection amount of -3 dB or less in the band between the first frequency fr1 and the second frequency fr2. The -3 dB band BW_5 of the spectrum was about 6.7 GHz. The value obtained by dividing the -3 dB band BW_5 by fr (ave) × tan δ was about 4.2.

(Example 6)
Under the conditions that the area Sa of the first shape layer 31 and the area Sb of the second shape layer 32 of Example 5 were changed as follows, the absorption characteristics of the radio wave absorption sheet 10 were evaluated based on the simulation.
The shape of the first shape layer 31: a circle having an area Sa of 0.836 mm ²・ The shape of the second shape layer 32: a circle Sa / Sb having an area Sb of 0.792 mm ² was 1.056.

FIG. 21 shows a spectrum showing the absorption characteristics of the radio wave absorption sheet 10 calculated by simulation. The first frequency fr1 of the spectrum was about 78.4 GHz, the second frequency fr2 was about 79.8 GHz, and fr (ave) was about 79.1 GHz. The spectrum had a reflection amount of −10 dB or less in the band between the first frequency fr1 and the second frequency fr2. The -3 dB band BW_6 of the spectrum was about 4.8 GHz. The value obtained by dividing the -3 dB band BW_6 by fr (ave) × tan δ was about 3.0.

(Example 7)
Example 5: Area Sa of the first shape layer 31, area Sb of the second shape layer 32, first period P1, second period P2, thickness of the first dielectric layer 20, dielectric loss tangent tan δ of the first dielectric layer 20. Was changed as follows, and the absorption characteristics of the radio wave absorption sheet 10 were evaluated based on the simulation.
The shape of the first shape layer 31: a circle having an area Sa of 0.869 mm ²・ The shape of the second shape layer 32: a circle having an area Sb of 0.636 mm ²・ First period P1: 1.700 mm
・ Second period P2: 1.700 mm
-Thickness of the first dielectric layer 20: 150 μm
Dielectric loss tan δ of the first dielectric layer 20: 0.05
Sa / Sb was 1.366.

FIG. 22 shows a spectrum showing the absorption characteristics of the radio wave absorption sheet 10 calculated by simulation. The first frequency fr1 of the spectrum was about 74.7 GHz, the second frequency fr2 was about 84.6 GHz, and fr (ave) was about 79.7 GHz. The spectrum had a reflection amount of -3 dB or less in the band between the first frequency fr1 and the second frequency fr2. The -3 dB band BW_7 of the spectrum was about 17.5 GHz. The value obtained by dividing the -3 dB band BW_7 by fr (ave) × tan δ was about 4.4.

(Example 8)
Under the conditions that the area Sa of the first shape layer 31 and the area Sb of the second shape layer 32 of Example 7 were changed as follows, the absorption characteristics of the radio wave absorption sheet 10 were evaluated based on the simulation.
The shape of the first shape layer 31: a circle having an area Sa of 0.811 mm ²・ The shape of the second shape layer 32: a circle Sa / Sb having an area Sb of 0.688 mm ² was 1.179.

FIG. 22 shows a spectrum showing the absorption characteristics of the radio wave absorption sheet 10 calculated by simulation. The first frequency fr1 of the spectrum was about 77.2 GHz, the second frequency fr2 was about 81.3 GHz, and fr (ave) was about 79.2 GHz. The spectrum had a reflection amount of −10 dB or less in the band between the first frequency fr1 and the second frequency fr2. The -3 dB band BW_8 of the spectrum was about 12.8 GHz. The value obtained by dividing the -3 dB band BW_8 by fr (ave) × tan δ was about 3.2.

(Example 9)
The absorption characteristics of the radio wave absorption sheet 10 shown in FIG. 14 were evaluated based on a simulation. The plurality of first conductor layers 30 of the radio wave absorbing sheet 10 have a plurality of first shape layers 31 having a radius r1, a plurality of second shape layers 32 having a radius r2, and a plurality of third shapes having a radius r3. Includes layer 33 and. The first shape layer 31, the second shape layer 32, and the third shape layer 33 are all arranged in the first direction D1 with a period P1. In the second direction D2, the first shape layer 31, the second shape layer 32, and the third shape layer 33 are arranged in this order in the period P2. The parameters of the components of the radio wave absorbing sheet 10 are as follows.
-Thickness of the first conductor layer 30: 12 μm
The shape of the first shape layer 31: a circle having an area Sa of 1.548 mm ²・ The shape of the second shape layer 32: a circle having an area Sb of 1.445 mm ²・ The shape of the third shape layer 33: the area Sc 1.331mm ² circular, 1st cycle P1: 2.068mm
・ Second cycle P2: 2.068 mm
-Thickness of the second conductor layer 40: 12 μm
-Thickness of the first dielectric layer 20: 110 μm
Relative permittivity of the first dielectric layer 20 ε _r : 4.2
The dielectric loss tangent tan δ of the first dielectric layer 20: 0.02
Sa / Sb was 1.071, Sb / Sc was 1.086, and Sa / Sc was 1.163.

FIG. 23 shows a spectrum showing the absorption characteristics of the radio wave absorption sheet 10 calculated by simulation. The first frequency fr1 of the spectrum was about 58.4 GHz, the second frequency fr2 was about 60.0 GHz, and the third frequency fr3 was about 61.5 GHz. The spectrum had a reflection amount of -10 dB or less in the band between the first frequency fr1 and the third frequency fr3.

10 Radio wave absorbing sheet 12 Unit cell 14 Primary circuit 15 Secondary circuit 16 Transformer 20 First dielectric layer 21 First surface 22 Second surface 30 First conductor layer 31 First shape layer 32 Second shape layer 33 Third Shape layer 35 Distribution area 37 Resist layer 40 Second conductor layer 50 Overcoat layer 51 Resin layer 52 Cover film 60 Adhesive layer 65 Peeling film 100 Communication device 110 Housing 120 Communication mechanism 130 Control mechanism D1 First direction D2 Second direction P1 1st cycle P2 2nd cycle

Claims (16)

A first dielectric layer including a first surface and a second surface located on the opposite side of the first surface,
A plurality of first conductor layers located on the first surface and arranged in the first direction,
A second conductor layer located on the second surface is provided.
The plurality of first conductor layers are a radio wave absorbing sheet including a plurality of first shape layers and a plurality of second shape layers having a shape different from the first shape layer in a plan view. The radio wave absorbing sheet according to claim 1, wherein the shape of the second shape layer is similar to the shape of the first shape layer. The radio wave absorbing sheet according to claim 1 or 2, wherein the area of the second shape layer is smaller than the area of the first shape layer. The following equation holds between the area Sa of the first shape layer and the area Sb of the second shape layer.
Sa / Sb ≦ 1 + 8 × tanδ
The radio wave absorption sheet according to any one of claims 1 to 3, wherein tan δ is a dielectric loss tangent of the first dielectric layer. The following equation holds between the area Sa of the first shape layer and the area Sb of the second shape layer.
Sa / Sb ≧ 1 + 1 × tanδ
The radio wave absorption sheet according to any one of claims 1 to 4, wherein tan δ is a dielectric loss tangent of the first dielectric layer. The spectra showing the absorption characteristics of the radio wave absorbing sheet include a first peak appearing at the first frequency fr1 corresponding to the first shape layer and a second peak appearing at the second frequency fr2 corresponding to the second shape layer. And, including
The -3 dB band BW of the spectrum satisfies the following equation.
BW> 2 x fr (ave) x tanδ
fr (ave) = (fr1 + fr2) / 2
The radio wave absorption sheet according to any one of claims 1 to 5, wherein tan δ is a dielectric loss tangent of the first dielectric layer. The radio wave absorbing sheet according to any one of claims 1 to 6, which is located on the first surface side and includes an overcoat layer that covers the first conductor layer. The radio wave absorbing sheet according to any one of claims 1 to 7, further comprising an adhesive layer located on the second conductor layer. The radio wave absorption sheet according to any one of claims 1 to 8, wherein the occupancy rate of the first conductor layer on the first surface is 0.85 or less. The radio wave absorption sheet according to any one of claims 1 to 9, wherein the dimension of the first conductor layer in the first direction is less than 5.0 mm. The radio wave absorbing sheet according to any one of claims 1 to 10, wherein the thickness of the first dielectric layer is 300 μm or less. The radio wave absorption sheet according to any one of claims 1 to 11, wherein the relative permittivity of the first dielectric layer is less than 20. The radio wave absorption sheet according to any one of claims 1 to 12, wherein the dielectric loss tangent of the first dielectric layer is 0.2 or less. The radio wave absorbing sheet according to any one of claims 1 to 13, wherein the thickness of the radio wave absorbing sheet is 350 μm or less. The spectra showing the absorption characteristics of the radio wave absorbing sheet include a first peak appearing at the first frequency fr1 corresponding to the first shape layer and a second peak appearing at the second frequency fr2 corresponding to the second shape layer. And, including
The radio wave absorption sheet according to any one of claims 1 to 14, wherein both the first frequency fr1 and the second frequency fr2 are 20 GHz or more and 110 GHz or less. A communication mechanism that transmits or receives radio waves,
A communication device comprising the radio wave absorbing sheet according to any one of claims 1 to 15.

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