JP2003309395A - Radio wave absorption material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To more easily and inexpensively provide a radio wave absorption material, having high absorption effect and wide range of applications. <P>SOLUTION: The radio wave absorption material comprises a sheet made of short conductive fibers. In this material, at least one point of a complex relative dielectric constant, in the range of 1 to 20 GHz of frequency, lies within a region surrounded by formulae (1) εR=2, (2) εR=50, (3) εR=0.7εR<SP>1/2</SP>, and (4) εJ=6.5εR<SP>1/2</SP>, wherein the real part of the dielectric constant is εR, and the imaginary part is εJ. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a radio wave absorber, particularly a microwave or millimeter wave band radiowave absorber, and more specifically, it is attached to an inner wall of an anechoic chamber or mounted on a moving body such as a ship aircraft. In addition to being attached to the outer walls of structures such as bridges, wireless LA has expanded rapidly in recent years.
N, installed on the inside and outside of equipment related to DSRC (narrow zone road-to-vehicle communication) and ITS (advanced traffic information system) such as ETC on highways, and structures around the system, such as outer walls, walls, ceilings and floors The present invention relates to a radio wave absorber that is used to prevent radio wave interference by being used.

[0002]

2. Description of the Related Art A radio wave absorber is designed to take an incoming radio wave into the body without reflecting it and rapidly attenuate it in the body. Therefore, the wave absorber is constructed so as to satisfy the matching effect and the absorbing effect at the same time. That is, the standardized impedance seen from the surface is set to 1 or as close to 1 as possible to prevent the reflection of the incoming radio wave, and the radio wave is taken into the body and absorbed by utilizing its own electric loss and magnetic loss. .

As such an electromagnetic wave absorber, one that has conventionally mixed magnetic and conductive powders such as ferrite and metal pieces and obtains a predetermined electromagnetic wave absorption by the mixture ratio (for example,
Patent Documents 1 and 2), one having a two-layer effect of a magnetic loss layer and an electrical loss layer (for example, see Patent Document 3),
Various things are known. However, in these conventional electromagnetic wave absorbers, it is very troublesome to mix the electrical loss material and the magnetic loss material in the base material so that the manufacturing cost is high, and the weather resistance and There are drawbacks such as poor strength.

As a method for solving these problems, so-called low-temperature carbonized yarn obtained by firing polyacrylonitrile fiber, pitch-based infusible fiber or the like at a temperature of 500 to 1,000 ° C., which is lower than in the case of obtaining ordinary carbon fiber. Or 1,300-2
By using a fiber such as silicon carbide fiber fired at 000 ° C., which has an electric conductivity in the semiconductor region of 10 ⁻⁶ to 10 ³ S / cm, a high absorption effect in the microwave band and a frequency band A wide range of radio wave absorbers having excellent weather resistance and strength have been proposed (see, for example, Patent Document 4). These are precursors such as polyacrylonitrile fiber or pitch-based infusible fiber or silicon carbide fiber formed into a sheet such as a woven fabric shape, and there is no need to bother to mix the loss material in order to obtain the semiconductor fiber by firing it. . Moreover, it can be made in the form of a fiber sheet from the beginning, and because it is lightweight and flexible, it can be wrapped around a curved surface for use, or it can be impregnated with resin to form a flat or cylindrical composite form, which makes it extremely useful. Easy and wide range of applications. However, the production of these fibers requires advanced firing temperature control technology and is limited to production only with advanced equipment.

[0005]

[Patent Document 1] JP-A-51-58046

[0006]

[Patent Document 2] Japanese Patent Laid-Open No. 58-71698

[0007]

[Patent Document 3] Japanese Patent Publication No. 504-2423

[0008]

[Patent Document 4] JP-A-62-183599

[0009]

In view of such background of the prior art, the present invention has a high absorption effect comparable to that of a low temperature carbonized yarn or a silicon carbide fiber sheet, and is easy to use for a wide variety of applications. Material
It is intended to be provided more easily and cheaply.

[0010]

The present invention employs the following means in order to solve the above problems. That is, the radio wave absorber of the present invention is configured to include a fiber sheet having short fibers of conductive fibers, and at least one of the complex relative permittivities in the frequency range of 1 to 20 GHz is the complex relative permittivity. When the real part of the ratio is ε _R and the imaginary part is ε _J , it is in the region surrounded by the following formulas (1) to (4).

(1) ε _R = 2 (2) ε _R = 50 (3) ε _J = 0.7 ε _R ^1/2 (4) ε _J = 6.5 ε _R ^1/2

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The radio wave absorber of the present invention is configured to include a fiber-containing sheet having conductive short fibers, and has at least one complex relative permittivity within a frequency range of 1 to 20 GHz. However, when the real part of the complex relative permittivity is ε _R and the imaginary part is ε _J , it is within the region surrounded by the following formulas (1) to (4).

(1) ε _R = 2 (2) ε _R = 50 (3) ε _J = 0.7 ε _R ^1/2 (4) ε _J = 6.5 ε _R ^{1/2 As} a short fiber of conductive fiber Is one or more of carbon fibers or fibers such as metal fibers such as gold, silver, copper, nickel, aluminum and iron. Even if the glass fiber, silicon carbide fiber, boron fiber, organic high-elasticity fiber, synthetic fiber such as polyester fiber or polyamide fiber has no or almost no electrical conductivity, Fibers to which conductivity is imparted by plating, vapor deposition, thermal spraying or the like with a metal as described above may be used. Nippon Silkworm Dyeing Co., Ltd. which dyes and absorbs metallic copper on acrylic fiber
Thunderron (R) can also be preferably used.

In the present invention, a plurality of fibrous sheets containing the above-mentioned conductive fiber short fibers are laminated, laminated with a resin or impregnated with a resin to form a composite, a honeycomb shape, a corrugated plate shape, or the like. And a complex electric permittivity within a frequency range of 1 to 20 GHz is controlled within the above range.

When the complex permittivity is controlled within the above range within the frequency range of 1 to 20 GHz,
Although it can be said that it has excellent electromagnetic wave absorption characteristics, it is particularly preferable to define a complex dielectric constant with a target frequency of 10 GHz. Because the radio wave of 10 GHz has a wavelength of 3 cm, which is neither too short nor too long, so that the complex relative permittivity can be easily measured and the measurement accuracy is high. Further, in the configuration of the concrete embodiment of the present invention, the values of the real part and the imaginary part of the complex relative permittivity are often substantially constant at 1 to 20 GHz, and the value of 10 GHz is representative of the frequency of 1 to 20 GHz. This is because it can be treated as a value.

The real part ε _R of the complex permittivity is a term corresponding to a normal relative permittivity. If it is too small, the radio wave cannot be efficiently attenuated because the compression of the wavelength of the radio wave taken into the absorber is small. . On the other hand, if it is large, the radio wave will be reflected on the surface, so that it must be in the range of 2 to 50 in order to be efficiently taken in and absorbed inside.

The imaginary part ε _J of the complex permittivity is a term caused by electrical loss, and this term converts radio wave energy into heat energy and attenuates the radio wave. ε _J is
It must be in the range of the square root of ε _R multiplied by a coefficient of 0.7 to 6.5. If it is too small, the attenuation of radio waves will be small, and if it is too large, the surface reflection will be large, and in order to avoid it, the radio wave absorber must be made extremely thin, which makes it difficult to control the thickness.

As the conductive fiber, carbon fiber is preferably used, which has conductivity by itself without any special treatment, and has the excellent features of high strength, high elasticity and small specific gravity. Further, carbon fibers can be used by cutting unnecessary long fibers, which have been discharged in the manufacturing process of carbon fiber long-fiber woven fabric and have been conventionally treated as industrial waste, into short fibers. The short fiber of the carbon fiber as described above preferably has a length of 0.1 to 20 mm and an aspect ratio (fiber length / fiber diameter) of 5 or more. If the fiber length is too short or the aspect ratio is too small, it becomes difficult for the fibers to overlap with each other, and the number of contacts will decrease.If you try to compensate for the decrease in contacts, the manufacturing cost will increase. Become. Also, when the average fiber length becomes long, it seems that the fibers are overlapped with each other and the usage amount is small, but on the contrary, since it is easy to break, it cannot be reduced so much. If it exists, you may contain the non-conductive short fiber. As for the ratio, the conductive short fibers are non-conductive short fibers 10
It is preferably 0.1 to 10 parts by weight with respect to 0 parts by weight. If the non-conductive fiber also has a similar shape to the conductive fiber,
Not only can the conductive fibers be easily coordinated uniformly, but the electrical loss can be controlled by the ratio of the two. In that case, if the amount of the conductive fibers is too small or too large, the uniformity of either of them is deteriorated, and the radio wave absorption tends to vary. With such a structure, it becomes easy to obtain a good electromagnetic wave absorber having a complex dielectric constant within the range of the present invention. In addition, various characteristics such as rigidity and strength of the mixed fiber material can be reflected.

The non-conductive fiber is at least one selected from glass fiber, aromatic polyamide fiber, polyphenylene sulfide fiber, polyether ether ketone fiber, and polyparaphenylene benzobisoxazole fiber.
It is preferred to be a kind of short fiber. These fibers are excellent in strength and flame retardancy. In particular, when the fiber is used in combination with a resin, “Kevlar (R)”, which is a kind of aromatic polyamide fiber and is a para-aramid fiber, has high strength,
Since it has high elasticity, it has a large reinforcing effect on the resin and is particularly preferably used. In addition, biodegradable fiber such as polylactic acid fiber,
It is preferably used to reduce environmental load.

There is no particular limitation on the form of the fiber sheet having the conductive short fibers, and the spun yarn obtained by mixing the conductive short fibers and the non-conductive short fibers into a woven fabric or a knitted fabric may be mixed with both fibers. You may make it into a nonwoven fabric and make it into a sheet. But,
In order to manufacture at a lower cost, a non-woven fabric is particularly preferable. The nonwoven fabric is not particularly limited as long as it has a form in which short fibers are uniformly dispersed in a sheet shape, but it is preferable that the conductive fibers are uniformly dispersed as randomly as possible. Further, the non-woven fabric here includes paper. As a method for manufacturing a non-woven fabric, any method such as a method of mixing short fibers and arranging them in a sheet shape and then interlacing the fibers with a needle punch, or a method of mixing short fibers in water and making paper is preferably used. To be done. If necessary, an inorganic binder such as aluminum hydroxide or an organic binder such as starch, polyvinyl alcohol, polyethylene, paraffin or acrylic fiber may be added.

[0021] Further, since the radio wave absorber is often used for the peripheral use of electronic parts, the non-woven fabric is preferably made of
Flammability test UL-94 for plastic materials for equipment parts
Flame retardant standard V-0,1 or VTM- stipulated in safety standards
Those satisfying 0 and 1 are preferable.

In order to satisfy the above-mentioned flame-retardant standard, it is preferable that the fiber component other than the conductive fiber is a flame-retardant fiber such as glass fiber, aromatic polyamide fiber, polyphenylene sulfide fiber or the like. However, even when the fiber component other than the conductive fibers is a non-flame retardant polyester fiber or the like, the resin mixture containing the flame retardant may be applied by impregnating the non-woven fabric. It is preferable that the flame retardant does not contain a halogen element having a large environmental load, and it is at least one selected from condensed phosphoric acid ester, phosphoric acid ester, aromatic diphosphate, magnesium hydroxide, aluminum hydroxide and red phosphorus. However, even if the addition amount is small, a high effect can be obtained, which is preferable.

Among the methods for producing non-woven fabric, wet papermaking is one of the effective methods for producing a large amount of non-woven fabric at a low cost at one time. At this time, in order to wind up cleanly, it is preferable to add a pulp component which functions as a "link" for facilitating entanglement of fibers. As the pulp component, any of cellulose type and synthetic fiber type can be used. When flame retardancy is required, it is preferably flame retardant pulp. Among them, the aromatic polyamide fibers, "Kevlar (R)" of para-aramid fiber and "Nomex (R)" of meta-aramid fiber, are excellent in having the characteristics of each fiber while having a pulp form. It is a material.

It is preferable that the flame-retardant pulp is contained in the range of 10 to 30% because the finished non-woven fabric is good.

The electrical conductivity of the non-woven fabric is preferably 1 S / cm or less. The electrical conductivity of the non-woven fabric may fluctuate slightly depending on the dispersed state of the conductive fibers, but those having the electrical conductivity in this range tend to have a drastically increased absorption amount.

The basis weight of the non-woven fabric is preferably in the range of 50 to 200 g / m ² . Further, the thickness is 50 μm to 30
It is desirable to be in the range of 0 μm. Since a certain amount of thickness is required to absorb radio waves, a plurality of non-woven fabrics may be arranged in layers and used. However, if the fabric weight per sheet is too small, many sheets must be laminated in a large amount of time, and conversely if the fabric weight is too large, it becomes heavy and difficult to handle. It is preferably in the range.

As described above, it is also preferable that the non-woven fabric is formed by compounding a resin. This is because the effect of reinforcing the resin by the fibers can be expected. Resins include thermosetting resins such as epoxy resin, unsaturated polyester resin, phenol resin, polyimide resin, polybismaleimide resin, polyester resin, polyamide resin, polyethylene resin, vinyl chloride resin, polyether ether ketone resin, polylactic acid resin. A thermoplastic resin such as the above can be preferably used. For applications requiring heat resistance, it is preferable to use a polyimide resin, polybismaleimide resin, or polyether ether ketone resin.

The thickness of the radio wave absorber is preferably 1 to
It is preferable to be in the range of 30 mm in order to have absorption in the microwave band.

On the other hand, in a place where the electromagnetic wave absorber does not have to have a flat appearance, for example, in a place where an anechoic chamber or an OA room is installed, the above-mentioned non-woven fabric may be used in a honeycomb shape or a filter unit. By adopting a corrugated plate or pyramid three-dimensional shape, a structure in which radio waves are absorbed by internal reflection can be obtained, and a high radio wave absorption effect can be obtained with a small amount of non-woven fabric used. There is no particular limitation on the method of forming a three-dimensional shape, and a method of embossing the non-woven fabric to make a crease or the like is included in this in order to facilitate assembling into a three-dimensional shape.

The radio wave absorber according to the present invention is preferably used not only as a one-layer type radio wave absorber but also as an absorption layer of a two-layer type radio wave absorber by providing a matching layer in the direction in which radio waves are incident.

The matching layer has a target frequency of 1 to 20 GHz.
In the range of, in particular complex relative permittivity at 10GHz is, the real part of the complex relative permittivity epsilon _R, when the imaginary part was epsilon _J epsilon _R
= 3 to 5ε _J ≦ 0.2 is preferable. By providing such a matching layer to form a two-layer type electromagnetic wave absorber,
It is possible to provide a radio wave absorber having a wider absorption band particularly in the microwave band and a relatively wider absorption in the absorption band than the one-layer type radio wave absorber.

The matching layer preferably contains at least one fabric selected from woven fabrics, knitted fabrics and non-woven fabrics. The cloth itself may be laminated with the absorbent layer, or the resin containing cloth may be laminated with no problem on the absorption effect.

The fabric preferably contains at least one fiber selected from glass fiber, aromatic polyamide fiber and polyphenylene sulfide fiber when flame retardancy is required. It is also preferable to use polylactic acid fiber in order to reduce the environmental load.

The thickness of the matching layer is preferably in the range of 1 to 20 mm in order to have a wide absorption width in the microwave band.

Each of the above-mentioned radio wave absorbers or radio wave absorbers is preferably used alone. However, by having the radio wave absorbers or radio wave absorbers on at least one surface of the conductive base material, not only the absorption performance is improved. It is possible to create a more excellent radio wave shielding structure that also has shielding performance.

When the electromagnetic wave absorber or the electromagnetic wave absorber is provided on one side of the base material, the electromagnetic wave can be absorbed and shielded from one direction, but when it is provided on both sides, the electromagnetic wave can be absorbed and shielded from both directions, and both sides can be absorbed. Since it is possible to reduce the interference of the above, it can be particularly preferably used for applications such as partitions in hospitals and offices.

[0037]

The present invention will be described in more detail with reference to the following examples. The performance values shown in Examples and Comparative Examples were measured by the following methods. <Complex Permittivity> Using a rectangular waveguide, the complex permittivity was determined by the S parameter method. The S-parameter method refers to reflection (S11) and transmission (S2) of a sample inserted in the middle of a transmission line.
This is a method of measuring 1) with a network analyzer to obtain a complex permittivity.

As the rectangular waveguide, a sample holder for permittivity measurement by S-parameter method manufactured by Kanto Electronics Co., Ltd. was used, and a network analyzer manufactured by Agilent Technology Co., Ltd. was used.

<Reflection Loss> After measuring the reflection level when a radio wave was vertically applied to an aluminum plate having a size of 30 cm × 30 cm and a thickness of 1 mm, the radio wave was applied to a sample having the same area, and the reflection loss was calculated from the difference between the two. (DB) was measured. The reflection loss was measured by transmission (S21) of the network analyzer.

Example 1 Fiber length of 12 m from polyacrylonitrile-based material
m, fiber diameter 7 μm, short carbon fiber, fiber length 1
A glass chopped yarn having a diameter of 7 mm and a diameter of 7 mm and Nomex (R) pulp having an average fiber length of 0.7 mm were mixed so that their weight ratios were 1% by weight, 79% by weight and 20% by weight, respectively, and wet papermaking was performed. . The obtained mixed paper has an electric conductivity of 3.3 × 10 ⁻³ S / cm or less and a thickness of 0.5 m.
m, the basis weight was 100 g / m ² , and this mixed paper satisfied the flame retardancy standard V-0.

The above mixed paper was impregnated with a room temperature curing type epoxy resin and three layers were laminated to obtain a composite material plate having a complex dielectric constant of 18-4j at a frequency of 10 GHz. The thickness of the obtained composite material plate was 1.8 mm. The complex relative permittivity of this composite material plate at 1, 5, 15, and 20 GHz was also measured and is shown in Table 1.

Next, this plate was used as an absorption layer, and an aluminum plate having a thickness of 1 mm was attached as a reflection layer on the back surface to obtain a radio wave absorber. The reflection loss in the said absorber was measured at 7-13 GHz. The results are shown in Fig. 1.

Example 2 Short fibers of carbon fiber, glass chop yarn and Nomex (R) pulp (manufactured by DuPont) similar to those in Example 1 were used in a weight ratio of 3 respectively.
Mix so that weight%, 77%, and 20% by weight,
Wet papermaking. The electric conductivity of the obtained mixed paper is 3.3 ×.
10 ^-3 S / cm or less, thickness 0.5 mm, basis weight 100
It was g / m ² . In addition, this mixed paper is flame retardant standard V-0.
Was satisfied.

The above mixed paper was impregnated with a room temperature curable epoxy resin and two layers were laminated to obtain a composite material plate (A) having a complex dielectric constant of 19-24j at a frequency of 10 GHz. The thickness of the obtained composite material plate (A) was 1.2 mm.
In addition, 1, 5, 15, 20 GH of this composite material plate (A)
The complex relative permittivity at z was also measured and is shown in Table 1.

Next, an aramid fiber Kevlar (R) fabric (plain weave, basis weight 460 g / m ² ) was impregnated with a room temperature curing type epoxy resin to laminate 6 layers, and a complex dielectric constant at a frequency of 10 GHz was 4-0.1 j. A composite material plate (B) was obtained. The thickness of the obtained composite material plate (B) was 2.6 mm.

Next, a composite material plate (A) serving as a radio wave absorbing layer and an composite material plate (B) serving as a matching layer are bonded on an aluminum plate having a thickness of 1 mm serving as a reflection layer and further bonded thereon with an adhesive. It was used as a radio wave absorber. The reflection loss in the said absorber was measured at 7-13 GHz. The results are shown in Figure 2.

Example 3 Short fibers of the same carbon fiber as in Example 1, Kevlar (R) chop yarn having a fiber length of 6 mm (manufactured by Toray DuPont) and Nomex pulp (R) having an average fiber length of 0.7 mm (Dupont) Manufactured by Mitsui Chemicals Co., Ltd., each having a weight ratio of 0.4 wt%,
6% by weight and 20% by weight were mixed, and wet papermaking was performed. The electric conductivity of the obtained mixed paper is 3.3 × 10 ⁻³ S /
The thickness was 0.5 cm or less, the thickness was 0.5 mm, and the basis weight was 100 g / m ² . Bond 6 sheets above with adhesive and
A fiber sheet plate (C) having a complex relative dielectric constant in z of 25-28j was obtained. The complex relative permittivity of the fiber sheet plate (C) at 1, 5, 15, and 20 GHz was also measured and shown in Table 1. The thickness of the obtained fiber sheet board is 1 m.
It was m.

Next, glass fiber woven fabric (plain weave, fabric weight 430)
g / m ² ) was impregnated with a room temperature curing type epoxy resin to obtain a composite material plate (D) having a complex dielectric constant of 5 to 0.1 j at a frequency of 10 GHz.

Next, an aluminum plate having a thickness of 1 mm, which serves as a reflection layer, is bonded with a fiber sheet plate (C), which serves as a radio wave absorbing layer, and a composite material plate (D), which serves as a matching layer.
It was used as a radio wave absorber. About the absorber 7-13 GHz
The reflection loss at was measured. The results are shown in Fig. 3.

Comparative Example 1 A non-woven fabric was obtained with 100% of the short carbon fibers of Example 1.
The electric conductivity of this non-woven fabric is 10 ² S / cm and the thickness is 0.5.
mm and basis weight was 100 g / m ² . The above nonwoven fabric was impregnated with a room temperature curable epoxy resin and three layers were laminated to obtain a composite material plate having a complex dielectric constant of 90-70j at a frequency of 10 GHz. In addition, 1, 5, 1 of this composite material plate
The complex relative permittivity at 5 and 20 GHz was also measured and shown in Table 1. The thickness of the obtained composite material plate is 1.8 mm. Next, this plate is used as an absorption layer and a reflection layer on the back surface of 1 m.
A m-thick aluminum plate was attached to make a radio wave absorber. The reflection loss in the said absorber was measured at 7-13 GHz. The results are shown in Fig. 4.

Comparative Example 2 The glass chopped yarn and Nomex (R) pulp of Example 1 were mixed so that their weight ratio was 80% by weight and 20% by weight, and wet papermaking. The resulting mixed paper has an electric conductivity of 10 ⁻³ S / cm or less, a thickness of 0.5 mm, a basis weight of 100 g / m ² , and a transmission attenuation of 0 d at 10 GHz.
It was B. The above mixed paper is impregnated with epoxy resin and three layers are laminated, and the complex dielectric constant at a frequency of 10 GHz is 5
A composite plate of 0.01 j was obtained. Further, the complex relative permittivity of this composite material plate at 1, 5, 15, and 20 GHz was also measured and shown in Table 1. The thickness of the obtained composite material plate is 1.8 mm.

Next, this plate was used as an absorption layer and an aluminum plate having a thickness of 1 mm was attached as a reflection layer on the back surface to obtain a radio wave absorber. The reflection loss in the said absorber was measured at 7-13 GHz. Results are shown in FIG.

[0053]

[Table 1]

As a result, it can be said that the embodiment has a higher radio wave absorption effect in the microwave region and has a wider bandwidth than the comparative example.

[0055]

According to the present invention, a fiber sheet having conductive short fibers is included, and a complex permittivity suitable for electromagnetic wave absorption can be controlled. The fiber sheet is easy to use and has a wide range of applications. A wide wave absorber can be obtained.

[Brief description of drawings]

1 is a measurement result of reflection loss of a radio wave absorber according to Example 1. FIG.

FIG. 2 is a measurement result of reflection loss of the radio wave absorber according to the second embodiment.

FIG. 3 is a measurement result of reflection loss of the radio wave absorber according to the third embodiment.

4 is a measurement result of reflection loss of the radio wave absorber according to Comparative Example 1. FIG.

5 is a measurement result of reflection loss of the radio wave absorber according to Comparative Example 2. FIG.

Claims (23)

[Claims] 1. A complex sheet including a fiber sheet having short fibers of conductive fibers, wherein at least one point of the complex relative permittivity within a frequency range of 1 to 20 GHz is the real part of the complex relative permittivity. When ε _R and imaginary part are ε _J , the following (1)
~ A radio wave absorber in the area surrounded by the formula (4). (1) ε _R = 2 (2) ε _R = 50 (3) ε _J = 0.7 ε _R ^1/2 (4) ε _J = 6.5 ε _R ^1/2 2. A complex relative permittivity at a frequency of 10 GHz is constituted by including a fiber sheet having short fibers of conductive fibers, and the real part of the complex relative permittivity is ε _R and the imaginary part is ε _J.
Then, the electromagnetic wave absorber within the region surrounded by the following formulas (1) to (4). (1) ε _R = 2 (2) ε _R = 50 (3) ε _J = 0.7 ε _R ^1/2 (4) ε _J = 6.5 ε _R ^1/2 3. The radio wave absorber according to claim 1, wherein the conductive fiber is carbon fiber. 4. The electromagnetic wave absorber according to claim 3, wherein the carbon fiber has a length within a range of 0.1 to 20 mm and an aspect ratio (fiber length / fiber diameter) of 5 or more. 5. A conductive fiber and a non-conductive short fiber are contained, and the conductive fiber has a content of 0.1 to 100 parts by weight of the non-conductive fiber.
The radio wave absorber according to any one of claims 1 to 4, which is contained within a range of 1 to 10 parts by weight. 6. The non-conductive short fiber is at least one selected from glass fiber, aromatic polyamide fiber, polyphenylene sulfide fiber, polyether ether ketone fiber, polyparaphenylene benzobisoxazole fiber, and polylactic acid fiber. The radio wave absorber according to claim 5, which is a short fiber of. 7. The fiber sheet is in the form of a non-woven fabric,
The radio wave absorber according to any one of claims 1 to 6. 8. A non-woven fabric is a flame-retardant standard V-defined by UL-94 safety standard for flammability test of plastic materials for equipment parts.
0,1 or VTM-0,1 is satisfied, 7.
Electromagnetic wave absorber described in. 9. The radio wave absorber according to claim 7, wherein the non-woven fabric contains flame-retardant pulp. 10. The radio wave absorber according to claim 9, wherein the flame-retardant pulp is at least one kind of pulp selected from aromatic polyamide pulp. 11. The radio wave absorber according to claim 9, which contains a flame-retardant pulp in the range of 10 to 30% by weight. 12. The radio wave absorber according to claim 7, wherein the non-woven fabric has an electric conductivity of 1 S / cm or less. 13. The non-woven fabric has a basis weight of 50 to 200 g / m ^2.
13. The radio wave absorber according to claim 7, which is within the range. 14. The radio wave absorber according to claim 7, wherein the non-woven fabric comprises a plurality of non-woven fabrics arranged in layers. 15. The radio wave absorber according to claim 1, wherein a resin is compounded and molded. 16. The radio wave absorber according to claim 1, having a thickness in the range of 1 to 30 mm. 17. A shape selected from a three-dimensional shape such as a honeycomb shape, a corrugated plate shape, and a pyramid shape.
The electromagnetic wave absorber according to any one of 1. 18. A radio wave absorber comprising the radio wave absorber according to claim 1. 19. A complex relative permittivity at a target frequency on at least one surface of a radio wave absorber, where ε _{R is} a real part of the complex relative permittivity and ε _J is an imaginary part, ε _R = 3 to 5 ε _J ≦ The radio wave absorber according to claim 18, which has a matching layer in the range of 0.2. 20. The matching layer contains at least one kind of fabric selected from woven fabric, knitted fabric and non-woven fabric.
9. The radio wave absorber according to item 9. 21. The electromagnetic wave absorber according to claim 20, wherein the cloth contains at least one fiber selected from glass fiber, aromatic polyamide fiber, polyphenylene sulfide fiber, and polylactic acid fiber. 22. The electromagnetic wave absorber according to claim 19, wherein the matching layer has a thickness within a range of 1 to 20 mm. 23. A radio wave shield structure having the radio wave absorber according to any one of claims 1 to 17 or the radio wave absorber according to any one of claims 18 to 22 on at least one surface of a conductive base material. body.