JP2008270582A - Radio wave absorber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a radio wave absorber capable of absorbing radio waves having a center frequency in a frequency range of, especially, about 2 to 5.8 GHz more effectively than a conventional λ/4 type absorber while maintaining superior heat insulating property and sound absorptitivty. <P>SOLUTION: Disclosed is the radio wave absorber 1 formed as a λ/4 type absorber by stacking a resistance film layer 2 of 150 to 330 Ω in surface resistance value, a fiber-based heat insulating sound absorber layer 3, and a radio wave reflecting layer 4 in order. For the resistance film layer 2, paper can be used which contains a dielectric loss body. Radio waves having its center frequency in the frequency range can effectively be absorbed by setting the thickness of the fiber-based heat insulating sound absorber layer 3 to 10 to 30 mm. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a radio wave absorber that has excellent heat insulation and sound absorption properties and can effectively absorb radio waves in a specific frequency range.

  In recent years, electronic devices using electromagnetic waves such as mobile communication and wireless LAN have become widespread, and problems have arisen that electromagnetic waves emitted from these electronic devices and reflected electromagnetic waves cause other electronic devices to malfunction. Yes. Therefore, the development of a radio wave absorber that can solve such problems is demanded.

  A λ / 4 type wave absorber has been developed as one of such wave absorbers. As shown in FIG. 2, the λ / 4 type wave absorber has a resistive film layer at a position separated by a quarter wavelength of a radio wave (plane wave) to be absorbed from a radio wave reflection layer 10 such as a metal plate that does not transmit radio waves. 11 is provided.

The principle of radio wave absorption of the λ / 4 type radio wave absorber will be described as follows with reference to FIG. In this figure, assuming that the radio wave propagating in the free space 13 in the direction of the arrow R is incident on the conductive radio wave reflection layer 10 in the absence of the resistance film layer 11, at the entrance slope of the radio wave reflection layer 10. The reflection coefficient S is given by the following equation.
Γ = (Z ₁₀ −Z ₁₂ ) / (Z ₁₀ + Z ₁₂ )
Here, Z10 is the characteristic impedance of the radio wave reflection layer 10, and Z12 is the characteristic impedance of the medium 12. However, since the radio wave reflection layer 10 is a conductor and has a characteristic impedance Z ₁₀ = 0, Γ = −1, and the radio wave is completely reflected by the incident slope, interferes with the incident radio wave, and a standing wave is generated. .

At this time, the load impedance Z in the medium 12 is 0 on the incident surface, and is infinite at a position away from the surface by a quarter of the wavelength λ of the radio wave incident in the direction opposite to the R direction in the figure. (∞). By installing a resistance film 11 of impedance (surface resistance) R at a position separated by λ / 4 in parallel with the radio wave reflection layer 10, the load impedance at this position is obtained by combining R and ∞ in parallel. As a result, it becomes equal to R. In this case, if the characteristic impedance in the free space 13 is Z ₀ , the reflection coefficient Γ _{λ / 4} at this position is
Γλ _{/ 4} = (R−Z ₀ ) / (R + Z ₀ )
Therefore, if the sheet resistance R of the resistance film 11 can be made to completely match the characteristic impedance Z ₀ (= 377Ω / □) of the free space, the reflection coefficient Γ _{λ / 4} = 0 can be obtained. The amount of radio wave absorption S (= −20ln | Γλ _{/ 4} |) in the absorber is maximized.

For this λ / 4 type absorber, many proposals have been made, such as a material in which various materials are sandwiched as a spacer between the radio wave absorption layer and the resistance film layer. For example, in Patent Document 1, in order to have light weight, excellent handleability, and at the same time, sound absorption performance, a sound absorber holding powder is formed in a gap between fibers via a binder, and one side of the sound absorber is formed. A radio wave absorber has been proposed in which a conductive layer is formed and a resistive film layer is formed on the other surface. Further, this document discloses that it is possible to have substantially the same sheet resistance value to the characteristic impedance Z ₀ of the as the resistive coating layer (about 377 ± 38Ω).
JP-A-8-181383

  However, the resistance film layer and the conductive layer in the radio wave absorber of Patent Document 1 are used, and a fiber-based heat-absorbing sound absorber having excellent heat insulation and sound absorption properties such as general glass wool is used instead of the sound absorber described in the document. When a radio wave absorber is formed, for example, there are cases where sufficient radio wave absorption characteristics cannot be obtained for radio waves for a wireless LAN (center frequency 2.45 GHz). In addition, it is possible to use the sound absorber described in Patent Document 1 as a radio wave absorber having the same configuration as the radio wave absorber described in the same document, but holding the powder inside the fiber-based heat-absorbing sound absorber, This is not preferable in terms of manufacturing cost and workability.

  Therefore, in order to solve the above problems, the present invention maintains a high heat insulating property and sound absorbing property even when a fiber heat insulating sound absorbing member is used, and has a frequency of several GHz band, particularly about 2 to 5.8 GHz. An object of the present invention is to provide a radio wave absorber capable of absorbing a radio wave having a central frequency in the region more effectively than a conventional λ / 4 type absorber.

  As a result of intensive studies to solve the above problems, the present inventors have found that when a fiber-based heat-absorbing sound absorber is used as a spacer of a λ / 4 type wave absorber, the sheet resistance of the resistive film layer laminated on the one surface is used. By setting the value within a predetermined range, it was found that radio waves in the specific frequency range can be effectively absorbed, thereby achieving the object, and the present invention has been completed.

  That is, the object is to form a λ / 4 absorber by sequentially laminating a resistance film layer having a sheet resistance value of 150 to 330Ω, a fiber-based heat-absorbing sound absorber layer, and a metal reflector layer. Achieved by the featured wave absorber.

  According to the radio wave absorber of the present invention, by using a fiber-based heat-absorbing sound absorber layer as a spacer of the λ / 4 type radio wave absorber, it has excellent heat insulating properties and sound absorbing properties, and the surface resistance value of the resistive coating layer is reduced. By setting it within the predetermined range, sufficiently superior radio wave absorption characteristics can be obtained as compared with the conventional λ / 4 type absorber. As a result, it is possible to eliminate the influence on the other electronic devices and the like due to the radio waves radiated from the electronic devices and the reflected waves.

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a side sectional view showing a radio wave absorber according to the present invention. In this figure, a radio wave absorber 1 according to the present invention is formed by laminating a resistive film layer 2, a fiber-based heat-absorbing sound absorber 3 and a radio wave reflection layer 4 in this order and forming a λ / 4 type absorber. .

  The resistance film layer 2 has a sheet resistance value of 150 to 330Ω / □. The resistance film layer having a surface resistance value in this range is particularly suitable when combined with the fiber-based heat-absorbing sound absorber 3, and by setting the fiber-based heat-absorbing sound absorber 3 to a predetermined thickness, for example, a wireless LAN 2.45 GHz radio waves and the like can be effectively absorbed. When the sheet resistance value is less than 150Ω / □, the surface itself reflects radio waves, and when it exceeds 330Ω / □, the radio wave absorption characteristics deteriorate.

  The resistance film layer 2 is not particularly limited in its material and the like as long as it has the above surface resistance value. For example, in addition to a known thin film prepared by ion plating, vapor deposition, sputtering, or the like using a metal oxide, metal nitride, or a mixture thereof, a paper containing a dielectric loss material may be used. As the metal oxide, ITO (indium oxide / tin oxide), tin oxide, zinc oxide, or the like can be used. As the metal nitride, titanium nitride or the like can be used. The thin film includes an organic polymer sheet, an organic polymer film, a coating film of an organic polymer resin, and the like. Examples of the paper containing the dielectric loss material include paper in which a dielectric loss material such as carbon particles is supported (infiltrated) on paper as a base material, and paper having a dielectric loss material attached to the surface. In the latter case, a paint containing a dielectric loss material may be prepared and applied to the entire surface of the paper, or a coating film of this paint may be dispersed and adhered to the paper surface in a predetermined shape. .

  In particular, the paper containing the dielectric loss body not only improves sound absorption by combining with a fiber-based heat-absorbing sound absorber layer, but can be easily manufactured by appropriately changing the content of the dielectric loss body. Therefore, it is more preferable because it is easy to change the surface resistance value according to the application within the range of the surface resistance value in the present invention. Here, the paper used as the base material is not particularly limited to the material, and examples thereof include pulp, organic fiber, inorganic fiber, and the form may be any of a nonwoven fabric and a woven fabric. From the viewpoint of manufacturing an inexpensive radio wave absorber, it is preferable to use a paper containing this dielectric loss body as the resistive film layer 2.

  The fiber-based heat-absorbing sound absorber layer 3 is formed in a mat shape by intertwining organic fibers and inorganic fibers, and has heat insulating properties and sound absorbing properties. Examples of such organic fibers include polyester fibers, nylon fibers, polyacrylonitrile fibers, polypropylene fibers, polyethylene fibers, polyvinyl chloride fibers, polyphenylene sulfide fibers, polyether ether ketone fibers, polyparaphenylene benzobisoxazole fibers, and polylactic acid. Synthetic fibers such as fibers; natural fibers such as silk and wool; cellulose fibers (cotton, hemp fiber), cellulose fibers (wool) such as bamboo and linter (sum of flower), and regenerated cellulose fibers (wool) such as rayon It is done. Examples of inorganic fibers include glass fibers such as glass wool, rock wool, alumina fibers, and silicon carbide fibers. These fibers can be used alone or in combination of two or more. The diameter and aspect ratio of these fibers are not particularly limited and can be set as appropriate. The fiber-based heat-absorbing sound absorber having such fibers formed in a mat shape may be coated at least on the front and back surfaces with a coating material such as a resin sheet. Among these, rock wool, glass wool, and cellulose wool are preferable because they are excellent in heat insulation and sound absorption.

  The thickness of the fiber-based heat-absorbing sound absorber layer 3 needs to be changed depending on the frequency of the radio wave to be absorbed and the relative dielectric constant of the layer 3, and cannot be generally stated, but a frequency range of about 2 to 5.8 GHz. For a radio wave having a central frequency, it is preferably set within a range of 10 to 30 mm, and more preferably set within a range of 15 to 30 mm. When this thickness is set to less than 10 mm, the fiber meter heat insulating sound absorber is excellent in flexibility and suitable workability is obtained, but it does not conform to a desired absorption frequency. For example, in the case of a radio wave of 2.45 GHz used for an indoor LAN, the wavelength is 122.4 mm. Therefore, the fiber-based heat insulating sound absorber layer 3 theoretically has a thickness of 30.6 mm, which is 1/4 of this wavelength. In the present invention, in consideration of the thickness of the resistance film layer 2 and the thickness of the adhesive layer described later, the thickness of the fiber-based heat-absorbing sound absorber layer 3 is smaller than the theoretical thickness. Can be set. Further, for example, two of the same type of the same type of heat insulating sound absorbers of the same type may be laminated to have the predetermined thickness.

The density (weight) of the fiber-based heat-absorbing sound absorber layer 3 is not particularly limited as long as the layer 3 has sound absorbing properties and heat insulating properties in addition to the radio wave absorption capability in the specific frequency region at the predetermined thickness. For example, when using glass wool to the layer 3, although the density of the mat a predetermined thickness can be appropriately set in the range of 10kg ^/ cm ³ ~96kg ^/ cm 3 setting, more preferably 15~35kg / cm ³ It is good to do. If the density is less than 10 kg / cm ³ , not only a sufficient heat insulation performance cannot be obtained, but also a uniform thickness may not be obtained, and a desired radio wave absorption characteristic cannot be obtained, and the density exceeds 96 kg / cm ³ . In this case, the weight of the fiber-based heat-absorbing sound absorber is increased, and workability is inferior.

  The radio wave reflection layer 4 can be used without particular limitation as long as it has conductivity and does not transmit radio waves incident from the resistance film layer 2 side to the radio wave absorber of the present invention. Examples of such radio wave reflection layers include metals (plating, vapor deposition films, foils, plates, etc.), carbon fiber cloths, conductive inks, conductive plastics, etc., but preferably metal foils such as aluminum foils and aluminum It is preferable to use a metal sheet (plate) such as a sheet (plate). The radio wave reflection layer 4 may also be one obtained by pasting paper such as kraft paper on the metal foil or metal sheet (plate). When an aluminum foil is used as the metal foil, there are various thicknesses of the aluminum foil, but it is preferable to use a thickness of 7 μm or more in order to completely reflect radio waves. Further, when an aluminum sheet is used as the metal sheet, it is preferable that the sheet itself has flexibility, and for that purpose, the thickness is preferably about 100 μm or less. As a result, the radio wave reflection layer can be easily bent, and can be securely adhered to the fiber-based heat-absorbing sound absorber layer.

  The fiber-based heat insulating sound absorber 3 of the present invention, the resistance film layer 2 and the radio wave reflection layer 4 can be bonded and laminated using a known adhesive. When bonding them, if necessary, paper such as kraft paper may be interposed between them. Examples of such adhesives include vinyl acetate resin adhesives, polyvinyl alcohol resin adhesives, vinyl acetal resin adhesives, vinyl chloride resin adhesives, vinylidene chloride resin adhesives, acrylic resin adhesives, Methacrylic acid resin adhesive, styrene resin adhesive, polyethylene resin adhesive, polyisobutylene resin adhesive, coumarone-indene resin adhesive, polyamide resin adhesive, polyamideimide resin adhesive, polyimide resin In addition to known adhesives such as adhesives, urea resin adhesives, melamine resin adhesives, phenol resin adhesives, resorcinol resin adhesives, silicone resin adhesives, epoxy resin adhesives, these resins or Examples include hot melt adhesives included in other resin series. These adhesives can be used alone or in combination of two or more. Among these, it is preferable to use a hot melt adhesive because it is excellent in continuous productivity in the production line.

  Hot melt adhesives include polyesters; polyester copolymers; polyolefins such as ethylene-vinyl acetate copolymers and ethylene-ethyl acrylate copolymers; modified polyolefins; polyurethanes; polyvinyl chlorides; Prepolymer systems; polyamide systems; thermoplastic rubber systems, styrene-butadiene copolymer systems, styrene-isoprene copolymer systems, and the like. These can be used alone or in combination of two or more.

The adhesive is preferably applied so as to adhere to ²⁰ to 300 g / m ^{2 with} respect to each surface of the fiber-based heat-absorbing sound absorber layer 3. When it is applied beyond this range, the relative dielectric constant of the adhesive adversely affects the radio wave absorption performance of the radio wave absorber of the present invention. Adhesiveness between the coating layer 2 and the radio wave reflection layer 4 is inferior and cannot be laminated integrally as a radio wave absorber. As a result, not only is the radio wave absorption performance lowered, but peeling is likely to occur between the respective layers. .

  In the radio wave absorber 1 of the present invention formed by bonding and laminating each of the above layers to a λ / 4 type absorber, the surface resistance value of the resistance film layer 2 is set within a predetermined range. By changing the thickness of 3 within the predetermined range, the conventional λ / 4 type wave absorber having a resistance film having a surface resistance value of about 377Ω / □ is several GHz band, particularly 2-5. It has excellent absorption characteristics for radio waves having a center frequency in the frequency range of 8 GHz.

  In addition, the radio wave absorber 1 of the present invention can change its excellent heat insulating performance and sound absorbing performance by appropriately selecting the material, particularly the type and density of the fiber heat insulating sound absorber. In particular, the fiber-based heat-absorbing sound absorber layer 3 has the predetermined thickness and density, and the resistance film layer 2 is made of paper as a base material. The thermal conductivity is 0.034 W / (m · K) or less, and the sound absorption performance is preferably NRC (average value of reverberation method sound absorption coefficient at 250, 500, 1000, 2000 Hz) 0.9 or more.

Next, the present invention will be specifically described with reference to examples.
[Examples 1 to 6]
500 mm square glass wool with different thicknesses of 15 mm, 16 mm, 18 mm, 20 mm, 22 mm and 25 mm thickness (each density is 32 kg / m ³ ) is prepared as the fiber-based heat-absorbing sound absorber layer 3 and dielectric is applied to one side 3a of each glass wool. A paper carrying a lossy body (resistive film layer 2) and an aluminum kraft paper (aluminum foil thickness 7 μm) (radio wave reflecting layer 4) were adhered to the other surface 3b with a hot melt adhesive. 6 radio wave absorber samples 6 were obtained (samples of Examples 1 to 6 in the order of the thickness of the glass wool). The sheet resistance value of the polymer film carrying the dielectric loss material was 210Ω / □. As the hot melt adhesive, a vinyl acetate resin adhesive was used, and the amount of application (adhesion) was 30 g per 1 m ^{2 of the} glass wool adhesive surface.

[Comparative Example 1]
A glass wool having a thickness of 25 mm and a square of 500 mm was used, and a Teflon (registered trademark) polymer film carrying a dielectric loss body instead of the paper carrying the dielectric loss body of Example 1 on one side 3a was used on the other side. The same aluminum kraft paper as used in Example 1 was bonded to 3b with the hot melt adhesive used in Example 1 at the same coating amount to produce one sample of wave absorber. The sheet resistance value of the polymer film carrying this dielectric loss body was 377 Ω / □.

[Measurement method of radio wave absorption characteristics]
For the radio wave absorber samples obtained in Examples 1 to 6 and Comparative Example 1, these radio wave absorption characteristics were measured in an anechoic chamber according to the reflected power method. The distance between the surface of the sample and the horn antenna was 820 mm, and the measurement frequency range was 0.1 to 6 GHz. The incident angle of radio waves was 10 °. The measurement results are shown in Table 1 and FIG. Table 1 shows the maximum return loss and the radio frequency at which the maximum return loss is extracted from the measurement results for each radio wave absorber sample. FIG. It is a graph which shows the return loss amount when a radio wave enters, the horizontal axis is the frequency (GHz) of the radio wave incident on the sample, and the vertical axis is the return loss (dB).

[Measurement of sound absorption coefficient]
The sound absorption coefficient of the radio wave absorber obtained in Example 1 and the glass wool used in Example 1 (reference example) were measured by a reverberation chamber method based on JIS A1408. The measurement results are shown in FIG.

According to Table 1 and FIG. 2 showing the radio wave absorption characteristics of each radio wave absorber sample, the radio wave absorber samples of Examples 1 to 6 are all specified with a certain relationship with the thickness of the glass wool used. For the radio wave of the frequency, the return loss shows a maximum peak of 20 dB or more, and for the radio wave in the frequency range of 2.3 to 4.5 GHz, the thickness of the glass wool is changed to at least reduce the return loss. It has been shown that it has excellent absorption characteristics of 18 dB or more. In particular, with respect to the 2.45 GHz radio wave for wireless LAN, it is clear that an absorption characteristic of 20 dB or more can be obtained in terms of return loss by using the radio wave absorber sample of Example 6.
On the other hand, the sample of Comparative Example 1 barely exhibits an absorption characteristic of 16.2 dB with respect to a radio wave with a wavelength of 2.6 GHz, but the radio wave absorption characteristic in the measurement frequency range is not good as a whole.

  In addition, according to FIG. 3, it was found that the radio wave absorber of the present invention is excellent in sound absorption particularly in a low frequency region of 1600 Hz or less, compared to glass wool having excellent sound absorption. In addition, NRC of the sample of Example 1 and the reference example is 0.61 and 0.51, respectively.

  As described above, according to the present invention, it has excellent heat insulating properties and sound absorbing properties, and by changing the thickness of the fiber-based heat insulating sound absorbing layer, it is mainly a few GHz band, particularly 2 to 5.8 GHz. For a radio wave having a center frequency in the frequency range of λ / 4, a radio wave absorber having better absorption characteristics than a conventional λ / 4 type absorber provided with a resistive film having a surface resistance of about 377 Ω / □ can be obtained.

It is the sectional side view which showed the electromagnetic wave absorber of this invention. It is a figure which shows the electromagnetic wave absorption characteristic result of one Embodiment of the electromagnetic wave absorption state of this invention. It is a figure which shows the sound absorption performance of one Embodiment of the electromagnetic wave absorption state of this invention. It is a figure which shows the principle of operation of a (lambda) / 4 type | mold electromagnetic wave absorber.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Radio wave absorber 2 Resistance membrane | film | coat layer 3 Fiber type heat insulation sound absorber layer 3a, 3b Surface of fiber type heat insulation sound absorber layer 4 Radio wave reflection layer

Claims (8)

  1. A radio wave absorber characterized in that a resistive film layer having a sheet resistance value of 150 to 330Ω, a fiber-based heat insulating sound absorber layer, and a radio wave reflection layer are sequentially laminated to form a λ / 4 type absorber.   The radio wave absorber according to claim 1, wherein the resistance film layer is made of paper containing a dielectric loss body.   The radio wave absorber according to claim 1 or 2, wherein a thickness of the fibrous heat-absorbing sound absorber layer is 10 to 30 mm.   The radio wave absorber according to any one of claims 1 to 3, wherein the radio wave reflection layer has a thickness of 7 µm or more and is flexible.   The radio wave absorber according to any one of claims 1 to 4, wherein the fiber-based heat-absorbing sound absorber layer, the resistance film layer, and the radio wave reflection layer are bonded to each other with a hot melt adhesive.   The radio wave absorber according to any one of claims 1 to 5, wherein the heat insulation performance is 0.034 W / (m · K) or less in terms of thermal conductivity, and the sound absorption performance is NRC 0.9 or more.   The radio wave absorber according to any one of claims 1 to 6, wherein an absorption capability of a radio wave having a center frequency in a frequency range of 2 to 5.8 GHz is 18 dB or more in terms of return loss.   The radio wave absorber according to any one of claims 1 to 7, wherein an absorption capability of radio waves having a center frequency of 2.45 GHz is 20 dB or more in terms of return loss.

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