JP6089625B2 - Sheet material for radio wave absorber and radio wave absorber using the same - Google Patents

Description

The present invention is used in a wireless communication system using high-frequency radio waves such as a mobile phone, RFID, wireless LAN, ETC, etc., and is used for absorbing unnecessary radio waves and reducing radio interference. The present invention relates to an absorbent sheet material and a radio wave absorber using the same.

  With the widespread use of wireless communication devices and wireless communication systems that use radio waves in the microwave and ultra-high-frequency region, unnecessary radio waves generated from wireless communication devices, etc. cause problems such as malfunction of other electronic devices, unnecessary radio waves, and multiple reflections There is a problem of poor communication due to radio wave interference with waves. Recently, RFID systems and the like have been widely used, and are used not only in distribution warehouses and factories, but also in offices, stores, and public commercial facilities. Specific usage frequencies of these wireless communications include mobile phones (800 MHz / 900 MHz band, 1.5 GHz band, etc.), RFID (860-960 MHz band), wireless LAN (2.5 GHz band, 5.6 GHz band), There are ETC and DSRC (700 MHz band, 5.8 GHz band), and radio waves in a high frequency region are widely used. In the future, these wireless communication systems are expected to be used in various fields, and the need for a radio wave absorber for realizing a stable communication environment is increasing. It is important for such a wave absorber to be excellent in handleability and workability such as being thin and lightweight, like a general building material such as a wall material, a ceiling material, or a partition. Further, there is an increasing demand for radio wave absorbers and radio wave absorbers that can be widely applied to any radio communication system having different utilization frequencies and that are inexpensive.

  As a radio wave absorber for the purpose of improving the communication environment, for example, Patent Document 1 proposes a radio wave absorber in which a flat soft magnetic powder having a predetermined aspect ratio and a ferrite powder are blended in a resin binder. . In such a radio wave absorber, since the magnetic powder having a large specific gravity is filled at a high blending ratio, there is a problem that the radio wave absorber has a high specific gravity and handling properties and workability are deteriorated.

  Patent Document 2 proposes a radio wave absorber composed of a pattern layer having a conductor element, a loss layer having at least one of a magnetic loss material and a dielectric loss material, and a dielectric layer and a conductive reflection layer. Has been. In this case, since the frequency that causes radio wave absorption is limited to a specific frequency depending on the pattern of the conductor element, the conductor pattern must be designed by each wireless communication system, and deployed in various wireless communication systems. However, there are problems such as a decrease in productivity and an increase in the cost of the radio wave absorber, such as requiring patterns having various types of conductor elements.

  Furthermore, the invention described in patent documents 3 and 4 is mentioned as a prior art relevant to this invention. In these prior arts, a radio wave absorber in which a resistance film layer containing conductive fibers, a spacer layer, and a reflector layer are combined is proposed. The purpose of these prior arts is to improve the oblique incident radio wave absorption performance of the invention of Patent Document 3, and to improve the anisotropy of the radio wave absorption performance of the invention of Patent Document 4. However, each of them has high radio wave absorption performance in a wide frequency range, can be widely applied to various radio communication systems, and the radio wave absorber cannot be significantly reduced in thickness.

JP 2002-15905 A JP 2009-59972 A JP 2010-157696 A JP 2008-205447 A

  The present invention has been made to solve the above-described problems of the prior art, and does not use many types of materials, and uses a small number of types of radio wave absorber sheet materials to support a wide frequency range. An object of the present invention is to provide a radio wave absorber sheet material capable of producing a body and a radio wave absorber comprising the same.

  In order to achieve the above object, the sheet material for a radio wave absorber of the present invention is a radio wave absorber sheet material containing conductive fibers, wherein the complex relative permittivity of the material is measured by a free space method, and the frequency n When the imaginary part of the complex relative permittivity at [GHz] is B (n), 0.5 GHz ≦ n ≦ 6.5 GHz with respect to the imaginary part B (0.5) of the complex relative permittivity at a frequency of 0.5 GHz. The ratio of the imaginary part B (n) of the complex relative dielectric constant in the frequency range, B (n) / B (0.5) is 0.6 or more.

  The radio wave absorber of the present invention is a radio wave absorber having the above-described radio wave absorber sheet material, a dielectric layer, and a radio wave reflector layer, and at least one layer on at least one side of the radio wave reflector layer. The dielectric layer and the wave absorber sheet material are sequentially laminated.

  According to the present invention, by adjusting the imaginary part B (n) of the complex relative dielectric constant of the radio wave absorber sheet material to a certain range in a wide frequency range, without using many kinds of materials, A radio wave absorber corresponding to a wide frequency range, particularly a frequency range of 0.5 to 6.5 GHz can be made with a small number of types of radio wave absorber sheet materials. As a result, the present invention can be widely applied to various wireless communication systems, and in addition, since the radio wave absorber can be significantly reduced in thickness and weight, it is possible to obtain a radio wave absorber sheet material and a radio wave absorber that are easy to handle.

Hereinafter, embodiments for carrying out the invention will be described.
(Radio wave absorber sheet material)
The sheet | seat material for electromagnetic wave absorbers of this invention contains a conductive fiber. When the conductive fiber functions as an electrical loss material, radio wave energy is converted into a minute current and converted into thermal energy, thereby attenuating the radio wave.

  The conductive fibers used for the sheet material for the radio wave absorber include carbon fibers, metal fibers such as stainless steel, copper, gold, silver, nickel, aluminum, iron, and non-conductive fibers such as the metals described above. Examples thereof include a metal imparted with conductivity by plating, vapor deposition, or thermal spraying a representative metal. Among these conductive fibers, carbon fibers are more preferable because the fibers themselves are rigid and can be easily oriented in the base material, and there is almost no change in performance over a long period of use.

  The electromagnetic wave absorber sheet material can contain other materials in addition to the conductive fibers, and can further contain a material to be a base material. Examples of the substrate include resin films and films made of various synthetic resins, and dry or wet nonwoven fabrics containing various fibers as main components. When the substrate is a resin film or film, synthetic rubber materials such as natural rubber, isoprene rubber, butadiene rubber, ethylene propylene rubber, urethane rubber, silicone rubber, polyolefin resin, polyamide resin, polyester resin, polyether resin, Various resins such as a polyimide resin, a polyurethane resin, a polysiloxane resin, a phenol resin, an epoxy resin, an acrylic resin, a urea resin, a melamine resin, a fluororesin, a polyvinyl chloride resin, and a polyphenylene sulfide resin can be given.

  In addition, when the base material is a non-woven fabric or other structure having fibers as main components, inorganic fibers such as glass fibers and ceramic fibers, synthetic fibers, natural fibers such as cotton, hemp, wool, and wood pulp, rayon, etc. Semi-synthetic fibers can be used. Furthermore, examples of the polymer forming the synthetic fiber include polyethylene terephthalate, polybutylene terephthalate, polylactic acid, and copolymer polyester obtained by copolymerizing the acid component of those polyesters with isophthalic acid, 5-sodium sulfoisophthalic acid, adipic acid, and the like. Polyester, nylon 6, nylon 66, nylon 12, nylon 46, polyamide such as copolymerized polyamide of nylon 6 and nylon 66, polyvinyl alcohol, aromatic polyamide, polyetheretherketone, poly Examples thereof include paraphenylene benzobisoxazole, polyphenylene sulfide, polyethylene, and polypropylene.

  In the sheet material for radio wave absorber of the present invention, the complex relative permittivity of the material is measured by the free space method, and 0.5 GHz ≦ n with respect to the imaginary part B (0.5) of the complex relative permittivity at a frequency of 0.5 GHz. It is important that the ratio of the imaginary part B (n) of the complex relative permittivity in the frequency range of ≦ 6.5 GHz, B (n) / B (0.5) is 0.6 or more. Here, the complex dielectric constant will be described. When the complex dielectric constant at the frequency n [GHz] is expressed by ε (n) = A (n) −j · B (n), the real part is A (n) and the imaginary part is B (n). . The real part A (n) is an index indicating the accumulation degree of radio wave energy, and the imaginary part B (n) is an index indicating the loss degree of radio wave energy. If B (n) / B (0.5) is smaller than 0.6, the radio wave energy loss effect of the radio wave absorber sheet material is reduced. It becomes difficult to absorb radio waves in a wide frequency range of 5 to 6.5 GHz. It is more preferable that B (n) / B (0.5) is in the range of 0.7 to 2.5. This is because a radio wave absorber corresponding to the above frequency range can be designed more easily.

  The range of B (n) / B (0.5) described above is to adjust the content and dispersion state of the conductive fibers in the base material, specifically, the contact state between the conductive fibers in the base material. Or it can achieve by adjusting the fiber length of a conductive fiber appropriately.

  In order to obtain the optimal dispersion state of the length of the conductive fiber as described above, the sheet material for a radio wave absorber of the present invention has one peak of the fiber length distribution of the conductive fiber in the range of 1 to 7 mm. Is preferred. Furthermore, when the peak center of the fiber length is L, the ratio of having a fiber length of 0.9 L to 1.1 L with respect to the total number of conductive fibers is preferably 70% or more. Thereby, the dispersion | distribution state of electroconductive fiber can be made uniform, and when it is set as a sheet material, the dispersion | distribution nonuniformity of electroconductive fiber can be decreased. The ratio of 0.9L to 1.1L of fiber length is more preferably 80% or more, and still more preferably 90% or more with respect to the total number of conductive fibers. Furthermore, the content is preferably 0.05 to 0.75 mass%, more preferably 0.1 to 0.6 mass%. Further, by making the fiber length and content of the conductive fiber within this range, when the conductive fiber is included in the base material of the radio wave absorber sheet material, there is little problem of breaking of the conductive fiber, It becomes easy to adjust appropriately to realize the ratio of the imaginary part of the complex relative dielectric constant, B (n) / B (0.5). Here, the average fiber length refers to the length average fiber length, and the length average fiber length is calculated by measuring a predetermined number in a plurality of fields of view using a length measuring device under an optical microscope.

  If the conductive fiber length is too short, it is difficult to obtain a sufficient radio wave energy loss effect. When long conductive fibers are used, if the conductive fibers are in contact with each other, even if a small amount of contact is made, it tends to be in the same state as being connected for a long time, and B (n) / B (0.5) becomes smaller than 0.6. This is because there is a risk of losing.

  Moreover, it becomes easy to implement | achieve the optimal dispersion state of an electroconductive fiber by making content of an electroconductive fiber into said range.

  When the content of the conductive fiber is small, it is difficult to obtain a sufficient radio wave energy loss effect in the frequency band of 0.5 to 6.5 GHz. When the content is large, the contact between the conductive fibers increases, and the conductive fibers connected for a long time increase, and the ratio of the imaginary part of the complex dielectric constant, B (n) / B (0.5) ≧ 0. .6 is difficult to achieve.

  Desirably, the optimal dispersion state of the conductive fibers is such that when the radio wave absorber sheet is observed from the surface with a microscope, the conductive fibers are present in one piece without contacting other conductive fibers, It is preferable that the two or three are in contact with each other in the range of 5 to 40% as a ratio to the total number of conductive fibers.

  By setting it as this range, the conductive fibers are in contact with each other, there is a state where the conductive fibers are connected for a long time, and there is a state where the conductive fibers are present in one, and two or three are in contact with a small number. When connected for a long time, radio waves with a long wavelength, that is, low-frequency radio waves can be absorbed as resistance loss (radio wave energy is lost as thermal energy due to the resistance of the conductive fiber), and one of them is isolated. In addition, in the state where only a small number of two or three are in contact, high-frequency radio waves having a short wavelength can be absorbed as dielectric loss (lost as thermal energy due to polarization vibration of the conductive fiber itself). Thereby, in the frequency band of 0.5 to 6.5 GHz, the value of the imaginary part B (n) of the complex relative permittivity is less changed from the low frequency to the high frequency, and B (n) / B (0.5 ) Can be easily set to 0.6 or more.

In addition, the ratio of the contact state of electroconductive fiber is measured with the optical microscope within the surface 5cm x 5cm square of the sheet | seat material for electromagnetic wave absorbers.
The average fiber length of the conductive fibers is more preferably 1 to 7 mm. In this case, the content of the conductive fiber is preferably 0.2 to 0.7% by mass when the fiber length of the conductive fiber is less than 4 mm. Moreover, when the fiber length of a conductive fiber is 4 mm-7 mm, it is preferable that content is 0.1-0.5 mass%.

The average fiber diameter of the conductive fibers is preferably in the range of 5 to 15 μm. The specific gravity of the conductive fibers is preferably in the range of 1.4 to 2.3 g / cm ³ .

  Furthermore, it becomes easy to achieve an optimal dispersion state when a sheet is formed by uniformly dispersing conductive fibers in the substrate.

  In order to disperse the conductive fibers uniformly, it is preferable that the radio wave absorber sheet of the present invention is made of paper such as paper, paperboard or cardboard. This is because the paper material is obtained by mixing the constituent materials in water to form a sheet, and thus the fibrous constituent materials are easily and uniformly mixed. In addition, it is easy to perform functional processing such as applying a flame retardant or a waterproofing material after paper making or increasing the strength by impregnating a resin.

  Moreover, as above-mentioned, it is preferable that an electroconductive fiber is a carbon fiber. Compared to metal fibers, it has a light specific gravity, so when dispersed in paper, especially when mixed with other materials in water, it does not sink into water like metal fibers and become non-uniform. This is because it is easy to uniformly disperse in the material.

  The dispersibility of this carbon fiber in water is such that 1 g of a carbon fiber bundle cut to a predetermined length is poured into 200 ml of water stirred at a speed of 500 rpm, and then the fiber bundle lump is dispersed and disappears. It is preferable that the average of the time measured by measuring the time required for the measurement and repeating this operation 10 times is within 20 seconds. Moreover, in order to implement | achieve this good dispersibility to water, it is preferable to use the thing in which the sizing process to the carbon fiber surface is not performed.

  As an example of the method for producing a radio wave absorber sheet material of the present invention, a sheet material made of a paper material will be described. First, a material to be a base material such as a fiber material such as pulp is mixed in water, and carbon fiber is added thereto and stirred to obtain a slurry. The slurry is made into a sheet to produce a sheet material for a radio wave absorber made of a paper material. When this method is employed, the stirring time after the carbon fiber is charged is preferably 10 to 120 seconds. Thereby, the carbon fiber can be prevented from being damaged or bent due to the impact of stirring, and a sheet having a stable complex relative dielectric constant can be produced when the sheet material is used.

  In addition, the manufacturing method of the sheet | seat material for electromagnetic wave absorbers of this invention is not limited only to said method, In addition to this, it knead | mixes a conductive fiber with a rubber material or a resin material with a kneader etc., and is rolled. Various methods such as a method of forming a sheet by melt extrusion are selectable.

  The sheet material for a radio wave absorber of the present invention has a real part A (n) of complex relative dielectric constant of 10 to 250 and an imaginary part within a frequency n [GHz] of 0.5 to 6.5 GHz. B (n) is preferably 5 to 150, and the ratio of real part A (n) to imaginary part B (n), and A (n) / B (n) is preferably in the range of 1.5 to 10.

  If the real part A (n) and the imaginary part B (n) of the complex relative permittivity are within these ranges, a sufficient electrical loss effect can be obtained, and reflection of radio waves by the radio wave absorber sheet material itself. Can be suppressed and a high level of radio wave absorptivity can be realized. A (n) is more preferably within the range of 10 to 200, and even more preferably within the range of 10 to 150. Further, B (n) is more preferably within a range of 5 to 100, and further preferably within a range of 5 to 70. If A (n) / B (n) is in the range of 1.5 to 10, when the radio wave absorber sheet material is used as the radio wave absorber, the thickness can be sufficiently reduced. In addition, as A (n) / B (n), it is more preferable that it is the range of 1.5-6.0.

  The thickness of the radio wave absorber sheet material is preferably 0.05 to 2.0 mm.

(Radio wave absorber)
The radio wave absorber of the present invention comprises the radio wave absorber sheet material, a dielectric layer, and a radio wave reflector layer, and at least one dielectric layer and the radio wave absorber on at least one side of the radio wave reflector layer. A sheet material layer is sequentially laminated.

  In the radio wave absorber of the present invention, the radio wave absorber sheet material is disposed on the radio wave arrival side, transmits a part of the incident radio wave, reflects a part thereof, and converts a part into heat energy to attenuate it. have. By laminating a dielectric layer and a radio wave reflector layer, which will be described later, the reflected radio wave on the surface of the radio wave absorber sheet material, and the radio wave transmitted through the radio wave absorber sheet material and reflected from the radio wave reflector layer on the back side Absorbs radio waves by canceling interference. In addition, a part of the radio wave energy is converted into heat energy inside the radio wave absorber sheet material to absorb radio waves.

  Next, the dielectric layer will be described. The dielectric layer is generally made of a material having a real part having a complex relative dielectric constant of 1.0 to 5.0 and an imaginary part having a value of 0.01 to 0.5. The shape is generally board-shaped or sheet-shaped. This is provided for the purpose of holding the radio wave absorber sheet material and the radio wave reflector layer at a constant interval in a substantially parallel state, and the thickness of the dielectric layer is dependent on the wavelength of the radio wave to be absorbed. And by setting the sheet material for radio wave absorber and a radio wave reflector layer to be described later, it can function as a radio wave absorber.

Materials for the dielectric layer include natural rubber, isoprene rubber, butadiene rubber, ethylene propylene rubber, urethane rubber, silicone rubber and other synthetic rubber materials, polyolefin resin, polyamide resin, polyester resin, polyether resin, polyimide resin , Polyurethane resins, polysiloxane resins, phenol resins, epoxy resins, acrylic resins, urea resins, melamine resins, fluororesins, polyvinyl chloride resins, polyphenylene sulfide resins, and other foamed resin materials. Fiber, paper, wood, plaster, etc. can be used. These materials may be used alone or in combination of two or more. The material for the dielectric layer is not particularly limited, but from the viewpoint of lightness, a foamed resin such as foamed polystyrene or foamed polyurethane having a density of 0.2 g / cm ³ or less is preferable. The foaming ratio of the foamed resin is preferably 15 to 70 times from the viewpoint of strength and lightness. In addition to the foamed resin, a honeycomb structure, a cardboard structure, or a resin structure having a high porosity may be used.

  Next, the radio wave reflector layer will be described. The radio wave reflector layer is a layer that reflects radio waves. It plays a role of reflecting the radio wave transmitted through the sheet material for the radio wave absorber and returning it to the surface. The electromagnetic wave shielding rate of the radio wave reflector layer is preferably 20 dB or more. The electromagnetic wave shielding rate is the ratio between the radio wave incident on the radio wave reflector layer and the radio wave transmitted through the radio wave reflector layer. As a material of the radio wave reflector layer, a metal plate or metal foil such as aluminum, copper, titanium, iron, nickel, gold, silver, and stainless steel, or a film provided with the above metal on the surface by processing such as plating or vapor deposition, Examples include woven fabric, paper, and non-woven fabric. In addition, woven fabrics, knitted fabrics, nonwoven fabrics, and the like made of conductive fibers such as carbon fibers and metal fibers can also be used. Furthermore, what made the content of the conductive fiber of the sheet material for radio wave absorbers higher, preferably 2% by mass or more, more preferably 5% by mass or more can be used. As the material for the radio wave reflection layer, metal foil, metal-deposited paper or film is preferable in terms of lightness, workability, and cost.

  The stacking order of the layers in the radio wave absorber of the present invention is the order of layer / dielectric layer / radio wave reflector layer made of the radio wave absorber sheet material in order from the radio wave arrival side. Similarly, a layer made of a dielectric layer and a wave absorber sheet material is laminated on the other surface of the radio wave reflector layer, and a layer made of the wave absorber sheet material / dielectric layer / radio wave reflector layer. The layers may be laminated in the order of / dielectric layer / wave absorber sheet material. With this configuration, incoming radio waves from both directions can be absorbed. A plurality of dielectric layers and radio wave absorber sheet materials may be laminated on the radio wave absorber sheet material on the surface on the radio wave arrival side.

  In the radio wave absorber of the present invention, the ratio of the thickness d [mm] of the radio wave absorber sheet material to the sum T [mm] of the radio wave absorber sheet material and the dielectric layer, d / T is 0.02. It is preferable to be within the range of ~ 0.15. Within this range, a sufficiently thin radio wave absorber can be obtained.

  Further, when the wavelength of the radio wave to be absorbed is λ [mm], the ratio of the sum T of the thickness of the radio wave absorber sheet material and the dielectric layer to the wavelength λ of the radio wave to be absorbed, λ / T is 10 or more. Is preferred. It is possible to achieve a sufficient thickness reduction of the radio wave absorber, improve the handleability of the radio wave absorber, and use a wide space area in which the radio wave absorber is disposed.

  Furthermore, it is preferable that the amount of radio wave absorption is 10 dB or more in at least a part of the frequency range of 0.5 to 6.5 GHz because a sufficient effect for improving the communication performance of the wireless communication system can be obtained. More preferably, it is 15 dB or more, More preferably, it is 20 dB or more.

Moreover, it is preferable that the density of a radio wave absorber is 1.5 g / cm < ³ > or less from the point of weight reduction. More preferably, it is preferably 0.8 g / cm ³ or less.

  The layer made of the wave absorber sheet material may be a laminate of a plurality of wave absorber sheet materials. As the laminating method, laminating can be suitably used when the electromagnetic wave absorber sheet material can be rolled up. In addition to this, when the object to be laminated is a paper material, a method of processing a cut sheet with a slip sheet can be employed, but the present invention is not limited to this.

  As a method of laminating a plurality of sheet materials for a radio wave absorber of the present invention, and a method of joining each layer of a radio wave absorber sheet material, a dielectric layer, and a radio wave reflector layer, which are constituent materials of a radio wave absorber, There is no particular limitation as long as the layers are bonded, bonded, or fixed. For example, a method of applying an adhesive between the layers and bonding them, or mounting a frame material around the laminate of each layer without bonding the layers. It is possible to adopt a fixing method, a method in which a heat-fusible film is disposed between the layers, and fixed by hot pressing. When using an adhesive, starch, vinyl acetate, acrylic, polyester, polyvinyl alcohol, epoxy, rubber, and the like can be used.

  As an application to which the radio wave absorber of the present invention is applied, there is an application in which the radio wave absorber is installed on the opposite surface of a radio wave transmission source to suppress the generation of reflected radio waves. It can also be installed in front of an object that is a reflection source of radio waves so as to suppress radio wave irregular reflection. For example, in a space where a radio wave transmission device is used, there is an application in which a radio wave absorber is installed on at least one surface to prevent radio wave irregular reflection in the space. In addition to this, when the radio wave transmission device emits radio waves radially, it is also used for the purpose of restricting the radio wave transmission area by arranging radio wave absorbers on the top, bottom, left and right sides, top and bottom, left and right sides of the radio wave source. It can be used suitably.

  Although the specific form of an electromagnetic wave absorber is not specifically limited, It can also process and use for the form of a self-supporting partition or the interior material for building materials.

  Examples of the present invention will be described below. The performance values shown in the examples were measured by the following method.

(Complex relative permittivity of sheet material for electromagnetic wave absorber)
Vector network analyzer (model: N5230, manufactured by Agilent Technologies) and radio transmission / reception antennas using horn antennas, free space method, complex relative permittivity of radio wave absorber sheet material and radio wave absorber absorption Was measured.

(Measurement of material complex permittivity for normal incidence)
A sheet material sample for a radio wave absorber is arranged on the front surface of an aluminum plate having a length of 60 cm, a width of 60 cm, and a thickness of 5 mm via a polystyrene foam spacer having a foaming ratio of 70 times, and a transmitting and receiving antenna is placed at a position 3 m away from the sample. Set so that the incident angle of the radio wave is 7 degrees, input the radio wave in the frequency range of 0.5 to 7 GHz to the sample, and use a vector network analyzer (model: N5230, manufactured by Agilent Technologies) to set the input impedance. It was measured. Thereafter, the sample was removed, the input impedance of the spacer alone was measured in the same manner as described above, and the complex relative dielectric constant of the sample was calculated by back calculation from the difference in input impedance between when the sample was present and when there was no sample. The frequency range of 0.5-7 GHz is divided into four ranges of 0.5-2.5 GHz, 2.0-4.0 GHz, 3.5-5.5 GHz, 5.0-7.0 GHz, Measurement was performed for each range (all ranges were measured in increments of 0.1 GHz). In addition, the polystyrene foam spacer used in the measurement of each range was 70 mm at a frequency of 0.5 to 2.5 GHz, 25 mm at 2.0 to 4.0 GHz, 15 mm at 3.5 to 5.5 GHz, 5.5 to 7.7. The thing of 10 mm thickness was used at 0 GHz.

(Radio wave absorption amount of radio wave absorber)
The transmitting and receiving antennas were set so that the incident angle of radio waves was 7 degrees and the distance between the antenna and the sample was 3 m. In this measurement system, first, as a blank, an aluminum plate 60 cm long × 60 cm wide × 5 mm thick is placed at a sample position, a radio wave is incident, and a reflection level of the radio wave from the aluminum plate is measured by a vector network analyzer (model: N5230). , Manufactured by Agilent Technologies). Next, a radio wave absorber sample was placed on the aluminum plate, and the reflection level was also measured. The radio wave absorption amount of the radio wave absorber was obtained from the reflection levels of both by the following equation.
Radio wave absorption (dB) = Radio wave absorber reflection level (dB) −Aluminum plate reflection level (dB)
The frequency range of 0.5-7 GHz is divided into four ranges of 0.5-2.5 GHz, 2.0-4.0 GHz, 3.5-5.5 GHz, 5.0-7.0 GHz, Measurement was performed for each range (all ranges were measured in increments of 0.1 GHz).

(Measurement of average fiber length and number of fibers of conductive fibers)
The average fiber length of the conductive fibers is such that the surface of the radio wave absorber sheet material is photographed with a 20x objective lens using a laser microscope manufactured by Keyence Corporation, and image concatenation processing is performed. A surface photograph was obtained. Based on this observation photograph, the length of 200 conductive fibers was measured, and a length average fiber length Ln was calculated from the following equation.
Length average fiber length Ln = (Σ (Li ² )) / ΣLi
Here, Li is the measured fiber length (i = 1, 2,... 200).

(Each component material and thickness of electromagnetic wave absorber)
The thickness of the radio wave absorber sheet material and the radio wave reflector was calculated by measuring the thickness of 40 points using a digital thickness gauge (manufactured by Mitutoyo Corporation) and calculating the average value.
The thickness of the dielectric layer was calculated by measuring the thickness of 40 points using a digital caliper (manufactured by Mitutoyo Corporation) and calculating the average value.
The sum T of the thickness of the sheet material for the radio wave absorber and the dielectric layer is 10 points on each side of the radio wave absorber cut into 60 cm square, which will be described later, and 40 points are measured with digital calipers, and the average value is The average value was calculated by subtracting the thickness of the radio wave reflector layer from this average value.
(Radio wave absorber density)
It calculated from the weight of the wave absorber cut | judged in the 60 cm square, and the thickness of the above-mentioned wave absorber.

<Radio wave absorber sheet material>
[Example 1]
Glass fibers having an average fiber length of 4 mm, wood pulp, and aluminum hydroxide were mixed at a ratio of 19.8 mass%, 10 mass%, and 70 mass%, respectively, to form an underwater slurry. A sizing agent-untreated polyacrylonitrile-based carbon fiber having a set fiber length of 3 mm was adjusted so that the content of the sheet base material was 0.2% by mass, and a liquid dispersed in water in advance was added to the slurry, followed by stirring for 60 seconds. After that, wet papermaking was performed at a winding speed of 100 m / min to obtain a sheet material for a radio wave absorber having a basis weight of 98 g / m ² and a density of 0.75 g / cm ³ . Complex relative dielectric constant of the obtained wave absorber sheet material, measured fiber length and content of the conductive fiber used, peak center L and fiber length peak of the average fiber length of carbon fibers in the sheet substrate The ratio of 0.9 L to 1.1 L with respect to the center L is shown in Table 1. The ratio of the imaginary part B (n) of the complex dielectric constant in the frequency range of 0.5 GHz ≦ n ≦ 6.5 GHz to the imaginary part B (0.5) of the complex dielectric constant at a frequency of 0.5 GHz, B (n ) / B (0.5) is 1.0 to 2.1, the value of the imaginary part of the complex dielectric constant is stable, and the radio wave absorber can be applied to a radio wave absorber that supports a wide frequency range. Sheet material.

[Example 2]
The same wave absorber sheet material as that of Example 1 was obtained except that the carbon fiber content was 0.4 mass% and the glass fiber content was 19.6 mass%. The rice basis weight was 96 g / m ² and the density was 0.74 g / cm ³ . Complex relative dielectric constant of the obtained wave absorber sheet material, measured fiber length and content of the conductive fiber used, peak center L and fiber length peak of the average fiber length of carbon fibers in the sheet substrate The ratio of 0.9 L to 1.1 L with respect to the center L is shown in Table 1. The ratio of the imaginary part B (n) of the complex dielectric constant in the frequency range of 0.5 GHz ≦ n ≦ 6.5 GHz to the imaginary part B (0.5) of the complex dielectric constant at a frequency of 0.5 GHz, B (n ) / B (0.5) is 0.9 to 1.6, the value of the imaginary part of the complex dielectric constant is stable, and the radio wave absorber can be applied to a radio wave absorber that supports a wide frequency range. Sheet material.

[Example 3]
The same radio wave absorber sheet material as in Example 2 was obtained except that the basis weight of rice was 150 g / m ² . The rice basis weight was 150 g / m ² and the density was 0.75 g / cm ³ . Complex relative dielectric constant of the obtained wave absorber sheet material, measured fiber length and content of the conductive fiber used, peak center L and fiber length peak of the average fiber length of carbon fibers in the sheet substrate The ratio of 0.9 L to 1.1 L with respect to the center L is shown in Table 1. The ratio of the imaginary part B (n) of the complex dielectric constant in the frequency range of 0.5 GHz ≦ n ≦ 6.5 GHz to the imaginary part B (0.5) of the complex dielectric constant at a frequency of 0.5 GHz, B (n ) / B (0.5) is 1.0 to 1.4, indicating that the value of the imaginary part of the complex relative permittivity is more stable than those of Examples 1 and 2, corresponding to a wide frequency range. The sheet material for a radio wave absorber is easier to apply to the radio wave absorber.

[Example 4]
The same radio wave absorber sheet material as in Example 1 was obtained except that the carbon fiber content was 0.6 mass% and the glass fiber content was 19.4 mass%. The rice basis weight was 103 g / m ² and the density was 0.80 g / cm 3. Complex relative dielectric constant of the obtained wave absorber sheet material, measured fiber length and content of the conductive fiber used, peak center L and fiber length peak of the average fiber length of carbon fibers in the sheet substrate The ratio of 0.9 L to 1.1 L with respect to the center L is shown in Table 1. The ratio of the imaginary part B (n) of the complex dielectric constant in the frequency range of 0.5 GHz ≦ n ≦ 6.5 GHz to the imaginary part B (0.5) of the complex dielectric constant at a frequency of 0.5 GHz, B (n ) / B (0.5) is 0.8 to 1.0, indicating that the value of the imaginary part of the complex relative permittivity is more stable than those of Examples 1 and 2, and corresponds to a wide frequency range. The sheet material for a radio wave absorber is easier to apply to the radio wave absorber.

[Example 5]
The same radio wave absorber sheet material as in Example 1 was obtained except that the carbon fiber content was 0.7 mass% and the glass fiber content was 19.3 mass%. The rice basis weight was 99 g / m ² and the density was 0.76 g / cm ³ . Complex relative dielectric constant of the obtained wave absorber sheet material, measured fiber length and content of the conductive fiber used, peak center L and fiber length peak of the average fiber length of carbon fibers in the sheet substrate The ratio of 0.9 L to 1.1 L with respect to the center L is shown in Table 1. The ratio of the imaginary part B (n) of the complex dielectric constant in the frequency range of 0.5 GHz ≦ n ≦ 6.5 GHz to the imaginary part B (0.5) of the complex dielectric constant at a frequency of 0.5 GHz, B (n ) / B (0.5) is 0.6 to 1.0, the value of the imaginary part of the complex relative permittivity is stable, and the radio wave absorber can be applied to a radio wave absorber corresponding to a wide frequency range. Sheet material.

[Example 6]
The same radio wave absorber sheet material as in Example 1 was obtained except that the set fiber length of the carbon fiber was 6 mm, the carbon fiber content was 0.3 mass%, and the glass fiber content was 19.7 mass%. . The basis weight was 98 g / m ² and the density was 0.70 g / cm ³ . Complex relative dielectric constant of the obtained wave absorber sheet material, measured fiber length and content of the conductive fiber used, peak center L and fiber length peak of the average fiber length of carbon fibers in the sheet substrate The ratio of 0.9 L to 1.1 L with respect to the center L is shown in Table 1. The ratio of the imaginary part B (n) of the complex dielectric constant in the frequency range of 0.5 GHz ≦ n ≦ 6.5 GHz to the imaginary part B (0.5) of the complex dielectric constant at a frequency of 0.5 GHz, B (n ) / B (0.5) is 1.0 to 1.6, the value of the imaginary part of the complex dielectric constant is stable, and the radio wave absorber can be applied to a radio wave absorber that supports a wide frequency range. Sheet material.

[Comparative Example 1]
The same radio wave absorber sheet material as in Example 1 was obtained except that the carbon fiber content was 1.0 mass% and the glass fiber content was 19.0 mass%. The rice basis weight was 99 g / m ² and the density was 0.76 g / cm ³ . Complex relative dielectric constant of the obtained wave absorber sheet material, measured fiber length and content of the conductive fiber used, peak center L and fiber length peak of the average fiber length of carbon fibers in the sheet substrate The ratio of 0.9 L to 1.1 L with respect to the center L is shown in Table 1. The ratio of the imaginary part B (n) of the complex dielectric constant in the frequency range of 0.5 GHz ≦ n ≦ 6.5 GHz to the imaginary part B (0.5) of the complex dielectric constant at a frequency of 0.5 GHz, B (n ) / B (0.5) is 0.3 to 1.0, and the change in the imaginary part of the complex dielectric constant is large, and it is a sheet material for a radio wave absorber that is difficult to cope with a wide frequency range. .

[Comparative Example 2]
The same radio wave absorber sheet material as in Example 1 was obtained except that the set fiber length of the carbon fiber was 6 mm, the carbon fiber content was 0.8 mass%, and the glass fiber content was 19.2 mass%. . The rice basis weight was 96 g / m ² and the density was 0.74 g / cm ³ . Complex relative dielectric constant of the obtained wave absorber sheet material, measured fiber length and content of the conductive fiber used, peak center L and fiber length peak of the average fiber length of carbon fibers in the sheet substrate The ratio of 0.9 L to 1.1 L with respect to the center L is shown in Table 1. The ratio of the imaginary part B (n) of the complex dielectric constant in the frequency range of 0.5 GHz ≦ n ≦ 6.5 GHz to the imaginary part B (0.5) of the complex dielectric constant at a frequency of 0.5 GHz, B (n ) / B (0.5) is 0.2 to 1.1, and the imaginary part of the complex dielectric constant is large, and it is a sheet material for a radio wave absorber that is difficult to cope with a wide frequency range. .

[Comparative Example 3]
The same radio wave absorber sheet material as in Example 1 was obtained except that the set fiber length of carbon fiber was 12 mm, the carbon fiber content was 0.4 mass%, and the glass fiber content was 19.6 mass%. . The basis weight was 101 g / m ² and the density was 0.78 g / cm ³ . Complex relative dielectric constant of the obtained wave absorber sheet material, measured fiber length and content of the conductive fiber used, peak center L and fiber length peak of the average fiber length of carbon fibers in the sheet substrate The ratio of 0.9 L to 1.1 L with respect to the center L is shown in Table 1. The ratio of the imaginary part B (n) of the complex dielectric constant in the frequency range of 0.5 GHz ≦ n ≦ 6.5 GHz to the imaginary part B (0.5) of the complex dielectric constant at a frequency of 0.5 GHz, B (n ) / B (0.5) is 0.2 to 1.0, and the imaginary part of the complex relative dielectric constant is large, and it is a radio wave absorber sheet material that is difficult to cope with a wide frequency range. .

<Radio wave absorber>
[Example 7]
(Radio wave absorber sheet material)
Ten sheets of the radio wave absorber sheet material prepared in Example 2 were laminated using an acetic acid-vinyl emulsion adhesive.

(Dielectric layer)
Styrofoam having a thickness of 23 mm and a density of 0.02 g / cm ³ was used.

(Radio wave reflector layer)
An aluminum sheet in which an aluminum foil having a thickness of 7 μm was bonded to paper (rice basis weight 33 g / m ² ) was used.

(Radio wave absorber)
After applying 93g / m2 of a two-component mixed epoxy resin adhesive (manufactured by Aika Co., Ltd.) on both sides of the dielectric layer, a sheet material for a radio wave absorber is applied on one side and a radio wave reflector on the other side. The layers were stacked and bonded by pressing for 1 day at a pressure of 0.16 kgf / cm ² under a press machine. The laminate was cut to 60 cm × 60 cm to obtain a radio wave absorber. Table 2 shows the measurement results such as the performance of the obtained radio wave absorber. The obtained radio wave absorber had an absorption peak of 18 dB at 0.9 GHz and was suitable as a radio wave absorber. Further, the sum T (hereinafter referred to as thickness T) of the wave absorber sheet material and the thickness of the dielectric layer is 24.3 mm, which is the wave absorber sheet material and the dielectric with respect to the wavelength λ of the absorbed radio wave. The ratio of the thickness of the body layer to the sum T, λ / T, was very thin at 13.7, and the wave absorber had a light density of 0.08 g / cm ³ .

[Example 8]
(Radio wave absorber sheet material)
The same radio wave absorber sheet material as the radio wave absorber sheet material of Example 7 was used except that the number of laminated sheets was 6.
(Dielectric layer)
The same thing as Example 7 was used, and thickness was 8 mm.
(Radio wave reflector layer)
The same one as in Example 7 was used.
(Radio wave absorber)
A radio wave absorber was prepared in the same manner as in Example 7. Table 2 shows the measurement results such as the performance of the obtained radio wave absorber. The obtained radio wave absorber had an absorption peak of 20 dB at 2.5 GHz and was suitable as a radio wave absorber. Further, the thickness T was 8.78 mm, λ / T was 13.7 and the thickness was extremely thin, and the density of the radio wave absorber was 0.13 g / cm ³ , which was lightweight.

[Example 9]
(Radio wave absorber sheet material)
The same radio wave absorber sheet material as the radio wave absorber sheet material of Example 7 was used except that the number of laminated sheets was 2.
(Dielectric layer)
The same thing as Example 7 was used, and thickness was 4 mm.
(Radio wave reflector layer)
The same one as in Example 7 was used.
(Radio wave absorber)
A radio wave absorber was prepared in the same manner as in Example 7. Table 2 shows the measurement results such as the performance of the obtained radio wave absorber. It had a 16 dB absorption peak at 5.8 GHz, and was suitable as a radio wave absorber. Further, the thickness T was 4.26 mm, λ / T was very thin as 12.2, and the density of the radio wave absorber was light as 0.13 g / cm ³ .
As a result, by using the radio wave absorber sheet material prepared in Example 2 and adjusting the thicknesses of the radio wave absorber sheet material and the dielectric layer, 0.9 GHz, 2.5 GHz, 5.8 GHz In either case, a radio wave absorber having a radio wave absorption peak of 10 dB or more was obtained. This indicates that by using one type of sheet material for a radio wave absorber, a radio wave absorber corresponding to a wide range of frequencies can be easily provided with a small amount of material.

[Example 10]
(Radio wave absorber sheet material)
Eight sheets of the electromagnetic wave absorber sheet material produced in Example 4 were laminated using an acetic acid-vinyl emulsion adhesive.
(Dielectric layer)
The same thing as Example 7 was used, and thickness was 20 mm.
(Radio wave reflector layer)
The same one as in Example 7 was used.
(Radio wave absorber)
A radio wave absorber was prepared in the same manner as in Example 7. Table 2 shows the measurement results such as the performance of the obtained radio wave absorber. The obtained radio wave absorber had an absorption peak of 23 dB at 0.9 GHz, and was suitable as a radio wave absorber. Further, the thickness T was 21.04 mm and λ / T was 15.8, which was extremely thin, and the density of the radio wave absorber was 0.08 g / cm ³ , which was lightweight.

[Example 11]
(Radio wave absorber sheet material)
The same wave absorber sheet material as that of Example 10 was used except that the number of laminated sheets was four.
(Dielectric layer)
The same thing as Example 7 was used, and thickness was 8 mm.
(Radio wave reflector layer)
The same one as in Example 7 was used.
(Radio wave absorber)
A radio wave absorber was prepared in the same manner as in Example 7. Table 2 shows the measurement results such as the performance of the obtained radio wave absorber. The obtained radio wave absorber had an absorption peak of 19 dB at 2.5 GHz, and was suitable as a radio wave absorber. Further, the thickness T was 8.52 mm, λ / T was 14.1 and extremely thin, and the density of the wave absorber was 0.11 g / cm ³ , which was lightweight.

[Example 12]
(Radio wave absorber sheet material)
One sheet for a radio wave absorber produced in Example 4 was used.
(Dielectric layer)
The same thing as Example 7 was used, and thickness was 5 mm.
(Radio wave reflector layer)
The same one as in Example 7 was used.
(Radio wave absorber)
A radio wave absorber was prepared in the same manner as in Example 7. Table 2 shows the measurement results such as the performance of the obtained radio wave absorber. The obtained wave absorber had an absorption peak of 17 dB at 5.8 GHz and was suitable as a wave absorber. Further, the thickness T was 5.13 mm, λ / T was as thin as 10.1, and the density of the radio wave absorber was 0.09 g / cm ³ and lightweight.
As a result, in the case of using the radio wave absorber sheet material created in Example 4, by adjusting the thicknesses of the radio wave absorber sheet material and the dielectric layer, 0.9 GHz, 2.5 GHz, A radio wave absorber having a radio wave absorption peak of 10 dB or more at any of 5.8 GHz was obtained. This indicates that by using one type of sheet material for a radio wave absorber, a radio wave absorber corresponding to a wide range of frequencies can be easily provided with a small amount of material.

[Example 13]
(Radio wave absorber sheet material)
Six sheets of the radio wave absorber sheet material prepared in Example 6 were laminated using an acetic acid-vinyl emulsion adhesive.
(Dielectric layer)
The same thing as Example 7 was used, and thickness was 22 mm.
(Radio wave reflector layer)
The same one as in Example 7 was used.
(Radio wave absorber)
A radio wave absorber was prepared in the same manner as in Example 7. Table 2 shows the measurement results such as the performance of the obtained radio wave absorber. The obtained radio wave absorber had an absorption peak of 23 dB at 0.9 GHz, and was suitable as a radio wave absorber. Further, the thickness T was 22.84 mm, λ / T was very thin at 14.6, and the wave absorber density was 0.06 g / cm ³ , which was lightweight.

[Example 14]
(Radio wave absorber sheet material)
The same wave absorber sheet material as that of Example 13 was used except that the number of laminated sheets was three.
(Dielectric layer)
The same thing as Example 7 was used, and thickness was 10 mm.
(Radio wave reflector layer)
The same one as in Example 7 was used.
(Radio wave absorber)
A radio wave absorber was prepared in the same manner as in Example 7. Table 2 shows the measurement results such as the performance of the obtained radio wave absorber. The obtained radio wave absorber had an absorption peak of 25 dB at 2.5 GHz and was suitable as a radio wave absorber. Further, the thickness T was 10.42 mm, λ / T was as thin as 11.5, and the density of the radio wave absorber was 0.08 g / cm ³ , which was lightweight.

[Example 15]
(Radio wave absorber sheet material)
One sheet for a radio wave absorber produced in Example 6 was used.
(Dielectric layer)
The same thing as Example 7 was used, and thickness was 5 mm.
(Radio wave reflector layer)
The same one as in Example 7 was used.
(Radio wave absorber)
A radio wave absorber was prepared in the same manner as in Example 7. Table 2 shows the measurement results such as the performance of the obtained radio wave absorber. The obtained wave absorber had an absorption peak of 15 dB at 5.8 GHz and was suitable as a wave absorber. Further, the thickness T was 5.14 mm, λ / T was as thin as 10.1, and the density of the radio wave absorber was 0.09 g / cm ³ , which was light.
As a result, in the case of using the radio wave absorber sheet material created in Example 6, by adjusting the thicknesses of the radio wave absorber sheet material and the dielectric layer, 0.9 GHz, 2.5 GHz, A radio wave absorber having a radio wave absorption peak of 10 dB or more at any of 5.8 GHz was obtained. This indicates that by using one type of sheet material for a radio wave absorber, a radio wave absorber corresponding to a wide range of frequencies can be easily provided with a small amount of material.

[Comparative Example 4]
(Radio wave absorber sheet material)
Two sheets of the radio wave absorbing material prepared in Comparative Example 1 were laminated using an acetic acid-vinyl emulsion adhesive.
(Dielectric layer)
The same thing as Example 7 was used and it was 35 mm in thickness.
(Radio wave reflector layer)
The same one as in Example 7 was used.
(Radio wave absorber)
A radio wave absorber was prepared in the same manner as in Example 7. Table 2 shows the measurement results such as the performance of the obtained radio wave absorber. The obtained radio wave absorber had an absorption peak of 22 dB at 0.9 GHz and was suitable as a radio wave absorber, but the thickness T was 35.26 mm and λ / T was 9.5. However, it did not satisfy the intended thickness reduction of the wave absorber.
[Comparative Example 5]
(Radio wave absorber sheet material)
One sheet material for a radio wave absorber produced in Comparative Example 1 was used.
(Dielectric layer)
The same thing as Example 7 was used, and it was 15 mm in thickness.
(Radio wave reflector layer)
The same one as in Example 7 was used.
(Radio wave absorber)
A radio wave absorber was prepared in the same manner as in Example 7. Table 2 shows the measurement results such as the performance of the obtained radio wave absorber. The obtained radio wave absorber had an absorption peak of 17 dB at 2.5 GHz and was suitable as a radio wave absorber. The thickness T was 15.13 mm and λ / T was 7.9. However, it did not satisfy the intended thickness reduction of the wave absorber.
[Comparative Example 6]
(Radio wave absorber sheet material)
One sheet material for a radio wave absorber produced in Comparative Example 1 was used.
(Dielectric layer)
The same thing as Example 7 was used, and it was 7 mm in thickness.
(Radio wave reflector layer)
The same one as in Example 7 was used.
(Radio wave absorber)
A radio wave absorber was prepared in the same manner as in Example 7. Table 2 shows the measurement results such as the performance of the obtained radio wave absorber. The obtained radio wave absorber had an absorption peak at 5.8 GHz, but the radio wave absorption amount was as low as 8 dB, and a sufficient effect for improving the communication performance of the radio communication system was not obtained.

  As a result, when the radio wave absorber sheet material prepared in Comparative Example 1 is used, it has a radio wave absorption peak of 10 dB or more at 0.9 GHz and 2.5 GHz. It was not to achieve the thinning. Further, at 5.8 GHz, sufficient radio wave absorption performance could not be obtained, and a radio wave absorber corresponding to a wide range of frequencies was not provided by using one type of radio wave absorber sheet material.

[Comparative Example 7]
(Radio wave absorber sheet material)
Two sheets of the radio wave absorbing material produced in Comparative Example 2 were laminated using an acetic acid-vinyl emulsion adhesive.
(Dielectric layer)
The same thing as Example 7 was used, and it was 20 mm in thickness.
(Radio wave reflector layer)
The same one as in Example 7 was used.
(Radio wave absorber)
A radio wave absorber was prepared in the same manner as in Example 7. Table 2 shows the measurement results such as the performance of the obtained radio wave absorber. The obtained radio wave absorber had an absorption peak at 0.9 GHz, but it was as low as 9 dB, and a sufficient effect for improving the communication performance of the radio communication system was not obtained.

[Comparative Example 8]
(Radio wave absorber sheet material)
One sheet material for a radio wave absorber produced in Comparative Example 2 was used.
(Dielectric layer)
The same thing as Example 7 was used, and it was 13 mm in thickness.
(Radio wave reflector layer)
The same one as in Example 7 was used.
(Radio wave absorber)
A radio wave absorber was prepared in the same manner as in Example 7. Table 2 shows the measurement results such as the performance of the obtained radio wave absorber. The obtained radio wave absorber had an absorption peak of 22 dB at 2.5 GHz and was suitable as a radio wave absorber, but the thickness T was 13.13 mm and λ / T was 9.1. However, it did not satisfy the intended thickness reduction of the wave absorber.

[Comparative Example 9]
(Radio wave absorber sheet material)
One sheet material for a radio wave absorber produced in Comparative Example 2 was used.
(Dielectric layer)
The same thing as Example 7 was used, and it was 5 mm in thickness.
(Radio wave reflector layer)
The same one as in Example 7 was used.
(Radio wave absorber)
A radio wave absorber was prepared in the same manner as in Example 7. Table 2 shows the measurement results such as the performance of the obtained radio wave absorber. The obtained wave absorber had an absorption peak at 5.8 GHz, but it was as low as 8 dB, and a sufficient effect for improving the communication performance of the wireless communication system was not obtained.
As a result, when the radio wave absorber sheet material prepared in Comparative Example 2 is used, it has a radio wave absorption peak at 2.5 GHz of 10 dB or more. It was not achieved. Moreover, at 0.9 GHz and 5.8 GHz, sufficient electromagnetic wave absorption performance cannot be obtained, and by using one type of electromagnetic wave absorber sheet material, an electromagnetic wave absorber that supports a wide range of frequencies cannot be provided. There wasn't.

[Comparative Example 10]
(Radio wave absorber sheet material)
Two sheets of the radio wave absorbing material produced in Comparative Example 3 were laminated using an acetic acid-vinyl emulsion adhesive.
(Dielectric layer)
The same thing as Example 7 was used, and it was 30 mm in thickness.
(Radio wave reflector layer)
The same one as in Example 7 was used.
(Radio wave absorber)
A radio wave absorber was prepared in the same manner as in Example 7. Table 2 shows the measurement results such as the performance of the obtained radio wave absorber. The obtained radio wave absorber had an absorption peak of 12 dB at 0.9 GHz, and was suitable as a radio wave absorber. However, the thickness T was 30.26 mm and λ / T was 11.0 and was thin. Furthermore, the density of the radio wave absorber was as light as 0.04 g / cm ³ .

[Comparative Example 11]
(Radio wave absorber sheet material)
One sheet material for a radio wave absorber produced in Comparative Example 3 was used.
(Dielectric layer)
The same thing as Example 7 was used, and it was 18 mm in thickness.
(Radio wave reflector layer)
The same one as in Example 7 was used.
(Radio wave absorber)
A radio wave absorber was prepared in the same manner as in Example 7. Table 2 shows the measurement results such as the performance of the obtained radio wave absorber. The obtained wave absorber had an absorption peak of 25 dB at 2.5 GHz and was suitable as a wave absorber, but the thickness T was 18.13 mm and λ / T was 6.6. However, it did not satisfy the intended thickness reduction of the wave absorber.

[Comparative Example 12]
(Radio wave absorber sheet material)
One sheet material for a radio wave absorber produced in Comparative Example 1 was used.
(Dielectric layer)
The same thing as Example 7 was used, and it was 5 mm in thickness.
(Radio wave reflector layer)
The same one as in Example 7 was used.
(Radio wave absorber)
A radio wave absorber was prepared in the same manner as in Example 7. Table 2 shows the measurement results such as the performance of the obtained radio wave absorber. The obtained radio wave absorber had an absorption peak at 5.8 GHz, but the radio wave absorption amount was as low as 8 dB, and a sufficient effect for improving the communication performance of the radio communication system was not obtained.
As a result, when the radio wave absorber sheet material prepared in Comparative Example 3 is used, it has a radio wave absorption peak of 10 dB or more at 0.9 GHz and 2.5 GHz. It was not to achieve the thinning. Further, at 5.8 GHz, sufficient radio wave absorption performance could not be obtained, and a radio wave absorber corresponding to a wide range of frequencies was not provided by using one type of radio wave absorber sheet material.

  The radio wave absorber of the present invention can make a radio wave absorber corresponding to a wide frequency range with a small number of types of radio wave absorber sheet materials without using many kinds of materials, and is thin and lightweight. An easy-to-handle radio wave absorber that suppresses poor communication due to multiple reflections of radio waves and realizes stable communication in various radio communication systems using high-frequency radio waves such as mobile phones, RFIDs, wireless LANs, and ETCs. Can do.

Claims (5)

A sheet material for a radio wave absorber containing conductive fibers, wherein a complex relative permittivity of the material is measured by a free space method, and an imaginary part of the complex relative permittivity at a frequency n [GHz] is defined as B (n) The ratio of the imaginary part B (n) of the complex dielectric constant in the frequency range of 0.5 GHz ≦ n ≦ 6.5 GHz to the imaginary part B (0.5) of the complex dielectric constant at a frequency of 0.5 GHz, B (N) / B (0.5) is 0.6 or more,
There is one peak of the fiber length distribution of the conductive fiber in the range of 1 to 7 mm, the length average fiber length is 1 to 7 mm, and the content is 0.05 to 0.75% by mass. Absorbent sheet material.   A sheet material for a radio wave absorber containing conductive fibers, wherein a complex relative permittivity of the material is measured by a free space method, and an imaginary part of the complex relative permittivity at a frequency n [GHz] is defined as B (n) The ratio of the imaginary part B (n) of the complex dielectric constant in the frequency range of 0.5 GHz ≦ n ≦ 6.5 GHz to the imaginary part B (0.5) of the complex dielectric constant at a frequency of 0.5 GHz, B (N) / B (0.5) is 0.6 or more,
  Radio waves characterized in that the ratio of having a fiber length of 0.9 L to 1.1 L is 70% or more with respect to the total number of conductive fibers, where L is the peak center of the fiber length of the conductive fibers. Absorbent sheet material.   The complex relative permittivity of the material is measured by the free space method, and the real part A (n) of the complex relative permittivity is 10 to 250 and the imaginary number within the frequency n [GHz] in the range of 0.5 to 6.5 GHz. The part B (n) is 5 to 150, the ratio of the real part A (n) to the imaginary part B (n), and A (n) / B (n) is in the range of 1.5 to 10. The sheet material for a radio wave absorber according to claim 1 or 2. A radio wave absorber comprising the radio wave absorber sheet material according to any one of claims 1 to 3 , a dielectric layer, and a radio wave reflector layer, wherein at least one dielectric layer is provided on at least one surface of the radio wave reflector layer. 1. A radio wave absorber comprising: a layer, and at least one or more layers of the radio wave absorber sheet material sequentially laminated. The ratio of the thickness d [mm] of the radio wave absorber sheet material to the sum T [mm] of the thickness of the radio wave absorber sheet material and the dielectric layer, and d / T is in the range of 0.02 to 0.15. Yes,
And the ratio of the sum T of the thickness of the sheet material for the radio wave absorber and the dielectric layer to the wavelength λ [mm] of the radio wave to be absorbed, λ / T is 10 or more,
The radio wave absorber according to claim 4 , wherein the radio wave absorption amount is 10 dB or more in at least a part of a frequency range of 0.5 to 6.5 GHz.