Radio wave absorber and radio wave absorbing device

Abstract

Provided are: a radio wave absorber which has sufficient radio wave absorption characteristics in a higher frequency band of 20-300 GHz, and is capable of self-standing while having an area of at least a certain size; and a radio wave absorbing device using said radio wave absorber. This radio wave absorber comprises: a radio wave absorbing layer 1; and a reinforcement layer 2 disposed on the radio wave 10-incident surface side of the radio wave absorbing layer, wherein the radio wave absorbing layer contains resin binders 1c and at least one among magnetic iron oxide powders 1a, which magnetically resonate in a frequency band of 20-300 GHz, and carbon-based fine particles 1b, the reinforcement layer 2 is formed of a dielectric material, and said radio wave absorber has a plate shape in which the thickness is smaller than the principal surface, and can be self-standing.

Description

[Technical Field]

[0001] This disclosure relates to a radio wave absorber that absorbs radio waves and a radio wave absorbing device using this radio wave absorber, and more particularly to a radio wave absorber that absorbs radio waves in a high frequency band from 20 gigahertz (GHz) to 300 gigahertz (GHz), which is known as the millimeter wave band. [Background technology]

[0002] Radio wave absorbers are used to prevent the effects of leaked radio waves emitted from electrical circuits and other sources, as well as unwanted reflected radio waves, from entering the receiving device.

[0003] In recent years, research has been progressing on technologies that utilize high-frequency radio waves, such as centimeter waves with a frequency band of several gigahertz (GHz), and even millimeter waves with frequencies ranging from 30 gigahertz to 300 gigahertz, for mobile communications such as cell phones, wireless LANs, and automated toll collection systems (ETC).

[0004] In response to this technological trend of utilizing higher frequency bands of radio waves, there is a growing demand for radio wave absorbers that can absorb radio waves from tens of gigahertz to millimeter-wave bands, even for absorbing unwanted radio waves.

[0005] As an electromagnetic wave absorber that absorbs electromagnetic waves in high frequency bands of 20 GHz or above the millimeter wave band (30 GHz), an electromagnetic wave absorber having a particle-packed structure with epsilon iron oxide (ε-Fe2O3) crystals as the magnetic phase that exhibits electromagnetic wave absorption performance in the range of 25 to 100 gigahertz has been proposed (see Patent Document 1). In addition, a flat electromagnetic wave absorber formed by applying a paste made by kneading fine particles of epsilon iron oxide together with a binder onto a substrate made of a metal plate has been proposed (see Patent Document 2).

[0006] Furthermore, the inventors have proposed various types of radio wave absorbing sheets, which are thin, sheet-like radio wave absorbing materials with a thin surface area relative to their thickness, as radio wave absorbing materials that effectively absorb radio waves in high-frequency bands above the millimeter wave band (Patent Documents 3 and 4). [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2008-60484 [Patent Document 2] Japanese Patent Publication No. 2016-111341 [Patent Document 3] Re-tabled publication 2017 / 221992 [Patent Document 4] International Publication No. 2018 / 084235 [Overview of the project] [Problems that the invention aims to solve]

[0008] The conventional radio wave absorbing sheets described above are made by dispersing and compounding radio wave absorbing materials in a resin binder. By selecting the binder material and manufacturing method, it is possible to produce radio wave absorbing sheets that are flexible and elastic. Radio wave absorbing sheets have high convenience because they can be easily placed in the desired position facing the direction of incidence of the radio waves to be absorbed. For example, they can be attached to the inner surface of a housing that covers equipment that is a noise source to prevent radio wave leakage to the outside, or attached to the outer surface of a container that houses equipment to be protected to avoid the influence of external radio waves.

[0009] However, because radio wave absorbing sheets are thin relative to their surface area, it is difficult to use them in a self-supporting manner, even if they use a plastic resin binder. On the other hand, while solid block-shaped radio wave absorbers can be processed into a self-supporting shape, their manufacturing process becomes large and heavy, making it difficult to obtain one that is sufficiently easy to handle.

[0010] This disclosure aims to solve the above-mentioned conventional problems and to realize a radio wave absorber that has sufficient radio wave absorption characteristics in a high frequency band from 20 GHz to 300 GHz, is capable of being self-supporting while having a certain area or more, and a radio wave absorbing device using this radio wave absorber. [Means for solving the problem]

[0011] To solve the above problems, the radio wave absorber disclosed in this application comprises a radio wave absorbing layer and a reinforcing layer disposed on the surface of the radio wave incident side of the radio wave absorbing layer, wherein the radio wave absorbing layer contains at least one of magnetic iron oxide powder and carbon-based fine particles that resonate magnetically in the frequency band of 20 GHz to 300 GHz, and a resin binder, the reinforcing layer is made of a dielectric material, and the radio wave absorber is characterized in that it is a plate-like shape with a small thickness relative to the main surface and is self-supporting.

[0012] Furthermore, the radio wave absorbing device disclosed in this application is a radio wave absorbing device using the radio wave absorbing material disclosed in this application, comprising the radio wave absorbing material and a support member capable of maintaining the surface of the radio wave absorbing material on the radio wave incident surface side at a predetermined angle, characterized in that the radio wave absorbing material is arranged such that the normal direction of the portion of the surface on the radio wave incident surface side located in the direction of propagation of the radio waves to be absorbed intersects the direction of propagation of the radio waves at a predetermined angle. [Effects of the Invention]

[0013] The radio wave absorber disclosed herein absorbs radio waves of a desired frequency through the radio wave absorption effect due to magnetic resonance of magnetic iron oxide powder contained in the radio wave absorption layer and the effect of increased dielectric loss due to carbon-based fine particles. Furthermore, because it has a reinforcing layer positioned on the radio wave incident surface side, it can stand on its own despite being a plate-like structure with a small thickness relative to the main surface. For this reason, it can be easily placed on the path of the radio waves to be absorbed, and the adverse effects of undesirable radio waves can be prevented.

[0014] In addition, the radio wave absorber disclosed in the present application includes a support member capable of maintaining the radio wave absorber at a predetermined angle, and can keep the surface on the radio wave incident surface side inclined with respect to the traveling direction of the radio waves to be absorbed. Therefore, a radio wave absorber with a high reflection attenuation amount that suppresses the influence of radio waves reflected on the surface of the reinforcing layer can be realized.

Brief Description of Drawings

[0015] [Figure 1] It is a cross-sectional configuration diagram for explaining the configuration of a radio wave absorber according to an embodiment. <000009 / 4>It is an image diagram for explaining a method of an evaluation test for evaluating the self-supportability of a radio wave absorber according to an embodiment. [[ID=1 / 2]] [Figure 3] It is a diagram showing the usage state of a radio wave absorber according to an embodiment. [Figure 4] It is an image diagram showing the measurement situation of the relationship between the inclination angle of the radio wave incident surface of a radio wave absorber and the radio wave absorption characteristics. [Figure 5] It is a diagram showing the change in radio wave absorption characteristics according to the inclination angle of the radio wave incident surface of a radio wave absorber. [Figure 6] It is a diagram showing the change in radio wave absorption characteristics according to the inclination angle of the radio wave incident surface of a radio wave absorber. [Figure 7] It is a diagram showing the change in radio wave absorption characteristics according to the inclination angle of the radio wave incident surface of a radio wave absorber.

Embodiments for Carrying Out the Invention

[0016] The radio wave absorber disclosed in the present application is a radio wave absorber including a radio wave absorption layer and a reinforcing layer disposed on the surface on the radio wave incident surface side of the radio wave absorption layer, wherein the radio wave absorption layer includes at least one of magnetic iron oxide powder and carbon-based fine particles that resonate magnetically in a frequency band of 20 GHz to 300 GHz, and a resin binder, the reinforcing layer is made of a dielectric material, the radio wave absorber is in a plate shape with a small thickness with respect to the main surface, and is self-supportable.

[0017] In this way, the radio wave absorber disclosed in this application can be easily positioned to block the path of the radio waves to be absorbed, and for example, adverse effects from undesirable radio waves can be easily avoided when measuring the radio wave characteristics of equipment.

[0018] In the radio wave absorber with the above configuration, it is preferable that the magnetic iron oxide powder is either a magnetoplanbite-type ferrite powder or an epsilon magnetic iron oxide powder, and that the carbon-based fine particles are at least one of carbon black, carbon nanotubes, and graphene. By using these materials as radio wave absorbing members that absorb radio waves in the radio wave absorbing layer, high radio wave absorption characteristics can be achieved in the frequency band from 20 GHz to 300 GHz.

[0019] Furthermore, it is preferable that the resin binder is a rubber-based material. By using a rubber binder, a radio wave absorbing layer with a large surface area can be easily created.

[0020] Furthermore, in the self-supporting capability test of the electromagnetic wave absorber, it is preferable that the Δ value, which indicates the degree of deformation of the radio wave absorber sample, is 0.5 mm or less. In this case, the radio wave absorber can be evaluated as being self-supporting.

[0021] Furthermore, it is preferable that the radio wave attenuation for transmitted radio waves passing through the radio wave absorber is -10 dB or more. By having radio wave absorption characteristics such as a radio wave attenuation of -10 dB or more for transmitted radio waves, that is, an absolute value of the transmitted attenuation (dB) of 10 or more, it can be used as a radio wave absorber that can sufficiently reduce the influence of undesirable radio waves.

[0022] The reinforcing layer may consist of one selected from a reinforcing plate having a honeycomb structure, a foamed plate, plastic corrugated cardboard, and a plastic plate.

[0023] The radio wave absorbing device disclosed herein is a radio wave absorbing device using the radio wave absorbing material disclosed herein, comprising the radio wave absorbing material and a support member capable of maintaining the surface of the radio wave absorbing material on the radio wave incident surface side at a predetermined angle, wherein the radio wave absorbing material is arranged such that the normal direction of the portion of the surface on the radio wave incident surface side located in the direction of propagation of the radio waves to be absorbed intersects the direction of propagation of the radio waves at a predetermined angle.

[0024] In this specification, "the normal direction of a portion of the surface of the radio wave incident surface and the direction of propagation of the radio wave intersect at a predetermined angle" means that the normal direction of a portion of the surface of the radio wave incident surface and the direction of propagation of the radio wave do not coincide, that is, they intersect at an angle greater than 0°.

[0025] In this way, the radio wave absorbing device disclosed in this application can maintain a state in which the radio wave incident surface of the radio wave absorber is tilted at a desired angle, and in particular, it is possible to realize a radio wave absorbing device with high return loss that suppresses the reflection of radio waves on the surface of the reinforcing layer.

[0026] In the radio wave absorbing device with the above configuration, it is preferable that the surface portion is a curved surface that curves in at least one direction. If the surface is a curved surface, the radio waves reflected in the direction of incidence of the radio waves can be reduced, and the amount of reflection attenuation can be improved.

[0027] Furthermore, it is preferable that the angle at which the radiation direction of the surface portion intersects with the direction of propagation of the radio waves is 2° or more and 20° or less, and more preferably that the angle at which they intersect is 3° or more and 7° or less.

[0028] The radio wave absorber disclosed in this application will be described below with reference to the drawings.

[0029] (Embodiment) As an example of the radio wave absorber disclosed in this application, we will explain a radio wave absorber that includes strontium ferrite as magnetic iron oxide powder and carbon black as carbon-based fine particles as radio wave absorbing members in the radio wave absorbing layer, and also includes silicone rubber as a resin binder, and a plastic sheet having a honeycomb structure made of polypropylene as a reinforcing layer.

[0030] [Configuration of the radio wave absorber] Figure 1 is a cross-sectional view showing the configuration of the radio wave absorber described in this embodiment.

[0031] As shown in Figure 1, the radio wave absorber according to this embodiment has a radio wave absorbing layer 1 in which strontium ferrite powder 1a and carbon black fine particles 1b as radio wave absorbing members are dispersed in a silicone rubber binder 1c, and a reinforcing layer 2 which is a plastic sheet having a honeycomb structure and is arranged on the surface of the radio wave absorbing layer on the side into which the radio waves 20 are incident.

[0032] Furthermore, in this embodiment, the radio wave absorber has a sufficiently small thickness compared to the area (main area) of the radio wave absorbing layer 1 and the reinforcing layer 2, which is the sum of the thicknesses of the radio wave absorbing layer 1 and the reinforcing layer 2. The radio wave absorber as a whole has a plate-like shape, which can be called a radio wave absorbing board. More specifically, the thickness of the radio wave absorbing layer 1 is approximately 1 mm to 5 mm, while the thickness of the reinforcing layer 2 is approximately 5 mm to 30 mm. The main surfaces of the radio wave absorbing layer 1 and the reinforcing layer 2 are, for example, configured as rectangles (rectangles or squares) with sides of several centimeters to several tens of centimeters or several meters.

[0033] The radio wave absorber according to this embodiment has a reinforcing layer 2 positioned on the radio wave incidence side of the radio wave absorption layer 1, and is self-supporting. Here, "self-supporting" means that when the radio wave absorber is placed upright with its main surface facing sideways, that is, when the main surface of the radio wave absorber is positioned vertically on a horizontal, flat surface such that the thickness of the portion corresponding to one side of the main surface of the radio wave absorber becomes the base, the shape of the radio wave absorber does not change. It is not a problem to use support members or legs to maintain the radio wave absorber in a self-supporting state, and "self-supporting" in this specification does not mean that the radio wave absorber can stand on its own with its main surface vertical. Furthermore, it is sufficient that the main surface of the radio wave absorber is maintained approximately vertically, and even when the radio wave absorber is leaning against something, that is, when a support member abuts against a part of the upper end or back portion (the side different from the side on which the reinforcing layer 2 into which radio waves are incident), and the main surface of the radio wave absorber is maintained in a state where it is slightly inclined with respect to the vertical, the radio wave absorber is also considered to be self-supporting.

[0034] In the radio wave absorber according to this embodiment, the radio wave absorbing layer 1 and the reinforcing layer 2 are integrated and configured such that their unity is not compromised even when the radio wave absorber is standing upright. For this reason, the radio wave absorber can be constructed by manufacturing the radio wave absorbing layer 1 and the reinforcing layer 2 separately and bonding them together using adhesive means such as a silicone-based adhesive or double-sided tape. Alternatively, the radio wave absorber can be constructed by integrating the radio wave absorbing layer 1 and the reinforcing layer 2 by pinning, riveting, screwing them together at multiple points, or by sandwiching them around a frame, while the two layers are in close contact and overlapping.

[0035] [Radio wave absorption layer] The radio wave absorbing layer of the radio wave absorber according to this embodiment is composed of strontium ferrite powder 1a and carbon black powder 1b, which are radio wave absorbing materials, dispersed and mixed within a resin binder 1c.

[0036] In this embodiment, we have illustrated a configuration that includes both strontium ferrite powder, which is magnetic iron oxide powder, and carbon black powder, which is carbon-based fine particles. However, the resin binder can be configured to contain only one of either magnetic iron oxide powder or carbon-based fine particles.

[0037] The main surface of the radio wave absorbing layer is set to have an area that can block unwanted radio waves by arranging one or more layers, taking into consideration the path of the radio waves to be absorbed by the radio wave absorber, the radiation angle of unwanted radio waves from equipment that acts as a noise source, and the incidence angle of external radio waves to the equipment to be protected from unwanted radio waves. The thickness of the radio wave absorbing layer is set to be greater than or equal to the thickness that can sufficiently absorb unwanted radio waves, based on the type of radio wave absorbing material contained and the density contained in the radio wave absorbing layer. Generally, it can be considered that the minimum radio wave absorption effect has been achieved if the radio wave absorber as a whole can attenuate unwanted radio waves to one-tenth, so it is preferable to set the radio wave absorption characteristics of the radio wave absorber so that the transmission attenuation, which is the amount of attenuation of radio waves that pass through the radio wave absorber, can be achieved at 10 dB.

[0038] Specifically, as an example, if a radio wave absorbing layer that absorbs 76.5 GHz radio waves is fabricated using strontium ferrite and carbon black as radio wave absorbing materials and silicone rubber as a binder, its thickness can be approximately 1 mm to 4.5 mm.

[0039] In the radio wave absorber according to this embodiment, a reinforcing layer, described later, is placed on the radio wave incident side of the radio wave absorbing layer to enable it to stand on its own, so there are no constraints on the rigidity or strength of the radio wave absorbing layer alone. For this reason, even if the radio wave absorbing layer alone is easily deformed, it is permissible to use a soft rubber-based material such as silicone rubber or natural rubber as a binder.

[0040] (Magnetic iron oxide powder) As the magnetic iron oxide powder used in the radio wave absorber according to this embodiment, powders of magnetoplumbite-type ferrite or epsilon magnetic iron oxide are preferably used as those that cause magnetic resonance with respect to radio waves in the frequency band of 20 GHz to 300 GHz.

[0041] As the magnetoplumbite-type (M-type) ferrite, magnetic powders of strontium ferrite (Sr-Fe) or barium ferrite (Ba-Fe) can be used.

[0042] In the magnetoplumbite-type ferrite, the imaginary part (μr'') of the complex magnetic permeability related to radio wave absorption becomes high at the frequency where resonance occurs when the magnetic material is magnetized at high frequencies. The natural resonance frequency f is proportional to the anisotropy magnetic field H 3+ , , 3+ , , ,

[0044] , 12 ,

[0045] , , , A , 3+ , A , ,

[0046] , , 19 , ,

[0043] Therefore, the higher the value of the anisotropy magnetic field H A , the higher the value of the natural resonance frequency f. The natural resonance frequency f of barium ferrite (BaFe 12 O 19 ) is calculated to be 48 GHz from its H A value of 1.35 MA / m, and it can absorb high-frequency electromagnetic waves in the GHz band.

[0043] Also, by substituting a part of Fe 3+ with (TiMn) 3+ or Al 3+ etc., the natural resonance frequency f can be controlled in the range of 5 to 150 GHz by controlling the value of the anisotropy magnetic field H A .

[0044] For example, as strontium ferrite (SrFe 12 O 19 ), by adding Al, a radio wave absorber corresponding to a 60 GHz band wireless LAN can be obtained.

[0045] Also, as the magnetic iron oxide powder used in the radio wave absorber according to this embodiment, epsilon magnetic iron oxide (ε-Fe2O3) can be used.

[0046] Epsilon magnetic iron oxide is a phase that appears between the alpha phase (α-Fe2O3) and the gamma phase (γ-Fe2O3) in ferric oxide (Fe2O3). It is a magnetic material that can be obtained in a single-phase state by a nanoparticle synthesis method that combines the reverse micelle method and the sol-gel method. Despite being fine particles ranging from a few nanometers to tens of nanometers, epsilon magnetic iron oxide possesses the highest coercivity among metal oxides, approximately 20 kOe at room temperature. Furthermore, because spontaneous resonance due to the gyromagnetic effect based on precession occurs in the so-called millimeter-wave frequency band of tens of gigahertz or more, it is a good radio wave absorbing material that absorbs radio waves in the millimeter-wave band.

[0047] Furthermore, epsilon magnetic iron oxide can have its magnetic resonance frequency altered by substituting some of the Fe sites in the crystal with trivalent metal elements such as aluminum (Al), gallium (Ga), rhodium (Rh), and indium (In). Therefore, by adjusting the type and amount of metal used for substitution, it is possible to match the frequency of radio waves that the radio wave absorber is intended to absorb.

[0048] Furthermore, epsilon magnetic iron oxide is available, including in which some Fe sites are metal-substituted. Epsilon magnetic iron oxide can be obtained as particles with an average particle size of approximately 30 nm, which are roughly spherical or short rod-shaped.

[0049] (Carbon-based fine particles) In the radio wave absorber according to this embodiment, the radio wave absorbing layer contains carbon-based fine particles together with the magnetic iron oxide powder described above, or on its own.

[0050] Suitable carbon-based nanoparticles include carbon black (CB), carbon nanotubes (CNT), or graphene. These carbon-based nanoparticles may be used individually or in combination of two or more types.

[0051] More specifically, various conductive carbon blacks can be used as carbon black, such as furnace-processed conductive carbon black, acetylene black, and Ketjen black. Either single-walled nanotubes (SWNTs) or multi-walled nanotubes (MWNTs) can be used as carbon nanotubes. Furthermore, graphene is a carbon material having a sheet-like structure with a thickness of one atom, formed by arranging a honeycomb-like hexagonal lattice created by sp2 bonds of carbon atoms in a planar arrangement. While strictly speaking graphene refers to a single sheet, the graphene used in the radio wave absorbing layer described in this embodiment also includes, for example, carbon films stacked in 2 to 1000 layers. Moreover, it also includes graphite in which graphene is stacked three-dimensionally.

[0052] As carbon-based fine particles, the specific surface area is 30 to 2300 m². 2 A material with a specific surface area of ​​300 to 2000 m² can be used. 2 More preferably, a material with a specific surface area of ​​800 to 1800 m² is used. 2 It is preferable to use carbon black with a concentration of 10-60 nm and a BET value of 300-1500 nm. 2 A value of / g is preferred. Furthermore, the carbon nanotubes should have a diameter of 3-50 nm, a length of 3-100 μm, and a BET value of 10-1200 m. 2 A product containing 1g is preferable.

[0053] (binder) As the binder used in the radio wave absorbing layer of the radio wave absorber according to this embodiment, resin materials such as epoxy resins, polyester resins, polyurethane resins, acrylic resins, phenolic resins, melamine resins, and rubber resins can be used.

[0054] More specifically, as epoxy resins, compounds in which the hydroxyl groups at both ends of bisphenol A are epoxidized can be used. As polyurethane resins, polyester urethane resins, polyether urethane resins, polycarbonate urethane resins, epoxy urethane resins, etc., can be used. As acrylic resins, methacrylic resins can be used, such as functional group-containing methacrylic polymers obtained by copolymerizing alkyl acrylates and / or alkyl methacrylates having alkyl groups with 2 to 18 carbon atoms, functional group-containing monomers, and other modifying monomers copolymerizable with these, if necessary.

[0055] Furthermore, various rubber materials can be used as rubber-based resins, including natural rubber (NR), isoprene rubber (IR), butadiene rubber (BR), styrene-butadiene rubber (SBR), butyl rubber (IIR), nitrile rubber (NBR), ethylene-propylene rubber (EPDM), chloroprene rubber (CR), acrylic rubber (ACM), chlorosulfonated polyethylene rubber (CSR), urethane rubber (PUR), silicone rubber (Q), fluororubber (FKM), ethylene-vinyl acetate rubber (EVA), epichlorohydrin rubber (CO), polysulfide rubber (T), and urethane rubber (U).

[0056] Furthermore, given the definition of rubber as a material that has rubber elasticity at room temperature, thermoplastic elastomers such as styrene-based thermoplastic elastomers (SIS), for example, can also be used as a binder for the radio wave absorbing layer of the radio wave absorber described in this embodiment, because they are fluid at high temperatures but have rubber elasticity at room temperature.

[0057] Among these rubber materials, acrylic rubber and silicone rubber are suitable due to their high heat resistance. Acrylic rubber exhibits excellent oil resistance even in high-temperature environments and is relatively inexpensive, offering excellent cost performance. Silicone rubber, in addition to its heat resistance, also has high cold resistance. Furthermore, it exhibits the least temperature dependence of its physical properties among synthetic rubbers and has excellent solvent resistance, ozone resistance, and weather resistance. Moreover, it has excellent electrical insulation properties and is materially stable over a wide temperature and frequency range.

[0058] Furthermore, when using a heat-resistant, high-melting-point thermoplastic resin as the thermoplastic resin for forming the radio wave absorber as a molded body, aromatic polyamides and their alloys such as 6T nylon (6TPA), 9T nylon (9TPA), 10T nylon (10TPA), 12T nylon (12TPA), and MXD6 nylon (MXDPA), as well as polyphenylene sulfide (PPS), liquid crystal polymer (LCP), polyether ether ketone (PEEK), polyetherimide (PEI), polyphenyl sulfone (PPSU), polystyrene (PS), styrene-butadiene-acrylonitrile copolymer (ABS), polypropylene (PP), polyacetal (POM), polybutylene terephthalate (PBT), and polycarbonate (PC) can be used.

[0059] Furthermore, from an environmental perspective, it is preferable to use halogen-free resins as binders. These resin materials are readily available as they are common binder materials for resin sheets.

[0060] Furthermore, in order to properly disperse the electromagnetic wave absorbing material, such as magnetoplanbite-type ferrite or epsilon magnetic iron oxide, within the binder, the binder may contain phosphoric acid compounds such as phenylphosphonic acid, aryl sulfonic acids such as phenylphosphonic acid dichloride, alkylphosphonic acids such as methylphosphonic acid, ethylphosphonic acid, octylphosphonic acid, and propylphosphonic acid, or polyfunctional phosphonic acids such as hydroxyethanediphosphonic acid and nitrotrismethylenephosphonic acid as dispersants.

[0061] More specifically, as a dispersant, phenylphosphonic acid (PPA) manufactured by Wako Pure Chemical Industries, Ltd. or Nissan Chemical Industries, Ltd., or oxidized phosphate ester "JP-502" (product name) manufactured by Johoku Chemical Industry Co., Ltd. can be used.

[0062] When using magnetoplanbite-type ferrite as the composition of the radio wave absorbing layer, for example, the composition can be 2 to 120 parts of resin binder and 0.1 to 15 parts of phosphoric acid compound per 100 parts of magnetoplanbite-type ferrite. When using epsilon iron oxide, for example, the composition can be 2 to 50 parts of resin binder and 0.1 to 15 parts of phosphoric acid compound per 100 parts of epsilon magnetic iron oxide powder. If the amount of resin binder is too small, the magnetic iron oxide cannot be dispersed well. Also, the magnetic layer will not be able to maintain a sheet-like shape. If the amount of resin binder is too large, the volume content of magnetic iron oxide in the radio wave absorbing layer will decrease, the permeability will decrease, and the radio wave absorption effect will be reduced.

[0063] Furthermore, when using carbon black as the carbon-based nanoparticle, for example, the ratio of resin binder can be 300 to 700 parts per 100 parts of carbon black. Also, when using carbon nanotubes or graphene, for example, the ratio of resin binder can be 300 to 2500 parts per 100 parts of carbon black or carbon nanotubes.

[0064] (Method of manufacturing a radio wave absorbing layer) The radio wave absorbing layer of the radio wave absorber in this embodiment can be formed, for example, by preparing a magnetic coating containing magnetic iron oxide powder and a resin binder, applying it to a predetermined thickness, drying it, and then calendering it.

[0065] Furthermore, magnetic paint can also be obtained by preparing a mixture by rapidly mixing magnetic iron oxide powder, a phosphate compound acting as a dispersant, and a binder resin in a high-speed stirrer, and then dispersing the resulting mixture in a sand mill.

[0066] A radio wave absorbing layer is fabricated using the magnetic paint produced in this manner.

[0067] For example, the magnetic coating prepared above is applied to a resin substrate sheet. As an example of the resin sheet, a 38 μm thick polyethylene terephthalate (PET) sheet whose surface has been treated with a silicone coating can be used. The magnetic coating is then applied to this resin sheet using a coating method such as a table coater or a bar coater.

[0068] Subsequently, the wet magnetic paint is dried and then calendered to form a sheet-like radio wave absorbing layer on the support. The thickness of the radio wave absorbing layer can be controlled by the coating thickness, calendering conditions, etc. The radio wave absorbing layer after calendering is peeled off from the resin sheet to obtain a radio wave absorbing layer of the desired thickness.

[0069] Furthermore, calendering may be performed as needed, and if the volume content of the radio wave absorbing material is within the specified range after the magnetic paint has dried, calendering may not be necessary.

[0070] Another method for manufacturing the radio wave absorbing layer involves kneading magnetic iron oxide powder and / or carbon-based fine particles with a resin binder, adjusting the viscosity of the resulting mixture by adding a crosslinking agent, and then crosslinking and molding the resulting magnetic compound into a sheet at a temperature of 165°C using, for example, a hydraulic press. Subsequently, a secondary crosslinking treatment is performed in a constant temperature bath to form the radio wave absorbing layer.

[0071] In addition to the press molding method described above, molding can also be performed by extrusion molding or injection molding. Specifically, the radio wave absorbing material, a resin binder, and, if necessary, a dispersant are blended in advance using a pressurized kneader, extruder, or roll mill, and these blended materials are supplied into the plasticizing cylinder from the resin supply port of the extrusion molding machine. As the extrusion molding machine, a standard extrusion molding machine equipped with a plasticizing cylinder, a die provided at the tip of the plasticizing cylinder, a screw rotatably disposed inside the plasticizing cylinder, and a drive mechanism for driving the screw can be used. The molten material, plasticized by the band heater of the extrusion molding machine, is fed forward by the rotation of the screw and extruded in a sheet-like manner from the tip, thereby obtaining a radio wave absorbing layer of a predetermined thickness.

[0072] Furthermore, by pre-blending the radio wave absorbing material, dispersant, and binder as needed, supplying these blended materials from the resin supply port of the injection molding machine into the plasticizing cylinder, melting and kneading them with a screw inside the plasticizing cylinder, and then injecting the molten resin into a mold connected to the tip of the injection molding machine, a molded body can be formed.

[0073] Furthermore, when using a rubber-based resin material as the resin binder, a roll-to-roll method can be used, in which the material is molded while moving between two rolls. By using the roll-to-roll method, a sheet-like radio wave absorbing layer with a width of approximately 1m to 3m can be produced, and compared to molding methods such as the extrusion molding described above, a large-area radio wave absorbing layer can be easily realized.

[0074] [Reinforcement layer] The reinforcing layer is a molded body of dielectric material, a plate-like member with a small thickness relative to the area of ​​its main surface. However, when placed on the radio wave incident side of the radio wave absorbing layer and configured as a radio wave absorber, it needs to be able to stand on its own with its main surface as a side surface, so a certain thickness is required to ensure sufficient rigidity for self-support. On the other hand, if the reinforcing layer is heavy, the overall weight of the radio wave absorber will increase, so it is preferable that it be as lightweight as possible while maintaining a certain level of rigidity.

[0075] As a dielectric material that meets these requirements, the radio wave absorber according to this embodiment can suitably use Texcel (product name, manufactured by Gifu Plastics Industry Co., Ltd.), a honeycomb core material made of polypropylene (PP), as a reinforcing layer. Because Texcel has a honeycomb structure sandwiched between two thin planes, it is lightweight yet possesses high strength (rigidity). Furthermore, Texcel is commercially available in thicknesses from 5 mm to 30 mm, widths up to 1250 mm, and lengths up to 2500 mm, so it is easy to obtain the desired shape to match the surface shape and size of the radio wave absorbing layer.

[0076] In addition to the honeycomb core material described above, various resin boards can be used as reinforcing layers for the radio wave absorber according to this embodiment. For example, Paronia (registered trademark, manufactured by Mitsui Chemicals Tohcello Co., Ltd.), which is made by foaming polypropylene, is produced by extruding polypropylene to approximately three times its original volume and foaming it. It is used as a substitute material for plywood and as a sheet material for container cases. Therefore, as a sheet material made of a dielectric with a low specific gravity and a certain level of strength, it can be effectively used as a reinforcing layer.

[0077] Furthermore, corrugated plastic cardboard made from polypropylene can also be used as a reinforcing layer because it is lightweight yet possesses a certain degree of rigidity.

[0078] Furthermore, the aforementioned honeycomb-structured reinforcing plates, foamed plates, and plastic corrugated cardboard all achieve weight reduction through the presence of cavities within the plate material, and also possess the characteristic of having a low dielectric constant as a plate material due to the inclusion of air.

[0079] In the radio wave absorber according to this embodiment, a reinforcing layer is placed on the surface of the radio wave absorption layer that is incident on the radio wave side. In this case, by having a sufficiently low dielectric constant of the reinforcing layer, the reflection of incident radio waves at the surface of the reinforcing layer is reduced, and more radio waves are incident on the radio wave absorption layer and absorbed by the radio wave absorption layer. Furthermore, by suppressing reflection at the surface of the reinforcing layer, for example, when measuring the radio wave characteristics of transmitting and receiving equipment such as radar, it is possible to effectively suppress the reflection of radio waves emitted from the transmitting unit at the surface of the radio wave absorber and received at the receiving unit, thereby enabling measurement of radio wave characteristics with a high signal-to-noise ratio.

[0080] Furthermore, although they do not contain cavities, plastic sheets such as acrylic and polycarbonate can also be used as reinforcing layers. However, in the case of non-cavity sheets, their specific gravity is high, which increases the weight when used as a reinforcing layer to which a large-area radio wave absorbing layer is fixed. Also, when the radio wave absorber is made to stand on its own, there is a risk of it bending due to its own weight, so it is an effective material as a reinforcing layer used in conjunction with a radio wave absorbing layer that has a relatively small main surface area.

[0081] According to the inventors' studies, if the reinforcing layer has a bending strength of 3 MPa or more as determined by a bending fracture test, it was confirmed that even if the radio wave absorbing layer is flexible and easily bendable, such as a radio wave absorbing layer using rubber material as a binder, when combined with the above-mentioned reinforcing layer, sufficient practical rigidity for the radio wave absorber to stand on its own can be obtained.

[0082] Furthermore, from the perspective of ensuring that the radio wave absorption characteristics of the radio wave absorber do not change even after long-term use, it is desirable that the dielectric material used as the reinforcing layer has a water absorption rate of 1.5% or less.

[0083] [Measuring the possibility of independence] As described above, the radio wave absorber according to this embodiment is considered self-supporting if its main surface can maintain a state in which it is positioned substantially vertically. Therefore, the inventors have devised an evaluation method to quantify and evaluate the degree of self-supporting capability of the radio wave absorber according to this embodiment.

[0084] Figure 2 is a model diagram illustrating a measurement method for evaluating the self-sufficiency of a radio wave absorber according to this embodiment.

[0085] The method for measuring self-supporting ability shown in Figure 2 uses a radio wave absorber 21 with a width of 25 mm and a length of 100 mm as a sample. This sample 21 is placed on a hard base 22 made of metal or plastic, with the reinforcing layer 2 facing downwards, so that the length direction of the sample side 21 is perpendicular to the edge of the base 22, and so that it extends by a length a (=30 mm) beyond the edge of the base 22. Then, a weight 23 is placed on the remaining portion of the base 22 with a length b (=70 mm). The weight 23 is a rectangular prism with a width of 25 mm and a length c=50 mm, and its height is arbitrary, but its weight is 5 kg. The end of this weight 23 is positioned to coincide with the edge of the base 22. With the weight 23 placed on the sample 21, it is left at room temperature (=25°C) for 10 minutes.

[0086] After 10 minutes, the difference Δ between the height of the leading edge of the top surface of sample 21 (B' in the figure) and the height of B if sample 21 had not deformed (the same height as the top surface A of sample 21 at the end of the stand 22) is measured.

[0087] If the Δ value, which indicates the degree of deformation of the radio wave absorber sample obtained in this way, is 0.5 mm or less, the sample is evaluated as being able to stand on its own.

[0088] [Measurement results of the radio wave absorber] The following describes the findings of a study in which the radio wave absorption characteristics and self-sufficiency of a radio wave absorber according to this embodiment were actually fabricated and measured.

[0089] <Radio wave absorption layer> As the radio wave absorbing layer, strontium ferrite magnetic powder was used as magnetic iron oxide powder, and variations were created using different types of carbon-based nanoparticles. In all cases, silicone rubber was used as the binder.

[0090] The electromagnetic wave absorbing layer was fabricated by press-molding a magnetic compound to a predetermined thickness. The magnetic compound was obtained by kneading magnetic iron oxide powder, a rubber binder, and carbon-based fine particles, and then adjusting the viscosity by mixing a crosslinking agent into the resulting mixture. The magnetic compound thus fabricated was crosslinked and molded into a sheet at a temperature of 165°C using a hydraulic press, and then subjected to a secondary crosslinking treatment at a temperature of 170°C in a constant temperature bath to obtain the desired electromagnetic wave absorbing layer with the thickness shown below.

[0091] The materials and quantities of the magnetic iron oxide powder, carbon-based fine particles, and rubber binder used to form the magnetic compound were as follows.

[0092] Radio wave absorption layer 1 Magnetic iron oxide strontium ferrite magnetic powder 65 parts by weight Carbon-based fine particles, carbon black, 1.5 parts by weight Binder, silicone rubber: KE-541-U, 33 parts by weight Crosslinking agent 0.5 parts by weight Radio wave absorption layer 2 Magnetic iron oxide strontium ferrite magnetic powder 65 parts by weight Carbon-based microparticles (carbon nanotubes): 1.5 parts by weight Binder, silicone rubber: KE-541-U, 33 parts by weight Crosslinking agent 1 part by weight Radio wave absorption layer 3 Magnetic iron oxide strontium ferrite magnetic powder 65 parts by weight Binder, silicone rubber: KE-541-U, 34 parts by weight Crosslinking agent 1 part by weight Radio wave absorption layer 4 Carbon-based fine particles, carbon black, 6 parts by weight Binder, Silicone Rubber: KE-541-U, 92 parts by weight Crosslinking agent: 2 parts by weight.

[0093] As for the materials of the above-mentioned radio wave absorbing layer, strontium ferrite magnetic powder has an average particle size of 2.2 μm and a BET value of 1.5 m 2 The carbon black used was 34 nm in primary particle size and 1400 mB in BET value. 2 The sample used was Ketjenblack EC600JD (product name) manufactured by Lion Specialty Chemicals Co., Ltd. at a concentration of / g. The carbon nanotubes had a fiber diameter of 150nm and a BET value of 13m. 2 We used VGCF-H (product name) manufactured by Showa Denko Corporation, in a quantity of / g.

[0094] The silicone rubber KE-541-U (product name) used as a binder is a silicone rubber manufactured by Shin-Etsu Chemical Co., Ltd. Furthermore, 2,5-dimethyl-2,5-bis(tert-butylperoxy)hexane (C-8A (product name) manufactured by Shin-Etsu Chemical Co., Ltd.) was used as the crosslinking agent.

[0095] For the reinforcing layers, we prepared the following: 9.7mm thick (reinforcing layer 1) and 7.7mm thick (reinforcing layer 2) polypropylene honeycomb core material (Texcel (product name, manufactured by Gifu Plastic Industry Co., Ltd.)), 6.0mm thick (reinforcing layer 3) polypropylene foam molded product (Paronia (registered trademark, manufactured by Mitsui Chemicals Tohcello Co., Ltd.)), 10.0mm thick acrylic sheet (reinforcing layer 4), and 10.0mm thick corrugated plastic sheet (reinforcing layer 5).

[0096] For each of the eight examples of radio wave absorbers combining these radio wave absorbing layers 1 to 4 and reinforcing layers 1 to 5, and for Comparative Example 1 which lacked a reinforcing layer, the self-sufficiency evaluation value (Δ) and the radio wave absorption characteristics of the radio wave absorber were measured, specifically the reflection attenuation characteristics at the surface of the reinforcing layer (reflection attenuation) and the attenuation amount when radio waves incident on the radio wave absorber pass through the radio wave absorber (transmission attenuation).

[0097] The specific combinations for each example were as follows: Example 1 used radio wave absorbing layer 1 and reinforcing layer 1; Example 2 used radio wave absorbing layer 1 and reinforcing layer 2; Example 3 used radio wave absorbing layer 1 and reinforcing layer 3; Example 4 used radio wave absorbing layer 3 and reinforcing layer 1; Example 5 used radio wave absorbing layer 1 and reinforcing layer 4; Example 6 used radio wave absorbing layer 2 and reinforcing layer 1; Example 7 used radio wave absorbing layer 1 and reinforcing layer 5; and Example 8 used radio wave absorbing layer 4 and reinforcing layer 1. Each of these combinations, measuring 25 mm in width and 100 mm in length, was attached to form a radio wave absorber sample using double-sided tape. In Comparative Example 1, radio wave absorbing layer 1 was used as the radio wave absorbing layer.

[0098] The reflection and transmission attenuation of radio waves were both measured using the free-space method. Specifically, an Anritsu ME7838A millimeter-wave network analyzer was used to irradiate the front side of the radio wave absorber (the incident surface side, i.e., the side where the reinforcing layer is located) with a 76.5 GHz input wave from a transmitting antenna through a dielectric lens. At this time, the surface reflected wave reflected from the front of the radio wave absorber and the transmitted wave that penetrates to the back side of the radio wave absorber (i.e., the side with the radio wave absorption layer) were measured by placing receiving antennas. The intensity of the radio waves irradiated from the transmitting antenna and the intensity of the radio waves received by the receiving antenna were measured as voltage values, and the reflection attenuation at the surface of the radio wave absorber was determined from the "irradiated wave - reflected wave" ratio, and the transmission attenuation was determined from the "irradiated wave - transmitted wave" ratio, both in dB.

[0099] The measurement results are shown in Table 1.

[0100] [Table 1]

[0101] As shown in Table 1, in all of Examples 1 to 8, the self-sufficiency evaluation value Δ was 0.5 or less, indicating sufficient self-sufficiency. Furthermore, the transmission attenuation when passing through the radio wave absorber exhibiting radio wave absorption characteristics was -10 dB or more in all examples, confirming that the radio wave absorption characteristics are practically sufficient.

[0102] On the other hand, the radio wave absorber of Comparative Example 1, which does not have a reinforcing layer, has an extremely large self-sufficiency evaluation value Δ of 12 mm, indicating that it is impossible to stand the radio wave absorbing layer upright with its sides facing vertically using only the radio wave absorbing layer.

[0103] Thus, it has been confirmed that the radio wave absorber according to this embodiment can be made self-supporting by providing a reinforcing layer on the radio wave incidence side of the radio wave absorption layer, resulting in a radio wave absorber with excellent handling properties that can be easily placed at a desired location.

[0104] Furthermore, we were able to confirm that placing the aforementioned dielectric reinforcing layer on the side of the radio wave absorber where the radio waves are incident improves the amount of reflection attenuation at the surface of the radio wave absorber.

[0105] Specifically, in Table 1, when comparing the reflection attenuation of the radio wave absorbers in Examples 1, 2, 3, 5, and 7, which have the same radio wave absorbing layer (radio wave absorbing layer 1), with the transmission attenuation of the radio wave absorber in Comparative Example 1, which does not have a reinforcing layer, that is, the reflection attenuation at the surface of the radio wave absorbing layer (-5.7 dB), it can be seen that the reflection attenuation at the surface of the reinforcing layer is large.

[0106] Thus, by arranging a reinforcing layer on the radio wave incident side of the radio wave absorbing layer to enhance its self-supporting ability, reflection at the surface of the radio wave absorber can also be suppressed. According to the inventors' studies, when the reflection loss (dB) at the surface of the reinforcing layer, which is the front of the radio wave absorber, is A, and the reflection loss (dB) at the surface of the radio wave absorbing layer, which is the back of the radio wave absorber (the amount of attenuation in radio waves reflected at the surface of the radio wave absorbing layer when radio waves are irradiated from the back side) is B, it was confirmed that the ratio A / B is preferably between 1.5 and 6.5.

[0107] [Radio wave absorbing device] The following describes a radio wave absorbing device that absorbs unwanted radio waves using the radio wave absorber according to this embodiment, with specific examples.

[0108] As described above, the radio wave absorber disclosed in this application is self-supporting because it includes a reinforcing layer having a predetermined thickness and rigidity along with the radio wave absorbing layer. By utilizing this, for example, when measuring the radio wave characteristics of a device to be measured, the radio wave absorber can be placed in the path between the measuring device and unwanted radio waves reflected by walls, etc., thereby effectively preventing the measuring instrument from detecting radio waves other than those from the device to be measured, and enabling measurement of radio wave characteristics with a high signal-to-noise ratio.

[0109] Furthermore, when placing a radio wave absorber in the path between unwanted radio waves and a measuring instrument, although the radio wave absorber can stand on its own, considering the need to easily move the radio wave absorber to the desired position and maintain its position and orientation, it is preferable to use a radio wave absorbing device in which support members such as legs or pillars are fixed to the radio wave absorber rather than using the radio wave absorber alone, as this greatly improves practicality. The support members should be any members that can maintain the radio wave absorber in a state where it can effectively absorb unwanted radio waves, such as the position of the radio wave absorber, the orientation of the radio wave incidence surface, and the inclination angle. For this reason, in addition to the legs and pillars exemplified above, the support members can be of any appropriate form, such as those that suspend the radio wave absorber from the ceiling or wall, or those that clamp the ends of the radio wave absorber.

[0110] Incidentally, in the radio wave absorber disclosed in this application, a reinforcing layer formed of a dielectric material is arranged on the radio wave incident surface side of the radio wave absorption layer. As described in the above embodiment, a portion of the radio waves incident on the radio wave absorber are reflected by the surface of the reinforcing layer. The inventors have found that the reflection characteristics of radio waves at the surface of this reinforcing layer change depending on the inclination angle of the radio wave incident surface relative to the radio waves incident on the radio wave absorber. Based on this finding, they also investigated the inclination angle of the radio wave incident surface that improves the radio wave absorption characteristics of the radio wave absorber. The details of this investigation are described below.

[0111] [Regarding the relationship between the tilt angle of a radio wave absorber and its radio wave absorption characteristics] Figure 3 shows an example of a radio wave absorbing device according to this embodiment, in which the radio wave absorber is tilted.

[0112] The radio wave absorbing device 30 illustrated in Figure 3 consists of a flat radio wave absorber 31 and two triangular support members 32 attached to each of the sides of the radio wave absorber 31 so as to extend perpendicularly to the radio wave incident surface, which is the main surface of the radio wave absorber 31. By providing the support members 32 in this way, the radio wave absorbing device 30 can be moved and easily positioned in a predetermined location and orientation while maintaining the radio wave incident surface of the radio wave absorber 31 inclined at a predetermined angle.

[0113] In the following, the inclination angle of the radio wave absorber 31 in the radio wave absorbing device 30 according to this embodiment refers to the angle formed by the direction of propagation of the radio waves 34 to be absorbed (direction of incidence to the radio wave absorber 31) and the direction of the normal (Z direction) of the portion 31a on the radio wave incidence surface side of the radio wave absorber 31 located in the direction of propagation 34 of the radio waves, i.e., the angle shown as θ in Figure 3. Furthermore, the radio wave absorbing device disclosed in this application is characterized in that the direction of radiation on the surface portion on the radio wave incidence surface side of the radio wave absorber intersects with the direction of propagation of the radio waves at a predetermined angle, i.e., θ is not 0°.

[0114] Here, the direction of propagation of the absorbed radio waves 34 can be understood as the direction of the straight line that connects the radio wave emission source (device 33 in the case of Figure 3) and the radio wave absorber 31 in such a way that the distance between them is minimized. Furthermore, the portion 31a of the radio wave absorber 31 on the radio wave incident surface side is a small area located in the direction of propagation of the radio waves as determined above, and its size can be considered, for example, as a circular portion with a diameter of about 1 / 100th of the distance between the radio wave emission source 33 and the radio wave absorbing device 30 (if the distance is 3m, it is a circular portion with a diameter of 3cm).

[0115] Based on the above definition, for example, as shown in Figure 3, if the radio waves 34 absorbed by the radio wave absorber 31 are emitted horizontally from the equipment 33 which is the source of the radio waves, then the inclination angle θ of the radio wave absorber 31 is the same as the angle between the radio wave incident surface of the flat radio wave absorber 31 and the vertical direction.

[0116] Furthermore, if the radio wave absorber is not flat, that is, if the surface of the radio wave absorber is curved rather than flat, the angle between the normal direction of the portion of the radio wave absorber on the radio wave incident surface side in the direction of propagation of the absorbed radio wave and the direction of propagation of the radio wave is similarly defined as the inclination angle θ of the radio wave absorber.

[0117] Furthermore, when the surface of a radio wave absorber is curved, the curvature direction can be horizontal (X direction in Figure 3), vertical (Y direction in Figure 3), or a direction that is neither X nor Y. Additionally, it can be approximately spherical, meaning it is curved in both the X and Y directions. However, in all of these cases, the normal direction of the portion of the radio wave absorber on the radio wave incident surface side, located in the direction of radio wave propagation, can be determined, and therefore the tilt angle of the radio wave absorber can be determined by the above definition.

[0118] Furthermore, when the surface of the radio wave absorber is a curved surface, there are two possible shapes: a convex shape where it is convex towards the side where the radio waves are incident, and a concave shape where it is concave towards the side where the radio waves are incident. In either case, the normal direction of the portion of the surface of the radio wave absorber can be determined, and therefore the tilt angle of the radio wave absorber can be determined using the definition described above.

[0119] Furthermore, if the surface of the radio wave absorber has an uneven or wave-like shape, resulting in varying heights, the average value of these heights can be used to define the surface of the radio wave absorber as a plane. By determining the normal direction of the portion on the radio wave incident side in the direction of radio wave propagation, the tilt angle of the radio wave absorber can be determined.

[0120] The return loss, a radio wave characteristic of the radio wave absorber discussed here, is a numerical value (dB) that indicates how much the intensity of the radio waves received by the receiving device after they are reflected off the surface of the radio wave absorber decreases compared to the intensity of the radio waves transmitted from the transmitting device. As described above, the measurement method is performed with the transmitting device and the receiving device of the reflected radio waves in the same position. Considering that radio waves travel in a straight line but spread radially, a curved or uneven surface of the radio wave absorber reduces the rate at which reflected radio waves scatter and return to the source compared to a flat surface. Therefore, in order to reduce reflected waves on the surface of the radio wave absorber on the side of the radio wave incidence surface and increase the return loss, it is preferable that the portion of the surface of the radio wave absorber in the direction of radio wave propagation is a curved surface that curves in at least one direction rather than being flat. In other words, it is considered that the return loss can be reduced even more if the normal direction of the surface portion of the radio wave absorber is a curved surface that is inclined with respect to the direction of radio wave propagation rather than being an inclined flat surface.

[0121] <Changes in radio wave absorption characteristics due to the tilt angle of the radio wave absorber> As an example, the inventors actually fabricated three types of radio wave absorbers equipped with reinforcing layers of different configurations and measured how the radio wave absorption characteristics changed by varying the tilt angle θ.

[0122] Figure 4 is a schematic diagram showing the measurement system used to measure the relationship between the tilt angle of a radio wave absorber and its radio wave absorption characteristics.

[0123] The change in radio wave characteristics at different tilt angles of the radio wave absorber was measured using the free-space method with an Anritsu ME7838A millimeter-wave network analyzer (product name: reference numeral 41), the same as described in the above embodiment of the radio wave absorber.

[0124] Specifically, as shown in Figure 4, a 76.5 GHz input wave (reference numeral 46) was irradiated from the transmitting antenna 42 through the dielectric lens 43 to the front side of the radio wave absorber 40, i.e., the side where the reinforcing layer is located. At this time, the surface reflected wave S reflected from the front surface of the radio wave absorber 40 was observed. 11 (Reference numeral 47) was received by the receiving antenna (Reference numeral 42) via the dielectric lens 43. In addition, the transmitted wave S that passed through to the back side of the radio wave absorber 40, i.e., to the radio wave absorption layer side, was also received. 21 (Reference numeral 48) was measured using a dielectric lens 44 and a receiving antenna 45 located on the back side.

[0125] Then, while gradually tilting the radio wave absorber 40 from an angle θ of 0°, i.e., vertically positioned, to 20°, the intensity of the radio waves irradiated from the transmitting antenna 42 and the intensity of the radio waves received by the receiving antennas 42 and 45 were measured as voltage values, and the reflection attenuation at the surface of the radio wave absorber and the transmission attenuation through the radio wave absorber were determined in dB.

[0126] Figures 5 to 7 show the measurement results for the relationship between tilt angle and return attenuation, and tilt angle and transmission attenuation, for each radio wave absorber. In each graph from Figures 5 to 7, the change in return attenuation with respect to the change in tilt angle of the radio wave absorber is represented as (a) in each figure, and the change in transmission attenuation with respect to the change in tilt angle of the radio wave absorber is represented as (b) in each figure. In each graph, the horizontal axis represents the tilt angle θ (°) of the radio wave absorber, and the vertical axis represents the radio wave attenuation (dB).

[0127] Figure 5 shows the measurement results when the aforementioned polypropylene honeycomb core material (Texcel T10-2000 (product name, manufactured by Gifu Plastic Industry Co., Ltd., 9.7 mm thick: reinforcing layer 1)) is used as the reinforcing layer, and the aforementioned "radio wave absorbing layer 1" is used as the radio wave absorbing layer (the same as the radio wave absorber in Example 1).

[0128] As shown in Figure 5(a), in both cases where the polarization direction of the irradiated wave is a TE (electric field) wave (solid line 51) or a TM (magnetic field) wave (dashed line 52), the return loss is greater up to a tilt angle of 20° compared to when the tilt angle is 0°, i.e., when the wave is incident perpendicularly to the surface of the radio wave absorber. However, as the tilt angle increases, the value of the return loss does not increase uniformly, but rather fluctuates in small waves, indicating that there is a range in which a larger return loss can be obtained. In particular, at tilt angles around 5° and 13°, the return loss of the TE wave exceeds 50dB, demonstrating extremely large radio wave absorption characteristics.

[0129] On the other hand, Figure 5(b), which shows the change in transmission attenuation, shows almost no change between tilt angles of 0° and 20°, and in both cases where the polarization direction of the irradiated wave is a TE (electric field) wave (solid line 53) and a TM (magnetic field) wave (dashed line 54), it shows a constant value of about -15 dB.

[0130] Figure 6 shows the measurement results when a resin board (Plapal® PGPPZ-200 (product name, manufactured by Kawakami Sangyo Co., Ltd., 9 mm thick)) is used as a reinforcing layer, with an air cap (bubble cushioning material) having rows of fine cylindrical protrusions filled with air sandwiched between resin plates, and the aforementioned "Radio wave absorbing layer 1" is used as the radio wave absorbing layer.

[0131] As shown in Figure 6(a), in both cases where the polarization direction of the irradiated wave is represented by the solid line 61 indicating a TE (electric field) wave and the dashed line 62 indicating a TM (magnetic field) wave, the return loss is greater up to a tilt angle of 20° compared to when the tilt angle is 0°, i.e., when the wave is incident perpendicularly to the surface of the radio wave absorber. Furthermore, as the tilt angle increases, the value of the return loss fluctuates in a small wave-like pattern, but changes to a generally larger value. It can be seen that there is a region around a tilt angle of 5° to 7° where both the solid line 61, which represents the electric field, and the dashed line 62, which represents the electric field, have large values.

[0132] On the other hand, Figure 6(b), which shows the change in transmission attenuation, shows almost no change between tilt angles of 0° and 20°, similar to the measurement results for the polypropylene honeycomb core material shown in Figure 5(b). In both cases, the solid line 63, which indicates the polarization direction of the irradiated wave is a TE (electric field) wave, and the dashed line 64, which indicates the polarization direction is a TM (magnetic field) wave, a constant value of about -15dB is observed.

[0133] Figure 7 shows the measurement results when a resin hollow structure plate (Danplate® J-10-180 (product name, manufactured by Ube Eximo Co., Ltd., 10 mm thick)) made of polypropylene plastic corrugated cardboard, in which vertical walls arranged in one direction are covered with upper and lower flat plates, is used as the reinforcing layer, and the above-mentioned "Radio wave absorbing layer 1" is used as the radio wave absorbing layer.

[0134] As shown in Figure 7(a), in both cases where the polarization direction of the irradiated wave is represented by the solid line 71 indicating a TE (electric field) wave and the dashed line 72 indicating a TM (magnetic field) wave, the return loss is greater up to a tilt angle of 20° compared to when the tilt angle is 0°, i.e., when the wave is incident perpendicularly to the surface of the radio wave absorber. When the hollow structure plate shown in Figure 7(a) is used as a reinforcing layer, the change in return loss with respect to the change in tilt angle is the largest, and it changes in a wave-like manner. It can be seen that there is a region around a tilt angle of 4° to 7° where both the solid line 71 indicating the electric field and the dotted line 72 indicating the electric field have large values.

[0135] On the other hand, Figure 7(b), which shows the change in transmission attenuation, shows almost no change between tilt angles of 0° and 20°, similar to the measurement results for the polypropylene honeycomb core material shown in Figure 5(b) and the resin board using air caps shown in Figure 6(b). The solid line 73, where the polarization direction of the irradiated wave is a TE (electric field) wave, shows a constant value of about 12 dB, and the dashed line 74, where the polarization direction is a TM (magnetic field) wave, shows a constant value of about -14 dB.

[0136] Thus, it can be seen that the amount of reflected radio waves at the surface of the reinforcing layer placed on the radio wave incident side of the radio wave absorber is greater when the reinforcing layer is tilted at an angle between 2° and 20° than when it is not tilted (tilt angle is 0°), regardless of the type of reinforcing layer. Furthermore, although the magnitude of the change in reflected attenuation differs depending on the shape of the space inside the reinforcing layer, it was confirmed that the reflected attenuation does not gradually increase with the tilt angle, but rather there are regions of tilt angles where the reflected attenuation is large and regions where the reflected attenuation is relatively small. As shown in Figures 5 to 7, it was confirmed that the reflected attenuation is large in the region of tilt angles between 3° and 7°.

[0137] The reason why the amount of reflection loss changes with the tilt angle, resulting in an overall increase in attenuation, is not entirely clear. However, when radio waves pass through honeycomb core materials, hollow structures, or structures containing air bubbles, the distance they travel through each material with different dielectric constants and through the air varies with the tilt angle. Therefore, it is presumed that the amount of reflection loss changes in a wave-like manner with the tilt angle, increasing in the direction of increase.

[0138] As described above, it was confirmed that the tendency of the change in the amount of reflected attenuation with respect to the angle of incident radio waves differs depending on the configuration of the reinforcing layer, and in particular, depending on the arrangement of the air contained in the reinforcing layer. Furthermore, it is thought that configurations in which the space between the upper and lower flat plates is not divided in the thickness direction, such as the honeycomb core material shown in Figure 5 and the plastic corrugated cardboard shown in Figure 7, result in a more drastic increase or decrease in the amount of reflected attenuation with respect to the tilt angle than configurations in which air is confined within a space limited in the thickness direction, such as the resin board of the air cap shown in Figure 6.

[0139] Therefore, when a dividing member that does not divide in the thickness direction is placed between the upper and lower plates, it is assumed that the tendency for the change in the amount of reflection loss with respect to the inclination angle will be similar regardless of the shape of the dividing member, such as a honeycomb shape, a linear shape in one direction, a grid shape where vertical and horizontal lines intersect at right angles, a grid shape where lines intersect diagonally to form a rhombus, or a shape in which multiple cylinders are lined up. Furthermore, for example, in the case where the tips of conical members placed on both the upper and lower plates are connected to form a dividing member, or in the case where a dividing member with a wavy shape in the thickness direction is placed, it is estimated that a tendency for the change in reflection absorption characteristics with respect to the inclination angle will be small, as shown in the resin board in Figure 6.

[0140] Furthermore, if the reinforcing layer is composed of foam, the degree of change in the amount of reflection loss with respect to changes in the tilt angle is thought to be smallest. However, by setting the tilt angle to between 2° and 20°, it is thought that the amount of reflection loss can be increased.

[0141] As described above, the radio wave absorber shown in this embodiment comprises a radio wave absorbing layer in which at least one of magnetic iron oxide and carbon-based fine particles that magnetically resonate in the frequency band of 20 GHz to 300 GHz is dispersed and mixed in a resin binder, and a reinforcing layer made of a dielectric material and arranged on the radio wave incident side of the radio wave absorbing layer. As a result, it effectively absorbs radio waves in the frequency band of 20 GHz to 300 GHz and is self-supporting as a radio wave absorber. For this reason, for example, when measuring radio wave characteristics, it can be placed like a screen in a position that blocks unwanted radio waves to protect measuring equipment and the equipment being measured from unwanted radio waves, and radio wave characteristics can be measured in a good environment with little noise.

[0142] Furthermore, the radio wave absorbing device shown in this embodiment is a radio wave absorbing device using the radio wave absorber disclosed herein, comprising a radio wave absorber and a support member capable of maintaining the surface of the radio wave absorber on the radio wave incident surface side at a predetermined angle, and is arranged such that the normal direction of the portion of the surface on the radio wave incident surface side located in the direction of propagation of the radio waves to be absorbed intersects with the direction of propagation of the radio waves at a predetermined angle. As a result, it is possible to easily maintain the state in which the radio wave absorber is tilted at a predetermined inclination angle, thereby suppressing the reflection of radio waves on the surface of the reinforcing layer, and realizing a radio wave absorbing device that has good reflection attenuation characteristics and can be easily placed at a desired location.

[0143] In the above embodiment, the main surfaces of the radio wave absorbing layer and the reinforcing layer of the radio wave absorber were described as being rectangular in shape and of the same size. However, the shape and size of the main surfaces of the radio wave absorbing layer and the reinforcing layer do not necessarily have to be identical. As long as the shape can sufficiently block the path of unwanted radio waves that the radio wave absorber wants to block, it is not a problem if one is larger than the other and a part of it protrudes. Furthermore, even if the planar shape of the radio wave absorbing layer is complex with protrusions, recesses, or voids, a reinforcing layer with a planar shape such as a rectangle, circle, or polygon that is larger than the radio wave absorbing layer can be used, as long as the reinforcing layer can maintain its overall shape in a substantially vertical direction. [Industrial applicability]

[0144] The radio wave absorber disclosed herein effectively absorbs radio waves from 20 GHz to 300 GHz and is self-supporting, allowing it to be easily placed in the path of the radio waves to be absorbed. Furthermore, a radio wave absorbing device that can maintain the radio wave absorber inclined at a predetermined angle can be easily used in a state that reduces radio wave reflection on the surface of the reflective layer. For this reason, it is useful as a radio wave absorbing member that can create a good space with the influence of undesirable radio waves suppressed. [Explanation of symbols]

[0145] 1. Radio wave absorption layer 1a Magnetic iron oxide powder 1b Carbon-based fine particles 1c Resin Binder 2 Reinforcement layer 10 Radio waves (incident waves) 30 Radio wave absorption device 31 Radio wave absorber 32 Support member

Claims

1. A radio wave absorber comprising a radio wave absorbing layer and a reinforcing layer disposed on the surface of the radio wave absorbing layer on the radio wave incident side, The aforementioned radio wave absorbing layer comprises at least one of magnetic iron oxide powder and carbon-based fine particles that resonate magnetically in the frequency band of 20 GHz to 300 GHz, and a resin binder. The reinforcing layer is made of a dielectric material having a honeycomb structure and a thickness of 5 mm to 30 mm. The aforementioned radio wave absorber is in the form of a plate with a small thickness relative to its main surface, A radio wave absorber characterized in that, when a self-supporting ability test is performed using the radio wave absorber with a width of 25 mm and a length of 100 mm as a sample, the sample is placed on a rigid stand with the reinforcing layer facing downwards, the length of the sample is perpendicular to the edge of the stand, and the sample extends 30 mm beyond the edge of the stand, and a rectangular parallelepiped with a width of 25 mm and a length of 5 mm and a weight of 5 kg is placed on the remaining 70 mm portion of the sample on the stand with its ends aligned with the edge of the stand, and left for 10 minutes at room temperature, the Δ value indicating the degree of deformation of the radio wave absorber is 0.5 mm or less.

2. The radio wave absorber according to claim 1, wherein the magnetic iron oxide powder is either a powder of magnetoplanbite-type ferrite or a powder of epsilon magnetic iron oxide.

3. The radio wave absorber according to claim 1 or 2, wherein the carbon-based fine particles are at least one of carbon black, carbon nanotubes, and graphene.

4. The radio wave absorber according to any one of claims 1 to 3, wherein the resin binder is a rubber-based material.

5. The radio wave absorber according to any one of claims 1 to 4, wherein the radio wave attenuation for transmitted radio waves that pass through the radio wave absorber is -10 dB or more.

6. A radio wave absorbing device using a radio wave absorber described in any one of claims 1 to 5, The system comprises the radio wave absorber and a support member capable of maintaining the surface of the radio wave absorber on the radio wave incident surface side at a predetermined angle. A radio wave absorbing device characterized in that the radio wave absorber is arranged such that the normal direction of the portion of the surface on the radio wave incident surface side, which is located in the direction of propagation of the radio wave to be absorbed, intersects with the direction of propagation of the radio wave at a predetermined angle.

7. The radio wave absorbing device according to claim 6, wherein the portion of the surface is a curved surface that is curved in at least one direction.

8. The radio wave absorbing device according to claim 6 or 7, wherein the angle at which the radiation direction of the surface portion intersects with the propagation direction of the radio waves is 2° or more and 20° or less.

9. The radio wave absorbing device according to claim 8, wherein the angle of intersection is 3° or more and 7° or less.

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