Radio wave reflector

Abstract

To achieve a radio wave reflector capable of satisfactorily reflecting radio waves of a predetermined frequency.SOLUTION: Provided is a radio wave reflector 10 in which a first dielectric layer 11, a resistive layer 12, a second dielectric layer 13, and a reflective layer 14 are stacked in order from a radio wave 1 incident surface side. When the center wavelength of radio waves reflected by the radio wave reflector is represented by λ, the thickness d of the second dielectric layer satisfies d=λ / 2.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to a radio wave reflector that reflects radio waves, and particularly to a radio wave reflector that can selectively reflect radio waves in a predetermined frequency band.

Background Art

[0002] In recent years, in mobile communications such as mobile phones, wireless LANs, toll collection systems (ETC), etc., centimeter waves having a frequency band of several gigahertz (GHz), and further, millimeter wave bands having frequencies from 30 gigahertz to 100 gigahertz, and radio waves having frequencies in the terahertz (THz, 100 GHz~) band, which are radio waves in a higher frequency band exceeding the millimeter wave band, are also being actively studied.

[0003] In response to such a technological trend of using radio waves of such high frequencies, there is an increasing demand for radio wave absorbers that can absorb radio waves in the millimeter wave band and higher frequency bands, rather than those that can only absorb unnecessary radio waves.

[0004] As a radio wave absorber that suppresses and absorbs the reflection of unnecessary radio waves, a resistive film is provided on the radio wave incident side surface of the dielectric layer, and a reflection layer that reflects radio waves is provided on the opposite back surface. By shifting the phase of the radio waves reflected by the reflection layer and radiated to the outside by 1 / 2 wavelength from the phase of the radio waves reflected on the surface of the resistive film, the radio waves reflected from the radio wave absorber are canceled out and absorbed. A so-called radio wave interference type (also called reflection type) is known. The radio wave interference type radio wave absorber is lighter than a type of radio wave absorber that magnetically absorbs radio waves by magnetic particles, and has the advantage that it can be easily manufactured and thus can be cost-reduced.

[0005] The inventors have proposed a radio wave absorbing sheet, which is a thin, radio wave interference-type radio wave absorber, that employs a conductive organic polymer film as the resistive film formed on the surface of the dielectric layer. This sheet is capable of effectively absorbing radio waves in a desired frequency band and possesses high flexibility, making it easy to handle (see Patent Document 1). [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] International Publication Number WO2018 / 088492 [Overview of the project] [Problems that the invention aims to solve]

[0007] By using the conventional radio wave absorber described above, unwanted radio waves can be absorbed, reducing noise factors and enabling the deployment of radio communication technology in a favorable environment.

[0008] On the other hand, unwanted radio waves can also be filtered out by effectively reflecting radio waves in the desired frequency band and suppressing the reflection of radio waves at unwanted frequencies. In particular, as the frequency of radio waves increases to the millimeter wave and terahertz bands, the directivity of the radio waves increases. Therefore, in order to deliver high-frequency radio waves to the desired location, it is important to place reflectors that reflect the radio waves within the radio wave propagation path.

[0009] This disclosure aims to solve the above-mentioned problems and realize a radio wave reflector that can effectively reflect radio waves of a predetermined frequency. [Means for solving the problem]

[0010] To solve the above problems, the radio wave reflector disclosed in this application is a radio wave reflector in which a first dielectric layer, a resistive layer, a second dielectric layer, and a reflective layer are sequentially stacked from the radio wave incident surface side, characterized in that when the central wavelength of the radio wave reflected by the radio wave reflector is λ, the thickness d of the second dielectric layer is d = λ / 2. [Effects of the Invention]

[0011] The radio wave reflector disclosed in this application has a configuration in which a first dielectric layer, a resistive layer, a second dielectric layer, and a reflective layer are sequentially stacked. Since the thickness d of the second dielectric layer is d = λ / 2 with respect to the central wavelength λ of the reflected radio wave, the phases of the radio waves reflected by the resistive layer and the radio waves reflected by the reflective layer after they enter the radio wave reflector overlap and are superimposed. For this reason, radio waves of a desired frequency can be strongly reflected. [Brief explanation of the drawing]

[0012] [Figure 1] This is a partial cross-sectional view illustrating the first configuration of the radio wave reflective sheet according to this embodiment. [Figure 2] These are the model diagram and equivalent circuit diagram used in the simulation to confirm the radio wave reflection characteristics of the radio wave reflective sheet according to this embodiment. Figure 2(a) shows the transmission circuit diagram of the radio wave reflective sheet according to this embodiment, and Figure 2(b) shows the model diagram of the radio wave reflective sheet used in the study. [Figure 3] This is a model diagram illustrating the configuration of the radio wave reflective sheet used in the simulation and the radio wave reflective sheet that was actually fabricated. [Figure 4] This figure compares the reflection attenuation characteristics of the radio wave reflective sheet with those of the first configuration, which was actually fabricated, with the simulation results. [Figure 5] This is a partial cross-sectional view illustrating the second configuration of the radio wave reflective sheet according to this embodiment. [Figure 6] This figure compares the reflection and attenuation characteristics of the actually fabricated radio wave reflective sheet with the simulation results. [Modes for carrying out the invention]

[0013] The radio wave reflector disclosed in the present application is a radio wave reflector in which a first dielectric layer, a resistance layer, a second dielectric layer, and a reflection layer are sequentially laminated from the radio wave incident surface side. When the center wavelength of the radio wave reflected by the radio wave reflector is λ, the thickness d of the second dielectric layer is d = λ / 2.

[0014] Here, the center wavelength λ of the radio wave reflected by the radio wave reflector refers to the wavelength when the radio wave propagates inside the second dielectric layer.

[0015] With such a configuration, the phases of the radio wave reflected by the resistance layer and the radio wave reflected by the reflection layer after passing through the second dielectric layer among the radio waves incident on the radio wave reflector overlap, and the reflected radio wave can be made stronger. In addition, due to the combined action of the radio wave reflected from the surface of the radio wave reflector, the radio wave reflected by the resistance layer, and the radio wave reflected by the reflection layer, radio waves in a wide frequency band can be reflected. On the other hand, it has sharpness in the bands of about 0.5 times and 1.5 times the center frequency of the reflection band and can have filter characteristics.

[0016] The center frequency of the radio wave to be reflected by the radio wave reflector is preferably 100 GHz or more and 450 GHz or less.

[0017] Also, the resistance value of the resistance layer is preferably 80 Ω / sq or more and 250 Ω / sq or less. By setting the resistance value of the resistance layer in the range of 80 to 250 Ω / sq, the balance between the radio wave reflected by the resistance layer and the radio wave transmitted through the resistance layer is improved, and radio waves in the particularly several hundred GHz band can be reflected well.

[0018] Furthermore, it is preferable that the second dielectric layer has adhesiveness. By doing so, the resistance layer, the second dielectric layer, and the reflection layer can be adhered by the adhesiveness provided by the second dielectric layer itself, eliminating the need for an adhesive for adhering each layer, and thus the radio wave reflector can be manufactured at a low cost.

[0019] Furthermore, it is preferable that the resistance layer is formed of any one of a conductive organic polymer film, a sputtered film, and a vapor-deposited film.

[0020] In addition, for the radio wave reflector disclosed in the present application, it is preferable that the first dielectric layer, the resistance layer, the second dielectric layer, and the reflection layer are all formed in a thin film shape and are formed as a flexible sheet as a whole. By making the radio wave reflector into a flexible sheet shape, it is possible to realize a radio wave reflector that is easy to handle when disposed at a desired location.

[0021] Hereinafter, the radio wave reflector disclosed in the present application will be described with reference to the drawings.

[0022] Here, as the radio wave reflector disclosed in the present application, a radio wave reflection sheet that can be grasped as a sheet with a thickness sufficiently small with respect to the main area will be exemplified and described. As described above, the radio wave reflector disclosed in the present application is a concept including both a radio wave reflection sheet that can be grasped as a sheet in terms of the relationship between its surface area and thickness, and a radio wave reflection block that has a relatively large thickness and is grasped as a block shape as a whole.

[0023] Note that, as will be described later, in a high-frequency band of millimeter wave band or higher, which is the main target of the radio wave reflector disclosed in the present application, the value of the wavelength λ, which is the reciprocal of the frequency, becomes small, and the thickness of the dielectric layer in the radio wave reflector that increases the reflection amount of a predetermined frequency using the interference of radio waves through the dielectric layer does not become very thick. In addition, as a radio wave reflector disposed at a predetermined position on the radio wave path, it is advantageous that it is in a thin sheet shape with a small thickness because it causes less interference to others. For this reason, it is considered that it is more common for the radio wave reflector disclosed in the present application to adopt a sheet shape having a certain surface area and a small thickness.

[0024] (Embodiment) <The First Configuration> FIG. 1 is a partial cross-sectional perspective view showing a first configuration of a radio wave reflection sheet (radio wave reflector) according to the present embodiment.

[0025] Note that Figure 1 and Figure 5, which illustrate the second configuration, are both diagrams included to facilitate understanding of the radio wave reflective sheet configuration according to this embodiment, and the sizes of the components shown in the figures, particularly the thickness of each layer, are not necessarily accurate representations of reality.

[0026] [Overall structure of the radio wave reflective sheet] The first configuration of the radio wave reflective sheet 10 illustrated in this embodiment is constructed by sequentially stacking a first dielectric layer 11, a resistive layer 12, a second dielectric layer 13, and a reflective layer 14 from the incident surface side of the radio wave 1 to be reflected.

[0027] In the first configuration of the radio wave reflective sheet 10 illustrated in Figure 1, a resin sheet used as a substrate when coating and forming the resistive layer 12 is used as the first dielectric layer 11.

[0028] The radio wave reflective sheet 10 according to this embodiment reflects radio waves in a predetermined frequency band by interfering with each other's radio waves, similar to radio wave interference type (also called reflective type) radio wave sheets, with the members arranged on either side of the dielectric layer.

[0029] [Details of each component] Next, we will describe each component that makes up the radio wave reflective sheet 10 according to this embodiment.

[0030] <Dielectric layer> The first dielectric layer 11 and the second dielectric layer 13 of the radio wave reflective sheet 10 according to this embodiment can both be formed from various dielectric materials such as titanium oxide, polyvinylidene fluoride, polyester resin, glass, and silicone rubber. The first dielectric layer 11 and the second dielectric layer 13 can both be formed as single layers made of the same material. Alternatively, they can be constructed by stacking two or more layers of the same or different materials. Furthermore, the first dielectric layer 11 and the second dielectric layer 13 can be formed using the same dielectric material, or they can be constructed using different dielectric materials, including the number of layers.

[0031] In the radio wave reflective sheet 10 of this embodiment shown in Figure 1, as described above, a 300 μm thick polyethylene terephthalate (PET) sheet, which is a resin substrate for forming the resistive layer 12, is used as the first dielectric layer 11.

[0032] The thicknesses of the first dielectric layer 11 and the second dielectric layer 13 can be appropriately determined based on the frequency of the radio wave 1 to be reflected by the radio wave reflection absorption sheet 10, taking into account the dielectric constant of the dielectric material constituting each dielectric layer. Specifically, when the center frequency of the radio wave 1 to be reflected by the radio wave reflection sheet 10 is between 100 GHz and 450 GHz, it is preferable to use a general dielectric material with a relative permittivity of about 2 to 3 for the first dielectric layer 11 and the second dielectric layer 12, and to set their thickness to 160 μm to 500 μm. In the case of dielectric layers with a relative permittivity of about 2 to 3, if the thickness is less than 160 μm, the center wavelength of the radio wave reflected by the radio wave reflection sheet will be higher than 450 GHz. On the other hand, if the thickness of the dielectric layer is greater than 500 μm, the center wavelength of the radio wave reflected by the radio wave reflection sheet will be lower than 100 GHz. In particular, it is preferable to set the thickness of the first dielectric layer 11 and the second dielectric layer 13 to 300 μm or more and 350 μm or less.

[0033] Furthermore, it is preferable that the difference between the thickness of the first dielectric layer 11 and the thickness of the second dielectric layer 13 be 100 μm or less.

[0034] In the first configuration of the radio wave reflective sheet 10 shown in Figure 1, the second dielectric layer 13 is made of an acrylic-based OCA (Optical Clear Adhesive) that is translucent and adhesive. By using an adhesive resin material for the second dielectric layer 13, the resistive layer 12, which includes the first dielectric layer 11 that forms the resistive layer, and the reflective layer 14 can be bonded together by the adhesive force of the second dielectric layer 13. This simplifies the structure of the radio wave reflective sheet 10, improves workability during manufacturing, reduces materials, and lowers the cost of producing the radio wave reflective sheet 10.

[0035] Of course, adhesive materials such as double-sided adhesive sheets can be used to bond the base layer, the first dielectric layer 11, the resistive layer 12, the second dielectric layer 13, and the reflective layer 14, and adhesive can be applied to the surfaces to which each layer is bonded to form a laminate as an electromagnetic wave reflective sheet 10.

[0036] Furthermore, if layers made of transparent or light-transmitting materials are used, such as the first dielectric layer 11 and the second dielectric layer 13 of the radio wave reflective sheet 10 exemplified as the first configuration in Figure 1, and if the resistive layer 12 and the reflective layer 14 are made of light-transmitting materials, then a radio wave reflective sheet 10 with light transmittance having a total light transmittance of a certain level or higher can be realized overall. The total light transmittance of the radio wave absorbing reflective sheet 10 is preferably 60% or higher, and more preferably 70% or higher.

[0037] <Resistance layer> The resistive layer 12 of the radio wave reflective sheet 10 shown in this embodiment is placed between the first dielectric layer 11 and the second dielectric layer 13, and performs the function of reflecting a portion of the radio waves 1 that have passed through the first dielectric layer 11 and transmitting the rest.

[0038] The proportion of radio waves 1 reflected by the resistive layer 12 is determined by the resistance value of the resistive layer 12; the higher the resistance value, the lower the proportion of reflected radio waves and the higher the proportion of transmitted radio waves. Furthermore, the proportion of radio waves transmitted through the resistive layer 12 also changes depending on the frequency of the incident radio waves; for the same resistance value of the resistive layer 12, the higher the frequency of radio waves 1, the higher the proportion transmitted through the resistive layer 12.

[0039] In the case of the radio wave reflective sheet 10 shown in this embodiment, the center frequency of the radio wave 1 to be reflected is set to 300 GHz, and the resistance value of the resistive layer 12 is set to 130 Ω / sq. When the center frequency of the radio wave 1 to be reflected by the radio wave reflective sheet 10 is set to 300 GHz, it is preferable that the resistance value of the resistive layer 12 be between 80 Ω / sq and 250 Ω / sq in order to reflect the radio wave 1 in the frequency band centered on 300 GHz well. If the resistance value of the resistive layer 12 is less than 80 Ω / sq, or if the resistance value of the resistive layer 12 is greater than 250 Ω / sq, the balance between the radio wave 1 reflected by the resistive layer 12 and the radio wave that passes through the resistive layer 12 and is reflected by the reflective layer 14 may be disrupted, which may result in a narrower frequency band of the radio wave reflected by the radio wave reflective sheet 10 or a reduction in the amount of reflection of the radio wave at the center frequency.

[0040] Furthermore, there are no particular restrictions on the resistive layer 12 used in the radio wave reflective sheet 10 according to this embodiment shown in Figure 1, as long as its surface resistance value falls within a predetermined range. Specifically, conductive organic polymer films, sputtered films, vapor-deposited films, etc., can be used effectively. Moreover, the above-mentioned conductive organic polymer films, sputtered films, and vapor-deposited films are preferable because their resistance values ​​can be controlled by the film thickness and formation density, making it easy to form a resistive layer 12 with a desired resistance value.

[0041] As the conductive organic polymer used as the resistive layer 12, a conjugated conductive organic polymer is used, and it is preferable to use polythiophene or its derivatives, or polypyrrole or its derivatives.

[0042] Furthermore, as the resistive layer 12, an organic polymer whose main chain is composed of a π-conjugated system can be used, and polyacetylene-based conductive polymers, polyphenylene-based conductive polymers, polyphenylene-vinylene-based conductive polymers, polyaniline-based conductive polymers, polyacene-based conductive polymers, polythiophene-vinylene-based conductive polymers, and copolymers thereof can be used.

[0043] Furthermore, a polyanion can be used as a counteranion for the conductive organic polymer used in the resistive layer 12. The polyanion is not particularly limited, but it is preferable that it contains an anionic group that can cause chemical oxidation doping in the conjugated conductive organic polymer used in the resistive layer 12 described above. Examples of such anionic groups include groups represented by the general formulas -O-SO3X, -O-PO(OX)2, -COOX, and -SO3X (wherein X represents a hydrogen atom or an alkali metal atom), and among these, the groups represented by -SO3X and -O-SO3X are particularly preferred because they have an excellent doping effect on the conjugated conductive organic polymer.

[0044] The above conductive organic polymers may be used individually or in combination of two or more. Among the materials exemplified above, polymers consisting of one or two selected from polypyrrole, poly(3-methoxythiophene), poly(3,4-ethylenedioxythiophene), poly(2-aniline sulfonic acid), and poly(3-aniline sulfonic acid) are preferred because they offer higher transparency and conductivity.

[0045] In particular, as a combination of a conjugated conductive organic polymer and a polyanion, it is preferable to use poly(3,4-ethylenedioxythiophene: PEDOT) and polystyrene sulfonic acid (PSS).

[0046] Furthermore, in the resistive layer 12 of the radio wave reflective sheet 10 according to this embodiment, a dopant can be used in combination to control the electrical conductivity of the conductive organic polymer and obtain a predetermined resistance value. As dopants, halogens such as iodine and chlorine, Lewis acids such as BF3 and PF5, protonic acids such as nitric acid and sulfuric acid, transition metals, alkali metals, amino acids, nucleic acids, surfactants, dyes, chloranil, tetracyanoethylene, TCNQ, etc. can be used. The content of the conductive organic polymer in the resistive layer 12 is preferably 10% by mass or more and 35% by mass or less, relative to the total mass of solids contained in the resistive layer 12 composition. If the content is less than 10% by mass, the conductivity of the resistive layer 12 tends to decrease. For this reason, if the surface electrical resistance value of the resistive layer 12 is set within a predetermined range to achieve impedance matching, the film thickness of the resistive layer 12 increases, which tends to make the entire radio wave reflective sheet 10 thicker, or, if it has light transmittance, the optical properties tend to decrease. On the other hand, if the content exceeds 35% by mass, the coating suitability when coating the resistive layer 12 decreases due to the structure of the conductive organic polymer, making it difficult to form a good resistive layer 12. Furthermore, if it is translucent, the haze of the resistive layer 12 increases, which also tends to reduce its optical properties.

[0047] The resistive layer 12 may also be configured to include carbon materials such as carbon microcoils, carbon nanotubes, and graphene.

[0048] Carbon microcoils are a type of vapor-grown carbon fiber obtained primarily by catalytically activated pyrolysis of acetylene, and are materials with a 3D-helical / spiral structure with a coil diameter on the order of microns. Preferably, the coil diameter is 1 to 10 μm, the carbon fiber diameter forming the coil is 0.1 to 1 μm, and the coil length is 1 to 10 mm.

[0049] Carbon nanotubes can be obtained by vapor phase growth methods such as arc discharge, laser evaporation, and thermal decomposition. The carbon nanotubes used as the resistive layer 12 of the radio wave reflective sheet 10 according to this embodiment may be single-layer or multi-layer.

[0050] Graphene can be obtained by methods such as exfoliation transfer, SiC pyrolysis, chemical vapor deposition, or by cutting carbon nanotubes. As the graphene used as the resistive layer 12 of the radio wave reflective sheet 10 according to this embodiment, it is preferable to use flaky powdered graphene, from the viewpoint of easily obtaining the desired aspect ratio and orientation in the radio wave reflective sheet 10.

[0051] Furthermore, a water-soluble polyester resin can be used as the resin for dispersing the carbon material mentioned above.

[0052] The resistive layer 12 can be formed by applying a coating composition, which serves as a paint for forming the resistive layer 12, onto a resin substrate and drying it, as described above.

[0053] As a method for applying the resistive layer-forming coating onto the substrate, for example, coating methods such as bar coating, reverse coating, gravure coating, microgravure coating, die coating, dipping, spin coating, slit coating, and spray coating can be used. Drying after coating should be carried out under conditions that allow the solvent components of the resistive layer-forming coating to evaporate, and is preferably done at 100 to 150°C for 5 to 60 minutes. If solvent remains in the resistive film, the strength tends to be inferior. Drying methods can be, for example, hot air drying, heating drying, vacuum drying, or natural drying. In addition, if necessary, the resistive layer 12 may be formed by curing the coating film by irradiating it with UV light (ultraviolet light) or EB (electron beam).

[0054] The substrate used to form the resistive layer 12 is not particularly limited, but a transparent substrate is preferred. Various materials can be used for such a transparent substrate, such as resin, rubber, glass, ceramics, acrylic resin, silicone resin, or dielectric materials such as OCA. As described above, the radio wave reflective sheet 10 exemplified in this embodiment uses a PET film with a thickness of 300 μm.

[0055] <Reflective layer> The reflective layer 14 is a layer that reflects the radio waves 1 that have passed through the second dielectric layer 13. Unlike the resistive layer 12, the reflective layer 12 does not need to transmit the radio waves 1. For this reason, it is preferable that it has the lowest possible surface resistance, and most preferably a surface resistance of 0 Ω / sq. Metal foil or metal plate can be used as such a reflective layer 14.

[0056] In order to make the radio wave reflective sheet 10 flexible, metal foil is more preferable as the material constituting the reflective layer 14, and various metal foils such as copper foil, aluminum foil, and gold foil can be used. Among these, aluminum foil is particularly preferable when considering cost and the effect of oxidation in air. Metal foil such as aluminum foil that forms the reflective layer 14 can be easily produced by rolling a metal material. Furthermore, when forming the reflective layer 14 with a vapor-deposited film in which metal is deposited on the surface of a non-metallic material, it is preferable to appropriately select a vapor deposition method that has been conventionally used to form various vapor-deposited films, taking into consideration the heat resistance temperature of the metal material to be deposited and the non-metallic material such as the resin that serves as the base material.

[0057] When using aluminum foil as the flexible radio wave reflective sheet 10, the thickness of the reflective layer 14 is preferably 1 μm to 20 μm.

[0058] Furthermore, in the radio wave reflective sheet 10 according to this embodiment shown in Figure 1, by directly forming a vapor-deposited film of a metal material on the surface of the second dielectric layer 13 opposite to the side on which the resistive layer 12 is formed, the reflective layer 14 can be formed solely from a vapor-deposited film of a conductive material such as metal. When a metal vapor-deposited film is formed on the back side of the second dielectric layer 13, no gap is created between the second dielectric layer 13 and the reflective layer 14, compared to the case where the second dielectric layer 13 and the reflective layer 14 are formed separately and placed in close contact. Therefore, radio waves 1 that have passed through the second dielectric layer 13 can be reflected at the back surface of the second dielectric layer 13, making it easy to realize a radio wave reflective sheet 10 with desired radio wave reflection characteristics.

[0059] On the other hand, compared to the case where a metal foil is used for the reflective layer 14, when a vapor-deposited film is used, it is necessary to form a uniform and sufficient density of the conductive material in the vapor-deposited film. According to the inventors' research, it is preferable to make the surface resistance of the reflective layer 1Ω / sq or less, and it is preferable to control the thickness of the metal vapor-deposited film sufficiently to make the surface resistance value less than or equal to the desired value.

[0060] Furthermore, in order to give the radio wave reflective sheet 10 both flexibility and light transmission, a conductive mesh made of conductive fibers can be used as the reflective layer 14. For example, the conductive mesh can be made conductive by attaching a metal to a mesh woven from polyester monofilament. As the metal, copper, silver, etc., which have high conductivity can be used, can be used. In addition, products have been commercialized that have a black anti-reflective layer added to the outside of the metal film covering the surface of the mesh in order to reduce reflection caused by the metal film.

[0061] As the reflective layer 14, a conductive metal grid can also be used, in which thin metal wires, such as copper wires with a diameter of several tens to several hundred micrometers, are arranged vertically and horizontally.

[0062] Furthermore, when the reflective layer 14 is constructed using the mesh or conductive metal grid described above, it will be configured to have the minimum thickness necessary to achieve the required surface resistance value for the reflective layer 14, in order to ensure flexibility and light transmission.

[0063] From the viewpoint of ensuring light transmission, a larger aperture ratio of the reflective layer 14, which is formed as a mesh or conductive metal grid, is preferable. From the viewpoint of reliably reflecting radio waves on its surface as the reflective layer 14 and improving the radio wave absorption characteristics of the radio wave reflective sheet 10, a smaller aperture ratio is preferable. According to the inventors' studies, an aperture ratio of 35% to 85% is preferable, and an aperture ratio of 35% to 75% is more preferable.

[0064] <Adhesive layer> Although not shown in Figure 1, an adhesive layer can be formed on the back surface of the reflective layer 14 so that the radio wave reflective sheet 10 according to this embodiment can be easily placed in a predetermined position.

[0065] As the adhesive layer, known materials used as adhesive layers in adhesive tapes, acrylic adhesives, rubber adhesives, silicone adhesives, etc., can be used. In addition, tackifiers and crosslinking agents can be used to adjust the adhesive strength to the substrate and reduce adhesive residue. The adhesive strength to the substrate is preferably 5N / 10mm to 12N / 10mm. If the adhesive strength is less than 5N / 10mm, the radio wave reflective sheet 10 may easily peel off or shift from the substrate. Conversely, if the adhesive strength is greater than 12N / 10mm, it becomes difficult to peel the radio wave reflective sheet 10 from the substrate.

[0066] Furthermore, the thickness of the adhesive layer is preferably 20 μm to 100 μm. If the thickness of the adhesive layer is less than 20 μm, the adhesive strength will be low, and the radio wave reflective sheet 10 may easily peel off or shift from the adherend. If the thickness of the adhesive layer is greater than 100 μm, it will be difficult to peel the radio wave reflective sheet 10 off the adherend. Also, if the cohesive strength of the adhesive layer is low, adhesive residue may be left on the adherend when the radio wave reflective sheet 10 is peeled off. In addition, this may reduce the overall flexibility of the radio wave reflective sheet 10.

[0067] Furthermore, the adhesive layer usable for the radio wave reflective sheet 10 according to this embodiment can be an adhesive layer that irremoves the radio wave reflective sheet 10 from the object to be adhered to, or an adhesive layer that allows for removable adhesion. It should be noted that in the radio wave reflective sheet 10 according to this embodiment, having an adhesive layer is not a mandatory requirement; the radio wave reflective sheet 10 can be adhered to the desired component using various conventional adhesive methods.

[0068] (Examples) The following describes the results of an investigation into the radio wave reflection characteristics of the radio wave reflection sheet 10 according to this embodiment.

[0069] [Regarding radio wave reflection characteristics] The inventors investigated the principle by which the radio wave reflection characteristics of the radio wave reflection sheet disclosed in this application, in which a first dielectric layer, a resistive layer, a second dielectric layer, and a reflective layer are sequentially laminated, particularly the reason why a radio wave reflector with high reflection characteristics in a predetermined frequency bandwidth can be obtained, and the possibility of controlling the radio wave reflection characteristics, by creating a model of the radio wave reflection sheet, conducting simulations, and actually fabricating a radio wave reflection sheet that conforms to the model and comparing the results.

[0070] Figure 2 shows the equivalent circuit diagram and the configuration model used when the inventors simulated the radio wave reflector (sheet) they were studying. Figure 2(a) is the equivalent circuit diagram, and Figure 2(b) is a diagram showing the model configuration of the radio wave reflector under consideration.

[0071] In the model of the radio wave reflective sheet according to this embodiment shown in Figure 2(b), a first dielectric layer 21, a resistive layer 22, a second dielectric layer 23, and a reflective layer 24 are sequentially stacked from the surface into which the radio waves are incident.

[0072] In this model, the impedance value of port P1, which is the surface of the first dielectric layer 21, is set to 377Ω, the same as the impedance in air, at a thickness of half the wavelength λ of the incident radio wave, taking into account the relative permittivity of the first dielectric layer. Let the surface resistance value of the resistive layer 22, which is placed on the back of the first dielectric layer 21, be XΩ. The thickness of the second dielectric layer 23 is half the wavelength λ of the radio wave, and the resistance value of the reflective layer 24, which becomes port P2, is set to 0Ω (= ground).

[0073] In this configuration, the thickness of the first dielectric layer 21 is set to λ / 2, so that radio waves of wavelength λ are seemingly absorbed. Furthermore, by setting the thickness of the second dielectric layer 23 to λ / 2, the phase of the radio waves reflected by the resistive layer 22 and the phase of the radio waves reflected by the reflective layer 24 overlap, resulting in strong reflection of radio wave 1. Here, the surface resistance value X of the resistive layer 22 is set to a value lower than the impedance value of the surface of the first dielectric layer, which is 377Ω (for example, 130Ω), so that the influence on the port 2 (P2) side is suppressed. In this way, the radio wave reflection effect of the first dielectric layer 21 and the radio wave reflection effect of the second dielectric layer 23 are combined to achieve strong radio wave reflection characteristics in a certain frequency bandwidth centered on the desired frequency. Therefore, we performed an Ansys HFSS simulation using the equivalent circuit shown in Figure 2(a) to determine the frequency characteristics of the radio wave reflection attenuation in the radio wave reflection sheet shown in this embodiment. In addition, we actually created the radio wave reflection sheet that served as the model for the Ansys HFSS simulation and measured its reflection attenuation frequency characteristics.

[0074] Figure 3 shows a specific model of the radio wave reflector used in the above simulation.

[0075] In the above simulation, the reflection attenuation at frequencies from 100 GHz to 500 GHz was calculated for a configuration matching the first radio wave reflection sheet shown in Figure 1, consisting of a first dielectric layer 31 with a relative permittivity of 3.2 and a thickness of 300 μm, a resistive layer 32 with a resistance of 130 Ω / sq, a second dielectric layer 33 with a relative permittivity of 2.55 and a thickness of 300 μm, and a reflective layer 34 with a resistance of 0 Ω / sq (= ground).

[0076] On the other hand, the actual radio wave absorbing sheet was constructed by forming a PEDOT resistive layer 32 with a surface resistance of 140 Ω / sq on a PET film with a relative permittivity of 3.2 and a thickness of 300 μm as the first dielectric layer 31, further using acrylic OCA with a relative permittivity of 2.55 and a thickness of 300 μm as the second dielectric layer 33, and using aluminum foil for the reflective layer 34.

[0077] The frequency characteristics of the return loss of this radio wave reflective sheet in the frequency range of 100 GHz to 500 GHz were measured using Advance Test's THZ-TDS TAS7500SP (product name). Similar to the simulation results described above, the return loss was determined by the attenuation of the reflected wave relative to the incident wave and expressed in dB.

[0078] Figure 4 shows the frequency characteristics of the reflection attenuation of the radio wave reflective sheet.

[0079] In Figure 4, the solid line indicated by reference numeral 41 shows the measurement results of the radio wave reflection sheet that was actually created, and the dashed line indicated by reference numeral 42 shows the results of the simulation described above.

[0080] Figure 4 shows that the measurement results of the actually created model and the simulation results closely match, confirming that it is possible to design a radio wave reflective sheet with the desired radio wave reflection characteristics through the above simulation.

[0081] <Second Structure> Here, as a second configuration of the radio wave reflector disclosed in this application, an example in which the first dielectric layer is composed of a laminate of multiple dielectric layers will be described.

[0082] Figure 5 is a partial cross-sectional perspective view showing the configuration of the radio wave reflective sheet in the second configuration shown in this embodiment.

[0083] The second configuration of the radio wave reflective sheet 50 shown in Figure 5 differs from the first configuration of the radio wave reflective sheet 10 shown in Figure 1 in that the first dielectric layer 51 is composed of two dielectric layers: a surface dielectric layer 51a formed of a dielectric material and a resin substrate 51b used when forming the reflective layer 52.

[0084] As described above, the radio wave reflector disclosed in this application can be constructed by laminating multiple dielectric films in either the first dielectric layer or the second dielectric layer. Therefore, as shown in the second configuration in Figure 5, a resin material having a predetermined dielectric constant can be used as the substrate when forming the resistive layer, and a dielectric film made of another dielectric material can be laminated to form the first dielectric layer.

[0085] The second configuration of the radio wave reflective sheet 50 shown in Figure 5 uses a 250 μm thick acrylic OCA film as the surface dielectric layer 51a and a 50 μm thick polyethylene terephthalate (PET) sheet as the resin substrate 51b for forming the resistive layer 52.

[0086] The second dielectric layer 53 is made of an acrylic-based OCA (Optical Clear Adhesive) that is transparent and adhesive, similar to the second dielectric layer 13 in the first configuration. The reflective layer 54 can also be made of various metal foils such as copper foil, aluminum foil, or gold foil, similar to the reflective layer 14 in the first configuration.

[0087] Figure 6 shows the frequency characteristics of the return loss when the second configuration shown in Figure 5 is used, i.e., when the first dielectric layer is composed of two dielectric films.

[0088] In Figure 6, the solid line indicated by reference numeral 61 shows the measurement results of the radio wave reflection sheet that was actually created, and the dashed line indicated by reference numeral 62 shows the simulation results.

[0089] The frequency characteristics of the second configuration shown in Figure 6 were obtained by the simulation described above. The first dielectric layer, with a total thickness of 300 μm, is constructed as a laminate of a surface dielectric layer with a relative permittivity of 2.55 and a thickness of 250 μm and a resin substrate with a relative permittivity of 3.2 and a thickness of 50 μm. The reflection attenuation at frequencies from 100 GHz to 500 GHz was calculated for a laminate consisting of a resistive layer with a surface resistance of 130 Ω / sq, a second dielectric layer with a relative permittivity of 2.55 and a thickness of 300 μm, and a reflective layer with a surface resistance of 0 Ω / sq (= ground).

[0090] On the other hand, the actual radio wave absorbing sheet was constructed using an acrylic OCA with a relative permittivity of 2.55 and a thickness of 250 μm as the first dielectric layer, and a PET film with a relative permittivity of 3.2 and a thickness of 50 μm as the resin substrate, to form a dielectric layer with a total thickness of 300 μm. A PEDOT with a surface resistance of 140 Ω / sq formed on the resin substrate was used as the resistive layer, an acrylic OCA with a relative permittivity of 2.55 and a thickness of 300 μm was used as the second dielectric layer, and aluminum foil was used as the reflective layer.

[0091] The frequency characteristics of the return loss of this radio wave reflective sheet in the frequency range of 100 GHz to 500 GHz were measured using Advance Test's THZ-TDS TAS7500SP (product name), similar to the first configuration shown in Figure 4. The radio wave reflection characteristics were determined as the attenuation of the reflected wave relative to the incident wave, as in the simulation results described above, and expressed in dB.

[0092] As shown in Figure 6, even when the first dielectric layer is configured as a laminate of two dielectric films, the trends of the measurement results and simulation results of the actually created model are in close agreement, similar to the case where a single dielectric layer forms the first dielectric layer as shown in Figure 4. This confirms that it is possible to design a radio wave reflective sheet with the desired radio wave reflection characteristics through the above simulation.

[0093] Furthermore, it was confirmed that in both cases—as shown in Figure 4, where the first dielectric layer is composed of a single dielectric film, and as shown in Figure 6, where it is composed of a laminate of two dielectric layers—a radio wave reflector with a return loss of less than -10 dB in the frequency band of 220 GHz to 370 GHz was obtained, meaning that it reflects more than 90% of the incident radio waves.

[0094] Furthermore, as shown in Figures 4 and 6, in the radio wave reflective sheet shown in this embodiment, the return attenuation increases for radio waves at frequencies outside the frequency band in which the radio waves are reflected well. For example, in the case of the radio wave reflective sheet with the first configuration shown in Figure 4, for radio waves with frequencies of 180 GHz or less, or radio waves with frequencies of 390 GHz or more, the value of the return attenuation is greater than -20 dB, meaning that more than 99% of the radio waves are absorbed, or in other words, less than 1% of the radio waves are reflected.

[0095] Thus, the radio wave reflective sheet according to this embodiment effectively absorbs radio waves other than those to be reflected, thereby reflecting only radio waves in the desired frequency band. According to the inventors' investigation, the sheet often exhibits frequency characteristics in which the reflection attenuation characteristics increase sharply at frequency bands of approximately 0.5 times and approximately 1.5 times the center frequency of the reflection band. As a result, it was confirmed that the sheet provides filter characteristics that effectively reflect radio waves near the center frequency of the reflection band and absorb (do not reflect) radio waves in the surrounding frequency bands.

[0096] Therefore, by placing the radio wave reflective sheet according to this embodiment in the path of radio waves, high-frequency bands, such as the millimeter wave band and above, which have particularly high directivity, can be propagated along a predetermined path. Furthermore, since radio waves outside the predetermined frequency band are absorbed by the radio wave reflector, the placement of the radio wave reflector according to this embodiment can avoid the risk of unwanted radio wave reflection. For example, in a space such as a conference room or office room, by placing the radio wave reflective sheet according to this embodiment in a predetermined part of the interior wall of the room that lies in the path of radio waves, an environment can be created in which radio waves can be received well in various parts of the room even if there are obstacles inside the room, without unwanted radio wave reflection, thereby creating a radio wave reception environment with a high C / N ratio.

[0097] As described above, in the radio wave reflective sheet shown in this embodiment, the first dielectric layer, the resistive layer, the second dielectric layer, and the reflective layer are sequentially stacked from the radio wave incident surface side, thereby combining the radio wave reflection characteristics through the first dielectric layer, the second dielectric layer, the first dielectric layer, and the second dielectric layer, respectively. Furthermore, by setting the thickness d of the second dielectric layer to d = λ / 2 with respect to the central wavelength λ of the radio wave reflected by the radio wave reflective sheet, the radio waves reflected through the second dielectric layer are superimposed, making it possible to realize a radio wave reflector with high reflection characteristics for radio waves in a wide frequency band centered on a predetermined frequency.

[0098] In particular, changing the relative permittivity and thickness of the first dielectric layer, and changing the resistance value of the resistive layer, alters the overall radio wave reflection characteristics of the radio wave reflective sheet. However, since it has been confirmed that the overall radio wave absorption characteristics (= frequency characteristics of the amount of reflection) of the radio wave reflective sheet can be calculated using the simulation described above, it is possible to design a radio wave reflective sheet with more desirable radio wave reflection characteristics.

[0099] Furthermore, although the above embodiment only described the configuration of a radio wave absorber (sheet) having two dielectric layers, a first dielectric layer and a second dielectric layer, the radio wave reflector (sheet) disclosed in this application is not limited to a configuration having two dielectric layers.

[0100] In the configuration of the radio wave reflective sheet shown in Figure 1 or Figure 5, it is possible to create a radio wave reflector that reflects radio waves in a desired band using three or more dielectric layers by providing two or more combinations of resistive layers and dielectric layers, such as a configuration in which a second resistive layer and a third dielectric layer are further laminated between the first dielectric layer and the resistive layer. Even when multiple resistive layers are formed and three or more dielectric layers are provided, by setting the thickness of the second dielectric layer adjacent to the reflective layer to a thickness of λ / 2 with respect to the center wavelength λ of the radio wave to be reflected, the amount of radio wave reflected at the center wavelength is increased, and a radio wave reflector (sheet) with better radio wave reflection characteristics can be realized.

[0101] Furthermore, the radio wave reflective sheet disclosed in this application can be made flexible as a whole by forming each of the multiple dielectric layers, resistive layers, and reflective layers from a flexible material, making it easy to handle when placing it in a desired location.

[0102] Furthermore, even when implemented not only in sheet form but also as a block-shaped radio wave reflector with a predetermined thickness relative to its surface area, if the entire structure can be flexible, the handling of the radio wave reflector when placing it in a predetermined position will be improved, resulting in a highly practical radio wave reflector.

[0103] Furthermore, by forming each of the multiple dielectric layers, resistive layers, and reflective layers from a translucent material, it is possible to create a radio wave reflective block that can be arranged in a tile-like pattern on, for example, a window or a transparent wall, allowing visibility to the other side. In this case as well, the combined radio wave absorption effect of the multiple dielectric layers makes it possible to realize a radio wave reflector with broad reflection attenuation characteristics that can selectively reflect radio waves across a wider frequency band. [Industrial applicability]

[0104] The radio wave absorber disclosed in this application has a first dielectric layer, a resistive layer, a second dielectric layer, and a reflective layer stacked sequentially. By setting the thickness d of the second dielectric layer to d = λ / 2 with respect to the central wavelength λ of the radio wave to be reflected, a radio wave reflector with high reflection characteristics in the wavelength region (frequency region) surrounding the central wavelength of the radio wave can be realized. The radio wave reflector disclosed in this application can be realized as a radio wave reflector that can reflect radio waves in a predetermined frequency band well and absorb radio waves in the surrounding frequency bands, thereby reducing the amount of reflection. [Explanation of Symbols]

[0105] 1 (incident) radio wave 10 Radio wave reflectors 11. First Dielectric Layer 12 resistance layer 13. Second dielectric layer 14 Reflective layer

Claims

1. A radio wave reflector in which a first dielectric layer, a resistive layer, a second dielectric layer, and a reflective layer are sequentially stacked from the radio wave incident surface side, The thickness of the first dielectric layer is 160 μm or more and 500 μm or less. A radio wave reflector characterized in that, when the central wavelength of the radio wave reflected by the radio wave reflector propagates inside the second dielectric layer is λ, the thickness d of the second dielectric layer is d = λ / 2.

2. The radio wave reflector according to claim 1, wherein the center frequency of the radio waves reflected by the radio wave reflector is 100 GHz or more and 450 GHz or less.

3. The radio wave reflector according to claim 1 or 2, wherein the surface resistance of the resistive layer is 80 Ω / sq or more and 250 Ω / sq or less.

4. The radio wave reflector according to any one of claims 1 to 3, wherein the second dielectric layer is adhesive.

5. The radio wave reflector according to any one of claims 1 to 4, wherein the resistive layer is formed of a conductive organic polymer film, a metal film, a sputtered film, or a vapor-deposited film.

6. The radio wave reflector according to any one of claims 1 to 5, wherein the first dielectric layer, the resistive layer, the second dielectric layer, and the reflective layer are all made in the form of thin films and are formed as a flexible sheet overall.

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