Patent Publication Number: US-2023144249-A1

Title: Radio wave absorber

Description:
TECHNICAL FIELD 
     The present invention relates to a radio wave absorber. 
     BACKGROUND ART 
     Hitherto, there have been attempts to provide transparent radio wave absorbers. 
     For example, Patent Literature 1 describes an electromagnetic wave absorber having transparency. In the electromagnetic wave absorber, a reflective layer composed of a thin-line mesh pattern is formed on one surface of a transparent substrate. A transparent solid dielectric layer lies along the reflective layer with an adhesive agent layer interposed therebetween. Furthermore, a frequency selective shielding layer lies along the solid dielectric layer with an adhesive agent layer interposed therebetween. 
     The frequency selective shielding layer is composed of a thin-line pattern of an FSS element formed on one surface of a transparent substrate. A transparent solid dielectric layer lies along the frequency selective shielding layer with an adhesive agent layer interposed therebetween. A frequency selective shielding layer lies along the solid dielectric layer with an adhesive agent layer interposed therebetween. The frequency selective shielding layer is composed of a thin-line pattern of an FSS element formed on one surface of a transparent substrate. The thin-line mesh pattern of the reflective layer and the thin-line pattern of each frequency selective shielding layer have a line width of 15 to 80 μm. 
     Patent Literature 2 describes a radio wave absorber having a dielectric, and a radio wave absorbing surface having a phase adjustment function is formed on a surface of the dielectric. In addition, a radio wave reflecting surface is formed on a surface of the dielectric opposite to the radio wave absorbing surface. A plurality of independent metal wire elements are provided on the radio wave absorbing surface. In addition, a plurality of independent metal wire elements are provided on the radio wave reflecting surface. Therefore, by using a material having a high light transmittance as the dielectric, the light transmittance of the radio wave absorber is increased, and for example, the radio wave absorber can be attached to window glass. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: JP 2009-170887 A 
         Patent Literature 2: JP 2003-78276 A 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     In the electromagnetic wave absorber described in Patent Literature 1, a plurality of thin-line mesh patterns overlap each other. In addition, in the electromagnetic wave absorber described in Patent Literature 2, the plurality of independent metal wire elements on the radio wave absorbing surface and the plurality of independent metal wire elements on the radio wave reflecting surface overlap each other. It is known that, when geometrically regularly distributed patterns are caused to overlap each other, mottles called moiré are caused due to the magnitudes of the intervals of the patterns. Moiré may diminish the appearance of an electromagnetic wave absorber. In Patent Literatures 1 and 2, no specific countermeasures against moiré have been considered, and the technologies described in Patent Literatures 1 and 2 have room for reconsideration from the viewpoint of taking countermeasures against moiré. 
     In view of such circumstances, the present invention provides a radio wave absorber in which it is less likely to visually recognize moiré caused due to overlap of a plurality of layers having openings. 
     Solution to Problem 
     The present invention provides a radio wave absorber including: 
     a resistive layer having a first main surface and having a plurality of first openings formed at equal intervals in a first direction along the first main surface; 
     an electroconductive layer having a second main surface and having a plurality of second openings formed at equal intervals in a second direction along the second main surface; and 
     a dielectric layer disposed between the resistive layer and the electroconductive layer in a thickness direction of the resistive layer, wherein 
     a value obtained by dividing a larger value out of a first ratio and a second ratio by a smaller value out of the first ratio and the second ratio is 1.3 or more, the first ratio being a ratio of a size of the first opening in the first direction to a distance between the nearest first openings, and the second ratio being a ratio of a size of the second opening in the second direction to a distance between the nearest second openings. 
     Advantageous Effects of Invention 
     In the above radio wave absorber, it is less likely to visually recognize moiré caused due to overlap of the resistive layer and the electroconductive layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1 A  is a plan view of one main surface of an example of a radio wave absorber according to the present invention. 
         FIG.  1 B  is a plan view of another main surface of the radio wave absorber shown in  FIG.  1 A . 
         FIG.  2 A  is a cross-sectional view of the radio wave absorber taken along a line IIa-IIa shown in  FIG.  1 A . 
         FIG.  2 B  is a plan view showing a modification of the radio wave absorber shown in  FIG.  1 A . 
         FIG.  2 C  is a plan view showing another modification of the radio wave absorber shown in  FIG.  1 A . 
         FIG.  3 A  is a plan view showing another example of the radio wave absorber according to the present invention. 
         FIG.  3 B  is a plan view showing still another example of the radio wave absorber according to the present invention. 
         FIG.  3 C  is a plan view showing still another example of the radio wave absorber according to the present invention. 
         FIG.  4    is a cross-sectional view showing still another example of the radio wave absorber according to the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     As for a radio wave absorber, a configuration of including a resistive layer, an electroconductive layer, and a dielectric layer disposed between the resistive layer and the electroconductive layer has been known. In such a radio wave absorber, if the resistive layer and the electroconductive layer each have a plurality of openings, this is advantageous in terms of imparting transparency to the radio wave absorber. In addition, in order to suppress spatial variations in the transparency of the radio wave absorber and the radio wave absorption performance of the radio wave absorber, it is advantageous to form the plurality of openings at equal intervals in each of the resistive layer and the electroconductive layer. Meanwhile, in this case, moiré can be caused due to overlap of the resistive layer and the electroconductive layer. Therefore, the present inventors have thoroughly studied countermeasures against moiré. As a result, the present inventors have newly found that, when the plurality of openings are formed in each of the resistive layer and the electroconductive layer such that a predetermined condition is satisfied, it is less likely to visually recognize moiré in the radio wave absorber. On the basis of this new finding, the present inventors have conceived of a radio wave absorber according to the present invention. As used herein, “transparency” means transparency to visible light, unless otherwise described. 
     Embodiments of the present invention will be described with reference to the drawings. The present invention is not limited to the following embodiments. 
     As shown in  FIG.  1 A ,  FIG.  1 B , and  FIG.  2 A , a radio wave absorber  1   a  includes a resistive layer  10 , an electroconductive layer  30 , and a dielectric layer  20 . The resistive layer  10  has a first main surface  12 , and has a plurality of first openings  11  formed at equal intervals in a first direction along the first main surface  12 . The electroconductive layer  30  has a second main surface  32 , and has a plurality of second openings  31  formed at equal intervals in a second direction along the second main surface  32 . The dielectric layer  20  is disposed between the resistive layer  10  and the electroconductive layer  30  in the thickness direction of the resistive layer  10 . In the radio wave absorber  1   a , a value D obtained by dividing a larger value out of a first ratio Ra and a second ratio Rb by a smaller value out of the first ratio Ra and the second ratio Rb is 1.3 or more. The first ratio Ra is the ratio (G R /W R ) of a size G R  of the first opening  11  in the first direction to a distance W R  between the nearest first openings  11 . The second ratio Rb is the ratio (G C /W C ) of a size G C  of the second opening  31  in the second direction to a distance W C  between the nearest second openings  31 . 
     In the radio wave absorber  1   a , the resistive layer  10  having the plurality of first openings  11  and the electroconductive layer  30  having the plurality of second openings  31  overlap each other. The plurality of first openings  11  are formed at equal intervals in the first direction, and the plurality of second openings  31  are formed at equal intervals in the second direction. Therefore, it is considered that moiré occurs in the radio wave absorber  1   a . However, in the radio wave absorber  1   a , since the above value D is 1.3 or more, it is less likely to visually recognize moiré. The reason for this is not clear, but it is considered that, when the value D is 1.3 or more, moiré occurs at a narrow pitch that makes it difficult to recognize the moiré with the naked eye. 
     The upper limit of the value D is not limited to a specific value. The upper limit of the value D can be adjusted, for example, such that the radio wave absorber  1   a  has desired radio wave absorption performance. 
     The magnitude relationship between the first ratio Ra and the second ratio Rb is not limited to a specific relationship as long as the value D is 1.3 or more in the radio wave absorber  1   a . The first ratio Ra may be higher than the second ratio Rb, or may be lower than the second ratio Rb. 
     The first ratio Ra is not limited to a specific value as long as the value D is 1.3 or more in the radio wave absorber  1   a . The first ratio Ra is, for example, 5 or more. Accordingly, G R  is less likely to be small with respect to the distance W R , so that it is less likely to visually recognize a frame that is in contact with the first openings  11 . The first ratio Ra may be 10 or more, or may be 20 or more. The first ratio Ra is, for example, 100 or less. Accordingly, the sheet resistance of the resistive layer  10  is easily adjusted in a desired range. The first ratio Ra may be 70 or less, or may be 50 or less. 
     The second ratio Rb is not limited to a specific value as long as the value D is 1.3 or more in the radio wave absorber  1   a . The second ratio Rb is, for example, 5 or more. Accordingly, G C  is less likely to be small with respect to the distance W C , so that it is less likely to visually recognize a frame that is in contact with the second openings  31 . The second ratio Rb may be 10 or more, or may be 20 or more. The second ratio Rb is, for example, 100 or less. Accordingly, the sheet resistance of the electroconductive layer  30  is easily adjusted in a desired range. The second ratio Rb may be 70 or less, or may be 50 or less. 
     The distance W R  between the nearest first openings  11  is not limited to a specific value as long as the value D is 1.3 or more in the radio wave absorber  1   a . The distance W R  is, for example, 100 μm or less, and may be 50 μm or less. On the other hand, the distance W R  is desirably 10 μm or less. Accordingly, it is less likely to visually recognize the frame that is in contact with the first openings  11 , when the resistive layer  10  is viewed in a plan view. In addition, the opening ratio of the resistive layer  10  is easily increased, so that the resistive layer  10  easily has high transparency. The distance W R  is, for example, 5 μm or more. 
     The size G R  of each first opening  11  in the first direction is not limited to a specific value as long as the value D is 1.3 or more in the radio wave absorber  1   a . The size G R  is, for example, 50 μm or more, and may be 100 μm or more, or may be 400 μm or more. The size G R  is, for example, 1000 μm or less, and may be 700 μm or less, or may be 500 μm or less. 
     The thickness of the resistive layer  10  is not limited to a specific value. The thickness of the resistive layer  10  is, for example, 10 nm or more, and may be 15 nm or more, or may be 20 nm or more. The thickness of the resistive layer  10  is, for example, 500 nm or less. Accordingly, the resistive layer  10  is less likely to warp, so that cracks are less likely to occur in the resistive layer  10 . The thickness of the resistive layer  10  may be 450 nm or less, or may be 400 nm or less. 
     The sheet resistance of the resistive layer  10  is not limited to a specific value. The sheet resistance of the resistive layer  10  is, for example, 350 to 600Ω/□, and may be 100 to 700Ω/□. The sheet resistance of the resistive layer  10  can be measured, for example, according to the eddy current method. 
     A specific resistance ρ 1  of the material forming the resistive layer  10  is not limited to a specific value. The specific resistance of the material forming the resistive layer  10  is, for example, 4×10 −5  to 1×10 −4  Ω·cm. The specific resistance of the material forming the resistive layer  10  may be 5×10 −5  to 1×10 −4  Ω·cm. 
     The specific resistance ρ 1  can be determined on the basis of a relationship of Rf=(ρ 1 /t 1 ){(G R +W R )/W R }, for example, by taking a fragment having a predetermined dimension from the resistive layer  10  and measuring a sheet resistance Rf of the fragment, the size G R  of the first opening  11 , the distance W R  between the nearest first openings  11 , and a thickness t 1  of the resistive layer  10 . The sheet resistance Rf can be measured according to the eddy current method using a non-contact resistance meter. The size G R  and the distance W R  can be determined by observing the fragment using an optical microscope. In addition, the thickness t 1  of the resistive layer  10  can be determined, for example, by observing a cross-section of the resistive layer  10  using a transmission electron microscope (TEM). Moreover, the specific resistance ρ 1  of the material forming the resistive layer  10  may be determined by analyzing the material composition of the material, forming a film having the same composition as the material composition, and measuring the sheet resistance and the thickness of the film. 
     The material forming the resistive layer  10  is not limited to a specific material. The material forming the resistive layer  10  may be an inorganic material such as metals, alloys, and metal oxides, or may be an organic material such as electroconductive polymers and carbon nanotubes. 
     The resistive layer  10  may be a film having a plurality of through holes formed therein and having a uniform thickness, or may be a woven fabric. The fiber forming the woven fabric may be an organic material such as electroconductive polymers and carbon nanotubes, or may be an inorganic material such as metals and alloys. 
     The opening ratio of the resistive layer  10  is not limited to a specific value as long as the value D is 1.3 or more in the radio wave absorber  1   a . The resistive layer  10  has, for example, an opening ratio of 65% or more. Accordingly, the resistive layer  10  easily has high transparency. The opening ratio of the resistive layer  10  is a ratio Saf/(Saf+Sbf) of an opening area Saf of the plurality of first openings  11  to a sum Saf+Sbf of the opening area Saf of the plurality of first openings  11  and an area Sbf of the non-opening portion of the resistive layer  10  when the resistive layer  10  is viewed in a plan view. 
     The opening ratio of the resistive layer  10  is desirably 70% or more and more desirably 75% or more. The opening ratio of the resistive layer  10  is, for example, 99% or less, and may be 98% or less, or may be 97% or less. 
     The arrangement of the plurality of first openings  11  is not limited to a specific arrangement as long as the value D is 1.3 or more in the radio wave absorber  1   a . For example, the first direction may include a plurality of alignment directions intersecting each other. For example, in the resistive layer  10 , the plurality of first openings  11  are arranged such that the centers thereof form a square lattice on the first main surface  12 . In other words, in the resistive layer  10 , the first direction includes alignment directions orthogonal to each other. 
     The shape of each first opening  11  is not limited to a specific shape as long as the value D is 1.3 or more in the radio wave absorber  1   a . For example, each first opening  11  has a square shape in a plan view. 
     The distance W C  between the nearest second openings  31  is not limited to a specific value as long as the value D is 1.3 or more in the radio wave absorber  1   a . The distance W C  is, for example, 100 μm or less, and may be 50 μm or less. On the other hand, the distance W C  is desirably 10 μm or less. Accordingly, it is less likely to visually recognize the frame that is in contact with the second openings  31 , when the electroconductive layer  30  is viewed in a plan view. In addition, the opening ratio of the electroconductive layer  30  is easily increased, so that the electroconductive layer  30  easily has high transparency. The distance W R  is, for example, 5 μm or more. 
     The size G C  of each second opening  31  in the second direction is not limited to a specific value as long as the value D is 1.3 or more in the radio wave absorber  1   a . The size G C  is, for example, 50 μm or more, and may be 100 μm or more, or may be 400 μm or more. The size G C  is, for example, 1000 μm or less, and may be 700 μm or less, or may be 500 μm or less. 
     The thickness of the electroconductive layer  30  is not limited to a specific value. The thickness of the electroconductive layer  30  is, for example, 50 nm or more, and may be 100 nm or more, or may be 500 nm or more. The thickness of the electroconductive layer  30  is, for example, 2000 nm or less. Accordingly, the electroconductive layer  30  is less likely to warp, so that cracks are less likely to occur in the electroconductive layer  30 . The thickness of the electroconductive layer  30  may be 1000 nm or less, or may be 500 nm or less. 
     The sheet resistance of the electroconductive layer  30  is not limited to a specific value. The sheet resistance of the electroconductive layer  30  is typically lower than the sheet resistance of the resistive layer  10 . The sheet resistance of the electroconductive layer  30  is, for example, 100Ω/□ or less, and may be 50Ω/□ or less, or may be 30Ω/□ or less. The sheet resistance of the electroconductive layer  30  is, for example, 0.1Ω/□ or more, may be 0.5Ω/□ or more, or may be 1Ω/□ or more. The sheet resistance of the electroconductive layer  30  can be measured, for example, according to the eddy current method. 
     A specific resistance ρ 2  of the material forming the electroconductive layer  30  is not limited to a specific value. The specific resistance of the material forming the electroconductive layer  30  is, for example, 2×10 −5  Ω·cm or less. The specific resistance of the material forming the electroconductive layer  30  may be 1×10 −5  Ω·cm or less. The specific resistance of the material forming the electroconductive layer  30  is, for example, 1×10 −6  Ω·cm or more. The specific resistance ρ 2  can be determined, for example, in the same manner as the specific resistance ρ 1 . 
     The material forming the electroconductive layer  30  is not limited to a specific material. The material forming the electroconductive layer  30  may be an inorganic material such as metals, alloys, and metal oxides, or may be an organic material such as electroconductive polymers and carbon nanotubes. 
     The electroconductive layer  30  may be a film having a plurality of through holes formed therein and having a uniform thickness, or may be a woven fabric. The fiber forming the woven fabric may be an organic material such as electroconductive polymers and carbon nanotubes, or may be an inorganic material such as metals and alloys. 
     The opening ratio of the electroconductive layer  30  is not limited to a specific value as long as the value D is 1.3 or more in the radio wave absorber  1   a . The electroconductive layer  30  has, for example, an opening ratio of 65% or more. Accordingly, the electroconductive layer  30  easily has high transparency. The opening ratio of the electroconductive layer  30  is a ratio Sas/(Sas+Sbs) of an opening area Sas of the plurality of second openings  31  to a sum Sas+Sbs of the opening area Sas of the plurality of second openings  31  and an area Sbs of the non-opening portion of the electroconductive layer  30  when the electroconductive layer  30  is viewed in a plan view. 
     The opening ratio of the electroconductive layer  30  is desirably 70% or more and more desirably 75% or more. The opening ratio of the electroconductive layer  30  is, for example, 99% or less, and may be 98% or less, or may be 97% or less. 
     The arrangement of the plurality of second openings  31  is not limited to a specific arrangement as long as the value D is 1.3 or more in the radio wave absorber  1   a . For example, the second direction may include a plurality of alignment directions intersecting each other. For example, in the electroconductive layer  30 , the plurality of second openings  31  are arranged such that the centers thereof form a square lattice on the second main surface  32 . In other words, in the electroconductive layer  30 , the second direction includes alignment directions orthogonal to each other. In the radio wave absorber  1   a , the second direction is, for example, a direction extending parallel to the first direction. 
     The shape of each second opening  31  is not limited to a specific shape as long as the value D is 1.3 or more in the radio wave absorber  1   a . For example, each second opening  31  has a square shape in a plan view. 
     The dielectric layer  20  has, for example, a visible light transmittance of 80% or more. Accordingly, the radio wave absorber  1   a  easily has high transparency. As used herein, the visible light transmittance is the average value of spectral transmittances in a wavelength range of 380 nm to 780 nm. 
     The dielectric layer  20  has, for example, a relative permittivity of 2.0 to 20.0. In this case, it is easy to adjust the thickness of the dielectric layer  20 , and it is easy to adjust the radio wave absorption performance of the radio wave absorber  1   a . The relative permittivity of the dielectric layer  20  is, for example, a relative permittivity at 10 GHz measured according to the cavity resonance method. 
     The dielectric layer  20  is formed, for example, from a predetermined polymer. The dielectric layer  20  contains, for example, at least one polymer selected from the group consisting of ethylene-vinyl acetate copolymer, vinyl chloride resin, urethane resin, acrylic resin, acrylic urethane resin, acrylic-based elastomer, polyethylene, polypropylene, silicone, polyethylene terephthalate, polyethylene naphthalate, polycarbonate, polyimide, and cycloolefin polymer. In this case, it is easy to adjust the thickness of the dielectric layer  20 , and the production cost of the radio wave absorber  1   a  can be kept low. The dielectric layer  20  can be produced, for example, by hot-pressing a predetermined resin composition. 
     The dielectric layer  20  may be formed as a single layer, or may be formed of a plurality of layers made of the same material or different materials. In the case where the dielectric layer  20  has n layers (n is an integer equal to or greater than 2), the relative permittivity of the dielectric layer  20  is determined as follows, for example. A relative permittivity ε i  of each layer is measured (i is an integer from 1 to n). Next, ε i ×(t i /T) is obtained by multiplying the measured relative permittivity ε i  of each layer by the ratio of a thickness t i  of the layer to a total thickness T of the dielectric layer  20 . The relative permittivity of the dielectric layer  20  can be determined by adding up ε i ×(t i /T) of all the layers. 
     As shown in  FIG.  2 A , the dielectric layer  20  includes, for example, a first layer  21 , a second layer  22 , and a third layer  23 . The first layer  21  is disposed between the second layer  22  and the third layer  23 . The first layer  21  contains, for example, at least one member selected from the group consisting of ethylene-vinyl acetate copolymer, vinyl chloride resin, urethane resin, acrylic resin, acrylic urethane resin, polyethylene, polypropylene, silicone, polyethylene terephthalate, polyethylene naphthalate, polycarbonate, polyimide, and cycloolefin polymer. 
     In the radio wave absorber  1   a , the second layer  22  serves as a substrate for the resistive layer  10 , for example. In this case, the resistive layer  10  can be produced, for example, by forming the plurality of first openings  11  by laser processing, etching, or the like in a non-porous film formed on one main surface of the second layer  22  by a film forming method such as sputtering. In some cases, a non-porous film for the resistive layer  10  may be formed by a film forming method such as ion plating or coating (for example, bar coating). 
     The second layer  22  is disposed, for example, at a position closer to the electroconductive layer  30  than the resistive layer  10  is. As shown in  FIG.  2 B , the second layer  22  may be disposed at a position farther from the electroconductive layer  30  than the resistive layer  10  is. In this case, the dielectric layer  20  is composed of the first layer  21  and the third layer  23 . In this case, the resistive layer  10  and the dielectric layer  20  are protected by the second layer  22 , and the radio wave absorber  1   a  has high durability. In this case, for example, the resistive layer  10  may be in contact with the first layer  21 . The material of the second layer  22  is, for example, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), acrylic resin (PMMA), polycarbonate (PC), polyimide (PI), or cycloolefin polymer (COP). Among them, the material of the second layer  22  is desirably PET in terms of the balance among good heat resistance, dimensional stability, and manufacturing cost. 
     In the radio wave absorber  1   a , the third layer  23  serves as a substrate for the electroconductive layer  30 , for example. In this case, the electroconductive layer  30  can be produced, for example, by forming the plurality of second openings  31  by laser processing, etching, or the like in a non-porous film formed on one main surface of the third layer  23  by a film forming method such as sputtering. In some cases, a non-porous film for the electroconductive layer  30  may be formed by a film forming method such as ion plating or coating (for example, bar coating). As shown in  FIG.  2 A , the third layer  23  is disposed, for example, at a position closer to the resistive layer  10  in the radio wave absorber  1   a  than the electroconductive layer  30  is. As shown in  FIG.  2 C , the third layer  23  may be disposed at a position farther from the resistive layer  10  than the electroconductive layer  30  is. In this case, for example, the electroconductive layer  30  is in contact with the first layer  21 . 
     As the material of the third layer  23 , for example, the materials exemplified as the material of the second layer  22  can be used. The material of the third layer  23  may be the same as or different from the material of the second layer  22 . The material of the third layer  23  is desirably PET in terms of the balance among good heat resistance, dimensional stability, and manufacturing cost. 
     The third layer  23  has, for example, a thickness of 10 to 150 μm, and desirably has a thickness of 15 to 100 μm. Accordingly, the flexural rigidity of the third layer  23  is low, and it is possible to suppress wrinkling or deformation of the third layer  23  when forming the electroconductive layer  30 . The third layer  23  may be omitted in some cases. 
     The first layer  21  may be composed of a plurality of layers. In particular, in the case where the first layer  21  is in contact with at least one of the resistive layer  10  and the electroconductive layer  30  as shown in  FIG.  2 B  or  FIG.  2 C , the first layer  21  can be composed of a plurality of layers. 
     The first layer  21  may have adhesiveness, or may not necessarily have adhesiveness. In the case where the first layer  21  has adhesiveness, an adhesive layer may be disposed in contact with at least one of both main surfaces of the first layer  21 , or adhesive layers may not necessarily be disposed in contact with both main surfaces of the first layer  21 , respectively. In the case where the first layer  21  does not have adhesiveness, adhesive layers are desirably disposed in contact with both main surfaces of the first layer  21 , respectively. In the case where the dielectric layer  20  includes the second layer  22 , even if the second layer  22  does not have adhesiveness, adhesive layers may not necessarily be disposed in contact with both main surfaces of the second layer  22 , respectively. In this case, an adhesive layer can be disposed in contact with one main surface of the second layer  22 . In the case where the dielectric layer  20  includes the third layer  23 , even if the third layer  23  does not have adhesiveness, adhesive layers may not necessarily be disposed in contact with both main surfaces of the third layer  23 , respectively. In this case, an adhesive layer can be disposed in contact with at least one main surface of the third layer  23 . Each adhesive layer contains, for example, a rubber-based adhesive agent, an acrylic-based adhesive agent, a silicone-based adhesive agent, or a urethane-based adhesive agent. The thickness of each adhesive layer containing the adhesive agent is not limited to a specific value, and is, for example, 3 to 50 μm, and desirably 5 to 30 μm. 
     The radio wave absorber  1   a  has, for example, a visible light transmittance of 50% or more. 
     The radio wave absorber  1   a  is, for example, a λ/4 radio wave absorber. The radio wave absorber  1   a  is designed such that, when radio waves of a wavelength λ 0  to be absorbed by the radio wave absorber  1   a  are incident on the radio wave absorber  1   a , radio waves resulting from reflection on the front surface of the resistive layer  10  (front surface reflection) and radio waves resulting from reflection on the electroconductive layer  30  (back surface reflection) interfere with each other. In the λ/4 radio wave absorber, as shown in the following equation (1), the wavelength λ 0  of the radio waves to be absorbed is determined according to a thickness t of the dielectric layer  20  and a relative permittivity ε r  of the dielectric layer  20 . That is, the radio waves of the wavelength to be absorbed can be set by adjusting the relative permittivity and the thickness of the dielectric layer as appropriate. In the equation (1), sqrt(ε r ) means the square root of the relative permittivity ε r . 
       λ 0 =4 t ×sqrt(ε r )  Equation (1)
 
     The radio wave absorber  1   a  may contain at least one of a dielectric loss material and a magnetic loss material. In other words, the radio wave absorber  1   a  may be a dielectric loss radio wave absorber or a magnetic loss radio wave absorber. The dielectric layer  20  may contain at least one of a dielectric loss material and a magnetic loss material. The material forming the resistive layer  10  may be magnetic. 
     As for the arrangement and the shapes of the plurality of first openings  11 , the radio wave absorber  1   a  may be modified into a radio wave absorber  1   b  shown in  FIG.  3 A , a radio wave absorber  1   c  shown in  FIG.  3 B , or a radio wave absorber  1   d  shown in  FIG.  3 C . Each of the radio wave absorber  1   b , the radio wave absorber  1   c , and the radio wave absorber  1   d  is configured in the same manner as the radio wave absorber  1   a , except for the portions that are particularly described. The components, of the radio wave absorber  1   b , the radio wave absorber  1   c , and the radio wave absorber  1   d , identical to or corresponding to the components of the radio wave absorber  1   a  are indicated by the same reference characters, and the detailed descriptions thereof are omitted. The descriptions given for the radio wave absorber  1   a  are also applicable to the radio wave absorber  1   b , the radio wave absorber  1   c , and the radio wave absorber  1   d  unless there is a technical inconsistency. 
     As shown in  FIG.  3 A , in the radio wave absorber  1   b , the plurality of first openings  11  each have a circular shape in a plan view. In addition, the plurality of first openings  11  are arranged such that the centers thereof form a parallelogram lattice on the first main surface  12 . The plurality of first openings  11  may be arranged so as to form a square lattice. The radio wave absorber  1   a  may be modified such that the plurality of second openings  31  each have a circular shape in a plan view and the centers thereof form a parallelogram lattice or a square lattice on the second main surface  32 . 
     As shown in  FIG.  3 B , in the radio wave absorber  1   c , the plurality of first openings  11  each have a regular hexagonal shape in a plan view. In addition, the plurality of openings  11  are arranged such that the centers thereof form a parallelogram lattice on the first main surface  12 . The radio wave absorber  1   a  may be modified such that the plurality of second openings  31  each have a regular hexagonal shape in a plan view and the centers thereof form a parallelogram lattice on the second main surface  32 . 
     As shown in  FIG.  3 C , in the radio wave absorber  1   d , the plurality of first openings  11  each have an equilateral triangle shape in a plan view. In addition, a plurality of the first openings  11  having equilateral triangle shapes having the same orientation are arranged such that the centers thereof form a parallelogram lattice on the first main surface  12 . The radio wave absorber  1   a  may be modified such that the plurality of second openings  31  each have an equilateral triangle shape in a plan view and the centers of a plurality of the second openings  31  having equilateral triangle shapes having the same orientation form a parallelogram lattice on the second main surface  32 . Herein, the center of gravity of a planar figure is regarded as the center of an opening having a planar figure shape. 
     Each of the first openings  11  and the second openings  31  may have another polygonal shape such as a rectangular shape, or an elliptical shape in a plan view. Each of the plurality of first openings  11  and the plurality of second openings  31  may be arranged such that the centers thereof form another planar lattice such as a rectangular lattice on the first main surface  12  or the second main surface  32 . As used herein, the planar lattice means an array of points on a plane that are unchanged as a result of parallel shift for a constant distance in each of two independent directions. 
     The radio wave absorber  1   a  may be modified into a radio wave absorber  1   e  shown in  FIG.  4   , for example. The radio wave absorber  1   e  is configured in the same manner as the radio wave absorber  1   a  except for the portions that are particularly described. 
     The radio wave absorber  1   e  further includes an adhesive layer  40 . In the radio wave absorber  1   b , the electroconductive layer  30  is disposed between the dielectric layer  20  and the adhesive layer  40 . 
     For example, the radio wave absorber  1   b  can be adhered to a predetermined article by pressing the radio wave absorber  1   b  against the article with the adhesive layer  40  brought into contact with the article. Accordingly, a radio wave absorber-attached article can be obtained. 
     The adhesive layer  40  contains, for example, a rubber-based adhesive agent, an acrylic-based adhesive agent, a silicone-based adhesive agent, or a urethane-based adhesive agent. The radio wave absorber  1   b  may further include a release liner (not shown). In this case, the release liner covers the adhesive layer  40 . The release liner is typically a film that can maintain the adhesive strength of the adhesive layer  40   a  when covering the adhesive layer  40  and that can easily be peeled from the adhesive layer  40 . The release liner is, for example, a film made of polyester resin such as PET. By peeling the release liner, the adhesive layer  40  becomes exposed, allowing the radio wave absorber  1   b  to be adhered to an article. 
     EXAMPLES 
     Hereinafter, the present invention will be described in more detail by means of Examples. The present invention is not limited to the following Examples. First, evaluation methods for the Examples and Comparative Examples will be described. 
     [D Value] 
     A resistive layer-attached film according to each of the Examples and the Comparative Examples was observed using an optical microscope, and the size W R  of the opening and the distance G R  between the nearest openings in the direction in which the plurality of openings were arranged at equal intervals were determined. Similarly, an electroconductive layer-attached film according to each of the Examples and the Comparative Examples was observed, and the size W C  of the opening and the distance G C  between the nearest openings in the direction in which the plurality of openings were arranged at equal intervals were determined. In each of the Examples and the Comparative Examples, a D value was determined by dividing a larger value out of the ratio G R /W R  and the ratio G C /W C  by a smaller value out of the ratio G R /W R  and the ratio G C /W C . The results are shown in Table 1. 
     [Tem Observation] 
     Cross-sectional observation samples of a non-porous film for a resistive layer according to each of the Examples and the Comparative Examples and a non-porous film for an electroconductive layer according to each of the Examples and the Comparative Examples, the resistive layer-attached film according to each of the Examples and the Comparative Examples, and an alloy film in an electroconductive layer-attached film according to each of the Examples and the Comparative Examples were prepared using a focused ion beam processing observation apparatus (product name: FB-2000A, manufactured by Hitachi High-Tech Corporation). Then, the cross-sectional observation samples were observed using a field emission transmission electron microscope (product name: HF-2000, manufactured by Hitachi High-Tech Corporation), and the thicknesses of the non-porous film for a resistive layer according to each of the Examples and the Comparative Examples and the non-porous film for an electroconductive layer according to each of the Examples and the Comparative Examples were measured. The thicknesses of the non-porous films were regarded as the thicknesses of the resistive layer and the electroconductive layer according to each of the Examples and the Comparative Examples. The results are shown in Table 1. 
     [Appearance Check] 
     A sample according to each of the Examples and the Comparative Examples was observed with the naked eye, and whether moiré can be visually recognized was determined. When moiré was not visually recognized, the sample was evaluated as “A”, and when moiré was visually recognized, the sample was evaluated as “X”. 
     [Radio Wave Absorption Performance] 
     With reference to JIS R 1679: 2007, radio waves having frequencies of 60 to 90 GHz were made incident at an incident angle of 0° on the sample according to each of the Examples and the Comparative Examples fixed to a sample holder, using a vector network analyzer manufactured by ANRITSU CORPORATION, and a return loss |S| at each frequency was determined according to the following equation (2). In the equation (2), P 0  is the power of transmitted radio waves when radio waves are incident on a measurement target at a predetermined incident angle, and P i  is the power of received radio waves in this case. Instead of the sample according to each of the Examples and the Comparative Examples, an aluminum plate was fixed to the sample holder, a return loss |S| when radio waves were incident on the plate at an incident angle of 0° was regarded as 0 dB, and the return loss |S| of each sample was determined. The plate had a face dimension of 30 cm square, and the thickness of the plate was 5 mm. For each sample, the maximum value of the return loss |S| was determined. The results are shown in Table 1. 
         S  [dB]=10×log| P   i   /P   0 |  Equation (2)
 
     [Visible Light Transmittance] 
     The visible light transmittance of each sample was measured using a spectrophotometer U-4100 manufactured by Hitachi, Ltd. The results are shown in Table 1. 
     [Specific Resistance and Sheet Resistance] 
     The sheet resistances of the resistive layer and the electroconductive layer according to each of the Examples and the Comparative Examples were measured by the eddy current method according to JIS Z 2316 using a non-contact type resistance measurement device NC-80LINE manufactured by NAPSON CORPORATION. Meanwhile, the sheet resistances of the non-porous film for a resistive layer and the non-porous film for an electroconductive layer according to each of the Examples and the Comparative Examples were measured in the same manner. The products of the thicknesses of the non-porous films measured as described above and the sheet resistances of the non-porous films measured as described above were obtained to determine the specific resistances of the materials forming the non-porous films. The specific resistance of the material forming the non-porous film for a resistive layer was regarded as the specific resistance of the material forming the resistive layer according to each of the Examples and the Comparative Examples, and the specific resistance of the material forming the non-porous film for an electroconductive layer was regarded as the specific resistance of the material forming the electroconductive layer according to each of the Examples and the Comparative Examples. The results are shown in Table 1. 
     Example 1 
     DC magnetron sputtering was performed using an Al (aluminum) target material and an Si (silicon) target material and using argon gas as a process gas, to form an Al—Si alloy film on a PET film. In the DC magnetron sputtering, discharge involving the Al (aluminum) target material and discharge involving the Si (silicon) target material were performed simultaneously. Thus, a non-porous film for a resistive layer according to Example 1 was formed on the PET film. The specific resistance of the material forming the non-porous film for a resistive layer according to Example 1 was 5.0×10 −5  Ω·cm. The non-porous film had a thickness of 35 nm. Next, using a metal laser patterning machine, a plurality of square-shaped openings were formed at equal intervals in the non-porous film for a resistive layer according to Example 1 so as to form a square lattice, to obtain a resistive layer-attached film according to Example 1. In a plan view of the resistive layer-attached film according to Example 1, the size of each opening in the direction in which the plurality of openings were arranged at equal intervals was 240 μm, and the distance between the nearest openings was 10 μm. 
     DC magnetron sputtering was performed using a copper (Cu) target material and using argon gas as a process gas, to form a Cu film on a PET film. Thus, a non-porous film for an electroconductive layer according to Example 1 was formed on the PET film. The specific resistance of the material forming the non-porous film for an electroconductive layer according to Example 1 was 5.0×10 −6  Ω·cm. The non-porous film had a thickness of 500 nm. Next, using a metal laser patterning machine, a plurality of square-shaped openings were formed at equal intervals in the non-porous film for an electroconductive layer according to Example 1 so as to form a square lattice, to obtain an electroconductive layer-attached film according to Example 1. In a plan view of the electroconductive layer-attached film according to Example 1, the size of each opening in the direction in which the plurality of openings were arranged at equal intervals was 490 μm, and the distance between the nearest openings was 10 μm. 
     Next, an acrylic resin having a relative permittivity of 2.6 was molded so as to have a thickness of 480 μm, to obtain an acrylic resin layer A. The visible light transmittance of the acrylic resin layer A was 85.7%. The resistive layer-attached film according to Example 1 was put on the acrylic resin layer A such that the resistive layer of the resistive layer-attached film according to Example 1 was in contact with the acrylic resin layer A. Next, the electroconductive layer-attached film according to Example 1 was put on the acrylic resin layer A such that the electroconductive layer in the electroconductive layer-attached film was in contact with the acrylic resin layer A. Thus, a sample according to Example 1 was obtained. 
     Example 2 
     DC magnetron sputtering was performed using a copper (Cu) target material and using argon gas as a process gas, to form a Cu film on a PET film. Thus, a non-porous film for an electroconductive layer according to Example 2 was formed on the PET film. The specific resistance of the material forming the non-porous film for an electroconductive layer according to Example 2 was 1.0×10 −5  Ω·cm. The non-porous film had a thickness of 400 nm. Next, using a metal laser patterning machine, a plurality of square-shaped openings were formed at equal intervals in the non-porous film for an electroconductive layer according to Example 2 so as to form a square lattice, to obtain an electroconductive layer-attached film according to Example 2. In a plan view of the electroconductive layer-attached film according to Example 2, the size of each opening in the direction in which the plurality of openings were arranged at equal intervals was 450 μm, and the distance between the nearest openings was 50 μm. 
     An acrylic resin having a relative permittivity of 2.6 was molded so as to have a thickness of 540 μm, to obtain an acrylic resin layer B. A sample according to Example 2 was produced in the same manner as Example 1, except that the acrylic resin layer B was used instead of the acrylic resin layer A, and the electroconductive layer-attached film according to Example 2 was used instead of the electroconductive layer-attached film according to Example 1. 
     Example 3 
     A non-porous film for a resistive layer according to Example 3 was formed in the same manner as Example 1, except that the conditions of the DC magnetron sputtering were adjusted such that the thickness of the non-porous film was 50 nm. Next, using a metal laser patterning machine, a plurality of square-shaped openings were formed at equal intervals in the non-porous film for a resistive layer according to Example 3 so as to form a square lattice, to obtain a resistive layer-attached film according to Example 3. In a plan view of the resistive layer-attached film according to Example 3, the size of each opening in the direction in which the plurality of openings were arranged at equal intervals was 450 μm, and the distance between the nearest openings was 50 μm. 
     A non-porous film for an electroconductive layer according to Example 3 was formed on a PET film in the same manner as Example 1. The specific resistance of the material forming the non-porous film for an electroconductive layer according to Example 3 was 5.0×10 −6  Ω·cm. The non-porous film had a thickness of 1500 nm. Next, using a metal laser patterning machine, a plurality of square-shaped openings were formed at equal intervals in the non-porous film for an electroconductive layer according to Example 3 so as to form a square lattice, to obtain an electroconductive layer-attached film according to Example 3. In a plan view of the electroconductive layer-attached film according to Example 3, the size of each opening in the direction in which the plurality of openings were arranged at equal intervals was 490 μm, and the distance between the nearest openings was 10 μm. 
     Next, an acrylic resin having a relative permittivity of 2.6 was molded so as to have a thickness of 550 μm, to obtain an acrylic resin layer C. The resistive layer-attached film according to Example 3 was put on the acrylic resin layer C such that the resistive layer of the resistive layer-attached film according to Example 3 was in contact with the acrylic resin layer C. Next, the electroconductive layer-attached film according to Example 3 was put on the acrylic resin layer C such that the electroconductive layer in the electroconductive layer-attached film was in contact with the acrylic resin layer C. Thus, a sample according to Example 3 was obtained. 
     Comparative Example 1 
     A non-porous film for a resistive layer according to Comparative Example 1 was formed on a PET film and a resistive layer-attached film according to Comparative Example 1 was obtained, in the same manner as Example 1 except for the following. In DC magnetron sputtering, the ratio of the discharge power of the discharge involving the Si (silicon) target material to the discharge power of the discharge involving the Al (aluminum) target material was adjusted such that the specific resistance of the material forming the resistive layer according to Comparative Example 1 was 1.0×10 −4  Ω·cm. In addition, the conditions of the DC magnetron sputtering were adjusted such that the thickness of the alloy film in the resistive layer-attached film according to Comparative Example 1 was 30 nm. Next, using a metal laser patterning machine, a plurality of square-shaped openings were formed at equal intervals in the non-porous film for a resistive layer according to Comparative Example 1 so as to form a square lattice, to obtain a resistive layer-attached film according to Comparative Example 1. In a plan view of the resistive layer-attached film according to Comparative Example 1, the size of each opening in the direction in which the plurality of openings were arranged at equal intervals was 90 μm, and the distance between the nearest openings was 10 μm. 
     An electroconductive layer-attached film according to Comparative Example 1 was obtained in the same manner as Example 2, except that the thickness of the non-porous film was adjusted to 1000 nm. An acrylic resin having a relative permittivity of 2.6 was molded so as to have a thickness of 500 μm, to obtain an acrylic resin layer D. 
     A sample according to Comparative Example 1 was obtained in the same manner as Example 2, except that the acrylic resin layer D was used instead of the acrylic resin layer B, the resistive layer-attached film according to Comparative Example 1 was used instead of the resistive layer-attached film according to Example 1, and the electroconductive layer-attached film according to Comparative Example 1 was used instead of the electroconductive layer-attached film according to Example 2. 
     Comparative Example 2 
     A non-porous film for a resistive layer according to Comparative Example 2 was formed in the same manner as Comparative Example 1. Using a metal laser patterning machine, a plurality of square-shaped openings were formed at equal intervals in the non-porous film for a resistive layer according to Comparative Example 2 so as to form a square lattice, to obtain a resistive layer-attached film according to Comparative Example 2. In a plan view of the resistive layer-attached film according to Comparative Example 2, the size of each opening in the direction in which the plurality of openings were arranged at equal intervals was 91.6 μm, and the distance between the nearest openings was 8.3 μm. 
     A non-porous film for an electroconductive layer according to Comparative Example 2 was formed in the same manner as Example 2, except that the thickness of the non-porous film was adjusted to 1000 nm. Using a metal laser patterning machine, a plurality of square-shaped openings were formed at equal intervals in the non-porous film for an electroconductive layer according to Comparative Example 2 so as to form a square lattice, to obtain an electroconductive layer-attached film according to Comparative Example 2. In a plan view of the electroconductive layer-attached film according to Comparative Example 2, the size of each opening in the direction in which the plurality of openings were arranged at equal intervals was 448.8 μm, and the distance between the nearest openings was 51.2 μm. 
     A sample according to Comparative Example 2 was produced in the same manner as Comparative Example 1, except that the resistive layer-attached film according to Comparative Example 2 was used instead of the resistive layer-attached film according to Comparative Example 1, and the electroconductive layer-attached film according to Comparative Example 2 was used instead of the electroconductive layer-attached film according to Comparative Example 1. 
     Comparative Example 3 
     A non-porous film for an electroconductive layer according to Comparative Example 3 was formed in the same manner as Example 2, except that the thickness of the non-porous film was adjusted to 1000 nm. Using a metal laser patterning machine, a plurality of square-shaped openings were formed at equal intervals in the non-porous film for an electroconductive layer according to Comparative Example 3 so as to form a square lattice, to obtain an electroconductive layer-attached film according to Comparative Example 3. In a plan view of the electroconductive layer-attached film according to Comparative Example 3, the size of each opening in the direction in which the plurality of openings were arranged at equal intervals was 90 μm, and the distance between the nearest openings was 10 μm. 
     An acrylic resin having a relative permittivity of 2.6 was molded so as to have a thickness of 590 μm, to obtain an acrylic resin layer E. A sample according to Comparative Example 3 was produced in the same manner as Comparative Example 1, except that the acrylic resin layer E was used instead of the acrylic resin layer D, and the electroconductive layer-attached film according to Comparative Example 3 was used instead of the electroconductive layer-attached film according to Comparative Example 1. 
     As shown in Table 1, the sample according to each Example had a high visible light transmittance and had good radio wave absorption performance. In addition, moiré was not visually recognized in the sample according to each Example. On the other hand, moiré was visually recognized in the sample according to each Comparative Example. The comparison between the Examples and the Comparative Examples suggests that the D value being 1.3 or more is advantageous in making it less likely to visually recognize moiré. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
            
               
                   
                 Resistive layer 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Distance W R   
                   
                   
                   
                 Specific 
                   
               
               
                   
                 between 
                 Size G R  of 
                   
                   
                 resistance 
                 Sheet 
               
               
                   
                 openings 
                 opening 
                   
                 Thickness 
                 of material 
                 resistance 
               
               
                   
                 [μm] 
                 [μm] 
                 G R /W R   
                 [nm] 
                 [Ωcm] 
                 [Ω/□] 
               
               
                   
               
               
                 Ex. 1 
                 10 
                 240 
                 24 
                 35 
                 5.0 × 10 −5   
                 357 
               
               
                 Ex. 2 
                 10 
                 240 
                 24 
                 35 
                 5.0 × 10 −5   
                 357 
               
               
                 Ex. 3 
                 50 
                 450 
                 9 
                 50 
                 5.0 × 10 −5   
                 100 
               
               
                 Comp. Ex. 1 
                 10 
                 90 
                 9 
                 30 
                 1.0 × 10 −4   
                 333 
               
               
                 Comp. Ex. 2 
                 8.3 
                 91.6 
                 10.9 
                 30 
                 1.0 × 10 −4   
                 387 
               
               
                 Comp. Ex. 3 
                 10 
                 90 
                 9 
                 30 
                 1.0 × 10 −4   
                 333 
               
               
                   
               
            
           
           
               
               
            
               
                   
                 Electroconductive layer 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Distance W C   
                   
                   
                   
                 Specific 
                   
               
               
                   
                 between 
                 Size G C  of 
                   
                   
                 resistance 
                 Sheet 
               
               
                   
                 openings 
                 opening 
                   
                 Thickness 
                 of material 
                 resistance 
               
               
                   
                 [μm] 
                 [μm] 
                 G C /W C   
                 [nm] 
                 [Ω · cm] 
                 [Ω/□] 
               
               
                   
               
               
                 Ex. 1 
                 10 
                 490 
                 49 
                 500 
                 5.0 × 10 −6   
                 5.0 
               
               
                 Ex. 2 
                 50 
                 450 
                 9 
                 400 
                 1.0 × 10 −5   
                 2.5 
               
               
                 Ex. 3 
                 10 
                 490 
                 49 
                 1500 
                 5.0 × 10 −6   
                 1.67 
               
               
                 Comp. Ex. 1 
                 50 
                 450 
                 9 
                 1000 
                 1.0 × 10 −5   
                 0.50 
               
               
                 Comp. Ex. 2 
                 51.2 
                 448.8 
                 8.8 
                 1000 
                 1.0 × 10 −5   
                 0.49 
               
               
                 Comp. Ex. 3 
                 10 
                 90 
                 9 
                 1000 
                 1.0 × 10 −5   
                 0.50 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Dielectric 
                   
                   
                   
                   
               
               
                   
                 layer 
                   
                   
                 Visual 
               
               
                   
                 Thickness 
                   
                 Return loss 
                 recognition 
                 Visible light 
               
               
                   
                 [μm] 
                 D value 
                 [dB] 
                 of moiré 
                 transmittance 
               
               
                   
               
               
                 Ex. 1 
                 480 
                 2.04 
                 16.7 
                 A 
                 70.8 
               
               
                 Ex. 2 
                 540 
                 2.67 
                 22.0 
                 A 
                 59.7 
               
               
                 Ex. 3 
                 550 
                 5.44 
                 18.2 
                 A 
                 62.2 
               
               
                 Comp. Ex. 1 
                 500 
                 1.00 
                 17.1 
                 X 
                 52.5 
               
               
                 Comp. Ex. 2 
                 500 
                 1.24 
                 25.6 
                 X 
                 53.4 
               
               
                 Comp. Ex. 3 
                 590 
                 1.00 
                 20.7 
                 X 
                 52.5