Patent Publication Number: US-2023140996-A1

Title: Resin sheet, laminate, and radar system

Description:
TECHNICAL FIELD 
     The present invention relates to a resin sheet, a laminate, and a radar system. 
     BACKGROUND ART 
     Radars have been used to detect objects. Adjusting members allowing transmission of radio waves from radars therethrough have been known. 
     For example, a system including a radar sensor and an adjusting member is described in Patent Literature 1. This system is to be mounted on a motor vehicle so that the adjusting member will allow transmission of a radio wave from the radar sensor therethrough. The adjusting member includes at least one layer that reflects a portion of the radio wave. The adjusting member includes an additional layer configured based on a thickness and a permittivity suitable for reducing reflection of the radio wave. The adjusting member is a bumper. The radar sensor is, for example, a 77-GHz radar. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: US 2015/0109162 A1 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     In the technique described in Patent Literature 1, the adjusting member includes the additional layer configured to reduce reflection of the radio wave. However, in Patent Literature 1, adjustment of transmission of a millimeter wave from a millimeter-wave radar using a frequency-modulated continuous wave (FMCW) is not taken into consideration. 
     Therefore, the present invention provides a resin sheet that is advantageous in view of adjusting transmission of a millimeter wave from a millimeter-wave radar using an FMCW. 
     Solution to Problem 
     The present invention provides a resin sheet including: 
     a porous structure configured to adjust transmission of a millimeter wave, wherein 
     the porous structure has a relative permittivity varying in stages in a thickness direction of the resin sheet from a plane on which the millimeter wave is incident, the relative permittivity varying such that a difference between average relative permittivities in two adjacent layer portions is a predetermined value or less, the layer portions each having a particular thickness smaller than a wavelength of the millimeter wave, and 
     the porous structure has, as pores, only pores each having a pore diameter equal to or less than 10% of the wavelength of the millimeter wave. 
     The present invention also provides a laminate configured to adjust transmission of a millimeter wave, including: 
     a member; and 
     the above resin sheet covering a surface of the member, wherein 
     the resin sheet has a surface in contact with air and a boundary surface in contact with the member, 
     the porous structure has: a first principal surface being the surface in contact with air or being closest to the surface in contact with air in the thickness direction of the resin sheet; and a second principal surface being the boundary surface or being closest to the boundary surface in the thickness direction of the resin sheet, and 
     a relative permittivity of the porous structure at the second principal surface is closer to a relative permittivity of the member than a relative permittivity of the porous structure at the first principal surface. 
     The present invention also provides a radar system including: 
     a millimeter-wave radar that uses a frequency-modulated continuous wave; and 
     the above resin sheet allowing transmission of a millimeter wave emitted from the millimeter-wave radar through the resin sheet. 
     Advantageous Effects of Invention 
     The above resin sheet is advantageous in view of adjusting transmission of a millimeter wave from a millimeter-wave radar using an FMCW. With the use of the above laminate and radar system, transmission of a millimeter wave from a millimeter-wave radar using an FMCW can be adjusted to a desired state. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a cross-sectional view showing an example of the resin sheet according to the present invention. 
         FIG.  2    is a cross-sectional view showing an example of using the resin sheet shown in  FIG.  1   . 
         FIG.  3    is a graph showing an example of a varying relative permittivity of a porous structure. 
         FIG.  4    is a cross-sectional view showing another example of the resin sheet according to the present invention. 
         FIG.  5    is a cross-sectional view showing an example of using the resin sheet shown in  FIG.  3   . 
         FIG.  6    is a graph showing an example of a varying relative permittivity of a porous structure. 
         FIG.  7    is a cross-sectional view showing an example of the laminate according to the present invention. 
         FIG.  8    shows an example of the radar system according to the present invention. 
         FIG.  9 A  is an X-ray CT image showing an example of a cross-section of a resin sheet according to Example 3. 
         FIG.  9 B  is an X-ray CT image showing an example of a cross-section of the resin sheet according to Example 3. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     It is conceivable to use, for sensing a given object, a millimeter-wave radar using an FMCW. In this case, a given structure such as a housing may be needed to be disposed in a transmission-reception path of a millimeter wave to protect the millimeter-wave radar. It is thought that widening a frequency range of a millimeter wave used for sensing an object using the above millimeter-wave radar is advantageous in improving the accuracy of object sensing. Therefore, the present inventors conducted intensive studies to invent a sheet capable of being included in a structure disposed in an transmission-reception path to protect a millimeter-wave radar, the sheet being capable of improving transmission of a millimeter wave in a wide frequency range. Through much trial and error, the present inventors have newly found that a resin sheet having a given porous structure is advantageous in enhancing transmission of a millimeter wave in a wide frequency range and have completed the present invention. 
     Hereinafter, embodiments of the present invention will be described with reference to the drawings. The following description describes examples of the present invention, and the present invention is not limited to the following embodiments. 
     As shown in  FIG.  1   , a resin sheet  1   a  includes a porous structure  10 . The porous structure  10  is configured to adjust transmission of a millimeter wave. As shown in  FIG.  2   , when used, the resin sheet  1   a  is disposed, for example, between air and a solid  2  in a thickness direction of the resin sheet  1   a . In other words, the resin sheet  1   a  has a surface  11   a  in contact with air and a boundary surface  11   b  in contact with the solid  2 . In this case, for example, a millimeter wave is incident on the surface  11   a . The porous structure  10  has, as pores, only pores  12  each having a pore diameter equal to or less than 10% of a wavelength of the millimeter wave. This makes it possible to prevent transmission of the millimeter wave from being affected by the sizes of the pores  12 . The pore diameter of each pore  12  refers to, for example, the maximum diameter thereof observed in a scanning electron microscope (SEM) image of a cross-section of the porous structure  10 . The pore diameter of each pore  12  may be determined based on the X-ray CT measurement result for the porous structure  10 . 
       FIG.  3    shows an example of a varying relative permittivity of the porous structure  10 . As shown in  FIG.  3   , the porous structure  10  has a relative permittivity varying in stages from a plane on which a millimeter wave is incident. The relative permittivity of the porous structure  10  is adjusted such that a difference Δε between average relative permittivities in two adjacent layer portions in the thickness direction of the resin sheet  1   a  is a predetermined value Ve or less, the layer portions each having a particular thickness t L  smaller than the wavelength of the millimeter wave. Because of this, in the porous structure  10 , a relative permittivity variation rapid enough to affect millimeter-wave propagation can be prevented at an interface between the porous structure  10  and another substance and inside the porous structure  10 . This also allows the relative permittivity of the porous structure  10  to vary in stages in the thickness direction of the resin sheet  1   a . Consequently, the resin sheet  1   a  can reduce reflection of the millimeter wave and can have an increased millimeter-wave transmittance. In particular, the resin sheet  1   a  can have an increased transmittance of a wideband millimeter wave from a millimeter-wave radar using an FMCW. With the resin sheet  1   a , a transmission loss [dB] of a perpendicularly incident millimeter wave in a bandwidth of 60 to 80 GHz can be reduced by 50% or more, compared to the case without the resin sheet  1   a.    
     Typically, the entire porous structure  10  satisfies the requirement that the difference Δε between average relative permittivities in the two adjacent layer portions is the predetermined value Ve or less, the layer portions each having the particular thickness t L  smaller than the wavelength of a millimeter wave. 
     In  FIG.  3   , a distance T from the surface  11   a  corresponds to the thickness of the porous structure  10 . In  FIG.  3   , ε air  represents a relative permittivity of air, ER represents a relative permittivity of the solid  2 , relative permittivities ε 1 , ε 2 , ε 3 , ε 4 , and ε 5  are the average relative permittivities of the layer portions each having the particular thickness t L  in the porous structure  10 . A boundary between each two adjacent layer portions may be real or imaginary. 
     An average relative permittivity ε L  in each layer portion of the porous structure  10  can be determined, for example, by the following equation (1). In the equation (1), ε K  is a relative permittivity of the material of a skeleton of the porous structure  10 , ε air  is the relative permittivity of air, and p is a porosity (0&lt;p&lt;1) in the layer portion. The porosity in each layer portion can be determined, for example, based on the X-ray CT measurement result for the porous structure  10 . The relative permittivity of the material of the skeleton of the porous structure  10  is, for example, a relative permittivity measured at 10 GHz by a cavity resonance method. 
       ε L   =p×ε   air +(1− p )ε K   Equation (1)
 
     The particular thickness t L  is not limited to a particular value as long as the particular thickness t L  is smaller than the wavelength of a millimeter wave. The particular thickness t L  is, for example, 100 μm. The predetermined value Ve is not limited to a particular value as long as the resin sheet  1   a  can have an increased transmittance of a millimeter wave. The predetermined value Ve is, for example, 0.3. The predetermined value Ve is desirably 0.25. 
     The largest value of Δε in the porous structure  10  is not limited to a particular value. The largest value of Δε is, for example, 0.1 or more and 0.3 or less. This prevents an increase of the thickness of the porous structure  10  when the porous structure  10  is formed so as to have a relative permittivity varying in stages in the thickness direction of the resin sheet  1   a  from the plane on which a millimeter wave is incident. 
     The thickness of the porous structure  10  is not limited to a particular value as long as the resin sheet  1   a  can have an increased transmittance of a millimeter wave. The porous structure  10  has a thickness of, for example, 1 mm or more. In this case, the resin sheet  1   a  can have an increased transmittance of a millimeter wave and can protect a millimeter-wave radar as appropriate. 
     The thickness of the porous structure  10  is desirably 2 mm or more, and more desirably 3 mm or more. The thickness of the porous structure  10  is, for example, 10 mm or less. This makes it easy to reduce the weight of the resin sheet  1   a.    
     The relative permittivity of the porous structure  10  is not limited to a particular value as long as the relative permittivity varies in stages from the plane on which a millimeter wave is incident. The porous structure  10  has a relative permittivity of, for example, 1.01 or more and 4.99 or less. In this case, the resin sheet  1   a  can have an increased transmittance of a millimeter wave more reliably. 
     The relative permittivity ε K  of the material of the skeleton of the porous structure  10  is not limited to a particular value as long as the resin sheet  1   a  can have an increased transmittance of a millimeter wave. The relative permittivity ε K  is, for example, 2.2 to 5.0. 
     The material of the skeleton of the porous structure  10  is not limited to a particular material as long as the resin sheet  1   a  can have an increased transmittance of a millimeter wave. Examples of the material include polyethylene, polypropylene, polystyrene, polyester, polyamide, polyvinyl chloride, polyvinylidene chloride, polybutene, polyacetal, polyphenylene oxide, polymethylmethacrylate, polysulfone, polyethersulfone, polyetherketone, polyetheretherketone, polyamide-imide, polycarbonate, polyarylate, polyimide, fluorine resin, ethylene-propylene resin, ethylene-ethylacrylate, epoxy resin, urethane resin, imide resin, acrylic resin, and norbornene resin. The skeleton of the porous structure  10  may include only one resin material or may include a plurality of resin materials. The skeleton of the porous structure  10  may be formed of a polymer alloy or may be formed of a composite material of a resin matrix and a filler. 
     As described above, the resin sheet  1   a  has the surface  11   a  and the boundary surface  11   b . The porous structure  10  has, for example, a first principal surface  11   f  being the surface  11   a  and a second principal surface  11   s  being the boundary surface  11   b . The porous structure  10  has, for example, a relative permittivity of 1.75 or less at the first principal surface  11   f  and a relative permittivity of 1.8 or more at the second principal surface  11   s . In this case, the relative permittivity of the porous structure  10  at the first principal surface  11   f  is likely to be close to the relative permittivity ε air  of air, and the relative permittivity of the porous structure  10  at the second principal surface  11   s  is likely to be close to the relative permittivity ER of the solid  2 . 
     For example, by changing the porosity in stages in the thickness direction of the porous structure  10 , the relative permittivity of the porous structure  10  can be adjusted so as to vary in stages from the plane on which a millimeter wave is incident. 
     As shown in  FIG.  3   , the relative permittivity of the porous structure  10  varies, for example, in the thickness direction of the resin sheet  1   a  in a way that can be regarded as discrete. Such a porous structure  10  can be formed, for example, by laminating a plurality of homogeneous porous layers having different porosities. Alternatively, such a porous structure  10  may be formed by 3D printing. 
     The pore diameter of each pore  12  of the porous structure  10  is not limited to a particular value as long as the pore diameter is equal to or less than 10% of the wavelength of a millimeter wave. The pore diameter of each pore  12  is desirably equal to or less than 7% of the wavelength of a millimeter wave. 
     The pores  12  each have a pore diameter of, for example, 20 to 500 μm. In this case, the resin sheet  1   a  can have an increased transmittance of a millimeter wave more reliably. The pore diameter of each pore  12  may be 450 μm or less, 400 μm or less, or 350 μm or less. 
     A resin sheet  1   b  shown in  FIG.  4    can also be provided. The resin sheet  1   b  is configured in the same manner as the resin sheet  1   a , unless otherwise described. The components of the resin sheet  1   b  that are the same as or correspond to those of the resin sheet  1   a  are denoted by the same reference characters, and detailed descriptions of such components are omitted. The description given for the resin sheet  1   a  can apply to the resin sheet  1   b , unless there is technical inconsistency. 
     The resin sheet  1   b  further includes a skin layer  20  in addition to the porous structure  10 . The skin layer  20  has a thickness equal to or less than 10% of the wavelength of a millimeter wave and forms the surface  11   a  in contact with air. Since the thickness of the skin layer  20  is equal to or less than 10% of the wavelength of a millimeter wave, the effect of the skin layer  20  on transmission of a millimeter wave is easily reduced even when the skin layer  20  has a relatively high relative permittivity. 
     The skin layer  20  is typically a solid layer. The thickness of the skin layer  20  is, for example, 500 μm or less, desirably 450 μm or less, and more desirably 400 μm or less. The skin layer  20  has, for example, a relative permittivity of 1.5 or more. 
     As shown in  FIG.  5   , as in the case of the resin sheet  1   a , the resin sheet  1   b  is, for example, disposed between air and the solid  2  in a thickness direction of the resin sheet  1   b  when used. As shown in  FIG.  4   , in the resin sheet  1   b , the porous structure  10  has the first principal surface  11   f  and the second principal surface  11   s . The first principal surface  11   f  is a principal surface of the porous structure  10 , the principal surface being closest to the surface  11   a  in the thickness direction of the resin sheet  1   b . The second principal surface  11   s  is a principal surface being the boundary surface  11   b . The porous structure  10  has a relative permittivity of 1.75 or less at the first principal surface  11   f  and a relative permittivity of 1.8 or more at the second principal surface  11   s.    
       FIG.  6    shows an example of a relation between the distance from the surface  11   a  and the relative permittivity in the thickness direction of the resin sheet  1   b . In  FIG.  6   , a distance t S  corresponds to the thickness of the skin layer  20 , the distance T corresponds to the thickness of the resin sheet  1   b , and T−t S  corresponds to the thickness of the porous structure  10 . In  FIG.  6   , ε S  represents the relative permittivity of the skin layer  20 . The skin layer  20  has a higher relative permittivity, for example, than that of the porous structure  10  at the first principal surface  11   f.    
     As shown in  FIG.  6   , in the resin sheet  1   b , the relative permittivity of the porous structure  10  varies, for example, in the thickness direction of the resin sheet  1   a  in a way that can be regarded as continuous. Such a porous structure  10  can be formed, for example, by foam injection molding. Alternatively, such a porous structure  10  may be formed by 3D printing. 
     As shown in  FIG.  7   , for example, a laminate  5  configured to adjust transmission of a millimeter wave can be provided using the resin sheet  1   a . The laminate  5  includes a member  3  and the resin sheet  1   a . The resin sheet  1   a  covers a surface of the member  3 . In the laminate  5 , the resin sheet  1   a  has the surface  11   a  in contact with air and the boundary surface  11   b  in contact with the member  3 . The porous structure  10  has the first principal surface  11   f  being the surface  11   a  and the second principal surface  11   s  being the boundary surface  11   b . A relative permittivity LA of the porous structure  10  at the second principal surface  11   s  is a value closer to a relative permittivity ε C  of the member  3  than a relative permittivity ε B  of the porous structure  10  at the first principal surface  11   f . In other words, the laminate  5  satisfies a relation |ε C −ε A |&lt;|ε C −ε B |. Because of this, a relative permittivity variation rapid enough to affect millimeter-wave propagation is prevented at the boundary surface  11   b , and the millimeter-wave transmittance can be increased. 
     In the laminate  5 , the resin sheet  1   a  may be joined, for example, to the member  3  by welding, adhesion, or pressure-sensitive adhesion. The laminate  5  may include an adhesive layer or a pressure-sensitive adhesive layer between the resin sheet  1   a  and the member  3 . 
     The laminate  5  may be modified to include the resin sheet  1   b  instead of the resin sheet  1   a . In this case, in the porous structure  10 , a principal surface closest to the surface  11   a  of the resin sheet  1   a  is the first principal surface  11   f.    
     The laminate  5  may be modified such that the surface  11   a  is in contact with an additional member. In this case, the additional member has a relative permittivity equal to or less than the relative permittivity ε B  of the porous structure  10  at the first principal surface  11   f.    
     As shown in  FIG.  8   , for example, a radar system  100  can be provided using the resin sheet  1   a . The radar system  100  includes a millimeter-wave radar  50  and the resin sheet  1   a . The millimeter-wave radar  50  uses an FMCW. The resin sheet  1   a  allows transmission of a millimeter wave emitted from the millimeter-wave radar  50  through the resin sheet  1   a . As described above, the resin sheet  1   a  can increase the transmittance of a wideband millimeter wave. Because of this, the radar system  100  tends to have a high accuracy of object sensing. 
     As shown in  FIG.  8   , the radar system  100  includes, for example, a housing  30 . The housing  30  protects the millimeter-wave radar  50 . For example, the housing  30  is formed at least partly of the resin sheet  1   a . The housing  30  is, for example, formed of the laminate  5 . 
     The radar system  100  may include the resin sheet  1   b  instead of the resin sheet  1   a.    
     EXAMPLES 
     The present invention will be described in more detail by examples. The present invention is not limited to the examples given below. First, methods for evaluation of Examples will be described. 
     &lt;Transmission Loss Improvement Rate&gt; 
     Resin sheets according to Examples and Comparative Examples were each adhered using CS986440A to one principal surface of a resin substrate A having a relative permittivity of 2.4 and a thickness of 2 mm to produce a sample for transmission loss measurement. In the sample, the resin sheet had a principal surface having a relatively low relative permittivity and being in contact with the resin substrate A. A transmission coefficient S 21  obtained by allowing a millimeter wave with a frequency of 60 to 80 GHz to be perpendicularly incident on the resin sheet of the sample was measured with reference to a method described in JIS R 1679 (Measurement methods for reflectivity of electromagnetic wave absorber in millimeter wave frequency), and an average transmission loss T A   S  [dB] in the frequency range of 60 to 80 GHz was determined. Moreover, a transmission coefficient S 21  obtained by allowing a millimeter wave with a frequency of 60 to 80 GHz to be perpendicularly incident on another resin substrate A to which no resin sheet was adhered was measured, and an average transmission loss T A   R  [dB] in the frequency range of 60 to 80 GHz was determined. A transmission loss improvement rate R of each sample was determined by the following equation (2). Table 1 shows the results. It should be noted that the wavelength of a millimeter wave with a frequency of 60 Hz is 5 mm, and the wavelength of a millimeter wave with a frequency of 80 GHz is 3.75 mm. As a measurement apparatus was used a millimeterwave-microwave transmission loss measurement system, Model No. RTS01, manufactured by KEYCOM Corporation or a 30 mmΦ transmission loss-return loss measurement system for millimeter waves, Model No. RTS06, manufactured by KEYCOM Corporation. 
       Improvement rate  R=|T   A   S   −T   A   R   |/|T   A   R |  Equation (2)
 
     &lt;Pore Diameters of Pores&gt; 
     Cross-sections of each of the resin sheets according to Examples and Comparative Examples were measured by X-ray CT scanning using an X-ray CT scanner SKYSCAN 1272 manufactured by Bruker Corporation. The image analysis software Image J provided by the National Institutes of Health, USA, was used for reconstruction from cross-sectional images of the porous structure of each resin sheet, and a smallest pore diameter and a largest pore diameter were calculated. The average of the diameters of inscribed circles of pores in cross-sectional images taken between an interface and a position was determined as the smallest pore diameter, the interface being between a skin layer forming one principal surface of the resin sheet and the porous structure, the position being 200 μm away from the interface in the thickness direction of the resin sheet toward the other principal surface of the resin sheet. On the other hand, the average of the diameters of inscribed circles of pores in cross-sectional images taken between the other principal surface of the resin sheet and a position was determined as the largest pore diameter, the position being 500 μm away from the other principal surface in the thickness direction of the resin sheet toward the skin layer. Table 1 shows the results. 
     &lt;Relative Permittivity&gt; 
     A relative permittivity of a resin forming the skeleton of the porous structure of each of the resin sheets according to Examples and Comparative Examples was measured at 10 GHz by a cavity resonance method. Table 1 shows the results. Each of the resin sheets according to Examples and Comparative Examples was measured by X-ray CT scanning using an X-ray CT scanner SKYSCAN 1272 manufactured by Bruker Corporation. On the basis of this measurement result, the porosity p in each of a plurality of 100-μm-thick layer portions forming the porous structure and arranged in the thickness direction of the resin sheet was determined. The average relative permittivity ε L  of each of the layer portions was calculated by the above equation (1) from the thus-determined porosity p, the relative permittivity ε K  of the resin measured as described above, and the relative permittivity ε air  (=1.0) of air. The difference Δε between average relative permittivities ε L  in each two adjacent layer portions of the plurality of layer portions was calculated. Table 1 shows the results. Table 1 also shows an average relative permittivity ε LS  as of one of the plurality of layer portions and an average relative permittivity ε LA  of another one of the plurality of layer portions, the one layer portion being disposed closest to the resin substrate A in the sample, the other layer portion being disposed farthest from the resin substrate A in the sample. 
     &lt;Thickness&gt; 
     Thicknesses of the resin sheets according to Examples and Comparative Examples were measured using a micrometer. Table 1 shows the results. 
     Example 1 
     Four homogeneous porous resin layers each having a thickness of 500 μm were laminated to obtain a resin sheet according to Example 1. These porous resin layers included a polypropylene resin and an ethylene propylene rubber and had different relative permittivities. The four porous resin layers were laminated in order of increasing relative permittivity from one principal surface of the resin sheet according to Example 1 to the other principal surface thereof. Consequently, the resin sheet according to Example 1 had a relative permittivity increasing in stages from the one principal surface to the other principal surface. The porous resin layer forming the one principal surface of the resin sheet according to Example 1 had a 50-μm-thick solid skin layer. 
     Example 2 
     A mixture of supercritical nitrogen and polypropylene was molded by foaming and injection using a MuCell foaming injection molding apparatus to obtain a resin sheet according to Example 2. The polypropylene had a relative permittivity of 2.2. The resin sheet according to Example 2 had a 150-μm-thick solid skin layer forming one principal surface of the resin sheet. The resin sheet according to Example 2 had a relative permittivity increasing in stages from the one principal surface to the other principal surface. 
     Example 3 
     A mixture of supercritical nitrogen and polypropylene was molded by foaming and injection using a MuCell foaming injection molding apparatus to obtain a resin sheet according to Example 3. The resin sheet according to Example 3 had a 250-μm-thick solid skin layer forming one principal surface of the resin sheet. The resin sheet according to Example 3 had a relative permittivity increasing in stages from the one principal surface to the other principal surface.  FIGS.  9 A and  9 B  show examples of X-ray CT scan images of the resin sheet according to Example 3. 
     Example 4 
     A mixture of supercritical nitrogen and polyamide 6 was molded by foaming and injection using a MuCell foaming injection molding apparatus to obtain a resin sheet according to Example 4. The polyamide 6 had a relative permittivity of 4.9. The resin sheet according to Example 4 had a 350-μm-thick solid skin layer forming one principal surface of the resin sheet. The resin sheet according to Example 4 had a relative permittivity increasing in stages from the one principal surface to the other principal surface. 
     Example 5 
     A mixture of supercritical nitrogen and polypropylene was molded by foaming and injection using a MuCell foaming injection molding apparatus to obtain a resin sheet according to Example 5. The polypropylene had a relative permittivity of 2.2. The resin sheet according to Example 5 had a 500-μm-thick solid skin layer forming one principal surface of the resin sheet. The resin sheet according to Example 5 had a relative permittivity increasing in stages from the one principal surface to the other principal surface. 
     Comparative Example 1 
     A resin sheet according to Comparative Example 1 having a porous structure and made from a UV-curable acrylic resin was produced by stereolithography using a 3D printer ProJet HD 3000 manufactured by 3D Systems, Inc. The resin sheet according to Comparative Example 1 had a relative permittivity increasing in stages from one principal surface to the other principal surface. 
     Comparative Example 2 
     A homogeneous polyethylene foam was cut to a given size to obtain a resin sheet according to Comparative Example 2. The resin sheet according to Comparative Example 2 had a relative permittivity being constant from one principal surface to the other principal surface. 
     Comparative Example 3 
     A resin sheet according to Comparative Example 3 having a porous structure and made from a UV-curable acrylic resin was produced by stereolithography using a 3D printer ProJet HD 3000 manufactured by 3D Systems, Inc. The resin sheet according to Comparative Example 3 had a relative permittivity increasing in stages from one principal surface to the other principal surface. 
     Comparative Example 4 
     A homogeneous polyethylene foam was cut to a thickness of 400 μm to obtain a resin sheet according to Comparative Example 4. The resin sheet according to Comparative Example 4 had a 30-μm-thick solid skin layer forming one principal surface of the resin sheet. The resin sheet according to Comparative Example 4 had a relative permittivity being constant from the one principal surface to the other principal surface. 
     As shown in Table 1, the improvement rate R was 50% or more for each of the samples including the resin sheets according to Examples. On the other hand, the improvement rate R was less than 50% for each of the samples including the resin sheets according to Comparative Examples. These indicate that it is advantageous for the porous structure of the resin sheet to have a relative permittivity varying in stages from the plane on which a millimeter wave is incident, the relative permittivity varying such that Δε is 0.30 or less, and to have only pores each having a pore diameter equal to or less than 10% of the wavelength of a millimeter wave. 
     
       
         
           
               
               
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                   
                   
                   
                 Compar- 
                 Compar- 
                 Compar- 
                 Compar- 
               
               
                   
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 ative 
                 ative 
                 ative 
                 ative 
               
               
                   
                 ple 1 
                 ple 2 
                 ple 3 
                 ple 4 
                 ple 5 
                 Example 1 
                 Example 2 
                 Example 3 
                 Example 4 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Difference Δε between 
                 0.09 to 0.30 
                 0.01 to 0.30 
                 0.01 to 0.30 
                 0.01 to 0.30 
                 0.01 to 0.30 
                 0.3 
                 — 
                 0.34 to 0.40 
                 — 
               
               
                 average relative 
               
               
                 permittivities 
               
               
                 Pore diameters [μm] 
                  20 to 300 
                  30 to 150 
                  30 to 250 
                  30 to 150 
                  30 to 150 
                 300 to 600 
                 300 
                  20 to 300 
                 20 to 150 
               
               
                 of pores 
               
               
                 Thickness [mm] 
                 2 
                 1.5 
                 3 
                 4 
                 2 
                 1.5 
                 1 
                 1.5 
                 0.4 
               
               
                 Relative permittivity 
                 — 
                 2.2 
                 2.2 
                 4.9 
                 2.2 
                 2.2 
                 2.2 
                 2.2 
                 2.2 
               
               
                 of resin in porous 
               
               
                 structure 
               
               
                 Thickness [μm] 
                 50 
                 150 
                 250 
                 350 
                 500 
                 N/A 
                 100 
                 N/A 
                 30 
               
               
                 of skin layer 
               
               
                 Average relative 
                 1.06 
                 1.72 
                 1.12 
                 1.39 
                 1.72 
                 1.2 
                 1.18 
                 1.2 
                 1.18 
               
               
                 permittivity ε LA   
               
               
                 Average relative 
                 1.84 
                 1.96 
                 1.96 
                 4.12 
                 1.96 
                 1.7 
                 1.18 
                  2.15 
                 1.18 
               
               
                 permittivity ε LS   
               
               
                 Improvement rate 
                 74 
                 68 
                 79 
                 75 
                 51 
                 0   
                 30 
                 30   
                 5 
               
               
                 R [%]