Radio wave absorber

Abstract

To provide a radio wave absorber which solves the problems that a conventional radio wave absorber of a foamed resin is not suitable for outdoor use in aspects of water resistance, humidity resistance and impact resistance, a conventional flat-surface and multi-layer radio wave absorber has a narrow band and is not suitable for applications of an outdoor inspection device and the like by high-resolution millimeter-wave radar, therefore, can simultaneously achieve suitability for outdoor use, wide band property and good reflection characteristic and has a simple structure.SOLUTION: A layer of a magnetic material is provided on the surface of a pyramid shape or a wedge shape using a non-foamed resin containing a carbon fiber.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a broadband millimeter wave band radio wave absorber for high-resolution millimeter wave radar, which has been put into practical use in recent years, and particularly to a radio wave absorber which has a simple structure yet good reflection characteristics over a wide band, and which is suitable for outdoor use, where moisture absorption and water resistance are problems with conventional radio wave absorbers using foamed resin. [Background technology]

[0002] In recent years, high-resolution millimeter-wave radars that use a wide 4 GHz band from 77 to 81 GHz have become widespread, and there is a demand for broadband radio wave absorbers for use in inspection equipment for these millimeter-wave radars. A radio wave absorber converts radio wave energy into heat and absorbs it. When wideband characteristics are required, radio wave absorbers in which a loss material is dispersed in pyramidal foam resin as described in Patent Document 1 or a wedge foam resin as described in Patent Document 2 are often used. In the radio wave absorbers in Patent Documents 1 and 2, the surface reflection of the material constituting the radio wave absorber is suppressed by forming the material from a foam resin, and the impedance is equivalently changed from the space where the radio wave arrives toward the inside of the radio wave absorber by forming the surface shape into a pyramidal or wedge shape, thereby obtaining wideband characteristics. By dispersing the loss material in the foam resin, radio waves are absorbed throughout the inside of the radio wave absorber, thereby obtaining large return attenuation. However, when moisture penetrates into the foam resin, the dielectric properties change significantly and the reflection properties deteriorate, so there are problems with water resistance and moisture resistance, and further, pyramidal and wedge foam resins are not suitable for outdoor use because the tip is easily deformed when an impact is applied from the outside, and are often used mainly inside an anechoic chamber or an anechoic box. On the other hand, when broadband characteristics are not required, the planar multi-layered radio wave absorber of Patent Document 3 may be used. The radio wave absorber of Patent Document 3 provides large return attenuation to incident radio waves by utilizing the resonance phenomenon, by arranging a resistive film on the front surface of a dielectric material with a thickness of 1 / 4 wavelength and a metal reflector on the rear surface. Radio wave absorbers with such a structure do not require foaming of the material, so that they have improved water resistance and moisture resistance, and furthermore, because the surface is planar, they have improved impact resistance, making them suitable for outdoor use, but they have narrowband characteristics because they utilize the resonance phenomenon. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 63-200600 [Patent Document 2] Japanese Patent Application Publication No. 02-050497 [Patent Document 3] Japanese Patent Application Publication No. 05-114813 Summary of the Invention [Problem to be solved by the invention]

[0004] As described above, the above-mentioned conventional pyramidal or wedge-shaped radio wave absorbers using foamed resin are not suitable for outdoor use from the viewpoints of water resistance, moisture resistance and impact resistance, and further, the above-mentioned conventional planar multi-layered radio wave absorbers have narrow band characteristics and are therefore not suitable for use in high-resolution millimeter wave radar which requires wide band characteristics. [Means for solving the problem]

[0005] In order to solve the problems associated with the conventional wave absorbers, the wave absorber of the present invention has a pyramidal or wedge-shaped structure made of non-foamed resin containing carbon fiber, on the surface of which a layer of magnetic material is provided, and further has a wall provided inside the pyramidal or wedge shape with a cavity surrounding the wall. Effect of the Invention

[0006] The above-mentioned configuration allows the wave absorber of the present invention to simultaneously achieve the following effects. In other words, by using a non-foamed resin in which the strength of the material itself is improved by including carbon fiber, moisture does not enter the inside of the wave absorber and the reflection characteristics do not deteriorate, and the strength of the wave absorber against external impacts is significantly improved, making it suitable for outdoor use. Furthermore, the impedance of the resin is reduced by including carbon fiber, and by providing a magnetic layer in contact with the surface of the resin with reduced impedance, the magnetic field in the magnetic layer is increased, and thus the magnetic loss against the wave can be increased. Furthermore, for the wave that passes through the magnetic layer and enters the resin, the dielectric loss against the wave can be increased due to the dielectric loss tangent increased by including carbon fiber. Furthermore, the magnetic loss on the surface of the pyramidal or wedge shape and the dielectric loss inside the resin do not utilize resonance, so that a wideband reflection characteristic can be obtained. As described above, with just a simple structure of providing a layer of magnetic material on the surface of a pyramidal or wedge-shaped material made of resin containing carbon fiber, this is an excellent invention that can simultaneously achieve three effects: improving the water resistance, moisture resistance, and impact resistance required for outdoor use, broadening the reflection characteristics, and improving the reflection characteristics by increasing the magnetic loss in the magnetic material layer and the dielectric loss inside the resin. Furthermore, by providing a wall inside the pyramid or wedge shape and leaving the periphery hollow, the weight of the radio wave absorber can be reduced while maintaining the above three effects. [Brief description of the drawings]

[0007] [Figure 1] FIG. 1 is a perspective view of a radio wave absorber according to a first embodiment. [Diagram 2] FIG. 1 is a diagram showing an incident wave to a radio wave absorber according to the first embodiment. [Diagram 3] Diagram showing the electromagnetic field at the boundary between free space and a dielectric material [Figure 4] FIG. 1 is a perspective view including a cross section of a radio wave absorber according to a first embodiment. [Diagram 5]FIG. 1 is a diagram showing the reflection characteristics of a radio wave absorber according to the first embodiment and a radio wave absorber other than the present invention. [Figure 6] FIG. 13 is a perspective view of a radio wave absorber according to the second embodiment. [Figure 7] FIG. 11 is a perspective view including a cross section of a radio wave absorber according to a second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] Hereinafter, an embodiment of the present invention will be described. EXAMPLES

[0009] (Embodiment 1) FIG. 1 shows a first embodiment of the radio wave absorber of the present invention. In FIG. 1, 101 shows a radio wave absorber in which a layer of a magnetic material is provided on the surface of a pyramid shape using a non-foamed resin containing carbon fiber of the present invention, 102 shows a main part of the pyramid shape, 103 shows a peripheral part of the tip of the pyramid shape, and 104 shows a flat part of the tip of the pyramid shape. In general, a pyramid shape is called a three-dimensional structure consisting of four isosceles triangles and one base surface sharing a sharp tip portion with an infinitesimal area, but a practical pyramid-shaped radio wave absorber needs to have a finite size at the tip portion, and therefore the radio wave absorber 101 in FIG. 1 shows a feasible and practical pyramid-shaped radio wave absorber. Here, the radio wave absorber 101 has the first effect of the present invention that it is suitable for outdoor use because it uses a non-foamed resin containing carbon fiber, and thus has increased impact resistance and does not allow moisture to penetrate into the absorber. Next, the second effect of the present invention will be described. Fig. 2 shows the state of incident radio waves when radio waves are incident from free space on the radio wave absorber 101 of Fig. 1, from the cross-sectional direction of the radio wave absorber. In Fig. 2, 201 is a non-foamed resin containing carbon fiber, and 202 is a layer of a magnetic material. Also, 203 indicates the incident radio wave on the main part 102 of the pyramid shape of Fig. 1, 204 indicates the incident radio wave on the tip peripheral part 103 of the pyramid shape of Fig. 1, and 205 indicates the incident radio wave on the tip flat part 104 of the pyramid shape of Fig. 1. As shown in Fig. 2, the incident radio wave 203 on the main part of the pyramid shape is reflected multiple times between the opposing pyramid side surfaces and then returns in a direction close to the incident direction, the incident radio wave 204 on the tip peripheral part is reflected only once at the tip peripheral part and then returns in a direction different from the incident direction, and the incident radio wave 205 on the tip flat part is reflected only once and returns to the incident direction. That is, the radio waves incident on the radio wave absorber that return in the direction of incidence are components reflected by the main part of the pyramid shape and the flat end part. Here, the ratio of each incident radio wave to the entire incident radio wave depends on the occupied area of ​​each part when the radio wave absorber is viewed from the direction of incidence, but since the main part of the pyramid shape occupies the majority of the area, most of the radio waves that return in the direction of incidence are also radio waves reflected by the main part of the pyramid shape. The radio waves reflected by the main part of the pyramid shape are multiple-reflected between the walls of the adjacent pyramid shapes, and the reflection loss increases due to accumulation, and the number of multiple reflections does not depend on the frequency, so that the radio wave absorber 101 in FIG. 1 has a wideband reflection characteristic. The second effect of the present invention is that such a wideband reflection characteristic can be realized. Next, the third effect of the present invention will be described. Figure 3 shows the electromagnetic fields of incident and reflected radio waves at the boundary between free space and dielectric material, where Figure 3(a) shows the case where radio waves are incident from the dielectric material side to the free space side, and Figure 3(b) shows the case where radio waves are incident from the free space side to the dielectric material side, where εo and μo are the permittivity and permeability of the free space, εd and μo are the permittivity and permeability of the dielectric material, Wi is the incident radio wave, Ei is the electric field of the incident radio wave, Hi is the magnetic field of the incident radio wave, Wr is the reflected radio wave, Er is the electric field of the reflected radio wave, and Hr is the magnetic field of the reflected radio wave. As shown in Figure 3(a), when radio waves are incident from the dielectric material side to the free space side, the electric fields of the incident radio wave and the reflected radio wave on the boundary are in the same direction, and the magnetic fields are in the opposite directions. Therefore, the electric fields are strengthened and the magnetic fields are weakened on the boundary. On the other hand, as shown in FIG. 3(b), when radio waves are incident from the free space side to the dielectric material side, the electric fields of the incident radio waves and the reflected radio waves on the boundary are in the opposite directions, and the magnetic fields are in the same direction. Therefore, the electric fields weaken each other on the boundary, and the magnetic fields strengthen each other. This means that in a multi-layer structure in which a magnetic layer is provided on the boundary, the loss of the magnetic material differs depending on which side the radio waves are incident from. In other words, when radio waves are incident from the free space side, the loss of the magnetic material increases more than when the radio waves are incident from the dielectric material side, and the reflection loss also increases, and the effect of increasing the magnetic material loss increases as the dielectric constant of the dielectric material increases. Note that, according to the principle of reciprocity, the transmission characteristics are the same regardless of which side the radio waves are incident from. The present invention makes the most of this phenomenon. In other words, the present invention focuses on the fact that the situation in which radio waves are incident on the radio wave absorber from the free space shown in FIG. 2 corresponds to FIG. 3(b), and provides a magnetic layer in contact with the surface of the resin constituting the radio wave absorber 101 shown in FIG. 1, and contains carbon fiber in the resin. In other words, by incorporating resin into the carbon fiber, the dielectric constant is increased, thereby increasing the magnetic material loss, and at the same time, by incorporating carbon fiber into the resin, the dielectric tangent is increased, thereby increasing the dielectric loss for radio waves that pass through the magnetic material and enter the resin containing the carbon fiber.In other words, the third effect of the present invention is that not only is a layer of magnetic material provided on the surface of the resin that constitutes the radio wave absorber, but also the resin that constitutes the radio wave absorber contains carbon fiber, thereby increasing both the loss inside the magnetic material and the loss inside the dielectric material. As described above, the radio wave absorber according to the present invention simultaneously achieves three effects, namely suitability for outdoor use, broadband performance, and improved reflection characteristics, with a simple structure. Here, we will explain an example of using a resin containing 20% ​​carbon fiber in polylactic acid as the resin containing carbon fiber, and a paint containing iron oxide powder dispersed in modified alkyd as the magnetic material. It goes without saying that the same qualitative effect can be obtained even if acrylonitrile, polycarbonate, or polyethylene terephthalate is used instead of polylactic acid, or if carbon fiber is contained in a different ratio, although there are quantitative differences. Fig. 4 shows the structure of a radio wave absorber produced to confirm the effect of the present invention, with a cross section of a part of the pyramid shape being visible. Here, the radio wave absorber shown in Fig. 4 is made light enough to confirm the effect of the present invention, and has a wall 401 provided inside the pyramid shape, with a cavity 402 around the wall. Fig. 5 shows the reflection characteristics of a radio wave absorber not according to the present invention and a radio wave absorber according to the present invention. Here, all the radio wave absorbers have the shape shown in Fig. 4, and the period of the pyramid shape is 10 mm and the height is 20 mm. The relative dielectric constant of the polylactic acid used in the calculation is 3.1 and the dielectric tangent is 0.01. For the polylactic acid containing carbon fiber, the radio wave absorber was produced by a 3D printer, and therefore anisotropy appears in the material. Therefore, the calculation was performed using values ​​obtained by separately measuring a material sample from which anisotropy due to the 3D printer was extracted. For the magnetic material used in the calculation, the calculation was performed using values ​​obtained by separately measuring a material sample. 5, 501 and 502 are reflection characteristics of a radio wave absorber not according to the present invention, and 503 and 504 are reflection characteristics of a radio wave absorber according to the present invention. More specifically, 501 indicates a calculated value of the reflection characteristic of a radio wave absorber in which a magnetic material layer is provided on non-foamed polylactic acid resin containing no carbon fiber, 502 indicates a calculated value of the reflection characteristic of a radio wave absorber in which a magnetic material layer is not provided on non-foamed polylactic acid resin containing carbon fiber, 503 indicates a calculated value of the reflection characteristic of a radio wave absorber in which a magnetic material layer is provided on non-foamed polylactic acid resin containing carbon fiber, and 504 indicates an actual measurement value of the reflection characteristic of the radio wave absorber of 503 that was actually manufactured. As shown in Fig. 5, in a 12 GHz bandwidth from 77 to 81 GHz used by high-resolution millimeter wave radar and three times that, 73 to 85 GHz, reflection characteristics 501 and 502 of a radio wave absorber not according to the present invention are in the range of about -20 dB to -30 dB, whereas it can be confirmed that a calculated value 503 and an actual measured value 504 of the reflection characteristic of a radio wave absorber according to the present invention are close to each other and are in the range of about -30 dB to -40 dB. In other words, it is shown that even if a magnetic layer is provided on unfoamed resin not containing carbon fiber, or even if a magnetic layer is not provided on unfoamed resin containing carbon fiber, improvement in reflection characteristics is not realized, and the third effect of the present invention that the reflection characteristics are improved only when a magnetic layer is provided on the surface of unfoamed resin containing carbon fiber is shown. Since the third effect of the present invention has been demonstrated as described above, by combining it with the first and second effects of the present invention described above, it can be seen that suitability for outdoor use, broadband performance, and improved reflection characteristics can be simultaneously achieved only when a layer of magnetic material is provided on the surface of a pyramid shape made of non-foamed resin containing carbon fiber of the present invention. In the first embodiment, an example in which the frequency band used is from 77 to 81 GHz has been shown, but the radio wave absorber, which is a passive component, can be designed and manufactured for other frequency bands, including fractional bands, and similar effects can be expected as long as the dielectric characteristics are the same and the shape dimensions and wavelength are manufactured under proportional conditions. (Embodiment 2) FIG. 6 shows a second embodiment of the radio wave absorber of the present invention, which is a wedge-shaped one instead of a pyramid-shaped one. In FIG. 6, 601 shows a radio wave absorber in which a layer of a magnetic material is provided on the surface of a wedge-shaped one made of a non-foamed resin containing carbon fiber of the present invention, 602 shows a main part of the wedge shape, 603 shows both ends of the wedge shape, and 604 shows a flat end part of the wedge shape. That is, the wedge-shaped radio wave absorber of the present invention shown in FIG. 6 has a practical shape of a radio wave absorber having a tip and both ends with a finite area. FIG. 7 shows a perspective view of the radio wave absorber of FIG. 6, in which a wall 702 is provided inside the wedge shape to reduce weight, and a cavity 701 is formed around the wall, and the part is cut away to show a cross section of the inside. The effects of the radio wave absorber of the present invention shown in FIG. 6 are the same as those of the first embodiment, so the description will be omitted. However, as in the first embodiment, the three effects of suitability for outdoor use, wide band characteristics, and improved reflection characteristics can be simultaneously achieved with a simple structure. Two embodiments of the radio wave absorber of the present invention have been described above, but the present invention can also be applied to other similar embodiments. For example, the effects of the present invention can be maintained even if a protective coating is applied to the surface of the radio wave absorber of the present invention in a thickness that has a negligible effect on the wavelength of the frequency used to improve weather resistance, and it goes without saying that the present invention also applies. [Industrial Applicability]

[0010] As described above, the radio wave absorber of the present invention has a simple structure and yet simultaneously achieves three effects: suitability for outdoor use, broadband performance, and improved reflection characteristics. In other words, since it is suitable for outdoor use, it is a radio wave absorber suitable for inspection devices and the like that inspect automobiles, construction machines, small boats, agricultural machines, and the like outdoors, which are equipped with high-resolution millimeter-wave radar that has become increasingly widespread in recent years. [Explanation of symbols]

[0011] 101 Radio wave absorber according to the first embodiment 102 Pyramid-shaped main part 103 Pyramid-shaped tip periphery 104 Pyramid-shaped flat tip 201 Non-foamed resin containing carbon fiber 202 Magnetic layer 203 Radio waves incident on the main part of a pyramid 204 Incident waves on the periphery of the pyramid tip 205 Incident waves on the flat top of a pyramid 401 Pyramid-shaped inner wall 402 Pyramid-shaped internal cavity 501 Calculated value when a magnetic layer is provided on non-foamed polylactic acid resin that does not contain carbon fiber 502 Calculated value when no magnetic layer is provided for non-foamed polylactic acid resin containing carbon fiber 503 Calculated value when a magnetic layer is provided on a non-foamed polylactic acid resin containing carbon fiber 504 503 Actual measurements when the radio wave absorber is manufactured 601 Radio wave absorber according to the second embodiment 602 Wedge-shaped main part 603 Wedge-shaped ends 604 Wedge-shaped tip flat surface 701 Wedge-shaped inner wall 702 Wedge-shaped internal cavity

Claims

1. A radio wave absorber characterized by having a layer of magnetic material on the surface of a pyramid or wedge shape made of non-foamed resin containing carbon fiber.

2. 2. The radio wave absorber according to claim 1, wherein a wall is provided inside said pyramidal or wedge-shaped body, and a cavity is formed around said wall.

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