Radio wave absorber

Abstract

To provide a wave absorber which is excellent in wave absorption characteristics in a wide band up to 40 GHz.SOLUTION: A wave absorber 100 has a plate-like base part 10, and a plurality of upper absorbers 20 which are provided on the base part 10 and have quadrangular pyramid shapes, wherein a width W1 of the base part exposed between the adjacent upper absorbers is 4 mm or less, an apex 22 of the upper absorber has a curved surface with a radius of curvature of 2 mm or less, the upper absorber 20 has a solid structure, and the base part 10 and the upper absorbers 20 contain ferrite, a non-polar resin and a carbon fiber.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a radio wave absorber. [Background technology]

[0002] Electronic devices are required to be electromagnetically compatible (EMC) to prevent electromagnetic waves generated by them from causing malfunctions in other devices, and conversely, to prevent electronic devices from malfunctioning due to external electromagnetic waves. To perform EMC evaluation, a measurement room called an anechoic chamber is required. The exterior walls of the anechoic chamber are covered with metal plates to prevent external electromagnetic waves from entering the chamber and electromagnetic waves generated by measuring equipment inside the chamber from radiating outward. Furthermore, radio wave absorbers are installed inside the anechoic chamber to prevent unwanted electromagnetic wave reflections.

[0003] Traditionally, the frequency range measured in an anechoic chamber was 30 MHz to 1 GHz. However, with the diversification of communication devices such as mobile phones and RF tags, the upper limit of the measurement frequency has expanded. This has led to a demand for radio wave absorbers with excellent radio wave absorption properties across a wide frequency band from 30 MHz to over 1 GHz. Along with this trend toward higher frequencies, the allowable frequency limits specified in the standards have been revised to 18 GHz or less. Furthermore, with the transition to an era of high-capacity, high-speed communications and the introduction of 5G to the market, the frequency band used by electronic devices has shifted to 28 GHz, in the millimeter wave range. Accordingly, radio wave absorbers are also required to have excellent radio wave absorption properties across a frequency band above 18 GHz and up to at least 40 GHz.

[0004] Patent Document 1 describes a composite radio wave absorber (claim 1) in which a ferrite tile and a flat, wedge-shaped, or pyramidal upper absorber are joined together, the upper absorber being characterized in that ferrite powder is dispersed in a general-purpose resin having a relative dielectric constant of 4.9 or less at frequencies of 1 MHz or more. An example of Patent Document 1 describes an upper absorber in which ferrite powder and polypropylene are mixed together in a predetermined ratio, then pelletized with a pelletizer, and then injection-molded into the pyramidal shape. The shape is a hollow pyramid with a base of 100 mm x 100 mm, a height of 100 mm, and a wall thickness of 20 mm.

[0005] Patent Document 2 describes a radio wave absorber (claim 1) that is to be combined with a ferrite tile, "a radio wave absorber whose main components are ferrite and a non-polar resin, and in which conductive powder is dispersed in the radio wave absorber." It describes that the conductive powder is carbon, and that the amount of carbon added is 0.1 to 6.0 volume %. Furthermore, the shape of the radio wave absorber in Patent Document 2 (see Figures 1 and 2) is a hollow pyramid with a hollow structure having a base of 5 cm x 5 cm, a height of 10 cm, and a wall thickness of 0.75 cm, and the tip of the pyramid is cut flat. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-188513 [Patent Document 2] Japanese Patent Application Laid-Open No. 2002-9482 Summary of the Invention [Problem to be solved by the invention]

[0007] In the technologies of Patent Documents 1 and 2, electromagnetic waves in the low frequency band of 100 to 400 MHz are absorbed by ferrite tiles, and electromagnetic waves in the frequency band of 400 MHz or higher are absorbed by a pyramidal wave absorber. However, with the composite wave absorber described in Patent Document 1, even if the compounding ratio of ferrite powder is optimized, it is only possible to achieve a return loss of 20 dB or more, which is normally required for anechoic chambers, up to 10 GHz at most, and the wave absorption characteristics in a wide band up to 40 GHz are insufficient.

[0008] In the combination of ferrite tiles and a radio wave absorber described in Patent Document 2, the radio wave absorption characteristics are improved in the frequency band above 10 GHz by incorporating carbon as conductive powder into the radio wave absorber. However, even with this, a return loss of 20 dB or more can only be achieved up to 20 GHz at most, and the radio wave absorption characteristics are insufficient in a wide band up to 40 GHz.

[0009] In view of the above problems, the present invention has an object to provide a radio wave absorber that has excellent radio wave absorption characteristics over a wide band up to 40 GHz. [Means for solving the problem]

[0010] To solve the above problems, the present inventors conducted extensive research and discovered the following. Specifically, they discovered that for a radio wave absorber having a flat base and multiple upper absorbers with a quadrangular pyramid shape provided on the base, improving the shape of the upper absorbers and improving the material of the radio wave absorbers can improve the radio wave absorption characteristics over a wide frequency range up to 40 GHz. The shape improvements require: (I) setting the width of the base exposed between adjacent upper absorbers to 4 mm or less; (II) making the top of the upper absorber a curved surface with a curvature radius of 2 mm or less; and (III) making the upper absorber a solid structure. The material improvements require adding carbon fiber in addition to ferrite and non-polar resin.

[0011] The present invention, which was completed based on the above findings, has the following essential features. [1] A radio wave absorber having a flat base portion and a plurality of upper absorbers each having a quadrangular pyramid shape provided on the base portion, The width of the base portion exposed between adjacent upper absorbent bodies is 4 mm or less, The top of the upper absorbent body has a curved surface with a curvature radius of 2 mm or less, The upper absorbent body has a solid structure, The radio wave absorber, wherein the base portion and the upper absorber contain ferrite, a non-polar resin, and carbon fiber.

[0012] [2] The radio wave absorber according to the above [1], wherein the carbon fiber is a pitch-based carbon fiber having a fiber length of 3 mm or less and a fiber diameter of 10 μm or more.

[0013] [3] The radio wave absorber according to the above [1], wherein the carbon fibers are PAN-based carbon fibers having a fiber length of 4 mm or less and a fiber diameter of 5 μm or more.

[0014] [4] The radio wave absorber according to any one of the above [1] to [3], wherein the non-polar resin is one or both of a polypropylene resin and a polyethylene resin.

[0015] [5] The radio wave absorber according to any one of the above [1] to [4], wherein in the base portion and the upper absorber, the content of the non-polar resin is 15 to 30 parts by mass and the content of the carbon fiber is 0.1 to 1.0 part by mass per 100 parts by mass of the ferrite.

[0016] [6] The length of the bottom side of the upper absorbent body is 46 to 50 mm, The height of the upper absorbent body is 75 to 90 mm, The radio wave absorber according to any one of the above [1] to [5], wherein the curved surface formed by the top of the upper absorber has a width of 2 to 4 mm when viewed from above the upper absorber, and a height of 1 to 2 mm when viewed from the side of the upper absorber. [Effects of the Invention]

[0017] The radio wave absorber of the present invention has excellent radio wave absorption characteristics over a wide band up to 40 GHz. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a perspective view of a radio wave absorber 100 according to an embodiment of the present invention. [Figure 2] 1A to 1C are a plan view, a front view, and a side view of a radio wave absorber 100 according to an embodiment of the present invention. [Figure 3] 1 is a graph showing the radio wave absorption characteristics in the frequency range of 2.6 to 40 GHz in Comparative Examples 1 to 6. [Figure 4] 1 is a graph showing the radio wave absorption characteristics in Examples 1 and 2 of the present invention at frequencies of 2.6 to 40 GHz. [Figure 5] 1 is a graph showing the radio wave absorption characteristics at frequencies of 2.6 to 40 GHz in Example 3 of the invention and Comparative Example 7. DETAILED DESCRIPTION OF THE INVENTION

[0019] A radio wave absorber 100 according to one embodiment of the present invention will be described with reference to Figures 1 and 2. The radio wave absorber 100 has a flat base portion 10 and a plurality of upper absorbers 20, each having a quadrangular pyramid shape, provided on the base portion 10.

[0020] The base portion 10 has a pair of main surfaces, an upper surface 10A and a lower surface 10B. Of the pair of main surfaces of the base portion 10, the surface on which the upper absorbent body 20 is provided is referred to as the upper surface 10A, and the surface opposite thereto is referred to as the lower surface 10B. The shapes and dimensions of the upper surface 10A and the lower surface 10B of the base portion 10 are the same as each other, and it is sufficient that the upper surface 10A of the base portion 10 can encompass the bottom surfaces of multiple upper absorbent bodies 20. The thickness T1 of the base portion 10 is not particularly limited, but can be approximately 5 to 20 mm.

[0021] The upper absorber 20 has a quadrangular pyramid shape and is provided on the upper surface 10A of the base 10. Electromagnetic waves arrive from the apex 22 of the upper absorber 20 toward the bottom. The upper absorber 20 has a shape in which the area of ​​a cross section perpendicular to the direction of arrival of the electromagnetic waves (a cross section perpendicular to a perpendicular line from the apex to the bottom) gradually increases along the direction of arrival of the electromagnetic waves. Therefore, the amount of ferrite and carbon fiber gradually increases toward the base of the pyramid, thereby achieving high magnetic loss and conductive loss effects in the radio wave absorption characteristics. The quadrangular pyramid shape is preferably a right pyramid (i.e., a pyramid in which the perpendicular line from the apex to the base passes through the center of gravity of the base). Furthermore, the quadrangular pyramid shape is preferably a rectangular pyramid (i.e., a pyramid with a rectangular base), and particularly preferably a square pyramid (i.e., a pyramid with a square base). For this reason, the quadrangular pyramid shape is most preferably a regular square pyramid (a right pyramid and a square pyramid), as shown in Figures 1 and 2. 1 and 2, the multiple upper absorbent bodies 20 preferably have the same bottom shape, and adjacent upper absorbent bodies 20 are positioned so that the nearest base sides 26 are parallel to each other. The number of upper absorbent bodies 20 provided on the upper surface 10A of the base portion 10 is not particularly limited as long as there is a plurality of them, and may be, for example, 8 upper absorbent bodies in 2 rows x 4 rows, 9 upper absorbent bodies in 3 rows x 3 rows, or 16 upper absorbent bodies in 4 rows x 4 rows, as shown in FIGS.

[0022] 1 and 2, in this embodiment, it is important that the width W1 of the base portion 10 exposed between adjacent upper absorbers 20 is 4 mm or less. If the width W1 exceeds 4 mm, radio waves are more likely to be reflected at the exposed base portion (flat surface), making it impossible to obtain excellent radio wave absorption characteristics over a wide band up to 40 GHz. A narrower width W1 is preferable from the perspective of radio wave absorption characteristics, so there is no particular lower limit, and the width W1 may be 0 mm.

[0023] 2, the length L2 of the base 26 of the upper absorber 20 is preferably 46 to 50 mm, and the height H2 of the upper absorber 20 is preferably 75 to 90 mm. By making the dimensions of the upper absorber 20 small in this way, a large effective area can be secured within the anechoic chamber.

[0024] 1 and 2, in this embodiment, it is important that the apex 22 of the upper absorber 20 has a curved surface with a radius of curvature of 2 mm or less. If the apex is flat, high-frequency electromagnetic waves will be reflected at the apex, resulting in insufficient radio wave absorption characteristics. By making the apex 22 of the upper absorber 20 a sharp curved surface as in this embodiment, it is possible to suppress the reflection of high-frequency electromagnetic waves. Note that the radius of curvature of the curved surface is preferably 1 mm or more, from the viewpoint of the accuracy of molding the apex 22.

[0025] 2, it is preferable that the curved surface formed by the top portion 22 of the upper absorbent body 20 has a width W3 of 2 to 4 mm when viewed from above the upper absorbent body 20, and a height H3 of 1 to 2 mm when viewed from the side surface 24 of the upper absorbent body 20. By setting the dimensions of the curved surface of the top portion in this manner, the molding process of the top portion 22 becomes easier.

[0026] It is important that the upper absorber 20 has a solid structure. If the upper absorber has a hollow structure, the amount of ferrite and carbon fiber will be insufficient, making it impossible to obtain excellent radio wave absorption characteristics over a wide frequency band up to 40 GHz.

[0027] 1 and 2, due to the draft angle of the injection molding mold, the side edge portion 28 of the upper absorbent body 20 may be cut to have a predetermined width. In this case, the width W4 of the side edge portion 28 is preferably 1 to 2 mm.

[0028] The base portion 10 and the upper absorbent body 20 may be separate bodies that are fixed together with adhesive, but are preferably integrally molded.

[0029] Next, we will explain the materials that make up the base portion 10 and the upper absorbent body 20. In this embodiment, the base portion 10 and the upper absorbent body 20 contain ferrite, a non-polar resin, and carbon fiber, and preferably consist of these components and other additives that are optionally contained.

[0030] The ferrite material is not particularly limited, and one or more materials selected from the group consisting of NiO / ZnO, LiO / ZnO, NiO / ZnO / CuO, MnO / ZnO, etc. can be used. The ferrite material is the main radio wave absorbing material in the radio wave absorber 100 of this embodiment. From the viewpoint of obtaining good radio wave absorption characteristics over a wide band, it is preferable to use a NiO / ZnO or LiO / ZnO material. The particle size of the ferrite powder in the raw material stage can be about 1 to 150 μm.

[0031] The non-polar resin refers to a resin composed of molecules that have no electric dipole, a molecule that has a polar bond but whose dipole moment is canceled out due to molecular symmetry, or a resin composed of molecules with a polar bond of similar low polarity. In this embodiment, the non-polar resin is preferably one or more selected from the group consisting of polypropylene resin, polyethylene resin, fluorine resin, Teflon (registered trademark), allyl resin, epoxy resin, vinyl chloride resin, vinyl acetate resin, styrene resin, acrylic resin, polyamide resin, polyacetal resin, polycarbonate resin, and acetyl cellulose resin, and more preferably one or both of polypropylene resin and polyethylene resin.

[0032] In this embodiment, it is important that the base portion 10 and the upper absorber 20 contain carbon fiber. The carbon fiber imparts conductivity to the radio wave absorber 100, improving the radio wave absorption characteristics at high frequencies. Moreover, by incorporating carbon fiber instead of general carbon powder as the conductive powder, it is possible to obtain excellent radio wave absorption characteristics over a wide frequency band up to 40 GHz.

[0033] The carbon fiber may be any known or arbitrary carbon fiber, such as a PAN-based carbon fiber or a pitch-based carbon fiber. It is particularly preferable to use a pitch-based carbon fiber having a fiber length of 3 mm or less and a fiber diameter of 10 μm or more. By using such short and thick carbon fibers, it is possible to more fully improve radio wave absorption characteristics over a wide frequency band up to 40 GHz. The fiber length of the pitch-based carbon fiber is preferably 3 mm or less, more preferably 1 mm or less. The lower limit of the fiber length of the pitch-based carbon fiber is not particularly limited, but the fiber length may be 0.1 mm or more. The upper limit of the fiber diameter of the pitch-based carbon fiber is not particularly limited, but the fiber diameter may be 30 μm or less. It is also preferable to use a PAN-based carbon fiber having a fiber length of 4 mm or less and a fiber diameter of 5 μm or more. The use of such PAN-based carbon fiber also makes it possible to suitably improve radio wave absorption characteristics over a wide frequency band up to 40 GHz. The fiber length of the PAN-based carbon fiber is preferably 4 mm or less, more preferably 3 mm or less. The lower limit of the fiber length of the PAN-based carbon fiber is not particularly limited, but the fiber length can be 1 mm or more. The upper limit of the fiber diameter of the PAN-based carbon fiber is not particularly limited, but the fiber diameter can be 20 μm or less. In this specification, the "fiber length" and "fiber diameter" of the carbon fiber may be the nominal values ​​provided by the manufacturer.

[0034] In the base portion 10 and the upper absorber 20, the content of the non-polar resin is preferably 15 to 30 parts by mass, and more preferably 20 to 25 parts by mass, per 100 parts by mass of ferrite. If the content of the non-polar resin per 100 parts by mass of ferrite is less than 15 parts by mass, the amount of non-polar resin will be too little and the amount of ferrite will be too much, making injection molding difficult, and if it exceeds 30 parts by mass, the amount of ferrite will be too little and the amount of non-polar resin will be too much, making it difficult to obtain the target radio wave absorption characteristics.

[0035] In the base portion 10 and the upper absorber 20, the carbon fiber content is preferably 0.1 to 1.0 part by mass, and more preferably 0.3 to 0.7 part by mass, relative to 100 parts by mass of ferrite. If the non-polar resin content is less than 0.1 part by mass relative to 100 parts by mass of ferrite, the amount of carbon fiber is too small, making it impossible to obtain a sufficient effect of improving radio wave absorption characteristics over a wide band up to 40 GHz, while if it exceeds 1.0 part by mass, the amount of carbon fiber is too large, causing radio wave reflection in the MHz band and making it impossible to obtain good radio wave absorption characteristics.

[0036] The base portion 10 and the upper absorber 20 may contain other additives such as antioxidants, ultraviolet absorbers, and light stabilizers. However, from the viewpoint of not impairing the radio wave absorption characteristics, the content of the other additives is preferably 1 part by mass or less in total per 100 parts by mass of ferrite.

[0037] The radio wave absorber 100 of this embodiment is combined with a ferrite tile, which is a sintered ferrite, to form a composite radio wave absorber, which can obtain excellent radio wave absorption characteristics over a wide frequency band from a low frequency band of 30 to 400 MHz to 40 GHz. Specifically, the ferrite tile is provided on the lower surface 10B of the base portion 10 of the radio wave absorber 100. The ferrite tile may be fixed to the base portion according to a conventional method.

[0038] The radio wave absorber 100 according to the embodiment of the present invention can be manufactured, for example, by mixing and stirring raw materials such as ferrite powder, non-polar resin, carbon fiber, and any other additives, and then injection molding the mixture into a predetermined shape. [Example]

[0039] The compositions of Comparative Example Formulations 1 and 2 and Invention Example Formulations 1 to 3 shown in Table 1 were mixed and stirred, and then injection molded into the shapes of the Comparative Examples or Invention Examples shown in Table 2, to produce the wave absorbers of Comparative Examples 1 to 7 and Invention Examples 1 to 3 shown in Table 3.

[0040] [Table 1]

[0041] [Table 2]

[0042] [Table 3]

[0043] [Measurement of radio wave absorption characteristics] Eighteen radio wave absorbers each of Comparative Examples 1 to 7 and Invention Examples 1 to 3 were prepared, and the size of the base portion was 600 mm x 600 mm, the number of upper absorbers was 144 in 12 rows x 12 rows, and ferrite tiles (600 mm x 600 mm x 5.2 mm) were fixed to the underside of the base portion to form a composite radio wave absorber, and the radio wave absorption characteristics were measured in the frequency band of 2.6 to 40 GHz. The measurements were carried out using a reflection amount measuring device using a dielectric lens. The results are shown in Figures 3 to 5.

[0044] In Comparative Example 1, there were many frequencies in the 10 to 40 GHz frequency band where a return loss of 20 dB or more could not be achieved. Even when only material improvements were made as in Comparative Examples 3, 4, and 7, or when only shape improvements were made as in Comparative Example 5, there were many frequencies in the 10 to 40 GHz frequency band where a return loss of 20 dB or more could not be achieved. Even when ordinary carbon powder (Ketjen Black) was used as the conductive material as in Comparative Examples 2 and 6, there were frequencies in the 10 to 40 GHz frequency band where a return loss of 20 dB or more could not be achieved. In contrast, in Invention Examples 1 to 3, in which both material and shape improvements were made, a return loss of 20 dB or more could be achieved over the entire frequency range of 1 to 40 GHz. [Industrial Applicability]

[0045] The radio wave absorber of the present invention has excellent radio wave absorption characteristics over a wide band up to 40 GHz, and is therefore suitable for installation in an anechoic chamber to perform EMC evaluation over a wide band up to 40 GHz. [Explanation of symbols]

[0046] 100 Radio wave absorber 10 Base 10A Top of the base 10B Underside of base 20 Upper absorbent body 22 Top of upper absorber (curved surface) 24 Side of upper absorbent body 26 Bottom edge of upper absorber 28 Side edge of upper absorbent body W1 Width of the base exposed between adjacent upper absorbent bodies T1 Base thickness L2 Length of the base of the upper absorber H2 Height of upper absorber W3 Width of the curved surface formed by the top of the upper absorbent body as seen from above the upper absorbent body H3 Height of the curved surface formed by the top of the upper absorbent body as seen from the side of the upper absorbent body W4 Width of the side of the upper absorbent body

Claims

1. A radio wave absorber having a flat base portion and a plurality of upper absorbers each having a quadrangular pyramid shape provided on the base portion, The width of the base portion exposed between the adjacent upper absorbent bodies is 4 mm or less, The top of the upper absorbent body has a curved surface with a curvature radius of 2 mm or less, The upper absorbent body has a solid structure, The radio wave absorber, wherein the base portion and the upper absorber contain ferrite, a non-polar resin, and carbon fiber.

2. 2. The radio wave absorber according to claim 1, wherein the carbon fibers are pitch-based carbon fibers having a fiber length of 3 mm or less and a fiber diameter of 10 μm or more.

3. 2. The radio wave absorber according to claim 1, wherein the carbon fibers are PAN-based carbon fibers having a fiber length of 4 mm or less and a fiber diameter of 5 μm or more.

4. 2. The radio wave absorber according to claim 1, wherein the non-polar resin is one or both of a polypropylene resin and a polyethylene resin.

5. 5. The radio wave absorber according to claim 1, wherein, in the base portion and the upper absorber, a content of the nonpolar resin is 15 to 30 parts by mass, and a content of the carbon fiber is 0.1 to 1.0 part by mass, relative to 100 parts by mass of the ferrite.

6. The length of the bottom side of the upper absorbent body is 46 to 50 mm, The height of the upper absorbent body is 75 to 90 mm, 5. The radio wave absorber according to claim 1, wherein the curved surface formed by the top of the upper absorber has a width of 2 to 4 mm when viewed from above the upper absorber, and a height of 1 to 2 mm when viewed from a side of the upper absorber.

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