JP6488602B2 - Ferrite composition for radio wave absorber and radio wave absorber - Google Patents

Description

  The present invention relates to a ferrite composition for a radio wave absorber and a radio wave absorber.

  In recent years, electric and electronic devices that transmit and receive various types of radio waves, such as televisions, radios, and mobile phones, have become widespread. Accordingly, there is an increasing demand for radio wave absorbers that absorb radio waves. As an application of the radio wave absorber, for example, a wall material of an anechoic chamber or the like can be mentioned. Examples of the material of the radio wave absorber include a ferrite sintered body. Mn—Zn-based ferrite, Ni—Zn-based ferrite, and the like have been developed as materials for radio wave absorbers.

  Patent Document 1 describes a radio wave absorber made of Ni-Zn ferrite. However, a radio wave absorber made of Ni—Zn ferrite is expensive because Ni is expensive.

  The wave absorber composed of Mn-Zn ferrite is less expensive than the wave absorber composed of Ni-Zn ferrite, and has excellent reflection attenuation characteristics even when the thickness is thin. Can be demonstrated. Therefore, the radio wave absorber composed of Mn—Zn ferrite can be manufactured at a lower cost than the radio wave absorber composed of Ni—Zn ferrite.

  Moreover, the electromagnetic wave absorber comprised with a Mn-Zn type ferrite is normally used for the low frequency band below 30 MHz. There is a demand for a radio wave absorber composed of Mn—Zn-based ferrite and capable of exhibiting sufficient reflection attenuation characteristics in a high frequency band of about 400 MHz.

 Furthermore, when the electromagnetic wave absorber is formed in a tile shape and used, for example, on the inner wall of an electromagnetic wave anechoic chamber, processing is necessary because a certain dimensional accuracy is required, but chipping and cracking during processing and construction are due to yield reduction. It is desirable to have excellent chipping resistance because it causes high costs.

JP-A-5-129123

  The present invention has been made in view of such a situation, and an object of the present invention is to provide a radio wave absorber having high reflection attenuation characteristics in a high frequency band of 100 MHz to 400 MHz, excellent chipping resistance, and low cost. It is providing the ferrite composition for electromagnetic wave absorbers.

In order to achieve the above object, the ferrite composition for a radio wave absorber according to the present invention comprises 39.0 to 45.0 mol% of iron oxide in terms of Fe ₂ O ₃ and 16.0 in terms of zinc oxide of zinc oxide. contained 20.0 mol%, the balance has a main component consisting of manganese oxide, with respect to 100 wt% of said main component as an auxiliary component, 50 to 200 weight of at least silicon oxide in terms of SiO ₂ It is characterized by containing 200 to 2500 ppm by weight of ppm and calcium oxide in terms of CaO.

  An electromagnetic wave absorber composed of the ferrite composition according to the present invention by containing the oxide constituting the main component in the above range and further containing at least silicon oxide and calcium oxide as subcomponents in the above range. Is excellent in chipping resistance and has a high return loss at 100 MHz to 400 MHz. Furthermore, since the radio wave absorber composed of the ferrite composition according to the present invention can keep the matching thickness thin, the material thickness can be reduced and the cost can be reduced.

  The reason why such an effect can be obtained is not necessarily clear, but the combined effect obtained by making the composition of the main component in the above range and coexisting at least silicon oxide and calcium oxide as subcomponents in the above range is large. It is thought to have influenced.

Furthermore, the ferrite composition for a radio wave absorber according to the present invention preferably contains niobium oxide as a subcomponent in an amount of 50 to 500 ppm by weight in terms of Nb ₂ O ₅ .

  By including niobium oxide in the above range, the return loss at 100 MHz to 400 MHz of the radio wave absorber constituted by the ferrite composition according to the present invention can be further improved.

Furthermore, the ferrite composition for a radio wave absorber according to the present invention preferably contains 50 to 500 ppm by weight of vanadium oxide as a subcomponent in terms of V ₂ O ₅ .

  By containing vanadium oxide in the above range, the chipping resistance of the radio wave absorber constituted by the ferrite composition according to the present invention can be further improved.

  The radio wave absorber according to the present invention is composed of a ferrite composition having the above composition.

  In the radio wave absorber according to the present invention, it is preferable that the return loss for a 100 MHz radio wave, the return loss for a 200 MHz radio wave, and the return loss for a 400 MHz radio wave are all 20 dB or more.

  The radio wave absorber according to the present invention preferably has a matching thickness of 5.5 mm or less.

  The radio wave absorber according to the present invention preferably has a plate-like tile shape.

FIG. 1 is a graph showing the relationship between frequency and return loss in Examples 6, 8 and Comparative Example 3.

  Hereinafter, modes for carrying out the present invention will be described.

  The radio wave absorber according to the present embodiment is characterized by being composed of the ferrite composition shown below.

  The ferrite composition constituting the radio wave absorber according to the present embodiment is a Mn—Zn-based ferrite and contains iron oxide, manganese oxide, and zinc oxide as main components.

In the main component 100 mol%, the content of iron oxide, calculated as _Fe 2 _{O 3,} 39.0 to 45.0 mol%, preferably 39.0 to 43.0 mol%. When the content of iron oxide is less than 39.0 mol%, the return loss at 400 MHz decreases and the matching thickness becomes too thick. When the iron oxide content exceeds 45.0 mol%, the return loss at 100 MHz decreases. In particular, when the content of iron oxide is 39.0 to 43.0 mol% in terms of Fe ₂ O ₃ , the matching thickness is easily set to 5.0 to 5.5 mm.

  In 100 mol% of the main component, the content of zinc oxide is 16.0 to 20.0 mol%, preferably 16.0 to 18.5 mol% in terms of ZnO. When the zinc oxide content is less than 16.0 mol%, the return loss at 100 MHz decreases. When the content of zinc oxide exceeds 20.0 mol%, the return loss at 400 MHz decreases. In particular, when the zinc oxide content is 16.0 to 18.5 mol% in terms of ZnO, the return loss at 400 MHz can be easily set to 21.0 dB or more.

  The balance of the main component is manganese oxide.

  Here, the definition of the matching thickness of the electromagnetic wave absorber, how to find it, etc. will be described below.

The normalized input impedance Z _{in on the} surface of the radio wave absorber is expressed by the following formula (1), the reflection coefficient S in the radio wave absorber is expressed by the following formula (2), and the return loss Ref in the radio wave absorber (DB) is represented by the following formula (3).

In addition, λ ₀ in the equation (1) is an electromagnetic wavelength (= c / f), a complex magnetic permeability μ _r = μ ′ _r −jμ ″ _r , a complex dielectric constant ε _r = ε ′ _r −jε ″ _r , d = Material thickness. The matching thickness is the material thickness d when there is a complete non-reflection, that is, the frequency f at which the return loss Ref (dB) is infinite.

  Cost can be reduced by reducing the material thickness of the radio wave absorber. When the reflection attenuation amount of the radio wave absorber is increased by changing the material thickness of the radio wave absorber, the material thickness of the radio wave absorber is preferably equal to the matching thickness. Therefore, the matching thickness of the radio wave absorber is preferably thin. In this embodiment, it is preferable that the matching thickness is 5.5 mm or less.

  Moreover, it is difficult to technically manufacture a radio wave absorber having a material thickness of less than 5.0 mm. Therefore, the matching thickness in the present embodiment is more preferably 5.0 mm or greater and 5.5 mm or less.

  The ferrite composition according to this embodiment contains at least silicon oxide and calcium oxide as subcomponents in addition to the main components (iron oxide, manganese oxide, and zinc oxide).

The content of silicon oxide is 50 to 200 ppm by weight, preferably 60 to 150 ppm by weight in terms of SiO _{2 with} respect to 100% by weight of the main component. When the content of silicon oxide is less than 50 ppm by weight, chipping resistance and return loss at 100 to 400 MHz are deteriorated. If it exceeds 200 ppm by weight, abnormal grain growth tends to occur during the sintering of the ferrite composition.

  The content of calcium oxide is 200 to 2500 ppm by weight, preferably 500 to 1500 ppm by weight in terms of CaO with respect to 100% by weight of the main component. When the content of calcium oxide is less than 200 ppm by weight, chipping resistance and return loss at 100 to 400 MHz are deteriorated. If it exceeds 2500 ppm by weight, abnormal grain growth tends to occur during the sintering of the ferrite composition.

  In the ferrite composition according to the present embodiment, in addition to the composition range of the main component being controlled to the above range, the above silicon oxide and calcium oxide are contained as subcomponents. As a result, the radio wave absorber constituted by the ferrite composition according to the present embodiment becomes a radio wave absorber excellent in chipping resistance while maintaining a high return loss in the frequency band of 100 to 400 MHz. Furthermore, since the radio wave absorber constituted by the ferrite composition according to the present embodiment has a small matching thickness, the cost can be significantly reduced.

  When silicon oxide or calcium oxide is contained alone, or when silicon oxide and calcium oxide are contained at the same time, if at least one of the contents is too small, the above effect is sufficiently obtained. Absent. That is, the above-described effect is a composite effect obtained only when a specific amount of silicon oxide and calcium oxide is contained at the same time.

In the ferrite composition according to the present embodiment, niobium oxide is further preferably contained as an auxiliary component in an amount of 50 to 500 ppm by weight, more preferably 100 to 400 ppm by weight in terms of Nb ₂ O ₅ . An electromagnetic wave absorber composed of a ferrite composition containing a specific amount of niobium oxide has a return loss in a frequency band of 100 to 400 MHz, compared with an electromagnetic wave absorber composed of a ferrite composition not containing niobium oxide. It tends to improve.

In the ferrite composition according to the present embodiment, it is preferable to further contain vanadium oxide as a subcomponent in an amount of 50 to 500 ppm by weight in terms of V ₂ O ₅ , and more preferably 100 to 400 ppm by weight. A wave absorber composed of a ferrite composition containing a specific amount of vanadium oxide tends to have improved chipping resistance as compared to a wave absorber composed of a ferrite composition not containing vanadium oxide.

  In addition, you may contain both niobium oxide and vanadium oxide simultaneously within the range of said content, respectively.

  In the ferrite composition according to the present embodiment, further, subcomponents other than the main component and subcomponent described above do not hinder the performance of the radio wave absorber composed of the ferrite composition according to the present embodiment. May be included. Although there is no restriction | limiting in particular in content of the said subcomponent, For example, it is about several weight ppm-several hundred weight ppm with respect to 100 weight% of main components. The subcomponent may be included as an inevitable impurity in the raw material, for example.

  The subcomponents are specifically B, C, P, S, Cl, As, Se, Br, Te, I, Li, Na, Mg, Al, K, Ga, Ge, Sr, Cd, In , Sn, Sb, Ba, Pb, Bi, etc., and simple metals including transition metal elements such as Sc, Ti, Cr, Co, Ni, Cu, Y, Zr, Mo, Pd, Ag, Hf, Ta Or a compound is mentioned.

  Next, an example of a method for producing a ferrite composition according to this embodiment will be described.

  First, starting materials (raw materials of main components and raw materials of subcomponents) are weighed and mixed so as to have a predetermined composition ratio to obtain a raw material mixture. Examples of the mixing method include wet mixing using a ball mill and dry mixing using a dry mixer. It is preferable to use a starting material having an average particle size of 0.1 to 3 μm.

As a raw material for the main component, iron oxide (α-Fe ₂ O ₃ ), zinc oxide (ZnO), manganese oxide (Mn ₃ O ₄ ), or a composite oxide can be used. Furthermore, various compounds that become the above-described oxides or composite oxides by firing can be used. Examples of the oxide that becomes the above-mentioned oxide upon firing include simple metals, carbonates, oxalates, nitrates, hydroxides, halides, organometallic compounds, and the like. In addition, although the content of manganese oxide in the main component is calculated in terms of MnO, Mn ₃ O ₄ is preferably used as the main component material.

As the raw material of the subcomponent, as in the case of the raw material of the main component, not only an oxide but also a composite oxide or a compound that becomes an oxide after firing can be used. It is preferable to use SiO ₂ as a raw material for silicon oxide and CaCO ₃ as a raw material for calcium oxide. Further, it is preferable to use Nb ₂ O ₅ as a raw material for niobium oxide and V ₂ O ₅ as a raw material for vanadium oxide.

  Next, the raw material mixture is calcined to obtain a calcined material. Calcining causes thermal decomposition of raw materials, homogenization of ingredients, formation of ferrite, disappearance of ultrafine powder due to sintering and grain growth to an appropriate particle size, and converts the raw material mixture into a form suitable for the subsequent process. Done for. Such calcination is preferably performed at a temperature of 800 to 1100 ° C. for about 1 to 3 hours. The calcination may be performed in the air (air), or may be performed in an atmosphere having a higher oxygen partial pressure or in a pure oxygen atmosphere than in the air. The mixing of the main component raw material and the subcomponent raw material may be performed before calcining or after calcining.

  Next, the calcined material is pulverized to obtain a pulverized material. The pulverization is performed in order to break down the coagulation of the calcined material to obtain a powder having appropriate sinterability. There is no particular limitation on the pulverization method, but when the calcined material forms a large lump, it is preferable to perform wet pulverization using a ball mill or an attritor after coarse pulverization. When wet pulverization is performed, the average particle size of the calcined material is preferably about 1 to 2 μm.

  Next, the pulverized material is granulated (granular) to obtain a granulated product. The granulation is performed in order to convert the pulverized material into aggregated particles having an appropriate size and convert it into a form suitable for molding. The granulation method is not particularly limited, and examples thereof include a pressure granulation method and a spray drying method. The spray drying method is a method in which a commonly used binder such as polyvinyl alcohol is added to a pulverized material, and then atomized in a spray dryer and dried at a low temperature to obtain a granulated product.

  Next, the granulated product is molded into a predetermined shape to obtain a molded body. The molding method of the granulated product is not particularly limited, and examples thereof include a dry molding method, a wet molding method, and an extrusion molding method. The dry molding method is a molding method in which a granulated product is filled in a mold and compressed and pressed (pressed).

Next, the compact is fired to obtain a sintered body (a radio wave absorber composed of the ferrite composition of the present embodiment). This firing is performed in order to obtain a dense sintered body by causing sintering in which the powder adheres at a temperature below the melting point between the powder particles of the molded body containing many voids. Such firing is preferably performed at a temperature of 900 to 1300 ° C. for usually 2 to 5 hours. It is preferable to control the oxygen partial pressure during cooling after the main calcination. Specifically, it is preferable to control the oxygen partial pressure so that a nitrogen (N ₂ ) atmosphere is obtained at 1000 ° C. or lower. Incidentally, in the case of performing the sintering at 1000 ° C. or less is preferably before the start cooling to control the oxygen partial pressure so that nitrogen (N ₂₎ atmosphere.

  Through such a process, the radio wave absorber composed of the ferrite composition according to the present embodiment is manufactured.

  The radio wave absorber according to the present embodiment is preferably configured to have a plate-like tile shape in terms of radio wave absorption performance, but the shape is not limited and is cylindrical, pyramidal, conical, It may be configured to have a grid tile shape, a cross shape, or the like.

  As mentioned above, although embodiment of this invention was described, this invention is not limited to such embodiment at all, Of course, in the range which does not deviate from the summary of this invention, it can implement in various aspects. .

  Hereinafter, although this invention is demonstrated based on a more detailed Example, this invention is not limited to these Examples.

Experimental example 1
First, Fe ₂ O ₃ , ZnO, and Mn ₃ O ₄ were prepared as main components. SiO ₂ , CaCO ₃ , Nb ₂ O ₅ and V ₂ O ₅ were prepared as subcomponent materials.

  Next, the prepared raw material powders of the main component and subcomponent were weighed and then wet mixed by a ball mill for 5 hours to obtain a raw material mixture.

  Next, the obtained raw material mixture was calcined in air at 950 ° C. for 2 hours to obtain a calcined material, and then wet pulverized with a ball mill for 20 hours to obtain a pulverized material having an average particle diameter of 1.5 μm. Obtained.

Next, this pulverized material was dried, and then granulated by adding 1.0% by weight of polyvinyl alcohol as a binder to 100% by weight of the pulverized material, and granulated with a 20 mesh sieve to obtain granules. This granule was pressure-molded at a pressure of 196 MPa ( ² ton / cm ² ) to form a toroidal-shaped (dimension = outer diameter 24 mm × inner diameter 12 mm × thickness 6 mm) compact and columnar (dimension = diameter 25 mm, height 12 mm). ) Was obtained.

  Next, each of these molded bodies was fired at 1300 ° C. for 3 hours while appropriately controlling the oxygen partial pressure to obtain a toroidal core sample and a columnar sample as a sintered body. The obtained toroidal core sample was subjected to fluorescent X-ray analysis, and the composition of the toroidal core sample was measured. At this time, the presence or absence of abnormal grain growth was observed for each sample. For samples in which abnormal grain growth was observed, the following physical property values were not measured as unnecessary. The observation of abnormal grain growth was performed with a microscope (× 100 to 500). The results are shown in Table 1.

Latra value About the above-mentioned columnar sample, the Latra test was conducted to obtain the Latra value. The results are shown in Table 1. The ratra value is an index representing the chipping resistance of the ferrite composition. The smaller the ratra value, the better the chipping resistance. In this example, the ratra value was good at 0.50% or less.

The ratra test was performed as follows. First, the weight (W1) before the test of the three cylindrical samples used for the test was measured. Next, the three cylindrical samples were put in a pot (Ratra tester) having a diameter of about 10 cm having a baffle plate inside. Then, the pot was rotated under the conditions of a rotation speed of 100 rpm and a rotation time of 10 minutes. Thereafter, the weight (W2) of the three cylindrical samples was measured. The reduction rate of the weight of the columnar sample was determined, and this was used as the ratra value. That is, the ratra value was calculated by the following formula (4).
Latra value (%) = 100 × (W1-W2) / W1 (4)

Matching thickness A sample cut out from the above cylindrical sample into a toroidal shape having an outer diameter of 19.9 mm, an inner diameter of 8.7 mm, and a thickness of 7 mm was inserted into the coaxial tube, and a room temperature analyzer (Agilent 8753C) was used. 25 ± 1 ° C.) with physical constants μ _'r, μ' of each of the examples and comparative examples _{'r, ε' r, ε} ' was measured' _r. The normalized input impedance (Z _in ) was obtained by substituting the physical constants μ ′ _r , μ ″ _r , ε ′ _r , ε ″ _r into the equation (1).

The normalized input impedance (Z _in ) obtained from the physical property constants was plotted on a Smith chart. The material thickness d when the plot passes through the center of the Smith chart, that is, when there is a theoretically non-reflective frequency, is defined as the matching thickness d ₀ (mm). In this example, the matching thickness d ₀ was set to be 5.5 mm or less.

Return loss amount For each example and comparative example, the sample was cut into a toroidal shape having an outer diameter of 19.9 mm, an inner diameter of 8.7 mm, and a thickness of d ₀ (mm) from the above cylindrical sample, and inserted into the coaxial tube. In the state, the reflection coefficient S at 100 MHz, 200 MHz, and 400 MHz was measured at room temperature (25 ± 1 ° C.) using a network analyzer (Agilent 8753C).

  The obtained reflection coefficient S was substituted into equation (3) to calculate the return loss Ref (dB). In the present example, the return loss was good at 20.0 dB or more within the range of 100 to 400 MHz.

Initial permeability (1 kHz)
The toroidal core sample as the sintered body obtained by sintering the toroidal shaped (size = outer diameter 24 mm × inner diameter 12 mm × thickness 6 mm) room temperature (25 ± 1 ° C.) using an LCR meter. The initial permeability μi was measured at a frequency of 1 kHz.

As shown in Table 1, in the ferrite composition, the contents of Fe ₂ O ₃ , MnO and ZnO as main components, and the contents of SiO ₂ and CaO as subcomponents are within the scope of the present invention ( In Examples 1 to 20), it was confirmed that a high return loss of 20.0 dB or more was maintained in the frequency band of 100 MHz to 400 MHz.

In contrast, as shown in Table 1, in the ferrite composition, when the content of Fe ₂ O ₃ was smaller than the predetermined range (Comparative Example 1), the return loss at 400 MHz was deteriorated. Moreover, when the ZnO content was smaller than the predetermined range (Comparative Example 2), the return loss at 100 MHz deteriorated. Furthermore, when the ZnO content was larger than the predetermined range (Comparative Example 3), the return loss at 400 MHz deteriorated. Moreover, when the content of Fe ₂ O ₃ was larger than the predetermined range (Comparative Example 4), the return loss at 100 MHz was deteriorated.

Further, in the ferrite composition, when the content of SiO ₂ is smaller than the predetermined range (Comparative Example 5) and when the content of CaO is smaller than the predetermined range (Comparative Example 7), 100 MHz to 400 MHz. The return loss and ratra value in the area deteriorated. Abnormal grain growth was observed when the content of SiO ₂ was larger than the predetermined range (Comparative Example 6) and when the content of CaO was larger than the predetermined range (Comparative Example 8).

Furthermore, Example 5 (ZnO = 17.0 mol%), Example 6 (ZnO = 18.0 mol%), Example 7 (ZnO = ZnO =) in which only the concentration of ZnO was changed with Fe ₂ O ₃ = 43.0 mol%. 18.5 mol%), when the ZnO concentration was decreased, the return loss at a frequency of 400 MHz increased and the absorption characteristics tended to improve.

Further, as shown in Table 1, when Nb ₂ O ₅ was added within a predetermined range in the ferrite composition (Examples 14 to 16), the return loss at 100 to 400 MHz was Nb ₂ O ₅ . It improved from the case where it does not add (Example 6). Further, by adding _Nb 2 _{O 5} within a predetermined range, and you can adjust the matching thickness 5.0Mm～5.5Mm. When V ₂ O ₅ was added within a predetermined range (Examples 17 to 19), the ratra value was improved as compared with the case where V ₂ O ₅ was not added (Example 6). Further, by adding V ₂ O ₅ within a predetermined range, the alignment thickness could be adjusted to 5.0 mm to 5.5 mm. When both Nb ₂ O ₅ and V ₂ O ₅ are added (Example 20), the reflection attenuation at 100 to 400 MHz is compared with the case where Nb ₂ O ₅ and V ₂ O ₅ are not added (Example 6). The amount became equal or better, and the Latra value improved.

Experimental example 2
Example 6 (Fe ₂ O ₃ = 43.0 mol%, ZnO = 18.0 mol%), Example 8 (Fe ₂ O ₃ = 43.5 mol%, ZnO = 19.0 mol%) and Comparative Example 3 (Fe ₂ With respect to a sintered body sample having a composition of O ₃ = 45.5 mol% and ZnO = 20.5 mol%) and a matching thickness, the return loss was measured while changing the frequency between 10 and 1000 MHz, and the return loss was measured. Changes were observed. The measurement was performed under the same conditions as in Experimental Example 1 except for the frequency. The results are shown in FIG.

As shown in FIG. 1, when the concentrations of the main components Fe ₂ O ₃ and ZnO are decreased, the graph representing the change in the return loss shifts to the high frequency side. Then, the graph representing the change in the return loss shifts to the high frequency side, whereby the return loss at the frequency of 400 MHz increases and the absorption characteristics are improved.

Experimental example 3
From Example 8 of Experimental Example 1, a toroidal core sample and a columnar sample as a sintered body were obtained by changing the amount of subcomponents and the like so that the initial permeability at 1 kHz was increased, and Ref (dB) was matched. Thickness and ratra values were measured. The measurement results are shown in Table 2.

  In the ferrite composition according to Comparative Example 11, the absorption characteristics at the frequencies of 100 MHz and 400 MHz are significantly lower than 20 dB, and the Latra value is significantly higher than 0.5%. Therefore, the ferrite composition according to Comparative Example 11 is not preferable as a ferrite composition for a radio wave absorber.

Claims (7)

Have the iron oxide in terms _{of Fe} 2 _{O 3} at 39.0 to 45.0 mol%, zinc oxide containing 16.0 to 20.0 mol% in terms of ZnO, the main component consisting of the balance being manganese oxide And
With respect to the main component as 100 wt%, as an auxiliary component, at least 50 to 200 ppm by weight of silicon oxide in terms of SiO _2, a calcium oxide containing 200 to 2500 ppm by weight calculated as CaO,
Niobium oxide and / or vanadium oxide may be contained as an auxiliary component,
A ferrite composition for a radio wave absorber, wherein the content of subcomponents other than the main component and subcomponent described above is about several ppm to several hundred ppm by weight with respect to 100 wt% of the main component. The ferrite composition for a radio wave absorber according to claim 1, wherein vanadium oxide is contained as an auxiliary component in an amount of 50 to 500 ppm by weight in terms of V ₂ O ₅ . As an auxiliary component, the ferrite composition for electromagnetic wave absorber according to claim 1 or 2 containing 50 to 500 ppm by weight of niobium oxide calculated as _Nb 2 _{O 5.}   The electromagnetic wave absorber comprised from the ferrite composition in any one of Claims 1-3.   5. The radio wave absorber according to claim 4, wherein the reflection attenuation for a 100 MHz radio wave, the reflection attenuation for a 200 MHz radio wave, and the reflection attenuation for a 400 MHz radio wave are all 20 dB or more.   The radio wave absorber according to claim 4 or 5, wherein the matching thickness is 5.5 mm or less. The radio wave absorber according to any one of claims 4 to 6, which has a plate-like tile shape.

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