JP2006203147A - Radio wave absorption material and wave absorber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a radio wave absorption material with large frequency bandwidth, and to provide a 1/4 wavelength type wave absorber by using the electric wave absorption material. <P>SOLUTION: A conductor arrangement sheet, in which conductors, such as linear conductor 1 and cross-shaped conductor, are arranged on a dielectric sheet 2, and a conductive sheet 3 are laminated to form an radio wave absorption material. The radio wave absorption material is provided on one side face of dielectric layer with 1/4 wavelength thickness, and a metallic plate C is mounted on the other face to form a 1/4 wavelength type wave absorber. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a material that absorbs radio waves and a radio wave absorber.

Various methods of constructing the radio wave absorber are known. For example, a metal plate is attached to one side of a dielectric layer made of a dielectric material having a thickness d and no radio wave loss, and the other side surface of the dielectric layer is provided with a surface. A configuration method in which a conductive sheet having a resistance of 377 [Ω] is mounted as a radio wave absorbing material is well known as a quarter wavelength absorber (for example, Non-Patent Document 1).
Absorption and shielding of electromagnetic waves, Yasutaka Shimizu, Yuki Sugiura, Takeshi Ishino, page 135, Nikkei Technical Books. The conductive sheet refers to a sheet-like conductive material, and the sheet resistance refers to resistance between opposing sides of a 1 m square conductive sheet. When radio waves are incident on the surface of the conductive sheet, reflection becomes zero at a frequency at which d is ¼ of the wavelength, and all of the incident radio waves are absorbed by the conductive sheet. Called an absorber. The wavelength refers to the wavelength inside the dielectric layer having a thickness d. The dielectric layer is hereinafter referred to as a quarter wavelength layer.

  While the above-described ¼ wavelength absorber has an advantage that the configuration is simple, it has a disadvantage that the frequency band in which the reflection coefficient is small is narrow. An object of the present invention is to provide a radio wave absorber for forming a ¼ wavelength type radio wave absorber having a small reflection coefficient in a wide frequency band, and a radio wave absorber using the same.

Means for Solving the Invention

  As shown in FIG. 1, the radio wave absorbing material A has a conductor arrangement sheet in which linear conductors 1 are regularly arranged on one side surface of a dielectric sheet 2 and a sheet resistance of 400 [ A configuration in which conductive sheets 3 of Ω] to 4000 [Ω] are stacked as shown in FIG. 2, or a conductor arrangement sheet formed by regularly arranging linear conductors 1 inside the dielectric sheet 2 and a sheet resistance of 400. A structure in which the conductive sheets 3 of [Ω] to 4000 [Ω] are laminated as shown in FIG. 3, or two conductive sheets having a combined sheet resistance of 400 [Ω] to 4000 [Ω] are combined with the conductor array sheet. 4 is one of the configurations of FIG. 4 sandwiched between the sheets 3-1 and 3-2, and the radio wave absorption material A is laminated on the quarter wavelength layer B as shown in FIG. A radio wave absorber having a configuration in which a metal plate C is mounted on the other surface of the wavelength layer B is used. The linear conductor refers to a conductor having an aspect ratio (length / thickness) of 10 or more and can be regarded as a substantially straight line.

  The linear conductor 1 may be randomly distributed on the surface of the dielectric sheet or inside. Alternatively, the linear conductors 1 arranged on the surface or inside of the dielectric sheet 2 may be replaced with a cross-shaped conductor, or a quadrangular or circular conductor.

The invention's effect

  The effects of the present invention will be described below with reference to the drawings.

となる。An equivalent circuit when the radio wave absorbing material A having the configuration of FIG. 2 is viewed as a transmission line is as follows. It is assumed that the electric field of the incident wave radio wave is parallel to the linear conductor 1 as shown in FIG. Since the conductor array sheet its thickness is sufficiently thinner than the free-space wavelength in the transmission line theory can be expressed by the admittance Y _A lumped. For the same reason, the conductive sheet 3 can also be expressed by a concentrated constant resistance R _S. Since the radio wave absorption material A is a laminate of the conductor array sheet and the conductive sheet 3, when the characteristics of the radio wave absorption material A are expressed by a lumped constant Y _T , Y _T is a parallel circuit of Y _A and R _S ,

It becomes.

となる。Frequency characteristics of Y _A is as follows. When radio waves are incident on the conductor array sheet, a resonance phenomenon occurs at a frequency f ₀ at which the length L of the linear conductor 1 is approximately ½ of the wavelength, and the conductor array sheet on which the linear conductors 1 are arrayed is viewed as a homogeneous medium. It is known that the relative dielectric constant ε _A (= ε _A ′ −jε _A ″) at this time exhibits resonant dispersion characteristics as shown in FIG. 6 (for example, Patent Document 2).
Japanese Patent Application No. 2004-37487. When Y _A is expressed using ε _A ,

It becomes.

Number 2

Λ ₀ is the free space wavelength and d _A is the thickness of the conductor array sheet. Y ₀ is a characteristic admittance of free space, and Y ₀ = 1/377 [S].

となる。ε_０は空気の誘電率である。ε_Ｓ”は図６に点線で示すように周波数に反比例する特性を持つ。The relative dielectric constant ε _S (= ε _S ′ −jε _S ″) of the conductive sheet 3 is ε _S ′ << ε _S ″. The conductivity σ _S of the conductive sheet is σ _S = 1 / (R _S d _S ) where the thickness of the conductive sheet is d _S, and ε _S ″ and σ _S are the angular frequency (2π × frequency) ω Then, since there is a relational expression of ε _S “= σ _S / (ωε ₀ ),

It becomes. ε ₀ is the dielectric constant of air. As shown by the dotted line in FIG. 6, ε _S ″ has a characteristic inversely proportional to the frequency.

That is, when the radio wave absorbing material according to the present invention is viewed as a dielectric constant, the characteristic ε _S ″ that monotonically decreases with respect to the frequency and the characteristic ε _A that resonates at the frequency f ₀ , as shown in FIG. There is a feature with the characteristics that combine.

The effect of the radio wave absorption material according to the present invention on the radio wave absorption characteristics will be described using a standardized admittance chart. An equivalent circuit in the configuration of the radio wave absorber shown in FIG. 5 is shown in FIG. The admittance Y _T representing the radio wave absorption material A is connected in parallel to the input admittance Y _B of the quarter wavelength layer B, and the input impedance Y _in viewed from the surface of the radio wave absorption material A is Y _in = Y _T + Y _B .

In normalized admittance chart of FIG. 8, b is the characteristic of Y _B whose frequency is normalized in f ₂ from f _1. Since the quarter-wave layer B is a layer made of a dielectric material with no radio wave loss, the standardized Y _B is on the outer periphery of the chart, and the thickness of the quarter-wave layer B is exactly 1 at the frequency f ₀ . If it becomes / 4 wavelength, Y _{B at} f ₀ is zero.

で与えられる。規格化アドミッタンスチャートの中心の値は１、すなわちＹ_ｉｎ＝Ｙ_０であり、Γ＝０となる。ロに示される特性はｆ_０でΓ＝０となり、入射電波は反射されず全て導電シートに吸収される。図８にロで示される特性の反射係数「は図９に点線で示すようになり、周波数がｆ_０から離れるにつれてΓは増加する。First, the case where only the conductive sheet which is the structure of the conventional electromagnetic wave absorber is attached to the quarter wavelength layer B will be described. B is the normalized Y _in characteristic when a conductive sheet having a surface resistance R _S of R _S = 377 [Ω] is mounted on the quarter wavelength layer B. Y _in is given by Y _B + 1 / _RS , and since Y _B = 0 at f ₀ , Y _in = 1/377 [S], that is, Y _in = Y ₀ . The relationship between Y _in and the reflection coefficient Γ is

Given in. The value at the center of the normalized admittance chart is 1, that is, Y _in = Y ₀ and Γ = 0. The characteristic indicated by B is Γ = ₀ at f ₀ , and the incident radio wave is not reflected and is completely absorbed by the conductive sheet. The reflection coefficient “of the characteristic shown by B in FIG. 8 becomes as shown by the dotted line in FIG. 9, and Γ increases as the frequency goes away from f ₀ .

  The electromagnetic wave absorbing material A according to the present invention has an equivalent circuit.

Number 1

Represented by the _{Y T.}

Number 1

,

Number 2

As can be seen from the equation, the real part of Y _T is given by Re (Y _A ) + 1 / R _S. Since Re (Y _A )> 0, in order to set Y _in = Y ₀ , (1 / R _S ) <Y ₀ , that is, R _S > (1 / Y ₀ ) = 377 [Ω] must be satisfied.

When the input admittance Y _in when only the conductive sheet 3 of _{RS in} the above range is laminated on the quarter wavelength layer B is shown _{in the} standardized admittance chart, for example, as shown in FIG. When added with Y _A is the effect of the conductor array sheet to the characteristics indicated by C, that is, when the radio wave absorbing material A according to the present invention was laminated to a quarter wavelength layer B is as shown in two of FIG.

When the radio wave absorbing material according to the present invention is used, the input admittance changes so as to surround the center of the normalized admittance chart, that is, the point where Γ = 0 near f ₀ , and the frequency range where Γ becomes a small value is widened. This characteristic is caused by the frequency characteristic of Y _A due to the frequency change of ε _A shown in FIG. The values obtained by converting the characteristics shown in FIG. 8D into the reflection coefficient Γ are shown in FIG. Obviously the gamma is W of W _R and two properties of b characteristic is a frequency range of gamma ₀ or less _A is W _{R <W A.}

The imaginary part ε _A ″ of the equivalent dielectric constant of the conductor array sheet can be set in the range of about 10 to 1000 by changing the array density of the linear conductors 1 according to experiments. Suppose ε _A ″ = 100 and d _A = 0.1 mm

Number 2

The real part of 5GH _Z in _{Y A} from equation is approximately _{Y 0.} Therefore, R _S = ∞ [Ω] can be designed only to obtain Γ = 0 at f ₀ , but as a result of repeated experimental studies, R _S is 400 [Ω] <R _S <4000 [Ω. ], It has been found that a desirable result can be obtained that a frequency range in which Γ is small can be widened.

As described above, it has been explained that a ¼ wavelength type wave absorber having a broadband wave absorption characteristic can be formed by taking the wave absorbing material having the configuration of FIG. 2 as an example. However, as shown in FIG. 2 is equivalent to ε _A shown in FIG. 6 because the equivalent dielectric constant due to the resonance phenomenon of the linear conductor is also equivalent to the ε _A shown in FIG. The same effect can be obtained.

As shown in FIG. 4, in the configuration in which the conductor arrangement sheet is sandwiched between the two conductive sheets 3-1 and 3-2, the conductive sheets 3-1 and 3-2 are arranged in parallel in the transmission line. Since it becomes an equivalent circuit to be connected, the combined sheet resistance is 1 / (1 / R _S1 + 1 / R _S2 ) when the sheet resistances of the conductive sheets 3-1 and 3-2 are R _S1 and R _S2. It is. Therefore, if the composite surface resistance is in the range of 400 [Ω] <1 / (1 / R _S1 + 1 / R _S2 ) <4000 [Ω], the two conductive sheets shown in FIG. The same effect can be obtained.

Linear conductor 1 conductor array sheet formed by arranging on the surface of the dielectric sheet 2 must align the resonance frequency of the linear conductor 1 on f _0. The resonance frequency of the linear conductor 1 can be estimated as a frequency at which the length L is approximately ½ of the free space wavelength. Strictly speaking, not only the length L of the linear conductor but also the circumference of the linear conductor It varies depending on the thickness and relative dielectric constant of the dielectric sheet 2 as the medium, and the sheet resistance and thickness of the conductive sheet. In order to set the resonance frequency of the linear conductor 1 to f ₀ , L is determined experimentally.

  All of the conductor arrangement sheets described in the above description are obtained by arranging the linear conductors 1 and the electric field of the incident wave radio wave is assumed to be parallel to the linear conductors 1 as shown in FIG. When an actual application is considered, the electric field of the incident wave radio wave is not always parallel to the linear conductor 1. In order to obtain the effect of the present invention regardless of the direction of the electric field of the incident wave radio wave, for example, as shown in FIG. 10, a cross-shaped conductor in which two linear conductors are orthogonal to each other is arranged to arrange the conductor. There is a method for forming a sheet, or a method for forming a conductor arrangement sheet by arranging linear conductors in random directions as shown in FIG. 11, both of which are the same as the conductor arrangement sheet shown in FIG. Since a resonance phenomenon occurs, the radio wave absorbing material of the present invention can be configured.

  Furthermore, the shape of the conductor that causes the resonance phenomenon is not limited to a linear shape. For example, even if a rectangular or circular conductor is arranged, it is known that the resonance phenomenon occurs at a specific frequency determined by the length or diameter of the side. It has been. Therefore, the shape of the conductor used for the radio wave absorbing material of the present invention is not limited to a linear shape or a cross shape, and for example, a rectangular or circular foil conductor may be arranged.

  The material of the conductor arranged in the dielectric sheet 2 may be substantially a good electrical conductor, and various metals, fibrous materials plated with various metals, carbon fibers, or metal powder or carbon powder as a main component. Conductive paint can be used.

In the above description, the quarter wavelength layer B is configured by using a dielectric material having no radio loss. However, even a dielectric material having some loss may be used as the quarter wavelength layer B of the present invention. Is possible. When a dielectric having radio loss is used as the ¼ wavelength layer B, the input impedance Y _B is inside the admittance chart, unlike the a on the outer periphery of the admittance chart shown in FIG. Re (Y _B ), which is the value of the real part of Y _B , increases as the radio loss increases. The maximum value of the sheet resistance of the electromagnetic wave absorbing conductive sheet 3 according to the present invention is 4000 [Ω], which is approximately 0.1 when converted to the real part of the standardized admittance. Further, the real part of the admittance Y _A due to ε _A ″ of the conductor array sheet is approximately 0.1 when ε _A ″ = 10 and f ₀ = 5 GHz, and Y _in / Y ₀ = 1 which is the condition of Γ = 0. To achieve this, Re (Y _B ) = 1−0.1−0.1 = 0.8. Therefore, any quarter wavelength layer satisfying Re (Y _B / Y ₀ ) <0.8 can be used.

  A cross-shaped conductor made of a metal having good electrical conductivity is arranged on a dielectric sheet such as polyethylene to form a conductor arrangement sheet, and a conductive paint mainly composed of carbon powder is applied to the dielectric sheet to provide a sheet resistance of 400 [ The conductive sheet 3 of Ω] to 4000 [Ω] is formed, and the radio wave absorbing material A is configured by laminating the conductor arrangement sheet and the conductive sheet 3.

  A dielectric layer such as foamed resin having a low radio wave loss is used as the quarter-wave layer B, a metal plate is provided on one side of the quarter-wave layer B, and the above-described radio wave absorption is provided on the other side. A quarter wavelength absorber is formed by providing a material.

  A linear conductor obtained by cutting a fibrous conductor having a thickness of 110 μm plated with silver into a length of 12 mm was regularly arranged on a polyethylene film having a thickness of 120 μm to obtain a conductor arrangement sheet. The arrangement interval in the horizontal direction of the fibrous conductor was 20 mm, and the arrangement interval in the fibrous conductor axial direction was 35 mm. Further, a conductive paint mainly composed of carbon was applied to a polyethylene film having a thickness of 0.2 mm to produce a conductive sheet having a surface resistance of 698 [Ω]. The conductor array sheet and the conductive sheet were laminated to obtain a radio wave absorbing material.

  A quarter-wave absorber was constructed by using a foamed resin having a thickness of 7 mm as a quarter-wave layer, mounting the above-described radio wave absorbing material on one side, and mounting a metal plate on the other side.

In FIG. 12, Y _in / Y ₀ that is a value obtained by standardizing the measured value of the input admittance Y _in viewed from the surface of the radio wave absorbing material is indicated by the symbol “black square”. The measurement frequency was 4 GHz to 12 GHz. It can be seen that Y _in / Y ₀ changes so as to surround the center of the chart as in FIG. FIG. 13 shows the characteristics shown in FIG. 12 in terms of the reflection coefficient Γ. However, it is expressed in units of dB. f ₀ is 8 GHz, and has the characteristic that the reflection coefficient becomes small at two frequencies in the same manner as the characteristics shown in FIG. 9. For example, when looking at the frequency band where the reflection coefficient is -10 dB or less, 5 GHz to 12 GHz or more. It was realized over a wide frequency band, and good radio wave absorption performance was obtained in a wide band.

The figure of the conductor arrangement | sequence sheet | seat which comprises the electromagnetic wave absorption material of this invention. The figure which shows the structure of the electromagnetic wave absorption material of this invention. The figure which shows the structure of the electromagnetic wave absorption material of this invention. The figure which shows the structure of the electromagnetic wave absorption material of this invention. The figure which shows the structure of the electromagnetic wave absorber of this invention. The figure which shows the equivalent dielectric constant of a conductor arrangement | sequence sheet. The figure which shows the transmission line equivalent circuit of the electromagnetic wave absorber of this invention. The figure which shows the normalized input admittance of the electromagnetic wave absorber of this invention. The figure which shows the reflection coefficient of the electromagnetic wave absorber of this invention. The figure which shows the conductor arrangement | sequence sheet which arranged the cross-shaped conductor. The figure which shows the conductor arrangement | sequence sheet which arranged the linear conductor in the random direction. The figure which shows the normalized input admittance of the electromagnetic wave absorber of an Example. The figure which shows the reflection coefficient of the electromagnetic wave absorber of an Example.

Explanation of symbols

1: Linear conductor 2: Dielectric sheet 3: Conductive sheet A: Electromagnetic wave absorbing material B: 1/4 wavelength layer C: Metal plate A: 1/4 wavelength layer normalized input admittance slot: 1/4 wavelength layer Normalized input admittance slot when a conductive sheet is attached Reflection coefficient when a conductive sheet is attached to a quarter wavelength layer C: Normalized input admittance when a conductive sheet is attached to a quarter wavelength layer: 1 / 4 standardized input admittance when the wave absorbing material of the present invention is mounted on the ¼ wavelength layer: reflection coefficient when the wave absorbing material of the present invention is mounted on the ¼ wavelength layer

Claims (5)

A conductor arrangement sheet in which linear conductors or cross-shaped conductors are regularly arranged on or inside a dielectric sheet and a conductive sheet having a sheet resistance of 400 [Ω] to 4000 [Ω] are laminated. Radio wave absorbing material. It is a configuration in which a conductor arrangement sheet in which linear conductors or cross-shaped conductors are regularly arranged inside a dielectric sheet is sandwiched between two conductive sheets each having sheet resistance R _S1 and R _S2 , An electromagnetic wave absorbing material, wherein the sheet resistance of the two conductive sheets is 400 [Ω] <1 / (1 / R _S1 + 1 / R _S2 ) <4000 [Ω]. 3. The radio wave absorbing material according to claim 1, wherein the conductor array sheet is formed by randomly dispersing linear conductors on the surface or inside of the dielectric sheet. 2. The radio wave absorbing material according to claim 1, wherein the electromagnetic wave absorbing material is a conductor array sheet in which rectangular or circular foil conductors are arrayed on the surface or inside of the dielectric sheet. A radio wave absorption comprising the radio wave absorbing material according to claim 1, 2 and 3 mounted on one side of a dielectric layer having a constant thickness, and a metal plate mounted on the other side of the dielectric layer. body.

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