JP2023000572A - Electromagnetic wave absorber - Google Patents

Abstract

To provide an electromagnetic wave absorber that is easy to be manufactured without substantially affecting the electromagnetic wave absorption action by the presence or absence of a backing metal plate.SOLUTION: An electromagnetic wave absorber 1 that absorbs radio waves includes a dielectric layer 2, and a conductive material layer 3 provided on the dielectric layer 2, and the thickness of the conductive material layer 3 is greater than the skin depth with respect to the frequency of the radio waves.SELECTED DRAWING: Figure 1

Description

The present invention relates to a radio wave absorber that prevents reflection of radio waves, and more particularly to a sheet-like radio wave absorber that prevents reflection of millimeter waves.

An example of the structure of a conventional radio wave absorber 101 is shown in FIG. Radio wave absorber 101 includes resistive film 102 provided on the radio wave incident side, dielectric layer 103 provided on resistive film 102 , and metal plate 104 provided on dielectric layer 103 . That is, the radio wave absorber 101 is lined with a metal plate 104 on the back side when viewed from the incident surface. Since the metal plate 104 has the property of totally reflecting radio waves and does not pass through them, the absorption characteristics of the radio wave absorber 101 will not be affected regardless of the material of the object to which the radio wave absorber 101 is attached. Therefore, it is possible to maintain stable radio wave absorption characteristics. In other words, the radio wave absorber 101 is designed on the assumption that the backing metal plate 104 having the properties described above exists, and on this precondition, it is possible to obtain good radio wave absorption characteristics.

Among radio-related technologies that are being put to practical use in various bands such as IoT and 5G, the millimeter-wave band is attracting attention because it can be used for applications that require large amounts of information and speed, such as radar. As radio wave absorbers for the millimeter wave band, characteristics such as lightness, thinness, and ease of handling are emphasized, and for reasons such as weight, simplification of the manufacturing process, cost, and appearance, it is desirable that there be no backing metal plate. . However, if the backing metal plate 104 is removed from the radio wave absorber 101 shown in FIG. 11, the above preconditions are not satisfied. There is a drawback that the characteristics change.

On the other hand, Patent Literature 1 discloses a transmissive electromagnetic wave absorber that does not have a backing metal plate. FIG. 12 shows the structure of a transmissive radio wave absorber 201 disclosed in Patent Document 1. As shown in FIG. The transmission-type radio wave absorber 201 includes a dielectric layer 202 formed on the radio wave incident side, and an impedance layer 203 formed by forming a conductor into a thin film. The dielectric constant and thickness of the dielectric layer 202 and the impedance of the impedance layer 203 cancel out the radio waves reflected on the surface of the dielectric layer 202 facing the impedance layer 203 with the radio waves reflected on the surface of the dielectric layer 202. is set to A part of the radio wave transmitted from the dielectric layer 202 to the impedance layer 203 is absorbed by the impedance layer 203 and the rest is transmitted to the region of the impedance layer 203 opposite to the dielectric layer 202 .

Patent No. 3556618

As is clear from the equivalent circuit shown in FIG. 2 of Patent Document 1, in the transmissive radio wave absorber 201, the impedance layer 203 is a thin film. It is a transparent structure. Therefore, if there is an object (such as a metal plate) that reflects radio waves on the back side of the transmissive radio wave absorber 201, radio waves are reflected on the radio wave incident side of the transmissive radio wave absorber 201, resulting in radio wave absorption. It lacks practicality as a body.

On the other hand, when suppressing radio waves passing through the transmissive radio wave absorber 201 so that the presence or absence of the backing metal plate does not substantially affect the radio wave absorption action, the dielectric constant of the dielectric layer 202 is made very high and the dielectric Layer 202 should be made very thin. For example, as shown in FIG. 3 of Patent Document 1, when suppressing the radio wave (transmission power coefficient T _p ) passing through the transmissive radio wave absorber 201 to 1/100 (−20 dB), the relative dielectric of the dielectric layer 202 The modulus must be 100 and the thickness must be 1/40 of the radio wave wavelength. If so, the degree of freedom in selecting the material for forming the dielectric layer 202 is reduced, making it difficult to manufacture the transmissive electromagnetic wave absorber 201 .

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above problems, and to provide a radio wave absorber that is easy to manufacture and that the presence or absence of a backing metal plate does not substantially affect the radio wave absorption action. .

In order to solve the above problems, the present invention includes the following aspects.
Section 1.
A radio wave absorber that absorbs radio waves,
a dielectric layer;
a conductive material layer provided on the dielectric layer;
A radio wave absorber, wherein the thickness of the conductive material layer is larger than the skin depth with respect to the frequency of the radio wave.
Section 2.
Item 2. The radio wave absorber according to Item 1, wherein the electrical conductivity of the conductive material layer is 10 S/m to 200 S/m.
Item 3.
The dielectric layer is
a resin base material;
Item 3. The electromagnetic wave absorber according to Item 1 or 2, further comprising a metal thin film having a specific shape disposed on the surface of the resin base material opposite to the conductive material layer.
Section 4.
Item 4. The radio wave absorber according to any one of Items 1 to 3, further comprising an additional dielectric layer provided on a surface of the conductive material layer opposite to the dielectric layer.
Item 5.
Item 5. The radio wave absorber according to any one of Items 1 to 4, wherein the radio waves are millimeter waves.

According to the present invention, the presence or absence of the backing metal plate does not substantially affect the radio wave absorption action, and a radio wave absorber that is easy to manufacture can be provided.

1 is a cross-sectional view of a radio wave absorber according to one embodiment of the present invention; FIG. (a) to (d) are configuration examples of artificial dielectrics. FIG. 4 is an explanatory diagram of a reflection coefficient chart; _{10 is a graph showing the relationship between the thickness d1 of the conductive material layer and the electrical conductivity σ at which the transmission coefficient |T|=0.07 when the radio wave frequency f is set to 40 GHz and the relative magnetic permeability is set to 1} ; It is an example of drawing a reflection coefficient on a reflection coefficient chart. (a) and (b) are chart examples of reflection coefficients at frequencies of 10 GHz to 100 GHz when the conductivity σ is 10 S/m and 100 S/m, respectively. (a) to (c) show the conductivity σ, the skin depth δ, and the thickness d ₁ at which the transmission coefficient |T|=0.07 when the radio wave frequencies are 30 GHz, 40 GHz, and 100 GHz, respectively It is a graph showing the relationship. FIG. 10 is a cross-sectional view of a radio wave absorber according to a modified example; (a) and (b) show a reflection coefficient chart at frequencies of 40 GHz to 54 GHz and the frequency dependence of the reflectance when the metal plate is on the back surface of the radio wave absorber. (a) and (b) show the reflection coefficient chart at frequencies from 40 GHz to 54 GHz and the frequency dependence of the reflectance without the metal plate. 1 is a cross-sectional view of a conventional radio wave absorber; FIG. FIG. 10 is a cross-sectional view of another conventional radio wave absorber;

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In addition, this invention is not limited to the following embodiment.

[Outline configuration]
FIG. 1 is a cross-sectional view of a radio wave absorber 1 according to one embodiment of the invention. A radio wave absorber 1 includes a dielectric layer 2 and a conductive material layer 3 . The radio wave absorber 1 is formed in a rectangular sheet shape in plan view, but the shape and size can be appropriately changed according to the installation location. Further, the radio wave absorber 1 is designed mainly to absorb millimeter waves, but applicable radio waves are not limited to millimeter waves.

The dielectric layer 2 is provided on the radio wave incident side and has a function of reducing reflection of radio waves on the conductive material layer 3 . If the wave impedance of the space where the radio waves arrive and the characteristic impedance of the material into which the radio waves enter are greatly different, the reflection on the surface of the material into which the radio waves enter becomes large. If the dielectric layer 2 were not provided, the difference between the wave impedance (377Ω) of the space and the characteristic impedance of the conductive material layer 3 would increase the reflection at the conductive material layer 3, and would not function as a radio wave absorber. The dielectric layer 2 functions as a layer that matches the difference between the spatial wave impedance and the characteristic impedance of the conductive material layer 3 and reduces the reflection on the conductive material layer 3 .

The dielectric layer 2 may be an artificial dielectric. Fig. 2 shows a configuration example of an artificial dielectric.

The dielectric layer 2a shown in FIG. 2(a) is an artificial dielectric obtained by mixing a resin base material 20 with a metal powder 21 having a particle size of 1/10 or less of the radio wave wavelength in a weight ratio of 5% to 60%. Thereby, even if the dielectric constant of the resin base material 20 itself is not high, the dielectric layer 2a can realize a high dielectric constant, which is advantageous in manufacturing.

In the above method, the metal powder 21 is evenly dispersed in the resin base material 20 to achieve a high dielectric constant. As another method, as shown in FIGS. 2(b) to 2(d), metal thin films 22, 23, 24 having a specific shape are applied to the surface of the resin base material 20 (conductive material layer) instead of the metal powder 21. 3) and forming a thin high dielectric constant layer can also increase the dielectric constant of the entire dielectric layer. In this case, by making the metal thin films 22, 23, 24 sufficiently thin with respect to the wavelength, they can be regarded as part of the dielectric layer.

The dielectric layers 2b, 2c, and 2d shown in FIGS. 2(b) to 2(d) are artificial dielectrics in which metal thin films having a specific shape are periodically arranged on the surface of the resin base material 20. As shown in FIG. In the dielectric layer 2b shown in FIG. 2B, circular metal thin films 22 are arranged on the surface of the resin base material 20. As shown in FIG. The diameter D of the metal thin film 22 is preferably 1/5 or less of the radio wave wavelength. In the dielectric layer 2c shown in FIG. 2(c), cross-shaped metal thin films 23 are arranged on the surface of the resin base material 20. As shown in FIG. The side length L1 of the metal thin film 23 is preferably _1/5 or less of the radio wave wavelength. In the dielectric layer 2c shown in FIG. 2D, square metal thin films 24 are arranged on the surface of the resin base material 20. As shown in FIG. _The side length L2 of the metal thin film 24 is preferably 1/5 or less of the radio wave wavelength.

These dielectric layers 2b, 2c, and 2d are also advantageous in terms of manufacturing because they can achieve a high dielectric constant even if the dielectric constant of the resin base material 20 itself is not high. Note that the dielectric constant _εr2 of the dielectric layer 2 will be described later.

The conductive material layer 3 is provided on the dielectric layer 2 and has a function of reflecting and absorbing radio waves incident from the dielectric layer 2 . A radio wave incident on the conductive material layer 3 generates a current in the conductive material layer 3 . The conductive material layer 3 functions as electrical resistance to convert current into heat and absorb the energy of incident radio waves. The higher the conductivity of the conductive material layer 3, the thinner the layer thickness can be while maintaining the radio wave absorption function. becomes larger. Therefore, in order to realize a high-performance radio wave absorber, it is necessary to appropriately design the thickness and conductivity of the conductive material layer 3 .

In this embodiment, the thickness d1 of the conductive material layer ₃ is designed to be larger than the skin depth δ with respect to the frequency f of radio waves. The skin depth δ is the length in the thickness direction at which the electromagnetic field incident on the conductive material layer 3 is attenuated to 1/e. The conductivity σ of the conductive material layer 3 is 10 S/m to 200 S/m, preferably 20 S/m to 100 S/m, more preferably 30 S/m to 70 S/m. A specific design method will be described below.

[Design method]
A method of designing a combination of the conductivity σ (S/m) and thickness d ₁ (m) of the conductive material layer 3 and the dielectric constant ε _r2 of the dielectric layer 2 will be described below. For convenience, it is effective to design using a chart, so the chart will be outlined first.

(chart)
A reflection coefficient is used as an index for evaluating the function of a radio wave absorber. Since the reflection coefficient is represented by a complex number including phase, it is convenient to use a reflection coefficient chart as shown in FIG.

In the chart, the radius of the circle (maximum 1) represents the magnitude (absolute value) of the reflected wave, and the angle represents the phase. In other words, at the right end (Γ=1−j0) of the circle, the phase is reflected as it is. 0±j1), it indicates that the reflection was performed with a phase difference of ±π/2. The performance of the radio wave absorber is superior when the reflection coefficient Γ=0−j0, that is, closer to the center of the circle.

The electromagnetic wave absorber 1 has a structure in which the presence or absence of the backing metal plate does not substantially affect the electromagnetic wave absorption action. Therefore, in this embodiment, the radio wave absorber 1 is designed so that the radio wave absorption amount is maintained at 20 dB or more regardless of the presence or absence of the backing metal plate, and the transmission amount is -20 dB or less in the absence of the backing metal plate. to design.

(Examination of conductivity range)
The conductivity σ can be set from the transmission coefficient when the dielectric layer 2 does not exist (that is, only the conductive material layer 3 exists). The reason why the transmission coefficient is used as a reference is that by setting the conductivity σ so that the transmission coefficient is low, the structure can be approximated to a configuration without a backing metal plate.

Since the dielectric layer 2 is laminated as a matching layer on the conductive material layer 3, in addition to the "component transmitted through the dielectric layer 2", although slight, "the component reflected by the conductive material layer 3 and the dielectric A component re-reflected from the layer 2 also enters the conductive material layer 3 . Therefore, it is expected that the transmission amount will increase as compared with the case where only the conductive material layer 3 is considered. Therefore, a margin is provided and the transmission coefficient |T| of only the conductive material layer 3 is set to 0.07 (-23 dB) instead of 0.1 (-20 dB).

First, the transmission coefficient T is derived from the conductivity σ. The dielectric constant ε _r1 of the conductive material layer 3 is represented by the following formula (1).

となり、以下の式（２）のように近似できる。

Note that ω=2πf. Furthermore, by setting the conductivity σ of the conductive material layer 3 to be large,

and can be approximated by the following equation (2).

At this time, radio wave propagation inside the conductive material layer 3 is considered. The propagation constant γ is expressed by the following equation (3) using the magnetic permeability _μ0 of the vacuum and the magnetic permeability _μr1 of the conductive material layer 3 when the dielectric constant _εr1 is given by the above equation (2). be.

と表される。 Here, the skin depth δ of the conductive material layer 3 is

is represented.

The specific impedance Z1 of the conductive material layer ₃ is expressed by the following equation (4) using the wave impedance _Z0 in free space.

ただし、Ａ＝Ｄ＝ｃｏｓｈ（γｄ_１）、Ｂ＝Ｚ_１ｓｉｎｈ（γｄ_１）、Ｃ＝ｓｉｎｈ（γｄ_１）／Ｚ_１である。 According to transmission line theory, the transmission coefficient T is expressed by the following equation ( ₅ ) using the propagation constant γ and the specific impedance Z1.

However, A=D=cosh(γd ₁ ), B=Z ₁ sinh(γd ₁ ), C=sinh(γd ₁ )/Z ₁ .

Therefore, by setting a target |T|, the thickness d1 can be derived as _a function of the conductivity σ. As an example, FIG. 4 shows the result of calculating d1 at |T|=0.07 when the radio frequency f is set to 40 GHz and the relative magnetic permeability is set to _1. In FIG.

From FIG. ₄ , it can be seen that the thickness d1 can be reduced (thinner) as the conductivity σ of the conductive material layer 3 is increased. That is, the larger the conductivity _σ , the more the electromagnetic wave absorber 1 is made thinner. /m to 200 S/m, more preferably 20 S/m to 100 S/m, and more preferably 30 S/m to 70 S/m. Thereby, the influence of the backing metal plate can be avoided.

(permittivity setting)
Next, the setting of the dielectric layer 2 that maximizes the absorption performance of the radio wave absorber 1 with respect to the set conductivity σ will be described.

First, by using the same coefficients A to D as in the above equation (5), the reflection coefficient (complex number) Γ _L ' in the conductive material layer 3 (when the dielectric layer 2 does not exist) is calculated from the following equation (6): can be derived.

FIG. 5 is an example of the reflection coefficient Γ _L ' plotted on a reflection coefficient chart. _Phase control ( rotation) is performed. Therefore, assume a circle passing through Γ _L ′ and the center of the chart, and let Γ _L be the point where the circle intersects the real axis of the chart. The midpoint between Γ _L and the center of the chart is represented by Γ _L /2, which is Γ _L0 . As can be seen from the positional relationship, _ΓL0 is a real number.

At this time, the dielectric constant ε _r2 of the dielectric layer 2 is represented by the following equation (7).

Since Γ _L is a real number, the permittivity ε _r2 is also a real number. This corresponds to assuming a lossless material for the dielectric layer 2 . However, this is a matter of handling values for designing, and although the dielectric layer 2 actually has even a slight loss, it does not impair the performance of the radio wave absorber 1 according to this embodiment.

FIGS. 6(a) and 6(b) are chart examples of reflection coefficients Γ _L ' at frequencies of 10 GHz to 100 GHz when the conductivity σ is 10 S/m and 100 S/m, respectively. In the figure, numbers shown with arrows represent frequencies (GHz). Comparing these figures, it can be seen that increasing the conductivity σ increases the dielectric constant _εr2 of the dielectric layer 2 derived from the equation (7).

In FIG. 6A, when the frequency is 40 GHz, the point where the circle passing through Γ _L ′ and the center of the chart intersects the real axis is Γ _L =−0.5, and the dielectric constant ε _r2 =3 from equation (7). .0. Also, in FIG. 6B, when the frequency is 40 GHz, Γ _L =−0.78, and the dielectric constant ε _r2 =8.1 from equation (7).

Thus, once the conductivity σ and the frequency f are set, the dielectric constant _εr2 is derived by plotting the reflection coefficient _ΓL ′ on a chart to obtain _ΓL and applying equation (7) be able to. Table 1 shows the dielectric constant ε _r2 when the conductivity σ=10, 20, 30, 40, 70, 100, 200 S/m and the frequency f=30, 40, 60, 80, 100 GHz.

From this result, when the radio waves are generally used millimeter waves of 30 GHz to 100 GHz, by setting the conductivity σ of the conductive material layer 3 to 10 S / m to 200 S / m, the dielectric constant ε _r2 is 2. A dielectric layer 2 of 0.35 to 13.93 can be used. Furthermore, by setting the conductivity σ of the conductive material layer 3 to 20 S/m to 100 S/m, the dielectric layer 2 having a dielectric constant _εr2 of 2.87 to 9.96 can be used. Furthermore, by setting the conductivity σ of the conductive material layer 3 to 30 S/m to 70 S/m, the dielectric layer 2 having a dielectric constant _εr2 of 3.35 to 8.39 can be used. A dielectric layer having such a dielectric constant _εr2 can be manufactured relatively easily by forming the dielectric layers 2a to 2d shown in FIG. 2, for example. In particular, when the conductivity σ of the conductive material layer 3 is set to 30 S/m, there is an advantage that a commercially available glass epoxy material (FR-4) can be used as the dielectric layer.

(Setting the thickness of the conductive material layer)
The thickness d1 of the conductive material layer ₃ can be set as follows.

7A to 7C show the conductivity σ, the skin depth δ, and the thickness d at which the transmission coefficient |T| = 0.07 when the radio frequency f is 30 GHz, 40 GHz, and 100 GHz, respectively. ₁ . _If the thickness d1 is too small, radio waves will not be attenuated in the conductive material layer 3, so that |T| will become large and the influence of the backing metal plate cannot be ignored. Therefore, if d ₁ >δ, the radio wave is sufficiently attenuated in the conductive material layer 3, and the conductivity σ is in the range of 10 S/m to 200 S/m, at any frequency of 30 GHz, 40 GHz, and 100 GHz. The transmission coefficient |T| can be made small enough not to be affected by the backing metal plate.

Also, as the thickness _d1 increases, the transmission coefficient |T| decreases. Therefore, if d ₁ <5δ, it is considered that there is no practical problem.

As described above, the thickness d ₁ of the conductive material layer 3 is preferably δ<d ₁ <5δ based on the skin depth δ. Of course, the thickness d ₁ is not limited to this range as long as there is no problem with the manufacturing process or installation location, and if there is no problem with manufacturing or practical use, d ₁ ≧5δ may be set.

(Brief Summary)
In the radio wave absorber ₁ according to this embodiment, since the thickness d1 of the conductive material layer 3 is larger than the skin depth with respect to the frequency of radio waves, the radio waves are sufficiently attenuated in the conductive material layer 3 . Therefore, even if there is a backing metal plate, it is possible to prevent the radio wave transmitted through the radio wave absorber 1 from being reflected on the backing metal plate and adversely affecting other places. Moreover, it is not necessary to set the dielectric constant of the dielectric layer 2 to a range that limits the degree of freedom in material selection. Therefore, the presence or absence of the backing metal plate does not substantially affect the radio wave absorption action, and a radio wave absorber that is easy to manufacture can be provided.

On the other hand, in Patent Document 1, since the impedance layer 203 is a thin film, radio waves are hardly attenuated in the impedance layer 203 . Therefore, in order to prevent the presence or absence of the backing metal plate from substantially affecting the radio wave absorption action, it is necessary to increase the dielectric constant of the dielectric layer 202 and to form the dielectric layer 202 very thin. be.

Furthermore, in the radio wave absorber 1 according to this embodiment, the dielectric constant ε _r2 of the dielectric layer 2 is set to 2.35 to 13.5 by setting the conductivity σ of the conductive material layer 3 to 10 S/m to 200 S/m. 93 can be set. Therefore, manufacturing of the dielectric layer 2 becomes easier.

In the millimeter wave band, radio wave absorbers are installed and used in various places on the order of millimeters, such as inside vehicles and between electronic devices. . The radio wave absorber 1 according to this embodiment meets those needs, and has a large economic effect.

(Modification)
Although the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments, and various modifications are possible without departing from the scope of the invention.

FIG. 8 is a cross-sectional view of a radio wave absorber 1' according to a modification. A radio wave absorber 1' comprises a dielectric layer 2, a conductive material layer 3 and an additional dielectric layer 2'. An additional dielectric layer 2 ′ is provided on the side of the conductive material layer 3 opposite to the dielectric layer 2 . As described above, since the radio wave absorber 1' has a structure in which the dielectric layers 2 and 2' are provided on both sides of the conductive material layer 3, the radio wave can be absorbed regardless of whether the radio wave is incident from the front or the back.

The inventors of the present invention laminated a dielectric layer 2 (FR-4, commercially available product) made of glass epoxy resin and a conductive material layer 3 formed by kneading a conductive material into a rubber material. A radio wave absorber 1 shown in FIG. The dielectric layer 2 had a dielectric constant _εr2 of 2.37, the conductive material layer 3 had a conductivity σ of 30 S/m, and _a thickness d1 of 2.0 mm.

9(a) and (b) are reflection coefficient charts at frequencies of 40 GHz to 54 GHz when the metal plate is on the back surface of the radio wave absorber 1 (on the conductive material layer 3 side), and the frequency dependence of the reflectance. is shown. Also, FIGS. 10(a) and 10(b) show a reflection coefficient chart at frequencies of 40 GHz to 54 GHz and the frequency dependence of the reflectance when there is no metal plate.

From FIGS. 9A and 10A, regardless of the presence or absence of the metal plate, the reflection coefficient Γ _L ' changed so as to pass near the center of the chart as the frequency changed. Also, comparing FIG. 9(b) and FIG. 10(b), the reflection coefficient Γ _L ' decreased to -10 dB (1/10 in power conversion) or less at frequencies of 42 to 45 GHz regardless of the presence or absence of the metal plate. . From these results, it could be verified that the electromagnetic wave absorber 1 according to the present invention shows good absorption performance with little change in performance depending on the presence or absence of the backing metal plate.

1 radio wave absorber 1' radio wave absorber 2 dielectric layer 2' additional dielectric layers 2a to 2d dielectric layer 20 resin base material 21 metal powder 22 metal thin film 23 metal thin film 24 metal thin film 3 conductive material layer

Claims (5)

A radio wave absorber that absorbs radio waves,
a dielectric layer;
a conductive material layer provided on the dielectric layer;
A radio wave absorber, wherein the thickness of the conductive material layer is larger than the skin depth with respect to the frequency of the radio wave. 2. The radio wave absorber according to claim 1, wherein said conductive material layer has a conductivity of 10 S/m to 200 S/m. The dielectric layer is
a resin base material;
3. The radio wave absorber according to claim 1, further comprising a metal thin film having a specific shape disposed on the surface of the resin base material opposite to the conductive material layer. 4. The radio wave absorber according to claim 1, further comprising an additional dielectric layer provided on a surface of said conductive material layer opposite to said dielectric layer. The radio wave absorber according to any one of claims 1 to 4, wherein the radio waves are millimeter waves.

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