Electromagnetic wave absorber

Abstract

An electromagnetic wave absorber according to the present invention comprises: a dielectric layer; a resistor layer that is provided on an electromagnetic wave incidence side of the dielectric layer; and a reflective layer that is provided on a side, of the dielectric layer, opposite of the resistor layer, the electromagnetic wave absorber being characterized in that the dielectric layer is adjusted to a thickness obtained by subtracting a thickness, that corresponds to the amount of phase shift generated when an electromagnetic wave to be absorbed by the electromagnetic wave absorber passes through the resistor layer, from a theoretical value of the thickness in the case of a λ / ４ electromagnetic wave absorber.

Description

Electromagnetic wave absorber

[0001] This invention relates to an electromagnetic wave absorber.

[0002] In an electromagnetic wave absorber comprising a dielectric layer, a resistive layer, and a reflective layer, the thickness of the dielectric layer can be adjusted so that the phase of the reflected wave is shifted by half a wavelength relative to the incident wave (see Patent Document 1). Such an electromagnetic wave absorber may generally be referred to as a λ / 4 type electromagnetic wave absorber.

[0003] Patent No. 7457690

[0004] Yuki Kawakami, Toshikazu Hori, Mitoshi Fujimoto, Ryo Yamaguchi and Keizo Cho, “Low-Profile Design of Metasurface Considering FSS Filtering Characteristics”, IEICE Transactions on Communications, Vol. E95-B, No. 2, pp. 477-483, February 1, 2012

[0005] A phase shift may occur when electromagnetic waves pass through a resistive layer (see Non-Patent Document 1). To ensure proper thin-film construction of electromagnetic wave absorbers, design techniques that take such phase shifts into account are generally required.

[0006] The present invention aims to provide a novel technique for appropriately thinning electromagnetic wave absorbers.

[0007] One aspect of the present invention relates to an electromagnetic wave absorber, the electromagnetic wave absorber comprising a dielectric layer, a resistor layer provided on the incident side of the dielectric layer to the electromagnetic wave, and a reflective layer provided on the opposite side of the dielectric layer from the resistor layer, wherein the dielectric layer is adjusted to a thickness obtained by subtracting a thickness corresponding to the amount of phase shift that occurs when the electromagnetic wave to be absorbed by the electromagnetic wave absorber passes through the resistor layer from the theoretical value of the thickness in the case of a λ / 4 type electromagnetic wave absorber.

[0008] According to the present invention, it becomes possible to appropriately thin down an electromagnetic wave absorber.

[0009] This is a schematic diagram showing an example of the structure of an electromagnetic wave absorber according to the embodiment. This is a diagram for explaining the analysis results regarding the amount of phase shift of electromagnetic waves that have passed through the resistive layer. This is a schematic diagram for explaining an example of a method for adjusting the thickness of the dielectric layer. This is a schematic diagram for explaining an example of a method for adjusting the thickness of the dielectric layer.

[0010] The embodiments will be described in detail below with reference to the attached drawings. Note that the following embodiments do not limit the invention as defined in the claims, and not all combinations of features described in the embodiments are essential to the invention. Two or more features from among those described in the embodiments may be arbitrarily combined. Furthermore, identical or similar configurations will be given the same reference numeral, and redundant descriptions will be omitted.

[0011] <Overall Structure> Figure 1 is a schematic cross-sectional view showing an example of the structure of an electromagnetic wave absorber 1 according to an embodiment. The electromagnetic wave absorber 1 comprises a reflective layer 11, a dielectric layer 12, and a resistive layer 13, which are stacked in this order. Here, the resistive layer 13 side is considered the upper side, and the reflective layer 11 side is considered the lower side.

[0012] The reflective layer 11 only needs to be configured to reflect electromagnetic waves, and for example, it may be made by laminating a metal foil such as copper onto a resin film.

[0013] The resistive layer 13 is configured to allow electromagnetic waves of a wavelength to be absorbed to pass through, and includes, for example, an element pattern 131 in which a plurality of elements 131a are arranged. Typically, resistors or conductors are used for the elements 131a, and the elements 131a may be formed on a substrate (not shown) from known metallic materials such as copper, aluminum, or tungsten, or compounds thereof. Such an element pattern 131 allows electromagnetic waves of a predetermined wavelength to selectively pass through the resistive layer 13. The elements 131a may also be called FSS (Frequency Selective Surface) elements. Similarly, the resistive layer 13 may also be called an FSS layer. Furthermore, the resistive layer 13 is not limited to an FSS layer, and may be, for example, a layer in which a mesh-like or grid-like pattern having a predetermined conductivity is continuously formed over substantially the entire surface, or a layer in which a conductive film is uniformly formed.

[0014] The dielectric layer 12 is configured to allow electromagnetic waves to propagate and is provided between the reflective layer 11 and the resistor layer 13. The dielectric layer 12 may be a single layer or a plurality of layers, and may be made of known dielectric materials such as plastic, rubber, or foamed resin formed in a sheet shape. Furthermore, by incorporating high dielectric materials such as barium titanate or strontium titanate into the dielectric layer 12, further increasing its dielectric properties may be achieved. Electromagnetic waves that pass through the resistor layer 13 propagate downward through the dielectric layer 12, and these electromagnetic waves are reflected upward by the reflective layer 11. The reflected electromagnetic waves propagate upward through the dielectric layer 12. That is, the resistor layer 13 is provided on the side of the dielectric layer 12 where the electromagnetic waves are incident, and the reflective layer 11, which can reflect the electromagnetic waves, is provided on the side of the dielectric layer 12 opposite to the resistor layer 13.

[0015] <About λ / 4 type electromagnetic wave absorber> At the interface E0 between the dielectric layer 12 and the resistor layer 13, when the electromagnetic wave from above (incident wave) and the electromagnetic wave from below (reflected wave) are in opposite phases, the electromagnetic wave is effectively absorbed by the electromagnetic wave absorber 1. Here, let λ be the wavelength of the electromagnetic wave to be absorbed in air, and let ε be the relative permittivity of the dielectric layer 12. ｒLet k be a natural number greater than or equal to 0 (k = 0, 1, 2...), and the dielectric layer 12 be (λ / 2 × k + λ / 4) × ε ｒ －１／２ By providing a thickness (theoretical value) of ε, the incident wave and reflected wave at the interface E0 can be made to be in opposite phase. In many cases, the minimum value of this theoretical value is (λ / 4) × ε ｒ －１／２ This is used for the thickness of the dielectric layer 12. Such an electromagnetic wave absorber 1 may be called a λ / 4 type electromagnetic wave absorber (for distinction, electromagnetic wave absorber 1) Ｒ (Let's assume that.)

[0016] <Regarding the phase shift of electromagnetic waves that may occur when passing through the resistive layer> Incidentally, a phase shift may occur in electromagnetic waves passing through the resistive layer 13 (see Non-Patent Literature 1). The amount of phase shift generally depends on the configuration of the resistive layer 13. Figure 2 shows an example of the analysis results of the amount of phase shift of electromagnetic waves, based on the inventor's diligent research, for the case where the frequency of the electromagnetic wave to be absorbed is 28 GHz (gigahertz). Here, as an example, plots of the amount of phase shift when element 131a is formed as a rectangular loop (plots of □ in the figure) and plots of the amount of phase shift when it is formed as a circular loop (plots of ○ in the figure) are shown, respectively. The horizontal axis is the size of element 131a (size of one side of the rectangular loop, or the size of the diameter of the circular loop), and the vertical axis is the amount of phase shift that occurs when passing through the resistive layer 13.

[0017] According to Figure 2, in the case of the rectangular loop element 131a, a phase lag occurs in the size range of 0.5 to 2 mm, and a phase lead occurs in the size range of 2 mm or more. In the case of the circular loop element 131a, a phase lag occurs in the size range of 0.5 to 2.7 mm, and a phase lead occurs in the size range of 2.7 mm or more.

[0018] When such a phase shift occurs, the thickness (for example, (λ / 4) × ε) is the theoretical value mentioned above. ｒ －１／２Even when the dielectric layer 12 is formed with the thickness), the antiphase relationship between the incident wave and the reflected wave at the interface E0 is not established. Therefore, such a phase shift can cause a reduction in electromagnetic wave absorption efficiency.

[0019] <Design in the Case Where Phase Shift Occurs> Figures 3A to 3B are schematic diagrams for explaining an example of a method for adjusting the thickness of the dielectric layer 12. For ease of explanation, let the amount of phase shift be Δθ. Note that Δθ>0 indicates a phase advance, and Δθ<0 indicates a phase lag. Further, for ease of understanding, ε in the figures ｒ the waveform when = 1 is schematically shown.

[0020] -Case of Phase Advance Figure 3A is a schematic diagram for explaining the antiphase relationship between an incident wave and a reflected wave at the interface E0 when Δθ>0 (phase advance). In the figure, as an example, an incident wave for comparison (without phase shift), an incident wave when Δθ=π / 4 (that is, a phase advance of 45 degrees), and a reflected wave when Δθ=π / 4 are shown side by side. Note that, to assist understanding, the waveform when the reflective layer 11 is not provided is also shown by a broken line in the figure.

[0021] That is, in the example of Figure 3A, when an incident wave passes through the resistor layer 13, its phase advances by π / 4, and when a reflected wave passes through the resistor layer 13, its phase advances by π / 4. When a phase advance occurs, it can be said that by reducing the thickness T of the dielectric layer 12 by an amount corresponding to the advanced phase, the incident wave and the reflected wave at the interface E0 can be brought into an antiphase relationship. However, when the amount of phase advance is equal to or greater than one half wavelength, the thickness T of the dielectric layer 12 becomes T=0 or a negative value. Therefore, when Δθ>0, as shown in Figure 3A, when the amount of phase advance is smaller than one half wavelength, it is possible to reduce the thickness T of the dielectric layer 12.

[0022] According to intensive studies by the inventors, when the phase advance (Δθ>0) when an electromagnetic wave passes through the resistor layer 13 is taken into consideration, the ideal value T of the thickness T of the dielectric layer 12 ０ is represented by: (λ / 4)×ε ｒ －１／２ ×{1−Δθ / (π / 2)}=T ０However, it is calculated that 0 < Δθ < π / 2. In this case, the dielectric layer 12 is 0.8 × T ０ ≤ T ≤ 1.2 × T ０ ...By adjusting the thickness T to such that (Equation 1) holds true, the electromagnetic wave absorption efficiency can be maintained. Preferably, 0.9 × T ０ ≤ T ≤ 1.1 × T ０ ...It is good if (Equation 2) holds, and 0.95 × T ０ ≤ T ≤ 1.05 × T ０ ...It would be even better if (Equation 3) were true.

[0023] - In the case of phase lag, Figure 3B is a schematic diagram illustrating the out-of-phase relationship between the incident wave and the reflected wave at interface E0 when Δθ < 0 (phase lag). As an example, the figure shows, side by side, the incident wave for the case Δθ = (-π) × 3 / 4 (i.e., a phase lag of 135 degrees), and the reflected wave for the case Δθ = (-π) × 3 / 4. For further understanding, the waveform without reflection by the reflective layer 11 is also shown as a dashed line.

[0024] That is, in the example shown in Figure 3B, the phase of the incident wave is delayed by π × (3 / 4) when it passes through the resistive layer 13, and the phase of the reflected wave is delayed by π × (3 / 4) when it passes through the resistive layer 13. When a phase delay occurs, it can be said that it is possible to make the incident wave and the reflected wave at interface E0 be in opposite phases by increasing the thickness T of the dielectric layer 12 by the amount of the delayed phase. However, since it is sufficient for the incident wave and the reflected wave at interface E0 to be in opposite phases, when Δθ < 0, as shown in Figure 3B, it is possible to reduce the thickness T of the dielectric layer 12 when the amount of phase delay |Δθ| is half a wavelength or more.

[0025] According to the inventor's diligent research, when considering such a phase delay (Δθ < 0), the ideal value T ０ is (λ / 4) × ε ｒ －１／２×{1-(π+Δθ) / (π / 2)}=T ０ where -π<Δθ<-(π / 2). In this case, it is also preferable that the thickness T of the dielectric layer 12 satisfies the above (Formula 1), preferably satisfies the above (Formula 2), and more preferably satisfies the above (Formula 3).

[0026] <Manufacturing Method> The electromagnetic wave absorber 1 can be manufactured by using a known manufacturing process. For example, the thickness T of the dielectric layer 12 formed on the reflective layer 11 may be adjusted in advance (before forming the resistor layer 13) based on the structure of the resistor layer 13 to be subsequently formed on the dielectric layer 12. The amount of phase shift that may occur when passing through the resistor layer 13 can be calculated by evaluating the resistor layer 13 through simulation analysis based on a predetermined calculation model. Alternatively, the amount of phase shift can be identified through actual measurement using the actually manufactured resistor layer 13. The thickness T of the dielectric layer 12 may be adjusted based on the amount of phase shift calculated or identified in this way.

[0027] The electromagnetic wave absorber 1 manufactured in this way is widely applicable to various electric products that require prevention of electromagnetic interference, and examples thereof include in-vehicle components, road installations, communication equipment, evaluation rooms, and the like.

[0028] <Summary> As described above, according to the present embodiment, the dielectric layer 12 is a λ / 4-type electromagnetic wave absorber 1 Ｒ adjusted to a thickness T obtained by subtracting, from the theoretical thickness value in this case, a thickness corresponding to the amount Δθ of phase shift generated when the electromagnetic wave to be absorbed passes through the resistor layer 13. Thereby, the anti-phase relationship between the incident wave and the reflected wave at the interface E0 is appropriately maintained, and the reduction in absorption efficiency of electromagnetic waves that may occur in the λ / 4-type electromagnetic wave absorber 1 Ｒ (the reduction in absorption efficiency of electromagnetic waves caused by the above-mentioned phase shift) can be appropriately suppressed. Along with the adjustment of the thickness T, the dielectric layer 12 can be made thinner than the theoretical thickness value of the λ / 4-type electromagnetic wave absorber 1 Ｒ (in the case of the minimum value, (λ / 4)×ε ｒ －１／２ ), specifically, the ideal value T ０, in the case of a phase advance where 0 < Δθ < (π / 2), (λ / 4) × ε ｒ －１／２ × {1 − Δθ / (π / 2)} = T ０ , and in the case of a phase delay where −π < Δθ < −(π / 2), (λ / 4) × ε ｒ －１／２ × {1 − (π + Δθ) / (π / 2)} = T ０ . At this time, the thickness T of the dielectric layer 12 satisfies T ０ × 0.8 ≦ T ≦ T ０ × 1.2. Therefore, according to the present embodiment, the electromagnetic wave absorber 1 can be appropriately thinned.

[0029] In the above description, each element is indicated by a name related to its functional aspect for ease of understanding. However, each element is not limited to those having the content described in the embodiment as a main function, and may have the content as an auxiliary function. Therefore, each element is not strictly limited to the expression thereof, and the expression can be replaced with another similar expression.

[0030] The invention is not limited to the above embodiments, and various modifications and changes can be made within the scope of the gist of the invention.

Claims

1. An electromagnetic wave absorber comprising a dielectric layer, a resistive layer provided on the incident side of the dielectric layer to the electromagnetic wave, and a reflective layer provided on the opposite side of the dielectric layer from the resistive layer, wherein the dielectric layer is adjusted to a thickness obtained by subtracting a thickness corresponding to the amount of phase shift that occurs when the electromagnetic wave to be absorbed by the electromagnetic wave absorber passes through the resistive layer from the theoretical value of the thickness in the case of a λ / 4 type electromagnetic wave absorber.

2. The electromagnetic wave absorber according to claim 1, characterized in that the dielectric layer is adjusted to a thickness obtained by subtracting a thickness corresponding to the amount of phase shift from the minimum value of the theoretical value.

3. Let λ be the wavelength of the electromagnetic wave to be absorbed, and let ε be the relative dielectric constant of the dielectric layer ｒ , let Δθ be the amount of phase shift, wherein, in the case of phase advance where 0 < Δθ < (π / 2), (λ / 4) × ε ｒ －１／２ × {1 - Δθ / (π / 2)} = T ０ , and in the case of phase delay where -π < Δθ < -(π / 2), (λ / 4) × ε ｒ －１／２ × {1 - (π + Δθ) / (π / 2)} = T ０ , when the thickness T of the dielectric layer satisfies ０ × 0.8 ≦ T ≦ T ０ × 1.2 The electromagnetic wave absorber according to claim 1, characterized in that:

4. The electromagnetic wave absorber according to any one of claims 1 to 3, characterized in that the resistive layer includes an element pattern in which a plurality of elements are arranged.

5. The electromagnetic wave absorber according to claim 4, characterized in that the amount of the phase shift is determined by the element pattern.

6. A method for manufacturing an electromagnetic wave absorber comprising a dielectric layer, a resistor layer provided on the incident side of the dielectric layer to the electromagnetic wave, and a reflective layer provided on the opposite side of the dielectric layer from the resistor layer, the method comprising: identifying the amount of phase shift that occurs when the electromagnetic wave to be absorbed by the electromagnetic wave absorber passes through the resistor layer; calculating a theoretical value for the thickness of the dielectric layer in the case of a λ / 4 type electromagnetic wave absorber; and forming the dielectric layer with a thickness obtained by subtracting the thickness corresponding to the identified amount of phase shift from the calculated theoretical value.

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