Electromagnetic wave absorber

Abstract

It suppresses changes in the resonant frequency of electromagnetic waves, thereby efficiently absorbing them. [Solution] In the electromagnetic wave absorber 1, incident electromagnetic waves incident on multiple openings 11 of the first metal layer 10 pass through the dielectric 30. Multiple inclined surfaces 22a, 22b retroreflect the incident electromagnetic waves D1 that have passed through the dielectric 30. The reflected electromagnetic waves D3 retroreflected by the multiple inclined surfaces 22a, 22b pass through the dielectric 30 and are then reflected by the first metal layer 10. These reflected electromagnetic waves D4 pass through the dielectric 30 and are then retroreflected by the multiple inclined surfaces 22a, 22b. These retroreflected electromagnetic waves D6 pass through the dielectric 30 and are then reflected by the first metal layer 10. The electromagnetic waves resonate when the wavefront of these reflected electromagnetic waves D7 coincides with the wavefront of the incident electromagnetic wave D1. The dielectric 30 absorbs the electromagnetic waves by converting them into heat through dielectric loss when the electromagnetic waves are in a resonant state.

Description

Technical Field

[0001] The present disclosure relates to an electromagnetic wave absorber.

Background Art

[0002] Conventionally, several basic forms of electromagnetic wave absorbers are known. In a form called a quarter-wave electromagnetic wave absorber, a first member as a semi-reflector that transmits a part of incident electromagnetic waves incident from one side in a predetermined direction, and a second member arranged on the other side in the predetermined direction with respect to the first member are provided (see, for example, Patent Document 1). λ is the wavelength of the electromagnetic wave. An intermediate material as a dielectric is arranged between the first member and the second member. The second member reflects the electromagnetic wave that has passed through the intermediate material. In the electromagnetic wave absorber, in a state where the electromagnetic wave is multiply reflected between the second member and the first member, the electromagnetic wave is attenuated by the resistance component of the first member, the dielectric loss due to the intermediate material, and the magnetic loss due to the intermediate material.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] The present inventor has studied reducing unnecessary electromagnetic waves that originally interfere with electromagnetic waves used for exploring the vehicle surroundings among the electromagnetic waves emitted from the in-vehicle radar device by the electromagnetic wave absorber. The in-vehicle radar device explores the vehicle surroundings using electromagnetic waves having frequencies within a predetermined frequency range. For example, when unnecessary electromagnetic waves having frequencies within the predetermined frequency range are incident as incident electromagnetic waves on the electromagnetic wave absorber, if the electromagnetic wave resonates at the frequency within the predetermined frequency range, the unnecessary electromagnetic waves can be efficiently attenuated by the resistance component of the first member, the dielectric loss due to the intermediate material, and the magnetic loss due to the intermediate material. Here, when an incident electromagnetic wave that has passed through the first member is reflected by the second member, and this reflected electromagnetic wave is reflected by the first member, if the wavefront of the reflected electromagnetic wave coincides with the wavefront of the incident electromagnetic wave that has passed through the first member, then the electromagnetic wave can be made to resonate in the electromagnetic wave absorber.

[0005] In this case, the resonant frequency of the electromagnetic wave is determined by the positional relationship between the wavefront of the reflected electromagnetic wave reflected by the first member and the wavefront of the incident electromagnetic wave that has passed through the first member. Therefore, if the angle of incidence of the incident electromagnetic wave to the electromagnetic wave absorber changes, the positional relationship between the wavefront of the reflected electromagnetic wave and the wavefront of the incident electromagnetic wave that has passed through the first member changes. Therefore, if the angle of incidence of the incident electromagnetic wave changes, the resonant frequency may change, and the resonant frequency may fall outside the predetermined frequency range. In this case, it becomes impossible to efficiently attenuate unwanted electromagnetic waves with frequencies within the predetermined frequency range. In view of the above points, this disclosure aims to provide an electromagnetic wave absorber that suppresses changes in the resonant frequency even when the incident angle of electromagnetic waves changes. [Means for solving the problem]

[0006] According to one aspect of this disclosure, an electromagnetic wave absorber, A first member (10) that transmits electromagnetic waves incident from one side in a predetermined direction (Ya), A dielectric (30) is positioned on the other side in a predetermined direction relative to the first member, The device comprises a second member (20) positioned on the other side of a predetermined direction relative to the dielectric, and having a plurality of inclined surfaces (22a, 22b, 22i, 22j, 24a, 24b, 24c) whose normal directions (hs1, hs2, hsa, hsb, hsc) are inclined with respect to the predetermined direction, causing electromagnetic waves transmitted through the dielectric to retroreflect, Electromagnetic waves that have passed through the first member undergo multiple reflections between the multiple inclined surfaces and the first member, resonating and then being attenuated by the dielectric. Therefore, the second member retroreflects electromagnetic waves on multiple inclined surfaces. As a result, when the angle of incidence of electromagnetic waves changes, the positional relationship between the electromagnetic waves transmitted through the first member and the electromagnetic waves that have been multiple-reflected can be suppressed. Thus, the positional relationship between the wavefront of the electromagnetic waves transmitted through the first member and the wavefront of the electromagnetic waves that have been multiple-reflected can be suppressed. Therefore, an electromagnetic wave absorber can be provided that suppresses changes in the resonant frequency even when the angle of incidence of electromagnetic waves changes. The reference numerals in parentheses attached to each component indicate an example of the correspondence between that component and the specific components described in the embodiments described later. [Brief explanation of the drawing]

[0007] [Figure 1] This is a perspective view of an electromagnetic wave absorber in the first embodiment, illustrating a plurality of openings formed in the first metal layer, a plurality of triangular prisms formed in the second metal layer, and a dielectric material disposed between the first and second metal layers. [Figure 2] Figure 1 is a perspective view of the electromagnetic wave absorber in the first embodiment, and is a diagram illustrating the positional relationship between the multiple openings formed in the first metal layer and the multiple triangular prisms formed in the second metal layer. [Figure 3] Figure 1 shows a part of the electromagnetic wave absorber in the first embodiment, illustrating the shape of the side surface of a triangular prism formed in the second metal layer, and the two angles that constitute this side surface. [Figure 4] This is a schematic diagram illustrating a specific example in the electromagnetic wave absorber of the first embodiment shown in Figure 1, in which electromagnetic waves incident from one side in the stacking direction through an opening are multiple-reflected between the first metal layer and the second metal layer. [Figure 5] This is a schematic diagram showing the positional relationship between the wavefront of the incident electromagnetic wave outside the dielectric, the wavefront of the incident electromagnetic wave inside the dielectric, and the wavefront of the reflected electromagnetic wave in the proportional electromagnetic wave absorber of the first embodiment. [Figure 6]This is a schematic diagram illustrating a specific example of multiple reflections of electromagnetic waves incident through an opening between the first and second metal layers in a proportional electromagnetic wave absorber according to the first embodiment. [Figure 7] This is a schematic diagram showing the positional relationship between the wavefront of the incident electromagnetic wave outside the dielectric, the wavefront of the incident electromagnetic wave inside the dielectric, and the wavefront of the reflected electromagnetic wave in the electromagnetic wave absorber of the first embodiment shown in Figure 1 when the angle of incidence changes. [Figure 8] This is a schematic diagram illustrating a specific example of multiple reflections of electromagnetic waves between the first and second metal layers in a proportional electromagnetic wave absorber according to the first embodiment, before the angle of incidence changes. [Figure 9] This is a schematic diagram illustrating a specific example of multiple reflections of electromagnetic waves between the first and second metal layers after the angle of incidence changes, in the proportional electromagnetic wave absorber of the first embodiment. [Figure 10] This is a schematic diagram illustrating a specific example in the electromagnetic wave absorber of the first embodiment, where, when the incident angle is 0°, the incident electromagnetic wave is retroreflected by a pair of inclined surfaces of the second metal layer. [Figure 11] This is a schematic diagram illustrating a specific example in the electromagnetic wave absorber of the first embodiment, in which, when the incident angle is 0°, the incident electromagnetic wave is retroreflected by the effective reflective surface formed by a pair of inclined surfaces of the second metal layer. [Figure 12] This is a schematic diagram illustrating a specific example in the electromagnetic wave absorber of the first embodiment, where, when the incident angle is 5°, the incident electromagnetic wave is retroreflected by a pair of inclined surfaces of the second metal layer. [Figure 13] This is a schematic diagram illustrating a specific example in the electromagnetic wave absorber of the first embodiment, in which, when the incident angle is 5°, the incident electromagnetic wave is retroreflected by the effective reflective surface formed by a pair of inclined surfaces of the second metal layer. [Figure 14] This is a schematic diagram illustrating a specific example in the electromagnetic wave absorber of the first embodiment, where, when the angle of incidence is 0°, electromagnetic waves undergo multiple reflections between a pair of inclined surfaces of the second metal layer and the first metal layer. [Figure 15]In the electromagnetic wave absorber according to the first embodiment, when the incident angle is 5°, it is a schematic diagram showing a specific example in which electromagnetic waves are multiply reflected between the pair of inclined surfaces of the second metal layer and the first metal layer. [Figure 16] In the electromagnetic wave absorber according to the first embodiment, when the incident angle is 5°, it is a schematic diagram showing the wavefront of the incident electromagnetic wave and the wavefront of the reflected electromagnetic wave when multiple reflections occur between the pair of inclined surfaces of the second metal layer and the first metal layer. [Figure 17] In the electromagnetic wave absorber according to the first embodiment, it is a schematic diagram showing a specific example of the effective reflection surface formed by two pairs of adjacent inclined surfaces in the second metal layer. [Figure 18] In the electromagnetic wave absorber according to the first embodiment, when the incident angle is 10°, it is a schematic diagram showing the wavefront of the incident electromagnetic wave and the wavefront of the reflected electromagnetic wave when multiple reflections occur between the pair of inclined surfaces of the second metal layer and the first metal layer. [Figure 19] In the electromagnetic wave absorber in the comparative ratio of the first embodiment, it is a diagram showing the relationship between the reflection loss of electromagnetic waves and the resonance frequency of electromagnetic waves when the incident angles of electromagnetic waves are 0°, 20°, 40°, and 60°. [Figure 20] In the electromagnetic wave absorber according to the first embodiment, it is a diagram showing the relationship between the reflection loss of electromagnetic waves and the resonance frequency of electromagnetic waves when the incident angles of electromagnetic waves are 0°, 20°, 40°, and 60°. [Figure 21] In the electromagnetic wave absorber in the comparative ratio of the first embodiment, it is a diagram showing the relationship between the reflection loss of electromagnetic waves and the incident angle of electromagnetic waves when the frequencies of electromagnetic waves are 75.5 GHz, 76.0 GHz, 76.5 GHz, and 77.0 GHz. [Figure 22] In the electromagnetic wave absorber according to the first embodiment, it is a diagram showing the relationship between the reflection loss of electromagnetic waves and the incident angle of electromagnetic waves when the frequencies of electromagnetic waves are 75.5 GHz, 76.0 GHz, 76.5 GHz, and 77.0 GHz. [Figure 23]In the electromagnetic wave absorber according to the first embodiment, it is a diagram showing the relationship between the absorption frequency and the height of the triangular prism when the bottom surface between the two triangular prisms is not provided and the incident angles are 0° and 40°. [Figure 24] In the electromagnetic wave absorber according to the first embodiment, taking the wavelength of the electromagnetic wave traveling in the dielectric as λ, it is a diagram for explaining a plurality of triangular prisms when the bottom surface between the two triangular prisms is not provided and the height is 0.15λ. [Figure 25] In the electromagnetic wave absorber according to the first embodiment, taking the wavelength of the electromagnetic wave traveling in the dielectric as λ, it is a diagram for explaining a plurality of triangular prisms when the bottom surface between the two triangular prisms is not provided and the height is 0.55λ. [Figure 26] In the electromagnetic wave absorber according to the first embodiment, it is a diagram showing the relationship between the absorption frequency and the height of the triangular prism when the bottom surface between the two triangular prisms is provided and the incident angles are 0° and 40°. [Figure 27] In the electromagnetic wave absorber according to the first embodiment, taking the wavelength of the electromagnetic wave traveling in the dielectric as λ, it is a diagram for explaining a plurality of triangular prisms when the bottom surface between the two triangular prisms is provided and the height is 0.15λ. [Figure 28] In the electromagnetic wave absorber according to the first embodiment, taking the wavelength of the electromagnetic wave traveling in the dielectric as λ, it is a diagram for explaining a plurality of triangular prisms when the bottom surface between the two triangular prisms is provided and the height is 0.5λ. [Figure 29] It is a side view of the electromagnetic wave absorber according to the second embodiment, and it is a diagram for explaining the positional relationship between the plurality of openings formed in the first metal layer and the plurality of triangular prisms formed in the second metal layer. [Figure 30] It is an inclined view of the electromagnetic wave absorber according to the third embodiment, and it is a perspective view for explaining the opening formed in the first metal layer and the inclined surface of the quadrangular prism formed in the second metal layer. [Figure 31]This is a perspective view of the electromagnetic wave absorber in the fourth embodiment, illustrating the opening formed in the first metal layer and the inclined surface of the rectangular prism formed in the second metal layer. [Figure 32] This is a perspective view of the electromagnetic wave absorber in the fourth embodiment, illustrating the opening formed in the first metal layer and the inclined surfaces of the three triangular prisms formed in the second metal layer. [Figure 33] This is a front view of the electromagnetic wave absorber in the fourth embodiment, illustrating the two inclined surfaces of the triangular prism of the second metal layer and the two inclined surfaces positioned vertically to this triangular prism, with the first metal layer removed. [Figure 34] This is a perspective view of the electromagnetic wave absorber in the fourth embodiment, illustrating the two inclined surfaces of one triangular prism of the second metal layer and the two triangular prisms arranged vertically, with the first metal layer removed. [Figure 35] This is a perspective view of an electromagnetic wave absorber in the fifth embodiment, illustrating the opening formed in the first metal layer, the two inclined surfaces of a rectangular prism formed in the second metal layer, and two rectangular prisms arranged vertically to this rectangular prism. [Figure 36] This is a front view of the electromagnetic wave absorber in the fifth embodiment, illustrating the opening formed in the first metal layer and the two inclined surfaces of the rectangular prism formed in the second metal layer. [Figure 37] This is a side view of the electromagnetic wave absorber in the fifth embodiment, illustrating the inclined surface of a rectangular prism formed in the second metal layer, and two inclined surfaces arranged vertically to this rectangular prism. [Figure 38] This is a perspective view of the electromagnetic wave absorber in the fifth embodiment, illustrating the inclined surface of a rectangular prism formed in the second metal layer, and two rectangular prisms arranged vertically relative to this rectangular prism. [Figure 39] This is a perspective view of the electromagnetic wave absorber in the sixth embodiment, and is a diagram illustrating the two reflective surfaces, which consist of curved surfaces formed on a triangular prism of the second metal layer. [Figure 40]This is a diagram of an electromagnetic wave absorber in the sixth embodiment, illustrating two reflective surfaces consisting of curved surfaces formed on a triangular prism of the second metal layer. [Figure 41] This is a diagram of the electromagnetic wave absorber in the seventh embodiment, and is a side view illustrating two reflective surfaces consisting of curved surfaces formed on a triangular prism of the second metal layer. [Figure 42] This is a perspective view of the electromagnetic wave absorber in the seventh embodiment, and is a diagram illustrating the two reflective surfaces, which consist of curved surfaces formed on the triangular prism of the second metal layer. [Figure 43] This is a front view of the electromagnetic wave absorber in the seventh embodiment, and is a diagram illustrating the inclined surfaces that constitute each of the multiple recesses formed in the second metal layer. [Figure 44] This is a side view of the electromagnetic wave absorber in the seventh embodiment, and is a diagram illustrating the inclined surfaces that constitute each of the multiple recesses formed in the second metal layer. [Figure 45] This is a front view of the second metal layer of the electromagnetic wave absorber in the seventh embodiment, and is a front view for explaining the inclined surfaces that constitute each of the multiple recesses formed in the second metal layer when the first metal layer is removed. [Figure 46] This is a schematic diagram showing the three inclined surfaces of a single triangular pyramidal recess formed in the second metal layer of the electromagnetic wave absorber in the seventh embodiment, and the recess is shown with an opening that is formed diagonally upward. [Modes for carrying out the invention]

[0008] The embodiments of this disclosure will be described below with reference to the drawings. In the following embodiments, parts that are the same or equivalent to each other are denoted by the same reference numerals in the drawings in order to simplify the explanation. (First Embodiment) This first embodiment of the electromagnetic wave absorber 1 of the present disclosure will be described with reference to Figures 1, 2, 3, etc. The electromagnetic wave absorber 1 of this embodiment attenuates unwanted electromagnetic waves that interfere with the electromagnetic waves used when searching the area around the vehicle, among the electromagnetic waves emitted from the vehicle-mounted radar device.

[0009] Figure 1 is a perspective view showing the overall configuration of the electromagnetic wave absorber 1, and Figure 2 is a perspective view of the electromagnetic wave absorber 1. As shown in Figures 1 and 2, the electromagnetic wave absorber 1 comprises a first metal layer 10, a second metal layer 20, and a dielectric 30. The first metal layer 10 is a first member formed as a thin film, with the stacking direction (i.e., a predetermined direction) Ya as the thickness direction, and formed along the transverse direction Yb and the longitudinal direction Yc, respectively. The transverse direction Yb is a second direction perpendicular to the stacking direction Ya, and the longitudinal direction Yc is a direction perpendicular to both the transverse direction Yb and the stacking direction Ya.

[0010] The first metal layer 10 is made of a conductive metallic material such as copper or silver. The first metal layer 10 is provided with a plurality of openings 11. Each of the plurality of openings 11 is formed, for example, in an elliptical shape. The plurality of openings 11 are arranged in the vertical direction Yc and the horizontal direction Yb. As a result, the first metal layer 10 constitutes an opening pattern having a plurality of openings 11 arranged in the vertical direction Yc and the horizontal direction Yb. Figure 1 shows an example in which the first metal layer 10 is provided with, for example, eight openings 11.

[0011] As shown in Figures 1 and 2, the second metal layer 20 is a second member positioned on the other side of the stacking direction Ya relative to the first metal layer 10. The second metal layer 20 is made of a conductive metallic material such as copper or silver. The second metal layer 20 comprises a base layer 21 and a plurality of triangular prisms 22. The base layer 21 is formed as a thin film with the stacking direction Ya as the thickness direction, and is formed along the transverse direction Yb and the longitudinal direction Yc, respectively. The plurality of triangular prisms 22 are reflectors positioned between the base layer 21 and the first metal layer 10. As shown in Figure 1, each of the plurality of triangular prisms 22 is formed in a triangular prism shape and has an axis Sa extending in the longitudinal direction Yc.

[0012] Multiple triangular prisms 22 are arranged at equal intervals in the lateral direction Yb. Each of the multiple triangular prisms 22 is formed to protrude from the base layer 21 to one side in the stacking direction Ya. Each of the multiple triangular prisms 22 is positioned to face one of the multiple openings 11. Each of the multiple triangular prisms 22 has inclined surfaces 22a and 22b, as shown in Figures 2 and 3. In the multiple triangular prisms 22, inclined surface 22a is a first inclined surface positioned on the other side in the lateral direction Yb relative to inclined surface 22b. As a result, the second metal layer 20 constitutes a pattern structure having multiple inclined surfaces 22a and 22b.

[0013] As shown in Figure 3, the inclined surface 22a is positioned such that its normal direction hs1 intersects the stacking direction Ya. The inclined surface 22a is formed so that as you move toward one side of the lateral direction Yb, you advance toward one side of the stacking direction Ya. The inclined surface 22a is formed so that as you move toward the other side of the lateral direction Yb, you advance toward the other side of the stacking direction Ya. On the other hand, the inclined surface 22b is positioned such that its normal direction hs2 intersects the stacking direction Ya. The inclined surface 22b is a second inclined surface formed so that as you move toward one side of the lateral direction Yb, you advance toward the other side of the stacking direction Ya.

[0014] The inclined surface 22b is formed so that it moves toward one side of the stacking direction Ya as it moves toward the other side of the horizontal direction Yb. As a result, when multiple triangular prisms 22 are arranged in the horizontal direction Yb, multiple inclined surfaces 22a and multiple inclined surfaces 22b are arranged alternately one by one in the horizontal direction Yb. As shown in Figure 3, a triangular side surface 23 is provided on one side of the vertical direction Yc of each of the multiple triangular prisms 22. The base side 23a of the side surface 23 is formed to follow the horizontal direction Yb. The side edges 23b and 23c of the side surface 23 are each positioned on one side of the stacking direction Ya with respect to the base side 23a. In this embodiment, the angle θa formed between the base side 23a and the side edge 23b and the angle θb formed between the base side 23a and the side edge 23c are set to the same angle.

[0015] Side 23b constitutes one side of the inclined surface 22a in the vertical direction Yc. Side 23c constitutes one side of the inclined surface 22b in the vertical direction Yc. Furthermore, in the base layer 21 of Figures 1 and 2, a bottom surface 21a is provided between two adjacent triangular prisms 22 among the multiple triangular prisms 22. Each of the multiple bottom surfaces 21a is formed to align with the horizontal direction Yb and the vertical direction Yc. Thus, the second metal layer 20 is provided with multiple inclined surfaces 22a, multiple inclined surfaces 22b, and multiple bottom surfaces 21a.

[0016] For the sake of explanation, the multiple inclined surfaces 22a and the multiple inclined surfaces 22b will be collectively referred to as multiple inclined surfaces 22a and 22b below. In this embodiment, two adjacent triangular prisms 22 have a pair of inclined surfaces 22a and 22b facing each other via the dielectric 30. As a result, the multiple triangular prisms 22 are provided with multiple pairs of inclined surfaces 22a and 22b facing each other via the dielectric 30. As will be described later, the multiple pairs of inclined surfaces 22a and 22b in this embodiment retroreflect electromagnetic waves that have passed through the dielectric 30.

[0017] The dielectric 30 is placed between the first metal layer 10 and the second metal layer 20. Specifically, the dielectric 30 is placed between the multiple inclined surfaces 22a, 22b and multiple bottom surfaces 21a of the second metal layer 20 and the first metal layer 10. As will be described later, the dielectric 30 attenuates electromagnetic waves by converting the electromagnetic waves incident through the multiple openings 11 into heat through dielectric loss. In this embodiment, for example, PPS, i.e., polyphenylene sulfide, is used as the dielectric 30.

[0018] Next, the operation of the electromagnetic wave absorber 1 of this embodiment will be described with reference to Figures 2, 4, and 5. Figure 4 is a schematic diagram showing multiple reflections of electromagnetic waves between the first metal layer 10 and the second metal layer 20. Figure 5 is a schematic diagram showing the positional relationship between the respective wavefronts of the incident electromagnetic waves D0 and D1, and the respective wavefronts of the reflected electromagnetic waves D3, D4, and D5. First, an incident electromagnetic wave D0 (i.e., unwanted electromagnetic wave) with a frequency within a predetermined frequency range arrives at the first metal layer 10 from one side in the stacking direction Ya. This incoming incident electromagnetic wave D0 passes through a plurality of openings 11 in the first metal layer 10 and is incident on the dielectric 30 as an incident electromagnetic wave D1. This incident electromagnetic wave D0 propagates within the dielectric 30 as an incident electromagnetic wave D1 and is retroreflected by a plurality of pairs of inclined surfaces 22a and 22b of the second metal layer 20.

[0019] Here, a pair of adjacent inclined surfaces 22a and 22b face each other via a dielectric 30. For example, one of the pair of inclined surfaces 22a and 22b reflects the incident electromagnetic wave D1 as a reflected electromagnetic wave D2. The other inclined surface of the pair of inclined surfaces 22a and 22b reflects this reflected electromagnetic wave D2 as a reflected electromagnetic wave D3 (i.e., the first reflected electromagnetic wave). At this time, the reflected electromagnetic wave D3 propagates parallel to the direction of propagation of the incident electromagnetic wave D1 and in the opposite direction to the incident electromagnetic wave D1. That is, the reflected electromagnetic wave D3 propagates along the direction of propagation of the incident electromagnetic wave D1 and in the opposite direction to the incident electromagnetic wave D1.

[0020] As a result, the incident electromagnetic wave D1 is retroreflective by a pair of inclined surfaces 22a and 22b. The reflected electromagnetic wave D3, which travels parallel to the direction of propagation of the incident electromagnetic wave D1 and in the opposite direction to the incident electromagnetic wave D1, travels through the dielectric 30 and is then reflected by the first metal layer 10. This reflected electromagnetic wave D3, as reflected electromagnetic wave D4 (i.e., second reflected electromagnetic wave), travels through the dielectric 30 and is again retroreflective by multiple pairs of inclined surfaces 22a and 22b of the second metal layer 20. For example, one of the pair of inclined surfaces 22a and 22b reflects the reflected electromagnetic wave D4 as reflected electromagnetic wave D5.

[0021] Of the pair of inclined surfaces 22a and 22b, the other inclined surface reflects the reflected electromagnetic wave D5 as a reflected electromagnetic wave D6. This incident reflected electromagnetic wave D6 (i.e., the third reflected electromagnetic wave) propagates parallel to the direction of propagation of the reflected electromagnetic wave D4 and in the opposite direction to the reflected electromagnetic wave D4. That is, the reflected electromagnetic wave D6 propagates along the direction of propagation of the reflected electromagnetic wave D4 and in the opposite direction to the reflected electromagnetic wave D4. After propagating within the dielectric 30, this propagating reflected electromagnetic wave D6 is reflected by the first metal layer 10. At this time, the wavefront of the reflected electromagnetic wave D4 and the wavefront of the reflected electromagnetic wave D6 intersect as shown in Figure 5.

[0022] The reflected electromagnetic wave D6 is reflected by the first metal layer 10 as reflected electromagnetic wave D7 (i.e., the fourth reflected electromagnetic wave). In this way, the incident electromagnetic wave D1 that has passed through the first metal layer 10 and the dielectric 30 undergoes multiple reflections between the multiple inclined surfaces 22a, 22b and the first metal layer 10. At this time, as will be described later, when the wavefront of the reflected electromagnetic wave D4 coincides with the wavefront of the incident electromagnetic wave D1, the electromagnetic wave enters a resonant state. On the other hand, even when the wavefront of the reflected electromagnetic wave D7 coincides with the wavefront of the incident electromagnetic wave D1, the electromagnetic wave enters a resonant state between the multiple inclined surfaces 22a, 22b and the first metal layer 10. At this time, when resonance of the electromagnetic wave occurs in the dielectric 30 at a frequency within a predetermined frequency range, the dielectric 30 attenuates the unwanted electromagnetic wave by converting it into heat through dielectric loss.

[0023] Next, a specific example of electromagnetic wave resonance in the proportional electromagnetic wave absorber 1A of this embodiment will be described with reference to Figures 6 and 7. Figure 7 is a schematic diagram showing the general configuration of the electromagnetic wave absorber 1A. In the proportional electromagnetic wave absorber 1A of this embodiment, flat surfaces are formed in the second metal layer 20 instead of the multiple inclined surfaces 22a and multiple inclined surfaces 22b. The flat surfaces are formed parallel to the lateral direction Yb and parallel to the vertical direction Yc. In the first metal layer 10 of the electromagnetic wave absorber 1A in Figure 7, the multiple openings 11 are not shown.

[0024] Figure 7 shows an example in which electromagnetic waves incident on the electromagnetic wave absorber 1A undergo multiple reflections between the first metal layer 10 and the second metal layer 20. Figure 7 shows the positional relationship between the wavefront of the incident electromagnetic wave Da1 and the wavefront of the reflected electromagnetic wave Da2 between the first metal layer 10 and the second metal layer 20. First, in the electromagnetic wave absorber 1A of Figure 7, an incident electromagnetic wave Da0 arrives on the first metal layer 10 from one side in the stacking direction Ya, and this incoming incident electromagnetic wave Da0 passes through multiple openings 11 in the first metal layer 10 and is incident on the dielectric 30.

[0025] The incident electromagnetic wave Da0, as incident electromagnetic wave Da1, propagates within the dielectric 30 and is then reflected by the second metal layer 20. The incident electromagnetic wave Da1 reflected by the second metal layer 20 is then reflected by the first metal layer 10 as reflected electromagnetic wave Da2, which passes through the dielectric 30 and is then reflected by the first metal layer 10. The reflected electromagnetic wave Da2 reflected by the first metal layer 10 is then reflected electromagnetic wave Da3, which passes through the dielectric 30 towards the second metal layer 20. At this time, when the wavefront of the reflected electromagnetic wave Da3 coincides with the wavefront of the incident electromagnetic wave D1, the electromagnetic waves resonate within the dielectric 30.

[0026] Here, in the electromagnetic wave absorber 1A, the dimension in the stacking direction Ya between the first metal layer 10 and the second metal layer 20 is defined as the thickness dimension a. Among the multiple intersections where the wavefront of the reflected electromagnetic wave Db and the wavefront of the incident electromagnetic wave D1 coincide, the intersection furthest to one side in the stacking direction Ya is defined as intersection Ka. Among the multiple intersections where the wavefront of the reflected electromagnetic wave Db and the wavefront of the incident electromagnetic wave D1 coincide, the intersection furthest to the other side in the stacking direction Ya is defined as intersection Kb. Between intersection Ka and intersection Kb, there exists an intersection Kc where the wavefront of the reflected electromagnetic wave Db and the wavefront of the incident electromagnetic wave D1 coincide.

[0027] Let distance X be the distance between one side of the electromagnetic wave absorber 1A in the stacking direction Ya and the intersection point Ka, and let distance X be the distance between the other side of the electromagnetic wave absorber 1A in the stacking direction Ya and the intersection point Kb. Furthermore, let the angle of incidence of the incident electromagnetic wave Da0 to the dielectric 30 be the angle of incidence θ1. The angle of incidence θ1 is the narrow angle formed between the wavefront of the incident electromagnetic wave Da0 and the transverse direction Yb. Let the angle of refraction of the incident electromagnetic wave Da0 to the dielectric 30 be the angle of refraction θ2. The angle of refraction θ2 is the narrow angle formed between the wavefront of the incident electromagnetic wave Da1 and the transverse direction Yb. Here, if the relative permittivity of the dielectric 30 is εr, then the angle of refraction θ2 is approximated by θ1 / √εr.

[0028] Let the wavelength of the incident electromagnetic wave Da0 in the atmosphere be λ0, and the wavelength of the incident electromagnetic wave Da1 in the lateral direction Yb within the dielectric 30 be λg. Here, if we let the frequency be f0 and the speed of light in the atmosphere be c0, then the incident angle θ1, wavelength λg, wavelength λ0, frequency f0, and speed of light c0 have the relationship shown in Equation 1 below. λg·sinθ=λ0=f0·c0 ····(Equation 1) Within the dielectric 30 of the electromagnetic wave absorber 1A, when the wavefront of the incident electromagnetic wave Da1 coincides with the wavefront of the reflected electromagnetic wave Da3, the electromagnetic waves enter a resonant state. In this case, the wavelength λg, thickness a, distance X, and refraction angle θ2 are related by the following equation 2.

[0029] (a-2·X) / λg=tan(θ2) ····(Equation 2) Next, based on equations 1 and 2, the frequency f0 shown in equation 3 can be determined. That is, the frequency f0 can be determined by the speed of light c0, the angle of refraction θ2, the angle of incidence θ1, the thickness dimension a, and the distance X. In this way, the resonant frequency of the electromagnetic wave in the electromagnetic wave absorber 1A is determined as frequency f0. f0 = c0·tan(θ²) / {(a-2·X)·sinθ} ····(Equation 3)

[0030] Here, when the angle of incidence θ1 of the incident electromagnetic wave Da0 to the electromagnetic wave absorber 1A changes, the angle of refraction θ2 changes. Consequently, the distance X changes, and therefore (a-2·X) changes. Thus, when the angle of incidence θ1 changes, the frequency f0, which is the resonant frequency, changes. For example, when the angle of incidence θ1 increases, (a-2·X) decreases, so the frequency f0, which is the resonant frequency, increases. However, in the electromagnetic wave absorber 1A, which is proportional, the distance X is determined by the positional relationship between the incident electromagnetic wave Da1 and the reflected electromagnetic wave Da2.

[0031] Therefore, as the incident angle θ1 increases, the reflected electromagnetic wave Da3 shifts significantly to the right in the figures relative to the incident electromagnetic wave Da1, as shown in Figures 8 and 9. Consequently, when the incident angle θ1 changes, the positional relationship between the wavefront of the incident electromagnetic wave Da1 and the wavefront of the reflected electromagnetic wave Da3 changes significantly, causing a large change in (a-2·X). This results in a large change in frequency f0 when the incident angle θ1 changes. Figure 8 shows the positional relationship between the wavefront of the incident electromagnetic wave Da1 and the wavefront of the reflected electromagnetic wave Da2 before the incident angle θ1 changes. Figure 9 shows the positional relationship between the wavefront of the incident electromagnetic wave Da1 and the wavefront of the reflected electromagnetic wave Da2 after the incident angle θ1 changes.

[0032] In contrast, the electromagnetic wave absorber 1 of this embodiment can suppress changes in the positional relationship between the incident electromagnetic wave D1 and the reflected electromagnetic wave D3 when the incident angle θ1 changes, as follows. First, in this embodiment, as shown in Figures 10, 11, 12, and 13, the pair of inclined surfaces 22a and 22b retroreflect the incident electromagnetic wave D1 that has passed through the multiple openings 11 of the first metal layer 10 and the dielectric 30 as reflected electromagnetic wave D3. At this time, as shown in Figures 11 and 13, the incident electromagnetic wave D1 is retroreflected by the effective reflective surface 25. The effective reflective surface 25 is a virtual reflective surface formed by the pair of adjacent inclined surfaces 22a and 22b. The effective reflective surface 25 is provided for the convenience of explanation. Here, the angle formed between the wavefront of the incident electromagnetic wave D1 and the transverse direction Yb is defined as the refraction angle θ2.

[0033] As shown in Figure 14, when an incident electromagnetic wave D1 propagates within the dielectric 30 in the stacking direction Ya and the refraction angle θ2 is 0°, the electromagnetic wave resonates when the wavefront K1 of the incident electromagnetic wave D1 coincides with the wavefront K4 of the reflected electromagnetic wave D4. Figure 14 shows a specific example where four vectors G1 to G4 act in parallel. As shown in Figures 15 and 16, when the refraction angle θ2 of the incident electromagnetic wave D1 is 5°, the electromagnetic wave resonates when the wavefront K1 of the incident electromagnetic wave D1 and the wavefront K4 of the reflected electromagnetic wave D4 partially coincide.

[0034] Figure 15 shows an example where the incident electromagnetic wave D1 and the reflected electromagnetic wave D4 have locations where the wavefronts K1 and K4 perfectly coincide and locations where the wavefronts K1 and K4 are misaligned. Figure 16 shows a specific example where the wavefront K1 is formed by connecting and combining the wavefronts of four vectors G1, and the wavefront K4 is formed by connecting and combining the wavefronts of four vectors G4, with two locations where the wavefronts K1 and K4 coincide. In this case, the incident electromagnetic wave D1 and the reflected electromagnetic wave D4 are retroreflective by the effective reflecting surface 25, respectively. On the other hand, when the refraction angle θ2 of the incident electromagnetic wave D1 is greater than 5°, as shown in Figure 17, the incident electromagnetic wave D1 and the reflected electromagnetic wave D4 are retroreflective by the effective reflecting surface 25a, respectively.

[0035] The effective reflective surface 25a is a virtual reflective surface formed by two adjacent pairs of inclined surfaces 22a and 22b. The effective reflective surface 25a is provided for the sake of explanation. As shown in Figure 18, when the refraction angle θ2 of the incident electromagnetic wave D1 is 10°, the incident electromagnetic wave D1 and the reflected electromagnetic wave D4 are retroreflected by the effective reflective surface 25a, respectively. When the wavefront K1 of the incident electromagnetic wave D1 and the wavefront K4 of the reflected electromagnetic wave D4 partially coincide, the electromagnetic waves enter a resonant state. Figure 18 shows a specific example in which the wavefront K1 is formed by connecting and combining the wavefronts of two vectors G1, and the wavefront K4 is formed by connecting and combining the wavefronts of two vectors G4, with five points where the wavefronts K1 and K4 coincide.

[0036] Thus, even if the incident angle θ1 of the incident electromagnetic wave D1 changes and the refraction angle θ2 changes, the incident electromagnetic wave D1 and the reflected electromagnetic wave D4 are retroreflected by the effective reflecting surface 25 or the effective reflecting surface 25a, respectively. Therefore, when the refraction angle θ2 changes, the change in the relative positions of the incident electromagnetic wave D1 and the reflected electromagnetic wave D4 is suppressed. In other words, when the refraction angle θ2 changes, the change in the relative positions of the wavefronts of the incident electromagnetic wave D1 and the reflected electromagnetic wave D4 is suppressed. As a result, when the incident angle θ1 changes, the change in the resonant frequency that occurs when the wavefronts of the incident electromagnetic wave D1 and the reflected electromagnetic wave D4 coincide is suppressed.

[0037] Furthermore, when the incident angle θ1 of the incident electromagnetic wave D1 changes, causing the refraction angle θ2 to change, the change in the relative positions of the incident electromagnetic wave D1 and the reflected electromagnetic wave D7 is suppressed. In other words, when the refraction angle θ2 changes, the change in the relative positions of the wavefronts of the incident electromagnetic wave D1 and the reflected electromagnetic wave D7 is suppressed. This makes it possible to suppress the change in the resonant frequency that occurs when the incident angle θ1 changes and the wavefronts of the incident electromagnetic wave D1 and the reflected electromagnetic wave D7 coincide. As described above, in this embodiment, the electromagnetic wave absorber 1 can suppress changes in the resonant frequency when the incident angle θ1 of the incident electromagnetic wave arriving from one side in the stacking direction Ya changes. In other words, when the incident angle θ1 changes, it is possible to suppress the resonant frequency of the electromagnetic wave from falling outside the predetermined frequency range. Next, the electromagnetic wave attenuation of the electromagnetic wave absorber 1 of this embodiment will be described with reference to Figures 19, 20, 21, and 22.

[0038] Figure 19 is a graph in which a 76 GHz electromagnetic wave is incident on the electromagnetic wave absorber 1A, with the vertical axis representing the reflection loss of the electromagnetic wave by the electromagnetic wave absorber 1A in a proportional manner [dB] and the horizontal axis representing the frequency of the electromagnetic wave [GHz]. Figure 20 is a graph in which a 76 GHz electromagnetic wave is incident on the electromagnetic wave absorber 1 of this embodiment, with the vertical axis representing the reflection loss of the electromagnetic wave by the electromagnetic wave absorber 1 of this embodiment [dB] and the horizontal axis representing the frequency of the electromagnetic wave [GHz]. In Figures 19 and 20, graph NK1 shows the reflection loss of the electromagnetic wave when the incident angle θ1 is 0°. Graph NK2 shows the reflection loss of the electromagnetic wave when the incident angle θ1 is 20°. Graph NK3 shows the reflection loss of the electromagnetic wave when the incident angle θ1 is 40°. Graph NK4 shows the reflection loss of the electromagnetic wave when the incident angle θ1 is 60°.

[0039] In a proportional electromagnetic wave absorber 1A, the electromagnetic wave reflection loss is expressed in dB as the ratio of the emitted electromagnetic wave to the incident electromagnetic wave in the electromagnetic wave absorber 1A. A larger absolute value of the electromagnetic wave reflection loss indicates a greater attenuation of the electromagnetic wave by the electromagnetic wave absorber 1A. The incident electromagnetic wave is the electromagnetic wave incident on the electromagnetic wave absorber 1A, and the emitted electromagnetic wave is the electromagnetic wave emitted from the electromagnetic wave absorber 1A. In the electromagnetic wave absorber 1 of this embodiment, the electromagnetic wave reflection loss is expressed in dB as the ratio of the emitted electromagnetic wave to the incident electromagnetic wave. The incident electromagnetic wave is the electromagnetic wave incident on the electromagnetic wave absorber 1, and the emitted electromagnetic wave is the electromagnetic wave emitted from the electromagnetic wave absorber 1. A larger absolute value of the electromagnetic wave reflection loss indicates a greater attenuation of the electromagnetic wave by the electromagnetic wave absorber 1.

[0040] In Figures 19 and 20, the resonant frequency is the frequency at which the absolute value of the electromagnetic wave reflection loss is maximum in graphs NK1, NK2, NK3, and NK4. In Figures 19 and 20, the resonant frequency at an incident angle θ1 of 20° is higher than the resonant frequency at an incident angle θ1 of 0°. The resonant frequency at an incident angle θ1 of 40° is higher than the resonant frequency at an incident angle θ1 of 20°. The resonant frequency at an incident angle θ1 of 60° is higher than the resonant frequency at an incident angle θ1 of 40°.

[0041] In the proportional electromagnetic wave absorber 1A, if the resonant frequency when the incident angle θ1 is 0° is taken as the reference resonant frequency, then when the incident angle θ1 becomes greater than 0°, the resonant frequency changes significantly with respect to the reference resonant frequency. On the other hand, in the electromagnetic wave absorber 1 of this embodiment, when the incident angle θ1 becomes greater than 0°, the resonant frequency also becomes greater with respect to the reference resonant frequency. However, in this embodiment, when the incident angle θ1 becomes greater than 0°, the amount of change in the resonant frequency with respect to the reference resonant frequency can be suppressed compared to the proportional electromagnetic wave absorber 1A.

[0042] This makes it possible to suppress the change in the resonant frequency when the incident angle θ1 changes in this embodiment. Therefore, it is possible to suppress the resonant frequency from falling outside the predetermined frequency range when the incident angle θ1 changes. Figure 21 is a graph with the reflection loss of electromagnetic waves by the proportionally proportional electromagnetic wave absorber 1A [dB] on the vertical axis and the incident angle of the electromagnetic wave [°] on the horizontal axis. Figure 22 is a graph with the reflection loss of electromagnetic waves by the electromagnetic wave absorber 1 of this embodiment [dB] on the vertical axis and the incident angle of the electromagnetic wave [°] on the horizontal axis. In Figures 15 and 16, graph FN1 shows the reflection loss of electromagnetic waves when the frequency of the electromagnetic wave is 75.5 GHz. Graph FN2 shows the reflection loss of electromagnetic waves when the frequency of the electromagnetic wave is 76.0 GHz.

[0043] Graph FN3 shows the reflection loss of electromagnetic waves when the frequency of the electromagnetic wave is 76.5 GHz. Graph FN4 shows the reflection loss of electromagnetic waves when the frequency of the electromagnetic wave is 77.0 GHz. As shown in Figure 21, in the proportional electromagnetic wave absorber 1A, as can be seen from graphs FN1, FN2, FN3, and FN4, the reflection loss of electromagnetic waves changes significantly depending on the incident angle θ1. On the other hand, as shown in Figure 22, in this embodiment, as can be seen from graphs FN1, FN2, FN3, and FN4, the amount of change in the reflection loss of electromagnetic waves with respect to each incident angle θ1 is small.

[0044] Next, specific examples of the multiple triangular prisms 22 in the second metal layer 20 of this embodiment will be described with reference to Figures 23, 24, 25, 26, 27, and 28. Figure 23 is a graph with the vertical axis representing the absorption frequency of the electromagnetic wave absorber 1 and the horizontal axis representing the height [λ] of the triangular prism 22. The absorption frequency is the frequency at which the electromagnetic wave absorber 1 can attenuate electromagnetic waves by -10 dB or more (for example, the resonant frequency). Here, when the wavelength of the electromagnetic wave propagating within the dielectric 30 is λ, the height [λ] of the triangular prism 22 in Figure 16 is the value obtained by dividing the dimension Tk in the stacking direction Ya on the inclined surfaces 22a and 22b of the triangular prism 22 in Figure 3 by λ. The height [λ] of the triangular prism 22 is information indicating the dimension Tk in the stacking direction Ya on the inclined surfaces 22a and 22b. λ is the wavelength of the electromagnetic wave propagating within the dielectric 30.

[0045] Figure 23 shows the relationship between the absorption frequency and the height of the triangular prism 22 in an electromagnetic wave absorber 1 that does not have a base surface 21a, as shown in Figures 24 and 25. Graph Ta in Figure 23 shows the relationship between the absorption frequency and the height of the triangular prism 22 when the incident angle θ is 0°. Graph Tb shows the relationship between the absorption frequency and the height of the triangular prism 22 when the incident angle θ is 40°. Here, |ΔF| is defined as the absolute value of the difference between the absorption frequency in graph Ta and the absorption frequency in graph Tb. As can be seen from graphs Ta and Tb, when the height [λ] of the triangular prism 22 is 0.15 and 0.55, |ΔF| can be made to be 1 GHz or less.

[0046] Therefore, in the electromagnetic wave absorber 1, if the dimension Tk in the stacking direction Ya of the inclined surfaces 22a and 22b is 0.15·λ or more and 0.55·λ or less, it becomes possible to attenuate electromagnetic waves by -10dB or more while keeping the change in absorption frequency below 1GHz. Figure 24 shows the triangular prism 22 in the electromagnetic wave absorber 1 when the height [λ] of the triangular prism 22 is 0.15, and the angles θa and θb of the inclined surfaces 22a and 22b are 20°, respectively. Figure 25 shows the triangular prism 22 in the electromagnetic wave absorber 1 when the height [λ] of the triangular prism 22 is 0.55, and the angles θa and θb of the inclined surfaces 22a and 22b are 60°, respectively.

[0047] Figure 26 is a graph in which the vertical axis represents the absorption frequency at which the electromagnetic wave absorber 1 can attenuate electromagnetic waves by -10 dB or more, and the horizontal axis represents the height [λ] of the triangular prism 22. Here, when the wavelength of the electromagnetic wave propagating within the dielectric 30 is λ, the height [λ] of the triangular prism 22 in Figure 27 is the value obtained by dividing the dimension Tk in the stacking direction Ya on the inclined surfaces 22a and 22b of the triangular prism 22 in Figure 3 by λ. Figure 26 is a diagram showing the relationship between the absorption frequency in the electromagnetic wave absorber 1, which has a base surface 21a, as shown in Figures 27 and 28, and the height [λ] of the triangular prism 22.

[0048] Graph Tc in Figure 26 shows the relationship between the absorption frequency and the height [λ] of the triangular prism 22 when the incident angle θ is 0°. Graph Td shows the relationship between the absorption frequency and the height [λ] of the triangular prism 22 when the incident angle θ is 40°. Here, |ΔF| is defined as the absolute value of the difference between the absorption frequencies in graph Tc and graph Td. As can be seen from graphs Tc and Td, when the height [λ] of the triangular prism 22 is 0.15 or greater and 0.5 or less, |ΔF| can be made 2GHz or less.

[0049] As a result, in the electromagnetic wave absorber 1, if the height [λ] of the triangular prism 22 is 0.15 or greater and 0.5 or less, it becomes possible to attenuate electromagnetic waves by -10 dB or more while keeping the change in absorption frequency below 2 GHz. Figure 27 shows the triangular prism 22 in the electromagnetic wave absorber 1 when the height [λ] of the triangular prism 22 is 0.15, and the angles θa and θb of the inclined surfaces 22a and 22b are 20°, respectively. Figure 28 shows the triangular prism 22 in the electromagnetic wave absorber 1 when the height [λ] of the triangular prism 22 is 0.5, and the angles θa and θb of the inclined surfaces 22a and 22b are 60°, respectively.

[0050] Incidentally, radar devices mounted on automobiles have become well-known in recent years as obstacle sensors. When radar is mounted on an automobile, radio waves reflected multiple times between the radar substrate and the bumper, and between the radome and the bumper, interfere with the radar's original transmission and reception signals, degrading the radar's performance (for example, maximum detection range and direction estimation accuracy). There is a need to address these multiple reflections of radio waves at low cost and in a compact size. In particular, in recent years, with improvements in processing technology, there has been an increase in cases where antenna sections are formed using metal-coated resin waveguides, and the need for measures to absorb reflected electromagnetic waves (i.e., unwanted electromagnetic waves) on the surface of the radar substrate has increased.

[0051] In response to this, a possible method for absorbing reflected electromagnetic waves on the radar substrate surface is to place a dummy antenna on the substrate surface and convert the reflected electromagnetic waves into heat using a resistor while resonating it with a resonant element on the substrate. Although such a method is useful in principle, if one attempts to implement it using an antenna made of a metal-coated resin waveguide, the waveguide routing becomes long, and the size of the electromagnetic wave absorber becomes large, which presents a challenge.

[0052] Furthermore, as an electromagnetic wave absorber that absorbs reflected electromagnetic waves on the surface of a radar substrate, a radio wave absorber can be considered that has a structure in which a full-surface conductive layer, a dielectric layer consisting of one or multiple dielectric layers, and a pattern layer having multiple conductive patterns are sequentially stacked. In this radio wave absorber, each pattern in the pattern layer differs from other adjacent patterns in at least one of its size and shape. This makes it possible to broaden the bandwidth and widen the angle of incident electromagnetic waves, but since it is in principle a multilayer resin structure, there is a problem that applying the radio wave absorber to the structure of a resin waveguide with a metal coating results in high costs.

[0053] Furthermore, in the field of electromagnetic wave absorbers, a sheet-like electromagnetic wave absorber has been proposed that absorbs electromagnetic waves incident at an oblique angle, i.e., from a direction inclined relative to the direction perpendicular to the sheet surface, in the same way as perpendicular incidence, provided the inclination is within a certain range. However, such electromagnetic wave absorbers present challenges, such as the need to control the particle shape of the soft magnetic metal powder, the packing density, and the thickness of the sheet.

[0054] In contrast, in this embodiment, none of the problems associated with electromagnetic wave absorbers arise. This electromagnetic wave absorber 1 is designed to be easily positioned around an antenna using a metal-coated resin waveguide, can be implemented at low cost, and attenuates electromagnetic waves within a predetermined frequency range even when the incident angle θ changes. According to this embodiment, the electromagnetic wave absorber 1 comprises a first metal layer 10, a second metal layer 20, and a dielectric 30. The first metal layer 10 has a plurality of openings 11 that are opened in the stacking direction Ya and through which incident electromagnetic waves D0 with a frequency within a predetermined frequency range arriving from one side of the stacking direction Ya are transmitted. The dielectric 30 is located on the other side of the stacking direction Ya relative to the first metal layer 10 and transmits the incident electromagnetic waves D1 that have passed through the plurality of openings 11. The second metal layer 20 is located on the other side of the stacking direction Ya relative to the dielectric 30 and has a plurality of inclined surfaces 22a, 22b that retroreflect the incident electromagnetic waves D1 that have passed through the dielectric 30.

[0055] Here, the incident electromagnetic wave D1 that has passed through the dielectric 30 is reflected multiple times between the multiple inclined surfaces 22a, 22b and the first metal layer 10, causing the electromagnetic wave to resonate at a frequency within a predetermined frequency range, and the dielectric 30 attenuates the electromagnetic wave. Specifically, the incident electromagnetic wave D1 that has passed through the multiple openings 11 of the first metal layer 10 and the dielectric 30 is retroreflected by the multiple inclined surfaces 22a, 22b as reflected electromagnetic wave D3. This reflected electromagnetic wave D3 passes through the dielectric 30 and is then reflected by the first metal layer 10 as reflected electromagnetic wave D4. This reflected electromagnetic wave D4 propagates through the dielectric 30. At this time, the wavefront of the reflected electromagnetic wave D4 coincides with the wavefront of the incident electromagnetic wave D1, causing the electromagnetic wave to resonate. Therefore, the dielectric 30 attenuates the electromagnetic wave by converting it into heat through dielectric loss while the electromagnetic wave is in a resonant state.

[0056] Therefore, in this embodiment, as described above, the incident electromagnetic wave D1 is retroreflected by the multiple inclined surfaces 22a and 22b. As a result, when the incident angle θ changes, the change in the positional relationship between the incident electromagnetic wave D1 and the reflected electromagnetic wave D4 can be suppressed. Thus, when the incident angle θ changes, the change in the positional relationship between the wavefront of the incident electromagnetic wave D1 and the wavefront of the reflected electromagnetic wave D4 can be suppressed. Consequently, even if the incident angle θ changes, the change in the resonant frequency of the electromagnetic wave can be suppressed.

[0057] Therefore, even if the angle of incidence θ changes, the electromagnetic waves are resonated at frequencies within a predetermined frequency range, thereby converting electromagnetic waves at frequencies within that predetermined frequency range into heat. As a result, it is possible to provide an electromagnetic wave absorber 1 that efficiently attenuates electromagnetic waves with frequencies within a predetermined frequency range, even when the incident angle θ changes. The electromagnetic wave absorber of this embodiment, configured in this manner, provides the following effects (a), (b), (c), and (d).

[0058] (a) The second metal layer 20 includes an inclined surface 22a whose normal direction hs1 is inclined toward the stacking direction Ya and which becomes more inclined toward one side of the stacking direction Ya as it proceeds toward one side of the lateral direction Yb. The second metal layer 20 also includes an inclined surface 22b located toward one side of the lateral direction Yb with respect to the inclined surface 22a, which is inclined whose normal direction hs2 is inclined toward the stacking direction Ya and which becomes more inclined toward one side of the stacking direction Ya as it proceeds toward the other side of the lateral direction Yb. When one of the inclined surfaces 22a and 22b reflects the incident electromagnetic wave D1, the other inclined surface of the inclined surfaces 22a and 22b reflects the incident electromagnetic wave D1 reflected by the other inclined surface as a reflected electromagnetic wave D2. At this time, the other inclined surface retroreflects the incident electromagnetic wave D1 by reflecting the reflected electromagnetic wave D2 along the direction of propagation of the incident electromagnetic wave D1 and in the opposite direction to the incident electromagnetic wave D1.

[0059] When one of the inclined surfaces 22a and 22b reflects the reflected electromagnetic wave D4, the other inclined surface 22a or 22b reflects the reflected electromagnetic wave D4 that was reflected by the other inclined surface as reflected electromagnetic wave D5. At this time, the other inclined surface retroreflects the reflected electromagnetic wave D4 by reflecting the reflected electromagnetic wave D5 along the direction of propagation of the reflected electromagnetic wave D4 and in the opposite direction to the reflected electromagnetic wave D4. In this way, the incident electromagnetic wave D1 and the reflected electromagnetic wave D4 can be appropriately retroreflected with a simple configuration of inclined surfaces 22a and 22b.

[0060] (b) The second metal layer 20 is provided with a plurality of inclined surfaces 22a and 22b, and the plurality of inclined surfaces 22a and 22b are arranged alternately one by one in the lateral direction Yb. Therefore, incident electromagnetic waves D1 and reflected electromagnetic waves D4 can be appropriately retroreflected. (c) Each of the multiple triangular prisms 22 is positioned to face one of the multiple openings 11. This allows for proper retroreflection of the incident electromagnetic wave D1 and the reflected electromagnetic wave D4. (d) In the electromagnetic wave absorber 1, when the wavelength of the electromagnetic wave propagating within the dielectric 30 of the electromagnetic wave absorber 1 is λ, the dimension Tk of the inclined surfaces 22a and 22b in the stacking direction Ya is set to a dimension of 0.15·λ or more and 0.55·λ or less.

[0061] Therefore, even when the incident angle θ1 of the electromagnetic wave is changed from 0° to 40°, it is possible to attenuate by -10dB or more while keeping the change in absorption frequency below 1GHz. The absorption frequency is the frequency of the electromagnetic wave that resonates in the electromagnetic wave absorber 1 (for example, the resonant frequency). As a result, even when the incident angle θ1 of the electromagnetic wave is changed from 0° to 40°, the change in the resonant frequency can be suppressed, and the resonant frequency can be kept within a predetermined frequency range. Therefore, a sufficient amount of attenuation of the electromagnetic wave can be ensured in the electromagnetic wave absorber 1.

[0062] (e) The second metal layer 20 is provided with a plurality of triangular prisms 22 having inclined surfaces 22a and 22b, which serve as a plurality of reflectors according to this disclosure. In the second metal layer 20, the plurality of triangular prisms 22 are arranged in the lateral direction Yb, so that the inclined surfaces 22a and 22b are arranged alternately one by one in the lateral direction Yb. Therefore, in the second metal layer 20, a plurality of reflectors that retroreflect incident electromagnetic waves D1 and reflected electromagnetic waves D4 can be appropriately configured.

[0063] (Second Embodiment) In the first embodiment described above, in the electromagnetic wave absorber 1, each of the multiple triangular prisms 22 is arranged to face one of the multiple openings 11. However, instead, in this second embodiment, in the electromagnetic wave absorber 1, as shown in Figure 29, each of the multiple triangular prisms 22 is arranged to face two adjacent openings 11 from the multiple openings 11.

[0064] Specifically, the multiple triangular prisms 22 are arranged such that each vertex 22c faces an intermediate portion 11a between two adjacent openings 11. The multiple triangular prisms 22 are arranged such that each inclined surface 22b faces one of the two adjacent openings 11. The multiple triangular prisms 22 are arranged such that each inclined surface 22a faces the other opening 11 of the two adjacent openings 11. Thus, each of the multiple triangular prisms 22 is arranged to face two adjacent openings 11 from the multiple openings 11. That is, each of the multiple triangular prisms 22 is arranged to face a number of openings 11 that is an integer of 2 or more (for example, 2). In this embodiment, the number of openings 11 is an integer multiple of the number of triangular prisms 22 (for example, twice the number of openings).

[0065] (Third embodiment) In the first and second embodiments described above, examples were given in which a triangular prism 22 having inclined surfaces 22a and 22b was used as a plurality of reflectors of the present disclosure. However, instead, in the third embodiment, as shown in Figures 30 and 31, a rectangular prism 22A formed in a rectangular prism shape with inclined surfaces 22a and 22b may be used as a plurality of reflectors of the present disclosure. Figure 30 is a perspective view showing the rectangular prism 22A of the electromagnetic wave absorber 1 of this embodiment. Figure 31 is a perspective view showing one side of the rectangular prism 22A of the electromagnetic wave absorber 1 of this embodiment in the vertical direction Yc.

[0066] In this embodiment, the rectangular prism 22A has a cross-sectional view that forms a rectangle when cut by a virtual plane parallel to the stacking direction Ya and parallel to the transverse direction Yb. Each rectangular prism 22A is positioned to face one of the multiple openings 11. Between the inclined surfaces 22a and 22b of the rectangular prism 22A, an upper surface 22d is formed that is parallel to the transverse direction Yb and extends in the vertical direction Yc. The upper surface 22d is positioned to face the opening 11 of the first metal layer 110.

[0067] (Fourth Embodiment) In this fourth embodiment of the electromagnetic wave absorber 1, an example of retroreflecting radio waves by using triangular prisms 22X and 22Y arranged in the vertical direction Yc in addition to the triangular prism 22 in the electromagnetic wave absorber 1 of the first embodiment described above, with reference to Figures 32, 33, and 34. Figure 32 is a perspective view showing the triangular prisms 22, 22X, and 22Y of the electromagnetic wave absorber 1 of this embodiment, and is an enlarged perspective view of a part of the electromagnetic wave absorber 1 of this embodiment.

[0068] Figure 33 is a front view of the triangular prisms 22, 22X, and 22Y of the electromagnetic wave absorber 1 of this embodiment with the first metal layer 10 removed. Figure 34 is a perspective view of the electromagnetic wave absorber 1 of this embodiment. As shown in Figures 32, 33, and 34, the second metal layer 20 of the electromagnetic wave absorber 1 of this embodiment is provided with triangular prisms 22X and 22Y for each triangular prism 22. The triangular prism 22X is a first reflector positioned on one side of the triangular prism 22 in the vertical direction Yc (i.e., the third direction) relative to the triangular prism 22.

[0069] The triangular prism 22X is a reflector formed in the shape of a triangular prism. The triangular prism 22X is formed such that its axis Sb extends in the lateral direction Yb. The triangular prism 22X is connected to one side of the triangular prism 22 in the vertical direction Yc. The triangular prism 22X is positioned on one side of the stacking direction Ya with respect to the base layer 21. The triangular prism 22X has an inclined surface 22e and an upper surface 22f. The inclined surface 22e is positioned on one side of the vertical direction Yc with respect to the inclined surfaces 22a and 22b. The vertical direction Yc is a direction perpendicular to the stacking direction Ya and perpendicular to the lateral direction Yb, and is the direction connecting the inclined surfaces 22a and 22b. The inclined surface 22e is a third inclined surface positioned such that its normal direction ks1 is inclined with respect to the stacking direction Ya.

[0070] The inclined surface 22e is formed such that as it moves toward one side in the vertical direction Yc, it moves toward one side in the stacking direction Ya. The inclined surface 22e is formed such that as it moves toward the other side in the vertical direction Yc, it moves toward the other side in the stacking direction Ya. The top surface 22f is positioned on one side in the vertical direction Yc relative to the inclined surface 22e. The top surface 22f is formed parallel to the stacking direction Ya and parallel to the horizontal direction Yb. The triangular prism 22Y is a second reflector positioned on the other side in the vertical direction Yc relative to each triangular prism 22. The triangular prism 22Y is connected to the other side of the triangular prism 22 in the vertical direction Yc (i.e., the third direction).

[0071] The triangular prism 22Y is positioned on one side of the stacking direction Ya relative to the base layer 21. The triangular prism 22Y has an inclined surface 22g and a bottom surface 22h. The triangular prism 22Y is a reflector formed in the shape of a triangular prism. The triangular prism 22Y is formed such that its axis Sc extends in the lateral direction Yb. The inclined surface 22g is a fourth inclined surface positioned such that its normal direction ks2 is inclined with respect to the stacking direction Ya. The inclined surface 22g is positioned on the other side of the vertical direction Yc relative to the inclined surfaces 22a and 22b. The inclined surface 22g is formed such that as it moves toward the other side of the vertical direction Yc, it moves toward one side of the stacking direction Ya.

[0072] The inclined surface 22g is formed such that as it progresses along one side in the vertical direction Yc, it progresses along the other side in the stacking direction Ya. The lower surface 22h is located on the other side of the vertical direction Yc relative to the inclined surface 22g. The lower surface 22h is formed parallel to the stacking direction Ya and parallel to the horizontal direction Yb. In this embodiment, the inclined surface 22e of the triangular prism 22X and the inclined surface 22g of the triangular prism 22Y face each other via the dielectric 30 for each triangular prism 22. The inclined surfaces 22e and 22g, like the inclined surfaces 22a and 22b, retroreflect the incident electromagnetic wave D1 and the reflected electromagnetic wave D4, as will be described later.

[0073] Next, the operation of the electromagnetic wave absorber 1 in this embodiment will be described. In this embodiment, when one of the inclined surfaces 22e and 22g reflects the incident electromagnetic wave D1, the other inclined surface 22e and 22g reflects the incident electromagnetic wave D1 reflected by the other inclined surface as reflected electromagnetic wave D2. The other inclined surface reflects the reflected electromagnetic wave D2 reflected by the other inclined surface as reflected electromagnetic wave D3, parallel to the direction of propagation of the incident electromagnetic wave D1 and in the opposite direction to the incident electromagnetic wave D1. That is, the other inclined surface reflects the reflected electromagnetic wave D2 reflected by the other inclined surface as reflected electromagnetic wave D3, along the direction of propagation of the incident electromagnetic wave D1 and in the opposite direction to the incident electromagnetic wave D1. As a result, the inclined surfaces 22e and 22g retroreflect the incident electromagnetic wave D1.

[0074] Furthermore, when one of the inclined surfaces 22e and 22g reflects the reflected electromagnetic wave D4, the other inclined surface 22e and 22g reflects the reflected electromagnetic wave D4 reflected by the other inclined surface as reflected electromagnetic wave D5. The other inclined surface reflects the reflected electromagnetic wave D5 reflected by the other inclined surface as reflected electromagnetic wave D6, parallel to the direction of propagation of the reflected electromagnetic wave D4 and in the opposite direction to the reflected electromagnetic wave D4. In other words, the other inclined surface reflects the reflected electromagnetic wave D5 reflected by the one inclined surface as reflected electromagnetic wave D6, along the direction of propagation of the reflected electromagnetic wave D4 and in the opposite direction to the reflected electromagnetic wave D4. As a result, the inclined surfaces 22e and 22g retroreflect the reflected electromagnetic wave D4.

[0075] According to the embodiment described above, the second metal layer 20 includes an inclined surface 22e located on one side of the vertical direction Yc relative to the inclined surfaces 22a and 22b. The inclined surface 22e is formed such that its normal direction ks1 is inclined toward the stacking direction Ya, and as it approaches one side of the vertical direction Yc, it advances toward one side of the stacking direction Ya. The second metal layer 20 also includes an inclined surface 22g located on the other side of the vertical direction Yc relative to the inclined surfaces 22a and 22b. The inclined surface 22g is formed such that its normal direction ks2 is inclined toward the stacking direction Ya, and as it approaches the other side of the vertical direction Yc, it advances toward one side of the stacking direction Ya. As a result, the inclined surfaces 22e and 22g can appropriately retroreflect incident electromagnetic waves D1 or reflected electromagnetic waves D4.

[0076] In this embodiment, the second metal layer 20 includes a triangular prism 22X, which is positioned on one side in the vertical direction Yc relative to the inclined surfaces 22a and 22b and has an inclined surface 22e, forming a triangular prism shape. The second metal layer 20 also includes a triangular prism 22Y, which is positioned on the other side in the vertical direction Yc relative to the inclined surfaces 22a and 22b and has an inclined surface 22g, forming a triangular prism shape. In the second metal layer 20, the triangular prisms 22X and 22Y are provided for each triangular prism 22. Therefore, the triangular prisms 22X and 22Y make it easy to realize inclined surfaces 22e and 22g for each triangular prism 22 that appropriately retroreflect incident electromagnetic waves D1 or reflected electromagnetic waves D4.

[0077] (Fifth embodiment) In the fourth embodiment described above, an example was described in which triangular prisms 22X and 22Y were used in the electromagnetic wave absorber 1 to retroreflect incident electromagnetic waves D1 or reflected electromagnetic waves D4. However, instead, this fifth embodiment of the electromagnetic wave absorber 1, in which rectangular prisms 22S and 22T are used to retroreflect incident electromagnetic waves D1 or reflected electromagnetic waves D4, will be described with reference to Figures 35, 36, 37, and 38.

[0078] Figure 35 is a perspective view showing a part of the structure of the electromagnetic wave absorber 1 of this embodiment. Figure 36 is a front view showing a part of the structure of the electromagnetic wave absorber 1 of this embodiment. Figure 37 is a side view showing a part of the structure of the electromagnetic wave absorber 1 of this embodiment. Figure 38 is a perspective view showing a part of the structure of the electromagnetic wave absorber 1 of this embodiment. In the second metal layer 20 of the electromagnetic wave absorber 1 of this embodiment, as shown in Figures 28, 29, 30, and 31, rectangular prisms 22S and 22T are provided for each rectangular prism 22A. The rectangular prism 22S is a first reflector that is positioned on one side of the rectangular prism 22A in the vertical direction (i.e., the third direction) Yc relative to the rectangular prism 22A.

[0079] The rectangular prism 22S is connected to one side of the rectangular prism 22A in the vertical direction Yc. The rectangular prism 22S is positioned on one side of the stacking direction Ya relative to the base layer 21. The rectangular prism 22S is a reflector formed in the shape of a rectangular prism. The rectangular prism 22S is formed such that its axis Sd extends in the horizontal direction Yb. The rectangular prism 22S has an inclined surface 22e. The inclined surface 22e is positioned such that its normal direction ks1 is inclined with respect to the stacking direction Ya. The inclined surface 22e is formed such that as it moves toward one side of the vertical direction Yc, it moves toward one side of the stacking direction Ya. The inclined surface 22e is formed such that as it moves toward the other side of the vertical direction Yc, it moves toward the other side of the stacking direction Ya.

[0080] The rectangular prism 22T is a second reflector positioned on the other side of the rectangular prism 22A in the vertical direction (i.e., the third direction) Yc relative to the rectangular prism 22A. The rectangular prism 22T is connected to the other side of the rectangular prism 22A in the vertical direction Yc. The rectangular prism 22T is positioned on one side of the stacking direction Ya relative to the base layer 21. The rectangular prism 22T is a reflector formed in the shape of a rectangular prism. The rectangular prism 22T is formed such that its axis Se extends in the horizontal direction Yb. The rectangular prism 22T has an inclined surface 22g. The inclined surface 22g is positioned such that its normal direction ks2 is inclined with respect to the stacking direction Ya. The inclined surface 22g is formed such that as it proceeds toward the other side of the vertical direction Yc, it moves toward the one side of the stacking direction Ya. The inclined surface 22g is formed such that as it proceeds toward the one side of the vertical direction Yc, it moves toward the other side of the stacking direction Ya.

[0081] In this embodiment, the inclined surface 22e of the rectangular prism 22S and the inclined surface 22g of the rectangular prism 22T face each other via a dielectric 30 for each rectangular prism 22A. The inclined surfaces 22e and 22g, like the inclined surfaces 22a and 22b, retroreflect the incident electromagnetic wave D1 and the reflected electromagnetic wave D4, as will be described later. Next, the operation of the electromagnetic wave absorber 1 in this embodiment will be described. In this embodiment, when one of the inclined surfaces 22e and 22g reflects the incident electromagnetic wave D1, the other inclined surface 22e and 22g reflects the incident electromagnetic wave D1 that was reflected by the other inclined surface as reflected electromagnetic wave D2.

[0082] The other inclined surface reflects the reflected electromagnetic wave D2, which is reflected by one of the inclined surfaces, as reflected electromagnetic wave D3, parallel to the direction of propagation of the incident electromagnetic wave D1 and in the opposite direction to the incident electromagnetic wave D1. In other words, the other inclined surface reflects the reflected electromagnetic wave D2, which is reflected by one of the inclined surfaces, as reflected electromagnetic wave D3, along the direction of propagation of the incident electromagnetic wave D1 and in the opposite direction to the incident electromagnetic wave D1. As a result, the inclined surfaces 22e and 22g retroreflect the incident electromagnetic wave D1.

[0083] Furthermore, when one of the inclined surfaces 22e and 22g reflects the reflected electromagnetic wave D4, the other inclined surface 22e and 22g reflects the reflected electromagnetic wave D4 reflected by the other inclined surface as reflected electromagnetic wave D5. The other inclined surface reflects the reflected electromagnetic wave D5 reflected by the other inclined surface as reflected electromagnetic wave D6, parallel to the direction of propagation of the reflected electromagnetic wave D4 and in the opposite direction to the reflected electromagnetic wave D4. In other words, the other inclined surface reflects the reflected electromagnetic wave D5 reflected by the one inclined surface as reflected electromagnetic wave D6, along the direction of propagation of the reflected electromagnetic wave D4 and in the opposite direction to the reflected electromagnetic wave D4. As a result, the inclined surfaces 22e and 22g retroreflect the reflected electromagnetic wave D4.

[0084] According to the embodiment described above, the second metal layer 20 is located on one side of the vertical direction Yc with respect to the inclined surfaces 22a and 22b, and includes an inclined surface 22e whose normal direction is inclined in the stacking direction, and which is formed to advance toward one side of the stacking direction Ya as it moves toward one side of the vertical direction Yc. The second metal layer 20 is located on the other side of the vertical direction Yc with respect to the inclined surfaces 22a and 22b, and includes an inclined surface 22g whose normal direction is inclined in the stacking direction, and which is formed to advance toward one side of the stacking direction Ya as it moves toward the other side of the vertical direction Yc. As a result, the inclined surfaces 22e and 22g can appropriately retroreflect incident electromagnetic waves D1 or reflected electromagnetic waves D4.

[0085] In this embodiment, the second metal layer 20 includes a rectangular prism 22S, which is positioned on one side in the vertical direction Yc relative to the inclined surfaces 22a and 22b and has an inclined surface 22e, forming a rectangular prism shape. The second metal layer 20 also includes a rectangular prism 22T, which is positioned on the other side in the vertical direction Yc relative to the inclined surfaces 22a and 22b and has an inclined surface 22g, forming a rectangular prism shape. In the second metal layer 20, the rectangular prisms 22S and 22T are provided for each triangular prism 22. Therefore, the rectangular prisms 22S and 22T can easily create inclined surfaces 22e and 22g that appropriately retroreflect incident electromagnetic waves D1 or reflected electromagnetic waves D4.

[0086] (Sixth Embodiment) In the first embodiment described above, an example was described in which the second reflective surface of the second metal layer 20 of the electromagnetic wave absorber 1 was formed by the inclined surfaces 22a and 22b of a triangular prism 22. However, instead, this fifth embodiment, in which the inclined surfaces 22i and 22j of the triangular prism 22 in the second metal layer 20 of the electromagnetic wave absorber 1 are formed by curved surfaces, will be described with reference to Figures 39, 40, 41, and 42.

[0087] Figure 39 is a perspective view showing the inclined surfaces 22i and 22j of the triangular prism 22 of the electromagnetic wave absorber 1 of this embodiment. Figure 40 is a front view showing the inclined surfaces 22i and 22j of the triangular prism 22 of the electromagnetic wave absorber 1 of this embodiment. Figure 41 is a side view showing the inclined surfaces 22i and 22j of the triangular prism 22 of the electromagnetic wave absorber 1 of this embodiment. Figure 42 is a perspective view showing the inclined surfaces 22i and 22j of the triangular prism 22 of the electromagnetic wave absorber 1 of this embodiment. The triangular prism 22 of this embodiment is formed similarly to the triangular prism 22 of the first embodiment described above, such that its axis Sa extends in the vertical direction Yc. The inclined surface 22i is formed in a curved shape that is concave on the other side in the stacking direction Ya. The inclined surface 22i is positioned on one side in the lateral direction Yb relative to the inclined surface 22j.

[0088] The inclined surface 22i is a first inclined surface formed such that as it progresses toward one side in the lateral direction Yb, it moves toward the other side in the stacking direction Ya. The inclined surface 22i is formed such that as it progresses toward the other side in the lateral direction Yb, it moves toward one side in the stacking direction Ya. The inclined surface 22j is a first inclined surface formed in a curved shape that is concave toward the other side in the stacking direction Ya. The inclined surface 22j is located on the other side in the lateral direction Yb relative to the inclined surface 22i. The inclined surface 22j is formed such that as it progresses toward the other side in the lateral direction Yb, it moves toward the other side in the stacking direction Ya. The inclined surface 22j is formed such that as it progresses toward one side in the lateral direction Yb, it moves toward one side in the stacking direction Ya. The inclined surfaces 22i and 22j reflect incident electromagnetic waves D1 and reflected electromagnetic waves D4, respectively, similar to the inclined surfaces 22a and 22b of the first embodiment described above.

[0089] (Seventh Embodiment) In the first embodiment described above, an example was described in which retroreflection occurs at two inclined surfaces 22a and 22b in the second metal layer 20 of the electromagnetic wave absorber 1. However, instead, this seventh embodiment, in which retroreflection occurs at three inclined surfaces 24a, 24b, and 24c in the second metal layer 20 of the electromagnetic wave absorber 1, will be described with reference to Figures 43, 44, 45, and 46. Figure 43 is a front view of the electromagnetic wave absorber 1 of this embodiment, and Figure 44 is a side view of the electromagnetic wave absorber 1 of this embodiment. Figure 45 is a front view of the second metal layer 20 alone, with the first metal layer 10 and dielectric 30 removed from the electromagnetic wave absorber 1 of Figure 37. Figure 46 is a perspective view showing the positional relationship of the three inclined surfaces 24a, 24b, and 24c.

[0090] The electromagnetic wave absorber 1 of this embodiment differs from the electromagnetic wave absorber 1 of the first embodiment in the second metal layer 20. In contrast, the second metal layer 20 of the electromagnetic wave absorber 1 of this embodiment will be described below. The second metal layer 20 is provided with a reflective layer 24 that replaces the plurality of triangular prisms 22. The reflective layer 24 is located on one side of the stacking direction Ya with respect to the base layer 21. The reflective layer 24 is formed as a film that extends in the lateral direction Yb and the vertical direction Yc, with the stacking direction Ya being the thickness direction. The reflective layer 24 is made of a conductive metal material such as copper or silver.

[0091] In this embodiment, a plurality of recesses 24U are provided on one side of the reflective layer 24 in the stacking direction Ya. Each of the plurality of recesses 24U is formed to open on one side in the stacking direction Ya and recess on the other side in the stacking direction Ya. In Figure 46, the recesses 24U are shown opening diagonally upward. Each of the plurality of recesses 24U forms a triangular pyramidal recess with inclined surfaces 24a, 24b, and 24c. Inclined surface 24a is a reflective surface whose normal direction hsa is inclined with respect to the stacking direction Ya. Inclined surface 24b is a reflective surface whose normal direction hsb is inclined with respect to the stacking direction Ya. Inclined surface 24c is a reflective surface whose normal direction hsc is inclined with respect to the stacking direction Ya. The inclined surfaces 24a, 24b, and 24c are three reflective surfaces having a first reflective surface, a second reflective surface, and a third reflective surface.

[0092] Next, the operation of the electromagnetic wave absorber 1 of this embodiment will be described. First, among the inclined surfaces 24a, 24b, and 24c, for example, the first inclined surface reflects the incident electromagnetic wave D1. Then, among the inclined surfaces 24a, 24b, and 24c, the second inclined surface other than the first inclined surface reflects the incident electromagnetic wave D1 reflected by the first inclined surface as reflected electromagnetic wave D2. In this case, among the inclined surfaces 24a, 24b, and 24c, the third inclined surface other than the first and second inclined surfaces reflects the reflected electromagnetic wave D2 reflected by the second inclined surface as reflected electromagnetic wave D3, parallel to the incident electromagnetic wave D1 and in the opposite direction to the incident electromagnetic wave D1. That is, the third inclined surface reflects the reflected electromagnetic wave D2 reflected by the second inclined surface as reflected electromagnetic wave D3, along the incident electromagnetic wave D1 and in the opposite direction to the incident electromagnetic wave D1. As a result, the incident electromagnetic wave D1 is retroreflected by the inclined surfaces 24a, 24b, and 24c.

[0093] Furthermore, among the inclined surfaces 24a, 24b, and 24c, for example, the first inclined surface reflects the reflected electromagnetic wave D4. Then, among the inclined surfaces 24a, 24b, and 24c, the second inclined surface other than the first inclined surface reflects the reflected electromagnetic wave D4 reflected by the first inclined surface as reflected electromagnetic wave D5. In this case, among the inclined surfaces 24a, 24b, and 24c, the third inclined surface other than the first and second inclined surfaces reflects the reflected electromagnetic wave D5 reflected by the second inclined surface as reflected electromagnetic wave D6, parallel to the reflected electromagnetic wave D4 and in the opposite direction to the reflected electromagnetic wave D4. That is, the third inclined surface reflects the reflected electromagnetic wave D5 reflected by the second inclined surface as reflected electromagnetic wave D6, along the reflected electromagnetic wave D4 and in the opposite direction to the reflected electromagnetic wave D4. As a result, the reflected electromagnetic wave D4 is retroreflected by the inclined surfaces 24a, 24b, and 24c.

[0094] According to the embodiment described above, the second metal layer 20 has inclined surfaces 24a, 24b, and 24c in the reflective layer 24 that form a triangular pyramidal recess 24U that is recessed from one side to the other in the stacking direction Ya. The inclined surfaces 24a, 24b, and 24c can appropriately retroreflect the incident electromagnetic wave D1 and the reflected electromagnetic wave D4. (Other embodiments)

[0095] (1) In the first embodiment described above, an example was described in which the electromagnetic wave absorber 1 retroreflects using two inclined surfaces 22a and 22b. Furthermore, in the seventh embodiment described above, an example was described in which the electromagnetic wave absorber 1 retroreflects using three inclined surfaces 24a, 24b, and 24c. Alternatively, in the first to seventh embodiments described above, the electromagnetic wave absorber 1 may retroreflect using four or more inclined surfaces.

[0096] (2) In the second embodiment described above, an example was described in which each of the multiple openings 11 is arranged to face two adjacent openings 11 from among the multiple triangular prisms 22. However, instead, each of the multiple openings 11 may be arranged to face three or more adjacent triangular prisms 22 from among the multiple triangular prisms 22. Here, the number of openings 11 is an integer multiple of the number of triangular prisms 22.

[0097] (3) In the second embodiment described above, an example was given in which a triangular prism 22, formed in the shape of a triangular prism, was used as the reflector of the present disclosure. However, the present disclosure is not limited to this, and a rectangular prism 22A, as described in the third embodiment described above, may also be used as a plurality of reflectors of the present disclosure. The rectangular prism 22A is a reflector formed in the shape of a rectangular prism, and a plurality of rectangular prisms 22A are arranged in the lateral direction Yb. In this case, the plurality of openings 11 are arranged so as to face two or more adjacent rectangular prisms 22A from the plurality of rectangular prisms 22A. Here, the number of openings 11 is an integer multiple of the number of rectangular prisms 22A.

[0098] (4) In the first to seventh embodiments described above, an example was given in which a first metal layer 10 having a plurality of openings 11 was used as the first member of the present disclosure. However, instead, a semi-reflective member located on one side of the stacking direction Ya with respect to the dielectric 30 may be used as the first member of the present disclosure. In this case, when electromagnetic waves are incident on the semi-reflective member from one side of the stacking direction Ya, a portion of the incident electromagnetic waves are transmitted through the semi-reflective member.

[0099] The transmitted electromagnetic waves, after passing through the dielectric 30, are retroreflected by the multiple inclined surfaces 22a and 22b. These retroreflected electromagnetic waves, after passing through the dielectric 30, are incident on the semi-reflective member. A portion of these incident electromagnetic waves is reflected by the semi-reflective member and emitted from the semi-reflective member to the other side in the stacking direction Ya. In this way, the electromagnetic waves undergo multiple reflections between the semi-reflective member and the multiple inclined surfaces 22a and 22b.

[0100] (5) In the first to seventh embodiments described above, an example was given in which a first metal layer 10 having a plurality of openings 11 was used as the first member of the present disclosure. However, instead, a plurality of metal patches arranged on one side of the stacking direction Ya with respect to the dielectric 30 may be used as the first member of the present disclosure. The plurality of metal patches are formed in the shape of plates from, for example, a conductive metal material. The plurality of metal patches function as antennas that absorb electromagnetic waves arriving from one side of the stacking direction Ya and re-emit electromagnetic waves to the other side of the stacking direction Ya.

[0101] As a result, the multiple metal patches allow a portion of the electromagnetic waves arriving from one side in the stacking direction Ya to pass through the multiple electrodes. On the other hand, when electromagnetic waves that have passed through the dielectric 30 are incident on the multiple metal patches, the multiple metal patches absorb the incident electromagnetic waves and re-emit them to the other side in the stacking direction Ya. As a result, the multiple metal patches reflect a portion of the electromagnetic waves arriving from the other side in the stacking direction Ya. As the first member of this disclosure, multiple conductive loops may be used instead of multiple metal patches. The multiple conductive loops may be annular loops or angular annular loops.

[0102] (6) In the sixth embodiment described above, an example was given in which the inclined surfaces 22i and 22j were curved surfaces. However, in addition to this, the following may also be done. That is, in the second embodiment described above, the inclined surfaces 22a and 22b are curved surfaces. Similarly, in the third embodiment described above, the inclined surfaces 22a and 22b of the rectangular prism 22A are curved surfaces. In the fourth embodiment described above, the inclined surface 22e of the triangular prism 22X and the inclined surface 22g of the triangular prism 22Y are curved surfaces. In the fifth embodiment described above, the inclined surface 22e of the rectangular prism 22S and the inclined surface 22g of the rectangular prism 22T are curved surfaces.

[0103] (7) In the fourth embodiment described above, an example was described in which a triangular prism 22X was used as the first reflector positioned on one side in the vertical direction Yc with respect to the inclined surfaces 22a and 22b. An example was described in which a triangular prism 22Y was used as the second reflector positioned on the other side in the vertical direction Yc with respect to the inclined surfaces 22a and 22b. However, instead, a rectangular prism formed in the shape of a rectangular prism may be used as at least one of the first and second reflectors.

[0104] (8) In the fifth embodiment described above, an example was described in which a rectangular prism 22S was used as the first reflector positioned on one side in the vertical direction Yc with respect to the inclined surfaces 22a and 22b. An example was described in which a rectangular prism 22T was used as the second reflector positioned on the other side in the vertical direction Yc with respect to the inclined surfaces 22a and 22b. However, instead, a triangular prism formed in the shape of a triangular prism may be used as at least one of the first and second reflectors.

[0105] (9) This disclosure is not limited to the embodiments described above, and can be modified as appropriate within the scope of the claims. Furthermore, the embodiments described above are not unrelated to each other and can be combined as appropriate, except in cases where the combination is clearly impossible. In addition, it goes without saying that the elements constituting the embodiments are not necessarily essential, except in cases where they are explicitly stated to be particularly essential or where they are clearly considered essential in principle. Furthermore, in the embodiments described above, when numerical values ​​such as the number, numerical values, quantities, or ranges of the components of the embodiments are mentioned, they are not limited to those specific numbers, except in cases where they are explicitly stated to be particularly essential or where they are clearly limited to a specific number in principle. Furthermore, in the embodiments described above, when the shape, positional relationship, etc., of the components are mentioned, they are not limited to those shapes, positional relationships, etc., except in cases where they are explicitly stated to be particularly essential or where they are clearly limited to a specific shape, positional relationship, etc., in principle.

[0106] The above disclosure can be understood from the following perspectives, for example. (Perspective of this disclosure) [First point of view] It is an electromagnetic wave absorber, A first member (10) that transmits electromagnetic waves incident from one side in a predetermined direction (Ya), A dielectric (30) disposed on the other side in the predetermined direction relative to the first member, A second member (20) is positioned on the other side of the predetermined direction relative to the dielectric, and has a plurality of inclined surfaces (22a, 22b, 22i, 22j, 24a, 24b, 24c) whose normal directions (hs1, hs2, hsa, hsb, hsc) are inclined with respect to the predetermined direction, retroreflecting the electromagnetic waves that have passed through the dielectric, An electromagnetic wave absorber in which the dielectric attenuates the electromagnetic wave when the electromagnetic wave that has passed through the first member undergoes multiple reflections between the plurality of inclined surfaces and the first member and resonates. [Second perspective] The incident electromagnetic wave (D1), which is the electromagnetic wave that has passed through the first member and the dielectric, is retroreflected by the plurality of inclined surfaces. The retroreflected incident electromagnetic wave, as the first reflected electromagnetic wave (D3), passes through the dielectric. The first reflected electromagnetic wave that has passed through the dielectric is reflected by the first member as a second reflected electromagnetic wave (D4). An electromagnetic wave absorber according to the first aspect, wherein the electromagnetic wave resonates when the wavefront of the second reflected electromagnetic wave (D4) reflected by the first member coincides with the wavefront of the incident electromagnetic wave (D1) transmitted through the dielectric. [Third perspective] When the predetermined direction is defined as the first direction (Ya) and the direction perpendicular to the first direction is defined as the second direction (Yb), The plurality of reflective surfaces include a first inclined surface (22a) formed such that as it proceeds toward one side of the second direction, it moves toward one side of the first direction, and a second inclined surface (22b) positioned on the other side of the second direction relative to the first inclined surface, and formed such that as it proceeds toward the other side of the second direction, it moves toward one side of the first direction. An electromagnetic wave absorber according to the first or second aspect, wherein when one of the first and second inclined surfaces reflects electromagnetic waves (D1, D4) that have passed through the dielectric, the other inclined surface, other than the first and second inclined surfaces, reflects the electromagnetic waves (D2, D5) reflected by the one inclined surface along the direction of propagation of the electromagnetic waves that have passed through the dielectric and in the opposite direction to the electromagnetic waves that have passed through the dielectric, thereby retroreflecting the electromagnetic waves that have passed through the dielectric. [Fourth perspective] The plurality of inclined surfaces comprises a plurality of first inclined surfaces and a plurality of second inclined surfaces, An electromagnetic wave absorber according to any third view, wherein the plurality of first inclined surfaces and the plurality of second inclined surfaces are arranged alternately one by one in the second direction. [Fifth perspective] The second member is provided with a plurality of reflectors (22) that are formed in a prismatic shape, each having a first inclined surface (22b) and a second inclined surface (22a) positioned on the other side in the second direction relative to the first inclined surface. The electromagnetic wave absorber according to the fourth view, wherein the plurality of reflectors are arranged in the second direction, so that the plurality of first inclined surfaces and the plurality of second inclined surfaces are arranged alternately one by one in the second direction. [Sixth perspective] The first member is provided with a plurality of openings (11) that allow electromagnetic waves incident from one side in a predetermined direction (Ya) to pass through, The electromagnetic wave absorber according to the fifth aspect, wherein each of the plurality of reflectors is arranged to face one of the plurality of openings. [Seventh perspective] The first member is provided with a plurality of openings (11) that allow electromagnetic waves incident from one side in a predetermined direction (Ya) to pass through, Each of the plurality of reflectors is arranged to face two or more of the plurality of openings, The electromagnetic wave absorber according to the fifth aspect, wherein the number of openings is an integer multiple of the number of reflectors. [Perspective 8] The electromagnetic wave absorber according to the fifth view, wherein the reflector is formed in a triangular prism shape having the first inclined surface and the second inclined surface. [Perspective 9] The electromagnetic wave absorber according to the fifth view, wherein the reflector is formed in a rectangular prism shape having the first inclined surface and the second inclined surface. [Perspective 10] When the third direction (Yc) is a direction that is perpendicular to both the first and second directions, The aforementioned multiple inclined surfaces are, A third inclined surface (22e) is positioned on one side of the third direction relative to the first and second inclined surfaces, and is formed such that its normal direction (ks1) is inclined in the first direction, and that as it proceeds toward one side of the third direction, it moves toward one side of the first direction. The present invention comprises a fourth inclined surface (22g) positioned on the other side of the third direction relative to the first and second inclined surfaces, having a normal direction (ks2) inclined in the first direction and formed such that as it proceeds toward the other side of the third direction, it moves toward one side of the first direction, An electromagnetic wave absorber according to any third aspect, wherein when one of the third and fourth inclined surfaces reflects electromagnetic waves (G1, G4) that have passed through the dielectric, the other inclined surface of the third and fourth inclined surfaces reflects the electromagnetic waves reflected by the one inclined surface in the direction of propagation of the electromagnetic waves that have passed through the dielectric and in the opposite direction to the electromagnetic waves that have passed through the dielectric, thereby retroreflecting the electromagnetic waves that have passed through the dielectric. [Perspective 11] The second member is, A first reflector (22X, 22S) is positioned on one side in the third direction relative to the first and second inclined surfaces, and is formed in a prismatic shape having the third inclined surface, The electromagnetic wave absorber according to the tenth aspect, comprising: a second reflector (22Y, 22T) disposed on the other side in the third direction with respect to the first and second inclined surfaces, and having a fourth inclined surface and formed in a prismatic shape. [Perspective 12] The electromagnetic wave absorber according to the eleventh aspect, wherein at least one of the first reflector and the second reflector is formed in the shape of a triangular prism. [Perspective 13] The electromagnetic wave absorber according to the eleventh aspect, wherein at least one of the first reflector and the second reflector is formed in the shape of a rectangular prism. [Perspective 14] The electromagnetic wave absorber according to the third aspect, wherein, if the wavelength of the electromagnetic wave propagating within the dielectric is λ, the dimension (Tk) of the first inclined surface and the second inclined surface in the first direction is set to be 0.15·λ or more and 0.55·λ or less. [Perspective 15] The second member has three reflective surfaces (24a, 24b, 24c) that form a triangular pyramidal recess (24U) that is recessed from one side to the other in the predetermined direction. An electromagnetic wave absorber according to the first aspect, wherein, in the case where the first of the three reflective surfaces reflects electromagnetic waves transmitted through the dielectric, and the second of the three reflective surfaces other than the first reflecting surface reflects the electromagnetic waves reflected by the first reflecting surface, the third of the three reflective surfaces other than the first and second reflecting surfaces reflects the electromagnetic waves reflected by the second reflecting surface along the direction of propagation of the electromagnetic waves transmitted through the dielectric and in the opposite direction to the electromagnetic waves transmitted through the dielectric, thereby retroreflecting the electromagnetic waves transmitted through the dielectric. [Perspective 16] The electromagnetic wave absorber according to any of the first to fifteenth viewpoints, wherein each of the plurality of inclined surfaces is a curved surface. [Explanation of Symbols]

[0107] 1. Electromagnetic wave absorber 10 1st metal layer 13 Reflective surface 20 Second metal layer 22 Triangular prisms 22A square prism 22S square prism 22T square prism 22Y Triangular Prism 22X Triangular prism 22a Slope 22b Slope 30 Dielectrics

Claims

1. It is an electromagnetic wave absorber, A first member (10) that transmits electromagnetic waves incident from one side in a predetermined direction (Ya), A dielectric (30) is disposed on the other side in the predetermined direction relative to the first member, A second member (20) is positioned on the other side of the dielectric in the predetermined direction and has a plurality of inclined surfaces (22a, 22b, 22i, 22j, 24a, 24b, 24c) whose normal directions (hs1, hs2, hsa, hsb, hsc) are inclined with respect to the predetermined direction, retroreflecting the electromagnetic waves that have passed through the dielectric. An electromagnetic wave absorber in which the dielectric attenuates the electromagnetic wave when the electromagnetic wave that has passed through the first member undergoes multiple reflections between the plurality of inclined surfaces and the first member and resonates.

2. The incident electromagnetic wave (D1), which is the electromagnetic wave that has passed through the first member and the dielectric, is retroreflected by the plurality of inclined surfaces. The retroreflected incident electromagnetic wave, as a first reflected electromagnetic wave (D3), passes through the dielectric. The first reflected electromagnetic wave that has passed through the dielectric is reflected by the first member as a second reflected electromagnetic wave (D4). The electromagnetic wave absorber according to claim 1, wherein the electromagnetic wave resonates when the wavefront of the second reflected electromagnetic wave (D4) reflected by the first member coincides with the wavefront of the incident electromagnetic wave (D1) transmitted through the dielectric.

3. When the predetermined direction is defined as the first direction (Ya) and the direction perpendicular to the first direction is defined as the second direction (Yb), The plurality of reflective surfaces include a first inclined surface (22a) formed such that as it proceeds toward one side of the second direction, it moves toward one side of the first direction, and a second inclined surface (22b) positioned on the other side of the second direction relative to the first inclined surface, and formed such that as it proceeds toward the other side of the second direction, it moves toward one side of the first direction. The electromagnetic wave absorber according to claim 1, wherein when one of the first and second inclined surfaces reflects electromagnetic waves (D1, D4) that have passed through the dielectric, the other inclined surface other than the first and second inclined surfaces reflects the electromagnetic waves (D2, D5) reflected by the one inclined surface along the direction of propagation of the electromagnetic waves that have passed through the dielectric and in the opposite direction to the electromagnetic waves that have passed through the dielectric, thereby retroreflecting the electromagnetic waves that have passed through the dielectric.

4. The plurality of inclined surfaces comprises a plurality of first inclined surfaces and a plurality of second inclined surfaces, The electromagnetic wave absorber according to claim 3, wherein the plurality of first inclined surfaces and the plurality of second inclined surfaces are arranged alternately one by one in the second direction.

5. The second member is provided with a plurality of reflectors (22) that are formed in a prismatic shape, each having a first inclined surface (22b) and a second inclined surface (22a) positioned on the other side in the second direction relative to the first inclined surface. The electromagnetic wave absorber according to claim 4, wherein the plurality of reflectors are arranged in the second direction, so that the plurality of first inclined surfaces and the plurality of second inclined surfaces are arranged alternately one by one in the second direction.

6. The first member is provided with a plurality of openings (11) that allow electromagnetic waves incident from one side in a predetermined direction (Ya) to pass through, The electromagnetic wave absorber according to claim 5, wherein each of the plurality of reflectors is arranged to face one of the plurality of openings.

7. The first member is provided with a plurality of openings (11) that allow electromagnetic waves incident from one side in a predetermined direction (Ya) to pass through, Each of the plurality of reflectors is arranged to face two or more of the plurality of openings, The electromagnetic wave absorber according to claim 5, wherein the number of openings is an integer multiple of the number of reflectors.

8. The electromagnetic wave absorber according to claim 5, wherein the reflector is formed in a triangular prism shape having the first inclined surface and the second inclined surface.

9. The electromagnetic wave absorber according to claim 5, wherein the reflector is formed in a rectangular prism shape having the first inclined surface and the second inclined surface.

10. When the third direction (Yc) is defined as the direction that is perpendicular to the first direction and also perpendicular to the second direction, The aforementioned multiple inclined surfaces are, A third inclined surface (22e) is positioned on one side of the third direction relative to the first and second inclined surfaces, and is formed such that its normal direction (ks1) is inclined in the first direction, and that as it proceeds toward one side of the third direction, it moves toward one side of the first direction. The device comprises a fourth inclined surface (22g) positioned on the other side of the third direction relative to the first and second inclined surfaces, having a normal direction (ks2) inclined in the first direction and formed such that it moves toward one side of the first direction as it progresses toward the other side of the third direction, The electromagnetic wave absorber according to claim 3, wherein when one of the third and fourth inclined surfaces reflects electromagnetic waves (G1, G4) that have passed through the dielectric, the other inclined surface of the third and fourth inclined surfaces reflects the electromagnetic waves reflected by the one inclined surface in the direction of propagation of the electromagnetic waves that have passed through the dielectric and in the opposite direction to the electromagnetic waves that have passed through the dielectric, thereby retroreflecting the electromagnetic waves that have passed through the dielectric.

11. The second member is, A first reflector (22X, 22S) is positioned on one side of the third direction relative to the first and second inclined surfaces, and is formed in a prismatic shape having the third inclined surface, The electromagnetic wave absorber according to claim 10, further comprising: a second reflector (22Y, 22T) disposed on the other side in the third direction with respect to the first and second inclined surfaces, and having a fourth inclined surface and formed in a prismatic shape.

12. The electromagnetic wave absorber according to claim 11, wherein at least one of the first reflector and the second reflector is formed in the shape of a triangular prism.

13. The electromagnetic wave absorber according to claim 11, wherein at least one of the first reflector and the second reflector is formed in the shape of a rectangular prism.

14. The electromagnetic wave absorber according to claim 3, wherein, if the wavelength of the electromagnetic wave propagating within the dielectric is λ, the dimension (Tk) of the first inclined surface and the second inclined surface in the first direction is set to be 0.15·λ or more and 0.55·λ or less.

15. The second member has three reflective surfaces (24a, 24b, 24c) that form a recess (24U) that is concave in a triangular pyramidal shape from one side to the other in the predetermined direction. The electromagnetic wave absorber according to claim 1, wherein, in the case where the first of the three reflective surfaces reflects electromagnetic waves transmitted through the dielectric, and the second of the three reflective surfaces other than the first reflecting surface reflects the electromagnetic waves reflected by the first reflecting surface, the third of the three reflective surfaces other than the first and second reflecting surfaces reflects the electromagnetic waves reflected by the second reflecting surface along the direction of propagation of the electromagnetic waves transmitted through the dielectric and in the opposite direction to the electromagnetic waves transmitted through the dielectric.

16. The electromagnetic wave absorber according to claim 1, wherein each of the plurality of inclined surfaces is a curved surface.

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