Radio wave absorber

Abstract

The objective is to accurately absorb multiple radio waves of different frequencies. [Solution] The absorption unit 1, which comprises three or more conductive layers 4 arranged at intervals in the thickness direction and intermediate dielectric 6 which is a dielectric material provided in the intervals, absorbs multiple radio waves of different frequencies, and in a front view, each conductive layer 4 is completely contained within the range of the conductive layer 4 located directly behind it.

Description

Technical Field

[0001] The present invention relates to a radio wave absorber that absorbs a plurality of radio waves having different frequencies.

Background Art

[0002] Devices that transmit and receive information by wireless communication have become widespread. When a plurality of devices perform wireless communication using radio waves in the same frequency band, the radio waves may become mixed, and the devices may receive unnecessary radio waves and malfunction. Therefore, as shown in Patent Document 1, a radio wave absorber that absorbs radio waves of a specific frequency is provided around the device so that radio waves that are not assumed by the device do not propagate.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, although the number of devices that communicate using a plurality of radio waves having different frequencies has been increasing, it has been difficult for conventional radio wave absorbers to accurately absorb a plurality of radio waves having different frequencies.

[0005] In order to solve the above problems, an object of the present invention is to accurately absorb a plurality of radio waves having different frequencies.

Means for Solving the Problems

[0006] In order to achieve the above object, a radio wave absorber according to an embodiment of the present invention absorbs a plurality of radio waves having different frequencies by an absorption unit including three or more conductive layers arranged at intervals in the thickness direction and an intermediate dielectric that is a dielectric provided in the interval, and in a front view, each of the conductive layers is completely included within the range of existence of the conductive layer located immediately behind.

[0007] In this way, by stacking three or more conductive layers, two conductive layers that are adjacent to each other in the stacking direction (thickness direction) and the dielectric material placed between them can be treated as a pair to absorb radio waves of one frequency, and three conductive layers can absorb two types of radio waves with different frequencies. Furthermore, by stacking four or more conductive layers, it is possible to absorb three or more types of radio waves with different frequencies.

[0008] Furthermore, in a pair of conductive layers that absorb radio waves of a predetermined frequency, the front conductive layer (upward in the stacking direction) is included within the range of the rear conductive layer (downward in the stacking direction) when viewed from the front (looking from upward to downward in the stacking direction). This allows each pair of conductive layers to absorb radio waves of a specific frequency with high precision.

[0009] As a result, the radio wave absorber with the above configuration can accurately absorb multiple radio waves of different frequencies.

[0010] Furthermore, multiple absorption portions may be provided along the planar direction of the conductive layer, which is located directly behind the conductive layer when viewed from the front.

[0011] In this way, multiple conductive layers for absorbing radio waves of different frequencies can be provided above (in front of) a pair of conductive layers that absorb radio waves of a predetermined frequency. The frequency of radio waves that can be absorbed depends on the size of the upper conductive layer. Depending on the frequency of radio waves that can be absorbed, the difference in size between the conductive layers arranged in front and behind (upper and lower) may become large. In such cases, the spacing of the conductive layers on the front (upper) side in the width direction (planar direction) may become too large due to the spacing of the conductive layers on the rear (lower) side, which may reduce the accuracy of radio wave absorption. Even in such cases, by arranging multiple conductive layers, it becomes easier to adjust the spacing, and the reduction in the accuracy of radio wave absorption can be suppressed. As a result, multiple radio waves of different frequencies can be absorbed with high accuracy.

[0012] The system may also include a substrate and a plurality of absorption units arranged on and along the substrate.

[0013] With this configuration, the radio wave absorber has multiple absorbing sections on a substrate, each capable of absorbing multiple radio waves of different frequencies (multiple types of radio waves). This allows for the creation of a radio wave absorber of a size (area) corresponding to the number of absorbing sections while absorbing multiple types of radio waves. The radio wave absorber can be made, for example, in the shape of a wall (flat plate). This allows the radio wave absorber to absorb multiple types of radio waves within a predetermined range. As a result, it can accurately absorb multiple radio waves of different frequencies. Furthermore, the absorbing sections can be arranged at arbitrary intervals, allowing for efficient radio wave absorption depending on the situation.

[0014] The system may also include a substrate and a plurality of absorption units arranged periodically on and along the substrate.

[0015] With this configuration, the radio wave absorber has multiple absorbing sections on a substrate, each capable of absorbing multiple radio waves of different frequencies (multiple types of radio waves). This allows for the creation of a radio wave absorber of a size (area) corresponding to the number of absorbing sections while absorbing multiple types of radio waves. In particular, by arranging the absorbing sections periodically, the radio wave absorber can absorb radio waves uniformly. The radio wave absorber can be made, for example, in the shape of a wall (flat plate). This allows the radio wave absorber to absorb multiple types of radio waves within a predetermined range. As a result, it can absorb multiple radio waves of different frequencies with high precision.

[0016] Furthermore, the rear dielectric, which is the dielectric material provided behind the rearmost conductive layer in a front view, or the rearmost conductive layer and the rear dielectric material may function as the substrate, and in a front view, all the conductive layers located directly in front of the rearmost conductive layer may be completely included within the range of the presence of the rear dielectric material.

[0017] With such a configuration, the lower layer portion of the absorption part can be diverted as a substrate, and while efficiently configuring a radio wave absorber having a plurality of absorption parts, a plurality of radio waves having different frequencies can be accurately absorbed.

[0018] Also, the dielectric may have translucency.

[0019] With such a configuration, while accurately absorbing a plurality of radio waves having different frequencies, visibility through the radio wave absorber can be ensured.

[0020] Also, the dielectric may be glass.

[0021] With such a configuration, while accurately absorbing a plurality of radio waves having different frequencies, visibility through the radio wave absorber can be easily ensured.

[0022] Also, the thickness of the dielectric may be 0.2 mm or more and 15 mm or less.

[0023] The thickness of the dielectric depends on the relationship between the frequency of the radio wave to be absorbed and the dielectric constant of the dielectric. According to the above configuration, radio waves of a wide range of frequencies can be absorbed, and the absorption characteristics are maintained, and a plurality of radio waves having different frequencies can be accurately absorbed.

[0024] Also, in the absorption part, the central axes of the respective conductive layers may coincide or substantially coincide.

[0025] With such a configuration, while accurately absorbing a plurality of radio waves having different frequencies, the absorption efficiency of each radio wave can be made uniform.

[0026] Also, the conductive layer may be square or circular.

[0027] With such a configuration, while accurately absorbing a plurality of radio waves having different frequencies, the radio wave absorber can be configured efficiently and with a high degree of freedom.

[0028] Furthermore, the absorption portion may include a first conductive layer, a second conductive layer provided behind the first conductive layer, a third conductive layer provided behind the second conductive layer, a first intermediate dielectric provided between the first conductive layer and the second conductive layer, a second intermediate dielectric provided between the second conductive layer and the third conductive layer, and an adhesive layer that bonds the first intermediate dielectric and the second intermediate dielectric, wherein, in a front view, the adhesive layer may be located between the second conductive layer and the second intermediate dielectric or between the first intermediate dielectric and the second conductive layer.

[0029] According to the above configuration, dielectrics adjacent to each other vertically in the stacking direction (front and back in the thickness direction) can be precisely bonded together by the adhesive layer. Furthermore, when the adhesive layer is located between the second conductive layer and the second intermediate dielectric, multiple radio waves of different frequencies can be absorbed with greater precision.

[0030] Furthermore, the conductive layer may include at least one of a silver layer and a conductive metal oxide.

[0031] By making the conductive layer a multilayer structure containing silver or a conductive metal oxide, and more preferably a silver layer or a conductive metal oxide, light transmission can be easily achieved, and radio wave radiation can be suppressed, allowing for more precise absorption of radio waves.

[0032] It may also be in the shape of a flat plate.

[0033] The flat shape of the radio wave absorber allows for more precise absorption of multiple radio waves of different frequencies, while also improving usability. In particular, a configuration in which multiple absorbing sections are arranged in a flat shape makes it easy to create a space where the influence of external radio waves is suppressed. [Brief explanation of the drawing]

[0034] [Figure 1] These are side and front views illustrating the configuration of the absorbing section that makes up the radio wave absorber. [Figure 2] This is a side view illustrating the configuration of a radio wave absorber. [Figure 3] This is a perspective view illustrating the configuration of a radio wave absorber. [Figure 4] This diagram illustrates a planar configuration of a square conductive layer. [Figure 5] This diagram illustrates a planar configuration of a circular conductive layer. [Figure 6] This is a side view illustrating the configuration of an absorption section comprising a single conductive layer, which is a reflective layer, and multiple conductive layers, which are radio wave resonant layers. [Figure 7] This diagram illustrates the main components of an absorption section, in which an adhesive layer is provided below the conductive layer. [Figure 8] This figure shows a graph comparing the radio wave absorption rate with respect to the placement of the adhesive layer. [Figure 9] This figure shows a graph comparing the absorption rate of radio waves with respect to sheet resistance. [Modes for carrying out the invention]

[0035] [Absorbent part] First, using Figure 1, we will explain the absorption section 1, which is the main part of the radio wave absorber that absorbs multiple radio waves of different frequencies (wavelengths). In the following explanation, the direction of arrow U in Figure 1 is considered up, and the direction of arrow D is considered down. Also, assuming that the radio waves are irradiated from above the radio wave absorber, the upper side from which the radio waves are irradiated will be called the front side, and the up and down directions in the radio wave absorber (absorbing section 1) may be called the front and back directions. Furthermore, the radio wave absorber (absorbing section 1) may be configured to absorb electromagnetic waves of a predetermined frequency, not just radio waves, but the following explanation will use radio waves as an example.

[0036] The absorption section 1 comprises three or more conductive layers 4 arranged with gaps between them in the thickness direction (vertical direction), and an intermediate dielectric 6, which is a dielectric material, provided in these gaps. In other words, the absorption section 1 is configured in which the conductive layers 4 and the intermediate dielectric 6 are stacked alternately multiple times in the vertical direction. Two adjacent conductive layers 4 (one pair of conductive layers 4 and the intermediate dielectric 6 sandwiched between them) on either side of the intermediate dielectric 6 absorb radio waves of a predetermined frequency, and by stacking three or more conductive layers 4, two or more pairs of conductive layers 4 absorb two or more radio waves of different frequencies. In each pair, the upper conductive layer 4 functions as a radio wave resonant layer, and the lower conductive layer 4 functions as a reflective layer, and the intermediate dielectric 6 sandwiched between them absorbs radio waves, thereby absorbing radio waves of a predetermined frequency.

[0037] Specifically, when the frequency of the incident radio wave (electromagnetic wave) matches the resonance frequency of the conductive layer 4 (radio wave resonant layer) in front (above), a large energy concentration is observed between the radio wave resonant layer and the conductive layer 4 (reflecting layer) directly behind (below) it. In this case, the energy flow in the space in front of the radio wave resonant layer is perpendicular (vertical direction) to the intermediate dielectric 6 sandwiched between the radio wave resonant layer and the reflecting layer, but the direction of the energy flow inside the intermediate dielectric 6 is parallel to the intermediate dielectric 6 (approximately perpendicular to the vertical direction), and the directions are opposite between the conductive layers 4 that are adjacent to each other in the vertical direction. As a result, the energy of the incident radio wave loses its outlet at the intermediate dielectric 6 and becomes stagnant. Consequently, due to the losses in the intermediate dielectric 6, the energy of the incident radio wave is consumed without being reflected in the direction of incidence, and radio waves of a predetermined frequency are absorbed by the absorption section 1.

[0038] In the example shown in Figure 1, the conductive layer 4 is arranged from top to bottom as a first conductive layer 4A, a second conductive layer 4B, and a reflector 4C (reflecting layer). The intermediate dielectric 6 is also arranged from top to bottom as a first intermediate dielectric 6A and a second intermediate dielectric 6B. Radio waves of a predetermined frequency are absorbed by the first conductive layer 4A, the second conductive layer 4B, and the first intermediate dielectric 6A. In this case, the first conductive layer 4A functions as a radio wave resonant layer, and the second conductive layer 4B functions as a reflecting layer. Radio waves of a different frequency are absorbed by the second conductive layer 4B, the reflector 4C (reflecting layer), and the second intermediate dielectric 6B. In this case, the second conductive layer 4B functions as a radio wave resonant layer, and the reflector 4C functions as a reflecting layer.

[0039] As described above, the absorption section 1 can absorb multiple radio waves of different frequencies by stacking multiple pairs of conductive layers 4 with a dielectric in between.

[0040] In this embodiment, when viewed from the front (from above to below), each conductive layer 4 is completely contained within the range of the conductive layer 4 located directly behind it. In the absorbent section 1 illustrated in Figure 1, when viewed from the front, the first conductive layer 4A is completely contained within the range of the second conductive layer 4B located directly behind it (directly below), and the second conductive layer 4B is completely contained within the range of the reflector 4C located directly behind it (directly below).

[0041] With this configuration, the entire conductive layer 4 (radio wave resonant layer) of the front layer (upper layer) can resonate with radio waves of a predetermined frequency, allowing the dielectric to absorb the radio waves, thus enabling efficient and precise absorption of radio waves of a predetermined frequency.

[0042] In addition, it is preferable that the central axes of each conductive layer 4 in the absorption section 1 coincide or nearly coincide. This allows the central and peripheral parts of the conductive layer 4 to absorb radio waves uniformly, and the absorption section 1 can absorb radio waves with high precision.

[0043] Here, the absorption section 1 is configured by stacking multiple radio wave resonant layers (conductive layers 4) that resonate with radio waves of different frequencies. Multiple radio wave resonant layers (different types of radio wave resonant layers) that resonate with radio waves of different frequencies can be arranged offset (aligned in the plane), but stacking different types of radio wave resonant layers allows for more efficient and precise absorption of radio waves. Specifically, the bandwidth of the radio wave frequencies that can be absorbed differs depending on the size of the radio wave resonant layer. In order to make the bandwidth of the radio waves absorbed by different types of radio wave resonant layers and reflective layers the same, it is necessary to adjust the size of the radio wave resonant layers, but if the size of radio wave resonant layers arranged offset in the plane will vary in the overall size. In contrast, when radio wave resonant layers are stacked, each radio wave resonant layer can be designed individually, and the thickness of the absorption section 1 does not vary in the plane, so the absorption rate can be easily made the same. Furthermore, when the radio wave resonant layers are arranged in a staggered manner (aligned in the plane), the absorption regions for different types of radio waves are aligned in the plane. As a result, other radio wave resonant layers can get in between the same type of resonant layers, increasing the spacing between them and reducing the absorption efficiency of radio waves of each frequency. In contrast, when the radio wave resonant layers are stacked, different types of resonant layers are stacked and can be densely arranged in the plane. Therefore, the spacing between the same type of resonant layers does not increase, and the absorption unit 1 can absorb radio waves of each frequency efficiently and accurately.

[0044] Specifically, the size of the radio wave resonant layer (conductive layer 4) is determined by the frequency of the radio waves to be absorbed and the dielectric constant and thickness of the rear dielectric 11. Therefore, by having a dielectric (rear dielectric 11) thickness of 0.2 mm to 15 mm, it is possible to absorb radio waves of commonly used frequencies. The absorption rate of radio waves absorbed by the radio wave resonant layer is determined by the conductivity of the radio wave resonant layer (conductive layer 4) and the dielectric loss of the dielectric provided between the radio wave resonant layer and the reflective layer.

[0045] [Radio wave absorber] Next, using Figures 2 and 3, we will describe a radio wave absorber equipped with an absorption section 1 that absorbs multiple radio waves of different frequencies.

[0046] The radio wave absorber of this embodiment comprises a plurality of absorbing units 1. Each absorbing unit 1 absorbs multiple radio waves of different frequencies (wavelengths). Therefore, the radio wave absorber absorbs multiple radio waves of different frequencies (wavelengths).

[0047] In a radio wave absorber, the absorbing section 1 is provided on a substrate 9. That is, the radio wave absorber comprises a plurality of absorbing sections 1 arranged on and along the substrate 9. Multiple absorbing sections 1 are provided on the substrate 9 along the substrate 9.

[0048] Specifically, the substrate 9 consists of a rear dielectric 11 and a reflector 4C laminated on the rear dielectric 11, and the radio wave absorber has multiple absorption parts 1 arranged on the rear dielectric 11. Furthermore, the reflector 4C of the substrate 9 is configured as a series of reflectors 4C of multiple absorption parts 1, and preferably, all the reflectors 4C of all the absorption parts 1 are connected to form an integrated structure. In other words, in the radio wave absorber, all absorption parts 1 share the same reflector 4C which becomes the rearmost conductive layer 4, and are laminated on the rear dielectric 11. This reduces the need to design the size of the reflector 4C, and allows for the efficient construction of the radio wave absorber.

[0049] The substrate 9 may have any shape, such as a curved surface, but it is preferably flat, and the radio wave absorber is preferably flat. This allows the radio wave absorber to function as a wall or screen, and it is possible to surround the device with a flat radio wave absorber to create a device configuration that is not affected by external radio waves, or to place a screen-shaped radio wave absorber between two devices to create a configuration in which two devices are not affected by each other's radio waves.

[0050] In a radio wave absorber, the absorption rate of radio waves varies depending on the distance between adjacent radio wave resonant layers (conductive layers 4) of the same type in the planar direction. Therefore, in a radio wave absorber, it is preferable that the distance (distance G) between adjacent absorption parts 1 in the planar direction is determined considering the absorption rate of radio waves. Here, distance G may be the length between any two positions of absorption parts 1. For example, distance G may be the length between the first conductive layers 4A, the length between the second conductive layers 4B, the length between any of the intermediate dielectrics 6, or the length between the centers (centroids) of the absorption parts 1 in the planar direction.

[0051] [Radio wave resonant layer] Next, referring to Figure 1, we will explain the shape and size of the conductive layer 4, which is the radio wave resonant layer, using Figures 4 and 5.

[0052] The conductive layer 4, which is the radio wave resonant layer, can be any shape, for example, a square (configuration in Figure 4) or a circle (configuration in Figure 5). The conductive layer 4 in a square shape may also be rectangular.

[0053] In such a radio wave resonant layer, the size of the conductive layer 4 is determined according to the frequency (wavelength) of the radio waves it absorbs (resonates). For example, the size of the conductive layer 4 is the diagonal dimension L1 in the case of a rectangular layer, and the diameter dimension L2 in the case of a circular layer. Here, the diagonal dimension L1 and the diameter dimension L2 together are referred to as dimension L.

[0054] When the wavelength of the radio wave to be absorbed is λ and the shortening factor of the dielectric that forms the reflective layer is α, the dimensions L of the conductor preferably satisfy the following relationship. 0.3 ≤ L / (λ·α) ≤ 0.7

[0055] For example, the dimension L of the conductive layer 4, which is a radio wave resonant layer that absorbs 2.5 GHz radio waves, can be determined as follows.

[0056] The wavelength of a 2.5GHz radio wave is 120mm. Assuming a shortening factor of 0.7, one wavelength resonates in a conductive layer 4 with L=84mm, and half a wavelength resonates in a conductive layer 4 with L=42mm. Based on this, and considering that resonance occurs at a quarter wavelength and the shape of the conductive layer 4, the diagonal dimension L1 for a square conductive layer 4 can be set to 24.5mm, and the diameter dimension L2 for a circular conductive layer 4 can be set to 20mm.

[0057] In this way, by designing the dimensions L of the radio wave resonance layer (conductive layer 4) to resonate with the radio waves to be absorbed, the absorption section 1 (radio wave absorber) can absorb radio waves with high precision.

[0058] Furthermore, it is desirable that the resistance of the conductive layer 4 be, for example, 10.0 Ω / □ or less in sheet resistance. This increases the absorption efficiency for radio waves of the desired frequency, and allows for absorption characteristics of 10 dB or more to be obtained.

[0059] [Another embodiment] (1) In the above embodiment, as shown in Figure 2, the substrate 9 may consist of a reflector 4C which is the rearmost conductive layer 4, and a rear dielectric 11 which is a dielectric provided behind the rearmost conductive layer 4 (reflector 4C) when viewed from the front. However, the substrate 9 may also function with only the rear dielectric 11 (dielectric) and without the reflector 4C. In this case, an independent reflector 4C is provided behind each individual absorption part 1 arranged on the substrate 9 which is the rear dielectric 11. The reflector 4C is separated for each absorption part 1, and the reflector 4C is configured such that, when viewed from the front, the second conductive layer 4B is completely contained within the range of the reflector 4C located directly behind it. In other words, in this case, the radio wave absorber is configured such that a plurality of absorption parts 1 shown in Figure 1 are provided on the substrate 9 which is the rear dielectric 11.

[0060] With this configuration, multiple absorption units 1 as shown in Figure 3 can be manufactured in advance, and a radio wave absorber can be produced simply by arranging the absorption units 1 on a rear dielectric 11 such as a glass substrate. Furthermore, by preparing a rear dielectric 11 as a wall or screen and arranging the absorption units 1 on the rear dielectric 11, walls or screens that absorb multiple radio waves of different frequencies can be easily manufactured.

[0061] Furthermore, in such a radio wave absorber, when viewed from the front, the second conductive layer 4B in all absorbing sections 1 located directly in front of the reflector 4C, which is the rearmost conductive layer 4, is completely contained within the range of the reflector 4C, which is the rear dielectric. As a result, the second conductive layer 4B and the reflector 4C can absorb radio waves of a predetermined frequency with high precision.

[0062] (2) In each of the above embodiments, the radio wave absorber may have a front dielectric 14 which is a dielectric material at its foremost layer (uppermost layer). This protects the front surface, which is the absorbing surface that absorbs radio waves of the absorbing part 1 in the radio wave absorber, from the front dielectric 14.

[0063] Furthermore, when the rear dielectric 11 functions as a substrate 9, the front dielectric 14 is provided to cover all of the absorption parts 1, so that the radio wave absorber is configured to sandwich the absorption parts 1 between the rear dielectric 11 and the front dielectric 14. As a result, the radio wave absorber can accurately absorb multiple radio waves of different frequencies while protecting the absorption parts 1, and can easily be used to form a wall or screen.

[0064] (3) In the radio wave absorbers of each embodiment described above, as shown in Figure 6, the absorption portion 1 may be provided in multiple locations along the planar direction of the conductive layer 4, which is located directly behind the conductive layer 4 in a front view. For example, in the example shown in Figure 6, the first conductive layer 4A, which is a conductive layer 4 located directly in front of one second conductive layer 4B, is provided in multiple locations along the plane of the second conductive layer 4B (first intermediate dielectric 6A). Note that the radio wave absorber is not limited to the first conductive layer 4A, but may also be provided with multiple second conductive layers 4B or other radio wave resonant layers (conductive layers 4).

[0065] The size of the conductive layer 4 depends on the frequency of the radio waves it absorbs. When the first conductive layer 4A and the second conductive layer 4B function as radio wave resonant layers and there is a large difference in the frequency of the radio waves they absorb, the size of the first conductive layer 4A becomes much smaller than that of the second conductive layer 4B. If there is one first conductive layer 4A for each absorption section 1, the spacing of the first conductive layers 4A in the planar direction becomes larger than the spacing of the second conductive layers 4B when viewed as a whole radio wave absorber. When the spacing of the radio wave resonant layers becomes larger, the efficiency of absorbing radio waves may decrease. However, by providing multiple first conductive layers 4A on the second conductive layer 4B, the increase in the spacing of the first conductive layers 4A is suppressed, and the decrease in the efficiency of absorbing radio waves is suppressed. As a result, the radio wave absorber can absorb radio waves with high precision.

[0066] (4) In each of the above embodiments, as shown in Figure 7, the absorption portion 1 may have a first conductive layer 4A, a second conductive layer 4B provided behind the first conductive layer 4A, a third conductive layer which is a reflector 4C provided behind the second conductive layer 4B, a first intermediate dielectric 6A provided between the first conductive layer 4A and the second conductive layer 4B, a second intermediate dielectric 6B provided between the second conductive layer 4B and the third conductive layer, and an adhesive layer 16 that bonds the first intermediate dielectric 6A and the second intermediate dielectric 6B.

[0067] In other words, the absorption section 1 may include an adhesive layer 16 that adheres dielectrics that move back and forth in the stacking direction (front-to-back direction). The dielectric may be an intermediate dielectric 6 sandwiched between any of the conductive layers 4, a rear dielectric 11 positioned directly behind the conductive layer 4 (reflector 4C), or a front dielectric 14 positioned directly in front of the frontmost conductive layer 4 (first conductive layer 4A) among the conductive layers 4.

[0068] Because the dielectric is bonded by the adhesive layer 16, the absorption section 1 is firmly constructed, preventing misalignment of the conductive layer 4, and enabling accurate absorption of radio waves.

[0069] Furthermore, in such a configuration, it is preferable that the adhesive layer 16 is located between the second conductive layer 4B and the second intermediate dielectric 6B when viewed from the front. In other words, it is preferable that the adhesive layer 16 is located between the conductive layer 4 and the dielectric located directly behind the conductive layer 4. To put it another way, the adhesive layer 16 may be placed directly in front of the conductive layer 4, but it is preferable that it be placed directly behind the conductive layer 4. By placing the adhesive layer 16 directly behind the conductive layer 4, it is possible to suppress the reduction in the absorption rate of radio waves.

[0070] The experimental results regarding the difference in radio wave absorption rate depending on the placement position of the adhesive layer 16 are explained using Figure 8.

[0071] The experiment shown in Figure 8 involved measuring the radio wave absorption rate using the same predetermined method on two samples with the same planar dimensions and thickness (same conditions) for the conductive layer 4 and dielectric. The difference between the two samples was the placement of the adhesive layer 16. In one sample, the adhesive layer 16 was placed between the second conductive layer 4B and the second intermediate dielectric 6B (the adhesive layer 16 was placed directly behind the conductive layer 4), while in the other sample, the adhesive layer 16 was placed between the first intermediate dielectric 6A and the second conductive layer 4B (the adhesive layer 16 was placed directly in front of the conductive layer 4). These samples also have an absorption section 1 that absorbs two radio waves of frequencies Fx and Fy.

[0072] In Figure 8, the graph shown as experimental result 18 shows the relationship between frequency and radio wave absorption rate in a sample where the adhesive layer 16 is placed between the second conductive layer 4B and the second intermediate dielectric 6B, and the graph shown as experimental result 19 shows the relationship between frequency and radio wave absorption rate in a sample where the adhesive layer 16 is placed between the first intermediate dielectric 6A and the second conductive layer 4B.

[0073] As shown in Figure 8, in experimental result 18, the absorption rate is -20 dB or higher at both frequencies Fx and Fy, whereas in experimental result 19, the absorption rate at frequency Fy is below -20 dB.

[0074] From this, it can be seen that when the adhesive layer 16 is placed between the second conductive layer 4B and the second intermediate dielectric 6B, the reduction in radio wave absorption is suppressed compared to when it is placed between the first intermediate dielectric 6A and the second conductive layer 4B. As a result, when the adhesive layer 16 is placed between the second conductive layer 4B and the second intermediate dielectric 6B, radio waves can be absorbed with greater precision.

[0075] Furthermore, as described above, if the radio wave absorber is configured such that the absorbing portion 1 is sandwiched between the rear dielectric 11 and the front dielectric 14, an adhesive layer 16 may be provided between the absorbing portion 1 and in at least a portion of the area around the absorbing portion 1 in the region sandwiched between the rear dielectric 11 and the front dielectric 14. This makes the absorbing portion 1 more rigid, suppresses misalignment of the conductive layer 4, and enables accurate absorption of radio waves.

[0076] In this case, the radio wave absorber may further include a pair of radio wave resonant layer and reflective layer inside the adhesive layer 16 in the region between the absorbing parts 1. For example, in the configuration shown in Figure 6, the first conductive layer 4A is smaller than the second conductive layer 4B. Therefore, the gap between the first conductive layers 4A, which function as radio wave resonant layers, tends to be larger than the gap between the second conductive layers 4B in the region between the absorbing parts 1. As a result, the gap in the planar direction between the first conductive layers 4A is larger in the region between the absorbing parts 1 than inside the absorbing parts 1, and the radio wave absorption efficiency of the radio wave absorber may not be uniform. By including a first conductive layer 4A (radio wave resonant layer) and a second conductive layer 4B (reflective layer) between the absorbing parts 1, the radio wave absorption efficiency can be made uniform.

[0077] (5) In each of the above embodiments, the absorbing units 1 may be arranged on the substrate 9 at any interval, or they may be arranged periodically (in a matrix). In other words, the radio wave absorber may have the absorbing units 1 arranged at any interval (distance G) in the planar direction, or the absorbing units 1 may be arranged in the planar direction such that the distance (distance G) between each absorbing unit 1 and adjacent absorbing units 1 is the same.

[0078] If the spacing (distance G) between the absorption units 1 becomes too wide, the efficiency of the radio wave absorber in absorbing radio waves may decrease compared to the region where the spacing (distance G) between the absorption units 1 is appropriate. By arranging the absorption units 1 uniformly at appropriate spacing (distance G) in the planar direction, the radio wave absorber can absorb radio waves uniformly across its entire surface, enabling accurate radio wave absorption.

[0079] (6) In each of the above embodiments, the radio wave absorber may be equipped with any number of absorption units 1, or it may be configured to be equipped with only one absorption unit 1. This makes it easy to define the area in which the radio wave absorber absorbs radio waves, and allows for efficient absorption of radio waves.

[0080] (7) In each of the above embodiments, the conductive layer 4 may contain silver (silver layer) or a conductive metal oxide, or it may be a multilayer structure containing a silver layer or a conductive metal oxide. Furthermore, the conductive layer 4 may be composed of a low-emission multilayer material (Low-E). By including a low-emission material such as silver in the conductive layer 4, radio waves can be absorbed efficiently.

[0081] (8) In each of the above embodiments, the conductive layer 4 may have a translucent configuration. For example, by making the conductive layer 4 a translucent conductive material such as a low-emissivity multilayer material (Low-E), the conductive layer 4 can easily be made translucent.

[0082] Because the conductive layer 4 is translucent, selecting a translucent dielectric material makes it easy to make the radio wave absorber transparent (with a translucent configuration). By surrounding the device with such a radio wave absorber, the device can be visually inspected while the absorber accurately absorbs multiple radio waves of different frequencies, suppressing the propagation of unwanted radio waves to the device. In particular, when suppressing interference from radio waves used by devices in confined spaces such as rooms, the device can be surrounded by the radio wave absorber in an open state, suppressing interference while minimizing the feeling of confinement caused by the radio wave absorber.

[0083] (9) In each of the above embodiments, it is preferable to optimize the sheet resistance of the conductive layer 4. Below, using Figure 9, experimental results verifying the difference in radio wave absorption rate when the sheet resistance is changed will be described.

[0084] The experiment shown in Figure 9 involved measuring the radio wave absorption rate using the same predetermined method for three samples with identical planar dimensions and thickness (same conditions) for the conductive layer 4 and dielectric. The difference between the three samples was the difference in the sheet resistance values ​​of the first conductive layer 4A and the second conductive layer 4B. The sheet resistance values ​​of the first conductive layer 4A and the second conductive layer 4B in each sample were 2Ω / □, 4Ω / □, and 15Ω / □. The sheet resistance value of the reflector 4C was common to all three samples. These samples each have an absorption section 1 that absorbs two radio waves of frequencies Fx and Fy.

[0085] As shown in Figure 9, in both the case of a sheet resistance of 2Ω / □ and 4Ω / □, the absorption rate at both frequencies Fx and Fy is -10dB or higher, whereas when the sheet resistance is 15Ω / □, the absorption rate at frequency Fy is below -10dB. Furthermore, at frequency Fx, the absorption rate of radio waves is higher when the sheet resistance is 2Ω / □ and 4Ω / □ compared to when the sheet resistance is 15Ω / □.

[0086] From this, it can be seen that, under the conditions described above, setting the sheet resistance to 2Ω / □ or 4Ω / □ allows for a higher radio wave absorption rate compared to the case where the sheet resistance is 15Ω / □. [Industrial applicability]

[0087] This invention can be applied to a radio wave absorber capable of absorbing multiple radio waves of different frequencies. [Explanation of Symbols]

[0088] 1 Absorbent part 4. Conductive layer 4A First conductive layer (conductive layer) 4B Second conductive layer (conductive layer) 4C reflector (conductive layer) 6. Intermediate dielectric 6A First Intermediate Dielectric 6B Second Intermediate Dielectric 9 circuit boards 16 Adhesive layer

Claims

1. The absorption section comprises three or more conductive layers arranged at intervals in the thickness direction, and an intermediate dielectric, which is a dielectric, provided in the intervals, thereby absorbing multiple radio waves of different frequencies. A radio wave absorber in which, when viewed from the front, each of the aforementioned conductive layers is completely contained within the range of the conductive layer located immediately behind it.

2. The radio wave absorber according to claim 1, wherein the absorbing portion is provided in a plurality along the planar direction of the conductive layer, which is located directly behind the conductive layer when viewed from the front.

3. circuit board and The radio wave absorber according to claim 1, further comprising a plurality of absorbing parts arranged on and along the substrate.

4. circuit board and The radio wave absorber according to claim 1, further comprising a plurality of absorbing parts periodically arranged on and along the substrate.

5. A rear dielectric, which is the dielectric material located behind the rearmost conductive layer in a front view, or the rearmost conductive layer and the rear dielectric material function as the substrate, The radio wave absorber according to claim 3 or 4, wherein, in a front view, all of the conductive layers located directly in front of the rearmost conductive layer are completely contained within the range of the rear dielectric.

6. The aforementioned dielectric is a light-transmitting radio wave absorber according to claim 1.

7. The radio wave absorber according to claim 1, wherein the dielectric is glass.

8. The radio wave absorber according to claim 1, wherein the thickness of the dielectric material is 0.2 mm or more and 15 mm or less.

9. The radio wave absorber according to claim 1, wherein the central axes of each of the conductive layers in the absorption section coincide or substantially coincide.

10. The radio wave absorber according to claim 1, wherein the conductive layer is rectangular or circular.

11. The absorption portion comprises a first conductive layer, a second conductive layer provided behind the first conductive layer, a third conductive layer provided behind the second conductive layer, a first intermediate dielectric provided between the first conductive layer and the second conductive layer, a second intermediate dielectric provided between the second conductive layer and the third conductive layer, and an adhesive layer that bonds the first intermediate dielectric and the second intermediate dielectric. The radio wave absorber according to claim 1, wherein, in a front view, the adhesive layer is located between the second conductive layer and the second intermediate dielectric or between the first intermediate dielectric and the second conductive layer.

12. The radio wave absorber according to claim 1, wherein the conductive layer comprises at least one of a silver layer and a conductive metal oxide.

13. The radio wave absorber according to claim 1, which is in the shape of a flat plate.

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