JP6481991B2 - Radio wave absorber - Google Patents

Description

  The present invention relates to a radio wave absorber in which a plate-like loss layer and a conductor are laminated.

  The radio wave absorber functions by converting incident radio wave energy into heat energy by magnetic loss and dielectric loss. In this case, in order to approximate the impedance of the surface of the radio wave absorber to the wave impedance of air in order to suppress the reflection of the surface of the radio wave, the electrical length of the radio wave absorber is at resonance when the electromagnetic wavelength to be absorbed is ¼. Limited to only. Therefore, in the uniform material type radio wave absorber, the corresponding frequency band is very narrow. Therefore, in order to enable radio wave absorption in a wide band, a radio wave absorber having a laminated type or pyramid type has been developed. However, since the impedance is gently inclined, there is a disadvantage that it becomes a thick material. there were. As a radio wave absorber that gently slopes the impedance, Patent Document 1 discloses a magnetic powder / resin composite that is dispersed so that the packing density of the magnetic powder increases continuously or stepwise along the traveling direction of the radio wave. The thing arrange | positioned in front of a conductor reflector is proposed.

  For the purpose of further reducing the overall layer thickness, Patent Document 2 describes that a porous magnetic layer, a resistor layer (resistor), a porous magnetic layer, and a conductor layer are stacked in this order as viewed from the incident direction of electromagnetic waves. An electromagnetic wave absorber having a structured structure has been proposed. Incidence surface impedance Zi in such a plate-shaped wave absorber is described in Patent Document 2 as equations (1) and (2) as follows.

In equation (1), Z ₀ is the atmospheric wave impedance, 377Ω, μ _r is the complex permeability of the material of the wave absorber, μ _r = μ′−jμ ″, and ε _r is the wave absorber The complex permittivity of the material, ε _r = ε′−jε ″, and λ is the wavelength of the incident electromagnetic wave. Then, in order to simplify the handling in Equation (1), if j√ (μ _r ε _r ) is (α + jβ), Equation (1) is expressed as Equation (2). Here, α is a loss term per unit thickness of the radio wave absorber, and β is a phase term per unit thickness of the radio wave absorber. Further, in Patent Document 2, the loss term α per unit thickness of the radio wave absorber made of a single material is approximated by α = √ (μ ”ε”) from the equations (1) and (2). ing. Hereinafter, the loss term α and the phase term β per unit thickness of the radio wave absorber are simply referred to as a loss term α and a phase term β. Further, Tanh {(2π / λ) · (α + jβ)} in the expression (2) increases its loss term α, that is, √ (μ ”ε”), and its behavior is suppressed to about “1”. from converges to the frequency dependence of the incidence surface impedance Zi is reduced, it is possible to maintain the state of being approximated and wave impedance Z ₀ of the incident surface impedance Zi and the atmosphere over a wide frequency range, to a wide band It is explained that the attenuation in the radio wave absorber can be increased.

  As an example of this, Patent Document 2 describes a porous magnetic layer having a thickness of 10 mm, a porous magnetic layer having a thickness of 5 mm, and an overall thickness of about 15 mm. In this embodiment, the material of the resistor layer sandwiched between the two porous magnetic layers is a resistor sheet having a resistance value of 370Ω formed using carbon or the like, and the thickness thereof is shown in FIG. As can be seen from FIG. 5, it is a thin layer compared to both porous magnetic layers. With such a configuration, an attenuation of about 10 dB or more is achieved in a frequency band of 2 GHz to 10 GHz. In the examples of Patent Document 2, a loss layer α is not obtained by mixing and dispersing a fine powder of a resistor, such as carbon, in an integrated body of both porous magnetic layers without providing a resistor layer. There is a statement that it may be increased.

JP 2009-188322 A JP 2007-59456 A

  In the radio wave absorber described in Patent Document 1, it is necessary to manufacture a magnetic powder / resin composite dispersed so that the packing density of the magnetic powder increases continuously or stepwise along the traveling direction of the radio wave.

  In addition, in the radio wave absorber provided with a resistor layer made of a thin resistor sheet mixed with fine powder of a resistor such as carbon described in Patent Document 2, an attenuation of 10 dB or more is obtained in a wide band. However, the range in which attenuation of 15 dB or more that is more significant in practical use is obtained is only 1 GHz from 2.5 GHz to 3.5 GHz. In addition, the thickness is, for example, about 15 mm from the relationship with λ / 4 wavelength, and further thinning is required from the viewpoint of improving versatility. Furthermore, according to each figure, the thickness of both porous magnetic layers occupies almost the whole, and the resistor layer is described as a thin layer that is not used or has no dimensional value.

  The present invention has been made in view of the above. An air-like layer is provided between the dielectric loss layer and the magnetic loss layer, and the air-absorbing ability of the shape of the air-like layer or the magnetic loss layer in contact with the air-like layer is improved. In view of frequency dependence, an object is to provide a radio wave absorber capable of adjusting the absorption capacity for the absorption frequency and making the band wider while being reduced in weight and thickness.

  The present invention provides a plate-shaped radio wave absorber in which a loss layer and a conductor are laminated from the incident direction of radio waves, wherein the loss layer is a laminate of a dielectric loss layer and a magnetic loss layer from the incident direction of the radio waves, and An air-like layer is interposed between the dielectric loss layer and the magnetic loss layer.

  According to the present invention, when the radio wave is in a resonance state inside the absorber, the magnetic field component of the electromagnetic wave is maximized on the surface where the current near the conductor layer is maximized. On the other hand, the electric field whose phase is shifted by 90 ° is maximized in the vicinity of the incident surface. Focusing on the behavior of the electromagnetic field, a magnetic loss layer, which is a resin molded body filled with or highly filled with magnetic powder, is arranged on the surface where the magnetic field is maximized in order to efficiently convert it into heat energy by magnetic loss. In the vicinity of the incident surface, a dielectric loss layer which is a resin molded body containing a material having dielectric loss is disposed. Furthermore, an air-like layer, which is a material having characteristics similar to air, is interposed between the two layers so that the resonance frequency matches the target frequency, thereby reducing absorption while reducing weight and thickness. It is possible to adjust the absorptivity with respect to frequency and widen the bandwidth.

  The air-like layer has a thickness corresponding to the bandwidth of the radio wave to be absorbed. When the material or the structure (for example, honeycomb structure) of the air-like layer is determined, the bandwidth varies depending on the thickness of the layer. That is, as the thickness of the layer increases, the absorption capacity with respect to the absorption frequency gradually increases, and the band tends to gradually narrow, so by setting the layer thickness according to the target to be applied and the application, It is possible to obtain a suitable radio wave absorber according to the application target.

  The magnetic loss layer has a hole in the thickness direction. According to this configuration, electromagnetic wave resonance easily occurs, the electromagnetic wave absorption ability increases, and reflection loss increases.

  Moreover, the said hole has a corner | angular part in cross-sectional shape. According to this configuration, the resonance of the electromagnetic wave is more likely to occur, the electromagnetic wave absorption ability is increased, and the reflection loss is increased.

  Moreover, the said hole has a cone-shaped hole from the electromagnetic wave incident direction. According to this configuration, it is possible to reduce the weight and shift the absorption frequency, thereby improving versatility.

  Further, the cone-shaped inclined wall angle of the hole is several tens of degrees or more with respect to the radio wave incident direction. According to this configuration, the larger the angle, the larger the shift amount can be obtained.

  ADVANTAGE OF THE INVENTION According to this invention, the electromagnetic wave absorber which can adjust the absorptivity with respect to an absorption frequency and can be broadened while being reduced in weight and thickness can be provided.

It is sectional drawing which shows one Embodiment of the electromagnetic wave absorber which concerns on this invention. It is a characteristic view which shows the reflection loss characteristic of the electromagnetic wave absorber which concerns on 1st Example of this invention, and the conventional impedance gradient type laminated | stacked electromagnetic wave absorber as a comparative example. FIG. 6 is a characteristic diagram showing reflection loss characteristics when an aramid honeycomb is used for an air-like layer as a second embodiment. It is a characteristic view which shows the reflection loss characteristic at the time of using the same material for resin which is a base material of a dielectric loss layer and a magnetic loss layer as 3rd Example. FIG. 10 is a characteristic diagram showing reflection loss characteristics when the thickness of the air-like layer is changed as a fourth embodiment. In 5th Example, it is a figure explaining the simulation result of the intensity distribution of the electromagnetic field in case the hole is not formed in the magnetic loss layer, (A) is the image perspective view for demonstrating the structure of a radio wave absorber, (B) is a plan view for explaining an electric field component, and (C) is a plan view for explaining a magnetic field component. In a 5th Example, it is a figure explaining the simulation result of the intensity distribution of an electromagnetic field when a circular hole is formed in a magnetic loss layer, (A) is an image perspective view for demonstrating the structure of a radio wave absorber. (B) is a top view explaining an electric field component, (C) is a top view explaining a magnetic field component. In 5th Example, it is a figure explaining the simulation result of the intensity distribution of an electromagnetic field when a square hole is formed in a magnetic loss layer, (A) is an image perspective view for demonstrating the structure of a radio wave absorber. (B) is a top view explaining an electric field component, (C) is a top view explaining a magnetic field component. In a 5th Example, it is a figure explaining the simulation result of the intensity distribution of an electromagnetic field when a hexagonal star-shaped hole is formed in a magnetic loss layer, (A) is an image perspective view for demonstrating the structure of a radio wave absorber. FIG. 4B is a plan view for explaining the electric field component, and FIG. 4C is a plan view for explaining the magnetic field component. In 5th Example, it is a characteristic view which shows the reflection loss characteristic of each electromagnetic wave absorber in FIGS. 6-9, respectively. In a 5th Example, it is a figure explaining the simulation result of the intensity distribution of an electromagnetic field when a hexagonal hole is formed in a magnetic loss layer, (A) is the image perspective for demonstrating the structure of a radio wave absorber. FIG. 4B is a plan view for explaining the electric field component, and FIG. 4C is a plan view for explaining the magnetic field component. In 5th Example, it is a figure explaining the simulation result of the intensity distribution of an electromagnetic field when a triangular hole is formed in a magnetic loss layer, (A) is an image perspective view for demonstrating the structure of a radio wave absorber. (B) is a top view explaining an electric field component, (C) is a top view explaining a magnetic field component. In a 5th Example, it is a figure explaining the simulation result of the intensity distribution of an electromagnetic field when a cross-shaped hole is formed in a magnetic loss layer, (A) is the image perspective for demonstrating the structure of a radio wave absorber. FIG. 4B is a plan view for explaining the electric field component, and FIG. 4C is a plan view for explaining the magnetic field component. In 5th Example, it is a characteristic view which shows the reflection loss characteristic of each electromagnetic wave absorber in FIG. 6, FIG. In a 6th Example, it is a figure explaining the simulation result of the intensity distribution of an electromagnetic field when a conical hole is formed in a magnetic loss layer, (A) is sectional drawing for demonstrating the structure of a radio wave absorber. (B) is sectional drawing which shows an electric field component, (C) is sectional drawing which shows a magnetic field component. In a 6th Example, it is a characteristic view which shows the reflection loss characteristic in the electromagnetic wave absorber of FIG. In a 6th Example, it is a figure explaining the simulation result of the intensity distribution of an electromagnetic field when a magnetic loss layer is formed in the shape which has a conical protrusion, (A) is for demonstrating the structure of a radio wave absorber. (B) is sectional drawing which shows an electric field component, (C) is sectional drawing which shows a magnetic field component. In a 6th Example, it is a characteristic view which shows the reflection loss characteristic in the electromagnetic wave absorber of FIG. In 6th Example, it is a figure explaining the simulation result of the intensity distribution of an electromagnetic field when not providing a hole in a magnetic loss layer, (A) is sectional drawing for demonstrating the structure of a radio wave absorber, (B) Is a sectional view showing an electric field component, and (C) is a sectional view showing a magnetic field component.

  FIG. 1 is a cross-sectional view showing an embodiment of a radio wave absorber according to the present invention. The radio wave absorber has a configuration in which a dielectric loss layer 1, an air-like layer 2, a magnetic loss layer 3, and a conductor layer 4 are laminated in this order from the incident direction of an electromagnetic wave indicated by an arrow. The conductor layer 4 is a reflective conductor made of a plate-like conductor, such as an aluminum metal plate.

  Further, in FIG. 1, the electric field component of the incident electromagnetic wave is short-circuited by the conductor layer 4 as indicated by the electric field strength characteristic line Le indicated by a broken line, and thus becomes zero on the surface, and λ ( It becomes maximum at a position separated by (wavelength) / 4. On the other hand, the magnetic field component becomes maximum on the surface of the conductor layer 4 as indicated by a magnetic field strength characteristic line Lh indicated by a broken line.

  The magnetic loss layer 3 is, for example, a sheet member in which ferromagnetic fine particles are dispersed and disposed in a predetermined weight% (wt%) in a dielectric material made of, for example, resin as a binder (base material). The electromagnetic wave is efficiently absorbed in a region where the strength of the magnetic field component of the electromagnetic wave is high. By dispersing and arranging fine particles that are ferromagnetic materials, microwaves can penetrate into the inside as an insulator as a whole. In addition, since the surface area of the ferromagnetic material is increased by dispersing the fine particles, the incident electromagnetic wave is attenuated by the magnetic loss by increasing the volume of the skin layer that functions as a magnetic material even if the skin effect acts. be able to. The fine particles of the ferromagnetic material are brought into a non-contact state by interposing a binder between them, and become a dielectric (insulator) as a whole. As the ferromagnetic material, for example, iron is used, and fine particles made of a ferromagnetic material such as ferrite fine powder and amorphous alloy can be used. As the dielectric material, for example, an epoxy resin or a silicone resin may be employed, or a material molded into a plate-like porous material using a foamable synthetic resin material such as foamed polystyrene or foamed urethane may be employed.

  The dielectric loss layer 1 is, for example, a sheet member in which fine particles of a resistor, for example, a carbon-based material, are dispersed in a dielectric material made of resin, for example, and efficiently transmits electromagnetic waves in a region where the electric field component intensity of incident electromagnetic waves is high. To absorb. For example, a resin material containing a predetermined wt% of ketjen black (manufactured by Lion Corporation, model name EC300J), which is a carbon material, such as a molded body of an epoxy resin or a urethane resin is used.

  The air-like layer 2 flattens (that is, broadens) by suppressing the behavior of the absorption characteristics of the incident electromagnetic wave, and the loss term α (= √ (μ ”ε”) per unit thickness is used to suppress the behavior. A material or structure having a large value of)) is preferred. As the air-similar layer 2, a resin containing a predetermined amount% of a microballoon (manufactured by Nippon Ferrite Co., Ltd., model name EMC40), for example, a urethane resin molded body is used. Alternatively, a structure of an aramid honeycomb (a complex relative dielectric constant real part value of 1.14 and a imaginary part value of 0.003) formed by impregnating aramid paper with resin and forming into a honeycomb shape may be used. Moreover, you may employ | adopt a foamable synthetic resin material like a polystyrene foam, for example. Each example will be described below.

(First embodiment)
FIG. 2 is a characteristic diagram showing reflection loss characteristics of the radio wave absorber according to the first embodiment of the present invention and a conventional impedance gradient laminated radio wave absorber as a comparative example.

  The magnetic loss layer 3 is prepared as a molded body by mixing spherical iron magnetic powder (manufactured by BASF, ES powder) obtained by thermal decomposition of an iron carbonyl complex with an epoxy resin. The air similar layer 2 is produced as a urethane resin molded body containing a microballoon (manufactured by Nippon Philite Co., Ltd., model name EMC40). The dielectric loss layer 1 is produced as an epoxy resin molded body containing ketjen black (manufactured by Lion Corporation, model name EC300J). An aluminum metal plate was used as the conductor layer 4. The magnetic loss layer 3, the air-like layer 2, and the dielectric loss layer 1 were laminated in this order, and the radio wave absorption characteristics were evaluated. For measurement, a horn antenna BBHA9120D manufactured by SchwarzbeckMess-Electronik and a vector network analyzer E8363A manufactured by Agilent were used, and the reflection loss (radio wave absorption ability) in the frequency band of 1.0 GHz to 18 GHz was evaluated by the free space method. did. The measurement uses a time domain gating method, and the gate span is 500 picoseconds.

As a more specific configuration of the radio wave absorber, a urethane resin molded body containing 80 wt% of the magnetic powder in the resin molded body serving as the magnetic loss layer 3 and 30 wt% of the microballoon as the air-like layer 2 is used. An epoxy resin molding containing 2 wt% of the ketjen black was used for the dielectric loss layer 1. The thicknesses of the magnetic loss layer 3, the air-like layer 2 and the dielectric loss layer 1 are 0.65 mm, 1.97 mm, 0.85 mm, and the weight of the absorber per 1 m ² is 4500 g.

  In FIG. 2, the first absorption (a concave first absorption peak, for example, see FIG. 5) appears around 10 GHz. Although not shown in FIG. 2, the air-similar layer 2 has a function of moving and adjusting the second absorption located on the higher frequency side (the concave second absorption peak, for example, see FIG. 5) mainly in the frequency direction. . That is, the second absorption peak position can be controlled by adjusting the electrical lengths of the magnetic loss layer 3, the dielectric loss layer 1, and the air-like layer 2 with the thickness or material constant. For example, when the electrical length is shortened, the position of the second absorption peak approaches the first absorption peak. Conversely, when the electrical length is increased, the reflection loss characteristic in the absorption band is sacrificed ( Although the reflection loss tends to decrease), radio wave absorption in a wide band is possible. This phenomenon is the same in the mechanisms of the following embodiments.

On the other hand, as a comparative example, a conventional laminated radio wave absorber (Patent Document 1) in which the dielectric constant and the magnetic permeability are gradually increased stepwise from the radio wave incident surface to the inside of the absorber. From the impedance matching, the surface layer is urethane foam with 70 vol% pores, the intermediate layer is an epoxy resin molded body containing 20 wt% metallic iron magnetic powder, and the lowermost layer is the same magnetic powder. A molded body containing 82.5 wt% was obtained. The thickness of each layer was 2.94 mm, 1.16 mm, and 0.83 mm, respectively, and the weight of the absorber per 1 m ² was 6100 g.

  In the characteristic diagram of FIG. 2, in the radio wave absorber according to the first example, an absorber having a reflection loss of −15 dB or less can be obtained for radio waves in a frequency band of 8 GHz or more (˜18 GHz). On the other hand, in the comparative example, although the reflection loss is relatively low, it has a reflection loss of −15 dB or less at 8 GHz to 18 GHz.

  However, the radio wave absorber according to the first embodiment is lighter and thinner than the comparative example, and has a large reflection loss. This is because the air-like layer 2 matches the resonance frequency to the target frequency, and incident radio waves are converted into thermal energy by the magnetic loss layer 3 in the vicinity of the reflector where the magnetic field component is maximized, and the electric field component is maximized. It can be said that this corresponds to efficient conversion into thermal energy by the dielectric loss layer 1 in the vicinity of the incident surface.

  According to such a thin layer, lightweight structure, and broadband and high radio wave absorption characteristics, it is used for microwave band, mainly X band (8GHz-12GHz) used in marine radar, ie mechanical or other in application site It is possible to suppress the generation of pseudo echoes and unnecessary echoes by attaching them with the pasting method. The present invention is also applicable to a Ku band (12 to 18 GHz) radar (for example, for weather). In addition, Wi-max (Worldwide Interoperability for microwave Access) use frequency (2 GHz to 11 GHz) and UWB standard (Ultra Wide Band) use frequency (3 GHz to 10 GHz) used as broadband communication suppress interference caused by unnecessary reflection. It becomes possible. Furthermore, it can also be used in a wide range of applications, such as consumer devices that use the microwave band, and can be expected to be applied to various fields such as home appliances and building materials. The same applies to the following embodiments.

(Second embodiment)
FIG. 3 is a characteristic diagram showing the reflection loss characteristic of the radio wave absorber when an aramid honeycomb is used for the air-like layer 2 as the second embodiment. For the air-like layer 2, instead of the microballoon-containing urethane resin molding used in the first example, the aramid honeycomb (value of complex relative permittivity real part 1.14, value of imaginary part 0.003) A radio wave absorber having a reflection loss of 8 GHz or more and -15 dB or less was prototyped. Specifically, the thicknesses of the magnetic loss layer 3, the air-like layer 2, and the dielectric loss layer 1 were 0.64 mm, 2.2 mm, and 0.87 mm, respectively, and the weight of the absorber per 1 m ² was 4100 g. . By using a honeycomb having a small specific gravity of 0.05 g / cm ^{3 for} the air-like layer 2, the weight of the absorber can be further reduced.

(Third embodiment)
FIG. 4 is a characteristic diagram showing reflection loss characteristics when the same material is used as the base material of the dielectric loss layer 1 and the magnetic loss layer 3 as the third embodiment. Since the magnetic loss layer 3 is also accompanied by a dielectric loss component derived from the capacitance of the magnetic powder, it can be applied to the surface layer, that is, the dielectric loss layer 1. Therefore, as the magnetic loss layer 3 and the dielectric loss layer 1, an epoxy resin molded body having a magnetic powder content of 80 wt% was manufactured, and a radio wave absorber was made as a trial from a combination of the air-like layer 2 and the aramid honeycomb. The thicknesses of the magnetic loss layer 3, the air-like layer 2, and the dielectric loss layer 1 were 0.92 mm, 1.80 mm, and 0.21 mm, respectively, and the weight of the absorber per 1 m ² was 4350 g. As shown in FIG. 4, a reflection loss of −15 dB or less was obtained in a frequency band of 8 GHz or more. By using the same kind of material for the magnetic loss layer 3 and the dielectric loss layer 1, the cost can be reduced.

(Fourth embodiment)
FIG. 5 is a characteristic diagram showing each reflection loss characteristic of the radio wave absorber in which the thickness of the air-like layer 2 is changed as the fourth embodiment. A 0.89 mm thick epoxy resin molded body containing 82 wt% metallic iron magnetic powder was produced as the magnetic loss layer 3, and a 0.34 mm thick epoxy resin molded article containing 4 wt% ketjen black as the dielectric loss layer 1. The body was made. Then, a urethane resin molded body containing 30 wt% of microballoons was prepared as the air-similar layer 2, and the reflection loss characteristics were evaluated when the thickness was changed from 2.0 mm to 3.5 mm by 0.5 mm. .

  It was found that the thinner the air-like layer 2 is, the ultra-wideband electromagnetic wave absorption characteristics are obtained. As the thickness of the air-like layer 2 is increased, the absorption band is narrowed but the reflection loss amount is increased. Further, when the thickness of the air-like layer 2 is about 2.5 mm, a reflection loss of 8 GHz or more and −15 dB or less is obtained.

  The radio wave absorber is a radio wave absorber that uses interference due to multiple reflection of λ / 4 type resonance, and the first absorption peak seen in the vicinity of 8 GHz is the reflection on the incident surface of the radio wave and the metal reflector of the conductor layer 4. It is obtained from the interference of reflections. The position of the second absorption peak adjusted by changing the thickness of the air-like layer 2 is derived from interference between the reflection from the surface of the radio wave absorber and the reflection from the surface of the magnetic loss layer 3. That is, the radio wave absorption band can be controlled by adjusting the thickness of the air-like layer 2. In addition, how much the thickness of the air-like layer 2 is set can be set according to the amount of radio wave absorption and the application target, and in any case, the thickness is 2.5 mm to 3.5 mm (as a whole ) Thinner and lighter. Further, the thickness of the magnetic loss layer 3 and the dielectric loss layer 1 is made thinner than that of, for example, the conventional one, while the thickness of the air-like layer 2 is made thicker than the thickness of the magnetic loss layer 3 and the dielectric loss layer 1. By setting, it becomes possible to realize a required amount of radio wave absorption in a required wide band. That is, in the case of the resistance layer described in Patent Document 2, the oscillation behavior of the impedance derived from the property of the tanh function is suppressed, but the air-like layer 2 in this embodiment is described in Patent Document 2. Is different in function and functions as a partial reflector for absorbing radio waves by dielectric loss and canceling radio wave energy by interference. Therefore, unlike Patent Document 2, if the thickness is too thin, surface reflection necessary for interference is reduced, that is, transmission is increased, which is not preferable.

(5th Example)
In the fifth embodiment, an epoxy resin molded body having a thickness of 0.89 mm containing 82% by weight of metal iron magnetic powder as the magnetic loss layer 3 was produced, and the aramid honeycomb having a thickness of 5.5 mm as the air-like layer 2 and dielectric As a loss layer 1, an epoxy resin molded body having a thickness of 0.38 mm containing 2 wt% of the ketjen black was prepared. FIG. 6A shows an image in which these layers are stacked. As a result of evaluating the reflection loss characteristics of the radio wave absorber shown in FIG. 6A, it was found that the UWB standard (3 GHz to 10 GHz) radio wave can be reduced by 90% or more (reflection loss of −10 dB or less) (in FIG. 10). , See "No holes").

  Here, in the radio wave absorber shown in FIG. 6 (A), 80% or more of the weight is due to the magnetic loss layer 3 except for the conductor layer 4. 3 is considered effective.

  7, 8, 9, 11, 12, and 13 are holes of various shapes corresponding to a predetermined wt%, in this example, 15 wt%, for example, circular holes 31 (see FIG. 7), square hole 32 (FIG. 8), hexagonal star hole 33 (FIG. 9), hexagonal hole 34 (FIG. 11), triangular hole 35 (FIG. 12), cross hole 36 (FIG. 13), Each reflection loss characteristic was evaluated. The size of each hole 31-36 is as follows. That is, the circular hole 31 has a radius of 10 mm, and the others are holes having the same volume. (B) and (C) of each figure show the electric field strength distribution and the magnetic field strength distribution. The electric field strength of (B) represents 0.0000e + 000 to 5.0000e + 002 in 12 steps, and the magnetic field strength of (C). Represents 0.0000e + 000 to 5.0000e + 000 in 12 steps.

  In FIG. 7, a pair of high electric field strength portions appear at the arc positions of the circular holes 31 facing each other (see FIG. 7B), and a slightly higher magnetic field strength portion is seen on the orthogonal side thereof (see FIG. 7C). ). In FIG. 8, a slightly higher magnetic field strength portion appears at a pair of opposite corners of the square hole 32. In FIG. 9, high electric field strength is observed on two opposing sides of the hexagonal star hole 33, and a slightly higher magnetic field strength portion appears at a pair of opposite corners in the same direction. In FIG. 11, high electric field strength is observed at a pair of opposite corners of the hexagonal hole 34 and sides on both sides thereof, and a slightly higher magnetic field strength portion appears on the pair of opposite sides on the orthogonal side. In FIG. 12, relatively high electric field strength and magnetic field strength appear in each side of the triangular hole 35 as compared with the surroundings, but the strength is partially higher than the other holes 31 to 34 and 36. The site is not found. That is, no resonance phenomenon has occurred. In FIG. 13, high electric field strength is seen on each pair of sides of the cross-shaped hole 36 facing in the left-right direction, and high magnetic field strength portions appear in a long space on the left and right.

  FIG. 10 is a characteristic diagram showing the reflection loss characteristic of each radio wave absorber in FIGS. 6 to 9 in the fifth embodiment. FIG. 14 is a characteristic diagram showing the reflection loss characteristic of each radio wave absorber in FIGS. 6 and 11 to 13 in the fifth embodiment.

  In any shape of holes 31 to 36, although the absorption band is slightly narrower than in the case where the hole shown in FIG. 6 is not formed, the reflection loss amount is 4 GHz to 10 GHz and the reflection loss is -15 dB or less. Is obtained. As a result of calculating the behavior of the electromagnetic field at the frequency of the second absorption peak in each absorption line by the three-dimensional electromagnetic field simulation, the radio wave resonates inside the holes 31 to 36 in the vicinity of the conductor layer 4, and the electric field component, It was confirmed that both magnetic field components were amplified. This effect is relatively less advantageous in the case of a triangular hole 35 that does not have enough space for resonance, while other hexagonal holes 33, which can utilize other relatively long distances, particularly for resonance. The cross-shaped hole 36 works more advantageously, and the degree of narrowing of the absorption band due to the formation of the hole can be reduced. In addition, although the hole drilled in the magnetic loss layer 3 has been described with 15 wt%, it is not limited to this value, and is smaller and larger wt% depending on the radio wave absorption characteristics and the band required for the application target. It may be adopted.

(Sixth embodiment)
15 to 19 are diagrams for explaining the sixth embodiment. FIG. 15A shows a three-dimensional structure of the cross section of the magnetic loss layer 3. In the sixth embodiment, as the magnetic loss layer 3, a silicone resin liquid containing 84 wt% of metal iron magnetic powder is selected and formed into a plate shape, and a conical hole 301 having a radius of 15 mm and a height of 0.9 mm is used. A molded body on which was formed with a mold (so-called concave cone). In FIG. 15A, the black portion corresponds to the cross section of the magnetic loss layer 3. In addition, you may produce using what is called a three-dimensional printer.

  Further, as shown in FIG. 19A, a flat-type magnetic loss resin molded body 3B having the same volume and having a thickness of 0.74 mm was also produced.

  Then, the magnetic loss layer 3 and the magnetic loss resin molding 3B have a thickness of 0.35 mm containing 3 wt% of the aramid honeycomb having a thickness of 2.47 mm as the air-like layer 2 and the ketjen black as the dielectric loss layer 1. The epoxy resin molded body was laminated.

  FIG. 15B shows an electric field component intensity distribution, and the intensity represents 0.0000e + 000 to 6.0000e + 002 in 12 steps. FIG. 15C shows a magnetic field component intensity distribution, and the intensity represents 0.0000e + 000 to 3.0000e + 000 in 12 steps. As shown in FIG. 15 (B), the electric field strength is higher toward the radio wave incident surface side and lower toward the lower layer, and the low electric field strength portion of the waveform is steadily corresponding to the layer thickness of the magnetic loss layer 3. Appears. In addition, as shown in FIG. 15C, the magnetic field intensity is high corresponding to the space portion of the hole 301. Note that FIG. 19B shows an electric field component intensity distribution, and the intensity represents 0.0000e + 000 to 6.0000e + 002 in 12 steps. FIG. 19C shows the magnetic field component intensity distribution, and the intensity represents 1.5233e−002 to 3,2661e + 000 in 17 steps. As shown in FIG. 19B, the electric field strength is higher toward the radio wave incident surface side, lower toward the lower layer, and a low electric field strength portion appears in the middle. In FIG. 19C, a low magnetic field strength region appears in the middle of the thickness direction.

  FIG. 16 is a characteristic diagram evaluating the reflection loss characteristics of the radio wave absorber (concave cone) shown in FIG. 15A and the radio wave absorber shown in FIG. 19A (no processing). When the resin molded body having the conical hole 301 shown in FIG. 15 (A) is used, the same volume (same weight) as that of the magnetic powder resin molded body 3B of FIG. 19 (A) is used. It has been found that the absorption frequency band is shifted to the low frequency side as shown in FIG. 16 as compared with the magnetic powder resin molded body 3B. More specifically, in the magnetic resin molded body 3B, the absorption loss of −15 dB or more is 9 GHz to 18 GHz, whereas in the radio wave absorber of the sixth embodiment, the frequency is 8 GHz to 17 GHz, which is a low frequency of about 1 GHz. Shift to the side. Further, it was found that the radio wave absorber in which the conical hole 301 is formed absorbs the radio wave by taking in the electromagnetic field derived from the structure. According to this, by forming a concave cone in the magnetic loss layer 3 in a conical shape other than the above-mentioned dimensions and in various shapes, the absorption frequency band is compared with the case of “no processing”. Thus, it is possible to shift to the low frequency side, and by adopting a shape according to the application target, it is possible to realize a frequency shift by a desired amount while maintaining a desired band.

  FIG. 17 (A) shows a magnetic powder-containing resin molded body 3A (convex cone) having a conical protrusion opposite to that shown in FIG. 15 (A). Similarly to the above, a dielectric loss layer 1, an air-like layer 2, and a conductor layer 4 (not shown) are laminated. The hole 302 is a space part in which the magnetic powder-containing resin is drilled. Note that the portion of the aramid honeycomb that comes into contact with the cone does not affect the reflection loss characteristics even if it is cut into a cylindrical shape because the aramid honeycomb is made of an air-like material. FIGS. 17B and 17C correspond to FIGS. 15B and 15C. As shown in FIG. 17C, the magnetic field intensity is low overall.

  As shown in the characteristic diagram of FIG. 18, in the case of the magnetic powder-containing resin molded body 3A (convex cone), in contrast to the case where the hole 301 is provided in a conical shape (FIG. 15A), the radio wave absorption ability is Declined. As a result of examining the behavior of the electromagnetic field for the laminated wave absorber of FIGS. 15A and 17A by simulation, the structure of the wave absorber of FIG. 15A in which the hole 301 is formed in a conical shape is shown. The radio wave absorber 3A in FIG. 17 (A) absorbs radio waves by taking in the electromagnetic field derived from it, whereas the absorption frequency band is shifted to the high frequency side and becomes narrower, and the impedance is reduced. It was found that the radio wave energy was converted to thermal energy due to loss while matching. Moreover, it turned out that the efficiency of the heat conversion is lower than "no processing". In addition, the shape of the hole of the magnetic loss layer 3 of FIG. Further, the slant wall angle of the conical shape of the hole is preferably a moderate change in impedance, and is preferably several tens of degrees or more, for example, 45 degrees or more, more preferably 60 degrees or more with respect to the radio wave incident direction.

DESCRIPTION OF SYMBOLS 1 Dielectric loss layer 2 Air-like layer 3 Magnetic loss layer 31-36 Hole 301 Conical hole 4 Conductor layer

Claims (4)

In the plate-shaped wave absorber in which the loss layer and the conductor are laminated from the incident direction of the radio wave,
The loss layer is formed by laminating a dielectric loss layer and a magnetic loss layer from the incident direction of the radio wave, and an air-like layer is interposed between the dielectric loss layer and the magnetic loss layer,
The magnetic loss layer has a hole in the thickness direction, and the hole has a shape having the same area as that of a circle having a radius of 10 mm and corresponds to 15% by weight of the entire magnetic loss layer. wave absorber characterized in that it is. The radio wave absorber according to claim 1, wherein the hole has a corner portion in a cross-sectional shape. The radio wave absorber according to claim 1, wherein the hole has a conical hole from a radio wave incident direction. The radio wave absorber according to claim 3, wherein the inclined wall angle of the conical shape of the hole is 45 degrees or more with respect to the radio wave incident direction.