JP2000059068A - Transparent radio wave absorbing body and manufacture thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provided a new transparent radio wave absorbing body, which is capable of efficiently absorbing radio waves of continuous wide frequency band and superior in transparency. SOLUTION: A transparent radio wave absorbing body that absorbs and reflects radio waves is of a multilayered structure composed of a transparent reflecting film 1 that reflects radio waves, N (where N>=2) layers 2a and 2b which are transparent and absorb radio waves, and N dielectric layers 3a and 3b which are transparent and arranged, being interposed between the absorbing film layers 2a and 2b so as to keep the layers 2a and 2b arranged at a certain interval, where the absorbing film layers 2a and 2b have of peak absorption frequencies different from each other, and the transparent radio wave absorbing body of this constitution is capable of absorbing radio waves of continuous frequency band that contain the above peak absorption frequencies.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a transparent radio wave absorber and a method for manufacturing the same. More specifically, the invention of this application relates to a wide-area radio wave that can be attached to windows, such as indoor and outdoor windows, and can effectively absorb radio noise and the like emitted from mobile phones, wireless LANs, broadcast radio waves, radars, and various electronic devices. The present invention relates to a novel transparent radio wave absorber having an absorption band and a high degree of transparency, and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, cellular phones and wireless LANs using about 1 GHz, marine radars using about 10 GHz, automobile radars to be used in the 35 to 100 GHz band, next-generation wireless LANs, etc. Various radio wave communication devices have been developed, and the environment in which they are used simultaneously has been increasing. Unwanted radio noise is also emitted from electronic devices that are used nearby, such as microwave ovens and computers, and the radio wave environment is getting worse.
When these radio waves are reflected by indoor and outdoor walls, serious problems such as communication failures, ghosts, and radar false images often occur.

[0003] Therefore, various types of radio wave absorbers have been conventionally proposed in order to prevent reflected waves of radio waves from a wall, and some of them have already been produced and sold. However, these conventional radio wave absorbers use metal,
Since a magnetic material or a carbon material is used, the material is opaque, and it cannot be used satisfactorily in a portion requiring transparency, such as an indoor or outdoor glass window.

In order to evaluate the performance of electronic devices that generate various radio waves as described above, it is necessary to prepare a non-reflective environment such as an anechoic chamber. The doors at the entrances and exits all had to use conventional opaque radio wave absorbers. In order to eliminate such opacity, a radio wave absorber having a metal or carbon material processed into a lattice shape or the like and partially provided with an opening has been proposed. In addition to not being able to obtain transparency, if the aperture is enlarged to improve transparency, the radio wave absorption will be impaired, and if importance is placed on radio wave absorption, transparency will be sacrificed. There is a problem that the property and the radio wave absorption are in an opposite relationship, which is not preferable for practical use.

[0005] The inventors of the present invention have already developed and disclosed a radio wave absorber having sufficient transparency to solve the problems of the conventional radio wave absorber.
(Refer to the IEICE 1997 IEICE Communication Society Conference Proceedings 1, p. 235, issued on August 13, 2007). This transparent radio wave absorber has a configuration in which a transparent absorption film that absorbs radio waves and a transparent reflection film that reflects radio waves are held at regular intervals by transparent spacers, and all constituent materials are transparent. Therefore, even if it is mounted on a glass window or the like, it does not disturb the field of view, and has sufficient radio wave absorption performance.

However, subsequent studies by the inventors have found that there is still something to be improved in this transparent radio wave absorber. This is because the radio wave absorption performance is localized only in a single narrow frequency band, and therefore, in a plurality of radio wave environments as described above, a frequency band that cannot be absorbed may be left. I understand.

Accordingly, the invention of this application has been made in view of the above circumstances, and a new transparent radio wave absorption device that efficiently absorbs radio waves in a continuous wide frequency band and has a high degree of transparency. It is intended to provide a body and a method of making it.

[0008]

Means for Solving the Problems The present invention solves the above-mentioned problems by providing a transparent radio wave absorber that absorbs and reflects radio waves, and includes a single-layer transparent reflector that reflects radio waves. It has a multilayer structure in which the film and an N-layer (where N ≧ 2) transparent absorbing film that absorbs radio waves are held at regular intervals by an N-layer transparent dielectric material interposed between these layers. Wherein the N-layer absorbing film has a different peak absorption frequency and absorbs a radio wave in a continuous frequency band including each peak absorption frequency. I do.

Further, the invention of this application provides the above-mentioned transparent radio wave absorber, wherein the reflection loss of radio waves by the absorbing film is 15 dB or more (claim 2), and the absorbing film has a transparent protective film. That the reflective film has a surface resistance of 5
0 Ω / □ or less (Claim 4), the reflection film has a transparent protective film (Claim 5), the reflection film and the absorption film are not oxidized, and indium oxide or oxidized A thin film containing indium tin as a main component (Claim 6) is provided as an embodiment thereof.

Further, the invention of this application is a method for producing the above-mentioned transparent radio wave absorber, wherein the outermost absorption film is formed at each of the peak absorption frequencies when a reflection film having a known surface resistance is used. A transparent radio wave absorber characterized by selecting the surface resistance value of each absorbing film and the thickness of each dielectric so that the radio wave impedance viewed from the radio wave incident surface on the side becomes equal to the radio wave impedance in free space. (Claim 7) is also provided.

[0011]

Embodiments of the present invention will be described below in more detail with reference to the accompanying drawings.

[0012]

(Embodiment 1) FIG. 1 illustrates a layer configuration of a transparent radio wave absorber according to an embodiment of the present invention. For example, the transparent electromagnetic wave absorber of the present invention shown in FIG.
One reflecting film (1) and two different absorbing films, that is, a first absorbing film (2a) and a second absorbing film (2b) are provided between two dielectrics, namely, a first absorbing film (2a) and a second absorbing film (2b). It has a multilayer structure that is held at a certain distance from each other by the dielectric (3a) and the second dielectric (3b).

Each of the reflection film (1), the first absorption film (2a) and the second absorption film (2b) is a transparent and conductive film. For example, an ITO (indium tin oxide) deposited film is used. Can be In the first embodiment, the reflection film (1), the first absorption film (2a) and the second absorption film (2b)
Are deposited on one surface of a PET (polyethylene terephthalate) film (6a) (6b) (6c) as a protective film, and on the other surface, plate-like glass films (4a) (4b) (4c) ) Each is adhered by a transparent double-sided adhesive tape (5). The transparent double-sided pressure-sensitive adhesive tape (5) is, for example, a PET film (5) as shown in FIG.
The acrylic resin-based pressure-sensitive adhesives (52) and (53) are applied to both surfaces of 1).

In the reflection film (1), the first absorption film (2a) and the second absorption film (2b), adjacent films are mutually adjacent to each other with the first dielectric (3a) and the thickness d1. It is held at regular intervals by the second dielectric (3b) having a thickness of d2. In the first embodiment, the first dielectric (3a) and the second dielectric (3b) are formed of an air layer. In addition, the second absorption film (2b) is a glass film (4b) for the purpose of protection from scratches when used for a glass window or the like.
c) is provided to be the outermost position.

Subsequently, the surface resistance of each absorbing film and the thickness of the dielectric in the transparent electromagnetic wave absorber having the above-mentioned multilayer structure are selected by using the manufacturing method of the present invention. First, the layer configuration illustrated in FIG. 1 is replaced with an equivalent circuit as illustrated in FIG. 2, and the radio wave incident surface (AA ′) on the outermost absorption film side of the transparent radio wave absorber, that is, the second absorption film (2b) The surface resistance value of each absorbing film and the thickness of each dielectric are calculated such that the input radio wave impedance viewed from the surface of the glass film (4c) in (2) becomes equal to the radio wave impedance in free space. For this calculation, a known point matching method is used.

According to the distributed constant line theory, the input radio wave impedance of the multilayered absorber viewed from the radio wave incident surface (AA ′) is given by the following equation.

[0017]

(Equation 1)

In the equation (1), k is the outer side (B−B) of each layer or film constituting the transparent radio wave absorber on the reflection film side.
B ′). Further, from equation 1, the input radio wave impedance and the radio wave impedance Z in free space
_{Using 0} , the absorption amount S [dB] of the radio wave incident from the radio wave incident surface (AA ′) on the outermost absorption film side of the transparent radio wave absorber is
In the case of normal incidence of a plane wave, it is given by the following equation.

[0019]

(Equation 2)

The point matching method refers to an absorption amount at a frequency of N points (f1 to fn) in an absorber having a multilayer structure composed of one reflecting film and N absorbing films as illustrated in FIG. (I.e., these frequencies are peak absorption frequencies) and that the minimum value of the amount of absorption between each of these frequencies is a desired constant value Amin. This is a method of calculating and determining the surface resistance value and the thickness of each of the other constituent layers.

That is, in a transparent electromagnetic wave absorber composed of a single reflective film having a known surface resistance value and an N-layer absorbing film having an unknown surface resistance value, the dielectric of the N layers having thicknesses d1 to dN is determined. If the thickness of the body is also unknown, the unknowns are the surface resistance (r1, r2,..., RN) and the thickness of the dielectric (d
1, d2,..., DN), and the lower limit absorption amount A when the lower limit peak absorption frequency f1 illustrated in FIG. 3 is arbitrarily determined.
bottom frequency f′i at min = (fi + fi−1) /
2 is the sum of the peak absorption frequencies (f2, f3,..., FN) other than the lower limit peak absorption frequency f1.
N-1.

On the other hand, the conditional expression is

[0023]

(Equation 3)

2N for each of the real and imaginary parts of
Also

[0025]

(Equation 4)

Since the number of unknowns is equal to the number of conditional expressions, the number of unknowns is equal to the number of conditional expressions.
Unknowns can be obtained by solving the original simultaneous linear equations. Therefore, in the transparent radio wave absorber of FIG. 1, the reflection film (1), the PET films (6a), (6b), (6c), and the PET film (5) in the transparent radio wave absorber are used.
1), acrylic resin-based pressure-sensitive adhesives (52) and (53), and glass films (4a) (4b) and (4c) having the values shown in Table 1 below were used, and the lower limit peak absorption frequency f1 = 1 2 GHz, lower limit absorption amount Amin = 25.85
When dB (voltage standing wave ratio VSWR = 1.1), the unknown thickness of each dielectric and the surface resistance of each absorbing film were calculated as described above, and the first dielectric (3a) Thickness d1 = 38.54 mm, thickness d2 of the second dielectric (3b)
= 26.35 mm, surface resistance value r1 of the first absorption film (2a)
= 252Ω / □, surface resistance value r2 of the second absorption film (2b)
= 2870Ω / □ was obtained.

[0027]

[Table 1]

FIG. 4 exemplifies theoretical values of the radio wave absorption characteristics obtained in the transparent radio wave absorber of the present invention having such numerical values. As is clear from FIG. 4, the transparent radio wave absorber of the present invention has two peak absorption frequencies of 1.2 GHz (set as the lower limit peak frequency) and about 1.75 GHz (value calculated from theoretical calculation).
Has a strong absorption peak in a wide frequency band of about 0.88 GHz to 2.11 GHz including a peak absorption frequency of 1.2 GHz and about 1.75 GHz, and a radio wave absorption amount of 15 dB or more, that is, a reflection loss of 15.
It is possible to perform radio wave absorption of dB or more. Further, for radio waves of about 0.99 GHz to 2.00 GHz, 2
It has a reflection loss of 0 dB or more, and is an excellent wide frequency band radio wave absorber.

This transparent electromagnetic wave absorber has high transparency because all the constituent materials are transparent, and has a total thickness of about 74 mm in the first embodiment, so that it can be easily mounted on a window or a wall. It is possible. Of course, in addition to the above-mentioned radio wave absorption in the wide frequency band, the reflection of the radio wave incident from the outside (BB ′) of the reflection film is reflected by the reflection film (1) which is a transparent conductive film having a surface resistance of 50Ω / □ or less. ) Is performed effectively.

(Embodiment 2) In Embodiment 2, based on the result calculated in Embodiment 1, the reflection film (1), the first absorption film (2a) and the second absorption film (2b) are A transparent electromagnetic wave absorber having a multi-layer structure held at regular intervals by each of the first dielectric (3a) and the second dielectric (3b) was actually manufactured. The size of the produced transparent radio wave absorber is length x width = 60 cm x 60 cm, and the total thickness is 7
4 mm.

The constituent materials used in the present invention may vary depending on the manufacturing conditions of each material.
Each has a slight quality error. Table 2 below shows the surface resistance [Ω / □] of the reflective film (1), the thickness [μm] of the glass films (4a), (4b), and (4c) used in the calculation in Example 1 and the examples. The surface resistance value r1 [Ω / □] of the first absorption film (2a), the surface resistance value r2 [Ω / □] of the second absorption film (2b), which are the design values calculated in Step 1, and the first dielectric ( 3a) thickness d1 [mm] and the second dielectric (3
The thickness d2 [mm] of b) was compared with each design value and each actually measured value of the transparent electromagnetic wave absorber produced in Example 2.

[0032]

[Table 2]

From Table 2, it can be seen that each constituent material has a value close to the design value. Further, the absorption characteristics of the transparent electromagnetic wave absorber made of each constituent material having such actually measured values were actually measured. The absorption characteristics were measured in an anechoic chamber having a non-reflective environment around which a commercially available non-transparent radio wave absorber (13) as shown in FIG. 5 was provided. In this anechoic chamber, the transparent radio wave absorber (15) is placed at a position about 4 m away from the transmitting / receiving antennas (11) and (12) at the support (1).
4) is installed on the front side, and the transmission antenna (11) radiates radio waves toward the transparent radio wave absorber (15);
The reflected radio wave from the transparent radio wave absorber (15) is received by the receiving antenna (12). Transmission / reception antenna (11)
The control of (12), the calculation of the reflection loss, and the like are performed by the network analyzer (10).

FIG. 6 illustrates the theoretical absorption characteristics at the time of design of the transparent electromagnetic wave absorber, the calculated absorption characteristics recalculated based on the actually measured values of the constituent materials of the manufactured transparent electromagnetic wave absorber, and the actually measured absorption characteristics. is there. As is apparent from FIG. 6, the theoretical absorption characteristic at design, the calculated absorption characteristic and the measured absorption characteristic are as follows:
Despite some errors that may have been affected by differences in complex permittivity due to variations in glass composition, differences in the thickness of each component material, and deviations in surface resistance, etc., performance was almost consistent. , About 1.13 G in measured absorption characteristics
Hz and about 1.73 GHz, and has a strong absorption band of 15 dB or more in the range of about 0.88 GHz to 2.00 GHz.
A wide-band, high-performance transparent radio wave absorber having an absorption band of 20 dB or more in the range of GHz to 1.88 GHz was obtained.

Further, since the theoretical value and the measured value are almost the same, according to the method of the present invention, the radio wave absorption performance is high at two frequencies and the radio wave is transmitted with good performance in the continuous band. It can be seen that a transparent radio wave absorber to be absorbed can be easily manufactured. (Embodiment 3) FIG. 7 shows another embodiment of the transparent radio wave absorber of the present invention.

In the transparent radio wave absorber illustrated in FIG.
In addition to the first absorption film (2a) and the second absorption film (2b),
The third absorption film (2c) is held at a constant interval from the second absorption film (2b) by the third dielectric (3c), and has a multilayer structure of three absorption films. The third absorbing film (2c) in Example 3 is the same as the first absorbing film (2c) in Examples 1 and 2.
a) and the second absorbing film (2b), the transparent conductive ITO (indium tin oxide) is made of PET film (6).
c) and further adhered to the glass film (4d) via the transparent double-sided adhesive tape (5).

In FIG. 7, unlike FIG. 1 described above,
Since the third absorption film (2c) is the outermost layer, the glass film (4d) of the third absorption film (2c) is provided so as to be at the outermost position, and is protected from scratches and the like. I have.
In the transparent electromagnetic wave absorber of FIG. 7 having such a multilayer structure, by using the manufacturing method of the present invention, after replacing FIG. 7 with the equivalent circuit of FIG. , The outermost absorption film side, that is, the third absorption film (2
Using the point matching method, the unknown surface resistance value and dielectric value of the absorbing film are determined so that the input radio wave impedance viewed from the radio wave incident surface (AA ′) on the c) side becomes equal to the radio wave impedance in free space. Determine the thickness of the body.

The radio wave incident surface (A-
The input radio wave impedance looking inside from A ') is
Is expanded to 27 layers in FIG. Also, the absorption amount S of the transparent radio wave absorber on the radio wave incident surface (AA ′)
And the reflection coefficient are also given by an extension equation of Expression 2.
The unknowns in this case are the surface resistance values r1, r of each absorption film.
2, r3 and the thicknesses d1, d2, and d3 of each dielectric, and the remaining unknown peak absorption frequencies f2 and f3 when the lower limit peak absorption frequency f1 is determined, for a total of eight. On the other hand, the conditional expression

[0039]

(Equation 5)

Six for each of the real and imaginary parts of

[0041]

(Equation 6)

The number of unknowns is equal to the number of conditional expressions, and the number of unknowns is equal to the number of conditional expressions. Therefore, the unknowns can be obtained by solving an eight-element simultaneous linear equation. In Example 3, the reflection film (1) and the PET film (6a)
(6b) (6c) (6d), PET film (51),
The acrylic resin-based pressure-sensitive adhesives (52) and (53) and the glass films (4a), (4b) and (4c) having the values shown in Table 1 above are used, and the glass film (4d) has a thickness of 1110.
μ, using a complex dielectric constant of 6.96-j0.0,
Lower limit peak absorption frequency f1 = 1.2 GHz, lower limit absorption amount Amin = 25.85 dB (voltage standing wave ratio VSWR = 1.
1).

In this case, the result of the calculation of the unknown value is that the thickness d1 of the first dielectric (3a) = 26.7 mm, the thickness d2 of the second dielectric (3b) = 20.7 mm, and the thickness of the third dielectric (3c) )
Thickness d3 = 25.8 mm, surface resistance r1 of the first absorption film (2a) = 228 Ω / □, surface resistance value r2 of the second absorption film (2b) = 885 Ω / □, third absorption film (2c) Was found to have a surface resistance value r3 = 3500Ω / □.

FIG. 9 exemplifies the radio wave absorption characteristics of the transparent radio wave absorber of the present invention having the above-mentioned values. This transparent radio wave absorber has a wider continuous wave of about 1.0 to 2.9 GHz. The radio wave in the frequency band is absorbed at a radio wave absorption (= radio wave reflection loss) of 20 dB or more, and the radio wave of a wider frequency band of about 0.8 to 3.0 GHz or more is absorbed at a radio wave absorption of 15 dB or more. , And realizes very excellent broadband radio wave absorption characteristics. Of course, the reflection film (1) having a surface resistance value of 50 Ω / □ or less can also perform excellent reflection of radio waves incident from outside (BB ′) of the reflection film.

The transparent radio wave absorber of the third embodiment having such excellent radio wave absorption characteristics and reflection characteristics is also
Like the transparent radio wave absorbers in Examples 1 and 2, it has a high degree of transparency, and its total thickness is about 83 mm, so that it can be easily mounted on windows and walls. (Embodiment 4) As another embodiment of the invention of this application, unlike Embodiments 1 to 3, in order to obtain the surface resistance value of each absorbing film and the thickness of each dielectric in the manufacturing method of the present invention. Using a two-dimensional Newton method instead of the point matching method described above, a broadband transparent radio wave absorber having two absorption peaks in the X band (8 to 12 GHz band) was designed and manufactured.

In the fourth embodiment, the same materials and the same thicknesses as in Table 1 were used except that the thickness of the glass films (4a), (4b) and (4c) was changed to 1770 μm in the structure of the transparent electromagnetic wave absorber of FIG. It is assumed that materials having the same surface resistance value (only the ITO reflection film (1)) and the same complex dielectric constant are used. When the equivalent circuit of FIG. 2 is replaced with the equivalent circuit of the first embodiment, the input radio wave impedance viewed from the radio wave incident surface (AA ′) is given by Equation 1, and the input radio wave impedance and the characteristic impedance in free space are given by When used, the absorption amount S [dB] of the radio wave incident from the radio wave incident surface (AA ′)
Is given by Equation 2 for normal incidence of a plane wave. In the two-dimensional Newton method, the equation to be solved is f (x, y) =
Assuming that g (x, y) = 0, first, the initial value (x ₀ , y
₀ ) and sequentially find a solution. here,
Equation 7 is obtained by performing the successive approximation i times.

[0047]

(Equation 7)

Then, in equation (7), x _{i + 1} −x _i
= Δx, y _{i + 1} −y _i = Δy, and further expressed using a matrix, Equation 8 is obtained.

[0049]

(Equation 8)

The solution of this simultaneous equation (x _{i + 1} , y _i ) =
Using (x _i + Δx, y _i + Δy), it is successively converged to an optimal solution. In the equation of the complex function,
When a complex number is represented by z and the equation to be solved is f (z) = 0, f (z) = Re [f (z)] + Im [f
(Z)]. Here, to make f (z) = 0, the real part Re [f (z)] of f (z) and the imaginary part Im
Since both [f (z)] must be zero, the simultaneous equations of Equation 9 need only be solved.

[0051]

(Equation 9)

Since the two equations in Equation 9 are defined by real numbers, the two-dimensional Newton method described above can be applied. Then, the two-dimensional Newton method is applied to the absorption amount s (dB) obtained by Expression 2, and the design is performed so that the absorption amount of the radio wave becomes maximum at both of the two target peak frequencies. The unknown values of the transparent electromagnetic wave absorber, the surface resistance values r ₁ and r _{2 of} the absorption films (2a) and (2b), and the dielectric material (3a) as an air layer
The thicknesses d ₁ and d ₂ of (3b) are determined.

In this embodiment, the lower limit first matching frequency f _{1 is set.}
Is fixed to 8.5 GHz, the upper limit second matching frequency f ₂ is variously changed in the range of 10 GHz to 11.5 GHz, the theoretical absorption characteristics are examined, and the widest possible absorption band is secured. to ensure a high absorption amount in its entirety band was determined second matching frequency f ₂ of an upper limit to 11 GHz.

As a result, the thicknesses d ₁ = 17.31 mm and d ₂ = 1.21 mm of the first dielectric (3a) and the second dielectric (3b), respectively, the first absorbing film (2a) and the second absorbing film (2a) Surface resistance r _{1 of} each film (2b) = 95Ω /
True solutions of □ and r ₂ = 441 Ω / □ were obtained.
Next, a transparent electromagnetic wave absorber was actually produced. Table 3 shows the resistance value [Ω / □] of the reflective film (1) and the thickness [μm] of the glass films (4a) (4b) (4c) used in the calculation and the design values calculated in Example 1. Certain absorbing films (2a) (2
The surface resistance values r1 and r2 [Ω / □] of b) and the thicknesses d1 and d2 [mm] of the dielectrics (3a) and (3b) were compared with actual measured values of actually manufactured samples.

[0055]

[Table 3]

Here, as shown in Table 3, the absorption characteristics of the transparent electromagnetic wave absorber made of each constituent material having a value substantially equal to the design value were measured in an anechoic chamber exemplified in FIGS. 10 (a) and 10 (b). did. In the anechoic chambers illustrated in FIGS. 10A and 10B, unlike the case of FIG. 5, the support (14) of the sample (15) and the transmitting / receiving antenna (11) (11) ( Two further radio wave absorbers (13) are disposed at a distance of about 1 m from the transparent radio wave absorber (15) between the support and the support (16) of (12). Also, not the network analyzer (10) but the oscillator (1)
7), a controller (18) and a spectrum analyzer (19). The sample (15) and the radio wave absorber (13) behind the support stand (14) are about 1.5 m apart, and the radio wave absorber (about the center of the support stand (14) and the side of the support stand (14)). 13) is about 1.5 m away. The transmitting / receiving antennas (11) and (12) are installed at a height of 1.8 m.

FIG. 11 illustrates the theoretical absorption characteristics at the time of design of the transparent electromagnetic wave absorber, the calculated absorption characteristics recalculated based on the actually measured values of the constituent materials of the manufactured transparent electromagnetic wave absorber, and the actually measured absorption characteristics. is there. As is clear from FIG.
The theoretical absorption characteristics at the time of design, the calculated absorption characteristics after fabrication, and the actually measured absorption characteristics are accurately matched with each other, although there are some errors, and especially in the actually measured absorption characteristics, the values are about 8.7 GHz and about 1.2 GHz. A broadband and high-performance transparent radio wave absorber having two strong absorption peaks and having an absorption band of 15 dB or more in the range of about 8.4 GHz to 11.8 GHz was obtained.

Further, since the theoretical value and the measured value are in agreement, the method of the invention of this application has a high radio wave absorption performance at two frequencies and a transparent radio wave which absorbs radio waves in a continuous band with good performance. It can be seen that the radio wave absorber was easily manufactured. Of course, since all the materials used for the produced transparent radio wave absorber are transparent, they have high transparency, and the total thickness is about 24.4 mm,
It goes without saying that it can be installed on windows and walls.

In the transparent electromagnetic wave absorber of the present invention in the above-described embodiments 1 to 4, the number of the absorbing films is two or three, but the number of the absorbing films is further increased in accordance with the desired absorption frequency band. It goes without saying that a transparent radio wave absorber having excellent radio wave absorption characteristics can be manufactured by the manufacturing method of the present invention, regardless of the number of layers, in the same manner as described above.

The absorption frequency band is also the above 1 GHz band or X band.
(8 to 12 GHz), for example, higher Ku band (12 to 18 GHz), V band (5
0-70 GHz) or lower MHz band.
It can absorb radio waves of various wide continuous frequency bands. Of course, in the transparent radio wave absorber of the present invention, it is assumed that all the constituent materials are transparent as described above,
For example, a transparent acrylic resin plate may be used instead of the glass films (4a) to (4d), and the protective film is made of the aforementioned PE.
For example, a polycarbonate resin protective film or the like other than the T films (6a) to (6d) may be used. Each dielectric may be a liquid such as glass, a transparent resin plate, or oil instead of the air layer. Instead of the transparent double-sided adhesive tape (5), any existing adhesive / adhesive technology which does not impair the transparency can be used.

The transparent electromagnetic wave absorber of the present invention requires only a reflection film, an absorption film, and a dielectric, and includes PET films (6a) to (6d), glass films (4a) to (4d), and a double-sided adhesive tape. May be omitted, added, or changed as necessary. As the reflection film and the absorption film, any existing transparent conductive film can be used, but the stability of the surface resistance value, the development of high transparency, the ease of manufacture, the ease of obtaining in the market, and the economic efficiency In view of this, tin oxide, indium oxide or films based on indium tin oxide are for example recommended.

In order to further improve the transparency, it is recommended to perform a light reflection preventing treatment. by the way,
Although the reflecting film, the absorbing film and the dielectric are single-layered in each of the above-described embodiments, they have a desired film thickness and surface resistance, and each of the absorbing films has a different desired peak absorption frequency. As long as it has a multi-layer structure composed of a plurality of thin films.

As described above, the present invention can be implemented in various modes with respect to the number of layers, the absorption frequency band, the material, the configuration, and the like, as long as the effects achieved by the invention are not hindered.

[0064]

As described above in detail, the transparent radio wave absorption band according to the present invention can be used for, for example, a 1G mobile phone.
In the Hz band, X band, Ku band, etc. used for radar, radio waves in a desired wide frequency band can be effectively absorbed on one surface and effectively reflected on the other surface. It can be easily mounted on windows and walls with sufficient transparency.

With such excellent transparency and radio wave absorption,
The transparent radio wave absorber of the present invention having reflection characteristics can be easily manufactured by the manufacturing method of the present invention. In other words, it is a multilayer even if a transparent conductive film having a surface resistance of 50 Ω / □ or less is used as the reflection film without using a metal film having a surface resistance of substantially 0 Ω / □ as in a conventional radio wave absorber. Each surface resistance value of the absorption film and each thickness of the dielectric can be accurately determined, and by using the absorption film and the dielectric having these values, continuous high performance absorption performance in a wide frequency band was achieved. A transparent radio wave absorber can be easily manufactured.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a transparent radio wave absorber according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating an equivalent circuit of the transparent radio wave absorber of FIG. 1;

FIG. 3 is a diagram exemplifying radio wave absorption characteristics of an absorber having a multilayer structure including a single-layer reflecting film and an N-layer absorbing film.

FIG. 4 is a diagram illustrating an example of a radio wave absorption characteristic of the transparent radio wave absorber of FIG. 1;

FIG. 5 is a plan view exemplifying a schematic configuration of an anechoic chamber for which absorption characteristics are actually measured and a measurement system.

FIG. 6 is a diagram exemplifying a theoretical absorption characteristic, a calculated absorption characteristic, and an actually measured absorption characteristic of a transparent electromagnetic wave absorber at design time.

FIG. 7 is a diagram illustrating a transparent radio wave absorber according to another embodiment of the present invention.

8 is a diagram illustrating an equivalent circuit of the transparent radio wave absorber of FIG. 7;

FIG. 9 is a diagram illustrating an example of a radio wave absorption characteristic of the transparent radio wave absorber of FIG. 7;

FIGS. 10 (a) and (b) are a plan view and a side view respectively illustrating the schematic configuration of an anechoic chamber and a measurement system in which absorption characteristics are actually measured.

FIG. 11 is a diagram exemplifying a theoretical absorption characteristic, a calculated absorption characteristic, and an actually measured absorption characteristic at the time of design of a transparent electromagnetic wave absorber.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 reflection film 2a first absorption film 2b second absorption film 2c third absorption film 3a first dielectric 3b second dielectric 3c third dielectric 4a, 4b, 4c, 4d glass film 5 transparent double-sided adhesive tape 51 PET film 52, 53 Acrylic resin adhesive 6a, 6b, 6c, 6d PET film 10 Network analyzer 11 Transmitting antenna 12 Receiving antenna 13 Radio wave absorber 14 Support 15 Transparent radio wave absorber 16 Antenna support 17 Oscillator 18 Controller 19 Spectrum analyzer

Claims (7)

[Claims] 1. A transparent radio wave absorber for absorbing and reflecting radio waves, comprising one transparent reflection film for reflecting radio waves and a transparent absorption film for N layers (N ≧ 2) for absorbing radio waves. Have a multilayer structure held at regular intervals by an N-layer transparent dielectric material interposed between these layers,
A transparent radio wave absorber, wherein the N-layer absorbing films have different peak absorption frequencies and absorb radio waves in a continuous frequency band including each peak absorption frequency. 2. The transparent radio wave absorber according to claim 1, wherein the reflection loss of radio waves by the absorbing film is 15 dB or more. 3. The transparent electromagnetic wave absorber according to claim 1, wherein the absorbing film has a transparent protective film. 4. The transparent electromagnetic wave absorber according to claim 1, wherein the reflection film has a surface resistance of 50Ω / □ or less. 5. The transparent electromagnetic wave absorber according to claim 1, wherein the reflection film has a transparent protective film. 6. The transparent electromagnetic wave absorber according to claim 1, wherein the reflection film and the absorption film are tin oxide, and are thin films containing indium oxide or indium tin oxide as a main component. 7. The method for producing a transparent radio wave absorber according to claim 1, wherein the outermost absorption film is formed at each of the peak absorption frequencies when a reflection film having a known surface resistance is used. A transparent radio wave absorber characterized by selecting the surface resistance value of each absorbing film and the thickness of each dielectric so that the radio wave impedance viewed from the radio wave incident surface on the side becomes equal to the radio wave impedance in free space. Method of manufacturing.