JPH04167602A - Wave absorber - Google Patents

Abstract

PURPOSE:To provide a thin radio wave absorber superior in radio wave absorptive characteristic over a broad band by arranging a resistance film at the front of a loss material whose magnetic loss tandelta is mu<2 and thickness is 1/4 of the in-medium wavelength whose lambdag, and furthermore, arranging the loss material at the front. CONSTITUTION:The wave absorptive characteristic which compensates a resistance component and that is thin and superior in a low frequency area can be obtained by arranging the resistance film 11 at the front of the loss material 12 with magnetic loss tandeltamu<2, and furthermore, and the making of it into the broad band can be attained by providing a loss material layer with thickness thinner than that of the loss material 12 at the front. The resistance R component of impedance can be mainly adjusted by attaching the resistance film 11 on the loss material 12, and matching in the neighborhood of 1/4(lambda0/Re(mugamma.epsilongamma)) that is 1/4 the in-medium wavelength lambdag can be obtained. Where, wavelength in free space is assumed as lambda0, the complex relative permeability of the loss material as mugamma, the relative complex dielectric constant of the loss material as epsilongamma, and the (mugamma, epsilongamma)<1/2> real number part of complex number as Re(mugamma.epsilongamma)<1/2>. In such a way, superior wave absorptive characteristic can be obtained by widely adjusting sheet resistivity (impedance).

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application) The present invention relates to a radio wave absorber used for preventing television ghosts and a radio wave absorber used for train radio/airport/ship communication.

(Prior Art) A ferrite sintered body is used as a radio wave absorber in a frequency band of several MHz to several thousand MHz for TV ghost interference countermeasures and radio anechoic chambers.

This has a magnetic loss tan δμ of more than 3 and is very thin, and also has an extremely wide band characteristic. However, the size is 100 mm x 100 m
It is in the form of a tile with a size of +A, and there are problems with the installation process, such as the hassle of pasting it up and the fact that it is easily broken. On the other hand, composite materials in which ferrite powder or carbon powder is mixed with rubber or resin are used in frequency bands of several thousand MHz or more.

An example of the structure of this radio wave absorber (conventional example) is shown in FIG.

This has a magnetic loss tan δμ of 2 or less and is effective in a high frequency region. However, when used at low frequencies, there is a problem in that it becomes very thick. Another disadvantage is that the effective frequency band is relatively narrow. On the other hand, there is a λ/4 type radio wave absorber in which a resistive film is formed at a wavelength of 174 in free space from the reflector, but it has the problem that it is thick in the long wavelength region of low frequencies.

(Problem to be solved by the invention) A composite magnetic material made by mixing ferrite powder with rubber, which solves the problem of the trouble of attaching ferrite sintered bodies and the tendency to break, has a small tan δμ of 2 or less, so it can be used in low frequency regions. In order to have effective radio wave absorption characteristics, the thickness must be thick, so it is desired to make it thinner. Furthermore, it is desired to solve the problem of the thick thickness of the λ/4 type radio wave absorber in order to obtain radio wave absorption characteristics at low frequencies, and to reduce the thickness of the λ/4 type radio wave absorber. In either case, it is desired to reduce the thickness in the low frequency region and to provide effective radio wave absorption characteristics in a wide frequency range.

The present invention is intended to solve these problems, and an object of the present invention is to provide a radio wave absorber that is thin and has broadband characteristics even in a low frequency region, using a material with low loss.

(Means for Solving the Problems) In order to solve the above problems, the present invention provides magnetic loss tan δ
By arranging a resistive film on the front surface of a lossy material with μ of 2 or less, it is thin and has excellent radio wave absorption characteristics in the low frequency region without compensating for the resistance component, and in addition, a lossy material layer thinner than the thickness of the lossy material is provided in front of it. It was devised to provide a wide band by providing the following.

(Function) When a loss material having a loss coefficient tan δμ (μr゛/μr') of 2 or less has a relatively thin thickness, the resistance R component of impedance is large and matching is difficult to obtain. Therefore, in order to obtain matching, the thickness must be around 3/4λ (medium wavelength), and for example, for 100MH2, the thickness is approximately 3
It becomes 0c+a or more. However, if tan δμ is smaller than 0.1, no loss can be obtained and effective radio wave absorption characteristics cannot be obtained.

By adding a resistor (film) to the lossy material, the resistance R component of impedance is mainly adjusted,
1/4 (λo
/ Re (T]-71-) ) can be matched. Here λ. is the wavelength in free space, pr is the complex relative permeability of the lossy material, εr is the complex relative dielectric constant of the lossy material, and Re(m 2 ) is the real part of the complex number 2. The concept of this idea is the impedance (377Ω) seen from the metal plate.
It applies the principle of a λ/4 type radio wave absorber, which has a structure in which a resistor (111) equal to Excellent radio wave absorption characteristics are obtained by adjusting the sheet resistance value (impedance).

The above will be explained using an impedance Smith chart. In the case of a structure in which a material with a small loss coefficient is attached to the reflector, the input impedance seen from the front changes to a high impedance region as shown in a of FIG. By attaching the resistive film, the resistance R component becomes smaller and the impedance changes as shown by the arrow, so that a condition for passing Z=1, which is a matching condition, is obtained. Also, the frequency characteristics of complex relative magnetic permeability and complex relative permittivity are taken into consideration to optimize the thickness and sheet resistance value.

Furthermore, in order to widen the band of the thinned radio wave absorber,
The impedance can be reduced by using a resistive film with a lower sheet resistance value than in the above case, making the input impedance seen from the front lower than Z = 1, and by placing a lossy material on the front side to create a two-layer structure. 1, and the movement of impedance with respect to frequency changes is made small as shown in FIG. 6, thereby realizing broadband characteristics.

Note that when the sheet resistance value used is smaller than 50Ω, it becomes close to metallic reflection and sufficient radio wave absorption characteristics cannot be obtained;
If it is larger than 000 mouths, almost no effect will be obtained.

In addition, by mixing carbon, metal fibers, etc. with a magnetic loss material made by mixing ferrite particles with concrete, a high dielectric constant can be obtained, and further thinning can be achieved.

It is possible to construct the radio wave absorber by using concrete mixed with carbon, metal fibers, etc. instead of the magnetic loss material and utilizing the ohmic loss thereof.

On the other hand, by forming a multilayer combination of lossy material and resistive film, it is possible to obtain excellent radio wave absorption characteristics over a wider frequency range. An example of its configuration is shown in FIG.

Embodiment (1) FIG. 1 is a structural diagram of a radio wave absorption wall according to an embodiment of the present invention. Approximately 4 ferrite particles of 31111 in diameter are added to concrete.
A magnetic material mixed with 0vol.1% was prepared.

The results of the measurement of material constants are shown in FIG. The material constants at 130MHz are μr'=8 μr” =3 tr’=8 t
r''=0.1, and tan δμ is 2 or less.

Further, the wavelength λg in the medium is approximately 300 . that λ
A material with a thickness of 70 mm and a weight of 70 mm was manufactured. In front of it, wire resistance value 35 and 07m resistance wire with interval 11.17.23■
Three types of resistance cloths with sheet resistance values of 400, 600, and 800 Ω were prepared in a lattice pattern, each was attached to the front of a magnetic material, and the radio wave absorption characteristics were measured. The results are shown in FIG. still,
The solid line indicates the case where no resistance cloth is attached. From these results, the reflection coefficient of 130 M) Iz was less than -30 dB with a sheet resistance of 600 Ω, and excellent characteristics were obtained. When the resistance cloth is not attached, the reflection coefficient is less than -20dB at a thickness of 300H, and when the resistance cloth is attached, the reflection coefficient is about 7.
Since the same or better characteristics can be obtained at 0.0 cm, the radio wave absorber according to the present invention is thinner to about 1/4 of the thickness of the conventional one.

Example (2) On the front surface of the magnetic material produced in Example (1), a sheet resistance wire of about 500 Ω made of the resistance wires woven in a lattice shape at 14 mm intervals was attached, and a composite made of concrete mixed with ferrite was attached to the front surface. An example of a two-layer radio wave absorber with attached materials is shown in FIG.

As a result, we measured the characteristics when the thickness of the composite material made of concrete and ferrite mixed on the front surface was 20 m1i, and the radio wave absorption characteristics were obtained as shown in FIG. From this, a broadband characteristic of -15 dB or less was obtained from 100 MHz to 450 MHz.

In addition, as a result of measuring the radio wave absorption characteristics by attaching an 8 mm thick porcelain tile to the front of this two-layered radio wave absorber,
A radio wave absorber was obtained which exhibited excellent characteristics with only a 2 dB deterioration in characteristics at high frequencies and was also excellent in appearance. Figure 3 shows its structural diagram.

(Effects of the Invention) As explained above, the present invention has an extremely thin structure and excellent radio wave absorption performance by arranging a resistive film in front of the 174-wavelength loss material having a thickness of tan δμ less than 2 and having an internal wavelength λg. In addition, by arranging a lossy material on the front side of the structure, it has an effect of having excellent radio wave absorption characteristics over a wide band.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a radio wave absorber according to an embodiment (1) of the present invention, FIG. 2 is a diagram showing a radio wave absorber according to an embodiment (2) of the present invention, and FIG. FIG. 4 is a diagram showing a radio wave absorber having a different structure in Example (2); FIG. 4 is a diagram showing a multilayer radio wave absorber of the present invention; FIG. 5 is a Smith chart diagram showing the principle of operation of the present invention; The figure is a Smith chart diagram showing another principle of operation of the present invention. Figure 7 is a diagram showing an example of the material constant of a lossy material. Figure 8 is a diagram showing the radio wave absorption characteristics of one embodiment (1) of the present invention. , FIG. 9 is a diagram showing the radio wave absorption characteristics of one embodiment (2) of the present invention, and FIG. 10 is a diagram showing a conventional radio wave absorber. 11.21, 31.41 ・Fist・ Resistive film 12, 22,
32.42.102 Hou... Loss material 13, 23.33
．． 43.103 ... Radio wave reflector 34 ... Exterior material A ... When resistance cloth is not attached (thickness 70+u+) B
・・・Loss body 70■　　+plane resistance value Rs = 400Ω mouth C* * *Loss body 70mm ten-plane resistance value Rs = 60
0Ω□D...Loss body 70+sm Decaface resistance Rs÷80
00 mouth E 11 e @ loss body 70mm decahedral resistance value Rs
= 400 Ω port + loss body 20 mm F...loss body 70 mm Decaface resistance value Rs = 5000
Mouth + Loss body 20iu G...Loss body 70 ■■ Decaface resistance value Rs = 600Ω
□Ten loss bodies 20 ■Otori

Claims (8)

[Claims] (1) Magnetic loss composite material in which magnetic material particles such as ferrite are mixed with concrete, mortar, resin, etc., or resistance loss composite material in which conductive material such as carbon or metal fiber is mixed in concrete, mortar, resin, etc., or magnetic A radio wave absorber characterized by a combination of a composite material containing both particles and a conductive material and a layer that essentially serves as a resistive film. (2) The magnetic loss composite material has a magnetic loss coefficient ta
The radio wave absorber according to claim 1, characterized in that nδμ (=μr''/μr') is 0.1 or more and 2 or less. (3) The radio wave absorber according to claim 1 or 2, wherein the layer that substantially becomes a resistive film has a sheet resistance value of 50 Ω□ or more and 3000 Ω□ or less. (4) The radio wave absorber according to any one of claims 1 to 3, characterized in that a dielectric layer is attached on the resistive film to form a two-layer structure. (5) A radio wave absorber, wherein the dielectric layer is the magnetic loss composite material, the resistive loss composite material, or a composite material containing these. (6) A radio wave absorber according to claim 5, wherein at least one of the layer structures of the radio wave absorber has a multilayer structure in which a layer substantially serving as a resistive film and the composite material are combined. (7) A radio wave absorber characterized in that a surface layer of rubber, resin, concrete, porcelain tile, stone, etc. is further provided on the front surface of the radio wave absorber according to any one of claims 1 to 6. (8) Claim 1 characterized in that a lossy material including a resistive film is provided on a radio wave reflecting material such as a metal plate, wire mesh, or reinforcing bar.
8. The radio wave absorber according to any one of .