JPH07302991A - Porous ferrite radio-wave absorber - Google Patents

Abstract

PURPOSE:To obtain a radio-wave absorber which is provided with a radio-wave absorption characteristic at 20dB in a high-frequency region of 1 to 20GHz and which is nonflammable, small and lightweight by a method wherein the shape of a porous ferrite is formed to be a pyramid shape or a triangular prism shape. CONSTITUTION:The frequency characteristic of the permeability of an Ni-Zn ferrite becomes a higher-frequency characteristic the more an Ni ratio is increased. Consequently, when a composition whose Ni ratio is increased is used as a material, the radio- wave absorption characteristic in a high-frequency region of a porous ferrite can be improved. For example, when NixZn1-xFe2O4 (where x=0.36 to 1.00) is used as the material of a porous ferrite, its radio-wave absorption characteristic in a high-frequency region can be increased. In addition, pyramid-shaped radio-wave absorbers 1 whose bottom face is 100X100mm and whose height is 100mm are formed of a porous ferrite at a porosity of 60%, they are attached to one face of a reflection board 2 without leaving any gap, radio waves are made to impinge, and an absorption factor is measured. As a result, it is possible to obtain a radio-wave absorber whose absorption band at 20dB in a Ni ratio of x=0.36 or higher is 1 to 20GHz and which is small and excellent.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lightweight, compact, nonflammable electromagnetic wave absorber having excellent electromagnetic wave absorption characteristics in a high frequency band.

[0002]

2. Description of the Related Art As an electromagnetic wave absorber, a ferrite that uses magnetic loss, carbon that has a dielectric loss, or the like is impregnated in urethane, or one that is formed into a pyramid shape or a triangular prism shape. We call it "carbon impregnated type"). The electromagnetic wave absorbing wall, which is constructed by arranging the electromagnetic wave absorbers on the reflector surface without any gap,
It is applied to anechoic chambers where there is no reflection of radio waves.

The above-mentioned carbon impregnated type has a problem in that the tip portion is deformed with time and is easily burned because urethane is molded into a pyramid shape or a triangular prism shape. Although a flame retardant has been applied to increase the flame resistance, the change over time is large and replacement is required within several years.

By the way, a radio wave absorber is usually required to have an absorption rate of 20 dB or more as a radio wave absorption characteristic. However, in the case of only a carbon impregnated type, the radio wave absorption characteristic has a low frequency band (30 M to 50 MHz). It has problems such as low absorption characteristics and a thickness of the electromagnetic wave absorber of 1 m or more.
In the case of ferrite tile, the frequency bandwidth showing the absorption rate of 20 dB or more at a thickness of 6 to 8 mm is 30 M to 6 at the maximum.
The frequency is around 00 MHz, and there is a problem that the thickness becomes 60 cm or more even when the ferrite and the carbon-impregnated type are combined.

An anechoic chamber is roughly classified into a semi-anechoic chamber for measuring electromagnetic noise emitted from electronic devices and a fully anechoic chamber for testing noise resistance of electronic devices. In the noise resistance test, since a high electric field is applied, there is a problem that the radio wave absorber generates heat, and it is extremely important that the radio wave absorber is nonflammable.

Further, the increase in the thickness of the electromagnetic wave absorber means that the effective area in the room such as the anechoic chamber is narrowed, the size of the building is increased, and the site and the construction cost are increased. There is.

Further, in recent years, the diversification of the use of radio waves has spread to the microwave range as represented by microwave ovens and mobile radio, and as one of the measures against radio wave pollution, in the microwave range from 10 GHz to about 20 GHz. The need for effective electromagnetic wave absorbers and anechoic chambers is increasing.

[0008]

SUMMARY OF THE INVENTION In consideration of the above-mentioned circumstances, the present invention has been proposed in the high frequency band of 1 to 20 GHz.
It is an object of the present invention to provide a non-flammable, small-sized, lightweight, and excellent radio wave absorber having a radio wave absorption characteristic of dB.

[0009]

In order to solve the above problems, the electromagnetic wave absorber of the present invention is made of a material in which ferrite is made porous, and has an electromagnetic wave absorption characteristic of 20 dB or more in the frequency band of 1 GHz to 20 GHz. I tried to have it.

The above-mentioned porous ferrite is advantageously shaped like a pyramid or a triangular prism.

The material of the above-mentioned porous ferrite is Ni _x Zn.
It is preferable to use ferrite having a composition of _1-x Fe ₂ O ₄ (X = 0.36 to 1.00) in order to improve radio wave absorption characteristics in a high frequency band.

The porosity of the above porous ferrite must be 30 to 90% from the viewpoint of weight and processing.

[0013]

In general, complex relative permeability μr = μ'-jμ "(or loss angle tan δ = μ" / μ ') and complex relative permittivity εr
= Ε′-jε ″ (or loss angle tanδ = ε ″ / ε ′)
Is the basic property of a material at high frequencies. If the complex relative permeability and complex relative permittivity are known, the reflectance of the substance (the ratio of the incident radio wave reflected by the surface of the substance when vertically incident on the substance) can be obtained, and also known as the absorption characteristics of the substance. You can

Since the porous ferrite utilizes the magnetic characteristics of ferrite to improve the absorption effect, its radio wave absorption characteristics depend on the frequency characteristics of the relative permeability of the material ferrite. Therefore, in order to obtain the absorption effect in the frequency band higher than the microwave, the higher the frequency is, the higher the peak of the loss term of the relative permeability is. In addition, the relative permittivity can be lowered by making it porous, and the radio wave absorption characteristics at high frequencies are improved.

The present invention improves radio wave absorption characteristics at high frequencies by forming a radio wave absorber with porous ferrite made of Ni-Zn ferrite having high frequency characteristics. Furthermore, by making the shape of the radio wave absorber into a pyramid shape or a triangular prism shape, the radio wave absorption characteristics at high frequencies can be further improved, and the size and weight can be reduced.

The frequency characteristic of the relative permeability of the Ni-Zn ferrite changes depending on the composition. As shown in FIG. 1, the composition having a higher Ni ratio has a higher frequency characteristic. Therefore, by using a material having a high Ni ratio as a material, it is possible to improve the radio wave absorption characteristics of the porous ferrite in the high frequency range. Therefore, the material of the porous ferrite forming the electromagnetic wave absorber should be Ni _x Zn.
_{By using 1-x} Fe ₂ O ₄ (X = 0.36 to 1.00), it is possible to enhance the radio wave absorption effect in the high frequency range. Ni ratio is
Below 0.36, the electromagnetic wave absorption characteristics in the low frequency range are good, but the characteristics in the high frequency range are poor.

The porous ferrite made of these materials is
Absorption bandwidth of 20 dB or more is 1G with a thickness of 100 mm or less
Since the frequency is up to 20 GHz, it is possible to obtain a compact and excellent electromagnetic wave absorber.

If the porosity of the porous ferrite is 30% or less, the weight becomes heavy, and if it is 90% or more, it cannot be formed. Therefore, when the porosity is 30 to 90%, the porous ferrite becomes lightweight and can be formed.

Since the radio wave absorber of the present invention is a sintered body of porous ferrite, it has not only incombustibility but also sufficient strength and little deformation over time.

Further, the porous ferrite according to the present invention is
30M-20GHz when combined with ferrite tile
It exhibits an absorption characteristic of 20 dB or more in the frequency band, and can be used as an absorber for an anechoic chamber compatible with a wide frequency band.

[0021]

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

<Example 1> As described above with reference to FIG. 1, the frequency characteristic of the magnetic permeability of the Ni-Zn ferrite becomes the higher frequency characteristic as the Ni ratio increases. As shown in FIG. 3, a pyramid-shaped radio wave absorber 1 having a bottom surface of 100 mm × 100 mm and a height of 100 mm is formed from a porous material having a porosity of 60% using Ni-Zn ferrites having different Ni ratios,
This was attached to one surface of the reflection plate 2 without any gap, and radio waves were made to enter the surface of the reflection plate 2 perpendicularly to measure the absorption rate. A radio wave absorption characteristic curve based on the measurement result is shown in FIG. In the figure, the Ni ratio X is 0.30, 0.36, 0.64 and 1.00 4
Regarding the case of seeds, the abscissa indicates the frequency and the ordinate indicates the radio wave absorption rate, and the radio wave absorption characteristic curve is shown. As is clear from this figure, the higher the Ni ratio, the better the absorption characteristics in the high frequency range, and the absorption band of 20 dB at the Ni ratio X = 0.36 or more is a wide band of 1 G to 20 GHz, which is excellent. Shows the radio wave absorption characteristics. When the Ni ratio is less than 0.36, the characteristics in the low frequency range are good, but the characteristics in the high frequency range deteriorate.

<Embodiment 2> A pyramid-shaped porous body 1 having a Ni ratio X = 0.36 composition of Embodiment 1 is attached to one surface of a reflector 2 in combination with a ferrite tile 3 as shown in FIG. The radio wave absorption rate was measured by injecting radio waves perpendicularly to the surface. The result is shown in FIG. As can be seen by comparing the electromagnetic wave absorption characteristic curve shown in this figure with the electromagnetic wave absorption characteristic curve of X = 0.36 in FIG.
The absorption characteristics in the low frequency region of 1 GHz or less are improved, and the absorption band of 20 dB extends from 30 MHz to 20 GHz.

[0024]

As described above, according to the present invention, 1
In the high frequency band of G to 20 GHz, it is possible to obtain a non-combustible, small-sized, lightweight, and excellent radio wave absorber having a radio wave absorption characteristic of 20 dB, and to economically construct an anechoic chamber for the high frequency band. it can.

[Brief description of drawings]

FIG. 1 is a curve diagram showing frequency characteristics of complex relative permeability with respect to various Ni ratios of Ni-Zn ferrite.

FIG. 2 is a curve diagram showing a radio wave absorption characteristic of each radio wave absorber according to the first embodiment of the present invention.

FIG. 3 is a cross-sectional view showing the configuration of an example of the present invention.

FIG. 4 is a cross-sectional view showing the configuration of another embodiment of the present invention.

5 is a curve diagram showing a radio wave absorption characteristic according to the configuration of the embodiment shown in FIG. 4 of the present invention.

[Explanation of symbols]

 1 Pyramid porous ferrite wave absorber 2 Reflector 3 Ferrite tile

Claims (4)

[Claims] 1. A radio wave absorber formed of a material in which ferrite is made porous and having a radio wave absorption characteristic of 20 dB or more in a frequency band of 1 GHz to 20 GHz. 2. The radio wave absorber according to claim 1, wherein the shape of the porous ferrite is a pyramid shape or a triangular prism shape. 3. The material of the porous ferrite is Ni _x Zn.
The radio wave absorber according to claim 1 or 2, which is a ferrite having a composition of _1-x Fe ₂ O ₄ (X = 0.36 to 1.00). 4. The porosity of the porous ferrite is 30.
% Or more and 90% or less, The radio wave absorber according to any one of claims 1 to 3, wherein.