JPH05299872A - Wave absorber for 900mhz-band - Google Patents

Abstract

PURPOSE:To enable effective unwanted wave absorption thereby for achieving a thin wave absorber by using as a ferrite powder a manganese-zinc ferrite material whose permeability and dielectric constant are greater than a fixed value, respectively, and causing the ferrite powder to be contained within a specific range. CONSTITUTION:A manganese-zinc ferrite material having a permeability of 2000 or greater (at 100kHz) and a dielectric constant of 1000 or greater (at 100kHz) is used, and the ferrite powder is contained within the range of 55 to 95wt%. Further, a carbon powder or non-magnetic metal powder is contained by 5wt% or less, and an adjustment may be made so that the total content of the ferrite powder and the carbon powder or non-magnetic metal powder is 85 to 95wt%. As the non-magnetic metal powder, for instance, an aluminum or copper powder is used. The use of the high-permeability ferrite powder enhances the magnetic loss, thereby enabling the wave absorber to be thin. Moreover, the dielectric constant of the whole wave absorber increases, and the absorption peak shifts to the low frequency side.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin ferrite / rubber-based electromagnetic wave absorber that absorbs unnecessary radio waves of 800 MHz to 1 GHz. This radio wave absorber is a material suitable for being attached to the inner surface of the housing of a mobile communication device, for example, to prevent intrusion of unnecessary radio waves into the housing and leakage of unnecessary radio waves to the outside of the housing.

[0002]

2. Description of the Related Art A radio wave absorber formed by kneading ferrite powder and synthetic rubber into a plate shape has been known. Such ferrite / rubber type electromagnetic wave absorbers are widely used in the GHz band (microwave). But 800MHz-1
In the relatively low frequency band of GHz, a sufficient electromagnetic wave absorption effect cannot be obtained unless the electromagnetic wave absorber itself is thick (about 12 to 15 mm). Therefore, when a thin electromagnetic wave absorber is required in this frequency band, an electromagnetic wave absorber made of a ferrite sintered plate has been used.

Further, in connection with the ferrite / rubber-based radio wave absorber, there is known a technique for effectively absorbing the radio wave by adjusting the particle size of the ferrite powder according to the frequency band of the radio wave to be absorbed. .. Focusing on the fact that the electromagnetic wave absorption effect is related to both the frequency and the ferrite particle size, this method uses a ferrite powder having a larger particle size as the frequency becomes lower (see Japanese Patent Publication No. 55-35002). In that patent publication, in the frequency band of 500 MHz to 1.5 GHz, the magnetic permeability is 300 or more and the particle size is 1.
It is described that a ferrite powder of 65 mm to 701 μm is used.

[0004]

Problems to be Solved by the Invention 900 MHz band (800
Radio waves of MHz to 1 GHz are used for mobile communication.
Therefore, in mobile communication devices, it is necessary to prevent unwanted radio waves in this frequency band from entering the inside of the device housing and from leaking to the outside of the device housing. Moreover, mobile communication devices are becoming smaller and lighter year by year, and accordingly, it is strongly demanded that the electromagnetic wave absorber be thin. However,
Conventional radio wave absorbers cannot be used for such applications. Since the ferrite / rubber-based electromagnetic wave absorber needs to have a thickness of 10 mm or more as described above, the size of the communication device becomes large especially when it is incorporated into a portable device, and the size of the communication device is small. This is because it will impair the conversion. If you intentionally make it thinner, you cannot expect a sufficiently large electromagnetic wave absorption effect. When a radio wave absorber made of a ferrite sintered plate is incorporated, the radio wave absorber itself can be made thin, but it has the drawback of being difficult to process and costly, and it is also unsatisfactory in terms of impact resistance such as dropping. Not suitable for portable devices.

If ferrite powder having a large particle size is used in order to improve the effect of absorbing radio waves in a relatively low frequency band, the kneading property with rubber deteriorates, and the amount of ferrite powder filled is restricted, resulting in a thin and uniform powder. The plate shape has a problem that it is difficult to form.

It is an object of the present invention to provide a ferrite / rubber-based radio wave absorber which can effectively absorb unnecessary radio waves in the frequency band of 800 MHz to 1 GHz and can be made thin.

[0007]

DISCLOSURE OF THE INVENTION According to the present invention, 900M molded into a desired shape by mixing ferrite powder in a synthetic rubber.
It is a ferrite / rubber type electromagnetic wave absorber for the Hz band (800 MHz to 1 GHz). The feature of the present invention is to use a manganese-zinc ferrite material having a magnetic permeability of 2000 or more (at 100 kHz) and a dielectric constant of 1000 or more (at 100 kHz) as the ferrite powder, and containing the ferrite powder in the range of 85 to 95% by weight. There is a point. Incidentally, the permittivity of general ferrite used conventionally is about 20.

The present invention further contains carbon powder or non-magnetic metal powder in an amount of 5% by weight or less, and the total content of ferrite powder and carbon powder or non-magnetic metal powder is 85 to 95% by weight.
Including the case of adjusting so that Examples of non-magnetic metal powders include aluminum and copper powders.

The electromagnetic characteristics of the manganese-zinc ferrite material are set within the above range because the electromagnetic wave absorber cannot be made thin if the magnetic permeability is low, and the dielectric constant of the entire electromagnetic wave absorber cannot be increased if the dielectric constant is low. This is because the absorption peak cannot be moved to the low frequency side. The ferrite filling amount is 8
The reason why the content is 5 to 95% by weight is that the absorption peak exceeds 1 GHz when the content is less than 85% by weight, and the mixing and molding with rubber becomes difficult when the content exceeds 95% by weight.

In order to manufacture this radio wave absorber, an appropriate amount of crushed ferrite powder is added to synthetic rubber, and further an appropriate amount of carbon powder or non-magnetic metal powder is added as necessary so that each powder becomes uniform. After kneading, it is formed into a plate having a thickness of about 7 mm. As the synthetic rubber, for example, ethylene propylene rubber (dielectric constant 3 to 4) is used, but the synthetic rubber is not limited thereto. The particle size of each powder is preferably 100 μm or less. This is because if the particle size is too large, it will be difficult to knead with the rubber, and the uniform dispersibility of each powder when molded will be impaired.

[0011]

In the present invention, the use of ferrite powder having a high magnetic permeability increases the magnetic loss, which allows the electromagnetic wave absorber to be thin. Further, the magnetic permeability of ferrite also affects the frequency band that absorbs radio waves, and the higher the magnetic permeability, the more the frequency band that absorbs radio waves shifts to the lower frequency side. Further, by using ferrite powder having a high dielectric constant, the dielectric constant of the entire radio wave absorber is increased, and the absorption peak moves to the low frequency side. By these actions, a good electromagnetic wave absorption characteristic (reflection loss of 20 dB or more) can be obtained by the ferrite / rubber type electromagnetic wave absorber having a thickness of about 7 mm in the 900 MHz band.

The addition of carbon powder or non-magnetic metal powder serves to control the dielectric constant of the radio wave absorber as a whole. This makes it possible to adjust the absorption peak position.

[0013]

[Example] Using the three types of ferrite materials shown in Table 1, a radio wave absorber was produced under the same conditions. Sample A is a material having electromagnetic characteristics within the scope of the present invention. Samples B and C
Is a comparative example, and sample B has a small magnetic permeability,
Indicates the case where the dielectric constant is small. The magnetic permeability and the dielectric constant are both values at 100 kHz.

[Table 1]

The ferrite powder of each sample has a particle size of 5 to 20.
It was pulverized to a size of μm. 90% by weight of each ferrite powder was added to ethylene propylene rubber having a dielectric constant of 3 to 4,
After kneading so as to be uniform, all were formed into a plate shape having a thickness of 7 mm. Then, a metal plate was placed on the back surface of each of the molded plate-shaped electromagnetic wave absorbers and placed in a coaxial tube to measure the reflection loss. The measurement result is shown in FIG. 1A to 1C correspond to the samples A to C, and show that the absorption peak of the radio wave changes due to the change in the electromagnetic characteristics of the ferrite. Whether the ferrite powder mixed in has a low magnetic permeability or a low dielectric constant, the absorption peak is on the higher frequency side than 1 GHz and cannot be used for the 900 MHz band. In FIGS. 1A and 1B, the reason why the absorption peak value of B in FIG. 1 is large even though the magnetic permeability of sample B is lower than that of sample A is the same for data comparison. It depends on the thickness. This is because the thickness at which the absorption peak value is highest deviates slightly from 7 mm, and A in FIG. 1 does not show the largest absorption peak.

FIG. 2 shows the relationship between the filling amount of ferrite powder and the reflection characteristics. In all cases, the sample A was used as the ferrite material. In FIG. 2, the filling amount of the ferrite powder is 93% by weight for A, 90% by weight for B, and 80 for C.
% By weight, the rest being all rubber. The absorption peak shifts to the high frequency side as the filling amount decreases, and exceeds 1 GHz at less than 85% by weight. 2C is outside the scope of the present invention.
On the other hand, if the amount exceeds 95% by weight, the amount of rubber becomes so small that kneading cannot be sufficiently performed, and it becomes impossible to form into a plate shape.

FIG. 3 shows the relationship between the change in the carbon powder filling amount and the reflection loss. The ferrite material used was the sample A, and the filling amount thereof was all 85% by weight. In FIG. 3, A is the case where the carbon powder is not filled (filling amount 0
%), B is the case where the filling amount of the carbon powder is 2% by weight, and C is 5% by weight. It can be seen that the absorption peak and its position can be controlled by adjusting the filling amount of the carbon powder even if the same ferrite material is used and the same ratio is mixed. Carbon is lightly conductive and controls the permittivity of the entire wave absorber. Therefore, the same effect can be obtained by adding aluminum or copper powder instead of carbon powder.

[0017]

As described above, the present invention uses a manganese-zinc ferrite material having a magnetic permeability of 2000 or more and a dielectric constant of 1000 or more as the ferrite powder, and the ferrite powder is contained in the synthetic rubber in an amount of 85 to 95% by weight. As a result, it is possible to obtain a radio wave absorber having a good characteristic of a reflection loss of 20 dB or more in the frequency band of 800 MHz to 1 GHz, while being thin with a thickness of about 7 mm. As a result, it can be easily mounted on the housing of a portable 900 MHz band communication device or the like, and when mounted, the device can be made much smaller than conventional products. Also, unlike the sintered type because it is a rubber type, it can be easily processed and installed even with a complicated housing shape, and it has excellent impact resistance.

Further, in the present invention, an appropriate amount of carbon powder or non-magnetic metal powder may be added, whereby the dielectric constant of the entire radio wave absorber can be adjusted to control the radio wave absorption characteristics. Furthermore, this electromagnetic wave absorber is in the 900 MHz band, but the particle size is 10
Since powder having a particle size of 0 μm or less can be used, it can be easily kneaded with rubber and uniformly dispersed, and a good plate-shaped molded product can be easily manufactured.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between ferrite material and reflection characteristics.

FIG. 2 is a graph showing a relationship between a ferrite powder filling amount and reflection characteristics.

FIG. 3 is a graph showing a relationship between a carbon powder filling amount and reflection characteristics.

Claims (3)

[Claims] 1. A ferrite-rubber radio wave absorber formed by mixing ferrite powder in synthetic rubber, wherein the ferrite powder is a manganese-zinc ferrite material having a magnetic permeability of 2000 or more and a dielectric constant of 1000 or more. An electromagnetic wave absorber for the 900 MHz band containing 85 to 95% by weight of powder. 2. Carbon powder or non-magnetic metal powder 5% by weight
The total content of ferrite powder and carbon powder or non-magnetic metal powder is 85 to 95% by weight.
The described 900 MHz band electromagnetic wave absorber. 3. The electromagnetic wave absorber for the 900 MHz band according to claim 1, wherein a powder having a particle size of 100 μm or less is used.