JPS648480B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a novel radio wave absorbing curtain using carbon-based fiber cloth. Specifically, the present invention relates to a novel radio wave absorbing curtain using carbon-based fiber cloth. A radio wave absorbing curtain with improved radio wave absorption properties and high flexibility is provided by laminating the same wave heights with different wave heights and fixing them to the carbon fiber base fabric surface. It is something to do.

Traditionally, research on antennas and radio wave propagation, as well as tests and measurements on noise radio waves from electronic devices and other radio waves, have been carried out in anechoic chambers, where radio wave absorption is used to suppress unnecessary radiation and reflection of radio waves from measurement instruments. Screens and radio wave absorbing curtains are used.

The radio wave absorbing screen is made of the same material as the solid radio wave absorber used on the inner surface of the anechoic chamber, but the radio wave absorbing curtain has a small standing wave ratio (high return attenuation coefficient) in a wide frequency range. , it is necessary to be flexible and lightweight for handling, and the solid radio wave absorber mentioned above cannot be used.

As this type of radio wave absorbing curtain, fiber cloth fixed with carbon powder and plastic foam sheet mixed with carbon powder are available, but their radio wave absorption characteristics are limited to a return loss of about -25 dB at normal incidence. Moreover, the important oblique incidence characteristics are extremely inadequate, and there has been a demand for the development of something that has sufficient reflection attenuation characteristics and is easy to handle.

The present invention provides a flexible and lightweight radio wave absorbing curtain that can obtain sufficient return loss for vertically and obliquely incident radio waves to meet the above requirements.

Hereinafter, the present invention will be explained with reference to Examples.

Fig. 1 is a perspective view of a part of the product of this example, in which a carbon fiber cloth with a thickness of 0.8 mm obtained by carbonizing an acrylonitrile synthetic fiber nonwoven fabric at a temperature of 800°C is heated at a substantially constant wavelength. Formed by bending, the one with the highest wave height 1, the one with the lowest wave height in order 2,
3, 4, and 5 with gaps 8 between them and stacked to form a bottom surface.A zigzag-shaped radio wave absorbing surface is bonded to a flat carbon fiber cloth 6 that forms the bottom surface. A metal foil 7 is adhered thereto.

Because of the above structure, the cross-sectional area of the carbon fiber cloth is sequentially from the top to the bottom in a plane parallel to the bottom 6, in the case of only 1 at the top, in the case of 1 and 2 below, and further. In the cases 1, 2, and 3 below, the number increases step by step almost exponentially.

Conventionally, if a radio wave absorber is configured such that the number of attenuation bands increases sequentially from the front surface to the rear surface of the radio wave incident surface, the reflection suppression effect will be improved both at normal incidence and at oblique incidence. This is already known (written by Ono, Ikuta, and Katagiri, "A construction method for a radio wave absorber with excellent properties for both normal incidence and oblique incidence," Transactions of the Institute of Electronics and Communication Engineers (B), October 1976 issue). The technology to achieve such a damping constant with a single material had not yet been developed.

Figure 2 is a distribution diagram of the attenuation constant showing this state, where the horizontal axis is the distance measured in the thickness direction from the front of the radio wave absorbing curtain, and the vertical axis is the attenuation constant of the material.
The one-dot chain line and the two-dot chain line indicate the distribution of the attenuation constant of a commercially available product that has been put into practical use, the dotted line indicates the distribution of the attenuation constant in the present invention, and the solid line indicates the stepwise attenuation constant.

On the other hand, in a structure where a part of a dielectric material is cut out, if the attenuation constant of that material is small, it is already known that the attenuation constant of the part where the material and voids coexist is proportional to the cross-sectional area of the material. However, the present invention is Utilizing this property, as a result of studying and researching the shape of carbon fiber cloth, we have found that the shape shown in Figure 1 is most desirable.

Figure 3 shows the measurement results of the frequency characteristics of the standing wave ratio (return loss) at normal incidence in this example, with the frequency on the horizontal axis and the standing wave ratio (return loss) on the vertical axis.
As shown in Figure 4, when the radio wave absorbing curtain is planar, a is marked with an ○, when it is a convex surface with a radius of curvature R = 20 cm, b is marked with a ●, and when it is a concave surface with a radius of curvature R = 20 cm, c is marked with an ×. The measured values are plotted as marks.

As is clear from this figure, at a center frequency of 5 GHz, the return loss reaches 25 to 30 dB, and when the allowable standing ratio is 1.2 (return loss -20.8 dB), the return loss reaches 4 to 6 G.
Hz frequency band, the fractional bandwidth at this time is 0.4, and a favorable result was shown in which this bandwidth hardly changes not only in the case of a flat surface but also in the case of a convex surface and a concave surface. Radio wave absorbing curtains are useful as medium band types.

FIG. 5 shows the measurement results of the frequency characteristics of the standing wave ratio (return loss) at oblique incidence in this example,
The sample was made into a flat shape as shown in Figure 4a, and as shown in Figure 5a.
is a transverse electric field (TE) wave when the electric field vector of the radio wave is polarized perpendicular to the plane of incidence, and b is a transverse magnetic field (TE) wave when the electric field vector is polarized parallel to the plane of incidence. TM) waves.

In the figure, the respective values are indicated by ● when the incident angle is 30°, × when it is 50°, and ○ when the incident angle is 70°.

In all cases, up to an angle of incidence of 70°, the bandwidth at an allowable standing wave ratio of 1.2 or less was almost the same as that at normal incidence.

The above is an explanation of an example of a radio wave absorbing curtain with a center frequency of 5 GHz, but the present invention is not limited thereto. 850℃
Select the bending angle of the topmost surface as appropriate between
It can also be adapted to other frequency bands by appropriately selecting the number, size and shape of the cloth, with the angle being 15° to 45°, particularly preferably about 30°.

FIG. 6 is a perspective view of another embodiment, in which a part of the zigzag structure shown in FIG. increases almost exponentially,
As shown in FIG. 7, the attenuation constant has a sloped stepwise shape as shown by a solid line, and the tendency of increase in the attenuation constant has an exponential function shape as shown by a broken line. However, in this case, carbon fiber cloths 1, 2, 3, 4, and 5 are heat-treated at 1000°C and have a thickness of 0.8 mm, 6 is aluminum foil, and 7 is aluminum foil.
indicates a void.

Figure 8 shows the measurement results of the frequency characteristics of the standing wave ratio (return loss) at normal incidence in this example, where 〇 marks are planar, ● marks are convex, × are concave, and This is a value obtained under the same conditions as in Example 1. However, in this case, part of the oblique incidence characteristic is
Since there is a tendency for a range in which the return loss is rather small to occur, the frequency band to be used may have to be determined taking this point into account. The radio wave absorbing curtain of the present invention is made of a flexible material as a whole, and by folding the bottom flat part together with the zigzag part, the whole can be folded to have a small volume.
It can also be stored, and when used, by stretching it over the required surface and installing it, it can be made into a planar, convex, or concave system, and the angle of incidence can be varied over a wide range of 0° to 70°. It has excellent performance in that the frequency characteristics of the standing wave ratio hardly change over time, and it will greatly contribute to research and testing of radio wave technology and prevention of radio wave interference.

[Brief explanation of drawings]

Fig. 1 is a perspective view showing an embodiment of the present invention, Fig. 2 is an attenuation constant distribution diagram of a radio wave absorber with a small standing wave ratio, and Fig. 3 is a diagram of the standing wave ratio (return loss) of the embodiment. Figure 4 is a diagram showing the frequency characteristics. Figure 4 is a diagram showing the shape of the radio wave absorbing curtain when measuring its characteristics. Figure 5 is the standing wave ratio (return loss) at oblique incidence in the example.
FIG. 6 is a perspective view of another embodiment, FIG. 7 is a diagram showing its attenuation constant distribution, and FIG. 8 is a diagram showing the frequency characteristics of another embodiment.
The figure also shows the standing wave ratio (return loss) at normal incidence.
It is a figure showing the frequency characteristic of.

Claims (1)

[Scope of Claims] 1 A carbon fiber cloth having a plurality of folded zigzag waveforms having sequentially different wave heights at a constant wavelength is placed on a carbon fiber base cloth surface, and the bottom edge of each cloth is polymerized. A flexible radio wave absorbing curtain characterized by being fixed in place. 2. A patent in which the carbon-based fiber base fabric and the carbon-based fiber fabric are obtained by subjecting woven or non-woven fabrics made of polyacrylonitrile synthetic fibers or viscose rayon fibers to carbonization heat treatment at a temperature range of 700° to 1100°C. A flexible radio wave absorbing curtain according to claim 1. 3 The bending apex angle of the zigzag waveform on the outermost surface is 15°~
The flexible radio wave absorbing curtain according to claim 1, which has an angle of 45°.