JPH02239700A - Radio wave absorber - Google Patents

Abstract

PURPOSE:To enable absorbing radio waves in a high-frequency band and those in a low-frequency band and maintain the stable electric wave absorption characteristic for a long time by making conductive films of surface resistivity gradually decreasing toward the surface of a tile. CONSTITUTION:Conductive films 3 are installed approximately perpendicularly to the surface of a tile 2 and arranged in parallel crosses. The surface resistivity of the conductive films 3 continuously changes to prevent the characteristic impedance from rapidly changing and reflect few electromagnetic waves. The surface resistivity gradually decreases from the ends (a) on the sides of incidence of radio waves generated in a room to the tile-side edges (b). Thereby radio waves over a wide band from a low-frequency band to a high-frequency band are efficiently absorbed without moisture absorption and the stable radio wave absorption characteristic is maintained for a long time.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a radio wave absorber, and more particularly to a radio wave absorber used in constructing an anechoic chamber. <Prior Art> In today's world where electronic technology, that is, information transmission technology using weak electrical energy, has developed, the influence of electromagnetic waves generated from electrical equipment and the like on these electronic technologies has become a serious problem.

In other words, wireless communication technology that uses electromagnetic waves is becoming increasingly difficult, and wired circuits are no longer able to function as intended due to the intrusion of jamming waves. Therefore, there is a need for a method to minimize the energy of jamming waves generated by electrical equipment, etc., and for this purpose, it is essential to accurately measure the energy of the jamming waves generated.

Ideally, such measurements should be performed in a wide, flat place without radio wave reflectors such as mountains or buildings, but it is difficult to secure such a wide, flat place, and it is necessary to carry out each measurement. It is also inconvenient to move to a location that requires For this reason, anechoic chambers are used in which radio wave absorbers are placed on the walls of the chamber to ensure no or low reflection. Ferrite tiles, which are made by molding ferrite, a magnetic material with high magnetic permeability, into tiles are known as radio wave absorbers. Such ferrite tiles efficiently absorb electromagnetic waves of about 30M}Iz. However, it is not possible to sufficiently absorb electromagnetic waves of 300 MHz or higher, for example, 1000 Hz or higher.

On the other hand, a radio wave absorber is known in which a conductive powder such as carbon black is molded together with a binder into a cone shape, and has excellent radio wave absorption ability in the above-mentioned high frequency band. Foamed polystyrene is suitably used as a binder because its characteristic impedance is close to that of air and it is lightweight. The radio wave absorber is placed so that its apex faces indoors, and its overall impedance decreases continuously as it approaches the wall. As a result, electromagnetic waves generated indoors are absorbed near the wall, reducing reflectance. .

<Problems to be Solved by the Invention> However, carbon vines are naturally highly hygroscopic, and the carbon vines that have broken down into polystyrene foam tend to absorb moisture due to their large contact area with the atmosphere, and the absorption of moisture causes them to emit radio waves. The absorption rate fluctuates, making its properties as a radio wave absorber unstable. An object of the present invention is to provide a radio wave absorber that can handle radio waves in both the above-mentioned high frequency band and low frequency band, and can maintain stable radio wave absorption characteristics over a long period of time. Means for Solving the Problems> In order to achieve this object, the present invention provides a radio wave absorber in which a conductive film is arranged on the surface of a magnetic tile substantially perpendicularly to the surface, the conductive film Provided is a radio wave absorber characterized in that it is a conductive film whose surface resistivity decreases continuously as it approaches the tile surface.

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

FIG. 1 is an explanatory cross-sectional view when a radio wave absorber is placed on the inner wall surface of an anechoic chamber.

In FIG. 1, (1) indicates the wall body. The wall body may be made of any material, but it is preferable to have a built-in metal plate to prevent radio waves from entering from outside the darkroom. Also, in order to prevent the reflection of radio waves due to the gap between the metal plate and the tile (2), it is desirable to have a metal plate on the side that contacts the tile (2). (2) indicates a magnetic tile, (
3) indicates a conductive film. The radio wave absorber of the present invention consists of this tile (2) and a conductive film (3). The magnetic tile (2) is especially for efficiently absorbing low frequency radio waves of 300 and 12 or less, and preferably has a large hysteresis loss. Further, it is desirable that the side end portion (d) of the wall body (1) has a characteristic impedance Zd as small as possible in order to prevent reflection at the interface between the wall body (1) and the metal temporary. Ideally it is OΩ.

At the conductive film (3) side end (C) of the tile (2), in order to prevent reflection at the interface with the conductive film (3) at the tile side end (C), the characteristic impedance Zc of both ends is
It is desirable that zb and zb be equal.

Such a magnetic tile (2) has a high i3f/I
A product obtained by sintering ferrite powder with a ratio of i and molding it into a tile shape can be used. Ferrite is MO・q'. o. (M and M° are the same or different metal elements, and at least one of M and M is Fe) and is a metal oxide having a spinel type or inverted Subinel type crystal structure. In order to obtain a large hysteresis loss, in addition to Fe80, N i O
+ Z n O is preferably included in the crystal structure. The conductive film (3) is especially for efficiently absorbing high frequency radio waves of 300 MHz or more, and is provided on the surface of the tile (2).

Providing the conductive film (3) on the surface of the tile (2) means that there is virtually no gap between them. Due to the presence of gaps, the characteristic impedance changes rapidly,
This is to prevent reflectance from increasing. Further, the conductive film (3) is provided approximately perpendicularly to the surface of the tile (2). This means that the angle h of the conductive film (3) on the surface of the tile (2) is perpendicular to the normal line h2 of the surface of the tile (2). i.e. h3 and h! The inner product of is expressed as O. However, construction errors are allowed.

The electric field component of the radio wave incident on the radio wave absorber is absorbed by the conductive film (
3), that is, when the magnetic field component ■ and the vector h
, are parallel, the radio waves are efficiently transmitted through the conductive skin Ill(3
It is absorbed into 1. In order to efficiently absorb radio waves having an electric field component or a magnetic field component (2) in any direction, it is desirable that the conductive film (3) is arranged in a grid pattern.

The conductive film (3) has a surface resistivity that changes continuously. No sudden changes in characteristic impedance
This is because reflection of itm waves is also less likely to occur.

The surface resistivity is measured at the incident side edge (a) of the radio waves generated indoors.
It is provided so that it decreases as it approaches the tile side edge (b). This is to absorb radio waves efficiently. In particular, it is desirable that the surface resistivity be an exponential function of the distance Nx from the incident end (a). The characteristic impedance is 2
Then, the reflectance d' inside the part where the conductive film (3) is arranged is proportional to dZ/Z, but since Z is roughly proportional to the surface resistivity, the surface resistivity is an exponential function of the distance Mx. This is because d' becomes a constant value and the overall reflectance becomes small.When the exponential function is expanded by Taylor, it becomes a linear function, so it is a quadratic or cubic function of distance In addition, in the present invention, the surface resistivity is determined by the conductive film (3).
It does not mean the surface resistance of one point of the tile (2)
Cut the conductive skin* (3) into strips of a fixed area at a fixed distance along the direction of the surface normal vector h8, measure and average the surface resistance at each point of the strip, It means the surface resistance calculated per unit area (1 inch). In other words, it is the overall surface resistivity. At the incident end (a), it is desirable that the characteristic impedance of the conductive film be close to the impedance of air in order to prevent reflections at the interface between the free space and the conductive film end (a). Practically speaking, a surface resistivity of 5×10”Ω or more is sufficient.

At the tile side end (b), it is better to have as low a surface resistivity as possible in order to increase the absorption rate of radio waves. Practically speaking, it is 3 x 10" Ω or less. As described later, the conductive film (3) can be formed on a suitable support. The support may be of any type, but an insulating one is preferable. Plastic, paper, wood, etc.Means for changing the surface resistivity include the following (i) to (v).
). (i) A method of partially overprinting conductive ink.

The surface resistivity of the overprinted area is low, and the surface resistivity of the nonoverprinted area is relatively low.

(ii) A method of printing conductive ink in a mesh pattern to form a mesh-like conductive film and continuously changing the mesh spacing.

(Mountain) A method in which conductive ink is printed in a mesh pattern to form a mesh-like conductive film, and the thickness of the cotton that makes up the mesh is continuously changed. (iv) A method of continuously changing the amount of conductive ink by controlling the depth of the printing plate. (v) A method of continuously changing the printing area by printing conductive ink in a triangular shape, for example.

The conductive ink may be mixed with a conductive charging agent such as conductive carbon black, metal powder, flakes or fibers, copper iodide, or a metal film formed on the surface of fibers or mica flakes. Ink can be used. Printing can also be done by gravure printing or silk screen printing. Although it may be printed on a support, it is also possible to print on a releasable sheet and transfer using an adhesive. Alternatively, the printed film may be laminated onto a support. When the support is made of a prismatic or prismatic cylindrical molded product with a square cross section, after forming a conductive film (3) on the side surface, the bottom of the molded product is fixed to the surface of the tile (2) to form a conductive film. A membrane (3) can be placed. If the support is made of a thick plastic sheet or paper, it may be bent into a rectangular parallelepiped shape, and then placed and fixed on the surface of the tile (2) using an adhesive, adhesive tape, or the like.

Alternatively, a large number of pillars having swant or recessed portions are set up on the surface of the tile (2), and a conductive film (3
) can be inserted and held, or a conductive film (3
), the conductive film (3) can be placed. Effect> The present invention is as described above, and since it is composed of magnetic tiles and a conductive film, it can efficiently absorb radio waves in a wide range from low frequency bands to high frequency bands, and does not absorb moisture. , it is possible to maintain stable radio wave absorption characteristics over a long period of time.

[Brief explanation of drawings]

FIG. 1 is an explanatory cross-sectional view when a radio wave absorber is placed on the inner wall surface of an anechoic chamber. (1) Wall body (2) Magnetic tiles (3) Conductive film (100) Bointing vector patent applicant Toppan Printing Co., Ltd. Representative Kazuo Suzuki No. 1 figure

Claims (1)

[Claims] (1) A radio wave absorber in which a conductive film is arranged approximately perpendicular to the surface of a magnetic tile, and the conductive film has a conductive property in which the surface resistivity decreases continuously as it approaches the tile surface. A radio wave absorber characterized by being a film.