JPH0221678B2 - - Google Patents

Description

[Detailed description of the invention] (Technical field) The present invention relates to a radio wave absorber.

(Background Art) As a conventional radio wave absorber, for example, a rubber sheet made of a mixture of ferrite and rubber is known. However, this rubber sheet has the disadvantage that it is heavy, and due to this, installation using ordinary adhesives is unreliable and construction is difficult.

To improve this drawback, approximately λ/4 (λ is the free space wavelength of the operating center frequency) is applied to the conductor surface.
A foam with a thickness of is placed on its surface whose characteristic impedance is the characteristic impedance of free space.
A radio wave absorber provided with a carbon coating film having a resistance approximately equal to 377Ω has been proposed. However, there is a drawback that the characteristic impedance changes greatly with changes in the thickness of the resistive film applied, and the manufacturing process must be strictly controlled.

In order to improve this drawback, the present applicant previously filed Japanese Patent Application No. 57-92964 for the purpose of providing a radio wave absorber that is easy to manufacture, lightweight, has excellent workability, and has excellent impedance matching. , its feature is that it provides a radio wave absorber in which a resistive film formed by arranging conductive fibers in a planar manner is placed at a distance of λ/4 or less from the conductor surface.

(Purpose of the Invention) This invention is a further development of the radio wave absorber proposed by the applicant mentioned above, and its purpose is to provide a radio wave absorber with particularly good radio wave absorption characteristics. The feature is that a radio wave absorber is provided with a resistive cloth made of conductive fibers or conductive fibers and synthetic fibers woven on the surface opposite to the one surface of a foamed dielectric material provided with a conductor on one surface, wherein the conductive fibers are The reason is that the interval between the two is λ/4 (λ is the free space wavelength of the operating center frequency) or less.

(Structure and operation of the invention) The present invention will be explained below based on the drawings.

FIG. 1 is a longitudinal sectional view of an embodiment of a radio wave absorber previously proposed by the applicant. In the same figure, 1
0 is a resistance fabric, 12 is an adhesive, 14 is a foam dielectric, and 16 is a conductor. As shown in Figure 2, the resistance woven fabric is made up of vertical conductive fibers 18 and horizontal conductive fibers 20, which are made up of dozens of conductive single fibers, woven in a lattice shape with approximately equal spacing. It is a woven fabric. FIG. 3 shows a longitudinal cross-sectional view of the resistance fabric body 10.
In FIG. 2, a woven fabric made of synthetic fibers such as nylon or acrylic may be provided between each conductive fiber. Here, as the conductive fiber, for example, CuS is applied on the surface of synthetic fiber such as nylon or acrylic.
A conductive film such as the one attached using a dyeing technique is used. As shown in FIG. 3, the surface and inside of the resistance woven body 10 are coated with or impregnated with a resin 21 having a dielectric constant of 5 or less, or a paint or an adhesive. The above radio wave absorber was previously proposed by the applicant,
This makes it possible to obtain excellent radio wave absorption characteristics.

Next, one embodiment of the present invention will be described in detail with reference to FIGS. 4 to 7.

FIG. 4 shows a longitudinal sectional view of a radio wave absorber according to an embodiment of the present invention, which has the same structure as the radio wave absorber shown in FIG. 1 except for the structure of the resistance cloth 28. That is, the conductor 26 is placed on one surface of the foam dielectric 24.
A resistance cloth 28 is provided with an adhesive 22 on the surface of the foam dielectric 24 opposite to the surface on which the conductor 26 is provided. The feature of this invention lies in the resistance cloth body 28, which consists of vertical conductive fibers 30 and horizontal conductive fibers 32, which are made up of dozens of conductive fibers. The woven fabric bodies are equally woven in a lattice shape at predetermined intervals of λ/4 (λ is the free space wavelength of the operating center frequency) or less, approximately 8 mm in this embodiment. This is because the distance between the conductive fibers and the resistance value are approximately proportional, and the resistance value of the resistive fabric when the distance is approximately 8 mm is approximately equal to the free space impedance of 377Ω.

Note that a woven fabric made of synthetic fibers such as nylon and acrylic may be provided between each conductive fiber. Further, as the conductive fiber, for example, a synthetic fiber such as nylon or acrylic fiber with a conductive film such as CuS adhered to the surface by a dyeing technique may be used. The surface and interior of the resistance cloth body 28 may be coated with or impregnated with a resin, paint, or adhesive having a dielectric constant of 5 or less.

FIG. 6 shows the above resistance cloth 28 each having a thickness of 15 mm.
The frequency characteristics of the amount of reflection when adhered to the surface of polyethylene sheets of mm, 10 mm, and 8 mm are shown. The matching frequency is 5GHz when the thickness is 15mm, and 9G when the thickness is 10mm.
Hz, 13GHz when the thickness is 8mm.

(1) Shows good characteristics at 5GHz when the thickness is 15mm. Regarding the relationship with the thickness, λ/4 at 5 GHz is 15 mm, which is consistent with theory.

(2) Shows good characteristics at 9GHz when the thickness is 10mm. λ/4 at 9GHz is 8.3mm, which is thicker than the theoretical value.

(3) When the thickness is 8 mm, the reflection amount is about -15 dB at 13 GHz.
The characteristics have significantly deteriorated compared to the 10mm case. λ/4 at 13GHz is 5.8mm.

As described above, when the spacing between the conductive fibers approaches λ/4, the thickness of the absorber must be made thicker than the theoretical value, and when it becomes larger than λ/4, the amount of reflection increases and the performance of the absorber deteriorates. Therefore, it can be seen that in order to obtain a thin absorber with good characteristics, it is preferable to set the spacing between the conductive fibers to λ/4 or less.

Next, we will examine the relationship between the amount of reflection and the frequency when a resistance cloth with the number of conductive single fibers contained in the conductive fibers is halved and the distance between the conductive fibers is 4 mm is adhered to a foamed polyethylene sheet with a thickness of 6 mm. It is shown in Figure 7. Note that λ/4 for a thickness of 12.5 GHz is 6 mm. As can be seen from the figure, it shows good characteristics at 12.5GHz. In this way, a good absorber can be obtained by keeping the amount of conductive single fibers contained in a unit area constant and by setting the interval between the conductive fibers to λ/4 or less.

(Effects of the Invention) As explained above, according to the present invention, it is possible to provide a lightweight radio wave absorber with extremely excellent radio wave absorption characteristics, which can be used to prevent false images from masts of ships and other structures, and to prevent parabolic antennas. This is effective for purposes such as sidelobe reduction.

[Brief explanation of drawings]

FIG. 1 is a longitudinal sectional view of a radio wave absorber previously proposed by the applicant, FIGS. 2 and 3 are diagrams showing the structure of a resistive woven body, and FIG. 4 is a radio wave absorber according to an embodiment of the present invention. FIG. 5 is a diagram for explaining the structure of the resistive fabric of the radio wave absorber in FIG. 4, and FIGS. 6 and 7 show the absorption characteristics of the radio wave absorber according to the present invention. FIG. 24... Foamed dielectric material, 26... Conductor, 28...
Resistance cloth.

Claims (1)

[Claims] 1. A radio wave absorber in which a resistive cloth made of conductive fibers or conductive fibers and synthetic fibers is provided on the surface opposite to the one surface of a foamed dielectric material provided with a conductor on one surface, A radio wave absorber characterized in that the distance between the conductive fibers is λ/4 (λ is the free space wavelength of the operating center frequency) or less. 2. The radio wave absorber according to claim 1, wherein the conductive fibers are woven in a grid pattern. 3. The radio wave absorber according to claim 1 or 2, wherein the conductive fiber is a nylon or acrylic fiber whose surface has been surface-treated with copper sulfide.