JPH0212896A - Radio wave absorber - Google Patents

Abstract

PURPOSE:To provide a radio wave absorber having improved polarized wave characteristics to obliquely incident radio waves by stacking sheets having conductive fibers dispersed principally in parallel with the surface of the sheets and sheets having conductive fibers dispersed principally in the direction of thickness, each sheet having thickness sufficiently smaller than wavelength of incident electromagnetic waves. CONSTITUTION:A non-woven cloth sheet is produced by mixing insulating fibers and conductive fibers consisting of elongated linear resin coated with a good conductor. Sheets of such non-woven cloth 2 in which the conductive fibers are dispersed principally in parallel with the surface of the sheet and sheets of non-woven cloth 3 in which the conductive fibers are dispersed principally in the direction of thickness are stacked alternately, each of the sheets 2, 3 having a thickness sufficiently smaller than wavelength of incident electromagnetic waves. According to an embodiment, sheets 2 of conventional non-woven cloth having conductive fibers dispersed in the direction of the plane (x-y plane) and sheets 3 having conductive fibers dispersed principally in the direction of thickness of the sheets (direction of the z axis) are stacked alternately, the sheet 3 being produced by superposing a sheet on another obtained by cutting the stacked sheets 2 along the vertical plane 4 such that interfaces 5 between the layers intersect orthogonally.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a radio wave absorber that suppresses unnecessary reflection and scattering of electromagnetic waves.

(Prior technology) Radio wave absorbers used in anechoic chambers and outdoors for purposes such as improving antenna performance and preventing false images of radar are made of plate-shaped magnetic materials such as ferrite, carbon, graphite, etc. Electrical loss materials mixed with foamed resin or impregnated with polyurethane foam were often used. Among them, a radio wave absorber made of a non-woven fabric made by mixing fibers coated with a good conductor with long thin linear resin, which the present applicant has already applied for, is lightweight and has good fir wave absorption characteristics (patent application). 63-011957). Since non-woven fabric is originally cotton-like, in order to construct a radio wave absorber, a structure 7 in which several layers of sheet-shaped non-woven fabric are stacked as shown in Fig. 5 is used.
It had become the property of

(Problem to be solved by the invention) However, in a structure in which sheets are stacked, the conductive fibers that contribute to absorption characteristics are dispersed only in the direction along the sheet surface, so when electromagnetic waves are incident obliquely, As shown in FIG. 6, the absorption characteristics deteriorate in the TM wave (b) with an electric field component E parallel to the incident plane. An object of the present invention is to improve the polarization characteristics of a radio wave absorber made of a nonwoven fabric upon oblique incidence.

(Means for Solving the Problems) In order to achieve the above object, in the radio wave absorber of the present invention, a conductive fiber whose surface is coated with a good conductor such as metal on the surface of an elongated linear resin, and a fiber which remains as an insulator are used. In the sheet of nonwoven fabric obtained by mixing the above, sheets in which the conductive fibers are mainly dispersed in the direction parallel to the sheet surface and sheets in which the conductive fibers are mainly dispersed in the thickness direction are stacked alternately to form a multilayer structure,
The structure is such that the thickness of each sheet is sufficiently small compared to the wavelength of the incident electromagnetic wave.

(Function) In a structure in which sheets of nonwoven fabric in which conductive fibers are distributed in the direction along the surface and sheets of nonwoven fabric in which conductive fibers are distributed in the thickness direction are stacked alternately, the thickness of each sheet corresponds to the wavelength of the incident electromagnetic wave. In comparison, when it is sufficiently small, it becomes equivalent to conductive fibers being uniformly dispersed in each direction in terms of electromagnetic waves, so the absorption characteristics at oblique incidence are constant regardless of polarization.

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

As shown in FIG. 1, the radio wave absorber 1 according to the present invention has a structure in which sheets of nonwoven fabric are stacked in multiple layers. 2 is a sheet made of a conventional nonwoven fabric in which conductive fibers are dispersed in the plane (xy plane) direction. 3 stacks the sheets 2 in multiple layers as shown in FIG. 2, and cuts the obtained sheet along a plane 4 perpendicular to the sheet so that the stacked seams 5 are perpendicular to each other as shown in FIG. 3. The conductive fibers are mainly dispersed in the thickness direction (Z-axis direction) of the sheet. The thickness d of each of these sheets is sufficiently small with respect to the wavelength λ of the incident electromagnetic wave.

FIG. 4 shows the absorption characteristics of this radio wave absorber at oblique incidence. When the electric field E is a TE wave perpendicular to the plane of incidence, the conductive fibers dispersed in the sheet 2 and the component in the direction along the sheet surface in the conductive fibers dispersed in the sheet 3 ( When the electric field E is a TM wave parallel to the incident plane (X-axis direction or y-axis direction), the conductive fibers dispersed in the sheet 3 mainly act to absorb the energy of the electromagnetic wave. Since the thickness d of each laminated sheet is sufficiently smaller than the wavelength λ of the incident electromagnetic wave, it is equivalent to a state in which these conductive fibers are uniformly dispersed in each of the X, y, and z directions. Good absorption characteristics are obtained for both TE and TM polarized waves.

(Effects of the Invention) According to the radio wave absorber of the present invention, it is possible to provide a radio wave absorber that is made of a non-woven fabric, is lightweight, maintains good absorption properties, and has improved polarization properties at oblique incidence. It becomes possible.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing a radio wave absorber according to the present invention, Fig. 2 is a diagram showing a conventional multilayered structure made by stacking sheets of nonwoven fabric, and Fig. 3 is a diagram showing a structure obtained by cutting the structure in Fig. 2. Figures 4 and 6 are absorption characteristic diagrams including the polarization characteristics of the radio wave absorber; 1 is a diagram showing a radio wave absorber obtained by stacking nonwoven fabric sheets in which conductive fibers are dispersed in a direction parallel to the sheet surface. 1... Radio wave absorber according to the present invention, 2... Nonwoven fabric sheet in which conductive fibers are dispersed in a direction parallel to the sheet surface, 3... Nonwoven fabric in which conductive fibers are also dispersed in the thickness direction. Sheet, 4...Cut surface for obtaining sheet 3, 5
...Overlapping stitch when sheets 2 are stacked, 610. Electromagnetic wave incident surface when obliquely incident, 7... Radio wave absorber made by stacking nonwoven fabric sheets 2.

Claims (1)

[Claims] (1) In a nonwoven fabric sheet obtained by mixing conductive fibers coated with a good conductor on the surface of a long thin linear resin and insulating fibers, the conductive fibers are mainly dispersed in a direction parallel to the sheet surface. A radio wave absorber is composed of alternating stacks of sheets that are distributed in the thickness direction and sheets that are distributed mainly in the thickness direction, and the thickness of each sheet is sufficiently smaller than the wavelength of the incident electromagnetic wave.