JPH0212897A - Radio wave absorber - Google Patents

Abstract

PURPOSE:To improve polarized wave characteristics to obliquely incident waves of a radio wave absorber formed from non-woven cloth by providing the radio wave absorber with a box formed of polymer resin plates having a wall thickness sufficiently smaller than wavelength while filling the box with non-woven cloth produced by mixing insulator fibers and conductive fibers consisting of elongated linear resin coated with a good conductor. CONSTITUTION:A radio wave absorber, comprises a box 1 formed of polymer resin plates having a thickness (d) that is sufficiently smaller than wavelength 1 of electromagnetic waves and non-woven cloth 2 filling the inside of the box 1, the non-woven cloth 2 being produced by mixing insulator fibers and conductive fibers consisting of elongated linear resin coated with a good conductor. According to an embodiment, the box 1 of a high polymer, resin is arranged in contact with the surface of a conductor 4 such as a metal or the like, and the inside of the box is filled with linear non-woven cloth pieces 2 produced by mixing conductive fibers consisting of elongated linear resin coated with a good conductor and insulator fibers. In a structure 3 thus obtained, the conductive fibers contributing to electromagnetic wave absorbing properties are dispersed in the x-, y- and z-directions substantially uniformly within the box 1 of the high polymer resin, Accordingly, it is possible to realize a radio wave absorber having absorbing properties independent from direction or polarization of obliquely incident radio waves.

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 nonwoven fabric made of a mixture of elongated linear resin and fibers coated with a good conductor, proposed by the applicant earlier, is lightweight and has good radio wave absorption properties (Patent Application No. 63- 011957). Since nonwoven fabric is originally cotton-like, in order to construct a radio wave absorber, a structure 6 in which sheet-shaped nonwoven fabrics 2 are stacked in several layers is used as shown in FIG.

(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. 7, this method had the disadvantage that the absorption characteristics deteriorated in TM waves 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 nonwoven fabric upon oblique incidence.

(Means for Solving the Problems) In order to achieve the above object, the radio wave absorber of the present invention includes (1) a box made of a polymer resin whose wall thickness is sufficiently thin compared to the wavelength λ; The interior is filled with a nonwoven fabric obtained by mixing conductive fibers, which are made by coating the surface of elongated linear resin with a good conductor such as metal, and fibers that remain as insulators.

(2) The inside of the rectangular parallelepiped box is divided into multiple layers, and each layer is filled with nonwoven fabrics containing different proportions of conductive fibers.

(Function) If a box with a wall thickness sufficiently small compared to the wavelength is provided and the inside of the box is filled with cotton-like nonwoven fabric before being formed into a sheet, the conductive fibers in the nonwoven fabric will be uniformly dispersed in all directions, so that The absorption characteristics remain constant regardless of polarization. The wall surface of the box is made of a polymer resin that is sufficiently thin compared to the wavelength, so it is electromagnetically transparent, and the influence of reflection on the surface of the box is extremely small.

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

FIG. 1 shows a radio wave absorber according to the present invention.

A box 1 made of polymer resin such as acrylic or ABS resin is provided so as to be in contact with a conductor surface 4 such as metal, and inside the box 1 is made of conductive fibers and insulators covered with a good conductor such as metal on the surface of elongated linear resin. This is a structure 3 filled with cotton-like nonwoven fabric 2 obtained by mixing raw fibers. Box 1 made of polymer resin
Since the plate thickness d is sufficiently smaller than the wavelength λ0 of the incident electromagnetic wave, there is almost no reflection on the surface of the box or multiple reflection between the front and back surfaces of the plates that make up the box. The height h of the box is 1/4 of the wavelength of electromagnetic waves propagating within the nonwoven fabric 2.

Since the nonwoven fabric 2 is cotton-like, the direction of the conductive fibers that contribute to the electromagnetic wave absorption characteristics is almost evenly distributed in all three directions (x, y, and z) within the polymer resin box 1. . Therefore, it is possible to realize a radio wave absorber that has absorption characteristics that are independent of directionality and polarization when incident obliquely. Figure 2 shows the characteristics of this radio wave absorber. Normal incidence is (a), and TE wave where the electric field E at oblique incidence is perpendicular to the electromagnetic wave incidence plane 5 (b).
, parallel TM waves are shown in (c). The same absorption characteristics are obtained for TE waves and TM waves.

The radio wave absorber shown in Fig. 1 utilizes the resonance characteristics between the conductor surface 4 and the absorber surface, so its frequency band is narrow, whereas the radio wave absorber shown in Fig. 3 has a narrow frequency band. The interior of the box 1 made of molecular resin is divided into multiple layers, and the layer close to the conductor surface 4 is filled with nonwoven fabric 2 containing a large amount of conductive fibers, creating a radio wave absorber 3 with a multilayered structure, thereby achieving a wide band. can be converted into FIGS. 4(a) and 4(b) show a radio wave absorber 3 in which a box 1 has a tapered shape such as a wedge shape or a pyramid shape, and the inside thereof is filled with a nonwoven fabric 2 to achieve a wide band. These absorption characteristics are shown in FIG. 5 for a multilayer absorber and a Taber type absorber. It has better absorption characteristics at high frequencies than a resonant type radio wave absorber, and also has good absorption characteristics at oblique incidence. However, in the case of a Taber-shaped absorber, when the shape has directionality as in the wedge shape shown in FIG. 4(a), the absorption characteristics also have directionality due to the influence of this. In these broadband type radio wave absorbers, the higher the height h, the better the characteristics at low frequencies.

(Effects of the Invention) According to the radio wave absorber of the present invention, the radio wave absorber has polarization-independent absorption properties particularly at oblique incidence while maintaining the lightweight and good absorption properties of the absorber made of nonwoven fabric. It becomes possible to donate the body. Also, multi-layered,
By adopting a tapered shape such as a wedge or pyramid, it is possible to provide a broadband radio wave absorber that exhibits good absorption characteristics on the high frequency side.

[Brief explanation of the drawing]

Fig. 1 is a cross-sectional view of the radio wave absorber according to the present invention, Fig. 2 is a diagram showing its absorption characteristics for vertical incidence and oblique incidence including polarization characteristics, and Fig. 3 is a diagram showing the multilayer radio wave absorber according to the present invention. , FIGS. 4(a) and 4(b) are diagrams showing examples of the radio wave absorber of the present invention,
FIG. 5 is an absorption characteristic diagram of a multilayer radio wave absorber and a Taber type radio wave absorber. FIG. 6 is a diagram showing a radio wave absorber obtained by stacking conventional nonwoven fabrics, and FIG. 7 is a diagram showing its absorption characteristics. 1... Box made of polymer resin, 2... Non-woven fabric, 3...
- Radio wave absorber according to the present invention, 4... conductor surface of metal, etc.
5... Conventional radio wave absorber made of nonwoven fabric.

Claims (2)

[Claims] (1) A box made of a polymer resin plate whose thickness is sufficiently thin compared to the wavelength λ of electromagnetic waves, and a mixture of conductive fibers coated with a good conductor and insulating fibers on the surface of the elongated linear resin that fills the inside of the box. A radio wave absorber comprising a nonwoven fabric obtained by. (2) The inside of the rectangular parallelepiped box is divided into multiple layers,
A radio wave absorber characterized in that each layer is filled with a nonwoven fabric having a different mixing ratio of conductive fibers.