JPH0212998A - Radio wave absorber - Google Patents

Abstract

PURPOSE:To realize a radio wave absorber which has satisfactory oblique incident characteristic and wide band by alternately laminating nonwoven fabrics having different conductivities, and forming a cavity in the nonwoven fabric having large conductivity. CONSTITUTION:Nonwoven fabrics having different conductivities are alternately laminated, and a cavity is formed in the nonwoven fabric having large conductivity. First, nonwoven fabric made of conductive fiber and polymer resin fiber is employed as a medium. This nonwoven fabric has means as stereoscopically disposed medium of the conductive fiber, becoming locally irregular in electromagnetic wave manner. As a result, radio wave characteristic which cannot be obtained in the homogeneous medium is provided, and Fn (n=1, 2,...) sheet corresponds thereto. Second, the nonwoven fabric sheet having larger conductivity than that of the Fn sheet is formed, and a medium formed with punched holes is introduced thereinto. It has the same characteristic in the electromagnetic wave manner, but its operating region is increased, and Mn (n=1, 2,...) sheet corresponds thereto. The hole has an arbitrary shape and size, thereby variously altering the characteristics of the Mn sheet.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a radio wave absorber that is used for suppressing unnecessary radio waves, improving antenna characteristics, radio anechoic chambers, etc., and preventing reflection and scattering of electromagnetic waves.

(Prior Art) In response to the remarkable development of radar technology and microwave communication technology since World War II, development of radio wave absorbers for suppressing reflection and scattering of electromagnetic waves has progressed. The characteristics of the radio wave absorber for this purpose are evaluated by the radio wave absorption characteristics when the radio waves are vertically and obliquely incident, and the frequency change of the radio wave absorption characteristics.

In other words, as shown in Fig. 3, when the radio wave 1 is incident on the radio wave absorber 2 on the metal plate 3 at an angle θ1, the radio wave reflected or scattered toward the angle θ2 is attenuated relative to the incident radio wave. The radio wave absorption ability is defined, and the case where θ1=02=0 is called the normal incidence characteristic, and the other cases are called the oblique incidence characteristic. When θ1 becomes large, the radio wave absorption characteristics differ greatly from those when 01=0, and in actual use, such cases are often the case, so
Oblique incidence characteristics are important. Further, it is preferable that the radio wave absorber can be used at any frequency. However, in actual radio wave absorbers, usable frequencies are limited, and those whose frequency band is less than 20% of the fractional band are called narrow-band types, and those whose frequency bands are above 20% are called wide-band types.

Radio wave absorbers can be roughly divided into sheet-like radio wave absorbers and those with an uneven surface shape, typified by pyramids. The former has the advantage of being easy to use because it has a flat surface and is thin, but it has narrow band characteristics and the radio wave absorption characteristics for oblique incidence of radio waves are defined by definition, and the absorption becomes more pronounced as the angle of incidence increases. Characteristics deteriorate. This includes rubber ferrite type, ferrite tile, rubber carbon type, and urethane carbon type sheet-like radio wave absorbers. On the other hand, the latter type of pyramid-shaped radio wave absorber has broadband characteristics and has the advantage of having a complex surface shape, making it a radio wave absorber with excellent oblique incidence characteristics. Since the length needs to be at least 1/4 wavelength, it is a radio wave absorber with a large thickness (or large size), which is disadvantageous in handling, and when used in an anechoic chamber, the work space is significantly reduced. The present invention is basically a sheet-like radio wave absorber, but its poor narrow band characteristics and oblique incidence characteristics can be overcome by using a nonwoven fabric containing conductive fibers as the material and by applying a special configuration. This has been improved.

Conductive fibers have been introduced into the electronics field in the form of conductive plastics, conductive nonwoven fabrics, etc. based on the recent demands for shielding technology. However, the purpose of these materials is to mix as much conductive fiber as possible to obtain electrical conductivity close to that of a metal body. If such a conductive plastic is used as the Mn sheet or Fn sheet shown in FIG. 1 or 2, the layer itself will reflect radio waves and will not exhibit any radio wave absorption effect.

An example of a radio wave absorber having a structure similar to that of the present invention is a Thanosbury screen type (or resistive film type) radio wave absorber in which a resistive film 4 is provided in front of the metal plate 3 (at the J4 wavelength). That is, it has a structure as shown in FIG. However, in this case, it is an M4 wavelength resonator, and its characteristics are different from those of the radio wave absorber of the present invention, in that it has narrow band characteristics and its oblique incidence characteristics are not good. The difference between the present invention and the Salisbury screen type radio wave absorber is that the Salisbury screen type uses a uniform medium (air or foam) as the medium;
In addition, no innovations have been made to the screen, whereas the present invention uses a non-uniform medium with the intention of improving the frequency band and oblique incidence characteristics, and has made improvements to the screen to create a new radio wave absorber. It lies in the realization of its composition and characteristics.

(Problem that the invention attempts to solve) Figure 5(a) shows typical characteristics of the sheet-like radio wave absorber.

Shown in (b). (a) shows the frequency characteristics, and (b) shows the oblique incidence characteristics. The above characteristics are narrow band characteristics, and the oblique incidence characteristics are not good. The present invention attempts to improve this drawback.

(Means for solving the problem) In a radio wave absorber constructed by laminating nonwoven fabrics in which conductive fibers and polymer resin are entangled, the nonwoven fabrics having different conductivities are alternately laminated to form a nonwoven fabric with high conductivity. is a radio wave absorber in which a cavity is formed.

The incidence and reflection of electromagnetic waves on a plane that forms the boundary between two homogeneous media has already been described in “
As described in ``Electromagnetic Wave Theory'' (Goupa Shoten, 1950, 1st printing), a unique solution is given as a boundary value problem, and as long as the medium is homogeneous, the narrow narrowing of the sheet-like radio wave absorber is Deterioration of band characteristics and oblique incidence characteristics cannot be avoided.

In the present invention, the above drawbacks are improved by making the medium of the sheet-like radio wave absorber locally non-uniform. For this purpose, two methods are used. First, a nonwoven fabric made of conductive fibers and polymer resin fibers is used as the medium.

The electrical (electromagnetic wave) characteristics of this nonwoven fabric are determined by the material, shape, and dimensions of the conductive fibers, and the degree of entanglement of the fibers.
Polymer resin fibers, which are other constituents of the nonwoven fabric, serve as a support medium for three-dimensionally arranging the conductive fibers. Therefore, this nonwoven fabric serves as a medium for the three-dimensional arrangement of conductive fibers, and is a medium that is electrically (electromagnetic waves) locally non-uniform. As a result, - for example, Figure 6 (a
) and (b), the entanglement of conductive fibers creates a state in which R, (resistance), C, (capacitance), and C, (inductance) are not uniformly scattered in space, and each R, , C1, L, to locally create various frequency characteristics, and the electromagnetic waves incident on the conductive fiber at various angles have R,, C1,
Scattered by L. That is, this nonwoven fabric has electromagnetic wave characteristics that cannot be obtained with a homogeneous medium. The Fn (n=1°2...) sheet in FIG. 1 corresponds to this.

As a second method, a nonwoven fabric sheet having higher conductivity than the Fn sheet is made, and a medium with punched holes is introduced into the nonwoven fabric sheet. The characteristics for electromagnetic waves are shown in Figure 7 (a) and (b).
As shown in FIG. 6, the function is the same as that shown in FIG. 6, but the area of action is larger than that shown in FIG. 6, and therefore the distribution of Rm, Cm, and Lm becomes non-uniform over a wide range. Mn(n
=1°2...) sheet corresponds to this. In FIG. 1, the holes are drawn as squares, but they can have any shape and size, thereby making it possible to vary the properties of the Mn sheet.

(Example) Nickel-coated acrylic fibers (conductive fibers) and polyester fibers (polymer resin fibers) were mixed at a mixing ratio of 1:99, and the fibers were mixed at a basis weight of 150 gr/cm2, laminated and compressed to a thickness of 11 cm. A nonwoven fabric sheet (Fl) was produced.

The radio wave absorption characteristics of this nonwoven fabric sheet are shown in FIG.

While the characteristics of the sheet-like radio wave absorber in a uniform medium are narrowband as shown in Figure 5, Figure 8 shows broadband characteristics.
This demonstrates the invention.

Nickel-coated acrylic fibers (conductive fibers) and acrylic fibers (polymer resin fibers) were mixed at a mixing ratio of 10:90 and 2:98, with a basis weight of 150 gr/cm2, and laminated and compressed into thicknesses of 2 mm and 2 cm. Non-woven fabric sheet (F
l) was produced. The impedance characteristics of this nonwoven fabric sheet exhibit unique frequency characteristics that cannot be seen in a homogeneous medium, as shown in FIGS. 9(a) and 9(b). This is due to the fact that the conductive fibers locally have a capacitance and
This is the result of changing the inductance distribution and producing frequency characteristics.

Using nickel-coated acrylic fibers (conductive fibers) and polyester fibers (polymer resin fibers), the Fn sheet in Figure 1 has a mixing ratio of 3:97 and a basis weight of 1.
30gr/cm2 fibers were mixed 3 times, the Mn sheet was mixed at a mixing ratio of 5:95, and the basis weight was 100gr/cm2 fibers were mixed 1 time to form a sheet, and the Ml and M2 sheets were
Drill a through hole as shown in Figure 0 and stack the layers as shown in Figure 2'! '1 and F2 are 7mm, F3 is 15mm, M
l and M2 were set to 2 mm. The frequency characteristics of this radio wave absorber are shown in FIG. At 2.5 GHz to 25 GHz, it exhibits an absorption characteristic of about 20 dB and exhibits an ultra-wideband characteristic.

Using steel fibers (conductive fibers) and polyester fibers (polymer resin fibers) with a length of 5 mm and a diameter of 2011 m, the Fn sheet in Figure 1 was made with a mixing ratio of 2:98 and a basis weight of 130 gr/cm2. The number of fibers was 3, and the Mn sheet had a mixing ratio of 3 to 97, and the basis weight was 100 gr/cm2.The number of fibers was 1 to form a sheet, and through holes were made in the Ml and M2 sheets as shown in Figure 12. When laminated as shown in Figure 2, Fl and F2 are 3mm, F3 is 6mm, and M
l and M2 were set to 2 mm.

This radio wave absorber exhibits broadband characteristics as shown in FIG. FIG. 14 shows the oblique incidence characteristics for a 15 GHz TE wave. The measured values when the electric field is parallel to the slit and when it is perpendicular to the slit are plotted from 01=02 to 5° to 60°. This shows that the oblique incidence characteristics are good and that even though the structure of the absorber has slits in a specific direction, the effect of this is not a problem in practical terms.

A pyramid made of the same material as the Fn sheet (8 cm long, bottom 3
cm x 3 cm), the absorption amount was 30 dB or more at frequencies above 3 GHz.

(Effect of the invention) We have achieved a broadband radio wave absorber with good oblique incidence characteristics.

[Brief explanation of the drawing]

Fig. 1 is a configuration diagram of the radio wave absorber of the present invention, Fig. 2 is an assembled diagram of the radio wave absorber of the invention, Fig. 3 is a diagram defining the incident angle θ1 and reflection angle θ2 of radio waves, and Fig. 4 is a diagram defining the radio wave incident angle θ1 and reflection angle θ2. A diagram explaining the Saris Variscreen type radio wave absorber. Figures 5 (a) and (b) are diagrams showing the frequency characteristics and oblique incidence characteristics of a normal sheet-shaped radio wave absorber. Figures 6 (a) and (b) ) is a diagram explaining that conductive fibers give spatially nonuniform resistance (R), capacitance (C), and inductance (L), and Figures 7 (a) and (b) are diagrams in which conductive fibers are mixed. A wider range of R and C can be achieved by opening a through hole in the seat. Figure 8 is a radio wave absorption characteristic diagram of the example, Figure 9 is an impedance characteristic diagram of the example, and Figure 10 is the Ml and M2 sheets of the example. Figure 11 is a diagram showing the radio wave absorption characteristics of the example. Figure 12 is a diagram showing the hole dimensions of Ml and M2 sheets in the example. Figure 13 is the radio wave absorption characteristic diagram of the example. Characteristic diagram: Figure 14 is an oblique incidence characteristic diagram of the embodiment. In the diagram, 1 is a radio wave, 2 is a radio wave absorber, 3 is a metal plate, and 4 is a resistive film.

Claims (1)

[Claims] (1) In a radio wave absorber constructed by laminating nonwoven fabrics in which conductive fibers and polymeric resin are entangled, the nonwoven fabrics with different conductivities are alternately laminated, and a cavity is formed in the nonwoven fabric with high conductivity. A radio wave absorber characterized by: