JPH02107000A - Radio wave absorber - Google Patents

Abstract

PURPOSE:To improve wave absorption characteristics to oblique incident waves by making the spaces L1 of the thread of conductive fibers larger (L1>L0) than an optimum space L0 to vertical incident waves. CONSTITUTION:In a wave absorber with a foamed dielectric 10 having a specified thickness, a conductor 12 installed onto one surface of the foamed dielectric 10 and resistance cloth 14, which is mounted onto the other surface of the foamed dielectric and in which conductive fibers or conductive fibers and nonconductive fibers are woven, the space L1 of the thread of conductive fibers is made larger (L1>L0) than an optimum space L0 to vertical incident waves. Since a specification required is approximately -20dB normally when the thickness d1 of the dielectric =11.1mm holds, a thread space L1 is L1=7.0mm, 7.5mm or 8.0mm, thus satisfying the specification when an incident angle is approximately 25 deg.-30 deg. or less.

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 for oblique incidence that has excellent oblique incidence characteristics, is lightweight, and has excellent workability.

(Prior Art) In conventional radio wave absorbing walls, emphasis has been placed mainly on the amount of return loss for vertically incident waves that are perpendicularly incident on the wall surface.

As radio wave absorbing walls, those made of composite materials of ferrite and rubber or plastic, and those made of conductive fiber cloth are known. These radio wave absorption walls are designed mainly based on reflection MRffi for vertically incident waves.

(Problem to be solved by the invention) However, when these radio wave absorption walls are installed on bridges or buildings as a measure to prevent radar false images, oblique incidence characteristics may become a problem due to the positional relationship between the radar and the reflecting object. be.

Although conventional radio wave absorbers exhibit sufficient attenuation for vertically incident waves, the amount of attenuation for obliquely incident waves obliquely incident on a wall surface is not necessarily large.

An object of the present invention is to improve the above-mentioned drawbacks of the prior art and provide a radio wave absorber with excellent oblique incidence characteristics.

(Means for Solving the Problems) The features of the present invention for achieving the above object include a foamed dielectric having a predetermined thickness, a conductor formed on one surface of the foamed dielectric, and a conductor formed on the other surface of the foamed dielectric. In a radio wave absorber having conductive fibers or a resistance cloth woven with conductive fibers and non-conductive fibers, the spacing L1 between the conductive fiber threads is the optimum spacing for vertically incident waves. Greater than (L + > L
o) is a radio wave absorber.

Another feature of the invention is that the thickness dl of the foam dielectric is an optimum thickness d for normal incident waves. (d, >do) in a radio wave absorber.

(Example) FIG. 1 shows an example of the structure of a radio wave absorber according to the present invention, in which lO is a dielectric with a thickness of d (for example, a foamed dielectric such as foamed polyethylene), and 12 is aluminum bonded to one surface of the dielectric 10. A foil-like conductor, 14 is a resistance cloth (woven from conductive fibers and non-conductive fibers (polyester fibers)),
The spacing between warp and weft yarns, 16 indicates caulking at the ends, and 18 indicates polyurethane coating.

In order to sufficiently absorb vertically incident radio waves, the thickness of the dielectric should be dl/4 (where d is the wavelength), the spacing between the threads, and the characteristic impedance of the resistive cloth should be the same as the characteristic impedance of free space (. 120π).

Table 1 shows the experimental values of the amount of reflection of the 9.4 GHz wave for various dl and beam angles for the vertically incident wave,
It can be seen that the amount of reflection is small and consistent with sample 8 (d+ = 11.1 mm, L, = 6.5 mm) or sample 8 (d+ = 11.1 mm, L + 7 mm).

(Leaving space below) Table 1 Table 2 On the other hand, Table 2 shows the actual measured values of return loss (9.4 GHz) when the incident angle is O', 30°, and 45°, and Figure 2 shows the measured values of return loss (9.4 GHz) when the incident angle is O', 30°, and 45°. Figure 3 shows the amount of reflection versus the angle of incidence for various thread spacings for the thickness d = 11.1 mm of the dielectric, and Figure 3 shows the amount of reflection versus the angle of incidence for various thread spacings for the dielectric thickness d = 11.9 mm. Note that FIGS. 2 and 3 are graphs of the contents of Table 2, and the plane of polarization is vertical polarization.

In FIG. 2, the dielectric thickness d + (41.1 mm
1 is the consistent value in Table 1 (sample number 7 or 8.d,
11.1mm). Here, since the standard value required is usually about -20 dB, the thread spacing L1 should be set to Ll・7.
, 0 mm, 7.5 mm, or 8.0 mm, it can be seen that the standard value is satisfied if the incident angle is less than 25° to 30°. From Table 1, the consistency value of thread spacing is
= 5.5 mm or Lo = 7.0 mm, it can be seen that the characteristics against oblique incidence waves are improved when the thread spacing is larger than the matching value Lb for vertical incidence.

On the other hand, the dielectric thickness d + (=11.9 mm
) is greater than the matching value (do=11.1 mm) in Table 1. At this time, the thread spacing is 6.5mm or 7.0mm.
mm (equal to the matching value in Table 1), the characteristics are improved near the incident angle of 30°. In other words,
When the thread spacing is a matching value for vertically incident waves, the dielectric thickness is the matching value for vertically incident waves (d, 11.1 mm
), the characteristics against obliquely incident waves are improved.

(Effects of the Invention) As described above, according to the present invention, the radio wave absorption characteristics for obliquely incident waves are improved, so it can be used when it is necessary to absorb radio waves from any direction, for example, to prevent radio wave interference caused by buildings. It is highly effective when applied.

[Brief explanation of the drawing]

FIG. 1 is a configuration example of a radio wave absorber according to the present invention, and FIGS. 2 and 3 are diagrams showing characteristics of the radio wave absorber according to the present invention. 10; dielectric 12; conductor 14; resistance cloth

Claims (2)

[Claims] (1) A foamed dielectric material having a predetermined thickness, a conductor formed on one side of the foamed dielectric material, and a conductive fiber or a resistance cloth woven from conductive fibers and non-conductive fibers formed on the other surface of the foamed dielectric material. A radio wave absorber having: an interval L_1 between the conductive fiber threads is larger than an optimum interval L_0 for vertically incident waves (L_1>L_0). (2) A foamed dielectric material having a predetermined thickness, a conductor formed on one side of the foamed dielectric material, and a conductive fiber or a resistance cloth woven from conductive fibers and non-conductive fibers formed on the other surface of the foamed dielectric material. A radio wave absorber having: the thickness d_1 of the foamed dielectric material is larger than the optimal thickness d_0 for vertically incident waves (d_1>d_0).