JPS625360B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a broadband radio wave absorber that is excellent in both normal incidence and oblique incidence characteristics.

Recently, many anechoic chambers have been constructed with elongated shapes to increase the propagation distance within them, and as a result, there have been an increase in the number of cases in which radio waves enter at a large angle of incidence on the walls of the anechoic chamber. There is a need for a radio wave absorber that has excellent properties for both normal incidence and oblique incidence. However, conventional radio wave absorbers have the disadvantage that when the incident angle is large, the thickness of the absorber increases to obtain the same characteristics as normal incidence, and the effective space of the radio wave anechoic chamber is narrowed.

The present invention eliminates the above-mentioned drawbacks and provides a radio wave absorber that is extremely thin and has excellent characteristics not only in the case of normal incidence but also in the case of oblique incidence with a large angle of incidence.

Embodiments of the present invention will be explained with reference to the accompanying drawings. In FIG. 1, 1 is a metal plate (shorting plate); 2;
3, 4, 5, 6, and 7 are plate-shaped absorbent materials mainly made of expanded polystyrene, etc., and 8 is a tapered protrusion made of the same material as the plate-shaped absorbent material, and its cross section is square. The surface is tapered so that its cross-sectional area increases almost exponentially in the thickness direction of the absorbent material, as shown in FIG.

Before explaining the absorptivity of the radio wave absorber of the present invention, FIG. 2 showing the distribution of the attenuation constant of the dielectric material will be explained. In FIG. 2, the horizontal axis is the distance measured from the front surface of the absorber, and the vertical axis is the attenuation constant of the material. Thus, if the attenuation constant of the material is distributed so that it increases stepwise with respect to the distance measured from the front surface of the absorber, generally approximately exponentially, that is, along the dashed line in Figure 2, then normal incidence and It is already known that a radio wave absorber with excellent properties for both oblique incidence and oblique incidence can be obtained. (Ono, Ikuta, and Katagiri, “A construction method for a radio wave absorber with excellent properties for both normal incidence and oblique incidence,” Transactions of the Institute of Electronics and Communication Engineers (B), October 1978 issue) However, it is difficult to realize a portion with a small attenuation constant. No suitable materials have been developed to date.

By the way, in a structure in which a part of a flat absorber is cut out, if the attenuation constant of that part is small, the attenuation constant for radio waves is proportional to the cross-sectional area of the absorber. (Shimizu, Suetake, “Equivalent permittivity when a dielectric is inserted in a part”, Institute of Electronics and Communication Engineers Microwave Study Group, MW70-21, June 1972)
Month. ) Therefore, if the shape of the absorber is determined using this property, a stepped laminate as shown in FIG. 3 will be obtained.

As shown in FIG. 4, the vertical cross-sectional view of the step-like tapered portion in FIG. 3 has a step-like width that increases exponentially, that is, along the broken line in the figure.

However, such a step-like shape complicates the manufacturing process and increases the manufacturing cost, so in the present invention, the step-like portion is formed into a smooth taper shape as indicated by the broken line in FIG.

That is, the tapered part of the absorbent body according to the present invention has a square cross section, and the width of the vertical cross section increases almost exponentially with the distance measured from the front surface of the absorbent body, as shown in FIG. It is. The reason why the cross section of this tapered portion is square is to eliminate polarization characteristics.

By the way, conventional absorbers have pyramids (square pyramids).
There is an absorber of the form, but the damping constant in this case is the second
As shown by the dotted line in the figure, the increase is parabolic, and the difference from the present invention is quite obvious from the difference in absorption characteristics of the present absorber.

FIG. 6 shows the frequency characteristics of an absorber according to an embodiment of the present invention obtained as described above, with the frequency on the horizontal axis and the standing wave ratio on the vertical axis.
Figure A shows the theoretical and measured values when the incident angle is θ = 0° and θ = 70° (TE wave), and Figure B shows the theoretical and actual values when the incident angle is θ = 70° (TM wave). As is clear from this figure, the absorber of the present invention has a frequency of 1.2 GHz or more when the incident angle θ=0°, and a frequency of 70° or less.
A standing wave ratio of 1.1 at 2.9 GHz (TE wave), 2.5 GHz (TM wave) or higher can be achieved with a thickness of 18.6 cm, and the normal incidence characteristics are on the same level as conventional absorbers. However, it has extremely excellent oblique incidence characteristics at an incident angle of 70° or less, and there is little difference in polarization characteristics.

Although an example of an absorber for a frequency band of 1.2 GHz (θ=0°) or higher has been described here, the present invention is not limited to this, and the dielectric material that is the absorber can be appropriately selected. Absorbers for other frequency bands can also be obtained by

As described above, according to the present invention, the characteristics are good not only at normal incidence but also at oblique incidence, and the thickness is thin.
In addition, since an absorber with small polarization characteristics can be obtained, when used in an anechoic chamber, the effective range of the room can be increased.

[Brief explanation of the drawing]

FIG. 1 is a perspective view of a radio wave absorber according to the present invention, FIG. 2 is a distribution diagram of the attenuation constant of a general absorbing material, FIG. 3 is a perspective view of a radio wave absorber with a stepped tapered part, and FIG. Figure 4 is the same side view, and Figure 5 is the first one.
6 is a side view of the tapered portion in the figure, and FIG. 6 is a chart showing the frequency characteristics of the standing wave ratio of the absorber of the present invention. 2, 3, 4, 5, 6...Plate radio wave absorbing material, 8...
...Tapered part.

Claims (1)

[Claims] 1. A radio wave absorber characterized by disposing a radio wave absorber having a square cross section and an area increasing almost exponentially in the thickness direction of the absorber on the front surface of the plate-shaped radio wave absorber.