JP2014150194A - Electromagnetic wave absorber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a compact electromagnetic wave absorber capable of effectively absorbing electromagnetic waves in a frequency band from 30 MHz to 1,000 MHz.SOLUTION: An electromagnetic wave absorber of the present invention includes a dielectric loss body layer 20 including dielectric loss bodies 11 each of which has a bottom face placed on a plane. The distribution in a direction vertical to the plane of an average dielectric constant in each cross section of the dielectric loss body layer 20 in parallel with the plane has two or more minimum points, and the average dielectric constant at the minimum points is 0.5 times or less of the average dielectric constant of the entire of the dielectric loss body layer 20.

Description

  The present invention relates to a radio wave absorber, and more particularly to a radio wave absorber that effectively absorbs electromagnetic waves in a frequency band of 30 MHz to 1000 MHz.

  Conventionally, as a radio wave absorber for an anechoic chamber, a composite radio wave absorber in which a pyramid-shaped or wedge-shaped dielectric loss body 11 is installed on the front surface of a magnetic absorber plate 10 as shown in FIG. 1 is used. I came. In general, ferrite tiles are used as the magnetic absorber plate, and materials obtained by mixing conductive materials such as carbon in synthetic resins such as foamed urethane, foamed styrene, and foamed polypropylene have been used as dielectric loss bodies. In this composite type wave absorber, electromagnetic waves in a high frequency region of 100 MHz or higher are absorbed by a dielectric loss material, and electromagnetic waves in a low frequency region are absorbed by a magnetic absorber plate. It is known that excellent radio wave absorption characteristics can be obtained.

  However, in recent years, there has been a demand for a more compact and high-performance anechoic chamber, and there is a need for an electromagnetic wave absorber that can be applied to such an anechoic chamber. It is known that the radio wave absorption performance of a conventional composite wave absorber is determined by the interaction between the magnetic absorber plate and the dielectric loss body, and in particular, the design of the shape of the dielectric loss body is important. For this reason, many studies have been made on the shape of the dielectric loss body of the composite wave absorber.

  For example, Patent Document 1 includes a magnetic absorber and a tapered structure formed of a dielectric loss material provided in front of the magnetic absorber, and the tapered structure is a dielectric loss material that operates as a radio wave absorber. A composite electromagnetic wave absorber having a shape in which the cross-sectional area ratio changes logarithmically with respect to the length from the front end to the bottom surface of the tapered structure is disclosed. In Patent Document 1, this configuration allows a change in dielectric constant to obtain good characteristics even when the length of the radio wave absorber is shortened, thereby reducing the size of the radio wave absorber, the size of the anechoic chamber, and reducing the cost. It is described that it becomes possible.

  Further, in Patent Document 2, in a radio wave absorber formed of a radio wave absorbing material and a conductor, when radio waves are incident on the radio wave absorbing material backed by a conductor from a space, the complex dielectric constant in the radio wave absorbing material is There has been disclosed a radio wave absorber whose size is once reduced from the space side toward the conductor side to become a minimum, and increases from here toward the conductor. It is described that the radio wave absorber having such a configuration can realize a radio wave absorber thinner than a conventional pyramid type in a relatively wide frequency range.

Japanese Patent No. 3291855 JP 2000-261240 A

  However, in spite of being required to have excellent radio wave absorption characteristics in both a low frequency band of 100 MHz or lower and a high frequency band of 1 GHz or higher, the low frequency band of 50 MHz or lower is sufficient in the configuration of Patent Document 1. Radio wave absorption characteristics cannot be obtained, and there is a limit to miniaturization of the radio wave absorber. On the other hand, the configuration of Patent Document 2 can reduce the thickness of the radio wave absorber, but it cannot be said that the radio wave absorption characteristics in the low frequency band are sufficient.

  In view of the above problems, an object of the present invention is to provide a small wave absorber that can effectively absorb electromagnetic waves in a frequency band from 30 MHz to 1000 MHz.

  As a result of earnest research in view of the above object, the present inventors have found that in a radio wave absorber having a dielectric loss body layer including a dielectric loss body whose bottom surface is set on a plane, the direction from the bottom surface to the top of the dielectric loss body layer (Ie, in the thickness direction of the dielectric loss body layer), by providing two or more minimum locations where the average relative dielectric constant is 0.5 times or less of the average relative dielectric constant of the entire dielectric loss body, 30 MHz to 1000 MHz The inventors have found that the radio wave absorption characteristics in the frequency band are greatly increased, and have completed the present invention.

  That is, the radio wave absorber of the present invention has a dielectric loss body layer including a dielectric loss body whose bottom surface is set on a plane, and has an average relative dielectric constant in a cross section of the dielectric loss body layer parallel to the plane. The distribution in the direction perpendicular to the plane has two or more local minimum points, and the average relative dielectric constant at the local minimum points is 0.5 times or less of the entire average relative dielectric constant of the dielectric loss layer. It is characterized by that.

  As one embodiment for realizing such a distribution, the dielectric loss layer may have a different cross-sectional shape in a direction perpendicular to the plane. As another embodiment, the dielectric loss layer may be formed by stacking layers made of materials having different relative dielectric constants in a direction perpendicular to the plane.

  According to the present invention, a small wave absorber having excellent radio wave absorption characteristics in a frequency band of 30 MHz to 1000 MHz can be obtained, and a small and high performance anechoic chamber can be realized.

It is sectional drawing which shows an example of the structure of the conventional electromagnetic wave absorber. It is sectional drawing which shows the structure of the electromagnetic wave absorber 100 by one Embodiment of this invention. (A)-(C) are sectional drawings which show the structure of the electromagnetic wave absorber by other embodiment of this invention. BRIEF DESCRIPTION OF THE DRAWINGS It is drawing which shows the electromagnetic wave absorber produced in the Example (Example 1 of this invention), (A) is a side view of a radio wave absorber, (B) is the top view, (C) is a photograph. It is a graph which shows the result of having measured the electromagnetic wave absorption characteristic of the electromagnetic wave absorber in this invention example 1 and a comparative example. It is a graph which shows the thickness direction distribution of the value X which remove | divided the average relative dielectric constant of the cross section of the dielectric loss body layer by the average relative dielectric constant of the whole dielectric loss body layer in this invention example 1 and a comparative example. It is a graph which shows the thickness direction distribution of X in the case where X in the position whose distance from a magnetic absorber board is 35 cm is changed (Examples 2 and 3 of the present invention) and Example 1 of the present invention. It is a graph which shows the increase in the return loss of this invention example 2 and 3 on the basis of the return loss (dB) of the example 1 of this invention.

  Hereinafter, embodiments of the radio wave absorber of the present invention will be described.

  FIG. 2 is a cross-sectional view showing the structure of the radio wave absorber 100 according to one embodiment of the present invention. The radio wave absorber 100 includes a magnetic absorber plate 10 and a dielectric loss body layer 20 formed on the magnetic absorber plate 10. The dielectric loss body layer 20 includes a plurality of (three in the figure) columnar dielectric loss bodies 11 and spaces (air) therebetween. The bottom surface of the dielectric loss body 11 is installed on the plane of the magnetic absorber plate 10. Each of the columnar dielectric loss bodies 11 is provided with two portions where the thickness is reduced from the bottom surface to the top portion.

Therefore, this dielectric loss body layer 20 has an average relative dielectric constant X1 in the cross section of the dielectric loss body layer 20 parallel to the plane of the magnetic absorber plate 10 and a direction perpendicular to the plane (hereinafter referred to as “dielectric loss body”). It is a structure that changes to “the thickness direction of the layer” or simply “the thickness direction”. The average relative dielectric constant X1 takes a minimum value in the thin portion (position 12 in FIG. 2) in the thickness direction. Therefore, the thickness direction distribution of the average relative dielectric constant X1 has two local minimum points. Furthermore, the present embodiment is characterized in that the average relative dielectric constant X1 _lmin at the minimum point is 0.5 times or less of the entire average relative dielectric constant X2 of the dielectric loss layer 20. A value obtained by dividing the average relative permittivity X1 by the average relative permittivity X2 of the entire dielectric loss layer 20 is referred to as “ratio X”. In this embodiment, the ratio X ≦ 0.5.

  As described above, by configuring the dielectric loss body layer 20 so as to have a minimum value at two or more locations in the thickness direction distribution of the average relative dielectric constant X1, a resonance phenomenon occurs between a large number of electromagnetic waves having different frequencies. The present inventors have found that excellent radio wave absorption characteristics can be obtained in a wide frequency band. When the minimum value is less than two places, sufficient absorption performance can be obtained only in the local frequency band. Further, it was also found that when the ratio X exceeds 0.5, the resonance effect is reduced, and sufficient radio wave absorption performance cannot be obtained.

  Here, the average relative dielectric constant X1 in each cross section is defined as follows. That is, when the entire cross section is occupied by the dielectric loss body 11, the relative dielectric constant of the dielectric loss body 11 is the average relative dielectric constant X1. On the other hand, as shown in FIG. 2, when the dielectric loss body 11 occupies a part of the cross section and the remainder is a space (ie, air), “the relative dielectric constant of the dielectric loss body and the area ratio occupied by the dielectric loss body” The sum of “the product of” and “the product of the relative dielectric constant of air (1.0) and the area ratio occupied by the space” is the average relative dielectric constant X1.

  The average relative dielectric constant X2 of the entire dielectric loss layer 20 is defined as follows. That is, when there is no space in the dielectric loss body layer 20, the relative dielectric constant of the dielectric loss body 11 becomes the average relative dielectric constant X2. On the other hand, as shown in FIG. 2, when the dielectric loss body is partially disposed in the dielectric loss body layer 20 and the remainder is a space (ie, air), “the relative dielectric constant of the dielectric loss body and The sum of “the product of the volume ratio occupied” and “the product of the air non-dielectric constant (1.0) and the volume ratio occupied by it” is the average relative dielectric constant X2. In addition, when it consists of a dielectric material of multiple types of materials, what is necessary is just to add sequentially the product of each dielectric constant and its volume ratio.

  As the magnetic absorber plate 10, a ferrite tile is generally used. The material of the dielectric loss body 11 is not particularly limited. Carbon-containing foamed urethane, carbon-containing foamed styrene, carbon-containing foamed polypropylene, or inorganic material such as plastic containing silica or coated with a carbon layer, silica, alumina, or the like. Can be mentioned. Examples of the structure of the dielectric loss body 11 include a sheet structure, a cardboard structure, and a honeycomb structure.

  With reference to FIG. 3 (A)-(C), the other aspect of the dielectric loss body layer 20 is shown.

In the radio wave absorber 200 of FIG. 3A, the dielectric loss body layer 20 is formed by laminating a plurality of dielectric loss bodies 11 made of materials having different relative dielectric constants on the magnetic absorber plate 10 in the thickness direction. Do it. For example, the dielectric loss body 11 at the position indicated by reference numeral 12 in the figure may be a resin layer to which carbon or the like is not added, or may be a layer having a low carbon content, while the dielectric loss body at other positions. 11 can be a resin material in which a conductive material such as carbon is dispersed. This dielectric loss body layer 20 also has local minimum values at two locations (positions indicated by reference numeral 12 in the figure) in the thickness direction distribution of the average relative dielectric constant X1, and the average relative dielectric constant X1 _{lmin at the} minimum point is the dielectric loss body. It can be designed to be 0.5 times or less of the overall average dielectric constant X2 of the layer 20. Thus, by stacking layers made of materials having different relative dielectric constants in the thickness direction of the dielectric loss body layer 20, the thickness direction distribution of the average relative dielectric constant X1 can be designed as desired.

In the radio wave absorber 300 of FIG. 3B, the dielectric loss body layer 20 is formed by laminating a plurality of dielectric loss bodies 11 in the thickness direction on the magnetic absorber plate 10, and some layers ( In the figure, the third layer and the sixth layer from the side of the magnetic absorber plate 10 are provided with a space in a part so that the volume of the dielectric loss body 11 is reduced, and the average relative dielectric constant X1 in that layer. Was made smaller. This dielectric loss body layer 20 also has local minimum values at two locations (positions indicated by reference numeral 12 in the figure) in the thickness direction distribution of the average relative dielectric constant X1, and the average relative dielectric constant X1 _{lmin at the} minimum point is the dielectric loss body. It can be designed to be 0.5 times or less of the overall average dielectric constant X2 of the layer 20. The shape of the dielectric loss layer 20 is not particularly limited as long as it is within such a design range, and may be changed stepwise or continuously. Further, a dielectric having no conductivity may be inserted in the space in FIG.

In the radio wave absorber 400 of FIG. 3C, the dielectric loss body layer 20 has holes in the dielectric loss body 11 at two locations in the thickness direction, and the average relative permittivity X1 at the thickness position is reduced. . This dielectric loss body layer 20 also has local minimum values at two locations (positions indicated by reference numeral 12 in the figure) in the thickness direction distribution of the average relative dielectric constant X1, and the average relative dielectric constant X1 _{lmin at the} minimum point is the dielectric loss body. It can be designed to be 0.5 times or less of the overall average dielectric constant X2 of the layer 20. If it is in the range of such a design, the method of providing a hole will not be specifically limited, The thickness direction distribution of the desired average dielectric constant X1 can be designed by changing the magnitude | size and number of holes.

  As shown in FIGS. 3B and 3C, the distribution of the average relative dielectric constant X1 in the thickness direction can be designed as desired by making the cross-sectional shape different in the thickness direction of the dielectric loss layer 20. . However, the width of the block of the dielectric loss body in FIG. 3B (symbol W in the figure) and the diameter of the hole in FIG. 3C must be sufficiently small with respect to the wavelength of the radio wave. 1/2 or less is desirable. If the frequency is 1000 MHz, it becomes 15 cm. If this size is exceeded, the assumption of an equivalent average relative dielectric constant is not valid.

  When the propagation direction of the radio wave is oblique, a structure that satisfies the conditions of the present invention may be formed according to the propagation direction of the radio wave instead of the thickness direction of the dielectric loss layer 20. In that case, a radio wave absorber having good oblique incidence characteristics can be obtained.

(Invention Example 1)
The radio wave absorber shown in FIGS. 4A to 4C was produced. Specifically, as shown in FIGS. 4A and 4B, the cross section of the radio wave absorber is divided into cells of 100 mm × 100 mm, and dielectric loss bodies stacked in 8 layers for each of the cells are towered. It is in shape. The thickness of each layer was uniformly 50 mm, and the ratio of the cross-sectional area occupied by the dielectric loss body in each cell was changed as shown in FIGS. 4 (A) and 4 (B). Note that, as shown in FIG. 4B, the central portion of the radio wave absorber is an opening, which is for measuring the return loss rate described later. As the magnetic absorption plate 10, a ferrite tile was used. The dielectric loss body 11 was made of foamed polypropylene containing carbon. However, foamed polypropylene not containing carbon was used for the first and seventh layers from the bottom of the dielectric loss body.

(Comparative example)
For comparison, a conventional pyramid shaped wave absorber shown in FIG. 1 was produced.

  The electromagnetic wave absorbers of the inventive example 1 and the comparative example were set in coaxial tubes each having a cross-sectional area of 300 mm square, and the return loss was measured with a network analyzer. IEEE Recommended Practice for RF Absorber Evaluation in the Range of 30 MHz to 5 GHz ", IEEE Std. 1128 (1998). The measurement results are shown in FIG. It can be seen that Example 1 of the present invention has better radio wave absorption characteristics in the frequency band of 30 MHz to 1000 MHz than the comparative example.

  Further, in the present invention example 1 and the comparative example, the thickness direction of “ratio X” which is a value obtained by dividing the average relative dielectric constant X1 in each cross section perpendicular to the thickness direction by the overall average relative dielectric constant X2 of the dielectric loss layer. The distribution is shown in FIG. Thus, in Example 1 of the present invention, there are three minimum points where the ratio X is 0.5 or less (locations where the distance from the magnetic absorber plate is 5 cm, 25 cm, and 35 cm, respectively).

  As described above, it was confirmed that good radio wave absorption characteristics can be obtained by providing two or more minimum points where the ratio X is 0.5 or less in the dielectric loss layer. Furthermore, it can be seen from this experimental result that the distance between the minimum points is preferably 10 to 20 cm.

(Invention Examples 2 and 3)
Further, by adjusting the area occupied by the seventh layer of the carbon-containing foamed polypropylene in the radio wave absorber of FIG. 4, the ratio X at a distance of 35 cm from the magnetic absorber plate 10 is 0.1 as shown in FIG. To 0.8 (Invention Example 2) and 1.6 (Invention Example 3) were prepared.

The return loss of the radio wave absorbers of Invention Examples 2 and 3 was measured in the same manner. FIG. 8 is a graph showing an increase in the return loss of Invention Examples 2 and 3 based on the return loss (dB) of Example 1 of the present invention. The return loss is an average value of 30 to 1000 MHz.
Thus, when the ratio X exceeds 0.5, the increase in the return loss becomes close to 1 dB. Therefore, the ratio X is desirably 0.5 or less.

  According to the present invention, a small wave absorber having excellent radio wave absorption characteristics in a frequency band of 30 MHz to 1000 MHz can be obtained, and a small and high performance anechoic chamber can be realized.

100, 200, 300, 400 Radio wave absorber 10 Magnetic absorber plate (ferrite tile)
11 Dielectric Loss Body 12 Position where Distribution of Average Relative Permittivity in Thickness Direction is Minimal 20 Dielectric Loss Layer

Claims (3)

A radio wave absorber having a dielectric loss body layer including a dielectric loss body whose bottom surface is set on a plane,
The distribution of the average relative dielectric constant in the cross section of the dielectric loss layer parallel to the plane, in the direction perpendicular to the plane, has two or more minimum points,
The radio wave absorber according to claim 1, wherein an average relative dielectric constant at the minimum point is 0.5 times or less of an entire average relative dielectric constant of the dielectric loss layer.   The radio wave absorber according to claim 1, wherein the dielectric loss body layer has a shape of the cross section that is different in a direction perpendicular to the plane. The radio wave absorber according to claim 1, wherein the dielectric loss body layer is formed by stacking layers made of materials having different relative dielectric constants in a direction perpendicular to the plane.