JPH1154981A - Electromagnetic wave absorber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form an electromagnetic wave absorber which readily uses good characteristics of a dielectric and a ferroelectric, by providing a resistance film to a surface of a dielectric whose rear is shortcircuited by a metal. SOLUTION: An electromagnetic wave absorber is constituted by providing a resistance film 3 to a surface of a dielectric 2 whose rear is shortcircuited by a metal 1. Such a wave absorber absorbes electromagnetic wave 4 injected by the dielectric 2 through the resistance film 3 and reflects the rest of electromagnetic wave 4 absorbed by the dielectric 2 by the metal 1. As a result, even if a combination of a real number part and an imaginary number part of dielectric constant of a dielectric does not satisfy matching condition, the electromagnetic wave absorber with good wave absorption characteristics can be obtained by combining with the resistance film 3, if a specified value is smaller than that of matching condition in a dielectric only.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a radio wave absorber used for measures against unnecessary electromagnetic waves.

[0002]

2. Description of the Related Art Currently used radio wave absorbers are mainly made of ferrite, and those using a dielectric have been developed, but the materials to be used and the application area are extremely limited. Ferrite radio wave absorbers are heavy, and have a problem that an absorber of several mm or less cannot be obtained because the magnetic permeability decreases at high frequencies. For this reason, development of more various radio wave absorbers is required.

On the other hand, there are a variety of dielectric materials, such as a resin-based light-weight material and a material having a very high dielectric constant such as a ferroelectric material and maintaining its characteristics up to a high frequency. However, among them, only very limited materials such as rubber carbon and carbon-mixed expanded polystyrene are used as the radio wave absorber. The reason will be described below.

[0004] As shown in FIG. 8, when the back surface of the radio wave absorber 11 is short-circuited with the metal plate 12, the incident electromagnetic wave 13
Among them, the remaining electromagnetic wave absorbed by the radio wave absorber 11 is reflected by the metal plate 12, and the small reflectance at this time is a measure of the performance of the radio wave absorber. The reflectivity の of vertically incident electromagnetic waves is

[0005]

(Equation 1)

[0006]

(Equation 2) Given by Here, Z _A is the impedance measured from the surface of the radio wave absorber 11 toward the metal plate 12, and Z 1, γ, and l are the radio wave absorber 11, respectively.
Characteristic impedance, propagation constant, and thickness. Also,
Z ₀ is the characteristic impedance of vacuum. The characteristic impedance and the propagation constant are represented by the complex dielectric constant ε and the complex magnetic permeability μ of the radio wave absorber 11, where ω is the angular frequency of the incident electromagnetic wave 13.
Has the following relationship:

[0007]

(Equation 3)

[0008]

(Equation 4) Considering that the radio wave absorber 11 is a dielectric material having no magnetism, equations (3) and (4) can be rewritten as follows.

[0009]

(Equation 5)

[0010]

(Equation 6) Here, ε ₀ is the magnetic permeability in vacuum, ε is the relative magnetic permeability of the dielectric, and c is the speed of light. After all, as the reflectance Γ

[0011]

(Equation 7) Is obtained.

FIG. 9 is a characteristic diagram showing a result obtained by solving the equation (7) to obtain a matching condition at which the reflectance becomes zero. In order to obtain a matching condition, dielectrics satisfying a condition that indicates that tan δ is uniquely determined for one relative permittivity value are very limited, and tan δ is Since it varies with frequency, it has been difficult to develop a dielectric-type radio wave absorber having desired characteristics.

[0013]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has as its object to provide a radio wave absorber that easily makes use of the excellent characteristics of dielectrics and ferroelectrics. .

[0014]

In order to achieve the above object, the radio wave absorber of the present invention is characterized in that a resistive film is provided on the surface of a dielectric whose back surface is short-circuited with a metal.
Further, the radio wave absorber according to the present invention is characterized in that a ferroelectric is used as the dielectric.

The radio wave absorber of the present invention is characterized in that a dielectric having a dielectric constant of 4 or more is used as the dielectric. Further, the radio wave absorber of the present invention is characterized in that a resistance film having a sheet resistance of 300Ω or more is used as the resistance film.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a configuration diagram showing an embodiment of the present invention. That is, it is a radio wave absorber in which the resistance film 3 is provided on the surface of the dielectric 2 whose back surface is short-circuited with the metal 1. Reference numeral 4 denotes an incident electromagnetic wave. Thus, the incident electromagnetic wave 4 is absorbed by the dielectric 2 through the resistive film 3, and the remaining electromagnetic wave absorbed by the dielectric 2 is reflected by the metal 1. In the electromagnetic wave absorber having the structure shown in FIG.
Is rewritten as the following equation.

[0017]

(Equation 8)

Here, R is the sheet resistance of the resistive film 3. As a result, the degree of freedom of the condition for providing the matching is increased by the amount of the variable R as compared with the case of the equation (7). FIG. 2 is a characteristic diagram showing an example of a result obtained by solving the equation (8) to obtain a matching condition. When the resistive film 3 exists on the surface, the dielectric 2
The tendency is basically the same as in the case only. That is,
The value of tan δ giving the matching is inversely proportional to the square root of the dielectric constant of the dielectric 2, and becomes smaller as the dielectric constant of the dielectric 2 increases. The value of tan δ also decreases as the resistance of the resistance film 3 decreases, and becomes zero at 377Ω. This result indicates that even if the combination of the real part and the imaginary part of the permittivity does not satisfy the matching condition, if the value of tan δ is smaller than the matching condition of the dielectric alone, the resistance film 3 It means that a radio wave absorber can be manufactured by combining with. When the resistance of the resistive film 3 is smaller than 377Ω, perfect matching cannot be obtained.
Up to about 00Ω, the return loss is 20 dB, that is, Γ =
It can be set to 0.1.

The matching thickness is constant irrespective of the presence or absence of the resistive film 3 and is １ ／ of the wavelength of the electromagnetic wave in the dielectric 2. When the dielectric constant of the dielectric 2 is close to 1, it is the same as that of a conventional so-called λ / 4 type absorber, but as the dielectric constant increases, the matching thickness becomes thinner. It is possible.

FIG. 3 is a characteristic diagram showing an example of the relationship between the thickness of the absorber and the frequency of the electromagnetic wave estimated from the above results. When a ferroelectric material having a relative dielectric constant of 10,000 is used as the dielectric 2, the matching thickness becomes thinner than 8 mm of the ferrite-based absorber at a frequency of 100 MHz or more. 5GH for dielectric
1.5m of carbonyl iron rubber-based absorber at frequencies above z
An electromagnetic wave absorber thinner than m can be obtained.

[0021]

【Example】

Embodiment 1 FIG. 4 is a characteristic diagram showing an example of characteristics (reflection attenuation) of a radio wave absorber using rubber carbon. Rubber carbon used as a dielectric has a dielectric constant of 16, tan
Since δ = 0.2, which is a value out of the characteristic value giving the matching condition, good radio wave absorption characteristics are not obtained. In order to obtain rubber carbon that can be used as a radio wave absorber, it is necessary to precisely control the amount of carbon to be blended.

FIG. 5 is a characteristic diagram showing an example of the characteristics of the radio wave absorber using rubber carbon according to the first embodiment of the present invention. By combining a resistive film having a sheet resistance of 1000Ω with the dielectric of FIG. 4 as the resistive film 3, a good radio wave absorption characteristic of 40 dB or more can be exhibited and the matching frequency can be freely adjusted.

[Embodiment 2] FIG. 6 shows a dielectric material having a dielectric constant of 1
FIG. 9 is a characteristic diagram showing radio wave absorption characteristics when a single layer of a ferroelectric material having a tan δ = 0.005 is used. With this material, even if the thickness is changed, a return loss of several dB or more cannot be obtained, and it cannot be used as a radio wave absorber.

FIG. 7 is a characteristic diagram showing characteristics of the radio wave absorber according to the second embodiment of the present invention. By combining the resistive film 3 having a surface resistance of 390Ω with the dielectric of FIG. 6, it is possible to produce a thin radio wave absorber utilizing the large dielectric constant of the ferroelectric, and the matching thickness at 10 GHz is approximately It is 0.7 mm, which is about half the value of the carbonyl iron rubber-based absorber.

[0025]

As described above, according to the present invention, it is possible to easily provide a radio wave absorber utilizing the excellent characteristics of dielectrics and ferroelectrics.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing an embodiment of the present invention.

FIG. 2 is a characteristic diagram showing the relationship between relative permittivity and tan δ under matching conditions when a resistive film according to one embodiment of the present invention is present.

FIG. 3 is a characteristic diagram illustrating a relationship between a thickness of an absorber and a frequency of an electromagnetic wave according to an embodiment of the present invention.

FIG. 4 is a characteristic diagram showing an example of a return loss frequency characteristic of a radio wave absorber using rubber carbon out of matching conditions.

FIG. 5 is a characteristic diagram showing an example of a return loss frequency characteristic of a radio wave absorber in which rubber carbon and a resistive film are combined according to the first embodiment of the present invention.

FIG. 6 is a characteristic diagram showing an example of a return loss frequency characteristic of a radio wave absorber using a ferroelectric substance alone.

FIG. 7 is a characteristic diagram showing an example of a return loss frequency characteristic of a radio wave absorber combining a ferroelectric and a resistive film according to a second embodiment of the present invention.

FIG. 8 is a configuration diagram showing a conventional radio wave absorber.

FIG. 9 is a characteristic diagram showing an example of a relationship between a relative dielectric constant and tan δ under a matching condition where the reflectance of a conventional radio wave absorber is zero.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 metal 2 dielectric 3 resistive film 4 electromagnetic wave 11 radio wave absorber 12 metal plate 13 electromagnetic wave

Claims (4)

[Claims] 1. A radio wave absorber characterized in that a resistive film is provided on the surface of a dielectric whose back surface is short-circuited with a metal. 2. The radio wave absorber according to claim 1, wherein a ferroelectric is used as the dielectric. 3. The radio wave absorber according to claim 1, wherein a dielectric having a dielectric constant of 4 or more is used as the dielectric. 4. A resistance film having a sheet resistance of not less than 300Ω is used as the resistance film.
The radio wave absorber described.