JP2000133983A - Radio wave absorbing body - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a matching radio wave absorbing body which can be easily regulated in matching frequencies and whose matching frequencies can be widened in band width. SOLUTION: A matching-type radio wave absorbing body is equipped with a radio wave absorbing medium lined with a metallic body. The radio wave absorbing medium is a magnetic material layer, and a laminate composed of a radio wave absorbing medium layer and the metallic layer is laminated into a laminate which functions as a resonator possessed of two resonant modes of two different resonant frequencies to the radio wave absorbing medium layer of the same thickness as that of the laminate. The resonant frequency f1 of one resonant mode out of two different resonant modes that belong to the resonator is less than frequencies where the frequency dispersion of the permeability of the radio wave absorbing medium becomes a constant, the other resonant frequency f2 is more than the frequencies where the frequency dispersion of the permeability of the radio wave absorbing medium becomes a constant. By a combination of these two modes, the radio wave absorption characteristics of a radio wave absorbing body of this constitution is determined on a load Q in a state where two modes are combined, and the radio wave absorbing body is widened in the band width of radio waves to absorb. A resonator is regulated in operating frequency band by adjusting a radio wave absorbing medium layer in thickness and permittivity.

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, and more particularly, to a matched radio wave absorber in which a radio wave absorbing medium layer is lined with a metal body.

[0002]

2. Description of the Related Art A matched electromagnetic wave absorber in which a medium layer is lined with a metal body has conventionally been used for attaching to a large bridge or a ship mast, or for use as a wall surface of a small dark room, for the purpose of suppressing the reflection of radio waves. Have been. Such a matched-type radio wave absorber is composed of a laminate of a radio wave absorption medium layer and a metal body, and the radio wave absorption characteristics thereof are, as shown in FIG. 1, the dielectric constant of the medium layer (ε = ε′− jε ′) and the magnetic permeability (μ = μ′−j)
μ ″) and thickness (d).

Conventionally, a mixture of a conductive carbon as a dielectric loss substance and an organic resin or rubber, and a metal magnetic powder such as carbonyl iron or alnico as a magnetic loss substance are solidified in an electromagnetic wave absorbing medium layer with an organic resin. Or a mixture of a ferrite sintered body and a powder thereof with an organic resin or rubber.

[0004] The matched type electromagnetic wave absorption characteristic is expressed by the following equation (1).

(Equation 1) In, ε, μ, and d are adjusted such that the input normalized impedance Z / Z ₀ on the medium surface on which the radio wave enters becomes 1. Here, Z is the wave impedance on the radio wave incident surface, Z ₀ is the intrinsic impedance of the medium, and ε and μ are
The complex dielectric constant and the complex magnetic permeability of the medium, d is the thickness of the medium layer, and λ is the free space wavelength of the incident electromagnetic wave.

The radio wave absorber having the structure shown in FIG.
4) It can be considered that the wavelength resonator is critically coupled to the external circuit. Therefore, the absorption characteristics reflect the load Q. The prior art is based on taking up a single resonance mode of the absorber and evaluating Expression 1 for the same thickness (d).

Therefore, to broaden the radio wave absorption characteristic determined by the equation 1 means that the degree (band characteristic) of keeping Z / Z ₀ close to 1 with respect to the frequency change is ε and μ.
By selecting the frequency dependence of

[0007] The dielectric loss material used for the radio wave absorber is
Since it is based on the ohmic loss of a conductor such as carbon, it is almost monotonically proportional to the frequency. On the other hand, in some magnetic materials typified by spinel ferrite, when the frequency increases, the dispersion of μ ′ occurs, and a peak occurs in μ ″.

This dispersion characteristic is closely related to the magnetization mechanism of the ferrite, and is caused by relaxation of the domain wall motion or natural resonance in the magnetic domain in the rotation magnetization mechanism. At many frequencies, μ ′ often approaches 1. In the VHF and UHF bands, this dispersion is used to develop a ferrite radio wave absorber having a wide band characteristic that cannot be normally achieved with a dielectric medium. Such a ferrite radio wave absorber has been used, for example, as a measure against television ghosts.

Further, hexagonal ferrite is known as ferrite that breaks the snake limit, and its use is being promoted in the GHz band.

[0010]

By the way, in the spinel ferrite electromagnetic wave absorber which has been widely used conventionally,
By using ferrite having a relatively large magnetic permeability, the frequency dispersion characteristics are skillfully used, and absorption characteristics having a wide band in the VHF and UHF bands can be obtained. When spinel ferrite is used in the GHz band, the magnetic permeability decreases rapidly as the frequency increases, and the imaginary part μ "effect takes precedence in the matched absorber in that frequency region, and the operating ratio band is at most 15%. It is usually on the order.

Such a narrow operating ratio band means that even when hexagonal ferrite is used, the frequency at which dispersion converges is shifted to the high frequency side, and is essentially unchanged, and the operating frequency band is 15%. It is about. In this case, an attempt to broaden the band by appropriately selecting the gradient of the μ ″ dispersion immediately before the frequency at which μ ′ approaches 1 is described in, for example, Japanese Patent Publication No. 7-1205.
No. 74 discloses this.

However, this method is based on the assumption that μ 'is substantially equal to 1 in principle, and therefore, there is a possibility that the selection of the medium is excessively restricted, and the limitation of widening the band is obvious. It was not done. In addition, as an attempt to improve the radio wave absorption characteristics to broaden the band, there is an attempt to change the magnetic permeability by applying a magnetic field to the soft magnetic material, but this method also has an effect instead of requiring an external magnetic field device. Is less.

It is an object of the present invention to provide a matched radio wave absorber which facilitates adjustment of a matching frequency and broadening of a band.

[0014]

Means for Solving the Problems To achieve the above object, a radio wave absorber according to the present invention is a matched radio wave absorber having a radio wave absorbing medium lined with a metal body, wherein the radio wave absorbing medium comprises: The magnetic material layer, which is a laminate of a radio wave absorbing medium and a metal body, is a resonator that couples two or more resonance modes to a radio wave absorbing medium having the same thickness.

Further, the lamination of the radio wave absorbing medium and the metal body forms a resonator having two or more resonance modes having different resonance frequencies.

The operating band of the resonator formed by the laminate of the radio wave absorbing medium and the metal body is adjusted by the thickness of the radio wave absorbing medium and its dielectric constant.

The resonance frequency of at least one of the two or more resonance modes of the resonator formed by the laminate of the radio wave absorbing medium and the metal body is the frequency dispersion of the magnetic permeability of the radio wave absorbing medium. Is set to a frequency equal to or lower than the frequency at which is a constant, and the resonance frequency of at least one other resonance mode is a frequency set to be equal to or higher than the frequency at which the frequency dispersion of the magnetic permeability of the electromagnetic wave absorbing medium becomes a constant.

The radio wave absorbing medium is a ferrite sintered body or a layer formed of a mixture of ferrite and rubber or ferrite and resin.

The radio wave absorbing medium is a composite of a transition metal or an alloy thereof, or fine particles of a transition metal, and a resin or rubber.

The dielectric constant of the radio wave absorbing medium is determined within the radio wave absorbing medium or between the radio wave absorbing medium and the lamination of the metal body.
It is adjusted by providing air or a dielectric layer.

The dielectric constant of the radio wave absorbing medium is adjusted by arranging a dielectric layer in an island shape or a honeycomb shape in the radio wave absorbing medium.

The radio wave absorbing medium has irregularities on the surface, and the irregularities on the surface improve the band and oblique incidence characteristics of the radio wave incident on the radio wave absorbing medium.

In a single-layer radio wave absorber, two or more resonance modes that are simultaneously and equally excited for a certain thickness (d) of a radio wave absorption medium are used to improve radio wave absorption characteristics. Attempts to do so have hitherto not received any attention.

The present invention shows a procedure for obtaining a frequency band corresponding to a difference between two (or more) resonance frequencies in a synthesized resonance mode (absorber), and a wide band to the limit of a medium to be used. Is to do it automatically.

The present invention is intended for all magnetic materials such as sintered compacts and rubber ferrites as the material used for the radio wave absorbing medium, and gives the possibility and limitation of widening the band of the radio wave absorber using those media. At the same time, it provides guidelines for improving the material for broadband.

In order to realize the radio wave absorber of the present invention, the radio wave absorber is configured as a single-layer type absorber as shown in FIG. 1, and the magnetic permeability of the radio wave absorbing medium has the frequency dispersion characteristic shown in FIG. At the same time, the first absorption (the first resonance) and the second absorption (the second resonance) as shown in FIG. 3 defined by the thickness d of the layer of the radio wave absorbing medium are simultaneously excited, and these are coupled. By using the resonance mode obtained, the operating frequency band is made much wider than the first or second operating frequency band alone (it is possible to obtain a bandwidth of one octave).

Here, some of the Y-type hexagonal ferrites having in-plane anisotropy were used as a medium, and it was shown that the present invention is effective.

[0028]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with reference to the drawings. In FIG. 1, a radio wave absorber according to the present invention is a matched radio wave absorber having a radio wave absorbing medium 1 lined with a metal body 2 as a reflection plate. The radio wave absorber having the structure shown in FIG. 1 is considered to be one in which a (1/4) wavelength resonator having the metal body 2 as a short-circuited end and the front surface of the radio-wave absorbing medium 1 as an open end is coupled to an external circuit.

That is, the thickness of the radio wave absorbing medium 1 at the time of resonance is exactly １ ／ of the wavelength in the radio wave absorbing medium 1. Normally, the frequency change of the medium constant around one resonance frequency (matching frequency) is analyzed in connection with the band characteristic.
In the present invention, starting from the fact that there can be two resonance modes deviating from the resonance state in a medium having a thickness d constituting the radio wave absorber, the radio wave absorber of the present invention has a thickness of the radio wave absorption medium. It is treated as a resonator that couples two or more resonance modes existing in.

According to the radio wave absorber of the present invention, the radio wave absorption characteristic is determined by the load Q in a state where the two resonance modes are coupled to each other, and the radio wave absorber becomes a wide band absorber. In the present invention, the resonance frequency of one of the two resonance modes of the resonator formed by the stack of the radio wave absorbing medium and the metal body has a constant frequency dispersion of the magnetic permeability of the radio wave absorbing medium. The frequency is set to be equal to or lower than the frequency, and the resonance frequency of another resonance mode is set to be equal to or higher than the frequency at which the frequency dispersion of the magnetic permeability of the radio wave absorbing medium becomes a constant.

FIG. 2 is a schematic diagram showing the magnetic permeability of the radio wave absorbing medium used in the present invention. In order to use the radio wave absorber of the present invention in the GHz band, a hexagonal ferrite having in-plane anisotropy is desirable. In FIG. 2, 3 is μ ′, 4 is μ ″
It is. The frequency at which the magnetic permeability μ is dispersed and becomes almost flat is fF. As shown in FIG. 3, by adjusting the thickness (d) of the radio wave absorbing medium, the two absorptions (the first absorption and the second absorption) can be excited to the same extent.

The respective absorption (resonance) frequencies are f1 and f
Let it be 2. The first absorption f1 is on the lower frequency side than fF, and the second absorption f2 is on the higher frequency side than fF. FIG. 4 shows a Smith diagram on the absorber incident surface at this time. In FIG. 4, the thickness (d) adjustment and the real part of the permittivity (ε ′)
Is reduced, impedance matching is performed at an intermediate frequency between the first absorption and the second absorption, whereby a broadband absorber can be produced.

If the radio wave absorbing medium 1 has ε ′ suitable for widening the band from the beginning, it is only necessary to adjust the thickness (d). Up to this point, the frequency characteristics of μ as shown in FIG. 2 are measured together with the dielectric constant, the reflection characteristics are simulated using equation (1), and the absorption characteristics shown in FIG. 3 are obtained.
Determine 2.

If the simulation is further advanced while changing the thickness (d) of the radio wave absorbing medium 1 so that sufficient absorption characteristics can be obtained by changing the ε 'of the medium, the radio wave absorption having a wide band as shown in FIG. Characteristics are obtained. In FIG. 5, the two resonance modes are coupled to each other and changed to a new resonance mode, and the difference (f2−f1) of the almost initial resonance frequency is −2.
A 0 dB bandwidth has been obtained.

FIG. 6 shows a Smith diagram at this time. In this method, when an existing electromagnetic wave absorbing medium (material) is used, the adjustment of the ε value becomes the most important. The method of lowering the dielectric constant of the ferrite material itself is to appropriately select the firing temperature, atmosphere (oxygen partial pressure), etc. so as to suppress hopping conduction between Fe ²⁺ and Fe ³⁺ , It is possible by introducing a low ε dielectric layer, and it is possible to lower ε and fine-tune the phase by introducing a void or a void layer in the medium.

Assuming that the two resonances are present simultaneously, assuming that the frequency of the magnetic permeability changes as shown in FIG. 2, the resonance wavelength on the low frequency side is much shorter than the resonance wavelength on the high frequency side. This is because the required thickness of the radio wave absorbing medium approaches. In a radio wave absorbing medium having no frequency dispersion, higher-order resonance appears as an integral multiple, and it is difficult to achieve matching at an intermediate frequency, and it is necessary to use ferrite or a magnetic material having frequency dispersion.

The one-dimensional electric field distribution of the two resonances is at the front of the medium, shortly before the low frequency side reaches the antinode, and after the high frequency side reaches the antinode. Coupling with an external circuit is performed by adjusting the electric field distribution by adjusting the thickness of the radio wave absorbing medium and adjusting the dielectric constant.

In the adjustment of the thickness, the effect of the magnetic permeability comes into play. Further, since the coupling with the external circuit is closely related to the phase advance on the front surface of the radio wave absorbing medium 1, the metallic body 2
It can be adjusted even if a gap is made in the front of the. The electromagnetic wave absorbing medium 1 includes a ferrite sintered body, a layer formed of a mixture of ferrite and rubber, or a mixture of ferrite and resin, a transition metal or an alloy thereof, or fine particles of a transition metal, and a resin or rubber. Can be used.

In the above embodiment, an example has been shown in which a laminate of a radio wave absorbing medium and a metal body forms a resonator for coupling two resonance modes to a radio wave absorbing medium having the same thickness. However, it is also possible to form a resonator that couples three or more resonance modes to a radio wave absorbing medium of the same thickness. In this case, a laminated resonator of a radio wave absorbing medium and a metal body is used. Of two or more resonance modes of
The resonance frequency of at least one resonance mode is set to a frequency equal to or lower than the frequency at which the frequency dispersion of the magnetic permeability of the radio wave absorbing medium becomes a constant, and the resonance frequency of the other at least one resonance mode is the magnetic permeability of the radio wave absorbing medium. The frequency is set to be equal to or higher than the frequency at which the frequency dispersion becomes a constant.

FIGS. 10 (a), 10 (b), and 10 (b) show examples of the configuration of a radio wave absorber formed by laminating a radio wave absorbing medium and a metal body.
Although shown in (c), (d), and (e), the embodiment of the present invention is not limited to these examples. FIG.
(A) is an example of a radio wave absorber having a basic structure in which a radio wave absorption layer 1 made of a sintered ferrite or rubber ferrite is lined with a metal body 2 and has a flat surface.

Radio waves enter from the layer surface of the radio wave absorbing medium 1 and are absorbed. (B) shows an example in which an air layer-like void or a low ε dielectric layer 5 is provided between the radio wave absorbing medium 1 and the metal body 2 to adjust the dielectric constant necessary for matching, and (c) shows an example in which This is an example in which an air layer-shaped void or a low ε dielectric layer 5 is provided in the absorption medium 1.

The layered structure here does not have a meaning in a usual multilayer type absorber, and has both the adjustment of the dielectric constant and the phase adjustment of the detailed reflection characteristics. (D) and (e) show the structure for adjusting the permittivity, (d) shows a low dielectric layer 6 scattered in an island shape in the radio wave absorption medium 1, and (e) shows a radio wave absorption medium. This is an example in which the medium 1 is configured in a honeycomb structure 7. The dielectric constant of the radio wave absorbing medium is adjusted by providing air or a dielectric layer between the radio wave absorber or the laminate of the radio wave absorber and the metal body.

(F) is an example in which unevenness 8 is formed on the surface layer of (a). Due to the surface irregularities, the band and oblique incidence characteristics are greatly improved as compared with (a).

[0044]

Examples of the present invention will be described below. (Example 1) A Ni ₂ Y sintered body (ε ′ =
FIG. 7 shows the absorption characteristics of the electromagnetic wave absorber using 6). In the figure, (a) shows two resonance modes with a thickness of 2.8 mm when ε '= 8.8, and (b) shows a thickness of 3.2 with ε' = 6.
Absorption characteristics of 4 mm ferrite sintered body, (c)
Shows the characteristic that ε ′ = 6 is broadened. In this embodiment, the center frequency is 7 GHz and the relative bandwidth is 50.
% Characteristics, thickness: 3.6 mm (0.5 mm void / 3.
1 mm ferrite).

(Embodiment 2) Co _0.5 Ni is used as a radio wave absorbing medium.
FIG. 8 shows the radio wave absorption characteristics of the radio wave absorber using the _1.5 Y sintered body (ε ′ = 5). (A) in the figure
In the case of ε ′ = 6, two resonance modes at a thickness of 2.5 mm are shown, (b) is an absorption characteristic of a ferrite sintered body of 2.8 ′ and ε ′ = 5, and (c) is This shows the characteristic of ε ′ = 5 having a wide band. In this embodiment, the characteristic having a center frequency of 11 GHz and a relative bandwidth of 45% is changed to a thickness of 2.8 mm (0.15 mm gap / 2.7 mm).
Ferrite).

Example 3 Ni ₂ Y silicon rubber ferrite (ferrite; 80 wt%) (ε ′)
= 6) shows the absorption characteristics of the radio wave absorber using (6). In the figure, (a) shows two resonance modes at a ferrite thickness of 2.4 mm, and (b) shows a broadened characteristic at a ferrite thickness of 3.6 mm. In this embodiment, a characteristic having a relative bandwidth of 38% at a center frequency of 8 GHz was obtained only by changing the thickness of the radio wave absorbing medium.

[0047]

As described above, according to the present invention, 1 GHz
Regardless of whether the magnetic material layer used for the radio wave absorption medium is a ferrite sintered body or a mixture of ferrite and resin, the matching frequency of the matched type ferrite radio wave absorber is adjusted and the band is broadened for the above frequencies. Has the effect of facilitating and greatly improving the bandwidth.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram of a single-layer radio wave absorber.

FIG. 2 is a graph illustrating frequency dispersion of magnetic permeability.

FIG. 3 is a graph showing radio wave absorption characteristics in which first absorption and second absorption are simultaneously and equally excited.

FIG. 4 is a Smith diagram in the case of FIG. 3;

FIG. 5 adjusts the intermediate frequency by adjusting the dielectric constant,
5 is a graph showing radio wave absorption characteristics when the band is widened.

FIG. 6 is a Smith diagram in the case of FIG. 5;

FIG. 7 is a graph showing broadband characteristics of a Ba ₂ Ni _1.5 Co _0.5 Fe ₁₂ O ₂₂ sintered body.

FIG. 8 is a graph showing broadband characteristics of a Ba ₂ Ni ₂ Fe ₁₂ O ₂₂ sintered body.

FIG. 9 is a graph showing broadband characteristics of a silicon rubber in which Ba ₂ Ni ₂ Fe ₁₂ O ₂₂ is mixed at 80 wt%.

FIGS. 10A to 10F are configuration diagrams of a radio wave absorber of the present invention.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 radio wave absorbing medium 2 metal body 3 μ ′ 4 μ ″ 5 void or low ε dielectric 6 low dielectric layer in island form 7 honeycomb structure 8 unevenness

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Kiichi Oda 5-Communitas S-715 5-chome, Chiyogaoka, Chigusa-ku, Nagoya-shi, Aichi (72) Inventor Koichi Konishi 3-2-1 Sakado, Takatsu-ku, Kawasaki-shi, Kanagawa Stock Company 5E040 AB04 AB09 AC05 BD01 CA13 5E321 AA44 BB01 BB25 BB51 GG05 GG07 GG12 5J020 BD02 EA02 EA10

Claims (9)

[Claims] 1. A matched radio wave absorber having a radio wave absorbing medium backed by a metal body, wherein the radio wave absorbing medium is a magnetic material layer, and the radio wave absorbing medium and the metal body have the same thickness. A radio wave absorber characterized in that it is a resonator that couples two or more resonance modes to the radio wave absorption medium. 2. The laminated structure of a radio wave absorbing medium and a metal body forms a resonator having two or more resonance modes having different resonance frequencies.
The radio wave absorber described in 1. 3. The operating band of a resonator formed by laminating a radio wave absorbing medium and a metal body is adjusted by the thickness of the radio wave absorbing medium and its dielectric constant. Or the radio wave absorber according to 2. 4. The resonance frequency of at least one of two or more resonance modes of a resonator formed by stacking a radio wave absorbing medium and a metal body has a frequency dispersion of magnetic permeability of the radio wave absorbing medium. Is set to a frequency equal to or lower than the frequency at which the constant becomes a constant, and the resonance frequency of at least one other resonance mode is a frequency set to be equal to or higher than the frequency at which the frequency dispersion of the permeability of the electromagnetic wave absorbing medium becomes a constant. The radio wave absorber according to claim 2. 5. The radio wave absorber according to claim 1, wherein the radio wave absorption medium is a ferrite sintered body or a layer formed of a mixture of ferrite and rubber or ferrite and resin. 6. The radio wave absorber according to claim 1, wherein the radio wave absorption medium is a composite of a transition metal, an alloy thereof, or fine particles of the transition metal, and a resin or rubber. 7. The dielectric constant of the radio wave absorbing medium is adjusted by providing air or a dielectric layer in the radio wave absorbing medium or between the radio wave absorbing medium and the lamination of the metal body. The radio wave absorber according to claim 3, wherein: 8. The radio wave absorbing medium according to claim 3, wherein the dielectric constant is adjusted by disposing a dielectric layer in an island shape or a honeycomb shape in the radio wave absorbing medium.
The radio wave absorber described in 1. 9. The radio wave absorbing medium has irregularities on its surface, and the irregularities on the surface improve the band and oblique incidence characteristics of radio waves incident on the radio wave absorbing medium. The radio wave absorber described in 1.