JPH0225279B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a radio wave absorbing coating, and more particularly to a radio wave absorbing coating suitable for preventing or reducing reflection of electromagnetic waves in the radar frequency range from a radio wave reflecting surface.

Radio-absorbing coatings are necessary, for example, to make radio-reflecting structures "invisible" to radar measurement equipment. Conventional radio-absorbing coatings are based on the principle of destructive interference between radio waves reflected from the coating surface and radio waves reflected from the radio-reflecting surface below, which reduces the intensity of the incident radio wave at the frequency at which absorption is desired. It uses a radio-absorbing coating with a thickness that corresponds to a quarter of the wavelength.

For normal incidence, the optical path difference between the resulting reflected wave from the radio-absorbing coating and the reflected wave from the surface of the coated radio-reflecting surface is 1/2 wavelength and 2 If the amplitude of each of the two waves is substantially equal, the two waves cancel each other out.

Furthermore, an alternative to this is radio-absorbing coatings using coating materials with high values of magnetic permeability μ _r , dielectric constant ε _r and high loss tangents tan δ〓, tan δ〓.

Magnetic permeability μ _r is μ _r = μ′ _r −jμ″ _r permittivity ε _r is ε _r = ε
′ _r −jε″ _r
μ′ _r , μ′ _r and μ″ _r ε″ _r are the real and imaginary parts of the magnetic permeability μ _r and the permittivity ε _r , respectively. The magnetic and dielectric loss tangents are tanδ〓=μ″ _r /μ′ _r and tanδ〓, respectively.
=
ε″ _r /ε″ _r . The radio wave absorbing coating is used to increase the attenuation by reducing the real parts μ′ _r and ε′ _r of the magnetic permeability μ _r and permittivity ε _r , respectively, and the magnetic and dielectric loss tangent tanδ〓tanδ〓
must be as large as possible and exhibit minimal impedance discontinuities at the interface with the propagation medium, typically air, around the propagating wave to reduce reflections at the outer surface of the wave absorbing coating. It won't happen. To achieve this, the outer surface of the radio wave absorbing coating is
The ratio of the real part of magnetic permeability μ′ _r to the real part of permittivity ε′ _r must be substantially equal to that of the surrounding medium.

Coating materials that meet the requirements for high loss tangent include ferrimagnetic materials, such as ferrite, which have both dielectric and magnetic permeability. However, in important frequency ranges, i.e. from 1 to 20 GHz and above, the real part of magnetic permeability μ' _r and the magnetic loss tangent tan δ of most ferrimagnetic materials can be used as effective radio-absorbing coatings. It's not that expensive. Conventional radio wave absorbing coatings for radars therefore usually operate on the principle of destructive interference.

Conventional radio wave absorbing coatings suffer from the problem that they are only effective over a narrow frequency band and are less effective for obliquely incident radio waves due to scattering and the breakdown of destructive interference conditions.

An object of the present invention is to provide a radio wave absorbing coating that can reduce or eliminate reflection of incident radio waves over a wide frequency band.

According to the present invention, the object includes a plurality of laminated layers each having a different substantial magnetic permeability for absorbing radio waves in a predetermined frequency band, and each of the layers has a different substantial magnetic permeability. This is achieved by a radio wave absorbing coating characterized in that it is arranged so that the radio waves to be absorbed gradually increase from the outermost layer toward the innermost layer.

According to the radio wave absorbing coating of the present invention, the coating includes a plurality of laminated layers having mutually different substantial magnetic permeabilities, and each of the layers has a different substantial magnetic permeability as the outermost layer on which the radio wave to be absorbed is incident. Since they are arranged so as to gradually increase from the top to the innermost layer, they can absorb the incident radio waves in a predetermined frequency band, and as a result, the reflection of the incident radio waves can be reduced or eliminated.

The plurality of laminated layers in the radio wave absorbing coating of the present invention include an outermost layer having a substantial magnetic permeability that matches the impedance between the outermost layer and the surrounding medium through which the radio waves to be absorbed have propagated. It's good to be prepared. This may reduce the reflection of incident radio waves on the outer surface of the radio wave absorbing coating.

The thickness of the plurality of laminated layers of the radio wave absorbing coating of the present invention is set so that the incident radio waves to be absorbed and the radio waves transmitted through the layers and reflected by at least one of the layers can interfere with each other. It is better. This can eliminate reflections of incident radio waves that should be absorbed.

The plurality of laminated layers of the radio wave absorbing coating of the present invention are preferably set so that the dielectric constant and thickness of each layer gradually increase from the outermost layer to the innermost layer. This can reduce the reflection of incident radio waves at the interface between each layer.

The plurality of laminated layers of the radio wave absorbing coating of the present invention preferably include ferrimagnetic materials that cooperate with each other to induce ferrimagnetic resonance over a wide frequency band. Thereby, incident radio waves can be absorbed over a wide frequency band.

The plurality of laminated layers of the radio wave absorbing coating of the present invention are preferably set so that the frequency of ferrimagnetic resonance gradually increases from the outermost layer to the innermost layer. This makes it possible to reduce the reflection of incident radio waves at the interface between each layer over a wide frequency band.

The ferrimagnetic material used in the radio wave absorbing coating of the present invention preferably contains ion-substituted hexagonal ferrite. This can cause Ferri magnetic resonance in the microwave frequency band.

The hexagonal ferrite used in the radio wave absorbing coating of the present invention preferably has the general formula BaCo _x Ti _x Fe _12-2x O ₁₉ . Thereby, by changing the value of x, a change in the Ferrimagnetic resonance frequency can be achieved over the most important microwave frequency bands.

The hexagonal ferrite used in the radio wave absorbing coating of the present invention has the general formula SrO _x Al ₂ O ₃ (6-x)
It is preferable to have Fe ₂ O ₃ . Thereby, by changing the value of x, it is possible to achieve a change in the Ferrimagnetic resonance frequency over a high frequency band above the microwave frequency band.

Hereinafter, the present invention will be explained in detail using an embodiment shown in the drawings.

Figure 1 shows the outermost layer on the surface of radio wave reflecting surface 1.
A radio wave absorbing coating 2 is shown consisting of a number n of layers including C ₁ to innermost layer C _o . Each layer from layer C ₁ to layer C _o adjacent to the surface of radio wave reflecting surface 1 consists of a different ferrimagnetic material exhibiting ferrimagnetic resonance at different microwave frequencies. Ferrimagnetic resonance produces increased magnetic permeability and loss tangent, a radio wave absorption property, over a relatively narrow frequency range in each layer. Each layer has a different ferrimagnetic resonance frequency, so that the materials of each layer are selected so that the different layers cooperate as a whole to absorb incident radio waves over a wide frequency band.

Figure 2 shows 5G in the microwave frequency band.
Figure 2 is a plot of the magnetic permeability versus frequency for different materials of layers C ₁ to C _o with ferrimagnetic resonance frequencies ranging from Hz to 9 GHz. Although the maximum values of the ferrimagnetic resonance permeability of each layer are shown to be substantially equal, in reality the maximum values are not equal due to the need for impedance matching.

Returning to Figure 1 again, in order to achieve the desired wide frequency band radio wave absorption characteristics, layer C ₁ to layer C _o
Each layer up _to
It is made from a Ti _x Fe _12-2x O ₁₉ composition, and each layer has a radio wave absorption peak in a different frequency range, that is, a peak at the ferrimagnetic resonance frequency, and these frequency ranges cover the desired radio wave absorption frequency band. They are arranged in a stacked manner. The spacing between the ferrimagnetic resonance frequencies of the different layers is selected depending on the number of layers and the width of the desired frequency band of radio wave absorption; the narrower the frequency band of radio wave absorption, the more The Ferri magnetic resonance frequency spacing approaches.

A number of parameters can be adjusted so that the coating 2 has the best properties in order to obtain maximum radio wave absorption. This includes varying the total thickness of the coating 2 by varying the number n of layers, varying the thickness of each layer, varying the spacing of the ferrimagnetic resonance frequencies of the material of each layer, and This includes varying the amplitude of the peak at the Ferrimagnetic resonance frequency of the real part μ' _r of the material's magnetic permeability μ _r .

In order to minimize reflections from the outer surface of coating 2 and reflections from the interface between successive layers of coating 2, the magnetic permeability μ _r and permittivity _{ε r} of the outermost layer C ₁ and the surrounding medium are
The ratio μ' _r of the real part of should be made as close as possible, and likewise the μ' _r /ε' _r ratio of the adjacent layers of material at each interface should be made as close as possible. Also, loss tangent ratio
It is necessary to adapt tanδ〓/tanδ〓 in the same way.

Materials used in the present invention, namely Co, Ti
- The substituted barium ferrite has a real and imaginary part of the magnetic permeability μ _r measured at the Ferrimagnetic resonance frequency of the order μ′ _r = 4, μ″ _r = 1, and a real permittivity ε _{r .} The part and the imaginary part are of the order of ε′ _r =10, ε″ _r =0.5, and are substantially frequency independent, at least within the frequency range of interest.

If the surrounding medium is air, perfect impedance matching at the outer surface of the coating 2 is achieved by making the μ′ _r /ε′ _r ratio of the layer C ₁ equal to 1 and the magnetic loss tangent tanδ therein. This is possible only by making 〓 substantially equal to the dielectric loss tangent tan δ〓, but this cannot be easily achieved with the material of this example in practice. However, low magnetic permeability μr
Impedance discontinuities at the interface can be achieved by diluting the material of layer _C1 using a suitable binder such as polyvinyl alcohol or epoxy resin with real part μ′ _r of and real part ε′ _r of dielectric constant εr. It is possible to substantially reduce the size of.

The proportion of binder in each layer is then shifted towards layer C _o in order to reduce the magnitude of the impedance discontinuities at the interface between layers C ₁ and C ₂ and between each of the remaining layers. By progressively reducing, the real part μ' _{r of the magnetic permeability μ r and} the real part ε′ _r of the dielectric constant ε _r of each layer can be gradually and progressively increased toward the layer _Co.

A gradual change in impedance is thereby achieved.

In order to achieve a gradual impedance change on the one hand and to maximize the degree of absorption for a given total thickness of the coating 2, the thickness of the outer layer is reduced by the thickness of the relatively better absorbing inner layer. This is achieved by progressively increasing the thickness of the layers from layer C ₁ to layer C ₅ as shown in FIG. 3, where it is advantageous to be thinner than the thickness. Typical values in the case of a five-layer coating include a layer C ₁ having a thickness of 1 mm, increasing in successive layers by 1 mm, and a layer C ₅ having a thickness of 5 mm.

Figure 4 shows the reflection loss characteristics of coating 2 with respect to frequency (the ratio of the amplitude of the reflected radio wave to the amplitude of the incident radio wave expressed in %) as a solid line.
Layer C ₁ to Layer respectively for the frequency range ~15GHz
The real part μ′ _r of the magnetic permeability μ _r of C ₅ is shown by broken lines 1 to 5.

The reflection loss characteristics in Fig. 4 show that the value of the real part ε' _r of the dielectric constant ε _r is 3 for layer C ₁ , and 6, 9, 12, and 15 for the remaining layers C ₂ to C ₅ , respectively. and is based on a material in which the imaginary part ε'' _r of the dielectric constant ε _r of each layer is equal to ε' _r /10, that is, one-tenth of the real part ε _{' r of} the dielectric constant ε r.Magnetic permeability μ The imaginary part μ″ _r of _r has a frequency characteristic similar to that of the real part μ′ _r of the magnetic permeability μ _r , except that the frequency is shifted upward by 1 GHz, i.e., to determine the return loss characteristics. μ″ _r (fGHz)=μ′ _r ((f−1)G
Hz).

The calculation results shown in Figure 4 are based on the layer thickness,
The number of layers, the dielectric and magnetic properties of each layer, and the spacing of the Ferrimagnetic resonance frequencies of each layer (in this example
1GHz was chosen). For a given total coating 2 thickness using a material with given properties,
Mathematical optimization methods may be used in which one or more of the above parameters are varied to obtain a frequency band width of minimum reflection and maximum radio wave absorption.

The values of permeability μ _r quoted for the ferritic materials above refer to the values measured for unsintered materials, but for sintered materials also for the peak amplitude at the ferrimagnetic resonance frequency: Higher values are also obtained for the peak amplitude at non-Ferimagnetic resonance frequencies.

The coating 2 can be provided on the radio wave reflecting surface 1 by suitable means.

For example, each layer can be formed as a separate flexible sheet by mixing a suitable magnetic compound with a suitable binder, such as a rubber matrix, and then a suitable adhesive can be used to bond the successive layers to a radio-reflecting layer. Adhere to the surface of side 1. If the layers are relatively thin, the radio-absorbing coating can also be provided by painting, in which case a suitable magnetic compound is first mixed with a suitable carrier liquid.

In this example, the coating 2 is in the form of a multilayer structure consisting of a plurality of different layers with different Ferrimagnetic resonance frequencies, but this need not necessarily be the case. The reason for using a multilayer structure is due to the need to minimize the impedance mismatch between the barium ferrite material used and the surrounding medium and also to achieve a gradual impedance change across the coating 2. do. However, if the dielectric and magnetic properties of the materials used are such that they form an acceptable impedance match with the surrounding medium, the coating 2 can be offset over the desired frequency band of radio wave absorption. It may also be a single layer consisting of a homogeneous mixture of several materials with different ferrimagnetic resonance frequencies that overlap each other. In some cases, a combination of multiple layers and uniform single layers in one coating 2 may be used.

Furthermore, although not taken into account in the results of FIG. 4, there is quarter-wave absorption in order to maximize the absorption of coating 2.

If the effective thickness of coating 2 at a particular frequency is an odd multiple of a quarter wavelength, then
The incident radio waves reflected by the outer surface of the coating 2 and the incident radio waves reflected by the coated surface, that is, the radio wave reflecting surface 1, destructively interfere with each other,
This improves the absorption properties of the coating 2 at this frequency.

Furthermore, if this condition is met for a large number of different frequencies in the frequency band of radio wave absorption in question by appropriate selection of the material of the coating 2, then the A fractional wavelength absorption is achieved.

Layer C ₁ to Layer C _o of the multilayer coating in Figure 1
Assume that _the thicknesses up _to t are _{t 1 ,} t _{2 ,} .

If the magnetic and dielectric properties of the material of the i-th layer at frequency f are μ _ri (f) and ε _ri (f), respectively, i.e. the values of complex numbers, then the electrically effective properties of the i-th layer at frequency f are The thickness is t _i・Re√ _ri (f)・_ri (f).

The coating 2 acts as a quarter-wavelength absorber when its total electrically effective thickness corresponds to a quarter-wavelength in free space, i.e. _o 〓 ⁱ⁼¹ t _i・When Re√ _ri (f)・_ri (f)=300/4f. In the above equation, it is assumed that the frequency f is in GHz, the thickness t _i is in mm, and the amplitude of the Ferri magnetic resonance peak is the same for all materials.

If no binder is used to dilute the magnetic properties of layer C ₁ of coating 2, the value of the dielectric constant ε _ri (f) of each layer of the hexagonal ferrite material used above is approximately ε′ It can be assumed to be equal to _r and independent of frequency. Therefore, the above equation becomes: _o 〓 ⁱ⁼¹ t _i Re√ _ri (f)=300/4f√′ _r To determine the thickness t _i of each layer of coating 2, this equation holds. Write out this equation n times for each of n different frequencies, and write out t ₁ , t ₂ ... t _o
This is done by solving these n independent equations to determine the value for . The magnetic permeability μ _ri (f) of each layer of the coating 2 differs for each of the n frequencies and has a maximum value at a specific ferrimagnetic resonance frequency, so the solutions of these equations for different frequencies are can get.

If the n different frequencies are substantially evenly distributed over the range of the frequency band of desired radio wave absorption, the coating 2 is substantially equal to or less than a quarter wavelength over the desired frequency band. absorption will occur.

The selection of the ferrimagnetic resonance frequency for the material of each layer and the respective thickness of the layers is determined by the following construction steps to obtain a specific frequency band of radio wave absorption ranging from the lowest frequency F _L to the highest frequency F _H (GHz). You can use it to decide.

The arrangement of each layer from layer C _{1 to} layer Co of coating 2 is preferably done to increase the ferrimagnetic resonance frequency, and using the graph of the real part μ′ _r of magnetic permeability μ _r versus frequency, the layer The Ferrimagnetic resonance frequency F _o of C _o is such that the real part μ′ _r of its magnetic permeability μ _r has a minimum value (i.e., for the material used) at the frequency F _H within the frequency band of desired radio wave absorption. It is preferable to choose to descend to 1).

This is because the quarter wavelength in free space at frequency _FH , ie, λ/4, has the minimum value.

The practical increase in λ/4 in free space as the frequency decreases from frequency F _H to frequency F _o is due to the layer
As the magnetic permeability of C _o increases as frequency F _o is approached, this is compensated, at least in part, by a corresponding increase in the electrically effective thickness of layer C _o .

After selecting the value of frequency F _o , the remaining (n-1 pieces)
The ferrimagnetic resonance frequency of each layer of is selected based on the following relationship: (F _o −F _o-1 )＞(F _o-1 −F _o-2 ＞……(F ₂ −F ₁ ) then Write the equation _o 〓 ⁱ⁼¹ t _i・Re√ _ri (f)=300/4f√′ _r for each of these frequencies, insert an appropriate value into μ _ri , and solve the resulting equation. The thickness of each layer is determined by determining the thickness of each layer.If the thickness values obtained by such a method are all positive, the configuration is workable.

However, if any of these values are negative, choose a lower value of n and continue this process until you get a solution where all thickness values from t ₁ to t _o are positive. repeat.

Since the quarter-wave absorbing coating relies on destructive interference between the waves reflected by the surface of the coating 2 and the waves reflected by the coated surface, i.e. the radio wave reflective surface 1, the layer There is no need to eliminate the impedance mismatch between the outer surface of the coating 2 and the surrounding medium by diluting the dielectric properties of C ₁ .

In fact, the effectiveness of the quarter-wave absorption of coating 2 depends on the relative magnitude of destructively interfering waves at any given frequency, whereas it is this mismatch that determines this magnitude. is the magnitude of , and is the absorption loss that occurs within the coating material at this frequency. In this way, mainly 4
When constructing a radio wave absorbing coating that aims to take advantage of the 1/4 wavelength absorption effect, the 1/4 wavelength absorption of Coating 2 is determined by the magnitude of the impedance mismatch on the surface of Coating 2. The relationship between the destructively interfering waves reflected from the surface of the coating 2 and the coated surface, i.e. the radio wave reflecting surface 1, at different frequencies selected in relation to the quarter wavelength interference condition is satisfied. Optimization can be achieved by selecting such that the target sizes are approximately equal. However, if it is desired to maximize absorption due to absorption losses within the coating material, it may not be possible to simultaneously optimize quarter-wave absorption in this manner.

Furthermore, quarter-wave absorbing coatings do not need to have a multilayer structure, and can be used at many different frequencies distributed over the desired frequency band of radio wave absorption to meet the quarter-wave conditions. A single wavelength absorbing coating can also be provided by a homogeneous mixture of magnetic materials having different Ferrimagnetic resonance frequencies.

Furthermore, the invention is not limited to the use of the particular magnetic materials described above.

Any suitable material exhibiting narrow band ferrimagnetic resonance resulting in increased radio wave absorption properties in the desired frequency band may be used.
The coating materials need not be selected from the same group or homologous magnetic compounds; the exemplary M-type hexagonal barium ferrites for different layers may be selected from different types of materials within the scope of the invention.

In the above description, the particular hexagonal ferrites shown are Co, Ti, -substituted BaFe ₁₂ O ₁₉
It is. A class of substituted compounds suitable for use in radio wave absorption in the frequency band above 20 GHz is, for example, SrO.
_{xAl2O3 .} (6-x) _Fe2O3 , and in this case, by changing x from 0 to 1.35 _, the frequency band of radio wave absorption changes from 58GHz to 111GHz. This class of compounds is particularly suitable for use in the frequency band surrounding the atmospheric window at 90 GHz. Absorption centered at 90 GHz is achieved by setting x=1.07 in the above equation.

A coating 2 using the latter compound can be made into a multilayer coating, for example, using the same configuration as described for the above-mentioned materials. The thickness of a quarter-wave coating using the latter compound at frequencies around 90 GHz is approximately 0.2 mm.

This thickness of coating 2 may be applied in the form of a paint by brushing or spraying.

Both of the above materials are uniaxial hexagonal ferrites, but planar hexagonal ferrites may also be used.

The coating 2 may thus consist of a number of different compounds of the same basic type, each with a different degree of ionic substitution.

When radio waves pass through each layer of coating 2,
In order to effectively absorb radio waves, the coating material must have fairly high values such as the real part μ′ _r of the magnetic permeability μ _r , the real part ε′ _r of the permittivity ε _r , and the magnetic and dielectric loss tangent as described above.
It must have tanδ〓, tanδ〓. This condition is necessary to minimize the thickness of the coating material.

In the most important frequency ranges, i.e. in the microwave frequency band 1-20 GHz and above, the real part μ′ _r of the permeability μ _r of most ferrimagnetic materials, e.g. spinel ferrites, cannot be used as absorbers. reduced to a degree that is impractical. However, certain materials exhibit an increased real part μ′ _r of permeability μ _r in the microwave range, and these materials include hexagonal ferrite and NiMnO ₃ and
Ilmenite type like CoMnO ₃ (general formula
There are ABO ₃ ) materials.

The real and imaginary parts μ′ _r of the magnetic permeability μ _r of such materials,
The value of μ″ _r increases in a narrow frequency range due to the Ferrimagnetic resonance effect. This can be explained qualitatively by considering the microwave incident on a Ferrimagnet under the influence of a magnetic field. The latter Causes precession due to the atomic moments (sum of spin and orbital moments).

It was found that this precession frequency can be varied by changing the static magnetic field in the material, and that the strongest absorption (maximum permeability) occurs when the precession frequency coincides with the microwave frequency. . Magnetic anisotropy also produces a torque on the spin system if it is oriented in a direction other than the direction in which it is easily magnetized, and thus the Ferrimagnetic resonance frequency depends on the magnetic anisotropy of the material.

Hexagonal ferrite materials are particularly suitable for this application because they have relatively high magnetic anisotropy in the frequency band of radio wave absorption of interest.

In particular, so-called uniaxial hexagonal ferrites, especially those in oriented polycrystalline form, exhibit Ferrimagnetic resonance in the millimeter wave range. A suitable material of this type is an M-type hexagonal ferrite such as barium ferrite (BaFe ₁₂ O ₁₉ ).

As mentioned above, the ferrimagnetic resonance frequency at which radio wave absorption occurs can be changed by changing the magnetic anisotropy of the material, which in the case of M-type barium ferrite is caused by iron (Fe ³⁺ ) ions in the crystal lattice. This is done by replacing .

Unsubstituted polycrystalline BaFe ₁₂ O ₁₉ has a Ferri magnetic resonance at about 50 GHz, but the iron ion can be removed from two sources such as Co ²⁺ Ti ⁴⁺ , Ni ²⁺ Ti ⁴⁺ or Zn ²⁺ Ti ⁴⁺ By substitution by pairs consisting of valent and tetravalent ions, the magnetic anisotropy and thus the Ferrimagnetic resonance frequency can be changed according to the degree of substitution, the greater the degree of substitution the lower the Ferrimagnetic resonance frequency. . Other forms of ion substitution, e.g. iron ions with other 3
Replacement with valent ions, or replacement of iron ions with divalent ions such as Co ²⁺ and replacement of the same number of divalent oxygen ions with monovalent ions such as fluorides to maintain electrical neutrality. It may also be replaced by

In the case of CO ²⁺ Ti ⁴⁺ -substituted BaFe ₁₂ O ₁₉ , the general formula of the substituted compound is BaCo _x Ti _x Fe _12-2x O ₁₉ , and by varying the value of x the most important microwave frequency range can be adjusted. ,
That is, variations in the Ferri magnetic resonance frequency over the entire range of 1 to 20 GHz can be achieved.

Different compositions can be made by the following methods.
The stoichiometric distribution of powdered BaCO ₃ , CO ₃ O ₄ , TiO ₂ and Fe ₂ O ₃ is determined gravimetrically for each composition.

The above powder is homogeneously mixed in water for about 2 hours in a wet whole mill using stainless steel balls, and then the powder is poured from the mill through a coarse sieve.

This slurry is then allowed to settle, excess water is drained off and the powder is dried to approximately 100°C. The dried powder was then passed through a 150 μm sieve to act as a binder.
Add 5% by weight of water dropwise. Next, add 1000 materials
Press at Kgcm ^-2 and dry at 90 °C, then react at 1300 °C for 10 h using oxygen bleed.

The reaction formula is as follows: BaCO ₃ +xTiO ₂ +x/3CO ₃ O ₄ + (6-x)Fe ₂ O ₃ 1300℃ ---→ BaCoxTi _x Fe _12-2x O ₁₉ +CO ₂ +x/6O ₂ after cooling The reacted material is crushed, milled in an edge follower for 10 minutes, and wet milled in water for 4 hours. Then drain off the excess water and dry the powder at 90°C. Polyvinyl alcohol (PVA) is then added to the powder as a solution and milled in using an edge fluorore, typically to a composition of 5% by weight. However, for reasons discussed below, namely impedance matching, the proportions of various suitable binders with low permittivity and magnetic permeability can be adjusted to vary the magnetic permeability and dielectric constant of the coating layer.

[Brief explanation of drawings]

1 is a schematic cross-sectional perspective view of an n-layer radio wave absorbing coating according to the present invention; FIG. 2 is a graph showing changes in magnetic permeability characteristics with respect to frequency for typical materials for various layers in the coating of FIG. 1;
The figure is a cross-sectional view of the five-layer radio wave absorbing coating according to the present invention, and FIG. 4 is the change in the characteristic of reflection loss with respect to frequency (solid line) of the coating illustrated in FIG. 2 is a graph showing the frequency change (dashed line) of the magnetic permeability of the material of the layer; C ₁ , C ₂ , ...C _o ... layer, 1 ... radio wave reflective surface, 2
...Coating.

Claims (1)

[Scope of Claims] 1. A device comprising a plurality of laminated layers having different substantial magnetic permeabilities for absorbing radio waves in a predetermined frequency band, and each of the layers having a different substantial magnetic permeability for absorbing radio waves in a predetermined frequency band. 1. A radio wave absorbing coating characterized in that the coating is arranged in such a manner that it gradually increases from the outermost layer where radio waves are incident to the innermost layer. 2. The radio wave absorbing coating according to claim 1, wherein the outermost layer has a substantial magnetic permeability that matches the impedance between the layer and the surrounding medium through which the radio waves to be absorbed have propagated. . 3. The thickness of the layer may cause interference between the radio wave to be absorbed and the radio wave transmitted through the layer and reflected by at least one of the layers, in order to eliminate reflection of the radio wave to be absorbed. The radio wave absorbing coating according to claim 1 or 2, which is set as follows. 4. The layer according to any one of claims 1 to 3, wherein the layers are arranged such that the dielectric constant and thickness of each layer gradually increase from the outermost layer to the innermost layer. Radio wave absorbing coating as described. 5. The layers include different ferrimagnetic materials that cooperate with each other to induce ferrimagnetic resonance over the frequency band.
The radio wave absorbing coating according to any one of Items 1 to 4. 6. The layers are arranged such that the frequency of the Ferrimagnetic resonance gradually increases from the outermost layer to the innermost layer.
The radio wave absorbing coating described in section. 7. The radio wave absorbing coating according to claim 4, wherein the ferrimagnetic material includes ion-substituted hexagonal ferrite. 8 The hexagonal ferrite has the general formula BaCo _x
Radio wave absorbing coating according to claim 6, comprising Ti _x Fe _12-2x O ₁₉ . 9 The hexagonal ferrite has the general formula SrO _x
The radio wave absorbing coating according to claim 6, comprising Al ₂ O ₃ (6-x)Fe ₂ O ₃ .