JPH0225280B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a radio wave absorbing coating for reducing or preventing reflection of incident radio waves from an antenna.

The profile of an antenna as a radio wave reflecting structure that may be applied in radar camouflage can be reduced by reducing the physical dimensions or reflectance of the reflecting surface of the antenna.

The physical length of a monopole or dipole antenna with a given electrically effective length is reduced considerably by radio-absorbing coatings using materials with both dielectric and magnetic properties, such as ferrite materials. be able to. This type of material is discussed in J.R. James and A. Henderson, "Electrically short monopole antennas with dielectric or ferrite coatings," Proc.
Discussed in IEEE, Vol. 125, No. 9, September 1978, pp. 793-803.

By using such a radio wave absorbing coating, the length of the antenna can be reduced by a factor √′ _r ′ _r . Here, μ′ _r and ε′ _r are the real parts of the magnetic permeability μ _r and the permittivity ε _r , and in general, the magnetic permeability μ _r = μ′ _r −
jμ″ _r and dielectric constant ε _r =ε′ _r −jε″ _r are complex quantities.
The higher the values of the real part μ′ _r of the magnetic permeability μ _r and the real part ε′ _r of the dielectric constant ε _r , the larger the reduction coefficient can be obtained. However, in order to minimize losses in the radio wave absorbing coating, the imaginary part μ″ _r of the magnetic permeability μ r and the imaginary part ε″ _r of the dielectric constant ε _r of the radio wave absorbing coating at the operating frequency of the _antenna must be adjusted as much as possible. It needs to be made smaller. This is because the magnetic loss tangent and dielectric loss tangent are tanδ〓=μ″ _r /μ′ _r and tanδ〓=ε″ _r /
This is because it can be expressed by ε′ _r .

The reflectance of the radio wave reflective surface of the antenna can be reduced by coating the radio wave reflective surface with a radio wave absorbing coating.

Radio-absorbing coatings are necessary to make the antenna "invisible" to radar measurements.

Conventional radio-absorbing coatings operate on the principle of destructive interference between radio waves reflected from the surface of the radio-absorbing coating and radio waves reflected from the radio-reflecting surface coated with the radio-absorbing coating. , has a thickness corresponding to a quarter wavelength of the incident radio wave at the frequency where absorption is required.

For normal incidence, the optical path difference between the resulting reflected wave from the surface of the radio-absorbing coating and the reflected wave from the surface of the coated radio-reflecting surface is one-half of a wavelength at the surface of the radio-absorbing coating. and when the amplitudes of the two reflected waves are substantially equal, the two reflected waves cancel each other out. This state also occurs when the optical path difference is an odd multiple of 1/2 wavelength.

It is also known to perform radio wave absorbing coatings using coating materials with high magnetic permeability μ _r , permittivity ε _r , and high magnetic and dielectric loss tangents tan δ〓, tan δ〓, such as ferrite, in the desired absorption frequency band. It is being However, in high frequency bands above 1 GHz, the magnetic permeability μ _r and magnetic loss tangent tan δ of most ferrite materials are not high enough to be used as effective absorbers.

Conventional radio wave absorbing coatings are effective only in low frequency bands and are less effective for obliquely incident radio waves due to scattering and the collapse of destructive interference conditions.

It is an object of the present invention to substantially reduce or eliminate the reflection of incident radio waves from an antenna in one or more frequency bands above 1 GHz, and to
(1~30MHz), VHF (30~300MHz), or UHF
(0.3 to 1.0 GHz) band and the antenna's physical dimensions for a given operating frequency or frequency band. An object of the present invention is to provide a radio wave absorbing coating that can be effectively reduced in size.

According to the invention, the said object of the invention is to
The layer includes a plurality of laminated layers having mutually different effective magnetic permeabilities that do not absorb radio waves in a frequency band below 1 GHz but absorb radio waves in a frequency band above 1 GHz, and the layers each have a different effective magnetic permeability. This is achieved by a radio wave absorbing coating characterized in that the radio waves to be absorbed are arranged so as to 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. Because it is arranged so that it gradually increases from can reduce or eliminate reflections of incident radio waves in the frequency band.

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 plurality of laminated layers of the radio wave absorbing coating of the present invention are 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. good.
This makes it possible to 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 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 results in 1GHz
Ferrimagnetic resonance can occur in the above 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, it is possible to change the Ferri magnetic resonance frequency in a frequency band of 1 GHz or more.

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 change the Ferri magnetic resonance frequency in a frequency band of 1 GHz or more.

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

FIG. 1 shows a quarter-wave monopole antenna adapted for operation in the HF, VHF or UHF frequency bands. The antenna consists of a conductive rod 1, one end of the rod 1 is open circuit, the other end is connected to the central electrode 3 of a coaxial connector 4, the ground conductor 5 of the connector 4 is attached with a monopole antenna. It is connected to a ground plane 6 (not shown).

The surface of the rod 1 is coated with a radio wave absorbing coating 2 to reduce the reflection of incoming radio waves at a desired frequency or frequency band in the radar frequency band (above 1 GHz) without substantially affecting its operation as an antenna. be done. The coating 2 consists of one or more ferrimagnetic materials, each of which exhibits a ferrimagnetic resonance effect producing an increased magnetic permeability μ _r in a narrow frequency bandwidth, thereby producing a desired absorption frequency or frequency band. However, it exhibits low loss characteristics at the antenna operating frequency.

Coating 2 can also reduce the physical length of the antenna for a given operating frequency.

As mentioned above, the amount of energy absorbed by the coating 2 in a particular frequency band is determined by the real part μ′ _r of the high value of magnetic permeability μ _r in the desired absorption frequency band, the magnetic loss tangent tanδ〓, and the dielectric constant ε _r Real part ε′ _r
and dielectric loss tangent tanδ〓. however,
A low or zero value of the magnetic and dielectric loss tangent tan δ〓, tan δ〓 is required at the operating frequency of the antenna.

Also, by coating the antenna with a material that has both dielectric and magnetic properties, the physical length of the antenna for the same frequency is reduced by a factor of √′ _r with respect to the length without the coating 2. ′ can be shortened to _r . The thickness reduction that can be achieved will depend on the values of the real part of the magnetic permeability μ _r and the real part of the permittivity ε _r μ′ _r , ε′ _r at the operating frequency of the antenna, and to obtain the maximum performance of the antenna. is the real part μ′ _r of the magnetic permeability μ _r at the operating frequency.
It is ideal that the values of the real part ε′ _r of the dielectric constant ε _r be as close as possible.

Another factor that affects the performance of the coating 2 as a radio wave absorber is the amount of energy of the incident radio wave in the desired absorption frequency band that passes through the interface between the coating 2 and the surrounding medium and enters the coating 2. It is quantity. In order to minimize the amount of incident radio wave energy reflected at this interface, it is necessary to minimize the impedance mismatch between the coating surface and the surrounding medium. To this end, the ratio of the real part μ′ _r of the material's magnetic permeability μ _r and the real part ε′ _r of the permittivity ε _r at this interface in the desired absorption frequency band is determined by the ratio of the magnetic permeability μ _r of the surrounding medium The real part μ′ _r
and the real part ε' _r of the dielectric constant ε _r . That is, when the medium is air, μ′ _r /
It is necessary to make ε′ _r as close to 1 as possible together with tanδ〓tanδ〓. However, if the coating 2 is used as a quarter-wave absorber, it may be preferable to allow some reflection to occur at this interface, which is due to the ferrimagnetic nature in the desired absorption frequency band. 1 showing resonance effect
This is possible by using a coating 2 containing one or more ferrimagnetic materials of hexagonal ferrite and ilmenite type (general formula
ABO ₃ ) materials such as NiMnO ₃ and CoMnO ₃ are suitable.

The permeability μ _r of most ferrite materials, such as spinel ferrite, decreases to such an extent that they cannot act as radar absorbers at frequencies above 1 GHz, whereas the permeability μ _r of some of the aforementioned materials decreases to the extent that they cannot act as radar absorbers at frequencies above 1 GHz. Increases in a narrow frequency range above 1 GHz due to magnetic resonance effects. Other frequencies, e.g. HF, VHF
or in UHF, such materials have relatively high values of the real part μ′ _r of the magnetic permeability μ _r and the real part ε′ _r of the permittivity ε _r
and the imaginary part μ″ _r of the magnetic permeability μ _r and the imaginary part ε″ _r of the permittivity ε _r with low or zero values, which gives HF,
A substantial reduction in length or thickness is obtained with no substantial loss at VHF or UHF frequencies.

The Ferrimagnetic resonance effect, which gives rise to the value of the real part μ′ _r of the magnetic permeability μ _r at the operating frequency of the antenna, is explained by considering the microwaves entering a Ferrimagnetic material under the influence of a magnetic field. The magnetic field causes precession due to atomic moments (sum of spin and orbital moments).

This precession frequency is varied by changing the static magnetic field in the material, and the strongest absorption, ie the highest permeability μ _r, occurs when the precession frequency coincides with the microwave frequency. Magnetic anisotropy also causes a torque in the spin system when it is oriented in a direction other than the direction in which it is easily magnetized, so the Ferrimagnetic resonance frequency depends on the magnetic anisotropy of the material. Become.

Hexagonal ferrite materials are particularly suitable because they have relatively high magnetic anisotropy at the frequencies of interest. Among them, uniaxial hexagonal ferrites, especially those in the oriented polycrystalline form, exhibit ferrimagnetic resonance in the _{millimeter wave} range; It is a type of hexagonal ferrite.

As mentioned above, the Ferrimagnetic resonance frequency at which radio wave absorption occurs can be changed by changing the magnetic anisotropy of the material, and in the case of M-type barium ferrite materials, iron ions (Fe ³⁺ )
This can be achieved by replacing .

Unsubstituted polycrystalline BaFe ₁₂ O ₁₉ has a Ferrimagnetic resonance of about 50 GHz. However, iron ions
By substituting with divalent or tetravalent ion pairs such as Co ²⁺ Ti ⁴⁺ , Ni ²⁺ Ti ⁴⁺ or Zn ²⁺ Ti ⁴⁺ , changes in magnetic anisotropy and the associated Ferrimagnetic resonance frequency can be achieved. A change in is obtained according to the degree of substitution; the greater the degree of substitution, the lower the Ferrimagnetic resonance frequency.

Other forms of ion substitution are also possible, such as replacing iron ions with trivalent ions, or replacing iron ions with divalent ions such as Co ²⁺ and maintaining electrical neutrality. It is also possible to replace with the same number of divalent oxygen ions and monovalent ions such as fluoride at the same time.

For BaFe ₁₂ O ₁₉ substituted by Co ²⁺ Ti ⁴⁺ , the general formula of the substituted compound is BaCo _x Ti _x Fe _12-2x O ₁₉
By changing the value of x, the Ferrimagnetic resonance frequency can be changed over the entire micro frequency range, that is, from 1 to 20 GHz.

FIG. 2 shows one preferred form of coating 2 in which different coating materials are arranged in each layer from the outermost layer C _{1 to} the innermost layer Co to provide a multilayer coating.

Figure 3 shows a typical hexagonal ferrite material exhibiting approximately 15 GHz Ferri magnetic resonance between 10 and 100 GHz.
The real part μ′ _r of the magnetic permeability μ _r for frequencies in the range of
It is a graph showing changes in magnetic loss tangent tanδ〓.

Returning again to Figure 2, each layer from layer C ₁ to layer C _o is composed of BaCo _x Ti _x Fe _12-2x with different substitutions in order to obtain the desired wide frequency bandwidth absorption characteristics.
Composed of O ₁₉ composition, each layer has radio wave absorption peaks in different frequency ranges, and these radio wave absorption ranges are stacked and arranged to cover a desired absorption frequency band.

The spacing between the ferrimagnetic resonance frequencies of different layers is selected depending on the number of layers and the width of the desired absorption frequency band, the narrower the absorption frequency band, the lower the ferrimagnetic resonance frequency in a given number of layers. The distance becomes closer.

Figure 4 shows layer C ₁ in the microwave frequency band.
Fig. 3 is a plot of the real part μ' _r of the magnetic permeability μ _r of different layer materials in an n-layer coating with ferrimagnetic resonance frequencies ranging from 5 GHz to 9 GHz, ranging from to layer C _o , versus frequency. Although the magnitude of the real part μ′ _r of the magnetic permeability μ _r at each Ferrimagnetic resonance frequency is shown to be equal for each layer, they are not substantially equal due to impedance matching.

In a multilayer configuration, a number of parameters can be adjusted to optimize the properties of the coating 2 in order to obtain maximum radio wave absorption. The parameters include
Varying the number of layers to vary the overall thickness of the coating 2, varying the thickness of each layer, varying the spacing of the ferrimagnetic resonance frequencies of the material of each layer, and changing the magnetic permeability μ _r of the material of each layer. This involves changing the amplitude of the peak at the Fereri magnetic resonance frequency of the real part μ' _r of .

In order to minimize reflections from the outer surface of the coating 2 and reflections from the interface between successive layers of the coating 2, the magnetic permeability μ _r and permittivity _{ε r} of the layer C ₁ and the surrounding medium are
The ratio μ′ _r /ε′ _r of the real parts of the material should be made as close as possible, and similarly the μ′ _r /ε′ _r ratio of the adjacent layers of material at each interface should be matched as closely as possible. There is.
It is also necessary to connect the loss tangent ratio tan δ/tan δ in the same way.

Materials used in the present invention, namely Co, Ti
- Substituted barium ferrite has real and imaginary parts of magnetic permeability μ _r measured at the Ferrimagnetic resonance frequency of μ′ _r =4, μ″ _r =1.0, and real and imaginary parts of permittivity ε _r and The imaginary part is of the order of ε′ _r =10, ε″ _r =0.5, and is substantially independent of frequency, at least in the absorption frequency band of interest.

If the surrounding medium is air, coating 2
A perfect impedance matching at the outer surface of is only possible by making the μ′ _r /ε′ _r ratio of layer C ₁ equal to 1 and tan δ〓tan δ〓, In practice, this cannot be easily achieved with the materials of this example. However, diluting the material of layer C ₁ using a suitable binder such as polyvinyl alcohol or epoxy resin with a low real part of permeability μ _r and real part of permittivity ε _r μ′ _r , ε′ _r By this, it is possible to substantially reduce the magnitude of the impedance discontinuity at the interface.

The proportion of binder in each layer is then progressively increased towards layer C _o 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 reducing the
The magnetic permeability μ _r and permittivity ε _r of the successive layers can be increased progressively towards 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 radio wave absorption for a given total thickness of the coating 2, the thickness of the outer layer is reduced by the relatively better absorbing inner layer. It is advantageous to be thinner than the layer thickness, which is achieved by progressively increasing the layer thickness from the outermost layer C ₁ to the innermost layer C ₅ as shown in FIG.

Typical values in the case of a five-layer coating are that the thickness of layer C ₁ is 1 mm, increasing by 1 mm in subsequent layers, and the thickness of layer C ₅ is 5 mm.

Figure 6 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.
The real part μ′ _r of the magnetic permeability μ _r of layers C ₁ to C ₅ is expressed by the dashed line 1 to 5.
This is shown in . Figure 6 shows frequency band 1~
15 GHz, and the reflection loss characteristics show that the real part ε' _{r of the dielectric constant ε r is} 3 for layer C ₁ , and 6, 9, 12, and 15 for layers C ₂ to C ₅ . It 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 . To determine the reflection loss characteristics, the imaginary part μ″ _r of the magnetic permeability μ _r
has a frequency characteristic similar to that of the real part μ′ _r of the magnetic permeability μ _r , but with the frequency shifted upward by 1 GHz, μ″ _r (fGHz) = μ′ _r ((f−1)GHz ).

The results shown in FIG.
Hz is selected). Using materials with given properties,
Using a mathematical optimization method, one or more of the above parameters are varied to obtain a minimum reflection and a maximum radio absorption frequency bandwidth for a given overall coating 2 thickness. Good too.

Considering the normal operation of the antenna at HF, VHF, or UHF frequencies, the values of the imaginary part of the magnetic permeability μ _r and the imaginary parts of the permittivity ε _r μ″ _r , ε″ _r of the aforementioned barium ferrite materials are these becomes negligible in frequency. In particular, for the real part μ' _r of the magnetic permeability μ _r and the real part ε′ _r of the dielectric constant ε _r , which are about 2 and 10, respectively, they can be ignored and the losses are also very small. The relatively high values of the real part of the magnetic permeability μ _r and the real part of the permittivity ε _r , μ′ _r , ε′ _r make the physical length of the antenna a factor.
It can be downsized to 4.5, further reducing the radar profile.

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.

Returning again to FIG. 1, coating 2 may be applied to rod 1 by any 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 the successive layers can be bonded together using a suitable adhesive. Glue to. Alternatively, if the layer is relatively thin, it can also be applied 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 it does not necessarily have to be in the form of a multilayer structure. The reason for using a multilayer structure is 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 throughout the coating 2. to cause. However, if the dielectric and magnetic properties of the materials used are such that they form an acceptable impedance match with the surrounding medium, then the coating 2 will have offset and overlapping differences over the desired absorption frequency range. It may also consist of a single layer consisting of a homogeneous mixture of several materials having magnetic resonance frequencies. In some cases, a combination of multiple and uniform layers in one coating 2 may be used.

Furthermore, although not taken into account in the results shown in Figure 6,
A factor that can be used to maximize the radio wave absorption of coating 2 is the quarter wavelength absorption of coating 2.

If the effective thickness of the coating 2 at a particular frequency is an odd multiple of a quarter wavelength, the incident radio waves reflected by the outer surface of the coating 2 and the reflected waves by the coated surface, i.e. the radio wave reflecting surface 1. They destructively interfere with the incident radio waves and improve the radio wave absorption characteristics of the coating 2 at specific frequencies.

Furthermore, if the above-mentioned conditions are met for a large number of different frequencies of the frequency band of interest, by appropriate selection of the material of the coating 2, the 1
Wavelength absorption is achieved.

Returning to Figure 2, the layers of the multilayer coating
The thicknesses from C ₁ to layer C _o are t ₁ , t ₂ , ... t _o ,
The Ferrimagnetic resonance frequencies of each layer are F ₁ , F ₂ ,...
...Assume that F _o .

If the permittivity, which indicates the magnetic and dielectric properties of the material of the i-th layer at frequency f, is μ _ri (f) and the permittivity is ε _ri (f), then the electrical property of the i-th layer at frequency f is The effective thickness is t _i・Re√ _ri (f)・_ri (f).

The coating 2 acts as a quarter-wave absorber when its total electrically effective thickness corresponds to a quarter-wavelength in free space, i.e. _o 〓 ⁱ⁼¹ t _i・Re When √ _ri (f)・_ri (f)=300/4f.
In the formula, the unit of frequency f is GHz, and the unit of thickness t _i is mm.
It is also assumed that the amplitude of the peak at the Ferrimagnetic resonance frequency 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 equal to its real part. equal to ε′ _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 Write out this equation n times for each of the n different frequencies that hold, and write out t ₁ , t ₂ , ... t _o
This is done by solving these n independent equations to determine the value for . Since the magnetic permeability μ _ri (f) of each layer of the coating 2 differs depending on each of the n frequencies and has a maximum value only at a specific ferrimagnetic resonance frequency, it is difficult to solve these equations for different frequencies. is obtained.

If the n different frequencies are distributed substantially evenly over the desired absorption frequency band, coating 2 will provide substantially quarter-wave absorption over the absorption frequency band. .

The selection of the Ferri resonance frequency for the material of each layer and the respective thickness of the layers is done using the following construction procedure to obtain a specific absorption frequency band ranging from the lowest frequency F _L to the highest frequency F _H (GHz). You may 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 Fn of C _o is such that the real part μ' _r of the magnetic permeability μ _r reaches its minimum value (i.e. 1 for the material used) at the highest frequency F _H in the desired absorption frequency band. Preferably, it is selected to descend. This is one quarter of the incident radio wave in free space at frequency F _H
This is because the thickness of the wavelength, ie, λ/4, has the minimum value. The increase in the practical length of λ/4 in free space as the frequency descends from frequency _F to frequency F is because the magnetic permeability _{μ of} the layer C increases as _it approaches frequency _F , is at least partially compensated for by a corresponding increase in the electrically effective thickness of layer _Co.

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 For each of these frequencies the equation _o 〓 ⁱ⁼¹ t _i・
Write Re√ _ri (f)=300/4f√′ _r , insert an appropriate value into μ _ri (f), and solve the resulting equation to determine the thickness of each layer. If the thickness values obtained by such a method are all positive, the configuration is viable. However, if any of these values is negative, choose the lower value of h and t ₁
Repeat this process until you get a solution where all thickness values from to _to are positive.

Since quarter-wave absorbing coatings rely on destructive interference between the waves reflected by the outer surface of the coating 2 and the waves reflected by the coated surface, i.e. the radio wave reflecting 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 constant ε _r of C ₁ .

In fact, the effectiveness of the quarter-wave absorption of coating 2 depends on the relative magnitude of the destructively interfering waves at any given frequency, whereas it is this mismatch that determines this magnitude. It is the size,
It is also the absorption loss that occurs within the coating material at this frequency. In this way, when configuring a radio wave absorbing coating mainly aimed at utilizing the quarter wavelength absorption effect, the coating 2
The quarter-wave absorption of the coating 2 is determined by selecting the absorption properties of the coating material in relation to the magnitude of the impedance mismatch at the coating surface and at different frequencies where the quarter-wave interference condition is satisfied. Optimization can be achieved by selecting such that the relative magnitudes of the two destructively interfering waves reflected from the radio wave reflecting surface 1 are approximately equal. However, if it is desired to maximize the absorption due to absorption losses within the coating material, the
It may not be possible to simultaneously optimize wavelength absorption.

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

The invention is not limited to the use of the particular magnetic materials described above. Any suitable material exhibiting Ferrimagnetic resonance resulting in enhanced absorption properties in the desired absorption 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 the different layers may be selected from different types of materials within the scope of the present invention. If the coating 2 is required to absorb only in a narrow frequency bandwidth, e.g. a specific frequency, the coating 2 need only contain a single Ferrimagnetic resonance at the specific frequency, and preferably this It may be a quarter wavelength resonance in frequency.

In the foregoing description, certain hexagonal ferrites disclosed are Co, Ti-substituted
_{BaFe12O19 .} A class of substituted compounds suitable for use above 20 GHz is, for example, SrO x Al ₂ O ₃ (6
-x) Fe ₂ O ₃ , in which case the Ferri magnetic resonance frequency changes by varying x from 0 to 1.35.
Varies 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.

Coating 2 using the latter compound can be made into a multilayer coating using the same configuration as described for the above materials. Quarter-wave coating 2 using the latter compound at frequencies around 90 GHz
The thickness will be approximately 0.2mm. This thickness of coating 2 may be applied in the form of a paint by brushing or spraying.

Both the former and latter chemicals are uniaxial hexagonal ferrites, but planar hexagonal ferrites may also be used.

The coating 2 is made of a plurality of different ferrimagnetic materials, each having a ferrimagnetic resonance effect,
The materials have Ferrimagnetic resonances in different narrow frequency bands, and thus can act as a coating that cooperates to provide absorption over an extended frequency band.
All of the various ferrimagnetic coating materials are
It may also consist of compounds of the same basic group with different degrees of ionic substitution.

The coating 2 may consist of a homogeneous mixture of different materials, each having a different ferrimagnetic resonance frequency. However, such a coating 2 may only be suitable if the magnetic and dielectric properties of the constituent materials form a good impedance match with the surrounding medium.

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 powder is mixed as homogeneously as possible in water for about 2 hours in a wet ball mill using stainless steel balls, and then the powder is drained from the mill through a coarse sieve. This slurry is then allowed to settle, excess water is drained off and the powder is dried at approximately 100°C. The dried powder is then passed through a 150 μm sieve and 5% by weight of water is added dropwise to act as a binder. Next, the material is pressed at 1000 Kgcm ^-2 and dried at 90°C, and then reacted at 1300°C for 10 hours while performing oxygen bleed. The reaction formula is as follows: BaCO ₃ + xTiO ₂ + ^x ₃ CO ₃ O ₄ + (6-x)Fe ₂ O ₃ ¹³⁰⁰ ℃BaCo _x Ti _x Fe _12-2x C ₁₉ + CO ₂ + ^x ₆ O ₂ cooling Afterwards, the reacted material is crushed and 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. Then polyvinyl alcohol (PVA) is added to the powder as a solution,
Milling is carried out using an edge fluorore, typically to a composition of 5% by weight. However, the proportions of various suitable binders with low permittivity ε _r and magnetic permeability μ _r can be varied to vary the permeability μ _r and dielectric constant ε _r of each layer of coating 2 for reasons discussed below.

[Brief explanation of drawings]

Figure 1 is a front view of a monopole antenna coated with the radio wave absorbing coating according to the present invention, Figure 2 is a schematic cross-sectional view of the radio wave absorbing coating according to the present invention, and Figure 3 is Ferrimagnetic resonance in the microwave frequency band. Figure 4 shows the changes in the real part μ' _r of the magnetic permeability μ _r and the magnetic loss tangent tanμ _r with respect to the frequency of the ferrimagnetic _material . _FIG . 5 is a cross-sectional view of the five-layer coating according to the present invention, and FIG. 6 is a diagram showing changes in reflection loss characteristics (solid line) with respect to frequency of the coating shown in FIG. 5. 6 shows the variation (dashed line) of the real part μ′ _r of the magnetic permeability μ _r of the materials of the various layers of the coating of FIG. 5 with respect to frequency; FIG. 1... Conductive rod, 2... Radio wave absorbing coating, 3... Center electrode, 4... Connector, 5... Earth conductor, 6... Earth surface.

Claims (1)

[Claims] 1. 1G without absorbing radio waves in the frequency band below 1GHz
The layer includes a plurality of laminated layers each having a different substantial magnetic permeability for absorbing radio waves in a frequency band of Hz or higher, and each of the layers has a different substantial magnetic permeability,
A radio wave absorbing coating characterized in that the radio wave absorption coating is arranged so that the radio waves to be absorbed gradually increase from the outermost layer toward the innermost layer. 2. The radio wave absorbing code according to claim 1, wherein the outermost layer has a substantial magnetic permeability that bridges the impedance between the layer and the surrounding medium through which the radio waves to be absorbed have propagated. Ing. 3. The thickness of the layer is set such that the radio wave to be absorbed and the radio wave to be absorbed are
The radio wave absorbing coating according to claim 1 or 2, which is configured to interfere with radio waves transmitted through the layer and reflected by at least one of the layers. 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 .