EP2898568B1 - Electromagnetic absorber - Google Patents

Description

The present invention relates to an electromagnetic absorber.

The document US 7 826 504 B2 describes an electromagnetic absorber.The document US-2011/0175672 discloses an electromagnetic absorber comprising a set of metal elements disposed on a semiconductor substrate. An electrical control is used to modulate the conductivity of the semiconductor substrate, thereby adjusting the electromagnetic absorption band of the absorbent.

A disadvantage of the electromagnetic absorbent described in this document is that it requires the use of an electrical control, which complicates its manufacture and use.

There is therefore a need for an electromagnetic absorber which is simpler to manufacture and use, and which can be used on shaped surfaces without losing its properties. The present invention improves the situation.

For this purpose, the invention provides an electromagnetic absorber according to claim 1.

Thus, the electromagnetic absorbent according to the invention makes it possible to obtain a passively desired electromagnetic absorption band. As a result, the electromagnetic absorber is simpler to implement.

According to embodiments of the invention, an elementary pattern comprising a plurality of resonant elements of different dimensions is periodically repeated on the insulating dielectric substrate.

A resonant element may for example have a square, rectangular, polygonal or circular shape.

The thickness of the insulating dielectric substrate can be determined according to an electromagnetic resonance frequency of the predicted electromagnetic absorption band and / or a desired absorption level.

où :

• f_r désigne la fréquence de résonance électromagnétique d'ordre zéro de l'élément résonant,
• c ₀ désigne la vitesse de la lumière dans le vide,
• µ _r désigne la perméabilité relative du substrat diélectrique,
• ε_r désigne la permittivité relative du substrat diélectrique, et
• L' désigne la longueur d'un côté de l'élément résonant.

The electromagnetic resonance frequency of a square-shaped resonant element can be adjusted by adapting the length of one side of the resonant element so that: $$f_r=\frac{{\mathrm{vs}}_0}{2\mathrm{The}\text{'}\sqrt{{\mathrm{\mu }}_r{\epsilon }_r}}\pm 5\%$$

or :

• f _r denotes the zero-order electromagnetic resonance frequency of the resonant element,
• c ₀ denotes the speed of light in a vacuum,
• μ _r denotes the relative permeability of the dielectric substrate,
• ε _r denotes the relative permittivity of the dielectric substrate, and
• The designates the length of one side of the resonant element.

où :

• f⁽⁰⁾ désigne la fréquence de résonance électromagnétique d'ordre zéro de l'élément résonant,
• a désigne le rayon de l'élément résonant,
• c ₀ désigne la vitesse de la lumière dans le vide,
• z ₀ = 1,841 désigne le premier maximum de la fonction de Bessel J ₁(z) d'ordre 1,
• µ _r désigne la perméabilité relative du substrat diélectrique,
• ε_r désigne la permittivité relative du substrat diélectrique,
• µ = µ_rµ₀
• ε = ε_rε₀
• µ₀ = 4π.10^-7 H/m, et
• ε₀ = 8,854187.10^-12 F/m.

The electromagnetic resonance frequency of a ring-shaped resonant element can be adjusted by adapting the radius of the resonant element so that: $${\mathrm{f}}^{\left(0\right)}=\frac{{\mathrm{z}}_0}{2\mathrm{\pi a}\sqrt{\mathrm{\mu \epsilon }}}=\frac{z_0\cdot {\mathrm{vs}}_0}{2\mathrm{\pi a}\sqrt{{\mathrm{\mu }}_{\mathrm{r}}{\mathrm{\epsilon }}_{\mathrm{r}}}}$$

or :

• f ⁽⁰⁾ denotes the zero-order electromagnetic resonance frequency of the resonant element,
• a denotes the radius of the resonant element,
• c ₀ denotes the speed of light in a vacuum,
• z ₀ = 1.841 denotes the first maximum of the Bessel function J ₁ ( z ) of order 1,
• μ _r denotes the relative permeability of the dielectric substrate,
• ε _r denotes the relative permittivity of the dielectric substrate,
• μ = μ _r μ ₀
• ε = ε _r ε ₀
• μ ₀ = 4π.10 ^-7 H / m, and
• ε ₀ = 8.854187.10 ^-12 F / m.

The electromagnetic absorber may further comprise a plurality of stacked absorption layers, each absorption layer having a set of metal resonant elements.

The invention also proposes a manufacturing method according to claim 8.

• La Figure 1 est une vue en perspective d'un absorbant électromagnétique selon un mode de réalisation de l'invention ;
• La Figure 2 est une vue en perspective d'une portion de l'absorbant électromagnétique de la Figure 1 ;
• La Figure 3 est une vue en coupe transversale de la portion d'absorbant électromagnétique de la Figure 2 ;
• La Figure 4 est un graphe représentant le coefficient de réflexion d'une onde électromagnétique incidente sur la portion d'absorbant électromagnétique des Figures 2 et 3 en fonction de la fréquence de l'onde électromagnétique incidente ;
• La figure 5 est une vue agrandie d'un motif élémentaire de l'absorbant électromagnétique de la Figure 1 ;
• La Figure 6 est un graphe représentant le coefficient de réflexion d'une onde électromagnétique incidente sur l'absorbant électromagnétique de la Figure 1 en fonction de la fréquence de l'onde électromagnétique incidente ;
• La Figure 7 est une vue en coupe transversale d'un absorbant électromagnétique selon un autre mode de réalisation dans lequel l'absorbant électromagnétique comporte plusieurs couches d'absorption empilées ; et
• la Figure 8 est un organigramme illustrant les étapes d'un procédé de fabrication d'un absorbant électromagnétique selon un mode de réalisation de l'invention.

Other features and advantages of the invention will become apparent on reading the description which follows. This is purely illustrative and should be read in conjunction with the attached drawings in which:

• The Figure 1 is a perspective view of an electromagnetic absorber according to one embodiment of the invention;
• The Figure 2 is a perspective view of a portion of the electromagnetic absorber of the Figure 1 ;
• The Figure 3 is a cross-sectional view of the electromagnetic absorbent portion of the Figure 2 ;
• The Figure 4 is a graph representing the reflection coefficient of an electromagnetic wave incident on the electromagnetic absorber portion of Figures 2 and 3 according to the frequency of the incident electromagnetic wave;
• The figure 5 is an enlarged view of a basic pattern of the electromagnetic absorber of the Figure 1 ;
• The Figure 6 is a graph representing the reflection coefficient of an electromagnetic wave incident on the electromagnetic absorber of the Figure 1 according to the frequency of the incident electromagnetic wave;
• The Figure 7 is a cross-sectional view of an electromagnetic absorber according to another embodiment wherein the electromagnetic absorbent has a plurality of stacked absorption layers; and
• the Figure 8 is a flowchart illustrating the steps of a method of manufacturing an electromagnetic absorber according to one embodiment of the invention.

The Figure 1 represents an electromagnetic absorber 1 according to one embodiment of the invention. The electromagnetic absorber 1 here has a planar shape. Alternatively, the electromagnetic absorbent 1 could have a curved shape, to allow the integration of the absorbent 1 in any curvature system.

An orthogonal coordinate system (0, X, Y, Z) is defined whose X and Y axes extend in the plane of the electromagnetic absorber 1, and whose Z axis is perpendicular to the plane of the absorbent 1.

The Figures 2 and 3 represent a portion of the electromagnetic absorber 1, respectively in perspective and in cross section.

The electromagnetic absorbent 1 comprises a metal ground plane 2.

The electromagnetic absorbent 1 also comprises an insulating dielectric substrate 3, disposed on the ground plane 2. The substrate 3 is for example a fiberglass-reinforced epoxy resin composite (FR4 epoxy).

The electromagnetic absorbent 1 also comprises a set of resonant elements 4 metal, arranged on the dielectric substrate 3. The resonant elements 4 are for example made of copper. Each element resonant 4 may have any shape, for example a polygonal or circular shape.

The electromagnetic absorber 1 represented on the Figure 1 comprises resonant elements 4 of square shape and resonant elements 4 of rectangular shape. The portion of electromagnetic absorber 1 represented on the Figures 2 and 3 comprises a single resonant element 4 of square shape.

The resonant frequency of a resonant element 4 depends in particular on the dimensions of the resonant element 4 and on the thickness of the dielectric substrate 3. The absorption level depends in particular on the thickness of the dielectric substrate 3 and the periodicity of the the set of resonant elements 4.

Où :

• f_r désigne la fréquence de résonance électromagnétique d'ordre zéro de l'élément résonant 4,
• c ₀ désigne la vitesse de la lumière dans le vide,
• µ _r désigne la perméabilité relative du substrat diélectrique,
• ε_r désigne la permittivité relative du substrat diélectrique 3, et
• L' désigne la longueur d'un côté de l'élément résonant 4.

For example, in the case of a resonant element 4 of square shape, the electromagnetic resonance frequency of the resonant element 4 can be adjusted by adapting the length L ' of one side of the resonant element 4 so that: $$f_r=\frac{{\mathrm{vs}}_0}{2\mathrm{The}\text{'}\sqrt{{\mathrm{\mu }}_r{\epsilon }_r}}\pm 5\%$$

Or :

• f _r denotes the zero-order electromagnetic resonance frequency of the resonant element 4,
• c ₀ denotes the speed of light in a vacuum,
• μ _r denotes the relative permeability of the dielectric substrate,
• ε _r denotes the relative permittivity of the dielectric substrate 3, and
• The designates the length of one side of the resonant element 4.

The equation above makes it possible to obtain an adjustment of the electromagnetic resonance frequency to within a few percent.

Ce qui donne : $$L=\frac{c_0}{2f_r\sqrt{{\mathrm{\mu }}_r{\epsilon }_r}}-2\mathit{\Delta L}$$

Avec : $$\frac{\mathit{\Delta L}}{h}=\mathrm{0,412}\frac{\left({\epsilon }_{\mathit{reff}}+\mathrm{0,3}\right)\left(\frac{W}{h}+0.264\right)}{\left({\epsilon }_{\mathit{reff}}-\mathrm{0,258}\right)\left(\frac{W}{h}+\mathrm{0,8}\right)}$$

Où :

• W désigne la largeur de l'élément résonant 4, c'est-à-dire que dans le cas d'un élément résonant de forme carrée W=L', et
• h désigne l'épaisseur du substrat diélectrique 3,

Et où : $${\epsilon }_{\mathit{reff}}=\frac{{\epsilon }_r+1}{2}+\frac{{\epsilon }_r+1}{2}{\left(1+12\frac{h}{W}\right)}^{-1/2}$$

A more precise adjustment of the electromagnetic resonance frequency of the resonant element 4 can be obtained by considering that the length L ' is an approximation of the length of one side of the resonant element 4 and adapting the length L of one side of the resonant element 4 so that: $$\mathrm{The}\text{'}=\mathrm{The}+2\mathrm{\Delta }\mathrm{The}$$

Which give : $$\mathrm{The}=\frac{{\mathrm{vs}}_0}{2f_r\sqrt{{\mathrm{\mu }}_r{\epsilon }_r}}-2\mathit{.DELTA.L}$$

With: $$\frac{\mathit{.DELTA.L}}{h}=0.412\frac{\left({\epsilon }_{\mathit{reff}}+0.3\right)\left(\frac{W}{h}+0264\right)}{\left({\epsilon }_{\mathit{reff}}-0.258\right)\left(\frac{W}{h}+0.8\right)}$$

Or :

• W denotes the width of the resonant element 4, that is to say that in the case of a resonant element of square shape W = L ', and
• h denotes the thickness of the dielectric substrate 3,

And or : $${\epsilon }_{\mathit{reff}}=\frac{{\epsilon }_r+1}{2}+\frac{{\epsilon }_r+1}{2}{\left(1+12\frac{h}{W}\right)}^{-1/2}$$

The Figure 4 shows a curve representing the calculated reflection coefficient of an incident electromagnetic wave on an infinite array of 4-square resonant elements as a function of the frequency of the incident electromagnetic wave.

Each resonant element 4 here has a square shape of 7mm side. The grating is therefore periodic and formed of a set of identical resonant elements 4 with a period of 8 mm in the directions of the X and Y plane. The substrate 3 is a FR4 epoxy substrate 0.3 mm thick. An incident electromagnetic wave propagating along the Z direction is considered.

We observe on the figure 4 that the portion of electromagnetic absorbent 1 has a reflection less than 100%, and therefore an absorption, around the frequency 9.45 GHz, which corresponds to the resonant frequency of the resonant element 4. The absorption is carried out by a plasmonic resonance effect of the resonant element 4 at its resonant frequency.

Où :

• f⁽⁰⁾ désigne la fréquence de résonance électromagnétique d'ordre zéro de l'élément résonant 4,
• a désigne le rayon de l'élément résonant 4,
• c ₀ désigne la vitesse de la lumière dans le vide,
• z ₀ = 1,841 désigne le premier maximum de la fonction de Bessel J ₁(z) d'ordre 1,
• µ _r désigne la perméabilité relative du substrat diélectrique,
• ε_r désigne la permittivité relative du substrat diélectrique 3,
• µ = µ_rµ₀
• ε = ε_rε₀
• µ₀ = 4π.10^-7 H/m, et
• ε₀ = 8,854187.10^-12 F/m.

In the case of a resonant element 4 of circular shape, the electromagnetic resonance frequency can be adjusted by adapting the radius of the resonant element 4 so that: $${\mathrm{f}}^{\left(0\right)}=\frac{{\mathrm{z}}_0}{2\mathrm{\pi a}\sqrt{\mathrm{\mu \epsilon }}}=\frac{{\mathrm{z}}_0\cdot {\mathrm{vs}}_0}{2\mathrm{\pi a}\sqrt{{\mathrm{\mu }}_r{\mathrm{\epsilon }}_{\mathrm{r}}}}$$

Or :

• f ⁽⁰⁾ denotes the zero-order electromagnetic resonance frequency of the resonant element 4,
• a denotes the radius of the resonant element 4,
• c ₀ denotes the speed of light in a vacuum,
• z ₀ = 1.841 denotes the first maximum of the Bessel function J ₁ ( z ) of order 1,
• μ _r denotes the relative permeability of the dielectric substrate,
• ε _r denotes the relative permittivity of the dielectric substrate 3,
• μ = μ _r μ ₀
• ε = ε _r ε ₀
• μ ₀ = 4π.10 ^-7 H / m, and
• ε ₀ = 8.854187.10 ^-12 F / m.

As shown on the Figure 1 , the set of resonant elements 4 of the absorbent 1 comprises resonant elements 4 of different dimensions and / or shapes. The juxtaposition of the electromagnetic resonance frequencies of the different resonant elements 4 thus makes it possible to obtain one or more electromagnetic absorption band (s).

Several resonant elements 4 of different dimensions and / or shapes may be arranged on the substrate 3 so as to form an elementary pattern ME making it possible to cover the predetermined electromagnetic absorption band (s).

The Figure 5 shows an enlargement of the elementary pattern ME of the Figure 1 . This elementary pattern ME comprises four square-shaped resonant elements 4a having a length L _a of the side, four rectangular-shaped resonant elements 4b having a length L _b and a width l _b , four square-shaped resonant elements 4c having a length L _c four rectangularly resonant elements 4d having a length L _d and a width l _d , four square-shaped resonant elements 4e having a length L _e of side, four rectangular-shaped resonant elements 4f having a length L _f and a rectangular width l _f , and a central square resonant element 4g having a length L _g of side.

The elementary unit ME may then be periodically repeated over the entire surface of the insulating dielectric substrate 3, or on part of the surface of the insulating dielectric substrate 3. The number of periodic repetitions depends on the surface on which absorption is desired.

The figure 6 shows a graph representing the reflection coefficient of an electromagnetic wave incident on the electromagnetic absorber 1 of the Figure 1 according to the frequency of the incident electromagnetic wave.

The curve Cs is obtained by a simulation, and the curve Cm by a measurement. A minimum absorption threshold set at -10 dB is considered. We thus observe on the Figure 6 a first absorption band around the frequency 7 GHz, and a second absorption band in a frequency range from 12.5 to 14.3 GHz.

The passive metamaterial electromagnetic absorbent 1 described above has the advantage of being lightweight, thin, and conformable. It allows polarization-independent operation over a wide frequency band and wide range of incidence.

The electromagnetic absorbent 1 has in addition a very small thickness in front of the wavelength λ for which it is calibrated. It is thus possible to achieve an absorption band with a simple structure of approximate thickness λ / 45. For example, the thickness of the absorbent 1 is about 0.5 mm for a wavelength of 2.24 cm.

Since this thickness is very small, the absorption can be increased by using stacks of identical layers of reduced thickness in front of the wavelength. In other words, the absorbent 1 then comprises several stacked absorption layers, each absorption layer comprising a set of metal resonant elements 4.

The Figure 7 shows an embodiment of an absorbent 1 having four stacked absorption layers. The electromagnetic absorber 1 here comprises a ground plane 2, on which is disposed a first insulating dielectric substrate 3 ₁ . A first set of resonant April ₁ metal elements is disposed on the first dielectric substrate 3 _1. A second dielectric substrate 3 ₂ is disposed on the first set of elements resonant April _1. A second set of resonant 4 ₂ metal elements is disposed on the second dielectric substrate 3 _2. A third dielectric substrate 3 ₃ is disposed on the second set of resonant elements 4 ₂ . A third set of resonant elements 4 ₃ metal is disposed on the third dielectric substrate 3 ₃ . A fourth dielectric substrate 3 ₄ is disposed on the third set of resonant 4 ₃ sections. A fourth set of resonant members 4 metal ₄ is disposed on the fourth dielectric substrate 3 _4.

The number of stacked absorption layers depends on the desired absorption and is not limiting.

In addition, the small thickness of the absorbent 1 makes it possible to produce a conformable absorbent 1 on surfaces of revolution with a small radius of curvature.

The electromagnetic absorbent 1 can mainly be used in the field of electromagnetic compatibility.

Referring to the Figure 8 the steps of a method of manufacturing an electromagnetic absorber 1 according to one embodiment of the invention are described.

In step S1, an insulating dielectric substrate 3 is disposed on a metal ground plane 2. The substrate 3 is for example a composite of epoxy resin reinforced with glass fiber (FR4 epoxy).

In step S2, a set of metal resonant elements 4 is disposed on the insulating dielectric substrate 3. As described above, the dimensions of the resonant elements 4 are adapted as a function of one or more band (s) of desired electromagnetic absorption (s).

This process makes it possible in particular to simplify the manufacture of the absorbent 1, thus reducing its manufacturing cost.

Of course, the present invention is not limited to the embodiments described above as examples; it extends to other variants.

Claims (8)

1. Electromagnetic absorber (1) comprising:

   - a metal ground plane (2),

   - an insulating dielectric substrate (3), arranged on said metal ground plane,

   - an array of resonating metal elements (4), arranged on said insulating dielectric substrate, the electromagnetic resonant frequency of a resonating element being adjusted by adapting the dimensions of the resonating element, characterised in that the array of resonating elements comprising resonating elements of various sizes arranged on the substrate in such a way as to form a pattern making it possible to cover a preset electromagnetic absorption band via the plasmon resonance effect of the resonating elements.
2. Electromagnetic absorber according to claim 1, wherein an elementary pattern comprising several resonating elements of various sizes is repeated periodically on the insulating dielectric substrate.
3. Electromagnetic absorber according to claim 1 or 2, wherein a resonating element has a square, rectangular, polygonal or circular shape.
4. Electromagnetic absorber according to any of claims 1 to 3, wherein the insulating dielectric substrate has a thickness determined according to an electromagnetic resonant frequency of the preset electromagnetic absorption band and/or of a desired absorption level.
5. Electromagnetic absorber according to any of claims 1 to 4, wherein the electromagnetic resonant frequency of a resonating element of square shape is adjusted by adapting the length of one side of the resonating element in such a way that: $$f_r=\frac{c_0}{2L\prime \sqrt{{\mathrm{\mu }}_{\mathrm{r}}{\epsilon }_r}}\pm 5\%$$

   

   where:

   f_r designates the zero-order electromagnetic resonant frequency of the resonating element,

   C₀ designates the speed of light in a vacuum,

   µ_r , designates the relative permeability of the dielectric substrate,

   ε_r designates the permittivity of the dielectric substrate, and

   L' designates the length of one side of the resonating element.
6. Electromagnetic absorber according to any of claims 1 to 4, wherein the electromagnetic resonant frequency of a resonating element of circular shape is adjusted by adapting the radius of the resonating element in such a way that: $${\mathrm{f}}^{\left(0\right)}=\frac{{\mathrm{z}}_0}{2\mathrm{\pi a}\sqrt{\mathrm{\mu \epsilon }}}=\frac{{\mathrm{z}}_0\cdot {\mathrm{c}}_0}{2\mathit{\pi a}\sqrt{{\mathrm{\mu }}_{\mathrm{r}}{\mathrm{\epsilon }}_{\mathrm{r}}}}$$

   

   where:

   f⁽⁰⁾ designates the zero-order electromagnetic resonant frequency of the resonating element,

   a designates the radius of the resonating element,

   c₀ designates the speed of light in a vacuum,

   Z₀ = 1,841 designates the first maximum of the order-1 Bessel function J₁ (z),

   µ_r designates the relative permeability of the dielectric substrate,

   ε_r designates the permittivity of the dielectric substrate,

   µ = µ_rµ₀

   ε = ε_rε₀

   µ₀ = 4π.10^-7 H/m, and

   ε₀ = 8.854187.10^-12 F/m.
7. Electromagnetic absorber according to any of claims 1 to 6, comprising several stacked absorption layers, each absorption layer comprising an array of resonating metal elements (4).
8. Method for manufacturing an electromagnetic absorber, comprising steps consisting in:

   - arranging an insulating dielectric substrate on a metal ground plane, and

   - arranging an array of resonating metal elements on said insulating dielectric substrate, the electromagnetic resonant frequency of a resonating element being adjusted by adapting the dimensions of the resonating element, characterised in that the array of resonating elements comprising resonating elements of various sizes arranged on the substrate in such a way as to form a pattern making it possible to cover a preset electromagnetic absorption band via the plasmon resonance effect of the resonating elements.

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