Electromagnetic Absorber

Abstract

The invention concerns an electromagnetic absorbent comprising: a metal earth plane, an insulating dielectric substrate, disposed on said metal earth plane, a set of metal resonant elements disposed on said insulating dielectric substrate, the electromagnetic resonant frequency of a resonant element being adjusted by adapting the dimensions of the resonant element, the set of resonant elements comprising resonant elements with different dimensions so as to enable the production, by juxtaposition of different electromagnetic resonant frequencies, of a predetermined electromagnetic absorption band. An elementary pattern formed by a plurality of metal resonant elements can be replaced periodically.

Description

Other features and advantages of the invention will also emerge from a reading of the following description. The latter is purely illustrative and must be read with regard to the accompany drawings, in which:

FIG. 1 is a perspective view of an electromagnetic absorbent according to one embodiment of the invention;

FIG. 2 is a perspective view of a portion of the electromagnetic absorbent of FIG. 1;

FIG. 3 is a view in cross section of the portion of electromagnetic absorbent of FIG. 2;

FIG. 4 is a graph showing the coefficient of reflection of an incident electromagnetic wave on the portion of electromagnetic absorption of FIGS. 2 and 3 according to the frequency of the incident electromagnetic wave;

FIG. 5 is an enlarged view of an elementary pattern of the electromagnetic absorbent of FIG. 1;

FIG. 6 is a graph showing the coefficient of reflection of an incident magnetic wave on the electromagnetic absorption of FIG. 1 as a function of the frequency of the incident electromagnetic wave;

FIG. 7 is a view in cross section of an electromagnetic absorbent according to another embodiment in which the electromagnetic absorbent comprises several stacked absorption layers; and

FIG. 8 is a flow diagram illustrating the steps of a method for manufacturing an electromagnetic absorbent according to an embodiment of the invention.

FIG. 1 shows an electromagnetic absorbent 1 according to an embodiment of the invention. The electromagnetic absorbent 1 has here a flat shape. In a variant, the electromagnetic absorbent 1 could have a curved shape, to enable the absorbent 1 to be integrated in a system with any curvature.

An orthogonal reference frame (0, X, Y, Z) is defined, the X and Y axes of which lie in the plane of the electromagnetic absorbent 1, and the Z axis of which is perpendicular to the plane of the absorbent 1.

FIGS. 2 and 3 show a portion of the electromagnetic absorbent 1, respectively in perspective and in cross section.

The electromagnetic absorbent 1 comprises a metal earth plane 2.

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

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

The electromagnetic absorbent 1 depicted in FIG. 1 comprises square-shaped resonant elements 4 and rectangular-shaped resonant elements 4. The portion of electromagnetic absorbent 1 depicted in FIGS. 2 and 3 comprises a single square-shaped resonant element 4.

The resonant frequency of a resonant element 4 depends in particular on the dimensions of the resonant element 4 and 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 set of resonant elements 4.

For example, in the case of a square-shaped resonant element 4, the electromagnetic resonant frequency of the resonant element 4 may be adjusted by adapting the length L′ of one side of the resonant element 4 so that:

$f_r=\frac{c_0}{2L^{\prime }\sqrt{{\mu }_r{\varepsilon }_r}}\pm 5\%$

where:

• • ƒ_r designates the zero-order electromagnetic resonant frequency of the resonant element 4,
  • c₀ designates the speed of light in a vacuum,
  • μ_r designates the relative permeability of the dielectric substrate,
  • ε_r designates the relative permittivity of the dielectric substrate 3, and
  • L′ designates the length of one side of the resonant element 4.

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

A more precise adjustment of the electromagnetic resonant frequency of the resonant element 4 can be obtained by considered that the length L′ is an approximation of the length of one side of the resonant element 4 and by adapting the length L of one side of the resonant element 4 so that:

L′=L+2ΔL

which gives:

$L=\frac{c_{0}}{2f_r\sqrt{{\mu }_r{\varepsilon }_r}}-2\Delta L$

with:

$\frac{\Delta L}{h}=0.412\frac{\left({\varepsilon }_{\mathrm{reff}}+0,3\right)\left(\frac{W}{h}+0.264\right)}{\left({\varepsilon }_{\mathrm{reff}}-0,258\right)\left(\frac{W}{h}+0,8\right)}$

where:

• • W designates the width of the resonant element 4, that is to say, in the case of a square-shaped resonant element, W=L′, and
  • h designates the thickness of the dielectric substrate 3,and where:

${\varepsilon }_{\mathrm{reff}}=\frac{{\varepsilon }_r+1}{2}+\frac{{\varepsilon }_r-1}{2}{\left(1+12\frac{h}{W}\right)}^{-1/2}$

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

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

It is observed in FIG. 4 that the portion of electromagnetic absorbent 1 has a reflection of 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 effected by a plasmon resonance effect of the resonant element 4 at its resonant frequency.

In the case of a circular-shaped resonant element 4, the electromagnetic resonant frequency can be adjusted by adapting the radius of the resonant element 4 so that:

$\begin{array}{c}{f}^{\left(0\right)}=\frac{z_0}{2\pi a\sqrt{\mathrm{\mu \varepsilon }}}\\ =\frac{z_0·c_0}{2\pi a\sqrt{{\mu }_r{\varepsilon }_r}}\end{array}$

where:

• • f⁽⁰⁾ designates the zero-order electromagnetic resonant frequency of the resonant element,
  • a designates the radius of the resonant element 4,
  • c₀ designates the speed of light in a vacuum,
  • z₀=1.841 designates the first maximum of the Bessel function of the first kind J₁(z),
  • μ_r designates the relative permeability of the dielectric substrate,
  • ε_r designates the relative permittivity of the dielectric substrate, and
  • μ=μ_rμ₀
  • ε=ε_rε₀
  • μ₀=4π.10⁻⁷ H/m, and
  • ε₀=8.854187×10⁻¹² F/m.

As depicted in FIG. 1, the set of resonant elements 4 of the absorbent 1 comprises resonant elements 4 with different dimensions and/or shapes. The juxtaposition of the electromagnetic resonant frequencies of the various resonant elements 4 thus makes it possible to obtain one or more electromagnetic absorption bands.

Several resonant elements 4 with different dimensions and/or shapes can be arranged on the substrate 3 so as to form an elementary pattern ME covering the predetermined electromagnetic absorption band or bands.

FIG. 5 shows an enlargement of the elementary pattern ME of FIG. 1. This elementary pattern ME comprises four square-shaped resonant elements 4a having sides with a length of L_a, four rectangular-shaped resonant elements 4b having a length L_b and a width I_b, four square-shaped resonant elements 4c having sides with length of L_c, four rectangular-shaped resonant elements 4d having a length L_d and a width I_d, four square-shaped resonant elements 4e having sides with a length of L_e, four rectangular-shaped resonant elements 4f having a length L_f and a width I_fand a square-shaped central resonant element 4g having a sides with the length of L_g.

The elementary pattern ME can then be repeated periodically over the entire surface of the insulating dielectric substrate 3, or over part of the surface of the insulating dielectric substrate 3. The number of periodic repetitions depends on the surface on which it is desired to effect an absorption.

FIG. 6 shows a graph depicting the coefficient of reflection of an incident electromagnetic wave on the electromagnetic absorption 1 of FIG. 1 as a function of 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 fixed a −10 dB is considered. Thus, in FIG. 6, a first absorption band is observed around the frequency 7 GHz, and a second absorption band in a frequency range from 12.5 to 14.3 GHz.

The electromagnetic absorption 1 with passive metamaterial described above has the advantage of being light, thin and conformable. It affords identical functioning independent of the polarisation over a large frequency band and a wide range of angles of incidence.

The electromagnetic absorbent 1 also has a very low thickness compared with the wavelength λ for which it is calibrated. It is thus possible to implement an absorption band with a simple structure with an approximate thickness λ/45. For example, the thickness of the absorbent 1 is approximately 0.5 mm for a wavelength of 2.24 cm.

As this thickness is very small it is possible to increase the absorption by using stacks of identical layers of reduced thickness compared with 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.

FIG. 7 shows an example embodiment of an absorbent 1 comprising four stacked absorption layers. The electromagnetic absorbent 1 here comprises an earth plane 2 on which a first insulating dielectric substrate 3₁ is disposed. A first set of metal resonant elements 4₁ is disposed on the first dielectric substrate 3₁. A second dielectric substrate 3₂ is disposed on the first set of resonant elements 4₁. A second set of metal resonant elements 4₂ is disposed on the second dielectric substrate 3₂. A third dielectric substrate 3₃ is disposed on the second set of resonant elements 4₂. A third set of metal resonant elements 4₃ is disposed on the third dielectric substrate 3₃. A fourth dielectric substrate 3₄ is disposed on the third set of resonance elements 4₃. A fourth set of metal resonant elements 4₄ is disposed on the fourth dielectric substrate 3₄.

The number of stacked absorption layers depends on the required absorption and is not limitative.

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 FIG. 8, the steps of a method for manufacturing an electromagnetic absorbent 1 according to an embodiment of the invention is described.

At step S1, an insulating dielectric substrate 3 is disposed on a metal earth plane 2. The substrate 3 is for example a glass fibre reinforced epoxy resin composite (FR4 epoxy).

At 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 according to one or more required electromagnetic absorption bands.

This method in particular simplifies the manufacture of the absorbent, and therefore reduces its manufacturing cost.

Naturally the present invention is not limited to the embodiments described above by way of examples; it extends to other variants.

Claims

1. An electromagnetic absorbent comprising: a metal earth plane,an insulating dielectric substrate, disposed on said metal earth plane,a set of metal resonant elements disposed on said insulating dielectric substrate, the electromagnetic resonant frequency of a resonant element being adjusted by adapting the dimensions of the resonant element, the set of resonant elements comprising resonant elements with different dimensions so as to enable the production, by juxtaposition of different electromagnetic resonant frequencies, of a predetermined electromagnetic absorption band.

2. The electromagnetic absorbent according to claim 1, in which an elementary pattern comprising several resonant elements with different dimensions is repeated periodically on the insulating dielectric substrate.

3. The electromagnetic absorbent according to claim 1, in which a resonant element has a square, rectangular, polygonal or circular shape.

4. The electromagnetic absorbent according to claim 1, in which the insulating dielectric substrate has a thickness determined according to an electromagnetic resonant frequency of the predetermined electromagnetic absorption band and/or a desired absorption level.

5. The electromagnetic absorbent according to claim 1, in which the electromagnetic resonant frequency of a square-shaped resonant element is adjusted by adapting the length of one side of the resonant element so that:

6. The electromagnetic element according to the claim 1, in which the electromagnetic resonant frequency of a circular-shaped resonant element can be adjusted by adapting the radius of the resonant element so that:

7. The electromagnetic absorbent according to claim 1, comprising several stacked absorption layers, each absorption layer comprising a set of metal resonant elements.

8. A method for manufacturing an electromagnetic absorbent, comprising steps consisting of: disposing an insulating dielectric substrate on a metal earth plane, anddisposing a set of metal resonant elements on the insulating dielectric substrate the electromagnetic resonant frequency of a resonant element being adjusted by adapting the dimensions of the resonant element, the set of resonant elements comprising resonant elements with different dimensions so as to enable the production, by juxtaposition of different electromagnetic resonant frequencies, of a predetermined electromagnetic absorption band.