EP2636096A1

EP2636096A1 - Artificial magnetic conductor, and antenna

Info

Publication number: EP2636096A1
Application number: EP11775953.0A
Authority: EP
Inventors: François GRANGE; Christophe Delaveaux; Bernard Viala
Original assignee: Commissariat a lEnergie Atomique CEA; Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 2010-11-03
Filing date: 2011-10-27
Publication date: 2013-09-11
Anticipated expiration: 2031-10-27
Also published as: US20130285858A1; WO2012059391A1; FR2966985B1; FR2966985A1; EP2636096B1; US9397406B2

Abstract

The invention relates to an artificial magnetic conductor having a surface impedance of greater than 100 Ω, said conductor comprising: a ground plane, at least one first frequency-selective surface that is transparent to certain wavelengths and reflective of a range of wavelengths, said frequency-selective surface including an array of resonant conductive elements arranged side-by-side in at least two different directions parallel to the ground plane, wherein each resonant element consists of at least one sub-layer (38, 42) of ferromagnetic material, the relative permeability of which is greater than 10 for a frequency of 2 GHz, and the thickness of which is strictly less than the thickness of the skin of said ferromagnetic material.

Description

ARTIFICIAL MAGNETIC DRIVER AND ANTENNA

[Ooi] The invention relates to an artificial magnetic conductor and an antenna incorporating this artificial magnetic conductor.

[002] Artificial magnetic conductors are better known by the acronym AMC ("Artificial Magnetic Conductor"). For more information on the operating principles of these artificial magnetic conductors and their physical properties, it is possible to refer to the patent application WO 99 509 29 filed by Sievenpiper.

[003] Typically, artificial magnetic conductors have two characteristic properties:

a surface impedance Z _{s that is} high in a frequency range called "bandwidth", and

- A resonance frequency f ₀ , included in the bandwidth, for which the phase shift is zero between an electromagnetic wave incident on the artificial magnetic conductor and the reflected electromagnetic wave.

[004] The surface impedance Z _s is defined by the following ratio: Z _s = E _ta n / Htan, where:

Etan is the component of the electric field of the tangential electromagnetic wave tangential to the face of the artificial magnetic conductor, and

- Htan is the component of the magnetic field of the tangential electromagnetic wave tangential to the surface of the artificial magnetic conductor.

[005] A surface impedance Z _s is said to be "high" if its modulus is greater than the vacuum wave impedance (module of Zs> 377 Ohms) and, preferably, several times greater than the impedance of vacuum wave.

[006] Known artificial magnetic conductors include:

- a plan of mass,

at least one first frequency selective surface, transparent at certain wavelengths and reflective for a range of wavelengths, this frequency selective surface comprising an array of conducting resonant elements arranged next to each other in at least two different directions parallel to the ground plane.

[007] These artificial magnetic conductors are used to make antennas. For example, known antennas include:

an artificial magnetic conductor having a resonance frequency f ₀ ,

a radiating conductor capable of radiating or receiving electromagnetic waves at a working frequency f _T between 0.5f ₀ and 2f ₀ , this conductor extending in a plane parallel to the artificial magnetic conductor and being separated from the selective surface in frequency closest to this driver artificial magnet by a distance less than λ _0/10 , where λ ₀ is the wavelength of an electromagnetic wave of frequency f ₀ .

[008] There is a strong demand to miniaturize the antennas. Today, it is possible to use radiating conductors whose length is less than λ _τ / 4 or λτ / 10, where λ _τ is the wavelength at the working frequency f _T of the antenna. It is therefore also desirable to reduce the size and bulk of artificial magnetic conductors. For this, it is necessary to reduce the dimensions of the resonant elements. However, when the dimensions of the resonant elements are reduced, the bandwidth of the artificial magnetic conductor also decreases. Which is not desirable.

Furthermore, the lower the resonant frequency f ₀ desired for the artificial magnetic conductor, the greater the size of the resonant elements is important. Thus, in order to miniaturize an artificial magnetic conductor, it is also desirable to have resonant elements which, at equal size with the resonant elements of the known artificial magnetic conductors, make it possible to obtain a lower resonance frequency f ₀ .

The invention aims to remedy these problems by proposing an artificial magnetic conductor which, at equal size for the resonant elements, has an enlarged bandwidth or which, at equal bandwidth, uses smaller resonant elements.

[001 1] It therefore relates to an artificial magnetic conductor in which each resonant element is formed of at least one ferromagnetic material underlayer whose relative permeability is greater than 10 for a frequency of 2GHz and whose thickness is strictly lower than the skin thickness of this ferromagnetic material.

The use of a ferromagnetic material to achieve the resonant elements reduces the resonance frequency f ₀ compared to the case of artificial magnetic conductors made only with resonant metal elements.

In addition, the use of a ferromagnetic material increases the bandwidth of the artificial magnetic conductor relative to an identical artificial magnetic conductor but in which the resonant elements are made only of metal.

Thus, the resonant elements provided with a ferromagnetic sub-layer allow to miniaturize the artificial magnetic conductor.

The fact that the underlayer of ferromagnetic material has a thickness less than the skin thickness avoids the relaxation mechanisms of the magnetization, which can lead to a drop in permeability and to high losses. magnetic, in the artificial magnetic conductor and makes possible the use of this sublayer of ferromagnetic material. Embodiments of this artificial magnetic conductor may include one or more of the following features:

At least each resonant element of the first frequency selective surface is formed of a stack of several sub-layers, each sub-layer having a thickness of less than 10 μι in a direction perpendicular to the ground plane;

At least one of the sub-layers of each resonant element is an underlayer of dielectric material having a relative permittivity greater than 10 for a frequency of 2 GHz;

At least one of the sub-layers of each resonant element is an antiferromagnetic sub-layer directly deposited on or under the ferromagnetic sublayer;

At least one of the underlays of each resonant element is a metal underlayer;

The artificial magnetic conductor comprises n frequency selective surfaces stacked one above the other in a direction perpendicular to the ground plane, each of these frequency-selective surfaces comprising an array of conductive resonant elements arranged next to one another in at least two different directions parallel to the ground plane and separated from each other by a layer of dielectric material whose thickness is strictly greater than 10 μm, where n is an integer greater than or equal to two;

Each resonant element of each of the n frequency selective surfaces is formed of at least one sublayer of ferromagnetic material whose relative permeability is greater than 10 for a frequency of 2 GHz and whose thickness is strictly less than skin thickness of this ferromagnetic material;

Each resonant element is electrically isolated from the ground plane by a layer of dielectric material;

Each resonant element extends mainly in a plane that forms an angle with the ground plane between 5 ° and 45 °.

These embodiments of the artificial magnetic conductor furthermore have the following advantages:

the use of an underlayer of dielectric material having a relative permittivity greater than 10 makes it possible to increase the miniaturization of the artificial magnetic conductor;

the use of an antiferromagnetic sub-layer also makes it possible to increase the bandwidth; the use of a metal underlayer makes it possible to limit the ohmic losses in the artificial magnetic conductor;

the use of several frequency-selective surfaces makes it possible to reduce the resonance frequency f ₀ of the artificial magnetic conductor without increasing the size of the resonant elements;

the use of sublayers of ferromagnetic material to form each of the resonant elements of each of the stacked frequency selective surfaces increases the bandwidth and further decreases the resonance frequency f ₀ ;

- Electrically isolating the resonant elements of the ground plane avoids having to make vertical conductive pads connecting the resonant elements to the ground plane which simplifies the manufacture of artificial magnetic conductor.

The invention also relates to an antenna comprising the artificial magnetic conductor above.

The invention will be better understood on reading the description which follows, given solely by way of nonlimiting example and with reference to the drawings in which:

FIG. 1 is a schematic and perspective illustration of an antenna comprising an artificial magnetic conductor,

FIG. 2 is a diagrammatic and perspective illustration of the artificial magnetic conductor of the antenna of FIG. 1;

FIG. 3 is a diagrammatic illustration in vertical section of a portion of the artificial magnetic conductor of FIG. 2,

FIG. 4 is a schematic illustration in vertical section of a resonant element of the artificial magnetic conductor of FIG. 2;

FIG. 5 is a graph illustrating the increase in the bandwidth and the decrease in the resonance frequency of the artificial magnetic conductor when its resonant elements comprise a ferromagnetic sublayer;

FIG. 6 is a graph illustrating the evolution of the modulus of the reflection coefficient of the artificial magnetic conductor of FIG. 2 as a function of frequency;

FIG. 7 is a graph illustrating the phase of the reflection coefficient as a function of frequency in two different situations;

FIG. 8 is a diagrammatic illustration in vertical section of a second embodiment of a resonant element;

- Figures 9 and 10 are schematic illustrations, in vertical section, of two other embodiments of the resonant elements of an artificial magnetic conductor.

In these figures, the same references are used to designate the same elements.

In the following description, the features and functions well known to those skilled in the art are not described in detail. The real part of the relative permeability and relative permittivity are physical quantities that vary as a function of frequency. Here, unless otherwise indicated, when we speak of the "relative permeability" / "relative permittivity", we denote the value of the real part of this physical quantity for a frequency of 2 GHz. However, what is described here also applies to the case where these values of relative permeability and relative permittivity are given for a frequency of 1 GHz.

Figure 1 shows an antenna 2 equipped with a radiating conductor 4 disposed above an artificial magnetic conductor 6 extending horizontally.

In this description, the figures are oriented relative to a reference 8 having two orthogonal horizontal directions X and Y and a vertical direction Z. The terms "up" / "down", "above" / "below And "higher" / "lower" are defined relative to this direction Z.

The antenna 2 is able to emit and / or receive electromagnetic waves at a working frequency f _T corresponding to a wavelength λ _τ . Typically, the frequency f _T is between 100 MHz and 20 GHz and preferably between 1 GHz and 10 GHz.

The antenna 2 emits essentially electromagnetic waves in the half-space above the plane XY. Here, the main transmit / receive direction is perpendicular to the XY plane and coincides with the Z direction.

Here, the artificial magnetic conductor 6 is in the form of a plate extending mainly horizontally. This plate has a front face 10 facing upwards and a rear face 12 facing downwards. Here, these faces 10 and 12 are horizontal. The face 12 is contained in the XY plane. In the particular case described here, the artificial magnetic conductor 6 is in the form of a horizontal rectangular plate.

The artificial magnetic conductor 6 has a frequency band, called "bandwidth", in which the electromagnetic waves are reflected without phase inversion (phase ≠ 180 °). Thus, in the bandwidth, the interferences between the incident and reflected waves on the conductor 6 are constructive while they are destructive outside this bandwidth. More specifically, here, the bandwidth of an artificial magnetic conductor is defined as being the frequency range for which the phase of the electromagnetic wave reflected on this artificial magnetic conductor is out of phase by an angle β between -90 ° and + 90 ° with respect to the electromagnetic wave incident on this same artificial magnetic conductor.

For a particular frequency of this bandwidth, called resonance frequency f ₀ , the angle β is zero. For this frequency f ₀ , the reflection coefficient of the component of the tangential electric field to the face 10 is equal to +1. By comparison, on a metal plane, this reflection coefficient is equal to -1.

The artificial magnetic conductor 6 limits or prevents the propagation of electromagnetic waves in the half-space below the XY plane for the transmission and reception frequencies located in the bandwidth of the artificial magnetic conductor 6.

In the remainder of this description, the given examples of dimensions for the various constituent elements of the artificial magnetic conductor 6 are for a resonance frequency f ₀ equal to or close to 6 GHz.

[0032] The artificial magnetic conductor 6 also has a Z _s impedance of high surface preventing or limiting the appearance of surface current. This limits the losses of the antenna 2. Here, the impedance module Z _s is greater than the vacuum wave impedance (Zs module> 377 Ohms) and, preferably, two or ten times greater than the vacuum wave impedance. The impedance Z _s of the artificial magnetic conductor 6 is raised mainly within its bandwidth.

[0033] The height h of the conductor 6, that is to say the shortest distance separating the faces 10 and 12 is strictly less than λ _0/4 and preferably less than λ _0/50 where λ ₀ is the wavelength corresponding to the resonance frequency f ₀ . For example, the height h is equal to 4 mm.

The radiating conductor 4 here extends essentially in a horizontal plane. It is spaced from the front face 10 by a height h _c less than λ _τ / 4 and, preferably, less than λ _τ / 10 or λ _τ / 100.

For example, the space between the radiating conductor 4 and the front face 10 is filled with a dielectric to maintain the radiating conductor 4 above this face 10.

Here, the radiating conductor 4 is shown in the form of a rectangular conductive element better known by the English term "patch". The radiating conductor 4 is sized to transmit and receive at the working frequency f _T. This working frequency f _T is between 0.5 f ₀ and ₂ f ₀ .

FIG. 2 represents in more detail the artificial magnetic conductor 6.

The rear face 12 is a ground plane or a substrate whose function is the mass. This face 12 is formed of a sheet of metal uniformly and continuously distributed in the XY plane. Typically it is a metallization layer. For example, the ground plane is made of copper. For example, its thickness is 35 m.

The face 10 is separated from the face 12 by one or more dielectric layers collectively referenced by the reference numeral 16.

The front face 10 is a frequency selective surface more known by the acronym FSS ("Selective Frequency Surface"). This face 10 is transparent for the electromagnetic plane waves whose frequency is located outside the bandwidth of the artificial magnetic conductor 6 and reflective for the electromagnetic plane waves whose frequency is within this bandwidth. However, the face 10 does not necessarily have a prohibited photonic band.

The face 10 is formed of a two-dimensional network of resonant elements 14. To simplify FIG. 2, the reference 14 is only indicated for some of these resonant elements. This network of elements 14 is said to be two-dimensional because the elements 14 are aligned next to each other along two different horizontal directions. Here, the elements 14 are aligned along the X and Y directions.

Here, the resonant elements 14 are arranged periodically along the X and Y directions. The period along the X and Y directions is denoted D. This period D is less than λ _0/10 and, preferably, less than λ _0/50. In the particular case described here, the periodicities along the X and Y directions are equal. For example, the period D is equal to 4.1 mm.

Each resonant element 14 has a front face exposed to electromagnetic radiation. Here, the front faces of the different radiating elements 14 are located in the same horizontal plane.

To explain the operation of each resonant element 14, it can be considered that it functions as a resonant LC circuit. For this, each resonant element 14 is adjacent to another resonant element 14 and capacitively coupled to the other adjacent elements 14. We denote "d" the shortest distance between two consecutive resonant elements 14 along the direction X or Y. This distance d is for example equal to 100 pm.

Each resonant element 14 is also inductively coupled to the ground plane 12. Here, this inductive coupling is through the dielectric layers 16.

In this embodiment, the resonant elements 14 are electrically isolated from the ground plane 12 by the dielectric layers 16. This means, in particular, that there are no vertical conductive pads, known by the term " vias ", directly electrically connecting all or only part of the resonant elements 14 to the ground plane 12.

Each resonant element is made of a conductive material whose conductivity is greater than 100 S / m and, preferably, greater than 1000 S / m or 1 MS / m. Here, the conductivity of the resonant elements 14 is greater than or equal to 5 MS / m.

[0048] The horizontal dimensions of the resonant elements 14 are less than λο / 10 and, preferably, less than λ _0/50 and λ _0/100 to appear as a homogeneous material to the incident electromagnetic waves. In addition, this makes it possible to repeat a large number of times each resonant element in the X or Y direction. The thickness of each resonant element is typically less than ten micrometers.

Here, each resonant element 14 is in the form of a solid pellet. Here, each pellet has a vertical axis 18 of symmetry. For example, in the particular case shown here, each resonant element 14 is a square pellet.

FIG. 3 represents a vertical section of the artificial magnetic conductor 6. This vertical section shows that the artificial magnetic conductor 6 comprises n frequency selective surfaces stacked one above the other in the direction Z, where n is an integer greater than or equal to two. In the particular case shown in FIG. 3, n is equal to three so that the artificial magnetic conductor 6 comprises three frequency-selective surfaces, 10, 20 and 22, respectively. The surfaces 10, 20 and 22 are separated from each other. others by layers of dielectric materials. More specifically, the surface 22 is separated from the ground plane 12 by a layer 24 of dielectric material of thickness ei.

The surface 20 is stacked above the surface 22 and separated from the surface 22 by a layer 26 of dielectric material of thickness e ₂ .

Finally, the surface 10 is stacked above the surface 20 and separated from this surface 20 by a layer of dielectric material 28 of thickness e ₃ .

The thickness of the layers 24, 26 and 28 is strictly greater than 10 μm and preferably greater than 50 μm. These thicknesses are less than λ _0/10 and preferably less than λ _0/100 to λ _0/1000.

In Figure 3, the thicknesses e ₂ and e ₃ are equal to and much smaller than the thickness ei.

The dielectric materials of the layers 26 and 28 are identical.

The dielectric material of the layer 24 is not necessarily the same as that used to form the layers 26 and 28. For example, here, the dielectric material of the layer 24 is glass.

In the particular case described here, the surfaces 20 and 22 are identical to the surface 10 with the exception that they are not arranged at the same height inside the artificial magnetic conductor 6. In addition, the resonant elements 14 of each surface 10, 20 and 22 are vertically aligned one above the other. Thus, the axes of symmetry 18 of the resonant elements of the different surfaces 10, 20 and 22 are merged.

The resonance frequency f ₀ of the artificial magnetic conductor 6 is in particular fixed by the following parameters:

the number n of frequency selective surfaces stacked one above the other,

the period D of the networks of resonant elements, the height h of the artificial magnetic conductor 6,

the dimensions of the resonant elements 14, and

the relative permittivity of the dielectric layers 20, 22 and 24.

Among these various parameters, the resonance frequency f ₀ is particularly sensitive to the number n of frequency-selective surfaces and to the period D.

Here, these different parameters are adjusted experimentally so that the resonance frequency f ₀ is between 100 MHz and 20 GHz and, preferably, between 1 GHz and 10 GHz. For example, these parameters are determined by electromagnetic simulation for different values of these parameters.

Each resonant element 14 is formed by a stack of fine sub-layers. "Thin" sublayer means an underlayer whose thickness is less than 10 μm and preferably less than 1 μm in the vertical direction. This stack of underlays is here called composite material.

To increase the bandwidth of the artificial magnetic conductor 6 and reduce its resonance frequency f ₀ , at least one of these sub-layers is made of a ferromagnetic material whose relative permeability is greater than 10 and, preferably, greater than at 100 to 2 or 3 GHz.

The interest of a high permeability for reducing the size of a resonant element is explained from the following equation:

physical ^

electric j

^■ ^ number ^ effective

or :

"(electric is the electrical length of the resonant element,

- Physical is the physical or actual length of the resonant element,

μβίίβοϋί is the relative effective permeability of the material of the resonant element, and

Eeffective is the relative effective permittivity of the material of the resonant element.

Thus, for the same electrical electric length, the greater the permeability, the smaller the physical length of the resonant element.

More specifically, each resonant element is made of a composite material simultaneously having the following properties without recourse to an artificial external magnetic field, that is to say a magnetic field other than the terrestrial magnetic field:

its conductivity is greater than 100 S / m and, preferably, greater than 1000 S / m or 1 MS / m at 25 ° C,

its relative permeability is greater than 10 and, preferably, greater than 100 in at least one horizontal direction for a frequency of 2 or 3 GHz,

its relative permittivity is greater than 10 and, preferably, greater than 100 at 2 or 3 GHz in the same direction as that where the relative permeability is greater than Typically the relative permittivity is the same regardless of the horizontal direction considered.

Such composite materials having these properties and their manufacture are described in more detail in the patent application FR 2 939 990.

In the particular case described here, this composite material comprises a first group 30 of ferromagnetic fine sublayers superimposed on a thin insulating sub-layer 32 itself superimposed on a second group 34 of ferromagnetic fine sub-layers.

The first group 30 of ferromagnetic fine sub-layers is composed of the stack from top to bottom:

an intermediate sublayer 36 providing the interface between a ferromagnetic sublayer and a dielectric underlayer,

a ferromagnetic sublayer 38,

an antiferromagnetic underlayer 40,

a ferromagnetic sublayer 42, and

an intermediate underlayer 44.

The sub-layer 36 is for example made of ruthenium (Ru), tantalum (Ta) or platinum (Pt). Its thickness is less than 10 nm.

The underlayer 38 has a thickness less than the skin thickness of the ferromagnetic material and, preferably, less than half or one third of this skin thickness. Here, its thickness is less than 100 nm and preferably less than 50 or 25 nm. Such a choice of the thickness of the ferromagnetic sublayer limits the magnetic losses of the material.

Typically, the underlayer 38 is made of an alloy of iron and / or cobalt and / or nickel. It may especially be a FeCo alloy or a FeCoB alloy. Here it is an alloy Fe ₆ 5Co ₃ 5.

The antiferromagnetic sublayer 40 is for example made of a manganese alloy and in particular an alloy of manganese and nickel. For example, here, it is a Ni _{5 5} oMn ₅ alloy. The presence of the antiferromagnetic layer makes it possible to create an exchange coupling so that the material is self-polarized and does not therefore require the presence of an artificial external magnetic field.

Typically, the thickness of this sublayer 40 is less than 100 nm and, for example, less than 50 nm.

The ferromagnetic sublayer 42 is for example identical to the underlayer 38.

Similarly, the intermediate sublayer 44 is for example identical to the underlayer 36.

The insulating sub-layer 32 is made of a dielectric material having a relative permittivity greater than 10 and, preferably, greater than 10. 100 to 2 or 3 GHz. This sublayer is typically made using a strontium (Sr) oxide and titanium (Ti). For example, it is titanium strontium (SrTi0 ₃ ). The thickness of the underlayer 32 is less than 10 μm or 1 μm. It is generally thicker than ferromagnetic and antiferromagnetic sublayers.

The second group 34 is for example identical to the first group 30 and will therefore not be described in more detail.

The radiating elements 14 are for example produced by depositing on the dielectric layer 20, 22 or 24 thin sub-layers one after the other. These sub-layers extend over the entire surface of the dielectric layer. Then, the resonant elements 14 are individualized by etching of this stack of fine sub-layers.

FIG. 5 illustrates the evolution of the phase of the reflection coefficient of four different artificial magnetic conductors corresponding to the curves, respectively, 50, 52, 54 and 56, as a function of the frequency of the incident electromagnetic wave. The curves 50 and 52 correspond to artificial magnetic conductors for which the number n of frequency-selective surfaces is equal to four. The curves 54 and 56 correspond to artificial magnetic conductors for which the number n of frequency-selective surfaces is equal to three.

The curves 50 and 54 correspond to artificial magnetic conductors made with resonant elements comprising at least one ferromagnetic sublayer. The curves 52 and 56 correspond to artificial magnetic conductors in which the resonant elements are only made using a metal layer such as copper.

As evidenced by the results of simulations illustrated in the graph of FIG. 5, the presence of a ferromagnetic sublayer makes it possible to reduce the resonance frequency f ₀ with respect to the case where such an underlayer is absent. In addition, the curves 50 and 54 are sloping less than the curves 52 and 56 so that the bandwidth of the corresponding artificial magnetic conductors is wider than those of the artificial magnetic conductors corresponding to the layers 52 and 56. Thus, the presence of at least one ferromagnetic sub-layer makes it possible to widen the bandwidth and to reduce the resonance frequency f ₀ .

The graph of FIG. 6 represents the evolution of the modulus, expressed in decibels, of the reflection coefficient of different artificial magnetic conductors as a function of the frequency of the incident electromagnetic wave.

The curves 60 and 62 each correspond to artificial magnetic conductors comprising only a stack of three frequency selective surfaces. The curves 64 and 66 correspond to artificial magnetic conductors comprising only a stack of four frequency selective surfaces.

The curves 60 and 66 correspond to artificial magnetic conductors in which the resonant elements are formed solely of a metallic material such as copper.

The curves 62 and 64 correspond to artificial magnetic conductors in which the resonant elements comprise at least one ferromagnetic sublayer.

As illustrated in this graph, the presence of the ferromagnetic sublayer decreases the frequency for which the modulus of reflection coefficient is minimal.

FIG. 7 represents a graph illustrating the evolution of the phase of the reflection coefficient (expressed in degrees) as a function of frequency (expressed in GHz). Curve 70 corresponds to an artificial magnetic conductor having only one frequency-selective surface while curve 72 corresponds to an artificial magnetic conductor comprising a stack of several frequency-selective surfaces. As illustrated in this graph, the use of a stack of several frequency-selective surfaces significantly reduces the resonance frequency f ₀ of the artificial magnetic conductor. This decrease in the frequency f ₀ is particularly noticeable for a number n of frequency selective surfaces between two and ten.

FIG. 8 represents a resonant element 80 that can be used in place of the resonant element 14. The resonant element 80 is formed of a stack of several fine sub-layers, at least one sub-layer of which layer made of ferromagnetic material. For example, here, the resonant element 80 comprises an underlayer 82 of ferromagnetic material superimposed on a sub-layer 84 of dielectric material itself superimposed on a metal underlayer 86.

The sub-layers 82 and 84 have, respectively, a relative permeability and a permittivity greater than 10 for a frequency of 2 or 3 GHz. These sub-layers are for example made as described opposite the resonant element 14.

The sub-layer 86 is for example made of copper so as to limit the ohmic losses of the antenna.

[0094] Figures 9 and 10 show two other embodiments of an artificial magnetic conductor. More precisely, FIGS. 9 and 10 show artificial magnetic conductors, respectively, 90 and 100. To simplify FIGS. 9 and 10, only the ground plane 12 has been shown and a single frequency-selective surface. The artificial magnetic conductor 90 comprises a frequency-selective surface 92 provided with an array of resonant elements 94. These resonant elements 94 are aligned along a horizontal axis 96. Each resonant element 94 extends in a plane forming an angle Θ with the ground plane 12. The angle Θ is typically between -45 ° and + 45 ° and preferably between -45 °; -5 °] and [+ 5 °, + 45 °].

The artificial magnetic conductor 100 comprises a frequency-selective surface 102 made using resonant elements 104 and 106 aligned along a horizontal direction 108. As in the embodiment of FIG. 9, the elements 104 and 106 extend in planes making angles, respectively a and β, with the ground plane 12. Here, the angles a and β are between -45 ° and + 45 ° and preferably between -45 °; -5 °] and [+ 5 °, + 45 °]. However, in this embodiment, the angles a and β are different from each other. Preferably, they are chosen so that each resonant element 104 is symmetrical with a resonant element 106 with respect to a vertical plane.

Many other embodiments are possible.

For example, the periodicity of the resonant elements is not necessarily the same in each frequency selective surface. Similarly, the periodicity of the resonant elements in one direction of the network is not necessarily the same as the periodicity in another direction.

The materials used to make the resonant elements of a frequency-selective surface are not necessarily the same as those used to make the resonant elements of another frequency-selective surface of the same artificial magnetic conductor.

The thicknesses of the dielectric layers separating the frequency-selective surfaces may all be different or, on the contrary, all the same. Similarly, the dielectric material forming these dielectric layers may be the same for all layers or different for one or more of these dielectric layers.

[00101] Many shapes are possible for each resonant element. For example, it may be a square pellet, orthogonal, diamond-shaped or a dipole. Generally, this shape has an axis of symmetry with respect to an axis orthogonal to the plane in which the essence of this resonant element extends.

The resonant elements of a frequency-selective surface are not necessarily stacked rigorously one above the other. For example, the axes of symmetry of the resonant elements of a lower frequency selective surface can be shifted in a horizontal direction relative to the axes of symmetry of the resonant elements of a higher frequency selective surface. All or only some of the resonant elements can be electrically connected to the ground plane by vertical metal studs known as "vias".

The resonant elements are not necessarily arranged periodically along one or two horizontal directions.

In a simplified embodiment, the second group and the dielectric sub-layer of the resonant element 14 are omitted. In an even more simplified embodiment, the resonant element consists of a single thin sublayer of ferromagnetic material whose thickness is less than the skin thickness of this ferromagnetic material.

Alternatively, other materials may be used as dielectric. For example, it may be a barium (Ba) and titanium (Ti) oxide, in particular barium titanium BaTiO ₃ , hafnium oxide (Hf), especially HfO ₂ , or tantalum (Ta), especially Ta ₂ 0 ₅ (ferroelectric). Perovskites, however, are preferred, such as BaTiO ₃ or SrTiO ₃ for example, which have a higher relative permittivity (of the order of 100 versus 10 for barium or halfnium oxides at 2 or 3 GHz).

Other materials are also possible for the antiferromagnetic layer such as a PtMn or IrMn alloy and more generally any manganese-based alloy or iron oxides or cobalt or nickel.

For the ferromagnetic layer, CoFeB, FeN and CoFeN alloys will be preferred, but other materials are possible, especially all alloys combining two or three of the elements selected from iron, cobalt and nickel. These alloys may optionally be doped, for example boron or nitrogen. They may also be associated with other elements such as Al, Si, Ta, Hf, Zr.

The radiating conductor may be a single wire. The driver can substitute for one of the radiating elements of the front face.

The ground plane may be a second artificial magnetic conductor identical to the first artificial magnetic conductor and arranged symmetrically with the first artificial magnetic conductor with respect to a plane of symmetry to form an electrical image of the first artificial magnetic conductor. Under these conditions, the first artificial magnetic conductor functions as if there existed a metal layer in place of the plane of symmetry. Thus, here, the term "ground plane" means a metal layer uniformly distributed in a plane that a second artificial magnetic conductor symmetrical to the first artificial magnetic conductor relative to this plane. Note, however, that the second artificial magnetic conductor radiates in the lower half-space below the plane of symmetry. Therefore, such an antenna radiates throughout the space.

Claims

An artificial magnetic conductor having a surface impedance greater than 100 Ω, said conductor comprising:

a ground plane (12),

at least one first frequency selective surface (10, 20, 22), transparent at certain wavelengths and reflective for a range of wavelengths, this frequency selective surface comprising an array of resonant elements (14; 94, 104, 106) conductors arranged next to each other in at least two different directions parallel to the ground plane,

characterized in that each resonant member (14; 94; 104,106) is formed of at least one ferromagnetic material underlayer (38,42; 82) having a relative permeability greater than 10 at a frequency of 2 GHz and whose thickness is strictly less than the skin thickness of this ferromagnetic material.

A conductor according to claim 1, wherein at least each resonant element (14) of the first frequency-selective surface is formed of a stack of a plurality of sub-layers, each sub-layer having a thickness of less than 10 μm in a direction perpendicular to the ground plane.

3. Conductor according to claim 2, wherein at least one of the sub-layers of each resonant element is an underlayer (32; 84) of dielectric material having a relative permittivity greater than 10 for a frequency of 2 GHz.

4. A conductor according to any of claims 2 to 3, wherein at least one of the sub-layers of each resonant element is an antiferromagnetic sub-layer (40) directly deposited on or under the ferromagnetic sublayer (38). , 42).

The conductor of any one of claims 2 to 4, wherein at least one of the underlays of each resonant element is a metal underlayer (86).

A conductor according to any one of the preceding claims, wherein the artificial magnetic conductor has n frequency selective surfaces (10, 20, 22) stacked one above the other in a perpendicular direction. to the ground plane, each of these frequency selective surfaces comprising an array of conductive resonant elements arranged next to each other in at least two different directions parallel to the ground plane (12) and separated from one another by a layer (24, 26, 28) dielectric material whose thickness is strictly greater than 10 pm, where n is an integer greater than or equal to two.

7. Conductor according to claim 6, wherein each resonant element of each of the n frequency selective surfaces is formed of at least one sublayer of ferromagnetic material (38, 42; 82) whose relative permeability is greater than 10 a frequency of 2 GHz and whose thickness is strictly less than the skin thickness of this ferromagnetic material.

8. A conductor according to any one of the preceding claims, wherein each resonant element (14) is electrically isolated from the ground plane (12) by a layer of dielectric material (24, 26, 28).

9. A conductor according to any one of the preceding claims, wherein each resonant element (94; 104, 106) extends mainly in a plane which forms an angle with the ground plane between 5 ° and 45 °.

10. Antenna comprising:

an artificial magnetic conductor (6; 90; 100) having a resonance frequency f ₀ ,

- A radiating conductor (4) capable of radiating or receiving electromagnetic waves at a working frequency f _T between 0.5f ₀ and 2f ₀ , the conductor extending in a plane parallel to the artificial magnetic conductor and being separated from the frequency selective surface (10) closest to this artificial magnetic conductor by a distance less than λ _0/10 , where λ ₀ is the wavelength of an electromagnetic wave of frequency f ₀ ,

characterized in that the artificial magnetic conductor (6; 90; 100) is in accordance with any one of the preceding claims.