EP3417507B1 - Electromagnetically reflective plate with a metamaterial structure and miniature antenna device including such a plate - Google Patents

Description

The present invention relates to an electromagnetic reflection plate with a metamaterial structure for a miniature antenna device. It also relates to a miniature antenna device comprising such an electromagnetic reflection plate and an antenna arranged at a short distance from this plate.

It generally fits into applications for telecommunications systems or communicating objects in which radiofrequency devices, including antennas and electronic circuits, are present and must be as compact as possible. In particular, the use of antennas in communication systems for aeronautics, surveillance or satellite navigation is essential. However, in this type of device, the space is reduced and generates a need for miniaturization of the antennas. An antenna is generally placed there in front of a reflector plane to have unidirectional radiation and to allow the integration of an electronic circuit close behind the reflector plane without appreciable interference. The radiation is thus directed in a direction of interest, making it possible, on the one hand, to improve the gain of the antenna and, on the other hand, to reduce the sensitivity of the antenna in a half-space.

According to a first possible production technology of the reflector plane, the latter is of a type approaching the model of a perfect electrical conductor with reflection of the electromagnetic field in phase opposition. The antenna must then be placed at a distance from the reflector plane as close as possible to a quarter of its average operating wavelength to compensate for the phase opposition in reflection and obtain constructive interference between an incident wave coming directly from the antenna and a wave reflected by the reflector plane. At VHF and HF wavelengths, this technology has the drawback of being bulky. For example for an average operating frequency f ₀ = 100 MHz of the antenna, it should be placed 75 cm from the reflective plane and for f ₀ = 1 GHz this still represents a distance of 7.5 cm.

According to a second possible technology for producing the reflector plane, the latter is of the artificial magnetic conductor type approaching the model of a perfect magnetic conductor with reflection of the electromagnetic field without phase shift. The antenna can then be placed very close to the reflective plane, in particular well below a quarter of its average operating wavelength, or even below a tenth of this wavelength. This considerably reduces the size of the antenna device and allows its advantageous integration into the design of miniature antennas. A reflector plane according to this technology can be produced using an electromagnetic reflection plate with a metamaterial structure which is precisely the subject of the present invention. A method was also proposed in 1999 in Sievenpiper's thesis paper, entitled “High-impedance electromagnetic surfaces”, PhD from the University of California, Los Angeles (USA ), to characterize artificial magnetic conductors by a method known as the phase diagram. This method consists of illuminating the surface to be characterized using a plane wave and at normal incidence. The phase difference which exists between the incident wave and the reflected wave is then compared. Interference is considered to be constructive when the phase difference is between ―π / 2 and + π / 2, which thus defines the bandwidth of use of the artificial magnetic conductor.

• une pluralité d'éléments conducteurs séparés les uns des autres et gravés sur une face supérieure d'une couche de substrat diélectrique,
• un plan de masse disposé en face inférieure de cette couche de substrat diélectrique, et
• un ensemble de vias métalliques traversants formés dans l'épaisseur du substrat, chacun comportant une extrémité supérieure en contact avec l'un des éléments conducteurs.

In accordance with this second artificial magnetic conductor technology, the present invention therefore relates more precisely to an electromagnetic reflection plate with a metamaterial structure for a miniature antenna device, comprising:

• a plurality of conductive elements separated from each other and etched on an upper face of a dielectric substrate layer,
• a ground plane disposed on the lower face of this dielectric substrate layer, and
• a set of through metal vias formed in the thickness of the substrate, each comprising an upper end in contact with one of the conductive elements.

However, the miniaturization of such an electromagnetic reflection plate is limited by the minimum number of conductive elements necessary to obtain a metamaterial structure, often organized in a matrix of at least four rows and four columns. In fact, each of these resonant conductive elements is generally of dimensions close to a quarter of the average operating wavelength of the antenna. Consequently, a metamaterial structure very quickly imposes a large reflecting surface facing the antenna to guarantee its operation in the frequency band of interest of the antenna.

Different solutions, such as US2011 / 061925 A1 Where US2010 / 252319 A1 , were made to improve the miniaturization of metamaterial structures, in particular by playing on the geometry of the conductive elements (interdigitated capacitors, spiral inductors, etc.), on a multiplication of layers coupled in capacitance or by the use of passive discrete electronic components or active. However, all of these solutions have drawbacks in terms of costs, of significant reduction in the bandwidth of the antenna or of overall thickness.

In a use of the metamaterial structures different from that envisaged in the present invention, that of the inter-antenna filtering components with electromagnetic band gap known as EBG (standing for “Electromagnetic Band-Gap”), innovative approaches have been proposed.

For example, in the patent application US 2014/0354502 A1 , it is proposed a ground plane pierced with holes through which pass the metal vias, these not being in contact with the ground plane but connected between them two by two using dedicated electrical connections located from the other side of the ground plane with respect to the conductive elements.

• une pluralité d'éléments conducteurs séparés les uns des autres et gravés sur une face supérieure d'une première couche de substrat diélectrique,
• un plan de masse disposé entre une face inférieure de la première couche de substrat diélectrique et une face supérieure d'une deuxième couche de substrat diélectrique, avec des trous aménagés dans ce plan de masse,
• un ensemble de vias métalliques traversants formés dans l'épaisseur des première et deuxième couches de substrat, chacun comportant une extrémité supérieure en contact avec l'un des éléments conducteurs, une extrémité inférieure atteignant une face inférieure de la deuxième couche de substrat diélectrique, et traversant le plan de masse sans contact électrique par l'un de ses trous,
  dans laquelle :
  • chaque élément conducteur est en contact avec plusieurs vias métalliques, et
  • chaque via métallique de chaque élément conducteur est connectable à un autre via métallique d'un élément conducteur voisin, à l'aide d'une connexion électrique correspondante en contact avec l'extrémité inférieure de ce via métallique.

For example also, in the patent application US 2009/0236141 A1 , FIG. 8 shows a structure of a metamaterial for EBG filtering comprising:

• a plurality of conductive elements separated from each other and etched on an upper face of a first layer of dielectric substrate,
• a ground plane disposed between a lower face of the first layer of dielectric substrate and an upper face of a second layer of dielectric substrate, with holes made in this ground plane,
• a set of through metal vias formed in the thickness of the first and second substrate layers, each having an upper end in contact with one of the conductive elements, a lower end reaching a lower face of the second dielectric substrate layer, and crossing the ground plane without electrical contact through one of its holes,
  in which :
  • each conductive element is in contact with several metal vias, and
  • each metallic via of each conductive element is connectable to another metallic via of a neighboring conductive element, using a corresponding electrical connection in contact with the lower end of this metal via.

But in these last two documents, the reflecting properties of the structure obtained are not observed, only the decoupling between neighboring antennas and the filtering of parasitic signals being studied. In addition, such a structure does not appear sufficient to obtain additional miniaturization in the case of an application of electromagnetic reflection.

It may thus be desired to design an electromagnetic reflection plate with a metamaterial structure for a miniature antenna device which allows additional miniaturization while overcoming at least some of the aforementioned problems and constraints.

There is therefore proposed an electromagnetic reflection plate with a metamaterial structure for a miniature antenna comprising the features of claim 1.

Thanks to the invention, it is possible to increase the phase shift between interconnected metal vias without increasing the size of the conductive elements of the metamaterial structure by cleverly exploiting, using one or more meanders on at least part of the electrical connections between vias, the surface located under the ground plane. However, it has been observed that, surprisingly, while the fact of multiplying the number of metal vias per conductive element does not in itself make it possible to reduce the overall size of an electromagnetic reflection plate, the fact of doing so in combination with an increase in phase shift using one or more meander connections allows such a size reduction.

Optionally, each electrical connection for connecting one metallic via to another is etched on the underside of the second layer of dielectric substrate. Thus, the arrangement of the meanders is optimal.

Also optionally, each of said electrical connections has several meanders.

• les éléments conducteurs sont répartis en matrice sur la face supérieure de la première couche de substrat diélectrique, et
• chaque élément conducteur est en contact avec quatre vias métalliques, chacun de ces quatre vias métalliques étant connectable à un autre via métallique d'un élément conducteur adjacent en ligne ou en colonne dans la matrice.

Also optionally:

• the conductive elements are distributed in a matrix on the upper face of the first layer of dielectric substrate, and
• each conductive element is in contact with four metal vias, each of these four metal vias being connectable to another metal via of an adjacent conductive element in a row or in a column in the matrix.

This option is advantageous in the case where it is desired to obtain axial symmetry along two orthogonal axes.

Also optionally, the metal vias of each conductive element and their respective electrical connections are distributed in a central symmetry around a central axis of symmetry of this conductive element.

Also optionally, at least part of the meandering electrical connections etched on the underside of the second dielectric substrate layer is further provided with adjustable phase shift devices.

According to the invention, each meandering electrical connection etched on the underside of the second layer of dielectric substrate gradually widens from its end in contact with the corresponding metal via towards one of the edges of the conductive element under which it is engraved.

Also optionally, each of the conductive elements has one of the shapes of the assembly consisting of a square shape, a shape rectangular, of a spiral shape, of a fork shape, of a cross shape with crutches and a dual shape of a cross with crutches known as the UC-EBG shape.

Also optionally, the conductive elements are periodically distributed over the upper face of the first layer of dielectric substrate.

There is also provided a miniature antenna device according to claim 9.

• la figure 1 représente en perspective transparente la structure générale d'une portion de plaque de réflexion électromagnétique à structure de métamatériau pour dispositif miniature d'antenne, selon un mode de réalisation de l'invention,
• la figure 2A représente, selon une même perspective transparente, une cellule élémentaire de la portion de plaque de la figure 1,
• les figures 2B, 2C et 2D sont respectivement des vues de face, de dessus et de dessous de la cellule élémentaire de la figure 2A,
• les figures 3A et 3B illustrent, en vues de dessus et de dessous, un exemple de réalisation d'un dispositif miniature d'antenne comportant une plaque de réflexion électromagnétique à structure de métamatériau, selon un mode de réalisation de l'invention,
• la figure 4 est un diagramme illustrant une relation entre la fréquence de fonctionnement d'un dispositif d'antenne tel que celui des figures 3A, 3B et certains paramètres de configuration propres à l'invention, et
• la figure 5 est un diagramme comparatif de phases de coefficients de réflexion en fonction de fréquences de fonctionnement pour un dispositif selon l'invention et un dispositif de l'état de la technique.

The invention will be better understood with the aid of the description which will follow, given solely by way of example and made with reference to the appended drawings in which:

• the figure 1 shows in transparent perspective the general structure of a portion of an electromagnetic reflection plate with a metamaterial structure for a miniature antenna device, according to one embodiment of the invention,
• the figure 2A represents, according to the same transparent perspective, an elementary cell of the plate portion of the figure 1 ,
• the figures 2B, 2C and 2D are respectively front, top and bottom views of the elementary cell of the figure 2A ,
• the figures 3A and 3B illustrate, in top and bottom views, an exemplary embodiment of a miniature antenna device comprising an electromagnetic reflection plate with a metamaterial structure, according to one embodiment of the invention,
• the figure 4 is a diagram illustrating a relationship between the operating frequency of an antenna device such as that of figures 3A, 3B and certain configuration parameters specific to the invention, and
• the figure 5 is a comparative phase diagram of reflection coefficients as a function of operating frequencies for a device according to the invention and a device of the state of the art.

The portion of electromagnetic reflection plate 10 with a metamaterial structure, shown schematically in transparent perspective on the figure. figure 1 can be considered as composed of several elementary cells repeating themselves in two main directions x and y. In the example of this figure, for for the sake of simplicity, only four elementary cells 12, 14, 16 and 18 are illustrated, one of which, for example cell 12, is shown alone on the diagram. figure 2A .

According to an overall description by layers in a z direction perpendicular to the x and y directions of the plate portion 10, several conductive elements 20, 22, 24, 26 separated from each other are etched on an upper face 28 of a first layer 30 of dielectric substrate. These conductive elements are for example rectangular or square but could be of any shape already studied in the state of the art. In particular, they could be in the form of a spiral, fork, cross with crutches or dual cross with crutches known as the UC-EBG form. In particular also, they could have interdigitated capacitors or spiral inductances, known to allow a certain miniaturization of the reflector plate as specified previously. They are also, for example, distributed in a matrix by periodically repeating their shape in the x and y directions on the upper face 28 of the first layer 30 of dielectric substrate. As a variant, the conductive elements could be of different shapes for a non-uniform distribution on the upper face 28, for example of increasing surfaces when moving away from a center, or any other topology relevant to the person skilled in the art in depending on the application context.

The plate portion 10 further comprises a ground plane 32 disposed between a lower face 34 of the first layer 30 of dielectric substrate and an upper face 36 of a second layer 38 of dielectric substrate, with holes 40 formed in this plane. mass 32.

Furthermore, through metal vias 42 are formed in the thickness of the first and second layers 30, 38 of substrate, each having an upper end in contact with one of the conductive elements 20, 22, 24, 26, and an end lower reaching a lower face 44 of the second layer 38 of dielectric substrate. Each of these vias 42 passes through the ground plane 32 without electrical contact through one of the holes 40.

More precisely, in the non-limiting example of figure 1 , each conductive element 20, 22, 24 or 26 is in electrical contact with four vias 42. Furthermore, according to the invention, each via 42 of each conductive element 20, 22, 24 or 26 is connectable to another via d 'a neighboring conductive element, using a corresponding electrical connection 46 etched on the lower face 44 of the second layer 38 of dielectric substrate and in contact with the lower end of this via 42. Also according to the invention , so as to increase the phase shift between any two vias interconnected by their lower ends, at least part of the electrical connections 46 etched on the lower face 44 of the second layer 38 of dielectric substrate has one or more meanders to optimize the occupation of this lower face 44. In the non-limiting example of figure 1 , each of these electrical connections 46 is meandering.

The elementary cell 12, shown alone in transparent perspective on the figure 2A and in front, top and bottom views on the figures 2B, 2C and 2D , is formed of the conductive member 20 and the entire thickness of the substrate below in the z direction. It is for example square with sides of length P. The conductive element 20 is also square with sides of length W slightly less than P so that two conductive elements of two adjacent elementary cells do not touch each other.

Four vias 42 are in contact with the conductive element 20 via their upper ends. They are more precisely referenced 42 (12) _a , 42 (12) _b , 42 (12) _c and 42 (12) _d on figures 2A to 2D . They are off-center with respect to the center of symmetry of the conductive element 20 but remain on its axes of symmetry. More precisely, the two vias 42 (12) _a and 42 (12) _d are on the x-directional axis of symmetry of the conductive element 20 but off-center with respect to its center of symmetry. More precisely also, the two vias 42 (12) _b and 42 (12) _c are on the y direction axis of symmetry of the conductive element 20 but off-center with respect to its center of symmetry. We denote by d the common distance between each via and the corresponding edge closest to the elementary cell 12.

Four meandering electrical connections 46 are etched on the lower face 44 of the second layer 38 of dielectric substrate in the elementary cell 12. They are more precisely referenced 46 (12) _a , 46 (12) _b , 46 (12) _c and 46 (12) _d on figures 2A to 2D and correspond respectively to vias 42 (12) _a , 42 (12) _b , 42 (12) _c and 42 (12) _d while being in respective contact with their lower ends. The meandering electrical connection 46 (12) _a has four prominent meanders on the 2D figure and gradually widens from its end in contact with the corresponding metallic via 42 (12) _a towards one of the edges of the elementary cell 12. It thus has a length much greater than the distance which separates the via 42 (12). _a of this edge and allows its electrical connection with another via a conductive element (not shown on the figure 1 ) adjacent in the negative direction of the x direction. The meandering electrical connection 46 (12) _b has four meanders clearly visible on the 2D figure and gradually widens from its end in contact with the corresponding metallic via 42 (12) _b towards another of the edges of the elementary cell 12. It thus has a length much greater than the distance which separates the via 42 (12) _b of this edge and allows its electrical connection with another via a conductive element (not shown on the figure 1 ) adjacent in the negative direction of the y direction. The meandering electrical connection 46 (12) _c has four clearly visible meanders on the 2D figure and gradually widens from its end in contact with the corresponding metal via 42 (12) _c towards another of the edges of the elementary cell 12. It thus has a length much greater than the distance which separates the via 42 (12) _c of this edge and allows its electrical connection with another via the adjacent conductive element in the positive direction of the y direction, that is to say via 42 (14) _b of the elementary cell 14. Finally, the electrical connection with meanders 46 (12) _d has four meanders clearly visible on the 2D figure and gradually widens from its end in contact with the corresponding metal via 42 (12) _d towards another of the edges of the elementary cell 12. It thus has a length much greater than the distance which separates the via 42 (12) _d from this edge and allows its electrical connection with another via the adjacent conductive element in the positive direction of the x direction, i.e. via 42 (16) _a of the elementary cell 16.

Note that the four vias 42 (12) _a , 42 (12) _b , 42 (12) _c and 42 (12) _d of the conductive element 20 and their respective meandering electrical connections 46 (12) _a , 46 ( 12) _b , 46 (12) _c and 46 (12) _d are distributed in a central symmetry around the center of symmetry of this conductive element 20. In addition, the surface of the lower face 44 of the second layer 38 of dielectric substrate is largely occupied by the respective meandering electrical connections 46 (12) _a , 46 (12) _b , 46 (12) _c and 46 (12) _d between vias 42 (12) _a , 42 (12) _b , 42 ( 12) _c , 42 (12) _d and the four edges of elementary cell 12.

Such a metamaterial structure described with reference to figures 1, 2A , 2B, 2C and 2D can advantageously be used for the design of a miniature antenna device such as that shown in top and bottom views on the figures 3A and 3B .

This device comprises a reflector plate 50 with a metamaterial structure composed of 25 elementary cells such as that illustrated in figure 2A distributed in a matrix of 5 rows and 5 columns. It also comprises a dipole antenna 52, visible in top view on the figure 3A , placed at a distance from the plate reflector 50. More precisely, if this dipole antenna 52 has an average operating wavelength denoted A, it can be placed at a distance from the reflector plate 50 less than one tenth of this average operating wavelength, or even even at a distance close to λ / 20, since the reflector plate 50 can behave like an artificial magnetic conductor when it is dimensioned to reflect the waves with zero phase shift at the average operating frequency of the antenna.

The figure 3B illustrates a bottom view of the vias interconnection network using the meander connections described above. It is shown that for a dipole antenna 52 with a length of 149 mm and a width of 3.5 mm arranged at a distance λ / 20 from the reflector plate 50, an antenna device of total dimensions 0.63.λ x 0.63 is obtained. .λ x 0.071.λ, where 0.071.λ is the thickness, i.e. a low profile antenna device since its total thickness is less than λ / 10.

In an alternative embodiment, at least part of the meandering connections engraved on the lower face 44 may be provided with adjustable phase shift devices well known to those skilled in the art, for example with diodes, for the interconnection of the conductive elements between them. This makes it possible to adjust the phase shifts according to the application to be optimized by simply varying the behavior of the active or passive elements employed while maintaining the structure of the metamaterial 10 or 50 and without the need to modify the length of the meandering connections.

In accordance with the invention and as illustrated in figure 4 , a miniaturization of the elementary cells can be obtained by optimally adjusting the position of the four vias of each elementary cell and the phase shift Δϕ between interconnected vias, this phase shift Δϕ being adjusted by the length of the meandering connections. Denote by k the parameter equal to P / d. This parameter k is necessarily strictly greater than 2 in order to be able to have four eccentric vias. Ultimately, the presence of a single centered via gives k = 2. The three curves of the figure 4 are: the curve in solid lines for k = 2, the curve in long dashed lines for k = 3 and the curve in short dashed lines for k = 4. We notice by the experiment that the more the vias are centered and therefore far from the edges of the elementary cell 12, k decreasing while tending towards 2, the more the operating frequency with reflection with zero phase shift, denoted F, decreases. However, the operating frequency is inversely proportional to the antenna size. So at given operating frequency and antenna size, a rapprochement of the vias towards the center induces a miniaturization of the elementary cells. It is also noted through experimentation that, under this configuration of elementary cells with several vias, the more the phase shifts Δϕ and therefore the lengths of meandering connections increase, the more the operating frequency with reflection at zero phase shift decreases, which also induces a miniaturization of elementary cells. But we also notice that the closer the vias are to the center, the less the effect of the phase shifts Δϕ on the operating frequency. Ultimately, for k = 2, there is no effect of the increase in phase shifts Δϕ on the operating frequency as shown by the horizontal straight line in the solid line of the diagram of the figure 4 . There is therefore a compromise to be found between the distance d of the vias with respect to the edges of each elementary cell and the implementable phase shifts Δϕ which are directly related to the possible size of the meandering connections on the lower face 44 of the second layer 38 dielectric substrate. This search for the optimal compromise depends on the intended application and is within the abilities of those skilled in the art.

• P = 53 mm,
• W = 51 mm,
• épaisseur de la première couche 30 de substrat diélectrique = 5 mm,
• épaisseur de la deuxième couche 38 de substrat diélectrique = 1,6 mm,
• permittivité relative du substrat diélectrique = 4,4,
• pertes diélectriques = 0,02.

The results of the figure 4 were obtained by varying the parameters k and Δϕ using simulations established on the basis of the miniature antenna device of the figures 3A and 3B with the following parameters:

• P = 53 mm,
• W = 51 mm,
• thickness of the first layer 30 of dielectric substrate = 5 mm,
• thickness of the second layer 38 of dielectric substrate = 1.6 mm,
• relative permittivity of the dielectric substrate = 4.4,
• dielectric losses = 0.02.

The figure 5 is a comparative phase diagram of reflection coefficients as a function of operating frequencies for a miniature antenna device according to the invention (in solid line) and a miniature antenna device of the prior art of the same dimensions ( in short dashed lines). The device of the prior art chosen has a reflector plate of the mushroom type, that is to say with square conductive elements connected to a solid ground plane by means of a single via each (without a second layer substrate).

• P = 42 mm,
• W = 40 mm,
• épaisseur de la première couche 30 de substrat diélectrique = 5 mm,
• épaisseur de la deuxième couche 38 de substrat diélectrique = 1,6 mm (pour le dispositif selon l'invention uniquement),
• permittivité relative du substrat diélectrique = 4,4,
• pertes diélectriques = 0,02,
• rayon des vias = 0,5 mm,
• k = 4 (pour le dispositif selon l'invention uniquement),
• Δϕ = 186° (pour le dispositif selon l'invention uniquement).

More precisely, for this comparison the following parameters were imposed:

• P = 42 mm,
• W = 40 mm,
• thickness of the first layer 30 of dielectric substrate = 5 mm,
• thickness of the second layer 38 of dielectric substrate = 1.6 mm (for the device according to the invention only),
• relative permittivity of the dielectric substrate = 4.4,
• dielectric losses = 0.02,
• vias radius = 0.5 mm,
• k = 4 (for the device according to the invention only),
• Δϕ = 186 ° (for the device according to the invention only).

The curves of the figure 5 were obtained by 3D electromagnetic simulation. While the dotted curve obtained with the miniature antenna device of the state of the art has a zero phase shift around 1.35 GHz, that in solid line obtained with the miniature antenna device according to the invention not only has a zero phase shift slightly below 1.35 GHz but above all an inflection point at 940 MHz. However, experience shows that the presence of this inflection point makes it possible to use the reflector plate of the miniature antenna device according to the invention as an artificial magnetic conductor at 940 MHz instead of 1.35 GHz. To use a reflector plate device of the mushroom type as an artificial magnetic conductor at 940 MHz, it would be necessary to impose P = 64 mm to have a zero phase shift around 940 MHz.

A gain in miniaturization of approximately 35% per dimension is thus demonstrated, which results in a gain of more than 57% in surface area. However, comparisons on other properties such as antenna adaptation and radiation efficiency at a chosen operating frequency, or directivity, show that miniature antenna devices according to the invention and with a mushroom reflector plate exhibit quite comparable performance in terms of improvement over reflector plane type devices approaching the perfect electrical conductor model. The gain in miniaturization is therefore all the more appreciable.

It clearly appears that an electromagnetic reflection plate with a metamaterial structure such as that described above makes it possible to miniaturize an antenna device including it without, however, exhibiting cost drawbacks, significant reduction in the bandwidth of the antenna. or substantial bulk in thickness. Only the surface available under the ground plane is exploited to obtain the advantageous technical effects resulting from the meandering connections.

It will also be noted that the invention is not limited to the embodiments described above.

In particular, although a miniature antenna device with a dipole antenna has been detailed previously, the invention is applicable to an antenna device whose antenna is of the ZOR type (standing for “Zeroth-Order Resonator”). ), wire-plate, broadband, circularly polarized or otherwise, arranged parallel or perpendicular to the reflective plane.

Also in a variant, each conductive element of the metamaterial may be in electrical contact with a number of vias other than four: for example two, six, etc. The vias are not necessarily all identical either.

Also in a variant, the invention also applies to a reflector plate with a metamaterial structure, the conductive elements of which are distributed over several layers, which may or may not be offset.

Also in a variant, the electrical connections between vias may not all be identical. It is in particular possible to vary the values of k and Δϕ from one elementary cell to another.

Also as a variant, the electrical connections between vias can be etched on several layers, not only on the underside of the second layer of dielectric substrate.

Also as a variant, each conductive element of the metamaterial may be in electrical contact with vias and / or corresponding electrical connections which are not distributed in a central and / or axial symmetry with respect to the center and / or to one or more axes of symmetry of the conductive element.

Claims (9)

1. An electromagnetically reflective plate (10; 50) with a metamaterial structure for a miniature antenna device including:

   - a dielectric substrate having at least two dielectric substrate layers;

   - a plurality of conductive elements (20, 22, 24, 26) separated from each other and etched on an upper face (28) of a first dielectric substrate layer (30),

   - a ground plane (32) placed between a lower face (34) of the first dielectric substrate layer (30) and an upper face (36) of a second dielectric substrate layer (38), with apertures (40) arranged in this ground plane (32),

   - a set of metal through-vias (42) formed in the thickness of the first (30) and second (38) substrate layers, each one including an upper end making contact with one of the conductive elements (20, 22, 24, 26), a lower end reaching a lower face (44) of the second dielectric substrate layer (38), and passing through the ground plane (32) without electrical contact in one of its apertures (40),
   wherein:

   - each conductive element (20, 22, 24, 26) makes contact with a plurality of metal vias (42; 42(12)_a, 42(12)_b, 42(12)_c, 42(12)_d), and

   - each metal via (42; 42(12)_a, 42(12)_b, 42(12)_c, 42(12)_d) of each conductive element is connectable to another metal via of a neighboring conductive element, using a corresponding electrical connection (46; 46(12)_a, 46(12)_b, 46(12)_c, 46(12)_d) making contact with the lower end of this metal via,
   characterized in that at least some of said electrical connections (46; 46(12)_a, 46(12)_b, 46(12)_c, 46(12)_d) has several meanders, wherein each electrical connection with meanders (46; 46(12)_a, 46(12)_b, 46(12)_c, 46(12)_d) is etched on the lower face (44) of the second dielectric substrate layer (38) and progressively expands from the end thereof making contact with the corresponding metal via (42; 42(12)_a, 42(12)_b, 42(12)_c, 42(12)_d) towards one of the edges of the conductive element (20, 22, 24, 26) under which it is etched.
2. The electromagnetically reflective plate (10; 50) as claimed in claim 1, wherein each electrical connection (46; 46(12)_a, 46(12)_b, 46(12)_c, 46(12)_d) for connecting a metal via to another is etched on the lower face (44) of the second dielectric substrate layer (38).
3. The electromagnetically reflective plate (10; 50) as claimed in claim 1 or 2, wherein each one of said electrical connections (46; 46(12)_a, 46(12)_b, 46(12)_c, 46(12)_d) has a plurality of meanders.
4. The electromagnetically reflective plate (10; 50) as claimed in any one of claims 1 to 3, wherein:

   - the conductive elements (20, 22, 24, 26) are distributed in a matrix on the upper face (28) of the first dielectric substrate layer (30), and

   - each conductive element (20, 22, 24, 26) makes contact with four metal vias (42; 42(12)_a, 42(12)_b, 42(12)_c, 42(12)_d), each one of these four metal vias being connectable to another metal via of an adjacent conductive element in line or in column in the matrix.
5. The electromagnetically reflective plate (10; 50) as claimed in any one of claims 1 to 4, wherein the metal vias (42; 42(12)_a, 42(12)_b, 42(12)_c, 42(12)_d) of each conductive element (20, 22, 24, 26) and the respective electrical connections (46; 46(12)_a, 46(12)_b, 46(12)_c, 46(12)_d) thereof are distributed according to a central symmetry around a central symmetry axis of this conductive element.
6. The electromagnetically reflective plate (10; 50) as claimed in any one of claims 1 to 5, wherein at least some of the electrical connections with meanders (46; 46(12)_a, 46(12)_b, 46(12)_c, 46(12)_d) etched on the lower face (44) of the second dielectric substrate layer (38) is further equipped with adjustable dephasing devices.
7. The electromagnetically reflective plate (10; 50) as claimed in any one of claims 1 to 6, wherein each one of the conductive elements (20, 22, 24, 26) has one of the shapes of the set consisting of a square shape, a rectangular shape, a spiral shape, a fork shape, a crutch cross shape and a dual crutch cross shape referred to as a UC-EBG shape.
8. The electromagnetically reflective plate (10; 50) as claimed in any one of claims 1 to 7, wherein the conductive elements (20, 22, 24, 26) are periodically distributed over the upper face (28) of the first dielectric substrate layer (30).
9. A miniature antenna device including:

   - an electromagnetically reflective plate (50) as claimed in any one of claims 1 to 8, and

   - an antenna (52), having an average functioning wavelength and placed at a distance from the reflective plate (50) less than one tenth of this average functioning wavelength.

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