EP2039021B1 - Method and device of transmission of waves - Google Patents

Description

The present invention relates to methods and devices for transmitting electromagnetic waves.

More particularly, the invention relates to a method for transmitting electromagnetic waves, for focusing a wave of wavelength λ (wavelength corresponding to the central frequency of the wave) in at least one focusing point of index i, the wave being emitted by index antennas j belonging to a first network to at least one antenna located at the point of focus i and belonging to a second network.

The document EP-A-0 803 991 describes an example of such a method, which allows a good focus on the point i.

The object of the present invention is in particular to improve the methods of this type, in order to make it possible to further improve the accuracy of the focusing on point i.

• l'antenne du deuxième réseau utilisée au point de focalisation est réactive, de façon à générer un champ évanescent,
• on utilise, au voisinage du point de focalisation i, au moins un diffuseur (qui peut lui-même être une antenne) pour l'onde, situé à une distance inférieure à une distance prédéterminée dudit point de focalisation, ladite distance prédéterminée étant au plus égale à λ/10.

For this purpose, according to the invention, a method of the kind in question is characterized in that:

• the antenna of the second network used at the point of focus is reactive, so as to generate an evanescent field,
• in the vicinity of the focusing point i, at least one diffuser (which may itself be an antenna) for the wave, located at a distance less than a predetermined distance from said focusing point, is used, said predetermined distance being at most equal to λ / 10.

• on produit une onde évanescente au point i, de sorte que le ou les diffuseurs convertissent cette onde évanescente en onde propagative, laquelle peut se propager jusqu'aux antennes du premier réseau,
• puis on détermine, à partir des signaux captés par les antennes j, les réponses impulsionnelles hij(t) entre le point i et les antennes j,
• puis on fait émettre par les antennes j du premier réseau une onde correspondant à un signal S_ji (t)=S_i (t)⊗h_ij (-t), où S_i(t) est une fonction du temps et h_ij(-t) est l'inversion temporelle de la réponse impulsionnelle h_ij(t) : le ou les diffuseurs recréent alors des ondes évanescentes à partir de l'onde propagative reçue, et ces ondes évanescentes peuvent se focaliser sur le point i avec une grande précision, la tache focale produite étant de dimension très inférieure à la longueur d'onde du signal. Ainsi, la largeur de la tache focale peut par exemple être de l'ordre de λ/30.

Thanks to these arrangements, it is possible to obtain a high focus accuracy, for example by implementing a method in which:

• an evanescent wave is produced at the point i, so that the diffuser or diffusers convert this evanescent wave into a propagating wave, which can propagate to the antennas of the first network,
• then, from the signals picked up by the antennas j, the impulse responses hij (t) between the point i and the antennas j are determined,
• then the antennas j of the first network transmit a wave corresponding to a signal S _ji ( t ) = S _i ( t ) ⊗ h _ij ( -t ), where S _i (t) is a function of time and h _ij (-t) is the time inversion of the impulse response h _ij (t): the diffuser or diffusers then recreate evanescent waves from the received propagating wave, and these evanescent waves can focus on the point i with a high accuracy, the focal spot produced being much smaller than the wavelength of the signal. Thus, the width of the focal spot may for example be of the order of λ / 30.

• le procédé comprend au moins :
  1. (a) une étape d'apprentissage dans laquelle on détermine à partir de signaux échangés entre les antennes j du premier réseau et au moins une antenne appartenant au deuxième réseau (le deuxième réseau peut être limité éventuellement à une seule antenne), une réponse impulsionnelle h_ij(t) entre le point de focalisation i et chaque antenne j du premier réseau,
  2. (b) une étape de focalisation au cours de laquelle on fait émettre depuis lesdites antennes j du premier réseau, des ondes correspondant à des signaux $$S_{\mathit{ji}}\left(t\right)=S_i\left(t\right)\otimes h_{\mathit{ij}}\left(-t\right),$$
     
  où S_i(t) est une fonction du temps et h_ij(-t) est une inversion temporelle de la réponse impulsionnelle h_ij(t) entre le point de focalisation i et l'antenne j, au moins les diffuseurs restant présents autour du point de focalisation i lors de l'étape de focalisation (le signal reçu au point i est alors proche de S_i(t)). On notera qu'au cours de l'étape de focalisation, on peut dans certains cas être amené à supprimer l'antenne située au point i, par exemple dans des applications visant à traiter une zone autour du point i ;
• au cours de l'étape d'apprentissage :
  • on fait émettre, par l'antenne du deuxième réseau, située audit point de focalisation i, une onde correspondant à un signal prédéterminé,
  • on capte des signaux générés par ladite onde sur les antennes d'indices j du premier réseau,
  • et on détermine à partir des signaux captés une réponse impulsionnelle h_ij(t) entre le point de focalisation i et chaque antenne j (2) du premier réseau ;
• l'antenne du deuxième réseau est présente au point de focalisation i lors de l'étape de focalisation, et on établit une communication entre ladite antenne et les antennes du premier réseau ;
• l'étape d'apprentissage est réalisée pour plusieurs points de focalisation d'indices i où sont disposées respectivement des antennes du deuxième réseau ayant chacune au moins un diffuseur situé à une distance inférieure à ladite distance prédéterminée par rapport au point de focalisation i correspondant,
  et au cours de l'étape de focalisation, on fait émettre à chaque antenne j du premier réseau, des ondes correspondant au moins à des signaux $$S_{\mathit{ji}}\left(t\right)=S_i\left(t\right)\otimes h_{\mathit{ij}}\left(-t\right),\mathrm{où\; i\; est\; lʹindice\; dʹun\; des\; points\; de}$$
  
  où i est l'indice d'un des points de focalisation souhaités ;
• au cours de l'étape de focalisation, on fait émettre par chaque antenne j du premier réseau, des ondes correspondant à une superposition de signaux $$S_{\mathit{ji}}\left(t\right)=S_i\left(t\right)\otimes h_{\mathit{ij}}\left(-t\right),\mathrm{pour\; plusieurs\; valeurs\; de\; i};$$
  
  pour plusieurs valeurs de i ;
• les antennes du deuxième réseau sont présentes aux points de focalisation i lors de l'étape de focalisation et au cours de l'étape de focalisation, on établit une communication sélective entre les antennes j du premier réseau et au moins certaines desdites antennes du deuxième réseau ;
• on utilise plusieurs diffuseurs, préférentiellement au moins 10 diffuseurs, situés à une distance inférieure à ladite distance prédéterminée du point de focalisation i ;
• la distance prédéterminée est au plus égale à λ/50 ;
• l'onde présente une fréquence f (fréquence centrale) comprise entre 0.7 et 50 GHz.
• l'antenne du deuxième réseau utilisée au point de focalisation souhaité présente une impédance ayant une partie imaginaire supérieure à la partie réelle, de façon à générer essentiellement un champ réactif ;
• la partie imaginaire de l'impédance de l'antenne du deuxième réseau est supérieure à 50 fois la partie réelle ;
• on utilise des diffuseurs métalliques.

In embodiments of the method according to the invention, one or more of the following provisions may also be used:

• the process comprises at least:
  1. (a) a learning step in which is determined from signals exchanged between the antennas j of the first network and at least one antenna belonging to the second network (the second network may be optionally limited to a single antenna), an impulse response h _ij (t) between the focal point i and each antenna j of the first network,
  2. (b) a focusing step during which said antennas j of the first network transmit waves corresponding to signals $$S_{\mathit{ji}}\left(t\right)=S_i\left(t\right)\otimes h_{\mathit{ij}}\left(-t\right),$$
     
  where S _i (t) is a function of time and h _ij (-t) is a time inversion of the impulse response h _ij (t) between the focusing point i and the antenna j, at least the diffusers remaining around from the point of focusing i during the focusing step (the signal received at the point i is then close to S _i (t)). Note that during the focusing step, it may in some cases be necessary to remove the antenna at the point i, for example in applications to treat an area around the point i;
• during the learning stage:
  • the antenna of the second network, located at said point of focus i, transmits a wave corresponding to a predetermined signal,
  • signals generated by said wave are collected on the antennae of indices j of the first network,
  • and from the picked-up signals is determined an impulse response h _ij (t) between the focusing point i and each antenna j (2) of the first grating;
• the antenna of the second network is present at the focal point i during the focusing step, and a communication is established between said antenna and the antennas of the first network;
• the learning step is carried out for several index focusing points i where are arranged respectively antennas of the second network each having at least one diffuser located at a distance less than said predetermined distance from the corresponding focal point i,
  and during the focusing step, each antenna j of the first network is sent waves corresponding at least to signals $$S_{\mathit{ji}}\left(t\right)=S_i\left(t\right)\otimes h_{\mathit{ij}}\left(-t\right),\mathrm{where\; i\; is\; the\; index\; of\; one\; of\; the\; points\; of}$$
  
  where i is the index of one of the desired focus points;
• during the focusing step, each antenna j of the first network transmits waves corresponding to a superposition of signals $$S_{\mathit{ji}}\left(t\right)=S_i\left(t\right)\otimes h_{\mathit{ij}}\left(-t\right),\mathrm{for\; several\; values\; of\; i};$$
  
  for several values of i;
• the antennas of the second network are present at the focusing points i during the focusing step and during the focusing step, a selective communication is established between the antennas j of the first network and at least some of said antennas of the second network ;
• several diffusers are used, preferably at least 10 diffusers, located at a distance less than said predetermined distance from the focusing point i;
• the predetermined distance is at most equal to λ / 50;
• the wave has a frequency f (center frequency) between 0.7 and 50 GHz.
• the antenna of the second network used at the desired focus point has an impedance having an imaginary portion greater than the real part, so as to essentially generate a reactive field;
• the imaginary part of the impedance of the antenna of the second network is greater than 50 times the real part;
• metal diffusers are used.

• une antenne située au point i, appartenant à un deuxième réseau (le deuxième réseau peut être limité éventuellement à une seule antenne), est réactive, de façon à générer un champ évanescent, et
• au moins un diffuseur métallique pour l'onde électromagnétique, situé à une distance inférieure à une distance prédéterminée du point i, ladite distance prédéterminée étant au plus égale à λ/10, où λ est la longueur d'onde de l'onde électromagnétique.

Furthermore, the invention also relates to a device for receiving an electromagnetic wave of wavelength λ in at least one index point i, this device comprising:

• an antenna located at the point i, belonging to a second network (the second network may be optionally limited to a single antenna), is reactive, so as to generate an evanescent field, and
• at least one metal diffuser for the electromagnetic wave, located at a distance less than a predetermined distance from the point i, said predetermined distance being at most equal to λ / 10, where λ is the wavelength of the electromagnetic wave.

• le dispositif comprend plusieurs diffuseurs métalliques, préférentiellement au moins 10 diffuseurs métalliques, à une distance inférieure à la distance prédéterminée du point i ;
• la distance prédéterminée est au plus égale à λ/50 ;
• l'antenne du deuxième réseau présente une impédance ayant une partie imaginaire supérieure à la partie réelle, de façon à générer essentiellement un champ évanescent ;
• la partie imaginaire de l'impédance est supérieure à 50 fois la partie réelle ;
• le dispositif comporte plusieurs antennes d'indices j appartenant à un premier réseau, et une unité centrale électronique commandant lesdites antennes j du premier réseau pour faire émettre depuis lesdites antennes j du premier réseau, des ondes électromagnétiques correspondant à des signaux $$S_{\mathit{ji}}\left(t\right)=S_i\left(t\right)\otimes h_{\mathit{ij}}\left(-t\right),$$
  
  où S_i(t) est une fonction du temps et h_ij(-t) est une inversion temporelle de la réponse impulsionnelle h_ij(t) entre le point i et chaque antenne j du premier réseau ;
• le deuxième réseau comporte plusieurs antennes situées en plusieurs points d'indices i et entourées par des diffuseurs métalliques situés respectivement à une distance inférieure à ladite distance prédéterminée par rapport au point i correspondant,
  et l'unité centrale électronique est adaptée pour faire émettre à chaque antenne j du premier réseau, des ondes électromagnétiques correspondant au moins à des signaux S_ji (t)=S_i (t)⊗h_ij (-t) ;
• l'unité centrale électronique est adaptée pour faire émettre à chaque antenne j du premier réseau, des ondes électromagnétiques correspondant à une superposition de signaux $$S_{\mathit{ji}}\left(t\right)=S_i\left(t\right)\otimes h_{\mathit{ij}}\left(-t\right),$$
  
  pour plusieurs valeurs de i.

In embodiments of the device according to the invention,

• the device comprises several metal diffusers, preferably at least 10 metal diffusers, at a distance less than the predetermined distance from the point i;
• the predetermined distance is at most equal to λ / 50;
• the antenna of the second network has an impedance having an imaginary part greater than the real part, so as essentially to generate an evanescent field;
• the imaginary part of the impedance is greater than 50 times the real part;
• the device comprises several index antennas j belonging to a first network, and an electronic central unit controlling said antennas j of the first network to emit from said antennas j of the first network, electromagnetic waves corresponding to signals $$S_{\mathit{ji}}\left(t\right)=S_i\left(t\right)\otimes h_{\mathit{ij}}\left(-t\right),$$
  
  where S _i (t) is a function of time and h _ij (-t) is a time inversion of the impulse response h _ij (t) between the point i and each antenna j of the first network;
• the second network comprises several antennas located at several points of indices i and surrounded by metal diffusers located respectively at a distance less than said predetermined distance from the corresponding point i,
  and the electronic central unit is adapted to cause each antenna j of the first network to transmit electromagnetic waves corresponding at least to signals S _ji ( t ) = S _i ( t ) ⊗ h _ij ( -t );
• the electronic central unit is adapted for transmitting to each antenna j of the first network, electromagnetic waves corresponding to a superposition of signals $$S_{\mathit{ji}}\left(t\right)=S_i\left(t\right)\otimes h_{\mathit{ij}}\left(-t\right),$$
  
  for several values of i.

Other features and advantages of the invention will become apparent from the following description of one of its embodiments, given by way of non-limiting example, with reference to the accompanying drawings.

• la figure 1 est un schéma de principe d'un dispositif mettant en oeuvre le procédé de focalisation selon une forme de réalisation de l'invention,
• la figure 2 est une vue de dessus d'une antenne, entourée de diffuseurs, appartenant à l'un des réseaux d'antennes du dispositif de la figure 1,
• et la figure 3 est une vue en perspective montrant l'antenne et les diffuseurs métalliques de la figure 2, dans un exemple de réalisation.

On the drawings:

• the figure 1 is a block diagram of a device implementing the focusing method according to one embodiment of the invention,
• the figure 2 is a top view of an antenna, surrounded by diffusers, belonging to one of the antenna arrays of the device of the figure 1 ,
• and the figure 3 is a perspective view showing the antenna and metal diffusers of the figure 2 , in an exemplary embodiment.

In the different figures, the same references designate identical or similar elements.

The figure 1 represents a radio communication device operating with electromagnetic waves having a central frequency generally between 0.7 and 50 GHz, for example of the order of 2.45 GHz (corresponding to a wavelength of 12.25 cm ). This device comprises a first network 1 of antennas 2, connected to a first electronic central unit 3 (UC1) and a second network 4 of antennas 5, connected to a second electronic central unit 6 (UC2).

Antennas 2, 5 are here in number of 8 for each network 1, 4 but could be different in number. In particular, the second network 4 could possibly include a single antenna 5.

The antennas 5 of the second network are separated from each other by a distance L (identical or not depending on the antenna pairs 5 considered), which is less than the wavelength λ of the electromagnetic waves. The distance L may for example be of the order of 4 mm, slightly less than λ / 30.

The first and second networks 1, 4, however, are distant from each other by a relatively large distance from λ, this distance being generally greater than 3λ.

As shown on the figure 2 each antenna 5 of the second network is surrounded by a plurality of metal diffusers 5, which are located in a radius R around the antenna 5. The radius R is less than λ / 2, preferably less than λ / 10 and in particular less than λ / 50.

Each antenna 5 is of reactive type. In other words, the imaginary part of the impedance of the antenna is not negligible, so that the antenna 5 creates an evanescent field when it receives an electrical signal.

Advantageously, the imaginary part of the impedance of the reactive antenna is greater than the real part.

For example, the imaginary part of the impedance is greater than 50 times the real part of the impedance.

In the particular example considered here, the real part of the impedance is 10 Ω and the imaginary part of 100 Ω.

In this way, the reactive antenna 5 essentially generates a reactive field when it receives an electrical signal, so that it then generates an evanescent electromagnetic wave located only around said reactive antenna (unlike a propagating wave which propagates relatively far away from the antenna 5). The metal diffusers 7 are in number greater than 10, for example greater than 20, in the zone of diameter R.

These metal diffusers are for example simple conductive elements, for example copper wires.

As is known, these diffusers, when they receive the evanescent electromagnetic wave coming from the reactive antenna 5, transform this evanescent wave into a propagating wave. Conversely, when they receive a electromagnetic propagating wave, these diffusers 7 transform said propagating wave evanescent wave.

By way of non-limiting example, the figure 3 shows an embodiment of the reactive antenna 5 and the reactive diffusers 7. In this example, the reactive antenna 5 may be constituted for example by a coaxial cable whose core 8 and the dielectric 12 pass through a resin plate 10 whose lower part has a layer 11 of metal in electrical connection with the shield 9 of the coaxial cable, the core 8 protruding from the plate 10 by a small distance e, for example of the order of 2 mm.

The distance e is preferably small relative to the wavelength λ. The core 8 can thus emit or receive electromagnetic waves on its short section that protrudes from the plate 10.

The metal diffusers 7 are here for example in the form of fine copper wires all parallel to each other and parallel to the core 8 mentioned above. These copper son have for example a length l of the order of 4 to 5 cm, and they can be fixed on the plate 10, for example by overmoulding by the resin forming this plate.

In the example described here, the antennas 2 of the first network 1 are conventional antennas arranged relatively far apart from each other with respect to the antennas of the second network 4, but of course the first network 1 could be identical or similar. to the second network 4.

The device which has just been described can be used for example to selectively communicate (simultaneously or not) the first network 1 with each antenna 5 of the second network 4.

For this purpose, during an initial learning step, an electromagnetic wave corresponding to a pulse signal having, for example, a duration of the order of 10 ns, is emitted successively by each reactive antenna 5.

This electromagnetic wave is received by the different antennas 2 of the first network 1, and the signals thus received by the antennas 2 respectively correspond to the impulse responses h _ij (t) between the reactive antenna 5 which has emitted the signal and each antenna 2 of the first network, i being an index which designates the reactive antenna 5 and j being an index which designates the antenna 2 concerned.

It will be noted that the impulse response h _ij (t) could be determined differently, for example by sending predetermined signals by the antennas j of the first network, by sensing the signals received by the antennas i of the second network, by transmitting the signals received. to the first CPU 3 (this transmission can be done by wire, radio or other) and processing these signals captured. An example of a process of this type is given in the document WO-2004/086557 .

The first CPU 3 then performs a time inversion of these impulse responses to thereby obtain h _ij (-t) signals.

This time inversion step can be performed for example as described in the publication of Leryose et al. (Physical review letters - May 14, 2004 - The American Physical Society - Vol.92, No. 19, pages 193904-1 at 193904-3).

Subsequently, when it is desired to transmit a signal S (t) to one of the reactive antennas 5 of index i, the first central unit 3 causes each antenna 2 of index j to transmit a signal S _ji (t). = If (t) ⊗h _ij (-t).

It will be noted that, in this way, the first central unit 3 can possibly transmit several signals S _i (t) in parallel, respectively to several reactive antennas 5 of indices i ₁ , i ₂ , i ₃ , etc.

For this purpose, during the focusing step, each antenna j of the first grating transmits electromagnetic waves corresponding to a superposition of signals S _ji (t) for several values of i (the signals S _ji (t) corresponding to the different reactive antennas i are summed before emission of the electromagnetic wave by each antenna index j).

Note that the bidirectional communication between the central units 3 and 6 can be further improved, if one proceeds to the initial learning step also by sending each antenna 2 a pulse signal during the step of learning so as to calculate then impulse responses h _ji (t) between each antenna 2 of index j and each antenna 5 index i. In this case, the second central unit 6 is also adapted to calculate and memorize the time inversions h _ji (-t) of these impulse responses. In this case, when the second central unit 6 has to transmit a signal S _j (t) to the antenna 2 _j of the first network 1, it causes the set of reactive antennas 5 to generate signals i $$S_{\mathit{ji}}\left(t\right)=S_i\left(t\right)\otimes h_{\mathit{ij}}\left(-t\right)\mathrm{.}$$

As explained above, these signals S _ij (t) may possibly be superimposed for several values of j, so as to transmit in parallel different messages to the different antennas 2 from the first central unit 6.

The device that has just been described can be used for example to communicate with each other electronic devices such as microcomputers or others at the scale of a room or a building, or even to communicate between them different circuits to inside the same electronic device, without physical connection between its circuits.

It should be noted that in the communication applications, the above-mentioned focus could be replaced by a correlation-based method or a method using registration and inversion of the transfer matrix to selectively transmit a signal to one of the reactive antennas 5.

Moreover, the invention can also be used to focus the electromagnetic waves on a weak focusing spot for the purpose of processing a material located at this focusing spot. In this case, the reactive antenna 5 may possibly be removed during the focusing step, the reactive diffusers remaining however present during this step.

Claims (21)

1. Method for the transmission of electromagnetic waves, in order to focus a wave of wavelength λ at at least one focal point of index i, the wave being emitted by antennas (2) of index j belonging to a first array (1), towards at least one antenna (5) located at the focal point i and belonging to a second array (4), characterized in that:

   - the antenna (5) belonging to the second array used at the focal point is reactive, so as to generate an evanescent field,

   - at least one diffuser (7) for the wave is used close to the focal point i, said diffuser being located at a distance smaller than a predetermined distance (R) from said focal point, said predetermined distance being at most equal to λ/10.
2. Method as claimed in claim 1, comprising at least:

   (a) a learning step in which an impulse response h_ij(t) between the focal point i and each antenna j (2) of the first array is determined from signals exchanged between the antennas j (2) of the first array and at least one antenna (5) belonging to the second array (4) ;

   (b) a focusing step during which waves corresponding to signals S_ji(t)=S_i(t)⊗h_ij(-t), are emitted from said antennas j (2) of the first array, where S_i(t) is a function of the time and h_ij(-t) is a temporal inversion of the impulse response h_ij(t) between the focal point i and the antenna j (2), at least the diffuser (7) remaining present around the focal point i during the focusing step.
3. Method as claimed in claim 2, in which, during the learning step:

   - a wave corresponding to a predetermined signal is emitted by the antenna (5) of the second array, said antenna being located at said focal point i,

   - signals generated by said wave are picked up on the antennas (2) of index j of the first array (1), and

   - an impulse response h_ij(t) between the focal point i and each antenna j (2) of the first array is determined from the signals picked up.
4. Method as claimed in claim 2 or claim 3, in which the antenna (5) of the second array is present at the focal point i during the focusing step and a communication is established between said antenna (5) and the antennas (2) of the first array.
5. Method as claimed in any one of claims 2 to 4, in which the learning step is carried out for several focal points of index i where antennas of the second array are placed respectively, each having at least one diffuser located at a distance smaller than said predetermined distance relative to the corresponding focal point i, and, during the focusing step, electromagnetic waves corresponding to at least signals S_ji(t)=S_i(t)⊗h_ij(-t), are emitted at each antenna j of the first array, where i is the index of one of the desired focal points.
6. Method as claimed in claim 5, in which, during the focusing step, electromagnetic waves corresponding to a superposition of signals S_ji(t)=S_i(t)⊗h_ij(-t), for several values of i, are emitted by each antenna j of the first array.
7. Method as claimed in any one of claims 5 and 6, in which the antennas (5) of the second array are present at the focal points i during the focusing step and, during the focusing step, a selective communication is established between the antennas j (2) of the first array and at least certain of said antennas (5) of the second array.
8. Method as claimed in any one of the preceding claims, in which several diffusers, preferably at least 10 diffusers, located at a distance smaller than said predetermined distance from the focal point i, are used.
9. Method as claimed in any one of the preceding claims, in which the predetermined distance (R) is at most equal to λ/50.
10. Method as claimed in any one of the preceding claims, in which the wave has a frequency f of between 0.7 and 50 GHz.
11. Method as claimed in any one of claims 1 and 10, in which the antenna (5) of the second array used at the focal point has an impedance having an imaginary part greater than the real part, so as to essentially generate a reactive field.
12. Method as any one of the preceding claims, in which the imaginary part of the impedance of the antenna (5) of the second array is greater than 50 times the real part.
13. The method as claimed in any one of preceding claims, in which metallic diffusers are used.
14. Device for receiving an electromagnetic wave of wavelength λ at at least one point of index i, this device comprising:

    - an antenna (5) located at the focal point i and belonging to a second array (4) is reactive, so as to generate an evanescent field, and

    - at least one metallic diffuser (7) for the electromagnetic wave, said diffusers being located at a distance smaller than a predetermined distance (R) from the point i, said predetermined distance being at most equal to λ/10, where λ is the wavelength of the electromagnetic wave.
15. Device as claimed in claim 14, comprising several metallic diffusers, preferably at least 10 metallic diffusers (7), at a distance smaller than the predetermined distance (R) from the point i.
16. Device as claimed in any one of claims 14 and 15, in which the predetermined distance (R) is at most equal to λ/50.
17. Device as claimed in claim 14, in which the antenna (5) of the second array has an impedance having an imaginary part greater than the real part, so as to essentially generate an evanescent field.
18. Device as claimed in claim 17, in which the imaginary part of the impedance is greater than 50 times the real part.
19. Device as claimed in any one of claims 14 to 18, comprising several antennas (2) of index j belonging to a first array (1), and an electronic central processing unit (3) controlling said antennas j (2) of the first array in order for electromagnetic waves corresponding to signals S_ji(t) = S_i (t)⊗h_ij(-t), to be emitted by said antennas j of the first array, where S_i(t) is a function of the time and h_ij(-t) is a temporal inversion of the impulse response h_ij(t) between the point i and each antenna j of the first array.
20. Device as claimed in claim 19, in which the second array comprises several antennas (5) that are located at several points of index i and are surrounded by metallic diffusers (7) located respectively at a distance smaller than said predetermined distance relative to the corresponding point i and the electronic central processing unit (3) is designed to make each antenna j (2) of the first array emit electromagnetic waves corresponding to at least signals $${\mathit{S}}_{\mathit{ji}}\left(\mathit{t}\right)={\mathit{S}}_{\mathit{i}}\left(\mathit{t}\right)\otimes {\mathit{h}}_{\mathit{ij}}\left(-\mathit{t}\right)\mathrm{.}$$

    
21. Device as claimed in claim 20, in which the electronic central processing unit (3) is designed to make each antenna j (2) of the first array emit electromagnetic waves corresponding to a superposition of signals S_ji(t)=S_i(t)⊗h_ij(-t), for several values of i.

Images