EP2047558B1 - Isotropic antenna and associated measurement sensor - Google Patents

Description

Technical field and prior art

The invention relates to an isotropic antenna capable of transmitting or receiving an electromagnetic field over a wide frequency spectrum. The invention also relates to a physical magnitude measuring sensor which comprises an antenna according to the invention.

The invention applies to communicating objects whose size is small compared to the wavelengths used for communication. Typically, the objects concerned by the invention are terminals having dimensions of the order of a few centimeters operating in the ISM (Industrial Scientific Medical), UHF (UHF for Ultra High Frequency), VHF ( VHF for "Very High Frequency"), SHF (SHF for "Super High Frequency"), EHF (EHF for "Extremly High Frequency").

The antennas that equip such terminals have reduced dimensions compared to operating wavelengths λ (typically less than 0.5 λ). This specificity of antennas defines a category of antennas commonly called "miniature antennas".

• les réseaux sans fil de capteurs disséminés : surveillance de bâtiments, surveillance de l'environnement, capteurs utilisés en milieu industriel ;
• la domotique : interrupteurs, télécommandes, etc. ;
• les accessoires pour réseaux personnels tels que kits mains libres, souris d'ordinateur, stylos intelligents, etc. ;
• les capteurs de capture de mouvements (objets, êtres vivants) ;
• les sondes de mesures de champ électromagnétique.

The proposed antenna is an antenna that applies, among other things, to low-range, low-speed and low-power applications such as, for example:

• wireless sensor networks disseminated: building surveillance, environmental monitoring, sensors used in industry;
• home automation: switches, remote controls, etc. ;
• accessories for personal networks such as handsfree kits, computer mouse, smart pens, etc. ;
• motion capture sensors (objects, living beings);
• the electromagnetic field measurement probes.

The applications mainly concerned by the invention are applications for which the orientation of one or more devices intended to communicate together is random and changing. The quality of the radio link must however remain constant regardless of the orientation. Ideally, an antenna whose radiation characteristics are substantially isotropic is therefore ideally sought. The proposed invention aims to answer this problem.

Conventionally, the antennas used to date in the applications mentioned above are omnidirectional type but it is noted, however, that they always have directions in which the radiation is zero. Transmission is impossible in these directions.

A second aspect affecting the quality of the transmission is the polarization mismatch of the waves transmitted or received by the antenna. When the polarization of the waves is linear, tilting the antennas relative to each other can lead to orthogonal polarization directions. In such a case, the transmitted power becomes zero.

The search for antenna structures with isotropic radiation began in the 1960s and 1970s for space applications. It continued until the 1990s. The problem then was: how to keep a constant radio link with a satellite or a space probe whose orientation can vary in any way during a transmission? All the solutions proposed were antennas of large dimensions, that is to say whose dimensions are equal to several times the wavelength of operation. Their operating principle does not allow miniaturize such antennas. For this reason and because of their unsuitable form factor, they are not transposable in the fields of application of the communicating objects of the invention.

With regard to the miniature antennas, two examples of antenna structure of the known art and their operating principle are presented below.

The figure 1 represents a first example of miniature antenna structure of the known art. Two dipoles D1, D2 of half-wave length are arranged orthogonally. The supply signals V1 and V2 respective dipoles D1 and D2 are applied at the intersection of the two dipoles. The power supplies are in quadrature phase: $$\mathrm{<figure-callout\; id="2"\; label="V"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">V</figure-callout>}2=\mathrm{V}1e^{\mathrm{j\pi }/2}$$

• l'écart entre le minimum et le maximum de puissance émise est typiquement de 4.7dB (ce qui est considéré comme une « bonne » isotropie en puissance ;
• la polarisation des ondes émises est circulaire dans la direction perpendiculaire au plan des dipôles et rectiligne dans le plan des dipôles ;
• la bande passante typique des ondes émises est sensiblement égale à 10% de la fréquence centrale.

The radiation of a dipole is generated by a current distribution which is established along the dipole, in a half-wave resonance mode. The produced radiation is then maximum in the direction orthogonal to the dipole and it is zero in the direction of the dipole. Due to the cross arrangement of the two dipoles and their phase quadrature power supply, the direction of the maximum radiation of one corresponds to the zero radiation direction of the other. The set of two dipoles radiates in all directions. The radiation is thus almost isotropic in power. In fact, the characteristics of the emitted radiation are as follows:

• the difference between the minimum and maximum power emitted is typically 4.7 dB (which is considered a "good" isotropy in power;
• the polarization of the transmitted waves is circular in the direction perpendicular to the plane of the dipoles and rectilinear in the plane of the dipoles;
• the typical bandwidth of the transmitted waves is substantially equal to 10% of the central frequency.

The Figures 2A and 2B represent a second example of miniature antenna structure of the known art. The antenna represented Figures 2A and 2B is an inverted F antenna commonly called IFA antenna (IFA for "Inverted F - Antenna").

An IFA antenna consists of an electrically conductive plane 1 (ground plane), a wired or planar metal part 2, commonly called "roof" of the antenna, arranged most often parallel to the ground plane (but can also not being parallel to the ground plane), an electrically conductive connection 3 placed at a first end of the roof, in a first plane perpendicular to the ground plane and which bypasses the roof and the ground plane, and an excitation means 4, for example a wire probe, placed in a second plane perpendicular to the ground plane and which is connected to an RF radiofrequency source which creates a potential difference between the roof and the ground plane. The second end of the roof 2 is in open circuit. The ground plane 1 preferably has dimensions larger than the roof so that, from a geometric point of view, the projection of the roof on the ground plane lies entirely within the ground plane.

The roof 2, the short-circuit 3 and the excitation means 4 draw, seen in profile, an inverted F which is at the origin of the name of the antenna (cf. Figure 2A ). The length 12 of the roof 2 is substantially equal to λg / 4, where λg is the guided wavelength of the antenna. The distance h separating the roof 2 from the ground plane 1 is on average equal to a small fraction of the wavelength λg, for example λg / 20, and the distance d which separates the plane in which the ground is placed. circuit the plane in which the excitation means is placed is chosen to match the impedance of the antenna to the RF source. A quarter-wave resonance mode is established between the roof 2 and the ground plane.

Such an antenna is not isotropic. It has a direction that has a high attenuation and this attenuation is all the more important that the ground plane is large. The difference between the minimum and the maximum power emitted by the antenna varies from 9.5 dB to 28 dB. The value of 9.5 dB is obtained for a small mass plane (i.e. l1 = 0.22λg) and the value of 28 dB for a large mass plane (i.e. l1 = 0.4 λg).

As far as the polarization is concerned, it is close to a linear state on the whole radiation pattern, except for two reduced aperture lobes for which the polarization is quasi-circular. The uniformity in circular polarization is therefore quite bad. The bandwidth is typically equal to 1.25% of the center frequency.

The documents US2006 / 0145926 , US6618016 FR2751471 and WO 2005/004283 describe antennas.

Miniature antennas of the known art have many disadvantages. The miniature antenna of the invention does not have these disadvantages.

Presentation of the invention

Indeed, the invention relates to an antenna intended for short range applications, and capable of guiding a wavelength λ, which comprises four elementary IFA antennas, each IFA antenna. elementary element comprising a ground plane, a roof defining a pattern, a short circuit between the ground plane and the roof and an excitation means, the four elementary IFA antennas being distributed around a reference axis in a first set two IFA antennas having substantially equivalent far-field elemental radiations and a second set of two IFA antennas having substantially equivalent far-field elementary radiations, the two excitation means of the two IFA antennas of the first set defining a first axis of alignment perpendicular to said reference axis, the two excitation means of the two IFA antennas of the second set defining a second alignment axis, perpendicular to said reference axis, the first alignment axis and the second alignment axis intersecting at right angle at a point of the reference axis, the excitation means of the four elementary IFA antennas being food s by radio-frequency signals of the same amplitude whose phases follow a progressive law substantially in quadrature by rotation about the reference axis (0 °, 90 °, 180 °, 270 °),
The antenna being remarkable in that the first set and the second set each having a length (L) of the roofs (2) less than λ / 4 so that the IFA antennas of the first set are coupled together, and the IFA antennas of the second set are also coupled together.

According to a further characteristic of the invention, the roofs of the four IFA antennas elementals are distributed on a flat surface substantially perpendicular to the reference axis.

According to another additional characteristic of the invention, the shape of the roofs of two elementary IFA antennas of the same set of two antennas is deduced by symmetry with respect to the point of intersection between the reference axis and the plane surface.

According to yet another additional feature of the invention, the roofs of the four elementary IFA antennas are substantially inscribed in a circle.

According to yet another additional feature of the invention, the roofs of the four elementary IFA antennas have an identical shape.

According to yet another additional characteristic of the invention, the roofs of the four elementary IFA antennas are substantially inscribed in an ellipse.

According to yet another additional feature of the invention, the roofs of the four elementary IFA antennas are distributed over a substantially conical closed surface.

According to yet another additional characteristic of the invention, the roofs of the four elementary IFA antennas are distributed on a cylindrical surface whose generator is parallel to the reference axis.

According to yet another additional feature of the invention, the cylindrical surface is a cylindrical surface whose directrix curve draws a circle, or a square, or a rectangle.

According to yet another additional characteristic of the invention, the roofs of the four elementary IFA antennas are formed by metallizations carried out on the same substrate.

According to yet another additional feature of the invention, the ground planes of the four elementary IFA antennas are formed by the same conductive layer.

According to yet another additional characteristic of the invention, the antenna comprises means for switching the progressive quadrature law between a first direction of rotation about the reference axis and a second direction of rotation about the opposite axis. in the first sense.

The invention also relates to a physical quantity measuring sensor comprising means for measuring the physical quantity and a transmitter equipped with an antenna able to transmit the measurement of the physical quantity in the form of a modulation of an electromagnetic wave. emitted by the transmitter, characterized in that the antenna is an antenna according to the invention.

An antenna according to the invention consists of a combination of four elementary IFA antennas. Preferably, an antenna according to the invention comprises a single ground plane, four electrically conductive patterns placed above the plane of each forming an IFA antenna roof, four short-circuit connections and four excitation means.

The four elementary IFA antennas are grouped according to two sets of two antennas, the two IFA antennas of the same set being designed so that their elementary radiations in far field are equivalent.

Two IFA antennas have equivalent far-field elemental radiations when, being placed independently in the same frame with the same orientation, they radiate in the band of useful frequencies, a wave of the same amplitude and of the same phase in each direction of space.

A simple way to obtain two IFA antennas with equivalent elementary radiations consists in producing identical antennas, that is to say having the same geometry (same shape and same dimensions). It is this embodiment that will be mainly described in the following patent application, as a preferred embodiment of the invention.

However, it is possible to make two IFA antennas having different shapes or dimensions and still having equivalent elemental radiations. Examples of such antennas will be described later, with reference to Figures 10A and 10B .

The ground plane of an antenna of the invention consists of a conductive element whose surface may admit, if necessary, metallization spares and electronic components. The surface of the ground plane may be a circular, elliptical, square, rectangular planar surface, a conical surface, a cylindrical, cubic, or parallelepipedic, cylindrical surface, and so on. In general, the surface that defines the ground plane has a symmetry with respect to an axis. The surface of the ground plane is of dimension greater than or equal to the surface in which the electrically conductive patterns forming roofs are integrated so that, from a geometrical point of view, the projection, on the ground plane, of the surface in which the motifs are integrated electrically conductive roofs lies entirely within the ground plane. The radiation of the antenna is all the more isotropic in power that the ground plane is small. This is why the ground plane will preferably be chosen of dimensions equal to the dimensions of the surface in which the electrically conductive patterns forming roofs are integrated. The ground plane will most often be larger when it has, for integration reasons, a circuit support function such as, for example, the RF circuit that supplies the elementary IFA antennas.

The RF circuit that supplies the four power supply connections can indeed be made on the upper or lower side of the ground plane. The influence of its presence on the radiation of the antenna is negligible when properly designed. Different possibilities of realization of the supply circuit are possible in the form of a parallel network or series of microstrip line including or not localized elements (couplers, phase shifters, etc.).

The patterns forming roofs may be son or flat elements whose contours can have very varied shapes: rectangular, trapezoidal, elliptical, arched or not, rounded at the ends or not, the general shape of a pattern and its dimensions strongly determining the radiation characteristics of the antenna, in particular its operating frequency. The patterns are arranged either parallel to the ground plane, or inclined at an angle to it (the angle of inclination of the patterns can be equal, for example, to 30 ° and can reach 45 ° or more). The patterns may be made on substrate by printed circuit techniques or by machining conductive parts, for example metal.

According to the preferred embodiment of the invention, the patterns are grouped into a first pair of identical patterns and a second pair of identical patterns. The patterns of a pair of identical patterns are aligned along an alignment axis perpendicular to the Oz axis of the antenna, the two alignment axes of the two pairs of patterns intersecting at right angles to the axis of the antenna. 'antenna. Also, the two conductive connections forming a short circuit between the ground plane and the ends of the conductive patterns of a pair of conductive patterns are arranged symmetrically with respect to the axis Oz. It is the same of the two excitation means associated with the two conductive patterns of the same pair of conductive patterns.

The four excitation means feed the four IFA antennas with signals of substantially equal amplitude, phase-shifted according to a progressive phase-quadrature law, so that for antennas a1-a4 which follow one another about the axis Oz (in clockwise or anti-clockwise), it comes: No. a1 a2 a3 a4 Phase shift 0 ° 90 180 ° 270 °

Two IFA antennas aligned along an axis perpendicular to the axis of the antenna are strongly coupled (typically -3 to -4 dB). Their power supplies are in phase opposition (180 °) but, because of their opposite orientations, their resonances are in phase. The coupling phenomenon is beneficial here because it advantageously allows a reduction in the length L of the roofs of the two IFA antennas which are facing each other compared to the case of a single isolated IFA having the same operating frequency. The dimension L can thus be less than λ / 4. The set is smaller than the simple combination of dipoles cross, which is an advantage of the invention.

Similarly, unlike the combination of cross dipoles for which the coupling between dipoles is weak (<-40dB), the coupling between two elementary IFA antennas of the invention whose roofs are perpendicular to each other is important. (-2 to -3dB). The concentrated electric field between the ground plane and the roof of the antenna is oriented in the normal direction to the ground plane. When two IFA antennas are arranged on the same ground plane, their field lines are oriented in the same direction perpendicular to the ground plane. There is then a strong coupling between them. This coupling is a function of the distance between the antennas and depends little on their orientations. For this reason, it is impossible to have two IFA antennas crosswise according to the principle of operation of the dipoles in cross. The strong coupling would not allow to feed the IFA antennas independently in quadrature phase.

In the context of the invention, the coupling between the pairs of orthogonal IFA antennas is reduced because of the central spacing left between them. The coupling is thus typically reduced between -7dB and -10dB, which allows a supply with a phase shift of 90 ° between adjacent IFA antennas. The spacing of the IFA antennas between them tends to increase the total dimension of all the antennas and therefore constitutes a limit to the miniaturization of the antenna. However, this is partially compensated by the coupling phenomenon mentioned above, thus making it possible to reduce the length of each elementary IFA antenna.

• Typiquement 3 à 6 dB d'écart entre le maximum et le minimum de puissance rayonnée sur l'ensemble du diagramme de rayonnement ;
• Polarisation circulaire dans la direction normale au plan de l'antenne ;
• Polarisation rectiligne dans le plan de l'antenne ;
• Les coordonnées polaires E_θ et E_ϕ du champ électrique émis ont des amplitudes égales ;
• La bande passante relative à -10dB est comprise entre 1 et 20% en fonction, notamment, du circuit RF d'alimentation utilisé et des caractéristiques des antennes IFA élémentaires.

From the point of view of the electromagnetic performances, an isotropic antenna according to the invention advantageously has the following characteristics:

• Typically 3 to 6 dB difference between the maximum and minimum radiated power over the entire radiation pattern;
• Circular polarization in the normal direction to the plane of the antenna;
• Rectilinear polarization in the plane of the antenna;
• The polar coordinates E _θ and E _φ of the emitted electric field have equal amplitudes;
• The relative bandwidth at -10 dB is between 1 and 20% depending, in particular, on the RF circuit used power supply and the characteristics of elementary IFA antennas.

Brief description of the figures

• La figure 1 déjà décrite représente un premier exemple de structure d'antenne miniature de l'art connu ;
• Les figures 2A et 2B déjà décrites représentent un deuxième exemple de structure d'antenne miniature de l'art connu ;
• La figure 3 représente une vue de dessus d'un premier exemple d'antenne selon le mode de réalisation préférentiel de l'invention ;
• La figure 4 représente une vue d'un deuxième exemple d'antenne selon le mode de réalisation préférentiel de de l'invention ;
• La figure 5 représente une vue en perspective d'un troisième exemple d'antenne selon le mode de réalisation préférentiel de l'invention ;
• La figure 6 représente une vue en perspective d'un quatrième exemple d'antenne selon le mode de réalisation préférentiel de l'invention ;
• La figure 7 représente une vue en perspective d'un cinquième exemple d'antenne selon le mode de réalisation préférentiel de l'invention ;
• Les figure 8A et 8B représentent, respectivement, une vue en perspective et une vue de dessus d'un sixième exemple d'antenne selon le mode de réalisation préférentiel de l'invention ;
• Les figures 9A et 9B représentent, respectivement, une vue en perspective et une vue de dessus d'un septième exemple d'antenne selon le mode de réalisation préférentiel de l'invention ;
• Les figures 10A et 10B représentent, respectivement, une vue en perspective et une vue de dessus d'exemples d'antennes miniatures selon une mode de réalisation différent du mode de réalisation préférentiel de l'invention ;
• Les figures 11A et 11B représentent des courbes comparatives de couvertures d'antennes de l'art antérieur et d'une antenne selon l'invention ;
• La figure 12 représente un histogramme comparatif du gain de couverture à 90% en polarisation rectiligne d'antennes de l'art antérieur et d'une antenne de l'invention ;
• La figure 13 représente une vue de profil d'un exemple de réalisation de capteur selon l'invention ;
• La figure 14 représente une application du capteur de l'invention à la capture de mouvement.

Other features and advantages of the invention will appear on reading a preferred embodiment with reference to the appended figures, among which:

• The figure 1 already described represents a first example of miniature antenna structure of the prior art;
• The Figures 2A and 2B already described represent a second example of miniature antenna structure of the prior art;
• The figure 3 represents a top view of a first antenna example according to the preferred embodiment of the invention;
• The figure 4 represents a view of a second antenna example according to the preferred embodiment of the invention;
• The figure 5 is a perspective view of a third antenna example according to the preferred embodiment of the invention;
• The figure 6 is a perspective view of a fourth antenna example according to the preferred embodiment of the invention;
• The figure 7 is a perspective view of a fifth antenna example according to the preferred embodiment of the invention;
• The Figure 8A and 8B represent, respectively, a perspective view and a top view of a sixth antenna example according to the preferred embodiment of the invention;
• The Figures 9A and 9B represent, respectively, a perspective view and a top view of a seventh example antenna according to the preferred embodiment of the invention;
• The Figures 10A and 10B represent, respectively, a perspective view and a top view of examples of miniature antennas according to a different embodiment of the preferred embodiment of the invention;
• The Figures 11A and 11B represent comparative curves of antenna covers of the prior art and of an antenna according to the invention;
• The figure 12 represents a comparative histogram of the 90% coverage gain in rectilinear polarization of antennas of the prior art and of an antenna of the invention;
• The figure 13 represents a side view of an exemplary embodiment of a sensor according to the invention;
• The figure 14 represents an application of the sensor of the invention to motion capture.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The Figures 3-9B illustrate different examples of antennas according to the preferred embodiment of the invention. According to the preferred embodiment of the invention, the roof patterns of the IFA antennas are identical in pairs, two identical patterns being aligned along an alignment axis perpendicular to the axis of the antenna.

The figure 3 represents a first example of an antenna according to the preferred embodiment of the invention. The four conductive patterns 2 forming roofs IFA antennas are all identical (for example, in the form of rectangle) and inscribed in a circle C. The conductive connections that connect the conductive patterns forming roofs to the ground plane are placed at the outer ends of the grounds (ie substantially on the periphery of the circle C), in planes perpendicular to the plane of the figure. The roof patterns may be discrete metallic elements or conductive elements made on the same substrate.

The figure 4 represents a top view of a second antenna example according to the preferred embodiment of the invention. The four rectangular-shaped conductor patterns 2 are distributed over an ellipse E. The conductive patterns 2 may be discrete elements or elements made on the same substrate.

The figure 5 represents a perspective view of a third antenna example according to the preferred embodiment of the invention. The conductive patterns forming roofs 2 are in the form of parallelepipeds. Patterns 2 are here formed on a same substrate S. They could also be discrete elements.

The figure 6 represents a perspective view of a fourth antenna example according to the preferred embodiment of the invention. The ground plane 1 has a conical surface and the conductive patterns 2 are arranged on a substrate which also has a conical shape. The axis of symmetry Oz is here the axis of the cones.

The figure 7 represents a perspective view of a fifth antenna example according to the preferred embodiment of the invention. The roof patterns of IFA antennas are distributed on a cylindrical surface whose generator is parallel to the axis of symmetry of the antenna and whose directing curve draws a square.

The Figures 8A and 8B represent two views of a sixth antenna example according to the preferred embodiment of the invention. The roof patterns of the IFA antennas are located in the same plane perpendicular to the axis of the antenna and are bent to be inscribed in a square surface.

The Figures 9A and 9B represent two views of a seventh antenna example according to the preferred embodiment of the invention. The roof patterns of the IFA antennas are located in the same plane perpendicular to the axis of the antenna and are folded to be inscribed in a circular surface. The patterns 2 are folded, for example, in the form of spirals. The patterns 2 are distributed on a substrate Circular S placed next to a circular mass plane. The circles defined by the ground plane and the substrate S are parallel and their centers are aligned along the axis Oz.

The Figures 10A and 10B represent, respectively, a perspective view and a top view of examples of miniature antennas according to a different embodiment of the preferred embodiment of the invention. The two IFA antennas of a set of two aligned antennas have substantially equivalent far-field radiations but their geometries are not identical.

The figure 10A represents an example where two aligned elementary IFA antennas have roofs of different lengths la, lb and different heights ha, hb with respect to the ground plane. The figure 10B is another example where each pair of two aligned elementary IFA antennas comprises an antenna whose roof is rectangular (2a, 2c) and another antenna whose roof is elliptical (2b, 2d).

By way of nonlimiting example, a detailed description of an antenna corresponding to the seventh example of the preferred embodiment of the invention is given below.

The roof patterns of the IFA antennas are made on an epoxy glass substrate (ε _r = 4.4, tgδ = 0.018 = loss tangent) 0.38 mm thick covered with a copper metallization of 17 μm thick . The roof patterns are made by photolithography. The ground connections 3 are located at the outer ends of the patterns 2. The connections 3 are copper wires 0.6 mm in diameter, one end of which is welded to the pattern 2 and the other end to the ground plane. The supply wires 4 are also 0.6mm diameter copper wires. The ends of the ground wires 3 and the supply wires 4 which are located on the side of the substrate S are distributed on a circle X.

The distance separating, on the same pattern 2, the end of the ground wire 3 from the end of the feed wire 4 is substantially equal to 3.6 mm. The distance separating the ground plane 1 from the substrate S is substantially equal to 4 mm. The diameter of the substrate S is substantially equal to 25 mm and the diameter of the ground plane is greater than the diameter of the substrate S, for example equal to 30 mm. As has already been mentioned above, other values of the diameter of the ground plane can be envisaged if the condition is respected by a diameter greater than or equal to the diameter of the substrate S.

The antenna described above has an operating frequency substantially equal to 2.5GHz. In a manner known per se, the bandwidth and the exact frequency of impedance matching also depend on the power supply network used.

The difference between the minimum and the maximum of the power emitted by the antenna is typically 5.6dB, which corresponds to a good power isotropy. The polarization of transmitted waves is circular along the axis Oz and rectilinear in the plane of the patterns 2. The The average axial ratio diagram is substantially 49%.

By way of comparison, the table below shows the typical performances of the difference between maximum and minimum of the directivity and mean diagram on the axial ratio diagram for the antenna of the invention and two antennas of the prior art. that is, the combination of cross dipoles and the IFA antenna alone.

The difference between the maximum and the minimum of the directivity diagram makes it possible to quantify the isotropy in power. The weaker this is, the better the isotropy in power. The average of the axial ratio diagram makes it possible to quantify the uniformity of the polarization with respect to the circular state. An average of 100% means that the antenna radiates with a perfectly circular polarization in all directions. Difference between maximum and minimum of the directivity diagram (dB) Average on the axial report diagram Combination of dipoles in cross 4.7 dB 46% IFA antenna only > 9.5 dB 21% Antenna according to the invention 5.6 dB 49%

Another significant criterion makes it possible to compare antennas with each other. This criterion is the coverage of antennas. The coverage of an antenna is the proportion of orientation / inclination covered by the antenna as a function of the minimum power it receives when it is illuminated by an incident plane wave of unit power density. The coverage curves of the three antennas mentioned above (combination of cross dipoles, IFA antenna alone and antenna according to the invention) are represented in FIGS. Figures 11A and 11B . The ordinates of the curves 11A and 11B are expressed in percentages and the abscissas in decibels. The Figure 11B is a detail view of the figure 11A in the area corresponding to cover greater than 60%. Moreover, the figure 12 represents a comparative histogram of the 90% coverage gain, in linear polarization, for the three antennas considered: the gain G1 corresponds to the half-wave dipoles, the gain G2 corresponds to a single antenna IFA and the gain G3 corresponds to an antenna according to the invention.

The curves C1, C2, C3 of Figures 11A and 11B are the typical typical coverage curves of an antenna according to the invention (typical size λ / 5), an IFA antenna alone and a combination of cross dipoles (typical size λ / 2).

It emerges from these figures that the antenna according to the invention makes it possible to find all the advantages of the combination of cross dipoles in the field of wide covers despite its reduced size.

The figure 13 represents a side view of an exemplary embodiment of sensor provided with an antenna according to the invention. The antenna is, for example, an antenna as described in Figures 9A-9B .

The sensor comprises a multilayer printed circuit board CI consisting of an insulating layer 5 on which are deposited, on one side, a conductive layer 6 which constitutes the ground plane and, on the other side, a substrate 7 on which are integrated different circuits x1, x2, x3 such as integrated circuits, battery, sensor, RF power supply network, etc. The dimensions of the sensor are small, so that the antenna is the largest component. The diameter D of the sensor is thus typically equal to λ / 5 or λ / 4. This dimension is to be compared to the diameter λ / 2 half-wave dipoles cross. The realization of the sensor in printed circuit technology advantageously allows mass production at low costs.

The combination of electronic circuits and the antenna advantageously allows the realization of an autonomous sensor. Components and devices placed under the ground plane disturb the radiation very little.

An example of use of the isotropic antenna of the invention will now be described, in the context of a TDMA network (TDMA for Time Division Multiple Access), with reference to the figure 14 .

• le noeud N1 est un point d'une raquette de tennis ;
• le noeud N2 est un point d'une balle de tennis ;
• les noeuds N3-N14 sont des points du corps d'un joueur de tennis.

The TDMA network is a star network for motion capture that includes a master node NM and a set of slave nodes N1-N14 that are moving relative to the master node. Each slave node of the network is placed a sensor which comprises an antenna according to the invention. The slave nodes are distributed as follows:

• the node N1 is a point of a tennis racket;
• node N2 is a point of a tennis ball;
• N3-N14 knots are points in the body of a tennis player.

This star network, orchestrated by the master node, makes it possible to recover, at determined time intervals, the data delivered by the various sensors whose positions vary over time.

Each sensor located at a slave node is optimized in terms of size, integration and power consumption. It consists of a physical measurement sensor and its conditioning, a processing unit and a radio transmitter / receiver connected to an isotropic antenna according to the invention. Autonomous, it has an on-board power source.

The sensor located at the master node is less subject to constraints of size and consumption but also has a radio transmitter / receiver and a processing unit. The antenna that equips the sensor located at the master node may be an isotropic antenna according to the invention or a dipole antenna.

All the advantage of the antenna according to the invention in this context lies in its radiation pattern which covers the entire space, in its circular polarization state which optimizes the radio transmission whatever the inclination of the sensors and in its low volume requirement.

The antenna according to the invention which equips each sensor located at a slave node has an isotropic radiation power in all directions and an optimized circular polarization so that there is no direction for which the transmission between a slave node and the master node would be interrupted. The antenna according to the invention equipping the slave nodes is circularly polarized, and the antenna equipping the master node is polarized rectilinearly. Thus, the transmission can not be interrupted due to polarization mismatch.

The antenna according to the invention increases very little the overall dimensions of the sensors because its planar form factor with a ground plane on one of these faces allows easy integration on the sensor. The antenna can be made with the same printed technology as the rest of the sensor circuit. The functions of the sensor and the battery integrate multilayer under the ground plane of the antenna as previously mentioned.

A description of the operation of the TDMA protocol connecting the master node to the slave nodes will now be given.

During a TDMA network nominal cycle, the master node transmits a time synchronization word and information addressed to the slave nodes, as well as a cyclic redundancy code also known as CRC code (CRC for "Cyclic Redundancy Code"). "). After which the slave nodes transmit, one after another, their data to the master node and a CRC code to detect communication errors. When all slave nodes have transmitted their data, they can go dormant until the next cycle to increase their autonomy. During this period of time, network management can then be ensured: detection of new slave node, management of communication channels, configuration of slave nodes.

Due to the isotropy of the antenna which equips them, the sensors of the invention advantageously make it possible to ensure a robust radio frequency communication link to the variations of positions. Fewer errors are detected and the use of the information retrieval procedure is much less necessary, helping to optimize real-time throughput and limit sensor consumption.

Different variants of antennas can be made in the context of the invention, namely, for example, reconfigurable antennas, diversity antennas or antennas with limited coverage at half-spaces.

Reconfigurable antennas include means for switching phase states. A first phase state can then correspond to a 0 ° → 90 ° → 180 ° → 270 ° phase progression between the different elementary antennas, whereas a second phase state corresponds to a 0 ° → -90 ° phase progression. → -180 ° → -270 ° between these same elementary antennas. Phase switching advantageously makes it possible to go from waves in right circular polarization to waves in left circular polarization and reciprocally.

In the context of the invention, the diversity antennas are made, when the coupling level between elementary IFA antennas allows it, by feeding them by two channels or by four independent channels.

Claims (13)

1. An antenna for low-range application able to guide a wavelength λ, which comprises four elementary IFA antennae, each elementary IFA antenna comprising a ground plane (1), a roof (2) defining a pattern, a short-circuit (3) between the ground plane and the roof and an excitation means (4), the four elementary IFA antennae being distributed around an reference axis (z) in a first set of two IFA antennae having substantially equivalent far field elementary radiations and a second set of two IFA antennae having substantially equivalent fair field elementary radiations, the two excitation means of the two IFA antennae of the first set defining a first alignment axis perpendicular to said reference axis, the two excitation means of the two antennae of the second set defining a second alignment axis perpendicular to said reference axis, the first alignment axis and the second alignment axis crossing at a right angle at one point of the axis, the excitation means (4) of the four elementary IFA antennae being fed by radiofrequency signals of same amplitude whereof the phases follow a law which is substantially progressive in quadrature by rotation around the axis (0°, 90°, 180°, 270°)
   the antenna being characterized in that roof of the first set and the second set each have a length (L) inferior to λ/4 so that IFA antennae of the first set are coupled together, and the IFA antennae of the second set are also coupled together.
2. The antenna according to claim 1, in which the roofs of the four elementary IFA antennae are distributed on a flat surface substantially perpendicular to the axis.
3. The antenna according to claim 2, in which the roof shape of two antennae of a same set is deduced by symmetry with respect to the intersection of the reference axis (z) and the flat surface
4. The antenna according to claim 2 or 3, in which the roofs of the four elementary IFA antennae are substantially inscribed in a circle.
5. The antenna according to claim 4, in which the four elementary IFA antennae are all identical.
6. The antenna according to claim 4, in which the roofs of the four elementary IFA antennae are substantially inscribed in an ellipsis.
7. The antenna according to claim 1, in which the roofs of the four elementary IFA antennae are distributed on a substantially conical closed surface.
8. The antenna according to claim 1, in which the roofs of the four elementary IFA antennae are distributed on a cylindrical surface whereof the generatrix is parallel to the axis (Oz).
9. The antenna according to claim 8, in which the cylindrical surface is a cylindrical surface whereof the directing curve draws a circle, or a square, or a rectangle.
10. The antenna according to any one of the preceding claims, in which the roofs of the four elementary IFA antennae are formed by metallizations realized on a same substrate (S).
11. The antenna according to any one of the preceding claims, in which the ground planes of the four elementary IFA antennae are formed by a same conductive layer.
12. The antenna according to any one of the preceding claims which comprises means for switching the progressive law in quadrature between a first direction of rotation around the axis and a second direction of rotation around the axis, opposite the first direction.
13. A sensor for measuring measurable quantity comprising means for measuring a measurable quantity and a transmitter provided with an antenna able to transmit the measurement of the measurable quantity in the form of a modulation of an electromagnetic wave emitted by the transmitter, wherein the antenna is an antenna according to any one of claims 1 to 12.

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