EP1225655B1 - Dual-band planar antenna and apparatus including such an antenna device - Google Patents

Description

The present invention relates generally to radio transmission devices, in particular portable radiotelephones, and more particularly to antennas which are produced according to the microstrip technique to be included in such devices.

Such an antenna comprises a patch which is typically formed by etching a metal layer. It is called in English by specialists "microstrip patch antenna" for "microstrip pellet antenna".

The microstrip technique is a planar technique that applies both to the realization of lines transmitting signals and to that of antennas coupling between such lines and radiated waves. It uses ribbons and / or conductive pads formed on the upper surface of a thin dielectric substrate. A conductive layer extends over the lower surface of this substrate and constitutes a ground of the line or antenna. Such a pellet is typically wider than such ribbon and its shapes and dimensions are important features of the antenna. The shape of the substrate is typically that of a rectangular flat sheet of constant thickness and the pellet is also typically rectangular. But a variation of the thickness of the substrate can widen the bandwidth of such an antenna and its pellet can take various forms and for example be circular. The electric field lines extend between the ribbon or pellet and the ground layer as they pass through the substrate.

Antennas made according to these techniques typically, although not necessarily, resonant structures capable of being the seat of standing waves for coupling with radiated waves in space.

Various types of resonant structures may be made according to the microstrip technique and may be the seat of various resonance modes, such modes being more briefly referred to hereinafter as "resonances". In schematic form, each such resonance can be described as being a standing wave formed by the superposition of two progressive waves propagating in two opposite directions on the same path, these two waves resulting from the alternative reflection of the same progressive electromagnetic wave at both ends of this path. In the context of such a description, it is considered that the latter wave propagates in an electromagnetic line which would be constituted by the mass, the substrate and the pellet and which would define a linear path devoid of width. In fact such a wave has wavelengths that extend transversely over the entire section that is offered to them by the antenna so that this description simplifies the reality in a sometimes excessive manner. Since it can be considered linear this path can be rectilinear or curved. It will be referred to hereinafter as the "resonance path". The frequency of the resonance is inversely proportional to the time taken by the traveling wave considered above to travel this path.

A first type of resonance can be called a "half wave". In this type, the length of the resonance path is typically substantially equal to half a wavelength, that is to say half the wavelength of the progressive wave considered above. The antenna is then called "half-wave". This type of resonance can be defined in a general manner by the presence of an electrical current node at each of the two ends of such a path, the length of which can therefore also be equal to said half-wavelength multiplied by an integer other than one. This number is typically odd. The coupling with the radiated waves is at least one of the two ends of this path, these ends being located in the regions where the amplitude of the electric field prevailing in the substrate is maximum.

A second type of resonance that can be obtained in the context of this same technique can be called "quarter wave". It differs from said half-wave type on the one hand in that the resonance path typically has a length substantially equal to one quarter wave, that is to say one quarter of the wavelength defined above. For this the resonant structure must include a short circuit at one end of this path, the word short circuit designating a connection connecting the mass and the chip. In addition this short circuit must have an impedance sufficiently small to be able to impose such a resonance. This type of resonance can be defined in a general manner by the presence of an electric field node fixed by such a short circuit in the vicinity of an edge of the chip and by an electrical current node located at the other end of the resonance path. The length of the latter can also be equal to an integer number of half-wavelengths adding to said quarter wavelength. The coupling with the spatially radiated waves is at an edge of the pellet in a region where the magnitude of the electric field across the substrate is sufficiently large.

Resonances of other more or less complex types can be established in antennas of this kind, each resonance being characterized by a distribution of electric and magnetic fields oscillating in a zone of space including the antenna and the immediate vicinity of this one. They depend in particular on the configuration of the pellets, the latter may in particular have slots, possibly radiative. They also depend on the possible presence and localization of short-circuits as well as electrical models representative of these short-circuits when they are imperfect short circuits, that is to say when they are not assimilated, even approximately, to perfect short circuits whose impedances would be zero.

The present invention finds application in various types of devices such as portable radiotelephones, base stations for the latter, automobiles and aircraft or air missiles. In the case of a portable radiotelephone, the continuous nature of the lower ground layer of a microstrip antenna limits the radiation power that is intercepted by the user's body of the device when the device operates program. In the case of automobiles and especially in the case of airplanes or missiles whose outer surface is metallic and has a curved profile making it possible to obtain low aerodynamic drag, the antenna may be shaped to this profile so as not to reveal additional aerodynamic drag annoying.

• elle doit être bi-fréquence c'est à dire qu'elle doit pouvoir émettre et/ou recevoir efficacement des ondes rayonnées sur deux fréquences séparées par un écart spectral important,
• elle doit pouvoir être raccordée à un organe de traitement de signal à l'aide d'une seule ligne de raccordement pour l'ensemble des fréquences de travail d'un dispositif de transmission sans donner naissance dans cette ligne à un taux d'ondes stationnaires parasites gênant,
• et il ne doit pas être nécessaire pour cela d'utiliser un multiplexeur ou démultiplexeur en fréquence.

This invention relates more particularly to the case where an antenna made according to the microstrip technique must have the following qualities:

• it must be bi-frequency, that is to say it must be able to emit and / or efficiently receive waves radiated on two frequencies separated by a large spectral difference,
• it must be capable of being connected to a signal processing unit by means of a single connecting line for all the working frequencies of a transmission device without giving rise in this line to a standing wave ratio troublesome parasites,
• and it must not be necessary for this purpose to use a frequency multiplexer or demultiplexer.

• Une première telle antenne connue est décrite dans le document de brevet US- A 4,766,440 (Gegan ). La pastille 10 de cette antenne a une forme générale rectangulaire permettant à cette antenne de présenter deux résonances demi onde dont les trajets s'établissent selon une longueur et une largeur de cette pastille. Par ailleurs elle présente une fente incurvée en forme de U qui est entièrement intérieure à cette pastille. Cette fente est radiative et fait apparaître un mode de résonance supplémentaire s'établissant selon un autre trajet. Elle permet en outre, par un choix convenable de sa forme et de ses dimensions, d'amener les fréquences des modes de résonance à des valeurs souhaitées ce qui donne la possibilité d'émettre une onde à polarisation circulaire grâce à l'association de deux modes ayant une même fréquence et des polarisations linéaires croisées et déphasées de 90°. Le dispositif de couplage présente la forme d'une ligne qui est réalisée selon la technique des microrubans mais dont il est aussi dit qu'elle est coplanaire, ceci parce que le microruban s'étend dans le plan de la pastille et pénètre entre deux encoches de cette dernière. Ce dispositif est muni de moyens de transformation d'impédance pour l'adapter aux différentes impédances d'entrée respectivement présentées par la ligne aux différentes fréquences de résonance utilisées comme fréquences de travail.

Many known antennas have been made or proposed as part of the microstrip technique so as to have these three qualities. They differ from each other by the means used to obtain several resonant frequencies. Three such antennas will be examined:

• A first such known antenna is described in the patent document US-A 4,766,440 (Gegan ). The patch 10 of this antenna has a generally rectangular shape allowing this antenna to present two half-wave resonances whose paths are established along a length and a width of this patch. In addition, it has a shaped curved slot of U which is entirely interior to this pellet. This slot is radiative and shows an additional resonance mode established in another path. It also allows, by a suitable choice of its shape and its dimensions, to bring the frequencies of the resonance modes to desired values which gives the possibility of emitting a circularly polarized wave through the combination of two modes having the same frequency and linear polarizations crossed and out of phase by 90 °. The coupling device has the form of a line which is made according to the microstrip technique but which is also said to be coplanar, because the microstrip extends in the plane of the pellet and penetrates between two notches of the latter. This device is provided with impedance transformation means to adapt it to the different input impedances respectively presented by the line at the different resonance frequencies used as working frequencies.

• La nécessité de prévoir des moyens de transformation d'impédance complique la réalisation.
• L'ajustement précis des fréquences de résonance à des valeurs souhaitées est difficile à réaliser.

This first known antenna has the following drawbacks:

• The need to provide impedance transformation means complicates the implementation.
• Accurate adjustment of the resonance frequencies to desired values is difficult to achieve.

A second known antenna is described in the patent document US-A 4,692,769 (Gegan ). In a first mode of implementation the pellet of this antenna has the shape of a circular disc 10 allowing this antenna to present two half-wave resonances. The coupling system is in the form of a line 16 constituting a quarter-wave transformer and connected at an inner point to the area of the wafer so as to give the real part of the input impedance of the antenna of substantially equal values for these two resonances. Line 16 is made according to the microstrip technique. Two slots are formed in the conductive layer of this pastille and enter the area thereof from its periphery to delimit between them the ribbon of a terminal segment of this line. One of these two slots is continued by an extension which constitutes an impedance matching slot 28.

• La réalisation des moyens de transformation d'impédance est délicate.
• L'ajustement précis des fréquences de résonance à des valeurs souhaitées est difficile à réaliser.

This second known antenna has the following drawbacks:

• The realization of the impedance transformation means is tricky.
• Accurate adjustment of the resonance frequencies to desired values is difficult to achieve.

A third known dual-frequency antenna differs from the previous ones by the use of a quarter-wave resonance. It is described in an article: IEEE ANTENNAS AND PROPAGATION SOCIETY INTERNATIONAL DIGEST SYMPOSIUM, NEWPORT BEACH, JUNE 18-23, 1995, pages 2124-2127 Boag et al "Dual Band Cavity-Backed Quarter-Wave Patch Antenna A first resonance frequency is defined by the dimensions and characteristics of the substrate and the pellet of this antenna.A resonance of substantially the same type is obtained at a second frequency on the same resonance path thanks to the use of an adaptation system.

• L'écart entre les deux fréquences de résonance est trop petit dans certains cas d'application.
• La nécessité d'utiliser un système d'adaptation complique la réalisation de l'antenne.
• Il peut en être de même de la réalisation du dispositif de couplage de l'antenne sous la forme d'une ligne coaxiale.

This third known antenna has the following drawbacks:

• The difference between the two resonance frequencies is too small in some application cases.
• The need to use an adaptation system complicates the realization of the antenna.
• It may be the same for the realization of the coupling device of the antenna in the form of a coaxial line.

• permettre de réaliser simplement une antenne bi-fréquence,
• permettre de choisir plus librement que précédemment le rapport des fréquences centrales de deux bandes de travail d'un dispositif de transmission, et plus particulièrement de réaliser pour ce dispositif une antenne telle que le rapport de deux fréquences de résonance utiles de cette antenne soit compris entre 1,25 et 5 environ et notamment voisin de 2,
• donner à cette antenne une bande passante suffisamment large autour de chacune de ces deux fréquences de résonance pour permettre de situer dans chacune de ces deux bandes une fréquence d'émission et une fréquence de réception de ce dispositif,
• permettre un ajustement facile et précis de ces deux fréquences de résonance,
• permettre d'utiliser un dispositif de couplage unique et facilement adaptable en impédance pour chacune de ces deux fréquences de résonance, et
• limiter les dimensions de cette antenne.

A fourth known antenna is described in the document US5926139 (Ilya A. Korisch ). This document describes a dual-band planar antenna having a separating slot. The present invention has the following aims in particular:

• allow to realize simply a dual-frequency antenna,
• allow to choose more freely than before the ratio of the center frequencies of two working bands of a transmission device, and more particularly to achieve for this device an antenna such that the ratio of two useful resonant frequencies of this antenna is between 1.25 and 5 approximately and in particular close to 2,
• give this antenna a bandwidth sufficiently wide around each of these two resonance frequencies to allow to locate in each of these two bands a transmission frequency and a reception frequency of this device,
• enable easy and precise adjustment of these two resonance frequencies,
• allow to use a single coupling device and easily adaptable impedance for each of these two resonant frequencies, and
• limit the dimensions of this antenna.

• une masse conductrice
• un substrat diélectrique formé sur cette masse, et
• une pastille formée sur ce substrat. Cette pastille a une aire et une périphérie. Elle présente une fente séparatrice ayant de préférence une origine située sur cette périphérie et un fond situé dans cette aire, ce fond laissant un passage entre lui et cette périphérie. Cette fente pénètre dans cette aire à partir de cette origine et coopère avec cette périphérie pour délimiter dans cette aire une première plage et une deuxième plage, ces deux plages étant conductrices et étant électriquement mutuellement séparées par cette fente et raccordées par ce passage.

For these purposes it has for particular a planar antenna. This antenna includes mutually superposed layers respectively constituting:

• a conductive mass
• a dielectric substrate formed on this mass, and
• a pellet formed on this substrate. This pellet has an area and a periphery. It has a separating slot preferably having an origin located on this periphery and a bottom located in this area, this background leaving a passage between it and this periphery. This slot enters this area from this origin and cooperates with this periphery to define in this area a first range and a second range, these two areas being conductive and being electrically mutually separated by this slot and connected by this passage.

• une couche diélectrique formée sur cette pastille, et
• une armature conductrice formée sur cette couche diélectrique.

According to this invention this antenna further includes a reactive element mutually coupling these two conductive pads, and this element is preferably flat, for example of the kind called "Surface Mounted Component", not to form a significant projection on the planar structure of the antenna. It is for example constituted by a capacitor having a lower area to each of said areas of the first and second ranges. This area is internal to the area of the pellet and it extends continuously on the first beach, on the separating slot away from the bottom thereof, and on the second beach. This capacitor is formed by said mutually superimposed layers cooperating with said pellet and constituting respectively:

• a dielectric layer formed on this chip, and
• a conductive reinforcement formed on this dielectric layer.

A flat reactive element may, however, have another form for coupling according to this invention. It may for example be constituted by an interdigitated capacitance integrated in the layout of the separating slot by adequate cutting of the edges opposite the two areas of the pellet.

Preferably, this antenna further includes a short circuit electrically connecting the first conductive pad to the ground in the vicinity of the origin of the separating slot.

More preferably the area of the capacitor is between 1% and 25% of that of the pellet.

Finally, the origin of the separating slot is preferably close to the short-circuit so as to give the two resonances two respective resonance paths extending both from this short circuit, one of these two paths s' extending only in the first range and the other extending in this first range and in the second range.

• La figure 1 représente une vue en perspective d'un dispositif de transmission réalisé selon cette invention.
• La figure 2 représente une vue de dessus d'une antenne réalisée selon cette invention et analogue à celle du dispositif de la figure 1.
• La figure 3 représente une vue partielle en coupe verticale de l'antenne de la figure 2.
• La figure 4 reproduit la vue de la figure 2 pour permettre de désigner diverses dimensions de la même antenne.

Various aspects of the present invention will be better understood from the description below and the accompanying schematic figures. When elements are represented on several of these figures, they are designated by the same numbers and / or letters of reference.

• The figure 1 is a perspective view of a transmission device made according to this invention.
• The figure 2 represents a view from above of an antenna made according to this invention and similar to that of the device of the figure 1 .
• The figure 3 represents a partial view in vertical section of the antenna of the figure 2 .
• The figure 4 reproduces the view of the figure 2 to allow to designate various dimensions of the same antenna.

• Un substrat diélectrique 2 présentant deux surfaces principales mutuellement opposées s'étendant selon des directions horizontales constituant une direction longitudinale DL et une direction transversale DT. Ce substrat peut présenter des formes diverses comme précédemment exposé. Ses deux surfaces principales constituent respectivement une surface inférieure et une surface supérieure.
• Une couche conductrice inférieure s'étendant par exemple sur la totalité de cette surface inférieure et constituant une masse 4 de cette antenne.
• Une couche conductrice supérieure s'étendant sur une aire de cette surface supérieure au-dessus de la masse 4 de manière à constituer une pastille 6. De manière générale cette pastille a une longueur et une largeur s'étendant respectivement selon les directions DL et DT, et sa périphérie peut être considérée comme constituée par quatre bords. L'un de ces bords s'étend de manière générale selon la direction DT et constitue un bord arrière incluant deux segments 10 et 11. Un bord avant 12 est opposé à ce bord arrière. Un premier et un deuxième bords latéraux 14 et 16 s'étendent de manière générale selon la direction DL et joignent ce bord arrière à ce bord avant.
• Enfin un court-circuit raccordant électriquement la pastille 6 à la masse 4 à partir du segment 10 du bord arrière de cette pastille. Dans le mode représenté de mise en oeuvre de cette invention, ce court-circuit est formé par une couche conductrice S s'étendant sur une surface de tranche du substrat 2, surface qui est typiquement plane et constitue alors un plan de court-circuit. Il impose à au moins une résonance de l'antenne de présenter au moins approximativement un noeud de champ électrique au voisinage du segment 10 de manière à être sensiblement du type quart d'onde. Les bords arrière, avant et latéraux et les directions longitudinale et transversale sont définis par la position d'un tel court-circuit dans la mesure où ce court-circuit est suffisamment important, c'est à dire notamment où son impédance est suffisamment basse pour imposer à l'antenne l'existence d'une résonance présentant un tel noeud de champ électrique.

In accordance with Figures 1 to 3 and in a manner known per se, the resonant structure of an antenna according to this invention comprises the following elements:

• A dielectric substrate 2 having two mutually opposite main surfaces extending in horizontal directions constituting a longitudinal direction DL and a transverse direction DT. This substrate may have various shapes as previously stated. Its two main surfaces constitute respectively a lower surface and an upper surface.
• A lower conductive layer extending for example over the entire lower surface and constituting a mass 4 of this antenna.
• An upper conductive layer extending over an area of this upper surface above the mass 4 so as to form a pellet 6. In general this pellet has a length and a width respectively extending along the directions DL and DT , and its periphery can be considered as consisting of four edges. One of these edges extends generally in the direction DT and constitutes a rear edge including two segments 10 and 11. A front edge 12 is opposite this rear edge. First and second side edges 14 and 16 extend generally in the DL direction and join this trailing edge to this leading edge.
• Finally a short circuit electrically connecting the chip 6 to the ground 4 from the segment 10 of the rear edge of this chip. In the illustrated embodiment of this invention, this short circuit is formed by a conductive layer S extending on a wafer surface of the substrate 2, which surface is typically flat and then constitutes a short-circuit plane. It requires at least one resonance of the antenna to have at least approximately one electric field node in the vicinity of the segment 10 so as to be substantially of the quarter-wave type. The rear, front and lateral edges and the longitudinal and transverse directions are defined by the position of such a short-circuit in so far as this short-circuit is sufficiently large, that is to say in particular where its impedance is sufficiently low for impose on the antenna the existence of a resonance having such an electric field node.

The antenna further comprises a coupling system. This system is in the form of a microstrip line. This line comprises on the one hand a main conductor consisting of a coupling strip C1 extending on the upper surface of the substrate. This ribbon is connected to the chip 6 at a connection point 18 which may for example be located on the first lateral edge 14. The distance from the rear edge 10 at this point constitutes a connection dimension L4. This line comprises, on the other hand, a ground conductor constituted by layer 4. On the figure 1 it is only to simplify the drawing that the substrate 2 is not represented under the ribbon C1 and that this line is represented with a very small length. This system is part of a connection assembly which connects the resonant structure of the antenna to a signal processing member T, for example to excite one or more resonances of the antenna from this organ in the case where it is a transmitting antenna. In addition to this system the connection assembly typically includes a connecting line which is external to the antenna. This line may in particular be of the coaxial type, of the microstrip type or of the coplanar type. On the figure 1 it has been symbolically represented in the form of two conducting wires C2 and C3 respectively connecting the mass 4 and the ribbon C1 to the two terminals of the signal processing unit T. But it must be understood that this line would in practice be made of preferably in the form of a microstrip line or a coaxial line.

The signal processing unit T is adapted to operate at predetermined working frequencies which are at least close to useful frequencies of the antenna, ie which are included in passbands centered on such useful frequencies, these frequencies being those of at least some of the resonance modes of the antenna. It can be composite and then include an element permanently assigned to each of these working frequencies. It can also include a tunable element on the various working frequencies.

A separating slot 17 enters the area of the pellet 6 from an origin 40 separating two segments 10 and 11 from the rear edge thereof. It extends to a bottom 15 located at a distance from the lateral edges 14 and 16 and the front edge 12. It partially separates from one another a first and a second beaches 31 and 33 which are connected beyond this bottom by a passage 32. It comprises for example three rectilinear segments of neighboring lengths a first segment extending from the origin 40 towards the front edge 12, approaching the second lateral edge 16, a second segment extending parallel to this front edge towards this lateral edge, and a third segment s' extending parallel to this first segment to the bottom 15. The distances from this bottom to these two edges are respectively less than half the length and width of the pellet. A width of this slot is defined at each point of its length. It is for example, although not necessarily, uniform.

The presence of this slot reveals two resonance modes respectively constituting a primary resonance having a frequency of primary resonance and secondary resonance having a secondary resonant frequency. The primary resonance extends over the whole of the pellet 6. It is approximately of the quarter-wave type, its resonance path extending from the short-circuit S to the segment 11 of the trailing edge. Its coupling with radiated waves is mainly from this segment and from the adjacent portion of the second lateral edge 16. The secondary resonance extends over the single range 31. It is also approximately of the quarter-wave type, its resonance path extending from the short circuit S to the front edge 12. Its coupling with radiated waves is mainly from this edge and from the adjacent part of the first lateral edge 14.

As represented to the only figure 1 , the first region 31 may have an outgrowth 34 extending in the plane of the pellet 6, projecting from the first lateral edge 14, in the vicinity of the front edge 12. It has indeed been found that such an outgrowth could facilitate adjusting the resonant frequencies of the antenna.

• une couche diélectrique CD formée sur cette pastille, et
• une armature conductrice CA formée sur cette couche diélectrique.

In the context of this invention, the antenna 1 further includes a coupling reactive element which is preferably flat and which is for example constituted by a capacitor CR. This capacitor has an area less than each of the areas of the first and second areas 31 and 33, this area being internal to the area of the pellet 6 and extending continuously over this first area, on the separating slot 17 away from the bottom 15 and on this second beach. As represented in figure 3 it is formed by mutually superposed layers cooperating with the pellet 6 and constituting respectively:

• a dielectric layer CD formed on this chip, and
• a conductive reinforcement CA formed on this dielectric layer.

The area of this capacitor is for example rectangular and close to 5% of that of this pellet. It preferably extends in contact with or in the immediate vicinity of the periphery of this pellet.

• Lors de la réalisation de l'antenne, il est facile d'ajuster la longueur et la largeur du condensateur, ce qui permet d'ajuster ce couplage et d'agir ainsi sur les paramètres électriques de l'antenne.
• La présence de ce condensateur allonge les longueurs électriques de l'antenne. C'est à dire qu'elle permet de diminuer l'encombrement de l'antenne tout en conservant des valeurs souhaitées pour les fréquences de résonance.
• Elle élargit les bandes passantes des deux résonances sans augmenter sensiblement les taux d'ondes stationnaires.

The coupling reactive element constituted by the capacitor CR creates a coupling between the first and second conductive pads 31 and 33, which provides the following three advantages:

• During the production of the antenna, it is easy to adjust the length and the width of the capacitor, which makes it possible to adjust this coupling and thus act on the electrical parameters of the antenna.
• The presence of this capacitor lengthens the electrical lengths of the antenna. That is to say, it reduces the size of the antenna while maintaining desired values for the resonant frequencies.
• It widens the bandwidths of the two resonances without significantly increasing the standing wave rates.

• La masse de l'antenne recouvre la face inférieure du substrat.
• Le court-circuit S occupe toute la largeur du segment 10 qui constitue un bord arrière de la première plage 31.
• Composition du substrat 2: mousse ayant une permittivité relative de 1,07 et un facteur de dissipation de 0,0002,
• Epaisseur de ce substrat : H1 = 7 mm,
• Composition des couches conductrices : cuivre,
• Epaisseur de ces couches : 17 microns,
• Largeur du conducteur C1 : 5 mm,
• Cote de raccordement : L4 = 10 mm,
• Longueur de la pastille : L1 = 35 mm,
• Largeur de la pastille : W1 = 24 mm,
• Largeur du segment 11: W5 = 16 mm,
• Largeur de la fente 17: 0,75 mm,
• Tracé de cette fente : L5 = 13.mm, W2 = 9 mm, W3 = 8 mm, L2 = 6.mm, W4 = 3 mm,
• Permittivité relative de la couche CD du condensateur CR égale à 2,2,
• Epaisseur de cette couche: H2 = 0,1 mm,
• Longueur de ce condensateur: L3 = 6 mm,
• Largeur de ce condensateur: W6 = 7 mm,
• Impédance d'entrée : 50 ohms,
• Fréquence de résonance primaire : F1 = 965 MHz,
• Fréquence de résonance secondaire : F2 = 1860 MHz,
• Largeurs des bandes passantes autour des fréquences primaire et secondaire : 9,1% et 19% de ces fréquences, respectivement, ces largeurs étant mesurées à-6dB.

In the context of the mode of implementation shown in figure 2 various arrangements, compositions and values will be given below as examples:

• The mass of the antenna covers the underside of the substrate.
• The short circuit S occupies the entire width of the segment 10 which constitutes a rear edge of the first area 31.
• Composition of substrate 2: foam having a relative permittivity of 1.07 and a dissipation factor of 0.0002,
• Thickness of this substrate: H1 = 7 mm,
• Composition of conductive layers: copper,
• Thickness of these layers: 17 microns,
• Conductor width C1: 5 mm,
• Connection dimension: L4 = 10 mm,
• Length of the pellet: L1 = 35 mm,
• Width of the pellet: W1 = 24 mm,
• Width of the segment 11: W5 = 16 mm,
• Width of the slot 17: 0.75 mm,
• Plot of this slot: L5 = 13.mm, W2 = 9mm, W3 = 8mm, L2 = 6.mm, W4 = 3mm,
• Relative permitivity of the capacitor CD layer CR equal to 2.2,
• Thickness of this layer: H2 = 0.1 mm,
• Length of this capacitor: L3 = 6 mm,
• Width of this capacitor: W6 = 7 mm,
• Input impedance: 50 ohms,
• Primary resonance frequency: F1 = 965 MHz,
• Secondary resonant frequency: F2 = 1860 MHz,
• Bandwidths around the primary and secondary frequencies: 9.1% and 19% of these frequencies, respectively, these widths being measured at -6dB.

In the absence of capacitor CR, these resonant frequencies and these bandwidth widths would have been, respectively, F1 = 1120 MHz, F2 = 2270 MHz, 16% and 10%.

Claims (14)

1. A planar antenna, which antenna includes mutually superimposed layers respectively constituting:

   - a conductive mass (4),

   - a dielectric substrate (2) formed of that mass, and

   - a dot (6) formed on that substrate, that dot having an area

   and a periphery (10,12,14,16), that dot having a separating slot (17) having an origin (40) located on that periphery and a back (15) located in that area, that back allowing a passage (32) between it and that periphery, that slot penetrating into that area from the point of that origin (40) and cooperating with that periphery to delimit within that area a first zone (31) and a second zone (33), those two zones being conductive and being mutually electrically separated by that slot and connected by that passage, those zones having respective areas, that antenna being characterized by the fact that it further comprises a flat reactive element (CR) mutually coupling said two conductive zones (31, 33).
2. An antenna according to claim 1, that antenna being characterized by the fact that said flat reactive element is a capacitor (CR) having an area smaller than each of said areas of the first and second zones, that area being inside said area of the dot (6) and extending continuously onto said first zone (31), onto said separating slot (17) a distance away from said back (15) and onto said second zone (33), that capacitor being formed by mutually superimposed layers cooperating with said dot (6) and respectively constituting:

   - a dielectric layer (CD) formed on said dot, and

   - a conductive reinforcement (CA) formed on that dielectric layer.
3. An antenna according to claim 2, that antenna being characterized by the fact that said area of the capacitor (CR) is between 1% and 25% of said area of the dot (6).
4. An antenna according to claims 2 or 3, wherein the dielectric layer (CD) has a relative permittivity of 2.2 and a thickness of 0.1 mm.
5. An antenna according to claim 1, that antenna being characterized by the fact that said flat reactive element (CR) extends to be in contact with or the immediate vicinity of the periphery of the dot (6).
6. An antenna according to claim 5, that antenna being characterized by the fact that it further includes a short circuit (S) electrically connecting said first conductive zone (31) to said mass (4) in the vicinity of said origin (40) of the separating slot (17).
7. An antenna according to one of the claims 1 to 6, wherein the substrate (2) comprises a foam having a relative permittivity of 1.07 and a dissipation factor of 0.0002, the thickness of the substrate being 7 mm.
8. An antenna according to one of the claims 1 to 7, wherein the slot (17) comprises a first segment extending from the origin (40) to a front edge (12) and approaching a side edge (16), a second segment extending parallel to the front edge (12) to the side edge (16), and a third segment extending parallel to the first segment all the way to the back (15).
9. An antenna according to one of the claims 1 to 8, wherein the dot is a plate whose length (L1) is equal to 35mm and whose width (W1) is equal to 24mm.
10. A dual-band transmission device, which device comprises:

    - a signal processing unit (T) adapted to be frequency-tuned in two working bands extending respectively around two predetermined central frequencies to send and/or receive an electrical signal in each of those two bands,

    - an antenna connection assembly including electrical conductors (C1, 4, C2, C3) connecting that processing unit (T) to an antenna (1) according to one of the claims 1 to 9 to couple said electrical signal to waves radiated around each of two said central frequencies, some of those conductors (C1, 4) being directly connected to that antenna (1);

    - the antenna (1) according to one of the claims 1 to 9, the reactive element (CR) being external to said antenna connection assembly, the antenna (1) comprising two resonances that differ by the area they occupy respectively on the dot (6) of the antenna (1), those two resonances being respectively centred in the two so-called working bands.
11. A transmission device according to claim 10, said antenna further including a short-circuit (S) formed in said first zone on said periphery of the dot (6), that short-circuit and said separating slot (17) allowing two resonances to be established in that antenna, at least one of those two resonances being of the quarter-wave type, with an electric field node, at least a virtual one, set by that short-circuit, one of those two resonances forming a primary resonance and having a primary frequency (F1) substantially equal to one of said two central frequencies, the other of those two resonances forming a secondary resonance having a secondary frequency (F2) substantially equal to the other of those two central frequencies, said connection assembly coupling said antenna (1) to said signal processing unit (T) around each of said two central frequencies,
    said transmission device being characterized by the fact that said reactive element (CR) is a flat element extending substantially in the plane of said dot (6).
12. A transmission device according to claim 11, that device being characterized by the fact that said flat reactive element is a capacitor (CR), that capacitor having an area less than each of said areas of the first and second zones, that area being less than said area of the dot (6) and extending continuously on said first zone (31), on said separating slot (17) a distance away from said back (15) and on said zone (33), that capacitor being formed by said mutually superimposed layers cooperating with said dot (6) and respectively constituting:

    a dielectric layer (CD) formed on said dot, and

    a conductive reinforcement (CA) formed on that dielectric layer.
13. A transmission device according to claim 11, said conductors (C1, 4) included in the antenna connection assembly and connected directly to the antenna being only:

    - a strip (C1) formed in the same conductive layer as said dot (6) and

    - a ground formed in the same conductive layer (4) as said ground of the antenna so as to constitute, with that strip, a microstrip-type line.
14. A transmission device according to one of the claims 10 to 13, wherein the primary resonance frequency (F1) is equal to 965 MHz and the secondary resonance frequency (F2) is equal to 1850 MHz.

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