Basic microstrip antenna and improved array antenna

Abstract

Elementary antenna (1) of the micro-strip type comprising a stack of layers, stacked along a direction, said stack comprising: a first conducting radiating element (P1) in the shape of a disk having a first center (C1), an axis along the direction and passing through said first center being called central axis (AC);a coupling assembly adapted for coupling an excitation device and the first radiating element, said coupling assembly comprising: a first slot (F1) having a center called the slot center (CF) located on said central axis; a second slot (F2) having a center coinciding with the slot center, and substantially perpendicular to the first slot, the first and second slots each having the ends (E1, E1', E2, E2') in arcs of a circle about the same circle (CL) centered on said slot center; said slots and said stacked layers being adapted so that a transverse footprint of said elementary antenna is in the shape of a disk. Figure for the abbreviation: Fig. 1A;

Description

Title of the invention: Improved micro-strip type elementary antenna and array antenna Technical field

[0001] The present invention relates to the field of network-type electromagnetic antennas and in particular active antennas. It applies in particular to radars, electronic warfare systems (such as radar detectors and radar jammers) as well as communication systems or other multifunction systems. Prior art

[0002] A so-called network antenna comprises a plurality of elementary antennas which may preferably be of the microstrip type, also called patch antennas or “patch” antennas in English terminology. These patch antennas comprise a stack of layers of dielectric substrates provided with metal tracks, spaced where appropriate by non-etched materials or substrates.

[0003] The technology of microstrip elementary antennas makes it possible to produce very compact antennas, in particular compact electronically scanned array antennas, which are simpler to produce and therefore less expensive than other types of antennas (waveguides, Vivaldi, etc.).

[0004] A microstrip elementary antenna conventionally comprises a radiating element placed on a dielectric layer arranged above a conductive plane serving as ground so as to constitute a resonator. The elementary antenna also comprises a power distribution device for exciting the radiating element from an input signal. The radiating device is coupled to its excitation by a metallized hole (called via) or by a slot. Electromagnetic coupling by slot makes it easier to generate a wide frequency band. It also makes it possible to avoid any connecting via between the radiating elements and the excitation, which simplifies the manufacture of the elementary antenna.

[0005] Conventionally, excitation means are provided capable of simultaneously exciting the radiating device according to two orthogonal linear polarizations. This makes it possible to choose the polarization most suited to a given environment (by combining these 2 polarizations with an appropriate amplitude-phase relationship). An example of such an antenna is given in patent FR 1873475.

[0006] The array antennas form an array of elementary antennas whose pitch is of the order of XFmax / 2, where XFmax is the smallest wavelength corresponding to the maximum frequency Fmax of the band, which is a strong constraint on the dimensions of the elementary antennas.

[0007] Such an elementary antenna has an overall size of square (or rectangular) shape that is entirely suitable for a square (or rectangular) mesh array antenna, for a double linear polarization h and v. [Fig.lA] illustrates an AR array antenna comprising a plurality of AE elementary antennas arranged in a square mesh. Obtaining +45° and -45° polarizations with the same square mesh pitch (therefore for the same beam pointing capacity) is impossible, due to the size of the patch because this would require an overlap of the elementary antennas. [Fig.lB] illustrates the overlap that would exist between the AE elementary antennas of an AR array antenna with a square mesh with the same pitch as that of [Fig.lA], for two polarizations +45° and -45°.Similarly, the use of this elementary antenna in a triangular mesh aiming for a similar pointing capacity along the two main axes is impossible because of the overlap that would exist between the elementary antennas AE (see [Fig.lC]).

[0008] Furthermore, an elementary antenna typically has a footprint of half a wavelength in the equivalent dielectric medium considered at the central frequency of use. This is then referred to as operation in fundamental mode TM 10 for a given linear polarization.

[0009] However, if this bandwidth is reduced around the central frequency of use, an elementary antenna will naturally become resonant at frequencies where its size becomes a multiple of half a wavelength at the frequencies considered. The elementary antenna becomes impedance-matched again and will be able to radiate again. Thus, the antenna will be able to radiate at twice the frequency of the central frequency of use, called harmonic 2 (H2) or octave frequency, or at triple the frequency, called harmonic 3 (H3). The distribution of currents and fields inside the elementary antenna is different from those of the fundamental mode (TM20 mode for H2 for example), giving a different radiation (for example hollow in the axis and radiation on the sides for H2).

[0010] Now these harmonics, particularly H2, are generated by the non-linear active circuits of the modules of the active electronically scanned array antennas. For reasons of frequency allocation, or reasons of electromagnetic discretion, or to avoid interference phenomena, it is necessary to block the possibility of radiation outside the antenna's operating band. For this it is possible to include filtering at the output of the active modules, but this solution has the disadvantages of ohmic losses, installation and size.

[0011] The invention aims to overcome certain problems of the prior art. To this end, an object of the invention is a versatile microstrip type antenna, which can be used in any network antenna intended to operate in a given frequency band and whose double linear polarization is adjustable by orientation of the elementary antennas (for example example Horizontal and Vertical, +45° and -45°, or left and / or right circular polarization). The elementary antenna of the invention is excited via slots in order to couple them to one or more conductive lines, without physical connection. Advantageously, the elementary antenna integrates a frequency filtering function intended to eliminate parasitic signals which may pass through during transmission or reception, without altering the radiation performance. In addition, the elementary antenna has a geometry compatible with any networking, therefore in particular compatible with a rectangular mesh or a triangular mesh. Summary of the invention

[0012] For this purpose, an object of the invention is an elementary antenna (1) of micro-strip type comprising a stack of layers, stacked in a z direction, said stack comprising: - a first disc-shaped conductive radiating element having a first center, an axis in the z direction and passing through said first center being called the central axis; - an excitation device coupled to the first radiating element and adapted to excite the first radiating element according to two orthogonal linear polarizations, said excitation device comprising: - a first elementary excitation device coupled to the first radiating element and comprising a first conductive line and a first power distribution device adapted to excite the first radiating element from a first input signal; - a second elementary excitation device coupled to the first radiating element and comprising a second conductive line and a second power distribution device adapted to excite the first radiating element from a second input signal; - a coupling assembly adapted to couple the excitation device and the first radiating element, said coupling assembly comprising: - a first slot comprising a center called the slot center located on said central axis - a second slot comprising a center coincident with the slot center, and substantially perpendicular to the first slot, the first and second slots each comprising ends in an arc of a circle along the same circle centered on said slot center - said slots and said stacked layers being adapted so that a transverse size of said elementary antenna is in the form of a disc.

[0013] According to one embodiment, the excitation device comprises: - a first (SI) and a second conductive line segment, coplanar and connected to the first conductive line, and not perpendicular to the first conductive line, an angle between the first segment and the first conductive line being opposite to an angle between the second segment and the first conductive line. - a third and a fourth conductive line segment, coplanar and connected to the second conductive line, and not perpendicular to the second conductive line, an angle between the third segment and the second conductive line being opposite to an angle between the fourth segment and the second conductive line.

[0014] Preferably, in this embodiment, the first and second segments are parallel to the first slot and in which the third and fourth segments are parallel to the second slot. Preferably, an assembly formed by the first, the second segments and a portion of the first conductive line connecting the first segment to the second segment, forms the letter Z, and in which an assembly formed by the third, the fourth segments and a portion of the second conductive line connecting the third segment to the fourth segment, forms the letter Z. Preferably, the first segment is opposite an edge of the first slot and the third segment is opposite an edge of the second slot. Preferably, the conductive line segments are substantially of the same length.Preferably, the length of the conductive line segments is approximately equal to λc / 4 with a wavelength corresponding to a central frequency of a bandwidth of said elementary antenna.

[0015] According to one embodiment, a middle of said first line is located on said central axis, wherein a middle of said second line is located on said central axis, and wherein the first and second lines are perpendicular to each other. Preferably, the first line and the second line are formed on two opposite faces of a first dielectric substrate.

[0016] According to one embodiment, said transverse size is fixed from a diameter of said circle.

[0017] According to one embodiment, an angle between the first slot and the first conductive line and an angle between the second slot and the second conductive line is equal to approximately 45°.

[0018] According to one embodiment, the first and second conductive lines are interposed between the first radiating element and the first and second slots, the first and second slots being formed from a metal plane, forming a ground plane for the first and second conductive lines and for the first radiating element.

[0019] According to one embodiment, the device comprises a second radiating element in the form of a disc superimposed on the first radiating element, and having a second center located on said central axis.

[0020] Another object of the invention is an array antenna comprising a plurality of elementary antennas according to the invention.

[0021] According to one embodiment, the plurality of elementary antennas is arranged with a square or rectangular mesh.

[0022] According to one embodiment, the plurality of elementary antennas is arranged with a triangular mesh. Brief description of the drawings

[0023] Other characteristics, details and advantages of the invention will emerge from reading the description given with reference to the appended drawings given by way of example and which represent, respectively:

[0024] [Fig.lA], a schematic illustration of an array antenna known from the prior art comprising a plurality of elementary antennas arranged in a square mesh,

[0025] [Fig.lB], a schematic illustration of an array antenna with a square mesh with the same pitch as that of [Fig.lA], for two polarizations +45° and -45°

[0026] [Fig.lC], a schematic illustration of an array antenna with a triangular mesh with the same pitch as that of [Fig.lA]

[0027] [Fig.2A], an exploded schematic view of an elementary antenna according to the invention,

[0028] [Fig.2B], a schematic sectional view of the elementary antenna according to the invention,

[0029] [Fig.2C], an exploded schematic view of an elementary antenna according to a mode of realization of the invention,

[0030] [Fig.3], a top view and in transparency of a stack formed by elements of the elementary antenna of [Fig.2C],

[0031] [Fig.4]], a top view and in transparency of a stack formed by elements of an advantageous embodiment of the elementary antenna of the invention,

[0032] [Fig.5], a graphical representation of the parameter SI 1 of two embodiments of the elementary antenna of the invention comprising line segments S1-S4 according to the embodiment of [Fig.4]

[0033] [Fig.6], an array antenna according to an embodiment of the invention in which the elementary antennas are arranged in a square mesh,

[0034] [Fig.7], an array antenna according to an embodiment of the invention

[0035] [Fig.8], an array antenna according to one embodiment of the invention.

[0036] In the figures, unless otherwise indicated, the elements are not to scale.

[0037] By two “parallel elements”, in the rest of the text, we mean that an angle between the elements are between 0° and 10°. Description of the embodiments

[0038] [Fig.2A] schematically represents an exploded view of an elementary antenna 1 according to the invention, of the planar type - also called microstrip antenna -.

[0039] The elementary antenna is capable of being in a planar configuration in which the stack comprises a stack of layers that are substantially planar and perpendicular to a stacking direction represented by the z axis. The elementary antenna may be flexible and be capable of having a curved configuration in which the layers are curved. In the remainder of the text, for simplicity, we will describe the arrangement of the antenna in its planar configuration.

[0040] The stack comprises parallel conductive planes, spaced along the z axis which is orthogonal to them. [Fig. 2B] shows a sectional view of the elementary antenna. For greater readability, only the conductive planes are shown in Figures 2A and 2B, except those which are explicitly mentioned in the description. Intervals are provided between the successive conductive planes. These intervals each comprise at least one layer of a dielectric substrate which may, for example, be a layer of air or foam.

[0041] According to one embodiment, the elementary antenna 1 comprises a first disc-shaped conductive radiating element PI having a first center CL. A central axis AC is defined passing through the first center Cl and along the z direction. Alternatively, according to another embodiment and as illustrated in FIGS. 2A and 2B, the antenna 1 comprises a second radiating element P2, also disc-shaped, superimposed on the first radiating element and having a second center C2 located on the central axis AC. The presence of two resonant elements allows the bandwidth of the antenna to be widened. The resonant elements are adjusted to produce a double resonator.

[0042] The elementary antenna 1 comprises an excitation device DE coupled to the first radiating element and adapted to excite the first radiating element according to two orthogonal linear polarizations h, v. The excitation device DE overcomes a lower ground plane PMI which is the overall ground plane of the antenna 1. By ground plane, we mean a conductive plane acting as a ground plane.

[0043] The excitation device DE comprises: - a first elementary excitation device coupled to the first radiating element and comprising a first conductive line L1 and a first power distribution device DR1 adapted to excite the first radiating element from a first input signal, via a first access point PA1 of the first line (not visible in [Fig.2A] but represented in [Fig.3]). - a second elementary excitation device coupled to the first radiating element and comprising a second conductive line L2 and a second power distribution device DR2 adapted to excite the first radiating element from a second input signal, via a second access point PA2 of the second line (not visible in [Fig.2A] but represented in [Fig.3]).

[0044] The lines are lines printed using microstrip or triplate technique. A double linear polarization is obtained in a conventional manner with two crossed excitations. Conventionally, a left or right circular polarization is achieved by phase quadrature excitation of the access points PA 1, PA2 of the crossed linear polarizations, without modification of the overall size. Preferably, the lines are not coplanar to avoid parasitic interactions between them. Preferably, as illustrated in [Fig.2B], for greater compactness, the first line and the second line are formed on two opposite faces of a first dielectric substrate PI2.

[0045] In addition, the elementary antenna of the invention comprises a coupling assembly adapted to couple the excitation device and the first radiating element. The coupling assembly comprises: - a first slot Fl comprising a center called slot center CF located on the central axis AC - a second slit F2 substantially perpendicular to the first slit and comprising a center coincident with the slit center. By "substantially perpendicular" is meant that an angle between the first and second slits is equal to 90° at ± 10°. The symmetry of the slits ensures for each linear polarization the absence of generation of penalizing cross-polarization. The use of a perpendicular slit makes it possible to decouple the two linear polarizations.

[0046] Preferably, as illustrated in figures 2A and 2B, the slots F1, F2 are coplanar and made in the same plane PII in order to limit the number of layers printed in the elementary antenna.

[0047] In order to widen the bandwidth of the elementary antenna, the slots F1, F2 each comprise ends E1, E1', E2, E2' in an arc of a circle along the same circle CL centered on the center of the slot.

[0048] In the elementary antenna of the invention, the slots and the stacked layers are adapted so that a transverse footprint of the elementary antenna is disc-shaped. This is made possible, among other things, by the judicious choice of the geometry of the ends of the slots. By "transverse footprint" is meant here the footprint on the ground, along a plane perpendicular to the direction ", formed by the layer with the largest transverse dimension. Thus, the elementary antenna with a disk-type transverse bulk can be used in an array antenna with a square, rectangular or triangular mesh without modifying the geometry of the elementary antenna and without modifying the pitch of the array mesh (see figures 6, 7). This implies that, via a simple rotation of the elementary antennas, a user adjusts the linear polarization of the radiation, without modifying a beam pointing capability of the array antenna. In addition, a total transverse bulk of the array antenna is not modified by the rotation of the elementary antennas.

[0049] Preferably, the transverse size is fixed from a diameter of the circle. That is to say that, in this embodiment, the slots are the elements of the layer of the stack having the largest transverse dimension.

[0050] A slit F2 and its ends have a length of the order of a wavelength in the dielectric medium considered.

[0051] The elementary antenna comprises a ground plane of the radiating elements PI, P2. Preferably, the first and second conductive lines are interposed between the first radiating element PI and the first and second slots and the first and second slots are formed from a metallic plane PII, forming the ground plane for the conductive lines L1, L2 and for the radiating elements PI, P2. By "from", it is meant that the slots are for example etched or cut on the metallic plane. This embodiment allows greater compactness of the antenna and good electromagnetic coupling between conductive lines L1, L2 and the radiating elements PI, P2.

[0052] Preferably, according to one embodiment, a middle of the first line and a middle of the second line are located on the central axis, and the first and second lines are perpendicular to each other to be correctly decoupled. Preferably, an angle between the first slot and the first conductive line and an angle between the second slot and the second conductive line is equal to approximately 45°. By "approximately 45°" is meant 45° to plus or minus 10°. These characteristics make it possible to excite, via the slots F1, F2, the first radiating element at the center of the first radiating element P1 in order to ensure good excitation symmetry over the entire frequency band of the elementary antenna.

[0053] Preferably, as illustrated in [Fig.2C], the elementary antenna comprises a multilayer CIP triplate printed circuit for excitation of the lines which is common to a plurality of elements of the elementary antenna. In this embodiment, preferably, the transverse size of the elementary antenna is defined by the shielding produced in the triplate printed feed circuit, typically produced by a succession of metallized holes (or vias) TM short- circuiting the metal plane PII comprising the cross slots and the lower ground plane PMI. That is to say that in this embodiment, the transverse dimension is equal to the dimension of the circle CL formed by the metallized vias surrounding the rounded edges of the slots and shielding the elementary antenna from the other adjacent elementary antennas when the elementary antenna is arranged in an array.

[0054] [Fig. 3] schematically illustrates a top view and in transparency of a stack formed by elements of the elementary antenna of [Fig. 2C], a layer comprising the slots F1, F2, the conductive line L1, and the line L2. The metallized vias TM defining the circle CL and therefore the transverse size are also represented in [Fig. 3].

[0055] According to an embodiment, different from that illustrated in [Fig.2A], each slot is excited at two excitation points on either side of the center of the slot CF by two respective printed lines which are joined on another printed layer by a balanced wideband coupler. In this embodiment, the elementary antenna 1 therefore comprises four printed lines. These characteristics allow very wideband operation with high excitation symmetry over the entire frequency band.

[0056] Preferably, the first radiating element comprises an optical sight placed at its first center C1 to facilitate the centering of the different elements and the different planes of the stack with each other.

[0057] [Fig.4] schematically illustrates a top view and in transparency of a stack formed by elements of an advantageous embodiment of the elementary antenna of the invention, a layer comprising the slots F1, F2, the conductive line L1, and the line L2. In this embodiment, the antenna 1 makes it possible to filter the radiation of the elementary antenna outside its bandwidth and makes it possible to eliminate parasitic signals which may pass through during transmission or reception, without altering the radiation performance. For this, the excitation device DE of the elementary antenna 1 comprises: - a first S1 and a second S2 coplanar conductive line segments and connected to the first conductive line Ll and not perpendicular to the first conductive line LL An angle between the first conductive line segment SI and the first conductive line is opposite to an angle between the second conductive line segment S2 and the first conductive line. This implies that the segments SI and S2 are parallel to each other. - a third S3 and a fourth S4 conductive line segments, coplanar and connected to the second conductive line and not perpendicular to the second conductive line L2. An angle ®2i between the third segment S3 and the second conductive line is opposite an angle between the fourth segment S4 and the second conductive line. This implies that segments SI and S2 are parallel to each other.

[0058] According to one embodiment, the segments are straight line segments. Alternatively, according to another embodiment, the segments are curve segments. In this embodiment, by "angle between a segment and a conductive line" is meant an angle between a main direction of elongation of the segment and the conductive line.

[0059] These conductive line segments S1-S4 (or "stubs" in English) are propagation line segments connected to the propagation line L1 or L2, and intended to bring back a certain pure susceptance at the junction point with the main propagation line L1 or L2. A pair of two segments in cascade allows a filtering of order 2, improving the band of the filter formed by the segments and the rejection level. The shape, positioning and length of these line segments have been optimized by the inventors in order to comply with the implantation constraint in the elementary antenna which has a transverse disk-shaped footprint.After numerous calculations, the inventors arrived at a clever arrangement that limits the impact of the line segments on the operation of the elementary antenna 1 in its bandwidth, while optimizing the filtering of the harmonic 2 (H2) of the central frequency of the bandwidth of the elementary antenna. The optimization of the segments (shape, length) was carried out "manually" rather than automatically due to the prohibitive simulation times when working in both the useful frequency band and the H2 band on the simulation software, and the difficulty in converging. The impact of the line segments was observed on the TOS (Standing Wave Ratio) levels in the bandwidth and the H2 filtering was observed by the rejection level and frequency band.Note that the use of line segments inclined relative to the line to which they are connected is different from the geometry usually used for frequency filtering by stubs.

[0060] In the case of a printed line alone (i.e.: without slits F1, F2), the optimal geometry for frequency filtering would correspond to straight line segments perpendicular to the line. However, the inventors have noted that in the elementary antenna of the invention, this geometry leads to a spatial overlap between one of the slits and a segment, which significantly degrades the operation of the slit. Similarly, significant folding (i.e. a small angle) of the segments relative to the conductive line to which they are connected creates significant parasitic coupling between the line and the segments. This is why, for optimal operation, it is preferable for the first and second segments to be parallel to the first slit and for the third and fourth segments are parallel to the second slit.

[0061] The optimal geometry therefore corresponds to that illustrated in [Fig. 4]. In this embodiment, an assembly formed by the first segment S1, the second segment S2 and a portion of the first conductive line connecting the first segment to the second segment, forms the letter Z. Similarly, an assembly formed by the third segment S3, the fourth segment S4 and a portion of the second conductive line connecting the third segment to the fourth segment, forms the letter Z. Preferably, in the embodiment of [Fig. 4], in order to optimize the frequency filtering without degrading the TOS level of the elementary antenna too much, the first segment is opposite an edge of the first slot and the third segment is opposite an edge of the second slot. According to another embodiment, the position of the segments S1-S4 is different from that illustrated in [Fig.4] but segments SI and S2 are arranged to be symmetrical to segments S3 and S4 respectively, by a rotation of central axis AC.

[0062] To optimize the frequency filtering, preferably, the conductive line segments are of substantially the same length. By substantially the same length, it is meant that their length is equal to ± 10%, preferably 5%. Preferably, in order to optimize the filtering of the 2nd harmonic, the length of the conductive line segments is substantially equal to λcl4, with the wavelength in the medium dielectric printed considered corresponding to the central frequency of the frequency band to be filtered (here for example the band around the harmonic frequency H2). This length has been optimized to limit the parasitic interactions segments / slot, and segments / triplate printed lines, the parasitic interactions between the segments S1-S4 and lines L1, L2 being the most critical in terms of degradation of the TOS because it causes a capacitive effect which tends to modify the electrical length of the lines.

[0063] Thanks to the S1-S4 segments, the TOS levels are controlled in the antenna bandwidth, for the entire angular range of electronic beam pointing. The filtering function allows a rejection of the order of - 20 dB on the H2 harmonic band compared to the elementary antenna without the S1-S4 segments.

[0064] Another object of the invention is an array antenna 2 comprising a plurality of elementary antennas 1 according to the invention. As mentioned previously, the array 2 formed from the elementary antenna of the invention allows easy control of the polarization of the radiation emitted by each elementary antenna of the array via the control of their orientation, without modification of the mesh of the array and without modification of the total size of the array.

[0065] [Fig.5] is a graphical representation of the parameter SI 1 of two embodiments of the elementary antenna of the invention comprising line segments S1-S4 according to the embodiment of [Fig.4]. The abscissa is in GHz, F cor corresponding to the minimum frequency of the useful frequency band of the network antenna, and the ordinate is in dB. As a reminder, the SI 1 parameter of an antenna is the input reflection coefficient of the elementary antenna (ratio between reflected background and incident background at the input).

[0066] Curves C1 and C2 are the parameter SI 1 of a first embodiment M1 in which the stack of the elementary antenna comprises an upper dielectric layer for mechanical protection of the antenna, for excitation along the line L1 and L2 respectively. This upper dielectric layer is a layer of Rohacell foam 6 mm thick, covered with a ceramic-filled PTFE dielectric film 500 pm thick.

[0067] Curves C1' and C2' are the parameter SI 1 of a second embodiment M2 without upper protection layer, for excitation along line L1 and L2 respectively. In mode M2, the value of the reflection coefficient remains lower than -16dB throughout the bandwidth for excitation via line L2 and higher than -19dB via line L2. The presence of the upper protection layer in mode M1 significantly degrades the broadband operation of the elementary antenna. However, the value of the reflection coefficient remains acceptable over the entire bandwidth (higher than -27dB).

[0068] [Fig. 5] demonstrates that the elementary antenna of the invention has a broadband operation in both modes M1 and M2, although mode M2 ​​exhibits a more stable behavior over the entire bandwidth of the antenna for both excitations along L1 and L2 and that the presence of segments S1-S4 does not significantly degrade the operation of the antenna.

[0069] Figure 6 schematically illustrates an array antenna 2 according to an embodiment of the invention in which the elementary antennas 1 are arranged in a square mesh so as to be able to excite a horizontal polarization and a vertical polarization. By square mesh, it is meant that a distance between the center of two adjacent elementary antennas (mesh pitch Pm) is identical for all the antennas 1 of the array 2. The array of [Fig.6] makes it possible to obtain an array antenna 2 with a directivity fixed by the pitch of the array.

[0070] Alternatively, according to another embodiment, the elementary antennas 1 of the network 2 are arranged in a rectangular mesh. That is to say that the network antenna 2 has a different mesh pitch according to the horizontal direction and the vertical direction. This is useful when the beam radiated by the network antenna is to be pointed by electronic scanning in a rectangular Azimuth-Elevation angular window. This is the case for example of a SAR Radar or an Airborne Surveillance Radar. In this case the mesh of the network can be relaxed along the axis corresponding to the lesser angular exploration.

[0071] Figure 7 schematically illustrates an array antenna 2 according to an embodiment of the invention, in which the orientation of the elementary antennas 1 is modified relative to the array of Figure 6. In the embodiment of Figure 7, the elementary antennas are oriented so as to be able to excite polarizations at +45° and -45° relative to those excited in the array of Figure 6. As in the array of Figure 6, the mesh of the array of Figure 7 is square and of pitch Pm identical to that of the array of [Fig.6]. In addition, the total size of the array 2 of [Fig.7] is identical to that of the array of [Fig.6].

[0072] Figure 8 schematically illustrates an array antenna 2 according to an embodiment of the invention, in which the elementary antennas 1 of the array 2 are arranged in a triangular mesh. In the embodiment, by way of non-limiting example, the mesh is in an equilateral triangle, also called a hexagonal mesh and the orientation of the polarizations is arbitrary. By hexagonal mesh, it is meant that the center of an antenna forms an equilateral triangle of mesh pitch Pm with the center of two adjacent antennas. This is useful for an array antenna whose beam points by electronic scanning inside an angular cone centered on the normal to the plane of the array. This is the case, for example, of an airborne radar at the front of an aircraft (airborne weather radar, or airborne combat radar).The advantage compared to a square mesh is that, for a given network antenna surface, we need approximately 15% fewer unit elements to "pave" the radiating surface, therefore 15% fewer active circuits (modules), therefore 15% reduction in cost, at the cost, however, of a less simple mechanical design.

Claims

1. Claims Elementary antenna (1) of micro-strip type comprising a stack of layers, stacked in a z direction, said stack comprising: - a first conductive radiating element (PI) in the form of a disc having a first center (Cl), an axis in the direction '• and passing through said first center being called the central axis (AC); - an excitation device (DE) coupled to the first radiating element and adapted to excite the first radiating element according to two orthogonal linear polarizations (h, v), said excitation device comprising: • a first elementary excitation device coupled to the first radiating element and comprising a first conductive line (L1) and a first power distribution device (DR1) adapted to excite the first radiating element from a first input signal; • a second elementary excitation device coupled to the first radiating element and comprising a second conductive line (L2) and a second power distribution device (DR2) adapted to excite the first radiating element from a second input signal; - a coupling assembly adapted to couple the excitation device and the first radiating element, said coupling assembly comprising: • a first slot (Fl) comprising a center called the slot center (CF) located on said central axis • a second slot (F2) comprising a center coincident with the slot center, and substantially perpendicular to the first slot, the first and second slots each comprising ends (El, El', E2, E2') in an arc of a circle along the same circle (CL) centered on said slot center said slots and said stacked layers being adapted so that a transverse size of said elementary antenna is disc-shaped.

2. Elementary antenna according to the preceding claim, in which the excitation device comprises: - a first (SI) and a second (S2) conductive line segments, coplanar and connected to the first conductive line, and not perpendicular to the first conductive line (L1), an angle between the first segment (SI) and the first conductive line being opposite to an angle between the second segment (S2) and the first conductive line. - a third (S3) and a fourth (S4) conductive line segments, coplanar and connected to the second conductive line, and not perpendicular to the second conductive line (L2), an angle between the third segment (S3) and the second conductive line being opposite to an angle between the fourth segment (S4) and the second conductive line.

3. Elementary antenna according to the preceding claim, in which the first and second segments are parallel to the first slot and in which the third and fourth segments are parallel to the second slot.

4. Elementary antenna according to the preceding claim, in which an assembly formed by the first, the second segments and a portion of the first conductive line connecting the first segment to the second segment, forms the letter Z, and in which an assembly formed by the third, the fourth segments and a portion of the second conductive line connecting the third segment to the fourth segment, forms the letter Z.

5. Elementary antenna according to claim 3 or 4, in which the first segment faces an edge of the first slot and the third segment faces an edge of the second slot.

6. Elementary antenna according to any one of claims 2 to 5, in which said conductive line segments are substantially of the same length.

7. Elementary antenna according to the preceding claim, in which the length of the conductive line segments is approximately equal to / 4 with Àc a wavelength corresponding to a central frequency of a bandwidth of said elementary antenna.

8. An elementary antenna according to any one of the preceding claims, wherein a middle of said first line is located on said central axis, wherein a middle of said second line is located on said central axis, and wherein the first and second lines are perpendicular to each other.

9. Elementary antenna according to the preceding claim, in which the first line and the second line are formed on two opposite faces of a first dielectric substrate (PI2).

10. Elementary antenna according to any one of the preceding claims, in which said transverse size is fixed from a diameter of said circle.

11. An elementary antenna according to any preceding claim, wherein an angle between the first slot and the first conductive line and an angle between the second slot and the second conductive line is equal to approximately 45°.

12. An elementary antenna according to any one of the preceding claims, wherein the first and second conductive lines are interposed between the first radiating element (PI) and the first and second slots, the first and second slots being formed from a metal plane, forming a ground plane for the first and second conductive lines and for the first radiating element.

13. Elementary antenna according to any one of the preceding claims, comprising a second radiating element (P2) in the form of a disc superimposed on the first radiating element, and having a second center (C2) located on said central axis.

14. Array antenna comprising a plurality of elementary antennas according to any one of the preceding claims.

15. Array antenna according to the preceding claim, in which the plurality of elementary antennas is arranged with a square or rectangular mesh.

16. An array antenna according to claim 14, wherein the plurality of elementary antennas are arranged with a triangular mesh.

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