ACTIVE ANTENNA WITH RADIATION ELEMENTS MOUNTED ON A CONICAL SURFACE

Description

[0001] The present invention relates to active antennas, in particular active antennas for aeronautics, especially fighter jets.

[0002] Today, active antennas are planar antennas. They consist of a platform carrying radiating elements arranged in a regular pattern.

[0003] In particular, the forward-facing RADAR of a fighter jet transmits and receives on a front antenna, which is capped with a conical radome for aerodynamic reasons.

[0004] To increase the exploration of the space around the aircraft, it is known to carry additional antennas on board the aircraft, the plates of which are arranged perpendicularly to the front antenna.

[0005] We would like to increase the capabilities of a front antenna by using a support surface for the radiating elements that makes maximum use of the internal volume of the conical radome.

[0006] This would allow, for example for a fighter jet, to increase capabilities in terms of RADAR detection (in particular by increasing the angular scanning range of the antenna), but also in terms of electronic warfare and telecommunications.

[0007] The article by JAECK V et al., "Design and manufacturing of conformal antenna array on a conical surface at 5.2 GHz", 2017 EUROPEAN RADAR CONFERENCE (EURAD), EUROPEAN MICROWAVE ASSOCIATION, October 11, 2017 (2017-10-11), pages 359-362, XP033292855, DOI: 10.23919 / EURAD.2017.8249221, presents an antenna array for radio communication. It is a 3D conical array. It discusses the possibility of operating the different antennas in beam steering.

[0008] Document EP 0 512 487 B1 discloses a network antenna arranged on a surface having an axis of symmetry and an arbitrary profile (conical, spherical, elliptical, parabolic, hyperbolic, etc.). A phase shifter compensates for the propagation delay for the different radiating elements located along the same generatrix of the surface.

[0009] Document DE 26 50 603 A1 discloses a phased-array radar antenna comprising a primary feed system and several individual radiators. An individual radiator consists of a collector element, which receives radiation from the primary feed system, and a transmitter element, which radiates into space. An electronically controlled phase shifter is provided between the collector and transmitter elements of an individual radiator. The phase shifters are used to focus and deflect the antenna beam into a particular sector.

[0010] The article by YUNXIANG ZHANG et al. "Full-polarisation three-dimensional pattern synthesis for conformal conical arrays with dynamic range ratio constraint by using the initialisations based on equivalence theorem", IET MICROWAVES, ANTENNAS & PROPAGATION, THE INSTITUTION OF ENGINEERING AND TECHNOLOGY, UNITED KINGDOM, vol. 9, no. 15, December 1, 2015 (2015-12-01), pages 1659-1666, XP006106536, ISSN: 1751-8725, DOI: 10.1049 / IET-MAP.2015.0273, presents a mathematical model for beam synthesis for a radar antenna array arranged on a conical surface.

[0011] Document WO 2018 / 036009 A1 discloses an antenna for a base station of a mobile radio communication network.

[0012] The aim of the present invention is to address this problem.

[0013] For this purpose the invention relates to an active antenna according to the attached claims.

[0014] The invention and its advantages will be better understood upon reading the following detailed description of a particular embodiment, given solely by way of non-limiting example, this description being made with reference to the accompanying drawings in which: There figure 1 is a representation along an axial plane and a transverse plane of a first embodiment of a conical active antenna according to the invention; The figure 2 is a representation of the transmission gain of antennas conforming to the first embodiment of the figure 1 compared to a flat antenna; The figure 3 is a representation of a variant of the first embodiment of a conical active antenna; The figure 4 is a representation along an axial plane of a second embodiment of a conical active antenna not forming part of the invention; and, The figure 5 is a representation according to a transverse plane of the antenna of the figure 4 .

[0015] From a general point of view, the present invention relates to the placement of radiating elements on a non-planar support surface in order to create a functional active antenna suitable for being positioned inside a radome of essentially conical shape constituting the nose of an aircraft.

[0016] Thus, the radome is a cone S, with axis A, vertex S0, and semi-vertical angle σ. The support surface is denoted S'. First embodiment: reference direction parallel to the cone axis and radiating elements arranged in flat steps

[0017] In a first embodiment of the invention, illustrated in the figure 1 The active antenna 100 allows observation of a domain located in front of the nose of the aircraft, that is to say along a reference direction V which is parallel to the axis A of the conical radome.

[0018] A reference plane PRef is defined as perpendicular to the reference direction V, passing, for example, through the vertex S0 of the conical radome. The reference plane is equipped with an XY coordinate system.

[0019] In this first embodiment, the radiating elements ERij are arranged so as to be parallel to the reference plane PRef.

[0020] Given the symmetry around axis A, the radiating elements ERij should be arranged on a plurality of concentric, circular rings Ci staggered along axis A. The active antenna 100 is thus arranged, along axis A, to present annular tiers. Each ring has radiating elements on its face oriented along axis A.

[0021] To best occupy the internal volume of the conical radome, the radius Ri of the outer edge of a ring Ci is preferably equal to (or slightly less than) the radius of the radome for this position along the axis A. The support surface S' of the active antenna 100 is therefore inscribed in the cone S formed by the radome.

[0022] For example, the active antenna has a plurality of rings Ci, indexed by the integer i, between 1 and N. For example, on the figure 1 , it has four crowns (N=4).

[0023] Each ring carries a plurality of radiating elements ERij indexed by the integer i and the integer j, between 1 and M. The value M depends on the ring considered. M is therefore a function of i: M(i).

[0024] The ERij radiating elements are identical to each other. They are planar. For example, they have a square shape. The center of each ERij radiating element is denoted CRij. Alternatively, a radiating element can have another shape (circular, elliptical, etc.) or be based on a different technology than a planar radiating element, such as a dipole antenna, a Vivaldi antenna, etc.

[0025] In a transverse plane, they are oriented so as to be parallel to each other, to facilitate, for example, the emission of polarized waves. Alternatively, to maintain symmetry by rotation around axis A, the radiating elements are rotated, in the transverse plane, towards axis A. For the antenna to emit a polarized wave, a relative orientation between the radiating elements must then be taken into account.

[0026] The normal vector Vij defining the normal direction to the surface of the radiating element ERij is parallel to the axis A, whatever i and whatever j, to allow observation of the domain in front of the antenna.

[0027] The distance between two successive crowns evaluated along axis A is denoted L.

[0028] The radiating elements ERij carried by a ring Ci must radiate with phase compensation ϕ ij proportional to the path difference dij, that is to say to the distance between the corona Ci and the reference plane PRef, so that the total radiation of the active antenna is equivalent to that of a planar active antenna located in the reference plane PRef.

[0029] In the implementation of the figure 1 we have: ϕ ij = i . L λ .2 . π Or λ is the wavelength under consideration.

[0030] This phase shift in the electronic activation command of the radiating element is referred to here as the "geometric" phase shift. It can be complemented by an "electronic" phase shift to form a beam in a pointing direction D outside the reference direction V.

[0031] There figure 2 is a simulation of the radiation from an array antenna consisting of a matrix of 3x3 radiating elements.

[0032] Curve G1 corresponds to a planar antenna.

[0033] The curve G2 corresponds to an antenna according to the first embodiment, with two rings and such that the first ring, carrying the central radiating element, is arranged in front of the second ring, carrying the eight other radiating elements, by a distance L which is taken as equal to one times the wavelength.

[0034] Curve G3 corresponds to an antenna according to the first embodiment, but this time the distance L is taken as equal to five times the wavelength.

[0035] For curve G1, the 3 dB width of the main lobe is approximately 34°, the gain is 14.6 dB, and the rejection of the side lobes is 16 dB.

[0036] For the G2 curve, the 3 dB width is approximately 30°, the gain is 13.4 dB, and the sidelobe rejection is 12 dB.

[0037] For the G3 curve, the 3 dB width is approximately 30°, a gain of 14.5 dB (therefore similar to the planar arrangement), and the rejection of the sidelobes of 17.5 dB (i.e. an improvement of 1.5 dB compared to the planar arrangement).

[0038] This improvement can be explained by the fact that the radiating elements are not all in the same transverse plane, they do not interact with each other as much as in the case of a planar configuration.

[0039] To optimize the position of the radiating elements on the different rings of the antenna, one can start from a regular pattern in the reference plane and project it onto the different rings.

[0040] For example, the regular pattern can be a Cartesian matrix delimited externally by a circle whose radius corresponds to that of the first (largest) ring and comprising columns of elements radiating along the reference direction X of the PRef plane and rows along the reference direction Y of the PRef plane.

[0041] To preserve this distribution, it is enough to delimit annular zones in the regular pattern and project the radiating elements of each zone onto the corresponding ring.

[0042] Radiating elements that straddle two annular zones are eliminated, leading to a punctually lacunary network.

[0043] Alternatively, the regular pattern exhibits radial symmetry around vertex S. This variant is the one represented on the figure 1 . Variant of the first embodiment: polygonal crown antenna.

[0044] In a variant embodiment of this first embodiment, represented in the figure 3The radiating elements ERij of the active antenna 200 are distributed not on circular rings, but on polygonal rings C'i, specifically hexagonal ones. The vertices of the outer edge of each polygonal ring rest on the cone defined by the radome. The support surface S' of the active antenna 200 is therefore inscribed within the cone S formed by the radome.

[0045] This variant allows for consideration of the geometry of the radiating elements and for optimization of the number and placement of the radiating elements.

[0046] In a transverse plane, the antenna can be divided into sectors. Two radiating elements located in the same sector are oriented parallel to each other, while two radiating elements in different sectors have a relative orientation. This relative orientation must be taken into account in order to emit radiation with a polarity common to all radiating elements.

[0047] A relative orientation between radiating elements in the transverse plane can be compensated for example by rotating the radiating elements so that they are all oriented parallel to each other, or by physical or electronic means (such as multiport radiating elements). Second embodiment: reference direction presenting an angle of inclination with the axis of the cone / radiating elements arranged in inclined steps

[0048] In a second embodiment not forming part of the invention, shown in the figures 4 And 5 The ERij radiating elements of the active antenna 300 are also arranged in a ring around axis A, but they are inclined relative to axis A towards the outside of cone S. This is to increase the angular coverage towards the rear of the aircraft's nose. The support surface S' on which the radiating elements are mounted is essentially a cone similar to cone S.

[0049] The embodiment presented here is particularly simple, in that all the radiating elements ERij are parallel to each other. They have the same angle of inclination with the axis A. Alternatively, the vector Vij normal to the surface of the radiating element ERij makes an angle of inclination with the axis A that depends on the radiating element considered.

[0050] To be able to observe an angular domain no longer centered on the axis A of the cone S, a reference direction V1 is chosen making an angle α 0 with axis A.

[0051] A reference plane PRef1 is now chosen perpendicular to the reference direction V1.

[0052] In the axial plane defined by axis A and the reference direction V1 (i.e., the plane of the figure 4 ), we choose to tilt the radiating elements ERij by the angle α 0 so that they are arranged parallel to the reference plane PRef1.

[0053] The angleα 0 is advantageously chosen to be able to depoint the beam along a direction D making a depointing angle φ with respect to the reference direction V1 so as to retain the possibility of observing the domain along the axis of the aircraft, i.e. along the axis A.

[0054] Given that the pointing angle φ is limited by a maximum value φmax (for example, approximately 60°) relative to the reference direction V1 (i.e., the direction without pointing), in order to be able to observe along axis A while increasing the observation area to the rear as much as possible, α 0 is chosen to be equal to φmax.

[0055] The maximum value φmax The angle of pointing is defined as a function of the degradation of the radiation pattern that can be tolerated.

[0056] In the plane transverse to axis A (that is, the plane of the figure 5), the radiating elements ERij that can potentially participate in the observation along the reference direction V1, are the radiating elements of the semi-cone resulting from the section of the cone S by an axial plane P0, orthogonal to the plane defined by the directions A and V1.

[0057] However, the further the radiating element ERij is located from the plane defined by the directions A and V1, the more its projection onto the reference plane PRef1 is distorted. The center CRij of the element ERij projects to the point CEij, which is the center of the equivalent radiating element EEij, resulting from the projection of the element ERij onto the reference plane PRef1.

[0058] This distortion affects the overall radiation pattern and the power radiated by each radiating element.

[0059] The power radiated by the radiating element must then be increased to compensate for this deformation so that the active antenna on the conical surface behaves like a planar active antenna in the reference plane.

[0060] It is therefore necessary to compensate for the gain of each radiating element according to its position relative to the plane defined by the directions A and V1.

[0061] However, it is not possible to compensate for the gain for the most distant radiating elements, i.e., those near the edge of the semi-cone (i.e., near plane P0). Therefore, it is necessary to perform a cut, retaining only the radiating elements closest to the plane defined by directions A and V1.

[0062] For example, the active network is reduced to the section of the cone between two axial planes P1 and P2 making an angle β 0 of + / -60° relative to the plane defined by directions A and V1.

[0063] Radiating elements that cannot contribute to radiation in the reference direction V1 must be switched off.

[0064] Restricting the number of radiating elements degrades the antenna's characteristics, particularly sidelobe rejection. Furthermore, it reduces the Equivalent Isotropically Radiated Power (EIRP) or antenna gain compared to a planar antenna. Therefore, it may be preferable to relax the constraint on sidelobe rejection to maintain maximum EIRP and minimum main lobe width.

[0065] It should be noted that the active antenna according to this second embodiment is symmetrical around the axis A. Thus, it is possible, rather than activating the radiating elements associated with a first reference direction, and then depointing the beam in a depointing direction relative to this first reference direction, to activate the radiating elements associated with a second reference direction corresponding to the initially sought depointing direction.

[0066] Active radiating elements for operation along the selected reference direction must radiate with phase compensation ϕ ij proportional to the distance dij between their center CRij and the reference plane so that the total radiation of the active antenna is equivalent to that of a planar active antenna located in the reference plane.

[0067] Note that the inclination of the radiating elements limits the radiating surface due to the masking of the radiating elements from each other when the beam is close to the axis A. The shadow area of ​​a radiating element is proportional to the masking angle and the shape of the radiating element (square, disk, ...).

[0068] One way to increase the radiating surface area of ​​a radiating element is to give it a spherical shape.

[0069] Another way to limit this masking is to arrange the radiating elements in a staggered pattern from one ring to the other.

[0070] Regardless of the embodiment, it is essential to ensure that all radiating elements emit with the same polarization relative to a reference frame associated with the reference plane used for beam formation. To achieve this, it is necessary to adjust the polarizations of each radiating element according to its position and orientation relative to the reference plane.

[0071] As with the first embodiment, in a variant of this second embodiment, the radiating elements could be arranged not on circular rings, but on polygonal rings. Benefits

[0072] The present invention presents different ways of arranging the radiating elements of a non-planar antenna in order to increase their number, therefore the EIRP or antenna gain and to control the quality of the radiation patterns to get as close as possible to a planar antenna.

[0073] In particular, the arrangement of an active antenna on a conical support surface allows for the creation of antenna subarrays. The radiating elements of a subarray are aligned in a reference direction specific to that subarray. It is then possible to orient the beam by electronic phase shifting in any direction within an extended RADAR scanning domain. Such an active antenna allows scanning not only forward but also laterally, up to an angle of view relative to the cone axis, for example, on the order of 120°.

[0074] The invention therefore makes it possible to arrange the radiating elements on a non-planar support surface, preferably conical, to best occupy the internal volume of the conical radome of the nose of an aircraft, while optimizing the number of radiating elements, the angular coverage of the active antenna thus obtained, and the quality of the radiation pattern.

Claims

1. An active antenna (100) for aeronautics including a plurality of radiating elements, the radiating elements (ERij) being arranged on a non-flat support surface that is inscribed inside a cone (S), a radiating element being positioned at a point (CRij) of the support surface (S') so that a normal direction (Vij) to said radiating element (ERij) forms an angle of inclination relative to an axis (A) of the cone, characterized in that the angle of inclination of each radiating element is zero and the support surface is tiered to present a plurality of concentric crowns (Ci) spaced from each other along the axis (A) of the cone, the radiating elements being positioned on the different crowns, the active antenna being thus adapted to be positioned inside a radome constituting the nose of an aircraft, the radome defining said cone.

2. The active antenna according to claim 1, wherein said angle of inclination is such that a set of active radiating elements among said plurality of radiating elements can, by electronic deflection, observe a domain located in front of the support surface, along the axis (A) of the cone.

3. The active antenna (100) according to claim 1 or claim 2, wherein the crowns of the plurality of crowns are circular or polygonal.

4. The active antenna (100) according to any one of claims 1 to 3, wherein, in a reference plane perpendicular to the axis (A) of the cone, the projections (EEij) of the radiating elements (ERij) follow a regular pattern, with Cartesian symmetry or radial symmetry.

5. The active antenna (100) according to any one of claims 1 to 4, adapted so that, when a radiating element (ERij) is active, a so-called geometric phase shift is introduced to compensate for a path difference between said radiating element and a common reference plane to all active radiating elements, so that the total radiation of the active antenna is equivalent to that of a flat active antenna located in the reference plane.

6. The active antenna (100) according to claim 5 adapted so that, when a radiating element is active, a power correction is introduced to compensate for a difference between the surface of said radiating element (ERij) and a surface of a projection (EEij) of said radiating element in the common reference plane to all active radiating elements.