Omnidirectional antenna and associated antenna assembly.

Abstract

Omnidirectional antenna and associated antenna assembly. The present invention relates to an omnidirectional antenna (20) comprising a radiating element (26), a metal layer (22) brought to a reference potential so as to constitute a ground layer, and means (40) for exciting the radiating element, the radiating element being arranged above the ground layer, which is characterized in that the radiating element (26) is constituted by a metal strip conforming to a curved surface, the curved surface having a strictly positive Gaussian curvature at each of its points. Figure for the abstract: Figure 3

Description

Title of the invention: Omnidirectional antenna and associated antenna assembly.

[0001] The present invention relates to an omnidirectional radiation antenna and an antenna assembly comprising such an antenna.

[0002] Various applications may require the implementation of an omnidirectional radiating antenna. In this document, the specific application in which an omnidirectional antenna is used as a common radio frequency antenna at the center of a hemispherical array antenna is considered.

[0003] A hemispherical array antenna comprises a plurality of radiating elements carried by the external surface of a hemispherical radome. The internal surface of this radome carries a plurality of transmission / reception modules, each transmission / reception module addressing an associated radiating element.

[0004] To transmit the transmission, reception and / or control signals of the active components, between low-level electronics and each of the transmission / reception modules, it is preferable to implement a wireless link, for example a radiofrequency - RF link.

[0005] While each transmit / receive module is equipped with an individual RF antenna, the low-level electronics are connected to a common RF antenna.

[0006] The common antenna is placed at the center of the half-sphere formed by the array antenna, or in the vicinity of the center, in particular slightly set back from the center along the axis of symmetry of the antenna to compensate for an opening that is too small in the main lobe of the antenna.

[0007] We seek to use an antenna which is capable of producing hemispherical radiation (i.e. having a solid opening angle of 2ir) to excite the transmission / reception modules distributed over the hemispherical radome.

[0008] Antennas are known which are presented as being omnidirectional, such as dipole antennas, monopole antennas, collinear antennas, helical antennas, etc.

[0009] However, in fact, these antennas do not generate an omnidirectional radiation pattern: if at a very high attenuation (-12dB), the effective radiation pattern can approach an omnidirectional radiation pattern, the more the attenuation is reduced (-6dB) and the more the effective radiation pattern deviates from an omnidirectional radiation pattern.

[0010] Now, for the application as a common RF antenna of a hemispherical array antenna, the radome, which carries a metal layer, internally delimits a cavity, which confines the electromagnetic waves. The RF signals emitted by the common antenna and each of the individual antennas of the transmitting / receiving modules are confined inside this cavity, so that the background noise level inside the cavity is high.

[0011] Consequently, the implemented antenna must be able to operate well above the background noise level, i.e. it must exhibit an omnidirectional radiation pattern even at low attenuation (around -3dB).

[0012] The aim of the invention is then to propose an antenna making it possible to respond to this problem.

[0013] To this end, the invention relates to an omnidirectional antenna comprising a radiating element, a metal layer brought to a reference potential so as to constitute a ground layer, and means for exciting the radiating element, the radiating element being arranged above the ground layer, characterized in that the radiating element is constituted by a metal strip conforming to a curved surface, the curved surface having a strictly positive Gaussian curvature at each of its points.

[0014] According to other advantageous aspects of the invention, the antenna comprises one or more of the following characteristics, taken in isolation or in all technically possible combinations:

[0015] - the curved surface is a sphere, with center O and radius R.

[0016] - the radiating element is symmetrical both with respect to a longitudinal plane and a transverse plane, the longitudinal and transverse planes being orthogonal to each other, their intersection defining a central axis, an apex of the radiating element being located on the central axis.

[0017] - in each plane passing through the central axis, a radiation pattern at -3dB having a main lobe opening of at least 160°.

[0018] - the radiating element is oblong in shape.

[0019] - the radiating element is delimited by longitudinal edges and by edges lateral, the curvature of the edges allowing the radiation pattern of the antenna to be adjusted.

[0020] - the antenna excitation means makes it possible to excite the metallic element in at least one excitation port, the excitation port being located away from the apex of the radiating element.

[0021] - the radiating element is carried on an external face of a support layer in a dielectric material, an inner face of the support layer carrying the ground layer.

[0022] - the outer and inner surfaces of the support layer are half-spheres concentric, the support layer having a constant thickness.

[0023] The invention also relates to an antenna assembly comprising an omnidirectional antenna conforming to the preceding antenna and a support element, the support element supporting the omnidirectional antenna, the support element having a metallic external wall brought to the reference potential, the presence of the support element modifying the radiation pattern of the omnidirectional antenna to adjust its aperture.

[0024] The invention will appear more clearly on reading the description which follows, given solely by way of non-limiting example, and made with reference to the drawings in which:

[0025] [Fig-1] [Fig.l] is a top view of an antenna assembly according to the invention;

[0026] [Fig.2] [Fig.2] is a side view of the antenna assembly of [Fig.l];

[0027] [Fig.3] [Fig.3] is an axial section of the antenna assembly of [Fig.l]; and,

[0028] [Fig.4] [Fig.4] represents the radiation pattern of the antenna assembly of [Fig.l] in two orthogonal axial planes.

[0029] Figures 1, 2 and 3 illustrate a preferred embodiment of an antenna assembly 10 according to the invention.

[0030] The antenna assembly 10 comprises an omnidirectional antenna 20 and a support element 30 for this omnidirectional antenna.

[0031] The omnidirectional antenna 20 has the shape of a half-sphere with center O and radius R.

[0032] An XYZ reference frame is attached to the center O of the half-sphere.

[0033] The antenna element 10 is symmetrical with respect to the YZ plane and with respect to the XZ plane.

[0034] As visible in section in [Fig.3], the omnidirectional antenna 20 comprises successively, from the inside to the outside, a metal layer 22 forming the electrical mass, a support layer 24 and a metal radiating element, or “patch”, 26.

[0035] The support layer 24 forms a hemispherical dome, the outer surface of which is at the distance R from the center O. The support layer 24 is made of a dielectric material whose relative permittivity is adapted.

[0036] The metal layer 22 covers the inner surface of the support layer 24.

[0037] If in the state of the art a patch is a planar radiating element, according to the present invention, it is a three-dimensional radiating element, shaped on the outer surface of the support layer 24, and consequently constituting a curved surface. At the apex point B for example, this surface has both a non-zero curvature in the YZ plane and a non-zero curvature in the XZ plane.

[0038] The patch 26 is delimited by two longitudinal edges, 33 and 34, and by lateral edges, 35 and 36.

[0039] For example, the longitudinal edge 33, respectively the longitudinal edge 34, correspond to the intersection of the sphere of center O and radius R and a horizontal plane, parallel to the plane XY, but at a height h above it. In top view ([Fig.l]), the longitudinal edges have a concavity oriented towards the center O of the antenna 20.

[0040] For example, the lateral edge 31, respectively the lateral edge 32, correspond to the intersection of the sphere of center O and radius R and a cylinder of axis C2, respectively a cylinder of axis Cl, and radius Rc, the axis C2, respectively the axis Cl, being parallel to the axis Z and lying in the plane XZ. In top view ([Fig.l]), the longitudinal edges have a concavity oriented away from the center O of the antenna 20.

[0041] The excitation of the patch 26 is carried out by a port P. In the embodiment shown in the figures, the port P is connected to the core 42 of a coaxial cable 40 allowing the connection of the radiating element of the omnidirectional antenna 20 to the low-level electronics (not shown in the figures). The core circulates through a via 25 arranged through the metal layer 22 and the support layer 24.

[0042] The sheath 44 of the coaxial cable 40 is electrically connected to the metal layer 22, i.e. to the ground potential.

[0043] Advantageously, the point P is located in the YZ plane of symmetry of the patch 26, but outside the Z axis. The offset of the point P relative to the apex B of the patch 26 makes it possible to adjust the impedance of the patch to, for example, 50 Ohms while operating the patch in a mode similar to the TM 10 mode of a planar radiating element.

[0044] The support element 30 is of truncated cone shape around the axis Z. Its large base is circular, of radius R0, and its small base is circular with center 0 and radius R. The support element 30 has a height h0. It has a half-opening at the apex A of the cone equal to 0.

[0045] As visible in the section of [Fig.3], the support element 30 consists of a metallic side wall 23. To ensure electrical continuity between the side wall 23 of the support element 30 and the metallic layer 22 of the antenna 20, the small base of the support element 30 has an annular collar 31 also metallic. Thus, the metallic layer 22, the collar 31 and the side wall 32 form an equipotential brought to the ground potential.

[0046] The radiation pattern of the antenna element 10 is shown in [Fig. 4]. More precisely, graph A is the gain as a function of the emission angle in the XZ plane, while graph B is the gain as a function of the emission angle in the YZ plane.

[0047] If we define the angular aperture between the emission directions where the amplitude is reduced by 3dB compared to the maximum amplitude (along the direction of the lobe principal DX on the diagram of Figure 4A and DY on the diagram of Figure 4B) then we obtain an angle aX of approximately 165 deg in the plane of Figure 4A, and an angle aY of approximately 200 deg in the plane of Figure 4B.

[0048] [Fig.4] results from a simulation made for an antenna element operating at 10 GHz, i.e. a characteristic wavelength in vacuum of 30 mm. The support element has the characteristic dimensions: h0 = 8 mm; Ro = 10 mm. The patch has a curvilinear length edge to edge in the Y direction of 7 mm and in the X direction of 2.5 mm.

[0049] In Figures 2 and 3 the electric field E radiated to infinity (Rj by the antenna element 10 has been shown in an illustrative manner at different points. The emitted wave is linearly polarized.

[0050] The antenna element just presented can have a large number of variants.

[0051] First of all, the antenna element can be limited to the omnidirectional antenna, without a support element. However, simulations show that in this case, the opening angle is greater than 180°. Such an opening angle is too large for the common RF antenna application in a hemispherical array antenna. The metal support element then makes it possible to "push back" the electric field and thus reduce the opening of the radiating element compared to that of the isolated omnidirectional antenna. By playing on the shape of the support element, the radiation pattern can thus be adapted. For example, by playing on the value of the half-opening angle. For example, by playing on the contour of the large base of the support element, which instead of being circular could be elliptical or "peanut" shaped.

[0052] The contour of the radiating element can also be adapted to adjust the shape of the radiation pattern. In particular, the concavity of the side edges, instead of being negative, can be reduced, cancelled, or even made positive.

[0053] Instead of one excitation port, the radiating element could be excited by a plurality of ports, including a pair of differentially powered ports.

[0054] The excitation of the radiating element can use any known technique (via feed, slot feed, etc.).

[0055] The value of the relative permittivity of the dielectric material constituting the support can also be adjusted.

[0056] The shape of the curved surface to which the radiating element conforms may be something other than a sphere.

[0057] Generally speaking, it is a curved surface presenting at each of its points a strictly positive Gaussian curvature (or total curvature). The Gaussian curvature is defined by the product of the curvature along a principal direction and a curvature in a secondary direction, normal to the main direction, the main and secondary directions defining a plane tangent to the surface considered at the point considered.

[0058] The omnidirectional antenna has the advantage of generating an electromagnetic wave that is homogeneous in amplitude, with linear polarity throughout the half-space.

[0059] It is easy to make. It is composed of a single radiating element (no matrix to maintain coherence and phase).

[0060] The antenna with or without its support element remains compact, which facilitates its integration, in particular in the case of use as a central RF antenna housed inside a hemispherical radome.

Claims

Claims

1. Omnidirectional antenna (20) comprising a radiating element (26), a metal layer (22) brought to a reference potential so as to constitute a ground layer, and means (40) for exciting the radiating element, the radiating element being arranged above the ground layer, characterized in that the radiating element is constituted by a metal strip conforming to a curved surface, the curved surface having a strictly positive Gaussian curvature at each of its points.

2. An antenna according to claim 1, wherein the curved surface is a sphere, with center 0 and radius R.

3. An antenna according to claim 2, wherein the radiating element is symmetrical about both a longitudinal plane (YZ) and a transverse plane (XZ), the longitudinal and transverse planes being orthogonal to each other, their intersection defining a central axis (Z), an apex (B) of the radiating element being located on the central axis.

4. Antenna according to claim 3, having, in each plane passing through the central axis (Z), a radiation pattern at -3dB having a main lobe opening of at least 160°.

5. An antenna according to any preceding claim, wherein the radiating element (26) is oblong in shape.

6. An antenna according to claim 5, wherein the radiating element (26) is delimited by longitudinal edges (33, 34) and by lateral edges (35, 36), the curvature of the edges making it possible to adjust the radiation pattern of the antenna.

7. Antenna according to any one of the preceding claims, in which the means for exciting the radiating element of the antenna makes it possible to excite the metallic element at at least one excitation port (P), the excitation port being located away from an apex (B) of the radiating element.

8. An antenna according to any preceding claim, wherein the radiating element is carried on an outer face of a support layer (24) made of a dielectric material, an inner face of the support layer carrying the ground layer.

9. An antenna according to claim 8, wherein the outer and inner surfaces of the support layer are concentric hemispheres, the support layer having a constant thickness.

10. Antenna assembly (10), characterized in that it comprises an omnidirectional antenna (20) according to any one of the preceding claims and a support element (30), the support element supporting the omnidirectional antenna, the support element having a metallic external wall brought to the reference potential, the presence of the support element modifying the radiation pattern of the omnidirectional antenna to adjust its aperture.

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