Omnidirectional antenna and associated antenna assembly.

Abstract

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.

Description

[0001] The present invention relates to an omnidirectional radiation antenna and an antenna assembly comprising such an antenna. More specifically, the invention relates to a hemispherical antenna having an omnidirectional radiation pattern.

[0002] Various applications may require the implementation of an omnidirectional radiation pattern antenna, or omnidirectional 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 transmitting / receiving modules, each transmitting / receiving module addressing one or more associated radiating elements.

[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 radio frequency - RF link.

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

[0006] The common RF antenna is placed at the center of the hemisphere formed by the array antenna, or near the center, in particular slightly set back from the center along the antenna's axis of symmetry to compensate for too small an opening of the antenna's main lobe.

[0007] The aim is to use an omnidirectional antenna capable of producing hemispherical radiation, i.e. one whose main lobe of the radiation pattern has a solid opening angle of 2π, to excite the transmission / reception modules distributed over the hemispherical radome.

[0008] There are known antennas that are presented as omnidirectional, such as dipole antennas, monopole antennas, collinear antennas, helical antennas, etc.

[0009] For example, the paper FERREIRA DB et al., "An efficient approach to the analysis and synthesis of spherical-circular thin microstrip antennas", ANTENNAS AND PROPAGATION SOCIETY INTERNATIONAL SYMPOSIUM (APSURSI), 2010 IEEE, IEEE, PISCATAWAY, NJ, USA, July 11, 2010 (2010-07-11), pages 1-4, XP032146197, DOI: 10.1109 / APS.2010.5562184,ISBN: 978-1-4244-4967-5 discloses an antenna whose radiating element forms a spherical cap.

[0010] HUFF GH et al, "A Spherical Inverted-F Antenna (SIFA)", IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, IEEE, PISCATAWAY, NJ, US, vol. 8, January 1, 2009 (2009-01-01), pages 649-652, XP011257495, ISSN: 1536-1225, discloses an antenna whose radiating element takes the form of a spindle on the surface of a sphere, which extends between two meridians and between two parallels.

[0011] Document US 6,281,847 B1 discloses an antenna comprising a plurality of radiating elements, but shaped on a cylinder.

[0012] However, in practice, 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.

[0013] However, 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.

[0014] 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).

[0015] The aim of the invention is therefore to propose an antenna that can address this problem.

[0016] To this end, the invention relates to an omnidirectional antenna and an antenna assembly comprising such an omnidirectional antenna in accordance with the appended claims.

[0017] 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: [ Fig. 1 ] there figure 1 is a top view of an antenna assembly according to the invention; [ Fig. 2 ] there figure 2 is a side view of the antenna assembly of the Figure 1 ; [ Fig. 3 ] there figure 3 is an axial section of the antennal assembly of the Figure 1 ; And, [ Fig. 4 ] there figure 4 represents the radiation pattern of the antenna assembly of the Figure 1 in two orthogonal axial planes.

[0018] THE figures 1 , 2 And 3illustrate a preferred embodiment of an antenna assembly 10 according to the invention.

[0019] The antenna assembly 10 comprises an omnidirectional antenna 20 and a support element 30 for this omnidirectional antenna.

[0020] The omnidirectional antenna 20 is hemispherical. It has the shape of a half-sphere with center O and radius R.

[0021] An XYZ coordinate system is attached to the center O of the hemisphere.

[0022] The antenna element 10 is symmetrical with respect to the YZ plane and with respect to the XZ plane.

[0023] As seen in section on the Figure 3 , the omnidirectional antenna 20 successively comprises, 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.

[0024] 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.

[0025] The metal layer 22 covers the inner surface of the support layer 24.

[0026] 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.

[0027] Patch 26 is bounded by two longitudinal edges, 33 and 34, and by lateral edges, 35 and 36.

[0028] According to the invention, a low attenuation omnidirectional radiation pattern is obtained by adapting the curvature of the edges of the radiating element.

[0029] In the preferred embodiment, 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 XY plane, but at a height h above it. In other words, the lateral edges are on a circle of radius Rp whose center is on the Z axis. In top view ( Figure 1 ), the longitudinal edges have a concavity oriented towards the center O of the antenna 20. They have an adapted curvilinear length L2.

[0030] In the preferred embodiment, the lateral edge 35, respectively the lateral edge 36, correspond to the intersection of the sphere of center O and radius R and a cylinder of axis C2, respectively a cylinder of axis C1, and radius Rc, the axis C2, respectively the axis C1, being parallel to the axis Z and lying in the plane XZ. In top view ( Figure 1 ), the longitudinal edges have a concavity oriented away from the center O of the antenna 20. They have a suitable curvilinear length L3. Alternatively, the lateral edges correspond to the intersection of the sphere with center O and radius R and a cylinder with elliptical section.

[0031] The distance between the C1 axis, respectively C2, and the Z axis also allows, by modifying the position of the corners of the patch, to reduce the length L3 by increasing the length L2.

[0032] 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.

[0033] The sheath 44 of the coaxial cable 40 is electrically connected to the metal layer 22, i.e. to the ground potential.

[0034] Advantageously, point P is located in the YZ plane of symmetry of patch 26, but outside the Z axis. The offset of point P relative to apex B of 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 TM10 mode of a planar radiating element.

[0035] According to the invention, adjusting the curvature of the edges of the “patch” makes it possible to play on the diffraction phenomena at the edges of the patch to obtain hemispherical radiation.

[0036] In TM10 mode the field is maximum at the longitudinal edges 33 and 34.

[0037] A convex shape of the lateral edges 33 and 34 advantageously follows a line of iso amplitude of the electric field.

[0038] The field evolves between its maximum values ​​along the lateral edges 35 and 36. A concave shape of the lateral edges makes it possible to compensate for the asymmetry of the field according to the XOZ plane.

[0039] It should be noted that the aim is to adjust the radiation pattern for a given resonance frequency F0. A person skilled in the art might believe that reducing the radius R of the hemispherical antenna can lead to the desired hemispherical radiation pattern. However, by reducing the radius, the resonance frequency increases. In addition, diffraction phenomena become predominant and, counterintuitively, actually lead to a reduction in the aperture of the main lobe of the antenna and not to its aperture.

[0040] The support element 30 is of truncated cone shape around the Z axis. 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 θ.

[0041] As visible on the cut of the Figure 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 which is also metallic. Thus, the metallic layer 22, the collar 31 and the side wall 32 form an equipotential brought to the ground potential.

[0042] The radiation pattern of the antenna element 10 is shown in the Figure 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.

[0043] 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 main DX lobe on the diagram of the Figure 4A and DY on the diagram of the Figure 4B ) then we obtain an angle αX of approximately 165 deg in the plane of the Figure 4A , and an angle αY of approximately 200 deg in the plane of the Figure 4B .

[0044] There Figure 4 results from a simulation made for an antenna element operating at 10 GHz, i.e. a characteristic wavelength in vacuum of 30 mm.

[0045] The antenna has the following characteristic dimensions: the radius of the patch support hemisphere: R= 1.16.λ 0 ; The distance between the patch and the ground plane: e = 0.025. λ 0 ; The radius of the lateral edges: Rp=0.12. λ 0 ; The curvilinear length edge to edge in the Y direction, L3 = 0.25.λ 0 , or 7 mm and in the X direction, L2 L2 = 0.1.λ or 2.5 mm ; Distance between C1 and C2 is 0.95.λ 0 ;

[0046] The support element has the following characteristic dimensions: h 0 = 0.28.λ 0 or 8 mm; Ro = 0.35.λ 0 or 10 mm (i.e. a half angle θ at the apex of approximately 30°) /

[0047] On the figures 2 And 3 the electric field E radiated to infinity ( R ∞ ) by the antenna element 10 has been shown illustratively at different points. The emitted wave is linearly polarized.

[0048] The antennal element just presented can have a large number of variants.

[0049] 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 metallic support element then advantageously allows to "repel" the electric field and thus reduce the aperture 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.

[0050] Instead of one excitation port, the radiating element could be excited by a plurality of ports, including a pair of differentially fed ports.

[0051] The excitation of the radiating element can use any known technique (via feed, slot feed, etc.).

[0052] The value of the relative permittivity of the dielectric material constituting the support can also be adjusted.

[0053] The omnidirectional antenna has the advantage of generating an electromagnetic wave that is homogeneous in amplitude, with linear polarity throughout the half-space.

[0054] It is easy to make. It is composed of a single radiating element (no matrix to maintain coherence and phase).

[0055] The antenna with or without its support element remains compact, which facilitates its integration, particularly in the case of use as a central RF antenna housed inside a hemispherical radome.

Claims

1. Hemispherical antenna (20) having an omnidirectional radiation pattern, the antenna 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 thatthe radiating element is constituted by a metal lamella conforming to a curved surface which is a sphere with center O and radius R, the radiating element being symmetrical both with respect to 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, the radiating element (26) being delimited by longitudinal edges (33, 34) and by lateral edges (35, 36), the curvature of the lateral edges and / or the longitudinal edges being adapted so that the radiation pattern of the antenna is omnidirectional even at low attenuation.

2. Antenna according to claim 1, for which the low attenuation radiation pattern is a -3dB radiation pattern, the antenna having, in each plane passing through the central axis (Z), a low attenuation radiation pattern with a main lobe opening of at least 160°.

3. Antenna according to any one of the preceding claims, in which the longitudinal edges (33, 34) are concave.

4. Antenna according to claim 3, in which the longitudinal edges are on a circle whose center is on the central axis.

5. Antenna according to any one of the preceding claims, in which the side edges (35, 36) are convex.

6. Antenna according to claim 5, in which the lateral edges are each at the intersection of the sphere of center O and radius R and a cylinder whose axis is parallel but distinct from the central axis.

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. Antenna according to any one of the preceding claims, in which the radiating element is carried on an external face of a support layer (24) made of a dielectric material, an internal 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 half-spheres, the support layer having a constant thickness.

10. Antenna assembly (10), characterized in thatit comprises a hemispherical antenna (20) having an omnidirectional radiation pattern according to any one of the preceding claims, and a support element (30), the support element supporting the hemispherical 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 hemispherical antenna to adjust its aperture.

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