ANTENNA WITH IMPROVED COVERAGE OVER AN EXTENDED FREQUENCY RANGE

Description

Technical Field

[0001] The present invention relates to an antenna capable of transmitting with wide coverage in several frequency bands, allowing it to perform several distinct communication functions on its own. Prior art

[0002] Space vehicles are equipped with antennas which ensure communication between these vehicles and ground stations during the flight phases.

[0003] These antennas are used in particular for telemetry, trajectography, or the satellite positioning system (“Global Navigation Satellite System”, “GNSS”).

[0004] Achieving these functions may require the use of a complex system with several antennas, each associated with a particular function, these antennas each transmitting in distinct frequency ranges.

[0005] Document FR 2 841 390 describes a dual-band, dual-polarization radiating device for base stations of GSM or UMTS type cellular radiocommunication networks.

[0006] Document FR 2 854 737 describes a multi-beam and / or multi-frequency BIP material antenna allowing workers to work around two distinct working frequencies.

[0007] US 4,042,935 describes a broadband antenna comprising annular cavities excited by dipoles.

[0008] US 5,548,299 describes a high-frequency dipole antenna with nested cavities.

[0009] It is therefore desirable to have an antenna capable of transmitting with improved coverage in several distinct frequency bands for the realization of several communication functions while maintaining a limited footprint and a simple system. Statement of the invention

[0010] The present invention relates to an antenna as recited in claim 1. The antenna comprising: a radiating antenna element capable of emitting a signal in at least a first frequency band and in a second frequency band, separate from the first band and at a higher frequency than the latter, and a waveguide covered by the radiating antenna element, comprising at least a first resonant cavity and single-mode or predominantly single-mode in the first frequency band, and a second resonant cavity distinct from the first resonant cavity and located inside the latter, said second resonant cavity being single-mode or predominantly single-mode in the second frequency band.

[0011] By "single-mode", it is meant that only the fundamental mode of the resonant cavity considered can propagate. By "mostly single-mode", it is meant that the resonant cavity considered is single-mode over at least 50%, for example at least 75%, of the frequency band considered. In this case, the resonant cavity may not be single-mode on at least one end of the frequency band, it may be modeless or dual-mode on this end.

[0012] The use of the first resonant cavity alone makes it possible to broaden the emission frequency spectrum but allows the excitation of higher-order resonant modes which can disturb the radiation pattern in the second frequency band, in particular at low elevation angles. The addition of the second resonant cavity inside the first resonant cavity advantageously makes it possible to obtain an improved gain in the second frequency band, in particular at low elevation angles, by not allowing the propagation of these higher-order modes while maintaining a limited footprint and a simple system, without modifying the dimensions of the antenna compared to the presence of the first cavity alone. The invention thus provides an antenna with improved coverage in several distinct frequency bands for the performance of several communication functions.

[0013] In an exemplary embodiment, the radiating antenna element is present on a substrate covering the waveguide, and the antenna has one or more openings between a wall delimiting the first cavity and the substrate.

[0014] Such a feature allows to obtain an improved gain at low elevation angles in the second frequency band.

[0015] In particular, an edge of said wall located on the substrate side may have a crenellated shape thus defining a plurality of openings between said wall and the substrate.

[0016] Such a feature allows for improved gain at low elevation angles in the second frequency band while limiting gain decline in the first frequency band.

[0017] According to the invention described in claim 1, a ratio RA1 H1 / H2 is between 1 and 3.25, where H1 denotes a height of the first cavity and H2 a height of the second cavity.

[0018] Such a feature allows to further improve the gain at low elevation angles in the second frequency band.

[0019] An optimal height for the second cavity H2 providing optimal gain at low elevation angles in the second frequency band can be determined by a parametric study. As such, the ratio RA1 is preferably between 1.28 and 2.2.

[0020] In an exemplary embodiment, the first frequency band corresponds to frequencies between 1164 MHz and 1591 MHz and the second frequency band corresponds to frequencies between 2200 MHz and 2290 MHz.

[0021] According to thisFor example, the first frequency band corresponds to the Global Positioning System (“GNSS”) application and the second frequency band to telemetry applications.

[0022] The invention also relates to a vehicle equipped with at least one antenna as described above. The vehicle may be a space vehicle, such as a space launcher, an exploration vehicle or a satellite. The use of the antenna described is not limited to a space application, it can be used on other vehicles such as a train, a car or an aircraft. Brief description of the drawings

[0023] [ Fig. 1 ] There figure 1 represents a first example of an antenna according to the invention. Fig. 2 ] There figure 2 represents a second example of an antenna according to the invention. Fig. 3 ] There figure 3 represents a third example of an antenna according to the invention. [ Fig. 4 ] There figure 4is a comparative diagram showing the gain obtained as a function of frequency for antennas according to the invention and an antenna outside the invention. Fig. 5 ] There Figure 5 is a diagram showing the gain obtained in the second frequency band for an antenna outside the invention. [ Fig. 6 ] There figure 6 is a diagram showing the gain obtained in the second frequency band for an antenna according to the invention. Fig. 7 ] There figure 7 is a diagram showing the gain obtained in the second frequency band for another antenna according to the invention. Description of the embodiments

[0024] There figure 1represents a first example of an antenna 1 according to the invention comprising a substrate 3 on which a radiating antenna element 5 is present. The substrate 3 may be a dielectric substrate. The substrate 3 may be made of a composite material, for example reinforced with glass. A substrate 3 marketed under the reference RO3210 ®< by the company ROGERS Corporation may be used. The substrate 3 may have a planar shape. It will be noted, however, that the presence of the substrate 3 is not necessary, the radiating antenna element being able to alternatively be formed by a self-supporting metal part.

[0025] The radiating element 5 is, in the illustrated example, of the double crossed dipole or dual-band asymmetric crossed dipole type and comprises a first pair of dipoles 5a and 5b which are perpendicular to each other and fed with a phase shift of 90°, and a second pair of dipoles 5c and 5d, distinct from the first pair, which are perpendicular to each other and fed with a phase shift of 90°. The dipoles 5a and 5b of the first pair have a different length from the dipoles 5c and 5d of the second pair. The dipoles 5a-5d of the first and second pairs each have a trapezoidal shape in the illustrated example. The dipoles 5a-5d form in the illustrated example a radiating element 5 having a bow-tie structure. Dipoles 5a-5d are present on both sides of the substrate (on the upper face and on its opposite lower face).The radiating element 5 is capable of emitting a signal in the radiofrequency spectrum, this signal having a circular polarization in at least a first frequency band and in a second frequency band, separate from the first band and at a higher frequency than the latter. For example, the first frequency band may correspond to frequencies between 1164 MHz and 1591 MHz and the second frequency band to frequencies between 2200 MHz and 2290 MHz. A coaxial cable 6 feeds the radiating element 5. The radiating element 5 of the double crossed dipole or dual-band asymmetric crossed dipole type is known per se, the invention is nevertheless not limited to this type of radiating element, it is possible as a variant to use a radiating element formed by a crossed dipole or other types of dual-band or wideband radiating elements such as crossed dipoles coupled to resonators for example, for example.The radiating element 5 may have a planar shape, as illustrated. The radiating element 5 may be devoid of vertical elements, directed along the direction Z, perpendicular to the plane P containing the radiating element 5 and the substrate 3 in the illustrated example.

[0026] The radiating element 5 covers a waveguide 7. The waveguide 7 comprises in its lower part, or at its base, a reflector 9. In the absence of the waveguide 7 and the reflector 9, the radiating element 5 emits a signal having an upward circular polarization along the direction Z shown but also downward with an opposite polarization direction. The reflector 9 makes it possible to obtain a unidirectional circular polarization signal along the direction Z, here only directed upwards (in the direction opposite to the reflector 9), by reflecting the emitted signal component downwards and reversing its polarization direction due to this reflection.

[0027] The waveguide 7 comprises a first resonant cavity 11 which is single-mode or predominantly single-mode in the first frequency band. The first resonant cavity 11 may not be single-mode, nor predominantly single-mode, in the second frequency band. The waveguide 7 further comprises a second resonant cavity 13 which is distinct from the first cavity 11 and nested in the latter. The second resonant cavity 13 is single-mode or predominantly single-mode in the second frequency band. The second cavity 13 may not be single-mode, nor predominantly single-mode, in the first frequency band.The first cavity 11 helps to increase the gain in the upper hemisphere, thanks to the presence of the reflector 9 which reflects the waves upwards, thus increasing the gain in the upper hemisphere, and to broaden the frequency band in which the antenna 1 emits by allowing the generation of a second circularly polarized signal in addition to the signal generated by the radiating element and corresponding to a distinct frequency range. If only the first cavity 11 is used, there is generation of higher order modes beyond the cut-off frequency of the second mode TM01 which disturbs the gain in the second frequency band, in particular at low elevation angles. The addition of the second cavity 13 allows a significant improvement in the gain at low elevation angles in the second frequency band by only allowing the first modes to be excited in the second frequency band.

[0028] The radiating element 5 is located above the first 11 and second 13 cavities on the side opposite the reflector 9. The waveguide 7 is, in the example illustrated, closed in its lower part by the reflector 9 which defines a base common to the first 11 and second 13 cavities and delimits the latter. The reflector 9 is in contact with the first 11 and second 13 cavities. The waveguide 7 is open in its upper part, opposite the reflector 9, in the absence of the radiating element 5 and the substrate 3. The first 11 and second 13 cavities are closed in their lower part by the reflector 9 and closed laterally, and are open in their upper part opposite the reflector 9, in the absence of the radiating element 5 and the substrate 3. The first 11 and second 13 cavities are located below the radiating element 5.In the illustrated example, the substrate 3 positioned on the waveguide 7 closes the latter and the first cavity 11 by coming into contact with the latter. The invention does not impose such contact as will be described below. The assembly of the first 11 and second 13 cavities and the reflector 9 may be entirely metallic. The first cavity 11 has dimensions greater than the second cavity 13. The second cavity 13 has a height H2 less than or equal to the height H1 of the first cavity 11. The largest dimension D1 of the first cavity 11 is greater than the largest dimension D2 of the second cavity 13. These largest dimensions D1 and D2 may be diameters in the illustrated example of a circular geometry for the first 11 and second 13 cavities. The second cavity 13 is centered relative to the first cavity 11.In the illustrated example, the first and second cavities each have a circular shape, but it is not outside the scope of the invention if the latter have a different shape, such as a polygonal shape, for example rectangular or octagonal, as will be described below. The walls of the first 11 and second 13 cavities may be solid, that is to say without a slot or lack of material. The coaxial cable 6 extends inside the first 11 and second 13 cavities through the latter.

[0029] As stated above, the RA1 H1 / H2 ratio is between 1 and 3.25, for example between 1.28 and 2.2.

[0030] According to one example, the ratio RA2 D1 / D2 can be between 1.19 and 2.1. Changing the ratio RA2 makes it possible to modulate the frequency bands in which the antenna 1 transmits depending on the desired application.

[0031] The first 11 and second 13 cavities are sized so as to be single-mode or predominantly single-mode in the first frequency band and in the second frequency band respectively. The choice of the dimensions to be adopted for this purpose is part of the general knowledge of the person skilled in the art. For example, the radii of the cavities 11 and 13 can be defined as a function of the cut-off frequencies of a circular waveguide calculated using the formula below. f c , mn = p ′ nm 2 πa εμ

[0032] In the above formula p' nm denote the roots of the Bessel functions of the first kind, a the radius of the desired waveguide, ε and µ the dielectric permittivity and the magnetic permeability of the medium respectively. The parameters n and m correspond to the order of the mode guided by the section of the cavity, here circular.

[0033] As an example for a first frequency band ranging from 1164 MHz to 1591 MHz and a second frequency band ranging from 2200 MHz to 2290 MHz, it is possible to use a waveguide 7 having a radius R1 of between 125 mm and 155 mm, for example between 135 mm and 150 mm, a radius R2 of between 75 mm and 105 mm, for example between 80 mm and 95 mm, a height H1 of between 35 mm and 60 mm, for example between 45 mm and 55 mm, and a height H2 of between 25 mm and 40 mm, for example between 25 mm and 35 mm. Unless otherwise stated, the radii R1 and R2 are respectively taken as being equal to half of the largest dimension of the first and second cavities and do not necessarily imply that the waveguide is of circular geometry. These values ​​were determined by taking a dielectric permittivity and a magnetic permeability of the medium filling the cavities equal to 1 (permittivity and permeability of the vacuum).

[0034] As an example for a first frequency band ranging from 1164 MHz to 1591 MHz and a second frequency band ranging from 2200 MHz to 2290 MHz, it is possible to use a waveguide 7 having a radius R1 of 140 mm, a radius R2 of 90 mm, a height H1 of between 35 mm and 60 mm, for example of between 45 mm and 55 mm, and a height H2 of between 25 mm and 40 mm, for example of between 25 mm and 35 mm.

[0035] Still as an example for a first frequency band ranging from 1164 MHz to 1591 MHz and a second frequency band ranging from 2200 MHz to 2290 MHz, it is possible to use a waveguide 7 having a height H1 of 50 mm, a height H2 of 25 mm, a radius R1 between 125 mm and 155 mm, for example between 135 mm and 150 mm, and a radius R2 between 75 mm and 105 mm, for example between 80 mm and 95 mm.

[0036] We represented at the figure 2a second example of antenna 10 according to the invention which does not differ from the example of the figure 1that an opening 20 is present between the first cavity 11 and the substrate 3. The same reference symbols have been used for similar elements. The opening 20 extends 360° around the axis of the first 11 and second 13 cavities, corresponding to the Z axis. The height H3 of the opening 20 may be less than or equal to H1-H2, for example less than or equal to 25 mm, for example between 0.25 mm and 25 mm. Increasing H3 makes it possible to further improve the gain for low elevation angles in the second frequency band. It is nevertheless preferable not to increase H3 too much so as not to lower the gain too much in the first frequency band. Depending on the gain requirements of the two frequency bands, this parameter H3 offers an additional degree of freedom to optimize the antenna. In this example, the substrate 3 is not in contact with the first cavity 11 and is present at a predetermined non-zero distance from the latter.

[0037] We represented at the figure 3 a third example of antenna 100 according to the invention which does not differ from the example of the figure 1that an edge 22 of the wall 110 of the first cavity has a crenellated shape defining a plurality of openings 24 between the substrate 3 and the wall 110. The openings 24 may each have the same shape and / or the same dimensions. Alternatively, the openings 24 differ in terms of shape and / or dimensions. The openings 24 may, as illustrated, be present all around the Z axis of the first and second cavities (at 360° around this Z axis). The openings 24 may or may not be regularly distributed around the Z axis of the first and second cavities. As described above, the height H4 of the openings 24 may be less than or equal to H1-H2, for example less than or equal to 25 mm, for example between 0.25 mm and 25 mm. As above for the case of aperture 20, increasing H4 allows for further improvement of the gain for low elevation angles in the second frequency band.It is nevertheless preferable not to increase H4 too much so as not to lower the gain in the first frequency band too much. Note that the 24 openings have the same effects as the 20 opening but with a lesser impact on the gain of the first frequency band (increasing the height of the 24 openings lowers the gain in the first frequency band less).

[0038] Examples of waveguides having a circular geometry have just been described, but it does not depart from the scope of the invention when the waveguide has another geometry such as a polygonal shape, for example rectangular or octagonal. A person skilled in the art knows how to size the resonant cavities for geometries other than circular by using formulas other than the formula [Math. 1] indicated above for the circular case. Furthermore, the examples which have just been described comprise only two resonant cavities 11 and 13, but it does not depart from the scope of the invention if the waveguide comprises more than two resonant cavities, for example three nested resonant cavities, the third resonant cavity being single-mode or predominantly single-mode in a third frequency band separate from the first and second frequency bands. This makes it possible to have an antenna transmitting with improved gain in more than two frequency bands.

[0039] We represented at the figure 4 a diagram showing the gain as a function of frequency for θ = 90° relative to the Z direction, corresponding to the horizon and therefore to an elevation angle of 0°. This figure shows the minimum value of the gain for any azimuth angle combined. The diagram is a comparative diagram showing the effect of adding the second cavity 13 in a first cavity 11 or 110, and showing the influence of the presence of the openings 20 or 24. The first 11 or 110 and second 13 cavities are both in this test of circular geometry as in figures 1 to 3 , with a radius R1 = 140 mm, a radius R2 = 90 mm, a height H1 = 50 mm and a height H2 = 25 mm. Opening 20 has a height H3 of 10 mm and openings 24 a height H4 of 15 mm. Curve A1 corresponds to the gain obtained with the first 11 and second 13 cavities without opening as in figure 1 , the curve A2 at the gain obtained with the opening 20 as at the figure 2, the A3 curve at the gain obtained with the 24 openings as at the figure 3 and curve B corresponds to the gain obtained without the second cavity 13, only with the first cavity 11. A significant improvement in the gain is observed in the second frequency band between 2200 MHz and 2290 MHz when the second cavity 13 is present. In addition, an improvement is observed additional of the gain in the second frequency band when apertures 20 and 24 are present.

[0040] We represented at the Figure 5a gain diagram at a frequency of 2300 MHz as a function of the angle θ relative to the Z direction on the abscissa, θ = 90° corresponding to the horizon therefore to an elevation angle of 0°, and the azimuth on the ordinate. The antenna evaluated included a radiating antenna element of the double crossed dipole type present on a substrate marketed under the reference RO3210 ®< by the company ROGERS Corporation and had only a first resonant cavity (no second cavity) of square shape with a side of 140 mm and a height of 50 mm.

[0041] There figure 6 shows the gain diagram obtained at this frequency for an antenna identical to that of the Figure 5 but which also included a second cavity inside the first cavity. The second cavity had a square shape with a side of 90 mm and a height of 25 mm. A significant improvement in gain is observed for low elevation angles.

[0042] There figure 7shows the gain diagram obtained at this frequency for an antenna which had a first and a second octagonal-shaped cavities. The first cavity had a larger dimension of 140 mm and a height of 50 mm and the second cavity a larger dimension of 90 mm and a height of 25 mm. We also see a significant improvement in gain for low elevation angles compared to the case of the Figure 5 .

[0043] The expression "between ... and ..." must be understood as including the limits.

Claims

1. An antenna (1, 10, 100) comprising: - a radiative antenna element (5) able to emit a signal in at least a first frequency band and in a second frequency band, disjoint from the first band and at a higher frequency than the latter, and - a waveguide (7) covered by the radiative antenna element, comprising at least a first resonant and single mode or considered single-mode over at least 50% cavity (11) in the first frequency band, and a second resonant cavity (13) separate from the first resonant cavity and located inside the latter, said second resonant cavity being single-mode or considered single-mode over at least 50% in the second frequency band, characterized in that a ratio RA1 H1 / H2 is between 1 and 3.25, where H1 denotes a height of the first cavity (11) and H2 a height of the second cavity (13).

2. The antenna (10, 100) as claimed in claim 1, wherein the radiative antenna element (5) is present on a substrate (3) covering the waveguide (7), and wherein the antenna has one or more openings (20, 24) between a wall delimiting the first cavity (11) and the substrate (3).

3. The antenna (100) as claimed in claim 2, wherein an edge (22) of said wall located on the side of the substrate (3) has a castellation shape thus defining a plurality of openings (24) between said wall and the substrate.

4. The antenna (1, 10, 100) as claimed in any of claims 1 to 3, wherein the first frequency band corresponds to the frequencies between 1164 MHz and 1591 MHz and the second frequency band corresponds to the frequencies between 2200 MHz and 2290 MHz.

5. The antenna as claimed in any of claims 1 to 4, wherein the ratio RA1 H1 / H2 is between 1.28 and 2.2.

6. The antenna as claimed in any of claims 1 to 5, wherein a ratio RA2 D1 / D2 is between 1.19 and 2.1, where D1 is the greatest dimension of the first cavity and D2 is the greatest dimension of the second cavity.

7. The antenna as claimed in any of claims 1 to 6, wherein the waveguide comprises in its mower part a reflector (9) defining a base shared by the first and second cavities.

8. A craft equipped with at least one antenna (1, 10, 100) as claimed in any of claims 1 to 7.

9. The craft as claimed in claim 8, wherein the craft is a spacecraft.

10. The craft as claimed in claim 9, wherein the craft is a space launcher, an exploration craft or a satellite.