Antenna having a defocused source

Abstract

The present invention relates to an offset antenna (1) comprising: a source (10) for transmitting or receiving an electromagnetic wave; a reflector (11) a generating surface of which is a paraboloid (20), arranged so as to reflect the electromagnetic wave; characterized in that the source (10) is arranged in a defocused position with respect to a focal point (P1) of the reflector, such that a radiation level of the antenna is homogeneous over a field of view (40) of the antenna (1), the defocused position being defocused by a first distance (Dz) along a first axis (z) joining the focal point (P1) and a center (C) of the reflector (11) and by a second distance (Dx) along a second axis (x) orthogonal to the first axis (z), the second distance (Dx) being equal to at least 5% of a focal length (F1) associated with the focal point (P1).

Description

Defocused feed antenna Technical field

[0001] The present invention relates to a reflector antenna, a phased array antenna and a method of generating a wave beam. State of the art

[0002] Parabolic reflector offset antennas whose source is arranged at a focal point of the reflector are known in the prior art. Locating the source at the focal point of the reflector implies that the radiant cone emitted by the source is reflected so as to form a beam of planar electromagnetic waves.

[0003] In particular, offset antennas are obtained by offsetting the reflector relative to the axis supporting the focal point so that the source does not obstruct the antenna's field of vision.

[0004] The radiation pattern of such antennas is typically circular or elliptical. The gain function of these antennas usually resembles a Gaussian of relatively low variance, i.e. the signal intensity is strong at the center of the antenna's field of view (i.e. around 0°) and decreases rapidly until it becomes negligible from + / - 8° from the center of the field of view. A problem resulting from this rapid decrease in intensity is that the distribution of the intensity over the signal reception area is very inhomogeneous. Indeed, the signal intensity at the center of the area covered by the antenna is strong while at the periphery of the covered area, the intensity is very weak.

[0005] There is therefore a need for antennas that can achieve consistent signal strength over a large part, or even the entire area of the antenna's field of view. Brief summary of the invention

[0006] An object of the present invention is to provide a reflector antenna free from the limitations of the prior art.

[0007] Another aim of the invention is to propose a reflector antenna whose radiation intensity over its field of vision is more homogeneous than that of antennas of the prior art.

[0008] According to the invention, these aims are achieved in particular by means of an offset antenna comprising: a source for emitting or receiving an electromagnetic wave; a reflector, a generating surface of which is a paraboloid, arranged so as to reflect the electromagnetic wave; characterized in that the source is arranged in a defocused position relative to a focal point of the reflector, so that a radiation level of the antenna is homogeneous over a field of vision (40) of the antenna, the defocused position being defocused by a first distance along a first axis (z) joining the focal point (P1) and a center (C) of the reflector (11) and by a second distance (Dx) along a second axis (x) orthogonal to the first axis (z), the second distance (Dx) being equal to at least 5% of a focal length (F1) associated with the focal point (P1).

[0009] Defocusing the source along both the first axis and the second axis surprisingly allows the production of a beam, after reflection on the reflector, whose radiation pattern has a rectangular profile corresponding to a homogeneous intensity over the field of vision of the antenna.

[0010] According to one embodiment, the defocused position is located on a third axis comprising the focal point and forming an angle of at least 20° with the first axis. Defocusing the source along this third axis makes it possible to obtain a particularly homogeneous radiation pattern over the field of vision of the antenna.

[0011] According to one embodiment, the second distance is between 5% of the focal length associated with the focal point and 30% of this focal length, and preferably between 10% and 20% of the focal length.

[0012] According to one embodiment, the first distance (Dz) is between 10% of the focal length associated with the focal point and 75% of this focal length.

[0013] An average radiation level across the entire field of view of the antenna can be at least 20dB.

[0014] The reflector can have a diameter between 500mm and 5000mm, preferably between 2000mm and 4000mm.

[0015] The reflector can have a diameter between 50Å and 350Å, where Å represents the nominal wavelength for which the antenna is intended.

[0016] According to one embodiment, the ratio of the focal length divided by the diameter of the reflector is less than 0.7.

[0017] According to one embodiment, the generating surface of the reflector is an elliptical paraboloid, such that the focal point is a first focal point having a first focal length and the reflector comprises a second focal point having a second focal length, the defocused position of the source being further defocused by a third distance along a third direction orthogonal to the first and second directions.

[0018] This defocusing of the source in relation to the two focal points of the reflector makes it possible in particular to optimize the geometry of the antenna's field of vision and / or to optimize the homogeneity of the signal intensity over the antenna's field of vision when the signal is cross-polarized.

[0019] The ratio of the second focal length divided by the first focal length can be between 0.3 and 1.2.

[0020] The ratio of the second focal length divided by the first focal length can be such that the field of view of the antenna is circular or elliptical.

[0021] According to one embodiment, the antenna may further comprise a plurality of sources arranged in an array around the source so as to be able to emit or receive a beam of electromagnetic waves, a radiation level of each source of the plurality of sources being homogeneous over its respective field of vision.

[0022] This embodiment makes it possible in particular to use all the radiating elements of the reflector's power supply network to generate the beam. The number of radiating elements thus used can be of the order of several hundred, or even several thousand.

[0023] It also allows the use of excitation coefficients of equal amplitude and the beam to be steered only by adapting the phase.

[0024] It also allows the use of sparse and irregularly distributed power supply networks to control the level of the secondary lobes by means of a reduced spatial distribution.

[0025] A pitch of the array formed by the plurality of feeds may be between 0.5λ and 0.9λ, where λ represents the nominal wavelength for which the antenna is intended.

[0026] The network formed by the plurality of sources can be triangular. Brief description of the figures

[0027] Examples of implementation of the invention are indicated in the description illustrated by the appended figures in which: • Figure 1 schematically illustrates an offset antenna whose reflector is generated by a paraboloid. • Figure 2 schematically illustrates an offset antenna whose reflector is generated by a paraboloid • Figure 3a shows a Gaussian profile radiation pattern of a focused feed antenna. • Figure 3b shows a rectangular profile radiation pattern of a defocused feed antenna. • Figure 4a illustrates a side view of a reflector generated by an elliptical paraboloid having two focal points. • Figure 4b illustrates a top view of a reflector generated by an elliptical paraboloid having two focal points. • Figure 5 illustrates an antenna according to the invention comprising a plurality of sources arranged in an array. • Figure 6 illustrates the terrestrial coverage area of a multibeam antenna according to the invention. Example(s) of embodiment of the invention

[0028] One aspect of the present invention relates to a particular configuration of an offset antenna 1 with a reflector 11 whose radiation source 10 is arranged in an unconventional manner relative to the reflector 11. A notable technical effect resulting from this particular configuration between the source and the reflector is the obtaining of a radiation pattern of the antenna, that is to say a footprint of the emitted beam, of essentially rectangular shape. In other words, the radiation level of the antenna 1 is homogeneous over the field of vision of the antenna.

[0029] As illustrated in Figure 1, the reflector 11 of the antenna 1 is formed by a portion of a paraboloid (the surface of the paraboloid is said to be a generator for the reflector). This paraboloid may be invariant by rotation around an axis or may be an elliptical paraboloid, that is to say a paraboloid whose intersection with a secant plane perpendicular to the axis of the paraboloid forms an ellipse. In all cases, the paraboloid has at least one axis of symmetry which may coincide with the axis of rotation if the paraboloid is invariant by rotation. Since the antenna is of the offset type, the axis of symmetry of the paraboloid generally does not intersect the reflector.

[0030] In the case of a rotationally invariant paraboloid, the reflector 11 has a focal point P1 located on the axis of rotation of the paraboloid. In the case of an elliptical paraboloid, the reflector has two focal points P1, P2 corresponding to the two principal directions of the profile elliptical of the paraboloid. These two focal points also lie on the axis of symmetry of the paraboloid.

[0031] As illustrated in Figures 1 and 2, a suitable coordinate system for locating and measuring the quantities relating to the present invention is given by a first z-axis joining the focal point P1 and a center C of the reflector 11 and a second x-axis orthogonal to the first z-axis and passing through the focal point P1 so that the origin of this coordinate system is at the focal point P1. This coordinate system is typically oriented as illustrated in Figures 1 and 2, that is, if one divides in thought the 3-dimensional space into two half-spaces by a plane comprising the axis of symmetry of the paraboloid and the first z-axis, then the second x-axis points in the same half-space as the z-axis.

[0032] The antenna 1 comprises a source 10 capable of transmitting and receiving electromagnetic waves. In transmission, a wave is transmitted by the source 10 towards the reflector 11, then is reflected by the reflector towards a reception area. When the antenna is operating in reception, an electromagnetic wave coming from a transmission area outside the antenna is reflected by the reflector towards the source. The source 10 can be connected to other passive and / or active radiofrequency elements making it possible to generate, propagate, divide / combine, process, filter, etc., signals. The transmitted and received signals can be single or dual polarized.

[0033] Surprisingly, a significant defocusing of the source 10 relative to the focal point P1 of the surface generating the reflector 11 makes it possible to obtain a gain function having the characteristics of a plateau function, that is to say that the gain of the antenna and / or the intensity of the signal over the field of view (FoV) of the antenna is essentially homogeneous.

[0034] The field of view of the antenna designates the projection on a plane orthogonal to the direction of propagation of the waves of the area of antenna coverage. For example, if the antenna is in operation on a satellite, the field of view refers to the projection onto a plane orthogonal to the direction of wave propagation of the terrestrial area covered by the antenna.

[0035] The expression "defocusing of the source" means that the position of the source 10 does not coincide with the focal point P1 of the reflector 11. In the coordinate system described above, the defocusing is carried out both in a first direction corresponding to the z axis (positive) and in a second direction corresponding to the x axis (positive). As illustrated in FIG. 1, the source 10 is defocused by a first distance Dz along the z axis and by a second distance Dx along the x axis relative to the focal point P1 of the reflector 11. The second distance Dx is equal to at least 5% of the focal length F1 associated with the focal point P1, that is to say at least 5% of the distance separating the vertex of the paraboloid generating the reflector 11 from the focal point P1.

[0036] Unlike a focused arrangement of the source 10 relative to the focal point P1, this defocusing along the two axes x and z makes it possible to obtain a homogeneous antenna radiation pattern over the field of vision of the antenna.

[0037] Figure 3a illustrates the radiation pattern of a conventional antenna in which the position of the wave source coincides with a focal point of the reflector. The horizontal axis corresponds to the angle 0 in degrees relative to the main propagation direction which coincides with 0°. The vertical axis corresponds to the magnitude (or intensity) of the signal in decibels (dB). It can be seen that over the angular range between -8° and +8°, corresponding approximately to the field of view of an antenna, the magnitude of the signal is very heterogeneous with a peak around 0° and a rapid decrease.

[0038] Unlike a focused arrangement of the source 10 relative to the focal point of the reflector 11, a defocusing according to the two The x and z axes provide a more homogeneous antenna radiation pattern across the antenna's field of view. Such a radiation pattern is illustrated in Figure 3b. Again, the vertical axis corresponds to the signal magnitude (or intensity) in decibels (dB). This time, we see that the signal magnitude is very homogeneous between -8° and +8°. The antenna gain function approaches a plateau function with an almost rectangular profile.

[0039] The antenna of the present invention is typically intended for the transmission of signals by satellite in the L, S, C, X, Ku, K, Ka, Q, V, or even W frequency bands.

[0040] It has thus been demonstrated that it is thus possible to obtain an average signal intensity of at least 20 dB over the field of vision of the antenna 40.

[0041] As illustrated in Figure 2, the source may be placed, particularly advantageously, such that the defocused position is along a third axis L comprising the focal point P1 and forming an angle a equal to at least 20° with the first axis z. Preferably, the angle a is between 20° and 40°.

[0042] Advantageously, the second defocusing distance Dx along the x axis is less than the focal length F1 of the reflector. The focal length F1 of the reflector corresponds to the distance between the vertex of the paraboloid generating the reflector and its focal point P1.

[0043] According to one embodiment, the second distance Dx is between 5% and 30% of the focal length F1 of the reflector, preferably between 10% and 20% of the focal length F1 of the reflector 11.

[0044] According to one embodiment, the first distance Dz is advantageously between 10% and 75% of the focal length F1 of the reflector 11.

[0045] Generally, the first and second distances Dx, Dz are significantly larger than the tolerances inherent in antenna manufacturing and feed placement relative to the reflector.

[0046] Although the use of the antenna according to the present invention is not limited to the satellite field, the dimensions of the antenna components such as the source and the reflector advantageously correspond to the usual dimensions of antennas used on satellites in low and / or geostationary orbit. These dimensions must in particular take into account numerous constraints limiting, for example, the diameter and / or the focal length of the reflector.

[0047] According to one embodiment, the antenna comprises a reflector with a diameter between 500mm and 5000mm, preferably between 2000mm and 4000mm.

[0048] Generally, the diameter of the reflector depends on the particular application of the antenna. Thus, if the antenna is intended for a nominal wavelength X, the diameter of the reflector is typically between 50 X and 350 X according to one embodiment.

[0049] Alternatively or additionally, the ratio of the focal length F1 divided by the diameter of the reflector 11 is less than 0.7. Thus, when the second distance Dx is less than the focal length F1, the second distance Dx is therefore less than 70% of the diameter of the reflector.

[0050] Advantageously, the larger the reflector aperture, the more homogeneous the radiation pattern profile is across the field of vision.

[0051] According to one embodiment, the generating surface of the reflector 11 of the antenna is an elliptical paraboloid that is not rotationally invariant about the vertical axis z. This means that the intersection of the paraboloid with a plane orthogonal to the vertical axis of the paraboloid is an ellipse. Accordingly, a reflector 11 having such a generating surface has two focal points (P1, P2) along the vertical axis corresponding to the axis of symmetry of the paraboloid corresponding to the respective focal points of the two extreme parabolas lying in orthogonal planes and comprising the principal axes (a, b) of the ellipses generating the paraboloid.

[0052] For the remainder of this text, the first focal point P1 designates the focal point corresponding to the parabola given by the intersection of the paraboloid with the plane generated by the coordinate system Oxz, while the second focal point P2 designates the focal point corresponding to the parabola given by the intersection of the paraboloid with the plane perpendicular to the plane Oxz and including the z axis, subsequently designated as the plane Oyz.

[0053] A top view of a reflector 11 and its generating paraboloid is schematically illustrated in Figure 4b. The two focal points (P1, P2) of the reflector 11 are located on an axis perpendicular to the illustrated ellipse and passing through its center corresponding to the axis of symmetry of the paraboloid. These focal points correspond to the focal lengths of the parabolas included in the perpendicular planes comprising respectively the semi-major axis a of the ellipse and the semi-minor axis b of the ellipse.

[0054] According to an embodiment in which the reflector 11 comes from an elliptical paraboloid, the source 10 is defocused both with respect to the first and second focal points P1, P2. Such defocusing This allows for uniform radiation intensity in two polarization directions orthogonal to each other. This makes it possible to use this antenna for dual-polarization signals while maintaining uniform intensity across the antenna's field of view for each of the two polarizations.

[0055] As illustrated in Fig. 4b, the source 10 can be offset by a distance d1 > 0 along an axis corresponding to the major axis of the ellipse (, and by a distance d2 > 0 along an axis corresponding to the minor axis of the ellipse, and have a coordinate along the vertical axis (i.e. corresponding to the axis of symmetry of the elliptical paraboloid) different from the first and second focal lengths F1 and F2.

[0056] According to one embodiment, the ratio of the second F2 to the first F1 focal length is between 0.3 and 1.2, or in other words: 0.3 Fl < F2 < 1.2 Fl.

[0057] The case of a paraboloid invariant by rotation around the axis of symmetry of the paraboloid mentioned above corresponds to the case where F1 = F2. Thus, the above equation includes this case when F2 / F1 = 1.

[0058] As illustrated in Figure 4a, the position of the source along the axis of symmetry of the paraboloid can for example be located between the first and second focal points.

[0059] The ratio between the first and second focal lengths (F1, F2) influences in particular the geometry of the field of view. Thus, advantageously, the ratio between the second and first focal lengths is such that the field of view of the antenna is elliptical or circular. The closer the ratio of the focal lengths is to 1, the more circular the field of view of the antenna, while the further the ratio is from 1, the more elliptical the field of view is.

[0060] The present invention also relates to an antenna as described above, further comprising a plurality of sources 10 arranged in an array around the original source, so as to be able to emit or receive a beam of electromagnetic waves. Due to the defocused position of the original source 10, each source of the plurality of sources is also defocused so that all these sources have the characteristics of homogeneity of radiation intensity over the field of vision of each source.

[0061] Fig. 5 illustrates an embodiment in which the antenna 1 comprises a plurality of sources 10 arranged in an array so as to form a beam directed towards the reflector 11.

[0062] By using a small array antenna, instead of a single feed, it is possible to operate as a larger direct-radiating array antenna, where the beam is oriented in any direction within the antenna's field of view, with increased performance in terms of maximum directivity.

[0063] According to one embodiment, the pitch of the network formed by the plurality of sources is regular. Typically this pitch can be between 0.5λ and 0.9λ, where λ represents the nominal wavelength for which the antenna is intended.

[0064] As illustrated in Figure 6, the field of view of an antenna comprising a plurality of sources arranged in an array (eg as illustrated in Fig. 5) consists of all the individual fields of view of each source. Since the magnitude of each source is homogeneous over its field of view, it follows that the magnitude of the antenna is homogeneous over its field of view. The antenna may comprise, for example, several tens or even several hundreds of sources arranged in an array.

[0065] According to one embodiment, the network formed by the plurality of sources is triangular, that is to say that the centers of three adjacent sources form a triangle. The footprint of such an arrangement is for example illustrated in Figure 6. However, any other arrangement deemed relevant by the person skilled in the art can be implemented without departing from the scope of the present invention. References I Offset Antenna 10 Source II Reflector 20 Paraboloid 40 Field of vision P1 First focal point F1 First focal length P2 Second focal point F2 Second focal length z First direction x Second direction y Third direction Dz First distance Dx Second distance Dy Third distance

Claims

Claims 1. Offset antenna (1) comprising: a source (10) for emitting or receiving an electromagnetic wave; a reflector (11), a generating surface of which is a paraboloid (20), arranged so as to reflect the electromagnetic wave; characterized in that the source (10) is arranged in a defocused position relative to a focal point (P1) of the reflector, so that a radiation level of the antenna is homogeneous over a field of vision (40) of the antenna (1), the defocused position being defocused by a first distance (Dz) along a first axis (z) joining the focal point (P1) and a center (C) of the reflector (11) and by a second distance (Dx) along a second axis (x) orthogonal to the first axis (z), the second distance (Dx) being equal to at least 5% of a focal length (F1) associated with the focal point (P1).

2. Antenna (1) according to claim 1, the defocused position being on a third axis (L) comprising the focal point (P1) and forming an angle (a) of at least 20° with the first axis (z).

3. Antenna (1) according to one of the preceding claims, the second distance (Dx) being between 5% of a focal length (F1) associated with the focal point (P1) and 30% of the focal length (F1), preferably between 10% and 20% of the focal length (F1).

4. Antenna (1) according to one of the preceding claims, the first distance (Dz) being between 10% of the focal length (F1) associated with the focal point (P1) and 75% of this focal length (F1).

5. Antenna (1) according to one of the preceding claims, in which an average radiation level over the entire field of vision of the antenna (40) is at least 20 dB.

6. Antenna (1) according to one of the preceding claims, the reflector (11) having a diameter between 500mm and 5000mm, preferably between 2000mm and 4000mm.

7. Antenna (1) according to one of the preceding claims, the reflector (11) having a diameter between 50Å and 350Å, where Å represents the nominal wavelength for which the antenna (1) is intended.

8. Antenna (1) according to one of the preceding claims, the ratio of the focal length (F1) divided by the diameter of the reflector (11) being less than 0.

7.

9. Antenna (1) according to one of the preceding claims, in which the generating surface of the reflector (11) is an elliptical paraboloid, such that the focal point (P1) is a first focal point having a first focal length (F1) and the reflector comprises a second focal point (P2) having a second focal length (F2), the defocused position of the source (10) being further defocused by a third distance (Dy) in a third direction (y) orthogonal to the first and second directions.

10. Antenna (1) according to the preceding claim, the ratio of the second focal length (F2) divided by the first focal length (F1) being between 0.3 and 1.

2.

11. Antenna (1) according to one of claims 8 or 9, the ratio of the second focal length (F2) divided by the first focal length (F1) is such that the field of vision (40) of the antenna is circular or elliptical.

12. Antenna (1) according to one of the preceding claims, further comprising a plurality of sources arranged in an array around the source (10) so as to be able to transmit or receive a beam of waves electromagnetic, a radiation level of each source of the plurality of sources being homogeneous over its respective field of vision (40).

13. Antenna (1) according to the preceding claim, a pitch of the network formed by the plurality of sources being between 0.5À and 0.9À, where À represents the nominal wavelength for which the antenna (1) is intended.

14. Antenna (1) according to one of claims 12 to 13, the network formed by the plurality of sources being triangular.

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