EP0656671B1 - Orientable antenna with maintenance of the polarisations axes - Google Patents

Description

The field of the invention is that of antennas for the emission and / or reception of electromagnetic radiation, and more particularly directive and orientable antennas, capable of emitting and / or receiving radiation in a determined and variable direction. Such an antenna may consist of a radiation source and one or more reflector (s), the shape of the reflector (s) and the arrangement of the DU system / reflectors relative to the source determining the directivity of the antenna thus formed as well as the shape of the beam emitted or received.

Numerous examples of directional antennas known to those skilled in the art can be concerned with the present invention, such as so-called parabolic antennas, Cassegrain antennas, Gregory et cetera antennas, with their variants having an illumination either axial or "offset". An offset system is a system comprising a main reflector, the cut of which is eccentric relative to the axis of the surface considered. In the case of a single reflector, the primary source located on this axis is inclined to target the center of the reflector.

The invention relates more particularly to antennas capable of transmitting and / or receiving according to two orthogonal linear polarizations, and the success of their mission of which depends on this capacity. This is the case for some telecommunications antennas, for example, which use polarization diversity to allow reuse of the spectrum in a given frequency band. Another example concerns antennas for satellite broadcasting in DBS (Direct Broadcast by Satellite) or DTH (Direct to the Home) systems. Independent measurements according to orthogonal polarizations are also carried out by certain equipment radar, to determine the radar signature of a complex target, for example, or for weather or earth observation radars.

In the known art, such embodiments have been, for the most part, fixed terrestrial systems or even embedded on mobile terrestrial or airborne platforms.

The present invention, for its part, will be particularly advantageous when deployed in space, on board a satellite, an orbital station, a probe, or any other space platform.

Indeed, a new problem can appear when one wants to extrapolate from terrestrial systems of the known art, to conceive a space system using the diversity of polarization, namely: the implicit axes of reference which we enjoy on the terrestrial surface, vertical and horizontal do not exist in space. Consequently, the conservation of these axes as references is questioned.

This problem is not insurmountable, and one can even very easily solve it by accepting different constraints on the system.

For example, a geostationary telecommunications satellite, in most cases, must be able to communicate with a relatively limited number of fixed ground stations. The orientations of the axes of orthogonal polarizations used in such a system can be arbitrary provided that some initial adjustments are made to the equipment on the ground before the transmission of the useful information. The constraint to accept is that in this case, no temporal variation of the geometric parameters of the link can be tolerated, without causing the need for a new sequence of settings. In the known art, this poses almost no drawback, since the geometric parameters of connection with a geostationary satellite are in principle invariable.

The situation is different for a satellite in low orbit, in polar orbit, or in inclined orbit (Walker, Molnya, et cetera); these orbits can be elliptical or circular. Satellites placed in such orbits parade through the sky when seen by an observer from a fixed point on the earth. Consequently, a link between such a so-called "traveling" satellite and a fixed station on the ground will be in a direction which undergoes a variation continuously due to the movement of the satellite.

Still, for these traveling satellites, there is not necessarily an insurmountable problem in using orthogonal linear polarizations provided that certain constraints are accepted in the design of the system. For example, a linear polarization can be chosen parallel to the trajectory of the satellite, known a priori from ephemerides, and with the other polarization chosen perpendicular to this trajectory and to the nadir. Each fixed station on the ground can know in advance the orientations of the polarization axes used by the satellite, and the antenna on the ground can be adjusted accordingly.

The importance and frequency of such adjustments will depend on the freedom which one wishes to grant to the geometric parameters of the link established between the traveling satellite and the ground station. Insofar as the link is only used when these parameters are almost identical (small variations in the values which can be tolerated in a range whose width is determined by the link budget in cross polarizations), there is no interference problem to be expected between two transmission channels operated at the same frequency in orthogonal polarizations (polarization diversity).

However, this constraint is a problem in the systems known from the prior art, insofar as the possibility of orienting the on-board antenna is limited by the radio performance specifications issued by national and international administrations (FCC, CCITT, ITU, et cetera) for radio transmissions. In known systems, the orientation of the antenna can vary performance outside the narrow range allowed by these standards and specifications.

The reuse of frequencies by diversity of polarizations can also provide advantages for direct broadcasting by satellite. A ground user will not be obliged to reorient his antenna to target a second satellite in order to receive a second "bouquet" of emissions, if a first satellite can provide the programs of this second bouquet, with those of the first bouquet, from the only orbital position of the first satellite, in cross polarizations.

The invention seeks to remedy the drawbacks of the prior art for telecommunications satellites (transmit and / or receive antenna) and direct broadcast satellites (transmit antenna only).

In on-board weather radar and earth observation systems, the polarization of the wave received by the equipment can be used to better probe the target. For example, backscattering and depolarization of a polarized wave emitted by the satellite can reveal the nature of atmospheric precipitation, since depolarization depends on the size, concentration, and phase state (ice, liquid in droplets). , vapor) of the probed compounds. In another example, radar backscatter from the sea surface can reveal the state of sea agitation through polarization measurements.

The sensitivity to polarization is variable depending on the mission. For these last two examples, the polarization of the initial wave can be arbitrary without influencing the result obtained, precisely because the targets themselves are not fixed but on the contrary of arbitrary orientation.

The situation is different in the case where one would like to observe a fixed target, illuminated by a polarized wave at separate times in time. Such successive measurements can be used to observe the evolution of the target over time, or to improve the signal-to-noise ratio and the resolution of the fixed image by correlation of the successive images (subtraction of the background). A typical case is the observation of the same geographical area or the same object on the ground, during the successive passages of a traveling satellite. The successive orbits of such a satellite are not generally closed when viewed from the earth's surface, but rather describe a spiral whose step advances in longitude. These are for example heliosynchronous orbits.

A problem with such a system of the prior art is that the vectors of orthogonal polarizations, while they can be arbitrary for isolated observations, must be kept to perform the correlation of successive measurements. However, these vectors tend to evolve for at least two reasons: on the one hand, the precession of the orbit introduces variable but predictable geometric factors; on the other hand the aiming on the ground of a same place from successive orbits generates other variations of geometric parameters, which must be taken into account in the correlations to be made.

European patent application No. 0.139.482 describes an antenna intended to be used on board a satellite, this antenna being of the Cassegrain type and comprising a source, a primary reflector and a secondary reflector. The two reflectors are integral with one another and the source is fixed. Thus, the source is constantly maintained at the focal point of the antenna, regardless of the position of the reflectors. This document does not however deal with the conservation of the orthogonal axes of polarization as a function of the pointing performed. In fact, it does not pose the problem linked to the targeting of non-circular spots, as will be seen later.

Expressed in the most general way, the new problem addressed by the invention is the following: we would like an antenna whose elements can be oriented at will to allow the arbitrary orientation of the beam of radiation emitted or received, while allowing conservation axes of orthogonal linear polarizations, whatever the orientation of the beam. In addition, the antenna according to the invention must allow the conservation of the axes of orthogonal linear polarizations even in the case of a rotation of the beam around its main direction of propagation.

To solve this problem, the invention proposes an antenna according to claims 1 or 14.

The nature of the source will be determined by the designer according to the mission to be accomplished. For example, the source can be a simple horn, a microstrip radiator ("patch" in English), a slot, ... or the source can be a complex or extended source, for example a network of patches or slots, possibly in association with cavities. The complex source can be a plurality of separate sources, with a selective polarization reflector or with a plurality of frequency selective reflectors. The source can be a direct source or a periscopic source. In short, the invention can be carried out using any source known to those skilled in the art for such applications.

According to a characteristic of the invention, the movement of at least one reflector comprises a rotation of the reflector around the preferred direction of radiation. According to another characteristic, this movement comprises an angular displacement (deflection) of the preferred direction around a point which represents the position of the source. According to a variant, the movement comprises a rotation of the reflector around the direction of propagation of radiation which connects the source and the reflector.

According to a particular characteristic, the direction of propagation between the source and the reflector coincides with the preferred direction of radiation.

According to a particular embodiment of the invention, the reflector is a single reflector having parabolic generators, this reflector being illuminated by a source arranged substantially in its focus, and the reflector can be rotated around the direction of radiation while the source is kept fixed. The geometry of the assembly is in this case centered.

Alternatively, the single parabolic reflector is illuminated by a source arranged in an "offset" geometry and the reflector can be rotated around the direction of radiation while the source is held stationary.

According to another particular embodiment, the antenna comprises at least two reflectors arranged in a geometry called "Gregory", offset or centered. The two reflectors are arranged with their concave surfaces facing each other, each of them being illuminated either in offset or in center.

According to another particularly advantageous embodiment, the antenna comprises at least two reflectors arranged in a Cassegrain geometry, including a main reflector which reflects the beam, and an auxiliary reflector which is illuminated by the source. At least the main reflector can be rotated around the preferred direction of radiation while the source is kept stationary. Alternatively, all of the reflectors can be rotated around the preferred direction of radiation while the source is held stationary. According to an additional characteristic, the antenna further comprises mechanical means for deflecting all of the constituents, without modification of their relative arrangement, in addition to the mechanical means previously described.

In all the embodiments the focusing reflectors are of an arbitrary shape; however, the invention will be particularly advantageous if at least one reflector does not have axial symmetry (of rotation about an axis).

The reflector can be simple or complex.

A complex reflector can for example be a bigrille reflector consisting of two reflectors arranged one in front of the other in a direction of propagation of the beam, the first reflector having to be reflective for a first linear polarization, and transparent for a second orthogonal linear polarization , which will be reflected by the second reflector located behind said first reflector. Such a bigrille reflector is well known to those skilled in the art. In a variant of the invention using a bigrille reflector, the mechanical means allow the rotation of the source, of any shape, while keeping the reflector (s) fixed.

• la figure 1 montre schématiquement un satellite avec un faisceau orientable sur orbite terrestre ;
• la figure 2 montre schématiquement les tracés au sol d'un faisceau orientable d'une antenne orientable selon l'invention avec conservation de la polarisation ;
• la figure 3 montre schématiquement et en coupe latérale une antenne parabolique selon l'art antérieur ;
• les figures 4A, 4B, 4C montrent respectivement en coupe AA', en plan, et en coupe BB', un réflecteur parabolique asymétrique selon une variante de l'invention ;
• la figure 5 montre schématiquement et en coupe la géométrie Cassegrain centrée ;
• la figure 6 montre schématiquement en trois dimensions et en perspective, le réflecteur parabolique des figures 4A, 4B, 4C avec un système de coordonnées qui permet de décrire les mouvements de l'antenne selon l'invention ;
• la figure 7 montre schématiquement et en coupe une géométrie Gregory à l'illumination en offset ;
• la figure 8 montre schématiquement et vue de côté, un exemple d'une réalisation d'une antenne Cassegrain selon l'invention ;
• la figure 9 montre schématiquement en trois dimensions et en vue plongeante, l'exemple de réalisation de la figure 8 ;
• la figure 10 montre un autre exemple en coupe axiale d'une réalisation d'antenne selon l'invention, dans une géométrie Cassegrain centrée avec adjonction d'un réflecteur périscopique auxiliaire et la source déportée ;
• la figure 11 montre schématiquement et en coupe partielle, un autre exemple d'une réalisation d'antenne selon l'invention, dans une géométrie Cassegrain offset.

Other characteristics and advantages of the invention will appear on reading the detailed description which follows, with its appended drawings, including:

• FIG. 1 schematically shows a satellite with an orientable beam in Earth orbit;
• FIG. 2 schematically shows the traces on the ground of a steerable beam of a steerable antenna according to the invention with conservation of the polarization;
• Figure 3 shows schematically and in side section a parabolic antenna according to the prior art;
• FIGS. 4A, 4B, 4C show respectively in section AA ', in plan, and in section BB', an asymmetrical parabolic reflector according to a variant of the invention;
• Figure 5 shows schematically and in section the centered Cassegrain geometry;
• FIG. 6 schematically shows in three dimensions and in perspective, the parabolic reflector of FIGS. 4A, 4B, 4C with a coordinate system which makes it possible to describe the movements of the antenna according to the invention;
• Figure 7 shows schematically and in section a Gregory geometry at offset illumination;
• Figure 8 shows schematically and side view, an example of an embodiment of a Cassegrain antenna according to the invention;
• Figure 9 shows schematically in three dimensions and in a plan view, the embodiment of Figure 8;
• FIG. 10 shows another example in axial section of an embodiment of the antenna according to the invention, in a centered Cassegrain geometry with the addition of an auxiliary periscopic reflector and the remote source;
• Figure 11 shows schematically and in partial section, another example of an antenna embodiment according to the invention, in a Cassegrain offset geometry.

The drawings represent nonlimiting examples of embodiments according to the invention. The same references designate the same elements in the different figures. The scale is not always respected for reasons of clarity.

Figure 1 schematically shows a satellite Q in Earth orbit.

The satellite has a steerable antenna; according to the position of the reflector 11, the beam can be directed in different directions, to illuminate different places on the earth E. In the example of figure 1, we see the beam F directed according to the nadir illuminate the "spot" 1, while the beams respectively F ', F' 'illuminate the spots 1', 1 '' (spot is the English word used by those skilled in the art to designate the path on the ground of a narrow beam directed towards the earth E).

The beam can be oriented either mechanically by positioning a main reflector 11 as shown schematically in this figure, or electronically in the case of a network antenna by playing on the phases applied to the elementary sources of the network.

In all the description which follows, we expose the operation of an antenna in emission only. However, a person skilled in the art knows the reciprocity of the theory of passive antennas according to which an antenna acts in the same way in transmission and in reception by means of an inversion of the sign of time (t) in the equations which describe the electromagnetic propagation. (Maxwell equations).

The description of the antenna of the invention will be made in transmission but it is understood that the invention also relates to a reception antenna having the same characteristics, as well as a transmission / reception antenna such as a radar or telecommunications antenna. Among these different variants, the amplification electronics associated with the antenna must be adapted: either to the power amplification for an antenna at transmission, or to the low noise amplification at reception, or both for a transmit / receive antenna.

In FIG. 2, we see the traces on the ground of a steerable antenna according to the invention with conservation of the linear polarization vectors along the x, y axes. In this example, the spot 1 has the shape of an ellipse having axes a, b; the ellipse being elongated along the axis a. The axes x, y of polarization coincide with the axes a, b of the elliptical spot 1.

The elliptical spots 1 ', 1' 'are illuminated for example by the beams F', F '' of FIG. 1, obtained by orientation of the orientable antenna 11. The relative orientation between the spots (1, 1 ', 1 '') can be obtained by a combination of antenna deflection which provides a translation of the spot, and a rotation of the antenna around the main axis of the emitted beam, to obtain a rotation of the axes of the ellipse .

In a steerable antenna of the known art, a rotation of the antenna around the main axis of the beam is obtained by mechanical means which physically rotate the antenna around this main axis. In the case where this antenna is supplied by one or more sources along two axes of linear orthogonal polarization, the axes of polarization undergo the same rotation as the axes of the spot on the ground. For the envisaged applications of the invention, the rotation of the axes of polarization cannot be tolerated, since it would inevitably cause interference between the signals conveyed by channels which are distinct and separated only by their polarization.

The antenna of the invention makes it possible to solve this problem and to obtain the result illustrated in FIG. 2. We note that the spots 1 ', 1''can be illuminated by a translation and a rotation of the elliptical spot 1, but that the axes of polarization (x, y) are preserved whatever the orientation of the axes (a ', b'; a '', b '') of the elliptical spot (1 ', 1''respectively). In this example, the elliptical spots are oriented to better cover the geographic areas indicated on a geopolitical map of Europe.

To better understand how the invention makes it possible to solve the problem posed, FIG. 3 shows schematically and in side section a parabolic antenna of the prior art. The essential elements of this antenna are the focusing reflector 11 having the shape of a paraboloid of revolution around the axis of symmetry z, and the source 10 placed at the focus of the reflector 11.

The source of this example is a horn 10 supplied by a waveguide 12. Mechanical means 13 are provided to maintain the source 10 at the focus of the reflector 11, in a fixed and optimal geometric arrangement. The electromagnetic radiation emitted by the source 10 at the focus is reflected by the reflector 11 according to parallel rays which form a beam F of radiation along the main axis z.

In the case of a main reflector 10 having a symmetry of revolution, there is no need to rotate the antenna around the main axis z because the spot at the nadir will be circular.

In FIGS. 4A, 4B, 4C are shown different views of an asymmetrical parabolic reflector, capable of making an elongated spot on the ground. The shape of the reflector 11 when viewed in plan in FIG. 4B is almost rectangular. The sections AA ', BB' shown respectively in FIGS. 4A, 4C, are arcs of paraboloids of different lengths. The arcs can have the same focal length, despite their different lengths, and the reflector 11 will have a single focus. The beam resulting from a source at the focus will have a rectangular section.

FIG. 5 shows in axial section a conventional Cassegrain geometry, which comprises a source 10 which illuminates an auxiliary reflector 21 through a hole 20 in a main parabolic reflector 11. The conventional geometry is axisymmetric around the z axis which corresponds to the direction of propagation of the beam F. The source 10 is either arranged on the z axis, or (in a variant not shown) imaged on the axis using a third periscope reflector (not shown).

The auxiliary reflector 21 in the form of a hyperboloid, the first focus C of which coincides with the focal point of the main parabolic reflector 11, while the phase center of the source 10 is imaged at the second focus C 'of the hyperboloid.

In this way, a ray emitted by the source 10 of the point C 'at an angle of θ with respect to the axis z will be reflected from the surface of the auxiliary reflector 21 towards the main reflector 11 in a direction which will have for its origin the focal point C of the main parabolic reflector 11. The rays arriving from the focal point C are reflected by the main parabolic reflector by a reflection angle θ 'to form a beam F whose all the rays are parallel to the axis z.

The vector N represents the normal to the surface of the auxiliary reflector 21. and the vector N 'represents the normal to the surface of the main reflector 11.

Figure 6 shows schematically and in three dimensions in perspective the parabolic reflector (11) of Figures 4A, 4B, 4C, with a coordinate system which makes it possible to describe the movements of the antenna according to the invention. The top of the reflector 11 is located at the origin 0, and the axis z represents the direction of propagation of the reflected waves (not shown).

The parabolic reflector 11 has an approximate rectangular shape when viewed in projection on a flat surface perpendicular to the z axis, for example the plane (x, y).

D is its width in the direction x, and D 'is its height in the direction y. A section AA 'in the plane (x, z) describes a parabola, and a section B'B in the plane (y, z) describes a parabola, in accordance with FIGS. 4A 4B and 4C.

qui est orienté selon les angles de directions (α, β, γ, ) pour aboutir à un point P en dehors de l'axe z. L'angle γ peut être exprimé comme une fonction des deux variables indépendantes (α, β).The system has three degrees of freedom of movement: rotation by an angle ϕ around the axis principal z; and a depointing which can be described by two angles (α, β) in two orthogonal planes whose intersection is the main axis z. The depointing can be represented by the unit vector $\overrightarrow{\text{u}}$

which is oriented along the angles of directions (α, β, γ,) to arrive at a point P outside the z axis. The angle γ can be expressed as a function of the two independent variables (α, β).

sur le plan (x, z) et le point M' la projection du point P sur ce même plan (x, z).The angle α represents the projection of the vector $\overrightarrow{\text{u}}$

on the plane (x, z) and the point M 'the projection of the point P on this same plane (x, z).

sur le plan (x, y), et le point M la projection du point P sur ce même plan (x, y). l'angle β représente la projection du vecteur $\overrightarrow{\text{u}}$

sur le plan (y, z). La projection du point P sur ce plan n'est pas montrée pour des raisons de clarté du dessin.The angle γ represents the projection of the vector $\overrightarrow{\text{u}}$

on the plane (x, y), and the point M the projection of the point P on this same plane (x, y). the angle β represents the projection of the vector $\overrightarrow{\text{u}}$

on the plane (y, z). The projection of point P on this plane is not shown for reasons of clarity of the drawing.

; ces angles ne sont pas indépendants l'un de l'autre.A rotation of the reflector can be represented either by the angle ϕ around the main axis z, or by the angle ϕ 'around the unit vector $\overrightarrow{\text{u}}$

; these angles are not independent of each other.

Figure 7 shows schematically and in section a Gregory geometry with offset illumination. The main parabolic reflector 11 is illuminated by the source 10 via an elliptical auxiliary reflector 13 disposed outside the main axis z of the beam F of the parallel rays. The source 10 placed at the first focus of the ellipse emits towards the auxiliary reflector 13 along the z axis "and the waves are reflected towards the main reflector 11 and focused at a point C" (focus of the parabola and second focus of the 'ellipse), from where they diverge to illuminate the whole of the main reflector 11. This system therefore has two axes (z, z ") around which one can perform either a rotation ϕ around the z axis, or a rotation ϕ "around the z axis", respectively.

Figure 8 shows schematically and in plan an example of an embodiment according to the invention of an antenna Cassegrain orientable with polarization conservation. As in FIG. 5, the main parabolic reflector 11 is illuminated by the source 10 via the auxiliary hyperbolic reflector 21, one of the focal points of which is disposed at the focal point of the main parabolic reflector 11. The two reflectors (11, 21) are maintained mechanically in relative position by means of supports S ₁ .

The assembly comprising the source (10), the reflectors (11, 21) and the mechanical positioning means (depointing, rotation) is fixed by means of the supports S ₃ to the platform Q, a satellite for example.

The positioning means comprise three stepping motors (Rϕ, Rα, Rβ) capable of effecting angular displacements (ϕ, α, β) explained in FIG. 6. These means are mounted on a small platform Q 'which rests on supports S ₃ .

The deflection means (Rα, Rβ) are fixed on the small platform Q 'and drive the support S ₂ which supports the axial rotation motor Rϕ. This axial rotation motor Rϕ is mechanically fixed to the main reflector 11 to perform a rotation (ϕ) of the latter around the main axis z. Unlike the antennas known in the prior art, the rotation of the main reflector 11 does not cause the rotation of the source 10, which is not fixed to the reflector 11.

The source 10 is supplied with two orthogonal polarizations which also remain fixed relative to the source 10 during a rotation ϕ of the main reflector.

In FIG. 9, the same embodiment in FIG. 8 is shown in three dimensions and in perspective seen from above. The elements already described in Figure 8 have the same references. We see the hole 20 in the main reflector 11 to allow the passage of the source 10, without mechanical contact with the latter. This characteristic, already present in the centered Cassegrain geometry, is used according to the invention to isolate the source 10 of rotations ϕ of the main reflector and of the auxiliary reflector linked to the main 11 around the z axis.

The orthogonal sections (A, A '; B, B') of the main reflector 11 are parabolas as in FIGS. 4A, 4B, 4C and 6.

The projections of points A, A '; B, B 'on the x plane, y are the points a, a'; b, b 'respectively, and gives the lateral dimensions of the main reflector 11 and of the auxiliary reflector 21 fixed to the main reflector 11 by the support rods S ₁ . In the most general case, and as shown in FIG. 6, these lateral dimensions (aa ', bb') are unequal, and the section of the beam F (not shown) can have an arbitrary shape determined by the shape of the perimeter of the main reflector 11, elliptical in this example.

In this FIG. 9, the source 10 of this example is a horn, but can be produced according to any other technology known to those skilled in the art. For example, the source 10 can be a network of elementary sources produced in microstrip technology.

FIG. 10 schematically shows in axial section another embodiment according to the invention which represents a variant of the antenna shown in FIGS. 8 and 9.

It is a Cassegrain geometry antenna centered with the addition of an auxiliary periscopic reflector 14 which receives the radiation from the source 10 offset on the z 'axis parallel to the x axis and perpendicular to the main z axis . This auxiliary reflector 14 is arranged in such a way that it reflects the radiation from the source 10 along the z axis to illuminate the hyperbolic auxiliary reflector 21. Everything then takes place according to the description which has been made of FIGS. 8 and 9.

The source 10 remains stationary relative to the platforms Q and Q ', even during a rotation φ of the main and auxiliary reflector 11 by the motor Rφ. During a deflection α in the plane x, z, the position of the auxiliary reflector 14 is adjusted to hold the reflection of the radiation from the source 10 along the main axis z to illuminate the auxiliary reflector 21.

FIG. 11 shows schematically and in partial section another example of an embodiment according to the invention of an adjustable offset Cassegrain antenna with polarization conservation. As in the previous figures, the main parabolic reflector 11 is illuminated by the source 10 via an auxiliary reflector 15. The main reflector is offset offset by the auxiliary reflector at an angle of δ relative to the normal N 'of the main reflector 11 at its summit; the beam F (not shown) is reflected at the same angle δ of the normal N 'along the main axis z.

The beam deflection is obtained in this example by positioning the main reflector by the means Rα, Rβ. Different mechanical means of static support are shown (S ₅ , S ₆ , S ₇ ), as well as a removable support S ₄ which supports the platform Q '' along the main axis z, while allowing its movement in a perpendicular plane to z. Different means of thermal insulation (I ₁ , I ₂ ) are also shown in this figure.

In the example of FIG. 11, the main axis z is distant from the illumination axis z 'of the auxiliary reflector 15, and the two axes are parallel. A mobile platform Q '' on which the main reflector 11 and the means of the support (S ₅ , S ₆ , S ₇ ) and of depointing (Rα, Rβ) of the latter are mounted, can be moved by the means Rφ d ' an angle φ around the z axis of primary illumination. Since the source 10 remains fixed relative to the platform Q (a satellite for example) during a rotation φ around the axis z ', the axes of polarization remain invariant with respect to the platform Q.

The support means S ₈ of the auxiliary reflector 15 connects the latter to the mobile platform Q '', which means that a rotation of the latter does not cause any modification the relative geometry of the two main 11 and auxiliary 15 reflectors.

These few examples of embodiments according to the invention serve to illustrate its principles and some of these variants from which those skilled in the art will be able to decline the invention according to the specific needs of a given mission. In these examples, the depointing means are mechanical and act on the main reflector, but the invention can also make use of electronic depointing (by phase shifts of the elementary sources in the network) or else, to a depointing effected by mechanical means which act on an auxiliary or auxiliary periscope reflector.

. Dans tous les cas, un découplage des moyens de dépointage et de moyens de rotation autour de l'un des axes (z, z', $\overrightarrow{\text{u}}$

) de propagation de rayonnement électromagnétique permet l'orientation du faisceau avec conservation de la polarisation. Il est évident, inversement, que ce même découplage permet à l'antenne selon l'invention, moyennant des adaptations de mécanismes, d'effectuer une rotation des axes de polarisation, tout en maintenant l'orientation du faisceau fixe, bien que cette capacité ne soit pas nécessaire pour les applications envisagées pour les exemples donnés.The rotation of the spot formed on the ground, without rotation of the polarizations, can be obtained either by a rotation ϕ around the main axis (z), or by a rotation φ of the reflector system (s) around the axis of primary illumination z ', or by a rotation ϕ' around a depointed main axis $\overrightarrow{\text{u}}$

. In all cases, a decoupling of the deflection means and of rotation means around one of the axes (z, z ', $\overrightarrow{\text{u}}$

) propagation of electromagnetic radiation allows the orientation of the beam with conservation of the polarization. Conversely, it is obvious that this same decoupling allows the antenna according to the invention, by means of adaptations of mechanisms, to rotate the axes of polarization, while maintaining the orientation of the fixed beam, although this capacity is not necessary for the applications envisaged for the examples given.

Claims (14)

1. Satellite antenna of the type including at least one reflector (11) and at least one source (10) of electromagnetic radiation, at last one of said sources (10) including at least one radiating element and means for exciting said element with two characteristic orthogonal linear polarizations, at least one reflector being a focusing reflector, said antenna further including mechanical means (S₁, S₂, ...) linking said sources (10) to said reflectors (11) and positioning them, said antenna being adapted to transmit or to receive a beam F of electromagnetic radiation in a preferred radiation direction (z), said beam F being adapted to illuminate a spot on the surface of the Earth, said mechanical positioning means (S₁, S₂, Rϕ, ...) enabling movement of at least one reflector (11) relative to said preferred radiation direction,
      characterized in that said spot is non-circular and in that said mechanical means (S₁, S₂, Rϕ) are adapted to enable said reflector (11) to rotate (ϕ, φ, ϕ') about a propagation axis (z, z', u) of said electromagnetic radiation to rotate said spot about the axis defined by said preferred radiation direction while holding said source (10) in a position such that the polarization axes remain invariant in said spot during said rotation (ϕ, φ, ϕ').
2. Antenna according to claim 1 characterized in that said rotation is a rotation ϕ about the main axis (z) which represents said preferred direction of radiation, said rotation being effected by mechanical rotation means Rϕ which operate on the disposition of at least one of said reflectors (11), leaving the position of said source (10) unchanged.
3. Antenna according to claim 1 characterized in that said rotation is a rotation φ about an auxiliary axis (z') which joins said source (10) and an auxiliary first reflector (15), said rotation being effected by mechanical rotation means Rφ which operate on the disposition of at least one reflector (11) while leaving the position of said source (10) unchanged.
4. Antenna according to any one of claims 1 to 3 characterized in that said auxiliary axis (z') is the same as that defining said preferred direction (z) and in that said antenna has a coaxial configuration.
5. Antenna according to any one of claims 1 to 4 characterized in that it has an offset or centered Cassegrain configuration.
6. Antenna according to claim 1, claim 2 or claim 4 having a parabolic main reflector (11) illuminated by a source (10) disposed at its focus characterized in that said parabolic main reflector (11) can be turned about said preferred radiation direction (z) while said source (10) is held fixed.
7. Antenna according to any one of claims 1 to 4 characterized in that it has an offset or centered Gregorian configuration.
8. Antenna according to any one of claims 1 to 7 characterized in that it further includes squinting means (Rα, Rβ) which enable said preferred direction (z) to be changed while holding the polarization axes invariant in said spot.
9. Antenna according to any one of claims 1 to 8 characterized in that it constitutes a transmit and/or receive antenna.
10. Antenna according to any one of claims 1 to 9 characterized in that it further includes a complex primary source (10).
11. Antenna according to claim 10 characterized in that said complex primary source (10) includes a plurality of separate sources and in that said antenna further includes at least one polarizationselective reflector.
12. Antenna according to claim 11 characterized in that said complex primary source (10) includes a plurality of separate sources and in that said antenna further includes a plurality of frequency-selective reflectors.
13. Antenna according to claim 10 characterized in that said complex primary source (10) includes at least one periscopic source.
14. Satellite antenna of the type including at least one reflector (11) and at least one source (10) of electromagnetic radiation, at least one of said sources (10) including at least one radiating element and means for exciting said element, at least one reflector being a dual gridded reflector, said antenna further including mechanical means (S₁, S₂, ...) which link said sources (10) and said reflectors (11) and position them, said antenna being adapted to transmit or to receive a beam F of electromagnetic radiation in a preferred direction z of radiation, said beam F having orthogonal polarization axes conferred on it by the orientation of the grids of said reflector (11), said beam F being adapted to illuminate a spot on the surface of the Earth, said mechanical positioning means (S₁, S₂, R_ϕ, ...) enabling movement of at least one reflector (11) relative to said preferred radiation direction z,
    characterized in that said spot is non-circular and in that said mechanical positioning means (S₁, S₂, R, ...) are adapted to effect rotation (ϕ, φ, ϕ') of said source (10) to rotate said spot about the axis defined by said preferred radiation direction z while holding said dual gridded reflector (11) in a position such that said polarization axes remain invariant in said spot during said rotation.

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