Abstract:
The invention concerns an antenna with high scanning capacity. The antenna comprises a panel ( 30 ) of static radiating elements which are controlled to transmit in variable directions relative to a direction ( 38 ) perpendicular to the plane of the panel. Reflectors ( 34, 44 ) amplify the scanning effected by the panel ( 30 ) of radiating elements. The reflectors ( 34, 44 ) are segments of paraboloids with the same axis ( 38 ) and the same focus ( 40 ), for example.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an antenna with high scanning capacity. It relates more particularly to an antenna for use in a telecommunications system, in particular a telecommunications system using satellites. 
     Antennas are frequently needed, in various applications, to receive signals from a mobile source and/or to transmit signals to a mobile receiver or target. Such transmit and/or receive antennas are usually active antennas made up of stationary radiating elements in which the direction of the radiation pattern can be varied by varying the phase of the signals feeding the radiating elements. 
     That technique cannot produce satisfactory radiation patterns for large squint angles, i.e. for directions departing significantly from the mean transmit and/or receive direction. 
     A source or a receiver can also be tracked using a conventional antenna moved by a motor. That type of antenna with mechanically movable elements and a motor is not suitable for all applications. In particular, it is preferable to avoid the use of any such antenna in space applications, for reasons of reliability, overall size, and weight. 
     The invention remedies those drawbacks. It provides an antenna with a high scanning capacity and with a satisfactory radiation pattern at large squint angles, and which does not require any moving parts. 
     The antenna of the invention comprises a set of static radiating elements commanded to perform scanning and reflector means to amplify the scanning angle of the radiating elements. The reflector means include two reflectors having a common focus, the first reflector receiving the beam transmitted by the set of radiating elements and the second reflector receiving the beam reflected by the first reflector. 
     SUMMARY OF THE INVENTION 
     According to the invention, the focal length of the first reflector is greater than the focal length of the second reflector so that the exit beam of the antenna has an inclination to a predetermined direction which is greater than the inclination Θ relative to the given direction of the beam transmitted by the radiating elements. 
     The scanning angle of the radiating elements can therefore be reduced in proportion to the amplification provided by the reflector means. Thus the radiating elements are not used for squint angles that are too large. Also, the constraints imposed on radiating elements to scan a small angle are much less severe. In particular, the dimensions of the system are less restricted, which enables a sufficiently large pitch (distance between two adjacent radiating elements) to prevent array lobes without compromising the propagation of the radiation. 
     The reflector means are in fact analogous to those usually employed to increase the size of the beam, for example in Cassegrain antennas. However, with the invention, the reflector means are used in the opposite way to usual. In a Cassegrain antenna, an increase in the size of the beam corresponds to a reduction in the scanning angle. 
     In one embodiment of the invention, each reflector is a paraboloid, for example. The scanning amplification gain depends on the ratio between the focal lengths of the two reflectors. 
     This ratio is 4/1, for example. 
     The reflectors are disposed so that the output beam is not even partly masked by the first reflector, i.e. the reflector receives the beam from the radiating elements directly. 
     A preferred application of the invention relates to an antenna for communicating with a plurality of sources or receivers in an extended area, communication having to remain confined within the area despite the changing position of the antenna relative to the area. 
     This problem arises in particular in a telecommunications system using a network of satellites in low Earth orbit. A system of this kind has already been proposed for high bit rate communication between terrestrial mobile or fixed stations in a particular geographical area covering several hundred kilometers. The altitude of the satellites is in the range from 1000 km to 1500 km. 
     In the above system, each satellite includes groups of transmit and receive antennas and each group is dedicated to a given area. The receive antennas in each group receive signals from a station in the area and the transmit antennas retransmit the received signals to another station in the same area. The antennas of a particular group point towards the area for all the time it remains within the field of view of the satellite. Accordingly, for each satellite, a region of the Earth is divided into n areas, and as the satellite moves over a region, each area is allocated a group of transmit and receive antennas which point towards it continuously. 
     While the satellite is moved over a region, which takes around twenty minutes, for example, assigning a single group of transmit and receive antennas to an area avoids switching between antennas, which could compromise the speed or quality of communication. 
     The low altitude of the satellites also minimizes propagation times, which is beneficial for interactive communications, in particular in multimedia applications. 
     Clearly, with the above telecommunications system it is preferable for an antenna intended for one area not to suffer from interference due to signals from other areas and for it not to interfere with other areas. Also, the radiation pattern has a shape which varies with the position of the satellite relative to the area. When the areas on the Earth are all circular, the antenna sees the area in the form of a circle when the satellite is at the nadir of the area; in contrast, as the satellite moves away from this position the antenna sees the area in the form of an ellipse that is progressively flattened as the satellite approaches the horizon. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     It has been found that an antenna in accordance with the invention in which the reflectors are paraboloids can match the trace of the pattern on the ground to the position of the antenna relative to the area without it being necessary to modify the radiation pattern produced by the radiating elements. 
     Also, the antenna has a high gain when the satellite is close to the horizon relative to the area. This is when the distance from the satellite to the area is the greatest; accordingly, the increasing gain compensates for the increase in the distance, which is favorable to maintaining calls. 
     In one embodiment, two antennas of the above type are used to track an area, each antenna effecting an even smaller scan. 
     An antenna of the invention can be used to track more than one area, the radiating elements being able to receive signals from or send signals to more than one area. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Other features and advantages of the invention become apparent from the following description of embodiments of the invention given with reference to the accompanying drawings, in which: 
     FIG. 1 is a diagram showing a telecommunications system linking terrestrial mobile or fixed stations using a system of satellites; 
     FIG. 2 is a diagram showing a telecommunications system; 
     FIG. 3 is a sectional diagram of an antenna of the invention; 
     FIG. 4 is a sectional diagram of a variant antenna; 
     FIG. 5 is a diagram showing the region that the antenna shown in FIG. 4 can cover; 
     FIG. 6 is a diagram showing two associated antennas covering all the areas shown in FIG. 6; and 
     FIG. 7 is a perspective diagram of an embodiment using associated antennas. 
     The example of an antenna to be described is intended for a telecommunications system using a constellation of satellites in low Earth orbit, approximately 1300 km above the surface  10  of the Earth. 
     The system must set up calls between users  12 ,  14 ,  16  and one or more connection stations  20  to which service providers such as databases are connected (see FIG.  1 ). Calls between users are also set up via the connection station  20 . 
     These calls employ a satellite  22 . 
     In the system, the satellite  22  can see a region  24  of the Earth at all times and this region is divided into areas  26   1 ,  26   2 , . . . ,  26   n  (see FIG.  2 ). 
     Each area  26   i  is in the form of a circle having a diameter of approximately 700 km. Each region  24  is defined by a cone  70  centered on the satellite and having a cone angle determined by the altitude of the satellite (see FIG.  1 ). A region is therefore that part of the Earth which is visible from the satellite. When the altitude of the satellite is 1300 km, the cone angle is approximately 110°. 
     Terrestrial means are used for communication between areas, for example cables between the connecting stations of the various areas that are part of the same region or different regions. 
     The number and the disposition of the satellites are such that at any time two or three satellites can be seen from an area  26   i . When an area  26   i  leaves the field of view of the satellite assigned to calls in that area, there is therefore another satellite ready to take over, and switching from one satellite to the other is instantaneous. 
     However, such switching occurs only about once every twenty minutes. In practice this switching occurs when, for the area  26   i  in question, the elevation of the satellite drops below 20°. 
     As the satellite crosses a region  24 , the antennas of the invention always point towards the same area or the same set of areas. They must therefore have a high capacity for scanning or squinting. 
     To this end, as shown in FIG. 3, the antenna comprises a panel  30  of radiating elements associated with a beam-forming network (not shown) controlling the phase of the signals feeding the radiating elements. A beam  32  transmitted by the panel  30  is directed towards a first reflector  34  having the form of a paraboloid with a circular cut-off. The reflector is part of an imaginary surface  36  whose axis  38 , on which the focus  40  lies, is far away from the reflector  34 . 
     The axis  38  is perpendicular to the plane of the panel  30 . 
     The reflector  34  reflects the beam  42  towards a second reflector  44  on the side of the axis  38  opposite to the reflector  34  and the panel  30 . The reflector  44  is also part of an imaginary surface  46  in the plane of FIG. 3, which is a parabola with the same focus  40  and the same axis  38  as the parabola  36 . The surface  46  is also a paraboloid. 
     The concave side of the reflector  44  faces towards the concave side of the reflector  34 . 
     The focal length of the reflector  44  is one quarter that of the reflector  34 , for example. 
     The axis  38  does not intersect the reflector  34  or  44 . The edge  44   1  of the reflector  44  nearest the axis  38  is at a distance from that axis substantially less than the distance from the corresponding edge  34   1  of the reflector  34  to the axis  38 . 
     In the example shown in FIG. 3 the array  30  has a generally circular exterior shape with a diameter of approximately 30 cm (12λ) with 37 radiating elements separated from each other by 42 mm (1.7λ), where λ is the wavelength of the radiation. 
     Each of the reflectors has a circular cut-off. In this example, the diameter of the circle defining the reflector  34  is in the order of 28λ. The diameter of the circle defining the reflector  44  is in the order of 30λ. The distance between the edge  34   1  and the axis  38  is 24λ and the distance between the edge  44   1  of the reflector  44  and the axis  38  is 4′. 
     When the array  30  transmits a beam  32   1  parallel to the axis  38 , i.e. perpendicular to its plane, the beam reflected by the reflector  34  is focused at the focus  40 . The reflector  44  therefore reflects the beam  32   2  parallel to the axis  38 , as represented by the beam  32   3 . 
     If the array  30  transmits a beam  32   1  inclined at a relatively small angle Θ to the axis  38 , the beam  32   6  reflected by the reflector  34  converges at a point  50  near the focus  40 , and the beam  32   7  reflected by the reflector  44  is inclined at an angle which is approximately n times the angle Θ, n being the ratio of the focal length f of the reflector  34  to the focal length f′ of the reflector  44 . In the example, the ratio between the focal lengths being 4/1, the beam  32   7  is inclined at an angle of 4Θ to the axis  38 . 
     However, this amplification in the ratio of the focal lengths does not occur for beams  32   10  transmitted by the array  30 , which beams have a large angle of inclination to the axis  38 . 
     Accordingly, FIG. 3 shows that the beam  32   10  is reflected as a beam  32   11  by the reflector  34  and this beam converges at a point  52  far away from the focus  40 . The beam  32   11  is reflected by the reflector  44  as a beam  32   12 . 
     For example, for a beam with azimuth φ=90° and inclination Θ of 4.5° to the axis  38 , i.e. to the normal to the plane of the array  30 , the beam  32   7 , also with an azimuth of 90°, is inclined at 18° to the axis  38 . This value is indeed 4Θ. 
     On the other hand, for an inclination, or squint, of −14° (beam  32   10 ), again with an azimuth of 90°, the beam  32   12  has an inclination of 38° to the axis  38 , which is significantly less than four times the inclination of the beam  32   10 . The azimuth of the beam  32   12  is also 90°. 
     In the example, for an azimuth of 90°, the beam transmitted by the array  30  can scan an angle Θ in the range from 4.5° to −14°. These limits are imposed, in the first instance, by geometry because the beam reflected by the reflector  34  must reach the reflector  44  and the beam reflected by the reflector  44  must not be masked by the reflector  34 . Secondly, the radiation performance of the beams converging in front of the focus  40  (in the direction of the exit beam) also limits the scan angle because, for these inclined beams, operation is far from nominal. 
     FIG. 4 relates to a variant of FIG. 3 in which the reflector  44 ′ is generally oval in shape, i.e. longer in one direction than in the orthogonal direction, and the reflector  34 ′ has a circular cut-off, like the reflector  34 . 
     The greatest dimension of the reflector  44 ′ is in the plane of symmetry perpendicular to the axis  38  common to the two paraboloids. In this example, this greatest dimension is approximately 48λ. 
     The other features are the same as in FIG.  3 . 
     The geometry shown in FIG. 4 yields the same performance for an azimuth of 90° as the antenna shown in FIG.  3 . 
     For a beam transmitted by the array  30  with an azimuth of 0°, and for an inclination Θ of −5° to the axis  38 , the exit beam is inclined at −20° with an azimuth of 2.3°. For a squint Θ of −15° and an azimuth of 0°, the squint of the exit beam is −45° with an azimuth angle of 31.5°. 
     With this reflector, and for an azimuth of 90°, the squint of the beam transmitted by the array  30  can be varied in the range from +4° to −14° in the plane containing the center of the array  30  and the axis  38  and in the range from +15° to −15° in the plane of symmetry. 
     With these squint angles the antenna cannot cover all of the region seen by the satellite but only the portion  80  of that region which is shaded in FIG.  5 . The portion  80  represents approximately 60% of the region. 
     To be able to cover all of the region, a pair of antennas arranged as shown in FIG. 6 is used. In this example, one antenna  90  transmits more towards the West and one antenna  92  transmits more towards the East. 
     The two antennas  90  and  92  are fastened to a plane support  94  whose normal  96  is directed towards the center of the Earth. In other words, the axis  96  always points towards the point  100  in FIG.  5 . 
     The antennas  90  and  92  transmit towards regions which are symmetrical about the axis  102  (FIG.  5 ). Thus the antenna  90  transmits towards the region  80  and the antenna  92  transmits towards the region symmetrical to the region  80  about the axis  102 . The axis  38   1  of the antenna  90  is inclined to the axis  96  so that it is directed towards an area  26   p  corresponding substantially to the center of the region  80  (see FIG.  5 ). The axis  38   2  of the antenna  92  is of course inclined symmetrically. 
     It should be noted that the same array of radiating elements  30  can be used to transmit a plurality of beams. In other words, the same array  30  associated with the reflectors  34  and  44  or  34 ′ and  44 ′ can be used to transmit towards more than one area or to receive signals from more than one area. 
     In the example shown in FIG. 7 a common support  94  carries two pairs of antennas  90   1 ,  92   1  and  90   2 ,  92   2 . Each antenna, for example the antenna  92   1 , comprises two panels of radiating elements, a transmit panel  30   1  and a receive panel  30   2 . 
     It can be seen that in all the embodiments the gain is greater at the limit of the region  24  than at the nadir. The limits of the region correspond to the greatest inclinations, for which the area concerned of the exit reflector (radiating aperture) is greatest and therefore for which the resolution is the highest. This property is apparent in FIG. 3 where it can be seen that the reflector  44  of the beam  32   12  corresponds to a larger area than the beam  32   3 . In this way, for the areas with the greatest inclination, i.e. those at the greatest distance, the increase in the gain compensates for the increase in the distance. 
     It has also been found that the shape of the trace on the ground matches the target area.