Abstract:
The antenna with shared feeds for generating multiple beams includes a plurality of identical individual feeds spaced apart by a constant pitch, wherein the individual feeds are associated in identical subnetworks around a central individual feed, each subnetwork being intended to synthesize a beam, the individual feeds of each subnetwork being phase-coupled together by electromagnetic coupling, and two successive subnetworks include at least one individual feed in common and are offset by a distance corresponding to a predetermined number of pitches greater than or equal to one, being applicable in particular to the field of satellite telecommunications.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of foreign French patent application no. FR 0806841, filed Dec. 5, 2008, the disclosure of which is hereby incorporated by reference in its entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to an antenna with shared feeds and a method of producing an antenna with shared feeds for generating multiple beams. It applies notably to the field of satellite telecommunications. 
     BACKGROUND OF THE INVENTION 
     In the context of satellite telecommunications, the needs of the operators are moving towards smaller and smaller terminals, of A4 or A5 size, or terminals of PDA (Personal Digital Assistant) type or telephones, to reach roaming users. 
     Numerous applications also relate to the trend among existing systems for internet or high bit rate applications. 
     To achieve these missions, the antenna architectures are increasingly complex, comprise a large number of feeds associated with numerous amplifiers to ensure the generation of around a hundred or several hundreds of increasingly narrow beams and provide continent-wide or worldwide coverage. The beams are formed either with a digital beam forming network BFN on board the satellite, or via a ground-based beam forming GBBF device. 
     For narrowband applications and for low frequency bands below 10 GHz, such as the L or S bands, it is difficult to multiply the number of antennas given their large size, generally 9 to 15 m, or even 20 m, in diameter for the deployable reflectors. In this case, it is known to use one or two active antennas placed in front of the beam forming reflectors being produced either on board in an analogue or digital way or by a device on the ground. 
     For wideband applications and for high frequency bands greater than 10 GHz, the digitization techniques are limited in the on-board band processing capacity, in the complexity and technological feasibility of the beam forming network BFN and above all in the excess energy consumption at the payload level. Generally, the payload is configured passively with a structure whereby a feed corresponds to a particular beam. For reasons of inter-beam isolation constraints and gain performance reasons, this means multiplying the number of antennas on the satellite according to the frequency reuse scenario from one spot to another. This scenario is called three- or four-colour code and generally necessitates an antenna for each frequency subband, or colour, used. 
     It is known to use beam synthesis with a network of individual feeds placed at the focus of a parabolic reflector and by reusing feeds from one spot to another as described notably in the patents EP 0340429 and EP0333166. These architectures allow for the use of compact antennas with a low F/D ratio, F being the focal distance and D the diameter of the reflector, and make it possible to tighten the beams, but give strong aberrations in the case of unfocused operation. 
     It is also known to produce radiant structures comprising a set of individual feeds coupled together by electromagnetic radiations and making it possible to radiate waves in phase, as described notably in the patents EP0899814 and EP0617480. These structures are excited by an individual feed powered by a radio frequency-type excitation signal originating from a channel of the satellite&#39;s payload. 
     SUMMARY OF THE INVENTION 
     The aim of the present invention is to remedy the problems of the known beam forming systems and propose an antenna with feed sharing making it possible to generate a large number of narrow beams simply without using complex beam forming circuits, the beams ensuring a positioning of the spots with no coverage holes, the spots representing efficient C/I ratios, the ratio C/I being the ratio between the wanted signal C and the interfering signals I. 
     To this end, the subject of the invention is an antenna with shared feeds for generating multiple beams, comprising a plurality of identical individual feeds spaced apart by a constant pitch, wherein:
         the individual feeds are associated in identical subnetworks around a central individual feed, each subnetwork being intended to synthesize a beam, the individual feeds of each subnetwork being phase-coupled together by electromagnetic coupling,   and wherein two successive subnetworks include at least one individual feed in common and are offset by a distance corresponding to a predetermined number of pitches greater than or equal to one.       

     Advantageously, each subnetwork includes a single excitation feed. 
     According to one embodiment of the invention, the excitation feed is the central individual feed of each subnetwork. 
     According to one embodiment of the invention, the individual feeds are metallic patches deposited on a first face of a dielectric substrate comprising a second opposite face, parallel to the first face and mounted on a metallic plane forming a ground plane and the exciting feed is mounted on the second face of the dielectric, in line with the central individual feed of each subnetwork. The exciting feed can include an excitation slot etched into the ground plane in line with the central individual feed of each subnetwork. 
     According to another embodiment of the invention, the individual feeds are dielectric antennas. 
     The invention also relates to a method of producing an antenna with shared feeds as defined above for generating multiple beams, the antenna comprising a plurality of identical individual feeds spaced apart by a constant pitch, the method comprising the following steps:
         choosing a diameter D for the parabolic reflector,   choosing a type of individual feed,   dimensioning and producing a subnetwork of several individual feeds around a central feed, the subnetwork being intended to synthesize a beam having a predetermined angular aperture, the feeds of the subnetwork being phase-coupled together with electromagnetic coupling,   choosing a number of offset pitches between two successive subnetworks, the number of pitches being greater than or equal to one and such that the two successive subnetworks include at least one individual feed in common,   determining a focal distance F for the parabolic reflector,   calculating the angular aperture of the beams corresponding to the choices made in the preceding steps and comparing theoretical performance of the antenna to desired performance.       

     Advantageously, if the theoretical performance does not correspond to the desired performance, the method furthermore consists in modifying the dimensions of the reflector to a value greater than the ratio F/D, in increasing the dimensions of the subnetwork, and therefore its directivity, proportionally to the focal distance of the reflector and in recalculating the new theoretical performance of the antenna with these new dimensions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other particular features and benefits of the invention will become clearly apparent hereinafter in the description given by way of purely illustrative and nonlimiting example, with reference to the appended diagrammatic drawings which represent: 
         FIG. 1 : a diagram of an example of coverage obtained with a configuration of around fifty spots, according to the invention; 
         FIGS. 2   a ,  2   b ,  2   c : diagrams of three examples of hexagonal-mesh feed configuration, according to the invention; 
         FIGS. 3   a  and  3   b : diagrams of two examples of rectangular-mesh feed configuration, according to the invention; 
         FIG. 4 : an example of theoretical radiation pattern obtained with a subnetwork comprising seven feeds distributed in a hexagonal mesh, according to the invention; 
         FIG. 5 : an example of coverage of Europe with a focal network configuration with 43 beams corresponding to 43 spots, according to the invention; 
         FIG. 6 : a first example of excitation of a subnetwork comprising a number of patch-type individual feeds, according to the invention; 
         FIGS. 7   a  and  7   b : two schematic views, respectively in perspective and from the front, of an example of subnetwork comprising a number of individual feeds of dielectric antenna type, according to the invention; 
         FIGS. 8   a  and  8   b : two schematic views respectively in transverse cross section and in plan view, of an example of focal network comprising a number of subnetworks of dielectric antennas, according to the invention; 
         FIG. 9 : an example of antenna comprising a multiple-feed network placed in the focal plane of a parabolic reflector. 
     
    
    
     DETAILED DESCRIPTION 
     The antenna architecture according to the invention must allow the transmission of information to terminals of very small size. The size of the terminals targeted dictates the values that the antenna parameters must have such as the ratio of the gain to the noise temperature G/T, called figure of merit, and the equivalent isotropic radiated power EIRP. For a given geographic coverage and a given frequency, the G/T parameter directly depends on the number of beams to be generated. Consequently, a fixed frequency band and G/T value have a corresponding number of beams and a beam size needed to produce a given coverage.  FIG. 1  shows an example of coverage obtained with a configuration of forty-three spots  30 , the spots  30  corresponding to beam angles ⊖ equal to 0.5°. To produce optimum coverage, all the spots must touch, or even be interlaced, and the gain of the antenna must be optimized to the hollows of each mesh corresponding to the intersection between three spots in the case of a triangular or hexagonal mesh. 
     The size required for the spots demands a minimum dimensioning for the diameter of the parabolic reflector  90  to be used to focus the energy. It then remains to define the feed or the network of individual feeds that will be placed in the focal area F of the reflector and that will have to illuminate this reflector in a multiple-spot configuration.  FIG. 9  shows an example of antenna comprising a multiple-feed network  91  placed in the focal plane of a parabolic reflector. 
     The feed must synthesize a focal spot induced by the parabolic reflector in the vicinity of its focal plane. However, the bulk of a conventional feed, like that used in the antennas where a feed corresponds to a beam, does not allow the spots to be positioned so that they touch. For the spots to touch, the size of the feeds must be reduced in order to tighten the beams, but to the detriment of the overall efficiency of the antenna, because in this case, the feeds illuminate the edge of the reflector at levels close to or less than −3 dB relative to the maximum. The efficiencies obtained are then of the order of 25 or 30%, which is unacceptable. 
     So as not to multiply the number of antennas on the satellite, the invention consists in using a new configuration of feeds making it possible to reuse individual feeds from one spot to another spot and to thus tighten the beams.  FIGS. 2   a ,  2   b ,  2   c  represent plan view diagrams of three examples of hexagonal mesh feed configuration according to the invention. 
     These figures show a number of patch-type individual feeds. The individual feeds are all identical, spaced apart by a constant pitch P, and are grouped together in two subnetworks of seven individual feeds, some shared between the two subnetworks. In  FIG. 2   a , the first subnetwork consists of the feeds  1  to  7  and the second subnetwork consists of the feeds  4  to  10 , with the four individual feeds  4  to  7  being shared between the two subnetworks, the other individual feeds  1  to  3  and  8  to  10  being independent. In  FIG. 2   b , a single feed  11  is shared; in  FIG. 2   c , two feeds  12 ,  13  are shared. In these examples, each subnetwork consists of a central individual feed surrounded by six peripheral individual feeds forming a hexagonal mesh. The two subnetworks are offset from one another by a translation corresponding to a selected number of pitches P, for example the offset can be chosen to be equal to a pitch as represented in  FIG. 2   a  or to two pitches as represented in  FIGS. 2   b  and  2   c . Each subnetwork is intended to synthesize a beam corresponding to a spot on the ground. Thus, for example in the case of  FIG. 2   a , each spot will be produced by three independent individual feeds and four shared individual feeds, all the individual feeds of one and the same subnetwork being powered in phase and with the same amplitude. The reuse of shared individual feeds from one subnetwork to another subnetwork makes it possible to tighten the beams and obtain spots that touch. In the example represented in  FIG. 2   a , the synthesized beams will be the tightest because there are four shared individual feeds out of the seven used to generate a spot. 
     The configurations and the number of individual feeds used to synthesize a beam are not limited to the examples represented in  FIGS. 2   a ,  2   b ,  2   c . It is also possible to choose a square mesh or a rectangular mesh, as represented for example in  FIGS. 3   a  and  3   b , with a number of individual feeds different from seven and a number of shared individual feeds different from one, two or four. Thus, in  FIG. 3   a , each subnetwork comprises nine individual feeds of which six individual feeds  14  to  19  are shared, and in  FIG. 3   b , each subnetwork comprises nine individual feeds of which three individual feeds  20  to  22  are shared. 
     When the geometry of the subnetworks is chosen, the offset distance between two consecutive subnetworks makes it possible to determine the relative positions of the centres of the two spots that will be radiated by these two subnetworks and deduce therefrom, in a known manner, the focal distance F of the parabolic reflector  90  to be used. In the case where the beam angle ⊖ is chosen to be equal to 0.5°, so that the spots touch, it is essential for the distance between the centres of the two spots to correspond to an angular distance of 0.5°. These conditions make it possible to achieve, in a known manner, possibly after a number of iterations, a particular value of the F/D ratio to be produced for the reflector  90 . 
     As a nonlimiting example, to sample Europe with fifty or so beams having a beam angle ⊖ equal to 0.5°, with a choice of hexagonal mesh with seven patch-type individual feeds, the centres of two consecutive patches being spaced apart by a pitch equal to 0.85λ, λ being the operating wavelength, the patches having a directivity equal to 9 dBi (9 dB relative to an isotrope antenna), the offset between two consecutive subnetworks being equal to two pitches and for a frequency equal to 6 GHz, a reflector is needed that has a focal length of 9260 mm, or an F/D ratio equal to 1.42. In this case, the pitch separating the centres of two consecutive patches is equal to 42.7 mm and the offset between two subnetworks is equal to 85.4 mm and corresponds to a pointing of a second beam at 0.5° from a first beam. The theoretical radiation pattern obtained with a subnetwork whose geometrical parameters correspond to the above example and represented in  FIG. 4 , shows that the subnetwork lights the reflector with an amplitude equal to −7 dB relative to the maximum over an angle of 38° approximately. The subnetwork produced in this way is therefore satisfactorily adapted relative to the reflector. The various dimensioning parameters thus defined in this example make it possible to produce a coverage of Europe with a focal network configuration with 43 beams corresponding to 43 spots, as represented in  FIG. 5 . Since each beam is produced by a subnetwork consisting of a group of seven individual feeds distributed in a hexagonal mesh and two consecutive subnetworks being offset by two individual feeds, the number of individual feeds needed to synthesize the 43 beams is equal to 209 individual feeds. For comparison purposes, it would have been necessary to use 301 individual feeds if there had not been any shared feeds between two consecutive beams, the 301 feeds corresponding to the simple product of the number of beams by the number of individual feeds needed to synthesize a beam (43×7=301). The sharing of the individual feeds therefore makes it possible to reduce the complexity of the antenna and of the beamforming circuits. 
     The sharing of the feeds can be done in different ways such as, for example, by means of orthogonal BFNs, by means of a change of polarization between a first beam and a second beam that have one or more shared feeds, or by power supply fault for the shared feed when generating the second beam. In the case of a power supply fault for the shared feed, a first beam is generated by seven individual feeds and a second beam adjacent to the first beam is generated by six individual feeds. 
     To achieve the sharing of the feeds in a simple way, the invention consists in coupling together the individual feeds of one and the same subnetwork by electromagnetic coupling. 
     With this approach, and in a very effective manner, the individual feeds coupled together in one and the same subnetwork form one feed, called composite feed, with directivity suited to the radiation towards the reflector, while observing the physical and geometric constraints associated with the positioning of the beams over the coverage area, that is to say corresponding to pitches less than those recommended by the desired directivity. 
     To couple the feeds together, the individual feeds of each subnetwork can be excited by an exciting feed powered by an excitation signal as represented for example in  FIG. 6 . In this figure, the exciting feed  60  is a patch deposited on a first face  61  of a dielectric substrate  62  and the individual feeds of the subnetwork are patches  63 ,  64 ,  65  deposited on a second opposite face  66  of the dielectric substrate  62  and parallel to the first face  61 . All the patches  63 ,  64 ,  65  are identical and the exciting feed  60  is arranged in line with the central individual feed  64  of the subnetwork, which means that the centres of the two patches  60 ,  64  are on the same normal to the plane of the parallel faces of the dielectric substrate  62 . The height H of the structure supporting the feeds excited by coupling is approximately at a distance equal to λ/2, λ being the excitation wavelength in a void. The exciting feed  60  is at a short distance from a conductive surface  67  forming a ground plane. In this example, the exciting feed  60  is a metallic patch, but the excitation could also be produced by a slot produced in the ground plane  67  or by a dielectric resonator. The assembly consisting of the dielectric substrate  62  and the patches  60 ,  63 ,  64 ,  65  constitutes a wave guide operating at a particular frequency of wavelength λ. Accurate management of the impedance of the structure as a whole can be achieved for example by irises, not represented, implanted in the dielectric substrate to allow it to be matched. The electrical power signal for the exciting feed  60  can be routed for example via a coaxial or microstrip or tri-plate transmission line that is not represented. 
     This structure makes it possible, by powering a single exciting feed for each subnetwork, the exciting feed being arranged under the central individual feed, to phase-couple the individual feeds of the subnetwork, for example seven of them for a hexagonal mesh, and thus generate beams with increased directivity without using complex BFN circuits, the topology of which soon becomes inextricable when the degree of sharing of the feeds between a number of beams become high, such as, for example, in the case of the central spots of  FIG. 1 . The sharing of the feeds is then done naturally by electromagnetic coupling between the individual feeds of the subnetwork concerned. 
     The coupling together of the feeds can also be produced by powering the central individual feed of each subnetwork. The other individual feeds of the subnetwork surrounding the central individual feed are then powered by electromagnetic radiation from the central individual feed to the peripheral individual feeds. This electromagnetic radiation induces currents on the peripheral individual feeds which causes the individual feeds to be coupled together. When the feeds are in phase, the overall radiation of all the individual feeds of the subnetwork synthesizes a beam with increased directivity without using BFN circuits. 
     The invention is not limited to patch antennas. It is also possible to use horn antennas or dielectric antennas, also called dielectric-rod antennas.  FIGS. 7   a  and  7   b  show two diagrammatic views, respectively in perspective and from the front, of an example of subnetwork comprising a number of individual feeds  80 ,  81  of dielectric antenna type and  FIGS. 8   a  and  8   b  two diagrammatic views, respectively in transverse cross section and in plan view, of an example of focal network with shared feeds comprising a number of subnetworks of seven individual feeds of dielectric antenna type arranged in a hexagonal mesh. In the same way as for the patches, the invention consists in powering, at a bottom end  83 , the central dielectric antenna  81  with a mesh that is for example hexagonal, and the peripheral dielectric antennas  80 ,  82  belonging to this same hexagonal mesh are naturally coupled together and to the central dielectric antenna  81 , by electromagnetic coupling. For example, for an internal diameter L of each individual dielectric antenna equal to 7.4 mm or to 10 mm, the pitch P between the centres of two consecutive dielectric-rod antennas can then be equal to 16 mm. In the context of dielectric-rod antennas, the wave, generated by the power supply to the central dielectric-rod antenna  81 , is propagated along an axis z of the dielectric antenna by being contained within the dielectric-filled wave guide with a diameter of 7.4 mm or 10 mm and delimited by the metallic walls  84  securing the dielectric rod. The electromagnetic coupling between the dielectric rods is obtained by a decontainment of the wave within a decontainment area situated at a top end  85 , level with the output cone of the dielectric rods where the metallic walls  84  disappear. In the decontainment area, the fields deriving from the central exciting dielectric antenna  81  will naturally be spread in transverse mode, be coupled to the surrounding dielectric antennas  80 ,  82  and induce mutual couplings that contribute to the ultimate radiation of all the antennas of the mesh. The closure impedance of the induced dielectric antennas  80 ,  82  is determined so as to give, in overall behaviour, a desired increase in directivity. The whole then behaves as a subnetwork of seven dielectric bars consisting of one central bar  81  forming an exciting feed and six peripheral bars  80 ,  82  forming coupled feeds whose impedance conditions are determined so as to control the coupling and re-radiation coefficients of the dielectric antennas in accordance with the desired objectives of the duly obtained coupled subnetwork. In the examples of networks represented in  FIGS. 8   a  and  8   b , each subnetwork of seven feeds, arranged in a hexagonal mesh, comprises two peripheral feeds  82  shared with an adjacent subnetwork. 
     Although the invention has been described in relation to particular embodiments, it is obvious that it is by no means limited thereto and that it comprises all the technical equivalents of the means described and their combinations provided that the latter fall within the context of the invention. In particular, to further improve the performance of the antenna, the subnetwork can comprise a number of rings of peripheral feeds powered by electromagnetic coupling around a central feed.