Abstract:
Dual polarisation antenna having low side lobes, useful in a satellite radio communication system. The antenna includes a reflector assembly ( 12 ) illuminated by a feed source ( 11 ). The reflector assembly ( 12 ) includes a front reflector ( 15 ) that reflects two electromagnetic waves orthogonally polarized and each at a different frequency, an auxiliary reflector ( 16 ) that reflects one of the orthogonally polarized electromagnetic waves, and a deflecting surface ( 13 ) that totally diffracts the orthogonally polarized electromagnetic wave that passes through the auxiliary reflector ( 16 ).

Description:
OBJECT OF THE INVENTION 
     The present invention relates to a dual polarisation antenna adapted to reflect electromagnetic waves of two orthogonal polarisations with low radiation side lobes. 
     The present antenna can more particularly be utilised in a satellite radiocommunication system that employs orthogonal polarisation, one polarisation being horizontal and the other vertical. 
     STATE OF THE ART 
     The technique is well known in the art, for instance in U.S. Pat. No. 4,757,323, which is incorporated herein by reference. It discloses how to manufacture a dual polarisation same-zone two-frequency antenna for telecommunications satellites. The antenna serves to focus and to direct electromagnetic energy along a communication link. 
     The antenna has a reflector and a source that is of the horn type. The reflector reflects two electromagnetic waves which are polarised orthogonally to each other and which are at different frequencies in such a manner as to obtain the some geographical coverage on the surface of the globe. 
     The central portion of the reflector, constituted by the area common to both first and second orthogonal grids, reflects both orthogonally polarised waves; whereas the peripheral portion outside the central second grid only reflects the low frequency polarised wave. The same zone coverage is obtained by determining the area and shape of the central grid in such a manner as to obtain the some zone coverage with the high frequency wave as is obtained by the first grid for the low frequency wave. 
     It is usually necessary to preserve a direction of polarisation of the electromagnetic energy, and to prevent the generation of both grating and side lobe components that produce interference on said desired direction of polarisation and in the rest of the satellite useful payload. 
     It is a disadvantage of prior art techniques that undesired grating lobes in the radiation pattern, namely in the desired direction, of an antenna are generated by antenna with many antenna elements, e.g. with several antenna elements. Further, grating lobes are undesired side lobes in the radiation pattern of an antenna. 
     On the other hand, wherein two reflectors are to be employed for the reflection of electromagnetic waves of differing polarisation, it is desirable to construct a single supporting structure for both reflectors, thereby conserving overall weight of the antenna. Such a sharing of support structure requires a positioning of a source such that its respective polarised electromagnetic waves impinge upon the desired reflectors for directing the waves of the respective polarisations in the desired directions. 
     In addition, the assembly of reflectors must be configured in a fashion such that the presence of one reflector does not interfere with the propagation of electromagnetic energy between a second reflector and the source associated therewith. 
     A problem arises in that constructional methods presently available for a composite antenna structure having plural reflectors entail a greater weight for the support structure than is desirable. 
     In brief, the prior art presents a central zone with two grids to reflect two orthogonal polarisations on different frequency, respectively, for the some coverage zone, which presents both grating and side lobes arising from the reflection of the orthogonal polarisations on the double grid. Likewise, a reflection zone formed by a single grid also favours the formation of side lobes that can have a harmful effect on the rest of the satellite useful payload. 
     There is therefore a need to develop a dual polorisation antenna with reduced grating and side lobes. The dual polarisation antenna has a central reflection zone for reflecting two orthogonal polarisations on different frequency and polarisation, respectively, for the same coverage zone, and a peripheral zone that reflects one polarisation and is transparent to its orthogonal polarisation. The antenna must be mounted on a single structure in order to reduce the overall weight of the assembly. 
     CHARACTERISATION OF THE INVENTION 
     To overcome the disadvantages mentioned above, the present invention provides a dual polarisation antenna with low image lobes and grating lobes, which is capable of being used in radiocommunication system. 
     The antenna mentioned comprises a reflector assembly illuminated by a feed source of the horn type, for example. The reflector assembly comprises a front reflector that is adopted for reflecting two orthogonally polarised electromagnetic waves and each one on a different frequency and polarisation; an auxiliary reflector that is adopted for reflecting one of the arthogonally polarised electromagnetic waves; and a deflecting surface that is adapted for diffracting the orthogonally polarised electromagnetic wave that passes through the auxiliary reflector. 
     The front reflector is a continuous metallic surface deposited on a supporting surface, which is capable of reflecting two orthogonal polarisations without permitting the formation of side lobes that could produce interference on the rest of the satellite useful payload and without dissipating thermal energy in the reflector. RF losses are reduced at the front reflector due to that this reflector is continuous. 
     The auxiliary reflector is formed by a set of uniformly spaced metallic wires, positioned to form a ring around the frontal reflector, with the purpose of permitting the reflection of the polarisation parallel to the wires, and filtering the wave polarised orthogonally thereto. 
     The electromagnetic wave cited is deflected by the associated deflecting surface, which deflects totally the orthogonal polarisation that traverses the auxiliary reflector into free space, permitting the formation of side lobes to be minimised. 
     The antenna is equipped with a single grid that simplifies its construction. The thermo-elastic behaviour of the reflector assembly is more appropriate as it admits strengthening at will. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more detailed explanation of the invention is given in the following description based on the attached drawings in which: 
     FIG. 1 shows a sectional view taken along the line AA′ in FIG. 2 of an antenna according to the invention, 
     FIG. 2 shows a plan view of the antenna according to the invention, 
     FIG. 3 shows a detail of the support assembly according to the invention, 
     FIG. 4 shows a vertical section of a second embodiment of the antenna according to the invention, and 
     FIG. 5 shows a vertical section of a third embodiment of the antenna according to the invention. 
    
    
     DESCRIPTION OF THE INVENTION 
     The antenna with dual polorisation of the invention shown in the FIG. 1 comprises a reflector assembly  12  that is illuminated by a feed horn  11  of two orthogonally polarised electromagnetic waves, that is, one wave polarised along a vertical axis and the other polarised along a horizontal axis. 
     The reflector assembly  12  may be mounted, for example, on a structure  20 , which can be a satellite or earth station adapted for use in a radiocommunication system. 
     The feed horn  11  is situated opposite the lowest part (focus) of the reflector assembly  12  by means of a bracket arrangement (not shown in FIG.  1 ). The feed horn  11  is separated by a determined distance from the supporting surface  14 . The location of the feed horn  11  permits two faces to be distinguished in the reflector assembly  12 . Thus, one face, concave, is that opposite the feed horn  11 , and the other face, convex, is opposite the first. 
     The reflector assembly  12  has a supporting surface  14 , generally in the form of a parabolic, against which rests a front reflector  15 , specifically on its concave face. The front reflector  15  has the mission of reflecting two orthogonally polarised electromagnetic waves and each on a different frequency, in such a manner that it is possible to cover a some geographical area on the surface of the earth&#39;s globe. 
     From the aforementioned, it can be deduced that in an embodiment the supporting surface  14  has a certain degree of curvature, for example it is parabolic in shape. In other embodiment, the supporting surface  14  is a flat disk, for example. 
     A central, circular region of the supporting surface  14  is imprinted with a continuous and conductive surface such as a metallic deposit. This region forms the front reflector  15 , which is centered on the focus of the parabola. This surface  15  offers the advantage that it reflects perfectly the two orthogonally polarised waves. Therefore, the front reflector  15  prevents that image lobes will be generated and thermal dissipation is reduced. Likewise, RF parasitic radiation is reduced, which could interfere with the rest of the satellite useful payload. 
     Around the periphery of the front reflector  15  there is an auxiliary reflector  16  in the form of a concentric annulus abutting on the front reflector  15 . The auxiliary reflector  16  is formed by a plurality of wires aligned in parallel forming a single grid reflector. The spacing between wires is chosen with the object of reflecting one of the two orthogonal polarisations, namely, the main polarisation and, therefore, is transparent to its orthogonal polarisation. 
     It is desired to make the supporting surface  14  as thin as possible, consistent with sufficient rigidity for maintaining dimensional stability of the reflector assembly  12 . In FIG. 2 it can be seen that the auxiliary reflector  16  is a surface concentric with the front reflector  15 . The single grid reflector  16  rests on the concave face of the supporting surface  14 , precisely on the zone free of front reflector  15 . 
     The alignment of the grid ensures the filtering of one of the two orthogonal polarisations and the reflection of the pertinent desired orthogonal polorisation. 
     Returning now to FIG. 1, the auxiliary reflector  16  has an associated deflecting surface  13  that minimises the formation of side lobes, which are associated with the configuration of the grid  16 . The deflecting surface  13  can present different shapes to improve the diffraction: for example embossed MLI may be used. 
     FIG. 3 shows in detail how the deflecting surface  13  is fixed to the supporting surface  14 , with the aim that the deflecting surface  13  is firmly fastened to the antenna. Thus, the deflecting surface  13  is fastened by the convex face to the supporting surface  14 , underneath the separation edge that is formed by the front reflector  15  and the auxiliary reflector  16 . 
     The disposition of the deflecting surface  13  is such that it is at an angle with respect to an axis that passes through the feed horn  11  and the focus of the supporting surface  14 , with the object of making possible the dissipation of thermal energy into free space, since it reflects outwards the filtered orthogonal polarisation wave, that is, the undesired polarisation. Thus, the supporting surface  14  is transparent to the orthogonal polarisation that is deflected by means of the deflecting surface  13 . 
     The deflecting surface  13  is contiguous with the joining edge of the front reflector  15  with the auxiliary reflector  16 . Therefore the deflecting surface  13  is collocated at the rear side of the supporting surface  14  and, likewise, it is a continuous surface; that is, it is not a grid. 
     Returning to FIG. 3, it shows a detail of the mounting of the reflector assembly  12 , specifically, the zone in which the front reflector  15  joins with the auxiliary reflector  16  and the deflecting surface  13 . 
     Returning to FIG. 1, in the case where the reflector assembly  12  of the invention is situated on board a satellite, said assembly is protected against heat effects by a first thermal control means  17 , that is, a heat shield  17  that envelopes the supporting assembly  12 . 
     Another embodiment of the invention is shown in FIG. 4, in which the reflector assembly  12  comprises a first mechanical supporting assembly  41 , having the particular task of ensuring the stability of the reflector assembly  12 . 
     The first mechanical supporting assembly  41  is joined to the convex face of the supporting surface  14 . Likewise, the first mechanical supporting assembly  41  is enveloped by a second thermal control means  42  that provides the first mechanical supporting assembly  41  and the convex face of the supporting surface  14  with a heat shield. 
     Likewise, FIG. 5 shows a further embodiment of the invention. In this case the reflector assembly  12  comprises a second mechanical supporting assembly  51 , having also the particular task of ensuring the stability of the reflector assembly  12 . 
     The second mechanical supporting assembly  51  is also joined to the convex face of the supporting surface  14  and, in like manner, a third thermal control means  52  provides the heat shielding for the convex face of the reflector assembly  12 . 
     The above mentioned applies to centered antenna design, feed at center of supporting surface  14 , but also to offset design, in which the horn  11  is offset from the reflector assembly  12  and does not mask the wave. It is well known in the art. 
     The two waves could be of very close frequencies. In the latter case there is a dual polarisation antenna with the same advantages. The only difference is that it would not benefit from the some coverage for both polarisations. 
     In other embodiment, the feed horn  11  can include several independent horns. So, the horn set generates multibeam coverages, implying several independent feeds in the focal plane, instead of just single feed. It also applies to more complex feeds, for example BFNs (beam forming networks) instead of a single feed. 
     The design of the outer part of the reflector assembly  12  could be of dichroic type (frequency filter). Such design has an additional advantage, that the two waves would not be necessarily orthogonal. They can be of the same polarisation if requested by the system design. 
     It is to be understood that the above-described embodiments of the invention ore illustrative only, and that modifications thereof may occur to those skilled in the art. Accordingly, this invention is not to be regarded as limited to the embodiments disclosed herein, but is to be limited only as defined by the appended claims.