GB2262387A - Multibeam antenna - Google Patents

Abstract

A multibeam antenna having shaped reflector(s) (11) shaped so as to optimize antenna gain at the intersect points between beams and/or over the composite coverage, each distinct beam being generated by a distinct source block (12, 13). Grids of mutually parallel wires may be used to form two distinct shaped reflective surfaces (14, 15) for different linear polarizations. <IMAGE>

Description

A MULTIBEAM PASSIVE ANTENNA WITH SHAPED REFLECTOR(S) The invention relates to a multibeam passive antenna with shaped reflector(s).

BACKGROUND OF THE INVENTION Known multibeam passive antennas are: either conventional focusing systems constituted by one or more reflectors illuminated by as many distinct source blocks as there are distinct beams. The drawback is then that since the spacing between the beams is directly governed by the spacing between the sources and by the equivalent focal length of the focusing system, the gain at the point where adjacent beams intersect is mediocre (typically 3 dB down on the optimum gain); either because the beams are spaced too far apart so the level where the beams intersect is very low (typically 8 dB to 12 dB below the maximum gain of the beam), which means that gain cannot be optimized at that point;; or because the aperture of each of the beam generating sources is limited by the spacing between the beams so that a large fraction of the energy radiated by the sources is lost by spilling past the reflector system, thereby leading to poor antenna efficiency and thus to low gain at the intersect point; or else they are "orthogonal beam" focusing systems constituted by one or more reflectors illuminated by a multiple source made up of radiating elements shared between adjacent beams. The gain at the point where beams intersect can then be optimized by overdimensioning the diameter of the main reflector so as to obtain proper sampling of the coverage by all of the radiating elements and by feeding each element of the multiple source by means of an energy distributor having orthogonal accesses.In addition to the need for overdimensioning the reflector, major drawbacks then reside: in the complexity and the weight of the multiple source which includes a very large number of radiating elements, far more than the number of beams; and in the complexity of the orthogonal beam formatter or energy distributor. In addition, the need to use said beam formatter gives rise to additional resistive losses that are troublesome when optimizing gain over the coverage.

An object of the invention is to optimize antenna gain at the intersect point between adjacent beams, and over the composite coverage formed by the set of beams.

SUMMARY OF THE INVENTION To this end, the present invention provides a multibeam passive antenna having shaped reflector(s), wherein the reflector(s) of the antenna are shaped so as to optimize antenna gain at the intersect points between beams or over the composite coverage, each distinct beam being generated by a distinct source block.

Advantageously, such an antenna makes it possible to optimize the gain at the intersect point between adjacent beams without requiring sources to be shared between beams and without the need for a beam formatter having orthogonal accesses, since a distinct source block is allocated to each beam. The source block thus remains extremely simple and light, for optimum gain of the antenna over the entire composite coverage. It is nevertheless necessary to overdimension the reflector.

In an advantageous embodiment, each reflector is constituted by two distinct shaped surfaces; each of the two surfaces being provided with grids of mutually parallel wires.

The advantage of the invention thus lies in the shaping of the reflector(s) of the antenna, said shaping making it possible to associate a distinct beam with each distinct source block, each beam having optimum gain without there being any reutilization or sharing of sources between two adjacent beams.

The invention thus makes it possible to achieve high gain multibeam coverage while using only one reflector system common to all of the beams, each beam being generated by a single source block or source that is dedicated solely to the beam.

BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention are described by way of example with reference to the accompanying drawings, in which: Figures 1 to 4 illustrate a first embodiment of an antenna of the invention; and Figures 5 to 8 illustrate a second embodiment of an antenna of the invention.

DETAILED DESCRIPTION The invention relates to an antenna having shaped reflector(s) enabling multibeam coverage to be generated, each beam being similar in shape and having the special feature of presenting optimum gain at the point where adjacent beams intersect, corresponding to a high intersect level; the intersect level being about 3 dB to about 5 dB below the maximum gain of each beam. Each beam is generated from a source block which is specific thereto, each source block being capable of being a single source such as an open waveguide, a horn, a helix, a printed circuit element, a dipole, etc.

or else a collection of such sources.

The reflector system may: either be a single reflector having a centered or an offset shape for use with linear or circular polarization; or it may be a two-grid reflector having a centered or an offset shape for use with double linear focusing. Under such circumstances, the surfaces of the grids may be shaped differently or the same depending on the nature of the beams, with the grids making it possible to separate orthogonally polarized signals onto two distinct sets of primary sources and to improve the cross polarization characteristics. Each of the two sets of sources provides multibeam operation; or else it may be a double reflector system of centered or offset shape derived from a Cassegrain type or a Gregorian type optical system, with the reflectors being shaped as plane shells or as grids.Although more bulky and more complex, this system makes it possible to reduce losses and deformation in beams that are distant from the center of coverage due to the beams being off-boresight.

In each case, the main reflector and the auxiliary reflector (if there is one) are shaped so as to optimize gain at the edge of the coverage of each beam that is to be produced or so as to optimize the gain at an arbitrary point in the composite coverage illuminated by the set of beams. The parameters enabling gain to be optimized at the edge of the coverage of each beam are the following: the diameter of the reflector; the sizes of the sources; and the shape of the surface of the reflector(s), said shape being modified and optimized by software relative to the original optical shape for focusing such as a one- or two-grid parabolic reflector or a Cassegrain or a Gregorian optical system Figure 1 shows a satellite 10 having a multibeam antenna of the invention for operating with two linear polarizations using band C.The antenna is constituted by a two-grid reflector 11 illuminated by two sets of primary sources 12 and 13. Each shell 14 and 15 of the reflector 11 is shaped to optimize the gain of the antenna at the point where two adjacent beams intersect. The use of a two-grid reflector 11 makes it possible to separate signals that are polarized in directions or or oJ onto the sources 12 and 13. The shell 14 has a grid of printed wires 16 that are parallel to the vector ox (for an observer at infinity) and it focuses waves that are linearly polarized in the ox direction onto primary source set 12. The shell 15 may also be provided with a grid of printed wires 17, but they are parallel to the vector oy, and it focuses waves that are linearly polarized parallel to the vector oy (orthogonal to the vector ox) onto primary source set 13.

This two-grid system thus separates waves onto two distinct primary source sets. The first set receives and/or transmits waves that are linearly polarized in the direction of the front grid, while the other set receives and/or transmits waves that are linearly polarized orthogonally to the front grid. Each of the primary source sets is made up of as many distinct source blocks as there are distinct beams providing multibeam operation in each of the two linear polarizations under consideration. Thus, in the example described, each of the primary source sets 12 and 13 is constituted by distinct pyramid-shaped horns belonging to distinct beams. The primary source sets 12 and 13 may be constituted respectively by horns 20 and 21 and by horns 22, 23, and 24 as shown in Figures 2 and 3 which are views perpendicular to the respective focal planes of the shells 14 and 15.

Such horns 20 and 21 generate the beams A and B of Figure 4 by reflection on the shaped grid 16. The beams C, D, and E of Figure 4 are generated by the horns 22, 23, and 24 of Figure 3 after reflection on the rear shaped shell 15 of the two-grid reflector 11.

The calculated minimum gain over the entire area to be covered by the five beams is 26 dB, losses included, which corresponds to an optimum for beams of this size.

For example, the shells 14 and 15 may be 2100 mm in diameter. The horns 20, 21, 22, 23, and 24 may, for example, have the following respective dimensions in the planes of Figures 2 and 3: 120 mm by 160 mm; 120 mm by 160 mm; 100 mm by 170 mm; 110 mm by 120 mm; and 120 mm by 180 mm.

Figure 5 shows a multibeam antenna having a shaped reflector 25 for use with two circular polarizations and in band C. The antenna comprises a solid shell reflector 26 which is shaped relative to the original parabolic surface 27 and a focus F (as shown in Figure 6), in such a manner as to optimize gain per beam over the entire coverage area. The reflector system is illuminated by a set of sources 28 constituted, for example, by five independent horns 30, 31, 32, 33, and 34 of identical apertures that may be disposed as shown in Figure 7, which is a view perpendicular to the focal plane. After wave transformation on the shaped reflector, this generates five adjacent beams X, Y, Z, T, and U as shown in Figure 8.Both directions of circular polarization (right and left) can be used when the horns are themselves fitted with twin polarization systems as is well known to the person skilled in the art.

The gain minimum calculated over the entire five-beam composite coverage area is likewise 26 dB when using horns having an aperture diameter equal to about 120 mm.

Figures 4 and 8 are respective radiation diagrams for the two above-described antennas for providing multibeam coverage of Africa.

Naturally, the present invention has been described and shown only by way of preferred example and the coverage, the disposition, the shape and number of beams, the number and type of radiating element, and the geometrical characteristics of the reflectors can all be modified without going beyond the ambit of the invention. In particular, the number of beams generated by the antenna although limited to five in the abovedescribed examples, could naturally be any number. Similarly, the antenna may be used in any other frequency band.

Similarly, arbitrary coverage can be achieved.

Claims (1)

1/ A multibeam passive antenna having shaped reflector(s), wherein the reflector(s) of the antenna are shaped so as to optimize antenna gain at the intersect points between beams and/or over the composite coverage, each distinct beam being generated by a distinct source block.

2/ An antenna according to claim 1, wherein each reflector is constituted by two distinct shaped surfaces; each of the two surfaces being provided with grids of mutually parallel wires.

3/ An antenna according to claim 2, wherein the front and rear surfaces of the two-grid reflector system are shaped differently to produce beams that are differently shaped or differently organized depending on the linear polarization under consideration.

4/ An antenna according to any preceding claim, wherein the beam producing radiating elements are aimed in non-parallel directions in front of the reflector(s) and have the centers of their apertures placed in a non-plane surface so as to optimize the gain at the edges of the beams as well as possible.

5/ An antenna according to any preceding claim, wherein the shaped reflector(s) is/are associated with a multiple source having orthogonal access so as to reduce the number of radiating elements in the multiple source, and thereby simplify the distributor for formatting the orthogonal beams.

6/ A multibeam passive antenna substantially as herein described with reference to and as illustrated in the accompanying drawings.