Antenna with single or double reflector, with shaped beams and linear polarisation

Abstract: Shaped beam antenna, with single or double reflector, gridded or non-gridded, with shaped beams, which can rotate the polarisation, independent of the polarisation of the feed cluster, for use preferably aboard satellites. Field of application: satellite telecommunications and technical field: micro-wave antennas. The antenna basically consists of (Fig. 1): a reflector (1), one or more polarisers (2), (2a), one or more feed clusters (3), (3a). microwave circuits (4), (4a) to set-up the BFN, input port (5), (5a), connections (C) and (C1) which to obtain the antenna configuration desired should be configured as follows: mono- or multimode BFN; rectangular feed elements (6) oriented according to the coverage required; feed elements excited by the fundamental mode plus some higher modes; polarisation rotator with three or more grids; parabolic or slightly shaped reflector.

Description: PRESENTATION The invention relates to an antenna with multiple shaped beams, with single or double reflector, gridded or not gridded, with one or more separated arrays of multimode (TE mo , m=1,2...5) rectangular apertures fed by mono- or multimode reconfigurable Beam Forming Networks (BFN). It can be applied in the field of telecommunication via satellite and in the scientific field of telecommunication, more specifically in that of microwave antennas. The most significant aspect of the invention is that in order to obtain a good shape of the antenna beams, the feed system is composed by a set of rectangular apertures having the electrical field E on their mouths parallel to the principal direction of the array alignment (see Fig.2), having each rectangular aperture shaped in the most appropriate manner and having each rectangular aperture excited by a particular series of TE mo modes (where typically m=1,2,...5) to achieve the most appropriate field distribution on the antenna focal plane. ADVANTAGES The aim of our invention is to improve the performance provided by other antenna configurations already known: multifeed parabolic reflector antennas using an array of identical radiating elements and shaped reflector antennas fed by a single feed. STATE OF ART The first contoured beam antenna solutions were parabolic reflector antennas fed by an array of identical, rectangular or circular, feeds. For convenience we will call this class of antennas as family a). In these antennas, which can use mono- or multimode BFNs, the field distribution on the reflector focal plane tends to resemble the image of the desired shaped beam. Reference should be made to the following patents of the same Patentee: EP 0 727 839 filed on 07/01/1994 and published on 21/08/1996 with the title " Multishaped beam direct radiating array antenna", and EP 0 683 541 filed on 17/05/1994 and published on 22/11/1995 with the title "Shaped-beam or scanned beams reflector or lens antenna". Another family, called for convenience family b), consists of antennas with (highly) shaped reflectors fed by one or few feed elements. In these antennas the field distribution on the focal plane does not resemble the image of the desired shaped beams. The reflector shaping introduces a phase modulation of the field distribution on the antenna aperture that allows the beam contouring. Additional feeds can be present to radiate more than one beam, but it must be observed that the reflector shaping can be made freely only for one shaped beam. Family a), which also includes the antenna system described on U.S. 4.090.203, with a feed array of identical radiating elements excited only by the fundamental mode, was the first contoured beam antenna solution and was used for about ten years. Because of its complexity due to the large number of array elements and because of its low focal-plane aperture efficiency (presence of grating lobes) family a) was slowly replaced by family b). This latter family achieves better gain performance with the same diameter of the main reflector and with a simpler feed system. On the other hand family b) has some disadvantages, some of which are listed below: presence of spurious radiation due to the phase variation of the antenna aperture field introduced by the shaped reflector; difficulty in reconfiguring the antenna beam, i.e., changing the beam shape on command; difficulty in obtaining simultaneous multiple beams. THE INVENTION The antenna presented here arose from the need to obtain antenna gain values better to those obtained by using antennas of these two families maintaining the above said reconfiguration capability, multiple shaped-beams generation and a limited complexity of the feed system. This better performance are a consequence of the possibility to shape the field distribution on the focal plane as an almost perfect image of the desired shaped-beams, so avoiding the grating lobes loss presents in family a) antennas and without the inconvenience of spurious radiation presents in family b) antennas. The antenna, whose invention is being filed, consists of the following essential elements (in dual-gridded reflector systems some items result doubled): mono/multimode reconfigurable BFN (see Fig.3); array of rectangular apertures radiating a field with a polarisation parallel to the principal direction of array alignment (with no regard to the desired antenna polarisation), the geometry arranged in the most appropriate manner (see Fig.2) and the aperture excitations composed by a particular series of TE mo modes (where m=1,2,...5) to achieve the most appropriate field distribution on the antenna focal plane; parabolic or slightly shaped main reflector, gridded or not-gridded, an ellipsoid or hyperbolic subreflector not-shaped or slightly shaped, gridded or not-gridded; a polarisation rotator to transform the feed array linear polarisation into a different linear polarisation. EMBODIMENT The invention is now described, by way of illustration and not limitation, considering a simple application consisting of a single reflector antenna. Figure I - Antenna layout. Figure 2 - Array of rectangular apertures. Figure 3 - Layout of a typical reconfigurable multimode BFN. The antenna comprises according to Fig.1 a reflector 1; a polarisation rotator 2; a feed cluster 3; a reconfigurable mono- multimode BFN 4; input ports 5; connections C. The feed cluster according to Fig. 2 consists of an array of rectangular multimode apertures 6 excited by a series of TE mo modes (m=1,2...5) having the electric field E parallel to the principal direction of the array alignment. The excitations of these series of TE mo modes inside the feeds (or horns) can be achieved in different ways. According to Fig. 3, the reconfigurable mono- multimode BFN 4 comprises Hybrid Dividers (7, HD ); fixed or variable Phase Shifters (8, PS ); fixed or variable Power Dividers (9, VPD ) and Switches (10, S ). The presence of variable power dividers, variable phase shifter and switches allows the reconfigurability of the radiated shaped beams.

Claims: Antenna with multiple shaped beams, comprising single or double reflector, with main and subreflector and of one or more separated feed arrays whereby the main reflector can be parabolic or slightly shaped, gridded or not-gridded, and the subreflector can be ellipsoid or hyperbolic not-shaped or slightly shaped, gridded or not-gridded; characterised by the fact that the feed array (3 - Fig.1) consists of a cluster of multimode rectangular apertures (6-Fig. 2) generating a field distribution on the antenna focal plane; the rectangular apertures (6 - Fig. 2), are aligned according to the direction of the aperture electric field ( E - Fig. 2), each rectangular aperture is shaped in the most appropriate manner in accordance with the orientation of the coverage and is excited by a particular series of TE mo modes (m = 1,2...5) to achieve the optimum field distribution on the antenna focal plane; a polarisation rotator (2 - Fig. 1) in front of the feed array (3 - Fig. 1) allows to align the polarisation of the electric field independently of the alignment of the feed array (3 - Fig. 1) and each feed array (3 - Fig. 1) is connected to a mono- or multimode reconfigurable BFN (4 - Fig. 1) whose reconfigurability is achieved by means of hybrid dividers (7), fixed or variable power dividers (9), fixed or variable phase shifters (8) and switch (10).

Inventors: CAPECE PASQUALE, LO FORTI RAIMONDO, RUGGERINI GIANFRANCO, CAPECE, PASQUALE, LO FORTI, RAIMONDO, RUGGERINI, GIANFRANCO
Applicants: FINMECCANICA SPA, SPACE ENGINEERING SPA, FINMECCANICA S.P.A., SPACE ENGINEERING S.P.A.
Application year: 1997
Main classification: H01Q19
IPC codes: H01Q21, H01Q19, H01Q15