RECONFIGURABLE, ZOOMABLE, TURNABLE, ELLIPTICAL-BEAM ANTENNA

Abstract: The present invention is a double-reflector microwave antenna (fig. 1), which can be classified as belonging to the Gregorian optics family, capable to provide an elliptical beam with a major axis which can be oriented in any required direction of space (fig. 2), through the simple rotation of sub-reflector (2) around axis (4 of fig. 1). By adding a further degree of freedom, that is, an independent translation of the same sub-reflector (2) and/or of main reflector (3) along axes (5) and/or (6), it is possible to obtain a considerable degree of reconfigurability of the shape of the beam. In particular, it is possible to widen the axes of the beam, to vary the ratio between the axes of the elliptical beam for any orientation of the major axis, or to obtain a circular beam (figs. 3a and 3b). These characteristics are such as to make the present invention suitable for use as antenna on board satellites with frequency re-use in an operational environment with one or more simultaneously active beams. The invention may be classified as belonging to the technical field of microwave antennas and to the application field of reconfigurable antennas for use on board satellites or space stations or in ground-based radar systems.

Description: / Description econfigurable. zoomable r turnable. elliptical-beam antenna The invention described here is a double-reflector microwave antenna (which can be classified as belonging to the family of Gregorian antennas) which, through the rotation of its sub-reflector and/or through the axial movement of this sub-reflector or of the main reflector, achieves the rotation of an elliptical beam (Fig. 2) without any variation to the beamwidth and polarisation and/or the re-configurability of this same beam into a circular, expanded ellipse (zoom effect) or intermediate ellipse between the original beam and the circular (variation of the beam shape, figures 3a, 3b). Moreover, through another sub-reflector profile, it is also possible to achieve the widening (zoom) of the circular beam into another circular beam (Fig. 3c). The invention belongs to the technical field of microwave antennas and to the application field of reconfigurable antennas for use on artificial satellites or space stations or in ground radar systems. The moderate reconfigurability requirements of future antenna systems are: (a) beam re-pointing; (b) turnable elliptical beam without rotation of polarisation; (c) circular or elliptical beam with zoom possibility, that is, broadening of the coverage with no variation to the ratio between the ellipse axes and of the area x gain product; (d) possibility to convert a circular beam into an elliptical one (and vice versa) with no variation to the area x gain product. Of these only function (a) is normally available for Ku band communication satellite antennas. The other functions are highly desirable, together with the first of them, and in combinations set, in principle, only by the capacity of the type of antenna considered not to degrade the quality of service as a conse¬ quence of the greater flexibility so assured. The practical implementation of functions (b), (c) .and (d), for what we have said above, should meet the following requirements: (I) minimum increase of the dimensions and mass of the antenna; (II) absence of movements of large masses; (III) absence of movement of the illuminators (inadvisable in the presence of high power levels); (IV) no movement of parts internal to the illuminators (this is not advisable as it can potentially generate intermodulation products); (V) maximum reliability and simplicity, minimum number of actuators; (VI) minimum sensitivity to alignment errors and thermal excursions. The requirements indicated above lead to research solutions which can achieve the reconfigurability functions by acting upon system optics and, as far as practicable, by trying to avoid movements of the illumination system or of large masses. The solution herein proposed meets these requirements. It consists of an antenna configuration (Fig. 1) which can implement a turnable elliptical beam with constant beam width or with variable contour, with electri¬ cal radiating characteristics typical of the double offset reflector of the Gregorian type. These latter can be defined as high antenna lobe efficiency, low cross-polarisation values and sidelobes. These characteristics are essential requirements for use as on-board communi¬ cation satellite antennas with dual polarisation capability in an operational environment which has more than one simultaneously active beams. The innovative aspects of the invention described here, compared to the class of Gregorian antennas already known, are the following: (a) Movements to achieve reconfigurability (rotation of the sub-reflector; translation of the main reflector and/or of the sub-reflector itself) have never been suggested or implemented in the past. This is because classi- cal Gregorian optics do not allow, for instance, for rotation of the sub-reflector. (b) The profiles of the surfaces and the method through which such surfaces are shaped allow the rotation of the beam keeping practically constant the electrical radiating characteristics of the co-polar and cross-polar compo- nents through a simple rotation of the sub-reflector. It is also to be noted that the orientation of the electrical field is unchanged during rotation. This is an essential feature for performance of the antenna within an operational environment which includes a number of simultaneous beams, (c) A further original feature of this invention is in its capability to combine the rotation of the sub-reflector with a further additional motion (transla¬ tion of the sub-reflector and/or of the main reflector) along pre-deter- mined axes, which provides a remarkable re-configuring capability of the starting elliptical beam, for any desired orientation of the beam itself, with an efficiency, polarisation purity and sidelobes comparable with those of a fixed beam Gregorian type. In particular, with this movement it is possible to progressively vary the ratio of the main axes of the ellip- tical beam for any orientation of the axes or to obtain an elliptical coverage shaped gradually into that of a circular beam. Antennas capable of electrical performance adequate for the present require¬ ments for satellite telecommunications belong to the group of double reflector Gregorian optics. These optics provide high coverage efficiency, low sidelobes and when some geometric relations are met, very high polarisation purity with size and mass compatible with their installation on board satellites (antennas of these types are in fact to be fitted on board the Intelsat Vm satellites). As is obvious from Fig. 1, the geometry proposed belongs to the Gregorian optics family, shown in Fig. 4. These optics are composed by the same ele- ments which make up the antenna proposed here (except for the movements and the surface profiles) such as a main reflector 3, a sub-reflector 2 and a suitable feed l. The design of classical Gregorian antennas usually starts from the canonical surfaces (with reference to Fig. 4, sub-reflector 2 is ellipsoidal and reflector 3 is parabolic). These surfaces provide extremely low cross-polarisation levels when the geometric requirement for maximum purity shown in Fig. 4 is met. This condition is met when eccentricity e of the ellipsoid of sub-reflector 2 satisfies the geometric relation of angles * and β shown in Fig. 4. In this figure, β is the angle between the symmetry axis 9 of the illuminator 1, whose phase centre is in point 7 which coincides with one of the foci of the ellipsoidal sub-reflector 2 and propagation axis Z. Angle β is the angle between such axis 9 and axis 10 which crosses both foci of the ellipsoid. It is to be noted that sub-reflector 2 of Fig. 4 is an ellipsoid obtained by revolution around axis 10, while the optics of this invention (Fig. 1) has a sub-reflector surface which cannot be obtained by revolution around the axis crossing points 7 and 8. The optical system so obtained can generate a circular beam. The procedure which is commonly adopted starting from the standard optics of Fig. 4, when an elliptical beam contour is required, consists in shaping the sub-reflector surface 2 and/or the main reflector 3 numerically and to accept the electrical degradations in terms of polarisation purity which derive from this upset system. These degradations are normally acceptable as the deviations introduced onto the surfaces are small. Clearly the optical system thus generated cannot provide a rotation of the elliptical beam by turning the sub-reflector. There .are not in fact to-date solutions which allow for the reconfiguration of the beam in terms of rotation and/or reconfiguration and/or widening (zoom) of the contour of the beam on such type of antennas, by using a single feed. It is a fact that the only function which is today available on these antennas is beam re-pointing, a function which is normally performed through a system of biaxial actuators within a cone of ±. 11 degrees (useful field of view of the Earth from a geostationary orbiting satellite). The systems normally adopted to obtain shaped antenna beams, when a recon¬ figurable contour and high polarisation purity are required, belong to another class and type of antennas. In particular, two feedarrays are used, located in the focal plane of an optical system of dual-gridded reflector type. Such systems have a rear reflector and a front reflector. The front reflector is realised by application of linear metal strips onto the dielectric surface of the front shell as shown as an example in Fig. 27. The back reflector can be either solid or gridded with strips orthogonal to those of the front reflector. In particular, figures 27a (front view), 27b (top view) and 27c (side view) provide the three main elevations of this optical system. In these figures, a schematic outline of the group of feeds 1 for the polarisation of the electrical field along axis X is provided together with the corresponding group of illuminators 1' for the polarisation along Y. The gridded front reflec¬ tor 3 is sensitive to polarisation X and the rear one 3' (solid or grid) is sensitive to polarisation Y. The characteristics of this optical system are such that each reflector operates in single polarisation and benefits of the space filtering effect of the other reflector on the cross-polarisation components which would otherwise be radiated over the service coverage. The radiating elements are normally excited by a beamforming network which contains microwave components capable of changing the excitation of the radiating elements placed in the focal plane through power dividers and/or phase shifters. As already noted, the technique described above, based on reconfigurable feedarrays, belongs to another class of antenna families which are not of interest here as what we are concerned with are reconfigurable single feed antennas, extremely simple and lightweight, which can exploit the optics degree of freedom to achieve better electrical performance as compared with multifeed antennas having the same main reflector aperture. The invention will now be described with reference to its presently preferred form of implementation, which is provided as an illustration only, without any limitation and with reference to the drawings provided: Figure 1: Geometry of the optical system proposed. It includes the following elements: 1 Feed; 2 Sub-reflector; 3 Main reflector; 4 Axis A-A around which to rotate the sub-reflector to rotate the elliptical beam; 5 Axis B-B along which to translate the sub-reflector for beam recon¬ figuration; 6 Axis C-C along which to translate main reflector for reconfiguration of the beam (as an alternative to translation along axis B-B); 7 Geometric point corresponding to the phase centre of feed 1 ; 8 Focus of main reflector 3; 9 Symmetry axis of feed 1; 20 Caustic point or pseudo focus into which the rays from feed 1 converge following reflection by sub-reflector 2 (such locus coincides with focus 8 of main reflector 3). Figure 2a: Cartesian axis tern which shows the angle co-ordinates (Azi¬ muth =Az, Elevation =E1) of a generic observation direction. Figure 2b: Schematic outline of the elliptical beam and of its possible orienta¬ tions on the Azimuth-Elevation plane. Figure 3a: Schematic reconfiguration of the elliptical beam into circular beam and vice versa. Figure 3b: Schematic outline of the zoom effect on the elliptical beam. Figure 3c: Schematic outline of the zoom effect on a circular beam. Figure 4: Classical geometry of a Gregorian optical system which highlights the conditions for maximum polarisation purity. It includes: 1 Feed; 2 Sub-reflector; 3 Main reflector; 7 Geometric point corresponding to the phase centre of feed 1 ; 8 Focus of the main reflector; 9 Symmetry axis of illuminator 1; 10 Rotation axis of symmetry of the sub-reflector surface (ellipsoid); 21 First focus of the sub-reflector which coincided with the focus of the main reflector; 22 Second focus of the sub-reflector. Figure 5: Detail of the geometry of the proposed optical system which shows the starting geometry of the sub-reflector and the maximum polarisation purity condition on the antenna symmetry plane. It includes: 1 Feed; 2 Sub-reflector with spherical profile; 3 Parabolic main reflector; 4 Sub-reflector rotation axis; 7 Phase centre of feed 1 ; 8 Focus of main reflector 3; 9 Symmetry axis of feed 1; 11 Centre of the sphere to which the spherical sub-reflector belongs; 12 Intersection of symmetry axis 9 of feed 1 with surface of sub-reflector 2; 13 Geometric extension of the spherical surface of the sub-reflector; 14 Direction of a ray from infinite along direction (-Z) after reflection on the geometric extension of the sphere in point 16; 15 Axis which describes the perpendicular to the sphere in point 16; 16 Point of intersection of axis Z with the geometric extension of spherical surface 13; 20 Caustic locus or pseudo focus into which the rays from feed 1 converge following reflection by sub-reflector 2 (such locus coincides with focus 8 of main reflector 3). Figure 6: Detail of the final geometry of the proposed optical system which highlights the main planes of symmetry of the sub-reflector. It includes: 1 Feed; 2 Shaped sub-reflector; 3 Shaped main reflector; 4 Rotation axis of the sub-reflector; 5 Translation axis of the sub-reflector; 6 Translation axis of the main reflector 17 First symmetry plane of the shaped sub-reflector; 18 Second symmetry plane (perpendicular to the first) of the shaped sub- reflector. Figure 7a: Spherical profile sub-reflector. It includes: 1 Feed; 2 Spherical sub-reflector; 7 Phase centre of feed 1 ; 8 Focus of main reflector; 11 Centre of the sphere to which the sub-reflector belongs; 13 Geometric extension of the spherical sub-reflector; 20 Point into which the feed rays converge following reflection by the sub-reflector (it coincides with the focus of the main reflector). Figure 7b: Shaped sub-reflector with elliptical profile with curvature on the antenna symmetry plane greater than the initial spherical sub-reflector. It includes: 1 Feed; 2 Sub-reflector with elliptical profile; 7 Phase centre of feed 1; 8 Focus of main reflector; 11 Centre of sphere 13 ; 13 Original spherical profile of the sub-reflector; 20 Point onto which the rays of feed 1 converge following reflection by sub-reflector 2; 21 First focus of the sub-reflector with elliptical profile; 22 Second focus of the sub-reflector with elliptical profile; 23 Geometric extension of the elliptical sub-reflector. Figure 7c: Shaped sub-reflector with elliptical profile with curvature (in the antenna symmetry plane) smaller than that of the initial spherical sub-reflector. It includes: 1 Feed; 2 Sub-reflector with elliptical profile; 7 Phase centre of feed 1 ; 8 Focus of main reflector; 11 Centre of sphere 13; 13 Original spherical profile of the sub-reflector; 20 Point onto which the rays from feed 1 converge following reflection by sub-reflector 2; 21 First focus of sub-reflector 2 with elliptical profile; 22 Second focus of sub-reflector 2 with elliptical profile; 23 Geometric extension of the elliptical sub-reflector. Figure 8: Initial geometry of the optical system pertaining to examples No. 1 and No. 2 to demonstrate the rotation and reconfiguration capability of an elliptical beam or the zoom capability of a circular beam. It includes: 1 Feed; 2 Sub-reflector; 3 Parabolic main reflector; 4 Axis around which the sub-reflector rotates; 5,6 Coincident axes along which the main or sub-reflectors translate; F Focal length of main reflector 3; D Diameter projected along the propagation direction of main reflector 3; C Distance from the vertex of the main reflector to the lower edge of the reflector itself; d Diameter of the sub-reflector. Figure 9a: Co-polar radiation pattern of the antenna of Figure 8 with the initial .spherical sub-reflector which shows the isolevels in dBi compared to the isotropic value. The value of each level is shown in the figure itself. Figure 9b: Cross polar radiation pattern of the antenna of Figure 8 with initial spherical reflector which shows the isolevels in dB with respect to the peak of the co-polar diagram. The value of each level is shown in the figure itself. Figure 10a: Example of a rotation of the elliptical beam. Co-polar radiation pattern of Figure 8 antenna with elliptically shaped sub-reflector, rotation angle of the sub-reflector: 0 degrees. It shows the isolevels in dBi referred to the isotropic value. The value of each level is shown in the figure itself. Figure 10b: Example of a rotation of the elliptical beam. Cross polar radiation pattern of Figure 8 antenna with elliptically shaped sub-reflector. Sub-reflector rotation angle: 0 degrees. It shows the isolevels in dB relevant to the peak of the co-polar diagram. The value of each level is shown in the figure itself. Figure Ila: Example of the rotation of the elliptical beam. Co-polar radiation pattern of the Figure 8 antenna with elliptically shaped sub-reflector, sub-re¬ flector rotation angle: 45 degrees. It shows the isolevels in dBi referred to the isotropic value. The value of each level is shown in the figure itself. Figure lib: Example of rotation of the elliptical beam. Cross polar radiation pattern of the Figure 8 antenna with elliptically shaped sub-reflector, sub-re¬ flector rotation angle: 45 degrees. It shows the isolevels in dB relevant to the peak of the co-polar diagram. The value of each level is shown in the figure itself. Figure 12a: Example of rotation of the elliptical beam. Co-polar radiation pattern of Figure 8 antenna with elliptically shaped sub-reflector, rotation angle of the sub-reflector: 90 degrees. It shows the isolevels in dBi referred to the isotropic value. The value of each level is shown in the same figure. Figure 12b: example of elliptical beam rotation. Cross-polar pattern of Figure 8 antenna with elliptically shaped sub-reflector. Sub-reflector rotation angle: 90 degrees. It shows the isolevels in dB relevant to the peak of the co-polar pattern. The value of each level is shown in the same figure. Figure 13a: Example of rotation and zoom of the elliptical beam. Co-polar radiation pattern of Figure 8 antenna with elliptically shaped sub-reflector and translation of the sub-reflector by 50 mm, sub-reflector rotation angle: 0 degrees. It shows the isolevels in dBi referred to the isotropic value. The value of each level is provided in the same figure. Figure 13b: Example of elliptical beam rotation and zoom. Cross-polar radia¬ tion pattern of Figure 8 antenna with elliptically shaped sub-reflector and 50 mm translation of the sub-reflector. Rotation angle of the sub-reflector : 0 degrees. It shows the isolevels in dB relevant to te peak of the co-polar pattern. The value of each level is provided in the same figure. Figure 14a: Example of elliptical beam rotation and zoom. Figure 8 antenna co-polar radiation pattern with elliptically shaped sub-reflector and 50 mm translation of the sub-reflector, rotation angle of the same: 45 degrees. It shows the isolevels in dBi referred to the isotropic value. The value of each level is shown in the same figure. Figure 14b: Example of elliptical beam rotation and zoom. Cross-polar radia¬ tion pattern of Figure 8 antenna with elliptical sub-reflector. Sub-reflector translation: 50 mm, rotation angle: 45 degrees. It shows the isolevels in dB compared to the peak of the copolar pattern. The value of each level is shown in the same figure. Figure 15a: Example of rotation and zoom of the elliptical beam. Co-polar radiation pattern of the Figure 8 antenna with elliptically shaped sub-reflector. The sub-reflector is translated by 50 mm and rotated by 90 degrees. It shows the isolevels in dBi referred to the isotropic value. The value of each level is shown in the same figure. Figure 15b: Example of the rotation and zoom of the elliptical beam. Cross- polar radiation pattern of Figure 8 antenna with elliptically shaped sub-reflec¬ tor. The sub-reflector translation is 50 mm and the rotation angle is 90 degrees. U It shows the isolevels in dB with respect to the peak of the co-polar pattern. The value of each level is shown in the same figure. Figure 16a: Example of the rotation and zoom of the elliptical beam. Figure 8 antenna co-polar radiation pattern with elliptically shaped sub-reflector and 100 mm translated main reflector. The orientation of the sub-reflector is 0 degrees. It shows the isolevels in dBi referred to the isotropic value. The value of each level is shown in the same figure. Figure 16b: Example of the rotation and zoom of the elliptical beam. Cross- polar radiation pattern of Figure 8 antenna with elliptically shaped sub-reflector and 100 mm translated main reflector. Orientation of the sub-reflector: 0 degrees. It shows the isolevels in dB compared to the co-polar peak value. The value of each level is shown in the same figure. Figure 17a: Example of the rotation and zoom of the elliptical beam. Figure 8 antenna co-polar radiation pattern with elliptically shaped sub-reflector and 100 mm translated main reflector. Orientation of the sub-reflector: 45 degrees. It shows the isolevels in dBi referred to the isotropic v.alue. The value of each level is shown in the same figure. Figure 17b: Example of the rotation and zoom of the elliptical beam. Cross polar radiation pattern of Figure 8 antenna with elliptically shaped sub-reflector and 100 mm translated main reflector. Orientation of the sub-reflector: 45 degrees. It shows the isolevels in dB compared to the peak of the co-polar pattern. The value of each level is shown in the same figure. Figure 18a: Example of the rotation and zoom of the elliptical beam. Figure 8 antenna co-polar radiation pattern with elliptically shaped sub-reflector and 100 mm translated main reflector. Orientation of the sub-reflector: 90 degrees. It shows the isolevels in dBi referred to the isotropic value. The value of each level is shown in the same figure. Figure 18b: Example of the rotation and zoom of the elliptical beam. Cross- polar radiation pattern of Figure 8 antenna with elliptically shaped sub-reflector and 100 mm translated main reflector. Orientation of the sub-reflector: 90 degrees. It shows the isolevels in dB compared to the peak of the co-polar pattern. The value of each level is shown in the same figure. Figure 19a: Example of the rotation and reconfigurability of the elliptical beam. Figure 8 antenna co-polar radiation pattern with elliptically shaped sub-reflector and -50 mm translation of the m∑rin reflector. Orientation of the sub-reflector: 0 degrees. It shows the isolevels in dBi referred to the isotropic value. The value of each level is shown in the same figure. Figure 19b: Example of the rotation and reconfigurability of the elliptical beam. Cross-polar radiation pattern of Figure 8 antenna with elliptically shaped sub-reflector and -50 mm translation of main reflector. Orientation of the sub-reflector: 0 degrees. It shows the isolevels in dB compared to the peak of the co-polar pattern. The value of each level is shown in the same figure. Figure 20a: Example of the rotation and reconfigurability of the elliptical beam. Figure 8 antenna co-polar radiation pattern with elliptically shaped sub-reflector and -50 mm translation of the main reflector. Orientation of the sub-reflector: 45 degrees. It shows the isolevels in dBi referred to the isotropic v.alue. The value of each level is shown in the same figure. Figure 20b: Example of the rotation and reconfigurability of the elliptical beam. Cross-polar radiation pattern of Figure 8 antenna with elliptically shaped sub-reflector and -50 mm translation of the main reflector. Orientation of the sub-reflector: 45 degrees. It shows the isolevels in dB compared to the peak of the co-polar pattern. The value of each level is shown in the same figure. Figure 21a: Example of the rotation and reconfigurability of the elliptical beam. Figure 8 antenna co-polar radiation pattern with elliptically shaped sub-reflector and -50 mm translation of the main reflector. Orientation of the sub-reflector: 90 degrees. It shows the isolevels in dBi referred to the isotropic value. The value of each level is shown in the same figure. Figure 21b: Example of the rotation and reconfigurability of the elliptical beam. Cross-polar radiation pattern of Figure 8 antenna with elliptically shaped sub-reflector and -50 mm translation of the main reflector. Orientation of the sub-reflector: 90 degrees. It shows the isolevels in dB compared to the peak of the co-polar pattern. The value of each level is shown in the same figure. Figure 22a: Example of the zoom of a circular beam. Figure 8 antenna co-polar radiation pattern with a rotational symmetric sub-reflector with respect to axis 4. Axial position of the sub-reflector: nominal (circular non-expanded beam). It shows the isolevels in dBi referred to the isotropic value. The value of each I level is shown in the same figure. Figure 22b: Example of the zoom of a circular beam. Figure 8 antenna cross- polar radiation pattern with a rotational symmetric sub-reflector with respect to axis 4. Axial position of the sub-reflector: nominal (circular non-expanded beam). It shows the isolevels in dB compared to the peak of the co-polar pattern. The value of each level is shown in the same figure. Figure 23a: Example of the zoom of a circular beam. Figure 8 antenna co-polar radiation pattern with a rotational symmetric sub-reflector with respect to axis 4. Axial position of the sub-reflector: +60 mm (circular expanded beam). It shows the isolevels in dBi referred to the isotropic value. The value of each lev¬ el is shown in the same figure. Figure 23b: Example of the zoom of a circular beam. Figure 8 antenna cross- polar radiation pattern with a rotational symmetric sub-reflector with respect to axis 4. Axial position of the sub-reflector: +60 mm (circular expanded beam). It shows the isolevels in dB compared to the peak co-polar pattern. The value of each level is shown in the same figure. Figure 24: Canonic Gregorian optics which shows the geometric parameters to exercise a zoom of a circular beam. It includes: 1 Feed; 2 Ellipsoidal sub-reflector; 3 Parabolic main reflector; 6 Axis along which the translation of the main reflector is effected to widen the beam; 7 Centre of phase of feed 1 ; 8 Focus of the parabolic main reflector 3; 9 Symmetry axis of feed 1; 10 Rotational symmetry axis of the sub-reflector surface (ellipsoid); 21 First focus of the ellipsoid of the sub-reflector; 22 Second focus of the ellipsoid of the sub-reflector; 24 Propagation axis of main reflector 3 (or Z axis); F Focal length of parabolic reflector 3; D Diameter (projected along the direction of propagation) of main reflector 3; m Cl Distance of the vertex of reflector 3 from the lower edge of the reflector itself; C Distance between the two foci of sub-reflector 2; e Eccentricity of ellipsoidal sub-reflector 2; β Angle between axis 10 and axis 24; β Angle between axis 10 and axis 9. Figure 25: Example of the zoom of a circular beam. Figure 24 antenna radia¬ tion pattern with canonic ellipsoidal sub-reflector. Main reflector in its nominal position .and sub-reflector in nomin.al position. Co-polar diagram which shows the isolevels in dB related to the peak of the beam. The value of each level is shown in the same figure. Figure 26: Example of the zoom of a circular beam. Figure 24 antenna radia¬ tion pattern with canonic ellipsoidal sub-reflector. Main reflector translated by 128 mm along axis 6, toward the sub-reflector. Co-polar diagram which shows the isolevels in dB related to the peak of the beam. The value of each level is shown at the side of the same figure. Figure 27: Geometry of a dual gridded reflector system. It includes: 1 Feed (or array of feeds) related to polarisation X; 1 ' Feed (or array of feeds) related to polarisation Y; 3 Grid surface sensitive (or reflective) to polarisation X; 3' Solid or grid surface sensitive to polarisation Y. Figure 28: Geometry of a Gregorian dual gridded reflector system capable of rotating the elliptical beam through simultaneous rotation of the two sub-re- ilectors. It includes: 1 Feed for polarisation X; 1 ' Feed for polarisation Y; 2 Sub-reflector related to feed 1; 2' Sub reflector related to feed 1'; 3 Grid surface sensitive to polarisation X; 3' Grid (or solid) surface sensitive to polarisation Y; 4 Rotation axis of the sub-reflector related to feed 1; 4' Rotation axis of the sub-reflector related to feed 1M Figure 29: Geometry of a classical Gregorian optical system which shows the /r degrees of freedom to translation of the sub-reflector and/or of the main reflector. It includes: 1 Feed; 2 Sub-reflector; 3 Main reflector; 5 Axis B-B along which to translate the sub-reflector for beam recon¬ figuration; 6 Axis C-C along which to translate the main reflector for the reconfigu¬ ration of the beam (as an alternative to translation along B-B); 7 Geometric point corresponding to the centre of phase of feed 1; 8 Focus of the main reflector; 9 Symmetry axis of feed 1; 10 Rotational symmetry axis of the sub-reflector surface (ellipsoid); 21 First focus of the sub-reflector coincident with the focus of the main reflector; 22 Second focus of the sub-reflector. The operation of the invention will now be described with reference to the figures listed above. The optical system proposed is illustrated qualitatively in Fig. 1. This figure also shows the .axes involved in the movement of the surfaces. With reference to Fig. 1, the antenna optics include: a feed 1 with adequate primary radiation characteristics (rotational symmetry pattern and low level of cross-polarisation). Such feed has its centre of phase indicated by point 7. a shaped sub-reflector 2 with a surface having two orthogonal symmetry planes (see Fig. 6) which cross along the rotation axis 4 (axis A- A). This rotation axis bisects the angle between axis 9 of feed 1 and the offset axis 5 of the m∑rin reflector 3 (axis B-B). a main reflector 3 with a suitably shaped profile. a rotation axis 4 of the sub-reflector (axis A- A). Rotating the sub-re- flector around this axis it is possible to achieve the rotation of the elliptical beam (Fig. 2). a translation axis 5 of the sub-reflector (axis B-B). Translating the sub-reflector along this axis and combining this movement with the l i rotation of the sub-reflector, it is possible to reconfigure the initial elliptical beam (figures 3a, 3b). translation axis 6 of the main reflector (axis C-C). This axis is the alternative axis along which to shift the main reflector instead of the sub-reflector to reconfigure the antenna beam, according to the version of this invention preferred at present by the inventors. In this version, axes 5 and 6 coincide with the offset axis of the main reflector, but more in general they may differ. In a version with three independent motors, all movements can be used to achieve the rotation of the elliptical beam, the zoom of the same elliptical beam into a wider one and/or the reconfigura¬ tion of the antenna beam into an elliptical beam with a major axis which can be gradually shortened to achieve a circular beam (Fig. 3a). Recon¬ figuration is also possible through two movements only, but with differ¬ ent excursion limits for surface translation and with similar but not identical performance. The proposed optical system adopts a design technique which can be simplified as follows. Fig. 5 shows the starting geometry. Centre of phase 7 of the illuminator 1 is suitably displaced with respect to the centre of the sphere 13 which generates sub-reflector 2. Orientation 9 of feed 1 is such as to assure an optimal polarisation purity characteristics to the optical system, which takes place when axis 9 coincides with ray 14 reflected (in point 16) by the geometric extension of sphere 13 for a source ray coming from infinite in axial direction -Z. In particular, therefore, axis Z must form an angle with the perpendicular 15 to the sphere in point 16 equal to that formed by the axis of the feed with the perpendicular 15. The scanning properties of a spherical surface are such as to collimate the rays of the feed placed outside the centre of the sphere (point 7), approximately in point 20, which coincides with focus 8 of the main parabolic reflector 3. By suitable selection of the optics parameters it is possible to implement a slightly aberrated system, with the result of an almost circularly symmetrical radiation pattern at secondary level. To recover the residual aberrations of the optics, which are due to the spherical IT- sub-reflector, the main reflector 3 is suitably shaped so as to achieve a perfectly focused and symmetrical circular beam at main reflector output. By rotating the spherical sub-reflector 2 around axis 4 passing through centre 11 of sphere 13 and through point 12 formed by the intersection of axis 9 of feed 1 with spherical sub-reflector 2, the invariance of secondary radiation pattern will result, as nothing has changed from a geometrical viewpoint. The next step is to suitably shape the sub-reflector spherical surface so as to generate the required asymmetry in the secondary radiation pattern. With reference to Fig. 6, which depicts the same optical system, the shaping of sub-reflector 2 is achieved by maintaining the symmetry of the reflecting sub-reflector surface with respect to main planes 17 and 18. Such planes are perpendicular to each other and intersect along the rotation axis 4 of the sub-reflector 2. The respect of the design principles indicated above assures to achieve with a good approximation that, for any arbitrary rotation of sub-reflector 2 with respect to its original rotational symmetry axis 4, an equal rotation occurs of the secondary radiation pattern of the antenna. The possible translation of sub-reflector 2 along axis 5 or of the main reflector 3 along axis 6 (these actually coincide with the offset angle of the main reflec¬ tor) can assure the reconfiguration and zoom functions of the beam too. The shaping of the sub-reflector will normally be effected numerically, keeping the symmetry of the sub-reflector with respect to its main planes 17, 18 in Fig. 6. However, to better highlight the high number of possibilities, it is best to represent the various possible profiles of the sub-reflector analytically and in a qualitative manner, as shown in Fig. 7. In particular, Fig. 7a is shown again for clarity of the geometries involved in the starting spherical sub-reflector already shown in Fig. 5. With reference to such Fig. 7a, the centre of phase 7 of feed 1 is displaced with respect to centre 11 of sphere 13 to which spherical sub-reflector 2 belongs. Following reflection on the sub-reflector 2, rays are collimated in point 20 which actually coincides with the focus 8 of main reflector. The geometric extension of sphere 13 is also shown for clarity in the same figure. Figures 7b and 7c show how the initially spherical section of the sub-reflector may be analytically shaped so as to obtain two different kinds of elliptical IS profile. In particular, Fig. 7b shows the case in which sub-reflector 2, initially spherical 13, is shaped with a curvature radius greater than that of the spherical sub-re¬ flector. With reference to Fig. 7b, the rays leaving the centre of phase 7 of feed 1 are now collected, following reflection on the elliptical sub-reflector 2 (with foci 21 and 22) into point 20 which differs from original point 8, so as to achieve the required asymmetrical illumination of the main reflector, which keeps its focus in point 8. Figure 7c, on the contrary, shows the case of sub-reflector 2 with elliptical profile with curvature radius shorter th.an that of the initial sphere 13. In this case, rays leaving centre of phase 7 of the feed 1 .are collimated, following reflection on sub-reflector 2, in point 20 which is this time closer to the surface 2 of the sub-reflector itself. With reference to Fig. 6, as the sub-reflector shape may be different along the two main planes 17 and 18, according to the profile types described above, it appears evident that the asymmetry generated at sub-reflector level may be exploited to generate the required elliptical beam following reflection by the main reflector. The three different possibilities which arise cover analytically the main types of shaping for elliptical and circular coverages. We shall now show, as practical examples obtained through analytical shaping of the sub-reflector and not shaping (for brevity) the main reflector, the typical performance and the functions of reconfiguration which can be obtained following each movement. As the geometric parameters of the optics have not been subjected to a fine optimisation procedure, and as the surface profiles have not been used to their best, the performance shown can be substantially improved. Two examples with the initial parameters of Fig. 8 are proposed. The first example is aimed at showing the rotation, zoom and reconfiguration capability of an elliptical beam. The .second case instead is aimed at demonstrating the zoom capacity of a circular beam. Example No. 1: Rotation, zoom and reconfigurability of an elliptical beam. The example is proposed at the frequency of 12.75 GHz. Fig. 8 illustrates the initial geometry of the optical system. It is of the same type and includes the same elements as already shown in Figures 1 and 6. With reference to Fig. 8, the following items can be identified: feed 1, sub-re¬ flector 2, main reflector 3, sub reflector rotation axis 4 and main or sub-reflec¬ tor translation axis 5 (here assumed coincident). The same figure shows the numeric values which define the optics and the equation of the sphere which defines the sub-reflector. The main reflector is parabolic with geometric data shown in Fig. 8. The radiation pattern of the co-polar component obtained at secondary level is shown in Fig. 9a in terms of isolevels in dBi related to the isotropic value. The corresponding pattern of the cross-polar component is instead shown in Fig. 9b, through isolevels in dB normalised to the peak value of the co-polar (the values of the levels are shown at the side of the figure). In particular, are to be noted the co-polar beam with quasi-circular symmetry (the main reflector has not been shaped for brevity) and the low value of cross-polarisation (<-37 dB compared to the peak of the co-polar) corre¬ sponding to the initial optical system, (la) Elliptical beam rotation function. With reference to Fig. 8, the surface of the sub-reflector is now shaped analyti¬ cally through the following parameters: A = 570 mm, B = 640 mm, C = 570 mm. The radiation patterns obtained for three positions of the sub-reflector, rotated by 0 degrees, 45 degrees and 90 degrees respectively, around axis 4, are shown in Figures 10, 11 and 12. The same representation in decibel through isolevels, compared to the isotropic, .already described in Fig. 9, has also been adopted for these figures for the co-polar (Figures 10a, Ila, 12a). Their cross-polar values are also shown in Figures 10b, lib, and 12b with the same representation in relative decibel, already adopted in Fig. 9. These figures show the substantial invariance to rotation of the co-polar ellipti¬ cal beam, although an analytical shaping for the surfaces of the sub-reflector and main reflector has been resorted to. Moreover, the cross-polarisation levels are kept at extremely low values (in line with the initial figures). These charac¬ teristics make it possible to use the antenna on board satellites, with re-use of the polarisation within an operational environment with one or more si¬ multaneously active beams. 2 θ (lb) Elliptical beam rotation and zoom functions. Examples of zooming of the elliptical beam of nominal dimensions 1.6 deg x 3.0 deg (already shown in figures 10, 11 and 12) are now shown with transla¬ tion of main reflector 3 of Fig. 8 or sub-reflector 2 along axis 5. The elliptical beam is widened to achieve a nominal coverage of 1.9 deg x 4.3 deg. Figures 13, 14, and 15 show the zoom effect of the elliptical beam obtained by combining the rotation of the sub-reflector with its translation for a distance of 50 mm along axis 5 (of Fig. 8) towards main reflector 3. Figures 13, 14, and 15 show the radiation patterns of the co-polar and cross-po- lar components for three rotations of the sub-reflector already analysed in figures 10, 11 and 12 respectively. In particular, figures 13a, 14a and 15a show the radiation patterns of the co-polar with the same representation in decibel referred to the isotropic level already used for the other co-polar patterns shown till now. The orientations of the sub-reflector with respect to axis 4 of Fig. 8 are respec¬ tively 0 degrees, 45 degrees and 90 degrees, as for those already shown for the original turnable elliptical beam. Figures 13b, 14b and 15b show the corresponding cross-polar values for figures 13a, 14a, 15a. The representation is in decibel related to the peak of the co-polar, as already effected for the other cross-polar patterns shown. It appears evident from these radiation patterns that the elliptical beam zoom function has been implemented with a good rotational invariance of the elliptical beam at extremely controlled cross-polar values and in line with the initial cross-polarisation of the original beam. The same widening effect of the elliptical beam can be obtained by translating the main reflector 3 of Fig. 8 along axis 5 instead of sub-reflector 2. The results, which are obtained through translation of main reflector 3 by 100 mm along axis 5 of Fig. 8, are shown in figures 16, 17 and 18 respectively for the same three orientations (0 degrees, 45 degrees, 90 degrees). In particular, figures 16a and 16b show the co-polar radiation pattern in dBi (absolute values with respect to the isotropic) and the cross-polar (with levels in dB related to the peak of the co-polar) for orientation 0 degrees of the sub-reflector. The same representations are therefore provided in figures 17a and 17b for rotation 45 degrees and in figures 18a and 18b for the 90 degree position of the sub-re¬ flector. Also in this case it can be seen how the widening effect of the elliptical beam takes place, for any orientation of the sub-reflector, with extremely low cross- polar values. (lc) Elliptical beam rotation and reconfiguration function. The continuous variation of the elliptical contour into a circular contour may be obtained through the same optical system of figure 8, by combining rotation with translation of the sub-reflector or of the main reflector, however in opposite direction to that used for the widening of the elliptical beam already demonstrated in point (lb). A practical example of such reconfigurability is shown in figures 19, 20 and 21, which show the radiation patterns at secondary level obtained by translating the main reflector 3 of Fig. 8 along axis 5 by 50 mm, in the opposite direction to the one followed previously, for three orientations of the sub-reflector. In par¬ ticular, Fig. 19a shows the isolevels in dBi with respect to the isotropic value of the co-polar pattern, for orientation 0 degrees of the sub-reflector around axis 4 of Fig. 8. Figure 19b shows the corresponding radiation pattern of the cross-po¬ lar with dB levels related to the peak of the co-polar. Similar patterns are shown in figures 20a and 20b for sub-reflector orientation by 45 degrees. The case related to the orientation of the sub-reflector by 90 degrees is finally shown in figures 21a and 21b, which show the co-polar and cross-polar radiation patterns respectively. The figures clearly show the capabil¬ ity of the movements to reconfigure the nominal elliptical beam continuously into an elliptical beam with ratio of minor and major axes smaller than the initial value, for whatsoever orientation of the elliptical beam. In particular, as an extreme case, it is possible to obtain the circular beam shown as a circle in figures 19a, 20a and 21a. Example No. 2: Circular beam zoom. The profile of sub-reflector 2 of Fig. 8 is here analytically modified to demon¬ strate the possibility of widening (zooming) a circular beam through translation along axis 5 of the sub or main reflectors. For this example, the parameters of the sub-reflector are the following: A = B = 640 mm, C = 570 mm. The corresponding initial radiation pattern is shown in figures 22a (co-polar) and 22b (cross-polar). The co-polar beam obtained is almost circular with a beam width at -3 dB of about 2 degrees. More specifically, Fig. 22a shows the co-polar pattern in dBi with absolute levels referred to the isotropic, while Fig. 22b shows the cross- polar pattern with levels in dB related to the co-polar. The effect of a translation of the sub-reflector by 60 mm along axis 5 of Fig. 8, in the direction of main reflector 3, is shown in Fig. 23. As can be seen, the co-polar radiation pattern shown in Fig. 23a has been widened to reach a beam width at -3 dB of 3.2 degrees. The slightly elliptical contour can be improved by suitably optimising the focal of main reflector 3 of Fig. 8 or by numerical shaping of the surface of the same reflector. In Fig. 23a the values of the isolevel curves are shown in dBi with respect to the isotropic, while Fig. 23b shows the isolevel values in dB related to the peak of the co-polar. It is possible to obtain a similar widening effect (or zoom) by translating main reflector 3 of Fig. 8 along axis 5 towards sub-reflector 2. The zoom function maintains extremely satisfactory radiation characteristics for both components co-polar and cross-polar, so that the system can be used as an antenna on board a satellite with frequency re-use and with more than one beam operating simultaneously. The performance shown in the example must be understood as demonstrative of the possibilities offered by the movements mentioned above and not as the effective performance obtainable in a detail design. The zoom or widening function of the circular beam is compatible, with excellent performance, even with canonic Gregorian optics, the geometry of which has already been illustrated in Fig. 4, as will also be illustrated based upon Fig. 24. With illustrative intent, the widening function of a circular beam by a factor 1.6: 1 will be shown through translation of the main reflector along axis 6 of Fig. 24, which provides the geometric parameters of the canonic optics consid¬ ered in the example itself. The nominal radiation pattern is shown in Fig. 25a. These figures show the isolevels in dB related to the antenna peak. The beam width at -3 dB is 2 degrees. Figure 26 shows the beam widened to 3.2 degrees at -3 dB obtained through translation of the reflector by 128 mm towards the sub-reflector along axis 6 of Fig. 24. Remarkable is the capability to maintain a circular symmetry of the beam and very low levels of sidelobes. The cross-polarisation levels (not shown here for brevity) in both cases are kept at extremely low levels within the useful coverage area (-34 dB with respect to the local value of the co-polar), so that the system may be used as an antenna on board satellites with frequency re-use. It is to be noted that, although the zoom function is compatible with sub-re¬ flector and main reflector translation, the latter is actually preferred because it appears to better optimise the electrical performance of the widened circular beam. The rotation function of the elliptical beam may be extended also to other optics. In particular, an extension to gridded optics (described in section 4, Fig. 27) of the rotation of the elliptical beam by rotation of the sub-reflector is possible and it implies the introduction of two sub-reflectors as shown in Fig. 28. With reference to this figure, the following items can be identified: (a) feed 1 related to polarisation X; (b) Feed 1' related to polarisation Y; (c) Sub-reflector 2 related to feed 1; (d) Sub-reflector 2' related to feed 1'; (e) Gridded main front reflector 3 sensitive to polarisation X; (f) Gridded (or solid) main rear reflector 3' related to polarisation Y; (g) Rotation axis 4 of sub-reflector 2 related to feed 1. By rotating sub- reflector 2 around this axis it is possible to obtain the rotation of the elliptical beam with polarisation along axis X; (h) Rotation axis 4' of sub-reflector 2' related to feed 1M By rotating sub-reflector 2' around this axis it is possible to obtain the rotation of the elliptical beam with polarisation along axis Y. It is to be noted that the criteria for the implementation of the geometric system for each polarisation are the same as already described. The idea of rotating the sub-reflector is possible on any type of double gridded reflector. The ZH advantages which are intrinsic to this optical system are mainly two: (1) it is possible to obtain turnable elliptical coverages with polarisation purity values far greater than those obtainable with solid reflector Gregorian antennas. Such values are typical of gridded reflectors; (2) it is possible to obtain two independent elliptical coverages with arbitrary orientation (one for X polarisation, the other for Y polarisation) in a double linear polarisation system. The zooming of a circular or elliptical beam may also be applied to other types of optics. In particular, the extension of the beam-widening from circular to circular and from elliptical to elliptical, through translation of main or sub-re¬ flectors, is applicable as we have already seen, to classical Gregorian optics with standard surfaces (shown in Fig. 4 and in Fig. 24). In particular, the circular beam optics may include an ellipsoidal sub-reflector and a parabolic main reflector. With reference to Fig. 24, by translation of the sub-reflector or of the main reflector along axis 6, we can obtain, with excellent co-polar and cross-polar performance, a zoom function of a circular beam, as described above. By modifying the profile of the main reflector analytically (bifocal parabolic reflector) or equivalently by numerical methods, it is possible to obtain an elliptical beam from the same antenna shown in Fig. 24. Through sub-reflector 2 .and/or main reflector 3 translation along axis 6 of Fig. 24, it is possible to achieve a zoom and/or reconfiguration of the elliptical beam. The main features of this invention are: - The concept of elliptical beam rotation. In particular it is possible, through a simple rotation of the sub-reflector of an antenna of the Gregorian family, to obtain the rotation of the radiation pattern keeping the orientation of the electrical field and the shape of the beam invariant during rotation. - The methodology with which the optical system herein described is built and the shaping of the surfaces, through which the rotation of the elliptical coverage pattern is obtained by rotating the surface of the sub-reflector. This rotation is not at present compatible with known zr Gregorian antenna systems. The compatibility of the optical system herein described with the widen¬ ing (zoom) and/or reconfigurability functions of the beam through the translation of the sub-reflector or of the main reflector along well determined axes. The unique combination of degrees of freedom of rotation and transla¬ tion, which makes it possible to rotate an elliptical beam, to zoom it, and to reconfigure it from an elliptical into a circular beam, whatever the orientation of the elliptical beam may be. - The separate or combined translation of the sub-reflector or of the main reflector, through which, even without a rotation of the beam, it is possible to have the zoom of a circular beam into another circular beam widened by a considerable factor (≥2:1) or of an elliptical beam into another widened elliptical beam. It is also possible to reconfigure an elliptical beam into a circular beam (with a diameter close to the minor axis of the elliptical beam) or into an elliptical beam oriented by 90 degrees with respect to the one initially available. The possibility to obtain a zoom, without rotation, of the beam even with classical Gregorian type of optics. - The possibility to obtain the rotation of the elliptical beam with gridded optics, through two turnable sub-reflectors, which can be rotated inde¬ pendently. The main feature of this invention is the compatibility of the functions men¬ tioned above with radiation electrical performance typical of antennas of the double offset reflector type such as the Gregorian family of antennas. These performances can be summarised in high efficiency of the beam and extremely low cross-polarisation and sidelobe levels. These characteristics assure that the system can be used as an antenna on board a satellite with reutilisation of polarisation within an operational environment with one or more simultaneously active beams. It is not a subject of claim the beam-scan function, which is compatible with the antenna configuration described herein and which can be implemented through already known meth¬ ods, such as the rotation of the entire antenna with twin orthogonal axes 7 L motors, or through the independent rotation of only the main reflector around any selected point.

Claims: CLAIMS 1. Gregorian, double-reflector, microwave antenna, shown in Fig. 1, said antenna comprising the following essential parts: a feed (1), a sub-re¬ flector (2) with a shaped surface which has two symmetry planes with respect to a suitable axis (4), and a main reflector (3), characterised in that, by rotating sub-reflector (2) around axis (4), it is possible to rotate the elliptical beam of said antenna by keeping the orientation of the electrical field (polarisation) unchanged while also keeping the shape of the beam itself unchanged, all with extremely low resulting cross-po- larisation values. 2. Gregorian, double-reflector, microwave antenna, shown in Fig. 1, as claimed in claim 1, characterised in that said antenna, further to the rotation of the beam defined in claim 1, offers other degrees of freedom, such as the translation of m∑dn reflector (3) along axis (6) and/or the translation of sub-reflector (2) along axis (5), so as to obtain a high re¬ configurability of the antenna beam. In particular it is possible to obtain a wider elliptical beam or to vary the ratio of the major and minor axes of the elliptical beam with any required orientation of such axes or even to obtain a circular beam. 3. Gregorian, double-reflector, microwave antenna, shown in Fig. 1, as claimed in claim 2, characterised in that the translation of sub-reflector (2) along axis (5) and/or the translation of main reflector (3) along axis (6) make it possible to widen the circular beam by exploiting a suitably shaped sub-reflector. 4. Gregorian, double-reflector, microwave antenna, shown in Fig. 1, as claimed in claim 1, characterised in that the main reflector (3) may be single, as in Fig. 1, or doubled into two main reflectors (3 and 3') which are polarisation-selective, as in Fig. 28. In this latter case, sub-reflector (2) is doubled into two separate sub-reflectors (2 and 2'), and each of said two sub-reflectors is coupled with its own feed (1 and 1'). This configuration provides a level of polarisation purity in the service coverage which is typical of gridded reflectors, for .any orientation of the elliptical beam. 2.Θ Classical Gregorian, double-reflector, microwave antenna, shown in Fig. 29, said antenna comprising the following essential parts: a feed (1), a sub-reflector (2) with shaped or ellipsoidal surface and a main reflector (3), characterised in that said antenna, through translation of sub-re¬ flector (2) along axis (5) and/or translation of main reflector (3) along axis (6), produces the widening of the circular or elliptical beam and also provides the reconfiguration of the elliptical beam into a circular beam.

Inventors: CONTU SALVATORE, MESCHINI ALBERTO, MIZZONI ROBERTO, CONTU, SALVATORE, MESCHINI, ALBERTO, MIZZONI, ROBERTO
Applicants: ALENIA SPAZIO SPA, CONTU SALVATORE, MESCHINI ALBERTO, MIZZONI ROBERTO, ALENIA SPAZIO S.P.A., CONTU, SALVATORE, MESCHINI, ALBERTO, MIZZONI, ROBERTO
Application year: 1995
Main classification: H01Q19
IPC codes: H01Q19