Title of Invention

SINGLE GRIDDED DUAL REFLECTOR ANTENNA

Abstract

A single-gridded dual-reflector antenna A single-gridded dual-reflector antenna comprises a front reflector shell (1) of non-conductor materials with a gridded reflector surface at the aperture plane thereof, a rear reflector shell (2) of conductor materials held behind of the front reflector shell (1) by dielectric spacers (5) at an axial inclination with the front reflector shell (1), and two feed horns (3, 4) located one each at the focal centre/axis of front and rear reflector shells (1, 2). The method of producing the antenna comprises: (a) constructing the front and rear reflector shells according to design particulars determined by the mathematical formulations developed for the purpose, (b) constructing grids (6) of required shape, size and orientation, (c) fixing grids (6) at the aperture plane of front reflector shell, and (d) positioning feed horns (3, 4) at the focal centre/axis of front and rear reflector shells (3,4) by brackets 10). (Figure 1)

Full Text

The present invention relates to a single-gridded dual-reflector antenna for communication satellites and method of producing the same. BACKGROUND OF THE INVENTION The invention relates, more particularly, to an antenna for use in communication satellites, which is of relatively simple design and low-cost construction, capable of transmitting as well as receiving orthogonally polarized radio signals of same frequency or of widely different frequencies at a reduced level of cross-polarization degradation of the desired signals by the undesired signals to and from expanded coverage areas on the Earth's surface with increased antenna gain over a wide band of frequencies in the GHz range without producing any coverage hole. At present communication satellites are being used extensively for rendering services like voice/data transfer and long-distance television coverage. The antenna systems installed in satellite vehicles play an important role for rendering such services to selected coverage areas on the Earth's surface with saving of the available frequency channels and without interference of the desired signals by undesired signals. The design and construction of antenna systems for use in communication satellites have therefore received considerable attention of late. GB 2166001A discloses a dual-gridded antenna reflector system, comprising a pair of reflector dishes each having a grid of parallel conductors. The pair of reflector dishes are mounted one over the other using linear support means. US Patent 6, 052, 095 discloses a dual-gridded reflector antenna in which a first parabolic reflector illuminated by a first signal source and a second parabolic reflector illuminated by a second signal source are provided. The parabolic reflectors are positioned one behind the other with their focal points lying on a common axis and the two signal sources are located at different offset positions with respect to the common axis. EP 1059 689 A2 discloses an antenna system for communication satellites, comprising a sub-reflector without having any gridded surface at the aperture plane, and a main reflector fitted with two gridded reflector surfaces located one behind the other at the aperture plane, for reflecting the two components of orthogonally polarized radio waves impingent upon the front gridded surface from two feed horns. EP 1 184 939 A2 discloses a dual-gridded single-reflector antenna for communication satellites, in which the orientation of grids of two reflector surfaces, placed one behind the other, at the aperture plane of a reflector shell is so modified that the grids are non-parallel when viewed from the focal axis of the reflector shell, and parallel when viewed from the directions of the radiated beams. Dual-gridded, single reflector antennas are used in many communication satellites, for transmitting two orthogonally polarized radio signals of the same frequency to cover two different geographical regions on the Earth's surface, each comprising a parabolic reflector shell of conductor materials, two gridded reflector surfaces, located one behind two other, and adjacent to each other, at the aperture plane of the parabolic reflector shell in a manner such that the linear orientation of grids in the front reflector surface is perpendicular to that of grids in the rear reflector surface. Two feed horns, one each for impinging two orthogonal i.e. horizontal and vertical, components of a polarized radio signal upon the front gridded reflector surface, are located near the focal center and axis of the parabolic reflector shell. If the grid orientation of the front reflector shell is horizontal and that of the rear reflector surface is vertical, then the horizontally polarized component of the radio signal is mainly reflected by the front reflector surface, and the vertically polarized component of radio signal passes through the front reflector surface and is reflected mainly by the rear reflector surface. Because of the finite width of the individual grids and gap between the adjacent grids of both the front and the rear reflector surfaces, a fraction each of the vertically and horizontally polarized components of the radio signal is also reflected respectively by the front and rear reflector surfaces, causing cross-polarization degradation of the quality of the horizontally and vertically polarized components of the radio signal mainly reflected by the front and rear reflector surfaces. Thus the radio signal reflected from each of the front and rear gridded reflector surfaces contain mainly the component of orthogonally polarized signals having orientation in the linear direction of grids, called co-polar component, and a part of the component of the orthogonally polarized signals, having orientation across the linear direction of grids, called cross-polar component, of the respective gridded reflector surfaces. The presence of the cross-polar component degrades the quality of the co-polar component of the signal radiated from each of the front and rear reflector surfaces and is therefore required to be minimized as much as possible, for satisfactory quality of transmission and reception of the desired signal. The directions of radiation of the co-polar components of the signals from the front and rear gridded reflector surfaces are controlled by adjusting the location and orientation of the feed horns, used for impinging the polarized radio signals thereupon, with respect to the focal centre and axis of the parabolic reflector shell. The level of cross-polarization degradation of the co-polar components of the signals radiated from the antenna is lowered partly by shaping of the paraboloid surface of the reflector shell and partly by adjustment of the geometrical layout of grids of the front and rear gridded reflector surfaces. The conventional dual-gridded single-reflector antenna has, however, certain drawbacks, which are required to be removed. These drawbacks are: (a) the cross-polarization degradation level of the transmitted signal is difficult to be lowered to the permissible level of -30db. (b) for allowing shaping of the paraboloid surface of the reflector shell with a view to reducing the level of cross-polarization degradation of the transmitted signal, the reflector shell is required to be made, of increased size and thickness, and hence relatively heavy, the typical dimensions being around 1270 mm x 1270 mm x 1270 mm. (c) the antenna shaped and adjusted for installation in one satellite is not useable for another satellite without re-shaping and re-adjustment and; (d) the time required for carrying out shaping and adjustment for lowering cross-polarization degradation of transmitted signals to the specified level of -30db is considerably long. SUMMARY OF THE INVENTION One object of the present invention is to provide a communication satellite antenna of relatively simple design and low-cost construction. The other object is to make the antenna suitable for achieving the specified -30db level of cross polarization degradation of the transmitted signals with relatively simple and rapid adjustment of the component parts of the antenna. Yet another object is to widen the coverage area of the signals, transmitted by the antenna, on the Earth's surface. A still further object is to eliminate presence of coverage holes in the co-polar gain contours on the Earth's surface. The single-gridded dual reflector communication satellite antenna according to the present invention comprises two parabolic reflector shells, disposed one behind the other. The front reflector shell is made of non-conductor materials such as moulded plastics, and is therefore transparent to radio waves impingent thereupon. A gridded reflector surface is fixed at the aperture plane of the shell. The rear reflector shell is made of conductor materials like aluminium, which, therefore reflects the radio waves impingent thereupon. No gridded reflector surface is fixed at the aperture plane of the rear shell. The focal axis of rear reflector shell is rotated from that of the front reflector shell by a preselected angle. The front and rear reflector shells are spaced apart by a given distance using inter-shell spacers made of dielectric materials like moulded plastics. Two feed horns are located one each at and near the focal centre and axis of front and rear parabolic shells for impinging the co-and cross-polarization components of orthogonally polarized radio signals upon the gridded reflector surface of the front parabolic shell. Thus the present invention provides a single-gridded dual reflector antenna for communication satellites, characterised in that the antenna comprises a front parabolic reflector shell made of non-conductor materials like moulded plastics; a gridded reflector surface containing horizontally or vertically oriented grids of conductor materials like duraluminium fitted at the aperture plane of the front reflector shell; a rear parabolic reflector shell of conductor materials like aluminium, held at a given distance behind the front reflector shell by spacers of dielectric materials like moulded plastics, with the focal axis of rear reflector shell being inclined at a pre-selected angle to the focal axis of front reflector shell; and two feed horns for impinging orthogonally polarized radio signals upon grids of the gridded reflector surface, being located at or near the focal centres and axes of front and rear reflector shells respectively, and arranged to function inter-dependently therewith. The present invention provides also a method of producing the antenna, comprising the following steps: (a) constructing the front reflector shell of nonconductor and rear reflector shell of conductor materials, according to design particulars obtained by the mathematical formulations developed for the purpose; (b) constructing grids of conductor materials and of required geometrical shape and size, and making the same free from wrinkles; (c) fixing the grids at the aperture plane of the front shell with linear orientation thereof in the horizontal or vertical direction to form the reflector surface of front shell; and (d) positioning two feed horns, one each at the focal centres and axes of the front and rear reflector shells, in brackets with respective rectangular, and rectangular-to-circular-transition waveguides. BRIEF DESCRIPTION OF THE DRAWINGS The invention is described in detail without restricting the scope of the invention in any manner with reference to the accompanying drawings, in which Figure 1 (a) shows the perspective view and fig. l(b) shows the side view of an embodiment of the invented single-gridded dual-reflector antenna; Figure 2 (a) depicts horizontally oriented and figure 2 (b) depicts vertically oriented grids of the gridded reflector surface provided at the aperture plane of the front parabolic shell of the invented antenna; Figure 3 (a) shows the side view and figure 3 (b) the front view of a feed horn for impinging orthogonally polarized radio signals on the gridded reflector surface of the front shell of the invented antenna ; Figure 4 (a) shows the contour of -32db cross-polarization degradation level obtained by the conventional dual-gridded single-reflector antenna and figure 4 (b) shows the same obtained by the invented single-gridded dual reflector antenna; Figure 5 (a) shows the contour of +32 dbi gain level obtained by the conventional antenna and figure 5(b) shows the same obtained by the invented antenna. DETAILED DESCRIPTION OF THE INVENTION Referring to Figs. 1 (a) and l (b), the rear reflector shell (2) of conductor materials is held in position at a predetermined distance behind the front reflector shell of non-conductor materials, by using inter inter-shell spacers (5) of dielectric materials, in a manner such that the focal axis of the rear shell (2) is rotated at a pre-selected angle with respect to that of the front shell (1). Feed horns (3 and 4) are located at the focal centre and directed along the focal axis of the front and rear reflector shells (1 and 2) respectively, for impinging orthogonally polarized radio signals on the gridded reflector surface of the front shell (1). Fig. 2 (a) and 2 (b) depict respectively the horizontal and vertical orientations of grids (6) fitted at the aperture plane of front shell (1) of the invented antenna, for reflecting respectively the horizontally polarized and vertically polarized components of the orthogonally polarized radio signals impingent upon grids (6) from the feed horns (3 and 4). The grids (6), being made of conductor materials like duraluminium, reflect the radio signals impingent from feed horn (3) having horizontal polarization, in the direction of focal axis of front reflector shell (1), if the grid orientation is horizontal as shown in Fig. 2(a), and allow the radio signals impingent from feed horn (4) having vertical polarization, to pass through grids (6) as well as through front shell (1) of non-conductor materials, and get reflected from the rear shell (2) of conductor materials in the direction of the focal axis of the rear shell (2). Similarly, if the grid orientation is vertical as shown in Fig. 2 (b), then vertically polarized radio signals impingent from feed horn (3) upon grids (6) are reflected in the direction of focal axis of front shell (1) and the horizontally polarized radio signals impingent from feed horn (4) pass through grids (6) as well as through front shell (1) of non-conductor materials, and get reflected from the rear shell (2) of conductor materials in the direction of focal axis of rear shell (2). The cross-polarization degradation level of the desired signals is minimized to be lower than -32 db, partly by (a) modifying the two- dimensional(2D) shape and size of grids (6), (b) by optimizing the three-dimensional (3D) orientation of grids (6) in relation to the shaped paraboloid surface of rear reflector shell (2), (c) by making the grids wrinkle-free and (d) by adjusting the location and orientation of axes of feed horns(3 and 4) with respect to the focal centres and axes of parabolic reflector shells (1 and 2) respectively. For facilitating rapid production of the invented antenna, suitable mathematical formulations have been developed for determining the design particulars of the front and rear reflector shells (1 and 2) and grids (6). Referring to Fig. 3 (a), the feed horns (3,4) are each of corrugated shape (7). The orthogonally polarized radio waves are fed into each feed horn through rectangular waveguide (9) and rectangular-to-circular-transition waveguide (8). Each feed horn is held in position, using bracket (10) in which the position of the phase centre (11) of the feed horn can be adjusted as required. Referring to Fig. 3 (b), the lateral side (12) of rectangular waveguide (9) is shown to be inclined to the axis (14) of feed horn at angle (13) for matching the polarized signal orientation with grid orientation of the gridded reflector surface fitted at the aperture plane of front shell (2). Referring to Fig. 4 (a) and 4 (b), it is noted that the contour (15) of-32 db cross-polarization degradation level on the earth's surface cover a larger area for the invented antenna as shown in fig. 4(b), compared with that for the conventional antenna as shown in fig. 4 (a). Referring to figs. 5 (a) and 5 (b), it is noted that the contour (16) of +32 dbi gain level has wider coverage area on the Earth's surface without any coverage hole for the invented antenna as shown in fig. 5 (b), while the same for the conventional antenna shown in Fig. 5 (a) has narrower coverage area with adjoining coverage holes. The typical functional features of the invented antenna are presented in Table I. Table I: Typical Functional Features of the Invented Antenna The invented antenna can be used both for radiation and reception of radio signals over a relatively wide band of frequencies in the GHz range. A saving in the cost of construction as well as in the space required for installation in the satellite of the invented antenna in spacecrafts is achieved, because of the capability of invented antenna to receive as well as to transmit radio signals of the same frequency and also of a wide frequency band. The invented single-grid dual-reflector antenna has the following advantageous features for which it is expected to find extensive applications in communication satellites: (a) Relatively simple design and low-cost construction; (b) Capability of lowering cross-polarization degradation level of the desired signals to the specified level of -30 db by relatively simple and rapid adjustments of the location and orientation of feed horns; (c) Usability for both radiation and reception of radio signals of the same frequency or of different frequencies covering a wide band in GHz range; (d) Suitable for covering relatively wide areas on the Earth's surface without producing any coverage hole in the gain contour of+32 dbi. (e) Has a high antenna gain of +23.5 dbi for both radiation and reception of radio signals. We claim :- 1. A single-gridded dual-reflector antenna for communication satellites, characterized in that the antenna comprises a front parabolic reflector shell (1) made of non-conductor materials like moulded plastics; a gridded reflector surface containing horizontally or vertically oriented grids (6) of conductor materials like duraluminium fitted at the aperture plane of the front reflector shell (1); a rear parabolic reflector shell (2) of conductor materials like aluminium, held at a given distance behind the front reflector shell (1) by spacers (5) of dielectric materials like moulded plastics, with the focal axis of rear reflector shell being in climed at a pre-selected angle to the focal axis of front reflector shell; and two feed horns (3 and 4) for impinging orthogonally polarized radio signals upon grids (6) of the gridded reflector surface, being located at or near the focal centres and axes of front and rear reflector shells (1 and 2) respectively, and arranged to function inter-dependently therewith. 2. The antenna as claimed in claim 1, wherein the paraboloid surface of the rear reflector shell is capable of being shaped as required. 3. The antenna as claimed in claims 1 and 2, wherein the grids are wrinkle- free and of two -and three-dimensional geometry as required for matching the same with the shaped paraboloid surface of the rear shell. 4. The antenna as claimed in any preceding claim, wherein the feed horns (3,4) are of corrugated shape (7). 5. The antenna as claimed in claim 4, wherein the feed horns are held in position by brackets (10) of dielectric materials like moulded plastics. 6. The antenna as claimed in claims 4 and 5, wherein the feed horns are provided with rectangular waveguide (9) and rectangular-to-circular-transition waveguide(8) for feeding orthogonally polarized radio signals thereinto. 7. The antenna as claimed in claims 4 to 6, wherein the lateral side (12) of rectangular waveguide (9) is held at an angle (13) with the axis (14) of a feed horn. 8. The antenna as claimed in claims 4 to 6, wherein the location of phase centre (11) of a feed horn is adjustable in bracket (10). 9. The method of producing the antenna as claimed in the preceding claims, comprising the following steps: (a) constructing the front reflector shell of non conductor and rear reflector shell of conductor materials, according to design particulars obtained by mathematical formulations developed for the purpose; (b) constructing grids of conductor materials and of required geometrical shape and size, and making the same free from crinkles; (c) fixing the grids at the aperture plane of the front shell with linear orientation thereof in the horizontal or vertical direction to form the reflector surface of front shell; and (d) positioning two feed horns, one each at the focal centres and axes of the front and rear reflector shells, in brackets with respective rectangular, and rectangular- to-circular-transition waveguides.

Documents:

80-CHE-2007 EXAMINATION REPORT REPLY RECIEVED 23-09-2014.pdf

80-CHE-2007 AMENDED CLAIMS 23-09-2014.pdf

80-che-2007-abstract.pdf

80-che-2007-abstractimage.jpg

80-che-2007-claims.pdf

80-che-2007-correspondence-others.pdf

80-che-2007-description-comnplete.pdf

80-che-2007-drawings.pdf

80-che-2007-form 1.pdf

80-che-2007-form 26.pdf

80-che-2007-form 3.pdf

80-che-2007-form 8.pdf