Abstract:
A multi-beam antenna system including a parabolic reflector with a parallelogram perimeter, which is illuminated by a plurality of feed elements; so that the parabolic reflector reflects a first beam corresponding to one frequency that illuminates a first parallelogram spot, a second beam corresponding to the same frequency that illuminates a second parallelogram spot, such that the first and second spots are parallelepiped arranged one un-adjacent another of the same frequency are generally un-adjacent.

Description:
OBJECT OF THE INVENTION  
         [0001]    The present invention relates to a multibeam antenna system for focusing and concentrating incident electromagnetic waves at a preselected direction that impinges a predetermined coverage area on the earth.  
           [0002]    More specifically, to an antenna system for producing communication beams, which have improved isolation characteristics around the spots at a prescribed coverage zone, covered by the antenna. This type of geometry is implemented when the purpose is to reuse the same frequency multiple times over a given coverage area.  
         STATE OF THE ART  
         [0003]    A multibeam antenna system for producing spots on the ground comprises reflector means that has a circular shape and a plurality of feed elements. The plurality of the feed elements is usually arranged in a hexagonal arrangement. So, the parabolic reflector cooperates with the plurality of the feed elements to transmit and/or receive electromagnetic waves.  
           [0004]    Therefore, the antenna system illuminates a group of spots with a circular cross-section, shown in FIG. 1, by use of a group of feeds arranged in a hexagonal array feeds.  
           [0005]    In addition, the plurality of the beams is generated simultaneously by the provision of different frequencies of electromagnetic radiation in each of the beams. The mechanical accommodation of feeds a to produce adjacent spots requires usually 3, 4 or more antenna. Each feed provides a spot at the coverage area and there are several spots of the same frequency.  
           [0006]    Unfortunately, the radiating diagram of the spots produced through a circular reflector is isotropic, shown in FIG. 2. Due to that fact side lobes of each beam radiate into the spots provide by other feeds of the same frequency, namely it causes interference between signals of the same frequency. So, these side lobes reduce the carrier over interference ratio C/I, which is one of the main parameter to assess efficiency at a transmission.  
           [0007]    This interference is undesirable because it reduces the overall efficiency of the system to transmit information. An isolation among spots of the same frequency of greater than 15 dB would be highly desirable.  
           [0008]    In view of the foregoing, there is a need for a multibeam parabolic antenna system that improves isolation among beams of the same frequency, resulting in a more efficient satellite communication system.  
         CHARACTERISATION OF THE INVENTION  
         [0009]    The technical problems mentioned above are resolved by the invention by constituting a multi-beam antenna system including a parabolic reflector with a parallelogram perimeter, which is illuminated by a plurality of feed elements; so that the parabolic reflector reflects a first beam corresponding to one frequency that illuminates a first elliptical spot, a second beam corresponding to the some frequency that illuminates a second elliptical spot, such that the first and second spots are generally un-adjacent.  
           [0010]    Any number of spots may be defined by projecting additional beams from the multi-beam antenna.  
           [0011]    The antenna system provides for improved uniformity of signal gain with a simplified mechanical structure of the antenna system.  
           [0012]    The present invention therefore introduces a spot-clustering scheme wherein spots are elliptical arranged one un-adjacent another of the same frequency.  
           [0013]    One of the main advantages of the spot-clustering scheme of the present invention is that, side lobes corresponding to one beam of one predetermined frequency are out of main lobe of the another beam of the same frequency. Therefore, the isolation among beam of the same frequency is improved. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    A more detailed explanation of the invention is given in the following description based on the attached drawings in which:  
         [0015]    [0015]FIG. 1 is a plot of typical spot beams formed on the surface of the earth by a circular parabolic antenna illuminated by a set of feeds,  
         [0016]    [0016]FIG. 2 is a plot of typical radiation diagram of one beam produced by one circular parabolic antenna,  
         [0017]    [0017]FIG. 3 shows in cross-section on the line AA′, in plan view and in cross-section on the line BB′ one embodiment of a rectangular parabolic reflector for an antenna system in accordance with the present invention,  
         [0018]    [0018]FIG. 4 is a diagrammatic representation in plan view of a radiation diagram of the rectangular antenna in accordance with the present invention, and  
         [0019]    [0019]FIG. 5 shows the spot beams formed on the ground from a set of four rectangular parabolic antennas, each antenna being illuminated by a set of feeds radiating with the same frequency and same polarisation, and  
         [0020]    [0020]FIG. 6 shows so-called four colour schemes where one colour symbolises a frequency and a polarisation system in accordance with the present invention. 
     
    
     DESCRIPTION OF THE INVENTION  
       [0021]    In general an antenna produces a beam of predetermined intensity over a designated geographic area, also called coverage.  
         [0022]    The multi-beam antenna system of the present invention is used for communications between a satellite and the earth, for example. The multibeam antenna system is adapted to transmit a group of beams as required for specific applications.  
         [0023]    Referring initially to FIG. 4, a multi-beam antenna embodying of the present invention is shown. In this embodiment, the multi-beam antenna system includes a reflector means  12  that is illuminated by a plurality of the feeds  13 ,  14 ,  15  and  16  with an offset geometry and generally designated by the reference numeral  13 .  
         [0024]    The reflector means has a surface of parabolic shape, formed of a material that reflects RF. The rectangular reflector  12  forms an antenna beam in a preselected direction that impinges a predetermined coverage area on the Earth.  
         [0025]    In another embodiment of the invention the single parabolic reflector is illuminated by feeds  13  disposed substantially at its focus, no shown.  
         [0026]    Mechanical means (no shown) are provided to hold the feeds  13  at the focus of the reflector  12  in a fixed and optimal geometrical arrangement.  
         [0027]    Depending on the position of the reflector  12 , the beams illuminate different places on the Earth. The beams illuminate the spots  23 ,  24 ,  25  and  26  and generally designated by the reference numeral  23 . The reflector  12  reflects incident RF energy propagating and forms an ellipse lying on ground, shown in FIG. 5  
         [0028]    Turning now to FIG. 3, the reflector  12  based on a parabolic reflector surface has a parallelogram perimeter, namely, rectangular rim when is projected onto the x-y plane, lines AA′ and BB′, respectively.  
         [0029]    It is noted that the shape of the reflector  12  as seen in plan view in FIG. 3 is virtually rectangular. The cross-sections on AA′, BB′ in FIG. 3 respectively, are parabolic arcs of different length.  
         [0030]    Referring now to FIG. 5, the beam resulting from a feed  13  will have an elliptical cross-section and its side lobes are mainly on the axis related to greater parabolic arc. This means that each spot  23  is anisotropic.  
         [0031]    In addition, the antenna system has one parabolic surface  12  and can be used with different feeds  13  which can be clustered by frequency f 1 , f 2 , f 3  and f 4 , see FIG. 6. Depending on the position of the each feed  13 , the beams of the same frequency f 1  can be directed in specific directions to illuminate closed areas on the Earth, shown in FIG. 6. So, the spots  23  of the same frequency f 1  are arranged at parallelepiped pattern, for example, hexagonal shape.  
         [0032]    Thus, the side lobes of a first spot  23  corresponding to one frequency f 1  are out of a second spot  23  of the same frequency f 1 . The second spot  23  is closed to the first spot  23 .  
         [0033]    The isolation is improved, up to 20 dB, among spots  23  of the same frequency f 1 . As a result, the signal to noise ratio of the offset signal is decreased and there is no need to provide means to the antenna system to increase the isolation among the beams of the same frequency f 1 .  
         [0034]    In the most general case, and as shown in FIG. 3, these parabolic arcs (AA′, BB′) are not the same and the cross-section of the beam can have a different length to width ratio dictated by the shape of the reflector  12 . Therefore, the parabolic reflector  12  is adapted to form an elongated spot  23  on the ground.  
         [0035]    Therefore an improvement layout of the spots  23  is provided, as shown in FIG. 6. This layout of spots  23  allows reducing the interference produced by another spots  23  on a given spot  23  of the same frequency f 1 . Since side lobes of each beam radiate out of the spots  23  provide by other feeds  13  of the same frequency f 1 , namely it prevents interference among spots  23  of the some frequency f 1 .  
         [0036]    Two layouts of elliptical spots  23  can be provided, firstly one layout is obtained by aligning of the spots  23  over the short axis of the elliptical shape. The other one by aligning of the spots over the long axis of the elliptical shape. Other layouts can be designed by rotating the axis of the spots.  
         [0037]    Therefore, the disposition and orientation of the reflector  12  and the feeds  13  determine the preferred direction and the shape of the beam of radiation.  
         [0038]    As the plurality of the feeds  13  are arranged according hexagonal pattern or parallelogram pattern. This means that the feeds  13  of the same frequency f 1  form a hexagon. Accordingly, other feeds of different frequency f 2 , f 3  and f 4  on another antenna are interleaved as so the spots  23  are also interleaved.