Patent Publication Number: US-7218902-B2

Title: Antenna for communication with a satellite

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application is a national phase application based on PCT/IT2002/000752, filed Nov. 29, 2002, the content of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention refers to telecommunication systems and it concerns in particular a satellite telecommunication system. 
   2. Description of the Related Art 
   Within the sphere of telecommunications, stationary satellites cover an ever more important role, both for television connections and for all the other applications, where they are used as transponders, for example in telephone communications. 
   As it is known, these satellites are located at a height of approximately 36,000 km in an orbit contained in the plane passing by the terrestrial equator. At this height, the speed, which this satellite must have to keep its position in the orbit, nullifying the gravitational pull force coincides with the earth rotation speed. The final result for an observer on the Earth, is that of seeing the fixed satellite in a precise position on the horizon. The choice of the orbital position (15 ° East, 28° West etc.) is made to stay, as much as possible, over the regions on the earth surface where the television signal is to be sent. By way of example, the position of one of the Hot Bird™(13° East) satellites, used to broadcast television signals, is optimal to cover Central Europe. Similarly, orbital positions shifted westwards are used for the Americas, and orbital positions shifted eastwards for Australia. 
   To ensure the best quality of signals received by earth stations, satellites are equipped with antennas having a radiation pattern shaped to concentrate transmitted power towards areas to be served. 
   Satellite position on the celestial vault, as seen by the land station is defined by the azimuth and the elevation angles, referred to the geographic North and to the horizon plane respectively, and it obviously varies according to the geographical co-ordinates of the land station itself. In particular, moving towards the poles, the angle between the horizon and the satellite direction, is gradually reduced, while the latitude increases (both Northwards and Southwards). 
   For this reason, to receive signals from a stationary satellite in the regions near the Equator, it is necessary to aim the antenna almost to the zenith (should the orbital position not coincide with the zenith of the land station, this angle must be recovered by slightly tilting the antenna). 
   Referring to a nation like Italy and to a stationary satellite like Hot Bird (13° East), the elevation required at Bolzano is of 36.3° and at Ragusa it is of 47.1° (example of maximum and minimum latitude for Italy), with a total excursion of approximately 10 degrees. The use of antennae with a sufficiently symmetrical radiation diagram with respect to the axis, entails therefore the necessity of having to set, at the time of the installation, both the elevation and the azimuth. This is currently performed at the installation of antennae, such as parabolas, both at a position that is fixed or a position that can be remote-controlled by means of mechanical positioning, see by way of example Patent EP0838876 application, or electronic controlled antennae at array with scanning beam, see by way of example U.S. Pat. No. 6,184,827. 
   Were it is possible to manage just the azimuth angle, installation operation would be simpler, and hence cheaper, and it would be possible to aim at different satellites without changing the elevation. 
   SUMMARY OF THE INVENTION 
   These problems are solved by the satellite communication system, being the purpose of this invention, and since it does not require aiming the antenna elevation, it can be installed in any place in a nation like Italy and it therefore enables using, and hence producing and distributing only one antenna model, entailing the reduction of production and storage costs. 
   The antenna can be installed in a vertical position, adhering to a wall, and this therefore means that it is not so showy, and that it complies with the regulations protecting the inner city urban decor. 
   A satellite telecommunications system is the particular purpose of this invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other characteristics of the present invention shall be clarified better by the following description of a preferred embodiment, given only by way of non limiting example, and by the enclosed drawings in which: 
       FIG. 1  represents the main lobe of a radiation diagram of an antenna realised according to the invention; 
       FIG. 2  represents a possible embodiment of an antenna realised according to the invention; 
       FIG. 3  represents a power divider and an antenna supply network phase shifter. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The system being the purpose of the invention includes a satellite receiver, to which a set for displaying television programmes can be connected, and an antenna, which as usual shows a sufficiently narrow radiation lobe on the horizontal plane, to discriminate two adjacent satellites. As it is known, television satellites are as a matter of fact spaced from one another by a minimum angle of approximately 3 degrees. 
   On the other hand on the vertical plane, the antenna shows a lobe which is:
         Wide enough to enable the reception of signal from a certain number of satellites in adjacent orbital positions, by land stations positioned on a certain meridian arc, without the necessity for changing the antenna elevation angle;   Sufficiently narrow to ensure an adequate gain, by way of example of 30 db.       

     FIG. 1  schematically shows the main lobe of the required radiation diagram, in side view a and in front view b, emitted by a plane surface antenna R. Plane p p cuts the lobe so as to highlight section p—p, having a vertical amplitude VA greater than the horizontal amplitude HA. 
   A planar antenna having such a radiation diagram, applied to a convenient external wall which “sees” the satellite, can enable the reception of the satellite transmissions after orienting, mechanically or electronically, the main lobe in the direction of the concerned satellite, acting only on one degree of freedom, that is the azimuth. 
   For what concerns both the movement and the shape of the radiation diagram, it can be useful to resort to array antennae with scanning beam technology. These antennae have a planar structure and are achieved with a large number of radiant components, all of which are equal and equally oriented. They are individually supplied with proper amplitude and phase signals, in order to obtain a radiation diagram showing the main lobe complying with the required elevation and azimuth direction. 
   The available project tools enable achieving an antenna with the main lobe in the direction of a wide range of azimuth and elevation angles. The lobe itself can moreover be modelled to show the required amplitudes on both the horizontal plane and on the vertical plane. By way of example, a suitable antenna can be used in a nation like Italy, covering a similar meridian arc and showing an equal average latitude, can show a main lobe amplitude on the vertical plane of approximately 10° and on the horizontal plane of 1 to 2°. 
   Each individual antenna radiant element is placed according to lines and columns in matrix structure, and it is supplied by proper amplitude and phase coefficients through transmission lines arranged according to columns. 
   Calculation of such coefficients sets the main lobe elevation and amplitude angle, which must be similar to that in  FIG. 1 . 
   Each column of elements makes up a vertical sub-array featured by a set elevation angle, which can be repeated in an identical way all over the whole antenna. 
   All the transmission line inputs supplying the aforesaid sub-arrays are combined in just one input in order to obtain the required main lobe direction on the azimuth plane. There are different techniques for performing this combination. Should just one direction be sufficient, a fixed controller can be used, while should rather the scanning be performed on the azimuth plane, numerical, electronic, RF devices etc. can be used. 
   The antenna can conveniently be achieved by using the micro-strip technique, according to which both the radiant elements and the supply leads can be made up of metal pads having a more or less wide or thin shape, achieved on a dielectric support. 
     FIG. 2  shows a possible embodiment of the antenna. The radiant structure is achieved on the rectangular plane surface R and it consists of a two-dimension planar array of 8×8 radiant elements on micro-strip P, arranged to make a regular matrix structure. Horizontal spacing So between the elements is not necessarily equal to the vertical spacing Sv. 
   All the radiant elements belonging to the same column C, achieve a sub-array, supplied by only one line in micro-strip and individual power dividers and phase shifters for each element, in order to generate a radiation diagram, similar to that shown in  FIG. 1  in the vertical plane. 
     FIG. 3  shows two basic devices of the antenna supply network. They are a power divider PD, provided with one input I with two outputs O 1  and O 2  at different power sizes, and a phase shifter PH, introducing a phase displacement φ along path L. 
   The just described satellite antenna is only one of the possible achievements suitable to the system being the purpose of the invention. The same functionality can be obtained by means of other technologies. By way of example, two further achievements are proposed. The first achievement consists in actuating the radiant elements by means of horns supplied by proper wave-guides. The second achievement, in lieu of the traditional parabola uses a reflector antenna shaped to generate a diagram similar to that in  FIG. 1  and driven by a simple powered positioner to select the azimuth and hence the required satellite. 
   Obviously the above descriptions are given by way of non limiting examples. Variants and amendments are possible without for this exiting the claim protection field.