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.
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.
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.
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.
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.
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.
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.
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.
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.
Any number of spots may be defined by projecting additional beams from the multi-beam antenna.
The antenna system provides for improved uniformity of signal gain with a simplified mechanical structure of the antenna system.
The present invention therefore introduces a spot-clustering scheme wherein spots are elliptical arranged one un-adjacent another of the same frequency.
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.