ANTENNA SYSTEM FOR CIRCULARLY POLARIZED WAVES

Abstract: An antenna system comprises a reflector which is a part of a paraboloid of revolution or parabolic cylinder, a primary radiator for clockwise circulatory polarized wave, and a primary radiator for counterclockwise circularly polarized wave. The reflector is of geometrically asymmetrical shape to effect different reflection properties for clockwise and counterclockwise circular polarizations. The primary radiators are fixed at two different positions in the vicinity of the focus of the paraboloid reflector.

Description: Background of the Invention The present invention relates to an antenna system for receiving and transmitting clockwise and counterclockwise circularly polarized wave signals. Recently, many countries have been participating in satellite communication and various kinds of antenna for receiving broadcast waves from satellites have been developed. Satellite communication, on 12GHz band particularly, uses circularly polarized waves to avoid crosstalk between channels and between broadcast waves of various countries. Each of these countries are allocated with a particular frequency band and either of clockwork or counterclockwise circularly polarized wave. In addition, the positions of satellites on stationary orbits are also fixed for each country. In some cases, two or more satellites are positioned at one location to transmit clockwise and counterclockwise circularly polarized waves. In such a situation, if one satellite communication-receiving antenna system can receive clockwise and counterclockwise circular polarized waves simultaneously or at different times, it would be extremely useful because more broadcast waves could be received. In general, a satellite communication-receiving antenna system is composed of a reflector and a primary radiator fixed on the focus of the reflector. The primary radiator is usually designed, and used, for receiving either clockwise or counterclockwise circularly polarized waves. To receive clockwise and counterclockwise circularly polarized waves sent from different broadcasting satellites by the conventional antenna system, a system must be equipped with a plurality of reflectors and primary radiators. As a result, the system construction and labour costs increase accordingly. It is still, however, desirable to provide an antenna system for satellite communication of simple construction capable of receiving both clockwise and counterclockwise circularly polarized waves. French patent 1,212,148 describes an antenna having a parabolic reflector in the vicinity of whose focus there are two sources located as close together as possible. Summary of the Invention It is an object of the present invention to provide an antenna system that alleviates some of the aforementioned problems. Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only; various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. To achieve the above objects, an antenna system comprises a transmitting and/or receiving electromagnetic radiation antenna system comprising: a parabolic reflector having a focal region, the optical path of the radiation transmitted or received by the reflector being offset from the focal region and radiation means adapted to transmit and/or receive said electromagnetic radiation and positioned in said focal region, said radiator means comprising first and second radiator elements respectively adapted for clockwise and counterclockwise circularly polarized radiation and characterized in that said first and second radiator elements are separated by a predetermined distance in accordance with the differential direction of reflection of clockwise and counterclockwise radiation by the parabolic reflector. That is, paying attention to the fact that the asymmetry of the reflector causes the beams of the clockwise and counterclockwise circularly polarized waves to be reflected in different directions, the primary radiators for clockwise and counterclockwise circularly polarized waves are fixed in different positions, so that clockwise and counterclockwise circularly polarized waves coming from the exterior with the same or different incident angles are reflected by the reflector and taken out separately by the respective primary radiators. Each of the primary radiators used in the invention may be of any desired type if it is designed either for clockwise or counterclockwise circularly polarized wave. To make the antenna system structure simple, a simple antenna such as a helical or patch antenna may be used. The present invention is not only to the receiving antenna system but also applicable to a transmitting antenna system based on the same principle. Brief Description of the Drawings The present invention will be better understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention in which: Figure 1 shows an offset parabolic antenna of an embodiment of the present invention viewed from the top; Figure 2 illustrates radiation characteristic of another embodiment of the invention; Figure 3 shows a typical offset parabolic antenna; Figure 4 shows the reflection characteristic of circularly polarized wave in an offset parabolic antenna; Figure 5 is a plan view showing the antenna of still another embodiment of the invention; Figure 6 shows the reflected beam characteristic of circularly polarized wave in a typical offset parabolic antenna viewed from the top; and Figure 7 is a side view of the reflector for showing the reflection characteristic of the antenna system of the invention. Detailed Description of the Invention The invention will be described using an antenna system that contains an asymmetrical offset parabolic antenna formed by a part of the paraboloid of revolution, a typical embodiment of the invention. Figure 3 shows an ordinary offset parabolic antenna. 1 is a paraboloid of revolution, 2 is a reflector formed by a part of the paraboloid of revolution 1, 3 is a primary radiator, B is an incident wave beam, and F is the focus of the paraboloid of revolution 1. The primary radiator 3 is fixed at the position of the focus F. As shown in the figure, the offset paraboloid antenna uses the asymmetrical reflector 2. As a result, the primary radiator 3 is positioned outside the aperture of the reflector, avoiding aperture blocking. With this antenna system, linearly polarized excitation results in a cross polarized component due to the asymmetrical reflecting surface. On the other hand, circularly polarized excitation does not result in cross polarized componentsbecause the circularly polarized waves becomes positively polarized component through 90° phase shift. The direction of reflected principal beam is different for clockwise and counterclockwise circularly polarized waves. Figure 4 shows the directions of reflected principal beams, assuming that polarized wave is fed from the position of the focus F. The Figure 4 is a top view of the offset parabolic antenna shown in Figure 2. Clockwise circularly polarized wave radiation from the position of the focus F is reflected by the reflector 2 so that the principal beam is directed as shown by the solid line a ○. Counterclockwise circularly polarized wave radiation from the focus F is reflected by the reflector so that the principal beam is directed as shown by the broken line b ○. For linearly polarized wave radiation, the principal beam is directed as shown by the chain line c ○ which is parallel to z axis of the offset parabolic antenna. The present invention is based on the above mentioned difference in the reflection characteristic between clockwise and counterclockwise circularly polarized waves. Figure 1 shows an embodiment of the offset parabolic antenna of the present invention, viewed from the top. In Figure 1, 2 is the same reflector as shown in Figure 3, F is the focus of the paraboloid of revolution (referred to as 1 in Figure 2), 3R is a clockwise circular polarization primary radiator, and 3L is a counterclockwise circular polarization primary radiator. The clockwise circular polarization primary radiator 3R is fixed at a position to the right of the focus F (above the focus F in Figure 1) on the plane defined by z axis and y axis. The counterclockwise circular polarization primary radiator 3L is fixed at a position to the left of the focus F (above the focus F in Figure 1) on the plane defined by z axis and y axis. The primary radiators 3R, 3L are offset from the axis of symmetry by the angle ϑ to compensate the beam displacement by circular polarization. This angle ϑ is equivalent to the angle ϑ between the solid line a ○ or broken line b ○ and the z axis shown in Figure 3. In the antenna system with the above construction, clockwise and counterclockwise circularly polarized waves coming from the same direction in the front (that is, from the direction along z axis) are reflected by the reflector 2 into different directions. Then, the principal beams of the circularly polarized waves are simultaneously or individually received by the primary radiators 3R, 3L respectively. When the antenna system is being used for transmission, clockwise and counterclockwise circularly polarized radiations from the respective primary radiators 3R, 3L are reflected by the reflector, so that the principal beams of the circularly polarized radiations are sent off in the same direction to the front (that is, in the direction along z axis). The primary radiators 3R, 3L may be of any type as long as they are specially designed for clockwise and counterclockwise circular polarizations respectively. Compact antenna system can be achieved by employing small elements such as helical elements or micro strip elements for the primary radiators 3R, 3L. As shown in Figure 2, a part of the paraboloid of revolution 1 which constitutes the reflector 2 may be away from the axis of symmetry, and the focus F may be closer to the symmetrical center of the paraboloid of revolution 1 to increase the asymmetry of the reflector 2. In this case, the angle ϑ is made larger than that shown in Figure 1, which is convenient in installing the primary radiators 3R, 3L (See Figure 1). In the above embodiments, partial paraboloid of revolution is used for the reflector. Partial parabolic cylinder used for the reflector also provides the same effect as the partial paraboloid of revolution. According to the present invention, as described above, paying attention to the fact that the beams for the clockwise and counterclockwise circularly polarized radiations shift in opposite directions, the primary radiators for clockwise and counterclockwise circularly polarized waves are arranged in different positions with respect to the geometrically asymmetrical reflector such as an offset parabolic antenna, so that clockwise and counterclockwise circularly polarized waves coming from the same direction (from the broadcasting satellites on the same stationary orbit) are separatedly received or transmitted by the respective primary radiators. Since signals with different circular polarization properties sent by one or more broadcasting satellites are received simultaneously by one reflector, the present invention is extremely useful when applied to satellite communication receiving antennas. Another embodiment of the present invention is now described with reference to Figures 5 through 7. Figure 5 is a plan view of the antenna system of another embodiment of the present invention, Figure 6 shows the beam reflection characteristics of circularly polarized waves in a typical offset parabolic antenna viewed from the top, and Figure 7 is a side view of the reflector of this embodiment for describing beam reflection characteristics. In this embodiment as well, a part of the paraboloid of revolution is used for an asymmetrical offset parabolic antenna reflector. In Figure 5, 11 is a reflector, 12 is a clockwise circular polarization primary radiator, 13 is a counterclockwise circular polarization primary radiator, 14 is a satellite transmitting clockwise circularly polarized wave, 15 is a satellite transmitting counterclockwise circularly polarized wave, and 16 is the focus of the reflector 11. The reflector 11 is of the shape of a partial paraboloid of revolution. Which part of the paraboloid of revolution should be used is described below with reference to Figures 6 and 7. Suppose a primary radiator is located at the focus 18 of the offset parabolic antenna reflector 17 as shown in Figure 6. The principal beams of clockwise circularly polarized wave 19 and counterclockwise circularly polarized wave 20 shift in different directions because of the asymmetry of the reflector 17. The amount of each beam shift varies depending on which part of the paraboloid of revolution is selected for the reflector 17. When a reflector 22 is such part of a paraboloid of revolution 21 as shown in Figure 7, for instance, the amount of beam shift increases with the angle ϑc between z axis and the line connnecting the focus 23 with the end 22a of the reflector 22 as well as with the angle ϑo between the above line and the line connecting the focus 23 with the end 22b of the reflector 22. Accordingly, the reflector 11 (Figure 5) of the present invention is formed by the part of the paraboloid of revolution so that the angles ϑc and ϑo are large. As shown in Figure 5, the clockwise circular polarization primary radiator 12 is positioned to the right of the focus 16 and the counterclockwise circular polarization primary radiator 13 to the left of the focus 16 as viewed from the top. The offset angle ϑ' of each of the primary radiators 12, 13 from z axis is determined so that the angle ϑ' + ϑ₁, in Figure 5 is equivalent to the beam shift. With such arrangement of the primary radiators 12, 13, the principal beams of clockwise and counterclockwise circularly polarized waves from the respective primary radiators 12, 13 are directed to a clockwise circular polarization satellite 14 and counterclockwise circular polarization satellite 15, respectively. Because of the theory of reversibility for antennas, the primary radiators 12, 13 can receive circularly polarized waves from broadcasting satellites with small gain loss. According to the present invention, as understood from the above description, two primary radiators having clockwise and counterclockwise circular polarization properties respectively are arranged in different positions with respect to a geometrically asymmetric reflector such as an offset parabolic antenna, so that clockwise and counterclockwise circularly polarized wave signals sent from satellites on one of more stationary orbits are separatedly received by the respective primary radiators or transmitted therefrom. Accordingly, signals with different circular polarization characteristics sent from a plurality of broadcasting satellites can be received by one reflector, which is extremely convenient for a satellite communication-receiving antenna system.

Claims: A transmitting and/or receiving electromagnetic radiation antenna system comprising: a parabolic reflector (2;11) having a focal region (F;16), the optical path of the radiation transmitted or received by the reflector being offset from the focal region and radiation means adapted to transmit and/or receive said electromagnetic radiation and positioned in said focal region, said radiator means comprising first and second radiator elements (3R,3L;12,13) respectively adapted for clockwise and counterclockwise circularly polarized radiation and characterized in that said first and second radiator elements are separated by a predetermined distance in accordance with the differential direction of reflection of clockwise and counterclockwise radiation by the parabolic reflector. An antenna system according to claim 1 wherein said first and second radiator elements are positioned to receive respective clockwise and counterclockwise circularly polarized light reflected by the reflector and incident thereon from substantially the same direction. An antenna system according to claim 1 wherein said first and second radiator elements are positioned so that clockwise and counterclockwise circularly polarized light respectively transmitted by them is emitted from the reflector in substantially the same direction. An antenna system according to claim 1 wherein said first and second radiator elements are positioned to receive respective clockwise and counterclockwise circularly polarized light reflected by the reflector and incident thereon from different respective directions. An antenna system according to claim 1 wherein said first and second radiator elements are positioned so that clockwise and counterclockwise circularly polarized light respectively transmitted by them is emitted from the reflector in different respective directions.

Inventors: HIGASHI, KAZUTADA, OHTA, TOMOZO, YAMAMOTO, HIROHIKO
Applicants: SHARP KABUSHIKI KAISHA
Application year: 1985
Main classification: H01Q25
IPC codes: H01Q19, H01Q25