Communications: radio wave antennas – Antennas – With polarization filter or converter
Reexamination Certificate
2001-05-17
2003-05-13
Wong, Don (Department: 2821)
Communications: radio wave antennas
Antennas
With polarization filter or converter
C343S771000, C343S776000
Reexamination Certificate
active
06563470
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to an antenna system employing a dual band frequency polarizer and, more particularly, to a satellite antenna system employing a dual band frequency polarizer, where the polarizer includes a waveguide having opposing corrugated structures that operate to convert a linearly polarized signal to a circularly polarized signal for a satellite downlink and convert a circular polarized signal to a linearly polarized signal for a satellite uplink, and vice versa.
2. Discussion of the Related Art
Various communications systems, such as certain telephone systems, cable television systems, internet systems, military communications systems, etc., make use of satellites orbiting the Earth to transfer signals. A satellite uplink communications signal is transmitted to the satellite from one or more ground stations, that retransmits the signal to another satellite or to the Earth as a satellite downlink communications signal to cover a desirable reception area depending on the particular use. The uplink and downlink signals are typically transmitted at different frequency bands. For example, the uplink signal may be transmitted at 30 GHz band and the downlink signal may be transmitted at 20 GHz band. The satellite is equipped with antenna systems including a number of antenna feeds that receive the uplink signals and transmit the downlink signals to the Earth.
For most of these satellite communications systems, one antenna system is provided for receiving the uplink signals and another antenna system is provided for transmitting the downlink signals. Each antenna system typically employs an array of antenna feed horns and one or more reflectors to collect and direct the signals. The uplink and downlink signals are circularly polarized so that the orientation of the reception antenna can be arbitrary relative to the incoming signal. To provide signal discrimination, one of the signals may be left hand circularly polarized (LHCP) and the other signal may be right hand circularly polarized (RHCP), where the signals rotate in opposite directions. Polarizers are employed in the antenna systems to convert the circularly polarized signals to linearly polarized signals suitable for propagation through a waveguide with low signal losses, and vice versa.
Because there are important weight and real estate limitations on a satellite, it is desirable to use the same antenna system for both transmitting the downlink signals and receiving the uplink signals. Because the uplink and downlink signals are at different frequency bands, the feed horns would have to be designed to transmit and receive the signals at both the uplink and downlink frequency bands. It would also be necessary to employ a dual band polarizer that could effectively convert the downlink signal from a linearly polarized signal to a circularly polarized signal and convert the uplink signal from a circularly polarized signal to a linearly polarized signal. However, known polarizers are only optimized for a single frequency band, making them unsuitable for polarizing signals of different frequency bands.
High frequency polarizers employing corrugated profiles are known in the art for converting a linearly polarized signal to a circularly polarized signal, and vice versa. For example, see U.S. Pat. No. 4,228,410 issued Oct. 14, 1980 to Goudey et al. However, the known corrugated polarizers of this type are not dual band polarizers that are able to polarize signals at two different frequency bands.
What is needed is a polarizer for an antenna system capable of transmitting a satellite downlink signal and receiving a satellite uplink signal, that is able to effectively provide polarization conversion in two separate frequency bands. It is therefore an object of the present invention to provide such a polarizer and antenna system.
SUMMARY OF THE INVENTION
In accordance with the teachings of the present invention, an antenna system is disclosed that employs antenna elements for both transmit and receive functions and a dual band polarizer to convert linearly polarized signals to circularly polarized signals and circularly polarized signals to linearly polarized signals for two separate frequency bands. The dual band polarizer is a waveguide device that includes a corrugated structure extending from opposing sidewalls, where ridges in the structures extend transverse to the propagation direction of the signals. The width of the ridges, the spacing between the ridges and the number of ridges are selected so that the polarization conversion is optimized for two frequency bands. Additionally, the height of the ridges taper from a lowest height at the ends of the waveguide to a largest height at the middle of the waveguide to minimize reflections. The corrugated structures interact with the field components of the signal in the direction perpendicular to the ridges to cause that component to be delayed relative to the field component parallel to the ridges, so that the polarization of the signal is changed accordingly.
Additional objects, advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.
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Chandler Charles W.
Em Makkalon
Wilson Louis C.
Chen Shih-Chao
Harness & Dickey & Pierce P.L.C.
Northrop Grumman Corporation
Wong Don
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