Communications: radio wave antennas – Antennas – Wave guide type
Reexamination Certificate
2001-08-28
2003-01-07
Le, Hoanganh (Department: 2821)
Communications: radio wave antennas
Antennas
Wave guide type
C343S786000, C343SDIG002
Reexamination Certificate
active
06504514
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to a dual-band equal-beam reflector antenna system and, more particularly, to a reflector antenna system for a satellite that employs a dual-band feed horn, including two different sizes of alternating corrugations, to create circularly symmetric beams at two different frequencies.
2. Discussion of the Related Art
Various communications systems, such as certain cellular 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, and is then retransmitted by the satellite to another satellite or to the Earth as a downlink communications signal to cover a desirable reception area depending on the particular use. The uplink and downlink signals are typically transmitted at different frequencies. For example, the uplink communications signal may be transmitted at 30 GHz and the downlink communications signal may be transmitted at 20 GHz.
The satellite is equipped with an antenna system including a configuration of antenna feeds that receive the uplink signals and transmit the downlink signals to the Earth. Typically, the antenna system includes one or more arrays of feed horns and one or more antenna reflectors for collecting and directing the signals. The uplink and downlink signals are typically 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 system to convert the circularly polarized signals to linearly polarized signals suitable for propagation through a waveguide with low signal losses, and vice versa.
For a satellite system, the coverage area on the Earth is broken into cells. The antenna coverage gain requirement for each cell then uniquely determines the reflector size. The combination of the reflector diameter and the reflector focal length will specify the feed locations and pointing angles. Further, each feed horn aperture must have a certain size for the frequency band of interest in order to provide a desirable antenna gain for that feed horn. Thus the feed horn size is much bigger than the cell size requires. Therefore, feed horns for the neighboring cells mechanically interface with each other when packaging as one feed array. In other words, because the feed horns must be a certain size to provide the desirable antenna gain, it is generally not possible to use the feed horns in the same array for contiguous cells on the Earth.
To provide the desirable antenna gain and still provide contiguous coverage on the Earth, it is therefore necessary to provide multiple antenna systems, each including a plurality of feed horns using the same reflectors, with each feed horn corresponding to a separate set of non-contiguous coverage areas, as designated, for example, by one of the letters A, B, C or D in FIG.
1
. In one design, the satellite includes four separate antenna systems (A, B, C and D antennas in
FIG. 2
) for the uplink communications signals and another four separate antenna systems for the downlink communications signals.
FIG. 2
illustrates this system. Because the uplink signals are typically at a higher frequency than the downlink signals, the size of the feed horn, and thus the size of the receive antenna system, is typically smaller than the size of the feed horns for the transmit arrays.
In order to reduce weight, conserve satellite real estate and decrease satellite production, integration and test costs, some satellite communications systems use the same antenna system and array of feed horns to receive the uplink signals and transmit the downlink signals. For example, if each antenna system on a satellite is a dual-band antenna system, then the number of antenna systems can be reduced from eight to four in the example being discussed herein. Combining satellite uplink signal reception and downlink signal transmission functions for a particular coverage area using a reflector antenna system requires specialized feed systems capable of supporting dual frequencies and providing dual polarization, and thus requires specialized feed system components. These specialized feed system components include signal orthomode couplers, such as four-arm turnstile junctions, known to those skilled in the art, in combination with each feed horn to provide signal combining and isolation to separate the uplink and downlink signals. Also, the downlink signal, transmitted at higher power (60-100 W) at the downlink bandwidths (18.3 GHz-20.2 GHz), requires low losses due to the cost/efficiency of generating the power and heat when losses are present.
One example of an antenna system providing both receive and transmit functions is referred in the industry as the MILSTAR dual band feed. The MILSTAR dual-band feed employs a co-axial design where concentric inner and outer conductive walls define an outer waveguide cavity and an inner waveguide cavity. The downlink signal is transmitted through the outer waveguide cavity and out of a tapered feed horn, and the uplink signal is received by the tapered feed horn and is directed through the inner waveguide cavity. A tapered dielectric is positioned at the aperture of the inner waveguide cavity to provide impedance matching between the feed horn and the inner waveguide cavity, and also launches the uplink signal into the inner waveguide cavity so that it is above the waveguide cut-off frequency. The inner surface of the feed horn is corrugated to provide a symmetrical pattern for both the uplink and downlink signals for equal E-plane and H-plane matching. The feed horn is tapered to provide an aperture suitable for illuminating the reflector associated with the antenna system.
Improvements can be made to those antenna systems that provide both transmit and receive functions. For example, because the uplink and downlink communications signals are at different frequencies, the cell coverage area for the uplink and downlink signals in the known dual-band antenna feeds have different beamwidths or cell size. Thus, the higher frequency uplink signal has a reduced coverage area than the lower frequency downlink signal when using a dual-band feed horn that affects antenna performance and uplink coverage capabilities.
What is needed is a dual-band antenna system for satellite communications where the uplink and downlink signals have the same beamwidths for optimal coverage capabilities. It is therefore an object of the present invention to provide such an antenna system.
SUMMARY OF THE INVENTION
In accordance with the teachings of the present invention, a satellite antenna system is disclosed that employs a dual-band feed horn and a dual-band beam forming network. The dual-band feed horn provides a common aperture for both a satellite uplink and a satellite downlink communications signal. The feed horn includes corrugations on its inside surface that define two sets of alternating channels having different depths to create circularly symmetric beams for the uplink and downlink signals. The antenna system includes at least one reflector, where the reflector size and position, and the configuration of the feed horn, is optimized so that the mainlobe of the lower frequency downlink feed signal illuminates the entire reflector, and the higher frequency uplink feed signal covers an inner portion of the reflector. The first sidelobes of the higher frequency feed signal illuminate the outer portion of the reflector so that the uplink and downlink antenna signals have the same beamwidth, and thus cover the same cell size on the Earth.
Additional objects, advantages and features of the present invention will become apparent from the fol
Chan Ronald Y.
Choung Youn H.
Hamada James S.
Minassian Vrage
Toland Brent T.
Le Hoang-anh
TRW Inc.
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