A portion of the disclosure of this patent document contains material which is subject to copyright protection. This patent document may show and/or describe matter which is or may become trade dress of the owner. The copyright and trade dress owner has no objection to the facsimile reproduction by anyone of the patent disclosure as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright and trade dress rights whatsoever.
1. Field
This disclosure relates to antennas for satellite communications earth stations.
2. Description of the Related Art
Satellite communications systems use one or more orbiting satellite to relay communications between a pair of earth stations. Each earth station typically consists of a transmitter and a receiver coupled to a highly directional antenna. A common form of antenna for transmitting to and receiving from a satellite consists of a parabolic dish reflector and a feed network. Given the large distance between each earth station and the satellite, each earth station must be configured to transmit a relatively high power signal and to receive a very low power signal. To ensure that transmission from a first earth station does not interfere with reception at a second proximate earth station, earth station antennas must be designed to have very low side lobe and back lobe radiation.
Earth station antennas typically have either a center-feed or an offset-feed. In a typical center-feed antenna, the feed network is located along the axis of the parabolic reflector, and thus blocks a portion of the reflector aperture. In an offset-feed antenna, the reflector is an off-axis portion of a parabolic dish and the feed network is located to one side where it does not block a portion of the reflector aperture. Center feeds are commonly used with large diameter reflectors, since the feed network may block only a negligible portion of the reflector aperture. Offset feeds are commonly used with small reflectors where a center feed network would block a substantial portion of the reflector aperture.
Since the feed network of an offset-feed antenna is located to the side of the reflector, an offset-feed antenna occupies a larger volume than a center-feed antenna for equivalent reflector aperture. In some applications, such as portable or mobile earth stations, an antenna may be mounted on a gimbal configured to point the antenna at any desired angle within a hemisphere. In this case, an offset-feed antenna will sweep a substantially larger volume than a center-feed antenna of equivalent aperture, and thus require a substantially larger radome.
In this patent, the term “circular waveguide” means a waveguide segment having a circular cross-sectional shape. Similarly, the term “annular waveguide” means a waveguide segment having a cross-sectional shape of an annulus between two concentric circles. In this patent, the term “port” refers generally to an interface between devices or between a device and free space. A port of a waveguide device may be formed by an aperture in an interfacial surface to allow microwave radiation to enter or exit a waveguide within the device.
Elements in the drawings are assigned three-digit reference numbers where the most significant digit indicates the figure number where the element was introduced. An element not described in conjunction with a figure may be presumed to be the same as a previously-described element having the same reference number.
The feed network 120 may include a circular waveguide 130, a sub-reflector 140, and a stem 150, each of which may be rotationally symmetric about the antenna axis 105. The circular waveguide 130 may have a first end forming a port 132 for introduction of signals to be transmitted from the antenna and for extraction of signals received by the antenna. The port 132 may be coupled, for example, to a diplexer and/or an ortho-mode transducer for separating the transmitted and received signals, neither of which is shown in
The subreflector 140 may comprise a generally conical central portion 142, and a curved outer portion 144. The stem 150 may extend from the conical central portion 142 of the sub-reflector 140 into the circular waveguide 130, thus forming a short length of annular waveguide 152. While the element 140 has been termed the “sub-reflector” in consideration of common practice, the sub-reflector 140 is not purely a reflector. Rather, the sub-reflector 140, the stem 150, and the second end 134 of the circular waveguide 130 collectively form a waveguide structure 148 that causes energy propagating in the annular waveguide 152 to bend radially outward through an angle approaching 180 degrees and thus be directed towards the primary reflector 110. The curved outer portion 144 of the sub-reflector 140 may have a rim that lies in a third plane 146 parallel to the first plane 116 and the second plane 136.
The “generally conical” center portion 142 of the sub-reflector 140 may be a surface generated by rotating a line passing through a fixed vertex. The “generally conical” center portion 142 of the sub-reflector 140 may be generated by rotating a straight line to form a right circular cone. The “generally conical” center portion 142 of the sub-reflector 140 may be generated by rotating a curved line, in which case the center portion 142 will deviate from a true cone.
In the example of
Referring now to
The feed network 220 may include a feed body 260 enclosing a circular waveguide 230, a sub-reflector 240, and a stem 250. The primary reflector 210, the feed body 260, the sub-reflector 240, and the stem 250 may all be rotationally symmetric about an antenna axis 205 (also the axis of the circular waveguide 230). Although section lines are not shown in
The circular waveguide 230 may have a first end forming a port 232 for introduction of signals to be transmitted from the antenna and for extraction of signals received by the antenna. The sub-reflector 240 may comprise a generally conical central portion 242, and a curved outer portion 244. The stem 250 may extend from the conical central portion 242 of the sub-reflector 240 into the circular waveguide 230, thus forming a short length of annular waveguide 252.
The curved outer portion 244 of the sub-reflector 240 may have the shape of a warped ring-focus parabola. As shown in
4F(z+αz2)=(r−r0)2+β(r−r0)4 (1)
Returning now to
The sub-reflector 240 may be formed with continuously curved surfaces, as shown in
An outer surface of the feed body 260 may be corrugated, which is to say the outer surface of the feed body 260 may include ribs 262 having relatively larger diameters separated by regions 264 having relatively smaller diameter. The ribs may be configured to concentrate energy radiated from the waveguide structure 248 close to the feed body 260. The ribs closest to the subreflector 240 also help control the match of the input waveguide, and antenna pattern properties such as cross polarization and side lobes.
The feed body 260, the sub-reflector 240, and the stem 250 may be formed of a conductive metal material such as aluminum or copper. In this case, the feed body 260, the sub-reflector 240, and the stem 250 may be fabricated by machining operations such as turning on a lathe or milling on a milling machine. The feed body 260, the sub-reflector 240, and/or the stem 250 may be fabricated by casting or some other metal working process. The sub-reflector 240 and the stem 250 may be fabricated as a single piece. The sub-reflector 240 and the stem 250 may be fabricated as two pieces assembled by, for example, soldering, brazing, bonding, or mating a threaded portion of the stem with a threaded hole in the sub-reflector.
The feed body 260, the sub-reflector 240, and/or the stem 250 may be formed of a nonconductive material, such as a ceramic or plastic material, coated with a conductive coating. For example, the feed body 260, the sub-reflector 240, and/or the stem 250 may be formed by casting, injection molding, or machining a plastic material. Subsequently, the plastic component may be coated with a conductive layer such as gold or aluminum by plating, sputtering, evaporation, or some other process.
The sub-reflector support 470 may be configured to mechanically support the sub-reflector 240 in a desired position relative to the food body 260. The sub-reflector support 470 may also provide a seal between the sub-reflector 240 and the feed body 260 to prevent moisture, dirt, and other environmental contaminants from entering the circular waveguide 230. The sub-reflector support 470 may be formed with continuously curved surfaces or, as shown in
The sub-reflector support 470 may be configured to press-fit over the feed body 260 and the sub-reflector 240. The sub-reflector support 470 may be bonded to one or both of the feed body 260 and the sub-reflector 240 using a suitable adhesive.
The stem support 480 may be configured to mechanically support the stem 250 centered within the circular waveguide 230. The stem support 480 may be shaped as a bobbin with two flanges, as shown in
The stem support 480 may be configured to press-fit over the stem 250 and slip-fit within the circular waveguide 230. The stem support 480 may be bonded to one or both of the stem 250 and the interior of the feed body 260 using a suitable adhesive.
Referring now to
The feed network 520 may include a feed body 560 enclosing a circular waveguide 530, a sub-reflector 540, and a stem 550. The primary reflector 510, the feed body 560, the sub-reflector 540, and the stem 550 may all be rotationally symmetric about an antenna axis 505 (also the axis of the circular waveguide 530). Although section lines are not shown in
The primary reflector 510 may have a substantially larger diameter that the diameter of the primary reflector 210 of the antenna 200. The larger diameter of the primary reflector 510 may necessitate a correspondingly longer feed body 560.
The circular waveguide 530 may have a first end forming a port 532 for introduction of signals to be transmitted from the antenna and for extraction of signals received by the antenna. The sub-reflector 540 may comprise a generally conical central portion 542, and a curved outer portion 544. The curved outer portion 544 may have the shape of a warped ring-focus parabola as previously described. The stem 550 may extend from the conical central portion 542 of the sub-reflector 540 into the circular waveguide 530, thus forming a short length of annular waveguide 552. The sub-reflector 540 may be formed with continuous or stepped surfaces as previously described.
The sub-reflector 540, the stem 550, and the feed body 560 may collectively form a waveguide structure 548 that causes energy propagating in the annular waveguide 552 to bend radially outward through an angle approaching 180 degrees and thus be directed towards the primary reflector 510.
An outer surface of the feed body 560 may be corrugated, which is to say the outer surface of the feed body 560 may include ribs 562 having relatively larger diameters separated by regions 564 having relatively smaller diameter. The corrugations may be configured to concentrate energy radiated from the waveguide structure 548 close to the feed body 560.
The feed body 560, the sub-reflector 540, and the stem 550 may be formed of a conductive metal material such as aluminum or copper, and may be fabricated by machining, casting, or some other metal working process as previously described. The feed body 560, the sub-reflector 540, and/or the stem 550 may be formed of a nonconductive material, such as a ceramic or plastic material, coated with a conductive coating, as previously described.
The choke groove 646 may be disposed around a perimeter of the subreflector 540. The presence of the choke groove 646 may help control antenna pattern properties such as side lobes.
An outside diameter of the stem 550 may change in steps 654, and an inside diameter of the circular waveguide 530 may change in steps 634 to provide impedance matching from the circular waveguide 530 through the annular waveguide section to the waveguide structure 548.
The sub-reflector support 670 may be configured to mechanically support the sub-reflector 540 in a desired position relative to the food body 560. The sub-reflector support may mechanically connect the perimeter of the sub-reflector 540 with the outside of the feed body 560. The sub-reflector support 670 may be formed with continuously curved surfaces or, as shown in
The sub-reflector support 670 may be configured to press-fit over the feed body 560 and the sub-reflector 540. The sub-reflector support 670 may be configured to engage the choke groove 646 around the perimeter of the sub-reflector 540. The sub-reflector support 670 may be bonded to one or both of the feed body 560 and the sub-reflector 540 using a suitable adhesive. A surface 672 of the sub-reflector support 670 may be adjacent to, and mechanically supported by, a top rib 668 of the feed body 560. Mechanically supporting the surface 672 of the sub-reflector support 670 may increase the physical robustness of the feed network 520. The feed network 520 may be suitable for use in portable applications where an antenna may encounter substantial shock and vibration during transportation and handling.
The sub-reflector support 670 may also provide a seal between the sub-reflector 540 and the feed body 560 to prevent moisture, dirt, and other environmental contaminants from entering the circular waveguide 530.
The stem support 680 may be configured to mechanically support the stem 550 centered within the circular waveguide 530. The stem support 680 may be shaped, for example, as a bobbin with three flanges, as shown in
The stem support 680 may be configured to press-fit over the stem 550 and slip-fit within the circular waveguide 530. The stem support 680 may be bonded to one or both of the stem 550 and the interior of the feed body 560 using a suitable adhesive.
An antenna, such as the antennas 100, 200, and 500, may be designed using a commercial software package such as CST Microwave Studio. An initial model of the antenna may be generated with estimated dimensions for the primary reflector and the feed network. The initial model may then be analyzed, and parameters such as the reflection coefficient at the antenna input port, antenna gain, and side lobe and back lobe radiation may be determined. The parameters and dimensions of the model may then be iterated manually or automatically to minimize the reflection coefficient, side lobe energy and back lobe radiation across an operating frequency band. Parameters that may be automatically optimized may include, for example, the warping coefficients α, β, that determine the shape of the curved outer potion of the sub-reflector and the shape of the generally conical center portion of the sub-reflector. As previously described,
Throughout this description, the embodiments and examples shown should be considered as exemplars, rather than limitations on the apparatus and procedures disclosed or claimed. Although many of the examples presented herein involve specific combinations of method acts or system elements, it should be understood that those acts and those elements may be combined in other ways to accomplish the same objectives. With regard to flowcharts, additional and fewer steps may be taken, and the steps as shown may be combined or further refined to achieve the methods described herein. Acts, elements and features discussed only in connection with one embodiment are not intended to be excluded from a similar role in other embodiments.
As used herein, “plurality” means two or more. As used herein, a “set” of items may include one or more of such items. As used herein, whether in the written description or the claims, the terms “comprising”, “including”, “carrying”, “having”, “containing”, “involving”, and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of” and “consisting essentially of”, respectively, are closed or semi-closed transitional phrases with respect to claims. Use of ordinal terms such as “first”, “second”, “third”, etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements. As used herein, “and/or” means that the listed items are alternatives, but the alternatives also include any combination of the listed items.
This patent claims priority from Provisional Patent Application No. 61/771,622, filed Mar. 1, 2013, entitled COMPACT LOW SIDELOBE ANTENNA AND FEED NETWORK.
Number | Date | Country | |
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61771622 | Mar 2013 | US |