High performance multi-band frequency selective reflector with equal beam coverage

[0002] The present invention relates to reflector antennas, and more particularly, to multi-band antennas for spacecraft having frequency selective absorber surfaces or frequency selective surfaces

[0003] Spacecraft, particularly satellite communication systems, use reflector antennas to transmit and to receive microwave frequency signals, typically, to and from land based communication stations. Advanced satellite communication systems typically require low mass, low volume, low cost, multi-band antennas. Examples of multi-band antennas where the same reflector is used for both uplink and downlink frequencies, which are different, are disclosed in U.S. Pat. No. 6,169,524 B1. The multi-band antennas disclosed in this patent utilize frequency selective or polarization sensitive zones to enable a single reflector to provide the plurality of antenna patterns for the different uplink and downlink frequencies.

[0004] Some applications require that different frequency bands have the same beamwidth or beam coverage cell size. In such applications, the antenna reflector for a multi-band antenna must provide the same cell size for the different frequency bands.

[0005] A multi-band antenna reflector for use with first and second different frequency bands to produce a comparable beam cell size for each frequency band with high performance, low sidelobe and cross polarization levels, and a superior carrier-to-interference ratio (C/I) in accordance with an embodiment of the invention has a concave reflector. The concave reflector has central and outer concentric zones. The central concentric zone reflects signals in the first and second frequency bands and the outer concentric zone is configured as a frequency selective absorber that reflects signals in the first frequency band and absorbs signals in the second frequency band. The frequency selective absorber has a finite conducting pattern or lossy element pattern dimensioned to reflect signals in the first frequency band with a predetermined equivalent reactance value and absorb the signals in the second frequency band with a pre-determined sheet resistance (ohms/square) and absorber rings disposed at the periphery of the outer concentric zone.

[0006] A multi-band antenna reflector for use with first and second different frequency bands to produce a comparable beam cell size for each frequency band with high performance, low sidelobe and cross polarization levels, and a superior carrier-to-interference ratio (C/I) in accordance with another embodiment of the invention has central and outer concentric zones. The central concentric zone has a reflective layer on an inner surface that reflects signals in the first and second frequency bands. The outer concentric zone is a frequency selective surface formed of a dielectric core having a finite conducting pattern on top and bottom sides. The conductive pattern is dimensioned to reflect signals in the first frequency band and pass signals in the second frequency band. Resistance cards overlay the top and bottom of the junction between the reflective layer of the central zone and the frequency selective surface of the outer zone.

[0007] In an embodiment of the invention, dual frequency feeds transmit signals in the first and second frequency bands to the concentric zones.

[0008] Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.

[0009] The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:

[0010] FIG. 1ais a top plan view of a multi-band antenna reflector in accordance with an embodiment of the invention;

[0011] FIG. 1bis a side plan view of the multi-band antenna reflector of FIG. 1a;

[0012] FIG. 2 is top view, partially broken away, of a frequency selective absorber of the multi-band antenna reflector of FIG. 1;

[0013] FIG. 3 is a graph showing specified reflected power of the frequency selective absorber of the multi-band antenna reflector of FIG. 1;

[0014] FIG. 4aa top plan view of a multi-band antenna reflector in accordance with an embodiment of the invention;

[0015] FIG. 4bis a side plan view of the multi-band antenna reflector of FIG. 4a;

[0016] FIG. 5ais top view, partially broken away, of a frequency selective surface of the multi-band antenna reflector of FIG. 4a;

[0017] FIG. 5bis a section view of a frequency selective surface of the multi-band antenna reflector of FIG. 4ataken along the line 5b—5b;

[0018] FIG. 6 is a side view of a junction between a conductive layer of a central zone and the frequency selective surface outer zone of the multi-band antenna of FIG. 4a;

[0019] FIG. 7 is a graph showing the specified transmission coefficient of the frequency selective surface of the multi-band antenna reflector of FIG. 4a;

[0020] FIG. 8 is a graph showing predicted radiation patterns at 30 GHz for the multi-band antenna reflectors of FIGS. 1aand 4a; and

[0021] FIG. 9 is a graph comparing the radiation patterns of a reflector in accordance with FIGS. 1aand 4awith a reflector that has neither a frequency selective absorber or a frequency selective surface.

[0022] The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.

[0023] Referring to FIGS. 1aand 1b, an embodiment of the invention utilizing a frequency selective absorber 20 is shown that provides comparable beam coverage cell size for two frequency bands, such as the Ku frequency band (10.95-14.5 GHz) and Ka frequency band (18.2-30 GHz) with high performance, low sidelobe and crosspolarization levels, and a superior C/I ratio (for example, better than 17 dB). A concave reflector 10, illustratively a parabolic reflector, has a two concentric zones, a central zone 12 and an outer concentric zone 14. Central zone 12 is configured to reflect both Ku and Ka band signals while outer zone 14 is formed as a frequency selective absorber that reflects only Ku band signals and absorbs Ka band signals. Reflector 10 has a lightweight core 16, preferably formed of a material reflective at the Ku and Ka bands, preferably graphite. If lightweight core 16 is formed of a material that is not reflective of the Ku and Ka bands, then a reflective coating must be applied to an inner or active surface 31 of at least the central zone 12 portion of lightweight core 16.

[0024] Referring to FIGS. 1band 2, outer zone 14 is a frequency selective absorber 20 comprising a finite conducting pattern or lossy element pattern 22, illustratively a metallic pattern, on a substrate film 24, such as teflon. The pattern 22 and substrate film 24 are affixed in spaced relationship to the lightweight core 16 of reflector 10, such as by spacers 26. The total thickness of substrate film 24 and spacers 26 should be approximately one-quarter wavelength at the Ka band. Spacers 26 can illustratively be a frame structure or a layer of foam such as Rohacell™ foam. Rohacell™ foam is fabricated by Richmond Corporation located in Norwalk, Calif. A ring 18 of taper shaped absorber may be added to an outer periphery 28 of outer zone 14 and to the boundary between inner zone 12 and outer zone 14. The absorptive material used for the absorber rings is an absorptive material that is absorptive at Ka band frequencies, such as an Eccosorb® absorber that is absorptive at Ka band frequencies, for example, Eccosorb® AN-72. Eccosorb® absorbers are manufactured by Emerson & Cuming located in Canton, Mass. A resistance card, such as resistance card 122 discussed below, can also be applied to the boundary between inner zone 12 and outer zone 14 and to the boundary between outer zone 14 and outer periphery 28.

[0025] Pattern 22 is preferably a pattern of resonant lossy square loops to provide sharp roll-off. Pattern 22 is dimensioned to reflect signals in the Ku band with a predetermined equivalent reactance value and absorb signals in the Ka band with a predetermined sheet resistance (ohms/square). FIG. 3 is a graph showing the specified reflected power of zone 14.

[0026] While resonant square loops are preferably used for pattern 22, it should be understood that other configurations for pattern 22 can be used, such as dipole, tripole or cross. Various configurations that can be used for pattern 22 are disclosed in U.S. Pat. Nos. 6,169,524 and 6,054,967, which are herein incorporated by reference.

[0027] For use with the Ku and Ka band frequencies, reflector 10 has a diameter of sixty-seven inches with the diameter of zone 12 being 38.3 inches and the width of zone 14 being 14.3 inches. While reflector 10 is described in the context of use with the Ku and Ka band frequencies, it should be understood that its use is not limited to these frequency bands and that it can be used for other frequency bands with appropriate dimensional changes as would be appreciated by one skilled in the art.

[0028] Illustratively describing the operation of reflector 10 in the context of a downlink, dual frequency feeds 30 transmit Ku band and Ka band signals toward reflector 10. Ku band signals 32 are reflected by both zones 12 and 14 of reflector 10. Ka band signals 34 are reflected by central zone 12 and absorbed by outer zone 14. Reflector 10 reflects both the Ka band signals and the Ku band signals with the same beam cell size, illustratively, one degree. Dual frequency feeds 30 can illustratively be the dual-band feed horn disclosed in U.S. Ser. No. 09/941,413 for a Dual-Band Equal-Beam Reflector Antenna System filed Aug. 28, 2001, which is incorporated by reference herein.

[0029] Turning to FIGS. 4aand 4b, an embodiment of the invention utilizing a frequency selective surface 106 is shown. A reflector 100, illustratively a parabolic reflector, has two concentric zones, a central zone 102 and an outer concentric zone 104. Central zone 102 is configured to reflect both Ku and Ka band signals while outer zone 104 has a frequency selective surface 106 that reflects only Ku band signals and passes Ka band signals. Central zone 102 illustratively includes a support core 108 having a reflective layer 110 on an inner surface 112, such as support core 108 having a metallized surface, that is reflective at the Ku and Ka bands.

[0030] Zone 104 comprises frequency selective surface 106 that is reflective at the Ku band and transparent at the Ka band. Referring to FIGS. 4b, 5aand 5b, zone 104 is comprised of a finite conducting pattern 114, illustratively a metallic pattern, over both top and bottom sides of a dielectric core 116. The top and bottom patterns 114 are illustratively formed on a substrate film 118, such as teflon, which is then bonded to the top and bottom sides of dielectric core 116. Dielectric core 116 is fabricated from materials such as Kevlar™, Nomex™, ceramic foam, Rohacell™ foam, or the like which are known in the art to pass microwave signals. Kevlar™ and Nomex™ are fabricated by Hexcel Corporation located in Huntington Beach, Calif. Conductive patterns 114 are dimensioned so that Ku band signals are reflected while Ka band signals are passed through. In this regard, a layer 120 of absorptive material can be disposed on the back side of frequency selective surface 106 to absorb the Ka band signals. The absorptive material can be the above referenced Eccosorb® AN-72 absorber.

[0031] Resistance cards 122 (FIG. 6) are applied to the top and bottom of all junctions between the frequency selective surface 106 and the reflective surface 110 of zone 102 of reflector 100. Resistance cards are known in the art and comprise a thin membrane onto which a thin, graded low density layer of copper is deposited. The resistance of the resistance card is determined by the density of the copper. A resistance card that could be utilized for resistance cards 112 is the R-card™ manufactured by Southwall Technologies located in Palo Alto, Calif.

[0032] For use with Ka and Ku band frequencies, the diameter of reflector 100 is 67 inches with the diameter of central zone 102 being 38.3 inches and the width of outer zone 104 being 14.3 inches. While reflector 100 is again described in the context of use with Ku and Ka band frequencies, it should be understood that its use is not limited to these frequency bands and it can be used for other frequency bands with appropriate dimensional changes as would be appreciated by one skilled in the art.

[0033] FIG. 7 is a graph showing the specified transmission coefficient of zone 104 using resonant square loops for conductive patterns 114. It should be understood that while resonant square loops are preferred for conductive patterns 114 to provide for sharp roll-off, other configurations can be used, such as dipole, tripole or cross. Various configurations that can be used for conductive patterns 114 are disclosed in the above referenced U.S. Pat. Nos. 6,169,524 and 6,054,967.

[0034] Illustratively describing the operation of reflector 100 in the context of a downlink, dual frequency feeds 124 transmit Ku and Ka band signals toward reflector 100. Ku band signals 126 are reflected by both zones 102 and 104 of reflector 100. Ka band signals 128 are reflected by zone 102 and pass through zone 14. Reflector 100 reflects both the Ka band signals and the Ku band signals with the same beam cell size, illustratively, one degree. Dual frequency feeds 124 can illustratively be the dual-band feed horn disclosed in the above referenced U.S. Ser. No. 09/941,413 for a Dual-Band Equal-Beam Reflector Antenna System.

[0035] FIG. 8 is a graph that shows the predicted radiation patterns at 30 GHz for the frequency selective absorber and the frequency selective surface embodiments of the invention described above.

[0036] FIG. 9 is a graph comparing the radiation patterns for a reflector in accordance with either the frequency selective absorber embodiment or the frequency selective surface embodiment of the invention to a reflector that has neither a frequency selective absorber or a frequency selective surface. As can been seen from FIG. 9, a reflector in accordance with either the frequency selective absorber embodiment or frequency selective surface embodiment of the invention has the same edge of coverage gain for a cell size of 0.5 degree radius as a reflector without a frequency selective absorber or frequency selective surface, but has lower sidelobe levels.

[0037] The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.