Patent Publication Number: US-6714163-B2

Title: Structurally-integrated, space-fed phased array antenna system for use on an aircraft

Description:
FIELD OF THE INVENTION 
     The present invention relates to phased array antenna systems, and more particularly to a space-fed phased array antenna system specifically adapted for use on an exterior portion of a fuselage of an aircraft. 
     BACKGROUND OF THE INVENTION 
     With the growing interest in providing live television programming and Internet connectivity with aircraft in flight, the design and implementation of phased array antenna systems has become extremely important. The Boeing Company is a leading innovator in the design and manufacture of phased array antennas systems. U.S. Pat. No. 5,886,671 to Riemer et al and U.S. Pat. No. 5,276,455 to Fitzsimmons et al both disclose packaging architectures for phased array antennas, and are each hereby incorporated by reference into the present application. 
     When implementing a phased array antenna on an aircraft, such as may be employed in the Connexion By Boeing SM  system, an important consideration is that the antenna be able to scan close to the horizon when the aircraft is at high latitudes. This heretofore required a very large antenna aperture to be disposed on a top area of the fuselage of an aircraft. However, a very large aperture of a phased array antenna is quite expensive to manufacture. Accordingly, it would be highly desirable to provide some form of antenna system which is able to make use of an antenna having a smaller aperture but which still is able to project a beam close to the horizon when the aircraft is traveling at high latitudes. 
     SUMMARY OF THE INVENTION 
     The above and other drawbacks are overcome by a structurally-integrated, space-fed phased array antenna system. The antenna system of the present invention makes use of a conformal antenna aperture which is mounted on an exterior surface of a mobile platform. In one preferred embodiment the mobile platform comprises an aircraft. In this embodiment the conformal antenna aperture is mounted on one of the port or starboard sides of the fuselage of the aircraft along the rear one-half of the fuselage. More preferably, the conformal antenna aperture is mounted on one of the port or starboard sides of the fuselage closely adjacent to one of the horizontal stabilizers of the aircraft. That horizontal stabilizer includes an antenna, such as a feedhorn, which is structurally integrated therewith. In a preferred embodiment the feedhorn is integrally formed at a leading edge of the horizontal stabilizer such that it does not produce any additional aerodynamic drag on the aircraft when the aircraft is in flight. The feedhorn provides a space feed to illuminate the conformal antenna aperture disposed on the fuselage, which in turn is able to reflect the beam at an angle that allows the beam to be directed close to the horizon when the aircraft is traveling at high latitudes. 
     In another preferred embodiment the conformal antenna aperture is divided into two antenna apertures: a receive antenna aperture and a transmit antenna aperture. In other preferred embodiments the feedhorn is also presented as two independent feedhorns mounted closely adjacent one another on a leading edge of a horizontal stabilizer of the aircraft. 
     The above described preferred embodiments all enable a conformal phased array antenna system used on an aircraft to readily project a beam below the horizon when the aircraft is flying at high latitudes. Moreover, the antenna system of the present invention does not add significantly to the aerodynamic drag experienced by the aircraft during flight. 
     The present invention is equally applicable to any mobile platform such as a land vehicle, ship or satellite. A principal advantage of the present invention is that the use of a space feed allows a much larger antenna aperture to be illuminated. 
     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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
     FIG. 1 is a perspective view of an aircraft illustrating a coverage pattern of a large aperture phased array antenna mounted on a top area of a fuselage of the aircraft, and further illustrating the limitations in scanning a beam from such a mounted antenna down to the horizon or below; 
     FIG. 2 is a perspective view of a portion of an aircraft illustrating a preferred embodiment of the present invention including a conformal phased array antenna mounted on a port side of the fuselage of the aircraft and a feedhorn mounted within a leading edge of a horizontal stabilizer of the aircraft; 
     FIG. 3 is a perspective view of the antenna system of FIG. 2 illustrating the feedhorn illuminating the conformal antenna aperture; 
     FIG. 4 is an alternative preferred embodiment of the present invention illustrating separate transmit and receive antennas being used along with separate transmit and receive feed horns; and 
     FIG. 5 is a perspective view of an alternative preferred embodiment of the present invention wherein the feed horn is incorporated into a blade mounted on an exterior surface of the aircraft. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     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. 
     Referring to FIG. 1, there is shown an aircraft  10  with a large aperture phased array antenna  12  mounted on a top portion  14  of a fuselage  16  of the aircraft. Arrows  18  indicate the approximate limitations in terms of scanning a beam from the antenna  12  down to the horizon or below the horizon. As will be appreciated from the drawing of FIG. 1, it is difficult for present day phased array antenna systems to operate at scan angles which project the beam therefrom down to angles close to or parallel with the wings of an aircraft. In practice, present day phased array antennas have significantly reduced performance at scan angles of greater than about 60° from a boresight of the antenna. Accordingly, when the aircraft  10  is operating at high latitudes and is attempting to communicate with a space based device, such as a satellite based transponder orbiting over the equator, communication can be difficult or impossible with a phased array antenna mounted on a top portion of a fuselage of an aircraft. This limitation has necessitated the use of very large antenna apertures which add significantly to the cost of the antenna system. 
     Referring now to FIG. 2, there is shown an antenna system  20  in accordance with a preferred embodiment of the present invention. The antenna system  20  includes a conformal phased array antenna  22  and a second antenna  24 , such as a feedhorn. The conformal phased array antenna  22  is mounted on either a port or a starboard side of the fuselage  16  of the aircraft  10 , and further along a rear one-half portion of the fuselage  16 . More preferably, the conformal antenna  22  is located on one of the port or starboard sides of the fuselage  16  closely adjacent a horizontal stabilizer  26  of the aircraft  10 . The feedhorn  24  is disposed on the horizontal stabilizer  26 , and more preferably on a leading edge  28  of the horizontal stabilizer  26 . The conformal phased array antenna  22  is disposed within a line of sight of the feedhorn  24  such that it can be irradiated by the feedhorn  24 . The antenna system  20  thus forms a space fed antenna system. 
     The conformal phased array antenna  22  could be formed as a completely passive conformal antenna with conventional phase-shifters used for beam steering. In this instance, the aperture of the antenna  22  must be large enough to overcome the phase shifter losses and meet the required transmission link margins with the other subsystems (such as satellite-based transponders) that the antenna  22  is in communication with. Sidelobe level suppression of the side lobes produced by the beam of the antenna  22  and/or density tapering of the beam could also be included to reduce stray radiation and thereby help to reduce the possibility of interference with other devices. Such other devices might include non-target satellite transponders orbiting in close proximity to a target satellite transponder. 
     FIG. 4 illustrates an antenna system  30  in accordance with an alternative preferred embodiment of the present invention. The antenna system  30  has a conformal phased array antenna  32  and a feedhorn assembly  34 . The conformal antenna  32  includes a transmit antenna  36  and a receive antenna  38  which are disposed closely adjacent to one another on the port side of the fuselage  16  of the aircraft  10 . The feedhorn assembly  34  includes a transmit feedhorn  40  and a receive feedhorn  42  disposed closely adjacent one another on the leading edge  28  of the horizontal stabilizer  26 . The transmit and receive antennas  36  and  38 , respectively, could form purely passive antenna apertures. The passive configuration, for each of the antenna systems  20  and  30 , can be implemented provided the conformal antenna apertures have the needed bandwidth and scanning properties. 
     It is anticipated that each of the phased array antenna apertures  22  and  32  will be most cost effectively constructed by implementing the teachings of U.S. application Ser. No. 10/007,067, entitled “Antenna Integrated Printed Wiring Board Assembly For A Phased Array Antenna”, filed Dec. 5, 2001, which discloses an apparatus and method for manufacturing an integrated, low cost phased array antenna system with a significantly reduced number of component parts, and which is hereby incorporated by reference. It is also anticipated that the use of GaAs may not be necessary in the antenna system  22  or  32 . Using SiGe or other low cost alternative to GaAs, the cost of an oversized antenna aperture could be reduced to the materials needed to form printed wiring board material, phase shifters and ASICs to control the beam steering. 
     In the preferred embodiments described above, it is further preferred that critical electronic components associated with such phased array antenna systems are housed with the horn  24  or  34  within the horizontal stabilizer  26 . For a passive phased array antenna aperture, a single horn can be employed with a circulator. Alternatively, two separate horns can be employed as described herein. In either case, suitable transmit and receive filters are included with up and down converters. 
     It will also be appreciated that with the transmit phased array antenna  36  shown in FIG. 4, a smaller transmit aperture could be employed which uses suitable power amplifiers therewith. Similarly, on the receive antenna aperture  38 , a smaller aperture could be employed if low noise amplifiers are included for use therewith. In this instance, the electrical and mechanical complexity increases along with some additional costs. 
     The use of a proven phased array antenna manufactured by the assignee of the present invention, utilizing loaded circular wave guides with wide-angle impedance matching (WAIM) sheets, provides excellent performance over a large field of view. Such phased array antennas and their constructions are disclosed in U.S. Pat. No. 5,886,671 to Riemer et al and U.S. Pat. No. 5,276,455 to Fitzimmons et al, both of which are hereby incorporated by reference. Pending U.S. patent application Ser. No. 09/915,836, entitled “Antenna-integrated Ceramic Chip Carrier For Phased Array Antennas”, assigned to the assignee of the present application and is also hereby incorporated by reference, also discloses manufacturing techniques for significantly reducing the cost of phased array antennas. 
     It will also be appreciated that the antenna apertures  22  and  32  can be made doubly conformal, if desired, to the inside or outside of a structure. This makes the antenna apertures  22  and  32  ideally suited for an aircraft. Since the phased array antenna apertures  22  and  32  have an extremely low profile, they do not protrude significantly above the skin of the fuselage  16  and therefore do not produce any significant additional aerodynamic drag on the aircraft  10  when the aircraft is in flight. Also, it will be appreciated that the antenna apertures  22  and  32  could also be integrated into the fuselage  16  during construction of the aircraft  10  such that the antenna apertures  22  and  32  are essentially flush with the skin of the fuselage. 
     Concerning the feedhorns  24  (FIG. 3) and  34  (FIG.  4 ), it will be appreciated that in the embodiments illustrated, these components are also integrally formed into the leading edge  28  of the horizontal stabilizer  26  such that they do not cause any significant additional aerodynamic drag on the aircraft  10 . The electronics associated with the feedhorns  24  and  34  are also preferably disposed within the horizontal stabilizer  26 . 
     It will also be appreciated, however, that the feed horn  24  could be incorporated into a blade  44  mounted at any point on the aircraft  10 , as indicated in FIG.  5 . FIG. 5 also shows transmit and receive antenna apertures  32  and  34  mounted atop the aircraft  10 . Thus, it will be appreciated that the space feed arrangement disclosed herein allows the use of larger antenna apertures at a variety of locations on the aircraft  10 , and not just on the sides of the fuselage  16 . The space feed approach allows larger antenna apertures to be used on virtually any type of moving platform such as a land vehicle, a ship or even satellite. 
     The preferred embodiments of the present invention are thus particularly effective in providing an antenna system which allows a beam from a conformal phased array antenna mounted on an aircraft to be directed down close to or below the horizon when the aircraft is traveling at high latitudes. Advantageously, the preferred embodiments do not form significant protrusions on the fuselage of the aircraft, and thereby do not negatively impact the aerodynamic performance of the aircraft when the aircraft is in flight. 
     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, specification and following claims.