Abstract:
An antenna array of parabolic rectangular reflectors for use in satellite communications. The antenna comprises two parabolic reflectors disposed contiguously on a common outer surface. The common surface forms a continuous antenna aperture. The parabolic reflectors have rectangular side edges which permit the adjacent edges of the parabolic reflectors to be spaced closely. The mouth of each parabolic reflector is focussed on a separate feed. The focus of the feed is not located at the center of the reflector but rather offset. The antenna feeds and the reflector foci are displaced toward the center of the array such that the spacing between the antenna feeds is less than half the length of the antenna. The present invention provides the displacement of each reflector focal point and each antenna feed toward the center of the array.

Description:
[0001]    This application relates to U.S. Provisional Patent Application No. 60/270,193 filed Feb. 22, 2001. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to the use of parabolic reflectors in an antenna system for use in broadband satellite communications. More specifically, the invention relates to an antenna array of parabolic rectangular reflectors having antenna feeds which are offset in order to reduce antenna sidelobe levels.  
         BACKGROUND OF THE INVENTION  
         [0003]    In the field of satellite communications, antenna systems for satellite communication are required to have a broad bandwidth while having a narrow antenna beam width. The broad bandwidth enables the antenna system to both transmit and receive signals over frequency bands of several GHz. The narrow antenna beam width provides a high gain for signals that are received and transmitted over a particular frequency to and from a particular satellite, and provides discrimination between satellites.  
           [0004]    Although the antenna beam width is usually focussed on a particular satellite, it may also be necessary to alter the focus of the antenna beam toward another satellite.  
           [0005]    Due to the high speed at which aircraft travel, antenna systems which are mounted on aircraft are required to maintain a low profile. The low profile minimizes drag. Typically, an antenna system is placed within a radome that has a height restriction in the range of 4 inches to 12 inches depending on the type of aircraft.  
           [0006]    Single parabolic reflectors are not ideal for use in applications requiring a low profile. This is due in part to the fact that a parabolic reflector has a low aspect ratio—it is difficult to optimally illuminate the entire reflector surface when the ratio of the aperture width to height is large. In order to illuminate the entire surface of the parabolic reflector, the reflector itself must be distanced from the reflector feed. For example, a parabolic reflector having a surface width of 28 inches would typically require the feed to be placed at least 10 inches from the reflector. This is well beyond the height restriction of the radome on an aircraft. Regardless of whether the feed is axial or offset, inside the radome, the geometry of a single parabolic reflector is less than ideal for use on an aircraft fuselage.  
           [0007]    The use of contiguously disposed parabolic reflectors produces a high gain and a narrow central beamwidth. However, two large sidelobes are produced—one on either side of the antenna beam peak. The sidelobes are introduced due to the modulation of the aperture illumination resultant from the radiation pattern of the antenna feeds. Techniques are required to minimize the impact of modulation, resulting from the aperture illumination, and provide lower sidelobes on either side of the main antenna beam when utilizing an array of contiguously disposed parabolic reflectors.  
           [0008]    U.S. Pat. No. 6,049,312, issued to Lord, discloses an antenna system with a plurality of reflectors for generating a plurality of beams. Lord teaches an antenna system comprising a first reflector and a second reflector, as well as corresponding first and second feeds. While the two feeds are offset from their respective reflectors, the first and the second reflector are in a substantially tandem arrangement and not contiguously disposed in array. Rather, Lord teaches a compact antenna configuration whereby the first reflector and the first feed cooperate to form a first antenna beam and the second reflector and the second feed form a second beam. Lord does not discuss the formation of a main antenna beam in which the antenna sidelobe levels may be reduced by displacing the feeds and the foci of the respective reflectors.  
           [0009]    U.S. Pat. No. 6,262,689, issued to Yamamoto, discloses an antenna system for communicating with low earth orbit satellites from the ground. In one embodiment, Yamamoto teaches the use of two reflectors separated by a predetermined distance, each reflector having a primary feed for radiating a beam onto its respective reflector, and a switching means to switch the antenna focus between various satellites. However, Yamamoto teaches the tracking of two satellites, one by each of the reflector/feed systems. The Yamamoto patent does not disclose an antenna system which reduces the sidelobe level of the antenna beam.  
           [0010]    In view of the above shortcomings of the prior art, the present invention seeks to provide an array of two antenna elements, wherein each antenna element has a feed that is displaced toward the center of the antenna array. Furthermore, the present invention seeks to provide an antenna system utilizing feedhorns, parabolic reflectors, a common aperture surface, and several pairs of contiguously disposed reflectors having displaced feeds to reduce antenna sidelobe levels. Moreover, the present invention seeks to provide an antenna array of parabolic reflectors with lower sidelobes adjacent to the main antenna beam.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention is an antenna array of parabolic rectangular reflectors for use in satellite communications. The antenna comprises two parabolic reflectors disposed contiguously on a common outer surface. The common surface forms a continuous antenna aperture. The parabolic reflectors have rectangular side edges which permit the adjacent edges of the parabolic reflectors to be spaced closely. The mouth of each parabolic reflector is focussed on a separate feed. The focus of the feed is not located at the center of the reflector but rather offset. The antenna feeds and the reflector foci are displaced toward the center of the array such that the spacing between the antenna feeds is less than half the length of the antenna. The present invention provides the displacement of each reflector focal point and each antenna feed toward the center of the array.  
           [0012]    According to the present invention, the antenna feeds are excited coherently in order to produce a narrow well focussed beam. Support struts, located between the feeds and their respective parabolic reflector, are designed such that they minimize the blockage of the antenna aperture. In one embodiment, the antenna array may be mounted on the fuselage of an aircraft. The antenna is steered mechanically in elevation and azimuth to maintain the antenna attitude directed toward a particular satellite at all times. Finally, the displacement of the antenna feeds and reflector foci result in lower sidelobes adjacent to the main antenna beam. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    The invention will now be described with reference to the drawings, in which:  
         [0014]    [0014]FIG. 1 is a side view of the antenna system having parabolic reflectors disposed contiguously in a linear array of the prior art;  
         [0015]    [0015]FIG. 2 is a bottom view of the antenna system of FIG. 1 of the prior art;  
         [0016]    [0016]FIG. 3 is a bottom view of the antenna system of FIG. 1, further including a power splitter/combiner, of the prior art;  
         [0017]    [0017]FIG. 4 is a schematic side view of an antenna system having two parabolic reflectors with offset foci and antenna feeds located at each of the offset foci according to the present invention;  
         [0018]    [0018]FIG. 5 is a bottom view of the antenna system of FIG. 4 of the present invention; and  
         [0019]    [0019]FIG. 6 is a front view of an antenna system having a plurality of parabolic reflectors with offset foci and antenna feeds displaced toward the center of the antenna array according to an alternative of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0020]    [0020]FIG. 1 illustrates a side view of the antenna system  5  of the prior art. The antenna system  5  consists of four antenna elements  10 ,  20 ,  30 ,  40 , and four antenna element feeds  50 ,  60 ,  70 ,  80 , respectively. The antenna elements are identical. The antenna element  10  is comprised of a rectangular parabolic reflector  90  and a support strut  100 . The antenna element  20  has both a rectangular parabolic reflector  110  and a support strut  120 . The antenna element  30  has both a rectangular parabolic reflector  130  and a support strut  140 . Finally, the antenna element  40  has both a rectangular parabolic reflector  150  and a support strut  160 .  
         [0021]    It should be further explained that the rectangular parabolic reflectors  90 ,  110 ,  130 ,  150  have a rectangular side edge configuration. The rectangular parabolic reflector differs from the conventional parabolic reflectors which have a circular or an elliptical edge configuration. The rectangular edge configuration permits the parabolic reflectors  90 ,  110 ,  130 ,  150 , to be adjacent, without gaps, forming a larger common rectangular aperture. The contiguous disposition of the parabolic reflectors  90 ,  110 ,  130 ,  150  is one factor which contributes to an optimal illumination of the antenna array and to the antenna system  5  having a low profile. Each rectangular parabolic reflector shown in FIG. 1 has a central focus point that is facing directly in line with a corresponding antenna feed.  
         [0022]    The support struts  100 ,  120 ,  140 ,  160  are support members for the feeds. However, the support struts are non-essential elements in that the element feeds  50 ,  60 ,  70 ,  80  may be attached to the parabolic reflectors  90 ,  110 ,  130 ,  150  by other means. The support struts  100 ,  120 ,  140 ,  160  are designed to provide for minimal blockage of the paraboloidal apertures so as not to interfere with the element feeds  50 ,  60 ,  70 ,  80 .  
         [0023]    The element feeds  50 ,  60 ,  70 ,  80  each transmit a guided wave deriving, for instance, from a coaxial cable. Alternatively, the element feeds receive an unguided wave propagating through space. An unguided wave reflects off the parabolic reflector surface and would then be received at the element feed. To transmit a guided wave, each element feed is excited in phase through a power splitting/combining means, shown in FIG. 3. As each element feed is excited, the combined radiation pattern of the antenna elements produces a narrow beam.  
         [0024]    The “front” of each parabolic reflector  90 ,  110 ,  130 ,  150  forms part of the common aperture surface  170 . The concave surface of each parabolic reflector  90 ,  110 ,  130 ,  150  faces the common aperture surface  170 . This common aperture surface  170  enables the rectangular parabolic reflectors to form a continuous antenna aperture in order to further narrow and focus the antenna beam.  
         [0025]    [0025]FIG. 2, of the prior art, illustrates a bottom view of the antenna system  5  described in FIG. 1. In FIG. 2, the common aperture surface  170  is attached to each of the support struts  100 ,  120 ,  140 ,  160  each of which are attached to the element feeds  50 ,  60 ,  70 ,  80 . The central foci of each reflector is directly above the element feeds  50 ,  60 ,  70 ,  80 .  
         [0026]    [0026]FIG. 3 illustrates the antenna system  5  of FIG. 1 and  2  of the prior art in combination with a power splitter/combiner. In FIG. 3, the power splitter/combiner is shown as two separate elements, although they may be one element. The power divider  300  has four connections  310 A,  310 B,  310 C,  310 D, which are connected to the antenna feeds  50 ,  60 ,  70 ,  80 , respectively. The four connections  310 A,  310 B,  310 C,  310 D may be a coaxial cable or any other connecting means. The power divider  300  also has an input beam port  320 . The use of four connections  310 A,  310 B,  310 C,  310 D enables the antenna system  5  to form an antenna beam which utilizes all of the parabolic reflectors.  
         [0027]    The power combiner  330  also has four connections  340 A,  340 B,  340 C,  340 D, each of which are connected to antenna feeds  50 ,  60 ,  70 ,  80 , respectively. The antenna feeds each have two connections. The antenna feed  50  is attached to the power combiner  330  through a connection  340 A and to the power splitter  300  through a connection  310 A. The antenna feed  60  is attached to the power combiner  330  through a connection  340 B and to the power splitter  300  through a connection  310 B. The antenna feed  70  is attached to the power combiner  330  through a connection  340 C and to the power splitter  300  through a connection  310 C. Accordingly, the antenna feed  80  is attached to the power combiner  330  through a connection  340 D and to the power splitter  300  through a connection  310 D.  
         [0028]    Also, each antenna feed  50 ,  60 ,  70 ,  80  has two connections which are attached at respective input/output ports. In FIG. 3, the antenna feed  50  has an input port  350 A which is coupled to the connection  310 A and in turn connected to the power splitter  300 . The power splitter sends a signal and the required input power to the antenna feed  50 . The antenna feed  50  has an output port  350 B which is coupled to the connection  340 A and in turn connected to the power combiner  330 . There may be more than one output port at each antenna feed. Each output port represents a particular horizontal or vertical polarisation. The horizontal and vertical polarisation permits the antenna feeds  50 ,  60 ,  70 ,  80  to excite the antenna elements at various phases. As such, through the appropriate phase and amplitude combining of each of the element feeds  50 ,  60 ,  70 ,  80 , the antenna elements  10 ,  20 ,  30 ,  40  may be excited in combination such that they produce an antenna beam that may be focussed in various directions.  
         [0029]    While FIG. 3 only shows two connections to each element feed  50 ,  60 ,  70 ,  80 , there may be more than one output connection to the power combiner  330 . Each additional output connection would be coupled to a separate power combiner. Each additional power combiner would also be connected to the main transceiver equipment located on the aircraft. In a dual-band system each element feed would have four connections corresponding to a horizontal and a vertical polarisation for each of the two bands.  
         [0030]    Also, an output beam port  360  is connected to the power combiner  330 . Both the input beam port  320  and the output beam port  360  may be coupled to the aircraft transceiver equipment that uses the antenna system.  
         [0031]    [0031]FIG. 4 illustrates an antenna array  400  similar to the prior art, yet in contrast, the antenna elements, belonging to the antenna array  400 , have offset antenna element foci and antenna feeds which are displaced in order to reduce antenna sidelobe levels. According to the present invention, the antenna array  400  of FIG. 4 consists of two antenna elements  410 ,  415  and two antenna feeds  420 ,  425 . The antenna element  410  further comprises a rectangular parabolic reflector  430  and a support strut  440 . Similarly, the antenna element  420  comprises a rectangular parabolic reflector  450  and a support strut  460 .  
         [0032]    In contrast to FIG. 1, FIG. 4 illustrates the use of an offset reflector focus point. The antenna feed  420  and the focus point  470  of the parabolic reflector  430  are not at the centre of the antenna element  410 . Rather, the antenna feed  420  and the focus point  470  are displaced toward the centre of the rectangular aperture of the parabolic reflector  430 (shown clearly in FIG. 5). The antenna feed  425  and the focus point  480  are also displaced toward the centre of the rectangular aperture of the parabolic reflector  450 . In fact, both antenna feeds  420 ,  425  and correspondingly both focus points  470 ,  480  have been displaced such that they are closer to the centre point  490  of the antenna array  400 .  
         [0033]    [0033]FIG. 5 is a bottom view of the antenna array  400  which illustrates the spacing between antenna feeds  420 ,  425  according to the present invention. Similar to the prior art, the “front” of the each parabolic reflector  430 ,  450  forms part of a common aperture surface  500 . The common The common aperture surface  500  is comprised of two rectangular aperture surfaces  500 A,  500 B and having a particular antenna system length  510 . Each of the two rectangular aperture surfaces  500 A,  500 B correspond to each of the two antenna elements  410 ,  415 , respectively. As opposed to the antenna feed  420  being located in the centre of the rectangular aperture  500 A it is instead displaced toward the centre of the common aperture surface  500 . The antenna feed  430  is also displaced toward the centre of the common aperture surface  500 . The antenna feeds  420 ,  425 , are displaced towards the centre of the antenna array  400  such that the spacing between the antenna feeds  420 ,  425 , is less than half the antenna system length  510 . The displacement of the parabolic reflector foci  470 ,  480 , correspond to the offset antenna feed positions. As such, the parabolic reflector foci  470 ,  480  are displaced towards the centre of the antenna array  400  such that the spacing  520  between the reflector foci  470 ,  480  is less than half the antenna system length  510 .  
         [0034]    According to the present invention, the displacement of the antenna feeds  420 ,  425  and the reflector foci  470 ,  480  reduces the antenna sidelobes adjacent to the main antenna beam of the antenna radiation pattern. In a dual-parabolic antenna system, the beamwidth of each individual parabolic reflector remains constant while the phase centers of their antenna beam are moved closer together. Thus, the first sidelobes, also termed grating lobes, are pushed further from the main antenna beam and suppressed by the narrow radiation pattern of the individual parabolic reflectors  430 ,  450 .  
         [0035]    [0035]FIG. 6 is a frontal view of an antenna array  600  according to an alternative embodiment of the present invention. The antenna array  600  consists of four antenna elements  610 ,  620 ,  630 ,  640  and four antenna feeds  650 ,  660 ,  670 ,  680 . Each of the four antenna elements are comprised of both a parabolic reflector (similar to that of FIG. 1) and a support strut. Each of the four support struts  700 ,  710 ,  720 ,  730  are each connected to the antenna feeds  650 ,  660 ,  670 ,  680 , respectively.  
         [0036]    According to this embodiment, the feed spacings are not uniform, in that the feed spacing  740 , between the antenna feeds  660  and  670 , is closer than the feed spacing  750 , between the antenna feeds  650  and  660 . Each of the four antenna feeds  650 ,  660 ,  670 ,  680  are displaced toward the centre of the antenna array  600 . In this alternative embodiment, the feed spacing between antenna feeds, in an array of more than two antenna elements, would be less than the length  760  of a rectangular aperture surface  770  for a single antenna element. Typically, the average spacing between antenna feeds would be lower than that obtained with conventional feed spacings since at the very least the two outer feeds  650 ,  680  would be displaced towards the centre of the array  600 . FIG. 6 further illustrates an antenna array in which all of the antenna feeds are displaced towards the centre of the array. The reflector foci of each of the four antenna elements  610 ,  620 ,  630 ,  640  are displaced toward the centre of the array. As such, the sidelobe levels of the main antenna beam are suppressed by the narrow radiation pattern of the individual antenna elements  610 ,  620 ,  630 ,  640 .  
         [0037]    It should be mentioned that the antenna feeds of both the antenna array  400  and the antenna array  600  may be connected to a power splitter  300  and power combiner  330  of FIG. 3. However, the power splitter  300  and the power combiner  330  need not be two separate units but rather a single power splitting/combining unit.  
         [0038]    Although the antenna system is advantageous for use on an aircraft, the present invention also lends itself to applications on vehicles or at various stations on the ground that are in communication with satellites.