Abstract:
An antenna steering mount includes two basic building blocks which are joined together to form any of the known steering-axis combinations. Each block includes a cylinder cut at an angle to form a cylindrical wedge. The wedges are joined together by bearings at their interface, and motors are used to counter rotate the two wedges relative to each other. One end of the complete assembly is attached to a mounting platform, and the other end includes mounting features for attaching an antenna dish thereto. Various two- and three-axis steering configurations are disclosed including combinations of azimuth, elevation, cross-elevation, and cross-level steering.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present invention claims priority from U.S. Patent Application No. 61/035,584 filed Mar. 11, 2008, which is incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a mount for an antenna, and in particular to a rotating two-part antenna mount with mating angled surfaces for steering the antenna in a desired direction. 
       BACKGROUND OF THE INVENTION 
       [0003]    Conventional antenna mounts are normally required to mechanically steer high-gain antenna systems in two dimensions. In some mobile applications, such as ship-mounted antennas, the required steering range can be up to full hemispheric; however, in other applications, e.g. forward-looking radar antennas in the nose of aircraft, the steering range is limited to a narrower region. Similarly, multiple shipboard antennas, each with limited steering range, which in combination cover a large steering range, are disclosed in a paper by E. Barry Felstead, entitled “Combining multiple sub-apertures for reduced-profile shipboard satcom-antenna panels,” in Proc. IEEE Milcom 2001, unclassified paper 19.6, Vienna, Va., 28-31 Oct. 2001; and in a paper by E. Barry Felstead, Jafar Shaker, M. Reza Chaharmir and Aldo Petosa, entitled “Enhancing multiple-aperture Ka-band navy satcom antennas with electronic tracking and reflectarrays,” in Proc. IEEE Milcom 2002, paper U105.7, Anaheim, Calif., 8-10 Oct. 2002. 
         [0004]    Regardless of the application, the steerable antenna mounts are preferably made as compact as possible by minimizing the size of the motors, and the profile depth, mass, and volume of the combined antenna and mounting structure. Moreover, it is also desirable to make the antenna mounts relatively simple and inexpensive to build. 
         [0005]    Steering or pointing of the antenna involves a rotation about a single axis or about a plurality of axes, e.g. a variety of different axes used in various combinations depending upon the application of the antenna. Typically, the basic axes are referred to as azimuth, elevation, cross-elevation, and cross-level, as is well known in the art. Driving motors are usually used for actuating the rotation about the different axes. The different axes can be coupled together in a variety of ways including the use of gimbals. 
         [0006]    With reference to  FIG. 1 , for discussion purposes, the reference coordinates are (x r ,y r ,z r ) with the antenna system located at (0, 0, 0). The zenith is considered to be in the direction of the z r  axis, and the x r  and y r  coordinates lie in the horizontal plane. For mobile applications, the y r  axis could be pointed in the direction of forward motion. Spherical coordinates (φ,θ,ρ) are also illustrated in  FIG. 1 , in which the angle φ corresponds to the azimuth angle, and the angle θ corresponds to the complement of the elevation angle, ε, i.e. ε=90°−θ. 
         [0007]    With reference to  FIG. 2(   a ), a common two-axis antenna mount is an elevation-over-azimuth mount  1  for antenna  2 , which uses a first motor (not shown) providing up to full azimuth rotation (360°) about a vertical axis V, and a second motor (not shown) providing full elevation rotation (90°) about a horizontal axis H. The center of gravity of the antenna  2  is usually offset from the pivot points, thereby requiring that the first and second motors have increased torque. These disadvantages can be reduced in certain applications in which the elevation range of the antenna is more limited, such as with the KVH series of satellite-dish antennas. Corey Pike and Claude Desormeaux, disclosed the adaptation of a type G3 KVH antenna for a vehicle-mounted application in the reference entitled “Ka-band land-mobile satellite communications using ACTS”, 7th Ka-Band Utilization Conf., September 2001, and Richard S. Wexler, D. Ho, and D. N. Jones, disclosed the adaptation of a type G6 by MITRE in the reference entitled “Medium data rate (MDR) satellite communications on the move (SOTM) prototype terminal for the Army warfighters,” in Proc. IEEE Milcom 2005, Atlantic City, Oct. 17-20, 2005. Unfortunately, the elevation-over-azimuth mount also has problems with cable wrap and with the keyhole effect in the zenith direction, as will be discussed later. 
         [0008]    A less-common type of mount is the cross-elevation-over-elevation mount  5 , as illustrated in  FIG. 2(   b ), sometimes referred to as an “X-Y” mount. An elevation motor (not shown) is used to rotate an antenna  6  about a first horizontal elevation axis, and a cross-elevation motor (not shown) is used to rotate the antenna  6  about a cross-elevation axis. The mass of both the antenna  6 , and the cross-elevation motor must be supported by the elevation motor, thereby adding to the motor-torque requirements; however, the X-Y mount does not have a keyhole problem in the zenith direction and does not have a cable wrap problem. Unfortunately, the X-Y mount tends to have a reduced steering range compared to the elevation-over-azimuth mount. 
         [0009]    In certain applications, such as on naval ships, a third axis of steering is sometimes added to the antenna mount to get around the keyhole problem that the standard azimuth-elevation mount exhibits in the zenith direction. Another purpose is to add what is sometimes called a “cross-level” axis to simplify the compensation for ship roll and pitch. 
         [0010]    An alternative approach to antenna steering is disclosed in U.S. Pat. No. 6,911,950 issued Jun. 28, 2005 to Harron, referred to as the “universal-joint gimbaled antenna mount” (or the “GiAnt” mount). As illustrated in  FIG. 3 , the antenna  7  plus the feed system is placed so that the center of mass is at, or near, the center of the universal joint  8 , such as a ball joint. A yoke  9  driven by a motor (Motor  2 ) scans the antenna  7  about the elevation axis EA, and another motor (Motor  1 ) mounted on the yoke  9  pivots the antenna  7  about the ends of the yoke  9 , i.e. scans the antenna  7  about the cross-elevation axis XEA. Since the center of mass of the system rests on the ball joint  8 , the motors (Motor  1  and Motor  2 ) can be very small, i.e. small digitally driven stepper motors with built in shaft encoders can be use. Such a system can scan to over ±50° in both elevation and cross elevation, and with careful mechanical design could be slightly extend. As a result of the “X-Y” form of scanning, there is no problem with cable wrap, and the keyhole has been pushed far from boresight. Moreover, the GiAnt mounting system is relatively inexpensive to manufacture. 
         [0011]    Unfortunately, the GiAnt mounting structure exhibits vibration in the form of twisting of the yoke  9  when mounted on a platform undergoing severe movements, e.g. ship mounted. The yoke  9  could be strengthened, but difficulties arise when making it sufficiently rigid for the steering accuracies likely to be encountered. 
         [0012]    Rotating-wedges were disclosed by G. Maral and M. Bousquet, in  Satellite Communications Systems: systems, Techniques and Technology,  Fourth ed., by John Wiley &amp; Sons, Chichester UK, 2002, pages 392 to 394, for supplementing a standard steering system to give a slight offset “bias”, which is used to avoid the keyhole problem, but were not intended to be used as the means of steering in one of the major axes. 
         [0013]    An object of the present invention is to overcome the shortcomings of the prior art by providing an antenna steering mount comprised of two counter-rotating wedged bodies. 
       SUMMARY OF THE INVENTION 
       [0014]    Accordingly, the present invention relates to an antenna mount comprising: 
         [0015]    abase; 
         [0016]    a first bearing structure supported by the base; 
         [0017]    a first wedge-shaped body having a first end mounted on the first bearing structure and a second end at a first acute wedge angle to the first end; 
         [0018]    a second bearing structure mounted on the second end of the first wedge-shaped body; 
         [0019]    a second wedge-shaped body mounted on the second bearing structure, having a first end parallel to the second end of the first wedge-shaped body and a second end at a second acute wedge angle to the first end of the second wedge-shaped body; 
         [0020]    a first motor for rotating the first wedge-shaped body relative to the base; and 
         [0021]    a second motor for rotating the second wedge-shaped body relative to the first wedge-shaped body. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    The invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, wherein: 
           [0023]      FIG. 1  illustrates the reference coordinates for an antenna steering mount; 
           [0024]      FIGS. 2   a  and  2   b  are schematic diagrams of conventional antenna steering mounts; 
           [0025]      FIG. 3  illustrates another prior art antenna steering mount; 
           [0026]      FIG. 4  is a side view of an antenna mount in accordance with the present invention; 
           [0027]      FIG. 5  illustrates front, side and top views of the base wedge-shaped body of the antenna mount of  FIG. 4 ; 
           [0028]      FIG. 6  is a side view of an antenna mount in accordance with another embodiment of the present invention with up to four wedge-shaped bodies; 
           [0029]      FIG. 7  is a side view of a portion of an antenna mount in accordance with a modification of the embodiment of  FIG. 6 ; 
           [0030]      FIG. 8  is a schematic diagram of the antenna mount of  FIG. 4  with an antenna mounted thereon illustrating the transmission of power and data signals; 
           [0031]      FIG. 9  is a schematic diagram of the antenna mount of  FIG. 4  with an antenna mounted thereon illustrating an alternative path for the transmission of power and data signals; 
           [0032]      FIG. 10  is a plot of elevation angle vs torque for conventional antenna mounts and the antenna mount of the present invention; 
           [0033]      FIG. 11  is a schematic illustration of the factors affecting the torque on an antenna mount with a moving antenna; 
           [0034]      FIG. 12   a  is a side view of an elevation-over-azimuth steering configuration for a 90° elevation angle looking along the elevation axis, 
           [0035]      FIG. 12   b  is a side view of an elevation-over-azimuth steering configuration for a 90° elevation angle looking perpendicular to the elevation axis; 
           [0036]      FIG. 13  is a side view of an elevation-over-azimuth steering configuration for a 0° elevation angle looking along the elevation axis; and 
           [0037]      FIG. 14  is an embodiment of an elevation-over-cross-level-over-azimuth antenna mount in accordance with another embodiment of the present invention rotating plates and a single rotating-wedge assembly looking along the azimuth axis with the elevation set to 90°. 
       
    
    
     DETAILED DESCRIPTION 
       [0038]    With reference to  FIG. 4 , an antenna mount  10  in accordance with the present invention includes two rotating wedges, e.g. wedge-shaped blocks or bodies, from which the various forms of antenna steering can be implemented. In the preferred embodiment the two wedge-shaped bodies are comprised of two cylindrical wedges  11  and  12 , with the first cylindrical wedge  11  rotatably mounted on a mounting structure  13 , and the second cylindrical wedge  12  rotatably mounted on the first cylindrical wedge  11 . The first and second wedges  11  and  12 , respectively, are preferably cylindrical; however, any other shapes are within the scope of the invention. 
         [0039]    In the illustrated embodiment, the mounting structure  13  is comprised of a mounting post  14  and a bottom plate  15  fixed on the end of the mounting post  14 ; however, other structures are within the scope of the invention. The first cylindrical wedge  11  is defined by a base  16  mounted for rotation on the mounting structure  13 , and an upper surface  17  with a flange  20  at a first acute wedge angle to the base  16 . The second cylindrical wedge  12  is defined by an upper mounting plate  18 , and a lower surface  19  parallel to the upper surface  17 . The upper mounting plate  18  is at a second acute wedge angle to the lower surface  19 . 
         [0040]    A first bearing structure  21 , e.g. a ring of ball bearings between corresponding bearing surfaces, is disposed at the interface between the first wedge  11  and the mounting structure  13  to enable free rotation therebetween. A gear set is used to drive the first wedge  11  relative to the mounting structure  13 , e.g. a 360° ring gear  22  with teeth extending diametrically inwardly thereof fixed to the base  16  is rotated by a spur gear  23 , which is driven by a first or lower motor  24 . The first wedge  11  is rotatable about a first axis perpendicular to the base  16 , the first axis being the same as the central longitudinal axis of the first wedge  11 . However, the second wedge  12  is rotatable about a second axis perpendicular to the lower surface  19  thereof and the upper surface  17  of the first wedge  11 , which is not the longitudinal axis of the second wedge, but at an acute angle, e.g. the wedge angle, thereto. 
         [0041]    In the illustrated embodiment, the base plate  15  is mounted horizontally on the earth; however, in practice, the base plate  15  can be mounted in any orientation. With reference to  FIG. 5 , the reference axes, (x r ,y r ,z r ), are centered in the middle of the base circular flange  20 . The z r  axis points vertically and the x r  axis is horizontal and in the plane that contains the z r  axis and cuts through the first wedge  11  between its lowest and highest part. The positive direction of the x r  axis is toward the small end of the first wedge  11 . The base plate  15  is shown as square so that it can more easily be distinguished from the first wedge  11 . 
         [0042]    Similarly, a second bearing structure  26 , such as seen in  FIG. 4 , e.g. a ring of ball bearings between corresponding bearing surfaces, is disposed at the interface between the first wedge  11  and the second wedge  12  to enable free rotation therebetween. A 360° circular rack gear  27  with teeth extending diametrically outwardly is mounted on the lower surface  19  of the second wedge  12 , for engaging a spur gear  28 , which is driven by a second or upper motor  29  mounted on the first wedge  11  via bracket  30 . 
         [0043]    The upper mounting plate  18  includes suitable fasteners for mounting an antenna dish or flat reflect array, as is well known in the art. Rotation of the first and second wedges  11  and  12  causes tilting of the upper mounting plate  18 , so as to steer the antenna in a motion like that of the elevation motors in  FIGS. 2(   a ) and  2 ( b ). The rotating-wedges  11  and  12  can be viewed as a replacement for the commonly used rotating axes or gimbals. For example, such a wedge pair  11  and  12  can be used to replace the elevation steering device in an elevation-over-azimuth configuration. In another example, the two wedges  11  and  12  can be used to replace the elevation and the cross-elevation units for the cross-elevation-over-elevation configuration. The two relatively rotating wedges  11  and  12  can be combined inline in various combinations of steering operations. 
         [0044]    The objective of the rotating wedge antenna mount in accordance with the present invention is to point an antenna over a two-dimensional region; accordingly, it is necessary to convert the desired pointing direction, such as azimuth, elevation and cross elevation, into the relative rotation angles of the various rotating-wedge, and rotating-plate blocks. 
         [0045]    The differential angle between the second wedge  12  and the first wedge  11  gives the elevation angle. To change the elevation without changing the azimuth, the lower and upper motors  24  and  29 , respectively, must rotate by an equal angle but in the opposite direction. To change the azimuth angle alone, the upper motor  29  is used to lock the first wedge  11  to the second wedge  12 , and the lower motor  24  rotates the combined wedges  11  and  12 , so as to steer to the new azimuth angle. For the second wedge  12 , the upper motor  29  causes the two wedges  11  and  12  to rotate differentially giving the elevation scanning. For elevation scanning with fixed azimuth scanning, the first wedge  11  must rotate equally and oppositely to the rotation of the second wedge  12 . For combined azimuth and elevation scanning, both lower and upper motors  24  and  29  must be operated. 
         [0046]    In the illustrated embodiment in  FIG. 4 , the maximum wedge angle, α max , is chosen as 30°, whereby the elevation-complement scan range is ±60°. A range of maximum wedge angles are within the scope of the invention, e.g. when both wedges  11  and  12  have a wedge angle, α max , of 45° the elevation scan can go from 90° (straight up) to 0° (pointing horizontally as seen in  6 ). Typically, when the wedge angles for both the first and second wedges  11  and  12  are the same, the wedge angles, α max , ideally vary between 20° and 45°; however, when the wedge angles are different, the range of wedge angles can vary between 20° and 70°, and typically add up to between 40° and 90°. The range in azimuth is 360°. 
         [0047]    For optimum operation, the central longitudinal axis of the first wedge  11 , shown as a dashed line in  4 , should intersect the central longitudinal axis of the second wedge  12  at the center of the interface of the second bearing  26 . Otherwise the second wedge  12  will experience an undesired mutation. 
         [0048]    In the antenna mount described above, the lower motor  24  does a combined action for both elevation and azimuth steering. In alternative embodiments, illustrated in  FIG. 6 , the azimuth and elevation steering is decoupled using a three-motor configuration, including a first (or bottom) wedge  31 , a second (or middle) wedge  32 , and a third (or top) wedge  33 , or a four-motor configuration, which also includes a fourth wedge  34 . The mounting structure  13 , including the mounting post  14  and the bottom plate  15  can be identical to those hereinbefore described with reference to  FIG. 4 . Similarly, the first wedge  31  can be rotatably mounted on the mounting structure  13  utilizing the first bearing structure  21 , and rotated utilizing the first (lower) motor  24  driving the spur gear  23  and the ring gear  22 , mounted on the bottom of the first wedge  31 . The second wedge  32  can be rotatably mounted on the first wedge  31  utilizing the second bearing structure  26 , and rotated utilizing the second upper motor  29  driving the spur gear  28  and the circular rack gear  27 . The third wedge  33  is mounted on the second wedge  32  utilizing a third bearing structure  36 , as hereinbefore defined. A third motor  37 , mounted on the second wedge  32  drives a third spur gear  38  on an upper circular rack gear  39 , which extends from around the bottom of the third wedge  33 . The third wedge  33  is rotated about an axis perpendicular to one end of the third wedge  33 , which is also the longitudinal axis thereof. 
         [0049]    If necessary, the fourth wedge  34  can be mounted on the third wedge  33  utilizing a fourth bearing structure  41 , similar to those hereinbefore described, and rotated by a fourth motor  42 , which drives a fourth spur gear  43  on a top circular rack gear (not shown) extending from around the bottom of the fourth wedge  34 . The fourth wedge  34  is rotated about an axis perpendicular to one end of the fourth wedge  34  adjacent to the outer end of the third wedge  33 , which is at an acute angle to the longitudinal axis thereof. 
         [0050]    The second and third motors  29  and  37  of the middle and top wedges  32  and  33 , perform elevation steering only. The azimuth steering could be performed by rotating the first wedge  31  or simply by rotating the mounting post  14 . The advantage of the three or four-motor systems over the two-motor systems is that the controls for driving the azimuth and elevation axes are decoupled enabling simpler control systems to be developed. 
         [0051]    For applications in which the requirement of scanning is over a relatively small two-dimensional angular range centered on a particular direction, e.g. radar antenna in the nose of an airplane, steering of the antenna mounts can be performed using a cross-elevation-over elevation configuration. 
         [0052]    Cross-elevation-over-elevation steering can be implemented with the four-wedge system illustrated in  FIG. 6 , which includes two complementary pairs of rotating-wedge blocks  31 / 32  and  33 / 34  rotatable on the mounting structure  13 . The lower pair of wedges  31 / 32  performs elevation steering, while the upper wedge pair  33 / 34  performs the cross-elevation steering, and is therefore oriented so that the plane of scanning of the upper pair of wedges  33 / 34  is at 90° (orthogonal) to the scanning plane of the lower pair of wedges  31 / 32 . The fourth (cross-elevation) motor  42  (shown in dashed lines) is hidden behind the third and fourth wedges  33  and  34 . Both the elevation wedges  31 / 32  and the cross-elevation wedges  33 / 34  were chosen for the example in  5  to have wedge angles of α max =45°; however, other wedge angles are within the scope of the invention, as hereinbefore described. 
         [0053]    With reference to  FIG. 7 , it is possible to implement the cross-elevation-over-elevation configuration of  FIG. 6  with only two motors, e.g. a first elevation motor  51 , with two drive shafts,  200  and  201  for the elevation wedge pair  31 / 32 , and a second cross-elevation motor (not shown) for the cross-elevation wedge pair  33 / 34 . The first elevation motor  51  drives both the first and second spur gears  23  and  28 , simultaneously, either directly, as with the first spur gear  23 , or indirectly via an angled gear box  52 . In this embodiment, the ring gear  22  is replaced by another rack gear  53  extending from around the bottom of the first wedge  31 . The lower drive shaft  200  drives the first spur gear  23 , which rotates the wedge  31  about an axis perpendicular to the base plate  15 , while the upper drive shaft  201  drives the second spur gear  28  through a 45° turn gear box. For the wedge angle of 45° used in this example, the shaft angle must also be turned by 45°. The gearing of the gear box  52  must be such that the rotation angle of the second wedge  32  is exactly equal to in magnitude, but opposite to in direction, the rotation angle of the first wedge  31 . 
         [0054]    In the embodiments illustrated in  FIGS. 4 and 6 , the scan range in both angular directions can be sufficiently small that elevation-over-azimuth steering can be operated in an approximation to a cross-elevation-over-elevation format. The usual elevation range for the two-wedge system is for θ=0° to 2α max , α max  being the wedge angle. However, in the region around θ=α max , i.e. in the center of the elevation steering range, the azimuth and elevation steering are approximately orthogonal. Therefore, X-Y (cross-elevation-over-elevation) steering can be achieved with the two-wedge mount  10  illustrated in  FIG. 4  via an elevation-over-azimuth system operating within a certain angular range around this central direction. The range can be extended by conversion of the desired cross-elevation-over-elevation coordinates to values of rotation of the wedges. 
         [0055]    With reference to  FIGS. 8 and 9 , a first cable  81  is required to transmit DC power and motor control signals between the two (or more) motors  24  and  29  and a electrical control box  82  disposed adjacent to the antenna support structure  13  or some other remote location. Moreover, a second cable  83  is required to transmit data, e.g. RF signals between RF control boxes  84  and an antenna feed  86  extending from an antenna  87  mounted on the mount  10 . 
         [0056]    In the illustrated embodiments, the antenna  87  is a dish antenna with a direct feed  86  held by struts  88 ; however, various other forms can be used including a Cassegrain system with a secondary reflector, and a flat reflectarray in place of the dish. The RF cables  83  between the feed  86  and the cable  92  or the RF control boxes  84 , e.g. the high power amplifier (HPA) and the low-noise block converter (LNB), are fixed to the dish  87  as illustrated in small dashed lines in  FIGS. 8 and 9 , and can be either co-axial cable or waveguide. 
         [0057]    In  FIG. 8 , the data control boxes  84 , such as the HPA, block up converter (BUC), and LNB, are located at the back of the antenna  87 . The data signals are then carried to and from the feed  86  by means of the second cable  83 , e.g. coaxial cable or waveguide, fixed in some manner to the dish  87  and struts  88 . In  FIG. 9 , the data control boxes  84  are placed at the base of the mounting structure  13  or some other remote location, and must be connected to the fixed coaxial cable  83  or waveguide at the dish  87  via a third and fourth connector cables  91  and  92 , which extend down through the mount  10 . 
         [0058]    For both layouts, the DC power and motor control distribution is the same. The distribution of power and control signals is relatively simple for the first motor  24 , since it is fixed relative to the mounting structure  13 . However, the second motor  28  rotates with the first wedge  11 , as it performs the azimuth steering. Such rotation can cause the first cable  81  to have unacceptable amounts of twist. In a preferred embodiment, the twisting is eliminated with the use of an electrical slip-ring  89  device placed at the center of the interface between the bottom plate  15  and the first wedge  11 . Slip rings  89  are relatively inexpensive and can be obtained “off-the-shelf.” Note that the cables  93  coming out of the top of the slip ring  89  rotate with the first wedge  11  and do not flex. 
         [0059]    The term “slip ring” might also be called by a variety of other names including “electrical rotary joint”, etc. We use the term “slip ring” here to apply to DC or low frequency control signal applications. It may also be possible to put data through slip rings if the data rate is sufficiently low. The term “rotary joint” is hereinafter used to apply to joints that handle IF or RF data signals. 
         [0060]    The distribution of the RF and data signals is more complex than for the DC and motor control. With reference to  FIG. 8 , the DC power and the data transfer must be brought from the electrical control box  82  to the RF control box  84  at the back of the antenna  87  via the cables  81  and  93 , which branches off from the cables running to the first and second motors  24  and  29 . Note that the cables  93  from the first wedge  11  that split off to the back of the antenna  87  do not twist so that there is no wire-wrap problem. Instead, the cables  93  flex as the mount  10  steers in elevation, because the first and second wedges  11  and  12  rotate equally but oppositely, so that there is no net rotation (twist) of the cables  93 . 
         [0061]    In  FIG. 9 , the RF control boxes  84  are mounted at the base of the mounting structure  13  or some other remote location so that RF power has to be carried to and from the back of the antenna  87 . In this configuration, there is no need for separate lines to transfer data. The RF cables  91  and  92  are shown in long-dashed lines in  FIG. 9 . In order to bring the RF line  91  from the RF control boxes  84  through the first wedge  11 , it is necessary to minimize the effects of the azimuth rotation of the first wedge  11  to prevent the RF line  91  from twisting. A commercial rotary joint  89  can be used for this transition; however, it is possible to have both a rotary joint  89  within a slip-ring assembly, whereby the DC and control electronics for the upper motor  29  and the RF line  91  can be simultaneously accommodated. 
         [0062]    The cable  92  between the rotary joint  89  and the cables  83  fixed to the antenna  87  is a flexible cable, which only flexes back and forth, without twisting, as the elevation steering is performed. Both transmit and a receive data, e.g. RF, signals can be accommodated on a single line, if some form of isolator is provided the back of the antenna  87 , where the cable  92  splits between transmit and receive. 
         [0063]    Alternatively, the data control boxes  84  are placed on top of the mount  10 , and connected to the antenna  87  by a flexible cable  83 . The DC power is provided to the control boxes  84  and the motors  24  and  29  through slip ring  89 , while the data is transferred between the a remote source and the data control boxes  84  by an inexpensive commercial off-the-shelf computer wireless link. 
         [0064]    In mechanical steering of antennas, there can arise a condition, called the “keyhole effect”, which requires a very large steering angle change for a relatively small angular change in the satellite direction. For example, in a steering system that uses elevation-over-azimuth pointing in which the elevation angle, ε, is close to 90°, i.e. pointing to the nadir, and the platform, such as on a ship, has a small roll or pitch that is at 90° to the elevation arc, it would be necessary for the azimuth steering to be changed by 90° very rapidly thereby requiring very large angular accelerations. 
         [0065]    For the elevation-over-azimuth steering with the rotating wedge antenna mount in accordance with the present invention, the keyhole problem can be eliminated by replacing the top plate  18  by a wedge-shaped mounting plate oriented so as to rotate the beam pointing by a small amount, Δε, along the elevation direction. The wedge angle of the wedge-shaped mounting plate would be relatively small, typically in the order of about 5° to 15°, preferably 10°. If the original range of elevation scanning was, 0° to 90°, then the new range is from Δε to 90°+Δε. The keyhole would be shifted to ε=90°+Δε where it would be out of the range of operation. The addition of the wedge-shaped mounting plate would require a more complex algorithm for computing the required wedge-rotation angles. 
         [0066]    For the cross-elevation-over-elevation (X-Y) configuration, the keyhole has been shifted from the zenith location down to the 0° elevation location. Therefore, the X-Y configuration can be operated over all of a hemisphere except near 0° elevation. In this region of operation, a third steering axis could be added to eliminate this problem. 
         [0067]    The size of the first and second motors  24  and  29  depends upon the torque required. The motor torque overcomes two forces: the first force is the static holding force of gravity exerted on the center of mass of the antenna  87 ; the second force arises from angular acceleration of the center of mass of the antenna  87 . The antenna  87  undergoes two angular accelerations: the first is the angular acceleration needed to steer the antenna  87  to a new position; and the second is the angular acceleration arising from motion of the mounting structure  13 , such as would be experienced on a ship. The force needed to overcome bearing friction is usually low relative to the other forces. 
         [0068]    The following analysis relates to the torque requirements for an elevation-over-azimuth mount in relation to the static force of gravity. Moreover, the analysis concentrated on an assembly mounted with a horizontal base plate, such as is shown in  FIG. 2   a.  For other mounting angles, the analysis would have to be correspondingly changed; however, the range of values of torque factor (to be defined) would be no larger. 
         [0069]    The torque required by the antenna mount  10  to support the mass of antenna  87  is compared to that required by the standard elevation-over-azimuth system, illustrated in  FIG. 2   a.  The orientation of the antenna in  FIG. 2   a  is the same as was used for determining the torque for the antenna mount  10 . The center of mass of the antenna and feeds, plus the elevation mounting assembly is at a distance r cm  from elevation axis. The elevation motor must provide a torque of 
         [0000]        T   el   =r   cm   F   g  sin θ= r   cm mg sin θ  (2) 
         [0070]    where m is the mass of the antenna and feeds. 
         [0071]    The torque factor for both the mount  10  of the present invention and the standard elevation over azimuth system is plotted in  FIG. 10 . For the mount  10  of the present invention, the value of the wedge angle α max =45° was chosen. The torque factor is zero for both systems for the elevation complement at θ=0°, i.e., for the antenna pointing at the zenith, and for all other values of elevation angle θ, the torque factor is less for the mount  10  of the present invention, and goes to zero for θ=90°. Overall, the rotating-wedges technique of the present invention requires somewhat less holding torque than the standard elevation-over-azimuth mounts. 
         [0072]    Unlike the acceleration due to gravity on a fixed platform, the motion-induced accelerations can be at any angle so that analysis would require extensive work to cover all possibilities. The torque on the bearing structures  21  and  26  arise from an acceleration of magnitude a exerted on the center of mass of the antenna  87 , which has a mass m. As illustrated in  FIG. 11 , the direction in which the acceleration is directed can be anywhere over a sphere. Therefore, the computation becomes much more complex than that for the force of gravity, where the direction of the gravitational force is confined to a plane. It is hypothesized that there is again a torque factor that helps reduce the torque that the motors  24  and  29  must supply. There will likely be zeros and maxima similar to those shown in  FIG. 10 . Note that the acceleration force can add or subtract from the force of gravity analyzed earlier depending upon the direction of the two forces. 
         [0073]    With reference to  FIGS. 12   a,    12   b  and  13 , counterweights  101  can be used to reduce the torque required from the first and second drive motors  24  and  29  in the antenna mount system in accordance with the present invention in an elevation-over-azimuth configuration for full hemispheric coverage. The counterweights  101  hang down to the opposite side of the elevation axis from the antenna structure  87 . In  FIG. 13 , the antenna  87  is positioned to point along the elevation axis to the horizon, whereby the counterweights  101  provide the most counter torque. In principle, the counterbalancing can be implemented so that there is no torque about the elevation axis over the full elevation range and for any roll and pitch of the antenna  87 . 
         [0074]    In the aforementioned embodiments, the emphasis was primarily on elevation-over-azimuth and cross-elevation-over-elevation stabilized platforms; however, the use of a third axis, i.e. three-axis steering, to compensate for the keyhole effect was mentioned. 
         [0075]    In defining the stabilization axes, there are a variety of terms used including the terms azimuth, elevation, and cross-elevation, as hereinbefore defined; however, other terms, such as “level”, “cross-level”, and “rolling and pitching axes” are sometimes employed. The cross-level angle is “the angle measured about the line of sight, between the vertical plane through the line of sight and the plane perpendicular to the deck through the line of sight”, and the “cross-level” and “rolling and pitching axis” are primarily applied to use on ship decks. The term “cross-level” is used when an axis of rotation is added between an azimuth and an elevation axis in accordance with the present invention. 
         [0076]    The third axis normally only has a relatively small offset steering capability that is just large enough to move the main two axes away from the keyhole. Another use of a third axis arises primarily in shipboard applications. For example, when a ship borne antenna that is originally pointing straight forward at some elevation angle with the ship level, undergoes a change in alignment due to the ship rolling or pitching a certain amount, it is necessary to find solutions to three-dimensional vector equations in order to determine the new pointing settings for the usual forms of two- or three-axis steering. However, with an antenna mount system with a third, cross-level axis, all that is required is for the cross-level structure to be rotated. Typically, not only are the computations much simpler but more accurate antenna pointing results. 
         [0077]    For a standard elevation-over-azimuth system, a cross-level axis must be inserted between the elevation and azimuth axes; however, for an antenna mount in accordance with the present invention it is only necessary to use an appropriate combination of rotating wedge pairs and rotating plates. For example, to perform the functions of a three-axes system an antenna mount  111 , illustrated in  FIG. 14 , comprising three sub-assemblies can be used. The first sub-assembly comprises a first rotating plate  112  for the azimuth steering about the azimuth (vertical) axis. The rotating plate  112  includes a circular rack gear  113  extending outwardly from around the bottom thereof for engaging a spur gear  114 , which is driven by an azimuth motor  115 . A bearing structure  116 , including opposed bearing surfaces with some form of bearing material therebetween, is mounted on a supporting structure  117 , which includes a horizontal plate  118  and a vertical post  119 . The supporting structure  117  also supports the azimuth motor  115 . The bearing structure  116  enables the rotating plate  112  to rotate relative to the supporting structure  117  about a first, e.g. vertical or azimuth, axis when the azimuth motor  115  is engaged to drive the spur gear  114  and the rack gear  113 . 
         [0078]    The second sub-assembly comprises a second rotating plate  121  extending from and perpendicular to the plane of the first rotating plate  112  for the cross-level steering. A cross-level motor  122  is mounted on the rotating plate  121  for driving a spur gear  123 . A second bearing structure  124  is mounted on the second rotating plate  121  for rotating the wedge pair, hereinafter described, about a horizontal cross-level axis, which is perpendicular to the first axis. 
         [0079]    The third sub-assembly comprises first and second wedges  131  and  132  with a third bearing structure  133  therebetween. The first wedge  131  includes a rack gear  134  extending around one end thereof for engaging the second spur gear  123 . The second wedge  132  includes a rack gear  135  extending around one end thereof for engaging a third spur gear  136 , which is driven by an elevation motor  137  mounted on the first wedge  131 . 
         [0080]    As above, the wedge angles ideally vary between 20° and 45°; however, when the wedge angle are different, the range of wedge angles can vary between 20° and 70°, and typically add up to between 40° and 90°. 
         [0081]    The cross-level motor  122  is used to perform the cross-level steering, and, in combination with the elevation motor  137 , is used to perform the elevation steering. In principle, this pointing system could be mounted upon a fixed post  119  with all the moving mechanisms clustered together near the back of the antenna (not shown). With the configuration illustrated in  FIG. 14 , the range of elevation steering need not be much more that the 90° to 0°. Full hemispheric coverage is achieved by appropriate azimuth steering provided by the first sub-assembly. In some special applications, a fourth steering axis is provided by either an additional rotating-wedge pair, or an additional rotating disk.