Patent Publication Number: US-6707432-B2

Title: Polarization control of parabolic antennas

Description:
This application relates to U.S. Provisional Patent Application No. 60/256,937 filed Dec. 21, 2000. 
    
    
     FIELD OF INVENTION 
     The present invention relates to antenna systems and, more particularly, to devices for mechanically changing the polarisation of such antenna systems. 
     BACKGROUND TO THE INVENTION 
     The revolution in telecommunications in the past decade has led to numerous developments in all fields of communications and data processing. It has also led to a corresponding increase in not only data traffic but also in the need for individuals to be constantly in communication with their colleagues. Such a need has been so pervasive that even while individuals are in transit, such as when travelling by air, data communications with their office computers, staff, and colleagues is vital. 
     To meet the above need for such communications, onboard data communications systems for aircraft have been developed. Such systems include antenna subsystems that track and communicate with satellites that relay data signals from the aircraft to the ground. Such data signals are ideally transmitted to the satellites with as little loss as possible to maintain the integrity of the signal. One source of signal degradation is polarisation loss. When an antenna receives linear polarisation from a satellite, the local polarisation in the coordinates of the antenna is dependent on the location of the antenna relative to the satellite as well as the orientation of the antenna relative to the satellite. If the antenna is mounted on an aircraft, then the position and orientation of the antenna will vary as the aircraft moves. This motion requires that the polarisation of the antenna also varies with time to ensure that polarisation loss is minimized. 
     While polarisation synthesis techniques may be used to compensate for the polarisation loss, in order to use such techniques it is necessary to have two orthogonal polarisation components excited in the antenna feed at each frequency of interest. This precludes the implementation of systems where the feed has only inputs that correspond to a single polarisation in each ban of interest. 
     Other polarisation synthesis techniques require more complexity and, hence, added cost. These complex implementations use two polarisation components in each band of interest. It is noted that some systems may have a single broadband feed channel in each of the two polarisation components. However, for full-duplex operation, each of the channels would have to be split into transmit and receive paths after the feed, resulting in a complex system having four polarisation/frequency ports. 
     Another possible solution to the polarisation loss problem is the use of ferrite devices for Faraday rotation of the electrical fields. Ferrite devices are attractive for polarisation control where they can be used. Unfortunately, such devices suffer from narrow bandwidth and high loss, making them unsuitable for simultaneous operation at 12 GHz and 14 GHz and also resulting in high noise temperature and reduced EIRP (Effective Isotropic Radiated Power) for a given input power. 
     Based on the above, there is a need for a low-cost and simple solution for compensating for polarisation loss. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to overcome the above problems by providing systems and devices for rotating the polarisation of a signal emanating from or being received by an antenna system through mechanical means. The rotation of the polarisation is achieved, as in the prior art, by mechanically rotating the feed receiving or transmitting the signal. A non-metallic drive cord or belt is used to transfer motion from a motor located outside or behind the aperture to the feed polarisation axis. For a linear array of multiple antenna elements, each feed for each antenna element is rotated simultaneously and by an equal amount through the use of a drive system common to all the feeds. The drive system is coupled to each feed and to a drive motor. When the motor is activated, the drive system simultaneously rotates each feed by a given amount. By rotating the feed, the polarisation of the signal is correspondingly rotated and compensation for polarisation loss is provided. 
     In one aspect the present invention provides, an antenna element including: 
     a reflective element having a reflecting surface; 
     a feed rotatable about an axis; 
     rotating means for rotating the feed about the axis, the rotating means being coupled to the feed, 
      wherein the reflecting surface faces the feed and rotation of the feed changes a polarisation signal emanating from the antenna element or being received by the antenna element. 
     In a second aspect the present invention provides an antenna element including: 
     a reflective element having a reflecting surface; 
     a non-metallic drive cord or belt; 
     a feed rotatable about an axis; 
     a drive motor located outside or behind the radiating aperture; 
      wherein the reflecting surface faces the feed and rotation of the feed changes a polarisation signal emanating from the antenna element or being received by the antenna element, and rotation of the motor shaft moves the drive cord or belt causing the feed to rotate. 
     In a third aspect the present invention provides an array of at least two antenna elements, each antenna element including: 
     a reflective element having a reflecting surface; 
     a feed rotatable about an axis; the array including a common rotating means for rotating each feed of each antenna element, the common rotating means being coupled to each feed, 
      wherein each reflecting surface faces a corresponding feed and activation of the common rotating means rotates each feed simultaneously. 
     an antenna element including: 
     a reflective element having a reflecting surface; 
     a feed rotatable about an axis; 
     a non-metallic cord or belt for rotating the feed about the axis, the rotating means being coupled to the feed, 
     a drive motor located outside or behind the radiating aperture and connected to the cord or belt, 
      wherein the reflecting surface faces the feed and rotation of the feed changes a polarisation signal emanating from the antenna element or being received by the antenna element. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the invention may be obtained by reading the detailed description of the invention below, in conjunction with the following drawings, in which: 
     FIG. 1 is a side view of a linear antenna array illustrating an embodiment of the present invention; 
     FIG. 2 is a top plan view of the linear antenna array of FIG. 1 illustrating the rotational motion of the feeds that is caused by the linear motion of the drive means; 
     FIG. 3 is a side view of a linear antenna array similar to that in FIG. 1 but with the use of a sub-reflector; and 
     FIG. 4 is side view of a linear antenna array using a different drive arrangement to that illustrated in FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, a linear array  10  of antenna elements  20  is illustrated. As can be seen from FIG. 1, each antenna element  20  has a reflective element  30  and a feed  40 . Each reflecting element has a reflective surface  50  that faces the feed  40 . Each feed  40  is coupled to a non-metallic drive cord or belt  60  and the drive cord or belt  60  is in turn coupled to a motor  70 . 
     Upon activation of the drive motor  70 , the drive cord or belt  60  is correspondingly activated and thereby simultaneously rotating each feed  40  by the same amount. In one embodiment, the rotational motion of the shaft  80  of the motor  70  is translated into linear motion by the drive means  60  through a capstan  90 . 
     Referring to FIG. 2, a top plan view of the linear array  10  is illustrated. As can be seen, each of the feeds  40  is free to spin on its axis through the use of the drive cord or belt  60 . As can also be seen in FIG. 2, the rotation of the feeds  40  can be clockwise of anti-clockwise as shown by the arrows  100 . As can also be seen in FIG. 2, the amount of rotation for each feed  40  is substantially equal among all the feeds. This is accomplished by having the drive cord or belt  60  being coupled and arranged to each feed similarly. Thus, a rotation of 10 degrees clockwise for a first antenna feed will be duplicated for all the other antenna feeds. 
     It should be noted that each of the antenna elements  20  are each independently excited by its own feed. Each of the feeds provide a transition between a guided wave on a coaxial wave guide or other transmission line to a wave propagating unguided through space. This unguided wave reflects off of the reflecting surface of the antenna element  20 . The coupling of the rotatable feed  40  to a signal source or to a receiver is accomplished through well known means. 
     Regarding the drive cord or belt  60 , the drive cord or belt  60  take the form of a drive cord that is wrapped around a feed pulley such that linear motion of the cord causes each of the feed pullies and thereby each of the feeds linear motion of the cord in one direction causes clockwise rotation of each of the feeds while linear motion of the cord in the other direction causes each of the feeds to rotate in an anti-clockwise direction. It should be quite clear that each of the feeds is mounted on a pulley that allows the feed to rotate when the pulley rotates. While the above description contemplates using a drive cord as the drive means  60 , other implementations may be used. As an example a cable or a string may take the place of the drive cord as in a similar arrangement as explained above. The cable or string may be wrapped around the feed pulley such that linear motion of the cable or string causes rotational motion of the pulley and thereby, the feed. 
     Regarding the capstan  90  and the motor  70 , the drive means  60  is coupled to the shaft  80  of the drive means in conjunction with the capstan  90 . As noted above, the rotation of the drive shaft  80  causes linear motion of the drive means  60 . This is accomplished by either looping or wrapping the drive means  60  around the shaft  80 . If the motor  70  is placed in a position such that the shaft  80  is substantially parallel to the feeds  40 , then the capstan  90  may not be required. In such an embodiment, the rotation of the shaft  80  directly translates into rotation into each of the feeds  40 . 
     Regarding the reflective element  30 , as noted above this reflective element  30  has a reflecting surface that faces the feed  40 . The reflective element  30  has been found to be most effective when it takes the form of a parabolic. As a parabolic, the concave inner surface of the parabolic serves as the reflecting surface  50  for the reflective element  30 . With the reflecting surface  50  facing the feed  40 , a plain wave incident on the mouth of the parabolic is thereby focussed onto the feed. As can also be seen in FIGS. 1 and 2, the adjacent edges of the parabolics forming the different reflective elements  30  are parallel and that a mouth of each parabolic in nominally rectangular in shape. 
     Referring to FIG. 3, a second embodiment of the linear array is illustrated. As can be seen the structure for each antenna element  20 A in FIG. 3 is similar to the structure of the each antenna element  20  in FIG.  1 . The main difference between the two structures is the presence of a sub-reflector  110  for the antenna element  20 A. The useful surface of the sub-reflector  110  either the concave or convex side depending on other design details. The sub-reflector  110  is placed such that feed  40  is between the reflective element  30  and the sub-reflector  110 . Also, the reflector is placed such that it faces the reflecting surface of the reflective element  30 . The drive means for the embodiment illustrated in FIG. 3 is similar to that explained above and illustrated in FIG.  1 . The use of the sub-reflector  110  in the antenna element  20 A allows the energy from the feed to be reflected off the sub-reflector prior to being reflected off of the primary reflecting surface  50  of the reflective element  30 . 
     While the above designs illustrate systems where the input of the feed  40  is also the access for the reflective element  30  and the sub-reflector  110 , this need not be the case. Other designs where the feed  40  does not share a common access with a reflector surface, either the reflector  120  or the reflective surface  50  of the reflective element  30  is possible. As another alternative, feeds  40  need not be rotated merely by means of a cord and pulley system. If the feeds or its pullies were equipped with outwardly extending teeth, a chain drive system could be implemented in place of the cord or belt drive system illustrated and explained above. It is important however that the drive be non-metallic in order that it does not alter the radiation pattern of the antenna system. 
     Referring to FIG. 4, an alternative drive arrangement for the invention is illustrated. For this embodiment, the feeds  40  are supported by dielectric support tubes  130 . The feeds  40  are rotated by rotating the dielectric support tubes  130 . The dielectric support tubes  130  are coupled to the drive means  60  and thereby tot he drive motor  70  in an arrangement similar to that explained above. The arrangement in FIG. 4 avoids the need for a bearing ring around the feeds. Such a bearing ring could block some the antenna radiation thereby reducing the achieved gain and possibly distorting the shape of the radiation pattern for the antenna array. Furthermore, as explained above the shaft  80  of the motor  70  in the embodiment illustrated in FIG. 4 is substantially parallel to the access of the feeds  40  and this arrangement allows for the dispensing of the capstan  90 . Such an arrangement is thereby simpler and may provide better performance for the antenna system. 
     While the embodiments illustrated and explained above contemplate an antenna array, it is also possible to use a single antenna element using the mechanically rotated feed is illustrated and explained above. While the single antenna element may not provide the performance and the results of a complete linear antenna array, other applications may be suited for such a single antenna element.