Abstract:
A method for aligning a feel horn in an antenna system ( 100 ) is disclosed. The antenna system ( 100 ) includes at least one reflector surface and one or more feed horns ( 141, 142, . . . 145 ). The method includes the steps of determining a desired reflection point of the central ray from the feedhorn of the reflector surface, configuring a laser beam source to be mounted on the feed horn to enable a laser beam to travel substantially coincidently along the axis of transmission of the feed horn in a direction towards the reflector surface, and adjusting the azimuth and elevation of the feed horn to align the laser beam with the desired reflection point on the reflector surface. A laser aligning apparatus ( 200 ) for practising the above method and an antenna system aligned by means of the laser alignment apparatus ( 200 ) and/or method are also disclosed.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to reflector antennas and more specifically to alignment of an antenna feed horn with respect to the reflecting surfaces.  
         BACKGROUND  
         [0002]    The ever increasing density of geostationary satellites demands increasing numbers of antennas for tracking and communication purposes. This situation can be somewhat alleviated by the use of Multibeam Antennas (MBA), whereby one antenna system can be used to receive from, and transmit to, many satellites simultaneously. As satellite antenna systems tend to be large in volumetric size, reduced real estate requirements represent a significant advantage. Each MBA has many feed horns for reception and/or transmission and the number of feed horns determines the number of satellites that can be accessed.  
           [0003]    Alignment of a feed horn in single beam axisymmetric antennas can be achieved relatively easily by centralising the feed with respect to the main surface of the antenna and levelling the feed aperture with the antenna pointing to zenith. An example of a single beam antenna is the circularly symmetrical Cassegrain type.  
           [0004]    MBAs, like the classical Cassegrain or Gregorian reflectors, typically employ a pair of reflector surfaces, namely a main reflector and a sub-reflector. The shape and size of the reflector surfaces are different, however, and typically a MBA has only one plane of symmetry. Multiple reflections of the beam and the lack of symmetry between the reflecting surfaces demand an alternative and more complex method of aligning the feed horns than is necessary in the case of a single beam axisymmetric antenna. Accurate alignment of each feed horn is necessary to prevent or reduce interference between adjacent beams.  
           [0005]    Consequently, a need exists for a method and apparatus for the alignment of one or more feed horns in a multibeam antenna system.  
         SUMMARY  
         [0006]    According to a first aspect of the present invention, there is provided a method for aligning a feed horn in an antenna system. The antenna system includes at least one reflector surface and one or more feed horns. The method includes the steps of determining a desired reflection point of the central ray from the feed horn on the reflector surface, configuring a laser beam source to be mounted on the feed horn to enable a laser beam to travel substantially coincidently along the axis of transmission of the feed horn in a direction towards the reflector surface, and adjusting the azimuth and elevation of the feed horn to align the laser beam with the desired reflection point on the reflector surface.  
           [0007]    According to another aspect of the present invention, there is provided a laser alignment apparatus for aligning a feed horn relative to a reflector surface in an antenna system. The apparatus includes a device for generating a laser beam, a mounting plate whereon the device is mounted such that the laser beam generated by the device is transmitted substantially perpendicularly to the surface of the mounting plate, and means for mounting the mounting plate to the feed horn such that the mounting plate is substantially perpendicular to the axis of transmission of the feed horn.  
           [0008]    According to another aspect of the present invention, there is provided an antenna system including at least one reflector surface, at least one feed horn, wherein the transmission axis of the feed horn is aligned with a reflection point on the reflector surface, and a laser alignment apparatus mounted on the feed horn and configured to transmit a laser beam substantially coincidently along the axis of transmission of the feed horn in a direction towards the reflector surface. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0009]    Features and preferred embodiments of the present invention are hereinafter described with reference to the accompanying drawings in which:  
         [0010]    [0010]FIG. 1 is a perspective view of a Multibeam Antenna (MBA) with which embodiments of the invention can be practiced;  
         [0011]    [0011]FIG. 2 is a perspective view of a laser alignment apparatus in accordance with an embodiment of the present invention;  
         [0012]    [0012]FIG. 3 is a plan view illustrating alignment of a feed horn with respect to the sub-reflector of the MBA of FIG. 1, using the laser alignment apparatus of FIG. 2; and  
         [0013]    [0013]FIG. 4 is a flow diagram showing a method of alignment of a feed horn with respect to the sub-reflector of a Multibeam Antenna (MBA) in accordance with the embodiment shown in FIG. 2 
     
    
       [0014]    Like reference numerals are representative of the same elements or items across the different figures.  
       DETAILED DESCRIPTION  
       [0015]    A laser alignment apparatus and a laser alignment method are disclosed hereinafter. The principles of the method and/or apparatus in accordance with the embodiments of the invention have general applicability to the alignment of point sources and/or point collectors. The included arrangements describe the application of the method and/or apparatus to align a feed horn in an asymmetrical Multibeam Antenna (MBA). However, it is not intended that the present invention be limited to the described method and/or apparatus. For example, aspects of the invention have application to the alignment of feed horns in symmetrical antenna systems or offset fed antennas that are not MBAs.  
         [0016]    [0016]FIG. 1 shows a Multibeam Antenna (MBA)  100  that includes a main reflector  110 , a sub-reflector  120 , and a number of feed horns  141 ,  142 , . . .  145 , mounted on a support arm  130 . The MBA  100  is asymmetrical, in that the main reflector  110  and the sub-reflector  120  are of different dimensions and shapes. Radio wave signals are transmitted and/or received by the feed horns  141 ,  142 , . . .  145  via reflections off the surfaces of the sub-reflector  20  and the main reflector  110 .  
         [0017]    During installation of the MBA  100 , the feed horns  141 ,  142 , . . .  145  must be accurately aligned with respect to the sub-reflector  120  to facilitate selective illumination of the sub-reflector  120  and the main reflector  110 . Selective illumination of the main reflector  110  by each one of feed horns  141 ,  142 , . . .  145  can facilitate simultaneous communication with a number of satellites, corresponding to the number of feed horns  141 ,  142  . . .  145 . Hence, each of the feed horns  141 ,  142 , . . .  145  illuminates a distinct portion of the sub-reflector  120  which is turn illuminates part or all of the main reflector  110 . These distinct portions to be illuminated are calculated to enable selective transfer of radio signals between a specific feed horn and a specific satellite. Careful determination thereof minimises the amount of interference between adjacent radio signal channels. The MBA  100  can preferably support up to 19 feed horns, thus providing 19 individual beams or channels for simultaneously communicating with 19 separate satellites in space. However, differing numbers of feed horns can be practiced without departing from the scope and spirit of the invention.  
         [0018]    [0018]FIG. 2 shows a laser alignment apparatus  200  in accordance with an embodiment of the present invention. The laser alignment apparatus  200  includes a mounting plate  210 , the dimensions of which are selected to facilitate mounting of the apparatus  200  on the circular front-end of an antenna feed horn  141 , . . . , 145 . Preferably, the mounting plate  10  is triangularly shaped, although different shapes can be practiced without departing from the scope and spirit of the invention.  
         [0019]    A disc-shaped levelling flange  220 , mounted in the centre of the mounting plate  210 , supports a cylindrical holder  230  that holds a laser source  240 . The levelling flange  220  includes levelling screws  222 ,  224  and  226 , the adjustment of which enables the angle of the laser source  240  to be adjusted such that a laser beam emitted from the laser source  240  is emitted perpendicularly to the surface of the mounting plate  210 . Laser beam emission occurs from a 1.5 mm emission aperture  242  in the tip of the laser source  240 . The size of the emission aperture  242  can be varied without departing from the scope and spirit of the invention.  
         [0020]    The mounting plate,  210  preferably includes locating pins  212 ,  214  and  216 , all of which protrude from a surface of the mounting plate  210 . The laser source  240  is mounted on the opposite surface of the mounting plate  210 . The locating pins  212 ,  214  and  216  enable the laser alignment apparatus  200  to be mounted on a feed horn in such a manner that a laser beam emission from the laser source  240  travels along the axis of transmission of the feed horn on which the apparatus  200  is mounted. Each of the locating pins  212 ,  214  and  216  are located on the circumference of a circle with centre located on the mounting plate  210  and co-incident with the axis of emission of the laser  30  source  240 . Furthermore, the apparatus  200  is mounted on the feed horn  141 , . . . , 145  such that the locating pins  212 ,  214  and  216  contact the outer circular casing of the feed horn  141 , . . . , 145 , thus permitting circular rotation of the apparatus  200  on the feed horn  141 , . . . , 145 . It will be apparent to those skilled in the art, in view of this disclosure, that variations to the locating pin mounting arrangement can be made without departing from the scope and spirit of the invention. For example, alternative mounting arrangements might include rollers, adjustable clamps, etc.  
         [0021]    During manufacture of the main reflector  110  and sub-reflector  120 , accurately positioned control marks are placed on the surface of the main reflector  110  and sub-reflector  120 . The main reflector  110  and sub-reflector  120  comprise a number of panels and the control marks are typically located at the corners thereof. Alignment of the two reflecting surfaces, being the sub-reflector  120  and the main reflector  110 , can be performed by use of the control marks. Alignment of a feed horn, with respect to the sub-reflector  120 , can also be performed using at least three position control marks, making use of the well known method of “triangulation”. It is not essential that the position control marks be located on the sub-reflector  120 , however, the position control marks should be fixed in position with respect to the sub-reflector  120 . Accordingly, the position control marks can be located on another part of the MBA  100 , such as the frame thereof. Furthermore, it is possible to produce position control marks without producing permanent marks on the MBA  100 . For example, the position control marks can be produced by means of a second laser that is setup at a reference position on the support arm  130  or an another part of the MBA  100 .  
         [0022]    [0022]FIG. 3 shows geometric alignment of a feed horn  145  with respect to a sub-reflector  120 , assuming that the sub-reflector  120  has already been aligned with respect to the main reflector  110 . Two position control marks  342  and  344  are shown on the sub-reflector  120 . A further two position control marks (not shown) are typically located directly below the position control marks  342  and  344 , and at the other end of the sub-reflector  120 . The position control marks typically comprise holes of 1.5 mm diameter, drilled through the sub-reflector  120  or indicated by a second laser. A laser alignment apparatus  200  is shown mounted over the aperture of the feed horn  145 . The laser alignment apparatus  200  is mounted on the feed horn  145  in a manner such that a laser beam can be transmitted substantially coincidently along the axis of transmission  320  of the feed horn  145  in a direction towards the sub-reflector  120 . The laser beam, representing the axis of transmission  320  of the feed horn  145 , is thus visible at a point  330  on the surface of the sub-reflector  120 . Dimensions  352  and  354  represent the distance between the aperture  242  of the laser alignment apparatus  200  and the position control marks  342  and  344 , respectively.  
         [0023]    [0023]FIG. 4 shows a flow diagram of a method of alignment of a feed horn with respect to a sub-reflector of a multibeam antenna.  
         [0024]    At step  410 , the desired reflection point, of the central ray from the feed horn to be aligned, is located and marked on the surface of the sub-reflector  120 . The desired location of the reflection point on the sub-reflector  120  is determined in accordance with the design configuration of the multi beam antenna  100 , preferably using a geometric modelling computer program. Based on the design configuration of the antenna (eg. specific curvature of the reflector surface, location and number of feed horns, etc) and the geostationary location of a particular satellite to be tracked, the computer program is used to determine the location of the reflection point relative to at least three position control marks on the surface of the sub-reflector  120 . The desired reflection point, on the surface of the sub-reflector  120 , is located by chordal measurement from at least three position control marks  342 ,  344  . . .  
         [0025]    At step  420 , the feed horn  145  is mounted on the support arm  130  of the MBA  100 . The laser alignment apparatus  200  is mounted on the surface of the feed horn  145  closest to the sub-reflector  120 . The laser apparatus  200  is mounted in such a manner that a laser beam emitted therefrom is transmitted substantially coincidently along the axis of transmission  320  of the feed horn  145 .  
         [0026]    At step  430 , the position of the feed horn is adjusted relative to at least three of the control marks on the sub-reflector  120 . Such adjustment entails measurement of the distances between the emission aperture  242  of the laser alignment apparatus  200  and the at least three position control marks on the surface of the sub-reflector  120 . The dimensions  352  and  354 , in FIG. 3, show the distance to be measured between the laser emission aperture  242  and two position control marks  342  and  344 , respectively. The specific distance values are calculated according to the design configuration of the MBA  100 , by the geometric modelling computer program. The critical distances extend between the actual radio wave emission point, in the feed horn  145 , to the position control marks  342  and  344  on the surface of the sub-reflector  120 . The distance between the actual radio wave emission point, in the feed horn  145 , to the emission aperture  242  constitutes a fixed offset that is compensated for in the geometric modelling computer program. For purposes of these measurements, measuring tapes of exact length have been used. Contact tips from conventional dial gauges are preferably used on each end of the measuring tapes as the tips located perfectly in the 1.5 mm laser emission aperture  242  and the 1.5 mm drilled position control marks.  
         [0027]    At step  440 , the axis of the feed horn  145  is aligned to coincide with the marked reflection point on the sub-reflector  120  by aligning the laser beam to illuminate the marked reflection point. The azimuth and elevation of the feed horn  145  are adjusted to perform this alignment. Once the laser beam is aligned to coincide with the marked reflection point, the laser alignment apparatus  200  can be rotated on the feed horn  145 , as earlier described. If the laser source  240  is not mounted perpendicularly to the mounting plate  210  and/or the surface of the feed horn  145 , such rotation of the apparatus  200  causes the laser beam to trace a circle on the surface of the sub-reflector  120 . The centre of the traced circle, which can be determined by bisection of the circle, should then be aligned with the marked reflection point.  
         [0028]    At step  450  the distances between the emission aperture  242  of the laser alignment apparatus  200  and at least three position control marks on the surface of the sub-reflector  120  are measured. If any of these measurements are not within a desired tolerance (N), the position of the feed horn  145  is again adjusted at step  430 . Once it is determined that the feed horn  145  is correctly positioned relative to the control marks on the sub-reflector  120  (Y) and points towards the reflection point on the sub-reflector  120 , the alignment procedure is complete.  
         [0029]    The foregoing describes only a few arrangements and/or embodiments of the present invention, and modifications and/or changes can be made thereto without departing from the scope and spirit of the invention, the arrangements and/or embodiments being illustrative and not restrictive.