Patent Publication Number: US-7724198-B2

Title: System and method for path alignment of directional antennas

Description:
TECHNICAL FIELD OF THE INVENTION 
   This invention relates to radio frequency antennas, and more specifically to path alignment of directional antennas, such as tower-mounted parabolic antennas. 
   BACKGROUND OF THE INVENTION 
   In a microwave communications network, wherever a transmission path (link) is to exist, accurate antenna path alignment is required to insure proper communications. Typically, links are between tower-mounted antennas up to 25 miles apart, and an initial alignment process requires tower crews to physically align the antennas using sophisticated test equipment to monitor the results. Using today&#39;s techniques, initial alignment can be off-path by several degrees to either side of the target antenna, resulting in the target being in a null or side lobe of the pattern of the antenna being aimed. 
   More specifically, one current practice of initial alignment of tower-mounted antennas requires that the two antennas be installed on their towers to provide a signal link for power measurements. A compass bearing to the distant end is taken and the antenna is visually aimed at a ground-based reference along that direction, typically a marker or a natural reference such as a tree. Radios are installed at each site and used to optimize the path. 
   Some antenna alignment methods use out-of-network radio devices, which permit tower installation crews to perform the alignment process before network radios are installed. One example is the Path Align-R™ test set from XL Microwave. Two identical test sets are used, one at each tower site. Each test set drives its respective antenna directly, while receiving the signal from the other test set. During alignment, the test sets provide continuous duplex voice communication over the antenna link, allowing the two technicians to communicate with each other. Both units indicate the received path loss, and each antenna&#39;s azimuth and elevation is physically adjusted, until minimum loss (maximum alignment) has been reached. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the present embodiments and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings, in which like reference numbers indicate like features, and wherein: 
       FIG. 1  illustrates two tower-mounted antennas, one being aligned to the other using a reflector and an alignment device in accordance with the invention. 
       FIG. 2  illustrates a parabolic antenna having a reflector installed in accordance with the invention. 
       FIG. 3  illustrates an antenna aligning device in accordance with the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  illustrates two antenna towers  10   a  and  10   b , each having a tower-mounted antenna  20   a  and  20   b , respectively. An alignment device  30  in accordance with the invention is shown being used for aligning the transmission path of antenna  20   a  to antenna  20   b . It is assumed that the direction to antenna  20   b  is known, that is, its compass bearing. 
   Alignment device  30  is placed on the ground to the side of tower  10   a , perpendicular to the desired transmission path. Typically, alignment device  30  is placed about 50 to 100 yards away from the tower base. 
   As explained below, antenna  20   a  has a special reflector  25  installed behind the antenna face. Aiming of antenna  10   a  is accomplished indirectly by using alignment device  30 , which sends a laser beam to reflector  25 , and receives the reflected beam when the position of antenna  20   a  provides a desired reflection path. The use of alignment device  30  facilitates and improves the accuracy of antenna alignment. 
   Although this description is in terms of parabolic tower-mounted antennas, the same concepts could be used to align any antenna, whether or not tower-mounted, whose transmission path is to be directed along a desired direction. With appropriate elevational adjustments, the alignment system and method described herein could be used to aim an antenna at a satellite. 
   In general, various types of directional antennas could require path alignment between fixed sites, mobile sites, or a mixture of fixed and mobile sites (referred to herein as “antenna sites”). Parabolic antennas have a relatively narrow focus (and high gain) as compared to other directional antennas, such as yagi and patch antennas, and are thus more susceptible to misalignment. 
     FIG. 2  illustrates antenna  20   a  in further detail. A feature of the invention is the installation of a reflector  25 , which has a circumferential reflecting surface. As explained below, the curved reflecting surface ensures a reflection to the alignment device  30 , which is not necessarily at the same elevation as the reflector  25 . In some embodiments, reflector  25  could be implemented as a semicircular surface or as having some other surface curvature that is less than fully round, so long as it is capable of reflecting back to alignment device  30  without undue repositioning. 
   In the example of  FIG. 2 , reflector  25  is attached to the waveguide flange  21  input to the feedhorn  22 . In general, reflector  25  is attached at some point on the antenna&#39;s transmission axis. In the case of a parabolic antenna, reflector  25  is behind the centerpoint of the antenna&#39;s reflecting surface. This is convenient in the case of parabolic antennas, because the reflector  25  can be easily attached for alignment and then removed prior to installation of antenna cabling. 
     FIG. 3  illustrates alignment device  30  in further detail. Alignment device  30  comprises an instrument unit  33  mounted atop a tripod  32 . Instrument unit  33  comprises a magnetic compass  31 , angular heading display  36 , and a telescopic scope/laser unit  34 , (referred to herein as the “optical unit”  33 ), all mounted atop a tripod  32 . The scope and a detector  35  are positioned to receive a laser beam reflected from reflector  25 . 
   Compass  31  is maintained in the desired heading of the signal path. The optical unit  34  and detector  35  have a common line of sight, and as explained below, this line of sight is perpendicular to the desired heading. During the alignment process, the line of sight is directed toward the reflector  25  on antenna  20   a . An elevation adjuster  34   a  permits the elevation angle of optical unit  34 , and thus the elevational direction of the optical path (upward toward the antenna) to be adjusted. 
   Alignment device  30  is equipped with various adjustment mechanisms. For leveling tripod  32 , its legs may be adjusted in length, using telescoping adjustment mechanisms such as are familiar with camera tripods. To conveniently accomplish leveling, a level  37  may be mounted on the surface of instrument unit  33 . 
   For rotating instrument unit  33  relative to tripod  32  so that compass  31  is pointed along a desired direction, tripod  32  has a swivel platform  32   a . Instrument unit  33  is mounted on a slide platform, which permits instrument unit  33  to translate back and forth relative to tripod  32  so that the line of sight of optical unit  34  is aimed at reflector  25 . Once device  30  is placed in an approximately correct location for aligning a particular antenna, these rotational and translatable adjustment mechanisms permit minor repositioning of the compass bearing (azimuthally) and optical path (horizontally) to be made without repositioning the entire device  30 . 
   In operation, detection of a laser beam, emitted from the laser of optical unit  34 , and reflected from reflector  25  in the same vertical plane of the laser, indicates alignment of the antenna along the correct heading. The tripod  32  is leveled, and the instrument unit  33  is rotated, using swivel platform  32   a , so the readout on display  36  matches the desired heading to the distant end. Using the scope of optical unit  34 , elevated to point toward the antenna, the operator checks how far the unit is forward of or behind the reflector  25 . If necessary, tripod  32  is relocated to be within a few inches of perpendicular relative to the reflector  25 , and the unit is re-leveled and reset to the desired heading. 
   Looking through the scope, the operator translates instrument unit  33  forward or back on the tripod  32  (using the sliding motion of platform  33   a ) as needed to view the crosshairs of the scope against the reflecting surface of reflector  25 . The laser, elevated together with the scope of optical unit  34 , is activated to illuminate the reflector  25 , and antenna  20   a  is moved until the laser beam returns to detector  35  and the scope. A visible light on instrument unit  34  or an audible tone can be used to indicate antenna alignment along the correct heading. 
   The above-described equipment and method for antenna alignment are expected to achieve alignment within one-half of a degree of the direction to the target antenna site, so the distant end is within the main lobe of the antenna pattern. Because terrestrial position and Earth&#39;s magnetic field are used to determine the direction to the target location, installation of a distant end antenna on tower  10   b  is not required. In fact, so long as the location and bearing of a desired target tower (the location of tower  10   b ) is known, tower  10   b  need not be actually installed. 
   Placing the direction finding equipment on the ground has two main advantages. Separation of the compass  31  from tower  10   a  avoids distortion of the Earth&#39;s magnetic field due to proximity of the tower&#39;s metal structure. Furthermore, hauling cumbersome equipment up the tower is unnecessary. Only the reflector  25  is required to be carried up and installed behind the antenna  20   a.    
   The tripod and fixture for the direction finding equipment could be constructed from a rigid non-metallic material to prevent distortion of Earth&#39;s magnetic field near the compass  31 . Alternatively, the compass could be elevated about 1 meter above the fixture on a non-metallic shaft to allow using a metal fixture and tripod. 
   The above-described concept is expected to achieve initial antenna alignment within one-half of a degree of the target and is based on the precision of geographic location and angular bearing between the two sites relative to true north. Using the described equipment, the antenna is aimed at the target location within the 3-dB beam width of the antenna main lobe. Confusing signal measurements due to nulls and sidelobes in the antenna pattern are avoided, improving safety and efficiency by reducing man-hours spent in hazardous conditions on a tower. With initial alignment on the antenna main lobe, final antenna alignment can then progress quickly. Because terrestrial position and Earth&#39;s magnetic field are used to determine the direction to the target location, installation of the distant end antenna or tower is not required. The reflector  25  is expected to be smaller and lighter than radio equipment currently used for antenna alignment, so carrying it up the tower and installing it on the antenna flange would be less cumbersome.