Abstract:
A physical angle of a portion of a variable element, such as a phase shifter, is used to determine a desired antenna beam attribute, such as beam downtilt. In one example, a variable element includes a stationary circuit board and a rotatable circuit board. The stationary circuit board has at least one transmission line having a first output and a second output. The rotatable circuit board includes an input and a coupling section, the coupling section located to capacitively couple an input signal to the at least one transmission line between the first output and the second output, and the accelerometer being oriented such that it provides a signal indicative of a physical angle of the rotatable circuit board with respect to vertical.

Description:
FIELD OF INVENTION 
     The present invention relates generally to base station antennas. More particularly, the present invention relates to a rotating wiper-type phase shifter used in a variable element of a feed network. 
     BACKGROUND 
     Wireless mobile communication networks continue to evolve given the increased traffic demands on the networks, the expanded coverage areas for service and the new systems being deployed. Cellular (“wireless”) communications networks rely on a network of base station antennas for connecting cellular devices, such as cellular telephones, to the wireless network. Many base station antennas include a plurality of radiating elements in a linear array. Various attributes of the antenna array, such as beam elevation angle, beam azimuth angle, and half power beam width may be adjusted by electrical-mechanical controllers. See, for example, U.S. Pat. Nos. 6,573,875 and 6,603,436, both of which are incorporated by reference. For example, with respect to U.S. Pat. No. 6,573,875, a plurality of radiating elements may be provided in an approximately vertical alignment. A feed network may be provided to supply each of the radiating elements with a signal. The phase angle of the signals provided to the radiating elements may be adjusted to cause a radiated beam angle produced by the antenna array to tilt up or down from a nominal or default beam angle. 
     Phase angles may be adjusted by mechanical phase shifters. In the example of the &#39;875 patent, phase shifters are coupled by a common mechanical linkage. An expected phase angle may be ascertained from markings on a linkage rod or by a sensor in an electro-mechanical actuator located off the antenna panel extending beyond a bottom edge of the panel. However, markings on a linkage rod may not be determined remotely from the site, and known sensors in an electromechanical actuator typically comprise potentiometers. Furthermore, such potentiometers may have degraded performance as they age, and determine a position of the linkage, which for various reasons, may not necessarily be consistently correlated to a position of a phase shifter arm itself. What is needed is a more robust sensor which may be closely integrated into an electrical control circuit and which more directly measures the position of a phase shifter arm. 
     Also, previously known antenna arrays were known to have adjustable mounts which allowed default tilt angle to be set. In this arrangement, a mounting hardware allows the antenna array to be mechanically inclined with respect to a vertical axis to set a default phase angle (e.g., the angle at which a radiated beam would propagate if electrical tilt was set to zero). Default phase angle must be recorded during installation, and typically is, not remotely detectable. 
     SUMMARY OF THE INVENTION 
     According to an example of the present invention, a physical angle of a portion of a variable element, such as a phase shifter, is used to determine a desired antenna beam attribute, such as beam downtilt. In one example, a variable element includes a stationary circuit board and a rotatable circuit board. The stationary circuit board has at least one transmission line having a first output and a second output. The rotatable circuit board includes an input and a coupling section, the coupling section located to capacitively couple an input signal to the at least one transmission line between the first output and the second output, and the accelerometer being oriented such that it provides a signal indicative of a physical angle of the rotatable circuit board with respect to vertical. 
     The variable element also includes a look-up table having a plurality of physical angles of the rotatable circuit board correlated to a plurality of beam attributes and a controller configured to access the lookup table to obtain a physical angle of the rotatable circuit board that corresponds to a desired beam attribute, and to access the accelerometer to obtain the signal indicative of a physical angle of the rotatable circuit board. The look up table and the controller may be physically located on the rotatable circuit board. In alternate embodiments, the look up table and the controller may be physically located on a circuit board associated with an actuator assembly. 
     In one illustrated example, the stationary circuit board and the rotatable circuit board comprise a phase shifter and the beam attribute comprises a beam downtilt angle. In this example, the controller may be configured to operate an actuator to cause the rotatable circuit board to move to a physical angle corresponding to a desired beam downtilt angle. In another example, the actuator includes the controller, a non-volatile memory coupled to the controller. The look-up table, a motor coupled to the controller, and the controller are further configured to operate the motor to cause the rotatable circuit board to move to a physical angle corresponding to a desired beam attribute. 
     While a single axis may be used to determine the orientation of the rotatable circuit board with respect to ground in some examples (e.g., antennas mounted vertically and/or with fixed azimuth angles), another example of the present invention includes an accelerometer which provides signals indicative of an angle with respect to vertical with respect to a plurality of axes. The look-up table further comprises a plurality of default tilt angles and beam downtilt angles correlated to the physical angle with respect to vertical with respect to a plurality of axes. 
     In another example of the present invention, the variable element is incorporated in a feed network of a panel antenna. The panel antenna includes a plurality of radiating elements, an input, and a first feed network coupling the input to a first set of dipoles of the plurality of radiating element. The first feed network includes a plurality of transmission lines and at least a first variable element as set forth above, and an actuator coupled to the variable element. In another example, the panel antenna may excite portions of the radiating elements separately, such as when cross dipoles are exited with differently phased signals. In this example the panel antenna would further include a second feed network coupling a second input to a second set of dipoles of the plurality of radiating elements, the second feed network comprising a plurality of transmission lines and at least a second variable element, the second variable element being mechanically coupled to the first variable element. Because the first and second variable elements are mechanically coupled, and move together, a second accelerometer is not needed on the second variable element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  comprises a schematic diagram of a panel antenna including a feed network, which may be used in combination with the present invention. 
         FIG. 2  illustrates one example of a variable element according to the present invention. 
         FIG. 3  comprises a block diagram according to one example of the present invention. 
         FIG. 4  is a flow diagram of a method for setting the angle of a wiper PCB of a phase shifter according to a desired beam downtilt angle according to one example of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While this invention is susceptible of an embodiment in many different forms, there are shown in the drawings and will be described herein in detail specific embodiments thereof with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention. It is not intended to limit the invention to the specific illustrated embodiments. 
     A typical antenna array  10  may include an input  11 , a plurality of radiating elements  12  and a feed network  14  coupling the input  11  to the radiating elements  12 . A schematic diagram of a typical feed network  14  for an antenna array  10  is provided in  FIG. 1 . The feed network  14  may include a plurality of transmission lines  16  and one or more variable elements  18 . The transmission lines  16  have nominal impedance which may be selected to match an impedance of an RF line that couples the antenna array  10  to a Low Noise Amplifier (not shown). Transmission lines  16  may be implemented as microstrip transmission lines, coaxial cables, or other impedance-controlled transmission media. The variable elements  18  may comprise one or more phase shifters, power dividers, a combination of the two, or another type of variable element. The variable elements  18  may comprise differential variable elements. In one example, first and second feed networks  14  are provided, with a first feed network  14  driving a first set of dipoles on radiating elements  12 , and a second feed network  14  driving a second set of dipoles on radiating elements  12 . 
     In one example of the invention, the variable elements  18  comprise rotating wiper-type phase shifters  20  connected to transmission lines  16  and input  11  as shown in  FIG. 2 . A phase shifter  20 , in one example, may be implemented with first and second printed circuit boards (PCBs). In one illustrated example, as seen in  FIG. 2 , the first PCB may comprise a stationary PCB  22 , and the second PCB may comprise a rotatable wiper PCB  24 . 
     The stationary PCB  22  includes a plurality of transmission line traces  26 ,  28 . The transmission line traces  26 ,  28  are generally arcuate. The transmission line traces  26 ,  28  may be disposed in a serpentine pattern to achieve a longer effective length. In an illustrated example, there are two transmission line traces  26 ,  28  on the stationary PCB  22 , one transmission line trace  26  being disposed along an outer circumference of a PCB  22 , and one transmission line trace  28  being disposed on a shorter radius concentrically within the outer transmission line trace  26 . A third transmission line trace  29  connects an input on the stationary PCB  22  to an unshifted output. 
     In the illustrated example, the stationary PCB  22  may include one or more input traces  40  leading from an input pad  42  near an edge of the PCB  22  to where the pivot of the wiper PCB  24  is located. (The use of “input” and “output” herein refers to the radio frequency signal path as the panel antenna transmits. Radio frequency signals received by the panel antenna flow in the reverse direction.) Electrical signals on an input trace  40  are coupled to the wiper PCB  24 . The wiper PCB  24  couples the electrical signals to the transmission line traces  26 ,  28 . Transmission line traces  26 ,  28  may be coupled to output pads  44  to which a coaxial cable may be connected. Alternatively, the stationary PCB  22  may be coupled to stripline transmission lines on a panel without additional coaxial cabling. As the wiper PCB  24  moves, an electrical length from the wiper PCB  24  to each radiating element served by the transmission lines  26 ,  28  changes. For example, as the wiper PCB  24  moves to shorten the electrical length from the input transmission line trace  40  to a first radiating element, the electrical length from the input transmission line trace end to a second radiating element increases by a corresponding amount. 
     In one example illustrated in  FIG. 2  two phase shifters  20  and  20   a  are illustrated, with phase shifter  20  stacked on top of phase shifter  20   a  as illustrated. Transmission lines  16   a  and input  11   a  are connected to phase shifter  20   a . Phase shifter  20  and transmission lines  16  comprise the first feed network  14  driving the first set of dipoles on radiating elements  12  ( FIG. 1 ), and phase shifter  20   a  and transmission lines  16   a  comprise the second feed network  14  driving the second set of dipoles on radiating elements  12  ( FIG. 1 ). The wiper PCBs  24  are coupled by slider  30  such that the wiper PCBs  24  move in unison. In this example, only one wiper PCB  24  requires an accelerometer. A throw rod (not shown) may be coupled to a pin on slider  30  by way of a slotted component (not shown). The throw rod may be actuated by hand or by an electrical-mechanical actuator. 
     In one example, at least one wiper PCB  24  includes an accelerometer  50 . Preferably, the accelerometer  50  comprises a multiple-axis digital accelerometer, such as Digital Accelerometer ADXL345, from Analog Devices, Inc. In this example, the accelerometer  50  is a digital 3-axis accelerometer. However, other accelerometers may be acceptable in alternate embodiments. The accelerometer  50  provides acceleration information for the three axes of rotation as serial data. In one example, the serial data conforms to the standard Inter-Integrated Circuit, or I 2 C, digital interface. X-axis data, Y-axis data, and Z-axis data may be obtained by reading appropriate registers in the accelerometer  50 . 
     As seen in  FIG. 3 , a microcontroller  52  may interface with the accelerometer  50  and read the data registers. In one example, a microcontroller  52  is included in an actuator  60 , which is mechanically coupled to the phase shifter  20 . The actuator  60  includes a motor  54  and an AISG connector  56 . The microcontroller  52 , through operation of the motor  54 , controls the location of wiper PCB  24 . The microcontroller  50  receives and transmits control information through AISG connectors  56 . In an alternate embodiment, the microcontroller  52  may be located on the wiper PCB  24 . 
     The accelerometer  50  is mounted on the wiper PCB  24  as shown in  FIG. 2  such that it may detect an actual angle of the wiper PCB  24  with respect to vertical. Wiper PCB  24  physical angle θ may be determined by a first axis of the accelerometer  50 . If wiper PCB  24  angle with respect to vertical is the only angle to be determined, the solution may be had with a single axis of the accelerometer  50  and the following trigonometry relationship:
 
 V   OUTX   =V   OFF   +S ×sin θ
 
Where V OUTX  is the voltage output from the X-axis of the accelerometer, V OFF  is the offset voltage and S is the sensitivity of the accelerometer. The acceleration on the X-axis due to gravity is:
 
 A   X =( V   OUTX   −V   OFF )÷ S  
 
In this case, the solution for wiper arm angle is:
 
θ=sin −1 (A X )
 
     In another example, the phase shifter is mounted such that the axis of rotation of the default angle of the panel antenna is on a different axis (e.g., the Y-axis) from an axis of rotation of the wiper PCB  24 . A default angle of the antenna panel may be determined by a second axis of the accelerometer  50  in the same manner as above. 
     In another example, three axes of the accelerometer  50  may be employed to determine the wiper PCB  24  angle and default angle. This embodiment is especially applicable when a portion of the panel antenna (e.g., the reflectors and radiating elements) is rotatable to adjust beam azimuth angle. This solution takes into account the delta angle of the azimuth from boresight to determine true mechanical tilt and wiper arm angle. 
     As seen in  FIG. 4 , once the physical angles are determined, a beam downtilt angle may be determined as in  70 . The actual physical angle of the wiper PCB  24  may be correlated to, but is not the same as, the downtilt angle of the radiated beam of the panel antenna. A correlation of a physical angle of a wiper arm to a downtilt angle of a radiated antenna beam may be determined empirically and stored in a look-up table in non-volatile memory  58  ( FIG. 3 ). When microcontroller  52  receives an instruction to set the antenna to a specified beam downtilt angle, the microcontroller would access a look up table to retrieve the corresponding wiper PCB  24  physical angle as in  72 . 
     When the microcontroller  52  receives an instruction to adjust downtilt, the microcontroller  52  may actuate the motor while monitoring phase shifter wiper PCB  24  position. During movement of the wiper PCB  24 , the registers of the accelerometer  50  may be read a number of times as in  74  to determine a position of the wiper PCB  24 . The microcontroller  52  can determine if the actual physical angle matches the desired physical angle as in  76  and if not (i.e., “No”), actuate the motor as in  78 . The microcontroller  52  may be configured to stop movement of the actuator  60  when the wiper PCB  24  reaches a desired physical angle as in  80  (i.e., “Done”). The registers may also be read while the phase shifters are stationary to confirm phase angle, to determine default mechanical angle, or act as a level and installation of the panel antenna. 
     From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope of the invention. It is to be understood that no limitation with respect to the specific system or method illustrated herein is intended or should be intended. It is, of course, intended to cover by the appended claims all such modifications as fall within the spirit and scope of the claims.