Patent Publication Number: US-2022231413-A1

Title: Remote electronic tilt base station antennas and mechanical calibration for such antennas

Description:
FIELD OF THE INVENTION 
     The present invention relates to communication systems and, in particular, to base station antennas having remote electronic tilt capabilities. 
     BACKGROUND 
     Cellular communications systems are used to provide wireless communications to fixed and mobile subscribers. A cellular communications system may include a plurality of base stations that each provide wireless cellular service for a specified coverage area that is typically referred to as a “cell.” Each base station may include one or more base station antennas that are used to transmit radio frequency (“RF”) signals to, and receive RF signals from, the subscribers that are within the cell served by the base station. Base station antennas are directional devices that can concentrate the RF energy that is transmitted in or received from certain directions. The “gain” of a base station antenna in a given direction is a measure of the ability of the antenna to concentrate the RF energy in that direction. The “radiation pattern” of a base station antenna—which is also referred to as an “antenna beam”—is a compilation of the gain of the antenna across all different directions. Each antenna beam may be designed to service a pre-defined coverage area such as the cell or a portion thereof that is referred to as a “sector.” Each antenna beam may be designed to have minimum gain levels throughout the pre-defined coverage area, and to have much lower gain levels outside of the coverage area to reduce interference between neighboring cells/sectors. Base station antennas typically comprise a linear array of radiating elements such as patch, dipole or crossed dipole radiating elements. Many base station antennas now include multiple linear arrays of radiating elements, each of which generates its own antenna beam. 
     Early base station antennas generated antenna beams having fixed shapes, meaning that once a base station antenna was installed, its antenna beam(s) could not be changed unless a technician physically reconfigured the antenna. Many modern base station antennas now have antenna beams that can be electronically reconfigured from a remote location. The most common way in which an antenna beam may be reconfigured electronically is to change the pointing direction of the antenna beam (i.e., the direction in which the antenna beam has the highest gain), which is referred to as electronically “steering” the antenna beam. An antenna beam may be steered horizontally in the azimuth plane and/or vertically in the elevation plane. An antenna beam can be electronically steered by transmitting control signals to the antenna that cause the antenna to alter the phases of the sub-components of the RF signals that are transmitted and received by the individual radiating elements of the linear array that generates the antenna beam. Most modern base station antennas are configured so that the elevation or “tilt” angle of the antenna beams generated by the antenna can be electronically altered. Such antennas are commonly referred to as remote electronic tilt (“RET”) antennas. 
     In order to electronically change the down tilt angle of an antenna beam generated by a linear array of radiating elements, a phase taper may be applied across the radiating elements of the array. Such a phase taper may be applied by adjusting the settings on a phase shifter that is positioned along the RF transmission path between a radio and the individual radiating elements of the linear array. One widely-used type of phase shifter is an electromechanical “wiper” phase shifter that includes a main printed circuit board and a “wiper” printed circuit board that may be rotated above the main printed circuit board. Such wiper phase shifters typically divide an input RF signal that is received at the main printed circuit board into a plurality of sub-components, and then couple at least some of these sub-components to the wiper printed circuit board. The sub-components of the RF signal may be coupled from the wiper printed circuit board back to the main printed circuit board along a plurality of concentric arc-shaped traces, where each arc has a different diameter. Each end of each arc-shaped trace may be connected to a respective sub-group of radiating elements that includes at least one radiating element. By physically (mechanically) rotating the wiper printed circuit board above the main printed circuit board, the locations where the sub-components of the RF signal couple back to the main printed circuit board may be changed, which thus changes the lengths of the transmission paths from the phase shifter to the respective sub-groups of radiating elements. The changes in these path lengths result in changes in the phases of the respective sub-components of the RF signal, and since the arcs have different radii, the phase changes along the different paths will be different. Typically, the phase taper is applied by applying positive phase shifts of various magnitudes (e.g., +X°, +2X° and +3X°) to some of the sub-components of the RF signal and by applying negative phase shifts of the same magnitudes (e.g., −X°, −2X° and −3X°) to additional of the sub-components of the RF signal. Exemplary phase shifters of this variety are discussed in U.S. Pat. No. 7,907,096 to Timofeev, the disclosure of which is hereby incorporated herein in its entirety. The wiper printed circuit board is typically moved using an electromechanical actuator such as a DC motor that is connected to the wiper printed circuit board via a mechanical linkage. These actuators are often referred to as “RET actuators.” Both individual RET actuators that drive a single mechanical linkage and “multi-RET actuators” that have a plurality of output members that drive a plurality or respective mechanical linkages are commonly used in base station antennas. 
     SUMMARY 
     Pursuant to embodiments of the present invention, a RET adjuster comprises a drive assembly including a rotatable drive member operatively connected to a phase shifter assembly such that rotation of the drive member adjusts the phase shifter assembly. A first connector is coupled to the drive member where the first connector occupies a first rest positon when the drive member is at rest. A drive system comprises a second connector occupying a second rest positon when the drive system is at rest. The second connector is releasably engageable with the first connector such that actuation of the drive system selectively rotates the drive member. A mechanical calibration system locates the second connector at the second rest position. 
     In some embodiments, the drive system may comprise a drive shaft connected to the second connector and a motor for rotating the drive shaft. The calibration system may comprise a stop member rotatable with the second connector. The calibration system may comprise a hard stop that is engageable by the stop member. When the stop member is engaged with the hard stop, the position of the second connector may be a known angular position relative to the second rest position. A base station control system may comprise a processor and a memory for storing the known angular position. The drive member may comprise a lead screw and a drive nut threadably engaged with the lead screw where the drive nut is operatively connected to the phase shifter assembly. A plurality of drive assemblies may be provided that are operatively connected to a plurality of phase shifter assemblies such that each drive assembly of the plurality of drive assemblies adjusts at least one phase shifter assembly of the plurality of phase shifter asemblies. The second connector may be engageable with the first connector by a linear movement of the drive system relative to the drive member. The first connector may comprise a coupling member mounted on the end of the drive member for reciprocating motion relative thereto where the coupling member rotates with the drive member. A spring may exert a force on the coupling member that tends to move the coupling member toward the second connector. The coupling member may comprise a plurality of first engagement structures arranged in spaced relationship about an axis of rotation of the coupling member. The plurality of first engagement structures may be equally spaced about the rotational axis of the coupling member. The coupling member may comprise a locking member that engages a stationary locking member to fix the drive member in position. The spring may move the locking member into engagement with the stationary locking member. The engagement of the first connector with the second connector may disengage the locking member from the stationary locking member. The first connector may comprise a plurality of first engagement structures arranged about an axis of rotation of the first connector. The plurality of first engagement structures may be equally spaced about the axis of rotation of the first connector. The second connector may comprise a plurality of second engagement structures that matingly engage the plurality of first engagement structures on the first connector. The plurality of second engagement structures may be arranged about an axis of rotation of the second connector. The plurality of second engagement structures may be equally spaced about the axis of rotation of the second connector. The drive system may be supported for reciprocating movement transverse to the plurality of drive assemblies such that the drive system may be aligned with any one of the plurality of drive assemblies. The drive system may comprise a drive shaft connected to the second connector and a motor for rotating the drive shaft wherein the drive shaft is supported in a bearing block. The drive shaft may have a first bevel gear on one end thereof remote from the second connector where the first bevel gear may engage a second bevel gear on an output shaft of the motor. The drive shaft may have a first section and a second section and the first section may be arranged for movement relative to the second section. The second bevel gear may be mounted for reciprocating movement on the output shaft such that the bevel gear may reciprocate along the the output shaft. A gear may rotate with the drive shaft where the gear selectively engages a fixed rack such that the actuation of the motor reciprocates the drive shaft transversely relative to the plurality of drive assemblies. A mode selection system may move the drive system between a drive mode, an index mode and a calibration mode. A mode selection system may move the drive system such that the first connector engages the second connector. A mode selection system may move the gear into engagement with the rack. A mode selection system may move the drive system between a first position where the stop member is positioned to engage the hard stop and a second position where the stop member is not positioned to engage the hard stop. The first connector may engage the second connector when the drive system is in the second position. A plurality of drive assemblies may be operatively connected to a plurality of phase shifter assemblies such that each drive assembly adjusts at least one phase shifter assembly of the plurality of phase shifter assemblies such that the mode selection system moves the drive system between a first position, a second position and a third position. In the first position, the stop member may engage the hard stop, in the second position the first connector may engage the second connector and in the third position the drive shaft may be supported for reciprocating movement transverse to the plurality of drive assemblies such that the drive system may be aligned with any one of the plurality of drive assemblies. The mode selection system may comprise a linearly reciprocating sled that supports the drive system. Movement of the sled may reciprocate the drive system between the first position, the second position and the third position. A rack may be fixed to the sled that is engaged by a pinion driven by a mode selection motor. 
     Pursuant to embodiments of the present invention, a multi-RET adjuster comprises a plurality of rotatable drive members where each of the plurality of rotatable drive members is operatively connected to an associated phase shifter assembly such that rotation of the drive member adjusts the associated phase shifter assembly. A first connector is coupled to each of the plurality of drive members where the first connector occupies a first rest positon when the drive member is at rest. A drive system comprises a second connector occupying a second rest positon when the drive system is at rest where the second connector is releasably engageable with the first connector such that actuation of the drive system selectively moves the drive member. A mechanical calibration system locates the second connector at the second rest position. 
     The drive system may comprise a rotatable drive shaft where the second connector is mounted on the drive shaft. A stop member may be mounted for rotation with the drive shaft. A stationary stop may be positioned such that the stop member contacts the stationary stop when the drive system is in a first position. A plurality of stationary stops may be positioned such that the stop member contacts one of the stationary stops when the drive system is in a first position. The drive shaft may be in the second rest position when the stop member contacts the stationary stop. A mode selection system may move the drive system between a first position, a second position and a third position. In the first position the stop member may engage the hard stop, in the second position the first connector may engage the second connector and in the third position the drive shaft may be supported for reciprocating movement transverse to the plurality of drive members such that the drive system may be aligned with any one of the plurality of drive members. The second rest position may be a known angular distance from the position of the drive system when the stop member contacts the stationary stop. 
     Pursuant to embodiments of the present invention, a method of calibrating a RET adjuster comprising: a movable drive member operatively connected to a phase shifter assembly such that movement of the drive member adjusts the phase shifter assembly; a first connector coupled to the drive member, the first connector occupying a first rest positon when the drive member is at rest; a drive system comprising a movable drive shaft and a stop member mounted for movement with the drive shaft, the drive shaft supporting a second connector, the second connector occupying a second rest positon when the drive system is at rest, and the second connector being releasably engageable with the first connector such that actuation of the drive system moves the drive member. The method comprises actuating the drive system to rotate the stop member into engagement with a stationary stop, the stationary stop positioned such that when the stop member contacts the stationary stop the second connector is in a known position; and using the known position to locate second connector at the second rest position. 
     The known position may be the second rest position. The known position may be a known angular distance from the second rest position. Actuating the drive system may rotate the drive shaft and the stop member. The second rest position may be stored in a memory of a base station control system. 
     Pursuant to embodiments of the present invention, a method of operating a multi-RET adjuster comprising a plurality of rotatable drive members wherein each of the plurality of rotatable drive members is operatively connected to an associated phase shifter assembly such that rotation of the drive member adjusts the associated phase shifter assembly; a first connector coupled to each of the plurality of drive members, the first connector occupying a first rest positon when the drive member is at rest; a drive system comprising a second connector occupying a second rest positon when the drive system is at rest, the second connector being releasably engageable with the first connector such that actuation of the drive system moves the drive member. The method comprises positioning the drive system adjacent to one of the plurality of rotatable drive members, the one of the plurality of rotatable drive members being operatively coupled to a phase shifter assembly to be adjusted; moving the drive system such that the second connector engages the first connector of the one of the plurality of rotatable drive members; and actuating the drive system to adjust the phase shifter assembly. 
     The step of positioning the drive system adjacent to one of the plurality of rotatable drive members may comprise actuating a first motor to move the drive assembly to an index mode position, and actuating a second motor to index the drive system transversely relative to the plurality of rotatable drive members. A rotating drive shaft may be connected to the second connector and a gear may be mounted on the drive shaft for rotation therewith. The gear may engage a rack when the drive system is in the index mode position. The step of moving the drive system may comprise actuating a first motor to move the drive assembly to a drive position. The step of actuating the drive system may comprise actuating a second motor to rotate a drive shaft that is connected to the second connector. The step of actuating the drive system may comprise rotating a lead screw to move a drive nut along the lead screw where the drive nut is operatively connected to the phase shifter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a perspective view of an example base station antenna according to embodiments of the present invention. 
         FIG. 1B  is a perspective view of the base station antenna of  FIG. 1A  with the radome thereof removed. 
         FIG. 2  is a schematic block diagram illustrating the electrical connections between various of the components of the base station antenna of  FIGS. 1A-1B . 
         FIG. 3  is a front perspective view of a pair of electromechanical phase shifters that may be included in the base station antenna of  FIGS. 1A-1B . 
         FIG. 4  is perspective view of an embodiment of the multi-RET actuator of the invention that may be included in the base station antenna of  FIGS. 1A-1B . 
         FIG. 5  is a rear view of a portion of the base station antenna of  FIGS. 1A-1B  that shows how mechanical linkages are used to connect the output members of the multi-RET actuator of  FIG. 4  to respective ones of the phase shifters illustrated in  FIGS. 2 and 3 . 
         FIG. 6  is a plan view of the multi-RET actuator of  FIG. 4  illustrating the drive mode. 
         FIG. 7  is a section view taken along line  7 - 7  of  FIG. 6 . 
         FIG. 8  is a plan view of the multi-RET actuator of  FIG. 4  illustrating the free mode. 
         FIG. 9  is a plan view of the multi-RET actuator of  FIG. 4  illustrating the index mode. 
         FIG. 10  is a plan view of the multi-RET actuator of  FIG. 4  illustrating the calibration mode. 
         FIG. 11  is a detailed section view taken along line  11 - 11  of  FIG. 10 . 
         FIG. 12  is a section view similar to  FIG. 7  illustrating the drive mode. 
         FIG. 13  is a section view similar to  FIG. 7  illustrating the free mode. 
         FIG. 14  is a section view similar to  FIG. 7  illustrating the calibration mode. 
         FIG. 15  is a section view similar to  FIG. 7  illustrating the index mode. 
         FIG. 16  is a block diagram illustrating a method of operating the system of the invention. 
         FIG. 17  is a block diagram illustrating a method of operating the calibration system of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Modern base station antennas often include two, three or more linear arrays of radiating elements, where each linear array has an electronically adjustable down tilt. The linear arrays typically include cross-polarized radiating elements, and a separate phase shifter is provided for electronically adjusting the down tilt of the antenna beam for each polarization, so that the antenna may include, for example, twice as many phase shifters as linear arrays. Moreover, in many antennas, separate transmit and receive phase shifters are provided so that the transmit and receive radiation patterns may be independently adjusted. This again doubles the number of phase shifters. Thus, it is not uncommon for a base station antenna to have eight, twelve, sixteen or more phase shifters for applying remote electronic down tilts to the linear arrays. As described above, RET actuators are provided in the antenna that are used to adjust the phase shifters. While the same downtilt is typically applied to the phase shifters for the two different polarizations, allowing a single RET actuator and a single mechanical linkage to be used to adjust the phase shifters for both polarizations, modern base station antennas still often include four, six, eight or more RET actuators (or, alternatively, one or two multi-RET actuators) and associated mechanical linkages. 
     In order to change the downtilt angle of an antenna beam generated by a linear array on a base station antenna, a control signal may be transmitted from a base station control system to the antenna that causes a RET actuator associated with the linear array to generate a desired amount of movement in an output member thereof. The movement may comprise, for example, linear movement or rotational movement. A mechanical linkage is used to translate the movement of the output member of the RET actuator to movement of a moveable element of a phase shifter (e.g., a wiper arm) associated with the linear array. Accordingly, each mechanical linkage may extend between the output member of the RET actuator and the moveable element of the phase shifter. 
     Because the adjustment of the phase shifter requires precise movement of the wiper arm of the phase shifter, the accuracy of the RET actuator must be controlled in order to ensure that the downtilt angle of the antenna beam is correct. The repeated actuation of the RET actuator can result in inaccuracies being introduced into the system. Embodiments of the present invention provide a RET actuator and a mechanical calibration system for a RET actuator that provides for accurate adjustment of the phase shifter. Embodiments of the present invention also provide a low profile, scalable and mechanically reliable RET actuator. 
     Embodiments of the present invention will now be discussed in greater detail with reference to the drawings. In some cases, two-part reference numerals are used in the drawings. Herein, elements having such two-part reference numerals may be referred to individually by their full reference numeral (e.g., linear array  120 - 2 ) and may be referred to collectively by the first part of their reference numerals (e.g., the linear arrays  120 ). 
       FIG. 1A  is a perspective view of a RET base station antenna  100  according to embodiments of the present invention.  FIG. 1B  is a perspective view of the base station antenna  100  with the radome removed to show the four linear arrays of radiating elements that are included in antenna  100 . 
     As shown in  FIG. 1A , the RET antenna  100  includes a radome  102 , a mounting bracket  104 , a bottom end cap  106  and a top end cap  120 . A plurality of input/output ports  110  are mounted in the end cap  106 . Coaxial cables (not shown) may be connected between the input/output ports  110  and the RF ports on one or more radios (not shown). These coaxial cables may carry RF signals between the radios and the base station antenna  100 . The input/output ports  110  may also include control ports that carry control signals to and from the base station antenna  100  from a base station control system  250  ( FIG. 5 ) that may be located remotely from base station antenna  100 . These control signals may include control signals for electronically changing the tilt angle of the antenna beams generated by the base station antenna  100 . 
     For ease of reference,  FIG. 1A  includes a coordinate system that defines the length (L), width (T) and depth (V) axes (or directions) of the base station antenna  100  that will be discussed throughout the application. The length axis may also be referred to as the longitudinal axis. 
       FIG. 1B  is a perspective view of the base station antenna of  FIG. 1A  with the radome  102  removed. As shown in  FIG. 1B , the base station antenna  100  includes two linear arrays  120 - 1 ,  120 - 2  of low-band radiating elements  122  (i.e., radiating elements that transmit and receive signals in a lower frequency band) and two linear arrays  130 - 1 ,  130 - 2  of high-band radiating elements  132  (i.e., radiating elements that transmit and receive signals in a higher frequency band). Each of the low-band radiating elements  122  is implemented as a cross-polarized radiating element that includes a first dipole that is oriented at an angle of −45° with respect to the azimuth plane (a horizontal plane) and a second dipole that is oriented at an angle of +45° with respect to the azimuth plane. Similarly, each of the high-band radiating elements  132  is implemented as a cross-polarized radiating element that includes a first dipole that is oriented at an angle of −45° with respect to the azimuth plane and a second dipole that is oriented at an angle of +45° with respect to the azimuth plane. Since cross-polarized radiating elements are provided, each linear array  120 - 1 ,  120 - 2 ,  130 - 1 ,  130 - 2  will generate two antenna beams, namely a first antenna beam generated by the −45° dipoles and a second antenna beam generated by the +45° dipoles. The radiating elements  122 ,  132  extend forwardly from a backplane  112  which may comprise, for example, a sheet of metal that serves as a ground plane for the radiating elements  122 ,  132 . 
       FIG. 2  is a schematic block diagram illustrating various additional components of the RET antenna  100  and the electrical connections therebetween. It should be noted that  FIG. 2  does not show the actual location of the various elements on the antenna  100 , but instead is drawn to merely show the electrical transmission paths between the various elements. 
     As shown in  FIG. 2 , each input/output port  110  may be connected to a phase shifter  150 . The base station antenna  100  performs duplexing between the transmit and receive sub-bands for each linear array  120 ,  130  within the antenna (which allows different downtilts to be applied to the transmit and receive sub-bands), and hence each linear array  120 ,  130  includes both a transmit (input) port  110  and a receive (output) port  110 . A first end of each transmit port  110  may be connected to the transmit port of a radio (not shown) such as a remote radio head. The other end of each transmit port  110  is coupled to a transmit phase shifter  150 . Likewise, a first end of each receive port  110  may be connected to the receive port of a radio (not shown), and the other end of each receive port  110  is coupled to a receive phase shifter  150 . Two transmit ports, two receive ports, two transmit phase shifters and to receive phase shifters are provided for each linear array  120 ,  130  to handle the two different polarizations. 
     Each transmit phase shifter  150  divides an RF signal input thereto into five sub-components, and applies a phase taper to these sub-components that sets the tilt (elevation) angle of the antenna beam generated by an associated linear array  120 ,  130  of radiating elements  122 ,  132 . The five outputs of each transmit phase shifter  150  are coupled to five respective duplexers  140  that pass the sub-components of the RF signal output by the transmit phase shifter  150  to five respective sub-arrays of radiating elements  122 ,  132 . In the example antenna  100  shown in  FIGS. 1A, 1B and 2 , each low-band linear array  120  includes ten low-band radiating elements  122  that are grouped as five sub-arrays of two radiating elements  122  each. Each high-band linear array  130  includes fifteen high-band radiating elements  132  that are grouped as five sub-arrays of three radiating elements  132  each. 
     Each sub-array of radiating elements passes received RF signals to a respective one of the duplexers  140 , which in turn route those received RF signals to the respective inputs of an associated receive phase shifter  150 . The receive phase shifter  150  applies a phase taper to each received RF signal input thereto that sets the tilt angle for the receive antenna beam and then combines the received RF signals into a composite RF signal. The output of each receive phase shifter  150  is coupled to a respective receive port  110 . 
     While  FIGS. 1B and 2  show an antenna having two linear arrays  120  of ten low-band radiating elements  122  each and two linear arrays  130  of fifteen high-band radiating elements  132  each, it will be appreciated that the number of linear arrays  120 ,  130  and the number of radiating elements  122 ,  132  included in each of the linear array  120 ,  130  may be varied. It will also be appreciated that duplexing may be done in the radios instead of in the antenna  100 , that the number(s) of radiating elements  122 ,  132  per sub-array may be varied, that different types of radiating elements may be used (including single polarization radiating elements) and that numerous other changes may be made to the base station antenna  100  without departing from the scope of the present invention. 
     As can be seen from  FIG. 2 , the base station antenna  100  may include a total of sixteen phase shifters  150 . While the two transmit phase shifters  150  for each linear array  120 ,  130  (i.e., one transmit phase shifter  150  for each polarization) may not need to be controlled independently (and the same is true with respect to the two receive phase shifters  150  for each linear array  120 ,  130 ), there still are eight sets of two phase shifters  150  that should be independently controllable. Accordingly, eight mechanical linkages may be required to connect the eight sets of phase shifters  150  to respective RET actuators. 
     Each phase shifter  150  shown in  FIG. 2  may be implemented, for example, as a rotating wiper phase shifter. The phase shifts imparted by a phase shifter  150  to each sub-component of an RF signal may be controlled by a mechanical positioning system that physically changes the position of the rotating wiper of each phase shifter  150 , as will be explained with reference to  FIG. 3 . It will be appreciated that other types of phase shifters may be used instead of rotating wiper phase shifters such as, for example, trombone phase shifters, sliding dielectric phase shifters and the like. For convenience, the movable element for the different types of phase shifters are referred to herein collectively as wiper printed circuit boards. 
     Referring to  FIG. 3 , a dual rotating wiper phase shifter assembly  200  is illustrated that may be used to implement, for example, two of the phase shifters  150  of  FIG. 2 . The dual rotating wiper phase shifter assembly  200  includes first and second phase shifters  202 ,  202   a.  In the description of  FIG. 3  that follows it is assumed that the two phase shifters  202 ,  202   a  are each transmit phase shifters that have one input and five outputs. It will be appreciated that if the phase shifters  202 ,  202   a  are instead used as receive phase shifters then the terminology changes, because when used as receive phase shifters there are five inputs and a single output. 
     As shown in  FIG. 3 , the dual phase shifter  200  includes first and second main (stationary) printed circuit boards  210 ,  210   a  that are arranged back-to-back as well as first and second rotatable wiper printed circuit boards  220 ,  220   a  (wiper printed circuit board  220   a  is barely visible in the view of  FIG. 3 ) that are rotatably mounted on the respective main printed circuit boards  210 ,  210   a.  The wiper printed circuit boards  220 ,  220   a  may be pivotally mounted on the respective main printed circuit boards  210 ,  210   a  via a pivot pin  222 . The wiper printed circuit boards  220 ,  220   a  may be joined together at their distal ends via a bracket  224 . 
     The position of each rotatable wiper printed circuit boards  220 ,  220   a  above its respective main printed circuit board  210 ,  210   a  is controlled by the position of a drive shaft  228  (partially shown in  FIG. 3 ), the end of which may constitute one end of a mechanical linkage. The other end of the mechanical linkage (not shown) may be coupled to an output member of the RET actuator. 
     Each main printed circuit board  210 ,  210   a  includes transmission line traces  212 ,  214 . The transmission line traces  212 ,  214  are generally arcuate. In some cases the arcuate transmission line traces  212 ,  214  may be disposed in a serpentine pattern to achieve a longer effective length. In the example illustrated in  FIG. 3 , there are two arcuate transmission line traces  212 ,  214  per main printed circuit board  210 ,  210   a  (the traces on printed circuit board  210   a  are not visible in  FIG. 3 ), with the first arcuate transmission line trace  212  being disposed along an outer circumference of each printed circuit board  210 ,  210   a,  and the second arcuate transmission line trace  214  being disposed on a shorter radius concentrically within the outer transmission line trace  212 . A third transmission line trace  216  on each main printed circuit board  210 ,  210   a  connects an input pad  230  on each main printed circuit board  210 ,  210   a  to an output pad  240  that is not subjected to an adjustable phase shift. 
     The main printed circuit board  210  includes one or more input traces  232  leading from the input pad  230  near an edge of the main printed circuit board  210  to the position where the pivot pin  222  is located. RF signals on the input trace  232  are coupled to a transmission line trace (not visible in  FIG. 3 ) on the wiper printed circuit board  220 , typically via a capacitive connection. The transmission line trace on the wiper printed circuit board  220  may split into two secondary transmission line traces (not shown). The RF signals are capacitively coupled from the secondary transmission line traces on the wiper printed circuit board  220  to the transmission line traces  212 ,  214  on the main printed circuit board. Each end of each transmission line trace  212 ,  214  may be coupled to a respective output pad  240 . A coaxial cable  260  or other RF transmission line component may be connected to input pad  230 . A respective coaxial cable  270  or other RF transmission line component may be connected to each respective output pad  240 . As the wiper printed circuit board  220  moves, an electrical path length from the input pad  230  of phase shifter  202  to each output pad  240  changes. For example, as the wiper printed circuit board  220  pivots to the left, as viewed in  FIG. 3 , it shortens the electrical length of the path from the input pad  230  to the output pad  240  connected to the left side of transmission line trace  212  (which connects to a first sub-array of radiating elements), while the electrical length from the input pad  230  to the output pad  240  connected to the right side of transmission line trace  212  (which connects to a second sub-array of radiating elements) increases by a corresponding amount. These changes in path lengths result in phase shifts to the signals received at the output pads  240  connected to transmission line trace  212  relative to, for example, the output pad  240  connected to transmission line trace  216 . 
     The second phase shifter  202   a  may be identical to the first phase shifter  202 . As shown in  FIG. 3 , the rotating wiper printed circuit board  220   a  of phase shifter  202   a  may be controlled by the same drive shaft  228  as the rotating wiper printed circuit board  220  of phase shifter  202 .  FIG. 5  is a rear view of a portion of the base station antenna  100  that shows how mechanical linkages  160  are used to connect the output members of the RET actuator  300  to moveable elements of respective pairs of phase shifters  150 . In  FIG. 5 , only a few of the elements have been given reference numerals to simplify the drawing (e.g., only one of the mechanical linkages and two of the phase shifters are given reference numerals). 
     As shown in  FIG. 5 , the multi-RET actuator  300  is mounted in the antenna  100  behind the backplane  112 . Eight pairs of phase shifters  150  are also mounted rearwardly of the backplane  112  (only four pairs of phase shifters are visible in  FIG. 5 ). Since the base station antenna  100  has linear arrays  120 ,  130  that are formed of dual-polarized radiating elements  122 ,  132 , the phase shifters  150  are mounted in pairs since the phase shifter  150  for each polarization will be adjusted the same amount. In  FIG. 5 , phase shifters  150 - 1  and  150 - 2  are used to adjust the phase tapers applied to the first and second polarization radiators of the radiating elements  122  of linear array  120 - 1 . It should be noted that the phase shifters a positioned in a side-by-side arrangement in  FIG. 5  as opposed to the front-to-back arrangement as shown in  FIG. 3 . 
     As is further shown in  FIG. 5 , a plurality of mechanical linkages  160  are provided that connect each output member of the multi-RET actuator  300  to a respective pair of phase shifters  150 . For example, mechanical linkage  160 - 1  is connected between one of the outputs of RET actuator  300  and a slider  154  of the phase shifter assembly that engages and rotationally moves the respective wiper arms  152  of phase shifters  150 - 1  and  150 - 2 . As shown in  FIG. 5 , the mechanical linkage  160 - 1  includes a first RET rod  162  that is attached to the output of multi-RET actuator  300 , a second RET rod  166 , a first RET linkage  164  that connects the first RET rod  162  to the second RET rod  166 , and the slider  154  that engages the wiper arms  152  of the phase shifters  150 - 1 ,  150 - 2 . The other mechanical linkages  160  shown in  FIG. 5  include similar combinations of RET rods  162 ,  166  and RET linkages  164 . The RET rods  162 ,  166  typically extend in a longitudinal direction of the antenna  100 , while the RET linkages  164  typically extend along the width and/or depth axes to connect RET rods  162 ,  166  together, and/or to connect a RET rod  162 ,  166  to an output member of the RET actuator or to a moveable element of a phase shifter assembly such as the slider  154  that engages the wiper arms  152 . Each mechanical linkage  160  is used to transfer a linear movement of the outputs of the RET actuator  300  to a phase shifter assembly. 
     Referring to  FIGS. 4 through 15 , the RET actuator  300  is used to drive the moveable element of the phase shifter  150  such as the wiper printed circuit board  220 ,  220   a  as described above. As shown in the figures, a housing  310  contains and supports the components of a multi-RET actuator where multiple outputs are provided that can drive multiple respective mechanical linkages  160 . The housing  310  is shown in  FIG. 4  as transparent and in  FIGS. 6, 8, 9 and 10  the housing  310  is open to better illustrate the internal structure of the multi-RET actuator; however, in actual construction the housing  310  may be an enclosed, opaque housing and may be made of a suitable rigid material, such as plastic, metal or combinations of materials. 
     A base station control system  250  ( FIG. 5 ) controls operation of the antenna  100  as is known in the art. The base station control system  250  also controls the multi-RET actuator  300  as will hereinafter be described. Communications cables  302  may be used to deliver control signals from the base station control system  250  to the multi-RET actuator  300  and from the multi-RET actuator  300  to the base station control system  250 . 
     In some embodiments, the base station control system  250  may comprise a processor  252  communicably coupled to such devices as a memory  254  and a user interface  256 . The processor  252  generally includes circuitry for implementing communication and/or logic functions of the antenna. The processor  252  may include functionality to operate one or more software programs, which may be stored in the memory  254 . The base station control system  250  may be located remotely from the antenna  100 , may be collocated with the antenna  100  or various functions of the base station control system  250  may be allocated between the antenna and a remote location. 
     As used herein, a “processor” refers to a device or combination of devices having circuitry used for implementing the communication and/or logic functions of the system. For example, the processor may include a digital signal processor device, a microprocessor device, and various analog-to-digital converters, digital-to-analog converters, and other support circuits and/or combinations of the foregoing. Control and signal processing functions of the system are allocated between these processing devices according to their respective capabilities. The processor may further include functionality to operate one or more software programs based on computer-executable program code thereof, which may be stored in memory  254 . As the phrase is used herein, a processor may be “configured to” perform a certain function in a variety of ways, including, for example, by having one or more general-purpose circuits perform the function by executing particular computer-executable program code embodied in computer-readable medium, and/or by having one or more application-specific circuits perform the function. 
     As used herein, a “memory” generally refers to a device or combination of devices that store one or more forms of computer-readable media for storing data and/or computer-executable program code/instructions. For example, in one embodiment, the memory  254  as described herein includes any computer memory that provides an actual or virtual space to temporarily or permanently store data and/or commands provided to the processor  252  when the processor carries out its functions described herein. As used herein, “memory” includes any computer readable medium configured to store data, code, or other information. The memory may include volatile memory, such as volatile Random Access Memory (RAM) including a cache area for the temporary storage of data. The memory may also include non-volatile memory, which can be embedded and/or may be removable. The non-volatile memory can additionally or alternatively include an electrically erasable programmable read-only memory (EEPROM), flash memory or the like. 
     The user interface  256  may be made up of user output devices and/or user input devices, which are operatively coupled to the processor  210 . The user output devices may include a visual display, audio device and/or the like. The user input devices may include any of a number of devices allowing the base station control system  250  to receive data, such as a keypad, keyboard, touch-screen, touchpad, microphone, mouse, joystick, other pointer device, button, soft key, and/or other input device(s). 
     Referring again to  FIGS. 4 through 15 , a multi-RET actuator  300  having four drive assemblies  306   a - 306   d  is shown. The four drive assemblies  306   a - 306   d  may move four or more mechanical linkages where each drive assembly  306   a - 306   d  is operatively connected to at least one phase shifter assembly by a mechanical linkage. While four drive assemblies  306   a - 306   d  are shown, the multi-RET actuator  300  is scalable such that the multi-RET actuator  300  may include a greater or fewer number of drive assemblies  306  to drive a greater or fewer number of mechanical linkages. Drive assemblies  306   a - 306   d  each comprise a drive mechanism for converting a rotational input into a linear output. In one embodiment, the drive assemblies  306   a - 306   d  comprise a rotary drive member that is operatively connected to a linear output. The linear output is operatively coupled to the wiper printed circuit board  220 ,  220   a  of the phase shifter assembly  200  by the mechanical linkage such that movement of the linear output adjusts the phase shifter assembly. The drive assemblies  306   a - 306   d  are identical such that a single drive assembly  306   a  will be described in detail. In one embodiment, the drive member comprises a lead screw  312  that is rotatably supported in the housing  310  such that lead screw  312  is rotatable along its longitudinal axis. The drive member may also comprise a belt drive, chain drive, ball drive, gear train, linkage, or the like, or combinations of such devices. 
     The distal end of lead screw  312  may be supported in a suitable bearing  313  in a first wall  315  of the housing  310 . The proximal end of lead screw  312  includes a screw connector  314  that is mounted to the lead screw  312  for rotation therewith. Screw connector  314  comprises a coupling member  324  that includes a nub  316  that is rotationally supported in and extends through an aperture  318  in wall  319  of housing  310 . The lead screws  312  of the drive assemblies  306   a - 306   d  are disposed parallel to one another. 
     A linear output is provided for transmitting the rotation of the drive members to the mechanical linkage. In one embodiment, the linear output comprises a drive nut  320  that threadably engages the lead screw  312  such that rotation of the lead screw  312  causes the drive nut  320  to reciprocate along the length of the lead screw  312 . The drive nut  320  includes a clip structure  321  for connecting the drive nut to the mechanical linkage. The clip structure  321  may comprise any suitable connection mechanism. A stationary rod  322  is supported in the housing  310  that extends parallel to the lead screw  312 . The rod  322  may extend through a bearing sleeve  323  in the drive nut  320  to prevent rotation of the drive nut  320  and to guide the drive nut  320  in a linear path of travel along the length of the lead screw  312 . The direction of rotation of the lead screw  312  may be reversed to change the direction of travel of the drive nut  320  along the lead screw. The mechanical linkage that connects to the wiper printed circuit board  220 ,  220   a  of the phase shifter assembly  200  is connected to the drive nut  320  such that the reciprocating movement of the drive nut  320  causes the adjustment of the phase shifter assembly as previously described. 
     The screw connector  314  releasably connects the lead screw  312  to the drive connector  400  of the drive system  401 . The screw connector  314  is configured such that the drive connector  400  may be selectively connected to, and released from, the screw connector  314  by a linear movement of the drive connector  400  relative to the screw connector  314  along the rotational axis of the lead screw  312 . The screw connector  314  also functions to lock the drive assemblies  306   a - 306   d  in position during use of the multi-RET actuator, as will be described. 
     The coupling member  324  of screw connector  314  is mounted on the end of the lead screw  312  for reciprocating motion relative thereto along the longitudinal axis of the lead screw  312 . In the illustrated embodiment, the coupling member  324  has a cup-shaped configuration where the interior recess  326  fits over the end of the lead screw  312  such that the coupling member  324  can reciprocate relative to the lead screw  312  but is prevented from rotating relative to the lead screw. In the illustrated embodiment, the proximal end of the lead screw  312  and the recess  326  of the coupling member  324  are provided with a series of flat faces, such as a hex-style connector, that allows reciprocating motion between the coupling member  324  and the lead screw  312  but that prevents relative rotational movement therebetween. While mating flat faces are used in the illustrated embodiment to connect the coupling member  324  to the end of the lead screw  312 , other mechanisms may be used such as a keyed connection, pin and slot arrangement or the like. A spring  328  exerts a force on the coupling member  324  that tends to move the coupling member  324  away from the lead screw  312  toward the drive connector  400 . The spring  328  may comprise a compression spring that is located in a longitudinally extending recess  330  formed in the end of the lead screw  312 . 
     The nub  316  of the coupling member  324  faces the drive connector  400 . The nub  332  includes a series of engagement structures  334  that are arranged in spaced relationship about the rotational axis of the nub  316 . The engagement structures  334  may comprise a series of external teeth, flat surfaces, splines, a star receptacle or the like that matingly and releasably engage with corresponding engagement structures formed on the drive connector  400  and that transfer rotary movement of the drive connector  400  to the lead screw  312 . While specific engagement structures comprising a plurality of teeth are shown and described herein, any suitable engagement structures that provide a releasable rotating connector to the drive connector  400  may be used provided that the engagement structures allow for angular positioning of the lead screw  312  relative to the drive system  401 , as will hereinafter be described. In this regard, the engagement structures  334  comprise a plurality of discrete elements arranged in known angular positions around the rotational axis of the lead screw. For example, a series of equally spaced teeth may be used as shown in the figures. 
     The coupling member  324  also includes an outwardly projecting flange or lip  344 . The flange  344  includes a series of spaced locking members  346  ( FIG. 6 ) arranged in an annular configuration about the longitudinal axis of the coupling member  324  that face the wall  319 . The wall  319  includes a series of stationary locking members  350  that extend from the wall  319  and face the locking members  346  on the flange  344 . The locking members  350  and the locking members  346  interengage with one another to fix the lead screw  312  in position when the lead screw is not being adjusted and is in the rest position. The locking members  346  and  350  may be formed as a series of projecting teeth and, in one embodiment, are formed as a series of generally triangular shaped teeth that allow the locking members to engage with one another even if slightly out of alignment. While the locking members  346  and  350  may be formed as a series of teeth, the locking members may be configured other than as teeth, such as roughened friction surfaces, ridges, pins and apertures, or the like, provided the interengagement of the locking members  346  and  350  locks the lead screw  312  in position. 
     Absent a counteracting force, the spring  328  extends the coupling member  324  away from the lead screw  312  such that the locking members  346  on the flange  344  engage the locking members  350  on the wall  319 . The engagement of locking members  346  with the locking members  350  prevents the coupling member  324  from rotating. The keyed engagement of the coupling member  324  with the lead screw  312  prevents the lead screw  312  from rotating relative to the coupling member  324 . Thus, the engagement of the locking members  346  and the locking members  350  prevents the lead screw  312  from rotating to thereby lock the drive nut  320  in positon and fix the position of the mechanical linkage and the corresponding wiper printed circuit board  220 ,  220   a  of the phase shifter assembly  220 . 
     The drive/index mechanism will now be described. The drive/index mechanism comprises a drive system  401  that rotates the lead screws  312  to adjust the position of the associated phase shifter assemblies  220  and an index system  422  that changes the position of the drive system relative to the lead screws  312  of the drive assemblies  306   a - 306   d.  The drive system  401  comprises a drive shaft  406  that is rotated along its longitudinal axis. The drive connector  400  is supported at one end of the drive shaft  406  such that the drive connector  400  rotates with the drive shaft  406 . The screw connector  314  and drive connector  400  are configured such that when they are engaged with one another in in the drive mode as shown in  FIGS. 6, 7 and 12 , the rotation of the drive shaft  406  is transferred to the lead screw  312 . 
     The drive connector  400  comprises a generally socket shaped member  402  that receives the nub  316  of the screw connector  314 . The socket  402  includes a plurality of engagement structures  408  that matingly engage with the corresponding engagement structures  334  formed on the screw connector  314  and that transfer rotary movement of the drive shaft  406  to the drive screw  312 . The engagement structures  408  may be formed as a series of internal flats, splines, teeth, star connector or the like. While a specific connector is shown in the drawings comprising a series of angularly spaced teeth, any suitable connector that provides a releasable rotating connector and that allows the connectors  314  and  400  to be connected using a relative linear movement between the connectors may be used provided that the engagement structures allow for angular positioning of the drive connector  400  relative to the screw connector  314  as will hereinafter be described. In this regard, an engagement structure that comprises a plurality of discrete elements arranged in known angular positions around the rotational axis of the drive shaft  406  may be used. For example, a series of equally spaced teeth may be used as shown in the figures. 
     The drive shaft  406  comprises telescoping sections  406   a,    406   b  that slide relative to one another along the longitudinal axis of the drive shaft  406  ( FIGS. 12-15 ). While the sections  406   a  and  406   b  can telescope relative to one another, they are constrained to rotate together. For example, the interior bore of section  406   a  has flat surfaces that engage flat surfaces on the exterior of section  406   b  such that the section  406   b  may slide into and out of section  406   a  along the length thereof but the sections  406   a,    406   b  are constrained to rotate together. Section  406   a  is rotatably supported in a bearing block  409  and section  406   b  has a bevel gear  410  formed at the end thereof remote from the connector  400 . The bevel gear  410  engages a bevel gear  412  mounted on the output shaft  414  of drive/index motor  416 . The output shaft  414  and the end of the drive shaft  406  may be supported in a common bearing block  419 . Energization of the drive/index motor  416  rotates output shaft  414  to thereby rotate the drive shaft  406  and drive connector  400 . 
     The bearing block  409  is supported on a trolley system comprising a rod  418  and a parallel track  420  that allow the bearing block  409  to reciprocate transversely relative to the lead screws  312  of the drive assemblies  306   a - 306   d  such that the drive shaft  406  may be aligned with any of the lead screws  312 . The bevel gear  412  is mounted for reciprocating movement along the length of output shaft  414  such that the bevel gear  412  may reciprocate along the length of the output shaft  414  with the drive shaft  406  and bearing block  419 . Specifically, the output shaft  414  has flat surfaces  414   a  that engage flat surfaces on the bevel gear  412  such that the bevel gear  412  may slide along the length of the output shaft  414  but is constrained to rotate with the output shaft  414 . As a result, when the bearing blocks  409  and  419  reciprocate transversely relative to the lead screws  312  of the drive assemblies  306   a - 306   d,  the bevel gear  412  slides along the output shaft  414 . 
     The drive system  401  may be indexed by an index system  422  that uses the same drive/index motor  416  that rotates the drive shaft  406 . The index system  422  comprises a circular gear  430  that is coaxially arranged with, and is fixed to, the drive shaft  406  such that rotation of the drive shaft  406  causes simultaneous rotation of the gear  430 . A rack  432  is fixed in the housing  302  such that it is matingly engaged by the gear  430  when the device is in the index mode as shown in  FIGS. 4, 9 and 15 . In the index mode, the mode selection system  500  positions the gear  430  such that it engages the rack  432 , as will be hereinafter described. With the gear  430  engaged with the rack  432 , the actuation of the drive/index motor  416  rotates the drive shaft  406  to thereby rotate gear  430 . Due to the engagement of gear  430  with the rack  432 , the drive shaft  406 , bearing blocks  409  and  419  and bevel gear  412  are translated transversely relative to the lead screws  312  of the drive assemblies  306   a - 306   d  to index the drive system and align the drive shaft  406  with a selected one of the lead screws  312 . In the index mode, the drive connector  400  is disengaged from the screw connector  314  such that rotation of the drive shaft does not rotate any of the lead screws  312 . 
     Information relating to the position of the drive shaft  406  relative to the drive assemblies  306   a - 306   d  is stored in the memory  254  of the base station control system  250 . The base station control system  250  actuates drive/index motor  416  over communication links  302  to rotate drive shaft  406  a predetermined angular rotational distance (number of rotations) and direction to position the drive system  401  opposite the selected one of the drive assemblies  306   a - 306   d.  In some embodiments, the base station control system  250  stores in memory  254  the current position of the drive system  401 . The base station control system  250  also stores the direction and angular rotational distance of the motor  416  to move the drive shaft  406  from the current position to each of the three positions aligned with the other drive assemblies  306   a - 306   d.  When a phase shifter to be adjusted is selected at the base station control system  250 , via user interface  256 , for example, the base station control system  250  controls motor  416  to rotate the shaft  406  the predetermined number of rotations in the proper direction to move from the stored current position to the selected position. The new selected position is then stored in memory  254  as the current position. The indexing process may be repeated to move the drive system  401  to align with any of the drive assemblies  306   a - 306   d.    
     The mode selection system  500  will now be described. The mode selection system  500  moves the drive system  401  between one of four positions or modes—drive mode, index mode, calibration mode and free mode. In the drive mode, the drive system  401  is moved fully toward the lead screws  312  of the drive assemblies  306   a - 306   d  such that the screw connector  314  is engaged by the drive connector  400  as shown in  FIGS. 6, 7 and 12 . In the drive mode position, rotation of the drive shaft  406  rotates the lead screw  312  to adjust the position of the wiper printed circuit board  220 ,  220   a  of the phase shifter assembly  200  operatively coupled to that lead screw  312 . In the index mode, the drive system  401  is moved to the index mode position in which gear  430  engages the rack  432  as shown in  FIGS. 4, 9 and 15 . In this position, rotation of the drive shaft  406  indexes the drive system  401  transversely relative to the lead screws  312  to align the drive shaft  406  with one of the lead screws  312  of the drive assemblies  306   a - 306   d.  In the calibration mode, the drive system  401  is moved to a calibration position in which gear  430  is disengaged from the rack  432  and the drive connector  400  is disengaged from the screw connector  314  as shown in  FIGS. 10, 11 and 14 . In the calibration mode position, rotation of the drive shaft  406  is used to calibrate the system as will hereinafter be described. The system may also be indexed to a fourth free mode position. In the free mode, the drive system  401  is moved to a position in which gear  430  is disengaged from the rack  432 , the drive connector  400  is disengaged from the screw connector  314  and the device is not in the calibration mode as shown in  FIGS. 8 and 13 . In the free mode, the drive shaft  406  can turn freely without affecting operation of the system. 
     The mode selection system  500  comprises a linearly reciprocating sled  502  that supports the drive shaft  406  and bearing block  409  and trolley  418 ,  420 . Movement of the sled  502  reciprocates the drive system  401  between the index mode, drive mode, calibration mode and free mode, described above. The sled  502  comprises a generally planar platform  506  that supports the drive shaft  406  and bearing block  409  and trolley  418 ,  420 . The sled  502  includes bearing blocks  508  that are connected to the platform  506  and are slidably supported on rods  510 . The rods  510  extend parallel to the lead screws  312  such that the sled  502  and drive system  401  may be reciprocated linearly between the index mode, drive mode, calibration mode and free mode as the bearing blocks  508  slide on the rods  510 . To effectuate movement of the sled  502 , a rack  512  is fixed to the platform  506  that extends in the direction of travel of the sled  502 , parallel to the rods  510 . A pinion  514  is supported on and rotates with output shaft  518  of mode selection motor  520 . The mode selection motor  520  and output shaft  518  are fixed in the housing  302  such that rotation of the output shaft  518  rotates the pinion  514  and causes the sled  502  to reciprocate linearly towards and away from the lead screws  312  of the drive assemblies  306   a - 306   d.  As the sled  502  reciprocates, the bearing block  409  moves with the sled and the shaft sections  406   a  and  406   b  telescopically move relative to one another to accommodate movement of the sled  502  and the bearing block  409  relative to the bearing block  419 . 
     Information relating to the relative position of sled  502  is stored in the memory  254  of the base station control system  250  for each of the four positions or modes—drive mode, index mode, calibration mode and free mode. The base station control system  250  actuates the mode selection motor  520  to rotate output shaft  518  a predetermined angular distance to position the drive system  401  in the selected one of the four modes. In some embodiments, the base station control system  250  saves the position of the motor  520  correlating to the four mode positions and rotates the motor  520  to the correct position based on the selected mode. The motor  520  may provide feedback to the base station control system indicative of the motor position over links  302 . In other embodiments, the base station control system  250  saves in memory the current position of the mode system and the direction and angular rotation of the shaft  518  to move the sled  502  to each of the four mode positions such that when a mode is selected, the base station control system  250  rotates the output shaft  518  the predetermined number of rotations in the proper direction to move from the saved current position to the selected position. The new selected position is then saved as the current position. 
     The operation of the multi-RET actuator  300  to adjust a phase shifter will be described. It is to be understood that the system operates in a repetitive manner such that any position of the system may be considered the starting point and the system may move between the four modes of operation based control signals from the base station control system  250 . To adjust the position of a wiper printed circuit board  220 ,  220   a  of a selected one of the phase shifter assemblies  200 , the desired adjustment is received as an input by the user interface  256  of the base station control system  200  (Block  1601 ). The input may include an identification of the selected phase shifter assembly and the adjustment level. The adjustment level may be input as direction and angle of change of the wiper printed circuit board  220 ,  220   a  or it may be input as a phase taper or using other data. The adjustment level is used by the base station control system  250  to identify the wiper printed circuit board/drive assembly  306   a - 306   b  to be adjusted and the amount and direction of rotation of the lead screw  312  that corresponds to the adjustment level. The base station control system  250  controls the index system  422  and the mode selection system  500  to position the drive system  401  adjacent the lead screw  312  of the drive assembly  306   a - 306   d  that is operatively coupled to the phase shifter assembly to be adjusted. The base station control system  250  determines if the drive system  401  is positioned at the selected drive assembly  306   a - 306   d  (Block  1602 ). If the drive system  401  is positioned at the selected drive assembly  306   a - 306   d,  no indexing of the drive system is required. If the drive system  401  is not positioned at the selected drive assembly  306   a - 306   d,  the base station control system  250  actuates the mode selection motor  520  to move the sled  502  to the index mode position where the circular gear  430  is engaged with the rack  432  as shown in  FIGS. 4, 9 and 15  (Block  1603 ). If the system is in the index mode position, the mode selection motor is not actuated. The index/drive motor  416  is then actuated to rotate the drive shaft  406  and circular gear  430  to move the drive system to the selected drive assembly  306   a - 306   d  (Block  1604 ). Engagement of the rotating circular gear  430  with the stationary rack  432  indexes the drive system  401  transversely relative to the lead screws  312  of drive assemblies  306   a - 306   d.  The index/drive motor  416  is energized until the drive shaft  406  and drive connector  400  are aligned with the lead screw  312  that is operatively coupled to the selected phase shifter assembly  200 . The index/drive motor  416  is deactivated. 
     The mode selection system  500  is then used to move the RET actuator to the drive mode where the drive connector  400  engages the screw connector  314  (Block  1605 ). Specifically, the, the base station control system  250  actuates mode selection motor  520  to move the sled  502  toward the selected drive assembly such that the drive connector  400  engages the screw connector  314  of the lead screw  312  of the selected drive assembly  306   a - 306   d  as shown in  FIGS. 6, 7 and 12 . The engagement structures  408  on the drive connector  400  engage the engagement structures  334  on the screw connector  314 . As the drive connector  400  is moved into engagement with the screw connector  314 , a force is applied to the coupling member  324  that moves the coupling member  324  against the bias of spring  328  such that the teeth  346  on the screw connector  314  are disengaged from the teeth  350  on the wall  319 , as shown with drive assembly  306   a  in  FIG. 6 . In this positon, the screw connector  314  and the lead screw  312  are free to rotate. 
     The base station control system  250  actuates the drive system  401  to move the wiper printed circuit board of the phase shifter assembly to the desired position (Block  1606 ). Drive/index motor  416  is energized to rotate drive shaft  406  and the coupled lead screw  312 . As the lead screw  312  rotates, the drive nut  320  is moved linearly along the length of the lead screw. Linear movement of the drive nut  320  is transmitted to the mechanical linkage that is connected to the wiper printed circuit board  220 ,  220   a  of the selected phase shifter assembly such that movement of the drive nut  320  results in a rotational and/or linear adjustment of the wiper printed circuit board. The adjustment of the wiper printed circuit board may be made in angle of adjustment for pivoting wiper printed circuit boards or in linear distance for linearly moveable wiper printed circuit boards. In either event, the movement of the wiper printed circuit board is correlated to a number of revolutions of the lead screw  312 . Accordingly, the base station control system  250  controls the rotation of the drive shaft  406  to rotate the lead screw  312  in the proper direction and number of revolutions to make the selected adjustment. The base station control system  250  stores in memory  254  the current position of each of the wiper printed circuit boards. When an adjustment to the position of one of the wiper printed circuit boards is required, the control system  250  determines (by calculation, look up table or the like) the number of rotations and direction of rotation of the lead screw  312  and drive shaft  406  that corresponds to the selected position. The index/drive motor  416  is actuated to rotate the drive shaft  406  and lead screw  312  the determined number of rotations in the desired direction of rotation to move the wiper printed circuit board to the selected position. The selected position is then stored in memory  254  as the current position. 
     It will be appreciated that a relationship exists between the number of rotations of the lead screw  312  and the linear distance traversed by the drive nut  320  along the lead screw  312  and the corresponding distance moved by the mechanical linkage. The distance moved by the mechanical linkage corresponds to a known movement of the wiper printed circuit board. Thus, a rotational movement of the lead screw  312  correlates to a known movement of the associated wiper printed circuit board. Therefore, to adjust the position of the wiper printed circuit board of a phase shifter assembly, the lead screw  312  operatively coupled to that wiper printed circuit board is rotated by the drive motor  416  the corresponding number of rotations in the correct direction. In systems where the wiper printed circuit board of the phase shifter assembly moves linearly rather than rotationally, the rotational movement of the lead screw  312  correlates to a linear movement of the wiper printed circuit board. In all events, the rotational movement of the lead screw  312  corresponds to a known movement of the wiper printed circuit board of the phase shifter assembly such that the movement and position of the wiper printed circuit board may be controlled by a known rotational movement of the lead screw  312 . To position the wiper printed circuit board, the lead screw  312  is rotated by the index/drive motor  416  a known number or rotations or degrees of rotation in the correct direction. When the desired position of the lead screw  312  is reached, the index/drive motor  416  is turned off. 
     The drive system  401  may then be retracted to the index mode position, free mode position or calibration mode position (Block  1607 ). The drive system  401  is retracted by energizing the mode selection motor  520  to drive gear  514  in engagement with rack  512 . When the drive system  401  is retracted, the force exerted on the spring  328  by the drive system is removed such that the spring  328  moves the coupling member  324  to the extended position where the teeth  346  on the coupling member  324  engage the stationary teeth  350  on the wall  319  to hold the lead screw  312  in the selected position. The process may be repeated for any phase shifter assembly that needs to be adjusted and may be repeated for each phase shifter assembly. 
     In one embodiment, each screw connector  314  is provided with eight engagement structures  334 , spaced 45 degrees apart. Each drive connector  400  is also provided with eight mating engagement structures  408  spaced 45 degrees apart. The lead screws  312  are always located at rest in one of eight positions with the eight engagement structures  334  in the same relative angular positions. The lead screws  312  may be rotated a minimum of 45 degrees, i.e. rotating the lead screw the angular distance between two adjacent engagement structures  334 , and may be rotated through angles that are multiples of 45 degrees. In one embodiment, an angular rotation of 45 degrees of the lead screw  312  may translate to a 0.2 mm adjustment of the wiper printed circuit board based on the geometry of the system. However, the relationship between the angular rotation of the lead screws  312  and the adjustment of the wiper board may be varied. Moreover, a greater number or fewer number of engagement structures may be used to thereby increase or decrease the minimum angular adjustment of the lead screws  312 . 
     The drive system  401  may rotate the lead screw  312  in multiples of 45 degrees of rotation based on the desired adjustment of the wiper printed circuit board. As a result, the screw connector  314  is always positioned at rest with the engagement structures  334  positioned in the same relative angular position. The screw connector  314  and associated lead screw  312  are held in the rest position by the engagement of the locking members  346  and stationary locking members  350 . It will be appreciated that in the rest position, the screw connector  314  is always positioned with the engagement members  334  occupying the same relative angular position; however, the position of the wiper printed circuit board will vary based on the rotation of the associated lead screw. Thus, if the lead screw  312  is rotated 360 degrees, the engagement members  334  are in the same angular position but the drive nut  320  will have been moved up or down the length of the lead screw  312  and the position of the wiper printed circuit board will have been adjusted. Similarly, if the lead screw  312  is rotated  540  degrees the engagement members  334  are in the same relative angular position, but rotated 180 degrees in absolute position, and the drive nut  320  will have been moved a corresponding distance up or down the length of the lead screw  312  and the position of the wiper printed circuit board will have been adjusted accordingly. 
     The drive connector  400  must be aligned with the screw connector  314  to allow the engagement members  408  of the drive connector  400  to engage the engagement members  334  of the screw connector  314  without interference. Thus, the drive connector  400  must be, at rest, at the same rest position as the screw connector  314 . It is to be understood that the rest position of the drive connector may be any one of a plurality of angular positions of the drive connector where the drive connector is aligned with the screw connector and that any one of the angular positions may be the rest position. The engagement structures on the drive connector are aligned with the engagement structures on the screw connector when the drive connector is in the rest position. As previously explained with respect to the screw connector, the rest position of the drive connector may correspond to anyone of a plurality of angular positions of the drive connector. For example, each drive connector  400  may be provided with eight mating engagement structures  408  spaced 45 degrees apart where the drive connector  400 , when in the rest position, may be located in anyone of the eight positions with the eight engagement structures  408  in the same relative angular positions. If the drive connector  400  is offset from the rest position, when the drive connector  400  is moved into engagement with the screw connector  314 , the engagement members  408  of the drive connector  400  may interfere with the engagement members  334  of the screw connector  314  as the coupling between the connectors is made. If this interference between the engagement members  408  of the drive connector  400  and the engagement members  334  of the screw connector  314  occurs, the drive connector  400  may cause the screw connector  314 , and the associated lead screw  312 , to inadvertently rotate. Repeated engagement and disengagement of the screw connector  314  with an out of alignment drive connector  400  can, over time, introduce errors in the position of the wiper printed circuit board of the phase shifter assembly. These positioning errors may also be compounded because the next adjustment of an out of position wiper printed circuit board is based on an inaccurate starting position. These errors in the positioning and repositioning of the wiper printed circuit board may be undetected by the system operator until performance issues arise. 
     The drive shaft  406  and drive connector  400  may come to a stop at a position other than the rest position over repeated actuations due to inherent inaccuracies in the drive motor  416  and transmission assembly to the drive connector  400 . The system of the invention provides a calibration mode to ensure that the drive system  401  and drive connector are properly aligned in the rest position. The calibration process may be performed during system set up to ensure that the drive connector  400  is properly positioned. The calibration process may also be performed periodically during use of the system to ensure that the drive system and drive connector remain properly aligned. For example, the calibration process may be performed as directed by the system operator or after a predetermined number of cycles, e.g.  1000  cycles, to maintain the proper alignment of the system in the field. 
     To calibrate the system, the system operates in the calibration mode. To initiate calibration of the system, a calibration command may be received by the user interface  256  of the base station control system  200 . Alternatively, the base station control system  250  may automatically calibrate the system based on cycles of operation, time or other factors stored in memory  254 . In either event, the base station control system  200  initiates the calibration process (Block  1701 ). The base station control system  250  determines if the drive system is positioned at the calibration mode position (Block  1702 ). If the drive system is positioned at the calibration mode position, the mode selection system is not utilized. If the drive system is not positioned at the calibration mode position, the base station control system  250  actuates the mode selection motor  520  to move the sled  502  to the to the calibration position (Block  1703 ). In the illustrated embodiment, the calibration position is between the drive position and the indexing position. The calibration position is shown in  FIGS. 10, 11 and 14  where the drive connector  400  is not engaged with the screw connector  314  and the circular gear  430  is not engaged with the rack  432 . While in the illustrated embodiment, the calibration position is an intermediate position that is physically between the drive position and the index position, in some embodiments, the calibration position may be, for example, behind the index position provided that the drive connector  400  is not engaged with the screw connector  314  and the circular gear  430  is not engaged with the rack  432 . In the calibration position, actuation of the drive motor  416  rotates the drive shaft  406  but does not index the drive system and does not rotate any of the lead screws  312 . 
     Referring more particularly to  FIG. 11 , a positioning member  600  is mounted for rotation with the drive shaft  406 . In the illustrated embodiment, the positioning member  600  is in the form of a ring  602  mounted on the shaft  406  for rotation therewith. A stop member  604  in the form of a tab or ear extends from the ring  602 . When the drive shaft  406  is rotated and the drive system is in the calibration mode, the stop member  604  is positioned to strike a hard stop  608 . In the illustrated embodiment, the hard stop  608  comprises a projection that extends upwardly from the housing  310 . In the illustrated embodiment, one hard stop  608  is associated with each lateral position of the drive system  401  such that calibration may be performed with the drive system aligned with any of the lead screws  312 . In other embodiments, a single hard stop  608  may be provided where the calibration process may only be performed after the drive system is indexed to that position. 
     The base station control system  250  energizes the index/drive motor  416  until the stop member  604  hits the hard stop  608  (Block  1704 ). This position is a known angular position of the drive connector  400 . This known position may be the rest position of the drive connector  400  or it may be a position at a known or predetermined angular distance from the rest position of the drive connector  400 . If the known position is the rest position of the drive connector  400 , the drive connector is properly located and is positioned at the rest position. If the known position is a known angular distance from the rest position, the drive shaft  406  is rotated by the drive/index motor  416  the known or predetermined angular distance to locate the drive shaft  406  and drive connector  400  at the rest position (Block  1705 ). The software registers the calibrated position as the rest position and future rotation of the drive shaft to adjust the lead screws  312  is calculated from this position (Block  1706 ). The next adjustment of the wiper printed circuit board is made from the true rest position thereby eliminating errors due to an out of alignment drive system. 
     The system of the invention is scalable such that a greater or fewer number of drive assemblies  306  may be provided. In some embodiments, the RET adjuster may be a single RET adjuster with a single drive assembly where the index system may be eliminated. The calibration system as described herein may be used with a single RET system having a single drive assembly. The RET adjuster of the invention also has a narrow profile such that it is better able to fit in the limited space of an antenna. The low profile is provided, in part, because the drive system elements are in a common plane. As shown in the figures, the motors  416 ,  520  shafts  406 ,  414 , and  518  and the drive assemblies  306  are all in a common plane such that the RET adjuster has a very small depth dimension. 
     The present invention has been described above with reference to the accompanying drawings. The invention is not limited to the illustrated embodiments; rather, these embodiments are intended to fully and completely disclose the invention to those skilled in this art. In the drawings, like numbers refer to like elements throughout. Thicknesses and dimensions of some elements may not be to scale. 
     Spatially relative terms, such as “under”, “below”, “lower”, “over”, “upper”, “top”, “bottom” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “under” or “beneath” other elements or features would then be oriented “over” the other elements or features. Thus, the exemplary term “under” can encompass both an orientation of over and under. The device may be otherwise oriented (rotated  90  degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. 
     Well-known functions or constructions may not be described in detail for brevity and/or clarity. As used herein the expression “and/or” includes any and all combinations of one or more of the associated listed items. 
     It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the present invention. 
     It will be understood that when an element is referred to as being “on” or extending “onto” another element, it can be directly on or extend directly onto the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” or extending “directly onto” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present.