Patent Publication Number: US-2023147936-A1

Title: Actuators for controlling multiple phase shifters of remote electronic downtilt base station antennas

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/746,387, filed Jan. 17, 2020, which is a continuation of U.S. patent application Ser. No. 15/622,407, filed Jun. 14, 2017, which in turn claims priority to U.S. Provisional Patent Application Ser. No. 62/350,252, filed Jun. 15, 2016, U.S. Provisional Patent Application Ser. No. 62/370,065, filed Aug. 2, 2016, and U.S. Provisional Patent Application Ser. No. 62/420,773, filed Nov. 11, 2016, the entire contents of each of which is incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to communication systems and components, and in particular, to actuators for electromechanical phase shifters used in base station antennas. 
     BACKGROUND 
     Base station antennas for wireless communication systems are used to transmit radio frequency (“RF”) signals to, and receive RF signals from, cellular. Base station antennas are directional devices that can concentrate the RF energy that is transmitted in certain directions (or received from those 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 particular direction. The “radiation pattern” of a base station antenna is compilation of the gain of the antenna across all different directions. The radiation pattern of a base station antenna is typically designed to service a pre-defined coverage area, which refers to a geographic region in which mobile users can communicate with the cellular network through the base station antenna. The base station antenna may be designed to have minimum gain levels throughout this pre-defined coverage area, and it is typically desirable that the base station antenna have much lower gain levels outside of the coverage area. Early base station antennas typically had a fixed radiation pattern, meaning that once a base station antenna was installed, its radiation pattern could not be changed unless a technician physically reconfigured the antenna. Unfortunately, such manual reconfiguration of base station antennas after deployment, which could become necessary due to changed environmental conditions or the installation of additional base stations, was typically difficult, expensive and time-consuming. 
     More recently, base station antennas have been deployed that have radiation patterns that can be reconfigured from a remote location. For example, base station antennas have been developed for which settings such as the down tilt angle, beam width and/or azimuth angle of the antenna can be reconfigured from a remote location by transmitting control signals to the antenna. Base station antennas that can have their down tilt or “elevation” angle changed from a remote location are typically referred to as remote electrical tilt (“RET”) antennas, although the term “RET antenna” is now also commonly used to cover antennas that can have their azimuth angle and/or beam width adjusted from a remote location. RET antennas allow wireless network operators to remotely adjust the radiation pattern of the antenna through the use of electro-mechanical actuators that may adjust phase shifters or other devices in the antenna to affect the radiation pattern of the antenna. Typically, the radiation pattern of a RET antenna is adjusted using actuators that are controlled via control signal specifications promulgated by the Antenna Interface Standards Group (“AISG”). 
     Base station antennas typically comprise a linear array or a two-dimensional array of radiating elements such as dipole or crossed dipole radiating elements. In order to change the down tilt angle of these antennas, a phase taper may be applied across the radiating elements, as is well understood by those of skill in the art. Such a phase taper may be applied by adjusting the settings on an adjustable phase shifter that is positioned along the RF transmission path between a radio and the individual radiating elements of the base station antenna. One known 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 capacitively couple at least some of these sub-components to the wiper printed circuit board. These sub-components of the RF signal may be capacitively coupled from the wiper printed circuit board back to the main printed circuit board along a plurality of arc-shaped traces, where each arc has a different diameter. Each end of each arc-shaped trace may be connected to a radiating element or to a sub-group of radiating elements. By physically rotating the wiper printed circuit board above the main printed circuit board, the location where the sub-components of the RF signal capacitively couple back to the main printed circuit board may be changed, which thus changes the path lengths from the phase shifter to the radiating elements. This change in the path lengths results in a change in the phase of the sub-components of the RF signal, and since the arcs have different radii, the change in phase experienced along each path differs. Typically, the phase taper is applied by applying positive phase shifts of various magnitudes (e.g., +1°, +2° and +3°) to some of the sub-components of the RF signal and by applying negative phase shifts of the same magnitudes (e.g., −1°, −2° and −3°) to additional of the sub-components of the RF signal. Thus, the above-described wiper phase shifters may be used to apply a phase taper to the sub-components of an RF signal that are applied to each radiating element (or sub-group of radiating elements). 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 since they are used to apply the remote electronic down tilt. 
     SUMMARY 
     Pursuant to embodiments of the present invention, an actuator for a plurality of phase shifters is provided. The actuator includes a plurality of shafts having respective axially-drivable members mounted thereon, each axially-drivable member configured to be connected with a respective one of the phase shifters; a motor having a drive shaft; and a gear system that is configured to selectively couple the motor to the respective shafts. The gear system is configured so that rotation of the drive shaft in a first rotative direction creates a mechanical linkage between the motor and a first of the shafts, and rotation of the drive shaft in a second rotative direction that is opposite the first rotative direction rotates the first of the shafts. 
     In some embodiments, the gear system includes a forward-direction primary drive gear that is connected to the drive shaft and a reverse-direction primary drive gear that is connected to the drive shaft. 
     In some embodiments, the forward-direction primary drive gear and the reverse-direction primary drive gear are each ratcheted gears that rotate in response to rotation of the drive shaft in the second rotative direction and which do not rotate in response to rotation of the drive shaft in the first rotative direction. 
     In some embodiments, the actuator further includes a reversing gear that is configured to engage the reverse-direction primary drive gear and rotate in a direction opposite a direction of rotation of the reverse-direction primary drive gear. 
     In some embodiments, the gear system further includes a plurality of secondary drive members mounted on respective ones of the shafts, each secondary drive member mounted so that rotation thereof will result in rotation of a respective one of the shafts. 
     In some embodiments, the gear system includes an engagement mechanism that is configured to rotate to selectively and exclusively engage one or more of the shafts to move a selected one of the secondary drive members into engagement with one of the forward-direction primary drive gear or the reversing gear. 
     In some embodiments, the engagement member includes a rotating cam plate. 
     Pursuant to further embodiments of the present invention, a method of adjusting a phase shifter is provided, the method including rotating a drive shaft in a first rotative direction to connect a first of a plurality of gears to a drive mechanism; rotating the drive shaft in a second rotative direction to rotate the drive mechanism, where rotation of the drive mechanism causes rotation of the first of the plurality of gears, and where rotation of the first of the plurality of gears mechanically adjusts a physical position of a component of the phase shifter. 
     In some embodiments, the plurality of gears comprises a plurality of secondary drive gears that are configured to rotate respective shafts, and the drive mechanism includes a forward-direction primary drive gear that is connected to the drive shaft and a reverse-direction primary drive gear that is connected to the drive shaft. 
     In some embodiments, the forward-direction primary drive gear is a ratcheted gear that only rotates in response to rotation of the drive shaft in a first rotative direction. 
     In some embodiments, the reverse-direction primary drive gear is a ratcheted gear that only rotates in response to rotation of the drive shaft in the first rotative direction. 
     In some embodiments, rotating the drive shaft in the first rotative direction to connect the first of the plurality of gears to the drive mechanism includes using the rotating drive shaft to rotate a cam to move the first of the plurality of gears into operative engagement with one of the forward-direction primary drive gear or the reverse-direction primary drive gear. 
     In some embodiments, at least one of the forward-direction primary drive gear or the reverse-direction primary drive gear is configured to engage the first of the plurality of gears through an intervening reversing gear. 
     Pursuant to further embodiments of the present invention, an actuator for a plurality of phase shifters is provided. The actuator includes a motor that is configured to rotate a primary rotary member; a plurality of axially-drivable members, each axially-drivable member mounted on a respective shaft, each axially-drivable member configured to be connected with a respective one of the phase shifters; a plurality of secondary rotary members, each secondary rotary member mounted so that rotation thereof will result in rotation of a respective one of the shafts; and a plurality of micro-motors, each micro-motor configured to rotate a respective one of the shafts. 
     In some embodiments, the shafts include worm gear shafts. 
     In some embodiments, the primary rotary member is a central gear and each of the secondary rotary members are gears. 
     In some embodiments, the axially-drivable members include pistons. 
     In some embodiments, the actuator further includes a plurality of springs that are mounted on the respective shafts, each spring configured to bias a respective one of the secondary rotary member toward a disengaged position where the secondary rotary member does not engage the primary drive member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1 A  is a perspective view of a base station antenna that includes a single motor multi-RET actuator according to embodiments of the present invention. 
         FIG.  1 B  is an end view of the base station antenna of  FIG.  1 A  that illustrates the input/output ports thereof 
         FIG.  1 C  is a schematic plan view of the base station antenna of  FIG.  1 A  that illustrates the three linear arrays of radiating elements thereof. 
         FIG.  2    is a schematic block diagram illustrating the connections between various components of the base station antenna of  FIG.  1   . 
         FIG.  3    is a front perspective view of a pair of electromechanical phase shifters that may be included in the base station antenna of  FIG.  1   . 
         FIG.  4 A  is a perspective view of a single motor multi-RET actuator assembly according to embodiments of the present invention. 
         FIG.  4 B  is a front perspective view of the multi-RET actuator included in the multi-RET actuator assembly of  FIG.  4 A  with one of the base plates removed therefrom. 
         FIG.  4 C  is a side perspective view of the multi-RET actuator of  FIG.  4 B . 
         FIG.  4 D  is a partial side perspective view of the multi-RET actuator included in the assembly of  FIG.  4 A  with one of the base plates and the motor removed that illustrates one of the secondary drive gears engaging the primary drive gear of the actuator. 
         FIG.  4 E  is a partial side view of the multi-RET actuator of  FIG.  4 B  that illustrates one of the secondary drive gears engaging the primary drive gear of the actuator. 
         FIG.  5 A  is a schematic block diagram of a single motor multi-RET actuator according to further embodiments of the present invention. 
         FIGS.  5 B and  5 C  are schematic block diagrams of a single motor multi-RET actuator according to still further embodiments of the present invention that illustrate a secondary drive gear thereof in its disengaged and engaged positions, respectively. 
         FIG.  5 D  is a schematic block diagram of a single motor multi-RET actuator according to yet additional embodiments of the present invention. 
         FIG.  5 E  is a schematic block diagram of a single motor multi-RET actuator according to yet further embodiments of the present invention. 
         FIG.  6    is a schematic block diagram of a single motor multi-RET actuator according to embodiments of the present invention in which the primary drive gear is moved as opposed to the secondary drive gears. 
         FIG.  7    is a schematic block diagram of a single motor multi-RET actuator according to further embodiments of the present invention that has a primary drive gear that can be moved in two different directions via application of electromagnetic force. 
         FIG.  8    is a schematic block diagram of a single motor multi-RET actuator according to further embodiments of the present invention that uses a piezoelectric actuator to connect a selected mechanical linkage to a motor. 
         FIG.  9 A  is a side view of a multi-RET actuator according to further embodiments of the present invention. 
         FIG.  9 B  is a partial side view of the multi-RET actuator of  FIG.  9 A  with one of the secondary drive gears engaged with the primary drive gear. 
         FIG.  9 C  is a partial side sectional view of the multi-RET actuator of  FIG.  9 A . 
         FIG.  9 D  is a partial side perspective view of the multi-RET actuator of  FIG.  9 A  with none of the secondary drive gears engaged with the primary drive gear. 
         FIG.  9 E  is a partial side perspective view of the multi-RET actuator of  FIG.  9 A  with one of the secondary drive gears engaged with the primary drive gear. 
         FIG.  10 A  is perspective view of a multi-RET actuator assembly according to further embodiments of the invention. 
         FIG.  10 B  is a perspective view of the multi-RET actuator of  FIG.  10 A  with the housing removed therefrom. 
         FIG.  10 C  is a perspective view of the actuator included in the multi-RET actuator assembly of  FIGS.  10 A- 10 B . 
         FIG.  10 D  is a perspective view of the actuator of  FIG.  10 C  with the motors, cam plate and one base plate removed. 
         FIG.  10 E  is a side view of the actuator of  FIG.  10 C . 
         FIG.  10 F  is another perspective view of the actuator of  FIG.  10 C  with the motors, cam plate and one base plate removed. 
         FIG.  11 A  is a schematic front view illustrating operation of a multi-RET actuator according to still further embodiments of the present invention. 
         FIG.  11 B  is a schematic top view of a portion of the multi-RET actuator of  FIG.  11 A . 
         FIG.  11 C  is conceptual diagram illustrating operation of the gears attached to the drive shaft of the actuator of  FIGS.  11 A- 11 B . 
     
    
    
     DETAILED DESCRIPTION 
     Modern base station antennas often include two, three or more linear arrays of cross-polarized radiating elements. Thus, it is not uncommon for a base station antenna to have eight, twelve or even more adjustable phase shifters for applying remote electronic down tilts to the linear arrays. Such a large number of phase shifters and associated RET actuators and mechanical linkages can significantly increase the size, weight and cost of the base station antenna. 
     Conventionally, a separate RET actuator has been provided for each phase shifter (or pair of phase shifters if dual polarized radiating elements are used in a linear array, as the same phase shift is typically applied to each polarization). More recently, RET actuators have been proposed that may be used to move the wiper printed circuit board on as many as twelve phase shifters. For example, U.S. Patent Publication No. 2013/0307728 (“the &#39;728 publication”) discloses a RET actuator that may be used to drive six different mechanical linkages for purposes of adjusting six different phase shifters using one multi-RET actuator. 
     Pursuant to embodiments of the present invention, fully automated multi-RET actuators are provided. The multi-RET actuators according to embodiments of the present invention may be controlled from a remote location to independently adjust the settings of one or more of a plurality of phase shifters. In some embodiments, the multi-RET actuators include two motors. In these embodiments, the first motor may operate to select one of a plurality of mechanical linkages that is to be moved, and the second motor may be used to move the selected mechanical linkage. In other embodiments, single motor multi-RET actuators are provided. In some of these single-motor embodiments, a ratcheted gear system may be provided that allows the motor to both select the mechanical linkage that is to be moved and to then move the selected mechanical linkage. In other embodiments, a separate actuator system such as, for example, remotely controlled electromagnets may be used to select the mechanical linkage that is to be moved, and the single motor may then be used to move the selected mechanical linkage. In still other embodiments, multi-RET actuators are provided that use a main drive motor and a plurality of micro-motors. 
     The multi-RET actuators according to embodiments of the present invention may be used to rotate a primary drive gear (or a pair of primary drive gears in one embodiment) that is mounted on the drive shaft of a motor. A plurality of worm gear shafts are provided, each of which has a respective secondary drive gear associated therewith. A selected one or more of the secondary drive gears may be moved to engage the primary drive gear. Each secondary drive gear may be connected to its associated worm gear shaft so that rotation of the primary drive gear causes the selected secondary drive gear to rotate, which in turn imparts rotational movement to the worm gear shaft on which the selected secondary drive gear is mounted. Rotation of the worm gear shaft causes a piston mounted thereon to move along the longitudinal axis of its worm gear shaft. Each piston may be connected via a mechanical linkage to a wiper arm on an adjustable phase shifter so that movement of the piston may be used to adjust the setting of the phase shifter. 
     In order to allow the adjustable phase shifters that are connected to each respective mechanical linkage to be independently adjusted, the multi-RET actuators according to embodiments of the present invention can select which of the secondary drive gears contacts the primary drive gear so that movement of the primary drive gear results in corresponding rotation of only the selected secondary drive gear. In some embodiments, electromagnets may be used to move selected ones of the secondary drive gears into engagement with the primary drive gear in response to control signals from a remote location. In other embodiments, piezoelectric actuators may be used to move selected ones of the secondary drive gears into engagement with the primary drive gear. In still other embodiments, other electro-mechanical mechanisms may be provided that move selected ones of the secondary drive gears into engagement with the primary drive gear. In further embodiments, micro-motors may be used to move selected ones of the secondary drive gears into engagement with the primary drive gear. In still other embodiments, an indexing cam plate may be used to move selected ones of the secondary drive gears into engagement with the primary drive gear. Moreover, while the embodiments discussed below primarily (but not exclusively) discuss actuators in which the selected secondary drive gears are moved to engage the primary drive gear, it will be appreciated that in other embodiments the primary drive gear may be moved to engage one or more selected secondary drive gears. 
     Embodiments of the present invention will now be discussed in greater detail with reference to the drawings. 
       FIG.  1 A  is a perspective view of a RET base station antenna  100  that may include any of the multi-RET actuators according to embodiments of the present invention that are disclosed herein.  FIG.  1 B  is an end view of the base station antenna  100  that illustrates the input/output ports thereof.  FIG.  1 C  is a schematic plan view of the base station antenna  100  that illustrates the three linear arrays of radiating elements thereof.  FIG.  2    is a schematic block diagram illustrating various internal components of the RET antenna  100  and the connections therebetween. It should be noted that  FIG.  2    does not show the actual location of the various elements on the antenna, but instead is drawn to shown the connections between the various elements with a minimal number of connection lines crossing. It will also be appreciated that the connection lines in  FIG.  2    represent paths for electrical signals (e.g., RF transmission lines). 
     Referring to  FIGS.  1 A- 1 C  and  FIG.  2   , the RET antenna  100  includes, among other things, input/output ports  110 , a plurality of linear arrays  120  of radiating elements  130 , duplexers  140 , phase shifters  150  and control ports  170 . As shown in  FIGS.  1 C and  2   , the antenna  100  includes a total of three linear arrays  120  (labeled  120 - 1  through  120 - 3 ) that each include five radiating elements  130 . It will be appreciated, however, that the number of linear arrays  120  and the number of radiating elements  130  included in each of the linear arrays  120  may be varied. It will also be appreciated that different linear arrays  120  may have different numbers of radiating elements  130 . 
     Referring to  FIG.  2   , the connections between the input/output ports  110 , radiating elements  130 , duplexers  140  and phase shifters  150  are schematically illustrated. Each set of an input port  110  and a corresponding output port  110 , and their associated phase shifters  150  and duplexers  140 , may comprise a corporate feed network  160 . A dashed box is used to illustrate one such corporate feed network  160  in  FIG.  2   . Each corporate feed network  160  connects the radiating elements  130  of one of the linear arrays  120  to a respective pair of input/output ports  110 . 
     As shown schematically in  FIG.  2    by the “X” that is included in each box, the radiating elements  130  may be cross-polarized radiating elements  130  such as +45° /−45° slant dipoles that may transmit and receive RF signals at two orthogonal polarizations. Any other appropriate radiating element  130  may be used including, for example, single dipole radiating elements or patch radiating elements (including cross-polarized patch radiating elements). When cross-polarized radiating elements  130  are used, two corporate feed networks  160  may be provided per linear array  120 , a first of which carries RF signals having the first polarization (e.g., +45°) between the radiating elements  130  and a first pair of input/output ports  110  and the second of which carries RF signals having the second polarization (e.g., −45°) between the radiating elements  130  and a second pair of input/output ports  110 , as shown in  FIG.  2   . 
     As shown in  FIG.  2   , an input port of each transmit (“TX”) phase shifter  150  may be connected to a respective one of the input ports  110 . Each input port  110  may be connected to the transmit output port of a radio (not shown) such as a remote radio head. Each transmit phase shifter  150  has five output ports that are connected to respective ones of the radiating elements  130  through respective duplexers  140 . The transmit phase shifters  150  may divide an RF signal that is input to an input port  110  into a plurality of sub-components and may effect a phase taper to the sub-components of the RF signal that are provided to the radiating elements  130 . In a typical implementation, a linear phase taper may be applied to the radiating elements  130 . As an example, the first radiating element  130  in a linear array  120  may have a phase of Y°+2X°, the second radiating element  130  in the linear array  120  may have a phase of Y°+X°, the third radiating element  130  in the linear array  120  may have a phase of Y°, the fourth radiating element  130  in the linear array  120  may have a phase of Y°−X°, and the fifth radiating element  130  in the linear array  120  may have a phase of Y°−2X°, where the radiating elements  130  are arranged in numerical order. 
     Similarly, each receive (“RX”) phase shifter  150  may have five input ports that are connected to respective ones of the radiating elements  130  through respective duplexers  140  and an output port that is connected to one of the output ports  110 . The output port  110  may be connected to the receive port of a radio (not shown). The receive phase shifters  150  may effect a phase taper to the RF signals that are received at the five radiating elements  130  of the linear array  120  and may then combine those RF signals into a composite received RF signal. Typically, a linear phase taper may be applied to the radiating elements as is discussed above with respect to the transmit phase shifters  150 . 
     The duplexers  140  may be used to couple each radiating element  130  to both a transmit phase shifter  150  and to a receive phase shifter  150 . As is well known to those of skill in the art, a duplexer is a three port device that (1) passes signals in a first frequency band (e.g., the transmit band) through a first port while not passing signals in a second band (e.g., a receive band), (2) passes signals in the second frequency band while not passing signals in the first frequency band through a second port thereof and (3) passes signals in both the first and second frequency bands through the third port thereof, which is often referred to as the “common” port. 
     As can be seen from  FIG.  2   , a base station antenna  100  that includes three linear arrays  120  of radiating elements  130  may include a total of twelve phase shifters  150 . While the two transmit phase shifters  150  for each linear array  120  (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 ), there still are six sets of two phase shifters  150  that should be independently controllable. Accordingly, six RET actuators would conventionally be used in a base station antenna having the linear array arrangement of base station antenna  100 . 
     The base station antenna  100  may include various other components such as low noise amplifiers, one or more processors, etc. that are not pictured in  FIGS.  1 A- 1 C  and  FIG.  2   . 
     Each phase shifter  150  shown in  FIG.  2    may be implemented as a rotating wiper phase shifter. The phase shifts imparted by the phase shifter  150  to each sub-component of the 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   . 
     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 transmit phase shifters  150  of  FIG.  2    (that are associated with the same linear array  120 ) or two of the receive phase shifters  150  of  FIG.  2    (that, again, are associated with the same linear array  120 ). 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 will be 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 two rotatable 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 linkage shaft  228 , the end of which may constitute one end of a mechanical linkage  226 . The other end of the mechanical linkage  226  (not shown) may be coupled to a multi-RET actuator according to embodiments of the present invention, as will be discussed in further detail below. A position sensor  250  may be provided on one of the rotatable wiper printed circuit boards  220 ,  220   a  to detect the position of the rotatable wiper printed circuit boards  220 ,  220   a.    
     Each main printed circuit board  210 ,  210   a  includes a plurality of 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 the transmission line traces on the wiper printed circuit board  220  (not visible in  FIG.  3   ). The RF signals are coupled from the 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 coaxial cable  260   a  is also coupled to the corresponding input pad on the main printed circuit board  210   a  of phase shifter  202   a ). A respective coaxial cable  270  or other RF transmission line component may be connected to each respective output pad  240  (coaxial cables  270   a  may likewise be coupled to the corresponding output pads on the main printed circuit board  210   a  of phase shifter  202   a ). Connections other than coaxial cables  260 ,  270  may be used in other embodiments. For example, in other embodiments, the main printed circuit board  210  may be coupled to stripline transmission lines on a panel without additional coaxial cabling. As the wiper printed circuit board  220  moves, an electrical path length from the input pad  230  of phase shifter  202  to each radiating element  130  served by the transmission lines  212 ,  214  changes. For example, as the wiper printed circuit board  220  moves to the left 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 radiating element  130 ), 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 radiating element) 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 linkage shaft  228  as the rotating wiper printed circuit board  220  of phase shifter  202 . For example, if a linear array  120  includes dual polarized radiating elements  130 , typically the same phase shift will be applied to the RF signals transmitted at each of the two orthogonal polarizations. In this case, a single mechanical linkage  226  may be used to control the positions of the wiper printed circuit boards  220 ,  220   a  on both phase shifters  202 ,  202   a.  In other cases, the wiper printed circuit boards  220 ,  220   a  of the two phase shifters  202 ,  202   a  may be connected to separate linkage shafts  228 . 
     As noted above, various physical and/or electrical settings of a RET antenna such as antenna  100  including the elevation angle can be controlled from a remote location by transmitting control signals to the antenna  100  that cause electromechanical actuators to adjust the settings on the electro-mechanical phase shifters  150 . Conventionally, a separate actuator was provided for each phase shifter  150  (or for a pair of phase shifters  150  associated with cross-polarized radiating elements  130 ). As discussed above, more recently multi-RET actuators have been suggested that may be used to control a plurality of different phase shifters. These multi-RET actuators use a first “drive” motor to drive the mechanical linkages and a second “indexing” motor to selectively connect one of the mechanical linkages to the first drive motor. 
     Pursuant to embodiments of the present invention, multi-RET actuator assemblies are provided that include a single motor that actuates multiple mechanical linkages. By eliminating one of the two motors from the above-discussed multi-RET actuator, the size, cost and weight of the multi-RET actuator assembly may be significantly reduced.  FIGS.  4 A- 4 E  illustrate a single motor multi-RET actuator assembly  300  according to embodiments of the present invention. In particular,  FIG.  4 A  is a perspective view of the single motor multi-RET actuator  300 ,  FIGS.  4 B and  4 C  are a front perspective view and a side view, respectively, of the single motor multi-RET actuator  300  with the housing removed therefrom, and  FIGS.  4 D and  4 E  are partial perspective and side views of the single motor multi-RET actuator  300  with the housing removed that illustrate how one of a plurality of secondary drive gears may be selectively connected to a primary drive gear. 
     As shown in  FIG.  4 A , the multi-RET actuator assembly  300  includes a housing  310  having a pair of connectors  320  mounted on one end wall  312  of the housing  310 . The housing  310  may be formed of any appropriate material, such as a metal or polymeric material. The housing  310  may be omitted in some embodiments. The connectors  320  may be mounted on a printed circuit board (not shown) in some embodiments. Each connector  320  may extend through a respective aperture  314  in the end wall  312 . The connectors  320  may connect to communications cables that may be used to deliver control signals from a base station control system to the multi-RET actuator assembly  300 . 
     Referring now to  FIGS.  4 B- 4 E , an actuator  330  is mounted within the housing behind the end wall  312 . The actuator  330  includes a pair of circular base plates  332 ,  334  that are mounted within the housing  310 . A third base plate  336  may be provided at the distal end of the actuator  330 . Six generally parallel worm gear shafts  340  are provided that extend along respective axes R 1 -R 6  between base plates  334  and  336  (see  FIG.  4 D ). Each worm gear shaft  340  includes a worm gear extension  342  that extends through the base plate  334  so that each worm gear shaft  340  is rotatably mounted in the base plate  334 . The worm gear shafts  340  are distributed generally circumferentially equidistant from each other. The worm gear extensions  342  may be formed integrally with their corresponding worm gear shafts  340 . Respective secondary drive gears  344  are axially aligned with the worm gear extensions  342 . Each worm gear extension  342  may extend partially into an internal cavity  347  of its respective secondary drive gear  344 . In some embodiments, each worm gear extension  342  may extend into the internal cavity  347  of its respective secondary drive gear  344  when the secondary drive gear  344  is in its resting (disengaged) position. In other embodiments, the worm gear extension  342  may only extend into the internal cavity  347  of its respective secondary drive gear  344  when the secondary drive gear  344  is in its engaged position. Each internal cavity  347  extends deeper into the secondary drive gear  344  than necessary to receive the worm gear extension  342  of its mating worm gear shaft  340 , which allows each secondary drive gear  344  to move axially towards its respective worm gear shaft  340 , in the manner discussed below. A rear portion  345  of each secondary drive gear  344  is mounted in a respective opening in the base plate  332  so that each secondary drive gear  344  is held in place on the worm gear extension  342  of its respective worm gear shaft  340 . 
     A spring  346  is mounted on the worm gear extension  342  of each worm gear shaft  340  between the base plate  334  and the respective secondary drive gears  344 . Each secondary drive gear  344  may move axially along its respective worm gear extension  342  between the base plates  332 ,  334  relative to its associated worm gear shaft  340 , and may also rotate in concert with its associated worm gear shaft  340 , at least when the secondary drive gear  344  is in its engaged position. The springs  346  bias the secondary drive gears  344  toward base plate  332  and away from base plate  334 , such that a gap exists between each secondary drive gear  344  and the base plate  334 . The spring loading of the secondary drive gears  344  by the springs  346  may assist in returning the secondary drive gears  344  to their resting (disengaged) positions after the secondary drive gears  344  are moved into their engaged positions in the manner discussed below. 
     A piston  350  is mounted on each worm gear shaft  340 . Each piston  350  may be connected to one end of a respective mechanical linkage (not shown). The mechanical linkage may prevent each piston  350  from rotating in response to rotation of its respective worm gear shaft  340 . Each piston  350  may be internally threaded to mate with the external threads on its corresponding worm gear shaft  340 . Each piston  350  may thus be configured to move axially relative to its associated worm gear shaft  340  along its respective axis R 1 -R 6  upon rotation of the worm gear shaft  340 . The far end of each mechanical linkage may be connected to a wiper arm of a phase shifter or a pair of phase shifters as is discussed above with reference to  FIG.  3   . Consequently, rotation of a worm gear shaft  340  may result in axial movement of the piston  350  mounted thereon, and this axial movement is transferred via the mechanical linkage  226  to a phase shifter in order to rotate a wiper arm of the phase shifter. 
     A motor  360  is mounted forward of the base plate  332 . A drive shaft  362  extends from the motor  360 . The motor  360  may be used to turn the drive shaft  362  to rotate about an eccentric axis R 7 . A primary drive gear  364  is mounted on the drive shaft  362  and may be formed integrally with the drive shaft  362  in some embodiments. The primary drive gear  364  is positioned in the center of a circle defined by the worm gear shafts  340 , and is axially offset along axis R 7  from the secondary drive gears  344  that are mounted on the respective worm gear extensions  342 . As will be discussed in detail below, one or more of the secondary drive gears  344  may be moved axially to engage the primary drive gear  364 , so that rotation of the primary drive gear  364  causes each such engaged secondary drive gear  344  to rotate, which in turn rotates the associated worm gear shafts  340 , thereby resulting in axial movement of the pistons  350 . Herein, when a particular secondary drive gear  344  is engaged with the primary drive gear  364 , the worm gear shaft  340  that the secondary drive gear  344  that is associated therewith is said to be “selected.” The primary drive gear  364  may be rotated in a first direction (e.g., clockwise) to move the pistons  350  on any selected worm gear shaft  340  away from the motor  360 , and may be rotated in a second direction (e.g., counter-clockwise) to move the pistons  350  on any selected worm gear shaft  340  toward the motor  360 . In this fashion, the rotational movement of the drive shaft  362  may be transformed into axial movement by one or more of the pistons  350 . 
     As is further shown in  FIGS.  4 B- 4 E  a magnet  370  and an electromagnet  372  may be mounted on (or adjacent) each worm gear extension  342 , on opposite sides of the springs  346 . An electromagnet refers to a magnet whose strength may be adjusted by application of an electric control signal. The polarity of an electromagnet may be reversed by reversing the polarity of the control signal. In an example embodiment, the electromagnets  372  may be connected to the secondary drive gears  344  and the magnets  370  may be connected to the base plate  334 . An electric control signal may be applied to a selected one of the electromagnets  372  in response to a control signal in order to increase the strength of the “selected” electromagnet  372 . As the magnetic strength is increased, the electromagnet  372  may be strongly attracted to its associated magnet  370 , thereby pulling the “selected” secondary drive gear  344  toward the base plate  334  (and compressing the spring  346 ) so that the secondary drive gear  344  engages the primary drive gear  364 . The remaining secondary drive gears  344  may remain in their “resting” (disengaged) positions and hence are spaced apart from the primary drive gear  364 , and therefore are not in position to drive any of the worm gear shafts  340 . 
     As noted above, an internal cavity  347  is provided in the rear portion  345  of each secondary drive gear  344 . As the secondary drive gear  344  moves axially toward the base plate  334  in response to the electromagnet force, the worm gear extension  342  is received within this internal cavity  347 . The cross-sectional shape of the internal cavity  347  may be the same as the cross-sectional shape of the portion of the worm gear extension  342  that is received therein (with the cross-sectional area of the worm gear extension  342  being slightly smaller so that the worm gear extension  342  may be received within the internal cavity  347 ). Accordingly, rotation of the secondary drive gear  344  will result in rotation of the worm gear extension  342 , which in turn causes rotation of the worm gear shaft  340 . 
       FIGS.  4 B and  4 C  illustrate the default position for the actuator  330  where none of the secondary drive gears  344  are engaged with the primary drive gear  364 .  FIGS.  4 D and  4 E  illustrate the positions of the gears when one of the six secondary drive gears  344  is engaged with the primary drive gear  364 . Notably, since the electromagnets  372  can be controlled independently, any number of the secondary drive gears  344  may be engaged with the primary drive gear  364  at the same time. This may allow phase shifts to be implemented more quickly. 
     Upon receiving a signal from a controller that a phase shift in the antenna is desired, the motor  360  may be activated to rotate the primary drive gear  364  about the axis R 7 . Rotation of the primary drive gear  364  rotates the engaged secondary drive gear  344  about its respective axis (in the example of  FIGS.  4 D- 4 E , axis R 6 ), which in turn rotates the worm gear shaft  340  associated with the secondary drive gear  344  about the axis R 6 . Rotation of the worm gear shaft  340  drives the piston  350  axially along its associated worm gear shaft  340  until the piston  350  reaches a desired position, at which point the motor  360  deactivates. 
     Notably, the actuator assembly  300  is capable of adjusting up to six phase shifters  150 , which is a typical number for a base station antenna, which often include two high band arrays and one low band array, with each array having a transmit phase shifter and a receive phase shifter for each of two polarizations, for a total of four phase shifters per linear array or twelve phase shifters total. Since a single RET actuator may control both polarizations, a total of six RET actuators are required for such an antenna. 
     It will be appreciated that numerous modifications may be made to the actuator assembly  300 . For example, the one or more of the pistons  350  may be replaced by another axially-drivable member. The primary drive gear  364  may be any type of central drive gear, or even another variety of a central drive member, such as a wheel or disc that frictionally engages the secondary drive gears  344 . Similarly, the secondary drive gears  344  may be replaced with another rotary member, such as a wheel or disc that engages the primary drive member  364 . The number of worm gear shafts  340  (and associated structures) may be increased or decreased from six as appropriate depending upon the number of phase shifters that need to be controlled. Numerous other modifications are possible. 
       FIGS.  5 A- 5 E,  6  and  7    illustrate single motor multi-RET actuators according to further embodiments of the present invention. 
       FIG.  5 A  is a schematic block diagram of a portion of a single motor multi-RET actuator  400  that is similar to the single motor multi-RET actuator  330  that is discussed above with reference to  FIGS.  4 A- 4 E . However, in the multi-RET actuator  400 , the positions of one or more of the electromagnets  372  and the permanent magnets  370  are reversed. This is shown schematically in  FIG.  5 A , which uses a block diagram format to illustrate the base plates  332 ,  334 , the drive shaft  362  with the primary drive gear  364  mounted thereon, one of the worm gear shafts  340  with a secondary drive gear  344  mounted on the extension  342  thereof. Various other elements of the multi-RET actuator  400  are not depicted in  FIG.  5 A  such as the other worm gears  340  and their associated secondary drive gears  344  and springs  346 , the motor  360 , the pistons  350 , etc. in order to simplify the drawing. The multi-RET actuator  400  may move a selected one of the secondary drive gears  344  into an engagement with the primary drive gear  364  by applying a control signal to the electromagnet  372  that increases the magnetism of the electromagnet  372  in order to attract the permanent magnet  370  toward the electromagnet  372 , thereby moving a selected one of the secondary drive gears  344  into engagement with the primary drive gear  364 . 
     It will also be appreciated that the electromagnet  372  may be configured to repel the permanent magnet  370  by switching the polarity of the control signal supplied to the electromagnet. When a repelling force is used as opposed to an attractive force, the configuration of the electromagnet  372 , the permanent magnet  370  and each secondary drive gear  344  may be changed.  FIGS.  5 B and  5 C  are schematic block diagrams of a portion of a single motor multi-RET actuator  500  according to still further embodiments of the present invention that illustrate how a repelling force may be used in other embodiments of the present invention. 
     As shown in  FIG.  5 B , the multi-RET actuator  500  may be similar to the multi-RET actuator  400 , except that the electromagnet  372  and the permanent magnet  370  are moved to the other side of the secondary drive gear  344 . The permanent magnet  370  may be mounted on or otherwise connected to the secondary drive gear  344  so that axial movement of the permanent magnet  370  results in axial movement of the secondary drive gear  344 . The spring  346  may bias the permanent magnet  370  (and hence the secondary drive gear  344 ) toward the electromagnet  372 . As shown in  FIG.  5 B , in this position, the secondary drive gear  344  is disengaged from the primary drive gear  364 . When a control signal is applied to the electromagnet  372 , a magnetism of the electromagnet  372  may be greatly increased. The electromagnet  372  is oriented so that the magnetic force repels the permanent magnet  370 . This repulsive magnetic force may exceed the counter-acting bias force applied by the spring  346 , and hence, as shown in  FIG.  5 C , when the electromagnet  372  is activated by the control signal, the secondary drive gear  344  is moved into engagement with the primary drive gear  364  so that rotational movement of the primary drive gear  364  results in rotational movement of the secondary drive gear  344  (and hence rotation of the worm gear shaft  340 ). 
       FIG.  5 D  is a schematic block diagram of a single motor multi-RET actuator  600  that is very similar to the multi-RET actuator  500 , with the only difference being that the permanent magnet  370  has been moved to the other side of the secondary drive gear  344 . The multi-RET actuator  600  may operate identically to the multi-RET actuator  500 , but this modified embodiment is depicted to make clear that the positions of the electromagnet  372  and/or the permanent magnet  370  may be changed without materially effecting operation of the device. It will also be appreciated that if the secondary drive gear  344  (or something attached thereto) is formed of a ferromagnetic material, the permanent magnet  370  may be omitted in any of the embodiments disclosed herein. Alternatively, the permanent magnets  370  in any of the embodiments disclosed herein may be replaced with a structure that is formed of or includes a ferromagnetic material that is attracted (or repelled, depending upon the orientation) from the electromagnet  372  when the electromagnet is activated. The ferromagnetic structure may have the same shape as the permanent magnet  370  or may have a different shape. The use of such ferromagnetic materials may be advantageous in some embodiments as it may reduce or eliminate any crosstalk between magnets that are in close proximity to each other, and also will reduce the possibility that other structures in the actuator are unintentionally magnetized such as the lead screw. 
       FIG.  5 E  is a schematic block diagram of a single motor multi-RET actuator  700  that is very similar to the multi-RET actuator  600  of  FIG.  5 D , with the only difference being that an additional electromagnet  372  is provided adjacent the base plate  334 . The two electromagnets  372  are labelled  372 - 1  and  372 - 2  for ease of description of this embodiment. The electromagnet  372 - 1  may impart a repulsive force on the permanent magnet  370  in response to a control signal, while the electromagnet  372 - 2  may impart an attractive force on the permanent magnet  370  so that the two electromagnets  372 - 1 ,  372 - 2  work together to overcome the bias force of the spring  346  that is mounted on the worm gear extension  342  in order to move the secondary gear  344  into engagement with the primary drive gear  364 . 
     In the above-described embodiments, electromagnets are provided that are used to selectively move one or more of the secondary drive gears  344  into engagement with the primary drive gear  364 . Pursuant to further embodiments of the present invention, the primary drive gear  364  may instead be moved into engagement with a selected one of the secondary drive gears  344 .  FIG.  6    is a schematic block diagram of a single motor multi-RET actuator  800  according to embodiments of the present invention in which the primary drive gear  364  is moved as opposed to the secondary drive gears  344 . To simplify the figure, only two of the worm gear shafts  340  and their associated extensions  342  and secondary drive gears  344  are illustrated in  FIG.  6   . It will be appreciated, that more than two worm gear shafts  340  and their associated elements may be provided. As shown in  FIG.  6   , the two secondary drive gears  344  are axially offset from each other so that when the primary drive gear  364  is engaged with one of the secondary drive gears  344  it is not engaged with the other of the secondary drive gears  344 . If more than two secondary drive gears  344  are provided, the additional secondary drive gears  344  may likewise be axially offset from each of the other secondary drive gears  344 . 
     As shown in  FIG.  6   , the electromagnet  372  is mounted on the primary drive gear  364  while the permanent magnet  370  is mounted on or adjacent the base plate  334 . A control signal may be applied to the electromagnet  372  to increase the magnetism thereof so that the electromagnet  372  is attracted to the permanent magnet  370 , thereby pulling the electromagnet  372  (and the primary drive gear  364 ) axially along the drive shaft  362 . The drive shaft  362  may, for example, have a transverse cross-section that is non-circular such as, for example, a square transverse cross-section. This may allow the primary drive gear  364  to move axially along the drive shaft  362  while also ensuring that rotation of the drive shaft  362  will result in rotation of the primary drive gear  364 . Different control signals may be used depending upon which of the secondary drive gears  344  is to be selected. For example, if the primary drive gear  364  is to engage the secondary drive gear  344 - 1 , then the electromagnet  372  may be caused to exhibit a first level of electromagnetic force that is sufficient to move the primary drive gear  364  to compress the spring  366  a first amount so that the primary drive gear  364  engages secondary drive gear  344 - 1 . If the primary drive gear  364  is to engage the secondary drive gear  344 - 2 , then the electromagnet  372  may be caused to exhibit a second, greater, level of electromagnetic force that is sufficient to move the primary drive gear  364  to compress the spring  366  a second amount so that the primary drive gear  364  engages secondary drive gear  344 - 2 . The secondary drive gears  344  may be offset by axial amounts that are sufficient so that variation in the attraction force between the electromagnet  372  and the permanent magnet  370  and or variation in the bias force of the spring  366  that may occur over time due to aging of components or due to other magnetic, friction or other forces is sufficient so that the primary drive gear  364  will always engage the selected one of the secondary drive gears  344 . 
     In the embodiment of  FIG.  6   , it may be necessary for the primary drive gear  364  to move a greater distance, particularly if the multi-RET actuator  800  includes a relatively large number of secondary drive gears  344  (e.g., 6). This may require the use of a more powerful electromagnet  372  and/or a more powerful permanent magnet  370 . Additionally, the technique described above where two electromagnets  372  may also be used. It will also be appreciated that the positions of the electromagnets  372  and the permanent magnets  370  may be varied in the manner discussed above with reference to  FIGS.  5 A- 5 E  in the embodiment of  FIG.  6   . 
       FIG.  7    is a schematic block diagram of a single motor multi-RET actuator  900  according to further embodiments of the present invention that has a primary drive gear  364  that may be moved in two different directions along the drive shaft  362  in order to reduce the amount of electromagnetic force that may be necessary in operation. 
     As shown in  FIG.  7   , the single motor multi-RET actuator  900  is similar to the single motor multi-RET actuator  800  of  FIG.  6   , except that the multi-RET actuator  900  includes an additional spring  366  (the two springs are labeled  366 - 1  and  366 - 2  in  FIG.  7   ), an additional electromagnet  372  (the two electromagnets are labeled  372 - 1  and  372 - 2  in  FIG.  7   ) and an additional permanent magnet  370  (the two permanent magnets are labeled  370 - 1  and  370 - 2  in  FIG.  7   ). In  FIG.  7   , the multi-RET actuator is illustrated as including a total of six worm gear shafts  340  and associated elements (e.g., secondary drive gears  344 ) to better illustrate the operation thereof. Note that only four of the worm gear shafts  340  and worm gear extensions  342  are visible in  FIG.  7    because of the side view, although the secondary drive gears  344  that are associated with the hidden worm gear shafts  340  are visible. It will be appreciated that the multi-RET actuator  900  may include a different number of worm gear shafts  340 . 
     In its resting position, the primary drive gear  364  may be axially located at approximately a midpoint between the base plates  332 ,  334 . As shown in  FIG.  7   , three of the secondary drive gears  344  are located axially to the left of the midpoint, while the other of the secondary drive gears  344  are located axially to the right of the midpoint. A spring  366 - 1  is mounted on the drive shaft  362  to the right of the midpoint, and a spring  366 - 2  is located on the drive shaft  362  to the left of the midpoint. The electromagnets  372 - 1 ,  372 - 2  are mounted on the primary drive gear  364  while the permanent magnets  370 - 1 ,  370 - 2  are mounted at the far ends of the respective springs  366 - 1 ,  366 - 2  from the primary drive gear  364 . 
     If, for example, a phase shifter attached via a mechanical linkage to a worm gear shaft  340  associated with one of the secondary drive gears  344  that is to the left of the midpoint needs adjustment, a controller (not shown) may send a control signal to the electromagnet  372 - 2  to increase the attractive force between electromagnet  372 - 2  and permanent magnet  370 - 2 . As a result, the primary drive gear  364  may move to the left, compressing spring  366 - 2  to a degree, so that the primary drive gear  364  engages the desired secondary drive gear  344 . If instead a phase shifter attached via a mechanical linkage to the worm gear shaft  340  associated with one of the secondary drive gears  344  that are to the right of the midpoint needs adjustment, then electromagnet  372 - 1  may be supplied a control signal so that a magnetic force is generated that moves the primary drive gear  364  to the right to engage the desired secondary drive gear  344 , which is the situation shown in  FIG.  7   . In each of the above cases, both electromagnets  372 - 1  and  372 - 2  may be used to move the primary drive gear  364  by controlling one of the electromagnets  372  to generate an attractive magnetic force and the other to generate a repelling magnetic force in a manner similar to the discussion of the embodiment of  FIG.  5 E  above. 
     While electromagnetic force provides one mechanism for moving the primary drive gear  364  into engagement with a selected one of the secondary drive gears  344 , or vice versa, it will be appreciated that embodiments of the present invention are not limited to the use of such electromagnetic forces. Instead, embodiments of the present invention extend to any mechanical force that may be applied in response to a control signal. For example,  FIG.  8    is a schematic block diagram of a single motor multi-RET actuator  1000  according to still further embodiments of the present invention that uses a piezoelectric actuator to connect a selected mechanical linkage to a motor. 
     As shown in  FIG.  8   , the multi-RET actuator  1000  may be similar to the multi-RET actuator  400  of  FIG.  5 A , except that the electromagnet  372  and permanent magnet  370  are replaced with a piezoelectric actuator  380 . Piezoelectric actuators are known in the art and use the piezoelectric effect to effect a physical movement. The piezoelectric effect refers to an electric charge that may accumulate in certain solid materials such as crystals in response to applied mechanical stress. The piezoelectric effect is thus a linear electromechanical interaction between the mechanical and the electrical state in crystalline materials. The piezoelectric effect is a reversible process in that materials exhibiting the direct piezoelectric effect (the internal generation of electrical charge resulting from an applied mechanical force) also exhibit the reverse piezoelectric effect (the internal generation of a mechanical strain resulting from an applied electrical field). Piezoelectric actuators apply an electrical field to generate a mechanical strain. 
     A separate piezoelectric actuator  380  may be provided for each of the secondary drive gears  344  and may be configured to move the respective secondary drive gears  344  into engagement with the primary drive gear  364  in response to respective control signals. While only one embodiment of the present invention is illustrated in the figures that includes a piezoelectric actuator  380 , it will be appreciated that the electromagnets/permanent magnets  372 / 370  of each of the other embodiments disclosed herein may be replaced with piezoelectric actuators to provide a plurality of additional embodiments. 
     Piezoelectric actuators tend to only provide a small amount of physical, mechanical movement (often referred to as “stroke”), which can be a limitation in some applications. For example, a typical piezoelectric material may only provide 0.1% strain, meaning that a 1 meter piece of piezoelectric material may be required to obtain a stroke of 1 mm. Amplified piezoelectric actuators may be used to mitigate this limitation in some embodiments. 
     It will be appreciated that numerous modifications may be made to the above-described example embodiments without departing from the scope of the present invention. As one example, the above described embodiments implement the primary drive gear as a central gear and the secondary drive gears as gears that circumferentially surround the central primary drive gear. It will be appreciated that in other embodiments the secondary drive gears may only partially circumferentially surround a central primary drive gear, or that the drive gears may have a different arrangement such as the secondary drive gears being linearly arranged. In such an embodiment the central drive gear could move to engage a respective one of the secondary drive gears or an intermediate gear that is engaged with the primary drive gear could move to engage a selected on of the secondary drive gears. Numerous other arrangements are possible. In each case an electromagnetic engagement mechanism and/or a piezoelectric engagement mechanism may be used to move one or more of the gears so that the primary drive gear may rotate a selected one (or more) of the secondary drive gears. 
     Pursuant to further embodiments of the present invention, multi-RET actuator assemblies are provided that include a main motor and a plurality of small “micro-motors” that are used together to, for example, serially actuate multiple mechanical linkages. While these multi-RET assemblies increase the total number of motors used, the six micro-motors may be less expensive than a single conventional motor, and the micro-motors may be highly reliable and hence may involve less risk of failure in the field as compared to some other options.  FIGS.  9 A- 9 E  illustrate a multi-RET actuator  1130  that may be used as part of such a multi-RET actuator assembly. While  FIGS.  9 A- 9 E  only depict the multi-RET actuator  1130 , it will be appreciated that the multi-RET actuator may be incorporated, for example, into the multi-RET actuator assembly  300  of  FIG.  4 A  in place of the multi-RET actuator  330 . 
     Referring now to the figures,  FIG.  9 A  is a side view of the multi-RET actuator  1130 ,  FIG.  9 B  is an enlarged, partial side view of the multi-RET actuator  1130  with one of the secondary drive gears thereof engaged with the primary drive gear,  FIG.  9 C  is a partial side sectional view of the multi-RET actuator  1130 ,  FIG.  9 D  is a partial side perspective view of the multi-RET actuator  1130  with none of the secondary drive gears engaged with the primary drive gear, and  FIG.  9 E  is a partial side perspective view of the multi-RET actuator  1130  with one of the secondary drive gears engaged with the primary drive gear. 
     Referring first to  FIGS.  9 A,  9 C and  9 D , the multi-RET actuator  1130  includes a pair of circular base plates  1132 ,  1134  that may be mounted within a housing (not shown) of the multi-RET actuator assembly (e.g., within housing  310  of multi-RET actuator assembly  300 ). A third base plate  1136  is provided at the distal end of the actuator  1130 . The base plates  1132 ,  1134 ,  1136  may be identical to the base plates  332 ,  334 ,  336  of multi-RET actuator  330  and hence further description thereof will be omitted herein. Six generally parallel worm gear shafts  1140  are provided that extend along respective generally parallel axes between base plates  1134  and  1136 . Each worm gear shaft  1140  includes a worm gear extension  1142  that is rotatably mounted in the base plate  1134 . A secondary drive gear  1144  is axially aligned with each worm gear extension  1142 . As shown best in  FIG.  9 C , each worm gear extension  1142  may extend partially into an internal cavity  1147  of its associated secondary drive gear  1144 . Each internal cavity  1147  extends deeper into the secondary drive gear  1144  than necessary to receive the worm gear extension  1142  of its mating worm gear shaft  1140 , which allows each secondary drive gear  1144  to move axially towards its associated worm gear shaft  1140 . A rod-like rear portion of each secondary drive gear  1144  is mounted in a respective opening in the base plate  1132 . A spring  1146  is mounted on each worm gear extension  1142 . Each secondary drive gear  1144  may move axially along its respective worm gear extension  1142 , and may also rotate in concert with its associated worm gear shaft  1140  when the secondary drive gear  1144  is in its engaged position so that it engages the primary drive gear  1164 . The springs  1146  bias the secondary drive gears  1144  toward base plate  1132 . The worm gear shafts  1140 , worm gear extensions  1142 , secondary drive gears  1144  and springs  1146  may be identical to the corresponding worm gear shafts  340 , worm gear extensions  342 , secondary drive gears  344  and springs  346  of multi-RET actuator  330  and hence further description thereof will be omitted herein. 
     An internally threaded piston  1150  is mounted on each externally threaded worm gear shaft  1140 . Each piston  1150  may be connected to a respective mechanical linkage (not shown). When a selected one of the worm gear shafts  1140  is rotated, the mechanical linkage that is connected to the piston  1150  that is mounted on the selected worm gear shaft  1140  prevents the piston  1150  from rotating. As the externally threaded worm gear shaft  1140  rotates, the piston  1150  moves axially relative to the worm gear shaft  1140  along the axis of rotation of the worm gear shaft  1140 , which in turn imparts the same axial movement to the mechanical linkage that is connected to the piston  1150 . The far end of each mechanical linkage may be connected to a phase shifter or a pair of phase shifters such as the phase shifters of  FIG.  3   . Thus, rotation of a worm gear shaft  1140  may impart axial movement to the piston  1150  and its associated mechanical linkage  226  that is used to rotate a wiper arm of a phase shifter. 
     A main motor  1160  is mounted forward of the base plate  1132 . A drive shaft  1162  extends from the main motor  160 . The main motor  1160  may be used to rotate the drive shaft  1162 . A primary drive gear  1164  is mounted on the drive shaft  1162  and may be formed integrally with the drive shaft  1162 . The primary drive gear  1164  is positioned in the center of a circle defined by the worm gear shafts  1140 , and is axially offset from the secondary drive gears  1144 . The secondary drive gears  1144  may be moved axially to engage the primary drive gear  1164 , so that rotation of the primary drive gear  1164  rotates each engaged secondary drive gear  1144 , which in turn rotates the associated worm gear shafts  1140 , thereby resulting in axial movement of the pistons  1150 . 
     As is further shown in  FIGS.  9 A,  9 C and  9 D  a micro-motor  1170  is mounted on each of the secondary drive gears  1144  forwardly of base plate  1132 . The micro-motors  1170  may be small and relatively inexpensive. Each micro-motor  1170  has an associated externally threaded drive shaft  1172  that rotates when its associated micro-motor  1170  is activated. The drive shafts  1172  may be rotated clockwise or counter-clockwise by the micro-motors  1170 . An internally threaded piston  1174  is mounted on each externally threaded drive shaft  1172 . A rear end of each piston  1174  is attached to a front portion of a respective one of the secondary drive gears  1144 . When one of the micro-motors  1170  rotates in, for example, the clockwise direction, the piston  1174  mounted thereon moves rearwardly along the axis of the drive shaft  1172 . This can best be seen in  FIG.  9 C , where the piston  1174 - 1  is shown in its retracted position while piston  1174 - 2  has been moved rearwardly into an extended positon by activation of micro-motor  1170 - 2 . As piston  1174 - 2  moves rearwardly, it pushes secondary drive gear  1144 - 2  rearwardly as well, compressing the spring  1146 - 2 , so that the geared portion of secondary drive gear  1144 - 2  engages the primary drive gear  1164 . As the secondary drive gear  1144 - 2  is pushed axially toward the base plate  1134  by the micro-motor  1170 - 2 , the worm gear extension  1142 - 2  is received within the internal cavity  1147  in secondary drive gear  1144 - 2 . The remaining secondary drive gears  1144  may remain in their “resting” (disengaged) positions and hence are spaced apart from the primary drive gear  1164 . 
     Upon receiving a signal from a controller that a phase shift in the antenna is desired, the motor  1160  may be activated to rotate the primary drive gear  1164 . Rotation of the primary drive gear  1164  rotates the engaged secondary drive gear  1144 - 2  about its respective axis. The cross-sectional shape of the internal cavity  1147  may be the same as the cross-sectional shape of the portion of the worm gear extension  1142 - 2  that is received therein so that rotation of the selected secondary drive gear  1144 - 2  by the primary drive gear  1164  results in rotation of the worm gear extension  1142 - 2 , which in turn causes rotation of the worm gear shaft  1140 - 2 . Rotation of the worm gear shaft  1140 - 2  drives the piston  1150  mounted thereon axially until it reaches a desired position, at which point the motor  1160  is deactivated. 
     It should be noted that multiple of the secondary drive gears  1144  may be moved into their engaged positions at the same time so that the main drive gear  1164  may move multiple of the pistons  1150  simultaneously. This may allow phase shifts to be implemented more quickly. 
       FIGS.  9 A and  9 D  illustrate the default position for the multi-RET actuator  1130  where none of the secondary drive gears  1144  are engaged with the primary drive gear  1164 .  FIGS.  9 B,  9 C and  9 E  illustrate the positions of the gears when one of the six secondary drive gears  1144  is engaged with the primary drive gear  1164 . 
     It will be appreciated that numerous modifications may be made to the multi-RET actuator  1130 , including the modifications discussed above with respect to multi-RET actuator  330 . 
     Pursuant to yet additional embodiments of the present invention, multi-RET actuator assemblies are provided that use a drive motor and a stepper motor to actuate multiple mechanical linkages. Examples of such embodiments are depicted in  FIGS.  10 A- 10 F . In particular,  FIG.  10 A  is perspective view of a multi-RET actuator assembly  1200  according to further embodiments of the invention.  FIG.  10 B  is a perspective view of the multi-RET actuator  1200  with the housing removed therefrom.  FIG.  10 C  is a perspective view of a multi-RET actuator  1230  that is included in the multi-RET actuator assembly  1200  of  FIGS.  10 A- 10 B .  FIG.  10 D  is a perspective view of the multi-RET actuator  1230  with the motors, cam plate and one base plate removed.  FIG.  10 E  is a side view of the multi-RET actuator  1230 .  FIG.  10 F  is another perspective view of the actuator  1230  with the motors, cam plate and one base plate removed. 
     The multi-RET actuator assembly  1200  is shown in  FIG.  10 A . The actuator assembly  1200  includes a housing  1210  with a pair of connectors  1220  mounted on one end wall  1212  thereof and a multi-RET actuator  1230  is mounted within the housing  1210 . The housing  1210  may be formed of any appropriate material, such as a metal or polymeric material. 
     Referring to  FIG.  10 B , the connectors  1220  may be mounted on a printed circuit board  1222  in some embodiments. The circuit board  1222  is mounted next to the end wall  1212  so that the connectors  1220  extend through the end wall  1212 . The connectors  1220  may connect to communications cables that may be used to deliver control signals from a base station control system to the multi-RET actuator assembly  1200 . 
     Referring now to  FIGS.  10 B- 10 F , the actuator  1230  includes a pair of circular base plates  1232 ,  1234  that are mounted within the housing  1210 . A third base plate  1236  may be provided at the distal end of the assembly  1200 . Six generally parallel worm gear shafts  1240  are provided that extend along respective axes between base plates  1234  and  1236 . The worm gear shafts  1240  are distributed generally circumferentially equidistant from each other. 
     Each worm gear shaft  1240  has a worm gear extension  1242  extending from the forward end thereof through base plate  1234 . Each worm gear extension  1242  may be formed integrally with its corresponding worm gear shaft  1240 . Each worm gear shaft  1240  and its corresponding worm gear extension  1242  are rotatably mounted in the base plate  1234 . A selector gear  1244  is mounted axially on each work gear extension  1242  so that each worm gear extension extends axially into an internal cavity within the selector gear  1244 . A spring  1246  is mounted on each worm gear extension between the base plate  1234  and the selector gear  1244 . Each spring  1246  biases its associated selector gear  1244  away from the base plate  1234  and toward base plate  1232 , such that a gap exists between each selector gear  1244  and the base plate  1234 . The spring loading of the selector gears  1244  by the springs  1246  may assist in returning the selector gears  1244  to their resting (disengaged) positions after the selector gears  1244  are moved into their engaged positions in the manner discussed below 
     Each selector gear  1244  is mounted onto its respective worm gear extension  1242  so that the selector gear  1244  can move axially between the base plates  1232 ,  1234  relative to the worm gear extension  1242 . The end of each worm gear extension  1242  may have a cross-section that corresponds to the cross-section of the internal cavity of its corresponding selector gear  1244  so that rotation of the selector gear  1244  causes corresponding rotation of the worm gear extension  1242  and the worm gear shaft  1240  that the worm gear extension  1242  extends from. 
     A piston  1250  is mounted on each worm gear shaft  1240  and is configured (e.g., via threads) to move axially relative to the worm gear shaft  1240  along its respective axis upon rotation of the worm gear shaft  1240 . Each piston  1250  is connected to a mechanical linkage (not shown) that associates the piston  1250  with one or more phase shifters of an antenna, such that axial movement of the piston  1250  can cause at least one phase shift in the antenna. For example, axial movement of the piston  1250  can be used to move the wiper arm of the phase shifter  150  of  FIG.  3   . 
     Referring now to  FIGS.  10 B- 10 D , a ringed cam plate  1270  is mounted forwardly and spaced apart from base plate  1232 . The cam plate  1270  has a nubbed cam  1272  that extends toward the base plate  1232 . A ring gear  1274  with teeth on its inner diameter extends axially from the cam plate  1270  and is positioned for rotation about a central axis that extends generally in parallel and in the center of the axes defined by the worm gear shafts  1240 . A cam plate drive motor  1276  is eccentrically mounted to rotate about an eccentric axis R; a gear (not shown) on a shaft (not shown) attached to the cam plate drive motor  1276  engages the teeth of the ring gear  1274 . 
     Referring again to  FIGS.  10 B- 10 F , a stepper gear motor  1260  is mounted collinearly with the ring gear  1274  forward of the base plate  1232 . A stepper gear  1264  is mounted to a drive shaft  1262  of the stepper gear motor  1260  and is positioned adjacent the base plate  1232  for rotation about the central axis. The stepper gear  1264  may be formed integrally with the drive shaft  1262 . The stepper gear  1264  is positioned in the center of a circle defined by the worm gear shafts  1240 , and is axially offset from the stepper gears  1244  that are mounted on the respective worm gear extensions  1242  when the stepper gears  1244  are in their resting (disengaged) positions. The stepper gear  1264  is sized so that its teeth can engage the teeth of a selector gear  1244  when the selector gear  1244  is in position adjacent the base plate  1234 . 
     In operation, the cam plate  1270  is rotated about the central axis to an orientation in which the cam  1272  is positioned between the forward ends of two the selector gears  1244 . When the cam  1272  is in this position, all of the selector gears  1244  are positioned to be spaced from the base plate  1234 . Accordingly, all of the selector gears  1244  are disengaged from the stepper gear  1264 , and therefore are not in position to drive any of the worm gear shafts  1240 . As such, in this disengaged position, all of the pistons  1250  remain in place on their respective worm gear shafts  1240 . 
     Upon a signal from a controller that a phase shift in the antenna is desired, the cam plate drive motor  1276  is activated and begins to rotate the cam plate  1270  about the central axis through interaction between the gear of the cam plate drive motor  1276  and the teeth of the ring gear  1274 . As the cam plate  1270  rotates about the central axis, the cam  1272  serially engages each of the forward ends of the stepper gears  1244  and forces them toward the base plate  1234  and into position for engagement with the stepper gear  1264 . Continued rotation of the cam plate  1270  about the central axis moves the cam  1272  past the forward end of a respective one of the selector gears  1244 , allowing the spring loading of the selector gear  1244  to return the selector gear  1244  to its rest position. 
     When the cam  1272  reaches the forward end of the selector gear  1244  associated with the piston  1250  that is to be moved to induce the phase shift in the antenna, the cam plate drive motor  1276  ceases to move, thereby allowing cam  1272  to remain in engagement with the forward end of the selector gear  1244 . Engagement of the forward end of the selector gear  1244  by the cam  1272  moves the selector gear  1244  rearwardly toward the base plate  1234  and into engagement with the stepper gear  1264  (this is shown in  FIGS.  10 D and  10 F ). The stepper gear motor  1260  then activates and rotates the stepper gear  1264  about the central axis. Rotation of the stepper gear  1264  rotates the engaged selector gear  1244  about its respective axis, which in turn rotates the worm gear shaft  1240  associated with the selector gear  1244  about the axis of the worm gear shaft  1240 . Rotation of the worm gear shaft  1240  drives the piston  1250  axially along the worm gear shaft  1240  until the piston  1250  reaches a desired position, at which point the stepper gear motor  1260  deactivates. The cam plate  1270  can either remain in position or move to a rest position to await the next phase shift instruction. The stepper gear  1264  may be rotated in a first direction (e.g., clockwise) to move the pistons  1250  on any selected worm gear shaft  1240  away from the stepper motor  1260 , and may be rotated in a second direction (e.g., counter-clockwise) to move the pistons  1250  on any selected worm gear shaft  1240  toward the stepper motor  1260 . 
     The actuator  1230  is capable of adjusting up to six mechanical linkages via the six pistons  1250 , each of which controls one or more phase shifters. In other embodiments, more or fewer linkages may be included. 
     Those of skill in this art will recognize that other variations of the actuator  1230  may be employed. For example, the pistons  1250  may be replaced by another axially-drivable member. The stepper gear  1264  may be any type of central drive gear, or even another variety of a central drive member, such as a wheel or disc that frictionally engages the selector gears  1244 . The selector gears  1244  may be replaced with another rotary member, such as a wheel or disc that engages the central drive member. The cam plate  1270  and ring gear  1274  may be replaced with another engagement mechanism that selectively and exclusively engages one shaft at a time. The cam plate  1270  may have a recess rather than a cam  1272 , such that a respective selector gear  1244  moves toward the base plate  1232  when the recess rotates in front of the selector gear, with engagement of the selector gear  1244  or other rotary member with the stepper gear  1264  occurring at a position spaced apart from, rather than adjacent to, the base plate  1234 . Drive units other than the stepper gear motor  1260  and the cam plate drive motor  1276  may be employed. Other variations may also be apparent to those of skill in this art. 
     Pursuant to still further embodiments of the present invention, multi-RET actuators are provided that use a single motor and a ratchet-based gear system to actuate multiple mechanical linkages. Examples of such embodiments are depicted in  FIGS.  11 A- 11 C . These multi-RET actuators may be similar to the single-motor multi-RET actuator  330  discussed above with reference to  FIGS.  4 A- 4 E , except the electromagnetic system for moving the secondary drive gears included in the multi-RET actuator  330  is replaced in multi-RET actuator  1330  with a ratchet based gear system. The ratchet based gear system is similar to the gear system included in the multi-RET actuator  1230  discussed above, but the use of ratcheted gears eliminates any need for a second motor. 
     Referring first to  FIG.  11 A , which is a schematic front view of the multi-RET actuator  1330  that illustrates various gears thereof, it can be seen that the multi-RET actuator  1330  includes a plurality of secondary drive gears  1344 , a forward-direction primary drive gear  1364 , a reverse direction primary drive gear  1366  and a reversing gear  1368 . The multi-RET actuator  1330  may include circular base plates, worm gear shafts, worm gear extensions, springs and pistons that may be identical in both structure and arrangement to the base plates  1132 ,  1134 ,  1136 , the worm gear shafts  1140 , the worm gear extensions  1142 , the springs  1146  and the pistons  1150  of multi-RET actuator  1130 , and hence further description thereof will be omitted herein. 
       FIG.  11 B  is a schematic top view of the various gears included in multi-RET actuator  1330 . A portion of one of the six worm gear shafts  1340 - 1  and its associated worm gear extension  1342 - 1  and spring  1346 - 1  are also illustrated in  FIG.  11 B , as is the circular base plate  1334  that abuts the forward ends of the worm gear shafts  1340 . 
     As shown in  FIG.  11 B , a drive shaft  1362  of the single motor (not shown) of multi-RET actuator  1330  has three gears mounted thereon, namely the forward-direction primary drive gear  1364 , the reverse direction primary drive gear  1366  and an indexing gear  1374 . The forward-direction primary drive gear  1364  and the reverse direction primary drive gear  1366  are each ratcheted gears that only rotate in response to clockwise rotation of the drive shaft  1362  and which do not rotate in response to counter-clockwise rotation of the drive shaft  1362 . A ringed cam plate  1370  is provided that may be located in the same position as the cam plate  1270  of multi-RET actuator  1230 , and which is similar in design thereto. The ringed cam plate  1370  includes a circular channel  1378  on the rear surface thereof (shown in dotted lines in  FIG.  11 B  which illustrates what a cross-section of the cam plate  1370  would look like), although it will be appreciated that the channel  1378  may be omitted in other embodiments. The ringed cam plate  1370  includes a fixed cam plate gear  1376  on a front surface thereof. The cam plate gear  1376  is positioned such that it is permanently engaged with the indexing gear  1374  that is mounted on drive shaft  1362 . The cam plate  1370  further includes a nubbed cam  1372  on its rear surface that extends toward the base plate  1334 . The cam  1372  is located in the channel  1378  so that the cam fills the channel  1378  and extends out of the channel  1378  as shown in  FIG.  11 B . 
     The cam plate  1370  is mounted for rotation about a central axis thereof (which may be the axis defined by the drive shaft  1362 ). The indexing gear  1374  is a ratchet gear that only rotates when the drive shaft rotates in a particular direction. For purposes of the discussion herein, it is assumed that the ratcheted indexing gear  1374  only rotates when the drive shaft rotates in the counter-clockwise direction, and that the forward-direction primary drive gear  1364  and the reverse-direction primary drive gear  1366  only rotate when the drive shaft rotates in the clockwise direction. It will be appreciated, however, that these directions may be reversed in other embodiments. 
     When the motor  1360  (not shown) rotates the drive shaft  1362  in the counter-clockwise direction, the indexing gear  1374  rotates in the clockwise direction. As noted above, a toothed cam plate gear  1376  is formed on the cam plate  1370 . As the indexing gear  1374  is mounted so that the teeth thereof are in permanent engagement with the teeth of cam plate gear  1376 , rotation of the indexing gear in the clockwise direction causes counter-clockwise rotation of the cam plate  1370  (since the cam plate  1370  is fixed to the cam plate gear  1376 ). Thus, by rotating the drive shaft  1362  in the counter-clockwise direction it is possible to rotate the cam plate  1370  in the counter-clockwise direction. The nubbed cam  1372  on cam plate  1370  may then be used to “select” one of the secondary drive gears  1344  in the same manner that the nubbed cam  1272  may be used to select one of the secondary drive gears  1244  of multi-RET actuator  1230 . Accordingly, further description of the operation of cam plate  1370  and cam  1372  will be omitted. 
     As is also shown in  FIG.  11 B , the reversing gear  1368  is mounted for rotation on a shaft  1369  that extends rearwardly from the cam plate  1370 . The reversing gear  1368  is axially aligned with each secondary drive gear  1344  and with the reverse-direction primary drive gear  1366  (i.e., they are each at the same distance from the circular base plate  1334 ). The reversing gear  1368  is positioned so that the teeth thereof permanently engage the teeth of the reverse-direction primary drive gear  1366 , and so that the teeth of the reversing gear  1368  engage the teeth of each secondary drive gear  1344  when the reverse-direction primary drive gear  1366 , the reversing gear  1368  and the secondary drive gear  1344  at issue are radially aligned. 
     The multi-RET actuator  1330  may operate as follows. In order to move a piston (not shown) that is mounted on a first of the worm gear shafts  1340 - 1  in a first direction (which we will assume here is the forward direction toward base plate  1334 ), the motor is activated to move the drive shaft  1362  in the counter-clockwise direction. As discussed above, this causes the indexing gear  1374  to rotate in the counter-clockwise direction which, via its interaction with the cam plate gear  1376 , causes the cam plate  1370  to rotate in the counter-clockwise direction. The cam plate  1370  is rotated until the cam  1372  engages the forward end of secondary drive gear  1344 - 1  (i.e., the secondary drive gear that is associated with the piston that is to be moved). As cam  1372  engages secondary drive gear  1344 - 1 , the secondary drive gear is pushed rearwardly so that the toothed section thereof engages for the forward-direction primary drive gear  1364 . When this occurs, the motor is shut off. The cam plate  1370  may then be left in place or may be rotated further. When the cam plate  1370  is further rotated, the cam  1372  disengages from the selected secondary drive gear  1344 , and the spring  1346  associated with the selected secondary drive gear  1344  pushes the selected secondary drive gear  1344  back into its resting position. 
     In order to move the piston in the forward direction, the motor is turned back on in the opposite direction so that the drive shaft  1362  rotates in the clockwise direction. As discussed above, the indexing gear  1374  is ratcheted and hence does not rotate in response to the clockwise rotation of the drive shaft  1362 . However, the forward-direction and reverse-direction primary drive gears  1364 ,  1366  are oppositely ratcheted, and hence both of these gears  1364 ,  1366  rotate in the clockwise direction in response to the clockwise rotation of the drive shaft  1362 . 
     As the secondary drive gears  1344  are circumferentially spaced at equal distances, the secondary drive gears  1344  may be radially spaced apart from each other at 60° intervals. As shown schematically in  FIG.  11 A , the reversing gear  1368  and the cam  1372  may be spaced apart from each other by about 30°. As a result, when the cam  1372  is used to select one of the secondary drive gears  1344  in the manner described above, the reversing gear  1368  may be radially positioned about midway between two of the secondary drive gears  1344 , and hence is not in contact with any of the secondary drive gears  1344 . 
     As the drive shaft  1362  rotates in the clockwise direction, both the forward-direction primary drive gear  1364  and the reverse-direction primary drive gear  1366  rotate in the clockwise direction. The reverse-direction primary drive gear  1366  rotates the reversing gear  1368 , but as the reversing gear  1368  does not engage any of the secondary drive gears  1344 , this rotation has no effect. The clockwise rotation of the forward-direction primary drive gear  1364  results in counter-clockwise rotation of the selected secondary drive gear  1344 - 1 . The counter-clockwise rotation of the selected secondary drive gear  1344 - 1  results in counter-clockwise rotation of the worm gear shaft  1340 - 1 , which causes the piston mounted thereon to move in the forward direction toward base plate  1334 . 
     In order to move the piston associated with secondary drive gear  1344 - 1  in the rearward direction (i.e., away from base plate  1334 ), the motor is activated to move the drive shaft  1362  in the counter-clockwise direction. As discussed above, this causes the cam plate  1370  to rotate in the counter-clockwise direction. The cam plate  1370  is rotated until the reversing gear  1368  is radially aligned with the selected secondary drive gear  1344 - 1  so that the teeth on the reversing gear  1368  engage the teeth on the reverse-direction drive gear  1366  and the teeth of the selected secondary drive gear  1344 - 1 . Note that when the cam plate  1370  is rotated to this position, the cam  1372  is radially positioned between two of the secondary drive gears  1344 , and hence all six of the secondary drive gears  1344  remain in their resting positions (i.e., the position shown in  FIG.  11 B ). 
     Once the reversing gear  1368  has been rotated to engage the selected secondary drive gear  1344 - 1 , the motor reverses direction to rotate the drive shaft  1362  in the clockwise direction. As the indexing gear  1374  is ratcheted, it does not rotate in response to the clockwise rotation of the drive shaft  1362  and hence the cam plate  1370  remains stationary. The forward-direction and reverse-direction primary drive gears  1364 ,  1366  rotate in the clockwise direction in response to the clockwise rotation of the drive shaft  1362 . 
     As all of the secondary drive gears  1344  are in their respective resting positions, the rotation of the forward-direction primary drive gear  1364  does not have any effect. However, the clockwise rotation of the reverse-direction primary drive gear  1366  results in counter-clockwise rotation of the reversing gear  1368 , which in turn results in clockwise rotation of the selected secondary drive gear  1344 - 1 . The clockwise rotation of the selected secondary drive gear  1344 - 1  results in clockwise rotation of the worm gear shaft  1340 - 1 , which causes the piston mounted thereon to move in the rearward direction, away from base plate  1334 . Thus, as described above, the motor in conjunction with the ratcheted gear system described above may be used to select any of the worm gear shafts  1340  and move a piston mounted thereon in either direction. 
       FIG.  11 C  conceptually illustrates the operation of the drive shaft  1362  and the ratcheted gears  1364 ,  1366 ,  1374  attached thereto. Note that to avoid undesired movements of non-selected ones of the secondary drive gears  1344  when the index gear  1374  is being moved, the torque of each secondary drive gear  1344  should be greater than the torque of the reversing gear  1368  plus the torque of the drive reverse-direction primary drive gear  1366 . 
     It should be noted that the forward-direction primary drive gear  1364  and the reverse-direction primary drive gear  1366  need only move the pistons  1150  in opposite directions. The actual direction (i.e., forward or reverse along the worm gear shafts  1140 ) of movement of the pistons is arbitrary. 
     The multi-RET actuator  1330  of  FIGS.  11 A- 11 C  may be viewed as comprising a plurality of shafts (e.g., the worm gear shafts  1340  and their associated worm gear extensions  1342 ) that have respective axially-drivable members (e.g., the pistons  1350 ) mounted thereon. Each of axially-drivable member may be configured to be connected to a respective one of a plurality of phase shifters. The multi-RET actuator  1330  further includes a motor  1360  having a drive shaft  1362  and a gear system that is configured to selectively couple the motor  1360  to the respective shafts  1340 / 1342 . The gear system is configured so that rotation of the drive shaft  1362  in a first direction creates a mechanical linkage between the motor  1360  and a first of the shafts  1340 / 1342 , and rotation of the drive shaft  1362  in a second direction that is opposite the first direction rotates the first of the shafts  1340 / 1342 . 
     The gear system may include a forward-direction primary drive gear  1364  that is connected to the drive shaft  1362  and a reverse-direction primary drive gear  1366  that is connected to the drive shaft  1362 . The forward-direction primary drive gear  1364  and the reverse-direction primary drive gear  1366  are each ratcheted gears that rotate in response to rotation of the drive shaft  1362  in the second direction and which do not rotate in response to rotation of the drive shaft  1362  in the first direction. The gear system may further include a reversing gear  1368  that is configured to engage the reverse-direction primary drive gear  1366  and rotate in a direction opposite a direction of rotation of the reverse-direction primary drive gear  1366 . The gear system may also include a plurality of secondary drive members (e.g., the secondary drive gears  1344 ) that are mounted on respective ones of the shafts  1340 / 1342 , each secondary drive member  1344  mounted so that rotation thereof will result in rotation of a respective one of the shafts  1340 / 1342 . The gear system may also include an engagement mechanism (e.g., the cam plate  1370 ) that is configured to rotate to selectively and exclusively engage one or more of the shafts  1340 / 1342  to move a selected one of the secondary drive members  1344  into engagement with one of the forward-direction primary drive gear  1364  or the reversing gear  1368 . 
     Pursuant to further embodiments of the present invention, methods of adjusting a phase shifter are provided. These methods may be implemented using, for example, the multi-RET actuator  1330  of  FIGS.  11 A- 11 C . Pursuant to these methods, a drive shaft (e.g., drive shaft  1362 ) is rotated in a first direction to connect a first of a plurality of gears (e.g., secondary drive gear  1344 - 1 ) to a drive mechanism. The drive shaft  1362  is then rotated in a second direction to rotate a gear of the drive mechanism, wherein rotation of the gear of the drive mechanism causes rotation of the first of the plurality of gears  1344 , and rotation of the first of the plurality of gears  1344  mechanically adjusts a physical position of a component of the phase shifter. 
     The plurality of gears may be secondary drive gears  1344  that are configured to rotate respective shafts such as worm gear shafts  1340 . The drive mechanism may include a forward-direction primary drive gear  1364  that is connected to the drive shaft  1362  and a reverse-direction primary drive gear  1366  that is connected to the drive shaft  1362 . The forward-direction primary drive gear  1364  may be a ratcheted gear that only rotates in response to rotation of the drive shaft in a first direction, and the reverse-direction primary drive gear  1366  may be a ratcheted gear that only rotates in response to rotation of the drive shaft  1362  in the first direction. The plurality of gears may further include a reversing gear  1368 . At least one of the forward-direction primary drive gear  1364  or the reverse-direction primary drive gear  1366  may be configured to engage the first of the plurality of gears  1344 - 1  through the reversing gear  1368 . 
     While  FIG.  3    above depicts a conventional wiper-arc type phase shifter, numerous other types of electromechanical phase shifters are known in the art. It will be appreciated that the actuators disclosed herein are suitable for use with a wide variety of different phase shifters. 
     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 components may be exaggerated for clarity. 
     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. 
     Herein, the terms “attached”, “connected”, “interconnected”, “contacting”, “mounted” and the like can mean either direct or indirect attachment or contact between elements, unless stated otherwise. 
     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. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising”, “includes” and/or “including” when used in this specification, specify the presence of stated features, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, operations, elements, components, and/or groups thereof.