Patent Publication Number: US-2023139300-A1

Title: Remote electronic tilt actuators for controlling multiple phase shifters and base station antennas with remote electronic tilt actuators

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority from and the benefit of U.S. Provisional Patent Application No. 62/947,595, filed Dec. 13, 2019, the entire content of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to communication systems and, in particular, to base station antennas having remote electronic tilt capabilities. 
     BACKGROUND 
     Cellular communications systems are used to provide wireless communications to fixed and mobile subscribers (herein “users”). A cellular communications system may include a plurality of base stations that each provide wireless cellular service for a specified coverage area that is typically referred to as a “cell.” Each base station may include one or more base station antennas that are used to transmit radio frequency (“RF”) signals to, and receive RF signals from, the users that are within the cell served by the base station. Base station antennas are directional devices that can concentrate the RF energy that is transmitted in 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 such as the cell or a portion thereof that is typically referred to as a “sector.” The base station antenna may be designed to have minimum gain levels throughout its pre-defined coverage area, and it is typically desirable that the base station antenna have much lower gain levels outside of the coverage area to reduce interference between sectors/cells. 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 by transmitting control signals to the antenna. Base station antennas having such capabilities are typically referred to as remote electronic tilt (“RET”) antennas. The most common changes to the radiation pattern are changes in the down tilt angle (i.e., the elevation angle) and/or the azimuth angle. RET antennas allow wireless network operators to remotely adjust the radiation pattern of the antenna by transmitting control signals to the antenna that electronically alter the RF signals that are transmitted and received by the antenna. 
     Base station antennas typically comprise a linear array or a two-dimensional array of radiating elements such as patch, dipole or crossed dipole radiating elements. In order to electronically change the down tilt angle of these antennas, a phase taper may be applied across the radiating elements of the array, 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 widely-used type of phase shifter is an electromechanical “wiper” phase shifter that includes a main printed circuit board and a “wiper” printed circuit board that may be rotated above the main printed circuit board. Such wiper phase shifters typically divide an input RF signal that is received at the main printed circuit board into a plurality of sub-components, and then capacitively couple at least some of these sub-components to the wiper printed circuit board. The 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 (mechanically) rotating the wiper printed circuit board above the main printed circuit board, the locations where the sub-components of the RF signal capacitively couple back to the main printed circuit board may be changed, which thus changes the length of the respective transmission path from the phase shifter to an associated radiating element for each sub-component of the RF signal. The changes in these path lengths result in changes in the phases of the respective sub-components of the RF signal, and since the arcs have different radii, the phase changes along the different paths will be different. 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 by reference 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 OF THE INVENTION 
     In some embodiments, a base station antenna comprises a remote electronic tilt (“RET”) actuator. A first mechanical linkage is connected between the RET actuator and a first phase shifter, and a second mechanical linkage is connected between the RET actuator and a second phase shifter. The RET actuator comprises a rotary drive element movable in a first rotary direction and a second rotary direction. A first drive system is connected between the rotary drive element and the first mechanical linkage where the first drive system moves the first mechanical linkage in a first linear direction and a second linear direction when the rotary drive element is moved in the first rotary direction. A second drive system is connected between the rotary drive element and the second mechanical linkage where the second drive system moves the second mechanical linkage in a third linear direction and a fourth linear direction when the rotary drive element is moved in the second rotary direction. 
     The rotary drive element may comprise a motor having a rotary output. A first one-way clutch may selectively connect the rotary drive element to the first drive system and a second one-way clutch may selectively connect the rotary drive element to the second drive system. The first one-way clutch and the second one-way clutch may each comprise a cam gear supporting a pivoting pawl where the cam gear is operably coupled to the rotary drive element; and a ratchet wheel having a plurality of teeth operably coupled to a clutch output where the pawl engages the teeth such that rotation of cam gear in a first direction causes the ratchet wheel to rotate with the cam gear and rotation of cam gear in a second direction allows the ratchet wheel to rotate independently of the cam gear. A first worm gear may be mounted for rotation with a clutch output of the first one-way clutch for transmitting rotation of the clutch output of the first one-way clutch to the first drive system and a second worm gear may be mounted for rotation with a clutch output of the second one-way clutch for transmitting rotation of the clutch output of the second one-way clutch to the second drive system. The first drive system may comprise a belt. The belt may be wound over a first pulley and a second pulley. The first pulley and the second pulley may include teeth that engage teeth on the belt. The belt may include a first run and a second run between the first pulley and the second pulley. The first run may move in an extension direction and the second run may move in a retraction direction. The first run and the second run may be selectively operably coupled to the first mechanical linkage by a first linkage system. The first linkage system may comprise a first stopper plate and a second stopper plate where the distance between the first stopper plate and the second stopper plate sets the maximum distance of travel of the first mechanical linkage. The first stopper plate may be positioned adjacent the first pulley and the second stopper plate may be positioned adjacent the second pulley. The first stopper plate may comprise a first curved track and the second stopper plate may comprise a second curved track where the first curved track faces the second curved track. At least one of the first stopper plate and the second stopper plate may comprise a longitudinally extending track. A drive rod may be mounted for reciprocating movement where the drive rod may be operatively coupled to the first mechanical linkage. The drive rod may be mounted for slidable movement in the longitudinally extending track. The drive rod may have a generally T-shape with the longitudinal leg of the drive rod supported in the longitudinally extending track. The drive rod may comprise a first arm and a second arm where the first arm extends over the first run and the second arm extends over the second run. A belt connector may releasably connect the drive rod to the first run of the belt and to the second run of the belt. The first arm may include a first engagement structure positioned to engage a belt connector that is mounted on and carried by the belt, and the second arm may include a second engagement structure positioned to engage the belt connector. The first engagement structure may comprise a first aperture positioned to receive a pin on the belt connector and the second engagement structure may comprise a second aperture positioned to receive the pin. The belt connector may be biased toward the drive rod. A first camming plate may be positioned at the leading edge of the first stopper plate and a second camming plate may be positioned at the leading edge of the second stopper plate. The first camming plate and the second camming plate may drive the belt connector away from the drive rod. The first camming plate may disengage the first engagement structure from the connector and the second camming plate may disengage the second engagement structure from the connector. A linkage connector may be rotatably mounted to the drive rod about a rotational axis. The linkage connector may comprise a stub. The stub may be aligned with the rotational axis of the linkage connector. The stub may engage the first stopper plate and the second stopper plate to set a first stop position and a second stop position of the first mechanical linkage. The linkage connector may comprise a shaft aligned with the rotational axis of the linkage connector. A linkage arm may be connected between the shaft and the belt. The linkage arm may be extensible and retractable between the shaft and the belt. The linkage connector may comprise a cam pin. The cam pin may be disposed such that the cam pin can enter and traverse the first curved track and the second curved track. When one of the first stop position and the second stop position is reached, the belt may be free to travel. When the drive rod reaches the first stop position and the second stop position, the cam pin may be positioned directly outside of one end of the first track and the second track, respectively. When the drive rod reaches the first stop position and the second stop position, the cam pin may traverse the first track and the second track, respectively, as the belt travels. When the drive rod reaches the first stop position and the second stop position, the linkage connector and cam pin may rotate about the shaft. When the drive rod reaches the first stop position and the second stop position, the linkage arm may follow the path of travel of belt and may rotate the shaft about its longitudinal axis to propel the cam pin through the first track and the second track, respectively. When the drive rod reaches the first stop position and the second stop position and the cam pin traverses the first track and the second track, respectively, the belt connector may follow the path of the belt. When the drive rod reaches the first stop position and the second stop position and the cam pin reaches an end of the first track and the second track, respectively, the belt connector may connect the drive rod to the belt. 
     In some embodiments, a RET actuator comprises a rotary drive element movable in a first rotary direction and a second rotary direction. A first drive system having a first linear output connected to the rotary drive element such that the first drive system moves the first linear output in a first linear direction and a second linear direction when the rotary drive element is moved in the first rotary direction. A second drive system having a second linear output connected to the rotary drive element such that the second drive system moves the second linear output in a third linear direction and a fourth linear direction when the rotary drive element is moved in the second rotary direction. 
     In some embodiments, a method of adjusting a phase shifter of a base station antenna comprising a remote electronic tilt (“RET”) actuator, a plurality of phase shifters, a first mechanical linkage connected between the RET actuator and a first phase shifter, and a second mechanical linkage connected between the RET actuator and a second phase shifter is provided. The method comprises rotating a rotary drive element in one of a first rotary direction and a second rotary direction; actuating a first drive system connected between the rotary drive element and the first mechanical linkage in response the rotary drive element rotating in the first rotary direction, the first drive system moving the first mechanical linkage in a first linear direction and a second linear direction when the rotary drive element is moved in the first rotary direction; and actuating a second drive system connected between the rotary drive element and the second mechanical linkage in response the rotary drive element rotating in the second rotary direction, the second drive system moving the second mechanical linkage in a third linear direction and a fourth linear direction when the rotary drive element is moved in the second rotary direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1 A  is a perspective view of an example base station antenna according to embodiments of the present invention. 
         FIG.  1 B  is an end view of the base station antenna of  FIG.  1 A . 
         FIG.  1 C  is a schematic plan view of the base station antenna of  FIG.  1 A  that illustrates three linear arrays of radiating elements thereof 
         FIG.  2    is a schematic block diagram illustrating the electrical connections between various components of the base station antenna of  FIGS.  1 A-  1 C . 
         FIG.  3    is a front perspective view of a pair of electromechanical phase shifters that may be included in the base station antenna of  FIGS.  1 A- 2   . 
         FIG.  4    is a rear view of a portion of the RET base station antenna of  FIGS.  1 A- 3    that shows one embodiment of how mechanical linkages are used to connect the output members of the RET actuator to respective ones of the phase shifters illustrated in  FIG.  3   . 
         FIG.  5    is a schematic top view of an embodiment of the RET actuator of the invention. 
         FIG.  6    is a side view of a one-direction clutch used in the RET actuator of  FIG.  4   . 
         FIG.  7    is a perspective view of an embodiment of a drive system used in the RET actuator of  FIG.  4   . 
         FIG.  8    is another perspective view of the drive system of  FIG.  7   . 
         FIG.  9    is another perspective view of the drive system of  FIG.  7    shown in partial phantom lines. 
         FIG.  10    is another perspective view of the drive system of  FIG.  7    shown in partial phantom lines. 
         FIG.  11    is a perspective view of an embodiment of a stopper plate used in the drive system of  FIG.  7   . 
         FIG.  12    is a perspective view of an embodiment of the connector used in the drive system of  FIG.  7   . 
         FIG.  13    is a perspective view of an embodiment of the linkage connector used in the drive system of  FIG.  7   . 
         FIG.  14    is a perspective view of an embodiment of a drive rod used in the drive system of  FIG.  7   . 
         FIG.  15    is a perspective view of an embodiment of a drive belt used in the drive system of  FIG.  7   . 
         FIG.  16    is a perspective view of an embodiment of a camming plate used in the drive system of  FIG.  7   . 
         FIG.  17    is a schematic view illustrating the operation of the drive system of  FIG.  7   . 
         FIG.  18    is a block diagram illustrating a method of operating the RET actuator of  FIG.  5   . 
     
    
    
     DETAILED DESCRIPTION 
     Modern base station antennas often include two, three or more arrays of radiating elements. If the arrays include cross-polarized radiating elements, then a separate phase shifter is provided for each polarization (i.e., two phase shifters per linear array). Moreover, separate transmit and receive phase shifters are often provided for each array so that the transmit and receive radiation patterns may be independently adjusted, which may again double the number of phase shifters. Additionally, in some cases, some (or all) of the arrays may be formed using wideband radiating elements that support service in multiple frequency bands (e.g., the 700 MHz and 800 MHz frequency bands or two or more frequency bands within the 1.7-2.7 GHz frequency range). When such wideband arrays are used, separate phase shifters may be provided for each frequency band within the broader operating frequency range of the radiating elements. Since base station antennas with two to as many as eight arrays of cross-polarized radiating elements are being deployed, it is not uncommon for a base station antenna to have eight, twelve or even twenty-four adjustable phase shifters for applying remote electronic down tilts to the arrays. As described above, RET actuators are provided in the antenna that are used to move elements on the phase shifters to adjust the down tilt angle of the antenna beams formed by the various arrays. While the same down tilt is typically applied to the phase shifters for the two different polarizations, allowing a single RET actuator and a single mechanical linkage to be used to adjust the phase shifters for both polarizations, modern base station antennas still often need four, six, twelve or even more RET actuators. 
     Conventionally, a separate RET actuator was provided for each phase shifter (or each pair of phase shifters if dual polarized radiating elements are used in a linear array). 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 (or twelve) different phase shifters using one so-called “multi-RET actuator.” U.S. Patent Publication No. 2017/0365923 (“the &#39;923 publication”) discloses a number of additional multi-RET actuator designs. 
     As more complex base station antennas are introduced, requiring ever increasing numbers of independently controlled phase shifters, it can become difficult to design base station antennas that fit within customer-demanded limitations on the size of the antenna. RET actuators also include expensive components, such as motors, such that as the number of independently controlled phase shifters increases the cost of providing the RET actuators also increases. 
     Pursuant to embodiments of the present invention, base station antennas are provided that include RET actuators that are less expensive to manufacture and may have a smaller physical footprint. In some embodiments, the RET actuators may include a single motor that controls more than one phase shifter and that can adjust the phase shifters in two different linear directions. The base station antennas pursuant to some embodiments of the present invention may include, among other things, a RET actuator, a plurality of phase shifters and a plurality of mechanical linkages, where each mechanical linkage is connected between the RET actuator and a respective one, or two, of the phase shifters. The RET actuator may comprise a drive element, a single motor that is selectively operably connected to one of a plurality of drive systems to move the selected one of the mechanical linkages in opposite linear directions. 
     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 base station antenna  100  that may include one or more of the RET actuators according to embodiments of the present invention.  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  (with the radome thereof removed) that illustrates three arrays of radiating elements thereof.  FIG.  2    is a schematic block diagram illustrating various components of the base station antenna  100  and the electrical connections therebetween. It should be noted that  FIG.  2    does not show the actual location of the various elements on the antenna, but instead is drawn to merely show the electrical transmission paths between the various elements. 
     Referring to  FIGS.  1 A- 1 C and  2   , the base station antenna  100  includes, among other things, input/output ports  110 , a plurality of arrays  120  of radiating elements  130 , duplexers  140 , phase shifters  150  and control ports  160 . As shown in  FIGS.  1 C and  2   , the base station antenna  100  may include a total of three arrays  120  (labeled  120 - 1  through  120 - 3 ) that each include five radiating elements  130 . It will be appreciated, however, that the number of arrays  120  and the number of radiating elements  130  included in each of the arrays  120  may be varied. It will also be appreciated that different 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. A dashed box is used in  FIG.  2    to illustrate one of the six corporate feed networks included in antenna  100 . Each corporate feed network 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 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 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 port of a radio (not shown) such as a remote radio head. Each transmit phase shifter  150  has five outputs 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 thereto 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 sub-component of the RF signal fed to the first radiating element  130  in a linear array  120  may have a phase of Y°+2X°, the sub-component of the RF signal fed to the second radiating element  130  in the linear array  120  may have a phase of Y°+X°, the sub-component of the RF signal fed to the third radiating element  130  in the linear array  120  may have a phase of Y°, the sub-component of the RF signal fed to the fourth radiating element  130  in the linear array  120  may have a phase of Y°- X°, and the sub-component of the RF signal fed to 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 inputs that are connected to respective ones of the radiating elements  130  through respective duplexers  140  and an output 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  130  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   , the base station antenna  100  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. 
     The RET actuators that are used to physically adjust the settings of the phase shifters  150  are typically spaced apart from the phase shifters  150 . So-called mechanical linkages  170  are used to transfer the motion of a RET actuator to a moveable element of a phase shifter. Each RET actuator may be controlled to generate a desired amount of movement of an output member thereof. The movement may comprise, for example, linear movement or rotational movement. A mechanical linkage  170  is used to translate the movement of the output member of the RET actuator to movement of a moveable element of a phase shifter  150  (e.g., a wiper arm, a sliding dielectric member, etc.). The mechanical linkage  170  may comprise, for example, one or more plastic or fiberglass RET rods  172  that extend between the output member of the RET actuator and the moveable element of the phase shifter  150 . 
     Each phase shifter  150  shown in  FIG.  2    may be implemented, for example, as a rotating wiper phase shifter. The phase shifts imparted by a phase shifter  150  to each sub-component of an RF signal may be controlled by a mechanical positioning system that physically changes the position of the rotating wiper of each phase shifter  150 , as will be explained with reference to  FIG.  3   . 
     Referring to  FIG.  3   , a dual rotating wiper phase shifter assembly  200  is illustrated that may be used to implement, for example, two of the phase shifters  150  of  FIG.  2    (one for each of the two polarizations). 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 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 mechanical linkage  170  (with a RET rod  172  partially shown in  FIG.  3   ) that extends between an output member of a RET actuator and the phase shifter  200 . 
     Each main printed circuit board  210 ,  210   a  includes transmission line traces  212 ,  214 . The transmission line traces  212 ,  214  are generally arcuate. In some cases the arcuate transmission line traces  212 ,  214  may be disposed in a serpentine pattern to achieve a longer effective length. In the example illustrated in  FIG.  3   , there are two arcuate transmission line traces  212 ,  214  per main printed circuit board  210 ,  210   a  (the traces on printed circuit board  210   a  are not visible in  FIG.  3   ), with the first arcuate transmission line trace  212  being disposed along an outer circumference of each printed circuit board  210 ,  210   a,  and the second arcuate transmission line trace  214  being disposed on a shorter radius concentrically within the outer transmission line trace  212 . A third transmission line trace  216  on each main printed circuit board  210 ,  210   a  connects an input pad  230  on each main printed circuit board  210 ,  210   a  to an output pad  240  that is not subjected to an adjustable phase shift. 
     The main printed circuit board  210  includes an input trace  232  leading from the input pad  230  near an edge of the main printed circuit board  210  to the position where the pivot pin  222  is located. RF signals on the input trace  232  are coupled to a transmission line trace (not visible in  FIG.  3   ) on the wiper printed circuit board  220 , typically via a capacitive connection. The transmission line trace on the wiper printed circuit board  220  may split into two secondary transmission line traces (not shown). The RF signals are capacitively coupled from the secondary transmission line traces on the wiper printed circuit board  220  to the transmission line traces  212 ,  214  on the main printed circuit board. Each end of each transmission line trace  212 ,  214  may be coupled to a respective output pad  240 . A coaxial cable  260  or other RF transmission line component may be connected to input pad  230 . A respective coaxial cable  270  or other RF transmission line component may be connected to each respective output pad  240 . As the wiper printed circuit board  220  moves, an electrical path length from the input pad  230  of phase shifter  202  to each 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 mechanical linkage  170  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  170  may be used to control the positions of the wiper printed circuit boards  220 ,  220   a  on both phase shifters  202 ,  202   a.    
       FIG.  4    is a rear view of a portion of the base station antenna  100  that shows how mechanical linkages  160 - 1  and  160 - 2  are used to connect the output members of the RET actuator  300  to moveable elements  220 ,  220   a  of respective pairs of phase shifters  202 - 1  through  202 - 4 . The mechanical linkages  160 - 1  and  160 - 2  are shaded in  FIG.  4    to better show the connection of the mechanical linkage  160 - 1  to the phase shifters  202 - 1  and  202 - 2  and the connection of mechanical linkage  160 - 2  from the RET actuator  300  to the phase shifters  202 - 3  and  202 - 4 . The RET actuator  300  is mounted in the antenna  100  behind the backplane  112 . Multiple pairs of phase shifters may be mounted rearwardly of the backplane  112  (only four pairs of phase shifters are visible in  FIG.  4   ). Since the base station antenna  100  has linear arrays  120 ,  130  that are formed of dual-polarized radiating elements  122 ,  132 , the phase shifters  202  are mounted in pairs since the phase shifter  202  for each polarization will be adjusted the same amount. The RET actuator  300  is connected to the phase shifters  202 - 1  to  202 - 4  by mechanical linkages  160 - 1  and  160 - 2 . The mechanical linkages 160 - 1  and  160 - 2  are provided to connect each output member of the RET actuator  300  to a respective pair of phase shifters  202 . The other phase shifters shown in  FIG.  4    are connected to the RET actuator  300  by additional mechanical linkages and additional phase shifters and mechanical linkages may be provided in the antenna; however, only mechanical linkages  160 - 1 ,  160 - 2  are specifically referenced to simplify the illustrated system. Each mechanical linkage  160 - 1 ,  160 - 2  may comprise a plurality of RET rods  166  connected by linkages  164 . The RET rods  166  may comprise, for example, generally rigid fiberglass or plastic longitudinally-extending rods. The RET rods  166  typically extend in a longitudinal direction of the antenna  100 , while the RET linkages  164  typically extend along the width and/or depth axes to connect two RET rods  166  together, and/or to connect a RET rod  166  to an output member of the RET actuator or to a moveable element of a phase shifter assembly. Each mechanical linkage  160 - 1 ,  160 - 2  is used to transfer a linear movement of the output member of the RET actuator  300  to a wiper board  220  of a phase shifter, although in other embodiments rotational movement may be transferred by the mechanical linkage. In some embodiments, a single RET rod may comprise the mechanical linkages  160 - 1 ,  160 - 2  while in other embodiments, a greater number of RET rods and linkages may be used. Other mechanical linkages shown in  FIG.  4    may include similar combinations of RET rods  166  and RET linkages  164  which may be operatively coupled between additional RET actuators  300  and phase shifters. 
       FIGS.  5  through  17    illustrate the RET actuator  300  of  FIG.  4    in greater detail. The RET actuator  300  may comprise a housing  301  ( FIG.  4   ) that houses the components of the RET actuator  300 . Referring to  FIG.  5   , the RET actuator  300  comprises a single rotary drive element  303  comprising a reversible motor  302  such as a reversible electric motor having a rotary output  304 . Output  304  from motor  302  is coupled to a drive gear  312  such that the drive gear  312  may be rotated in either direction as represented by arrows A and B in  FIG.  5   . In one embodiment, the drive gear  312  comprises a bevel gear. 
     The output of the reversible rotary drive element  303  is selectively connected to one of a plurality of drive systems  306 - 1 ,  306 - 2  that are in turn connected to mechanical linkages  160 - 1 ,  160 - 2  that transmit the output of the drive systems  306 - 1 ,  306 - 2  to phase shifters  202 . Each of the drive systems  306 - 1 ,  306 - 2  is connected to one phase shifter  202  through a mechanical linkage  160 - 1  and  160 - 2 , respectively, such that the illustrated RET actuator  300  controls two phase shifters  202 . One-way clutch drives  310 - 1 ,  310 - 2  selectively connect the output of the rotary drive element  303  to one of the drive systems  306 - 1 ,  306 - 2 . 
     Because the drive systems  306 - 1 ,  306 - 2  and the one-way clutch drives  310 - 1 ,  310 - 2  are substantially identical to one another, drive system  306 - 1  and one-way clutch drive  310 - 1  will be described in detail with it being understood that drive system  306 - 2  and one-way clutch drive  310 - 2  are structured and operate in substantially the same way. 
     Drive gear  312  is coupled to a driven gear  314 - 1  that also comprises a bevel gear. Rotation of drive gear  312  in the direction of arrow A by motor  302  rotates driven gear  314 - 1  in the direction of arrow C, while rotation of drive gear  312  in the direction of arrow B by motor  302  rotates driven gear  314 - 1  in the direction of arrow D. 
     The output  316 - 1  of driven gear  314 - 1  is coupled to clutch  310 - 1 . Clutch  310 - 1  is configured to transmit rotation of driven gear  314 - 1  in direction C to the clutch output  320 - 1  but to not transmit rotation of driven gear  314 - 1  in direction D to the clutch output  320 - 1 . Referring more specifically to  FIG.  6   , in one embodiment, clutch  310 - 1  comprises a cam gear  324 - 1  operably coupled to the driven gear output  316 - 1  such that the cam gear  324 - 1  and the driven gear output  316 - 1  rotate together on a common axis. Clutch  310 - 1  also comprises a toothed ratchet wheel  322 - 1  operably coupled to the clutch output  320 - 1  such that the ratchet wheel  322 - 1  and the clutch output  320  rotate together on a common axis. The cam gear  324 - 1  and the ratchet wheel  322 - 1  are independently rotatable relative to one another about a common axis of rotation. 
     As shown in  FIG.  6   , the ratchet wheel  322 - 1  comprises a plurality of teeth  325  spaced about the periphery thereof. The cam gear  324 - 1  supports a pawl  326  on a pivot pin  328  such that the pawl  326  can pivot relative to the cam gear  324 - 1 . A spring  330  biases the pawl  326  into engagement with the teeth  325  on ratchet wheel  322 - 1 . A stop  332  is positioned such that when the pawl  326  is engaged with the stop  330  the pawl  326  is prevented from rotating counterclockwise about pivot pin  328  as viewed in  FIG.  6   . 
     In operation of the clutch  310 - 1 , when the driven gear  314 - 1  and cam gear  324 - 1  are rotated in the direction of arrow C (by the rotation of motor  302  in direction A), the pawl  326 , which is carried by the cam gear  324 - 1 , is moved into engagement with the tooth  325 a immediately forward of the pawl  326 . Pawl  326  is driven into engagement with stop  332  such that continued rotation of cam gear  324 - 1  in direction C causes the ratchet wheel  322 - 1  to rotate with the cam gear  324 - 1 . When the driven gear  314 - 1  and cam gear  324 - 1  are rotated in the direction of arrow D (by the rotation of motor  302  in direction B), the pawl  326 , which is carried by the cam gear  324 - 1 , is moved into engagement with the tooth  325   b  immediately behind the pawl  326 . The engagement of tooth  325   b  with pawl  326  rotates pawl  326  away from stop  332  such that tooth  325   b  and each successive tooth  325  can pass the pawl  326  such that rotation of cam gear  324 - 1  in direction D does not rotate the ratchet wheel  322 - 1 . The spring  330  returns the pawl  326  to the engaged position when movement of the cam gear  324 - 1  stops. Thus, rotation of the motor  302  in a first direction A causes rotation of ratchet wheel  322 - 1  and clutch output  320 - 1  while rotation of the motor  302  in the second opposite direction B, does not result in movement of the ratchet wheel  322 - 1  and clutch output  320 - 1 . 
     Clutch  310 - 2  is arranged with the opposite orientation such that rotation of the motor in direction B, causes the rotation of ratchet wheel  322 - 2  and clutch output  320 - 2  while rotation of the motor  302  in the direction A, does not cause movement of the ratchet wheel  322 - 2  and clutch output  320 - 2 . As a result, the drive systems  306 - 1  and  306 - 2  may be selectively actuated based on the direction of rotation of motor  302 . While the motor  302  is driven in a first direction to actuate drive system  306 - 1  and in a second direction to actuate drive system  306 - 2 , the drive systems  306 - 1  and  306 - 2  are configured such that rotation of the motor  302  in either of the directions A and B controls movement of the associated mechanical linkages  160 - 1 ,  160 - 2  in two linear directions as will hereinafter be described. 
     A worm gear  340 - 1  is mounted for rotation with the clutch output  320 - 1  and engages a mating gear  342 - 1  to transmit rotation of the output  320 - 1  to the two-way drive system  306 - 1 . The use of a worm gear  340 - 1  to drive mating gear  342 - 1  has the advantage that, while the worm gear  340 - 1  can rotate the mating gear  342 - 1 , the mating gear  342 - 1  cannot rotate the worm gear  340 - 1 . As a result, when the system is not activated, the worm gear  340 - 1  locks the drive system  306 - 1  in place such that inadvertent movement of the drive system and associated mechanical linkage does not occur. 
     The mating gear  342 - 1  is operatively coupled to a toothed pulley  344 - 1  of belt drive  346 - 1  such that the rotation of mating gear  342 - 1  causes the rotation of the pulley  344 - 1 . A toothed belt  348 - 1  runs over toothed pulley  344 - 1  and a second toothed pulley  350 - 1  such that the belt  348 - 1  may be driven in a first rotational direction when motor  302  is driven in the direction of arrow A. The belt  348 - 1  has two runs  347 - 1  and  349 - 1  ( FIG.  8   ) between the pulleys  344 - 1 ,  350 - 1 . A top run  347 - 1  moves in a first linear direction E away from the phase shifter  202  and a bottom run  349 - 1  moves in a second linear direction F toward from the phase shifter  202 . The first linear direction E may be considered the retraction direction and the second linear direction F may be considered the extension direction. The linkage system  360 - 1  allows the mechanical linkage  160 - 1  to be moved in a first linear direction when the linkage system  360 - 1  is coupled to the top run  347 - 1  of belt  348 - 1  and in a second linear direction when the linkage system  360 - 1  is coupled to the bottom run  349 - 1  of belt  348 - 1 . The terms “top run” and “bottom run” are used to distinguish the two portions of belt  348 - 1  between pulleys  344 - 1  and  350 - 1  and are not intended to describe a spatial orientation of the belt drive in use. In actual use, the RET drive  300  may have any spatial orientation such that either run may be above or below the other run and the runs may also be disposed in a side-by-side orientation or at any angle relative to the horizon. 
     The linkage system  360 - 1  will be described in greater detail with reference to  FIGS.  7  through  17   . The linkage system  360 - 1  comprises a first stopper plate  362  and a second stopper plate  364  ( FIG.  11   ). The first stopper plate  362  is positioned over the toothed pulley  344 - 1  and the second stopper plate  364  is positioned over the toothed pulley  350 - 1 . The stopper plates  362 ,  364  have substantially semicircular tracks  366 ,  367 , respectively, formed therein where the tracks  366 ,  367  face one another and open inwardly toward one another. The stopper plate  364  also includes a longitudinally extending track  368  for receiving a drive rod  370  such that the drive rod  370  can linearly reciprocate in the track  368 . While only the second stopper plate  364  requires the longitudinally extending track  368 , both stopper plates  362 ,  364  may be identical to reduce component count. The distance T ( FIG.  5   ) between the facing sides  364   a  and  364   a  of stopper plates  362 ,  364 , respectively, sets the distance of travel of the drive rod  370  and the mechanical linkage  160 - 1 . This distance T may be varied by increasing or decreasing the distance between stopper plates  362 ,  364 . The maximum range of the adjustment of the phase shifter is determined by the length of the belts  348 - 1 ,  348 - 2 . In some embodiments, the maximum travel range is 68 mm but 0-10 degrees of tilt may be obtained for an antenna with the phase shifter moving by 30 mm (these values depend on antenna length, tilt value etc.). 
     Drive rod  370  ( FIG.  14   ) has a generally T-shape with the longitudinal leg  372  of drive rod  370  supported in the track  368 . The leg  372  is reciprocally mounted in the track  368  such that it can move linearly along its length relative to stopper plate  364 . The linear movement of leg  372  corresponds to the linear movement of the mechanical linkage  160 - 1  that is connected to the drive rod  370 . The leg  372  may be directly coupled to a RET rod or other linkage to transfer movement of the drive rod  370  to the mechanical linkage  160 - 1 . 
     The drive rod  370  also includes two extending arms  374 ,  376  that extend from leg  372  and are disposed at substantially right angles relative thereto. One of the arms  374 ,  376  extends over one of the runs  347 - 1 ,  349 - 1  of the belt  348 - 1 , respectively. Each arm  374 ,  376  includes an engagement structure  378  located near the distal end of the arm and positioned to engage a belt connector  380  that is mounted on and carried by belt  348 - 1 . In the illustrated embodiment the engagement structure comprises an aperture. A through-hole  382  is formed between the arms  374 ,  376  along the longitudinal axis of the drive rod  370  for receiving a linkage connector  384 . 
     The belt connector  380  ( FIG.  12   ) comprises a flat flange  390  having a first engagement structure  386  extending from one side thereof. The engagement structure  386  can releasably connect to the engagement structure  378  on the arms  374 ,  376  of the drive rod  370 . The engagement structure  386  in the illustrated embodiment comprises pin that fits into the apertures  378  on arms  374 ,  376 . The aperture  378  and pin  386  may be reversed such that the pin is formed on the arm and the aperture is formed on the belt connector in other embodiemnts. Moreover, other engagement structures may be used provided that the engagement structures may be released by moving the belt connector  380  away from the arms  374 ,  376  as will be described. A second pin  388  extends from the opposite side of flange  390 . While the flange  390  is show as circular, the flange  390  may have any shape. The first pin  386  may be slidably received in either of the apertures  378  on arms  374 ,  376  such that the first pin  386  may be removed from the apertures  378  by moving the pin  386  along its longitudinal axis away from arms  374 ,  376  and out of the apertures  378 . The second pin  388  is received in an aperture  392  formed on the belt  348 - 1  ( FIG.  15   ). The belt  348 - 1  may include a thickened portion  395  that defines the aperture  392 . A biasing mechanism such as a compression spring  393  is disposed in the aperture  392  and engages the belt connector  380  to bias the belt connector  380  away from the belt  348 - 1  and toward the drive rod  370 . When the first pin  386  is engaged with one of the apertures  378  on arms  374 ,  376  and the second pin  388  is engaged with the belt  348 - 1 , the drive rod  370  is connected to and moves with the belt  348 - 1 . 
     The linkage connector  384  ( FIG.  13   ) comprises a shaft  394  that extends from a first side of flange  396 . The shaft  394  is rotatably supported in the through-hole  382  in drive rod  370  such that the linkage connector  384  may rotate about shaft  394  relative to the drive rod  370 . A stub  396  extends from a second side of the disk  396  opposite to shaft  394 . The stub  396  is coaxially disposed with the shaft  394  along the rotational axis of the linkage connector  384 . A cam pin  398  also extends from the second side of the disk  396  radially spaced from the stub  396 . The cam pin  398  is disposed such that it can enter and traverse the track  366  in the first stopper plate  362  and the track  367  in the second stopper plate  364 . A linkage arm  400  is connected between the shaft  394  and the belt  348 - 1 . The distal end of the linkage arm  400  remote from shaft  394  connects to the belt  348 - 1  at the location of aperture  392  by a suitable pivoting connector  395 . The pivoting connector may connect to the underside of belt  348 - 1 . The linkage arm  400  is extensible along its longitudinal axis between the shaft  394  and the belt  348 - 1 . The linkage arm  400  may comprise a telescoping arm, spring, spring cylinder, pneumatic or hydraulic cylinder where the length of the linkage arm  400  is freely extensible and retractable. 
     A camming plate  402  ( FIG.  16   ) is positioned at the leading edge of each of the stopper plates  362 ,  364 . Each camming plate  362 ,  364  is aligned with the belt connector  380 . The camming plate  402  may be mounted to the bottom of stopper plates  362 ,  364 , as shown, or the camming plates  402  may be mounted on other structures. Each camming plate  402  may comprise a member having a tapered or beveled leading edge  402   a  that is positioned to engage the first side of the flange  390  of belt connector  380  as the belt connector  380  approaches the stopper plates  362 ,  364 . The beveled leading edge  402   a  of camming plate  402  is inserted between the flange  390  and the arms  374 ,  376  to drive the belt connector  380  against the bias of spring  393  and away from arms  374 ,  376 . The camming plate  402  is dimensioned such that the first pin  386  of belt connector  380  is withdrawn from apertures  378  of drive rod  30  as the flange  390  traverses the camming plate  402 . 
     The operation of the drive system  306 - 1  will now be described. As previously described, the drive system  306 - 1  is actuated based on the direction of rotation of the motor  302 . For purposes of explanation, it is assumed that the motor  302  is rotated in the direction of arrow A such that drive system  306 - 1  is activated. Referring to  FIGS.  7  -  10   , when the motor  302  is rotated in the direction of arrow A, belt  348  is moved as shown by arrow M. Belt  348 - 1  of drive system  306 - 1  is always moved in direction M. As shown in  FIGS.  7  -  10   , the drive system  306 - 1  is positioned with the drive rod  370  and arms  374 ,  376  positioned midway between the stopper plates  362  and  364 , although the drive rod  370  may be positioned at any point between the stopper plates  362 ,  364  when the motor  302  is actuated. The belt connector  380  connects drive rod  370  to run  349 - 1  of belt  348 - 1 . When the motor  302  is actuated and belt  348 - 1  rotates in direction M, run  349 - 1  moves linearly in the direction of arrow F. The drive rod  370  is also extended linearly from the RET drive  300  in the direction of arrow F and the mechanical linkage  160 - 1 , attached to the drive rod  370 , is also extended linearly to thereby move the adjustable member of the phase shifter in a first direction. Specifically, as the belt  348 - 1  moves in the direction M, the engagement of the belt connector  380  with belt  348 - 1  and arm  374  of drive rod  370 , transmits movement of belt  348 - 1  to the drive rod  370  and the associated mechanical linkage  160 - 1 . 
     When the drive rod  370  reaches stopper plate  364 , movement of drive rod  370  and its associated mechanical linkage  160 - 1  in direction F is stopped. It is noted that the system may be stopped at any position before this end stop position to position the drive rod  370  and associated mechanical linkage  160 - 1  at any intermediate position between the stopper plates  362 ,  364 . The end stop positions are the fullest extended and retracted positions of the drive rod  370  and associated mechanical linkage  160 - 1 . Specifically, the end stop positions occurs when stub  396  engages either stopper plate  364  or stopper plate  362  and prevents further extension or retraction, respectively, of the drive rod  370 . When the stub engages stopper plate  362 , the end stop position is the fullest retracted position of the drive rod  370  and associated mechanical linkage  160 - 1 . When the stub engages stopper plate  364 , the end stop position is the fullest extended position of the drive rod  370  and associated mechanical linkage  160 - 1 . However, even though movement of the drive rod  370  is stopped, the belt  348  may continue to travel in the rotational direction of arrow M as explained below. 
     When the drive rod  370  reaches the extended end stop position, the cam pin  398  is positioned directly outside of the input end of track  367  in stopper plate  364 . The cam pin  398  enters the track  367  as the belt  348 - 1  continues to move in the rotational direction M. As the belt  348 - 1  continues to move, the linkage connector  384  and cam pin  398  rotate about shaft  394 . Referring more specifically to  FIG.  17   , the movement of the linkage connector  384  and cam pin  398  will be described. The center of track  367  is aligned with the axis of rotation of pulley  350 - 1  on plane A-A. The end  364   a  of stopper plate  364  is disposed offset from the axis of rotation of pulley  350 - 1  and the center of track  366  by distance D. Track  367  has a semicircular shape that follows the size and shape of the path of belt  348 - 1  about pulley  350 - 1 . As belt  348  moves in direction M, cam pin  398  moves in track  366 . Simultaneously, the end  395  of linkage arm  400  that is connected to the belt  348  follows the path of travel of belt  348  around pulley  350 . As the end  395  of linkage arm  400  traverses the path about pulley  350 , the linkage arm  400  rotates shaft  394  about its longitudinal axis to propel cam pin  398  in a circular arc through circular track  367 . 
     Because the end  364   a  of the stopper plate  364  is positioned offset from the axis of rotation of pulley  350 , the distance between the axis of rotation of shaft  394  and the end  395  of linkage arm  400  connected to belt  348 - 1  increases as the belt  348 - 1  winds around pulley  350 - 1 . To accommodate this change in distance, the linkage arm  400  can extend and retract as previously described. As the cam pin  398  traverses the semi-circular track  367 , the engagement of the cam pin  398  with the track  367  holds the linkage connector  384  in position against the stopper plate  364  and holds the drive rod  370  in the end position. 
     The cam pin  398  traverses track  367  until it reaches the opposite output end of track  367 . Simultaneously, the belt connector  380  follows the path of the belt  348  as it winds about pulley  350 . The end of pin  386  may be biased against the underside of stopper plate  364  by spring  393  as the belt connector  380  rotates about shaft  394 . As the belt connector  380  follows the path of belt  348 , the belt connector  380  reaches the front edge of the stopper plate  364  at the same time that the cam pin  384  reaches the output end of track  366 . The belt connector  380  with pin  386  passes under arm  376  of the drive rod  370  where the pin  386  on belt connector  380  is inserted into the aperture  378  formed on the bottom of arm  376  under the bias force of spring  393 . In this positon, the belt connector  380  connects the drive rod  370  to the belt  348 - 1  on the second run  347 - 1  of the belt  348 - 1 . If the motor  302  continues to drive the belt  348  in the direction M, the belt  348 - 1  moves the drive rod  370  and the associated mechanical linkage  160 - 1  in the opposite linear direction E to retract the drive rod  370  and the associated mechanical linkage  160 - 1 . The retracting movement of the drive rod  370  and associated mechanical linkage moves the movable element of the phase shift in the opposite direction to that of the extending movement of the drive rod. 
     The motor  302  can actuate belt  348  to move the drive rod  370  to any positon between the stopper plate  364  and the stopper plate  362  to thereby adjust the phase shifter to the desired position. When the drive rod  370  and associated mechanical linkage are properly positioned, the motor  302  is deactivated and the drive rod  370  is stopped in the desired position. 
     If it is necessary to extend the drive rod  370  to move the phase shifter in the opposite direction F, the motor  30  is again actuated to move the linkage system  360 - 1  into engagement with the stopper plate  362 . The linkage system  360 - 1  reverses direction as previously described with respect to stopper plate  364  such that the connection between the drive rod  370  and the belt  348  is again made on the first run  349 - 1  of the belt  348 . The motor  302  and belt  348  may be driven to position the drive rod  370  at any position along the first run  349 - 1  to extend the drive rod  370  and associated mechanical linkage  160 - 1  to any extended position. 
     Once the drive rod  370  and the associated mechanical linkage are properly positioned, the motor  302  is deactivated and movement of the belt  348  is halted. The worm gear  340 - 1  holds the system in the desired position to lock the movable element of the phase shifter in the selected position. 
     The motor may be rotated in direction A to adjust the first drive system  306 - 1  and its associated mechanical linkage as described. Alternatively, the motor  302  may be rotated in the opposite direction B to adjust the second drive system  306 - 2 . Linkage system  360 - 2  operates in the same manner as linkage system  360 - 1  to move its drive rod and associated mechanical linkage  160 - 2  in either of two linear directions. Thus, a single, motor  302  may be used to selectively drive either one of the two drive systems  306 - 1 ,  306 - 2  in both linear directions (retracting and extending) simply by reversing the rotational direction of the motor  302 . 
     The RET actuators according to embodiments of the present invention have various advantages over conventional RET actuators. The RET actuators use a single reversible motor to control movement of multiple phase shifters in two linear directions. The RET actuators may be very compact, and may have a low profile which allows them to readily be installed in a wide variety of different base station antennas. 
     The RET actuators according to embodiments of the present invention are suitable for use in base station antennas. The base station antennas may include any number of arrays of radiating elements (which can, but do not have to be, linear arrays of radiating elements), and the RET actuators may be used to control phase shifters that are associated with the arrays of radiating elements. 
     Pursuant to further embodiments of the present invention, methods of adjusting a phase shifter of a base station antenna are provided.  FIG.  18    is a flow chart that illustrates one such method according to embodiments of the present invention. As shown in  FIG.  18   . Operations of the RET actuator are initiated by rotating a rotary drive element in one of a first rotary direction and a second rotary direction (Block  1801 ). A first drive system, connected between the rotary drive element and a first mechanical linkage, is actuated in response the rotary drive element rotating in the first rotary direction (Block  1802 ). The first drive system moves the first mechanical linkage in a first linear direction and a second linear direction when the rotary drive element is moved in the first rotary direction (Block  1803 ). A second drive system, connected between the rotary drive element and a second mechanical linkage, is actuated in response the rotary drive element rotating in the second rotary direction (Block  1804 ). The second drive system moves the second mechanical linkage in a third linear direction and a fourth linear direction when the rotary drive element is moved in the second rotary direction (Block  1805 ). 
     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 
     Components of the various embodiments of the present invention discussed above may be combined to provide additional embodiments. Thus, it will be appreciated that while a component or element may be discussed with reference to one embodiment by way of example above, that component or element may be added to any of the other embodiments.