Patent Publication Number: US-2023164588-A1

Title: Multi-beam base station antennas having wideband radiating elements

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 U.S.C. § 120 to, and is a continuation of, U.S. patent application Ser. No. 17/241,271, filed Apr. 27, 2021, which claims priority under 35 U.S.C. § 120 to, and is a continuation of, U.S. patent application Ser. No. 16/956,229, filed Jun. 19, 2020, which is a 35 U.S.C. § 371 national stage application of PCT Application No. PCT/CN2019/092788, filed on Jun. 25, 2019, the entire content of each of which is incorporated herein by reference as if set forth in their entireties. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to radio communications and, more particularly, to multi-beam base station antennas utilized in cellular and other communications systems. 
     BACKGROUND 
     Cellular communications systems are well known in the art. In a typical cellular communications system, a geographic area is divided into a series of regions that are referred to as “cells,” and each cell is served by a base station. The base station may include baseband equipment, radios and base station antennas that are configured to provide two-way radio frequency (“RF”) communications with subscribers that are positioned throughout the cell. In many cases, the cell may be divided into a plurality of “sectors,” and separate base station antennas provide coverage to each of the sectors. The antennas are often mounted on a tower or other raised structure, with the radiation beam (“antenna beam”) that is generated by each antenna directed outwardly to serve a respective sector. Typically, a base station antenna includes one or more phase-controlled arrays of radiating elements, with the radiating elements arranged in one or more vertical columns when the antenna is mounted for use. Herein, “vertical” refers to a direction that is perpendicular relative to the plane defined by the horizon. 
     A common base station configuration is a “three sector” configuration in which the cell is divided into three 120° sectors in the azimuth plane, and the base station includes three base station antennas that provide coverage to the three respective sectors. The azimuth plane refers to a horizontal plane that bisects the base station antenna and that is parallel to the plane defined by the horizon. In a three sector configuration, the antenna beams generated by each base station antenna typically have a Half Power Beam Width (“HPBW”) in the azimuth plane of about 65° so that the antenna beams provide good coverage throughout a 120° sector. Typically, each base station antenna will include a vertically-extending column of radiating elements that is typically referred to as a “linear array.” Each radiating element in the linear array may have a HPBW of approximately 65° so that the antenna beam generated by the linear array will provide coverage to a 120° sector in the azimuth plane. In many cases, the base station antenna may be a so-called “multi-band” antenna that includes different arrays of radiating elements that operate in different frequency bands. 
     Sector-splitting refers to a technique where the coverage area for a base station is divided into more than three sectors in the azimuth plane, such as six, nine or even twelve sectors. A six-sector base station will have six 60° sectors in the azimuth plane. Splitting each 120° sector into two sub-sectors increases system capacity because each antenna beam provides coverage to a smaller area, and therefore can provide higher antenna gain and/or allow for frequency reuse within a 120° sector. In six-sector sector-splitting applications, a single twin beam antenna is typically used for each 120° sector. The twin beam antenna generates separate antenna beams that each have a reduced size in the azimuth plane (typically about half the size of a normal sector antenna beam) and that each point in different directions in the azimuth plane (typically about −30° and 30° from the boresight pointing direction of the antenna) for at least one frequency band, thereby splitting the sector into two smaller sub-sectors. 
     SUMMARY 
     Pursuant to embodiments of the present invention, twin beam base station antennas are provided that are configured to split a sector of a three-sector base station into first and second sub-sectors. These antennas include a first array that has a plurality of columns of first frequency band radiating elements. The first array is configured to form a first antenna beam that provides coverage throughout the first sub-sector. The radiating elements in a first of the columns in the first array have a first azimuth boresight pointing direction and the radiating elements in a second of the columns in the first array have a second azimuth boresight pointing direction that is offset from the first azimuth boresight pointing direction by at least 10°. The radiating elements in the second of the columns in the first array are electrically steered. 
     In some embodiments, the antenna may further include a second array that has a plurality of columns of first frequency band radiating elements, the second array configured to form a second antenna beam that provides coverage throughout the second sub-sector. In such embodiments, the antenna may also include a column of second frequency band radiating elements, the column of second frequency band radiating elements configured to generate a third antenna beam having an azimuth half power beamwidth that is about twice an azimuth half power beamwidth of the first antenna beam. 
     In some embodiments, the antenna may further include an additional column of first frequency band radiating elements that is configured to generate a fourth antenna beam having an azimuth half power beamwidth that is about twice an azimuth half power beamwidth of the first antenna beam. 
     In some embodiments, the antenna may further include a backplane that includes a reflector, where the first of the columns in the first array is on a first section of the reflector that is perpendicular to an azimuth boresight pointing direction of the twin beam base station antenna and the second of the columns in the first array is on a second section of the reflector that is angled at least 15° from the first section. 
     In some embodiments, a first of the columns in the second array is on a third section of the reflector that is perpendicular to the azimuth boresight pointing direction of the twin beam base station antenna and a second of the columns in the second array is on a fourth section of the reflector that is angled at least 15° from the third section. 
     In some embodiments, a peak gain of the first antenna beam may occur at an azimuth angle that is offset by at least 30° from an azimuth angle where a peak gain of the second antenna beam occurs. 
     In some embodiments, the radiating elements in the second of the columns in the first array may be electrically steered toward the first azimuth boresight pointing direction. 
     In some embodiments, the radiating elements in the second of the columns in the first array may be electrically steered in the azimuth direction by a fixed amount. 
     In some embodiments, the antenna may further include a shared column of radiating elements that is part of the first array and the second array. 
     In some embodiments, the antenna may further include first and second columns of second frequency band radiating elements. The first column of second frequency band radiating elements may be on a first portion of the reflector that is perpendicular to an azimuth boresight pointing direction of the twin beam base station antenna and the second column of second frequency band radiating elements may be on a second portion of the reflector that is perpendicular to an azimuth boresight pointing direction of the twin beam base station antenna. In some embodiments, the first portion of the reflector and the second portion of the reflector may each be positioned rearwardly of a forward most portion of the reflector. 
     In some embodiments, the antenna may further include a backplane that includes a reflector, where the reflector includes a first longitudinally-extending section that is perpendicular to an azimuth boresight pointing direction of the twin beam base station antenna, a second longitudinally-extending section that directly connects to a left side of the first longitudinally-extending section, the second longitudinally-extending section being oriented at a first oblique angle with respect the first longitudinally-extending section, and a third longitudinally-extending section that directly connects to a right side of the first longitudinally-extending section, the third longitudinally-extending section being oriented at a second oblique angle with respect to the first longitudinally-extending section. In some such embodiments, the first angle may be within 5° of the second angle. 
     In some embodiments, the first array may be on a right side of the twin beam base station antenna and the first sub-sector may be the left side of the sector. 
     Pursuant to further embodiments of the present invention, base station antennas are provided that include a first RF port, a first array that includes first and second columns of radiating elements that are both coupled to the first RF port, and a backplane that includes a reflector. The reflector has a first longitudinally extending section that is perpendicular to the azimuth boresight pointing direction of the base station antenna and a second longitudinally extending section that is angled in the azimuth plane by at least 15° from the first longitudinally extending section of the reflector. The radiating elements in the first column are mounted to extend forwardly from the first longitudinally extending section of the reflector, and the radiating elements of the second column are mounted to extend forwardly from the second longitudinally extending section of the reflector. The first and second columns of radiating elements are configured to generate respective first and second radiation patterns that have pointing directions that are within about 10° of each other in the azimuth plane. 
     In some embodiments, the first and second radiation patterns may form at least part of a first antenna beam that provides coverage to half of a sector of a three-sector base station. 
     In some embodiments, the antenna may further include a second RF port and a second array that includes third and fourth columns of radiating elements that are coupled to the second RF port. In such embodiments, the reflector may further include a third longitudinally extending section that is spaced apart from the first longitudinally-extending section and that is perpendicular to the azimuth boresight pointing direction of the base station antenna, and a fourth longitudinally extending section that is angled in the azimuth plane by at least 15° from the third longitudinally extending section of the reflector. The radiating elements in the third column may be mounted to extend forwardly from the third longitudinally extending section of the reflector, and the radiating elements of the fourth column may be mounted to extend forwardly from the fourth longitudinally extending section of the reflector. 
     In some embodiments, the radiating elements in the first and second columns of radiating elements may be first frequency band radiating elements, and the base station antenna may further include a column of second frequency band radiating elements, the column of second frequency band radiating elements configured to generate a third antenna beam having an azimuth half power beamwidth that is about twice an azimuth half power beamwidth of the first antenna beam. 
     In some embodiments, the antenna may further include an additional column of first frequency band radiating elements, the additional column of first frequency band radiating elements configured to generate a fourth antenna beam having an azimuth half power beamwidth that is about twice an azimuth half power beamwidth of the first antenna beam. 
     In some embodiments, the radiating elements in the second column of radiating elements may be electrically steered in the azimuth direction and the radiating elements in the first column of radiating elements may not be electrically steered in the azimuth direction. 
     In some embodiments, the antenna may further include a shared column of radiating elements that is coupled to the first RF port and to the second RF port, the shared column of radiating elements being part of the first array and part of the second array. 
     In some embodiments, the first array may on a right side of the twin beam base station antenna and the first sub-sector may be the left side of the sector. 
     Pursuant to additional embodiments of the present invention, base station antennas are provided that include a reflector that has first through fifth longitudinally-extending sections and first through fifth columns of first frequency band radiating elements that are mounted to extend forwardly from the respective first through fifth longitudinally-extending sections of the reflector. The first longitudinally-extending section of the reflector directly connects to the second longitudinally-extending section of the reflector at a first angle that is at least 5°, the second longitudinally-extending section of the reflector directly connects to the third longitudinally-extending section of the reflector at a second angle that is at least 5°, the third longitudinally-extending section of the reflector directly connects to the fourth longitudinally-extending section of the reflector at a third angle that is at least 5°, and the fourth longitudinally-extending section of the reflector directly connects to the fifth longitudinally-extending section of the reflector at a fourth angle that is at least 5°. Each of the first through fifth longitudinally-extending sections of the reflector defines a respective plane, and axes that are perpendicular to each respective plane are each angled less than 45° in an azimuth plane with respect to a boresight pointing direction of the base station antenna. 
     In some embodiments, the antenna may further include first and second RF ports, and the first through third columns of radiating elements may be are coupled to the first RF port, and the fourth and fifth columns of radiating elements are coupled to the second RF port. In some embodiments, the third column of radiating elements may also be coupled to the second RF port. 
     In some embodiments, the first through third columns of radiating elements may be part of a first array that is configured to form a first antenna beam that provides coverage throughout a first sub-sector of a sector of a three-sector base station and the fourth and fifth columns of radiating elements may be part of a second array that is configured to form a second antenna beam that provides coverage throughout a second sub-sector of the sector. 
     In some embodiments, a peak gain of the first antenna beam may occur at an azimuth angle that is offset by at least 30° from an azimuth angle where a peak gain of the second antenna beam occurs. 
     In some embodiments, the radiating elements in the second column of radiating elements may be electrically steered in the azimuth direction and the radiating elements in the first column of radiating elements may not be electrically steered in the azimuth direction. 
     In some embodiments, the antenna may further include a column of second frequency band radiating elements that is configured to generate a third antenna beam having an azimuth half power beamwidth that is about twice an azimuth half power beamwidth of the first antenna beam. 
     In some embodiments, the antenna may further include first and second columns of second frequency band radiating elements, and the first column of second frequency band radiating elements may be on a first portion of the reflector that is perpendicular to an azimuth boresight pointing direction of the twin beam base station antenna and the second column of second frequency band radiating elements may be on a second portion of the reflector that is perpendicular to an azimuth boresight pointing direction of the twin beam base station antenna. 
     In some embodiments, the radiating elements in at least some of the first through fifth columns of radiating elements may be both mechanically and electrically steered in the azimuth direction. 
     Pursuant to further embodiments of the present invention, twin beam base station antennas are provided that include first and second RF ports, an array that includes a plurality of columns of first frequency band radiating elements that are configured to form an antenna beam that provides coverage to a predefined area, and a beamforming network coupled between the first and second RF ports and the array, the beamforming network including a first frequency dependent power divider. 
     In some embodiments, the beamforming network includes a 90° hybrid coupler having a first input coupled to the first RF port, a second input coupled to the second RF port, a first output coupled to the first frequency dependent power divider and a second output coupled to the second frequency dependent power divider. 
     In some embodiments, a first output of the first frequency dependent power divider is coupled to a first of the first frequency band radiating elements through a 180° phase shifter and a second output of the first frequency dependent power divider is coupled to a second of the first frequency band radiating elements. 
     In some embodiments, a first output of the second frequency dependent power divider is coupled to a third of the first frequency band radiating elements through a 180° phase shifter and a second output of the second frequency dependent power divider is coupled to a fourth of the first frequency band radiating elements. 
     In some embodiments, the first frequency dependent power divider may comprise a power divider having a filter on a first output thereof. 
     In some embodiments, the first frequency dependent power divider may comprise a power divider that has a first output coupled to a first input of a 90° hybrid coupler and a second output that is coupled to a second input of the 90° hybrid coupler via a delay line. 
     In some embodiments, the array may include a total of four columns or a total of five columns of first frequency band radiating elements. 
     Pursuant to yet additional embodiments of the present invention, twin beam base station antennas are provided that are configured to provide coverage to a sector of a three-sector base station. These antennas include first and second RF ports, a first array that includes at least two columns of radiating elements that are coupled to the first RF port via a first feed network, a second array that includes at least two columns of radiating elements that are coupled to the second RF port via a second feed network, and a shared column of radiating elements that is coupled to the first RF port via the first feed network and to the second RF port via the second feed network. The first array is configured to generate a first antenna beam that provides coverage throughout a first sub-sector of the sector, and the second array is configured to generate a second antenna beam that provides coverage throughout a second sub-sector of the sector. 
     In some embodiments, the shared column of radiating elements may be between the at least two columns of radiating elements of the first array and the at least two columns of radiating elements of the second array. 
     In some embodiments, the first feed network may include a plurality of variable power dividers that are configured to scan a first antenna beam that is generated by the first array between 20° and 40° in the azimuth plane. 
     Pursuant to still further embodiments of the present invention, twin beam base station antennas are provided that are configured to provide coverage to a sector of a three-sector base station. These antennas include a first low-band RF port, second and third high-band RF ports, a backplane that includes a reflector, the reflector having a first longitudinally extending section that is perpendicular to the azimuth boresight pointing direction of the base station antenna, a second longitudinally extending section that is angled at least 15° from the first section, and a third longitudinally extending section that is angled at least 15° from the first section, the first longitudinally extending section positioned between the second longitudinally extending section and the third longitudinally extending section, a first column of low-band radiating elements that is coupled to the third RF port via a third feed network, a second array that includes at least three columns of high-band radiating elements that are coupled to the first RF port via a second feed network, the second array mounted to extend forwardly from the second longitudinally extending section of the reflector, and a third array that includes at least three columns of high-band radiating elements that are coupled to the second RF port via a second feed network, the third array mounted to extend forwardly from the third longitudinally extending section of the reflector. 
     In some embodiments, the first linear array of low-band radiating elements the first array may be mounted to extend forwardly from the first longitudinally extending section of the reflector. 
     In some embodiments, the reflector may be a generally V-shaped reflector. 
     In some embodiments, the second and third longitudinally extending sections of the reflector may extend forwardly from the first longitudinally extending section of the reflector. 
     In some embodiments, the second and third longitudinally extending sections of the reflector may extend rearwardly from the first longitudinally extending section of the reflector. 
     In some embodiments, the first array of radiating elements may be configured to form a first antenna beam that provides coverage throughout a sub-sector having a first azimuth half power beamwidth, and the base station antenna may further include an additional column of high-band radiating elements that is configured to generate a fourth antenna beam having an azimuth half power beamwidth that is about twice the first azimuth half power beamwidth. 
     Pursuant to further embodiments of the present invention, base station antennas are provided that include first and second RF ports, first through fifth columns of radiating elements, and a beamforming network. Three of first through fifth columns of radiating elements are coupled to both the first RF port and the second RF port through the beamforming network, another one of the first through fifth columns of radiating elements is coupled to the first RF port and not to the second RF port, and the last of the first through fifth columns of radiating elements is coupled to the second RF port and not to the first RF port. 
     In some embodiments, the first through fifth columns of radiating elements may be arranged in numerical order on a reflector, and the three of the first through fifth columns of radiating elements that are coupled to both the first RF port and the second RF port through the beamforming network are the second, third and fourth columns of radiating elements. 
     In some embodiments, the beamforming network may comprise a 90° hybrid coupler. 
     In some embodiments, the second and fourth columns of radiating elements may be coupled to a first output of the beamforming network, and the third column of radiating elements may be coupled to a second output of the beamforming network. 
     In some embodiments, a frequency dependent power divider may be coupled between the first RF port and the first column of radiating elements. 
     In some embodiments, the reflector may include first through third longitudinally-extending sections, and the first and third longitudinally-extending sections of the reflector are each angled with respect to the second longitudinally-extending section of the reflector. The first column of radiating elements may be mounted to extend forwardly from the first longitudinally-extending section of the reflector and the fifth column of radiating elements may be mounted to extend forwardly from the third longitudinally-extending section of the reflector. 
     In some embodiments, the second through fourth columns of radiating elements may be mounted to extend forwardly from the second longitudinally-extending section of the reflector. 
     In some embodiments, the second longitudinally-extending section of the reflector may be forward of the first and third longitudinally-extending sections of the reflector. 
     In some embodiments, the second longitudinally-extending section of the reflector may be rearward of the first and third longitudinally-extending sections of the reflector. 
     Pursuant to yet additional embodiments of the present invention, twin beam base station antennas are provided that are configured to split a sector of a three-sector base station into first and second sub-sectors. These antennas include a first array that includes a plurality of columns of first frequency band radiating elements that are mechanically steered in a first direction from an azimuth boresight pointing direction of the antenna and that are electrically steered further in the first direction. The first array is configured to form a first antenna beam that provides coverage throughout the first sub-sector. 
     In some embodiments, the antenna may further include a second array that includes a plurality of columns of first frequency band radiating elements that are mechanically steered in a second direction from an azimuth boresight pointing direction of the antenna and that are electrically steered further in the second direction. In such embodiments, the second array may be configured to form a second antenna beam that provides coverage throughout the second sub-sector. 
     In some embodiments, the antenna may further include a reflector that includes a first longitudinally-extending section that is angled between 5° and 20° in the azimuth plane from a plane that is perpendicular to the azimuth boresight pointing direction of the twin beam base station antenna. 
     In some embodiments, the first array may include first through third columns of radiating elements, each of which is mounted to extend forwardly from the first longitudinally-extending section of the reflector. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is an azimuth pattern illustrating how the azimuth pointing direction and azimuth HPBW of the antenna beams formed by a conventional twin beam base station antenna that includes a Butler Matrix style beamforming network change with increasing frequency. 
         FIG.  2    is a schematic transverse cross-sectional view of a conventional multi-column mechanically-steered twin beam base station antenna. 
         FIG.  3 A  is a perspective view of a twin beam base station antenna according to embodiments of the present invention that includes a beamforming network having frequency dependent power dividers. 
         FIG.  3 B  is a schematic block diagram illustrating the columns of radiating elements and the feed network of the twin beam base station antenna of  FIG.  3 A . 
         FIG.  3 C  is a cross-sectional view illustrating a row of radiating elements included in the twin beam base station antenna of  FIGS.  3 A- 3 B . 
         FIG.  3 D  is a block diagram of one of the beamforming networks shown in  FIG.  3 B . 
         FIG.  4 A  is a block diagram of a conventional Butler Matrix style beamforming network. 
         FIG.  4 B  is a block diagram of another conventional beamforming network. 
         FIG.  5 A  is a plan view of a printed circuit board based frequency dependent variable power divider that may be used in implementing the beamforming network of  FIG.  3 D . 
         FIG.  5 B  is a graph showing the simulated response of the frequency dependent variable power divider circuit of  FIG.  5 A . 
         FIG.  5 C  is a block diagram of another frequency dependent variable power divider that may be used in implementing the beamforming network of  FIG.  3 D . 
         FIG.  6 A  is a transverse cross-sectional view of a base station antenna according to further embodiments of the present invention. 
         FIG.  6 B  is a block diagram of a beamforming network according to embodiments of the present invention that may be used in the base station antenna of  FIG.  6 A . 
         FIG.  7 A  is a transverse cross-sectional view of an electrically-steered twin beam base station antenna according to embodiments of the present invention. 
         FIG.  7 B  is a transverse cross-sectional view of an electrically-steered twin beam base station antenna according to further embodiments of the present invention that includes a shared column of radiating elements. 
         FIG.  8 A  is a transverse cross-sectional view of a mechanically-steered dual-band twin beam base station antenna according to embodiments of the present invention that further includes a single linear array of low-band radiating elements. 
         FIG.  8 B  is a transverse cross-sectional view of another mechanically-steered dual-band twin beam base station antenna according to an alternative embodiment of the present invention that also includes a single linear array of low-band radiating elements. 
         FIG.  8 C  is a transverse cross-sectional view of a mechanically-steered dual-band twin beam base station antenna according to embodiments of the present invention that also includes two linear arrays of low-band radiating elements. 
         FIG.  8 D  is a transverse cross-sectional view of another mechanically-steered dual-band twin beam base station antenna according to an alternative embodiment of the present invention that includes two linear arrays of low-band radiating elements. 
         FIGS.  9 A- 9 I  are a series of transverse cross-sectional views of dual-band twin beam base station antennas according to embodiments of the present invention that include sector-splitting arrays that are both electrically-steered and mechanically-steered. 
         FIGS.  10 A- 10 B  are transverse cross-sectional views of dual-band twin beam base station antennas according to further embodiments of the present invention that include columns of radiating elements that are both electrically and mechanically-steered. 
         FIG.  11 A  is a transverse cross-sectional view of a twin beam base station antenna according to embodiments of the present invention that uses a Butler Matrix style beamforming network on only some of the columns of radiating elements that are used to generate the twin beams. 
         FIG.  11 B  is a block diagram of the base station antenna of  FIG.  11 A . 
         FIG.  12 A  is a transverse cross-sectional view of a modified version of the base station antenna of  FIGS.  11 A- 11 B  that includes mechanical steering for two of the columns of radiating elements. 
         FIG.  12 B  is a transverse cross-sectional view of another modified version of the base station antenna of  FIGS.  11 A- 11 B  that includes mechanical steering for two of the columns of radiating elements. 
         FIG.  13    is a transverse cross-sectional view of a tri-beam base station antenna according to embodiments of the present invention. 
         FIG.  14 A  is a schematic perspective view of a dual-band twin beam base station antennas according to still further embodiments of the present invention that include two staggered columns of high-band radiating elements that are mechanically-steered and which may optionally also be electrically-steered, with the optional two low-band columns of radiating elements omitted. 
         FIG.  14 B  is a schematic transverse cross-sectional of the base station antenna of  FIG.  14 A . 
         FIG.  14 C  is an isometric sectional view of the base station of  FIGS.  14 A- 14 B  with a low-band radiating element from each column included in the view. 
     
    
    
     DETAILED DESCRIPTION 
     Several approaches have been used to implement twin beam antennas that generate first and second antenna beams that provide coverage to respective first and second sub-sectors of a 120° sector in the azimuth plane. In a first approach, first and second linear arrays of radiating elements are mounted within an antenna. The linear arrays are mechanically positioned or “steered” to have different azimuth pointing angles, typically by bending the reflector of the antenna on which the linear arrays are mounted. The first linear array is mechanically steered so that the peak radiation therefrom will be at the middle of the first sub-sector, and the second linear array is mechanically steered so that the peak radiation therefrom will be at the middle of the second sub-sector. Since the azimuth beamwidth of typical radiating elements is usually appropriate for covering a full 120° sector, an RF lens is mounted in front of the two linear arrays of radiating elements that narrows the azimuth beamwidth of each antenna beam by a suitable amount for providing service to a 60° sub-sector. Unfortunately, however, the use of RF lenses may increase the size, weight and cost of the base station antenna, and the amount that the RF lens narrows the beamwidth is a function of frequency, making it difficult to obtain suitable coverage when wideband radiating elements are used that operate over a wide frequency range (e.g., radiating elements that operate over the full 1.7-2.7 GHz cellular frequency range). 
     In a second approach, two or more linear arrays of radiating elements (typically 2-4 linear arrays) are mounted within an antenna where each linear array points toward the boresight pointing direction for the antenna. Two RF ports (per polarization) are coupled to the two or more linear arrays through a beamforming network such as a Butler Matrix. The beamforming network generates two separate antenna beams (per polarization) based on the RF signals input at the two RF ports, and the antenna beams are electrically steered off boresight at angles of about −30° and 30° to provide coverage to the two sub-sectors. With beamforming network based twin beam antennas, the pointing angle in the azimuth plane of each antenna beam and the HPBW of each antenna beam may vary as a function of the frequency of the RF signals input at the two RF ports. In particular, the azimuth pointing direction of the antenna beams (i.e., the azimuth angle where peak gain occurs) tends to move toward the boresight pointing direction of the antenna and the azimuth HPBW tends to get smaller with increasing frequency, as shown in  FIG.  1   . This can lead to a large variation as a function of frequency in the power level of the antenna beam at the outside edges of the sub-sectors, which is undesirable. 
     In a third approach, several linear arrays of radiating elements are mounted on each panel of a V-shaped reflector to provide a sector-splitting twin beam antenna. A schematic transverse cross-sectional view of a conventional antenna  1  using this technique is provided in  FIG.  2   . As shown in  FIG.  2   , the antenna  1  includes six linear arrays of  20 - 1  through  20 - 6  of radiating elements  22  (only one radiating element of each array is visible in the cross-sectional view of  FIG.  2   ) that are mounted to extend forwardly of a V-shaped reflector  10 . While the antenna beams generated by the antenna  1  of  FIG.  2    may vary less as a function of frequency as compared to the lensed and beamforming based twin beam antennas discussed above, the inclusion of six columns of radiating elements  22  may significantly increase the required width for the antenna  1 , which is undesirable, and the V-shaped reflector  10  may not provide suitable mounting locations and/or room for one or more lower-band linear arrays of radiating elements that are often required by network operators. 
     Generally speaking, cellular operators desire twin beam antennas that have azimuth HPBW values of anywhere between 30°-38°, so long as the azimuth HPBW does not vary significantly (e.g., more than 12°) across the operating frequency band of the linear arrays that generate the twin beams. Likewise, the azimuth pointing angle of the antenna beam peak may vary anywhere between +/−26° to +/−33°, so long as the azimuth pointing angle does not vary significantly (e.g., more than 4°) across the operating frequency band of the linear arrays that generate the twin beams. The crossover point of the two antenna beams at the boresight pointing direction of the antenna (0°) may be about 9-12 dB below the peak gain. The azimuth roll off at the outer edges of the two 60° sub-sectors is preferably about 10-15 dB below the peak gain. The peak azimuth sidelobe levels should be at least 15 dB below the peak gain value. 
     Pursuant to embodiments of the present invention, improved twin beam (and other multi-beam) base station antennas are provided that overcome or mitigate various of the difficulties with the above-described conventional twin beam antennas. A number of different techniques may be used in the improved twin beam designs disclosed herein. These approaches can be mixed and matched, as will be discussed below, to meet desired performance, cost and/or size requirements for the twin beam base station antenna. 
     According to some embodiments of the present invention, twin beam base station antennas are provided that have multiple columns of radiating elements mounted on a backplane. The columns of radiating elements are connected to two RF ports (per polarization) through a beamforming network. The beamforming network is implemented using frequency-dependent power dividers in order to reduce the variation in the azimuth pointing direction and azimuth HPBW of the twin antenna beams. 
     According to further embodiments, twin beam antennas are provided that have multiple columns of radiating elements mounted on a backplane that are used to form two antenna beams per polarization. In some embodiments, the multiple columns may be divided into two sets of columns, with each set of columns being used to generate an independent antenna beam. In some implementations, the columns of radiating elements may be mounted on a flat backplane, and the two antenna beams formed by the columns of radiating elements may be electrically steered to point in the desired directions (e.g., −30° and 30°) in the azimuth plane. In other implementations, the columns of radiating elements may be mounted on a V-shaped (or generally V-shaped) backplane that mechanically steers the antenna beams so that electrical scanning of the antenna beams is not required. In other embodiments, the multiple columns of radiating elements may be divided into two sets of columns, with each set of columns being used to generate an independent antenna beam, where at least one, but less than all, of the columns is part of both sets of arrays. In some embodiments, one or more columns of lower-band radiating elements may also be included in the antenna. In still other embodiments, the antennas may include backplanes having complex shapes that use a combination of mechanical steering and electrical scanning to form a pair of sector-splitting antenna beams per polarization. 
     Pursuant to further embodiments, twin beam base station antennas are provided that have multiple columns of radiating elements mounted on a flat backplane. The columns of radiating elements are divided into two sets of columns that generate respective antenna beams (per polarization), where at least one, but less than all, of the columns of radiating elements is part of both sets of arrays. The beamforming network includes a 90° hybrid coupler as well as variable power dividers that are interposed between the RF ports and the 90° hybrid coupler. In yet further embodiments, the flat backplanes may be replaced with shaped backplanes so that a combination of mechanical steering and electrical steering is used to point the antenna beams in their desired azimuth pointing directions. 
     Embodiments of the present invention will now be discussed in greater detail with reference to  FIGS.  3 A- 13   , in which example embodiments are shown. 
     Reference is now made to  FIGS.  3 A- 3 D , which illustrate a twin beam Butler Matrix style base station antenna  100  according to an example embodiment of the present invention that fully shares all of the columns of radiating elements in generating the two antenna beams.  FIG.  3 A  is a perspective view of the twin beam base station antenna  100 , while  FIG.  3 B  is a block diagram of the base station antenna  100  that illustrates the feed network that is used to couple a pair of RF ports to the four columns of radiating elements included in the antenna  100 .  FIG.  3 C  is a transverse cross-sectional view of the twin beam base station antenna  100  that illustrates one of the rows of radiating elements included therein.  FIG.  3 D  is a block diagram of one of the beamforming networks included in the twin beam base station antenna  100 . 
     As shown in  FIG.  3 A , the twin beam Butler Matrix style base station antenna  100  includes a housing  110 . In the depicted embodiment, the housing  110  is a multi-piece housing that includes a radome  112 , a top end cap  114  and a bottom end cap  116 . A plurality of RF ports  120  and control ports are mounted in the bottom end cap  116 . The RF ports  120  may comprise RF connectors that may receive coaxial cables that provide RF connections between the base station antenna  100  and one or more radios (not shown). 
     The base station antenna  100  is an elongated structure that extends along a longitudinal axis A 1 . The azimuth boresight pointing direction of the base station antenna  100  refers to a horizontal axis extending from the base station antenna  100  to the center, in the azimuth plane, of the 120° sector served by the base station antenna  100 . When the base station antenna  100  is mounted for normal use, the longitudinal axis A 1  will typically extend along a vertical axis, although in some cases the base station antenna  100  may be tilted a few degrees from the vertical to impart a mechanical downtilt to the antenna beams formed by the base station antenna  100 . As is further shown in  FIG.  3 A , the beamforming antenna  100  has a length, a depth and a width. The length L refers to the distance that the antenna  100  extends along the longitudinal axis A 1 . The depth D refers to the distance that the antenna  100  extends along an axis A 2  that is perpendicular to the longitudinal dimension A 1  and that is collinear with the azimuth boresight pointing direction of the antenna  100 . The width dimension W refers to the distance that the antenna  100  extends along an axis A 3  that is perpendicular to both axis A 1  and axis A 2 . 
     Referring to  FIGS.  3 B and  3 C , the base station antenna  100  further includes four columns  130 - 1  through  130 - 4  of radiating elements  132 . Herein, when multiple of the same elements are included in an antenna, the elements may be referred to individually by their full reference numeral (e.g., column  130 - 3 ) and collectively by the first part of their reference numerals (e.g., the columns  130 ). Each column  130  includes a plurality of radiating elements  132 . While the radiating elements  132  included in each column  130  are illustrated in  FIGS.  3 B  and  3 C as slant −45°/+45° cross-polarized dipole radiating elements  132  that have a dipole arm printed circuit board  133  that includes four dipole arms (not shown) that is mounted on a pair of feed stalk printed circuit boards  135 , it will be appreciated that any appropriate radiating elements  132  may be used. For example, patch radiating elements may be used in other embodiments. The radiating elements  132  in each linear array  130  are mounted to extend forwardly from a backplane  140 . The backplane  140  may comprise a metallic sheet that serves as a ground plane for the radiating elements  132  and that also serves as a reflector that redirects forwardly much of the backwardly-directed radiation emitted by the radiating elements  132 . The backplane  140  may also include sidewalls (not shown) and may serve as a structural member of the antenna  100 . 
     As is also shown in  FIG.  3 B , the antenna  100  includes a pair of feed networks  150 - 1 ,  150 - 2  that each include phase shifters/power dividers  160  and a plurality of beamforming networks  170 . It will be appreciated that  FIG.  3 B  only shows the feed network  150 - 1  for the −45° radiators of each cross-polarized dipole radiating element  132 . The feed network  150 - 2  for the +45° radiators of each cross-polarized dipole radiating element  132  may be identical to feed network  150 - 1 , except that feed network  150 - 2  extends between the +45° RF ports  120 - 3 ,  120 - 4  and the +45° radiators of each cross-polarized dipole radiating element  132 . 
     As shown in  FIG.  3 B , an RF signal input at RF port  120 - 1  is passed to a first phase shifter/power divider  160 - 1 . The first phase shifter/power divider  160 - 1  is configured to split an RF signal input thereto into five sub-components (which may have the same or, more typically, different power levels) and to inject a phase taper across the five sub-components of the RF signal in order to electrically change the elevation angle of the antenna beam formed by the RF signal input at RF port  120 - 1  in a desired fashion. Similarly, an RF signal input at RF port  120 - 2  is passed to a second phase shifter/power divider  160 - 2 . The second phase shifter/power divider  160 - 2  is configured to split an RF signal input thereto into five sub-components (which may have the same or, more typically, different power levels) and to inject a phase taper across the five sub-components of the RF signal in order to electrically change the elevation angle of the antenna beam formed by the RF signal input at RF port  120 - 2  in a desired fashion. As the operation of phase shifter/power divider circuits is well understood in the art, further description thereof will be omitted here. 
     The five outputs of the first phase shifter/power divider  160 - 1  are coupled to respective first inputs of five Butler Matrix style beamforming networks  170 - 1  through  170 - 5 , and the five outputs of the second phase shifter/power divider  160 - 2  are similarly coupled to respective second inputs of the five Butler Matrix style beamforming networks  170 - 1  through  170 - 5 . Each Butler Matrix style beamforming network  170  includes four outputs, and each output is coupled to a respective one of the four radiating elements  132  in a row of radiating elements that are fed by the Butler Matrix style beamforming network  170  at issue. 
       FIG.  3 D  is a block diagram of one of the beamforming networks  170  shown in  FIG.  3 B . As shown in  FIG.  3 D , the outputs of the first and second phase shifters/power dividers  160 - 1 ,  160 - 2  are coupled to the two input ports of a 90° hybrid coupler  172 . The two output ports of the 90° hybrid coupler  172  are coupled to inputs of respective first and second variable power dividers  174 . The first and second variable power dividers  174  are frequency dependent power dividers, as will be discussed in greater detail below. The first output of the first variable power divider  174 - 1  is coupled to a first 180° phase shifter  176 - 1  (which may be implemented for example, as a delay line or by a 180° rotation of the dipole feed point), and the output of the first 180° phase shifter  176 - 1  is coupled to the first output  178 - 1  of the beamforming network  170 . The second output of the first variable power divider  174 - 1  is coupled to the third output  178 - 3  of the beamforming network  170 . Similarly, the first output of the second variable power divider  174 - 2  is coupled to a second 180° phase shifter  176 - 2  (which may be implemented for example, as a delay line or by a 180° rotation of the dipole feed point), and the output of the second 180° phase shifter  176 - 2  is coupled to the fourth output  178 - 4  of the beamforming network  170 . The second output of the second variable power divider  174 - 2  is coupled to the second output  178 - 2  of the beamforming network  170 . 
     The beamforming network  170  may be significantly simpler than a comparable conventional Butler Matrix beamforming network.  FIG.  4 A  is a schematic block diagram of such a conventional Butler Matrix beamforming network  180 . As shown in  FIG.  4 A , the first and second inputs to the conventional Butler Matrix beamforming network  180  are coupled to first input ports of respective first and second 90° hybrid couplers  182 - 1 ,  182 - 2 . The second inputs of the first and second 90° hybrid couplers  182 - 1 ,  182 - 2  are coupled to ground. The first output of each of the first and second 90° hybrid couplers  182 - 1 ,  182 - 2  is coupled to respective first and second 90° phase shifters  186 - 1 ,  186 - 2 , and the second output of each of the first and second 90° hybrid couplers  182 - 1 ,  182 - 2  is coupled to the first inputs of respective third and fourth 90° hybrid couplers  182 - 3 ,  182 - 4 . The outputs of first and second 90° phase shifters  186 - 1 ,  186 - 2  are coupled to the second inputs of the respective third and fourth 90° hybrid couplers  182 - 3 ,  182 - 4 . The first output of each of the third and fourth 90° hybrid coupler  182 - 3 ,  182 - 4  is coupled to a respective one of third and fourth 450 phase shifters  186 - 3 ,  186 - 4 , and the second output of each of the third and fourth 90° hybrid couplers  182 - 3 ,  182 - 4  is coupled to the first inputs of respective fifth and sixth 90° hybrid couplers  182 - 5 ,  182 - 6 . The outputs of the third and fourth 45° phase shifters  186 - 3 ,  186 - 4  are coupled to the second inputs of respective fifth and sixth 90° hybrid couplers  182 - 5 ,  182 - 6 . The four outputs of the fifth and sixth 90° hybrid couplers  182 - 5 ,  182 - 6  are coupled to the respective four outputs  188 - 1  through  188 - 4  of the beamforming network  180 . 
     As can be seen by comparing  FIG.  3 D  and  FIG.  4 A , the beamforming network  170  is significantly simpler than the conventional beamforming network  180 . In particular, the conventional beamforming network  180  incudes a total of six 90° hybrid couplers  182 , whereas the beamforming network  170  includes only a single 90° hybrid coupler  172  and two frequency dependent variable power dividers  174 . Additionally, the frequency dependent variable power dividers  174  decrease the variance in the azimuth HPBW as a function of frequency as compared to the conventional beamforming network  180 . Thus, the beamforming base station antenna  100  may be simpler than a comparable conventional beamforming base station antenna and may also exhibit improved performance. 
       FIG.  4 B  is a block diagram of another known beamforming network  190 . As shown in  FIG.  4 B , the beamforming network  190  has first and second inputs that are coupled to the two input ports of a 90° hybrid coupler  192 . The two output ports of the 90° hybrid coupler  192  are coupled to inputs of respective first and second variable power dividers  194 - 1 ,  194 - 2 . The first output of the first variable power divider  194 - 1  is coupled to a first 180° phase shifter  196 - 1  and the output of the first 180° phase shifter  196 - 1  is coupled to the first output  198 - 1  of the beamforming network  190 . The second output of the first variable power divider  194 - 1  is coupled to the third output  198 - 3  of the beamforming network  190 . Similarly, the first output of the second variable power divider  194 - 2  is coupled to a second 180° phase shifter  196 - 2 , and the output of the second 180° phase shifter  196 - 2  is coupled to the fourth output  198 - 4  of the beamforming network  190 . Finally, the second output of the second variable power divider  194 - 2  is coupled to the second output  198 - 2  of the beamforming network  190 . 
     In some embodiments, the frequency dependent variable power dividers  174  may be implemented using the circuit shown in  FIG.  5 A , which depicts a printed circuit board based power divider  200  having a low pass filter at one output thereof. As shown in  FIG.  5 A , the power divider/low pass filter circuit  200  is implemented on a microstrip printed circuit board  210 . The power divider/low pass filter circuit  200  includes a power divider  222  and a low pass filter  224 . The microstrip printed circuit board  210  may include a dielectric substrate that has a ground plane metallization layer (not shown) covering a back side of the substrate and a metal “trace” pattern on the front side of the substrate. The traces of the trace pattern form microstrip transmission line segments and resonating stubs  216 . The resonating stubs  216  form the low pass filter  224 . A pair of plated through holes  218  are provided at the distal end of the central resonating stub  216  that connect to the ground layer of the microstrip printed circuit board  210 . 
       FIG.  5 B  is a graph showing the simulated response of the frequency dependent variable power divider circuit  200  of  FIG.  5 A . Curves  230 - 1  and  230 - 2  in  FIG.  5 B  show the respective magnitudes of the signals output at the first and second output ports of frequency dependent variable power divider circuit  200  in response to an input RF signal, as a function of frequency. As shown in  FIG.  5 B , the relative magnitudes of the output signals vary as a function of frequency. The shapes of the curves  230 - 1 ,  230 - 2  in  FIG.  5 B  may be changed by increasing or decreasing the number of resonating stubs  216  included in the low pass filter  224 . If additional resonating stubs  216  are added, then the power at the output port that includes the low pass filter  224  decreases more quickly with increasing frequency, whereas if less resonating stubs  216  are included, then the power at that port decreases more slowly with increasing frequency. 
     In other embodiments, the frequency dependent variable power dividers  174  may be implemented using the circuit  250  illustrated in  FIG.  5 C , which is disclosed in PCT Publication No. 2015/006676. As shown in  FIG.  5 C , the frequency dependent variable power divider  250  that is disclosed in PCT Publication No. 2015/006676 includes a first power divider  252 , a delay line  254  and a 90° hybrid coupler  256 . The power divider  252  splits an input signal into two sub-components. The power divider  252  may be an equal or unequal power divider. A first output of the power divider  252  is coupled to a first input of the 90° hybrid coupler  256 . A second output of the power divider  252  is coupled to the delay line  254 . The output of the delay line  254  is coupled to the second input of the 90° hybrid coupler  256 . 
     The frequency dependent divider  250  may operate as follows. The delay line  254  imposes a phase delay to RF signals that are coupled to the second input of the 90° hybrid coupler  256 . However, if the delay line  254  is a fixed length, the phase delay that will be experienced by an RF signal varies with the frequency of the RF signal input thereto. Thus, for a fixed length delay line  254 , higher-frequency RF signals experience relatively more phase delay than do lower frequency RF signals. The 90° hybrid coupler  256 , therefore, receives equal amplitude signals, where the signals to the second input thereof experience increasing phase delay with increasing frequency. The 90° hybrid coupler  256  outputs equal phase, variable amplitude signals, where the amount of the amplitude difference increases with increasing frequency. With proper selection of the length of the delay line  254 , the azimuth HPBW can be stabilized over a desired frequency band, as is explained in further detail in PCT Publication No. 2015/006676, which is incorporated herein by reference 
     It should be noted that while the twin beam antenna  100  is illustrated as including four columns  130 - 1  through  130 - 4  of radiating elements  132 , different numbers of columns may be included in other embodiments. For example,  FIGS.  6 A and  6 B  illustrate a base station antenna  260  according to a further embodiment of the present invention that includes three shared columns  130 - 1  through  130 - 3  of high-band radiating elements  132  that are used to form first and second antenna beams.  FIG.  6 A  schematically illustrates one of the rows of high-band radiating elements  132  included in the base station antenna  260 , while  FIG.  6 B  illustrates a beamforming network  270  that may be used to connect first and second RF ports of the base station antenna  260  to at least one radiating element  132  in each column  130  of radiating elements  132 . 
     As shown in  FIG.  6 B , the beamforming network  270  includes first and second inputs  271 - 1 ,  271 - 2  that may be coupled to first and second RF ports of the antenna  260 . The inputs  271 - 1 ,  271 - 2  are coupled to the inputs of a 90° hybrid coupler  272 . A first output of the 90° hybrid coupler  272  is coupled to one or more radiating elements in a second (middle) column  130 - 2  of high-band radiating elements, while the second output of the 90° hybrid coupler  272  is coupled to a frequency dependent variable power divider  274 . The first output of the frequency dependent variable power divider  274  is coupled to one or more radiating elements in a first column  130 - 1  of high-band radiating elements, and the second output of the frequency dependent variable power divider  274  is coupled via a 180° phase shifter  276  to one or more radiating elements in a third column  130 - 3  of high-band radiating elements. 
     In still other embodiments (not depicted herein), the base station antenna could include five shared columns  130  of radiating elements  132 , and the 2×4 beamforming networks (i.e., 2 inputs, 4 outputs) shown in  FIG.  3 B  could be replaced with 2×5 beamforming networks. 
     Pursuant to further embodiments of the present invention, twin beam base station antennas are provided that use first and second multi-column arrays of radiating elements to form the twin beams. In some embodiments, the first and second multi-column arrays of radiating elements may be independent of each other, while in other embodiments the first and second multi-column arrays of radiating elements may share one or more columns of radiating elements. In some embodiments, the radiating elements in the first and second multi-column arrays may all “point” in the azimuth boresight pointing direction of the antenna (i.e., the radiating elements are positioned so that their peak radiation will be along the azimuth boresight pointing direction of the antenna), and electrical steering may be used to steer the first and second antenna beams in the azimuth plane. In other embodiments, mechanical steering may be used so that the radiating elements for the first multi-column array all have an azimuth pointing direction that corresponds to the peak gain of the first antenna beam in the azimuth plane, and the radiating elements for the second multi-column array similarly all have an azimuth pointing direction that corresponds to the peak gain of the second antenna beam in the azimuth plane. In still other embodiments, a combination of electrical and mechanical steering may be used. Example embodiments of twin beam base station antennas according to these aspects of the present invention are illustrated in  FIGS.  7 A- 13   .  FIGS.  7 A- 7 B,  8 A- 8 D,  9 A- 9 I,  10 A- 10 B,  11 A,  12 A- 12 B and  13    are transverse cross-sectional diagrams illustrating the backplane for the antennas as well as one radiating element from each column of radiating elements. It will be appreciated, therefore, that each radiating element shown in these figures represents a linear array or “column” of radiating elements that extend into the page in the drawings. 
     A first purely electrically steered twin beam base station antenna  300  according to embodiments of the present invention is illustrated in  FIG.  7 A . As shown in  FIG.  7 A , the twin beam base station antenna  300  includes six columns  130 - 1  through  130 - 6  of high-band radiating elements  132  that are mounted to extend forwardly from a flat backplane  140 A. The three left-most columns  130 - 1  through  130 - 3  of high-band radiating elements  132  are coupled to first and second RF ports (not shown) via a first common feed network (not shown) and are used to generate first antenna beams  302  (namely one for each of two polarizations) that have an azimuth pointing direction of about −30° (e.g., between −26° and −33°). The three right-most columns  130 - 4  through  130 - 6  of high-band radiating elements  132  are coupled to third and fourth RF ports (not shown) via a second common feed network (not shown) and are used to generate second antenna beams  304  (namely one for each of two polarizations) that have an azimuth pointing direction of about 30°. The first and second common feed networks each include frequency dependent variable power dividers that are used to scan the antenna beams  302 ,  304  in the azimuth plane. 
       FIG.  7 B  illustrates a twin beam base station antenna  310  according to embodiments of the present invention that is a modified version of the twin beam base station antenna  300  illustrated in  FIG.  7 A . The twin beam base station antenna  310  only has five columns  130 - 1  through  130 - 5  of high-band radiating elements  132  mounted on the backplane  140 B, and the middle column  130 - 3  is connected to both feed networks. The sharing of the middle column  130 - 3  of radiating elements  132  results in power loss, degrading the performance of the antenna  310  as compared to the antenna  300 , but eliminates one sixth of the radiating elements  132  and also results in an antenna that is perhaps 15% narrower in width. Columns  130 - 1  through  130 - 3  are coupled to first and second RF ports (not shown) and are used to generate first slant −45° and slant +45° polarization antenna beams  312  that have an azimuth pointing direction of about −30°, and columns  130 - 3  through  130 - 5  are coupled to third and fourth RF ports (not shown) and are used to generate second slant −45° and slant +45° polarization antenna beams  314  that have an azimuth pointing direction of about 30°. 
     A variety of different techniques may be used to share the radiating elements of column  130 - 3  so that the column is used in generating both antenna beam  312  and antenna beam  314 . For example, U.S. patent application Ser. No. 16/287,114, filed Feb. 27, 2019 (“the &#39;114 application”), discloses a variety of techniques for sharing a radiating element so that it is used in the generation of two different antenna beams. The entire contents of the &#39;114 application is incorporated herein by reference. 
     As discussed above with reference to  FIG.  2   , twin beam antennas are known in the art that have multiple columns of radiating elements mounted on the two planar panels of a V-shaped backplane to generate respective first and second antenna beams that split a sector. The V-shaped backplane “mechanically” steers the antenna beams in that it points each radiating element at the azimuth angle corresponding to the antenna beams that the radiating element is used to generate. One disadvantage of the antenna of  FIG.  2   , however, is that the backplane does not include a suitable location for an array of low-band radiating elements, and hence the antenna of  FIG.  2    is only suitable for use as a single band (high-band) antenna. 
     Pursuant to further embodiments of the present invention, twin beam, mechanically-steered base station antennas are provided that are configured as dual-band antennas.  FIGS.  8 A- 8 D  illustrate four example embodiments of such antennas. 
     Referring first to  FIG.  8 A , a dual-band base station antenna  320  is depicted that includes six linear arrays  130 - 1  through  130 - 6  of high-band radiating elements  132  as well as a single linear array  134 - 1  of low-band radiating elements  136 . The three left-most columns  130 - 1  through  130 - 3  of high-band radiating elements  132  are mounted to extend forwardly from a first face  142 - 1  of a generally V-shaped backplane  140 C and are coupled to first and second RF ports (not shown) via a first common feed network (not shown) to generate first slant −45° and slant +45° polarization antenna beams  322  that have an azimuth pointing direction of about −30°. The three right-most columns  130 - 4  through  130 - 6  of high-band radiating elements  132  are mounted to extend forwardly from a third face  142 - 3  of the generally V-shaped backplane  140 C and are coupled to third and fourth RF ports (not shown) via a second common feed network (not shown) to generate second slant −45° and slant +45° polarization antenna beams  324  that have an azimuth pointing direction of about 30°. As the first and third faces  142 - 1 ,  142 - 3  of the backplane  140 C are perpendicular to the azimuth pointing direction of the respective first and second antenna beams  322 ,  324 , no electrical steering of the radiation pattern is necessary. As is further shown in  FIG.  8 A , the backplane  140 C is only generally V-shaped as the apex of the “V” is flattened to form a narrow second face  142 - 2 . The linear array (column)  134  of low-band radiating elements  136  is mounted to extend forwardly from this second face  142 - 2 . The linear array  134  of low-band radiating elements  136  may be a conventional linear array with an azimuth HPBW of about 65° so that the linear array  134  of low-band radiating elements  136  serves the entire sector (i.e., there is no sector-splitting in the low-band). 
       FIG.  8 B  illustrates a dual-band base station antenna  330  that is identical to the dual-band base station antenna  320  of  FIG.  8 A  except that the generally V-shaped backplane  140 C of base station antenna  320  is inverted (flipped) to provide a backplane  140 D. As a result, the three left-most columns  130 - 1  through  130 - 3  of high-band radiating elements  132  are used to generate the antenna beam that points at an azimuth angle of about 30°, and the three right-most columns  130 - 4  through  130 - 6  of high-band radiating elements  132  are used to generate the antenna beam that points at an azimuth angle of about −30°. One potential advantage of the dual-band base station antenna  330  over the dual-band base station antenna  320  of  FIG.  3 A  is that the linear array  134 - 1  of low-band radiating elements  136  is mounted relatively further rearwardly due to the inverted backplane  140 D, which may reduce the overall depth of the antenna  330  as compared to the antenna  320 . 
       FIG.  8 C  illustrates a dual-band base station antenna  340  that may be used, for example, to (1) support two different low-band services (e.g., 700 MHz and 800 MHz), (2) perform sector-splitting in the high-band, and (3) support service in another frequency band within the high-band frequency range. The base station antenna  340  include a total of seven linear arrays  130 - 1  through  130 - 7  of high-band radiating elements  132  and two linear arrays  134 - 1 ,  134 - 2  of low-band radiating elements  136 . The three left-most columns  130 - 1  through  130 - 3  of high-band radiating elements  132  and the three right-most columns  130 - 5  through  130 - 7  of high-band radiating elements  132  may operate in the exact same manner as columns  130 - 1  through  130 - 3  and  130 - 4  through  130 - 6 , respectively, of base station antenna  320 , and hence further description thereof will be omitted. The backplane  140 E may have a shape that is similar to the backplane  140 C of antenna  320 . 
     The antenna  340  further includes two linear arrays  134 - 1 ,  134 - 2  of low-band radiating elements  136 . Low-band array  134 - 1  may be generally collinear with high-band array  130 - 2 , and low-band array  134 - 2  may be generally collinear with high-band array  130 - 6 , where the radiating elements of each pair of collinear arrays  134 - 1 ,  130 - 2 ;  134 - 2 ,  130 - 6  are interleaved along the vertical direction when the antenna  340  is mounted for use. The two low-band linear arrays  134 - 1 ,  134 - 2  may operate as sector (as opposed to sector-splitting) arrays that provide service in different frequency bands or that operate as a 4×MIMO antenna in the same frequency band. The linear array  130 - 4  of high-band radiating elements  132  may be used to support service in a different portion of the high-band. For example, the linear arrays  130 - 1  through  130 - 3  and  130 - 5  through  130 - 7  may comprise diplexed linear arrays that support service in two frequency bands in the 1695-2180 MHz frequency range, while the linear array  130 - 4  may be used to support service in the 2400-2690 MHz frequency band in an example embodiment. 
       FIG.  8 D  illustrates a dual-band base station antenna  350  that is identical to the dual-band base station antenna  340  of  FIG.  8 C , except that the linear arrays  134 - 1 ,  134 - 2  of low-band radiating elements are moved to outside positions on each side of the antenna. 
     The base station antennas  300  and  310  of  FIGS.  7 A- 7 B  include respective flat backplanes  140 A,  140 B that are perpendicular to the boresight pointing direction of the respective antennas, and electrically steer the sector-splitting antenna beams in the appropriate directions in the azimuth plane. In contrast, the base station antennas  320 ,  330 ,  340 ,  350  of  FIGS.  8 A- 8 D  each instead use shaped backplanes to mechanically steer the sector-splitting antenna beams in the appropriate directions in the azimuth plane, thereby avoiding any need to electrically steer the antenna beams from boresight. Pursuant to still further embodiments of the present invention, dual-band, twin beam base station antennas are provided that use a combination of mechanical steering and electrical steering to potentially achieve improved performance.  FIGS.  9 A- 9 I  are a series of transverse cross-sectional views of dual-band twin beam base station antenna according to embodiments of the present invention that include sector-splitting arrays that are both electrically and mechanically-steered. 
     Referring to  FIG.  9 A , a dual-band base station antenna  400  is depicted that includes seven columns  130 - 1  through  130 - 7  of high-band radiating elements  132  as well as two columns  134 - 1 ,  134 - 2  of low-band radiating elements  136 . All of the columns of radiating elements are mounted to extend forwardly from a shaped reflector  140 G that includes a plurality of longitudinally-extending sections that are angled with respect to adjacent longitudinally-extending sections. One or more columns of radiating elements may be mounted to extend forwardly from each longitudinally-extending section of the reflector  140 G. Some of the longitudinally extending sections are mounted to extend perpendicularly to an azimuth boresight pointing direction of the base station antenna  400 , while others of the longitudinally-extending sections are angled with respect to the azimuth boresight pointing direction of the base station antenna  400  to mechanically steer the column(s) of radiating elements mounted thereon at least part of the way toward the desired azimuth pointing direction of the antenna beam formed by the column(s) of radiating elements. 
     In the embodiment of  FIG.  9 A , the reflector  140 G includes first through seventh longitudinally-extending sections  142 - 1  through  142 - 7  that are arranged in numerical order across the width of the base station antenna  400 . Longitudinally-extending sections  142 - 2 ,  142 - 4  and  142 - 6  extend perpendicularly to an azimuth boresight pointing direction of the base station antenna  400 , while longitudinally-extending sections  142 - 1 ,  142 - 3 ,  142 - 5  and  142 - 7  are angled so that they do not extend perpendicularly to the azimuth boresight pointing direction of the base station antenna  400 . Longitudinally-extending sections  142 - 1  and  142 - 3  are angled to the left in the azimuth plane from the azimuth boresight pointing direction of the base station antenna  400 , while longitudinally-extending sections  142 - 5  and  142 - 7  are angled to the right in the azimuth plane from the azimuth boresight pointing direction of the base station antenna  400 . In some embodiments, longitudinally-extending sections  142 - 1  and  142 - 3  are angled about −30° in the azimuth plane from the sections  142 - 2 ,  142 - 4 ,  142 - 6  that extend perpendicularly to the azimuth boresight pointing direction of the base station antenna  400  so that the first and third columns  130 - 1 ,  130 - 3  of high-band radiating elements  132  need not be electrically steered in the azimuth plane. Similarly, longitudinally-extending sections  142 - 5  and  142 - 7  are also angled about 30° in the azimuth plane from the sections  142 - 2 ,  142 - 4 ,  142 - 6  that extend perpendicularly to the azimuth boresight pointing direction of the base station antenna  400  so that the fifth and seventh columns  130 - 5 ,  130 - 7  of high-band radiating elements  132  need not be electrically steered in the azimuth plane. 
     The three left-most columns  130 - 1  through  130 - 3  of high-band radiating elements are coupled to first and second RF ports (not shown) via a first common feed network (not shown). The second column  130 - 2  of high-band radiating elements  132  may be electrically steered about −30° in the azimuth plane so that the boresight pointing direction for the radiation patterns emitted by each of the three left-most columns  130 - 1  through  130 - 3  of high-band radiating elements is at an azimuth angle of about −30° with respect to the azimuth boresight pointing direction of the base station antenna  400 . Thus, the three left-most columns  130 - 1  through  130 - 3  of high-band radiating elements  132  will generate first slant −45° and slant +45° polarization antenna beams  402  that have an azimuth pointing direction of about −30°. The three right-most columns  130 - 5  through  130 - 7  of high-band radiating elements  132  are coupled to third and fourth RF ports (not shown) via a second common feed network (not shown). The sixth column  130 - 6  of high-band radiating elements  132  may be electrically steered about 30° in the azimuth plane so that the boresight pointing direction for the radiation patterns emitted by each of the three right-most columns  130 - 5  through  130 - 7  of high-band radiating elements is at an azimuth angle of about 30° with respect to the azimuth boresight pointing direction of the base station antenna  400 . Thus, the three right-most columns  130 - 5  through  130 - 7  of high-band radiating elements will generate second slant −45° and slant +45° polarization antenna beams  404  that have an azimuth pointing direction of about 30°. 
     The linear array  130 - 4  of high-band radiating elements  132  may be used to support service in a different portion of the high-band. The azimuth half power beamwidth of the antenna beam formed by the fourth linear array  130 - 4  of high-band radiating elements  132  may be about twice the azimuth half power beamwidth of the antenna beam formed by the first through third linear arrays  130 - 1  through  130 - 3  of high-band radiating elements  132 . 
     The antenna  400  further includes two linear arrays  134 - 1 ,  134 - 2  of low-band radiating elements  136 . Low-band array  134 - 1  may be generally collinear with high-band array  130 - 2 , and low-band array  134 - 2  may be generally collinear with high-band array  130 - 6 , where the radiating elements of each pair of collinear arrays  134 - 1 ,  130 - 2 ;  134 - 2 ,  130 - 6  are interleaved along the vertical direction when the antenna  400  is mounted for use. The two low-band linear arrays  134 - 1 ,  134 - 2  may operate as sector (as opposed to sector-splitting) arrays that provide service in different frequency bands or that operate as a 4×MIMO antenna in the same frequency band. 
       FIG.  9 B  depicts a dual-band base station antenna  410  that is a modified version of base station antenna  400  of  FIG.  9 A . Base station antenna  410 , however, only includes five columns  130 - 1  through  130 - 5  of high-band radiating elements  132  as well as the two columns  134 - 1 ,  134 - 2  of low-band radiating elements  136 . In the embodiment of  FIG.  9 B , the reflector  140 H includes first through fifth longitudinally-extending sections  142 - 1  through  142 - 5  that are arranged in numerical order, with longitudinally-extending sections  142 - 1 ,  142 - 3  and  142 - 5  extending perpendicularly to an azimuth boresight pointing direction of the base station antenna  410 , while longitudinally-extending section  142 - 2  is angled to the left in the azimuth plane (e.g., by about −30°) from the longitudinally-extending section  142 - 3 , and longitudinally-extending section  142 - 4  is angled to the right in the azimuth plane (e.g., by about 30°) from the longitudinally-extending section  142 - 3 . Thus, the second and fourth columns  130 - 2 ,  130 - 4  of high-band radiating elements  132  need not be electrically steered in the azimuth plane. 
     Columns  130 - 1  through  130 - 3  of high-band radiating elements are coupled to first and second RF ports via a first common feed network, with columns  130 - 1  and  130 - 3  of high-band radiating elements  132  electrically steered about −30° in the azimuth plane. Similarly, columns  130 - 3  through  130 - 5  of high-band radiating elements are coupled to third and fourth RF ports via a second common feed network, with columns  130 - 3  and  130 - 5  of high-band radiating elements  132  electrically steered about 30° in the azimuth plane. Column  130 - 3  of high-band radiating elements  132  is a shared column that is coupled to all of the first through fourth RF ports. 
     The antenna  410  again includes two columns  134 - 1 ,  134 - 2  of low-band radiating elements  136 . As the columns  134 - 1 ,  134 - 2  of low-band radiating elements  136  have already been described above with reference to  FIG.  9 A , further description thereof will be omitted. 
       FIG.  9 C  depicts a dual-band base station antenna  420  that is another modified version of base station antenna  400  of  FIG.  9 A . Base station antenna  420  is identical to base station antenna  400  except that the reflector  140 I of base station antenna  420  is shaped differently than the reflector  140 G of base station antenna  400 . In particular, in base station antenna  420 , the longitudinally-extending section of the reflector  140 I that includes the column of high-band radiating elements  132  that generates an antenna beam that provides coverage to the full sector is moved to an outside edge of the antenna (here it is longitudinally extending section  142 - 7 ) instead of being positioned in the middle of the antenna as is the case in base station antenna  400 . Additionally, in the antenna of  FIG.  9 C , the two columns  134 - 1 ,  134 - 2  of low-band radiating elements  136  are not quite collinear with columns  130 - 2 ,  130 - 5  of high-band radiating elements  132  in order to maintain proper spacing between the two low-band columns  134 - 1 ,  134 - 2  of radiating elements  136  for isolation purposes. 
       FIG.  9 D  depicts a dual-band base station antenna  430  that is another modified version of base station antenna  400  of  FIG.  9 A . Base station antenna  430  is identical to base station antenna  400  except that the reflector  140 J of base station antenna  430  is again shaped differently than the reflector  140 G of base station antenna  400 . In particular, the reflector  140 J of base station antenna  430  only includes five longitudinally-extending sections  142 - 1  through  142 - 5 , and only longitudinally-extending sections  142 - 2  and  142 - 4  are angled such that they are not perpendicular to the azimuth boresight pointing direction of antenna  430 . Additionally, longitudinally-extending section  142 - 3  is widened and has three columns  130 - 3  through  130 - 5  of high-band radiating elements  132  and both columns  134 - 1 ,  134 - 2  of low-band radiating elements  136  mounted thereon. Thus, base station antenna  430  only mechanically steers two columns  130 - 2 ,  130 - 6  of high-band radiating elements  132  and electrically steers four columns  130 - 1 ,  130 - 3 ,  130 - 5 ,  130 - 7  of high-band radiating elements  132  to generate the twin antenna beams  432 ,  434  that split the sector, whereas base station antenna  400  mechanically steers four columns  130 - 1 ,  130 - 3 ,  130 - 4 ,  130 - 7  of high-band radiating elements  132  and only electrically steers two  130 - 2 ,  130 - 6  columns of high-band radiating elements  132  to generate the twin antenna beams that split the sector. Generally speaking, base station antenna  400  would be expected to provide better performance than base station antenna  430 , but base station antenna  400  also may have a larger footprint in the depth direction. 
       FIG.  9 E  depicts a dual-band base station antenna  440  that is a modified version of base station antenna  430  of  FIG.  9 D . Base station antenna  440  is identical to base station antenna  430  except that the reflector  140 J of base station antenna  430  is inverted (as was the case for base station antenna  330  of  FIG.  8 B , discussed above) to provide a reflector  140 K so that the three columns  130 - 1  through  130 - 3  of high-band radiating elements  132  on the left side of antenna  440  are used to form the antenna beam  444  that covers the right side of the sector and the three columns  130 - 5  through  130 - 7  of high-band radiating elements  132  on the right side of antenna  440  are used to form the antenna beam  442  that covers the left side of the sector. 
       FIG.  9 F  depicts a dual-band base station antenna  450  that is a modified version of base station antenna  410  of  FIG.  9 B . In particular, base station antenna  450  is identical to base station antenna  410  except that the reflector  140 L of base station antenna  450  is inverted so that the three columns  130 - 1  through  130 - 3  of high-band radiating elements  132  on the left side of antenna  450  are used to form the antenna beam  454  that covers the right side of the sector and the three columns  130 - 3  through  130 - 5  of high-band radiating elements  132  on the right side of antenna  450  are used to form the antenna beam  452  that covers the left side of the sector. Column  130 - 3  of high-band radiating elements  132  is a shared column of radiating elements that is coupled to all of the first through fourth RF ports (not shown). 
       FIG.  9 G  depicts a dual-band base station antenna  460  that is a modified version of base station antenna  450  of  FIG.  9 F . In particular, base station antenna  460  only includes a single column  134 - 1  of low-band radiating elements  136  that is mounted on the middle longitudinally-extending section  142 - 3  of a reflector  140 M. Base station antenna  460  thus can only support service in a single frequency band in the low-band. Base station antenna  460 , however, may be thinner in the depth direction than base station antenna  450  as the column  134 - 1  of low-band radiating elements  136  is mounted on a more rearwardly positioned section of the reflector. 
       FIG.  9 H  depicts a dual-band base station antenna  470  that is a modified version of base station antenna  460  of  FIG.  9 G . In particular, base station antenna  470  includes a sixth column  130 - 6  of high-band radiating elements  132  by mounting two columns  130 - 3 ,  130 - 4  of high-band radiating elements  132  on the third longitudinally extending section  142 - 3  of the reflector  140 N. As a result, the base station antenna  470  does not need to share a column  130  of high-band radiating elements  132  and instead uses columns  130 - 1  through  130 - 3  to form the first sector-splitting high-band antenna beam  474  and columns  130 - 4  through  130 - 6  to form the second sector-splitting high-band antenna beam  472 . 
       FIG.  9 I  depicts a dual-band base station antenna  480  that is a modified version of base station antenna  470  of  FIG.  9 H . In particular, base station antenna  480  is identical to base station antenna  470  except that the reflector  140 O of base station antenna  480  is not inverted as was the case with the reflector  140 N of base station antenna  470 . Thus, in base station antenna  480 , the three left-most columns  130 - 1  through  130 - 3  of high-band radiating elements  132  form the antenna beam  482  that covers the left side of the sector and the three right-most columns  130 - 4  through  130 - 6  of high-band radiating elements  132  form the antenna beam  484  that covers the right side of the sector. 
     Thus,  FIGS.  9 A- 9 I  illustrate twin beam base station antennas that are configured to split a sector of a three-sector base station into first and second sub-sectors. These antennas each include a first array that includes a plurality of columns of first frequency band radiating elements, the first array configured to form a first antenna beam that provides coverage throughout the first sub-sector. These antennas may also include a second array that includes a plurality of columns of first frequency band radiating elements, the second array configured to form a second antenna beam that provides coverage throughout the second sub-sector. As an example, in the base station antenna  400  of  FIG.  9 A , the first array comprises columns  130 - 1  through  130 - 3  that are used to generate the “left” antenna beam  402  and the second array comprises columns  130 - 4  through  130 - 6  that are used to generate the “right” antenna beam  404 . As another example, in the base station antenna  410  of  FIG.  9 B , the first array comprises columns  130 - 1  through  130 - 3  that are used to generate the “left” antenna beam  412  and the second array comprises columns  130 - 3  through  130 - 5  that are used to generate the “right” antenna beam  414 . A peak gain of the first antenna beam (e.g., antenna beam  402  in  FIG.  9 A ) may occur at an azimuth angle that is offset by at least 30° from an azimuth angle where a peak gain of the second antenna beam (e.g., antenna beam  404  in  FIG.  9 A ) occurs. 
     The radiating elements in a first of the columns in the first array have a first azimuth boresight pointing direction and the radiating elements in a second of the columns in the first array have a second azimuth boresight pointing direction that is offset from the first azimuth boresight pointing direction by at least 10°. Moreover, the radiating elements in the second of the columns in the first array are electrically steered. For example, as shown in  FIG.  9 A , the radiating elements in column  130 - 3  have a first azimuth boresight pointing direction as shown by arrow Az 1  and the radiating elements in column  130 - 2  have a second azimuth boresight pointing direction as shown by arrow Az 2 . The first and second azimuth boresight pointing directions Az 1  and Az 2  differ by about 30°. Additionally, the radiating elements in column  130 - 2  are electrically steered. 
     The columns of radiating elements in the first and second arrays may be mounted on a backplane that includes a reflector that has a plurality of longitudinally-extending sections. Some of these sections, such as sections  142 - 2 ,  142 - 4  and  142 - 6  in base station antenna  400  of  FIG.  9 A , may be perpendicular to an azimuth boresight pointing direction of the twin beam base station antenna. Other of the columns in the first and second arrays may be on sections of the reflector that are angled in the azimuth plane with respect to the sections that are perpendicular to the azimuth boresight pointing direction of the antenna, such as sections  142 - 1 ,  142 - 3 ,  142 - 5  and  142 - 7  in base station antenna  400  of  FIG.  9 A . In some embodiments, these sections may be angled at least 15° from the sections that are perpendicular to the azimuth boresight pointing direction of the antenna. 
     The base station antennas may also include one or more columns of second frequency band radiating elements. Typically, the first frequency band radiating elements may comprise “high-band” radiating elements that operate, for example, in all or a portion of the 1.7-2.7 GHz frequency band (or higher frequency bands) and the second frequency band radiating elements may comprise low-band radiating elements that operate in all or a portion of the 617-960 MHz frequency band. Embodiments of the present invention, however, are not limited thereto. In embodiments where the second frequency band radiating elements are low-band radiating elements, it may be advantageous to mount these radiating elements on portions of the reflector that are more rearwardly located than other portions of the reflector in order to reduce the overall depth of the antenna. 
       FIGS.  9 A- 9 I  also illustrate twin beam base station antennas that include a first RF port, a first array that includes first and second columns of radiating elements that are both coupled to the first RF port, and a backplane that includes a reflector, the reflector having a first longitudinally extending section that is perpendicular to the azimuth boresight pointing direction of the base station antenna and a second longitudinally extending section that is angled in the azimuth plane by at least 15° from the first longitudinally extending section of the reflector. The radiating elements in the first column are mounted to extend forwardly from the first longitudinally extending section of the reflector, and the radiating elements of the second column are mounted to extend forwardly from the second longitudinally extending section of the reflector. The first and second columns of radiating elements are configured to generate respective first and second radiation patterns that have pointing directions that are within about 10° of each other in the azimuth plane. With reference to base station antenna  400  of  FIG.  9 A , the first column of radiating elements may be column  130 - 2  and the second column of radiating elements may be column  130 - 1  (or column  130 - 3 ). Typically, columns  130 - 1  through  130 - 3  will each generate respective radiation patterns that point in the same direction in the azimuth plane, although some variation (although less than 10° variation) may be desired in some cases. 
     The base station antennas of  FIGS.  9 A- 9 I  also each have a reflector that includes at least first through fifth longitudinally-extending sections and first through fifth columns of first frequency band radiating elements, the first through fifth columns of first frequency band radiating elements mounted to extend forwardly from the respective first through fifth longitudinally-extending sections of the reflector. Each of the first through fifth longitudinally-extending section of the reflector directly connect to one or two other of the first through fifth longitudinally-extending sections of the reflector at angles of at least 5° in the azimuth plane. Additionally, each of the first through fifth longitudinally-extending sections of the reflector defines a respective plane, and axes that are perpendicular to each respective plane are each angled less than 45° in an azimuth plane with respect to a boresight pointing direction of the base station antenna. 
     Pursuant to still further embodiments of the present invention, twin beam sector splitting base station antennas are provided that use a combination of mechanical steering and electrical steering to create the twin beams, but less than full mechanical steering is used. In other words, the reflector may be shaped to partially mechanically steer some or all of the columns of radiating elements in the azimuth direction toward the middle of a sub-sector, and electrical steering may be used to steer the antenna beam the remainder of the way in the azimuth plane. This approach may advantageously allow all of the columns  130  of high-band radiating elements  132  to be mechanically steered without significantly increasing the depth of the antenna. Moreover, while all of the columns  130  of high-band radiating elements may also need to be electrically steered, the amount of electrical steering may be significantly reduced (e.g., to less than 20° in example embodiments). As the most significant distortions in the antenna beams tend to occur when large amounts of electrical steering are applied, the approach of both mechanically and electrically steering some or all of the columns  130  of high-band radiating elements  132  may provide excellent performance without significantly increasing the size of the base station antenna in the depth direction.  FIGS.  10 A and  10 B  illustrate two example embodiments of base station antennas according to embodiments of the present invention that have both mechanically and electrically steered columns of high-band radiating elements. 
     As shown in  FIG.  10 A , a base station antenna  500  includes seven columns  130 - 1  through  130 - 7  of high-band radiating elements  132  as well as two columns  134 - 1 ,  134 - 2  of low-band radiating elements  136 . All of the columns of radiating elements are mounted to extend forwardly from a shaped reflector  140 P that includes three longitudinally-extending sections. The middle longitudinally-extending section  142 - 2  extends perpendicularly to an azimuth boresight pointing direction of the base station antenna  500 . The longitudinally-extending sections  142 - 1 ,  142 - 3  on either side of the middle longitudinally-extending section  142 - 2  each extend rearwardly from longitudinally-extending section  142 - 2  at an angle that is significantly less than 30° (i.e., by an angle that is less than the angle required to mechanically steer the columns of radiating elements  130  that are mounted on longitudinally-extending sections  142 - 1 ,  142 - 3  so that electrical steering is not required). In the depicted embodiment, each longitudinally-extending section  142 - 1 ,  142 - 3  extends rearwardly at an angle of about 12.5°, although any appropriate angle may be used. Angles between about 5° and about 20° may be preferred in some applications. 
     Since each column  130 - 1  through  130 - 3  of high-band radiating elements  132  that is used to form the antenna beam  502  covering the left half of the sector is only mechanically steered by 12.5°, these columns are electrically steered in the azimuth plane by, for example, about 15.5°-17.5° in order to have a total scan in the azimuth plane of about 27°-30°. Notably, all three columns  130 - 1  through  130 - 3  of high-band radiating elements  132  that are used to form the antenna beam  502  covering the left half of the sector are mounted on the same longitudinally-extending section  142 - 1  of the reflector  140 P. Antenna columns  130 - 5  through  130 - 7  on the right side of the antenna  500  have a mirror image design and operate in the same fashion to form a second sector-splitting high-band antenna beam  504 , and hence further description thereof will be omitted. The linear array  130 - 4  of high-band radiating elements  132  may be used to support service in a different portion of the high-band, as discussed above, for example, with respect to base station antenna  400  of  FIG.  9 A . 
     Base station antenna  500  further includes first and second columns  134 - 1 ,  134 - 2  of low-band radiating elements  136 . One potential complication in the design of base station antenna  500  is that the low-band columns  134 - 1 ,  134 - 2  are mounted to extend forwardly from angled sections  142 - 1 ,  142 - 3  of the reflector  140 P, and hence the feed stalks of the low-band radiating elements  136  will not be perpendicular to the underlying sections of the reflector  140 P from which they extend. Plastic clips may be used to support the low-band radiating elements  136  so that they extend forwardly from the reflector  140 P at the correct angle (which here would be at angles of about 77.5° in light of the 12.5° angling of the reflector  140 P. 
       FIG.  10 B  depicts a dual-band base station antenna  510  that is a modified version of base station antenna  500  of  FIG.  10 A . Base station antenna  510  differs from base station antenna  500  in that (1) the middle longitudinally-extending section  142 - 2  of reflector  140 P is omitted, (2) antenna  510  only includes a single column  134 - 1  of low-band radiating elements  136  that is mounted in the middle of the antenna, and (3) antenna  510  does not include the extra column  130  of high-band radiating elements  132  that is used in antenna  500  to form a sector antenna beam in a different portion of the high-band frequency range. The primary advantage of base station antenna  510  over base station antenna  500  is that it has a reduced width, although it supports service in less frequency bands. 
     Pursuant to still further embodiments of the present invention, twin beam base station antennas are provided that use a beamforming network with some, but not all, of the columns of radiating elements that are used to form the two sector-splitting antenna beams. It has been found that including one or more additional columns of radiating elements that are not connected to the beamforming network may improve the overall performance of the antenna for some applications.  FIGS.  11 A and  11 B  schematically illustrate a base station antenna  600  that has such a design. 
     As shown in  FIG.  11 A , the base station antenna  600  includes five columns of high-band radiating elements  132  that are mounted on a flat reflector  140 R. Columns  130 - 1  through  130 - 4  may be used to generate a first sector-splitting antenna beam  602 , while columns  130 - 2  through  130 - 5  may be used to generate a second sector-splitting antenna beam  604 .  FIG.  11 B  schematically illustrates the feed network for one of the two polarizations supported by base station antenna  600 . As shown in  FIG.  11 B , the antenna  600  includes a first RF port  610 - 1  and a second RF port  610 - 2 . RF port  610 - 1  is coupled to a first power divider  612 - 1 , and RF port  610 - 2  is coupled to a second power divider  612 - 2 . The power dividers  612 - 1 ,  612 - 2  may be unequal power dividers in some embodiments. For example, the power dividers  612  may provide more power to their outputs  614 - 2 ,  614 - 4  (that feed columns  130 - 2  through  130 - 4 ) than to their outputs  614 - 1 ,  614 - 3  (that feed columns  130 - 1  and  130 - 5 , respectively). 
     Output  614 - 2  of power divider  612 - 1  and output  614 - 4  of power divider  612 - 2  are coupled to the respective inputs  622 - 1 ,  622 - 2  of a beamforming network  620 . The beamforming network  620  may comprise, for example, a 90° hybrid coupler in some embodiments. The first output  624 - 1  of beamforming network  620  is coupled to a power divider  612 - 3 . The first output  614 - 5  of power divider  612 - 3  is coupled to column  130 - 2  of high-band radiating elements  132 , and the second output  614 - 6  of power divider  612 - 3  is coupled to column  130 - 4  of high-band radiating elements  132  through a 180° phase shifter  630 . The second output  624 - 2  of beamforming network  620  is coupled to column  130 - 3  of high-band radiating elements  132 . The first output  614 - 1  of power divider  612 - 1  is coupled to column  130 - 1  of high-band radiating elements  132 , and the first output  614 - 3  of power divider  612 - 2  is coupled to column  130 - 5  of high-band radiating elements  132 . The power and phase of the sub-component of an RF signal input at RF port  610 - 1  that is passed to column  130 - 1  may be adjusted to shape the first antenna beam independently of the second antenna beam, while the power and phase of the sub-component of an RF signal input at RF port  610 - 2  that is passed to column  130 - 5  may be adjusted to shape the second antenna beam  604  independently of the first antenna beam  602 . This can provide improved performance in terms of azimuth beamwidth, azimuth pointing angle and azimuth sidelobes. While not shown in  FIG.  11 B , a phase shifter/power divider may be coupled between the beamforming network  620  and each column  130  of radiating elements  132 . Each phase shifter/power divider splits the RF signal that is fed to the column  130  of radiating elements  132  in order to feed sub-components of the RF signal to individual radiating elements  132  (or sub-arrays of radiating elements  132 ) and to apply an electrical downtilt, if desired, in the elevation plane. 
     Referring next to  FIGS.  12 A and  12 B , modified versions  700  and  710 , respectively, of the base station antenna  600  are provided that add mechanical steering to the columns  130 - 1 ,  130 - 5  of radiating elements  132  that are not coupled to the beamforming network  620 . In particular, as shown in  FIG.  12 A , the base station antenna  700  includes a reflector  140 S that has five longitudinally extending sections  142 - 1  through  142 - 5 . Longitudinally-extending section  142 - 3  is perpendicular to the azimuth boresight pointing direction of antenna  700 , and has the three columns  130 - 2  through  130 - 4  of radiating elements that are coupled to the beamforming network  620  mounted thereon. Longitudinally-extending section  142 - 2  is angled rearwardly (e.g., by an angle of about 27°-30° in the azimuth plane) from the left side of longitudinally-extending section  142 - 3 , and longitudinally-extending section  142 - 4  is angled rearwardly (e.g., by an angle of about 27°-30° in the azimuth plane) from the right side of longitudinally-extending section  142 - 3 . This design may be used to eliminate the need to electrically steer columns  130 - 1  and  130 - 5  (or to reduce the amount of electrical steering required), which may result in improved antenna patterns. 
     One problem with using Butler Matrix style beamforming networks to implement a twin beam, sector-splitting antenna is that there may be considerable change in the azimuth pointing angle (i.e., the azimuth angle where peak gain occurs) as a function of frequency. For example, one conventional Butler Matrix style twin beam base station antenna that includes four columns of high-band radiating elements that operate in the 1.7-2.7 GHz frequency band may generate antenna beams having azimuth pointing directions of 29.5° at 1.7 GHz, 26.1° at 2.2 GHz and 22.7° at 2.7 GHz, for a spread in azimuth pointing direction of 6.8°. For such an antenna, the azimuth HPBW is 38.8° at 1.7 GHz, 33.2° at 2.2 GHz and 28.0° at 2.7 GHz, for a spread of 10.8°. The use of the antenna  300  of  FIG.  7 A  according to embodiments of the present invention may provide some improvement (reduction) in the variation in the azimuth pointing angle, but at the expense of a corresponding increase in the azimuth HPBW. In particular, the antenna  300  of  FIG.  7 A  generates antenna beams having azimuth pointing directions of 21.3° at 1.7 GHz, 24.5° at 2.2 GHz and 26.6° at 2.7 GHz, for a spread in azimuth pointing direction of 5.3°. For such an antenna, the azimuth HPBW is 43.2° at 1.7 GHz, 36.7° at 2.2 GHz and 31.9° at 2.7 GHz, for a spread of 11.3°. 
     Pursuant to further embodiments of the present invention, the phase slope of the power divider output transmission lines may be adjusted using Shiffman phase shifters, phase balance lines or the like to improve (reduce) the variation in azimuth pointing direction and the azimuth HPBW as a function of frequency. For example, a Shiffman phase shifter may be added at the output of the beamforming network. With this change, the antenna may generate antenna beams having azimuth pointing directions of 26.9° at 1.7 GHz, 25.7° at 2.2 GHz and 24.2° at 2.7 GHz, for a spread in azimuth pointing direction of only 2.7°. For such an antenna, the azimuth HPBW is 42.0° at 1.7 GHz, 36.6° at 2.2 GHz and 31.8° at 2.7 GHz, for a spread of 10.2°. 
     It will also be appreciated that the techniques described herein may be used in tri-beam antennas (or quad-beam antennas) as well as in twin beam antennas. For example,  FIG.  13    as a schematic diagram illustrating a tri-beam antenna  800  according to embodiments of the present invention that may be viewed as a tri-beam implementation of the twin beam base station antenna  410  of  FIG.  9 B . The base station antenna  800  includes seven columns  130 - 1  through  130 - 7  of high-band radiating elements  132  that are mounted on a reflector  140 U that includes five longitudinally-extending sections  142 - 1  through  142 - 5 . Columns  130 - 3  and column  130 - 5  are shared columns of radiating elements. Thus, columns  130 - 1  through  130 - 3  may be used to generate a first antenna beam  802  that provides coverage to one third of a 120° sector, columns  130 - 3  through  130 - 5  may be used to generate a second antenna beam  804  that provides coverage to another third of the 120° sector, and columns  130 - 5  through  130 - 7  may be used to generate a third antenna beam  806  that provides coverage to the remaining third of the 120° sector. Columns  130 - 2  and  130 - 6  are mechanically steered (by about 35°-40°) which may help improve the shape of the first and third antenna beams  802 ,  806 . The tri-beam antenna  800  further includes a pair of columns  134 - 1 ,  134 - 2  of low-band radiating elements  136 . It will be appreciated that others of the twin beam antennas disclosed herein may similarly be modified to provide a variety of different tri-beam base station antennas. 
     The base station antennas described above include “columns” of radiating elements. Most typically, each “column” may comprise a vertically-oriented linear array of radiating elements where the radiating elements extend along a vertical axis. However, it will be appreciated that in some cases the columns may be so-called “staggered” linear arrays of radiating elements in which some of the radiating elements are offset horizontally from other of the radiating elements by a small amount. As explained in U.S. Provisional Patent Application Ser. No. 62/722,238, filed Aug. 24, 2018, the entire content of which is incorporated herein by reference, such staggered linear arrays may be included in base station antennas to, for example, improve the stability of the azimuth beamwidth across the frequency band of operation. 
       FIGS.  14 A- 14 C  illustrate a dual-band twin beam base station antenna  900  according to further embodiments of the present invention which includes such staggered columns of radiating elements. In particular,  FIG.  14 A  is a schematic perspective view of the dual-band twin beam base station antenna  900  (with the columns of low-band radiating elements omitted),  FIG.  14 B  is a schematic transverse cross-sectional of  FIGS.  14 A, and  14 C  is an isometric sectional view of the base station  900  with a low-band radiating element from each column included in the view. 
     As shown best in  FIG.  14 A , the base station antenna  900  includes five columns  130 - 1  through  130 - 5  of high-band radiating elements  132 . The columns  130  of high-band radiating elements  132  are mounted to extend forwardly from a shaped reflector  140 U. The high-band radiating elements  132  forming columns  130 - 1 ,  130 - 2 ,  130 - 4  and  130 - 5  are each staggered columns of radiating elements  132  in which every other radiating element  132  in each column  130  is offset in the horizontal direction from the adjacent radiating elements  132  in the column  130  by a small amount. Columns  130 - 1  and  130 - 2  are coupled to first and second RF ports (not shown) via a first common feed network (not shown) and together form a first array of high-band radiating elements  132  that generate first slant −45° and slant +45° polarization antenna beams. Columns  130 - 4  and  130 - 5  are coupled to third and fourth RF ports (not shown) via a second common feed network (not shown) and together form a second array of high-band radiating elements  132  that generate second slant −45° and slant +45° polarization antenna beams. Column  130 - 3  is coupled to fifth and sixth RF ports (not shown) via a third feed network (not shown) and generates third slant −45° and slant +45° polarization antenna beams. The staggering of columns  130 - 1  and  130 - 2  acts to narrow the azimuth beamwidth of the first array so that the first antenna beams generated by the first array each have an azimuth HPBW of about 33°. Similarly, the staggering of columns  130 - 4  and  130 - 5  acts to narrow the azimuth beamwidth of the first array so that the first antenna beams generated by the first array each have an azimuth HPBW of about 33°. Thus, the first and second antenna beams are each sized to cover a sub-sector of a 120° sector. The third antenna beams may have an azimuth HPBW of about 65° and may cover the full sector. 
     The reflector  140 U includes first through third longitudinally-extending sections  142 - 1  through  142 - 3 . Longitudinally-extending section  142 - 2  is mounted to extend perpendicularly to an azimuth boresight pointing direction of the base station antenna  900 , while longitudinally-extending sections  142 - 1  and  142 - 2  are angled with respect to longitudinally-extending section  142 - 2 . As a result, the first antenna beams generated by columns  130 - 1 ,  130 - 2  of high-band radiating elements  132  are mechanically steered to the left in the azimuth plane to cover the left sub-sector, and the second antenna beams generated by columns  130 - 4 ,  130 - 5  of high-band radiating elements  132  are mechanically steered to the right in the azimuth plane to cover the right sub-sector. In some embodiments, the staggered columns  130 - 1 ,  130 - 2 ;  130 - 4 ,  130 - 5  of high-band radiating elements  132  forming the first and second arrays may be mechanically steered by about 27° (by angling the reflector sections  142 - 1 ,  142 - 3  with respect to reflector section  142 - 2 ) so that electrical steering of the first and second antenna beams is not required. In other embodiments, the staggered columns  130 - 1 ,  130 - 2 ;  130 - 4 ,  130 - 5  of high-band radiating elements  132  forming the first and second arrays may be mechanically steered by less than, for example, 27° (e.g., between 10°-25°) and the first and second antenna beams may be electrically steered the remaining distance in the azimuth plane in order to point toward the middle of their respective sub-sectors. 
     The antenna  900  further includes two linear arrays  134 - 1 ,  134 - 2  of low-band radiating elements  136 , as shown in  FIG.  14 C . Low-band array  134 - 1  may be generally aligned with staggered high-band array  130 - 2 , and low-band array  134 - 2  may be generally aligned with staggered high-band array  130 - 4 . The two low-band linear arrays  134 - 1 ,  134 - 2  may operate as sector (as opposed to sector-splitting) arrays that provide service in different frequency bands or that operate as a 4×MIMO antenna in the same frequency band 
     It will be appreciated that the present specification only describes a few example embodiments of the present invention and that the techniques described herein have applicability beyond the example embodiments described above. It should also be noted that the antennas according to embodiments of the present invention may be used in applications other than sector-splitting such as, for example, in venues such as stadiums, coliseums, convention centers and the like. In such applications, the multiple beams are more usually configured to cover a 60°-90° sector. 
     Embodiments of the present invention have been described above with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. 
     It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the present invention. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     It will be understood that when an element is referred to as being “on” another element, it can be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. Other words used to describe the relationship between elements should be interpreted in a like fashion (i.e., “between” versus “directly between”, “adjacent” versus “directly adjacent”, etc.). 
     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 herein, 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. 
     Aspects and elements of all of the embodiments disclosed above can be combined in any way and/or combination with aspects or elements of other embodiments to provide a plurality of additional embodiments.