Patent Publication Number: US-11038272-B2

Title: Configurable antenna array with diverse polarizations

Description:
TECHNICAL FIELD 
     The present disclosure relates to configurable antenna arrays with diverse polarizations. 
     BACKGROUND 
     Wireless Local Area Networks (WLANs) are utilized for providing users with access to services and/or network connectivity. As a result, compact antenna modules are desirable to provide adaptive beams and multiple beams in WLANs. Many base station or access point antennas deploy arrays of antenna elements to achieve advanced antenna functionality, e.g., beam forming, etc. Thus, solutions for reducing the profile of individual antenna elements as well as for reducing the size (e.g., width, etc.) of the antenna element arrays are desired, while maintaining key performance features such as polarization diversity, high gain in a particular direction, and wide frequency bandwidths. 
     SUMMARY 
     Typical existing antennas face challenges in respect of the number of radio frequency streams, peak gain, polarizations and frequency bandwidths they can effectively support within a compact antenna package. Examples described herein can address one or more of these challenges in at least some applications. In at least some examples, an antenna configuration is provided that can support different frequency bands with multiple antenna units, each of which provide selectable polarization diversity. 
     According to one example aspect is a radio frequency (RF) antenna unit that includes a first antenna and a second antenna. The first antenna is positioned on a reflector element, and includes at least three inverted-F antenna (IFA) elements that are electrically connected to a first RF signal port and that each have an associated tunable element that controls excitation of the IFA element, the tunable elements being operative to control a radiation pattern direction of the first antenna. The second antenna is co-located on the reflector element with the first antenna, and includes a plurality of antenna elements. 
     In some examples, the tunable elements are operative to control excitation of the IFA elements to enable a first mode in which the first antenna has an omni-directional radiation pattern and a second mode in which the first antenna has a directional radiation pattern. Furthermore, the IFA elements may be arranged symmetrically around a central axis, on a printed surface board (PCB) substrate, and are spaced apart from and parallel to the reflector element. 
     In some examples, the first RF port is centrally located relative to the IFA elements, each IFA element being electrically connected to the first RF signal port through the tunable element associated with the IFA element such that the tunable element can selectively couple and decouple the IFA element to the first RF signal port. In some configurations, each IFA element may have an associated gain enhancing parasitic conductor that is located adjacent the IFA element on the PCB substrate a further distance from the RF signal port than the IFA element. 
     In some examples, the antenna elements of the second antenna are each connected to a second RF signal port and each have an associated tunable element that controls excitation of the antenna element, the tunable elements being operative to control a radiation pattern direction of the second antenna. The antenna elements of the second antenna may be centrosymetrically arranged around the central axis, and the antenna elements are each folded monopole antenna elements that extend perpendicular to the reflector element. 
     In some examples of the first aspect, the first antenna and the second antenna are configured to operate in the same frequency band, for example a 2.4 GHz band or a 5 GHz band. In some examples, the first antenna and the second antenna are configured to operate in different frequency bands, for example one in the 2.4 GHz band and one in the 5 GHz band. 
     In some examples, the first antenna comprises four IFA elements and the second antenna comprises four folded monopole antenna elements. In some examples, a shorting line of each monopole antenna element is connected to ground through the tunable element associated with the monopole antenna element. 
     In some alternative configurations, the antenna elements of the second antenna are IFA elements arranged symmetrically around the central axis, on a further PCB substrate, and are spaced apart from and parallel to the reflector element and the PCB substrate of the first antenna. 
     According to a further aspect, an antenna array is provided that includes a planar reflector element and first and second antenna units that respectively include a first antenna and a second antenna positioned on the reflector element. The first antenna is configured to operate in a first frequency range, and has at least three inverted-F antenna (IFAs) elements electrically connected to a first RF signal port and that each have an associated tunable element that controls excitation of the IFA element. The second antenna is configured to operate in a second frequency range and has at least three inverted-F antenna (IFAs) elements that are electrically connected to a second RF signal port. All of the IFA elements have an associated tunable element that controls excitation of the IFA element. A controller is operatively connected to the tunable elements associated with each of the IFA elements for selectively controlling radiation pattern directions of the first antenna and the second antenna. 
     In some examples configurations, the tunable elements are responsive to the controller to control excitation of the IFA elements to selectively enable a first and second mode for each of the first and second antennas, wherein in the first mode the IFA elements are excited collectively to provide an omni-directional radiation pattern and in the second mode the IFA elements are selectively excited to provide a directional radiation pattern. 
     In some examples, the first antenna unit includes a further antenna co-located on the reflector element with the first antenna and comprising at least three antenna elements electrically connected to a third RF signal port and that each have an associated tunable element that controls excitation of the antenna element. Similarly, the second antenna unit includes a further antenna co-located on the reflector element with the second antenna and comprising at least three antenna elements electrically connected to a forth RF signal port and that each have an associated tunable element that controls excitation of the antenna element. The controller is operatively connected to the tunable elements associated with each of the antenna elements for selectively controlling radiation pattern directions of the further antennas of the first antenna unit and the second antenna unit. 
     In some embodiments of the antenna array, each of the first antenna and the second antenna have their IFA elements arranged symmetrically around a central axis, on a printed surface board (PCB) substrate, and are spaced apart from and parallel to the reflector element. For the first antenna the first RF signal port is centrally located relative to the IFA elements, and each IFA element of the first antenna is connected to the first RF signal port through the tunable element associated with the IFA element. For the second antenna the second RF signal port is centrally located relative to the IFA elements, and each IFA element of the second antenna is connected to the second RF signal port through the tunable element associated with the IFA element. 
     In some embodiments the antenna array includes two of the first antenna units and two of the second antenna units located symmetrically around a central area of the reflector element, enabling 8 RF signals to be independently polarized. 
     In some examples of the antenna array, the first antenna and second antenna each include at least four IFA elements and the further antennas of the first antenna unit and the second antenna unit each comprise at least four folded monopole antenna elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a perspective view of an antenna array according to example embodiments; 
         FIG. 2  is a top plan view of the antenna array of  FIG. 1 ; 
         FIG. 3  is a perspective view of a 5 GHz band antenna unit of the antenna array of  FIG. 1 ; 
         FIG. 4A  is a perspective view of a first antenna of the antenna unit of  FIG. 3 ; 
         FIG. 4B  is a top view of the first antenna element of the antenna unit of  FIG. 3 ; 
         FIG. 4C  is a side view of the first antenna element of  FIG. 3 ; 
         FIG. 5A  is a perspective view of a second antenna of the antenna unit of  FIG. 3 ; 
         FIG. 5B  is a front side view of one leg of the second antenna of the antenna unit of  FIG. 3 ; 
         FIG. 5C  is a back side view of the second antenna leg of  FIG. 5B ; 
         FIG. 5D  is a front side view of another leg of the second antenna of the antenna unit of  FIG. 3 ; 
         FIG. 5E  is a back side view of the second antenna leg of  FIG. 5D ; 
         FIG. 6  is a top view of an antenna that can be used with the antenna unit of  FIG. 3  according to an alternative example embodiment; 
         FIG. 7A  is a perspective view of a stacked antenna unit that can be used in the antenna array of  FIGS. 1 and 2  according to further example embodiments; 
         FIG. 7B  is a top view of the stacked antenna unit of  FIG. 7A ; 
         FIG. 7C  is a side view of the stacked antenna unit of  FIG. 7A ; 
         FIG. 8  shows an example of an omni-directional radiation patterns for IFA elements of a 5 GHz antenna unit; 
         FIG. 9  shows directional radiation patterns of the IFA elements of a 5 GHz antenna unit; 
         FIG. 10  shows an example of omni-directional radiation patterns of the folded monopole antenna elements of a 5 GHz antenna unit; and 
         FIG. 11  shows and example of directional radiation patterns of the folded monopole antenna elements of the 5 GHz antenna unit. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     Multiple input and multiple output (MIMO) antenna technology produces significant increases in spectral efficiency and link reliability, and these benefits generally increase as the number of transmission antennas within the MIMO system increases. System operators require more and more capacity for multiple input and multiple output (MIMO) antennas. One way to increase the capacity of such a system is to provide an antenna array that includes multiple antenna units to support dual bands with high gain in diverse radiation pattern directions. 
       FIGS. 1 and 2  illustrate perspective and top views of an independently configurable dual band antenna array  100  with configurable radiation patterns, in accordance with example embodiments. The antenna array  100  includes a planar reflector element  114  that supports a set of first antenna units  110 ( 1 ),  110 ( 2 ) (referred to generically as first antenna units  110 ) and a set of second antenna units  120 ( 1 ),  120 ( 2 ) (referred to generically as second antenna units  120 ). The antenna units  110  and  120  all extend from the same side (referred to herein as the top surface  115 ) of the reflector element  114  and are centrosymmetrically arranged in alternating fashion around a central area of the top surface  115  of reflector element  114 . In an example embodiment the reflector element  114  is a multi-layer printed circuit board (PCB) that includes a conductive ground plane layer with a ground connection, one or more dielectric layers, and one or more layers of conductive traces for distributing control and power signals throughout the reflector element  114 . By way of non-limiting example, in one possible configuration the reflector element is a 200 mm by 200 mm square, although several other shapes and sizes are possible. 
     In example embodiments the first antenna units  110  are configured to emit or receive wireless radio frequency (RF) signals within a first RF band and the second antenna units  120  are configured to emit or receive wireless RF signals within a second RF band. For example, in some embodiments the antenna array  100  is used to support WiFi communications, with the first antenna units  110  configured to operate in the 5 GHz frequency band and the second antenna units  120  configured to operate in the 2.4 GHz frequency band. 
     In the illustrated example, the antenna array  100  includes two 5 GHz antenna units  110 ( 1 ),  110 ( 2 ), positioned at two corners of the reflector element  114  along a diagonal of the front surface  115 , and two 2.4 GHz antenna units  120 ( 1 ),  120 ( 2 ), positioned at the other two corners of the reflector element  114  along the other diagonal of the front surface  115 . The 2.4 GHz antenna units  120  are substantially centrosymmetrical with respect to each other about the central area of the front surface  115  and the 5 GHz antenna units  110  are centrosymmetrical with respect to each other about the central area of the front surface  115 , as illustrated in  FIGS. 1 and 2 . In different example embodiments, the number of antenna units operating at each frequency band could be less than or greater than 2, and the relative locations and orientations could be different than that shown in the Figures. Furthermore, the operating frequency bands could be different than the 2.4 GHz and 5 GHz bands that are referenced herein. 
     In the illustrated embodiment the configuration of the 5 GHz band antenna units  110 ( 1 ),  110 ( 2 ) is substantially identical to that of 2.4 GHz band antenna units  120 ( 1 ),  120 ( 2 ), except that the dimensions of each antenna unit  120  are scaled-up compared to those of each antenna unit  110  in order to target the larger wavelength of the 2.4 GHz band as opposed to the shorter wavelength of the 5 GHz band. In this regard  FIG. 3  shows an example architecture that can be applied to both antenna units  110  and  120  according to example embodiments. Each antenna unit  110 ,  120  includes co-located, electrically isolated first and second antennas  310  and  320  that are disposed on reflector element  114 . As will be explained in greater detail below, in example embodiments the first antenna  310  includes four inverted-F antenna (IFA) elements  311  that are disposed on a planar, horizontal substrate  312 . The substrate  312  is supported by a support structure  313  in a plane spaced apart from and parallel to the top surface  115  of reflector element  114 . The second antenna  320  includes two legs  320 A,  320 B that each support a pair of folded monopole-type antenna elements  314 . The legs  320 A,  320 B intersect at right angles at a central antenna unit axis A 1  that is normal to the reflector element  114  (e.g. the axis A 1  extends in the vertical Z direction in the coordinate system illustrated in the Figures). 
     The first and second antennas  310  and  320  provide independently configurable radiation patterns, with the four IFA elements  311  of the first antenna element  310  being configurable to emit or receive RF signals polarized with either omni-directional radiation pattern or directional radiation pattern, and the four monopole elements  314  of second antenna element  320  are also configurable to emit or receive RF signals polarized with either omni-directional radiation pattern or directional radiation pattern. Thus, both of the antennas  310 ,  320  of antenna unit  110 ,  120  can be configured into either omni-directional radiation pattern or directional radiation pattern modes independently of each other. 
     In the embodiment shown in  FIGS. 1 and 2 , the two 5 GHz antenna units  110 ( 1 ),  110 ( 2 ) and the two 2.4 GHz antenna units  120 ( 1 ),  120 ( 2 ) all have a similar orientation on the reflector element  114 . However, in other embodiments one or more of the units may have different radiation pattern orientations—for example one of the antenna units  110 ( 1 ) may be rotated 90 degrees about its vertical axis relative to the unit  110 ( 2 ). 
     Accordingly, in the illustrated embodiment of  FIGS. 1 and 2 , the antenna array  100  includes a total of eight independent antennas. In one embodiment, as shown in  FIG. 1 , eight independent conductive RF lines (RFL( 1 )-RFL( 8 )) are connected to the antenna array  100  to provide each antenna  310 ,  320  of each antenna unit  110 ( 1 ),  110 ( 2 ),  120 ( 1 ),  120 ( 2 ) with its own respective RF line. For example, the first antenna  310  of the antenna unit  110 ( 1 ) is connected to RF line RFL( 1 ) and the second antenna  320  of the antenna unit  110 ( 1 ) is connected to RF line RFL( 2 ). In example embodiments, the RF lines RFL( 1 )-( 8 ) each include a coaxial line having a signal conductor that is electrically connected to a respective signal path that extends through the reflector element  114  and is connected to an RF port for a corresponding antenna  310 ,  320 . 
     Configuring the two antennas  310 ,  320  of the antenna units  110 ,  120  to emit or receive RF signals with either omni-directional radiation pattern or directional radiation pattern is controlled by an antenna controller  140  ( FIG. 1 ). The antenna controller  140  could for example include a microprocessor and a storage element that stores instructions that configure the microprocessor to operate to selectively control tunable elements that, as described in greater detail below, are provided at each of the antennas  310 ,  320 . 
     The antenna units  110 ,  120  can take a number of different possible configurations. An example configuration for a horizontally oriented first antenna  310  that can be used in antenna units  110 ,  120  will now be described in greater detail with reference to  FIGS. 4A to 4C . As previously noted, in example embodiments the first antenna  310  includes four inverted-F antenna (IFA) elements  311  that are disposed on a horizontal substrate  312  that is supported by support structure  313 . In example embodiments, the support structure  313  is formed from co-located, vertical support legs  313 A and  313 B, that are perpendicular to each other and bisect each other at vertical axis A 1 . 
     In examples, substrate  312  and support legs  313 A and  313 B are each formed from printed circuit boards (PCBs) that include a dielectric substrate that support one or more conductive regions. In at least some example embodiments, the PCBs may be 0.5 mm thick, although thicker and thinner substrates could be used. Conventional PCB materials such as those available under the Taconic™ or Arlon™ brands can be used. In some examples, the PCBs may be formed from a thin film substrate having a thickness thinner than around 600 μm in some examples, or thinner than around 500 μm, although thicker substrate structures are possible. Typical thin film substrate materials may be flexible printed circuit board materials such as polyimide foils, polyethylene naphthalate (PEN) foils, polyethylene foils, polyethylene terephthalate (PET) foils, and liquid crystal polymer (LCP) foils. Further substrate materials include polytetrafluoroethylene (PTFE) and other fluorinated polymers, such as perfluoroalkoxy (PFA) and fluorinated ethylene propylene (FEP), Cytop® (amorphous fluorocarbon polymer), and HyRelex materials available from Taconic. In some embodiments the substrates are a multi-dielectric layer substrate. 
     As shown in  FIGS. 4A-4C , the four IFA elements  311  are each formed from a conductive material printed on an upper surface  402  of the horizontal substrate  312  that is parallel to and faces away from the upper surface  115  of reflector element  114 . A conductive ground plane  402  is formed on the opposite, bottom surface  404  of the substrate  312 , facing towards the reflector element  114 . In the Figures, substrate  312  is shown as being transparent for the purpose of illustrating the components of the described embodiment. The four IFA elements  311  are disposed centrosymmetrically on the substrate  312  around a central RF port  401 , with each IFA element  311  rotated 90 degrees relative to its adjacent IFA elements. Arrows  408  in  FIG. 4B  illustrate the directions of electric field radiation pattern of the IFA elements  311 . The RF signal line  410  of each IFA element  311  is connected by a respective microstrip signal path  414  formed on substrate  312  to the central RF port  401 . A tunable element  412  is provided on each of the signal paths  414  that enables each of the IFA elements  311  to be selectively coupled to or decoupled from the RF port  401 . The shorting lines  416  of each of the elements are connected by respective conductive paths that extend through the substrate  312  to the ground plane  406 . 
     In example embodiments, the tunable element  412  may selectively couple or decouple the IFA elements  311  by creating a virtual, RF open circuit or closed circuit, such as with the use of PIN diodes. Alternatively, in example embodiments, the tunable element  412  may selectively couple or decouple the IFA elements  311  by creating a physical open circuit or closed circuit, such as with the use of MEMS devices. 
     In example embodiments, the ground plane  406  is centrosymmetrical about and electrically isolated from the central RF port  401 . In the illustrated embodiment, the ground plane  406  is rectangular and includes slots that extend inward on each of its four sides in order to reduce coupling between the IFA elements  311 . Each side edge of the ground plane  406  runs parallel to the elongate resonating element of a respective IFA element  311 . 
     The IFA elements  311  and the microstrip signal paths  414  may be formed from conductive material such as copper or a copper alloy, or alternatively, aluminum or an aluminum alloy, that have been printed onto the first surface  402  of the substrate  312 . Additionally, the centrosymmetrically shaped ground plane  406  may be formed from conductive material such as copper or a copper alloy, or alternatively, aluminum or an aluminum alloy, that have been printed onto the second surface  404  of the substrate  312 . In example embodiments, tunable elements  412  may include PIN diodes or Micro-Electro-Mechanical System (MEMS) devices. 
       FIG. 4C  shows a side view of legs  313 A and  313 B of the support structure  313  of antenna  310 . The PCBs that form support legs  313 A and  313 B each include a conductive ground layer, as well as conductive control lines  420  and one or more conductive RF signal paths  422 . The conductive ground layer connects ground plane  406  of the horizontal substrate  312  to a ground layer of reflector element  114 . In an example, the support structure  313  supports four independent control lines  420 , each of which is operatively connected at an upper end to a respective one of the tunable elements  412  and at its opposite end to a respective control line provided on the reflector element  114  and electrically connected to controller  140 . In some examples, each support leg  313 A and  313 B includes two control lines  420 . The RF signal paths  422  in support structure  313  are electrically coupled to RF port  401  at an upper end, and coupled at their opposite ends through a signal path in the reflector element  114  to one of the eight RF lines (for example RFL( 1 ). 
     In an example embodiment, the vertical support legs  313 A and  313 B have cooperating slots along the central axis A 1  that allows them to connect to each other, and they also each include centrally located a downwardly opening void or slot  424  that allows the structure of the first antenna  312  to be placed over a central part of the structure of the second antenna  320 . The ground planes, control lines  420  and RF signal path  422  on the substrate  400  of the support legs  313 A,  313 B are electrically isolated with respect to each other, and may be formed from conductive material such as copper or a copper alloy, or alternatively, aluminum or an aluminum alloy, that have been printed onto the substrate of the antenna support legs  313 A,  313 B. 
     Accordingly, in example embodiments, each of the four IFA elements  311  of the antenna  310  are connected to a common RF line (for example RFL( 1 )) through a respective tunable element  412 . The four tunable elements  412  are in turn each individually connected to controller  140 , such that each of the four IFA elements  311  of the antenna  310  can be selectively activated by coupling them to or decoupling them from the RF signal line, enabling the antenna  310  to be controlled to emit or receive RF signals using all of the IFA elements  311  together in an omnidirectional mode or selectively using the IFA elements  311  in a directional mode. In the illustrated example, controller  140  is used to control a connection between each IFA element  311  and the central RF port  401 , exciting the IFA elements  311  to emit or receive signals with diverse radiation pattern in either omni-directional radiation pattern direction or directional radiation pattern. As illustrated by the electric field radiation pattern arrows  408 , the four symmetrical IFA elements  311  facilitate electric field vectors that form a circle, cancelling the radiation in the direction normal to the ground plane of the reflector element  114  as well as increasing radiation at angles close to the ground plane of the reflector element  114 . Such a configuration can be beneficial for increasing antenna radiation range. 
     Referring to  FIG. 4B , in example embodiments, the IFA elements  311  of an antenna  320  are each identical and each have a combined back length L 1  plus shorting line length L 2  of about ¼ of the operating wavelength λ 1 , and the rectangular ground plane  406  has a side edge length of about ½ of the operating wavelength λ 1 . Additionally, in example embodiments, the antenna support structure  313  supports the substrate  312  of antenna  310  a distance H 1  from the reflector element  114 , where H 1  is about H 1 ≈λ 1 /2 for a 5 GHz frequency band antenna and about H 1 ≈λ 1 /4 for a 2.4 Ghz frequency band antenna. λ 1  is the operating wavelength near the lower end of the 5 GHz or 2.4 GHz frequency band for antenna unit  110  or  120  respectively. In some example embodiments, “about” can include a range of +/−15%. 
     An example embodiment of second antenna  320  will now be described in greater detail with reference to  FIGS. 5A to 5E . As indicated above, the second antenna  320  includes two legs  320 A,  320 B that each support a pair of folded monopole-type antenna elements  314 . The legs  320 A,  320 B each have a generally U-shaped profile and intersect at right angles at a central antenna unit axis A 1  that is normal to the reflector element  114 . The legs  320 A and  320 B are each formed from a respective PCB that includes a dielectric substrate  502 A,  502 B. Regarding the leg  320 A, as best seen in  FIG. 5B , a conductive pattern or region  501  is formed on one side of the generally U-shaped dielectric substrate  502 A that is symmetrical about antenna unit axis A 1 . The substrate  504  has mounting tabs  508 ,  510  formed along its back edge  511  for mating with corresponding slots that are formed in the reflector element  114 . The conductive region  501  is a conductive layer formed on a surface of the substrate  502 A that is perpendicular to the front surface  115  of reflector element  114 . Conductive region  501  is connected to a central microstrip RF signal port  506  that is electrically isolated from the ground plane of the reflector element  114 . 
     Conductive region  501  includes two identical portions that extend in opposite directions outward from central connector  506 . Each portion forms one of the folded ¼ wavelength monopole antenna elements  314 , with each antenna element  314  including: a first elongate RF signal line  512  that extends along surface  503  generally parallel to back edge  511  to a RF resonating section  514  that extends at a right angle from the first section  512  towards a top edge  516  of the substrate  504  to a connecting line section  518  that extends generally parallel to the front edge  516 . The connecting line section  518  extends to a shorting line  520  that folds back to extend to the back edge  511  of the substrate  502 A. In example embodiments, RF resonating section  514  has a height H 2  of about ¼ of the operating wavelength λ 1 , and each U-shaped leg  320 A has a width of about ½ of the operating wavelength λ 1 . 
     Leg  320 B has a similar configuration to leg  320 A, with the exception of the central regions of the legs that are respectively slotted to cooperate with each other so that the legs can bisect each other at a perpendicular angle along central axis A 1 . In this regard, as seen in  FIG. 5C , the first monopole leg  320 A includes a conductive pad  5308  on its reverse surface that is electrically connected to RF signal port  506 , and an upwardly opening slot  5304  along the central axis A 1  for receiving a portion of the second monopole leg  320 B. The second monopole leg  320 B has the corresponding downwardly opening slot  5306  along central axis A 1  for receiving a portion of the first monopole leg. When the monopole legs  320 A and  320 B are connected at 90 degree angle along axis A 1 , the conductive regions  502 A,  502 B are located at right angles to each other and are bisected along axis A 1 . One antenna element  314  of leg  320 B is electrically and physically connected (for example by solder) to the conductive region  518  of the leg  320 A, and the other antenna element of the second leg  320 B is electrically and physically connected (for example by solder) to the conductive pad  5308 , such that all four antenna elements  314  are electrically connected to RF signal port  306 . 
     Antenna elements  314  and the other conductive portions on legs  320 A,  320 B may be formed from a conductive material such as copper or a copper alloy, or alternatively, aluminum or an aluminum alloy, that have been printed onto the substrate  502 A,  502 B. 
     Referring to  FIG. 5A , when antenna element  320  is mounted on reflector element  114 , the central RF signal port  506  is connected to one of the RF lines (for example RFL( 2 ), such that all four antenna elements  314  of antenna  320  are electrically connected to the same RF feed. In the illustrated example, the ground line  520  of each antenna element  314  is connected through a respective tunable element  530  to the ground plane layer of the reflector element  114 , and the respective tunable elements  530  are each connected by a respective control line  532  that extends through the reflector element  114  to controller  140 . The tunable elements  530  enable each of the antenna elements  314  to be selectively coupled to or decoupled from ground, and may include for example PIN diodes or MEMS devices. 
     Accordingly, in example embodiments, the ground line  520  of each of the four folded monopole antenna elements  314  of the antenna  320  are connected to a common ground plane through a respective tunable element  530 . The four tunable elements  530  are in turn each individually connected to controller  140 , such that each of the four antenna elements  314  can be selectively activated by coupling them to or decoupling them from ground, enabling the antenna  314  to be controlled in an omni-directional mode or in a directional mode. In the illustrated example, controller  140  is used to control a connection between each antenna element  314  and ground, exciting the elements  314  to emit or receive signals with diverse radiation pattern in either omni-directional radiation pattern direction or directional radiation pattern. 
     In example embodiments, the tunable element  530  may selectively couple or decouple the antenna elements  314  by creating a virtual, RF open circuit or closed circuit, such as with the use of PIN diodes. Alternatively, in example embodiments, the tunable element  530  may selectively couple or decouple the antenna elements  314  by creating a physical open circuit or closed circuit, such as with the use of MEMS devices. 
     As shown in  FIG. 3 , first and second antennas  310  and  320  are co-located on the surface  115  of reflector element  114  to form an antenna unit  110 ,  120 . In the illustrated example, the support legs  313 A and  313 B of first antenna  310  meet at a right angle at the axis A 1  with one leg  313 A rotated clockwise +45 degrees relative to the second antenna leg  320 A and the other first antenna leg  313 B is rotated clockwise +45 degrees relative to the second antenna leg  320 B such that the legs are symmetrically spaced round the common antenna unit axis A 1 . The upwardly U-shaped configuration of the second antenna legs  320 A,  320 B provides space that cooperates with the downwardly opening U-shaped voids  424  in first antenna legs  313 A,  313 B to physically isolate the first antenna  310  and the second antenna  320  from each other. 
     In example embodiments the antenna elements  314  of antenna unit  310 ,  320  are vertically oriented at a right angle relative to reflector element  114 , with the pair of antenna elements  310  on leg  320 A and the antenna elements on leg  320 B being perpendicular planes relative to each other. The IFA elements  311  extend in a horizontal plane parallel to reflector element  114 . 
     In the embodiment described above, the antenna array  100  can support up to 8 RF streams or channels using the four antenna units  110 ( 1 ),  110 ( 2 ),  120 ( 1 ),  120 ( 2 ), with 4 of the streams operating in a first frequency band and 4 of the streams operating in a second frequency band. Furthermore, by controlling the tunable elements that are attached to each of antenna elements  311 ,  314 , the radiation pattern of each RF stream can be controlled, providing independently selectable directive patterns for each RF stream and each operating frequency. In addition, configurations of the antenna array not only reduce gain at boresight but also increase high performance with high gain near horizontal plane for each stream. 
     In the examples described above, the selective excitability of the antenna elements is provided in first antenna  310  by the use of tunable elements that operatively connect the RF signal lines of IFA elements  311  to RF signal port, whereas in second antenna  320 , the selective excitability is provided by the use of tunable elements that operatively connect the shorting lines of the folded monopole antenna elements  314  to ground. In alternative example embodiments, the location of the tunable elements in antennas  310 ,  320  can be changed—for example the tunable elements could be moved to the IFA element shorting line from the RF signal line in the case of first antenna  310 , and from the shorting line to the RF signal line in the case of second antenna  320 . 
     In example embodiments, the number of antenna elements used in each of the first and second antennas  310 ,  320  could be more then or less than four controllable antenna elements. For example, in an alternative embodiment, second antenna  320  could be formed from three folded monopole elements  314  spaced at 120 degree intervals about central axis A 1 . Similarly, first antenna  310  could also include only three IFA elements  311 , and in this regard  FIG. 6  shows an alternative example of a first antenna  610  that is substantially identical to antenna  310  except that antenna  610  only includes three individually controllable IFA elements  311  rather than four. In the example of  FIG. 6 , the IFA elements are centrosymetrically located about axis A 1  at 120 degree spacing relative to each other, and ground plane  406  is triangular with each side running parallel to the elongate resonating element of a respective IFA element  311 . 
     As illustrated in  FIG. 6 , in some example embodiments, outboard parasitic conductors  602  are provided on the substrate  312  to provide enhanced horizontal pattern gain. In the example of  FIG. 6 , three electrically isolated parasitic conductors  602  are located on the upper surface of substrate  312  to function as a parasitic director. As shown in  FIG. 6 , each parasitic conductors  602  is an elongate conductive strip that is located outward (relative to central axis A 1  and RF port  401 ) of a respective IFA element  311  and parallel to the radiation pattern direction of the respective IFA element  311 . Although shown in the context of a three IFA element antenna  610 , parasitic conductors  602  could also be used in the four IFA element antenna  310  described above, with a respective parasitic conductor  602  being located outward of and parallel to each of the four IFA elements  311 . 
     In the embodiments described above, each antenna unit  110 ,  120  has included two co-located antennas  310 ,  320  that both operate in the same band (for example 5 GHz for antenna unit  110  and 2.4 GHz for antenna unit  120 ), with the IFA elements  311  in antenna  310  being oriented in an orthogonal plane relative to the folded monopole antenna elements  314  in antenna  320 . However, in alternative example embodiments the co-located antennas in each antenna unit may be configured to operate in different bands or have antenna elements that are oriented in parallel planes, or both. In this regard,  FIGS. 7A, 7B and 7C  show an example embodiment of an alternative structure for a co-located antenna unit  700  that can be used in array  100  in place of one or more antenna units  110 ,  120 . Co-located antenna unit  700  is a stacked antenna unit that includes a first antenna  710  that operates at a first frequency band, and a second antenna  720  that operates at a second frequency band. Each of first antenna  710  and second antenna  720  has a configuration similar to that of first antenna  310  or  610  described above. In the illustrated example, first antenna  710  includes at least three horizontally oriented IFA elements  311  arranged on a PCB substrate  7101  centrosymetrically around a central RF port  701  that is located at central antenna axis A 1 , with each RF element  311  connected to the central RF port  701  through a respective tunable element  412 . Similarly, second antenna  710  includes at least three horizontally oriented IFA elements  311  arranged on a PCB substrate  7201  centrosymetrically around a central RF port  702  that is located at central axis A 1 , with each RF element  311  connected to the central RF port  702  through a respective tunable element  412 . 
     As best seen in  FIG. 7C , The PCB substrates  7101 ,  7201  of antennas  710 ,  720  are arranged in a horizontally oriented stacked configuration parallel to each other and parallel to the upper surface  115  of reflector element  114 . The second antenna  720  is spaced above the reflector element  114  by a distance H 3  and the first antenna  710  spaced above the reflector element  114  by a larger distance H 4 . The PCB substrate  7101  of second antenna  720  is secured to and supported above the reflector element  114  by a PCB support structure  7202 , and the PCB substrate  7101  of first antenna  710  is secured to and supported above the PCB substrate  7201  by a further PCB support structure  7102 . The PCB support structure  7202  includes a ground plane that connects the ground plane  406  on the under side of PCB substrate  7201  of second antenna  720  to the ground plane of the reflector element  114 . The PCB support structure  7102  also includes a ground plane that electrically connects the ground plane  406  on the under side of PCB substrate  7101  of first antenna  710  to the ground plane of the substrate  7202 . A first RF signal path RF 1  is provided through PCB support structures  7102 ,  7201  that connects the RF signal port  701  of the first antenna  710  to a respective one of the RF lines RFL( 1 ) to ( 8 ), and a second RF signal path RF 2  is provided through PCB support structure  7201  that connects the RF signal port  702  of the second antenna  720  to a further respective one of the RF lines RFL( 1 ) to ( 8 ). Although not shown in  FIG. 7C , controls paths  420  for the tunable elements  412  are also provided through the PCB support structures  7102 ,  7201  to allow the antenna controller  140  to selectively excite each of the IFA elements  311 . 
     In the example of  FIGS. 7A-7C  the first upper antenna  710  is rotated 60 degrees relative to second antenna  720  so that the IFA elements  311  on the upper first antenna  710  are not in vertical alignment with the IFA elements  311  on the lower second antenna  720 . 
     In the example shown in  FIGS. 7A-7C , first antenna  710  is configured to operate in the 5 GHz band and accordingly and the dimensions of second antenna  720  are scaled up relative to the first antenna  710  to operate in the 2.4 GHz band. However, in other embodiments, both antennas  710  and  720  could be configured to operate in the same band. Furthermore, in some embodiments, additional antennas for additional RF signals could be added to the antenna unit  700 . 
     In example embodiments, antenna units  700  can be used to replace some or all of the antenna units  110 ,  120  in antenna array  100 , or be added as additional antenna units in antenna array  100 . In at least some configurations, embodiments of the antenna array  100  can advantageously accomplish one of more of the following: increase the capacity of a MIMO antennal; efficiently use available real estate and space; reduce the size of an antenna required; reduce gain at boresight; and detect a wide range of RF signals. 
       FIGS. 8 and 9  show example radiation patterns for the antenna elements of a three IFA 5 GHz antenna unit  610 . In particular:  FIG. 8  shows an example of a omni-directional radiation pattern for all three IFAs being excited;  FIG. 9  shows an example of directional radiation patterns for two of three IFAs being excited.  FIGS. 10 and 11  shows example radiation patterns for the folded monopole antenna  320  in the presence of the three IFA 5 GHz antenna unit  610 :  FIG. 10  shows an omni-directional radiation pattern for the monopole elements  314 ; and  FIG. 11  shows a directional radiation pattern for the monopole elements  314 . 
     For each antenna elements of the antenna units, omni-directional radiation patterns as well as directional radiation patterns are independently configurable on any stream. Embodiments of the invention may be applied to radar system such as automotive radar or telecommunication applications such as transceiver applications in base stations or user equipment (e.g., hand held devices) or access point (AP). In one example embodiment, antenna array  100  is incorporated into a low profile wireless local area network (WLAN) access point (AP). The dimensions described in this application for the various elements of the antenna array  100  are non-exhaustive examples and many different dimensions can be applied depending on both the intended operating frequency bands and physical packaging constraints. 
     While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.