CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation-in-part of co-pending U.S. application Ser. No. 13/907,962, entitled “Antenna system providing simultaneously identical main beam radiation characteristics for independent polarizations”, filed Jun. 2, 2013. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of Invention 
     The invention generally is related to the area of antennas, and more particularly related to integrated antenna arrays structured in a way and controlled electronically to form a desired antenna pattern, wherein the desired antenna pattern can be controlled to demonstrate identical main beam radiation characteristics for independently polarized antennal elements. 
     Related Art 
     An antenna system is an indispensable component in communication systems. In conventional wireless communications, a single antenna is used at the source, and another single antenna is used at the destination. Physically, an antenna (or aerial) is an electrical device which converts electric power into radio frequency (RF) energy or waves, and vice versa. It is usually used with a radio transmitter or radio receiver. In transmission, a radio transmitter supplies an oscillating radio frequency electric current to the antenna, and the antenna radiates the energy from the current as electromagnetic waves (radio waves). In reception, an antenna intercepts some of the power of an electromagnetic wave in order to produce a tiny voltage to be supplied to a receiver to be amplified. 
     Antennas are essential components of all equipment that uses radio. They are used in systems such as radio broadcasting, broadcast television, two-way radio, communications receivers, radar, cell phones, and satellite communications, as well as other devices such as garage door openers, wireless microphones, Bluetooth enabled devices, wireless computer networks, baby monitors, and RFID tags on merchandise. Typically an antenna is an arrangement of metallic conductors and electrically connected (often through a transmission line) to a receiver or a transmitter. An oscillating current of electrons forced through the antenna by a transmitter creates an oscillating magnetic field around the antenna conductors, while the charge of the electrons also creates an oscillating electric field along the antenna conductors. These time-varying fields, when created in the proper proportions, radiate away from the antenna into space as a moving transverse electromagnetic field wave. Conversely, during reception, the oscillating electric and magnetic fields of an incoming radio wave exert force on the electrons in the antenna conductors, causing them to move back and forth, creating oscillating currents in the antenna. 
     It is well known that the RF energy radiated from an antenna system has its very unique polarization which depends on the geometry and the orientation of the antenna system. Typically, the polarization can be linear or elliptical. More specifically, linear polarization covers vertical polarization and horizontal polarization, and elliptical polarization covers circular polarization. Regardless, the very fundamental polarizations are vertical and horizontal polarizations. All other polarizations are simply the linear combinations of these two fundamental polarizations. For any transmission and reception of the RF energy, if the polarization of the transmitting antenna and the polarization of the receiving antenna do not line up, the RF energy would be lost, resulting in a weak RF link between two communicating devices. 
     In the Wi-Fi arena, the antenna systems at client ends are typically structurally simple and respond only to linearly polarized RF signals. Moreover, the orientations of the polarization at the client ends (e.g., a communicating device) are often unpredictable. It is therefore desirable for the Wi-Fi service providers to provide a system that is capable of offering both horizontally and vertically polarized RF links simultaneously for each individual channel in order to establish reliable respective RF links with the clients. Operationally, the clients can be best served when the main beam radiation characteristics of both the vertically polarized and the horizontally polarized RF energy are identical or substantially similar. Although it is not difficult to design an antenna system which provides both vertically polarized and horizontally polarized RF links, it is not trivial to provide both vertically polarized and horizontally polarized RF links that have the substantially similar main beam radiation characteristics. 
     One embodiment of the present invention is to provide designs of antennas that can provide both vertically polarized and horizontally polarized RF links that have substantially similar main beam radiation characteristics. With the compact designs, the antennas can be very well used for mobile devices for wireless communications. 
     SUMMARY OF THE INVENTION 
     This section is for the purpose of summarizing some aspects of the present invention and to briefly introduce some preferred embodiments. Simplifications or omissions in this section as well as in the abstract may be made to avoid obscuring the purpose of this section and the abstract. Such simplifications or omissions are not intended to limit the scope of the present invention. 
     The present invention generally pertains to designs of antenna arrays structured in a way to form a desired antenna pattern with substantially similar main beam radiation characteristics for both horizontally and vertically polarized RF links. According to one aspect of the present invention, the antenna arrays or an antenna system includes an array of antenna units. Each of the antenna units includes two antennas disposed orthogonally or in parallel disposed. These antenna units are arranged in a pre-defined geometric pattern to create two substantially similar main beam radiation characteristics for independent polarizations. 
     According to another aspect of the present invention, the antennas in an antenna unit are implemented on two printed circuit (PC) boards with metal (e.g., copper) strips etched thereon according to a predefined geometric design. Depending on implementation, each of the PC boards may have the metal strips on one side or both sides thereof. When there are metal strips or complete antennas on both sides of a PC board, they are coupled by a connection through the PC board. 
     According to still another aspect of the present invention, an antenna element includes a substrate (e.g., a PC board) with metal lines on one side and a set of metal elements on another side of the substrate, where the metal elements are provided to match a desired impedance (e.g., 50 ohms). When a plurality of such antennas are arranged in a predefined configuration, a desirable radiation pattern is formed to provide optimized links between an antenna system and respective clients. 
     According to still another aspect of the present invention, besides the metal strips for the antenna, a PC board is also used to support other circuits used to control the antenna thereon or for impedance matching. According to yet another aspect of the present invention, the antenna units in an array can be selectively energized to form a desired antenna pattern in accordance with a signal determined from radio signals communicated between a device equipped with the antenna system and another device (e.g., a Wi-Fi router in communication with a mobile device), where the desired antenna pattern provides an optimized antenna pattern to facilitate seamless or QoS communication between the two devices. 
     Depending on implementation, the present invention may be implemented as a method, an apparatus or part of a system. According to one embodiment, the present invention is an antenna system that comprises: a substrate; and a plurality of antenna units bonded to the substrate, each of the antenna units including a first printed circuit board and a second printed circuit board, wherein the first and the second printed circuit boards are disposed orthogonally or in parallel, each of the first and the second printed circuit boards includes metal strips etched on a first side according to a first predefined pattern and metal strips etched on a second side according to a first predefined pattern, wherein the first side and the second side is coupled by a connector going through the each of the first and the second printed circuit boards. The metal strips etched on the first side form two rounded halves, each of the halves having a metal strip going a certain pattern to form two enclosed loops and one channel between two segments of the metal strip. 
     According to another embodiment, the present invention is an antenna system that comprises: a substrate; and a plurality of antenna units arranged in a predefined geometric pattern and bonded to the substrate, each of the antenna units including a horizontally polarized antenna and a vertically polarized antenna, wherein either horizontally polarized antennas or vertically polarized antennas in the antenna units are energized, in accordance with a signal indicating a particular type of antenna a communication device is equipped with, to provide a better wireless link to the communication device communicating with an equipment employing the antenna system. 
     One of the objects, features and advantages of the present invention is to provide an antenna array or system that is amenable to small footprint, broad operating wavelength range, enhanced antenna pattern, lower cost, and easier manufacturing process. Other objects, features, benefits and advantages, together with the foregoing, are attained in the exercise of the invention in the following description and resulting in the embodiment illustrated in the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       These and other features, aspects, and advantages of the present invention will be better understood with regard to the following description, appended claims, and accompanying drawings where: 
         FIG. 1A  shows a perspective view of an antenna element with metal (e.g., copper) conductors etched on a printed circuit board; 
         FIG. 1B  shows a corresponding antenna element perceived as vertically mounted; 
         FIG. 1C  and  FIG. 1D  respectively show corresponding horizontal and vertical radiation patterns of the antenna element shown in  FIG. 1A  and  FIG. 1B ; 
         FIG. 1E  shows that the RF energy can reach the inverted “J” through a coupling provided in a printed circuit board; 
         FIG. 2A  shows an exemplary embodiment of the present invention using the basic antenna elements of  FIG. 1A  and  FIG. 1B  to structure an antenna unit configured to provide both vertically polarized and horizontally polarized RF links that have substantially similar main beam radiation characteristics; 
         FIG. 2B  shows that two antenna units are arranged vertically or one on top of the other with an equal space between them; 
         FIG. 2C  shows that two antenna units are arranged horizontally or next to each other; 
         FIG. 2D ,  FIG. 2E ,  FIG. 2F ,  FIG. 2G , and  FIG. 2H  shows respectively some additional exemplary structures using more such antenna units; 
         FIG. 2I - FIG. 2L  show respectively the radiation patterns of the antenna (unit) as shown in  FIG. 2B .  FIG. 2M - FIG. 2P  show respectively the radiation patterns of the antenna (unit) as shown in  FIG. 2C ; 
         FIG. 3A  shows another antenna element according to one embodiment of the present invention; 
         FIG. 3B  shows a back side of a substrate of  FIG. 3A , where there is a metal strip formed in a particular pattern to match with the antenna pattern on a front side to provide a desired impedance of the antenna element of  FIG. 3A ; 
         FIG. 3C  shows corresponding radiating characteristics of the antenna element in the E-plane, and  FIG. 3D  shows corresponding radiating characteristics of the antenna element in the H-plane; 
         FIG. 3E  shows a 3D perspective of the antenna element of  FIG. 3A  and  FIG. 3B ; 
         FIG. 3F  shows one exemplary antenna unit employing two antenna elements arranged perpendicular to each other; 
         FIG. 3G  shows one exemplary antenna unit employing two antenna elements arranged in parallel; 
         FIG. 3H  shows that an antenna unit including two such antenna elements arranged horizontally or next to each other; 
         FIGS. 3I and 3J  show respectively some exemplary structures using more such antenna units; and 
         FIG. 4  shows a system block diagram of an antenna system according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The detailed description of the invention is presented largely in terms of procedures, steps, logic blocks, processing, and other symbolic representations that directly or indirectly resemble the operations of communication devices coupled to networks. These process descriptions and representations are typically used by those skilled in the art to most effectively convey the substance of their work to others skilled in the art. 
     Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Further, the order of blocks in process flowcharts or diagrams representing one or more embodiments of the invention do not inherently indicate any particular order nor imply any limitations in the invention. 
     Service providers for wireless communication are looking for antenna systems that provide high power gain with small physical size. Further, it is desirable to deploy an antenna system that is capable of delivering optimal radio frequency (RF) power covering a known span of azimuthal angles. One embodiment of the present invention provides designs of antenna arrays structured in a way to form a desired antenna pattern with substantially similar main beam radiation characteristics for both horizontally and vertically polarized RF links. 
     Referring now to the drawings, in which like numerals refer to like parts throughout the several views. According to one embodiment,  FIG. 1A  shows a perspective view of an antenna element  100  with metal (e.g., copper) conductors etched on a printed circuit board, PC board or PCB  102 . The copper strips on one side of the PC board  102  can clearly be seen in the embodiment shown in  FIG. 1A . Depending on implementation, the other side of the PC board  102  may also be used to have the copper strips arranged similar to that on the first side. The antenna element  100  is perceived as horizontally mounted in  FIG. 1A .  FIG. 1B  shows a corresponding antenna element  104  perceived as vertically mounted. 
     According to one embodiment, the PC board  102  is double-sided. The copper trace on the other side of the PC board  102  is not visible in  FIG. 1A  but may be seen in  FIG. 1B . In general, the copper strips on both sides of a PC board (e.g., the PC board  102 ) do not look similar. On one side of the PC board, it is an antenna element, such as an inverted “J” shape as shown in  FIG. 1A  or  FIG. 1B , so it radiates when in transmission. On the other side of the PC board, the copper strips are serving as feed and impedance matching thus do not radiate when in transmission. Thus, the antenna on the PC board  102  or  106  is defined herein as an antenna element. 
     The PC board  102  is mounted perpendicularly on a metallic ground plane  110 . The geometries of the copper strips and the size of the ground plane  110  are uniquely designed to achieve a desirable beam width in a horizontal plane. This horizontal plane is also known as the E-plane if the antenna element is oriented in the way as it is shown in  FIG. 1A . The antenna element in  FIG. 1B  can be designed to achieve a desirable beam width in a horizontal plane, known as the H-plane if the antenna element is oriented in the way as it is shown in  FIG. 1B .  FIG. 1C  and  FIG. 1D  respectively show horizontal radiation patterns of the antenna element shown in  FIG. 1A  and  FIG. 1B . In other words, this basic antenna element in  FIG. 1A  or  FIG. 1B  offers substantially similar radiation characteristics in the horizontal plane. Furthermore, the antenna element shown in  FIG. 1A  and the antenna element shown in  FIG. 1B  are essentially identical. They are simply mounted in different orientations, one horizontally and the other one vertically. In other words, this basic antenna element essentially offers similar radiation characteristics in its E-pane and in its H-plane. 
     As it is shown in  FIG. 1A , the copper strips on one side of the PC board  102  take the form of two inverted “J” shapes.  FIG. 1B  shows the copper strips on the other side of the PC board  106  with additional functions as a feed system and a circuit for impedance matching. The feed system follows the leg of one of the inverted “J” traces and crosses over to the leg of the other inverted “J” and couple the RF energy to the inverted “J” on the other side of the PC board  106 . The RF energy can reach the inverted “J” through a direct connection as shown in  FIG. 1E . 
     Referring now to  FIG. 2A , it shows an exemplary embodiment of the present invention using the basic antenna elements shown in  FIG. 1A  and  FIG. 1B  to structure an antenna unit  200  configured to provide both vertically polarized and horizontally polarized RF links that have substantially similar main beam radiation characteristics. As shown in  FIG. 2A , two antenna elements  202  and  204  are isolated from each other as far as the RF energy is concerned. In other words, these two antenna elements  202  and  204  can function independently and provide both vertically polarized and horizontally polarized RF links simultaneously, where the main beam radiation characteristics for each of the RF links is substantially similar. 
       FIG. 2B  shows that two antenna units are arranged vertically or one on top of the other.  FIG. 2C  shows that two antenna units are arranged horizontally or next to each other. Multiple such antenna units may be arranged differently.  FIG. 2D ,  FIG. 2E ,  FIG. 2F ,  FIG. 2G , and  FIG. 2H  shows respectively some additional exemplary structures using more such antenna units. Those skilled in the art shall come up with additional structure based on the symmetric arrangement to form different antenna systems given the detailed description of the present invention herein. 
     Nevertheless, in all the possible configurations, the antenna units can be horizontally or vertically stacked or arrayed to function as one antenna system. According to one embodiment, all the antenna elements that are vertically arranged can be arrayed together and function as one antenna unit, and all the antenna elements that are horizontally arranged can be arrayed together and function as another antenna unit. In any case, the antenna units or arrays are fully independent from each other and provide both vertically polarized and horizontally polarized RF links that have substantially similar main beam radiation characteristics. 
       FIG. 2I - FIG. 2L  show respectively the radiation patterns of the antenna (unit) as shown in  FIG. 2B .  FIG. 2M - FIG. 2P  show respectively the radiation patterns of the antenna (unit) as shown in  FIG. 2C . These figures clearly demonstrate that the main beam radiations provided by the vertically polarized antenna unit and the horizontally polarized antenna unit are substantially similar. 
     Referring now to  FIG. 3A , it shows another antenna element  300  according to one embodiment of the present invention. The antenna element  300  is formed on a substrate (e.g., a PCB) and has two sides. On the first side as shown in  FIG. 3A , there are metal strips (e.g., copper) formed in a particular pattern  302 , two rounded halves  302 , each having a metal strip going a certain pattern to form two enclosed loops and one channel between two segments of the metal strip. On the second side of the substrate as shown in  FIG. 3B , there is also a metal strip formed in a particular pattern to match with the pattern  302  on the first side to provide a desired impedance. The metal strip on the second side is referred to as a driving or feeding metal trace. It appears to fork after going along a straight line and ends with a 90-degree turn segment. The metal strip on the second side is coupled by a conductor  306  to the first side of the substrate. According to one embodiment, the conductor goes through the substrate. 
     In one embodiment, the shapes of the copper trace as shown in  FIG. 3A  and  FIG. 3B  are so designed that (A) the RF radiating characteristics in both the H-plane and the E-plane of the antenna are substantially similar or identical, where the E-plane is the plane that is in parallel with the substrate and the H-plane is the plane that is perpendicular to the substrate; and (B) the input impedance of the antenna element is optimized to be matched with 50-ohm. 
       FIG. 3C  shows the radiating characteristics of the antenna element in the E-plane, and  FIG. 3D  shows the radiating characteristics of the antenna element in the H-plane.  FIG. 3E  shows a 3D perspective of the antenna element of  FIG. 3A  and  FIG. 3B . 
       FIG. 3F  shows one exemplary antenna unit employing two antenna elements as shown in  FIG. 3A  and  FIG. 3B . The two identical antenna elements are arranged perpendicular to each other as shown in  FIG. 3F  or vertically (one on top of the other) as shown in  FIG. 3G .  FIG. 3H  shows that two such antenna elements are arranged horizontally or next to each other. Multiple such antenna units may be arranged differently.  FIGS. 3I and 3J  show respectively some exemplary structures using more such antenna units. Those skilled in the art shall come up with additional structure based on the symmetric arrangement to form different antenna systems given the detailed description of the present invention herein. 
     Similar to the antenna elements shown in  FIG. 1A , in all the possible configurations, the antenna units of  FIG. 3F  or  FIG. 3F  can be horizontally or vertically stacked or arrayed to function as one antenna system. According to one embodiment, all the antenna elements that are vertically arranged can be arrayed together and function as one antenna unit, and all the antenna elements that are horizontally arranged can be arrayed together and function as another antenna unit. In any case, the antenna units or arrays are fully independent from each other and provide both vertically polarized and horizontally polarized RF links that have substantially similar main beam radiation characteristics. 
       FIG. 4  shows a system block diagram of an antenna system  400  used in a device (e.g., a wireless router) according to one embodiment of the present invention. The antenna units  402  may be housed in an enclosure. As shown in  FIG. 4 , the antenna system  400  includes a plurality of antenna units  402 - 1 , . . .  402 -N, where N is a positive integer. As described above, the antenna units  402 - 1 , . . .  402 -N are arranged in accordance with a predefined geometry and mounted onto a substrate. Each of the antennas units  402 - 1 , . . .  402 -N includes at least two antennas disposed orthogonally with each other or in parallel. In one embodiment, one antenna is a horizontally polarized antenna and the other is a vertically polarized antenna. 
     When the antenna units  402  are engaged to communicate with a communication device  404  (e.g., a laptop computer), both of the horizontally polarized antennas and the vertically polarized antennas therein are energized to exchange RF signals with the device  404 . As the antenna system  400  receives the RF signals via the horizontally polarized antennas or the vertically polarized antennas, it can be configured to detect what type of antenna  406  the device  404  is being equipped with. Without obscuring the aspects of the present instant invention, the details of how to detect the signal strength from an antenna or antennas are not to be provided herein. Those skilled in the art shall know that there are ways to do so. 
     According to one embodiment, when it is detected that the device  404  is equipped with a horizontally polarized antenna, there is no need to energize the vertically polarized antennas in the antenna units  402 . Likewise, when it is detected that the device  404  is equipped with a vertically polarized antenna, there is no need to energize the horizontally polarized antennas in the antenna units  402 . It is assumed that the feedback signal  408  is generated from the detection (e.g., through a signal detecting or measuring circuit) indicating which polarized antennas are better to sustain a reliable wireless link between the equipment employing the antenna system  400  and the device  404 . Either the horizontally polarized antennas or the vertically polarized antennas are energized by the engine  410 . 
     According to another embodiment, the antenna  406  of the device  404  may not be fully detected as to which of the horizontally polarized antennas and the vertically polarized antennas are better for the wireless link there between. The engine  410  may be configured to partially energize the horizontally polarized antennas while fully energizing the vertically polarized antenna or based on a ratio statically or dynamically determined from the RF signals being exchanged. 
     While the present invention has been described with reference to specific embodiments, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications to the present invention can be made to the preferred embodiments by those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claim. Accordingly, the scope of the present invention is defined by the appended claims rather than the forgoing description of embodiments.
Techniques of designing an antenna array or antenna system are described. The antenna system includes a plurality of antenna units structured in a way to form a desired antenna pattern. According to one aspect of the present invention, each of the antenna units includes two antennas disposed orthogonally or in parallel. These antenna units are arranged in a pre-defined geometric pattern to create two substantially similar main beam radiation characteristics for independent polarizations.