Abstract:
An antenna formable on a ground plane for wireless communications is disclosed. The antenna has a first structure spatially displaced from a ground plane. The antenna also has a second structure coupled to the first structure and extending away from the ground plane. The antenna further has a third structure configured for disposing at least a portion of the first structure between the third structure and the ground plane, the third structure being grounded to the ground plane and being spatially displaced from the second structure, the second structure and the third structure being inter-configured for electromagnetic coupling therebetween for forming a magnetic loop. More specifically, electromagnetic field generated by the first structure and the second structure electromagnetically couples the third structure for inducing generation of broadband electromagnetic waves from the first, second and third structures.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. provisional application Ser. No. 60/929,245, filed Jun. 19, 2007 and entitled “A Broadband Antenna Apparatus” incorporated herein by reference in its entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The invention relates generally to antennas. In particular, it relates to a broadband antenna for wireless communications. 
       BACKGROUND 
       [0003]    The use of broadband technology is becoming increasingly popular in wireless communication systems. In particular, broadband technology is used in multi-band applications to support WLAN, WiFi, WiMAX and UWB standards. Separate antennas are typically required for each of the WLAN, WiFi, WiMAX and UWB applications, as well as separate corresponding circuitries for supporting the applications. 
         [0004]    Additionally, there is an increasing demand for smaller and portable broadband wireless communication systems. This means that the antennas and corresponding circuitries of conventional wireless communication systems are required to have smaller dimensions in order to facilitate the miniaturization of such systems. 
         [0005]    However, conventional wireless communication systems either do not support multiband applications or are not sufficiently miniaturized for such systems to be portable. 
         [0006]    There is therefore a need for a broadband antenna that is dimensionally small and is capable of supporting multi-band applications for use in small portable broadband systems. 
       SUMMARY 
       [0007]    Embodiments of the invention are disclosed hereinafter for broadband applications having a small dimensional size and capable of supporting multi-band applications for use in small portable broadband systems. 
         [0008]    In accordance with a first embodiment of the invention, there is disclosed an antenna formable on a ground plane for wireless communications. The antenna has a first structure spatially displaced from a ground plane. The antenna also has a second structure coupled to the first structure and extending away from the ground plane. The antenna further has a third structure configured for disposing at least a portion of the first structure between the third structure and the ground plane, the third structure being grounded to the ground plane and being spatially displaced from the second structure, the second structure and the third structure being inter-configured for electromagnetic coupling therebetween for forming a magnetic loop. More specifically, electromagnetic field generated by the first structure and the second structure electromagnetically couples the third structure for inducing generation of broadband electromagnetic waves from the first, second and third structures. 
         [0009]    In accordance a second embodiment of the invention, there is disclosed a method for configuring an antenna formable on a ground plane for wireless communications. The method involves spatially displacing a first structure from a ground plane. The method also involves coupling a second structure to the first structure and extending the second structure away from the ground plane. The method further involves configuring a third structure for disposing at least a portion of the first structure between the third structure and the ground plane, the third structure being grounded to the ground plane and being spatially displaced from the second structure, the second structure and the third structure being inter-configured for electromagnetic coupling therebetween for forming a magnetic loop. More specifically, electromagnetic field generated by the first structure and the second structure electromagnetically couples the third structure for inducing generation of broadband electromagnetic waves from the first, second and third structures. 
         [0010]    In accordance with a third embodiment of the invention, there is disclosed an antenna array for wireless communications. The antenna array has a body having a plurality of surfaces and a plurality of antennas formed on the surfaces of the structure. Each of the plurality of antennas comprises a first structure spatially displaced from a ground plane and a second structure coupled to the first structure and extending away from the ground plane. Each of the plurality of antennas further has a third structure configured for disposing at least a portion of the first structure between the third structure and the ground plane, the third structure being grounded to the ground plane and being spatially displaced from the second structure, the second structure and the third structure being inter-configured for electromagnetic coupling therebetween for forming a magnetic loop. More specifically, electromagnetic field generated by the first structure and the second structure electromagnetically couples the third structure for inducing generation of broadband electromagnetic waves from the first, second and third structures. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0011]    Embodiments of the invention are described in detail hereinafter with reference to the drawings, in which: 
           [0012]      FIG. 1  shows a perspective view of an antenna having a shorting wall, radiator and feed plate structure according to a first embodiment of the invention; 
           [0013]      FIG. 2  shows a plan view of the antenna of  FIG. 1 ; 
           [0014]      FIG. 3  shows a side view of the antenna of  FIG. 1 ; 
           [0015]      FIG. 4  shows side views of the antenna of  FIG. 1 , including exemplary configurations of the feed plate structure according to other embodiments of the invention; 
           [0016]      FIG. 5  is a graph showing measured return loss characteristic of the antenna of  FIG. 1 ; 
           [0017]      FIGS. 6   a  to  6   c  are graphs showing measured radiating patterns of the antenna of  FIG. 1  at 5.25 GHz, 5.6 GHz, and 5.8 GHz respectively; and 
           [0018]      FIG. 7  shows an antenna array for providing an omni-directional coverage in a desired plane. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    With reference to the drawings, embodiments of the invention involving an antenna are described for broadband applications having a small dimensional size and capable of supporting multi-band applications for use in small portable broadband systems. 
         [0020]    For purposes of brevity and clarity, the description of the invention is limited hereinafter to broadband applications. This, however, does not preclude embodiments of the invention from other applications that require similar operating performance as the broadband applications. The functional principles fundamental to the embodiments of the invention remain the same throughout the various embodiments. 
         [0021]    Embodiments of the invention are described in greater detail in accordance with  FIGS. 1 to 7  of the drawings hereinafter, wherein like elements are identified with like reference numerals. 
         [0022]      FIG. 1  shows a perspective view of an antenna  100  according to a first embodiment of the invention. The following description of the antenna  100  is made with reference to an x-axis, a y-axis and a z-axis of a three-dimensional coordinate system. The x and y axes extend along a ground plane  110  and are coincident therewith. 
         [0023]    The antenna  100  comprises structures that are interconnected and inter-displaced for supporting applications in high gain broadband wireless communications. Specially, the antenna  100  comprises a shorting wall  102  formed substantially along the yz-plane. The shorting wall  102  has a first edge  104  that is attached to a radiator  106 . The radiator  106  is formed substantially parallel to the xy-plane and is connected along the first edge  104  of the shorting wall  102 . This means that the radiator  106  and the shorting wall  102  are substantially parallel and perpendicular to the ground plane  110  respectively. 
         [0024]    The shorting wall  102  has a second edge  108  that is opposite to the first edge  104 . Additionally, the shorting wall  102  is connected to the ground plane  110  along the second edge  108  substantially parallel to the y-axis. In other words, the second edge  108  and the radiator  106  are electrically shorted to ground via the ground plane  110  during operation of the antenna  100 . 
         [0025]    Each of the shorting wall  102  and the radiator  106  is preferably plate-like and has a rectangular shape. The shorting wall  102  is alternatively replaceable with shorting pins (not shown) for supporting the radiator  106 . The shorting wall  102  allows the radiator  106  to be miniaturizes. 
         [0026]    The antenna  100  further comprises a feed plate structure  112 . The feed plate structure  112  excites the radiator  106  during operation of the antenna  100 . The feed plate structure  112  comprises a first portion  114  formed substantially parallel to the xy-plane and a second portion  116  substantially parallel to the yz-plane. The second portion  116  is arranged substantially perpendicular to the first portion  116 . Specifically, the second portion  116  extends from one edge of the first portion  114  that is proximal the shorting wall  102  towards the radiator  106 . The first and second portions  114 ,  116  of the feed plate structure  112  are also known as the first and second structures of the antenna  100 , respectively. 
         [0027]    Alternatively, the second portion  116  extends from another edge of the first portion  114  that is distal the shorting wall  102 . The first portion  114  and the second portion  116  are preferably plate-like and are arranged to form an L-shaped feed plate structure  112 . 
         [0028]    More specifically, the feed plate structure  112  is preferably formed, but not limited to, within a space created by the radiator  106  and the ground plane  110 . 
         [0029]    The first portion  114  of the feed plate structure  112  has a feed point  118 . A feeding probe  120  is connected to the first portion  114  of the feed plate structure  112  at the feeding point  118 . The feed plate structure  112  is suspended above the ground plane  110  at one end of the feeding probe  120 . The other end of the feeding probe  120  is connected through the ground plane  110  to a radio frequency connector (not shown). The feeding probe  120  is preferably a 50Ωco-axial probe. 
         [0030]      FIGS. 2 and 3  show a plan view and a side view of the antenna  100  respectively. With reference to  FIG. 3 , the feed plate structure  112  is spaced apart from the shorting wall  102  and the radiator  106 . In particular, the second portion  116  of the feed plate structure  112  and the shorting wall  102  are substantially parallel to each other and are separated by a distance d. 
         [0031]    With reference to  FIG. 3 , the second portion  116  of the feed plate structure  112  has a free edge  122  that extends along a direction substantially parallel to the y-axis distal to the first portion  114 . The separation or gap between the free edge  122  of the second portion  116  and the radiator  106  is h 2 . 
         [0032]    Each of the radiator  106 , shorting wall  102 , and the first and second portions  114 ,  116  of the feed plate structure  112  has a geometrical shape such as rectangular, triangular, elliptical, semi-elliptical or other polygonal shapes. The radiator  106  and shorting wall  102  are also known as the third and fourth structures of the antenna  100 , respectively. 
         [0033]      FIG. 4  shows other embodiments of the invention and side views of exemplary configurations of the feed plate structure  112 . Additional portions  400  are shown to extend from one or both free edges of the first and second portions  114 ,  116  of the feed plate structure  112 . The additional portions  400  are preferably a plurality of plates sequentially inter-coupled. The second portion  116  preferably has a rectilinear shape. 
         [0034]    The electromagnetic coupling between the radiator  106  and the feed plate structure  112 , especially between the free edge  122  of the second portion  116  and the radiator  106  for forming a magnetic loop therebetween, allows the antenna  100  to be used for broadband applications. The presence of the first portion  114  of the feed plate structure  112  increases the capacitance at the feed point  118 . This is to compensate for the increase in inductance at the feed point  118  necessary for broad bandwidth operation. The manufacturing tolerance of the antenna  100  is advantageously high due to the broadband design. 
         [0035]      FIG. 5  is a graph showing measured return loss |S 11 | characteristic of the antenna  100 . The antenna  100  is capable of operating within a bandwidth of 3.5 GHz to more than 10 GHz for |S 11 | less than −10 dB. In addition, the antenna  100  has a well-matched impedance matching characteristic within a bandwidth of 5 GHz to 6 GHz for |S 11 | less than −14 dB. 
         [0036]    As shown in  FIG. 5 , the antenna  100  has a well-matched impedance matching characteristic to cover frequency bands of 5.15 to 5.35 GHz, 5.47 to 5.73 GHz and 5.73 to 5.88 GHz bands for |S 11 | less than −14 dB. This means that the antenna  100  is capable of supporting multi-band operation for each of WLAN, WiFi, WiMAX and UWB standards and thereby advantageously eliminates the need for separate antennas and corresponding base band circuitries. 
         [0037]      FIGS. 6   a  to  6   c  are graphs showing measured radiating patterns of electromagnetic waves generated by the antenna  100  in the xz-plane and yz-plane at 5.25 GHz, 5.6 GHz, and 5.8 GHz respectively.  FIGS. 6   a  to  6   c  also show stable radiating patterns across a broad bandwidth. The peak gain is found to be greater than 6 dBi in the xz-plane. 
         [0038]    Specifically, the gain is dependent on the size of the ground plane  110  while the antenna  100  has a 40 to 45° beam-squinting angle from the bore sight due to the asymmetrical structure of the antenna  100 . Beam-squinting here refers to a condition where peak gain is found along the z-axis, i.e. θ=0° or bore sight. The maximum squinted beam is conducive to indoor applications, especially when the antenna  100  is to be installed on a ceiling. 
         [0039]    The radiator  106 , shorting wall  102  and feed plate structure  112  are made of electrically conductive material with low ohmic loss such as copper, brass, sheet metal and aluminum. 
         [0040]    The various embodiments of the invention are designed to provide a compact antenna having a broad bandwidth and stable gain. The antenna  100  has low manufacturing cost and is suitable for implementation in portable devices, indoor or outdoor access points and MIMO applications that employs WiMAX, WLAN, WiFi, and UWB standards. 
         [0041]      FIG. 7  shows an antenna array  700  for providing an omni-directional coverage in a desired plane. The antenna array  700  comprises antenna elements  702  arranged on a body  701  having twelve sides  702 . Each of the twelve sides  102  of the body  701  has at least one and preferably four antenna elements  702  formed thereon for providing omni-directional coverage in that side. 
         [0042]    More specifically, the foregoing antenna  100  is used as the antenna element  702  of the antenna array  700 . Each antenna element  702  has a directional pattern that covers a certain sector. The use of a twelve-sector antenna array  700  is to ensure that the antenna array  700  has omni-directional coverage in a desired plane and substantially reducing blind spots, for example in the azimuth plane. 
         [0043]    With reference to  FIG. 7 , there are multiple antenna elements  702  in each sector  704  of the twelve-sector antenna array  700  for MIMO applications. The use of the antenna  100  as the antenna element  702  advantageously reduces the overall size of the antenna array  700  and allows the antenna array  700  to have a more compact design. 
         [0044]    Additionally, the configuration of each of the antenna elements  702  in the antenna array  700  provides the antenna array  700  with well-matched impedance response and a gain of more than 5 dBi. The antenna array  700  also provides a stable radiation performance across the entire WiFi and WiMAX bandwidths of 5.15 to 5.875 GHz and 5.725 to 5.875 GHz respectively. Given the compact size of the antenna array  700 , the mutual coupling between adjacent antenna elements  702  is significant and therefore requires suppression. The mutual coupling between adjacent antenna elements  702  of the antenna array  700  is reduced to less than −15 dB. 
         [0045]    In the foregoing manner, an antenna having a feed plate structure for wireless communications is disclosed. Although only a number of embodiments of the invention are disclosed, it becomes apparent to one skilled in the art in view of this disclosure that numerous changes and/or modifications can be made without departing from the scope and spirit of the invention.