Abstract:
An antenna for ultra-wideband applications is disclosed. The antenna has a first radiating element shaped for defining a first notch, the first radiating element having a first feeding structure. The antenna further has a second radiating element operatively couplable to the first radiating element and shaped for defining a second notch. The second radiating element having a second feeding structure, the first and second feeding structures being substantially orthogonal to each other and the first and second radiating elements having an inter-displacement. More specifically, the first radiating element and the first feeding structure is substantially symmetrical to the second radiating element and the second feeding structure respectively about a line of symmetry passing through the inter-displacement between the first and second radiating elements for achieving orthogonal polarization and radiating pattern diversity between the first and second radiating elements.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. provisional application Ser. No. 60/978,429, filed Oct. 9, 2007 and entitled “Antennas For Diversity Applications” incorporated herein by reference in its entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The invention relates generally to antennas. In particular, it relates to ultra-wideband antennas for diversity applications. 
       BACKGROUND 
       [0003]    Ultra-wideband (UWB) technology is now widely used in wireless communication systems. Wireless UWB systems have very wide operating bandwidth and fast data transfer rates. One such system is able to support a data transfer rate of 480 Mbps within an operating range of 3 m or 110 Mbps within an operating range of 10 m. 
         [0004]    The demand for wireless communication device miniaturization has created a need for wireless UWB systems that are sufficiently small to be contained in a USB thumb drive. However, antennas used in conventional wireless UWB systems require a large ground plane to achieve the desired UWB operating performance. 
         [0005]    Therefore, a space limitation exist in conventional UWB systems to be implemented in a device as small as a USB thumb drive while improving impedance matching and radiation performance across a broad bandwidth. 
         [0006]    There is therefore a need for a UWB antenna which is sufficiently small while improving impedance matching and radiation performance across a broad bandwidth for use in small portable UWB devices. 
       SUMMARY 
       [0007]    Embodiments of the invention are disclosed hereinafter for ultra-wideband (UWB) applications for improving impedance matching and radiation performance across a broad bandwidth and sufficiently small for use in small portable UWB devices. 
         [0008]    In accordance with one aspect of the invention, there is disclosed an antenna for ultra-wideband applications. The antenna has a first radiating element shaped for defining a first notch, the first radiating element having a first feeding structure. The antenna further has a second radiating element operatively couplable to the first radiating element and shaped for defining a second notch. The second radiating element having a second feeding structure, the first and second feeding structures being substantially orthogonal to each other and the first and second radiating elements having an inter-displacement. More specifically, the first radiating element and the first feeding structure is substantially symmetrical to the second radiating element and the second feeding structure respectively about a line of symmetry passing through the inter-displacement between the first and second radiating elements for achieving orthogonal polarization and radiating pattern diversity between the first and second radiating elements. 
         [0009]    In accordance with another aspect of the invention, there is disclosed a method for configuring an antenna for UWB applications. The method involves an initial step of providing a first radiating element shaped for defining a first notch, the first radiating element having a first feeding structure. The method then involves the step of providing a second radiating element operatively couplable to the first radiating element and shaped for defining a second notch. The second radiating element having a second feeding structure, the first and second feeding structures being substantially orthogonal to each other and the first and second radiating elements having an inter-displacement. More specifically, the first radiating element and the first feeding structure is substantially symmetrical to the second radiating element and the second feeding structure respectively about a line of symmetry passing through the inter-displacement between the first and second radiating elements for achieving orthogonal polarization and radiating pattern diversity between the first and second radiating elements. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0010]    Embodiments of the invention are described in detail hereinafter with reference to the drawings, in which: 
           [0011]      FIGS. 1   a  to  1   d  are schematic views of an antenna according to a first embodiment of the invention; 
           [0012]      FIG. 2  is a graph showing measured results of the return loss and isolation characteristics of the antenna  100  of  FIG. 1   a;    
           [0013]      FIGS. 3   a  to  3   c  are images showing current distribution during operation of antenna of  FIG. 1   a  at 3, 4 and 5 GHz respectively; 
           [0014]      FIGS. 4   a  to  4   c  are graphs showing measured radiation patterns across the bandwidth of the antenna of  FIG. 1   a  over three main planes; 
           [0015]      FIGS. 5   a  and  5   b  are schematic views of the antenna according to a second embodiment of the invention; 
           [0016]      FIG. 6  is a graph showing measured and simulated results of the impedance matching characteristics of the antenna  100  of  FIG. 5   a;    
           [0017]      FIGS. 7   a  to  7   c  are graphs showing measured radiation patterns across the bandwidth of the antenna of  FIG. 5   a  over three main planes; 
           [0018]      FIG. 8  is a schematic view showing important dimensional parameters of the antenna of  FIG. 5   a ; and 
           [0019]      FIG. 9  shows various examples of the geometrical shape of radiating elements of the antenna of  FIGS. 1   a  and  5   a.    
       
    
    
     DETAILED DESCRIPTION 
       [0020]    With reference to the drawings, antennas having substantially good impedance matching and radiation performance across a broad bandwidth and sufficiently small for use in small portable ultra-wideband (UWB) devices according to embodiments of the invention are disclosed. 
         [0021]    Various conventional antennas have been previously proposed for UWB applications. However, some of these conventional UWB antennas require ground planes for operation that are not suitable for use in small portable UWB devices. 
         [0022]    For purposes of brevity and clarity, the description of the invention is limited hereinafter to UWB applications. This, however, does not preclude embodiments of the invention from other applications that require similar operating performance as the UWB applications. The functional principles fundamental to the embodiments of the invention remain the same throughout the various embodiments. 
         [0023]    Embodiments of the invention are disclosed hereinafter for UWB applications having substantially good impedance matching and radiation performance across a broad bandwidth and sufficiently small for use in small portable UWB devices. Embodiments of the invention are described in greater detail in accordance with  FIGS. 1   a  to  9 , and the drawings hereinafter, wherein like elements are identified with like reference numerals. 
         [0024]      FIGS. 1   a  to  1   d  show the geometry of an antenna  100  according to a first embodiment of the invention for UWB applications.  FIG. 1   a  is a plan view of the antenna  100 .  FIG. 1   b  is a side view of the antenna  100  along line  1 - 1 .  FIG. 1   c  is a back view of the antenna  100  and  FIG. 1   d  is the plan view of the antenna  100  superimposed on the back view. 
         [0025]    The antenna  100  is formed on a first surface  104  of a substrate  102 , for example a printed circuit board (PCB) made of dielectric materials such as FR4, Rogers 4003 or RT Duroid. The antenna  100  comprises a first radiating element  106  and a second radiating element  108  for transmitting and receiving signals. In this first embodiment of the invention, each of the first and second radiating elements  106 ,  108  is formed in the shape of a triangle having a planar surface. The first and second radiating elements  106 ,  108  are symmetrically positioned with respect to a line of symmetry  110  therebetween. 
         [0026]    The antenna  100  has a first notch  112  and a second notch  114  formed in the first and second radiating elements  106 ,  108  respectively. The first and second notches  112 ,  114  extend from a respective portion of the periphery of the first and second radiating elements  106 ,  108  and thereinto. In particular, each of the first and second notches  112 ,  114  is open-ended along respective first edges  132 ,  134  of the first and second radiating elements  106 ,  108  and substantially segregates each of the first and second radiating elements  106 ,  108  into respective two portions  116 ,  118  and  122 ,  124  connected by respective interconnecting portions  120 ,  126 . The first and second notches  112 ,  114  are preferably but not limited to inwardly facing each other and having a substantially elongated shape. The periphery of the first and second radiating elements  106 ,  108  has a triangular shape but can be of any other geometrical shapes. 
         [0027]    A first feeding structure  128  or feed strip is connected along a second edge  136  of the first radiating element  106  while a second feeding structure  130  or feed strip is connected along a second edge  138  of the second radiating element  108 . Examples of the first and second feed strips  128 ,  130  include a co-planar waveguide (CPW), a co-planar stripe (CPS) and a coaxial cable. 
         [0028]    The second edges  136 ,  138  are adjacent to one end of the first edges  132 ,  134  respectively. Each of the first and second radiating elements  106 ,  108  has a third edge  140 ,  142  that interconnects the first  132 ,  134  and second  136 ,  138  edges thereof. The first and second radiating elements  106 ,  108  and the first and second feed strips  128 ,  130  are arranged as such to achieve pattern diversity. 
         [0029]    The first and second feed strips  128 ,  130  preferably extend outwardly from the second edges  136 ,  138  of the first and second radiating elements  106 ,  108  respectively, in a substantially 45° and 135° configuration with respect to the line of symmetry  110  as shown in  FIG. 1   a . Specifically, each of the first and second feed strips  128 ,  130  is substantially orthogonal to the respective second edges  136 ,  138  of the first and second radiating elements  106 ,  108  and substantially parallel to the respective first and second notches  112 ,  114 . This is so that the polarization direction of each radiating element  106 ,  108  is orthogonal to each other and substantially parallel to the respective notches  112 ,  114 . 
         [0030]    Each of the first and second feed strips  128 ,  130  further extends towards a first edge  144  of the substrate  104 . The first and second feed strips  128 ,  130  are preferably configured for facilitating connection to respective first and second feeds (not shown) via respective first and second feeding terminals or ports  146 ,  148 . An example of the first and second feeds is a co-axial probe. Each of the first and second feed strips  128 ,  130  is preferably but not limited to, for example a 50Ω micro-strip line. The first and second feed strips  128 ,  130  are preferably formed on the first surface  104  of the substrate  102 . 
         [0031]    A first stub  150  and a second stub  152  are formed on the first and second feed strips  128 ,  130  respectively for impedance matching purposes. Each of the first and second stubs  150 ,  152  is preferably formed proximal to the respective second edges  136 ,  138  of the first and second radiating elements  106 ,  108 . 
         [0032]    As illustrated in  FIG. 1   c , a ground plane  154  is preferably formed on a second surface  156  of the substrate  102 . The second surface  156  is outwardly opposite to the first surface  104  of the substrate  102 . The ground plane  154  has a central strip  158  extending from one portion thereof to a second edge  160  of the substrate  102  opposite the first edge  144  of the substrate  102 . The ground plane  154  and in particular the central strip  158  reduces mutual coupling between the first and second radiating elements  106 ,  108  on the first surface  104  of the substrate  102 . The ground plane  154  is geometrically shaped such that it does not overlap with the first and second radiating elements  106 ,  108  and has a geometrical shape not limited to the shape as shown in  FIG. 1   c.    
         [0033]    Additionally, the first and second feeds are preferably connected to the first and second feeding terminals  146 ,  148  respectively as well as to the ground plane for transmitting and receiving the signals. 
         [0034]    Each of the first and second notches  112 ,  114  formed respectively in the first and second radiating elements  106 ,  108  advantageously creates an electrical current path through which signals having UWB bandwidths travel. In addition, each of the first and second notches  112 ,  114  helps to concentrate electrical currents within the respective first and second radiating elements  106 ,  108 , especially at the lower operating frequencies. As a result, the effect of the ground plane  154  and the first and second feeds on the impedance matching and radiation performance of the antenna  100  is minimized. The operating frequency bandwidth and impedance response characteristics of the antenna  100  are modifiable by respectively varying the dimensions and configuration of the first and second notches  112 ,  114  and the first and second radiating elements  106 ,  108 . 
         [0035]      FIG. 2  shows the measured impedance performance of the antenna  100 . The measured results show the antenna  100  having a well-matched impedance matching characteristic achieving good return loss |S 11 |&lt;−10 dB throughout the frequency range of 3.1 GHz to 5 GHz and good isolation |S 21 |&lt;−20 dB over the same frequency range. 
         [0036]      FIGS. 3   a  to  3   c  show the current distributions on the antenna  100  at operating frequencies of 3, 4 and 5 GHz respectively. The current, which is in lighter shade, is mostly concentrated around the first  106  or second  108  notch and the central strip  158  of the ground plane  154  instead of the other parts of the ground plane  154 , especially at the lower frequency of 3 GHz. This allows the antenna  100  to consist of two radiating elements and yet maintains operational at the lower edge of the operating frequency at 3.1 GHz. Additionally, since there is a small current present in the ground plane  154 , mutual coupling between the first and second radiating elements  106 ,  108  is significantly reduced. 
         [0037]      FIG. 4  shows radiation patterns of the antenna  100  measured at 4 GHz and across three principal planes, namely the x-y plane of  FIG. 4   a , the φ=45°/225° plane of  FIG. 4   b  and the φ=135°/315° plane of  FIG. 4   c . During measurements, the second terminal  148  of the antenna  100  is terminated with a 50Ω load when the first terminal  146  of the antenna  100  is excited, and vice versa. The graphs of  FIGS. 4   a  to  4   c  shows the radiation patterns at the three principal planes when the first and second terminals  146 ,  148  are respectively excited cover complementary spatial regions. 
         [0038]    The degree of pattern overlap or correlation ρ is represented by the following equation: 
         [0000]    
       
         
           
             ρ 
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                           2 
                         
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         [0000]    where N is the number of data points, G P1  and G P2  represent the magnitude of the gain response at the first and second terminals  146 ,  148  respectively, θ and φ are angles of direction formed with reference to the z axis and x-y plane respectively. For pattern diversity, ρ should have a value less than 0.7 in order to achieve good diversity gain. At the operating frequency of 4 GHz, the value of ρ at each of the three principal planes is about 0.4. This demonstrates that the antenna  100  is suitable to be used for diversity applications. 
         [0039]    The performance of the antenna  100  is directly related to the structural dimensions thereof. With reference to  FIG. 1   d , antenna parameters f, g, and l g  affect the impedance matching, l, and l s  determine the lower edge of the operating frequency range, and s, d, and l g  control the mutual coupling. 
         [0040]      FIGS. 5   a  and  5   b  show a front view and a back view of the antenna  100  formable on the first and second surfaces  104 ,  156  of the substrate  102  respectively, according to a second embodiment of the invention. The antenna  100  comprises a radiating element  500  for transmitting and receiving signals for UWB applications, similar to the first embodiment of the invention. The radiating element  500  has a feeding structure  502  connected and substantially orthogonal thereto. The radiating element  500  and the feeding structure  502  are preferably formed on the first surface  104  of the substrate  104 . 
         [0041]    The feeding structure  502  has a feeding point  504  preferably positioned proximal to a first side  501  of the radiating element  500 . The radiating element  500  has a notch  506  formed therein. The notch  506  extends from a portion of the periphery of the radiating element  500  and into the radiating element  500 , wherein the periphery of the radiating element  500  can be of any shape. The notch  506  is therefore open-ended along a second side  509  of the radiating element  500 . The notch  506  is geometrically shaped and is preferably substantially elongated. 
         [0042]    The feeding structure  502  has a first portion  505  that extends outwardly from the radiating element  500 , substantially orthogonal to a first side  501  of the radiating element  500  where the feeding point  504  resides, as shown in  FIG. 5   a . The feeding structure  502  is preferably configured for facilitating connection of the radiating element  500  to a feed  503 . The feeding structure  502  is preferably but not limited to, for example a 50Ω micro-strip line. The feeding structure  502  has a second portion  507  that extends from the first portion  505  and substantially parallel to the longitudinal length of the substrate  102 . 
         [0043]    In this second embodiment of the invention as shown in  FIGS. 5   a  and  5   b , the radiating element  500  has an arm  508  that extends from a top corner  510  of the radiating element  500 . The arm  508  has a first section  512  that is connected to the top corner  510  of the radiating element  500  substantially proximal to the portion of the periphery of the radiating element  500  wherefrom the notch  506  extends. The arm  508  further has a second section  514  extending substantially perpendicularly from the free end of the first section  512 . 
         [0044]    With reference to  FIG. 5   b , a ground plane  516  is preferably formed on the second surface  156  of the substrate  102 . The ground plane  516  has a vertical strip  518  as well as a horizontal strip  520  extending substantially perpendicularly from the vertical strip  516 . The vertical strip  516  is used to control the impedance matching performance of the antenna  100 . 
         [0045]    The ground plane  516  has a geometrical shape not limited to that shown in  FIG. 5   b . The feed  503  is preferably connected at one terminal to the feeding structure  502  and the other terminal to the ground plane  516  for transmitting and receiving the signals. 
         [0046]    Similar to the first embodiment of the invention, the radiating element  500  with the notch  506  advantageously creates an electrical current path through which signals having UWB bandwidths travel. As with the first embodiment of the invention, the presence of the notch  506  helps to concentrate the electrical current within the radiating element  500  instead of the ground plane  516 , especially at the lower operating frequencies. Therefore, the effect of the ground plane  516  and the feed  503  on the impedance matching and radiation performance of the antenna  100  is substantially minimized. The operating frequency bandwidth and impedance response characteristics of the antenna  100  are modifiable by respectively varying the dimensions and configuration of the notch  506  and the radiating element  500 . 
         [0047]      FIG. 6  is a graph showing measured and simulated results of the impedance matching of the antenna  100  of  FIG. 5   a  in good agreement. The impedance matching frequency response of the antenna  100  is represented by |S 11 |. The measured and simulated results show the antenna  100  having a well-matched impedance matching characteristic throughout the frequency range of 3.1 GHz to 5 GHz and achieving good return loss |S 11 | of less than −10 dB over the same frequency range. 
         [0048]      FIGS. 7   a  to  7   c  show measured co-polarized radiation patterns of the antenna  100  of  FIG. 5   a  across three main planes, namely the x-z plane, the y-z plane, and the x-y plane, respectively. The radiation patterns are co-polarized across each of the three main planes are measured at three different frequencies, namely 3.1, 4 and 5 GHz. The co-polarized radiation patterns show that the radiation from the antenna  100  is omni-directional across the impedance bandwidth. 
         [0049]      FIG. 8  shows the plane view of the antenna  100  of  FIG. 5   a  superimposed with the back view of the antenna  100  of  FIG. 5   b . The performance of the antenna  100  is directly related to the structural dimensions thereof. With reference to  FIG. 8 , antenna parameters f, g, and l g  affect the impedance matching l, l s , l 1 , and l 2  determine the lower edge of the operating frequency range for miniaturizing the antenna  100 . 
         [0050]    In another aspect of the above-described embodiments of the invention, the radiating elements  106 ,  108 ,  500  have a geometrical shape not being limited to rectangular, elliptical, semi-elliptical or triangular, as shown in  FIG. 9 . The radiating elements  106 ,  108 ,  500  can be orientated towards any direction. In the case of the first embodiment of the invention, the first and second radiating elements  106 ,  108  are arranged on the same first side  104  of the substrate  102  and are symmetrically displaced about the line of symmetry  110 . As such, the shape of the first and second notches is dependable on the shape or orientation of the radiating elements. 
         [0051]    The antenna  100  is advantageously able to achieve a broad impedance bandwidth of 3.1 to 5 GHz or 6 to 10.6 GHz with good gain and radiation performance. The antenna  100  is also sufficiently miniaturized for use in wireless USB dongles or thumb drives and other portable mobile devices. 
         [0052]    In the foregoing manner, an antenna for UWB applications 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.