Abstract:
An antenna apparatus is disclosed that includes an antenna element having a teardrop shape and configured to be fed with electrical power from an external power source; and a ground element coupled to the antenna element, wherein the antenna element includes one or more nonconductive portions.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an antenna apparatus used in Ultra Wide Band (UWB) communication. 
         [0003]    2. Description of the Related Art 
         [0004]    UWB communication enables an ultra fast communication by using a GHz frequency band. In 2002, the U.S. Federal Communication Commission (FCC) approved usage of the UWB. Since then, an application of the UWB to a portable electronic device and a high precision positioning system has been expected. An ultra wide frequency band antenna apparatus, which includes a teardrop-shaped antenna element and is suitable for a GHz frequency band, has been developed. 
         [0005]    Patent Document 1: Japanese Patent Laid-Open Publication No. 2004-129209 
         [0006]    A conventional antenna apparatus which includes the teardrop-shaped antenna element provides uniform directivity in 360 degrees, because the antenna element has a uniform shape in planar view. 
         [0007]    However, in a case where a communication device with which the antenna apparatus communicates is settled and the direction or the location of the communication device is known, the uniform characteristics in 360 degrees may not be required. In such a case, the conventional antenna apparatus which provides the uniform directivity in 360 degrees may be influenced by noise from a noise source located in a direction which is not necessary for a UWB communication, or may cause noise in an electronic device which does not communicate with the conventional antenna apparatus. 
         [0008]    Although it is a technical feature of an antenna apparatus which includes an antenna element having a teardrop shape to provide uniform directivity in 360 degrees, the antenna apparatus can be used more widely if the antenna apparatus selects communication direction. 
       SUMMARY OF THE INVENTION 
       [0009]    It is a general object of the present invention to provide an antenna apparatus which includes an antenna element having a teardrop shape and provides selectivity of communication direction. 
         [0010]    Features and advantages of the present invention will be set forth in the description which follows, and in part will become apparent from the description and the accompanying drawings, or may be learned by practice of the invention according to the teachings provided in the description. Objects as well as other features and advantages of the present invention will be realized and attained by an antenna apparatus particularly pointed out in the specification in such full, clear, concise, and exact terms as to enable a person having ordinary skill in the art to practice the invention. 
         [0011]    To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, an embodiment of the present invention provides an antenna apparatus including an antenna element having a teardrop shape and configured to be fed with electrical power from an external power source; and a ground element coupled to the antenna element, wherein the antenna element includes one or more nonconductive portions. 
         [0012]    Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a schematic drawing of an antenna apparatus according to first embodiment shown in perspective view; 
           [0014]      FIG. 2A  is a schematic drawing of an antenna element according to the first embodiment shown in planar view; 
           [0015]      FIG. 2B  is a graph showing directivity of the antenna apparatus according to the first embodiment; 
           [0016]      FIG. 3  is a schematic drawing of an antenna apparatus according to second embodiment shown in perspective view; 
           [0017]      FIG. 4A  is a schematic drawing of an antenna element according to the second embodiment shown in planar view; 
           [0018]      FIG. 4B  is a graph showing directivity of an antenna apparatus according to the second embodiment; and 
           [0019]      FIG. 5  is a schematic drawing of an antenna apparatus according to third embodiment shown in perspective view. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    In the following, embodiments of the present invention will be described with reference to the accompanying drawings. 
       First Embodiment  
       [0021]      FIG. 1  is a schematic drawing of an antenna apparatus according to a first embodiment shown in perspective view. 
         [0022]    As shown in  FIG. 1 , an antenna apparatus  10  includes an antenna element  11  having an inverted teardrop shape, and a ground element  12  having a disc shape. 
         [0023]    The antenna element  11  is formed of a metal-plated, inverted-teardrop-shaped resin which is formed by plating a metal on a molded inverted teardrop-shaped resin. The antenna element  11  includes a metal layer  11 A and a nonconductive portion  11 B. The metal layer  11 A is formed on a part of the surface of the inverted teardrop-shaped resin. The rest of the surface on which the metal layer  11 A is not formed becomes the nonconductive portion  11 B. The inverted teardrop-shaped resin may be made from, for example, a resin such as ABS (Acrylonitrile Butadiene Styrene), PC (Polycarbonate), or PS (Polystyrene). The metal layer  11 A may be made from, for example, chromium, aluminum, or nickel. 
         [0024]    The inverted teardrop shape of the antenna element  11  may be formed by engaging a hemispherical shape and a conical shape so that the hemispherical shape and the conical shape form a continuous outer surface of the inverted teardrop shape. 
         [0025]    The nonconductive portion  11 B is a part of the surface of the inverted teardrop-shaped resin, and is formed in an area bounded by two virtual planes S 1  and S 2  which include a central axis  1  of the inverted teardrop shape and intersect at an angle θ. 
         [0026]    The nonconductive portion  11 B is formed, for example, by a mask process when the molded inverted teardrop-shaped resin is metal plated. 
         [0027]    The antenna element  11  includes a top portion  11 C (a top portion of the hemispherical shape) which becomes a null point of the antenna element  11 , and a feeding portion  11 D which is formed at an apex of the conical shape. A cable core  14 A of a coaxial cable  14  is connected to the feeding portion  11 D in order to feed electrical power thereto from an external power source. 
         [0028]    The antenna element  11  is a quarter wavelength high from the upper surface of the ground element  12  to the top portion  11 C when assembled to the ground element  12 . The quarter wavelength is determined by a communication frequency, and becomes, for example, about 25 centimeters if the communication frequency is 3 GHz. 
         [0029]    The ground element  12  includes an opening  12 A through which the cable core  14 A of the coaxial cable  14  is connected to the feeding portion  11 D in order to feed electrical power to the feeding portion  11 D. A shielded line  14 B of the coaxial cable  14  is connected to the back surface of the ground element  12 . Thus, in the illustrated embodiment, the electrical potential of the ground element  12  is kept to the ground potential. 
         [0030]    Although the ground element  12  in  FIG. 1  is shown as having a disc shape, the ground element  12  may be shaped in any shape in planar view, and the ground element  12  may be shaped to have any thickness. 
         [0031]    The coaxial cable  14  may be formed of a shielded coaxial cable of which the characteristic impedance is 50Ω. 
         [0032]    The antenna element  11  is insulated from the ground element  12 , and is connected to the ground element  12 . 
         [0033]      FIG. 2A  is a schematic drawing of the antenna element  11  according to the first embodiment shown in planar view.  FIG. 2B  is a graph showing directivity of the antenna apparatus according to the first embodiment. 
         [0034]    As shown in  FIG. 2A , the nonconductive portion  11 B is located in an area defined by the central angle θ in the circle of the antenna element  11  in planar view. As described above, the angle θ is formed by two virtual planes S 1 , S 2  which include the central axis  1  (shown in  FIG. 1 ). The antenna element  11  does not radiate an electrical wave and does not receive an electrical wave in a direction included in the angle θ in planar view. 
         [0035]    Referring to  FIGS. 2A and 2B , the directivity is lower in the direction included in the angle θ in which the nonconductive portion  11 B is formed. The directivity shown in  FIG. 2B  is different from those of an antenna element which does not include the nonconductive portion  11 B, e.g. a molded shaped resin of which a whole surface is metal plated. 
         [0036]    As described above, the antenna apparatus  10  according to the first embodiment provides selectivity of communication direction by forming the nonconductive portion  11 B. As shown in  FIG. 2B , the antenna apparatus  10  provides an excellent directivity except for the direction in which the nonconductive portion  11 B is formed. The electrical wave radiation is lower in the direction in which the nonconductive portion  11 B is formed. 
         [0037]    The antenna apparatus  10  including the antenna element  11  having the inverted teardrop shape as described above provides a VSWR value of about 2.0 to about 3.0 within the frequency band from about 3.0 to about 20.0 GHz. The antenna apparatus  10  is suitable for UWB communication. 
         [0038]    As described above, the antenna apparatus  10  according to first embodiment provides selectivity of communication direction by forming the nonconductive portion  11 B. Thus, an influence exerted by the noise source is greatly reduced by facing the nonconductive portion  11 B toward a noise source, and the antenna apparatus  10  provides a high quality UWB communication except for the direction in which the nonconductive portion  11 B is formed. 
         [0039]    In a case where the antenna apparatus  10  is disposed close to a wall, it is better not to radiate an electrical wave toward the wall, because a reflected wave from the wall causes a decrease of sensitivity of the antenna apparatus  10 . In this case, it is possible to reduce a reflected wave from the wall and to enhance a sensitivity of the antenna apparatus  10  by facing the nonconductive portion  11 B toward the wall. 
         [0040]    In a case where there is a direction in which it is better for the antenna apparatus  10  not to radiate an electrical wave, the antenna apparatus  10  provides a high quality UWB communication except for the direction such as a direction toward a wall by forming the nonconductive portion  11 B. 
         [0041]    The central angle θ of the area in which the nonconductive portion  11 B is formed may be set to any value depending on positional relationship between the antenna apparatus  10  and the noise source. 
         [0042]    Although the nonconductive portion  11 B is formed in an area bounded by two virtual planes S 1 , S 2 , which include the central axis  1  connecting the top portion  11 C and the feeding portion  11 D is shown in  FIG. 1 , the nonconductive portion  11 B may be formed into any shape. 
         [0043]    Although the molded inverted teardrop-shaped resin included in the antenna element  11  is described above, the inverted teardrop-shaped resin may be formed by any process except for molding. 
         [0044]    Furthermore, although the antenna element  11  described above is formed by plating a metal on a molded teardrop-shaped resin, the antenna element  11  is not limited to this. For example, the antenna element could be formed of any combination of a metal layer and a nonconductive portion, which together form a teardrop-shaped antenna element. The metal layer may comprise other than a metal plating, for example its own sector metal layer. The nonconductive portion may comprise other than a surface of a molded teardrop-shaped resin, for example a sector resin portion. 
       Second Embodiment  
       [0045]      FIG. 3  is a schematic drawing of an antenna apparatus according to a second embodiment shown in perspective view. Hereinafter, the same elements as or similar elements to those of the antenna apparatus  10  according to the first embodiment are referred to by the same reference numerals, and a description thereof is omitted. An antenna apparatus  20  according to the second embodiment is different from the antenna apparatus  10  according to the first embodiment in that an antenna element  21  of the antenna apparatus  20  includes a pair of metal layers  21 A, a pair of nonconductive portions  21 B, a top portion  21 C, and a feeding point  21 D. 
         [0046]      FIG. 4A  is a schematic drawing of the antenna element  21  according to the second embodiment shown in planar view.  FIG. 4B  is a graph showing directivity of the antenna apparatus  20  according to the second embodiment. 
         [0047]    The nonconductive portions  21 B are formed in areas respectively which are included in the surface of the molded inverted teardrop-shaped resin, and are disposed 180 degrees apart with respect to each other with regard to the central axis  1  in planar view. 
         [0048]    Each of the nonconductive portions  21 B is formed in an area defined by the central angle θ in the circle of the antenna element  21  in planar view, respectively. 
         [0049]    The antenna apparatus  20  according to the second embodiment does not radiate an electrical wave from the nonconductive portions  21 B, and does not receive an electrical wave from the nonconductive portions  21 B, because the nonconductive portions  21 B are not metal plated. 
         [0050]    As shown  FIG. 4B , the directivity is lower in the directions in which the nonconductive portions  21 B are formed. The directivity shown in  FIG. 4B  is different from those of an antenna element which does not include the nonconductive portions  21 B, e.g. a molded inverted teardrop-shaped resin of which a whole surface is metal plated. 
         [0051]    As described above, the antenna apparatus  20  according to the second embodiment provides selectivity of communication direction by forming the nonconductive portions  21 B. As shown in  FIG. 4B , the antenna apparatus  20  provides an excellent directivity except for the direction in which the nonconductive portions  21 B are formed. 
         [0052]    As described above, the antenna apparatus  20  according to the second embodiment provides selectivity of communication direction by forming the nonconductive portions  21 B. Thus, an influence exerted by the noise sources is greatly reduced by facing the nonconductive portions  21 B toward the noise sources, and the antenna apparatus  20  provides a high quality UWB communication except for the directions in which the nonconductive portions  21 B are formed. 
         [0053]    In a case where the antenna apparatus  20  is disposed close to a wall, it is better not to radiate an electrical wave toward the wall, because a reflected wave from the wall causes decrease of sensitivity of the antenna apparatus  20 . In this case, it is possible to reduce a reflected wave from the wall and to enhance a sensitivity of the antenna apparatus  20  by facing the nonconductive portions  21 B toward the wall. 
         [0054]    Although the nonconductive portions  21 B formed in areas disposed 180 degrees apart to each other in planar view are described, locations of the respective areas may be selected depending on a circumstance where the antenna apparatus  20  is disposed. 
         [0055]    The center angle θ of the respective areas in which the nonconductive portions  21 B are formed respectively may be set to any value depending on a positional relationship between the antenna apparatus  20  and the noise sources. 
       Third Embodiment  
       [0056]      FIG. 5  is a schematic drawing of an antenna apparatus according to a third embodiment shown in perspective view. Hereinafter, the same elements as or similar elements to those of the antenna apparatus  10  according to the first embodiment are referred to by the same reference numerals, and a description thereof is omitted. An antenna apparatus  30  according to the third embodiment is different from the antenna apparatus  10  according to the first embodiment in that an antenna element  31  of the antenna apparatus  30  includes a metal layer  31 A which is formed on a surface of a conical shaped portion of the molded inverted teardrop-shaped resin, a nonconductive portion  31 B which comprises the surface of a hemispherical shaped portion of the molded inverted teardrop-shaped resin, a top portion  31 C, and a feeding point  31 D. 
         [0057]    Since the antenna element  31  according to the third embodiment includes the metal layer  31 A formed on the surface of the conical shaped portion, the antenna apparatus  30  provides uniform directivity in 360 degrees. The antenna apparatus  30  does not radiate an electrical wave upwardly, because the nonconductive portion  31 B is not metal plated. The antenna apparatus  30  radiates an electrical wave mainly downwardly. 
         [0058]    As described above, the antenna apparatus  30  according to the third embodiment provides selectivity of communication direction by forming the nonconductive portion  31 B. Thus, an influence exerted by a noise source is greatly reduced by facing the nonconductive portion  31 B toward the noise source, and the antenna apparatus  30  provides a high quality UWB communication except for the direction in which the nonconductive portion  31 B is formed. 
         [0059]    In a case where the antenna apparatus  30  is disposed close to a wall, it is better not to radiate an electrical wave toward the wall, because a reflected wave from the wall causes a decrease of sensitivity of the antenna apparatus  30 . In this case, it is possible to reduce a reflected wave from the wall and to enhance a sensitivity of the antenna apparatus  30  by facing the nonconductive portion  31 B toward the wall. The antenna apparatus  30  according to the third embodiment provides a high quality UWB communication. 
         [0060]    Further, the present invention is not limited to these embodiments, but variations and modifications may be made without departing from the scope of the present invention. 
         [0061]    The present application is based on Japanese Priority Application No. 2008-235021 filed on Sep. 12, 2008 with the Japanese Patent Office, the entire contents of which are hereby incorporated by reference.