Abstract:
Provided is an antenna device having excellent antenna characteristics at desired frequencies by suppressing influences to a human body and the like to minimum. In the device, a power feeding section ( 102 ) is arranged on a ground plate ( 101 ). An antenna element ( 103 ) has a section ( 112 ) to be fed with power supplied from the power feeding section ( 102 ), a grounding section ( 111 ) grounded to the ground plate ( 101 ), and a main body section ( 113 ) formed by making the surfaces of a conductive plate face each other by folding the conductive plate midway. The section ( 112 ) to be fed with power and the grounding section ( 111 ) form a reverse F-shape by being connected to the main body section ( 113 ), and are arranged at an end portion of the ground plate ( 101 ).

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an antenna apparatus to constitute a wideband antenna that reduces the current flow on a ground plate. 
       BACKGROUND ART 
       [0002]    A planar-inverted F antenna (PIFA) is known as an antenna to mount on a small mobile terminal (see, for example, patent literature 1). With the antenna disclosed in patent literature 1, a ground plate is connected with a conductor ground plate through metallic wiring and is fed power from a power feeding point through metallic wiring. Also, one end of a conductor wall is electrically connected with the ground plate, and an electromagnetic coupling adjusting plate is electrically connected with the other end of the conductor wall. The electromagnetic coupling adjusting plate is placed a certain gap apart from the conductor ground plate, so that a capacitor is formed between the electromagnetic coupling adjusting plate and the conductor ground plate. By this means, it is possible to make an antenna&#39;s frequency characteristics to support wideband characteristics. 
       CITATION LIST 
     Patent Literature 
       [0000]    
       
         PTL 1 
         Japanese Patent Application Laid-Open No. 2002-223114 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0005]    However, with the antenna disclosed in patent literature 1, when this antenna oscillates, a large amount of current flows on the ground plate of a mobile terminal, due to the electromagnetic coupling adjusting plate. Consequently, when the user holds the casing of the mobile terminal, the current flowing on the ground plate changes significantly, and, furthermore, the human body influences as a dielectric and causes an impedance mismatch in the antenna. As a result of this, there is a problem that intended antenna characteristics cannot be achieved and the mobile terminal suffers deterioration of wireless communication quality. 
         [0006]      FIG. 1(   a ) shows voltage standing wave ratio (VSWR) characteristics in the event a mobile terminal is placed in free space, and  FIG. 1(   b ) shows VSWR characteristics in the event a mobile terminal is placed near the human body. In  FIG. 1(   a ) and  FIG. 1(   b ), the horizontal axis is frequency and the vertical axis is VSWR, which shows the level of antenna impedance matching. When VSWR is 1, the best impedance matching is shown. 
         [0007]    In  FIG. 1(   a ), an oscillation occurs at desired frequency f 0 . That is to say, the best impedance matching is shown at desired frequency f 0 . By contrast with this, in  FIG. 1(   b ), due to the influence of the human body, an oscillation occurs at frequency fs, shifted to the lower frequency side compared to above desired frequency f 0  in the event of free space. 
         [0008]    Consequently, even if design is made to match the desired frequency of a mobile terminal in free space, due to the influence of the human body and so on, impedance matching cannot be seen at that desired frequency. As a result of this, there is a problem of instability of communication, and communication might even be disconnected in the worst scenario. In particular, when a large amount of current flows on the ground plate, the above problem becomes more obvious. 
         [0009]    It is therefore an object of the present invention to provide an antenna apparatus that can minimize the influence of the human body and make antenna characteristics at a desired frequency good. 
       Solution to Problem 
       [0010]    An antenna apparatus according to the present invention adopts a configuration having: a ground plate; a power feeding section that is provided in the ground plate; and an antenna element having a power-fed section where power is fed from the power feeding section, a grounding section that is grounded to the ground plate, and a main body section that is formed by bending back a conductor plate in the middle so that the conductor plate has mutually opposing surfaces, the antenna element having an inverted F shape connecting the power-fed section, the ground section, and the main body section, and being placed in an edge section of the ground plate. 
       Advantageous Effects of Invention 
       [0011]    With the present invention, it is possible to reduce the influence of the human body and make antenna characteristics at a desired frequency good. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0012]      FIG. 1  shows conventional VSWR characteristics; 
           [0013]      FIG. 2  is a perspective view of an antenna apparatus according to embodiment 1 of the present invention; 
           [0014]      FIG. 3  is a plan view of an antenna apparatus according to embodiment 1 of the present invention; 
           [0015]      FIG. 4  is a side view of an antenna apparatus according to embodiment 1 of the present invention; 
           [0016]      FIG. 5  is a front view of an antenna apparatus according to embodiment 1 of the present invention; 
           [0017]      FIG. 6  shows current distribution characteristics on a ground plate according to embodiment 1 of the present invention; 
           [0018]      FIG. 7  shows VSWR characteristics according to embodiment 1 of the present invention; 
           [0019]      FIG. 8  shows the relationship between an unequal median value, and user throughput during MIMO communication; 
           [0020]      FIG. 9  is a perspective view of an antenna apparatus according to embodiment 2 of the present invention; 
           [0021]      FIG. 10  is a plan view of an antenna apparatus according to embodiment 2 of the present invention; 
           [0022]      FIG. 11  is a front view of an antenna apparatus according to embodiment 2 of the present invention; and 
           [0023]      FIG. 12  shows VSWR characteristics according to embodiment 2 of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0024]    Now, embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
       Embodiment 1 
       [0025]      FIG. 2  is a perspective view of antenna apparatus  100  according to embodiment 1 of the present invention. 
         [0026]    From  FIG. 2 , antenna apparatus  100  is configured primarily with ground plate  101 , power feeding section  102  and antenna element  103 . Antenna apparatus  100  has a pair of power feeding sections  102   a  and  102   b  and a pair of antenna elements  103   a  and  103   b.    
         [0027]    Ground plate  101  has power feeding section  102 . Ground plate  101  is formed with, for example, a conductive case, a circuit board on which a ground pattern is printed, and so on. 
         [0028]    Power feeding section  102  is provided on ground plate  101 . 
         [0029]    Antenna element  103  is provided in an edge section of ground plate  101 . Also, antenna element  103  is formed by a conductor plate and has an inverted-F shape connecting grounding section  111 , power-fed section  112  and main body section  113  as one, and functions as an inverted-F antenna. Also, antenna element  103  is formed by, for example, forming grounding section  111  and main body section  113  as one by processing conductive (metallic, for example) members and furthermore by welding power-fed section  112  to main body section  113 . 
         [0030]    Grounding section  111  is bent vertically from main body section  113  and is formed as one with main body section  113 , and attached vertically with respect to ground plate  101 . Also, ground section  111  is grounded by being electrically connected to the ground of ground plate  101 . 
         [0031]    Power-fed section  112  is attached vertically with respect to main body section  113 , electrically and mechanically connected with main body section  113 , and attached vertically with respect to ground plate  101 . Also, power-fed section  112  is fed power from power feeding section  102 . Also, the length of the long side of power-fed section  112  (the vertical direction with respect to ground plate  101 ) is virtually the same as in grounding section  111 , and power-fed section  112  is attached to ground plate  101   a  predetermined distance apart from grounding section  111  parallel to grounding section  111 . 
         [0032]    Main body section  113  is formed by bending a conductor plate in the middle so that the conductive plate has opposing surfaces. Also, main body section  113  is placed parallel to ground plate  101 . 
         [0033]    Next, the configuration of antenna element  103  will be described in detail using  FIGS. 3 to 5 .  FIG. 3  is a plan view of antenna apparatus  100 ,  FIG. 4  is a side view of antenna apparatus  100 , and  FIG. 5  is a front view of antenna apparatus  100 . 
         [0034]    As shown in  FIG. 3 , main body section  113   a  of antenna element  103   a  is formed in an L shape on a planar view, and main body section  113   b  of antenna element  103   b  is formed in an inverted-L shape on a planar view. Also, main body sections  113   a  and  113   b  are formed such that the long side that is 21 mm (0.15λ) long in the long direction and the short side that is 19 mm (0.13λ) in the short direction meet vertically. In ground plate  101 , long side  203  and short side  205  meet vertically and form corner section  201 , and long side  204  and short side  205  meet vertically and form corner section  202 . Also, main body section  103   a  of antenna element  103   a  is placed in corner section  201  along long side  203  and short side  205  forming corner section  201 . Likewise, main body section  113   b  of antenna element  103   b  is placed in corner section  202  along long side  204  and short side  205  forming corner section  202 . Also, short side  205  of ground plate  101  is 45 mm (0.31λ) long. 
         [0035]    Also, as shown in  FIG. 4 , main body section  113   a  of antenna element  103   a  is formed by bending back a conductor plate in the middle so that surface  302  of the conductor plate before the bending point and surface  301  of the conductor plate that is bent backward meet at a distance of 1 mm (0.007λ). Also, antenna element  103   a  is placed on ground plate  101  such that the distance between main body section  113   a  that is bent back, and ground plate  101 , is 7 mm (0.05λ). Although not shown in  FIG. 4 , the same applies to antenna element  103   b , and so its description will be omitted. 
         [0036]    Also, referring to  FIG. 5 , antenna element  103   a  is formed such that the distance between grounding section  111   a  and power-fed section  112   a  is 5 mm (0.03λ). Also, antenna element  103   b  is formed such that the distance between grounding section  111   b  and power-fed section  112   b  is 5 mm (0.03λ). 
         [0037]    The dimensions of the antenna elements shown in  FIG. 3  to  FIG. 5  are only examples but are by no means limited to these dimensions insofar as having the antenna element configuration shown in  FIG. 2 . 
         [0038]      FIG. 6  shows current distribution characteristics on ground plate  101  according to the present embodiment. Referring to  FIG. 6 , ground plate  101  of the casing of a communication terminal apparatus has a current distribution where the current value increases closer to antenna elements  103   a  and  103   b  (where P 1 &gt;P 2 &gt;P 3 &gt;P 4  and where P 1  through P 4  show current values (A/m)). That is to say, the current distribution according to the present embodiment is concentrated near antenna elements  103   a  and  103   b  in the event ground plate  101  is attached to the casing of the communication terminal apparatus (not shown) and does not distribute over the entirety of ground plate  101 . Consequently, the current value in area R 1 , by which the user holds the mobile terminal, is 4, which is the smallest current value. 
         [0039]    With the present embodiment, main body section  113   a  of antenna element  103   a  and main body section  113   b  of antenna element  103   b  are symmetric with respect to the line connecting between the middles of opposing short sides of ground plate  101  (the short side to oppose short side  205  is not shown in  FIG. 2  and  FIG. 3 ), and are arranged to along short side  205  and long side  203 , or short side  205  and long side  204 . By this means, provision is made to concentrate current distribution in an antenna element and prevent current distribution from showing in area R 1 , so that it is possible to reduce the current distribution in ground plate  101 . This is because the current that flows in main body sections  113   a  and  113   b  of the bent-back parts of antenna elements  103   a  and  103   b  and the current that flows in main body sections  113   a  and  113   b  of the unbent parts flow in directions to cancel each other. 
         [0040]      FIG. 7  shows VSWR characteristics according to the present embodiment. As shown in  FIG. 7 , with the present embodiment, antenna apparatus  100  functions as a multi-band antenna having oscillating frequencies of frequency f 1  and frequency f 2 . 
         [0041]    Incidentally, in MIMO communication, if part of a plurality of antennas is influenced by the human body, variations in receiving power are produced between antennas (i.e. unequal median value). As a result of this, in MIMO communication, communication performance (transmission rate) lowers due to deterioration of antenna characteristics or deterioration of reception. 
         [0042]    Conventionally, in a mobile terminal performing MIMO communication using a plurality of antennas, when a user holds the mobile terminal by his hand, for example, the user&#39;s hand is situated near one antenna (antenna #1), and therefore antenna characteristics deteriorate and the receiving power lowers. In this case, power difference ΔG [dB] is produced between antenna #1 and the other antenna (antenna #2). Power difference ΔG results in an unequal median value between antennas, and, if this power difference ΔG increases, MIMO communication performance lowers. 
         [0043]      FIG. 8  shows relationship between the unequal median value and user throughput during MIMO communication. Now, for example, user throughput [bps] represents the level of high-speed transmission in a quantitative manner, and, for example, downloading can be finished in a shorter time if throughput increases. As shown in  FIG. 8 , if unequal median value increases, user throughput during MIMO communication lowers. 
         [0044]    However, with the present embodiment, current distribution is concentrated in antenna elements and the current distribution near the part (area R 1 ) which the user holds by his hand lowers, so that an impedance mismatch due to the user&#39;s hand being located near an antenna element of a mobile terminal is less likely to occur. That is to say, with the present embodiment, the influence upon deterioration of antenna characteristics is less and the power difference that is produced between antenna element  103   a  and antenna element  103   b  can be alleviated, so that it is possible to achieve good antenna characteristics in MIMO communication and prevent communication performance from lowering. 
         [0045]    Antenna apparatus  100  of the present embodiment is applicable to a communication terminal apparatus such as a mobile telephone. By this means, it is possible to provide a communication terminal apparatus to support multi-band communication or MIMO communication. 
         [0046]    By this means, with the present embodiment, it is possible to reduce the influence of the human body and so on and make the antenna characteristic at desired frequency good by reducing the current distribution in the ground plate. Also, with the present embodiment, it is possible to achieve good communication performance when performing MIMO communication, by a simple configuration. Also, with the present embodiment, by making an antenna in a simple configuration, it is possible to save space and miniaturize the apparatus. 
       Embodiment 2 
       [0047]      FIG. 9  is a perspective view of antenna apparatus  800  according to embodiment 2 of the present invention. 
         [0048]    Antenna apparatus  800  shown in  FIG. 9  is configured by adding short-circuit section  801  to antenna apparatus  100  according to embodiment 1 of  FIG. 2 . Incidentally, parts in  FIG. 9  that are the same as in  FIG. 2  will be assigned the same reference codes as in  FIG. 2 , and their detailed descriptions will be omitted. 
         [0049]    Short-circuit section  801  is short-circuited by connecting main body section  113   a  of antenna element  103   a  and main body section  113   b  of antenna element  103   b  electrically. Also, short-circuit section  801  short-circuits the edge sections of the bent-back parts of main body sections  113   a  and  113   b  mutually. 
         [0050]    Next, a configuration of antenna element  103  and short-circuit section  801  will be described in more detail using  FIG. 10  and  FIG. 11 .  FIG. 10  is a plan view of antenna apparatus  800  and  FIG. 11  is a front view of antenna apparatus  800 . 
         [0051]    As shown in  FIG. 10 , short-circuit section  801  is 1 mm (0.007λ) long in the short direction (upper and lower directions in  FIG. 10 ). Incidentally, the rest of the configurations are the same as in  FIG. 3 , and their descriptions will be omitted. 
         [0052]    Also, as shown in  FIG. 11 , short-circuit section  801  short-circuits bent-back edge section  1001  of main body section  113   a  and bent-back edge section  1002  of main body section  113   b  mutually. The rest of the configurations are the same as in  FIG. 5 , and so their descriptions will be omitted. 
         [0053]      FIG. 12  shows the VSWR frequency characteristics of the present embodiment. As shown in  FIG. 12 , with the present embodiment, antenna apparatus  800  functions as a wideband antenna that can oscillate at frequency f 3  and frequency f 4 . 
         [0054]    Antenna apparatus  800  of the present embodiment is applicable to a mobile telephone and suchlike communication terminal apparatuses. By this means, it is possible to provide a communication terminal apparatus to support wideband communication. 
         [0055]    By this means, with the present embodiment, in addition to the above advantage of embodiment 1, it is possible to configure an antenna that can oscillate in wideband, and configure a multi-band antenna (wideband antenna) that supports various systems and frequencies, in a simple configuration. 
         [0056]    The disclosure of Japanese Patent Application No. 2008-269988, filed on Oct. 20, 2008, including the specification, drawings and abstract, is incorporated herein by reference in its entirety. 
       INDUSTRIAL APPLICABILITY 
       [0057]    An antenna apparatus according to the present invention is suitable to configure a wideband antenna that reduces the current to flow in the ground plate.