Abstract:
Disclosed is an antenna device that improves the antenna gain and reduces the correlation for a MIMO antenna. Said antenna device is provided with a first housing ( 101 ) and a second housing ( 102 ). In a folding handheld terminal wherein the first housing ( 101 ) and the second housing ( 102 ) are rotatably connected by a hinge ( 202 ), the first housing ( 101 ) is provided with a first power-supply unit ( 107 ) and the second housing ( 102 ) is provided with a second power-supply unit ( 108 ). An antenna element ( 201 ) is provided inside the first housing ( 101 ) near the hinge ( 202 ). One end of the antenna element is open, and the first power-supply unit ( 107 ) and second power-supply unit ( 108 ) supply power through the other end of the antenna.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an antenna apparatus. 
       BACKGROUND ART 
       [0002]    In recent years, as various applications become increasingly sophisticated, mobile radio terminals are required to enhance their functionality. Furthermore, 3G-LTE systems or the like need a MIMO (Multiple input Multiple Output) antenna and are required to suppress correlation characteristics of radiation directivity in each antenna of the MIMO antenna to a low level. 
         [0003]    Patent Literature 1 describes an antenna apparatus shown in  FIG. 1 . This antenna apparatus includes an antenna element and at least two waveguide assemblies (grounds), which are connected to connection points of the antenna element via a plurality of supply devices and insulated from each other regarding electromagnetic oscillation. Each waveguide assembly is made up of a conductor part connected to the supply device and the waveguide assembly is designed to absorb or radiate electromagnetic oscillation via the supply device. Furthermore, the conductor part is designed to absorb or radiate electromagnetic oscillation in a way similar to the way the antenna element operates. 
       CITATION LIST 
     Patent Literature 
       [0000]    
       
         PTL1 
         Japanese Patent Application Laid-Open No. HEI 10-79617 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0006]    However, it is structurally impossible for the antenna apparatus disclosed in Patent Literature 1 above to mount the two waveguide assemblies on a mobile radio terminal such as mobile phone. Furthermore, arranging the two waveguide assemblies in the same case generates interference between the waveguide assemblies, deteriorates the antenna gain and makes it difficult to achieve low correlation. 
         [0007]    The present invention has been implemented in view of such problems and it is an object of the present invention to provide an antenna apparatus that improves the antenna gain and achieves low correlation in a MIMO antenna. 
       Solution to Problem 
       [0008]    An antenna apparatus of the present invention is an antenna apparatus mounted on a flip mobile terminal provided with a first case and a second case, and a hinge section that rotatably connects the first case and the second case, and adopts a configuration including a first ground plate incorporated in the first case, a second ground plate incorporated in the second case, an antenna element provided in the vicinity of the hinge section, resonating at a desired frequency and having one open end, a first power supply section connected to the first ground plate to supply power from the other end of the antenna element and a second power supply section connected to the second ground plate to supply power from the other end of the antenna element. 
       Advantageous Effects of Invention 
       [0009]    The present invention can improve the antenna gain and achieve low correlation in a MIMO antenna. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0010]      FIG. 1  illustrates an antenna apparatus described in Patent Literature 1; 
           [0011]      FIG. 2  illustrates a configuration of a mobile terminal according to Embodiment 1 of the present invention; 
           [0012]      FIG. 3  illustrates a situation in which a three-dimensional coordinate system is provided; 
           [0013]      FIG. 4  illustrates a frequency characteristic of VSWR; 
           [0014]      FIG. 5  illustrates radiation directivity (horizontal polarized wave component) of an XZ plane; 
           [0015]      FIG. 6  illustrates a horizontal polarized wave phase characteristic; 
           [0016]      FIG. 7  illustrates a configuration of a mobile terminal according to Embodiment 2 of the present invention; 
           [0017]      FIG. 8  illustrates another configuration of the mobile terminal according to Embodiment 2 of the present invention; 
           [0018]      FIG. 9  illustrates a configuration of a mobile terminal according to Embodiment 3 of the present invention; 
           [0019]      FIG. 10  illustrates antenna current paths in the configuration (structure  1 ) shown in  FIG. 2 ; 
           [0020]      FIG. 11  illustrates antenna current paths in the configuration (structure  1 ) shown in  FIG. 9 ; 
           [0021]      FIG. 12  illustrates measurement results of S-parameters of structure  1  and structure  2 ; 
           [0022]      FIG. 13  illustrates another configuration of the mobile terminal according to Embodiment 3 of the present invention; 
           [0023]      FIG. 14  illustrates antenna current paths in the configuration (structure  3 ) shown in  FIG. 13 ; 
           [0024]      FIG. 15  illustrates measurement results of S-parameters in structures  1  to  3 ; 
           [0025]      FIG. 16  illustrates a configuration of a mobile terminal without using any coaxial cable; and 
           [0026]      FIG. 17  illustrates a configuration of a mobile terminal having a first ground plate and a second ground plate of different sizes. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0027]    Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, components having the same functions among the embodiments will be assigned the same reference numerals and overlapping descriptions will be omitted. 
       Embodiment 1 
       [0028]      FIG. 2  illustrates a configuration of a mobile terminal according to Embodiment 1 of the present invention. The mobile terminal shown in this figure has a folding shape, and first case  101  and second case  102  are rotatably connected together by hinge section  202 . 
         [0029]    First case  101  incorporates first ground plate  103 , second case  102  incorporates second ground plate  104 , and first ground plate  103  and second ground plate  104  are connected together via data communication line  112 . 
         [0030]    First ground plate  103  is provided with radio circuit  105  and signal processing section  106  connected to this radio circuit  105 . Furthermore, first ground plate  103  includes first power supply section  107  connected to radio circuit section  105  and having transmitting and receiving functions. 
         [0031]    Second ground plate  104  includes second power supply section  108  connected to radio circuit  105  via coaxial cable  109  and having transmitting and receiving functions. 
         [0032]    One end of first strip element  110  is connected to first power supply section  107  and the other end of first strip element  110  is connected to antenna element  201  provided near hinge section  202  inside first case  101 . Furthermore, one end of second strip element  111  is connected to second power supply section  108  and the other end of second strip element  111  is connected to antenna element  201 . 
         [0033]    One end of antenna element  201  is connected to first strip element  110  and second strip element  111  and the other end is left open. 
         [0034]    As specific sizes, suppose, for example, first ground plate  103  and second ground plate  104  have a width of 45 mm, a length of 100 mm, antenna element  201  has a length of 25 mm and the strip element between power supply points has a length of 30 mm. 
         [0035]      FIGS. 3 to 5  show results of an electromagnetic field simulation of the antenna having such a size.  FIG. 3  illustrates a situation in which a three-dimensional coordinate system is provided. As shown in  FIG. 3 , suppose the horizontal direction of the mobile terminal is the X-axis, the length direction is the Y-axis and the width direction is the Z-axis. Furthermore, the angle formed with respect to the XZ plane is φ and the angle formed with respect to the YZ plane is θ. 
         [0036]      FIG. 4  illustrates a frequency characteristic of VSWR. In  FIG. 4 , a thick solid line represents a frequency characteristic of VSWR when first power supply section  107  is excited and a broken line represents a frequency characteristic of VSWR when second power supply section  108  is excited. As is clear from this figure, the thick solid line and the broken line overlap each other and resonance centered on the same 2 GHz is obtained even when both power supply sections are excited. 
         [0037]      FIG. 5  illustrates a horizontal polarized wave component of radiation directivity of the mobile terminal of the XZ plane in  FIG. 3 .  FIG. 5(   a ) illustrates radiation directivity when first power supply section  107  is excited and  FIG. 5(   b ) illustrates radiation directivity when second power supply section  108  is excited. As is clear from these figures, the radiation directivity is symmetric, which allows the correlation characteristic of the radiation directivity to be suppressed low. 
         [0038]    Furthermore,  FIG. 6  illustrates a phase characteristic.  FIG. 6  illustrates the phase of the XZ plane in  FIG. 3 . In  FIG. 6 , a thick solid line represents the phase of a horizontal polarized wave when first power supply section  107  is excited and a broken line represents the phase of a horizontal polarized wave when second power supply section  108  is excited. As is clear from this figure, the phase characteristic also has a characteristic symmetric between the power supply sections bordered at θ=180° and the correlation characteristic of the phase characteristic can also be suppressed low. A possible reason for this may be that an unbalanced operation of the antenna causes a current flowing into the ground plate on which each power supply section is arranged to become dominant. 
         [0039]    Furthermore, in the aforementioned size, a correlation coefficient (CC) obtained using a Pearson product-moment correlation expressed in equation 1 shown below was as low as 0.3 in its horizontal polarized wave component. 
         [0040]    That is, it is possible to realize low correlation of antenna directivity radiating from the respective power supply sections. 
         [0000]    [1] 
         [0000]    
       
         
           
             
               
                 
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         [0041]    where, antenna complex directivity from each power supply section is represented by x={x i }, y={y i }, (i=1, 2, . . . , n) and x −  represents an average value of x={x i } and y −  represents an average value of y={y i }. 
         [0042]    Thus, Embodiment 1 provides a first power supply section in a first case and a second power supply section in a second case of a flip mobile terminal, arranges an antenna element in the vicinity of a hinge section that rotatably connects the first case and the second case, connects the first power supply section and the second power supply section to one end of the antenna element respectively, and can thereby realize a plurality of antenna effects in one antenna element, and further improve the antenna gain and realize low correlation of antenna directivity. 
       Embodiment 2 
       [0043]      FIG. 7  illustrates a configuration of a mobile terminal according to Embodiment 2 of the present invention.  FIG. 7  is different from  FIG. 2  in that case state sensor  114  and switching circuit  115  are added. 
         [0044]    Case state sensor  114  detects whether a flip mobile terminal is in a closed state or in an open state and reports the detection result to switching circuit  115  via signal processing section  106  and radio circuit  105 . 
         [0045]    Switching circuit  115  switches whether or not to connect radio circuit  105  and second power supply section  108 , that is, turns ON/OFF of the second power supply section according to the detection result outputted from case state sensor  114 . To be more specific, switching circuit  115  connects radio circuit  105  and second power supply section  108  when the detection result outputted from case state sensor  114  indicates an open state of the mobile terminal, and disconnects radio circuit  105  from second power supply section  108  when the detection result indicates a closed state of the mobile terminal. 
         [0046]    When the mobile terminal is in a closed state and continues to receive power supply from first power supply section  107  and second power supply section  108 , the antenna gain deteriorates due to a positional relationship between first ground plate  103  and second ground plate  104 , but by disconnecting radio circuit  105  from second power supply section  108  and stopping power supply from second power supply section  108 , it is possible to prevent the antenna gain from deteriorating. 
         [0047]    Thus, according to Embodiment 2, when the mobile terminal is in a closed state, the switching circuit disconnects the radio circuit from the second power supply section and stops power supply from the second power supply section, and can thereby prevent the antenna gain from deteriorating. 
         [0048]    Although a case has been described in the present embodiment where switching circuit  115  is controlled based on the result of case state sensor  114  detecting an open/closed state of the mobile terminal, as shown in  FIG. 8 , signal processing section  106  may be provided with a receiving power detection circuit in addition to case state sensor  114  so that switching circuit  115  disconnects radio circuit  105  from second power supply section  108  when receiving power detected by the receiving power detection circuit falls to or below a predetermined value. 
       Embodiment 3 
       [0049]      FIG. 9  illustrates a configuration of a mobile terminal according to Embodiment 3 of the present invention.  FIG. 9  is different from  FIG. 2  in that antenna element  201  is changed to antenna element  301 . However,  FIG. 9  only illustrates a portion of the mobile terminal in the vicinity of hinge section  202 . 
         [0050]    Antenna element  301  has a length of λ/4 and is arranged perpendicular to the plane of the ground plate (first ground plate  103  or second ground plate  104 ), and one end of the antenna element is connected to first strip element  110  and second strip element  111 , and the other end is left open. 
         [0051]    Antenna element  301  arranged in this way functions as an open stub at a desired frequency. This allows isolation performance to be secured between first power supply section  107  and second power supply section  108  at the desired frequency. 
         [0052]    Here,  FIG. 10  illustrates antenna current paths in the configuration (structure  1 ) shown in  FIG. 2 .  FIG. 10(   a ) illustrates antenna current paths in structure  1  when first power supply section  107  is excited. In this case, antenna current leakage not only to antenna element  201  but also to first strip element  110  occurs. Furthermore,  FIG. 10(   b ) illustrates antenna current paths in structure  1  when second power supply section  108  is excited. As shown in  FIG. 10(   b ), antenna current leakage not only to antenna element  201  but also to second strip element  111  occurs. 
         [0053]    On the other hand,  FIG. 11  illustrates antenna current paths in the configuration (structure  2 ) shown in  FIG. 9 .  FIG. 11(   a ) illustrates antenna current paths in structure  2  when first power supply section  107  is excited. In this case, an antenna current flows into antenna element  301  and substantially no antenna current leaks to first strip element  110 . Furthermore,  FIG. 11(   b ) illustrates antenna current paths in structure  2  when second power supply section  108  is excited. As shown in  FIG. 11(   b ), in this case, an antenna current flows into antenna element  301  and substantially no antenna current leaks to second strip element  111 . This is because the antenna current becomes 0 at an end of antenna element  301  and no reflected current is generated. 
         [0054]    Next,  FIG. 12  illustrates measurement results of S-parameters in structure  1  and structure  2  assuming a desired frequency is 2 GHz. In structure  1 , resonance takes place with S 11  falling below −10 dB at 2 GHz, but an isolation characteristic (S 21 ) between the power supply sections does not fall below −10 dB at 2 GHz, and it can be confirmed that both are not mutually compatible at the desired frequency. On the other hand, in structure  2 , a fractional bandwidth of the resonance frequency improves from that of structure  1  and the isolation characteristic (S 21 ) between the power supply sections falls below −10 dB, and it can be confirmed that both are mutually compatible at the desired frequency. 
         [0055]    Thus, since the isolation performance between first power supply section  107  and second power supply section  108  at a desired frequency can be secured, electrical interference between first power supply section  107  and second power supply section  108  is suppressed and it is thereby possible to improve the radiation gain characteristic at the time of excitation of first power supply section  107  and second power supply section  108  respectively. Furthermore, regarding also the performance of the radio circuit connected to first power supply section  107  and second power supply section  108  respectively, it is possible to suppress mutual leakage of transmission currents. 
         [0056]    Thus, Embodiment 3 provides an antenna element having a length of λ/4, arranged perpendicular to the surface of a ground plate, one end of which is connected to a first strip element and a second strip element and the other end of which is left open, and can thereby secure isolation performance between the first power supply section and the second power supply section at a desired frequency, thereby suppress electrical interference between the first power supply section and the second power supply section and improve the radiation gain characteristic at the time of excitation of the first power supply section and the second power supply section respectively. 
         [0057]    As shown in  FIG. 13 , it may also be possible to arrange an antenna element perpendicular to the long side of the ground plate (first ground plate  103  or second ground plate  104 ) so as to protrude from first case  101  (or second case  102 ) on the same plane as the ground plate (in the +X direction on the coordinate axis shown in  FIG. 3 ), connect one end thereof to first strip element  110  and second strip element  111  and leave open the other end. 
         [0058]      FIG. 14  illustrates an antenna current path in the configuration (structure  3 ) shown in  FIG. 13 .  FIG. 14(   a ) illustrates an antenna current path in structure  3  when first power supply section  107  is excited. In this case, an antenna current flows into the antenna element and no antenna current flows into first strip element  110 . Furthermore,  FIG. 14(   b ) illustrates an antenna current path in structure  3  when second power supply section  108  is excited. As shown in  FIG. 14(   b ), in this case, an antenna current flows into the antenna element and no antenna current flows into second strip element  111 . 
         [0059]    Thus, in the arrangement shown in  FIG. 13 , the antenna element also functions as an open stub at a desired frequency. Thus, isolation performance between first power supply section  107  and second power supply section  108  can be secured at the desired frequency. 
         [0060]      FIG. 15  illustrates measurement results of S-parameters in structure  3 . As is clear from the figure, a fractional bandwidth of the resonance frequency in structure  3  also improves from that in structure  1  and the isolation characteristic (S 21 ) between the power supply sections falls below −10 dB, and it can be confirmed that both are mutually compatible at the desired frequency. 
         [0061]    A case has been described in the above-described embodiments where radio circuit  105  inside the first case and the second power supply section are connected together via a coaxial cable, but as shown, for example, in  FIG. 16 , it may also be possible to provide radio circuit  113  connected to signal processing section  106  via data communication line  112  in the second ground plate so as to connect radio circuit  113  to the second power supply section. 
         [0062]    It has been assumed in the above-described embodiments that first ground plate  103  and second ground plate  104  have the same size, but the present invention is not limited to this, and first ground plate  103  may be greater or smaller than second ground plate  104 .  FIG. 17  illustrates a case where first ground plate  103  is greater than second ground plate  104 . 
         [0063]    When part of the first case or second case or the whole of the first case or second case is metallic, it may be possible to connect the metallic case to the first ground plate or second ground plate and thereby change the size of the ground. 
         [0064]    A case has been described in the above-described embodiments where antenna element  201  is provided in first case  101 , but antenna element  201  may also be provided in second case  102  as long as it is located in the vicinity of hinge section  202 . 
         [0065]    Antenna element  201  or antenna element  301  described above is also applicable to a metallic part (e.g., hinge metal of a flip terminal) having substantially the same length as the assumed antenna element when mounted on a mobile terminal. 
         [0066]    The disclosures of Japanese Patent Application No. 2009-168139, filed on Jul. 16, 2009 and Japanese Patent Application No. 2010-154865, filed on Jul. 7, 2010, including the specification, drawings and abstract are incorporated herein by reference in its entirety. 
       INDUSTRIAL APPLICABILITY 
       [0067]    The antenna apparatus according to the present invention is applicable to a radio communication terminal apparatus such as a flip mobile phone. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           101  First case 
           102  Second case 
           103  First ground plate 
           104  Second ground plate 
           105 ,  113  Radio circuit 
           106  Signal processing section 
           107  First power supply section 
           108  Second power supply section 
           109  Coaxial cable 
           110  First strip element 
           111  Second strip element 
           112  Data communication line 
           114  Case state sensor 
           115  Switching circuit 
           201 ,  301  Antenna element 
           202  Hinge section