Abstract:
An antenna device includes: a plurality of antenna elements of at least two or more; transmission lines connected to said antenna elements, respectively; a filter circuit connected between each of said transmission lines; a matching circuit connected, in said transmission lines, with an end opposite to said plurality of antenna elements; and an antenna port connected with said matching circuit. Isolation between all antenna ports is achieved about said plurality of antenna elements having a plurality of frequency bands in common with each other by choosing an electrical length of said transmission lines and a design condition of said filter circuit.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an antenna device in a radio communication apparatus using a MIMO system. 
       BACKGROUND ART 
       [0002]    In recent years, a plurality of antenna elements having identical frequencies have begun to be mounted in equipment like a mobile terminal along with the popularization of a MIMO (Multiple-Input Multiple-Output) system. However, when a plurality of antenna elements having identical frequencies are adjacent to each other in a terminal having a small mounting area, mutual coupling is caused, resulting in decline of an antenna radiation efficiency and degeneration of a space correlation coefficient. Thus, it becomes a factor which deteriorates the MIMO communication performance. 
         [0003]    As a solution for it, in an antenna structural object indicated in  FIG. 10B  of patent literature 1, for example, a connection device is provided between neighboring antenna elements, and the length of the connection device and its installation position are adjusted. There is being adopted a structure which can, by giving an appropriate susceptance value by the above, suppress isolation between antenna ports. 
       CITATION LIST 
     Patent Literature 
       [0004]    [PTL n] Patent literature 1: Japanese Patent Publication No. 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0005]    However, there is the following problem with the antenna structure disclosed in patent document 1. That is, isolation between antenna ports can be realized only in a specific frequency band because, in a case of a plurality of antenna elements having a plurality of frequency bands in common with each other, a connection device has only a monotonous frequency operation as an inductor only by inserting a connection device between each antenna element. Rather, there is a high possibility that mutual coupling between antenna ports becomes stronger in the other frequency bands, and thus it will be a cause to deteriorate the MIMO communication performance. 
         [0006]    The present invention tries to provide an antenna device which compensates, for a plurality of antenna elements having a plurality of frequency bands in common with each other, isolation between antenna ports in each of the frequency bands. 
       Solution to Problem 
       [0007]    An antenna device in the present invention includes: at least two antenna elements; transmission lines connected to each of said antenna elements, respectively; a filter circuit connected between each of said transmission lines; a matching circuit connected to an end part of each of said transmission lines; and each of said antenna elements having a plurality of frequency bands in common with each other. Said filter circuit has a passband in a specific frequency band of said antenna element. Said filter circuit is formed using a lumped constant circuit or a distributed constant circuit, and mutual coupling between antennas is suppressed by making a total of a phase difference between said antenna elements and an electrical length of said transmission lines be plus or minus 90 degrees, and by giving a non-coupling value to enable susceptance of said filter circuit to be decided by a coupling coefficient between antennas. 
       Advantageous Effects of Invention 
       [0008]    According to the present invention, there can be provided an antenna device in which, about a plurality of antenna elements having a plurality of frequency bands in common with each other, isolation can be achieved between all antenna ports in the plurality of frequency bands. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0009]      FIG. 1  is a circuit diagram of an antenna device according to a first exemplary embodiment of the present invention. 
           [0010]      FIG. 2  is a circuit diagram in which a matching circuit is added to an end of an antenna of the first exemplary embodiment of  FIG. 1 . 
           [0011]      FIG. 3  is a circuit diagram illustrating an example of a low-pass filter which can be applied to the first exemplary embodiment of the present invention. 
           [0012]      FIG. 4  is a diagram illustrating an antenna device in which the first exemplary embodiment of the present invention is embodied. 
           [0013]      FIG. 5  is a diagram describing details of the filter circuit and the matching circuit in  FIG. 4 . 
           [0014]      FIG. 6  is a diagram showing frequency characteristics of an S parameter of the antenna device of  FIG. 4 . 
           [0015]      FIG. 7  is a diagram showing frequency characteristics of a correlation coefficient of the antenna device of  FIG. 4 . 
           [0016]      FIG. 8  is a circuit diagram of an antenna device according to a second exemplary embodiment of the present invention. 
           [0017]      FIG. 9  is a circuit diagram showing an antenna device according to a third exemplary embodiment of the present invention. 
           [0018]      FIG. 10  is a circuit diagram showing an antenna device according to a fourth exemplary embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
     Structure 
       [0019]    The first exemplary embodiment of an antenna device according to the present invention will be described in detail with reference to a drawing. 
         [0020]      FIG. 1  is a circuit diagram illustrating the basic structure of an antenna device according to the first exemplary embodiment of the present invention. 
         [0021]    In  FIG. 1 , the numbers  101  and  102  indicate a first and a second antenna element,  103  and  104  a first and a second transmission line,  105  a filter circuit, and  106  and  107  a first and a second matching circuit. The first matching circuit  106  and the second matching circuit  107  perform impedance matching when taking a look toward the side of the first antenna element  101  and the second antenna element  102 . The first antenna element  101  and the second antenna element  102  may be of different structures, different kinds and different arrangements in so far as they have same frequency bands. 
         [0022]    The first transmission line  103  and the second transmission line  104  may be of any kind, any length and any shape in so far as they satisfy the condition of Formula (1) described later. As the filter circuit  105 , a low-pass filter, a high-pass filter, a band-pass filter and a band reject filter, for example, can be used, and a filter using lumped constant circuit or a distributed constant circuit can be used. By using a filter circuit in the circuit  105 , a complicated alteration can be given to the frequency characteristics of susceptance for determining a non-coupling value that is important when isolation between antenna ports is realized, and thus isolation between antenna ports is made easy in multiple bands. 
         [0023]      FIG. 2  is a circuit diagram of an antenna device made by adding, to the antenna device illustrated in  FIG. 1 , matching circuits  101 - 2  and  102 - 2  near antenna elements  101 - 1  and  102 - 1 , respectively. 
         [0024]    Although the matching circuit  101 - 2  and the matching circuit  102 - 2  in  FIG. 2  are not needed necessarily in designing, they are introduced expediently here because they can be mounted in order to make designing of an antenna device easy. 
         [0025]    The filter circuit  105  in  FIG. 1  or  FIG. 2  can be formed by a low-pass filter circuit in which an inductors (L+L m1 ) and L m2 /2 and capacitors C and  2 C m , for example, are put together like  FIG. 3 . Here, L is the inductance of an inductor forming a low-pass filter, C and C m  are the capacitance of capacitors forming the low-pass filter, and L m1  and L m2  are the inductance of inductors forming the low-pass filter. 
         [0026]      FIG. 4  is a diagram illustrating an antenna device in which the first exemplary embodiment of the present invention is realized specifically by a substrate. Here, an inverted F antenna is used as an antenna element, microstrip lines M 1  and M 2  are wired from antenna elements A 1  and A 2  in the both sides of a substrate, respectively, and are connected to a filter circuit F and a matching circuit arranged in the center of the substrate. The matching circuit  101 - 2  and the matching circuit  102 - 2  in  FIG. 2  correspond to a short line in an inverted F antenna. Both the two antenna elements A 1  and A 2  are designed to have frequency bands in 850 MHz band and 2.1 GHz band. 
         [0027]      FIG. 5  is a diagram describing details of the filter circuit F and the matching circuit in the antenna device of  FIG. 4 . L 1 -L 3  and C 1 -C 3  are the lumped constants making up the filter circuit F, and L 4 , L 5 , C 4  and C 5  represent the lumped constants making up the matching circuit. 
         [0028]    Specifically, L 1 -L 3  are the inductance of inductors forming a low-pass filter, L 4  and L 5  are the inductance of inductors forming the matching circuit, and C 1 -C 3  are the capacitance of capacitors forming the low-pass filter and C 4  and C 5  are the capacitance of capacitors forming the matching circuit. Here, L 1 =3.8 nH, L 2 =4.5 nH, L 3 =4.5 nH, L 4 =1 nH, L 5 =1 nH, C 1 =2.7 pF, C 2 =2.05 pF, C 3 =2.05 pF, C 4 =2 pF and C 5 =2 pF. 
         [0029]    The filter circuit F is designed to have a stop band in 2.1 GHz band and a path band in 850 MHz band. 
         [0030]    In order to realize isolation between antenna ports, electrical length θ 1  of the microstrip line M 1  that is the first transmission line, electrical length θ 2  of the microstrip line M 2  that is the second transmission line and a lumped constant value of the filter circuit F are designed so that the following conditional expression (1) may be filled in a plurality of desired frequency bands. 
         [0000]    
       
         
           
             
               
                 
                   
                     B 
                     = 
                     
                       
                         
                           
                             ± 
                             2 
                           
                            
                           α 
                         
                         
                           1 
                           + 
                           
                             α 
                             2 
                           
                         
                       
                        
                       
                         Y 
                         0 
                       
                     
                   
                    
                   
                     
 
                   
                    
                   
                     
                       
                         θ 
                         1 
                       
                       + 
                       
                         θ 
                         1 
                       
                       + 
                       φ 
                     
                     = 
                     
                       ± 
                       
                         π 
                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0031]    In the above Formula (1), B represents the susceptance of the filter circuit F, and α and θ represent a coupling coefficient and a phase difference, respectively, between the antenna elements A 1  and A 2  in a case where only the antenna elements A 1  and A 2  are mounted. Y 0  is a characteristic admittance. 
         [0032]    Designing is performed in a plurality of desired frequency bands so that the total of a phase difference between the antenna elements A 1  and A 2  and the electrical lengths of the micro strip lines M1 and M2 may be made to be plus or minus 90 degrees (π/2), and the susceptance of the filter circuit F to be the non-coupling value B decided by coupling coefficient α between the antenna elements A 1  and A 2 . By this, mutual coupling between the antenna elements A 1  and A 2  can be suppressed. Here, an event that the susceptance of the filter circuit F becomes a non-coupling value B in the above Formula (1) indicates a situation that S parameter S 21  (or, S 12 ) mentioned later becomes minimal. 
         [0033]    Therefore, a condition of the above Formula (1) can be found by taking lumped constants in the filter circuit F as a parameter. Specifically, L 1 , L 2 , L 3  and C 1  can be employed as a parameter. 
         [0034]      FIG. 6  is a diagram illustrating frequency characteristics of S parameters of the substrate in  FIG. 4 . Here, S parameters when making a power feeding part P 1  and a power feeding part P 2  shown in  FIG. 4  be antenna ports are indicated. Although only S 11  and S 21  are shown here, S 22  and S 12  also indicate the same result as S 11  and S 21 , respectively. This is because S 22  =S 11  and S 12 =S 21  are ensured from the symmetry and reversibility of the antenna substrate. Because both S 11  and S 21  indicate a dip in 850 MHz and 2.1 GHz which are the frequency bands of the antennas, it is found that, in the two bands, isolation between the antenna ports are achieved as well as resonance characteristics of the antennas. 
         [0035]      FIG. 7  is a correlation coefficient between the ports calculated from the S parameters of  FIG. 6 . Correlation coefficient ρ e  can be expressed by the following Formula (2) when two antenna structures have a symmetrical structure. 
         [0000]    
       
         
           
             
               
                 
                   
                     ρ 
                     e 
                   
                   = 
                   
                     
                       
                          
                         
                           
                             
                               S 
                               11 
                               * 
                             
                              
                             
                               S 
                               12 
                             
                           
                           + 
                           
                             
                               S 
                               21 
                               * 
                             
                              
                             
                               S 
                               22 
                             
                           
                         
                          
                       
                       2 
                     
                     
                       
                         ( 
                         
                           1 
                           - 
                           
                             ( 
                             
                               
                                 
                                    
                                   
                                     S 
                                     11 
                                   
                                    
                                 
                                 2 
                               
                               + 
                               
                                 
                                    
                                   
                                     S 
                                     21 
                                   
                                    
                                 
                                 2 
                               
                             
                             ) 
                           
                         
                         ) 
                       
                        
                       
                         ( 
                         
                           1 
                           - 
                           
                             ( 
                             
                               
                                 
                                    
                                   
                                     S 
                                     22 
                                   
                                    
                                 
                                 2 
                               
                               + 
                               
                                 
                                    
                                   
                                     S 
                                     12 
                                   
                                    
                                 
                                 2 
                               
                             
                             ) 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0036]    The correlation coefficient ρe is found out to be approximately zero at just 850 MHz and at 2.1 GHz, and this sufficiently meets a low correlation required to obtain good characteristics in MIMO communication. 
       Other Exemplary Embodiments 
       [0037]    By mounting filter circuits in a multiple-stage manner in the first exemplary embodiment mentioned above, isolation can be realized even in more multiple bands. A structure for this is shown in  FIG. 8  as the second exemplary embodiment. 
         [0038]    In  FIG. 8 , this antenna device is made to be a multiple-stage structure in which a filter circuit  105 - 1  is connected between a transmission line  103 - 1  connected with an antenna element  101  and a transmission line  104 - 1  connected with an antenna element  102 , and a filter circuit  105 - 2  is connected between a transmission line  103 - 2  connected with the transmission line  103 - 1  and a transmission line  104 - 2  connected with the transmission line  104 - 1 . Here, the numbers  106  and  107  indicate matching circuits. 
         [0039]    Although, in the second exemplary embodiment, the filter circuits  105 - 1  and  105 - 2  of two stages are inserted between the transmission lines as a specific example, there is no problem to mount filter circuits more than that. By inserting filter circuits in a multiple-stage manner, it becomes possible to realize isolation between ports in a plurality of bands. 
         [0040]    The present invention becomes capable of coping with a plurality of bands also by using, as the third exemplary embodiment, a filter circuit  105 - 4  and a susceptance element  105 - 3  including inductors or capacitors for isolation between ports in a combined manner as shown in  FIG. 9 . 
         [0041]    In  FIG. 9 , an antenna device according to the third exemplary embodiment is made to be of a structure that the susceptance element  105 - 3  is connected between the transmission line  103 - 1  connected with the antenna element  101  and the transmission line  104 - 1  connected with the antenna element  102  and the filter circuit  105 - 4  is connected between the transmission line  103 - 2  connected with the transmission line  103 - 1  and the transmission line  104 - 2  connected with the transmission line  104 - 1 . 
         [0042]    Although, in the third exemplary embodiment, a structure in which one filter circuit and one susceptance element are employed has been indicated as a specific example, these may be inserted in a multiple-stage manner. Here, the numbers  106  and  107  are matching circuits. 
         [0043]    The present invention is also applicable in a case of three antenna elements or more. A structure for that is shown in  FIG. 10  as the fourth exemplary embodiment. 
         [0044]    In  FIG. 10 , transmission lines  112 ,  122  and  132  are connected to antenna elements  111 ,  121  and  131 , respectively, and a filter circuit  141  is connected between the transmission line  112  and the transmission line  122 , a filter circuit  142  is connected between the transmission line  122  and the transmission line  132 , and a filter circuit  143  is connected between the transmission line  112  and the transmission line  132  A matching circuit  113  is connected to an end of the transmission line  112 , a matching circuit  123  to an end of the transmission line  122  and a matching circuit  133  to an end of the transmission line  132 , respectively. Although a case of three antenna elements is indicated for simplicity here, it is also applicable in the same way in a case of the number of antenna elements more than that. 
         [0045]    Although the present invention has been described with reference to a plurality of exemplary embodiments above, the present invention is not limited to the above-mentioned exemplary embodiments. Various changes which a person skilled in the art can understand can be made to the compositions and details of the present invention within the spirit and the scope of the present invention described in the claims. 
       INDUSTRIAL APPLICABILITY 
       [0046]    The present invention is applicable in wireless radio equipment using a plurality of antenna elements. It can be loaded into equipment such as a base station and terminals for mobile communication or wireless LAN (Local Area Network) routers. 
         [0047]    This application claims priority based on Japanese application Japanese Patent Application No. 2012-201884 filed on Sep. 13, 2012, the disclosure of which is incorporated herein in its entirety. 
       Reference Signs List 
       [0000]    
       
           101 ,  102 ,  101 - 1 ,  102 - 1 ,  111 ,  121  and  131  Antenna element 
           103 ,  104 ,  103 - 1 ,  103 - 2 ,  104 - 1 ,  104 - 2 ,  112 ,  122  and  132  Transmission line 
           101 - 2 ,  102 - 2 ,  106 ,  107 ,  113 ,  123  and  133  Matching circuit 
           105 ,  105 - 1 ,  105 - 2 ,  105 - 4 ,  141 ,  142 ,  143  Filter circuit 
           105 - 3  Susceptance element