Patent Publication Number: US-2021194122-A1

Title: Antenna device and wireless communication device

Description:
TECHNICAL FIELD 
     The present invention relates to an antenna device including a first radiating element and a second radiating element, and a wireless communication device including the antenna device. 
     BACKGROUND ART 
     The following Patent Literature 1 discloses a circularly polarized wave switching antenna for radiating a right-hand circularly polarized wave or a left-hand circularly polarized wave. 
     The circularly polarized wave switching antenna includes the following components (1) to (4): 
     (1) a radiating element having two feed points and configured to radiate two linearly polarized waves orthogonal to each other; 
     (2) a first phase shifter with one end connected to one feed point of the radiating element and configured to shift the phase of a signal by 0 degrees or 180 degrees; 
     (3) a second phase shifter with one end connected to the other feed point of the radiating element and configured to shift the phase of a signal by 0 degrees or 180 degrees; and 
     (4) a 90°-hybrid circuit for splitting an input signal into two signals with a phase difference of 90 degrees, outputting one split signal to the first phase shifter, and outputting the other split signal to the second phase shifter. 
     CITATION LIST 
     Patent Literatures 
     Patent Literature 1: JP 2000-223942 A 
     SUMMARY OF INVENTION 
     Technical Problem 
     Suppose an antenna device is obtained by removing the radiating element in (1) and the first phase shifter in (2) from the known circularly polarized wave switching antenna and adding a first radiating element and a second radiating element to it. 
     In such a supposed antenna device, further suppose that the first radiating element is connected to a first output terminal of the 90°-hybrid circuit, and that the second radiating element is connected to a second output terminal of the 90°-hybrid circuit through the second phase shifter. 
     Such a supposed antenna device can function as a four-branch diversity antenna by switching the amounts of phase shift of the second phase shifter. 
     However, in the supposed antenna device, in a case in which the distance between the first radiating element and the second radiating element is short, for example, equal to or less than one-half of the wavelength of operating frequency, mutual coupling between the first radiating element and the second radiating element is strengthened. As the mutual coupling between the first radiating element and the second radiating element becomes stronger, a larger part of one or more signals radiated from the first radiating element enters the second radiating element. There is a problem that by a large part of the one or more signals radiated from the first radiating element entering the second radiating element, signal reflection increases. 
     Some embodiments in this disclosure have been made to solve a problem such as that described above, and an object of some embodiments in this disclosure is to obtain an antenna device capable of suppressing signal reflection even if the distance between two radiating elements is short. 
     In addition, another object of the invention is to obtain a wireless communication device including an antenna device capable of suppressing signal reflection. 
     Solution to Problem 
     An antenna device according to this disclosure includes a directional coupler for splitting, when a signal is inputted to the directional coupler from a first terminal or a second terminal, the signal into signals, outputting one split signal to a third terminal, and outputting an other split signal to a fourth terminal; a first radiating element connected to the third terminal; a first phase shifter having one end connected to the fourth terminal; a second radiating element connected to an other end of the first phase shifter; a second phase shifter having one end connected to the first terminal; a third phase shifter having one end connected to the second terminal; a first matching circuit having one end connected to an other end of the second phase shifter and having an other end connected to a first input/output terminal; and a second matching circuit having one end connected to an other end of the third phase shifter and having an other end connected to a second input/output terminal. 
     Advantageous Effects of Invention 
     According to this disclosure, the antenna device is constructed to include the first phase shifter having one end connected to the fourth terminal of the directional coupler; the second phase shifter having one end connected to the first terminal of the directional coupler; the third phase shifter having one end connected to the second terminal of the directional coupler; the first matching circuit having one end connected to the other end of the second phase shifter and having the other end connected to the first input/output terminal; and the second matching circuit having one end connected to the other end of the third phase shifter and having the other end connected to the second input/output terminal. Therefore, the antenna device according to this disclosure can suppress signal reflection even if the distance between two radiating elements is short. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a configuration diagram showing a wireless communication device including an antenna device  4  according to Embodiment 1. 
         FIG. 2  is a configuration diagram showing the antenna device  4  according to Embodiment 1. 
         FIG. 3  is a configuration diagram showing a first phase shifter  24 , a second phase shifter  25 , and a third phase shifter  26 . 
         FIG. 4  is an illustrative diagram showing a relationship among two diversity modes, four branches, the amounts of phase shift of the first to third phase shifters, feed points, and the phase difference between the excitation phase of a first radiating element  21  and the excitation phase of a second radiating element  22 . 
         FIG. 5  is an illustrative diagram showing a coupling from a first input/output terminal  11  to a second input/output terminal  12 . 
         FIG. 6  is an illustrative diagram showing reflections of transmission signals at the first input/output terminal  11 . 
         FIG. 7  is an illustrative diagram showing a two-element antenna array. 
         FIG. 8A  is a Smith chart showing S-parameters and  FIG. 8B  is an illustrative diagram showing frequency characteristics of amplitude. 
         FIG. 9A  is a Smith chart showing S-parameters for a case of a mode ( 1 ) and  FIG. 9B  is a Smith chart showing S-parameters for a case of a mode ( 2 ). 
         FIG. 10A  is a Smith chart showing S-parameters for a case of the mode ( 1 ) and  FIG. 10B  is a Smith chart showing S-parameters for a case of the mode ( 2 ). 
         FIG. 11A  is a Smith chart showing S-parameters for a case of the mode ( 1 ) and  FIG. 11B  is a Smith chart showing S-parameters for a case of the mode ( 2 ). 
         FIG. 12  is an illustrative diagram showing simulation results of radiation patterns obtained when a branch ( 1 ) in the mode ( 1 ) is used and the feed point is the first input/output terminal  11 . 
         FIG. 13  is an illustrative diagram showing simulation results of radiation patterns obtained when a branch ( 2 ) in the mode ( 1 ) is used and the feed point is the second input/output terminal  12 . 
         FIG. 14  is an illustrative diagram showing simulation results of radiation patterns obtained when a branch ( 3 ) in the mode ( 2 ) is used and the feed point is the first input/output terminal  11 . 
         FIG. 15  is an illustrative diagram showing simulation results of radiation patterns obtained when a branch ( 4 ) in the mode ( 2 ) is used and the feed point is the second input/output terminal  12 . 
         FIG. 16  is an illustrative diagram showing simulation results for correlation coefficients between the branches ( 1 ) to ( 4 ). 
         FIG. 17  is a configuration diagram showing another antenna device  4  according to Embodiment 1. 
         FIG. 18  is a configuration diagram showing an antenna device  4  according to Embodiment 2. 
         FIG. 19  is a configuration diagram showing a branch-line 90°-hybrid circuit. 
         FIG. 20  is a configuration diagram showing a directional coupler  60  including capacitors and inductors. 
         FIG. 21  is a configuration diagram showing a directional coupler  60  including four capacitors in total. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     To describe the invention in more detail, embodiments for carrying out the invention will be explained below in accordance with the accompanying drawings. 
     Embodiment 1 
       FIG. 1  is a configuration diagram showing a wireless communication device including an antenna device  4  according to Embodiment 1. 
     In  FIG. 1 , a transmitter  1  is a communication device for outputting transmission signals to a transmission/reception switching switch  3 . 
     A receiver  2  is a communication device for performing reception processes on reception signals outputted from the transmission/reception switching switch  3 . 
     The transmission/reception switching switch  3  outputs the transmission signals outputted from the transmitter  1  to a first input/output terminal  11  or a second input/output terminal  12  of the antenna device  4 , and outputs the reception signals outputted from the first input/output terminal  11  or the second input/output terminal  12  to the receiver  2 . 
     The antenna device  4  has the first input/output terminal  11  and the second input/output terminal  12 . 
     The antenna device  4  functions as a four-branch diversity antenna, using two antennas. 
     The first input/output terminal  11  is a terminal for accepting, as input, transmission signals outputted from the transmission/reception switching switch  3 , or outputting reception signals of the antenna device  4  to the transmission/reception switching switch  3 . 
     The second input/output terminal  12  is a terminal for accepting, as input, transmission signals outputted from the transmission/reception switching switch  3 , or outputting reception signals of the antenna device  4  to the transmission/reception switching switch  3 . 
       FIG. 2  is a configuration diagram showing the antenna device  4  according to Embodiment 1. 
     In  FIG. 2 , a first radiating element  21  is an antenna connected to a third terminal  23   c  of a directional coupler  23 . 
     A second radiating element  22  is an antenna connected to a first phase shifter  24 . 
     The directional coupler  23  is, for example, a branch-line directional coupler and has a first terminal  23   a , a second terminal  23   b , the third terminal  23   c , and a fourth terminal  23   d.    
     The first terminal  23   a  is connected to one end of a second phase shifter  25 . 
     The second terminal  23   b  is connected to one end of a third phase shifter  26 . 
     The third terminal  23   c  is connected to the first radiating element  21 . 
     The fourth terminal  23   d  is connected to one end of the first phase shifter  24 . 
     The directional coupler  23  is implemented as, for example, a branch-line directional coupler or a rat-race directional coupler. 
     When, for example, a transmission signal is inputted to the directional coupler  23  from the first terminal  23   a  or the second terminal  23   b , the directional coupler  23  splits the transmission signal into two transmission signals. 
     Then, the directional coupler  23  outputs one split transmission signal to the third terminal  23   c , and outputs the other split transmission signal to the fourth terminal  23   d.    
     When the transmission signal is inputted from the first terminal  23   a , the phase difference of the other split transmission signal with respect to one split transmission signal is ϕ degrees. 
     When the transmission signal is inputted from the second terminal  23   b , the phase difference of one split transmission signal with respect to the other split transmission signal is (π−ϕ) degrees. 
     When, for example, a reception signal is inputted to the directional coupler  23  from the third terminal  23   c  or the fourth terminal  23   d , the directional coupler  23  splits the reception signal into two reception signals. 
     Then, the directional coupler  23  outputs one split reception signal to the first terminal  23   a , and outputs the other split reception signal to the second terminal  23   b.    
     When the reception signal is inputted from the third terminal  23   c , the phase difference of the other split reception signal with respect to one split reception signal is (π−ϕ) degrees. 
     When the reception signal is inputted from the fourth terminal  23   d , the phase difference of one split reception signal with respect to the other split reception signal is degrees. 
     In Embodiment 1, as the directional coupler  23 , for example, a directional coupler with a degree of coupling of √0.5 (3 dB) is used. 
     The first phase shifter  24  is connected at its one end to the fourth terminal  23   d  and connected at its other end to the second radiating element  22 . 
     The first phase shifter  24  is a phase shifter that can switch the amount of phase shift to 0 degrees or θ degrees. 
     When a transmission signal is outputted from the fourth terminal  23   d , the first phase shifter  24  shifts the phase of the transmission signal by 0 degrees or θ degrees, and outputs the phase-shifted transmission signal to the second radiating element  22 . 
     When a reception signal is outputted from the second radiating element  22 , the first phase shifter  24  shifts the phase of the reception signal by 0 degrees or θ degrees, and outputs the phase-shifted reception signal to the fourth terminal  23   d.    
     The second phase shifter  25  is connected at its one end to the first terminal  23   a  and connected at its other end to a first matching circuit  27 . 
     The second phase shifter  25  is a phase shifter that can switch the amount of phase shift to 0 degrees or one-half of θ (hereinafter, represented as “θ/2”) degrees. 
     When a transmission signal is outputted from the first matching circuit  27 , the second phase shifter  25  shifts the phase of the transmission signal by 0 degrees or θ/2 degrees, and outputs the phase-shifted transmission signal to the first terminal  23   a.    
     When a reception signal is outputted from the first terminal  23   a , the second phase shifter  25  shifts the phase of the reception signal by 0 degrees or θ/2 degrees, and outputs the phase-shifted reception signal to the first matching circuit  27 . 
     The third phase shifter  26  is connected at its one end to the second terminal  23   b  and connected at its other end to a second matching circuit  28 . 
     The third phase shifter  26  is a phase shifter that can switch the amount of phase shift to 0 degrees or θ/2 degrees. 
     When a transmission signal is outputted from the second matching circuit  28 , the third phase shifter  26  shifts the phase of the transmission signal by 0 degrees or θ/2 degrees, and outputs the phase-shifted transmission signal to the second terminal  23   b.    
     When a reception signal is outputted from the second terminal  23   b , the third phase shifter  26  shifts the phase of the reception signal by 0 degrees or θ/2 degrees, and outputs the phase-shifted reception signal to the second matching circuit  28 . 
     The first matching circuit  27  is connected at its one end to the other end of the second phase shifter  25  and connected at its other end to the first input/output terminal  11 . 
     The first matching circuit  27  is a circuit for matching the impedance seen from the first input/output terminal  11  toward a second phase shifter  25  to the impedance seen from the first input/output terminal  11  toward a transmission/reception switching switch  3 . 
     The second matching circuit  28  is connected at its one end to the other end of the third phase shifter  26  and connected at its other end to the second input/output terminal  12 . 
     The second matching circuit  28  is a circuit for matching the impedance seen from the second input/output terminal  12  toward a third phase shifter  26  to the impedance seen from the second input/output terminal  12  toward the transmission/reception switching switch  3 . 
     Although  FIG. 2  shows an example in which each of the first matching circuit  27  and the second matching circuit  28  is a Π-circuit including three lumped elements, each circuit is not limited thereto and may be a Π-circuit including two or less lumped elements. 
     In addition, each of the first matching circuit  27  and the second matching circuit  28  may be, for example, a T-circuit including three or less lumped elements. 
       FIG. 3  is a configuration diagram illustrating the first phase shifter  24 , the second phase shifter  25 , and the third phase shifter  26 . 
     Each of the first phase shifter  24 , the second phase shifter  25 , and the third phase shifter  26  can use a switched-line phase shifter such as that shown in  FIG. 3 . 
     In  FIG. 3 , each of a switch  31  and a switch  32  is implemented by, for example, a Single-Pole Double-Throw (SPDT) switch. 
     A line  33  is a line for connecting the switches  31  and  32 . The line  33  is a short line with a line length being able to be ignored. Thus, it is supposed that the line  33  does not affect the phase of a signal passing through the line  33 . 
     A bypass line  34  is a line with a length corresponding to an amount of phase shift of a phase shifter. 
     As for the case in which the phase shifter shown in  FIG. 3  is the first phase shifter  24 , the bypass line  34  has a length corresponding to the amount of phase shift θ. 
     In the case in which the phase shifter shown in  FIG. 3  is the first phase shifter  24 , each of the switch  31  and the switch  32  is connected to the line  33  to set the amount of phase shift to 0 degrees. By connecting each of the switch  31  and the switch  32  to the line  33 , the fourth terminal  23   d  is connected to the second radiating element  22 . 
     In the case in which the amount of phase shift is to be set to 0 degrees, each of the switch  31  and the switch  32  is connected to the bypass line  34 . By connecting each of the switch  31  and the switch  32  to the bypass line  34 , the fourth terminal  23   d  is connected to one end of the bypass line  34  and the other end of the bypass line  34  is connected to the second radiating element  22 . 
     Furthermore, as for the case in which the phase shifter shown in  FIG. 3  is the second phase shifter  25 , the bypass line  34  has a length corresponding to the amount of phase shift θ/2. 
     In the case in which the phase shifter shown in  FIG. 3  is the second phase shifter  25 , each of the switch  31  and the switch  32  is connected to the line  33  to set the amount of phase shift of the second phase shifter  25  to 0 degrees. By connecting each of the switch  31  and the switch  32  to the line  33 , the first terminal  23   a  is connected to the one end of the first matching circuit  27 . 
     In the case in which the amount of phase shift is to be set to one-half of 0 degrees, each of the switch  31  and the switch  32  is connected to the bypass line  34 . By connecting each of the switch  31  and the switch  32  to the bypass line  34 , the first terminal  23   a  is connected to the one end of the bypass line  34  and the other end of the bypass line  34  is connected to the one end of the first matching circuit  27 . 
     Furthermore, as for the case in which the phase shifter shown in  FIG. 3  is the third phase shifter  26 , the bypass line  34  has a length corresponding to the amount of phase shift θ/2. 
     In the case in which the phase shifter shown in  FIG. 3  is the third phase shifter  26 , each of the switch  31  and the switch  32  is connected to the line  33  to set the amount of phase shift to 0 degrees. By connecting each of the switch  31  and the switch  32  to the line  33 , the second terminal  23   b  is connected to the one end of the second matching circuit  28 . 
     In the case in which the amount of phase shift of the third phase shifter  26  is to be set to one-half of 0 degrees, each of the switch  31  and the switch  32  is connected to the bypass line  34 . By connecting each of the switch  31  and the switch  32  to the bypass line  34 , the second terminal  23   b  is connected to the one end of the bypass line  34  and the other end of the bypass line  34  is connected to the one end of the second matching circuit  28 . 
     Note that each of the switch  31  and the switch  32  may be operated by a control device which is not shown, or may be manually operated by a user. 
     Next, operations of the wireless communication device shown in  FIG. 1  will be described. 
     The antenna device  4  can function as a four-branch diversity antenna by switching the respective amounts of phase shift of the first phase shifter  24 , the second phase shifter  25 , and the third phase shifter  26 . 
       FIG. 4  is an illustrative diagram showing a relationship among two diversity modes, four branches, the amounts of phase shift of the first to third phase shifters, feed points, and the phase difference between the excitation phase of the first radiating element  21  and the excitation phase of the second radiating element  22 . 
     The antenna device  4  has the first input/output terminal  11  and the second input/output terminal  12  as feed points. 
     A mode ( 1 ) of the diversity mode includes a branch ( 1 ) and a branch ( 2 ), and a mode ( 2 ) of the diversity mode includes a branch ( 3 ) and a branch ( 4 ). 
     Although here an example in which the wireless communication device uses the antenna device  4  as a transmission antenna is explained, it is obvious that even if the wireless communication device uses the antenna device  4  as a reception antenna, the same advantageous effects can be obtained by the reversibility of the antenna device  4 . 
     The transmitter  1  outputs a transmission signal to the transmission/reception switching switch  3 . 
     When the transmission/reception switching switch  3  receives the transmission signal outputted from the transmitter  1 , if, for example, the diversity mode of the antenna device  4  is set to the mode ( 1 ) and the branch is set to the branch ( 1 ), then the transmission/reception switching switch  3  outputs the transmission signal to the first input/output terminal  11 . 
     When the diversity mode of the antenna device  4  is set to the mode ( 1 ) and the branch is set to the branch ( 2 ), the transmission/reception switching switch  3  outputs the transmission signal to the second input/output terminal  12 . 
     When the diversity mode of the antenna device  4  is set to the mode ( 2 ) and the branch is set to the branch ( 3 ), the transmission/reception switching switch  3  outputs the transmission signal to the first input/output terminal  11 . 
     When the diversity mode of the antenna device  4  is set to the mode ( 2 ) and the branch is set to the branch ( 4 ), the transmission/reception switching switch  3  outputs the transmission signal to the second input/output terminal  12 . 
     Each of the diversity mode and branch of the antenna device  4  is, for example, set by a control device which is not shown or set by a manual operation by the user. 
     For example, by the control device setting the branch to the branch ( 1 ) or the branch ( 3 ), a transmission signal outputted to the first input/output terminal  11  from the transmission/reception switching switch  3  reaches the second phase shifter  25  through the first matching circuit  27 . 
     As shown in  FIG. 4 , when the branch is the branch ( 1 ), the diversity mode is the mode ( 1 ), and thus, the amount of phase shift of the second phase shifter  25  is set to θ/2 degrees. 
     As shown in  FIG. 4 , when the branch is the branch ( 3 ), the diversity mode is the mode ( 2 ), and thus, the amount of phase shift of the second phase shifter  25  is set to 0 degrees. 
     Therefore, when the branch is the branch ( 1 ), the second phase shifter  25  shifts the phase of the transmission signal by θ/2 degrees, and outputs the transmission signal shifted in phase by θ/2 degrees to the first terminal  23   a.    
     When the branch is the branch ( 3 ), the second phase shifter  25  shifts the phase of the transmission signal by 0 degrees, and outputs the transmission signal shifted in phase by 0 degrees to the first terminal  23   a.    
     For example, by the control device setting the branch to the branch ( 2 ) or the branch ( 4 ), a transmission signal outputted to the second input/output terminal  12  from the transmission/reception switching switch  3  reaches the third phase shifter  26  through the second matching circuit  28 . 
     As shown in  FIG. 4 , when the branch is the branch ( 2 ), the diversity mode is the mode ( 1 ), and thus, the amount of phase shift of the third phase shifter  26  is set to θ/2 degrees. 
     As shown in  FIG. 4 , when the branch is the branch ( 4 ), the diversity mode is the mode ( 2 ), and thus, the amount of phase shift of the third phase shifter  26  is set to 0 degrees. 
     Therefore, when the branch is the branch ( 2 ), the third phase shifter  26  shifts the phase of the transmission signal by θ/2 degrees, and outputs the transmission signal shifted in phase by θ/2 degrees to the second terminal  23   b.    
     When the branch is the branch ( 4 ), the third phase shifter  26  shifts the phase of the transmission signal by 0 degrees, and outputs the transmission signal shifted in phase by 0 degrees to the second terminal  23   b.    
     When the transmission signal is outputted to the first terminal  23   a  from the second phase shifter  25  in a case in which the branch is the branch ( 1 ) or the branch ( 3 ), the directional coupler  23  accepts, as input, the transmission signal from the first terminal  23   a  and divides the power of the transmission signal into two parts, and thereby splits the transmission signal into two transmission signals. 
     At this time, the directional coupler  23  splits the transmission signal into two transmission signals in such a manner that the phase difference of a transmission signal outputted to the fourth terminal  23   d  with respect to a transmission signal outputted to the third terminal  23   c  is ϕ degrees. 
     The directional coupler  23  outputs one split transmission signal to the third terminal  23   c  and outputs the other split transmission signal to the fourth terminal  23   d.    
     When the transmission signal is outputted to the second terminal  23   b  from the third phase shifter  26  in a case in which the branch is the branch ( 2 ) or the branch ( 4 ), the directional coupler  23  accepts, as input, the transmission signal from the second terminal  23   b  and divides the power of the transmission signal into two parts, and thereby splits the transmission signal into two transmission signals. 
     At this time, the directional coupler  23  splits the transmission signal into two transmission signals in such a manner that the phase difference of a transmission signal outputted to the third terminal  23   c  with respect to a transmission signal outputted to the fourth terminal  23   d  is (π−ϕ) degrees. 
     The directional coupler  23  outputs one split transmission signal to the third terminal  23   c  and outputs the other split transmission signal to the fourth terminal  23   d.    
     The transmission signal outputted from the third terminal  23   c  reaches the first radiating element  21 . 
     The transmission signal outputted from the fourth terminal  23   d  reaches the first phase shifter  24 . 
     As shown in  FIG. 4 , when the diversity mode is the mode ( 1 ), the amount of phase shift of the first phase shifter  24  is set to 0 degrees, and when the diversity mode is the mode ( 2 ), the amount of phase shift of the first phase shifter  24  is set to 0 degrees. 
     Therefore, when the diversity mode is the mode ( 1 ), the first phase shifter  24  shifts the phase of the transmission signal outputted from the fourth terminal  23   d  by 0 degrees, and outputs the transmission signal shifted in phase by 0 degrees to the second radiating element  22 . 
     When the diversity mode is the mode ( 2 ), the first phase shifter  24  shifts the phase of the transmission signal outputted from the fourth terminal  23   d  by 0 degrees, and outputs the transmission signal shifted in phase by 0 degrees to the second radiating element  22 . 
     The first radiating element  21  radiates the transmission signal outputted from the third terminal  23   c  into space. 
     The second radiating element  22  radiates the transmission signal outputted from the first phase shifter  24  into space. 
     When the branch is the branch ( 1 ), if the phase of the transmission signal inputted from the first input/output terminal  11  is 0 degrees, then the excitation phase of the first radiating element  21  is θ/2 degrees, and the excitation phase of the second radiating element  22  is (θ/2+ϕ) degrees. Here, for simplification of description, the phase rotation of the transmission signal when passing through the first matching circuit  27  and the phase rotation of the transmission signal when passing from the first terminal  23   a  to the third terminal  23   c  are ignored. 
     Therefore, the difference of the excitation phase of the second radiating element  22  with respect to the excitation phase of the first radiating element  21  is degrees. 
     When the branch is the branch ( 2 ), if the phase of the transmission signal inputted from the second input/output terminal  12  is 0 degrees, then the excitation phase of the first radiating element  21  is θ/2+(π−ϕ)) degrees, and the excitation phase of the second radiating element  22  is θ/2 degrees. Here, for simplification of description, the phase rotation of the transmission signal when passing through the second matching circuit  28  and the phase rotation of the transmission signal when passing from the second terminal  23   b  to the fourth terminal  23   d  are ignored. 
     Therefore, the difference of the excitation phase of the second radiating element  22  with respect to the excitation phase of the first radiating element  21  is −(π−ϕ) degrees. 
     When the branch is the branch ( 3 ), if the phase of the transmission signal inputted from the first input/output terminal  11  is 0 degrees, then the excitation phase of the first radiating element  21  is 0 degrees, and the excitation phase of the second radiating element  22  is (ϕ+θ) degrees. 
     Therefore, the difference of the excitation phase of the second radiating element  22  with respect to the excitation phase of the first radiating element  21  is (ϕ+θ) degrees. 
     When the branch is the branch ( 4 ), if the phase of the transmission signal inputted from the second input/output terminal  12  is 0 degrees, then the excitation phase of the first radiating element  21  is (π−ϕ) degrees, and the excitation phase of the second radiating element  22  is 0 degrees. 
     Therefore, the difference of the excitation phase of the second radiating element  22  with respect to the excitation phase of the first radiating element  21  is (−(π−ϕ)+θ) degrees. 
     Thus, the antenna device  4  can form four different radiation patterns by switching the respective amounts of phase shift of the first phase shifter  24 , the second phase shifter  25 , and the third phase shifter  26  as shown in  FIG. 4 . 
     Here, if the distance between the first radiating element  21  and the second radiating element  22  is short, mutual coupling between the first radiating element  21  and the second radiating element  22  is increased. 
     If signal reflection at the first radiating element  21  is 0 and signal reflection at the second radiating element  22  is 0, as a coupling from the first input/output terminal  11  to the second input/output terminal  12 , there is a possible coupling between a transmission signal passing through a path R 1  and a transmission signal passing through a path R 2 , as shown in  FIG. 5   
       FIG. 5  is an illustrative diagram showing a coupling from the first input/output terminal  11  to the second input/output terminal  12 . 
     The path R 1  is a path through which a transmission signal inputted from the first input/output terminal  11  passes through the first matching circuit  27 , the second phase shifter  25 , the directional coupler  23 , the first radiating element  21 , the second radiating element  22 , the first phase shifter  24 , the directional coupler  23 , the third phase shifter  26 , and the second matching circuit  28  and reaches the second input/output terminal  12 . 
     The path R 2  is a path through which a transmission signal inputted from the first input/output terminal  11  passes through the first matching circuit  27 , the second phase shifter  25 , the directional coupler  23 , the first phase shifter  24 , the second radiating element  22 , the first radiating element  21 , the directional coupler  23 , the third phase shifter  26 , and the second matching circuit  28  and reaches the second input/output terminal  12 . 
     In the branch ( 1 ), the amount of phase shift of the first phase shifter  24  is 0 degrees and the amount of phase shift of the second phase shifter  25  is θ/2 degrees. 
     Therefore, when the phase of the transmission signal inputted from the first input/output terminal  11  is 0 degrees, the phase of the transmission signal passing through the path R 1  is θ/2 degrees at the second terminal  23   b  of the directional coupler  23 . 
     In addition, the phase of the transmission signal passing through the path R 2  is θ/2+ϕ+(π−ϕ)=(θ/2+π) degrees at the second terminal  23   b.    
     At the second terminal  23   b , the phase difference between the phase of the transmission signal passing through the path R 1  and the phase of the transmission signal passing through the path R 2  is π. 
     Thus, the transmission signal passing through the path R 1  and the transmission signal passing through the path R 2  have an equal amplitude and an opposite phase and cancel each other out at the second terminal  23   b , and thus, the coupling from the first input/output terminal  11  to the second input/output terminal  12  is reduced. 
     For a coupling from the second input/output terminal  12  to the first input/output terminal  11  when the branch is the branch ( 2 ), though not shown, as with the branch ( 1 ), there are two paths of transmission signals. Here, the two paths are a path R 3  and a path R 4 . 
     The path R 3  is a path through which a transmission signal inputted from the second input/output terminal  12  passes through the second matching circuit  28 , the third phase shifter  26 , the directional coupler  23 , the first phase shifter  24 , the second radiating element  22 , the first radiating element  21 , the directional coupler  23 , the second phase shifter  25 , and the first matching circuit  27  and reaches the first input/output terminal  11 . 
     The path R 4  is a path through which a transmission signal inputted from the second input/output terminal  12  passes through the second matching circuit  28 , the third phase shifter  26 , the directional coupler  23 , the first radiating element  21 , the second radiating element  22 , the first phase shifter  24 , the directional coupler  23 , the second phase shifter  25 , and the first matching circuit  27  and reaches the first input/output terminal  11 . 
     In the branch ( 2 ), the amount of phase shift of the first phase shifter  24  is 0 degrees and the amount of phase shift of the third phase shifter  26  is θ/2 degrees. 
     Therefore, when the phase of the transmission signal inputted from the second input/output terminal  12  is 0 degrees, the phase of the transmission signal passing through the path R 3  is θ/2 degrees at the first terminal  23   a  of the directional coupler  23 . 
     In addition, the phase of the transmission signal passing through the path R 4  is θ/2+(π−ϕ)+=(θ/2+π) degrees at the first terminal  23   a.    
     At the first terminal  23   a , the phase difference between the phase of the transmission signal passing through the path R 3  and the phase of the transmission signal passing through the path R 4  is π. 
     Thus, the transmission signal passing through the path R 3  and the transmission signal passing through the path R 4  have an equal amplitude and an opposite phase and cancel each other out at the first terminal  23   a , and thus, the coupling from the second input/output terminal  12  to the first input/output terminal  11  is reduced. 
     In the antenna device  4 , since the first matching circuit  27  and the second matching circuit  28  are mounted, signal reflection at the first input/output terminal  11  and the second input/output terminal  12  can be suppressed. 
     Suppose an antenna device without the first matching circuit  27  and the second matching circuit  28  in the antenna device  4  shown in  FIG. 2 . 
     In such a supposed antenna device, it is supposed that signal reflection at the first radiating element  21  is 0 and signal reflection at the second radiating element  22  is 0. 
     In the supposed antenna device, when the branch is the branch ( 1 ) or the branch ( 3 ), as shown in  FIG. 6 , a reflection of a transmission signal passing through a path R 5  and a reflection of a transmission signal passing through a path R 6  occur at the first input/output terminal  11 . 
       FIG. 6  is an illustrative diagram showing reflections of transmission signals at the first input/output terminal  11 . 
     The path R 5  is a path through which a transmission signal inputted from the first input/output terminal  11  passes through the second phase shifter  25 , the directional coupler  23 , the first radiating element  21 , the second radiating element  22 , the first phase shifter  24 , the directional coupler  23 , and the second phase shifter  25  and reaches the first input/output terminal  11 . 
     The path R 6  is a path through which a transmission signal inputted from the first input/output terminal  11  passes through the second phase shifter  25 , the directional coupler  23 , the first phase shifter  24 , the second radiating element  22 , the first radiating element  21 , the directional coupler  23 , and the second phase shifter  25  and reaches the first input/output terminal  11 . 
     The antenna device  4  shown in  FIG. 2  has the first matching circuit  27  and the second matching circuit  28  mounted thereon. 
     The first matching circuit  27  matches the impedance seen from the first input/output terminal  11  toward the second phase shifter  25  to the impedance seen from the first input/output terminal  11  toward the transmission/reception switching switch  3 . 
     Therefore, in the antenna device  4  shown in  FIG. 2 , when the branch is the branch ( 1 ) or the branch ( 3 ), reflection of the transmission signal passing through the path R 5  and reflection of the transmission signal passing through the path R 6  are suppressed by the operation of the first matching circuit  27 . 
     The second matching circuit  28  matches the impedance seen from the second input/output terminal  12  toward the third phase shifter  26  to the impedance seen from the second input/output terminal  12  toward the transmission/reception switching switch  3 . 
     Therefore, in the antenna device  4  shown in  FIG. 2 , when the branch is the branch ( 2 ) or the branch ( 4 ), signal reflection at the second input/output terminal  12  is suppressed by the operation of the second matching circuit  28 . 
     Suppose an antenna device without the second phase shifter  25  and the third phase shifter  26  in the antenna device  4  shown in  FIG. 2 . 
     In such a supposed antenna device, a reflection phase obtained in the mode ( 1 ) is smaller by θ than a reflection phase obtained in the mode ( 2 ). In the assumed antenna device, even without the second phase shifter  25  and the third phase shifter  26 , a reflection amplitude obtained in the mode ( 1 ) and a reflection amplitude in the mode ( 2 ) are identical. 
     The antenna device  4  shown in  FIG. 2  includes the second phase shifter  25  and the third phase shifter  26  to make the reflection phase in the mode ( 1 ) and the reflection phase in the mode ( 2 ) the same. 
     The amount of phase shift of each of the second phase shifter  25  and the third phase shifter  26  varies between the mode ( 1 ) and the mode ( 2 ). 
     The amount of phase shift in the mode ( 1 ) is θ/2 and the amount of phase shift in the mode ( 2 ) is 0. 
     In the antenna device  4  shown in  FIG. 2 , since the reflection phase in the mode ( 1 ) and the reflection phase in the mode ( 2 ) are identical, each of the first matching circuit  27  and the second matching circuit  28  can be used in either of the mode ( 1 ) and the mode ( 2 ). 
     Here, the effectiveness of the antenna device  4  shown in  FIG. 2  is considered using a two-element antenna array shown in  FIG. 7  as an example. 
     In general, it is known that when the distance between two radiating elements is equal to or less than one-half of the wavelength of a transmission signal, mutual coupling between two input/output terminals increases and an antenna device does not effectively operate. Here, it will be explained that the antenna device  4  shown in  FIG. 2  effectively operates even when the distance between the first radiating element  21  and the second radiating element  22  is equal to or less than one-half of the wavelength of a transmission signal. 
     The two-element antenna array shown in  FIG. 7  has two inverted-F antennas  41  and  42  placed on a square ground plate  40 . 
     In  FIG. 7 , λc is the free space wavelength of a transmission signal at the frequency (operating frequency) fc. 
       FIG. 8  is an illustrative diagram showing simulation results of S-parameters of the two-element antenna array shown in  FIG. 7 . S-parameter simulation is performed by, for example, a computer. 
       FIG. 8A  is a Smith chart showing S-parameters and  FIG. 8B  is an illustrative diagram showing frequency characteristics of amplitude. In  FIG. 8B , frequency is normalized by the operating frequency fc. 
     In an example of  FIG. 7 , the distance between the inverted-F antenna  41  and the inverted-F antenna  42  is 0.15 λc and is shorter than 0.5 λc. 
     It can be seen from  FIG. 8B  that coupling |S&lt;| between the inverted-F antenna  41  and the inverted-F antenna  42  is about −3 dB at the operating frequency fc and is very high. 
     Next, a case in which the two-element antenna array shown in  FIG. 7  is applied to an antenna device is considered. 
     First, an antenna device without the second phase shifter  25 , the third phase shifter  26 , the first matching circuit  27 , and the second matching circuit  28  in the antenna device  4  shown in  FIG. 2  is considered. 
     The antenna device considered uses the inverted-F antenna  41  as the first radiating element  21  and uses the inverted-F antenna  42  as the second radiating element  22 . 
       FIG. 9  is an illustrative diagram showing results of S-parameter simulation obtained when an inverted-F antenna  41 ,  42  is viewed from each of the first input/output terminal  11  and the second input/output terminal  12 . In the S-parameter simulation, θ=90° and ϕ=−90°. 
       FIG. 9A  is a Smith chart showing S-parameters for a case of the mode ( 1 ) and  FIG. 9B  is a Smith chart showing S-parameters for a case of the mode ( 2 ). 
     As shown in  FIGS. 9A and 9B , it can be seen that in either of the mode ( 1 ) and the mode ( 2 ) the coupling |S&lt;| between the inverted-F antenna  41  and the inverted-F antenna  42  is located at the center of the Smith chart and that the coupling is sufficiently low. 
     At the operating frequency fc, the distance from the center of the Smith chart is the same for both S 11  in the mode ( 1 ) and S 11  in the mode ( 2 ), but their locations are different. Likewise, at the operating frequency fc, the distance of S 22  from the center of the Smith chart in the mode ( 1 ) is the same as the distance of S 22  from the center of the Smith chart in the mode ( 2 ), but their locations are different. This indicates that the amplitude is the same for both the mode ( 1 ) and the mode ( 2 ), but the phase is different between the mode ( 1 ) and the mode ( 2 ). That is, it indicates that a matching circuit required in the mode ( 1 ) differs from a matching circuit required in the mode ( 2 ), and there is a need to mount different matching circuits for the mode ( 1 ) and the mode ( 2 ). 
     Therefore, the antenna device considered requires a first matching circuit  27  for the mode ( 1 ) and a second matching circuit  28  for the mode ( 1 ), and a first matching circuit  27  for the mode ( 2 ) and a second matching circuit  28  for the mode ( 2 ). 
     Next, an antenna device which is a version of the antenna device  4  shown in  FIG. 2  on which the second phase shifter  25  and the third phase shifter  26  are mounted, but the first matching circuit  27  and the second matching circuit  28  are not mounted is considered. 
     The antenna device considered uses the inverted-F antenna  41  as the first radiating element  21  and uses the inverted-F antenna  42  as the second radiating element  22 . 
       FIG. 10  is an illustrative diagram showing results of S-parameter simulation obtained when an inverted-F antenna  41 ,  42  is viewed from each of the first input/output terminal  11  and the second input/output terminal  12 . In the S-parameter simulation, θ=90° and ϕ=−90°. 
       FIG. 10A  is a Smith chart showing S-parameters for a case of the mode ( 1 ) and  FIG. 10B  is a Smith chart showing S-parameters for a case of the mode ( 2 ). 
     As shown in  FIGS. 10A and 10B , it can be seen that in either of the mode ( 1 ) and the mode ( 2 ) the coupling |S 21 | between the inverted-F antenna  41  and the inverted-F antenna  42  is located at the center of the Smith chart and that the coupling is sufficiently low. 
     At the operating frequency fc, by mounting the second phase shifter  25  and the third phase shifter  26 , the phase in the mode ( 1 ) rotates 90° and the location of S 11  in the mode ( 1 ) coincides with the location of S 11  in the mode ( 2 ). In addition, the location of S 22  in the mode ( 1 ) coincides with the location of S 22  in the mode ( 2 ). This indicates that a matching circuit required in the mode ( 1 ) and a matching circuit required in the mode ( 2 ) can be used in a sharing manner. 
     Next, the antenna device  4  shown in  FIG. 2  having the second phase shifter  25 , the third phase shifter  26 , the first matching circuit  27 , and the second matching circuit  28  mounted thereon is considered. 
     The antenna device  4  shown in  FIG. 2  uses the inverted-F antenna  41  as the first radiating element  21  and uses the inverted-F antenna  42  as the second radiating element  22 . 
     In the antenna device  4  shown in  FIG. 2 , the first matching circuit  27  using three lumped elements is shown. However, this is merely an example and a first matching circuit  27  using two lumped elements may be used. 
     For the two lumped elements, for example, a jumper element connected in series between the other end of the second phase shifter  25  and the first input/output terminal  11 , and a parallel capacitor connected at its one end to one end or the other end of the jumper element and grounded at its other end may be used. 
     In addition, in the antenna device  4  shown in  FIG. 2 , the second matching circuit  28  using three lumped elements is shown. However, this is merely an example and a second matching circuit  28  using two lumped elements may be used. 
     For the two lumped elements, for example, a jumper element connected in series between the other end of the third phase shifter  26  and the second input/output terminal  12 , and a parallel capacitor connected at its one end to one end or the other end of the jumper element and grounded at its other end may be used. 
       FIG. 11  is an illustrative diagram showing results of S-parameter simulation obtained when an inverted-F antenna  41 ,  42  is viewed from each of the first input/output terminal  11  and the second input/output terminal  12 . In the S-parameter simulation, θ=90° and ϕ=−90°. 
       FIG. 11A  is a Smith chart showing S-parameters for a case of the mode ( 1 ) and  FIG. 11B  is a Smith chart showing S-parameters for a case of the mode ( 2 ). 
     As shown in  FIGS. 11A and 11B , it can be seen that in both the mode ( 1 ) and the mode ( 2 ) the coupling |S 21 | between the inverted-F antenna  41  and the inverted-F antenna  42  is located at the center of the Smith chart and that the coupling is sufficiently low. 
     At the operating frequency fc, the location of S 11  in the mode ( 1 ) coincides with the location of S 11  in the mode ( 2 ). In addition, it can be seen that the location of S 11  in the mode ( 1 ) and the location of S 11  in the mode ( 2 ) are located at substantially the center of the Smith chart and reflection is sufficiently low. At the operating frequency fc, the location of S 22  in the mode ( 1 ) coincides with the location of S 22  in the mode ( 2 ). In addition, it can be seen that the location of S 22  in the mode ( 1 ) and the location of S 22  in the mode ( 2 ) are located at substantially the center of the Smith chart and reflection is sufficiently low. 
     The first matching circuit  27  of the antenna device  4  shown in  FIG. 2  is appropriate to both the mode ( 1 ) and the mode ( 2 ), and the second matching circuit  28  is appropriate to both the mode ( 1 ) and the mode ( 2 ). 
       FIGS. 12 to 15  are illustrative diagrams showing simulation results of radiation patterns of the antenna device  4  shown in  FIG. 2  in a z-x-plane shown in  FIG. 7  in the modes ( 1 ) and ( 2 ), and simulation results of radiation patterns of the antenna device  4  shown in  FIG. 2  in a z-y-plane shown in  FIG. 7  in the modes ( 1 ) and ( 2 ). 
       FIG. 12  shows simulation results of radiation patterns obtained when the branch ( 1 ) in the mode ( 1 ) is used and the feed point is the first input/output terminal  11 . 
       FIG. 13  shows simulation results of radiation patterns obtained when the branch ( 2 ) in the mode ( 1 ) is used and the feed point is the second input/output terminal  12 . 
       FIG. 14  shows simulation results of radiation patterns obtained when the branch ( 3 ) in the mode ( 2 ) is used and the feed point is the first input/output terminal  11 . 
       FIG. 15  shows simulation results of radiation patterns obtained when the branch ( 4 ) in the mode ( 2 ) is used and the feed point is the second input/output terminal  12 . 
     Comparing the simulation results shown in  FIGS. 12 to 15 , it can be seen that the radiation pattern varies between the branches ( 1 ) to ( 4 ). 
       FIG. 16  is an illustrative diagram showing simulation results for correlation coefficients between the branches ( 1 ) to ( 4 ). 
     The correlation between the first radiating element  21  and the second radiating element  22  is computed from a radiation pattern of the first radiating element  21  and a radiation pattern of the second radiating element  22 . 
       FIG. 16  shows that the correlation coefficient between the branch ( 1 ) and the branch ( 2 ) is 0.0, the correlation coefficient between the branch ( 1 ) and the branch ( 3 ) is 0.5, and the correlation coefficient between the branch ( 1 ) and the branch ( 4 ) is 0.5. 
     In addition,  FIG. 16  shows that the correlation coefficient between the branch ( 2 ) and the branch ( 3 ) is 0.5, and the correlation coefficient between the branch ( 2 ) and the branch ( 4 ) is 0.5. 
     Furthermore,  FIG. 16  shows that the correlation coefficient between the branch ( 3 ) and the branch ( 4 ) is 0.0. 
     When the radiation pattern of the first radiating element  21  and the radiation pattern of the second radiating element  22  are similar to each other, the correlation increases, and when they are not similar to each other, the correlation decreases. 
     It is known that when the correlation coefficient between the first radiating element  21  and the second radiating element  22  is equal to or less than 0.5, the antenna device can obtain substantially equivalent diversity performance to that obtained when the correlation coefficient is 0. 
     It can be seen from  FIG. 16  that in the antenna device  4  shown in  FIG. 2 , the correlation coefficients between the branches ( 1 ) to ( 4 ) are equal to or less than 0.5. 
     In the above-described Embodiment 1, the antenna device is constructed to include the first phase shifter  24  connected at its one end to the fourth terminal  23   d  of the directional coupler  23 ; the second phase shifter  25  connected at its one end to the first terminal  23   a  of the directional coupler  23 ; the third phase shifter  26  connected at its one end to the second terminal  23   b  of the directional coupler  23 ; the first matching circuit  27  connected at its one end to the other end of the second phase shifter  25  and connected at its other end to the first input/output terminal  11 ; and the second matching circuit  28  connected at its one end to the other end of the third phase shifter  26  and connected at its other end to the second input/output terminal  12 . Therefore, the antenna device according to Embodiment 1 can suppress signal reflection in a case in which the distance between the first radiating element  21  and the second radiating element  22  is short. 
     In Embodiment 1, the effectiveness of the antenna device is considered assuming that each of the first radiating element  21  and the second radiating element  22  is an inverted-F antenna. 
     However, each of the first radiating element  21  and the second radiating element  22  is not limited to an inverted-F antenna and may be a radiating element with large reflection. 
     When, for example, a radiating element with large reflection is used as each of the first radiating element  21  and the second radiating element  22 , the antenna device includes, as shown in  FIG. 17 , a third matching circuit  51  and a fourth matching circuit  52 . 
       FIG. 17  is a configuration diagram showing another antenna device  4  according to Embodiment 1. 
     In  FIG. 17 , the same reference signs as those in  FIG. 2  indicate the same or corresponding portions and thus description thereof is omitted. 
     The third matching circuit  51  is connected at its one end to the third terminal  23   c  and connected at its other end to the first radiating element  21 . 
     The third matching circuit  51  is a circuit for matching the impedance seen from the third terminal  23   c  toward a first radiating element  21  to the impedance seen from the third terminal  23   c  toward a directional coupler  23 . 
     The fourth matching circuit  52  is connected at its one end to the other end of the first phase shifter  24  and connected at its other end to the second radiating element  22 . 
     The fourth matching circuit  52  is a circuit for matching the impedance seen from the other end of the first phase shifter  24  toward a second radiating element  22  to the impedance seen from the other end of the first phase shifter  24  toward a first phase shifter  24 . 
     As with the first matching circuit  27  shown in  FIG. 2 , each of the third matching circuit  51  and the fourth matching circuit  52  may be a Π-circuit including three or less lumped elements, or may be a T-circuit including three or less lumped elements. 
     The antenna device  4  shown in  FIG. 2  is described as one used as a diversity antenna. The antenna device  4  shown in  FIG. 2  has low correlation between the first radiating element  21  and the second radiating element  22 , and thus, can also be used as a Multiple Input Multiple Output (MIMO) antenna. 
     Embodiment 2 
     In the antenna device  4  according to Embodiment 1, an example in which the directional coupler  23  is a branch-line directional coupler is shown. 
     In Embodiment 2, an antenna device  4  is explained that includes a directional coupler  60  being a 90°-hybrid circuit including a plurality of lumped elements. 
       FIG. 18  is a configuration diagram showing the antenna device  4  according to Embodiment 2. 
     In  FIG. 18 , the same reference signs as those in  FIG. 2  indicate the same or corresponding portions and thus description thereof is omitted. 
     The directional coupler  60  is a circuit having the same function as the directional coupler  23  shown in  FIG. 2 . 
     The directional coupler  60  is a 90°-hybrid circuit including first to twelfth lumped elements. 
     A first lumped element  61  is connected at its one end to the first terminal  23   a  and connected at its other end to the second terminal  23   b.    
     A second lumped element  62  is connected at its one end to the one end of the first lumped element  61  and grounded at its other end. 
     A third lumped element  63  is connected at its one end to the other end of the first lumped element  61  and grounded at its other end. 
     The first lumped element  61 , the second lumped element  62 , and the third lumped element  63  form a first Π-circuit. 
     A fourth lumped element  64  is connected at its one end to the first terminal  23   a  and connected at its other end to the third terminal  23   c.    
     A fifth lumped element  65  is connected at its one end to the one end of the fourth lumped element  64  and grounded at its other end. 
     A sixth lumped element  66  is connected at its one end to the other end of the fourth lumped element  64  and grounded at its other end. 
     The fourth lumped element  64 , the fifth lumped element  65 , and the sixth lumped element  66  form a second Π-circuit. 
     A seventh lumped element  67  is connected at its one end to the third terminal  23   c  and connected at its other end to the fourth terminal  23   d.    
     An eighth lumped element  68  is connected at its one end to the one end of the seventh lumped element  67  and grounded at its other end. 
     A ninth lumped element  69  is connected at its one end to the other end of the seventh lumped element  67  and grounded at its other end. 
     The seventh lumped element  67 , the eighth lumped element  68 , and the ninth lumped element  69  form a third Π-circuit. 
     A tenth lumped element  70  is connected at its one end to the second terminal  23   b  and connected at its other end to the fourth terminal  23   d.    
     An eleventh lumped element  71  is connected at its one end to the one end of the tenth lumped element  70  and grounded at its other end. 
     A twelfth lumped element  72  is connected at its one end to the other end of the tenth lumped element  70  and grounded at its other end. 
     The tenth lumped element  70 , the eleventh lumped element  71 , and the twelfth lumped element  72  form a fourth Π-circuit. 
     Components other than the directional coupler  60  are the same as those according to Embodiment 1, and thus, only the directional coupler  60  will be described here. 
     For example, suppose that a directional coupler is constructed as a branch-line 90°-hybrid circuit as shown in  FIG. 19 . 
       FIG. 19  is a configuration diagram showing the branch-line 90°-hybrid circuit. 
     The branch-line 90°-hybrid circuit is constructed of a ring-shaped transmission line arranged in a substantially square. 
     Each of four transmission lines included in the ring-shaped transmission line is about λg/4 in length. The parameter λg is the guide wavelength at the operating frequency fc. 
     Therefore, when the branch-line 90°-hybrid circuit is formed on a substrate, the length of one side of the 90°-hybrid circuit is shorter than the free space wavelength λc due to a wavelength reduction caused by a dielectric included in the substrate. 
     By replacing each of the four transmission lines with a Π-circuit including three lumped elements as shown in  FIG. 18 , further circuit miniaturization can be achieved. 
     Each of characteristic admittance Y 1  of the first Π-circuit, characteristic admittance Y 2  of the second Π-circuit, characteristic admittance Y 3  of the third H-circuit, and characteristic admittance Y 4  of the fourth Π-circuit is represented by the following equations (1) to (4): 
     
       
         
           
             
               
                 
                   
                     Y 
                     1 
                   
                   = 
                   
                     
                       1 
                       
                         
                           1 
                           - 
                           
                             k 
                             2 
                           
                         
                       
                     
                      
                     
                       
                         
                           G 
                           1 
                         
                          
                         
                           G 
                           2 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
             
               
                 
                   
                     Y 
                     2 
                   
                   = 
                   
                     
                       k 
                       
                         
                           1 
                           - 
                           
                             k 
                             2 
                           
                         
                       
                     
                      
                     
                       
                         
                           G 
                           2 
                         
                          
                         
                           G 
                           3 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
             
               
                 
                   
                     Y 
                     3 
                   
                   = 
                   
                     
                       1 
                       
                         
                           1 
                           - 
                           
                             k 
                             2 
                           
                         
                       
                     
                      
                     
                       
                         
                           G 
                           3 
                         
                          
                         
                           G 
                           4 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
             
               
                 
                   
                     Y 
                     4 
                   
                   = 
                   
                     
                       k 
                       
                         
                           1 
                           - 
                           
                             k 
                             2 
                           
                         
                       
                     
                      
                     
                       
                         
                           G 
                           4 
                         
                          
                         
                           G 
                           1 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     In equations (1) to (4), G 1  is the load conductance of the first terminal  23   a , G 2  is the load conductance of the second terminal  23   b , G 3  is the load conductance of the third terminal  23   c , and G 4  is the load conductance of the fourth terminal  23   d.    
     The parameter k is the degree of coupling of the directional coupler  60 . 
     Each of capacitance C 1  of the first Π-circuit, capacitance C 2  of the second Π-circuit, capacitance C 3  of the third Π-circuit, and capacitance C 4  of the fourth Π-circuit is represented by the following equation (5): 
     
       
         
           
             
               
                 
                   
                     C 
                     i 
                   
                   = 
                   
                     
                       1 
                       
                         ω 
                         c 
                       
                     
                      
                     
                       Y 
                       i 
                     
                      
                     
                         
                     
                      
                     
                       ( 
                       
                         
                           i 
                           = 
                           1 
                         
                         , 
                         2 
                         , 
                         3 
                         , 
                         4 
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     In equation (5), ω c  is angular frequency at the operating frequency fc. 
     Each of inductance L 1  of the first Π-circuit, inductance L 2  of the second Π-circuit, inductance L 3  of the third Π-circuit, and inductance L 4  of the fourth Π-circuit is represented by the following equation (6): 
     
       
         
           
             
               
                 
                   
                     L 
                     i 
                   
                   = 
                   
                     
                       1 
                       
                         ω 
                         c 
                       
                     
                      
                     
                       1 
                       
                         Y 
                         i 
                       
                     
                      
                     
                         
                     
                      
                     
                       ( 
                       
                         
                           i 
                           = 
                           1 
                         
                         , 
                         2 
                         , 
                         3 
                         , 
                         4 
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     Therefore, the directional coupler  60  shown in  FIG. 18  can be constructed by arranging capacitors and inductors of each of the first Π-circuit, the second Π-circuit, the third Π-circuit, and the fourth Π-circuit as shown in  FIG. 20 . 
       FIG. 20  is a configuration diagram showing the directional coupler  60  including capacitors and inductors. 
     Note, however, that each Π-circuit is not limited to one having two capacitors and an inductor arranged therein as shown in  FIG. 20 . 
     For example, although the directional coupler  60  shown in  FIG. 20  includes eight capacitors in total, the directional coupler  60  may include four capacitors in total by coupling two adjacent capacitors together. 
       FIG. 21  is a configuration diagram showing a directional coupler  60  including four capacitors in total. 
     The directional coupler  60  shown in  FIG. 21  includes a capacitor with capacitance C 12 , a capacitor with capacitance C 23 , a capacitor with capacitance C 34 , and a capacitor with capacitance C 41 . 
     The capacitor with capacitance C 12  is a capacitor obtained by coupling together a capacitor with capacitance C 1  shown in  FIG. 20  (a capacitor on the left side in the drawing) and a capacitor with capacitance C 2  shown in  FIG. 20  (a capacitor on the bottom side in the drawing). 
     The capacitor with capacitance C 23  is a capacitor obtained by coupling together a capacitor with capacitance C 2  shown in  FIG. 20  (a capacitor on the top side in the drawing) and a capacitor with capacitance C 3  shown in  FIG. 20  (a capacitor on the left side in the drawing). 
     The capacitor with capacitance C 34  is a capacitor obtained by coupling together a capacitor with capacitance C 3  shown in  FIG. 20  (a capacitor on the right side in the drawing) and a capacitor with capacitance C 4  shown in  FIG. 20  (a capacitor on the top side in the drawing). 
     The capacitor with capacitance C 41  is a capacitor obtained by coupling together a capacitor with capacitance C 4  shown in  FIG. 20  (a capacitor on the bottom side in the drawing) and a capacitor with capacitance C 1  shown in  FIG. 20  (a capacitor on the right side in the drawing). 
     Although here an example in which the directional coupler  60  includes four Π-circuits is shown, instead of each Π-circuit, a T-circuit including two series inductors and one parallel capacitor may be used. 
     Note that a free combination of the embodiments, modifications to any component in the embodiments, or omissions of any component in the embodiments are possible. 
     INDUSTRIAL APPLICABILITY 
     One or more embodiments in this disclosure are suitable for an antenna device including a first radiating element and a second radiating element. 
     Furthermore, one or more embodiments in this disclosure are suitable for a wireless communication device including the antenna device. 
     REFERENCE SIGNS LIST 
       1 : transmitter,  2 : receiver,  3 : transmission/reception switching switch,  4 : antenna device,  11 : first input/output terminal,  12 : second input/output terminal,  21 : first radiating element,  22 : second radiating element,  23 : directional coupler,  23   a : first terminal,  23   b : second terminal,  23   c : third terminal,  23   d : fourth terminal,  24 : first phase shifter,  25 : second phase shifter,  26 : third phase shifter,  27 : first matching circuit,  28 : second matching circuit,  31 ,  32 : switch,  33 : line,  34 : bypass line,  40 : ground plate,  41 ,  42 : inverted-F antenna,  51 : third matching circuit,  52 : fourth matching circuit,  60 : directional coupler,  61 : first lumped element,  62 : second lumped element,  63 : third lumped element,  64 : fourth lumped element,  65 : fifth lumped element,  66 : sixth lumped element,  67 : seventh lumped element,  68 : eighth lumped element,  69 : ninth lumped element,  70 : tenth lumped element,  71 : eleventh lumped element, and  72 : twelfth lumped element.