Patent Publication Number: US-8994606-B2

Title: Antenna and radio communication device

Description:
TECHNICAL FIELD 
     The present invention relates to an antenna and a radio communication device that are used for radio communication. 
     BACKGROUND ART 
     In recent years, much attention is focused on WBAN (Wireless Body Area Network) for performing short range radio communication in a relatively small area for an application such as medical care and health care. WBAN is a network for a user to perform communication while carrying or wearing a radio communication device with a built-in sensor or IC (Integrated Circuit) for biometric monitoring. In this case, WBAN is used for the purpose of improving real time performance and efficiency by collecting and transmitting data such as biometric information. Here, the biometric information indicates information such as a user&#39;s body temperature, pulse, and/or blood pressure. 
       FIG. 32  is an illustration showing an example of the WBAN system configuration. 
     In the WBAN system shown in  FIG. 32 , a sensor node  501  and a master node  502  communicate in a network NW 10  in the vicinity of a human body. Each of the sensor node  501  and the master node  502  is a radio communication device. The sensor node  501  and the master node  502  are attached to respective locations of a human body (user). Each sensor node  501  acquires biometric information, and transmits the biometric information to the master node  502 . 
     The master node  502  receives the biometric information from each sensor node  501 . 
     The master node  502  communicates with an external device  500 . The master node  502  transmits the biometric information received from each master node  502 , to the external device  500 . 
     The external device  500  notifies a user of his/her state of health in real time based on the received biometric information. Also, the external device  500  notifies the biometric information to a medical institution such as a hospital, thereby serving the purpose of early detection of disease for the user. 
     The sensor nodes attached to respective locations of a human body (user) may directly communicate with the external device  500  without utilizing the master node  502 . 
     The system using a conventional short range radio communication includes RFID (Radio Frequency Identification) system. The RFID system includes an IC card system which performs data recording and reading using radio waves for ticket gate management, entrance/exit management, and the like, and a product distribution system using labels or product tags. That is to say, the RFID system is currently utilized in many fields. 
     Patent Literature 1 discloses an antenna constituting a plurality of linear conductors (hereinafter referred to as a conventional antenna) formed on a planar housing, as an antenna to be mounted on a radio communication device used in these RFID systems. 
     CITATION LIST 
     Patent Literature 
     
         
         [PTL 1] 
         Japanese Unexamined Patent Application Publication No. 2005-244283 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     However, the conventional antenna is formed on a plane. That is to say, the shape of the conventional antenna is planar. Accordingly, on a plane perpendicular to the antenna, there is a large variation in the directivity of the radio waves emitted from the conventional antenna. That is to say, in the conventional antenna, there exists a location (null point) on a plane where the electric field strength is significantly reduced, depending on the position of the plane in relation to the conventional antenna. 
     Here, the conventional antenna is assumed to be used in the WBAN system. In this case, as shown in (a) in  FIG. 33 , the attachment position of each radio communication device (the sensor node  501 , the master node  502 ) is different for each user. In addition, as shown in (b) in  FIG. 33 , the attachment orientation of each radio communication device (the sensor node  501 , the master node  502 ) may vary for each user. Also, as shown in (c) in  FIG. 33 , the orientation of the radio communication device (the sensor nodes  501 ) may vary due to the user&#39;s movement. 
     Therefore, the directivity of the antenna may vary three-dimensionally, and the communication may be temporarily disconnected depending on a user&#39;s posture or movement. This is because, on a plane in the three-dimensional space, there exists a large variation in the directivity of the radio waves emitted from the conventional antenna. That is to say, there exists a location (null point) on the plane where the electric field strength is significantly reduced in the conventional antenna, depending on the position of the plane in relation to the conventional antenna. 
     The present invention has been made to solve the above-described problem, and it is an object of the invention to provide an antenna that prevents an occurrence of a location on the orthogonal planes in the three-dimensional space, where the electric field strength is significantly reduced. 
     Solution to Problem 
     In order to solve the above-described problem, an antenna according to one aspect of the present invention is used for radio communication. The antenna includes a planar conductor which is grounded; and a three-dimensional linear conductor in which at least a first linear conductor, a second linear conductor, and a third linear conductor are integrally formed, wherein the first linear conductor is provided on a major surface side of the planar conductor and perpendicularly to the major surface, the second linear conductor is provided on the major surface side and parallel to the major surface, the third linear conductor is provided on the major surface side, parallel to the major surface, and perpendicularly to the second linear conductor, one end of the second linear conductor and one end of the third linear conductor are electrically connected to each other, the planar conductor is provided with a power feed point, to which a high frequency current used for the radio communication is externally supplied, the power feed point being electrically disconnected to the planar conductor, the power feed point is electrically connected to one end of the first linear conductor of the three-dimensional linear conductor, the three-dimensional linear conductor has a flow of the high frequency current therethrough, a current flows through the planar conductor due to the flow of the high frequency current through the three-dimensional linear conductor, and a relationship of Mx=My=Mz is satisfied, where Mx denotes an electromagnetic moment Ix×Lx, My denotes an electromagnetic moment Iy×Ly, and Mz denotes an electromagnetic moment Iz 1 ×Lz 1 −Iz 2 ×Lz 2 , Ix denotes a current flowing along an x-axis out of the high frequency current flowing through the three-dimensional linear conductor where Ix is represented by a positive value when the current flows in +x direction, Iy denotes a current flowing along a y-axis out of the high frequency current flowing through the three-dimensional linear conductor where Iy is represented by a positive value when the current flows in +y direction, Iz 1  denotes a current flowing along a z-axis out of the current flowing through the planar conductor where Iz 1  is represented by a positive value when the current flows in +z direction, Iz 2  denotes a current flowing along the z-axis out of the high frequency current flowing through the three-dimensional linear conductor where Iz 2  is represented by a positive value when the current flows in +z direction, Lx denotes a length of the three-dimensional linear conductor in the x-axis direction, Ly denotes a length of the three-dimensional linear conductor in the y-axis direction, Lz 1  denotes a length of the planar conductor in the z-axis direction, Lz 2  denotes a length of the three-dimensional linear conductor in the z-axis direction, and in a three-dimensional coordinate system in which the x-axis, the y-axis and the z-axis are perpendicular to each other, the major surface of the planar conductor is parallel to the z-y plane of the three-dimensional coordinate system, the +x direction denotes one of two directions along the x-axis, −x direction denotes the other of the two directions along the x-axis, the +y direction denotes one of two directions along the y-axis, −y direction denotes the other of the two directions along the y-axis, the +z direction denotes one of two directions along the z-axis, −z direction denotes the other of the two directions along the z-axis. 
     That is to say, the antenna includes a planar conductor and a three-dimensional linear conductor in which at least a first linear conductor, a second linear conductor, and a third linear conductor are integrally formed. The first linear conductor is provided perpendicularly to the major surface of the planar conductor. The second linear conductor is parallel to the major surface. The third linear conductor is provided parallel to the major surface, and perpendicularly to the second linear conductor. 
     Also, the antenna is configured in such a manner that all the electromagnetic moments Mx, My, and Mz are equal where Mx denotes Ix×Lx, My denotes Iy×Ly, and Mz denotes Iz 1 ×Lz 1 −Iz 2 ×Lz 2 . 
     By the simulation and the measurement of a prototype antenna, the inventors have verified that an antenna, which is configured in such a manner that all the electromagnetic moments Mx, My, and Mz are equal, prevents an occurrence of a location on the orthogonal planes in the three-dimensional space, at which the electric field strength is significantly reduced where Mx denotes Ix×Lx, My denotes Iy×Ly, and Mz denotes Iz 1 ×Lz 1 −Iz 2 ×Lz 2 . 
     Accordingly, the antenna prevents an occurrence of a location on the orthogonal planes in the three-dimensional space, at which the electric field strength is significantly reduced. 
     Preferably, the planar conductor has a quadrilateral shape, and the power feed point is provided in the vicinity of an edge of the planar conductor. 
     Preferably, the three-dimensional linear conductor includes the first linear conductor, the second linear conductor, the third linear conductor, and a fourth linear conductor that are integrally formed, the fourth linear conductor is provided on the major surface side, the fourth linear conductor is parallel to the first linear conductor, the fourth linear conductor has the same length as the first linear conductor, and the other end of the second linear conductor and the planar conductor are electrically connected to each other via the fourth linear conductor. 
     Preferably, the length of the planar conductor in the z-axis direction, and respective lengths of the first linear conductor, the second linear conductor, the third linear conductor, and the fourth linear conductor are ¼ or less of the wavelength for the frequency of the high frequency current. 
     Preferably, the three-dimensional linear conductor includes the first linear conductor, the second linear conductor, the third linear conductor, the fourth linear conductor, and a fifth linear conductor electrically connected to the third linear conductor that are integrally formed, and the fifth linear conductor is provided on the major surface side. 
     Preferably, the length of the second linear conductor is less than or equal to the length of the planar conductor in the y-axis direction, and the length of the third linear conductor is less than or equal to the length of the planar conductor in the z-axis direction. 
     Preferably, the three-dimensional linear conductor includes the first linear conductor, the second linear conductor, the third linear conductor, and a sixth linear conductor provided on the opposite side to the major surface of the planar conductor that are integrally formed, the sixth linear conductor is provided such that the sixth linear conductor and the first linear conductor lie on the same line, one end of the sixth linear conductor is electrically connected to the power feed point, and one end of the first linear conductor electrically connected to the power feed point, and one end of the sixth linear conductor electrically connected to the power feed point are electrically connected to each other. 
     Preferably, a loading coil is inserted in at least one of the first linear conductor, the second linear conductor, and the third linear conductor. 
     Preferably, at least one of the first linear conductor, the second linear conductor, and the third linear conductor is meander-shaped. 
     Preferably, at least one of the first linear conductor, the second linear conductor, and the third linear conductor is connected to a loading capacitor. 
     Preferably, the planar conductor is further provided with a slit. 
     Preferably, the input impedance of the antenna and the output impedance of the antenna are matched to each other by an external matching circuit. 
     A radio communication device according to another aspect of the present invention performs radio communication using the antenna. 
     Advantageous Effects of Invention 
     The present invention can achieve an antenna that prevents an occurrence of a location on the orthogonal planes in the three-dimensional space, at which the electric field strength is significantly reduced. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram showing the configuration of a radio communication device in Embodiment 1. 
         FIG. 2  is an illustration showing a three-dimensional coordinate system. 
         FIG. 3  is an illustration showing the configuration of an antenna in Embodiment 1. 
         FIG. 4  is an illustration showing the location where a planar conductor is formed. 
         FIG. 5  is an illustration for explaining a power feed region. 
         FIG. 6  is a graph showing the emission characteristic of the electric field emitted from the antenna, as indicated by simulation A. 
         FIG. 7  is a graph showing the emission characteristic of each electric field. 
         FIG. 8  is a graph showing the emission characteristic of the electric field emitted from the antenna, as indicated by the simulation A. 
         FIG. 9  is a graph showing the emission characteristic of each electric field. 
         FIG. 10  is a graph showing the emission characteristic of the electric field emitted from the antenna, as indicated by the simulation A. 
         FIG. 11  is a graph showing the emission characteristic of each electric field. 
         FIG. 12  is a graph showing the emission characteristic of the electric field emitted from the antenna, as indicated by simulation J. 
         FIG. 13  is a graph showing the emission characteristic of each electric field. 
         FIG. 14  is a graph showing the emission characteristic of the electric field emitted from the antenna, as indicated by the simulation J. 
         FIG. 15  is a graph showing the emission characteristic of each electric field. 
         FIG. 16  is a graph showing the emission characteristic of the electric field emitted from the antenna, as indicated by the simulation J. 
         FIG. 17  is a graph showing the emission characteristic of each electric field. 
         FIG. 18  is a graph showing the emission characteristic of each electric field. 
         FIG. 19  is an illustration showing the configuration of another antenna for comparison. 
         FIG. 20  is a graph showing the emission characteristic of each electric field. 
         FIG. 21  is an illustration showing the configuration of an antenna. 
         FIG. 22  is an illustration showing the configuration of another antenna. 
         FIG. 23  is an illustration showing the configuration of an antenna in Modification 1 of Embodiment 1. 
         FIG. 24  is an illustration showing the configuration of the antenna in Modification 1 of Embodiment 1. 
         FIG. 25  is an illustration showing the configuration of an antenna in Modification 2 of Embodiment 1. 
         FIG. 26  is an illustration showing the configuration of an antenna in Modification 3 of Embodiment 1. 
         FIG. 27  is an illustration showing the configuration of an antenna in Modification 4 of Embodiment 1. 
         FIG. 28  is an illustration showing the configuration of an antenna in Modification 5 of Embodiment 1. 
         FIG. 29  is an illustration showing the configuration of an antenna in Modification 6 of Embodiment 1. 
         FIG. 30  is an illustration showing the configuration of an antenna in Modification 7 of Embodiment 1. 
         FIG. 31  is a diagram showing a matching circuit included in a radio communication device. 
         FIG. 32  is an illustration showing an example of a WBAN system configuration. 
         FIG. 33  is an illustration showing an example of how the radio communication device in the WBAN system is used. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present invention are described with reference to the drawings. In the following description, the same components are labeled with the same reference symbols. The names and functions of those components are also the same. For this reason, detailed description of them is not given in some cases. 
     Embodiment 1 
       FIG. 1  is a block diagram showing the configuration of a radio communication device  1000  in Embodiment 1. 
     As shown in  FIG. 1 , the radio communication device  1000  includes a radio IC (Integrated Circuit)  20 , a power feed line L 10 , and an antenna  200 . 
     The radio IC  20  is electrically connected to the antenna  200  via the power feed line L 10 , and the detail is described later. The radio IC  20  supplies high frequency current (electric power) used for radio communication to the antenna  200  via the power feed line L 10 . 
     Here, the three-dimensional coordinate system in the present description is described. 
       FIG. 2  is an illustration showing the three-dimensional coordinate system. 
     As shown in  FIG. 2 , respective axes of the x-axis, the y-axis, and the z-axis are perpendicular to each other in the three-dimensional coordinate system. Here, +x direction denotes one of two directions along the x-axis, and −x direction denotes the other of the two directions along the x-axis. Also, +y direction denotes one of two directions along the y-axis, and −y direction denotes the other of the two directions along the y-axis. Also, +z direction denotes one of two directions along the z-axis, and −z direction denotes the other of the two directions along the z-axis. 
     Hereinafter, the plane that includes the x-axis and the y-axis is referred to as the x-y plane. Also, hereinafter, the plane that includes the z-axis and the x-axis is referred to as the z-x plane. Also, hereinafter, the plane that includes the z-axis and the y-axis is referred to as the z-y plane. 
       FIG. 3  is an illustration showing the configuration of the antenna  200  in Embodiment 1. 
     (A) in  FIG. 3  is a perspective view of the antenna  200 . (B) in  FIG. 3  is a view of the antenna  200  projected onto the z-y plane of the three-dimensional coordinate system. 
     The antenna  200  includes a planar conductor M 20  and a three-dimensional linear conductor  201 . 
     The shape of the planar conductor M 20  is planar. Specifically, the shape of the planar conductor M 20  is quadrilateral. The shape of the planar conductor M 20  is not limited to quadrilateral, but may be another shape (for example, hexagonal). The planar conductor M 20  is grounded. 
     As shown in  FIG. 4 , the planar conductor M 20  is formed on a substrate SB 20 . 
     The plane size of the planar conductor M 20  is the same as that of the substrate  5 B 20 . However, the plane size of the planar conductor M 20  may be different from that of the substrate SB 20 . 
     Referring back to  FIG. 3  again, the three-dimensional linear conductor  201  is a linear conductor in which a linear conductor  210 , a linear conductor  220 , a linear conductor  230 , and a linear conductor  240  are integrally formed. The linear conductor  210 , the linear conductor  220 , the linear conductor  230 , and the linear conductor  240  are a first linear conductor, a second linear conductor, a third linear conductor, and a fourth linear conductor, respectively. 
     Each of the linear conductors  210 ,  220 ,  230 ,  240  is a conductor with a linear shape. However, each of the linear conductors  210 ,  220 ,  230 ,  240  is not limited to be a conductor with a linear shape, but may be a conductor with another shape. Each of the linear conductors  210 ,  220 ,  230 ,  240  is composed of metallic material such as tin or copper. 
     Each of the linear conductors  210 ,  220 ,  230 ,  240  is provided on the major surface side of the planar conductor M 20 . The major surface of the planar conductor M 20  is a rear surface that is on the opposite side to the surface of the planar conductor M 20  of  FIG. 4  that is in contact with the substrate SB 20 . 
     The linear conductor  210  is provided perpendicularly to the major surface of the plane conductor M 20 . Each of the linear conductors  220 ,  230  is parallel to the major surface of the planar conductor M 20 . The linear conductor  230  is provided perpendicularly to the linear conductor  220 . One end of the linear conductor  230  is electrically connected to the linear conductor  220  at a contact point N 10 . The linear conductor  230  is provided so as to extend in −z direction from the contact point N 10 . 
     The length of the linear conductor  240  is the same as that of the linear conductor  210 . The linear conductor  240  is parallel to the linear conductor  210 . 
     The length of the linear conductor  220  is equal to or less than that of the planar conductor M 20  in the y-axis direction. Also, the length of the linear conductor  230  is equal to or less than that of the planar conductor M 20  in the z-axis direction. 
     The gauges of the linear conductors  210 ,  220 ,  230 ,  240  are almost the same. The respective radii of the linear conductor  220 ,  230  are supposed to be shorter than the length of the linear conductor  210 . That is to say, the respective gauges of the linear conductors  220 ,  230  have such dimensions that the linear conductors  220 ,  230  are not in contact with the planar conductor M 20 . 
     One end of the linear conductor  240  is electrically connected to the planar conductor M 20 . As described above, one end of the linear conductor  220  is electrically connected to one end of the linear conductor  230 . The other end of the linear conductor  220  is electrically connected to the planar conductor M 20  via the linear conductor  240 . 
     Also, as shown in (b) in  FIG. 3 , the respective linear conductors  220 ,  230  are disposed perpendicularly above the corresponding ends of the planar conductor M 20 . The respective linear conductors  220 ,  230  may be disposed perpendicularly above the interior of the planar conductor M 20 . 
     Here, the major surface of the planar conductor M 20  is supposed to be parallel to the z-y plane of the three-dimensional coordinate system. In this case, the linear conductors  210 ,  240  are parallel to the x-axis of the three-dimensional coordinate system. Also, the linear conductor  220  is parallel to the y-axis of the three-dimensional coordinate system. Also, the linear conductor  230  is parallel to the z-axis of the three-dimensional coordinate system. 
       FIG. 3  shows a power feed region P 10  contains a power feed point PT 10  which is described later. 
       FIG. 5  is an illustration for explaining the power feed region P 10 . 
     (A) in  FIG. 5  is an illustration for showing in detail the configuration around the power feed region P 10 . 
     The power feed region P 10  is provided on the major surface of the planar conductor M 20 . The power feed region P 10  contains the power feed point PT 10 . The power feed point PT 10  is provided on the major surface of the planar conductor M 20 . The power feed point PT 10  is electrically disconnected to the planar conductor M 20  via an insulating film PX 20 . That is to say, the power feed point PT 10  is provided in the planar conductor M 20  so as to be disconnected thereto. 
     The power feed point PT 10  is provided in the vicinity of the edge of the planar conductor M 20  as shown in  FIG. 3 . The power feed point PT 10  may not be provided in the vicinity of the edge of the planar conductor M 20   
     Here, the detailed configuration of the power feed line L 10  is described. 
     (B) in  FIG. 5  is an illustration for showing in detail the configuration of the power feed line L 10 . 
     As shown in (b) in  FIG. 5 , the power feed line L 10  contains a power supply line PL 10 . The power supply line PL 10  is a conductive line which transmits a high frequency current. The power supply line PL 10  is covered with an insulating film PX 10 . A ground film G 10  is formed on the surface of the insulating film PX 10 . That is to say, the power supply line PL 10  and the ground film G 10  are electrically disconnected to each other. Also, the ground film G 10  is grounded. 
     The power feed point PT 10  is electrically connected to the power supply line PL 10  of the power feed line L 10 . The boundary of the power feed region P 10  provided in the planar conductor M 20  is electrically connected to the ground film G 10 . The power supply line PL 10  and the ground film G 10  are electrically connected to the radio IC  20 . 
     The radio IC  20  supplies a high frequency current (electric power) used for radio communication to the power feed point PT 10  via the power supply line PL 10 . That is to say, a high frequency current used for radio communication is supplied to the power feed point PT 10  from the outside. The power feed point PT 10  is electrically connected to one end of the linear conductor  210  of the three-dimensional linear conductor  201 . 
     Accordingly, the high frequency current supplied to the power feed point PT 10  flows through the three-dimensional linear conductor  201 . In this case, radio waves are emitted from the antenna  200  that includes the three-dimensional linear conductor  201 . The planar conductor M 20  is effectively used to emit the radio waves. 
     That is to say, the radio IC  20  performs radio communication using the antenna  200 . In other words, the radio communication device  1000  performs radio communication using the antenna  200 . 
     Also, a high frequency current flows through the three-dimensional linear conductor  201 , so that a current flows through the planar conductor M 20  to the power feed point PT 10 . 
     When the three-dimensional linear conductor  201  receives a radio wave from the outside, the radio wave is converted to a high frequency current, which flows through the radio IC  20  via the power feed point PT 10  and the power supply line PL 10 . 
     Also, the other end of the linear conductor  210  is electrically connected to a contact point N 11  of the linear conductor  220 . 
     The length of the planar conductor M 20  in the z-axis direction is ¼ or less of the wavelength λ of the frequency of the high frequency current that is used for radio communication. Also, each of the lengths of the linear conductors  210 ,  220 ,  230 ,  240  is ¼ or less of the wavelength λ for the frequency of the high frequency current that is used for radio communication. 
     Here, the following are defined in a state where a high frequency current which is supplied to the power feed point PT 10  flows through the three-dimensional linear conductor  201  to emit a radio wave from the antenna  200 . 
     The major surface of the planar conductor M 20  is defined to be parallel to the z-y plane of the three-dimensional coordinate system of  FIG. 2 . Also, Lx denotes the length of the three-dimensional linear conductor  201  in the x-axis direction. That is to say, Lx denotes the length of each of the linear conductors  210 ,  240 . Also, Ly denotes the length of the three-dimensional linear conductor  201  in the y-axis direction. That is to say, Ly denotes the length of the linear conductor  220 . Also, Lz 2  denotes the length of the three-dimensional linear conductor  201  in the z-axis direction. That is to say, Lz 2  denotes the length of the linear conductor  230 . Also, Lz 1  denotes the length of the planar conductor M 20  in the z-axis direction. 
     Furthermore, Ix denotes a current flowing along the x-axis out of the high frequency current flowing through the three-dimensional linear conductor  201  where Ix is represented by a positive value when the current flows in the +x direction, Iy denotes a current flowing along the y-axis out of the high frequency current flowing through the three-dimensional linear conductor  201  where Iy is represented by a positive value when the current flows in the +y direction, Iz 1  denotes a current flowing along a z-axis out of the current flowing through the planar conductor M 20  where Iz 1  is represented by a positive value when the current flows in the +z direction, Iz 2  denotes a current flowing along the z-axis out of the high frequency current flowing through the three-dimensional linear conductor  201  where Iz 2  is represented by a positive value when the current flows in the +z direction. 
     Also, an electromagnetic moment Mx is defined as Ix×Lx. Also, an electromagnetic moment My is defined as Iy×Ly. An electromagnetic moment Mz is defined as Iz 1 ×Lz 1 −Iz 2 ×Lz 2 . 
     In this case, a current Ix 1  flows in the +x direction through the linear conductor  210 . Also, in this case, a current Ix 2  flows in the −x direction through the linear conductor  240 . The current Ix is calculated as Ix 1 +(−Ix 2 ). 
     Also, in this case, a current Iy 1  flows from the contact point N 11  in the +y direction through the linear conductor  220 . Also, in this case, a current Iy 2  flows from the contact point N 11  in the −y direction through the linear conductor  220 . The current Iy is calculated as Iy 1 +(−Iy 2 ). 
     Also, in this case, a current Iz 2  flows in the −z direction through the linear conductor  230 . That is to say, the current flowing through the linear conductor  230  is expressed by −Iz 2  where the +z direction is assumed to be positive direction. 
     The inventors formulated a hypothesis (hereinafter referred to as a hypothesis A) that by satisfying the following Expression (1) regarding the electromagnetic moments Mx, My, Mz, it is possible to achieve an antenna that prevents an occurrence of a location (null point) in all directions in the three-dimensional space, where the electric field strength is significantly reduced.
 
 Mx=My=Mz   Expression (1)
 
     The electromagnetic moments Mx, My, and Mz are defined by the following Expressions (2), (3), and (4), respectively.
 
 Mx=Ix×Lx   Expression (2)
 
 My=Iy×Ly   Expression (3)
 
 Mz=IZ 1× Lz 1− Iz 2× Lz 2  Expression (4)
 
     In other words, the inventors formulated the hypothesis A that by designing the size and shape of an antenna so that all the electromagnetic moments Mx, My, and Mz are equal, it is possible to achieve an antenna that prevents an occurrence of a location (null point) in all directions on each of the orthogonal planes in the three-dimensional space, where the electric field strength is significantly reduced. Here, the orthogonal planes are the x-y plane, the z-y plane, and the z-x plane. In order to prove the validity of the hypothesis A, a simulation was performed using an electromagnetic field simulator which is operated by a computer. 
     Here, the antenna to be simulated is the antenna  200  of  FIG. 3 . The condition (hereinafter referred to as a condition A) for the simulation is as follows: 
     Each of the linear conductors  210 ,  240  has a length of 15 mm. The linear conductor  220  has a length of 40 mm. The linear conductor  230  has a length of 38 mm. The planar conductor M 20  has a length of 40 mm in the y-axis and the z-axis directions. The frequency of the high frequency current supplied to the power feed point PT 10  is 950 MHz. 
     Hereinafter, a simulation which is performed under the condition A is referred to as the simulation A. 
       FIG. 6  is a graph showing the emission characteristic of the electric field emitted from the antenna, as indicated by the simulation A. 
     The emission characteristic of the electric field of  FIG. 6  is the emission characteristic of the electric field in the x-y plane. 
     Hereinafter, the electric field is denoted by E. Also, hereinafter, θ-component of the electric field E is denoted by Eθ. Here, θ is the angle formed by the z-axis and the electric field direction as shown in  FIG. 3 . Also, hereinafter, Φ-component of the electric field E is denoted by EΦ. Here, Φ is the angle formed by the x-axis and the electric field direction as shown in  FIG. 3 . 
     The characteristic line Lθ 10  shows the emission characteristic of the electric field Eθ in the x-y plane. The characteristic line LΦ 10  shows the emission characteristic of the electric field EΦ in the x-y plane. The characteristic line LE 10  shows the emission characteristic of the electric field E in the x-y plane. The electric field E is the composite electric field of the electric field Eθ and the electric field EΦ. The electric field E is a value calculated by the following Expression (5).
 
[Math. 1]
 
 E =√{square root over (| EΦ|   2   +|E θ|)} 2   Expression (5)
 
       FIG. 7  is a graph showing the emission characteristic of each electric field shown in  FIG. 6 . In  FIG. 7 , the vertical axis shows the amplitude (gain) of each characteristic line, and the horizontal axis shows an angle. 
     The characteristic lines LE 11 , Lθ 11 , and LΦ 11  of  FIG. 7  correspond to the characteristic lines LE 10 , Lθ 10 , and LΦ 10 , respectively. 
     The difference between the maximum and minimum values of the amplitude (gain) of the characteristic line LE 11  of  FIG. 7  is equal to or less than 5 dB. 
     That is to say, based on the result in  FIGS. 6 and 7 , it can be safely said that there is not a point (null point) in all directions on the x-y plane, at which the strength of the electric field emitted from the antenna is significantly reduced. 
       FIG. 8  is a graph showing the emission characteristic of the electric field emitted from the antenna, as indicated by the simulation A. 
     The emission characteristic of the electric field in  FIG. 8  is the emission characteristic of the electric field in the z-y plane. 
     The characteristic line Lθ 20  shows the emission characteristic of the electric field EA in the z-y plane. The characteristic line LΦ 20  shows the emission characteristic of the electric field EΦ in the z-y plane. The characteristic line LE 20  shows the emission characteristic of the electric field E in the z-y plane. The electric field E is the composite electric field of the electric field Eθ and the electric field EΦ. 
       FIG. 9  is a graph showing the emission characteristic of each electric field shown in  FIG. 8 . The vertical axis and the horizontal axis are the same as those in  FIG. 7 . 
     The characteristic lines LE 21 , Lθ 21 , and LΦ 21  of  FIG. 9  correspond to the characteristic lines LE 20 , Lθ 20 , and LΦ 20 , respectively. 
     The difference between the maximum and minimum values of the amplitude (gain) of the characteristic line LE 21  of  FIG. 9  is equal to or less than 5 dB. 
     That is to say, based on the result in  FIGS. 8 and 9 , it can be safely said that there is not a point (null point) in all directions on the z-y plane, at which the strength of the electric field emitted from the antenna is significantly reduced. 
       FIG. 10  is a graph showing the emission characteristic of the electric field emitted from the antenna, as indicated by the simulation A. 
     The emission characteristic of the electric field in  FIG. 10  is the emission characteristic of the electric field in the z-x plane. 
     The characteristic line Lθ 30  shows the emission characteristic of the electric field Eθ in the z-x plane. The characteristic line LΦ 30  shows the emission characteristic of the electric field EΦ in the z-x plane. 
     The characteristic line LE 30  shows the emission characteristic of the electric field E in the z-x plane. The electric field E is the composite electric field of the electric field Eθ and the electric field E. 
       FIG. 11  is a graph showing the emission characteristic of each electric field shown in  FIG. 10 . The vertical axis and the horizontal axis are the same as those in  FIG. 7 . 
     The characteristic lines LE 31 , Lθ 31 , and LΦ 31  of  FIG. 11  correspond to the characteristic lines LE 30 , Lθ 30 , and LΦ 30 , respectively. 
     The difference between the maximum and minimum values of the amplitude (gain) of the characteristic line LE 31  of  FIG. 11  is equal to or less than 5 dB. 
     That is to say, based on the result in  FIGS. 10 and 11 , it can be safely said that there is not a point (null point) in all directions on the z-x plane, at which the strength of the electric field emitted from the antenna is significantly reduced. 
     Next, the result of a simulation is described where the simulation is performed for an antenna as a comparison target (hereinafter, referred to as an antenna for comparison) by using an electromagnetic field simulator, which does not satisfy the relationship of Expression (1). 
     Hereinafter, a simulation which is performed for the antenna for comparison is referred to as the simulation J. The condition (hereinafter referred to as the condition J) for the simulation J differs from the above-described condition A only in that the planar conductor M 20  has a length of 70 mm in the z-axis direction. Except this, the condition J is the same as the condition A. 
       FIG. 12  is a graph showing the emission characteristic of the electric field emitted from the antenna, as indicated by simulation J. 
     The emission characteristic of the electric field in  FIG. 12  is the emission characteristic of the electric field in the x-y plane. 
     The characteristic line Lθ 40  shows the emission characteristic of the electric field Eθ in the x-y plane. The characteristic line LΦ 40  shows the emission characteristic of the electric field EΦ in the x-y plane. The characteristic line LE 40  shows the emission characteristic of the electric field E in the x-y plane. The electric field E is the composite electric field of the electric field Eθ and the electric field EΦ. 
       FIG. 13  is a graph showing the emission characteristic of each electric field shown in  FIG. 12 . The vertical axis and the horizontal axis are the same as those in  FIG. 7 . 
     The characteristic lines LE 41 , Lθ 41 , and LΦ 41  of  FIG. 13  correspond to the characteristic lines LE 40 , Lθ 40 , and LΦ 40 , respectively. 
     The difference between the maximum and minimum values of the amplitude (gain) of the characteristic line LE 41  of  FIG. 13  is equal to or less than 5 dB. 
     That is to say, based on the result in  FIGS. 12 and 13 , it can be safely said that there is not a point (null point) in all directions on the x-y plane, at which the strength of the electric field emitted from the antenna is significantly reduced. 
       FIG. 14  is a graph showing the emission characteristic of the electric field emitted from the antenna, as indicated by the simulation J. 
     The emission characteristic of the electric field in  FIG. 14  is the emission characteristic of the electric field in the z-y plane. 
     The characteristic line Lθ 50  shows the emission characteristic of the electric field EA in the z-y plane. The characteristic line LΦ 50  shows the emission characteristic of the electric field EΦ in the z-y plane. The characteristic line LE 50  shows the emission characteristic of the electric field E in the z-y plane. The electric field E is the composite electric field of the electric field Eθ and the electric field EΦ. 
       FIG. 15  is a graph showing the emission characteristic of each electric field shown in  FIG. 14 . The vertical axis and the horizontal axis are the same as those in  FIG. 7 . 
     The characteristic lines LE 51 , Lθ 51 , and LΦ 51  of  FIG. 15  correspond to the characteristic lines LE 50 , Lθ 50 , and LΦ 50 , respectively. 
     The difference between the maximum and minimum values of the amplitude (gain) of the characteristic line LE 51  of  FIG. 15  is greater than 5 dB. 
     That is to say, based on the result in  FIGS. 14 and 15 , it can be safely said that there exists a point (null point) on the z-y plane, at which the strength of the electric field emitted from the antenna is significantly reduced. 
       FIG. 16  is a graph showing the emission characteristic of the electric field emitted from the antenna, as indicated by the simulation J. 
     The emission characteristic of the electric field in  FIG. 16  is the emission characteristic of the electric field in the z-x plane. 
     The characteristic line Lθ 60  shows the emission characteristic of the electric field Eθ in the z-x plane. The characteristic line LΦ 60  shows the emission characteristic of the electric field EΦ in the z-x plane. The characteristic line LE 60  shows the emission characteristic of the electric field E in the z-x plane. The electric field E is the composite electric field of the electric field Eθ and the electric field EΦ. 
       FIG. 17  is a graph showing the emission characteristic of each electric field shown in  FIG. 16 . The vertical axis and the horizontal axis are the same as those in  FIG. 7 . 
     The characteristic lines LE 61 , Lθ 61 , and LΦ 61  of  FIG. 17  correspond to the characteristic lines LE 60 , Lθ 60 , and LΦ 60 , respectively. 
     The difference between the maximum and minimum values of the amplitude (gain) of the characteristic line LE 61  of  FIG. 17  is greater than 5 dB. 
     That is to say, based on the result in  FIGS. 16 and 17 , it can be safely said that there exists a point (null point) on the z-x plane, at which the strength of the electric field emitted from the antenna is significantly reduced. 
     From the result of the above simulation, it can be inferred that by designing the size and shape of an antenna so that all the electromagnetic moments Mx, My, and Mz are equal, it is possible to achieve an antenna that prevents an occurrence of a location (null point) in all directions on each of the orthogonal planes in the three-dimensional space, where the electric field strength is significantly reduced. 
     The inventors produced a prototype of an antenna (hereinafter, referred to as a prototype antenna A) which satisfies Expression (1) and the above-described condition A, and measured the emission characteristic of the actual electric field. The prototype antenna A is the antenna  200  of  FIG. 3 . 
       FIG. 18  is a graph showing the emission characteristic of the electric field emitted from the prototype antenna A. 
     The emission characteristic of the electric field in (a) in  FIG. 18  is the emission characteristic of the electric field in the x-y plane. 
     The characteristic line Lθ 110  shows the emission characteristic of the electric field EA in the x-y plane. The characteristic line LΦ 110  shows the emission characteristic of the electric field EΦ in the x-y plane. The characteristic line LE 110  shows the emission characteristic of the electric field E in the x-y plane. The electric field E is the composite electric field of the electric field Eθ and the electric field EΦ. 
     The shape of the characteristic line LE 110  is substantially a circle. That is to say, from (a) in  FIG. 18 , it can be safely said that there is not a point (null point) in all directions on the x-y plane, at which the strength of the electric field emitted from the prototype antenna A is significantly reduced. 
     The emission characteristic of the electric field in (b) in  FIG. 18  is the emission characteristic of the electric field in the z-y plane. 
     The characteristic line Lθ 120  shows the emission characteristic of the electric field Eθ in the z-y plane. The characteristic line LΦ 120  shows the emission characteristic of the electric field Eθ in the z-y plane. The characteristic line LE 120  shows the emission characteristic of the electric field E in the z-y plane. The electric field E is the composite electric field of the electric field Eθ and the electric field EΦ. 
     The shape of the characteristic line LE 120  is substantially a circle. That is to say, from (b) in  FIG. 18 , it can be safely said that there is not a point (null point) in all directions on the z-y plane, at which the strength of the electric field emitted from the prototype antenna A is significantly reduced. 
     The emission characteristic of the electric field in (c) in  FIG. 18  is the emission characteristic of the electric field in the z-x plane. 
     The characteristic line Lθ 130  shows the emission characteristic of the electric field Eθ in the z-x plane. The characteristic line LΦ 130  shows the emission characteristic of the electric field EΦ in the z-x plane. 
     The characteristic line LE 130  shows the emission characteristic of the electric field E in the z-x plane. The electric field E is the composite electric field of the electric field Eθ and the electric field EΦ. 
     The shape of the characteristic line LE 130  is substantially a circle. That is to say, from (c) in  FIG. 18 , it can be safely said that there is not a point (null point) in all directions on the z-x plane, at which the strength of the electric field emitted from the prototype antenna A is significantly reduced. 
     In addition, the inventors produced an antenna (hereinafter, referred to as a comparison antenna  900 ) which does not satisfy Expression (1), and measured the emission characteristic of the actual electric field. The comparison antenna  900  is an antenna that is formed so as to satisfy the above-described condition J. 
       FIG. 19  is an illustration showing the configuration of the comparison antenna  900 . 
     As shown in  FIG. 19 , compared with the antenna of  FIG. 3 , the comparison antenna  900  has a different length of the planar conductor M 20  in the z-axis direction. Except for this difference, the configuration of the comparison antenna  900  is the same as that of the antenna  200 , thus detailed description is not repeated. The length Lz 1  of the planar conductor M 20  in the z-axis direction is, for example, 70 mm. 
     When Lz 1  is 70 mm, i.e., Lz 1  is increased, the electromagnetic moment Mz becomes greater than the electromagnetic moments Mx, My as seen from Expression (4). Consequently, Expression (1) is not satisfied. That is to say, in the comparison antenna  900 , the electromagnetic moments Mx, My, and Mz do not have the same value. 
       FIG. 20  is a graph showing the emission characteristic of the electric field emitted from the comparison antenna  900 . 
     The emission characteristic of the electric field in (a) in  FIG. 20  is the emission characteristic of the electric field in the x-y plane. The characteristic line LE 210  shows the emission characteristic of the electric field E in the x-y plane. 
     The shape of the characteristic line LE 210  is substantially a circle. That is to say, from (a) in  FIG. 20 , it can be safely said that there is not a point (null point) in all directions on the x-y plane, at which the strength of the electric field emitted from the comparison antenna  900  is significantly reduced. 
     The emission characteristic of the electric field in (b) in  FIG. 20  is the emission characteristic of the electric field in the z-y plane. 
     From (b) in  FIG. 20 , it can be safely said that there exists a point (null point) on the z-y plane, at which the strength of the electric field emitted from the antenna is significantly reduced. 
     The emission characteristic of the electric field in (c) in  FIG. 20  is the emission characteristic of the electric field in the z-x plane. 
     From  FIG. 20 , it can be safely said that there exists a point (null point) on the z-x plane, at which the strength of the electric field emitted from the antenna is significantly reduced. 
     That is to say, from  FIG. 18 , the prototype antenna A which satisfies Expression (1) and the above-described condition A serves to prevent an occurrence of a location (null point) in all directions on the orthogonal planes in the three-dimensional space, where the electric field strength is significantly reduced. In other words, the antenna designed to have equal electromagnetic moments of Mx, My, and Mz serves to prevent an occurrence of a location (null point) in all directions on the orthogonal planes in the three-dimensional space, where the electric field strength is significantly reduced. Therefore, the validity of the above-mentioned hypothesis A has been proved. 
     Thus, the antenna  200  in the present embodiment serves to prevent an occurrence of a location (null point) in all directions on the orthogonal planes in the three-dimensional space, where the electric field strength is significantly reduced. That is to say, the antenna  200  serves to prevent an occurrence of a location (null point) in all directions on the orthogonal planes in the three-dimensional space, where the electric field strength is significantly reduced. In other words, the antenna  200  has a small variation in its directivity on each of the orthogonal planes in the three-dimensional space. 
     Therefore, the radio communication device  1000  equipped with the antenna  200  can perform stable communication regardless of where or which direction the radio communication device  1000  is installed on a human body or at a location away from a human body. 
     That is to say, the radio communication device  1000  equipped with the antenna  200  can perform stable communication regardless of the install location, direction, or movement of a human body. That is to say, the antenna  200  is particularly effective when communication is performed among a plurality of radio communication devices attached to human bodies while the antenna  200  is used for each radio communication device. 
     In addition, the antenna  200  is particularly effective when communication is performed between a radio communication device attached to a human body and another radio communication device away from the human body while the antenna  200  is used for each radio communication device. 
     In addition, because the planar conductor M 20  is advantageously utilized for the emission of radio waves (electric field), the radio communication device  1000  equipped with the antenna  200  can be reduced in size. 
     In the three-dimensional linear conductor  201  of  FIG. 3 , a portion closer to the power feed point PT 10  has more current flowing through the portion. Accordingly, the length of the conductor in relation to each electromagnetic moment can be reduced. On the other hand, in the three-dimensional linear conductor  201 , a portion far from the power feed point PT 10  (for example, the linear conductor  230 ) has less current flowing therethrough than a portion near the power feed point PT 10  (for example, the linear conductor  210 ). 
     The distance between the linear conductor  210  and the linear conductor  240  is preferably such that the input impedance of the antenna  200  is 50Ω for the frequency of the high frequency current which flows through the antenna  200  and is used for radio communication. The input impedance of the antenna  200  is the impedance as the antenna  200  is viewed from the power feed point PT 10 . 
     However, in most cases, the input impedance of the antenna  200  is not set to 50Ω because of the effect of the shape or the like of the antenna  200 . Thus, a matching circuit (not shown) is used. Impedance matching is performed by the matching circuit so that the input impedance of the antenna  200  is set to 50Ω. The matching circuit is included in the radio communication device  1000 . 
     As described above, the power feed point PT 10  is provided in the vicinity of the edge of the planar conductor M 20 . Consequently, the lengths of the linear conductor  220  and the linear conductor  230  can be effectively secured. Accordingly, the radio communication device  1000  equipped with the antenna  200  can be reduced in size. 
     Also, as described above, the length of the planar conductor M 20  in the z-axis direction and the respective lengths of the linear conductors  210 ,  220 ,  230 ,  240  are ¼ or less of the wavelength λ for the frequency of the high frequency current that is used for radio communication. 
     The antenna  200  excites the high frequency current with the wavelength λ centered on the power feed point PT 10 . When the length of the planar conductor M 20  in the z-axis direction and the respective lengths of the linear conductors  210 ,  220 ,  230 ,  240  become λ/4 or more, a positive and a negative amplitudes occur simultaneously on the planar conductor M 20 . Accordingly, degradation of the emission characteristic is caused. 
     For this reason, the length of the planar conductor M 20  in the z-axis direction and the respective lengths of the linear conductors  210 ,  220 ,  230 ,  240  are set to λ/4 or less. Accordingly, degradation of the emission characteristic of the antenna  200  can be prevented and the performance of the antenna  200  can be improved. 
     Although the linear conductor  230  of  FIG. 3  has been assumed to be provided so as to extend from the contact point N 10  in the −z direction, however this is not always the case. The linear conductor  230  may be provided so as to extend from the contact point N 10  in the +z direction like an antenna  200 A shown in (a) and (b) in  FIG. 21 . 
     (A) in  FIG. 21  is a perspective view of the antenna  200 A. (B) in  FIG. 21  is a view of the antenna  200 A projected onto the z-y plane of the three-dimensional coordinate system. Also in the antenna  200 A, similarly to what has been described above, the size and shape of each component are defined so that the electromagnetic moments Mx, My, and Mz are equal. 
     In this case, a current flows through the linear conductor  230  in the +z direction. The current is denoted by Iz 2 . 
     In this case, the electromagnetic moment Mz is expressed by the following Expression (6).
 
 Mz=Iz 1 ×Lz 1 +Iz 2 ×Lz 2  Expression (6)
 
     From Expressions (4) and (6), it can be seen that the value of the electromagnetic moment Mz in the antenna  200 A is greater than that of the electromagnetic moment Mz in the antenna  200 . In this case, the length of the planar conductor M 20  in the z-axis direction of the antenna  200 A can be made shorter than that of the antenna  200 . 
     Also, as described above, the power feed point PT 10  does not need to be provided in the vicinity of the edge of the planar conductor M 20 . For example, the power feed point PT 10  may be disposed near the center of the planar conductor M 20  like the antenna  200 B of  FIG. 22 . (A) in  FIG. 22  is a perspective view of the antenna  200 B. (B) in  FIG. 22  is a view of the antenna  200 B projected onto the z-y plane of the three-dimensional coordinate system. 
     Also in the antenna  200 B, similarly to what has been described above, the size and shape of each component are defined so that the electromagnetic moments Mx, My, and Mz are equal. 
     Modification 1 of Embodiment 1 
     The radio communication device  1000  in Modification 1 of the present embodiment includes an antenna  200 C instead of the antenna  200 . Except for this, the configuration of the radio communication device  1000  is the same as that of the radio communication device  1000  of  FIG. 1 , thus detailed description is not repeated. 
       FIG. 23  is an illustration showing the configuration of the antenna  200 C in Modification 1 of Embodiment 1. 
     (A) in  FIG. 23  is a perspective view of the antenna  200 C. (B) in  FIG. 23  is a view of the antenna  200 C projected onto the z-y plane of the three-dimensional coordinate system. 
     As shown in  FIG. 23 , the antenna  200 C differs from the antenna  200  in that the antenna  200 C includes a three-dimensional linear conductor  201 C instead of the three-dimensional linear conductor  201 . Except for this, the configuration of the antenna  200 C is the same as that of the antenna  200 , thus detailed description is not repeated. 
     The three-dimensional linear conductor  201 C differs from the three-dimensional linear conductor  201  of  FIG. 3  in that the three-dimensional linear conductor  201 C further includes a linear conductor  250 . 
     The three-dimensional linear conductor  201 C is a linear conductor in which the linear conductor  210 , the linear conductor  220 , the linear conductor  230 , the linear conductor  240 , and the linear conductor  250  are integrally formed. The linear conductor  250  is a fifth linear conductor. 
     The linear conductor  250  is a conductor with a linear shape. The linear conductor  250  is not limited to be a conductor with a linear shape, but may be a conductor with another shape. The linear conductor  250  is provided on the major surface side of the planar conductor M 20 . 
     One end of the linear conductor  250  is electrically connected to the linear conductor  230  at a contact point N 21 . The linear conductor  250  is provided so as to extend in the −y direction from the contact point N 21 . 
     Also, the linear conductor  250  may be provided so as to extend in any one of the +y direction, the −z direction, and ±x direction from the contact point N 21 . 
     Also, like the antenna  200 D shown in  FIG. 24 , the linear conductor  250  may be provided so as not to be parallel to any one of the x-axis, the y-axis and the z-axis. (A) in  FIG. 24  is a perspective view of the antenna  200 D. (B) in  FIG. 24  is a view of the antenna  200 D projected onto the z-y plane of the three-dimensional coordinate system. 
     Also in the antenna  200 C and the antenna  200 D, similarly to Embodiment 1, the size and shape of each component are defined so that the electromagnetic moments Mx, My, and Mz are equal. 
     As described above, according to Modification 1 of the present embodiment, the electrical length of the three-dimensional linear conductor  201 C required to efficiently emit radio waves can be adjusted by the linear conductor  250 . Also, the magnitude of each electromagnetic moment can be flexibly adjusted by the linear conductor  250 . Consequently, the radio communication device  1000  equipped with the antenna  200 C or the antenna  200 D can be reduced in size. Also, flexible design of an antenna is possible. 
     Modification 2 of Embodiment 1 
     The radio communication device  1000  in Modification 2 of the present embodiment includes an antenna  200 E instead of the antenna  200 . Except for this, the configuration of the radio communication device  1000  is the same as that of the radio communication device  1000  of  FIG. 1 , thus detailed description is not repeated. 
       FIG. 25  is an illustration showing the configuration of the antenna  200 E in Modification 2 of Embodiment 1 
     As shown in  FIG. 25 , the antenna  200 E differs from the antenna  200  in that the antenna  200 E includes a three-dimensional linear conductor  201 E instead of the three-dimensional linear conductor  201 . Except for this, the configuration of the antenna  200 E is the same as that of the antenna  200 , thus detailed description is not repeated. 
     The three-dimensional linear conductor  201 E is a linear conductor in which the linear conductor  210 , the linear conductor  220 , the linear conductor  230 , and a linear conductor  260  are integrally formed. That is to say, the three-dimensional linear conductor  201 E does not include the linear conductor  240 . The linear conductor  260  is a sixth linear conductor. 
     The linear conductor  260  is provided on the opposite side to the major surface of the planar conductor M 20 . The linear conductor  260  is provided perpendicularly to the major surface of the planar conductor M 20 . Also, the linear conductor  260  is provided so that the linear conductor  260  and the linear conductor  210  lie on the same line. 
     One end of the linear conductor  260  is electrically connected to the power feed point PT 10  contained in the power feed region P 10 . That is to say, one end of linear conductor  210  which is electrically connected to the power feed point PT 10  and one end of the linear conductor  260  which is electrically connected to the power feed point PT 10  are electrically connected to each other. 
     Also in the antenna  200 E, similarly to Embodiment 1, the size and shape of each component are defined so that the electromagnetic moments Mx, My, and Mz are equal. 
     As described above, according to Modification 2 of the present embodiment, the length of the linear conductor  210  in the x-axis direction can be reduced because of the linear conductor  260 . Consequently, flexible design of an antenna can be supported. 
     The linear conductor  260  may be composed of the same metallic material as that for the linear conductor  210 . 
     Modification 3 of Embodiment 1 
     The radio communication device  1000  in Modification 3 of the present embodiment includes an antenna  200 F instead of the antenna  200 . Except for this, the configuration of the radio communication device  1000  is the same as that of the radio communication device  1000  of  FIG. 1 , thus detailed description is not repeated. 
       FIG. 26  is an illustration showing the configuration of the antenna  200 F in Modification 3 of Embodiment 1. 
     As shown in  FIG. 26 , the antenna  200 F differs from the antenna  200  in that the antenna  200 F includes a three-dimensional linear conductor  201 F instead of the three-dimensional linear conductor  201 . Except for this, the configuration of the antenna  200 F is the same as that of the antenna  200 , thus detailed description is not repeated. 
     The three-dimensional linear conductor  201 F differs from the three-dimensional linear conductor  201  of  FIG. 3  in that the three-dimensional linear conductor  201 F includes a linear conductor  220 F instead of the linear conductor  220 . Except for this, the configuration of the three-dimensional linear conductor  201 F is the same as that of the three-dimensional linear conductor  201 , thus detailed description is not repeated. 
     The three-dimensional linear conductor  201 F is a linear conductor in which the linear conductor  210 , the linear conductor  220 F, the linear conductor  230 , and the linear conductor  240  are integrally formed. 
     The linear conductor  220 F is a linear conductor in which a loading coil L 22  is inserted in all or part of the linear conductor  220  of  FIG. 3 . 
     Normally, the loading coil L 22  is used to have an efficient flow of a current through an antenna by eliminating a reactance component thereof when the electrical length of the antenna is insufficient, or the physical length of the antenna is intended to be reduced. 
     Here, the physical length of a linear conductor which extends in the x-axis, the y-axis, or z-axis direction means the length of the linear conductor in the corresponding direction. For example, the physical length of the linear conductor  210  which extends in the x-axis direction is the length of the linear conductor  210  along the x-axis direction. 
     That is to say, the physical length of the linear conductor  220 F which extends in the y-axis direction is the length of the linear conductor  220 F along the y-axis direction. 
     Also in the antenna  200 F, similarly to Embodiment 1, the size and shape of each component are defined so that the electromagnetic moments Mx, My, and Mz are equal. 
     As described above, according to Modification 3 of the present embodiment, the electrical length of the linear conductor  220 F of the three-dimensional linear conductor  201 F can be increased by using the loading coil L 22 , thus setting of a desired resonance frequency is made possible. Consequently, the emission characteristic of the antenna can be improved. Also, the antenna can be reduced in size because the physical length of the linear conductor in which the loading coil L 22  is inserted can be reduced. 
     The loading coil L 22  may be inserted in any one of the linear conductors  210 ,  230 , and  240 . 
     Modification 4 of Embodiment 1 
     The radio communication device  1000  in Modification 4 of the present embodiment includes an antenna  200 G instead of the antenna  200 . Except for this, the configuration of the radio communication device  1000  is the same as that of the radio communication device  1000  of  FIG. 1 , thus detailed description is not repeated. 
       FIG. 27  is an illustration showing the configuration of the antenna  200 G in Modification 4 of Embodiment 1. 
     As shown in  FIG. 27 , the antenna  200 G differs from the antenna  200  in that the antenna  200 G includes a three-dimensional linear conductor  201 G instead of the three-dimensional linear conductor  201 . Except for this, the configuration of the antenna  200 G is the same as that of the antenna  200 , thus detailed description is not repeated. 
     The three-dimensional linear conductor  201 G differs from the three-dimensional linear conductor  201  of  FIG. 3  in that the three-dimensional linear conductor  201 G includes a linear conductor  220 G instead of the linear conductor  220 . Except for this, the configuration of the three-dimensional linear conductor  201 G is the same as that of the three-dimensional linear conductor  201 , thus detailed description is not repeated. 
     The three-dimensional linear conductor  201 G is a linear conductor in which the linear conductor  210 , the linear conductor  220 G, the linear conductor  230 , and the linear conductor  240  are integrally formed. 
     The three-dimensional linear conductor  201 G is such that all or part of the linear conductor  220  of  FIG. 3  is replaced by a meander shape (zigzag shape). 
     A meander-shaped conductor normally can achieve the miniaturization of an antenna, while maintaining the electrical length thereof. For this reason, the meander-shaped conductor is utilized for a miniaturized antenna which is used in a mobile phone or the like. 
     Also in the antenna  200 G, similarly to Embodiment 1, the size and shape of each component are defined so that the electromagnetic moments Mx, My, and Mz are equal. 
     As described above, according to Modification 4 of the present embodiment, the electrical length of the antenna can be increased by using the meander-shaped conductor  201 G. That is to say, the electrical length of the antenna can be flexibly adjusted. Accordingly, the frequency of the high frequency current that is used in the antenna for radio communication can be set to a desired resonance frequency. Consequently, the emission characteristic of the antenna can be improved. Also, miniaturization of the antenna can be achieved because the physical length of the linear conductor can be reduced by replacing the linear conductor by a meander-shaped conductor. 
     All or part of each of the linear conductors  210 ,  230 ,  240  may be replaced by a meander-shaped conductor. 
     Modification 5 of Embodiment 1 
     The radio communication device  1000  in Modification 5 of the present embodiment includes an antenna  200 H instead of the antenna  200 . Except for this, the configuration of the radio communication device  1000  is the same as that of the radio communication device  1000  of  FIG. 1 , thus detailed description is not repeated. 
       FIG. 28  is an illustration showing the configuration of the antenna  200 H in Modification 5 of Embodiment 1. 
     As shown in  FIG. 28 , the antenna  200 H differs from the antenna  200  in that the antenna  200 H includes a three-dimensional linear conductor  201 H instead of the three-dimensional linear conductor  201 . Except for this, the configuration of the antenna  200 H is the same as that of the antenna  200 , thus detailed description is not repeated. 
     The three-dimensional linear conductor  201 H differs from the three-dimensional linear conductor  201  of  FIG. 3  in that the three-dimensional linear conductor  201 H further includes a linear conductor  270 . Except for this, the configuration of the three-dimensional linear conductor  201 H is the same as that of the three-dimensional linear conductor  201 , thus detailed description is not repeated. 
     The linear conductor  270  is provided parallel to the linear conductor  210 . The linear conductor  270  is provided perpendicularly to the major surface of the planar conductor M 20 . 
     The three-dimensional linear conductor  201 H is a linear conductor in which the linear conductor  210 , the linear conductor  220 , the linear conductor  230 , and the linear conductor  240  are integrally formed. 
     A loading capacitor C 22  is inserted in the linear conductor  270 . 
     Normally, the loading capacitor C 22  is used to have an efficient flow of a current through an antenna by eliminating a reactance component thereof when the electrical length of the antenna is insufficient, or the physical length of the antenna is intended to be reduced. 
     The contact point N 10  between the linear conductor  220  and the linear conductor  230  is connected to the planar conductor M 20  via the linear conductor  270 . That is to say, the loading capacitor C 22  is provided between the planar conductor M 20  and the contact point N 10  where the linear conductor  220  and the linear conductor  230  are in contact with each other. That is to say, the linear conductor  220  and the linear conductor  230  are electrically connected to the loading capacitor C 22 . 
     Also in the antenna  200 H, similarly to Embodiment 1, the size and shape of each component are defined so that the electromagnetic moments Mx, My, and Mz are equal. 
     As described above, according to Modification 5 of the present embodiment, miniaturization of the antenna can be achieved because the physical length of the linear conductor  220  which is electrically connected to the loading capacitor C 22  can be reduced by using the loading capacitor C 22 . 
     The loading capacitor C 22  may be inserted into any one of the linear conductors  210 ,  230 , and  240 . That is to say, the loading capacitor C 22  may be electrically connected to any one of the linear conductors  210 ,  230 , and  240 . 
     Modification 6 of Embodiment 1 
       FIG. 29  is an illustration showing the configuration of the antenna  200  in Modification 6 of Embodiment 1. For the purpose of description,  FIG. 29  shows a substrate SB 20  which is not included in the antenna  200 . 
     As shown in  FIG. 29 , the plane size of the planar conductor M 20  included in the antenna  200  is different from the plane size of the substrate SB 20 . 
     In order to achieve an antenna that prevents an occurrence of a location (null point) in all directions on each of the orthogonal planes, where the electric field strength is significantly reduced, the size and shape of the antenna may be determined so that Expressions (1) to (4) are satisfied. Accordingly, even when the plane size of the planar conductor M 20  is different from that of the substrate SB 20 , the size and shape of the antenna may be determined so that Expressions (1) to (4) are satisfied, thus flexible design of the antenna is possible. 
     Modification 7 of Embodiment 1 
     The radio communication device  1000  in Modification 7 of the present embodiment includes an antenna  200 J instead of the antenna  200 . Except for this, the configuration of the radio communication device  1000  is the same as that of the radio communication device  1000  of  FIG. 1 , thus detailed description is not repeated. 
       FIG. 30  is an illustration showing the configuration of the antenna  200 J in Modification 7 of Embodiment 1. 
     As shown in  FIG. 30 , the antenna  200 J differs from the antenna  200  in that the planar conductor M 20  is provided with a slit SL 22 . Except for this, the configuration of the antenna  200 J is the same as that of the antenna  200 , thus detailed description is not repeated. 
     By adjusting the shape and size of the slit SL 22 , the amount of the current flowing through the planar conductor M 20  can be controlled. 
     Also in the antenna  200 J, similarly to Embodiment 1, the size and shape of each component are defined so that the electromagnetic moments Mx, My, and Mz are equal. That is to say, in the antenna  200 J, the length of the planar conductor M 20  in the z-axis direction and the length of the linear conductor  230  are defined so that the electromagnetic moments Mx, My, and Mz are equal. Accordingly, flexible design of the antenna is made possible by providing the slit SL 22  in the planar conductor M 20 . 
     Matching Circuit 
       FIG. 31  is a diagram showing the above-described matching circuit  300  which is included in the radio communication device  1000 . The matching circuit  300  is mounted on the substrate SB 20 . 
     As shown in  FIG. 31 , the matching circuit  300  is disposed in the vicinity of the antenna  200 , on the power feed line L 10  interconnecting the antenna  200  and the radio IC  20 . 
     The matching circuit  300  performs impedance matching so that each of the input impedance and the output impedance of the antenna  200  is set to 50Ω. Because the matching circuit  300  is a known circuit, detailed description of the matching circuit  300  is not given. The matching circuit  300  is constituted by passive elements, for example, a resistor, an inductor, or a capacitor. 
     The input impedance of the antenna  200  is the impedance as the antenna  200  is viewed from the power feed point PT 10 . The output impedance of the antenna  200  is the impedance as the radio IC  20  is viewed from the power feed point PT 10 . 
     As described above, by matching the input impedance of the antenna  200  to the output impedance thereof, the high frequency signal outputted from the radio IC  20  is efficiently emitted from the antenna  200 . Also, the high frequency signal that is received by the antenna  200  can be efficiently transmitted to the radio IC. 
     The radio communication device  1000  may include any one of the above-described antennas  200 A,  200 B,  200 C,  200 D,  200 E,  200 F,  200 G,  200 H, and  200 J instead of the antenna  200  shown in  FIG. 31 . In this case, the input impedance and the output impedance of the antenna (for example, the antenna  200 A) provided in the radio communication device  1000  can be matched to each other by the matching circuit  300 . 
     In the above, the antenna (for example, the antenna  200 ) in the present invention has been described based on the embodiments, however, the present invention is not limited to these embodiments. As long as not departing from the spirit of the present invention, modified embodiments obtained by making various modifications, which occur to those skilled in the art, to the present embodiment, and the embodiments that are constructed by combining the components of different embodiments are also included in the scope of the present invention. 
     It should be understood that the embodiments disclosed herein are for illustrative purposes in every point rather than limiting purposes. It is contemplated that the scope of the present invention is defined by the CLAIMS rather than the above description, and includes all modifications within the meaning and the range of equivalency of the CLAIMS. 
     INDUSTRIAL APPLICABILITY 
     The present invention can be utilized as an antenna which prevents an occurrence of a location on the orthogonal planes in the three-dimensional space, where the electric field strength is significantly reduced. 
     REFERENCE SIGNS LIST 
     
         
           20  Radio IC 
           200 ,  200 A,  200 B,  200 C,  200 D,  200 E,  200 F,  200 G,  200 H,  200 J Antenna 
           201 ,  201 C,  201 E,  201 F,  201 G,  201 H Three-dimensional linear conductor 
           210 ,  220 ,  220 F,  220 G,  230 ,  240 ,  250 ,  260 ,  270  Linear conductor 
           300  Matching circuit 
           1000  Radio communication device 
         C 22  Loading capacitor 
         L 10  Power feed line 
         L 22  Loading coil 
         M 20  Planar conductor 
         P 10  Power feed region 
         PT 10  Power feed point 
         SB 20  Substrate 
         SL 22  Slit