Patent Publication Number: US-9905919-B2

Title: Antenna, antenna device, and wireless device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a continuation application filed under 35 U.S.C. 111(a) claiming benefit under 35 U.S.C. 120 and 365(c) of PCT International Application No. PCT/JP2014/066334 filed on Jun. 19, 2014 and designating the U.S., which claims priority to Japanese Patent Application No. 2013-131195 filed on Jun. 21, 2013. The entire contents of the foregoing applications are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to an antenna, an antenna device, and a wireless device such as a mobile phone. 
     2. Description of the Related Art 
     Techniques are known for controlling the directivity of an antenna by switching a connection destination of a feeding point. For example, Japanese Laid-Open Patent Publication No. 2012-186562 (Patent Document 1) discloses an antenna including a switch for switching the directivity of a radiating conductor by controlling a feeding point to come into contact with either one of two end points of the radiating conductor. 
     On the other hand, Japanese Patent No. 4422767 (Patent Document 2) discloses an antenna that is operable in multiple frequency bands by having a feeding element and a parasitic element that are coupled without being in contact. 
     However, it has been difficult to control the directivity of an antenna that has a contactless feeding system as disclosed in Patent Document 2 where a feeding element connected to a feeding point is coupled to a radiating element (parasitic element) without being in contact. For example, the switching technique disclosed in Patent Document 1 that involves controlling the directivity of an antenna by switching the connection point of a feeding element to a radiating element cannot be implemented in the antenna disclosed in Patent Document 2 because the feeding element and the radiating element have to be coupled without being in contact. Also, various constrains may be imposed on the arrangement and shape of the feeding element in order to prevent coupling at unintended locations when a connection point is switched, for example. 
     In view of the above, there is a demand for a technique for controlling the directivity of an antenna having a feeding element and a radiating element that are coupled without being in contact. 
     SUMMARY OF THE INVENTION 
     An aspect of the present invention relates to implementing a technique for controlling the directivity of an antenna, an antenna device, and a wireless device having a feeding element and a radiating element that are coupled without being in contact. 
     According to one aspect of the present invention, an antenna, an antenna device, and a wireless device are provided that include a feeding element that is connected to a feeding point; a first radiating element that is spaced apart from the feeding element and is fed by being coupled to the feeding element through electromagnetic field coupling to function as a radiating conductor; a second radiating element that is spaced apart from the feeding element and is fed by being coupled to the feeding element through electromagnetic field coupling to function as a radiating conductor; a first control element that is connected to the feeding element via a first impedance variable unit and is arranged such that when an impedance of the first impedance variable unit at a resonant frequency of the first radiating element is decreased, the electromagnetic field coupling between the feeding element and the first radiating element is weakened and the function of the first radiating element as the radiating conductor is degraded; a second control element that is connected to the feeding element via a second impedance variable unit and is arranged such that when an impedance of the second impedance variable unit at a resonant frequency of the second radiating element is decreased, the electromagnetic field coupling between the feeding element and the second radiating element is weakened and the function of the second radiating element as the radiating conductor is degraded; and a control unit that controls the first impedance variable unit to adjust the connection between the feeding element and the first control element, and controls the second impedance variable unit to adjust the connection between the feeding element and the second control element. 
     According to another aspect of the present invention, an antenna, an antenna device, and a wireless device are provided that include a feeding element that is connected to a feeding point; a first radiating element that is spaced apart from the feeding element and is fed by being coupled to the feeding element through electromagnetic field coupling to function as a radiating element; a second radiating element that is spaced apart from the feeding element and is fed by being coupled to the feeding element through electromagnetic field coupling to function as a radiating element; a first control element that is connected to the feeding element via a first impedance variable unit; a second control element that is connected to the feeding element via a second impedance variable unit; and a control unit that controls the first impedance variable unit to adjust the connection between the feeding element and the first control element, and controls the second impedance variable unit to adjust the connection between the feeding element and the second control element. The first control element is arranged such that a high impedance portion of the first control element having a high impedance at a resonant frequency of the first radiating element and a low impedance portion of the first radiating element having a low impedance at the resonant frequency of the first radiating element are positioned close to each other, and the second control element is arranged such that a high impedance portion of the second control element having a high impedance at a resonant frequency of the second radiating element and a low impedance portion of the second radiating element having a low impedance at the resonant frequency of the second radiating element are positioned close to each other. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an exemplary analysis model of an antenna according to a first embodiment of the present invention; 
         FIG. 2  is a diagram schematically illustrating an exemplary positional relationship between components of the antenna according to the first embodiment; 
         FIG. 3  is a diagram illustrating an exemplary configuration of an impedance control unit of the antenna according to the first embodiment; 
         FIG. 4  is a diagram illustrating a directivity of the antenna according to the first embodiment in one exemplary case; 
         FIG. 5  is a diagram illustrating a directivity of the antenna according to the first embodiment in another exemplary case; 
         FIG. 6  is a graph indicating S 11  measurements for illustrating an effect of a matching circuit of the antenna according to the first embodiment; 
         FIG. 7  is a perspective view of an exemplary analysis model of an antenna device including a plurality of antennas according to a second embodiment of the present invention; 
         FIG. 8  is a perspective view of an exemplary analysis model of an antenna according to a third embodiment of the present invention; 
         FIG. 9  is a diagram schematically illustrating an exemplary positional relationship between components of the antenna according to the third embodiment; 
         FIG. 10  is a diagram illustrating a directivity of the antenna according to the third embodiment in one exemplary case; 
         FIG. 11  is a diagram illustrating a directivity of the antenna according to the third embodiment in another exemplary case; 
         FIG. 12  is a graph indicating S 11  measurements for illustrating an effect of a matching circuit of the antenna according to the third embodiment; 
         FIG. 13  is a perspective view of an exemplary analysis model of an antenna device including a plurality of antennas according to a fourth embodiment of the present invention; 
         FIG. 14  is a graph indicating S 11  and S 22  measurements, and a correlation coefficient of the antenna device according to the fourth embodiment in one exemplary case; 
         FIG. 15  is a graph indicating S 11  and S 22  measurements, and a correlation coefficient of the antenna device according to the fourth embodiment in another exemplary case; 
         FIG. 16  is a graph indicating S 11  and S 22  measurements, and a correlation coefficient of the antenna device according to the fourth embodiment in another exemplary case; 
         FIG. 17  is a graph indicating S 11  and S 22  measurements, and a correlation coefficient of the antenna device according to the fourth embodiment in another exemplary case; 
         FIG. 18  is a diagram illustrating an exemplary directivity of a first antenna of the antenna device according to the fourth embodiment in one exemplary case; 
         FIG. 19  is a diagram illustrating a directivity of a second antenna of the antenna device according to the fourth embodiment in one exemplary case; 
         FIG. 20  is a diagram illustrating a directivity of the first antenna of the antenna device according to the fourth embodiment in another exemplary case; 
         FIG. 21  is a diagram illustrating a directivity of the second antenna of the antenna device according to the fourth embodiment in another exemplary case; 
         FIG. 22  is a diagram illustrating a directivity of the first antenna of the antenna device according to the fourth embodiment in another exemplary case; 
         FIG. 23  is a diagram illustrating a directivity of the second antenna of the antenna device according to the fourth embodiment in another exemplary case; 
         FIG. 24  is a diagram illustrating a directivity of the first antenna of the antenna device according to the fourth embodiment in another exemplary case; 
         FIG. 25  is a diagram illustrating a directivity of the second antenna of the antenna device according to the fourth embodiment in another exemplary case; 
         FIG. 26  is a perspective view of an exemplary analysis model of an antenna according to a fifth embodiment of the present invention; 
         FIG. 27  is a graph indicating S 11  measurements of the antenna according to the fifth embodiment; 
         FIG. 28  is a perspective view of an exemplary analysis model of an antenna according to a sixth embodiment of the present invention; 
         FIG. 29  is a diagram illustrating an exemplary configuration of an impedance control unit of the antenna according to the sixth embodiment; 
         FIG. 30  is a graph illustrating a continuous change in directivity; 
         FIG. 31  is a plan view schematically illustrating an antenna device according to a seventh embodiment of the present invention; and 
         FIG. 32  is a plan view schematically illustrating an antenna device according to an eighth embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, embodiments of the present invention will be described with reference to the accompanying drawings. 
     &lt;Antenna  1 &gt; 
       FIG. 1  is a perspective view of an exemplary computer simulation model for analyzing the operation of an antenna  1  according to a first embodiment of the present invention. Note that in the present example, CST Microwave Studio (registered trademark) by Computer Simulation Technology AG (CST) was used as an electromagnetic field simulator. 
     The antenna  1  includes a feeding point  11 , a ground plane  70 , a feeding element  20 , a first radiating element  30 , a second radiating element  40 , a first feeding portion  35 , a second feeding portion  45 , a first control element  50 , a second control element  60 , and an impedance control unit  120 . Note that in the following descriptions, the first radiating element  30 , the second radiating element  40 , the first feeding portion  35 , the second feeding portion  45 , the first control element  50 , and the second control element  60  may simply be referred to as “radiating element  30 ,” “radiating element  40 ,” “feeding portion  35 ,” “feeding portion  45 ,” “control element  50 ,” and “control element  60 ,” respectively. Note that the feeding portion  35  is a feeding portion for feeding the radiating element  30 , and the feeding portion  45  is a feeding portion for feeding the radiating element  40 . That is, the feeding portions  35  and  45  do not constitute feeding portions for the antenna  1 . The feeding point  11  constitutes the feeding portion for feeding the antenna  1 . 
     The feeding point  11  is a feeding portion that is connected to a predetermined transmission line or a feeding line that uses the ground plane  70 . Specific examples of a predetermined transmission line includes a microstrip line, a strip line, a coplanar waveguide with a ground plane (coplanar waveguide having a ground plane arranged on a surface opposing a conductor surface), and the like. Specific examples of a feeding line include a feeder line, a coaxial cable, and the like. The feeding point  11  may be arranged at a central portion of an outer edge  71  of the ground plane  70 . 
     The feeding element  20  is a conductor that is connected to the feeding point  11  that uses the ground plane  70  as a ground reference. The feeding element  20  may be connected to a feeding circuit that is mounted on a substrate  80  (e.g., an integrated circuit such as an IC chip, which is not shown) via the feeding point  11 , for example. The feeding element  20  and the feeding circuit may be interconnected via one or more of the various types of transmission lines and feeding lines described above. 
     The feeding element  20  is a conductor that is arranged a predetermined distance apart from the radiating element  30  and the radiating element  40 . For example, the feeding element  20  may be spaced apart from the radiating element  30  and the radiating element  40  by a gap having a parallel direction component in the Z-axis. 
     In the example illustrated in  FIG. 1 , the feeding element  20  overlaps with the radiating element  30  and the radiating element  40  in plan view from a direction parallel to the Z-axis. Note, however, that the feeding element  20  does not necessarily have to overlap with the radiating element  30  and the radiating element  40  in plan view from the direction parallel to the Z-axis as long as the feeding element  20  is adequately spaced apart from the radiating element  30  and the radiating element  40  to be capable of performing noncontact feeding of the radiating element  30  and the radiating element  40 . For example, the feeding element  20 , the radiating element  30 , and the radiating element  40  may overlap in plan view from any direction including a direction parallel to the X-axis or the Y-axis. 
     The feeding element  20  is capable of feeding the radiating element  30  via the feeding portion  35  of the radiating element  30  without being in contact with the radiating element  30 . Also, the feeding element  20  is capable of feeding the radiating element  40  via the feeding portion  45  of the radiating element  40  without being in contact with the radiating element  40 . For example, the feeding element  20  may be a linear conductor having at least a portion of the feeding element  20  and the ground plane  70  arranged to not overlap in plan view from a direction normal to the ground plane  70 . Note that a direction normal to the ground plane  70  corresponds to a direction parallel to the Z-axis in  FIG. 1 . 
     The feeding element  20  may be a linear conductor having a linear conductor portion extending from the feeding point  11 , as a starting point, to an end portion  21 , in a direction away from the outer edge  71  of the ground plane  70 , which is parallel to the XY plane. The end portion  21  is a portion located at the tip of the feeding element  20  in the direction away from the outer edge  71 . In  FIG. 1 , the feeding element  20  extends in a direction parallel to the ground plane  70  and perpendicular to the outer edge  71 . Note that a direction parallel to the ground plane  70  and perpendicular to the outer edge  71  corresponds to a direction parallel to the Y-axis in  FIG. 1 . 
     Note that although  FIG. 1  illustrates an exemplary case where a matching circuit  90  is arranged at the feeding element  20 , the matching circuit  90  may be omitted in other examples. Note that the matching circuit  90  will be described in greater detail below. 
     The feeding element  20  extends from the feeding point  11  to the end portion  21  in a direction toward a gap  130  formed between one end portion  33  of the radiating element  30  and one end portion  43  of the radiating element  40  in plan view from a direction normal to the ground plane  70 . The feeding element  20  includes the end portion  21 , which is spaced apart by a predetermined distance from the end portion  33  of the radiating element  30  and the end portion  43  of the radiating element  40 , and the end portion  21  is positioned in the vicinity of the gap  130 . 
     Note that although  FIG. 1  illustrates an exemplary case where the feeding element  20  corresponds to a T-shaped conductor element arranged within an XY plane, the feeding element  20  may be in other shapes such as an L-shape or an I-shape, for example. Also, the feeding element  20  may include a conductor portion extending in the XY plane and a conductor portion extending in a plane other than the XY plane, for example. 
     The radiating element  30  is a radiating conductor that includes the end portion  33  and another end portion  34 , and extends linearly from the end portion  33  to the other end portion  34 . Note that the end portion  33  and the end portion  34  are open ends that are not connected to another conductor. For example, the radiating element  30  may be a linear conductor having at least a portion of the radiating element  30  and the ground plane  70  arranged to not overlap in plan view from a direction normal to the ground plane  70 . 
     For example, the radiating element  30  may be a linear conductor having a linear radiating conductor portion arranged along the outer edge  71  of the ground plane  70 . The radiating element  30  may include a conductor portion  31  that is spaced apart from the outer edge  71  by a predetermined shortest distance and extends in a direction parallel to the outer edge  71  facing the outer edge  71  of the ground plane  70 , for example. Note that a direction parallel to the outer edge  71  corresponds to a direction parallel to the X-axis in  FIG. 1 . By having the radiating element  30  include the conductive portion  31  extending along the outer edge  71 , the directivity of the antenna  1  may be easily controlled, for example. 
     Note that although  FIG. 1  illustrates an exemplary case where the radiating element  30  corresponds to a linear radiating element arranged in the XY plane, the radiating element  30  may be in other shapes such as an L-shape, for example (see  FIG. 28  and descriptions below). Also, the radiating element  30  may include a conductor portion extending in the XY plane and a conductor portion extending in a plane other than the XY plane, for example. 
     The radiating element  40  may have a configuration identical or similar to the configuration of the radiating element  30  as described above. As such, detailed descriptions thereof are omitted. The radiating element  40  is an antenna conductor that includes one end portion  43  and another end portion  44 , and extends linearly from the end portion  43  to the other end portion  44 . The radiating element  40  may include a conductor portion  41  that is spaced apart from the outer edge  71  by a predetermined shortest distance and extends in a direction parallel to the outer edge  71  facing the outer edge  71  of the ground plane  70 , for example. 
     The radiating element  30  and the radiating element  40  are conductors that extend in different directions from each other. That is, the radiating element  30  and the radiating element  40  extend in directions away from each other from the feeding element  20 . Note that although  FIG. 1  illustrates an example where the radiating element  30  and the radiating element  40  correspond to conductors arranged on the same XY plane, the radiating element  30  and the radiating element  40  may alternatively be arranged in different planes, for example. Also, although  FIG. 1  illustrates an example where the radiating element  30  and the radiating element  40  are arranged along a single straight line, the radiating element  30  and the radiating element  40  may alternatively be arranged along different straight lines. For example, in plan view from a direction parallel to the Z-axis in  FIG. 1 , one of the radiating element  30  and the radiating element  40  may be arranged closer to the ground plane  70  and the other may be arranged farther away from the ground plane  70  with respect to the end portion  21  of the feeding element  20 . 
     The control element  50  is a conductor that is spaced apart by a predetermined distance from the radiating element  30 . For example, the control element  50  may be spaced apart from the radiating elements  30  by a gap having a parallel direction component in the Z-axis. The control element  50  is connected to the end portion  21  of the feeding element  20  via an impedance control unit  120  and extends linearly from the impedance control unit  120  to an end portion  51 . The end portion  51  is an open end that is not connected to another conductor. For example, the control element  50  may be a linear conductor having at least a portion of the control element  50  and the ground plane  70  arranged so as not to overlap in plan view from a direction normal to the ground plane  70 . 
     For example, the control element  50  may be a linear conductor having a linear conductor portion arranged along the radiating element  30 . Note that although  FIG. 1  illustrates an example where the control element  50  is a linear element arranged in the XY plane, the control element  50  may be in other shapes such as an L-shape (see  FIG. 28  and descriptions below). Also, the control element  50  may include a conductor portion extending in the XY plane and a conductor portion extending in a plane other than the XY plane, for example. 
     The control element  60  is a conductor that is spaced apart by a predetermined distance from the radiating element  40 . The control element  60  may have a configuration identical or similar to the configuration of the control element  50  as described above. As such, detailed descriptions thereof are omitted. The control element  60  is connected to the end portion  21  of the feeding element  20  via the impedance control unit  120  and extends linearly from the impedance control unit  120  to an end portion  61 . 
     Note that although  FIG. 1  illustrates an example where the control element  50  and the control element  60  correspond to conductors arranged within the same XY plane, the control element  50  and the control element  60  may alternatively be arranged in different planes, for example. Also, although  FIG. 1  illustrates an example where the control element  50  and the control element  60  are arranged along a single straight line, the control element  50  and the control element  60  may alternatively be arranged along different straight lines, for example. Also, although  FIG. 1  illustrates an example where the control element  50  and the control element  60  are arranged on the same XY plane together with the feeding element  20 , the control element  50  and the control element  60  may alternatively be arranged in a plane different from that on which the feeding element  20  is arranged. 
       FIG. 2  schematically illustrates a positional relationship between the components of the antenna  1  with respect to the Z-axis direction. An antenna according to an embodiment of the present invention may be installed in a wireless device (e.g., portable communication terminal). Specific examples of the wireless device include various types of electronic devices such as information terminals, mobile phones, smart phones, personal computers, game consoles, TVs, music/video players, etc. 
     For example, in  FIG. 2 , in a case where the antenna  1  is installed in a wireless communication device  100  including a display (as one example of the wireless device), a cover glass covering the entire image display surface of the display may be provided as a substrate  110 , for example, or the substrate  110  may be a housing (top cover, back cover, side wall, etc.) to which the substrate  80  is fixed, for example. The cover glass may be a flat member arranged on top of the display and may be a transparent or semi-transparent dielectric substrate that allows a user to view an image displayed by the display. 
     In a case where the radiating elements  30  and  40  are arranged on the surface of the cover glass, the radiating elements  30  and  40  may be formed by applying a conductive paste such as copper or silver on the surface of the cover glass and performing a firing process thereon, for example. Note that the conductive paste used in this case is preferably a type that can be fired at a sufficiently low temperature so as to not affect the reinforced properties of the chemically reinforced glass used for the cover glass. Also, a plating process may be performed in order to prevent deterioration of the conductors due to oxidation, for example. Also, a decorative printing process may be performed on the cover glass and the conductors may be formed on the decorative printed portions. Also, in a case where a black concealing layer is arranged at the peripheral edge of the cover glass for the purpose of concealing wiring and the like, the radiating elements  30  and  40  may be formed on the black concealing layer, for example. 
     Also, the positions of the feeding element  20 , the radiating elements  30  and  40 , the control elements  50  and  60 , and the ground plane  70  with respect to a height direction parallel to the Z-axis may be different from one another, partially the same, or all the same. 
     Also, in some examples, one feeding element may be configured to feed a plurality of radiating elements. By utilizing a plurality of radiating elements, multi-band operation, wide-band operation, and/or directivity control may be facilitated, for example. Also, in some examples, a plurality of antennas may be installed in one wireless communication device. 
     Note that although  FIG. 2  illustrates an example where the feeding element  20  and the control elements  50  and  60  are arranged on the surface of the substrate  80 , these elements may alternatively be arranged within the substrate  80 , for example. 
     In one example, a chip component including the feeding element  20  and a medium in contact with the feeding element  20  may be mounted to the substrate  80 . In this way, the feeding element  20  that is in contact with the medium may be easily mounted to the substrate  80 . 
     The substrate  80  may be made from a dielectric material, a magnetic material, or a combination of dielectric and magnetic materials. Specific examples of dielectric materials include resin, glass, glass ceramics, LTCC (Low Temperature Co-Fired Ceramics), alumina, and the like. Specific examples of a combination of dielectric and magnetic materials include hexagonal crystal system ferrites, spinel ferrites (Mn—Zn ferrites, Ni—Zn ferrites, etc.), garnet ferrites, permalloy, Sendust (registered trademark), and other materials including a transition metal element such as Fe, Ni, or Co, and a metal or an oxide including a rare earth element such as Sm or Nd, for example. 
     The substrate  80  includes the ground plane  70 , and the feeding point  11  that uses the ground plane  70  as a ground reference. Note that although  FIG. 2  illustrates an example where the ground plane  70  is formed on a surface layer of the substrate  80 , the ground plane  70  may alternatively be formed on an inner layer of the substrate  80 , for example. 
     The substrate  80  includes a transmission line having a strip conductor  82  that is connected to the feeding point  11 . The strip conductor  82  may be a signal line formed on the surface of the substrate  80  such that the substrate  80  may be interposed between the strip conductor  82  and the ground plane  70 , for example. 
     The radiating elements  30  and  40  are positioned apart from the feeding element  20  and the control elements  50  and  60 . For example, as illustrated in  FIG. 2 , the radiating elements  30  and  40  may be arranged on the substrate  110  that faces the substrate  80  and is spaced apart from the substrate  80  by a distance H 2 . The substrate  110  may be made from a dielectric material, a magnetic material, or a combination of dielectric and magnetic materials. Specific examples of the material of the substrate  110  may be the same as those of the substrate  80  as described above. Note that in  FIG. 2 , the radiating elements  30  and  40  are arranged on a surface of the substrate  110  facing the feeding element  20  and the control elements  50  and  60 . However, the radiating elements  30  and  40  may alternatively be arranged on the surface of the substrate  110  on the opposite side of the surface facing the feeding element  20  and the control elements  50  and  60 . The radiating elements  30  and  40  may also be arranged at a side face of the substrate  110 , for example. 
     The feeding element  20  and the radiating elements  30  and  40  may be spaced apart from each other by a distance that enables electromagnetic field coupling between the feeding element  20  and the radiating elements  30  and  40 , for example. By coupling the feeding element  20  and the radiating element  30  through electromagnetic field coupling, noncontact feeding of the radiating element  30  at the feeding portion  35  via the feeding element  20  may be realized. By feeding the radiating element  30  in this manner, the radiating element  30  may function as a radiating conductor of the antenna  1 . In the case where the radiating element  30  is a linear conductor connecting two points as illustrated in  FIG. 1 , a resonance current (standing wave current distribution) similar to that formed on a half wave dipole antenna may be formed on the radiating element  30 . That is, the radiating element  30  may function as a dipole antenna that resonates at half the wavelength of a predetermined frequency (hereinafter referred to as “dipole mode”). In another example, the radiating element may be a looped conductor. In a case where the radiating element is a looped conductor, a resonant current (standing wave current distribution) similar to that formed on a loop antenna may be formed on the radiating element. That is, the radiating element may function as a loop antenna that resonates at one wavelength of a predetermined frequency (hereinafter referred to as “loop mode”). Note that electromagnetic field coupling may be similarly established between the radiating element  40  and the feeding element  20  to enable noncontact feeding of the radiating element  40  at the feeding portion  45  via the feeding element  20 . However, detailed descriptions thereof are omitted because they may be substantially the same as the above descriptions relating to the radiating element  30 . 
     Electromagnetic field coupling refers to coupling that utilizes a resonance phenomenon of an electromagnetic field as disclosed, for example, in the following non-patent literature: A. Kurs et. al., “Wireless Power Transfer via Strongly Coupled Magnetic Resonances,” Science Express, Vol. 317, No. 5834, pp. 83-86, July 2007. Electromagnetic field coupling, also referred to as “electromagnetic field resonance coupling” or “electromagnetic field resonant coupling,” is a technique in which resonators that resonate at the same frequency are brought close to each other, one of the resonators is caused to resonate to generate a near field (non-radiation field area) between the resonators, and energy is transmitted to another one of the resonators via coupling at the near field. Also, electromagnetic field coupling refers to coupling via an electric field and a magnetic field at a high frequency excluding electrostatic capacitive coupling and electromagnetic induction coupling. Here, “excluding electrostatic capacitive coupling and electromagnetic induction coupling” does not necessarily mean electrostatic capacitive coupling and electromagnetic induction coupling are completely eliminated, but indicates that their influence is negligible. A medium between the feeding element  20  and the radiating elements  30  and  40  may be air or a dielectric material such as glass or resin. It is preferable to not place a conductor material such as a ground plane or a display between the feeding element  20  and the radiating elements  30  and  40 . 
     By coupling the feeding element  20  and the radiating elements  30  and  40  through electromagnetic field coupling, a durable structure that is resistant to impact may be obtained. That is, by utilizing electromagnetic field coupling, feeding of the radiating elements  30  and  40  may be implemented using the feeding element  20  without requiring physical contact between the feeding element  20  and the radiating elements  30  and  40 , and thus, a durable structure that is resistant to impact may be obtained as compared to a contact type feeding mechanism that requires physical contact between the feeding element and the radiating element. 
     Also, as compared with feeding using electrostatic capacitive coupling, when feeding using electromagnetic field coupling is implemented, the total efficiency (antenna gain) of the radiating elements  30  and  40  may be less likely to decrease even if the distance between the feeding element  20  and the radiating elements  30  and  40  (coupling distance) is increased. Note that the total efficiency is calculated based on the radiation efficiency×return loss of the antenna, and the total efficiency is defined as the efficiency of the antenna with respect to the input power. Therefore, by coupling the feeding element  20  and the radiating elements  30  and  40  through electromagnetic field coupling, a greater degree of freedom for determining the arrangement positions of the feeding element  20  and the radiating elements  30  and  40  may be obtained and position robustness may be increased. Note that when high position robustness is achieved, this means that the total efficiency of the radiating elements  30  and  40  may be less likely to be affected even when variations occur in the arrangement positions of the feeding element  20  and the radiating elements  30  and  40 . Also, by obtaining a greater degree of freedom for determining the arrangement positions of the feeding element  20  and the radiating elements  30  and  40 , the space required for installing the antenna  1  may be easily reduced. Also, by feeding the radiating elements  30  and  40  through electromagnetic field coupling as opposed to feeding through electrostatic capacitive coupling, for example, feeding of the radiating elements  30  and  40  may be performed via the feeding element  20  without the use of other components such as a capacitance plate, and as such, feeding may be realized through a simple structure. 
     Also, in  FIG. 1 , the feeding portion  35 , which corresponds to a part of the radiating element  30  that is fed by the feeding element  20 , is positioned at a region between the end portion  33  and the other end portion  34  of the radiating element  30  other than a central portion  32  (region between the central portion  32  and the end portion  33  or the end portion  34 ). By positioning the feeding portion  35  at a region of the radiating element  30  other than the region having the lowest impedance at the resonant frequency of the fundamental mode of the radiating element  30  (the central portion  32  in the present case), impedance matching of the antenna  1  may be facilitated. The feeding portion  35  is defined by a region at a conductor portion of the radiating element  30  (conductor portion of the radiating element  30  that is closest to the feeding element  20 ) that is closest to the feeding point  11 . 
     The impedance of the radiating element  30 , when in dipole mode, becomes higher as the distance from the central portion  32  toward the end portion  33  or the end portion  34  of radiating element  30  increases. In the case of coupling at high impedance by electromagnetic field coupling, even when slight variations occur in the impedance between the feeding element  20  and the radiating element  30 , its impact on impedance matching may be relatively small as long as the feeding element  20  and the radiating element  30  are coupled at a sufficiently high impedance of at least a certain level. Thus, to facilitate matching, the feeding portion  35  of the radiating element  30  is preferably positioned at a high impedance portion of the radiating element  30 . 
     For example, to facilitate impedance matching of the antenna  1 , the feeding portion  35  may be positioned at a region spaced apart from the region having the lowest impedance at the resonant frequency of the fundamental mode of the radiating element  30  (the central portion  32  in the present case) by a distance greater than or equal to ⅛ of the total length of the radiating element  30  (preferably greater than or equal to ⅙ of the total length, and more preferably greater than or equal to ¼ of the total length). In  FIG. 1 , the total length of the radiating element  30  is the same as a total length L 15  of the radiating element  40 , and the feeding portion  35  is positioned away from the central portion  32  toward the end portion  33 . 
     The feeding portion  45  corresponds to a part of the radiating element  40  at which feeding of the radiating element  40  is implemented. Note that because features of the feeding portion  45  may be substantially identical to those of the feeding portion  35 , detailed descriptions thereof will be omitted. Note that in the case where the fundamental mode of resonance of the radiating elements corresponds to the loop mode, the feeding portion may be positioned at a region spaced apart from the region having the highest impedance at the resonant frequency of the fundamental mode of the radiating element by a distance less than or equal to 3/16 of the inner circumference of the loop (preferably less than or equal to ⅛ of the inner circumference, and more preferably less than or equal to 1/16 of the inner circumference). 
     Also, assuming Le 20  denotes the electrical length that imparts the fundamental mode of resonance to the feeding element  20 , Le 30  and Le 40  respectively denote the electrical lengths that impart the fundamental mode of resonance to the radiating elements  30  and  40 , and λ denotes a wavelength on the feeding element  20  or the radiating element  30  or  40  at a resonant frequency f 1  of the fundamental mode of the radiating elements  30  and  40 , Le 20  is preferably less than or equal to (⅜)λ, Le 30  and Le 40  are preferably greater than or equal to (⅜)λ and less than or equal to (⅝)λ in the case where the fundamental mode of resonance of the radiating elements  30  and  40  corresponds to the dipole mode, and Le 30  and Le 40  are preferably greater than or equal to (⅞)λ and less than or equal to ( 9/8)λ in the case where the fundamental mode of resonance of the radiating elements  30  and  40  corresponds to the loop mode. 
     The electrical length Le 20  is preferably less than or equal to (⅜)λ. Also, in order to allow a greater degree of freedom in the configuration including the presence/absence of the ground plane  70 , the electrical length Le 20  may preferably be greater than or equal to (⅛)λ and less than or equal to (⅜)λ, and more preferably greater than or equal to ( 3/16)λ and less than or equal to ( 5/16)λ. By arranging the electrical length Le 20  to be within the above ranges, resonance of the feeding element  20  may occur at the design frequency (resonant frequency f 1 ) of the radiating elements  30  and  40 , and in this way, the feeding element  20  and the radiating elements  30  and  40  may resonate without depending on the ground plane  70  of the antenna  1  and desirable electromagnetic field coupling may be achieved. 
     Also, when the ground plane  70  is formed such that the outer edge  71  extends along the radiating elements  30  and  40 , a resonance current (standing wave current distribution) may be formed on the feeding element  20  and the ground plane  70  as a result of an interaction between the feeding element  20  and the outer edge  71 , and the feeding element  20  may resonate and be coupled with the radiating elements  30  and  40  through electromagnetic field coupling. For this reason, there is no specific lower limit for the electrical length Le 20  of the feeding element  20  as long as the feeding element  20  has a physical length that is sufficient to be coupled to the radiating elements  30  and  40  by electromagnetic field coupling. 
     Note that when electromagnetic field coupling is achieved this means that impedance matching is achieved. Also, in this case, the electrical length Le 20  of the feeding element  20  does not have to be designed to a suitable electrical length according to the resonant frequency of the radiating elements  30  and  40 , and the feeding element  20  may be freely designed as a radiating conductor. In this way, the antenna  1  may be easily designed to support multiple frequencies. Note that the sum of the length of the outer edge  71  of the ground plane  70  extending along the radiating elements  30  and  40  and the electrical length of the feeding element  20  is preferably greater than or equal to (¼)λ of the design frequency (resonant frequency f 1 ). 
     When the feeding element  20  does not include a component such as a matching circuit, a physical length L 20  of the feeding element  20  (L 14  in the case of  FIG. 1 ) is determined by λ g1 =Δ 0 k 1 , where λ 0  denotes the radio wave wavelength in vacuum at the resonant frequency of the fundamental mode of the radiating elements  30  and  40 , and k 1  denotes a shortening coefficient of a wavelength shortening effect in an actual environment. Here, k 1  is calculated based on, for example, a relative permittivity and a relative permeability of a medium (environment) such as an effective relative permittivity (∈ r1 ) and an effective relative permeability (μ r1 ) of the dielectric substrate at which the feeding element is arranged, a thickness of the medium (environment), and a resonant frequency. That is, L 20  is less than or equal to (⅜)λ g1 . The physical length L 20  of the feeding element  20  is a physical length that gives Le 20 . In an ideal case where no other factor is considered, the physical length L 20  is equal to Le 20 . When the feeding element  20  includes a matching circuit, for example, L 20  is preferably greater than zero and less than or equal to Le 20 . By using a matching circuit such as an inductor, L 20  can be reduced (i.e., the size of the feeding element  20  can be reduced). 
     In the case where the fundamental mode of resonance of the radiating elements  30  and  40  corresponds to the dipole mode (i.e., when the radiating elements  30  and  40  are linear conductors having open ends), Le 30  and Le  40  are preferably greater than or equal to (⅜)λ and less than or equal to (⅝)λ, more preferably greater than or equal to ( 7/16)λ and less than or equal to ( 9/16)λ, and more preferably greater than or equal to ( 15/32)λ and less than or equal to ( 17/32)λ. When a higher-order mode is taken into account, Le 30  and Le 40  are preferably greater than or equal to (⅜)λm and less than or equal to (⅝)λm, more preferably greater than or equal to ( 7/16)λm and less than or equal to ( 9/16)λm, and more preferably greater than or equal to ( 15/32)λm and less than or equal to ( 17/32)λm. Here, m denotes a mode number of a higher-order mode and is represented by a natural number. The value of m is preferably an integer between 1 through 5, and more preferably an integer between 1 through 3. In this case, m=1 represents the fundamental mode. When Le 30  and Le 40  are within the above ranges, the radiating elements  30  and  40  may function sufficiently as radiating conductors, and the efficiency of the antenna  1  may be desirably high. 
     Also, in the case where the fundamental mode of resonance of the radiating elements  30  and  40  corresponds to the loop mode (i.e., when the radiating elements  30  and  40  are looped conductors), Le 30  and Le  40  are preferably greater than or equal to (⅞)λ and less than or equal to ( 9/8)λ, more preferably greater than or equal to ( 15/16)λ and less than or equal to ( 17/16)λ, and more preferably greater than or equal to ( 31/32)λ and less than or equal to ( 33/32)λ. When a higher-order mode is taken into account, Le 30  and Le 40  are preferably greater than or equal to (⅞)λm and less than or equal to ( 9/8)λm, more preferably greater than or equal to ( 15/16)λm and less than or equal to ( 17/16)λm, and more preferably greater than or equal to ( 31/32)λm and less than or equal to ( 33/32)λm. 
     Note that physical lengths L 30  and L 40  of the radiating elements  30  and  40  (corresponding to length L 15  in the case of  FIG. 1 ) are determined by λ g2 =λ 0 k 2 , where λ 0  denotes the radio wave wavelength in vacuum at the resonant frequency of the fundamental mode of the radiating elements  30  and  40 , and k 2  denotes a shortening coefficient of a wavelength shortening effect in an actual environment. Here, k 2  is calculated based on, for example, a relative permittivity and a relative permeability such as an effective relative permittivity (∈ r2 ) and an effective relative permeability (μ r2 ) of a medium (environment) such as a dielectric substrate at which the radiating elements  30  and  40  are arranged, a thickness of the medium (environment), and a resonant frequency. That is, in the case where the fundamental mode of resonance of the radiating elements  30  and  40  is the dipole mode, L 30  and L 40  are greater than or equal to (⅜)λ g2  and less than or equal to (⅝)λ g2 , and in the case where the fundamental mode of resonance of the radiating elements  30  and  40  corresponds to the loop mode, L 30  and L 40  are greater than or equal to (⅞)λ g2  and less than or equal to ( 9/8)λ g2 . The physical lengths L 30  and L 40  of the radiating elements  30  and  40  are physical lengths that give Le 30  and Le 40 . In an ideal case where no other factors are considered, the physical lengths L 30  and L 40  are equal to Le 30  and Le 40 . Even when L 30  and L 40  are shortened by using a matching circuit such as an inductor, for example, L 30  and L 40  are preferably within a range greater than zero and less than or equal to Le 30  and Le 40 , and more preferably greater than or equal to 0.4×Le 30  and 0.4×Le 40  and less than or equal to Le 30  and Le 40 . 
     Also, in the case where the interaction between the feeding element  20  and the outer edge  71  of the ground plane  70  is utilized as illustrated in  FIG. 1 , the feeding element  20  may function as a radiating conductor as described above. By having the feeding element  20  implement noncontact feeding of the radiating elements  30  and  40  at their feeding portions  35  and  45  through electromagnetic field coupling, the radiating elements  30  and  40  may be radiating conductors that function as λ/2 dipole antennas, for example. Note that while the feeding element  20  is a linear conductor that is capable of feeding the radiating elements  30  and  40 , the feeding element  20  may also be capable of functioning as a monopole antenna (e.g., λ/4 monopole antenna) that is fed at the feeding point  11 , for example. By setting the resonant frequency of the radiating elements  30  and  40  to f 1 , setting the resonant frequency of the feeding element  20  to f 2 , and adjusting the length of the feeding element  20  to realize a monopole antenna that resonates at the frequency f 2 , the radiating function of the feeding element  20  may be utilized and the antenna  1  may be configured to support multiple frequencies with relative ease. 
     The physical length L 20  of the feeding element  20  when utilizing the radiating function of the feeding element  20 , assuming the feeding element  20  does not include a component such as a matching circuit, is determined by λ g3 =λ 1 k 1 , where λ 1  denotes the radio wave wavelength in vacuum at the resonant frequency f 2  of the feeding element  20 , and k 1  denotes a shortening coefficient of a wavelength shortening effect in an actual environment. Here, k 1  is calculated based on, for example, a relative permittivity and a relative permeability such as an effective relative permittivity (∈ r1 ) and an effective relative permeability (μ r1 ) of a medium (environment) such as a dielectric substrate at which the feeding element  20  is arranged, a thickness of the medium (environment), and the resonant frequency. That is, L 20  is greater than or equal to (⅛)λ g3  and less than or equal to (⅜)λ g3 , and preferably greater than or equal to ( 3/16)λ g3  and less than or equal to ( 5/16)λ g3 . The physical length L 20  of the feeding element  20  is a physical length that gives Le 20 . In an ideal case where no other factor is considered, the physical length L 20  is equal to Le 20 . When the feeding element  20  includes a matching circuit, for example, L 20  is preferably greater than zero and less than or equal to Le 20 . By using a matching circuit such as an inductor, L 20  can be reduced (i.e., the size of the feeding element  20  can be reduced). 
     Also, assuming λ 0  denotes the radio wave wavelength in vacuum at the resonant frequency f 1  of the fundamental mode of the radiating elements  30  and  40 , a shortest distance x between the feeding element  20  and the radiating elements  30  and  40  is preferably less than or equal to 0.2×λ 0  (more preferably less than or equal to 0.1×λ 0 , and more preferably less than or equal to 0.05×λ 0 ). By arranging the feeding element  20  and the radiating elements  30  and  40  to be spaced apart by the shortest distance x as described above, the total efficiency (antenna gain) of the radiating elements  30  and  40  may be improved. 
     Note that the shortest distance x refers to the linear distance between a portion of the feeding element  20  and portions of the radiating elements  30  and  40  that are closest to each other, is the linear distance between portions that are closest. Also, the orientations of the feeding element  20  and the radiating elements  30  and  40  are not particularly limited as long as the feeding element  20  and the radiating elements  30  and  40  are coupled through electromagnetic field coupling. That is, the feeding element  20  and the radiating elements  30  and  40  may or may not be intersecting one another as viewed from a given direction, and their intersection angles may be set to any arbitrary angle. 
     Also, in the dipole mode, a distance over which the feeding element  20  and the radiating elements  30  and  40  run parallel to each other spaced the shortest distance x apart is preferably less than or equal to ⅜ of the length of the radiating elements  30  and  40 . More preferably, the distance is less than or equal to ¼ of the length of the radiating elements, and more preferably less than or equal to ⅛ of the length of the radiating elements. In the loop mode, the distance is preferably less than or equal to 3/16 of the inner circumference of the loop formed by the radiating elements, more preferably less than or equal to ⅛ of the inner circumference, and more preferably less than or equal to 1/16 of the inner circumference. Also, in a monopole mode (described below), the distance is preferably less than or equal to ¾ of the length of radiating elements  160  and  170 , more preferably ½ of the length of the radiating element, and more preferably ¼ of the length of the radiating elements. The position where the feeding element  20  and the radiating elements  30  and  40  are spaced apart by the shortest distance x corresponds to where coupling between the feeding element  20  and the radiating elements  30  and  40  is strong, and when the distance over which the feeding element  20  and the radiating elements  30  and  40  run parallel to each other spaced the shortest distance x apart is too long, strong coupling may occur at both a high impedance portion and a low impedance portion of the radiating elements  30  and  40 , and as such, impedance matching may become difficult. Thus, to obtain strong coupling only at a region where there is little variation in the impedance of the radiating elements  30  and  40 , the distance over which the feeding element  20  and the radiating elements  30  and  40  run parallel to each other spaced the shortest distance x apart is preferably arranged to be relatively short, and in this way, advantageous effects may be achieved in terms of impedance matching. 
     In  FIG. 1 , the shortest distance x corresponds to the shortest distance between the end portion  21  of the feeding element  20  and the end portion  33  of the radiating element  30  and the shortest distance between the end portion  21  of the feeding element  20  and the end portion  43  of the radiating element  40 . The feeding portion  35  is positioned at the end portion  33  (and possibly a conductive portion of the radiating element  30  in the vicinity of the end portion  33 ), and the feeding portion  45  is positioned at the end portion  43  (and possibly a conductor portion of the radiating element  40  in the vicinity of the end portion  43 ). 
     In  FIG. 1 , the radiating element  30  is a radiating conductor that functions as an antenna that operates in diploe mode (e.g., λ/2 dipole antenna) by being fed at the feeding portion  35  by the feeding element  20  through noncontact feeding (in particular, feeding through electromagnetic field coupling). The same applies to the radiating element  40 . 
     On the other hand, the feeding element  20  is a linear feeding conductor that is capable of feeding the radiating elements  30  and  40 . Also, the feeding element  20  may be fed by the feeding point  11  and thereby function as an antenna operating in monopole mode (e.g., λ/4 monopole antenna). 
     The radiating element  30  has the feeding portion  35  positioned toward the end portion  33  with respect to the central portion  32 , and in this way, high impedance electromagnetic field coupling between the radiating element  30  and the feeding element  20  may be realized. Similarly, the radiating element  40  has the feeding portion  45  positioned toward the end portion  43  with respect to a central portion  42 , and in this way, high impedance electromagnetic field coupling between the radiating element  40  and the feeding element  20  may be realized. 
     In the state where the feeding element  20  is coupled to both the radiating elements  30  and  40  through high impedance electromagnetic field coupling, the directivity of the antenna  1  may be linearly symmetrical with respect to a YZ plane passing through the feeding element  20 , provided the environment is uniform. 
     The impedance control unit  120  includes an impedance variable unit that interconnects the feeding element  20  and the control element  50 , and an impedance variable unit that interconnects the feeding element  20  and the control element  60 . The impedance variable unit is for varying the impedance between the feeding element and the control element from low impedance to high impedance or from high impedance to low impedance. For example, an impedance adjusting unit that is capable of adjusting the impedance may be used as the impedance variable unit. 
     The impedance variable unit may be, for example, a switch that is capable of selectively switching the impedance between the feeding element and the control element to either low impedance or high impedance. For example, when the switch is turned on, the impedance between the feeding element and the control element may be switched to low impedance, and when the switch is turned off, the impedance between the feeding element and the control element may be switched to high impedance. Alternatively, the impedance variable unit may be configured to continuously change the impedance between the feeding element and the control element in an increasing direction or a decreasing direction, for example. 
     The control element  50  may be arranged such that when the impedance of the impedance variable unit between the control element  50  and the feeding element  20  at the resonant frequency of the radiating element  30  is decreased, the electromagnetic field coupling between the feeding element  20  and the radiating element  30  is weakened and the function of the radiating element  30  as a radiating conductor is degraded, for example. The control element  50  may be arranged such that when the impedance variable unit between the control element  50  and the feeding element  20  is set to low impedance, the electromagnetic field coupling between the feeding element  20  and the radiating element  30  may be weakened such that the radiating element  30  loses its function as a radiating conductor, for example. In  FIG. 1 , the control element  50  is arranged such that a high impedance portion of the control element  50  and a low impedance portion of the radiating element  30  at the resonant frequency of the radiating element  30  are positioned close to each other. Note that the high impedance portion of the control element  50  may correspond to an end portion  51 , for example, and the low impedance portion of the radiating element  30  may correspond to the central portion  32 , for example. 
     The control element  60  may be arranged in a manner similar to the control element  50 . For example, the control element  60  may be arranged such that when the impedance of the impedance variable unit between the control element  60  and the feeding element  20  at the resonant frequency of the radiating element  40  is decreased, the electromagnetic field coupling between the feeding element  20  and the radiating element  40  is weakened and the function of the radiating element  40  as a radiating conductor is degraded. For example, the control element  60  may be arranged such that when the impedance variable unit between the control element  60  and the feeding element  20  is set to low impedance, the electromagnetic field coupling between the feeding element  20  and the radiating element  40  is weakened such that the radiating element  40  loses its function as a radiating conductor, for example. In  FIG. 1 , the control element  60  is arranged such that a high impedance portion of the control element  60  and a low impedance portion of the radiating element  40  at the resonant frequency of the radiating element  40  are positioned close to each other. Note that the high impedance portion of the control element  60  may correspond to an end portion  61 , for example, and the low impedance portion of the radiating element  40  may correspond to the central portion  42 , for example. 
     In the antenna  1  having the feeding element  20  coupled to a high impedance portion of the radiating element  30  (feeding portion  35 ) through electromagnetic field coupling, the impedance control unit  120  establishes low impedance connection between the feeding element  20  and the control element  50 . By establishing low impedance connection between the feeding element  20  and the control element  50  via the impedance control unit  120 , the electromagnetic field coupling between the feeding element  20  and the radiating element  30  is weakened. That is, because the end portion  51  corresponding to a high impedance portion of the control element  50  and the central portion  32  corresponding to a low impedance portion of the radiating element  30  at the resonant frequency of the radiating element  30  are arranged close to each other, by establishing low impedance connection between the feeding element  20  and the control element  50 , the electromagnetic field coupling between the feeding element  20  and the radiating element  30  may be weakened. Similarly, in the antenna  1  having the feeding element  20  coupled to a high impedance portion of the radiating element  40  (feeding portion  45 ) through electromagnetic field coupling, the impedance control unit  120  establishes low impedance connection between the feeding element  20  and the control element  60 . By establishing low impedance connection between the feeding element  20  and the control element  60  via the impedance control unit  120 , the electromagnetic field coupling between the feeding element  20  and the radiating element  40  may be weakened. 
     Thus, when the feeding element  20  is coupled to both the radiating element  30  and the radiating element  40  through electromagnetic field coupling, the electromagnetic field coupling between the feeding element  20  and the radiating element  30  may be weakened by establishing low impedance connection between the feeding element  20  and the control element  50 . In this way, the antenna gain of the radiating element  30  may become smaller than the antenna gain of the radiating element  40  and the radiation from the radiating element  40  may become dominant such that the directivity of the antenna  1  may be altered and controlled. Similarly, when the feeding element  20  is coupled to both the radiating element  30  and the radiating element  40  through electromagnetic field coupling, by establishing low impedance connection between the feeding element  20  and the control element  60 , the electromagnetic field coupling between the feeding element  20  and the radiating element  40  may be weakened. In this way, the antenna gain of the radiating element  40  may become smaller than the antenna gain of the radiating element  30  and the radiation from the radiating element  30  may become dominant such that the directivity of the antenna  1  may be altered and controlled. 
     Also, by weakening the electromagnetic field coupling between the radiating element  30  and the feeding element  20  and the electromagnetic field coupling between the radiating element  40  and the feeding element  20 , the antenna gain of both the radiating element  30  and the radiating element  40  may be reduced. In this way, the SAR (Specific Absorption Rate) of the antenna  1  and a wireless device equipped with the antenna  1  may be reduced, and their impact on the human body may be reduced, for example. 
     Thus, by arranging the antenna  1  to have the above-described configuration, the directivity of the antenna  1  may be switched and controlled without having the feeding element  20  arranged in contact with the radiating element  30  or the radiating element  40 . 
     Note that in  FIG. 1 , the control element  50  overlaps with the radiating element  30  in plan view from a direction parallel to the Z-axis. However, the control element  50  does not necessarily have to be arranged to overlap with the radiating element  30  in plan view from the direction parallel to the Z-axis as long as the control element  50  is arranged at a suitable position such that low impedance connection may be established between the feeding element  20  and the control element  50  to thereby weaken the electromagnetic field coupling between the feeding element  20  and the radiating element  30 . For example, the control element  50  may be arranged to overlap with the radiating element  30  in plan view from any direction such as a direction parallel to the X-axis or the Y-axis. Note that the same applies to the overlapping relationship between the control element  60  and the radiating element  40 . 
     The impedance control unit  120  may include an impedance adjusting unit  121  that is configured to lower the impedance between the feeding element  20  and the control element  50  to thereby degrade the function of the radiating element  30  as a radiating conductor, for example. The impedance adjusting unit  121  may be configured to lower the impedance between the feeding element  20  and the control element  50  close to zero to thereby weaken the electromagnetic field coupling between the radiating element  30  and the feeding element  20 , for example. Note that the impedance adjusting unit  121  is an example of the impedance variable unit that is capable of increasing or decreasing the impedance between the feeding element  20  and the control element  50 , and may be implemented by an element such as a variable capacitance diode or a circuit including such an element, for example. The impedance adjusting unit  121  may be capable of gradually changing (decreasing or increasing) the impedance between the feeding element  20  and the control element  50  and thereby continuously change the directivity of the antenna  1 , for example. Note that the impedance control unit  120  may also be configured to switch and control the directivity of the antenna  1  by turning on/off a switch element such as a transistor included in the impedance adjusting unit  121 . 
     By controlling the impedance between the feeding element  20  and the control element  50  to low impedance (e.g., ON), the impedance adjusting unit  121  may increase the RF current flowing between the feeding element  20  and the control element  50 . In this way, the electromagnetic field coupling between the radiating element  30  and the feeding element  20  that is connected to the control element  50  with low impedance may be weakened, and the function of the radiating element  30  as a radiating conductor may be degraded. Conversely, by controlling the impedance between the feeding element  20  and the control element  50  to high impedance (e.g., OFF), the impedance adjusting unit  121  may reduce or stop the RF current flowing between the feeding element  20  and the control element  50 . In this way, the radiating elements  30  may be coupled to the feeding element  20  through electromagnetic field coupling. 
     Similarly, the impedance control unit  120  may include an impedance adjusting unit  122  that is configured to lower the impedance between the feeding element  20  and the control element  60  to thereby degrade the function of the radiating element  40  as a radiating conductor, for example. The impedance adjusting unit  122  may be configured to lower the impedance between the feeding element  20  and the control element  60  close to zero to thereby weaken the electromagnetic field coupling between the radiating element  40  and the feeding element  20 , for example. Note that features and functions of the impedance adjusting unit  122  may be substantially identical to those of the impedance adjusting unit  121 , and as such, detailed descriptions thereof will be omitted. 
       FIG. 3  is a diagram illustrating an exemplary configuration of the impedance control unit  120 . The impedance control unit  120  includes a capacitor  147 , inductors  143 ,  144 , and  148 , variable capacitances diode  145  and  146 , and DC voltage sources  141  and  142 . 
     The capacitor  147  and the inductor  148  are connected in series, one end of the capacitor  147  is connected to the end portion  21  of the feeding element  20 , and one end of the inductor  148  is connected to the ground plane  70 . One end of the control element  50  is connected to an intermediate connection point between the capacitor  147  and the inductor  148  via the variable capacitance diode  145 , and one end of the control element  60  is connected to the intermediate connection point between the capacitor  147  and the inductor  148  via the variable capacitance diode  146 . The inductor  143  and the DC voltage source  141  are connected in series, one end of the inductor  143  is connected to an intermediate connection point between the variable capacitance diode  145  and the control element  50 , and one end of the DC voltage source  141  is connected to the ground plane  70 . The inductor  144  and the DC voltage source  142  are connected in series, one end of the inductor  144  is connected to an intermediate connection point between the variable capacitance diode  146  and the control element  60 , and one end of the DC voltage source  142  is connected to the ground plane  70 . 
     When the DC voltage source  141  increases its DC voltage output, the capacitance of the variable capacitance diode  145  decreases, and as a result, the impedance between the feeding element  20  and the control element  50  increases such that the RF current flowing through the control element  50  may be reduced or stopped. In this way, the connection between the feeding element  20  and the control element  50  may be weakened or disconnected such that the radiating element  30  that is coupled to the feeding element  20  through electromagnetic field coupling may be able to implement its function as a radiating conductor. 
     Conversely, when the DC voltage source  141  decreases or stops its DC voltage output, the capacitance of the variable capacitance diode  145  increases, and as a result, the impedance between the feeding element  20  and the control element  50  decreases such that the RF current flowing through the control element  50  may be increased. In this way, the connection between the feeding element  20  and the control element  50  may be strengthened such that the function of the radiating element  30 , which is electromagnetically coupled to the feeding element  20 , as a radiating conductor may be suppressed or blocked, for example. 
     Similarly, when the DC voltage source  142  increases its DC voltage output, the capacitance of the variable capacitance diode  146  decreases, and as a result, the impedance between the feeding element  20  and the control element  60  increases, such that the RF current flowing through the control element  60  may be reduced or stopped. In this way, the connection between the feeding element  20  and the control element  60  may be weakened or disconnected such that the radiating element  40  that is coupled to the feeding element  20  through electromagnetic field coupling may implement its function as a radiating conductor. 
     Conversely, when the DC voltage source  142  decreases or stops its DC voltage output, the capacitance of the variable capacitance diode  146  increases, and as a result, the impedance between the feeding element  20  and the control element  60  decreases such that the RF current flowing through the control element  60  may be increased. In this way, the connection between the feeding element  20  and the control element  60  may be strengthened such that the function of the radiating element  40 , which is electromagnetically coupled to the feeding element  20 , as a radiating conductor may be suppressed or blocked, for example. 
     By using the impedance control unit  120  as illustrated in  FIG. 3 , the impedance between the feeding element  20  and the control element  50  and the impedance between the feeding element  20  and the control element  60  may be gradually changed (decreased or increased), for example. By gradually changing the impedance, the directivity of the antenna  1  may be controlled to gradually change according to changes in the surrounding environment, for example, rather than controlling the directivity through on/off switching. 
       FIGS. 4 and 5  are diagrams illustrating the directivity of the antenna  1 . In  FIGS. 4 and 5 , “directivity” represents the directional gain at the resonant frequency of the fundamental mode of the antenna  1  (1.485 GHz in the present example), θ represents an angle formed with respect to the extending direction of the feeding element  20  within a YZ plane that passes through the feeding portion  11  and a center point of the ground plane  70 , and φ represents an angle formed with respect to a normal direction of the ground plane  70  within the ZX plane passing through the center point of the ground plane  70  (see  FIG. 1 ). 
       FIG. 4  illustrates the directivity of the antenna  1  in a case where the impedance between the feeding element  20  and the control element  50  is high, and the impedance between the feeding element  20  and the control element  60  is also high.  FIG. 5  illustrates the directivity of the antenna  1  in a case where the impedance between the feeding element  20  and the control element  50  is high, and the impedance between the feeding element  20  and the control element  60  is low. As can be appreciated from  FIGS. 4 and 5 , the directivity of the antenna  1  can be switched. 
     The antenna  1  has a symmetrical configuration with respect to the YZ plane passing through the feeding point  11 . Thus, in a case where the impedance between the feeding element  20  and the control element  50  is low, and the impedance between the feeding element  20  and the control element  60  is high, as opposed to the case of  FIG. 5 , the antenna  1  may have a directivity that is line symmetrical, with respect to φ=180°, to the directivity illustrated in  FIG. 5 . 
     In  FIG. 1 , the antenna  1  may include a matching circuit  90  that operates in conjunction with the impedance control unit  120  to adjust the resonant frequency in the fundamental mode of the radiating element  30  and the radiating element  40 , for example. The matching circuit  90  may adjust the resonant frequency in conjunction with the operation of the impedance control unit  120  altering the coupling state between the radiating element  30  and the feeding element  20  or the coupling state between the radiating element  40  and the feeding element  20 , for example. The matching circuit  90  may be inserted into or connected to the feeding element  20 , for example. 
     By using the matching circuit  90 , even when the resonant frequency of the fundamental mode of the radiating element  30  or the radiating element  40  changes as a result of a change in the coupling state between the radiating element  30  and the feeding element  20  or the coupling state between the radiating element  40  and the feeding element  20 , the matching circuit  90  may correct such change in the resonant frequency, for example. 
       FIG. 6  is a graph indicating S 11  characteristic measurements of the antenna  1  for illustrating an effect of the matching circuit  90 . In the graph of  FIG. 6 , “a” represents a case where no matching circuit  90  is used, the impedance between the feeding element  20  and the control element  50  is high, and the impedance between the feeding element  20  and the control element  60  is high (i.e., impedance adjusting unit  121 : high impedance; impedance adjusting unit  122 : high impedance). Also, “b” represents a case where the matching circuit  90  is used, the impedance between the feeding element  20  and the control element  50  is high, and the impedance between the feeding element  20  and the control element  60  is high (i.e., impedance adjusting unit  121 : high impedance; impedance adjusting unit  122 : high impedance). Also, “c” represents a case where no matching circuit  90  is used, the impedance between the feeding element  20  and the control element  50  is high, and the impedance between the feeding element  20  and the control element  60  is low (i.e., impedance adjusting unit  121 : high impedance; impedance adjusting unit  122 : low impedance). 
     Note that  FIG. 6  illustrates an example where the matching circuit  90  includes an inductor (inductance: 15 nH) that is serially inserted to the feeding element  20  and an inductor (inductance: 15 nH) that is inserted between the end portion  21  of the feeding element  20  and the ground plane  70 . 
     If the matching circuit  90  is not operated when the impedance adjusting unit  122  is switched from ON to OFF, the resonant frequency of the fundamental mode of the radiating element  30  (1.485 GHz in the present example) may deviate in some cases (e.g., change from “c” to “a” in  FIG. 6 ). However, by operating the matching circuit  90  in conjunction with the operation of switching the impedance adjusting unit  122  from ON to OFF, such a deviation of the resonant frequency of the fundamental mode of the radiating element  30  may be prevented (e.g., change from “c” to “b”). 
     Note that upon obtaining the S 11  characteristic measurements, the dimensions L 11 -L 16  of the configuration illustrated in  FIG. 1  were set up as follows (in mm). 
     L 11 : 60 
     L 12 : 30 
     L 13 : 130 
     L 14 : 10.5 
     L 15 : 58 
     L 16 : 30 
     Also, the line widths of the feeding element  20 , the radiating elements  30  and  40 , the control elements  50  and  60  were set to 1 mm. 
     Also, upon obtaining the S 11  characteristic measurements, the dimensions of the configuration illustrated in  FIG. 2  were set up as follows. That is, the substrate  80  was set up to have a relative permittivity of ∈ r =3.3, a loss tangent of tan δ=0.003, and a thickness of H 1 =0.8 mm; and the substrate  110  was set up to have a relative permittivity of ∈ r =7.44, a loss tangent of tan δ=0.011, and a thickness of H 3 =1.1 mm. Also, the gap between the substrate  80  and the substrate  110  was set up to be H 2 =2 mm. 
     &lt;Antenna Device  201 &gt; 
       FIG. 7  is a perspective view of a computer simulation model for analyzing the operation of an antenna device  201  including antennas  1  and  2  according to a second embodiment of the present invention. Note that in the present example, CST Microwave Studio (registered trademark) by Computer Simulation Technology AG (CST) was used as an electromagnetic field simulator. Also, note that descriptions of features of the present embodiment that may be substantially identical to those of the embodiment described above may be simplified or omitted. 
     The antenna  2  of the antenna device  201  may have a configuration that is substantially identical to the configuration of the antenna  1 , and is arranged on the opposite side of the antenna  1  with respect to the ground plane  70 . The antenna  2  includes a feeding element  22 , a radiating element  36 , a radiating element  46 , a control element  52 , a control element  62 , an impedance control unit  125 , and a matching circuit  91 . 
     The feeding element  22  is a conductor that uses the ground plane  70  as a ground reference and is connected to a feeding point  12 . The feeding point  12  may be arranged at a central portion of an outer edge  72  of the ground plane  70 , for example. The outer edge  72  is located at the opposite side of the outer edge  71  with respect to the central portion of the ground plane  70 . 
     The radiating element  36  and the radiating element  46  are both coupled to the feeding element  22  through electromagnetic field coupling. The control element  52  is spaced apart from the radiating element  36  in a direction parallel to the Z-axis, and the control element  62  is spaced apart from the radiating element  46  in a direction parallel to the Z-axis. 
     The impedance control unit  125  is an example of a control unit that controls an impedance variable unit to establish low impedance connection between the feeding element  22  and the control element  52 , or between the feeding element  22  and the control element  62 . The impedance control unit  125  may include an impedance adjusting unit  123  that may be substantially identical to the impedance adjusting unit  121  described above. For example, the impedance adjusting unit  123  may be configured to lower the impedance between the feeding element  22  and the control element  52  to thereby weaken the electromagnetic field coupling between the radiating element  36  and the feeding element  22 . Similarly, the impedance control unit  125  may include an impedance adjusting unit  124  that may be substantially identical to the impedance adjusting unit  122  described above. For example, the impedance adjusting unit  124  may be configured to lower the impedance between the feeding element  22  and the control element  62  to thereby weaken the electromagnetic field coupling between the radiating element  46  and the feeding element  22 . 
     The matching circuit  91  may be similar to the matching circuit  90  as described above. That is, the matching circuit  91  may operate in conjunction with the operation of the impedance control unit  125  to adjust the resonant frequency of the fundamental mode of the radiating element  36  and the radiating element  46 . 
     By including the antennas  1  and  2 , the antenna device  201  may function as a MIMO (Multiple Input Multiple Output) antenna. Also, the antenna device  201  may be capable of switching and controlling the directivity of each of the antennas  1  and  2  while maintaining the correlation coefficient between the antenna  1  and the antenna  2  at a desirably low value regardless of the impedances set up by the impedance adjusting units  121 ,  122 ,  123 , and  124 . 
     &lt;Antenna  3 &gt; 
       FIG. 8  is a perspective view of a computer simulation model for analyzing the operation of an antenna  3  according to a third embodiment of the present invention. Note that in the present example, CST Microwave Studio (registered trademark) by Computer Simulation Technology AG (CST) was used as an electromagnetic field simulator. Also, note that descriptions of features of the present embodiment that may be substantially identical to those of the embodiments described above may be simplified or omitted. 
     The antenna  3  includes a ground plane  70 , a plate conductor  150 , a feeding element  20 , a radiating element  160 , a radiating element  170 , a control element  50 , a control element  60 , an impedance control unit  120 , and optionally, a matching circuit  90 . Note that the feeding element  20 , the control element  50 , the control element  60 , the impedance control unit  120 , and the matching circuit  90  may be substantially identical to those described above with reference to  FIG. 1 . 
     The ground plane  70  is a planar ground pattern having at least one side as an outer edge. In  FIG. 8 , the ground plane  70  is arranged into a rectangular shape extending in the XY plane. In  FIG. 8 , the ground plane  70  includes an outer edge  71  that extends linearly in the X-axis direction, an outer edge  73  that extends linearly in the Y-axis direction, an outer edge  72  opposing the outer edge  71  and extending in the X-axis direction, and an outer edge  74  opposing the outer edge  73  and extending in the Y-axis direction. 
     The plate conductor  150  is a flat conductor arranged parallel to the ground plane  70  and spaced apart from the ground plane  70  in a direction parallel to the Z-axis. In  FIG. 8 , the plate conductor  150  is arranged into a polygonal shape extending in the XY plane and having outer edges  151 ,  152 ,  153  and  154 . 
     By arranging the plate conductor  150  to have at least one outer edge extending along at least one outer edge of the ground plane  70 , resonance between the plate conductor  150  and the ground plane  70  may be facilitated, and the number of resonance of the antenna  3  may be increased. In  FIG. 8 , the outer edges  151 ,  152 ,  153  and  154  of the plate conductor  150  are respectively arranged to run parallel with the outer edges  71 ,  72 ,  73  and  74  of the ground plane  70 . Note that the outer edge  151  may be arranged to overlap with the position of the outer edge  71  in a plan view from a direction parallel to the Z-axis or be offset from the position of the outer edge  71 . The same applies to the outer edges  152 ,  153 , and  154 . 
     The plate conductor  150  includes a portion spaced apart from the ground plane  70  in a direction parallel to the Z-axis and facing the ground plane  70 . In  FIG. 8 , the plate conductor  150  is arranged into a rectangular conductor that is spaced apart from the ground plane  70  by a distance that enables high frequency coupling between the plate conductor  150  and the ground plane  70 . The plate conductor  150  includes the outer edge  151  that linearly extends in the X-axis direction, the outer edge  153  that linearly extends in the Y-axis direction, the outer edge  153  that opposes the outer edge  151  and linearly extends in the X-axis direction, and the outer edge  154  that opposes the outer edge  153  and linearly extends in the Y-axis direction. 
     The feeding element  20  is a conductor that is spaced apart from the radiating element  160  and the radiating element  170  by a predetermined distance. The feeding element  20  may be spaced apart from the radiating element  160  and the radiating element  170  by a gap having a direction component parallel to the Z-axis. 
     In  FIG. 8 , the feeding element  20  overlaps with the radiating element  160  and the radiating element  170  in plan view from a direction parallel to the Z-axis. However, the feeding element  20  does not necessarily have to overlap with the radiating element  160  and the radiating element  170  in plan view from a direction parallel to the Z-axis as long as the feeding element  20  is spaced apart from the radiating element  160  and the radiating element  170  by a distance that enables the feeding element to perform noncontact feeding of the radiating element  160  and the radiating element  170 . For example, the feeding element may be arranged to overlap with the radiating element  160  and the radiating element  170  in plan view from any direction such as a direction parallel to the X-axis or the Y-axis. 
     The feeding element  20  is a conductor that is capable of performing noncontact feeding of the radiating element  160  via the feeding portion  165  of the radiating element  160 , performing noncontact feeding of the radiating element  170  via the feeding portion  175  of the radiating element  170 . 
     In plan view from a direction normal to the ground plane  70 , the feeding element  20  extends from the feeding point  11  to the end portion  21  in a direction toward a gap  131  between one end portion  163  of the radiating element  160  and one end portion  173  of the radiating element  170 . The feeding element  20  includes the end portion  21  that is spaced apart from the end portion  163  of the radiating element  160  and the end portion  173  of the radiating element  170  by a predetermined distance, and the end portion  21  is positioned in the vicinity of the gap  131 . 
     The radiating element  160  is a linear radiating conductor that is connected to the plate conductor  150  and protrudes from the outer edge  151  of the plate conductor  150  in an opposite direction from the plate conductor  150 . The radiating element  160  is arranged such that at least a portion of the radiating element  160  does not overlap with the ground plane in plan view from a direction parallel the Z-axis. The radiating element  160  includes the end portion  163  and another end portion  164 , and is arranged into an L-shape that extends from one end portion  164  to the other end portion  163  via a bent portion  167 . The end portion  164  is a root portion that is connected to a portion of the plate conductor  150  in the vicinity of one end portion  155  of the outer edge  151  of the plate conductor  150 , and the end portion  163  is an open end that is not connected to another conductor. 
     The radiating element  160  may include a linear radiating conductor portion that is arranged along the outer edge  71  of the ground plane  70 , for example. The radiating element  160  may include a conductor portion  161  that is arranged opposite the outer edge  71  of the ground plane  70  and extends in a direction parallel to the outer edge  71  while being spaced apart from the outer edge  71  by a predetermined shortest distance, for example. Note that a direction parallel to the outer edge  71  corresponds to a direction parallel to the X axis in  FIG. 8 . By arranging the radiating element  160  to include the conductor portion  161  extending along the outer edge  71 , the directivity of the antenna  3  may be more easily controlled, for example. 
     Note that although  FIG. 8  illustrates an example where the radiating element  160  is arranged into an L-shape within the XY plane, the radiating element  160  may be in other shapes such as linear shape. Also, the radiating element  160  may include a conductor portion extending in the XY plane and a conductor portion extending in a plane other than the XY plane, for example. 
     The radiating element  170  may have the same or similar configuration as the radiating element  160 , and as such, descriptions thereof are simplified. The radiating element  170  is an antenna conductor that includes one end portion  174  and another end portion  173 , and is arranged into an L-shape extending from the end portion  174  to the end portion  173  via a bent portion  177 . The end portion  174  is a root portion that is connected to a portion of the plate conductor  150  in the vicinity of an end portion  156  of the outer edge  151  of the plate conductor  150 , and the end portion  173  is an open end that is not connected to another conductor. The radiating element  170  may include a conductor portion  171  that is arranged opposite the outer edge  71  of the ground plane  70  and extends in a direction parallel to the outer edge  71  while being spaced apart from the outer edge  71  by a predetermined shortest distance, for example. 
     The radiating element  170  and the radiating element  160  are conductors extending in different directions from each other, in directions toward the feeding element  20 . Note that although  FIG. 8  illustrates an example where the radiating element  170  and the radiating element  160  are conductors arranged in the same XY plane, the radiating element  170  and the radiating element  160  may alternatively be conductors arranged in different planes, for example. Also, although  FIG. 8  illustrates an example where the conductor portion  171  of the radiating element  170  and the conductor portion  161  of the radiating element  160  are arranged along a single straight line, the conductor portions  161  and  171  may alternatively be arranged on different straight lines, for example. 
     Also, although  FIG. 8  illustrates an example where the radiating element  160  and the feeding element  20  overlap in plan view from the Z-axis direction, the radiating element  160  and the feeding element  20  do not necessarily have to overlap in plan view from the Z-axis direction as long as the feeding element  20  is spaced apart from the radiating element  160  by an adequate distance to enable electromagnetic field coupling between the feeding element  20  and the radiating element  160 . For example, the radiating element  160  and the feeding element  20  may overlap in plan view from any direction such as the X-axis or the Y-axis direction. 
     The feeding element  20  and the radiating element  160  may be spaced apart from each other by a certain distance so as to enable high frequency coupling between the radiating element  160  and the feeding element  20 . Noncontact feeding of the radiating element  160  may be implemented via the feeding element  20 . By feeding the radiating element  160  in this manner, the radiating element  160  may function as a radiating conductor of an antenna. As illustrated in  FIG. 8 , in the case where the radiating element  160  is arranged into a linear conductor having one end connected to the plate conductor  150  having a large area and another end corresponding to an open end, a resonant current (standing wave current distribution) similar to that formed in a λ/4 monopole antenna may be formed on the radiating element  160 . That is, the radiating elements  160  may function as a monopole antenna that resonates at a quarter wavelength of a predetermined frequency (hereinafter referred to as “monopole mode”). 
     The radiating element  170  may have the same or similar configuration as the radiating element  160 , and as such descriptions thereof are simplified. The feeding element  20  and the radiating element  170  may be spaced apart by a certain distance so as to enable electromagnetic field coupling between these elements. Noncontact feeding of the radiating element  170  may be implemented via the feeding element  20 . By feeding the radiating element  170  in this manner, the radiating element  170  may function as a radiating conductor of an antenna. 
     Also, assuming Le 160  and Le 170  denote the electrical lengths that impart the fundamental mode of resonance to the radiating elements  160  and  170 , and λ denotes the wavelength on the radiating elements  160  and  170  at the resonant frequency f 1  of the fundamental mode of the radiating elements  160  and  170 , Le 160  and Le 170  are greater than or equal to (⅛)λ and less than or equal to (⅜)λ. 
     Also, in a case where the fundamental mode of resonance of the radiating elements corresponds to the monopole mode (i.e., the radiating elements are connected to the outer edge of the plate conductor and have open ends), Le 160  and Le 170  are preferably greater than or equal to (⅛)λ and less than or equal to (⅜)λ, more preferably greater than or equal to ( 3/16)λ and less than or equal to ( 5/16)λ, and more preferably greater than or equal to ( 7/32)λ and less than or equal to ( 9/32)λ. By arranging Le 160  and Le 170  to be within the above ranges, the radiating elements  160  and  170  may adequately function as radiating conductors, and the antenna  3  may achieve desirably high efficiency, for example. 
     Also, assuming L 160  and L 170  denote the physical lengths of the radiating elements  160  and  170  (corresponding to L 18 +L 19  in  FIG. 8 ), λ 0  denotes the radio wave wavelength in vacuum at the resonant frequency of the fundamental mode of the radiating elements, and k 2  denotes a shortening coefficient of a wavelength shortening effect in an actual environment, L 160  and L 170  are determined by λ g2 =λ 0 k 2 . Here, k 2  is calculated based on, for example, a relative permittivity and a relative permeability such as an effective relative permittivity (∈ r2 ) and an effective relative permeability (μ r2 ) of a medium (environment) such as a dielectric substrate at which the radiating elements  160  and  170  are arranged, the thickness of the medium (environment), and the resonant frequency. That is, in the case where the fundamental mode of resonance of the radiating elements corresponds to the monopole mode, L 160  and L 170  is greater than or equal to (⅛)λ g2  and less than or equal to (⅜)λ g2 . The physical lengths L 160  and L 170  of the radiating elements  160  and  170  are physical lengths that give Le 160  and Le 170 . In an ideal case where no other factor is considered, the physical lengths L 160  and  170  are equal to Le 160  and Le 170 . Note that even when a matching circuit such as an inductor is used to shorten the physical lengths L 160  and L 170  (i.e., reduce the size of the radiating elements  160  and  170 ), the physical lengths L 160  and L 170  are preferably greater than zero and less than or equal to Le 160  and Le 170 , and more preferably greater than or equal to 0.4×Le 160  and 0.4×Le 170 , and less than or equal to Le 160  and Le 170 . 
     In  FIG. 8 , the feeding portions  165  and  175 , which correspond to portions of the radiating elements  160  and  170  that are fed by the feeding element  20 , are arranged at positions toward the end portions  163  and  173  and away from the end portions  164  and  174 , which correspond to low impedance portions of the radiating elements  160  and  170  that are connected to the plate conductor  150  and have the lowest impedance at the resonant frequency of the fundamental mode of the radiating elements  160  and  170 . In this way, impedance matching of the antenna  3  may be facilitated. In particular, the feeding portions  165  and  175  are preferably arranged at positions toward the end portions  163  and  173  from the central portions  162  and  172 . Note that the feeding portions  165  and  175  are defined by portions closest to the feeding point  11  of the conductor portions of the radiating elements  160  and  170  closest to the feeding element  20 . Also, note that the feeding portions  165  and  175  are feeding portions for the radiating elements  160  and  170 , respectively, and are not feeding portions for the antenna  3 . That is, the feeding point  11  functions as the feeding portion for the antenna  3  in the present example. 
     In the monopole mode, the impedance of the radiating elements  160  and  170  increases from the end portions  164  and  174  toward the end portions  163  and  173  of the radiating elements  160  and  170 . In the case of implementing high impedance coupling between the feeding element  20  and the radiating elements  160  and  170  through electromagnetic field coupling, even when slight variations occur in the impedance between the feeding element  20  and the radiating elements  160  and  170 , their impact on impedance matching may be relatively small as long as the feeding element  20  and the radiating elements  160  and  170  are coupled at a sufficiently high impedance of at least a certain level. Thus, to facilitate matching, the feeding portions  165  and  175  of the radiating elements  160  and  170  are preferably positioned at high impedance portions of the radiating elements  160  and  170 . 
     For example, to facilitate impedance matching of the antenna  3 , the feeding portions  165  and  175  may be positioned at a region spaced apart from the region having the lowest impedance at the resonant frequency of the fundamental mode of the radiating elements  160  and  170  (end portions  164  and  174  in the present example) by a distance greater than equal to ¼ of the total length of the radiating elements  160  and  170  (preferably greater than or equal to ⅓ of the total length, and more preferably greater than or equal to ½ of the total length). Further, the feeding portions  165  and  175  are preferably arranged at positions toward the end portions  163  and  173  from the central portions  162  and  172 . In  FIG. 8 , the total length of the radiating elements  160  and  170  correspond to L 18 +L 19 , and the feeding portions  165  and  175  are positioned toward the end portions  163  and  173  from the central portions  162  and  172 . 
     In the antenna  3  having the above-described configuration, even when the plate conductor  150  having a relatively large area is arranged, because noncontact feeding of the radiating elements  160  and  170  by the feeding element  20  is implemented, restrictions on the configurations and layout of the radiating elements  160  and  170  and/or the feeding element  20  may be reduced. That is, as long as the feeding element  20  and the radiating elements  160  and  170  are spaced apart by a suitable distance that enables noncontact feeding of the radiating elements  160  and  170 , the positional relationship between the feeding element  20  and the radiating elements  160  and  170  may be freely designed and functions of the antenna  3  may be implemented with relative ease. 
       FIG. 9  schematically illustrates a positional relationship between components of the antenna  3  in the Z-axis direction. Note that descriptions of features and effects that may be substantially identical to those of the embodiments described above may be omitted or simplified. The feeding element  20  and the radiating elements  160  and  170  may be spaced apart from each other by a distance that enables electromagnetic field coupling, between these elements, for example. 
     The ground plane  70  and the plate conductor  150  may be DC coupled via a connection conductor  84 , for example. Note that any number of connection conductors  84  may be provided. In a case where a heating element  83  is arranged on the substrate  80 , heat emitted by the heating element  83  may be transferred to the plate conductor  150  via the substrate  80  and the connection conductor  84 . 
     The plate conductor  150  is capable of functioning as a heat sink that dissipates heat. The plate conductor  150  may release the heat generated by the heating element  83  mounted on the substrate  80 , or release heat generated by a heating element (not shown) mounted on the substrate  110 , for example. 
     Specific examples of the connection conductor  84  include a metal plate and wiring such as a via or a wire. Specific examples of the heating element  83  include circuit components mounted on the substrate  80  (transistor, IC, etc.). 
     In  FIG. 8 , an elongated metal plate that is connected to the outer edge  74  of the ground plane  70  and the outer edge  154  of the plate conductor  150  and an elongated metal plate that is connected to the outer edge  73  of the ground plane  70  and the outer edge  153  of the plate conductor  150  are illustrated as the connection conductors  84 . 
     The radiating element  160  has the feeding portion  165  arranged at a position toward the end portion  163  from the central portion  162 , and in this way, the feeding element  20  may be coupled to a high impedance portion of the radiating element  160  through electromagnetic field coupling. Likewise, the radiating elements  170  has the feeding portion  175  arranged at a position toward the end portion  173  from the central portion  172 , and in this way, the feeding element  20  may be coupled to a high impedance portion of the radiating element  170  through electromagnetic filed coupling. 
     In the case where the feeding element  20  is electromagnetically coupled to both the radiating element  160  and the radiating element  170  at high impedance portions and impedance matching with the radiating elements  160  and the radiating element  170  are achieved, the directivity of the antenna  3  may be linearly symmetrical with respect to the YZ plane that passes through the feeding element  20 , provided the surrounding environment is uniform. 
     The impedance control unit  120  is an example of a control unit that controls an impedance variable unit to connect the feeding element  20  to the control element  50  or the control element  60  and vary the impedance between the feeding element  20  and the control element  50  or the impedance between the feeding element  20  and the control element  60 . Note that the impedance control unit  120  of  FIG. 8  may have a configuration and functions substantially similar to those described above. 
       FIGS. 10 and 11  are diagrams illustrating the directivity of the antenna  3 . In  FIGS. 10 and 11 , “directivity” represents the directional gain at the resonant frequency in the fundamental mode of the antenna  3  (1.175 GHz in the present example), θ represents an angle formed with respect to the extending direction of the feeding element  20  within a YZ plane that passes through the feeding portion  11  and a center point of the ground plane  70 , and φ represents an angle formed with respect to a normal direction of the ground plane  70  within the ZX plane passing through the center point of the ground plane  70  (see  FIG. 8 ). 
       FIG. 10  illustrates the directivity of the antenna  3  in a case where the impedance between the feeding element  20  and the control element  50  is high, and the impedance between the feeding element  20  and the control element  60  is high.  FIG. 11  illustrates the directivity of the antenna  3  in a case where the impedance between the feeding element  20  and the control element  50  is high, and the impedance between the feeding element  20  and the control element  60  is low. As illustrated in  FIGS. 10 and 11 , the directivity of the antenna  3  may be switched. 
     The antenna  3  has a symmetrical configuration with respect to an YZ plane that passes through the feeding point  11 . Thus, in a case where the impedance between the feeding element  20  and the control element  50  is low, and the impedance between the feeding element  20  and the control element  60  is high, as opposed to the case of  FIG. 11 , the antenna  3  may have a directivity that is line symmetrical, with respect to φ=180°, to the directivity illustrated in  FIG. 11 . 
     In  FIG. 8 , the antenna  3  may include the matching circuit  90  that operates in conjunction with the impedance control unit  120  to adjust the resonant frequency in the fundamental mode of the radiating element  160  and radiating element  170 , for example. The matching circuit  90  may be configured to adjust the resonant frequency in conjunction with the operation of the impedance control unit  120  changing the coupling state between the feeding element  20  and the radiating element  160  or the coupling state between the feeding element  20  and the radiating element  170 , for example. The matching circuit  90  may be inserted or connected to the feeding element  20 , for example. 
     By using the matching circuit  90 , even when the resonant frequency of the fundamental mode of the radiating element  160  or the radiating element  170  is changed as a result of a change in the coupling state between the radiating element  160  and the feeding element  20  or the coupling state between the radiating element  170  and the feeding element  20 , the matching circuit  90  may be able to correct such a change in the resonant frequency, for example. 
       FIG. 12  is a graph indicating S 11  characteristic measurements of the antenna  3  for illustrating an effect of the matching circuit  90 . Note that in  FIG. 12 , “d” represents a case where the matching circuit  90  is not used, the impedance between the feeding element  20  and the control element  50  is high, and the impedance between the feeding element  20  and the control element  60  is high (impedance adjusting unit  121 : high impedance; impedance adjusting unit  122 : high impedance). Also, “e” represents a case where the matching circuit  90  is used, the impedance between the feeding element  20  and the control element  50  is high, and the impedance between the feeding element  20  and the control element  60  is high (impedance adjusting unit  121 : high impedance; impedance adjusting unit  122 : high impedance). Also, “f” illustrates a case where the matching circuit  90  is not used, the impedance between the feeding element  20  and the control element  50  is high, and the impedance between the feeding element  20  and the control element  60  is low (impedance adjusting unit  121 : high impedance; impedance adjusting unit  122 : low impedance). 
       FIG. 12  illustrates an example where the matching circuit  90  includes an inductor (inductance: 15 nH) that is serially inserted to the feeding element  20  and an inductor (inductance: 15 nH) inserted between the end portion  21  of the feeding element  20  and the ground plane  70 . 
     In the case where the matching circuit  90  is not operated, when the impedance adjusting unit  122  is switched from ON to OFF, the resonant frequency of the fundamental mode of the radiating element  160  (1.175 GHz in the present example) may deviate in some cases (e.g., change from “f” to “d” in  FIG. 12 ). However, by operating the matching circuit  90  in conjunction with the operation of switching the impedance adjusting unit  122  from ON to OFF, such a deviation of the resonant frequency in the fundamental mode of the radiating element  160  may be prevented (e.g., change from “f” to “e” in  FIG. 12 ). 
     Note that when measuring the S 11  characteristics of the antenna  3 , the dimensions of the configuration illustrated in  FIG. 8  were set up as follows (in mm). 
     L 11 : 120 
     L 12 : 80 
     L 13 : 60 
     L 14 : 10.5 
     L 16 : 29.5 
     L 17 : 80 
     L 18 : 10.5 
     L 19 : 26.5 
     L 22 : 60 
     Also, the line widths of the feeding element  20 , the radiating elements  160  and  170 , the control elements  50  and  60  were set to 1 mm. 
     Also, the dimensions of the configuration illustrated in  FIG. 9  upon measuring the S 11  characteristics of the antenna were set up as follows. That is, the substrate  80  was set up to have a relative dielectric constant of ∈ r =3.3, a loss tangent of tan δ=0.003, and a thickness of H 1 =0.8 mm; and the substrate  110  was set up to have a relative dielectric constant of ∈ r =7.44, a loss tangent of tan δ=0.011, and a thickness of H 3 =1.1 mm. Also, the gap H 2  between the substrate  80  and the substrate  110  was set to 2 mm. 
     &lt;Antenna Device  202 &gt; 
       FIG. 13  is a perspective view of a computer simulation model for analyzing the operation of an antenna device  202  including antennas  3  and  4  according to a fourth embodiment of the present invention. Note that in the present example, CST Microwave Studio (registered trademark) by Computer Simulation Technology AG (CST) was used as an electromagnetic field simulator. Also, note that descriptions of features of the present embodiment that may be substantially identical to those of the embodiments described above may be simplified or omitted. 
     The antenna  4  may have a configuration that is substantially identical to the configuration of the antenna  3 , and is arranged on the opposite side of the antenna  3  with respect to the ground plane  70 . The antenna  4  includes a feeding element  22 , a radiating element  166 , a radiating element  176 , a control element  52 , a control element  62 , an impedance control unit  125 , and a matching circuit  91 . 
     The radiating element  166  and the radiating element  176  are each coupled to the feeding element  22  through electromagnetic field coupling. The control element  52  is spaced apart from the radiating element  166  in a direction parallel to the Z-axis, and the control element  62  is spaced apart from the radiating element  176  in a direction parallel to the Z-axis. 
     By including the antennas  3  and  4 , the antenna device  202  can function as a MIMO (Multiple Input Multiple Output) antenna. Also, the antenna device  202  is capable of switching and controlling the directivity of each of the antennas  3  and  4  while maintaining the correlation coefficient between the antenna  3  and the antenna  4  to a desirably low value, regardless of the impedance of the impedance adjusting units  121 ,  122 ,  123 , and  124 . 
       FIGS. 14-17  are graphs indicating the reflection coefficient S 11  of the antenna  3 , the reflection coefficient S 22  of the antenna  4 , the correlation coefficient at the resonant frequency (1.175 GHz in the present example) in the antenna device  202 . Note that the correlation coefficient was calculated based on S-parameters.  FIGS. 18-25  are graphs indicating the directivity of the antenna device  202 . Note that in  FIGS. 18-25 , “directivity” represents the directional gain at the resonant frequency of the fundamental mode of the antenna device  202  (1.175 GHz in the present example); θ represents an angle formed with respect to the extending direction of the feeding element  20  within a YZ plane that passes through feeding portions  11  and  12 , and a center point of the ground plane  70 ; and φ represents an angle formed with respect to a normal direction of the ground plane  70  within the ZX plane passing through the center point of the ground plane  70  (see  FIG. 13 ). 
       FIGS. 14, 18, and 19  illustrate a case where the impedance between the feeding element  20  and the control element  50  is high, the impedance between the feeding element  20  and the control element  60  is high, the impedance between the feeding element  22  and the control element  52  is high, and the impedance between the feeding element  22  and the control element  62  is high. 
       FIGS. 15, 20, and 21  illustrate a case where the impedance between the feeding element  20  and the control element  50  is high, the impedance between the feeding element  20  and the control element  60  is high, the impedance between the feeding element  22  and the control element  52  is high, and the impedance between the feeding element  22  and the control element  62  is low. 
       FIGS. 16, 22, and 23  illustrate a case where the impedance between the feeding element  20  and the control element  50  is high, the impedance between the feeding element  20  and the control element  60  is low, the impedance between the feeding element  22  and the control element  52  is low, and the impedance between the feeding element  22  and the control element  62  is high. 
       FIGS. 17, 24, and 25  illustrate a case where the impedance between the feeding element  20  and the control element  50  is high, the impedance between the feeding element  20  and the control element  60  is low, the impedance between the feeding element  22  and the control element  52  is high, and the impedance between the feeding element  22  and the control element  62  is low. 
     In  FIGS. 14, 16, and 17 , the S 11  measurements and the S 22  measurements substantially overlap. The correlation coefficients in the cases of  FIGS. 14 to 17  are respectively 0.004, 0.005, 0.099, and 0.007. These correlation coefficient values all adequately satisfy the requirements of a MIMO antenna relating to the correlation between S 11  and S 22  characteristics. Also, note that  FIGS. 18, 20, 22, and 24  represent the directivity of the antenna  3 ; and  FIGS. 19, 21, 23, and 25  represent the directivity of the antenna  4 . As can be appreciated from these drawings, even when the antenna  3  and the antenna  4  share the same ground plane  70 , the directivity of the antenna  3  and the antenna  4  may be switched and controlled while maintaining the correlation coefficient between the antenna  3  and the antenna  4  to a desirably low value. 
     Note that the dimensions of the configurations illustrated in  FIGS. 13 and 9  upon measuring the S 11  and S 22  characteristics of the antenna device  202  were the same as the above dimensions that were used in measuring the S 11  characteristics of the antenna  3  of  FIG. 8 . 
     &lt;Antenna  5 &gt; 
       FIG. 26  is a perspective view of a computer simulation model for analyzing the operation of an antenna  5  according to a fifth embodiment of the present invention. Note that in the present example, CST Microwave Studio (registered trademark) by Computer Simulation Technology AG (CST) was used as an electromagnetic field simulator. Also, note that descriptions of features of the present embodiment that may be substantially identical to those of the embodiments described above may be simplified or omitted. 
     The antenna  5  corresponds to a modification of the antenna  3  that is obtained by cutting out the plate conductor  150  of the antenna illustrated in  FIG. 8  to form an opening  157 . In the antenna  5 , the substrate  80  is visible through the opening  157  in plan view from a direction parallel to the Z-axis. By providing the opening  157  in the plate conductor  150 , the height tolerance for components mounted on the substrate  80  may be increased such that other antennas and components such as IC tags may be mounted, for example. 
       FIG. 27  is a graph indicating S 11  characteristic measurements of the antenna  5  in four cases where the dimension L 21  (see  FIG. 26 ) of the opening  157  is, 0 mm, 20 mm, 40 mm, and 60 mm. Note that “L 21 =0 mm” corresponds to a case where the opening  157  is not provided. In  FIG. 27 , the S 11  characteristic measurements obtained in the four cases substantially overlap. As can be appreciated from  FIG. 27 , even when the opening  157  is formed in the plate conductor  150 , no substantial changes occur in the resonant frequency, and the antenna  5  may be suitably operated. 
     &lt;Antenna  6 &gt; 
       FIG. 28  is a perspective view of an antenna  6  according to a sixth embodiment of the present invention. Note that descriptions of features of the present embodiment that may be substantially identical to those of the embodiments described above may be omitted or simplified. 
     The antenna  6  has the same components as those of the antenna  1  of  FIG. 1 , and the positional relationship between the components of the antenna  6  may be substantially identical to the positional relationship between the components of the antenna  1 . The antenna  6  includes L-shaped radiating elements  30  and  40  that are arranged along the outer edge of the ground plane  70 , and L-shaped control elements  50  and  60  that are arranged along the outer edge of the ground plane  70 . The antenna  6  has a symmetrical configuration with respect to the YZ plane. 
     The radiating element  30  includes a conductive portion extending along the outer edge  71 , and a conductive portion extending along the outer edge  73 . The radiating element  40  includes a conductive portion extending along the outer edge  71 , and a conductive portion extending along the outer edge  74 . The ground plane  70  includes the outer edge  73  and outer edge  74  that oppose each other. 
     By arranging the radiating element  30  and the radiating element  40  such that the ground plane  70  may be interposed between the conductive portion of the radiating element  30  and the conductor portion of the radiating element  40 , directivity control of the antenna  6  may be facilitated. For example, by arranging the radiating element  30  to include a conductive portion that extends along the outer edge  73 , and by arranging the radiating element  40  to include a conductor portion that extends along the outer edge  74  opposing the outer edge  73 , the directivity control of the antenna  6  may be facilitated. 
       FIG. 29  illustrates an exemplary configuration of the impedance control unit  120 . In  FIG. 29 , the impedance control unit  120  includes inductors  243 ,  244 ,  247 ,  248 ,  251 , and  252 , capacitors  249 ,  250 ,  253 , and  254 , variable capacitance diodes  245  and  246 , and DC voltage sources  241  and  242 . 
     One end of the inductor  251  is connected to one end of the control element  50 , and the other end of the inductor  251  is connected to the end portion  21  of the feeding element  20 . A series circuit including the capacitor  253  and the inductor  243  is connected between the positive terminal of the DC voltage source  241  and a connection point between the inductor  251  and the control element  50 . A series circuit including the capacitor  249  and the inductor  247  is connected to a negative terminal of the DC voltage source  241  and a connection point between the inductor  251  and the feeding element  20 . The negative terminal of the DC voltage source  241  is connected to the ground plane  70 . The variable capacitance diode  245  includes a cathode that is connected to a connection point between the capacitor  253  and the inductor  243 , and an anode that is connected to a connection point between the capacitor  249  and the inductor  247 . 
     One end of the inductor  252  is connected to one end of the control element  60 , and the other end of the inductor  252  is connected to the end portion  21  of the feeding element  20 . A series circuit including the capacitor  254  and the inductor  244  is connected to a positive terminal of the DC voltage source  242  and a connection point between the inductor  252  and the control element  60 . A series circuit including the capacitor  250  and the inductor  248  is connected to the negative terminal of the DC voltage source  242  and a connection point between the inductor  252  and the feeding element  20 . The negative terminal of the DC voltage source  242  is connected to the ground plane  70 . The variable capacitance diode  246  includes a cathode that is connected to a connection point between the capacitor  254  and the inductor  244 , and an anode that is connected to a connection point between the capacitor  250  and the inductor  248 . 
     When the DC voltage source  241  controls the output of a DC voltage V 1 , adjusts the capacitance of the variable capacitance diode  245 , and increases the impedance between the feeding element  20  and the control element  50 , an RF current flowing through the control element  50  may be suppressed or stopped. In this way, the connection between the feeding element  20  and the control element  50  may be weakened or disconnected such that the radiating element  30  that is electromagnetically coupled to the feeding element  20  may be able to implement its function as a radiating conductor. 
     Conversely, when the DC voltage source  241  controls the output of the DC voltage V 1 , adjusts the capacitance of the variable capacitance diode  245 , and decreases the impedance between the feeding element  20  and the control element  50 , the RF current flowing though the control element  50  may be increased. In this way, the connection between the feeding element  20  and the control element  50  may be strengthened such that the function of the radiating element  30 , which is electromagnetically coupled to the feeding element  20 , as a radiating conductor may be suppressed or stopped. 
     Similarly, when the DC voltage source  242  controls the output of a DC voltage V 2 , adjusts the capacitance of the variable capacitance diode  246 , an increases the impedance between the feeding element  20  and the control elements  60 , the RF current flowing through the control element  60  may be suppressed or stopped. In this way, the connection between the feeding element  20  and the control element  60  may be weakened or disconnected such that the radiating element  40  that is electromagnetically coupled to the feeding element  20  may implement its function as a radiating conductor. 
     Conversely, when the DC voltage source  242  controls the output of the DC voltage V 2 , adjusts the capacitance of the variable capacitance diode  246 , and decreases the impedance between the feeding element  20  and the control element  60 , the RF current flowing through the control element  60  may be increased. In this way, the connection between the feeding element  20  and the control element  60  may be strengthened such that the function of the radiating element  40 , which is electromagnetically coupled to the feeding element  20 , as a radiating conductor may be suppressed or stopped. 
     By using the impedance control unit  120  as illustrated in  FIG. 29 , the impedance between the feeding element  20  and the control element  50  and the impedance between the feeding element  20  and the control element  60  may be gradually changed (decreased or increased). By gradually changing the impedance, the directivity of the antenna may also be gradually changed according to the surrounding environment rather than being switched on/off, for example. 
       FIG. 30  is graph illustrating an exemplary case where the directivity of the antenna  6  is continuously changed by the impedance control unit  120  as illustrated in  FIG. 29 . Note that in  FIG. 30 , “directivity” represents the directional gain at the resonant frequency of the fundamental mode of the antenna  6  (1.91 GHz in the present example), and φ represents an angle formed with respect to a normal direction of the ground plane  70  within the ZX plane passing through the center point of the ground plane  70  (see  FIG. 28 ). Note that the directivity when φ=0° represents the antenna gain of the antenna  6  in the Z-axis direction. 
     As illustrated in  FIG. 30 , provided the DC voltage V 1  of the DC voltage source  241  is fixed to a predetermined value (zero in the present example), as the DC voltage V 2  of the DC voltage source  242  increases, the angle φ at which the directional gain reaches its peak value continuously changes from an angle close to 0° to 90°. Although not illustrated in  FIG. 30 , in the converse case where the DC voltage V 2  of the DC voltage source  242  is fixed to a predetermined value (e.g., zero), as the DC voltage V 1  of the DC voltage source  241  increases, the angle φ at which the directional gain reaches its peak value continuously changes from an angle close to 0° to −90°. In this way, the impedance control unit  120  is capable of continuously changing the directivity of the antenna  6 . 
     Note that in measuring the directivity of the antenna  6  in  FIG. 30 , the dimensions of the configuration illustrated in  FIG. 28  were set up as follows (in mm). 
     L 11 : 120 
     L 12 : 68.2 
     L 13 : 38.75 
     L 14 : 8.525 
     L 15   a:  21.475 
     L 15   b:  34.1 
     L 16   a:  23.675 
     L 16   b:  8.525 
     L 23 : 60 
     Also, the line widths of the feeding element  20 , the radiating elements  30  and  40 , and the control elements  50  and  60  were set to 1 mm. 
     Also, in obtaining the measurements of  FIG. 30 , the dimensions of the configuration illustrated in  FIG. 2  were set up as follows. That is, the substrate  80  was set up to have a relative dielectric constant of ∈ r =3.3, a loss tangent of tan δ=0.003, and a thickness of H 1 =0.8 mm; and the substrate  110  was set up to have a relative dielectric constant of ∈ r =7.44, a loss tangent of tan δ=0.011, and a thickness of H 3 =1.1 mm. Also, the gap H 2  between the substrate  80  and the substrate  110  was set to 2 mm. 
     Also, in obtaining the measurements of FIG.  30  the component illustrated in  FIG. 29  were set up as follows. That is, the inductance of the inductors  251  and  252  were set to 1.5 nH, the inductance of the inductors  243 ,  244 ,  247 , and  248  were set to 15 nH, the capacitance of the capacitors  249 ,  250 ,  253 , and  254  were set to 2.2 pF. 
     &lt;Antenna Device  203 &gt; 
       FIG. 31  is a plan view of an antenna device  203  including four antennas  211 ,  212 ,  213  and  214  that have the same configuration as the antenna  1  of  FIG. 1 . The antenna  211  includes radiating elements having conductor portions arranged along the outer edge  71  of the ground plane  70 . The antenna  212  includes radiating elements having conductor portions arranged along the outer edge  72  opposing the outer edge  71 . The antenna  213  includes radiating elements having conductor portions arranged along the outer edge  73 . The antenna  214  includes radiating elements having conductor portions arranged along the outer edge  74  opposing the outer edge  73 . 
     By including the antennas  211 ,  212 ,  213 , and  214  in the antenna device  203 , the antenna device  203  may function as a four-channel MIMO (Multiple Input Multiple Output) antenna. Also, even when the antennas of the antenna device  203  share the same ground plane  70 , the antenna device  203  may be capable of switching and controlling the directivity of each of the antennas while maintaining the correlation coefficients between the antennas to desirably low values, regardless of the impedance of the impedance adjusting units  121  and  122  of the antennas. 
       FIG. 32  is a plan view of an antenna device  204  including four antennas  221 ,  222 ,  223 , and  224  having configurations similar to that of the antenna  1  of  FIG. 1 . The antenna  221  includes radiating elements having conductor portions arranged along the outer edges  71  and  73 . The antenna  222  includes radiating elements having conductor portions arranged along the outer edges s  72  and  73 . The antenna  223  includes radiating elements having conductor portions arranged along the outer edges  72  and  74 . The antenna  224  includes radiating elements having conductor portions arranged along the outer edges  71  and  74 . 
     The antenna device  204  may also function as a four-channel MIMO (Multiple Input Multiple Output) antenna in a manner similar to the antenna device  203  of  FIG. 31 . The antenna device  204  may also be capable of switching and controlling the directivity of each of the antennas while maintaining the correlation coefficients between the antennas to desirably low values. 
     Although an antenna, an antenna device, and a wireless device according to the present invention have been described above with respect to certain illustrative embodiments, the present invention is not limited to these embodiments and various modifications and improvements may be made without departing from the scope of the present invention. 
     For example, the configuration of the antenna is not limited to the specific embodiments described above. For example, the antenna may include a conductor portion that is directly connected to a radiating element or indirectly connected to the radiating element via a connection conductor. Also, the antenna may include a conductor portion that is coupled to a radiating element through high-frequency (e.g., capacitive) coupling. 
     Also, the feeding element, the radiating element, and the control element are not limited to linear conductors extending linearly but may include a curved conductor portion. For example, the feeding element, the radiating element, and/or the control element may include an L-shaped conductor portion, a meander-shaped conductor portion, or a conductor portion with branches spreading out from an intermediate point. 
     Also, the transmission line including the ground plane is not limited to a microstrip line. For example, a strip line or a coplanar waveguide with a ground plane (coplanar waveguide with a ground plane arranged on a surface on the opposite side of a conductor surface) may be used. 
     Also, the outer profile of the ground plane is not limited those illustrated in the drawings. That is, the ground plane may be a conductive pattern having other outer profiles. Also, the ground plane is not limited to a planar shape and may alternatively be arranged into a curved shape, for example. Similarly, the outer profile of the plate conductor is not limited to those illustrated in the drawings but it may be a conductor having other outer profiles. Also, the plate conductor is not limited to a planar shape and may alternatively be arranged into a curved shape. 
     Also, note that the term “plate” used above in describing the configuration of a conductor and the like may also encompass configurations arranged into a “foil” or a “film”, for example. 
     Also, note that by arranging the lengths of the radiating elements (e.g., radiating elements  30  and  40  in the case of  FIG. 1 ) running parallel to the outer edge of the ground plane to be equal to each other, the directivity control of the antenna may be facilitated. 
     Also, by controlling the directivity of the antennas provided in an antenna device to be directed in the same direction, the antenna device may function as a diversity antenna.