Patent Publication Number: US-10312589-B2

Title: Antenna directivity control system and radio 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/JP2015/051017 filed on Jan. 16, 2015 and designating the U.S., which claims priority of Japanese Patent Application No. 2014-008169 filed on Jan. 20, 2014. The entire contents of the foregoing applications are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an antenna directivity control system and a radio device provided with the antenna directivity control system (e.g., a mobile radio terminal, such as a cellular phone). 
     2. Description of the Related Art 
     As a method for enhancing communication speed, a MIMO spatial multiplexing communication technique by using a MIMO (Multiple Input Multiple Output) antenna has been utilized. A MIMO antenna is a multi-antenna capable of multiple inputs and multiple outputs at a predetermined frequency by using a plurality of antenna elements. However, for mobile communication, radio wave propagation environments for a terminal are diversified; and, in fact, environments where the MIMO spatial multiplexing communication can be utilized are limited. 
     For example, Non-Patent Document 1 (Tetsuro Imai, etc., “A Propagation Prediction System for Urban Area Macrocells Using Ray-tracing Methods” NTT DoCoMo Technical Journal, Vol. 6, No. 1, p. 41-51) discloses actually measured data of an angle spread of an incoming wave in an urban area. It shows that, even in an urban area where there are relatively many reflection objects, such as buildings, an angle spread of an incoming wave is less than or equal to 30 degrees, so that a sufficiently rich multi-path environment may not be obtained. 
     Because of the presence of such a fact, in the 3GPP standard that is indicated as Non-Patent Document 2 (3GPP TS 36.213 V10.1.0 3 rd  Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures (Release 10), p. 26-27), in addition to the MIMO spatial multiplexing mode, nine transmission modes, such as a beam forming mode, a transmit diversity mode, and a multi-user MIMO mode, are specified in total. A method has been adopted such that a radio wave environment where a terminal is located is measured based on a reference signal that is transmitted from a base station, and a proper transmission mode is selected. 
     However, since antenna characteristics required for an antenna are different for a case of transmitting by the MIMO spatial multiplexing mode and for a case of transmitting by the beam forming mode, it is difficult to commonize the antenna, so that, currently, these are supported by separate antennas. 
     There is a need for an antenna directivity control system where different antenna characteristics can be supported by a common antenna. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, there is provided an antenna directivity control system including an antenna including a plurality of antenna elements, feeding points for the plurality of antenna elements being mutually different; and a controller for controlling weight for each of the plurality of antenna elements, wherein each of the plurality of antenna elements includes a feed element connected to the feed point, and a radiating element that functions, upon power being fed by establishing electromagnetic field coupling with the feed element, as a radiating conductor, and wherein the controller controls a directivity of the antenna by adjusting an amplitude of a signal at each of the feeding points. 
     According to another aspect of the present invention, there is provided a radio device including an antenna directivity control system, wherein the antenna directivity control system includes an antenna including a plurality of antenna elements, feeding points for the plurality of antenna elements being mutually different; and a controller for controlling weight for each of the plurality of antenna elements, wherein each of the plurality of antenna elements includes a feed element connected to the feed point, and a radiating element that functions, upon power being fed by establishing electromagnetic field coupling with the feed element, as a radiating conductor, and wherein the controller controls a directivity of the antenna by adjusting an amplitude of a signal at each of the feeding points. 
     According to an embodiment, different antenna characteristics can be supported by a common antenna. 
     Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating an example of a configuration of an antenna directivity control system; 
         FIG. 2  is a plan view illustrating an example of an antenna including a plurality of antenna elements, where feeding points for the plurality of antenna elements are mutually different; 
         FIG. 3  is a diagram illustrating an example of a positional relationship among components of the antenna; 
         FIG. 4  is a characteristic diagram illustrating an example of a simulation result of a correlation coefficient of the antenna; 
         FIG. 5  is a characteristic diagram illustrating an example of directivity of the antenna; 
         FIG. 6  is a plan view illustrating an example of the antenna including the plurality of antenna elements, where the feeding points for the plurality of antenna elements are mutually different; 
         FIG. 7  is a characteristic diagram illustrating an example of an experimental result of S parameters of the antenna; and 
         FIG. 8  is a characteristic diagram illustrating an example of an experimental result of the correlation coefficient of the antenna. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     &lt;Configuration of an Antenna Directivity Control System  10 &gt; 
       FIG. 1  is a block diagram illustrating a configuration example of an antenna directivity control system  10  according to an embodiment of the present invention. The antenna directivity control system  10  is, for example, an antenna system that is installed in a radio communication device  100 . As an example of the radio communication device  100 , there is a mobile entity itself or a radio communication device that is installed inside the mobile entity. As examples of the mobile entity, there are a portable mobile terminal device, a vehicle, such as an automobile, a robot, and so forth. As specific examples of the mobile terminal device, there are electronic devices, such as a cellular phone, a smartphone, a tablet type computer, a gaming device, a television, an audio or video player, and so forth. 
     The antenna directivity control system  10  includes an antenna  13  including a plurality of antenna elements  11  and  12 ; a signal processing circuit  23 ; a controller  24 ; and a plurality of weight control circuits  21  and  22 . The antenna elements  11  and  12  are connected to mutually different feeding points. 
     The two antenna elements  11  and  12  can receive an incoming radio wave (an incoming wave) or transmit a signal of the radio communication device  100 ; and by adjusting amplitudes of electric currents flowing through the two antenna elements  11  and  12 , directivity of the antenna  13  can be controlled. 
     The signal processing circuit  23  is a circuit that processes a received signal that is obtained by receiving the incoming wave by the antenna elements  11  and  12 , or that processes the transmit signal of the radio communication device  100 . The signal processing circuit  23  is a circuit that applies, to the received signal obtained by the antenna elements  11  and  12 , a high frequency process, such as amplification and AD conversion, or a baseband process, for example. 
     The controller  24  is an example of a selector for selecting, as a transmission mode to be applied to the antenna  13 , a MIMO spatial multiplexing mode or a beam forming mode. The controller  24  outputs, to the weight control circuits  21  and  22 , control signals corresponding to the selected transmission mode. 
     The controller  24  selects, for example, a transmission mode to be applied to the antenna  13 , depending on a result of measuring, by the signal processing circuit  23 , a radio wave environment in the vicinity of the antenna elements  11  and  12  by using the antenna elements  11  and  12 . Upon detecting that a radio wave environment suitable for transmission by the MIMO spatial multiplexing mode is measured, the controller  24  selects the MIMO spatial multiplexing mode, as a transmission mode to be applied to the antenna  13 . For the case of the MIMO spatial multiplexing mode, if the antenna  13  includes a plurality of antenna elements, the antenna  13  becomes a MIMO antenna for the plurality of channels. For example, assuming that there are two antenna elements  11  and  12 , as illustrated in  FIG. 1 , the antenna  13  becomes the MIMO antenna for two channels. Whereas, upon detecting that a radio wave environment is suitable for transmission by the beam forming mode, the controller  24  selects the beam forming mode, as the transmission mode to be applied to the antenna  13 . For the case of the beam forming mode, the antenna  13  becomes the antenna capable of a directivity control by using the two antenna elements  11  and  12 . 
     The weight control circuits  21  and  22  are an example of a controller for controlling the directivity of the antenna  13  in accordance with a control signal from the controller  24 . The weight control circuit  21  and  22  control, for example, the directivity of the antenna  13 , which is based on maximum ratio combining of the antenna element  11  and the antenna element  12 , by controlling weight, such as amplitudes and phases of the signals received by the antenna elements  11  and  12 , respectively, or amplitudes and phases of the signals to be transmitted. In order to control the directivity of the antenna  13 , the weight control circuits  21  and  22  adjust, for example, a current value of an electric current that flows through each of the feeding points of the antenna elements  11  and  12 . 
     &lt;Configuration of an Antenna  1 &gt; 
       FIG. 2  is a plan view schematically illustrating an example of a configuration of an antenna  1  according to the embodiment of the present invention. The antenna  1  is an example of the antenna  13  illustrated in  FIG. 1 . The antenna  1  includes a ground plane  70 ; an antenna element  30 ; and an antenna element  40 . 
     The ground plane  70  is a planar conductor pattern; and, in the figure, the ground plane  70  is exemplified that has a rectangular shape extending in the XY-plane. The ground plane  70  includes, for example, outer edge portions  71  and  72  that linearly extend in the X-axis direction; and outer edge portions  73  and  74  that linearly extend in the Y-axis direction. The outer edge portion  72  is the opposite side of the outer edge portion  71 ; and the outer edge portion  74  is the opposite side of the outer edge portion  73 . The ground plane  70  is arranged, for example, parallel to the XY-plane; and the ground plane  70  has a rectangular outer shape with a lateral length of L 7 , which is parallel to the X-axis direction, and a vertical length of L 4 , which is parallel to the Y-axis direction. The ground plane  70  is laminated on a substrate  25  (cf.,  FIG. 3 ); the ground plane  70  may be installed on a surface layer (outer layer) of the substrate  25 , or the ground plate  70  may be installed on an inner layer of the substrate  25 . The ground plane  70  is a ground part with a ground potential. It is preferable that the ground plane  70  be a ground portion with an area that is greater than or equal to a predetermined value, so that impedance matching of the antenna can be easily achieved; however, the ground plane  70  may be a ground part to which components implemented on the substrate  25 , such as a capacitor, are electrically connected. 
     The antenna elements  30  and  40  are connected to mutually different feeding points. The antenna element  30  is connected to a feeding point  38  with a ground end, which is the outer edge portion  71 ; and the antenna element  40  is connected to a feeding point  48  with a ground end, which is the outer edge portion  71 , and which is the same as that of the feeding point  38 . The ground plane  70  is a ground reference that is in common with the feeding point  38  and the feeding point  48 . 
     The feeding point  38  and the feeding point  48  are arranged in close proximity to each other. The feeding point  38  is installed at a position closer to the feeding point  48 , compared to one end  71   a  of the outer edge portion  71  in the X-axis direction (for the depicted case, an intersection point between the outer edge portion  71  and the outer edge portion  74 ). The feeding point  48  is installed at a position closer to the feeding point  38 , compared to the other end  71   b  of the outer edge portion  71  in the X-axis direction (for the depicted case, an intersection point between the outer edge portion  71  and the outer edge portion  73 ). Since the feeding point  38  and the feeding point  48  are arranged in close proximity to each other, strip conductors that are connected to the feeding point  38  and the feeding point  48  can be approximated to each other, so that a space required for installing the antenna elements  30  and  40  can be easily reduced. 
     The antenna element  30  is an example of an antenna element with a feed element  37  and a radiating element  31 ; and the antenna element  40  is an example of an antenna element with a feed element  47  and a radiating element  41 . 
     It is preferable that the shapes of the antenna element  30  and the antenna element  40  be line symmetric with respect to an axis of symmetry that is a straight line parallel to the Y-axis (line symmetric with respect to the YZ-plane that passes through between the feeding point  38  and the feeding point  48 ), so that the directivity of the antenna  1  can be easily controlled. If these are line symmetric, the total length of the feed element  37  is equal to the total length of the feed element  47 ; and the total length of the radiating element  31  is equal to the total length of the radiating element  41 . 
     The feed element  37  is an example of a feed element connected to the feeding point  38  for which the ground plane  70  is the ground reference. The feed element  37  is a line shaped conductor that can feed power by being contactlessly coupled to the radiating element  31  in a high-frequency coupling. In the figure, the feed element  37  is exemplified, which is formed to have an L-shape by a linear conductor that extends in a direction perpendicular to the outer edge portion  71  and parallel to the Y-axis; and by a linear conductor that extends by running in parallel with the outer edge portion  71 , which is parallel to the X-axis. For the depicted case, the feed element  37  extends in the Y-axis direction from the feeding point  38 , as a starting point; and then the feed element  37  is bent in the X-axis direction, and extends in the X-axis direction until an end portion  39  of the extension in the X-axis direction. The end portion  39  is an open end to which no other conductor is connected. The feed element  37  is not limited to the depicted shape. 
     The feeding point  38  is a feeding part that is to be connected to a predetermined transmission line or a feeder line that utilizes the ground plane  70 . As specific examples of the predetermined transmission line, there are a microstripline, a strip line, a coplanar waveguide with a ground plane (a coplanar waveguide where the ground plane is installed on a surface that is opposite to a conductor surface), and so forth. As the feeder line, there are feeder wire and a coaxial cable. 
     The radiating element  31  is arranged to be separated from the feed element  37 ; and the radiating element  31  is an example of a radiating element that functions, upon power being fed by establishing electromagnetic field coupling with the feed element  37 , as a radiating conductor. The radiating element  31  is a linear conductor with a feeding part  36  at which power is contactlessly fed from the feed element  37 . 
     In the figure, the radiating element  31  is exemplified, which is formed to have an L-shape. The L-shaped radiating element  31  includes a conductor portion  31   a  that is arranged to be separated from the outer edge portion  71  and that extends in the X-axis direction to follow the outer edge portion  71 ; and a conductor portion  31   b  that is arranged to be separated from the outer edge portion  74  and that extends in the Y-axis direction to follow the outer edge portion  74 . In the figure, the L-shaped radiating element  31  is exemplified; however, the shape of the radiating element  31  may be another shape, such as a single straight-line shape or a meander shape. 
     By including, in the radiating element  31 , the conductor portion  31   a  along the outer edge portion  71 , or by including, in the radiating element  31 , the conductor portion  31   b  along the outer edge portion  74 , the directivity of the antenna element  30  can be easily adjusted, for example. 
     Further, by extending the conductor portion  31   a  in the X-axis direction so as to intersect the Y-axis direction in which the conductor portion  41   b  extends, the directivity of the antenna  1  can be easily controlled, for example. Similarly, by extending the conductor portion  31   b  in the Y-axis direction so as to intersect the X-axis direction in which the conductor portion  41   a  extends, the directivity of the antenna  1  can be easily controlled, for example. 
     Since the ground plane  70  that is used in common by the feeding point  38  and the feeding point  48  is located between the conductor portion  31   b  of the radiating element  31  and the conductor portion  41   b  of the radiating element  41 , the directivity of the antenna  1  can be easily controlled, for example. 
     The radiating element  31  and the feed element  37  may be overlapped or may not be overlapped in a plan view in any direction, such as the X-axis direction, the Y-axis direction, or the Z-axis direction, as long as the feed element  37  is separated from the radiating element  31  by a distance with which electromagnetic field coupling can be established, so that the feed element  37  can contactlessly feed power to the radiating element  31 . 
     The feed element  37  and the radiating element  31  are arranged to be separated by a distance with which they can mutually establish electromagnetic field coupling. The radiating element  31  includes the feeding part  36  at which power is fed from the feed element  37 . The radiating element  31  is contactlessly fed power at the feeding part  36  through the feed element  37  by electromagnetic field coupling. By being fed power in this manner, the radiating element  31  functions as a radiating conductor of the antenna element  30 . 
     As depicted, if the radiating element  31  is a linear conductor connecting the two points, a resonance current (distribution) similar to that of a half-wavelength dipole antenna is formed on the radiating element  31 . Namely, the radiating element  31  functions as a dipole antenna (which is referred to as a “dipole mode,” hereinafter) that resonates at a half-wavelength of a predetermined frequency. 
     The electromagnetic field coupling is coupling that utilizes a resonance phenomenon of an electromagnetic field; and the electromagnetic field coupling is disclosed, for example, in a non-patent document (A. Kurs, et al, “Wireless Power Transfer via Strongly Coupled Magnetic Resonances,” Science Express, Vol. 317, No. 5834, pp. 83-86, July 2007). The electromagnetic field coupling is also referred to as electromagnetic field resonant coupling or electromagnetic field resonance coupling; and the electromagnetic field coupling is a technique for transmitting energy, by placing resonators that resonate at the same frequency in close proximity to each other, and by causing one of the resonators to be resonated, to the other resonator through coupling in a near field (a non-radiation field area) that is formed between the resonators. Additionally, the electromagnetic field coupling means coupling by an electric field and a magnetic field at a high frequency excluding capacitive coupling and coupling by electromagnetic induction. Here, “excluding capacitive coupling and coupling by electromagnetic induction” does not mean that all of these couplings disappear, and it implies that these couplings are so small to the extent that no effect is caused. A medium between the feed element  37  and the radiating element  31  may be the air, or a dielectric, such as a glass and a resin. Note that it is preferable not to place a conductive material, such as a ground plane or a display, between the feed element  37  and the radiating element  31 . 
     By establishing the electromagnetic field coupling between the feed element  37  and the radiating element  31 , a structure that is robust against impact can be obtained. Namely, by using the electromagnetic field coupling, power can be fed to the radiating element  31  by using the feed element  37  without physically contacting the feed element  37  and the radiating element  31 , so that the structure can be obtained that is robust against the impact, compared to a contact power feeding method with which a physical contact is required. 
     By establishing the electromagnetic field coupling between the feed element  37  and the radiating element  31 , contactless power feeding can be implemented with a simple structure. Namely, by using the electromagnetic field coupling, power can be fed to the radiating element  31  by using the feed element  37  without physically contacting the feed element  37  and the radiating element  31 , so that power feeding can be achieved with the simple structure, compared to the contact power feeding method with which a physical contact is required. Additionally, by using the electromagnetic field coupling, power can be fed to the radiating element  31  by using the feed element  37  without including a redundant component, such as a capacitor plate, so that power feeding can be achieved with the simple structure, compared to a case where power is fed by capacitive coupling. 
     Furthermore, even if clearance (a coupling distance) between the feed element  37  and the radiating element  31  is increased, a total efficiency (an antenna gain) of the radiating element  31  tends not to be lowered for a case where power is fed by electromagnetic field coupling, compared to a case where power is fed by capacitive coupling or by magnetic field coupling. Here, the total efficiency is a quantity that is calculated as a product of radiation efficiency and a return loss of an antenna; and the total efficiency is a quantity that is defined as antenna efficiency with respect to input power. Thus, by establishing electromagnetic coupling between the feed element  37  and the radiating element  31 , degrees of freedom of determining installation positions of the feed element  37  and the radiating element  31  can be increased, whereby positional robustness can be enhanced. Note that high positional robustness means that even if the installation positions of the feed element  37  and the radiating element  31  are shifted, an effect that is caused to the total efficiency of the radiating element  31  is small. It is also advantageous in a point that, since the degrees of freedom of determining the installation positions of the feed element  37  and the radiating element  31  are large, a space required for installing the antenna element  30  can be easily reduced. 
     Further, for the depicted case, the feeding part  36  that is a part at which the feed element  37  feeds power to the radiating element  31  is located at a part other than a center portion  33  between one end portion  34  and the other end portion  35  of the radiating element  31  (the part between the center portion  33  and the end portion  34  or the end portion  35 ). In this manner, by locating the feeding part  36  at the part of the radiating element  31  other than the part with the lowest impedance at the resonance frequency of a fundamental mode of the radiating element  31  (the center portion  33  in this case), matching of the antenna element  30  can be easily achieved. The feeding part  36  is defined to be the part, which is closest to the feeding point  38 , of the conductor part of the radiating element  31  where the radiating element  31  and the feed element  37  are the closest to each other. 
     The impedance of the radiating element  31  increases, as a position separates from the center portion  33  of the radiating element  31  toward the end portion  34  or the end portion  35 . For a case of high impedance coupling of the electromagnetic field coupling, even if the impedance between the feed element  37  and the radiating element  31  is slightly changed, an effect caused to the impedance matching is small, as long as the coupling with the impedance that is greater than or equal to a certain level is maintained. Thus, the feeding part  36  of the radiating element  31  is preferably located at a high-impedance portion of the radiating element  31 , so that the matching can be easily achieved. 
     For example, in order to easily achieve impedance matching of the antenna element  30 , the feeding part  36  can be located at a portion that is separated from the portion with the lowest impedance at the resonance frequency of the fundamental mode of the radiating element  31  (the center portion  33 , in this case) by a distance that is greater than or equal to ⅛ of the entire length of the radiating element  31  (preferably greater than or equal to ⅙; and more preferably greater than or equal to ¼). For the depicted case, the entire length of the radiating element  31  corresponds to L 1 +L 5 ; and the feeding part  36  is located at the side of the end portion  34  with respect to the center portion  33 . 
     Further, for a case where the wavelength of the radio wave at the resonance frequency of the fundamental mode of the radiating element  31  in vacuum is λ 0 , the shortest distance D 1  between the feeding part  36  and the ground plane  70  is greater than or equal to 0.0034λ 0  and less than or equal to 0.21λ 0 . The shortest distance D 1  is more preferably greater than or equal to 0.0043λ 0  and less than or equal to 0.199λ 0 , and further more preferably greater than or equal to 0.0069λ 0  and less than or equal to 0.164λ 0 . It is advantageous to set the shortest distance D 1  to be within such a range in a point to enhance the total efficiency of the radiating element  31 . Furthermore, since the shortest distance D 1  is less than (λ 0 /4), the antenna element  30  generates a linearly polarized wave, instead of generating circularly polarized wave. 
     Note that the shortest distance D 1  corresponds to the distance of a straight line connecting the closest portions of the feeding part  36  and the outer edge portion  71 ; and the outer edge portion  71  for this case is the outer edge portion of the ground plane  70  that is the reference of the ground of the feeding point  38 , which is connected to the feed element  37  for feeding power to the feeding part  36 . Additionally, the radiating element  31  and the ground plane  70  may be on the same plane, or on different planes. Furthermore, the radiating element  31  may be installed on a plane that is parallel to a plane on which the ground plane  70  is installed; or the radiating element  31  may be installed on a plane that intersects the plane on which the feed element  37  in installed at any angle. 
     Furthermore, for a case where a wavelength of a radio wave at the resonance frequency of the fundamental mode of the radiating element  31  in vacuum is λ 0 , the shortest distance D 2  between the feed element  37  and the radiating element  31  is preferably less than or equal to 0.2×λ 0  (more preferably less than or equal to 0.1×λ 0 , and further more preferably less than or equal to 0.05×λ 0 ). It is advantageous to install the feed element  37  and the radiating element  31  to be separated by the shortest distance D 2  in a point to enhance the total efficiency of the radiating element  31 . 
     Note that the shortest distance D 2  corresponds to the distance of a straight line connecting the closest portions of the feeding part  36  and the feed element  37  for feeding power to the feeding part  36 . Further, when the feed element  37  and the radiating element  31  are viewed in any direction, the feed element  37  may or may not intersect the radiating element  31 , and the angle of the intersection may be any angle, as long as electromagnetic coupling is established between them. Additionally, the radiating element  31  and the feed element  37  may be on the same plane, or on different planes. Furthermore, the radiating element  31  may be installed on a plane that is parallel to a plane on which the feed element  37  is installed; or the radiating element  31  may be installed on a plane that intersects the plane on which the feed element  37  in installed at any angle. 
     Additionally, a distance with which the feed element  37  and the radiating element  31  are extended in parallel while separated by the shortest distance D 2  is preferably less than or equal to ⅜ of the physical length of the radiating element  31 . It is more preferably less than or equal to ¼, and further more preferably less than or equal to ⅛. 
     The position of the shortest distance D 2  is the portion where the coupling between the feed element  37  and the radiating element  31  is strong, so that, if the distance with which the feed element  37  and the radiating element  31  are extended in parallel while separated by the shortest distance D 2  is long, strong coupling is made at a high impedance portion and a low impedance portion of the radiating element  31 , and impedance matching may not be achieved. Thus, it is advantageous, in a point of impedance matching, that the distance with which these are extended in parallel while separated by the shortest distance D 2  is short, so that strong coupling is made only at a portion of the radiating element  31  where a variation of the impedance is small. 
     Furthermore, assuming that an electrical length that induces the fundamental mode of the resonance of the feed element  37  is Le 37 , an electrical length that induces the fundamental mode of the resonance of the radiating element  31  is Le 31 , and the wavelength on the feed element  37  or the radiating element  31  at the resonance frequency f 1  of the fundamental mode of the radiating element  31  is λ, it is preferable that Le 37  be less than or equal to (⅜)·λ; and that Le 31  be greater than or equal to (⅜)·λ and less than or equal to (⅝)·λ. 
     Additionally, since the ground plane  70  is formed in such a manner that the outer edge portion  71  follows the radiating element  31 , the feed element  37  can form, by the interaction with the outer edge portion  71 , a resonance current (distribution) on the feed element  37  and the ground plane  70 , and the feed element  37  resonates with the radiating element  31  to establish the electromagnetic field coupling. Thus, there is no specific lower limit value for the electrical length Le 37  of the feed element  37 , and the electrical length Le 37  may be a length with which the feed element  37  can physically establish electromagnetic field coupling with the radiating element  31 . 
     Additionally, if it is desirable to add a degree of freedom to the shape of the feed element  37 , Le 37  is more preferably greater than or equal to (⅛)·λ and less than or equal to (⅜)·λ, and especially preferably greater than or equal to ( 3/16)·λ and less than or equal to ( 5/16)·λ. It is preferable that Le 37  be within this range because the feed element  37  favorably resonates at a design frequency (the resonance frequency f 1 ) of the radiating element  31 , and consequently the feed element  37  and the radiating element  31  resonate without depending on the ground plane  70 , so that favorable electromagnetic field coupling can be obtained. 
     Here, the fact that electromagnetic field coupling is established implies that matching is achieved. Further, in this case, it is not necessary to design the electrical length of the feed element  37  to adjust to the resonance frequency f of the radiating element  31 , and the feed element  37  can be freely designed as a radiation conductor, so that the antenna element  30  to support multi-frequency can be easily achieved. 
     Note that, for a case where, for example, a matching circuit is not included, the physical length L 37  (which corresponds to L 2 +L 3  for the depicted case) of the feed element  37  is determined by λ g1 =λ 0 ·k 1 , where λ 0  is the wavelength of the radio wave at the resonance frequency of the fundamental mode of the radiating element in vacuum, and k 1  is a shortening coefficient of a wavelength shortening effect caused by an environment of implementation. Here, k 1  is a value that is calculated from a relative dielectric constant, relative permeability, thickness, a resonance frequency, and so forth of a medium (an environment) of, for example, a dielectric substrate, in which the feed element is installed, such as an effective dielectric constant (ε r1 ) and effective relative permeability (μ r1 ) of an environment of the feed element  37 . Namely, L 37  is less than or equal to (⅜)·λ g1 . Note that the shortening coefficient may be calculated from the above-described physical properties, or the shortening coefficient may be obtained by actual measurement. For example, a resonance frequency is measured for a target element installed in an environment for which a shortening coefficient is to be measured, and a resonance frequency is measured for the same element in an environment where a shortening coefficient for each of frequencies is known. Then, the shortening coefficient may be calculated from the difference between these resonance frequencies. 
     The physical length L 37  of the feed element  37  is a physical length providing Le 37 , and, for an ideal case where no other elements are included, L 37  is equal to Le 37 . For a case where the feed element  37  includes a matching circuit, L 37  is preferably greater than zero and less than or equal to Le 37 . L 37  can be shortened (the size is reduced) by using a matching circuit, such as an inductor. 
     Further, the fundamental mode of the resonance of the radiating element  31  is the dipole mode (a linear conductor such that both ends of the radiating element  31  are open ends); and Le 31  is 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 especially preferably greater than or equal to ( 15/32)·λ and less than or equal to ( 17/32)·λ. Additionally, when higher-order modes are considered, Le 31  is preferably greater than or equal to (⅜)·λ·m and less than or equal to (⅝)·λ·m; more preferably greater than or equal to ( 7/16)·λ·m and less than or equal to ( 9/16)·λ·m; and especially preferably greater than or equal to ( 15/32)·λ·m and less than or equal to ( 17/32)·λ·m. Note that m is a mode number of the higher-order mode, and it is a natural number. It is preferable that m be an integer from 1 to 5; and it is particularly preferable that it be an integer from 1 to 3. The case where m=1 is the fundamental mode. It is preferable that L 31  be within this range because the radiating element  31  sufficiently functions as a radiation conductor, and the antenna efficiency is favorable. 
     Note that the physical length L 31  of the radiating element  31  is determined by λ g2 =λ 0 ·k 2 , where λ 0  is the wavelength of the radio wave at the resonance frequency of the fundamental mode of the radiating element in vacuum, and k 2  is a shortening coefficient of a wavelength shortening effect caused by an environment of implementation. Here, k 2  is a value that is calculated from a relative dielectric constant, relative permeability, thickness, a resonance frequency, and so forth of a medium (an environment) of, for example, a dielectric substrate, in which the radiating element is installed, such as an effective dielectric constant (ε r2 ) and effective relative permeability (μ r2 ) of an environment of the radiating element  31 . Namely, L 31  is ideally (½)·λ g2 . The length L 31  of the radiating element  31  is preferably greater than or equal to (¼)·λ g2  and less than or equal to (¾)·λ g2 , and more preferably greater than or equal to (⅜)·λ g2  and less than or equal to (⅝)·λ g2 . 
     A physical length L 31  of the radiating element  31  is a physical length providing Le 31 , and, for an ideal case where no other elements are included, L 31  is equal to Le 31 . Even if L 31  is shortened by using a matching circuit, such as an inductor, L 31  is preferably greater than zero and less than or equal to Le 31 , and particularly preferably greater than or equal to 0.4 times Le 31  and less than or equal to 1 times Le 31 . It is advantageous to adjust the length L 31  of the radiating element  31  to be such a length in a point to enhance the total efficiency of the radiating element  31 . 
     Additionally, as depicted, for a case where the interaction between the feed element  37  and the outer edge portion  71  of the ground plane  70  can be used, the feed element  37  may be caused to function as a radiating conductor. The radiating element  31  is a radiating conductor that functions, upon power being contactlessly fed at the feeding part  36  by the feed element  37  through electromagnetic field coupling, as a λ/2 dipole antenna, for example. Whereas, the feed element  37  is a linear feed conductor capable of feeding power to the radiating element  31 ; however, the feed element  37  is a radiating conductor that can function, upon power being fed at the feeding point  38 , as a monopole antenna (e.g., a λ/4 monopole antenna). By setting the resonance frequency of the radiating element to be f 1  and the resonance frequency of the feed element  37  to be f 2 , and by adjusting the length of the feed element  37 , so that the feed element  37  is a monopole antenna that resonates at the frequency f 2 , the radiation function of the feed element can be utilized, whereby the antenna element  30  to support multi-frequency can be easily achieved. 
     Note that, for a case where, for example, a matching circuit is not included, the physical length L 37  of the feed element  37  for the case of using the radiation function is determined by λ g3 =λ 1 ·k 1 , where λ 1  is the wavelength of the radio wave at the resonance frequency f 2  of the feed element  37  in vacuum, and k 1  is a shortening coefficient of a wavelength shortening effect caused by an environment of implementation. Here, k 1  is a value that is calculated from a relative dielectric constant, relative permeability, thickness, a resonance frequency, and so forth of a medium (an environment) of, for example, a dielectric substrate, in which the feed element is installed, such as an effective dielectric constant (ε r1 ) and effective relative permeability (μ r1 ) of an environment of the feed element  37 . Namely, L 37  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 . 
     Here, power may be fed to a plurality of radiating elements from the single feed element  37 . By using the plurality of radiating elements, it can be facilitated to achieve multi-band, wide-band, directivity control, and so forth. Furthermore, a plurality of antennas  1  may be installed in a single radio communication device. 
     Since the antenna element  40  has a structure that is the same as that of the antenna element  30 , for the description of the antenna element  40 , a reference is made to the description of the antenna element  30 . 
       FIG. 3  is a diagram schematically illustrating a positional relationship (positional relationship in the height direction that is parallel to the Z-axis) of each of the components of the antenna  1  in the Z-axis direction. At least two of the feed element  37 , the radiating element  31 , and the ground plate  70  may be conductors with portions that are arranged at different heights from each other, or may be conductors with portions that are arranged at the same height. 
     The feed element  37  is installed on the surface of the substrate  25  facing the radiating element  31 . However, the feed element  37  may be installed on the surface of the substrate  25  that is opposite to the side facing the radiating element  31 , may be installed on a lateral surface of the substrate  25 , may be installed inside the substrate  25 , or may be installed in a member other than the substrate  25 . 
     The ground plane  70  is installed on the surface of the substrate  25  that is opposite to the side facing the radiating element  31 . However, the ground plane  70  may be installed on the surface of the substrate  25  facing the radiating element  31 , may be installed on a lateral surface of the substrate  25 , may be installed inside the substrate  25 , or may be installed in a member other than the substrate  25 . 
     The substrate  25  includes the feed element  37 ; the feeding point  38 , and the ground plane  70  that is the reference of the ground of the feed point  38 . Additionally, the substrate  25  includes a transmission line including a strip conductor  27  to be connected to the feeding point  38 . The strip conductor  27  is, for example, a signal line formed on the surface of the substrate  25 , so that substrate  25  is disposed between the strip conductor  27  and the ground plane  70 . 
     The radiating element  31  is arranged to be separated from the feed element  37 ; and, as depicted, the radiating element  31  is installed on the substrate  26  facing the substrate  25 , while the radiating element  31  is separated from the substrate  25  by a distance of H 2 , for example. The radiating element  31  is installed on the surface of the substrate  26  facing the feed element  37 . However, the radiating element  31  may be installed on the surface of the substrate  26  opposite to the side facing the feed element  37 , may be installed on a lateral surface of the substrate  26 , or may be installed in a member other than the substrate  26 . 
     The substrate  25  or the substrate  26  is arranged, for example, to be parallel to the XY-plane; and the substrate  25  or the substrate  26  is a substrate with a base material, which is dielectric, a magnetic material, or a mixture of dielectric and a magnetic material. As specific examples of the dielectric, there are a resin, glass-ceramics, LTCC (Low Temperature Co-Fired Ceramics), alumina, and so forth. As a specific example of the mixture of the dielectric and the magnetic material, it suffices if the mixture includes at least one of transition elements, such as Fe, Ni, and Co, metals including rare earth elements, such as Sm and Nd, and an oxide; and there are, for example, hexagonal ferrite, spinnel ferrite (e.g., Mn—Zn type ferrite, and Ni—Zn type ferrite)), garnet-based ferrite, permalloy, sendust (registered trademark), and so forth. 
     For example, for a case where the antenna  1  is to be installed in a mobile radio device with a display, the substrate  26  may be, for example, a cover glass that entirely covers an image display surface of the display; or the substrate  26  may be a casing (in particular, a bottom surface or a lateral surface, etc.) to which the substrate  25  is fixed. The cover glass is a dielectric substrate that is transparent, or semitransparent to the extent that a user can visually recognize an image displayed on the display; and the cover glass is a flat-plate like member that is laminated and installed on the display. 
     For a case where the radiating element  31  is installed on the surface of the cover glass, the radiating element  31  may be formed by spreading conductive paste, such as copper and silver, on the surface of the cover glass, and by sintering it. As the conductor paste for this case, conductor paste that can be sintered at a low temperature may be used, which can be sintered at a temperature at which strengthening of the chemically strengthened glass used for the cover glass is not to be weakened. Additionally, to prevent deterioration of the conductor due to oxidation, plating may be applied to it. Furthermore, decorative printing may be made on the cover glass; and the conductor may be formed on the portion where the decorative printing is made. Further, for a case where a black shielding film is formed at a periphery of the cover glass, for example, to hide wiring, the radiating element  31  may be formed on the black shielding film. 
     For the MIMO spatial multiplexing mode, it is preferable that the correlation coefficient between antennas be small. Note that, for the MIMO spatial multiplexing mode, it may not be true that it is better that the correlation coefficient is as small as possible, and it suffices if the correlation coefficient is smaller than a certain correlation coefficient because favorable communication can be ensured under an environment where sufficient multipath can be obtained. 
     The antenna  1  of  FIG. 2  has an antenna characteristic such that the correlation coefficient between the antenna element  30  and the antenna element  40  becomes small at the resonance frequency. The reason is that, even if the antenna element  30  and the antenna element  40  are in close proximity to each other, electromagnetic field coupling is established between the feed element  37  and the radiating element  31 , and electromagnetic field coupling is established between the feed element  47  and the radiating element  41 . 
     For example, for a case where it is designed so that the resonance frequency of the fundamental mode of the antenna  1  is in the vicinity of 1.8 GHz, the characteristic diagram, such as that of illustrated in  FIG. 4 , is obtained.  FIG. 4  is a diagram showing, for the antenna  1  that is designed so that the resonance frequency of the fundamental mode is in the vicinity of 1.8 GHz, a relationship between the correlation coefficient between the antenna element  30  and the antenna element  40  and the frequency. The correlation coefficient is calculated in accordance with the following expression, from the S parameter for a case where the feeding point  38  is the antenna port  1 , and the feeding point  48  is the antenna port  2 . 
     
       
         
           
             
               
                 
                   
                     correlation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     coefficient 
                   
                   = 
                   
                     
                       
                          
                         
                           
                             
                               S 
                               11 
                             
                             * 
                             
                               S 
                               12 
                             
                           
                           + 
                           
                             
                               S 
                               21 
                             
                             * 
                             
                               S 
                               22 
                             
                           
                         
                          
                       
                       2 
                     
                     
                       
                         ( 
                         
                           1 
                           - 
                           
                             ( 
                             
                               
                                 
                                    
                                   
                                     S 
                                     11 
                                   
                                    
                                 
                                 2 
                               
                               + 
                               
                                 
                                    
                                   
                                     S 
                                     21 
                                   
                                    
                                 
                                 2 
                               
                             
                             ) 
                           
                         
                         ) 
                       
                       ⁢ 
                       
                         ( 
                         
                           1 
                           - 
                           
                             ( 
                             
                               
                                 
                                    
                                   
                                     S 
                                     22 
                                   
                                    
                                 
                                 2 
                               
                               + 
                               
                                 
                                    
                                   
                                     S 
                                     12 
                                   
                                    
                                 
                                 2 
                               
                             
                             ) 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ] 
                 
               
             
           
         
       
     
     As it is apparent from  FIG. 4 , the correlation coefficient between the antenna element  30  and the antenna element  40  is decreased close to zero in the vicinity of the resonance frequency of 1.8 GHz. A similar result can be obtained for a case where the antenna  1  is designed so that the resonance frequency matches another frequency included in the UHF band or the SHF band. 
     Whereas, since the beam forming mode is a scheme such that the same information is simultaneously transmitted by the plurality of antenna elements, while directing the directivity in the maximum gain direction, it is preferable that the maximum value of a combined gain of the plurality of antenna elements be large. Thus, if the direction of the maximum combined gain of the plurality of antenna elements can be varied, a directivity pattern suitable for transmission by the beam forming mode can be formed. 
     The antenna  1  also has an antenna characteristic such that the direction of the maximum combined gain obtained by combining the antenna element  30  and the antenna element  40  can be varied by differentiating the amplitude of the signal flowing through the feeding point  38  from the amplitude of the signal flowing through the feeding point  48 . For example, for a case where it is designed so that the resonance frequency of the fundamental mode of the antenna  1  is in the vicinity of 1.8 GHz, the characteristic diagram, such as that of shown in  FIG. 5 , can be obtained.  FIG. 5  is a diagram illustrating the relationship between the directional gain and the azimuth angle for the main polarization (elevation angle θ=90 degrees) at the resonance frequency (which is adjusted to be in the vicinity of 1.8 GHz) of the fundamental mode of the antenna  1 . 
     The elevation angle θ represents, in the YZ-plane that passes through the middle point between the feeding point  38  and the feeding point  48 , and the center point of the ground plane  70 , an angle formed with respect to the Y-axis direction. The azimuth angle that is the horizontal axis of  FIG. 5  represents, in the ZX-plane that passes through the center point of the ground plane  70 , an angle formed with respect to the normal direction of the ground plane  70 . The directional gain that is the vertical axis of  FIG. 5  represents the combined gain of the antenna element  30  and the antenna element  40 . 
     In  FIG. 5 , each of the amplitude 1, the amplitude 0.8, the amplitude 0.5, the amplitude 0.3, and the amplitude 0.1 represents, for a case where the amplitude of the signal flowing through the feeding point  38  is set to be 1, the magnitude of the amplitude of the signal flowing through the feeding point  48 . Additionally, the phase of the signal flowing through the feeding point  38  and the phase of the signal flowing through the feeding point  48  are the same. 
     As it is apparent from  FIG. 5 , by differentiating the amplitude of the signal flowing through the feeding point  38  from the amplitude of the signal flowing through the feeding point  48 , the direction of the maximum combined gain of the antenna element  30  and the antenna element  40  (the direction in which the value of the directional gain is maximum) is varied. A similar result can be obtained for a case where it is designed so that the resonance frequency matches another frequency included in the UHF band or the SHF band. 
     Here, the sizes of the components illustrated in  FIGS. 2 and 3  at the time of measurement of  FIGS. 4 and 5  in units of mm are as follows: 
     L 1 : 20.975, 
     L 2 : 15.9, 
     L 3 : 8.025, 
     L 4 : 68.2, 
     L 5 : 33.6, 
     L 6 : 120, 
     L 7 : 38.75, 
     L 8 : 60, 
     conductor width of each the feed elements  37  and  38 : 1, 
     conductor width of each of the radiating elements  31  and  41 : 1, 
     H 1 : 0.8, 
     H 2 : 2.0, and 
     H 3 : 1.1. 
     The relative dielectric constant of each of the substrates  25  and  26  is 3.3, and tan δ=0.003. 
     Thus, in  FIGS. 1 and 2 , when the controller  24  selects the MIMO spatial multiplexing mode, as the transmit mode to be applied to the antenna  1 , the antenna  1  can be operated as a MIMO antenna with two channels such that the correlation coefficient between the antenna element  30  and the antenna element  40  is small, and that the antenna element  30  and the antenna element  40  can be used independently from each other. 
     Whereas, when the controller  24  selects the beam forming mode, as the transmit mode to be applied to the antenna  1 , the weight control circuits  21  and  22  adjust the directivity of the antenna  1  to be a pattern that matches the transmission by the beam forming mode. By adjusting the ratio between the amplitudes of the signals flowing through the feeding points  38  and  48  by the weight control circuits  21  and  22 , the direction of the maximum combined gain obtained by combining the antenna element  30  and the antenna element  40  can be varied. Thus, the antenna directivity control system  10  can operate the antenna  1 , as a single variable directivity antenna that uses the antenna element  30  and the antenna element  40 . 
     When the weight controller  24  selects the beam forming mode, as the transmit mode to be applied to the antenna  1 , the weight control circuits  21  and  22  adjust, for example, the amplitude of the signal flowing through the feeding point  48  to be greater or smaller, while fixing the amplitude of the signal flowing through the feeding point  38 . However, the weight control circuits  21  and  22  may adjust the amplitude of the signal flowing through the feeding point  38  to be greater or smaller, while fixing the amplitude of the signal flowing through the feeding point  48 ; or the weight control circuits  21  and  22  may simultaneously adjust the amplitude of the signal flowing through the feeding point  38  and the amplitude of the signal flowing through the feeding point  48  to be greater or smaller. 
     When the controller  24  selects the beam forming mode, as the transmit mode to be applied to the antenna  1 , the weight control circuits  21  and  22  adjust, for example, the amplitudes of the signals flowing through the feeding points  38  and  48 , while controlling the phases of the signals flowing through the feeding points  38  and  48  to be the same. However, the weight control circuits  21  and  22  may adjust the amplitudes of the signals flowing through the feeding points  38  and  48 , without controlling the phases of the signals flowing through the feeding points  38  and  48 , so as to leave these phases as different phases. 
     &lt;Configuration of Antenna  2 &gt; 
       FIG. 6  is a plan view schematically illustrating an example of a configuration of an antenna  2  according to another embodiment of the present invention. The antenna  2  is an example of the antenna  13  illustrated in  FIG. 1 . The description of the configurations that are the same as those of the above-described embodiment is omitted. The antenna  2  includes the ground plane  70 ; and four antenna elements  30 ,  40 ,  50 , and  60 . 
     The antenna  2  differs from the antenna  1  of  FIG. 2  in a point that the antenna elements  50  and  60  having the same configurations as those of the antenna elements  30  and  40  are arranged line symmetrically with respect to the ground plane  70 . 
     The antenna  2  has an antenna characteristic such that the correlation coefficient among the antenna elements  30 , the antenna element  40 , the antenna element  50 , and the antenna element  60  becomes small at the resonance frequency. In addition, the antenna  2  has an antenna characteristic such that by differentiating the amplitude of the signal flowing through the feeding point  38  from the amplitude of the signal flowing through the feeding point  48 , the direction of the maximum combined gain obtained by combining the antenna element  30  and the antenna  40  can be varied. Furthermore, the antenna  2  has an antenna characteristic such that by differentiating the amplitude of the signal flowing through a feeding point  58  from the amplitude of the signal flowing through a feeding point  68 , the direction of the maximum combined gain obtained by combining the antenna element  50  and the antenna  60  can be varied. 
     Consequently, the antenna directivity control system  10  can operate the antenna  2  as a MIMO antenna with four channels where the antenna elements  30 ,  40 ,  50 , and  60  are used independently from each other. Additionally, the antenna directivity control system  10  can operate the antenna  2  as two variable directivity antennas including a first variable directivity antenna that uses the antenna element  30  and the antenna element  40 ; and a second variable directivity antenna that uses the antenna element  50  and the antenna element  60 . 
     EXAMPLE 
     Next, results of an experiment are shown by referring to  FIGS. 7 and 8 , where the antenna  1  was actually produced, and the experiment was conducted as to whether the correlation coefficient between the antenna element  30  and the antenna element  40  was lowered at the resonance frequency. 
     Here, the sizes of the components illustrated in  FIGS. 2 and 3  at the time of  FIGS. 7 and 8  in units of mm were as follows: 
     L 1 : 14, 
     L 2 : 11, 
     L 3 : 5.7, 
     L 4 : 50, 
     L 5 : 25, 
     L 6 : 120, 
     L 7 : 28.5, 
     L 8 : 60, 
     conductor width of each the feed elements  37  and  38 : 0.5, 
     conductor width of each of the radiating elements  31  and  41 : 0.5, 
     the shortest distance between the end portion  34  of the radiating element  31  and the end portion  44  of the radiating element  41 : 4, 
     the shortest distance between the center of the conductor width of the feed element  37  and the center of the conductor width of the feed element  47  in the X-axis direction: 4 
     H 1 : 0.8, 
     H 2 : 2.0, and 
     H 3 : 1.0. 
     The relative dielectric constant of each of the substrates  25  and  26  was 3.3, and tan δ=0.003. The shapes of the antenna element  30  and the antenna element  40  were line symmetric with respect to the YZ-plane passing through the feeding point  38  and the feeding point  48 . 
       FIG. 7  shows an example of the results of measuring S 11  and S 12 , which represents the reflection coefficients at the two antenna ports of the antenna  1  in the experiment, and the antenna  1  in the experiment had a resonance frequency of approximately 2.5 GHz.  FIG. 8  shows an example of the correlation coefficient that was calculated from the S parameters between the two antenna ports of the antenna  1  in the experiment, in accordance with the above-described formula; and it is shown that the correlation coefficient between the antenna element  30  and the antenna element  40  was decreased close to zero in the vicinity of 2.5 GHz. Namely, the antenna  1  favorably functions as a MIMO antenna that operates in the vicinity of approximately 2.5 GHz. 
     The antenna directivity control system is described above by the embodiments; however, the present invention is not limited to the above-described embodiments. Various modifications and improvements, such as a combination with a part or all of another embodiment or replacement, may be made within the scope of the present invention.