CROSS-REFERENCE TO RELATED APPLICATIONS 
     The entire disclosure of Japanese Patent Application No. 2007-252029 filed on Sep. 27, 2007 including specification, claims, drawings and abstract is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     An aspect of the present invention relates to a radio system, radio apparatus, and an antenna device, and more particularly to an antenna device including a loop-type antenna element and radio apparatus and a radio system using the antenna device. 
     2. Description of the Related Art 
     An individual identification technique utilizing radio communication (Radio Frequency Identification which will be abbreviated hereinafter as “RFID”) is widely utilized for automatic ticket gates of a railway, management of times of arrival and departure of employees at and from a corporation or an office, various types of electronic money, and the like. In the RFID, information is exchanged through radio communication between a device (reader/writer) and an information medium (card or tag). A loop antenna built into the reader/writer and a loop antenna built into the card are held in a communicable state while being opposed each other, whereupon the reader/writer can write information into the card or read information from the card. 
     Some types of portable cellular phones are equipped with a function compatible with such RFID. In recent years, the portable cellular phones have not only the card function but also the reader/writer function. There is a strong request for miniaturization of the reader/writer including a portable cellular phone. However, there is a problem of a metallic portion of the device intended for miniaturization coming close to a loop antenna, thereby generating an eddy current induced by an A.C. magnetic field and shortening a communicable distance between the reader/writer and a card. 
     A technique of preventing generation of such an eddy current by utilization of a magnetic substance has been disclosed (see, e.g., JP-A-2007-122225 or JP-A-2006-178713). In a non-contact IC card reader described in JP-A-2007-122225, the decorative laminate panel is attached to the mount frame (made of metal), which is used to mount the module onto the housing, through a base. A loop antenna is laminated on the base by through the magnetic substance layer, to thus shield a space between the antenna and the mount frame with the magnetic substance layer and prevent occurrence of an eddy current in the mount frame. 
     An information processing device described in JP-A-2006-178713 has a structure in which a conductive plate, a magnetic substance, and an antenna substrate are sequentially arranged on a circuit substrate in a layered manner, thereby shielding the circuit substrate from a magnetic flux of the antenna and preventing occurrence of an eddy current. 
     In the RFID, loop antennas built into the reader/writer and the card constitute respectively resonators, and nominal values of resonance frequencies of the resonators are set equally to each other. In general, it is known that, when two resonators having the same resonance frequency come close to each other, frequencies of the two resonators are gradually separated toward two frequencies f 1  and f 2  (f 1 &lt;f 2 ) (for example, Kawaguchi et al., “Study of Equivalent Circuit Display of Electromagnetic Coupling between Distributed Constant Resonators”, Technical Research Report EMCJ2003-78/MW2003-175 of the IEICE, October, 2003; and Ito et al., “Relationship between a Dead Zone and a Coupling Coefficient in an HF band RFID”, General Convention B-1-143 of the IEICE, March, 2007). This phenomenon is called a frequency split. The frequency split arises when strong coupling occurs as a result of a space between the reader/writer and the card being reduced to a certain extent or more. When the value of the frequency split increases in excess of a limit, it may be the case where communication cannot be established between the reader/writer and the card. 
     The frequency split is described by reference to  FIGS. 11 through 13  while taking the RFID system as an example.  FIG. 11  is an exemplary measurement result of a frequency characteristic of a return loss of a card for an RFID system. A horizontal axis in the drawing represents a frequency; the center of the plot corresponds to 13.56 megahertz (MHz); and one tick mark corresponds to 1 MHz. A vertical axis in the drawing represents a return loss; the maximum value represents 0 dB; and one tick mark corresponds to 1 dB. 
       FIG. 12  shows an exemplary measurement result of a frequency characteristic of a return loss of a related-art reader/writer for an RFID system. A horizontal axis in  FIG. 12  is identical with the horizontal axis shown in  FIG. 11 . A vertical axis in  FIG. 12  represents a return loss; the maximum value corresponds to 0 dB; and one tick mark corresponds to 0.2 dB. 
       FIG. 13  is an exemplary measurement result of frequency characteristic of return losses performed when the loop antenna of the card whose characteristic is shown in  FIG. 11  and the loop antenna of the reader/writer whose characteristic shown in  FIG. 12  are brought closely to each other in a mutually-opposing manner. The horizontal axis and the vertical axis shown in the drawing are identical with their counterparts in  FIG. 12 . In the drawings, a left resonance point represents a resonance point of the card, and a right resonance point represents a resonance point of the reader/writer. In this case, the value of the foregoing frequency split has come to about 5.5 MHz, whereupon the frequency split manifests itself noticeably. Under such conditions, it is extreme difficult to perform radio communication between the card and the reader/writer. 
     The related-art technique described in JP-A-2007-122225 is intended for assuring a communicable distance by means of preventing occurrence of an eddy current, which would otherwise be arise in a metal mount frame, to thus lessen an adverse effect on an antenna characteristic. However, no consideration is given to the problem of the frequency split induced as a result of the reader/writer coming closely to the card. 
     The related-art technique described in JP-A-2006-178713 is directed toward preventing a magnetic flux from reaching a substrate by converting a magnetic flux leaked from an antenna into an eddy current by means of a conductive plate, thereby preventing occurrence of an eddy current also. However, no consideration is given to the problem of the frequency split induced as a result of the reader/writer coming closely to the card. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, there is provided a radio system including: a first radio apparatus that has a first loop antenna; and a second radio apparatus that has a second loop antenna and that performs a radio communication with the first radio apparatus when the second loop antenna is opposed with the first loop antenna, wherein the second radio apparatus has a shield member that is formed of a magnetic substance and disposed to shield at least a portion of the second loop antenna with respect to the first loop antenna when the radio communication is performed. 
     According to another aspect of the present invention, there is provided a radio apparatus including: a built-in loop antenna that is to be opposed with a loop antenna of an external apparatus for performing a radio communication between the radio apparatus and the external apparatus; and a shield member that is formed of a magnetic substance disposed to shield at least a portion of the built-in loop antenna with respect to the loop antenna of the external apparatus when the radio communication is performed. 
     According to still another aspect of the present invention, there is provided an antenna device that is included in a radio apparatus and that is to be opposed with a loop antenna of an external apparatus for performing a radio communication between the radio apparatus and the external apparatus, the antenna device including: an antenna element that is formed into a loop shape; and a shield member that is formed of a magnetic substance disposed to shield at least a portion of the antenna element with respect to the loop antenna of the external apparatus when the radio communication is performed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiment may be described in detail with reference to the accompanying drawings, in which: 
         FIG. 1  is a block diagram of a radio system of an embodiment; 
         FIG. 2  is a plan view showing a substrate of a radio apparatus of the embodiment and the configuration of a periphery of the substrate; 
         FIG. 3  is a side view showing the substrate of the radio apparatus and the configuration of the periphery of the substrate; 
         FIG. 4  is a side view showing a positional relationship between a loop antenna and a magnetic substance of the embodiment; 
         FIG. 5  is a perspective view showing a positional relationship between a loop antenna and a magnetic substance of the embodiment; 
         FIG. 6  is a plan view showing a substrate of a radio apparatus of a modified embodiment and the configuration of the periphery of the substrate; 
         FIG. 7  is a side view showing the substrate of the radio apparatus of the modified embodiment and the configuration of the periphery of the substrate; 
         FIG. 8  is a side view showing a positional relationship between a loop antenna and a magnetic substance of the modified embodiment; 
         FIG. 9  is an exemplary measurement result of a frequency characteristic of a return loss of the reader/writer in an RFID system of the embodiment; 
         FIG. 10  shows an exemplary measurement result of a frequency characteristic of return losses of the reader/writer and of the card in the RFID system of the embodiment acquired when loop antennas of the reader/writer and the card are opposed closely each other; 
         FIG. 11  is an exemplary measurement result of a frequency characteristic of a return loss of a card for an RFID system; 
         FIG. 12  is an exemplary measurement result of a frequency characteristic of a return loss of a related-art reader/writer for an RFID system; and 
         FIG. 13  shows an exemplary measurement result of a frequency characteristic of return losses of a related-art reader/writer and of a card for an RFID system acquired when loop antennas of the related-art reader/writer and the card are opposed closely each other; 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An embodiment of the present invention is described hereunder by reference to  FIGS. 1 through 10 . When up, down, right, and left, or a horizontal direction and a vertical direction (a normal direction) are referred to by reference to the drawings provided below, the directions signify up, down, right, and left, or a horizontal direction and a vertical direction (a normal direction) of a drawing sheet having a picture unless otherwise specified. Moreover, like reference numerals designate like configurations throughout the drawings. 
       FIG. 1  is a block diagram showing the configuration of a radio system  1  of the embodiment. The radio system  1  has radio apparatus  10  and radio apparatus  20 . The radio system  1  may be considered as an individual identification system (an RFID) utilizing a radio frequency; the radio apparatus  10  may be considered as a card in the RFID system; and the radio apparatus  20  may be considered as a reader/writer or a portable cellular phone having a built-in reader/writer function. 
     The radio apparatus  10  has a loop antenna  11  built thereinto. The radio apparatus  20  has a loop antenna  21  built thereinto. The radio apparatus  20  can perform radio communication with the radio apparatus  10  when the loop antenna  21  is positioned opposite the loop antenna  11 . Here, the term “communication” includes exchange of information, such as writing of information into an information medium typified by the RFID system or reading of information from the information medium. 
       FIG. 2  is a plan view showing a substrate  22  of the radio apparatus  20  and the configuration of the periphery of the substrate (including the loop antenna  21 ).  FIG. 3  is a side view showing the substrate  22  and the configuration of the periphery of the substrate when viewed from a right side in  FIG. 2 . As shown in  FIG. 3 , a battery  23  is attached to one surface of the substrate  22 . The battery  23  is housed in a metal package. A surface of the battery  23  opposite to the substrate  22  is provided with a magnetic substance  24 . The magnetic substance  24  is formed into; for example, a sheet, and affixed to the surface of the battery  23 .  FIG. 2  shows a state where the battery  23  is covered with the magnetic substance  24 , to thus remain hidden from view. 
     The loop antenna  21  has an antenna element that is formed into the shape of a loop and arranged to surround the battery  23 . In reality, an unillustrated feeding point is provided at any location on the antenna element and connected to and fed power from an unillustrated radio circuit of the radio apparatus  20 . A magnetic substance  25  is arranged while wrapping at least a portion of the loop antenna  21 . 
     The advantages realized by providing the magnetic substance  24  and the magnetic substance  25  will now be described by reference to  FIGS. 4 and 5 .  FIG. 4  is a view showing a positional relationship among the loop antenna  11 , the loop antenna  21 , the magnetic substance  24 , and the magnetic substance  25  when the loop antenna  21  is positioned opposite the loop antenna  11  as shown in  FIG. 1  and when viewed in the same direction as in  FIG. 3 .  FIG. 5  is a perspective view showing a similar positional relationship while the loop antenna  11  is positioned above. 
     When the loop antenna  21  is excited, an A.C. magnetic field is induced. The magnetic substance  24  suppresses an eddy current arising in the metal package of the battery  23  caused by the A.C. magnetic field. However, as is inferred by reference to  FIG. 5 , when the loop antenna  11  or the loop antenna  12  is excited, the density of a magnetic flux penetrating through the loops is intensified by the presence of the magnetic substance  24 . Hence, coupling between the loop antenna  11  and the loop antenna  21  is intensified (a Q value of the loop antenna  11  and a Q value of the loop antenna  21  are also increased at this time). Consequently, a frequency split becomes likely to arise. 
     In the meantime, the magnetic substance  25  is arranged to wrap at least a portion of the loop antenna  21 . In the portion of the loop antenna  21  where the magnetic substance  25  is wrapped, a magnetic field induced by an antenna current is shielded, thereby suppressing leaks to the outside. Therefore, influence on the loop antenna  11  can be diminished. In short, occurrence of the frequency split can be lessened by preventing coupling between the loop antenna  11  and the loop antenna  21 . 
     A location and a range on the loop antenna  21  wrapped with the magnetic substance  25  can be selected appropriately according to a design or usage conditions (an interval between the loop antenna  11  and the loop antenna  21 , transmission power, receiving sensitivity, and the like). 
     By reference to  FIGS. 6 through 8 , a modified embodiment will be described. In the modified embodiment, the magnetic substance is modified form the magnetic substance  25  of the foregoing embodiment. Therefore, respective elements of the structure are assigned the same reference numerals of the previously-described elements except the magnetic substance  25 .  FIG. 6  is a plan view showing the substrate  22  of the radio apparatus  20  and the configuration of the periphery of the substrate (including the loop antenna  21 ) in the modified embodiment.  FIG. 7  is a side view showing the substrate  22  and the configuration of the periphery of the substrate when viewed from a right side in  FIG. 6 . 
     In  FIGS. 6 and 7 , a positional relationship among the loop antenna  21 , the substrate  22 , the battery  23 , and the magnetic substance  24  is the same as that described by reference to  FIGS. 2 and 3 . In  FIG. 7 , a magnetic substance  26  is disposed to cover at least a portion of the loop antenna  21 . The magnetic substance  26  may also be attached to an interior surface of a housing case  27  of the radio apparatus  20  indicated by a broken line in  FIG. 7 . 
       FIG. 8  is a view showing, when viewed in the same direction as that in  FIG. 7 , a positional relationship among the loop antenna  11 , the loop antenna  21 , the magnetic substance  24 , and the magnetic substance  26  achieved when the loop antenna  21  is positioned opposite the loop antenna  11  as shown in  FIG. 1 . 
     The frequency split becomes more likely to arise because of presence of the magnetic substance  24  as already mentioned. In the meantime, the magnetic substance  26  is disposed to cover at least a portion of the loop antenna  21  from the loop antenna  11 . The magnetic substance  26  shields a magnetic field induced by the loop antenna  21  and prevents the magnetic filed from leaking toward a direction where the loop antenna  11  is to be positioned. Hence, influence on the loop antenna  11  can be diminished. In other words, coupling between the loop antenna  11  and the loop antenna  21  is suppressed, to thus lessen a frequency split. 
     A location and a range on the loop antenna  21  wrapped with the magnetic substance  26  can be selected appropriately according to a design or usage conditions (an interval between the loop antenna  11  and the loop antenna  21 , transmission power, receiving sensitivity, and the like). 
     An advantage of the embodiments verified by experiment is now described by reference to  FIGS. 9 and 10 . 
       FIG. 9  is an exemplary measurement result of a frequency characteristic of a return loss of a reader/writer to which one of the embodiments is applied. A horizontal axis in the drawing represents a frequency; the center of the plot corresponds to 13.56 megahertz (MHz); and one tick mark corresponds to 1 MHz. A vertical axis in the drawing represents a return loss; the maximum value represents 0 dB; and one tick mark corresponds to 0.2 dB. 
       FIG. 10  is an exemplary measurement result of frequency characteristic of return losses performed when a loop antenna of the card whose characteristic is shown in  FIG. 11  in connection with the descriptions of the related art and a loop antenna of the reader/writer whose characteristic is shown in  FIG. 9  are brought closely to each other in a mutually-opposing manner. In the drawings, a left resonance point represents a resonance point of the card, and a right resonance point represents a resonance point of the reader/writer. In this case, the present invention is applied to the reader/writer. Hence, as is evident from a comparison with  FIG. 13 , the value of the foregoing frequency split is improved to a little more than 3 MHz or thereabouts. Under such conditions, communication can be established between the card and the reader/writer in a more reliable manner than in the related art. 
     According to the foregoing embodiment, the magnetic substance is disposed to shield at least a portion of one loop antenna from another loop antenna, whereby a frequency split induced when both loop antennas are closely opposed is reduced, so that a communicable state can be maintained. 
     In the embodiment mentioned above, shapes, layouts, positional relationships, and the like, of the respective constituent elements and conditions set in experiment are mere illustrative. They are susceptible to various modifications without departing from the gist of the present invention. 
     According to an aspect of the present invention, in one piece of radio apparatus constituting a radio system, a magnetic substance is arranged to shield at least a portion of a loop antenna of the radio apparatus from a loop antenna of another piece of radio apparatus, thereby preventing occurrence of a frequency split, which would otherwise be caused when loop antennas of radio apparatus comes closely to each other, to thus ensure stable communication.
According to an aspect of the present invention, there is provided a radio system including: a first radio apparatus that has a first loop antenna; and a second radio apparatus that has a second loop antenna and that performs a radio communication with the first radio apparatus when the second loop antenna is opposed with the first loop antenna, wherein the second radio apparatus has a shield member that is formed of a magnetic substance and disposed to shield at least a portion of the second loop antenna with respect to the first loop antenna when the radio communication is performed.