Patent Publication Number: US-2016226146-A1

Title: Antenna device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an antenna device, and particularly to an antenna device that is suitable for NFC (Near Field Communication) system. The present invention also relates to a portable electronic device in which such an antenna device is used. 
     2. Description of Related Art 
     In recent years, a RFID (Radio Frequency Identification) system is incorporated in portable electronic devices such as smartphones. As a communication means for the system, an antenna is incorporated in portable electronic devices to perform near field communication with a reader/writer or the like. 
     Meanwhile, a metal shield is provided in the portable electronic device in order to protect an internal circuit from external noise and prevent unnecessary radiation of noise generated inside the device. In particular, in order to make the body thinner, lighter, and more resistant to shock such as when the body is dropped, and to improve the design and other factors, the housing of a recent portable electronic device itself has increasingly been made of metal instead of resin, with the housing doubling as a metal shield. However, in general, the metal shield blocks radio waves. Therefore, an antenna needs to be placed in such a way as not to overlap with the metal shield. If the metal shield is provided across a wide range, how to dispose the antenna becomes a problem. 
     To solve the above problem, for example, in an antenna device disclosed in Japanese Patent No. 4,687,832, Japanese Patent Application Laid-open No. 2002-111363, or Japanese Patent Application Laid-open No. 2012-162195, an opening is formed in a conductor layer, and a slit is formed in such a way as to connect the opening to an outer edge. An antenna coil is disposed in such a way that an inner diameter area overlaps with the opening. In the antenna device, current flows through the conductor layer in such a way as to block a magnetic field generated by a flow of current through a coil conductor. Then the current flows along with the slit that flows around the opening of the conductor layer, and the current also flows around the conductor layer due to the edge effect. As a result, a magnetic field is generated from the conductor layer, and the conductor layer makes a large loop of the magnetic flux, resulting in a longer communication distance between the antenna device and an antenna that the antenna device is communicating with. That is, the conductor layer functions as an accelerator that helps to increase the antenna coil&#39;s communication distance. 
     However, it is demanded that the conventional antenna device described above should be further improved to increase the communication range. Particularly, the communication range will be shorter as the planer size of the metal layer of the antenna device becomes smaller. The communication range should therefore have a desirable value even if the metal layer has a small planer size. 
     SUMMARY 
     It is therefore an object of the present invention to enhance the accelerating function of the metal layer in an antenna device, which is provided on a mobile electronic apparatus and which is used as accelerator to lengthen the communication range of the antenna coil. 
     To achieve the above-mentioned object, an antenna device according to the present invention comprises a first metal layer having a first slit, an antenna coil having an inner diameter area overlapping with the first slit in planar view and a coil axis perpendicular to the first metal layer, 
     and a first magnetic layer provided on the back surface of the first metal layer, which faces the antenna coil, and provided outside the antenna coil in planar view, wherein an upper surface of the first magnetic layer facing the back surface of the first metal layer is positioned closer than a far end of the antenna coil to the back surface of the first metal layer along the coil axis, as viewed from the back surface of the first metal layer, or is positioned in the same plane as the far end of the antenna coil. 
     In this invention, the first magnetic layer is provided at the back of the first metal layer and outside the antenna coil In planar view, and therefore does not overlap with the antenna coil as viewed in plane. Hence, the magnetic flux generated from the current flowing in the antenna coil and intersecting with the antenna coil can be changed in direction, and can be guided not to the first metal layer. This can suppresses the diamagnetic field and the loss of eddy current, ultimately increasing the communication range of the antenna. 
     In this invention, it is desired that the first magnetic layer is bonded to the back surface of the first metal layer. The first magnetic layer can thereby be easily arranged between the first metal layer and the antenna coil. 
     The antenna device according to the present invention preferably further comprises a second magnetic layer facing the first metal layer across the antenna coil. In most mobile electronic apparatuses, the antenna coil is mounted on a metal body such as the battery pack. If the second magnetic layer is interposed between a metal member and the antenna coil a magnetic fins path that intersects with the antenna coil can be easily provided. The influence the metal member imposes on the antenna coil can therefore be controlled. Hence, the antenna device can acquire desirable antenna characteristic. 
     In this indention, the first magnetic layer is preferably integrated with the second magnetic layer. Then, both the first magnetic layer and the second magnetic layer change the path of the magnetic flux generated from the current flowing in the antenna coil. The magnetic flux is therefore guided toward the inner diameter area of the antenna coil. A magnetic flux loop can therefore be formed more reliably than in the first embodiment. The communication range of the antenna can therefore be increased reliably. Further, the antenna coil and the magnetic layer can be easily positioned with respect to the slit. This can prevent the degradation of the antenna characteristic, which would be inevitable if applied to the first metal layer with possible displacement. 
     In this invention, it is desired that the antenna device further comprises a substrate arranged parallel to the first metal layer, wherein the antenna coil is formed on an upper surface of the substrate which opposes to the first metal layer, and the second magnetic layer is formed on a lower surface of the substrate. In this configuration, the antenna coil and the second magnetic layer can be easily forced, handled and mounted. 
     In this invention, it is desired that the antenna device further comprises a substrate arranged parallel to the first metal layer, wherein the antenna coil is formed on an upper surface of the substrate, the first magnetic layer is positioned not overlapping with the substrate in planar view, and an lower surface of the first magnetic layer is located below the upper surface of the substrate. This configuration enables the first magnetic layer to function as a positioning guide for the antenna coil, enhancing the mounting precision of the antenna coil with respect to the slit. 
     In this invention, it is desired that the antenna device further comprises a second metal layer having a second slit, wherein the second metal layer is provided opposite to the first metal layer as viewed from the antenna coil, and the inner diameter area of the antenna coil overlaps with the second slit in planar view. The magnetic flux intersecting with the antenna coil greatly extends around not only the first metal layer, but also the second, metal layer at the back, and then extends back to the inner diameter area of the antenna coil through the second slit. The loop size of the magnetic flux can therefore increase. This enhances the directivity of the antenna, ultimately further lengthening the communication range of the antenna. 
     In this invention, it is desired that a lower surface of the first magnetic layer is in contact with the second metal layer. If so configured, the first magnetic layer can be thick and arranged in the space between the first metal layer and the second meal layer. The first magnetic layer can therefore sufficiently suppress the generation of a diamagnetic field and the loss of eddy current. 
     In this invention, it is desired that the first metal layer has a first metal plate, a second metal plate located adjacent to the first metal plate, across the first slit, and a connecting portion bridging the first slit at one end thereof and connecting the first metal plate and the second metal plate, making them integral with each other. Since the connecting portion connects the first metal surface and the second metal surface in this configuration, the first and second metal surfaces can be treated as a single metal member. Hence, a cover having such metal surfaces can be easily manufactured. Further, the first metal surface and the second metal surface need not foe aligned with each other, and the width of the slit SL will never change. 
     In this invention, the first metal layer is preferably a housing of the mobile electronic apparatus that incorporates the antenna coil. If the housing of the mobile electronic apparatus is made of metal, not resin, and therefore functions as s metal shield, a part of the housing is used as an accelerator for the antenna coil. This helps to enhance the emission characteristic of the antenna, and ultimately to increase the communication range of the antenna coil. 
     In this invention, the first magnetic layer is preferably a magnetic sheet containing fiat metal grains. If the first magnetic layer contains fiat metal grains, the magnetic field emanating from the antenna coil can be orientated in the horizontal direction that intersects with the coil axis. Since polymer insulates the fiat metal particles from one another, the first magnetic layer can prevent the generation of an eddy current. The antenna device can therefore achieve both high magnetic permeability and low magnetic loss in the high RFID frequency band. 
     In the antenna device according to the present invention, the metal layer provided in the mobile electronic apparatus is used as accelerator to increase the communication range of the antenna coil. Therefore, the acceleration function can be enhanced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above features and advantages of the present invention will be more apparent from the following description of certain preferred embodiments taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a plan view showing a configuration of an antenna device according to a first embodiment of the present invention; 
         FIG. 2  is a schematic cross-sectional view of the antenna device, taken along line A-A shown in  FIG. 1 ; 
         FIG. 3  is a schematic plan view for explaining how the first metal layer and the first magnetic layer perform their functions; 
         FIG. 4  is a schematic cross-sectional view for explaining how the first metal layer and the first magnetic layer perform their functions; 
         FIG. 5  is a schematic cross-sectional view showing a of an antenna device according to a second embodiment of the present invention; 
         FIG. 6  is a schematic cross-sectional view showing a configuration of an antenna device according to a third embodiment of the present invention; 
         FIG. 7  is a schematic cross-sectional view showing a configuration of an antenna device according to a fourth embodiment of the present invention; 
         FIG. 8  is a schematic plan view showing a configuration of an antenna device according to a fifth embodiment of the present invention; and 
         FIG. 9  is a schematic plan view showing a configuration of an antenna device according to a sixth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Preferred embodiments of the present invention will be explained below in detail with reference to the accompanying drawings. 
       FIG. 1  is a plan view showing a configuration of an antenna device according to a first embodiment of the present invention.  FIG. 2  is a schematic cross-sectional view of the antenna device, taken along line A-A shown in  FIG. 1 . 
     As shown in  FIGS. 1 and 2 , the antenna device  1  comprises an antenna element  10  composed of a planer loop antenna, a first metal layer  20  covering the antenna element  10 , and a first magnetic layer  21  formed on a back surface of the first metal layer  20 . 
     The antenna element  10  comprises a substrate  11  and an antenna coil  12  formed on an upper surface of the substrate  11 . The substrate  11  is a flexible substrate made of, for example, PET resin, its planer size is, for example, 40×50 mm, and its thickness is, for example, 30 μm. The substrate  11  is arranged parallel to the first metal layer  20 . 
     The antenna coil  12  is composed of a spiral pattern which is substantially rectangular, and has a coil axis which is perpendicular to the main surface of the first metal layer  20 . Both ends of the spiral pattern constituting the antenna coil  12  are led to an edge of the substrate  11  by lead parts. Particularly, an inner end of the spiral pattern is led to the outside of the loops by crossing the spiral loops. Both ends of the antenna coil  12  are connected to, for example, an NFC chip (not shown). The antenna coil  12  may be formed by means of electroplating. Alternatively, it may be formed by means of etching (patterning) a metal layer formed on the entire surface of the substrate  11 . 
     As shown in  FIG. 2 , a second magnetic layer  13  is formed on a lower surface of the substrate  11 . In most cases, the antenna element  10  of the mobile electronic apparatus is mounted, on a surface of a battery pack  30 . However, if the second magnetic layer  13  is interposed between the battery pack  30  and the antenna coil  12  as shown in  FIG. 2 , a passage is provided for the magnetic flux generated from the current flowing in the antenna coil. Hence, the influence the metal body of the battery pack  30  imposed on the antenna coil can be suppressed and a desirable characteristic of the antenna device can be obtained. 
     The first metal layer  20  is, for example, a member constituting the housing of the mobile electronic apparatus incorporating the antenna, element  10 . The first metal layer  20  has therefore a larger planer size than that of the antenna coil  12 , and covers almost all surface of the antenna element  10 . The first metal layer  20  can be a member not integral with the housing of the mobile electronic apparatus. Preferably, the planer size of the first metal layer  20  is 4 to 5 times the planer size of the antenna coil  12 , but may be about 2 times the planer size of the antenna coil  12 . The smaller the planer size the first metal layer  20  has, the more its function of accelerating antenna coil  12  will decrease, inevitably shortening the communication range. Nonetheless, the first magnetic layer  21  enhances the accelerating function of the first metal layer  20 , compensating for the decrease of the accelerating function resulted from the small planer size of the first metal layer  20 . Hence, the antenna device can acquire a desirable communication range. 
     A slit SL (i.e., first slit) is formed in the first metal layer  20 . The slit SL is a linear slit that extends in X direction, from one edge of the first metal layer  20  to the opposite edge thereof. The slit SL therefore divides the first metal layer  20  into two parts. The first metal layer  20  is thus composed of first and second metal plates  20 A and  20 B that are adjacent in Y direction, across the slit SL. The first and second metal, plates  20 A and  20 B are rectangular patterns, preferably having the same width in X direction. The first and second metal plates  20 A and  20 B need not have the same size, and may have different sizes. The slit SL need not remain void. Rather, it may be filled with resin. 
     As shown in  FIG. 1 , the antenna element  10  is so positioned that the inner diameter area  12   a  of the antenna coil  12  overlaps with the slit SL in planar view. To make the first and second metal plates  20 A and  20 B overlap with the inner diameter area  12   a  of the antenna coil  12  in planar view, the width W 0  of the slit SL need be smaller than the width W 1  of the inner diameter area  12   a,  preferably W 1 /2 or less. The slit SL crosses the center of the inner diameter area  12   a  of the antenna coil  12 , and intersects with two parts E 1  and E 2  of the antenna coil  12 . That is, two parts of the antenna coil  12  intersect with the slit SL and are thereby exposed. Hence, the emission efficiency of the antenna device is higher than in the case where only one part of the antenna coil  12  is exposed. This can enhance the antenna characteristic. 
     As shown in  FIG. 2 , the first magnetic layer  21  is provided on the back surface of the first metal layer  20 , which opposes the antenna element  10 . The first magnetic layer  21  is secured to the first, metal layer  20  by means of, for example, adhesion. The first magnetic layer  21  is provided outside the antenna coil  12 , and does not overlap with the antenna coil  12  or the inner diameter area  12   a  thereof in planar view. The first magnetic layer  21  covers particularly those parts of the first metal layer  20  which are juxtaposed in Y direction. However, the first magnetic layer  21  does not cover those parts which are juxtaposed in X direction. 
     The first magnetic layer  21  need not be adhered to the back surface of the first metal layer  20 . The first magnetic layer  21  only need to be arranged on the space provided at the back surface  20   d  of the first metal layer  20  interposed between the back surface  20   d  of the first metal layer  20  and the plans including the mount surface Of the antenna coil  20  (i.e., upper surface of the substrate  11 ). In  FIG. 2 , the region F encircled by two-dotted, broken line is the space in which the first magnetic layer  21  can be arranged. The first magnetic layer  21  may be arranged in at least one part of the region F. In order to arrange the first magnetic layer  21  in the region F, at an appropriate position with respect to the antenna coil  12 , the upper surface  21   t  of the first magnetic layer  21 , which opposes the back surface  20   d  of the first metal layer  20 , must be closer to the back surface  20   d  of the first metal layer  20  than to the far-end surface  12   f  of the antenna coil  12  in the axial direction of the coil, as seen from the back surface  20   d  of the first metal layer  20 . Alternatively, the upper surface  21   t  of the first magnetic layer  21  must exist in the same plane as the far-end surface  12   f  of the antenna coil  21 . Preferably, the upper surface  21   t  of the first magnetic layer  21  exists closer to the back surface  20   d  of the first metal layer  20  along the coil axis than the near-end (opposing the far-end surface  12   f , i.e., the upper surface of the antenna coil  12 ), as viewed from the back surface  20   d  of the first metal layer  21 . 
       FIGS. 3 and 4  are diagrams for explaining how the first metal layer  20  and the first magnetic layer  21  perform their functions.  FIG. 3  is a schematic plan view, and  FIG. 4  is a schematic cross-sectional view. 
     As shown in  FIGS. 3 and 4 , when a current Ia flows counterclockwise in the antenna coil  12 , a magnetic flux is generated, passing through the inner diameter area  12   a  of the antenna coil  12 . The magnetic flux passes through the slit SL existing between the first and second metal plates  20 A and  20 B, and then flow around in the first and second metal plates  20 A and  20 B. Meanwhile, currents flow in the first and second metal plates  20 A and  20 B, respectively, to cancel out the magnetic flux. These currents become eddy currents Ib and Ic generated inside and outside of the antenna coil  12 , respectively, by virtue of edge effect. 
     As shown in  FIG. 4 , the current Ia flowing in the antenna coil  12  generates a magnetic flux φ 1 , which intersects with the antenna coil  12 . The magnetic flux φ 1  is a large magnetic loop that passes through the slit SL and extends around outside the first metal layer  20 . A part of the magnetic flux φ 1  passes through the first magnetic layer  21 . A magnetic flux φ 2  is generated from the eddy current Ic generated in the inner diameter area  12   a  of the antenna coil  12 , and forms a magnetic flux loop that boosts the magnetic flux φ 1 . 
     Without the first magnetic layer  21 , a part φ 1a  of the magnetic flux φ 1  emanating from the antenna coil  12  abuts on the back surface of the first metal layer  20 , generating a diamagnetic field. The part φ 1a  of the magnetic flux φ 1  also results in an eddy current loss in the first metal layer  20 , and cannot serve to increase the communication range of the antenna device, nonetheless, the first magnetic layer  21  guides the magnetic flux φ 1a  in a specific direction, not to the first metal layer  20  because the first magnetic layer  21  is provided on the back surface of the first metal layer  20  and outside the antenna coil  12 . This prevents generation of a diamagnetic field that does not work to increase the communication range of the antenna device, and suppresses the eddy current loss. As a result, the communication range of the antenna device can be increased. 
     The first magnetic layer  21  is preferably a composite magnetic sheet made of polymer containing flat magnetic metal grains having a large aspect ratio. Like the first magnetic layer  21 , the second magnetic layer  13  may foe a composite magnetic sheet. In the magnetic layer, the particles of flat metal grains overlap with one another in the thickness direction of the composite magnetic sheet, and are oriented with their planes parallel to the planer direction of the composite magnetic sheet. The composite magnetic sheet therefore has a high effective magnetic permeability with respect to its planer direction. Hence, the magnetic field generated by the antenna coil  12  can be introduced from outside into the magnetic layer to guide the magnetic flux in the horizontal direction, namely the direction intersecting at right angles with the coil axis. The particles of fiat metal grains are densely orientated in the polymer, but are insulated by the polymer. This prevents generation of an eddy current. Therefore, the antenna device can achieve both high magnetic permeability and low magnetic loss in the high RFID frequency band. 
     As described above, the antenna device according to this embodiment comprises an antenna coil  12  and a first metal layer  20  covering the antenna coil  12 . The first metal layer  20  has a slit SL that overlaps with the inner diameter area  12   a  of the antenna coil  12  in planar view. Further, a first magnetic layer  21  is formed on the back surface of the first metal layer  20 , which faces the antenna coil  12 , and in an outside region that does not overlap with the antenna coil  21  in planar view. Therefore, the first magnetic layer  21  can prevent the generation of a diamagnetic field and a loss of eddy current in that outside region. Hence, the antenna characteristic can be enhanced to lengthen the communication range of the antenna device. 
       FIG. 5  is a schematic cross-sectional view showing a configuration of an antenna device according to a second embodiment of the present invention. 
     As shown in  FIG. 5 , the antenna device  2  according to this embodiment is characterized in that the first magnetic layer  21  is thicker than that in the first embodiment. The lower surface  21   a  of the first magnetic layer  21  is therefore lies below the upper surface of the substrate  11  (i.e., the surface on which the antenna coil  12  is formed). In this embodiment, the lower surface  21   b  of the first magnetic layer  21  is flush with the lower surface of the second magnetic layer  13 . In any other respects, this embodiment is identical to the first embodiment in terms of configuration. 
     Preferably, the space in which the antenna element  10  interposed between a pair of first layers (i.e., left and right magnetic layers  21 ) should have a width almost equal to that of the width the substrate  11  has in Y direction. If the antenna device is so configured, the first magnetic layer  21  can be used as a guide for positioning the antenna element  10 . The antenna coil  12  can be positioned with respect to the slit SL more precisely than otherwise. 
     In the antenna device  2  according to this embodiment, the first magnetic layer  21  is large. Therefore, many magnetic fluxes generated from the current flowing in the antenna coil  12  can be guided not to be applied to the first metal layer  20 . Hence, a loop of magnetic fluxes can be formed more reliably than in the first embodiment, and the communication range of the antenna device can be reliably lengthened. 
       FIG. 6  is a schematic cross-sectional view showing a configuration of an antenna device according to a third embodiment of the present invention. 
     As shown in  FIG. 6 , the antenna device  3  according to this embodiment is characterized in that a first magnetic layer  21  of the type used in the first embodiment extends downwards and is integrated with the second magnetic layer  13 . That is, the antenna device  3  has a magnetic layer  22  formed by integrating the first magnetic layer  21  and second magnetic layer  13  used in the first embodiment. In any other respects, the third embodiment is identical to the first embodiment in terms of configuration. 
     According to this embodiment, the numerous magnetic fluxes generated from the current flowing in the antenna coil  12  can be guided not to foe applied to the first metal layer  20 . Hence, a loop of magnetic fluxes can be formed more reliably than in the first embodiment, and the communication range of the antenna device can be reliably lengthened. Further, the antenna element  10  and the magnetic layer  22  can be easily positioned with respect to the slit SL. The degradation of the antenna characteristic, which would otherwise occur, can therefore be prevented. Still further, this embodiment is advantageous in terms of the number of steps of arranging magnetic layers and the manufacturing cost. 
       FIG. 7  is a schematic cross-sectional view showing a configuration of an antenna device according to a fourth embodiment of the present invention. 
     As shown in  FIG. 7 , the antenna device  4  according to this embodiment is characterized in that it has no members equivalent to the second magnetic layer  13  used in the first embodiment. Namely, only the first magnetic layer  21  is used in this embodiment. The first magnetic layer  21  is thick, and its lower surface  21   b  lies at lower position and contacts a second metal layer  40 . Thus, the lower surface  21   b  of the first magnetic layer  21  is flush with the lower surface of the substrate  11 . This embodiment has no second magnetic layer  13 . Nonetheless, the magnetic path is not cut at the lower surface of the substrate  11  because the antenna element  10  is not mounted on a metal body such as the battery pack. 
     In this embodiment, the second metal layer  40  is provided on the back side of the antenna element  10  in place of the second magnetic layer  13  in the first embodiment. The second metal layer  40  has a slit SL′ (i.e., second slit). Like the slit SL, the slit SL′ overlaps with the inner diameter area  12   a  of the antenna coil  12  in planar view. The magnetic flux φ 1  intersecting the antenna coil  12  greatly extends around not only the first metal layer  20 , but also the second metal layer  40  at the back, and then extends back to the inner diameter area  12   a  of the antenna coil  12  through the slit SL′. The loop size of the magnetic flux φ 1  can therefore increase even more. As a result, the directivity of the antenna is enhanced, increasing the communication range of the antenna even more. 
       FIG. 8  is a schematic plan view showing a configuration of an antenna device  5  according to a fifth embodiment of the present invention. 
     As shown in  FIG. 8 , the antenna device  5  according to this embodiment is characterized in that the slit SL made in the first metal layer  20  does not completely divide the first metal layer  20  into two parts. Rather, the first metal layer  20  is composed of a first metal plate  20 A, a second metal plate  20 B and a connecting portion  20 C. The first and second metal plates  20 A and  20 B are adjacent to each other in Y direction, across the slit SL. The connecting portion  20 C bridges the slit SL and connects the first and second metal plates  20 A and  20 B, at one end thereof. In any other respects, this embodiment is identical to the first embodiment in terms of configuration. This embodiment may be combined with any one of the first to fourth embodiments. 
     The connecting portion  20 C prevents the slit SL from extending in X direction to cut the metal layer completely into two parts. The connecting portion  20 C exists at one end of the slit SL, filling up that end of the slit SL. The slit SL has a width uniform over its entire length. The connection part  20 C has an X-direction width that is preferably one-third or less, more preferably one-fifth or less, of the X-direction width of the first and second metal plates  20 A and  20 B. 
     The first and second metal plates  20 A and  20 B are almost isolated by the slit SL, but are connected by the connecting portion  20 C, respectively at the lower-right part and upper-right part. That is, they are not isolated physically or electrically. Hence, they can foe treated as a single metal member, and can be made by using one mold. Further, the first and the second metal plates  20 A and  20   b  are integrated, never displaced from each other, and the width of the slit SL will never change at all. 
       FIG. 9  is a schematic plan view showing a configuration of an antenna device according to a sixth embodiment of the present invention. 
     As shown in  FIG. 9 , the antenna device  6  according to this embodiment is characterized in that the first magnetic layer  21  is shaped like letter C as viewed in plane and is arranged outside the antenna coil  12 , covering the first and second metal layers  20 A and  20 B almost entirely. That is, of those parts of the magnetic layer  21  which overlap with the first metal layer  20 , not only the parts adjacent in Y direction as viewed from the antenna coil  12 , but also the parts  21 X adjacent as viewed in X direction from the antenna coil  12  are covered by the first magnetic layer  21 . Thus, the first magnetic layer  21  covers a broader region than in the first embodiment, and can therefore more suppress the diamagnetic field and the loss of eddy current, ultimately increasing the communication range of the antenna even more. 
     It is apparent that the present invention is not limited to the above embodiments, but may be modified and changed without departing from the scope and spirit of the invention. 
     In each embodiments described above, the antenna coil  12  is a spiral pattern composed of several turns. Nonetheless, it may be a loop pattern composed of less than one turn. That is, the antenna coil  12  may have a planer coil pattern shaped like either a loop or a spiral. The antenna coil  12  may be formed on the lower surface of the substrate  11 , or on both surfaces of the substrate  11 . The slit SL need not be a linear slit and may be curved or zig-zag slit. Moreover, the first, and second metal plates  20 A and  20 B may not be the thick metal layers constituting the housing, but may be metal foil bonded to the outer or inner surfaces of a resin case.