Patent Publication Number: US-2017373389-A1

Title: Antenna apparatus and rfid system

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of PCT/JP2016/051153 filed Jan. 15, 2016, which claims priority to Japanese Patent Application No. 2015-033807, filed Feb. 24, 2015, the entire contents of each of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to an antenna apparatus, as well as an RFID system having the antenna apparatus and an RFID tag that performs wireless communication with the antenna apparatus. 
     BACKGROUND 
     Conventionally, antenna apparatuses exist that uses electromagnetic induction to perform wireless communication with IC cards and RFID tags. 
     For example, the antenna apparatus described in JP11-282980A is utilized in a reader/writer for writing information to an IC card and reading information from the IC card, and includes a loop antenna for the writing or reading. The loop antenna is twisted so as to form two small loops (coils), that is, to be in an “8” shape. According to such a loop antenna, directions of magnetic fluxes respectively passing through the two coils are different. Therefore, the magnetic fluxes respectively generated by the two coils cancel each other at a position distant from the coil antenna. With this design, it is possible to suppress an influence of a magnetic field generated by the antenna apparatus on other wireless communication devices. 
     In general, elements within an antenna apparatus are also influenced by a magnetic field generated by an antenna of the antenna apparatus. 
     For example, a power supply circuit element that is connected to an antenna and supplies power to the antenna is influenced by a magnetic field generated by this antenna. 
     SUMMARY 
     Thus, an object of the present disclosure is to provide an antenna apparatus that is used, for example, for an RFID system and is capable of reducing an influence of a magnetic field generated by an antenna of the apparatus on a power supply circuit element connected to this antenna. 
     In order to overcome the technical problems and limitations in conventional antenna apparatuses discussed above, an antenna apparatus is disclosed that includes a substrate; an antenna provided on the substrate; and a power supply circuit element provided on the substrate and connected to the antenna. According to the exemplary aspect, the antenna includes a first coil antenna unit and a second coil antenna unit, each having a coil axis that intersects with the substrate, where the first coil antenna unit and the second coil antenna unit are arranged on the substrate such that a direction in which a current flows through one of the coil antenna units is clockwise and a direction in which a current flows through the other of the coil antenna units is counterclockwise. Moreover, in the exemplary aspect, the power supply circuit element is provided within a region between the first coil antenna unit and the second coil antenna unit. 
     Another aspect of the present disclosure is to provide an antenna apparatus that includes a substrate; an antenna provided on the substrate; and a power supply circuit element provided on the substrate and connected to the antenna. In this aspect, the antenna includes a first coil antenna unit and a second coil antenna unit, with each being coil-shaped and having a coil axis that intersects with the substrate. Moreover, the first coil antenna unit and the second coil antenna unit are arranged on the substrate such that a direction in which a current flows through one of the coil antenna units is clockwise and a direction in which a current flows through the other of the coil antenna units is counterclockwise. Furthermore, in this aspect, the power supply circuit element is provided on an imaginary straight line on the substrate, with the imaginary straight line being located equidistantly from the coil axis of the first coil antenna unit and the coil axis of the second coil antenna unit. 
     Yet another exemplary aspect provides an RFID system that includes a product having an RFID tag; and an antenna apparatus that performs wireless communication with the RFID tag of the product. In this aspect, the antenna apparatus includes a substrate; an antenna provided on the substrate; and a power supply circuit element provided on the substrate and connected to the antenna. In this aspect, the antenna includes a first coil antenna unit and a second coil antenna unit, each being coil-shaped and having a coil axis that intersects with the substrate, where the first coil antenna unit and the second coil antenna unit are arranged on the substrate such that a direction in which a current flows through one of the coil antenna units is clockwise and a direction in which a current flows through the other of the coil antenna units is counterclockwise. Moreover, the power supply circuit element is provided within a region between the first coil antenna unit and the second coil antenna unit. 
     Further another exemplary aspect is to provide an RFID system that includes a product having an RFID tag; and an antenna apparatus that performs wireless communication with the RFID tag of the product. In this aspect, the antenna apparatus includes a substrate; an antenna provided on the substrate; and a power supply circuit element provided on the substrate and connected to the antenna. Moreover, the antenna includes a first coil antenna unit and a second coil antenna unit, each being coil-shaped and having a coil axis that intersects with the substrate, the first coil antenna unit and the second coil antenna unit being arranged on the substrate such that a direction in which a current flows through one of the coil antenna units is clockwise and a direction in which a current flows through the other of the coil antenna units is counterclockwise. Furthermore, the power supply circuit element is provided on an imaginary straight line on the substrate, with the imaginary straight line being located equidistantly from the coil axis of the first coil antenna unit and the coil axis of the second coil antenna unit. 
     According to the exemplary embodiments disclosed herein, with an antenna apparatus used, for example, for an RFID system, it is possible to reduce an influence of a magnetic field generated by an antenna of the apparatus on a power supply circuit element connected to this antenna. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic perspective view of an RFID system according to a first exemplary embodiment 1. 
         FIG. 2  is an exploded view of the RFID system according to the embodiment 1. 
         FIG. 3  is a top view of an antenna apparatus according to the embodiment 1. 
         FIG. 4  is a bottom view of the antenna apparatus according to the embodiment 1. 
         FIG. 5  is a circuit diagram of the antenna apparatus according to the embodiment 1. 
         FIG. 6  is a sectional view of the antenna apparatus showing a distribution of a magnetic field generated by the antenna apparatus according to the embodiment 1. 
         FIG. 7  is a top view of an antenna apparatus for an RFID system according to a second exemplary embodiment 2. 
         FIG. 8  is a bottom view of the antenna apparatus according to the embodiment 2. 
         FIG. 9  is a sectional view of the antenna apparatus taken along line Q-Q in  FIG. 7 . 
         FIG. 10  is a sectional view of the antenna apparatus taken along line R-R in  FIG. 7 . 
         FIG. 11  is a block diagram illustrating a configuration of the antenna apparatus according to the embodiment 2. 
         FIG. 12  is a diagram for illustration of signal lines and return paths. 
     
    
    
     DETAILED DESCRIPTION 
     An antenna apparatus according to an exemplary aspect includes a substrate; an antenna provided on the substrate; and a power supply circuit element provided on the substrate and connected to the antenna. In this aspect, the antenna includes a first coil antenna unit and a second coil antenna unit, each having a coil axis that intersects with the substrate, where the first coil antenna unit and the second coil antenna unit are arranged on the substrate such that a direction in which a current flows through one of the coil antenna units is clockwise and a direction in which a current flows through the other of the coil antenna units is counterclockwise. Moreover, the power supply circuit element is provided within a region between the first coil antenna unit and the second coil antenna unit. 
     Further, an antenna apparatus according to another exemplary aspect is disclosed that includes a substrate; an antenna provided on the substrate; and a power supply circuit element provided on the substrate and connected to the antenna. In this aspect, the antenna includes a first coil antenna unit and a second coil antenna unit, with each being coil-shaped and having a coil axis that intersects with the substrate, where the first coil antenna unit and the second coil antenna unit are arranged on the substrate such that a direction in which a current flows through one of the coil antenna units is clockwise and a direction in which a current flows through the other of the coil antenna units is counterclockwise. In this aspect, the power supply circuit element is provided on an imaginary straight line on the substrate that is located equidistantly from the coil axis of the first coil antenna unit and the coil axis of the second coil antenna unit. 
     According to these aspects, with an antenna apparatus used, for example, for an RFID system, it is possible to reduce an influence of a magnetic field generated by an antenna of the apparatus on a power supply circuit element connected to this antenna. 
     In one exemplary aspect, the power supply circuit element may include an RFIC element that sends and receives signals via the antenna. 
     In addition, the power supply circuit element may include a matching element that is connected to the antenna and the RFIC element. With this configuration, the RFIC element is able to perform high-quality wireless communication via the antenna. 
     The power supply circuit element may further include a control IC element that is connected to the RFIC element and controls the RFIC element. With this configuration, an external device connected to the antenna apparatus does not need to have a function for controlling the RFID element. Therefore, it is possible to connect the antenna apparatus to an external device without a function for controlling the RFIC element, that is, a general-purpose external device such as a computer. 
     If a region between the first coil antenna unit and the second coil antenna unit forms a narrowest region, according to an exemplary aspect, the RFIC element may be disposed on one side with respect to the narrowest region, the control IC element may be disposed on the other side with respect to the narrowest region, and a conductor connecting the RFIC element with the control IC element may pass through the narrowest region. With this configuration, it is possible to reduce a distance between the first coil antenna unit and the second coil antenna unit, and in turn to reduce a size of the substrate. As a result, the antenna apparatus may be made smaller. 
     The power supply circuit element may include a conductor-for-signal through which a signal current flows, and a conductor-for-return through which a return current for the signal current flows. In this case, it is preferable that the conductor-for-signal and the conductor-for-return are in parallel with each other, and face each other in a direction perpendicular to the substrate. With this configuration, it is possible to suppress interlinkage between magnetic fluxes of a loop-shaped circuit having a conductor-for-signal and a conductor-for-return in which a current flows around and the first coil antenna unit and the second coil antenna unit. As a result, it is possible to suppress mixture of noises into signals that flows through the conductor-for-signal. 
     The first coil antenna unit and the second coil antenna unit may be helical-shaped according to one exemplary aspect. With this configuration, it is possible to make an area of an opening of a coil larger. As a result, when the RFID tag or the like that wirelessly communicates with the coil antenna is placed within an opening of the coil antenna unit, it is possible to increase an area for placement. 
     An RFID system according to yet another exemplary aspect includes a product having an RFID tag; and an antenna apparatus that performs wireless communication with the RFID tag of the product. In this aspect, the antenna apparatus includes a substrate; an antenna provided on the substrate; and a power supply circuit element provided on the substrate and connected to the antenna, where the antenna includes a first coil antenna unit and a second coil antenna unit, each being coil-shaped and having a coil axis that intersects with the substrate, the first coil antenna unit and the second coil antenna unit are arranged on the substrate such that a direction in which a current flows through one of the coil antenna units is clockwise and a direction in which a current flows through the other of the coil antenna units is counterclockwise. Moreover, the power supply circuit element is provided within a region between the first coil antenna unit and the second coil antenna unit. 
     An RFID system according to another exemplary aspect includes a product having an RFID tag; and an antenna apparatus that performs wireless communication with the RFID tag of the product. In this aspect, the antenna apparatus includes a substrate; an antenna provided on the substrate; and a power supply circuit element provided on the substrate and connected to the antenna. In addition, the antenna includes a first coil antenna unit and a second coil antenna unit, each being coil-shaped and having a coil axis that intersects with the substrate, where the first coil antenna unit and the second coil antenna unit is arranged on the substrate such that a direction in which a current flows through one of the coil antenna units is clockwise and a direction in which a current flows through the other of the coil antenna units is counterclockwise. Furthermore, the power supply circuit element is provided on an imaginary straight line on the substrate that is located equidistantly from the coil axis of the first coil antenna unit and the coil axis of the second coil antenna unit. 
     According to these aspects, with the antenna apparatus used, for example, for the RFID system, it is possible to reduce an influence of a magnetic field generated by the antenna of the apparatus on the power supply circuit element connected to this antenna. Further, it is possible to suppress generation of a position of the RFID tag at which the RFID tag is not able to wirelessly communicate with the antenna, i.e., a null point. 
     If the antenna apparatus includes placement portions on each of which the product is placed, it is preferable that the placement portions are provided respectively within the first coil antenna unit and the second coil antenna unit when viewed in a direction perpendicular to the substrate. With this configuration, the RFID tag of the product and the antenna apparatus may wirelessly communicate in a favorable manner. 
     Hereinafter, exemplar embodiments of the present disclosure will be described with reference to the drawings. 
     Embodiment 1 
       FIG. 1  schematically illustrates an RFID system according to a first exemplar embodiment 1.  FIG. 2  is an exploded view of the RFID system illustrated in  FIG. 1 . Here, an X-Y-Z coordinate system shown in the figures is employed merely to facilitate understanding of the exemplary embodiments, and not intended to limit the present disclosure. 
     An RFID system  10  shown in  FIG. 1  constitutes an HF band RFID system using an HF band frequency as a carrier frequency, and includes an RFID (Radio Frequency Identification) tag  12  attached to a product G, and an antenna apparatus  14  as an antenna of a reader/writer device that performs wireless communication with the RFID tag  12 . 
     While not shown, the RFID tag  12  includes an antenna that performs wireless communication with the antenna apparatus  14 , a control unit connected to the antenna, and a memory connected to the control unit. Based on a requesting signal from the antenna apparatus  14  received by its antenna, for example, the control unit of the RFID tag  12  obtains information (data) within the memory, and sends the obtained information to the antenna (that is, sends information to the antenna apparatus  14  via the antenna of the RFID tag  12 ). The control unit of the RFID tag  12  also writes information from the antenna apparatus  14  received by its antenna to the memory. 
     The antenna apparatus  14  that performs wireless communication with the RFID tag  12  includes a substrate  16 , an antenna  18  provided on the substrate  16 , and a cover  20  that protects the antenna  18  and on which the product G is placed. Here, the cover  20  is provided with marks  20   a  indicating positions of placement portions on the cover  20  on each of which the product G is placed. 
     The substrate  16  of the antenna apparatus  14  includes a main surface  16   a  and a back surface  16   b  that faces the main surface  16   a.  As an example of the substrate  16 , a printed circuit board made of an epoxy resin may be used. On the main surface  16   a  and the back surface  16   b  of the substrate  16 , coil antenna units  30  and  32  that constitute antennas, lands for mounting a capacitor  36  that provides a power supply circuit element  22  and an RFIC element  34 , and connecting conductors  38  and  40  for connecting the coil antenna units  30  and  32  with the power supply circuit element  22  are provided as conductor patterns. The conductor patterns are patterned, for example, by etching or like a copper foil over an entire surface of the printed circuit board in a predetermined shape. The main surface  16   a  of the substrate  16  is provided with the antenna  18 , and the power supply circuit element  22  connected to the antenna  18  and supplying power to the antenna  18 . 
     As shown in  FIG. 2 , according to the exemplary aspect the antenna  18  includes the first coil antenna unit  30  and the second coil antenna unit  32  that are in a coil shape. The first and the second coil antenna units  30 ,  32  respectively include coil axes (winding axes)  30   a,    32   a  that intersect with the main surface  16   a  of the substrate  16 , for example, perpendicularly (extend along a Z axial direction). Further, the first coil antenna unit  30  and the second coil antenna unit  32  are connected in series. 
     In the case of the embodiment 1, as shown in  FIG. 3  which is a top view of the antenna apparatus  14 , each of the first and the second coil antenna units  30 ,  32  is configured as a double loop conductor centered at corresponding one of the coil axes  30   a,    32   a.  Further, the shapes of the first and the second coil antenna units  30 ,  32  are symmetric with respect to an imaginary plane VP that passes a midpoint of a connecting straight line JL connecting the coil axes  30   a,    32   a  perpendicularly to the connecting straight line JL. 
     Here, the placement portions for products G on the cover  20 , that is, the marks  20   a  indicating the placement portions are provided respectively within the first and the second coil antenna units  30 ,  32  when viewed in a direction perpendicular to the main surface  16   a  of the substrate  16  (Z axial direction). More specifically, centers of the marks  20   a  are offset to a side of the imaginary plane VP from centers of the respective coil antenna units. With this configuration, the RFID tags  12  of the products G are respectively arranged within the first and the second coil antenna units  30 ,  32 , that is, respectively within magnetic fluxes passing the first and the second coil antenna units  30 ,  32 . As a result, the RFID tag  12  and the antenna apparatus  14  may wirelessly communicate in a favorable manner. 
     Specifically, as shown in  FIG. 3 , the first coil antenna unit  30  includes a substantially circular inner conductor  30   b  which is partially disconnected (“C” shape), and two semi-circular outer conductors  30   c,    30   d  arranged outside and along the inner conductor  30   b.  The two semi-circular outer conductors  30   c,    30   d  are arranged along a circumference of the same circle centered at the coil axis  30   a.    
     One end of the outer conductor  30   c  of the first coil antenna unit  30  is connected to the power supply circuit element  22 . The other end of the outer conductor  30   c  is connected to one end of the inner conductor  30   b.  The other end of the inner conductor  30   b  is connected to one end of the outer conductor  30   d  via a bridge conductor  30   e.  As shown in  FIG. 4 , which illustrates a bottom view of the antenna apparatus  14 , the bridge conductor  30   e  is provided on the back surface  16   b  of the substrate  16  according to the exemplary aspect. Here, the inner conductor  30   b  and the outer conductor  30   c  on the main surface  16   a  of the substrate  16  and the bridge conductor  30   e  are connected via an interlayer connecting conductor (a conductor that penetrates the substrate  16 ) such as a via hole conductor or a through hole conductor that is not illustrated. Further, the other end of the outer conductor  30   d  is connected to the second coil antenna unit  32 . 
     Similar to the first coil antenna unit  30 , the second coil antenna unit  32  includes a substantially circular inner conductor  32   b  which is partially disconnected (“C” shape), and two semi-circular outer conductors  32   c,    32   d  arranged outside and along the inner conductor  32   b.  The two semi-circular outer conductors  32   c,    32   d  are arranged along a circumference of the same circle centered at the coil axis  32   a.    
     One end of the outer conductor  32   c  of the second coil antenna unit  32  is connected to the other end of the outer conductor  30   d  of the first coil antenna unit  32 . The other end of the outer conductor  32   c  is connected to one end of the inner conductor  32   b.  The other end of the inner conductor  32   b  is connected to one end of the outer conductor  32   d  via a bridge conductor  32   e.  As also shown in  FIG. 4 , the bridge conductor  32   e  is provided on the back surface  16   b  of the substrate  16  according to the exemplary aspect. Further, the other end of the outer conductor  32   d  is connected to the power supply circuit element  22  via a bridge conductor  32   f.    
       FIG. 5  is a circuit diagram of the antenna apparatus  14 , showing the power supply circuit element  22  connected to the antenna  18  (the first coil antenna unit  30  and the second coil antenna unit  32 ). 
     In the case of the embodiment 1, the power supply circuit element  22  includes the RF (Radio Frequency) IC element  34  and the capacitor  36 . 
     The RFIC element  34  is connected to the antenna  18 . More specifically, the RFIC element  34  includes two input/output terminals, and the input/output terminals are respectively connected to one end and the other end of the antenna  18 . The RFIC element  34  is also configured to send and receive signals via the antenna  18 . For example, the RFIC element  34  receives information within the memory of the RFID tag  12  as a signal via the antenna  18  that communicates with the RFIC tag  12 . Alternatively, the RFIC element  34  sends information recorded in the RFID tag  12  as a signal to the RFID tag  12  via the antenna  18 . 
     Here, the RFIC element  34  is configured to be able to output information received from the RFID tag  12  to an external device (not shown) external to the antenna apparatus  14 , and to receive an input of information from the external device. With this configuration, the antenna apparatus  14  is able to function as a reader/writer device in the RFID system  10  that reads and writes information from and to the RFID tag  12 . The RFIC element includes an RFIC chip. 
     The capacitor  36  is connected in parallel to the coil-shaped antenna  18 . This constitutes a resonant circuit configured by the coil-shaped antenna  18  and the capacitor  36 . Capacitance of the capacitor  36  is determined such that a resonant frequency of the resonant circuit is a predetermined frequency (here, an HF band frequency). 
     According to such a configuration, as shown in  FIG. 3 , for example, a current I from the power supply circuit element  22  flows through the outer conductor  30   c,  the inner conductor  30   b,  and the outer conductor  30   d  of the first coil antenna unit  30  of the antenna  18 , and the outer conductor  32   c,  the inner conductor  32   b,  and the outer conductor  32   d  of the second coil antenna unit  32  of the antenna  18 , in that, order, according to the exemplary aspect. Further, when viewed from top of the antenna apparatus  14 , the current I flows, in the clockwise direction, through the first coil antenna unit  30 , and the current I flows, in the counterclockwise direction, into the second coil antenna unit  32 . 
     Thus, directions of a magnetic flux generated by the first coil antenna unit  30  and a magnetic flux generated by the second coil antenna unit  32  are different. A magnetic field distribution as shown in  FIG. 6  is generated according to the exemplary aspect. For example, in  FIG. 3 , the magnetic flux passing the first coil antenna unit  30  in which the current I flows in the clockwise direction is directed from top to bottom (toward a negative direction along a Z axis). On the other hand, the magnetic flux passing the second coil antenna unit  32  in which the current I flows in the counterclockwise direction is directed from bottom to top (toward a positive direction along the Z axis). 
     As the directions of a magnetic flux generated by the first coil antenna unit  30  and a magnetic flux generated by the second coil antenna unit  32  are different in this manner, there is a portion on the substrate  16  at which a magnetic flux density is relatively lower than a different portion. 
     Specifically, as shown in  FIG. 3 , as compared to the different portion, the magnetic flux density is low in a region between the first coil antenna unit  30  and the second coil antenna unit  32 , for example, a region A (cross-hatched region) within an imaginary circle VC centered at the midpoint of the connecting straight line JL connecting the coil axes  30   a,    32   a  and outside the first and the second coil antenna units  30 ,  32 . At any position within the low magnetic flux density region A, magnetic fluxes respectively generated by the first and the second coil antenna units  30 ,  32  cancel each other since a difference between a distance from the first coil antenna unit  30  to the any position and a distance from the second coil antenna unit  30  to the any position is small. As a result, the magnetic flux density becomes low. 
     By contrast, for example, in a region facing the second coil antenna unit  32  across the first coil antenna unit  30  (a region on a left side of the first coil antenna unit  30  in  FIG. 3 ), a difference between the distances from the first and the second coil antenna units  30 ,  32  is large, the magnetic flux of the second coil antenna unit  32  does not cancel the magnetic flux of the first coil antenna unit  30 . As a result, the magnetic flux density becomes high. 
     Here, the magnetic flux density is the lowest (substantially zero) at a position on the imaginary plane VP at which the distances from the first and the second coil antenna units  30 ,  32  are equal. 
     Further, as shown in  FIG. 1 , when a product G having the RFID tag  12  is placed above one of the first coil antenna unit  30  and the second coil antenna unit  32 , the magnetic field distributions of the coil antenna units  30 ,  32  are not symmetric with respect to the imaginary plane VP. In this case, the magnetic flux density in the low magnetic flux density region A changes as well. However, an amount of change in the magnetic flux density in this area is smaller than that in the region outside the low magnetic flux density region A. 
     As shown in  FIG. 3 , the connecting conductors  38  and  40  that provide the conductor patterns provided on the substrate  16  as well as the RFIC element  34  and the capacitor  36  of the power supply circuit element  22  are positioned within the low magnetic flux density region A. In the case of the embodiment 1, the power supply circuit element  22  is provided at a position on an imaginary straight line VL on the substrate  16  at which the distances from the first and the second coil antenna units  30 ,  32  (i.e., the coil axes  30   a,    32   a ) are equal. Here, the imaginary straight line VL is a line of intersection between the imaginary plane VP and the main surface  16   a  of the substrate  16 . 
     It should be noted that, in the case of the embodiment 1, as shown in  FIG. 3 , a terminal of the first coil antenna unit  30  and a terminal of the second coil antenna unit  32  of the antenna  18  (i.e., one ends of the outer conductor  30   c,    32   d ) are located within the low magnetic flux density region A. Therefore, similar to the power supply circuit element  22 , the connecting conductor  38  connecting the terminal of the first coil antenna unit  30  to the power supply circuit element  22  and the connecting conductor  40  connecting the terminal of the second coil antenna unit  32  to the power supply circuit element  22  are also provided within the low magnetic flux density region A. Specifically, connecting portions to the power supply circuit element  22  in the antenna  18  are also provided within the low magnetic flux density region A. 
     Therefore, the RFIC element  34  of the power supply circuit element  22  and the connecting conductors  38 ,  40  are insusceptible to the magnetic fluxes as compared to a case in which these components are provided outside the low magnetic flux density region A. With this configuration, noises attributed to the antenna  18  may not be easily mixed with signals output from the RFIC element  34 . Further, noises attributed to the antenna  18  may not be easily mixed with signals input to the RFIC element  34 . As a result, a communication quality of the RFID system  10  including the antenna apparatus  14  is highly reliable according to the configuration of the exemplary embodiment. 
     According to the embodiment 1 described above, with the antenna apparatus  14  used for the RFID system  10 , it is possible to reduce an influence of a magnetic field generated by the antenna  18  of the apparatus on the power supply circuit element  22  connected to the antenna  18 . 
     Further, In the case of the embodiment 1, the antenna  18  includes the two coil antenna units  30 ,  32 . With this configuration, it is possible to suppress generation of a position of the RFID tag  12  at which the RFID tag  12  is not able to wirelessly communicate with the antenna  18 , i.e., a null point. 
     When lengths of the conductors are the same, an area of an opening of one coil antenna unit (an area within the conductor of the coil) is larger than an area of an opening of each of the two coil antenna units. If the area of the opening is larger, a magnetic flux density at a center of the coil antenna unit that is distant form from the conductor is low, and a null point where antenna sensitivity is low occurs at this position. 
     By contrast, when the same currents flow, magnetic flux densities at centers of two coil antenna units are higher than the magnetic flux density at the center of the one coil antenna unit. Therefore, it is possible to suppress generation of a null point more effectively by two coil antenna units than by one coil antenna unit if the lengths of the conductors and the currents flowing therethrough are the same. 
     Embodiment 2 
     Differences between an RFID system of according to an embodiment 2 and the RFID system  10  according to the embodiment 1 lie in the antenna apparatus. In particular, configurations of an antenna and a power supply circuit element are different from those in the embodiment 1. Therefore, the embodiment 2 will be described focusing on the configurations of the antenna and the power supply circuit element different from the embodiment 1. 
       FIG. 7  is a top view of an antenna apparatus  114  according to the embodiment 2.  FIG. 8  is a bottom view of the antenna apparatus  114 .  FIG. 9  is a sectional view taken along line Q-Q in  FIG. 7 .  FIG. 10  is a sectional view taken along line R-R in  FIG. 7 .  FIG. 11  is a block diagram illustrating a configuration of the antenna apparatus  114 . Finally,  FIG. 12  is a circuit diagram of a part of the antenna apparatus  114 . 
     As shown in  FIG. 7 , an antenna  118  of the antenna apparatus  114  according to the embodiment 2 includes a first coil antenna unit  130  and a second coil antenna unit  132  respectively having the coil axes  130   a,    132   a  perpendicular to a main surface  116   a  of a substrate  116 . Further, the first coil antenna unit  130  and the second coil antenna unit  132  are connected in series. 
     The first and the second coil antenna unit  130 ,  132  of the embodiment 2 are helical-shaped, unlike the double loop first coil antenna units  30 ,  32  of the embodiment 1. 
     Specifically, the first coil antenna unit  130  includes a “C”-shaped main-side conductor  130   b  which is provided on the main surface  116   a  of the substrate  116 , and a substantially circular back-side conductor  130   c  which is provided on a back surface  116   b  of the substrate  116  and partially disconnected (“C” shape). 
     One end of the main-side conductor  130   b  of the first coil antenna unit  130  is connected to an RFIC element  134  of a power supply circuit element  122  that will be later described in detail. The other end of the main-side conductor  130   b  is connected to one end of the back-side conductor  130   c  via a hole conductor (not shown). The other end of the back-side conductor  130   c  is connected to the second coil antenna unit  132 . 
     On the other hand, the second coil antenna unit  132  includes a “C”-shaped main-side conductor  132   b  which is provided on the main surface  116   a  of the substrate  116 , and a substantially circular back-side conductor  132   c  which is provided on a back surface  116   b  of the substrate  116  and partially disconnected (“C” shape). 
     One end of the back-side conductor  132   c  of the second coil antenna unit  132  is connected to the other end of the back-side conductor  130   c  of the first coil antenna unit  130  via a connecting conductor  136 . The other end of the back-side conductor  132   c  is connected to one end of the main-side conductor  132   b  via a via hole conductor (not shown). Then, the other end of the main-side conductor  132   b  is connected to the RFIC element  134 . 
     According to such a configuration, for example, a current from the RFIC element  134  flows through the main-side conductor  130   b,  and the back-side conductor  130   c  of the first coil antenna unit  130 , and the back-side conductor  132   c,  and the main-side conductor  132   b  of the second coil antenna unit  132 , in that order according to the exemplary embodiment. Further, when viewed from top of the antenna apparatus  114 , in  FIG. 7 , the current I flows, in the clockwise direction, through the first coil antenna unit  130 , and the current I flows, in the counterclockwise direction, into the second coil antenna unit  32 . 
     Thus, a magnetic flux passing the first coil antenna unit  130  in which the current I flows in the clockwise direction is directed from top to bottom (toward a negative direction along a Z axis). On the other hand, the magnetic flux passing the second coil antenna unit  132  in which the current I flows in the counterclockwise direction is directed from bottom to top (toward a positive direction along the Z axis). As a result, as compared to the different portion, the magnetic flux density is low in a region between the first coil antenna unit  130  and the second coil antenna unit  132 , for example, a low magnetic flux density region A′ within an imaginary circle VC′ centered at a midpoint of a connecting straight line JL′ connecting the coil axes  130   a,    132   a  and outside the first and the second coil antenna unit  130 ,  132 . 
     Here, the magnetic flux density is the lowest (substantially zero) at a position on the imaginary plane VP′ at which the distances from the first and the second coil antenna units  130 ,  132  are equal. 
     As shown in  FIG. 7 , the power supply circuit element  122  is provided within the low magnetic flux density region A′. Here, as shown in  FIG. 8 , a grand pattern  138  is provided on a portion of the back surface  116   b  of the substrate  116  facing the power supply circuit element  122 , that is, a region between the first coil antenna unit  130  and the second coil antenna unit  132  on the back surface  116   b.  Further, a grand pattern  140  is provided on the main surface  116   a  of the substrate  116  so as to face the grand pattern  138  and to surround the power supply circuit element  122 . 
     As shown in  FIG. 7  and  FIG. 11 , the power supply circuit element  122  of the embodiment 2 includes the RFIC element  134  that sends and receives signals via the antenna  118  (the first and the second coil antenna units  130 ,  132 ), and an MCU (Micro Controller Unit)  142  as a control IC element that is connected to the RFIC element  134  and controls the RFIC element  134 . The power supply circuit element  122  also includes an RF (Radio Frequency) front-end circuit  144  that is provided between and connected to the antenna  118  and the RFIC element  134 . 
     The RF front-end circuit  144  includes a matching unit  146  (as shown in  FIG. 11 , for example) for impedance matching between the antenna  118  and the RFIC element  134 , and an EMI (Electro Magnetic Interference) filtering unit  148  for noise rejection. By the RF front-end circuit  144 , the RFIC element  134  is able to perform high quality wireless communication with an RFID tag via the antenna  118 . 
     The MCU  142  sends and receives signals (information) with the RFIC element  134  to control the RFIC element  134 . Therefore, a plurality of conductors  170  connecting the MCU  142  and the RFIC element  134  is arranged on the substrate  116 . 
     In the case of the embodiment 2, as shown in  FIG. 7 , the RFID element  134  and the MCU  142  are provided at a position on an imaginary straight line VL′ on the substrate  116  at which distances from the first and the second coil antenna units  130 ,  132  (i.e., coil axes  130   a,    132   a ) whose magnetic flux density due to the first and the second coil antenna units  130 ,  132  is substantially zero are equal. Here, the imaginary straight line VL′ is a line of intersection between an imaginary plane VP′ and the main surface  116   a  of the substrate  116 . Further, the RFIC element  134  is located on one side with respect to a narrowest region within a region between the first coil antenna unit  130  and the second coil antenna unit  132  (i.e., constriction in the low magnetic flux density region A′). On the other hand, the MCU  142  is located on the other side with respect to the narrowest region. Therefore, the plurality of conductors  170  connecting the RFIC element  134  and the MCU  142  pass the narrowest region. 
     According to such an arrangement, the plurality of conductors  170  connecting the RFIC element  134  and the MCU  142  are arranged at positions where the magnetic flux density is low, and thus insusceptible to the magnetic field. Therefore, noises are not easily mixed in signals carried by the conductors  170 . Further, as almost all of the grand patterns are arranged in the low magnetic flux density region A′, no significant influence is given to communication characteristics of the antenna. 
     Further, as compared to a case in which the RFIC element  134  and the MCU  142  are located on the same side with respect to the narrowest region within the region between the first coil antenna unit  130  and the second coil antenna unit  132 , it is possible to make a distance between the first coil antenna unit  130  and the second coil antenna unit  132  smaller. With this configuration, a size of the substrate  116  may be reduced, and as a result, it is possible to make the antenna apparatus  114  small. 
     According to the exemplary aspect of embodiment 2, the RFIC element  134  and the MCU  142  are connected to the grand pattern  138  provided on the back surface  116   b  of the substrate  116 . Further, as shown in  FIG. 10 , the plurality of conductors  170  arranged on the main surface  116   a  of the substrate  116  and connecting the RFIC element  134  and the MCU  142  faces the grand pattern  138  in a direction perpendicular to the main surface  116   a  of the substrate  116  (Z axial direction). Moreover, the plurality of conductors  170  and the grand pattern  138  are in parallel with each other according to the exemplary embodiment. Therefore, as shown in  FIG. 12 , when a signal current S flows through the conductors  170 , a return current R directed oppositely to the signal current S flows through the grand pattern  138  as a return path. 
     Specifically, a loop L including the RFIC element  134 , the conductors  170 , the MCU  142 , and the grand pattern  138  (return path) through which a current flows around is occurred. The loop L is perpendicular to the main surface  116   a  of the substrate  116 . With this configuration, interlinkages of the magnetic fluxes of the first and the second coil antenna units  130 ,  132  into the loop L may be suppressed. As a result, it is possible to suppress mixture of noises due to the magnetic fluxes of the first and the second coil antenna units  130 ,  132  into signals carried by the conductors  170 . 
     As shown in  FIG. 11 , the MCU  142  is connected to an external device outside the antenna apparatus  114 , for example, a computer  200 . The MCU  142  is configured to receive power supply for driving from the computer  200 , and to send and receive signals (information) with the computer  200 . For example, the MCU  142  receives information recorded in the memory of the RFID tag  12  from the computer  200 . As shown in  FIG. 7 , as an interface for connection to the computer  200 , the antenna apparatus  114  includes a plurality of connecting terminals  172  connected to the MCU  142  and arranged on the main surface  116   a  of the substrate  116 . 
     According to the embodiment 2 described above, similar to the embodiment 1, with the antenna apparatus  114  used for the RFID system, it is possible to reduce an influence of a magnetic field generated by the antenna  118  of the apparatus on the power supply circuit element  122  connected to the antenna  118 . Further, similar to the embodiment 1, it is possible to suppress generation of a position of the RFID tag  12  at which the RFID tag  12  is not able to wirelessly communicate with the antenna  118 , i.e., a null point. 
     Moreover, according to the exemplary aspect of embodiment 2, the MCU  142  as a control IC element, which controls the RFIC element  134 , is incorporated into the antenna apparatus  114 . Therefore, an external device connected to the antenna apparatus  114  does not need to have a function for controlling the RFID element  134 . Therefore, it is possible to connect the antenna apparatus  114  to an external device without a function for controlling the RFIC element  134 , that is, a general-purpose external device such as a computer. Specifically, the antenna apparatus  114  of the embodiment 2 is more versatile than that of the embodiment 1. 
     As described above, the present invention has been described with reference to the embodiments. However, embodiments of the present invention are not limited to the above examples. 
     For example, in the case of the embodiment 1, as shown in  FIG. 3 , the first and the second coil antenna units  30 ,  32  are in a double looped shape (spiral shape). Further, in the case of the embodiment 2, the first and the second coil antenna units  130 ,  132  are helical-shaped as shown in  FIG. 7  and  FIG. 8 . However, embodiments of the present invention are not limited to specific shapes of the plurality of coil antenna units. Specifically, as long as a region having a lower magnetic flux density than in other regions may be provided by magnetic fluxes respectively generated from the plurality of coil antenna units cancelling each other, between the coil antenna units, shapes and sizes of the coil antenna units, a winding number (i.e., a number of the loops) and a stacking number (the number of stacks of the loop) of the coils can be different according to various exemplary aspects. 
     However, when the coil antenna units are helical-shaped as in the embodiment 2, it is possible to make the area of an opening of the coil (an area within the conductor of the coil) larger as compared to the multi-loop shape as in the embodiment 1 (when sizes of the substrates for which the coil antenna units are provided are the same). Therefore, as shown in  FIG. 2 , it is possible to increase an area for placement on which a product having an RFID tag is placed and set within the opening of the coil antenna unit. Further, providing a larger number of winding of the coils (the number of the loops) is more preferable as it leads to generation of a stronger magnetic field, that is, a communication range of the antenna increases. Moreover, as shown in the above embodiments, it is preferable that the internal or external shape of the coil antenna units is circular, as this makes the magnetic field distribution even, and a null point may not easily be generated. 
     Furthermore, in the case of the embodiments 1 and 2, two coil antenna units for the antenna are provided, but 2 or more coil antenna units may be provided. For example, 2 or more coil antenna units may be arranged in series or in parallel. 
     Further, in the case of the embodiments 1 and 2 described above, the first coil antenna unit and the second coil antenna unit of the antenna are connected in series. Specifically, a current supplied from the power supply circuit element and passing one of the coil antenna units also passes the other of the coil antenna unit. Unlike the above example, the antenna apparatus may be configured such that a current is supplied from the power supply circuit element to each of the first coil antenna unit and the second coil antenna unit separately. 
     Moreover, in the case of the embodiments 1 and 2 described above, the antenna apparatus is able to wirelessly communicate with RFID tags respectively of two products, as shown in  FIG. 1 . However, it is understood that the antenna apparatus is able to wirelessly communicate with a RFID tag of one product as long as it is within the communication range of the antenna. Further, within the communication range of the antenna, the antenna apparatus is able to wirelessly communicate with the RFID tag without placing the product G on the placement portion of the antenna apparatus, that is, with a gap interposed between the antenna and the product. Therefore, a product having an RFID tag is not limited to a product such as a toy that can be placed on the antenna apparatus, and may be a card, for example. Preferably, a placement surface for the product G of the substrate  16  is the back surface  16   b,  instead of the main surface  16   a  on which the power supply circuit element  22  is provided. In other words, as design and convenience may be improved by flattening the placement surface for the product G, it is preferable to use a surface opposite to the mounting surface for the power supply circuit element  22  as the placement surface for the product G. 
     In addition, in the case of the embodiments 1 and 2 described above, the antenna apparatus is able to function as a reader/writer device in the RFID system that reads and writes information from and to the RFID tag. However, embodiments of the present invention are not limited to such an example. The antenna apparatus according to the embodiments of the present invention may be used, for example, in a communication system in which antenna apparatuses wireless communicates with each other. Further, the antenna apparatus is not limited to the use in the HF band RFID system, and may be used as an antenna apparatus for UHF band RFID system or the like. 
     Finally, as used herein, an “element” is not limited to a chip-like component, and may be interpreted as an individual component that constitutes an electrical circuit. Therefore, examples of the “element” are not limited to a chip-like component, and include circuits of patterns provided on the substrate. If the element is chip-like, the element may be mounted on the main surface or the back surface of the substrate, or built within the substrate. 
     Further, it is possible to achieve a new embodiment by partially combining the embodiments 1 and 2. For example, the MCU  142  of the embodiment 2 may be mounted on the antenna apparatus  14  of the embodiment 1. 
     The present invention may be applied to antenna apparatuses for sending and receiving information, as well as systems that employ such an antenna apparatus, for example, RFID systems and communication systems.