Abstract:
An antenna device includes a base including a planar conductor disposed thereon, and a coil antenna. The coil antenna includes a coil conductor wound around a magnetic core. The coil antenna is arranged such that a coil opening of the coil conductor is closed to an edge of the planar conductor. A current passing through the coil conductor induces a current in the planar conductor. Thus, a first magnetic flux occurs in the coil antenna, and a second magnetic flux occurs in the planar conductor. Therefore, a third magnetic flux occurs in an area of the planar conductor. Accordingly, the antenna device achieves a small footprint, a small-sized communication terminal apparatus and a desired communication distance.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to antenna devices and communication terminal apparatuses and, in particular, to an antenna device and communication terminal apparatus preferably for use in a communication system in the high-frequency (HF) range. 
         [0003]    2. Description of the Related Art 
         [0004]    A radio-frequency identification (RFID) system for exchanging information between a reader-writer and an RFID tag by non-contact communications between the reader-writer and the RFID tag and a near field communication (NFC) system for carrying out communications between two communication apparatuses at a short range are known. An RFID system and a near field communication system that use the HF range, for example, a 13.56 MHz range, as the communication frequency employ antennas coupled to each other mainly through an induction field. 
         [0005]    In recent years, some communication terminal apparatuses, such as cellular phones, have introduced an RFID system or a near field communication system, and the communication terminal apparatuses have been used as an RFID tag and a reader-writer used therefor or been used as terminals in near field communication. A magnetic antenna is known as an antenna device for use in transmitting and receiving a radio-frequency signal in the HF range. The magnetic antenna has a structure in which a coil conductor is wound around the surface of a magnetic core, as described in, for example, Japanese Unexamined Patent Application Publication No. 2005-317674 and Japanese Unexamined Patent Application Publication No. 2007-019891. 
         [0006]      FIG. 1  is an exploded perspective view of a magnetic antenna in Japanese Unexamined Patent Application Publication No. 2007-019891. The magnetic antenna is a laminate that includes a plurality of magnetic layers  5  in which a coil  4  made up of electrode layers  2  and through holes  1  is disposed, insulating layers  6  sandwiching the upper and lower surfaces of the coil  4 , and a conductive layer  7  disposed on the upper surface of one or more of the insulating layers. 
         [0007]    In a system that uses the HF range as the communication frequency, the communication distance between the antenna devices depends on magnetic flux passing through the coil antennas. That is, to ensure some communication distance between the antenna devices, it is necessary for each of the coil antennas to have a large size. The large size of the coil antenna hinders miniaturization of the communication terminal apparatus. In contrast, if the antenna is small, the effective area of the antenna is small and the communication distance is not sufficient. 
       SUMMARY OF THE INVENTION 
       [0008]    In light of the above-described circumstances, preferred embodiments of the present invention provide an antenna device having a small footprint and a small-sized communication terminal apparatus while a predetermined communication distance is ensured. 
         [0009]    An antenna device according to a preferred embodiment of the present invention includes a coil antenna and a booster antenna. The coil antenna includes a coil conductor wound around a winding axis and a magnetic body arranged at least inside a winding region of the coil conductor, the coil antenna being mounted such that a mounting surface thereof is a conductor aperture plane, the conductor aperture plane being a plane through which the winding axis passes. The booster antenna includes a planar conductor functioning as a booster coupled to the coil antenna through an electromagnetic field. A portion of the coil conductor and an edge of the planar conductor overlap each other at least partially when seen in plan view in a direction of the winding axis. 
         [0010]    A communication terminal apparatus according to a preferred embodiment of the present invention includes an antenna device and a communication circuit. The antenna device includes a coil antenna and a booster antenna, the coil antenna including a coil conductor wound around a winding axis and a magnetic body arranged at least inside a winding region of the coil conductor, the coil antenna being mounted such that a mounting surface thereof is a conductor aperture plane, the conductor aperture plane being a plane through which the winding axis passes, the booster antenna including a planar conductor functioning as a booster coupled to the coil antenna through an electromagnetic field. A portion of the coil conductor and an edge of the planar conductor overlap each other at least partially when seen in plan view in a direction of the winding axis. The communication circuit is connected to the antenna device. 
         [0011]    The antenna device according to a preferred embodiment of the present invention includes the coil antenna and the planar conductor. Accordingly, the antenna device having a small footprint while a predetermined communication distance is ensured can be achieved, and the small-sized communication terminal apparatus can also be achieved. 
         [0012]    The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is an exploded perspective view of a magnetic antenna in Japanese Unexamined Patent Application Publication No. 2007-019891. 
           [0014]      FIG. 2A  is a perspective view of an antenna device  201  according to a first preferred embodiment of the present invention,  FIG. 2B  is a plan view of the antenna device  201 , and  FIG. 2C  is a front view of the antenna device  201 . 
           [0015]      FIG. 3A  is a perspective view that illustrates the direction of each of a current passing through a coil conductor of a coil antenna  100  in the antenna device  201 , a current passing through a planar conductor  11 , a magnetic field resulting from the coil antenna  100 , and a magnetic field resulting from the planar conductor  11 , and  FIGS. 3B and 3C  illustrate the relationship between a current passing through the planar conductor  11  and magnetic flux produced by it. 
           [0016]      FIG. 4A  is a cross-sectional view of a communication terminal apparatus  301  including the antenna device  201 , and  FIG. 4B  is a see-through plan view of the communication terminal apparatus  301 . 
           [0017]      FIG. 5  is a see-through perspective view that illustrates a used state of a communication terminal apparatus according to a second preferred embodiment of the present invention. 
           [0018]      FIG. 6  is an exploded perspective view of an antenna device  203  according to a third preferred embodiment of the present invention. 
           [0019]      FIG. 7A  is a perspective view of an antenna device  204  according to a fourth preferred embodiment of the present invention, and  FIG. 7B  is a front view that illustrates a state in which the antenna device  204  is incorporated in a communication terminal apparatus. 
           [0020]      FIGS. 8A and 8B  are front views of two antenna devices  205 A and  205 B according to a fifth preferred embodiment of the present invention, respectively. 
           [0021]      FIG. 9A  is a perspective view of a resonant booster antenna  110 ,  FIG. 9B  is an exploded perspective view of the resonant booster antenna  110 , and  FIG. 9C  is a plan view of the resonant booster antenna  110 . 
           [0022]      FIG. 10  is an equivalent circuit diagram of the resonant booster antenna  110 . 
           [0023]      FIGS. 11A to 11D  are front sectional views of four communication terminal apparatuses  306 A,  306 B,  306 C, and  306 D according to a sixth preferred embodiment of the present invention, respectively. 
           [0024]      FIG. 12A  is an exploded perspective view of a resonant booster antenna  120  according to a seventh preferred embodiment of the present invention, and  FIG. 12B  is a plan view of the resonant booster antenna  120 . 
           [0025]      FIG. 13  is an equivalent circuit diagram of the resonant booster antenna  120 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0026]    Antenna devices and communication terminal apparatuses according to preferred embodiments described below are preferably used in a radio-frequency identification (RFID) system in the HF range, such as a near field communication (NFC) system, for example. 
       First Preferred Embodiment 
       [0027]      FIG. 2A  is a perspective view of an antenna device  201  according to a first preferred embodiment,  FIG. 2B  is a plan view thereof, and  FIG. 2C  is a front view thereof. 
         [0028]    The antenna device  201  includes a booster antenna including a planar conductor  11  and a coil antenna  100 . The coil antenna  100  includes a coil conductor  21  wound around a magnetic core  20 . 
         [0029]    The coil antenna  100  is surface-mounted on a base  10  including a printed wiring board made of, for example, epoxy resin, such that the mounting surface is a conductor aperture plane AP (see  FIG. 2B ), the conductor aperture plane AP being a plane through which the winding axis of the coil conductor  21  passes. 
         [0030]    Specifically, the coil antenna  100  has a structure in which the coil conductor  21  made of, for example, silver or copper, is wound around the magnetic core  20  made of, for example, ferrite. The coil conductor  21  is wound around four side surfaces (peripheral surfaces) perpendicular or substantially perpendicular to two major surfaces (one of which is the conductor aperture plane AP) of the magnetic core  20  preferably having the shape of a rectangular parallelepiped, for example. That is, the winding axis of the coil conductor  21  extends along the direction perpendicular or substantially perpendicular to the major surfaces of the magnetic core  20 . 
         [0031]    The magnetic core  20  in the coil antenna  100  includes a ferrite sinter or a resin body in which a ferrite material is distributed in resin. The coil conductor  21  may further be overlaid with a protective film made of an insulating material having low permeability. 
         [0032]    The coil antenna  100  is preferably configured as a so-called surface-mounted coil antenna (chip coil antenna). Two mounting terminal electrodes (not illustrated) connected to first and second ends of the coil conductor  21 , respectively, are disposed on the back surface of the coil antenna  100 . That is, the coil antenna  100  is configured such that it can be surface-mounted on various substrates, including a printed wiring board. 
         [0033]    The planar conductor  11  is preferably configured so as to be made of foil of metal, such as copper, silver, or aluminum, and have a rectangular or substantially rectangular shape and is disposed on the surface of the base  10  including a printed wiring board. The base  10  is not limited to a rigid printed wiring board and may be made of flexible resin. The planar shape of the planar conductor is not limited to a rectangular or substantially rectangular shape and can have any shape, such as a circular shape or a diamond shape. The planar conductor is not limited to a planar thin metal film and may be an integral portion of a metal item. 
         [0034]    The coil antenna  100  is arranged such that a portion of the coil conductor  21  and an edge of the planar conductor  11  overlap each other when seen in plan view in the direction of the winding axis. In the example illustrated in  FIG. 2C , the portion of the coil conductor  21  in the coil antenna  100  extends into the region where the planar conductor  11  is defined by the dimension G 1 . The outer surface of the planar conductor  11  and the lower end of the coil conductor  21  are separated from each other by the height G 2 . Smaller dimensions G 1  and G 2  may be preferable because they lead to a stronger coupling degree between the coil antenna  100  and the booster antenna. As described below, the booster antenna including the planar conductor  11  is coupled to the coil antenna  100  through an electromagnetic field. 
         [0035]      FIG. 3A  is a perspective view that illustrates the direction of each of a current passing through the coil conductor  21  of the coil antenna  100  in the antenna device  201  and a current passing through the planar conductor  11 .  FIGS. 3B and 3C  schematically illustrate states of the current passing through the coil conductor  21  of the coil antenna  100 , the current passing through the planar conductor  11 , and magnetic flux produced by them. 
         [0036]    When a current passes through the coil conductor  21  in the direction of the current “a”, a current is induced in the planar conductor  11  in the direction of the current “b”. That is, the current passing through the coil conductor  21  makes the induced current b circulate in the periphery of the planar conductor. As a result, as illustrated in  FIG. 3B , magnetic flux indicated by the arrow φa occurs to the coil antenna  100  and magnetic flux indicated by the arrow φb occurs to the planar conductor  11 . The magnetic flux φa′ illustrated in  FIG. 3B  indicates magnetic flux that does not pass through the planar conductor  11 . 
         [0037]      FIG. 3C  depicts the magnetic flux indicated in  FIG. 3B  more equivalently. The magnetic flux indicated by the arrow φc is the one in which the magnetic flux B occurring to the vicinity of the planar conductor  11  and the magnetic flux φa′ occurring to the coil antenna  100  are combined. 
         [0038]    When magnetic flux is received from a coil antenna of a communication partner, the phenomenon opposite to that described above arises. That is, when magnetic flux of the coil antenna of the communication partner flows in the vicinity of the planar conductor  11  and links the coil antenna  100 , the current b passes through the planar conductor  11  and the current a passes through the coil conductor  21 . 
         [0039]      FIG. 4A  is a cross-sectional view of a communication terminal apparatus  301  including the antenna device  201 , and  FIG. 4B  is a see-through plan view thereof. The base  10  is a printed wiring board. The planar conductor  11  is disposed on the surface of the base  10 . The coil antenna  100  is surface-mounted on the base  10 . 
         [0040]    As illustrated in  FIG. 3C , the magnetic flux resulting from the coil antenna  100  and the magnetic flux resulting from the planar conductor  11  are combined into the large magnetic flux in the direction of the arrow illustrated in  FIG. 4A . Therefore, the antenna device  201  has the directivity in the direction of the arrow illustrated in  FIG. 4A . That is, the antenna device  201  obtains a high gain in the direction from the vicinity of the front end FE of a terminal casing  320  of the communication terminal apparatus  301  toward the back surface BS. Accordingly, when a user grips the hand-held portion HP of the communication terminal apparatus  301  and holds the front end over the communication partner, communication can be carried out under the high gain. 
       Second Preferred Embodiment 
       [0041]      FIG. 5  is a see-through perspective view that illustrates a used state of a communication terminal apparatus according to a second preferred embodiment. The communication terminal apparatus  302  can be a cellular phone terminal, for example. The communication terminal apparatus  302  includes a main substrate  111  and the base  10  as a sub-substrate in the terminal casing  320 . An antenna device  202  is disposed on the surface of the base  10 . The antenna device  202  is arranged on the back surface BS side of the terminal casing  320  together with a battery pack  112 . The main substrate  111  is a large printed wiring board including a rigid resin substrate made of, for example, epoxy resin. Circuit elements that define, for example, a circuit that drives a display device and a circuit for controlling the battery are mounted on the main substrate  111 . The base  10  as the sub-substrate includes a flexible resin substrate made of, for example, a polyimide or liquid crystal polymer. In addition to the antenna device  202 , circuit elements that define a communication circuit (RF circuit) and other circuits are mounted on the base  10 . Alternatively, these circuit elements may be mounted on the main substrate  111 . 
         [0042]    The above-described communication circuit includes, for example, a wireless IC chip and is connected to (receives electricity from) the antenna device  202 . The wireless IC chip and the antenna device  202  form RFID. 
         [0043]    When the communication terminal apparatus  302  is held over a coil antenna  400 , such as a reader-writer, of a communication partner, as illustrated in  FIG. 5 , the antenna device  202  and the coil antenna  400  of the communication partner are coupled to each other mainly through an induction field and exchange predetermined information, and the communication terminal apparatus  302  functions as RFID. 
       Third Preferred Embodiment 
       [0044]      FIG. 6  is an exploded perspective view of an antenna device  203  according to a third preferred embodiment. The antenna device  203  includes a multilayer substrate in which base layers  10   a,    10   b,    10   c,    10   d,  and  10   e  including a magnetic body are stacked. Loop conductive patterns  21   a  to  21   d  are disposed on the base layers  10   a  to  10   d,  respectively. Input and output terminals  22   a  and  22   d  to be connected to a feeder circuit are disposed on a first major surface of the base layer  10   e.  Via conductors  21   v  are disposed in the base layers  10   a  to  10   e,  and the conductive patterns  21   a  to  21   d  and the via conductors  21   v  define a single coil conductor. 
         [0045]    The planar conductor  11  is disposed on a second major surface of the base layer  10   a.  The planar conductor  11  is disposed such that its edge is arranged in close vicinity of the coil opening of the coil conductor. Therefore, the antenna device is configured such that the coil antenna and the planar conductor are integrated in the multilayer substrate. 
         [0046]    It is not necessary that all of the base layers  10   a  to  10   e  are magnetic layers. For example, the base layer  10   a  may be a non-magnetic layer. When the base layer  10   a  is a non-magnetic layer, a high degree of coupling between the coil conductor and the planar conductor  11  (booster antenna) is obtainable. 
       Fourth Preferred Embodiment 
       [0047]      FIG. 7A  is a perspective view of an antenna device  204  according to a fourth preferred embodiment.  FIG. 7B  is a front view that illustrates a state in which the antenna device  204  is incorporated in a communication terminal apparatus  304 . 
         [0048]    As illustrated in  FIG. 7B , the antenna device  204  is arranged in a location adjacent to the front end FE in the terminal casing  320  of the communication terminal apparatus  304 . Thus, when the front end FE of the communication terminal apparatus  304  is made to approach (is held over) a communication partner, such as an antenna of a reader-writer, stable communication can be carried out. 
         [0049]    In the antenna device  204  according to the present preferred embodiment, the coil antenna  100  is arranged on an edge of a first planar conductor region  11 A. The first planar conductor region  11 A and a second planar conductor region  11 B are disposed on the planes intersecting at a predetermined angle θ, respectively. In this case, the directivity of the antenna device  204  occurs in an intermediate direction between the direction of the normal to the first planar conductor region  11 A and the direction of the normal to the second planar conductor region  11 B, and the communication distance in this direction can be increased. 
         [0050]    That is, as illustrated in  FIG. 7B , the antenna device  204  is arranged such that the second planar conductor region  11 B of the antenna device  204  is adjacent to the front end FE of the terminal casing  320  in the communication terminal apparatus. Thus, the antenna device  204  can have a high sensitivity in a range from the direction of the front end FE of the terminal casing  320  toward the direction of the back surface BS. 
         [0051]    To prevent an increase in the loss of a current passing through each of the planar conductor regions  11 A and  11 B, the angle θ formed between the first planar conductor region  11 A and the second planar conductor region  11 B may preferably be larger than about 90° and smaller than about 135°, for example. 
       Fifth Preferred Embodiment 
       [0052]      FIGS. 8A and 8B  are front views of two antenna devices  205 A and  205 B according to a fifth preferred embodiment, respectively. Each of the antenna devices  205 A and  205 B according to the fifth preferred embodiment is the one in which the antenna device  201  illustrated in the first preferred embodiment further includes a resonant booster antenna  110 . This resonant booster antenna corresponds to “planar coil antenna” according to a preferred embodiment of the present invention. The resonant booster antenna  110  is coupled to the coil antenna  100  through a magnetic field and acts as a booster antenna, and the detailed configuration of the resonant booster antenna  110  is described below. In the example illustrated in  FIG. 8A , the resonant booster antenna  110  preferably is parallel or substantially parallel to the planar conductor  11  and is arranged in a location adjacent to the coil antenna  100  with respect to the center of the planar conductor  11 . Thus, as illustrated in  FIG. 8A , the resonant booster antenna  110  is coupled to the magnetic flux φc resulting from the coil antenna  100  through a magnetic field and acts as an antenna device having the directivity in the direction of the arrow A. 
         [0053]    In the example illustrated in  FIG. 8B , the resonant booster antenna  110  preferably is parallel or substantially parallel to the planar conductor  11  and is arranged in a location remote from the coil antenna  100  with respect to the center of the planar conductor  11 . That is, the resonant booster antenna  110  is arranged in a location adjacent to a side of the planar conductor, the side being opposite to another side close to the coil antenna  100 . Thus, as illustrated in  FIG. 8B , the resonant booster antenna  110  is coupled to the magnetic flux φc resulting from the coil antenna  100  through a magnetic field and acts as an antenna device having the directivity in the direction of the arrow A. 
         [0054]      FIG. 9A  is a perspective view of the resonant booster antenna  110 , and  FIG. 9B  is an exploded perspective view of the resonant booster antenna  110 .  FIG. 9C  is a plan view of the resonant booster antenna  110 . The resonant booster antenna  110  includes a base  30  and rectangular or substantially rectangular spiral coil conductors L 1  and L 2  on the base  30 . The rectangular or substantially rectangular spiral coil conductor L 1  on the upper surface of the base  30  and the rectangular or substantially rectangular spiral coil L 2  on the lower surface of the base  30  are arranged such that their coil conductors face each other and their winding directions are opposite (the same when seen in plan view in one direction). 
         [0055]      FIG. 10  is an equivalent circuit diagram of the resonant booster antenna  110 . In  FIG. 10 , the inductors L 1  and L 2  correspond to the above-described rectangular or substantially rectangular spiral coils L 1  and L 2 . Because the rectangular or substantially rectangular spiral coils L 1  and L 2  face each other such that the base  30  is disposed therebetween, capacitances occur between them. The capacitances are indicated by capacitors C 1  and C 2  in  FIG. 10 . In such a way, the inductors L 1  and L 2  and the capacitors C 1  and C 2  enable the resonant booster antenna  110  to act as an LC resonant circuit. Its resonant frequency is the same as or near the carrier frequency of a communication signal. 
         [0056]    As described above, providing the resonant booster antenna arranged so as to be close to the planar conductor enables the communication sensitivity in a desired direction to be improved using the resonant booster antenna, without the use of an additional conductive plate, irrespective of the mounting position of the coil antenna. 
       Sixth Preferred Embodiment 
       [0057]      FIGS. 11A to 11D  are front sectional views of four communication terminal apparatuses  306 A,  306 B,  306 C, and  306 D according to a sixth preferred embodiment, respectively. In these drawings, the main substrate  111 , the coil antenna  100 , the resonant booster antenna  110 , and other elements are contained in the terminal casing  320  of each of the communication terminal apparatuses  306 A,  306 B,  306 C, and  306 D. The upper side of the terminal casing  320  in the drawings indicates the bottom of the terminal casing, and the lower side indicates the top (the surface where the display panel and the operating unit are disposed) of the terminal casing. 
         [0058]    The planar conductor  11  as a ground conductor is disposed inside the main substrate  111 . The coil antenna  100  and many other chip components are mounted on the main substrate  111 . The coil antenna  100  is disposed such that a portion of the coil conductor and an edge of the planar conductor  11  overlap each other at least partially when seen in plan view in the direction of the winding axis. The resonant booster antenna  110  is attached to or arranged along the inner surface of the terminal casing  320 . The resonant booster antenna  110  preferably is parallel or substantially parallel to the planar conductor  11  and is arranged in a location remote from the coil antenna  100  with respect to the center of the planar conductor  11 . 
         [0059]    In the communication terminal apparatus  306 A illustrated in  FIG. 11A , the resonant booster antenna  110  is adjacent to the surface of the main substrate  111  on which the coil antenna  100  is mounted. In the communication terminal apparatus  306 B illustrated in  FIG. 11B , the resonant booster antenna  110  is adjacent to a surface that is opposite to the surface of the main substrate  111  on which the coil antenna  100  is mounted. In the communication terminal apparatus  306 C illustrated in  FIG. 11C , a resonant booster antenna  110 F is adjacent to the surface of the main substrate  111  on which the coil antenna  100  is mounted, and a resonant booster antenna  110 B is adjacent to a surface that is opposite to the surface of the main substrate  111  on which the coil antenna  100  is mounted. In the communication terminal apparatus  306 D illustrated in  FIG. 11D , the resonant booster antenna  110  is arranged along two surfaces (over the ridge) of the terminal casing  320 . 
         [0060]    In the communication terminal apparatus  306 A illustrated in  FIG. 11A , the planar conductor  11  acts as a radiator and the resonant booster antenna  110  also acts as a radiator. Because the resonant booster antenna  110  has a high directivity in the direction of the arrow A, the maximum distance for communication in the direction of the arrow A can be increased. 
         [0061]    In the communication terminal apparatus  306 B illustrated in  FIG. 11B , because the resonant booster antenna  110  has a high directivity in the direction of the arrow B, the maximum distance for communication in the direction of the arrow B can be increased. Because the planar conductor  11  also acts as a radiator, a gain in the direction opposite to the direction of the arrow B can be ensured. 
         [0062]    In the communication terminal apparatus  306 C illustrated in  FIG. 11C , the planar conductor  11  acts as a radiator and the resonant booster antennas  110 F and  110 B also act as radiators. Because the resonant booster antenna  110 F has a high directivity in the direction of the arrow A and the resonant booster antenna  110 B has a high directivity in the direction of the arrow B, the maximum distance for communication can be increased in both the directions of the arrows A and B. 
         [0063]    In the communication terminal apparatus  306 D illustrated in  FIG. 11D , the planar conductor  11  acts as a radiator and the resonant booster antenna  110  also acts as a radiator. Because the resonant booster antenna  110  has a high directivity in the direction of the arrow C (direction of 45 degrees), the maximum distance for communication in the direction of the arrow C can be increased. 
       Seventh Preferred Embodiment 
       [0064]    Another example of the resonant booster antenna is described in a seventh preferred embodiment.  FIG. 12A  is an exploded perspective view of a resonant booster antenna  120  according to the seventh preferred embodiment.  FIG. 12B  is a plan view of the resonant booster antenna  120 .  FIG. 13  is an equivalent circuit diagram of the resonant booster antenna  120 . The resonant booster antenna  120  includes the base  30  and the rectangular or substantially rectangular spiral coil conductors L 1  and L 2  on the base  30 . The rectangular or substantially rectangular spiral coil conductor L 1  on the upper surface of the base  30  and the rectangular or substantially rectangular spiral coil conductor L 2  on the lower surface of the base  30  are arranged such that their coil conductors face each other and their winding directions are opposite (the same when seen in plan view in one direction). The inner end of the coil conductor L 1  is electrically connected to the inner end of the coil conductor L 2  with a via conductor disposed therebetween. The capacitor C 1  outside the illustration is connected between the outer end of the coil conductor L 1  and the outer end of the coil conductor L 2 . 
         [0065]    As illustrated in  FIG. 13 , the inductors L 1  and L 2  and the capacitor C 1  enable the resonant booster antenna  120  to act as an LC resonant circuit. Its resonant frequency is the same as or near the carrier frequency of a communication signal. 
       Other Preferred Embodiments 
       [0066]    In the above preferred embodiments, examples in which the planar conductor  11  preferably is exposed to the exterior of the base  10  are described. Alternatively, the planar conductor  11  may be disposed inside a printed wiring board, for example. 
         [0067]    Of the coil conductor ( 21 , which is the corresponding reference numeral in the first preferred embodiment; the same applies to the following), the winding axis may not be perpendicular or substantially perpendicular to the planar conductor ( 11 ). It is sufficient that the coil antenna ( 100 ) be mounted such that the mounting surface is the conductor aperture plane AP, which is a plane through which the winding axis of the coil conductor ( 21 ) passes, and that the booster antenna including the planar conductor ( 11 ) and the coil antenna ( 100 ) be coupled together through an electromagnetic field. In particular, when the winding axis of the coil conductor ( 21 ) is in a perpendicular or substantially perpendicular relation to the plane of the planar conductor ( 11 ), the magnetic flux resulting from the current passing through the coil conductor ( 21 ) of the coil antenna ( 100 ) and the magnetic flux resulting from the current passing through the planar conductor ( 11 ) are in the same direction, the directivity of the antenna device ( 201 ) can be increased. Typically, when the angle between the winding axis of the coil conductor ( 21 ) and the normal of the planar conductor ( 11 ) is in the range of about ±45 degrees, for example, satisfactory directivity and gain are obtainable. 
         [0068]    The antenna device according to various preferred embodiments of the present invention is not limited to an antenna device for use in the HF range and is also applicable to an antenna device for use in other frequency bands, such as the low frequency (LF) range or the ultrahigh frequency (UHF) range. 
         [0069]    While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.