Abstract:
An antenna device includes two conductor surfaces at opposing positions with a space interposed between the conductor surfaces, a first connection conductor connecting the two conductor surfaces galvanically at one location, and an antenna coil arranged in proximity to the first connection conductor. The antenna coil is arranged at a position at which the antenna coil causes an induced current to flow through the first connection conductor by electromagnetic induction. Currents in the opposite directions flow through peripheral edge portions of the two conductor surfaces, so that a magnetic field is radiated from A space across which the two conductor surfaces oppose each other. This enables a conductor surface defined by a metal plate or the like to be used as a radiation element without providing a slit or an opening in the metal plate so as to avoid problems of a decrease in mechanical strength, design restrictions, and a decrease in an electric field shielding effect.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an antenna device and a communication terminal device that are preferably used for a communication system using a high-frequency (HF) band or an ultra-high-frequency (UHF) band. 
         [0003]    2. Description of the Related Art 
         [0004]    In a device that is included in an electronic apparatus, such as a cellular phone, and performs HF-band communication, such as a near field communication (NFC), a radio-frequency integrated circuit (RFIC) and a matching element are typically mounted on a circuit substrate, an antenna is bonded to the inner side of a housing of the electronic apparatus, and the RFIC and the antenna are electrically connected to each other with a spring pin or the like interposed therebetween. 
         [0005]    Meanwhile, a reduction in the thickness of a recent wireless communication terminal such as the cellular phone terminal has advanced, and “metallization” by applying magnesium plating to a housing and so on is performed in increasing cases in order to compensate for insufficient strength associated with such reduction in thickness. 
         [0006]    However, when the “metallization” is performed on the terminal housing, an electromagnetic field around an antenna embedded in the terminal is shielded by metal. Due to the shielding of the electromagnetic field, there arises a problem that the antenna cannot make communication with a party-side antenna. 
         [0007]    In order to solve the above problem, as disclosed in Japanese Patent No. 4993045, an antenna device having a configuration in which a metal plate having a larger area than an antenna coil proximate (magnetic field coupling) to the antenna coil and the metal plate is used as a radiator has been proposed. 
         [0008]    By employing the antenna configuration disclosed in Japanese Patent No. 4993045, the antenna can make communication with a party-side antenna although the antenna is covered with metal. However, with provision of a slit or an opening in the metal plate, a decrease in the mechanical strength needs to be considered, and the number of processes in manufacturing increases. Further, when a slit or an opening is to be provided in a metal housing, in particular, design of the housing is restricted. In addition, the vicinity of the slit or the opening cannot be connected to the ground of a circuit. Therefore, the potential of the metal plate partially fluctuates in some cases, resulting in a problem that an electric field shielding effect of the metal plate decreases or a concern over interference of a first conductor surface and a second conductor surface with another high-frequency circuit is generated. 
       SUMMARY OF THE INVENTION 
       [0009]    Preferred embodiments of the present invention provide an antenna device that prevents problems of a decrease in mechanical strength, design restrictions, and a decrease in electric field shielding effect, and that also significantly reduces or prevents the problem of interference with another high-frequency circuit, if necessary, by enabling a conductor surface defined by a metal plate or the like to be used as a radiation element without a slit or an opening provided in the metal plate, and also provide a communication terminal device including the antenna device. 
         [0010]    An antenna device according to an aspect of various preferred embodiments of the present invention includes two conductor surfaces arranged at opposing positions with a space interposed between the conductor surfaces, a first connection conductor connecting the two conductor surfaces galvanically, or in terms of direct current at one location, and an antenna coil arranged in proximity to the first connection conductor, wherein the antenna coil is arranged at a position at which the antenna coil causes an induced current to flow through the first connection conductor by electromagnetic induction. 
         [0011]    With this configuration, the induced current generated by the electromagnetic induction of the antenna coil flows through the connection conductor to which the antenna coil is arranged in proximity and currents in opposite directions flow through the two conductor surfaces. Therefore, the antenna device acts as an antenna that radiates a magnetic field from a space across which the two conductor surfaces oppose each other. 
         [0012]    It is preferable that the antenna coil be arranged closer to a center of the two conductor surfaces than the first connection conductor is. With this configuration, a magnetic field generated by the antenna coil and a magnetic field radiated from the space across which the two conductor surfaces oppose each other do not cancel out each other. 
         [0013]    It is preferable that the conductor surfaces include a conductor portion of a housing of an electronic apparatus. With this configuration, the housing also defines and functions as a portion of a radiation element. 
         [0014]    It is preferable that the conductor surfaces include a ground electrode provided on a circuit substrate. With this configuration, the ground electrode on the circuit substrate defines and functions as a portion of the radiation element. 
         [0015]    It is preferable that the conductor surfaces include a ground electrode provided on a circuit substrate and a conductor portion of a housing of an electronic apparatus, and the first connection conductor be a ground connection pin connecting the ground electrode and the conductor portion of the housing. With this configuration, the ground connection pin also defines and functions as the first connection conductor. 
         [0016]    It is preferable that a second connection conductor connected to the two conductor surfaces with a capacitor interposed between the two conductor surfaces be further included, a carrier frequency of a communication signal be a frequency in an HF band, and the capacitor be an element of a low impedance at a frequency in a UHF band or higher. With this structure, when an antenna for the UHF band is provided in the same housing, a substrate current generated by the antenna for the UHF band is not easily influenced by the antenna coil and the antenna for the UHF band achieves predetermined antenna characteristics. 
         [0017]    A communication terminal device according to another aspect of various preferred embodiments of the present invention includes an antenna device and a power supply circuit connected to the antenna device, wherein the antenna device includes two conductor surfaces arranged at opposing positions with a space interposed between the conductor surfaces, a first connection conductor connecting the two conductor surfaces galvanically at one location, and an antenna coil arranged in proximity to the first connection conductor, and the antenna coil is arranged at a position at which the antenna coil causes an induced current to flow through the first connection conductor by electromagnetic induction. 
         [0018]    Various preferred embodiments of the present invention enables the two conductor surfaces to be used as radiation elements without providing a slit or an opening in a conductor surface. Therefore, problems of a decrease in mechanical strength, design restriction, and a decrease in an electric field shielding effect are prevented. 
         [0019]    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 DRAWINGS 
         [0020]      FIG. 1A  is a perspective view of an antenna device  101  according to a first preferred embodiment of the present invention, and  FIG. 1B  is a perspective view of an antenna coil  30  provided in the antenna device  101 . 
           [0021]      FIG. 2  is a front view of the antenna device  101 . 
           [0022]      FIG. 3  is a development view illustrating an example of electric currents that are induced in a first conductor surface  11  and a second conductor surface  12 . 
           [0023]      FIG. 4  is a perspective view of an antenna device  102  according to a second preferred embodiment of the present invention. 
           [0024]      FIG. 5  is a plan view illustrating a configuration of an inner portion of a communication terminal device according to a third preferred embodiment of the present invention. 
           [0025]      FIG. 6  is a cross-sectional view cut at positions passing through a first connection conductor  21  and a second connection conductor  22 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0026]    Hereinafter, a plurality of preferred embodiments of the present invention will be described using several specific examples with reference to the drawings. In the drawings, the same reference numerals denote the same portions. The preferred embodiments are illustrative and partial replacement or combination of configurations described in different embodiments can be made. 
       First Preferred Embodiment 
       [0027]      FIG. 1A  is a perspective view of an antenna device  101  according to a first preferred embodiment of the present invention, and  FIG. 1B  is a perspective view of an antenna coil  30  provided in the antenna device  101 .  FIG. 2  is a front view of the antenna device  101 . The antenna device  101  preferably is an antenna that is used in an HF band of 13.56 MHz or the like, for example, and generates proximity-type or vicinity-type electromagnetic field coupling (magnetic field coupling, mainly) with an antenna of a communication party. 
         [0028]    The antenna device  101  includes a first conductor surface  11  and a second conductor surface  12  opposing each other. The first conductor surface  11  and the second conductor surface  12  are connected with each other with a first connection conductor  21  (referred to as a “first” connection conductor in order to distinguish it from a second connection conductor, which will be described in another preferred embodiment later). The antenna coil  30  is arranged between the first conductor surface  11  and the second conductor surface  12  at a position in proximity to the first connection conductor  21 . When the first connection conductor  21  and the antenna coil  30  are seen in an in-plane direction of the two conductor surfaces  11  and  12  from any point of view, the antenna coil  30  is arranged closer to the center of the conductor surfaces  11  and  12  than the first connection conductor  21  is. That is to say, the antenna coil  30  is arranged toward the inner side relative to the position of the first connection conductor  21  schematically. 
         [0029]    As illustrated in  FIG. 1B , the antenna coil  30  preferably includes a magnetic core  31  and a coil conductor  32 , and the coil conductor  32  includes a pattern wound around the magnetic core  31 . For example, coil conductor patterns are provided on a plurality of resin sheets containing magnetic ferrite fillers dispersed therein. The resin sheets are laminated and subjected to thermal pressure bonding so as to define a chip-type antenna in which a coil conductor in a rectangular or substantially rectangular helical form is embedded in a multilayer body. The antenna coil  30  may be a chip-type antenna defined by magnetic ferrite ceramics as a base body. 
         [0030]    The antenna coil  30  is arranged in the vicinity of the first connection conductor  21  such that the portions of the coil conductor  32  included in the antenna coil  30  that are in proximity to the first connection conductor  21  are parallel or substantially parallel with the first connection conductor  21 . 
         [0031]    For example, the first conductor surface  11  is a ground electrode pattern of a circuit substrate. The second conductor surface  12  is a metal portion of a housing, for example. The first connection conductor  21  is a spring pin terminal and galvanically connects the first conductor surface  11  and the second conductor surface  12 . The pin terminal is originally a ground connection pin that causes the metal portion of the housing and the ground electrode of the circuit substrate to have the same potential. In the present preferred embodiment, the pin terminal also defines and functions as a coupling portion that induces currents in the first conductor surface  11  and the second conductor surface  12 , as will be described later. 
         [0032]    As illustrated in  FIG. 2 , the coil conductor  32  of the antenna coil  30  and the first connection conductor  21  are in proximity to each other, so that the first connection conductor  21  and the portions of the coil conductor  32  that are in proximity thereto generate induction coupling to each other. That is to say, an induced current flows through the first connection conductor  21  in the direction opposite to the direction of a current flowing through the coil conductor  32  of the antenna coil  30 . 
         [0033]      FIG. 3  is a development view illustrating an example of currents that are induced in the first conductor surface  11  and the second conductor surface  12 . This drawing is a view illustrating the two conductor surfaces  11  and  12  developed in a state in which they are connected with each other with the first connection conductor  21  interposed therebetween. When an induced current ic flows through the first connection conductor  21  by induction of the coil conductor  32  of the antenna coil  30 , currents flow through the peripheral edge portions of the first conductor surface  11  and the second conductor surface  12  as indicated by arrows in  FIG. 3 . 
         [0034]    As illustrated in  FIG. 3 , the currents flow through opposing portions of the two conductor surfaces  11  and  12  in the opposite directions. Therefore, a magnetic field is radiated from a space across which the two conductor surfaces  11  and  12  oppose each other. Dashed arrows in  FIG. 1  indicate directions of the radiated magnetic field with magnetic field lines. 
         [0035]    As described above, the antenna coil  30  is arranged toward the inner side relative to the position of the first connection conductor  21 . Therefore, the magnetic field that is radiated from the space across which the two conductor surfaces  11  and  12  oppose each other and the magnetic field that is radiated from the antenna coil  30  have the same polarity. In other words, these magnetic fields are not cancelled out by each other. 
       Second Preferred Embodiment 
       [0036]      FIG. 4  is a perspective view of an antenna device  102  according to a second preferred embodiment of the present invention. The antenna device  102  includes the first connection conductor  21  and a plurality of second connection conductors  22 ,  23 , and  24 . These connection conductors  21  to  24  are located at the edges of the first conductor surface  11  and the second conductor surface  12 . The first connection conductor  21  allows a land  11 L and the second conductor surface  12  to become conductive. A chip inductor  6  is mounted between the land  11 L and the first conductor surface  11 . That is to say, the first conductor surface  11  and the second conductor surface  12  are connected with each other with the chip inductor  6  interposed therebetween at a portion around the first connection conductor  21 . 
         [0037]    The second connection conductor  22  allows a land  11 L and the second conductor surface  12  to become conductive. A chip capacitor  5  is mounted between the land  11 L and the first conductor surface  11 . That is to say, the first conductor surface  11  and the second conductor surface  12  are connected with each other with the chip capacitor  5  interposed therebetween at a portion around the second connection conductor  22 . At portions around the second connection conductors  23  and  24 , the first conductor surface  11  and the second conductor surface  12  are connected with each other with chip capacitors  5  interposed therebetween in the same manner. 
         [0038]    The above-mentioned chip capacitors  5  are elements of high impedances in a frequency band (HF band) of a carrier frequency of a communication signal but of low impedances at a frequency in the UHF band or higher. Therefore, in the HF band, the portions at the connection conductors  22 ,  23 , and  24  are galvanically in an open state and operate in the same manner as in the antenna device  101  described in the first preferred embodiment. At the frequency in the UHF band or higher, the portions at the connection conductors  22 ,  23 , and  24  are in a connecting state in a high-frequency manner and ground connection is made at these portions reliably. Therefore, the two conductor surfaces  11  and  12  are overall at the stable ground potential. 
         [0039]    Further, the first conductor surface  11  and the second conductor surface  12  are connected with each other with the chip inductor  6  interposed therebetween at the portion around the first connection conductor  21 , so that the first conductor surface  11  and the second conductor surface  12  are directly connected in a high-frequency manner at the portion around the first connection conductor  21  in the HF band and a current is suppressed in the UHF band. Accordingly, a substrate current by an antenna for the UHF band does not flow through the first connection conductor  21  and has little influence on the ferrite of the antenna coil  30 . With this, antenna characteristics of the UHF band are maintained. 
       Third Preferred Embodiment 
       [0040]      FIG. 5  is a plan view illustrating a configuration of an inner portion of a communication terminal device according to a third preferred embodiment of the present invention. Circuit substrates  61  and  62 , a battery pack  90 , a camera module  76 , and so on are accommodated in an upper housing  91 . An RFIC  60  including a communication circuit, a resonance capacitor C, the antenna coil  30 , and so on are mounted on the circuit substrate  61 . A main UHF-band antenna  82  and so on are also provided on the circuit substrate  61 . Further, a sub UHF-band antenna  83  and so one are provided on the circuit substrate  62 . A circuit on the circuit substrate  61  and a circuit on the circuit substrate  62  are connected with each other with a cable interposed therebetween. The UHF-band antennas  82  and  83  are provided by mounting chip antennas, forming line patterns, or the like. 
         [0041]    A ground electrode is provided on substantially an overall area of the circuit substrate  61 , and the ground electrode defines and functions as a first conductor surface. A lower housing  92  is made of resin and the second conductor surface  12  defined by a metal film is provided on the inner surface of the lower housing  92 . The metal film may be formed by bonding an aluminum foil or a copper foil to the inner side of the lower housing  92  or may be formed by drawing it on the inner side of the lower housing  92  with a laser direct structuring (LDS) technology or the like. Further, the metal film preferably occupies an area equal to or larger than half the area of the main surface of the circuit substrate  61  in order to also define and function as a shield for various components mounted on the circuit substrate  61  and so on. In this example, substantially the overall area other than the regions occupied by the main antenna  82  and the sub antenna  83  is shielded by the metal film. An opening  12 A is provided in the lower housing  92 . A lens of the camera module  76  is arranged so as to be optically exposed through the opening  12 A. 
         [0042]    The first connection conductor  21  is mounted on the ground electrode of the circuit substrate  61 . The connection conductor  22  is mounted on the land  11 L. The chip capacitor  5  is mounted between the land  11 L and the ground electrode. 
         [0043]      FIG. 5  illustrates currents flowing through the first conductor surface (ground electrode formed on the circuit substrate  61 )  11  and the second conductor surface (metal portion of the housing)  12  by thick arrows. 
         [0044]      FIG. 6  is a cross-sectional view cut at positions passing through the first connection conductor  21  and the second connection conductor  22 . As in the antenna device illustrated in  FIG. 1A  in the first preferred embodiment, a current flows through the first conductor surface (ground electrode formed on the circuit substrate  61 )  11  and the second conductor surface (metal portion of the housing)  12 . Thus, the two conductor surfaces  11  and  12  and the space across which they oppose each other define and function as radiation elements. 
         [0045]    As illustrated in  FIG. 5 , the first conductor surface (ground electrode)  11  and the second conductor surface (metal portion of the housing)  12  are not required to oppose each other on the overall surfaces and may partially oppose each other. 
         [0046]    The above-described preferred embodiments are illustrative and the present invention is not limited to these preferred embodiments. The antenna coil  30  and the RFIC  60  may be integrated for modularization. With this configuration, electric conduction between the RFIC and the power supply coil does not need to be made by wiring of a substrate, such as a circuit substrate, and the degree of freedom in a mounting space is enhanced. 
         [0047]    Further, one of the first conductor surface and the second conductor surface according to various preferred embodiments of the present invention is not limited to be the ground electrode provided on the circuit substrate. In addition, one of the first conductor surface and the second conductor surface is not limited to be the metal portion of the housing. For example, a shield case, a shield plate, a battery pack, a liquid-crystal display (LCD) panel, or the like may be used as the first conductor surface or the second conductor surface. 
         [0048]    Although  FIG. 1A  and so on illustrate the planar first conductor surface  11  and the planar second conductor surface  12 , the shape of the second conductor surface  12  is not limited thereto. The second conductor surface may be a metal portion of the housing accommodating the circuit substrate. In addition, the metal portion of the housing may be a molded product of the metal plate. 
         [0049]    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.