Patent Publication Number: US-8528829-B2

Title: Wireless communication device and metal article

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a wireless communication device and a metal article, and in particular, relates to a wireless communication device used for a RFID (Radio Frequency Identification) system and a metal article including the wireless communication device. 
     2. Description of the Related Art 
     In recent years, as an information management system for articles, there has been put into practical use an RFID system in which communication is established between a reader/writer generating an induction magnetic field and an RFID tag (also referred to as a wireless communication device) attached to an article on the basis of a non-contact method utilizing an electromagnetic field and predetermined information is transmitted. This RFID tag includes a wireless IC chip that stores therein the predetermined information and processes a predetermined wireless signal and an antenna (radiator) that transmits and receives a high-frequency signal. 
     As an RFID tag that is operable even if it is disposed in proximity to a metal plate, a metal-compatible tag described in Japanese Unexamined Patent Application Publication No. 2007-272264 is known. In this metal-compatible tag, a loop antenna conductor is wound around a plate-shaped dielectric member and an RFID chip is mounted in a gap portion formed in a portion of the loop antenna conductor. In addition, a gap is also formed on a surface side opposite to the chip mounting surface of the loop antenna conductor. When this metal-compatible tag is stuck to a metal plate, a high-frequency signal current flows in both the loop antenna conductor and the metal plate through capacitive coupling between the conductor of the back surface of the dielectric member and the metal plate. 
     In the metal-compatible tag, while a radiation gain on the front surface side (tag mounting surface) of the metal plate is secured to some extent, there is a problem that a radiation gain on the back surface side of the metal plate is small and a communication distance is short. That trend becomes more noticeable with an increase in the thickness of the metal plate, and, for example, it has been hard to use the metal-compatible tag for a metal article such as a stepladder, a building material, or the like. 
     SUMMARY OF THE INVENTION 
     Therefore, preferred embodiments of the present invention provide a wireless communication device and a metal article in which a radiation gain is large not only on a surface mounted to a metal plate or a metal member but also on a surface opposite to the mounting surface. 
     A wireless communication device according to a preferred embodiment of the present invention includes a wireless IC device that processes a high-frequency signal, a radiation conductor coupled to the wireless IC device, a ground conductor connected to the radiation conductor, and a metal plate that includes first and second main surfaces arranged such that the ground conductor is coupled to the first main surface and a portion that defines a radiation element, wherein the metal plate includes a current path portion arranged to conduct a high-frequency signal current on a first main surface side to a second main surface side when a high-frequency signal is supplied from the wireless IC device through the radiation conductor and the ground conductor. 
     According to a second preferred embodiment of the present invention, a metal article includes a wireless communication device and a metal member, wherein the wireless communication device includes a wireless IC device that processes a high-frequency signal, a radiation conductor coupled to the wireless IC device, and a ground conductor connected to the radiation conductor, wherein the metal member includes first and second main surfaces, the ground conductor is coupled to the first main surface, and the metal member includes a current path portion arranged to conduct a high-frequency signal current on a first main surface side to a second main surface side when a high-frequency signal is supplied from the wireless IC device through the radiation conductor and the ground conductor. 
     In the wireless communication device, since the high-frequency signal current on the first surface side (the mounting surface side of the wireless communication device) of the metal plate or the metal member is conducted to the second surface side through the current path portion, a radiation gain becomes large not only on the first surface side of the metal plate or the metal member but also on the second surface side. Therefore, it is possible to secure a communication distance not only on the first surface side but also on the second surface side. 
     According to various preferred embodiments of the present invention, a radiation gain becomes large not only on a surface mounted to a metal plate or a metal member but also on a surface opposite to the mounting surface. 
     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 
         FIGS. 1A and 1B  illustrate a stepladder as a metal article equipped with a wireless communication device, wherein  FIG. 1A  is a perspective view, and  FIG. 1B  is a back surface view in a folding state. 
         FIGS. 2A-2C  illustrate a wireless communication device according to a preferred embodiment of the present invention, wherein  FIG. 2A  is a plan view,  FIG. 2B  is a cross-sectional view, and  FIG. 2C  is a bottom view. 
         FIG. 3  is a perspective view illustrating a radiation conductor and a ground conductor included in a wireless communication device according to a preferred embodiment of the present invention. 
         FIG. 4  is an explanatory diagram illustrating an operating principle of the wireless communication device according to a preferred embodiment of the present invention. 
         FIGS. 5A and 5B  are explanatory diagrams illustrating a directivity and a gain, wherein  FIG. 5A  illustrates a preferred embodiment of the present invention, and  FIG. 5B  illustrates a comparative example. 
         FIG. 6  is a perspective view illustrating a wireless IC chip defining a wireless IC device. 
         FIG. 7  is a perspective view illustrating a state in which the wireless IC chip is mounted, as the wireless IC device, on a feed circuit substrate. 
         FIG. 8  is an equivalent circuit diagram illustrating an example of a feed circuit. 
         FIG. 9  is a plan view illustrating a laminated structure of the feed circuit substrate. 
         FIGS. 10A-10C  illustrate a wireless communication device according to another preferred embodiment of the present invention, wherein  FIG. 10A  is a plan view,  FIG. 10B  is a cross-sectional view, and  FIG. 10C  is a bottom view. 
         FIGS. 11A-11C  illustrate a wireless communication device according to a further preferred embodiment of the present invention, wherein  FIG. 11A  is a cross-sectional view,  FIG. 11B  is an operating principle explanatory diagram, and  FIG. 11C  is a perspective view of a radiation conductor and a ground conductor. 
         FIGS. 12A-12C  illustrate a wireless communication device according to yet another preferred embodiment of the present invention, wherein  FIG. 12A  is a perspective view,  FIG. 12B  is a cross-sectional view, and  FIG. 12C  is a perspective view of a radiation conductor and a ground conductor. 
         FIGS. 13A-13C  illustrate a wireless communication device according to an additional preferred embodiment of the present invention, wherein  FIG. 13A  is a plan view,  FIG. 13B  is a cross-sectional view, and  FIG. 13C  is a perspective view of a radiation conductor and a ground conductor. 
         FIGS. 14A and 14B  illustrate a wireless communication device according to another preferred embodiment of the present invention, wherein  FIG. 14A  is a cross-sectional view, and  FIG. 14B  is an operating principle explanatory diagram. 
         FIG. 15  is a cross-sectional view illustrating a wireless communication device according to another preferred embodiment of the present invention. 
         FIG. 16  is a cross-sectional view illustrating a wireless communication device according to yet another preferred embodiment of the present invention. 
         FIG. 17  is a cross-sectional view illustrating a wireless communication device according to a further preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, preferred embodiments of a wireless communication device and a metal article according to the present invention will be described with reference to attached drawings. In addition, in each drawing, the same symbols are assigned to a common component and a common portion, and the redundant descriptions thereof will be omitted. 
     A stepladder  1  illustrated in  FIGS. 1A and 1B  is a non-limiting example of a building metal article, and includes a top board portion  2  and a foldable leg portion  3 . A wireless communication device  10  is firmly attached to the back side of the top board portion  2  with being stuck and crimped thereto, for example. As described later according to a first preferred embodiment to a ninth preferred embodiment of the present invention, the wireless communication device  10  communicates with a reader/writer of an RFID system, and performs information management for the stepladder  1 . In addition, a portion of the top board portion (metal plate)  2  functions as a radiation element of the wireless communication device  10 . Hereinafter, the wireless communication device  10  will be described in detail. 
     First Preferred Embodiment 
     A wireless communication device  10 A according to a first preferred embodiment is preferably used for the communication of a UHF band, and includes a wireless IC device  50 , a dielectric substrate  20 , and a metal plate  30 , as illustrated in  FIGS. 2A-2C . The wireless IC device  50  processes a high-frequency signal, and the detail thereof will be described in detail later with reference to  FIG. 6  to  FIG. 9 . The dielectric substrate  20  includes thermosetting resin such as epoxy resin or the like, thermoplastic resin such as polyimide or the like, or ceramic such as LTCC or the like (may be a magnetic material), and is preferably configured as a single-layer substrate or a multilayer substrate, for example. For example, the metal plate  30  is the top board portion  2  of the stepladder  1 . 
     The dielectric substrate  20  preferably has a rectangular parallelepiped shape including a first surface (front surface) and a second surface (back surface), a radiation conductor  25  is provided on the front surface, and a ground conductor  26  is provided on the back surface. As illustrated in  FIG. 3 , the radiation conductor  25  and the ground conductor  26  are electrically connected to each other through a plurality of interlayer connection conductors (via hole conductors)  27  provided in the dielectric substrate  20 . The radiation conductor  25  and the ground conductor  26  preferably are defined by thin-film conductor patterns including metal foils such as copper, aluminum, or the like, or alternatively, are preferably defined by thick-film conductor patterns that include conductive paste containing powder made of silver, copper, or the like. 
     The radiation conductor  25  and the ground conductor  26  are separated from each other by gaps  25   a  and  26   a  in the center portion of the dielectric substrate  20 . A projecting power feeding portion  25   b  is located in the gap  25   a  in the radiation conductor  25 , and the wireless IC device  50  is coupled to the power feeding portion  25   b . This coupling is electromagnetic field coupling or electrical direct coupling (DC connection). 
     In each of the dielectric substrate  20  and the metal plate  30 , through holes  21  and  31  are formed to penetrate from the front thereof to the back thereof. The back surface of the dielectric substrate  20  is caused to adhere to the front surface of the metal plate  30  through insulating adhesive  22 , for example. Furthermore, a conductive members  35  inserted into the through holes  21  and  31  is individually crimped at the front surface portion of the dielectric substrate  20  and the back surface portion of the metal plate  30 , and hence the dielectric substrate  20  is firmly fixed to the metal plate  30 . This conductive member  35  defines a current path portion electrically conducting the front surface and back surface of the metal plate  30  to each other. Furthermore, the conductive member  35  is also electrically conducted to the ground conductor  26 . It is preferable that the conductive member  35  is made of a material having the same electrical conductivity as or a higher electrical conductivity than the electrical conductivity of the metal plate  30 . 
     A loop-shaped electrode  28  is disposed in the dielectric substrate  20  (refer to  FIG. 2B ). More specifically, the loop-shaped electrode  28  includes the radiation conductor  25 , the ground conductor  26 , and the plural interlayer connection conductors  27  starting from the power feeding portion  25   b , and is capacitively coupled at the gap  26   a  portion. Namely, the ground electrode  26  is capacitively coupled at the gap  26   a  portion through the metal plate  30 . In this loop-shaped electrode  28 , a loop plane that is the circling plane thereof is disposed so as to be perpendicular or substantially perpendicular to the front surface of the metal plate  30 . Since the gap  26   a  portion is included, when, for example, the dielectric substrate  20  preferably is formed using flexible material, it is easy to cause the dielectric substrate  20  to bend. 
     In the wireless communication device  10 A having the above-described configuration, when a predetermined high-frequency signal is transmitted from the wireless IC device  50 , a high-frequency signal current a flows along the loop-shaped electrode  28 , as illustrated in  FIG. 4 . In addition, a high-frequency signal current b is excited to flow by the high-frequency signal current a, in a portion located outside of the interlayer connection conductor  27  of the ground conductor  26 . Owing to this high-frequency signal current b, a high-frequency signal current c flows in a region in the proximity of a surface boundary between the conductive member  35  and the metal plate  30 . More specifically, the high-frequency signal current a flowing through the ground conductor  26  is conducted to the back surface side of the metal plate  30  with the surface boundary portion between the conductive member  35  and the metal plate  30  serving as a current path portion. 
     As a result, as illustrated in  FIG. 5A , not only the radiation A of the high-frequency signal from the radiation conductor  25  to the front surface side of the metal plate  30  occurs but also the radiation B of the high-frequency signal to the back surface side of the metal plate  30  occurs. More specifically, it is possible to establish communication with the reader/writer from the front and back surfaces of the metal plate  30 . A high-frequency signal radiated from the reader/writer in the RFID system and received by the metal plate  30  is supplied to the wireless IC device  50  through the surface boundary portion between the conductive member  35  and the metal plate  30  and the loop-shaped electrode  28 , and the wireless IC device  50  operates. On the other hand, a response signal from the wireless IC device  50  is transmitted to the metal plate  30  through the loop-shaped electrode  28  and the surface boundary portion, and radiated to the reader/writer. 
     Incidentally, in a comparative example not including the conductive member  35 , since no high-frequency signal current is transmitted between the loop-shaped electrode  28  and the back surface of the metal plate  30 , the radiation A from the radiation conductor  25  only occurs, as illustrated in  FIG. 5B , and no radiation occurs from the back surface of the metal plate  30 . 
     The loop-shaped electrode  28  causes the wireless IC device  50  and the metal plate  30  to be coupled to each other, and functions as an impedance matching circuit. It is possible for the loop-shaped electrode  28  to perform impedance matching by adjusting the electrical length thereof or the like. In addition, since the loop plane of the loop-shaped electrode  28  is disposed so as to be perpendicular or substantially perpendicular to the front surface of the metal plate  30 , a magnetic field is generated with respect to the front surface of the metal plate  30 . Accordingly, an electric field is induced perpendicular or substantially perpendicular to the metal plate  30 , a magnetic field loop is induced owing to this electric field loop, and an electromagnetic field distribution spreads due to the concatenation thereof. With this unique configuration, it is possible to realize a wireless communication device including the metal plate  30 . 
     As illustrated in  FIG. 4 , it is desirable that roundness is assigned to the inner peripheral surfaces of the through holes  31  of the metal plate  30 , specifically, ridge line portions in which the through holes  31  open on the front and back surfaces of the metal plate  30 . This is because the high-frequency signal current c smoothly flows. In addition, it is desirable that the thickness of the metal plate  30  ranges from about 0.005 to about 0.5 times as thick as the wavelength of the high-frequency signal. More specifically, when the high-frequency signal is in a 900 MHz band, the thickness preferably is about from about 0.8 mm to about 8 cm, for example. Depending on the material (electrical conductivity) of the metal plate  30 , if the thickness is within this range, it is also possible to obtain a desirable radiation gain on the back surface side of the metal plate  30 . 
     As illustrated in  FIG. 6 , the wireless IC device  50  may be a wireless IC chip  51  processing a high-frequency signal, or alternatively, as illustrated in  FIG. 7 , the wireless IC device  50  may also be configured to include the wireless IC chip  51  and a feed circuit substrate  65  including a resonant circuit having a predetermined resonance frequency. 
     The wireless IC chip  51  illustrated in  FIG. 6  includes a clock circuit, a logic circuit, a memory circuit, and the like, and necessary information is stored therein. On the back surface of the wireless IC chip  51 , input-output-use terminal electrodes and  52  and mount-use terminal electrodes  53  and  53  are provided. The input-output-use terminal electrodes  52  and  52  are electrically connected to the power feeding portions  25   b  and  25   b  illustrated in the first preferred embodiment, through metal bumps or the like. In addition, as the material of the metal bump, Au, solder, or the like may be used. 
     As illustrated in  FIG. 7 , when the wireless IC device  50  is configured to include the wireless IC chip  51  and the feed circuit substrate  65 , it is possible to provide various kinds of feed circuits (a resonant circuit/a matching circuit are included) in the feed circuit substrate  65 . For example, as illustrated as an equivalent circuit in  FIG. 8 , there may be adopted a feed circuit  66  including inductance elements L 1  and L 2  that have inductance values different from each other and are subjected to magnetic coupling (indicated by mutual inductance M) with the phases thereof being opposite to each other. The feed circuit  66  has a predetermined resonance frequency, and establishes impedance matching between the impedance of the wireless IC chip  51  and the metal plate  30 . In addition, the wireless IC chip  51  and the feed circuit  66  may be electrically connected (DC-connected) to each other, or may be coupled to each other through an electromagnetic field. 
     The feed circuit  66  transmits, to the metal plate  30 , a high-frequency signal that is sent out from the wireless IC chip  51  and has a predetermined frequency, through the loop-shaped electrode  28 , and supplies, to the wireless IC chip  51 , a high-frequency signal received by the metal plate  30 , through the loop-shaped electrode  28 . Since the feed circuit  66  has a predetermined resonance frequency, it is easy to establish impedance matching with the metal plate  30 , and it is possible to shorten the electrical length of the loop-shaped electrode  28 . 
     Next, the configuration of the feed circuit substrate  65  will be described. As illustrated in  FIG. 6  and  FIG. 7 , the input-output-use terminal electrode  52  of the wireless IC chip is connected to feed terminal electrodes  142   a  and  142   b  located on the feed circuit substrate  65  and the mount-use terminal electrode  53  is connected to mounting terminal electrodes  143   a  and  143   b , through metal bumps or the like. 
     As illustrated in  FIG. 9 , the feed circuit substrate is preferably obtained by laminating, crimping, and firing ceramic sheets  141   a  to  141   h  including dielectric material or magnetic material. In this regard, however, insulation layers configuring the feed circuit substrate  65  are not limited to the ceramic sheets, and, for example, the insulation layers may be resin sheets such as thermosetting resin such as liquid crystal polymer or the like or thermoplastic resin. On the sheet  141   a  serving as an uppermost layer, the feed terminal electrodes  142   a  and  142   b , the mounting terminal electrodes  143   a  and  143   b , and via hole conductors  144   a ,  144   b ,  145   a , and  145   b  are provided. On each of the sheets  141   b  to  141   h  serving as a second layer to an eighth layer, wiring electrodes  146   a  and  146   b  configuring the inductance elements L 1  and L 2  are provided, and via hole conductors  147   a ,  147   b ,  148   a , and  148   b  are provided as necessary. 
     By laminating the sheets  141   a  to  141   h , the inductance element L 1  is provided such that the wiring electrode  146   a  is connected in a spiral shape through the via hole conductor  147   a  and the inductance element L 2  is provided such that the wiring electrode  146   b  is connected in a spiral shape through the via hole conductor  147   b . In addition, capacitance is generated between the lines of the wiring electrodes  146   a  and  146   b.    
     The end portion  146   a - 1  of the wiring electrode  146   a  on the sheet  141   b  is connected to the feed terminal electrode  142   a  through the via hole conductor  145   a , and the end portion  146   a - 2  of the wiring electrode  146   a  on the sheet  141   h  is connected to the feed terminal electrode  142   b  through the via hole conductors  148   a  and  145   b . The end portion  146   b - 1  of the wiring electrode  146   b  on the sheet  141   b  is connected to the feed terminal electrode  142   b  through the via hole conductor  144   b , and the end portion  146   b - 2  of the wiring electrode  146   b  on the sheet  141   h  is connected to the feed terminal electrode  142   a  through the via hole conductors  148   b  and  144   a.    
     In the above-mentioned feed circuit  66 , since the inductance elements L 1  and L 2  are individually wound in directions opposite to each other, magnetic fields occurring in the inductance elements L 1  and L 2  are cancelled out. Since the magnetic fields are cancelled out, it is necessary to lengthen the wiring electrodes  146   a  and  146   b  to some extent, in order to obtain a desired inductance value. Accordingly, since a Q-value is lowered, the steepness of a resonance characteristic disappears and the resonance characteristic has a wider bandwidth in the vicinity of a resonance frequency. 
     When the perspective plane of the feed circuit substrate  65  is viewed, the inductance elements L 1  and L 2  are located at right and left different positions. In addition, the directions of magnetic fields occurring in the inductance elements L 1  and L 2  are opposite to each other. Accordingly, when the feed circuit  66  is caused to be coupled to the loop-shaped electrode  28 , a reversed current is excited in the loop-shaped electrode  28  to enable a current to occur in the metal plate  30 , and owing to a potential difference due to this current, it is possible to cause the metal plate  30  to operate as a radiation element (antenna). 
     By embedding a resonance/matching circuit into the feed circuit substrate  65 , it is possible to suppress and prevent a characteristic fluctuation due to the influence of an external article, and it is possible to avoid the degradation of communication quality. In addition, when the wireless IC chip  51  configuring the wireless IC device  50  is disposed so as to be directed toward a central side in the thickness direction of the feed circuit substrate  65 , it is possible to avoid the destruction of the wireless IC chip  51 , and it is possible to improve a mechanical strength as the wireless IC device  50 . 
     Second Preferred Embodiment 
     As illustrated in  FIGS. 10A-10C , in a wireless communication device  10 B according to a second preferred embodiment, through holes  32 , which penetrate from a front surface to a back surface, are formed in a portion of the metal plate  30 , located directly below the ground conductor  26 . The other configuration is preferably the same or substantially the same as in the first preferred embodiment. In the present second preferred embodiment, the inner peripheral surface of the through hole  32  is also used as a current path portion. 
     More specifically, in the first preferred embodiment, since, in a region X (refer to  FIG. 4 ) between the conductive members  35  on the back surface of the metal plate  30 , a current flows whose direction is opposite to the direction of the current flowing through the loop-shaped electrode  28 , it is hard for a high-frequency signal current to flow, and it is hard for a high-frequency signal to be radiated from the region X. On the other hand, in the present second preferred embodiment, since the through holes  32  are located in the region X, a high-frequency signal current flowing along the front surface of the metal plate  30  is conducted to the back surface along the inner peripheral surfaces of the through holes  32 . Accordingly, since, from among the region X, a region is narrowed in which it is hard for the high-frequency signal current to flow, and a region is increased in which the high-frequency signal current flows (namely, the high-frequency signal current flows in the central portion of the region X), it is possible to cause a radiation characteristic to be improved. 
     In addition, since the high-frequency signal current propagates in the surface layer region of the metal plate  30 , this through hole  32  may be filled with conductive material or insulating material. In addition, in the same way as described above, it is desirable that roundness is assigned to ridge line portions in which the through holes  32  open on the front and back surfaces of the metal plate  30 . 
     Third Preferred Embodiment 
     As illustrated in  FIGS. 11A-11C , in a wireless communication device  10 C according to a third preferred embodiment of the present invention, the radiation conductor  25  provided on the front surface of the dielectric substrate  20  and the ground conductor  26  provided on the back surface thereof are connected to each other using interlayer connection conductors  29  located on the end surfaces of the dielectric substrate  20 , thereby defining the loop-shaped electrode  28 . Furthermore, in the metal plate  30 , the conductive members  36  are arranged to electrically conduct the front and back surfaces thereof to each other and are also electrically conducted to the ground conductor  26 . In the present third preferred embodiment, a high-frequency signal transmitted from the wireless IC device  50  flows, as the high-frequency signal current a, along the loop-shaped electrode  28 , and is conducted to the back surface of the metal plate  30  along the conductive member  36 , and a high-frequency signal is radiated from the back surface side. 
     In the present preferred embodiment, compared with the first preferred embodiment, since it is possible to shorten a distance between the conductive members  36 , it is possible to improve a radiation efficiency by narrowing a region in which it is hard for the high-frequency signal to be radiated. 
     Fourth Preferred Embodiment 
     As illustrated in  FIGS. 12A-12C , in a wireless communication device  10 D according to a fourth preferred embodiment of the present invention, an aperture portion  25   c  and a slit  25   d  are disposed in the radiation conductor  25  provided on the front surface of the dielectric substrate  20 , and a power feeding portion  25   b  is formed through the slit  25   d . The ground conductor  26  provided on the back surface of the dielectric substrate  20  is a sheet of conductor (the gap  26   a  is not formed), and is electrically connected to the radiation conductor  25  by the plural interlayer connection conductors  27 , thereby defining the loop-shaped electrode  28 . The conductive member defines a device to conduct the high-frequency signal current from the front surface of the metal plate  30  to the back surface thereof, in the same way as in the first preferred embodiment. 
     In the fourth preferred embodiment, the high-frequency signal transmitted from the wireless IC device  50  flows along the periphery of the aperture portion  25   c , and the periphery of the aperture portion  25   c  functions as a magnetic field antenna. Accordingly, the radiation conductor  25  has a potential difference with respect to the ground conductor  26 , and the radiation conductor  25  functions as a patch antenna with the ground conductor  26  serving as a ground electrode. According to such a simple configuration, it is also possible to realize a wireless communication device including the metal plate  30 . As described in the above-mentioned first preferred embodiment, a high-frequency signal is also radiated from the back surface side of the metal plate  30  connected to the ground conductor  26 . 
     Fifth Preferred Embodiment 
     As illustrated in  FIGS. 13A-13C , in a wireless communication device  10 E according to a fifth preferred embodiment, the ground conductor  26  is embedded in the interlayer of the dielectric substrate  20 , both end portions thereof are caused to be exposed from both end surfaces of the dielectric substrate  20 , and the ground conductor  26  is a sheet of conductor (the gap  26   a  is not formed). The other configuration in the present fifth preferred embodiment is preferably the same as in the first preferred embodiment, and the radiation state of the high-frequency signal is also the same as in the first preferred embodiment. In particular, in the present fifth preferred embodiment, since the ground conductor is embedded in the dielectric substrate  20 , the insulating adhesive  22  is not used when the dielectric substrate  20  is attached to the metal plate  30 , and it is possible to directly crimp the dielectric substrate  20  using the conductive member  35 . In addition, since both end portions of the ground conductor  26  are exposed from both end surfaces of the dielectric substrate  20 , a high-frequency signal current flowing along the front surface of the metal plate  30  is increased. 
     Sixth Preferred Embodiment 
     As illustrated in  FIGS. 14A and 14B , in a wireless communication device  10 F that is a sixth preferred embodiment, the loop-shaped electrode  28  including the radiation conductor  25 , the ground conductor  26 , and the interlayer connection conductor  29  has the same configuration as that of the third preferred embodiment, and the wireless communication device  10 F differs in that the conductive member  36  electrically conducting the front and back surfaces of the metal plate  30  to each other is capacitively coupled to the ground conductor  26 . In the present sixth preferred embodiment, a reversed current d is induced on the front surface of the metal plate  30  with respect to the high-frequency signal current a flowing through the ground conductor  26 , and the induced current d is conducted to the back surface of the metal plate  30  through the vicinity of the surface boundary between the conductive member  36  and the through hole  33 . By being subjected to capacitive coupling in this way, it is possible to cause the dielectric substrate  20  to easily adhere to the metal plate  30 , and it is possible to thermally insulate the ground conductor  26  and the metal plate  30  from each other while the ground conductor  26  and the metal plate  30  are electrically connected to each other. 
     Seventh Preferred Embodiment 
     As illustrated in  FIG. 15 , in a wireless communication device  10 G according to a seventh preferred embodiment of the present invention, the loop-shaped electrode  28  including the radiation conductor  25 , the ground conductor  26 , and the interlayer connection conductor  27  preferably has the same configuration as that of the first preferred embodiment, and in the metal plate  30 , through holes  32  that penetrate from a front surface to a back surface are formed in a portion located directly below the ground conductor  26 . The conductive member  35  is not provided, and the dielectric substrate  20  is fixed to the front surface of the metal plate  30  using the insulating adhesive  22 . 
     In the present seventh preferred embodiment, the reversed current d is induced on the front surface of the metal plate  30  with respect to the high-frequency signal current a flowing through the ground conductor  26 , and the induced current d is conducted to the back surface of the metal plate  30  through the vicinity of the inner peripheral surfaces of the through holes  32 . 
     In addition, since the high-frequency signal current d propagates in the surface layer region of the metal plate  30 , this through hole  32  may be filled with conductive material or insulating material. In addition, in the same way as described above, it is desirable that roundness is assigned to ridge line portions in which the through holes  32  open on the front and back surfaces of the metal plate  30 . 
     Eighth Preferred Embodiment 
     As illustrated in  FIG. 16 , in a wireless communication device  10 H that is an eighth preferred embodiment, a screw member severs as the conductive member  35 , and this screw member is screwed from the back surface of the metal plate  30  into the ground conductor  26 . Owing to this screw member, the front surface and back surface of the metal plate  30  are electrically connected to each other, and the leading end of the screw member is electrically connected to the ground conductor  26 . The other configuration preferably is the same as that of the first preferred embodiment, and the high-frequency signal current flowing through the ground conductor  26  is conducted to the back surface side of the metal plate  30  with the surface boundary portion between the screw member and the metal plate  30  serving as a current path portion. 
     Ninth Preferred Embodiment 
     As illustrated in  FIG. 17 , a wireless communication device  10 I that is a ninth preferred embodiment preferably has the same configuration as that of the first preferred embodiment, and the dielectric substrate  20  is fixed to the front surface of the metal plate  30  by crimping the conductive member  35  through no insulating adhesive. The ground conductor  26  is electrically in contact with the front surface of the metal plate  30 , and also electrically conducted to the conductive member  35 . In the present ninth preferred embodiment, the high-frequency signal current flowing through the ground conductor  26  is also conducted to the back surface side of the metal plate  30  with the surface boundary portion between the conductive member  35  and the metal plate  30  defining a current path portion. 
     Other Preferred Embodiments 
     In addition, a wireless communication device and a metal article according to the present invention are not limited to the above-mentioned preferred embodiments, and various modifications may occur insofar as they are within the scope thereof. 
     In particular, a metal article to which the wireless communication device is attached may be various scaffolding members used for a building site in addition to the above-mentioned stepladder or may be a metal article used for the wide range of application other than the scaffolding members. More specifically, a metal article that has not fundamentally functioned as an antenna may be used as a radiation element. 
     As described above, preferred embodiments of the present invention are useful for a wireless communication device and a metal article, and in particular, is superior in terms of the fact that a radiation gain becomes large not only on a surface mounted to a metal plate or a metal member but also on a surface opposite to the mounting surface. 
     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.