Abstract:
Provided is an antenna device, having a small and simple configuration, which reduces connection resistance in connections between a film antenna element and a power feeding unit. An antenna device comprises a nonconductive transparent film, which is an insulating substrate having a prescribed optical transmission factor, an antenna element that is formed, upon the transparent film, from transparent conductive material, which is a conductive material having a prescribed optical transmission factor, and a non-transparent conductive power feeding wire. The transparent film and the transparent conductive material are doubled over and pressed upon a portion of one side of the antenna element, which forms a square dipole antenna, such that the transparent conductive material is on the inside, and the power feeding wire is sandwiched therein. By being doubled over, the transparent conductive material has an overlap portion, the two sides whereof electrically connect to one other.

Description:
TECHNICAL FIELD 
     The present invention relates to an antenna apparatus using a film-like antenna element. 
     BACKGROUND ART 
     In recent years, proposal has been made to form an antenna element with a transparent conductive material formed on a non-conductive transparent film as an antenna apparatus that can be attached to glass of vehicles, stores, or offices, for example, and that does not impair the view and design. The transparent conductive material formed on the transparent film cannot be connected by soldering due to its low heat resistance. Therefore, a power feeding structure between a transparent conductive material and a power feed line, a metal conductor, is under consideration. 
     As an example of such a power feeding structure other than soldering, the antenna apparatus disclosed in Patent Literature 1 has been proposed, for example.  FIG. 1  shows a configuration of the antenna apparatus disclosed in Patent Literature 1.  FIG. 1A  is a plan view showing a state in which antenna body  10  and connecting apparatus  11  and power feed line  12  are divided into two, and  FIG. 1B  is a cross sectional view of antenna body  10  and connecting apparatus  11  that are mounted on window glass  17 . 
     Antenna body  10  is configured such that antenna element  14  is formed on antenna element holding sheet  13 , and connection terminals  15   a  and  15   b  of the antenna element are provided. Connecting apparatus  11  is provided with wiring substrate  18  in which a low-noise amplifier, for example, is mounted, and one side of wiring substrate  18  is connected to power feed line  12  and the other side of wiring substrate  18  is connected to connection terminals  16   a  and  16   b  of connection apparatus  11 . Connection terminals  16   a  and  16   b  of connecting apparatus  11  are formed with an elastic conductive member and are formed in a spring structure. 
     Connection terminals  15   a  and  15   b  of antenna body  10  and connection terminals  16   a  and  16   b  of connecting apparatus  11  are arranged on hard window glass  17  such that connection terminals  15   a  and  15   b  and connection terminals  16   a  and  16   b  overlap, and are fixed on window glass  17  using adhesive, for example. By this means, connection terminals  16   a  and  16   b  of connecting apparatus  11  are pressed by the spring structure and contact physically, leading to a conduction state. 
     Further, as another example of a power feeding structure, the antenna apparatus disclosed in Patent Literature 2 has been proposed, for example.  FIG. 2  shows a configuration of the antenna apparatus disclosed in Patent Literature 2. Antenna apparatus  20  is configured such that antenna pattern  22  formed on film  21  and power feed line  23  are sandwiched by hard cover  25 , hard front cover plate  26 , and back cover plate  27 , via power feed terminal plate  24 , and are pressed together by screw  28  to be electrically conducted. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL1 
         Japanese Patent Application Laid-Open No. 2000-196327 
         PTL2 
         Japanese Patent Application Laid-Open No. 63-254804 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     However, the antenna apparatus disclosed in Patent Literature 1 presumes pasting a transparent film, on which transparent conductive material is formed, on hard glass, and fixing connection terminals of an antenna element, and requires a lot of parts, such as a power feed line, a connecting apparatus for fixing connection terminals via a power feed substrate, and spring-shaped connection terminals. For these reasons, the antenna apparatus disclosed in Patent Literature 1 has problems with a complicated structure and a large power feed section that cannot be miniaturized. Further, there is another problem that, when a connection terminal of an antenna element and a connection terminal of a connecting apparatus contact, only one side of each connection terminal makes a contact surface, making it difficult to reduce contact resistance. 
     Further, the antenna apparatus disclosed in Patent Literature 2 is structured to sandwich an antenna element and a power feed line by a hard cover, a front cover plate, and a back cover plate via a power feed terminal plate, raising problems that the antenna apparatus requires a lot of parts and the structure is complicated, and a power feed section is large and cannot be miniaturized. Further, there is a problem that, as is the case with Patent Literature 1, only one side of the antenna element and the power feed line becomes a contact surface upon connection, making it difficult to reduce contact resistance. 
     It is therefore an object of the present invention to provide an antenna apparatus that is small and simple, and that reduces contact resistance when connecting a film-like antenna element and a power feed section. 
     Solution to Problem 
     An antenna apparatus comprising: a conductive power feed line; a film-like insulation base material that has a predetermined optical transmittance; and an antenna element that is formed with a conductive material having a predetermined optical transmittance on one surface of the insulation base material, folded together with the insulation base material to place the conductive material inside and sandwich an end of the power feed line, and electrically connected with the power feed line. 
     Advantageous Effects of Invention 
     According to the present invention, it is possible to realize an antenna apparatus that is small and simple, and that reduces contact resistance when connecting a film-like antenna element and a power feed section. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS. 1A and 1B  show a configuration of the antenna apparatus disclosed in Patent Literature 1 which is prior art; 
         FIG. 2  shows a configuration of the antenna apparatus disclosed in Patent Literature 2 which is prior art; 
         FIG. 3  shows a configuration of the antenna apparatus according to Embodiment 1 of the present invention; 
         FIG. 4  shows the view of the antenna element in  FIG. 3  seen in the direction of arrow A; 
         FIG. 5  shows a case where a cross section of the power feed line is flat; 
         FIG. 6  shows a configuration of the antenna apparatus according to Embodiment 2 of the present invention; 
         FIG. 7  shows a configuration of the antenna apparatus according to Embodiment 3 of the present invention; 
         FIGS. 8A and 8B  show a configuration of the non-transparent metal conductors in  FIG. 7 ; and 
         FIGS. 9A and 9B  show a configuration of the antenna apparatus according to Embodiment 4 of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Now, embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, components having the same functions in the embodiments will be assigned the same reference numerals, and overlapping explanations will be omitted. 
     Embodiment 1 
       FIG. 3  shows a configuration of antenna apparatus  100  according to Embodiment 1 of the present invention. Antenna apparatus  100  comprises: non-conductive transparent film  101 , which is an insulation base material having an optical transmittance of, for example, 90%; antenna element  103 , which is formed with transparent conductive material  102  having an optical transmittance of, for example, 80%; and non-transparent, conductive power feed line  104 . 
     In part of one side of antenna element  103  constituting a rectangular dipole antenna, transparent film  101  and transparent conductive material  102  are folded and pressed together, placing conductive material  102  inside and sandwiching one end of power feed line  104 . 
       FIG. 4  shows a view of the antenna element in  FIG. 3  seen in the direction of arrow A. By being folded, transparent conductive material  102  has overlapping parts  105 , electrically connected with each other and having a width of w 1 . Conductive adhesive is applied to overlapping parts  105  and contact surface  106  where transparent conductive material  102  and power feed line  104  contact, so that power feed line  104  and transparent conductive material  102  are electrically connected and mechanically fixed. 
     As shown in  FIG. 4 , when power feed line  104  is formed in a cylindrical shape, almost the entirety of the circumference of the section of the power feed line contacts transparent conductive material  102 . As a result of this, compared to the case where part of the circumference of the section of the power feed line contacts transparent conductive material  102 , it is possible to enlarge the area of contact surface  106  where antenna element  103  and power feed line  104  contact, and therefore reduce contact resistance. By this means, it is possible to improve the antenna radiation efficiency, and realize high antenna performance. 
     Even when a cross section of the power feed line is flat, for example, as shown in  FIG. 5 , it is possible to make contact surface  106  two-sided, and therefore reduce contact resistance. 
     Further, generally, the antenna current is the highest in the vicinity of a power feed section, and therefore it is possible to improve antenna performance by lowering the resistance value in the power feed section. 
     According to the present embodiment, by providing overlapping parts  105  where parts of transparent conductive material  102  overlap each other over a width of w 1 , it is possible to double the thickness of transparent conductive material  102  and reduce the resistance value of the transparent conductive material to a half. For example, when determining the radiation efficiency of a square dipole antenna of 950 MHz using a transparent conductive material having a sheet resistance of 1 ohm per square, the radiation efficiency is −1.6 dB when width w 1  of overlapping parts  105  is 0 mm (i.e. when antenna element  103  is not folded). By contrast, the radiation efficiency is −1 dB when width w 1  of overlapping parts  105  is 4 mm (0.013 wavelength), achieving an improvement by 0.6 dB. As described above, it is possible to significantly improve the radiation efficiency by providing overlapping parts  105  having about a 0.01 wavelength. 
     As described above, according to Embodiment 1, part of an antenna element, which is formed with a transparent conductive material on one surface of a film element, is folded to place the transparent conductive material inside and sandwich an end of a power feed line. By this means, it is possible to enlarge the contact area of the transparent conductive material and the power feed line, consequently reducing contact resistance and improving the antenna radiation efficiency. Further, by providing overlapping parts to be electrically connected with each other when a transparent conductive material is folded, it is possible to lower the resistance value of the transparent conductive material in the vicinity of a power feed section, where the density of antenna current is high, and improve the antenna radiation efficiency. Further, parts for fixing an antenna element and a power feed line are not necessary, so that it is possible to realize a simpler power feeding structure and a smaller antenna apparatus. Further, by fixing a power feed line and a transparent conductive material using conductive adhesive, it is possible to realize reliable power feeding with sufficient strength to prevent the transparent conductive material from being peeled off, for example. 
     Embodiment 2 
       FIG. 6  shows a configuration of antenna apparatus  200  according to Embodiment 2 of the present invention. In  FIG. 6 , cut part  201  having a cut width of w 2  is provided in antenna element  103  and is formed in the position of length L from an end face of antenna element  103 . Transparent film  101  and transparent conductive material  102 , covering distance L from an end face of antenna element  103  to cut part  201 , are folded and pressed together to place transparent conductive material  102  inside and sandwich an end of power feed line  104 . 
     Now, assume that, for example, the sheet resistance value of a transparent conductive material of a square dipole antenna of 950 MHz is 1 ohm per square, distance L is 32 mm (0.1 wavelength), and width w 2  is 5 mm (0.016 wavelength). By this means, the radiation efficiency improves by 0.5 dB, compared to the configuration which provides no cutting and which involves no folding. 
     With the configuration of the antenna shown in  FIG. 6 , when width w 2  is widened, the space between power-feeding points becomes larger and the operation of the dipole antenna becomes unstable, and therefore it is preferable to set width w 2  at 5 mm or less. As for distance L, because a lot of antenna current is distributed around an antenna element, antenna performance will be improved when distance L is longer. 
     As described above, even when a cut is provided in part of a transparent film and a transparent conductive material and only part in the vicinity of a power feed section is folded, it is possible to improve the radiation efficiency, as is the case with the configuration shown in  FIG. 3 . Further, compared to the configuration where one side of an antenna element is folded as shown in  FIG. 3 , it is possible to reduce the size of the fold, ensuring a uniform fold and realizing stable antenna performance. 
     As described above, according to Embodiment 2, a cut is provided in part of an antenna element formed with a transparent film and a transparent conductive material, and, in that cut part, the transparent film and the transparent conductive material are folded to place the transparent conductive material inside and sandwich an end of a power feed line. By this means, it is possible to improve the radiation efficiency and reduce the size of the fold, ensuring a uniform fold and realizing stable antenna performance. 
     Embodiment 3 
       FIG. 7  shows a configuration of antenna apparatus  300  according to Embodiment 3 of the present invention. In  FIG. 7 , antenna apparatus  300  comprises: non-conductive transparent film  101 , antenna element  303  formed on transparent film  101 , and non-transparent, conductive power feed line  104 . 
     Antenna element  303  constitutes a rectangular dipole antenna, in which, for example, super-thin, non-transparent metal conductors  307  having a width of 10 μm are arranged at 100 μm intervals in a mesh pattern with grids, as shown in  FIGS. 8A and 8B . As described above, by increasing a mesh interval compared to the width of a conductor, it is possible to configure an antenna element having a higher optical transmittance. For example, when the opening area of conductors is one tenth of the opening area of the non-conductive part, the transmittance will be 90% 
     Further, in part of one side of antenna element  303 , transparent film  101  and antenna element  303  are folded and pressed together to place non-transparent metal conductors  307  inside and sandwich an end of power feed line  104 . In this case, non-conductive metal conductors  307  have overlapping parts  305  to be electrically connected with each other, and these overlapping parts  305  are pressed together such that the mesh pattern matches itself (that is, the conductors overlap each other). Because the mesh pattern is a periodic geometric pattern, it is possible to make the pattern match itself in overlapping parts  305 . Further, conductive adhesive is applied to overlapping parts  305  and the contact surface where non-transparent metal conductors  307  and power feed line  104  contact, so that power feed line  104  and non-transparent metal conductors  307  are electrically connected. 
     As described above, according to Embodiment 3, given an antenna element formed by arranging super-thin, non-transparent metal conductors in a mesh pattern with grids, part of this antenna element and transparent film is folded to place the non-transparent metal conductors inside and sandwich an end of a power feed line, so that, in overlapping parts where the non-transparent metal conductors are electrically connected with each other, and the mesh pattern matches itself. By this means, it is possible to suppress the decrease of optical transmittance in the overlapping parts. Further, it is possible to enlarge the contact area where the non-transparent metal conductors and the power feed line contact, reducing contact resistance and improving the antenna radiation efficiency. 
     Although a case has been described with the above embodiment as an example where a transparent conductive material is formed with a mesh pattern of non-transparent metal conductors, the present invention is by no means limited to this, and it is equally possible to use a periodic geometric pattern such as stripe. 
     Although a case has been described with the above embodiment as an example where the width of each non-transparent metal conductor is 10 μm and the interval of the non-transparent metal conductor is 100 μm, the present invention is by no means limited to this, and it is equally possible to adopt any configuration with which an optical transmittance is obtained. 
     Embodiment 4 
       FIGS. 9A and 9B  show a configuration of antenna apparatus  400  according to Embodiment 4 of the present invention. In  FIGS. 9A and 9B , antenna apparatus  400  comprises: non-conductive transparent film  101 , antenna element  403  formed on transparent film  101 , and non-transparent, conductive power feed line  104 . 
     Antenna element  403  comprises non-transparent metal conductors  307  that are configured such that mesh pattern density is set higher in the vicinity of a connection with power feed line  104 , while the pattern density is set lower gradually where the distance from the vicinity of the power feed section is longer. In this case, although the pattern density is set to become lower linearly or exponentially, for example, there is almost no influence on the radiation efficiency depending on the difference of change of the pattern density. Therefore, it is possible to obtain equivalent antenna performance in any configurations. 
     As described above, according to Embodiment 4, by increasing the density of a mesh pattern formed with non-transparent metal conductors in the vicinity of a power feed section, it is possible to facilitate the non-transparent metal conductors to contact a power feed line, and it is also possible to enlarge contact area where the power feed line and the non-transparent metal conductors contact, consequently decreasing contact resistance. Further, by gradually decreasing the mesh pattern density where the distance from the power feed section is longer, it is possible to increase the optical transmittance gradually from the vicinity of the power feed section and prevent local changes of transmittance, making the vicinity of the power feed section less distinguishable. 
     Although cases have been described with the above embodiments as examples where the antenna element constitutes a rectangular dipole antenna, the present invention is by no means limited to this, and it is equally possible to use any antenna elements as long as it is configured with a transparent conductive material formed on a non-conductive transparent film. 
     Further, the present invention is by no means limited to the configuration parameter according to the above embodiments, and it is equally possible to set a configuration parameter so as to improve the antenna radiation efficiency. 
     The disclosure of Japanese Patent Application No. 2009-242575, filed on Oct. 21, 2009, including the specification, drawings and abstract, is incorporated herein by reference in its entirety. 
     INDUSTRIAL APPLICABILITY 
     The antenna apparatus according to the present invention has an effect of not impairing view and design, and is suitable for use in window glass for vehicles and stores, for example. 
     REFERENCE SIGNS LIST 
     
         
           101  Transparent film 
           102  Transparent conducive material 
           103 ,  303 ,  403  Antenna element 
           104  Power feed line 
           105 ,  305  Overlapping parts 
           201  Cut part 
           307  Non-transparent metal conductor