Abstract:
The present invention provides a small, thin, and cheap foldable broadband antenna that is valid in a wide band and, moreover, can be manufactured at low cost, and a method of using the same. A foldable broadband antenna includes: a plate conductor having a rectangular outer shape and in which a slit is formed from a long side so as be orthogonal to a longitudinal direction; a side conductor provided perpendicularly from a side parallel with the slit in the plate conductor; and a back conductor disposed in parallel with the plate conductor from an end of the side conductor toward the slit. In the plate conductor, a coaxial external conductor of a coaxial cable is connected to the side opposite to the side conductor with the slit therebetween, and a coaxial central conductor of the coaxial cable is electrically connected to the same side as that of the side conductor.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an antenna formed by a metal plate and a dielectric printed board and, more particularly, to a small-sized broadband antenna element and a method of using the same. 
         [0003]    2. Description of the Related Art 
         [0004]    In recent years, a TV reception antenna for digital terrestrial broadcasting and an antenna for a wireless TV using the UWB (Ultra Wide Band) technique require a very wide band. 
         [0005]    For example, communications using the UWB technique is expected to use a frequency band of 3.1 GHz to 4.9 GHz. 
         [0006]    There is a conventional broadband antenna having a structure as shown in  FIG. 1 . The antenna is disclosed in “plate antenna and television receiver having the antenna” of Japanese Patent Application Laid-Open No. 2005-203830 (document 1). A plate conductor 101 has a slit 102 on one side, and power is supplied through a coaxial cable 105. 
         [0007]    Recent electronic devices such as a device having a USB interface have to be compact as typified by a USB (Universal Serial Bus) memory. 
         [0008]    As a conventional technique related to miniaturization of an antenna, there is an antenna disclosed in “radio device” of Japanese Patent Application Laid-Open No. 2004-215132 (document 2). 
         [0009]    The antenna disclosed in document 1 has a broadband characteristic but has a drawback of large dimensions. The dimensions are 210 mm×210 mm at the lowest useful frequency of 470 MHz, which are equivalent to “0.3 wavelength×0.3 wavelength”. The dimensions do not satisfy the demand for miniaturization of an electronic device. 
         [0010]    The invention disclosed in document 2 relates to a so-called notch antenna which hardly covers a broad band. Further, since the circuit board itself is folded, it is difficult to reduce the thickness and the high manufacturing cost due to folding of the circuit board occurs. 
       SUMMARY OF THE INVENTION 
       [0011]    Disclosed herein are a small and thin foldable broadband antenna that covers a wide band and, moreover, can be manufactured at low cost and a method of using the same. 
         [0012]    The foldable broadband antenna comprises a plate conductor having a slit, a back conductor that is electrically connected with the plate conductor and is disposed parallel to the plate conductor with space, wherein one of a pair of conductors composing a line spans the slit and is electrically connected with the plate conductor on a conductive route from the slit to the back conductor. 
         [0013]    The present invention provides a small and thin foldable broadband antenna that is effective in a wide band and, moreover, is manufactured at low cost, and a method of using the same. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    Features of the disclosed embodiments will be described by way of the following detailed description with reference to the accompanying drawings in which: 
           [0015]      FIG. 1  is a diagram showing the configuration of a conventional antenna for a radio device; 
           [0016]      FIG. 2  is a diagram showing the configuration of a foldable broadband antenna according to a first exemplary embodiment; 
           [0017]      FIG. 3  is a diagram showing the return loss of the foldable broadband antenna; 
           [0018]      FIG. 4  is a diagram showing the configuration of a foldable broadband antenna according to a second exemplary embodiment; 
           [0019]      FIG. 5  is a top perspective view of a foldable broadband antenna according to a third exemplary embodiment; 
           [0020]      FIG. 6  is a bottom perspective view of a foldable broadband antenna; 
           [0021]      FIG. 7A  is a top view of the foldable broadband antenna; 
           [0022]      FIG. 7B  is a side view of the foldable broadband antenna; 
           [0023]      FIG. 8  is a top perspective view of a foldable broadband antenna according to a fourth exemplary embodiment; 
           [0024]      FIG. 9  is a bottom perspective view of the foldable broadband antenna; 
           [0025]      FIG. 10A  is a top view of the foldable broadband antenna; 
           [0026]      FIG. 10B  is a side view of the foldable broadband antenna; 
           [0027]      FIG. 11  is a top perspective view of a foldable broadband antenna according to a fifth exemplary embodiment; 
           [0028]      FIG. 12  is a top perspective view of a foldable broadband antenna according to a sixth exemplary embodiment; 
           [0029]      FIG. 13  is a top perspective view of a foldable broadband antenna according to a seventh exemplary embodiment; 
           [0030]      FIG. 14A  is a top view of the foldable broadband antenna; 
           [0031]      FIGS. 14B ,  14 C, and  14 D are diagrams showing examples of the shape of a connecting part; 
           [0032]      FIG. 15  is a diagram showing the configuration of a foldable broadband antenna according to an eighth exemplary embodiment; 
           [0033]      FIGS. 16A ,  16 B, and  16 C are diagrams showing an antenna, clothes to which the antenna is attached, and clothes to which the antenna is attached, in an example of using the foldable broadband antenna. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0034]    A foldable broadband antenna according to the present invention has a configuration that a slit  2  is formed on one end of the longitudinal direction of a plate conductor. A side conductor is added to one end orthogonal to the longitudinal direction and. further, a back conductor is added. By forming the plate conductor, the side conductor, and the back conductor by folding one metal plate, the antenna can be formed at low cost. 
         [0035]    Power is supplied by a coaxial cable. A coaxial external conductor is electrically connected across the plate conductor on one side of a slit, and a coaxial central conductor is connected to the conductor on the other side of the slit. Consequently, a loop antenna that is effective within a wide band is formed by the plate conductor and the coaxial cable. 
         [0036]    A similar power supply structure can be also formed electrically by using a printed board. 
         [0037]    Exemplary embodiments of the invention will be described below with reference to the drawings. 
       First Embodiment 
       [0038]      FIG. 2  shows the configuration of a foldable broadband antenna according to the first embodiment. The foldable broadband antenna has a configuration that the slit  2  is formed at one side of the shorter direction (at one of long sides) of a plate conductor  1 , a side conductor  3  is provided on one side of the longer direction (on one of short sides), and a back conductor  4  is added. 
         [0039]    The width of the plate conductor  1  is about 0.2 wavelength of the lowest useful frequency. The depth of the slit  2  is 85% (0.17 wavelength) of the width of the plate conductor  1 . The width of the slit  2  is about 0.02 wavelength of the lowest useful frequency. 
         [0040]    The length of the plate conductor  1  in the longer direction is about 0.3 wavelength of the lowest useful frequency. The length of the side conductor  3  is about 0.04 wavelength, and the length of the back conductor  4  is about 0.1 wavelength. 
         [0041]    The plate conductor  1 , the side conductor  3 , and the back conductor  4  are formed from one metal plate, so that the manufacture cost can be reduced. 
         [0042]    Power is supplied via a coaxial cable  5 . A coaxial external conductor  7  is electrically connected (by soldering or the like) across the plate conductor  1  on one side of the slit  2 , and a coaxial central conductor  6  is electrically connected (by soldering or the like) to a conductor part on the other side of the slit  2 . The coaxial external conductor  7  is soldered across the plate conductor  1  to come in front of the slit  2 . 
         [0043]    Although the coaxial cable  5  is attached onto a front surface of the plate conductor  1 , not facing the back conductor  4 , in  FIG. 2 , the coaxial capable  5  may be connected to a reverse surface of the plate conductor  1 , facing the back conductor  4 . 
         [0044]      FIG. 3  shows an example of a return loss characteristic of the foldable broadband antenna of the first embodiment. The width is 30 mm, the length is 20 mm, the width of the back conductor is 11 mm, the width of the side conductor is 4 mm, the length of the slit is 17 mm, and the width of the slit is 2 mm. 
         [0045]    Within a target bandwidth of 3.1 GHz to 4.9 GHz, a return loss of 7.5 dB or less (VSWR 2.5 or less) is obtained. The antenna has sufficient performance in the target bandwidth. 
         [0046]    As described above, when the target bandwidth is 3.1 GHz to 4.9 GHz, the antenna having approximately 20 mm in length, 30 mm in width, and 4 mm in height can present desired antenna characteristics. In this case, the size is 0.2×0.3×0.04 in wavelength equivalent, so that the area of the antenna can be reduced to about ⅔ of that of the conventional technique (document 1). 
         [0047]    Since the circuit board itself is not bent, the antenna is thinner than that disclosed in document 2. 
         [0048]    Further, since the bandwidth of 470 MHz to 620 MHz has been used conventionally, the normalized bandwidth is about 28%. In the embodiment. the antenna can be used from 3.1 GHz to 4.9 GHz, so that the normalized bandwidth is about 45%. 
         [0049]    As described above, the foldable broadband antenna of the first embodiment can effectively operate in the bandwidth that is twice as wide as that of the conventional technique. 
       Second Embodiment 
       [0050]      FIG. 4  shows the configuration of a foldable broadband antenna according to the second embodiment. The foldable broadband antenna has the configuration almost similar to that of the first embodiment except for the method of connecting the coaxial cable  5 . 
         [0051]    As shown in  FIG. 4 , only an end (upper end) of the coaxial external conductor  7  of the coaxial cable  5  is soldered near the slit  2  on the plate conductor  1 , thereby forming a loop antenna. 
         [0052]    In the first embodiment, the coaxial external conductor  7  and the plate conductor  1  are in line-contact. In the second embodiment, they are in point-contact. Also in this configuration, effects similar to those of the foldable broadband antenna of the first embodiment are obtained. 
         [0053]    Since the other configuration is similar to the first embodiment, repetitive description is omitted. 
       Third Embodiment 
       [0054]      FIG. 5  is a top perspective view of a foldable broadband antenna according to the third embodiment.  FIG. 6  is a bottom perspective view of the foldable broadband antenna.  FIGS. 7A and 7B  are top view and side view, respectively, of the foldable broadband antenna. 
         [0055]    The foldable broadband antenna is constructed using a printed board  10 . First, a plate conductor  11  formed by a copper foil pattern is disposed on the bottom surface of the printed board  10 . A slit  12  is formed in the plate conductor  11  in a manner similar to the first embodiment. At one end (the right end in  FIG. 5 ) of the short side of the plate conductor  11 , a conductor having a U shape in cross section formed by a surface conductor  15 , a side conductor  13 , and a back conductor  14  is disposed. 
         [0056]    In the conductor having the U shape in cross section, the surface conductor  15  is electrically connected (by solder or the like) to the end of the plate conductor  11 . 
         [0057]    Power is supplied via a microstrip line  16 . The microstrip line  16  disposed on the top surface of the printed board  10  and a ground  18  disposed on the bottom face of the printed board  10  function as a microstrip transmission line. Both of the microstrip line  16  and the ground  18  are formed as copper foil patterns on the printed board  10 . 
         [0058]    The tip of the microstrip line  16  extends over the slit  12  and is connected to the plate conductor  11  via a conductive through hole  17 . The plate conductor  11  and the microstrip lines form a loop antenna. 
         [0059]    Since the power supply structure is equivalent to that of the first embodiment from an electrical viewpoint, similar effects are obtained. 
       Fourth Embodiment 
       [0060]      FIG. 8  is a top perspective view of a foldable broadband antenna according to the fourth embodiment.  FIG. 9  is a bottom perspective view of the foldable broadband antenna.  FIGS. 10A and 10B  are top view and plan view, respectively, of the foldable broadband antenna. 
         [0061]    The fourth embodiment is different from the third embodiment in that a conductor is constructed using back conductors  22  and  23  and through holes  24  in place of the conductor having the U shape (the conductor formed by the surface conductor  15 , the side conductor  13 , and the back conductor  14 ). 
         [0062]    The back conductor  22  is a conductor having a U shape. Part of the back conductor  22  is soldered to the back conductor  23  formed by a copper foil pattern on the printed board. Further, the back conductor  23  is electrically connected to the end of the plate conductor  11  through a plurality of conductive through holes  24 . When the width of connection between the back conductors  22  and  23  is small, the structure is equivalent to that of the third embodiment from an electrical viewpoint, so that similar effects are obtained. 
       Fifth Embodiment 
       [0063]      FIG. 11  shows the configuration of a foldable broadband antenna according to the fifth embodiment. The foldable broadband antenna has a configuration similar to that of the case where the orientation of the slit  12  in the third embodiment is turned by 90 degrees. A plate conductor  31  corresponds to the plate conductor  11  in the third embodiment. 
         [0064]    In a manner similar to the third embodiment, power is supplied via a microstrip transmission line formed by a microstrip line  36  and a ground  36 . Since the orientation of the slit  12  is turned by 90 degrees, the microstrip line  36  is also turned by 90 degrees in this embodiment, and is electrically connected to the plate conductor  31  via the through hole  17  immediately after spanning the slit  12 . 
         [0065]    Since the foldable broadband antenna of the fifth embodiment is equivalent to the third embodiment from an electrical viewpoint, similar effects are obtained. 
       Sixth Embodiment 
       [0066]      FIG. 12  shows the configuration of a foldable broadband antenna according to the sixth embodiment. The foldable broadband antenna has a configuration similar to that of the case where the orientation of the slit  12  in the fourth embodiment is turned by 90 degrees. The plate conductor  31  corresponds to the plate conductor  11  in the fourth embodiment. 
         [0067]    In a manner similar to the fourth embodiment, power is supplied via a microstrip transmission line formed by a microstrip line  36  and a ground  38 . Since the orientation of the slit  12  is turned by 90 degrees, the orientation of the microstrip line  36  is also turned by 90 degrees. 
         [0068]    Since the foldable broadband antenna of the sixth embodiment is equivalent to the fourth embodiment from an electrical viewpoint, similar effects are obtained. 
       Seventh Embodiment 
       [0069]      FIG. 13  shows the configuration of a foldable broadband antenna according to the seventh preferred embodiment. The foldable broadband antenna has a configuration similar to that of the third embodiment but differs from the third embodiment in that the through holes are not provided. 
         [0070]    By adjusting the shape and size of one end of the microstrip line  16  and the length S of the projecting part of the line  16  from the slit  12 , impedance match is obtained. 
         [0071]      FIGS. 14A to 14D  show configuration examples of a power feeding part.  FIG. 14A  is a top view, and  FIGS. 14B ,  14 C, and  14 D show modifications of the tip portion of the microstrip line  16 .  FIG. 14B  shows a square tip portion  45 ,  FIG. 14C  shows a triangle tip portion  46 , and  FIG. 14D  shows an ellipse tip portion  47 . By adjusting the length S and the shape and size of the tip portion, impedance match can be obtained. The shape of the tip portion of the microstrip line  16  may be different from any of those shown in the figures. As long as a desired antenna characteristic is obtained, any shape can be adopted. 
       Eighth Embodiment 
       [0072]      FIG. 15  shows the configuration of a foldable broadband antenna of the eighth embodiment. The foldable broadband antenna is similar to that of the seventh embodiment but is constructed by using an insulator  51  in place of the printed board  10 . 
         [0073]    A plate conductor  52  having a slit  53  is disposed at the rear side of the insulator  51 , and an end of the plate conductor  52  is folded back to the front side of the insulator  51 . On the front of the insulator  51 , a bar-shaped conductor  54  is disposed so as to cross the slit  53 . A coaxial central conductor  6  of the coaxial cable  5  is electrically connected to the bar-shaped conductor  54 , and the coaxial external conductor  7  is electrically connected to the plate conductor  52 . 
         [0074]    The insulator  51  is a foldable insulator such as cloth, sponge, film, or FPC (flexible printed circuit board). A soft antenna  50  has such as velcro (registered trademark) straps  55  on the back, and it can be attached to clothes, bags, or the like. 
         [0075]      FIGS. 16A ,  16 B, and  16 C show a use example of the foldable broadband antenna of the embodiment. The soft antenna  50  with a velcro (registered trademark) straps  55  in  FIG. 16A  and clothes  60  with a velcro (registered trademark) straps  56  in  FIG. 16B  provide the soft antenna  50  attached to the wear  60  in  FIG. 16C . In such a manner, for example, the soft antenna  50  for receiving digital terrestrial broadcasting or the like is carried as the antenna  50  is attached to the clothes  60 . By connecting the soft antenna  50  to an antenna terminal of a portable terminal, users can view the broadcasting in an excellent reception state. 
       Ninth Embodiment 
       [0076]    The foldable broadband may further include a side conductor rising perpendicularly from a side parallel to the slit. The plate conductor has a rectangular outer shape where a slit is formed from a long side. The back conductor extends from a side of the side conductor in parallel with the plate conductor. One end of a line is electrically connected to one side of the slit, opposite to the side conductor, and the other end of the line is electrically connected to the other side of the slit. 
       Tenth Embodiment 
       [0077]    The line may be a coaxial cable. A coaxial external conductor of the coaxial cable may be electrically connected to one side of the slit, opposite to the side conductor, and a coaxial central conductor of the coaxial cable may be electrically connected to the other side of the slit. 
       Eleventh Embodiment 
       [0078]    The coaxial external conductor may be electrically connected at one point to the plate conductor. 
       Twelfth Embodiment 
       [0079]    The coaxial external conductor may be electrically connected in line contact with the plate conductor. 
       Thirteenth Embodiment 
       [0080]    The plate conductor, the side conductor, and the back conductor may be integrally formed by folding a single conductive plate. 
       Fourteenth Embodiment 
       [0081]    The plate conductor may be provided on one surface of a printed board. The back conductor may be disposed on the other side of the printed board and in parallel with the printed board with space left between the back conductor and the printed board. The line may be a microstrip line forming a microstrip transmission line with the plate conductor on the surface of the printed board opposite to the plate conductor. The microstrip line and the plate conductor may be electrically connected via a first through hole in a position closer to the back conductor than the slit. 
       Fifteenth Embodiment 
       [0082]    The back conductor, a side conductor and a surface conductor may be connected and form an almost U shape where the surface conductor is electrically connected to the plate conductor so as to cover part of the printed board. 
       Sixteenth Embodiment 
       [0083]    A rear conductor may be disposed on the other side of the printed board. The back conductor may form part of a U-shaped conductor that is on the rear conductor. The plate conductor and the rear conductor may be electrically connected via a second through hole. 
       Seventeenth Embodiment 
       [0084]    The slit may be formed in the same direction as a longitudinal direction of the printed board. 
       Eighteenth Embodiment 
       [0085]    The plate conductor may be provided on a surface of an insulator. A conductor having an almost U shape may be obtained by connecting a surface conductor, the back conductor and a side conductor so as to cover part of the insulator. A power supply conductor that crosses the slit on a surface of the insulator opposite to the plate conductor may form the line. 
       Nineteenth Embodiment 
       [0086]    A sheet of conductor is bent, and the plate conductor and the conductor having an almost U shape may be integrally formed from the sheet of conductor. 
       Twentieth Embodiment 
       [0087]    The insulator, the plate conductor and the conductor having an almost U shape may have flexibility. 
       Twenty-First Embodiment 
       [0088]    The insulator may be provided with an attaching means for being attached to another member. 
       Twenty-Second Embodiment 
       [0089]    The insulator may be a printed board. The power supply conductor may be disposed on a surface opposite to the plate conductor, and be a microstrip line that forms a microstrip transmission line with the plate conductor. 
       Twenty-Third Embodiment 
       [0090]    A return loss adjusting part may be formed by enlarging an end of the microstrip line. 
       Twenty-Fourth Embodiment 
       [0091]    A method may be provided for using the foldable broadband antenna where the insulator may be provided with an attaching means for being attached to another member. The foldable broadband antenna is attached to clothes by using the attaching means. 
         [0092]    The foregoing embodiments are exemplary embodiments of the invention and the invention is not limited to the embodiments. 
         [0093]    For example, although the configuration of using the coaxial cable for power supply has been described in the foregoing embodiments, similar effects can be also obtained by using a line having another structure such as a twist pair cable. 
         [0094]    It is to be appreciated that those skilled in the art can change or modify the embodiments without departing from the scope and spirit of the present invention.