Patent Publication Number: US-8125390-B2

Title: Small-size wide band antenna and radio communication device

Description:
TECHNICAL FIELD 
     The present invention relates to an antenna using a dielectric substrate and, more particularly, to a small-size antenna for use in wide band radio communication. 
     BACKGROUND ART 
     There is known a UWB (Ultra Wide Band) technique for a ultra wide band radio communication. In general, the UWB technique is used in a wireless TV, a wireless LAN system for a notebook PC (notebook personal computer) or portable information terminal (personal digital assistant), and the like. In general, communications using the UWB technique is expected to use a frequency band of 3.1 GHz to 4.9 GHz. To realize the communication using the UWB technique, an antenna compatible with UWB wireless communication is required. 
     As a conventionally known wide band antenna, there is a discone antenna  200 ′ as shown in  FIG. 25 . The discone antenna  200 ′ has a structure in which a disk  201 ′ and conical plate  202 ′ serving as a radiating element are fitted, in a manner as illustrated in  FIG. 25 , to a coaxial cable  203 ′ having a coaxial center conductor  204 ′ covered by a coaxial external conductor  205 ′. 
     Further, there is known, in addition to a 3D antenna as the discone antenna  200 ′, a planar antenna having a structure in which a radiating element is formed on a printed board. As an antenna technique of this type, the following Non-Patent Document 1 discloses a wide band antenna using a self-complementary radiating element. This antenna has a structure in which two patterns corresponding to two system-radiating elements of a dipole antenna are formed on a printed board. One of the two patterns is formed on the front surface of the printed board, and the other is formed on the back surface thereof in such a manner as not to face the pattern on the front surface.
     Non-Patent Document 1: Journal of Institute of Electronics, Information and Communication Engineers (B) Vol. J88-B No. 9, September 2005, pages 1,662 to 1,673   

     DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
     Nowadays, a technique for realizing USB (Universal Serial Bus) connection for a portable information terminal or notebook PC by radio using the above-mentioned UWB technique has been proposed. In general, it is desirable the size of USB devices attached to the portable information terminal or notebook PC be as small as possible, like a memory stick (typically, having a size of length×width×thickness of about 60 mm×15 mm×8 mm), in consideration of the size of the portable information terminal or notebook PC or portability. Therefore, in order to realize the USB connection based on the UWB technique, the size of a radio interface device attached to a terminal is required to be as small as that of the memory stick. 
     An antenna and a printed board implementing a communication circuit connected to the antenna are mounted on a stick-like USB device according to the UWB technique, that is, radio interface device attached to a terminal. The printed board has a size of length×width of about 50 mm×10 mm. Of the above entire area, a size of length×width of about 20 mm×10 mm is assigned to the antenna. 
     Although the discone antenna  200  described above can obtain wide band characteristics, it has a 3D shape as shown in  FIG. 25  and the size thereof tends to be large and, therefore, is not suitably used as the radio interface device to be attached to the portable information terminal. Although the antenna proposed in Non-patent document 1 has a planar shape, it requires a size of length×width of 65 mm×40 mm. Thus, it is difficult to apply the technique of Non-patent document 1 to the above-mentioned radio interface device in which the size assigned to an antenna is limited to a size of length×width of about 20 mm×10 mm. 
     The present invention has been made in view of the above problems, and an object thereof is to provide a technique for making an antenna for use in wide band radio communication into a smaller size for mounting on a printed board. 
     Means for Solving the Problems 
     A small-size wide band antenna of the present invention includes a radiating element formed on a dielectric substrate and a power supply unit for supplying dipole potential to the radiating element. The radiating element includes a ground potential section having a power supply point to which a ground potential is supplied from the power supply unit and an opposite-pole potential section having a power supply point to which a potential forming a pair with the ground potential is supplied from the power supply unit. Each of the ground potential section and opposite-pole potential section includes a pair of conductors which are formed in a tapered shape on front and rear surfaces of the dielectric substrate and are mutually capacitively coupled. The power supply points of the ground potential section and opposite-pole potential section are positioned at tapered apexes of the conductors formed on the same side (front or rear side) of the dielectric substrate. 
     The basic concept of the present invention is that each of the two-system radiating elements of a dipole antenna is divided, and the element portions obtained by the division are arranged on the front and rear sides of the dielectric substrate. Thus, two-system radiating elements exist on the same surface of the substrate. When a power is supplied to an antenna having such a configuration, the elements of the same system formed on the front and rear surfaces of the dielectric substrate are capacitively coupled to each other at the portions overlapping each other, i.e., facing each other via the dielectric substrate. As a result, the elements of the same system are electrically connected to each other via the substrate. 
     Advantages of the Invention 
     According to the present invention, each of the ground potential section and opposite-pole potential section constituting the radiation element is divided, and conductors serving as the element portions obtained by the division are arranged on the front and rear sides of the dielectric substrate. Thus, the size of the antenna can be reduced. Further, by forming each conductor in a tapered shape, wide band frequency characteristics can be obtained. Therefore, it is possible to apply the present invention to a technique for realizing USB connection by radio using the UWB technique. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a configuration view of a first embodiment of a small-size wide band antenna according to the present invention; 
         FIG. 2  is a configuration view showing both sides of the antenna according to the first embodiment; 
         FIG. 3  is a configuration view of a second embodiment of a small-size wide band antenna according to the present invention; 
         FIG. 4  is a configuration view showing both sides of the antenna according to the second embodiment; 
         FIG. 5  is a configuration view of a third embodiment of a small-size wide band antenna according to the present invention; 
         FIG. 6  is a configuration view showing both sides of the antenna according to the third embodiment; 
         FIG. 7  is a configuration view of a fourth embodiment of a small-size wide band antenna according to the present invention; 
         FIG. 8  is a configuration view showing both sides of the antenna according to the fourth embodiment; 
         FIG. 9  is a configuration view of a fifth embodiment of a small-size wide band antenna according to the present invention; 
         FIG. 10  is a configuration view showing both sides of the antenna according to the fifth embodiment; 
         FIG. 11  is a configuration view of a sixth embodiment of a small-size wide band antenna according to the present invention; 
         FIG. 12  is a configuration view showing both sides of the antenna according to the sixth embodiment; 
         FIG. 13  is a configuration view of a seventh embodiment of a small-size wide band antenna according to the present invention; 
         FIG. 14  is a configuration view showing both sides of the antenna according to the seventh embodiment; 
         FIG. 15  is a configuration view of an eighth embodiment of a small-size wide band antenna according to the present invention; 
         FIG. 16  is a configuration view showing both sides of the antenna according to the eighth embodiment; 
         FIG. 17  is a configuration view of a ninth embodiment of a small-size wide band antenna according to the present invention; 
         FIG. 18  is a configuration view showing both sides of the antenna according to the ninth embodiment; 
         FIG. 19  is a configuration view of a tenth embodiment of a small-size wide band antenna according to the present invention; 
         FIG. 20  is a configuration view of an eleventh embodiment of a small-size wide band antenna according to the present invention; 
         FIG. 21  is a configuration view of a twelfth embodiment of a small-size wide band antenna according to the present invention; 
         FIG. 22  is a configuration view of a thirteenth embodiment of a small-size wide band antenna according to the present invention; 
         FIG. 23  is a configuration view showing both sides of the antenna according to the thirteenth embodiment; 
         FIG. 24  is an explanatory view of return loss characteristics of the small-size wide band antenna according to the present invention; 
         FIG. 25  is a configuration view of a conventional discone antenna; 
         FIG. 26  is a configuration view showing both sides of the antenna according to a fourteenth embodiment; 
         FIG. 27  is an explanatory view of return loss characteristics of the antenna according to the fourteenth embodiment; 
         FIG. 28  is a configuration view showing both sides of the antenna according to a fifteenth embodiment; 
         FIG. 29  is a configuration view showing both sides of the antenna according to a sixteenth embodiment; 
         FIG. 30  is a configuration view showing both sides of the antenna according to a seventeenth embodiment; and 
         FIG. 31  is a block diagram schematically showing a radio communication device. 
     
    
    
     EXPLANATION OF REFERENCE SYMBOLS 
     
         
           101  to  117 : Antenna 
           1 ,  61 : Printed board (dielectric substrate) 
           2 : Coaxial cable 
           3 : Coaxial center conductor 
           4 : Coaxial external conductor 
           5 : Coaxial external conductor connecting wire 
           11  to  17 ,  31 ,  32 ,  41 ,  42 : Conductor 
           21 ,  51 ,  73 : Through hole 
           71 : Micro-strip line 
           72 : Ground 
           200 : Printed board (dielectric substrate) 
           201 : Ground 
           202 : Micro-strip line 
           203 ,  301 ,  401 : Stub 
           204 : Through hole 
       
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Explanation of Configurations of Embodiments—1 
       FIG. 1  shows a configuration of an antenna  101  according to a first embodiment of the present invention.  FIG. 2  collectively shows conductor patterns formed on the front and rear surfaces of the antenna  101 . In the antenna  101  according to the present embodiment, conductors  11  to  16  (to be described later) each serving as a radiating element and a conductor  17  (to be descried later) serving as an impedance matching section are patterned on a printed board  1 . The printed board  1  is a rectangular dielectric substrate having a dimension of “Y” in the longitudinal direction and “X” (X&lt;Y) in the traverse direction. That is, the printed board mentioned in the present and subsequent embodiments denotes a dielectric substrate on the outer surface of which the conductors are to be printed. 
     A coaxial cable  2  serving as a power supply unit for supplying a dipole potential to the radiating elements is connected to the antenna  101 . The coaxial cable  2  includes a coaxial external conductor  4  assuming a ground potential and a coaxial center conductor  3  which is covered by the coaxial external conductor  4  and supplies a potential forming a pair with the ground potential to the radiating element. 
     The printed board  1  has a rectangular shape, and radiating elements are formed in the rectangular antenna area defined by two longitudinal direction peripheral sides (straight peripheral sides each having a dimension of Y) and two traverse direction peripheral sides (straight peripheral sides each having a dimension of X). 
     The conductor  11  is a tapered conductor pattern which spreads from near the center of a first longitudinal direction peripheral side toward the traverse direction upper peripheral side on the front surface of the printed board  1 . The conductor  11  is formed into substantially a right triangle in which one upper apex of the printed board  1  is set as a right-angle apex and has a protruding portion protruding from the hypotenuse of the right triangle toward a second longitudinal direction peripheral side of the printed board  1 . The protruding portion is formed into a triangle or trapezoid at near the upper end portion of the printed board  1 . 
     The conductor  12  is a tapered conductor pattern which spreads from near the center of the second longitudinal direction peripheral side toward the traverse direction upper peripheral side on the rear surface of the printed board  1 . The conductor  12  is formed into substantially a right triangle in which one upper apex of the printed board  1  is set as a right-angle apex and has a protruding portion protruding from the hypotenuse of the right triangle toward the first longitudinal direction peripheral side of the printed board  1 . The protruding portion is formed into a triangle or trapezoid at near the upper end portion of the printed board  1 . The conductors  11  and  12  are components corresponding to an opposite-pole potential section to which a potential forming a pair with the ground potential is supplied. 
     The conductor  13  is a tapered conductor pattern which spreads from near the center of the first longitudinal direction peripheral side toward the traverse direction lower peripheral side on the front surface of the printed board  1 . The conductor  14  is a tapered conductor pattern which spreads from near the center of the second longitudinal direction peripheral side toward the traverse direction lower peripheral side on the rear surface of the printed board  1 . The conductors  13  and  14  are components corresponding to a ground potential section to which a ground potential is supplied and are formed into substantially right triangles in which different apexes of the printed board  1  are set as right-angle apexes. 
     The conductors  15  and  16  are formed on both side surfaces corresponding respectively to the second and first longitudinal direction peripheral sides of the printed board  1  and are each connected to both the conductors  11  and  12  to serve as a unit for short-circuiting between the conductors  11  and  12  which are positioned adjacently to the traverse direction upper peripheral side of the printed board  1 . The conductor  17  is a hook-like (L-shaped) stub conductor extending from the conductor  11  formed on the front surface of the printed board  1 . The bending direction of the conductor  17  is set such that the leading end of the stub conductor faces the conductor  11  (that is, such that the leading end thereof extends substantially in parallel to the diagonal line of the conductor  11 ). The conductors  15 ,  16 , and  17  are components corresponding to an impedance matching section for matching a characteristic impedance of the coaxial cable  2  and input impedance as viewed from the coaxial cable  2  to conductor  11 . 
     The shape of the conductor  17  serving as a stub is not limited to the hook-like shape as illustrated, but the conductor  17  may be formed into a linear strip shape as long as the leading end thereof is opened. Further, it is not always necessary to arrange the stub at near the tapered apex of the conductor  11 , as in the case of the conductor  17 , but the arrangement thereof may be changed in accordance with the impedance matching. 
     Power supply to the antenna  101  having the configuration described above is achieved by soldering the coaxial center conductor  3  of the coaxial cable  2  to the tapered apex of the conductor  11  and further soldering the coaxial external conductor  4  of the coaxial cable  2  uniformly along the first longitudinal direction peripheral side of the printed board  1 , starting from the tapered apex of the conductor  13 . As a result, the ground potential section and opposite-pole potential section have power supply points, respectively, at tapered apexes of the conductors  11  and  13  formed on the front surface of the dielectric substrate  1 . 
     As described above, the pair of conductors  13  and  14  serving as the ground potential section are arranged such that the areas in the vicinity of the tapered apexes of the respective conductors do not face each other via the dielectric substrate  1  and that the residual areas (areas adjacent to the traverse direction lower peripheral side) of the respective conductors face each other via the dielectric substrate  1 . Similarly, the pair of conductors  11  and  12  serving as the opposite-pole potential section are arranged such that the areas in the vicinity of the tapered apexes of the respective conductors do not face each other via the dielectric substrate  1  and that the residual areas (areas adjacent to the traverse direction upper peripheral side) of the respective conductors face each other via the dielectric substrate  1 . 
     The tapered apexes of the conductors  11  and  13  having, respectively, the power supply points of the ground potential section and opposite-pole potential section are positioned near the center of the first longitudinal direction peripheral side of the antenna area having a rectangular shape corresponding to the outer shape of the printed board  1 . Respective ones of the sides of the conductors  11  and  13  that form the tapered apexes correspond to the first longitudinal direction peripheral side of the antenna area. The tapered apexes of the conductors  12  and  14  paired respectively with the conductors having, respectively, the power supply points of the ground potential section and opposite-pole potential section are positioned near the center of the second longitudinal direction peripheral side of the antenna area. Respective ones of the sides of the conductors  12  and  14  that form the tapered apexes correspond to the second longitudinal direction peripheral side of the antenna area. Further, respective other ones (i.e., diagonal lines) of the sides of the conductors  13  and  14  serving as the ground potential section that form the tapered apexes cross each other; and respective other ones (i.e., diagonal lines) of the sides of the conductors  11  and  12  serving as the opposite-pole potential section that form the tapered apexes cross each other. Note that the above conductors do not actually cross each other but appear to cross each other when viewed in the normal line direction of the front or rear surface of the substrate. 
       FIG. 3  shows a configuration of an antenna  102  according to a second embodiment of the present invention.  FIG. 4  collectively shows conductor patterns formed on the front and rear surfaces of the antenna  102 . The antenna  102  of the present embodiment differs from the antenna  101  shown in  FIG. 1  in the unit for short-circuiting between the conductors  11  and  12 . Concretely, in the antenna  101  of  FIG. 1 , the conductors  15  and  16  formed on the side surfaces serve as the short-circuit unit, while in the antenna  102  of the present embodiment, through holes  21  shown in  FIGS. 3 and 4  serve as the short-circuit unit. 
     The through holes  21  are known short-circuit unit and also referred to as “via hole”. The through holes  21  each have a structure in which a conductor is formed on the inner wall of the hole penetrating the printed board  1  positioned between the conductors  11  and  12 . In the example of  FIGS. 3 and 4 , three through holes  21  are arranged at the upper portion of the printed board  1  along each of the both side surfaces, and thus a total of six through holes are formed. However, the number of the through holes  21  may arbitrarily be determined (e.g., two through holes each and a total of four, or one through hole each and a total of two, or three or more through holes each, etc.) as long as the size of each through hole  21  is sufficiently small enough in terms of high frequency characteristics, i.e., small enough relative to the wavelength used. Further, the arrangement of the through holes  21  is not limited to that shown in  FIGS. 3 and 4 . 
       FIG. 5  shows a configuration of an antenna  103  according to a third embodiment of the present invention.  FIG. 6  collectively shows conductor patterns formed on the front and rear surfaces of the antenna  103 . The antenna  103  of the present embodiment differs from the antenna  101  shown in  FIG. 1  in the presence/absence of the short-circuit unit and shape of the conductor pattern serving as the opposite-pole potential section. That is, the antenna  103  does not include the unit for short-circuiting between the conductors on the front and rear surfaces of the printed board  1  and includes conductors  31  and  32  as the opposite-pole potential section in place of the conductors  11  and  12  of  FIG. 1 . 
     The conductor  31  is a tapered conductor pattern which spreads from near the center of the first longitudinal direction peripheral side toward the traverse direction upper peripheral side on the front surface of the printed board  1 . The conductor  32  is a tapered conductor pattern which spreads from near the center of the second longitudinal direction peripheral side toward the traverse direction upper peripheral side on the rear surface of the printed board  1 . As shown in  FIGS. 5 and 6 , the conductors  31  and  32  are each formed into substantially a right triangle that does not have the protruding portion that the above-mentioned conductors  11  and  12  have. 
       FIG. 7  shows a configuration of an antenna  104  according to a fourth embodiment of the present invention.  FIG. 8  collectively shows conductor patterns formed on the front and rear surfaces of the antenna  104 . The antenna  104  according to the present embodiment has a structure obtained by adding a conductor  41  serving as a stub to the rear surface of the printed board  1  of the antenna  101  of  FIG. 1 . 
     The conductor  41  is formed on the rear surface of the printed board  1  such that a part thereof faces the conductor  13  formed on the front surface of the printed board  1  to serve as a second stub conductor constituting the impedance matching section for the ground potential section in the present invention. On the rear side of the printed board  1 , the conductor  41  shown in  FIGS. 7 and 8  extends from near the center of the first longitudinal direction peripheral side and is formed in an independent manner such that it is not connected to any other conductor patterns. The bending direction of the conductor  41  is symmetrical to the bending direction of the stub conductor  17  formed on the front surface of the printed board  1  with respect to the horizontal direction (direction parallel to the traverse direction peripheral side of the printed board  1 ). That is, the bending direction of the second stub conductor  41  is set such that the leading end thereof faces (that is, such that the leading end thereof extends in substantially parallel to the diagonal line of the conductor  13 ), on the front and rear sides of the dielectric substrate  1  (via the dielectric substrate  1 ), the conductor  13  serving as the ground potential section that is capacitively coupled to the second stub conductor  41 . The shape of the conductor  41  is not limited to the hook-like shape (L-shape) as illustrated, but the conductor  41  may be formed into a linear strip shape. 
       FIG. 9  shows a configuration of an antenna  105  according to a fifth embodiment of the present invention.  FIG. 10  collectively shows conductor patterns formed on the front and rear surfaces of the antenna  105 . 
     The antenna  105  of the present embodiment differs from the antenna  104  of  FIG. 7  in the short-circuit unit. Concretely, in the antenna  104  of  FIG. 7 , the conductors  15  and  16  formed on the side surfaces of the printed board  1  serve as the short-circuit unit, while as shown in  FIG. 9 , in the antenna  105  of the present embodiment, through holes  21  serve as the short-circuit unit. The configuration of the through holes  21  is the same as that shown in  FIG. 3 , and the description thereof is omitted here. 
       FIG. 11  shows a configuration of an antenna  106  according to a sixth embodiment of the present invention.  FIG. 12  collectively shows conductor patterns formed on the front and rear surfaces of the antenna  106 . The antenna  106  of the present embodiment has a structure obtained by adding the second stub conductor  41  that the antenna  104  of  FIG. 7  has to the rear surface of the antenna  103  of  FIG. 5  that does not have the short-circuit unit. 
       FIG. 13  shows a configuration of an antenna  107  according to a seventh embodiment of the present invention.  FIG. 14  collectively shows conductor patterns formed on the front and rear surfaces of the antenna  107 . The antenna  107  of the present embodiment has a structure obtained by adding a conductor  42  for short-circuiting between the conductor  13  formed on the front surface of the printed board  1  and second stub conductor  41  formed on the rear surface of the printed board  1  at the substrate side surface. 
       FIG. 15  shows a configuration of an antenna  108  according to an eighth embodiment of the present invention.  FIG. 16  collectively shows conductor patterns formed on the front and rear surfaces of the antenna  108 . The antenna  108  of the present embodiment has a structure obtained by adding a through hole  51  for short-circuiting between the conductor  13  formed on the front surface of the printed board  1  and second stub conductor  41  formed on the rear surface of the printed board  1  to the antenna  105  of  FIG. 9 . The configuration of the through hole  51  is the same as that of each of the through holes  21  formed at the upper end portion of the printed board  1 , and the description thereof is omitted here. 
       FIG. 17  shows a configuration of an antenna  109  according to a ninth embodiment of the present invention.  FIG. 18  collectively shows conductor patterns formed on the front and rear surfaces of the antenna  109 . The antenna  109  of the present embodiment has a structure obtained by adding the through hole  51  for short-circuiting between the conductor  13  formed on the front surface of the printed board  1  and second stub conductor  41  formed on the rear surface of the printed board  1  to the antenna  106  of  FIG. 11 . 
     Here, two embodiments concerning power supply to the small-size wide band antenna according to the present invention will be described.  FIG. 19  shows a configuration of an antenna  110  according to a tenth embodiment of the present invention. Although the conductor pattern of the first embodiment ( FIGS. 1 and 2 ) is applied to the antenna  110  for the sake of convenience, conductor patterns of any other embodiments may be employed. 
     The power supply method of the antenna  110  is as follows. That is, the coaxial center conductor  3  of the coaxial cable  2  is soldered to the tapered apex of the conductor  11 , and the coaxial external conductor  4  is connected to the tapered apex of the conductor  13  by a coaxial external conductor connecting wire  5 . More specifically, one end of the coaxial external conductor connecting wire  5  is soldered to the coaxial external conductor  4 , and the other end thereof is soldered to the tapered apex of the conductor  13 . 
     In the above-described first to ninth embodiments, the coaxial cable  2  is arranged along the longitudinal direction of the printed board  1  for connection, while in the present embodiment shown in  FIG. 19 , the coaxial center conductor  3  is bent such that the coaxial cable  2  is arranged in the direction substantially perpendicular to the longitudinal direction of the printed board  1 . 
       FIG. 20  shows an eleventh embodiment of the present invention as another embodiment concerning the power supply method. An antenna  111  of the present embodiment differs from the antenna  110  of  FIG. 19  in the connection configuration of the coaxial external conductor  4 . That is, in the antenna  110  of  FIG. 19 , the conductors  13  and coaxial external conductor  4  are connected to each other by the coaxial external conductor connecting wire  5 , while, in the antenna  111  of the present embodiment, the coaxial external conductor  4  is directly soldered to the tapered apex of the conductor  13  in a point contact manner. 
     As described above, in practicing the present invention, any one of the power supply methods as shown in  FIGS. 1 ,  19 , and  20  can be selected in accordance with the wiring direction of the coaxial cable  2 . 
       FIG. 21  shows a configuration of a twelfth embodiment of the present invention. In the above-mentioned embodiments, the dimension of the printed board  1  defines the outer peripheral dimension of the antenna, while in the present embodiment, an antenna  112  is formed on an area (antenna area) of a printed board  61  having a size larger than that of the printed board  1 . The printed board  61  is a dielectric substrate mounted in a radio communication device such as a USB compatible radio interface device attached to a portable information terminal. This printed board  61  is used to form a not shown communication circuit together with the antenna  112 . 
     That is, the dielectric substrate  61  has a rectangular shape, and the radiating elements are formed in the rectangular antenna area defined by a part of the longitudinal direction peripheral side of the dielectric substrate  61  and a part of the traverse direction peripheral side thereof. The longitudinal direction of the dielectric substrate  61  need not coincide with the longitudinal direction of the antenna area and, for example, they may be perpendicular to each other. 
     A radio communication device including a small-size wide band antenna and a radio communication circuit section which is formed using the printed board  61  on which the antenna is formed and electrically connected to the antenna is thus obtained. A block diagram schematically showing a configuration of such a radio communication device is shown in  FIG. 31 . 
     The antenna  112  shown in  FIG. 21  adopts the conductor pattern of the antenna  102  shown in  FIG. 3  and power supply method shown in  FIG. 19 . Any of the conductor patterns in the previously-described embodiments may be applied to the antenna to be formed on the printed board  61 . However, in the case where the short-circuit unit is provided, the conductor pattern having the through holes is preferably employed. 
       FIG. 22  shows a configuration of a thirteenth embodiment of the present invention.  FIG. 23  collectively shows conductor patterns formed on the front and rear surfaces of an antenna  113  according to the present embodiment. 
     The antenna  113  of the present embodiment has a structure obtained by forming, as the power supply unit, a micro-strip line  71  and a ground  72  on the front and rear surfaces of the printed board  1 , respectively, in place of the configuration of the antenna  112  of  FIG. 21  in which the coaxial cable  2  is connected as the power supply unit. Concretely, as shown in  FIG. 22 , the micro-strip line  71  corresponding to the coaxial center conductor  3  is connected to the conductor  31  formed on the front surface of the printed board  1 , and short-circuit between the ground  72  which corresponds to the coaxial external conductor  4  and is formed on the rear surface of the printed board  1  and conductor  13  formed on the front surface of the printed board  1  is made by the use of the through holes  73 . 
     The short-circuit configuration between the conductor  13  formed on the front surface and ground  72  formed on the rear surface is not limited to that shown in  FIGS. 22 and 23 . For example, the short-circuit between the conductor  13  and ground  72  may be achieved by soldering connection using a bar-like conductor or conducting wire. Alternatively, a configuration in which high frequency short-circuit between the conductor  13  and ground  72  is achieved by an electrostatic capacitance by forming a pattern of the ground  72  extended to below the conductor  13  may be employed. 
     Although, in the above embodiments, the conductors  15  and  16  (e.g.,  FIG. 1 ) formed on the side surfaces of the printed board  1  are used as the short-circuit unit, a configuration may be employed in which conductors for short-circuiting between the conductors  11  and  12  are formed on the upper end surface of the printed board  1 , i.e., on the traverse direction upper peripheral side of the circuit board  1 . In this case, as a conductor pattern, the conductors  31  and  32  as shown in  FIG. 5  may be used in place of the conductors  11  and  12  ( FIG. 1 ) having the rectangular part or protruding portion at the upper end portion of the printed board  1 . 
     Further, with regard to the small-size wide band antenna according to the present invention, the shape of the radiating element is not limited to that shown in the above embodiments. For example, each conductor pattern serving as the radiating element may be formed into substantially a triangle having no right angle. Further, each conductor pattern may be formed into not only a shape defined only by straight lines but also a shape including curved lines as long as it has a tapered shape including the apex at which the power supply point is set. Further, a configuration may be employed in which both of the two sides forming the tapered apex of each of the conductors serving as the ground potential section and opposite-pole potential section do not coincide with the peripheral side of the printed board. 
     &lt;Explanation of Electrical Action—1&gt; 
     Next, electrical action of the small-size wide band antenna according to the present invention will be described. A description will first be made by taking up the antenna  103  of  FIG. 5  that does not have the short-circuit unit as an example. The basic operation of the antenna  103  is based on a dipole antenna. In  FIG. 5 , the coaxial cable  2  is connected to the conductors  31  and  13  on the front surface of the printed board  1 . That is, each of the conductors  31  and  13  corresponds to a dipole element of the dipole antenna. 
     However, merely forming the conductors  31  and  13  on the front surface of the substrate is not enough to ensure absolute length as the element. Thus, the conductors  32  and  14  are formed in order to make up for the deficiency. That is, the opposite-pole potential section according to the present invention is formed using the front surface conductor  31  and rear surface conductor  32 , and ground potential section according to the present invention is formed using the front surface conductor  13  and rear surface conductor  14 . 
     Although the front surface conductor  31  and rear surface conductor  32  constituting the opposite-pole potential section are not galvanically brought into conduction, they can be regarded as being connected in a high frequency manner to each other. The connection in a high frequency manner denotes an action induced by capacitive coupling between the conductors  31  and  32 . More specifically, when a power is supplied from the coaxial cable  2 , the capacitive coupling occurs at the overlapping portion between the conductors  31  and  32  via the printed board  1 , whereby electrical connection between the conductors  31  and  32  is established. 
     Therefore, when viewing the antenna  103  as the dipole antenna, it is possible to regard the length of the radiating element connected to the coaxial center conductor  3  as one obtained by adding the lengths of the conductors  31  and  32 , and to consider that the conductors  31  and  32  are connected to each other at the upper end portion of the printed board  1  and the conductor  32  is folded to the rear side. 
     Since both the conductors  31  and  32  are formed into a tapered shape, when assuming a state where they are connected to each other on the same plane, the obtained shape is like a parallelogram. Thus, it is possible to ensure routes of various lengths as a propagation route of electricity from the tapered apex of the conductor  31  serving as the power supply point to conductor  32 . This means that various wavelengths can be distributed, that is, wide band characteristics can be obtained. 
     The electrical action in the ground potential section which is another element of the dipole antenna is the same as that obtained in the case where the above description is applied to the conductors  13  and  14 , and the description thereof is omitted here. The conductor  17  is, as described above, a stub which is formed at an appropriate position for achieving impedance matching. 
     Next, the electrical action of the present invention will be described by taking up the antenna  101  of  FIG. 1  that has the short-circuit unit as an example. The electrical action in the antenna  101  of  FIG. 1  is basically the same as that in the antenna  103  of  FIG. 5 . A difference between the antennas  101  and  103  is the presence/absence of the short-circuit unit for achieving impedance matching. That is, in the antenna  101 , the conductors  11  and  12  serving as the opposite-pole potential section each have the protruding portion, and the conductors  11  and  12  are short-circuited by the conductors  15  and  16  connected to the protruding portions. 
     Although there is such a structural difference between the antenna  101  of  FIG. 1  and antenna  103  of  FIG. 5 , they have the same configuration in that respective antenna elements are formed in a folded manner at the end portion of the printed board  1  and they are capacitively coupled through the overlapping portions obtained by the folding. It is convenient to think that the structural difference between the antennas  101  and  103  exists in the impedance matching unit, and it can be concluded that there is no difference, in principle, in the electrical action between them. 
     As described above, any of the small-size wide band antennas according to the present invention operate in the same manner in principle as the dipole antenna having dipole elements. 
     The actual dimension of the small-size wide band antenna according to the present invention will be described. The antenna dimension can be calculated using a minimum wavelength of the use frequency. For example, the traverse direction dimension of the antenna can be set to about 0.1 wavelengths, and the longitudinal direction dimension thereof can be set to about 0.2 wavelengths. In the example of  FIGS. 1 and 5 , X is set to about 0.1 wavelengths, and Y is set to about 0.2 wavelengths. 
     As described above, the antenna according to the present invention can be regarded as a structure in which each element of the dipole antenna having a wide center portion is folded. Thus, since the longitudinal length (Y) in the folded state is 0.2 wavelengths, the length of each element becomes 0.2 wavelengths in the extended state. Further, when considering the diagonal direction of the element, that is, considering that a current also flows in the diagonal line direction in the above-mentioned pseudo parallelogram, it can be considered that the entire length of each element is about 0.25 wavelengths. In view of this, it can be understood that the principle of the present invention is sufficiently practical and effective for the wide band communication. 
     In the case where the minimum value of the use frequency is, e.g., 3.1 GHz, the wavelength corresponding to the frequency is about 9.7 mm. In this case, it can be understood that when the size of 10 mm× about 20 mm can be ensured as the antenna dimension, the present invention can be practiced. Thus, the present invention can suitably be applied to a radio interface device for realizing USB connection based on the UWB technique. 
       FIG. 24  shows the actual measurement value of return loss characteristics in the configuration of  FIG. 1 . It is assumed that the printed board  1  shown in  FIG. 1  has a dimension of length (Y)×width (X)×thickness of about 20 mm×10 mm×0.8 mm. The material of the printed board  1  is an FR-4 substrate (glass-epoxy substrate). As shown in  FIG. 24 , the return loss between 3.1 GHz and 4.9 GHz is about −7.4 dB, and VSWR obtained is 2.5 or less. 
     According to the embodiments described above, it is possible to form a small-size antenna capable of meeting the requirement of wide band radio communication such as the UWB on the printed board. 
     Explanation of Configurations of Embodiments—2 
       FIG. 26  shows a configuration of a fourteenth embodiment of the present invention.  FIG. 26  collectively shows conductor patterns formed on the front and rear surfaces of an antenna  114  according to the present embodiment. The antenna  114  of the present embodiment is based on the antenna  113  ( FIGS. 22 and 23 ) having the micro-strip lines ( 71  and  72 ) as the power supply unit. 
     The antenna  114  includes a printed board  200 , in which the entire shape or at least the shape of the antenna area is formed into a rectangle, conductors  11 ,  12 ,  13  and  14  formed on the front and rear surfaces of the printed board  200  at its one end portion, and a micro-strip line  202  and a ground  201  which serve as the power supply unit. The micro-strip line  202  corresponds to a first conductor constituting a micro-strip line in the present invention, and the ground  201  corresponds to a second conductor thereof. 
     The shapes of the conductors  11  to  14  are basically the same as corresponding conductors shown in the above embodiments. However, the conductor  13  is connected to the ground  201  at its gradually-widening end portion and is substantially integrated with the ground  201 . The ground  201  is so-called a ground plate that is formed on the printed board  200  so as to supply components such as an LSI (not shown) for UWB implemented on the printed board  200  with a ground potential. In the present embodiment, the conductor  13  and ground  201  are integrated with each other so that the ground  201  is shared by the antenna  114  and implemented components. 
     As shown in  FIG. 26 , the antenna  114  has a stub  203  which is a hook-like stub conductor extending from the tapered apex of the conductor  13  formed on the front surface of the printed board  200 . The bending direction of the stub  203  is set such that the leading end of the stub faces the conductor  13  (that is, such that the leading end thereof extends in substantially parallel to the diagonal line of the conductor  13 ). The stub  203  is provided for adjusting electrical impedance of the antenna  114 , so that the arrangement and number of the stubs are not limited to those illustrated, but may be changed as needed. 
     The power supply to the antenna  114  is made by the micro-strip line  202  connected to the tapered apex of the conductor  11  via the though hole  204 . If needed, one end of the micro-strip line  202  is connected to a circuit such as the LSI for UWB implemented on the ground  201  side. 
     A radio communication device including a small-size wide band antenna and a radio communication circuit section which is formed using the printed board  200  on which the antenna is formed and electrically connected to the antenna is thus obtained. 
     &lt;Explanation of Electrical Action—2&gt; 
     The electrical action in the antenna  114  is the same in principle as that described with the antennas  101  and  103  ( FIGS. 1 and 5 ) taken as examples. Quoting the above description, the antenna  114  can be regarded as a vertical dipole antenna. Further, the conductor  13  and ground  201  are integrated with each other in the antenna  114 , so that the right end (in  FIG. 26 ) of the conductor  13  partially actions as a part of the other element of the dipole, as described in the explanation of the electrical action about the antenna of  FIG. 5 . Thus, by connecting the conductor  13  to the ground  201  so as to allow a current on the conductor  13  to freely flow into the ground  201  side, the effect of impedance matching can be enhanced. 
       FIG. 27  shows the return loss characteristics in the configuration of  FIG. 26 . It is assumed that the printed board  200  has a dimension of width×length×thickness of 10 mm×45 mm×0.8 mm. The material of the printed board  200  is an FR-4 substrate (glass-epoxy substrate). As shown in  FIG. 27 , the return loss in the user frequency band (between 3.1 GHz and 4.9 GHz) is about −11 dB, which corresponds to 1.8 or less in terms of VSWR. Such satisfactory VSWR can be obtained and therefore the power reflected by the antenna due to impedance mismatching is reduced, thereby enhancing the radiation efficiency and gain of the antenna. 
     With the configuration in which the ground plate ( 201 ) for the components such as the LSI for UWB implemented on the printed board ( 200 ) is shared by the antenna and implemented components as described above, it is possible to achieve more satisfactory VSWR characteristics, radiation efficiency, and gain. 
     Explanation of Configurations of Embodiments—3 
     As described above, the arrangement and number of the stub conductors like the stub  203  shown in  FIG. 26  are not limited to those illustrated. In the following, embodiments in which the arrangement or number of the stub conductors ( 203 ) has been modified from the configuration of the antenna  114  will be described with reference to  FIGS. 28 ,  29 , and  30 . 
       FIG. 28  shows a configuration of a fifteenth embodiment of the present invention.  FIG. 28  collectively shows conductor patterns formed on the front and rear surfaces of an antenna  115  according to the present embodiment. The above-mentioned antenna  114  ( FIG. 26 ) has the stub  203  extending from the conductor  13 , while the antenna  115  of the present embodiment has a stub  301  extending from the micro-strip line  202  on the rear surface of the printed board  200  as shown in  FIG. 28 . The stub  301  has a hook-like shape like the stub  203  and bending direction thereof is set such that the leading end of the stub faces the conductor  11  (that is, such that the leading end thereof extends in substantially parallel to the diagonal line of the conductor  11 ) via the printed board  200 . 
       FIG. 29  shows a configuration of a sixteenth embodiment of the present invention.  FIG. 29  collectively shows conductor patterns formed on the front and rear surfaces of an antenna  116  according to the present embodiment. The antenna  116  of the present embodiment has a hook-like stub  401  extending from the tapered apex of the conductor  11 , i.e., near the through hole  204  on the front surface of the printed board  200 . The bending direction of the stub  401  is set such that the leading end of the stub faces the conductor  11  (that is, such that the leading end thereof extends in substantially parallel to the diagonal line of the conductor  11 ) as shown in  FIG. 29 . 
       FIG. 30  shows a configuration of a seventeenth embodiment of the present invention.  FIG. 30  collectively shows conductor patterns formed on the front and rear surfaces of an antenna  117  according to the present embodiment. The antenna  117  of the present embodiment has the above-mentioned stub  203  ( FIG. 26 ) extending from the conductor  13  on the front surface of the printed board  200  and stub  301  ( FIG. 28 ) extending from the micro-strip line  202  on the rear surface of the printed board  200 . 
     The configuration obtained by modifying the arrangement or number of the stub conductors ( 203 ) from the antenna  114  shown in  FIG. 26  is not limited to those illustrated in  FIGS. 28 to 30  but may be changed as needed depending on the convenience of the impedance matching. 
     INDUSTRIAL APPLICABILITY 
     The small-size wide band antenna of the present invention can suitably be applied to usages requiring small-size and wide range frequency band, and suitably be used as an antenna for use in a UWB radio technique, antenna for wireless LAN, antenna for receiving terrestrial digital TV broadcasting, antenna for mobile telephone, and the like.