Patent Publication Number: US-11380997-B2

Title: Antenna

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. JP2019-196315 filed Oct. 29, 2019, the contents of which are incorporated herein in their entirety by reference. 
     BACKGROUND OF THE INVENTION 
     This invention relates to an antenna. 
     JPB6020451 (Patent Document 1) discloses a small wideband antenna 900. As shown in  FIG. 20 , the antenna 900 of Patent Document 1 has a split ring resonator  910  using a split ring  920  which is a ring-shaped conductor with a split portion  922 . Specifically, the antenna 900 of Patent Document 1 has a main portion  930  and a feeding portion  940 . The main portion  930  forms the split ring  920 . The feeding portion  940  is provided on the main portion  930 . 
     The antenna 900 of Patent Document 1 works at a resonance frequency of the split ring resonator  910 . In other words, the antenna 900 of Patent Document 1 can resonate at one operating frequency but cannot function over multiband. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an antenna having a structure which can resonate at a plurality of operating frequencies. 
     One aspect of the present invention provides an antenna comprising a split ring resonator. The antenna has a main portion, a feeding portion and at least one radiation element. The main portion forms a split ring. The feeding portion is provided on the main portion. The radiation element extends from the main portion. 
     The antenna of the present invention has at least one radiation element which extends from the main portion forming the split ring. Accordingly, the antenna of the present invention can resonate at both of operating frequencies of the split ring resonator and the radiation element. In other words, the antenna of the present invention has a structure which can resonate at a plurality of operating frequencies. 
     An appreciation of the objectives of the present invention and a more complete understanding of its structure may be had by studying the following description of the preferred embodiment and by referring to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing an antenna device according to an embodiment of the present invention. In the figure, an antenna is mounted on a circuit board. 
         FIG. 2  is a top view showing the antenna device of  FIG. 1 . 
         FIG. 3  is a front view showing the antenna device of  FIG. 1 . 
         FIG. 4  is a rear view showing the antenna device of  FIG. 1 . 
         FIG. 5  is a side view showing the antenna device of  FIG. 1 . 
         FIG. 6  is another side view showing the antenna device of  FIG. 1 . 
         FIG. 7  is an upper, perspective view showing the antenna which is included in the antenna device of  FIG. 1 . 
         FIG. 8  is a lower, perspective view showing the antenna of  FIG. 7 . 
         FIG. 9  is a top view showing the antenna of  FIG. 7 . 
         FIG. 10  is a bottom view showing the antenna of  FIG. 7 . 
         FIG. 11  is a front view showing the antenna of  FIG. 7 . 
         FIG. 12  is a rear view showing the antenna of  FIG. 7 . 
         FIG. 13  is a side view showing the antenna of  FIG. 7 . 
         FIG. 14  is another side view showing the antenna of  FIG. 7 . 
         FIG. 15  is a top view showing a modification of the antenna of  FIG. 7 . In the figure, the modification is schematically depicted. 
         FIG. 16  is a top view showing another modification of the antenna of  FIG. 7 . In the figure, the modification is schematically depicted. 
         FIG. 17  is a top view showing yet another modification of the antenna of  FIG. 7 . 
       In the figure, the modification is schematically depicted. 
         FIG. 18  is a top view showing still another modification of the antenna of  FIG. 7 . In the figure, the modification is schematically depicted. 
         FIG. 19  is a top view showing still yet another modification of the antenna of  FIG. 7 . In the figure, the modification is schematically depicted. 
         FIG. 20  is a top view showing an antenna of Patent Document 1. 
     
    
    
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. 
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     As shown in  FIG. 1 , an antenna device  10  according to an embodiment of the present invention comprises a circuit board  600  and an antenna  100 . 
     As shown in  FIG. 2 , the circuit board  600  of the present embodiment is formed with a feeding line  610  and a ground plane  620 . Specifically, the feeding line  610  is electrically connected with the antenna  100 . 
     As shown in  FIG. 1 , the antenna  100  of the present embodiment is formed of metal body  110  which is mounted on the circuit board  600  when used. In other words, the antenna  100  is a discrete member which is mounted on the circuit board  600  when used. However, the present invention is not limited thereto. The antenna  100  of the present invention may be formed of a plurality of conductive layers and vias which are included in a multilayer wiring substrate. Alternatively, the antenna of the present invention may be formed by another method, such as plating metal films on a resin body or sticking metal bodies on a resin body. The antenna  100  has a split ring resonator  200 . The antenna  100  has a plurality of operating frequencies. The antenna  100  has a split ring resonator structure which is made of metal plate. In other words, the antenna  100  of the present embodiment is a resonant antenna. 
     As shown in  FIG. 7 , the antenna  100  has a main portion  220 , a feeding portion  260 , a radiation element  300 , a first facing portion  432  and a second facing portion  436 . The main portion  220  forms a split ring  210 . However, the present invention is not limited thereto. The antenna  100  may be modified, provided that the antenna  100  has the main portion  220  forming the split ring  210 , the feeding portion  260  and one or more of the radiation elements  300 . 
     Referring  FIG. 7 , the main portion  220  of the present embodiment constitutes an inductance of the antenna  100 . The main portion  220  has a ring shape with a split portion  212 . The wording “ring shape” as used herein includes not only a substantially rectangular ring shape as the present embodiment and a circular shape but also an elliptical annular shape and a polygonal annular shape. 
     As shown in  FIG. 7 , the main portion  220  has a first portion  230 , a second portion  240 , a third portion  250 , a fourth portion  270 , a fifth portion  280 , a first end portion  222 , a second end portion  226 , two grounding portions  292 ,  296  and a fixed portion  294 . 
     As shown in  FIGS. 9 and 10 , the first portion  230  of the present embodiment has a flat-plate shape perpendicular to an up-down direction. In the present embodiment, the up-down direction is a Z-direction. Specifically, upward is a positive Z-direction while downward is a negative Z-direction. The first portion  230  extends in a right-left direction. The first portion  230  defines a right end of the main portion  220  in the right-left direction. In the present embodiment, the right-left direction is a Y-direction. Specifically, rightward is a negative Y-direction while leftward is a positive Y-direction. 
     As shown in  FIGS. 9 and 10 , the second portion  240  of the present embodiment has a flat-plate shape perpendicular to the up-down direction. The second portion  240  extends rearward in a front-rear direction from a rear end of the first portion  230 . In the present embodiment, the front-rear direction is an X-direction. Specifically, forward is a positive X-direction while rearward is a negative X-direction. 
     As shown in  FIGS. 9 and 10 , the third portion  250  of the present embodiment has a flat-plate shape perpendicular to the up-down direction. The third portion  250  extends leftward in the right-left direction from a rear end of the second portion  240 . The third portion  250  defines a rear end of the main portion  220  in the front-rear direction. The third portion  250  is positioned rearward of the first portion  230  in the front-rear direction. 
     As shown in  FIGS. 9 and 10 , the fourth portion  270  of the present embodiment has a flat-plate shape perpendicular to the up-down direction. The fourth portion  270  extends forward in the front-rear direction from a front end of the third portion  250 . The fourth portion  270  defines a left end of the main portion  220  in the right-left direction. The fourth portion  270  is positioned leftward of the second portion  240  in the right-left direction. 
     As shown in  FIGS. 7 and 8 , the fifth portion  280  of the present embodiment has an upper portion  282 , a middle portion  284  and a lower portion  286 . 
     As shown in  FIGS. 7 and 8 , the upper portion  282  of the present embodiment has a flat-plate shape perpendicular to the up-down direction. The upper portion  282  extends rightward in the right-left direction from a front end of the fourth portion  270 . As shown in  FIG. 9 , the upper portion  282  is positioned forward of the first portion  230  in the front-rear direction. 
     As shown in  FIG. 8 , the middle portion  284  of the present embodiment has a flat-plate shape perpendicular to the front-rear direction. The middle portion  284  extends downward in the up-down direction from a lower end of the upper portion  282 . The middle portion  284  is positioned forward of the radiation element  300  in the front-rear direction. The middle portion  284  defines a front end of the main portion  220  in the front-rear direction. 
     As shown in  FIG. 8 , the lower portion  286  of the present embodiment has a flat-plate shape perpendicular to the up-down direction. The lower portion  286  extends rearward in the front-rear direction from a lower end of the middle portion  284  and then extends rightward in the right-left direction. As shown in  FIG. 10 , the lower portion  286  has a substantially L-shape when the metal body  110  is viewed from below. 
     As shown in  FIG. 9 , the first end portion  222  of the present embodiment is provided on the first portion  230  of the main portion  220 . The first end portion  222  is positioned rightward of the radiation element  300  in the right-left direction. The first end portion  222  is positioned rearward of the radiation element  300  in the front-rear direction. 
     As shown in  FIG. 10 , the second end portion  226  of the present embodiment is provided on the lower portion  286  of the fifth portion  280  of the main portion  220 . The second end portion  226  is positioned at a right end of the lower portion  286  of the fifth portion  280  of the main portion  220  in the right-left direction. The second end portion  226  is positioned rearward of the radiation element  300  in the front-rear direction. The second end portion  226  is positioned at a position same as a position of the first end portion  222  in the front-rear direction. As shown in  FIG. 11 , the second end portion  226  is positioned below the first end portion  222  in the up-down direction. 
     Referring to  FIGS. 7 and 8 , the first end portion  222  and the second end portion  226  form the split portion  212  of the split ring  210 . In other words, the main portion  220  has the first end portion  222  and the second end portion  226  which form the split portion  212  of the split ring  210 . 
     As shown in  FIG. 11 , the split portion  212  of the present embodiment is a space which extends in a plane perpendicular to the up-down direction. The split portion  212  is positioned between the first end portion  222  and the second end portion  226  in the up-down direction. The split portion  212  is sandwiched between the first end portion  222  and the second end portion  226  in the up-down direction. In the up-down direction, the split portion  212  is positioned below the first end portion  222  and above the second end portion  226 . As shown in  FIG. 7 , the split portion  212  is positioned between the first facing portion  432  and the second facing portion  436  in the up-down direction. The split portion  212  is sandwiched between the first facing portion  432  and the second facing portion  436  in the up-down direction. In the up-down direction, the split portion  212  is positioned below the first facing portion  432  and above the second facing portion  436 . The split portion  212  is positioned between the second portion  240  and the fourth portion  270  in the right-left direction. The split portion  212  is positioned between the second portion  240  and the fifth portion  280  in the right-left direction. As understood from the  FIG. 8 , the split portion  212  is positioned between the second portion  240  and the lower portion  286  of the fifth portion  280  in the right-left direction. The split portion  212  is positioned below any of the first portion  230 , the second portion  240  and the third portion  250  and the fourth portion  270  in the up-down. The split portion  212  is positioned below the upper portion  282  of the fifth portion  280  in the up-down direction. The split portion  212  is positioned above the lower portion  286  of the fifth portion  280  in the up-down direction. 
     As shown in  FIG. 8 , the grounding portion  292  of the present embodiment is provided on the first portion  230  of the main portion  220  and the grounding portion  296  of the present embodiment is provided on the fourth portion  270  of the main portion  220 . In detail, each of the grounding portions  292 ,  296  has a rectangular plate-like shape. Each of the grounding portions  292 ,  926  are positioned at opposite ends, respectively, of the main portion  220  in the right-left direction. The grounding portion  292  is provided at a front end of a side edge of the first portion  230 . The grounding portion  296  is provided in the vicinity of a front end of a side edge of the fourth portion  270 . The grounding portion  292  extends downward from the first portion  230 . The grounding portion  296  extends downward from the fourth portion  270 . As shown in  FIG. 4 , the grounding portions  292 ,  296  are electrically connected with the ground plane  620  formed on the circuit board  600  when the antenna  100  is mounted on the circuit board  600 . 
     As shown in  FIG. 8 , the fixed portion  294  of the present embodiment is provided on the third portion  250  of the main portion  220 . In detail, the fixed portion  294  extends downward from a middle in the right-left direction of a rear edge of the third portion  250 . As shown in  FIG. 4 , when the antenna  100  is mounted on the circuit board  600 , the fixed portion  294  is fixed on the circuit board  600  and supports the main portion  220 . The fixed portion  294  may be electrically connected with the ground plane  620  but instead may not be connected with the ground plane  620 . Although the number of the fixed portion  294  of the present embodiment is one, the main portion  220  may have two or more of the fixed portions  294 . 
     As shown in  FIG. 2 , the feeding portion  260  of the present embodiment is electrically connected with the feeding line  610  of the circuit board  600  when the antenna  100  is mounted on the circuit board  600 . Here, an electrical connecting method between the feeding portion  260  and the feeding line  610  is not particularly limited. For example, the feeding portion  260  may be directly connected to the feeding line  610  by soldering or the like. Alternatively, the feeding portion  260  may be located near a part of the feeding line  610  with an interval left therebetween to be connected capacitively or electromagnetically. At any rate, the feeding portion  260  and the feeding line  610  should be electrically connected to each other so that the feeding portion  260  is supplied with electric power from the feeding line  610 . As shown in  FIG. 8 , the feeding portion  260  is provided on the main portion  220 . More specifically, the feeding portion  260  extends downward from the lower portion  286  of the fifth portion  280  of the main portion  220 . The feeding portion  260  is provided with a fixed portion  262  which is configured to be fixed to the feeding line  610  of the circuit board  600  as shown in  FIG. 2 . The fixed portion  262  of the present embodiment is a lower end of the feeding portion  260 . 
     As shown in  FIG. 8 , the radiation element  300  of the present embodiment extends from the main portion  220 . The radiation element  300  is formed integrally with other parts of the antenna  100 . However, the present invention is not limited thereto. The radiation element  300  may be distinct and separated from the other parts of the antenna  100 . The radiation element  300  forms a so-called inverted L-shape antenna. An electrical length of the radiation element  300  is defined with reference to one fourth of a wavelength of one of the operating frequencies of the antenna  100 . In other words, the radiation element  300  corresponds to one fourth of a wavelength of any one of the operating frequencies of the antenna  100 . 
     As shown in  FIG. 8 , the radiation element  300  has an extending portion  310  and a coupling portion  330 . 
     As shown in  FIG. 8 , the extending portion  310  of the present embodiment has a flat-plate shape perpendicular to the up-down direction. As shown in  FIG. 9 , the extending portion  310  extends in the right-left direction perpendicular to the up-down direction. The extending portion  310  is positioned away from the main portion  220  and extends along the main portion  220 . However, the present invention is not limited thereto. The extending portion  310  may be modified, provided that the extending portion  310  is positioned away from the main portion  220  while partially extending along the main portion  220 . The extending portion  310  and the lower portion  286  of the fifth portion  280  are positioned on a common plane perpendicular to the up-down direction. The extending portion  310  and a part of the lower portion  286  of the fifth portion  280  are arranged parallel to each other with an interval left therebetween. Thus, the radiation element  300  resonates with the split ring resonator  200  and enhances the function of the antenna  100 . 
     As shown in  FIG. 3 , the extending portion  310  is provided with a fixed portion  312  which is configured to be fixed on the circuit board  600 . 
     As shown in  FIG. 4 , the fixed portion  312  of the present embodiment is fixed on the circuit board  600  when the antenna  100  is mounted on the circuit board  600 . But, the fixed portion  312  is not connected with a conductive portion which is included in the circuit board  600 . In other words, the fixed portion  312  mechanically supports the radiation element  300 . The fixed portion  312  extends downward in the up-down direction. The fixed portion  312  is positioned at a right end of the extending portion  310  in the right-left direction. However, the present invention is not limited thereto. An arrangement of the fixed portion  312  may be modified accordingly. 
     As shown in  FIG. 11 , the coupling portion  330  of the present embodiment extends in the up-down direction perpendicular to both the front-rear direction and the right-left direction. The coupling portion  330  couples the extending portion  310  and the main portion  220  with each other. More specifically, the coupling portion  330  couples the extending portion  310  and the fifth portion  280  of the main portion  220  with each other. A direction in which the extending portion  310  extends intersects with a direction in which the coupling portion  330  extends. More specifically, the direction in which the extending portion  310  extends is perpendicular to the direction in which the coupling portion  330  extends. 
     As shown in  FIG. 7 , the first facing portion  432  of the present embodiment extends from the first end portion  222 . However, the present invention is not limited thereto. The first facing portion  432  may be modified, provided that the first facing portion  432  is provided on the first end portion  222  or extends from the first end portion  222 . The first facing portion  432  forms an open stub  410  in part. An electrical length of the first facing portion  432  defines an electrical length, or a predetermined electrical length, of the open stub  410 . The first facing portion  432  has a meander portion  433  and an extension portion  434 . 
     As shown in  FIG. 9 , the meander portion  433  of the present embodiment extends leftward in the right-left direction from the first end portion  222 . The meander portion  433  has a meandering shape when viewed along the up-down direction. The meander portion  433  is positioned between the first portion  230  and the third portion  250  in the front-rear direction. More specifically, in the front-rear direction, the meander portion  433  is positioned rearward of the first portion  230  and forward of the third portion  250 . The meander portion  433  is positioned between the fifth portion  280  and the third portion  250  in the front-rear direction. The meander portion  433  is positioned rearward of the fifth portion  280  in the front-rear direction. The meander portion  433  is positioned between the second portion  240  and the fourth portion  270  in the right-left direction. More specifically, in the right-left direction, the meander portion  433  is positioned leftward of the second portion  240  and rightward of the fourth portion  270 . The meander portion  433  is positioned rearward of the radiation element  300  in the front-rear direction. As shown in  FIG. 7 , the meander portion  433  is positioned above the lower portion  286  of the fifth portion  280  in the up-down direction. The meander portion  433  is positioned above the feeding portion  260  in the up-down direction. The meander portion  433  is positioned above the radiation element  300  in the up-down direction. The meander portion  433  is positioned rightward of the coupling portion  330  of the radiation element  300  in the right-left direction. As shown in  FIG. 10 , the meander portion  433  is positioned rightward of the feeding portion  260  in the right-left direction. 
     As shown in  FIG. 9 , the extension portion  434  of the resent embodiment extends leftward in the right-left direction from the meander portion  433 . The extension portion  434  is positioned between the first portion  230  and the third portion  250  in the front-rear direction. More specifically, in the front-rear direction, the extension portion  434  is positioned rearward of the first portion  230  and forward of the third portion  250 . The extension portion  434  is positioned between the fifth portion  280  and the third portion  250  in the front-rear direction. The extension portion  434  is positioned rearward of the fifth portion  280  in the front-rear direction. The extension portion  434  is positioned between the second portion  240  and the fourth portion  270  in the right-left direction. More specifically, in the right-left direction, the extension portion  434  is positioned leftward of the second portion  240  and rightward of the fourth portion  270 . The extension portion  434  is positioned rearward of the radiation element  300  in the front-rear direction. 
     As shown in  FIG. 9 , the main portion  220  is arranged to be partly parallel to the first facing portion  432 . More specifically, each of a part of the lower portion  286  of the fifth portion  280 , the fourth portion  270  and the third portion  250  of the main portion  220  is arranged to be partly parallel to a part of the extension portion  434  of the first facing portion  432 . Thus, the main portion  220  forms the open stub  410  in part. 
     As shown in  FIG. 10 , the extension portion  434  of the present embodiment has an extension main portion  438  and a fixed portion  437 . 
     As shown in  FIG. 9 , the extension main portion  438  of the present embodiment extends leftward from the meander portion  433 , and is bent to extend rearward, and is further bent to extends rightward. As shown in  FIG. 7 , the extension main portion  438  is positioned above the lower portion  286  of the fifth portion  280  in the up-down direction. The extension main portion  438  is positioned above the feeding portion  260  in the up-down direction. The extension main portion  438  is positioned rightward of the coupling portion  330  of the radiation element  300  in the right-left direction. The extension main portion  438  has an end  435 . 
     As shown in  FIG. 9 , the end  435  of the present embodiment is a free end. Specifically, the end  435  is not short-circuited with the second facing portion  436 . The end  435  is positioned between the first portion  230  and the third portion  250  in the front-rear direction. More specifically, in the front-rear direction, the end  435  is positioned rearward of the first portion  230  and forward of the third portion  250 . The end  435  is positioned between the fifth portion  280  and the third portion  250  in the front-rear direction. The end  435  is positioned rearward of the fifth portion  280  in the front-rear direction. The end  435  is positioned between the second portion  240  and the fourth portion  270  in the right-left direction. More specifically, in the right-left direction, the end  435  is positioned leftward of the second portion  240  and the rightward of the fourth portion  270 . As shown in  FIG. 7 , the end  435  is positioned above the feeding portion  260  in the up-down direction. The end  435  is positioned rightward of the coupling portion  330  of the radiation element  300  in the right-left direction. As shown in  FIG. 10 , the end  435  is positioned rearward of the feeding portion  260  in the front-rear direction. The end  435  is positioned leftward of the feeding portion  260  in the right-left direction. The end  435  is positioned rearward of the radiation element  300  in the front-rear direction. 
     As understood from  FIGS. 7 and 9 , the first portion  230 , the second portion  240 , the third portion  250 , the fourth portion  270 , the upper portion  282  of the fifth portion  280 , the meander portion  433  of the first facing portion  432  and the extension main portion  438  of the first facing portion  432  are positioned on a common plane perpendicular to the up-down direction. 
     As shown in  FIG. 4 , the fixed portion  437  of the present embodiment is fixed on the circuit board  600  when the antenna  100  is mounted on the circuit board  600 . The fixed portion  437  prevents a deformation of the first facing portion  432 . The fixed portion  437  is not connected with the conductive portion which is included in the circuit board  600  having the ground plane  620 . As shown in  FIG. 8 , the fixed portion  437  extends rearward from the extension main portion  438  and then extends downward. As shown in  FIG. 9 , the fixed portion  437  is positioned between the radiation element  300  and the third portion  250  in the front-rear direction. The fixed portion  437  is positioned between the first portion  230  and the third portion  250  in the front-rear direction. The fixed portion  437  is positioned between the second portion  240  and the fourth portion  270  in the right-left direction. The fixed portion  437  is positioned between the meander portion  433  and the end  435  in the right-left direction. The fixed portion  437  is positioned between the first portion  230  and the end  435  in the front-rear direction. As shown in  FIG. 10 , the fixed portion  437  is positioned between the feeding portion  260  and the fourth portion  270  in the right-left direction. The fixed portion  437  is positioned between the feeding portion  260  and the end  435  in the right-left direction. However, the present invention is not limited thereto. An arrangement of the fixed portion  437  may be modified accordingly. 
     As shown in  FIGS. 11 and 12 , lower ends of the grounding portions  292 ,  296 , a lower end of the fixed portion  294 , the fixed portion  262  of the feeding portion  260  and a lower end of the fixed portion  437  of the extension portion  434  are positioned at positions same as each other in the up-down direction. 
     As shown in  FIG. 8 , the second facing portion  436  of the present embodiment has a flat-plate shape perpendicular to the up-down direction. The second facing portion  436  extends from the second end portion  226 . However, the present invention is not limited thereto. The second facing portion  436  may be modified, provided that the second facing portion  436  is provided on the second end portion  226  or extends from the second end portion  226 . As shown in  FIG. 11 , the first facing portion  432  and the second facing portion  436  are spaced away from each other and face each other. More specifically, in the up-down direction, the first facing portion  432  and the second facing portion  436  are spaced away from each other and face each other. The second facing portion  436  is positioned below the first facing portion  432  in the up-down direction. As understood from  FIGS. 9 and 10 , the first facing portion  432  and the second facing portion  436  partly overlap with each other when the antenna  100  is viewed along the up-down direction. More specifically, the second facing portion  436  partly overlaps with the meander portion  433  of the first facing portion  432  when the antenna  100  is viewed along the up-down direction. The second facing portion  436  forms the open stub  410  in part. 
     As shown in  FIG. 12 , the lower portion  286  of the fifth portion  280  and the second facing portion  436  are positioned on a common plane perpendicular to the up-down direction. 
     The first facing portion  432 , the second facing portion  436 , the main portion  220  and the radiation element  300  of the present embodiment are formed from a single metal plate and are integrally formed with each other. However, the present invention is not limited thereto. The antenna  100  may be formed from a plurality of conductive members. 
     As shown in  FIG. 8 , the second facing portion  436  is provided with no fixed portion. The second facing portion  436  may, however, be provided with one of more fixed portions as with the first facing portion  432 . The fixed portion, which is provided to the second facing portion  436 , should not be connected with the conductive portion included in the circuit board  600 . 
     Referring to  FIGS. 7 and 8 , the first facing portion  432  and the second facing portion  436  of the present embodiment constitute a capacitor  400 . Since the main portion  220  constitutes the inductance of the antenna  100  as described above, the first facing portion  432 , the second facing portion  436  and the main portion  220  form an LC resonator circuit. An operating frequency of the LC resonator circuit is different from an operating frequency of the radiation element  300 . 
     Referring to  FIGS. 7 and 8 , the first facing portion  432  and the second facing portion  436  form the open stub  410 . More specifically, the first facing portion  432  and the second facing portion  436  form the open stub  410  in part. The first facing portion  432  and the second facing portion  436  form the open stub  410  at not only their parts identical with each other when seen along the up-down direction but also other parts of them. In other words, the first facing portion  432  and the second facing portion  436  form the stub by arranging them near each other. As described above, the main portion  220  forms the open stub  410  in part. Thus, in the antenna  100  of the present embodiment, the open stub  410  is formed by using not only the first facing portion  432  and the second facing portion  436  but also a part of the main portion  220 . However, the present embodiment is not limited thereto. The antenna  100  may have a short stub which is formed by short-circuiting the end  435  of the first facing portion  432  and the second facing portion  436  to each other. In other words, the first facing portion  432  and the second facing portion  436  may form the open stub  410  or short stub. In the case of the open stub, the electrical length of the open stub  410 , or the predetermined electrical length, must be equal to or longer than a half of a wavelength corresponding to one of the operating frequencies, wherein the half of the wavelength is 0.5λ. On the other hand, in the case of the short stub, an electrical length of the short stub, or a predetermined electrical length, must be equal to or longer than three fourths of a wavelength corresponding to one of the operating frequencies, wherein the three fourths of the wavelength is 0.75λ. Since any of the open stub  410  and the short stub has the predetermined electrical length as described above, the antenna  100  can have the plurality of operating frequencies. 
     As described above, the antenna  100  of the present embodiment has the single radiation element  300  extending from the main portion  220  which forms the split ring  210 . Thus, the antenna  100  can resonate at both of the operating frequencies of the split ring resonator  200  and the radiation element  300 . In other words, the antenna  100  of the present embodiment has a structure which can resonate at the plurality of operating frequencies. 
     More specifically, the antenna  100  of the present embodiment has the structure which can resonate at three operating frequencies, namely, the operating frequency of the LC resonator circuit which is formed by the first facing portion  432 , the second facing portion  436  and the main portion  220 , an operating frequency corresponding to the electrical length, or the predetermined electrical length, of the open stub  410  and the operating frequency of the radiation element  300 . 
     Where the present embodiment of the present invention is described above, the present embodiment may be modified as follows. 
     First Modification 
     As shown in  FIG. 15 , an antenna  100 A of a first modification is formed of metal body  110 A which is mounted on a circuit board (not shown) when used. However, the present invention is not limited thereto. The antenna  100 A may be formed of traces which are printed on a circuit board. 
     As shown in  FIG. 15 , the antenna  100 A of the present modification has a split ring resonator  200 A. The antenna  100 A has a plurality of operating frequencies. The antenna  100 A has a split ring resonator structure. In other words, the antenna  100 A is a resonant antenna. 
     As shown in  FIG. 15 , the antenna  100 A of the present modification has a main portion  220 A, a feeding portion  260 A, a radiation element  300 A, a first facing portion  432 A and a second facing portion  436 A. The main portion  220 A forms a split ring  210 A. 
     Referring  FIG. 15 , the main portion  220 A of the present modification constitutes an inductance of the antenna  100 A. As shown in  FIG. 15 , the main portion  220 A has a ring shape with a split portion  212 A. More specifically, the main portion  220 A has a substantially rectangular ring shape with four sides. The wording “ring shape” as used herein includes not only a substantially rectangular ring shape as the present modification and a circular shape but also an elliptical annular shape and a polygonal annular shape. 
     As shown in  FIG. 15 , the main portion  220 A of the present modification has a first portion  230 A, a second portion  240 A, a third portion  250 A, a fourth portion  270 A, a fifth portion  280 A, a first end portion  222 A and a second end portion  226 A. 
     As shown in  FIG. 15 , the first portion  230 A of the present modification extends in the right-left direction. The first portion  230 A defines a front end of the main portion  220 A in the front-rear direction. 
     As shown in  FIG. 15 , the second portion  240 A of the present modification extends rearward in the front-rear direction from a rear end of the first portion  230 A. The second portion  240 A defines a right end of the main portion  220 A in the right-left direction. 
     As shown in  FIG. 15 , the third portion  250 A of the present modification extends leftward in the right-left direction from a rear end of the second portion  240 A. The third portion  250 A defines a rear end of the main portion  220 A in the front-rear direction. The third portion  250 A is positioned rearward of the first portion  230 A in the front-rear direction. 
     As shown in  FIG. 15 , the fourth portion  270 A of the present modification extends forward in the front-rear direction from a front end of the third portion  250 A. The fourth portion  270 A defines a left end of the main portion  220 A in the right-left direction. The fourth portion  270 A is positioned leftward of the second portion  240 A in the right-left direction. 
     Referring to  FIG. 15 , any part of the second portion  240 A, the third portion  250 A and the fourth portion  270 A functions as a ground connecting point to be electrically connected with a ground plane (not shown) of the circuit board. 
     As shown in  FIG. 15 , the fifth portion  280 A of the present modification extends rightward in the right-left direction from a front end of the fourth portion  270 A. The fifth portion  280 A defines the front end of the main portion  220 A. 
     As shown in  FIG. 15 , the first end portion  222 A of the present modification is provided on the first portion  230 A of the main portion  220 A. 
     As shown in  FIG. 15 , the second end portion  226 A of the present modification is provided on the fifth portion  280 A of the main portion  220 A. 
     As shown in  FIG. 15 , the first end portion  222 A and the second end portion  226 A form the split portion  212 A of the split ring  210 A. In other words, the main portion  220 A has the first end portion  222 A and the second end portion  226 A which form the split portion  212 A of the split ring  210 A. 
     As shown in  FIG. 15 , the split portion  212 A of the present modification is a space which extends in the front-rear direction. The split portion  212 A is positioned between the first end portion  222 A and the second end portion  226 A in the right-left direction. The split portion  212 A is sandwiched between the first end portion  222 A and the second end portion  226 A in the right-left direction. The split portion  212 A is positioned between the first facing portion  432 A and the second facing portion  436 A in the right-left direction. The split portion  212 A is sandwiched between the first facing portion  432 A and the second facing portion  436 A in the right-left direction. 
     As shown in  FIG. 15 , the feeding portion  260 A is provided on the fifth portion  280 A of the main portion  220 A. 
     As shown in  FIG. 15 , the radiation element  300 A of the present modification extends from the main portion  220 A. In detail, dissimilar to the radiation element  300  of the aforementioned embodiment, the radiation element  300 A extends forward from the fifth portion  280 A of the main portion  220 A. The radiation element  300 A and the main portion  220 A are positioned on a common plane perpendicular to the up-down direction. The radiation element  300 A corresponds to one fourth of a wavelength of any one of the operating frequencies of the antenna  100 A. 
     As shown in  FIG. 15 , the first facing portion  432 A of the present modification is provided on the first end portion  222 A. The first facing portion  432 A extends rearward in the front-rear direction from the first end portion  222 A. 
     As shown in  FIG. 15 , the second facing portion  436 A of the present modification is provided on the second end portion  226 A. The second facing portion  436 A extends rearward in the front-rear direction from the second end portion  226 A. The first facing portion  432 A and the second facing portion  436 A are spaced away from each other and face each other. More specifically, in the right-left direction, the first facing portion  432 A and the second facing portion  436 A are spaced away from each other and face each other. 
     Dissimilar to the split ring  210  of the aforementioned embodiment, the split ring  210 A of the present modification is configured so that the main portion  220 A extends in a plane perpendicular to the up-down direction. Specifically, the first portion  230 A, the second portion  240 A, the third portion  250 A, the fourth portion  270 A, the fifth portion  280 A, the split portion  212 A, the first end portion  222 A and the second end portion  226 A, which are components of the main portion  220 A, are positioned on the common plane perpendicular to the up-down direction. The main portion  220 A, the first facing portion  432 A and the second facing portion  436 A are positioned on the common plane perpendicular to the up-down direction. 
     Referring to  FIG. 15 , the first facing portion  432 A and the second facing portion  436 A of the present modification constitute a capacitor  400 A. Since the main portion  220 A constitutes the inductance of the antenna  100 A as described above, the first facing portion  432 A, the second facing portion  436 A and the main portion  220 A form an LC resonator circuit. An operating frequency of the LC resonator circuit is different from an operating frequency of the radiation element  300 A. 
     As described above, the antenna  100 A of the present modification has the single radiation element  300 A extending from the main portion  220 A which forms the split ring  210 A. Thus, the antenna  100 A of the present embodiment can resonate at both of the operating frequencies of the split ring resonator  200 A and the radiation element  300 A. In other words, the antenna  100 A of the present modification has a structure which can resonate at the plurality of operating frequencies. 
     Second Modification 
     As shown in  FIG. 16 , an antenna  100 B of a second modification is formed of metal body  110 B which is mounted on a circuit board (not shown) when used. However, the present invention is not limited thereto. The antenna  100 B may be formed from traces which are printed on a circuit board. 
     As shown in  FIG. 16 , the antenna  100 B of the present modification has a split ring resonator  200 B. The antenna  100 B has a plurality of operating frequencies. The antenna  100 B has a split ring resonator structure. In other words, the antenna  100 B is a resonant antenna. 
     As shown in  FIG. 16 , the antenna  100 B of the present modification has a main portion  220 B, a feeding portion  260 B, a radiation element  300 B, a first facing portion  432 B and a second facing portion  436 B. The main portion  220 B forms a split ring  210 B. The main portion  220 B constitutes an inductance of the antenna  100 B. The main portion  220 B has a first portion  230 B, a second portion  240 B, a third portion  250 B, a fourth portion  270 B, a fifth portion  280 B, a first end portion  222 B and a second end portion  226 B. Any part of the second portion  240 B, the third portion  250 B and the fourth portion  270 B functions as a ground connecting point to be electrically connected with a ground plane (not shown) of the circuit board. The first end portion  222 B and the second end portion  226 B form a split portion  212 B of the split ring  210 B. Components of the antenna  100 B other than the radiation element  300 B have structures same as those of the first modification. Accordingly, detailed explanation thereabout is omitted. 
     As shown in  FIG. 16 , the radiation element  300 B of the present modification extends from the main portion  220 B. Specifically, dissimilar to the radiation element  300 A of the first modification, the radiation element  300 B extends forward from the fifth portion  280 B, which is provided with the feeding portion  260 B, and is then bent to extend rightward. However, the present invention is not limited thereto. The radiation element  300 B may be modified as follows: the radiation element  300 B extends forward from the fifth portion  280 B, which is provided with the feeding portion  260 B, and is then bent to extend leftward. However, the antenna  100 B with the original radiation element  300 B can, as a whole, have a reduced size as compared with an antenna  100 B with the modified radiation element  300 B. Thus, the original radiation element  300 B is preferred. The radiation element  300 B and the main portion  220 B are positioned on a common plane perpendicular to the up-down direction. The radiation element  300 B corresponds to one fourth of a wavelength of any one of the operating frequencies of the antenna  100 B. 
     Referring to  FIG. 16 , the first facing portion  432 B and the second facing portion  436 B of the present modification constitute a capacitor  400 B. Since the main portion  220 B constitutes the inductance of the antenna  100 B as described above, the first facing portion  432 B, the second facing portion  436 B and the main portion  220 B form an LC resonator circuit. An operating frequency of the LC resonator circuit is different from an operating frequency of the radiation element  300 B. 
     As described above, the antenna  100 B of the present modification has the single radiation element  300 B extending from the main portion  220 B which forms the split ring  210 B. Thus, the antenna  100 B of the present modification can resonate at both of the operating frequencies of the split ring resonator  200 B and the radiation element  300 B. In other words, the antenna  100 B of the present modification has a structure which can resonate at the plurality of operating frequencies. 
     Third Modification 
     As shown in  FIG. 17 , an antenna  100 C of a third modification is formed of metal body  110 C which is mounted on a circuit board (not shown) when used. However, the present invention is not limited thereto. The antenna  100 C may be formed of traces which are printed on a circuit board. 
     As shown in  FIG. 17 , the antenna  100 C of the present modification has a split ring resonator  200 C. The antenna  100 C has a plurality of operating frequencies. The antenna  100 C has a split ring resonator structure. In other words, the antenna  100 C is a resonant antenna. 
     As shown in  FIG. 17 , the antenna  100 C of the present modification has a main portion  220 C, a feeding portion  260 C, a radiation element  300 C, a first facing portion  432 C and a second facing portion  436 C. The main portion  220 C forms a split ring  210 C. The main portion  220 C constitutes an inductance of the antenna  100 C. The main portion  220 C has a first portion  230 C, a second portion  240 C, a third portion  250 C, a fourth portion  270 C, a fifth portion  280 C, a first end portion  222 C and a second end portion  226 C. Any part of the second portion  240 C, the third portion  250 C and the fourth portion  270 C functions as a ground connecting point to be electrically connected with a ground plane (not shown) of the circuit board. The first end portion  222 C and the second end portion  226 C form a split portion  212 C of the split ring  210 C. Components of the antenna  100 C other than the radiation element  300 C have structures same as those of the first modification. Accordingly, detailed explanation thereabout is omitted. 
     As shown in  FIG. 17 , the radiation element  300 C of the present modification extends from the main portion  220 C. In detail, dissimilar to the radiation element  300 A of the first modification, the radiation element  300 C extends forward from the first portion  230 C, which is not provided with the feeding portion  260 C, and is then bent to extend leftward. As understood from comparison of the present modification and the second modification, a position at which the radiation element  300 C is provided on the main portion  220 C does not depend on a position of the feeding portion  260 C. The radiation element  300 C and the main portion  220 C are positioned on a common plane perpendicular to the up-down direction. The radiation element  300 C corresponds to one fourth of a wavelength of any one of the operating frequencies of the antenna  100 C. 
     Referring to  FIG. 17 , the first facing portion  432 C and the second facing portion  436 C of the present modification constitute a capacitor  400 C. Since the main portion  220 C constitutes the inductance of the antenna  100 C as described above, the first facing portion  432 C, the second facing portion  436 C and the main portion  220 C form an LC resonator circuit. An operating frequency of the LC resonator circuit is different from an operating frequency of the radiation element  300 C. 
     As described above, the antenna  100 C of the present modification has the single radiation element  300 C extending from the main portion  220 C which forms the split ring  210 C. Thus, the antenna  100 C of the present modification can resonate at both of the operating frequencies of the split ring resonator  200 C and the radiation element  300 C. In other words, the antenna  100 C of the present modification has a structure which can resonate at the plurality of operating frequencies. 
     Fourth Modification 
     As shown in  FIG. 18 , an antenna  100 D of a fourth modification is formed of metal body  110 D which is mounted on a circuit board (not shown) when used. However, the present invention is not limited thereto. The antenna  100 D may be formed of traces which are printed on a circuit board. 
     As shown in  FIG. 18 , the antenna  100 D of the present modification has a split ring resonator  200 D. The antenna  100 D has a plurality of operating frequencies. The antenna  100 D has a split ring resonator structure. In other words, the antenna  100 D is a resonant antenna. 
     As shown in  FIG. 18 , the antenna  100 D of the present modification has a main portion  220 D, a feeding portion  260 D, a radiation element  300 D, a first facing portion  432 D and a second facing portion  436 D. The main portion  220 D forms a split ring  210 D. The main portion  220 D constitutes an inductance of the antenna  100 D. The main portion  220 D has a first portion  230 D, a second portion  240 D, a third portion  250 D, a fourth portion  270 D, a fifth portion  280 D, a first end portion  222 D and a second end portion  226 D. Any part of the second portion  240 D, the third portion  250 D and the fourth portion  270 D functions as a ground connecting point to be electrically connected with a ground plane (not shown) of the circuit board. The first end portion  222 D and the second end portion  226 D form a split portion  212 D of the split ring  210 D. Components of the antenna  100 D other than the radiation element  300 D have structures similar to those of the first modification. Accordingly, detailed explanation thereabout is omitted. 
     As shown in  FIG. 18 , the radiation element  300 D of the present modification extends from the main portion  220 D. Specifically, dissimilar to the radiation element  300 A of the first modification, the radiation element  300 D of the present modification extends forward from the third portion  250 D of the main portion  220 D and is then bent to extend rightward. The radiation element  300 D and the main portion  220 D are positioned on a common plane perpendicular to the up-down direction. The radiation element  300 D corresponds to one fourth of a wavelength of any one of the operating frequencies of the antenna  100 D. 
     Referring to  FIG. 18 , the first facing portion  432 D and the second facing portion  436 D of the present modification constitute a capacitor  400 D. Since the main portion  220 D constitutes the inductance of the antenna  100 D as described above, the first facing portion  432 D, the second facing portion  436 D and the main portion  220 D form an LC resonator circuit. An operating frequency of the LC resonator circuit is different from an operating frequency of the radiation element  300 D. 
     As described above, the antenna  100 D of the present modification has the single radiation element  300 D extending from the main portion  220 D which forms the split ring  210 D. Thus, the antenna  100 D of the present modification can resonate at both of the operating frequencies of the split ring resonator  200 D and the radiation element  300 D. In other words, the antenna  100 D of the present modification has a structure which can resonate at the plurality of operating frequencies. 
     Fifth Embodiment 
     As shown in  FIG. 19 , an antenna  100 E of a fifth modification is formed of metal body  110 E which is mounted on a circuit board (not shown) when used. However, the present invention is not limited thereto. The antenna  100 E may be formed of traces which are printed on a circuit board. 
     As shown in  FIG. 19 , the antenna  100 E of the present modification has a split ring resonator  200 E. The antenna  100 E has a plurality of operating frequencies. The antenna  100 E has a split ring resonator structure. In other words, the antenna  100 E is a resonant antenna. 
     As shown in  FIG. 19 , the antenna  100 E of the present modification has a main portion  220 E, a feeding portion  260 E, three radiation elements  300 E,  301 E and  302 E, a first facing portion  432 E and a second facing portion  436 E. The main portion  220 E forms a split ring  210 E. The main portion  220 E constitutes an inductance of the antenna  100 E. The main portion  220 E has a first portion  230 E, a second portion  240 E, a third portion  250 E, a fourth portion  270 E, a fifth portion  280 E, a first end portion  222 E, and a second end portion  226 E. Any part of the second portion  240 E, the third portion  250 E and the fourth portion  270 E functions as a ground connecting point to be electrically connected with a ground plane (not shown) of the circuit board. The first end portion  222 E and the second end portion  226 E form a split portion  212 E of the split ring  210 E. Components of the antenna  100 E other than the radiation elements  300 E,  301 E and  302 E have structures same as those of the fourth modification. Accordingly, detailed explanation thereabout is omitted. 
     As shown in  FIG. 19 , each of the radiation elements  300 E,  301 E and  302 E of the present modification extends from the main portion  220 E. Specifically, dissimilar to the radiation element  300 D of the fourth modification, the radiation element  300 E of the present modification extends forward from the third portion  250 E of the main portion  220 E and is then bent to extend leftward. The radiation element  301 E extends forward from around a right end of the fifth portion  280 E of the main portion  220 E and is then bent to extend leftward. The radiation element  302 E extends forward from around a left end of the fifth portion  280 E of the main portion  220 E and is then bent to extend rightward. The radiation elements  300 E,  301 E and  302 E and the main portion  220 E are positioned on a common plane perpendicular to the up-down direction. Each of the radiation elements  300 E,  301 E and  302 E corresponds to one fourth of a wavelength of any one of the operating frequencies of the antenna  100 E. 
     Referring to  FIG. 19 , the first facing portion  432 E and the second facing portion  436 E of the present modification constitute a capacitor  400 E. Since the main portion  220 E constitutes the inductance of the antenna  100 E as described above, the first facing portion  432 E, the second facing portion  436 E and the main portion  220 E form an LC resonator circuit. An operating frequency of the LC resonator circuit is different from any of operating frequencies of the radiation elements  300 E,  301 E and  302 E. 
     As described above, the antenna  100 E of the present modification has the three radiation elements  300 E,  301  and  302 E each extending from the main portion  220 E which forms the split ring  210 E. Thus, the antenna  100 E of the present modification can resonate at any of the operating frequencies of the split ring resonator  200 E and the radiation elements  300 A,  301 E and  302 E. In other words, the antenna  100 E of the present modification has a structure which can resonate at the plurality of operating frequencies. In particular, the number of the radiation elements  300 E,  301 E and  302 E of the antenna  100 E of the present modification is greater than that of the antenna  100 A,  100 B,  100 C and  100 D of the aforementioned first to fourth modifications. Accordingly, the number of the operating frequencies of the antenna  100 E can be increased with an increased number of the radiation elements. 
     Although the specific explanation about the present invention is made above referring to the embodiments, the present invention is not limited thereto and is susceptible to various modifications and alternative forms. 
     While there has been described what is believed to be the preferred embodiment of the invention, those skilled in the art will recognize that other and further modifications may be made thereto without departing from the spirit of the invention, and it is intended to claim all such embodiments that fall within the true scope of the invention.