Patent Publication Number: US-8537052-B2

Title: Antenna and electronic device equipped with the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2009-041470, filed on Feb. 24, 2009, the entire contents of which are incorporated herein by reference. 
     FIELD 
     A certain aspect of the embodiments discussed herein is related to an antenna and an electronic device equipped with the same. 
     BACKGROUND 
     Recently, an RFID (Radio Frequency IDentification) system has been applied for inventory management, merchandise management and distribution management. An exemplary RFID system is configured as follows. A host computer and a reader/writer are connected. A memory having a built-in antenna, called tag, is attached to a managed object. A variety of information related to the managed object (managed object information) is stored in the tag. The managed object information is transferred between the tag and the host computer via the reader/writer. The managed object information in the tag is read out to the host computer, and the managed object information in the host computer is written in the tag. Thus, the managed object information realizes the traceability of the managed object. 
     Preferably, the antenna employed in the RFD system has a wideband characteristic, a compact size and low profile. It is also preferred that the antenna performance is immune to the property of a member to which the antenna is attached. 
     There are various proposals for realizing antennas as described above. For example, a proposed antenna has planar antenna elements that are formed on a dielectric substrate and have different resonance frequencies, in which the antenna elements are coupled at a feed point via a transmission line for impedance matching (see Japanese Laid-Open Patent Publication No. 2006-287452). Another proposed antenna functions as a slot antenna in the vicinity of a metal surface and functions as an ordinary antenna away from the metal surface (see U.S. Pat. No. 6,914,562). 
     SUMMARY 
     According to an aspect of the present invention, there is provided an antenna including: a dielectric substrate; a ground electrode provided on a first surface of the dielectric substrate; a first antenna element and a second antenna elements provided to a second surface of the dielectric substrate, the first and second antenna elements having an identical resonance frequency and an identical Q value; a transmission line connecting the first and second antenna elements; and a feed part provided in the transmission line. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates a dipole antenna; 
         FIG. 2  is a graph of an exemplary antenna gain characteristic of the dipole antenna; 
         FIG. 3  is a graph of an exemplary feed point impedance of the dipole antenna; 
         FIG. 4  is a graph of an antenna gain characteristic of a downsized dipole antenna in order to use the dipole antenna as an antenna for an RFID tag; 
         FIG. 5  is a graph of an exemplary feed point impedance characteristic of the downsized dipole antenna; 
         FIG. 6  is a diagram of an antenna using a patch antenna for the RFID tag; 
         FIG. 7  is a graph of an exemplary antenna gain characteristic of the antenna illustrated in  FIG. 6 ; 
         FIG. 8  is a graph of an exemplary feed point impedance characteristic of the antenna illustrated in  FIG. 6 ; 
         FIG. 9  is a graph of an antenna gain characteristic of an RFID tag oriented patch antenna designed to have a broadened band; 
         FIG. 10  is a graph of an exemplary feed point impedance characteristic of the patch antenna described with reference to  FIG. 9 ; 
         FIG. 11  is a perspective view of a tag employed in an RFID system in accordance with a first embodiment; 
         FIG. 12  is a graph of an exemplary antenna gain characteristic of the antenna in accordance with the first embodiment; 
         FIG. 13  is a graph of an exemplary input impedance characteristic of the antenna in accordance with the first embodiment; 
         FIG. 14  is a perspective view of a tag employed in an RFID system in accordance with a second embodiment; 
         FIG. 15  is a graph of an exemplary antenna gain characteristic of the antenna in accordance with the second embodiment; 
         FIG. 16  is a graph of an exemplary input impedance characteristic of the antenna in accordance with the second embodiment; 
         FIG. 17  is a perspective view of a tag employed in an RFID system in accordance with a third embodiment; 
         FIG. 18  is a graph of an exemplary antenna gain characteristic of the antenna in accordance with the third embodiment; 
         FIG. 19  is a graph of an input impedance characteristic of the antenna in accordance with the third embodiment; and 
         FIG. 20  is a perspective view of another tag for an RFID system. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Nowadays, a wideband antenna and a strip antenna for the multi-band use have been developed for wireless LANs (Local Area Network), cellular phones and UWB (Ultra-Wide Band) systems. Preferably, the RFID system employs wideband, multi-band and downsized antennas. The antennas used in the RFID system are apt to be affected by the ambient environment and are designed to have different frequencies in different countries. More specifically, the RFID tag in the UHF band is assigned 915 MHz in the United States of America, 953 MHz in Japan and 860 MHz in Europe. In order to enable the RFID tag to be worldwide used in the different countries adopting the different frequencies, the antenna is preferably capable of covering the different frequencies. A dipole antenna and a patch antenna, which are typical microstrip antennas, have the following advantages and disadvantages. 
       FIG. 1  illustrates an exemplary dipole antenna  1  having a feed point  3  arranged between antenna elements  2   a  and  2   b .  FIG. 2  illustrates an exemplary antenna gain characteristic of the dipole antenna  1 , and  FIG. 3  illustrates a feed point impedance characteristic of the dipole antenna  1 . A wideband bandwidth is realized under the condition of the ideal antenna structure and environment. 
     If the dipole antenna  1  is bent or curved for downsizing, the dipole antenna  1  will have a narrowed band and a reduced gain. In addition, the curved or bent dipole antenna  1  will more easily be affected by the property of a member such as a metal to which the dipole antenna  1  is attached. 
       FIG. 4  illustrates an exemplary antenna gain characteristic of a downsized dipole antenna used as an antenna for the RFID tag, and  FIG. 5  illustrates a feed point impedance characteristic of the downsized dipole antenna.  FIGS. 4 and 5  illustrate that downsizing of the dipole antenna narrows the band and reduces the antenna gain. 
       FIG. 6  illustrates an antenna  4  for use in the RFID tag using an ordinary patch antenna.  FIG. 7  illustrates an exemplary antenna gain characteristic of the antenna  4 . The antenna  4  has a ground member  5 , a patch antenna part  6  and a feed point  7 . 
     The antenna  4  using the patch antenna has a narrow bandwidth of the radiation characteristic, as compared to the dipole antenna. The antenna  4  uses the antenna substrate with the ground member  5 , and the radiation pattern is thus obtained on the only one side of the antenna  4 . In a case where the ground member  5  is used to attach the antenna  5  to an attachment member, the member may be made of a metal. However, the antenna  4  has a narrow bandwidth. The bandwidth tends to be further narrowed by facilitating the low profile of the RFID tag, that is, by thinning the antenna substrate. Generally, the bandwidth of the patch antenna may be broadened by coupling multiple resonators in various ways and thickening the antenna substrate. For example, the antenna substrate is set equal to or greater than 3 mm.  FIG. 9  illustrates an exemplary antenna gain characteristic of a patch antenna for the RFID tag designed to broaden the bandwidth.  FIG. 10  illustrates an exemplary feed-point impedance characteristic. As illustrated in  FIG. 9 , the broadening of the bandwidth degrades the antenna gain characteristic. The antenna substrate is thick. 
     Generally, the antenna may be designed as follows. The strip antenna uses a resonator formed on the antenna substrate and has the feed point at a specific position on the resonator at which the antenna is conjugate-matched with the output impedance of a transmitter. More specifically, the antenna such as the dipole antenna or the patch antenna primarily uses one resonator, and has the feed point at a specific position on the resonator at which the antenna is conjugate-matched with the impedance of a signal source. A matching circuit for conjugate matching may be used. 
     The patch antenna may employ multiple resonators having different resonance frequencies in order to broaden the band. However, in some cases, a satisfactory wideband characteristic is not obtained. 
     As described above, it is difficult to realize the microstrip antenna for the RFID tag in the UHF band that simultaneously achieves a reduced size, a broader bandwidth, improved low profile and adaptation to a metal. 
     According to an aspect of embodiments, there is provided an antenna capable of achieving a reduced size, a broader bandwidth, improved low profile and metal attachment. 
     First Embodiment 
       FIG. 11  is a perspective view of a tag  100  used for the RFID systems. The tag  100  has an antenna  200  equipped with a circuit chip such as a large scale integration (LSI) chip  300 . The tag  100  is an exemplary example of an electronic device in accordance with an aspect of the present invention. In practice, the tag  100  may be covered with a protection member, which is not illustrated for the sake of simplicity. 
     The antenna  200  has a dielectric substrate  26  and a ground electrode  29  provided on a surface of the dielectric substrate  26 . The antenna  200  has a first antenna element  21  and a second antenna element  25 , which are provided on the other surface of the dielectric substrate  26 . Further, the antenna  200  has a first transmission line  22  and a second transmission line  24 , which are used to connect the first antenna element  21  and the second antenna element  25 . The first transmission line  22  extends from the first antenna element  21 , and the second transmission line  24  extends from the second antenna element  25 . An end of the first transmission line  22  and an end of the second transmission line  24  face each other. The ends of the transmission lines  22  and  24  that face each other form a feed part  23 . The first antenna element  21  is connected to the ground electrode  29  via an electrode  27  provided on an end of the dielectric substrate  26 . Similarly, the second antenna element  25  is connected to the ground electrode  29  via an electrode  28  provided on the end of the dielectric substrate  26 . 
     The antenna  200  thus configured may have the following exemplary dimensions. The length L 1  of the dielectric substrate  26  is equal to 38 mm, and the width W 1  thereof is equal to 40 mm. The thickness T 1  of the dielectric substrate  26  is equal to 1 mm. The length L 2  of the first antenna element  21  is equal to 36 mm, and the width W 2  thereof is equal to 12 mm. The second antenna element  25  has the same dimensions as those of the first antenna element  21 . The width W 3  between the first antenna element  21  and the second antenna element  25  is set equal to 12 mm. 
     The first antenna element  21  and the second antenna element  25  may have the following conditions. The first antenna element  21  and the second antenna element  25  are printed on the dielectric substrate  26  and have short-circuited ends and open-circuited ends. The first antenna element  21  having the short-circuited end and the open-circuited end functions as a λ/4 microstrip resonator that resonates at a frequency f R1  described below:
 
 f   R1   =c/ 4( L 2 +T 1)√{square root over (∈ r )}
 
where L 2 +T 1  denotes the length of the first antenna element  21 , c is the velocity of light and ∈ r  is the dielectric constant of the dielectric substrate  26 . Similarly, the second antenna element  25  functions as a λ/4 microstrip resonator that resonates at a frequency f R2  described below:
 
 f   R2   =c/ 4( L 2+ T 1)√{square root over (∈ r )}
 
where L 2 +T 1  denotes the length of the second antenna element  25 . Thus, the antenna  200  has a structure with the two λ/4 microstrip resonators. It is noted that the lengths L 2 +T 1  of the first and second antenna elements  21  and  25  consider the thickness of the dielectric substrate  26 .
 
     The first antenna element  21  and the second antenna element  25  have the following relations.
 
 f   R1   =f   R2  
 
 Q 1 =Q 2
 
Where Q 1  is the Q value of the first antenna element  21  and Q 2  is the Q value of the second antenna element  25 .
 
     The Q value can be written as a general expression as follows:
 
 Q =(1/ R )×( L/C ) 1/2  
 
The antenna element functioning as a resonator may be represented in the form of an equivalent circuit in which an inductive element L and a capacitive element C are combined. When the antenna element is considered as a resonator, multiple antenna elements connected by transmission lines function as follows.
 
     The antenna element operates as a capacitive element within a range shorter than λ/4 in the distance from the open-circuited end to the input/output port, and operates as an inductive element within a range shorter than λ/4 in the distance from the short-circuited end to the input/output port in accordance with the theory of distribution constant. The characteristic impedance of the antenna element arranged on the dielectric substrate is defined by the dimensions thereof and the thickness of the dielectric substrate. 
     Thus, the Q value of the first antenna element  21  is defined by the dimensions of the first antenna element  21 , the position of the input/output port, and the thickness of the dielectric substrate  26 . Similarly, the Q value of the second antenna element  25  is defined by the dimensions of the second antenna element  25 , the position of the input/output port and the thickness of the dielectric substrate  26 . 
     The lengths L 2  and the widths W 2  of the first and second antenna elements  21  and  25  and the thickness T 1  of the dielectric substrate  26  are determined so as to obtain a desired Q value. 
     The lengths of the first transmission line  22  and the second transmission line  24  used to connect the first antenna element  21  and the second antenna element  25  is λ/4 of the resonance frequencies fR 1  and fR 2  of the first and second antenna elements  21  and  25  (fR 1 =fR 2 ). 
     The position of the feed part  23  is selected so that the antenna is conjugate-matched with the impedance of the signal source. The feed part  23  includes the LSI chip  300  for RFID. The feed part  23  is supplied with power. The antenna  200  forms the tag  100  along with the LSI chip  300  arranged in the feed part  23 . 
       FIG. 12  illustrates an antenna gain characteristic of the antenna  200  configured as described above. The antenna  200  has a good gain characteristic over an extremely wide band, as compared to the antenna gain characteristics of the dipole antenna illustrated in  FIGS. 2 and 4  and the antenna gain characteristic of the patch antenna having the broadened band illustrated in  FIG. 7 . 
     The tag  100  for the RFID systems may be attached to, for example, goods distributed worldwide. Communications between the tag  100  and host computers take place in various areas in the world. The RFID system is assigned a frequency of 860 MHz in Europe, 915 MHz in the United States, and 953 MHz in Japan. The patch antenna illustrated in  FIG. 7  is designed to cover all the bands of the above frequencies. However, the antenna gain characteristic of the patch antenna is degraded. Further, the antenna substrate is thick. In contrast, the antenna  200  of the present embodiment covers all the bands and the dielectric substrate  26  is as very thin as 1 mm, and achieves the low profile. 
     The antenna  200  is the microstrip antenna having the ground electrode on the backside. The antenna  200  has the downsized and thinned structure, and may be attached to a metal member. 
       FIG. 13  illustrates an input impedance characteristic of the antenna  200  depicted in  FIG. 11 .  FIG. 13  may not illustrate any considerable improvement in the input impedance characteristic, as compared to the conventional antenna. However, it is to be noted that the radiation characteristic of antenna is determined by the current distribution on the antenna electrode. Thus, improvements in the antenna gain may not be related to improvements in the input impedance characteristic. 
     Second Embodiment 
     A second embodiment is described with reference to  FIGS. 14 through 16 .  FIG. 14  is a perspective view of a tag  101  used for the RFID systems. The tag  101  has an antenna  400  equipped with the LSI chip  300 . The tag  101  is an exemplary electronic device. In practice, the tag  101  may be covered with a protection member, which is not illustrated for the sake of simplicity. 
     The antenna  400  has a dielectric substrate  46  and a ground electrode  29  on a surface of the dielectric substrate  46 . The antenna  400  has a first antenna element  41  and a second antenna element  45  provided on the other surface of the dielectric substrate  46 . The antenna  400  has a first transmission line  42  and a second transmission line  44  used to connect the first antenna element  41  and the second antenna element  45 . The first transmission line  42  extends from the first antenna element  41 , and the second transmission line  44  extends from the second antenna element  45 . An end of the first transmission line  42  and an end of the second transmission line  44  face each other to thus form a feed part  43 . The first antenna element  41  is connected to a ground electrode  49  by an electrode  47  provided on an end of the dielectric substrate  46 , and the second antenna element  45  is connected to the ground electrode  49  by an electrode  48  provided on another end of the dielectric substrate  46 . The electrodes  47  and  48  are provided on the opposite ends of the dielectric substrate  46 . This arrangement of the electrodes  47  and  48  is different from that employed in the first embodiment. 
     The antenna  400  is similar to the antenna  200  of the first embodiment. However, the antenna  400  has different dimensions from those of the antenna  200 . The following are exemplary dimensions of the antenna  400 . The length L 3  of the dielectric substrate  46  is equal to 30 mm, and the width W 4  is equal to 52 mm. The thickness T 2  of the dielectric substrate  46  is 1 mm. The length L 4  of the first antenna element  41  is equal to 26 mm, and the width W 5  is equal to 18 mm. The second antenna element  45  is oriented in a direction opposite to the direction in which the first antenna element  41  is oriented. The first antenna element  41  and the second antenna element  45  have identical dimensions. The distance W 6  between the first antenna element  41  and the second antenna element  45  is set equal to 12 mm. 
     The first antenna element  41  and the second antenna element  45  may satisfy have the following conditions. The first antenna element  41  and the second antenna element  45  are printed on the dielectric substrate  46  and have short-circuited ends and open-circuited ends. The first antenna element  41  having the short-circuited end and the open-circuited end functions as a λ/4 microstrip resonator that resonates at a frequency f R1  described below:
 
 f   R1   =c/ 4( L 4+ T 2)√{square root over (∈ r )}
 
where L 4 +T 2  denotes the length of the first antenna element  41 , c is the velocity of light and ∈ r  is the dielectric constant of the dielectric substrate  46 . Similarly, the second antenna element  25  functions as a λ/4 microstrip resonator that resonates at a frequency f R2  described below:
 
 f   R2   =c/ 4( L 4+ T 2)√{square root over (∈ r )}
 
where L 4 +T 2  denotes the length of the second antenna element  45 . Thus, the antenna  400  has a structure with the two λ/4 microstrip resonators. It is noted that the lengths L 4 +T 2  of the first and second antenna elements  41  and  45  consider the thickness of the dielectric substrate  46 .
 
     The first antenna element  41  and the second antenna element  45  have the following relations.
 
 f   R1   =f   R2  
 
 Q 1 =Q 2
 
Where Q 1  is the Q value of the first antenna element  41  and Q 2  is the Q value of the second antenna element  45 .
 
     The Q value can be written as a general expression as follows:
 
 Q =(1/ R )×( L/C ) 1/2  
 
The antenna element functioning as a resonator may be represented in the form of an equivalent circuit in which an inductive element L and a capacitive element C are combined. When the antenna element is considered as a resonator, multiple antenna elements connected by transmission lines function as has been described.
 
     The Q value of the first antenna element  41  is defined by the dimensions of the first antenna element  41 , the position of the input/output port, and the thickness of the dielectric substrate  46 . Similarly, the Q value of the second antenna element  45  is defined by the dimensions of the second antenna element  45 , the position of the input/output port and the thickness of the dielectric substrate  46 . 
     The lengths L 4  and the widths W 5  of the first and second antenna elements  41  and  45  and the thickness T 2  of the dielectric substrate  46  are determined so as to obtain a desired Q value. 
     The lengths of the first transmission line  42  and the second transmission line  44  used to connect the first antenna element  41  and the second antenna element  45  is λ/4 of the resonance frequencies fR 1  and fR 2  of the first and second antenna elements  41  and  45  (fR 1 =fR 2 ). 
     The position of the feed part  43  is selected so that the antenna is conjugate-matched with the impedance of the signal source. The feed part  43  includes the LSI chip  300  for RFD. The feed part  43  is supplied with power. The antenna  400  forms the tag  101  along with the LSI chip  300  arranged in the feed part  43 . 
       FIG. 15  illustrates an antenna gain characteristic of the antenna  400  configured as described above. The antenna  400  has a good gain characteristic over an extremely wide band, as compared to the antenna gain characteristics of the dipole antenna illustrated in  FIGS. 2 and 4  and the antenna gain characteristic of the patch antenna having the broadened band illustrated in  FIG. 7 . 
     The antenna  400  is the microstrip antenna having the ground electrode on the backside. The antenna  400  has the downsized and thinned structure, and may be attached to a metal member. 
       FIG. 16  illustrates an input impedance characteristic of the antenna  400  depicted in  FIG. 14 .  FIG. 16  may not illustrate any considerable improvement in the input impedance characteristic, as compared to the conventional antenna. However, it is to be noted that the radiation characteristic of antenna is determined by the current distribution on the antenna electrode. Thus, improvements in the antenna gain may not be related to improvements in the input impedance characteristic. 
     Third Embodiment 
     A description will now be given, with reference to  FIG. 17 , of an antenna  600  in accordance with a third embodiment.  FIG. 17  is a perspective view of a tag  102  in which the antenna  600  is incorporated. The antenna  600  differs from the antenna  200  of the first embodiment in the following. In the antenna  200 , the first antenna element  21  and the second antenna element  25  are printed on the dielectric substrate  26 . One end of each of the first and second antenna elements  21  and  25  is short-circuited, and the other is open-circuited. In contrast, the antenna  600  of the third embodiment has a first antenna element  61  and a second antenna element  65 , each of which has both ends that are open-circuited. A ground electrode  69  is provided on a surface of the dielectric substrate  66  as in the case of the first embodiment. 
     For example, the antenna  600  may have the following dimensions. The length L 5  of the dielectric substrate  66  is equal to 70 mm, and the width W 7  is equal to 40 mm. The thickness T 3  of the dielectric substrate  66  is equal to 1 mm. The length L 6  of the first antenna element  61  is equal to 66 mm, and the width W 8  is equal to 8 mm. The second antenna element  65  has a length L 6  of 66 mm, and a width W 8  of 8 mm. The second antenna element  65  has the same dimensions as those of the first antenna element  61 . There is a distance W 9  of 10 mm between the first antenna element  61  and the second antenna element  65 . 
     The first antenna element  61  having the open-circuited ends functions as a λ/2 microstrip resonator that resonates at a frequency fR 1  described below:
 
 f   R1   =c/ 2 L 6√{square root over (∈ r )}
 
where L 6  denotes the length of the first antenna element  61 , c is the velocity of light and ∈ r  is the dielectric constant of the dielectric substrate  66 . Similarly, the second antenna element  45  functions as a λ/2 microstrip resonator that resonates at a frequency f R2  described below:
 
 f   R2   =c/ 2 L 6√{square root over (∈ r )}
 
where L 6  denotes the length of the second antenna element  65 . Thus, the antenna  600  has a structure with the two λ/2 microstrip resonators.
 
     The Q value can be written as a general expression as follows:
 
 Q =(1/ R )×( L/C ) 1/2  
 
The antenna element functioning as a resonator may be represented in the form of an equivalent circuit in which an inductive element L and a capacitive element C are combined. When the antenna element is considered as a resonator, multiple antenna elements connected by transmission lines function as follows. The antenna element operates as a capacitive element within a range shorter than λ/4 in the distance from the open-circuited end to the input/output port, and operates as an inductive element within a range shorter than λ/4 in the distance from the short-circuited end to the input/output port in accordance with the theory of distribution constant. The characteristic impedance of the antenna element arranged on the dielectric substrate is defined by the dimensions thereof and the thickness of the dielectric substrate.
 
     Thus, the Q value of the first antenna element  61  is defined by the dimensions of the first antenna element  61 , the position of the input/output port, and the thickness of the dielectric substrate  66 . Similarly, the Q value of the second antenna element  65  is defined by the dimensions of the second antenna element  65 , the position of the input/output port and the thickness of the dielectric substrate  66 . 
     The lengths L 6  and the widths W 8  of the first and second antenna elements  61  and  65  and the thickness T 3  of the dielectric substrate  66  are determined so as to obtain a desired Q value. 
     The lengths of the first transmission line  62  and the second transmission line  64  used to connect the first antenna element  61  and the second antenna element  65  is λ/4 of the resonance frequencies fR 1  and fR 2  of the first and second antenna elements  61  and  65  (fR 1 =fR 2 ). 
     The position of the feed part  63  is selected so that the antenna is conjugate-matched with the impedance of the signal source. The feed part  63  includes the LSI chip  300  for RFID. The feed part  63  is supplied with power. The antenna  600  forms the tag  102  along with the LSI chip  300  arranged in the feed part  63 . 
     The antenna  600  is the microstrip antenna having the ground electrode on the backside. The antenna  600  has the downsized and thinned structure, and may be attached to a metal member. 
       FIG. 18  illustrates an antenna gain characteristic of the antenna  600  configured as described above. The antenna  600  has a good gain characteristic over an extremely wide band, as compared to the antenna gain characteristics of the dipole antenna illustrated in  FIGS. 2 and 4  and the antenna gain characteristic of the patch antenna having the broadened band illustrated in  FIG. 7 . Further, the dielectric substrate  66  is as very thin as 1 mm, and achieves the low profile. 
       FIG. 19  illustrates an input impedance characteristic of the antenna  600  depicted in  FIG. 17 .  FIG. 19  may not illustrate any considerable improvement in the input impedance characteristic, as compared to the conventional antenna. However, it is to be noted that the radiation characteristic of antenna is determined by the current distribution on the antenna electrode. Thus, improvements in the antenna gain may not be related to improvements in the input impedance characteristic. 
       FIG. 20  illustrates an antenna  800  that corresponds to a variation of the antenna  600 . The antenna  800  has a first transmission line  82  and a second transmission line  84 , that are substituted for the first transmission line  62  and the second transmission line  64 . The other structural elements of the antenna  800  are the same as those of the antenna  600 . The first transmission line  82  and the second transmission line  84  are arranged alternately or are symmetrical about the feed part  63 . The antenna  800  thus configured exhibits the good antenna characteristic similar to that of the antenna  600  as long as the conditions related to the aforementioned resonance frequency and the Q value are met. The antenna  800  is capable of covering the different frequency bands of the RFID systems employed in the various countries. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various change, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.