Patent Abstract:
A radio frequency identification tag includes: a resilient base sheet; an electronic component; a reinforcing member having at least one concave portion at a periphery of the reinforcing member; and an antenna including a dipole portion and an inductance portion, the inductance portion having an impedance matching with that of the electronic component and being formed in a loop shape; the inductance portion being partly covered by the reinforcing member, the loop shape of the inductance portion being narrowed where the loop shape runs under the concave portion of the periphery of the reinforcing member and being widened outside of the reinforcing member.

Full Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2008-283651, filed on Nov. 4, 2008, the entire contents of which are incorporated herein by reference. 
     FIELD 
     A certain aspect of the embodiments discussed herein relates to a technique of a radio frequency identification tag. 
     BACKGROUND 
     A wireless tag includes an antenna and a circuit chip electrically connected to the antenna. The antenna includes an inductance section and a dipole section. The inductance section performs impedance matching with the circuit chip. The impedance matching is performed by approximating a radiation resistance value of the antenna with respect to the capacitance of the circuit chip, and resonating the inductance of the antenna and the capacitance of the circuit chip. 
     Japanese Laid-open Patent Publication No. 2006-268090 discloses a technique of a tag-use antenna. 
     SUMMARY 
     According to an aspect of an embodiment, a radio frequency identification tag includes: a resilient base sheet; an electronic component; a reinforcing member having at least one concave portion at a periphery of the reinforcing member; and an antenna including a dipole portion and an inductance portion, the inductance portion having an impedance matching with that of the electronic component and being formed in a loop shape; the inductance portion being partly covered by the reinforcing member, the loop shape of the inductance portion being narrowed where the loop shape runs under the concave portion of the periphery of the reinforcing member and being widened outside of the reinforcing member. 
     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  is a perspective view illustrating a configuration of a wireless tag. 
         FIG. 2  is a front view of the wireless tag. 
         FIG. 3  is an equivalent circuit diagram of an antenna and a circuit chip. 
         FIG. 4  is a front view of a wireless tag having a different structure from the wireless tag according to the embodiment. 
         FIG. 5  is a graph comparing communication distances of the wireless tag according to the embodiment and the wireless tag having the different structure from the wireless tag according to the embodiment. 
         FIG. 6  is a graph illustrating relationships among the inductance of the antenna, the width of the non-covered section, and the dielectric constant of the resin. 
         FIG. 7  is an enlarged diagram of an area around a reinforcing member in  FIG. 2 . 
         FIGS. 8A ,  8 B,  8 C are illustrations of modified examples of the antenna. 
         FIGS. 9A ,  9 B are illustrations of modified examples of the antenna. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an embodiment according to the present invention will be described with reference to the drawings. 
       FIG. 1  is a perspective view illustrating a configuration of a wireless tag. The wireless tag  1  includes a resilient base sheet  10 , an antenna  20 , a circuit chip  30 , a reinforcing member  40 , and resin  50 . 
     The resilient base sheet  10  is made of a PET (Polyethylene terephthalate) film and has flexibility. The size of the entire resilient base sheet  10  is about 60 mm in length and 15 mm in width. The antenna  20  is provided on the resilient base sheet  10 . The antenna  20  is arranged on the resilient base sheet  10  along the longitudinal direction of the resilient base sheet  10 . For example, the antenna  20  is formed by screen printing conductor paste on the resilient base sheet  10 . The size of the entire antenna  20  is about 53 mm in length and 7 mm in width. The circuit chip  30  is electrically connected to the antenna  20  and performs wireless communication. The reinforcing member  40  covers the circuit chip  30  provided on the resilient base sheet  10 . The reinforcing member  40  is made of a fiber-reinforced resin. The circuit chip  30  is covered by the reinforcing member  40 , so that the circuit chip is prevented from being damaged. The resin  50  covers both entire sides of the resilient base sheet  10  along with the antenna  20  and the circuit chip  30 . The reinforcing member  50  is made of a synthetic resin. Since the wireless tag  1  is covered by the resin  50 , the durability of the wireless tag  1  is improved. 
       FIG. 2  is a front view of the wireless tag  1 . The circuit chip  30  is omitted, and the reinforcing member  40  is represented by dashed lines. The antenna  20  includes an inductance section  21  and a dipole section  26 . The inductance section  21  is formed in a loop shape. The inductance section  21  is formed in a concave shape when seen from the front. A power supply section  25  for supplying electric current to the circuit chip  30  is formed in the center of the inductance section  21 . The power supply section  25  is electrically connected to the circuit chip  30 . The dipole section  26  is formed in a spiral shape. Details of the reinforcing member  40  will be described below. 
     The inductance section  21  includes a covered section  22  covered by the reinforcing member  40  and a non-covered section  23  not covered by the reinforcing member  40 . The minimum width W 1  of the covered section  22  is different from the maximum width W 2  of the non-covered section  23 . Specifically, the width W 2  of the non-covered section  23  is larger than the width W 1  of the covered section  22 . The width W 1  is, for example, about 1.5 mm, and the width W 2  is, for example, about 3.0 mm. The peripheral area  4 D of the reinforcing member  40  is an area for accommodating displacement of the reinforcing member  40  when bonding the reinforcing member  40  on the resilient base sheet  10 , and the width of the inductance section  21  in the area  4 D is set to the same width W 1  as that of the covered section  22 . In this way, by changing the width of the covered section  22  from the width of the non-covered section  23 , a sufficient length of the entire inductance section  21  is secured. The reason for securing the sufficient length of the entire inductance section  21  is to achieve impedance matching with the circuit chip  30  having a relatively small capacitance. 
     The impedance matching will be briefly described. 
       FIG. 3  is an equivalent circuit diagram of the antenna  20  and the circuit chip  30 . As illustrated in  FIG. 3 , the antenna  20  can be equivalently represented by a parallel circuit of a radiation resistance Rap and an inductance Lap. The circuit chip  30  can be equivalently represented by a parallel circuit of an internal resistance Rcp and a capacitance Ccp. The capacitance Ccp and the inductance Lap resonate due to the antenna  20  and the circuit chip  30  being connected in parallel. In this way, the impedance matching is achieved at a desired resonant frequency fo (=1/2π√(LC)), and received power at the antenna  20  is fully provided to the circuit chip  30 . A frequency around 953 MHz is used in Japan, a frequency around 868 MHz is used in Europe, and a frequency around 915 MHz is used in the USA. 
     To achieve impedance matching with the circuit chip  30  having a relatively small capacitance Ccp, the inductance Lap needs to be a large value. To increase the inductance Lap, the length of the inductance section  21  of the antenna  20  needs to be long. As illustrated in  FIG. 2 , to secure a sufficient length of the inductance section  21 , the width W 2  of the non-covered section  23  is set to be larger than the width W 1  of the covered section  22 . 
     Next, a wireless tag  1   x  having a different structure from the wireless tag  1  will be described.  FIG. 4  is a front view of the wireless tag  1   x  having a different structure from the wireless tag  1 . The circuit chip is omitted in the figure, and the reinforcing member  40  is represented by dashed lines. The shape of an antenna  20   x  of the wireless tag  1   x  is different from the shape of the antenna  20  of the wireless tag  1 . In the antenna  20   x , the covered section  22  and the non-covered section  23  have the same width. Specifically, the width of the covered section  22  and the non-covered section  23  is the same as the width W 1  of the covered section  22  of the antenna  20  of the wireless tag  1 . Therefore, when employing a circuit chip having a relatively small capacitance Ccp in a wireless tag  1   x , as illustrated in  FIG. 4 , the inductance section  21   x  needs to be long in order to achieve impedance matching. 
     When the inductance section  21   x  is long, it is necessary to secure a sufficient length of the dipole section  26   x  in an area smaller than that of the dipole section  26  of the wireless tag  1 . To secure a sufficient length of the inductance section  21  in a small area, as illustrated in  FIG. 4 , the dipole section  26   x  has a multi-folded shape. Since the dipole section  26   x  has such a shape, there is a risk that the antenna gain of the antenna  20   x  decreases and the communication distance becomes short. When making the wireless tag  1   x  compatible with the frequency (868 MHz) used in Europe, the length of the dipole section  26  needs to be 953/868 times as long, so that the entire wireless tag  1   x  needs to be up-sized. 
     However, in the same way as the wireless tag  1  illustrated in  FIG. 2 , the length of the covered section  22  and the non-covered section  23  of the inductance section  21  are different, so that it is possible to secure a sufficient area of the dipole section  26  while securing a sufficient length of the inductance section  21 . In this way, the dipole  26  can have a shape including relatively few folded portions. Therefore, the decrease of the antenna gain is suppressed, and the communication distance becomes long. In this way, the antenna  20  can be used for a circuit chip having a relatively small capacitance. Even when making the wireless tag  1  compatible with the frequency (868 MHz) used in Europe, it is possible to secure a sufficient length of the dipole section  26  while maintaining the small size of the wireless tag  1 . 
       FIG. 5  is a graph comparing communication distances of the wireless tag  1  and the wireless tag  1   x . The horizontal axis is frequency, and the vertical axis is communication distance ratio. The communication distance ratio of the horizontal axis is ratios of the communication distances of the wireless tags  1  and  1   x  when the communication distance of the wireless tag  1   x  whose frequency is set to 958 MHz is assumed to be the wireless tag  1 . The results illustrated in the graph are calculated for the wireless tags  1  and  1   x  both of which are not covered by the resin  50 . As illustrated in  FIG. 5 , the communication distance of the wireless tag  1  is longer than that of the wireless tag  1   x  between frequencies of 900 MHz and 1000 MHz. 
     Next, the resin  50  will be described. The resin  50  has a predetermined dielectric constant.  FIG. 6  is a graph illustrating relationships among the inductance Lap of the antenna  20 , the width W 2  of the non-covered section  23 , and the dielectric constant of the resin. For example, as the dielectric constant of the resin  50  gets larger compared with the value of the predetermined inductance Lap, the width W 2  of the non-covered section  23  needs to be smaller. In other words, as the dielectric constant of the resin  50  gets larger, the length of the inductance section  21  can be shorter. For example, to use a circuit chip having a capacitance Ccp of 1.0 pF, the inductance Lap should be 28 nH according to a resonance condition, and the width W 2  should be 3 mm when the dielectric constant εr of the resin  50  is 3. In this way, the width W 2  of the non-covered section  23  is set in accordance with the dielectric constant of the resin  50 . As a material of the resin  50 , for example, a polyester resin such as a polyethylene terephthalate (dielectric constant: 3.2) in thermoplastic resins, and an epoxy resin (dielectric constant: 4.0 to 4.6) and a polyurethane resin (dielectric constant: 4.2 to 7.6) in thermosetting resins can be used. 
     The relationship between the dielectric constant of the resin  50  and the length of the antenna  20  will be briefly described. The size of an antenna used in a frequency band such as the UHF band needs to have a length obtained by dividing the wavelength by an even integer, for example, a length obtained by dividing the wavelength by 2. Inside of the resin  50 , the wavelength of a radio wave is inversely proportional to the square root of the dielectric constant. Therefore, when using a material having a high dielectric constant as a material of the resin  50 , the size of the antenna can be downsized. As a result, the higher the dielectric constant of the resin  50 , the shorter the length of the antenna  20  can be. In other words, the higher the dielectric constant of the resin  50 , the shorter the length of the inductance section  21  can be. As described above, the higher the dielectric constant of the resin  50 , the smaller the width W 2  of the non-covered section  23  can be. 
     Next, the reinforcing member  40  will be described.  FIG. 7  is an enlarged diagram of the area around the reinforcing member  40  in  FIG. 2 . The reinforcing member  40  has a first edge portion  41  not crossing the inductance section  21  and a second edge portion  42  crossing the inductance section  21 . The first edge portion  41  and the second edge portion  42  shape the edge portion of the reinforcing member into a concave form. Consider a case in which the resilient base sheet  10  is bent along the extended line L on the first edge portion  41 . In this case, the bending radius of the resilient base sheet  10  around the first edge portion  41  is relatively small. On the other hand, the bending radius of the resilient base sheet  10  around the second edge portion  42  is relatively large. Since the inductance section  21  crosses the second edge portion  42 , the bending radius of the inductance section  21  can be suppressed to be small. In this way, a disconnection of the antenna  20  is prevented. 
     Next, modified examples of the antenna will be described with reference to the  FIGS. 8A ,  8 B,  8 C, and  FIGS. 9A ,  9 B. 
     As illustrated in  FIG. 8A , an inductance section  21   a  has a bent portion  24   s  which is bent roundly. Since a large electric current flows in the inductance section  21   a , the conductor loss can be suppressed by the bent portion  24   a  bent roundly. When the bent portion of the inductance section  21   a  is bent roundly as described above, the circumference length of the inductance section  21   a  becomes long. Therefore, the entire area of the inductance section  21   a  needs to be large, and when the inductance section  21   a  is large, the area of the dipole section  26  becomes small. 
     As illustrated in  FIG. 8B , an inductance section  21   b  has a convex portion  24   b  protruding downwardly. By such a shape, a sufficient length of the inductance section  21   b  can be secured. However, since the bent portion of the inductance section  21   b  increases, the loss in the inductance section  21   b  increases. 
     As illustrated in  FIG. 8C , a dipole section  26   c  includes a narrow width portion  26   c   1  having a narrow width and a large width portion  26   c   2  having a large width connected to the narrow width portion  26   c   1 . The length of the large width portion  26   c   2  is longer than the length of the narrow width portion  26   c   1 . The large width portion  26   c   2  is formed on the top end side of the dipole section  26   c . In this way, a sufficient area of the dipole section  26   c  can be secured, and the antenna gain increases because the dipole section  26   c  has fewer bent portions. 
     As illustrated in  FIG. 9A , a dipole section  26   d  of the antenna  20   d  has a meandering form. By such a shape, a sufficient length of the dipole section  26   d  can be secured. 
     As illustrated in  FIG. 9B , a non-covered section  23   e  of an inductance section  21   e  in an antenna  20   e  meanders. The non-covered section  23   e  corresponds to the meandering portion. The maximum width W 2  of the non-covered section  23   e  is larger than the minimum width W 1  of the covered section  22 . Also by this, a sufficient length of the inductance section  21   e  can be secured. 
     Although a preferred embodiment of the present invention is described above, the present invention is not limited to the specific embodiment, and various variations and modifications are possible within the scope of the gist of the present invention described in the claims. 
     Although the wireless tag  1  is covered by the resin  50 , the wireless tag  1  may not be covered by the resin  50 . The entire body including the resilient base sheet  10 , the antenna  20 , the circuit chip  30 , and the reinforcing member  40  may laminated. 
     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 condition, nor does the organization of such examples in the specification relate to a showing of superiority and inferiority of the invention. Although the embodiment of the present inventions have been described in detail, it should be understood that the various changes, substitutions, and alternations could be made hereto without departing from the spirit and scope of the invention.

Technology Classification (CPC): 6