Abstract:
A wireless identification tag having circular polarization planar antenna is disclosed. The wireless identification tag includes a conductive substrate and a wireless identification device. The conductive substrate includes a first slot portion, a second slot portion, and a third slot portion. The first slot portion, the second slot portion, and the third slot portion pass through the conductive substrate. The first slot portion and the second slot portion stretch in a first direction and a second direction, respectively. The angle between the first direction and the second direction is between 45 degrees and 135 degrees. The third slot portion is connected between the first slot portion and the second slot portion. The wireless identification device is disposed in the first slot portion or the second slot portion. The wireless identification device is used to transmit or receive an electric wave.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority to Taiwan Application Serial Number 100113925, filed Apr. 21, 2011, which is herein incorporated by reference. 
       BACKGROUND 
       [0002]    1. Field of Invention 
         [0003]    The present invention relates to a wireless identification tag having a circularly polarized planar antenna. More particularly, the present invention relates to a wireless identification tag applicable to variable coupling signal sources. 
         [0004]    2. Description of Related Art 
         [0005]    Because radio frequency identification (RFID) technology has the features of repeatable reading/writing, storing more information, accessing without needing to aim, simultaneously reading variable identification signals, fast reading, and easily implementing automatic operation, etc., the RFID technology is widely used in logistics and other industries, wherein the RFID technology applied on an ultra-high frequency band (860-960 MHz) is the most popular. 
         [0006]    In the RFID technology applying ultra-high frequency, a RFID tag is usually designed to have a structure of linear polarization to obtain the characteristic of isotropic radiation. However, the antenna having linear polarization structure still has problems about the limitations of the match for the polarization direction of the antenna, so that there is a great inconvenience in the practice of the antenna. 
         [0007]    For solving the problems about the polarization direction of the antenna, the industries provide variable methods to improve the antenna. 
         [0008]    For example, in a case of Taiwan Patent Number 1236318, two micro strip lines are disposed in orthogonal, thereby obtaining a relation that the included angle between two radiation bodies is equal to 90 degrees in space, and a transmission line of which the length is equal to ¼ wavelength is used to achieve the effect of phase difference equal to 90 degrees, so that a design of circularly polarized antenna is implemented. However, the circularly polarized antenna itself needs a grounding structure to enable the radiation pattern thereof simply shows the characteristic of single direction radiation, and when the antenna structure is designed to operate at UHF, the size of a finished product of the antenna is not easy to be decreased. 
         [0009]    According to the above analysis, a new wireless identification tag applicable to variable coupling signal sources and having circular radiation pattern is provided. 
       SUMMARY 
       [0010]    An aspect of the present invention is to provide a wireless identification tag having a circular polarization planar antenna, wherein the wireless identification tag is applicable to variable coupling signal sources and able to provide a radiation pattern of circular polarization. 
         [0011]    According to an embodiment of the present invention, the wireless identification tag comprises an electrically conductive substrate and a wireless identification device. The electrically conductive substrate comprises a first slot portion, a second slot portion, and a third slot portion. The first slot portion stretches along a first direction in parallel with the electrically conductive substrate, wherein the first slot portion substantially passes through the electrically conductive substrate and has a first width. The second slot portion stretches along a second direction in parallel with the electrically conductive substrate, wherein the second slot portion substantially passes through the electrically conductive substrate and has a second width, and an angle between the first direction and the second direction is between 45 degrees and 135 degrees. The third slot portion is connected between the first slot portion and the second slot portion, wherein the third slot portion substantially passes through the electrically conductive substrate and has a third width. The wireless identification device is used to receive or transmit an electromagnetic wave signal, wherein the wireless identification device is disposed in the first slot portion or the second slot portion. 
         [0012]    It is to be understood that the wireless identification tag provided by the embodiment of the present invention has two larger slot portions designed to be applicable to variable coupling sources and finished products of tag, and in the design of the wireless identification tag, single slot radiation body is designed fir variable coupling source, accordingly. Further, the wireless identification tag provided by the embodiment of the present invention can provide a radiation pattern of circular polarization to improve a problem about signal receiving. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The invention can be more fully understood by reading the following detailed description of the embodiment, with reference made to the accompanying drawings as follows, and single slot radiation body can be designed 
           [0014]      FIG. 1  is a diagram showing a structure of a wireless identification tag in accordance with an embodiment of the present invention; 
           [0015]      FIG. 2  is a diagram showing the relation between axial ratios and operation frequencies in accordance with an embodiment of the present invention; 
           [0016]      FIG. 3  is a diagram showing an equivalent circuit of the antenna of the wireless identification tag in accordance with an embodiment of the present invention; 
           [0017]      FIG. 4   a  is a diagram showing a structure of a wireless identification tag in accordance with an embodiment of the present invention; 
           [0018]      FIG. 4   b  is a diagram showing a radiation pattern of circular polarization in accordance with an embodiment of the present invention; 
           [0019]      FIG. 5   a  and  FIG. 5   b  are diagrams showing structures of electrically conductive substrates in accordance with an embodiment of the present invention; and 
           [0020]      FIG. 6  is a diagram showing a structure of a wireless identification tag in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    Reference will now be made in detail to the present embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 
         [0022]    Referring to  FIG. 1 ,  FIG. 1  is a diagram showing a structure of a wireless identification tag  100 . The wireless identification tag  100  includes an electrically conductive substrate  110  and a wireless identification device  120 . The electrically conductive substrate  110  includes a first slot portion  111 , a second slot portion  112 , and a third slot portion  113 . The first slot portion  111  and the second slot portion  112  are used for receiving the wireless identification device  120 . A user can select one of the first slot portion  111  and the second slot portion for receiving the wireless identification device  120  in accordance with the characteristics of the wireless identification device  120 . The electrically conductive substrate is made of metal materials having high electrical conductivity such as copper, iron, etc. 
         [0023]    The wireless identification device  120  includes an antenna  122  and a wireless radio frequency (RF) chip  124 . In this embodiment, the antenna  122  is a dipole antenna. More particularly, the antenna  122  is an indirect coupling feed-in antenna, and the wireless radio frequency chip  124  is a RFID chip (for example having impedance equal to 16-130 j ohm) for transmitting or receiving radio frequency signals. 
         [0024]    The first slot portion  111  passes through the electrically conductive substrate  110  and stretches along a first direction D 1  in parallel with the electrically conductive substrate  110 . The second slot portion  112  also passes through the electrically conductive substrate  110  and stretches along a second direction D 2  in parallel with the electrically conductive substrate  110 . It is can be understood from  FIG. 1  that there is an angle between the stretching direction D 1  of the first slot portion  111  and the stretching direction D 2  of the second slot portion  112 . In this embodiment, the stretching direction D 1  is perpendicular to the stretching direction D 2 , and it means that the angle is 90 degrees. However, the embodiments of the present invention are not limited thereto. In other embodiments of the present invention, the angle between the stretching direction D 1  and the stretching direction D 2  is between 45 degrees and 135 degrees. In addition, in this embodiment, when the angle is between 90±10 degrees, the stretching direction D 1  and the stretching direction D 2  are considered as being perpendicular to each other. The third slot portion  113  passes through the electrically conductive substrate  110  and lies between the first slot portion  111  and the second slot portion  112 , thereby connecting the first slot portion  111  with the second slot portion  112 . 
         [0025]    The first slot portion  111  has a first width W 1 , and the second slot portion  112  has a second width W 2 , and the third slot portion  113  has a third width W 3 . In this embodiment, the third with W 3  is smaller than the first path W 1  and the second width W 2 , and the length of the third slot portion  113  is equal to one fourth of the wavelength of an electromagnetic wave signal transmitted by the wireless radio frequency chip  124 . However, the embodiments of the present invention are not limited there to. In other embodiments of the present invention, the width and length of the third slot portion  113  can varied in accordance with actual demands. 
         [0026]    In this embodiment, the wireless identification tag  100  has a radiation characteristic of approximately circular polarization as shown in  FIG. 2 , and the radiation characteristic is produced by the shape of the slots, and the design and arrangement of slot locations described above. It can be understood from the analysis of axial ratios shown in  FIG. 2  that the wireless identification tag  100  of this embodiment has a good circular polarization characteristic and a good operable bandwidth of circular polarization. In addition, the wireless identification tag  100  of this embodiment can radiate from a backside and a front-side, so that the use of the tag  100  designed in this embodiment is more convenient than that of a conventional tag having a grounding structure and a characteristic of single radiation direction. 
         [0027]    In addition, this embodiment has a high degree of freedom about adjustment of the impedance of the tag  100 . Referring to  FIG. 3 ,  FIG. 3  is a diagram showing an equivalent circuit of the antenna of the wireless identification tag  100 , wherein an equivalent circuit of the dipole antenna is presented as a coupling source portion shown in the left side of  FIG. 3 , and the characteristic of a radiation body is equivalent to a radiation body portion shown in the right side of  FIG. 3 , and the coupling effect between the coupling source portion and the radiation body portion is equivalent to a mutual inductance L x  shown in  FIG. 3  through an operation principle of a transformer. The radiation body portion is constructed by an equivalent resistor R 2 , an equivalent capacitor C 2 , and an equivalent inductor L 2 . The coupling source portion is constructed by an equivalent capacitor C 10 , an equivalent inductor L 10 , an equivalent resistor R 1 , an equivalent capacitor C 1 , and an equivalent inductor L 1 . 
         [0028]    Regarding a resonant frequency of the coupling source, the resonant frequency f s  can be represented by f s =1/(2π√{square root over (L 1 ×C 1 )}), so that the value of the resonant frequency f s  of the coupling source can be controlled by the adjustments of a loop size (i.e., the value of L 1 ) and a stretching micro strip line length (i.e., the value of C 1 ) of the dipole antenna. Regarding the resonant frequency f r  of the radiation body, the resonant frequency f r  can be determined in accordance with the size of the radiation body and the type of the slots. Through the arrangement and the control of the relative position between the two resonant frequencies f s  and f r , a tag antenna featured in wide bandwidth can be developed, and the degree of freedom about the design of the antenna impedance is increased as well, so that a conjugate match between the impedance of the RFID chip (i.e., 16-130 j ohm in this embodiment) and the impedance of the dipole antenna is easily implemented. 
         [0029]    In addition, the first slot portion  111  and the second slot portion  112  have different sizes. The sizes of the first slot portion  111  and the second slot portion  112  are determined in accordance with the wireless identification device  120  applied by the user. In this embodiment, the first slot portion  112  is designed for receiving a customized wireless identification device, and the first slot portion  111  is designed for receiving a finished product of a RFID tag available on market. 
         [0030]    Referring to  FIG. 4   a ,  FIG. 4   a  is a diagram showing a structure of a wireless identification tag  400  in accordance with an embodiment of the present invention. The wireless identification tag  400  is similar to the wireless identification tag  100 , but the difference is in that the wireless identification tag  400  uses a finished product of a RFID tag available on market. In order to fix the RFID tag finished product in the first slot portion  111 , this embodiment uses a fixing element such as an adhesive tape to fix the RFID tag finished product in the first slot portion  111 , so that the wireless identification tag  400  can provide a radiation pattern of circular polarization (as shown in  FIG. 4   b ). 
         [0031]    Referring to  FIG. 5   a  and  FIG. 5   b ,  FIG. 5   a  and  FIG. 5   b  are diagrams showing structures of electrically conductive substrates  510   a  and  510   b . The electrically conductive substrates  510   a  and  510   b  are similar to the electrically conductive substrate  110 , but the difference is in that the shapes of the third slot portion  513   a  and  513   b  of the electrically conductive substrates  510   a  and  510   b  are different from that of the third slot portion  113  of the electrically conductive substrate  110 . As shown in  FIG. 5   a  and  FIG. 5   b , the third slot portion  513   a  of the electrically conductive substrate  510   a  has a plurality of corners, and the third slot portion  513   b  of the electrically conductive substrate  510   b  has a curved structure. Although the shapes of the slot portions  513   a  and  513   b  are different from that of the third slot portion  113  of the electrically conductive substrate  110 , wireless RFID tags utilizing the electrically conductive substrates  510   a  and  510   b  can still have a good circular polarization characteristic when the length thereof meet the requirement of one-fourth wavelength. 
         [0032]    Referring to  FIG. 6 ,  FIG. 6  is a diagram showing a structure of a wireless identification tag  600  in accordance with an embodiment of the present invention. The wireless identification tag  600  is similar to the wireless identification tag  100 , but the difference is in that a wireless identification device  620  of the wireless identification tag  600  utilizes a RFID chip having different impedance. In this embodiment, the impedance of the RFID chip of the wireless identification device  620  is equal to 27-180 j ohm. It can be understood from the equivalent circuit diagram and the relative descriptions that although the impedance of the RFID chip of the wireless identification device is varied, the wireless identification tag  600  can still provide a radiation pattern of circular polarization through a proper slot design. 
         [0033]    In accordance with the above descriptions, the wireless identification tags of the embodiments of the present invention can provide the radiation pattern of circular polarization, and the slot portions of the wireless identification tags can be designed for RFID chips sold by variable vendors or having different impedance, so that the slot radiation body can be applicable to variable in RFID tag chips or finished products of RFID tag. Further, the wireless identification tag can use a dipole antenna with a small size to be the coupling source, so that the degree of freedom about adjustment of the impedance of the antenna is increased and it is easy to design an antenna conjugate matching variable finished products of RFID tag in market. Furthermore, the wireless identification tags of the embodiments of the present invention can be produced in variable ways. For example, the radiation body and the coupling source thereof can be a monocoque structure. For another example, the radiation body and the coupling source can be fabricated in separate at first, and then combined. 
         [0034]    It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims.