Abstract:
An antenna and a radio frequency identification (RFID) tag using the same are disclosed. The antenna includes: a radiating unit having a helical structure; and a feeding unit having a loop structure on which a terminal connected to an element connected to the antenna is formed, wherein the feeding unit is electromagnetically induced and coupled with the radiating unit to be apart from each other.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an antenna and a radio frequency identification (RFID) tag using the same; and more particularly, to an antenna having loop and helical structure, and a radio frequency identification (RFID) tag using the same. 
       DESCRIPTION OF RELATED ARTS 
       [0002]    A radio frequency identification (RFID) tag is used in various fields, e.g., a resource management and security management with a radio frequency identification (RFID) reader. 
         [0003]    In general, if an object having an RFID tag is put on a read zone of the RFID reader, the RFID reader modulates a radio frequency (RF) signal having a specific carrier frequency, and transmits an interrogation signal to the RFID tag. The RFID tag responds to the interrogation signal of the RFID reader. That is, the RFID reader modulates a continuous electromagnetic wave having a specific frequency and transmits an interrogation signal to the RFID tag. The RFID tag performs back-scattering modulation for an electromagnetic wave transmitted from the RFID reader and returns the back-scattering modulated signal to the RFID reader for transmitting information stored in a memory of the RFID tag. 
         [0004]    When the RFID tag scatters the electromagnetic wave transmitted from the RFID reader and re-transmits the scattered electromagnetic wave to the RFID reader, the back-scattering modulation represents to transmit tag information by modulating a phase or amplitude of the scattered electromagnetic wave. 
         [0005]    A passive RFID tag not having an RF transmitter rectifies an electromagnetic wave transmitted from the RFID reader for obtaining an operational power and uses the rectified electromagnetic wave as a power source. In order to operate the passive RFID tag under a normal state, an intensity of the electromagnetic wave transmitted from the RFID reader should be larger than a threshold value at a location of the RFID tag. That is, a read zone of the RFID reader is limited by the intensity of the electromagnetic wave transmitted from the RFID reader to the RFID tag. However, a transmitting power of the RFID reader is limited by a local regulation or a federal communication commission (FCC) of U.S. Accordingly, the RFID tag has to efficiently receive the electromagnetic wave transmitted from the RFID tag in order to widen the read zone without increasing the transmitting power. 
         [0006]    A matching circuit is one of solutions for increasing an efficiency of the RFID tag. In general, a RFID tag includes an antenna, an RF front-end and a signal processing unit. The RF front-end and signal processing units are manufactured in one chip. A method using a matching circuit maximizes an intensity of a signal transmitted from the antenna to the RF front-end by conjugate-matching the antenna and the RF front-end through the matching circuit. However, the matching circuit which includes a combination of a capacitor and an inductor has miniaturization and cost problem since the matching circuit needs large area in a chip. 
         [0007]    Moreover, the intensity of the electromagnetic wave transmitted from the RFID reader to the RFID tag may be reduced by a path loss and an influence of an object having the RFID tag. Particularly, if the object having the RFID tag is a liquid, an electromagnetic distortion occurs by a coupling between the antenna and the liquid, and the intensity of the electromagnetic wave which is induced to the antenna is reduced. 
       SUMMARY OF THE INVENTION 
       [0008]    It is, therefore, an object of the present invention to provide an antenna and a radio frequency identification (RFID) tag using the same having the loop structure and the helical structure which may be efficiently matched to an antenna element having predetermined complex impedance; and more particularly, which are capable of being efficiently matched with the RF front-end having a larger capacitive reactance than a resistance element. 
         [0009]    It is, therefore, an another object of the present invention to provide an RFID tag which is capable of recognizing an object and not having influenced on the gain of the tag antenna although a liquid substance having electromagnetic wave interference characteristics exists under the tag antenna. 
         [0010]    In accordance with an aspect of the present invention, there is provided an antenna, including: a radiating unit having a helical structure; and a feeding unit having a loop structure on which a terminal connected to an element connected to the antenna is formed, wherein the feeding unit is electromagnetically induced and coupled with the radiating unit to be apart from each other. 
         [0011]    In accordance with another aspect of the present invention, there is provided a radio frequency identification (RFID) tag, including: an antenna receiving a radio frequency (RF) signal transmitted from an RFID reader; an RF front-end rectifying and detecting the RF signal; and a signal processing unit being connected to the RF front-end, wherein the antenna includes a radiating unit having a helical structure and a feeding unit having a loop structure on which a terminal connected to the RF front-end is formed, and the feeding unit and the radiating unit are electromagnetically induced and coupled to be apart each other. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The above and other objects and features of the present invention will become better understood with regard to the following description of the preferred embodiments given in conjunction with the accompanying drawings, in which: 
           [0013]      FIG. 1  is a block diagram showing an RFID system in accordance with an embodiment of the present invention; 
           [0014]      FIG. 2  is a circuit diagram showing an equivalent circuit which is modeling a tag antenna and an RF front-end in accordance with an embodiment of the present invention; 
           [0015]      FIG. 3  is a block diagram showing a tag antenna using a loop structure and a helical structure in accordance with an embodiment of the present invention; 
           [0016]      FIG. 4  is a circuit diagram showing an equivalent circuit which is modeling a tag antenna in accordance with an embodiment of the present invention; 
           [0017]      FIG. 5  shows a graph illustrating a change of an input impedance of a tag antenna according to a frequency change; 
           [0018]      FIG. 6  shows a graph illustrating a resistance change of input impedance of a tag antenna according to a gap between a feeding unit and a radiating unit; 
           [0019]      FIG. 7  shows a graph illustrating a reflective loss for a tag antenna in accordance with an embodiment of the present invention; 
           [0020]      FIGS. 8A to 8C  show the shapes of a feeding unit and a radiating unit of a tag antenna in accordance with an embodiment of the present invention; 
           [0021]      FIG. 9  shows a radiation pattern of a tag antenna according to an elevation direction and an azimuth direction; 
           [0022]      FIG. 10  shows a graph illustrating a read range of a tag antenna according to an azimuth direction; 
           [0023]      FIG. 11  shows a graph illustrating a read range of a tag antenna according to an elevation direction; and 
           [0024]      FIG. 12  illustrates a tag antenna in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    The advantages, features and aspects of the invention will become apparent from the following description of the embodiments with reference to the accompanying drawings, which is set forth hereinafter. Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
         [0026]      FIG. 1  is a block diagram showing an RFID system  100  in accordance with an embodiment of the present invention. The RF system  100  includes an RFID tag  120  having identification information, an RFID reader  110  having a reading function, and a host computer (not illustrated in  FIG. 1 ) processing data which is read from the RFID tag  120  through the RFID reader. 
         [0027]    The RFID reader  110  includes an RF transmitter  111 , an RF receiver  112 , and a reader antenna  113 . The read antenna  113  is electrically connected to the RF transmitter  111  and the RF receiver  112 . The RFID reader  110  transmits an RF signal to the RFID tag  120  through the RF transmitter  111  and the reader antenna  113 . The RFID reader  110  receives an RF signal from the RFID tag  120  through the reader antenna  113  and the RF receiver  112 . As described in U.S. Pat. No. 4,656,463, since a configuration of the RFID reader  110  is known to those skilled in the art, a description for the configuration of the RFID reader  110  will be omitted. 
         [0028]    The RFID tag  120  includes an RF front-end  121 , a signal processing unit  122  and a tag antenna  123 . If the RFID tag is a passive RFID tag, the RF front-end  121  converts a received RF signal to a DC voltage and provides power to the signal processing unit  122 . The RF front-end  121  extracts a base band signal from the received RF signal. As described in U.S. Pat. No. 6,028,564, since a configuration of the RF front-end  121  is known to those skilled in the art, a description for the configuration of the RFID reader  110  will be omitted. Moreover, as described in U.S. Pat. No. 5,942,987, since a configuration of the signal processing unit  122  is known to those skilled in the art, a description for the configuration of the signal processing unit  122  will be omitted. 
         [0029]    The RFID reader  110  modulates the RF signal having a specific carrier frequency and transmits an interrogation to the RFID tag  120 . The RF signal generated in the RF transmitter  111  of the RFID reader  110  is transmitted to an external apparatus through the reader antenna  113 . The electromagnetic wave  130  transmitted to the external apparatus is transmitted to the tag antenna  123 . The tag antenna  123  having a loop structure and a helical structure transmits the received electromagnetic wave to the RF front-end  121 . 
         [0030]    If an amplitude of the RF signal transmitted to the RF front-end  121  is larger than a minimum power to operate the RFID tag  120 , the RFID tag  120  performs a back-scattering modulation for the electromagnetic wave  130  transmitted from the RFID reader  110  and responds to the interrogation of the RFID reader  110 . 
         [0031]    In order to widen a read zone of the RFID reader  110 , the amplitude of the electromagnetic wave  130  transmitted from the RFID reader  110  is so large as to provide an operational power to the RFID tag  120 . Moreover, the electromagnetic wave  130  transmitted from the RFID reader  110  is transmitted to the RF front-end  121  through a high-efficient tag antenna  123  without a loss. Finally, in order to have the high-efficient tag antenna  123 , the tag antenna  123  has resonant characteristics in a carrier frequency of the RFID reader  110 , and becomes a complex conjugate matching with the RF front-end  121 . 
         [0032]      FIG. 2  is a circuit diagram showing an equivalent circuit which is modeling a tag antenna and an RF front-end in accordance with an embodiment of the present invention. The equivalent circuit includes of a voltage source (V oc ), antenna impedance (Z a ), and RF front-end impedance (Z c ). 
         [0033]    The voltage source (V oc ) and antenna impedance (Z a ) is equivalent circuit to the tag antenna  123 . The RF front-end impedance (Z c ) is an equivalent circuit to the RF front-end  121 . The antenna impedance (Z a ) has a real part (R a ) and an imaginary part (X a ). The real part (R a ) means an equivalent resistance to the tag antenna  123 , and the imaginary part (X a ) means an equivalent reactance of the tag antenna  123 . The RF front-end impedance (Z c ) has a real part (R c ) and an imaginary part (X c ). The real part (R c ) means an equivalent resistance to the RF front-end  121 , and the imaginary part (X c ) means an equivalent reactance of the RF front-end  121 . 
         [0034]    In general, if the antenna impedance (Z a ) becomes a complex conjugate matching with the RF front-end impedance (Z c ), a maximum power is transmitted from the tag antenna  123  to the RF front-end  121 . The complex conjugate matching means that two complex impedances have the same absolute value and an opposite phase. That is, if the impedance of the tag antenna  123  or the impedance of the RF front-end  121  is adjusted to be |R a =R c | and |X a =−X c |, the maximum power is transmitted from the tag antenna  123  to the RF front-end  121 . 
         [0035]    In general, the RF front-end  121  of a passive RFID tag includes rectifying and detecting circuits using a diode, and does not include a matching circuit for reducing an area of a chip. Accordingly, the impedance of the RF front-end  121  has complex impedance different from a normal 50Ω and has a small resistance element (R c ) and a large capacitive reactance element (X c ) in 900 MHz band due to characteristics of the rectifying circuit. 
         [0036]    Accordingly, the antenna impedance (Z a ) for the complex conjugate matching has a small resistance element (R a ) and a large inductive reactance element (X a ), and is resonant with the frequency of the electromagnetic wave transmitted from the RFID reader  110 . The tag antenna  123  according to an embodiment of the present invention is effectively matched to the RF front-end by adjusting the antenna impedance to have the large inductive reactance based on a loop structure. 
         [0037]      FIG. 3  is a block diagram showing a tag antenna using a loop structure and a helical structure in accordance with an embodiment of the present invention. 
         [0038]    A tag antenna  300  includes a feeding unit  310  of a loop structure and a radiating unit  320  of a helical structure. The feeding unit  310  is a loop shape of which a part connected to a front-end is removed. Both ends of the feeding unit  310  are connected to the front-end  121 . In an embodiment of the present invention, the tag antenna  300  is embodied on a bottle cap based on the fact that a tag read range is the same with a free space in a case that a distance between a tag and a liquid is above 3 cm. 
         [0039]    If a thickness (t f )  312  of a wire of the feeding unit  310  is reduced, the feeding unit  310  is easily attached to a tag chip, and has an efficient inductive coupling and a large input resistance. However, if the thickness (t f )  312  of the wire is increased, an inductive reactance of the feeding unit  310  is increased by increasing an efficient area of the loop, and a resonant frequency of the feeding unit  310  is increased. 
         [0040]    If a loop radius (R f )  311  of the feeding unit  310  is reduced, an inductive reactance of the feeding unit  310  is decreased by reducing an efficient area of the loop, and a resonant frequency of the feeding unit  310  is decreased. It is preferable that the radius (R f )  311  of the feeding unit  310  is larger than a size of the bottle cap by 0.8 mm to 1 mm. 
         [0041]    The radiating unit  320  has a helical structure and is loaded to be apart from the feeding unit at a certain distance (d)  340 . If the radiating unit  320  is loaded on the feeding unit  310  of a loop structure, a second harmonic resonant frequency is formed by a mutual coupling of the feeding unit  310  and the radiating unit  320  in addition to the resonant frequency of the feeding unit  310 . 
         [0042]    The tag antenna  300  according to an embodiment of the present invention is communicated with an RFID reader by using the second harmonic resonant frequency. That is, since each of the feeding unit  310  and the radiating unit  320  of the tag antenna  300  is a circle shape having a similar radius, the second harmonic resonant frequency is generated by a mutual inductive reactance. 
         [0043]    Meanwhile, if the thickness (t r )  322  of the radiating unit  320  is increased, an intensity of radiation is increased, and an input resistance is increased, and an inductive coupling of the feeding unit  310  and the radiating unit  320  is performed easily. A size of a radius (r r )  321  of the radiating unit  320  is determined so that a wire is wound to a certain bottle cap which is not made of a metallic material. It is preferable that the radius of the radiating unit  320  is the same with the radius of the bottle cap. Moreover, the radiating unit  320  is made of a conductive wire, a thin copper film, or a conductive line. 
         [0044]      FIG. 4  is a circuit diagram showing an equivalent circuit which is modeling a tag antenna in accordance with an embodiment of the present invention. 
         [0045]    The equivalent circuit includes an equivalent impedance (Z c ) of the RF front-end  121 , an equivalent impedance (Z f ) of the feeding unit  310 , an equivalent impedance (Z r ) of the radiating unit  320 , and a transformer having a mutual impedance (M). 
         [0046]    The equivalent impedance (Z c ) of the RF front-end  121  includes a resistance element (R c ) and a capacitive reactance (C c ), and is represented by Eq. (1). 
         [0000]        Z   C   =R   c +1 /jwC   c   Eq. (1) 
         [0047]    The equivalent impedance (Z f ) of the feeding unit  310  is represented by Eq. (2) based on an inductance of a loop (L f ). 
         [0000]      Z f =jwL f   Eq. (2) 
         [0048]    The equivalent impedance (Z r ) of the radiating unit  320  includes a resistance element (R r ), a parasitic capacitive reactance (C r ) according to a gap (d)  340  between the feeding unit  310  and the radiating unit  320  and a self-mutual inductance (L rp ) according to the overlapped region in the helical structure of the radiating unit  320 , and is represented by Eq. (3). 
         [0000]        Z   r   =jwL   r +( R   r   +jwL   rp )//(1 /jwC   r )  Eq. (3) 
         [0049]    The parasitic capacitive reactance (C r ) means a reactance element when the radiating unit  320  is regarded as a simple loop. The helical structure of the radiating unit  320  is represented by the self-mutual inductance (L rp ). The self-mutual inductance (L rp ) induces a current to the radiating unit  320  based on an electric field generated in the loop of the feeding unit  310 . Moreover, the radiating unit  320  is one line on which a current flows to the same direction according to the principle that a current flows to the same direction on parallel lines having a predetermined gap. The current element is called as “pair of parallel current element”. 
         [0050]    The resistance element (R r ) of the radiating unit  320  is adjusted by a gap (d) between the feeding unit  310  and the radiating unit  320 . Accordingly, the resistance element (R r ) of the radiating unit  320  is matched with the resistance element (R c ) of the RF front-end  121  by adjusting the gap between the feeding unit  310  and the radiating unit  320 . The self-mutual inductance (L rp ) is adjusted based on a cross length of the radiating unit  320 . 
         [0051]    The RF front-end  121  is connected to the loop of the feeding unit  310  for the complex conjugate matching with the tag antenna  300 . The equivalent circuit of the feeding unit  310  and the RF front-end  121  is converted into inductive circuit. An inductive reactance element induced by a connection of the feeding unit  310  with the RF front-end  121  is varied based on a mutual coupling coefficient (M), and determines a resonant frequency band along with the parasitic capacitive reactance (C r ) and self-mutual inductance (L rp ) In particular, if the number of winding in the helical structure of the radiating unit  320  is increased, the self-mutual inductance (L rp ) is increased but the parasitic capacitive reactance (C r ) decreases. The radiating frequency is transited by the increased self-mutual inductance (L rp ) and the decreased parasitic capacitive reactance (C r ). Accordingly, the tag antenna  300  shifts a radiating frequency to a requested band without a change of an input resistance by adjusting the number of the winding of the radiating unit  310 . 
         [0052]    The radiating unit  320  has a helical structure, and is manufactured by winding a wire so as to have an electric structure of a screw shape. The radiating unit  320  generates a frequency resonant by a pitch angle parameter having the diameter and the number of the winding, a gap between wound elements, and a line which is tangential element between a plane of a helical axis direction and a helical wire. 
         [0053]    The input impedance depends on a valve of the pitch angle and a radius of the helical wire. A normal mode helical antenna has an electrically small size, a low gain, and narrowband frequency characteristics. However, a radiating field is a maximum to a normal vector direction of a screw axis. 
         [0054]      FIG. 5  shows a graph illustrating a change in an input impedance of a tag antenna according to a frequency change. The loop radius (r f )  311  of the feeding unit  310  is 14.8 mm, and the thickness (t f )  312  of the helical wire is 0.38 mm respectively. The loop radius (r r )  321  of the radiating unit  320  is 0.95 mm, and the thickness (t f )  322  of the helical wire is 14.0 mm respectively. The gap (d) between the feeding unit  310  and the radiating unit  320  is 4 mm. 
         [0055]    The resistance element (R a ) and the reactance element (X a ) of the input impedance of the tag antenna  300  are conjugate-matched with the impedance (Z c =R c +jX c ) of the RF front-end  121  at a resonant frequency (w o =912 MHz). The resistance element (R a ) of the input impedance of the tag antenna  300  has a constant value near the resonant frequency. The reactance element (X a ) of the input impedance of the tag antenna  300  is proportion to the frequency. 
         [0056]      FIG. 6  shows a graph illustrating a resistance change of input impedance of a tag antenna according to a gap between a feeding unit and a radiating unit. 
         [0057]    The other values except the gap (d) between the feeding unit  310  and the radiating unit  320  are the same with the values shown in  FIG. 5 . Since the resistance element (X a ) is proportion to the gap (d) between the feeding unit  310  and the radiating unit  320 , the resistance element (X a ) is matched with the resistance element (R c ) of the front-end impedance by adjusting the gap (d). 
         [0058]      FIG. 7  shows a graph illustrating a reflective loss for a tag antenna in 900 MHz-930 MHz band in accordance with an embodiment of the present invention. The loop radius  311  of the feeding unit  310 , the thickness (t f )  312  of the wire, the loop radius  321  of the radiating unit  320  and the thickness  322  of the wire are the same with the values shown in  FIG. 5 . 
         [0059]    If a frequency bandwidth is defined to be centered on −5 dB based on the tag antenna having narrowband small antenna characteristics, the tag antenna  300  shown in the  FIG. 3  has a narrow bandwidth from 2.3 MHz to 4 MHz at a reference frequency of 910 MHz. Since the passive RFID system communicates through ten channels having a frequency bandwidth of 200 KHz, the tag antenna  300  needs the minimum bandwidth of 2 MHz. 
         [0060]    Meanwhile, the shape of the feeding unit  310  or radiating unit  320  may be changed to extend a bandwidth of the tag antenna  300 . 
         [0061]      FIGS. 8A to 8C  show the shapes of a feeding unit and a radiating unit of an antenna in accordance with an embodiment of the present invention. The bandwidth of the feeding units  810  and  820  may be increased by manufacturing the feeding units  810  and  820  to be a bow-tie shape as shown in  FIG. 8A . 
         [0062]    The bandwidth of the radiating units  830  and  840  may be increased by manufacturing the radiating units  830  and  840  to be step shape or a slanted step shape when the radiating units  830  and  840  are unfolded as shown in  FIG. 8B . 
         [0063]    Moreover, the bandwidth of the radiating units  850  and  860  may be increased by manufacturing the radiating units  850  and  860  to be radiating lines having a groove or a slot as shown in  FIG. 8C . 
         [0064]      FIG. 9  shows a radiation pattern of an antenna according to an elevation direction and an azimuth direction. A gain value of the tag antenna  300  is represented according to an azimuth angle  910  and an elevation angle  920  based on polarization characteristics of the reader antenna. 
         [0065]    When the reader antenna is in the state of a left hand circular polarization, the tag antenna  300  has a gain of −5 dB, and there is no null in a specific azimuth angle. Moreover, if the tag antenna  300  is located at 0°, a first maximum radiation occurs at 240°, and a second maximum radiation occurs at 60°. 
         [0066]    Since a long distance radiation of an electric field in the tag antenna  300  occurs at a cross portion of the radiating unit of the tag antenna  300 , a read range of the azimuth direction may be omni-directional if the cross portion of the radiating unit of the tag antenna  300  is extended. 
         [0067]    A maximum gain of the elevation direction  920  occurs at the inclination of 30° from the tag antenna  300 . Since a minimum gain of the elevation direction  920  occurs at the inclination of near −90°, although a liquid substance having electromagnetic wave interference characteristics exists under the tag antenna  300 , the liquid substance has not influenced on the gain of the tag antenna. 
         [0068]      FIG. 10  shows a graph illustrating a measured read range and a simulated result of an antenna when the tag antenna rotates to an azimuth direction.  FIG. 11  shows a graph illustrating a measured read range and a simulated result of an antenna when the tag antenna rotates to an elevation direction. 
         [0069]    The length of each element  311 ,  312 ,  321 ,  322  and  320  of the tag antenna  300  used in measurement is the same with the length of the tag antenna shown in  FIG. 5 . The RFID reader  110  using a circular polarization has a power of 1[W] and an antenna gain of 5.9[dB]. The read range in the simulation is normalized to a maximum value of the measured read range based on various measuring elements. 
         [0070]    Referring to  FIG. 10 , the read range of the tag antenna  300  having omni-directional characteristics has a maximum value of 1.7 [m] and a minimum value of 1.3 [m]. A maximum radiation occurs at a cross portion of the helical structure. 
         [0071]    Referring to  FIG. 11 , the radiation axis is slanted to 30° from the elevation direction. The tag antenna  100  has a read range of 1 [m] at the position which is slanted to 90° from the elevation direction. 
         [0072]    In general, the read range in a passive RFID system is under influence of a forward link which represents electromagnetic wave propagation from a reader antenna to a tag antenna through an air medium. The read range is calculated by substituting a gain value of the azimuth direction or an elevation direction by Friis transmission formula as Eq. (4). 
         [0000]      20 log  R   km   =P   reader(dBm)   +G   reader(dB)   −P   tag(dBm)   +G   tag(dB) −32.44− PLF   Eq. (4) 
         [0073]    The R km  denotes a distance between the RFID tag antenna and the reader antenna. The P reader(dBm)  denotes an output power of the RFID reader. The G reader(dBm)  denotes a gain of the reader antenna. The P tag(dBm)  denotes a minimum threshold power of the RF front-end. The G tag(dBm)  denotes a gain of the tag antenna. The f MHz  denotes a carrier frequency of the RFID reader. The PLF denotes a loss according to a polarization between the reader antenna and the tag antenna. The read range means a physical distance between the reader antenna and the RFID tag. 
         [0074]      FIG. 12  illustrates an antenna in accordance with an embodiment of the present invention. In the embodiment of the present invention, the tag antenna  300  is applied to a cork attached to a plastic bottle of polyethylene terephthalate (PET) material. 
         [0075]    Since the cost of the RF front-end  121  is higher than the cost of the tag antenna  300 , it is preferable that the radiating unit  320  and the feeding unit  310  connected to the RF front-end  121  are manufactured respectively. The radius of the feeding unit  310  is larger than the radius of the radiating unit  320 . 
         [0076]    Since the loop without ground is not related to a dielectric constant, a supporting unit  1210  uses a plastic material. The supporting unit  1210  is easily attached and detached by being inserted into the cork  1220 . An iron wire of circle shape which is attached to the cork  1220  is inserted into the supporting unit  1210 . Although the read range of the tag antenna  300  is reduced, the radiating unit  320  is manufactured easily by attaching a metallic tape or printing a conductive ink. 
         [0077]    If the metallic tape or conductive ink is used instead of the iron wire, a thickness of the radiating unit  320  becomes thin, and an input resistance of the tag antenna  300  increases. Thus, the height of the tag antenna  300  is reduced, and the length of the cross portion of the tag antenna  300  for the self-mutual inductance (L rp ) is lengthened. Accordingly, the read range of the tag antenna  300  may be dispersed in omni-direction. 
         [0078]    The antenna having the loop structure and the helical structure according to an embodiment of the present invention may be efficiently matched to an antenna element having predetermined complex impedance. 
         [0079]    In particular, the antenna and the RFID tag using the same according to the present invention are capable of being efficiently matched with the RF front-end having a larger capacitive reactance than a resistance element. 
         [0080]    Moreover, the antenna and the RFID using the same according to the present invention have a maximum read range in an azimuth direction and omni-directional characteristics using the loop and helical structure. Although an electromagnetic wave interference object exists under the object having the RFID tag, the RFID tag recognizes the object without an influence on the gain of the antenna. 
         [0081]    The antenna of the present invention transits a radiation frequency to a requested band through the number of the winding in the helical structure without the change of the input resistance, and may increase a maximum radiation angle from the antenna by increasing a cross portion in the helical structure. 
         [0082]    The present application contains subject matter related to Korean patent application No. 2006-61387, filed in the Korean Intellectual Property Office on Jun. 30, 2006, the entire contents of which being incorporated herein by reference. 
         [0083]    While the present invention has been described with respect to certain preferred embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirits and scope of the invention as defined in the following claims.