Patent Publication Number: US-7583235-B2

Title: Folded dipole loop antenna having matching circuit integrally formed therein

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit under 35 U.S.C. § 119(a) of Korean Patent Application No. 10-2006-0088238, filed Sep. 12, 2006, in the Korean Intellectual Property Office, the entire disclosure of which is incorporated herein by reference. 
   FIELD OF THE INVENTION 
   The present invention relates generally to a folded dipole loop antenna having a matching circuit integrally formed therein. More particularly, the present invention relates to a folded dipole loop antenna, which has a matching circuit integrally formed therein to adjust an input reactance thereof and an input impedance thereof and to reduce the size of a device to which it is mounted. 
   BACKGROUND OF THE INVENTION 
   Generally, a loop antenna is formed in the shape of a tetragonal loop, a circle loop, or the like and is used in various fields according to a length thereof. 
   The loop antenna has a characteristically low input resistance. In order to match a 50Ω input resistance of a general antenna, the length of the loop antenna should be taken into account in its design. 
   According to an impedance curve of a square-shaped loop antenna, an input resistance comes close to 50Ω and an input reactance comes close to 0 only when the length of the loop is near to one wavelength. That is, the loop antenna causes resonances only when it is designed to have the length of one wavelength. 
   Also, the loop antenna has a radiating pattern which changes according to the length thereof. For instance, the loop antenna radiates electromagnetic waves along a plane direction thereof when the length of the loop antenna is shorter than one wavelength, and along a direction vertical to the plane direction thereof when it is longer than one wavelength. Accordingly, the radiating pattern of the loop antenna can be adjusted by adjusting of the length of the loop antenna. 
   However, if the radiating pattern is adjusted by forming the length of the loop antenna to be shorter or longer than one wavelength as described above, it is difficult to match the input resistance and the input reactance due to characteristics of the loop antenna. Accordingly, a device to which the antenna is mounted should be equipped with a separate matching circuit for matching the input resistance and the input reactance. 
   However, if the device is equipped with the separate matching circuit, it requires a space for installing the matching circuit. Also, there is a disadvantage in that if a design of the matching circuit should be changed due to interference with other circuit elements after the matching circuit is mounted to the device, it is not easy to change the design of the matching circuit. 
   Thus, there is required a new method capable of minimizing the space which the matching circuit occupies thereby reducing the device in size, and easily changing the design of the matching circuit. 
   SUMMARY OF THE INVENTION 
   Exemplary embodiments of the present invention overcome the above disadvantages and other disadvantages not described above. Also, the present invention is not required to overcome the disadvantages described above, and an exemplary embodiment of the present invention may not overcome any of the problems described above. 
   ms According to an aspect of the present invention, there is provided a folded dipole loop antenna in which a matching circuit is integrally formed to adjust a change of an input reactance thereof and thus to change a resonant frequency thereof, as well as to reduce a size of a device to which the antenna is mounted. 
   According to another aspect of the present invention, there is provided a folded dipole loop antenna including a matching circuit integrally formed in the antenna, and a radiating unit formed in the shape of a loop. The matching circuit has an extended part projected and extended toward a central area of the radiating unit from an inner side surface of the radiating unit. 
   The radiating unit may include an inner loop and an outer loop, which are formed in the same shape. 
   The outer loop at one side thereof may be opened to have both ends, one of which forms a current supplying point and the other of which forms a shorting point. 
   The inner loop may be formed to be bent toward an inner side of the outer loop at an area thereof opposite to the current supplying point and the shorting point of the outer loop and then extended along an inner side surface of the outer loop. 
   The extended part may include a pair of extended lines disposed to face each other toward a central area of the inner loop, free ends of the extended lines being disposed in a spaced-apart relation to each other. 
   The extended lines may be formed on the same line. 
   A tuning part may be formed on each of free ends of the extended lines to be enlarged by a predetermined length in a direction vertical to a longitudinal direction thereof. 
   The more a length of the tuning part is reduced, the more a resonant frequency may be lowered. 
   The above and other aspects of the invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses exemplary embodiment of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above aspect and other features of the present invention will become more apparent by describing in detail exemplary embodiment thereof with reference to the attached drawing figures, wherein; 
       FIG. 1  is a top plan view illustrating a folded dipole loop antenna according to an exemplary embodiment of the present invention; 
       FIG. 2A  is a view illustrating traces on a Smith chart, which changes according to a length C 2  of a tuning part of  FIG. 1 ; 
       FIG. 2B  is a view illustrating traces on the Smith chart, which changes according to a distance L between an extended part and an inner loop of  FIG. 1 ; 
       FIG. 3A  is a view illustrating a trace on the Smith chart in case that a matching circuit is removed from the folded dipole loop antenna of  FIG. 1 ; 
       FIG. 3B  is a view illustrating a trace on the Smith chart of the folded dipole loop antenna of  FIG. 1 ; 
       FIG. 4  is a view illustrating a flow of electric current of the folded dipole loop antenna of  FIG. 1 ; 
       FIG. 5  is a graph illustrating an S11 characteristic of an example of the folded dipole loop antenna according to the exemplary embodiment of the present invention; and 
       FIGS. 6A through 6C  are graphs illustrating a radiating characteristic of the folded dipole loop antenna of  FIG. 1 . 
   

   Throughout the drawings, the same drawing reference numerals will be understood to refer to the same elements, features, and structures. 
   DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
   Hereinafter, a folded dipole loop antenna according to an exemplary embodiment of the present invention will be described in greater detail with reference to the accompanying drawings. 
     FIG. 1  is a top plan view exemplifying a folded dipole loop antenna according to an exemplary embodiment of the present invention. 
   The folded dipole loop antenna according to an exemplary embodiment of the present invention includes a radiating unit  10  to radiate electromagnetic waves, and a matching circuit  20  and  30  to adjust an input reactance of the loop antenna, and is mounted to a circuit board or the like in a spaced-apart relation therewith. 
   The radiating unit  10  is formed in the shape of a tetragonal loop, a circle loop, etc.  FIG. 1  illustrates a radiating unit  10  formed in the shape of the tetragonal loop as an example. 
   The radiating unit  10  includes an inner loop  15  and an outer loop  11 , which are formed of a single conductive wire or strip line bent several times. The outer loop  11  is opened at one side having both ends  5 ,  7  bent facing the circuit board (not illustrated). 
   The both ends of the outer loop  11  are connected with a resonating unit (not illustrated) installed in the circuit board, so that one end of the outer loop forms a current supplying point  5  and the other end of the outer loop forms a shorting point  7 . The current supplying point  5  receives an electric current from the resonating unit (not illustrated), and the shorting point  7  provides an electric current, which remains in the radiating unit  10 , to the resonating unit. The inner loop  15  is formed of a conductive wire or a strip line, which is bent toward an inner side of the outer loop  11  from one side portion of the outer loop  11  opposite to the current supplying point  5  and the shorting point  7  and then extended and disposed in the form of a loop spaced apart from the outer loop  11  along an inner side surface of the outer loop  11 . The inner loop  15  is formed in the same shape as that of the outer loop  11 . 
   The radiating unit  10  constructed as described above constitutes a folded dipole antenna having the same loop shape as that formed by bending a conventional dipole antenna several times. 
   In the radiating unit  10  is disposed the matching circuit  20  and  30 . 
   The matching circuit  20  and  30  includes an extended part  20  extended from an inner side surface of the inner loop  15  of the radiating unit  10 , and a tuning part  30  formed on free ends of the extended part  20 . 
   The extended part  20  is formed of a pair of extended lines extended toward a central area of the radiating unit  10  from the inner side surface of the inner loop  15 . To be more specific, the extended lines are extended from a pair of sides of the inner loop  15 , which are located adjacent to a side of the outer loop  11  on which the current supplying point  5  and the shorting point are formed and a side on which the outer loop  11  and the inner loop  15  are connected with each other, respectively. The extended lines are formed on the same line, and free ends of the extended lines are disposed in a spaced-apart relation to each other. 
   The tuning part  30  is formed of a pair of tuning lines, each of which is formed on one of the free ends of the extended lines. Each of the tuning lines are enlarged and formed by predetermined length and width along a longitudinal direction of the corresponding extended line. Such a tuning part  30  is formed in the form of a capacitor, and acts as the matching circuit  20  and  30  together with the extended part  20 . 
     FIG. 2A  is a view illustrating traces on a Smith chart, which change according to a length C 2  of the tuning part  30  of  FIG. 1 , and  FIG. 2B  is a view illustrating traces on the Smith chart, which change according to a distance L between the extended part  20  and the inner loop  15  of  FIG. 1 . 
   As illustrated in  FIG. 2A , as the length C 2  of the tuning part  30  is adjusted, the trace on the Smith chart is changed. That is, it can be seen that when traces on the Smith chart are measured after adjusting the length C 2  from 8 mm to 16 mm, with the distance L fixed, the greater the length of C 2 , the more a change width of the trace on the Smith chart is enlarged. The reason is that in a reactance curve of the antenna, the more a capacitance of the capacitor is decreased, the more a resonant frequency is lowered. 
   On the other hand, as illustrated in FIG,  2 B, it can be appreciated that when the distance L is adjusted with the length C 2  is fixed, there is almost no change between resultant traces on the smith chart. This means that what is important is not positions of the extended part  20  and the tuning part  30  in the loop, but the existence of the extended part  20  and the tuning part  30  disposed in the loop and the value of C2. 
     FIG. 3A  is a view illustrating a trace on the Smith chart in the case that the matching circuit  20  and  30  is removed from the folded dipole loop antenna of  FIG. 1 , and  FIG. 3B  is a view illustrating a trace on the Smith chart of the folded dipole loop antenna of  FIG. 1 . 
   The traces on the Smith chart shown in  FIGS. 3A and 3B  have almost the same shape. This means that irrespective of whether the matching circuit  20  and  30  exists, there is no change in a resistance value of the folded dipole loop antenna. 
   However, comparing  FIG. 3A  in case that the matching circuit  20  and  30  does not exist and  FIG. 3B  in case that the matching circuit  20  and  30  exists, it can be appreciated that resonant frequencies are different. That is, the resonant frequency of the antenna having the matching circuit  20  and  30  is lower than that of the antenna not having the matching circuit  20  and  30 . The reason is that in the case of having the matching circuit  20  and  30 , the matching circuit  20  and  30  abruptly changes an input reactance in the antenna and thus lowers the resonant frequency. 
     FIG. 4  illustrates a flow of electric current of the folded dipole loop antenna of  FIG. 1 . 
   As illustrated in the drawing, the folded dipole loop antenna according to the exemplary embodiment of the present invention has an electric current path divided into two parts by the matching circuit  20  and  30 . One part of the electric current path is a main current path flowing along the inner loop  15  and the outer loop  11 , and the other part of the electric current path is a subsidiary current path flowing along the extended lines and the tuning lines. Since the inner loop  15  and the outer loop  11  have the same shape as that formed by bending the conventional dipole antenna several times, similar to the antenna mode of the dipole antenna, they have a main current flow along the longitudinal direction of the dipole antenna, that is, a girth direction of the inner loop  15  and the outer loop  11 . The matching circuit  20  and  30  has a subsidiary current flow from one extended line adjacent to the current supplying point  5  to the other extended line adjacent to the shorting point  7 . Here, the subsidiary current acts as a feedback. 
   An amount of current flowing along the main current path is adjusted by an amount of feedback current flowing along the subsidiary current path. This means that the amount of current flowing along the main current path is adjusted according to the length of the tuning part  30 . Due to the adjustment of the amount of current and the change in phase as described above, the input reactance can be adjusted. 
     FIG. 5  is a graph illustrating an S11 characteristic of the folded dipole loop antenna according to the exemplary embodiment of the present invention. 
     FIG. 5  illustrates an S11 characteristic of an example of the folded dipole loop antenna according to the exemplary embodiment of the present invention in which lengths of the radiating unit  10 , the extended part  20 , and the tuning part  30  are designed in predetermined values. As illustrated in the graph, the folded dipole loop antenna according to the exemplary embodiment of the present invention forms a resonant frequency at a band of approximately 0.91 GHz. A bandwidth at −10 dB is approximately 10 MHz from 0.9035 GHz through 0.9135 GHz. That is, the folded dipole loop antenna according to the exemplary embodiment of the present invention is usable as an antenna at the band as described above, and particularly, is adapted to use as an antenna of a radio frequency identification (RFID) system. 
     FIGS. 6A through 6C  are graphs illustrating a radiating characteristic of the folded dipole loop antenna of  FIG. 1 . 
   Assuming that a longitudinal direction of the extended part  20  in a plane of the folded dipole loop antenna is an X axis, a longitudinal direction (C 2  direction) of the tuning part  30  in the plane of the folded dipole loop antenna is a Y axis, and a direction normal to the plane of the folded dipole loop antenna is a Z axis,  FIG. 6A  represents a radiating pattern as viewed from the X-Y axes,  FIG. 6B  represents a radiating pattern as viewed from the Z-X axes, and  FIG. 6C  represents a radiating pattern as viewed from the Z-Y axes. 
   Referring to the graphs of  FIGS. 6A through 6C , the folded dipole loop antenna has omnidirectional properties at the respective planes. From this, it can be appreciated that the matching circuit  20  and  30  formed in the loop antenna does not influence the radiating patterns of the loop antenna. 
   As is apparent from the foregoing description, according to the exemplary embodiment of the present invention, the folded dipole loop antenna has the matching circuit integrally formed therein. Accordingly, a device to which the loop antenna is mounted does not need a separate space for the matching circuit, so that it can be reduced in size. Also, the folded dipole loop antenna can change the resonant frequency by adjusting the change of the input reactance through simply adjusting the length of the tuning part. Accordingly, the folded dipole loop antenna can conveniently change a design of the matching circuit. 
   Although an exemplary embodiment of the present invention has been shown and described in order to exemplify the principle of the present invention, the present invention is not limited to the specific exemplary embodiment. It will be understood that various modifications and changes can be made by one skilled in the art without departing from the spirit and scope of the invention as defined by the appended claims. Therefore, it shall be considered that such modifications, changes and equivalents thereof are all included within the scope of the present invention.