Patent Publication Number: US-2013234909-A1

Title: Circularly polarized antenna

Description:
TECHNICAL FIELD 
     The present invention relates to a circularly polarized antenna, and more particularly, to a circularly polarized antenna which is miniaturized and ultralight and has excellent characteristics applicable to small sized communication modules and terminals. 
     BACKGROUND ART 
     In general, satellite utilizing broadcasting, communication, and Internet industries are growing explosively, to settle as core media of a 21C information oriented society. Starting with satellite antennae, a satellite broadcasting reception device industry has been developing, already. 
     Especially, the development has been outstanding in a GPS (Global Positioning System) field and a DMB (Digital Multimedia Broadcasting) field. 
     Moreover, as the device becomes smaller and lighter gradually owing to development of semiconductor and communication technologies, it is a trend that application ranges of the device become wider, generalizing application of the device, not only to a vehicle, but also to GPS and DMB reception functions in a PDA (Personal Digital Assistant). Besides the satellite communication field, a RFID (Radio Frequency Identification) field is a core field that will advance a ubiquitous society. A plurality of tags and reader antennae to be applied to a RFID system require high efficiency and high performance. 
     In order to make the antenna to receive high quality information from the satellite, or to make the RFID system to transmit accurate information between the tag and the reader, the antenna or the RFID system is required to meet numerous requirements, such as a circular polarization characteristic, a large beam width, a high F/B ratio (Front-Back ratio), minimization of performance variation caused by positions and shapes of ground and terminal, and so on. 
     A related art method for embodying circular polarization of a small sized antenna is supply of power to an appropriate position of a patch of a metal square patch antenna having a cut off corner mounted on a high dielectric constant ceramic piece with a coaxial line probe. 
     In general, such a structure has very wide applications, and enables to embody antennae of different sizes by controlling the dielectric constant of the ceramic piece. 
     The ceramic antenna has disadvantages in that weight is heavy in comparison to density of the piece itself, the bandwidth is very small, and individual tuning of the antenna is required during a fabrication process if the dielectric constant is high. 
     In order to solve the problems, usually two or four radiating elements arranged appropriately, and a power feed network for obtaining a circular polarization characteristic are required. At the time of embodying the power feed network for embodying an antenna circular polarization, a power divider and quadrature hybrid circuit are required, to require an additional space, causing an entire structure larger. 
     DISCLOSURE OF INVENTION 
     Technical Problem  
     To solve the problems, an object of the present invention is to provide a circularly polarized antenna by using small sized horizontal monopole radiating elements, coaxial connectors for supplying power to the radiating elements, and a series power divider. 
     Another object of the present invention is to provide a circularly polarized antenna which is miniaturized and ultralight applicable to small sized communication modules and terminals. 
     Technical Solution  
     To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, a circularly polarized antenna includes an upper printed circuit board having a plurality of horizontal monopole radiating elements arranged at fixed intervals, a lower printed circuit board spaced a fixed distance from the upper printed circuit board to have a configuration matched thereto and a power feed network formed thereon, and a plurality of coaxial connectors each for connecting each of the horizontal monopole radiating elements at the upper printed circuit board to the power feed network at the lower printed circuit board, electrically. 
     Advantageous Effects  
     The circularly polarized antenna of the present invention has the following advantages. 
     First, the antenna of the present invention can have a mechanically rigid structure owing to the coaxial connectors which connect the upper printed circuit board having a plurality of horizontal monopole radiating elements formed thereon to the lower printed circuit board having a power feed network formed thereon, electrically. 
     Second, a weight of a portable communication terminal can be reduced significantly by making a weight lighter compared to a related art ceramic patch. 
     Third, an antenna with excellent gain, axial ratio, and bandwidth characteristics can be embodied even under limitations of fabrication of a miniaturized and ultralight antenna. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a perspective view of a circularly polarized antenna in accordance with a preferred embodiment of the present invention. 
         FIGS. 2 and 3  illustrate plan views of the upper printed circuit board and the lower printed circuit board shown in  FIG. 1 , respectively. 
         FIG. 4  illustrates a back side view showing a back side of the lower printed circuit board shown in  FIG. 3 . 
         FIG. 5  illustrates a perspective view of the coaxial connector shown in  FIG. 1 . 
         FIGS. 6 to 8  illustrate graphs showing reflection coefficient, elevation direction pattern, and elevation direction axial ratio characteristics of a circularly polarized antenna in accordance with a preferred embodiment of the present invention, respectively. 
     
    
    
     BEST MODE 
     A circularly polarized antenna in accordance with a preferred embodiment of the present invention will be described with reference to the attached drawing, in more detail. 
       FIG. 1  illustrates a perspective view of a circularly polarized antenna in accordance with a preferred embodiment of the present invention,  FIGS. 2 and 3  illustrate plan views of the upper printed circuit board and the lower printed circuit board shown in  FIG. 1  respectively,  FIG. 4  illustrates a back side view showing a back side of the lower printed circuit board shown in  FIG. 3 , and  FIG. 5  illustrates a perspective view of the coaxial connector shown in  FIG. 1 . 
     Referring to  FIG. 1 , the circularly polarized antenna includes an upper printed circuit board  100  having a plurality of horizontal monopole radiating elements  110  arranged at fixed intervals along edges, a lower printed circuit board  200  spaced a fixed distance from the upper printed circuit board  100  to have a configuration matched thereto and a power feed network  210  formed thereon, and a plurality of coaxial connectors  300  each for connecting each of the horizontal monopole radiating elements  110  at the upper printed circuit board  100  to the power feed network  210  at the lower printed circuit board  200 , electrically. 
     In this instance, referring to  FIG. 2 , the upper printed circuit board  100  is square with the plurality of horizontal monopole radiating elements  110  arranged at fixed intervals along edges. 
     In embodying each of the horizontal monopole radiating elements  110 , the plurality of horizontal monopole radiating elements  110  are arranged at edges of the upper printed circuit board  100 , taking the circular polarization characteristic and a size of the antenna into account. 
     Each of the horizontal monopole radiating elements  110  has an one side final end with a shorted point  120  formed thereon for impedance match, and an end portion adjacent to the shorted point  120  with a power feed point  130  formed thereon. 
     Referring to  FIG. 2 , for making a size of each of the horizontal monopole radiating elements  110  smaller, though a meander shape may be applied, the present invention suggests application of different shapes of the horizontal monopole radiating element including a straight line type. 
     Each of the horizontal monopole radiating elements  110  has the same shape. Along with this, though the embodiment describes the horizontal monopole radiating elements  110  each having a rectangular structure and formed along four edges, the horizontal monopole radiating elements  110  are not limited to this, but the horizontal monopole radiating elements  110  may be arranged on a circular or polygonal structure at fixed intervals. 
     Referring to  FIGS. 3 and 4 , the lower printed circuit board  200  has power feed network  210  of a series power feed type which is suitable for miniaturization, with one input port  211  and four output ports  212 . 
     The power feed network  210  is configured to supply signals having the same magnitudes and sequential 90 degree phase differences from one another to each of the horizontal monopole radiating elements  110  formed at the upper printed circuit board  100 , respectively. 
     In this instance, the input port  211  is positioned at a center of the lower printed circuit board  200 , and each of the output ports  212  is positioned at a corner of the lower printed circuit board  200  around the input port  211 . 
     The input port  211  is a portion to be connected to the coaxial cable directly in a case the antenna is mounted to a small sized communication module and a terminal, and the output ports  212  are portions to be connected to the coaxial connectors  300  for feeding power to the radiating elements, respectively. 
     Power fed from the input port  211  positioned at the center is distributed to the plurality of output ports  212  such that the power is the same and has a sequential 90 degree phase difference. 
     The lower printed circuit board  200  has a narrow metal band  220  with a plurality of via holes  219  formed therein mounted along edges, and a plurality of coaxial connector fastening holes  218  formed around each of the output ports  212 . The metal band  220  and the via holes  219  are provided for suppressing radiation of a surface wave excitable at the lower printed circuit board  200  from corners of the lower printed circuit board  200 . 
     The power feed network  210  has series power feed type transmission lines  213 ,  214 ,  215 , and  216  having impedances different from one another. The transmission lines  213 ,  214 ,  215 , and  216  and the output ports  212  are connected with branch lines  217  having the same impedances, respectively. 
     The power fed from the input port  211  is divided into the same magnitude by means of a parallel structure of the transmission lines  213 ,  214 ,  215 , and  216  having impedances different from one another and the branch lines  217  respectively connected to the output ports  212 . 
     The branch lines  217  have characteristic impedances made the same with one another for application to the coaxial connectors  300  for connecting the horizontal monopole radiating elements  110 . 
     The transmission lines  213 ,  214 ,  215 , and  216  have lengths made to have a sequential 90 degree phase difference at the output ports  212 . 
     Each of the transmission lines  213 ,  214 ,  215 , and  216  may be embodied to have a meander structure for miniaturization of the antenna. 
     The coaxial connectors  300  performs to serve connecting the power feed points  130  of the horizontal monopole radiating elements  110  positioned at the upper printed circuit board  100  to the output ports  212  at the lower printed circuit board  200  respectively, and, at the same time with this, to serve as mechanical couplings. 
     Referring to  FIG. 5 , the coaxial connector  300  includes an inner core  310  and  320  passed through a center portion and projected beyond opposite sides, a cylindrical Teflon dielectric  330  surrounding the inner core  310  and  320 , a cylindrical outer conductor  340  surrounding the Teflon dielectric  330 , an upper conductor  350  and a lower conductor  360  respectively at a top and a bottom of the outer conductor  340  each having a square shape with an area larger than the outer conductor  340 , an impedance matching short pin  370  projected from a corner of the upper conductor  350 , and a plurality of board fastening pins  370  each projected from a corner of the lower conductor  360 . 
     In this instance, the outer conductor  340 , the upper conductor  350 , the lower conductor  360 , the impedance matching short pin  370 , and the board fastening pins  370  are formed as one unit. 
     Of the inner cores  310  and  320  of the coaxial connector  300 , the inner core  310  projected upward is coupled with the power feed point  130  of each of the horizontal monopole radiating elements  110  at the upper printed circuit board  100  with soldering. 
     In the meantime, the impedance match is achieved by connecting the impedance matching short pin  370  at the upper conductor  350  to the shorted point  120  at the end of each of the horizontal monopole radiating elements  110  with soldering, electrically. 
     And, the lower printed circuit board  200  is connected to the coaxial connector  300  by respectively connecting the output ports  212  to the inner cores  320  projected downward of the inner cores  310  and  320  with soldering, and respectively placing a plurality of supporting pins  380  at the lower conductor  360  in the fastening holes  218  and applying soldering thereto. In this instance, an appropriate space is maintained so that the lower conductor  360  is not in contact with the branch lines  217 . 
       FIGS. 6 to 8  illustrate graphs showing reflection coefficient, elevation direction pattern, and elevation direction axial ratio characteristics of a circularly polarized antenna in accordance with a preferred embodiment of the present invention, respectively. A size of the antenna fabricated as an example has width×length×height of 0.18×0.18×0.04 wavelength, with characteristics of a maximum 3 dBic gain, an axial ratio below 3 dB, and a bandwidth of 2.3%. 
     If an antenna larger than the antenna fabricated as an example of the present invention may have effects in which the maximum gain and bandwidth increase, and the axial ratio decreases in proportion to a size of the antenna. 
     The circularly polarized antenna of the present invention can have characteristics of a miniaturized and ultralight antenna having an excellent mounting effect in a small sized communication module and a terminal. 
     In the meantime, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention.