Abstract:
The present invention relates to techniques to excite a circularly polarized antenna and, more particularly, to a circularly polarized antenna having a QUAD-EMC unit structure. It comprises plural polarized antenna elements; a signal distributor; and a signal coupling element electrically coupled to the polarized antenna elements and electrically connected the signal distributor; wherein, when the circularly polarized antenna is in a transmitting state, the signal coupling element sends the electrical signal from the signal distributor to the polarized antenna elements, and the polarized antenna elements transform the electrical signal into the circularly polarized signal and transmit the circularly polarized signal thereafter; when the circularly polarized antenna is in a receiving state, the polarized antenna elements receive the circularly polarized signal and transform the circularly polarized signal into the electrical signal, and the signal coupling element sends the electrical signal from the polarized antenna elements to the signal distributor.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to techniques to excite a circularly polarized antenna and, more particularly, to a circularly polarized antenna having a QUAD-EMC unit structure. 
   2. Description of Related Art 
   In the application field of mobile communication (such as the communication between mobile phone and base station), the mobile communication end point (e.g. mobile phone) in any state (for example, mobile phone is held horizontally or vertically by the user) must completely receive the signal coming from the fixed part (e.g. the base station) so it usually makes use of a circular polarized (CP) signal. 
   The circularly polarized antenna nowadays has structure of a single polarized antenna element, as shown in  FIG. 1A . The polarized antenna element  11  electrically connects to the conducting bar  111  with a function of distributing signals, and the conducting bar  111  electrically connects to a signal distributor, which is not shown in the figure. Besides, the two diagonal corners of the polarized antenna element  11  are chamfered to transmit and receive the circular polarized signal, which is the so-called “chamfered-corner treatment”. 
   However, the CP antenna of this structure is used in conjunction with a narrow bandwidth, which can not be adjusted according to the need in reality. Besides, the gain of this kind of CP antenna is restricted, so it is unable to fit in with the strict requirement of a mobile communication module (e.g. the antenna module of a mobile phone). 
   Moreover, compared with the above mentioned circularly polarized antenna having the single polarized antenna element, the circularly polarized antenna with a QUAD-EMC unit structure has the following advantages: (1) the gain is better still; (2) adjusting the relative position among every composed polarized antenna can improve the directivity of the transmitted polarized signal and modify the distributing situation of bandwidth. 
     FIG. 1B  is a schematic drawing of a prior art polarized antenna having the QUAD-EMC unit structure. The polarized antenna comprises a grounding substrate  15  and four polarized antenna elements  121 ,  122 ,  123 , and  124  on its surface. These polarized antenna elements electrically couple with a square conducting plate  13 , which has a function of distributing signals, under the bottom surface of substrate  15 , and the conducting plate  13  electrically connects to a signal distributor (not shown in the figure) through the conducting bar  14 . When the polarized antenna is in a transmitting state, an electrical signal from the signal distributor is sent to the conducting bar  14 , where it passes through the square conducting plate  13 , and finally is sent to the polarized antenna elements  121 ,  122 ,  123 , and  124 . Then, the polarized antenna elements  121 ,  122 ,  123 , and  124  transform the electrical signal into a wireless linear polarized (LP) signal and transmit to the environment. When the polarized antenna is in a receiving state, the polarized antenna elements  121 ,  122 ,  123 ,  124  receive the wireless linear polarized (LP) signal from the environment and transform it into an electrical signal. Then the electrical signal is sent to the square conducting plate  13 , where it passes through the conducting bar  14 , and finally is sent to the signal distributor. 
   Although the polarized antenna with the QUAD-EMC unit has such advantages, it can transmit and receive only the linear polarized signal, not the circular polarized signal. Hence, this polarized antenna with the QUAD-EMC unit cannot be applied in a mobile phone or any antenna module of mobile communication apparatus. 
   Therefore, it is desirable for the industries to provide a circular polarized antenna, which not only can transmit and receive the circular polarized signal, but also has a structure of the QUAD-EMC unit, to improve the performance of the mobile phone or any antenna module of mobile communication apparatus. 
   SUMMARY OF THE INVENTION 
   The circularly polarized antenna (CP antenna) of the present invention comprises a plurality of polarized antenna elements for transmitting and receiving a circularly polarized signal (CP signal); a signal distributor for distributing an electrical signal; and a signal coupling element electrically coupled to the polarized antenna elements and electrically connected to the signal distributor. When the CP antenna is in a transmitting state, the signal coupling element sends the electrical signal from the signal distributor to the polarized antenna elements, and the polarized antenna elements transform the electrical signal into the CP signal and transmit the CP signal thereafter. When the CP antenna is in a receiving state, the polarized antenna elements receive the CP signal and transform the CP signal into the electrical signal, and the signal coupling element sends the electrical signal from the polarized antenna elements to the signal distributor. 
   Compared with the conventional CP antenna that has a single polarized antenna element, the gain of the CP antenna of the present invention is better. Moreover, the CP antenna having the QUAD-EMC unit structure still has the same advantage of the conventional CP antenna. Besides, by adjusting the relative position of the polarized antenna element, and adjusting the locations where the signal coupling element is electrically coupled to the polarized antenna elements (i.e. the locations of the coupling points), the directivity of CP signal transmitted by the CP antenna of the present invention can be improved, as well as improving the operating bandwidth region thereof, such as the 3-dB axial ratio bandwidth and the 10-dB return loss bandwidth. 
   The quantity of the polarized antenna elements that the CP antenna of the present invention comprises is not restricted. Preferably, the quantity of the polarized antenna elements is four. The shape of the polarized antenna element CP antenna of the present invention is not restricted. Preferably, the shape of the polarized antenna element is nearly squared in shape. The corners of the polarized antenna elements can be treated by any conventional method. Preferably, at least one corner of the polarized antenna element is a chamfered-corner. The signal coupling element of the CP antenna of the present invention can be a conductor with any shape. Preferably, the signal coupling element is a coupling-ring or a conductive plate. More preferably, the signal coupling element is a coupling-ring with a shape of a rectangle or a conductive plate with a shape of a square. The polarized antenna elements of the CP antenna of the present invention can be mounted on any suitable printed circuit board. Preferably, the printed circuit board is an FR-4 microwave substrate, a Duroid™ microwave substrate, or a Teflon™ microwave substrate. The signal coupling element of the CP antenna of the present invention can be mounted on any suitable printed circuit board. Preferably, the printed circuit board is an FR-4 microwave substrate, a Duroid™ microwave substrate, or a Teflon™ microwave substrate. The signal distributor of the CP antenna of the present invention can be electrically connected to any kind of signal line. Preferably, the signal line is a coaxial cable, or a copper strand wire. The CP antenna of the present invention can transmit and receive a CP signal at any frequency. Preferably, the frequency of the CP signal ranges from 5.15 to 5.825 GHz. 
   Other objects, advantages, and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  is a schematic drawing of a prior art circularly polarized antenna having a single polarized antenna element. 
       FIG. 1B  is a schematic drawing of a prior art polarized antenna having a QUAD-EMC unit. 
       FIG. 2A  is a schematic drawing of a circularly polarized antenna according to the first preferred embodiment of the present invention. 
       FIG. 2B  is a schematic drawing of a circularly polarized antenna in an operating state according to the first preferred embodiment of the present invention. 
       FIG. 3  is a schematic drawing of the axial ratio vs. the frequency as the tail length (l) as shown in  FIG. 2B  is 6 mm. 
       FIG. 4  is a schematic drawing of the return loss vs. the frequency as the tail length (l) as shown in  FIG. 2B  is 6 mm. 
       FIG. 5A  is a schematic drawing of a circularly polarized antenna according to the second preferred embodiment of the present invention. 
       FIG. 5B  is a schematic drawing of a circularly polarized antenna in an operating state according to the second preferred embodiment of the present invention. 
       FIG. 6  is a schematic drawing of the axial ratio vs. the frequency as the tail length (l) as shown in  FIG. 5B  is 6 mm. 
       FIG. 7  is a schematic drawing of the return loss vs. the frequency as the tail length (l) as shown in  FIG. 5B  is 6 mm. 
       FIG. 8  is a schematic drawing of a circularly polarized antenna in an operating state according to the third preferred embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 2A  is a schematic drawing of a circularly polarized antenna (CP antenna) according to the first preferred embodiment of the present invention. Polarized antenna elements  211 ,  212 ,  213 , and  214  are disposed on the upper surface  221  of the first substrate  22 . A coupling-ring  23  is disposed on the upper surface  241  of the second substrate  24 . The terminals of the coupling-ring are first terminal  231  and second terminal  232 , and the terminals  231 , and  232  are electrically connected to a converter  25 . A coaxial cable  25  is electrically connected to the converter  25  to send an electrical signal to a signal distributor  26  for signal treatment. 
   The first substrate  22  is mounted on the second substrate  24 . Afterwards, the first substrate  22  and the second substrate  24  are disposed on a surface of a grounded plate, and a CP antenna is obtained. As shown in  FIG. 2B , the polarized antenna elements  211 ,  212 ,  213 , and  214  are electrically connected to the coupling-ring through the coupling points  233 ,  234 ,  235 , and  236 , respectively. In this embodiment, the first substrate  22  and the second substrate  24  are identical substrates made of FR-4, and they have the same thickness of 1.6 mm. The dimension of the grounded plate is 5 cm×5 cm. 
     FIG. 2B  is a schematic drawing of a circularly polarized antenna in an operating state according to the first preferred embodiment of the present invention. The view of  FIG. 2B  is directed from the second substrate  24  to the first substrate  22 . Besides, in order to simplify the figure, the first substrate  22  and the second substrate  24  are not shown in  FIG. 2B . Therefore,  FIG. 2B  shows the relative position of the polarized antenna elements  211 ,  212 ,  213 , and  214  and the coupling-ring  23 . Tail length (l) and offset distance (d)—two parameters concerning efficiency—are defined in  FIG. 2B . The efficiency of the CP antenna of the first preferred embodiment is performed by the 3-dB axial ratio bandwidth and the 10-dB return loss bandwidth of the transmitted signal. 
   Referring to  FIG. 2B , the polarized antenna elements  211 ,  212 ,  213 , and  214  all have a dimension of 12 mm×11 mm, and are nearly squared in shape. Each polarized antenna element can support two degenerate states, and therefore, the CP antenna can transmit and receive circularly polarized signals (CP signal). The distance between two adjacent polarized antenna elements is 7 mm. 
   In another aspect, the width of the coupling-ring  23  is 1.5 mm, the length of the coupling-ring between coupling points  235  and  233  is 13 mm, and the length of the coupling-ring between coupling points  235  and  236  is 14 mm. As to the optimum location of the coupling points, the approximate location of the coupling points is decided by probe-feed reference design, and then tested by a software through a trial-and-error process to obtain the optimum location of the coupling points. 
   Table 1 shows the simulated results of the CP antenna of the first preferred embodiment as shown in  FIG. 2B  and it shows that the axial ratio and the return loss of the CP antenna variably depend on the increase of the tail length (l). 
   
     
       
             
             
             
           
             
             
             
             
             
           
             
             
             
             
             
           
         
             
                 
               TABLE 1 
             
           
           
             
                 
                 
             
             
                 
               Axial ratio 
               Return loss 
             
           
        
         
             
                 
               Center 
               3-dB 
               Center 
               10-dB 
             
             
                 
               frequency 
               bandwidth 
               frequency 
               bandwidth 
             
             
                 
                 
             
           
        
         
             
               l = 1 mm 
               6.043 GHz 
               112 MHz 
                5.45 GHz 
               700 MHz 
             
             
               l = 2 mm 
               5.943 GHz 
               129 MHz 
                5.4 GHz 
               600 MHz 
             
             
               l = 3 mm 
               5.813 GHz 
               157 MHz 
                5.35 GHz 
               600 MHz 
             
             
               l = 4 mm 
               5.657 GHz 
               195 MHz 
               5.325 GHz 
               550 MHz 
             
             
               l = 4.5 mm 
               5.455 GHz 
               140 MHz 
               5.325 GHz 
               550 MHz 
             
             
               l = 5 mm 
               5.373 GHz 
                97 MHz 
                5.3 GHz 
               600 MHz 
             
             
                 
             
           
        
       
     
   
   In table 1, the optimum frequency of the CP antenna of the first preferred embodiment is around 5.3 GHz, as the tail length (l) is 5 mm. Generally speaking, the longer the tail length, the longer the coupling-ring, the lower the center frequency within the axial ratio bandwidth region. However, the center frequency within the return loss bandwidth region has no obvious changes while tuning the tail length. Moreover, at certain specific tail lengths, the center frequency within the return loss bandwidth region has only slight changes. Therefore, the CP antenna of the first preferred embodiment can obtain very similar or identical center frequency within an axial ratio bandwidth and within a return loss bandwidth by tuning the tail length or the spacing between the polarized antenna elements. 
     FIG. 3  is a schematic drawing of the axial ratio vs. the frequency, which shows that the axial ratio of the transmitted CP signal variably depends on the increase of the offset distance (d), while the tail length (l) as shown in  FIG. 2B  is 6 mm.  FIG. 4  is a schematic drawing of the return loss vs. the frequency, which shows that the return loss of the transmitted CP signal variably depends on the increase of the offset distance (d), while the tail length (l) as shown in  FIG. 2B  is 6 mm. In  FIGS. 3 and 4 , the offset distance is increased from 0.12 mm to 0.72 mm. 
   Regardless of the offset distance,  FIGS. 3 and 4  also show that the 10-dB return loss bandwidth is wider than the 3-dB axial ratio bandwidth of the transmitted CP signal. Besides, both of the bandwidths are wider than the predetermined working frequency range of the CP antenna, i.e. the United State U-NII band. In this embodiment, the predetermined working frequency ranges from 5.15 GHz to 5.825 GHz. Therefore, the CP antenna of the first preferred embodiment can transmit and receive CP signals at its predetermined working frequency range. 
     FIG. 5A  is a schematic drawing of a CP antenna according to the second preferred embodiment of the present invention. Polarized antenna elements  511 ,  512 ,  513 , and  514  are disposed on the upper surface  521  of the first substrate  52 . A coupling-ring  53  is disposed on the upper surface  541  of the second substrate  24 . The terminals of the coupling-ring are first terminal  531  and second terminal  532 , and the terminals  531 , and  532  are electrically connected to a converter  55 . A coaxial cable  551  is electrically connected to the converter  55  to send an electrical signal to a signal distributor  56  for signal treatment. 
   The first substrate  52  is mounted on the second substrate  54 . Afterward, the first substrate  52  and the second substrate  54  are disposed on a surface of a grounded plate, and a CP antenna is obtained. As shown in  FIG. 2B , the polarized antenna elements  511 ,  512 ,  513 , and  514  are electrically connected to the coupling-ring through the coupling points  533 ,  534 ,  535 , and  536 , respectively. In this embodiment, the first substrate  52  and the second substrate  24  are identical substrates made of FR-4, and they have the same thickness of 1.6 mm. The dimension of the grounded plate is 5 cm×5 cm. 
     FIG. 5B  is a schematic drawing of a CP antenna in an operating state according to the second preferred embodiment of the present invention. The view of  FIG. 5B  is directed from the second substrate  54  to the first substrate  52 . Besides, in order to simplify the figure, the first substrate  52  and the second substrate  54  are not shown in  FIG. 5B . Therefore,  FIG. 5B  shows the relative position of the polarized antenna elements  511 ,  512 ,  513 , and  514  and the coupling-ring  53 . Tail length (l) and offset distance (d)—two parameters concerning efficiency—are defined in  FIG. 5B . The efficiency of the CP antenna of the second preferred embodiment is shown by the 3-dB axial ratio bandwidth and the 10-dB return loss bandwidth of the transmitted signal. 
   Referring to  FIG. 5B , the polarized antenna elements  211 ,  212 ,  213 , and  214  all have a dimension of 12 mm×11 mm, and are similar to a square in shape. The border-length of them is approximately half of the predetermined wavelength of the transmitting or receiving CP signal of the second preferred embodiment. The nearly squared antenna element can support two degenerate states, and the CP antenna can transmit and receive CP signals. The distance between two adjacent polarized antenna elements is 7 mm. 
   In another aspect, the width of the coupling-ring  53  is 1.5 mm, the length of the coupling-ring between coupling points  535  and  533  is 23 mm, and the length of the coupling-ring between coupling points  535  and  536  is 22 mm. Therefore, the length of the whole coupling-ring  53  is approximately four times the wavelength of the CP signal transmitted by the CP antenna of the second preferred embodiment. 
   Besides, compared with the coupling-ring  23  of the first preferred embodiment, the coupling-ring  53  of the second preferred embodiment is larger, and the coupling-ring  53  has more space to regulate the offset distance (d). Hence, the efficiency of the CP antenna of the second preferred embodiment, such as the 3-dB axial ratio bandwidth and the 10-dB return loss bandwidth, is better than that of the first preferred embodiment. 
   Table 2 shows the simulated results of the CP antenna of the second preferred embodiment as shown in  FIG. 5B  and it shows that the axial ratio and the return loss of the CP antenna variably depend on the increase of the tail length (l) while the offset distance is zero. 
   
     
       
             
             
             
           
             
             
             
             
             
           
             
             
             
             
             
           
         
             
                 
               TABLE 2 
             
           
           
             
                 
                 
             
             
                 
               Axial ratio 
               Return loss 
             
           
        
         
             
                 
               Center 
               3-dB 
               Center 
               10-dB 
             
             
                 
               frequency 
               bandwidth 
               frequency 
               bandwidth 
             
             
                 
                 
             
           
        
         
             
               L = 2 mm 
               No CP wave 
               No CP wave 
               5.436 GHz 
               920 MHz 
             
             
               L = 3 mm 
               No CP wave 
               No CP wave 
               5.376 GHz 
               826 MHz 
             
             
               L = 4 mm 
               No CP wave 
               No CP wave 
               5.326 GHz 
               750 MHz 
             
             
               L = 4.5 mm 
               No CP wave 
               No CP wave 
                5.31 GHz 
               733 MHz 
             
             
               L = 5 mm 
               5.61 GHz 
               372 MHz 
               5.298 GHz 
               720 MHz 
             
             
               l = 5.5 mm 
               5.54 GHz 
               425 MHz 
                5.29 GHz 
               715 MHz 
             
             
               l = 6 mm 
               5.335 GHz  
               230 MHz 
               5.286 GHz 
               712 MHz 
             
             
                 
             
           
        
       
     
   
   In table 2, the optimum frequency of the CP antenna of the second preferred embodiment is approximately 5.3 GHz, as the tail length (l) is 6 mm. 
     FIG. 6  is a schematic drawing of the axial ratio vs. the frequency, which shows that the axial ratio of the transmitted CP signal variably depends on the increase of the offset distance (d), while the tail length (l) as shown in  FIG. 5B  is 6 mm.  FIG. 7  is a schematic drawing of the return loss vs. the frequency, which shows that the return loss of the transmitted CP signal variably depends on the increase of the offset distance (d), while the tail length (l) as shown in  FIG. 5B  is 6 mm. In  FIGS. 6 and 7 , the offset distance is increased from 1.45 mm to 3.45 mm. 
   Regardless of the offset distance,  FIGS. 6 and 7  show that the 10-dB return loss bandwidth is wider than the 3-dB axial ratio bandwidth of the transmitted CP signal. Besides, both of the bandwidths are wider than the predetermined working frequency range of the CP antenna, i.e. the United State U-NII band. In this embodiment, the predetermined working frequency ranges from 5.15 GHz to 5.825 GHz. Therefore, the CP antenna of the second preferred embodiment can transmit and receive CP signals at its predetermined working frequency range. 
   Moreover, the CP antenna of the second preferred embodiment has two resonant frequencies at 5.3 GHz and 5.85 GHz as shown in  FIGS. 6 and 7 . It is also seen that equal return loss is obtainable at these resonant frequencies, and the axial ratio at these resonant frequencies is the lowest in  FIGS. 6 and 7 . Hence, the CP antenna of the second preferred embodiment can transmit CP signals at these two frequencies simultaneously. 
     FIG. 8  is a schematic drawing of a circularly polarized antenna in an operating state according to the third preferred embodiment of the present invention.  FIG. 8  shows the relative position of the polarized antenna elements  811 ,  812 ,  813 , and  814  and the conductive plate  82 . These polarized antenna elements  811 ,  812 ,  813 , and  814  are all similar to a square in shape, and they are all electrically connected to the conductive plate  82 , which has a function of coupling signals, and are electrically connected to a signal distributor  85  through a conducting strip  83 . Besides, the diagonal corners of each polarized antenna element  811 ,  812 ,  813 , and  814  are chamfered. 
   These chamfered polarized antenna elements  811 ,  812 ,  813 , and  814  can provide two degenerate states, and the CP antenna of the third preferred embodiment therefore can transmit and receive CP signals. Compared with the first and the second embodiments, the 3-dB axial ratio bandwidth and the 10-d-B return loss bandwidth of the CP signal transmitted by the CP antenna of the third preferred embodiment is narrower. However, these two bandwidths of the third preferred embodiment are wider than that of the conventional CP antenna having a single polarized antenna element. 
   Therefore, the CP antenna of the present invention can transmit and receive CP signals. Besides, the gain of the CP antenna of the present invention is better than that of the conventional CP antenna that has a single polarized antenna element. Moreover, the CP antenna having the QUAD-EMC unit structure still has the same advantage of the conventional CP antenna. By tuning the relative position of the polarized antenna element, and tuning the locations whereat the signal coupling element is electrically coupled to the polarized antenna elements (i.e. the locations of the coupling points), the bandwidth of CP signals transmitted by the CP antenna of the present invention can be improved. As shown in  FIG. 2B  and  FIG. 5B , the coupling points of these two embodiments are all located on four corners of the coupling-rings, besides, the coupling-ring of the first preferred embodiment is smaller than that of the second preferred embodiment. Therefore, the relative positions of the coupling point  232  in  FIG. 2B  and the coupling point  532  in  FIG. 5B  are almost symmetrical to the center point of the polarized antenna element  211  or  511 , and the relative positions of the coupling points ( 234 ,  534 ), ( 235 , 535 ), or ( 236 , 536 ) are also symmetrical to the center points of the polarized antenna element  212 ,  213 , or  214 , respectively. 
   Furthermore, the directivity of the CP signal transmitted by the CP antenna of the present invention is improved, and the operation bandwidth, such as the 3-dB axial ratio bandwidth and the 10-dB return loss bandwidth is increased. 
   Although the present invention has been explained in relation to its preferred embodiment, it is to be understood that many other possible modifications and variations can be made without departing from the scope of the invention as hereinafter claimed.