Abstract:
The present invention relates to a circularly polarized antenna and, more particularly, to a compact circularly polarized antenna for transmitting and receiving a circularly polarized signal. The circularly polarized antenna comprises a substrate having an upper surface and a lower surface; a signal distributor; an antenna for transmitting and receiving the circularly polarized signal; and a plurality of support units. The upper surface of the substrate comprises a plurality of slots. One end of each slot overlaps with the respective ends of the other slots at a central region. The lower surface of the substrate comprises a coupling unit being electrically connected with the signal distributor, and the center of the coupling unit corresponds to the central region.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a circularly polarized antenna and, more particularly, to a small-sized circularly polarized antenna for transmitting and receiving a circularly polarized signal. 
         [0003]    2. Description of Related Art 
         [0004]    In some electrical devices, such as the reader device of an RFID system, the antenna module of which must be able to transmit and receive a circularly polarized signal, in order to ensure that the electrical devices can operate normally in any kind of attitude. Besides, since the antenna module must be small enough to be portable, the size of the antenna module of the electrical device is also limited. 
         [0005]    Generally, the circularly polarized antenna of the prior art uses a straight coupling line to couple the electrical signal to the antenna unit, in order to transform the electrical signal into a circularly polarized signal. Then, the circularly polarized signal is transmitted outside. Thus, the substrate of the circularly polarized antenna of the prior art must have a size large enough to enclose the straight coupling line on the surface thereof. Moreover, since the length of the side of the antenna unit must be about half of the wavelength of the circularly polarized signal being transmitted, so if the frequency of the circularly polarized signal being transmitted is 915 MHz, the length of the antenna unit should be 164 mm in the free space. 
         [0006]    The methods to reduce the length of the side of the antenna unit are (1) forming some slots on the surface of the antenna unit or (2) changing the shape of the antenna unit, in order to increase current path. But, both the aforementioned methods are too complex. As a result, the structure of the circularly polarized antenna of the prior art is too complex to reach the requirement of easy-design. 
         [0007]    Therefore, it is desirable for the industries to provide a circularly polarized antenna with a small size, which can not only have the simple structure (the standard shape of square and circle), but also have same function to apply in any kind of the antenna module of the portable electrical device. 
       SUMMARY OF THE INVENTION 
       [0008]    The circularly polarized antenna for transmitting and receiving a circularly polarized signal of the present invention comprises a substrate having an upper surface and a lower surface; a signal distributor; an antenna for transmitting and receiving the circularly polarized signal; and a plurality of support units for supporting the antenna and maintaining a predetermined distance between the antenna and the upper surface of the substrate. The upper surface of the substrate comprises a plurality of slots, wherein one end of each slot overlaps with the respective ends of the other slots at a central region. The lower surface of the substrate comprises a coupling unit being electrically connected with the signal distributor, and the center of the coupling unit corresponds to the central region. 
         [0009]    Therefore, in the same range of the operating frequency (i.e. the operating frequency of the RFID ranges from 902 MHz to 928 MHz), the circularly polarized antenna of the present invention can reduce the size of the antenna and the substrate by forming some slots on the upper surface of the substrate and by changing the size of the coupling portion, so as to maintain the same operating ability as the circularly polarized antenna of the prior art (i.e. having the same return loss and the operating frequency bandwidth). Therefore, the circularly polarized antenna of the present invention can be compact and keep the shape of antenna simple, so as to facilitate the development of a small-sized, more convenient and portable electrical device having the circularly polarized antenna of the present invention, such as the reader device of an RFID system. 
         [0010]    The coupling unit of the circularly polarized antenna of the present invention can comprise any kind of coupling portion, but preferably the coupling portion is a coupling-ring portion with an opening or a polygon-shaped ring having fewer than thirty-six sides with an opening. The substrate of the circularly polarized antenna of the present invention can be made as any suitable printed circuit board, but 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 circularly polarized antenna of the present invention can use any kind of signal distributor, but preferably it is a coaxial cable connector. The signal distributor of the circularly polarized antenna of the present invention can be electrically connected with any kind of signal transmitting line, but preferably the signal transmitting line is a coaxial cable, or a copper strand wire. The upper surface of the substrate of the circularly polarized antenna of the present invention can have formed therein any quantity of the slots, but preferably the quantity of the slots ranges from 4 to 36. Besides, each slot formed on the upper surface of the substrate of the circularly polarized antenna of the present invention preferably has the same width. The size of the coupling portion of the lower surface of the substrate of the circularly polarized antenna of the present invention is not restricted, but preferably the width of the coupling portion is equal to the width of each slot. The shape of the end of each slot is preferably dumbbell-shaped or having a lateral pool. The antenna of the circularly polarized antenna of the present invention can be composed of any kind of metals, but preferably the antenna is composed of a copper alloy containing more than ninety-eight percent copper. The substrate of the circularly polarized antenna of the present can be formed in any kind of shape, but preferably the substrate is a square plate, a rectangular plate or a circular plate. The antenna of the circularly polarized antenna of the present invention preferably is a square plate, a rectangular plate, a square plate with chamfered corners, a rectangular plate with chamfered corners, a polygon-shaped plate, or a circular plate. The supporting unit of the circularly polarized antenna of the present invention preferably is composed of plastics or any electrically insulating materials. The circularly polarized signal of the present invention can transmit or receive circularly polarized signals in any frequency range, but preferably, the frequency ranges from 900 MHz to 930 MHz or from 400 MHz to 600 MHz. The length of the side-length of the antenna of the circularly polarized antenna of the present invention is not restricted, but preferably, the side-length of the antenna ranges from the one-quarter to three-quarters of the wavelength of the circularly polarized signal being transmitted or received by the circularly polarized antenna of the present invention. 
         [0011]    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 
         [0012]      FIG. 1  is a schematic drawing of the circularly polarized antenna according to the first preferred embodiment of the present invention. 
           [0013]      FIG. 2A  is a schematic drawing of the substrate of the circularly polarized antenna according to the first preferred embodiment of the present invention. 
           [0014]      FIG. 2B  is a schematic drawing of the substrate of the circularly polarized antenna according to the first preferred embodiment of the present invention. 
           [0015]      FIG. 3  is a schematic drawing showing the relation between the diameter of the coupling-ring and resonant frequency of the circularly polarized antenna according to the first preferred embodiment of the present invention. 
           [0016]      FIG. 4A  shows the simulated result and the measured result of the axial ratio of the circularly polarized signal transmitted by the circularly polarized antenna according to the first preferred embodiment of the present invention. 
           [0017]      FIG. 4B  shows the simulated result and the measured result of the gain of the circularly polarized antenna according to the first preferred embodiment of the present invention. 
           [0018]      FIG. 5A  is a schematic diagram of the upper surface of substrate of the circularly polarized antenna according to the second preferred embodiment of the present invention, wherein the quantity of the slots is 16. 
           [0019]      FIG. 5B  is a schematic diagram of the upper surface of substrate of the circularly polarized antenna according to the third preferred embodiment of the present invention, wherein the quantity of the slots is 36. 
           [0020]      FIG. 6A  shows the variation of the return loss of the circularly polarized antenna of the present invention regarding the changing of the operating frequency. 
           [0021]      FIG. 6B  shows the variation of the axial ratio of the circularly polarized antenna of the present invention regarding the changing of the operating frequency. 
           [0022]      FIG. 7  is a schematic drawing of the circularly polarized antenna according to the fourth preferred embodiment of the present invention. 
           [0023]      FIG. 8A  is a schematic diagram of the upper surface of substrate of the circularly polarized antenna according to the fourth preferred embodiment of the present invention. 
           [0024]      FIG. 8B  is a schematic diagram of the upper surface of substrate of the circularly polarized antenna according to the fourth preferred embodiment of the present invention. 
           [0025]      FIG. 9  shows the variation of the axial ratio of the circularly polarized signal and the gain of the circularly polarized antenna regarding the changing of the operating frequency. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0026]      FIG. 1  is a schematic diagram of the circularly polarized antenna according to the first preferred embodiment of the present invention. The substrate  21  is an FR-4 microwave substrate with the thickness of 0.8 mm, and the antenna  22  is composed of a copper alloy containing more than ninety-eight percent copper. Referring to  FIG. 1 , the antenna  21  connects with the supporting structure  24  supported by a first supporting rod  231 , a second supporting rod  232 , a third supporting  233 , and a fourth supporting rod  234 . Therefore, the antenna  22  maintains a predetermined distance between it and the upper surface  211  of the substrate  21 . By adjusting the predetermined distance, the gain of the circularly polarized antenna of the present invention can be raised and the circularly polarized characteristics of the circularly polarized antenna can also be improved. 
         [0027]    Since the predetermined distance between the antenna  22  and the upper surface  211  of the substrate  21  is essential for designing the operating frequency of the circularly polarized antenna  1 , when the operating frequency of the circularly polarized antenna  1  needs to be changed, the first supporting rod  231 , the second supporting rod  232 , the third supporting  233 , and the fourth supporting rod  234  must be adjusted to change the predetermined distance between the antenna  22  and the upper surface  211  of the substrate  21 . 
         [0028]      FIG. 2A  is a schematic diagram of the upper surface  211  of substrate  21  of the circularly polarized antenna according to the first preferred embodiment of the present invention.  FIG. 2B  is a schematic diagram of the lower surface  212  of substrate  21  of the circularly polarized antenna according to the first preferred embodiment of the present invention. Referring to  FIG. 2A , the upper surface  211  of the substrate  21  comprises eight slots  213 , and one end of each slot  213  overlaps with the respective ends of the other slots at a central region  214 . In addition, referring to  FIG. 2B , the lower surface  212  of the substrate  21  comprises a coupling-ring line  215  and a straight coupling line  216 , wherein the coupling-ring line  215  has an opening  217  on the edge. That is, the coupling-ring line  215  is not completely closed. In addition, the center of the coupling-ring line  215  corresponds to the center region  214  of the upper surface  211  of the substrate  21 , and the coupling-ring line  215  is electrically connected with a coaxial cable connector  25  through the straight coupling line  216 . 
         [0029]    Moreover, the frequency range of the circularly polarized signal being transmitted and received (i.e. the resonant frequency) by the circularly polarized antenna  1  can be controller by adjusting the diameter of the coupling-ring line  215 , while the shape of the antenna  21  still remains simple. 
         [0030]    Referring to  FIG. 3 , when the size of the antenna size of the present invention is equal to that of the prior art circularly polarized antenna. That is, the diameter of the coupling-ring line is about 138 mm, the resonant frequency of the circularly polarized antenna according to the first preferred embodiment of the present invention is about 500 MHz, which is suitable for the application of digital television. Besides, this resonance frequency is obviously lower than that of the circularly polarized antenna of the prior art (about 915 MHZ). Therefore, when the diameter of the coupling-ring line of the antenna of the present invention becomes shorter, the resonant frequency of the circularly polarized antenna will become larger, toward the high-frequency range. For this reason, the circularly polarized antenna of the present invention can use a substrate with smaller size to have the same resonant frequency range as the circularly polarized antenna of the prior art. 
         [0031]    In the present embodiment, the circularly polarized antenna of the present invention only uses the substrate (FR-4 microwave substrate) with a dimension of 130 mm×130 mm and the antenna (copper plate) with a dimension of 108 mm×108 mm to transmit and receive the circularly polarized signal, the frequency of which ranges from 902 MHz to 928 MHz. Obviously, the size of the antenna of the circularly polarized antenna of the present invention is smaller than that of the antenna of the circularly polarized antenna of the prior art (i.e., 164 mm×164 mm). Furthermore, the resonant distance between the substrate and the antenna of the circularly polarized antenna of the present invention is only 11.4 mm. 
         [0032]    Referring to  FIG. 2A , the upper surface of the substrate  21  of the circularly polarized antenna of the present invention comprises eight slots  213 , and one end of each slot overlaps with the respective ends of the other slots at a central region  214 . The width of each slot is 4 mm. In another aspect, referring to  FIG. 2B , the coupling-ring line  215  is electrically connected with the coaxial cable connector  25  through the straight coupling line  216 , wherein the width of the coupling-ring line  215  and the width of the straight coupling line  216  are both 4 mm, and the diameter of the coupling-ring line is 72 mm. 
         [0033]    Therefore, when the circularly polarized antenna of the present invention is in its “transmitting state”, the coaxial cable connector  25  receives an electrical signal from a coaxial cable (not shown), so as to transmit the electrical signal to the coupling-ring line  215  with an opening via the straight coupling line  216 . Then, the coupling-ring line  215  and the slots  213  on the upper surface  211  of the substrate  21  transform the electrical signal into a circularly polarized signal and then transmit it outside. In addition, while the circularly polarized antenna of the present invention is in its “receiving state”, the coupling-ring line  215  and the slots  213  on the upper surface  211  of the substrate  21  receive a circularly polarized signal and transform the circularly polarized signal into an electrical signal, Then, the electric signal is transmitted to a coaxial cable (not shown) via the straight coupling line  216  and the coaxial cable connector  25  for further signal processing processes. 
         [0034]      FIG. 4A  shows the simulated result and the measured result of the axial ratio of the circularly polarized signal transmitted by the circularly polarized antenna according to the first preferred embodiment of the present invention, wherein the simulated result and the measured result are represented by triangular dots and square dots, respectively. Referring to  FIG. 4A , the measured result of the center frequency of the circularly polarized signal (about 0.91 GHz) is slightly smaller than the simulated result of the center frequency of the circularly polarized signal (about 0.95 GHz). Besides, the impedance bandwidth (the −10 dB bandwidth) of the circularly polarized antenna according to the first preferred embodiment of the present invention is about 126 MHz, while the 3 dB axial ratio thereof is about 2.5%. 
         [0035]      FIG. 4B  shows the simulated result and measured result of the gain of the circularly polarized antenna according to the first preferred embodiment of the present invention, wherein the simulated result and the measured result are represented by triangular dots and square dots, respectively. Referring to  FIG. 4B , the simulated result of the gain of the circularly polarized antenna according to the first preferred embodiment of the present invention is bigger than the measured result of the gain of the circularly polarized antenna according to the first preferred embodiment of the present invention, since the simulated result is based on an assumption that the substrate is a substrate without any return loss. 
         [0036]    In addition, the substrate of the circularly polarized antenna of the present invention can have any quantity of the slots on the upper surface thereof, i.e., the quantity can be 12, 16, 36, and even 64. 
         [0037]      FIG. 5A  is a schematic diagram of the upper surface of substrate of the circularly polarized antenna according to the second preferred embodiment of the present invention, wherein there are 16 slots  51  formed on the upper surface of the substrate thereof. One end of each slot overlaps with the respective ends of the other slots at a central region  52 .  FIG. 5B  is a schematic diagram of the upper surface of substrate of the circularly polarized antenna according to the third preferred embodiment of the present invention, wherein there are 36 slots  53  formed on the upper surface of the substrate thereof. One end of each slot  53  overlaps with respective ends of the other slots at a central region  54 . By comparing  FIG. 5A  with  5 B, it is shown that as the quantity of the slots is raised (from 16 to 36), and the area of the center region has become larger. In addition, the characteristics of the circularly polarized antenna of the present invention, such as the return loss, and the characteristics of the circularly polarized signal being transmitted by the circularly polarized antenna of the present invention will be affected by the different quantities of the slots formed on the upper surface of the substrate, as described below. 
         [0038]      FIG. 6A  shows the variation of the return loss of the circularly polarized antenna of the present invention regarding the changing of the operating frequency, wherein the upper surface of the substrate comprises different quantities of slots (4, 8, 12, 16 and 36).  FIG. 6B  shows the variation of the axial ratio of the circularly polarized antenna of the present invention regarding the changing of the operating frequency, wherein the upper surface of the substrate comprises different quantities of slots (4, 8, and 36). 
         [0039]    As shown in  FIG. 6A , when the quantity of the slots is raised, the return loss of the circularly polarized antenna of the present invention becomes smaller. That is, the circularly polarized antenna of the present invention can transform the electrical signal into the circularly polarized signal more efficiently. As shown in  FIG. 6B , the signals transmitted by the circularly polarized antenna of the circularly polarized antenna are all circularly polarized, regardless of the quantity of the slots formed on the upper surface of the substrate thereof. Therefore, once the quantity of the slots is more than  8 , the circularly polarized antenna of the present invention can have enough efficiency to transmit or receive the circularly polarized signals, without the need of forming too many slots on the upper surface of the substrate of the circularly polarized antenna of the present invention. 
         [0040]      FIG. 7  is a schematic drawing of the circularly polarized antenna according to the fourth preferred embodiment of the present invention. In this preferred embodiment, the substrate  71  is an FR-4 microwave substrate with the thickness of 0.8 mm, and having a coaxial cable connector  75  mounting at the edge. The antenna  72  is composed of a copper alloy containing more than ninety-eight percent copper, and two of the corresponding corners are chamfered. As shown in  FIG. 7 , the antenna  71  connects with a supporting structure  74  which is supported by a first supporting rod  731 , a second supporting rod  732 , a third supporting  733 , and a fourth supporting rod  734 . Therefore, the antenna  72  maintains a predetermined between it and the upper surface  711  of the substrate  71 . By adjusting the predetermined distance, the gain of the circularly polarized antenna of the present invention can be raised and the circularly polarized characteristics of the circularly polarized antenna can also be improved. 
         [0041]    Since the predetermined distance between the antenna  72  and the upper surface  711  of the substrate  71  is essential for designing the operating frequency of the circularly polarized antenna  7 , while the circularly polarized antenna  7  is required to change its operating frequency, the first supporting rod  731 , the second supporting rod  732 , the third supporting  733 , and the fourth supporting rod  734  must be adjusted to change the predetermined distance between the antenna  72  and the upper surface  711  of the substrate  21 . Besides, in the present embodiment, the slots formed on the upper surface of the substrate of the circularly polarized antenna can have any kind of shape. Moreover, after the “end treatment” is executed on the ends, the ends of the slots formed on the upper surface of the substrate of the circularly polarized antenna can have any kind of shape, as shown in  FIG. 8A  and  FIG. 8B . 
         [0042]      FIG. 8A  is a schematic diagram of the upper surface of substrate of the circularly polarized antenna according to the fourth preferred embodiment of the present invention, wherein each slot  81  has a lateral slot  82  at one end, and the opposing end of each slot  81  overlaps with the respective ends of the other slots at a central region  83 .  FIG. 8B  is a schematic diagram of the upper surface of substrate of the circularly polarized antenna according to the fourth preferred embodiment of the present invention, wherein each slot  81  has a dumbbell-shaped part  85  at one end, and the opposing end of each slot  84  overlaps with the respective ends of the other slots at a central region  86 . In addition, the characteristics of the circularly polarized antenna of the present invention, such as the return loss, and the characteristics of the circularly polarized signal being transmitted by the circularly polarized antenna of the present invention will be affected by the different shape of the ends of the slots formed on the upper surface of the substrate, as described below. 
         [0043]      FIG. 9  shows the variation of the axial ratio of the circularly polarized signal and the gain of the circularly polarized antenna regarding the changing of the operating frequency, wherein the curve connecting the square points is the axial ratio curve and the curve connecting the circle points is the gain curve. 
         [0044]    Referring still to  FIG. 9 , in the present preferred embodiment, the center frequency of the circularly polarized signal is slightly higher than the operating frequency of an RFID system. Besides, the 3-dB bandwidth of the axial ratio of the circular polarized signal transmitted by the circularly polarized antenna according to the fourth preferred embodiment is wider than the 3-dB bandwidth of the axial ratio of the circular polarized signal transmitted by the circularly polarized antenna according to the first preferred embodiment. In addition, the gain of the circularly polarized antenna according to the fourth embodiment in the operating frequency range of an RFID system is always larger than 4 dB. Therefore, the circularly polarized antenna according to the fourth preferred embodiment of the present invention can be used in most of the applications of the circularly polarized antenna. 
         [0045]    In summary, in the same range of the operating frequency (i.e. the operating frequency of the RFID ranges from 902 MHz to 928 MHz), the circularly polarized antenna of the present invention can reduce the size of the antenna and the substrate by forming some slots on the upper and lower surface of the substrate and by changing the size of the coupling portion, so as to maintain the same operating ability as the circularly polarized antenna of the prior art (i.e. having the same return loss and the operating frequency bandwidth). Therefore, the circularly polarized antenna of the present invention can have a compact size and keep antenna as simple, so as to facilitate the development of a small-sized, more convenient and portable electrical device having the circularly polarized antenna of the present invention, such as the reader device of an RFID system. 
         [0046]    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.