Patent Publication Number: US-2010109962-A1

Title: Circularly polarized antenna and an electronic device having the circularly polarized antenna

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a circularly polarized antenna, and more particularly, the present invention relates to a circularly polarized antenna for fine tuning a resonant center and an axial ratio center. 
     2. Description of the Related Art 
     A circularly polarized antenna for receiving satellite signals has been disclosed in the prior art. Please refer to  FIG. 1A .  FIG. 1A  is an illustration of a circularly polarized antenna of the prior art. The circularly polarized antenna  90  of the prior art comprises a base board  91 , a grounding layer  92 , a dielectric body  93 , a radiating patch metal layer  94 , a feeding line  95 , a feeding point  96 , and a screw  97 . The grounding layer  92  is disposed on the base board  91 . The dielectric body  93  is disposed on the grounding layer  92 . The radiating patch metal layer  94  is disposed on the dielectric body  93 . A radiating electromagnetic field can be generated by matching the radiating patch metal layer  94  and the grounding layer  92 . The feeding line  95  can transmit signal to the antenna  90  via the feed point  96 . The screw  97  fixes the grounding layer  92  to the base board  91 , wherein there is a specific angle between the grounding layer  92  and the radiating patch metal layer  94 . 
     Generally, the direction of the radiating electromagnetic field will be affected by the dielectric body  93 , which is limited in size, and will be oblique to one side. In order to make the radiating electromagnetic field symmetric in all directions, the dielectric body  93  will be rotated. After rotation, there is a specific angle between the grounding layer  92  and the dielectric body  93  (also the radiating patch metal layer  94 ). On the other hand, in order to prevent interference from the screw  97 , the dielectric body  93  will also be rotated. When the grounding layer  92  is in a rectangular shape, the two perpendicular resonant lengths of the circularly polarized antenna  90  will be different. The condition abovementioned will cause the separation between the resonant center and the axial ratio center. 
     Please refer to  FIG. 1B  and  FIG. 1C  for characteristic diagrams of the circularly polarized antenna of the prior art.  FIG. 1B  is an input impedance diagram of the circularly polarized antenna of the prior art.  FIG. 1C  is a Smith chart of the circularly polarized antenna of the prior art. 
       FIG. 1B  is the input impedance diagram when a signal is reflected out after the feeding line  95  feeds the signal to the circularly polarized antenna  90 . A resonant center of the circularly polarized antenna  90  is the smallest frequency in  FIG. 1B  where the reflection coefficient is at its minimum value and the transmitting energy of the circularly polarized antenna  90  is at its maximum value. The Smith chart in  FIG. 1C  represents the input impedance of the circularly polarized antenna  90  in a polar coordinate system, which looks like a heart shape. The center of the heart shape is an axial ratio center of the circularly polarized antenna  90 . The axial ratio center represents the frequency when the circularly polarized antenna  90  has the best circularly polarized characteristic. The axial ratio is close to 1 or 0 dB, and the antenna has the best polarized characteristic then, wherein the polarization is a relationship between the space and time of the electromagnetic field of the antenna. An antenna generally has horizontal polarization and vertical polarization, and these two polarizations are independent of each other. Alternatively, the antenna may have left-hand circular polarization and right-hand circular polarization, and these two polarizations are also independent of each other. Antennas with the same polarization can transmit the greatest amount of energy. Therefore, the circularly polarized antenna  90  cannot transmit wireless signals effectively if it has bad polarization. 
     If the resonant center and the axial ratio center are at the same frequency, the circularly polarized antenna  90  has the best polarization and can transmit the greatest amount of energy. In other words, the circularly polarized antenna  90  has the best performance then. In the prior art, the resonant center of the circularly polarized antenna  90  is 2.329 GHz, and the axial ratio center of the circularly polarized antenna  90  is 2.345 GHz. There is a 16 MHz shift between these two centers. The circularly polarized antenna  90  cannot have the best performance because of the frequency shift between these two centers. 
     In addition, it will increase cost and manufacture time if the dielectric body  93  is designed to match the shape of the grounding layer  92 . 
     Therefore, a new circularly polarized antenna is needed to solve the problems of the prior art. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a circularly polarized antenna for tuning a resonant center and an axial ratio center. 
     It is another object of the present invention to provide an electronic device having a circularly polarized antenna. 
     In order to achieve the object mentioned above, an electronic device of the invention comprises a wireless module and a circularly polarized antenna electrically connected to the wireless module. The circularly polarized antenna comprises a base board, a grounding layer, a dielectric body, and a radiating patch metal layer. The grounding layer is disposed on the base board. The dielectric body is disposed on the grounding layer. The radiating patch metal layer is disposed on the dielectric body. The radiating patch metal layer comprises a first slot and a second slot disposed on opposite sides of the radiating patch metal layer separately and disposed along a first extension line substantially, wherein the first extension line passes through a center of the grounding layer and is parallel to a side of the grounding layer substantially. 
     Other objects, advantages, and novel features of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects and advantages of the present invention will become apparent from the following description of the accompanying drawings, which disclose several embodiments of the present invention. It is to be understood that the drawings are to be used for purposes of illustration only, and not as a definition of the invention. 
       In the drawings, wherein similar reference numerals denote similar elements throughout the several views: 
         FIG. 1A  is an illustration of a circularly polarized antenna of the prior art. 
         FIG. 1B  is an input impedance diagram of the circularly polarized antenna of the prior art. 
         FIG. 1C  is a Smith chart of the circularly polarized antenna of the prior art. 
         FIG. 2A  is an illustration of a circularly polarized antenna of the first embodiment of the invention. 
         FIG. 2B  is an input impedance diagram of the circularly polarized antenna according to the first embodiment of the invention. 
         FIG. 2C  is a Smith chart of the circularly polarized antenna according to the first embodiment of the invention. 
         FIG. 3A  is an illustration of a circularly polarized antenna of the second embodiment of the invention. 
         FIG. 3B  is an input impedance diagram of the circularly polarized antenna according to the second embodiment of the invention. 
         FIG. 3C  is a Smith chart of the circularly polarized antenna according to the second embodiment of the invention. 
         FIG. 4A  is an illustration of a circularly polarized antenna of the third embodiment of the invention. 
         FIG. 4B  is an input impedance diagram of the circularly polarized antenna according to the third embodiment of the invention. 
         FIG. 4C  is a Smith chart of the circularly polarized antenna according to the third embodiment of the invention. 
         FIG. 5  is a system block diagram of an electronic device of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Please refer to  FIG. 2A .  FIG. 2A  is an illustration of a circularly polarized antenna of the first embodiment of the invention. 
     In the first embodiment of the invention, the circularly polarized antenna  10   a  comprises a base board  11 , a grounding layer  12 , a dielectric body  13 , a radiating patch metal layer  14 , a feeding line  15 , a feeding point  16 , and a screw  17 . The base board  11  is a metal plate made of aluminum, tin or the like. The grounding layer  12  is a conductive layer of a printed circuit board installed on the base board by the screw  17  or welding. The dielectric body  13 , made of ceramic or other dielectric material, is disposed on the grounding layer  12 . The radiating patch metal layer  14 , made of copper, silver, or gold, is disposed on the dielectric body  13 . A radiating electromagnetic field can be generated to transmit wireless signals by matching the radiating patch metal layer  14  and the grounding layer  12 . The feeding line  15  is electrically connected to the grounding layer  12  for feeding an electric signal to an antenna  10  via the feeding point  16 . The feeding line  15  can be but is not limited to an RF cable. The screw  17  fixes the grounding layer  12  to the base board  11 . 
     In order to make the electromagnetic field of the circularly polarized antenna  10   a  symmetric in all directions or correspond to the shapes of the grounding layer  12  and the dielectric body  13 , there is a specific angle between the grounding layer  12  and the dielectric body  13  (also the radiating patch metal layer  14 ). 
     The radiating patch metal layer  14  comprises a first slot  21   a  and a second slot  21   b  disposed on opposites sides of the radiating patch metal layer  14  separately and disposed along a first extension line L 1  substantially. The direction of the first slot  21   a  and the second slot  21   b  is parallel to the first extension line L 1  substantially, wherein the first extension line L 1  passes through a center of the grounding layer  12  and is parallel to a side of the grounding layer  12  substantially. The resonant frequency of the circularly polarized antenna  10   a  can be fine tuned by the first slot  21   a  and the second slot  21   b  to make the resonant center close to the axial ratio center. 
     Please refer to  FIG. 2B  and  FIG. 2C .  FIG. 2B  is an input impedance diagram of the circularly polarized antenna according to the first embodiment of the invention.  FIG. 2C  is a Smith chart of the circularly polarized antenna according to the first embodiment of the invention. 
     As shown in  FIG. 2B  and  FIG. 2C , before fine tuning, the resonant center and the axial ratio center of the circularly polarized antenna  10   a  (without the first slot  21   a  and the second slot  21   b ) are 2.329 GHz and 2.345 GHz respectively. There is a 16 MHz shift between these two centers. After fine tuning by the first slot  21   a  and the second slot  21   b,  both the resonant center and the axial ratio center of the circularly polarized antenna  10   a  are around 2.3325 GHz; therefore, the circularly polarized antenna  10   a  has better performance after fine tuning. 
     More particularly, the use of tuning by the first slot  21   a  and the second slot  21   b  is not limited to the above condition. If the grounding layer  12  is in a rectangular shape or other unsymmetrical shape such that two perpendicular resonant lengths of the circularly polarized antenna  10   a  are different, it will cause a shift between the resonant center and the axial ratio center. The circularly polarized antenna  10   a  can be fine tuned by the first slot  21   a  and the second slot  21   b  under such conditions. 
     The first slot  21   a  and the second slot  21   b  are not limited by the above description. Please refer to  FIG. 3A .  FIG. 3A  is an illustration of a circularly polarized antenna of the second embodiment of the invention. 
     In contrast to the circularly polarized antenna  10   a,  both the first slot  21   a ′ and the second slot  21   b ′ of the circularly polarized antenna  10   b  of the second embodiment of the invention are also disposed along the first extension line L 1  substantially, but the direction of the first slot  21   a ′ and the second slot  21   b ′ is perpendicular to the disposed sides of the radiating patch metal layer  14  substantially. 
     The characteristics of the circularly polarized antenna  10   b  are shown in  FIG. 3B  and  FIG. 3C .  FIG. 3B  is an input impedance diagram of the circularly polarized antenna according to the second embodiment of the invention.  FIG. 3C  is a Smith chart of the circularly polarized antenna according to the second embodiment of the invention 
     As shown in  FIG. 3B  and  FIG. 3C , before fine tuning, the resonant center and the axial ratio center of the circularly polarized antenna  10   b  are 2.329 GHz and 2.33875 GHz, respectively. There is a 9.75 MHz shift between these two centers. After fine tuning by the first slot  21   a ′ and the second slot  21   b ′, both the resonant center and the axial ratio center of the circularly polarized antenna  10   b  are around 2.3325 GHz; therefore, the circularly polarized antenna  10   b  has better performance after fine tuning. 
     Please refer to  FIG. 4A .  FIG. 4A  is an illustration of a circularly polarized antenna of the third embodiment of the invention. 
     In the third embodiment of the invention, when the first slot  21   a ′ and the second slot  21   b ′ of the circularly polarized antenna  10   c  are too deep, the resonant center and the axial ratio center of the circularly polarized antenna  10   c  will be separated again after matching together. Therefore, a third slot  21   c  and a fourth slot  21   d  are needed in order to compensate. The third slot  21   c  and the fourth slot  21   d  are disposed on opposite sides of the radiating patch metal layer  14  separately and disposed along a second extension line L 2  substantially, wherein the second extension line L 2  passes through the center of the grounding layer  12  and is perpendicular to the first extension line L 1  substantially. The direction of the third slot  21   c  and the fourth slot  21   d  is perpendicular to the disposed sides of the radiating patch metal layer  14 . 
     The characteristics of the circularly polarized antenna  10   c  are shown in  FIG. 4B  and  FIG. 4C .  FIG. 4B  is an input impedance diagram of the circularly polarized antenna according to the third embodiment of the invention.  FIG. 4C  is a Smith chart of the circularly polarized antenna according to the third embodiment of the invention 
     As shown in  FIG. 4B  and  FIG. 4C , before fine tuning, the resonant center and the axial ratio center of the circularly polarized antenna  10   c  are 2.323 GHz and 2.3325 GHz, respectively. There is a 9.5 MHz shift between these two centers. After fine tuning by the first slot  21   a ′ and the second slot  21   b ′, both the resonant center and the axial ratio center of the circularly polarized antenna  10   c  are close to each other and then separated again. The third slot  21   c  and the fourth slot  21   d  are implemented in order to fine tune the resonant center and the axial ratio center of the circularly polarized antenna  10   c  back to 2.3325 GHz again. Therefore, the circularly polarized antenna  10   c  can have better performance by the above tuning. The above description explains how to tune the circularly polarized antenna with the slots. 
     Finally, please refer to  FIG. 5 .  FIG. 5  is a system block diagram of an electronic device of the invention. 
     In one of the embodiments of the invention, the electronic device  30  can be a GPS (global positioning system) device or the like. As shown in  FIG. 5 , the electronic device  30  of the invention comprises the circularly polarized antenna  10   a  and a wireless signal module  31 . The electronic device  30  can feed a signal to the circularly polarized antenna  10   a  via the feeding line  15  (such as an RF cable). The electronic device  30  is electrically connected to the wireless signal module  31  by the feeding line  15  for processing signals (emitting or receiving signals) from the circularly polarized antenna  10   a.  Therefore, the electronic device  30  can transmit or receive wireless signals by the circularly polarized antenna  10   a  for wireless communication with other devices (not shown). 
     Besides the circularly polarized antenna  10   a,  the electronic device  30  can also use the circularly polarized antenna  10   b  or the circularly polarized antenna  10   c  to achieve the same purpose. 
     Although the present invention has been explained in relation to its preferred embodiments, it is to be understood that many other possible modifications and variations can be made without departing from the spirit and scope of the invention as hereinafter claimed.