Abstract:
A switchable antenna includes a substrate, a first antenna element, a second antenna element, a first switch element, a second switch element, a first radiating portion on an upper surface of the substrate including a first center, a first bend section and a second bend section, and a second radiating portion on an lower surface of the substrate including a second center, a third bend section and a fourth bend section. The third and the fourth bend sections extending from the second center are respectively disposed corresponding to the first and the second bend sections extending from the first center. The first and the second antenna elements on the upper surface are disposed corresponding to the first and the second bend sections. The first and the second switch elements are respectively configured to switch the first and the second antenna elements between a reflector and a parasitic radiating element.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a switchable antenna, and more particularly, to a switchable antenna able to reduce interference, eliminate dead zones, and switch between an omnidirectional mode and a directional mode. 
     2. Description of the Prior Art 
     Antennas are utilized to emit and receive radio-frequency waves, thereby transmitting or exchanging radio-frequency signals. Basically, antennas can be divided into omnidirectional antennas and directional antennas according to radiation patterns. Omnidirectional antennas do not need to be pointed and provide equal coverage in all directions. Directional antennas point energy toward a specific direction for concentration within a targeted area, and hence are ideal to increase transmission efficiency covering specific area. 
     In general, directivity of an antenna is determined after the antenna has been designed. However, it is preferable to operate an antenna in different modes. Namely, it is a common goal in the industry to efficiently switch an electronic product between an omnidirectional mode and a directional mode. 
     SUMMARY OF THE INVENTION 
     Therefore, the present invention provides a switchable antenna able to switch between an omnidirectional mode and a directional mode, reduce interference, and eliminate dead zones. 
     An embodiment of the invention provides a switchable antenna, configured to transmit and receive radio-frequency signals, comprising a substrate comprising an upper surface and a lower surface; a first radiating portion formed on the upper surface of the substrate and comprising a first center, a first bend section and a second bend section respectively extending from the first center; a second radiating portion formed on the lower surface of the substrate and comprising a second center, a third bend section and a fourth bend section respectively extending from the second center, wherein the third bend section and the fourth bend section are disposed corresponding to the first bend section and the second bend section, respectively; a first antenna element disposed on the upper surface and corresponding to the first bend section; a first switch element electrically connected to the first antenna element and configured to switch the first antenna element between a reflector and a parasitic radiating element; a second antenna element disposed on the upper surface and corresponding to the second bend section; and a second switch element electrically connected to the second antenna element and configured to switch the second antenna element between a reflector and a parasitic radiating element. 
     Another embodiment of the invention further provides a switchable antenna configured to transmit and receive radio-frequency signals, comprising a substrate comprising an upper surface and a lower surface; a first radiating portion formed on the upper surface of the substrate and comprising a first bend section and a second bend section; a second radiating portion formed on the lower surface of the substrate and comprising a center; a third bend section extending from the center and electrically connected to the first bend section through a first via and disposed corresponding to the first bend section; and a fourth bend section extending from the center and electrically connected to the second bend section through a second via and disposed corresponding to the second bend section; a first switch element configured to control a connection between the first bend section and a radio signal processing module; and a second switch element configured to control a connection between the second bend section and the radio signal processing module. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are schematic diagrams respectively illustrating a top view of a front surface and a back surface of a switchable antenna according to an embodiment of the present invention. 
         FIG. 1C  is a schematic diagram illustrating a perspective view of the switchable antenna of  FIG. 1A . 
         FIGS. 1D and 1E  are schematic diagrams respectively illustrating current distribution of the switchable antenna of  FIG. 1A  operated in an omnidirectional mode and a directional mode. 
         FIG. 2  is a schematic diagram illustrating antenna resonance simulation results of the switchable antenna of  FIG. 1A  operated in an omnidirectional mode. 
         FIG. 3A  is a schematic diagram illustrating antenna pattern characteristic simulation results for the switchable antenna of  FIG. 1A  operated at 5500 MHz and calculated at 60 degrees with the switch elements all turned off. 
         FIG. 3B  is a schematic diagram illustrating antenna pattern characteristic simulation results for the switchable antenna of  FIG. 1A  operated at 5500 MHz and calculated at 60 degrees with merely one of the switch elements turned on. 
         FIG. 3C  is a schematic diagram illustrating antenna pattern characteristic simulation results for the switchable antenna of  FIG. 1A  operated at 5500 MHz and calculated at 60 degrees with merely one of the switch elements turned off. 
         FIG. 4A  is a schematic diagram illustrating antenna pattern characteristic measurement results for the switchable antenna of  FIG. 1A  measured at 60 degrees with the switch elements all turned off. 
         FIG. 4B  is a schematic diagram illustrating antenna pattern characteristic measurement results for the switchable antenna of  FIG. 1A  measured at 60 degrees with merely one of the switch elements turned on. 
         FIGS. 5A and 5B  are schematic diagrams respectively illustrating a top view of a front surface and a back surface of a switchable antenna according to an embodiment of the present invention. 
         FIG. 5C  is a schematic diagrams illustrating a perspective view of the switchable antenna of  FIG. 5A . 
         FIG. 5D  is a schematic diagram illustrating an equivalent circuit, which the switchable antenna of  FIG. 5A  may be modeled as. 
         FIG. 6A  is a schematic diagram illustrating antenna pattern characteristic simulation results for the switchable antenna of  FIG. 5A  operated at 2500 MHz with the adjustment elements. 
         FIG. 6B  is a schematic diagram illustrating antenna pattern characteristic simulation results for the switchable antenna of  FIG. 5A  operated at 2500 MHz without the adjustment elements. 
         FIG. 7A  is a schematic diagram illustrating current distribution of the switchable antenna of  FIG. 5A  operated in a directional mode. 
         FIG. 7B  is a schematic diagram illustrating antenna resonance simulation results of the switchable antenna of  FIG. 5A . 
         FIG. 8A  is a schematic diagram illustrating antenna pattern characteristic simulation results for the switchable antenna of  FIG. 5A  operated at 2450 MHz and calculated at 60 degrees with the switch elements all turned on. 
         FIG. 8B  is a schematic diagram illustrating antenna pattern characteristic simulation results for the switchable antenna  50  operated at 2450 MHz and calculated at 60 degrees with merely one of the switch elements  532 ,  534 ,  536  turned off. 
         FIG. 8C  is a schematic diagram illustrating antenna pattern characteristic simulation results for the switchable antenna of  FIG. 5A  operated at 2450 MHz and calculated at 60 degrees with merely one of the switch elements turned on. 
         FIG. 9A  is a schematic diagram illustrating antenna pattern characteristic measurement results for the switchable antenna of  FIG. 5A  measured at 60 degrees with the switch elements all turned off. 
         FIG. 9B  is a schematic diagram illustrating antenna pattern characteristic measurement results for the switchable antenna of  FIG. 5A  measured at 60 degrees with merely one of the switch elements turned off. 
         FIG. 9C  is a schematic diagram illustrating antenna pattern characteristic measurement results for the switchable antenna of  FIG. 5A  measured at 60 degrees with merely one of the switch elements turned on. 
         FIG. 10  is a schematic diagram illustrating a perspective view of a switchable antenna according to an embodiment of the present invention. 
         FIG. 11  is a schematic diagram illustrating a perspective view of a switchable antenna according to an embodiment of the present invention. 
         FIG. 12  is a schematic diagram illustrating a perspective view of a switchable antenna according to an embodiment of the present invention. 
         FIG. 13  is a schematic diagram illustrating a perspective view of a switchable antenna according to an embodiment of the present invention. 
         FIG. 14  is a schematic diagram illustrating a perspective view of a switchable antenna according to an embodiment of the present invention. 
         FIG. 15  is a schematic diagram illustrating a perspective view of a switchable antenna according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Please refer to  FIGS. 1A to 1E .  FIGS. 1A and 1B  are schematic diagrams respectively illustrating a top view of a front surface and a back surface of a switchable antenna  10  according to an embodiment of the present invention.  FIG. 1C  is a schematic diagram illustrating a perspective view of the switchable antenna  10 .  FIGS. 1D and 1E  are schematic diagrams respectively illustrating current distribution of the switchable antenna  10  operated in an omnidirectional mode and a directional mode. As shown in  FIGS. 1A to 1C , the switchable antenna  10  may be adapted to a wireless local area network (such as IEEE 802.11 wireless local area network) to transmit and receive radio-frequency signals. The switchable antenna  10  comprises a substrate  12 , radiating portions  100 ,  110 , antenna elements  122 ,  124 ,  126 , switch elements  132 ,  134 ,  136 , extension sections  142 ,  144 ,  146 , chokes  152   a ,  152   b ,  154   a ,  154   b ,  156   a ,  156   b , and resistors  162 ,  164 ,  166 . The radiating portion  100  is formed on an upper surface  12   a  of the substrate  12  and comprises a center  101  and upper surface bend sections  102 ,  104 ,  106  extending from the center  101 . The radiating portion  110  is formed on a lower surface  12   b  of the substrate  12  and comprises a center  111  and lower surface bend sections  112 ,  114 ,  116  extending from the center  111 . One end of the antenna elements  122 ,  124 ,  126  is respectively coupled to a control module  14 , which is used for providing direct-current (DC) power through the switch elements  132 ,  134 ,  136  and the extension sections  142 ,  144 ,  146 ; the other end of the antenna elements  122 ,  124 ,  126  is grounded through the resistors  162 ,  164 ,  166 , respectively. Therefore, when the control module  14  respectively turns on the switch elements  132 ,  134 ,  136 , the antenna elements  122 ,  124 ,  126  would respectively serve as a reflector; when the control module  14  respectively turns off the switch elements  132 ,  134 ,  136 , the antenna element  122 ,  124 ,  126  would respectively serve as a parasitic radiating element. The chokes  152   a ,  154   a ,  156   a  are respectively coupled between a system ground and the antenna elements  122 ,  124 ,  126 , and the chokes  152   b ,  154   b ,  156   b  are respectively coupled between the control module  14  and the antenna elements  122 ,  124 ,  126  in order to limit the resonating radio-frequency signals in the antenna elements  122 ,  124 ,  126  and in order to prevent radio-frequency signals from interfering the control module  14 . 
     In brief, by controlling the switch elements  132 ,  134 ,  136 , the antenna elements  122 ,  124 ,  126  can respectively switch between a reflector and a parasitic radiating element, such that the switchable antenna  10  can be operated in an omnidirectional mode or a directional mode, and directivity of the switchable antenna  10  can be adjusted to avoid interference. 
     Specifically, when all of the switch elements  132 ,  134 ,  136  are switched off, the antenna elements  122 ,  124 ,  126  would respectively serve as a parasitic radiating element to increase bandwidth. In such a situation, the switchable antenna  10  enters an omnidirectional mode to transmit and receive radio-frequency signals in all directions for detecting and searching stations or other operation requirements. When one of the switch elements  132 ,  134 ,  136  (such as the switch element  136 ) is turned on, the corresponding one of the antenna elements  122 ,  124 ,  126  (i.e., the antenna element  126 ) becomes a reflector, while the other the antenna elements still serve as a parasitic radiating element (i.e., the antenna elements  122 ,  124 ), respectively. Accordingly, the switchable antenna  10  changes into a directional mode such that radio-frequency signals are transmitted or received along a specific direction (for example, toward a direction Y) to increase transmission efficiency and to reduce power consumption. When one of the switch elements  132 ,  134 ,  136  (such as the switch element  136 ) is turned off, the corresponding one of the antenna elements  122 ,  124 ,  126  (i.e., the antenna element  126 ) serve as a parasitic radiating element while the other antenna elements respectively turn into a reflector (i.e., the antenna element  122 ,  124 ) in order to enhance directivity of the switchable antenna  10  toward a specific direction (for example, opposite to the direction Y) and in order to avoid interference by means of the transmitted or received radio-frequency signals of narrow beamwidth. 
     In order to improve quality of radio-frequency signals transmitted or received omnidirectionally, geometric structure of the switchable antenna  10  enables itself to form stable annular currents. Specifically, the upper surface bend section  102  comprises portions  102   a ,  102   b ; the upper surface bend section  104  comprises portions  104   a ,  104   b ; the upper surface bend section  106  comprises portions  106   a ,  106   b . With an enclosed angle θ 1  of 90 degrees enclosed by the portions  102   a ,  102   b , an enclosed angle θ 2  of 90 degrees enclosed by the portions  104   a ,  104   b , and an enclosed angle θ 3  of 90 degrees enclosed by the portions  106   a ,  106   b , the upper surface bend sections  102 ,  104 ,  106  respectively form a L-shaped structure with clockwise bending and are equally spaced apart. Similarly, the lower surface bend section  112  comprises portions  112   a ,  112   b ; the lower surface bend section  114  comprises portions  114   a ,  114   b ; the lower surface bend section  116  comprises portions  116   a ,  116   b . With an enclosed angle φ 1  of 90 degrees enclosed by the portions  112   a ,  112   b , an enclosed angle φ 2  of 90 degrees enclosed by the portions  114   a ,  114   b  and an enclosed angle φ 3  of 90 degrees enclosed by the portions  116   a ,  116   b , the lower surface bend sections  112 ,  114 ,  116  respectively form a L-shaped structure with counterclockwise bending and are spaced evenly around. As shown in  FIG. 1C , along a vertical projection direction Z, the centers  101  and  111  are aligned and the upper surface bend sections  102 ,  104 ,  106  with a L-shaped structure bent clockwise and the lower surface bend section  112 ,  114 ,  116  with a L-shaped structure bent counterclockwise respectively form a T-shaped structure. Accordingly, when the switchable antenna  10  transmits radio-frequency signals in an omnidirectional mode, currents flow in the radiating portion  100 ,  110  clockwise or counterclockwise as shown in  FIG. 1D , and hence the switchable antenna  10  can provide Alford loop antenna effect. A null can also occur in the radiation pattern in the vertical projection direction Z by means of geometry features of the switchable antenna  10 . Moreover, because of time delay, radio-frequency signals generated from a T-shaped structure of the switchable antenna  10  and radio-frequency signals generated from another T-shaped structure of the switchable antenna  10  add up in phase to enhance the total intensity and to form an omnidirectional radiation pattern. 
     In order to enhance directivity of the switchable antenna  10 , distances D 1 , D 2 , D 3  respectively between the center  111  and the antenna elements  122 ,  124 ,  126  may be in a range of 0.15 to 0.25 times operating wavelength corresponding to the center frequency (i.e., 0.15 times the operating wavelength) to ensure a front-to-back (F/B) ratio of the operating frequency (e.g., 5150 MHz to 5850 MHz) at 60 degrees (i.e., the elevation angle of 30 degrees from XY plane) greater than 5 dB. In other words, antenna resonance mechanism of the switchable antenna  10  functions as an annular antenna and therefore satisfies the requirements that distance between a reflector and a radiator of a Yagi antenna is in a range of 0.15 to 0.25 times the operating wavelength. 
     Simulation and measurement may be employed to determine whether radiation pattern of the switchable antenna  10  at different frequencies meets system requirements. Please refer to  FIGS. 2 to 4B .  FIG. 2  is a schematic diagram illustrating antenna resonance (Voltage Standing Wave Ratio, VSWR) simulation results of the switchable antenna  10  operated in an omnidirectional mode. In  FIG. 2 , antenna resonance simulation results of the switchable antenna  10  without the antenna elements  122 ,  124 ,  126  are presented by a dotted line, and antenna isolation simulation results of the switchable antenna  10  with the antenna elements  122 ,  124 ,  126  are presented by a solid line. As shown in  FIG. 2 , the antenna elements  122 ,  124 ,  126  of the switchable antenna  10  can effectively broaden bandwidth. In practical application, a vast metal plate is usually disposed below the switchable antenna  10  to provide shielding or other functions. However, the vast metal plate would cause the radiation pattern of the switchable antenna  10  to shift upward and thus generate a tilt angle. In order to properly present characteristics of the switchable antenna  10 , the switchable antenna  10  can be sampled at 60 degrees (i.e., the elevation angle of 30 degrees from XY plane).  FIG. 3A  is a schematic diagram illustrating antenna pattern characteristic simulation results for the switchable antenna  10  operated at 5500 MHz and calculated at 60 degrees with the switch elements  132 ,  134 ,  136  all turned off.  FIG. 3B  is a schematic diagram illustrating antenna pattern characteristic simulation results for the switchable antenna  10  operated at 5500 MHz and calculated at 60 degrees with merely one of the switch elements  132 ,  134 ,  136  turned on.  FIG. 3C  is a schematic diagram illustrating antenna pattern characteristic simulation results for the switchable antenna  10  operated at 5500 MHz and calculated at 60 degrees with merely one of the switch elements  132 ,  134 ,  136  turned off.  FIG. 4A  is a schematic diagram illustrating antenna pattern characteristic measurement results for the switchable antenna  10  measured at 60 degrees with the switch elements  132 ,  134 ,  136  all turned off.  FIG. 4B  is a schematic diagram illustrating antenna pattern characteristic measurement results for the switchable antenna  10  measured at 60 degrees with merely one of the switch elements  132 ,  134 ,  136  turned on. As shown in  FIG. 3A to 4B , when the number of the switch elements turned on grows the beamwidth is less divergent. 
     On the other hand, please refer to  FIGS. 5A to 5D .  FIGS. 5A and 5B  are schematic diagrams respectively illustrating a top view of a front surface and a back surface of a switchable antenna  50  according to an embodiment of the present invention.  FIG. 5C  is a schematic diagrams illustrating a perspective view of the switchable antenna  50 .  FIG. 5D  is a schematic diagram illustrating an equivalent circuit, which the switchable antenna  50  may be modeled as. As shown in  FIGS. 5A to 5C , the switchable antenna  50  may be adapted to a wireless local area network (such as IEEE 802.11 wireless local area network) to transmit and receive radio-frequency signals as well. The switchable antenna  50  comprises a substrate  52 , radiating portions  500 ,  510 , adjustment elements  522 ,  524 ,  526 , switch elements  532 ,  534 ,  536 , direct current blocks  542 ,  544 ,  546 , chokes  552 ,  554 ,  556 ,  558 , and resistor  562 ,  564 ,  566 . The radiating portion  500  is formed on an upper surface  52   a  of the substrate  52  and comprises upper surface bend sections  502 ,  504 ,  506 . The radiating portion  510  is formed on a lower surface  52   b  of the substrate  52  and comprises a center  511  and lower surface bend sections  512 ,  514 ,  516 , reflection sections  572 ,  574 ,  576  and vias  582 ,  584 ,  586  extending from the center  511 . The lower surface bend sections  512 ,  514 ,  516  correspond to the upper surface bend sections  502 ,  504 ,  506 , and are electrically connected to the upper surface bend sections  502 ,  504 ,  506  through the vias  582 ,  584 ,  586  which are disposed in the substrate  52 , respectively. 
     As shown in  FIG. 5D , one end of the switch elements  532 ,  534 ,  536  is respectively coupled to a radio signal processing module  56  which is used for providing alternating-current (AC) power and is coupled to a system ground through the choke  558 ; the other end of the switch elements  532 ,  534 ,  536  is electrically connected to the upper surface bend sections  502 ,  504 ,  506  and is coupled to a control module  54  which is used for providing direct-current (DC) power through the upper surface bend sections  502 ,  504 ,  506 , the chokes  552 ,  554 ,  556  and the resistors  562 ,  564 ,  566 . Therefore, when the control module  54  respectively turns on the switch elements  532 ,  534 ,  536 , the upper surface bend sections  502 ,  504 ,  506  can be respectively connected to the radio signal processing module  56  so as to transmit and receive radio-frequency signals; when the control module  54  respectively turns off the switch element  532 ,  534 ,  536 , the upper surface bend sections  502 ,  504 ,  506  cannot connect to the radio signal processing module  56 . The chokes  552 ,  554 ,  556 ,  558  can limit the resonating radio-frequency signals in the upper surface bend sections  502 ,  504 ,  506  and prevent radio-frequency signals from interfering the control module  54 . The direct current blocks  542 ,  544 ,  546  can prevent DC power in any of the upper surface bend sections  502 ,  504 ,  506  (e.g., the upper surface bend section  502 ) from being transmitted to other upper surface bend sections (e.g., the upper surface bend sections  504 ,  506 ) through vias  582 ,  584 ,  586 . The reflection sections  572 ,  574 ,  576  are respectively disposed between two adjacent lower surface bend sections so as to enhance directivity of the switchable antenna  50 . 
     Briefly, by controlling the switch elements  532 ,  534 ,  536 , the upper surface bend sections  502 ,  504 ,  506  can respectively be connected to the radio signal processing module  56 , such that the switchable antenna  50  can be operated in an omnidirectional mode or a directional mode. Moreover, with the reflection sections  572 ,  574 ,  576 , directivity of the switchable antenna  50  can be adjusted to avoid interference. 
     Specifically, when all of the switch elements  532 ,  534 ,  536  are switched on, the upper surface bend sections  502 ,  504 ,  506  are respectively connected to the radio signal processing module  56 , and the switchable antenna  50  can provide Alford loop antenna effect together with the lower surface bend sections  512 ,  514 ,  516  electrically connected. In such a situation, the switchable antenna  50  enters an omnidirectional mode to transmit and receive radio-frequency signals in all directions for detecting and searching stations or other operation requirements. When one of the switch elements  532 ,  534 ,  536  (such as the switch element  536 ) is turned off, only two of the upper surface bend sections (i.e., the upper surface bend sections  502 ,  504 ) are still connected to the radio signal processing module  56 , and the two upper surface bend sections respectively form a folded dipole antenna structure along with the corresponding lower surface bend section (i.e., the lower surface bend sections  512 ,  514 ). Furthermore, with the corresponding reflection sections (i.e., the reflection sections  574 ,  576 ), the switchable antenna  50  changes into a directional mode, such that radio-frequency signals are transmitted or received along a specific direction (for example, toward a direction Y) to increase transmission efficiency and to reduce power consumption. When one of the switch elements  532 ,  534 ,  536  (such as the switch element  536 ) is turned on, only one of the upper surface bend sections (i.e., the upper surface bend section  506 ) is still connected to the radio signal processing module  56 , and the upper surface bend section forms a folded dipole antenna structure along with the corresponding lower surface bend section (i.e., the lower surface bend section  516 ). Also, with the corresponding reflection sections (i.e., the reflection sections  574 ,  576 ), directivity of the switchable antenna  50  toward a specific direction (for example, opposite to the direction Y) is enhanced, and the beamwidth of the transmitted or received radio-frequency signals is narrower in order to avoid interference. 
     In order to improve quality of radio-frequency signals transmitted or received omnidirectionally, geometric structure of the switchable antenna  50  enables itself to form stable annular currents. Specifically, the upper surface bend section  502  comprises portions  502   a ,  502   b ,  502   c , the upper surface bend section  504  comprises portions  504   a ,  504   b ,  504   c , and the upper surface bend section  506  comprises portions  506   a ,  506   b , and  506   c . With enclosed angles α 1  to α 6  of 90 degrees enclosed respectively by the portions  502   a  to  506   c , the upper surface bend sections  502 ,  504 ,  506  respectively form a clockwise bending structure and are equally spaced apart. Similarly, the lower surface bend section  512  comprises portions  512   a  to  512   e , the lower surface bend section  514  comprises portions  514   a  to  514   e , and the lower surface bend section  516  comprises portions  516   a  to  516   e . With enclosed angles β 1  to β 12  of 90 degrees enclosed respectively by the portions  512   a  to  516   e , the lower surface bend sections  512 ,  514 ,  516  respectively form a counterclockwise bending structure and are equally spaced out. As shown in  FIG. 5C , the upper surface bend sections  502 ,  504 ,  506  and the lower surface bend sections  512 ,  514 ,  516  respectively form a closed folded dipole antenna structure along the vertical projection direction Z. In addition, the lower surface bend sections  512 ,  514 ,  516  can be electrically connected to the upper surface bend sections  502 ,  504 ,  506  through the vias  582 ,  584 ,  586 . Accordingly, when transmitting radio-frequency signals in an omnidirectional mode, the switchable antenna  50  can generate Alford loop antenna effect. 
     In order to enhance directivity of the switchable antenna  50 , the reflection sections  572 ,  574 ,  576  are respectively disposed between two adjacent lower surface bend sections and corresponds to the folded dipole antenna structure respectively formed from the upper surface bend sections  502 ,  504 ,  506  and the lower surface bend sections  512 ,  514 ,  516  so as to provide reflection characteristics as a Yagi antenna. The adjustment element  522  comprises portions  522   a ,  522   b ,  522   c , the adjustment element  524  comprises portions  524   a ,  524   b ,  524   c , and the adjustment element  526  comprises portions  526   a ,  526   b , and  526   c . With enclosed angles δ 1  to δ 6  enclosed respectively by the portions  522   a  to  526   c , the adjustment elements  522 ,  524 ,  526  respectively corresponding to the reflection sections  572 ,  574 ,  576  can form a bow structure and are equally spaced apart, thereby enhancing antenna gain around boundary of radiation pattern under a directional mode. In other words, the adjustment elements  522 ,  524 ,  526  can increase beamwidth and therefore eliminate dead zones. Specifically, please refer to  FIGS. 6A and 6B .  FIG. 6A  is a schematic diagram illustrating antenna pattern characteristic simulation results for the switchable antenna  50  operated at 2500 MHz with the adjustment elements  522 ,  524 ,  526 .  FIG. 6B  is a schematic diagram illustrating antenna pattern characteristic simulation results for the switchable antenna  50  operated at 2500 MHz without the adjustment elements  522 ,  524 ,  526 . As shown in  FIGS. 6A and 6B , beamwidth of the switchable antenna  50  with the adjustment elements  522 ,  524 ,  526  is wider. 
     Besides, the geometric structure of the switchable antenna  50  ensures resistance matching under both an omnidirectional mode and a directional mode. Specifically, when the switchable antenna  50  is operated in an omnidirectional mode, the upper surface bend sections  502 ,  504 ,  506  are all connected to the radio signal processing module  56 . When the switchable antenna  50  is operated in a directional mode, only some of the upper surface bend sections  502 ,  504 ,  506  (such as the upper surface bend section  506 ) are connected to the radio signal processing module  56 . However, because one of the upper surface bend sections (for example, the upper surface bend section  506 ) can be electrically connected to the corresponding lower surface bend section (i.e., the lower surface bend section  516 ) through the corresponding via (i.e., the via  586 ), and because the lower surface bend section (i.e., the lower surface bend section  516 ) can be electrically connected to the other lower surface bend sections (i.e., the lower surface bend sections  512 ,  514 ) through the center  511 ) and the corresponding upper surface bend sections (i.e., the upper surface bend sections  502 ,  504 ), when the switchable antenna  50  enters a directional mode to connect some of the upper surface bend sections  502 ,  504 ,  506  (i.e., the upper surface bend section  506 ) to the radio signal processing module  56 , reverse currents are conducted in the other upper surface bend section(s) and the other lower surface bend section(s) (i.e., the upper surface bend sections  502 ,  504  and the lower surface bend sections  512 ,  514 ), thereby achieving resistance matching. For example,  FIG. 7A  is a schematic diagram illustrating current distribution of the switchable antenna  50  operated in a directional mode.  FIG. 7B  is a schematic diagram illustrating antenna resonance simulation results of the switchable antenna  50 . In  FIG. 7B , antenna resonance simulation results of the switchable antenna  50  operated in an omnidirectional mode are presented by a thin dotted line; return loss (scattering parameters S 11 ) simulation results of the upper surface bend sections  502 ,  504 ,  506  are respectively presented by a thick dotted line, a thin dash-dotted line and a thick dash-dotted line; and antenna isolation simulation results of the upper surface bend sections  502 ,  504 ,  506  are respectively presented by a dashed line, a thick solid line and a thin solid line. 
     Simulation and measurement may be employed to determine whether radiation pattern of the switchable antenna  50  at different frequencies meets system requirements. In practical application, a vast metal plate is usually disposed below the switchable antenna  50  to provide shielding or other functions. However, the vast metal plate would cause the radiation pattern of the switchable antenna  50  to shift upward and thus generate a tilt angle. In order to properly present characteristics of the switchable antenna  50 , the switchable antenna  50  can be sampled at 60 degrees (i.e., the elevation angle of 30 degrees from XY plane). Please refer to  FIGS. 8A to 9C .  FIG. 8A  is a schematic diagram illustrating antenna pattern characteristic simulation results for the switchable antenna  50  operated at 2450 MHz and calculated at 60 degrees with the switch elements  532 ,  534 ,  536  all turned on.  FIG. 8B  is a schematic diagram illustrating antenna pattern characteristic simulation results for the switchable antenna  50  operated at 2450 MHz and calculated at 60 degrees with merely one of the switch elements  532 ,  534 ,  536  turned off.  FIG. 8C  is a schematic diagram illustrating antenna pattern characteristic simulation results for the switchable antenna  50  operated at 2450 MHz and calculated at 60 degrees with merely one of the switch elements  532 ,  534 ,  536  turned on.  FIG. 9A  is a schematic diagram illustrating antenna pattern characteristic measurement results for the switchable antenna  50  measured at 60 degrees with the switch elements  532 ,  534 ,  536  all turned on.  FIG. 9B  is a schematic diagram illustrating antenna pattern characteristic measurement results for the switchable antenna  50  measured at 60 degrees with merely one of the switch elements  532 ,  534 ,  536  turned off.  FIG. 9C  is a schematic diagram illustrating antenna pattern characteristic measurement results for the switchable antenna  50  measured at 60 degrees with merely one of the switch elements  532 ,  534 ,  536  turned on. As shown in  FIG. 8A to 9C , when the number of the switch elements turned on drops, the beamwidth is less divergent. 
     Please note that the switchable antennas  10 ,  50  are exemplary embodiments of the invention, and those skilled in the art can make alternations and modifications accordingly. For example, a switch element of a switchable antenna may be of various kinds such as a diode and a transistor. The number of switch elements may vary with the number of upper surface bend sections and an upper surface bend section may correspond to a plurality of switch elements. The switchable antenna in the aforementioned embodiments comprises three upper surface bend sections and three lower surface bend sections; however, the present invention is not limited herein and a switchable antenna can comprise a plurality of upper surface bend sections and a plurality of lower surface bend sections. Alternatively, it is also possible that a switchable antenna merely comprises two upper surface bend sections and two lower surface bend sections. Besides, the upper surface bend sections  102 ,  104 ,  106  are substantially of rotational symmetry to evenly distribute the space between the upper surface bend sections  102 ,  104 ,  106 . In such a situation, the corresponding lower surface bend sections  112 ,  114 ,  116  are symmetric with respect to rotations about the center  111 . Likewise, the upper surface bend sections  502 ,  504 ,  506  are substantially of rotational symmetry to space evenly around, such that the corresponding lower surface bend sections  512 ,  514 ,  516  have rotational symmetry. Nevertheless, the present invention is not limited to this, and the configuration may be non-symmetrical, rectangle arranged and mirror symmetrical. Sizes of the antenna elements  122 ,  124 ,  126 , the upper surface bend sections  102 ,  104 ,  106  and the lower surface bend sections  112 ,  114 ,  116  of the switchable antenna  10  may be respectively identical, and the upper surface bend sections  502 ,  504 ,  506  and the lower surface bend sections  512 ,  514 ,  516  of the switchable antenna  50  may also have the same size respectively, but not limited thereto—the exact size of each component is determined according to different system requirements or design considerations. Additionally, the antenna elements  122 ,  124 ,  126 , the portions  522   a  to  526   c  of the adjustment elements  522 ,  524 ,  526 , the portions  102   a  to  506   c  of the upper surface bend sections  102 ,  104 ,  106 ,  502 ,  504 ,  506  and the portions  112   a  to  516   e  of the lower surface bend sections  112 ,  114 ,  116 ,  512 ,  514 ,  516  are substantially linear, but the antenna elements, the upper surface bend sections and the lower surface bend sections can have the shape of a curve. 
     Furthermore, lengths of the antenna elements  122 ,  124 ,  126  of the switchable antenna  10  can be in a range of 0.4 to 0.475 times operating wavelength corresponding to the center frequency to increase bandwidth as a parasitic radiating element. However, if the switch elements  132 ,  134 ,  136  are not ideal switches and thus suffer effects of capacitance or inductance, when all of the switch elements  132 ,  134 ,  136  are turned off, currents can still flow through the switch elements  132 ,  134 ,  136 , respectively. In this case, the antenna elements  122 ,  124 ,  126  may be properly adjusted according to system requirements. For example, please refer to  FIG. 10 .  FIG. 10  is a schematic diagram illustrating a perspective view of a switchable antenna  60  according to an embodiment of the present invention. Since structure of the switchable antenna  60  is similar to that of the switchable antenna  10  in  FIG. 1A , the same numerals and symbols denote the same components in the following description, and the identical parts are not detailed redundantly. As shown in  FIG. 10 , the switch elements  132 ,  134 ,  136  of the switchable antenna  60  are respectively disposed between antenna elements  1022 ,  1024 ,  1026  and extension sections  1042 ,  1044 ,  1046 . The length of the antenna element  1022  is substantially equal to that of the extension section  1042 , the length of the antenna element  1024  is substantially equal to that of the extension section  1044 , and the length of the antenna element  1026  is substantially equal to that of the extension section  1046 . Please note that length ratios of an antenna element to the corresponding extension section in the present invention is not limited thereto and may be adjusted according to characteristics of the corresponding switch element and equivalent lengths of the antenna element corresponding to the resonating radio-frequency signals. The configuration of an antenna element and the corresponding extension section may be appropriately modified as well. Furthermore, an upper surface bend section may form a clockwise bent structure while the corresponding lower surface bend section may form a counterclockwise bend structure. Alternatively, an upper surface bend section may form a counterclockwise bend structure while the corresponding lower surface bend section may form a clockwise bent structure correspondingly. Bend structure may be a bent L-shaped structure, for example but not limited thereto. 
     The number of portions constituting an upper surface bend section or a lower surface bend section is not limited to a specific number. For example, please refer to  FIG. 11 .  FIG. 11  is a schematic diagram illustrating a perspective view of a switchable antenna  68  according to an embodiment of the present invention. Since structure of the switchable antenna  68  is similar to that of the switchable antenna  10  in  FIG. 1A , the same numerals and symbols denote the same components in the following description. As shown in  FIG. 11 , an upper surface bend section  1302  comprises portions  1302   a ,  1302   b ,  1302   c , an upper surface bend section  1304  comprises portions  1304   a ,  1304   b ,  1304   c , and an upper surface bend section  1306  comprises portions  1306   a ,  1306   b ,  1306   c . A lower surface bend section  1312  comprises portions  1312   a ,  1312   b ,  1312   c , a lower surface bend section  1314  comprises portions  1314   a ,  1314   b ,  1314   c , and a lower surface bend section  1316  comprises portions  1316   a ,  1316   b ,  1316   c . Please note that width ratios or length ratios of portions of an upper surface bend section or a lower surface bend section and the manner that widths and lengths vary depend on different system requirements, and are not limited thereto. 
     Structures of a lower surface bend section and an upper surface bend section of a switchable antenna can be properly adjusted, and configurations of a via vary correspondingly. For example, please refer to  FIG. 12 .  FIG. 12  is a schematic diagram illustrating a perspective view of a switchable antenna  80  according to an embodiment of the present invention. Since structure of the switchable antenna  80  is similar to that of the switchable antenna  50  in  FIG. 5A , the same numerals and symbols denote the same components in the following description. As shown in  FIG. 12 , an upper surface bend section  1402  comprises portions  1402   a  to  1402   d , an upper surface bend section  1404  comprises portions  1404   a  to  1404   d , and an upper surface bend section  1406  comprises portions  1406   a  to  1406   d . A lower surface bend section  1412  comprises portions  1412   a  to  1412   d , a lower surface bend section  1414  comprises portions  1414   a  to  1414   d , and a lower surface bend section  1416  comprises portions  1416   a  to  1416   d . Correspondingly, vias  1482 ,  1484 ,  1486  are respectively disposed between the upper surface bend sections  1402 ,  1404 ,  1406  and the lower surface bend sections  1412 ,  1414 ,  1416  to electrically connect the upper surface bend sections  1402 ,  1404 ,  1406  and the lower surface bend sections  1412 ,  1414 ,  1416 . 
     Besides, a direct current block of a switchable antenna may be disposed in any position between a choke and the center of a radiating portion. For example, please refer to  FIG. 13 .  FIG. 13  is a schematic diagram illustrating a perspective view of a switchable antenna  82  according to an embodiment of the present invention. Since structure of the switchable antenna  82  is similar to that of the switchable antenna  50  in  FIG. 5A , the same numerals and symbols denote the same components in the following description. As shown in  FIG. 13 , direct current blocks  1442 ,  1444 , and  1446  are respectively disposed at ends of the upper surface bend sections  502 ,  504 ,  506 . However, the present invention is not limited to these, for example, please refer to  FIG. 14 .  FIG. 14  is a schematic diagram illustrating a perspective view of a switchable antenna  84  according to an embodiment of the present invention. Since structure of the switchable antenna  84  is similar to that of the switchable antenna  50  in  FIG. 5A , the same numerals and symbols denote the same components in the following description. As shown in  FIG. 14 , direct current block  1492 ,  1494 ,  1496  are respectively disposed within the lower surface bend sections  512 ,  514 ,  516 . 
     Geometric structures of the adjustment elements  522 ,  524 ,  526  of the switchable antenna  50  may be properly adjusted according to system requirements. For example, the number of portions of the adjustment elements  522 ,  524 ,  526  is not limited to 3, and the adjustment elements  522 ,  524 ,  526  may respectively comprise a plurality of portions to enhance antenna gain around boundary of radiation pattern under a directional mode, thereby broadening beamwidth and eliminating dead zones. Moreover, enclosed angles enclosed by portions and width ratios or length ratios of the portions may also be adjusted correspondingly, which are not detailed redundantly. Similarly, the number of portions of an upper surface bend section and a lower surface bend section may be properly adjusted according to system requirements. For example, the upper surface bend sections  502 ,  504 ,  506  and the lower surface bend sections  512 ,  514 ,  516  may respectively comprise a plurality of portions such that the upper surface bend sections  502 ,  504 ,  506  and the lower surface bend sections  512 ,  514 ,  516  respectively form a closed folded dipole antenna structure. Please note that width ratios or length ratios of portions of an upper surface bend section or a lower surface bend section and the manner that widths and lengths vary depend on different system requirements, and are not limited thereto. 
     An enclosed angle enclosed by portions of an upper surface bend section or a lower surface bend section may be appropriately modified according to system requirements. For example, please refer to  FIG. 15 .  FIG. 15  is a schematic diagram illustrating a perspective view of a switchable antenna  92  according to an embodiment of the present invention. Since structure of the switchable antenna  92  is similar to that of the switchable antenna  10  in  FIG. 1A , the same numerals and symbols denote the same components in the following description. As shown in  FIG. 15 , an enclosed angle α 4 ′ enclosed by portions  1702   b ,  1702   c  of an upper surface bend section  1702  is greater than 90 degrees, an enclosed angle α 5 ′ enclosed by portions  1704   b ,  1704   c  of an upper surface bend section  1704  is greater than 90 degrees, and an enclosed angle α 6 ′ enclosed by portions  1706   b ,  1706   c  of an upper surface bend section  1706  is greater than 90 degrees. An enclosed angle β 4 ′ enclosed by portions  1712   b ,  1712   c  of a lower surface bend section  1712  is greater than 90 degrees, an enclosed angle β 7 ′ enclosed by portions  1712   c ,  1712   d  and an enclosed angle β 10 ′ enclosed by portions  1712   d ,  1712   e  are less than 90 degrees, an enclosed angle β 5 ′ enclosed by portions  1714   b ,  1714   c  of a lower surface bend section  1714  is greater than 90 degrees, an enclosed angle β 8 ′ enclosed by portions of the portions  1714   c ,  1714   d  and an enclosed angle β 11 ′ enclosed by portions  1714   d ,  1714   e  are less than 90 degrees, an enclosed angle β 6 ′ enclosed by portions  1716   b ,  1716   c  of a lower surface bend section  1716  is greater than 90 degrees, and an enclosed angle β 9 ′ enclosed by portions  1716   c ,  1716   d  and an enclosed angle β 12 ′ enclosed by portions of  1716   d ,  1716   e  are less than 90 degrees. Therefore, the upper surface bend sections  1702 ,  1704 ,  1706  and the lower surface bend sections  1712 ,  1714 , and  1716  respectively form a closed folded dipole antenna structure. 
     To sum up, by controlling switch elements, a switchable antenna can be operated in an omnidirectional mode or a directional mode. With antenna elements or reflection sections, directivity of the switchable antenna can be adjusted to avoid interference. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.