Abstract:
An antenna for receiving and transmitting radio signals, including a reflective unit, comprising a central reflective element; and a plurality of peripheral reflective elements, enclosing the central reflective element to form a frustum structure; and at least one radiation unit, disposed above the central reflective element; where the reflective unit is electrically isolated from the at least one radiation unit.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an antenna and a complex antenna, and more particularly, to an antenna and a complex antenna having smaller size to be disposed in a cylindrical radome and allowing both multiband and low-frequency operations. 
         [0003]    2. Description of the Prior Art 
         [0004]    Electronic products with wireless communication functionalities utilize antennas to emit and receive radio waves, to transmit or exchange radio signals, so as to access a wireless communication network. With the advance of wireless communication technology, an electronic product may be configured with an increasing number of antennas. Alternatively, a complex antenna equipped with a plurality of antennas may be used in an electronic product to transmit or receive radio signals. A complex antenna turns on its antenna (s) according to the direction of signal transmission, thereby effectively enhancing spectral efficiency and transmission rate for the wireless communication system, as well as improving communication quality. In such a situation, each of the antennas constituting a complex antenna is preferably a directional antenna, which point energy toward a specific direction for concentration within a targeted area. 
         [0005]    An ideal antenna should maximize its bandwidth within a permitted range, while minimizing physical dimensions to accommodate the trend for smaller-sized electronic products. Technically, a complex antenna is disposed in a cylindrical radome, which limits the sizes of the antennas constituting the complex antenna. However, the long term evolution (LTE) wireless communication system includes 44 bands which cover from 698 MHz to 3800 MHz. Because of the bands being separated and disordered, a mobile system operator may use multiple bands simultaneously in the same country or area. In the LTE wireless communication system, band  13  (covering from 746 MHz to 787 MHz) requires lower frequencies, and hence a complex antenna operated in band  13  would occupy larger space. Without adequate size, the complex antenna cannot meet the requirements of multiband or wideband transmission. What&#39;s worse, interference between antennas might occur to threaten normal operations of the antennas. 
         [0006]    Obviously, providing an antenna of small size that allows multiband and low-frequency operations is a significant objective in the field. 
       SUMMARY OF THE INVENTION 
       [0007]    Therefore, the present invention primarily provides an antenna and a complex antenna having small size and allowing both multiband and low-frequency operations. 
         [0008]    An embodiment of the present invention discloses an antenna for receiving and transmitting radio signals, comprising a reflective unit, comprising a central reflective element; and a plurality of peripheral reflective elements, enclosing the central reflective element to form a frustum structure; and at least one radiation unit, disposed above the central reflective element; wherein the reflective unit is electrically isolated from the at least one radiation unit. 
         [0009]    An embodiment of the present invention further discloses a complex antenna for receiving and transmitting radio signals, comprising a plurality of antennas, each of the plurality of antennas comprising a reflective unit, comprising a central reflective element; and a plurality of peripheral reflective elements, enclosing the central reflective element to form a frustum structure; and at least one radiation unit, disposed above the central reflective element; wherein the reflective unit is electrically isolated from the at least one radiation unit. 
         [0010]    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 
         [0011]      FIG. 1A  is a schematic diagram illustrating an antenna according to an embodiment of the present invention. 
           [0012]      FIG. 1B  is a lateral-view schematic diagram illustrating the antenna shown in  FIG. 1A . 
           [0013]      FIGS. 2A to 2C  are schematic diagrams illustrating antenna resonance simulation results of the antenna shown in  FIG. 1A  with the height set to 75 mm, 82 mm and 86 mm, respectively. 
           [0014]      FIG. 3  is a top-view schematic diagram illustrating an antenna according to an embodiment of the present invention. 
           [0015]      FIG. 4  is a schematic diagram illustrating antenna resonance simulation results of the antenna shown in  FIG. 3  with the width set to 25.5 mm. 
           [0016]      FIG. 5  is a schematic diagram illustrating an antenna according to an embodiment of the present invention. 
           [0017]      FIG. 6  is a schematic diagram illustrating antenna resonance simulation results of the antenna shown in  FIG. 5  with the width set to 25.5 mm. 
           [0018]      FIG. 7A  is a schematic diagram illustrating an antenna according to an embodiment of the present invention. 
           [0019]      FIG. 7B  is a top-view schematic diagram illustrating the antenna shown in  FIG. 7A . 
           [0020]      FIG. 7C  is a cross-sectional view schematic diagram taken along a cross-sectional line A-A′ in  FIG. 7B . 
           [0021]      FIGS. 8A and 8  B are schematic diagrams illustrating curves representing relationships between frequencies and the reflection phases of the reflective unit of the antenna shown in  FIG. 7A  when the height of the vias is set to 17.6 mm and 22 mm respectively. 
           [0022]      FIGS. 9A and 9B  are schematic diagrams illustrating antenna resonance simulation results of the antenna shown in  FIG. 7A  with the height set to 82 mm and 66.4 mm, respectively. 
           [0023]      FIG. 10  is a schematic diagram illustrating antenna pattern characteristic simulation results of one radiation unit of the antenna shown in  FIG. 9B  operated at 777 MHz. 
           [0024]      FIG. 11  is a schematic diagram illustrating antenna pattern characteristic simulation results of another radiation unit of the antenna shown in  FIG. 9B  operated at 777 MHz. 
           [0025]      FIG. 12A  is a schematic diagram illustrating an antenna according to an embodiment of the present invention. 
           [0026]      FIG. 12B  is a lateral-view schematic diagram illustrating the antenna shown in  FIG. 12A . 
           [0027]      FIG. 12C  is a schematic diagram illustrating radiation units of the antenna shown in  FIG. 12A . 
           [0028]      FIG. 13  is a schematic diagram illustrating antenna resonance simulation results of the antenna shown in  FIG. 12A . 
           [0029]      FIG. 14  is a schematic diagram illustrating antenna pattern characteristic simulation results of the radiation unit of the antenna shown in  FIG. 12A  operated at 777 MHz. 
           [0030]      FIG. 15  is a schematic diagram illustrating radiation units of an antenna according to an embodiment of the present invention. 
           [0031]      FIG. 16  is a schematic diagram illustrating antenna resonance simulation results of the antenna shown in  FIG. 15 . 
           [0032]      FIG. 17  is a schematic diagram illustrating antenna pattern characteristic simulation results of the radiation unit of the antenna shown in  FIG. 15  operated at 777 MHz. 
           [0033]      FIG. 18  is a schematic diagram illustrating a complex antenna according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0034]    Please refer to  FIG. 1A  and  FIG. 1B .  FIG. 1A  is a schematic diagram illustrating an antenna  10  according to an embodiment of the present invention.  FIG. 1B  is a lateral-view schematic diagram illustrating the antenna  10 . The antenna  10  includes a reflective unit  100 , radiation units  120 ,  140  and a supporting element  180 . The reflective unit  100  includes a central reflective element  102  and peripheral reflective elements  104   a  to  104   d  to reflect electromagnetic waves, thereby increasing gain of the antenna  10 . Each of the peripheral reflective elements  104   a  to  104   d  has a shape substantially conforming to an isosceles trapezoid with symmetry. Taken together, the peripheral reflective elements  104   a  to  104   d  enclose the central reflective element  102  symmetrically to form a frustum structure. The radiation units  120  and  140  are disposed above the central reflective element  102  with the supporting element  180 , and the radiation units  120  and  140  are electrically isolated from the reflective unit  100 —meaning that the radiation unit  120  or  140  is not electrically connected to or contacting the reflective unit  100 . The radiation unit  120  includes conductor plates  120   a  and  120   b  with symmetry to form a dipole antenna of 135-degree slant polarized. The conductor plates  120   a  and  120   b  include main sections  122   a ,  122   b , first arm sections  124   a ,  124   b  and feed-in points  126   a ,  126   b , respectively. The feed-in points  126   a  and  126   b , which are configured for feeding the antenna  10  with a transmission line (not shown) connected to the feed-in points  126   a  and  126   b , are disposed on and within the main sections  122   a  and  122   b , respectively. Ends of the first arm sections  124   a  and  124   b  are connected to ends of the main sections  122   a  and  122   b  respectively. However, the first arm section  124   a  is not coplanar to the main section  122   a  but extending toward the reflective unit  100 ; the first arm section  124   b  is not coplanar to the main section  122   b  but extending toward the reflective unit  100 . Similarly, the radiation unit  140  includes the conductor plates  140   a  and  140   b  with symmetry to form a dipole antenna of 45-degree slant polarized. The conductor plates  140   a  and  140   b  include main sections  142   a ,  142   b , first arm sections  144   a ,  144   b  and feed-in points  146   a ,  146   b , respectively. The feed-in points  146   a  and  146   b , which are configured for feeding the antenna  10  with another transmission line (not shown) connected to the feed-in points  146   a  and  146   b , are disposed on and within the main sections  142   a  and  142   b , respectively. Ends of the first arm sections  144   a  and  144   b  are connected to ends of the main sections  142   a  and  142   b  respectively. Nevertheless, the first arm section  144   a  is not coplanar to the main section  142   a  but extending toward the reflective unit  100 ; the first arm section  144   b  is not coplanar to the main section  142   b  but extending toward the reflective unit  100 . 
         [0035]    In short, when the total length DP_L of the main sections  122   a  and  122   b  and the total length DP_L of the main sections  142   a  and  142  b are less than half of an operating wavelength, an effective length of the radiation unit  120  and an effective length of the radiation unit  140  would be increased to improve return loss (i.e., S11 parameter value) by means of the first arm sections  124   a ,  124   b ,  144   a  and  144   b  respectively. This may minimize a size of the antenna  10 , meet transmission requirements of low frequency, and improve resonance effects of the antenna  10 . 
         [0036]    To enhance polarization isolation (i.e., common polarization to cross polarization parameters), the antenna  10  should be symmetrical. Therefore, as shown in  FIG. 1B , the reflective unit  100  and the main sections  122   a ,  122   b ,  142   a ,  142   b  are symmetric with respect to a centerline CENT of the reflective unit  100  extending along an axis Z respectively. If the radiation unit  140  is separated from the central reflective element  102  by a height DP_H, the radiation unit  120  is separated from the central reflective element  102  by the height DP_H substantially. Nevertheless, there may be a height difference between the radiation unit  140  and the radiation unit  120  to avoid a short circuit, and a value of the height difference is substantially less than one tenth of a height DP_H. Because of symmetry, the total length between the main sections  122   a  and  122   b  and between the main sections  142   a  and  142   b  will be the total length DP_L; the first arm sections  124   a ,  124   b ,  144   a  and  144   b  may have a length BN_L 1  respectively. Moreover, the antenna  10  may be disposed in a cylindrical radome RAD, which may have a radius R 1  less than one quarter of the operating wavelength. A centerline CEN 2  of the cylindrical radome RAD extending along an axis Y is determined after the peripheral reflective elements  104   b  and  104   d  are extended to intersect. In other words, because the antenna  10  is restricted by the radius R 1 , the height DP_H between the radiation unit  140  and the central reflective element  102  of the antenna  10  is less than one quarter of the operating wavelength, and the total length DP_L, of the main sections  142   a  and  142   b  is less than half of the operating wavelength. As the height DP_H increases, the total length DP_L must be reduced; when the total length DP_L becomes longer, the height DP_H must be shorten. In such a situation, to improve the return loss, the height DP_H is adjusted to a proper value first, and then the first arm sections  124   a ,  124   b ,  144   a  and  144   b  are utilized to increase the effective lengths of the radiation units  120  and  140 . 
         [0037]    For example, please refer to Table 1 and  FIGS. 2A to 2C .  FIGS. 2A to 2C  are schematic diagrams illustrating antenna resonance simulation results of the antenna  10  with the height DP_H set to 75 mm, 82 mm and 86 mm, respectively. Antenna resonance simulation results of a control group without the first arm sections  124   a ,  124   b ,  144   a  and  144   b  are also shown in  FIG. 2A  to be compared against. Antenna resonance simulation results of the radiation unit  120  of the antenna  10  and a radiation unit of an antenna of the control group are presented by a thin long dashed line and a thick long dashed line, respectively; antenna resonance simulation results of the radiation unit  140  of the antenna  10  and another radiation unit of the antenna of the control group are presented by a thin short dashed line and a thick short dashed line, respectively. Because antenna isolation simulation results are less than −60 dB, they are not illustrated in  FIGS. 2A to 2C . Table 1 lists dimensions and maximum return loss of the antenna  10  shown in  FIGS. 2A to 2C  and the antenna of the control group. In Table 1, the radius R 1  is set to 99 mm, and a base length W of the peripheral reflective elements  104   a  to  104   d  of the antenna  10  is set to 140 mm. Moreover, the radiation unit of the antenna of the control group also has the total length DP_L and is separated from a central reflective element of the antenna of the control group by the height DP_H. According to Table 1 and  FIGS. 2A to 2C , the return loss of the antenna  10  may be improved to −6.97 dB when the first arm sections  124   a ,  124   b ,  144   a  and  144   b  are disposed. 
         [0000]    
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 the 
                 the maximum 
                   
                   
               
               
                   
                 the 
                 total 
                 return loss 
                 the 
                 the maximum 
               
               
                 corresponding 
                 height 
                 length 
                 (the antenna of 
                 length 
                 return loss 
               
               
                 FIGS. 
                 DP_H 
                 DP_L 
                 the control group) 
                 BN_L1 
                 (the antenna 10) 
               
               
                   
               
             
             
               
                 FIG. 2A 
                 75 mm 
                 135 mm  
                 −0.18 dB 
                 25.0 mm 
                 −4.66 dB 
               
               
                   
                 78 mm 
                 113 mm  
                   
                 37.2 mm 
                 −6.12 dB 
               
               
                   
                 80 mm 
                 99 mm 
                   
                 44.8 mm 
                 −6.74 dB 
               
               
                   
                 81 mm 
                 91 mm 
                   
                 49.1 mm 
                 −6.91 dB 
               
               
                 FIG. 2B 
                 82 mm 
                 85 mm 
                   
                 52.3 mm 
                 −6.97 dB 
               
               
                   
                 83 mm 
                 79 mm 
                   
                 55.6 mm 
                 −6.87 dB 
               
               
                   
                 84 mm 
                 75 mm 
                 −0.01 dB 
                 57.9 mm 
                 −6.75 dB 
               
               
                 FIG. 2C 
                 86 mm 
                 45 mm 
                   
                 73.8 mm 
                 −4.03 dB 
               
               
                   
               
             
          
         
       
     
         [0038]    By adjusting the radiation units  120  and  140  shown in  FIG. 1A , the return loss may be improved further. Please refer to  FIG. 3 .  FIG. 3  is a top-view schematic diagram illustrating an antenna  30  according to an embodiment of the present invention. The structure of the antenna  30  is similar to that of the antenna  10  in  FIGS. 1A and 1B , and the same numerals and symbols denote the same components in the following description. Since the reflective unit  100  has the frustum structure, the distance from radiation unit  320  or  340  of the antenna  30  to the reflective unit  100  is tough to pin down—the central reflective element  102  of the reflective unit  100  is far from the radiation units  320  and  340 , but the peripheral reflective elements  104   a  to  104   d  of the reflective unit  100  are closer to the radiation units  320  and  340 . Therefore, main sections  322   a ,  322   b  of the radiation unit  320  and main sections  342   a ,  342   b  of the radiation unit  340  form a bishop hat dipole antenna, respectively, such that a geometrical center (for example, the center of mass) of the main section  322  a moves toward the centerline CEN 1 , and geometrical centers of the main sections  322   b ,  342   a  and  342   b  move toward the centerline CEN 1  likewise, thereby increase an effective distance between the radiation unit  320  and the reflective unit  100  or between the radiation unit  340  and the reflective unit  100 . Besides, a geometrical shape of the antenna  30  is symmetrical with respect to symmetrical axes SYM 1  and SYM 2 . The main sections  322   a  and  322   b  along the symmetrical axis SYM 2  reaching a length BS_L 1  has a width BS_W to the maximum; the main sections  342   a  and  342   b  along the symmetrical axis SYM 1  reaching the length BS_L 1  has the width BS_W to the maximum. When the length BS_L 1  is reduced to make the points, which correspond to the width BS_W and the length BS_L 1 , move toward the centerline CEN 1 , the geometrical centers of the main sections  322   a ,  322   b ,  342   a  and  342   b  also move toward the centerline CEN 1  and the return loss (S11) drops. By adjusting a ratio of the width BS_W to the length BS_L 1  and a ratio of the width BS_W to a width DP_W, the geometrical centers of the main sections  322   a ,  322   b ,  342   a  and  342   b  may become closer to the centerline CEN 1 . 
         [0039]    For example, please refer to Table 2 and  FIG. 4 .  FIG. 4  is a schematic diagram illustrating antenna resonance simulation results of the antenna  30  with the width BS_W set to 25.5 mm. In  FIG. 4 , antenna resonance simulation results for the radiation unit  320  of the antenna  30  is presented by a long dashed line, and the antenna resonance simulation result for the radiation unit  340  of the antenna  30  is presented by a short dashed line. Antenna isolation simulation results are not shown in  FIG. 4  because it is less than −60 dB. Table 2 lists the dimensions and the maximum return loss of the antenna  10  shown in  FIG. 2B  and those of the antenna  30  shown in  FIG. 4 , respectively. The total length DP_L and the height DP_H of the antenna  30  shown in  FIG. 4  are the same as those of the antenna  10  shown in  FIG. 2B  respectively; the width DP_W of the antenna  10  shown in  FIGS. 2A to 2C  is the same as that of the antenna  30  shown in  FIG. 4 . According to Table 2 and  FIG. 4 , the return loss of the antenna  30  may be effectively improved to −8.27 dB by adjusting the ratio of the width BS_W to the length BS_L 1  and the ratio of the width BS_W to the width DP_W. To prevent the isolation from being affected, it would be better to keep projections of the main sections  322   a ,  322   b ,  342   a ,  342   b  along the axis Z from overlapping as the width BS_W increases to improve the return loss. 
         [0000]    
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 corre- 
                 the 
                 the 
                 the 
                 the 
                 the 
               
               
                 sponding 
                 width 
                 width 
                 length 
                 length 
                 maximum 
               
               
                 FIGS. 
                 BS_W 
                 DP_W 
                 BN_L1 
                 BS_L1 
                 return loss 
               
               
                   
               
             
             
               
                 FIG. 2B 
                 5.15 mm 
                 5.15 mm 
                 52.3 mm 
                  0 mm 
                 −6.97 dB 
               
               
                   
                 12.75 mm  
                 5.15 mm 
                 55.4 mm 
                 17 mm 
                 −7.53 dB 
               
               
                 FIG. 4 
                 25.5 mm 
                 5.15 mm 
                 58.4 mm 
                 17 mm 
                 −8.27 dB 
               
               
                   
               
             
          
         
       
     
         [0040]    By adding a reflective plate, the return loss may be improved further. Please refer to  FIG. 5 .  FIG. 5  is a schematic diagram illustrating an antenna  50  according to an embodiment of the present invention. The structure of the antenna  50  is similar to that of the antenna  30  in  FIG. 3 , and the same numerals and symbols denote the same components in the following description. The antenna  50  further includes a reflective plate  560  to increase effective radiation area of the antenna  50  and to improve effective resonance results of the antenna  50 . The reflective plate  560  is disposed above the radiation unit  340  by means of the supporting element  180  and is separated from the central reflective element  102  by the height RF_H, such that the reflective plate  560  is not electrically connected to or contacting the reflective unit  100  or the radiation units  320 ,  340 . To improve common polarization to cross polarization (Co/Cx) parameter, a geometrical shape of the reflective plate  560  has symmetry, and may be a circle or a regular polygon with vertices whose number is a multiple of 4. As shown in  FIG. 5 , the reflective plate  560  (or its projection on the plane XY) is symmetrical with respect to the symmetrical axes SYM 1 , SYM 2  and the axes X, Y respectively. The centerline CEN 1  passes a center CEN 3  of the reflective plate  560 . Since the antenna  50  is disposed in the cylindrical radome RAD with the radius R 1  smaller than one quarter of the operating wavelength, a height RF_H is less than one quarter of the operating wavelength, and a length RF_R from the center CEN 3  to each of the vertices of the reflective plate  560  are quite limited. 
         [0041]    For example, please refer to Table 3 and  FIG. 6 .  FIG. 6  is a schematic diagram illustrating antenna resonance simulation results of the antenna  50  with the width BS_W set to 25.5 mm. In  FIG. 6 , antenna resonance simulation results for the radiation unit  320  of the antenna  50  is presented by a long dashed line, and antenna resonance simulation result for the radiation unit  340  of the antenna  50  is presented by a short dashed line. Antenna isolation simulation results are not shown in  FIG. 6  because it is less than −60 dB. Table 3 lists dimensions and maximum return loss of the antenna  50  shown in  FIG. 6  respectively. The total length DP_L, the length RF_R, the height DP_H, the height RF_H and the width DP_W of the antenna  50  are set to 85 mm, 29 mm, 82 mm, 85.5 mm and 5.15 mm respectively. Comparing  FIG. 6  and Table 3 with  FIGS. 2B, 4  and Table 2, return loss of the antenna  50  may be effectively improved to −9.38 dB by adding the reflective plate  560 . 
         [0000]    
       
         
               
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 corre- 
                 the 
                 the 
                 the 
                 the 
               
               
                 sponding 
                 width 
                 length 
                 length 
                 maximum 
               
               
                 FIGS. 
                 BS_W 
                 BN_L1 
                 BS_L1 
                 return loss 
               
               
                   
               
             
             
               
                   
                 5.15 mm 
                 52.3 mm 
                  0 mm 
                 −8.03 dB 
               
               
                   
                 12.75 mm  
                 55.4 mm 
                 17 mm 
                 −8.64 dB 
               
               
                 FIG. 6 
                 25.5 mm 
                 58.4 mm 
                 17 mm 
                 −9.38 dB 
               
               
                   
               
             
          
         
       
     
         [0042]    By properly designing the reflective unit  100 , the return loss may be improved further. Please refer to  FIG. 7A to 7C .  FIG. 7A  is a schematic diagram illustrating an antenna  70  according to an embodiment of the present invention.  FIG. 7B  is a top-view schematic diagram illustrating the antenna  70 .  FIG. 7C  is a cross-sectional view schematic diagram taken along a cross-sectional line A-A′ in  FIG. 7B . The structure of the antenna  70  is similar to that of the antenna  50  in  FIG. 5 , and the same numerals and symbols denote the same components in the following description. Peripheral reflective element  704   a  to  704   d  of a reflective unit  700  of the antenna  70  include conductor base plates MB_a to MB_d, vias V_a to V_d, spacer layers DL_a to DL_d and conductor patches MF_a to MF_d, respectively. Each of the conductor base plates MB_a to MB_d has a shape substantially conforming to an isosceles trapezoid with symmetry, and the conductor base plates MB_a to MB_d enclose the central reflective element  102  symmetrically to form a frustum structure. The shapes of the conductor patches MF_a to MF_d are similar to the shapes of the conductor base plates MB_a to MB_d respectively, meaning that they have the same shape or that one may be obtained from the other by uniformly scaling. The conductor patch MF_a is connected to the conductor base plate MB_a with the via V_a to form a mushroom-type structure, thereby ensuring magnetic conductor reflection effects (i.e., reflection effects of a magnetic conductor). Likewise, the conductor patches MF b  to MF_d are connected to the conductor base plates MB b  to MB_d with the vias V b  to V_d respectively. The spacer layers DL_a to DL_d are disposed to surround or encompass the vias V_a to V_d so that the conductor patches MF_a to MF_d are not electrically connected to or contacting the conductor base plates MB_a to MB_d. The spacer layers DL_a to DL_d may be made of various electrically isolation materials such as air, ceramic, plastic or microwave substrate materials. By properly increasing permittivity of the spacer layers DL_a to DL_d, a size of the antenna  70  may be minimized and the transmission requirements of low frequency may be met efficiently. 
         [0043]    Technically, a conventional artificial magnetic conductor has a periodic structure and thus may alter various reflection phases of electromagnetic waves. However, a conventional artificial magnetic conductor is basically of a plane structure, meaning that it is flat or made by sticking several flat layers together. Unlike a conventional artificial magnetic conductor, the conductor patches MF_a to MF_d of the present invention providing magnetic conductor reflection effects are regularly (or periodically) arranged above the conductor base plates MB_a to MB_d, which are not parallel to each other, thereby presenting the distinct frustum structure of the reflective unit  700 . Besides, a radio wave, when reflected from the reflective unit  700 , undergoes a phase shift, and this phase shift, which is referred to as a reflection phase of the reflective unit  700  hereafter, is in a range of −180° to 180° corresponding to different frequencies. Therefore, even if the radiation units  320  and  340  are quite close to the reflective unit  700 , a reflected radio signal bounced back from the reflective unit  700  may be in phase with its incident radio signal, which is transmitted or received by the radiation unit  320  or  340 , thereby achieving constructive interference. As a result, distances between the radiation unit  320  and the reflective unit  700  and between the radiation unit  340  and the reflective unit  700  may be reduced, the size of the antenna  70  may be minimized and the transmission requirements of low frequency may be met efficiently. For example, please refer to  FIGS. 8A and 8B .  FIGS. 8A and 8B  are schematic diagrams illustrating curves representing relationships between frequencies and the reflection phases of the reflective unit  700  of the antenna  70  when a height T_MR of the vias V_a to V_d is set to 17.6 mm and 22 mm respectively. In  FIGS. 7B and 7C , projection of edges of the conductor patches MF_a to MF_d projected on the conductor base plates MB_a to MB_d are separated from edges of the conductor base plates MB_a to MB_d by distances BT 1 , BT, BT 2  respectively. The vias V_a to V_d are separated from the central reflective element  102  by a distance PST_O. The distance BT 1 , BT, BT 2 , PST_O are set to 12.375 mm, 18.4 mm, 10 mm, 51.5 mm respectively; dielectric constant of the spacer layers DL_a to DL_d is set to 10. As shown in  FIGS. 8A and 8B , the reflection phases of the reflective unit  700  are in a range of −180° to 180° corresponding to different frequencies. When a structure or dimensions of the reflective unit  700  are adjusted, a reflection phase of the reflective unit  700  corresponding to a specific frequency is changed. In general, comparing with a conventional antenna having a normal metal plate to serve as its reflective unit, the reflective unit  700  with the reflection phases in a range of −180° to 0° allows reduction in height of the antenna  70  so as to minimize the size of the antenna  70 . When a reflection phase of the reflective unit  700  gets closer to 0 degrees, heights of the radiation units  320  and  340  of the antenna  70  becomes lower and the size of the antenna  70  is smaller. Obviously, the size of the antenna  70  may be minimized with the reflective unit  700  having adjustable reflection phases. The structure and the dimensions of the reflective unit  700  maybe adjusted appropriately according to the lowest frequency required by the antenna system, such that the reflection phase of the reflective unit  700  corresponding to the lowest frequency gets closer to 0 degrees so as to reduce the size of the antenna  70 . 
         [0044]    Simulation and measurement may be employed to determine whether the antenna  70  operated at different frequencies meets system requirements. Please refer to Table 4 and  FIGS. 9A, 9B .  FIGS. 9A and 9B  are schematic diagrams illustrating antenna resonance simulation results of the antenna  70  with the height DP_H set to 82 mm and 66.4 mm, respectively. In  FIGS. 9A and 9B , antenna resonance simulation results for the radiation unit  320  and  340  of the antenna  70  are presented by a long dashed line and a short dashed line respectively; antenna isolation simulation results between the radiation units  320  and  340  of the antenna  70  is presented by a solid line. Table 4 lists dimensions and maximum return loss of the antenna  70  shown in  FIGS. 9A and 9B  respectively. The distances BT 1 , BT, BT 2 , PST_O and the height T_MR are set to 12.3 mm, 18.4 mm, 11.9 mm, 51.5 mm and 17.5 mm respectively; the dielectric constant of the spacer layers DL_a to DL_d is set to 10. According to Table 4 and  FIGS. 9A and 9B , return loss of the radiation units  320  and  340  may be effectively improved to −11.9 dB to meet the requirements of having the return loss less than −10 dB. 
         [0000]    
       
         
               
               
               
               
               
               
             
           
               
                   
                 TABLE 4 
               
               
                   
                   
               
             
             
               
                   
                 the total length DP_L 
                 85 
                 mm 
                 137.3 
                 mm 
               
               
                   
                 the height DP_H 
                 82 
                 mm 
                 66.4 
                 mm 
               
               
                   
                 the length BN_L1 
                 58.4 
                 mm 
                 13.7 
                 mm 
               
               
                   
                 the width DP_W 
                 5.15 
                 mm 
                 3.28 
                 mm 
               
               
                   
                 the length BS_L1 
                 17 
                 mm 
                 34.1 
                 mm 
               
               
                   
                 the width BS_W 
                 25 
                 mm 
                 50.5 
                 mm 
               
               
                   
                 the length RF_R 
                 29 
                 mm 
                 55.3 
                 mm 
               
               
                   
                 the height RF_H 
                 85.5 
                 mm 
                 74.1 
                 mm 
               
               
                   
                 the maximum return loss 
                 −11.9 
                 dB 
                 −10.3 
                 dB 
               
               
                   
                   
               
             
          
         
       
     
         [0045]    Please refer to Tables 5 to 9 and  FIGS. 10, 11 . Tables 5 and 6 are field pattern characteristic tables for the radiation unit  320  of the antenna  70  in a horizontal plane (i.e., an H cross-sectional plane) and a vertical plane (i.e., a V cross-sectional plane) shown in  FIG. 7A , respectively. Tables 7 and 8 are field pattern characteristic tables for the radiation unit  340  of the antenna  70  in the horizontal plane and the vertical plane shown in  FIG. 7A , respectively. Table 9 is a simulation antenna characteristic table for the antenna  70  shown in  FIG. 7A .  FIG. 10  is a schematic diagram illustrating antenna pattern characteristic simulation results of the radiation unit  320  of the antenna  70  shown in  FIG. 7A  operated at 777 MHz.  FIG. 11  is a schematic diagram illustrating antenna pattern characteristic simulation results of the radiation unit  340  of the antenna  70  shown in  FIG. 7A  operated at 777 MHz. In  FIGS. 10 and 11 , a common polarization radiation pattern of the antenna  70  in the horizontal plane (i.e., at 0 degrees) is presented by a thick solid line, a common polarization radiation pattern of the antenna  70  in the vertical plane (i.e., at 90 degrees) is presented by a thick dashed line, a cross polarization radiation pattern of the antenna  70  in the horizontal plane is presented by a thin solid line, and a cross polarization radiation pattern of the antenna  70  in the vertical plane is presented by a thin dashed line. According to Table 9, within Band  13 , the return loss of the antenna  70  is at least −10.3 dB, a maximum gain is at least 5.96 dBi, and a common polarization to cross polarization parameter is at least 43.5 dB. Therefore, it is shown that the antenna  70  of the present invention meets LTE wireless communication system requirements of Band  13 . 
         [0000]    
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 5 
               
               
                   
               
               
                   
                   
                   
                   
                   
                 the common 
               
               
                   
                   
                   
                   
                   
                 polarization 
               
               
                   
                   
                   
                   
                 front-to- 
                 to cross 
               
               
                 corre- 
                   
                 the 
                 3 dB 
                 back 
                 polarization 
               
               
                 sponding 
                 fre- 
                 maximum 
                 beam- 
                 (F/B) 
                 (Co/Cx) 
               
               
                 FIGS. 
                 quency 
                 gain 
                 width 
                 ratio 
                 parameter 
               
               
                   
               
             
             
               
                   
                 746 MHz 
                 5.96 dBi 
                 94 degrees 
                 7.3 dB 
                 49.8 dB 
               
               
                   
                 756 MHz 
                 6.32 dBi 
                 94 degrees 
                 7.6 dB 
                 48.5 dB 
               
               
                 FIG. 10 
                 777 MHz 
                 6.45 dBi 
                 93 degrees 
                 8.2 dB 
                 45.9 dB 
               
               
                   
                 787 MHz 
                 6.31 dBi 
                 93 degrees 
                 8.5 dB 
                 44.9 dB 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 6 
               
               
                   
               
               
                   
                   
                   
                   
                   
                 the common 
               
               
                   
                   
                   
                   
                   
                 polarization 
               
               
                 corre- 
                   
                 the 
                 3 dB 
                 front-to- 
                 to cross 
               
               
                 sponding 
                 fre- 
                 maximum 
                 beam- 
                 back 
                 polarization 
               
               
                 FIGS. 
                 quency 
                 gain 
                 width 
                 ratio 
                 parameter 
               
               
                   
               
             
             
               
                   
                 746 MHz 
                 5.96 dBi 
                 94 degrees 
                 7.3 dB 
                 46.7 dB 
               
               
                   
                 756 MHz 
                 6.32 dBi 
                 94 degrees 
                 7.6 dB 
                 46.9 dB 
               
               
                 FIG. 10 
                 777 MHz 
                 6.45 dBi 
                 94 degrees 
                 8.2 dB 
                 45.6 dB 
               
               
                   
                 787 MHz 
                 6.31 dBi 
                 93 degrees 
                 8.5 dB 
                 45.0 dB 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 7 
               
               
                   
               
               
                   
                   
                   
                   
                   
                 the common 
               
               
                   
                   
                   
                   
                   
                 polarization 
               
               
                 corre- 
                   
                 the 
                 3 dB 
                 front-to- 
                 to cross 
               
               
                 sponding 
                 fre- 
                 maximum 
                 beam- 
                 back 
                 polarization 
               
               
                 FIGS. 
                 quency 
                 gain 
                 width 
                 ratio 
                 parameter 
               
               
                   
               
             
             
               
                   
                 746 MHz 
                 5.98 dBi 
                 94 degrees 
                 7.3 dB 
                 47.2 dB 
               
               
                   
                 756 MHz 
                 6.24 dBi 
                 94 degrees 
                 7.6 dB 
                 46.4 dB 
               
               
                 FIG. 11 
                 777 MHz 
                 6.31 dBi 
                 94 degrees 
                 8.2 dB 
                 44.4 dB 
               
               
                   
                 787 MHz 
                 6.20 dBi 
                 93 degrees 
                 8.5 dB 
                 43.5 dB 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 8 
               
               
                   
               
               
                   
                   
                   
                   
                   
                 the common 
               
               
                   
                   
                   
                   
                   
                 polarization 
               
               
                 corre- 
                   
                 the 
                 3 dB 
                 front-to- 
                 to cross 
               
               
                 sponding 
                 fre- 
                 maximum 
                 beam- 
                 back 
                 polarization 
               
               
                 FIGS. 
                 quency 
                 gain 
                 width 
                 ratio 
                 parameter 
               
               
                   
               
             
             
               
                   
                 746 MHz 
                 5.98 dBi 
                 94 degrees 
                 7.3 dB 
                 44.0 dB 
               
               
                   
                 756 MHz 
                 6.24 dBi 
                 94 degrees 
                 7.6 dB 
                 44.6 dB 
               
               
                 FIG. 11 
                 777 MHz 
                 6.31 dBi 
                 94 degrees 
                 8.2 dB 
                 45.5 dB 
               
               
                   
                 787 MHz 
                 6.20 dBi 
                 93 degrees 
                 8.5 dB 
                 45.8 dB 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 9 
               
               
                   
                   
               
               
                   
                 frequency band 
                 Band 13 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 the return loss 
                 &gt;10.3 
                 dB 
               
               
                   
                 isolation 
                 &gt;51.3 
                 dB 
               
               
                   
                 the maximum gain 
                 5.96-6.45 
                 dBi 
               
               
                   
                 front-to-back ratio 
                 7.3-8.5 
                 dB 
               
               
                   
                 3 dB beamwidth 
                 93-94 
                 degrees 
               
               
                   
                 the common polarization to cross 
                 43.5-49.8 
                 dB 
               
               
                   
                 polarization parameter 
               
               
                   
                   
               
             
          
         
       
     
         [0046]    Please note that the reflection phases of the reflective unit  700  are in a range of −180° to 180° corresponding to different frequencies while variation of the reflection phases corresponding to higher frequencies shown in  FIGS. 8A and 8  B is large. Taking full advantage of the characteristics of the reflective unit  700 , the structure of the antenna  70  is suitable for multiband applications. 
         [0047]    Please refer to  FIGS. 12A to 12  C.  FIG. 12A  is a schematic diagram illustrating an antenna  80  according to an embodiment of the present invention.  FIG. 12B  is a lateral-view schematic diagram illustrating the antenna  80 .  FIG. 12C  is a schematic diagram illustrating radiation units  820  and  840  of the antenna  80 . The structure of the antenna  80  is similar to that of the antenna  70  in  FIGS. 7A to 7C , and the same numerals and symbols denote the same components in the following description. The radiation unit  820  includes conductor plates  820   a  and  820   b  with symmetry to form a dipole antenna of 135-degree slant polarized. The conductor plates  820   a  and  820   b  include the main sections  322   a ,  322   b , the first arm sections  124   a ,  124   b , second arm sections  828   a ,  828   b  and the feed-in points  126   a ,  126   b , respectively. As shown in  FIGS. 12B and 12C , the ends of the first arm sections  124   a  and  124   b  (e.g., an endpoint B of the first arm section  124   a ) are connected to the ends of the main sections  322   a  and  322   b  (e.g., the endpoint B of the main section  322   a ) respectively, such that a distance between a positively charged side and a negatively charged side becomes longer during resonance so as to enhance radiation effects. Ends of the second arm sections  828   a  and  828   b  (e.g., an endpoint D of the second arm section  828   a ) are connected to different points of the main sections  322   a  and  322   b  (e.g., the point D of the main section  322   a ) respectively. The end of the second arm section  828   a  is separated from the end of the first arm section  124   a  by a distance D 1 ; the end of the second arm section  828   b  is separated from the end of the first arm section  124   b  by the distance D 1 . Similarly, the radiation unit  840  includes conductor plates  840   a  and  840   b  with symmetry to form a dipole antenna of 45-degree slant polarized. The conductor plates  840   a  and  840   b  include the main sections  342   a ,  342   b , the first arm sections  144   a ,  144   b , second arm sections  848   a ,  848   b  and the feed-in points  146   a ,  146   b , respectively. The ends of the first arm sections  144   a  and  144   b  are connected to the ends of the main sections  342   a  and  342   b  respectively. Ends of the second arm sections  848   a  and  848   b  are connected to different points of the main sections  342   a  and  342   b  respectively. The ends of the second arm sections  848   a  and  848   b  are separated from the ends of the first arm sections  144   a  and  144   b  by the distance D 1  respectively. The first arm sections  124   a ,  124   b ,  144   a ,  144   b  and the second arm sections  828   a ,  828   b ,  848   a ,  848   b  are not coplanar to the main sections  322   a ,  322   b ,  342   a  and  342   b  but extending toward the reflective unit  700  respectively. 
         [0048]    As shown in  FIG. 12C , comparing with a current path ODBA formed of the main section (e.g., from a point O to the endpoint B of the main section  322   a ) and the first arm section (e.g., from the endpoint B to an endpoint A of the first arm section  124   a ), a current path ODC formed of the main section (e.g., from the point O to the point D of the main section  322   a ) and the second arm section (e.g., from the endpoint D to an endpoint C of the second arm section  828   a ) is shorter. Consequently, only the first arm sections  124   a ,  124   b ,  144   a  and  144   b  may resonate at a first resonance frequency, which belongs to low frequency; the second arm sections  828   a ,  828   b ,  848   a  and  848   b  however cannot resonate at the first resonance frequency. In this way, the second arm sections  828   a ,  828   b ,  848   a  and  848   b  would have little or no influence on resonance of the first resonance frequency. Besides, although the first arm sections  124   a ,  124   b ,  144   a ,  144   b  and the second arm sections  828   a ,  828   b ,  848   a ,  848   b  may resonate at a second resonance frequency, which is higher than the first resonance frequency, the first arm sections  124   a ,  124   b ,  144   a  and  144   b  resonate at the second resonance frequency by means of higher order mode, and the second arm sections  828   a ,  828   b ,  848   a  and  848   b  resonate at the second resonance frequency using lower order mode. Because resistance of the lower order mode is smaller than resistance of the higher order mode, resonance of the second resonance frequency tends to occur within the current path formed of the main section and the second arm section (i.e., the current path ODC). In other words, the current path formed of the main section and the first arm section (i.e., the current path ODBA) corresponds to the first resonance frequency, the current path formed of the main section and the second arm section (i.e., the current path ODC) corresponds to the second resonance frequency. The two-arm structure may minimize the mutual influence of the first arm section and the second arm section and provide more design flexibility to structure parameters of multiband applications. 
         [0049]    Simulation and measurement may be employed to determine whether the antenna  80  operated at different frequencies meets system requirements. Please refer to Tables 10, 11 and  FIGS. 13, 14 .  FIG. 13  is a schematic diagram illustrating antenna resonance simulation results of the antenna  80 . In  FIG. 13 , the radius R 1  of the antenna  80 , the base length W of the peripheral reflective elements  704   a  to  704   d  and the height T_MR are set to 99 mm, 140 mm and 11.9 mm, respectively; the dielectric constant of the spacer layers DL_a to DL_d is set to 10. Besides, antenna resonance simulation results for the radiation units  820  and  840  of the antenna  80  are presented by a long dashed line and a short dashed line respectively; antenna isolation simulation results between the radiation units  820  and  840  of the antenna is presented by a solid line. According to  FIG. 13 , within Band  13  (covering from 746 MHz to 756 MHz and from 777 MHz to 787 MHz) and Band  4  (covering from 1710 MHz to 1755 MHz and from 2110 MHz to 2155 MHz), isolation between the radiation units  820  and  840  is at least 53.2 dB; return loss of the antenna  80  is improved to −8.3 dB.  FIG. 14  is a schematic diagram illustrating antenna pattern characteristic simulation results of the radiation unit  840  of the antenna  80  shown in  FIG. 12A  operated at 777 MHz. In  FIG. 14 , a common polarization radiation pattern of the antenna  80  in the horizontal plane (i.e., at 0 degrees) is presented by a thick solid line, a common polarization radiation pattern of the antenna  80  in the vertical plane (i.e., at 90 degrees) is presented by a thick dashed line, a cross polarization radiation pattern of the antenna  80  in the horizontal plane is presented by a thin solid line, and a cross polarization radiation pattern of the antenna  80  in the vertical plane is presented by a thin dashed line. Based on  FIG. 14 , at 777 MHz, front-to-back (F/B) ratio of the antenna  80  is at least 7.5 dB, a maximum gain is at least 5.67 dBi, and a common polarization to cross polarization parameter is at least 51.1 dB. Antenna pattern characteristic simulation results of the radiation unit  840  of the antenna  80  operated at other frequencies or antenna pattern characteristic simulation results of the radiation unit  820  are basically similar to aforementioned illustrations and hence are not detailed redundantly. Tables 10 and 11 are field pattern characteristic tables for the radiation units  820  and  840  of the antenna  80 , respectively. According to Tables 10 and 11, within Band  13  and Band  4 , the front-to-back ratio of the antenna  80  is at least 6.8 dB, the maximum gain is at least 5.35 dBi, and the common polarization to cross polarization parameter is at least 13.6 dB. 
         [0000]    
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 10 
               
               
                   
               
               
                   
                   
                   
                   
                   
                 the common 
               
               
                   
                   
                   
                   
                 the 
                 polarization 
               
               
                 corre- 
                   
                 the 
                 3 dB 
                 front-to- 
                 to cross 
               
               
                 sponding 
                 fre- 
                 maximum 
                 beam- 
                 back 
                 polarization 
               
               
                 FIGS. 
                 quency 
                 gain 
                 width 
                 ratio 
                 parameter 
               
               
                   
               
             
             
               
                   
                  746 MHz 
                 5.53 dBi 
                 100 degrees 
                 6.8 dB 
                 48.1 dB 
               
               
                   
                  756 MHz 
                 5.69 dBi 
                 100 degrees 
                 7.1 dB 
                 49.1 dB 
               
               
                 FIG. 14 
                  777 MHz 
                 5.67 dBi 
                 100 degrees 
                 7.5 dB 
                 51.1 dB 
               
               
                   
                  787 MHz 
                 5.55 dBi 
                 100 degrees 
                 7.7 dB 
                 51.8 dB 
               
               
                   
                 1710 MHz 
                 8.33 dBi 
                  69 degrees 
                 17.1 dB  
                 22.3 dB 
               
               
                   
                 1755 MHz 
                 8.13 dBi 
                  69 degrees 
                 17.2 dB  
                 22.3 dB 
               
               
                   
                 2110 MHz 
                 9.00 dBi 
                  57 degrees 
                 17.2 dB  
                 20.1 dB 
               
               
                   
                 2155 MHz 
                 10.20 dBi  
                  49 degrees 
                 9.8 dB 
                 13.6 dB 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 11 
               
               
                   
               
               
                   
                   
                   
                   
                   
                 the common 
               
               
                   
                   
                   
                   
                 the 
                 polarization 
               
               
                 corre- 
                   
                 the 
                 3 dB 
                 front-to- 
                 to cross 
               
               
                 sponding 
                 fre- 
                 maximum 
                 beam- 
                 back 
                 polarization 
               
               
                 FIGS. 
                 quency 
                 gain 
                 width 
                 ratio 
                 parameter 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                  746 MHz 
                 5.35 dBi 
                 100 degrees  
                 6.8 dB 
                 48.1 dB 
               
               
                   
                  756 MHz 
                 5.70 dBi 
                 100 degrees  
                 7.1 dB 
                 48.6 dB 
               
               
                   
                  777 MHz 
                 5.98 dBi 
                 99 degrees 
                 7.5 dB 
                 48.9 dB 
               
               
                   
                  787 MHz 
                 5.95 dBi 
                 99 degrees 
                 7.7 dB 
                 48.8 dB 
               
               
                   
                 1710 MHz 
                 8.34 dBi 
                 70 degrees 
                 16.7 dB  
                 22.2 dB 
               
               
                   
                 1755 MHz 
                 7.90 dBi 
                 70 degrees 
                 17.3 dB  
                 22.0 dB 
               
               
                   
                 2110 MHz 
                 9.33 dBi 
                 56 degrees 
                 17.6 dB  
                 19.6 dB 
               
               
                   
                 2155 MHz 
                 10.40 dBi  
                 48 degrees 
                 9.8 dB 
                 14.2 dB 
               
               
                   
                   
               
             
          
         
       
     
         [0050]    The antennas  10 ,  30 ,  50 ,  70  and  80  are exemplary embodiments of the invention, and those skilled in the art may make alternations and modifications accordingly. For example, each of the spacer layers DL_a to DL_d may be disposed behind a shield of one of the conductor patches MF_a to MF_d, or overlay one of the conductor base plates MB_a to MB_d to cover it completely. Above each of the conductor base plates MB_a to MB_d, there may be one conductor patch, whose shape is similar to the shape of its corresponding conductor base plate, or more than one conductor patches, which are regularly arranged above the conductor base plate. In addition, the ends of the first arm sections  124   a ,  124   b ,  144   a  and  144   b  of the antenna  80  (e.g., the endpoint B of the first arm section  124   a ) are connected to the ends of the main sections  322   a ,  322   b ,  342   a  and  342   b  (e.g., the endpoint B of the main section  322   a ) respectively; however, the present invention is not limited herein, and the first arm section may be connected to a center of the main section or other locations within the main section (e.g., the point D of the main section  322   a ). Moreover, the first arm sections  124   a ,  124   b ,  144   a ,  144   b  and the second arm sections  828   a ,  828   b ,  848   a ,  848   b  of the antenna  80  may be perpendicular to the main sections  322   a ,  322   b ,  342   a ,  342   b  respectively, such that the first arm sections  124   a ,  124   b ,  144   a ,  144   b  and the second arm sections  828   a ,  828   b ,  848   a ,  848   b  are not coplanar to the main sections  322   a ,  322   b ,  342   a  and  342   b . Alternatively, there may be an included angle larger or smaller than 90 degrees between each of the first arm sections  124   a ,  124   b ,  144   a ,  144   b  (or each of the second arm sections  828   a ,  828   b ,  848   a ,  848   b ) and each of the main sections  322   a ,  322   b ,  342   a ,  342   b  to keep them not coplanar. In  FIGS. 12B and 12C , the first arm sections  124   a ,  124   b ,  144   a ,  144   b  and the second arm sections  828   a ,  828   b ,  848   a ,  848   b  of the antenna  80  are in parallel with each other. Nevertheless, the present invention is not limited to this because the included angle between the first arm section and the main section maybe different from the included angle between the second arm section and the main section to make the first arm section and the second arm section unparalleled. As set forth above, the first arm sections  124   a ,  124   b ,  144   a ,  144   b  and the second arm sections  828   a ,  828   b ,  848   a ,  848   b  of the antenna  80  are not coplanar to the main sections  322   a ,  322   b ,  342   a  and  342   b , but the present invention is not limited herein. Alternatively, the first arm section or the second arm section may be coplanar to the main section; this however hinders minimization of antenna size. In  FIGS. 12B and 12C , a length BN_L 2  of the second arm section  828   a ,  828   b  is smaller than the length BN_L 1  of the first arm section  124   a ,  124   b  but those skilled in the art might make appropriate modifications or alterations according to different design considerations. 
         [0051]    To meet requirements of multiband or wideband transmission, the radiation units  820  and  840  of the antenna  80  need further modifications. Please refer to  FIG. 15 .  FIG. 15  is a schematic diagram illustrating radiation units  920  and  940  of an antenna  90  according to an embodiment of the present invention. The radiation units  920  and  940  may replace the radiation units  820  and  840  of the antenna  80  shown in  FIG. 12A . The structure of the antenna  90  is similar to that of the antenna  80  in  FIGS. 12A to 12C  so that the same numerals and symbols denote the same components in the following description. Unlike the radiation units  820  and  840 , the radiation unit  920  includes conductor plates  920   a  and  920   b  with symmetry, and the conductor plates  920   a  and  920   b  further include third arm sections  929   a  and  929   b  respectively. As shown in  FIG. 15 , the third arm sections  929   a  and  929   b  are connected to the main sections  322   a  and  322   b . An endpoint E of the third arm section  929   a  is separated from an endpoint F of the second arm section  828   a  by a distance D 2 ; an endpoint G of the third arm section  929   b  is separated from an endpoint H of the second arm section  828   b  by the distance D 2 . Similarly, the radiation unit  940  includes conductor plates  940   a  and  940   b  with symmetry, and the conductor plates  940   a  and  940   b  further include third arm sections  949   a  and  949   b  respectively. The third arm sections  949   a  and  949   b  are connected to the main sections  342   a  and  342   b . Endpoints I and K of the third arm sections  949   a  and  949   b  are separated from endpoints J and L of the second arm sections  848   a  and  848   b  by the distance D 2 , respectively. With the third arm sections  929   a ,  929   b ,  949   a  and  949   b , the antenna  90  may be operated at broader frequency bands to cover, for example, Band  4 . 
         [0052]    Simulation and measurement may be employed to determine whether the antenna  90  operated at different frequencies meets system requirements. Please refer to Tables 12, 13 and  FIGS. 16, 17 .  FIG. 16  is a schematic diagram illustrating antenna resonance simulation results of the antenna  90 . In  FIG. 16 , the radius R 1  of the antenna  90 , the base length W of the peripheral reflective elements  704   a  to  704   d  and the height T_MR are set to 99 mm, 140 mm and 11.9 mm, respectively; the dielectric constant of the spacer layers DL_a to DL_d is set to 10. Besides, antenna resonance simulation results for the radiation unit  920  and  940  of the antenna  90  are presented by a long dashed line and a short dashed line respectively; antenna isolation simulation results between the radiation units  920  and  940  of the antenna  90  is presented by a solid line. According to  FIG. 16 , within Band  13  and Band  4 , isolation between the radiation units  820  and  840  is at least 41.7 dB and return loss of the antenna  80  is improved to −8.4 dB.  FIG. 17  is a schematic diagram illustrating antenna pattern characteristic simulation results of the radiation unit  940  of the antenna  90  shown in  FIG. 15  operated at 777 MHz. In  FIG. 17 , a common polarization radiation pattern of the antenna  90  in the horizontal plane (i.e., at 0 degrees) is presented by a thick solid line, a common polarization radiation pattern of the antenna  90  in the vertical plane (i.e., at 90 degrees) is presented by a thick dashed line, a cross polarization radiation pattern of the antenna  90  in the horizontal plane is presented by a thin solid line, and a cross polarization radiation pattern of the antenna  90  in the vertical plane is presented by a thin dashed line. Based on  FIG. 17 , at 777 MHz, front-to-back ratio of the antenna  90  is at least 7.6 dB, a maximum gain is at least 5.62 dBi, and a common polarization to cross polarization parameter is at least 51.0 dB. Antenna pattern characteristic simulation results of the radiation unit  940  of the antenna  90  operated at other frequencies or antenna pattern characteristic simulation results of the radiation unit  920  are basically similar to aforementioned illustrations and hence are not detailed redundantly. Tables 12 and 13 are field pattern characteristic tables for the radiation units  920  and  940  of the antenna  90 , respectively. According to Tables 12 and 13, within Band  13  and Band  4 , the front-to-back ratio of the antenna  90  is at least 6.9 dB, the maximum gain is at least 5.41 dBi, and the common polarization to cross polarization parameter is at least 12.3 dB. 
         [0000]    
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 12 
               
               
                   
               
               
                   
                   
                   
                   
                   
                 the common 
               
               
                   
                   
                   
                   
                 the 
                 polarization 
               
               
                 corre- 
                   
                 the 
                 3 dB 
                 front-to- 
                 to cross 
               
               
                 sponding 
                 fre- 
                 maximum 
                 beam- 
                 back 
                 polarization 
               
               
                 FIGS. 
                 quency 
                 gain 
                 width 
                 ratio 
                 parameter 
               
               
                   
               
             
             
               
                   
                  746 MHz 
                 5.51 dBi 
                 100 degrees 
                 6.9 dB 
                 49.6 dB 
               
               
                   
                  756 MHz 
                 5.65 dBi 
                 100 degrees 
                 7.1 dB 
                 50.7 dB 
               
               
                 FIG. 17 
                  777 MHz 
                 5.62 dBi 
                 100 degrees 
                 7.6 dB 
                 51.0 dB 
               
               
                   
                  787 MHz 
                 5.50 dBi 
                 100 degrees 
                 7.8 dB 
                 50.0 dB 
               
               
                   
                 1710 MHz 
                 8.44 dBi 
                  68 degrees 
                 15.5 dB  
                 22.3 dB 
               
               
                   
                 1755 MHz 
                 8.29 dBi 
                  67 degrees 
                 15.6 dB  
                 21.7 dB 
               
               
                   
                 2110 MHz 
                 9.87 dBi 
                  50 degrees 
                 15.4 dB  
                 18.9 dB 
               
               
                   
                 2155 MHz 
                 10.70 dBi  
                  44 degrees 
                 9.7 dB 
                 12.3 dB 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 13 
               
               
                   
               
               
                   
                   
                   
                   
                   
                 the common 
               
               
                   
                   
                   
                   
                 the 
                 polarization 
               
               
                 corre- 
                   
                 the 
                 3 dB 
                 front-to- 
                 to cross 
               
               
                 sponding 
                 fre- 
                 maximum 
                 beam- 
                 back 
                 polarization 
               
               
                 FIGS. 
                 quency 
                 gain 
                 width 
                 ratio 
                 parameter 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                  746 MHz 
                 5.41 dBi 
                 100 degrees 
                 6.9 dB 
                 45.9 dB 
               
               
                   
                  756 MHz 
                 5.73 dBi 
                 100 degrees 
                 7.1 dB 
                 46.9 dB 
               
               
                   
                  777 MHz 
                 5.96 dBi 
                 100 degrees 
                 7.6 dB 
                 48.0 dB 
               
               
                   
                  787 MHz 
                 5.93 dBi 
                 100 degrees 
                 7.8 dB 
                 47.9 dB 
               
               
                   
                 1710 MHz 
                 8.45 dBi 
                  67 degrees 
                 15.9 dB  
                 21.4 dB 
               
               
                   
                 1755 MHz 
                 8.06 dBi 
                  66 degrees 
                 16.0 dB  
                 20.8 dB 
               
               
                   
                 2110 MHz 
                 10.10 dBi  
                  51 degrees 
                 14.6 dB  
                 20.0 dB 
               
               
                   
                 2155 MHz 
                 10.50 dBi  
                  44 degrees 
                 9.1 dB 
                 12.9 dB 
               
               
                   
                   
               
             
          
         
       
     
         [0053]    On the other hand, a dual-polarized beam switching antenna set may be derived from the antenna  10 ,  30 ,  50 ,  70 ,  80  or  90  with appropriate modifications. Please refer to  FIG. 18 .  FIG. 18  is a schematic diagram illustrating a complex antenna  18  according to an embodiment of the present invention. In  FIG. 18 , antennas ANT_ 1  to ANT_ 4  of identical structure constitute the complex antenna  18 . The structure of any of the antennas ANT_ 1  to ANT_ 4  share the same basic concept with or based on the structure of the antenna  10  shown in  FIGS. 1A, 1B , the structure of the antenna  30  shown in  FIG. 3 , the structure of the antenna  50  shown in  FIG. 5 , the structure of the antenna  70  shown in  FIGS. 7A to 7C , or the structure of the antenna  80  shown in  FIGS. 12A to 12B ; therefore, only the antenna ANT_ 1  is illustrated with full details. As shown in  FIG. 18 , the antenna ANT_ 1  includes the reflective unit  700 , the radiation units  320 ,  340 , the reflective plate  560  and the supporting element  180 . After combination of the antennas ANT_ 1  to ANT_ 4 , the complex antenna  18  forms a symmetric annular structure on the horizontal plane (i.e., the XZ plane), and the complex antenna  18  is disposed in the cylindrical radome RAD completely. In the complex antenna  18 , the peripheral reflective elements of the reflective units of the antennas ANT_ 1  to ANT_ 4  are electrically connected; namely, the antennas ANT_ 1  to ANT_ 4  share a common ground. In such a situation, it is possible to suitably adjust the reflective units of the antennas ANT_ 1  to ANT_ 4  to reduce manufacturing costs. For example, as shown in  FIG. 18 , the central reflective elements of the antennas ANT_ 2  and ANT_ 4  are only connected to the peripheral reflective elements of the antennas ANT_ 1  and ANT_ 3  without the peripheral reflective elements of the antennas ANT_ 2  and ANT_ 4  serving as two flanks of its central reflective element. However, the present invention is not limited thereto, and the structure of the antennas ANT_ 1  to ANT_ 4  may be slightly different from each other. During operations of the complex antenna  18 , one of the antennas ANT_ 1  to ANT_ 4  may be turned on while the rest of the antennas ANT_ 1  to ANT_ 4  are turned off, such that antenna pattern characteristic simulation results of the complex antenna  18  is the same as antenna pattern characteristic simulation results of one single antenna (shown in, for example,  FIGS. 10 and 11 ). When the antennas ANT_ 1  to ANT_ 4  are switched on in turn, antenna pattern characteristic simulation results of the antennas ANT_ 1  to ANT_ 4  overlap and are combined/superposed to form the antenna pattern characteristic simulation results of the complex antenna  18 . In addition, two adjacent antennas of the antennas ANT_ 1  to ANT_ 4  may form a combined beam to improve the distribution of antenna radiation pattern, thereby making the antenna radiation pattern more homogeneous and even. 
         [0054]    To sum up, the effective length of the radiation unit of the present invention would be lengthened with the main sections and the first arm sections, which are not coplanar to the main sections. By adjusting the ratios of the widths to the lengths of the radiation unit, the effective distance between the radiation unit and the reflective unit of the present invention would increase. The effective radiation area of the antenna of the present invention would be enlarged with the reflective plate. The conductor patches of the reflective unit in the present invention are regularly arranged to alter reflection phases of electromagnetic waves. In this way, antenna characteristics would be improved, the size of the antenna may be minimized and the transmission requirements of low frequency may be met efficiently. Besides, when the reflective unit providing magnetic conductor reflection effects matches the second arm section or the third arm section of the present invention, multiband transmission may be achieved. 
         [0055]    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.