Abstract:
A compact size antenna operating in LTE frequency bands includes a radiation element, a ground plane, a connecting piece, and a ground extension element. The radiation element at least includes a first radiation branch extending toward a first direction, wherein a connection end of the radiation element has a signal feeding point. The connecting piece is coupled to the ground plane. The ground extension element includes: a metal arm, coupled to the ground plane through the connecting piece; a first ground branch, coupled to the metal arm, and extending toward the first direction; a second ground branch coupled to the metal arm, and extending toward a second direction opposite to the first direction; and a third ground branch, coupled to the metal arm, coupled to the second ground branch, and extending toward the first direction.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of Taiwan Patent Application No. 100106162 filed on Feb. 24, 2011, the entirety of which is incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The disclosure generally relates to a compact size antenna, and more particularly, relates to a compact size antenna operating in LTE (Long Term Evolution) frequency bands. 
     2. Description of the Related Art 
     Nowadays, 2G or 3G communication system technology is applied in notebooks or tablet PCs. Telecommunication manufacturers all over the world have actively introduced LTE (Long Term Evolution) system. Therefore, it is required that in small spaces, an antenna of a communication device can operate in LTE and WWAN (Wireless Wide Area Network, WWAN) frequency bands. In prior art, a monopole antenna comprising two radiation branches and a PIFA (Planar Inverted F Antenna, PIFA) antenna can merely cover five WWAN system frequency bands, and do not cover LTE system frequency bands of 12, 13, 14, 17 (698 MHz-798 MHz) and 7, 38, 40 (2300-2690 MHz). 
     Thus, a compact size antenna operating in LTE frequency bands is required. 
     BRIEF SUMMARY OF THE INVENTION 
     In one exemplary embodiment, the disclosure is directed to a compact size antenna operating in LTE frequency bands, comprising: a radiation element, at least comprising a first radiation branch, wherein the first radiation branch extends toward a first direction, and a connection end of the radiation element comprises a signal feeding point; a ground plane; a connecting piece, coupled to the ground plane; and a ground extension element, comprising: a metal arm, coupled to the ground plane through the connecting piece; a first ground branch, coupled to the metal arm, and extending toward the first direction; a second ground branch, coupled to the metal arm, and extending toward a second direction opposite to the first direction; and a third ground branch, coupled to the metal arm, coupled to the second ground branch, and extending toward the first direction. 
     In another exemplary embodiment, the disclosure is directed to a compact size antenna, comprising: a radiation element, disposed on a first plane, and at least comprising a first radiation branch that extends toward a first direction, wherein a connection end of the radiation element comprises a signal feeding point; a ground plane; a connecting piece, coupled to the ground plane; and a ground extension element, disposed on a second plane perpendicular to the first plane, and comprising: a C-shaped element, coupled to the ground plane through the connecting piece; and a ground branch, coupled to the C-shaped element, and extending toward the first direction. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG. 1  is a pictorial drawing illustrating a compact size antenna according to an embodiment of the invention; 
         FIG. 2A  is a plan-view drawing illustrating a radiation element according to an embodiment of the invention; 
         FIG. 2B  is a plan-view drawing illustrating a ground extension element according to an embodiment of the invention; 
         FIG. 3  is a diagram illustrating return loss of the compact size antenna according to an embodiment of the invention; 
         FIG. 4  is a pictorial drawing illustrating a compact size antenna according to another embodiment of the invention; 
         FIG. 5  is a pictorial drawing illustrating a compact size antenna according to another embodiment of the invention; 
         FIG. 6  is a pictorial drawing illustrating a compact size antenna according to another embodiment of the invention; 
         FIG. 7  is a pictorial drawing illustrating a compact size antenna according to another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a pictorial drawing illustrating a compact size antenna  10  according to an embodiment of the invention. As shown in  FIG. 1 , the compact size antenna  10  comprises: a ground plane  11 , a substrate (not shown), a radiation element  12 , a ground extension element  13 , an appended ground branch  14 , and a connecting piece  15 . The ground plane  11  may be a large area of metal plane. The substrate may be a glass fiber substrate attached to the bottom of the ground plane  11 . In the embodiment, the radiation element  12  may be made of metal and may be disposed on an XZ-plane of a coordinate system, the ground extension element  13  may be made of metal and may be disposed on an XY-plane of the coordinate system, and the appended ground branch  14  may be made of metal and may be disposed on an XZ-plane of the coordinate system. It is noted that the embodiment is just utilized for illustration, and not limitation of the invention. The appended ground branch  14  may be coupled to the ground extension element  13 , and may be of an L-shape. In some embodiments of the invention, the appended ground branch  14  may be eliminated or merely a part of the extension of the ground extension element  13 . The connecting piece  15  may be made of metal, disposed on an YZ-plane of the coordinate system, and coupled to the ground plane  11  and ground extension element  13 . 
       FIG. 2A  is a plan-view drawing illustrating the radiation element  12  according to an embodiment of the invention. As shown in  FIG. 2A , the radiation element  12  may comprise radiation branches  121 ,  122  that are coupled to each other. The radiation branches  121 ,  122  both extend toward a positive X-axis of the coordinate system. A signal feeding point  120  of the compact size antenna  10  is disposed on the connection of radiation branches  121 ,  122 , i.e., a connection end  125  of the radiation element  12 , wherein energy or a signal can be fed into the compact size antenna  10  via the signal feeding point  120 . A length of the radiation branch  121  corresponding to a lower frequency is greater than a length of the radiation branch  122  corresponding to a higher frequency. Furthermore, as shown in  FIGS. 1 and 2A , a distance between the radiation branch  121  and the ground extension element  13  is smaller than a predetermined distance d 1 . In an embodiment of the invention, the predetermined distance d 1  is approximately equal to 2.3 mm, and the predetermined distance d 1  is approximately equal to a 0.05 wavelength, with the condition that a central frequency is 2300 MHz and a dielectric coefficient is 2.5. It is noted that the sizes of the elements in the above embodiment are not limited. A person of ordinary skill can adjust the sizes of the elements according to the central frequency and the dielectric coefficient. 
       FIG. 2B  is a plan-view drawing illustrating the ground extension element  13  according to an embodiment of the invention. As shown in  FIG. 2B , the ground extension element  13  may comprise ground branches  131 ,  132 ,  133 , a metal arm  134 , and a matching element  136 . The ground branches  131 ,  132  and the metal arm  134  constitute a C-shaped element  139 . The metal arm  134  may coupled to the connecting piece  15 . The ground branches  131 ,  133  are both coupled to the metal arm  134 , and both extend toward a positive X-axis of the coordinate system. The ground branch  132  is coupled to the metal arm  134 , and extends toward negative X-axis of the coordinate system. In addition, the ground branch  133  is coupled to the ground branch  132 . The matching element  136  is coupled to the metal arm  134 . If the length of the matching element  136  changes, impedance matching of the compact size antenna  10  will be adjusted. As shown in  FIG. 1 , there is a gap between the ground branches  131 ,  132 , wherein the gap is smaller than a predetermined distance d 2 . In an embodiment of the invention, the predetermined distance d 2  is approximately equal to 6.1 mm, and the predetermined distance d 2  is approximately equal to a 0.2 wavelength on the condition that a central frequency is 2300 MHz and a dielectric coefficient is 2.5. The appended ground branch  14  may be coupled to a free end of the ground branch  133 . It is noted that the sizes of the elements in the above embodiment is not limited. A person of ordinary skill can adjust the sizes of the elements according to the central frequency and the dielectric coefficient. 
       FIG. 3  is a diagram  300  illustrating return loss of the compact size antenna  10  according to an embodiment of the invention.  FIG. 3  is utilized for illustrating measured return loss (unit: dB) over frequency (unit: MHz) on the condition that energy or a signal is fed into the compact size antenna  10  via the signal feeding point  120 . As shown in  FIG. 3 , the compact size antenna  10  comprises operating frequency bands  31 ,  32  according to the criterion set as 6 dB. In an embodiment of the invention, the operating frequency band  31  covers from about 698 MHz to 960 MHz, covering the LTE700/GSM850/900, and the operating frequency band  32  covers from about 1710 MHz to 2690 MHz, covering the GSM1800/PCS1900/WCDMA Band1/LTE2700. Therefore, the compact size antenna  10  can meet frequency bands of 2G/3G/LTE communication systems. The compact size antenna  10  can cover LTE frequency bands except for “band 11” and “band 21” of LTE communication systems. 
     In an aspect of antenna theory, the radiation branch  121  may be excited to form a frequency point  311  in the operating frequency band  31 . The connecting piece  15 , the ground branch  132 , and the metal arm  134  may be excited to form a frequency point  322  in the operating frequency band  32 . The connecting piece  15 , the ground branch  131 , and a portion of the metal arm  134  may be excited to form a frequency point  323  in the operating frequency band  32 . The radiation branch  122  may be excited to form a frequency point  324  in the operating frequency band  32 . The appended ground branch  14 , the connecting piece  15 , the ground branch  133 , and the metal arm  134  may be excited to form a frequency point  315  in the operating frequency band  31 . The above frequency points can be ordered from low to high as follows, the frequency points  315 ,  311 ,  324 ,  322  and  323 . 
     In some embodiments of the invention, the sizes of the elements in the compact size antenna  10  are as follows: the length of the ground plane  11  is approximately equal to 246 mm; the width of the ground plane  11  is approximately equal to 150 mm; the thickness of the substrate is approximately equal to 1 mm; the length of the radiation branch  121  is approximately equal to 53 mm; the width of the radiation branch  121  is approximately equal to 2 mm; the length of the radiation branch  122  is approximately equal to 33 mm; the width of the radiation branch  122  is approximately equal to 2 mm; the total length of the appended ground branch  14 , the connecting piece  15 , the ground branch  133  and the metal arm  134  is approximately equal to 91.9 mm; the total length of the connecting piece  15 , the ground branch  132  and the metal arm  134  is approximately equal to 70.6 mm; and the total length of the connecting piece  15 , the ground branch  131 , the portion of the metal arm  14  is approximately equal to 17.4 mm. It is noted that the sizes of the elements in the above embodiment are not limited. A person of ordinary skill can adjust the sizes of the elements according to the central frequency and the dielectric coefficient. 
       FIG. 4  is a pictorial drawing illustrating a compact size antenna  40  according to another embodiment of the invention. The compact size antenna  40  of  FIG. 4  is similar to the compact size antenna  10  of  FIG. 1 , but the differences are as follows: (1) a radiation element  42  of the compact size antenna  40  only includes the radiation branch  121 , and a signal feeding point  420  is disposed on a connection end  425  of the radiation element  42 ; (2) a ground extension element  43  of the compact size antenna  40  does not include the matching element  136 ; and (3) the compact size antenna  40  does not include the appended ground branch  14 . 
       FIG. 5  is a pictorial drawing illustrating a compact size antenna  50  according to another embodiment of the invention. The compact size antenna  50  of  FIG. 5  is similar to the compact size antenna  10  of  FIG. 1 , but the differences are as follows: (1) a ground extension element  43  of the compact size antenna  50  does not include the matching element  136 ; and (3) the compact size antenna  50  does not include the appended ground branch  14 . 
       FIG. 6  is a pictorial drawing illustrating a compact size antenna  60  according to another embodiment of the invention. The compact size antenna  60  of  FIG. 6  is similar to the compact size antenna  10  of  FIG. 1 , but the differences are as follows: the compact size antenna  60  does not include the appended ground branch  14 . 
       FIG. 7  is a pictorial drawing illustrating a compact size antenna  70  according to another embodiment of the invention. The compact size antenna  70  of  FIG. 7  is similar to the compact size antenna  10  of  FIG. 1 , but the differences are as follows: a radiation element  72  and the ground extension element  43  of the compact size antenna  70  are disposed on the same plane, i.e., XY-plane of the coordinate system. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the invention. It is intended that the standard and examples be considered as exemplary only, with a true scope of the disclosed embodiments being indicated by the following claims and their equivalents.