Abstract:
To improve vertical efficiency, the present invention provides a loop antenna for a wireless network, which includes a feeding terminal for receiving a feeding signal, a shorting-to-ground terminal for providing grounding, a metal bar formed on a first plane and surrounding a center point, a first metal arm formed on a second plane associated with the first plane and coupled between an terminal of the metal bar and the feeding terminal, and a second metal arm formed on a third plane associated with the first plane and coupled between another terminal of the metal bar and the shorting-to-ground terminal.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention is related to a loop antenna for a wireless network, and more particularly, to a loop antenna capable of improving vertical efficiency. 
         [0003]    2. Description of the Prior Art 
         [0004]    In modern IT (Information Technology) society, various wireless communication networks have become one of the most important ways for exchanging voice, text, data, and video for many people. In the prior art, a computer system can access a wireless communication network via a wireless adapter through a plug-and-play interface, such as a Universal Serial Bus (USB). For example, in  FIG. 1 , when using a USB wireless adapter  10 , a user only needs to plug the USB wireless adapter  10  into one of USB ports of a notebook. In general, the USB wireless adapter  10  is composed of an antenna and related processing units, and utilized for emitting and receiving radio waves, to transmit or receive wireless signals. Therefore, in order to allow the user to access wireless communication networks more conveniently, a size of the antenna is required to be reduced, to fit a main stream of reducing a size of a portable wireless device. 
         [0005]    Planar Inverted-F Antenna (PIFA) is a monopole antenna commonly used in the wireless adapter. As implied in the name, a shape of PIFA is similar to an inverted and rotated “F”. PIFA has advantages of low production cost, high radiation efficiency, easily realizing multi-channel operations. However, a vertical gain of PIFA is not good enough for the example shown in  FIG. 2 . 
         [0006]    A loop antenna is an electric conductor surrounded as a closed curving shape (e.g. circular, square and triangular shapes) on a plane with an operating principle similar to a dipole antenna, or a resonant antenna. Please refer to  FIG. 2 .  FIG. 2  is a schematic diagram of a loop antenna  20  of the prior art. As shown in  FIG. 2 , the loop antenna  20  is a circular electric conductor formed on an x-y plane, with characteristics of low profile, such that the loop antenna  20  requires smaller forming space, to fit the application of the wireless adapter. However, the loop antenna  20  belongs to horizontal polarization. That is, a vertical gain (along a z-axis in  FIG. 2 ) is not good and not appropriate for the example of  FIG. 2 , which restricts the application range. 
         [0007]    In short, although the loop antenna has the feature of low profile, the property of horizontal polarization limits its performance in wireless network applications. Therefore, how to improve the abovementioned drawback has become a goal of the industry. 
       SUMMARY OF THE INVENTION 
       [0008]    It is therefore a primary objective of the claimed invention to provide a loop antenna for a wireless network. 
         [0009]    The present invention discloses a loop antenna for a wireless network, which comprises a feeding terminal for receiving a feeding signal, a shorting-to-ground terminal for providing grounding, a metal bar formed on a first plane and surrounding a center point, a first metal arm formed on a second plane associated with the first plane and coupled between an terminal of the metal bar and the feeding terminal, and a second metal arm formed on a third plane associated with the first plane and coupled between another terminal of the metal bar and the shorting-to-ground terminal. 
         [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. 1  is a schematic diagram of a wireless adapter applied in a notebook in the prior art. 
           [0012]      FIG. 2  is a schematic diagram of a loop antenna of the prior art. 
           [0013]      FIG. 3A  is a schematic diagram of a loop antenna according to an embodiment of the present invention. 
           [0014]      FIG. 3B  is a schematic diagram of the current paths of the loop antenna shown in  FIG. 3A . 
           [0015]      FIG. 3C  is a schematic diagram of a modification of the loop antenna shown in  FIG. 3A . 
           [0016]      FIG. 4  to  FIG. 6  are schematic diagrams of loop antennas according to the embodiments of the present invention. 
           [0017]      FIG. 7  is a schematic diagram of a wireless adapter according to an embodiment of the present invention. 
           [0018]      FIG. 8  is a return loss diagram of the wireless adapter shown in  FIG. 7 . 
           [0019]      FIG. 9A  is a schematic diagram of a horizontal gain on an x-y plane of the wireless adapter shown in  FIG. 7 . 
           [0020]      FIG. 9B  is a schematic diagram of a vertical gain on an x-y plane of the wireless adapter shown in  FIG. 7 . 
           [0021]      FIG. 9C  is a schematic diagram of a total gain on an x-y plane of the wireless adapter shown in  FIG. 7 . 
           [0022]      FIG. 10  is a schematic diagram of radiation efficiency of the wireless adapter shown in  FIG. 7 . 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    Please refer to  FIG. 3A , which illustrates a schematic diagram of a loop antenna  30  according to an embodiment of the present invention. The loop antenna  30  is utilized for transmitting and receiving wireless signals of a wireless network, and comprises a feeding terminal  300 , a shorting-to-ground terminal  302 , a metal bar  304 , a first metal arm  306 , and a second metal arm  308 . The feeding terminal  300  and the shorting-to-ground terminal  302  are utilized for receiving feeding signals and providing grounding respectively, and preferably formed on a printed circuit board (not shown in  FIG. 3 ). The metal bar  304  is formed on an x-y plane and surrounds a virtual center, with two ends connecting to the feeding terminal  300  and the shorting-to-ground terminal  302  via the first metal arm  306  and the second metal arm  308  respectively. The first metal arm  306  and the second metal arm  308  both extend along a z-axis. That is, the first metal arm  306  and the second metal arm  308  are formed on a plane perpendicular to the metal bar  304 . 
         [0024]    On the other hand, a total length of the metal bar  304 , the first metal arm  306  and the second metal arm  308  is preferably λ/2 (λ represents a wavelength of a feeding signal), and lengths of the first metal arm  306  and the second metal arm  308  are all 0.05λ. Under such circumstance, current paths of the loop antenna  30  can be illustrated as shown in  FIG. 3B . According to the electromagnetism theorem, when high frequency (HF) signals transmit on a conductor, a resonant point, namely open circuit, is formed on a position which is a quarter wavelength apart from a feeding terminal. Therefore, in the loop antenna  30 , a current L 1  from the feeding terminal  300  to a center of the metal bar  304  (the position of λ/4) counterbalances or nearly cancels out a current L 2  from the shorting-to-ground terminal  302  to the center of the metal bar  304 . In other words, as shown in  FIG. 3B , horizontal currents (on the x-y plane) nearly counterbalance each other (a small portion of currents along the x axis still exist), and vertical currents dominate radiation of an electric field. Since a radiating direction of the electric field is parallel to the direction of current flow, the vertical currents enhance a vertical gain, making the loop antenna  30  become vertically polarized. 
         [0025]    In short, since the total length of the metal bar  304 , the first metal arm  306  and the second metal arm  308  is λ/2, an open circuit is formed on the center of the metal bar  304  and (nearly) counterbalances horizontal currents. Meanwhile, by forming the first metal arm  306  and the second metal arm  308  on the plane perpendicular to the metal bar  304 , there are only the vertical currents in the loop antenna  30 , which enhances the vertical gain to reach the goal of vertical polarization. 
         [0026]    Note that, a total length of the loop antenna  30  is not limited to λ/2. Designs, which can (nearly) counterbalance the horizontal currents and make currents of the first metal arm  306  and the second metal arm  308  to flow in the same direction, conform to the inventive concept of the present invention. For instance, in  FIG. 3C , the first metal arm  306  and the second metal arm  308  are formed face to face, and the horizontal currents can also be counterbalanced to enhance the vertical gain. 
         [0027]    In  FIG. 3 , the shape of the metal bar  304  surrounded is a circular shape. In fact, the shape of the metal bar  304  is not a limitation of the present invention, and can be other shapes. For example,  FIG. 4  to  FIG. 6  are schematic diagrams of loop antennas  40 ,  50  and  60  according to embodiments of the present invention. The loop antennas  40 ,  50  and  60  derive from the loop antenna  30  with differences of square shape, triangular shape, and symmetrical meander shape. Certainly, other shapes can also be applied for the present invention. 
         [0028]    Therefore, the present invention can counterbalance the horizontal currents, and keep the vertical currents, to make the loop antenna to become vertically polarized. Under such circumstance, if the loop antenna of the present invention is applied to the wireless adapter shown in  FIG. 1 , the size can be reduced and the vertical gain can be enhanced. For example, please refer to  FIG. 7 .  FIG. 7  is a schematic diagram of a wireless adapter  70 . The loop antenna  60  shown in  FIG. 6  is formed in the wireless adapter  70  for transmitting and receiving signals of WLAN (Wireless Local Area Network), while other elements are omitted and not shown in  FIG. 7 . In such situation, when the wireless adapter  70  replaces the wireless adapter  10  for the example of  FIG. 1 , corresponding radiation characteristics are shown in  FIG. 8 ,  FIG. 9A to 9C , and  FIG. 10 .  FIG. 8  is a return loss diagram of the wireless adapter  70 .  FIG. 9A  is a schematic diagram of a horizontal gain (generally called Gain Phi) of the wireless adapter  70  on the x-y plane.  FIG. 9B  is a schematic diagram of a vertical gain (generally called Gain Theta) of the wireless adapter  70  on the x-y plane.  FIG. 9C  is a schematic diagram of a total gain of the wireless adapter  70  on the x-y plane.  FIG. 10  is a schematic diagram of radiation efficiency of the wireless adapter  70 . Therefore, as shown in  FIG. 8  to  FIG. 10 , via the loop antenna  60 , the forming space required by the wireless adapter  70  can be reduced, and vertical efficiency can be enhanced due to vertical polarization. 
         [0029]    Note that, the wireless adapter  70  shown in  FIG. 7  is an embodiment of the present invention, and those skilled in the art can make modifications and alterations according to different requirements. For instance, in order to enhance reception and transmission efficiency of the wireless adapter  70 , an area of a bottom board of the wireless adapter  70  should be larger than a projection area of the loop antenna  60 ; or, a hole or a slot on the bottom board is generated to make electric wavelength close to or greater than λ/4. These skills are well-known for the industry, and the main purpose is to further enhance antenna efficiency instead of confining the present invention. 
         [0030]    In summary, in the present invention, the total length of the loop antenna is λ/2 to form the open circuit at the center, so as to (nearly) cancel out horizontal currents. Meanwhile, via the metal arms in vertical, only vertical currents of the loop antenna are left, so as to enhance the vertical gain, and reach the purpose of vertical polarization. 
         [0031]    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.