Abstract:
A decoupling circuit for enhancing isolation of two antennas is disclosed. The two antennas are substantially symmetrically disposed on a substrate. The decoupling circuit includes a first and second metal strips parallel disposed between the two antennas and electrically connected to a ground, a connection strip electrically connected between terminals of the first and second metal strips, to substantially form a doorframe structure, a first comb structure comprising a plurality of metal segments parallel to each other, disposed on the substrate, electrically connected to and perpendicular to the first metal strip, and a second comb structure comprising a plurality of metal segments parallel to each other, disposed on the substrate, electrically connected to and perpendicular to the second metal strip.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention is related to a decoupling circuit and an antenna device, more particularly, to a decoupling circuit and an antenna device capable of reducing coupling effect between antennas, to enhance antenna isolation. 
         [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. Therefore, to facilitate a user&#39;s access to the wireless communication network, an ideal antenna should maximize its bandwidth within a permitted range, while minimizing physical dimensions to accommodate the trend for smaller-sized electronic products. Additionally, with the advance of wireless communication technology, electronic products may be configured with an increasing number of antennas. For example, a long term evolution (LTE) wireless communication system and a wireless local area network standard IEEE 802.11n both support multi-input multi-output (MIMO) technology, i.e. an electronic product is capable of concurrently receiving and transmitting wireless signals via multiple (or multiple sets of) antennas, to vastly increase system throughput and transmission distance without increasing system bandwidth or total transmission power expenditure, thereby effectively enhancing spectral efficiency and transmission rate for the wireless communication system, as well as improving communication quality. 
         [0005]    As can be seen, a prerequisite for implementing spatial multiplexing and spatial diversity in MIMO is to employ multiple sets of antenna to divide a space into many channels, in order to provide multiple antenna field patterns. When an electronic product is configured with multiple sets of antenna under a limited space, a basic requirement includes that these antennas are independent, do not affect each other, and have good isolation. Therefore, how to reduce mutual coupling between antennas becomes one of the industry goals. However, in the limited space, to enhance the isolation of the antennas and simultaneously maintain throughput of MIMO must increase design complexity. Therefore, it is a common goal in the industry to design antennas that suit both transmission demands, as well as dimension and functionality requirements. 
       SUMMARY OF THE INVENTION 
       [0006]    It is therefore an objective of the present invention to provide a decoupling circuit and an antenna device capable of reducing coupling effect between antennas, to enhance antennas isolation. 
         [0007]    The present invention discloses a decoupling circuit for enhancing isolation of two antennas substantially symmetrically disposed on a substrate. The decoupling circuit comprises a first metal strip, disposed between the two antennas on the substrate, and electrically connected to a ground; a second metal strip, disposed between the two antennas on the substrate, substantially parallel to the first metal strip, and electrically connected to the ground; a metal connection strip, disposed between the two antennas on the substrate, and electrically connected to a terminal of the first metal strip and a terminal of the second metal strip, to substantially form a doorframe structure with the first metal strip and the second metal strip; a first comb structure, comprising a plurality of metal segments parallel to each other, disposed on the substrate, and electrically connected to and perpendicular to the first metal strip; and a second comb structure, comprising a plurality of metal segments parallel to each other, disposed on the substrate, and electrically connected to and perpendicular to the second metal strip. 
         [0008]    The present invention further discloses an antenna device. The antenna device comprises a substrate; two antennas, substantially symmetrically disposed on the substrate; and a decoupling circuit, comprising: a first metal strip, disposed between the two antennas on the substrate, and electrically connected to a ground; a second metal strip, disposed between the two antennas on the substrate, substantially parallel to the first metal strip, and electrically connected to the ground; a metal connection strip, disposed between the two antennas on the substrate, and electrically connected to a terminal of the first metal strip and a terminal of the second metal strip, to substantially form a doorframe structure with the first metal strip and the second metal strip; a first comb structure, comprising a plurality of metal segments parallel to each other, disposed on the substrate, and electrically connected to and perpendicular to the first metal strip; and a second comb structure, comprising a plurality of metal segments parallel to each other, disposed on the substrate, and electrically connected to and perpendicular to the second metal strip. 
         [0009]    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 
         [0010]      FIG. 1  illustrates a schematic diagram of an antenna device according to an embodiment of the present invention. 
           [0011]      FIG. 2  illustrates a schematic diagram of an antenna device according to an embodiment of the present invention. 
           [0012]      FIG. 3  illustrates a schematic diagram of the antenna device in  FIG. 2  disposed vertically on a base. 
           [0013]      FIG. 4  illustrates a schematic diagram of an antenna device according to an embodiment of the present invention. 
           [0014]      FIG. 5  illustrates a schematic diagram of the antenna device in  FIG. 4  disposed vertically on a base. 
           [0015]      FIG. 6  illustrates a sectional-view diagram of an antenna device according to an embodiment of the present invention. 
           [0016]      FIG. 7A ,  7 B are schematic diagrams of voltage standing wave ratio when the antenna device is applied to a LTE system for performing multi-input multi-output operation. 
           [0017]      FIG. 8  is a schematic diagram of isolation when the antenna device in  FIG. 6  is applied to a LTE system for performing multi-input multi-output operation. 
           [0018]      FIG. 9A ,  9 B are diagrams of radiation efficiency of a first antenna and a second antenna when the antenna device in  FIG. 6  is applied to a LTE system for performing multi-input multi-output operation. 
           [0019]      FIG. 10A ,  10 B are field diagrams of E-plane of a first antenna and a second antenna when the antenna device in  FIG. 6  is applied to a LTE system for performing multi-input multi-output operation. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    Please refer to  FIG. 1 , which illustrates a schematic diagram of an antenna device  10  according to an embodiment of the present invention. The antenna device  10  comprises a substrate  100 , a first antenna  102 , a second antenna  104  and a decoupling circuit  106 . The substrate  100  may be a printed circuit board or a part of a housing of an electronic device. If the substrate  100  is the housing of the electronic device, the substrate  100  may include structures of corrugations/protrusions/holes for matching external design of mechanism or may have flexibility. However, to simply and clearly illustrate the concept of the present invention, it is assumed that the substrate  100  is flat. The first antenna  102  and the second antenna  104  are both monopole antennas, substantially symmetrical, and disposed on the substrate  100 . The decoupling circuit  106  is made of conducting materials, and disposed between the first antenna  102  and the second antenna  104  on the substrate  100  for reducing coupling effect between the first antenna  102  and the second antenna  104 , to enhance antenna isolation, such that a throughput of multi-input or multi-output (MIMO) can be maintained or increased. 
         [0021]    In detail, the decoupling circuit  106  comprises a first metal strip  108 , a second metal strip  110 , a metal connection strip  112 , a first comb structure  114  and a second comb structure  116 . The first metal strip  108  and the second metal strip  110  are parallel, disposed on the substrate  100 , and electrically connected to a ground. The metal connection strip  112  is disposed on the substrate  100 , and electrically connected to the first metal strip  108  and a top of the second metal strip  110 , to substantially form a doorframe structure with the first metal strip  108  and the second metal strip  110  (i.e. similar to a “π” shape), wherein a distance between the first metal strip  108  and the first antenna  102  is substantially equal to a quarter of a wavelength of wireless signals. In the same way, a distance between the second metal strip  110  and the second antenna  104  is also equal to a quarter of the wavelength of the wireless signals. On the other hand, the first comb structure  114  is composed of multiple metal segments  118  parallel to each other. The metal segments  118  are disposed on the substrate  100 , and electrically connected to the first metal strip  110 , wherein a distance between any two of juxtaposed metal segments  118  is between one-twentieth and one-tenth of the wavelength of the wireless signals. In the same way, the second comb structure  116  comprises multiple metal segments  120  parallel to each other. The metal segments  120  are disposed on the substrate  100  and electrically connected to the second metal strip  112 . A distance between any two of juxtaposed metal segments  120  is between one-twentieth and one-tenth of the wavelength of the wireless signals. In addition, the metal segments  118 ,  120  are respectively parallel to the first metal strip  110  and the second metal strip  112 . More specifically, the metal segments  118 ,  120  are orthogonal to a direction of vertical polarization of antennas. 
         [0022]    Therefore, since the first metal strip  108 , the second metal strip  110 , and the metal connection strip  112  form the doorframe structure which is on the same plane of the first antenna  102  and the second antenna  104  and also between the first antenna  102  and the second antenna  104 , coupling effect is effectively blocked by space. Simultaneously, the metal segments  118 ,  120  effectively avoid transmission of direct waves of corresponding frequency bands. Under such a situation, concerning frequency bands of LTE, a width covering the first antenna  102  and the second antenna  104  is less than 4 centimeters, such that spectral efficiency can be enhanced effectively. 
         [0023]    Note that,  FIG. 1  is an embodiment of the present invention. Those skilled in the art should make modifications or alterations accordingly. For example, please refer to  FIG. 2 , which illustrates a schematic diagram of an antenna device  20  according to an embodiment of the present invention. Structures and operations of the antenna device  20  and the antenna device  10  in  FIG. 1  are similar. Thus, the same components are presented by the same symbols for simplicity. A difference between the antenna device  20  and the antenna device  10  is that a decoupling circuit  206  of the antenna device  20  further comprises a third metal strip  200  and a fourth metal strip  202  in comparison with the decoupling circuit  106 . The antenna device  20  can also reduce coupling effect between antennas, to enhance isolation. In detail, the third metal strip  200  is disposed between the first metal strip  108  and the first antenna  102 , and electrically connected to the ground. The fourth metal strip  202  is disposed between the second metal strip  110  and the second antenna  104 , and also electrically connected to the ground. The third metal strip  200  and the fourth metal strip  202  can adjust a bandwidth to raise flexibility of design. 
         [0024]    Besides, as mentioned above, the substrate  100  can be a printed circuit board. Under such a situation, the antenna devices  10 ,  20  can be disposed on a base vertically. For example,  FIG. 3  illustrates a schematic diagram of the antenna device  20  disposed vertically on a base  300 . In  FIG. 3 , depending on different applications, the base  300  may comprise mechanism for fixing the antenna device  20 , radio frequency circuits for processing radio signals, processors, etc. 
         [0025]    On the other hand, in  FIG. 1  or  FIG. 2 , lengths of the first metal strip  108  and the second metal strip  110  can be either different or the same depending on system requirements. In the same way, lengths of the third metal strip  200  and the fourth metal strip  202  can also be either different or the same. Besides, in  FIG. 1  or  FIG. 2 , the first antenna  102  and the second antenna  104  are substantially parallel, and both formed by three monopole antennas with doorframe structures which can generate capacitor effect, to shorten lengths of antennas effectively. One of the monopole antenna structures is shorter than a quarter of a corresponding wavelength of the wireless signals. In addition, shapes or sizes of the first antenna  102  and the second antenna  104  can be adjusted according to system requirements, or other forms of antennas can be adopted. For example, please refer to  FIG. 4 , which illustrates a schematic diagram of an antenna device  40  according to an embodiment of the present invention. Structures and operations of the antenna device  40  and the antenna device  10  in  FIG. 1  are similar. The antenna device  40  comprises a substrate  400 , a first antenna  402 , a second antenna  404  and a decoupling circuit  406 . Structures and operations of the decoupling circuit  406  and the decoupling circuit  106  in  FIG. 1  are the same, and both are utilized for reducing coupling effect between antennas, to enhance antenna isolation, such that the throughput of MIMO can be maintained or increased. A difference between the antenna device  40  and the antenna device  10  is that the first antenna  402  and the second antenna  404  of the antenna device  40  form a planar inverted F antenna, which is also within the scope of the present invention. Certainly, the antenna device  40  may also add third and fourth medal strips such as the antenna device  20  shown in  FIG. 2 , or may be disposed on a base  500  as shown in  FIG. 5 . 
         [0026]    In the above-mentioned embodiments, the substrates  100 ,  400  are flat structures as examples. However, as mentioned above, the substrate  100  may be a part of a housing of an electronic device, and may include structures of corrugations/protrusions/holes for matching external design. Under such a situation, the decoupling circuit of the present invention can also reduce coupling effect between antennas and enhance antenna isolation. For example, please refer to  FIG. 6 , which illustrates a sectional-view diagram of an antenna device  60  according to an embodiment of the present invention. The antenna device  60  comprises a substrate  600 , a first antenna  602 , a second antenna  604 , and a decoupling circuit  606 . As can be seen by comparing the antenna device  60  in  FIG. 6  and the antenna device  20  in  FIG. 2  or  FIG. 3 , structures of the antenna device  60  and the antenna device  20  are similar and thus the antenna device  60  can also reduce coupling effect between antennas by the decoupling circuit  606 , to enhance antenna isolation, such that the throughput of MIMO can be maintained or increased. A difference between the antenna device  60  and the antenna device  20  is that the substrate  600  is a part of a housing of a top antenna on a car. In other words, the first antenna  602 , the second antenna  604 , and the decoupling circuit  606  are disposed or directly formed inside the housing of the top antenna on the car by means of laser direct structuring (LDS), or using a conductive coating material to coat, print, perform evaporation deposition, or produce on the surface of the housing of the product before coating or rubber coating to cut off from contact, etc., but are not limited to this. 
         [0027]    In addition, the size and material of the antenna device  60  can be adjusted according to different systems. When a LTE system is applied, a width covering the first antenna  602  and the second antenna  604  can be less than 4 centimeters, to enhance spectral efficiency effectively. In addition, when the antenna device  60  is applied to the LTE system, efficiency of multi-input multi-output can further refer to  FIGS. 7A ,  7 B,  8 ,  9 A,  9 B,  10 A and  10 B.  FIG. 7A ,  7 B are schematic diagrams of voltage standing wave ratio when the antenna device  60  is applied to the LTE system for performing multi-input multi-output operation (i.e. S11, S22 parameters).  FIG. 8  is a schematic diagram of isolation when the antenna device  60  is applied to the LTE system for performing multi-input multi-output operation (i.e. S21 parameter). In  FIG. 7A ,  7 B,  8 , dashed lines and solid lines respectively illustrate results of testing or simulation of the first antenna  602  and the second antenna  604 . As can be seen, the isolation of the antenna device  60  can be 20-35 dB within the frequency band 2.02-2.25 GHz. In addition,  FIG. 9A ,  9 B are diagrams of radiation efficiency of the first antenna  602  and the second antenna  604  when the antenna device  60  is applied to the LTE system for performing multi-input multi-output operation.  FIG. 10A ,  10 B are field diagrams of E-plane of the first antenna  602  and the second antenna  604  when the antenna device  60  is applied to the LTE system for performing multi-input multi-output operation. Therefore, as can be seen in  FIG. 7A ,  7 B to  FIG. 10A ,  10 B, even in a limited space, the antenna device  60  still has appropriate bandwidth, and the isolation and the radiation efficiency of the antenna device  60  maintain well when the antenna device  60  performs multi-input multi-output operation. 
         [0028]    To sum up, decoupling circuits of the present invention can effectively enhance the antenna isolation and spectral efficiency, and reduce coupling effect between antennas to enhance antenna isolation, such that the throughput of MIMO can be maintained or increased. 
         [0029]    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.