Patent Publication Number: US-7586445-B2

Title: MIMO antenna

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to wireless communication, and particularly to a Multi Input Multi Output antenna. 
   2. Description of Related Art 
   Recently, the Multi Input Multi Output (MIMO) technology has achieved significant growth due to the ever growing demand for wireless communication products. MIMO antennas are widely used in the field of wireless communication. Generally, a MIMO antenna includes at least two individual antennas. Each antenna should be designed as small as possible and the isolation between the antennas should be designed to satisfy space and radiation requirements of wireless local area network (WLAN) devices employing the antennas. 
   SUMMARY OF THE INVENTION 
   One aspect of the present invention provides a Multi Input Multi Output (MIMO) antenna. The MIMO antenna is disposed on a substrate. The substrate includes a first surface and a second surface. The MIMO antenna includes a first antenna and a second antenna set as mirror image to the first antenna, each of the first and the second antennas includes a radiation body, a feeding portion, and a grounded portion. The radiation portion is disposed on the first surface for transceiving electromagnetic signals. The radiation body includes a first radiation portion and a second radiation portion electronically connected to the first radiation portion. The first radiation portion is serpentine-shaped and the second radiation portion is rectangular-shaped. The feeding portion is disposed on the first surface, and electronically connected to the second radiation portion for feeding electromagnetic signals to the radiation body. The grounded portion is disposed on the second surface. 
   Other objectives, advantages and novel features of the present invention will be drawn from the following detailed description of preferred embodiments of the present invention with the attached drawings, in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a front view schematic diagram of a Multi Input Multi Output (MIMO) antenna in accordance with an embodiment of the invention; 
       FIG. 2  is a back view schematic diagram of the MIMO antenna of  FIG. 1 ; 
       FIG. 3  and  FIG. 4  are schematic diagrams illustrating dimensions of the MIMO antenna of  FIG. 1  and  FIG. 2 ; 
       FIG. 5  is a graph of test results showing voltage standing wave ratios (VSWRs) of a first antenna of the MIMO antenna of  FIG. 1 ; 
       FIG. 6  is a graph of test results showing the VSWRs of a second antenna of the MIMO antenna of  FIG. 1 ; and 
       FIG. 7  is a graph of test results showing isolation between the first antenna and the second antenna of the MIMO antenna of  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  and  FIG. 2  are respectively front and back views of a Multi Input Multi Output (MIMO) antenna  20  in accordance with an embodiment of the invention. 
   In this embodiment, the MIMO antenna  20  is disposed on a substrate  10 . The substrate  10  includes a first surface  102  (as shown in  FIG. 1 ) and a second surface  104  (as shown in  FIG. 2 ) opposite to the first surface  102 . The MIMO antenna  20  includes at least a first antenna  20   a  and a second antenna  20   b . The first antenna  20   a  is set as mirror image to the second antenna  20   b , that is, the first antenna  20   a  and the second antenna  20   b  are in axial symmetry. 
   The first antenna  20   a  includes a radiation body  22   a , a feeding portion  24   a , and a grounded portion  26   a . The radiation body  22   a  includes a first radiation portion  220   a , a second radiation portion  222   a , and a connecting portion  224   a.    
   The second antenna  20   b  similarly includes a radiation body  22   b , a feeding portion  24   b , and a grounded portion  26   b . The radiation body  22   b  includes a first radiation portion  220   b , a second radiation portion  222   b , and a connecting portion  224   b.    
   The radiation bodies  22   a ,  22   b  are disposed on the first surface  102 , for transceiving electromagnetic signals. The first radiation portions  220   a ,  220   b  are serpentine-shaped, and each includes an open end  2202   a  ( 2202   b ) and a connecting end  2204   a  ( 2204   b ) electronically connected to the second radiation portion  222   a  ( 222   b ). In this embodiment, the connecting end  2204   a  is disposed adjacent to the connecting end  2204   b . The open ends  2202   a  and  2202   b  are mirror images of each other and extend in opposite directions. In this way, the isolation between the first antenna  20   a  and the second antenna  20   b  is improved. The connecting portion  224   a  ( 224   b ) is electronically connected between the second radiation portion  222   a  ( 222   b ) and the feeding portion  24   a  ( 24   b ). The feeding portion  24   a  ( 24   b ) is disposed on the first surface  102 , and electronically connected to the second radiation portion  222   a  ( 222   b ). The feeding portion  24   a  ( 24   b ) is used for feeding electromagnetic signals to the radiation body  22   a  ( 22   b ). The grounded portions  26   a ,  26   b  are disposed on the second surface  104 . 
   In this embodiment, the first radiation portion  220   a  ( 220   b ) can reduce the rectilinear length of the radiation body  22   a  ( 22   b ) yet still keep the radiation body  22   a  ( 22   b ) resonating. A radiation field produced by a coupling effect of the first radiation portions  220   a ,  220   b  can improve the radiation efficiency of the MIMO antenna  20 . In other words, the first radiation portions  220   a  and  220   b  can reduce the area of the MIMO antenna  20 , and improve the radiation efficiency of the MIMO antenna  20 . In this embodiment, the first radiation portion  220   a  ( 220   b ) has a selected one of an s-shaped configuration, a w-shaped configuration, and a u-shaped configuration. 
   The second radiation portions  222   a ,  222   b  and the connecting portions  224   a ,  224   b  are rectangle-shaped. In this embodiment, a length and a width of the connecting portion  224   a  ( 224   b ) are smaller than those of the second radiation portion  222   a  ( 222   b ). The connecting portion  224   a  ( 224   b ) has matching impedance function. 
   The grounded portions  26   a ,  26   b  are step-shaped and in axial symmetry along an axis of the first surface  102 . In this embodiment, the grounded portions  26   a ,  26   b  can improve the radiation efficiency of the MIMO antenna  20 . 
     FIG. 3  and  FIG. 4  jointly illustrate dimensions of the MIMO antenna  20  of  FIG. 1  and  FIG. 2 . 
   In this embodiment, a total length d 1  of the MIMO antenna  20  is 27.5 millimeter (mm), and a total width d 2  of the MIMO antenna  20  is 9.5 mm. All dimensions of all parts of the first antenna  20   a  are the same as those of the second antenna  20   b . In order to describe succinctly, we just illustrate dimensions of the first antenna  20   a . The first radiation  220   a  is serpentine-shaped. A total length d 3  of the first radiation  220   a  is 12 mm, and a total width d 4  of the first radiation  220   a  is 2.4 mm. A length d 5  of the slot of the first radiation  220   a  is 10.4 mm, and a width d 6  of the slot of the first radiation  220   a  is 0.3 mm. The second radiation portion  222   a , the connecting portion  224   a , and the feeding portion  24   a  are rectangle-shaped. A length d 7  of the second radiation portion  222   a  is 12 mm, and a width d 8  of the second radiation portion  222   a  is 4.725 mm. A length d 9  of the connecting portion  224   a  is 6 mm, and a width d 10  of the connecting portion  224   a  is 0.5 mm. A length d 11  of the feeding portion  24   a  is 1.675 mm, and a width d 12  of the feeding portion  224   a  is 1.5 mm. The parallel distance d 15  between the first antenna  20   a  and the second antenna  20   b  is 3 mm. 
   In  FIG. 4 , a total width d 13  of the grounded portion  26   a  is 12 mm, and a total height d 14  of the grounded portion  26   a  is 1 mm. The grounded portion  26   a  is step-shaped and symmetrical along an axis, and the projection of the axis on the first surface  102  and the feeding portion  24   a  partially overlap. The grounded portion  26   a  has 5 steps, and a height of each step is about 0.2 mm. Widths of the fourth step and the fifth step are about 1 mm, and widths of the other steps are about 1.5 mm. In other embodiments, the grounded portion  26   a  may be other shaped so long as the overall dimensions remain at about 1 mm high by about 12 mm wide. 
     FIG. 5  is a graph of test results showing voltage standing wave ratios (VSWRs) of the first antenna  20   a  of the MIMO antenna  20  of  FIG. 1 . The horizontal axis represents the frequency (in GHz) of the electromagnetic signals traveling through the first antenna  20   a , and the vertical axis represents amplitude of the VSWRs. A curve shows the amplitude of the VSWRs of the first antenna  20   a  at operating frequencies. As shown in  FIG. 5 , the first antenna  20   a  performs well when operating at frequency bands of 2.3-2.7 GHz and 4.6-6.0 GHz. The amplitude values of the VSWRs in the band pass frequency range are smaller than a value of 2, indicating the first antenna  20   a  complies with application requirements of the MIMO antenna  20 . 
     FIG. 6  is a graph of test results showing VSWRs of the second antenna  20   b  of the MIMO antenna  20  of  FIG. 1 . The horizontal axis represents the frequency (in GHz) of the electromagnetic signals traveling through the second antenna  20   b , and the vertical axis represents amplitude of the VSWRs. A curve shows the amplitude of the VSWRs of the second antenna  20   b  at operating frequencies. As shown in  FIG. 6 , the second antenna  20   b  performs well when operating at frequency bands of 2.3-2.7 GHz and 4.6-6.0 GHz. The amplitude values of the VSWRs in the band pass frequency range are smaller than a value of 2, indicating the second antenna  20   b  complies with application requirement of the MIMO antenna  20 . 
     FIG. 7  is a graph of test results showing isolation between the first antenna  20   a  and the second antenna  20   b  of the MIMO antenna  20  of  FIG. 1 . The horizontal axis represents the frequency (in GHz) of the electromagnetic signals traveling through the MIMO antenna  20 , and the vertical axis represents the amplitude of the isolation. As shown in  FIG. 7 , a curve shows isolation between the first antenna  20   a  and the second antenna  20   b  is at most substantially −23 dB when the MIMO antenna  20  operates at frequency band of 2.3-2.7 GHz. Isolation between the first antenna  20   a  and the second antenna  20   b  is at most substantially −15.3 dB when the MIMO antenna  20  operates at frequency band of 4.6-6.0 GHz. The isolation values of the two bands are smaller than −10, indicating the MIMO antenna  20  complies with application requirement of a MIMO antenna. 
   In this embodiment, the first radiation portion  220   a  ( 220   b ) is serpentine-shaped. Therefore, the area of the MIMO antenna  20  is reduced. The grounded portion  26   a  ( 26   b ) improves the VSWRs of the MIMO antenna  20  operating at the pass bands.