Abstract:
A planar antenna includes first and second radiation elements. A first partial periphery of the first radiation element and a second partial periphery of the second radiation element face each other at a uniform gap equal to or less than a tenth of the length of the first partial periphery. The first radiation element includes a third partial periphery parallel to a straight line for connecting the both ends of the first partial periphery, a feeding point at a central portion of the first partial periphery, and a slit having an opened end and a closed end. A distance from the feeding point to the opened end along the slit through the closed end is longer than a sum of a half of the first partial periphery and a longer one of the other two partial peripheries.

Description:
[0001]    The present disclosure relates to the subject matter contained in Japanese Patent Application No. 2006-136977 filed May 16, 2006, which is incorporated herein by reference in its entirety. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a planar antenna, and more particularly, to a planar antenna capable of realizing multi-resonance and band widening. 
         [0004]    2. Background Art 
         [0005]    There is a wideband planar antenna. (For example, see JP-A-2006-033069.) In the planar antenna of JP-A-2006-033069, it is possible to obtain wideband characteristics in which a standing wave ratio is substantially flat between a first resonance frequency of a low frequency side and a second resonance frequency of a high frequency side. The wavelength λ 1  of the first resonance frequency of the low frequency side is associated with a dimension A obtained by adding both sides and a short side of a trapezoid of the planar antenna and a relationship of the dimension A=approximately λ 1 / 2  is satisfied. 
         [0006]    In the mobile wireless device having the antenna mounted thereon, multi-frequency and band widening are further required. Accordingly, in the planar antenna of JP-A-2006-033069, in order to widen a low frequency band, the dimension A, that is, the length of the sides of the trapezoid, need increase and thus the dimension of the antenna increases. 
         [0007]    In the planar antenna of JP-A-2006-033069, the strength against bending is weak. In the arrangement of the coaxial cable, the thickness of the mobile wireless device increases. 
       SUMMARY OF THE INVENTION 
       [0008]    The invention provides a planar antenna capable of realizing band widening without increasing the dimension of the antenna. The invention further provides a planar antenna capable of improving strength against bending. 
         [0009]    The invention may provide a planar antenna including: a first radiation element having a planar shape, the first radiation element including a first partial periphery, a third partial periphery, a fourth partial periphery, a fifth partial periphery, a feeding point positioned at a substantially center of the first partial periphery; and a second radiation element having a planar shape, the second radiation element including a second partial periphery having a length at least substantially equal to that of the first partial periphery; wherein the first partial periphery and the second partial periphery face each other at a substantially uniform gap that is substantially equal to or less than a tenth of the length of the first partial periphery; wherein the third partial periphery is substantially parallel to a straight line connecting both ends of the first partial periphery; wherein the fourth partial periphery connects one end of the first partial periphery to one end of the third partial periphery; wherein the fifth partial periphery connects the other end of the first partial periphery to the other end of the third partial periphery; wherein the first radiation element includes a slit having an opened end and a closed end, the opened end opening at a periphery of the first radiation element other than the first partial periphery, the closed end closed within the first radiation element; and wherein a distance from the feeding point to the opened end along the slit through the closed end is longer than a distance obtained by adding a half of the length of the first partial periphery and a longer one of a length of the fourth partial periphery and a length of the fifth partial periphery. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The present invention may be more readily described with reference to the accompanying drawings, in which: 
           [0011]      FIGS. 1(   a )-( e ) are views showing the configurations of planar antennas according to a first embodiment of the invention; 
           [0012]      FIG. 2  is a graph showing a simulation result of a VSWR of the planar antenna shown in  FIG. 1(   a ) according to the first embodiment; 
           [0013]      FIG. 3  is a graph showing a simulation result of the VSWR of the planar antenna shown in  FIG. 1(   a ) according to the first embodiment (A dimension L 1  is variable.); 
           [0014]      FIG. 4  is a graph showing a simulation result of the VSWR of the planar antenna shown in  FIG. 1(   a ) according to the first embodiment (an influence of a slit width); 
           [0015]      FIGS. 5(   a )-( d ) are simulation views of radiation patterns of a vertically polarized wave of the planar antenna shown in  FIG. 1(   a ) according to the first embodiment; 
           [0016]      FIG. 6  is a graph showing a simulation result of a VSWR of the planar antenna shown in  FIG. 1(   b ) according to the first embodiment; 
           [0017]      FIG. 7  is a simulation view of a VSWR of the planar antenna shown in  FIG. 1(   c ) according to the first embodiment; 
           [0018]      FIG. 8  is a simulation view of a VSWR of the planar antenna shown in  FIG. 1(   d ) according to the first embodiment; 
           [0019]      FIG. 9  is a simulation view of a VSWR of the planar antenna shown in  FIG. 1(   e ) according to the first embodiment; 
           [0020]      FIG. 10  is a view showing the configuration of a planar antenna according to a modified example of the first embodiment; 
           [0021]      FIG. 11  is a simulation view of a VSWR of the planar antenna according to the modified example of the first embodiment; and 
           [0022]      FIGS. 12(   a )-( c ) are views showing the configurations of planar antennas according to a second embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0023]    Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. 
       First Embodiment 
       [0024]    A first embodiment realizes band widening compared with the background art. 
         [0025]      FIGS. 1(   a )-( d ) are views showing the configurations of dipole type planar antennas according to the first embodiment.  FIGS. 1(   a ), ( b ), ( c ), ( d ) and ( e ) show different slit shapes. The antenna  100  includes a first radiation element  1 , a second radiation element  2  and a feeding point  3 . 
         [0026]    The first radiation element  1  is a trapezoid plane having a short side  4  (first partial periphery) and a long side  5  (third partial periphery) which are parallel sides of the trapezoid, a side  6  and a side  7 . The feeding point  3  is connected to a central portion of the short side  4  of the first radiation element  1  to supply power. The first radiation element  1  has a slit  8  which is the characteristic of the first embodiment. The second radiation element  2  has the same shape as the first radiation element  1  except the slit  8 , and the short side  4  of the first radiation element  1  and the short side (second partial periphery) of the second radiation element  2  face each other in parallel at a minute gap G. The length of the short side of the second radiation element  2  may be larger than that of the short side  4  of the first radiation element  1 . 
         [0027]    A dimension L 1  denoted by a dotted line is a dimension from the feeding point  3  to the end of the side  6  along the short side  4 . The lengths of the side  6  and the side  7  may be different from each other. In this case, the dimension L 1  is a dimension from the feeding point  3  to the end of the side having a larger length along the short side  4 . The length of the short side  4  is denoted by a dimension L 2 . The relationship among the dimension L 1 , the dimension L 2  and the resonance frequency will be described later ( FIGS. 2 and 3 ). 
         [0028]    Hereinafter, the shape of the slit  8  which is the characteristic of the first embodiment will be described with is reference to  FIG. 1(   a ),  1 ( b ),  1 ( c ),  1 ( d ) and  1 ( e ). 
         [0029]    In  FIG. 1(   a ), the slit  8  vertically extends from a closed end  8   a , which is positioned in the vicinity of the feeding point  3 , to the vicinity of the long side  5  and extends from the vicinity of the long side S parallel to the long side  5 , and an opened side  8   b  is opened at the side  7 . A dimension L 3  denoted by a dotted line is a dimension from the feeding point  3  to the right upper end  1   a  of the trapezoid through the closed end  8   a  of the slit  8  and the opened end  8   b  along the slit  8 . Since the opened end  8   b  and the right upper end  1   a  are adjacent to each other, the dimension L 3  may be defined to the dimension from the feeding point  3  to the opened end  8   b  through the closed end  8   a  of the slit  8  along the slit  8 . 
         [0030]    In  FIG. 1(   b ), the slit  8  extends in the vicinity of the long side  5  parallel to the long side  5 . One end of the slit  8  is the closed end  8   a  and the other end thereof is the opened end  8   b  which is opened at the side  7 . The dimension L 3  denoted by a dotted line is a dimension from the feeding point  3  to the right upper side  1   a  of the trapezoid through the closed end  8   a  of the slit  8  and the opened end  8   b  along the slit  8 , similar to  FIG. 1(   a ). Since the opened end  8   b  and the right upper end  1   a  are adjacent to each other, the dimension L 3  may be defined to the dimension from the feeding point  3  to the opened end  8   b  through the closed end  8   a  of the slit  8  along the slit  8 . 
         [0031]    In  FIG. 1(   c ), the slit  8  extends from the closed end  8   a  which is positioned within the trapezoid in a left direction and vertically extends, and the opened end  8   b  is opened at the long side  5 . The dimension L 3  denoted by a dotted line is a dimension from the feeding point  3  to the opened end  8   b  through the closed end  8   a  of the slit  8  along the slit  8 . 
         [0032]    The slit  8  of  FIG. 1(   d ) is substantially similar to that of  FIG. 1(   a ) except that the right upper end  1   a  of the trapezoid further extends in a right direction of the long side  5 . The dimension L 3  denoted by a dotted line is a dimension from the feeding point  3  to the right upper end  1   a  of the trapezoid through the closed end  8   a  of the slit  8  and the opened end  8   b  along the slit  8 . Since the opened end  8   b  and the right upper end  1   a  are separated from each other, the dimension L 3  is defined to the dimension from the feeding point  3  to the right upper end  1   a.    
         [0033]    The slit  8  of  FIG. 1(   e ) is similar to that of  FIG. 1(   a ) except that a slit  9  is provided. The slit  9  vertically extends from a closed end  9   a , which is positioned in the vicinity of the closed end  8   a , to the long side  5  along a vertical portion of the slit  8 , and an opened end  9   b  is opened at the long side  5 . The dimension L 3  denoted by a dotted line is a dimension from the feeding point  3  to the right upper end  1   a  of the trapezoid through the closed end  8   a  of the slit  8  and the opened end  8   b  along the slit  8 , similar to  FIG. 1(   a ). Since the opened end  8   b  and the right upper end  1   a  are adjacent to each other, the dimension L 3  may be defined to the dimension from the feeding point  3  to the opened end  8   b  through the closed end  8   a  of the slit  8  along the slit  8 . 
         [0034]    Although the closed end  8   a  of the slit  8  is positioned in the vicinity of the feeding point  3  in  FIGS. 1(   a ),  1 ( d ) and  1 ( e ), the closed end  8   a  may not be positioned in the vicinity of the feeding point  3 . For example, the vertical portion from the closed end  8   a  of the slit  8  may be shifted in a horizontal direction. Accordingly, the dimension L 3 , that is, the dimension from the feeding point  3  to the right upper end  1   a  through the closed end  8   a  of the slit  8  and the opened end  8   b  along the slit  8 , can be adjusted. Alternatively, the vertical portion from the closed end  8   a  may be sloped. 
         [0035]    Although the short side  4 , the side  6  and the side  7  of the trapezoid of the first radiation element  1  are straight lines, the coupling portion between the short side  4  and the side  6  and the coupling portion between the short side  4  and the side  7  may be curved without a singular point as shown. 
         [0036]    Next, the performance of the antenna  100  will be described with reference to  FIGS. 2 to 9 . 
         [0037]      FIG. 2  is a simulation view of a voltage standing wave ratio (VSWR) of the shape (a) of the antenna  100  ( FIG. 1(   a )) and shows comparison with a case where the slit  8  is not formed. The gap G between the first radiation element  1  and the second radiation element  2  is approximately equal to or less than a tenth of the dimension L 2  of the short side  4 . 
         [0038]    The case where the slit is not formed corresponds to the disclosure in JP-A-2006-033069. The dimension L 1  is approximately a fourth of the wavelength λ 1  of the first resonance frequency f 1  of a low frequency side. The dimension L 2  of the short side  4  is approximately 0.3 to 0.4 times the wavelength λ 2  of the second resonance frequency f 2  of a high frequency side. 
         [0039]    When the slit  8  of the present invention is formed, a third resonance frequency f 3  is generated in the lower frequency side than the first resonance frequency f 1  of the low frequency side, compared with the case where the slit is not formed. The first resonance frequency f 1  of the low frequency side and the second resonance frequency f 2  of the high frequency side are substantially similar those of the case where the slit is not formed. The relationship between the first resonance frequency f 1  and the dimension L 1  and the relationship between the second resonance frequency f 2  and the dimension L 2  are similar those of the case where the slit is not formed. The third resonance frequency f 3  which is newly generated is related to the dimension L 3  including the dimension of the slit  8 . This relationship will be described later. 
         [0040]      FIG. 3  is a simulation view of the VSWR of the shape (a) of the antenna  100 , which simulates the relationship between the dimension L 3  and the third resonance frequency f 3  by varying the dimension L 3 . The dimension L 3  varies by shifting the vertical portion from the closed end  8   a  of the slit  8  in the horizontal direction. 
         [0041]    There are seven dimensions L 3 . The larger the dimension L 3 , the lower the third resonance frequency f 3 . Although the dimension L 3  varies, the first resonance frequency f 1  of the low frequency side and the second resonance frequency f 2  of the high frequency side (not shown) do not vary. Even in any state, in the relationship between the dimension L 3  and the third resonance frequency f 3 , the dimension L 3  is approximately 0.2 to 0.3 times, that is, a fourth, of the wavelength λ 3  of the third resonance frequency f 3 . As described with reference to  FIG. 2 , the dimension L 1  is approximately a fourth of the wavelength λ 1  of the first resonance frequency f 1  of the low frequency side. Accordingly, when the dimension L 3  including the dimension of the slit  8  is larger than the dimension L 1 , the third resonance frequency f 3  can be generated in the lower frequency side than the first resonance frequency f 1  of the low frequency side. 
         [0042]    When the dimension L 3  of the slit  8  varies, only the third resonance frequency f 3  varies and the first resonance frequency f 1  of the low frequency side and the second resonance frequency f 2  of the high frequency side are not influenced. Accordingly, the third resonance frequency f 3  can be independently controlled. 
         [0043]      FIG. 4  is a simulation view of the VSWR of the shape (a) of the antenna  100  which simulates the influence of the slit width of the slit  8 . When the slit width varies from 0.5 mm to 2 mm, the third resonance frequency f 3  slightly varies and is substantially ignorable. Accordingly, as described with reference to  FIG. 3 , the third resonance frequency f 3  can be controlled by the dimension L 3  of the slit  8 . 
         [0044]      FIG. 5  is a simulation view of a radiation pattern of a vertically polarized wave in the shape (a) of the antenna  100 , which simulates the radiation pattern of the vertically polarized wave with respect to frequencies of 2 GHz, 3 GHz, 4 GHz and 5 GHz. Even in any case, the radiation pattern is uniform and a null state is not generated at a specific angle. That is, it can be seen that a uniform radiation pattern can be obtained in the wideband. 
         [0045]      FIG. 6  is a simulation view of the VSWR of the shape (b) of the antenna  100  ( FIG. 1(   b )). In this case, the same result as the shape (a) is obtained. The third resonance frequency f 3  is generated at the lower frequency side than the first resonance frequency f 1  of the low frequency side. The relationship between the dimension L 3  of the slit  8  and the third resonance frequency f 3  is similar to that of the shape (a) and thus their detailed description will be omitted. 
         [0046]      FIG. 7  is a simulation view of the VSWR of the shape (c) of the antenna  100  ( FIG. 1(   c )). In this case, the same result as the shape (a) is obtained. The third resonance frequency f 3  is generated at the lower frequency side than the first resonance frequency f 1  of the low frequency side. The relationship between the dimension L 3  of the slit  8  and the third resonance frequency f 3  is similar to that of the shape (a) and thus their detailed description will be omitted. 
         [0047]      FIG. 8  is a simulation view of the VSWR of the shape (d) of the antenna  100  ( FIG. 1(   d )), which simulates the relationship between the dimension L 3  and the third resonance frequency f 3  by varying the dimension L 3 . The dimension L 3  varies by extending the length of the long side  5  of the first radiation element  1  in the right direction and changing the position of the right upper end  1   a . In Figure, “H+22 mm” or the like is the dimension L 3 . Here, a reference character H is the height of the slit  8  in the vertical direction and is fixed, and 22 mm, 20 mm and 18 mm are distances from the corner of the slit  8  to the right upper end  1   a.    
         [0048]    There are three dimensions L 3 . The larger the dimension L 3 , the lower the third resonance frequency f 3 . Even in any state, the first resonance frequency f 1  of the low frequency side and the second resonance frequency f 2  of the high frequency side (not shown) do not vary. Even in any state, in the relationship between the dimension L 3  and the third resonance frequency f 3 , the dimension L 3  is approximately 0.2 to 0.3 times the wavelength λ 3  of the third resonance frequency f 3 . 
         [0049]      FIG. 9  is a simulation view of the VSWR of the shape (e) of the antenna  100  ( FIG. 1(   e )) and shows the comparison between the shape (e) and the shape (a) in a state that the dimension L 3  of the slit  8  is fixed. The entire dimension of the first radiation element  1  of the shape (e) is equal to that of the shape (a), but the shape (e) further includes the slit  9 . The third resonance frequency f 3  of the shape (e) is lower than the third resonance frequency f 3  of the shape (a). This is because a current distribution is further concentrated in the shape (e). 
         [0050]    Next, a modified example of the first embodiment will be described. 
         [0051]      FIG. 10  shows a modified example of  FIG. 1(   a ). While the slit  8  vertically extends from the closed end  8   a  to the vicinity of the long side  5  in  FIG. 1(   a ), the slit  8  vertically extends from the closed end  8   a  to a middle portion and extends from the middle portion parallel to the long side  5  and the opened end  8   b  is opened at the side  7  in  FIG. 10 . The dimension L 3  denoted by a dotted line is a dimension from the feeding point  3  to the right upper end  1   a  of the trapezoid through the closed end  8   a  of the slit  8  and the opened end  8   b  along the slit  8 . The opened end  8   b  and the right upper end  1   a  are separated from each other. 
         [0052]    Next, the performance of the antenna  100  will be described. 
         [0053]      FIG. 11  is a simulation view of the VSWR of the antenna  100  ( FIG. 10 ). The third resonance frequency f 3  can be generated in addition to the first resonance frequency f 1  and the second resonance frequency f 2 . The third resonance frequency f 3  is related to the dimension L 3  including the dimension of the slit  8 . The third resonance frequency f 3  can be determined by adjusting the dimension L 3 . 
         [0054]    However, the first resonance frequency f 1  is shifted to the higher frequency side, compared with that of  FIG. 2  showing the simulation view of  FIG. 1(   a ). Accordingly, like  FIG. 1(   a ), it is preferable that the horizontal portion of the slit  8  is disposed in the vicinity of the long side  5 . 
         [0055]    Although the first radiation element  1  and the second radiation element  2  are the trapezoid planes, a quadrangle plane such as a rectangle may be used. 
         [0056]    According to the first embodiment of the present invention, since the slit is provided, it is possible to generate the third resonance frequency f 3  at the low frequency side with the same dimension, compared with the case where the slit is not formed. When the dimension L 3  of the slit  8  varies, only the third resonance frequency f 3  varies and the first resonance frequency f 1  of the low frequency side and the second resonance frequency f 2  of the high frequency side are not influenced. Accordingly, the third resonance frequency f 3  can be independently controlled. 
       Second Embodiment 
       [0057]    A second embodiment improves a strength against bending. 
         [0058]      FIGS. 12(   a ) and ( b ) are views showing the configurations of dipole type planar antennas according to the second embodiment In  FIGS. 12(   a ) and  12 ( b ), an antenna  200  includes a first radiation element  21  formed of a copper plate, a second radiation element  22  formed of a copper plate and a feeding point  23 . The first radiation element  21  and the second radiation element  22  are adhered to each other using polyimide resin  24 . The first radiation element  21  has a notched concave portion  21   a  and the feeding point  23  is formed in the vicinity of the concave portion  21   a . The second radiation element  22  has a convex portion  22   a  in conformity with the concave portion  21   a  of the first radiation element  21 . 
         [0059]    The convex portion  22   a  of the second radiation element  22  is formed on a dashed line of a gap G in which the first radiation element  21  and the second radiation element  22  face each other. Accordingly, it is possible to improve the strength against bending. 
         [0060]      FIG. 12(   c ) shows a structure in which a coaxial cable  25  is attached to the antenna  200 . A core wire  25   a  of the coaxial cable  25  is soldered to the feeding point  23 . A GND  25   b  of the coaxial cable  25  is soldered to the convex portion  22   a  of the second radiation element  22  and the coaxial cable  25  extends in a horizontal direction. Accordingly, a main portion except the convex portion  22   a  of the second radiation element  22  does not overlap the coaxial cable  25 . Thus, when a LCD  26  or the other substrate is mounted in a mobile wireless device having the antenna  200  mounted thereon, the total thickness of the antenna  200 , the LCD  26  and so on can decrease. 
         [0061]    The dimension L 1  denoted by a dotted line of  FIG. 12(   a ) is longer than the corresponding dimension of the antenna disclosed in JP-A-2006-033069. As described in the first embodiment, the first resonance frequency f 1  of the low frequency side which is determined by the dimension L 1  can decrease and the low frequency band can be covered. In a case where the first resonance frequency f 1  may be equal to JP-A-2006-033069, it is possible to decrease the dimension of the antenna  200 . 
         [0062]    Although the first radiation element  21  and the second radiation element  22  are trapezoid planes in the second embodiment, a quadrangle plane such as a rectangle may be used. 
         [0063]    According to the second embodiment of the present invention, it is possible to increase the strength of the planar antenna against bending. It is possible to decrease the thickness of a mobile wireless device in a state that a coaxial cable is attached. It is possible to cover a low frequency band.