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:
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 
   1. Field of the Invention 
   The present invention relates to a planar antenna, and more particularly, to a planar antenna capable of realizing multi-resonance and band widening. 
   2. Background Art 
   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. 
   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. 
   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 
   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. 
   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 
     The present invention may be more readily described with reference to the accompanying drawings, in which: 
       FIGS. 1(   a )-( e ) are views showing the configurations of planar antennas according to a first embodiment of the invention; 
       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; 
       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.); 
       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); 
       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; 
       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; 
       FIG. 7  is a simulation view of a VSWR of the planar antenna shown in  FIG. 1(   c ) according to the first embodiment; 
       FIG. 8  is a simulation view of a VSWR of the planar antenna shown in  FIG. 1(   d ) according to the first embodiment; 
       FIG. 9  is a simulation view of a VSWR of the planar antenna shown in  FIG. 1(   e ) according to the first embodiment; 
       FIG. 10  is a view showing the configuration of a planar antenna according to a modified example of the first embodiment; 
       FIG. 11  is a simulation view of a VSWR of the planar antenna according to the modified example of the first embodiment; and 
       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 
   Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. 
   First Embodiment 
   A first embodiment realizes band widening compared with the background art. 
     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 . 
   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 . 
   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 ). 
   Hereinafter, the shape of the slit  8  which is the characteristic of the first embodiment will be described with is reference to  FIGS. 1(   a ),  1 ( b ),  1 ( c ),  1 ( d ) and  1 ( e ). 
   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  5  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 . 
   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 . 
   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 . 
   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.    
   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 . 
   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. 
   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. 
   Next, the performance of the antenna  100  will be described with reference to  FIGS. 2 to 9 . 
     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 . 
   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. 
   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. 
     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. 
   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. 
   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. 
     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 . 
     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. 
     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. 
     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. 
     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.    
   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 . 
     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). 
   Next, a modified example of the first embodiment will be described. 
     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. 
   Next, the performance of the antenna  100  will be described. 
     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 . 
   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 . 
   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. 
   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 
   A second embodiment improves a strength against bending. 
     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 . 
   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. 
     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. 
   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 . 
   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. 
   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.