Patent Publication Number: US-7903030-B2

Title: Planar antenna device and radio communication device using the same

Description:
TECHNICAL FIELD 
     The present invention relates to radio communication devices such as a mobile phone, or devices which measure a distance to a target or recognize the position of a target. The present invention particularly relates to a planar antenna device which transmits and receives radio waves used for the above devices. 
     BACKGROUND ART 
     As a conventional planar antenna device, a thin, lightweight microstrip patch antenna is widely available (For example, see Patent Reference 1). The patch antenna has a structure such that a ground conductor (ground) and a rectangular antenna conductor (also known as an “antenna element”) to serve as an antenna unit are formed on the back and the face side of a dielectric board, respectively. As electrical power is fed to the antenna conductor, radio waves are radiated at a frequency where resonance occurs according to the length of the longer sides of the antenna conductor. In order to feed electrical power to the antenna conductor, the coplanar feeding system is available in which a feed line provided as a microstrip line (also known as a “feed line”) is formed on the surface of the dielectric board, the surface where the antenna conductor is also formed. In this system, which forms the antenna conductor and the feed line on the same surface, a planar antenna device can be manufactured with ease at a low cost. 
     Typically, the antenna conductor and the feed line are connected at a position called a “feeding point”. The design is made such that the impedances of the antenna conductor and the feed line are matched at the feeding point. By matching the impedances so as to prevent reflections from occurring, electrical power can be fed to the antenna conductor with efficiency. 
     Here, a brief explanation is given as to how to match the impedances of the antenna conductor and the feed line. The impedance of the antenna conductor varies according to its position on the antenna conductor. The impedance is low around the central portion in the antenna, becomes higher toward the end portion, and reaches a value almost equal to infinity at the end. Therefore, a cut (also called as a “matching slit”) is made in the antenna conductor down to position of the feeding point, at which the antenna conductor and the feed line are connected, so that the antenna conductor has the same impedance as the feed line. The method thus matches the antenna conductor and the feed line. 
     In order to use the planar antenna device practically, a predetermined radiation pattern or radiant gain is required. A radiation pattern and a radiant gain characteristic depend on the entire effective aperture dimensions of the antenna conductor. With an antenna conductor of larger dimensions, the antenna&#39;s directivity increases and a higher radiant gain is obtained. In the case where the patch antenna is employed singly, since the size of the antenna conductor is determined by the frequency to be used, the radiation directivity decreases and the gain is low. Therefore, in order to adjust a radiation pattern and a radiant gain, the array structure is employed, in which a plurality of antenna conductors are arranged at a specific regular spacing, so as to adjust effective aperture dimensions. However, as the interval between the antenna conductors becomes wider, the side lobe of the radio waves radiated from the array antenna increases in level, the feed line has to be installed into the narrow interval limited by the antenna conductors. 
     Here, a description is given for the structure of a conventional planar array antenna employing the coplanar feeding system with reference to  FIG. 1 .  FIG. 1  is a top view showing a conventional planar array antenna  1000 . The planar array antenna  1000  shown in  FIG. 1  includes a dielectric board  1010 , four antenna elements  1001 , a feed line  1002 , and a ground conductor (not shown). Each of the antenna elements  1001  is connected to the power feeder  1005  via the feed line  1002  (in this case, the point where each of the antenna elements  1001  and the feed line  1002  are connected together, is referred to as a “feeding point”). Each of the antenna elements  1001  has a matching slit  1003  for matching the impedances of the antenna elements  1001  and the feed line  1002 . The ground conductor is provided at the back of the planar array antenna  1000 . 
     Generally, the array structure shown in  FIG. 1 , where the antenna elements  1001  are connected to the power feeder  1005  in parallel (tree-shaped structured connection), is available for radiation over a wide range of frequencies. 
     In the planar array antenna  1000 , the antenna elements  1001  have to be excited in phase. This is because the radio waves radiated from the antenna elements  1001  cancel each other thereby degrading their function as an array antenna, in the case where the radio waves from the antenna elements  1001  are out of phase each other. 
     The planar array antenna  1000 , therefore, is designed so that the electrical lengths from the power feeder  1005  to the respective antenna elements  1001 , or equivalently, the lengths of the respective sections of the feed line  1002  are equal. Furthermore, in order to excite the electric fields of the respective antenna elements  1001  in a same direction, the respective sections of the feed line  1002 , the respective feeding points  1004 , and the respective antenna elements  1001  are arranged in a same way. Specifically, the structure is such that the respective sections of the feed line  1002  connect to the respective antenna elements  1001  at the respective feeding points  1004  from a same side. 
     Patent Reference 1: Japanese Unexamined Patent Application Publication No. 2004-166043 
     DISCLOSURE OF INVENTION 
     Problems that Invention is to Solve 
     As has been described above, in the conventional planar array antenna  1000 , since the electric fields of the respective antenna elements  1001  have to be excited in a same direction, the structure is such that all of the feed lines  1002  connect to the respective antenna elements  1001  from a same side. Consequently, a bend  1011  is formed in each of the feed line  1002  as shown in  FIG. 1 . In the structure having such bends  1011 , the feed line  1002  becomes longer, and a larger space is required to lay out the feed line  1002  therein. 
     Thus, the array antenna which sends and receives high-frequency radio waves is disadvantageous in that losses become larger because of the bends  1011  in the feed lines  1002  and the increased length of the feed line. 
     Furthermore, in recent years, much attention has been given to a broadband radio communication system called “Ultra-wide Band”. In order for the planar antenna to obtain such wideband characteristics, it is effective to employ a thicker antenna board. 
     In the case where a thicker board is employed, however, the electric fluxes of the feed line  1002  do not terminate in the ground when there is not enough space between the respective feed lines or between the antenna element  1001  and the feed line  1002 , thereby causing interference in electromagnetic coupling between the feed lines or between the antenna element and the feed line. Due to such interference, impedance in the feed line  1002  partially changes, thereby causing an unsynchronized phase among the excited field in the antenna elements. Specifically, in the case where the antenna elements  1001  are not excited in phase, the radio waves radiated from the respective antenna elements  1001  cancel each other, thereby extremely worsening their radiation characteristics. 
     For the reason stated above, in the conventional structure where the feed line occupies a larger portion therein, it is impossible to achieve a planar array antenna with the coplanar feeding system for high-frequency or broadband use. 
     In order to solve the above problem, the planar array antenna shown in  FIG. 2(   a ) is conceivable, having no bend in a feed line, in which antenna elements can be arranged symmetrically. 
     However, the planar array antenna shown in  FIG. 2(   a ) is also impractical. With reference to  FIGS. 2(   a ) and ( b ), a reason is given as to the impracticality of a symmetrical array antenna, in which antenna elements are arranged to face each other and electrical power is fed from a feed line located between the facing sides of the antenna elements. 
       FIG. 2(   a ) is a top view showing the structure of a planar array antenna where a feed line, with a small bend  1011 , occupies a smaller portion therein. As shown in  FIG. 2(   a ), a planar array antenna  1100  includes a first antenna element  1101   a , a second antenna element  1101   b , and a feed line  1102  for feeding electrical power, which are arranged on a dielectric board  1110 . Similarly to the matching slit  1003  shown in  FIG. 1 , the first antenna element  1101   a  and the second antenna element  1101   b  include a first matching slit  1103   a  and a second matching slit  1103   b , respectively. The structure of the planar array antenna  1100  is such that a branch point  1109  at which the feed line  1002  branches off, is provided at the midpoint between the two antenna elements  1101   a  and  1101   b , so that electrical power is fed to each of the antenna elements through sections of the feed line  1102  of equal length. 
       FIG. 2(   b ) is a schematic diagram showing a current distribution  1107  and a voltage distribution  1108  in a cross section of the planar array antenna  1100  taken along the line A-A′ of  FIG. 2(   a ) in a perpendicular direction. Referring to  FIG. 2(   b ), a description is given for the respective states in which the first antenna element  1101   a  and the second antenna element  1101   b  are excited. 
       FIG. 2(   b ) shows current and voltage distributions with the centers of the first antenna element  1101   a  and the second antenna element  1101   b  arranged symmetrically. As shown in  FIG. 2(   b ), the radio waves radiated from the antenna elements partially cancel each other because of their phase difference, which means that it is impossible to obtain a preferable intensity of the radio waves (radiant gain) relative to the vertical plane of the antenna. This is verified by the radiation intensity characteristics shown in  FIG. 3 . 
       FIG. 3  shows the intensity of the radio waves radiated in the direction perpendicular to the antenna surface of the planar array antenna  1100  shown in  FIG. 2(   a ). In this case, a Teflon (registered trademark) board 0.5 mm thick, with a relative permittivity of 3.0 was employed as the dielectric board  1110  of the planar array antenna  1100 . Furthermore, in the planar array antenna  1100 , each of the antenna elements was formed into a square, 3.1 mm on a side, with the size of each feed-wire matching slit equivalent to 0.9 mm×0.8 mm (frequency: 26 GHz). The characteristic shown in  FIG. 3 , where the radiation intensity becomes low in the direction perpendicular to the antenna surface, is insufficient in performance as a planar array antenna. Therefore, the planar array antenna  1100  having the structure shown in  FIG. 2(   a ) cannot be employed. 
     In light of the above, it is an object of the present invention to provide a practical planar antenna device which has antenna elements facing each other, in which electrical power is fed from facing sides of the antenna elements. 
     Means to Solve the Problems 
     In order to solve the above described problem, a planar antenna device of the present invention includes at least one antenna pair and a feed line on a dielectric board having a ground conductor on a back surface. The antenna pair includes the first antenna element having the first slit, and the second antenna element having the second slit. The first antenna element and the second antenna element are arranged in the planar antenna device in such a way that the first slit and the second slit are oriented toward a center of the antenna pair. The first antenna element is connected to the feed line electrically or electromagnetically at the first feeding point positioned in an inmost recess of the first slit. The second antenna element is connected to the feed line electrically or electromagnetically at the second feeding point positioned in an inmost recess of the second slit. The first slit of the first antenna element and the second slit of the second antenna element are different in length. 
     This achieves an array antenna structure where the feed line occupies only a small portion therein without conventional bends, thereby achieving a planar antenna device of the coplanar feeding system, available at a high frequency or on a wide frequency band. 
     Further, the planar antenna device may be structured such that the distance from the first feeding point of the first antenna element to the branch point is approximately equal to the distance from the second feeding point of the second antenna element to the branch point. 
     This prevents the radio waves radiated from the antenna elements from canceling each other due to their phase difference, although the planar antenna device has antenna elements facing each other, in which electrical power is fed from facing sides of the antenna elements, thereby achieving a planar array antenna with a desired radiant gain. 
     Further, the antenna pair may include a plurality of the first antenna elements and a plurality of the antenna elements. 
     This achieves a planar antenna capable of preventing the radio waves radiated from the antenna elements from cancelling each other due to their phase difference, with space efficiency. 
     Note that it is preferable that the first antenna element has an area 1 to 1.3 times larger than an area of the second antenna element. 
     Further, in the planar antenna device, the first slit of the first antenna element is shorter than a length from an end of the first antenna element on a side facing the second antenna element to a center of the first antenna element, and the second slit of the second antenna element is longer than a length from an end of the second antenna element on a side facing the first antenna element to a center of the second antenna element. 
     Further, the feed line feeds electrical power fed from a power feeder, to the first antenna element and the second antenna element via a branch point, and the first antenna element and the second antenna element are arranged with the branch point in between. 
     Furthermore, The feed line feeds electrical power fed from a power feeder, to the first antenna element and the second antenna element via a branch point, and the feed line near the branch point is formed into a T-shape, a Y-shape or an arrow-shape. 
     Further, the planar antenna element of the present invention may be structured so as to include a radiating element conductor having a slit and a feed line on a dielectric board having a ground conductor on a back surface. The radiating element conductor is connected to the feed line electrically or electromagnetically at a feeding point positioned in an inmost recess of the slit. The slit is longer than a length from an end of the radiating element conductor on a side facing the feed line to a center of the radiating element conductor. In this case, the planar antenna element can be miniaturized also as an antenna device. 
     The present invention can also be achieved as a radio communication device employing the planar antenna device, such as a mobile phone. 
     EFFECTS OF THE INVENTION 
     According to the present invention, a bend does not have to be provided in the feed line, thereby achieving an array antenna structure where a smaller portion is occupied by the feed line. There are no losses due to bends, thereby achieving an array antenna with high radiation efficiency. Furthermore, the portion to be occupied by the feed line can become the smallest possible dimensions, thereby achieving a low-loss array antenna with high radiation efficiency, owing to a shorter feed line. Furthermore, the smaller portion to be occupied by the feed line allows the antenna elements to be arranged at a shorter spacing, thereby allowing a grating lobe of the radiated radio waves to be suppressed. 
     Furthermore, according to the present invention, only a smaller portion is occupied by the feed line, a larger space can be provided between the feed lines, or between the antenna and the feed line, thereby reducing interference between the feed lines. This achieves a broadband antenna which was unavailable due to its thicker antenna board and its larger interference between the feed lines. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a top view showing a conventional planar antenna device having a 2×2 array structure. 
         FIG. 2(   a ) is a top view showing another conventional planar array antenna; and  FIG. 2(   b ) shows current and voltage distributions in the planar array antenna, taken along the line A-A′ of  FIG. 2(   a ). 
         FIG. 3  shows a radiation characteristic of the conventional planar array antenna. 
         FIG. 4(   a ) is a top view showing a planar antenna device according to the present invention; and  FIG. 4(   b ) shows current and voltage distributions taken along the line A-A′ of  FIG. 4(   a ). 
         FIG. 5  shows a radiation characteristic of the radio waves radiated from the planar antenna device shown in  FIG. 4(   a ). 
         FIG. 6  is a top view showing a 2×4 array antenna according to the present invention. 
         FIG. 7  shows the appearance of a typical planar patch antenna. 
         FIG. 8  shows frequency characteristics of the voltage reflection coefficients of first and second antenna elements according to the present invention on an individual basis. 
         FIG. 9(   a ) shows the appearance of a planar antenna device including two pairs of antenna elements, in each pair of which a first antenna element and a second antenna element are arranged to face each other; and  FIG. 9(   b ) shows the appearance of a planar antenna device including two pairs of antenna elements, in each pair of which a first antenna element and a second antenna element are arranged to face each other as well as to orient a same direction. 
         FIG. 10(   a ) shows an exemplary antenna element according to the present invention, in which a matching slit is arranged off-center;  FIG. 10(   b ) shows an exemplary antenna element according to the present invention, in which a matching slit is provided in a slanting direction;  FIG. 10(   c ) shows an exemplary antenna element according to the present invention, in which a circular antenna element is employed;  FIG. 10(   d ) shows an exemplary antenna element according to the present invention, in which a feed line and an antenna element are connected electromagnetically;  FIG. 10(   e ) shows an example according to the present invention in which a feed line is formed into Y-shape near a branch point; and  FIG. 10(   f ) shows an example according to the present invention in which a feed line is formed like an arrow near a branch point. 
     
    
    
     NUMERICAL REFERENCES 
     
         
         
           
               10 ,  11 ,  12  Planar antenna device 
               101   a ,  301   a ,  1101   a  First antenna element 
               101   b ,  301   b ,  1101   b  Second antenna element 
               102 ,  302 ,  1002 ,  1102  Feed line 
               103   a ,  1103   a  First slit 
               103   b ,  1103   b  Second slit 
               1003 ,  1103   a ,  1103   b  Matching slit 
               104   a ,  304   a  First feeding point 
               104   b ,  304   b  Second feeding point 
               105 ,  305 ,  1005 ,  1105  Power feeder 
               106 ,  406 ,  1006  Ground conductor 
               107 ,  1007  Current distribution 
               108 ,  1008  Voltage distribution 
               109 ,  1009  Branch point 
               110 ,  310 ,  410 ,  1010 ,  1110  Dielectric board 
               401  Antenna conductor 
               501   a ,  501   b ,  501   c  Antenna element 
               501   d  Antenna element 
               502   a ,  502   b ,  502   c  Feed line 
               502   d  Feed line 
               503   a ,  503   b ,  503   c  Matching slit 
               503   d  Matching slit 
               504   a ,  504   b ,  504   c  Feeding point 
               504   d  Feeding point 
               1004 ,  1104   a ,  1104   b  Feeding point 
               1000 ,  1100  Planar array antenna 
               1011  Bend 
           
         
       
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, a planar antenna device according to the embodiment of the present invention is described with reference to accompanying drawings. Although the description is given for the present invention with the following embodiment and the accompanying drawings, the embodiment and the drawings are given for exemplification; the present invention is not limited to the embodiment and the drawings. 
       FIG. 4(   a ) is a top view showing a planar antenna device  10  according to the present invention. The planar antenna device  10  shown in  FIG. 4(   a ) includes a first antenna element  101   a , a second antenna element  101   b , a first feeding point  104   a , a second feeding point  104   b , impedance-matching first slit  103   a  and second slit  103   b , a feed line  102  (a branch point  109  included), a power feeder  105  and a dielectric board  110 . 
     The first feeding point  104   a  of the first antenna element  101   a  is provided on the side of the first antenna element having an edge facing the branch point  109  of the feed line  102  (specifically, right-sided end of the first antenna element  101   a ). On the other hand, the second feeding point  104   b  of the second antenna element  101   b  is provided near the right-sided end of the second antenna element  101   b , the end which is in the inmost position of recess of the second slit  103   b.    
     Specifically, the length of the first slit  103   a  “SL 1 ” is shorter than the distance from the right-sided end of the first antenna element  101   a  to its center (specifically, L 1 /2). The length of the second slit  103   b  “SL 2 ” is longer than the distance from the left-sided end of the second antenna element  101   b  to its center (specifically, L 2 /2). 
     Even in such a structure, each of the impedances of the first antenna element  101   a  and the second antenna element  101   b , which is symmetrical relative to the center of each antenna element, can be matched to the impedance of the feed line  102  either at the first feeding point  104   a  or at the second feeding point  104   b.    
     In this case, the feed line  102  has the same line width both for connecting to the first antenna element  101   a  and for connecting to the second antenna element  101   b . The sections of the feed line from the branch point  109  both to the first feeding point  104   a  and to the second feeding point  104   b , have almost the same length. This means that electrical power is fed to the first antenna element  101   a  and to the second antenna element  101   b  in phase. Furthermore, the first feeding point  104   a  and the second feeding point  104   b  are provided on the same side (in this case, on the right side) within each antenna element, which means that their electric fields are excited in the same way. Therefore, the radio waves radiated from the first antenna element  101   a  and the second antenna element  101   b  intensify each other. 
     Referring to  FIG. 4(   b ), a more detailed description is given for the phase relation between the electric fields excited by the respective antenna elements.  FIG. 4(   b ) shows a current distribution  107  and a voltage distribution  108  in each of the antenna element  101   a  and the antenna element  101   b , taken along the line A-A′ of the planar antenna device  10  shown in  FIG. 4(   a ). As shown in  FIG. 4(   b ), the planar antenna device  10  has the first antenna element  101   a , the second antenna element  101   b , the first feeding point  104   a , the second feeding point  104   b , and the like, arranged on the surface of the dielectric board  110 . A ground conductor  106  is provided at the back of the dielectric board  110 . 
     Since the second feeding point  104   b  of the second antenna element  101   b  is provided on the right side similarly to the feeding point  104   a  of the first antenna element  101   a , the electric field in the second antenna element  101   b  is excited in the same direction as in the first antenna element  101   a . Specifically, the electric fields in the first antenna element  101   a  and the second antenna element  101   b  are excited exactly the same as in the case when electrical power is fed from the same side. Therefore, similarly to the conventional technology shown in  FIG. 1 , although the feed line sections from the branch point  109  have almost the same length, the radio waves radiated from the respective antenna elements intensify each other. 
     Hereinafter, a specific example is given for the planar antenna device  10  according to the present invention. 
     As the dielectric board  110  of the planar antenna device  10  having the array structure shown in  FIG. 4(   a ), a Teflon (registered trademark) board of 0.5 mm thick, with a relative permittivity of 3.0 was employed. The first antenna element  101   a  is formed into a square having 3.1 mm on a side, (specifically “W 1 =L 1 ”). The second antenna element  101   b  has the width “W 2 ” of 3.1 mm and the length “L 2 ” of 2.8 mm. The first slit  103   a  has the width “SW 1 ” of 0.8 mm and the length “SL 1 ” of 0.9 mm. The second slit  103   b  has the width “SW 2 ” of 0.5 mm and the length “SL 2 ” of 2.4 mm. The feed line  102  has a width of 0.2 mm. Note that the first antenna element  101   a  preferably has dimensions 1 to 1.3 times larger than the dimensions of the second antenna element  101   b.    
       FIG. 5  shows the intensity of the radio waves radiated at a frequency of 26 [GHz] from the planar antenna device  10  shown in  FIG. 4(   a ). The angle given along the horizontal axis means the angle from the axis perpendicular to the antenna surface. 
     As shown in  FIG. 5 , since the electric fields in the first antenna element  101   a  and the second antenna element  101   b  are excited in phase in the same direction, the radiation intensity reaches a maximum value in the axis direction perpendicular to the antenna surface. 
     As has been described above, according to the planar antenna device of the present invention, even in the system where at least two antenna elements are arranged so that their matching slits face each other and electrical power is fed from facing sides of antenna elements, it is possible to excite the electric fields in the antenna elements in phase in the same direction. Furthermore, it is naturally possible to achieve a large-scale planar array antenna such as the 2×4 array structure shown in  FIG. 6 , by employing a plurality of the planar antenna devices according to the present invention. 
     Next, a description is given for a single antenna element of the planar antenna device  10  according to the present invention.  FIG. 7  shows the appearance of a typical planar patch antenna. The planar patch antenna shown in  FIG. 7  has an antenna conductor formed into a square having the length of “a” on a side, on the surface of the dielectric board  410 . The planar patch antenna has a ground conductor  406  at the back thereof. In this case, the resonance frequency fr is given by Equation (1). 
     
       
         
           
             
               
                 
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     In this case, the Qt value of the antenna is given by Equation (2) (for further explanation of Equations (1) and (2), see “SAISHIN HEIMEN ANTENA GIJYUTU” by Misao HANEISHI (1993)).
 
1 /Qt= 1 /Qr+ 1 /Qc+ 1 /Qd   (2)
 
     Qr: radiation loss; Qc: conductive loss; Qd: dielectric loss 
     As has been described above, the first antenna element  101   a  of the planar antenna device  10  shown in  FIG. 4  has a structure such that the first feeding point  104   a  is provided between the end of the first antenna element  101   a  on the side facing the feed line  102 , and the center of the antenna element  101   a . This structure is employed as the conventional offset feeding technique. 
     On the other hand, the second antenna element  101   b  shown in  FIG. 4  has a structure such that the feeding point  104   b  is provided between the right-sided end of the second antenna element  101   b , on the opposite side of the feed line  102 , and the center of the antenna element  101   b  (specifically, SL 1 &lt;SL 2 ). As has been described above, the impedances of the first antenna element  101   a  and the second antenna element  101   b  are symmetrical relative to their centers, thereby achieving a practical antenna. 
       FIG. 8  shows frequency characteristics of the voltage reflection coefficients of the first antenna element  101   a  and the second antenna element  101   b  on an individual basis. The second antenna element  101  has a matching slit of a shape different from the matching slit of the first antenna element  101   a  equivalent to the conventional antenna element. However,  FIG. 8  verifies that the second antenna element  101   b  shows a frequency characteristic equivalent to the first antenna element  101   a . Furthermore, it becomes possible to make the length “L 2 ” of the second antenna element  101   b  shorter than the first antenna element  101   a  equivalent to the conventional antenna element. 
     Although the embodiment relates to the case in which the electrical lengths and the feed line lengths are the same from the branch point to the respective feeding points, these lengths may vary. 
     Furthermore, the embodiment relates to the example in which the widths of the respective feed lines, as well as their impedances, are equal to each other. However, the widths and the impedances may be different from each other. 
     Furthermore, although the embodiment relates to linear polarization, circular polarization may also be employed. 
     Furthermore, although the embodiment relates to the case in which the antenna elements operate at the same central operating frequency, the antenna elements may operate at different central operating frequencies. 
     Furthermore, the embodiment relates to the case in which two patch antennas are employed, the present invention is achieved also as two array antennas or more. 
     Furthermore, the embodiment has described the antenna device which has a single pair of the first antenna element  101   a  and the second antenna element  101   b , with the branch point  1109  in between as shown in  FIG. 1 . However, the antenna device is also available, having a plurality of pairs of antenna elements, in which first antenna elements  101   a  and second antenna elements  101   b  of the same number are arranged with a branch point  1109  in between. 
       FIGS. 9(   a ) and  9 ( b ) show an exemplary planar antenna device including a plurality pairs of antenna elements, in which first antenna elements  101   a  and second antenna elements  101   b  are provided.  FIG. 9(   a ) shows the appearance of a planar antenna device  11  including two pairs of antenna elements, in each pair of which a first antenna elements  101   a  and a second antenna element  101   b  are arranged to face each other. Furthermore,  FIG. 9(   b ) shows the appearance of a planar antenna device  12  including two pairs of antenna elements, in each pair of which a first antenna element  101   a  and a second antenna element  101   b  are arranged to face each other as well as to orient a same direction. Both the planar antenna device  11  and the planar antenna device  12  have a structure such that the distances from the branch point to the respective feeding points are almost equal. 
     Furthermore, although the embodiment relates to the example in which the wires are connected to the end&#39;s center point of each antenna element, the wires may be connected at a position deviated from the center point as shown in  FIG. 10(   a ). 
     Furthermore, although the embodiment relates to the T-shaped structure in which the feed lines are connected or electrical power is fed perpendicularly to a side of the patch antenna, the structure may be such that the feed lines are connected or electrical power is fed at a given angle from the antenna side. For example, the structure may be formed into Y-shape as shown in  FIG. 10(   e ), or formed like an arrow as shown in  FIG. 10(   f ). 
     Furthermore, although the embodiment relates to the rectangular planar patch antenna, various shapes of planar antennas such as a circular planar antenna shown in  FIG. 10(   c ) and an antenna having a matching slit formed therein, are also applicable. For example, although not shown, a planar antenna of a polygonal shape may be employed, including a pentagon, hexagon, and an icosagon. Quadrilaterals other than a rectangle are also applicable. Examples of quadrilaterals other than a rectangle include a rhombus and a parallelogram. 
     Furthermore, although the embodiment relates to the case in which the feed line is connected in the direction perpendicular to the parallel ends of the planar patch antenna, the feed line may be connected at an angle other than perpendicular as shown in  FIG. 10(   b ). 
     Furthermore, as shown in  FIGS. 10(   a ) to  10 ( d ), the antenna element is electrically or electromagnetically connected to the feed line at the feeding point positioned in the inmost position of the recess of the slit. The matching slit is longer than the length from the end of the radiating element conductor on the feed line side to the center of the radiating element conductor, so that the antenna element can be downsized, thereby miniaturizing the whole antenna device. Note that the antenna element  101   a  shown in  FIG. 4(   a ) may be connected to the feed line  102  electromagnetically at the first feeding point  104   a  in the inmost position of the recess of the first slit  103   a.    
     Although linear in the embodiment, the feed line does not have to be linear. The embodiment relates to the case in which the feed line is connected to the antenna element through wiring, the feed line and the antenna element may be connected electromagnetically as shown in  FIG. 10(   d ). 
     Furthermore, the embodiment of the present invention relates to the case in which the matching slit has a rectangular structure, the structure may be formed into other shapes. 
     INDUSTRIAL APPLICABILITY 
     According to the antenna device of the present invention, matching slits of a antenna element pair are provided asymmetrically so that the positions of feeding points are on the same side of the antenna device. This allows the wiring portion to be the smallest possible dimensions, thereby improving the antenna characteristics. The antenna device of the present invention is extremely effective as an antenna device for high-frequency and broadband.