Abstract:
In order to have an antenna apparatus small in size and capable of switching its directivity pattern to be adaptive to multiple frequencies, the present invention provides an antenna apparatus having a first antenna element formed at an approximately center position of a planar printed circuit board and second antenna elements formed before and behind the first antenna element. It is possible to construct an antenna in which the first antenna element functions as a radiator and the second antenna elements function as a director or a reflector, respectively, by changing electrical length of the second antenna elements. The antenna becomes adaptive to multiple frequencies by feeding the second antenna elements at different phases to have the second antenna elements functioning as radiators.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
   The present document is based on Japanese Priority Document JP 2004-016185, filed in the Japanese Patent Office on Jan. 23, 2004, the entire contents of which being incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an antenna apparatus capable of performing a switching of a directivity pattern. 
   2. Description of Related Art 
   Conventionally, it is known that a use of an antenna having no directivity pattern leads to a degradation of communication quality with an interference wave caused by a reflection from a building wall etc. in a multi path propagation environment in which multiple radio waves are available. Thus, an antenna apparatus capable of turning a directivity pattern in a specific direction has attracted attention. 
   A phased array antenna apparatus shown in  FIG. 13  and an adaptive array antenna apparatus shown in  FIG. 14  are known as such an antenna apparatus capable of turning a directivity pattern in a specific direction. The phased array antenna apparatus shown in  FIG. 13  has N pieces of antenna elements  101 - 1 ,  101 - 2 , . . . and  101 -N. Then, an amplification of signals having been received by the N pieces of antenna elements  101 - 1 ,  101 - 2 , . . . and  101 -N is performed by amplifiers (AMP)  102 - 1 ,  102 - 2 , . . . and  102 -N. The received signals having been amplified by the amplifiers  102 - 1 ,  102 - 2 , . . . and  102 -N are outputted to a synthesizer  104  after a phase adjustment by variable phase shifters (phase shifters)  103 - 1 ,  103 - 2 , . . . and  103 -N. The synthesizer  104  performs a synthesis of the received signals from the respective variable phase shifters  103 - 1 ,  103 - 2 , . . . and  103 -N. A frequency converter (a down-converter)  105  is operated to output the resultant received signal obtained by the synthesizer  104  through a conversion into a signal of a lower frequency. 
   An adaptive array antenna shown in  FIG. 14  has N pieces of antenna elements  111 - 1 ,  111 - 2 , . . . and  111 -N. In the adaptive array antenna of this type, the amplification of signals having been received by the N pieces of antenna elements  111 - 1 ,  111 - 2 , . . . and  111 -N is performed by amplifiers (AMP)  112 - 1 ,  112 - 2 , . . . and  112 -N at the time of a receiving operation of the above antenna. Then, the received signals having been amplified by the amplifiers  112 - 1 ,  112 - 2 , . . . and  112 -N are respectively down-converted (DC) by frequency converters  113 - 1 ,  113 - 2 , . . . and  113 -N and subsequently undergo an analog signal-to-digital signal conversion by AD/DA converters  114 - 1 ,  114 - 2 , . . . and  114 -N. Following the conversion, an output of the obtained digital signals is performed through a so-called adaptive signal processing such as weighting and synthesizing with a digital signal processing unit  115 . 
   On the contrary, at the time of a transmitting operation, digital transmitting signals having been given a required signal processing by the digital signal processing unit  115  are converted into analog transmitting signals with the AD/DA converters  114 - 1 ,  114 - 2 , . . . and  114 -N and subsequently undergo an up-conversion (UC) with the frequency converters  113 - 1 ,  113 - 2 , . . . and  113 -N. Following the conversion, the amplification is performed by the amplifiers  112 - 1 ,  112 - 2 , . . . and  112 -N, leading to a transmission (a radiation) from the antenna elements  111 - 1 ,  111 - 2 , . . . and  111 -N. 
   However, the phased array antenna as shown in  FIG. 13  requires that a receiving system should be configured with a plurality of variable phase shifters  103 - 1  to  103 -N at a high frequency band. Further, the adaptive array antenna as shown in  FIG. 14  requires that the adaptive signal processing should be performed using a plurality of transmitting/receiving systems. For the above reasons, either of the above antenna apparatuses calls for a complicated system and costs much, resulting in a difficult application to a consumer apparatus requiring to be produced at low cost. 
   By the way, a Yagi-Uda antenna widely used for a reception of television broadcasting is well known as an antenna having a directivity pattern in a specific direction. The Yagi-Uda antenna shown in  FIG. 15A  comprises a radiator  121  that radiates a radio wave, a director  122  having an electrical length slightly smaller than an electrical length (2/λg, where λg is a guide wavelength) of the radiator  121  and a reflector  123  having an electrical length slightly larger than the electrical length of the radiator  121 , wherein the director  122  and the reflector  123  are disposed before and behind the radiator  121  to ensure that the directivity as shown in  FIG. 15B  is obtained. 
   Then, a patent document 1 proposes an antenna apparatus that is configured based on the above Yagi-Uda antenna to ensure that a switching of a direction of the directivity is performed. Further, a patent document 2 proposes an antenna apparatus in which a sharing of a director is applied to attain a reduction in antenna size, with reference to an antenna apparatus that performs the switching of a feed point to ensure that a formation of multi-beams is attained. Furthermore, a patent document 3 proposes a multi-beam antenna of multi-frequency sharable type.
     [Patent document 1] Japanese Patent Application Publication (KOKAI) No. Hei 11-27038   [Patent document 2] Japanese Patent Application Publication (KOKAI) No. 2003-142919   [Patent document 3] Japanese Patent Application Publication (KOKAI) No. Hei 11-168318   

   SUMMARY OF THE INVENTION 
   However, the antenna apparatus of the above patent document 1 is in the form of an array of multiple Yagi-Uda antennas, and thus requires more than one director and more than one reflector, resulting in a disadvantage of being difficult of a downsizing. Further, the antenna apparatus of the above patent document 1 is supposed to be of a structure in which a monopole antenna is projecting in a vertical direction of a ground plate, also resulting in a difficulty in attaining a reduction in thickness. Alternatively, it is also suggested that a dipole antenna should be used in place of the monopole antenna, for instance, to form the antenna on a printed circuit board, in which case, however, the ground plate fails to be disposed in the vicinity of the antenna, resulting in a difficult packaging of a selector switch etc. Further, the monopole antenna, even if formed with a dielectric substance, has little effect of shortening a wavelength, resulting in a disadvantage of being difficult to downsize. 
   The antenna apparatus of the above patent document 2 applies the sharing of the director to reduce an antenna size, so that there is a limitation to the downsizing. Further, the antenna apparatus of the above configuration needs a selector switch between transmitting and receiving systems for each beam direction to attain the formation of multi-beams, resulting in a disadvantage in that the selector switch leads to a degradation of efficiency as the antenna. Furthermore, the antenna apparatus of the above configuration is basically supposed to have one transmitting/receiving system, so that a one-to-multiple switching is required for the selector switch, resulting in a disadvantage of being very difficult of a manufacturing adaptive to an available frequency band of a radio communication. 
   Moreover, the antenna apparatus of each of the above patent documents 1 and 2 has been considered to be incapable of using a transmitting/receiving frequency at more than one frequency. On the contrary, the multi-frequency sharable multi-beam antenna of the above patent document 3 is supposed to be available at more than one frequency, in which case, however, the antenna of this type is merely in the form of the array of antennas to individual frequencies, resulting in a disadvantage of being difficult to downsize. 
   Thus, the present invention has been undertaken in view of the above problems, and is intended to realize that an antenna apparatus being small in size and capable of performing the switching of a directivity pattern is adaptive to multiple frequencies. 
   To attain the above object, an antenna apparatus according to the present invention comprises a first antenna element having a prescribed electrical length, first feed means capable of performing a feed to the first antenna element, second antenna elements respectively having an electrical length larger than the electrical length of the first antenna element and disposed at the opposite sides of the first antenna element, second feed means capable of performing, at respectively different phases, the feed to the second antenna elements disposed at the opposite sides of the first antenna element, and changing means of changing each electrical length of the second antenna elements. 
   According to the above configuration, a first antenna circuit may be formed by performing the feed from the first feed means to the first antenna element, for instance, and by changing, by the changing means, the electrical length of either of the second antenna elements disposed at the opposite sides of the first antenna element. Further, a second antenna circuit may be formed by performing the feed at the respectively different phases from the second feed means to the second antenna elements disposed at the opposite sides of the first antenna element. 
   Thus, according to the present invention, a formation of more than one antenna circuit ensures that a multi-frequency antenna being adaptive to more than one frequency and besides, capable of controlling the directivity pattern is realizable. Further, in this case, the second antenna elements may be used in common as the first antenna circuit and the second antenna circuit, so that the downsizing of the antenna apparatus is attainable. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1 , consisting of  FIG. 1A  and  FIG. 1B , is a view for illustrating a configuration of a Yagi slot antenna specified as an embodiment of the present invention. 
       FIG. 2 , consisting of  FIG. 2A  and  FIG. 2B , is a view showing directivity patterns of the Yagi slot antenna of the embodiment of the present invention. 
       FIG. 3 , consisting of  FIG. 3A  and  FIG. 3B , is a view showing the directivity patterns of the Yagi slot antenna of the embodiment of the present invention. 
       FIG. 4 , consisting of  FIG. 4A  and  FIG. 4B , is a view illustrating a different configuration of the Yagi slot antenna of the embodiment of the present invention. 
       FIG. 5 , consisting of  FIG. 5A  and  FIG. 5B , is a view showing the directivity patterns of the Yagi slot antenna of the embodiment of the present invention. 
       FIG. 6 , consisting of  FIG. 6A  and  FIG. 6B , is a view showing the directivity patterns of the Yagi slot antenna of the embodiment of the present invention. 
       FIG. 7 , consisting of  FIG. 7A  and  FIG. 7B  is a view showing a configuration of a switch provided for the Yagi slot antenna of the embodiment of the present invention. 
       FIG. 8 , consisting of  FIG. 8A ,  8 B, and  8 C, is a view showing the directivity patterns of the Yagi slot antenna shown in  FIG. 7 . 
       FIG. 9 , consisting of  FIG. 9A ,  FIG. 9B ,  FIG. 9C , and  FIG. 9D , is a view showing a mechanism of a phase-difference feed antenna. 
       FIG. 10  is a view showing a structure of a multi-frequency antenna specified as the embodiment of the present invention. 
       FIG. 11 , consisting of  FIG. 11A ,  FIG. 11B ,  FIG. 11C ,  FIG. 11D , is a view showing the directivity patterns of the multi-frequency antenna of the embodiment of the present invention. 
       FIG. 12 , consisting of  FIG. 12A  and  FIG. 12B , is a view showing an electronic apparatus mounted with the Yagi slot antenna of the embodiment of the present invention. 
       FIG. 13  is a block diagram showing the configuration of a conventional phased array antenna. 
       FIG. 14  is a block diagram showing the configuration of a conventional adaptive array antenna. 
       FIG. 15 , consisting of  FIG. 15A  and  FIG. 15B , is a view showing the configuration of a conventional Yagi-Uda antenna. 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
   A description on a basic structure of an antenna apparatus specified as an embodiment of the present invention is hereinafter given. Incidentally, the embodiment of the present invention is described by taking a case of an antenna apparatus suitable to a wireless LAN (Local Area Network) in which a radio wave of 5.2 GHz band, for instance, is available. 
     FIG. 1A  is a view showing a configuration of a slot antenna that forms the basis of the antenna apparatus specified as the embodiment of the present invention. A slot antenna  1  shown in  FIG. 1A  has, at an approximately center position of a planar printed circuit board  2 , a driven element  11  given a feed, and before and behind the driven element  11 , parasitic elements  12  and  13  respectively given no feed. Then, the slot antenna  1  having the above configuration is supposed to be capable of radiating radio waves from the driven element  11 . 
   The driven element  11  is in the form of a slot (a slit) provided in a conductor (a ground plate)  2   a  formed at one surface side of the planar printed circuit board  2 , for instance. The driven element  11  is given the feed with a micro-strip transmission line  14  formed at the other surface side of the planar printed circuit board  2 . Each of the parasitic elements  12  and  13  is also in the form of a slot provided in the conductor  2   a  of the planar printed circuit board  2 , for instance. 
   In this case, a slot length (an electrical length) of the driven element  11  is specified as a length equivalent to a ½ wavelength (0.5 λg) of a transmitting/receiving frequency required for the slot antenna  1  to perform a transmission and a reception. 
   Each slot length (the electrical length) of the parasitic elements  12  and  13  is supposed to be larger than the slot length (0.5 λg) of the driven element  11 . Further, the driven element  11  and the parasitic elements  12  and  13  are spaced at intervals of about ¼ wavelength (0.25 λo, where λo represents a free space wavelength), respectively. 
   Then, the antenna apparatus of the embodiment of the present invention ensures that the antenna apparatus is configured using the slot antenna  1  having the above structure.  FIG. 1B  is a view showing the configuration of a Yagi slot antenna available as the antenna apparatus of the embodiment of the present invention. A Yagi slot antenna  10  shown in  FIG. 1B  sets the driven element  11  of the slot antenna  1  shown in  FIG. 1A  to function as a radiator  21  as it is. As to the parasitic element  12  similarly shown in  FIG. 1A , a function as a director  22  is provided by means of making the electrical length thereof equal to or slightly smaller than the electrical length (the ½ wavelength) of the radiator  21 . As to the parasitic element  13 , a function as a reflector  23  is provided by means of taking advantage of the electrical length larger than the electrical length of the driven element  11  as it is. Thus, a directivity of the Yagi slot antenna  10  of the embodiment of the present invention as shown in  FIG. 1B  is directed as shown by an arrow, that is, in a direction from the radiator  21  toward the director  22 . 
   Incidentally, in the present specification, the electrical length required to set the parasitic elements  12  and  13  to function as the director  22  is hereinafter referred to as a director length. Further, the electrical length required to set the parasitic elements  12  and  13  to function as the reflector  23  is referred to as a reflector length. Further, in the slot antenna, there is a change of a resonant frequency also depending on a dielectric constant of a board material of the planar printed circuit board  2 , so that each electrical length of the driven element  11  and the parasitic element  12  is determined in consideration of the dielectric constant etc. of the planar printed circuit board  2 . 
     FIGS. 2 and 3  are views showing directivity patterns of the Yagi slot antenna  10  shown in  FIG. 1B . Incidentally, each of the directivity patterns shown in  FIGS. 2 and 3  is assumed to be one obtained when the planar printed circuit board  2  has thereon the director  22 , the radiator  21  and the reflector  23  that are 2 mm in slot width and respectively 18 mm, 17 mm and 20.5 mm in slot length. Further, a FR-4 board formed with a glass epoxy resin having a planar size of 40 mm×40 mm, a thickness of 1 mm and a dielectric constant of 4.2 as a material is used for the planar printed circuit board  2 . Further, the directivity pattern shown in  FIG. 2B  is assumed to be one obtained when a length direction of the slot, a width direction of the slot and a thickness direction of the printed circuit board  2  are specified as a X-direction, a Y-direction and a Z-direction, respectively. 
   Analytic values and measured values of the directivity patterns of a horizontal polarized wave Eφ and a vertical polarized wave Eθ in a YZ-plane of the above Yagi slot antenna  10  are given as shown in  FIG. 2A , wherein it may be appreciated that the direction of the directivity undergoes a control by the director  22  and the reflector  23 . Incidentally, the measured value of an average gain in this case is assumed to be −6.05 dBi, and an average gain in a radial direction is assumed to be −1.16 dBi. 
   For reference, the analytic values and the measured values of the directivity patterns of the horizontal polarized wave Eφ and the vertical polarized wave Eθ in an XY-plane and an XZ-plane of the Yagi slot antenna  10  are given as shown in  FIG. 3A , and the respective average gains (the measured values) are assumed to be −9.14 dBi and −10.3 dBi. 
     FIG. 3B  is a view showing an input feature of the Yagi slot antenna  10  shown in  FIG. 1B , wherein it may be appreciated from the input feature in  FIG. 3B  that the Yagi slot antenna  10  causes a resonance with the length of the radiator  21  assumed to be about a ½ wavelength of the guide wavelength. 
   The Yagi slot antenna  10  of the embodiment of the present invention ensures that an antenna apparatus having different directions of the directivity is configured by taking advantage of the above slot antenna  1 .  FIG. 4A  is a view showing the slot antenna  1  that forms the basis of the Yagi slot antenna  10  specified as the embodiment of the present invention, wherein the above slot antenna  1  is supposed to have the same configuration as the slot antenna in  FIG. 1A . 
   The Yagi slot antenna  10  in this case sets the driven element  11  shown in  FIG. 4A  to function as the radiator  21  as it is, as shown in  FIG. 4B . In addition to the above, the function as the reflector  23  is provided by means of setting the electrical length of the parasitic element  12  at the reflector length, while the function as the director  22  is provided by means of setting the electrical length of the parasitic element  13  at the director length. 
   In other words, the Yagi slot antenna  10  shown in  FIG. 4B  is supposed to set the parasitic element  12  having been set to function as the director  22  in  FIG. 1B  to function as the reflector  23 , and the parasitic element  13  having been set to function as the reflector  23  to function as the director  22 . Thus, the directivity of the Yagi slot antenna  10  of the embodiment of the present invention shown in  FIG. 4B  is directed as shown by an arrow in  FIG. 4B , resulting in the opposite direction to that shown in  FIG. 1B . 
     FIGS. 5 and 6  are views showing the directivity patterns of the Yagi slot antenna  10  shown in  FIG. 4B . 
   Incidentally, each of the directivity patterns shown in  FIGS. 5 and 6  is also assumed to be one obtained when the planar printed circuit board  2  has thereon the director  22 , the radiator  21  and the reflector  23  that are 2 mm in slot width and respectively 18 mm, 17 mm and 20.5 mm in slot length. Further, the FR-4 board formed with the glass epoxy resin having the planar size of 40 mm×40 mm, the thickness of 1 mm and the dielectric constant of 4.2 as the material is also used for the planar printed circuit board  2 . Further, the directivity pattern shown in  FIG. 5B  is assumed to be one obtained when the length direction of the slot, the width direction of the slot and the thickness direction of the planar printed circuit board  2  are specified as the X-direction, the Y-direction and the Z-direction, respectively. 
   The analytic values and the measured values of the directivity patterns of the horizontal polarized wave Eφ and the vertical polarized wave Eθ in the YZ-plane of the above Yagi slot antenna  10  are given as shown in  FIG. 5A , wherein it may be also appreciated that the direction of the directivity undergoes the control by the director  22  and the reflector  23 . Incidentally, the measured value of the average gain in this case is assumed to be −6.80 dBi, and the average gain in the radial direction is assumed to be −1.08 dBi. 
   For reference, the analytic values and the measured values of the directivity patterns of the horizontal polarized wave Eφ and the vertical polarized wave Eθ in the XY-plane and the XZ-plane of the Yagi slot antenna shown in  FIG. 4B  are given as shown in  FIG. 6A , wherein the respective average gains are assumed to be −11.5 dBi and −7.39 dBi. 
     FIG. 6B  is a view showing the input feature of the Yagi slot antenna  10  shown in  FIG. 4B , wherein it may be also appreciated from the input feature in  FIG. 6B  that the Yagi slot antenna  10  causes the resonance with the length of the radiator  21  assumed to be about the ½ wavelength of the guide wavelength. 
   As described the above, the Yagi slot antenna  10  of the embodiment of the present invention, provided that the driven element  11  of the basic slot antenna  1  as shown in  FIG. 1A  ( FIG. 4A ) is set to function as the radiator  21 , performs a change of the electrical length of either of the parasitic elements  12  and  13  to set the parasitic element  12  to function as the director  22  and the parasitic element  13  to function as the reflector  23 , or on the contrary, the parasitic element  12  to function as the reflector  23  and the parasitic element  13  to function as the director  22 . 
   Thus, the embodiment of the present invention is provided with switches SW 1  and SW 2  as changing means at prescribed positions of the parasitic elements  12  and  13  to change each electrical length of the parasitic elements  12  and  13 , provided that each electrical length of the parasitic elements  12  and  13  is preliminarily set at the reflector length as shown in  FIG. 7A . Then, the change of each electrical length of the parasitic elements  12  and  13  from the reflector length to the director length is performed with the switches SW 1  and SW 2 . In this case, the switches SW 1  and SW 2  are supposed to be at positions where each electrical length of the parasitic elements  12  and  13  reaches the director length. 
     FIG. 7B  is a view showing the configuration of the switch SW used for the above Yagi slot antenna  10 . Incidentally, in  FIG. 7B , there is shown the switch SW 1  provided for the parasitic element  12 . The switch SW 1  shown in  FIG. 7B  is specified as a switch that has one end connected to the conductor  2   a  of the planar printed circuit board  2  and allows the other end to be switched over to either of an on state (a short-circuited state) making a connection to the conductor  2   a  and an off state (an open-circuited state) making no connection to the conductor  2   a . Then, when the switch SW 1  is placed in the short-circuited state, the electrical length of the parasitic element  12 , for instance, may be changed from the reflector length to the director length. Incidentally, an MMIC (Monolithic Microwave IC) switch or a MEMS (Micro Electro Mechanical System) switch is supposed to be available for the switch SW 1 . 
   As described the above, the embodiment of the present invention is provided with the switches SW 1  and SW 2  respectively at the prescribed positions of the parasitic elements  12  and  13  to ensure that the electrical length of either of the parasitic elements  12  and  13  is changed from the reflector length to the director length by the switches SW 1  and SW 2 . 
     FIG. 8  is a view showing the directivity patterns of the Yagi slot antenna  10  shown in  FIG. 7A . Specifically, in  FIG. 8A , there is shown the directivity pattern obtained when only the switch SW 2  of the parasitic element  13  is set to the on state, and in  FIG. 8B , there is shown the directivity pattern obtained when only the switch SW 1  of the parasitic element  12  is set to the on state. Incidentally, each of the directivity patterns in this case is also assumed to be one obtained when the planar printed circuit board  2  has thereon the parasitic element  12 , the driven element  11  and the parasitic element  13  that are 2 mm in slot width and respectively 20.5 mm, 17 mm and 20.5 mm in slot length, as shown in  FIG. 8C . The FR-4 board formed with the glass epoxy resin having the planar size of 40 mm×40 mm, the thickness of 1 mm and the dielectric constant of 4.2 as the material is also used for the planar printed circuit board  2 . Further, each of the directivity patterns shown in  FIGS. 8A and 8B  is assumed to be one obtained when the length direction of the slot, the width direction of the slot and the thickness direction of the planar printed circuit board  2  are specified as the X-direction, the Y-direction and the Z-direction, respectively. 
   It may be appreciated from the directivity pattern of the Yagi slot antenna  10  shown in  FIG. 8A  that a setting of only the switch SW 2  to the on state enables the directivity to be directed as shown by an arrow A in  FIG. 8C . Further, it may be also appreciated that the setting of only the switch SW 1  to the on state enables the directivity to be changed to a direction as shown by an arrow B in  FIG. 8C . That is, it may be understood that the setting of either of the switches SW 1  and SW 2  to the on state enables the directivity pattern to be changed. 
   According to the Yagi slot antenna of the embodiment of the present invention, the parasitic elements  12  and  13  may be used in common as the director or the reflector, so that the antenna apparatus having two different directivities may be configured with the single Yagi slot antenna  10 . That is, the use of the parasitic elements  12  and  13  in common as the director and the reflector makes it possible to realize the antenna apparatus being small-sized and having the two different directivities. 
   Further, the Yagi slot antenna  10  of the embodiment of the present invention eliminates the need to provide the switch SW for the driven element  11 , resulting in no degradation of a radiation feature of the radiator. In addition, the Yagi slot antenna  10  of the embodiment of the present invention also eliminates the need to provide the phase shifter, unlike the conventional phased array antenna shown in  FIG. 13 , resulting in no degradation of the radiation feature of the radiator as well from this point of view. 
   Furthermore, according to the Yagi slot antenna  10  of the embodiment of the present invention, the driven element  11  operative as the radiator and the parasitic elements  12  and  13  operative as the director or the reflector may be formed directly on the conductor  2   a  of the planar printed circuit board  2 , so that the antenna may reduce the thickness down to a level of a board thickness of the planar printed circuit board  2 . 
   Moreover, the parasitic elements  12  and  13  operative as the director or the reflector are supposed to be formed on the conductor  2   a  of the planar printed circuit board  2 , so that there is also an advantage of easily performing a packaging of components such as the switches SW 1  and SW 2  for changing each electrical length of the parasitic elements  12  and  13 . In addition, the use of the dielectric substrate ensures that the effect of shortening the wavelength is obtained, resulting in an advantage of attaining a downsizing. 
   By the way, the Yagi slot antenna  10  having been described above is merely effective in controlling the directivity pattern on a single frequency. A multi-frequency antenna capable of controlling the directivity pattern on more than one frequency is, however, desired to meet a great variety of radio communications in recent years. 
   For the above reason, in the embodiment of the present invention, the above Yagi slot antenna (a first antenna circuit) and a phase-difference feed antenna (a second antenna circuit) are configured to ensure that the multi-frequency antenna capable of controlling the directivity pattern on more than one frequency is realized. 
   Then, a mechanism of the phase-difference feed antenna employing a hybrid coupler is now described with reference to  FIG. 9 , in advance of a description on the multi-frequency antenna specified as the embodiment of the present invention. A 3 dB-hybrid coupler  41  shown in  FIG. 9A  is in the form of a four-terminal circuit, and an S-matrix thereof may be expressed as follows. 
   
     
       
         
           
             
               
                 
                   [ 
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                   Expression 
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   Thus, an entry of (1, 0) into input terminals t 1  and t 2  of the hybrid coupler  41  shown in  FIG. 9A  is supposed to provide a phase difference of 90 degrees between output terminals t 3  and t 4  at an amplitude equal to [Expression 2]
 
(1,0)           (1/√{square root over (2)}, − j/ √{square root over (2)}).
 
Further, the entry of (0, 1) into the input terminals t 1  and t 2  is supposed to allow the output terminals t 3  and t 4  to invert phases to [Expression 3]
 
(0,1)         (− j/ √{square root over (2)}, 1/√{square root over (2)}).

   The use of a phase inversion of 90 degrees as described above enables the switching of the directivity to be performed, in which case, the phase inversion of two monopole antennas a and b spaced at an interval of ¼ λ as shown in  FIG. 9B , for instance, is supposed to provide the directivity in an xy-plane as follows. 
   
     
       
         
           
             
               
                 
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   The above directivity is in the form of two Cardioid patterns symmetrical with respect to a y-axis to ensure that an inverted directivity with respect to the y-axis is obtained as shown in  FIG. 9C . The phases of the monopole antennas a and b are switched over by the 3 dB-hybrid coupler  41 , so that a two-way switching of the beams is made possible. 
   While the two-way switching is supposed to be attainable with the 3 dB-hybrid coupler  41  and a non-directional antenna, the use of the directivity of the antenna contained in an antenna array may lead to a four-way switching of beams. 
   When four micro current elements each having a figure-8 pattern within a horizontal plane, for instance, are arranged as shown in  FIG. 9D , an excitation of the above elements with two 3 dB-hybrid couplers  41   a  and  41   b  is supposed to enable the four-way switching of the beams to be performed within the horizontal plane. 
     FIG. 10  shows a structure of the multi-frequency antenna specified as the embodiment of the present invention. A multi-frequency antenna  30  of the embodiment of the present invention as shown in  FIG. 10  has an antenna element  31  at the approximately center position of the planar printed circuit board  2 , and antenna elements  32  and  33  before and behind the antenna element  31 . The antenna element  31  is connected to a first feed unit  34  and is given the feed from the first feed unit  34 . One end of the antenna element  32  is connected to a second feed unit  35  to ensure that the feed is given with the second feed unit  35 . One end of the antenna element  33  is connected to a third feed unit  36  to ensure that the feed is given with the third feed unit. In this case, the slot length of the antenna element  31  is specified as the length equivalent to the ½ wavelength of the transmitting/receiving frequency. Further, each slot length of the antenna elements  32  and  33  is supposed to be larger than that of the antenna element  31 . 
   The antenna element  32  has switches SW 1  and SW 2 . Further, the antenna element  33  has switches SW 3  and SW 4 . The antenna element  31  and the antenna elements  32  and  33  are spaced at intervals of about ¼ wavelength respectively. 
   In the multi-frequency antenna  30  of the above configuration, when setting this antenna to operate at a first frequency F 1  of 5.2 GHz band, for instance, the feed from the first feed unit  34  only to the antenna element  31  is firstly performed. That is, only the antenna element  31  is set to function as the driven element (the radiator), while the antenna elements  32  and  33  are set as the parasitic elements. Then, a control of the switches SW 1  and SW 2  of the antenna element  32  or the switches SW 3  and SW 4  of the antenna element  33  is performed to control the electrical length of either of the antenna elements  32  and  33  to reach the director length. Thus, the antenna apparatus having the two-way directivity at the first frequency F 1  may be realized by setting the multi-frequency antenna  30  of the embodiment of the present invention to operate like the Yagi slot antenna  10  shown in  FIG. 7A . 
   On the contrary, when setting the multi-frequency antenna  30  of the embodiment of the present invention to operate at a second frequency F 2  of 2.45 GHz band, for instance, the feed from the second feed unit  35  and the third feed unit  36  is performed at different phases (0 degree and 90 degrees), provided that the switches SW 1  to SW 4  are placed in the open-circuited state. With this operation, the multi-frequency antenna  30  of the embodiment of the present invention may be set to operate as the above phase-difference feed antenna for reason that the antenna elements  32  and  33  are spaced at a fixed interval, thereby providing the antenna apparatus having the two-way directivity also at the second frequency F 2 . 
   That is, according to the multi-frequency antenna  30  of the embodiment of the present invention, the control of the directivity pattern of the radio waves at two different frequency bands of the first frequency F 1  and the second frequency F 2  may be ensured. 
   Further, in this case, the antenna elements  32  and  33  may be used in common as the parasitic element in the Yagi slot antenna and a radiation element in the phase-difference feed antenna, so that there is also an advantage of attaining the downsizing of the multi-frequency antenna. 
     FIG. 11  shows the directivity patterns of the multi-frequency antenna of the embodiment of the present invention shown in  FIG. 10 . It may be appreciated that when using the multi-frequency antenna  30  at the first frequency F 1 , the directivity of the multi-frequency antenna is made controllable by setting the switches SW 1  and SW 2  of the antenna element  32  to a short-circuited position (the short-circuited state) and the switches SW 3  and SW 4  of the antenna element  33  to an opened position (the open-circuited state) or by changing over the switches SW 1  and SW 2  of the antenna element  32  to the opened position (the open-circuited state) and the switches SW 3  and SW 4  of the antenna element  33  to the short-circuited position (the short-circuited state), as shown in  FIGS. 11A and 11B . 
   It may be also appreciated that when using the multi-frequency antenna  30  of the embodiment of the present invention at the second frequency F 2 , the directivity pattern of the multi-frequency antenna is made controllable by performing the feed, with the second feed unit  35  set to have the phase of 90 degrees and the third feed unit  36  set to have the phase of 0 degree or on the contrary, with the second feed unit  35  set to have the phase of 0 degree and the third feed unit  36  set to have the phase of 90 degrees, as shown in  FIGS. 11C and 11D . 
   Thus, a mounting of the multi-frequency antenna  30  of the embodiment of the present invention in an apparatus body  52  of a wireless LAN base station apparatus  51  available at any place irrespective of indoor and outdoor places as shown in  FIG. 12A , in a mobile information terminal  53  such as a notebook-sized personal computer as shown in  FIG. 12B  or in a non-illustrated wireless television receiver makes it possible to realize the multi-frequency antenna adaptive to more than one radio communication. Further, the multi-frequency antenna in this case enables the control of the directivity, leading to a possibility of restraining the degradation of the communication quality with the interference wave caused by the reflection from the wall etc. 
   Further, while the multi-frequency antenna  30  of the embodiment of the present invention limits the number of the antenna elements  32  and  33  available also as the director or the reflector to one, respectively, this is merely given as one instance, and it is also allowable to form each of the antenna elements  32  and  33  with more than one antenna element. Furthermore, while the embodiment of the present invention has been described by taking the case of the antenna configured on the basis of the slot antenna, it is a matter of course that the above antenna may be also configured on the basis of antennas other than the slot antenna.