Patent Publication Number: US-8525748-B2

Title: Variable directivity antenna apparatus provided with antenna elements and at least one parasitic element connected to ground via controlled switch

Description:
TECHNICAL FIELD 
     The present invention relates to a variable directivity antenna apparatus for use in a wireless communication system employing, for example, a MIMO (Multiple Input Multiple Output) wireless method. 
     BACKGROUND ART 
     Up to now, various array antenna apparatuses have been proposed as variable directivity antenna apparatuses for use in a wireless communication system employing, for example, the MIMO wireless method (See Patent Documents 1 and 2, for example). 
     The Patent Document 1 discloses an array antenna apparatus, which has a structure simpler than that of an antenna according to prior art and can easily form an excitation element and parasitic elements. The array antenna apparatus is characterized as follows. At least one dielectric substrate on which at least one of a plurality of parasitic elements is provided around an excitation element. Alternatively, the array antenna apparatus includes the excitation element and a first dielectric substrate on which at least one of the plurality of parasitic elements is formed, and at least one second dielectric substrate is provided around the excitation element, where at least one further parasitic element among the plurality of parasitic element is formed on the second dielectric substrate. 
     In addition, the Patent Document 2 proposes an antenna apparatus which can control directivity or omni-directivity, radiation polarization, and a radiation direction of the antenna apparatus to provide a desired state without increasing size and cost of the antenna apparatus, by devising a structure of each antenna element. The antenna apparatus includes a conductive excitation element, parasitic elements each made of semiconductive plastics, and control electrodes connected to these parasitic elements, respectively, where the conductive excitation element and the parasitic elements have predetermined lengths and arranged on a dielectric substrate, respectively. Direct-current bias voltages supplied to the control electrodes are controlled to change over the parasitic elements to have insulating properties or conductive properties. The antenna apparatus is characterized as follows. Two parasitic elements changed over to have the conductive properties are combined to configure a directional antenna apparatus including a wave director, a reflector and the like. In addition, the wave director and the reflector other than this excitation element (feeder) are made to have the insulating properties to configure an omni-directional antenna apparatus. 
     CITATION LIST 
     Patent Document 
     Patent Document 1: Japanese patent laid-open publication No. JP-2002-261532-A. 
     Patent Document 2: Japanese patent laid-open publication No. JP-2007-013692-A. 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, in all the environments, causes for unstable wireless communication are roughly classified into two problems. 
     The first problem is that an electric field level is low because of a too long distance between wireless apparatuses in a case of a predetermined outputted power of a radio wave. In regard of this problem, it is possible to receive the radio wave with a stable electric field level by configuring at least one of antenna elements of a base station and a terminal to have directivity and by orienting the directivity to the antenna element of the other party. 
     The second problem is that fading occurs in a band required for communication due to interference of reflected waves from walls and a ceiling. In this case, the problem becomes a severe one at a location where a level difference between a direct radio wave and the reflected wave is very small. Therefore, in a manner similar to that of the first problem, the interference can be suppressed by configuring an antenna element to have directivity so as not to receive radio waves other than a desired wave. This method is effective when SISO (Single Input Single Output) is employed and antenna selection diversity for changing over antenna elements of a receiver side is adopted. However, this method causes a problem when the receiver side executes MRC (Maximum Ratio Combination) processing instead of simply adopting the antenna selection diversity. For example, in a case of an OFDM (Orthogonal Frequency Division Multiplex) wireless communication system typified by IEEE802.11a/g Standards, when one of two antenna elements each having directivity receives a direct wave and another antenna element receives a reflected wave having a delay time longer than an assumed time of a guard interval of the direct wave, a signal deteriorates in a desired band. 
     In this case, the MIMO wireless communication method typified by IEEE802.11n Standards is provided for increasing a communication rate greatly by receiving a radio wave via a plurality of antennas and decomposing the radio wave into a plurality of streams according to propagation channels generated from path differences among the antennas. Namely, the MIMO wireless communication method positively uses propagation path differences among antenna elements. Generally speaking, a wireless apparatus employing this MIMO wireless communication method uses a plurality of omni-directional antennas such as dipole antennas or sleeve antennas. In this case, when the antennas are not away from each other by one wavelength or longer, correlation among the antennas becomes large, it is not possible to generate propagation channels enough to ensure a transmission quality. In addition, there has been known a method of reducing this antenna correlation by tilting respective antenna elements in directions different from each other to provide a combination of different polarized waves. However, this method has such a mounting problem that it is required to tilt the antenna elements physically. 
     In any case, there is such a problem that an antenna apparatus of a wireless apparatus employing the MIMO wireless method cannot be generally made small in size at present. 
     It is an object of the present invention to provide a variable directivity antenna apparatus capable of solving the above described problems, and capable of reducing the size thereof and improving a transmission quality of MIMO wireless method by making it possible to shorten the inter-element distance greatly, in the environment in which the fading tends to occur because of many reflected waves. 
     SOLUTION TO PROBLEM 
     A variable directivity antenna apparatus according to the present invention includes a first parasitic element, a plurality of antenna elements each provided in proximity to the first parasitic element so as to be electromagnetically coupled to the first parasitic element, first switch means connected to the first parasitic element, and changing over whether or not to ground the first parasitic element, and controller means. The controller means changes a radiation pattern from the variable directivity antenna apparatus by outputting a control signal for turning on or off the first switch means to change over whether or not the first parasitic element operates as a reflector. 
     The above-mentioned variable directivity antenna apparatus includes two antenna elements. 
     In addition, the above-mentioned variable directivity antenna apparatus further includes at least one second parasitic element each provided in proximity to the respective antenna elements so as to be electromagnetically coupled to the respective antenna elements, and at least one second switch means connected to the at least one second parasitic element, and changing over whether or not to ground each of the second parasitic elements. The controller means outputs a further control signal for selectively turning on or off each of the switch means to selectively change over whether or not each of the parasitic elements operates as a reflector. 
     Further, the above-mentioned variable directivity antenna apparatus includes two antenna elements and one second parasitic element. 
     Still further, the above-mentioned variable directivity antenna apparatus includes two antenna elements and four second parasitic elements. 
     In addition, the above-mentioned variable directivity antenna apparatus includes three antenna elements and three second parasitic elements. 
     Further, the above-mentioned variable directivity antenna apparatus includes four antenna elements and four second parasitic elements. 
     Still further, in the above-mentioned variable directivity antenna apparatus, each of the antenna elements is provided to be away from the first parasitic element by an electrical length of a quarter-wavelength. 
     In addition, in the above-mentioned variable directivity antenna apparatus, each of the antenna elements is provided to be away from the first parasitic element by an electrical length of a quarter-wavelength, and each of the second parasitic elements is provided to be away from each of the antenna elements by an electrical length of a quarter-wavelength. 
     Further, in the above-mentioned variable directivity antenna apparatus, each of the switch means is a PIN diode connected between each of the parasitic element and a ground conductor. 
     ADVANTAGEOUS EFFECTS OF INVENTION 
     Therefore, in the variable directivity antenna apparatus according to the present invention, the distance between each antenna element and each parasitic element is set so that the antenna element is electromagnetically coupled to the parasitic element. The variable directivity antenna apparatus includes the controller means for changing a radiation pattern from the variable directivity antenna apparatus by outputting a control signal for turning on or off the first switch means to change over whether or not the first parasitic element operates as a parasitic element. Therefore, it is possible to selectively change radiation pattern from the variable directivity antenna apparatus, and orient a main beam of the radiation pattern to a desired direction. Due to this configuration, it is possible to greatly shorten the inter-element distance in the environment in which the fading tends to occur because of many reflected waves, and this leads to the variable directivity antenna apparatus which has a small size and can improve a transmission quality of the MIMO wireless method. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1A  is a plan view showing a configuration of a variable directivity antenna apparatus  21  according to a first preferred embodiment of the present invention; 
         FIG. 1B  is a side view of the variable directivity antenna apparatus  21  of  FIG. 1A ; 
         FIG. 2  is a perspective view of the variable directivity antenna apparatus  21  of  FIGS. 1A and 1B ; 
         FIG. 3  is a block diagram showing a configuration of a wireless communication apparatus  20  using the variable directivity antenna apparatus  21  of  FIGS. 1A and 1B ; 
         FIG. 4  is a circuit diagram showing a configuration of a control circuit  30  for each of parasitic elements  12   a  to  12   d  of  FIGS. 1A and 1B ; 
         FIG. 5  is a diagram of radiation pattern characteristics in an XY plane, showing simulation results of the variable directivity antenna apparatus  21  of  FIGS. 1A and 1B  when the parasitic element  12   a  is turned off, the parasitic element  12   b  is turned off, the parasitic element  12   c  is turned off, and the parasitic element  12   d  is turned off; 
         FIG. 6  is a diagram of radiation pattern characteristics in the XY plane, showing simulation results of the variable directivity antenna apparatus  21  of  FIGS. 1A and 1B  when the parasitic element  12   a  is turned on, the parasitic element  12   b  is turned off, the parasitic element  12   c  is turned off, and the parasitic element  12   d  is turned off; 
         FIG. 7  is a diagram of radiation pattern characteristics in the XY plane, showing simulation results of the variable directivity antenna apparatus  21  of  FIGS. 1A and 1B  when the parasitic element  12   a  is turned off, the parasitic element  12   b  is turned on, the parasitic element  12   c  is turned off, and the parasitic element  12   d  is turned off; 
         FIG. 8  is a diagram of radiation pattern characteristics in the XY plane, showing simulation results of the variable directivity antenna apparatus  21  of  FIGS. 1A and 1B  when the parasitic element  12   a  is turned on, the parasitic element  12   b  is turned on, the parasitic element  12   c  is turned off, and the parasitic element  12   d  is turned off; 
         FIG. 9  is a diagram of radiation pattern characteristics in the XY plane, showing simulation results of the variable directivity antenna apparatus  21  of  FIGS. 1A and 1B  when the parasitic element  12   a  is turned off, the parasitic element  12   b  is turned off, the parasitic element  12   c  is turned on, and the parasitic element  12   d  is turned off; 
         FIG. 10  is a diagram of radiation pattern characteristics in the XY plane, showing simulation results of the variable directivity antenna apparatus  21  of  FIGS. 1A and 1B  when the parasitic element  12   a  is turned on, the parasitic element  12   b  is turned off, the parasitic element  12   c  is turned on, and the parasitic element  12   d  is turned off; 
         FIG. 11  is a diagram of radiation pattern characteristics in the XY plane, showing simulation results of the variable directivity antenna apparatus  21  of  FIGS. 1A and 1B  when the parasitic element  12   a  is turned off, the parasitic element  12   b  is turned on, the parasitic element  12   c  is turned on, and the parasitic element  12   d  is turned off; 
         FIG. 12  is a diagram of radiation pattern characteristics in the XY plane, showing simulation results of the variable directivity antenna apparatus  21  of  FIGS. 1A and 1B  when the parasitic element  12   a  is turned on, the parasitic element  12   b  is turned on, the parasitic element  12   c  is turned off, and the parasitic element  12   d  is turned off; 
         FIG. 13  is a diagram of radiation pattern characteristics in the XY plane, showing simulation results of the variable directivity antenna apparatus  21  of  FIGS. 1A and 1B  when the parasitic element  12   a  is turned off, the parasitic element  12   b  is turned off, the parasitic element  12   c  is turned off, and the parasitic element  12   d  is turned on; 
         FIG. 14  is a diagram of radiation pattern characteristics in the XY plane, showing simulation results of the variable directivity antenna apparatus  21  of  FIGS. 1A and 1B  when the parasitic element  12   a  is turned on, the parasitic element  12   b  is turned off, the parasitic element  12   c  is turned off, and the parasitic element  12   d  is turned on; 
         FIG. 15  is a diagram of radiation pattern characteristics in the XY plane, showing simulation results of the variable directivity antenna apparatus  21  of  FIGS. 1A and 1B  when the parasitic element  12   a  is turned off, the parasitic element  12   b  is turned on, the parasitic element  12   c  is turned off, and the parasitic element  12   d  is turned on; 
         FIG. 16  is a diagram of radiation pattern characteristics in the XY plane, showing simulation results of the variable directivity antenna apparatus  21  of  FIGS. 1A and 1B  when the parasitic element  12   a  is turned on, the parasitic element  12   b  is turned on, the parasitic element  12   c  is turned off, and the parasitic element  12   d  is turned on; 
         FIG. 17  is a diagram of radiation pattern characteristics in the XY plane, showing simulation results of the variable directivity antenna apparatus  21  of  FIGS. 1A and 1B  when the parasitic element  12   a  is turned off, the parasitic element  12   b  is turned off, the parasitic element  12   c  is turned on, and the parasitic element  12   d  is turned on; 
         FIG. 18  is a diagram of radiation pattern characteristics in the XY plane, showing simulation results of the variable directivity antenna apparatus  21  of  FIGS. 1A and 1B  when the parasitic element  12   a  is turned on, the parasitic element  12   b  is turned off, the parasitic element  12   c  is turned on, and the parasitic element  12   d  is turned on; 
         FIG. 19  is a diagram of radiation pattern characteristics in the XY plane, showing simulation results of the variable directivity antenna apparatus  21  of  FIGS. 1A and 1B  when the parasitic element  12   a  is turned off, the parasitic element  12   b  is turned on, the parasitic element  12   c  is turned on, and the parasitic element  12   d  is turned on; 
         FIG. 20  is a diagram of radiation pattern characteristics in the XY plane, showing simulation results of the variable directivity antenna apparatus  21  of  FIGS. 1A and 1B  when the parasitic element  12   a  is turned on, the parasitic element  12   b  is turned on, the parasitic element  12   c  is turned on, and the parasitic element  12   d  is turned on; 
         FIG. 21  is a plan view showing a configuration of a variable directivity antenna apparatus  21 A according to a second preferred embodiment of the present invention; 
         FIG. 22  is a plan view showing a configuration of a variable directivity antenna apparatus  21 B according to a third preferred embodiment of the present invention; 
         FIG. 23A  is a plan view showing a configuration of a variable directivity antenna apparatus  21 C according to a fourth preferred embodiment of the present invention; 
         FIG. 23B  is a side view of the variable directivity antenna apparatus  21 C of  FIG. 23A ; 
         FIG. 24A  is a plan view showing a configuration of a variable directivity antenna apparatus  21 D according to a fifth embodiment of the present invention; and 
         FIG. 24B  is a side view of the variable directivity antenna apparatus  21 D of  FIG. 24A . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Preferred embodiments according to the present invention will be described below with reference to the attached drawings. Components similar to each other are denoted by the same reference numerals and will not be described herein in detail. 
     First Preferred Embodiment 
       FIG. 1A  is a plan view showing a configuration of a variable directivity antenna apparatus  21  according to a first preferred embodiment of the present invention.  FIG. 1B  is a side view of the variable directivity antenna apparatus  21 .  FIG. 2  is a perspective view of the variable directivity antenna apparatus  21  of  FIGS. 1A and 1B . 
     In the variable directivity antenna apparatus according to the present preferred embodiment, a parasitic element  12   a,  an antenna element  11   a,  a parasitic element  12   d,  an antenna element  11   c,  a parasitic element  12   c,  an antenna element  11   b,  and a parasitic element  12   b  are provided on a dielectric substrate  10  having a back surface on which a ground conductor  13  is formed. The antenna element  11   a,  the parasitic element  12   d,  the antenna element  11   c,  the parasitic element  12   c,  the antenna element  11   b,  and the parasitic element  12   b  are arranged on a circumference of a circle in a clockwise order so as to be located at vertexes of a regular hexagon, respectively, where the circle has a radius of “d” and a center at which a parasitic element  12   a  is located. Each of the elements  11   a  to  11   c  and  12   a  to  12   d  has a circular patch antenna having a predetermined circumferential length and provided at a top portion thereof, and is supported by a support member  14  that has a feeding line and the like to the dielectric substrate  10  therein. It is to be noted that each of the elements  11   a  to  11   c  and  12   a  to  11   d  may be, for example, a quarter-wavelength whip antenna. In this case, an inter-element spacing “d” is set to 14 mm, which corresponds to an electrical length of about a quarter-wavelength (λ/4) for an operating frequency of 5.2 GHz so that the antenna element and the parasitic element adjacent to each other are electromagnetically coupled to each other. When communication is to be held in a 2.4 GHz band, it suffices to set the spacing to an electrical length of about 31 mm. As will be described later in detail, in the variable directivity antenna apparatus  21  configured as described above, it is possible to form a total of 16 (=2 4 ) directional patterns by turning on or off control signals for the four parasitic elements  12   a  to  12   d,  respectively. 
       FIG. 3  is a block diagram showing a configuration of a wireless communication apparatus  20  using the variable directivity antenna apparatus  21  of  FIGS. 1A and 1B .  FIG. 4  is a circuit diagram showing a configuration of a control circuit  30  for each of the parasitic elements  12   a  to  12   d  of  FIGS. 1A and 1B . Referring to  FIG. 3 , the wireless communication apparatus according to the present preferred embodiment is configured by including the variable directivity antenna apparatus  21  of  FIGS. 1A ,  1 B and  2 , three wireless transceiver circuits  22   a,    22   b,  and  22   c,  a MIMO modulator and demodulator circuit  23 , a baseband signal processing circuit  24 , a MAC (Media Access Control) circuit  26 , and a controller  25  for controlling the variable directivity antenna apparatus  21  and these circuits. In this case, each of the wireless transceiver circuits  22   a,    22   b,  and  22   c  is configured by including a duplexer, a wireless transmitter circuit, and a wireless receiver circuit. Using a well-known MIMO modulation and demodulation method, the MIMO modulator and demodulator circuit  23  executes a modulation processing on wireless signals transmitted by the three antenna elements  11   a  to  11   c  and the wireless transceiver circuits  22   a  to  22   c,  and executes a demodulation processing on wireless signals received by the three antenna elements  11   a  to  11   c  and the wireless transceiver circuits  22   a  to  22   c.  The baseband signal processing circuit  24  is connected to the MIMO modulator and demodulator circuit  23  and the MAC circuit  26 , executes a predetermined baseband signal processing on a data signal inputted from the MAC circuit  26 , and outputs a processed data signal to the MIMO modulator and demodulator circuit  23 . The baseband signal processing circuit  24  also executes a predetermined baseband signal processing on a demodulated signal from the MIMO modulator and demodulator circuit  23 , and outputs a processed demodulated signal to the MAC circuit  26 . The MAC circuit  26  generates a predetermined data signal by executing a predetermined signal processing for the MAC, and outputs a generated predetermined data signal to the baseband signal processing circuit  24 . The MAC circuit  26  inputs the data signal from the baseband signal processing circuit  24 , and executes a predetermined MAC processing on the data signal. 
     In the variable directivity antenna apparatus  21 , the antenna elements  11   a,    11   b,  and  11   c  are connected to the wireless transceiver circuits  22   a,    22   b,  and  22   c,  respectively. Each of the parasitic elements  12   a,    12   b,    12   c,  and  12   d  has the control circuit  30  of  FIG. 4 . Control signals for the parasitic elements  12   a,    12   b,    12   c,  and  12   d  are supplied to the respective control circuits  30  from the controller  25 . Referring to  FIG. 4 , each of the parasitic elements  12   a,    12   b,    12   c,  and  12   d  is connected to a connection point  36  via an impedance matching capacitor  33 . The connection point  36  is connected to a control signal input terminal  31  via a high frequency blocking inductor  32  having impedance high enough at the operating frequency, and an anode of a PIN diode  34 . A cathode of the PIN diode  34  is grounded via an inductor  35  for changing an electrical length of the parasitic element. By inputting a control signal having a predetermined positive direct-current voltage to the control signal input terminal  31 , the PIN diode  34  is turned on, and each of the parasitic elements  12   a,    12   b,    12   c,  and  12   d  operates as a parasitic element (reflector) having an electrical length longer than those of the antenna elements  11   a,    11   b,  and  11   c.  On the other hand, by inputting a control signal representing off and having, for example, a ground potential to the control signal input terminal  31 , the PIN diode  34  is turned off, and each of the parasitic elements  12   a,    12   b,    12   c,  and  12   d  does not operate as a parasitic element. Namely, the PIN diodes  34  operate as a plurality of switch means for changing over whether or not to ground the parasitic elements  12   a,    12   b,    12   c,  and  12   d,  respectively. 
       FIGS. 5 to 20  are diagrams of radiation pattern characteristics in an XY plane, showing simulation results of the variable directivity antenna apparatus  21  of  FIGS. 1A and 1B  when each of the parasitic element  12   a  to  12   d  is turned on or off. As apparent from  FIGS. 5 to 20 , by turning on or off each of the control signals corresponding to the four parasitic elements  12   a  to  12   d,  respectively, it is possible to form a total of 16 (=2 4 ) directional patterns by the variable directivity antenna apparatus  21 . Therefore, it is possible to change the radiation pattern of the wireless signal radiated from the variable directivity antenna apparatus  21 , and it is possible to orient a main beam direction to a desired direction. In particular, when the parasitic elements  12   b,    12   c,  and  12   d  are turned on, respectively, directivities of radiation from the antenna apparatus  21  are oriented to directions different from one another. Therefore, interference among the antenna elements is reduced, and a correlation value becomes smaller. 
     The wireless communication apparatus  20  including the variable directivity antenna apparatus  21 , and configured as described above can solve the following two problems. 
     First of all, even when the fading occurs in a band due to the reflected waves from the walls and the ceiling, it is possible to hold more effective MIMO wireless communication, by configuring so that one of the two antenna elements (two antenna elements selected from among the antenna elements  11   a,    11   b,  and  11   c ) receives a direct wave, and so that another antenna element receives a reflected wave having a longer delay time. 
     Secondly, it is possible to adjust an intensity of a signal inputted to the wireless receiver circuit of each of the wireless transceiver circuits  22   a  to  22   c  to some extent. Generally speaking, the wireless receiver circuit should lead in a signal using AGC (Auto Gain Control) at a preamble part of a packet. Therefore, in the wireless communication apparatus that receives signals simultaneously in a manner such as the MIMO communication method, it is difficult to execute the AGC on each of the wireless receiver circuits individually. In order to prevent signal saturation, the gain should be adjusted according to the largest signal level. For this reason, it is difficult to secure a signal having a small intensity in an environment in which received levels are different from each other greatly. In the present preferred embodiment, it is possible to adjust the intensities of signals to a uniform intensity to some extent by changing over directional patterns of the antenna apparatus. Therefore, even in the environment in which the received levels are greatly different from each other, the present preferred embodiment can exhibit the same advantageous effects. In addition, for this AGC problem, not only in the MIMO wireless communication apparatus, but also in a wireless communication apparatus receiving a plurality of wireless signals simultaneously such as a wireless communication apparatus performing the MRC (Maximum Ratio Combination) processing as described above, the advantageous effects similar to above can be exhibited. 
     Further, the other advantageous effects of the present preferred embodiment are as follows. The number of feeding paths to each of the antenna elements  11   a  to  11   c  is one per antenna element. Therefore, as compared with the selection diversity method of changing over antenna elements while preparing a plurality of antenna elements, the number of feeding paths can be reduced even when the antenna elements are connected to a wireless apparatus using a coaxial cable or a high frequency connector. The wireless communication apparatus  20  exhibits such an advantageous effect that it can be manufactured with a low cost. 
     Second Preferred Embodiment 
       FIG. 21  is a plan view showing a configuration of a variable directivity antenna apparatus  21 A according to a second preferred embodiment of the present invention. In the variable directivity antenna apparatus according to the present preferred embodiment, four parasitic elements  70 ,  71 ,  72 ,  73 , and  74 , and antenna elements  61 ,  62 ,  63 , and  64  are provided on the dielectric substrate  10  having the back surface on which the ground conductor  13  is formed. The parasitic elements  71 ,  72 ,  73 , and  74  are located at vertexes of a square, respectively, where the square has a center at which the parasitic element  70  is located. The antenna elements  61 ,  62 ,  63 , and  64  are located at midpoints of pairs of adjacent parasitic elements (midpoints of respective sides of the square), respectively. In this case, a distance between each antenna element and each of the parasitic elements adjacent to the antenna element is set to a distance “d” of a quarter-wavelength, so that the antenna element is electromagnetically coupled to the parasitic elements adjacent to the antenna element. It is to be noted that each of the parasitic elements  70  to  74  includes the control circuit  30  of  FIG. 4 . 
     According to the present preferred embodiment configured as described above, it is possible to configure the variable directivity antenna apparatus  21 A using the four antenna elements  61  to  64 , and the five parasitic elements  70  to  74 . The variable directivity antenna apparatus  21 A can be configured in a manner similar to that of the wireless communication apparatus according to the first preferred embodiment of  FIG. 3  except for the number of circuits connected to the antenna elements  61  to  64  and the number of control signals inputted to the parasitic elements  70  to  74 , and can exhibit the action and advantageous effects similar to those according to the first preferred embodiment. 
     Third Preferred Embodiment 
       FIG. 22  is a plan view showing a configuration of a variable directivity antenna apparatus  21 B according to a third preferred embodiment of the present invention. The configuration of the variable directivity antenna apparatus  21 B according to the present preferred embodiment is characterized by eliminating the antenna elements  63  and  64 , as compared with that of the variable directivity antenna apparatus  21 A of  FIG. 21 . 
     According to the present preferred embodiment configured as described above, it is possible to configure the variable directivity antenna apparatus  21 B using the two antenna elements  61  and  62 , and the five parasitic elements  70  to  74 . The variable directivity antenna apparatus  21 B can be configured in a manner similar to that of the wireless communication apparatus according to the first preferred embodiment of  FIG. 3  except for the number of circuits connected to the antenna elements  61  and  62  and the number of control signals inputted to the parasitic elements  70  to  74 , and can exhibit the action and advantageous effects similar to those according to the first preferred embodiment. 
     Fourth Preferred Embodiment 
       FIG. 23A  is a plan view showing a configuration of a variable directivity antenna apparatus  21 C according to a fourth preferred embodiment of the present invention.  FIG. 23B  is a side view of the variable directivity antenna apparatus  21 C of  FIG. 23A . The variable directivity antenna apparatus  21 C according to the present preferred embodiment includes two antenna elements  11   b  and  11   d  and one parasitic element  12   a.  The antenna elements  11   b  and  11   d  and one parasitic element  12   a  are arranged on a Y-axis. In this case, a distance between the antenna element  11   b  and the parasitic element  12   a,  and a distance between the antenna element  11   d  and the parasitic element  12   a  are set to a distance “d” of a quarter-wavelength, respectively. In addition, the parasitic element  12   a  includes the control circuit  30  of  FIG. 4 . 
     According to the present preferred embodiment configured as described above, it is possible to configure the variable directivity antenna apparatus  21 C using the two antenna elements  11   b  and  11   d,  and one parasitic element  12   a.  The variable directivity antenna apparatus  21 C can be configured in a manner similar to that of the wireless communication apparatus according to the first preferred embodiment of  FIG. 3  except for the number of circuits connected to the antenna elements  11   b  and  11   d  and the number of control signals inputted to the parasitic element  12   a,  and can exhibit the action and advantageous effects similar to those according to the first preferred embodiment. 
     Fifth Preferred Embodiment 
       FIG. 24A  is a plan view showing a configuration of a variable directivity antenna apparatus  21 D according to a fifth preferred embodiment of the present invention.  FIG. 24B  is a side view of the variable directivity antenna apparatus  21 D of  FIG. 24A . The configuration of the variable directivity antenna apparatus  21 D according to the present embodiment is characterized by eliminating the antenna element  11   b  and the parasitic elements  12   b  and  12   c,  as compared with that of the variable directivity antenna apparatus  21  of  FIG. 1A . 
     According to the present preferred embodiment configured as described above, it is possible to configure the variable directivity antenna apparatus  21 D using the two antenna elements  11   a  and  11   c,  and the two parasitic elements  12   a  and  12   d.  The variable directivity antenna apparatus  21 D can be configured in a manner similar to that of the wireless communication apparatus according to the first preferred embodiment of  FIG. 3  except for the number of circuits connected to the antenna elements  11   a  and  11   c  and the number of control signals inputted to the parasitic elements  12   a  and  12   d,  and can exhibit the action and advantageous effects similar to those according to the first preferred embodiment. 
     Industrial Applicability 
     As described above in detail, in the variable directivity antenna apparatus according to the present invention, the distance between each antenna element and each parasitic element is set so that the antenna element is electromagnetically coupled to the parasitic element. The variable directivity antenna apparatus includes the controller means for changing a radiation pattern from the variable directivity antenna apparatus by outputting a control signal for turning on or off the first switch means to change over whether or not the first parasitic element operates as a parasitic element. Therefore, it is possible to selectively change radiation pattern from the variable directivity antenna apparatus, and orient a main beam of the radiation pattern to a desired direction. Due to this configuration, it is possible to greatly shorten the inter-element distance in the environment in which the fading tends to occur because of many reflected waves, and this leads to the variable directivity antenna apparatus which has a small size and can improve a transmission quality of the MIMO wireless method. In particular, the present invention is applicable to a home electric product such as a wireless communication apparatus using an antenna apparatus employing the MIMO wireless communication method, and to any other industrial apparatus. 
     Reference Signs List 
     
         
           10  dielectric substrate, 
           11   a,    11   b,    11   c  and  11   d  antenna element, 
           12   a,    12   b,    12   c  and  12   d  parasitic element, 
           13  ground conductor, 
           14  support member, 
           20  wireless communication apparatus, 
           21 ,  21 A,  21 B,  21 C and  21 D variable directivity antenna apparatus, 
           22   a,    22   b  and  22   c  wireless transceiver circuit, 
           23  MIMO modulator and demodulator circuit, 
           24  baseband signal processing circuit, 
           25  controller, 
           26  MAC circuit, 
           30  control circuit, 
           31  control signal input terminal, 
           32  high frequency blocking inductor, 
           33  impedance matching capacitor, 
           34  PIN diode, 
           35  inductor, 
           36  connection point, 
           61 ,  62 ,  63  and  64  antenna element, and 
           71 ,  72 ,  73  and  74  parasitic element.