Abstract:
The antenna apparatus of the present invention places antenna element  302  that transmits or receives electromagnetic waves on basic plate  301 , places parasitic antenna elements  303  to  306  on basic plate  301  evenly spaced concentrically centered on antenna element  302 , places switch elements  307  to  310  and capacitances  311  to  314  in parallel between one end of each of antenna elements  303  to  306  and said basic plate and disconnects one of switch elements  307  to  310  and connects all the others. In this way, the present invention provides a small and high-gain antenna apparatus with directivity switching capability.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an antenna apparatus with directivity switching capability used for a communication terminal apparatus and base station apparatus, etc. in a radio communication system. 
     2. Description of the Related Art 
     In radio communications, it is desirable to radiate electromagnetic waves focused on a specific direction and one of the antennas that realize this objective is Yagi antenna. The Yagi antenna is an antenna that controls directivity (radiation direction) by means of the length of a conductor bar placed near a ½ wavelength dipole antenna. 
     This antenna utilizes the nature of radiation direction that inclines toward a parasitic conductor bar placed near an antenna element, which acts as a radiator, if this conductor bar is shorter than ½ wavelength, and inclines toward the opposite direction of the conductor bar if the conductor bar is longer than ½ wavelength. 
     Hereafter, an antenna element with directivity toward itself is called “director” and an antenna element with directivity toward its opposite direction is called “reflector”. The measure used to indicate the sharpness of directivity is called “gain”. 
     Here, in radio communications, there are cases where it is necessary to switch directivity, for example, to minimize a multipath phenomenon that the radio traveling direction varies depending on the transmission environment. As the apparatus with directivity switching capability, the one using an array of a plurality of Yagi antennas made up of 3 elements of reflector, radiator and director is already proposed. 
     Here, it is possible to achieve higher gain by forming directivity by setting the director and reflector at symmetric positions with respect to the radiator rather than forming directivity using either one of the director or reflector. 
     FIG.  1 A and FIG. 1B show a configuration of a conventional antenna apparatus whose directivity can be changed by 90 degrees. 
     As shown in FIG.  1 A and FIG. 1B, the conventional antenna apparatus consists of basic plate  1 , 4 arrays of 3 elements of reflector  2 , radiator  3  and director  4  placed in ¼ wavelength intervals on basic plate  1  and distributed in  90 -degree intervals on the horizontal plane, switch circuit  4  inserted into the output of radiator  3  of each antenna array and switching circuit  5  that switches connection/disconnection of switch circuit  4 . The reason that the antenna elements are placed in ¼ wavelength intervals is that the antenna element interval smaller that this would reduce impedance due to mutual coupling. 
     The conventional antenna apparatus above implements switching of directivity by 90 degrees by changing switching circuit  5  as shown in the directivity diagram in FIG.  2 . 
     However, the conventional antenna apparatus requires the same number of Yagi antenna arrays with antenna element intervals of approximately ¼ wavelength, as the number of directivities to be switched, causing a problem of increasing the size of the apparatus. 
     Furthermore, the conventional antenna apparatus has a switch circuit inserted into each radiator output, which will cause another problem that the antenna gain will be reduced due to loss in those switch circuits. 
     SUMMARY OF THE INVENTION 
     It is an objective of the present invention to provide a small, high-gain antenna apparatus with directivity switching capability. 
     The present invention achieves the objective above by placing a first antenna element that transmits/receives electromagnetic waves and a parasitic second antenna element on a basic plate, inserting a switching section between one end of the second antenna element and the basic plate, connecting or disconnecting the switching section and thereby making the second antenna element act as a reflector or director. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and features of the invention will appear more fully hereinafter from a consideration of the following description taken in connection with the accompanying drawing wherein one example is illustrated by way of example, in which; 
     FIG. 1A is diagrams showing a configuration of a conventional antenna apparatus; 
     FIG. 1B is diagrams showing a configuration of a conventional antenna apparatus; 
     FIG. 2 is a directivity diagram showing the conventional antenna apparatus; 
     FIG. 3 is a diagram showing a first configuration of an antenna apparatus according to Embodiment 1 of the present invention; 
     FIG. 4 is a diagram showing a configuration example of a switch circuit of the antenna apparatus according to Embodiment 1 of the present invention; 
     FIG. 5 is a directivity diagram of the antenna apparatus according to Embodiment 1 of the present invention; 
     FIG. 6 is a rear view of a printed circuit board of the antenna apparatus according to Embodiment 1 of the present invention; 
     FIG. 7 is a diagram showing a second configuration of the antenna apparatus according to Embodiment 1 of the present invention; 
     FIG. 8 is a diagram showing a first configuration of an antenna apparatus according to Embodiment 2 of the present invention; 
     FIG. 9 is a directivity diagram of the antenna apparatus according to Embodiment 2 of the present invention; 
     FIG. 10 is a diagram showing a second configuration of the antenna apparatus according to Embodiment 2 of the present invention; 
     FIG. 11 is a diagram showing a first configuration of an antenna apparatus according to Embodiment 3 of the present invention; 
     FIG. 12 is a diagram showing a second configuration of the antenna apparatus according to Embodiment 3 of the present invention; 
     FIG. 13 is a directivity diagram of the antenna apparatus according to Embodiment 3 of the present invention; 
     FIG. 14 is a diagram showing an internal configuration of a switch circuit of an antenna apparatus according to Embodiment 4 of the present invention; 
     FIG. 15 is a diagram showing an internal configuration of a switch circuit of an antenna apparatus according to Embodiment 5 of the present invention; 
     FIG. 16 is a diagram showing an internal configuration of a switch circuit of an antenna apparatus according to Embodiment 6 of the present invention; 
     FIG. 17 is a diagram showing an internal configuration of a switch circuit of an antenna apparatus according to Embodiment 7 of the present invention; 
     FIG. 18 is a diagram showing a first configuration of a radiator of an antenna apparatus according to Embodiment 8 of the present invention; 
     FIG. 19 is a diagram showing a second configuration of the radiator of the antenna apparatus according to Embodiment 8 of the present invention; 
     FIG. 20 is a diagram showing a first configuration of a radiator of an antenna apparatus according to Embodiment 9 of the present invention; 
     FIG. 21 is a diagram showing a second configuration of the radiator of the antenna apparatus according to Embodiment 9 of the present invention; 
     FIG. 22 is a diagram showing a third configuration of the radiator of the antenna apparatus according to Embodiment 9 of the present invention; 
     FIG. 23 is a diagram showing a fourth configuration of the radiator of the antenna apparatus according to Embodiment 9 of the present invention; 
     FIG. 24 is a diagram showing a first configuration of an inductance of an antenna apparatus according to Embodiment 10 of the present invention; 
     FIG. 25 is a diagram showing a second configuration of the inductance of an antenna apparatus according to Embodiment 10 of the present invention; 
     FIG. 26 is a diagram showing a first configuration of a capacitance of an antenna apparatus according to Embodiment 11 of the present invention; 
     FIG. 27 is a diagram showing a second configuration of the capacitance of the antenna apparatus according to Embodiment 11 of the present invention; 
     FIG. 28A is a top view of a basic plate of an antenna apparatus according to Embodiment 12 of the present invention; 
     FIG. 28B is a front sectional view of the basic plate of the antenna apparatus according to Embodiment 12 of the present invention; and 
     FIG. 29 is a diagram showing a configuration of an antenna apparatus according to Embodiment 13 of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference now to the attached drawings, the embodiments of the present invention are explained in detail below. 
     (Embodiment 1) 
     FIG. 3 is a diagram showing a configuration of an antenna apparatus according to Embodiment 1 of the present invention. 
     As shown in FIG. 3, the antenna apparatus according to the present embodiment comprises antenna element  102  acts as a radiator and parasitic antenna element  103  on basic plate  101 , and switch circuit  104  and capacitance  105  are connected in parallel between one end of antenna element  103  and basic plate  101 . Insertion of capacitance  105  allows the antenna element to act as a reflector even if the distance between antenna elements is narrowed from its conventional length of approximately ¼ wavelength. 
     FIG. 4 is a diagram showing an internal configuration of switch circuit  104  of the antenna apparatus according to Embodiment 1. 
     As shown in FIG. 4, switch circuit  104  mainly comprises switch  111 , diode element  112 , choke inductance  113 , capacitance  114  and capacitance  115 . Switch circuit  104  turns ON diode element  112  by closing switch  111  to apply a bias via choke inductance  113 , and turns OFF diode element  112  by opening switch Ill to apply no bias to diode element  112 . 
     Choke inductance  113  is inserted to produce high impedance on the power supply side to prevent a high frequency component entering from the antenna from entering into the power supply side. Capacitance  114  is inserted to prevent any current from flowing into the antenna side when a voltage is applied via choke inductance  113  to turn ON diode element  112  when switch  111  is closed. Capacitance  115  is inserted to short the high frequency component entering from the antenna to avoid the high frequency component from entering into the power supply side. 
     Here, when switch circuit  104  is ON, if antenna element  103  is electrically continuous with basic plate  101  and if antenna element  103  is a little longer than antenna element  102  acts as a radiator, antenna element  103  acts as a reflector. On the other hand, when switch circuit  104  is OFF, if capacitance  105  is set so that the phase of impedance produced by antenna element  103  and capacitance  105  lags behind antenna element  102 , antenna element  103  acts as a director. 
     FIG. 5 shows a directivity diagram showing actually measured values of directivity at 2 GHz of a specific example of the antenna apparatus in FIG. 3, with circular basic plate  101  of approximately 75 mm in diameter, antenna element  102  of approximately 34.5 mm in length, antenna element  103  of approximately 37 mm in length, distance between antenna element  102  and antenna element  103  of approximately ⅛ wavelength, capacitance  105  of approximately 2 pF when switch circuit  104  is OFF. 
     As shown in FIG. 5, when switch circuit  104  is OFF, the direction of maximum radiation is toward antenna element  103 . On the other hand, when switch circuit  104  is ON, the direction of maximum radiation is toward antenna element  102 . 
     Thus, the present embodiment provides a switch circuit and capacitance in parallel between one end of a parasitic antenna element placed near a radiator and a basic plate, makes the parasitic antenna element act as a reflector or director by turning ON/OFF the switch circuit and makes the parasitic antenna element act as a reflector even if the distance between antenna elements is ¼ wavelength or below, thus making it possible to implement a small antenna apparatus capable of switching directivity in 2 directions. Furthermore, since the switch circuit is not provided at the output of the radiator, the present embodiment provides a high-gain antenna apparatus without loss caused by the switch circuit. 
     Here, it is also possible to implement the basic plate using a printed circuit board and mount switch circuit  104  and capacitance  105  on the rear of the printed circuit board. This will facilitate manufacturing of an antenna in a normal manufacturing process and provide an antenna with high reproducibility in the characteristic aspect. 
     Furthermore, as shown in the rear view of the printed circuit board of the antenna apparatus in FIG. 6, it is also possible to use transmission line  116  of ¼ wavelength instead of choke inductance  113  to short between the power supply side of ¼ wavelength transmission line  116  and the basic plate by means of high frequency using capacitance  115  and open its opposite side, thus reducing influences on the power supply side. 
     This can solve a problem that with a choke inductance of approximately 2-GHz band, the inductance does not match its nominal value making it impossible to obtain sufficient impedance, and achieve sufficient impedance even in a high frequency band. 
     FIG. 7 shows a configuration of the antenna apparatus in FIG. 3 using inductance  106  instead of capacitance  105 . 
     In the case of FIG. 7, when switch circuit  104  is ON, antenna element  103  is electrically continuous with basic plate  101  and antenna element  103  acts as a director. When switch circuit  104  is OFF, inductance  106  is loaded and antenna  103  acts as a reflector. 
     In this way, the present embodiment can make the parasitic antenna element act as a reflector or director and make the parasitic antenna element act as a reflector even if the distance between the antenna elements is ¼ wavelength or below, thus making it possible to implement a small antenna apparatus capable of switching directivity in 2 directions. Furthermore, since the switch circuit is not provided at the output of the radiator, the present embodiment provides a high-gain antenna apparatus without loss caused by the switch circuit. 
     (Embodiment 2) 
     Embodiment 2 is an embodiment configuring an antenna apparatus with  3  antenna elements in order to achieve an antenna apparatus with higher gain than Embodiment 1. 
     FIG. 8 shows a configuration of the antenna apparatus according to Embodiment 2. 
     FIG. 8 is a diagram showing a configuration of the antenna apparatus according to Embodiment 2. 
     As shown in FIG. 8, the antenna apparatus according to the present embodiment comprises antenna element  202  that acts as a radiator at the center of the upper surface of basic plate  201 , antenna elements  203  and  204  that act as either a reflector or director arrayed on a straight line so that their respective distance from antenna element  202  is ¼ wavelength or less. The antenna apparatus according to the present embodiment provides switch circuits  205  and  206  and capacitances  206  and  207  in parallel between one end of each of antenna elements  203  and  204  and basic plate  201 , respectively. 
     Here, when switch circuit  205  is ON, if antenna element  203  is electrically continuous with basic plate  201  and if antenna element  203  is a little longer than antenna element  102  acts as a radiator, antenna element  203  acts as a reflector. On the other hand, when switch circuit  205  is OFF, if capacitance  207  is set so that the phase of impedance produced by antenna element  203  and capacitance  207  lags behind antenna element  202 , antenna element  203  acts as a director. Likewise, when switch circuit  206  is ON, antenna element  204  acts as a reflector and when switch circuit  206  is OFF, antenna element  204  acts as a director. 
     That is, it is possible to make one of antenna element  203  or antenna element  204  act as a director and the other act as a reflector by turning ON either of switch circuit  205  or switch circuit  206  and turning OFF the other. 
     FIG. 9 shows a directivity diagram showing actually measured values of directivity at 2 GHz of a specific example of the antenna apparatus in FIG. 8, with circular basic plate  201  of approximately 75 mm in diameter, antenna element  202  of approximately 34.5 mm in length, antenna elements  203  and  204  of approximately 37 mm in length, distance between antenna element  202  and antenna element  203  and distance between antenna element  202  and antenna element  204  of approximately ⅛ wavelength, capacitances  207  and  208  of approximately 2.7 pF when switch circuit  205  is OFF and switch circuit  206  is ON. 
     As shown in FIG. 9, when switch circuit  205  is OFF and switch circuit  206  is ON, the direction of maximum radiation is toward antenna element  203 . On the other hand, when switch circuit  205  is ON and switch circuit  206  is OFF, the direction of maximum radiation is toward antenna element  204 . 
     Thus, the present embodiment provides switch circuits and capacitances in parallel between one end of each of two parasitic antenna elements placed symmetrically with respect to a radiator at the center and a basic plate, respectively, makes one of the two parasitic antenna elements act as a reflector and the other as a director by switching ON/OFF of the switch circuits so that one of the switch circuits is ON and the other is OFF, and in this way can implement an antenna apparatus with higher gain than Embodiment 1. 
     By the way, according to FIG. 8, both antenna elements  203  and  204  act as reflectors or directors by turning ON or OFF both switch circuits  205  and  206 , and in this way it is possible to use this antenna apparatus as an isotropic antenna on a horizontal plane without performing complicated switching operations. 
     As opposed to the antenna apparatus in FIG. 8, FIG. 10 shows a configuration of the antenna apparatus using inductances  209  and  210  instead of capacitances  207  and  208 . 
     In FIG. 10, when switch circuit  205  is ON, antenna element  203  is electrically continuous with basic plate  201  and antenna element  203  acts as a director. When switch circuit  205  is OFF, inductance  209  is loaded and antenna  203  acts as a reflector. Likewise, when switch circuit  206  is ON, antenna element  204  is electrically continuous with basic plate  201  and antenna element  204  acts as a director. When switch circuit  206  is OFF, antenna element  204  is isolated from basic plate  201  and inductance  210  is loaded and antenna  204  acts as a reflector. 
     That is, in the antenna apparatus shown in FIG. 10, one of antenna elements  203  and  204  acts a director and the other acts as a reflector by turning ON one of either switch circuit  205  or switch circuit  206  and turning OFF the other, thus implementing an antenna apparatus with higher gain than Embodiment 1 as in the case of the antenna apparatus shown in FIG.  8 . 
     According to FIG. 10, both antenna elements  203  and  204  act as reflectors or directors by turning ON or OFF both switch circuits  205  and  206 , and in this way it is possible to use this antenna apparatus as an isotropic antenna on a horizontal plane without performing complicated switching operations. 
     (Embodiment 3) 
     Embodiment 3 is an embodiment configuring an antenna apparatus with 5 antenna elements in order to implement a small and high-gain antenna apparatus with the capability of switching directivity by 90 degrees. 
     FIG. 11 is a diagram showing a configuration of the antenna apparatus according to Embodiment 3. 
     As shown in FIG. 11, the antenna apparatus according to the present embodiment comprises antenna element  302  that acts as a radiator at the center of the upper surface of basic plate  301 , antenna elements  303  to  306  that act as reflectors or directors arrayed concentrically so that their respective distance from antenna element  302  is ¼ wavelength or less. The antenna apparatus according to the present embodiment provides switch circuits  307  to  310  and capacitances  311  to  314  in parallel between one end of each of antenna elements  303  to  306  and basic plate  301 , respectively. 
     Here, when switch circuit  307  is ON, if antenna element  303  is electrically continuous with basic plate  301  and if antenna element  303  is a little longer than antenna element  102  acts as a radiator, antenna element  303  acts as a reflector. On the other hand, when switch circuit  307  is OFF, if capacitance  311  is set so that the phase of impedance produced by antenna element  303  and capacitance  311  lags behind antenna element  302 , antenna element  303  acts as a director. 
     Likewise, when switch circuit  308  is ON, antenna element  304  acts as a reflector and when switch circuit  308  is OFF, antenna element  304  acts as a director. Furthermore, when switch circuit  309  is ON, antenna element  305  acts as a reflector and when switch circuit  309  is OFF, antenna element  305  acts as a director. Furthermore, when switch circuit  310  is ON, antenna element  306  acts as a reflector and when switch circuit  310  is OFF, antenna element  306  acts as a director. 
     That is, it is possible to make one of parasitic antenna elements act as a director and the others act as reflectors by switching ON/OFF of switch circuits so that one of switch circuits  307  to  310  is OFF and all the others are ON, making it possible to implement an antenna apparatus smaller than conventional apparatuses, capable of switching directivity by 90 degrees in  4  directions. 
     By the way, according to FIG. 11, all antenna elements  303   306  act as reflectors or directors by turning ON or OFF all switch circuits  307  to  310 , and in this way it is possible to use this antenna apparatus as an isotropic antenna on a horizontal plane without performing complicated switching operations. 
     As opposed to the antenna apparatus in FIG. 11, FIG. 12 shows a configuration of the antenna apparatus using inductances  315  to  318  instead of capacitances  311  to  314 . 
     In the antenna apparatus in FIG. 12, when switch circuit  307  is ON, antenna element  303  is electrically continuous with basic plate  301  and antenna element  303  acts as a director. When switch circuit  307  is OFF, inductance  315  is loaded and antenna  303  acts as a reflector. 
     Likewise, when switch circuit  308  is ON, antenna element  304  acts as a director. When switch circuit  308  is OFF, antenna element  304  acts as a reflector. Furthermore, when switch circuit  309  is ON, antenna element  305  acts as a director. When switch circuit  309  is OFF, antenna element  305  acts as a reflector. Furthermore, when switch circuit  310  is ON, antenna element  306  acts as a director. When switch circuit  310  is OFF, antenna element  306  acts as a reflector. 
     FIG. 13 shows a directivity diagram showing actually measured values of directivity at 2 GHz of a specific example of the antenna apparatus in FIG. 12, with circular basic plate  201  of approximately 75 mm in diameter, antenna element  302  of approximately 34.5 mm in length, antenna elements  303  to  306  of approximately 34 mm in length, inductances  314  to  318  configured with a line distance of approximately 1 mm and a distribution constant of approximately 24 mm when shorted at one end, when switch circuit  307  is ON and switch circuits  308  to  310  are OFF. 
     As shown in FIG. 13, when switch circuit  307  is ON and switch circuits  308  to  310  are OFF, the direction of maximum radiation is toward antenna element  303 . Likewise, when switch circuit  308  is ON and switch circuits  307 ,  309  and  310  are OFF, the direction of maximum radiation is toward antenna element  304 . When switch circuit  309  is ON and switch circuits  307 ,  308  and  310  are OFF, the direction of maximum radiation is toward antenna element  305 . When switch circuit  310  is ON and switch circuits  307  to  309  are OFF, the direction of maximum radiation is toward antenna element  306 . 
     That is, the present embodiment makes one of the parasitic antenna elements act as a director and the others as reflectors by switching ON/OFF of the switch circuits so that one of the switch circuits  307  to  310  is ON and all the others are OFF, and in this way can implement an antenna apparatus smaller than conventional apparatuses and capable of switching directivity by 90 degrees in 4 directions. 
     By the way, according to FIG. 12, all antenna elements  303  to  306  act as reflectors or directors by turning ON or OFF all switch circuits  307  to  310 , and in this way it is possible to use this antenna apparatus as an isotropic antenna on a horizontal plane without performing complicated switching operations. 
     Here, if the number of antenna elements is further increased compared to the present embodiment, it is possible to switch directivity in multiple directions according to the number of antenna elements by switching ON/OFF of switch circuits as in the case of the present embodiment. 
     (Embodiment 4) 
     Embodiment 4 adopts such a switch circuit configuration as to implement a high-gain antenna apparatus independent of impedance on the power supply side. 
     In FIG. 4 above, since the power supply section made up of switch  111 , choke inductance  113  and capacitance  115  is connected in parallel with the diode element, when diode element  112  is turned OFF by the impedance on the power supply side, the impedance may decrease. 
     FIG. 14 is a diagram showing a configuration example of switch circuit  104  of the antenna apparatus according to Embodiment 4 of the present invention. In FIG. 14, the components common to those in FIG. 4 are assigned the same codes as those in FIG.  4  and their explanations are omitted. 
     In the switch circuit shown in FIG. 14, the power supply is connected to the anode side of diode element  112  not directly but via inductance  106 , and capacitance  114  is inserted between inductance  106  and the basic plate. This makes it possible to sufficiently lower impedance by means of high frequency, preventing the impedance on the power supply side from influencing diode element  112 . 
     Thus, the present embodiment can improve the isolation characteristic when diode element  112  is turned OFF independently of the impedance on the power supply side, making it possible to achieve a high-gain antenna apparatus. Its capability of configuring the antenna independently of the impedance on the power supply side makes design easier. 
     (Embodiment 5) 
     Embodiment 5 adopts such a switch circuit configuration as to implement a high-gain antenna apparatus. 
     In FIG. 4 above, in order to achieve high gain for the antenna apparatus, when diode element  112  is turned ON, that is, when the antenna element is electrically continuous with the basic plate, it is ideal that the resistance of switch circuit  104  be 0Ω. However, because of the resistance component deriving from the characteristic of diode element  112  itself, it is impossible to reduce the resistance to 0Ω. 
     FIG. 15 is a diagram showing a configuration example of switch circuit  104  of the antenna apparatus according to Embodiment 5 of the present invention. In FIG. 15, the components common to those in FIG. 4 are assigned the same codes as those in FIG.  4  and their explanations are omitted. 
     The switch circuit shown in FIG. 15 is different from the one in FIG. 4 in that diode element  121  is connected in parallel with diode element  112 . Thus, connecting a plurality of diodes in parallel can reduce the resistance deriving from characteristics of diode elements themselves as a whole, making it possible to achieve higher gain than the antenna apparatus with the switch circuit in FIG.  4 . 
     By the way, Embodiment 5 can be combined with Embodiment 4. 
     (Embodiment 6) 
     Embodiment 6 adopts such a switch circuit configuration as to reduce power consumption of an antenna apparatus. 
     FIG. 16 is a diagram showing a configuration example of switch circuit  104  of the antenna apparatus according to Embodiment 6 of the present invention. In FIG. 16, the components common to those in FIG. 4 are assigned the same codes as those in FIG.  4  and their explanations are omitted. 
     The switch circuit shown in FIG. 16 is different from the one in FIG. 4 in that field-effect transistor  131  is used instead of diode element  112  and capacitance  114 . When a diode element is turned ON a current flows. The smaller its resistance, the greater the current. On the other hand, power consumption of a field-effect transistor when performing ON/OFF control is virtually zero. Using a field-effect transistor instead of a diode element can reduce power consumption of the antenna apparatus. 
     By the way, Embodiment 6 can be combined with Embodiment 4. In Embodiment 6, connecting field-effect transistors in parallel can achieve an antenna apparatus with higher gain for the same reason as in Embodiment 5. 
     (Embodiment 7) 
     Embodiment 7 adopts such a switch circuit configuration as to achieve a high-gain antenna apparatus without characteristic deterioration due to the connection of switch circuits. 
     In FIG. 4 above, when diode element  112  is turned OFF, leakage of high frequency wave is produced due to the capacitance component of diode element  112  itself, preventing sufficient isolation from being secured. 
     FIG. 17 is a diagram showing a configuration example of switch circuit  104  of the antenna apparatus according to Embodiment 7 of the present invention. In FIG. 17, the components common to those in FIG. 4 are assigned the same codes as those in FIG.  4  and their explanations are omitted. 
     The switch circuit shown in FIG. 17 is different from the one in FIG. 4 in that inductance  141  and capacitance  142  are added in parallel with diode element  112 . This cancels out the capacitance component of diode element  112  itself, making it possible to improve isolation characteristic and achieve a high-gain antenna apparatus without characteristic deterioration due to the connection of switch circuits. 
     By the way, Embodiment 7 can be combined with Embodiments 4 to 6. 
     (Embodiment 8) 
     The embodiments above described how to reduce the size of the apparatus by narrowing the distance between array antenna elements. However, narrowing the distance between array antenna elements involves a problem of reducing the impedance of radiators. Embodiment 8 of the present invention is an embodiment that solves this problem. 
     FIG. 18 is a diagram showing a first configuration of a radiator of the antenna apparatus according to the present embodiment. As shown in FIG. 18, the antenna apparatus according to the present embodiment has antenna element  402 , which is used as a radiator, folded at a length of ¼ wavelength from the power supply point with its end shorted to basic plate  401 , forming a folded antenna. The two antenna elements forming the folded antenna have a same wire diameter. 
     This increases the impedance by a factor of 4 compared with the case where a normal rectilinear antenna element is used as a radiator, making it easier to maintain consistency of impedance when the distance between array antenna elements is small and the impedance of the radiator decreases. 
     FIG. 19 is a diagram showing a second configuration of the radiator of the antenna apparatus according to the present embodiment. Antenna element  412  in FIG. 19 is different from antenna element  402  in FIG. 18 in that the two antenna elements forming a folded antenna have different wire diameters. 
     This allows the input impedance of the radiator to be arbitrarily changed, making it easier to maintain consistency of impedance. 
     By the way, Embodiment 8 can be combined with one of Embodiments 1 to 3 as appropriate. 
     (Embodiment 9) 
     Embodiment 9 adopts such a form of the antenna element used as a radiator as to reduce the size and widen the band of the radiator. 
     FIG. 20 is a diagram showing a first configuration of a radiator of the antenna apparatus according to the present embodiment. As shown in FIG. 20, the antenna apparatus according to the present embodiment has antenna element  502 , which is used as a radiator, folded at a length of ¼ wavelength from the power supply point with its end shorted to basic plate  501 , forming a folded antenna. Reactance  503  is inserted between the top ends of the two antenna elements forming the folded antenna. 
     This can shorten the antenna element compared with the case where a normal rectilinear antenna element is used as a radiator. This can also widen the band if antenna elements of the same length as antenna elements of a normal rectilinear form are used. 
     Moreover, as shown in FIG. 21, adopting antenna element  512  of a tabular form as a radiator can widen the band compared with the case where a normal rectilinear antenna element is used as a radiator. 
     Moreover, as shown in FIG. 22, adopting antenna element  522  of a zigzag form as a radiator can shorten the antenna element compared with the case where a normal rectilinear antenna element is used as a radiator. 
     Moreover, as shown in FIG. 23, adopting antenna element  532  of a spiral form as a radiator can shorten the antenna element compared with the case where a normal rectilinear antenna element is used as a radiator. 
     By the way, Embodiment 9 can be combined with one of Embodiments 1 to 3 as appropriate. 
     (Embodiment 10) 
     Embodiments 1 to 3 have no restrictions on the form of the inductances used for the antenna apparatus. However, if a concentrated constant type inductance is used, there remains a problem of loss caused by self-resonance. Embodiment 10 adopts such a form of the inductance used for the antenna apparatus as to reduce or eliminate loss caused by self-resonance. 
     FIG. 24 is a diagram showing a first configuration of an inductance of the antenna apparatus according to the present embodiment. As shown in FIG. 24, inductance  601  is formed on printed circuit board  602 . 
     This can implement an inductance with smaller loss and with a higher self-resonance frequency than chip parts, etc. 
     FIG. 25 is a diagram showing a second configuration of the inductance of the antenna apparatus according to the present embodiment. As shown in FIG. 25, a distribution type inductance is formed with two microstrip-figured wires  612  and  613  and one end of wire  613  is shorted to basic plate  611 . 
     This can implement an inductance without loss or self-resonance frequency. 
     By the way, Embodiment 10 can be combined with Embodiments 1 to 9 as appropriate. 
     (Embodiment 11) 
     Embodiments 1 to 3 have no restrictions on the form of the capacitance used for the antenna apparatus. However, if a concentrated constant type capacitance is used, there remains a problem of loss caused by self-resonance. Embodiment 11 adopts such a form of the capacitance used for the antenna apparatus as to reduce or eliminate loss caused by self-resonance. 
     FIG. 26 is a diagram showing a first configuration of a capacitance of the antenna apparatus according to the present embodiment. As shown in FIG. 26, a capacitance is formed between two conductor plates  701  and  702 . 
     This can implement a capacitance with smaller loss go and with a higher self-resonance frequency than chip parts, etc. 
     FIG. 27 is a diagram showing a second configuration of the capacitance of the antenna apparatus according to the present embodiment. As shown in FIG. 27, a distribution type capacitance is formed with two microstrip-figured wires  712  and  713  and one end of wire  713  is shorted to basic plate  711 . 
     This can implement a capacitance without loss or self-resonance frequency. 
     By the way, Embodiment 11 can be combined with Embodiments 1 to 9 as appropriate. 
     (Embodiment 12) 
     Embodiment 12 is an embodiment that adopts such a form of the basic plate as to improve antenna gain. 
     FIG. 28A is a top view of a basic plate of the antenna apparatus according to the present embodiment. FIG. 28B is a front sectional view of the basic plate of the antenna apparatus according to the present embodiment. As shown in FIG. 28, the antenna apparatus according to the present embodiment provides groove section  802  of approximately ¼ wavelength wide on the outer circumference of basic plate  801 . 
     This makes the impedance of groove section  802  with respect to basic plate  801  infinite, suppresses an antenna current flowing onto the back of the basic plate, reduces radiation to the back of the basic plate and improves the antenna gain. 
     By the way, Embodiment 12 can be combined Embodiments 1 to 11 as appropriate. 
     (Embodiment 13) 
     Embodiment 13 is an embodiment intended to further reduce the size of the apparatus. 
     FIG. 29 is a diagram showing a configuration of a basic plate of the antenna apparatus according to the present embodiment. As shown in FIG. 29, the antenna apparatus according to the present embodiment fills antenna elements  902  to  906  acting as directors or reflectors shorted to basic plate  901  with dielectric material  907 . 
     This produces a dielectric constant reducing effect, making it possible to shorten the antenna elements, narrow the distance between the antenna elements and further reduce the size of the apparatus. 
     By the way, Embodiment 13 can be combined with Embodiments 1 to 12 as appropriate. 
     As described above, the antenna apparatus of the present invention can reduce the size of the apparatus and switch directivity without reducing the antenna gain. 
     The present invention is not limited to the above described embodiments, and various variations and modifications may be possible without departing from the scope of the present invention. 
     This application is based on the Japanese Patent Application No.HEI 11-059449 filed on Mar. 5, 1999, the Japanese Patent Application No.HEI 11-139122 filed on May 19, 1999 and the Japanese Patent Application No.HEI 11-231381 filed on Aug. 18, 1999, entire content of which is expressly incorporated by reference herein.