Patent Publication Number: US-6211830-B1

Title: Radio antenna device

Description:
TECHNICAL FIELD 
     The present invention relates to a radio antenna apparatus, and in particular, to a radio antenna apparatus for use in a portable telephone or a mobile telephone for use in mobile communications. 
     BACKGROUND ART 
     A radio set comprising a conventionally publicly known radio antenna apparatus is shown in FIG. 17 so as to schematically show an antenna and related parts. The radio set of the prior art is constituted by an external antenna  602  such as a whip antenna or a helical antenna, a built-in antenna  603  such as a plane antenna, feeder lines  604  and  605 , a transceiver unit  606  including a transceiver, and a microphone  609  connected to the transceiver unit  606 , which are provided in a radio set housing  601 . The external antenna  602  and the built-in antenna  603  are arranged in proximity to each other so as to be electromagnetically coupled with each other, constitute a receiving space selective diversity antenna. The external antenna  602  is arranged so as to be electrically insulated from the radio set housing  601 , while a predetermined point of the built-in antenna  603  is grounded to the radio set housing  601  through a short-circuiting line  603   a , and the built-in antenna  603  constitutes an inverted-F antenna. 
     When a power is supplied to the external antenna  602 , a switch  607  is turned on so that the external antenna  602  is connected to the transceiver unit  606  provided in the radio set housing  601  through the feeder line  604 . At the same time, the switch  608  is turned off, and the feeder line  605  connected to the built-in antenna  603  is disconnected from the transceiver unit  606 . 
     On the other hand, When the built-in antenna  603  is supplied with power, the switch  608  is turned on so that the built-in antenna  603  is connected to the transceiver unit  606  through the feeder line  605 . At the same time, the switch  607  is turned off so that the feeder line  604  connected to the external antenna  602  is disconnected from the transceiver unit  606 . 
     In the radio set comprising the conventional radio antenna apparatus described above, the external antenna  602  and the built-in antenna  603  are designed to have a high gain primarily in a free space, and have a uniform horizontal plane directivity or radiation pattern along the x-y plane with a center of the external antenna  602  and the built-in antenna  603 . In other words, as shown in FIG. 17, in the case where the orthogonal coordinates are set so that the z-axis direction is coincident with the axial direction of the external antenna  602  and the x-axis direction is coincident with the direction of the normal to the built-in antenna  603 , the horizontal plane directivity pattern of the antenna of the conventional radio set in a free space is shown in FIG. 18, and it has a shape of a circle (as indicated by a thick solid line of FIG. 18) with the center of the z-axis on the x-y plane, as shown in FIG.  18 . It is to be noted that the microphone  108  is arranged under the radio set housing  101  on the side nearer to the whip antenna  102  in the x-axis direction. 
     The conventional radio antenna apparatus described above has the same horizontal plane directivity pattern in the x-y plane and hence a horizontal plane non-directivity pattern. Therefore, in a case where a human head or the like obstacle approaching the microphone  609  exists in proximity to the radio set comprising the conventional radio antenna apparatus described above, the radio wave is interrupted by the obstacle, and this leads to a problem of gain deterioration. 
     An object of the present invention is to solve the above-mentioned problems and to provide a radio antenna apparatus, in which the horizontal plane directivity pattern of the antenna is changed in a direction not affected by an obstacle, and radio wave interference by the obstacle is reduced so as to improve a radiation efficiency thereof. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided a radio antenna apparatus connected to a transceiver unit of a radio set, comprising an antenna element, a passive element arranged in proximity to the antenna element so as to be electromagnetically coupled with the antenna element, a load impedance element, connected to the passive element, and capable of changing an impedance value thereof, and control means for changing a directivity pattern of the antenna element by changing the impedance value of the load impedance element. 
     Also, the above-mentioned radio antenna apparatus preferably further comprises an impedance matching circuit, connected between the antenna element and the transceiver unit of the radio set, for matching the impedance of the antenna element with the impedance of the transceiver unit of the radio set. 
     Also, according to a radio antenna apparatus of the present invention, there is provided a radio antenna apparatus connected to the transceiver unit of a radio set, comprising at least two antenna elements including first and second antenna elements arranged close enough to each other so as to be electromagnetically coupled with each other and constituting a space selective diversity antenna, a load impedance element capable of changing an impedance value thereof, first switching means for selectively switching over so as to connect one of the first and second antenna elements to the transceiver unit of the radio set, and to connect another one thereof to the load impedance element, and control means for changing a directivity pattern of the antenna element by changing the impedance value of the load impedance element. 
     Further, the above-mentioned radio antenna preferably further comprises an impedance matching circuit, connected between the first or second antenna element connected to the transceiver unit of the radio set, and the transceiver unit of the radio set, for matching the impedance of the antenna element with the impedance of the transceiver unit of the radio set. 
     Still further, in the above-mentioned radio antenna apparatus, the control means preferably changes a correlation coefficient between the first antenna and the second antenna by changing the value of the load impedance element. 
     Also, in the above-mentioned radio antenna apparatus, preferably, one of the first and second antennas is at least one of a whip antenna and a helical antenna, and another one of the first and second antennas is a plane antenna. 
     Further, in the above-mentioned radio antenna apparatus, the control means preferably changes the directivity pattern of the antenna elements by selectively changing the value of the load impedance element between a standby mode and a speech mode of the transceiver unit of the radio set. 
     Still further, the above-mentioned radio antenna apparatus preferably further comprises first detecting means for detecting a strength of a received signal received by the transceiver unit of the radio set, wherein the control means changes the directivity pattern of the antenna elements by changing the value of the load impedance element in accordance with the strength of the received signal detected by the first detecting means at a standby mode of the transceiver unit of the radio set. 
     Also, in the above-mentioned radio antenna apparatus, the load impedance element preferably includes an impedance variable element. 
     Further, in the above-mentioned radio antenna apparatus, the load impedance element preferably includes a reactance element. 
     Still further, in the above-mentioned radio antenna apparatus, the load impedance element preferably includes a plurality of impedance elements, and second switching means for selectively switching the plurality of the impedance elements, wherein the control means changes the value of the load impedance element by controlling the switching of the second switching means. 
     Also, in the above-mentioned radio antenna apparatus, the impedance matching circuit preferably includes a plurality of impedance matching circuit units, and third switching means for selectively switching the plurality of the impedance matching circuit units. 
     Further, the above-mentioned radio antenna apparatus preferably further comprises second detecting means for detecting a supplied power supplied to the antenna element, wherein the control means matches the impedance of the antenna elements with the impedance of the transceiver unit of the radio set by controlling the impedance matching circuit so as to maximize the supplied power detected by the second detecting means. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view showing a configuration of a radio set comprising a radio antenna apparatus according to a first preferred embodiment of the present invention. 
     FIG. 2 is a perspective view showing a configuration of a radio set comprising a radio antenna apparatus according to a second preferred embodiment of the present invention. 
     FIG. 3 is a block diagram showing a configuration of a radio set comprising a radio antenna apparatus according to a third preferred embodiment of the present invention, and showing an extended state of an antenna unit. 
     FIG. 4 is a block diagram showing an contracted state of the antenna unit of the radio set of FIG.  3 . 
     FIG. 5 is a circuit diagram showing a first modified preferred embodiment in which a load impedance element of FIG. 1 is constituted by a variable capacitor. 
     FIG. 6 is a circuit diagram showing a second modified preferred embodiment in which the load impedance element of FIG. 1 is constituted by a variable capacitance diode. 
     FIG. 7 is a circuit diagram showing a third modified preferred embodiment in which the load impedance element of FIG. 1 is constituted by a variable inductor. 
     FIG. 8 is a circuit diagram showing a fourth modified preferred embodiment in which the load impedance element of FIG. 1 is constituted by a circuit for switching three capacitors having different electrostatic capacitances using a switch. 
     FIG. 9 is a circuit diagram showing a fifth modified preferred embodiment in which the load impedance element of FIG. 1 is constituted by a circuit for switching three inductors of different inductance using a switch. 
     FIG. 10 is a circuit diagram showing a first modified preferred embodiment of the impedance matching circuit of FIG.  1 . 
     FIG. 11 is a circuit diagram showing a second modified preferred embodiment of the impedance matching circuit of FIG.  1 . 
     FIG. 12 is a circuit diagram showing a third modified preferred embodiment of the impedance matching circuit of FIG.  1 . 
     FIG. 13 is a diagram showing an example of a horizontal plane directivity pattern of the radio antenna apparatus of FIGS. 1,  2  and  3 . 
     FIG. 14 is a diagram showing another example of a horizontal plane directivity pattern of the radio antenna apparatus of FIGS. 1,  2  and  3 . 
     FIG. 15 is a diagram showing still another example of a horizontal plane directivity pattern of the radio antenna apparatus of FIGS. 1,  2  and  3 . 
     FIG. 16 is a graph showing a change in a correlation coefficient between two antennas making up a space selective diversity antenna, to a reactance component of the load impedance element, in the case of the space selective diversity antenna of FIG.  2 . 
     FIG. 17 is a perspective view showing a configuration of a radio set comprising a conventional radio antenna apparatus. 
     FIG. 18 is a diagram showing an example of a horizontal plane directivity pattern of the radio antenna apparatus of FIG.  17 . 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Preferred embodiments of the present invention will be described below with reference to the accompanying drawings. 
     FIRST PREFERRED EMBODIMENT 
     FIG. 1 shows a radio set comprising a radio antenna apparatus according to a first preferred embodiment of the present invention, so as to schematically show an antenna and related parts. The radio set according to the first preferred embodiment of the present invention is constituted within a radio set housing  101  and comprises a whip antenna  102 , a passive or parasitic element  103 , a load impedance element  104 , a feeder line  105 , a transceiver unit  106  including a transceiver, an impedance matching circuit  107 , a microphone  108  connected to the transceiver unit  106 , and a controller  109  connected to the transceiver unit  106  and the load impedance element  104 . It is to be noted that the microphone  108  is arranged under the radio set housing  101  on the side nearer to the whip antenna  102  along the x-axis direction of FIG.  1 . 
     Referring to FIG. 1, the whip antenna  102  and the passive (no-power-supplied) element  103  making up a plane antenna are arranged so as to be electromagnetically coupled with each other and to be electrically isolated from the radio set housing  101 . In this case, in a manner similar to that of the prior art shown in FIG. 17, a predetermined point of the passive element  103  may be grounded to the radio set housing  101  through a short-circuiting line (not shown), and then, the passive element  103  constitutes an inverted-F antenna. The whip antenna  102  is connected to the transceiver unit  106  provided in the radio set housing  101 , through the feeder line  105  and the impedance matching circuit  107 . Also, the passive element  103  is grounded to the radio set housing  101  through the load impedance element  104 . 
     The impedance matching circuit  107  is a circuit for matching an impedance of the whip antenna  102  with an impedance of the transceiver unit  106 . Concretely speaking, the impedance matching circuit  107  is constituted by a circuit shown in one of FIGS. 10 to  12 , for example. 
     The impedance matching circuit  107  of FIG. 10 is constituted by an L-shaped circuit comprising an inductor  141 , and a variable capacitor of a trimmer capacitor  142  with one terminal thereof grounded. A supplied power detecting unit  145  detects a power supplied from the transceiver unit  106  through the impedance matching circuit  107  to the whip antenna  102 , and outputs the detected power to the controller  109 . In response thereto, the controller  109  changes the electrostatic capacitance of the variable capacitor  142  to maximize the detected supplied power, so that the impedance of the whip antenna  102  is matched with the impedance of the transceiver unit  106 . 
     As compared with the impedance matching circuit  107  of FIG. 10, the impedance matching circuit  107  of FIG. 11 has such a feature that the variable capacitor  142  is replaced with a parallel circuit including a variable capacitance diode  143  and a variable voltage DC power supply  144  for applying a reverse bias voltage Vb to the variable capacitance diode  143 . The controller  109  changes the reverse bias voltage Vb of the variable voltage DC power supply  144  so as to maximize the detected supplied power, and then, this leads to that the electrostatic capacitance of the variable capacitor  142  changes so as to match the impedance of the whip antenna  102  with the impedance of the transceiver unit  106 . 
     The impedance matching circuit  107  of FIG. 12 comprises three L-shaped circuits  181 ,  182  and  183 , each having a configuration similar to that of the impedance matching circuit of FIG. 10, and each having different output impedance on the side nearer to the antenna  102  from each other, and the impedance matching circuit  107  further comprises switches  151  and  152  for selectively switching the three L-shaped circuits in operatively interlocked relation with each other. In this case, the L-shaped circuit  181  is constituted by an L-shaped circuit comprising an inductor  161  having an inductance L11 and a capacitor  171  having an electrostatic capacitance C11. Also, the L-shaped circuit  182  is constituted by an L-shaped circuit comprising an inductor  162  having an inductance L12 and a capacitor  172  having an electrostatic capacitance C12. Further, the L-shaped circuit  183  is constituted by an L-shaped circuit comprising an inductor  163  having an inductance L13 and a capacitor  173  having an electrostatic capacitance C13. In this case, the controller  109  selectively switches over between the switches  151  and  152  in operatively interlocked relation to each other so as to maximize the supplied power detected, so that the impedance of the whip antenna  102  is substantially matched with the impedance of the transceiver unit  106 . 
     According to the present preferred embodiment, the load impedance element  104  preferably includes a reactance component, and in this case, as shown in FIG. 5, the load impedance element  104  is of a variable capacitor  110  of a trimmer or variable capacitor with one terminal thereof grounded. By changing the value of the variable capacitor  110  under the control of the controller  109 , namely, by changing the electrical length of the passive element  103  including the load impedance element  104  as compared with the electrical length of the whip antenna  102 , the horizontal plane directivity or radiation pattern is changed. Also, the following configuration can be employed in place of the variable capacitor  110  of FIG.  5 . 
     (a) The load impedance element  104 , as shown in FIG. 6, is constituted by a parallel circuit including a variable capacitance diode  111  and a variable voltage DC power supply  112  for applying a reverse bias voltage Vb to the variable capacitance diode  111 . In this case, the controller  109  changes the horizontal plane directivity pattern, as described in detail, by changing the reverse bias voltage Vb of the variable voltage DC power supply  112  and thus changing the electrostatic capacitance of the variable capacitance diode  111 . 
     (b) As shown in FIG. 7, the horizontal plane directivity pattern is changed, as described in detail later, by changing the inductance value of the variable inductor  113  under the control of the controller  109 . 
     (c) As shown in FIG. 8, the horizontal plane directivity pattern is changed, as described in detail later, by selectively switching among the capacitors  121 ,  122  and  123  with one terminal grounded and having different electrostatic capacitances C1, C2 and C3, respectively, by the switch  120 , so as to change the electrostatic capacitance value under the control of the controller  109 . 
     (d) As shown in FIG. 9, the horizontal plane directivity pattern is changed, as described in detail later, by selectively switching the inductors  131 ,  132  and  133  of a coil with one terminal grounded and having different inductance values L1, L2 and L3, respectively, by the switch  130 , so as to change the inductance value under the control of the controller  109 . 
     In the first preferred embodiment shown in FIG. 1, one end of the load impedance element  104  is grounded, however, the present invention is not limited to this. The end of the load impedance element  104  may be in an open state. 
     In addition, the horizontal plane directivity pattern of the whip antenna  102  is changed in dependence upon the electromagnetic coupling with the passive element  103 . Namely, the passive element  103  functions as a wave director or a reflector for the whip antenna  102  in dependence on the value of the load impedance element  104  connected to the passive element  103 . For example, in the case where the load impedance element  104  has a comparatively large electrostatic capacitance and the electrical length of the passive element  103  including the load impedance element  104  is shorter than the electrical length of the whip antenna  102 , the passive element  103  functions as a wave director, and the radiation toward the passive element  103  becomes much stronger. On the other hand, in the case where the load impedance element  104  has a comparatively large inductance and the electrical length of the passive element  103  including the load impedance element  104  is longer than the electrical length of the whip antenna  102 , the passive element  103  functions as a reflector, and the radiation in the direction opposite to the direction toward the passive element  103  becomes much stronger. 
     As a result, as shown in FIG. 1, in the case where orthogonal coordinates are set so that the z-axis direction is coincident with the axial direction of the antenna  102  and the x-axis direction is coincident with the direction of the normal to the passive element  103 , the horizontal plane directivity pattern of the antenna  102  in a free space as shown by a thick solid line in FIG. 13 is realized when the passive element  103  functions as a wave director. On the other hand, when the passive element  103  functions as a reflector, the horizontal plane directivity pattern indicated by the thick solid line in FIG. 14 is realized. Also, in the case where the electrical length of the passive element  103  including the load impedance element  104  is substantially the same as the electrical length of the whip antenna  102 , the horizontal plane directivity pattern of the whip antenna  102  is almost non-directional (or substantially non-directional pattern) as shown in FIG. 15 as the result of electromagnetic coupling with the passive element  103 . 
     While the transceiver unit  106  of the radio set is not in a speaking state, or busy state but in standby state communicating with the base station for position registration or the like, the controller  109  controls the horizontal plane directivity pattern to be that shown in FIG. 15 by changing the value of the load impedance element  104 . On the other hand, in the case where the transceiver unit  106  of the radio set is activated so that the operator is speaking, the controller  109  controls the horizontal plane directivity pattern to be that as shown in FIG. 13, for example. Namely, while the operator is speaking as in the latter case and the head of the operator is located in proximity to the side of the whip antenna  102  in the x-axis direction of the radio set housing  10 , the electromagnetic radiation is not directed to an obstacle of the head of the operator, and this leads to reducing the electromagnetic radiation to the operator while at the same time making it possible to reduce the radio wave interference by the particular obstacle. Therefore, even if an obstacle exists in proximity to the radio set in the direction of weakening radiation, the radio interference by such an obstacle can be reduced, so as to improve the radio wave radiation efficiency when an obstacle is in proximity to the radio set. 
     In the first preferred embodiment described above, a polarization diversity is also constituted by two antennas  102  and  103  having different polarizations. 
     In the preferred embodiment described above, a capacitor or an inductor is used as the load impedance element  104 . Alternatively, a distributed constant line such as a microstrip line, a coplanar line or the like can be used as the load impedance element. When using the distributed constant line, a similar effect can be obtained by setting a load impedance element based on the termination conditions and the line length. 
     In the preferred embodiment described above, the value of the load impedance element  104  can be easily changed as shown in FIGS. 5 to  9 , for example, and this leads to a result in which the directivity pattern of the radio set comprising the radio antenna apparatus according to the present preferred embodiment can be changed arbitrarily. 
     The preferred embodiment described above includes only one set of the passive element  103  and the load impedance element  104  connected to the passive element  103 , however, the present invention is not limited to this. Two or more sets of the passive element  103  and the load impedance element  104  can be provided. 
     SECOND PREFERRED EMBODIMENT 
     FIG. 2 shows a radio set comprising a radio antenna apparatus according to the second preferred embodiment of the present invention, so as to schematically show an antenna and related parts. The radio set of the second preferred embodiment is constituted within a radio set housing  201  and comprises a whip antenna  202 , a plane antenna  203 , load impedance elements  204  and  205 , feeder lines  206  and  207 , a transceiver unit  208  having a transceiver, switches  211 ,  212  and  213 , impedance matching circuits  221  and  222 , a microphone  250  connected to the transceiver unit  208 , and a controller  260  connected to the transceiver unit  208  and the load impedance elements  204  and  205 . The microphone  250  is arranged under the radio set housing  201  on the side nearer to the whip antenna  202  in the x-axis direction as shown in FIG.  1 . 
     Referring to FIG. 2, the whip antenna  202  and the plane antenna  203  are arranged so as to be electromagnetically coupled with each other and to be electrically insulated from the radio set housing  201 . The plane antenna  203  constitutes an inverted-F antenna with a predetermined point thereof grounded to the radio set housing  201  through a short-circuiting line (not shown). 
     The whip antenna  202  is connected to the transceiver unit  208  provided in the radio set housing  201  through the feeder line  206 , a contact “a” of the switch  211 , the impedance matching circuit  221 , and a contact “a” of the switch  213 . The whip antenna  202  is grounded to the radio set housing  201  through the feeder line  206 , a contact “b” of the switch  211  and the load impedance element  204 . Also, the plane antenna  203  is grounded through the feeder line  207 , a contact “a” of the switch  212  and the load impedance element  205 , and the plane antenna  203  is connected to the transceiver unit  208  through the feeder line  207 , a contact “b” of the switch  212 , the impedance matching circuit  222 , and a contact “b” of the switch  213 . 
     In the present preferred embodiment, the load impedance elements  204  and  205  are each preferably constituted of a reactance component, and in a manner similar to that of the first preferred embodiment, for example, they can each be the load impedance element shown in any one of FIGS. 5 to  9 . Also, in the present preferred embodiment, the impedance matching circuits  221  and  222  can be the impedance matching circuit shown in any one of FIGS. 10 to  12 , for example, in a manner similar to that of the first preferred embodiment. 
     In the radio antenna apparatus shown in FIG. 2, the whip antenna  202  and the plane antenna  203  constituting an inverted-F antenna are arranged so as to be electromagnetically coupled with each other and make up a space selective diversity antenna. When the whip antenna  202  is supplied with power from the transceiver unit  208 , the switches  211 ,  212  and  213  are switched over to the contact “a” thereof under the control of the controller  260 . At the same time, the whip antenna  202  is connected to the transceiver unit  208  through the impedance matching circuit  221 , while the plane antenna  203  is connected to the load impedance element  205 . On the other hand, when the power is supplied to the plane antenna  203  from the transceiver unit  208 , the switches  211 ,  212  and  213  are switched over to the contact “b” thereof under the control of the controller  260 . At the same time, the plane antenna  203  is connected to the transceiver unit  208  through the impedance matching circuit  222 , while the whip antenna  202  is connected to the load impedance element  204 . 
     In the radio antenna apparatus configured as described above, when the whip antenna  202  is supplied with power, the whip antenna  202  changes the horizontal plane directivity pattern thereof in dependence on the electromagnetic coupling with the plane antenna  203 . Then, the plane antenna  203  functions as a wave director or reflector for the whip antenna  202  according to the value of the load impedance element  205 . In the case where the electrical length of the plane antenna  203  including the load impedance element  205  is shorter than the electrical length of the whip antenna  202  and the plane antenna  203  functions as a wave director, the radiation in the direction toward the plane antenna  203  becomes much stronger as shown in FIG.  13 . On the other hand, in the case where the electrical length of the plane antenna  203  including the load impedance element  205  is longer than the electrical length of the whip antenna  202  and the plane antenna  203  functions as a reflector, the radiation becomes much stronger in the direction toward the whip antenna  202  as shown in FIG.  14 . 
     In a manner similar to that of above, when the plane antenna  203  is supplied with power, the horizontal plane directivity pattern of the plane antenna  203  changes in dependence on the electromagnetic coupling with the whip antenna  202 . At the same time, the whip antenna  202  functions as a wave director or a reflector for the plane antenna  203  according to the value of the load impedance element  204 . In the case where the electrical length of the whip antenna  202  including the load impedance element  204  is shorter than the electrical length of the plane antenna  203  and the whip antenna  202  functions as a wave director, as shown in FIG. 14, the radiation becomes much stronger in the direction toward the whip antenna  202 . On the other hand, in the case where the electrical length of the whip antenna  202  including the load impedance element  204  is longer than the electrical length of he plane antenna  203  and the whip antenna  202  functions as reflector, as shown in FIG. 13, the radiation becomes much stronger in the direction toward the plane antenna  203 . 
     As a result, as shown in FIG. 2, when the orthogonal coordinates are set so that the z-axis direction is coincident with the axial direction of the whip antenna  202  and the x-axis direction is coincident with the direction of the normal to the plane antenna  203 , the horizontal plane directivity pattern of the radio antenna apparatus in the free space is similar to that described in the first preferred embodiment. Thus, even in the presence of an obstacle in the vicinity of the radio set in the direction of a weakening radiation, the radio wave interference by such an obstacle can be reduced, and therefore, the radio wave radiation efficiency can be improved with an obstacle located in the vicinity of the radio set. 
     In the case where the transceiver unit  208  of the radio set is not in a speaking or busy state, but in standby state only communicating with the base station for position registration or the like, the controller  260  controls the horizontal plane directivity pattern to be that as shown in FIG. 15, for example, by changing the value of the load impedance element  204  or  205 . On the other hand, in the case where the transceiver unit  208  of the radio set is occupied in a speaking or busy state by the operator, the controller  260  controls the horizontal plane directivity pattern to be that as shown in FIG. 13, for example, by changing the value of the load impedance element  204  or  205 . Namely, while in the speaking or busy state when the head of the operator is located in proximity to the whip antenna  202  along the x-axis direction of the radio set housing  201 , the electromagnetic wave is not radiated in the direction toward the obstacle of the head of the operator, and this leads to not only a reduction in the electromagnetic radiation to the operator, but also a reduction in the radio wave interference by the obstacle. 
     FIG. 16 is a graph showing a change in a correlation coefficient ρ between the two antennas  202  and  203  making up the space selective diversity antenna of FIG. 2 with respect to the reactance component of the load impedance elements  204  and  205 . The correlation coefficient ρ can be expressed as follows:              ρ   =         ∫     -   π     π              G   1   *          (   φ   )              G   2          (   φ   )            P        (   φ   )                   -   j2π               cosφ     /   λ                            φ             [       ∫     -   π     π              G   1   *          (   φ   )              G   1          (   φ   )            P        (   φ   )                            φ     ·       ∫     -   π     π              G   2   *          (   φ   )              G   2          (   φ   )            P        (   φ   )                          φ               ]       1   /   2                 (   1   )                         
     where G i (φ) is a directivity pattern of the antennas  202  and  203  (i=1, i=2), P(φ) is an angular distribution of the multiple arriving waves, and the exponent term in the numerator on the right side of the equation (1) indicates a phase difference in the arriving wave between the antennas  202  and  203 . 
     As apparent from FIG. 16, when the reactance components of the load impedance elements  204  and  205  are changed, FIG. 16 shows that the correlation coefficient between the two antennas  202  and  203  constituting the space selective diversity antenna can be reduced from the maximum value. In this case, as apparent from the equation (1), the correlation coefficient indicates the degree to which the directivity patterns of the two antennas  202  and  203  are overlapped with each other. The larger the correlation coefficient, the larger the overlapped relation between the directivity patterns, so that the performance as a space selective diversity antenna is deteriorated. On the other hand, the smaller the correlation coefficient, the smaller the overlapped portion of the directivity patterns, so that the performance of the space selective diversity antenna can be improved. In other words, the performance of the space selective diversity antenna can be improved by changing the reactance components of the load impedance elements  204  and  205  so as to reduce the correlation coefficient. According to the second preferred embodiment, the two antennas  202  and  203  having different polarizations also make up a polarization diversity. 
     In the preferred embodiment described above, the whip antenna  202  and the plane antenna  203  are used as an antennas making up a space selective diversity antenna, however, the present invention is not limited to this. Similar advantageous effects can be obtained even in, for example, a helical antenna, the other linear antennas, a dielectric tip antenna, a spiral plane antenna or the like. Also, similar effects can be obtained with a further increased number of antennas making up a space selective diversity antenna. 
     The aforementioned configuration of the space selective diversity antenna according to the present preferred embodiment includes one passive plane antenna  203  connected with the load impedance element  205 , however, the present invention is not limited to this. Two or more passive antennas each connected with a load impedance element may be provided. 
     THIRD PREFERRED EMBODIMENT 
     FIG. 3 is a block diagram showing a configuration of a radio set comprising a radio antenna apparatus according to a third preferred embodiment of the present invention and shows an extended state of an antenna unit thereof. FIG. 4 is a block diagram showing a contracted state of the antenna unit of the radio set of FIG.  3 . In FIGS. 3 and 4, the component parts similar to the corresponding ones in FIG. 2 are designated by the same reference numerals, respectively. The radio set of the third preferred embodiment is different from the radio set of FIG. 2 in the following points. 
     (a) An antenna unit  210  comprising a helical antenna  209  and a whip antenna  202  is provided in place of the whip antenna  202 . 
     (b) An antenna position detecting unit  233  is further provided for detecting whether the antenna unit  210  is extended or contracted. 
     (c) The transceiver unit  208  further comprises a received signal strength detecting unit  242  for detecting a strength of a signal received from a base station. 
     The above-mentioned differences will be described in detail. 
     The antenna unit  210  is constituted by a helical antenna  209  and a whip antenna  202  which are electrically insulated from each other and longitudinally coupled with each other. The entire longitudinal surface of the whip antenna  202  is formed of an electrical conductor. Also, the surface portion nearer to the whip antenna  202  at one end of a predetermined length of the helical antenna  209  is formed of an electrical conductor, although the other surface portion except for the particular end is formed of an electrically insulating material such as a dielectric material or the like. 
     Therefore, when the operator speaks and the antenna unit  210  is extended as shown in FIG. 3, the two contacts  232  and  233  connected to the antenna position detecting unit  241  and supported in opposed contact with the surface of the antenna unit  210  are both connected to an electrical conductor formed on the surface of the whip antenna  202 , so that the contacts  232  and  233  are short-circuited. On the other hand, the contact  231  is connected to one end of the whip antenna  202 , while the whip antenna  202  is connected to the transceiver unit  208  through the contact  231 , the feeder line  206  and the switch  211 . The short-circuited state between the contacts  232  and  233  is detected by the antenna position detecting unit  241 , and the detection signal is outputted to the controller  260 . In response thereto, the controller  260  switches over both of the switches  212  and  213  to the contact “a” thereof, for example, while at the same time controlling the horizontal plane directivity pattern to be that as shown in FIG. 13 by changing the value of the load impedance element  205 . Namely, while the operator is speaking and the head of the operator is located in proximity to the antenna unit  210  along the x-axis direction, the radio wave is not radiated toward the head of the operator of an obstacle, so that the electromagnetic radiation to the operator can be reduced while at the same time reducing the radio wave interference by the obstacle. 
     On the other hand, when the operator does not speak and the antenna  210  is contracted in standby state communicating with the base station for position registration as shown in FIG. 4, the contact  233  connected to the antenna position detecting unit  241  is brought into contact with the electrical conductor formed on the surface of the helical antenna  209 , while the contact  232  is brought into contact with the electrical insulating member formed on the surface of the helical antenna  209 . On the other hand, the contact  231  is connected to one end of the helical antenna  209 , and the helical antenna  209  is connected to the transceiver unit  208  through the contact  231 , the feeder line  206  and the switch  211 . In this case, the contacts  232  and  233  are in a non-conductive state, which state is detected by the antenna position detecting unit  241  and the resulting detection signal is outputted to the controller  260 . The controller  260  switches all of the switches  211 ,  212  and  213  to the contact “a” thereof while at the same time controlling the horizontal plane directivity pattern to be that as shown in FIG. 15 by changing the value of the load impedance element  205 . 
     In addition, when the plane antenna  203  is used, the switches  211 ,  212  and  213  are switched over to the contact “b” thereof under the control of the controller  260 , and the horizontal plane directivity pattern is controlled by changing the value of the load impedance element  204  connected to the whip antenna  202 . 
     Further, when the antenna  210  is contracted and the transceiver unit  208  is in standby state communicating with the base station for position registration or the like as shown in FIG. 4, the received signal strength detecting unit  208  detects, for example, an AGC current of an intermediate frequency amplifier of a receiver provided in the transceiver unit  208 , and then, detects the strength of the received signal from the base station, which detection signal is outputted to the controller  260 . On the other hand, the controller  260  switches over all of the switches  211 ,  212  and  213  to the contact “a” thereof, for example, while at the same time controlling the horizontal plane directivity pattern to be that as shown in FIG. 13 or  14 , for example, by changing the value of the load impedance element  205  in accordance with the strength of the received signal. Namely, the controller  260  changes the value of the load impedance element  205  so as to maximize the strength of the received signal, for example, this leads to controlling the plane directivity pattern so that the main beam is substantially directed toward the base station. 
     As described above in detail, a radio antenna apparatus according to the present invention is connected to the transceiver unit of a radio set and comprises an antenna element, a passive element arranged in proximity to the antenna element so as to be electromagnetically coupled to the antenna element, a load impedance element connected to the passive element and capable of changing the impedance value, and control means for changing a directivity pattern of the antenna element by changing an impedance value of the load impedance element. 
     In other words, the passive element functions as a wave director or a reflector for the antenna in dependence on the value of the load impedance element connected to the passive element, so that when the passive element functions as a wave director, the radiation in the direction toward the passive element becomes much stronger. On the other hand, when the passive element functions as a reflector, the radiation becomes much stronger in the direction opposite to that toward the passive element. Thus, by changing the value of the load impedance element, the directivity pattern of the radio antenna apparatus can be controlled. In the presence of an obstacle nearby, therefore, the radio wave interference due to the obstacle can be reduced by reducing the radiation toward the obstacle, and this leads to an improvement in the radiation efficiency. 
     Also, a radio antenna apparatus according to the present invention is connected to the transceiver unit of a radio set and comprises at least two antenna elements including first and second antenna elements arranged in such a proximity so as to be electromagnetically coupled with each other and constituting a space selective diversity antenna, a load impedance element capable of changing the impedance value, first switching means for selectively switching over so as to connect one of said first and second antenna elements to the transceiver unit of said radio set, and to connect another one thereof to said load impedance element, and control means for changing a directivity pattern of said antenna element by changing the impedance value of said load impedance element. 
     In other words, the other antenna, which is passive and separated electrically from the transceiver unit, functions as a wave director or a reflector for one antenna connected to the transceiver unit in dependence on the value of the load impedance element connected to the other antenna. In this case, when the other passive antenna functions as a wave director, the radiation in the direction toward the other passive antenna becomes much stronger. On the other hand, when the other passive antenna functions as a reflector, the radiation in the direction opposite to that toward the passive other antenna becomes much stronger. Therefore, by changing the value of the load impedance element, the directivity pattern of the radio antenna apparatus can be controlled. Accordingly, in the presence of an obstacle nearby, the radiation toward that direction can be reduced so as to reduce the radio wave interference due to the obstacle, and this leads to improvement in the radiation efficiency.