Patent Publication Number: US-2009231219-A1

Title: Antenna device for vehicle

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is based on and claims priority to Japanese Patent Application No. 2008-67651 filed on Mar. 17, 2008, the contents of which are incorporated in their entirety herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an antenna device for a vehicle. 
     2. Description of the Related Art 
     Conventionally, an inter-vehicle communication system and a road-to-vehicle communication system are known. In the inter-vehicle communication system, the vehicle transmits and receives information about an actual location, a speed, and a running direction with another vehicle for preventing a collision with each other. In the road-to-vehicle system including an electric toll collection system (ETC system) and a vehicle information and communication system (VICS), the vehicle communicates with an apparatus placed in the vicinity of a road. 
     An antenna used for the above-described systems can be disposed on a roof of the vehicle or on a dashboard of the vehicle. Alternatively, the antenna can be built in a room mirror for securing a space of a vehicle interior and improving an appearance. In a case where the antenna is built in the room mirror, a direction of the antenna is changed when a direction of the room mirror is changed. Thus, a stable communication is difficult to be secured. 
     JP-A-2006-279881 discloses a room mirror with a phased array antenna. A directivity of the phased array antenna is adjusted in accordance with an angle of the room mirror. US 2003/0090820A (corresponding to JP-A-2003-146136) discloses a room mirror with a built-in antenna. When an angle of the room mirror is changed, an angle of the antenna is changed by a gear so that an angle of the antenna with respect to a vehicle is unchanged. 
     A polarization plane of a radio wave transmitted from a transmitting side may be changed until the radio wave reaches a receiving side due to a communication environment, for example, an existence of a reflecting object. Thus, when a vertically-polarized wave is transmitted from the transmitting side, the polarization plane of the radio wave received by the receiving side may be different from the vertically-polarized wave, for example, a horizontally-polarized wave. 
     When the polarization plane is changed, a sensitivity and a gain of a receiving antenna may be reduced. Thus, a receiving performance may be reduced. In a method disclosed in JP-A-2006-279881, the directivity can be changed by using the phased array antenna. However, the phased array antenna cannot respond to a change in the polarization plane. Thus, a stable communication may be difficult to be secured depending on the communication environment. 
     Therefore, an antenna device is required to have a high receiving performance even when a polarization plane of a radio wave is changed in addition to adjust a directivity. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing problems, it is an object of the present invention to provide an antenna device for a vehicle. 
     An antenna device for a vehicle according to an aspect of the present invention includes a substrate, a first antenna, a second antenna, a first ground pattern, a second ground pattern, an intermediate pattern, a phase control part, and a combining portion. The substrate extends in an approximately horizontal direction. The first antenna and the second antenna are disposed at respective horizontal end portions of the substrate. Each of the first antenna and the second antenna is a vertically-polarized antenna. The first ground pattern and the second ground pattern are disposed on the substrate. The first ground pattern functions as a ground plane of the first antenna and the second ground pattern functions as a ground plane of the second antenna. The intermediate pattern is disposed on the substrate and is located between the first ground pattern and the second ground pattern. The intermediate pattern functions as a horizontally-polarized non-feed element for the first antenna and the second antenna. The phase control part controls a phase difference between a signal of the first antenna and a signal of the second antenna by changing at least one of a phase of the signal of the first antenna and a phase of the signal of the second antenna. The combining portion combines the signals after the phase control part controls the phase deference. The present antenna device can have a high receiving performance even if an arrival direction and a polarization plane of a received signal are changed. 
     An antenna device for a vehicle according to another aspect of the present invention includes a first antenna, a second antenna, an intermediate conductive member, a phase control part, and a combining portion. The first antenna and a second antenna are disposed apart from each other in an approximately horizontal direction. Each of the first antenna and the second antenna is a vertically-polarized antenna. The intermediate conductive member is disposed between the first antenna and the second antenna. The intermediate conductive member functions as a horizontally-polarized non-feed element for the first antenna and the second antenna. The phase control part controls a phase difference between a signal of the first antenna and a signal of the second antenna by changing at least one of a phase the signal of the first antenna and a phase of the signal of the second antenna. The combining portion is configured to combine the signals after the phase control part controls the phase deference. The present antenna device can have a high receiving performance even if an arrival direction and a polarization plane of a received signal are changed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Additional objects and advantages of the present invention will be more readily apparent from the following detailed description of preferred embodiments when taken together with the accompanying drawings. In the drawings: 
         FIG. 1  is a diagram illustrating a room mirror including an antenna device according to a first embodiment of the present invention; 
         FIG. 2  is a diagram illustrating a rear surface of a substrate of the antenna device; 
         FIG. 3  is a diagram illustrating a relationship between a phase difference of two antennas and a directivity of the antenna device; 
         FIG. 4  is a graph illustrating a relationship between the phase difference of the two antennas and an angle of a peak gain; 
         FIG. 5  is a graph illustrating a relationship between the phase difference of the two antennas and the peak gain; 
         FIG. 6  is a diagram illustrating the peak gain and the angle of the peak gain with respect to the phase difference of the two antennas; 
         FIG. 7  is a diagram illustrating a room mirror including an antenna device according to a second embodiment of the present invention; 
         FIG. 8A  is a cross-sectional view illustrating the room mirror including the antenna device according to the second embodiment and  FIG. 8B  is a front view of the antenna device; 
         FIG. 9  is a diagram illustrating a directivity of horizontally-polarized wave in a case where a phase difference of two antennas is 180 degree; 
         FIG. 10  is a diagram illustrating a room mirror including an antenna device according to a modification; and 
         FIG. 11  is a diagram illustrating an inverted L-shaped antenna. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     An antenna device  2  according to a first embodiment of the present invention will be described with reference to  FIG. 1  and  FIG. 2 . The antenna device  2  is built in a room mirror  1 . The room mirror  1  is provided in a front upper portion of a vehicle interior. The room mirror  1  is used by a user of a vehicle for seeing the vehicle interior and an outside of the vehicle. The room mirror  1  includes a mirror housing  3 , a mirror body  6  fitted in the mirror housing  3 , and a resin cover  4  covering the mirror housing  3 . The antenna device  2  is disposed in the mirror housing  3 . 
     On a front surface of the mirror body  6 , a metal film  7  is formed for reflecting light. On a rear surface of the mirror body  6 , a mirror plane is formed. The mirror body  6  is fitted in the mirror housing  3  in such a manner that the rear surface is exposed from the mirror housing  3  toward a rear side of the vehicle. Thus, the metal film  7  of the mirror body  6  is located inside of the mirror housing  3 . The mirror housing  3  is fixed to a ceiling of the vehicle interior through a supporting member  5 . 
     The antenna device  2  can be used for an inter-vehicle communication and a road-to-vehicle communication. The antenna device  2  is configured to transmit and receive a radio wave having a frequency band of 700 MHz, for example. 
     The antenna device  2  includes a substrate  10 . The substrate  10  is disposed in the mirror housing  3  in such a manner that a plane surface of the substrate  10  is approximately vertical to a ground and is approximately parallel to the mirror body  6  and metal film  7 . The substrate  10  is apart from the metal film  7  of the mirror body  6 . For example, a distance between the substrate  10  and the metal film  7  is about a quarter of a wavelength λ corresponding to a communication frequency. In the present embodiment, the communication frequency band is 700 MHz. 
     The substrate  10  extends in an approximately horizontal direction. A horizontal length of the substrate  10  is substantially similar to a horizontal length of the mirror body  6 . At respective horizontal end portions of a front surface of the substrate  10 , a first antenna  20  and a second antenna  30  are disposed. 
     The first antenna  20  is an inverted F antenna having a plate shape. The first antenna  20  is disposed on a right end portion of the substrate  10 . The first antenna  20  includes an antenna element  22  and a ground plane  21 . Between the ground plane  21  and the antenna element  22 , a dielectric member  23  having a predetermined permittivity is disposed. The antenna element  22  includes a rising part, an extending part, and a shorted part. The rising part rises from a feeding point  24  approximately vertically to the ground plane  21 . The extending part extends from an end portion of the rising part approximately parallel to the ground plane  21 , that is, approximately vertically to the ground. The shorted part falls approximately vertically from a part of a lower side of the extending part to the ground plane  21 . An end of the shorted part is shorted to the ground plane  21 . The shorted part is not illustrated in  FIG. 1 . The shorted part can be seen when the antenna device  2  is seen from a lower side. The ground plane  21  is disposed on a rear surface of the substrate  10 . 
     The second antenna  30  is an inverted F antenna having a plate shape. The second antenna  30  is disposed on a left end portion of the substrate  10 . The second antenna  30  includes an antenna element  32  and a ground plane  31 . Between the ground plane  31  and the antenna element  32 , a dielectric member  33  having a predetermined permittivity is disposed. The antenna element  32  includes a rising part, an extending part, and a shorted part. The rising part rises from a feeding point  34  approximately vertically to the ground plane  31 . The extending part extends from an end portion of the rising part approximately parallel to the ground plane  31 , that is, approximately vertically to the ground. The shorted part falls approximately vertically from a part of a lower side of the extending part to the ground plane  31 . An end of the shorted part is shorted to the ground plane  31 . The shorted part is not illustrated in  FIG. 1 . The shorted part can be seen when the antenna device  2  is seen from a lower side. The ground plane  31  is disposed on the rear surface of the substrate  10 . 
     Each of the first antenna  20  and the second antenna  30  is a vertically-polarized wave antenna that transmits and receives a vertically-polarized wave strongly. A property of the first antenna  20  is substantially similar to a property of the second antenna  30 . 
     The substrate  10  is not required to be disposed strictly vertically to the ground. The substrate  10  may be disposed approximately vertically to the ground so that the first antenna  20  and the second antenna  30  can provide a desired performance. Similarly, in the above-described positional relationships between each of the antenna elements  22  and  23  and the substrate  10 , it is not required to be strictly vertical or strictly parallel. The positional relationships may be approximately vertical or approximately parallel so that the first antenna  20  and the second antenna  30  can provide the desired performance. 
     Between the first antenna  20  and the second antenna  30 , two phase shifters  12  and  13  and a combiner/distributor  14  are formed on the front surface of the substrate  10  by microstrip lines. A feeding line between each of the phase shifters  12  and  13  and the combiner/distributor  14  and a feeding line between each of the phase shifters  12  and  13  and corresponding one of the feeding points  24  and  34  are formed by microstrip lines. 
     The phase shifter  12  is coupled with the feeding point  24  of the first antenna  20 . The phase shifter  12  shifts a phase of a signal received by the first antenna  20  or a phase of a signal transmitted from the first antenna  20 . The phase shifter  12  is a variable phase shifter. A phase shifting amount of the phase shifter  12  is changed in accordance with the phase-control signal transmitted from an electronic control unit (ECU)  80  through a transmission cable  17 . The ECU  80  is provided aside from the room mirror  1 . 
     The phase shifter  13  is coupled with the feeding point  34  of the second antenna  30 . The phase shifter  13  shifts a phase of a signal received by the second antenna  30  or a phase of a signal transmitted from the second antenna  30 . The phase shifter  13  is a variable phase shifter. A phase shifting amount of the phase shifter  13  is changed in accordance with the phase-control signal transmitted from the ECU  80  through a transmission cable  18 . 
     Each of the phase shifters  12  and  13  may be a hybrid phase shifter (reflective variable phase shifter) including a hybrid coupler and a variable-capacitance element. The ECU  80  can be disposed in any position in the vehicle. For example, the ECU  80  may be disposed in the dashboard. The ECU  80  executes various controls of the inter-vehicle communication and the road-to-vehicle communication. The ECU  80  includes a microcomputer  81 , a transmitting/receiving control part  82 , and a phase control circuit  83 . The microcomputer  81  controls the transmitting/receiving control part  82 . The transmitting/receiving control part  82  controls a transmitting and receiving of a signal at the antenna device  2 . The phase control circuit  83  outputs a phase-control signal to the phase shifters  12  and  13  based on a phase-shifting order from the transmitting/receiving control part  82 . The ECU  80  can include various elements including a hardware and a software although only main components related to the antenna device  2  are illustrated in  FIG. 1 . 
     The phase control circuit  83  outputs the phase-control signal to the phase shifters  12  and  13  for changing a phase-shifting amount of each of the phase shifters  12  and  13 . Thereby, a phase difference of the received signals of the first antenna  20  and the second antenna  30  and a phase difference of the transmitting signals from the first antenna  20  and the second antenna  30  can be a predetermined value. The phase control circuit  83  may be disposed in the room mirror  1 , for example, on the substrate  10 . 
     The received signal of the first antenna  20  is phase-shifted at the phase shifter  12 . The received signal of the second antenna  30  is phase-shifted at the phase shifter  13 . The combiner/distributor  14  combines the phase-shifted received signals and transmits the combined signal to a communication device (not shown) through a coaxial cable  16 . In addition, the combiner/distributor  14  equally distributes a signal transmitted from the communication device through the coaxial cable  16  to the phase shifters  12  and  13 . 
     On the rear surface of the substrate  10 , the ground plane  21  of the first antenna  20 , the ground plane  31  of the second antenna  30 , and a circuit ground  11  are disposed, as shown in  FIG. 2 . The circuit ground  11  can function as a common ground of the phase shifters  12  and  13  and the combiner/distributor  14 . The ground planes  21  and  31  and the circuit ground  11  are physically-connected with each other and are made of one electric conductive film. The substrate  10  has a slit  26  between the ground plane  21  and the circuit ground  11 . In addition, the substrate  10  has a slit  36  between the circuit ground  11  and the ground plane  31 . Thus, the ground plane  21  and the ground plane  31  are apart from the circuit ground  11  at a predetermined region. 
     The slit  36  is provided in approximately vertical direction of the substrate  10  in such a manner that a feeding ground width Wg remains from a lower end of the substrate  10 . A feeding line  35  extends from the phase shifter  13  to the feeding point  34  of the second antenna  30 . The feeding ground width Wg is greater than or equal to three times a line width Ws of the feeding line  35 , for example. When the feeding ground width Wg is greater than or equal to three times the line width Ws of the feeding line  35 , a performance of the feeding line  35  can be maintained with certainty. However, the feeding ground width Wg may not be greater than or equal to three times the line width Ws of the feeding line  35 . 
     A relationship between a feeding ground width Wg and the line width Ws of the first antenna  20  is substantially similar to the relationship between the feeding ground width Wg and the width Ws of the second antenna  30 . A distance between a center portion of the first antenna  20  and a center portion of the second antenna  30  is about ½λ. When the communication frequency band is 700 MHz, ½λ is about 20 cm, which is approximately equal to a length of a general room mirror. Thus, the substrate  10  can be fitted in the mirror housing  3  of the room mirror  1 . 
     Due to the slits  26  and  36 , each of the first antenna  20  and the second antenna  30  can have a high gain for the vertically-polarized wave. If the slits  26  and  36  are not provided and the substrate  10  has a simple rectangular shape, a width of each of the ground planes  21  and  31  increases with respect to a width of corresponding one of the antenna elements  22  and  32 . Thus, each of the first antenna  20  and the second antenna  30  cannot have a high gain for the vertically-polarized wave. 
     In the antenna device  2  according to the present embodiment, the ground planes  21  and  31  are separated from the circuit ground  11  by the slits  26  and  36 . Thus, the gain of each of the first antenna  20  and the second antenna  30  for the vertically-polarized wave increases. 
     The slits  26  and  36  may be provided by making cuts in the substrate  10 . Alternatively, the silts  26  and  36  may be provided only in the electric conductive film on the rear surface of the substrate  10 , for example, by etching. 
     In the antenna device  2  according to the present embodiment, the electric conductive film is formed on the whole area of the rear surface of the substrate  10 . The electric conductive film is divided into the ground plane  21 , the circuit ground  11 , and the ground plane  31  by the slits  26  and  36 . That is, due to the slits  26  and  36 , the ground plane  21  and the ground plane  31  are apart from the circuit ground  11  at least at a region where a distance from the feeding point  24  and  34  is greater than a predetermined distance in the vertical direction. However, the ground plane  21 , the circuit ground  11 , and the ground plane  31  are physically and electrically connected with each other at a region corresponding to the feeding line  35 , that is, a region having the feeding ground width Wg. Therefore, the ground plane  21 , the circuit ground  11 , and the ground plane  31  are formed of the one electric conductive film that is physically integrated and is electrically conductive. 
     The circuit ground  11  located between the ground planes  21  and  31  can function as the ground of the phase shifters  12  and  13  and the combiner/distributor  14 . Furthermore, the circuit ground  11  can function as a horizontally-polarized non-feed element for the first antenna  20  and the second antenna  30 . 
     When the first antenna  20  receives a radio wave or when a transmitting signal is supplied to the first antenna  20 , an electric current flows in the first antenna  20 . Especially, a large electric current flows to the feeding point  24 . When the electric current flows in the first antenna  20 , the electric current also flows in the circuit ground  11  due to an electromagnetic coupling. The electric current due to the electromagnetic coupling with the first antenna  20  (first electric current) flows in the circuit ground  11  in an approximately horizontal direction. Thereby, the circuit ground  11  can function as the horizontally-polarized non-feed element. A relationship between the second antenna  30  and the circuit ground  11  is substantially similar to the above-described relationship between the first antenna  20  and the circuit ground  11 . Thus, an electric current due to the electromagnetic coupling with the second antenna  30  (second electric current) flows in the circuit ground  11  in an approximately horizontal direction. 
     A direction and the amount of the first electric current and the second electric current are changed in accordance with the phase difference of the received signals of the first antenna  20  and the second antenna  30  or the phase difference of the transmitting signals from the first antenna  20  and the second antenna  30 . 
     In theory, when the phase-shifting amount of the phase shifters  12  and  13  are controlled so that the phase difference of the signals of the first antenna  20  and the second antenna  30  becomes 0 degree (same phase), the first electric current and the second electric current weaken each other. When the phase-shifting amount of the phase shifters  12  and  13  are controlled so that the phase difference of the signals of the first antenna  20  and the second antenna  30  becomes 180 degrees, the first electric current and the second electric current have approximately the same phase and flows in approximately the same direction. Thus, the first electric current and the second electric current strengthen each other. Therefore, the antenna device  2  can have a high gain for the horizontally-polarized wave. 
     If the circuit ground  11  is not provided in the antenna device  2  and the first antenna  20  and the second antenna  30  are apart from each other so as to have a distance D therebetween, the antenna device  2  can function as only a phased array antenna that can control a directivity of the vertically-polarized wave by controlling the phase-shifting amount of the phase shifters  12  and  13 . 
     The antenna device  2  according to the present embodiment includes the circuit ground  11  located between the first antenna  20  and the second antenna  30 . The circuit ground  11  can function as the horizontally-polarized non-feed element for the first antenna  20  and the second antenna  30 . Thus, by controlling the phase difference of the signals of the first antenna  20  and the second antenna  30 , the antenna device  2  can change the polarization planes of the horizontally-polarized wave and the vertically-polarized wave and can control the directivities of each of the polarized wave. 
     Changes in the directivities and the gains of the vertically-polarized wave and the horizontally-polarized wave when the phase difference of the received signals of the first antenna  20  and the second antenna  30  is changed will be described with reference to  FIG. 3  to  FIG. 6 . The phase difference is changed in steps of 10 degrees or 5 degrees. The directivities and the gains of the vertically-polarized wave and the horizontally-polarized wave are measured at each of the phase differences when a radio wave of a predetermined level is received. 
     The directivities of the vertically-polarized wave (VW) and the horizontally-polarized wave (HW) of the received signals of the first antenna  20  and the second antenna  30  at a time when the phase difference is 0 degree, ±90 degrees, and ±180 degrees are shown in  FIG. 3 . The phase difference is a relative difference of the received signal of the first antenna  20  with respect to the received signal of the second antenna  30 . Thus, when the phase difference is a positive value, the phase of the received signal of the first antenna  20  is ahead of the phase of the received signal of the second antenna  30 . 
     As illustrated in  FIG. 3 , when the phase difference is ±180 degrees, the vertically-polarized wave does not have sufficient gain. When the phase difference is +90 degrees, the vertically-polarized wave has a sufficient peak gain to the left side of the vehicle. When the phase difference is 0 degrees, the vertically-polarized wave has a sufficient peak gain to the front side of the vehicle. When the phase difference is −90 degrees, the vertically-polarized wave has a sufficient peak gain to the right side of the vehicle. 
     Thus, by changing the phase difference from −90 degrees to +90 degrees, a direction of the peak gain can be changed from the left side to the right side. As illustrated in  FIG. 4 , when the phase difference is changed from −90 degrees to +90 degrees, a peak gain angle can be changed from −20 degrees to +20 degrees. When the absolute value of the phase difference exceeds 90, the peak gain decreases under −3 dB. Thus, when the phase difference is from −90 degrees to +90 degrees, the antenna device  2  can have a high receiving performance for the vertically-polarized wave. In this range, the peak gain angle can be changed from −20 degrees to +20 degrees. 
     On the other hand, the horizontally-polarized wave does not have sufficient gain when the phase difference is 0 degree. When the phase difference is ±90 degrees, the horizontally-polarized wave has a sufficient peak gain. When the phase difference is ±180 degrees, the peak gain of the horizontally-polarized wave further increases. The peak gain angle of the horizontally-polarized wave does not change apparently compared with the peak gain angle of the vertically-polarized wave. When the phase difference is −90 degrees and −180 degrees, the peak gain angle of the horizontally-polarized wave is left-leaning. When the phase difference is +90 degrees and the +180 degrees, the peak gain angle of the horizontally-polarized wave is right-leaning. 
     As illustrated in  FIG. 4  and  FIG. 6 , when the phase difference is from +10 degrees to +130 degrees, the peak gain angle of the horizontally-polarized wave is about −30±10 degrees and the direction of the peak angle is the left side. When the phase difference is other than above-described range, the direction of the peak gain angle changes to the right side and the peak gain angle is about +45±10 degrees. When the absolute value of the phase difference is less than 60, the peak gain decreases under −3 dB, as illustrated in  FIG. 5 . Thus, when the absolute value of the phase difference is greater than or equal to 60, the antenna device  2  can have a high receiving performance for the horizontally-polarized wave. 
     As described above, the antenna device  2  according to the present embodiment is disposed in the room mirror  1 . The antenna device  2  includes the first antenna  20  and the second antenna  30  disposed on the respective horizontal end portions of the substrate  10  and the circuit ground  11  disposed between the first antenna  20  and the second antenna  30 . Each of the first antenna  20  and the second antenna  30  is the vertically-polarized antenna. The circuit ground  11  can function as the horizontally-polarized non-feed element for the first antenna  20  and the second antenna  30 . The signals of the first antenna  20  and the second antenna  30  are controlled by the corresponding phase shifters  12  and  13  so that the phase difference of the signals becomes a desired value. As illustrated in  FIG. 3  to  FIG. 6 , the polarization planes and the directivities of the horizontally-polarized wave and the vertically-polarized wave can be changed by controlling the phase difference. 
     Thus, even if an arrival direction and the polarization plane of the received signal are changed, the antenna device  2  can have a high receiving performance. In addition, due to a reversibility of a transmitting/receiving property of an antenna, the antenna device  2  can have similar effects at a time of transmitting signal. 
     On the rear surface of the substrate  10 , the ground planes  21  and  31  and the circuit ground  11  are formed of the one electric conductive film. Thus, the electric conductive film can be formed easily and the gain of the horizontally-polarized wave is further improved. 
     The ground planes  21  and  31  are separated from the circuit ground  11  by the corresponding slits  26  and  36 . Thus, the gain of the vertically-polarized wave of each of the first antenna  20  and the second antenna  30  can be improved, and thereby the gain of the vertically-polarized wave of the antenna device  2  can be improved. 
     On the substrate  10 , the phase shifters  12  and  13  and the combiner/distributor  14  are formed of the microstrip lines in addition to the first antenna  20  and the second antenna  30 . Thus, a dimension of the antenna device  2  can be reduced. 
     The antenna device  2  is disposed in the mirror housing  3 . The substrate  10  is apart from the metal film  7  and is arranged in parallel with the metal film  7 . In the present embodiment, the distance between the substrate  10  and the metal film  7  is about ¼λ. Because the antenna device  2  is disposed in the mirror housing  3 , the space of the vehicle interior is not reduced. In addition, because the antenna device  2  is hidden from an outside of the room mirror  1 , the appearance of the vehicle interior can be improved. The substrate  10  is disposed in the mirror housing  3  in such a manner that the rear surface of the substrate  10  faces the metal film  7  of the mirror body  6 . Thus, the electric current that flows in the circuit ground  11  in the horizontal direction is increased due to a reflection by the metal film  7 . Therefore, the gain of the horizontally-polarized wave of the antenna device  2  can be further improved. Even if the direction of the mirror body  6  is changed by a user of the vehicle, the antenna device  2  can control the directivity by changing the phase difference. Therefore, the antenna device  2  can have a high receiving performance regardless of the direction of the mirror body  6 . 
     Each of the first antenna  20  and the second antenna  30  is the inverted F antenna having the plate shape. Thus, the dimension of each of the first antenna  20  and the second antenna  30  can be reduced. In addition, each of the first antenna  20  and the second antenna  30  can have a high gain of the vertically-polarized wave. 
     In the present embodiment, the phase control circuit  83  can function as a control-signal output portion. The phase shifters  12  and  13  can function as a phase changing portion. The combiner/distributor  14  can function as a combining portion and a distributing portion. The circuit ground  11  can function as an intermediate pattern. 
     Second Embodiment 
     An antenna device  41  according to a second embodiment of the present invention will be described with reference to  FIG. 7 ,  FIG. 8A  and  FIG. 8B . The antenna device  41  is disposed in a room mirror  40 . The room mirror  40  includes a mirror housing  3 , a resin cover  4 , a supporting member  5 , and a mirror body  6  similar to those of the room mirror  1  according to the first embodiment. 
     The antenna device  41  includes a first antenna  50  and a second antenna  60 . The first antenna  50  and the second antenna  60  are apart from each other in the approximately horizontal direction. Each of the first antenna  50  and the second antenna  60  is an inverted F antenna having a plate shape. 
     The first antenna  50  is disposed on the right side. The first antenna  50  includes a ground plane  51  and an antenna element  52 . The second antenna  60  is disposed on the left side. The second antenna  60  includes a ground plane  61  and an antenna element  62 . Each of the ground planes  51  and  61  are arranged approximately vertically to the ground. A distance D between a center portion of the first antenna  50  and a center portion of the second antenna  60  is about ½λ. 
     The antenna device  41  further includes a metal plate  42 . The metal plate  42  is disposed between the first antenna  50  and the second antenna  60 . The metal plate  42  is located on a rear-surface side of the ground planes  51  and  61 . Thus, the metal plate  42  is located on an opposite side of the ground planes  51  and  61  from the antenna elements  52  and  62 . The metal plate  42  can function as an intermediate conductive member. The metal plate  42  has cutout portions  56  and  66  at respective horizontal end portions. 
     As illustrated in  FIG. 8B , the first antenna  50  faces the metal plate  42  only at a predetermined section including a feeding point  54 . The other section of the first antenna  50  is located outside of the horizontal end portion of the metal plate  42 . Thus, the other section of the first antenna  50  does not face the metal plate  42 . The second antenna  60  faces the metal plate  42  only at a predetermined section including a feeding point  64 . The other section of the second antenna  60  does not face the metal plate  42 . 
     The metal plate  42  has the cutout portions  56  and  66  at the respective horizontal end portions so that only the predetermined sections of the first antenna  50  and the second antenna  60  adjacent to the corresponding feeding points  54  and  64  face the metal plate  42  and the other sections of the first antenna  50  and the second antenna  60  are located outside horizontal ends of the metal plate  42 . Thereby, each of the first antenna  50  and the second antenna  60  can function as the vertically-polarized antenna and the metal plate  42  coupled with the first antenna  50  and the second antenna  60  can function as a horizontally-polarized non-feed element. 
     On a front surface of the metal plate  42 , a phase shifter  44 , a phase control circuit  46 , and a combiner/distributor  45  are disposed between the first antenna  50  and the second antenna  60 . An ECU  90  includes a microcomputer  81  and a transmitting/receiving control part  82 . The phase control circuit  46  outputs a phase-control signal to the phase shifter  44  in accordance with a phase-shifting order from the transmitting/receiving control part  82 . 
     The whole area of the metal plate  42  faces the metal film  7  of the mirror body  6 , as illustrated in  FIG. 8A . A distance between the metal plate  42  and the metal film  7  is about ¼λ. 
     In the antenna device  41  shown in  FIG. 7 , the ground plane  51  and the antenna element  52  of the first antenna  50  have a clearance therebetween and the ground plane  61  and the antenna element  62  of the second antenna  60  have a clearance therebetween. A dielectric member may be interposed between the ground plane  51  and the antenna element  52  and between the ground plane  61  and the antenna element  62 . The phase shifter  44  and the combiner/distributor  45  may be disposed on a substrate instead of the metal plate  42 . In the present case, the substrate may be formed into a shape similar to the metal plate  42  so that the substrate can function as a horizontally-polarized non-feed element. 
     Thus, the antenna device  41  can control the polarization planes and the directivities of the horizontally-polarized wave and the vertically-polarized wave by controlling the phase difference of the signals of the first antenna  50  and the second antenna  60  in a manner similar to the antenna device  2  according to the first embodiment. For example, the antenna device  41  can have a high gain of the vertically-polarized wave by controlling the phase difference to ±90 degrees. In addition, the antenna device  41  control the directivity, that is, the peak gain angle of the vertically-polarized wave by changing the phase difference. Furthermore, the antenna device  41  can have a high gain of the horizontally-polarized wave by controlling the phase difference to ±180 degrees and can control the directivity of the horizontally-polarized wave by changing the phase difference. 
     The directivity of the horizontally-polarized wave at a time when the phase shifter  44  is controlled so that the phase difference of the signals of the first antenna  50  and the second antenna  60  becomes 180 degrees will be described with reference to  FIG. 9 . The directivity of the antenna device  41  according to the present embodiment, that is, in a case where both of the metal plate  42  and the mirror body  6  are provided is compared with a directivity of an antenna device according to a first comparative example and a directivity of an antenna according to a second comparative example. 
     In the antenna device according to the first comparative example, the metal plate  42  is not disposed between the first antenna  50  and the second antenna  60 , and the mirror body  6  is not provided. Thus, as illustrated by the dotted line IXa in  FIG. 9 , the antenna device can function only as a vertically-polarized phased array antenna and cannot have a gain of the horizontally-polarized wave. In the antenna device according to the second comparative example, although the metal plate  42  is not disposed between the first antenna  50  and the second antenna  60 , the mirror body  6  is provided. Thus, as illustrated by the dashed line IXb in  FIG. 9 , the metal film  7  of the mirror body  6  can function as a horizontally-polarized non-feed element. Therefore, the antenna device according to the second comparative example can have a gain of the horizontally-polarized wave at a high level. In the antenna device according to the present embodiment, both of the metal plate  42  and the mirror body  6  are provided. The function of the metal plate  42  as the horizontally-polarized non-feed element is further increased due to the reflection by the mirror body  6 . Thus, as illustrated by the solid line IXc, the antenna device according to the present embodiment can have a gain of horizontally-polarized wave at a level higher than the gain of the second comparative example. 
     As described above, the room mirror  40  according to the present embodiment includes the antenna device  41 . The antenna device  41  includes the first antenna  50 , the second antenna  60 , and the metal plate  42 . Each of the first antenna  50  and the second antenna  60  is the vertically-polarized antenna. The first antenna  50  and the second antenna  60  are apart from each other in the approximately horizontal direction. The metal plate  42  is arranged between the first antenna  50  and the second antenna  60  on the rear side of the ground planes  51  and  61 . The metal plate  42  can function as the horizontally-polarized non-feed element for both of the first antenna  50  and the second antenna  60 . The phase difference between the signal of the first antenna  50  and the signal of the second antenna  60  are controlled to be the desired amount. Thereby, the polarization plane can be changed between the horizontally-polarized wave and the vertically-polarized wave, and the directivities of each of the polarized wave can be changed in a manner similar to the first embodiment. 
     Thus, even if an arrival direction and the polarization plane of the received signal are changed, the antenna device  41  can have a high receiving performance. Also at a time of transmitting signal, the antenna device  41  can have effects similar to the time of receiving signal. The first antenna  50  faces the metal plate  42  only at the predetermined section including the feeding point  54  and the other section of the first antenna  50  does not face the metal plate  42 . The second antenna  60  faces the metal plate  42  only at a predetermined section including a feeding point  64  and the other section of the second antenna  60  does not face the metal plate  42 . Thus, each of the first antenna  50  and the second antenna  60  can have a high gain of the vertically-polarized wave. The first antenna  50  and the second antenna  60  are strongly coupled with each other centering on each of the feeding points  54  and  64 , and large horizontal electric current flows in the metal plate  42 . Thus, each of the first antenna  50  and the second antenna  60  can have high gain of the horizontally-polarized wave. As a result, the gain of the horizontally-polarized wave of the whole antenna device  41  can be increased. 
     In the present embodiment, the antenna device  41  is disposed in the mirror housing  3 . Thus, the metal plate  42  may be omitted and the metal film  7  of the mirror body  6  may be used as a non-feed element instead of the metal plate  42 . In such a case, a dimension of the antenna device  41  can be reduced while keeping the high gain of the vertically-polarized wave and the horizontally-polarized wave. 
     Other Embodiments 
     Although the present invention has been fully described in connection with the preferred embodiments thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications will become apparent to those skilled in the art. 
     In the second embodiment, the room mirror  40  includes the phase shifter  44 , the combiner/distributor  45 , and the phase control circuit  46  in addition to the first antenna  50  and the second antenna  60 . Alternatively, as illustrated in  FIG. 10 , the room mirror  40  may include the first antenna  50 , the second antenna  60 , and the metal plate  42 , and an in-vehicle ECU  100  may include the phase shifter  44 , the combiner/distributor  45 , and the phase control circuit  46 . 
     Alternatively, the room mirror  40  may include the phase shifter  44 , and the in-vehicle ECU  100  may include the combiner/distributor  45  and the phase control circuit  46 . Components disposed in the mirror housing  3  and components disposed outside the mirror housing  3  are appropriately determined so that the antenna device  41  can have a desired function. Also in the room mirror  1  according to the first embodiment, components disposed in the mirror housing  3  and disposed outside the mirror housing  3  can be determined so that the antenna device  2  can have a desired function. 
     In the above-described embodiments, the antenna device  2  or the antenna device  41  is disposed in the mirror housing  3 , as an example. Each of the antenna device  2  and the antenna device  41  may be coupled with the corresponding room mirror  1  or  40 , for example, by fixing each of the antenna device  2  and the antenna device  41  on the front side of the resin cover  4 . Alternatively, each of the antenna device  2  and the antenna device  41  may be separated from the corresponding room mirror  1  or  40 . For example, each of the antenna device  2  and the antenna device  41  may be disposed in or coupled with other device or a module. 
     In the first embodiment, each of the first antenna  20  and the second antenna  30  is the inverted F antenna having the plate shape, as an example. Each of the first antenna  20  and the second antenna  30  may have other shape as long as a desired property can be obtained. For example, each of the first antenna  20  and the second antenna  30  may be an inverted L antenna as illustrated in  FIG. 11 . The inverted L antenna includes a ground plane  71  and an antenna element  72  disposed on the ground plane  71 . The antenna element  72  has an inverted L shape. A portion of the ground plane  71  where the antenna element  72  rises becomes a feeding point  73 . 
     Each of the first antenna  20  and the second antenna  30  is not limited to a plate-shape antenna. For example, each of the first antenna  20  and the second antenna  30  may be an inverted F antenna or an inverted L antenna having a line shape. The plate shape antenna is preferable to the line shape antenna in view of matching impedance, forming an antenna element, and a stability of the antenna element fixed to a ground plane. The first antenna  20  and the second antenna  30  may be different types. For example, one of the first antenna  20  and the second antenna  30  may be an inverted F antenna and the other may be an inverted L antenna. The first antenna  50  and the second antenna  60  may be various types of antenna in a manner similar to the first antenna  20  and the second antenna  30 . 
     In the first embodiment, the circuit ground  11  that can function as a horizontally-polarized non-feed element is formed at the whole area of rear surface of the substrate  10  except of the regions corresponding to the ground planes  21  and  31 . The circuit ground  11  is not required to be formed at such a wide area. For example, the circuit ground  11  may be a metal pattern having a line shape connecting a portion located adjacent to the feeding point  24  of the first antenna  20  and a portion located adjacent to the feeding point  34  of the second antenna  30 . The circuit ground  11  may have various shapes as long as the circuit ground  11  is coupled with the first antenna  20  and the second antenna  30  and can function as the horizontally-polarized non-feed element. The metal plate  42  in the second embodiment may have various shapes as long as the metal plate  42  is coupled with the first antenna  50  and the second antenna  60  and can function as the horizontally-polarized non-feed element. 
     In the above-described embodiments, each of the phase shifters  12 ,  13  and  44  is a hybrid phase shifter (reflective variable phase shifter), as an example. Each of the phase shifters  12 ,  13 , and  44  may be other type of phase shifter as long as the phase shifters  12 ,  13 , and  44  can change the phase of the received signal or the phase of the transmitting signal into a desired phase. 
     In the first embodiment, the two phase shifters  12  and  13  are provided, as an example. Alternatively, only one of the phase shifters  12  and  13  may be provided in a manner similar to the second embodiment. On the other hand, in the second embodiment, the number of phase shifter may be two in a manner similar to the first embodiment. A phase shifter may be provided for each of two antennas or may be provided for one of two antennas as long as the phase shifter can control the phase difference of signals into a desired amount. 
     In the above-described embodiment, a distance between the metal film  7  and the substrate  10  and a distance between the metal film  7  and the metal plate are set to be about ¼λ. The distances may be shorter than about ¼λ. When each of the distances is longer than about ¼λ, an electric current that flows in a pattern functioning as non-feed element may counteract the reflection wave by the metal film  7 . 
     In the above-described embodiments, each of the antenna device  2  and the antenna device  41  changes the polarization plane and controls the directivity both at a time of receiving a signal and a time of transmitting a signal. Alternatively, each of the antenna device  2  and the antenna device  41  may be configured to change the polarization plane and control the directivity only at one of a time of receiving a signal and a time of transmitting a signal.