Abstract:
Since a front antenna  33  performs road-to-vehicle communication and vehicle-to-vehicle communication with oncoming vehicles, a nondirectional antenna enables effective communication in communication distances. Vehicle-to-vehicle communication with rearward vehicles by a rear antenna  37  needs long communication distances compared with road-to-vehicle communication and vehicle-to-vehicle communication with oncoming vehicles. The rear antenna  37  can also perform communication effectively with rear vehicles since what is selected is a directional one that can provide longer communication distances than a nondirectional one and that has directionality rearward of the vehicle. The front antenna  33  is disposed forward of a seat  7 , and the rear antenna  37  is disposed rearward of the seat  7 , which can inhibit an attenuation of electromagnetic waves due to the rider and the like. Consequently, while being capable of performing road-to-vehicle communication effectively, vehicle-to-vehicle communication with rearward vehicles can also be performed effectively.

Description:
TECHNICAL FIELD 
       [0001]    This invention relates to two-wheeled motor vehicles for performing communications specified in ASV (Advanced Safety Vehicle) using IT (Information Technology), which is a part of the ITS (Intelligent Transport System), and more particularly relates to antenna arranging structures. 
       BACKGROUND ART 
       [0002]    Conventionally, there exists a first apparatus that has an antenna disposed forward of a vehicle for performing communications with vehicles in front or with roadside units (see Patent Document 1, for example). Specifically, a nondirectional antenna is disposed forward of an inclined line extending between the middle of a bar handle and the axle of a front wheel. This prevents influences of wave absorption by the rider&#39;s body, and prevents lowering of electric field strength in up and down and right and left directions. 
         [0003]    Further, there exists a second apparatus that has a base in a storage box at the rear, and has a nondirectional antenna removably attached to the base (see Patent Document 2, for example). The antenna in this second apparatus is for receiving radio, and not for performing communications for an advanced safety vehicle ASV. However, a construction is employed to dispose the nondirectional antenna in the rear storage box. 
       PRIOR ART DOCUMENTS 
     Patent Documents 
       [0004]    [Patent Document 1] 
         [0005]    Unexamined Patent Publication No. 2001-352209 
         [0006]    [Patent Document 2] 
         [0007]    Unexamined Patent Publication No. 2008-239069 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0008]    However, the conventional examples with such constructions have the following problems. 
         [0009]    The first conventional apparatus is suitable for performing road-to-vehicle communication using spot communication (also called DSRC (Dedicated Short Range Communication)) with roadside units (also called ITS spots) installed at roadside, for example, for providing ITS spot service, in order to use an information providing service, charge settlement service and so on. On the other hand, there is a disadvantage where, of vehicle-to-vehicle communications which are communications between the rider&#39;s own vehicle and other vehicles, communication is made with another vehicle located rearward. This is because, although vehicle-to-vehicle communication with a rearward vehicle requires a longer communication distance than road-to-vehicle communication and vehicle-to-vehicle communication at a time of passing each other, a transmit-receive distance with a rearward vehicle becomes short under the influence of attenuation or reflection by the rider or the load located rearward of the antenna. 
         [0010]    The second conventional apparatus is also applicable to vehicle-to-vehicle communication with other vehicles located rearward, but is disadvantageous for vehicle-to-vehicle communication with other vehicles located in front. This is because a transmit-receive distance with forward locations becomes short under the influence of attenuation or reflection by the rider or the load located forward of the antenna. 
         [0011]    This invention has been made in view of the state of the art noted above, and its object is to provide two-wheeled motor vehicles which, while being capable of effectively performing road-to-vehicle communication and vehicle-to-vehicle communication with forward vehicles (preceding vehicles and oncoming vehicles), can also effectively perform vehicle-to-vehicle communication with rearward vehicles. 
       Solution to Problem 
       [0012]    To fulfill the above object, this invention provides the following construction. 
         [0013]    This invention provides a two-wheeled motor vehicle for performing road-to-vehicle communication with roadside units, and vehicle-to-vehicle communication with forward vehicles and rearward vehicles, comprising a front antenna disposed forward of a seat of the two-wheeled motor vehicle; and a rear antenna rearward disposed rearward of the seat of the two-wheeled motor vehicle; wherein the front antenna consists of a nondirectional antenna; and the rear antenna consists of a directional antenna having directionality rearward of a vehicle body. 
       Functions and Effects 
       [0014]    According to the invention, the front antenna mainly performs road-to-vehicle communication with roadside units and vehicle-to-vehicle communication with preceding vehicles and oncoming vehicles at times of passing each other. It is therefore possible to perform communications effectively by employing a nondirectional antenna which has wide-angle transmit-receive characteristics. On the other hand, vehicle-to-vehicle communication with rearward vehicles by means of the rear antenna needs a long communication distance compared with the road-to-vehicle communication and vehicle-to-vehicle communication with oncoming vehicles at times of passing each other. In this invention, the rear antenna has directionality that enables a long communication distance compared with nondirectionality, and that is rearward of the vehicle. It is therefore possible to perform communications effectively with rearward vehicles also. Since the front antenna is disposed forward of the seat, and the rear antenna rearward of the seat, the electromagnetic waves can be inhibited from being attenuated by the bodies of the rider and passenger. The rear antenna having a rearward directionality can inhibit bad influences of an interference with the electromagnetic waves of the front antenna. As a result, the road-to-vehicle communication and the vehicle-to-vehicle communication with forward vehicles (preceding vehicles and oncoming vehicles) can be performed effectively, and at the same time the vehicle-to-vehicle communication with rearward vehicles can also be performed effectively. 
         [0015]    In this invention, it is preferred that the rear antenna is attached below a taillight unit. 
         [0016]    There are a grab bar and a load carrier above the taillight unit, and when a load is placed thereon, the rear antenna may be blocked by the load, which may exert a bad influence on the emission of electromagnetic waves. However, since the rear antenna is located below the taillight unit, the rear antenna is not blocked by the load, and no bad influence is exerted on the emission of electromagnetic waves. Therefore, communication is not obstructed by the influence of the load. 
         [0017]    The taillight unit here includes one having only a taillight indicating a braking situation, and one having turn signals as well as the taillight in an integrated manner. 
         [0018]    In this invention, it is preferred that the rear antenna is disposed inward of side covers covering areas around a lower part of the taillight unit. 
         [0019]    Since the side covers are often formed of resin, by containing the rear antenna inward of the side covers, a fine appearance can be maintained without a bad influence on the emission of electromagnetic waves. 
         [0020]    In this invention, it is preferred that the rear antenna is attached through an anchor to a lower surface of the taillight unit located inward of the side covers. 
         [0021]    The rear antenna can be fixed stably since it is attached through the anchor to the lower surface of the taillight unit located inward of the side covers. 
         [0022]    In this invention, it is preferred that the rear antenna is disposed outward of side covers covering areas around a lower part of the taillight unit. 
         [0023]    The rear antenna, since it is disposed outward of the side covers, can assure a high degree of freedom of arrangement. Therefore, not only assuring a high degree of freedom of design for the two-wheeled motor vehicle, this allows the rear antenna to be installed afterward as an add-on. 
         [0024]    In this invention, it is preferred that the rear antenna is attached to a rear slope of a rear fender through an anchor. 
         [0025]    While being able to avoid interference with other components, a high degree of freedom can be afforded to an attaching method compared with a case of attachment to other portions. 
         [0026]    In this invention, it is preferred that the rear fender has a reinforcing member attached along a front slope thereof for supporting the rear antenna. 
         [0027]    The rear fender is often formed of resin, which makes it difficult to give strong support. So, by providing the reinforcing member along the front slope, the support for the rear antenna can be strengthened. Further, an outward appearance is not impaired since the reinforcing member is not exposed from the rear surface of the rear fender. 
         [0028]    In this invention, it is preferred that the anchor is formed such that an angle between a base seating surface attached to the rear slope of the rear fender and an antenna seating surface to which the rear antenna is attached cause the rear antenna to be in an upright posture as seen from a side. 
         [0029]    The angle between the base seating surface and antenna seating surface of the anchor is set to place the rear antenna in an upright posture. The rear antenna can therefore be attached in a posture well suited for vehicle-to-vehicle communication with rearward vehicles. 
         [0030]    In this invention, it is preferred that the rear fender has, formed integrally with the rear slope, an attaching portion for attaching the rear antenna; and the rear antenna is attached to the attaching portion. 
         [0031]    Since the rear fender and attaching portion are formed integrally, the rear antenna can be attached rigidly to the rear fender. Further, the number of parts for attachment can be reduced, thereby to keep the cost low. 
         [0032]    In this invention, it is preferred that the rear antenna is attached above a license plate. 
         [0033]    The license plate is prevented from exerting a bad influence on the emission of electromagnetic waves from the rear antenna. 
         [0034]    In this invention, it is preferred that the license plate is attached by a license plate mounting bracket having, integrated together, a license plate attaching portion to which the license plate is attached, and a rear antenna attaching portion to which the rear antenna is attached. 
         [0035]    The rear antenna can be attached easily since the license plate is attached to the license plate attaching portion of the license plate mounting bracket, and the rear antenna is attached to the rear antenna attaching portion. The rear antenna can therefore be attached efficiently. 
         [0036]    In this invention, it is preferred that the license plate mounting bracket is formed such that an angle between the license plate attaching portion and the rear antenna attaching portion causes the rear antenna to be in an upright posture as seen from a side. 
         [0037]    The angle between the license plate attaching portion and rear antenna attaching portion of the license plate mounting bracket is set to place the rear antenna in an upright posture. The rear antenna can therefore be attached in a posture well suited for vehicle-to-vehicle communication with rearward vehicles. 
         [0038]    In this invention, it is preferred that the rear antenna is attached in a rearward region marked by a vertical line drawn from a rear end of a seat frame continuous with and rearward of a main frame and an extended line along a rear slope of a rear fender. 
         [0039]    Stable communication can be performed since bad influences of other components can be prevented while preventing the bad influence of a load placed on the grab bar and the load carrier. 
         [0040]    In this invention, it is preferred that the front antenna is attached through an anchor to a stay extending from a main frame further forward of a head tube for fixing a front cover. 
         [0041]    The front antenna can be attached relatively easily by attaching the front antenna through the anchor to the stay which fixes the front cover. 
         [0042]    In this invention, it is preferred that the front antenna is attached to a front fork through an anchor. 
         [0043]    With the front antenna attached to the front fork through the anchor, the front antenna can be attached easily even to a two-wheeled motor vehicle without a front cover. 
         [0044]    In this invention, it is preferred that the front antenna is attached forward of a headlight and below the headlight. 
         [0045]    Lighting by the headlight is not obstructed, and besides a forward emission of electromagnetic waves from the front antenna is not obstructed by the headlight. 
       Advantageous Effects of Invention 
       [0046]    With the two-wheeled motor vehicle according to the invention, the front antenna mainly performs road-to-vehicle communication with roadside units and vehicle-to-vehicle communication with preceding vehicles and oncoming vehicles at times of passing each other. It is therefore possible to perform communication effectively by employing a nondirectional antenna which has wide-angle transmit-receive characteristics. On the other hand, vehicle-to-vehicle communication with rearward vehicles by means of the rear antenna needs a long communication distance compared with the road-to-vehicle communication and vehicle-to-vehicle communication with oncoming vehicles at times of passing each other. In this invention, the rear antenna has directionality enabling a long communication distance compared with an antenna has nondirectionality, and has the directionality rearward of the vehicle. It is therefore possible to also perform communication effectively with rearward vehicles. Since the front antenna is disposed forward of the seat, and the rear antenna rearward of the seat, electromagnetic waves can be inhibited from being attenuated by the bodies of the rider and the passenger. The rear antenna having rearward directionality can inhibit bad influences by preventing interference with the electromagnetic waves of the front antenna. As a result, the road-to-vehicle communication and the vehicle-to-vehicle communication with forward vehicles (preceding vehicles and oncoming vehicles) can be performed effectively, and at the same time the vehicle-to-vehicle communication with rearward vehicles can also be performed effectively. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0047]      FIG. 1  is a side view of a scooter according to Embodiment 1; 
           [0048]      FIG. 2  is a plan view of the scooter according to Embodiment 1; 
           [0049]      FIG. 3  is a schematic view showing emission patterns of electromagnetic waves in a plan view of the scooter of Embodiment 1; 
           [0050]      FIG. 4  is an enlarged side view of a portion of a scooter according to Embodiment 2; 
           [0051]      FIG. 5  is an enlarged side view of a portion of a scooter according to Embodiment 3; 
           [0052]      FIG. 6  is an enlarged side view of a portion of a scooter according to Embodiment 4; 
           [0053]      FIG. 7  is an enlarged side view of a portion of a scooter according to Embodiment 5; 
           [0054]      FIG. 8  shows a license plate mounting bracket, in which (a) is a side view, and (b) is a front view; 
           [0055]      FIG. 9  is a view illustrating a preferred region for attaching a rear antenna; 
           [0056]      FIG. 10  is a side view of a two-wheeled motor vehicle according to Embodiment 6; and 
           [0057]      FIG. 11  is a view illustrating a preferred region for attaching a rear antenna. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0058]    Description will be made hereinafter with reference to embodiments concerning two types of two-wheeled motor vehicles for example. 
       Embodiment 1 
       [0059]    Embodiment 1 of this invention will be described hereinafter with reference to the drawings. 
         [0060]      FIG. 1  is a side view of a scooter according to Embodiment 1.  FIG. 2  is a plan view of the scooter according to Embodiment 1.  FIG. 3  is a schematic view showing emission patterns of electromagnetic waves in a plan view of the scooter of Embodiment 1. 
         [0061]    The scooter which corresponds to the “two-wheeled motor vehicle” in this invention includes a main frame  3  having a head tube  1 , a seat frame  5 , a seat  7 , side covers  9 , a grab bar  11 , a taillight unit  13 , a rear fender  15 , a drive train  17 , a rear wheel  19 , a front fork  21 , a front wheel  23 , a front cover  25 , a headlight  27  and a steering handle  29 . 
         [0062]    The main frame  3  acts as a framework of the scooter. The seat frame  5  extends rearward from the main frame  3 , and supports the seat  7  for seating the rider and a passenger seat  7   a  for seating a passenger. The taillight unit  13  is attached to the rear of the seat frame  5 . The side covers  9  are attached to cover side and rear portions of the seat frame  5 , and cover front and lower portions of the taillight unit  13 . This taillight unit  13  has a taillight indicating a braking situation and turn signals in an integrated manner. The grab bar  11  is gripped by a passenger or is used when a load is carried. 
         [0063]    The drive train  17  is mounted rearward of the main frame  3 . The drive train  17  includes an engine, a suspension and so on. The rear wheel  19  is attached rearward of the drive train  17 . The rear fender  15  is spaced from the rear wheel  19  to cover an upper portion and an area obliquely upward thereof. The rear fender  15  is formed of resin, for example. A license plate  30  is attached in a tilted posture to the rear of the rear fender  15  and below the taillight unit  13 . 
         [0064]    The steering handle  29  is tunably attached to an upper portion of the head tube  1 . The front fork  21  is attached below the head tube  1 , and is interlocked with turning of the steering handle  29 . The front wheel  23  is rotatably attached to a lower portion of the front fork  21 . The front cover  25  is attached to the front of the head tube  1  through a stay  31  extending from the main frame  3 . The headlight  27  is provided on the front cover  25 . 
         [0065]    A front antenna  33  is mounted in the front cover  25 . The front antenna  33  has nondirectional characteristics with an emission pattern and reception sensitivity of electromagnetic waves uniform all around a horizontal plane. This front antenna  33  is erected on an anchor  35  extending toward a front center from the stay  31  serving to attach the front cover  25 . The front antenna  33  is located substantially at the middle transversely of the vehicle when seen in plan view, and is located above and rearward of the headlight  27  when seen from a side. 
         [0066]    A rear antenna  37  is mounted inward of the side covers  9 . The rear antenna  37  has directional characteristics with an emission pattern and reception sensitivity of electromagnetic waves biased to one direction. Specifically, this rear antenna  37  has directionality with the transmit and receive sensitivity directed rearward of the vehicle. The rear antenna  37 , which is located inward of the side covers  9 , is attached to a lower surface of the taillight unit  13  by an anchor  39 . The anchor  39  presents an inverted L-shape when seen from a side, with an upper surface thereof fixed to the lower surface of the taillight unit  13 , and the rear antenna  37  is attached to a rear side thereof. The rear antenna  37  is attached to the anchor  39  to assume an upright posture. The rear antenna  37  is located substantially at the middle transversely of the vehicle when seen in plan view, and is located below the taillight unit  13  and above the license plate  30  when seen from a side. Therefore, since no metal parts that would obstruct the emission of electromagnetic waves are arranged rearward of the rear antenna  37 , rearwardly directed emission of electromagnetic waves is free from obstruction. 
         [0067]    The scooter including the nondirectional front antenna  33  and the rear antenna  37  having rearward directionality as described above forms emission patterns of electromagnetic waves as shown in two-dot chain lines in  FIG. 3 . When the electromagnetic emission patterns of the front antenna  33  and rear antenna  37  are combined, the resulting shape connects a forward circle and a rearward ellipse shown in the dashed line in  FIG. 3 . 
         [0068]    According to this embodiment, the front antenna  33  mainly performs road-to-vehicle communication with roadside units and vehicle-to-vehicle communication with preceding vehicles and oncoming vehicles at times of passing each other. It is therefore possible to perform communications effectively by employing a nondirectional antenna which has wide-angle transmit-receive characteristics. On the other hand, vehicle-to-vehicle communication with rearward vehicles by means of the rear antenna  37  needs a long communication distance compared with the road-to-vehicle communication and vehicle-to-vehicle communication with oncoming vehicles at times of passing each other. In this invention, the rear antenna  37  has the directionality that enables a long communication distance compared with nondirectionality, and has the directionality rearward of the vehicle. It is therefore possible to also perform communications effectively with rearward vehicles. Since the front antenna  33  is disposed in front of the seat  7  and the rear antenna  37  behind the seat  7 , the electromagnetic waves can be inhibited from being attenuated by the bodies of the rider and the passenger. The rear antenna having the rearward directionality can inhibit bad influences of the interference with the electromagnetic waves of the front antenna. As a result, the road-to-vehicle communication and the vehicle-to-vehicle communication with forward vehicles (preceding vehicles and oncoming vehicles) can be performed effectively, and at the same time the vehicle-to-vehicle communication with rearward vehicles can also be performed effectively. 
         [0069]    There are the grab bar and a load carrier above the taillight unit  13 , and when a load is placed thereon, the rear antenna  37  may be blocked by the load, which may exert a bad influence on the emission of electromagnetic waves. In this embodiment, however, since the rear antenna  37  is located below the taillight unit  13 , the rear antenna  37  is not blocked by the load, and no bad influence is exerted on the emission of electromagnetic waves. Therefore, communication is not obstructed by the influence of the load. 
         [0070]    By containing the rear antenna  37  inward of the side covers  9  formed of resin, a fine appearance can be maintained without a had influence on the emission of the electromagnetic waves. Since the rear antenna  37  is attached above the license plate  30 , the license plate  30  can be prevented from exerting a bad influence on the emission of electromagnetic waves from the rear antenna  37 . 
         [0071]    The front antenna  33  can be attached relatively easily by attaching the front antenna  33  through the anchor  35  to the stay  31  which fixes the front cover  25 . 
         [0072]    In the above description, the anchor  39  which fixes the rear antenna  37  is attached to the lower surface of the taillight unit  13 . Alternatively, the anchor  39  may be attached to the seat frame  5 . 
       Embodiment 2 
       [0073]    Next, Embodiment 2 of this invention will be described with reference to the drawings. 
         [0074]      FIG. 4  is an enlarged side view of a portion of a scooter according to Embodiment 2. 
         [0075]    Components identical to those of foregoing Embodiment 1 are shown with the same signs, and will not particularly be described. This applies to each of the embodiments described subsequently. 
         [0076]    The rear antenna  37  is disposed outward of the side covers  9 . Specifically, the rear antenna  37  is attached by an anchor  41  having an inverted L-shape. The anchor  41  is fixed at an upper surface thereof to a lower surface of the taillight unit  13 , and has the rear antenna  37  attached to a vertical surface thereof. The anchor  41  is attached such that its vertical surface to which the rear antenna  37  is attached protrudes rearward from the taillight unit  13 . However, the rear antenna  37  is located above the license plate  30  and below the taillight unit  13  as seen from a side. 
         [0077]    According to this embodiment, besides the same effects as those of the foregoing Embodiment 1, the rear antenna  37  can be fixed stably since it is attached through the anchor  41  to the lower surface of the taillight unit  13  located inward of the side covers  9 . 
         [0078]    The rear antenna  37 , since it is disposed outward of the side covers  9 , can assure a high degree of freedom of arrangement. Therefore, not only assuring a high degree of freedom of scooter design, this allows the rear antenna  37  to be installed afterward as an add-on. 
         [0079]    In the above description, the anchor  41  which fixes the rear antenna  37  is attached to the lower surface of the taillight unit  13 . Alternatively, the anchor  41  may be attached to the seat frame  5 . 
       Embodiment 3 
       [0080]    Next, Embodiment 3 of this invention will be described with reference to the drawings. 
         [0081]      FIG. 5  is an enlarged side view of a portion of a scooter according to Embodiment 3. 
         [0082]    The rear antenna  37  is attached to a rear slope  43  of the rear fender  15  by an anchor  45 . The anchor  45  has a base seating surface  47  attached to the rear slope  43  of the rear fender  15 , and an antenna seating surface  49  to which the rear antenna  37  is attached. The anchor  45  is formed such that, as seen from a side, an angle between the base seating surface  47  and antenna seating surface  49  causes the rear antenna  37  attached to the antenna seating surface  49  to be in an upright posture. 
         [0083]    A reinforcing member  53  is disposed on a front slope  51  which corresponds to an opposite side of the rear slope  43  of the rear fender  15 . The reinforcing member  53  is formed of sheet metal having higher rigidity than the rear fender  15  formed of resin. The rear fender  15  is placed between the reinforcing member  53  and the anchor  45 , and the anchor  45  is supported stably. The reinforcing member  53  may have an upper end thereof extending upward to be fastened together with the rear fender  15  to the vehicle body. 
         [0084]    According to this embodiment, besides the same effects as those of the foregoing Embodiment 1, while being able to avoid interference with other components, a high degree of freedom can be afforded to an attaching method compared with a case of attachment to other portions. By providing the reinforcing member  53  along the front slope  51 , the support for the rear antenna  37  can be strengthened. Further, the angle between the base seating surface  47  and antenna seating surface  49  of the anchor  45  is set to place the rear antenna  37  in an upright posture. The rear antenna  37  can therefore be attached in a posture well suited for vehicle-to-vehicle communication with rearward vehicles. 
       Embodiment 4 
       [0085]    Next, Embodiment 4 of this invention will be described with reference to the drawings. 
         [0086]      FIG. 6  is an enlarged side view of a portion of a scooter according to Embodiment 4. 
         [0087]    The rear antenna  37  is attached to the fender  15  as in foregoing Embodiment 3. The rear fender  15  in this embodiment has, formed integrally with the rear slope  43 , an attaching portion  55  for attaching the rear antenna  37 . This attaching portion  55  has a recess  57  formed for attaching the rear antenna  37 . The recess  57  receives a back surface opposite to the electromagnetic waves emitting plane of the rear antenna  37 , and a bottom surface of the rear antennas  37 , and holds the rear antenna  37  in an upright posture. 
         [0088]    According to this embodiment, besides the same effects as those of the foregoing Embodiment 1, since the rear fender  15  and attaching portion  55  are formed integrally, the rear antenna  37  can be attached rigidly to the rear fender  15 . Further, the number of parts for attachment can be reduced, thereby to keep the cost low. 
       Embodiment 5 
       [0089]    Next, Embodiment 5 of this invention will be described with reference to the drawings. 
         [0090]      FIG. 7  is an enlarged side view of a portion of a scooter according to Embodiment 5.  FIG. 8  shows a license plate mounting bracket, in which (a) is a side view, and (b) is a front view. 
         [0091]    The rear antenna  37  is attached to the rear fender  15  through a license plate mounting bracket  59  which is formed of resin. The license plate mounting bracket  59  has a license plate attaching portion  61  and a rear antenna attaching portion  63  integrated together. The license plate attaching portion  61  has supporting projections  65  formed thereon which support an upper part and a lower part of the license plate  30 . The license plate  30 , in a state of being inserted inward of the upper and lower supporting projections  65  from a side (in a depth direction of the plane of  FIG. 8  ( a ), and a sidewise direction in  FIG. 8  ( b )), is fixed to the license plate attaching portion  61  with screws  66 . 
         [0092]    The rear antenna attaching portion  63  is formed to project from an upper middle position of the license plate attaching portion  61 , and has a recess  67  formed therein for attaching the rear antenna  37 . The rear antenna attaching portion  63  is formed to have an upper part thereof assuming a position tilting slightly downward and rearward of the vehicle (rightward in  FIG. 8  ( a ) and to the near side with respect to the plane of  FIG. 8  ( b )) when the license plate attaching portion  61  is in an upright posture. With the license plate mounting bracket  59 , since the license plate attaching portion  61  is attached in a forwardly tilted posture to the rear fender  15 , the rear antenna attaching portion  63  tilting rearward assumes an upright posture. 
         [0093]    In this embodiment, the rear antenna  37  can be attached easily since the license plate  30  is attached to the license plate attaching portion  61  of the license plate mounting bracket  59 , and the rear antenna  37  is attached to the rear antenna attaching portion  63 . The rear antenna  37  can therefore be attached efficiently. An angle between the license plate attaching portion  59  and rear antenna attaching portion  63  of the license plate mounting bracket  59  is set to place the rear antenna  37  in an upright posture. The rear antenna  37  can therefore be attached in a posture well suited for vehicle-to-vehicle communication with rearward vehicles. 
         [0094]    A mounting position of the rear antenna  37  in each of Embodiments 1-5 described hereinbefore is, preferably, in a region rg 1  (the hatched region) shown in  FIG. 9 . Specifically, it is preferable to position the rear antenna  37  in a rearward region rg 1  marked by a vertical line L 1  drawn from the rear end of the seat frame  5  and an extended line L 2  along the rear slope  43  of the rear fender  15 . What is still more desirable is a part of region rg 1  below the taillight unit  13 . 
         [0095]    With the rear antenna  37  positioned in the region rg 1 , stable communication can be performed since bad influences of the other components can be prevented while preventing the bad influence of a load placed on the grab bar  11 . 
       Embodiment 6 
       [0096]    Next, Embodiment 6 of this invention will be described with reference to the drawings. 
         [0097]      FIG. 10  is a side view of a two-wheeled motor vehicle according to Embodiment 6. 
         [0098]    Foregoing Embodiments 1-5 have been described taking a scooter as an example of two-wheeled motor vehicles, but this embodiment will be described taking what is called a naked type two-wheeled motor vehicle as the example of the two-wheeled motor vehicles. The naked type refers to a two-wheeled motor vehicle with an uncovered engine and frame. 
         [0099]    This naked type two-wheeled motor vehicle has an attachment form similar to foregoing Embodiment 3. Specifically, a directional rear antenna  37  is attached to a lower portion of a rear slope  43  of a rear fender  15  through an anchor  69 . The anchor  69  is formed to have such a shape that the antenna  37  assumes an upright posture when attached to the rear fender  15 . 
         [0100]    A nondirectional front antenna  33  is erected at the other end of an anchor  73  having one end thereof attached to a lower bracket  71  of a front fork  21 . The anchor  73  places the front antenna  33  in front of and below a headlight  27 . With the front antenna  33  attached to the lower bracket  71  through the anchor  73  in this way, the front antenna  33  can be attached easily as in Embodiment 1, even if this is the naked type motor vehicle without the front cover  25 . Lighting by the headlight  27  is not obstructed, and in addition, forward emission of the electromagnetic waves from the front antenna  33  is not obstructed by the headlight  27 . 
         [0101]    Also the case of the front antenna  33  and rear antenna  37  being attached as described above can obtain emission patterns of electromagnetic waves similar to those in foregoing Embodiment 1 ( FIG. 3 ). Therefore, effects similar to those in foregoing Embodiment 1 are produced. 
         [0102]    Also with the naked type two-wheeled motor vehicle, the rear antenna  37  may be attached in the modes of foregoing Embodiments 3, 4 and 5. As in foregoing Embodiment 2, the rear antenna  37  may be disposed outward of the side covers  9 . 
         [0103]    A mounting position of the rear antenna  37  in Embodiment 6 described above, preferably, is in a region rg 2  (the hatched region) shown in  FIG. 11 . That is, a preferred mounting position of the rear antenna  37  is in a rearward region rg 2  marked by a vertical line L 1  drawn from the rear end of the seat frame  5  and an extended line L 2  along the rear slope  43  of the rear fender  15 . What is still more desirable is a part of region rg 2  below the taillight unit  13 . The rear antenna  37  is therefore not limited to the above-described mounting position in  FIG. 10 , but may be disposed in any appropriate position within the region rg 2 . 
         [0104]    Although the front antenna  33  is attached to the lower bracket  71  through the anchor  73  in foregoing Embodiment 6, the anchor  73  may be attached to an upper portion of an inner tube  75  (upper tube) of the front fork  21 , for example. 
         [0105]    This invention is not limited to the foregoing embodiments, but may be modified as follows: 
         [0106]    (1) In Embodiments 1-6 described above, two-wheeled motor vehicles are exemplified by the scooter type and the naked type. However, this invention is not limited to these types of two-wheeled motor vehicles, but is applicable also to other types of two-wheeled motor vehicles. 
         [0107]    (2) In Embodiments 1-6 described above, the front antenna  33  and rear antenna  37  are attached using various types of anchors and mounting brackets. This invention does not provide these anchors or mounting brackets as indispensable. What is necessary is to provide the nondirectional front antenna  33  in a position forward of the seat  7 , and the directional rear antenna  37  in a position rearward of the seat  7 . 
         [0108]    (3) In Embodiments 1-6 described above, the front antenna  33  and the rear antenna  37  are arranged in substantially middle positions transversely of the vehicle. However, this invention may have the front antenna  33  and rear antenna  37  in positions shifted right or left from the middle transversely of the vehicle. For example, the front antenna  33  may be attached to a side surface of the front cover  25 , and the rear antenna  37  may be attached to a side surface of a side cover  9 . 
       REFERENCE SIGNS LIST 
       [0000]    
       
         
           
               1  . . . head tube 
               3  . . . main frame 
               5  . . . seat frame 
               7  . . . seat 
               9  . . . side covers 
               13  . . . taillight unit 
               15  . . . rear fender 
               21  . . . front fork 
               25  . . . front cover 
               27  . . . headlight 
               30  . . . license plate 
               33  . . . front antenna 
               35  . . . anchor 
               37  . . . rear antenna 
               39  . . . anchor