Patent Publication Number: US-10333205-B2

Title: On-vehicle radar device and vehicle

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an on-vehicle radar device and a vehicle. 
     2. Description of the Related Art 
     Research has been conducted in recent years into areas such as collision avoidance, driving assistance, and automatic driving, utilizing technology that uses radar to detect objects around a vehicle. In the case of a car, the radar has conventionally been provided on the front nose. A high-frequency oscillator needs to be placed in the vicinity of an antenna and requires water and weather proofing measures, such as protection using a radome (i.e., a radio dome), to avoid wind and rain. Meanwhile, more sophisticated detection technology has also been developed, using both radar detection and camera images. 
     U.S. Pat. No. 8,604,968 proposes a radar-camera sensor in which a radar and a camera are housed in a single housing. The radar-camera sensor is mounted on the front windshield of a car forward of the rear-view mirror. The radar waves used are either vertically or horizontally polarized radio waves. 
     A multifunctional sensor unit disclosed as an external-field-of-vehicle recognizing apparatus in International Publication No. WO/2006/035510 also has an image capturing part and a transmission/reception part that are mounted on a single sensor mounting board. The multifunctional sensor unit is installed in the interior of the vehicle. 
     Radar waves are attenuated due to being reflected and absorbed by the front windshield if a radar device is placed in the interior of a vehicle. The glass shows a greater influence in the case where short-wavelength radio waves are used to improve the resolution of the radar. Also, the output of the oscillator cannot be increased because there are statutory regulations governing the output of high-frequency oscillators that are available for use with vehicles. This consequently reduces the distance that can be monitored by the radar. 
     SUMMARY OF THE INVENTION 
     Preferred embodiments of the present invention provide an on-vehicle radar device, and are able to suppress or prevent a reduction in the efficiency of radio-wave transmission and reception when the on-vehicle radar device is arranged in the interior of a vehicle. 
     Front windshields used in vehicles such as cars are transparent and seemingly made of a single glass plate but are, in actuality, laminated glass having a three-layer structure in which two sheets of glass are laminated on inner and outer sides of a thin resin film to ensure the safety of passengers. Conventionally, it was not recognized that the amount of reflection between the resin layer which is the second layer and the outermost glass layer is large enough to affect the performance of the radar. Instead, it was thought that sufficiently accurate results could be obtained by treating the front windshield as a single glass plate for analysis purposes, as in the case where the front windshield is viewed with visible light. Under this assumption, even if a person considered an idea of reducing reflectivity by optimizing the incident angle of radio waves on the front windshield, the person would not have discerned any advantages in adjusting the incident angle to be greater than the Brewster angle. If the incident angle exceeds the Brewster angle, reflectivity increases rapidly. Accordingly, it was reasonable to select an installation angle that is slightly smaller than the Brewster angle because the installation angle could deviate from a predetermined angle due to limited accuracy of a mounting process. The inventor of preferred embodiments of the present invention discovered that the assumption described above was incorrect and conceived that the amount of reflection occurring between the resin layer which is the second layer and the outermost glass layer is too large to ignore, and that it is necessary to suppress the amount of reflection in this area. The inventor conceived of and developed various preferred embodiments of the present invention after having discovered that the reflectivity of the three-layered glass can be reduced as a whole by making the incident angle of radio waves on the innermost glass layer of the front windshield greater than the Brewster angle. 
     An on-vehicle radar device according to an exemplary preferred embodiment of the present invention includes a mount configured to be mounted on a bracket that is fixed to one of an innermost glass layer of laminated glass, a rear-view mirror placed on an inner side of the innermost glass layer, and a ceiling, the laminated glass including the innermost glass layer, an outermost glass layer, and an intermediate resin layer that is sandwiched between the innermost glass layer and the outermost glass layer, and an antenna configured to transmit a transmission wave from the inner side of the innermost glass layer to an outer side of the outermost glass layer and receiving a reflected wave that enters from the outer side of the outermost glass layer to the inner side of the innermost glass layer, the transmission wave being a radio wave in a millimeter wave band. 
     The antenna includes a transmitting antenna configured to transmit the transmission wave. A vertical polarization component of the transmission wave relative to the laminated glass is greater than a horizontal polarization component thereof. When the mount is mounted on the bracket, an incident angle of the transmission wave on the innermost glass layer at a center of a main lobe of the transmitting antenna is greater than a Brewster angle on an inner surface of the innermost glass layer, and an incident angle of the transmission wave on the outermost glass layer at the center of the main lobe is less than or equal to a Brewster angle between the outermost glass layer and the intermediate resin layer. 
     Preferred embodiments of the present invention are also intended for a vehicle that includes an on-vehicle radar device. 
     According to preferred embodiments of the present invention, it is possible to suppress a reduction in the efficiency of radio-wave transmission and reception in an on-vehicle radar device located in the interior of a vehicle. 
     The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified side view of a vehicle according to a preferred embodiment of the present invention. 
         FIG. 2  is a cross-sectional view of laminated glass according to a preferred embodiment of the present invention. 
         FIG. 3  is a cross-sectional view of a radar device mounted on the laminated glass according to a preferred embodiment of the present invention. 
         FIG. 4  is a perspective view of the radar device according to a preferred embodiment of the present invention. 
         FIG. 5  is a block diagram illustrating a schematic configuration of the radar device according to a preferred embodiment of the present invention. 
         FIG. 6A  illustrates a state of a near-field monitoring mode according to a preferred embodiment of the present invention. 
         FIG. 6B  illustrates a state of a far-field monitoring mode according to a preferred embodiment of the present invention. 
         FIG. 7  illustrates how a transmission wave enters the laminated glass according to a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a simplified side view of a vehicle  1  according to an exemplary preferred embodiment of the present invention. The vehicle  1  is preferably, for example, a passenger car and includes an on-vehicle radar device  11  (hereinafter, referred to as a “radar device”). 
     The radar device  11  is used for purposes such as, for example, collision avoidance, driving assistance, and automatic driving. The radar device  11  is mounted on the inner surface of a front windshield  12  of the vehicle  1  and located in a vehicle interior  13 . The vehicle interior  13  does not need to be a completely isolated space separated from the outside, and may be open-roofed, for example. The radar device  11  is located forward of a rear-view mirror  14  mounted on the front windshield  12 . The vehicle  1  preferably includes a drive mechanism  15  configured to move a vehicle body  10 . The drive mechanism  15  is defined by, for example, an engine, a steering mechanism, a power transmission mechanism, wheels and so on. 
     The front windshield  12  is fixed to the vehicle body  10  and located between the vehicle interior  13  and the vehicle exterior. The front windshield  12  is preferably made of laminated glass in which a film is sandwiched between two sheets of glass. The front windshield  12  is hereinafter also referred to as “laminated glass.” The radar device  11  is fixed directly to the inner surface of the laminated glass  12  or indirectly thereto via a mounting member such as a bracket. As another preferred mounting arrangement, the radar device  11  may be mounted on the rear-view mirror or the ceiling, for example. In the present preferred embodiment, the radar device  11  preferably is indirectly fixed to the laminated glass  12  via a bracket, for example. 
     As illustrated in  FIG. 2 , the laminated glass  12  preferably includes an innermost glass layer  121 , an outermost glass layer  122 , and an intermediate resin layer  123 . The intermediate resin layer  123  is sandwiched between the innermost glass layer  121  and the outermost glass layer  122 . That is, the innermost glass layer  121 , the intermediate resin layer  123 , and the outermost glass layer  122  are arranged in this order when viewed from the vehicle interior  13 . The laminated glass  12  may also include other layers as long as the above three layers are included as primary constituent elements. In the present preferred embodiment, the innermost glass layer  121  and the outermost glass layer  122  are preferably made of soda-lime glass, for example. The innermost glass layer  121  and the outermost glass layer  122  may have the same optical characteristics, or may have different optical characteristics. The intermediate resin layer  123  is preferably made of polyvinyl butyral (PVB). The intermediate resin layer  123  may be made of two or more laminated resin layers. 
       FIG. 3  is a cross-sectional view of the radar device  11  mounted on the laminated glass  12 . Hatching in some of the detailed portions of the cross section have been omitted for the sake of clarity. As described previously, the radar device  11  is fixed to the laminated glass  12  via a bracket  16 . The radar device  11  is freely detachable from the bracket  16 . 
     The bracket  16  includes two plates  161  and a connecting structure  162 . The two plates  161  are located, approximately overlapping with each other, and their front ends are rotatably coupled to each other by the connecting structure  162 . The upper surface of the upper plate  161  is preferably firmly fixed to the laminated glass  12  with an adhesion member  163 , for example. Other methods may also be used to fix the bracket  16  to the innermost glass layer  121 . The lower surface of the lower plate  161  is preferably fixed to the radar device  11  with screws  164 . The connecting structure  162  allows the lower plate  161  to be rotatable about an axis that extends in the right-left direction relative to the travel direction of the vehicle  1 . This mechanism enables selection of the angle of the lower plate  161  relative to the upper plate  161 . 
     The bracket  16  preferably further includes an adjusting bolt  165  and a spring  166 . The spring  166  gives the two plates  161  a force acting in such a direction that the two plates approach each other. The adjusting bolt  165  determines the position of the lower plate  161  relative to the upper plate  161 . The monitoring direction of the radar device  11  in the elevation direction is thus determined accurately. Instead of the adjustment mechanism of the bracket  16  in  FIG. 3 , other various mechanisms may be included or used. For example, a mechanism may be used in which a plurality of different types of brackets that have different angles of tilt between upper and lower surfaces are prepared, and a bracket having a suitable tilt angle is selected according to the required angle. 
     The radar device  11  preferably includes an antenna  21 , a camera  22 , a circuit  23 , and a cover  24 . The camera  22  is located above the antenna  21 . The cover  24  covers over the antenna  21 , the camera  22 , and the circuit  23 . The cover  24  is mounted on the antenna  21 . The camera  22  is also preferably mounted on the antenna  21  via a member, which is not shown. The arrangement of the antenna  21 , the camera  22 , and the circuit  23  may be appropriately changed. For example, the camera  22  may be located below or beside the antenna  21 . The cover  24  may cover the antenna  21 , the camera  22 , and the circuit  23  in various forms. For example, the cover  24  may cover the whole of the antenna  21 , the camera  22 , and the circuit  23 , or may cover only lower portions of these structural elements. 
       FIG. 4  is a perspective view of the radar device  11 . A mount  241  configured to be mounted on the bracket  16  is preferably provided on the top of the cover  24 . The mount  241  preferably includes a flat surface  242  and mounting holes  243 . The flat surface  242  is in contact with the lower plate  161  of the bracket  16 . The screws  164  are to be inserted into the mounting holes  243 . 
     As illustrated in  FIG. 3 , the circuit  23  includes a circuit board  23   a  configured to be mounted on the antenna  21 , and a circuit board  23   b  to be connected to the camera  22 . The circuit boards  23   a  and  23   b  are electrically connected to each other. The circuit board  23   a  mainly processes signals inputted from the antenna  21 , and the circuit board  23   b  mainly processes signals inputted from the camera  22 , but the distribution of these functions may be appropriately changed. 
     The antenna  21  transmits radio waves, which are radar waves, to the outside of the vehicle through the laminated glass  12  and receives reflected waves from the outside through the laminated glass  12 . That is, the antenna  21  transmits transmission waves from the inner side of the innermost glass layer  121  to the outer side of the outermost glass layer  122  and receives reflected waves that enter from the outer side of the outermost glass layer  122  to the inner side of the innermost glass layer  121 . 
     As illustrated in  FIG. 4 , the antenna  21  preferably includes a transmitting antenna  211  and a receiving antenna  212 . The transmitting antenna  211  transmits transmission waves. The receiving antenna  212  receives reflected waves resulting from the transmission waves. The transmitting antenna  211  includes a first transmitting antenna  213  and a second transmitting antenna  214 . The first transmitting antenna  213  and the second transmitting antenna  214  preferably are horn antennas, for example. The horns of the first transmitting antenna  213  and the second transmitting antenna  214  preferably have the same height in the elevation direction. The lateral width of the horn of the first transmitting antenna  213  is smaller than that of the horn of the second transmitting antenna  214 . Thus, the first transmitting antenna  213  transmits a first transmission wave that has a wide radiation range, and the second transmitting antenna  214  transmits a second transmission wave that has a different radiation pattern from that of the first transmission wave and a narrower radiation range than that of the first transmission wave. That is, the transmitting antenna  211  is configured to transmit both the first transmission wave and the second transmission wave. 
     The receiving antenna  212  preferably includes five receiving antennas  215 , for example. These receiving antennas  215  are arranged in the lateral direction. Each receiving antenna  215  preferably is a horn antenna. That is, every antenna included in the antenna  21  is preferably a horn antenna. The horns of the receiving antennas  215  preferably are of the same shape. Note that the “longitudinal direction” and the “lateral direction” referred to here are respectively a longitudinal direction and a lateral direction that are defined for the purpose of designing the vehicle  1 , and are not necessarily exactly parallel or perpendicular to the direction of gravity. 
     Each horn antenna of the antenna  21  is electrically or spatially connected to a structure configured to transmit and receive signals to and from a monolithic microwave integrated circuit (MMIC), a transmission line (specifically, a microstrip line, a transducer, and a waveguide), and a horn in this order. Use of the horn antennas makes it possible to ensure a gain while suppressing the width of the antennas in the height direction, and to reduce the forward projection area of the radar device  11 . It is thus possible to locate the radar device  11  in the vicinity of the front windshield without blocking the field of view of the passengers. 
     As illustrated in  FIG. 3 , the radar device  11  preferably further includes an antenna cover  25 . The antenna cover  25  is not shown in  FIG. 4 . The antenna cover  25  is located between the laminated glass  12  and the antenna  21  and covers a front portion of the antenna  21 . The antenna cover  25  is molded from a resin. The front surface, i.e., outer surface, of the antenna cover  25  is preferably black in color. This prevents the antenna  21  from standing out when viewed from the outside of the vehicle, and ensures the aesthetic appearance of the vehicle  1 . The antenna cover  25  is inclined at or approximately at 10 degrees from the vertical direction relative to the direction of transmission of the transmission waves, for example. 
     The camera  22  preferably includes a two-dimensional image sensor. The camera  22  observes the outside from the inner side of the laminated glass  12 . In other words, the camera  22  observes the vehicle exterior from the vehicle interior  13 . As illustrated in  FIGS. 3 and 4 , the cover  24  includes a camera window  244 . The camera window  244  is transparent. The camera  22  observes the vehicle exterior through the camera window  244  and the laminated glass  12 . 
       FIG. 5  is a block diagram illustrating a schematic configuration of the radar device  11 . The first transmitting antenna  213  and the second transmitting antenna  214  are connected to a selector circuit  311 . The selector circuit  311  is connected to a high-frequency oscillator  312 . This enables switching between the connection of the high-frequency oscillator  312  and the first transmitting antenna  213  and the connection of the high-frequency oscillator  312  and the second transmitting antenna  214 , allowing high-frequency electric power to be supplied to the first transmitting antenna  213  or the second transmitting antenna  214 . That is, the transmission of the first transmission wave and the transmission of the second transmission wave is able to be switched. The present preferred embodiment preferably uses a frequency-modulated continuous wave (FMCW) system that uses a relatively narrow frequency band, and the frequency of the high-frequency signal outputted by the high-frequency oscillator  312  varies upward and downward. 
     Each of the five receiving antennas  215  is preferably connected to a mixer  321  and an AD converter  322  in this order. The AD converter  322  is connected to a selector circuit  33 . The receiving antenna  215  receives a reflected wave that is obtained as a result of a transmission wave being reflected by an external object. A signal of the reflected wave obtained by the receiving antenna  215  and a circuit associated therewith is inputted to the mixer  321 . The mixer  321  also receives input of the signal from the high-frequency oscillator  312  and combines the obtained signals to acquire a beat signal that indicates a difference in frequency between the transmission wave and the reflected wave. The beat signal is converted into a digital signal by the AD converter  322  and inputted to the selector circuit  33 . 
     The selector circuit  33  selects at least some of the five beat signals and inputs the selected signals to a detector  35 . The detector  35  obtains position, speed or the like of the object by converting the beat signals through Fourier transformation and further performing computations on the transformed signals. Meanwhile, image signals from the camera  22  are also inputted to the detector  35 . Using the information received from the antenna  21  and the camera  22 , the detector  35  performs more advanced detection procedures of the type and state of the object. 
     The selector circuit  311 , the high-frequency oscillator  312 , the selector circuit  33 , and the detector  35  are connected to a controller  34 . The controller  34  controls these constituent elements to implement the detection operation of the detector  35 . The controller  34  and the detector  35  are provided in the circuit  23 . 
     The operation of the controller  34  includes a near-field monitoring mode and a far-field monitoring mode.  FIG. 6A  illustrates a state of the near-field monitoring mode, and  FIG. 6B  illustrates a state of the far-field monitoring mode. In  FIGS. 6A and 6B , the bottom side corresponds to the antenna side, and the top side corresponds to the forward side of the vehicle  1 . A range  41  indicates a radiation range of a transmission wave. The first transmitting antenna  213  and the second transmitting antenna  214  have side lobes that are sufficiently small relative to the main lobe. A pattern  42  indicates an antenna pattern of the receiving antenna  212 . Reference numeral  421  indicates the main lobe, and reference numeral  422  indicates side lobes other than the main lobe  421 . 
     In the near-field monitoring mode, the first transmission wave is transmitted from the first transmitting antenna  213  under the control of the controller  34  controlling the selector circuit  311 . Meanwhile, signals derived from the five receiving antennas  215  are inputted to the detector  35  under the control of the controller  34  controlling the selector circuit  33 . By using the signals from the five receiving antennas  215  arranged at narrow intervals, it is possible to enable the spread of the main lobe  421  of the receiving antenna  212  to be increased while sufficiently suppressing the spread of the side lobes  422 . Thus, in the near-field monitoring mode, the azimuth resolution is lower and the effective azimuth detection range is wider than in the far-field monitoring mode, which will be described later. As described previously, the first transmission wave has a wider radiation range  41  than the second transmission wave. Thus, objects can be detected over a wide range in the near-field monitoring mode. 
     In the far-field monitoring mode, the second transmission wave is transmitted from the second transmitting antenna  214  under the control of the controller  34  controlling the selector circuit  311 . Meanwhile, signals derived from only three of the five receiving antennas  215 , namely, the leftmost, central, and rightmost receiving antennas, are inputted to the detector  35  under the control of the controller  34  controlling the selector circuit  33 . By using only the signals from the three receiving antennas  215  arranged at wide intervals, it is possible the spread of the main lobe  421  of the receiving antenna  212  to be reduced. On the other hand, the spreads of the side lobes  422  increase. 
     However, since the second transmission wave has a narrow radiation range  41 , the second transmission wave is not transmitted in the directions of the side lobes  422  as illustrated in  FIG. 6B . In other words, in order to detect objects that exist far in front of the vehicle, radio waves are not transmitted in directions that deviate from the front side and do not need to be monitored. This enables detection of the reflected waves in the main lobe  421  while suppressing the influence of the side lobes  422 . In the far-field monitoring mode, the azimuth resolution is high, and the effective azimuth detection range is narrow. Thus, objects that exist in the distance within a narrow range are able to be detected in the far-field monitoring mode. 
     As described above, the radar device  11  executes two operating modes under the control of the controller  34  controlling constituent elements including the transmitting antenna  211  and the receiving antenna  212 . The radar device  11  uses a condition unique to vehicles, namely, that the receiving antenna  212  changes the range of the main lobe, and the resolution does not have to be increased across all azimuths in the far-field monitoring mode. This reduces the manufacturing cost of the radar device  11  while achieving both near- and far-field monitoring. The radar device  11  achieves adequate near- and far-field monitoring at low cost by providing a structure in which the two or more receiving antennas  215  used in the far-field monitoring mode are included in the plural receiving antennas  215  used in the near-field monitoring mode. 
     The antenna pattern of the receiving antenna  212  may be changed by the selector circuit  33  performing weighing on the signals from the receiving antennas  215 . As another alternative, instead of using the selector circuit  33 , a mechanism configured to turn on and off the actual receiving function of the receiving antennas  215  may be provided to select signals from the receiving antennas  215 . In this case, the mechanism configured to turn on and off the receiving functions serves as a selector circuit. 
     The near-field monitoring mode and the far-field monitoring mode are switched at high speed. That is, the first transmission wave and the second transmission wave are alternately transmitted under the control of the controller  34 . In actuality, in order to avoid needless transmission of radio waves during computations, a transmission stop period between the first transmission wave and the second transmission wave is longer than the transmission period of any one of the first transmission wave and the transmission period of the second transmission wave. For example, a single transmission period of a transmission wave is 2 milliseconds, and the transmission interval is about 50 milliseconds, for example. 
     The number of receiving antennas  215  arranged at equal intervals in the lateral direction is not limited to five. The number of receiving antennas  215  may be six or more if so desired. If the number of receiving antennas  215  is five or more, it is possible to use signals from three or more receiving antennas  215  arranged at wide intervals after the receiving antennas  215  to be used are made sparse, and to thus grasp the positions of objects that are located far away. When only one object needs to be detected, the number of receiving antennas  215  used may be two, for example. Accordingly, the minimum number of receiving antennas  215  included in the radar device  11  is three. The minimum number of selected receiving antennas  215  arranged at wide intervals is two. 
     If there is no need to detect the positions of objects in the near-field monitoring mode, the number of receiving antennas  215  to be used in the near-field monitoring mode may be two. For example, three receiving antennas  215  may be arranged at equal intervals, with the near-field monitoring mode using signals from adjacent two receiving antennas  215 , and the far-field monitoring mode using signals from the two receiving antennas  215  at either end. 
     More generally, in the near-field monitoring mode, the first transmission wave is transmitted from the transmitting antenna  211 , and signals from two or more narrowly spaced receiving antennas  215  among the plural receiving antennas  215  are preferably used. In the far-field monitoring mode, the second transmission wave is transmitted from the transmitting antenna  211 , and signals from two or more widely spaced receiving antennas  215  among the plural receiving antennas  215  are used. To reduce the number of receiving antennas  215 , at least one of the above two or more widely spaced receiving antennas  215  are included in the above two or more narrowly spaced receiving antennas. 
     The first and second transmission waves are vertically polarized waves relative to the lateral direction. The first and second transmission waves do not need to be exactly vertically polarized waves, and may be diagonally or elliptically polarized waves. More generally, vertical polarization components of the first and second transmission waves relative to the lateral direction are greater than the horizontal polarization components thereof. The laminated glass  12  is typically inclined such that the upper portion is located rearward of the lower portion. Thus, the vertical polarization components of the first and second transmission waves relative to the lateral direction are vertical polarization components relative to the laminated glass  12 . This improves the efficiency of the transmission waves passing through the laminated glass  12 . In particular, the efficiency of detection by the radar device  11  improves if the incident angles of the first and second transmission waves on the laminated glass  12  are close to the Brewster angle on the inner surface of the laminated glass  12 . Note that the vertically polarized waves are also referred to as transverse magnetic waves (TM waves), and indicate polarized waves in which electric-field components are perpendicular or substantially perpendicular to the plane of reflection such that their magnetic-field components are parallel or substantially parallel to the plane of reflection. The horizontally polarized waves are also referred to as “transverse electric waves (TE waves), and indicate polarized waves in which magnetic-field components are perpendicular or substantially perpendicular to the plane of reflection. At this time, their electric-field components are parallel or substantially parallel to the plane of reflection. 
     In the present preferred embodiment in which the horn of the first transmitting antenna  213  and the horn of the second transmitting antenna  214  are arranged in the lateral direction, the first transmitting antenna  213  and the second transmitting antenna  214  are preferably located on both right and left sides of the receiving antenna  212 . By arranging the first transmitting antenna  213 , the second transmitting antenna  214 , and the receiving antenna  215  side by side, plural horns can be provided in a single member. This reduces the manufacturing cost of the radar device  11 . Additionally, the orientation of each horn is able to be readily and accurately determined at the time of installing the radar device  11 . In particular, by arranging the horns of the first transmitting antenna  213  and the second transmitting antenna  214  in the lateral direction, it is possible to accurately match the orientation of the first transmitting antenna  213  in the elevation direction with the orientation of the second transmitting antenna  214  in the elevation direction. 
     The first transmitting antenna  213  and the second transmitting antenna  214  are preferably oriented so that the directions of the centers of their main lobes, i.e., the directions of the peaks of the main lobes are oriented in the horizontal direction. The first transmitting antenna  213  and the second transmitting antenna  214  may oriented so that the directions of the main lobes are oriented between the horizontal direction and a direction that is inclined at two degrees downward from the horizontal direction. 
     The first transmitting antenna  213 , the second transmitting antenna  214 , and the receiving antennas  215  may be antennas other than horn antennas, for example. They may be any type of antenna that transmits and receives millimeter waves. Examples that can be used include lens antennas, low-cost printed antennas, microstrip antennas, and slit antennas. Not every antenna included in the antenna  21  needs to be of the same type, and different types of antennas may be used together. 
     The following is an explanation of the orientations of the transmitting antennas with consideration given to the influence of both of the innermost glass layer  121  and the outermost glass layer  122  of the laminated glass  12 . A vertically polarized wave that has entered an object at the Brewster angle is guided into the object without being reflected under the ideal condition. However, in the case of the laminated glass  12 , a vertically polarized wave that has entered the innermost glass layer  121  at the Brewster angle enters the outermost glass layer  122  at an angle smaller than the Brewster angle, as will be described later. The laminated glass  12  in this case reflects the vertically polarized wave at the interface between the outermost glass layer  122  and the intermediate resin layer  123 . Thus, if the incident angle of a radio wave on the innermost glass layer  121  is made slightly greater than the Brewster angle, the incident angle of the radio wave on the outermost glass layer  122  is able to be made closer to the Brewster angle. Consequently, the total reflection by the laminated glass  12  decreases due to the decrease at the interface between the outermost glass layer  122  and the intermediate resin layer  123 , thus improving the efficiency of radio-wave transmission and reception. 
       FIG. 7  illustrates how a transmission wave enters the laminated glass  12 . Note that the incident angle of a transmission wave indicates the incident angle of the transmission wave on an object at the center of a main lobe of a transmitting antenna, i.e., the first transmitting antenna  213  or the second transmitting antenna  214  when the mount  241  is mounted on the bracket  16 . 
     In the following explanations, the refractive index of the air is referred to as n a , the refractive index of the innermost glass layer  121  is referred to as n g1 , the refractive index of the intermediate resin layer  123  is referred to as n r , the refractive index of the outer most glass layer  122  is referred to as n g2 , the incident angle of a radio wave on the inner most glass layer  121  is referred to as θ i1 , and the incident angle of a radio wave on the outer most glass layer  122  is referred to as θ i2 . The refractive indices n g1  and n g2  of the glass layers  121  and  122  are greater than the refractive index n r  of the intermediate resin layer  123 . 
     First, Formula 1 holds true according to the Snell&#39;s law.
 
 n   a  sin θ i1   =n   r  sin θ i2   Formula 1
 
     Thus, if the radio wave enters the innermost glass layer  121  at the Brewster angle θ b1  from air space, sin θ i2  can be expressed by Formula 2. 
     
       
         
           
             
               
                 
                   
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     Since tan θ b1  can be expressed by Formula 3 and sin θ b1  can be expressed by Formula 4 using tan θ b1 , sin θ i2  can be expressed by Formula 5. 
     
       
         
           
             
               
                 
                   
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                   Formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   4 
                 
               
             
             
               
                 
                   
                     
                       
                         
                           sin 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             θ 
                             
                               i 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               2 
                             
                           
                         
                         = 
                           
                         ⁢ 
                         
                           
                             
                               n 
                               a 
                             
                             
                               n 
                               r 
                             
                           
                           · 
                           
                             
                               
                                 n 
                                 
                                   g 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   1 
                                 
                               
                               
                                 n 
                                 a 
                               
                             
                             
                               
                                 1 
                                 + 
                                 
                                   
                                     ( 
                                     
                                       
                                         n 
                                         
                                           g 
                                           ⁢ 
                                           
                                               
                                           
                                           ⁢ 
                                           1 
                                         
                                       
                                       
                                         n 
                                         a 
                                       
                                     
                                     ) 
                                   
                                   2 
                                 
                               
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             
                               n 
                               
                                 g 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 1 
                               
                             
                             
                               n 
                               r 
                             
                           
                           · 
                           
                             1 
                             
                               
                                 1 
                                 + 
                                 
                                   
                                     ( 
                                     
                                       
                                         n 
                                         
                                           g 
                                           ⁢ 
                                           
                                               
                                           
                                           ⁢ 
                                           1 
                                         
                                       
                                       
                                         n 
                                         a 
                                       
                                     
                                     ) 
                                   
                                   2 
                                 
                               
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   Formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   5 
                 
               
             
           
         
       
     
     Tan θ b2  can be expressed by Formula 6, where θ b2  is the Brewster angle when the radio wave travels from the intermediate resin layer  123  to the outermost glass layer  122 . 
     
       
         
           
             
               
                 
                   
                     tan 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       θ 
                       
                         b 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         2 
                       
                     
                   
                   = 
                   
                     
                       n 
                       
                         g 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         2 
                       
                     
                     
                       n 
                       r 
                     
                   
                 
               
               
                 
                   Formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   6 
                 
               
             
           
         
       
     
     Thus, sin θ b2  can be expressed by Formula 7. 
     
       
         
           
             
               
                 
                   
                     sin 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       θ 
                       
                         b 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         2 
                       
                     
                   
                   = 
                   
                     
                       
                         n 
                         
                           g 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           2 
                         
                       
                       
                         n 
                         r 
                       
                     
                     · 
                     
                       1 
                       
                         
                           1 
                           + 
                           
                             
                               ( 
                               
                                 
                                   n 
                                   
                                     g 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     2 
                                   
                                 
                                 
                                   n 
                                   r 
                                 
                               
                               ) 
                             
                             2 
                           
                         
                       
                     
                   
                 
               
               
                 
                   Formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   7 
                 
               
             
           
         
       
     
     Since n g1  and n g2  are approximately equal to each other and n a  is smaller than n r , Formula 8 can be derived from a comparison of Formulas 5 and 7.
 
sin θ b2 &gt;sin θ i2   Formula 8
 
     That is, the radio wave that enters the innermost glass layer  121  at the Brewster angle θ b1  enters the outermost glass layer  122  at an angle smaller than the Brewster angle θ b2 . Accordingly, it is possible to cause a radio wave to enter the innermost glass layer  121  at an incident angle greater than the Brewster angle θ b1  and to enter the outermost glass layer  122  at an incident angle smaller than the Brewster angle θ b2 . 
     Using the above-described phenomenon occurring in the laminated glass  12 , the radar device  11  is designed such that when the mount  241  is mounted on the bracket  16 , the incident angle of the first transmission wave on the innermost glass layer  121  at the center of the main lobe of the first transmitting antenna  213  is greater than the Brewster angle θ b1  on the inner surface of the innermost glass layer  121 , and the incident angle of the first transmission wave on the outermost glass layer  122  at the center of the main lobe is less than or equal to the Brewster angle θ b2  between the outermost glass layer  122  and the intermediate resin layer  123 . 
     Similarly, when the mount  241  is mounted on the bracket  16 , the incident angle of the second transmission wave on the innermost glass layer  121  at the center of the main lobe of the second transmitting antenna  214  is designed to be greater than the Brewster angle θ b1  on the inner surface of the innermost glass layer  121 , and the incident angle of the second transmission wave on the outermost glass layer  122  at the center of the main lobe is designed to be less than or equal to the Brewster angle θ b2  between the outermost glass layer  122  and the intermediate resin layer  123 . 
     It is, however, noted that setting too high a value for the incident angle of the first and second transmission waves on the innermost glass layer  121  is not preferable because the reflectivity increases exponentially when the incident angle is greater than the Brewster angle. Thus, the difference between the Brewster angle and the incident angles of the first and second transmission waves on the innermost glass layer  121  at the centers of the main lobes of the first transmitting antenna  213  and the second transmitting antenna  214  is preferably about 25% or less of the difference between 90 degrees and the Brewster angle, for example. This condition may be applied to only one of the first transmitting antenna  213  and the second transmitting antenna  214 . 
     Note that the refractive indices for radio waves in the millimeter wave band have to be used when evaluating the above formulas since the refractive indices are significantly different from those at another wave band, which the inventor of the present invention observed at the wave band. The radio waves in the millimeter wave band referred to here are radio waves that have wavelengths of about 1 mm to about 10 mm in air, for example. 
     The radar device  11  and the vehicle  1  can be modified in various ways. 
     For example, the transmitting antenna and the receiving antenna may be the same antenna. Alternatively, a single transmitting antenna that is provided with a mechanism configured to change the antenna pattern may transmit both of the first transmission wave and the second transmission wave. As another alternative, a single receiving antenna that is provided with a mechanism configured to change the receiving antenna pattern may achieve both of the near-field monitoring mode and the far-field monitoring mode. In other words, the number of antennas included in the antenna  21  may be one, and the antenna  21  includes at least one antenna. It is, of course, preferable for the antenna  21  to include plural antennas. 
     The plural receiving antennas  215  may include antennas that are arranged in the longitudinal direction, as long as they include antennas that are arranged in the lateral direction. For example, the plural receiving antennas  215  may be arranged two dimensionally. 
     The object on which the radar device  11  is mounted is not limited to the front windshield. The radar device  11  may be mounted on the rear windshield for rearward monitoring. The position to mount the radar device  11  is not limited to positions on glass. 
     The vehicle  1  is not limited to a passenger car and may be other vehicles for use in various applications, such as, for example, a truck, a train, a plane, a boat, etc. In addition, the vehicle  1  is not limited to a man-driven vehicle, and may be an unattended vehicle such as an automated guided vehicle used in a factory. 
     The configurations of the above-described preferred embodiments and variations may be appropriately combined as long as there are no mutual inconsistencies. 
     While preferred embodiments of the present invention have been shown and described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is therefore to be understood that numerous modifications and variations can be devised without departing from the scope of the invention. 
     This application claims priority benefit under 35 U.S.C. Section 119 of Japanese Patent Application No. 2014-201870 filed in the Japan Patent Office on Sep. 30, 2014 and Japanese Patent Application No. 2015-098991 filed in the Japan Patent Office on May 14, 2015, the entire disclosures of which are incorporated herein by reference. 
     The radar devices according to various preferred embodiments of the present invention are able to be installed in vehicles for use in various applications. 
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.