Patent Publication Number: US-10322566-B2

Title: Vehicle

Description:
TECHNICAL FIELD 
     The present invention relates to a vehicle including an on-vehicle radar device. 
     BACKGROUND ART 
     Research into areas such as collision avoidance, driving assistance, and automatic driving has been conducted in recent years, using technology for detecting objects around a vehicle with radar. Automobiles conventionally have radar on their front noses. A high-frequency oscillator needs to be provided in the vicinity of an antenna, and water and weather proofing measures, such as using a Radome for protection, are also necessary against 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 windshield of an automobile forward of the rear-view mirror. A radar wave is either a vertically or horizontally polarized radio wave. 
     International Publication No. WO/2006/035510 discloses an external-field-of-vehicle recognizing apparatus that is a multifunctional sensor unit in which an image capturing part and a transmission/reception part are mounted on a single sensor mounting board. The multifunctional sensor unit is installed in the interior of a vehicle. 
     When a radar device is provided in the interior of a vehicle, a radar wave is attenuated by being reflected and absorbed by the windshield. The influence of the glass increases if a short-wavelength radio wave is used to improve the resolution of the radar. Besides, it is not possible to increase the output of the high-frequency oscillator because there are statutory regulations governing the output of high-frequency oscillators available for use with vehicles. As a result, the distance that can be monitored by the radar is reduced. 
     European Patent No. 888646 discloses a method in which, when a communication antenna is installed in the interior of a vehicle, a dielectric intermediate element is disposed between glass and the radiating surface of an antenna in order to suppress the reflection of a radio wave on the glass. According to European Patent No. 888646, the electrically effective distance between the glass and the antenna is adjusted to several times the half-wavelength of the radio wave. 
     The windshield used in a vehicle such as an automobile is transparent and seemingly made of a single glass plate, but in actuality, they are made of three-layer laminated glass in which two sheets of glass are laminated on inner and outer sides of an extremely thin resin film, in order to ensure the safety of passengers. It has conventionally been thought that treating the windshield as a single glass plate, as in the case of viewing the windshield with visible light, would be enough to obtain accurate results. However, the inventor of the present invention has found this assumption to be wrong and recognized the need to focus on each layer of the laminated glass and take measures to suppress reflection. The present invention has been achieved on the basis of the idea of dividing the laminated glass into each layer and taking measures to suppress reflection on a layer-by-layer basis. 
     SUMMARY OF INVENTION 
     The present invention is intended for a vehicle including an on-vehicle radar device, and it is an object of the present invention to suppress a reduction in the efficiency of transmission and reception of radio waves when the on-vehicle radar device is disposed in the interior of the vehicle. 
     A vehicle accordingly to an exemplary embodiment of the present invention includes a vehicle body, a drive mechanism for moving the vehicle body, laminated glass fixed to the vehicle body and located between a vehicle interior and an outside, and an on-vehicle radar device fixed directly or indirectly to an inner surface of the laminated glass, a rear-view mirror arranged inward of the inner surface, or a ceiling. The laminated glass includes an innermost glass layer, an outermost glass layer, and an intermediate resin layer sandwiched between the innermost glass layer and the outermost glass layer. 
     The on-vehicle radar device includes an antenna part for transmitting a transmission wave from inside the innermost glass layer to outside the outermost glass layer, the transmission wave being a radio wave in a millimeter waveband, and receiving a received wave that enters inside the innermost glass layer from outside the outermost glass layer. The antenna part includes a transmitting antenna for transmitting the transmission wave. The transmission wave has a vertical polarization component greater than a horizontal polarization component thereof with respect to the laminated glass. 
     Formulas 1 and 2 are satisfied, where θ a  is an angle formed by a normal to the laminated glass and a direction of propagation of the transmission wave at a center of a main lobe of the transmitting antenna, n 1 , n 2 , and n 3  are respectively refractive indices of three layers including inner, middle, and outer layers that are in contact with one another and that are selected from among an air layer located inward of the innermost glass layer, the innermost glass layer, the intermediate resin layer, the outermost glass layer, and an air layer located outward of the outermost glass layer, d 2  is a thickness of the middle layer, and N is an integer of 0 or more. 
     
       
         
           
             
               
                 
                   
                     
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                   , 
                   
                     
 
                   
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                               9 
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                                   - 
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                   , 
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                   , 
                   
                     
 
                   
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                           ( 
                           
                             
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                                 2 
                               
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                       2 
                     
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                           ( 
                           
                             
                               2 
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                               N 
                             
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                               5 
                               4 
                             
                           
                           ) 
                         
                         ⁢ 
                         
                           n 
                           2 
                         
                         ⁢ 
                         
                           λcos 
                           ⁡ 
                           
                             ( 
                             
                               
                                 sin 
                                 
                                   - 
                                   1 
                                 
                               
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     1 
                                     
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                                   ⁢ 
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                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   
                                     θ 
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                         4 
                         ⁢ 
                         
                           ( 
                           
                             
                               n 
                               2 
                               2 
                             
                             - 
                             
                               
                                 sin 
                                 2 
                               
                               ⁢ 
                               
                                 θ 
                                 a 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                     
                 
               
             
           
         
       
     
     If the horizontal polarization component of the transmission wave is greater than a vertical polarization component thereof with respect to the laminated glass, Formulas 3 and 4 are satisfied. 
     
       
         
           
             
               
                 
                   
                       
                   
                   ⁢ 
                   
                     
                       
                         If 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           n 
                           1 
                         
                       
                       &lt; 
                       
                         n 
                         2 
                       
                       &lt; 
                       
                         
                           n 
                           3 
                         
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         or 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           n 
                           1 
                         
                       
                       &gt; 
                       
                         n 
                         2 
                       
                       &gt; 
                       
                         n 
                         3 
                       
                     
                     , 
                   
                 
               
               
                 
                   [ 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     3 
                   
                   ] 
                 
               
             
             
               
                 
                   
                     
                       
                         ( 
                         
                           
                             2 
                             ⁢ 
                             N 
                           
                           + 
                           
                             3 
                             4 
                           
                         
                         ) 
                       
                       ⁢ 
                       
                         n 
                         2 
                       
                       ⁢ 
                       
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     According to the present invention, it is possible to suppress a reduction in the efficiency of transmission and reception of radio waves in the on-vehicle radar device disposed in the interior of the vehicle. 
     These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a simplified side view of a vehicle; 
         FIG. 2  is a cross-sectional view of laminated glass; 
         FIG. 3  is a cross-sectional view of a radar device mounted on the laminated glass; 
         FIG. 4  is a perspective view of the radar device; 
         FIG. 5  is a block diagram illustrating an outline of a configuration of the radar device; 
         FIG. 6A  illustrates a near-field monitoring mode; 
         FIG. 6B  illustrates a far-field monitoring mode; 
         FIG. 7  illustrates how a transmission wave enters the laminated glass; 
         FIG. 8  is a cross-sectional view illustrating reflection of the transmission wave; 
         FIG. 9  is a cross-sectional view of the laminated glass and a reflection suppression layer; 
         FIG. 10  is a front view of the reflection suppression layer; 
         FIG. 11  is a cross-sectional view of the radar device, the laminated glass, and the reflection suppression layer; 
         FIG. 12  illustrates the results of measuring loss of radio waves in the laminated glass; 
         FIG. 13  is a cross-sectional view of a reflection suppression layer that includes a plurality of dielectric layers; 
         FIG. 14  is a front view showing another example of the reflection suppression layer; and 
         FIG. 15  is a cross-sectional view showing another example of the reflection suppression layer. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1  is a simplified side view of a vehicle  1  according to an exemplary embodiment of the present invention. The vehicle  1  is 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 collision avoidance, driving assistance, and automatic driving. The radar device  11  is mounted on the inner surface of a 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 windshield  12 . The vehicle  1  includes a drive mechanism  15  for moving a vehicle body  10 . The drive mechanism  15  includes, for example, an engine, a steering mechanism, a power transmission mechanism, and wheels. 
     The windshield  12  is fixed to the vehicle body  10  and located between the vehicle interior  13  and the outside. The windshield  12  is laminated glass in which a film is sandwiched between two sheets of glass. The windshield  12  is hereinafter also referred to as “laminated glass.” The radar device  11  is fixed to the inner surface of the laminated glass  12  either directly or indirectly via a mounting member, such as a bracket. As another form of mounting, the radar device  11  may be mounted on the rear-view mirror or the ceiling. In the present embodiment, the radar device  11  is indirectly fixed to the laminated glass  12  via a bracket. 
     As illustrated in  FIG. 2 , the laminated glass  12  includes an innermost glass layer  121 , an outermost glass layer  122 , and an intermediate resin layer  123 . The intermediate resin layer  123  is located 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 the stated order when viewed from the vehicle interior  13 . The laminated glass  12  may also include other interlayers as long as these three layers serve as primary constituent elements. In the present embodiment, the innermost glass layer  121  and the outermost glass layer  122  are made of soda-lime glass. The innermost glass layer  121  and the outermost glass layer  122  may have the same optical properties, or may have different optical properties. The intermediate resin layer  123  is preferably made of polyvinyl butyrate (PVB), and may include a plurality of resin layers stacked on top of each other. 
       FIG. 3  is a cross-sectional view of the radar device  11  mounted on the laminated glass  12 . Hatching has been omitted from the details of the cross section. As described previously, the radar device  11  is fixed to the laminated glass  12  via a bracket  16 . The radar device  11  is removably attached to the bracket  16 . 
     The bracket  16  includes two plate parts  161  and a connecting part  162 . The two plate parts  161  are located approximately overlapping with each other, with their front ends rotatably coupled by the connecting part  162 . The upper surface of the upper plate part  161  is firmly fixed to the laminated glass  12  with an adhesion member  163 . The bracket  16  may be fixed by other methods to the innermost glass layer  121 . The lower surface of the lower plate part  161  is fixed to the radar device  11  with screws  164 . The connecting part  162  allows the lower plate part  161  to be rotatable about an axis that extends in the right-left direction relative to the travel direction of the vehicle  1 . With this mechanism, it is possible to select the angle of the lower plate part  161  relative to the upper plate part  161 . 
     The bracket  16  further includes an adjusting bolt  165  and a spring  166 . The spring  166  gives the two plate parts  161  a force acting in such a direction that the two plate parts approach each other. The adjusting bolt  165  determines the position of the lower plate part  161  relative to the upper plate part  161 . With this mechanism, the monitoring direction of the radar device  11  in the up-down direction is determined precisely. Alternatively, other various mechanisms may be employed, instead of the adjustment mechanism of the bracket  16  in  FIG. 3 . For example, a mechanism is conceivable in which a plurality of different types of brackets having different tilt angles 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  includes an antenna part  21 , a camera part  22 , a circuit part  23 , and a cover  24 . The camera part  22  is located above the antenna part  21 . The cover  24  covers the tops of the antenna part  21 , the camera part  22 , and the circuit part  23 . The cover  24  is mounted on the antenna part  21 . The camera part  22  is also mounted on the antenna part  21  via a member not shown. The arrangement of the antenna part  21 , the camera part  22 , and the circuit part  23  may be appropriately changed. For example, the camera part  22  may be located below or beside the antenna part  21 . The cover  24  may cover the antenna part  21 , the camera part  22 , and the circuit part  23  in various forms. For example, the cover  24  may cover the whole of the antenna part  21 , the camera part  22 , and the circuit part  23 , or may cover only the bottom of them. 
       FIG. 4  is a perspective view of the radar device  11 . The cover  24  has a mounting part  241  at the top, which is mounted on the bracket  16 . The mounting part  241  includes a flat surface  242  and mounting holes  243 . The flat surface  242  is in contact with the lower plate part  161  of the bracket  16 . The mounting holes  243  have the screws  164  in them. 
     As illustrated in  FIG. 3 , the circuit part  23  includes a circuit board  23   a  that is mounted on the antenna part  21 , and a circuit board  23   b  that is connected to the camera part  22 . The circuit boards  23   a  and  23   b  are electrically connected to each other. The circuit board  23   a  primarily processes signals received from the antenna part  21 , and the circuit board  23   b  primarily processes signals received from the camera part  22 , but this sharing of functions may be appropriately changed. 
     The antenna part  21  transmits a radio wave, which is a radar wave, to outside the vehicle through the laminated glass  12  and receives a received wave through the laminated glass  12 , the received wave being a reflected wave from the outside. That is, the antenna part  21  transmits a transmission wave from inside the innermost glass layer  121  to outside the outermost glass layer  122  and receives a reflected wave that enters inside the innermost glass layer  121  from outside the outermost glass layer  122 . 
     As illustrated in  FIG. 4 , the antenna part  21  includes a transmitting antenna part  211  and a receiving antenna part  212 . The transmitting antenna part  211  transmits transmission waves. The receiving antenna part  212  receives reflected waves resulting from the transmission waves. The transmitting antenna part  211  includes a first transmitting antenna  213  and a second transmitting antenna  214 . The first transmitting antenna  213  and the second transmitting antenna  214  are horn antennas. The horns of the first transmitting antenna  213  and the second transmitting antenna  214  have the same height in the up-down direction. The horn of the first transmitting antenna  213  has a smaller lateral width than the lateral width 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 the first transmission wave and has a narrower radiation range than the radiation range of the first transmission wave. That is, the transmitting antenna part  211  can transmit both the first transmission wave and the second transmission wave. 
     The receiving antenna part  212  includes five receiving antennas  215 . The receiving antennas  215  are arranged in the lateral direction. Each receiving antenna  215  is a horn antenna. That is, every antenna included in the antenna part  21  is a horn antenna. The horns of the receiving antennas  215  have the same shape. Note that the “longitudinal direction” and the “lateral direction” as used herein respectively refer to a longitudinal direction and a lateral direction that are defined in the design of the vehicle  1 , and respectively do not need to be exactly parallel to and perpendicular to the direction of gravity. 
     In each horn antenna of the antenna part  21 , constituents are electrically or spatially connected for transmitting and receiving signals in the order of a monolithic microwave integrated circuit (MMIC), a transmission line (specifically, a microstrip line, a transducer, and a waveguide), and a horn. Using the horn antenna allows gains to be secured while minimizing the width in the height direction of the antenna, and allows the forward projection area of the radar device  11  to be reduced. Thus, the radar device  11  can be installed in the vicinity of the windshield without limiting the vision of passengers. 
     As illustrated in  FIG. 3 , the radar device  11  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 part  21  and covers the front of the antenna part  21 . The antenna cover  25  is molded of a resin. The front surface, i.e., outer surface, of the antenna cover  25  is black in color. This prevents the antenna part  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 approximately 10 degrees from the vertical direction to the direction of transmission of a transmission wave. 
     The camera part  22  includes a two-dimensional image sensor. The camera part  22  observes the outside from inside the laminated glass  12 . In other words, the camera part  22  observes the outside of the vehicle 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 part  22  observes the outside of the vehicle through the camera window  244  and the laminated glass  12 . 
       FIG. 5  is a block diagram illustrating an outline of a configuration of the radar device  11 . The first transmitting antenna  213  and the second transmitting antenna  214  are connected to a selector  311 . The selector  311  is connected to a high-frequency oscillator  312 . This allows the high-frequency oscillator  312  to switch connection between the first transmitting antenna  213  and the second transmitting antenna  214  and to supply high-frequency electric power to either the first transmitting antenna  213  or the second transmitting antenna  214 . That is, the transmission wave is switched between the first transmission wave and the second transmission wave. In the present embodiment, a frequency-modulated continuous wave (FMCW) system that uses a relatively narrow frequency band is employed, and the frequency of the high-frequency signal received from the high-frequency oscillator  312  fluctuates. 
     Each of the five receiving antennas  215  is connected to a mixer  321  and an AD converter  322  in the stated order. The AD converter  322  is connected to a selector  33 . The receiving antenna  215  receives input of a reflected wave generated by reflection of a transmission wave on an external object. A signal of the reflected wave received by the receiving antenna  215  and a circuit associated therewith is input to the mixer  321 . The mixer  321  also receives input of a signal from the high-frequency oscillator  312  and combines the received 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 input to the selector  33 . 
     The selector  33  selects at least one or more of the five beat signals and inputs the selected signal(s) to a detector  35 . The detector  35  obtains, for example, the position and speed of the object by converting the beat signal(s) through Fourier transformation and further performing arithmetic processing on the signal(s). Meanwhile, the detector  35  also receives input of an image signal from the camera part  22 . Using the information received from the antenna part  21  and the camera part  22 , the detector  35  detects the type and condition of the object with higher precision. 
     The selector  311 , the high-frequency oscillator  312 , the selector  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 part  23 . 
     Operations of the controller  34  includes a near-field monitoring mode and a far-field monitoring mode.  FIG. 6A  illustrates the near-field monitoring mode, and  FIG. 6B  illustrates the far-field monitoring mode. In  FIGS. 6A and 6  B, 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 smaller than the main lobe. A pattern  42  indicates an antenna pattern of the receiving antenna part  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  311 . Meanwhile, signals derived from the five receiving antennas  215  are input to the detector  35  under the control of the controller  34  controlling the selector  33 . Using the signals from the five receiving antennas  215 , which are arranged at narrow intervals, increases the spread of the main lobe  421  of the receiving antenna part  212  and, on the other hand, suppresses the spread of the side lobes  422 . As a result, 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, the near-field monitoring mode achieves a wide range of detection of objects. 
     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  311 . Meanwhile, signals derived from only three of the five receiving antennas  215 , namely, the leftmost, central, and rightmost receiving antennas  215 , are input to the detector  35  under the control of the controller  34  controlling the selector  33 . Using only the signals from the three receiving antennas  215 , which are arranged at wide intervals, reduces the spread of the main lobe  421  of the receiving antenna part  212  and, on the other hand, increases the spread of the side lobes  422 . 
     However, the second transmission wave is not emitted in the directions of the side lobes  422  as illustrated in  FIG. 6B  because the second transmission wave has a narrow radiation range  41 . In other words, in order to detect objects that are located far in front of the vehicle, a radio wave is not emitted in such directions that deviate from the front side of the vehicle and thus do not need monitoring. This allows the main lobe  421  to detect a reflected wave 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, the far-field monitoring mode achieves a distant and narrow range of detection of objects. 
     As described above, the radar device  11  implements two modes of operation by the controller  34  controlling constituent elements including the transmitting antenna part  211  and the receiving antenna part  212 . The radar device  11  uses conditions peculiar to vehicles, that the receiving antenna part  212  changes the range of the main lobe, and there is no need in the far-field monitoring mode to increase the resolution in all directions. This reduces the manufacturing cost of the radar device  11  while achieving both near- and far-field monitoring. The radar device  11  implements proper near- and far-field monitoring at low cost by including a plurality of receiving antennas  215  used in the far-field monitoring mode in a plurality of receiving antennas  215  used in the near-field monitoring mode. 
     The antenna pattern of the receiving antenna part  212  may be changed by the selector  33  performing weighing on the signals received from the receiving antennas  215 . Moreover, the selection of signals from the receiving antennas  215  may be implemented by providing a mechanism for turning on and off the receiving functions themselves of the receiving antennas  215 , instead of using the selector  33 . In this case, this mechanism for turning on and off the receiving functions serves as a selector. 
     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 a radio wave during arithmetic processing, a transmission stop period between the first transmission wave and the second transmission wave is longer than both of the transmission period 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 50 milliseconds. 
     The number of receiving antennas  215  arranged at equal intervals in the lateral direction is not limited to five, and the antenna part  21  may include six or more receiving antennas  215 . Including five or more receiving antennas  215  makes it possible to use signals from three or more receiving antennas  215  arranged at wide intervals, even after the thinning-out of the receiving antennas  215  to be used, and to thereby grasp the position of an object that is located far away. If there is only one object to be detected, the number of receiving antennas  215  that remain after the thinning-out may be two. Thus, the minimum number of receiving antennas  215  in the radar device  11  is three. 
     If there is no need to detect the position of an object in the near-field monitoring mode, the number of receiving antennas  215  that are used in the near-field monitoring mode may be two. For example, a configuration is conceivable in which three receiving antennas  215  are arranged at equal intervals so that signals from two adjacent receiving antennas  215  are used in the near-field monitoring mode, and signals from two receiving antennas  215  located on both sides are used in the far-field monitoring mode. 
     More generally, in the near-field monitoring mode, the first transmission wave is transmitted from the transmitting antenna part  211 , and among the plurality of receiving antennas  215 , signals from two or more receiving antennas  215  arranged at narrow intervals are used. In the far-field monitoring mode, the second transmission wave is transmitted from the transmitting antenna part  211 , and among the plurality of receiving antennas  215 , signals from two or more receiving antennas  215  arranged at wide intervals are used. In order to reduce the number of receiving antennas  215 , at least one of the two or more receiving antennas  215  arranged at wide intervals is included in the two or more receiving antennas arranged at narrow intervals. 
     The first and second transmission waves are vertically polarized waves with respect to the lateral direction. The first and second transmission waves do not need to be completely vertically polarized waves, and may be diagonally or elliptically polarized waves. More generally, the first and second transmission waves have a vertical polarization component greater than the horizontal polarization component thereof with respect to the lateral direction. Since the laminated glass  12  is usually inclined with its upper part located rearward of its lower part, the vertical polarization components of the first and second transmission waves with respect to the lateral direction are vertical polarization components thereof with respect to the laminated glass  12 . Thus, the efficiency of the transmission waves passing through the laminated glass  12  improves. 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 wave is also called a transverse magnetic wave (TM wave) that indicates a polarized wave having an electric-field component perpendicular to the plane of reflection. At this time, the magnetic-field component thereof is parallel to the plane of reflection. The horizontally polarized wave is also called a transverse electric wave (TE wave) that indicates a polarized wave having a magnetic-field component perpendicular to the plane of reflection. At this time, the electric-field component thereof is parallel to the plane of reflection. As will be described later, the first transmission wave and the second transmission wave may have a horizontal polarization component greater than the vertical polarization component thereof. 
     The horns of the first transmitting antenna  213  and the second transmitting antenna  214  are arranged in the lateral direction. In the present embodiment, the first transmitting antenna  213  and the second transmitting antenna  214  are located on the right and left sides of the receiving antenna part  212 . By arranging the first transmitting antenna  213 , the second transmitting antenna  214 , and the receiving antenna  215  side by side, it is possible to house a plurality of horns in a single member and to thereby reduce the manufacturing cost of the radar device  11 . Additionally, the orientation of each horn can 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 align the orientations of the first transmitting antenna  213  and the second transmitting antenna  214  in the up-down direction. 
     The first transmitting antenna  213  and the second transmitting antenna  214  are preferably disposed such that the direction of the center of the main lobe, i.e., the direction of the peak of the main lobe, is oriented in the horizontal direction. The first transmitting antenna  213  and the second transmitting antenna  214  may be disposed such that the direction of the main lobe is oriented in a direction that is between the horizontal direction and a direction inclined 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. They may be any type of antenna that can transmit and receive millimeter waves. Examples of such antennas include lens antennas, low-cost printed antennas, microstrip antennas, and slit antennas. Every antenna included in the antenna part  21  does not need to be of the same type, and different types of antennas may be mixed. 
     Next is a description of the laminated glass  12  when the radar device  11  is provided in the vehicle interior.  FIG. 7  illustrates how a transmission wave enters the laminated glass  12 . Note that the incident angle of the transmission wave indicates an incident angle of the transmission wave on an object at the center of the main lobe of the transmitting antenna (i.e., the first transmitting antenna  213  and the second transmitting antenna  214 ) when the mounting part  241  is mounted on the bracket  16 . 
     Part of the transmission wave that enters the laminated glass  12  will be reflected at the boundary between the innermost glass layer  121  and an air layer  171  on the vehicle interior  13  side of the innermost glass layer  121 , at the boundary between the innermost glass layer  121  and the intermediate resin layer  123 , at the boundary between the intermediate resin layer  123  and the outermost glass layer  122 , and at the boundary between the outermost glass layer  122  and an air layer  172  on the vehicle exterior side of the outermost glass layer  122  when sequentially passing through the innermost glass layer  121 , the intermediate resin layer  123 , and the outermost glass layer  122 . Hereinafter, the air layer  171  is referred to as an “inner air layer,” and the air layer  172  is referred to as an “outer air layer.” 
     Focusing now on three layers that are in contact with one another and selected from among the inner air layer  171 , the innermost glass layer  121 , the intermediate resin layer  123 , the outermost glass layer  122 , and the outer air layer  172 , the three layers may be selected as desired as long as they are in contact with one another. The three layers are respectively referred to as a “first layer  51 ,” a “second layer  52 ,” and a “third layer  53 ” in the order from the vehicle interior  13  side to outside, i.e., in the order from the innermost layer. 
       FIG. 8  schematically illustrates the three layers  51 ,  52 , and  53 . As indicated by bold arrows in  FIG. 8 , part of the transmission wave that enters the second layer  52  from a point A on an interface  521  between the first layer  51  and the second layer  52  is reflected at a point B on an interface  522  between the second layer  52  and the third layer  53  and returns as a reflected wave to a point C on the interface  521 . At this time, if the reflected wave passing through the point C and a reflected wave generated by reflection of the transmission wave that enters the point C on the interface  521  from the antenna part  21  side are opposite in phase (i.e., the phases of the reflected waves are shifted by π), they will cancel out each other. As a result, the transmission wave component reflected on the second layer  52  will decrease. In other words, apparent reflection of the transmission wave that enters the interface  521  from the first layer  51  side, which is adjacent to the surface of the second layer  52 , will be suppressed. 
     The following describes the second layer  52  that suppresses reflection of a transmission wave by interference between a reflected wave generated by reflection of the transmission wave on the interface  522  and a reflected wave generated by reflection of a transmission wave on the interface  521 . In the following description, m is the refractive index of the first layer  51 , n 2  is the refractive index of the second layer  52 , n 3  is the refractive index of the third layer  53 , d 2  is the thickness of the middle, i.e., second layer  52 , θ 1  is the incident angle of the transmission wave on the second layer  52 , and θ 2  is the refraction angle in the second layer  52 . Formula 5 holds true according to Snell&#39;s law. 
     
       
         
           
             
               
                 
                   
                     
                       sin 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         θ 
                         1 
                       
                     
                     
                       sin 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         θ 
                         2 
                       
                     
                   
                   = 
                   
                     
                       n 
                       2 
                     
                     
                       n 
                       1 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     5 
                   
                   ] 
                 
               
             
           
         
       
     
     An optical path length L abc  from the point A via the point B to the point C in the second layer  52  is expressed by Formula 6. 
     
       
         
           
             
               
                 
                   
                     L 
                     abc 
                   
                   = 
                   
                     
                       2 
                       ⁢ 
                       
                         d 
                         2 
                       
                       ⁢ 
                       
                         n 
                         2 
                       
                     
                     
                       cos 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         θ 
                         2 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     6 
                   
                   ] 
                 
               
             
           
         
       
     
     An optical path length δ between the point A and the point C in the travel direction of the transmission wave entering the second layer  52  from the antenna part  21  is expressed by Formula 7.
 
δ=2d 2 n 1  tan θ 2  sin θ 1   [Formula 7]
 
     The condition under which the reflected wave generated by reflection of the transmission wave on the interface  522  and the reflected wave generated by reflection of the transmission wave on the interface  521  will be opposite in phase on the interface  521  is expressed by Formula 8, where N is an integer of 0 or more, and λ is the wavelength of the transmission wave in the air. Formula 8 is based on the assumption that the phases are inverted (i.e., the phases are shifted by π) by the reflection of the transmission wave on the point B and the reflection of the transmission wave that enters the point C from the adjacent layer, respectively. 
     
       
         
           
             
               
                 
                   
                     L 
                     abc 
                   
                   = 
                   
                     
                       
                         λ 
                         2 
                       
                       ⁢ 
                       
                         ( 
                         
                           
                             2 
                             ⁢ 
                             N 
                           
                           + 
                           1 
                         
                         ) 
                       
                     
                     + 
                     δ 
                   
                 
               
               
                 
                   [ 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     8 
                   
                   ] 
                 
               
             
           
         
       
     
     Transforming Formula 8 by using Formulas 5 to 7 yields Formula 9 that expresses the thickness d 2  of the second layer  52 . 
     
       
         
           
             
               
                 
                   
                     d 
                     2 
                   
                   = 
                   
                     
                       
                         ( 
                         
                           
                             2 
                             ⁢ 
                             N 
                           
                           + 
                           1 
                         
                         ) 
                       
                       ⁢ 
                       
                         n 
                         2 
                       
                       ⁢ 
                       
                         λcos 
                         ⁡ 
                         
                           ( 
                           
                             
                               sin 
                               
                                 - 
                                 1 
                               
                             
                             ⁡ 
                             
                               ( 
                               
                                 
                                   
                                     n 
                                     1 
                                   
                                   
                                     n 
                                     2 
                                   
                                 
                                 ⁢ 
                                 sin 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   θ 
                                   1 
                                 
                               
                               ) 
                             
                           
                           ) 
                         
                       
                     
                     
                       4 
                       ⁢ 
                       
                         ( 
                         
                           
                             n 
                             2 
                             2 
                           
                           - 
                           
                             
                               n 
                               1 
                               2 
                             
                             ⁢ 
                             
                               sin 
                               2 
                             
                             ⁢ 
                             
                               θ 
                               1 
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     9 
                   
                   ] 
                 
               
             
           
         
       
     
     Meanwhile, Formula 10 holds true according to Snell&#39;s law, where the refractive index of the air layer is assumed to be 1, and θ a  is the incident angle of the transmission wave on the innermost glass layer  121  from the inner air layer  171  as illustrated in  FIG. 7 . Thus, Formula 9 is expressed as Formula 11. Note that the incident angle θ a  is also an angle formed by the normal to the laminated glass  12  and the direction of propagation of the transmission wave at the center of the main lobe of the transmitting antenna.
 
n 1  sin θ 1 =sin θ a   [Formula 10]
 
     
       
         
           
             
               
                 
                   
                     d 
                     2 
                   
                   = 
                   
                     
                       
                         ( 
                         
                           
                             2 
                             ⁢ 
                             N 
                           
                           + 
                           1 
                         
                         ) 
                       
                       ⁢ 
                       
                         n 
                         2 
                       
                       ⁢ 
                       
                         λcos 
                         ⁡ 
                         
                           ( 
                           
                             
                               sin 
                               
                                 - 
                                 1 
                               
                             
                             ⁡ 
                             
                               ( 
                               
                                 
                                   1 
                                   
                                     n 
                                     2 
                                   
                                 
                                 ⁢ 
                                 sin 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   θ 
                                   a 
                                 
                               
                               ) 
                             
                           
                           ) 
                         
                       
                     
                     
                       4 
                       ⁢ 
                       
                         ( 
                         
                           
                             n 
                             2 
                             2 
                           
                           - 
                           
                             
                               n 
                               1 
                               2 
                             
                             ⁢ 
                             
                               sin 
                               2 
                             
                             ⁢ 
                             
                               θ 
                               a 
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     11 
                   
                   ] 
                 
               
             
           
         
       
     
     If the phase shift between the reflected wave generated by reflection of the transmission wave on the interface  522  and the reflected wave generated by reflection of the transmission wave on the interface  521  is within a range of (π±π/4), it is considered possible to suppress the reflection of the transmission wave on the second layer  52 . In this case, (2N+1) in Formula 11 is in the range of (2N+1±¼). Accordingly, a preferable condition for the thickness d 2  of the second layer  52  is expressed by Formula 12. 
     
       
         
           
             
               
                 
                   
                     
                       
                         ( 
                         
                           
                             2 
                             ⁢ 
                             N 
                           
                           + 
                           
                             3 
                             4 
                           
                         
                         ) 
                       
                       ⁢ 
                       
                         n 
                         2 
                       
                       ⁢ 
                       
                         λcos 
                         ⁡ 
                         
                           ( 
                           
                             
                               sin 
                               
                                 - 
                                 1 
                               
                             
                             ⁡ 
                             
                               ( 
                               
                                 
                                   1 
                                   
                                     n 
                                     2 
                                   
                                 
                                 ⁢ 
                                 sin 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   θ 
                                   a 
                                 
                               
                               ) 
                             
                           
                           ) 
                         
                       
                     
                     
                       4 
                       ⁢ 
                       
                         ( 
                         
                           
                             n 
                             2 
                             2 
                           
                           - 
                           
                             
                               sin 
                               2 
                             
                             ⁢ 
                             
                               θ 
                               a 
                             
                           
                         
                         ) 
                       
                     
                   
                   ≤ 
                   
                     d 
                     2 
                   
                   ≤ 
                   
                     
                       
                         ( 
                         
                           
                             2 
                             ⁢ 
                             N 
                           
                           + 
                           
                             5 
                             4 
                           
                         
                         ) 
                       
                       ⁢ 
                       
                         n 
                         2 
                       
                       ⁢ 
                       
                         λcos 
                         ⁡ 
                         
                           ( 
                           
                             
                               sin 
                               
                                 - 
                                 1 
                               
                             
                             ⁡ 
                             
                               ( 
                               
                                 
                                   1 
                                   
                                     n 
                                     2 
                                   
                                 
                                 ⁢ 
                                 sin 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   θ 
                                   a 
                                 
                               
                               ) 
                             
                           
                           ) 
                         
                       
                     
                     
                       4 
                       ⁢ 
                       
                         ( 
                         
                           
                             n 
                             2 
                             2 
                           
                           - 
                           
                             
                               sin 
                               2 
                             
                             ⁢ 
                             
                               θ 
                               a 
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     12 
                   
                   ] 
                 
               
             
           
         
       
     
     The above condition is based on the assumption that the incident angle θ 1  of the transmission wave on the second layer  52  is less than or equal to a Brewster angle in the case where a radio wave enters the interface  521  of the second layer  52  from the first layer  51 . As described previously, the transmission wave has a vertical polarization component greater than the horizontal polarization component thereof with respect to the laminated glass  12 . Thus, if the incident angle θ 1  is greater than the Brewster angle, the influence of the phase inversion of the vertical polarization component reflected on the interface  521  will increase. In this case, the condition under which the reflected wave generated by reflection of the transmission wave on the interface  522  and the reflected wave generated by reflection of the transmission wave on the interface  521  will be opposite in phase on the interface  521  is expressed by Formula 13, and a preferable condition for the thickness d 2  of the second layer  52  is expressed by Formula 14.
 
 L   abc =λ( N+ 1)+δ  [Formula 13]
 
     
       
         
           
             
               
                 
                   
                     
                       
                         ( 
                         
                           
                             2 
                             ⁢ 
                             N 
                           
                           + 
                           
                             7 
                             4 
                           
                         
                         ) 
                       
                       ⁢ 
                       
                         n 
                         2 
                       
                       ⁢ 
                       
                         λcos 
                         ⁡ 
                         
                           ( 
                           
                             
                               sin 
                               
                                 - 
                                 1 
                               
                             
                             ⁡ 
                             
                               ( 
                               
                                 
                                   1 
                                   
                                     n 
                                     2 
                                   
                                 
                                 ⁢ 
                                 sin 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   θ 
                                   a 
                                 
                               
                               ) 
                             
                           
                           ) 
                         
                       
                     
                     
                       4 
                       ⁢ 
                       
                         ( 
                         
                           
                             n 
                             2 
                             2 
                           
                           - 
                           
                             
                               sin 
                               2 
                             
                             ⁢ 
                             
                               θ 
                               a 
                             
                           
                         
                         ) 
                       
                     
                   
                   ≤ 
                   
                     d 
                     2 
                   
                   ≤ 
                   
                     
                       
                         ( 
                         
                           
                             2 
                             ⁢ 
                             N 
                           
                           + 
                           
                             9 
                             4 
                           
                         
                         ) 
                       
                       ⁢ 
                       
                         n 
                         2 
                       
                       ⁢ 
                       
                         λcos 
                         ⁡ 
                         
                           ( 
                           
                             
                               sin 
                               
                                 - 
                                 1 
                               
                             
                             ⁡ 
                             
                               ( 
                               
                                 
                                   1 
                                   
                                     n 
                                     2 
                                   
                                 
                                 ⁢ 
                                 sin 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   θ 
                                   a 
                                 
                               
                               ) 
                             
                           
                           ) 
                         
                       
                     
                     
                       4 
                       ⁢ 
                       
                         ( 
                         
                           
                             n 
                             2 
                             2 
                           
                           - 
                           
                             
                               sin 
                               2 
                             
                             ⁢ 
                             
                               θ 
                               a 
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     14 
                   
                   ] 
                 
               
             
           
         
       
     
     A Brewster angle θ b1  in the case where a radio wave enters the second layer  52  from the first layer  51  is tan −1 (n 2 /n 1 ). At this time, the incident angle θ a  on the innermost glass layer  121  is expressed by Formula 15 using Formula 10. 
     
       
         
           
             
               
                 
                   
                     sin 
                     
                       - 
                       1 
                     
                   
                   ⁡ 
                   
                     ( 
                     
                       
                         n 
                         1 
                       
                       ⁢ 
                       
                         sin 
                         ⁡ 
                         
                           ( 
                           
                             
                               tan 
                               
                                 - 
                                 1 
                               
                             
                             ⁢ 
                             
                               
                                 n 
                                 2 
                               
                               
                                 n 
                                 1 
                               
                             
                           
                           ) 
                         
                       
                     
                     ) 
                   
                 
               
               
                 
                   [ 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     15 
                   
                   ] 
                 
               
             
           
         
       
     
     Similarly, a Brewster angle θ b2  in the case where a radio wave enters the third layer  53  from the second layer  52  is tan −1 (n 3 /n 2 ). At this time, the incident angle θ a  on the innermost glass layer  121  is expressed by Formula 16. 
     
       
         
           
             
               
                 
                   
                     sin 
                     
                       - 
                       1 
                     
                   
                   ⁡ 
                   
                     ( 
                     
                       
                         n 
                         2 
                       
                       ⁢ 
                       
                         sin 
                         ⁡ 
                         
                           ( 
                           
                             
                               tan 
                               
                                 - 
                                 1 
                               
                             
                             ⁢ 
                             
                               
                                 n 
                                 3 
                               
                               
                                 n 
                                 2 
                               
                             
                           
                           ) 
                         
                       
                     
                     ) 
                   
                 
               
               
                 
                   [ 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     16 
                   
                   ] 
                 
               
             
           
         
       
     
     If the incident angle θ a  on the innermost glass layer  121  is smaller than both of the angles given by Formulas 15 and 16, both of the reflected wave generated by reflection of the transmission wave on the interface  522  and the reflected wave generated by reflection of the transmission wave on the interface  521  will not undergo a phase inversion due to the relationship with the Brewster angles θ b1  and θ b2 . If the incident angle θ a  is greater than both of the angles given by Formulas 15 and 16, both of the reflected wave generated by reflection of the transmission wave on the interface  522  and the reflected wave generated by reflection of the transmission wave on the interface  521  will undergo a phase inversion due to the relationship with the Brewster angles θ b1  and θ b2 . If the incident angle θ a  is a value between or equal to one of the angles given by Formulas 15 and 16, only one of the reflected wave generated by reflection of the transmission wave on the interface  522  and the reflected wave generated by reflection of the transmission wave on the interface  521  will undergo a phase inversion due to the relationship with the Brewster angles θ b1  and θ b2 . 
     If the refractive index n 2  of the second layer  52  is higher than the refractive index n 1  of the first layer  51 , the reflected wave generated by reflection of the transmission wave on the interface  521  will undergo a phase inversion due to a difference in the refractive index. If the refractive index n 3  of the third layer  53  is higher than the refractive index n 2  of the second layer  52 , the reflected wave generated by reflection of the transmission wave on the interface  522  will undergo a phase inversion due to a difference in the refractive index. Accordingly, the reflection of the transmission wave on the interface  521  will be suppressed if the thickness d 2  of the second layer  52  satisfies the relationships expressed by Formulas 17 and 18. 
     
       
         
           
             
               
                 
                   
                     
                       If 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         n 
                         1 
                       
                     
                     &lt; 
                     
                       n 
                       2 
                     
                     &lt; 
                     
                       
                         n 
                         3 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       or 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         n 
                         1 
                       
                     
                     &gt; 
                     
                       n 
                       2 
                     
                     &gt; 
                     
                       
                         n 
                         3 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       and 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         θ 
                         a 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       is 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       smaller 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       than 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       or 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       greater 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       than 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       both 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       of 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         
                           sin 
                           
                             - 
                             1 
                           
                         
                         ⁡ 
                         
                           ( 
                           
                             
                               n 
                               1 
                             
                             ⁢ 
                             
                               sin 
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     tan 
                                     
                                       - 
                                       1 
                                     
                                   
                                   ⁢ 
                                   
                                     
                                       n 
                                       2 
                                     
                                     
                                       n 
                                       1 
                                     
                                   
                                 
                                 ) 
                               
                             
                           
                           ) 
                         
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       and 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         
                           sin 
                           
                             - 
                             1 
                           
                         
                         ⁡ 
                         
                           ( 
                           
                             
                               n 
                               2 
                             
                             ⁢ 
                             
                               sin 
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     tan 
                                     
                                       - 
                                       1 
                                     
                                   
                                   ⁢ 
                                   
                                     
                                       n 
                                       3 
                                     
                                     
                                       n 
                                       2 
                                     
                                   
                                 
                                 ) 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   [ 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     17 
                   
                   ] 
                 
               
             
             
               
                 
                   
                     
                       
                         
                           ( 
                           
                             
                               2 
                               ⁢ 
                               N 
                             
                             + 
                             
                               3 
                               4 
                             
                           
                           ) 
                         
                         ⁢ 
                         
                           n 
                           2 
                         
                         ⁢ 
                         
                           λcos 
                           ⁡ 
                           
                             ( 
                             
                               
                                 sin 
                                 
                                   - 
                                   1 
                                 
                               
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     1 
                                     
                                       n 
                                       2 
                                     
                                   
                                   ⁢ 
                                   sin 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   
                                     θ 
                                     a 
                                   
                                 
                                 ) 
                               
                             
                             ) 
                           
                         
                       
                       
                         4 
                         ⁢ 
                         
                           ( 
                           
                             
                               n 
                               2 
                               2 
                             
                             - 
                             
                               
                                 sin 
                                 2 
                               
                               ⁢ 
                               
                                 θ 
                                 a 
                               
                             
                           
                           ) 
                         
                       
                     
                     ≤ 
                     
                       d 
                       2 
                     
                     ≤ 
                     
                       
                         
                           ( 
                           
                             
                               2 
                               ⁢ 
                               N 
                             
                             + 
                             
                               5 
                               4 
                             
                           
                           ) 
                         
                         ⁢ 
                         
                           n 
                           2 
                         
                         ⁢ 
                         
                           λcos 
                           ⁡ 
                           
                             ( 
                             
                               
                                 sin 
                                 
                                   - 
                                   1 
                                 
                               
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     1 
                                     
                                       n 
                                       2 
                                     
                                   
                                   ⁢ 
                                   sin 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   
                                     θ 
                                     a 
                                   
                                 
                                 ) 
                               
                             
                             ) 
                           
                         
                       
                       
                         4 
                         ⁢ 
                         
                           ( 
                           
                             
                               n 
                               2 
                               2 
                             
                             - 
                             
                               
                                 sin 
                                 2 
                               
                               ⁢ 
                               
                                 θ 
                                 a 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       and 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       if 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         n 
                         1 
                       
                     
                     &lt; 
                     
                       n 
                       2 
                     
                     &lt; 
                     
                       
                         n 
                         3 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       or 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         n 
                         1 
                       
                     
                     &gt; 
                     
                       n 
                       2 
                     
                     &gt; 
                     
                       
                         n 
                         3 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       and 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         θ 
                         a 
                       
                       ⁢ 
                       
                         
                             
                         
                         ⁢ 
                         
                             
                         
                       
                       ⁢ 
                       is 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       value 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       between 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       or 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       equal 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       to 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       one 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       of 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         
                           sin 
                           
                             - 
                             1 
                           
                         
                         ⁡ 
                         
                           ( 
                           
                             
                               n 
                               1 
                             
                             ⁢ 
                             
                               sin 
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     tan 
                                     
                                       - 
                                       1 
                                     
                                   
                                   ⁢ 
                                   
                                     
                                       n 
                                       2 
                                     
                                     
                                       n 
                                       
                                         1 
                                         ⁢ 
                                         
                                             
                                         
                                       
                                     
                                   
                                 
                                 ) 
                               
                             
                           
                           ) 
                         
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       and 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         
                           sin 
                           
                             - 
                             1 
                           
                         
                         ⁡ 
                         
                           ( 
                           
                             
                               n 
                               2 
                             
                             ⁢ 
                             
                               sin 
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     tan 
                                     
                                       - 
                                       1 
                                     
                                   
                                   ⁢ 
                                   
                                     
                                       n 
                                       3 
                                     
                                     
                                       n 
                                       2 
                                     
                                   
                                 
                                 ) 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       
                         ( 
                         
                           
                             2 
                             ⁢ 
                             N 
                           
                           + 
                           
                             7 
                             4 
                           
                         
                         ) 
                       
                       ⁢ 
                       
                         n 
                         2 
                       
                       ⁢ 
                       
                         λcos 
                         ⁡ 
                         
                           ( 
                           
                             
                               sin 
                               
                                 - 
                                 1 
                               
                             
                             ⁡ 
                             
                               ( 
                               
                                 
                                   1 
                                   
                                     n 
                                     2 
                                   
                                 
                                 ⁢ 
                                 sin 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   θ 
                                   a 
                                 
                               
                               ) 
                             
                           
                           ) 
                         
                       
                     
                     
                       4 
                       ⁢ 
                       
                         ( 
                         
                           
                             n 
                             2 
                             2 
                           
                           - 
                           
                             
                               sin 
                               2 
                             
                             ⁢ 
                             
                               θ 
                               a 
                             
                           
                         
                         ) 
                       
                     
                   
                   ≤ 
                   
                     d 
                     2 
                   
                   ≤ 
                   
                     
                       
                         ( 
                         
                           
                             2 
                             ⁢ 
                             N 
                           
                           + 
                           
                             9 
                             4 
                           
                         
                         ) 
                       
                       ⁢ 
                       
                         n 
                         2 
                       
                       ⁢ 
                       
                         λcos 
                         ⁡ 
                         
                           ( 
                           
                             
                               sin 
                               
                                 - 
                                 1 
                               
                             
                             ⁡ 
                             
                               ( 
                               
                                 
                                   1 
                                   
                                     n 
                                     2 
                                   
                                 
                                 ⁢ 
                                 sin 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   θ 
                                   a 
                                 
                               
                               ) 
                             
                           
                           ) 
                         
                       
                     
                     
                       4 
                       ⁢ 
                       
                         ( 
                         
                           
                             n 
                             2 
                             2 
                           
                           - 
                           
                             
                               sin 
                               2 
                             
                             ⁢ 
                             
                               θ 
                               a 
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       If 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         n 
                         1 
                       
                     
                     &gt; 
                     
                       n 
                       2 
                     
                     &lt; 
                     
                       
                         n 
                         3 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       or 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         n 
                         1 
                       
                     
                     &lt; 
                     
                       n 
                       2 
                     
                     &gt; 
                     
                       
                         n 
                         3 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       and 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         θ 
                         a 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       is 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       smaller 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       than 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       or 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       greater 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       than 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       both 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       of 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         
                           sin 
                           
                             - 
                             1 
                           
                         
                         ⁡ 
                         
                           ( 
                           
                             
                               n 
                               1 
                             
                             ⁢ 
                             
                               sin 
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     tan 
                                     
                                       - 
                                       1 
                                     
                                   
                                   ⁢ 
                                   
                                     
                                       n 
                                       2 
                                     
                                     
                                       n 
                                       1 
                                     
                                   
                                 
                                 ) 
                               
                             
                           
                           ) 
                         
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       and 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         
                           sin 
                           
                             - 
                             1 
                           
                         
                         ⁡ 
                         
                           ( 
                           
                             
                               n 
                               2 
                             
                             ⁢ 
                             
                               sin 
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     tan 
                                     
                                       - 
                                       1 
                                     
                                   
                                   ⁢ 
                                   
                                     
                                       n 
                                       3 
                                     
                                     
                                       n 
                                       2 
                                     
                                   
                                 
                                 ) 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   [ 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     18 
                   
                   ] 
                 
               
             
             
               
                 
                   
                     
                       
                         
                           ( 
                           
                             
                               2 
                               ⁢ 
                               N 
                             
                             + 
                             
                               7 
                               4 
                             
                           
                           ) 
                         
                         ⁢ 
                         
                           n 
                           2 
                         
                         ⁢ 
                         
                           λcos 
                           ⁡ 
                           
                             ( 
                             
                               
                                 sin 
                                 
                                   - 
                                   1 
                                 
                               
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     1 
                                     
                                       n 
                                       2 
                                     
                                   
                                   ⁢ 
                                   sin 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   
                                     θ 
                                     a 
                                   
                                 
                                 ) 
                               
                             
                             ) 
                           
                         
                       
                       
                         4 
                         ⁢ 
                         
                           ( 
                           
                             
                               n 
                               2 
                               2 
                             
                             - 
                             
                               
                                 sin 
                                 2 
                               
                               ⁢ 
                               
                                 θ 
                                 a 
                               
                             
                           
                           ) 
                         
                       
                     
                     ≤ 
                     
                       d 
                       2 
                     
                     ≤ 
                     
                       
                         
                           ( 
                           
                             
                               2 
                               ⁢ 
                               N 
                             
                             + 
                             
                               9 
                               4 
                             
                           
                           ) 
                         
                         ⁢ 
                         
                           n 
                           2 
                         
                         ⁢ 
                         
                           λcos 
                           ⁡ 
                           
                             ( 
                             
                               
                                 sin 
                                 
                                   - 
                                   1 
                                 
                               
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     1 
                                     
                                       n 
                                       2 
                                     
                                   
                                   ⁢ 
                                   sin 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   
                                     θ 
                                     a 
                                   
                                 
                                 ) 
                               
                             
                             ) 
                           
                         
                       
                       
                         4 
                         ⁢ 
                         
                           ( 
                           
                             
                               n 
                               2 
                               2 
                             
                             - 
                             
                               
                                 sin 
                                 2 
                               
                               ⁢ 
                               
                                 θ 
                                 a 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       and 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       if 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         n 
                         1 
                       
                     
                     &gt; 
                     
                       n 
                       2 
                     
                     &lt; 
                     
                       
                         n 
                         3 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       or 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         n 
                         1 
                       
                     
                     &lt; 
                     
                       n 
                       2 
                     
                     &gt; 
                     
                       
                         n 
                         3 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       and 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         θ 
                         a 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       is 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       a 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       value 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       between 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       or 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       equal 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       to 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       one 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       of 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         
                           sin 
                           
                             - 
                             1 
                           
                         
                         ⁡ 
                         
                           ( 
                           
                             
                               n 
                               1 
                             
                             ⁢ 
                             
                               sin 
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     tan 
                                     
                                       - 
                                       1 
                                     
                                   
                                   ⁢ 
                                   
                                     
                                       n 
                                       2 
                                     
                                     
                                       n 
                                       
                                         1 
                                         ⁢ 
                                         
                                             
                                         
                                       
                                     
                                   
                                 
                                 ) 
                               
                             
                           
                           ) 
                         
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       and 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         
                           sin 
                           
                             - 
                             1 
                           
                         
                         ⁡ 
                         
                           ( 
                           
                             
                               n 
                               2 
                             
                             ⁢ 
                             
                               sin 
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     tan 
                                     
                                       - 
                                       1 
                                     
                                   
                                   ⁢ 
                                   
                                     
                                       n 
                                       3 
                                     
                                     
                                       n 
                                       2 
                                     
                                   
                                 
                                 ) 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       
                         ( 
                         
                           
                             2 
                             ⁢ 
                             N 
                           
                           + 
                           
                             3 
                             4 
                           
                         
                         ) 
                       
                       ⁢ 
                       
                         n 
                         2 
                       
                       ⁢ 
                       
                         λcos 
                         ⁡ 
                         
                           ( 
                           
                             
                               sin 
                               
                                 - 
                                 1 
                               
                             
                             ⁡ 
                             
                               ( 
                               
                                 
                                   1 
                                   
                                     n 
                                     2 
                                   
                                 
                                 ⁢ 
                                 sin 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   θ 
                                   a 
                                 
                               
                               ) 
                             
                           
                           ) 
                         
                       
                     
                     
                       4 
                       ⁢ 
                       
                         ( 
                         
                           
                             n 
                             2 
                             2 
                           
                           - 
                           
                             
                               sin 
                               2 
                             
                             ⁢ 
                             
                               θ 
                               a 
                             
                           
                         
                         ) 
                       
                     
                   
                   ≤ 
                   
                     d 
                     2 
                   
                   ≤ 
                   
                     
                       
                         ( 
                         
                           
                             2 
                             ⁢ 
                             N 
                           
                           + 
                           
                             5 
                             4 
                           
                         
                         ) 
                       
                       ⁢ 
                       
                         n 
                         2 
                       
                       ⁢ 
                       
                         λcos 
                         ⁡ 
                         
                           ( 
                           
                             
                               sin 
                               
                                 - 
                                 1 
                               
                             
                             ⁡ 
                             
                               ( 
                               
                                 
                                   1 
                                   
                                     n 
                                     2 
                                   
                                 
                                 ⁢ 
                                 sin 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   θ 
                                   a 
                                 
                               
                               ) 
                             
                           
                           ) 
                         
                       
                     
                     
                       4 
                       ⁢ 
                       
                         ( 
                         
                           
                             n 
                             2 
                             2 
                           
                           - 
                           
                             
                               sin 
                               2 
                             
                             ⁢ 
                             
                               θ 
                               a 
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                     
                 
               
             
           
         
       
     
     If the phase shift between the reflected wave generated by reflection of the transmission wave on the interface  522  and the reflected wave generated by reflection of the transmission wave on the interface  521  is within a range of (π±π/6), the reflection of the transmission wave will more reliably be suppressed. In this case, Formulas 17 and 18 are respectively expressed by Formulas 19 and 20. 
     
       
         
           
             
               
                 
                   
                     
                       If 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         n 
                         1 
                       
                     
                     &lt; 
                     
                       n 
                       2 
                     
                     &lt; 
                     
                       
                         n 
                         3 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       or 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         n 
                         1 
                       
                     
                     &gt; 
                     
                       n 
                       2 
                     
                     &gt; 
                     
                       
                         n 
                         3 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       and 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         θ 
                         a 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       is 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       smaller 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       than 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       or 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       greater 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       than 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       both 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       of 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         
                           sin 
                           
                             - 
                             1 
                           
                         
                         ⁡ 
                         
                           ( 
                           
                             
                               n 
                               1 
                             
                             ⁢ 
                             
                               sin 
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     tan 
                                     
                                       - 
                                       1 
                                     
                                   
                                   ⁢ 
                                   
                                     
                                       n 
                                       2 
                                     
                                     
                                       n 
                                       1 
                                     
                                   
                                 
                                 ) 
                               
                             
                           
                           ) 
                         
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       and 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         
                           sin 
                           
                             - 
                             1 
                           
                         
                         ⁡ 
                         
                           ( 
                           
                             
                               n 
                               2 
                             
                             ⁢ 
                             
                               sin 
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     tan 
                                     
                                       - 
                                       1 
                                     
                                   
                                   ⁢ 
                                   
                                     
                                       n 
                                       3 
                                     
                                     
                                       n 
                                       2 
                                     
                                   
                                 
                                 ) 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   [ 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     19 
                   
                   ] 
                 
               
             
             
               
                 
                   
                     
                       
                         
                           ( 
                           
                             
                               2 
                               ⁢ 
                               N 
                             
                             + 
                             
                               5 
                               6 
                             
                           
                           ) 
                         
                         ⁢ 
                         
                           n 
                           2 
                         
                         ⁢ 
                         
                           λcos 
                           ⁡ 
                           
                             ( 
                             
                               
                                 sin 
                                 
                                   - 
                                   1 
                                 
                               
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     1 
                                     
                                       n 
                                       2 
                                     
                                   
                                   ⁢ 
                                   sin 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   
                                     θ 
                                     a 
                                   
                                 
                                 ) 
                               
                             
                             ) 
                           
                         
                       
                       
                         4 
                         ⁢ 
                         
                           ( 
                           
                             
                               n 
                               2 
                               2 
                             
                             - 
                             
                               
                                 sin 
                                 2 
                               
                               ⁢ 
                               
                                 θ 
                                 a 
                               
                             
                           
                           ) 
                         
                       
                     
                     ≤ 
                     
                       d 
                       2 
                     
                     ≤ 
                     
                       
                         
                           ( 
                           
                             
                               2 
                               ⁢ 
                               N 
                             
                             + 
                             
                               7 
                               6 
                             
                           
                           ) 
                         
                         ⁢ 
                         
                           n 
                           2 
                         
                         ⁢ 
                         
                           λcos 
                           ⁡ 
                           
                             ( 
                             
                               
                                 sin 
                                 
                                   - 
                                   1 
                                 
                               
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     1 
                                     
                                       n 
                                       2 
                                     
                                   
                                   ⁢ 
                                   sin 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   
                                     θ 
                                     a 
                                   
                                 
                                 ) 
                               
                             
                             ) 
                           
                         
                       
                       
                         4 
                         ⁢ 
                         
                           ( 
                           
                             
                               n 
                               2 
                               2 
                             
                             - 
                             
                               
                                 sin 
                                 2 
                               
                               ⁢ 
                               
                                 θ 
                                 a 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       and 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       if 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         n 
                         1 
                       
                     
                     &lt; 
                     
                       n 
                       2 
                     
                     &lt; 
                     
                       
                         n 
                         3 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       or 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         n 
                         1 
                       
                     
                     &gt; 
                     
                       n 
                       2 
                     
                     &gt; 
                     
                       
                         n 
                         3 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       and 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         θ 
                         a 
                       
                       ⁢ 
                       
                         
                             
                         
                         ⁢ 
                         
                             
                         
                       
                       ⁢ 
                       is 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       value 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       between 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       or 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       equal 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       to 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       one 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       of 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         
                           sin 
                           
                             - 
                             1 
                           
                         
                         ⁡ 
                         
                           ( 
                           
                             
                               n 
                               1 
                             
                             ⁢ 
                             
                               sin 
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     tan 
                                     
                                       - 
                                       1 
                                     
                                   
                                   ⁢ 
                                   
                                     
                                       n 
                                       2 
                                     
                                     
                                       n 
                                       
                                         1 
                                         ⁢ 
                                         
                                             
                                         
                                       
                                     
                                   
                                 
                                 ) 
                               
                             
                           
                           ) 
                         
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       and 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         
                           sin 
                           
                             - 
                             1 
                           
                         
                         ⁡ 
                         
                           ( 
                           
                             
                               n 
                               2 
                             
                             ⁢ 
                             
                               sin 
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     tan 
                                     
                                       - 
                                       1 
                                     
                                   
                                   ⁢ 
                                   
                                     
                                       n 
                                       3 
                                     
                                     
                                       n 
                                       2 
                                     
                                   
                                 
                                 ) 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       
                         ( 
                         
                           
                             2 
                             ⁢ 
                             N 
                           
                           + 
                           
                             11 
                             6 
                           
                         
                         ) 
                       
                       ⁢ 
                       
                         n 
                         2 
                       
                       ⁢ 
                       
                         λcos 
                         ⁡ 
                         
                           ( 
                           
                             
                               sin 
                               
                                 - 
                                 1 
                               
                             
                             ⁡ 
                             
                               ( 
                               
                                 
                                   1 
                                   
                                     n 
                                     2 
                                   
                                 
                                 ⁢ 
                                 sin 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   θ 
                                   a 
                                 
                               
                               ) 
                             
                           
                           ) 
                         
                       
                     
                     
                       4 
                       ⁢ 
                       
                         ( 
                         
                           
                             n 
                             2 
                             2 
                           
                           - 
                           
                             
                               sin 
                               2 
                             
                             ⁢ 
                             
                               θ 
                               a 
                             
                           
                         
                         ) 
                       
                     
                   
                   ≤ 
                   
                     d 
                     2 
                   
                   ≤ 
                   
                     
                       
                         ( 
                         
                           
                             2 
                             ⁢ 
                             N 
                           
                           + 
                           
                             13 
                             6 
                           
                         
                         ) 
                       
                       ⁢ 
                       
                         n 
                         2 
                       
                       ⁢ 
                       
                         λcos 
                         ⁡ 
                         
                           ( 
                           
                             
                               sin 
                               
                                 - 
                                 1 
                               
                             
                             ⁡ 
                             
                               ( 
                               
                                 
                                   1 
                                   
                                     n 
                                     2 
                                   
                                 
                                 ⁢ 
                                 sin 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   θ 
                                   a 
                                 
                               
                               ) 
                             
                           
                           ) 
                         
                       
                     
                     
                       4 
                       ⁢ 
                       
                         ( 
                         
                           
                             n 
                             2 
                             2 
                           
                           - 
                           
                             
                               sin 
                               2 
                             
                             ⁢ 
                             
                               θ 
                               a 
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       If 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         n 
                         1 
                       
                     
                     &gt; 
                     
                       n 
                       2 
                     
                     &lt; 
                     
                       
                         n 
                         3 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       or 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         n 
                         1 
                       
                     
                     &lt; 
                     
                       n 
                       2 
                     
                     &gt; 
                     
                       
                         n 
                         3 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       and 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         θ 
                         a 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       is 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       smaller 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       than 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       or 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       greater 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       than 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       both 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       of 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         
                           sin 
                           
                             - 
                             1 
                           
                         
                         ⁡ 
                         
                           ( 
                           
                             
                               n 
                               1 
                             
                             ⁢ 
                             
                               sin 
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     tan 
                                     
                                       - 
                                       1 
                                     
                                   
                                   ⁢ 
                                   
                                     
                                       n 
                                       2 
                                     
                                     
                                       n 
                                       1 
                                     
                                   
                                 
                                 ) 
                               
                             
                           
                           ) 
                         
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       and 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         
                           sin 
                           
                             - 
                             1 
                           
                         
                         ⁡ 
                         
                           ( 
                           
                             
                               n 
                               2 
                             
                             ⁢ 
                             
                               sin 
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     tan 
                                     
                                       - 
                                       1 
                                     
                                   
                                   ⁢ 
                                   
                                     
                                       n 
                                       3 
                                     
                                     
                                       n 
                                       2 
                                     
                                   
                                 
                                 ) 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   [ 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     20 
                   
                   ] 
                 
               
             
             
               
                 
                   
                     
                       
                         
                           ( 
                           
                             
                               2 
                               ⁢ 
                               N 
                             
                             + 
                             
                               11 
                               6 
                             
                           
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                           ⁡ 
                           
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                         ⁢ 
                         
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                           ( 
                           
                             
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                         ⁢ 
                         
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                       ⁢ 
                       
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                         n 
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                       ⁢ 
                       
                           
                       
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                       ⁢ 
                       
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                         ( 
                         
                           
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                             6 
                           
                         
                         ) 
                       
                       ⁢ 
                       
                         n 
                         2 
                       
                       ⁢ 
                       
                         λcos 
                         ⁡ 
                         
                           ( 
                           
                             
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                                 - 
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                               ( 
                               
                                 
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                                 ⁢ 
                                 
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                       4 
                       ⁢ 
                       
                         ( 
                         
                           
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                         ) 
                       
                     
                   
                 
               
               
                 
                     
                 
               
             
           
         
       
     
     The vertical polarization that enters an object at the Brewster angle is ideally guided into the object without reflection. However, in the case of the laminated glass  12 , even if the vertical polarization enters the innermost glass layer  121  at the Brewster angle or an angle close to the Brewster angle, the vertical polarization will not always enter the outermost glass layer  122  at an ideal angle. This phenomenon becomes a problem when the outermost glass layer  122  causes a considerable reduction in the efficiency of transmission of a radio wave. In this case, it is necessary to adjust the behavior of a radio wave within the outermost glass layer  122  while properly suppressing a reduction in the efficiency of transmission of the radio wave, caused by the presence of the innermost glass layer  121 . 
     Accordingly, the above conditional expressions are in particular suited to the case where the incident angle θ a  on the innermost glass layer  121  is close to the Brewster angle θ b  in the case where the transmission wave enters the innermost glass layer  121  from the inner air layer  171 , and the outermost glass layer  122  is used as the second layer  52 . In general, the efficiency of vertical polarization incidence on glass will suddenly drop if the incident angle becomes greater than the Brewster angle θ b  in the inner surface of the innermost glass layer  121 , and will gradually decrease if the incident angle becomes smaller than the Brewster angle θ b . Thus, if inside the innermost glass layer  121  is an air layer, the incident angle θ a  on the innermost glass layer  121  preferably satisfies Formula 21.
 
(θ b −10°)≤θ a ≤(θ b +5°)  [Formula 21]
 
     A preferable condition for the thickness d 2  of the second layer  52  described above is based on the assumption that the transmission wave has a vertical polarization component greater than the horizontal polarization component thereof with respect to the laminated glass  12 . If the transmission wave has a horizontal polarization component greater than or equal to the vertical polarization component thereof with respect to the laminated glass  12 , the influence of the phase inversion due to the relationship with the Brewster angles will decrease. Accordingly, the reflection of the transmission wave on the second layer  52  will be suppressed if the thickness d 2  of the second layer  52  satisfies the relationships expressed by Formulas 22 and 23. If the phase shift between the two reflected waves is within a range of (π±π/6), Formulas 22 and 23 are changed in accordance with Formulas 19 and 20. 
     
       
         
           
             
               
                 
                   
                       
                   
                   ⁢ 
                   
                     
                       
                         If 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           n 
                           1 
                         
                       
                       &lt; 
                       
                         n 
                         2 
                       
                       &lt; 
                       
                         
                           n 
                           3 
                         
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         or 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           n 
                           1 
                         
                       
                       &gt; 
                       
                         n 
                         2 
                       
                       &gt; 
                       
                         n 
                         3 
                       
                     
                     , 
                     
                       
 
                     
                     ⁢ 
                     
                       
                         
                           
                             ( 
                             
                               
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                               + 
                               
                                 3 
                                 4 
                               
                             
                             ) 
                           
                           ⁢ 
                           
                             n 
                             2 
                           
                           ⁢ 
                           
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                             ⁡ 
                             
                               ( 
                               
                                 
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                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     
                                       θ 
                                       a 
                                     
                                   
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                           4 
                           ⁢ 
                           
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                                   sin 
                                   2 
                                 
                                 ⁢ 
                                 
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                                   a 
                                 
                               
                             
                             ) 
                           
                         
                       
                       ≤ 
                       
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                         2 
                       
                       ≤ 
                       
                         
                           
                             ( 
                             
                               
                                 2 
                                 ⁢ 
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                               + 
                               
                                 5 
                                 4 
                               
                             
                             ) 
                           
                           ⁢ 
                           
                             n 
                             2 
                           
                           ⁢ 
                           
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                             ⁡ 
                             
                               ( 
                               
                                 
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                                     - 
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                                 ⁡ 
                                 
                                   ( 
                                   
                                     
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                                         n 
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                                     ⁢ 
                                     
                                         
                                     
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                           4 
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                   [ 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     22 
                   
                   ] 
                 
               
             
             
               
                 
                   
                       
                   
                   ⁢ 
                   
                     
                       
                         If 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           n 
                           1 
                         
                       
                       &gt; 
                       
                         n 
                         2 
                       
                       &lt; 
                       
                         
                           n 
                           3 
                         
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         or 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           n 
                           1 
                         
                       
                       &lt; 
                       
                         n 
                         2 
                       
                       &gt; 
                       
                         n 
                         3 
                       
                     
                     , 
                     
                       
 
                     
                     ⁢ 
                     
                       
                         
                           
                             ( 
                             
                               
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                               + 
                               
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                           ⁢ 
                           
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                             ⁡ 
                             
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                                     - 
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                                   ( 
                                   
                                     
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                           4 
                           ⁢ 
                           
                             ( 
                             
                               
                                 n 
                                 2 
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                                   2 
                                 
                                 ⁢ 
                                 
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                                   a 
                                 
                               
                             
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                       ≤ 
                       
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                         2 
                       
                       ≤ 
                       
                         
                           
                             ( 
                             
                               
                                 2 
                                 ⁢ 
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                               + 
                               
                                 9 
                                 4 
                               
                             
                             ) 
                           
                           ⁢ 
                           
                             n 
                             2 
                           
                           ⁢ 
                           
                             λcos 
                             ⁡ 
                             
                               ( 
                               
                                 
                                   sin 
                                   
                                     - 
                                     1 
                                   
                                 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     
                                       1 
                                       
                                         n 
                                         2 
                                       
                                     
                                     ⁢ 
                                     sin 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     
                                       θ 
                                       a 
                                     
                                   
                                   ) 
                                 
                               
                               ) 
                             
                           
                         
                         
                           4 
                           ⁢ 
                           
                             ( 
                             
                               
                                 n 
                                 2 
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                                 ⁢ 
                                 
                                   θ 
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                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     23 
                   
                   ] 
                 
               
             
           
         
       
     
     The innermost glass layer  121  and the outermost glass layer  122  preferably have the same thickness and the same refractive index. In this case, the design and manufacturing cost of the laminated glass  12  will be reduced. 
     As illustrated in  FIG. 3 , the antenna cover  25  is provided at the front of the horns of the antenna part  21 . The antenna cover  25  is provided substantially perpendicular to the central axes of the horns. Thus, the transmission wave enters substantially perpendicular to the antenna cover  25  at the center of the main lobe. To be more specific, the antenna cover  25  is often designed to intersect with the center of the main lobe of the antenna at an angle that is greater than or equal to 70° and less than or equal to 110°. 
     In the case of suppressing the reflection of a radio wave on the antenna cover  25 , the formulas given above are applicable, where the first layer  51  is an air layer located inward of the antenna cover  25 , the second layer  52  is the antenna cover  25 , and the third layer  53  is an air layer located outward of the antenna cover  25 . Specifically, Formulas 17 and 18 are applied if the transmission wave has a vertical polarization component greater than the horizontal polarization component thereof with respect to the laminated glass  12 , and Formulas 22 and 23 are applied if the transmission wave has a horizontal polarization component greater than the vertical polarization component thereof. The same also applies when a ⅙ wavelength shift is tolerable. 
     Accordingly, a reduction of the transmission wave in the antenna cover  25  will be suppressed, and a reduction in the efficiency of transmission and reception will also be suppressed. 
     As the inventor of the present invention has deduced from the observation of electromagnetic waves in the millimeter waveband, the refractive indices of electromagnetic waves in the millimeter waveband differ greatly from those in the other frequency bands. Thus, the reflective indices of radio waves in the millimeter waveband have to be used to evaluate the formulas described above. The “radio waves in the millimeter waveband” as used herein refer to radio waves having wavelengths of 1 mm to 10 mm in the air. 
       FIG. 9  shows an example in which a reflection suppression layer  54  is provided on the surface on the vehicle interior  13  side of the laminated glass  12 . The laminated glass  12  may have other structures as long as the surface on the vehicle interior  13  side is a surface of a glass layer, i.e., at least the surface on the vehicle interior  13  side is a surface of covering glass. The reflection suppression layer  54  includes a sheet-like dielectric layer  541 . The details of the dielectric layer  541  will be described later. 
       FIGS. 10 and 11  illustrate part of the radar device  11  mounted on the laminated glass  12  and the reflection suppression layer  54 . The radar device  11  is simplified in  FIGS. 10 and 11 . In  FIGS. 10 and 11 , constituent elements that are identical to those illustrated in  FIGS. 3 to 5  are given by the same reference numerals.  FIG. 10  illustrates the vehicle interior  13  as viewed from the front side of the laminated glass  12 . In the antenna part  21 , one single transmitting antenna is illustrated as the transmitting antenna part  211 , and two receiving antennas are illustrated as the receiving antenna part  212 .  FIG. 11  illustrates cross-sections of the radar device  11 , the laminated glass  12 , and the reflection suppression layer  54  that are approximately perpendicular to the laminated glass  12 . In  FIG. 11 , the laminated glass  12  is illustrated as a single layer without distinguishing among the innermost glass layer  121 , the intermediate resin layer  123 , and the outermost glass layer  122  illustrated in  FIG. 9 . 
     The dielectric layer  541  is bonded to the surface on the vehicle interior  13  side of the laminated glass  12 , i.e., the surface on the antenna part  21  side of the laminated glass  12 , and closely adheres to that surface. The dielectric layer  541  covers only part of the laminated glass  12 . The width of the dielectric layer  541  along the surface of the laminated glass  12  increases in the downward direction. The dielectric layer  541  is an amorphous resin sheet and made of, for example, denatured polyphenylene ether (PPE). The dielectric layer  541  may be made of other materials. The dielectric layer  541  is preferably transparent if the radar device  11  includes a camera. If there is no interference with the function of the radar device  11 , the dielectric layer  541  may be opaque. 
     Here, the refractive index of the reflection suppression layer  54 , i.e., the refractive index of the dielectric layer  541 , is lower than the refractive index of the innermost glass layer  121  of the laminated glass  12  and higher than the refractive index of the air. Thus, the reflectivity of a surface  641  on the antenna part  21  side of the dielectric layer  541  will be reduced to some extent, as compared to the reflectivity of the surface on the antenna part  21  side of the laminated glass  12  on the condition that no dielectric layer  541  is provided with the laminated glass  12 . The refractive index of the dielectric layer  541  may be adjusted by introducing air bubbles or other materials. 
     In the case of the dielectric layer  541  as well, the formulas given above, either Formulas 17 and 18 or Formulas 22 and 23, are applicable, where the first layer  51  is an air layer located inward, the second layer  52  is the dielectric layer  541 , and the third layer  53  is the innermost glass layer  121 . The same also applies to the case where a ⅙ wavelength shift is tolerable. Here, θ a  is the incident angle of the transmission wave on the reflection suppression layer  54  at the center of the main lobe of the transmitting antenna. Since the laminated glass  12  and the reflection suppression layer  54  are parallel to each other, i.e., they are inclined at the same angle, the incident angle θ a  is also an angle formed by the normal to the laminated glass  12  and the direction of propagation of the transmission wave at the center of the main lobe of the transmitting antenna. 
       FIG. 12  illustrates the results of measuring loss of radio waves due to the presence of the windshield. The vertical axis in  FIG. 12  indicates one-way loss of radio waves travelling through the windshield to the outside, and the horizontal axis indicates the azimuthal angle. Bold lines L 11  and L 12  in  FIG. 12  indicate loss due to the presence of the windshield with the dielectric layer  541 , and fine lines L 21  and L 22  indicate loss due to the presence of the windshield without the dielectric layer  541 . The lines L 11  and L 21  indicate the case where the incident angle is 55° when the azimuthal angle is 0°, and the lines L 12  and L 22  indicate the case where the incident angle is 85° when the azimuthal angle is 0°. Either incident angle is greater than the Brewster angle on the inner surface  641  of the dielectric layer  541 , and the dielectric layer  541  has a thickness that satisfies the condition expressed by Formula 14. 
     As can be seen from  FIG. 12 , the presence of the dielectric layer  541  on the windshield reduces loss. In actuality, loss increases as the incident angle changes with increasing azimuthal angle, but the presence of the dielectric layer  541  on the windshield will suppress loss, regardless of the azimuthal angle. Alternatively, the thickness of the dielectric layer  541  may be gradually changed in accordance with the incident angle at each azimuthal angle. In this case, loss will be reduced considerably over the entire range of predetermined azimuthal angles. The front side of the radar device  11 , i.e., when having an azimuthal angle of 0°, is required to have high sensitivity, whereas the sides thereof are not required to have such high sensitivity. From this viewpoint, there will be no problem if the dielectric layer  541  has a constant thickness. 
     As described above, the vehicle  1  includes the dielectric layer  541  located between the antenna part  21  and the laminated glass  12  and closely adhering to the surface of the laminated glass  12 . The dielectric layer  541  has a refractive index that is lower than the refractive index of the innermost glass layer  121  of the laminated glass  12  and higher than the refractive index of the air. The dielectric layer  541  has a thickness that allows reflection of a transmission wave to be suppressed by interference between a reflected wave generated by reflection of the transmission wave on the interface at which the dielectric layer  541  closely adheres to the surface of the laminated glass  12 , and a reflected wave generated by reflection of the transmission wave on the surface on the antenna part  21  side of the dielectric layer  541 . This structure will help reduce loss of the transmission wave passing through the laminated glass  12  and improve the efficiency of transmission and reception of radio waves. 
     The incident angle of the transmission wave on the reflection suppression layer  54  at the center of the main lobe of the transmitting antenna is preferably greater than 10°. In other words, the laminated glass  12  may be inclined by a large amount with respect to the radiating surface of the transmitting antenna. Accordingly, the radar device  11  can be mounted on various parts of vehicles  1  in various designs. 
     The reflection suppression layer  54  may include additional dielectric layers that closely adhere to the surface on the antenna part  21  side of the dielectric layer  541 . In the example in  FIG. 13 , two dielectric layers  542  and  543  are stacked on top of each other on the surface  641  of the dielectric layer  541 . Hereinafter, the dielectric layers  541 ,  542 , and  543  are respectively referred to as an “outer dielectric layer  541 ,” an “intermediate dielectric layer  542 ,” and an “inner dielectric layer  543 .” The intermediate dielectric layer  542  closely adheres to the surface  641  on the antenna part  21  side of the outer dielectric layer  541 . The refractive index of the intermediate dielectric layer  542  is preferably lower than the refractive index of the outer dielectric layer  541  and higher than the refractive index of the air. The inner dielectric layer  543  closely adheres to a surface  642  on the antenna part  21  side of the intermediate dielectric layer  542 . The refractive index of the inner dielectric layer  543  is preferably lower than the refractive index of the intermediate dielectric layer  542  and higher than the refractive index of the air. 
     The reflection suppression layer  54  may include four or more dielectric layers. If the reflection suppression layer  54  includes a plurality of dielectric layers, the refractive indices of all dielectric layers are preferably lower than the refractive index of the innermost glass layer  121  of the laminated glass  12  and higher than the refractive index of the air. Additionally, the refractive indices of these dielectric layers are preferably set such that the dielectric layers located closer to the antenna part  21  have lower refractive indices. 
     In the case of each of the outer dielectric layer  541 , the intermediate dielectric layer  542 , and the inner dielectric layer  543  as well, the formulas given above, namely, either Formulas 17 and 18 or Formulas 22 and 23, are applicable, where the first layer  51  is a layer located inward of the dielectric layer, the second layer  52  is the dielectric layer, and the third layer  53  is a layer located outward of the dielectric layer. For example, the intermediate dielectric layer  542  has a thickness that allows reflection of a transmission wave to be suppressed by interference between a reflected wave generated by reflection of the transmission wave on the interface at which the intermediate dielectric layer  542  closely adheres to the outer dielectric layer  541 , and a reflected wave generated by reflection of the transmission wave on the interface at which the intermediate dielectric layer  542  closely adheres to the inner dielectric layer  543 . The inner dielectric layer  543  has a thickness that allows reflection of a transmission wave to be suppressed by interference between a reflected wave generated by reflection of the transmission wave on the interface at which the inner dielectric layer  543  closely adheres to the intermediate dielectric layer  542 , and a reflected wave generated by reflection of the transmission wave on the surface on the antenna part  21  side of the inner dielectric layer  543 . 
     More generally speaking, including the laminated glass  12 , the reflection of the transmission wave can be suppressed by applying Formulas 17 and 18 or Formulas 22 and 23 to the intermediate layer among given three layers that are in contact with one another and selected from among the inner air layer  171 , at least one dielectric layer, the innermost glass layer  121 , the intermediate resin layer  123 , the outermost glass layer  122 , and the outer air layer  172 , in accordance with the properties of the transmission wave. The same applies to the case where a ⅙ wavelength shift is tolerable. It goes without saying that these conditions may be applied to only some of the layers. Here, θ a  is the incident angle of the transmission wave on the layer that is in contact with the inner air layer  171  at the center of the main lobe of the transmitting antenna. Since the laminated glass  12  and the reflection suppression layer  54  are parallel to each other, i.e., they are inclined at the same angle, θ a  can also be expressed as the angle formed by the normal to the laminated glass  12  and the direction of propagation of the transmission wave at the center of the main lobe of the transmitting antenna, irrespective of the presence or absence the reflection suppression layer  54 . 
       FIGS. 14 and 15  show another example of a reflection suppression layer  54   a,  and illustrate part of the radar device  11  mounted on the laminated glass  12  and the reflection suppression layer  54   a.    FIGS. 14 and 15  correspond respectively to  FIGS. 10 and 11 . 
     The reflection suppression layer  54   a  includes at least one dielectric layer and has a plate-like shape. The reflection suppression layer  54   a  is located between the antenna part  21  and the laminated glass  12  and covers the front of the opening of the antenna part  21 . The reflection suppression layer  54   a  also serves as an antenna cover of the radar device  11 . In other words, the antenna cover also serves as the reflection suppression layer  54   a.  Hereinafter, the reflection suppression layer  54   a  is referred to as a “dielectric cover  54   a. ” A dielectric layer(s) of the dielectric cover  54   a  may be made of an ABS resin, a polycarbonate resin, a syndiotactic polystyrene resin, or the like. The dielectric cover  54   a  has flexibility. 
     The dielectric cover  54   a  has two bearings  549 . The two bearings  549  are fixed at the upper part to the surface on the antenna part  21  side of the dielectric cover  54   a.  The antenna part  21  has one bearing  261 . The bearing  261  is provided at the upper part of the antenna part  21 . The bearing  261  is located between the two bearings  549 , which are arranged approximately in the horizontal direction. The two bearings  549  and the one bearing  261  share one shaft  262 . Thus, the upper part of the dielectric cover  54   a  is rotatably supported on the upper part of the antenna part  21 . For example, the angle of the dielectric cover  54   a  relative to the antenna part  21  may vary within a range of approximately ±10°. In actuality, the bearing  261  is arranged at a position that is in close proximity to the laminated glass  12 , and the shaft  262  applies pressure toward the laminated glass  12  to the top part of the dielectric cover  54   a.    
     The dielectric cover  54   a  includes a lower cover  544  and a rod  548 . The lower cover  544  extends toward the bottom of the antenna part  21 . The lower cover  544  includes a bearing  545 . The bearing  545  is connected to one end of the rod  548 . The bearing  545  rotatably supports the rod  548 . The rod  548  is inserted in a coil spring  546 . One end on the bearing  545  side of the coil spring  546  is fixed to the rod  548 . The other end of the coil spring  546  is in contact with a supporter  547  provided on the bottom of the antenna part  21 . The coil spring  546  applies pressure toward the laminated glass  12  to the bottom of the dielectric cover  54   a.  As a result, the dielectric cover  54   a  is brought into intimate contact with the surface on the antenna part  21  side of the laminated glass  12 , while being bent. 
     A dielectric layer of the dielectric cover  54   a  that closely adheres to the surface on the antenna part  21  side of the laminated glass  12  has a thickness that allows reflection of a transmission wave to be suppressed by interference between the reflected wave generated by reflection of the transmission wave on an interface at which the dielectric layer closely adheres to the surface of the laminated glass  12 , and the reflected wave generated by reflection of the transmission wave on the surface on the antenna part  21  side of the dielectric layer. In other words, the thickness of the dielectric layer satisfies the relationships expressed by Formulas 17 and 18 if the transmission wave has a vertical polarization component greater than the horizontal polarization component thereof with respect to the dielectric cover  54   a.  If the transmission wave has a horizontal polarization component greater than or equal to the vertical polarization component thereof with respect to the dielectric cover  54   a,  the thickness of the dielectric layer satisfies the relationships expressed by Formulas 22 and 23. As a result, this structure will help reduce loss of the transmission wave passing through the laminated glass  12  and improve the efficiency of transmission and reception of radio waves. The same applies to each dielectric layer if the dielectric cover  54   a  includes a plurality of dielectric layers. 
     As described previously, the refractive indices of electromagnetic waves in the millimeter waveband differ greatly from those in the other frequency bands. Thus, the refractive indices of radio waves in the millimeter waveband have to be used to evaluate the formulas described above. 
     The vehicle  1  described above may be modified in various ways. 
     For example, one antenna may serve as both a transmitting antenna and a receiving antenna. Alternatively, one transmitting antenna may be provided with a mechanism for changing the antenna pattern and transmit both of the first transmission wave and the second transmission wave. As another alternative, one receiving antenna may be provided with a mechanism for changing the receiving antenna pattern and implement both of the near-field monitoring mode and the far-field monitoring mode. In other words, the number of antennas included in the antenna part  21  may be one, and the antenna part  21  may include at least one antenna. It is, of course, preferable for the antenna part  21  to include a plurality of antennas. 
     The plurality of receiving antennas  215  may include portions arranged in the longitudinal direction, as long as they include portions arranged in the lateral direction. For example, the receiving antennas  215  may be arranged two dimensionally. 
     The position at which the radar device  11  is mounted is not limited to the windshield, and the radar device  11  may be mounted on the rear glass for the purpose of rearward monitoring. The installation position of the radar device is not limited to a position on glass. 
     The radar device  11  transmits a transmission wave in approximately the horizontal direction and receives a reflective wave that returns in approximately the horizontal direction. Thus, a similar reflection suppression layer may also be provided on the outer surface of the laminated glass  12 . 
     The vehicle  1  is not limited to a passenger car and may be other vehicles, such as a truck or a train, for use in various applications. 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 preferred embodiments and variations described above may be appropriately combined as long as there are no mutual inconsistencies. 
     While the invention has 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. 2016-008158 filed in the Japan Patent Office on Jan. 19, 2016, the entire disclosure of which is incorporated herein by reference. 
     INDUSTRIAL AVAILABILITY 
     The vehicle including the radar device according to the present invention can be used for various applications. 
     REFERENCE SIGNS LIST 
       1  Vehicle 
       10  Vehicle body 
       11  (On-vehicle) radar device 
       12  Laminated glass (windshield) 
       15  Drive mechanism 
       21  Antenna part 
       25  Antenna cover 
       54  Reflection suppression layer 
       121  Innermost glass layer 
       122  Outermost glass layer 
       123  Intermediate resin layer 
       171  Inner air layer 
       172  Outer air layer 
       213  First transmitting antenna 
       214  Second transmitting antenna 
       541  (Outer) dielectric layer 
       542  Intermediate dielectric layer