Patent Publication Number: US-2023155293-A1

Title: Antenna device for vehicle

Description:
TECHNICAL FIELD 
     The present invention relates to an antenna device for vehicle. 
     BACKGROUND ART 
     In recent years, there is an increasing demand for communication using a frequency band, such as Long Term Evolution (LTE), 4th Generation Mobile Communication System (4G), or 5th Generation Mobile Communication System (5G). There is also a demand for a small antenna stably usable with radiation efficiency over a broadband, such as a frequency band of 69 MHz to 6 GHz, 617 MHz to 5 GHz, or 5.9 GHz to 7.1 GHz. 
     Patent Document 1 describes an antenna having a polygonal conductor plate of which a lower side on a ground side is shorter than an upper side. The conductor plate includes a slit having an open end in the vicinity of a feeding point on a lower side of a conductor. In this antenna, a return loss equal to or less than −5 dB is obtained in a frequency band of 748 MHz to 960 MHz, a frequency band of 1450 MHz to 2175 MHz, and a frequency band of 2490 MHz to 2690 MHz. 
     Patent Document 2 describes an antenna having a triangular conductor. In this antenna, a voltage standing wave ratio (VSWR) equal to or less than 5 is obtained in a frequency band of about 700 MHz to 1000 MHz and a frequency band of about 1500 MHz to 3000 MHz. 
     RELATED DOCUMENT 
     Patent Document 
     
         
         Patent Document 1: International Publication No. WO2017/191811 
         Patent Document 2: U.S. patent Ser. No. 10/305,162 
       
    
     SUMMARY OF THE INVENTION 
     Technical Problem 
     An example of an object of the present invention is to enable an antenna to be used stably with high radiation efficiency over a broadband. 
     Solution to Problem 
     An aspect of the present invention is an antenna device for vehicle including 
     a first antenna element disposed on a ground, and 
     a second antenna element disposed on the ground, 
     in which at least a portion of the first antenna element and at least a portion of the second antenna element are capacitively coupled. 
     Advantageous Effects of Invention 
     According to the above-described aspect, capacitive coupling between the first antenna element and the second antenna element contributes to a low VSWR and high radiation efficiency in a low frequency band. Accordingly, it is possible to enable an antenna to be used stably with high radiation efficiency over a broadband. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a perspective view of an antenna device for vehicle according to Embodiment 1. 
         FIG.  2    is a perspective view of an antenna device for vehicle according to a comparative embodiment. 
         FIG.  3    is a graph showing VSWR characteristics of the antenna device for vehicle according to Embodiment 1 and the antenna device for vehicle according to the comparative embodiment. 
         FIG.  4    is a graph showing radiation efficiency characteristics of the antenna device for vehicle according to Embodiment 1 and the antenna device for vehicle according to the comparative embodiment. 
         FIG.  5    is a diagram showing a modification example of FIG.  1 . 
         FIG.  6    is a graph showing VSWR characteristics of the antenna device for vehicle according to the modification example and the antenna device for vehicle according to Embodiment 1. 
         FIG.  7    is a graph showing radiation efficiency characteristics of the antenna device for vehicle according to the modification example and the antenna device for vehicle according to Embodiment 1. 
         FIG.  8    is a perspective view of an antenna device for vehicle according to Embodiment 2. 
         FIG.  9    is a graph showing VSWR characteristics of the antenna device for vehicle according to Embodiment 2 and the antenna device for vehicle according to Embodiment 1. 
         FIG.  10    is a graph showing radiation efficiency characteristics of the antenna device for vehicle according to Embodiment 2 and the antenna device for vehicle according to Embodiment 1. 
         FIG.  11    is a perspective view of a first example of the whole of the antenna device for vehicle according to Embodiment 2. 
         FIG.  12    is a left side view of the first example of the whole of the antenna device for vehicle shown in  FIG.  11   . 
         FIG.  13    is a diagram with an antenna case removed from  FIG.  11   . 
         FIG.  14    is a perspective view of a second example with the antenna case removed from the whole of the antenna device for vehicle according to Embodiment 2. 
         FIG.  15    is a left side view of the second example of the antenna device for vehicle shown in  FIG.  14   . 
         FIG.  16    is a sectional view illustrating an example of mechanical joining of a first block and a second block shown in  FIG.  14   . 
         FIG.  17    is a perspective view of a third example with the antenna case removed from the whole of the antenna device for vehicle according to Embodiment 2. 
         FIG.  18    is a left side view of the third example of the antenna device for vehicle shown in  FIG.  17   . 
         FIG.  19    is a perspective view of an antenna device for vehicle according to Embodiment 3. 
         FIG.  20    is a graph showing VSWR characteristics of the antenna device for vehicle according to Embodiment 3 and the antenna device for vehicle according to Embodiment 1. 
         FIG.  21    is a graph showing radiation efficiency characteristics of the antenna device for vehicle according to Embodiment 3 and the antenna device for vehicle according to Embodiment 1. 
         FIG.  22    is a perspective view of an antenna device for vehicle according to Embodiment 4. 
         FIG.  23    is a graph showing VSWR characteristics of the antenna device for vehicle according to Embodiment 4 and the antenna device for vehicle according to Embodiment 1. 
         FIG.  24    is a graph showing radiation efficiency characteristics of the antenna device for vehicle according to Embodiment 4 and the antenna device for vehicle according to Embodiment 1. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described referring to the drawings. In all the drawings, the same components are represented by the same reference numerals and description thereof will not be repeated. 
     In the specification, ordinal numbers, such as “first”, “second”, and “third”, are attached only for distinguishing configurations to which the same names are attached unless specifically limited, and do not mean particular features (for example, an order or a degree of importance) of the configurations. 
     Embodiment 1 
       FIG.  1    is a perspective view of an antenna device  10  for vehicle according to Embodiment 1. 
     In  FIG.  1   , a first direction X, a second direction Y, and a third direction Z indicate a front-rear direction, a right-left direction, and an up-down direction of an antenna device  10  for vehicle, respectively. In detail, a positive direction of the first direction X that is a direction of an arrow indicating the first direction X indicates a front direction of the antenna device  10  for vehicle. A negative direction of the first direction X that is an opposite direction of the arrow indicating the first direction X indicates a rear direction of the antenna device  10  for vehicle. A positive direction of the second direction Y that is a direction of an arrow indicating the second direction Y indicates a left direction of the antenna device  10  for vehicle. A negative direction of the second direction Y that is an opposite direction of the arrow indicating the second direction Y indicates a right direction of the antenna device  10  for vehicle. A positive direction of the third direction Z that is a direction of an arrow indicating the third direction Z indicates an up direction of the antenna device  10  for vehicle. A negative direction of the third direction Z that is an opposite direction of the arrow indicating the third direction Z indicates a down direction antenna device  10  for vehicle. 
     “Front”, “rear”, “right”, “left”, “up”, and “down” regarding the first direction X, the second direction Y, and the third direction Z of the present embodiment are decided by an automobile on which the antenna device  10  for vehicle is mounted. That is, the front direction is a forward movement direction of the automobile, and the rear direction is a backward movement direction of the automobile. The left direction is a left direction as viewed from the rear side toward the front side of the automobile, and the right direction is a right direction as viewed from the rear side toward the front side of the automobile. The up direction is an up direction of the automobile, and the down direction is a down direction of the automobile. Note that the first direction X, the second direction Y, and the third direction Z may be different from the front-rear direction, the right-left direction, and the up-down direction of the automobile, respectively. For example, the antenna device  10  for vehicle may be used such that the first direction X is directed in the right-left direction of the automobile, and the second direction Y is directed in the front-rear direction of the automobile. Alternatively, for example, the antenna device  10  for vehicle may be used such that the positive direction of the first direction X is directed in the rear direction of the automobile, and the negative direction of the first direction X is directed in the front direction of the automobile. 
     Hereinafter, the first direction X, the second direction Y, and the third direction Z are referred to as a front-rear direction, a right-left direction, and an up-down direction of the antenna device  10  for vehicle or members that configure the antenna device  10  for vehicle, such as a first antenna element  100  and a second antenna element  200 , as necessary, respectively. The positive direction of the first direction X, the negative direction of the first direction X, the positive direction of the second direction Y, the negative direction of the second direction Y, the positive direction of the third direction Z, and the negative direction of the third direction Z are referred to as a front direction, a rear direction, a left direction, a right direction, an up direction, and a down direction of the antenna device  10  for vehicle or the members that configure the antenna device  10  for vehicle, such as the first antenna element  100  and the second antenna element  200 , as necessary, respectively. 
     The antenna device  10  for vehicle includes the first antenna element  100  and the second antenna element  200 . The first antenna element  100  and the second antenna element  200  are disposed on a ground  20 . The ground  20  is, for example, a roof of the automobile. 
     The first antenna element  100  is formed by bending sheet metal. Note that a method of forming the first antenna element  100  is not limited thereto. 
     The first antenna element  100  has a first end  102  and a second end  104 . 
     The first end  102  of the first antenna element  100  is a proximal end of the first antenna element  100 . The first end  102  has a feeding portion  102   a . The feeding portion  102   a  is capable of being fed through a connection member passing through a through-hole formed in the ground  20 . The second end  104  of the first antenna element  100  is a distal end of the first antenna element  100 . The second end  104  of the first antenna element  100  is an open end at a position away from the ground  20  with respect to the first end  102  of the first antenna element  100 . In the present embodiment, the first end  102  is positioned on a positive direction side of the first direction X with respect to the second end  104 , and the second end  104  is positioned on a negative direction side of the first direction X with respect to the first end  102 . The first end  102  is positioned on a negative direction side of the second direction Y with respect to the second end  104 , and the second end  104  is positioned on a positive direction side of the second direction Y with respect to the first end  102 . The first end  102  is positioned on a negative direction side of the third direction Z with respect to the second end  104 , and the second end  104  is positioned on a positive direction side of the third direction Z with respect to the first end  102 . 
     The first antenna element  100  has a substantially L shape as viewed from the positive direction or the negative direction of the second direction Y. Specifically, the first antenna element  100  has a first portion  112 , a second portion  114 , and a first step portion  116 . The first portion  112 , the first step portion  116 , and the second portion  114  are arranged in this order from the first end  102  to the second end  104 . The first portion  112  is a portion of the first antenna element  100  from the first end  102  to the first step portion  116 . Specifically, the first portion  112  includes a portion that extends from the first end  102  toward the positive direction side of the second direction Y, and a portion that extends from a front side of the first antenna element  100  toward a rear side of the first antenna element  100 , that is, the first step portion  116 . The first portion  112  is bent between the portion of the first portion  112  that extends from the first end  102  toward the positive direction side of the second direction Y and the portion of the first portion  112  that extends from the front side of the first antenna element  100  toward the rear side of the first antenna element  100 . Accordingly, an end of the first portion  112  on the first step portion  116  side is positioned on the positive direction side of the second direction Y with respect to an end of the first portion  112  on the first end  102  side. The first step portion  116  extends from the first portion  112  to the second portion  114  from the positive direction side of the second direction Y toward the negative direction side of the second direction Y. The second portion  114  extends from the first step portion  116  to the second end  104  toward the positive direction side of the third direction Z. In a case where the first step portion  116  is provided, it is possible to increase a total length between the first end  102  and the second end  104  of the first antenna element  100  compared to a case where the first step portion  116  is not provided and the first portion  112  and the second portion  114  are directly connected. 
     A width of the first antenna element  100  increases stepwise or gradually from the first end  102  toward the second end  104 . Accordingly, the width of the first antenna element  100  in the vicinity of the second end  104  is wider than the width of the first antenna element  100  in the vicinity of the first end  102 , that is, in the vicinity of the feeding portion  102   a . In this manner, the first antenna element  100  has a self-similar shape formed in a bent shape. 
     Examples of an antenna having a self-similar shape include an antenna that has a similar shape even though a scale (size ratio) changes, such as a biconical antenna or a bow-tie antenna. As a premise of the antenna having the self-similar shape, the electrical characteristics of the antenna show the same characteristics in principle even though an antenna size or a frequency changes. In actual design, for adjustment of impedance, or the like, an isosceles triangle shape of a radiating element, such as a biconical antenna or a bow-tie antenna, can be deformed and can be changed to a shape, such as the first antenna element  100  in the present embodiment. Even in such a case, it is possible to utilize a certain electrical characteristics that are obtained by the self-similar shape. In the present embodiment, the first antenna element  100  as a part of one radiating element having a self-similar shape is disposed to face the ground  20 , whereby the substantially same operational effects as a tapered slot antenna or a bow-tie antenna are obtained in a pseudo manner, and such an operational effect as if another radiating element is virtually disposed on an opposite side to face the radiating element is obtained due to the ground  20 . 
     The second antenna element  200  is formed of sheet metal. 
     The second antenna element  200  has a third end  202  and a fourth end  204 . 
     The third end  202  of the second antenna element  200  is a proximal end of the second antenna element  200 . The third end  202  has a short-circuit portion  202   a . The short-circuit portion  202   a  is short-circuited to the ground  20 . In the present embodiment, the third end  202  and the short-circuit portion  202   a  are positioned behind the first end  102  and the feeding portion  102   a . The third end  202  and the short-circuit portion  202   a  however may be positioned ahead of the first end  102  and the feeding portion  102   a . That is, the first end  102  and the feeding portion  102   a , and the third end  202  and the short-circuit portion  202   a  may be spaced apart from each other. The fourth end  204  of the second antenna element  200  is a distal end of the second antenna element  200 . The fourth end  204  of the second antenna element  200  is an open end at a position away from the ground  20  with respect to the third end  202  of the second antenna element  200 . In the present embodiment, the third end  202  is positioned on the positive direction side of the first direction X with respect to the fourth end  204 , and the fourth end  204  is positioned on the negative direction side of the first direction X with respect to the third end  202 . The third end  202  and the fourth end  204  are aligned in the second direction Y without deviating from each other. The third end  202  is positioned on the negative direction side of the third direction Z with respect to the fourth end  204 , and the fourth end  204  is positioned on the positive direction side of the third direction Z with respect to the third end  202 . 
     In a case where the third end  202  and the ground  20  are short-circuited by the short-circuit portion  202   a , it is possible to secure satisfactory characteristics in a low frequency band compared to a case where the third end  202  is electrically opened. The third end  202  however may not have the short-circuit portion  202   a . For example, the third end  202  may be electrically opened with respect to the ground  20 . 
     The second antenna element  200  has a substantially L shape as viewed from the positive direction or the negative direction of the second direction Y. Specifically, the second antenna element  200  has a third portion  212  and a fourth portion  214 . The third portion  212  extends from the third end  202  toward the positive direction side of the third direction Z. The fourth portion  214  extends from an end of the third portion  212  opposite to the third end  202  to the fourth end  204  toward the negative direction side of the first direction X. 
     A width of the second antenna element  200  increases stepwise or gradually from the third end  202  to the fourth end  204 . Accordingly, the width of the second antenna element  200  in the vicinity of the fourth end  204  is wider than the width of the second antenna element  200  in the vicinity of the third end  202 , that is, in the vicinity of the short-circuit portion  202   a . In this manner, the second antenna element  200  has a self-similar shape formed in a bent shape. 
     In the present embodiment, at least a portion of the first antenna element  100  and at least a portion of the second antenna element  200  are capacitively coupled. Specifically, at least a portion of the second end  104  of the first antenna element  100  and at least a portion of the fourth end  204  of the second antenna element  200  overlap in the second direction Y and are capacitively coupled. The second end  104  of the first antenna element  100  and the fourth end  204  of the second antenna element  200  are spaced apart from each other. 
     The second end  104  of the first antenna element  100  is inclined obliquely with respect to the ground  20 , that is, a plane parallel to an XY plane. More specifically, the second end  104  is inclined obliquely toward the positive direction side of the third direction Z from the positive direction side of the first direction X toward the negative direction side of the first direction X. An area where the second end  104  of the first antenna element  100  and the fourth end  204  of the second antenna element  200  are superimposed in the second direction Y is adjusted by the inclination of the second end  104 , that is, the shape of the second end  104 , whereby it is possible to adjust a capacitive component between the second end  104  of the first antenna element  100  and the fourth end  204  of the second antenna element  200 . An area where the second end  104  of the first antenna element  100  and the fourth end  204  of the second antenna element  200  are superimposed in the second direction Y is adjusted by the shape of the fourth end  204 , whereby it is possible to adjust a capacitive component between the second end  104  of the first antenna element  100  and the fourth end  204  of the second antenna element  200 . The capacitive component between the second end  104  of the first antenna element  100  and the fourth end  204  of the second antenna element  200  can also be adjusted by a distance between the second end  104  and the fourth end  204 . 
     The first antenna element  100  and the second antenna element  200  do not overlap in the second direction Y except for a portion in the vicinity of the first end  102 , a portion in the vicinity of the third end  202 , a portion in the vicinity of the second end  104 , and a portion in the vicinity of the fourth end  204 . 
     It is preferable that a length between the third end  202  and the fourth end  204  of the second antenna element  200  is substantially equal to a length between the first end  102  and the second end  104  of the first antenna element  100 . For example, a difference between the length between the first end  102  and the second end  104  of the first antenna element  100  and the length between the third end  202  and the fourth end  204  of the second antenna element  200  may be within ±25%. of the length between the first end  102  and the second end  104  of the first antenna element  100  or the length between the third end  202  and the fourth end  204  of the second antenna element  200 . In this manner, it is possible to enable the antenna to be used stably with high radiation efficiency over a broadband. Here, the length between the third end  202  and the fourth end  204  of the second antenna element  200  may be a length of an outer edge between the third end  202  and the fourth end  204  in the second antenna element  200  or a length of an inner edge between the third end  202  and the fourth end  204  in the second antenna element  200 . In the same manner, the length between the first end  102  and the second end  104  of the first antenna element  100  may be a length of an outer edge between the first end  102  and the second end  104  in the first antenna element  100  or a length of an inner edge between the first end  102  and the second end  104  in the first antenna element  100 . Alternatively, a length of a center line of the width of each of the first antenna element  100  and the second antenna element  200  may be used. 
     It is assumed that the antenna device  10  for vehicle according to the present embodiment operates in accordance with the following principle. 
     From a high frequency band to a medium frequency band of the operation frequency band of the antenna device  10  for vehicle, the first portion  112  of the first antenna element  100  functions as a self-similar shape or an equivalent tapered antenna, and the second portion  114  of the first antenna element  100  functions as a monopole antenna with the first portion  112  functioning as a transmission path. 
     From the medium frequency band to a low frequency band of the operation frequency band of the antenna device  10  for vehicle, the second portion  114  of the first antenna element  100  functions as a monopole antenna with the first portion  112  functioning as a transmission path, and the second portion  114  of the first antenna element  100  and the second antenna element  200  function as a loop antenna or a split ring antenna with the first portion  112  functioning as a transmission path. 
     With the above, the first antenna element  100  and the second antenna element  200  are operable over a broadband, and specifically, over at least 698 MHz to 6 GHz. Although the first antenna element  100  and the second antenna element are designed to operate over 698 MHz to 6 GHz in the present embodiment, it can be assumed from  FIGS.  3 ,  4   , and the like that the first antenna element  100  and the second antenna element  200  are operable over other frequency bands, such as 617 MHz to 5 GHz or 5.9 GHz to 7.1 GHz, in addition to 698 MHz to 6 GHz or instead of 698 MHz to 6 GHz. Accordingly, design can withstand a requirement for a broader frequency band or a higher frequency band. 
       FIG.  2    is a perspective view of an antenna device  10  for vehicle according to a comparative embodiment. The antenna device  10  for vehicle according to the comparative embodiment is the same as the antenna device  10  for vehicle according to Embodiment 1, except that the second antenna element  200  is not provided. 
       FIG.  3    is a graph showing VSWR characteristics of the antenna device  10  for vehicle according to Embodiment 1 and the antenna device  10  for vehicle according to the comparative embodiment.  FIG.  4    is a graph showing radiation efficiency characteristics of the antenna device  10  for vehicle according to Embodiment 1 and the antenna device  10  for vehicle according to the comparative embodiment. 
     A horizontal axis of the graph of  FIG.  3    indicates a frequency. A vertical axis of the graph of  FIG.  3    indicates a VSWR. A solid line in the graph of  FIG.  3    indicates the VSWR characteristic of the antenna device  10  for vehicle according to Embodiment 1. A broken line in the graph of  FIG.  3    indicates the VSWR characteristic of the antenna device  10  for vehicle according to the comparative embodiment. 
     A horizontal axis of the graph of  FIG.  4    indicates a frequency. A vertical axis of the graph of  FIG.  4    indicates radiation efficiency. A solid line in the graph of  FIG.  4    indicates the radiation efficiency characteristic of the antenna device  10  for vehicle according to Embodiment 1. A broken line in the graph of  FIG.  4    indicates the radiation efficiency characteristic of the antenna device  10  for vehicle according to the comparative embodiment. 
     As shown in  FIG.  3   , in a comparatively low frequency band of about 700 MHz to 1750 MHz, the VSWR of Embodiment 1 is lower than the VSWR of the comparative embodiment. As shown in  FIG.  4   , in a comparatively low frequency band of about 700 MHz to 1750 MHz, the radiation efficiency of Embodiment 1 is higher than the radiation efficiency of the comparative embodiment. Accordingly, capacitive coupling between the first antenna element  100  and the second antenna element  200  could contribute to a low VSWR and high radiation efficiency in a comparatively low frequency band. 
     As shown in  FIG.  3   , the VSWR of Embodiment 1 is as low as less than 3.5 over a broadband of 700 MHz to 6500 MHz. As shown in  FIG.  4   , the radiation efficiency of Embodiment 1 is as high as greater than 60% over a broadband of 700 MHz to 6500 MHz. In contrast, in the antenna of Patent Document 1, a return loss is equal to or greater than −5 dB in a frequency band of about 960 MHz to 1450 MHz. In the antenna of Patent Document 2, the VSWR is equal to or greater than 5 in a frequency band of about 1000 MHz to 1500 MHz. Accordingly, the antenna device  10  for vehicle according to Embodiment 1 can be used stably with high radiation efficiency over a broadband compared to the antennas of Patent Documents 1 and 2. 
       FIG.  5    is a diagram showing a modification example of  FIG.  1   . An antenna device  10  for vehicle according to the modification example is the same as the antenna device  10  for vehicle according to Embodiment 1, except for the following point. 
     The antenna device  10  for vehicle further includes a dielectric  150 . As described below in detail, the antenna device  10  for vehicle has the dielectric  150  in at least a portion of the first antenna element  100  or the second antenna element  200 . 
     The dielectric  150  is attached to at least a portion of the first antenna element  100 . Specifically, the dielectric  150  is attached to an inside surface of the first portion  112  of the first antenna element  100 . The dielectric  150  may be attached to at least one of the inside surface of the first portion  112  and an outside surface of the first portion  112 . The dielectric  150  may be attached to at least a portion of the second antenna element  200 . For example, the dielectric  150  may be attached to at least one of a surface of the second antenna element  200  on the positive direction side of the second direction Y and a surface of the second antenna element  200  on the negative direction side of the second direction Y. At least a portion of the dielectric  150  may be provided over at least a portion of the first antenna element  100  and at least a portion of the second antenna element  200 . 
       FIG.  6    is a graph showing VSWR characteristics of the antenna device  10  for vehicle according to the modification example and the antenna device  10  for vehicle according to Embodiment 1.  FIG.  7    is a graph showing radiation efficiency characteristics of the antenna device  10  for vehicle according to the modification example and the antenna device  10  for vehicle according to Embodiment 1. 
     A horizontal axis of the graph of  FIG.  6    indicates a frequency. A vertical axis of the graph of  FIG.  6    indicates a VSWR. A solid line in the graph of  FIG.  6    indicates the VSWR characteristic of the antenna device  10  for vehicle according to the modification example. A broken line in the graph of  FIG.  6    indicates the VSWR characteristic of the antenna device  10  for vehicle according to Embodiment 1. 
     A horizontal axis of the graph of  FIG.  7    indicates a frequency. A vertical axis of the graph of  FIG.  7    indicates radiation efficiency. A solid line in the graph of  FIG.  7    indicates the radiation efficiency characteristic of the antenna device  10  for vehicle according to the modification example. A broken line in the graph of  FIG.  7    indicates the radiation efficiency characteristic of the antenna device  10  for vehicle according to Embodiment 1. 
     As shown in  FIG.  6   , the VSWR of each of the modification example and Embodiment 1 is as low as less than 3.5 over a broadband of 700 MHz to 6500 MHz. As shown in  FIG.  7   , the radiation efficiency of each of the modification example and Embodiment 1 is as high as greater than 60% over a broadband of 700 MHz to 6500 MHz. 
     As shown in  FIG.  6   , the VSWR of the modification example is less than 3 at any of a frequency band equal to or higher than 1000 MHz. In contrast, the VSWR of Embodiment 1 is greater than 3 in the vicinity of 1250 MHz of the frequency band equal to or higher than 1000 MHz. Accordingly, the dielectric  150  of the modification example could contribute to smoothing of the VS % R characteristic. 
     As shown in  FIG.  7   , the radiation efficiency of the modification example is greater than 75, at any of the frequency band equal to or higher than 1000 MHz. In contrast, the radiation efficiency of Embodiment 1 is less than 75% in the vicinity of 1250 MHz of the frequency band equal to or higher than 1000 MHz. Accordingly, the dielectric  150  of the modification example could contribute to smoothing of the radiation efficiency characteristic. 
     Embodiment 2 
       FIG.  8    is a perspective view of an antenna device  10  for vehicle according to Embodiment 2. The antenna device  10  for vehicle according to Embodiment 2 is the same as the antenna device  10  for vehicle according to Embodiment 1, except for the following points. 
     A first antenna element  100  has a fifth end  106  in addition to a first end  102  and a second end  104 . The second end  104  and the fifth end  106  are on opposite sides of the first end  102 . The first end  102 , that is, a feeding portion  102   a  is positioned at the substantially center of the first antenna element  100 . 
     The fifth end  106  of the first antenna element  100  is a distal end of the first antenna element  100 . The fifth end  106  of the first antenna element  100  is an open end at a position away from the ground  20  with respect to the first end  102  of the first antenna element  100 . In the present embodiment, the first end  102  is positioned on the positive direction side of the first direction X with respect to the fifth end  106 , and the fifth end  106  is positioned on the negative direction side of the first direction X with respect to the first end  102 . The first end  102  is positioned on the positive direction side of the second direction Y with respect to the fifth end  106 , and the fifth end  106  is positioned on the negative direction side of the second direction Y with respect to the first end  102 . The first end  102  is positioned on the negative direction side of the third direction Z with respect to the fifth end  106 , and the fifth end  106  is positioned on the positive direction side of the third direction Z with respect to the first end  102 . 
     The first antenna element  100  has a substantially U shape in an expanded state. Specifically, the first antenna element  100  has a fifth portion  122 , a sixth portion  124 , and a second step portion  126  in addition to a first portion  112 , a second portion  114 , and a first step portion  116 . The fifth portion  122 , the sixth portion  124 , and the second step portion  126  have shapes substantially symmetrical to the first portion  112 , the second portion  114 , and the first step portion  116  about the first end  102 . The fifth portion  122 , the second step portion  126 , and the sixth portion  124  are arranged in this order from the first end  102  to the fifth end  106 . The fifth portion  122  extends from the first end  102  toward the negative direction side of the first direction X. The fifth portion  122  is bent between the first end  102  and the second step portion  126 . Accordingly, an end of the fifth portion  122  on a second step portion  126  side is positioned on the negative direction side of the second direction Y with respect to an end of the fifth portion  122  on a first end  102  side. The second step portion  126  extends from the fifth portion  122  to the sixth portion  124  from the negative direction side of the second direction Y toward the positive direction side of the second direction Y. The sixth portion  124  extends from the second step portion  126  to the second end  104  toward the positive direction side of the third direction Z. In a case where the second step portion  126  is provided, it is possible to increase a total length between the first end  102  and the fifth end  106  of the first antenna element  100  compared to a case where the second step portion  126  is not provided and the fifth portion  122  and the sixth portion  124  are directly connected. 
     In the same manner as the width of the first antenna element  100  from the first end  102  to the second end  104  described in Embodiment 1, a width of the first antenna element  100  increases stepwise or gradually from the first end  102  to the fifth end  106 . Accordingly, the width of the first antenna element  100  in the vicinity of the fifth end  106  is wider than the width of the first antenna element  100  in the vicinity of the first end  102 , that is, in the vicinity of the feeding portion  102   a . Here, “increase stepwise” means, for example, increase with a step, such as a step shape, and “increases gradually” means, for example, increases smoothly and steadily with no step. 
     The first antenna element  100  has a first region  110  including the first portion  112 , the second portion  114 , and the first step portion  116  positioned on one side of the second antenna element  200 , that is, on the positive direction side of the second direction Y of the second antenna element  200 . The first antenna element  100  has a second region  120  including the fifth portion  122 , the sixth portion  124 , and the second step portion  126  positioned on the other side opposite to the one side of the second antenna element  200 , that is, on the negative direction side of the second direction Y of the second antenna element  200 . 
     At least a portion of the first region  110  of the first antenna element  100  and at least a portion of the second antenna element  200  are capacitively coupled. At least a portion of the second region  120  of the first antenna element  100  and at least a portion of the second antenna element.  200  are capacitively coupled. Specifically, in the same manner as in Embodiment 1, at least a portion of the second end  104  of the first antenna element  100  and at least a portion of the fourth end  204  of the second antenna element  200  overlap in the second direction Y and are capacitively coupled. The second end  104  of the first antenna element  100  and the fourth end  204  of the second antenna element  200  are spaced apart from each other. At least a portion of the fifth end  106  of the first antenna element  100  and at least a portion of the fourth end  204  of the second antenna element  200  overlap in the second direction Y and are capacitively coupled. The fifth end  106  of the first antenna element  100  and the fourth end  204  of the second antenna element  200  are spaced apart from each other. 
     In the same manner as the second end  104  of the first antenna element  100  described in Embodiment 1, the fifth end  106  of the first antenna element  100  is inclined obliquely with respect to the ground  20 , that is, a plane parallel to an XY plane. More specifically, the fifth end  106  is inclined obliquely toward the positive direction side of the third direction Z from the positive direction side of the first direction X toward the negative direction side of the first direction X. 
     The first region  110  of the first antenna element  100  and the second antenna element  200  do not overlap in the second direction Y, except for portions in the periphery of the first end  102  and the third end  202  and portions in the periphery of the second end  104  and the fourth end  204 . The second region  120  of the first antenna element  100  and the second antenna element  200  do not overlap in the second direction Y, except for portions in the periphery of the first end  102  and the third end  202  and portions in the periphery of the fifth end  106  and the fourth end  204 . 
       FIG.  9    is a graph showing VSWR characteristics of the antenna device  10  for vehicle according to Embodiment 2 and the antenna device  10  for vehicle according to Embodiment 1. FIG. is a graph showing radiation efficiency characteristics of the antenna device  10  for vehicle according to Embodiment 2 and the antenna device  10  for vehicle according to Embodiment 1. 
     A horizontal axis of the graph of  FIG.  9    indicates a frequency. A vertical axis of the graph of  FIG.  9    indicates a VSWR. A solid line in the graph of  FIG.  9    indicates the VSWR characteristic of the antenna device  10  for vehicle according to Embodiment 2. A broken line in the graph of  FIG.  9    indicates the VSWR characteristic of the antenna device  10  for vehicle according to Embodiment 1. 
     A horizontal axis of the graph of  FIG.  10    indicates a frequency. A vertical axis of the graph of  FIG.  10    indicates radiation efficiency. A solid line in the graph of  FIG.  10    indicates the radiation efficiency characteristic of the antenna device  10  for vehicle according to Embodiment 2. A broken line in the graph of  FIG.  10    indicates the radiation efficiency characteristic of the antenna device  10  for vehicle according to Embodiment 1. 
     As shown in  FIG.  9   , the VSWR of Embodiment 2 is less than 2.5 at any of the frequency band equal to or higher than 1000 MHz. In contrast, the VSWR of Embodiment 1 is greater than 2.5 in the vicinity of 1250 MHz of the frequency band equal to or higher than 1000 MHz. Accordingly, the second region  120  of the first antenna element  100  of Embodiment 2 could contribute to smoothing of the VSWR characteristic. 
     As shown in  FIG.  10   , the radiation efficiency of Embodiment 2 is greater than 85% at any of the frequency band equal to or higher than 1000 MHz. In contrast, the radiation efficiency of Embodiment 1 is less than 85% in the vicinity of 1250 MHz of the frequency band equal to or higher than 1000 MHz. Accordingly, the second region  120  of the first antenna element  100  of Embodiment 2 could contribute to smoothing of the radiation efficiency characteristic. 
       FIG.  11    is a perspective view of a first example of the whole of the antenna device  10  for vehicle according to Embodiment 2.  FIG.  12    is a left side view of the first example of the whole of the antenna device  10  for vehicle shown in  FIG.  11   .  FIG.  13    is a diagram with an antenna case  530  removed from  FIG.  11   . In  FIGS.  11  and  12   , a left portion of the antenna case  530  is removed. 
     The antenna device  10  for vehicle includes an antenna base  510 , a substrate  520 , and an antenna case  530 . The antenna base  510  is, for example, a conductive base, such as a metal base. Alternatively, the antenna base  510  may have both a conductive base and an insulating base. The antenna base  510  may have a conductive base, an insulating base, and a metallic plate or may have a metallic plate and an insulating base. The substrate  520  is, for example, a printed circuit board (PCB). The substrate  520  is provided on an upper surface side of the antenna base  510 . On an upper surface side of the substrate  520 , two antennas, that is, a rear antenna having a first antenna element  100 A and a second antenna element  200 A, and a front antenna having a first antenna element  100 B and a second antenna element  200 B are provided. The antenna base  510  and the antenna case  530  form an accommodation space for accommodating the substrate  520  and the two antennas. 
     The rear first antenna element  100 A has a first end  102 A, a second end  104 A, and a fifth end  106 A in the same manner as the first antenna element  100  shown in  FIG.  8   . The rear second antenna element  200 A has a third end  202 A and a fourth end  204 A in the same manner as the second antenna element  200  shown in  FIG.  8   . 
     The front first antenna element  100 B has a first end  102 B, a second end  104 B, and a fifth end  106 B in the same manner as the first antenna element  100  shown in  FIG.  8   . The front second antenna element  200 B has a third end  202 B and a fourth end  204 B in the same manner as the second antenna element  200  shown in  FIG.  8   . 
     In the present embodiment, the antenna having the first antenna element  100 A and the second antenna element  200 A, and the antenna having the first antenna element  100 B and the second antenna element  200 B are arranged in the front-rear direction of the antenna device  10  for vehicle. An electric field is strong in capacitively coupled portions of the two antennas, that is, in capacitively coupled portions of the second end  104 A and the fourth end  204 A, and the fifth end  106 A and the fourth end  204 A of the rear antenna, and capacitively coupled portions of the second end  104 B and the fourth end  204 B, and the fifth end  106 B and the fourth end  204 B of the front antenna. Accordingly, in a case where the capacitively coupled portions of the second end  104 A and the fourth end  204 A, and the fifth end  106 A and the fourth end  204 A of the rear antenna, and the capacitively coupled portions of the second end  104 B and the fourth end  204 B, and the fifth end  106 B and the fourth end  204 B of the front antenna face and are disposed close to each other, the capacitively coupled portions may be coupled and the antennas may not operate as antennas (the rear antenna and the front antenna) independent of each other. For this reason, it is desirable that the capacitively coupled portions of the two antennas are separated from each other. In the present embodiment, it is possible to increase a distance between the capacitively coupled portions of the respective antennas compared to a case where the two antennas are arranged in the right-left direction of the antenna device  10  for vehicle. The above-described two antennas may be arranged in a direction different from the front-rear direction of the antenna device  10  for vehicle, such as the right-left direction of the antenna device  10  for vehicle. That is, any disposition may be applied as long as the disposition is made in which the distance between the capacitively coupled portions of the respective antennas is large. For example, the respective antennas may be disposed such that the capacitively coupled portions of the second end  104 A and the fourth end  204 A, and the fifth end  106 A and the fourth end  204 A of the rear antenna are toward the rear, and the capacitively coupled portions of the second end  104 B and the fourth end  204 B, and the fifth end  106 B and the fourth end  204 B of the front antenna are toward the front. 
     A height of the antenna case  530  is low in a region where the front antenna having the first antenna element  100 B and the second antenna element  200 B are disposed. It is preferable that a height of each of the first antenna element  100 B and the second antenna element  200 B is high. In the present embodiment, the second end  1048  and the fifth end  1068  are positioned behind the first end  102 B, and the fourth end  204 B is positioned behind the third end  202 B. In this case, it is possible to increase the height of the first antenna element  100 B and the second antenna element  200 B compared to a case where the second end  104 B and the fifth end  106 B are positioned ahead of the first end  102 B, and the fourth end  204 B is positioned ahead of the third end  202 B. The second end  104 B and the fifth end  106 B however may be positioned ahead of the first end  1021   i , and the fourth end  204 B may be positioned ahead of the third end  202 B. 
       FIG.  14    is a perspective view of a second example with the antenna case removed from the whole of the antenna device  10  for vehicle according to Embodiment 2.  FIG.  15    is a left side view of the second example of the antenna device  10  for vehicle shown in  FIG.  14   .  FIG.  16    is a sectional view illustrating an example of mechanical joining of a first block  310 A and a second block  320 A shown in  FIG.  14   . The antenna device  10  for vehicle according to the second example shown in  FIGS.  14  to  16    is the same as the antenna device  10  for vehicle according to the first example shown in  FIGS.  11  to  13   , except for the following points. 
     A corner of at least a portion of the first antenna element  100 A is rounded. Specifically, as shown in  FIGS.  14  and  15   , a corner between an upper end edge and a rear end edge of the second end  104 A, that is, a corner of a portion of the first antenna element  100 A capacitively coupled to at least a portion of the second antenna element  200 A is rounded. In this case, it is possible to suppress the occurrence of a defect, such as damage to a worker in assembling the antenna device  10  for vehicle due to the corner or damage to other members due to the corner compared to a case where the corner is sharp. A corner different from the corner between the upper end edge and the rear end edge of the second end  104 A, such as a corner between an upper end edge and a front end edge of the second end  104 A may also be rounded. 
     As shown in  FIG.  14   , in the same manner as the corner between the upper end edge and the rear end edge of the second end  104 A, a corner of at least a portion of the first antenna element  100 A, such as a corner between an upper end edge and a rear end edge of the fifth end  106 A is rounded. 
     As shown in  FIGS.  14  and  15   , in the same manner as the corner between the upper end edge and the rear end edge of the second end  104 A, a corner of at least a portion of the second antenna element  200 A, such as a corner between an upper end edge and a rear end edge of the fourth end  204 A is also rounded. 
     In the example shown in  FIGS.  14  and  15   , the front first antenna element  100 B and the front second antenna element  200 B also have the same configuration as the configuration of the rear first antenna element.  100 A and the second antenna element  200 A described above. 
     The antenna device  10  for vehicle includes a holder  300 A provided in the rear antenna having the first antenna element  100 A and the second antenna element  200 A. The holder  300 A is positioned between at least a portion of the first antenna element  100 A and at least a portion of the second antenna element  200 A. At least a portion in the first antenna element  100 A and the second antenna element  200 A is supported by the holder  300 A. In this case, it is possible to suppress an influence of vibration of an automobile on which the antenna device  10  for vehicle is mounted, on mechanical characteristics of at least one of the first antenna element  100 A and the second antenna element  200 A compared to a case where the holder  300 A is not provided. Fluctuation of a distance between the second end  104 A and the fourth end  204 A in the second direction Y or fluctuation of a superimposed area of the second end  104 A and the fourth end  204 A in the second direction Y due to vibration of the automobile in which the antenna device  10  for vehicle is mounted is suppressed, and it is possible to suppress fluctuation of capacitance between the second end  104 A and the fourth end  204 A, compared to a case where the holder  300 A is not provided. In the same manner, fluctuation of a distance between the fifth end  106 A and the fourth end  204 A in the second direction Y or fluctuation of a superimposed area of the fifth end  106 A and the fourth end  204 A in the second direction Y due to vibration of the automobile in which the antenna device  10  for vehicle is mounted is suppressed, and it is possible to suppress fluctuation of capacitance between the fifth end  106 A and the fourth end  204 A, compared to a case where the holder  300 A is not provided. 
     As shown in  FIG.  14   , the holder  300 A has the first block  310 A and the second block  320 A. The first block  310 A and the second block  320 A are, for example, resin blocks. The first block  310 A is positioned between the second antenna element  200 A and a first region  110 A of the first antenna element  100 A. The second block  320 A is positioned between the second antenna element  200 A and a second region  120 A of the first antenna element  100 A. 
     As shown in  FIG.  16   , a projection portion  330 A is provided on a surface of the first block  310 A on the negative direction side of the second direction Y. The projection portion  330 A passes through a hole provided in the second antenna element  200 A in the second direction Y. The projection portion  330 A is mechanically joined to, such as fitted into a recess portion provided in a surface of the second block  320 A on the positive direction side of the second direction Y. In this manner, the first block  310 A, the second block  320 A, and the second antenna element  200 A are integrated. In this case, it becomes easy to integrally assemble the first block  310 A, the second block  320 A, and the second antenna element  200 A compared to a case where the first block  310 A and the second block  320 A are mechanically spaced apart from each other without being mechanically joined through the projection portion  330 A. Fluctuation of the distance between the second end  104 A and the fourth end  204 A in the second direction Y or fluctuation of the superimposed area of the second end  104 A and the fourth end  204 A in the second direction Y due to vibration of the automobile in which the antenna device  10  for vehicle is mounted is suppressed, and it is possible to suppress fluctuation of capacitance between the second end  104 A and the fourth end  204 A, compared to a case where the first block  310 A and the second block  320 A are mechanically spaced apart from each other without being mechanically joined through the projection portion  330 A. In the same manner, fluctuation of the distance between the fifth end  106 A and the fourth end  204 A in the second direction Y or fluctuation of the superimposed area of the fifth end  106 A and the fourth end  204 A in the second direction Y due to vibration of the automobile in which the antenna device  10  for vehicle is mounted is suppressed, and it is possible to suppress fluctuation of capacitance between the fifth end  106 A and the fourth end  204 A, compared to a case where the first block  310 A and the second block  320 A are mechanically spaced apart from each other without being mechanically joined through the projection portion  330 A. The first block  310 A and the second block  320 A may not be mechanically joined through the projection portion  330 A and may be mechanically spaced from each other. 
     In the example shown in  FIG.  16   , the projection portion  330 A provided in the first block  310 A is mechanically joined to the recess portion provided in the second block  320 A. The projection portion  330 A however may be provided in the second block  320 A. In this case, the projection portion  330 A provided in the second block  320 A is mechanically joined to the recess portion provided in the first block  310 A. 
     As shown in  FIG.  14   , a surface of the first block  310 A on the positive direction side of the second direction Y is in a shape along unevenness of the first portion  112 A, the first step portion  116 A, and the second portion  114 A of the first region  110 A. In this case, it is possible to suppress vibration of the first region  110 A due to vibration at the time of traveling of the automobile on which the antenna device  10  for vehicle is mounted, compared to a case where the surface of the first block  310 A on the positive direction side of the second direction Y is not in a shape along unevenness of the first portion  112 A, the first step portion  116 A, and the second portion  114 A, for example when a gap is formed between the surface of the first block  310 A on the positive direction side of the second direction Y and the first portion  112 A, the first step portion  116 A, and the second portion  114 A. Fluctuation of the distance between the second end  104 A and the fourth end  204 A in the second direction Y or fluctuation of the superimposed area of the second end  104 A and the fourth end  204 A in the second direction Y due to vibration of the automobile on which the antenna device  10  for vehicle is mounted is suppressed, and it is possible to suppress fluctuation of capacitance between the second end  104 A and the fourth end  204 A, compared to a case where the surface of the first block  310 A on the positive direction side of the second direction Y is not in a shape along unevenness of the first portion  112 A, the first step portion  116 A, and the second portion  114 A. 
     In the same manner as the surface of the first block  310 A on the positive direction side of the second direction Y, a surface of the second block  320 A on the negative direction side of the second direction Y may also be in a shape along unevenness of the second region  120 A. 
     As shown in  FIGS.  14  and  15   , a first protrusion  342 A and a second protrusion  344 A are provided in an end portion on the positive direction side of the first direction X of at least one of the first block  310 A and the second block  320 A. The first protrusion  342 A is positioned on the positive direction side of the third direction Z with respect to the first end  102 A of the first antenna element  100 A. The second protrusion  344 A is positioned on the negative direction side of the third direction Z with respect to the first end  102 A of the first antenna element  100 A. The first end  102 A is pressed in the third direction Z by the first protrusion  342 A and the second protrusion  344 A. For this reason, it becomes easy to position and fix the first end  102 A in the third direction Z compared to a case where the first protrusion  342 A and the second protrusion  344 A are not provided. The first protrusion  342 A and the second protrusion  344 A may not be provided. 
     As shown in  FIGS.  14  and  15   , a third protrusion  352 A caught in a hole provided in the second portion  114 A is provided on the positive direction side of the second direction Y of the first block  310 A. The third protrusion  352 A is mechanically joined to the hole provided in the second portion  114 A by, for example, snap-fit. In a case where the third protrusion  352 A is provided, it becomes easy to position the first block  310 A and the second portion  114 A compared to a case where the third protrusion  352 A is not provided. A configuration in which the third protrusion  352 A of the holder  300 A is caught in the hole provided in the second portion  114 A of the first antenna element  100 A functions as fixing means of the first antenna element  100 A and the holder  300 A. The third protrusion  352 A may be caught in a hole provided in a portion of the first region  110 A different from the second portion  114 A instead of the hole provided in the second portion  114 A. A plurality of third protrusions  352 A may be provided on the positive direction side of the second direction Y of the first block  310 A. In this case, a plurality of third protrusions  352 A can be caught in a plurality of holes provided in at least a portion of the first portion  112 A, the first step portion  116 A, and the second portion  114 A. 
     In the same manner as the third protrusion  352 A provided in the first block  310 A, a protrusion caught in a hole provided in the second region  120 A may be provided also on the negative direction side of the second direction Y of the second block  320 A. 
     As shown in  FIGS.  14  and  15   , a support portion  362 A supporting the first region  110 A is provided on the negative direction side of the first direction X of the first block  310 A. The support portion  362 A includes a first support  362 Aa and a second support  362 Ab. 
     The first support  362 Aa is positioned on the negative direction side of the first direction X with respect to at least a portion of an end of the second portion  114 A on the negative direction side of the first direction X. For this reason, the first support  362 Aa can support the second portion  114 A from the negative direction side of the first direction X. In a case where the first support  362 Aa is provided, it becomes easy to position the first region  110 A in the first direction X compared to a case where the first support.  362 Aa is not provided. In a case where the first support  362 Aa is provided, it is possible to suppress an influence of vibration at the time of traveling of the automobile on which the antenna device  10  for vehicle is mounted, on the mechanical characteristics of the first region  110 A compared to a case where first support  362 Aa is not provided. In a case where the first support  362 Aa is provided, fluctuation of the superimposed area of the second end  104 A and the fourth end  204 A in the second direction Y due to vibration of the automobile on which the antenna device  10  for vehicle is mounted is suppressed, and it is possible to suppress fluctuation of capacitance between the second end  104 A and the fourth end  204 A, compared to a case where the first support  362 Aa is not provided. 
     The second support  362 Ab is positioned on the positive direction side of the second direction Y with respect to at least a portion of the surface of the second portion  114 A on the positive direction side of the second direction Y. For this reason, the second support  362 Ah can support the second portion  114 A from the positive direction side of the second direction Y. In a case where the second support  362 Ab is provided, it is possible to suppress deflection of the first region  110 A in the second direction Y compared to a case where the second support  362 Ab is not provided. In a case where the second support  362 Ab is provided, it is possible to suppress an influence of vibration at the time of traveling of the automobile on which the antenna device  10  for vehicle is mounted, on the mechanical characteristics of the first region  110 A compared to a case where the second support  362 Ab is not provided. In a case where the second support  362 Ab is provided, fluctuation of the distance between the second end  104 A and the fourth end  204 A in the second direction Y due to vibration of the automobile on which the antenna device  10  for vehicle is mounted is suppressed, and it is possible to suppress fluctuation of capacitance between the second end  104 A and the fourth end  204 A, compared to a case where the second support  362 Ab is not provided. 
     In the same manner as the support portion  362 A provided in the first block  310 A, a support portion supporting the second region  120 A may be provided also on the negative direction side of the first direction X of the second block  320 A. 
     A structure of the holder  300 A is not limited to the example shown in  FIGS.  14  to  16   . For example, the holder  300 A may have only one of the first block  310 A and the second block  320 A. In a case where the first antenna element  100 A has only the first region  110 A, for example, like the first antenna element  100  shown in  FIGS.  1  and  2   , or  5  described above or  FIG.  19  or  22    described below, the holder  300 A may have only the first block  310 A positioned between the second antenna element  200 A and the first region  110 A. 
     The antenna device  10  for vehicle further includes a holder  300 B provided in the front antenna having the first antenna element  100 B and the second antenna element  200 B. In the example shown in  FIGS.  14  and  15   , the front holder  300 B has the same configuration as the configuration of the rear holder  300 A described above. A height of a first block  310 B and a second block  320 B of the front holder  300 B in the third direction Z is lower than the height of the first block  310 A and the second block  320 A of the rear holder  300 A in the third direction Z according to the shape of the antenna case (not shown). 
       FIG.  17    is a perspective view of a third example with the antenna case removed from the whole of the antenna device  10  for vehicle according to Embodiment 2.  FIG.  18    is a left side view of the third example of the antenna device  10  for vehicle shown in  FIG.  17   . The antenna device  10  for vehicle according to the third example shown in  FIGS.  17  and  18    is the same as the antenna device  10  for vehicle according to the second example shown in  FIGS.  14  to  16   , except for the following points. 
     The antenna device  10  for vehicle further includes a first antenna portion  410  and a second antenna portion  420 . The first antenna portion  410  and the second antenna portion  420  are positioned between the rear antenna having the first antenna element  100 A and the second antenna element  200 A and the front antenna having the first antenna element  100 E and the second antenna element  200 B in the first direction X. The first antenna portion  410  is positioned on the positive direction side of the second direction Y with respect to a virtual line passing through the rear second antenna element  200 A and the front second antenna element  200 B in parallel to the first direction X. The second antenna portion  420  is positioned on the negative direction side of the second direction Y with respect to a virtual line passing through the rear second antenna element  200 A and the front second antenna element  200 B in parallel to the first direction X. 
     The first antenna portion  410  and the second antenna portion  420  are, for example, LTE antennas, Wi-Fi (Registered Trademark) antennas, or Multiple-input and Multiple-Output (MIMO) antennas. The first antenna portion  410  and the second antenna portion  420  may be antennas of the same type or may be antennas of different types. 
     As described referring to  FIGS.  11  to  13   , the rear antenna having the first antenna element  100 A and the second antenna element  200 A and the front antenna having the first antenna element  100 B and the second antenna element  200 B are disposed with an appropriate space in the first direction X such that both antennas function as antennas independent of each other. The first antenna portion  410  and the second antenna portion  420  are disposed in the space. Accordingly, it is possible to efficiently utilize a space in the antenna case to dispose the first antenna portion  410  and the second antenna portion  420 , compared to a case where the first antenna portion  410  and the second antenna portion  420  are disposed in a region different from the space. 
     The disposition of the first antenna portion  410  and the second antenna portion  420  is not limited to the example shown in  FIGS.  17  and  18   . For example, the first antenna portion  410  and the second antenna portion  420  may be arranged in the first direction X. One of the first antenna portion  410  and the second antenna portion  420  may not be provided. At least one another antenna portion may be provided in addition to the first antenna portion  410  and the second antenna portion  420 . 
     Embodiment 3 
       FIG.  19    is a perspective view of an antenna device  10  for vehicle according to Embodiment 3. The antenna device  10  for vehicle according to Embodiment 3 is the same as the antenna device  10  for vehicle according to Embodiment 1, except for the following points. 
     The third portion  212  of the second antenna element  200  includes, from the third end  202  to the fourth portion  214 , not only a portion extending from the third end  202  to the fourth portion  214  toward the positive direction side of the third direction Z, but also a portion extending from the third end  202  to the fourth portion  214  in another direction. Specifically, the third portion  212  includes, from the third end  202  to the fourth portion  214 , a portion extending toward the positive direction side of the third direction Z, a portion extending toward the positive direction side of the first direction X, a portion extending toward the positive direction side of the second direction Y, and a portion extending toward the positive direction side of the third direction Z, in order. In this case, it is possible to maintain the total length between the third end  202  and the fourth end  204  of the second antenna element  200  while reducing the length of the second antenna element  200  in the first direction X compared to a case where the third portion  212  includes only a portion extending from the third end  202  to the fourth portion  214  toward the positive direction side of the third direction Z, for example, as shown in  FIG.  1   . 
     The antenna device  10  for vehicle further includes a first dielectric  150 A and a second dielectric  150 B. The first dielectric  150 A is attached to at least a portion of the first antenna element  100 . Specifically, the first dielectric  150 A is attached to an inside surface of the first portion  112  of the first antenna element  100 . The dielectric  150  may be attached to at least one of the inside surface of the first portion  112  and an outside surface of the first portion  112 . The second dielectric  150 B is provided over at least a portion of the first antenna element  100  and at least a portion of the second antenna element  200 . Specifically, the second dielectric  150 B is provided over at least a portion of the second end  104  of the first antenna element  100  and at least a portion of the fourth end  204  of the second antenna element  200 . 
     In the present embodiment, the second end  104  of the first antenna element  100  and the fourth end  204  of the second antenna element  200  do not overlap in the second direction Y. Even in this case, since the second end  104  of the first antenna element  100  and the fourth end  204  of the second antenna element  200  are close to each other, the second end  104  and the fourth end  204  are capacitively coupled to each other. The second end  104  of the first antenna element  100  and the fourth end  204  of the second antenna element  200  do not overlap in the second direction Y, whereby a capacitive component between the second end  104  of the first antenna element  100  and the fourth end  204  of the second antenna element  200  can be adjusted to be smaller than a capacitive component in a case where the second end  104  of the first antenna element  100  and the fourth end  204  of the second antenna element  200  overlap in the second direction Y. 
     In the present embodiment, for example, as shown in  FIGS.  11  to  13   , in a case where the antenna having the first antenna element  100 A and the second antenna element  200 A and the antenna having the first antenna element  100 B and the second antenna element  200 B are arranged in the front-rear direction of the antenna device  10  for vehicle, it is possible to reduce the length of each antenna in the front-rear direction, for example, compared to the antenna having the first antenna element  100  and the second antenna element  200  according to Embodiment 1. Accordingly, it is possible to further secure the isolation of the above-described two antennas, for example, compared to the antenna having the first antenna element  100  and the second antenna element  200  according to Embodiment 1. An antenna of another medium may be disposed between the above-described two antennas. 
       FIG.  20    is a graph showing VSWR characteristics of the antenna device  10  for vehicle according to Embodiment 3 and the antenna device  10  for vehicle according to Embodiment 1.  FIG.  21    is a graph showing radiation efficiency characteristics of the antenna device  10  for vehicle according to Embodiment 3 and the antenna device  10  for vehicle according to Embodiment 1. 
     A horizontal axis of the graph of  FIG.  20    indicates a frequency. A vertical axis of the graph of  FIG.  20    indicates a VSWR. A solid line in the graph of  FIG.  20    indicates the VSWR characteristic of the antenna device  10  for vehicle according to Embodiment 3. A broken line in the graph of  FIG.  20    indicates the VSWR characteristic of the antenna device  10  for vehicle according to Embodiment 1. 
     A horizontal axis of the graph of  FIG.  21    indicates a frequency. A vertical axis of the graph of  FIG.  21    indicates radiation efficiency. A solid line in the graph of  FIG.  21    indicates the radiation efficiency characteristic of the antenna device  10  for vehicle according to Embodiment 3. A broken line in the graph of  FIG.  21    indicates the radiation efficiency characteristic of the antenna device  10  for vehicle according LO Embodiment 1. 
     As shown in  FIG.  20   , the VSWR of each of Embodiment 3 and Embodiment 1 is as low as less than 3.5 over a broadband of 700 MHz to 1000 MHz and 1500 MHz to 6500 MHz. As shown in  FIG.  21   , the radiation efficiency of each of Embodiment 3 and Embodiment 1 is as high as greater than 60% over a broadband of 700 MHz to 6500 MHz, except for near 1250 MHz of Embodiment 3. 
     Embodiment 4 
       FIG.  22    is a perspective view of an antenna device  10  for vehicle according to Embodiment 4. The antenna device  10  for vehicle according to Embodiment 4 is the same as the antenna device  10  for vehicle according to Embodiment 1, except for the following point. 
     The antenna device  10  for vehicle includes a substrate  160 , such as a printed circuit board (PCB). A first antenna element  100  is a conductive pattern formed on a surface of the substrate  160  on the positive direction side of the second direction Y. A second antenna element  200  is a conductive pattern formed on a surface of the substrate  160  on the negative direction side of the second direction Y. In the present embodiment, a second end  104  of the first antenna element  100  and a fourth end  204  of the second antenna element  200  do not overlap in a thickness direction of the substrate  160 , that is, in the second direction Y. Even in this case, since the second end  104  of the first antenna element  100  and the fourth end  204  of the second antenna element  200  are close to each other, the second end  104  and the fourth end  204  are capacitively coupled to each other. The second end  104  of the first antenna element  100  and the fourth end  204  of the second antenna element  200  do not overlap in the second direction Y, whereby a capacitive component between the second end  104  of the first antenna element  100  and the fourth end  204  of the second antenna element  200  can be adjusted to be smaller than a capacitive component in a case where the second end  104  of the first antenna element  100  and the fourth end  204  of the second antenna element  200  overlap in the second direction Y. 
       FIG.  23    is a graph showing VSWR characteristics of the antenna device  10  for vehicle according to Embodiment 4 and the antenna device  10  for vehicle according to Embodiment 1.  FIG.  24    is a graph showing radiation efficiency characteristics of the antenna device  10  for vehicle according to Embodiment 4 and the antenna device  10  for vehicle according to Embodiment 1. 
     A horizontal axis of the graph of  FIG.  23    indicates a frequency. A vertical axis of the graph of  FIG.  23    indicates a VSWR. A solid line in the graph of  FIG.  23    indicates the VSWR characteristic of the antenna device  10  for vehicle according to Embodiment 4. A broken line in the graph of  FIG.  23    indicates the VSWR characteristic of the antenna device  10  for vehicle according to Embodiment 1. 
     A horizontal axis of the graph of  FIG.  24    indicates a frequency. A vertical axis of the graph of  FIG.  24    indicates radiation efficiency. A solid line in the graph of  FIG.  24    indicates the radiation efficiency characteristic of the antenna device  10  for vehicle according to Embodiment 4. A broken line in the graph of  FIG.  24    indicates the radiation efficiency characteristic of the antenna device  10  for vehicle according to Embodiment 1. 
     As shown in  FIG.  23   , the VSWR of each of Embodiment 4 and Embodiment 1 is as low as less than 3.5 over a broadband of 700 MHz to 6500 MHz. As shown in  FIG.  24   , the radiation efficiency of each of the Embodiment 4 and Embodiment 1 is as high as greater than 60% over a broadband of 700 MHz to 6500 MHz. 
     In Embodiment 4, since a configuration is made in which the first antenna element  100  and the second antenna element  200  are provided using the conductive patterns on the substrate  160 , it is possible to restrain collision of the antenna elements due to vibration of a vehicle, to restrain change in interval between the antenna elements due to vibration of the vehicle or assembling work, and to stably uniformize capacitive coupling, compared to a case where the antenna elements are configured with sheet metal. 
     Although the embodiment and the modification examples of the present invention have been described referring to the drawings, these are examples of the present invention, and various configurations other than the embodiment and the modification examples may also be employed. 
     For example, in each embodiment and each modification example, the second end  104  of the first antenna element  100  has the upper end edge inclined obliquely with respect to the ground  20 . The upper end edge of the second end  104  of the first antenna element  100  however may have, for example, a triangular shape, a quadrangular shape, a semi-circular shape, or a semi-elliptical shape. The same also applies to the first antenna element  100 A, the first antenna element  100 B, the second antenna element  200 A, and the second antenna element  200 B described referring to  FIGS.  11  to  18   . 
     In each embodiment and each modification example, the first antenna element  100  has the feeding portion  102   a , and the second antenna element  200  has the short-circuit portion  202   a . The first antenna element  100  however may have the short-circuit portion  202   a , and the second antenna element  200  may have the feeding portion  102   a . The same also applies to the first antenna element  100 A, the first antenna element  100 B, the second antenna element  200 A, and the second antenna element  200 B described referring to  FIGS.  11  to  18   . 
     According to the specification, the following aspects are provided. 
     (Aspect 1) 
     Aspect 1 is an antenna device for vehicle including 
     a first antenna element disposed on a ground, and 
     a second antenna element disposed on the ground, 
     in which at least a portion of the first antenna element and at least a portion of the second antenna element are capacitively coupled. 
     According to Aspect 1, capacitive coupling between the first antenna element and the second antenna element contributes to a low VSWR and high radiation efficiency in a low frequency band. Accordingly, it is possible to enable an antenna to be used stably with high radiation efficiency over a broadband. 
     (Aspect 2) 
     Aspect 2 is the antenna device for vehicle according to Aspect 1, 
     in which the first antenna element has a feeding portion, and 
     the second antenna element has a short-circuit portion short-circuited to the ground. 
     According to Aspect 2, satisfactory characteristics can be obtained in a low frequency band compared to a case where the second antenna element is electrically opened to the ground. 
     (Aspect 3) 
     Aspect 3 is the antenna device for vehicle according to Aspect 1 or 2, 
     in which the first antenna element has a first region positioned on one side of the second antenna element, and a second region positioned on the other side opposite to the one side of the second antenna element, 
     at least a portion of the first region of the first antenna element is capacitively coupled to at least a portion of the second antenna element, and 
     at least a portion of the second region of the first antenna element is capacitively coupled to at least a portion of the second antenna element. 
     According to Aspect 3, capacitive coupling between the first region of the first antenna element and the second antenna element, and capacitive coupling between the second region of the first antenna element and the second antenna element contribute to a low VSWR and high radiation efficiency in a low frequency band. Accordingly, it is possible to enable an antenna to be used stably with high radiation efficiency over a broadband. 
     (Aspect 4) 
     Aspect 4 is the antenna device for vehicle according to any one of Aspects 1 to 3, 
     in which the first antenna element has a first end and a second end at a position away from the ground with respect to the first end, 
     the second antenna element has a third end and a fourth end at a position away from the ground with respect to the third end, and 
     at least a portion of the second end of the first antenna element and at least a portion of the fourth end of the second antenna element are capacitively coupled. 
     According to Aspect 4, capacitive coupling between the second end of the first antenna element and the fourth end of the second antenna element contributes to a low VSWR and high radiation efficiency in a low frequency band. Accordingly, it is possible to enable an antenna to be used stably with high radiation efficiency over a broadband. 
     (Aspect 5) 
     Aspect 5 is the antenna device for vehicle according to Aspect 4, 
     in which a width of the first antenna element increases stepwise or gradually from the first end to the second end. 
     According to Aspect 5, the first antenna element can function as an antenna having a self-similar shape. That is, since the first antenna element functions as a self-similar shape or an equivalent tapered antenna, functions as a monopole antenna, or functions as a loop antenna or a split ring antenna depending on a frequency band of an operation frequency band, it is possible to enable an antenna to be used stably with high radiation efficiency over a broadband. 
     (Aspect 6) 
     Aspect 6 is the antenna device for vehicle according to Aspect 4 or 5, 
     in which a width of the second antenna element increases stepwise or gradually from the third end to the fourth end. 
     According to Aspect 6, since the second antenna element functions as an antenna having a self-similar shape, it is possible to enable an antenna to be used stably with high radiation efficiency over a broadband. 
     (Aspect 7) 
     Aspect 7 is the antenna device for vehicle according to any one of Aspects 4 to 6, 
     in which the second end of the first antenna element is inclined obliquely with respect to the ground. 
     According to Aspect 7, a superimposed area of the second end of the first antenna element and the fourth end of the second antenna element is adjusted by the inclination of the second end, such that it is possible to adjust a capacitive component between the second end of the first antenna element and the fourth end of the second antenna element. 
     (Aspect 8) 
     Aspect 8 is the antenna device for vehicle according to any one of Aspects 1 to 7, 
     in which at least a portion of the first antenna element or the second antenna element is provided with a dielectric. 
     According to Aspect 8, it is possible to smooth the VSWR characteristics with the dielectric. 
     (Aspect 9) 
     Aspect 9 is the antenna device for vehicle according to any one of Claims  1  to  8 , 
     in which a difference between a length of the first antenna element and a length of the second antenna element is within ±25% of the length of the first antenna element or the length of the second antenna element. 
     According to Aspect 9, it is possible to enable an antenna to be used stably with high radiation efficiency over a broadband. 
     (Aspect 10) 
     Aspect 10 is the antenna device for vehicle according to any one of Aspects 1 to 9, 
     in which a corner of the at least a portion of the first antenna element is rounded. 
     According to Aspect 10, it is possible to suppress the occurrence of a defect, such as damage to a worker in assembling the antenna device for vehicle due to the corner or damage to other members due to the corner, compared to a case where the corner of the first antenna element is sharp. 
     (Aspect 11) 
     Aspect 11 is the antenna device for vehicle according to any one of Aspects 1 to 10, further including 
     a holder positioned between at least a portion of the first antenna element and at least a portion of the second antenna element, 
     in which at least a portion of the first antenna element and the second antenna element is supported by the holder. 
     According to Aspect 11, it is possible to suppress an influence of vibration of an automobile on which the antenna device for vehicle is mounted, on mechanical characteristics of at least one of the first antenna element and the second antenna element, compared to a case where the holder is not provided. Fluctuation of a distance between at least a portion of the first antenna element and at least a portion of the second antenna element capacitively coupled or fluctuation of a superimposed area of at least a portion of the first antenna element and at least a portion of the second antenna element capacitively coupled, due to vibration of an automobile on which the antenna device for vehicle is mounted is suppressed, and it is possible to suppress fluctuation of capacitance between the first antenna element and the second antenna element, compared to a case where the holder is not provided. 
     This application claims priority based on Japanese Patent Application No. 2020-053910, filed Mar. 25, 2020, the entire disclosure of which is incorporated herein. 
     REFERENCE SIGNS LIST 
     
         
         
           
               10  antenna device for vehicle 
               20  ground 
               100  first antenna element 
               100 A first antenna element 
               100 B first antenna element 
               102  first end 
               102 A first end 
               102 B first end 
               102   a  feeding portion 
               104  second end 
               104 A second end 
               104 B second end 
               106  fifth end 
               106 A fifth end 
               106 B fifth end 
               110  first region 
               110 A first region 
               112  first portion 
               112 A first portion 
               114  second portion 
               114 A second portion 
               116  first step portion 
               116 A first step portion 
               120  second region 
               120 A second region 
               122  fifth portion 
               124  sixth portion 
               126  second step portion 
               150  dielectric 
               150 A first dielectric 
               150 B second dielectric 
               160  substrate 
               200  second antenna element 
               200 A second antenna element 
               200 B second antenna element 
               202  Third end 
               202 A third end 
               202 B third end 
               202   a  short-circuit portion 
               204  fourth end 
               204 A fourth end 
               204 B fourth end 
               212  third portion 
               214  fourth portion 
               300 A holder 
               300 B holder 
               310 A first block 
               310 B first block 
               320 A second block 
               320 B second block 
               330 A projection portion 
               342 A first protrusion 
               344 A second protrusion 
               352 A third protrusion 
               362 A support portion 
               362 Aa first support 
               362 Ab second support 
               410  first antenna portion 
               420  second antenna portion 
               510  antenna base 
               520  substrate 
               530  antenna case 
             X first direction 
             Y second direction 
             Z third direction