Patent Publication Number: US-9837715-B2

Title: Antenna device

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a U.S. National Phase Application under 35 U.S.C. 371 of International Application No. PCT/JP2013/002239 filed on Apr. 1, 2013 and published in Japanese as WO 2013/153770 A1 on Oct. 17, 2013. This application is based on and claims priority to Japanese Patent Application No. 2012-092005 filed on Apr. 13, 2012. The entire disclosures of all of the above applications are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to an antenna device including multiple antenna elements having respective operating frequency bands different from one another. 
     BACKGROUND ART 
     For example, patent literature 1 discloses a configuration in which multiple antenna elements operating at different frequency bands are disposed on a ground plate having a planar shape in a standing manner adjacent to one another. 
     However, in the configuration disclosed in patent literature 1, induced currents having current vectors in opposite directions are generated in the antenna elements disposed adjacent to one another by a coupling of the antenna elements disposed adjacent to one another. As a result, a gain of the antenna element is decreased caused by the flow of the induced current, and a desired directivity and a desired gain of the antenna element cannot be acquired. 
     PRIOR ART LITERATURES 
     Patent Literature 
     
         
         Patent Literature 1: JP H9-181525 A 
       
    
     SUMMARY OF INVENTION 
     In view of the foregoing difficulties, it is an object of the present disclosure to provide an antenna device including multiple antenna elements that properly provide an improved directivity and an improved gain. 
     According to an aspect of the present disclosure, an antenna device includes a first antenna element and a second antenna element. The first antenna element operates at a first predetermined frequency band. The second antenna element operates at a second predetermined frequency band that is different from the first predetermined frequency band. The first antenna element includes a base end portion, a front end portion, a folded portion, a first side portion disposed between the base end portion and the folded portion, and a second side portion disposed between the folded portion and the front end portion. A direction of a current vector in the first side portion is equal to a direction of a current vector in the second side portion. The second antenna element is disposed adjacent to the first antenna element. 
     With the above device, an induced current that generated and flows in the first antenna element disposed adjacent to the second antenna element can be restricted. The induced current in the first antenna element flows in a direction opposite to a direction of a current flow in the second antenna element. The first antenna element is able to provide an improved directivity and an improved gain even with a configuration in which multiple antenna elements having different operating frequency bands are disposed adjacent to one another. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The above and other objects, features and advantages of the present disclosure will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings: 
         FIG. 1  is a diagram showing a perspective view of an antenna device according to a first embodiment of the present disclosure; 
         FIG. 2  is a diagram showing a front view of the antenna device according to the first embodiment; 
         FIG. 3  is a diagram showing a directivity of the antenna device according to the first embodiment on a horizontal plane; 
         FIG. 4  is a diagram showing a front view of a comparison example of an antenna device; 
         FIG. 5  is a diagram showing a directivity of the comparison example of the antenna device on a horizontal plane; 
         FIG. 6  is a diagram showing a perspective view of an antenna device according to a second embodiment of the present disclosure; 
         FIG. 7  is a diagram showing a front view of an antenna device according to a third embodiment of the present disclosure; 
         FIG. 8  is a diagram showing a back view of the antenna device according to the third embodiment; 
         FIG. 9  is a diagram showing a front view of an antenna device according to a fourth embodiment of the present disclosure; 
         FIG. 10  is a diagram showing a back view of the antenna device according to the fourth embodiment; 
         FIG. 11  is a diagram showing a front view of an antenna device according to a fifth embodiment of the present disclosure; 
         FIG. 12  is a diagram showing a front view of an antenna device according to a sixth embodiment of the present disclosure; and 
         FIG. 13  is a diagram showing a perspective view of an antenna device according to a seventh embodiment of the present disclosure. 
     
    
    
     EMBODIMENTS FOR CARRYING OUT INVENTION 
     First Embodiment 
     The following will describe an antenna device  1  according to a first embodiment of the present disclosure with reference to  FIG. 1  to  FIG. 5 . The antenna device  1  includes an antenna element  10  that is utilized in a vehicle-to-vehicle communication system operating at a frequency band of 5.9 gigahertz (GHz) as an operating frequency band and an antenna element  4  that is utilized in a mobile phone system operating at a frequency band of 900 megahertz (MHz) as an operating frequency band. The antenna device  1  further includes a ground plate  2  having a planar shape. The ground plate  2  is provided by, for example, a metal plate having an approximately rectangular shape. The ground plate  2  is electrically connected with a metal body having a sufficiently large size relative to a wavelength of an operating frequency of the antenna element  4 . Details of the antenna element  4  will be described later. A substrate  3  is disposed on an upper surface  2   a  of the ground plate  2  in a standing manner such that the substrate  3  is approximately perpendicular to the upper surface  2   a  of the ground plate  2 . The substrate  3  is made of resin material and has a planar shape. Herein, the approximately perpendicular to includes a state that is close to a perpendicular state. The antenna element  4 , which is provided by a folded monopole antenna element, is disposed on one face  3   a  of the substrate  3 . The antenna element  4  is formed by a conductive pattern (conductive film). The folded monopole antenna element  4  is also referred to as a first antenna element, and the operating frequency band of the first antenna element, for example, 900 megahertz (MHz) band is also referred to as a first frequency band. 
     The first antenna element  4  transmits and receives radio waves of the mobile phone system that operates at the frequency band of, for example, 900 MHz band. The first antenna element  4  is electrically connected with (conductive with) the ground plate  2 . The first antenna element  4  has a base end portion  4   a , a front end portion  4   b , and a folded portion  4   c . The first antenna element  4  further has a side portion  5  that extends from the base end portion  4   a  to the folded portion  4   c  and a side portion  6  that extends from the folded portion  4   c  to the front end portion  4   b . Hereinafter, the side portion  5  is also referred to as a first side portion, and the side portion  6  is also referred to as a second side portion. A length of each of the first side portion and the second side portion is electrically equal to a quarter wavelength. For example, the length of each of the first side portion and the second side portion is equal to a length acquired by multiplying a quarter wavelength of 900 MHz band radio wave by a wavelength shortening rate. Herein, the wavelength shortening rate is defined by a relative permittivity of material of the substrate  3 . That is, a distance from the base end portion  4   a  of the first antenna element  4  to the front end portion  4   b  of the first antenna element  4  is equal to a value acquired by multiplying half wavelength of 900 MHz band radio wave by the wavelength shortening rate defined by the relative permittivity of the material of the substrate  3 . Hereinafter, the distance from the base end portion  4   a  of the first antenna element  4  to the front end portion  4   b  of the first antenna element  4  is also referred to as an element length. An electrical potential of the front end portion  4   b  of the first antenna element  4  is equal to an electrical potential of the ground plate  2 . 
     A feeding point  7  that supplies power to the first antenna element  4  is disposed at the base end portion  4   a  of the first antenna element  4 . The ground plate  2  functions as an antenna ground of the first antenna element  4 . When the feeding point  7  is positioned apart from the first antenna element  4 , the feeding point  7  may be electrically connected with the base end portion  4   a  of the first antenna element  4  via a microstripline. For example, as shown in  FIG. 2 , the feeding point  7  may be provided by a configuration in which an internal conductor of a coaxial cable  8  is connected with the base end portion  4   a  of the first antenna element  4  and an external conductor of the coaxial cable  8  is connected with the ground plate  2 . As shown in  FIG. 1 , the first antenna element  4  is bended at the folded portion  4   c  such that a distance d (clearance) between the first side portion  5  and the second side portion  6  has a constant value along a direction perpendicular to the ground plate  2 . 
     The second side portion  6  of the first antenna element  4  has a broad portion  9  that is formed integrally with an upper half portion of the second side portion  6  as one body. The broad portion  9  has a width greater than a width of other portion of the second side portion  6 . The monopole antenna element  10  has a linear shape, and includes a base end portion  10   a  and a front end portion  10   b . The base end portion  10   a  of the monopole antenna  10  is electrically connected with (conductive with) an upper end portion  9   a  of the broad portion  9  so that the base end portion  10   a  of the monopole antenna  10  is electrically connected with the broad portion  9 . The monopole antenna element  10  is also referred to as a second antenna element, and the operating frequency band of the second antenna element, for example, 5.9 GHz band is also referred to as a second frequency band. The second antenna element  10  extends in an approximately vertical direction. That is, the second antenna element  10  is connected with the broad portion  9  such that the second antenna element  10  extends from the base end portion  10   a  to the front end portion  10   b  in a direction moving away from the broad portion  9 . A length of the second antenna element  10  is electrically equal to a quarter wavelength. Hereinafter, the length of the second antenna element  10  is also referred to as an element length. For example, the element length of the second antenna element  10  may be set equal to a length acquired by multiplying a quarter wavelength of 5.9 GHz band radio wave by the wavelength shortening rate defined by the relative permittivity of the material of the substrate  3 . For example, a width of the broad portion  9  in a direction horizontal to the ground plate  2  may be set greater than the length acquired by multiplying a quarter wavelength of 5.9 GHz band radio wave by the wavelength shortening rate defined by the relative permittivity of the material of the substrate  3 . 
     A feeding point  11  that supplies power to the second antenna element  10  is disposed at the base end portion  10   a  of the second antenna element  10 . The broad portion  9  functions as an antenna ground of the second antenna element  10 . When the feeding point  11  is positioned apart from the second antenna element  10 , the feeding point  11  may be electrically connected with the base end portion  10   a  of the second antenna element  10  via a microstripline. For example, as shown in  FIG. 2 , the feeding point  11  may be provided by a configuration in which an internal conductor of a coaxial cable  12  is connected with the base end portion  10   a  of the second antenna element  10  and an external conductor of the coaxial cable  12  is connected with the broad portion  9 . The second antenna element  10  may have a predetermined width. 
     The antenna device  1  having above-described configuration is disposed in a casing (not shown), and the antenna device  1  together with the casing is attached to a vehicle roof such that a plane on which the ground plate  2  is disposed extends along (parallel to) an upper surface of the vehicle roof. That is, the antenna device  1  is attached to the vehicle such that an axis of the second antenna element  10  is perpendicular to the ground plate  2 . Hereinafter, the axis perpendicular to the ground plate  2  is defined as a Z axis, an axis perpendicular to the Z axis and perpendicular to a plane on which the first antenna element  4  is disposed is defined as a Y axis, and an axis perpendicular to both the Z axis and the Y axis is defined as an X axis. That is, the X axis is parallel to the plane on which the first antenna element  4  is disposed. Further, a X-Y plane defined by the X axis and the Y axis is also referred to as a horizontal plane. That is, the horizontal plane refers to a plane that is parallel to the ground plate  2 . 
       FIG. 3  shows a simulation result of a directivity of the first antenna element  4  on the horizontal plane. The first antenna element  4  has the configuration shown in  FIG. 1  and is used in the mobile phone system operating at 900 MHz band.  FIG. 4  shows an antenna device  21  as a comparison example. The antenna device  21  includes an unfolded monopole antenna element  22  as an antenna element for the mobile phone system operating at 900 MHz band.  FIG. 5  shows a simulation result of a directivity of the antenna element  22  shown in  FIG. 4  on the horizontal plane. As shown in  FIG. 3  and  FIG. 5 , when the antenna element for the mobile phone system operating at 900 MHz band is provided by the folded monopole antenna element  4 , a gain on the horizontal plane is improved by 3 decibels (dB) on average compared with the configuration in which the antenna element for the mobile phone system operating at 900 MHz band is provided by the unfolded monopole antenna element. 
     In the configuration shown in  FIG. 4 , the unfolded monopole antenna element  22  electrically couples with a ground pattern  23  that functions as an antenna ground of the monopole second antenna element  10 , and induced currents having current vectors in opposite directions are generated and flow through the antenna elements. That is, when a current flows from the base end portion  22   a  of the antenna element  22  toward the front end portion  22   b  of the antenna element  22 , an induced current flows from an upper end portion  23   a  of the ground pattern  23  toward a lower end portion  23   b  of the ground pattern  23 . As a result, the gain of the antenna element  22  on the horizontal plane is decreased caused by a flow of the induced current. 
     Regarding above-described difficulty, in the antenna device  1  according to the present embodiment, the first antenna element  4  is provided by the folded monopole antenna element and the electrical potential of the front end portion  4   b  of the first antenna element  4  is set equal to the electrical potential of the ground plate  2 . Thus, the first antenna element  4  becomes equivalent to half of a roof antenna that has an element length equal to one wavelength of the operating frequency band radio wave. Accordingly, in the first antenna element  4 , a direction of the current vector in the base end portion  4   a  and in the front end portion  4   b  is opposite to a direction of the current vector in the folded portion (one corresponds to an anti-node and the other one corresponds to a node). As a result, a direction of the current vector in the first side portion  5  that extends from the base end portion  4   a  to the folded portion  4   c  is equal to a direction of the current vector in the second side portion  6  that extends from the folded portion  4   c  to the front end portion  4   b . That is, the induced currents, which have current vectors having opposite directions and are canceled by one another, are restricted, and a decrease in gain of the first antenna element  4  on the horizontal plane is restricted. 
     As described above, in the antenna device  1  according to the present embodiment, the first antenna element  4  for the mobile phone system operating at 900 MHz band is provided by the folded monopole antenna element disposed on the ground plate  2  in a standing manner. The electrical potential of the front end portion  4   b  of the first antenna element  4  is set equal to the electric potential of the ground plate  2  so that the ground plate  2  functions as the antenna ground of the first antenna element  4 . The second antenna element  10  for the vehicle-to-vehicle communication system operating at 5.9 GHz band is provided by the monopole antenna element, and the broad portion  9  of the first antenna element  4  functions as the antenna ground of the second antenna element  10 . With this configuration, the folded monopole first antenna element  4  becomes equivalent to half of the roof antenna that has the element length equal to one wavelength of the operating frequency band radio wave. Further, even with the configuration in which the first antenna element  4  and the antenna element  10  are disposed, an improved directivity and an improved gain can be properly provided by the first antenna element  4  that is disposed on the ground plate  2  in a standing manner. 
     The second antenna element  10  operates with the broad portion  9  of the first antenna element  4  as the antenna ground. Thus, the first antenna element  4  and the second antenna element  10  can be disposed adjacent to each other with ease and a size of the antenna device  1  can be reduced. The antenna ground of the second antenna element  10  has a broad width. Thus, current paths can be increased and a frequency band can be broadened compared with a case in which the antenna ground of the second antenna element  10  has a small width. 
     As shown in the following fifth embodiment, when multiple antenna elements are attached to the vehicle roof, the vehicle roof usually functions as a common antenna ground of multiple antenna elements. Details of the fifth embodiment will be described later. However, when the vehicle roof is used as the common antenna ground, a wire circuit from each antenna element to the vehicle roof is necessary and a wire pattern is necessary to be formed on the substrate. Thus, a size of the antenna device can be increased. In the present embodiment, the second antenna element  10  operates with the broad portion  9  of the first antenna element  4  as the antenna ground. That is, a part of the first antenna element  4  provides the antenna ground of the second antenna element  10 . Thus, there is no need to add the wire circuit or to form the wire pattern on the substrate. Thus, a size of the antenna device  1  can be reduced. 
     In the present embodiment, the antenna element (first antenna element  4 ) operating at a relatively low frequency is provided by the folded monopole antenna element, and the antenna element (second antenna element  10 ) operating at a relatively high frequency is provided by the monopole antenna element. As another example, the antenna element operating at a relatively low frequency may be provided by the monopole antenna element, and the antenna element operating at a relatively high frequency may be provided by the folded monopole antenna element. 
     However, in the antenna device according to the following fifth embodiment in which multiple antenna elements are attached to the vehicle roof, it is better that the antenna element (first antenna element  4 ) operating at a relatively low frequency is provided by the folded monopole antenna element, and the antenna element (second antenna element  10 ) operating at a relatively high frequency is provided by the monopole antenna element due to the following reasons. That is, the second antenna element  10  operating at a relatively high frequency may be disposed at an upper portion of the antenna device  1 , and the first antenna element  4  operating at a relatively low frequency may be disposed at a lower portion of the antenna device  1 . 
     A high frequency radio wave having a frequency around GHz band is more easily to be attenuated with an increase of a transmission distance. Thus, to secure a predetermined gain is especially important for the antenna device receiving the high frequency radio wave having the frequency around GHz band. However, the vehicle roof to which the antenna device  1  is attached is usually gently bended to have a gentle projected surface. When the antenna device is attached to a rear portion (REAR) of the vehicle roof, the projected portion of the vehicle roof exists at a front area (FRONT) of the antenna device as an obstacle. Thus, an improved directivity in the front direction of the vehicle is difficult to be secured. 
     As described in the present embodiment, the second antenna element  10  operating at a relatively high frequency is disposed on an upper side of the first antenna element  4  operating at a relatively low frequency. Thus, a visibility in front direction (field of front vision) of the vehicle viewed from the second antenna element  10  is improved and a directivity in the front direction of the vehicle can be properly secured compared with a case in which the first antenna element  4  operating at a relatively low frequency is disposed on an upper side of the second antenna element  10  operating at a relatively high frequency. 
     Second Embodiment 
     The following will describe an antenna device  31  according to a second embodiment of the present disclosure with reference to  FIG. 6 . A description of the same part with the above-described first embodiment will be omitted, and different parts will be described. The antenna device  31  according to the second embodiment includes a second antenna element  32 , and a position of the second antenna element  32  with respect to the broad portion  9  of the first antenna element  4  is different from the antenna device  1  according to the first embodiment. 
     In the antenna device  31 , the second antenna element  32  is provided by the monopole antenna element and has a linear shape. A base end portion  32   a  of the second antenna element  32  is electrically connected with (conductive with) a lower end portion  9   b  of the broad portion  9  such that the base end portion  32   a  is electrically connected with the broad portion  9 . The second antenna element  32  extends in an approximately vertical direction. That is, the second antenna element  32  is connected with the broad portion  9  such that the second antenna element  32  extends from the base end portion  32   a  to the front end portion  32   b  in a direction moving away from the broad portion  9 . A length of the second antenna element  32  is electrically equal to a quarter wavelength. For example, the length of the second antenna element  32  may be set equal to a length acquired by multiplying a quarter wavelength of 5.9 GHz band radio wave by the wavelength shortening rate defined by the relative permittivity of the material of the substrate  3 . A feeding point  32  that supplies power to the second antenna element  32  is disposed at the base end portion  32   a  of the second antenna element  32 . The broad portion  9  of the first antenna element  4  functions as an antenna ground of the second antenna element  32 . The antenna device  31  according to the second embodiment provides advantages similar to the advantages provided by the antenna device  1  according to the first embodiment. 
     Third Embodiment 
     The following will describe an antenna device  41  according to a third embodiment of the present disclosure with reference to  FIG. 7  and  FIG. 8 . A description of the same part with the above-described first embodiment will be omitted, and different parts will be described. In the antenna device  41  according to the third embodiment, the first antenna element includes two first sub antenna elements  42 ,  421  provided by the folded monopole antenna elements, and the two first sub antenna elements  42 ,  421  are disposed on opposite faces of the substrate. 
     In the antenna device  41 , the first sub antenna element  421  and the second antenna element  10  are disposed on one face  3   a  of the substrate  3 . The first sub antenna element  421  corresponds to the first antenna element  4  of the first embodiment, and the second antenna element  10  is provided by the monopole antenna element. The first sub antenna element  421  has a configuration similar to the configuration of the first antenna element  4  of the first embodiment. The other first sub antenna element  42  is disposed on the other face  3   b  of the substrate  3 , and the other first sub antenna element  42  is provided by a folded monopole antenna element and is formed by a conductive pattern. 
     The first sub antenna element  42  is an antenna element that transmits and receives radio waves for a mobile phone system operating at a frequency band of 2 gigahertz (GHz) band. A base end portion  42   a  of the first sub antenna element  42  is electrically connected with a base end portion  4   a  of the first sub antenna element  421  through a via hole  43 . A front end portion  42   b  of the first sub antenna element  42  is electrically connected with the ground plate  2 . For example, a length of each of a first side portion  44  and a second side portion  45  of the first sub antenna element  42  is electrically equal to a quarter wavelength. The first side portion  44  extends from the base end portion  42   a  of the first sub antenna element  42  to a folded portion  42   c  of the first sub antenna element  42 . The second side portion  45  extends from the folded portion  42   c  of the first sub antenna element  42  to the front end portion  42   b  of the first sub antenna element  42 . For example, the length of the first side portion and the second side portion is equal to a length acquired by multiplying a quarter wavelength of 2 GHz band radio wave by the wavelength shortening rate defined by the relative permittivity of the material of the substrate  3 . That is, a distance from the base end portion  42   a  of the first sub antenna element  42  to the front end portion  42   b  of the first sub antenna element  42  is equal to a value acquired by multiplying half wavelength of 2 GHz band radio wave by the wavelength shortening rate defined by the relative permittivity of the material of the substrate  3 . In the first sub antenna element  42 , a distance between the first side portion  44  and the second side portion  45  has a constant value along a perpendicular direction. The folded portion  42   c  is sandwiched between the first side portion  44  and the second side portion  45 . 
     The antenna device  41  according to the third embodiment provides advantages similar to the advantages provided by the antenna device  1  according to the first embodiment. That is, the first sub antenna element  421  disposed on the ground plate  2  in a standing manner can properly provide an improved directivity and gain. The other first sub antenna element  42  disposed on the ground plate  2  in a standing manner can also properly provide an improved directivity and gain. Further, in the present embodiment, the antenna device  41  corresponds to three different operating frequency bands, and can function as a multi frequency band antenna device. Further, the first sub antenna element  4  is disposed together with the other first sub antenna element  42  on the same substrate, and has the common feeding point  7 . Thus, a configuration of the antenna device  41  can be simplified. 
     Fourth Embodiment 
     The following will describe an antenna device  51  according to a fourth embodiment of the present disclosure with reference to  FIG. 9  and  FIG. 10 . A description of the same part with the above-described first embodiment will be omitted, and different parts will be described. In the antenna device  51  according to the fourth embodiment, a first side portion  55  and a second side portion  53  of the first antenna element provided by a folded monopole antenna element are disposed on opposite faces of the substrate. 
     In the antenna device  51 , a first antenna element  52  is disposed on the substrate  3 . The first antenna element  52  corresponds to the first antenna element  4  of the first embodiment, and is provided by a folded monopole antenna element. In the antenna device  51 , a second side portion  53  and a broad portion  54  are disposed on one face  3   a  of the substrate  3 . The second side portion  53  corresponds to the second side portion  6  of the first embodiment, and the broad portion  54  corresponds to the broad portion  9  of the first embodiment. On the other face  3   b  of the substrate  3 , a first side portion  55  is disposed. The first side portion  55  corresponds to the first side portion  5  of the first embodiment. The second side portion  53  is electrically connected with the first side portion  55  through a via hole  56 . A feeding point  57  is disposed at a base end portion of the first side portion  55 , and the second antenna element  10  is connected with the broad portion  54 . The antenna device  51  according to the fourth embodiment provides advantages similar to the advantages provided by the antenna device  1  according to the first embodiment. 
     Fifth Embodiment 
     The following will describe an antenna device  61  according to a fifth embodiment of the present disclosure with reference to  FIG. 11 . A description of the same part with the above-described first embodiment will be omitted, and different parts will be described. The antenna device  61  according to the fifth embodiment includes a substrate  64  and a first antenna element  65  provided by a folded monopole antenna element. The substrate  64  and the first antenna element  65  are shaped to track a shape of a casing to be attached to the vehicle roof. 
     The antenna device  61  is disposed in the casing  62 , and is attached to the vehicle roof under a state being disposed in the casing  62 . The antenna device  61  includes a ground plate  63 , the substrate  64 , the first antenna element  65 , a feeding point  66 , a second antenna element  67  provided by a monopole antenna element, a feeding point  68 , and coaxial cables  69 ,  70 . The ground plate  63 , the substrate  64 , the first antenna element  65 , the feeding point  66 , the second antenna element  67 , the feeding point  68 , and the coaxial cables  69 ,  70  have functions similar to the functions of ground plate  2 , the substrate  3 , the first antenna element  4 , the feeding point  7 , the second antenna element  10 , the feeding point  11 , and the coaxial cables  8 ,  12  described in the first embodiment, respectively. 
     The antenna device  61  according to the fifth embodiment provides advantages similar to the advantages provided by the antenna device  1  according to the first embodiment. The first antenna element  65  is shaped to track the shape of the casing  62  to be attached to the vehicle roof. Specifically, the first antenna element  65  is bended at a center portion. Thus, a dimension of the first antenna element  65  can be reduced, and accordingly, a size of the antenna device  61  can be reduced compared with a case in which the antenna element having the same antenna length (current path length) is disposed in the antenna device without being bended. Even when the size of the antenna device  61  is reduced, the necessary antenna length can be secured and an improved antenna characteristic can be provided with the antenna device  61  of the present embodiment. 
     Sixth Embodiment 
     The following will describe an antenna device  71  according to a sixth embodiment of the present disclosure with reference to  FIG. 12 . A description of the same part with the above-described first embodiment will be omitted, and different parts will be described. The antenna device  71  according to the sixth embodiment, includes an antenna element (first antenna element  73 ), which is used for transmitting and receiving radio waves of the mobile phone system. The antenna element is provided by a folded dipole antenna element. 
     The antenna device  71  includes the first antenna element  73  disposed on one face  72   a  of a substrate  72 . The first antenna element  73  is provided by a folded dipole antenna element, and is formed by a conductive pattern on one face  72   a  of the substrate  72 . The first antenna element  73  includes two side portions including a first side portion  74  and a second side portion  75 . A length of each of the first side portion  74  and the second side portion  75  is equal to a value acquired by multiplying half wavelength of 900 GHz band radio wave by a wavelength shortening rate. Herein, the wavelength shortening rate is defined by a relative permittivity of material of the substrate  72 . That is, an entire length of the antenna element  73  (element length) is equal to a value acquired by multiplying the wavelength of 900 GHz band radio wave by the wavelength shortening rate defined by the relative permittivity of the material of the substrate  72 . 
     The second side portion  75  has a broad portion  76  that is formed integrally with an upper half portion of the second side portion  75  as one body. The broad portion  76  of the second side portion  75  corresponds to the broad portion  9  of the first embodiment. A feeding point  77  is disposed at an approximately center portion of the first side portion  74 , and the broad portion  76  is connected with the monopole second antenna element  10 . The antenna device  71  according to the sixth embodiment provides advantages similar to the advantages provided by the antenna device  1  according to the first embodiment. The first antenna element  73  is provided by the folded dipole antenna element. Thus, the antenna device  71  according to the present embodiment is appropriate to be disposed on a portion that is separated from a body metal of the vehicle, for example, on a dashboard. 
     Seventh Embodiment 
     The following will describe an antenna device  81  according to a seventh embodiment of the present disclosure with reference to  FIG. 13 . A description of the same part with the above-described sixth embodiment will be omitted, and different parts will be described. In the antenna device  81  according to the seventh embodiment, a second antenna element  83  is disposed on a surface of a broad portion  76  of the first antenna element  73  in a standing manner, and is approximately perpendicular to the surface of the broad portion  76 . The second antenna element  83  is provided by a monopole antenna element, and the first antenna element  73  is provided by a folded dipole antenna element. 
     In the antenna device  81 , on one face  82   a  of a substrate  82 , the first antenna element  73  provided by the folded dipole antenna element described in the sixth embodiment is disposed along the one face  82   a . A base end portion  83   a  of the second antenna element  83 , which is provided by the monopole antenna element and corresponds to the second antenna element  10  of the first embodiment, is electrically connected with the surface of the broad portion  76 . The second antenna element  83  is disposed on the surface of the broad portion  76  in a standing manner and is approximately perpendicular to the surface of the broad portion  76 . That is, the second antenna element  83  is connected with the broad portion  76  such that the second antenna element  83  extends from the base end portion  83   a  to the front end portion  83   b  in a direction perpendicular to the surface of the broad portion  76  moving away from the broad portion  76 . A feeding point  84  is disposed at the base end portion  83   a  of the second antenna element  83 . The antenna device  81  according to the seventh embodiment provides advantages similar to the advantages provided by the antenna device  1  according to the sixth embodiment. 
     Other Embodiments 
     The antenna devices according to foregoing embodiments are used in the vehicle-to-vehicle communication system and the road-to-vehicle communication system. The antenna device according to the present disclosure may also be applied to a vehicle antenna device used in a different communication system other than the road-to-vehicle communication system and the vehicle-to-vehicle communication system. The antenna device according to the present disclosure may also be applied to an antenna device attached to a target other than the vehicle. 
     The substrate of the antenna device according to the present disclosure may be provided by a multi-layer substrate, and multiple antenna elements may be formed on different layers of the same multi-layer substrate. The substrate may be provided by a flexible substrate that is flexible at a predetermined level. Further, the substrate may be provided by a substrate on which electronic components can be mounted on a surface of the flexible substrate. That is, under a condition that multiple antenna elements can be formed on the substrate, the substrate may be provided by any type of substrate. Similarly, the ground plate on which the substrate is disposed in a standing manner can be provided by a plate that is curved at a predetermined level under a condition that the ground plate is able to function as the antenna ground of the antenna element. 
     The antenna element may be provided by a metal plate. 
     While the disclosure has been described with reference to preferred embodiments thereof, it is to be understood that the disclosure is not limited to the preferred embodiments and constructions. The disclosure is intended to cover various modification and equivalent arrangements. In addition, while the various combinations and configurations, which are preferred, other combinations and configurations, including more, less or only a single element, are also within the spirit and scope of the disclosure.