Patent Publication Number: US-2022239006-A1

Title: Antenna apparatus and wireless communication apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation application of International Application PCT/JP2019/040504 filed on Oct. 15, 2019 and designated the U.S., the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     The embodiments discussed herein are related to an antenna apparatus and a wireless communication apparatus. 
     BACKGROUND 
     There are demands for higher efficiency of antennas in wireless communication apparatuses such as vehicles equipped with car-mounted antennas, smartphones, tablet computers, and mobile phones. 
     For example, Japanese Laid-open Patent Publication No. 2016-165035 discloses a monopole antenna with a folded shape. With the monopole antenna, a conductor device preceding the fold and a conductor device after the fold are both provided on a same plane (at a same height from a ground pattern). In the monopole antenna, impedance adjustment of the antenna in a case of a downsized ground pattern is achieved by increasing a linewidth after being connected to the ground pattern and before being folded. 
     DOCUMENT OF PRIOR ART 
     Patent Document 
     
         
         [Patent document 1] Japanese Laid-open Patent Publication No. 2016-165035 
       
    
     SUMMARY 
     According to an aspect of the embodiments, an antenna apparatus includes: a ground substrate; a feeding point provided on the ground substrate; a first conductor device of which one end is electrically connected to the feeding point and which has a plate shape being parallel to the ground substrate; a second conductor device which is arranged between the first conductor device and the ground substrate, of which one end is electrically connected to the ground substrate, and which has a plate shape being parallel to the ground substrate; and a connecting portion which electrically connects another end of the first conductor device and another end of the second conductor device to each other, wherein a width of the first conductor device is wider than a width of the second conductor device. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view showing an example of an antenna according to an embodiment; 
         FIG. 2  is a diagram illustrating a positional relationship between a first conductor device and a second conductor device in a plan view of the antenna according to the embodiment from a +Y direction; 
         FIG. 3  is a side view showing an example of an antenna according to a first comparative example; 
         FIG. 4  is a side view showing an example of an antenna according to a second comparative example; 
         FIG. 5  is a diagram showing an example of an antenna according to a third comparative example; 
         FIG. 6  is a side view schematically showing an example of an antenna according to a fourth comparative example; 
         FIG. 7  is a side view schematically showing an example of an antenna according to a fifth comparative example; 
         FIG. 8  is a diagram illustrating a state in which a piece of metal is placed in a vicinity of a feeding point of an antenna; 
         FIG. 9  is a diagram illustrating a state in which a piece of metal is placed in a vicinity of a folding portion of the antenna; 
         FIG. 10  is a diagram illustrating a state in which a piece of metal is placed in both a vicinity of the feeding point and a vicinity of the folding portion of the antenna; 
         FIG. 11  is a diagram showing a result of a first evaluation; 
         FIG. 12  is a diagram showing a result of a second evaluation; 
         FIG. 13  is a diagram showing a result of a third evaluation; 
         FIG. 14  is a diagram showing a result of a fourth evaluation; 
         FIG. 15  is a diagram illustrating a relationship between a ratio of widths of the first conductor device and the second conductor device and radiation efficiency; 
         FIG. 16  is a diagram illustrating a current distribution of the antenna according to the first comparative example; 
         FIG. 17  is a diagram illustrating a current distribution of the antenna according to the embodiment; 
         FIG. 18  is a side view schematically showing an example of an antenna according to a sixth comparative example; 
         FIG. 19  is a diagram schematically showing an orientation of a current in the antenna according to the embodiment; 
         FIG. 20  is a diagram schematically showing an orientation of a current in the antenna according to the fourth comparative example; 
         FIG. 21  is a diagram schematically showing an orientation of a current in the antenna according to the fifth comparative example; 
         FIG. 22  is a diagram schematically showing an orientation of a current in the antenna according to the sixth comparative example; 
         FIG. 23  is a diagram schematically showing an antenna according to a first modification; 
         FIG. 24  is a diagram schematically showing an antenna according to a second modification; 
         FIG. 25  is a diagram schematically showing an antenna according to a third modification; 
         FIG. 26  is a diagram schematically showing an antenna according to a fourth modification; 
         FIG. 27  is a diagram schematically showing an antenna according to a fifth modification; 
         FIG. 28  is a diagram schematically showing an antenna according to a sixth modification; 
         FIG. 29  is a diagram schematically showing an antenna according to a seventh modification; 
         FIG. 30  is a diagram illustrating a configuration in which the antenna according to the embodiment is applied to a smartphone; 
         FIG. 31  is a diagram illustrating S 11  of an antenna mounted to a smartphone according to an application example; 
         FIG. 32  is a diagram illustrating a Smith chart of the antenna mounted to the smartphone according to the application example; 
         FIG. 33  is a diagram illustrating S 11  of an antenna mounted to a smartphone according to an application example; and 
         FIG. 34  is a diagram illustrating total efficiency of the antenna mounted to the smartphone according to the application example. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     With conventional antennas, there is a problem in that the presence of metal in a vicinity of an antenna reduces radiation efficiency of the antenna due to an effect of the metal. 
     An aspect of the embodiments relates to an antenna apparatus which suppresses a reduction in radiation efficiency of an antenna even when metal is present in a vicinity of the antenna and a wireless communication apparatus which is mounted with the antenna apparatus. 
     Hereinafter, an embodiment will be described. It is to be understood that configurations of the embodiment described below are illustrative and that the disclosed technique is not limited to the configurations of the embodiment. For example, an antenna apparatus according to the present embodiment is configured as described below. 
     The antenna apparatus according to the present embodiment includes: 
     a ground substrate; 
     a feeding point provided on the ground substrate; 
     a first conductor device of which one end is electrically connected to the feeding point and which has a plate shape being parallel to the ground substrate; 
     a second conductor device which is arranged between the first conductor device and the ground substrate, of which one end is electrically connected to the ground substrate, and which has a plate shape being parallel to the ground substrate; and 
     a connecting portion which electrically connects another end of the first conductor device and another end of the second conductor device to each other, wherein 
     a width of the first conductor device is wider than a width of the second conductor device, and 
     lengths of the first conductor device and the second conductor device are ½ of a wavelength of a radio wave for operating the antenna apparatus. 
     The ground substrate is a grounded substrate. The second conductor device is grounded by being electrically connected to the ground substrate. The second conductor device is arranged between the first conductor device and the ground substrate. In other words, the first conductor device and the second conductor device are arranged so as to overlap with each other in a plan view. Since the first conductor device is connected to a feeding point and the first conductor device is formed wider than the second conductor device, a stronger current tends to flow through the first conductor device than the second conductor device. Therefore, a current of which an orientation is opposite to that of the current flowing through the first conductor device also flows through the ground substrate. The orientation of the current with an opposite orientation to the current flowing through the first conductor device is a current with a same orientation as that of the current flowing through the second conductor device. Therefore, a current of which an orientation is opposite to that of the current flowing through the second conductor device on the ground substrate is weakened. An antenna apparatus with such features is capable of reducing a decline in radiation efficiency of an antenna due to heat loss. In addition, as will be explained in the present specification, even when metal is present in a vicinity of the antenna apparatus, a reduction in radiation efficiency of the antenna apparatus can be suppressed and, at the same time, a further improvement in radiation efficiency can also be expected. 
     In addition, a length of the connecting portion may be set to 1/50 or less of the wavelength of the radio wave for operating the antenna apparatus. In other words, by defining the length of the connecting portion in this manner, an interval between the first conductor device and the second conductor device may also be set to 1/50 or less of the wavelength of the radio wave for operating the antenna apparatus. 
     The present antenna apparatus may further include the following feature. At least one of an inductor and a capacitor is provided between the feeding point and the first conductor device. The antenna apparatus with such a feature can change a frequency which causes the antenna apparatus to resonate by appropriately adjusting a capacitance of the capacitor or an inductance of the inductor without changing physical lengths of the first conductor device and the second conductor device. In addition, the inductor or the capacitor may be provided between the second conductor device and the ground substrate. 
     The present antenna apparatus may further include the following feature. The antenna apparatus is mounted to a mobile terminal apparatus and at least a part of the first conductor device is formed by a metal frame which constitutes an exterior of the mobile terminal apparatus. By using a metal external frame which constitutes an exterior of the mobile terminal apparatus as at least a part of the first conductor device, the antenna apparatus with such a feature can reduce an area occupied by the antenna apparatus in a region defined by the metal frame. Therefore, the antenna apparatus with such a feature enables the mobile terminal apparatus to be downsized or enable a larger number of electronic components to be mounted to the mobile terminal apparatus. In addition, at least a part of the second conductor device may be formed using Laser Direct Structuring (LDS) or a flexible substrate. 
     The present antenna apparatus may further include the following feature. At least one of the first conductor device and the second conductor device is formed in a meander shape. By giving at least one of the first conductor device and the second conductor device a meander shape, the antenna apparatus can be further downsized. 
     The present antenna apparatus may further include the following feature. The antenna apparatus may further include a third conductor device which is connected to the other end of the first conductor device and which has a plate shape being parallel to the ground substrate, wherein a length of the third conductor device may be ¼ or less of a wavelength of a radio wave for operating the third conductor device as an antenna. In addition, the antenna apparatus may further include a fourth conductor device being connected to the ground substrate, wherein a length of the fourth conductor device may be ¼ or less of a wavelength of a radio wave for operating the fourth conductor device as an antenna. By having such features, the antenna apparatus can operate at a plurality of frequencies (multiband). 
     The present antenna apparatus may further include the following feature. A grounded metal member is further provided at a distance of 1/50 or less of the wavelength of the radio wave for operating the antenna apparatus from at least one of the one end of the first conductor device and the connecting portion. By providing the metal member in a vicinity of at least one of the one end of the first conductor device (a location connected to the feeding point) and the connecting portion, the radiation efficiency of the present antenna apparatus can be increased. 
     In addition, the disclosed technique may be a wireless communication apparatus mounted with an antenna apparatus having any of the features described above. In the present wireless communication apparatus, the metal member may be an exterior of the wireless communication apparatus. 
     Embodiment 
     Hereinafter, an embodiment will be further described with reference to the drawings.  FIG. 1  is a perspective view showing an example of an antenna according to the embodiment. An antenna  1  includes a feed line  11 , a first conductor device  12 , a second conductor device  13 , a folding portion  14 , a ground line  15 , and a ground substrate  3 . Hereinafter, in the present specification, a near side on a right-hand side of  FIG. 1  will be referred to as a +X direction, a far side on a left-hand side of  FIG. 1  will be referred to as a −X direction, above in  FIG. 1  will be referred to as a +Y direction, and below in  FIG. 1  will be referred to as a −Y direction. 
     The ground substrate  3  is a substrate including a grounded ground surface  3   a . The ground substrate  3  also includes the feeding point  2  for feeding power to the antenna  1 . The ground substrate  3  may be a printed substrate to which various electronic components are to be mounted. An entire surface of the ground substrate  3  may constitute the ground surface  3   a.    
     The first conductor device  12  is a conductor device which is formed in a flat shape parallel to the ground surface  3   a . A +X-side end of the first conductor device  12  is connected to the feeding point  2  by the feed line  11  and the folding portion  14  is connected to a −X-side end thereof. A length (a length from the +X-side end to the −X-side end) of the first conductor device  12  is ½ or less (for example, 0.43λ) of a wavelength λ of a radio wave which resonates the antenna  1 . The second conductor device  13  is arranged between the first conductor device  12  and the ground surface  3   a.    
     The second conductor device  13  is a conductor device which is formed in a flat shape parallel to the ground surface  3   a . A −X-side end of the second conductor device  13  is connected to a −X-side end of the first conductor device  12  by the folding portion  14 . A +X-side end of the second conductor device  13  is grounded by being connected to the ground surface  3   a  of the ground substrate  3  via the ground line  15 . A width of the second conductor device  13  is formed narrower than a width of the first conductor device  12 . The width of the second conductor device  13  is, for example, ⅕ of the width of the first conductor device  12 . In addition, a distance between the first conductor device  12  and the second conductor device  13  in the Y direction is preferably 1/50 or less of the wavelength λ of the radio wave which resonates the antenna  1 . 
     The folding portion  14  is a conductor device which extends from the −X-side end of the first conductor device  12  toward the −X-side end of the second conductor device  13 . The first conductor device  12  and the second conductor device  13  are electrically connected to each other by the folding portion  14 . 
       FIG. 2  is a diagram illustrating a positional relationship between the first conductor device and the second conductor device in a plan view of the antenna according to the embodiment from the +Y direction. In  FIG. 2 , the second conductor device  13  is shown by a dotted line. A center line that extends in a longitudinal direction of the first conductor device  12  and a center line that extends in a longitudinal direction of the second conductor device  13  overlap with each other in a plan view. An end in the −X direction of the first conductor device  12  and an end in the −X direction of the second conductor device  13  overlap with each other in a plan view. In addition, the second conductor device  13  is arranged so that an end in the +X direction of the second conductor device  13  approaches an end in the +X direction of the first conductor device  12  as much as possible. In other words, a length of the second conductor device  13  is designed so as to approach a length of the first conductor device  12  as much as possible. Therefore, the length of the first conductor device  12  can be described a length of the antenna  1 . 
     First Comparative Example 
     Comparative examples will now be considered.  FIG. 3  is a side view showing an example of an antenna according to a first comparative example. An antenna  100  illustrated in  FIG. 3  includes the feed line  11 , a first conductor device  102 , a second conductor device  103 , the folding portion  14 , the ground line  15 , and the ground substrate  3 . The antenna  100  differs from the antenna  1  according to the embodiment in that a width of the first conductor device  102  and a width of the second conductor device  103  are equal to each other due to the first conductor device  102  and the second conductor device  103  being formed in a linear shape instead of a plate shape. 
     Second Comparative Example 
       FIG. 4  is a side view showing an example of an antenna according to a second comparative example. An antenna  100   a  illustrated in  FIG. 4  includes the feed line  11 , the first conductor device  102 , the second conductor device  103 , the folding portion  14 , the ground line  15 , and the ground substrate  3 . The antenna  100   a  differs from the antenna  100  according to the first comparative example in that the first conductor device  102  is grounded by being connected to the ground surface  3   a  of the ground substrate  3  by the ground line  15  and the second conductor device  103  is connected to the feeding point  2  by the feed line  11 . In other words, the antenna  100   a  can be described an antenna created by swapping the feeding point and the ground of the antenna  100 . 
     Third Comparative Example 
       FIG. 5  is a diagram showing an example of an antenna according to a third comparative example. An antenna  110  illustrated in  FIG. 5  includes a feed line  111 , a first conductor device  112 , a second conductor device  113 , a folding portion  114 , a ground line  115 , and the ground substrate  3 . The antenna  110  differs from the antenna  1  according to the embodiment in that a distance between the first conductor device  112  and the ground surface  3   a  of the ground substrate  3  and a distance between the second conductor device  113  and the ground surface  3   a  are equal to each other, the first conductor device  112  is connected to the ground surface  3   a  via the ground line  115 , and the second conductor device  113  which is formed thinner than the first conductor device  112  is fed power from a feeding point (not illustrated) via the feed line  111 . Note that the antenna  110  according to the third comparative example is the antenna described in Japanese Patent Application Laid-open No. 2016-165035. 
     Fourth Comparative Example 
       FIG. 6  is a side view schematically showing an example of an antenna according to a fourth comparative example. An antenna  120  illustrated in  FIG. 6  includes the feed line  11 , the first conductor device  12 , the second conductor device  13 , the folding portion  14 , the ground line  15 , and the ground substrate  3 . In  FIG. 6 , widths of the first conductor device  12  and the second conductor device  13  are schematically shown by being replaced with heights of the first conductor device  12  and the second conductor device  13 . The antenna  120  differs from the antenna  1  according to the embodiment in that the first conductor device  12  is grounded by being connected to the ground surface  3   a  of the ground substrate  3  by the ground line  15  and the second conductor device  13  is connected to the feeding point  2  by the feed line  11 . In other words, the antenna  120  can be described an antenna created by swapping the feeding point and the ground of the antenna  1 . 
     Fifth Comparative Example 
       FIG. 7  is a side view schematically showing an example of an antenna according to a fifth comparative example. An antenna  130  illustrated in  FIG. 7  includes the feed line  11 , a first conductor device  132 , a second conductor device  133 , the folding portion  14 , the ground line  15 , and the ground substrate  3 . In  FIG. 7 , widths of the first conductor device  132  and the second conductor device  133  are schematically shown by being replaced with heights of the first conductor device  132  and the second conductor device  133  in a similar manner to  FIG. 6 . The antenna  130  differs from the antenna  1  according to the embodiment in that a width of the second conductor device  133  is five times a width of the first conductor device  132 . 
     Radiation Efficiency of Antennas 
     Radiation efficiency of the antenna  1  according to the embodiment and the antennas according to the comparative examples was evaluated. In the present evaluation, conductivity of the feed line  11 , the first conductor device, the second conductor device, the folding portion, and the ground line was set to 5.8×10 5  S/m and a distance between the first conductor device and the ground surface  3   a  was set to λ/30. 
     When mounting an antenna to a wireless communication apparatus, it is expected that a metal object such as a metal frame of the wireless communication apparatus, other electronic components, or the like is often present in a vicinity of the antenna. In consideration thereof, in the present evaluation, a case where a piece of metal is placed in a vicinity of the feeding point of the antenna (schematically shown in  FIG. 8 ), a case where a piece of metal is placed in a vicinity of the folding portion of the antenna (schematically shown in  FIG. 9 ), and a case where a piece of metal is placed in both a vicinity of the feeding point and a vicinity of the folding portion of the antenna (schematically shown in  FIG. 10 ) have also been evaluated. In this case, an interval D 1  between a piece of metal  401  placed in the vicinity of the feeding point of the antenna and the first conductor device is assumed to be 1 mm and an interval D 2  between a piece of metal  402  placed in the vicinity of the folding portion of the antenna and the folding portion is assumed to be 1 mm. The piece of metal  401  and the piece of metal  402  are grounded by being connected to the ground substrate  3 . 
     First Evaluation 
     In the first evaluation, an effect of forming a conductor device in a plate shape on radiation efficiency will be evaluated. In the first evaluation, a comparison of radiation efficiency was performed between the first comparative example and the embodiment.  FIG. 11  is a diagram showing a result of the first evaluation. Referring to  FIG. 11 , compared to the radiation efficiency of the antenna  100  according to the first comparative example being −4.5 dB, the radiation efficiency of the antenna  1  according to the embodiment is −3.0 dB. In other words, it can be understood that the antenna  1  according to the embodiment is capable of improving radiation efficiency by 1.5 dB as compared to the antenna  100  according to the first comparative example. 
     In addition, with the antenna  1  according to the embodiment, radiation efficiency is improved to −2.6 dB when placing the piece of metal  402  in the vicinity of the folding portion. Furthermore, it is to be understood that with the antenna  1  according to the embodiment, radiation efficiency hardly declines even when the piece of metal  401  is placed in the vicinity of the feeding point. 
     Second Evaluation 
     In a second evaluation, an effect of connecting the first conductor device to ground and connecting the second conductor device to the feeding point on radiation efficiency will be evaluated. In the second evaluation, a comparison of radiation efficiency was performed between the second comparative example and the fourth comparative example.  FIG. 12  is a diagram showing a result of the second evaluation. Referring to  FIG. 12 , the radiation efficiency of the antenna  100   a  according to the second comparative example is −4.9 dB and the radiation efficiency of the antenna  120  according to the fourth comparative example is −4.3 dB. In other words, it can be understood that the radiation efficiency is improved by 0.6 dB by forming the first conductor device and the second conductor device in a plate shape. However, it can be understood that placing the piece of metal  402  in the vicinity of the folding portion or placing the piece of metal  401  in the vicinity of the feeding point causes radiation efficiency of the antenna  120  according to a fourth modification in which the first conductor device and the second conductor device are formed in a plate shape to decline. In other words, it can be understood that, by connecting the first conductor device to ground and connecting the second conductor device to the feeding point, unlike the antenna  1  according to the embodiment, an improvement in radiation efficiency due to arranging the pieces of metal  401  and  402  in the vicinity of the folding portion or the vicinity of the feeding point is not to be expected. 
     Third Evaluation 
     In a third evaluation, an effect of making the width of the second conductor device larger than the width of the first conductor device on radiation efficiency will be evaluated. In the third evaluation, the first comparative example and the fifth comparative example were evaluated.  FIG. 13  is a diagram showing a result of the third evaluation. Referring to  FIG. 13 , the radiation efficiency of the antenna  100  according to the first comparative example is −4.5 dB and the radiation efficiency of the antenna  130  according to the fifth comparative example is −4.2 dB. In other words, it can be understood that the radiation efficiency is improved by 0.3 dB by making the width of the second conductor device larger than the width of the first conductor device. However, it can be understood that placing the piece of metal  402  in the vicinity of the folding portion or placing the piece of metal  401  in the vicinity of the feeding point causes radiation efficiency of the antenna  130  according to a fifth modification to decline. In other words, it can be understood that, by forming the width of the second conductor device being closer to the ground surface  3   a  to be larger than the width of the first conductor device, unlike the antenna  1  according to the embodiment, an improvement in radiation efficiency due to arranging the pieces of metal  401  and  402  in the vicinity of the folding portion or the vicinity of the feeding point is not to be expected. 
     Fourth Evaluation 
     In a fourth evaluation, radiation efficiency of the antenna  110  according to the third comparative example which has also been cited as Japanese Patent Application Laid-open No. 2016-165035 will be evaluated.  FIG. 14  is a diagram showing a result of the fourth evaluation. Referring to  FIG. 14 , it can be understood that the radiation efficiency of the antenna  110  according to the third comparative example is −4.3 dB. In other words, it can be understood that an improvement has been made from the radiation efficiency of the antenna  100  according to the first comparative example illustrated in  FIG. 11  or the radiation efficiency of the antenna  100   a  according to the second comparative example illustrated in  FIG. 12 . However, it can be understood that placing the piece of metal  402  in the vicinity of the folding portion or placing the piece of metal  401  in the vicinity of the feeding point causes radiation efficiency of the antenna  110  according to a third modification to decline. In other words, it can be understood that, by making the distance between the first conductor device and the ground surface  3   a  of the ground substrate  3  and the distance between the second conductor device and the ground surface  3   a  equal to each other, unlike the antenna  1  according to the embodiment, an improvement in radiation efficiency due to arranging the pieces of metal  401  and  402  in the vicinity of the folding portion or the vicinity of the feeding point is not to be expected. 
     Fifth Evaluation 
     In a fifth evaluation, with respect to the antenna  1  according to the embodiment, a relationship between a ratio of widths of the first conductor device  12  and the second conductor device  13  and the radiation efficiency of the antenna  1  will be evaluated.  FIG. 15  is a diagram illustrating a relationship between a ratio of widths of the first conductor device and the second conductor device and radiation efficiency. In  FIG. 15 , (width of first conductor device  12 : width of second conductor device  13 ) and radiation efficiency are associated with each other. As is evident from reference to  FIG. 15 , it can be understood that, making the width of the first conductor device  12  wider as compared to the width of the second conductor device  13  enables radiation efficiency of the antenna  1  to be improved. 
     According to the first to fourth evaluations, it can be understood that the radiation efficiency of the antenna  1  according to the embodiment illustrated in  FIG. 13  of −3.0 dB is higher than the radiation efficiency of the antennas according to any of the comparative examples. In addition, with the antennas according to any of the comparative examples, radiation efficiency declines when the pieces of metal  401  and  402  are arranged in a vicinity of the folding portion or a vicinity of the feeding point. On the other hand, with the antenna  1  according to the embodiment, radiation efficiency can be further enhanced by arranging the pieces of metal  401  and  402  in a vicinity of the folding portion or a vicinity of the feeding point. Note that, in order to enhance radiation efficiency, the interval D 1  between the piece of metal  401  and the first conductor device and the interval D 2  between the piece of metal  402  and the folding portion are preferably set to λ/50. The pieces of metal  401  and  402  represent an example of the “metal member”. 
     Relationship Between Current Intensity Distribution and Performance of Antenna  1   
     In order to evaluate a mechanism which enables the antenna  1  according to the embodiment to realize higher radiation efficiency than the antennas according to the comparative examples described above, a simulation of a current distribution in the antennas was performed. First, a current distribution in the antenna  100  according to the first comparative example and a current distribution in the antenna  1  according to the embodiment are compared. In the comparison, current distributions in a state where the pieces of metal  401  and  402  are arranged in the vicinity of the folding portion or the feeding point are compared. 
       FIG. 16  is a diagram illustrating a current distribution of the antenna according to the first comparative example, and  FIG. 17  is a diagram illustrating a current distribution of the antenna according to the embodiment.  FIGS. 16 and 17  illustrate that the larger a size of a triangle (Δ), the stronger the created current. In  FIG. 16 , a current distribution  402  illustrates a current distribution on the first conductor device  102 , a current distribution  403  illustrates a current distribution on the second conductor device  103 , and a current distribution  413  illustrates a current distribution on the ground substrate  3 . In addition, in  FIG. 17 , a current distribution  302  illustrates a current distribution on the first conductor device  12 , a current distribution  303  illustrates a current distribution on the second conductor device  13 , and a current distribution  313  illustrates a current distribution on the ground substrate  3 . 
     In the current distributions in the antenna  100  illustrated in  FIG. 16 , it can be understood that a current is distributed in a concentrated manner on the first conductor device  102  and the second conductor device  103  which are formed in a linear shape while a current of which an orientation is opposite to that of the current flowing through the second conductor device  103  is intensely distributed on the ground surface  3   a . On the other hand, in the current distributions in the antenna  1  illustrated in  FIG. 17 , it can be understood that a current distribution is dispersed on the first conductor device  12  which is formed wider than the second conductor device  13  and a current which is created on the ground surface  3   a  and of which an orientation is opposite to that of the current flowing through the second conductor device  13  is dispersed over a wider range than the antenna  100  illustrated in  FIG. 16 . 
     Furthermore, with respect to the first conductor device and the second conductor device, current distributions in a case where the conductor device to be made wider is swapped or a case where the conductor device to be connected to the feeding point and the conductor device to be connected to ground are swapped will be evaluated. For this evaluation, an antenna  130   a  (illustrated in  FIG. 18 ) according to a sixth comparative example will be considered which is created by, in the antenna  130  according to the fifth comparative example, connecting the first conductor device  132  to the ground surface  3   a  of the ground substrate  3  with the ground line  15  and connecting the second conductor device  133  to the feeding point  2  by the feed line  11 . 
     In other words, by comparing the antenna  1  according to the embodiment, the antenna  120  according to the fourth comparative example, the antenna  130  according to the fifth comparative example, and the antenna  130   a  according to the sixth comparative example, with respect to the first conductor device and the second conductor device, current distributions in a case where the conductor device to be made wider is swapped or a case where the conductor device to be connected to the feeding point and the conductor device to be connected to ground are swapped will be evaluated. 
       FIG. 19  is a diagram schematically showing an orientation of a current in the antenna according to the embodiment. In  FIG. 19 , an orientation of a current is illustrated by an arrow. As is evident from reference to  FIG. 19 , a current that flows through the first conductor device  12  and a current that flows through the second conductor device  13  have opposite orientations. In this case, the first conductor device  12  is designed wider than the second conductor device  13  and the first conductor device  12  is connected to the feeding point  2 . Consequently, a stronger current is to flow through the first conductor device  12  than the second conductor device  13 . Therefore, a current of which an orientation is opposite to that of the current flowing through the first conductor device  12  is also created on the ground surface  3   a  despite the first conductor device  12  being farther away from the ground surface  3   a  than the second conductor device  13 . In other words, in the antenna  1  according to the embodiment, a current with a same orientation as the current flowing through the second conductor device  13  is also created on the ground surface  3   a . On the ground surface  3   a , an intensity of the current flowing in an orientation which is opposite to that of the current flowing through the second conductor device  13  is to be weakened. 
       FIG. 20  is a diagram schematically showing an orientation of a current in the antenna according to the fourth comparative example. In the antenna  120  according to the fourth comparative example, the first conductor device  12  is connected to the ground surface  3   a  of the ground substrate  3  and the second conductor device  13  is connected to the feeding point  2 . Consequently, a stronger current is to flow through the second conductor device  13  than the first conductor device  12 . Since the second conductor device  13  is closer to the ground surface  3   a  and a stronger current is to flow through the second conductor device  13  than the first conductor device  12 , the influence of the first conductor device  12  with respect to the current flowing through the ground surface  3   a  is reduced as compared to the antenna  1  according to the embodiment. Therefore, due to the ground surface  3   a  being strongly affected by the current flowing through the second conductor device  13 , a current of which an orientation is opposite to that of the current flowing through the second conductor device  13  is created on the ground surface  3   a.    
       FIG. 21  is a diagram schematically showing an orientation of a current in the antenna according to the fifth comparative example. In the antenna  130  according to the fifth comparative example, the second conductor device  133  is designed wider than the first conductor device  132  as described earlier. Consequently, a stronger current more readily flows through the second conductor device  133 . In addition, the second conductor device  133  is provided at a position closer to the ground surface  3   a  than the first conductor device  132 . Therefore, due to the ground surface  3   a  being strongly affected by the current flowing through the second conductor device  133 , a current of which an orientation is opposite to that of the current flowing through the second conductor device  133  is created on the ground surface  3   a.    
       FIG. 22  is a diagram schematically showing an orientation of a current in the antenna according to the sixth comparative example. In the antenna  130   a  according to the sixth comparative example, the first conductor device  132  is connected to the ground surface  3   a  and the second conductor device  133  is connected to the feeding point  2  as described earlier. Consequently, in the antenna  130   a , a stronger current more readily flows through the second conductor device  133  than in the antenna  130  according to the fifth comparative example. Therefore, due to the ground surface  3   a  being strongly affected by the current flowing through the second conductor device  133 , a current of which an orientation is opposite to that of the current flowing through the second conductor device  133  is created on the ground surface  3   a.    
     In an antenna, when orientations of a current flowing through the second conductor device and a current flowing through the ground surface  3   a  of the ground substrate  3  are opposite orientations, generated heat loss causes radiation efficiency of the antenna to decline. In all of the fourth comparative example, the fifth comparative example, and the sixth comparative example, due to a current which flows through the ground surface  3   a  and of which an orientation is opposite to that of the current which flows through the second conductor device, radiation efficiency declines due to heat loss. On the other hand, in the antenna  1  according to the embodiment, since intensity of the current which flows in an orientation opposite to that of the current flowing through the second conductor device  13  is weakened as described above, a decline in radiation efficiency due to heat loss can be reduced as compared to the fourth comparative example, the fifth comparative example, and the sixth comparative example. In other words, the antenna  1  according to the embodiment is capable of realizing higher radiation efficiency than the antennas according to any of the fourth comparative example, the fifth comparative example, and the sixth comparative example. 
     Advantageous Effect of Embodiment 
     In the antenna  1  according to the embodiment, the first conductor device  12  is connected to the feeding point  2  and, at the same time, a width of the first conductor device  12  is designed wider than that of the second conductor device  13 . As a result, a stronger current is to flow through the first conductor device  12  than the second conductor device  13  and a current which flows in an orientation opposite to that of the current flowing through the first conductor device  12  can be created on the ground surface  3   a . Accordingly, on the ground surface  3   a , a current flowing in an orientation which is opposite to that of the current flowing through the second conductor device  13  is to be weakened and a decline in radiation efficiency due to heat loss can be suppressed. 
     In addition, according to the first to fourth evaluations, when the pieces of metal  401  and  402  are present in a vicinity of an antenna, while radiation efficiency declines in antennas according to comparative examples, the antenna  1  according to the embodiment is capable of further enhancing radiation efficiency. 
     First Modification 
     The antenna  1  according to the embodiment can be modified in various ways.  FIG. 23  is a diagram schematically showing an antenna according to a first modification. An antenna  1   a  according to the first modification differs from the antenna  1  according to the embodiment in that the antenna  1   a  further includes a third conductor device  12   a  which extends in a +X direction from the +X-side end of the first conductor device  12 . A dotted line L 1  in  FIG. 23  schematically shows a boundary between the first conductor device  12  and the third conductor device  12   a . A length (a length in the X direction) of the third conductor device  12   a  may be set to ¼ or less of a wavelength λ 3  of a radio wave which resonates the third conductor device  12   a . By adopting such a design, the third conductor device  12   a  can be operated as a monopole antenna which resonates with a radio wave with a wavelength of λ 3 . In the first modification, the piece of metal  402  may be arranged in a vicinity of the folding portion  14 . For example, the piece of metal  402  may be an antenna which differs from the antenna  1   a.    
     Second Modification 
       FIG. 24  is a diagram schematically showing an antenna according to a second modification. An antenna  1   b  according to the second modification differs from the antenna  1  according to the embodiment in that the antenna  1   b  further includes a fourth conductor device  17  which is connected to the first conductor device  12  via a connecting line  16  which extends in a +Y direction from the X-side end of the first conductor device  12  and which extends in the −X direction from the connecting line  16 . A length (a length in the X direction) of the fourth conductor device  17  may be set to ¼ or less of a wavelength λ 4  of a radio wave which resonates the fourth conductor device  17 . By adopting such a design, the fourth conductor device  17  can be operated as a monopole antenna which resonates with a radio wave with a wavelength of A. The fourth conductor device  17  is an example of the “third conductor device”. 
     Third Modification 
       FIG. 25  is a diagram schematically showing an antenna according to a third modification. An antenna  1   c  according to the third modification differs from the antenna  1  according to the embodiment in that the antenna  1   c  is provided with a fifth conductor device  18  which is connected to the ground surface  3   a  by a ground line  11   a . The fifth conductor device  18  is not in contact with any of the first conductor device  12 , the second conductor device  13 , the folding portion  14 , the feed line  11 , and the ground line  15 . A length (a length in the X direction) of the fifth conductor device  18  may be set to ¼ or less of a wavelength λ 5  of a radio wave which resonates the fifth conductor device  18 . By adopting such a design, the fifth conductor device  18  can be operated as a monopole antenna which resonates with a radio wave with a wavelength of λ 5 . The fifth conductor device  18  is an example of the “fourth conductor device”. 
     Fourth Modification 
       FIG. 26  is a diagram schematically showing an antenna according to a fourth modification. In an antenna  1   d  according to the fourth modification, a capacitor  71  is provided on the feed line  11 . In addition, in the feed line  11 , a ground line  15   a  is provided which branches from between the capacitor  11  and the first conductor device  12  and which is connected to the ground surface  3   a . An inductor  72  is provided on the ground line  15   a . For example, the capacitor  71  is a loading coil. In addition, for example, the inductor  72  is an extension inductor. By appropriately determining a capacitance of the capacitor  71  and an inductance of the inductor  72 , a wavelength of a radio wave with which the antenna  1   d  resonates can be changed. 
     Fifth Modification 
       FIG. 27  is a diagram schematically showing an antenna according to a fifth modification. In an antenna  1   e  according to the fifth modification, a capacitor  71   a  is provided on the ground line  15 . In the fifth modification, by appropriately determining a capacitance of the capacitor  71   a , a wavelength of a radio wave with which the antenna  1   e  resonates can be changed. 
     Sixth Modification 
       FIG. 28  is a diagram schematically showing an antenna according to a sixth modification. In an antenna  1   f  according to the sixth modification, an inductor  72   a  is provided on the ground line  15 . In the sixth modification, by appropriately determining an inductance of the inductor  72   a , a wavelength of a radio wave with which the antenna  1   f  resonates can be changed. 
     Seventh Modification 
       FIG. 29  is a diagram schematically showing an antenna according to a seventh modification. In an antenna  1   g  according to a seventh modification, the first conductor device  12   a  is given a meander shape. Although the first conductor device  12   a  is given a meander shape in  FIG. 29 , alternatively, the second conductor device  13  may be given a meander shape or parts of the first conductor device  12  and the second conductor device  13  may be given a meander shape. Adopting a meander shape enables the antenna  1   g  to be downsized. 
     Application Example 
       FIG. 30  is a diagram illustrating a configuration in which the antenna according to the embodiment is applied to a smartphone.  FIG. 30  illustrates a state where a display-side exterior of a smartphone  500  has been removed. In the smartphone  500 , a side surface is surrounded by a frame-like metal frame  510 . The ground substrate  3  is provided in a region defined by the metal frame  510 . In the smartphone  500 , a region partitioned by slits  511  and  512  among the metal frame  510  is used as the first conductor device  12 . In addition, the second conductor device  13  can be formed by a conductor pattern on a flexible substrate arranged inside the region defined by the metal frame  510  or by Laser Direct Structuring (LDS). 
     As described earlier, radiation efficiency of the antenna  1  can be enhanced due to the presence of metal in a vicinity of the feeding point  2  or a vicinity of the folding portion  14 . Therefore, in the smartphone  500  according to the application example, the radiation efficiency of the antenna  1  can be enhanced due to the present of the metal frame  510  in the vicinity of the antenna  1 . In addition to the smartphone  500 , the antenna  1  according to the embodiment can be applied to wireless communication apparatuses such as tablet computers, mobile phones, and vehicle-mounted antennas. 
       FIG. 31  is a diagram illustrating S 11  of an antenna mounted to a smartphone according to an application example. In addition,  FIG. 32  is a diagram illustrating a Smith chart of the antenna mounted to the smartphone according to the application example.  FIGS. 31 and 32  illustrate a case where an antenna is not provided with a matching circuit. For reference&#39;s sake,  FIGS. 31 and 32  also illustrate data of a case where the antenna  1  is not mounted to the smartphone  500  (in other words, a case where the metal frame  510  is not present in the vicinity of the antenna  1 ) and data of the antenna  100  according to the first comparative example in which widths of the first conductor device  12  and the second conductor device  13  are set equal to each other. In  FIGS. 31 and 32 , a double dot chain line illustrates data of a case where the antenna  1  is mounted to the smartphone  500 . In addition, a solid line illustrates data of a case where the metal frame  510  is not present in the vicinity of the antenna  1 . Among two dotted lines, a dotted line depicted by large dots illustrates data of a case where the antenna  100  is mounted to the smartphone  500 . Among the two dotted lines, a dotted line depicted by small dots illustrates data of a case where the metal frame  510  is not present in the vicinity of the antenna  100 . 
     As is evident from reference to  FIG. 31 , in a case where the antenna  1  is mounted to the smartphone  500  and a case where the metal frame  510  is not present in the vicinity of the antenna  1 , frequencies at which S 11  drops are both in the vicinity of 1.6 GHz. On the other hand, it can be understood that, in a case where the antenna  100  is mounted to the smartphone  500  and a case where the metal frame  510  is not present in the vicinity of the antenna  100 , frequencies at which S 11  drops have shifted to higher frequencies than 1.6 GHz. 
     In addition, as is evident from reference to  FIG. 32 , it can be understood that, in all of a case where the metal frame  510  is not present in a vicinity of the antenna  1 , a case where the antenna  100  is mounted to the smartphone  500 , and a case where the metal frame  510  is not present in a vicinity of the antenna  100 , radiation resistance has declined as compared to a case where the antenna  1  is mounted to the smartphone  500 . 
     From  FIGS. 31 and 32 , it can be understood that radiation resistance can be increased by placing a piece of metal in a vicinity of the antenna  1 . It can also be understood that, by designing the first conductor device  12  to be wider than the second conductor device  13 , a resonating frequency can be shifted and radiation resistance can be increased. 
       FIG. 33  is a diagram illustrating S 11  of an antenna mounted to a smartphone according to an application example. In addition,  FIG. 34  is a diagram illustrating total efficiency of the antenna mounted to the smartphone according to the application example.  FIGS. 33 and 34  illustrate a case where the antenna  1  is provided with a matching circuit. An ordinate in  FIG. 33  illustrates  511  (dB) while an abscissa illustrates frequency (GHz). An ordinate in  FIG. 34  illustrates total efficiency (dB) while an abscissa illustrates frequency (GHz). 
     With reference to  FIGS. 33 and 34 , with the antenna  1 , the graph of S 11  bottoms out near 1.5 GHz while the graph of total efficiency peaks near 1.5 GHz. In other words, it can be understood that the antenna  1  exhibits preferable performance with respect to frequencies near 1.5 GHz. Note that total efficiency near 1.5 GHz when the antennas cited as the respective comparative examples described earlier are applied to the smartphone  500  is around −8 dB. On the other hand, total efficiency near 1.5 GHz when the antenna  1  is applied to the smartphone  500  is −2 dB. In other words, it can be understood that radiation efficiency is improved by around 6 dB with the antenna  1  at frequencies near 1.5 GHz over the antennas according to the respective comparative examples. 
     The embodiment and the modifications disclosed above can be combined with each other. 
     The disclosed technique enables a reduction in radiation efficiency of an antenna to be suppressed even when metal is present in a vicinity of the antenna. 
     All examples and conditional language provided herein are intended for the pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although one or more embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.