Patent Publication Number: US-11031694-B2

Title: Antenna

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation application of International Application PCT/JP2018/018107, filed on May 10, 2018 which claims the benefit of priority from Japanese Patent application No.2017-149871 filed on Aug. 2, 2017 and designating the U.S., the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an antenna. 
     2. Description of the Related Art 
     Some conventional antennas receive circularly polarized waves. For example, Japanese Patent Application Laid-open No. 2007-128321 describes a patch antenna that receives a right-handed circularly polarized wave transmitted from an electronic toll collection system (ETC). 
     Unfortunately, the patch antenna of Japanese Patent Application Laid-open No. 2007-128321 occasionally receives a right-handed circularly polarized wave and a left-handed circularly polarized wave at the same time, which may reduce the level of discrimination between the circularly polarized waves. There remains room for improvement in this point. 
     SUMMARY OF THE INVENTION 
     To overcome the above problem, the present invention aims to provide an antenna capable of properly receiving a circularly polarized wave to be received. 
     In order to solve the above mentioned problem and achieve the object, an antenna according to the present invention includes an outer conductor formed of a first linear conductor, the first linear conductor having a length corresponding to one wavelength of either one of a right-handed circularly polarized wave and a left-handed circularly polarized wave, circularly extended from a first end to a second end, and causing current to flow between the first end and the second end; and an inner conductor disposed inside the outer conductor, the inner conductor including a curved portion formed with a second linear conductor curvedly extended between a starting point and an end point, the second linear conductor having a length determined based on one wavelength of another one of the right-handed circularly polarized wave and the left-handed circularly polarized wave, and being different from the first linear conductor, the inner conductor having the starting point connected to either one of the first end and the second end, having the end point kept free from connection at a location inside the outer conductor, and causing current to flow in a direction opposite to a flow in the outer conductor. 
     According to another aspect of the present invention, in the antenna, it is preferable that the outer conductor and the inner conductor are mounted on a mounting surface, when the outer conductor receives the right-handed circularly polarized wave, the inner conductor is extended counterclockwise from the starting point to the end point in a top-down view of the mounting surface, and when the outer conductor receives the left-handed circularly polarized wave, the inner conductor is extended clockwise from the starting point to the end point in a top-down view of the mounting surface. 
     According to still another aspect of the present invention, in the antenna, it is preferable that the inner conductor has a circular portion circularly formed as the curved portion. 
     According to still another aspect of the present invention, in the antenna, it is preferable that the inner conductor has a rectangular portion rectangularly formed as the curved portion. 
     According to still another aspect of the present invention, in the antenna, it is preferable that the inner conductor has an L-shaped portion formed in a shape of L, as the curved portion. 
     The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front view of an example configuration of an antenna according to a first embodiment; 
         FIG. 2  is a graph of cross-polarization discrimination (XPD) of the antenna according to the first embodiment; 
         FIG. 3  is a graph of the voltage standing wave ratio (VSWR) of the antenna according to the first embodiment; 
         FIG. 4  is a Smith chart that illustrates the characteristic impedance of the antenna according to the first embodiment; 
         FIG. 5  is a graph of the axial ratio of the antenna according to the first embodiment; 
         FIG. 6  is a chart that illustrates directivity of the antenna according to the first embodiment; 
         FIG. 7  is a front view of an example configuration of an antenna according to a first modification of the first embodiment; 
         FIG. 8  is a graph of XPD values of the antenna according to the first modification of the first embodiment; 
         FIG. 9  is a graph of the VSWR of the antenna according to the first modification of the first embodiment; 
         FIG. 10  is a Smith chart that illustrates the characteristic impedance of the antenna according to the first modification of the first embodiment; 
         FIG. 11  is a graph of the axial ratio of the antenna according to the first modification of the first embodiment; 
         FIG. 12  is a chart that illustrates directivity of the antenna according to the first modification of the first embodiment; 
         FIG. 13  is a front view of an example configuration of an antenna according to a second modification of the first embodiment; 
         FIG. 14  is a graph of XPD values of the antenna according to the second modification of the first embodiment; 
         FIG. 15  is a graph of the VSWR of the antenna according to the second modification of the first embodiment; 
         FIG. 16  is a Smith chart that illustrates the characteristic impedance of the antenna according to the second modification of the first embodiment; 
         FIG. 17  is a chart that illustrates directivity of the antenna according to the second modification of the first embodiment; 
         FIG. 18  is a front view of an example configuration of an antenna according to a third modification of the first embodiment; 
         FIG. 19  is a graph of XPD values of the antenna according to the third modification of the first embodiment; 
         FIG. 20  is a graph of the VSWR of the antenna according to the third modification of the first embodiment; 
         FIG. 21  is a Smith chart that illustrates the characteristic impedance of the antenna according to the third modification of the first embodiment; 
         FIG. 22  is a graph of the axial ratio of the antenna according to the third modification of the first embodiment; 
         FIG. 23  is a chart that illustrates directivity of the antenna according to the third modification of the first embodiment; 
         FIG. 24  is a front view of an example configuration of an antenna according to a fourth modification of the first embodiment; 
         FIG. 25  is a graph of XPD values of the antenna according to the fourth modification of the first embodiment; 
         FIG. 26  is a graph of the VSWR of the antenna according to the fourth modification of the first embodiment; 
         FIG. 27  is a Smith chart that illustrates the characteristic impedance of the antenna according to the fourth modification of the first embodiment; 
         FIG. 28  is a graph of the axial ratio of the antenna according to the fourth modification of the first embodiment; 
         FIG. 29  is a chart that illustrates directivity of the antenna according to the fourth modification of the first embodiment; 
         FIG. 30  is a front view of an example configuration of an antenna according to a second modification; 
         FIG. 31  is a graph of XPD values of the antenna according to the second embodiment; 
         FIG. 32  is a graph of the VSWR of the antenna according to the second embodiment; 
         FIG. 33  is a Smith chart that illustrates the characteristic impedance of the antenna according to the second embodiment; 
         FIG. 34  is a graph of the axial ratio of the antenna according to the second embodiment; 
         FIG. 35  is a chart that illustrates directivity of the antenna according to the second embodiment; 
         FIG. 36  is a front view of an example configuration of an antenna according to a modification of the second embodiment; 
         FIG. 37  is a graph of XPD values of the antenna according to the modification of the second embodiment; and 
         FIG. 38  is a graph of the VSWR of the antenna according to the modification of the second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will now be described in detail with reference to the drawings. The following description of the embodiments is not intended to limit the present invention. Components in the following description include what are easily conceived by the skilled person and what are substantially the same. The configurations described below can be combined as appropriate. Various omissions, substitutions, and changes in the configurations can be made without departing from the scope of the present invention. 
     First Embodiment 
     An antenna  1  according to a first embodiment will now be described. The antenna  1  is, for example, an antenna to receive a right-handed circularly polarized wave of a global positioning system (GPS). The right-handed circularly polarized wave of the GPS has, for example, a frequency of 1.575 GHz. The antenna  1  is made by, for example, printing conductor patterns in silver paste or the like on a polyethylene terephthalate (PET) film; however, without being limited thereto, the antenna  1  may be made using conductive ink, conductive thin film, and others. The antenna  1  is, for example, mounted on a vehicle, particularly, mounted on a dielectric mounting surface  2  such as the inside of the roof, the front windshield, the instrument panel (made of resin) of the vehicle. The antenna  1  will now be described in detail. 
     As illustrated in  FIG. 1 , the antenna  1  includes an outer conductor  10 , first and second feedlines  21  and  22 , and an inner conductor  30 . The outer conductor  10  is, for example, an antenna to receive a right-handed circularly polarized wave of a GPS. The outer conductor  10  is arranged on the mounting surface  2  and includes a first feed point  11  at an end thereof and a second feed point  12  at the other end thereof, and a body  13 . In the first embodiment, for example, the first feed point  11  is the negative electrode, and the second feed point  12  is the positive electrode. The body  13  is formed of a first linear conductor circularly extended from the first feed point  11  to the second feed point  12 . The first linear conductor has a length corresponding to one wavelength of the right-handed circularly polarized wave of a GPS. The body  13  has a gap between the first feed point  11  and the second feed point  12 . Current travels in the outer conductor  10 , specifically, between the first feed point  11  and the second feed point  12  along the circumferential direction of the body  13 . In the first embodiment, since the outer conductor  10  receives a right-handed circularly polarized wave of a GPS, current travels clockwise between the first feed point  11  and the second feed point  12  in the top-down view of the mounting surface  2 . In other words, when the outer conductor  10  receives the right-handed circularly polarized wave, current flows from the second feed point  12 , as the positive electrode, toward the first feed point  11 , as the negative electrode. 
     The first and second feedlines  21  and  22  are, for example, conductive wires to pass current received by the body  13 . The first feedline  21  has an end connected to the first feed point  11  of the outer conductor  10  and has the other end to a receiving circuitry (not illustrated). Likewise, the second feedline  22  has an end connected to the second feed point  12  of the outer conductor  10  and has the other end to the receiving circuitry. The first and second feedlines  21  and  22  pass current received by the body  13  to the receiving circuitry. 
     The inner conductor  30  is used to control receipt of a left-handed circularly polarized wave. The inner conductor  30  is mounted on the mounting surface  2 , inside the outer conductor  10 , and includes a circular portion  31  as a curved portion and a connection portion  32 . The circular portion  31  and the connection portion  32  are formed of a second linear conductor different from the first linear conductor. The second linear conductor has a length determined based on one wavelength of a left-handed circularly polarized wave of a GPS. The circular portion  31  is circularly formed with a starting point  31   a  of the second linear conductor connected to the first feed point  11  as the negative electrode through the connection portion  32  and with an end point  31   b  of the second linear conductor kept free from connection at a location inside the outer conductor  10 . The circular portion  31  has a gap between the starting point  31   a  and the end point  31   b.  The inner conductor  30  is designed such that current flows in a direction opposite to the current flow in the outer conductor  10 . Specifically, the circular portion  31  of the inner conductor  30  is extended counterclockwise from the starting point  31   a  to the end point  31   b  along the circumferential direction of the outer conductor  10 , in the top-down view of the mounting surface  2 . Current flows in the inner conductor  30  from the starting point  31   a  toward the end point  31   b  along the circumferential direction of the circular portion  31 . In other words, in the top-down view of the mounting surface  2 , current flows counterclockwise in the inner conductor  30  from the starting point  31   a  connected to the first feed point  11  toward the end point  31   b  kept free from connection. The connection portion  32  connects the starting point  31   a  of the circular portion  31  and the first feed point  11  of the outer conductor  10 . The connection portion  32  is extended along the radial direction of the outer conductor  10 . 
     Simulations have been conducted on the antenna  1  of the first embodiment, and the results of the simulations will now be described. In the first embodiment, the antenna  1  for the simulations was prepared by printing 1-mm width patterns of the antenna  1  on a 0.25-mm thick PET film using 0.01-mm thick silver paste and arranging the resulting film between 0.1-mm thick PET films in the vertical direction. The permittivity of the PET film is “3”, and the connection portion  32  for connecting the inner conductor  30  and the outer conductor  10  has a length of 1 mm.  FIG. 2  is a graph of values of cross polarization discrimination (XPD) of the antenna  1  of when the radius R of the inner conductor  30  is changed from 8 mm to 11 mm at intervals of approximately 0.5 mm. In  FIG. 2 , the y-axis represents the XPD value, and the x-axis represents the frequency. In  FIG. 2 , the simulations demonstrate that the antenna  1  has the largest XPD value, approximately 25 dB (P 1  in the graph), at a frequency of 1.6 GHz in use of the inner conductor  30  having a radius R of 8 mm. The result indicates that the gain of the left-handed circularly polarized wave is low.  FIG. 3  is a graph of the voltage standing wave ratio (VSWR) of the antenna  1  of when the radius R of the inner conductor  30  is changed from 8 mm to 11 mm at intervals of approximately 0.5 mm. In  FIG. 3 , the y-axis represents the VSWR, and the x-axis represents the frequency. In  FIG. 3 , the simulations demonstrate that the antenna  1  has a VSWR of approximately 5.6 (P 2  in the graph) at a frequency of 1.6 GHz, in use of the inner conductor  30  having a radius R of 8 mm. The result indicates that the electrical efficiency is relatively low.  FIG. 4  is a Smith chart that illustrates the characteristic impedance of when the inner conductor  30  has a radius R of 8 mm. In  FIG. 4 , the simulation using the inner conductor  30  having an 8-mm radius R demonstrates that the antenna  1  has a magnitude of reflection of approximately 0.69 and a phase of approximately −58 (P 3  in the graph) at a frequency of 1.6 GHz. The results indicate that reflection is relatively large.  FIG. 5  is a graph of the axial ratio (AR) of when the inner conductor  30  has a radius R of 8 mm. In  FIG. 5 , the y-axis represents the axial ratio, and the x-axis represents the frequency. In  FIG. 5 , the simulation using the inner conductor  30  having an 8-mm radius R demonstrates that the antenna  1  has an axial ratio of approximately 1.1 dB (P 4  in the graph) at a frequency of 1.6 GHz. The result indicates that the axial ratio is good.  FIG. 6  is a chart that illustrates directivity of when the inner conductor  30  has a radius R of 8 mm. In  FIG. 6 , the simulation using the inner conductor  30  having an 8-mm radius R demonstrates that a right-handed circularly polarized wave and a left-handed circularly polarized wave are symmetrical to each other and that there is a symmetry in directivity between the circularly polarized waves. The symmetry allows the outer conductor  10  to receive the left-handed circularly polarized wave with the antenna  1  turned over. In receiving the left-handed circularly polarized wave, the inner conductor  30  has the circular portion  31  extended clockwise from the starting point  31   a  to the end point  31   b , in the top-down view of the mounting surface  2 . 
     As described above, the antenna  1  according to the first embodiment includes the outer conductor  10  and the inner conductor  30 . The outer conductor  10  is formed of the first linear conductor having a length corresponding to one wavelength of the right-handed circularly polarized wave and circularly extended from the first feed point  11  to the second feed point  12 . Current flows between the first feed point  11  and the second feed point  12 . The inner conductor  30  is disposed inside the outer conductor  10 , and is formed of the second linear conductor. The second linear conductor is another conductor different from the first linear conductor and has a length determined based on one wavelength of the left-handed circularly polarized wave. The second linear conductor of the inner conductor  30  has the starting point  31   a  connected to the first feed point  11  and has the end point  31   b  kept free from connection at a location inside the outer conductor  10 . The inner conductor  30  has a circular portion  31  as a curved portion curvedly formed between the starting point  31   a  and the end point  31   b  and is designed such that current flows in a direction opposite to the current flow in the outer conductor  10 . 
     With the antenna  1  configured as above, current of the right-handed circularly polarized wave flows into the outer conductor  10 , and current of the left-handed circularly polarized wave flows into the inner conductor  30 . The antenna  1  configured as above can keep current of the left-handed circularly polarized wave from flowing into the outer conductor  10 . This flow control of the antenna  1  can increase the gain of the right-handed circularly polarized wave. Consequently, the antenna  1  can improve XPD and properly receive the right-handed circularly polarized wave. The circular shape of the outer conductor  10  of the antenna  1  is advantageous in acquiring good values of the axial ratio, which represents the roundness of the right-handed circularly polarized wave. The antenna  1  is produced, for example, by printing the first and the second linear conductors. The method can reduce the number of production processes and thus reduce the cost of production compared with a conventional method of assembling the antenna  1 . Since there is no necessity of using a member (fixing stay) to fix the antenna  1 , as used for a conventional antenna, the method of printing is beneficial in reducing the number of components of the antenna  1 . Furthermore, the antenna  1  is thinner and more flexible than a conventional patch antenna, which can increase conformability of the antenna  1  to the place of installation. For example, the antenna  1  can be installed inside the roof of a vehicle. 
     The above antenna  1  has the outer conductor  10  and the inner conductor  30  mounted on the mounting surface  2 . The outer conductor  10  receives a right-handed circularly polarized wave with the inner conductor  30  extended counterclockwise from the starting point  31   a  to the end point  31   b,  in the top-down view of the mounting surface  2 . The antenna  1  configured as above allows current of a left-handed circularly polarized wave to flow into the inner conductor  30  while keeping the current from flowing into the outer conductor  10 . This structure can improve XPD. 
     The inner conductor  30  of the antenna  1  has a circularly formed circular portion  31  as the curved portion. The antenna  1  configured as above allows current of a left-handed circularly polarized wave to flow into the circular portion  31  of the inner conductor  30  while keeping the current from flowing into the outer conductor  10 . This structure can improve XPD. 
     First Modification of First Embodiment 
     An antenna  1 A according to a first modification of the first embodiment will now be described. In the first modification, like reference numerals indicate like components of the first embodiment, and detailed description thereof will be omitted. The antenna  1 A of the first modification is different from the antenna of the first embodiment in that a length H of a connection portion  32 A, connecting the inner conductor  30  and the outer conductor  10 , is changed from 1 mm to 10 mm at intervals of 1 mm. Compared to the antenna  1  of the first embodiment, the antenna  1 A is configured such that the inner conductor  30  is located closer to the center of the outer conductor  10  by a distance consistent with an increase in the length of the connection portion  32 A from 1 mm to 10 mm along the radial direction.  FIG. 7  is a drawing of the antenna  1 A of when the connection portion  32 A has a length H of 8 mm.  FIG. 8  is a graph of XPD values of the antenna  1 A of when the length H of the connection portion  32 A is changed from 1 mm to 10 mm at intervals of 1 mm. In  FIG. 8 , the y-axis represents the XPD value, and the x-axis represents the frequency. In  FIG. 8 , the simulations demonstrate that the antenna  1 A has the largest XPD value, approximately 19 dB (P 5  in the graph), at a frequency of 1.6 GHz, in use of the connection portion  32 A having a length H of 1 mm. The result indicates that the gain of the left-handed circularly polarized wave is low.  FIG. 9  is a graph of the VSWR of the antenna  1 A of when the length H of the connection portion  32 A is changed from 1 mm to 10 mm at intervals of 1 mm. In  FIG. 9 , the y-axis represents the VSWR, and the x-axis represents the frequency. In  FIG. 9 , the simulations demonstrate that the antenna  1 A has a VSWR of approximately 4.5 (P 6  in the graph) at a frequency of 1.6 GHz, in use of the connection portion  32 A having a length H of 1 mm. The result indicates that the electrical efficiency is relatively low. At a frequency of 1.6 GHz and a length H of the connection portion  32 A of 8 mm, the VSWR is approximately 2.0 (P 7  in the graph), which indicates that the electrical efficiency is relatively high, and XPD has a relatively good value, approximately 11.5 dB (P 8  in the graph). These results indicate that the antenna  1 A is well balanced when the length H of the connection portion  32 A is 8 mm.  FIG. 10  is a Smith chart that illustrates the characteristic impedance of when the connection portion  32 A has a length H of 8 mm. In  FIG. 10 , the simulation using the connection portion  32 A having an 8-mm length H demonstrates that the magnitude of reflection is approximately 0.2 and the phase is approximately −74 (P 9  in the graph) at a frequency of 1.6 GHz. The results indicate that reflection is relatively small compared with the antenna  1  of the first embodiment.  FIG. 11  is a graph of the axial ratio in use of the connection portion  32 A having a length H of 8 mm. In  FIG. 11 , the y-axis represents the axial ratio, and the x-axis represents the frequency. In  FIG. 11 , the simulation using the connection portion  32 A having an 8-mm length H demonstrates that the antenna  1 A has an axial ratio of approximately 1.8 dB (P 10  in the graph) at a frequency of 1.6 GHz. The result indicates that the axial ratio is worse than that of the antenna  1  of the first embodiment.  FIG. 12  is a chart that illustrates directivity of when the connection portion  32 A has a length H of 8 mm. In  FIG. 12 , the simulation using the connection portion  32 A having an 8-mm length H demonstrates that the right-handed circularly polarized wave and the left-handed circularly polarized wave are symmetrical to each other and that there is a symmetry in directivity between the circularly polarized waves. The symmetry allows the outer conductor  10  to receive the left-handed circularly polarized wave with the antenna  1 A turned over. In receiving the left-handed circularly polarized wave, the inner conductor  30  has the circular portion  31  extended clockwise from the starting point  31   a  to the end point  31   b,  in the top-down view of the mounting surface  2 . 
     As described above, the antenna  1 A according to the first modification of the first embodiment includes the outer conductor  10  having a length corresponding to one wavelength of the right-handed circularly polarized wave of a GPS and includes the inner conductor  30  having a length determined based on one wavelength of the left-handed circularly polarized wave of the GPS and consisting of the circular portion  31  and the connection portion  32 A. The connection portion  32 A of the antenna  1 A has a length H of 8 mm. The above configuration allows the antenna  1 A to have a smaller VSWR than that of the antenna  1  of the first embodiment, which means that higher electrical efficiency is achieved with the antenna  1 A than with the antenna  1  of the first embodiment. Although the value of XPD of the antenna  1 A is smaller than that of the antenna  1  of the first embodiment, the value 11.5 dB is satisfactory to exert balanced performance of the antenna  1 A. Furthermore, the antenna  1 A has a symmetry in directivity, which allows the outer conductor  10  to receive the left-handed circularly polarized wave with the antenna  1 A turned over. 
     Second Modification of First Embodiment 
     An antenna  1 B according to a second modification of the first embodiment will now be described. In the second modification, like reference numerals indicate like components of the first embodiment and the first modification, and detailed description thereof will be omitted. As illustrated in  FIG. 13 , the inner conductor  30 B of the second modification is different from the inner conductors of the first embodiment and the first modification in that the circular portion  31  of the first embodiment is replaced by a C-shaped arcuate portion  31 B. The arcuate portion  31 B has the starting point  31   a  of the second linear conductor connected to the first feed point  11  as the negative electrode through the connection portion  32  and has the end point  31   b  of the second linear conductor kept free from connection at a location inside the outer conductor  10 . As described above, the second linear conductor has a length, for example, determined based on one wavelength of the left-handed circularly polarized wave of a GPS. The inner conductor  30 B is designed such that current flows in a direction opposite to the current flow in the outer conductor  10 . Specifically, the arcuate portion  31 B of the inner conductor  30 B is extended counterclockwise from the starting point  31   a  to the end point  31   b  along the circumferential direction of the outer conductor  10 , in the top-down view of the mounting surface  2 . With the radius of the outer conductor  10  defined as r, the arcuate portion  31 B of the inner conductor  30 B has a radius of ½ r and has a circumference of ¾ πr. The inner conductor  30 B has the center located at a distance of ¼ r from the first feed point  11 . Current flows in the inner conductor  30 B from the starting point  31   a  toward the end point  31   b  along the circumferential direction of the arcuate portion  31 B. In other words, in the top-down view of the mounting surface  2 , current flows in the inner conductor  30 B counterclockwise from the starting point  31   a  connected to the first feed point  11  toward the end point  31   b  kept free from connection. The connection portion  32  connects the starting point  31   a  of the arcuate portion  31 B and the first feed point  11  of the outer conductor  10 . The connection portion  32  is extended along the radial direction of the outer conductor  10 . 
     Simulations with the antenna  1 B of the second modification of the first embodiment demonstrate the following results.  FIG. 14  is a graph of XPD values of the antenna  1 B. In  FIG. 14 , the y-axis represents the XPD value, and the x-axis represents the frequency. In  FIG. 14 , the simulation demonstrates that the antenna  1 B has a value of XPD of approximately 12 dB (P 11  in the graph), at a frequency of 1.6 GHz. The result indicates that the gain of the left-handed circularly polarized wave is low.  FIG. 15  is a graph of the VSWR of the antenna  1 B. In  FIG. 15 , the y-axis represents the VSWR, and the x-axis represents the frequency. In  FIG. 15 , the simulation demonstrates that the antenna  1 B has a VSWR of approximately 2.0 (P 12  in the graph) at a frequency of 1.6 GHz. The result indicates that the electrical efficiency is relatively high.  FIG. 16  is a Smith chart that illustrates the characteristic impedance. In  FIG. 16 , the simulation demonstrates that the magnitude of reflection is approximately 0.35 and the phase is approximately −70 (P 13  in the graph) at a frequency of 1.6 GHz. The results indicate that reflection is relatively small.  FIG. 17  is a chart that illustrates directivity. In  FIG. 17 , the simulation demonstrates that the right-handed circularly polarized wave and the left-handed circularly polarized wave are symmetrical to each other and that there is a symmetry in directivity between the circularly polarized waves. The symmetry allows the outer conductor  10  to receive the left-handed circularly polarized wave with the antenna  1 B turned over. In receiving the left-handed circularly polarized wave, the inner conductor  30 B has the arcuate portion  31 B extended clockwise from the starting point  31   a  to the end point  31   b , in the top-down view of the mounting surface  2 . 
     As described above, the antenna  1 B according to the second modification of the first embodiment includes the outer conductor  10  having a length corresponding to one wavelength of a right-handed circularly polarized wave of a GPS and includes the inner conductor  30 B having a length determined based on one wavelength of a left-handed circularly polarized wave of the GPS and consisting of the arcuate portion  31 B and the connection portion  32 . The antenna  1 B configured as above is allowed to decrease the gain of the left-handed circularly polarized wave and to increase the electrical efficiency. Furthermore, the antenna  1 B has a symmetry in directivity, which allows the outer conductor  10  to receive the left-handed circularly polarized wave with the antenna  1 B turned over. 
     Third Modification of First Embodiment 
     An antenna  1 C according to a third modification of the first embodiment will now be described. In the third modification, like reference numerals indicate like components of the first embodiment, the first modification, and the second modification, and detailed description thereof will be omitted. As illustrated in  FIG. 18 , an inner conductor  30 C of the third modification is different from the inner conductors of the first embodiment and others in that the circular portion  31  of the first embodiment is replaced by a rectangularly formed rectangular portion  31 C. The rectangular portion  31 C is an example of the curved portion, and the shape is, for example, square (rhomboid). The rectangular portion  31 C has the starting point  31   a  of the second linear conductor connected to the first feed point  11  as the negative electrode through the connection portion  32  and has the end point  31   b  of the second linear conductor kept free from connection at a location inside the outer conductor  10 . As described above, the second linear conductor has a length, for example, determined based on one wavelength of the left-handed circularly polarized wave of a GPS. The rectangular portion  31 C has a gap between the starting point  31   a  and the end point  31   b.  The inner conductor  30 C is designed such that current flows in a direction opposite to the current flow in the outer conductor  10 . Specifically, the rectangular portion  31 C of the inner conductor  30 C is extended counterclockwise from the starting point  31   a  to the end point  31   b  along the circumferential direction of the outer conductor  10 , in the top-down view of the mounting surface  2 . Current flows in the inner conductor  30 C from the starting point  31   a  toward the end point  31   b  along the circumferential direction of the rectangular portion  31 C. In other words, in the top-down view of the mounting surface  2 , current flows in the inner conductor  30 C counterclockwise from the starting point  31   a  connected to the first feed point  11  toward the end point  31   b  kept free from connection. The connection portion  32  connects the starting point  31   a  of the rectangular portion  31 C and the first feed point  11  of the outer conductor  10 . The connection portion  32  is extended along the radial direction of the outer conductor  10 . 
     Simulations with the antenna  1 C of the third modification of the first embodiment demonstrate the following results.  FIG. 19  is a graph of XPD values of the antenna  1 C. In  FIG. 19 , the y-axis represents the XPD value, and the x-axis represents the frequency. In  FIG. 19 , the simulation demonstrates that the antenna  1 C has a value of XPD of approximately 16 dB (P 14  in the graph), at a frequency of 1.6 GHz. The result indicates that the gain of the left-handed circularly polarized wave is low.  FIG. 20  is a graph of the VSWR of the antenna  1 C. In  FIG. 20 , the y-axis represents the VSWR, and the x-axis represents the frequency. In  FIG. 20 , the simulation demonstrates that the antenna  1 C has a VSWR of approximately 2.6 (P 15  in the graph), at a frequency of 1.6 GHz. The result indicates that reflection is relatively small.  FIG. 21  is a Smith chart that illustrates the characteristic impedance. In  FIG. 21 , the simulation demonstrates that the magnitude of reflection is approximately 0.45 and the phase is approximately −69 (P 16  in the graph) at a frequency of 1.6 GHz. The results indicate that reflection is relatively small.  FIG. 22  is a graph of the axial ratio. In  FIG. 22 , the y-axis represents the axial ratio, and the x-axis represents the frequency. In  FIG. 22 , the simulation demonstrates that the antenna  1 C has an axial ratio of approximately 1.4 dB (P 17  in the graph) at a frequency of 1.6 GHz. The result indicates that the axial ratio is relatively good.  FIG. 23  is a chart that illustrates directivity. In  FIG. 23 , the simulation demonstrates that the right-handed circularly polarized wave and the left-handed circularly polarized wave are symmetrical to each other and that there is a symmetry in directivity between the circularly polarized waves. The symmetry allows the outer conductor  10  to receive the left-handed circularly polarized wave with the antenna  1 C turned over. In receiving the left-handed circularly polarized wave, the inner conductor  30 C has the rectangular portion  31 C extended clockwise from the starting point  31   a  to the end point  31   b,  in the top-down view of the mounting surface  2 . 
     As described above, the antenna  1 C according to the third modification of the first embodiment includes the outer conductor  10  having a length corresponding to one wavelength of a right-handed circularly polarized wave of a GPS and includes the inner conductor  30 C having a length determined based on one wavelength of a left-handed circularly polarized wave of the GPS and consisting of the rectangular portion  31 C and the connection portion  32 . The antenna  1 C configured as above is allowed to decrease the gain of the left-handed circularly polarized wave and to increase the electrical efficiency. Furthermore, the antenna  1 C has a symmetry in directivity, which allows the outer conductor  10  to receive the left-handed circularly polarized wave with the antenna  1 C turned over. 
     Fourth Modification of First Embodiment 
     An antenna  1 D according to a fourth modification of the first embodiment will now be described. In the fourth modification, like reference numerals indicate like components of the first embodiment, the first modification, the second modification, and the third modification, and detailed description thereof will be omitted. As illustrated in  FIG. 24 , an inner conductor  30 D of the fourth modification is different from the inner conductors of the first embodiment and others in that the circular portion  31  of the first embodiment is replaced by an L-shaped portion  31 D formed in the shape of L. The L-shaped portion  31 D is an example of the curved portion. The L-shaped portion  31 D has the starting point  31   a  of the second linear conductor connected to the first feed point  11  as the negative electrode through the connection portion  32  and has the end point  31   b  of the second linear conductor kept free from connection at a location inside the outer conductor  10 . As described above, the second linear conductor has a length, for example, determined based on one wavelength of the left-handed circularly polarized wave of a GPS. The inner conductor  30 D is designed such that current flows in a direction opposite to the current flow in the outer conductor  10 . Specifically, the L-shaped portion  31 D of the inner conductor  30 D is extended counterclockwise from the starting point  31   a  to the end point  31   b,  in the top-down view of the mounting surface  2 . The L-shaped portion  31 D, for example, has a first side with the starting point  31   a  extended along the radial direction of the outer conductor  10  to a substantial center of the outer conductor  10 , and has a second side with the end point  31   b  extended at a substantially right angle to the first side. The first side and the second side of the L-shaped portion  31 D have the same length. Current flows in the inner conductor  30 D from the starting point  31   a  toward the end point  31   b  of the L-shaped portion  31 D. In other words, in the top-down view of the mounting surface  2 , current flows in the inner conductor  30 D counterclockwise from the starting point  31   a  connected to the first feed point  11  toward the end point  31   b  kept free from connection. The connection portion  32  connects the starting point  31   a  of the L-shaped portion  31 D and the first feed point  11  of the outer conductor  10 . The connection portion  32  is extended along the radial direction of the outer conductor  10 . In this configuration, the connection portion  32  is an end of the first side closer to the starting point  31   a  in the direction in which the first side is extended. 
     Simulations with the antenna  1 D of the fourth modification of the first embodiment demonstrate the following results.  FIG. 25  is a graph of XPD values of the antenna  1 D. In  FIG. 25 , the y-axis represents the XPD value, and the x-axis represents the frequency. In  FIG. 25 , the simulation demonstrates that the antenna  1 D has a value of XPD of approximately 10 dB (P 18  in the graph), at a frequency of 1.6 GHz. The result indicates that the gain of the left-handed circularly polarized wave is low.  FIG. 26  is a graph of the VSWR of the antenna  1 D. In  FIG. 26 , the y-axis represents the VSWR, and the x-axis represents the frequency. In  FIG. 26 , the simulation demonstrates that the antenna  1 D has a VSWR of approximately 1.8 (P 19  in the graph) at a frequency of 1.6 GHz. The result indicates that reflection is relatively small.  FIG. 27  is a Smith chart that illustrates the characteristic impedance. In  FIG. 27 , the simulation demonstrates that the magnitude of reflection is approximately 0.29 and the phase is approximately −54 (P 20  in the graph) at a frequency of 1.6 GHz. The results indicate that reflection is relatively small.  FIG. 28  is a graph of the axial ratio. In  FIG. 28 , the y-axis represents the axial ratio, and the x-axis represents the frequency. In  FIG. 28 , the simulation demonstrates that the antenna  1 D has an axial ratio of approximately 1.9 dB (P 21  in the graph) at a frequency of 1.6 GHz. The result indicates that the axial ratio is worse than that of the antenna  1  of the first embodiment.  FIG. 29  is a chart that illustrates directivity. In  FIG. 29 , the simulation demonstrates that the right-handed circularly polarized wave and the left-handed circularly polarized wave are symmetrical to each other and that there is a symmetry in directivity between the circularly polarized waves. The symmetry allows the outer conductor  10  to receive the left-handed circularly polarized wave with the antenna  1 D turned over. In receiving the left-handed circularly polarized wave, the inner conductor  30 D has the L-shaped portion  31 D extended clockwise from the starting point  31   a  to the end point  31   b,  in the top-down view of the mounting surface  2 . 
     As described above, the antenna  1 D according to the fourth modification of the first embodiment includes the outer conductor  10  having a length corresponding to one wavelength of a right-handed circularly polarized wave of a GPS and includes the inner conductor  30 D having a length determined based on one wavelength of a left-handed circularly polarized wave of the GPS and consisting of the L-shaped portion  31 D and the connection portion  32 . The antenna  1 D configured as above is allowed to decrease the gain of the left-handed circularly polarized wave and to increase the electrical efficiency. Furthermore, the antenna  1 D has a symmetry in directivity, which allows the outer conductor  10  to receive the left-handed circularly polarized wave with the antenna  1 D turned over. 
     Second Embodiment 
     An antenna  1 E according to a second embodiment will now be described. In the second embodiment, like reference numerals indicate like components of the first embodiment, the first modification, the second modification, the third modification, and the fourth modification, and detailed description thereof will be omitted. An inner conductor  30 E of the second embodiment illustrated in  FIG. 30  is different from the inner conductors of the first embodiment and others in receiving a right-handed circularly polarized wave of an ETC. The right-handed circularly polarized wave of an ETC has, for example, a frequency of 5.8 GHz. The antenna  1 E of the second embodiment has the same shape as that of the antenna  1  of the first embodiment, and is smaller than the antenna  1  to receive radio waves having frequencies higher than the frequency of a GPS. The antenna  1 E according to the second embodiment includes an outer conductor  10 E, first and second feedlines  21  and  22 , and the inner conductor  30 E. The outer conductor  10 E is an antenna to receive a right-handed circularly polarized wave of an ETC. The outer conductor  10 E is mounted on the mounting surface  2  and includes a body  13 E and a first feed point  11  provided at an end thereof and a second feed point  12  at the other end thereof. In the second embodiment, the first feed point  11  is the negative electrode and the second feed point  12  is the positive electrode. The body  13 E is formed of the first linear conductor circularly extended from the first feed point  11  to the second feed point  12 . The first linear conductor has a length corresponding to one wavelength of the right-handed circularly polarized wave of an ETC. The body  13 E has a gap between the first feed point  11  and the second feed point  12 . Current travels in the outer conductor  10 E, between the first feed point  11  and the second feed point  12  along the circumferential direction of the body  13 E. In the second embodiment, since the outer conductor  10 E receives the right-handed circularly polarized wave of an ETC, current travels clockwise between the first feed point  11  and the second feed point  12  in the top-down view of the mounting surface  2 . 
     The inner conductor  30 E is used to control receipt of a left-handed circularly polarized wave. The inner conductor  30 E is disposed on the mounting surface  2 , inside the outer conductor  10 E, and consists of a circular portion  31 E and the connection portion  32 . The circular portion  31 E and the connection portion  32  are formed of the second linear conductor. The second linear conductor has a length, for example, determined based on one wavelength of the left-handed circularly polarized wave of an ETC. The circular portion  31 E is circularly formed with the starting point  31   a  of the second linear conductor connected to the first feed point  11  as the negative electrode through the connection portion  32  and with the end point  31   b  of the second linear conductor kept free from connection at a location inside the outer conductor  10 E. The circular portion  31 E has a gap between the starting point  31   a  and the end point  31   b.  The inner conductor  30 E is designed such that current flows in a direction opposite to the current flow in the outer conductor  10 E. Specifically, the circular portion  31 E of the inner conductor  30 E is extended counterclockwise from the starting point  31   a  to the end point  31   b  along the circumferential direction of the outer conductor  10 E, in the top-down view of the mounting surface  2 . Current flows in the inner conductor  30 E from the starting point  31   a  toward the end point  31   b  along the circumferential direction of the circular portion  31 E. In other words, in the top-down view of the mounting surface  2 , current flows in the inner conductor  30 E counterclockwise from the starting point  31   a  connected to the first feed point  11  toward the end point  31   b  kept free from connection. The connection portion  32  connects the starting point  31   a  of the circular portion  31 E and the first feed point  11  of the outer conductor  10 E. The connection portion  32  is extended along the radial direction of the outer conductor  10 E. 
     Simulations have been conducted on the antenna  1 E of the second embodiment, and the results of the simulations will now be described.  FIG. 31  is a graph of XPD values of the antenna  1 E. In  FIG. 31 , the y-axis represents the XPD value, and the x-axis represents the frequency. In  FIG. 31 , the simulation demonstrates that the antenna  1 E has a value of XPD of approximately 27 dB (P 22  in the graph), at a frequency of 5.8 GHz. The result indicates that the gain of the left-handed circularly polarized wave is low.  FIG. 32  is a graph of the VSWR of the antenna  1 E. In  FIG. 32 , the y-axis represents the VSWR, and the x-axis represents the frequency. In  FIG. 32 , the simulation demonstrates that the antenna  1 E has a VSWR of approximately 1.6 (P 23  in the graph), at a frequency of 5.8 GHz. The result indicates that reflection is relatively small.  FIG. 33  is a Smith chart that illustrates the characteristic impedance. In  FIG. 33 , the simulation demonstrates that the magnitude of reflection is approximately  0 . 23  and the phase is approximately −179 (P 24  in the graph) at a frequency of 5.8 GHz. The results indicate that reflection is relatively small.  FIG. 34  is a graph of the axial ratio. In  FIG. 34 , the y-axis represents the axial ratio, and the x-axis represents the frequency. In  FIG. 34 , the simulation demonstrates that the antenna  1 E has an axial ratio of approximately 1.1 dB (P 25  in the graph), at a frequency of 5.8 GHz. The result indicates that the axial ratio is relatively good.  FIG. 35  is a chart that illustrates directivity. In  FIG. 35 , the simulation demonstrates that the right-handed circularly polarized wave and the left-handed circularly polarized wave are symmetrical to each other and that there is a symmetry in directivity between the circularly polarized waves. The symmetry allows the outer conductor  10 E to receive the left-handed circularly polarized wave with the antenna  1 E turned over. In receiving the left-handed circularly polarized wave, the inner conductor  30 E has the circular portion  31 E extended clockwise from the starting point  31   a  to the end point  31   b , in the top-down view of the mounting surface  2 . 
     As described above, the antenna  1 E according to the second embodiment includes the outer conductor  10 E having a length corresponding to one wavelength of the right-handed circularly polarized wave of an ETC and includes the inner conductor  30 E having a length determined based on one wavelength of the left-handed circularly polarized wave of the ETC and consisting of the circular portion  31 E and the connection portion  32 . The antenna  1 E configured as above is allowed to decrease the gain of the left-handed circularly polarized wave and to increase the electrical efficiency. Furthermore, the antenna  1 E has a symmetry in directivity, which allows the outer conductor  10 E to receive the left-handed circularly polarized wave with the antenna  1 E turned over. 
     The first embodiment, the first to the fourth modifications of the first embodiment, and the second embodiment have presented examples in which the starting point  31   a  is connected to the first feed point  11  as the negative electrode; however, these examples are not limiting. As demonstrated by an antenna  1 F of a modification of the second embodiment, the starting point  31   a  of an inner conductor  30 F may be connected to the second feed point  12  as the positive electrode (see  FIG. 36 ). In this case, the antenna  1 F receives a left-handed circularly polarized wave with the gain characteristics of the right-handed and left-handed circularly polarized waves inverted.  FIG. 37  is a graph of XPD values of the antenna  1 F. In  FIG. 37 , the y-axis represents the XPD value, and the x-axis represents the frequency. In  FIG. 37 , the simulation demonstrates that the antenna  1 F has a value of XPD of approximately 22 dB (P 26  in the graph), at a frequency of 5.8 GHz. The result thus indicates that the gain of the right-handed circularly polarized wave is low.  FIG. 38  is a graph of the VSWR of the antenna  1 F. In  FIG. 38 , the y-axis represents the VSWR, and the x-axis represents the frequency. In  FIG. 38 , the simulation demonstrates that the antenna  1 F has a VSWR of approximately 1.6 (P 27  in the graph), at a frequency of 5.8 GHz. The result thus indicates that reflection is relatively small. 
     The antennas of the first embodiment, the first to the fourth modifications of the first embodiment, the second embodiment, and the modification of the second embodiment are capable of receiving GPS signals and ETC signals by changing the lengths of the outer conductors  10  and  10 E and the inner conductors  30 ,  30 B,  30 C,  30 D,  30 E, and  30 F. 
     An antenna according to the present embodiment includes an outer conductor the length of which corresponds to one wavelength of a right-handed circularly polarized wave and an inner conductor disposed inside the outer conductor and having a length determined based on one wavelength of a left-handed circularly polarized wave and causing current to flow therein in a direction opposite to the current flow in the outer conductor. The antenna configured as above can keep current of a left-handed circularly polarized wave from flowing to the outer conductor and to properly receive a right-handed circularly polarized wave. 
     Although the invention has been described with respect to specific embodiments for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.