Patent Publication Number: US-11664602-B2

Title: Lens, antenna, and device for vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is based on PCT filing PCT/JP2019/017062, filed Apr. 22, 2019, which claims priority to JP 2018-090595, filed May 9, 2018, the entire contents of each are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to a lens, an antenna, and a device for vehicle. 
     BACKGROUND ART 
     PTL 1 discloses a Luneburg lens. A typical Luneburg lens is a spherical lens that has a relative dielectric constant that changes in a radial direction. The lens disclosed in PTL 1 is hemispherical and has a relative dielectric constant that changes stepwise. 
     NPL 1 (Mushiake Yasuto, “antenna⋅radio wave propagation”, CORONA PUBLISHING CO., LTD., Jun. 25, 1983, P. 106) discloses that regarding a Luneburg lens, a relationship between a refractive index and a distance from the center of the lens satisfies: refractive index=square root of 2−(r/a){circumflex over ( )}2. In the above expression, r is the distance from the center of the lens, and a is the radius of the lens. 
     The relative dielectric constant is given as the square of the refractive index, and the relative dielectric constant of the Luneburg lens satisfies an expression: relative dielectric constant=2−(r/a){circumflex over ( )}2. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2009-516933 
       
    
     Non Patent Literature 
     
         
         NPL 1: Mushiake Yasuto, “antenna⋅radio wave propagation”, CORONA PUBLISHING CO., LTD., Jun. 25, 1983, P. 106 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     A lens according to the disclosure includes a dielectric having a first surface and a second surface that is spaced from the first surface and that faces the first surface in a direction of a reference axis intersecting the first surface. The dielectric has an equivalent relative dielectric constant that decreases in a direction from the reference axis toward outer circumferences of the first surface and the second surface. 
     Another aspect of the present disclosure is an antenna. An antenna according to the disclosure includes a lens including a dielectric having a first surface and a second surface that is spaced from the first surface and that faces the first surface in a direction of a reference axis intersecting the first surface, and a radio wave radiator that is disposed on outer circumferences of the first surface and the second surface. The dielectric has an equivalent relative dielectric constant that decreases in a direction from the reference axis toward the outer circumferences of the first surface and the second surface. 
     Another aspect of the present disclosure is a device for vehicle that includes an antenna. The antenna of the device for vehicle according to the disclosure includes a lens including a dielectric having a first surface and a second surface that is spaced from the first surface and that faces the first surface in a direction of a reference axis intersecting the first surface, and a radio wave radiator that is disposed on outer circumferences of the first surface and the second surface. The dielectric has an equivalent relative dielectric constant that decreases in a direction from the reference axis toward the outer circumferences of the first surface and the second surface. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    illustrates components in a usage example and devices for vehicle each of which includes an antenna according to a first embodiment. 
         FIG.  2    illustrates the structure of the antenna according to the first embodiment. 
         FIG.  3 A  illustrates the structure of a dielectric member according to the first embodiment. 
         FIG.  3 B  is a side view of a lens according to the first embodiment. 
         FIG.  3 C  is a sectional view of  FIG.  2    taken along line A-A. 
         FIG.  3 D  illustrates the distribution of the equivalent relative dielectric constant of the dielectric. 
         FIG.  3 E  illustrates focusing of radio waves in the lens. 
         FIG.  3 F  illustrates a radio wave radiation direction. 
         FIG.  4    is a side view of the structure of a modification to the dielectric member according to the first embodiment. 
         FIG.  5    illustrates the structure of a body portion of the dielectric member according to the first embodiment. 
         FIG.  6    is a graph illustrating a relationship between a distance from a reference axis of the dielectric member illustrated in  FIG.  5    and the equivalent relative dielectric constant of the dielectric member. 
         FIG.  7    illustrates a flowchart in which procedures for a method of manufacturing the antenna according to the first embodiment are defined. 
         FIG.  8    is a graph illustrating the horizontal plane directivity of horizontally polarized waves that are transmitted and received by the antenna according to the first embodiment. 
         FIG.  9    is a graph illustrating the horizontal plane directivity of vertically polarized waves that are transmitted and received by the antenna according to the first embodiment. 
         FIG.  10    illustrates the structure of a body portion of a dielectric member according to a first modification to the first embodiment. 
         FIG.  11    illustrates the structure of a body portion of a dielectric member according to a second modification to the first embodiment. 
         FIG.  12    illustrates the structure of a body portion of a dielectric member according to a third modification to the first embodiment. 
         FIG.  13    illustrates the structure of a body portion of a dielectric member according to a fourth modification to the first embodiment. 
         FIG.  14    illustrates the structure of a body portion of a dielectric member according to a fifth modification to the first embodiment. 
         FIG.  15    illustrates the structure of an antenna according to a second embodiment. 
         FIG.  16    is a side view of the structure of a dielectric member according to the second embodiment. 
         FIG.  17    illustrates the structure of a body portion of a dielectric member according to a first modification to the second embodiment. 
         FIG.  18    illustrates the structure of a body portion of a dielectric member according to a second modification to the second embodiment of the present disclosure. 
         FIG.  19    illustrates the structure of a body portion of a dielectric member according to a third modification to the second embodiment. 
         FIG.  20    is a perspective view of the structure of an antenna according to a third embodiment. 
         FIG.  21    illustrates a flowchart in which procedures for a method of manufacturing the antenna according to the third embodiment are defined. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Problem to be Solved by the Present Disclosure 
     A spherical or hemispherical Luneburg lens can three-dimensionally change a radio wave radiation direction by three-dimensionally changing the position of a radio wave radiator along a spherical surface. However, it is necessary for the spherical or hemispherical Luneburg lens to have a relative dielectric constant that three-dimensionally changes in a radial direction from the center of a sphere or a hemisphere such that a “relationship between a refractive index and a distance from the center of the lens” disclosed in NPL 1 is satisfied. For this reason, the spherical or hemispherical Luneburg lens is difficult to manufacture. 
     The present inventors have found that a structure can be simplified and manufacturing is facilitated in the case where it suffices that the radiation direction two-dimensional changes. 
     DESCRIPTION OF EMBODIMENTS OF PRESENT DISCLOSURE 
     The contents of embodiments of the present disclosure will now be listed and described. 
     (1) A lens according to an embodiment includes a dielectric having a first surface and a second surface that is spaced from the first surface and that faces the first surface in a direction of a reference axis intersecting the first surface. The dielectric has an equivalent relative dielectric constant that decreases in a direction from the reference axis toward outer circumferences of the first surface and the second surface. Since the dielectric is a structure having the first surface and the second surface that is spaced from the first surface and that faces the first surface in the direction of the reference axis intersecting the first surface, the structure is simpler than that in the spherical or hemispherical lens. In the case where the dielectric is composed of a single kind of substance, the equivalent relative dielectric constant is equal to the relative dielectric constant of the substance of which the dielectric is composed. In the case where the dielectric is composed of kinds of substances, the equivalent relative dielectric constant corresponds to a relative dielectric constant when the substances in the direction of the reference axis are regarded as a single substance, and is obtained as the weighted average of relative dielectric constants depending on the proportion of each substance in the direction of the reference axis. 
     (2) In the dielectric, a first substance that has a first relative dielectric constant and a second substance that has a second relative dielectric constant larger than the first relative dielectric constant are preferably adjacent to each other in the direction of the reference axis. In this case, the equivalent relative dielectric constant corresponds to a relative dielectric constant when the first substance and the second substance are regarded as a single substance. 
     (3) In the dielectric, a proportion of the second substance in the direction of the reference axis preferably decreases in the direction from the reference axis toward the outer circumferences. The equivalent relative dielectric constant can be decreased by decreasing the proportion of the second substance. 
     (4) The second substance preferably includes multiple components that are stacked in the direction of the reference axis. A structure that decreases the equivalent relative dielectric constant can be readily obtained by stacking the multiple components in a reference direction. 
     (5) The second substance is preferably subjected to a cutting process. The cutting process enables a structure that decreases the equivalent relative dielectric constant to be readily obtained. 
     (6) The second substances are preferably located on both sides of the first substance in the direction of the reference axis. In this case, a structure in which both sides of the first substance are interposed between the second substances is obtained. 
     (7) The first substance is preferably air. In this case, it is not necessary to process the first substance. 
     (8) The first relative dielectric constant is preferably less than 2. 
     (9) The second relative dielectric constant is preferably 2 or more. 
     (10) The lens preferably further includes a member that prevents a radio wave from leaking through the first surface, and a member that prevents a radio wave from leaking through the second surface. Radio waves can be prevented from leaking through the first surface and the second surface without increasing the length of the lens in the direction of the reference axis. 
     (11) The lens preferably further includes a waveguide that is disposed on the outer circumferences of the first surface and the second surface. In this case, a radio wave that propagates in a waveguide mode by using the waveguide can be incident on the dielectric, and the radio wave can efficiently propagate. 
     (12) The lens preferably further includes a member that prevents a radio wave from leaking through the first surface, a member that prevents a radio wave from leaking through the second surface, and a waveguide that is disposed on the outer circumferences of the first surface and the second surface. The waveguide is preferably integrally formed with the member that prevents the radio wave from leaking through the first surface and the member that prevents the radio wave from leaking through the second surface. In this case, a radio wave can be prevented from leaking with more certainty when the radio wave propagates from the waveguide to the lens. 
     (13) A length of the dielectric in the direction of the reference axis is preferably equal to or less than twice a wavelength of a radio wave that propagates in the dielectric. That is, the length of the dielectric in the direction of the reference axis is preferably 2λ or less where X is the wavelength. The length of the dielectric in the direction of the reference axis is more preferably 1.5λ or less, further preferably λ or less. 
     (14) A lens according to an embodiment is a two-dimensional Luneburg lens that changes a radio wave radiation direction into a direction parallel to a two-dimensional plane depending on a two-dimensional position of a radio wave radiator in the two-dimensional plane. The lens includes a first substance that has a first relative dielectric constant, and a second substance that is adjacent to the first substance in a direction perpendicular to the two-dimensional plane and that has a relative dielectric constant different from the first relative dielectric constant. As for the two-dimensional Luneburg lens, change in the radio wave radiation direction is preferably limited to the direction parallel to the two-dimensional plane. 
     (15) An antenna according to an embodiment is an antenna including a lens including a dielectric having a first surface and a second surface that is spaced from the first surface and that faces the first surface in a direction of a reference axis intersecting the first surface, and a radio wave radiator that is disposed on outer circumferences of the first surface and the second surface. The dielectric has an equivalent relative dielectric constant that decreases in the direction from the reference axis toward the outer circumferences of the first surface and the second surface. 
     (16) A length of the radio wave radiator in the direction of the reference axis is preferably equal to or less than a length of the dielectric in the direction of the reference axis. In this case, a radio wave can be inhibited from leaking near the boundary between the radio wave radiator and the dielectric when the radio wave is radiated. 
     (17) A length of the radio wave radiator in the direction of the reference axis is preferably equal to or more than a length of the dielectric in the direction of the reference axis. In this case, a radio wave can be inhibited from leaking near the boundary between the radio wave radiator and the dielectric when the radio wave is received. 
     (18) A length of the radio wave radiator in the direction of the reference axis is preferably equal to a length of the dielectric in the direction of the reference axis. In this case, radio waves can be inhibited from leaking near the boundary between the radio wave radiator and the dielectric when the radio wave is radiated and when the radio wave is received. 
     (19) A waveguide is preferably disposed between the radio wave radiator and the dielectric. In this case, a radio wave can propagate by using the waveguide between the radio wave radiator and the dielectric. 
     (20) A device for vehicle according to an embodiment is a device for vehicle including an antenna. The antenna includes a lens including a dielectric having a first surface and a second surface that is spaced from the first surface and that faces the first surface in a direction of a reference axis intersecting the first surface, and a radio wave radiator that is disposed on outer circumferences of the first surface and the second surface. The dielectric has an equivalent relative dielectric constant that decreases in the direction from the reference axis toward the outer circumferences of the first surface and the second surface. 
     Embodiments of the present disclosure will hereinafter be described with reference to the drawings. In the drawings, portions like or corresponding to each other are designated by like reference signs, and a description thereof is not repeated. At least parts of the embodiments described below may be freely combined. 
     First Embodiment 
     [Structure] 
     (Antenna) 
       FIG.  1    illustrates a usage example and the structures of devices for vehicle each of which includes an antenna according to a first embodiment of the present disclosure. 
     Referring to  FIG.  1   , each of devices for vehicle  401  is disposed in a vehicle Cr such as a bus and includes an antenna  301 . The device for vehicle  401  uses the antenna  301  to communicate with a wireless base station device  161  by wireless communication, for example, in accordance with a communication method of a fifth generation mobile communication system (referred to below as “5G). 
     More specifically, the device for vehicle  401  detects a direction from the vehicle Cr toward the wireless base station device  161  and adjusts a main radiation direction of radio waves that are transmitted from and received by the antenna  301 , based on the result of detection. The direction from the vehicle Cr toward the wireless base station device  161  can change into any direction in a horizontal plane as the vehicle Cr runs. For this reason, the antenna  301  can adjust a radio wave radiation direction and a radio wave reception direction into any direction in the horizontal plane. 
     The antenna  301  may be capable of vertically adjusting the radio wave radiation direction. An angle at which the radio wave radiation direction and the radio wave reception direction can be vertically adjusted may be small. 
     The antenna  301  is not limited to an antenna that is included in the device for vehicle  401 . 
     The antenna  301  can be used for wireless communication in accordance with a communication method other than 5G. However, the wireless communication in accordance with the communication method of 5G, in which the degree of straightness of radio waves is high, is more suitable to use the antenna  301  that can change the radio wave radiation direction and the radio wave reception direction. 
       FIG.  2    illustrates the structure of the antenna according to the first embodiment of the present disclosure. 
     Referring to  FIG.  2   , the antenna  301  is included in, for example, a device for vehicle of a mobile communication system. The antenna  301  includes a lens  201 , waveguides  151  that are coupled with the lens  201 , and one or more radio wave radiators  221  that are disposed around the lens  201 . 
     Examples of the radio wave radiators  221  include a horn antenna. In  FIG.  2   , the seven radio wave radiators  221  are illustrated by way of example. The seven radio wave radiators  221  are disposed, for example, equiangularly. In  FIG.  2   , the horn antennas  221  are illustrated as members that have a quadrangular pyramid shape. The horn antennas  221  are actually members that have, for example, a truncated quadrangular pyramid shape obtained by forming an opening in a vertex portion of a quadrangular pyramid. Each waveguide is connected to the opening near the vertex of the quadrangular pyramid. 
     The lens  201  includes a dielectric member  101 , an upper member  22 , and a lower member  23 . The dielectric member  101  is consist of a dielectric. The dielectric member  101  is, for example, a columnar member and has an upper surface  11  and a lower surface  12 . 
     The upper surface  11  and the lower surface  12  of the dielectric member  101  are circular. The radii R of the upper surface  11  and the lower surface  12  are designed to be 30 mm, for example, in the case where the antenna  301  transmits and receives radio waves in a band of 28 GHz. A relationship among the speed of light c, a frequency band f, and a wavelength λ satisfies an expression: c=f×λ, and the wavelength λ of a radio wave in a band of 28 GHz when the speed of light c satisfies c=3×10{circumflex over ( )}8 m/seconds is 10.7 mm. 
     In the following description, an XY plane in a direction in which the upper surface  11  and the lower surface  12  extend, that is, illustrated in  FIG.  2    is referred to as a horizontal plane. The direction of the normal to the upper surface  11  and the lower surface  12 , that is, a Z-axis direction illustrated in  FIG.  2    is referred to as a perpendicular direction. For example, in  FIG.  3 A  and  FIG.  3 C , an imaginary horizontal plane P parallel to the XY plane is illustrated. 
     In the case where the dielectric member  101  is composed of kinds of substances, the dielectric member  101  has an equivalent relative dielectric constant Ere that is equal to the weighted average of relative dielectric constants in a thickness direction at a position a distance r away from the reference axis S and that decreases in a direction from the reference axis S that passes through the upper surface  11  and the lower surface  12  toward the outside of the dielectric member  101 . An example of the reference axis S is an axis that passes through the center of the upper surface  11  and the center of the lower surface  12  and that extends in the perpendicular direction. 
     In the case where at the position the distance r away from the reference axis S, the dielectric member  101  is composed of a single kind of substance, the relative dielectric constant at the position of the distance r is referred to as the “equivalent relative dielectric constant ε re ”. In this case, the equivalent relative dielectric constant ε re  at the position the distance r away from the reference axis S is equal to the relative dielectric constant of the material thereof. 
     The number of the waveguides  151  is, for example, 7. The seven waveguides  151  are disposed at positions at which the waveguides  151  face the respective seven radio wave radiators  221 . Specifically, an angle θ that is formed between a straight line L 1  that passes through one of the waveguides  151  and the center of the upper surface  11  and a straight line L 2  that passes through another waveguide  151  adjacent to the one of the waveguides  151  and the center of the upper surface  11  is, for example, 20°. The waveguides  151  cause radio waves to propagate between the radio wave radiators  221  and the dielectric member  101 . 
     According to an embodiment, each waveguide  151  has a tubular shape that has a rectangular section perpendicular to a direction in which an inner space extends, that is, a waveguide direction. For example, the length of each of sides of the section is designed to be 7.112 mm in the case where the antenna  301  transmits and receives radio waves in a band of 28 GHz. The waveguide direction is a direction in which the waveguide  151  and the dielectric member  101  are connected to each other. The waveguide direction is parallel to the XY plane. 
     The lens  201  may not include the upper member  22 , or the lower member  23 , or both. In this case, the thickness of the dielectric member  101  is preferably set to a thickness equal to or more than a predetermined value. This predetermined value is a value that enables radio waves that propagate in the radial direction in the dielectric member  101  to pass through the inside of the dielectric member  101  before the radio waves leak out through the upper surface  11 , or the lower surface  12  of the dielectric member  101 , or both. 
       FIG.  3 A  illustrates the structure of the dielectric member according to the first embodiment of the present disclosure. For convenience of description,  FIG.  3 A  illustrates a side view of the lens  201  such that side surfaces of the waveguides  151  are illustrated at a left part of the figure and sections of the waveguides  151  and the radio wave radiator  221  are illustrated at a right part of the figure.  FIG.  3 B  plainly illustrates a side view of the lens  201 .  FIG.  3 C  illustrates a sectional view of the lens  201  illustrated in  FIG.  2    taken along line A-A.  FIG.  3 D  illustrates the structure of the dielectric member  101 . The radio wave radiator  221  illustrated in  FIG.  3 A  and  FIG.  3 C  has a truncated quadrangular pyramid shape obtained by forming an opening in a vertex portion of a quadrangular pyramid (see the horn antennas  221  in  FIG.  2   ). 
     Referring to  FIG.  3 A  and  FIG.  3 B , the dielectric member  101  includes a body portion  21  and a substance M. In the following description, the substance M is referred to as a first substance M, and a substance that is contained in the body portion  21  is referred to as a second substance in some cases. The body portion  21  and the substance M are provided between the upper member  22  and the lower member  23 . The relative dielectric constant ε rM  of the substance M is referred to as a “first relative dielectric constant ε rM1 ”, and the first relative dielectric constant ε rM1  is less than 2. Here, the substance M is air. The relative dielectric constant ε rM  of the air is 1. 
     The upper member  22  and the lower member  23  are composed of, for example, material containing metal or metal. As illustrated in  FIG.  3 C , the upper member  22  prevents radio waves B that propagate in the dielectric member  101  from leaking through the upper surface  11 . Similarly, the lower member  23  prevents the radio waves B that propagate in the dielectric member  101  from leaking through the lower surface  12 . That is, the upper member  22  and the lower member  23  prevent the radio waves from leaking through the upper surface  11  and the lower surface  12  and cause the radio waves B to propagate in a direction parallel to the horizontal plane P in the dielectric  10 . The upper member  22  and the lower member  23  are thus waveguide members that cause the radio waves to propagate in the dielectric member  101 . According to an embodiment, the upper member  22  and the lower member  23  are disposed on the upper surface  11  and the lower surface  12  of the dielectric member  101 , and a location from which the radio waves can enter and/or exit the dielectric member  101  is restricted to the outer circumference  18  of the dielectric member  101 . A distance a between the upper member  22  and the lower member  23  is designed to be 7.112 mm, for example, in the case where the antenna  301  transmits and receives radio waves in a band of 28 GHz. The upper member  22 , the lower member  23 , and the waveguides  151  are, for example, integrally formed. The distance a also corresponds to the thickness of the dielectric member  101 , that is, a length in the perpendicular direction. According to an embodiment, the thickness a of the dielectric member  101  is equal to or less than one wavelength (10.7 mm). The thickness of the dielectric member  101  is preferably equal to or less than twice the wavelength (2λ), more preferably equal to or less than 1.5 times the wavelength (1.5λ), further preferably equal to or less than one wavelength (λ). Even when there are substances in the thickness direction of the dielectric member  101 , sufficiently decreasing the thickness of the dielectric member  101  enables the substances to be regarded as a single kind of substance. In contrast, sufficiently increasing the thickness a of the dielectric member  101  enables a radio wave to be prevented from leaking out through the upper surface  11 , or the lower surface  12  of the dielectric member  101 , or both as described above. In the case where the thickness a of the dielectric member  101  is sufficiently increased, the thickness a of the dielectric member  101  is preferably equal to or more than twice the wavelength. 
     The height Hr of an opening portion of each radio wave radiator  221 , that is, a length Hr of the radio wave radiator  221  in the perpendicular direction is equal to the distance a between the upper member  22  and the lower member  23 , that is, the thickness of the dielectric member  101 . This enables a radio wave to be inhibited from leaking near the boundary between the dielectric member  101  and each radio wave radiator  221 . 
     In the case where it is not necessary to consider leakage of a radio wave near the boundary when the radio wave is received by the antenna  301 , it suffices that the height Hr of the opening portion of the radio wave radiator  221  is equal to or less than the thickness of the dielectric member  101 . 
     In the case where it is not necessary to consider leakage of a radio wave near the boundary when the radio wave is radiated from the antenna  301 , it suffices that the height Hr of the opening portion of the radio wave radiator  221  is equal to or more than the thickness of the dielectric member  101 . 
       FIG.  4    illustrates the structure of a modification to the dielectric member according to the first embodiment of the present disclosure.  FIG.  4    illustrates a side view of the lens  201  such that side surfaces of the waveguides  151  are illustrated at a left part of the figure and sections of the waveguides  151  and the radio wave radiator  221  are illustrated at a right part of the figure as in  FIG.  3 A . 
     Referring to  FIG.  4   , the upper member  22  and the lower member  23  are preferably coupled with the radio wave radiators  221  with members Mt that are composed of material that contains metal or metal and the waveguides  151  interposed therebetween. The members Mt may be integrally formed with the waveguides  151 . The members Mt may be integrally formed with the upper member  22  and the lower member  23 . That is, the members Mt may be tubular members that are disposed on the outer edges of the upper member  22  and the lower member  23 . 
     In this way, metal plates, for example, extend to positions nearer than the body portion  21  of the dielectric member  101  to the radio wave radiators  221 , and a radio wave is consequently prevented from leaking near the boundary between the dielectric member  101  and each radio wave radiator  221  with more certainty. 
     Referring to  FIG.  3 A  and  FIG.  3 B  again, the body portion  21  includes a first member  21   a  that is disposed near the upper member  22  and a second member  21   b  that is disposed near the lower member  23 . The air that is the first substance M exists between the first member  21   a  and the second member  21   b . In other words, the second substances are provided on both sides of the first substance M in the direction of the reference axis S. The first member  21   a  and the second member  21   b  are plane-symmetrical to each other with respect to the plane P. In  FIG.  3 A  and  FIG.  3 B , the plane P is the horizontal plane that is located at the center of the dielectric member  101  in the thickness direction. That is, the body portion  21  has a plane-symmetrical structure in the perpendicular direction. In  FIG.  3 C , as for the dielectric member  101 , only a region in which the dielectric member  101  is disposed is illustrated, and the body portion  21  and the substance M that are contained in the dielectric member  101  are not distinguished. According to an embodiment, the region in which the dielectric member  101  is disposed contains the upper surface  11  corresponding to a first surface and the lower surface  12  corresponding to a second surface. The second surface  12  is spaced from the first surface  11  in the perpendicular direction and faces the first surface  11 . According to an embodiment, the region in which the dielectric member  101  is disposed has a cylindrical shape. The outer circumference  18  of the dielectric member  101  that has a tubular shape corresponds to a radio wave entrance and exit surface. The sizes of the first surface  11  and the second surface  12  may differ from each other. 
     The relative dielectric constants ε rM  of the first member  21   a  and the second member  21   b  are referred to as “second relative dielectric constants ε rM2 ”, and the second relative dielectric constants ε rM2  are 2 or more. The first member  21   a  and the second member  21   b  are composed of, for example, resin that has a second relative dielectric constant ε rM2  of 3. 
     More specifically, the thicknesses h of the first member  21   a  and the second member  21   b  decrease in a direction from the reference axis S toward the outside of the dielectric member  101 . That is, as schematically illustrated in  FIG.  3 D , the proportion of the body portion  21  in the direction of the reference axis S concentrically decreases in the direction from the reference axis S toward the outer circumference  18  of the dielectric member  101  in a view of a vertical section of the dielectric member  101  (in a sectional view taken along line B-B in  FIG.  3 D ). The amount of the second substances (such as resin) that are contained in the body portion  21  is largest at the position of the reference axis S and decreases in the direction toward the outer circumference  18 . In contrast, the amount of the first substance M (air) is smallest at the position of the reference axis S and increases in the direction toward the outer circumference  18 . According to an embodiment, the first substance M and the second substances that are contained in the body portion  21  are thus adjacent to each other in the direction of the reference axis S. The proportion of the second substances in the direction of the reference axis S concentrically decreases in the direction from the reference axis S toward the outer circumference  18 . The proportion of the first substance M in the direction of the reference axis S concentrically increases in the direction from the reference axis S toward the outer circumference  18 . 
     Consequently, the equivalent relative dielectric constant ε re  of the dielectric member  101  decreases in the direction from the reference axis S toward the outside of the dielectric member  101 . For example, as illustrated in a diagram of a relationship between the equivalent relative dielectric constant and a relative radius in  FIG.  3 D , the equivalent relative dielectric constant ε re  of a portion of the dielectric member  101  through which the reference axis S passes is about 2, and the equivalent relative dielectric constant ε re  of the outer circumference  18  corresponding to an outer edge portion is about 1. 
     That is, the equivalent relative dielectric constant ε re  at the position the distance r away from the reference axis S has a value obtained by using a proportion between the material of the first member  21   a  and the second member  21   b  and the air that is the substance M and calculating the weighted average of the relative dielectric constant ε rM2  of the material and the relative dielectric constant ε rM1  of the air. For this reason, the dielectric member  101  can change the equivalent relative dielectric constant ε re  as in a spherical Luneburg lens. 
     In the case where at the position the distance r away from the reference axis S, the dielectric member  101  is composed of a single kind of substance as described above, the equivalent relative dielectric constant ε re  at the position the distance r away from the reference axis S is equal to the relative dielectric constant of the material, for example, the relative dielectric constant ε rM2  or the relative dielectric constant ε rM1 . 
     Specifically, the thicknesses of the first member  21   a  and the second member  21   b  at the position the distance r away from the reference axis S in the horizontal plane are referred to as thicknesses h r . 
     A relationship between the equivalent relative dielectric constant ε re  and the thicknesses h r  satisfies an expression:
 
ε re =ε rM1 +(ε rM2 −ε rM1 )×2 h   r   /a.   (1)
 
     Expression (1) is transformed, and the equivalent relative dielectric constant ε re  satisfies:
 
ε re =(2 h   r   /a )×ε rM2 +(( a− 2 h   r )/ a )×ε rM1 .  (2)
 
     In expression (1) described above and expression (2) described above, a is the distance between the upper member  22  and the lower member  23  in the direction Z of the reference axis and corresponds to the thickness of the dielectric member  101 . R is the radii of the upper surface  11  and the lower surface  12  of the dielectric member  101 , ε rM2  is the relative dielectric constant of a material of which the body portion  21  is composed, that is, the second substances, and ε rM1  is the relative dielectric constant of the air that is the first substance. 
     That is, the equivalent relative dielectric constant ε re  is the total value of a value obtained by multiplying 2h r /a by the second relative dielectric constant ε rM2  of the body portion  21  and a value obtained by multiplying (a−2hr)/a by the first relative dielectric constant ε rM1  of the air. 2h r /a represents the proportion of the thickness 2h r  of the body portion  21  at the position of the distance r to the thickness a of the dielectric member  101 . The thickness 2h r  of the body portion  21  is the sum of the thickness h r  of the first member  21   a  and the thickness h r  of the material of the second member  21   b . ((a−2h r )/a) represents the proportion of the thickness of the air at the position of the distance r to the thickness a of the dielectric member  101 . 
     (Body Portion) 
       FIG.  5    illustrates the structure of the body portion of the dielectric member according to the first embodiment of the present disclosure. 
     Referring to  FIG.  5   , the first member  21   a  and the second member  21   b  of the body portion  21  are plane-symmetrical with respect to the plane P as described above. The first member  21   a  has the upper surface  11 , and the second member  21   b  has the lower surface  12 . The structure of the first member  21   a  will now be described. 
     The first member  21   a  includes components  31  that are stacked in a direction parallel to the reference axis S. An example of each component  31  is a disk-shaped member that has a circular main surface, and the reference axis S passes through the center of the main surface. Here, the body portion  21  includes the eight components  31 , that is, components  31   a ,  31   b ,  31   c ,  31   d ,  31   e ,  31   f ,  31   g , and  31   h.    
     The components  31   a  to  31   h  contain the same substance and have the same relative dielectric constant ε rM . The components  31   a  to  31   h  are stacked downward from the upper member  22  in order of the components  31   h ,  31   g ,  31   f ,  31   e ,  31   d ,  31   c ,  31   b , and  31   a . The radii of the components  31   a  to  31   h  are referred to as radii r 1  to r 8 , and the radii r 1  to r 8  satisfy a relationship in magnitude: r 1 &lt;r 2 &lt;r 3 &lt;r 4 &lt;r 5 &lt;r 6 &lt;r 7 &lt;r 8 . That is, the sizes of the components  31   a  to  31   h  in the radial direction differ from each other. As a component of the components  31   a  to  31   h  is nearer to the plane P at the center of the dielectric member  101  in the thickness direction, the radius thereof is smaller than those of the others. 
     The second member  21   b  has the same structure as that of the first member  21   a  except that these are plane-symmetrical with respect to the plane P. That is, the second member  21   b  includes components that have different radii and that are stacked upward from the lower member  23 . As a component of the components that are included in the second member  21   b  is nearer to the plane P at the center of the dielectric member  101  in the thickness direction, the radius thereof is smaller than those of the others. The thickness h r  of each of the first member  21   a  and the second member  21   b  is equal to the total value of the thicknesses of the component or components  31  that are located at the position of the distance r. 
       FIG.  6    is a graph illustrating a relationship between a distance from the reference axis of the dielectric member illustrated in  FIG.  5    and the equivalent relative dielectric constant of the dielectric member. In  FIG.  6   , the vertical axis represents the equivalent relative dielectric constant ε re , and the horizontal axis represents the proportion r/R of the distance r from the reference axis S to the radius R of the dielectric member  101 . 
     In addition to a graph G 1  illustrating the relationship between the distance r from the reference axis S of the dielectric member  101  and the equivalent relative dielectric constant ε re  of the dielectric member  101 ,  FIG.  6    illustrates a graph G 2  illustrating a relationship between the distance r from the center of a Luneburg lens that has a radius R and a spherical shape and the dielectric constant ε r  of the Luneburg lens. 
     Referring to  FIG.  6   , as for the Luneburg lens that has a spherical shape, relationship between the distance r from the center of the lens and the relative dielectric constant ε r  satisfies an expression:
 
ε r =2−( r/R ) 2 ,  (3)
 
as illustrated in the graph G 2 . That is, the relative dielectric constant ε r  continuously changes in the radial direction. Expression (3) is referred to as a Luneburg lens relational expression. The Luneburg lens that has a spherical shape has relative dielectric constant distribution that satisfies the Luneburg lens relational expression of expression (3) in any radial direction in an XYZ three-dimensional space.
 
     Specifically, in the case where the radius R of the dielectric member  101  is 30 mm, the dielectric constant ε r  at the center of the lens, that is, a position at which the distance r satisfies r=0 mm is 2. The dielectric constant ε r  at a position near a surface of the lens, that is, a position at which the distance r satisfies r=30 mm is 1. 
     The dielectric member  101  of the lens  201  according to the present embodiment has relative dielectric constant distribution such that the distance r from the reference axis S and the relative dielectric constant ε r  satisfy the Luneburg lens relational expression given as expression (3) in the horizontal plane P that is the XY plane. The dielectric member  101  according to the present embodiment does not have the relative dielectric constant distribution that satisfies the Luneburg lens relational expression in the perpendicular direction that is a Z-direction. Thus, the lens  201  according to the present embodiment is a two-dimensional Luneburg lens that satisfies the Luneburg lens relational expression of expression (3) only in the radial direction in an XY two-dimensional space. 
     The lens  201  according to the present embodiment has the relative dielectric constant distribution that satisfies the Luneburg lens relational expression given as expression (3) in the horizontal plane and consequently defines focal points  171   a  to  171   g  on which radio waves are focused on the outer circumference  18  or near the outer circumference  18  of the lens  201  as illustrated in  FIG.  3 E . Waveguides  151   a  to  151   g  cause radio waves to enter the dielectric member  101  from the focal points  171   a  to  171   g  on the outer circumference  18  or near the outer circumference  18  when the radio waves are radiated. The waveguides  151   a  to  151   g  cause radio waves that reach the positions of the focal points  171   a  to  171   g  on the outer circumference  18  or near the outer circumference  18  to propagate toward the radio wave radiators  221  when the radio waves are received. 
     The lens  201  according to the present embodiment enables change into a direction parallel to the two-dimensional plane P depending on the two-dimensional position of the waveguides  151  or the radio wave radiators  221  in the two-dimensional plane P. That is, as illustrated in  FIG.  3 E , the direction of radio waves Bd that are transmitted and received via the waveguide  151   d  differs from the direction of radio waves Bg that are transmitted and received via the waveguide  151   g.    
     As for the dielectric member  101  according to the first embodiment of the present disclosure, the equivalent relative dielectric constant ε re  decreases in the direction from the reference axis S toward the outside of the dielectric member  101  as described above. That is, the thickness h r  of the body portion  21  of the dielectric member  101  is designed such that the desired equivalent relative dielectric constant ε re  is obtained. 
     For example, the thickness h r  is designed such that the equivalent relative dielectric constant ε re  of the dielectric member  101  satisfies expression (3) for the Luneburg lens. 
     Specifically, the thickness h r  is designed to satisfy an expression:
 
 h   r   ={a ×(2−( r/R ) 2 −ε rM1 )}/{(ε rM2 −ε rM1 )/2},  (4)
 
from the relationship of expression (2) described above and expression (3) described above, that is, a relationship:
 
ε re =(2 h   r   /a )×ε rM2 +(( a− 2 h   r )/ a )×ε rM1 =2−( r/R ) 2 .
 
     In addition, the thickness h r  is designed to satisfy, for example, expression (2) described above in the case where the equivalent relative dielectric constant ε re  of the dielectric member  101  is changed stepwise so as to approximate to the relative dielectric constant ε r  of the Luneburg lens as illustrated in the graph G 1  in  FIG.  6   . 
     For example, the thickness h r  is designed such that as the distance r increases, the equivalent relative dielectric constant ε re  decreases to 1.81, 1.74, 1.68, 1.62, 1.53, 1.39, 1.25, or 1.09, that is, decreases stepwise from 2 to 1. 
     The radii and thicknesses of the components  31   a  to  31   h  are designed, for example, to satisfy expression (1) described above such that the equivalent relative dielectric constant ε re  of the dielectric member  101  is the equivalent relative dielectric constant in the graph G 1  illustrated in  FIG.  6   . 
     Here, the radius R of the dielectric member  101  is 30 mm, the thickness a of the dielectric member  101  is 7.112 mm, and the relative dielectric constants ε rM2  of the first member  21   a  and the second member  21   b  are 2.2. 
     In this case, for example, the thickness h r  is designed such that ε re =1.81 is satisfied at positions at which the distance r satisfies 0 mm≤r≤7.9 mm, that is, 0≤r/R≤0.264, specifically is designed to be about 2.40 mm. For example, the thickness h r  is designed such that ε re =1.74 is satisfied at positions at which the distance r satisfies 7.9 mm≤r≤11.7 mm, that is, 0.264≤r/R≤0.389, specifically is designed to be about 2.19 mm. 
     For example, the thickness h r  is designed such that ε re =1.68 is satisfied at positions at which the distance r satisfies 11.7 mm≤r≤14.6 mm, that is, 0.389≤r/R≤0.486, specifically, is designed to be about 2.02 mm. For example, the thickness h r  is designed such that ε re =1.62 is satisfied at positions at which the distance r satisfies 14.6 mm≤r≤17.7 mm, that is, 0.486≤r/R≤0.589, specifically is designed to be about 1.84 mm. 
     For example, the thickness h r  is designed such that ε re =1.53 is satisfied at positions at which the distance r satisfies 17.7 mm≤r≤21.2 mm, that is, 0.589≤r/R≤0.708, specifically, is designed to be about 1.57 mm. For example, the thickness h r  is designed such that ε re =1.39 is satisfied at positions at which the distance r satisfies 21.2 mm≤r≤24.5 mm, that is, 0.708≤r/R≤0.816, specifically is designed to be about 1.16 mm. 
     For example, the thickness h r  is designed such that ε re =1.25 is satisfied at positions at which the distance r satisfies 24.5 mm≤r≤27.4 mm, that is, 0.816≤r/R≤0.913, specifically is designed to be about 0.74 mm. For example, the thickness h r  is designed such that ε re =1.09 is satisfied at positions at which the distance r satisfies 27.4 mm≤r≤30 mm, that is, 0.913≤r/R≤1, specifically is designed to be about 0.27 mm. 
     Two or more adjacent components of the components  31   a  to  31   h  may be integrally formed. 
     The lens  201  is not limited to a structure in which the reference axis S passes through the center of the upper surface  11  and the center of the lower surface  12 , provided that the lens  201  is located at a position at which the radio wave radiation direction is in a desired settable range, and the reference axis S may shift from the center of the upper surface  11 , or the center of the lower surface  12 , or both. 
     The dielectric member  101  is not limited to the columnar member, provided that the dielectric member  101  has the upper surface  11  and the lower surface  12 . 
     [Manufacturing Method] 
       FIG.  7    illustrates a flowchart in which procedures for a method of manufacturing the antenna according to the first embodiment of the present disclosure are defined. 
     Referring to  FIG.  7   , an operator first prepares a member that includes the components  31   a  to  31   h  of the first member  21   a , the components  31   a  to  31   h  of the second member  21   b , the upper member  22 , the lower member  23 , and the waveguides  151 , and the radio wave radiators  221  (step S 11 ). 
     Subsequently, the operator manufactures the first member  21   a  by stacking the components  31   a  to  31   h  in the direction parallel to the reference axis S (step S 12 ). 
     Subsequently, the operator manufactures the second member  21   b  by stacking the components  31   a  to  31   h  in the direction parallel to the reference axis S (step S 13 ). 
     Subsequently, the operator mounts the first member  21   a  and the second member  21   b  between the upper member  22  and the lower member  23 . Specifically, the operator mounts the first member  21   a  on the upper member  22  and mounts the second member  21   b  on the lower member  23  (step S 14 ). 
     The operator disposes the radio wave radiators  221  at positions at which the radio wave radiators  221  face the respective waveguides  151  around the lens  201  in which the first member  21   a  and the second member  21   b  are mounted (step S 15 ). 
     The order of stacking the components  31   a  to  31   h  (step S 12 ) and stacking the components  31   a  to  31   h  (step S 13 ) may be switched. 
     The first member  21   a  and the second member  21   b  may be integrally manufactured by a cutting process. In this case, at step S 11 , a component A that is used for the first member  21   a  and a component B that is used for the second member  21   b  are prepared instead of the components  31   a  to  31   h  of the first member  21   a  and the components  31   a  to  31   h  of the second member  21   b . At step S 12 , a cutting process is performed on the component A to manufacture the first member  21   a . At step S 13 , a cutting process is performed on the component B to manufacture the second member  21   b.    
     [Directivity of Antenna] 
     (Horizontal Plane Directivity of Horizontally Polarized Wave) 
       FIG.  8    is a graph illustrating the horizontal plane directivity of horizontally polarized waves that are transmitted and received by the antenna according to the first embodiment of the present disclosure. In the graph illustrated in  FIG.  8   , the vertical axis represents gain, and the horizontal axis represents the radio wave radiation direction of the horizontally polarized waves that are transmitted and received in the waveguides  151  illustrated in  FIG.  2    in the horizontal plane. The graph illustrated in  FIG.  8    represents the result of a simulation of the horizontal plane directivity of the horizontally polarized waves in the case where a relationship between the equivalent relative dielectric constant ε re  and the distance r from the reference axis S of the dielectric member  101  in the antenna  301  is the same as the relationship illustrated in  FIG.  6   , and radio waves in a band of 28 GHz are transmitted and received. The radius R and thickness a of the dielectric member  101  and the relative dielectric constants ε rM2  of the first member  21   a  and the second member  21   b  are equal to those in the case of  FIG.  6   , and a detailed description is not repeated herein. 
     Referring to  FIG.  8   , the seven waveguides  151  illustrated in  FIG.  2    are referred to herein as the waveguides  151   a ,  151   b ,  151   c ,  151   d ,  151   e ,  151   f , and  151   g . As illustrated in  FIG.  3 F , radio waves that are transmitted and received in the waveguide  151   a  are designated by Ba, radio waves that are transmitted and received in the waveguide  151   b  are designated by Bb, radio waves that are transmitted and received in the waveguide  151   c  are designated by Bc, radio waves that are transmitted and received in the waveguide  151   d  are designated by Bd, radio waves that are transmitted and received in the waveguide  151   e  are designated by Be, radio waves that are transmitted and received in the waveguide  151   f  are designated by Bf, and radio waves that are transmitted and received in the waveguide  151   g  are designated by Bg. Graphs illustrating the directivity of the horizontally polarized waves that are transmitted and received in the waveguides  151   a  to  151   g  in the horizontal plane are graphs Gh 1 , Gh 2 , Gh 3 , Gh 4 , Gh 5 , Gh 6 , and Gh 7 . The graph Gh 1  is related to the radio waves Bg, the graph Gh 2  is related to the radio waves Bf, the graph Gh 3  is related to the radio waves Be, the graph Gh 4  is related to the radio waves Bd, the graph Gh 5  is related to the radio waves Bc, the graph Gh 6  is related to the radio waves Bb, and the graph Gh 7  is related to the radio waves Ba. 
     As illustrated in the graph Gh 4  and  FIG.  3 F , the main radiation direction of the horizontally polarized waves of the radio waves Bd that are transmitted and received in the waveguide  151   d  is standard, that is, 0°. In this case, as illustrated in the graphs Gh 1  to Gh 7 , the main radiation directions of the horizontally polarized waves that are transmitted and received in the waveguides  151   a  to  151   g  in the horizontal plane are about −60°, −40°, −20°, 0°, +20°, +40°, and +60°. 
     The antenna  301  can thus change the radio wave radiation direction of the horizontally polarized waves with the gain ensured. 
     (Horizontal Plane Directivity of Vertically Polarized Wave) 
       FIG.  9    is a graph illustrating the horizontal plane directivity of vertically polarized waves that are transmitted and received by the antenna according to the first embodiment of the present disclosure. In the graph illustrated in  FIG.  9   , the vertical axis represents the gain, and the horizontal axis represents the radio wave radiation direction of the vertically polarized waves that are transmitted and received in the waveguides  151  illustrated in  FIG.  2    in the horizontal plane. The graph illustrated in  FIG.  9    represents the result of a simulation of the horizontal plane directivity of the vertically polarized waves in the case where it is assumed that the relationship between the equivalent relative dielectric constant ε re  and the distance r from the reference axis S of the dielectric member  101  in the antenna  301  is the same as the relationship illustrated in  FIG.  6   , and that radio waves in a band of 28 GHz are transmitted and received. The radius R and thickness a of the dielectric member  101  and the relative dielectric constants ε rM2  of the first member  21   a  and the second member  21   b  are equal to those in the case of  FIG.  6   , and a detailed description is not repeated herein. 
     Referring to  FIG.  9   , graphs illustrating the directivity of the vertically polarized waves that are transmitted and received in the waveguides  151   a  to  151   g  in the horizontal plane are graphs Gv 1 , Gv 2 , Gv 3 , Gv 4 , Gv 5 , Gv 6 , and Gv 7 . The graph Gv 1  is related to the radio wave Bg, the graph Gv 2  is related to the radio wave Bf, the graph Gv 3  is related to the radio wave Be, the graph Gv 4  is related to the radio wave Bd, the graph Gv 5  is related to the radio wave Bc, the graph Gv 6  is related to the radio wave Bb, and the graph Gv 7  is related to the radio wave Ba. 
     As illustrated in the graph Gv 4  and  FIG.  3 F , the main radiation direction of the vertically polarized waves of the radio waves Bd that are transmitted and received in the waveguide  151   d  is standard, that is, 0°. In this case, as illustrated in the graphs Gv 1  to Gv 7 , the main radiation directions of the vertically polarized waves that are transmitted and received in the waveguides  151   a  to  151   g  in the horizontal plane are about −60°, −40°, −20°, 0°, +20°, +40°, and +60°. 
     The antenna  301  can thus change the radio wave radiation direction of the vertically polarized waves with the gain ensured as in the horizontally polarized waves. That is, the antenna  301  can change the radio wave radiation direction by changing the waveguides  151  that are to be used to transmit and receive the radio waves. 
     The antenna  301  is not limited to a structure including the waveguides  151  but may include a single waveguide  151 . In this case, the radio wave radiation direction can be changed from Ba into Bg, for example, by changing the position or direction of the single waveguide  151  from that of the waveguide  151   a  into that of the waveguide  151   g  in  FIG.  2    and  FIG.  3 F . The antenna  301  according to the present embodiment enables the change into the direction parallel to the two-dimensional plane P depending on the two-dimensional position of the waveguides  151  or the radio wave radiators  221  in the two-dimensional plane P. However, change into a direction perpendicular to the two-dimensional plane P is restricted. 
     The components  31   a  to  31   h  illustrated in  FIG.  5    are not limited to a structure in which these have the same relative dielectric constant ε rM . For example, at least a component  31  of the components  31   a  to  31   h  may have a relative dielectric constant ε rM  that differs from those of the other components  31 . 
     For example, the relative dielectric constant ε rM  of the components  31   a  to  31   d  illustrated in  FIG.  5    may differ from the relative dielectric constant ε rM  of the components  31   e  to  31   h . In this case, the equivalent relative dielectric constant ε re  of the lens  201  is equal to, for example, the weighted average of the relative dielectric constant ε rM  of the material of the components  31   a  to  31   d , the relative dielectric constant ε rM  of the material of the components  31   e  to  31   h , and the relative dielectric constant ε rM  of the air. 
     Also, with this structure, the thicknesses of the components  31   a  to  31   h  can be designed such that the equivalent relative dielectric constant ε re  approximates to, for example, that in the graph G 1  illustrated in  FIG.  6    as in the case where the components  31   a  to  31   h  have the same relative dielectric constant ε rM . 
     The first member  21   a  and the second member  21   b  may be formed, for example, by performing a cutting process on an integral component instead of stacking the components  31 . 
     [First Modification] 
       FIG.  10    illustrates the structure of a body portion of a dielectric member according to a first modification to the first embodiment of the present disclosure. 
     Referring to  FIG.  10   , a body portion  41  of a dielectric member  102  according to the first modification includes a first member  41   a  and a second member  41   b . The first member  41   a  and the second member  41   b  are plane-symmetrical to each other with the plane P centered. 
     The first member  41   a  includes members that have different relative dielectric constants ε rM . For example, the first member  41   a  is composed of a material that has a relative dielectric constant ε rM  of about 3 at a position at which the distance r from the reference axis S is 0 mm to a predetermined value rx 1  and a material that has a relative dielectric constant ε rM  of about 2 at a position at which the distance r is more than the predetermined value rx 1 . The material that has a relative dielectric constant ε rM  of about 2 is, for example, polytetrafluoroethylene or polyethylene. 
     The thickness h of the first member  41   a  decreases stepwise in a direction from the reference axis S toward the outside of the dielectric member  102 . 
     The structure of the second member  41   b  is the same as that of the first member  41   a.    
     Consequently, the equivalent relative dielectric constant ε re  of the dielectric member  102  decreases stepwise in the direction from the reference axis S toward the outside of the dielectric member  102 . Specifically, the equivalent relative dielectric constant ε re  of a portion of the dielectric member  102  through which the reference axis S passes is about 2, and the equivalent relative dielectric constant ε re  of an outer edge portion thereof is about 1. 
     In the dielectric member  102  according to the first modification to the first embodiment of the present disclosure, a member that has a low relative dielectric constant ε rM  is thus used for the portion at which the distance r is more than the predetermined value rx 1 , and the thickness h of this portion is consequently more than that in the dielectric member  101  illustrated in  FIG.  3   . For this reason, the strength of the dielectric member  102  can increased. 
     [Second Modification] 
       FIG.  11    illustrates the structure of a body portion of a dielectric member according to a second modification to the first embodiment of the present disclosure. 
     Referring to  FIG.  11   , a body portion  42  of a dielectric member  103  according to the second modification includes a first member  42   a  and a second member  42   b . The first member  42   a  and the second member  42   b  are plane-symmetrical to each other with the plane P centered. 
     The structures of the first member  42   a  and the second member  42   b  are the same as the structures of the first member  21   a  and the second member  21   b  illustrated in  FIG.  5   . That is, the thicknesses h of the first member  42   a  and the second member  42   b  decrease stepwise in a direction from the reference axis S toward the outside of the dielectric member  103 . 
     The dielectric member  103  further includes a low relative dielectric constant member  51  that has a relative dielectric constant ε rM  of no less than 1 and less than 2 as the substance M that has a relative dielectric constant ε rM  of less than 2. Examples of the low relative dielectric constant member  51  include polystyrene containing bubbles, that is, polystyrene foam and is disposed so as to fill a space between the first member  42   a  and the second member  42   b.    
     In the lens  201  according to the second modification to the first embodiment of the present disclosure, the dielectric member  103  thus includes the low relative dielectric constant member  51  that has a relative dielectric constant ε rM  of more than 1 as the substance M that has a relative dielectric constant ε rM  of less than 2. 
     With this structure, for example, the body portion  42  is supported by using the low relative dielectric constant member  51 , and the strength of the dielectric member  103  can be increased. 
     The dielectric member  103  is not limited to a structure in which the low relative dielectric constant member  51  fills the space between the first member  42   a  and the second member  42   b . For example, the first member  42   a  and the second member  42   b  may be connected to each other along the plane P, and the low relative dielectric constant member  51  may surround the first member  42   a  and the second member  42   b.    
     [Third Modification] 
       FIG.  12    illustrates the structure of a body portion of a dielectric member according to a third modification to the first embodiment of the present disclosure. 
     Referring to  FIG.  12   , a body portion  43  of a dielectric member  104  according to the third modification includes a first member  43   a  and a second member  43   b . The first member  43   a  and the second member  43   b  are plane-symmetrical to each other with the plane P centered. 
     The structures of the first member  43   a  and the second member  43   b  are the same as the structures of the first member  21   a  and the second member  21   b  illustrated in  FIG.  5    except for a structure described below. That is, the thicknesses h of the first member  43   a  and the second member  43   b  decrease stepwise in a direction from the reference axis S toward the outside of the dielectric member  104 . 
     More specifically, the first member  43   a  and the second member  43   b  are composed of material that has a relative dielectric constant ε rM  of about 2 and are connected to each other at a portion through which the reference axis S passes. Consequently, the equivalent relative dielectric constant ε re  of the portion of the dielectric member  104  through which the reference axis S passes is about 2, and the equivalent relative dielectric constant ε re  of an outer edge portion of the dielectric member  104  is about 1. 
     In the lens  201  according to the third modification to the first embodiment of the present disclosure, the body portion  43  includes the first member  43   a  and the second member  43   b  that is connected to the first member  43   a  at the portion through which the reference axis S passes. 
     With this structure, the strength of the dielectric member  104  can be increased in the case where the body portion  43  includes the members. 
     Since the dielectric member  104  is composed of the material that has a relative dielectric constant ε rM  of about 2, the volume of the substance M can be smaller than that in the case where the dielectric member  104  is composed of material that has a relative dielectric constant ε rM  of about 3. That is, the length of the body portion  43  in the perpendicular direction, that is, the thickness b thereof is less than the distance a illustrated in  FIG.  5   , and the size of the dielectric member  104  can be decreased. 
     [Fourth Modification] 
       FIG.  13    illustrates the structure of a body portion of a dielectric member according to a fourth modification to the first embodiment of the present disclosure. 
     Referring to  FIG.  13   , a body portion  44  of a dielectric member  105  according to a fourth modification includes a first member  44   a  and a second member  44   b . The first member  44   a  and the second member  44   b  are plane-symmetrical to each other with the plane P centered. 
     The structures of the first member  44   a  and the second member  44   b  are the same as the structures of the first member  21   a  and the second member  21   b  illustrated in  FIG.  5    except for a structure described below. 
     That is, the first member  44   a  and the second member  44   b  are composed of material that has a relative dielectric constant ε rM  of about 3. The thicknesses h of the first member  44   a  and the second member  44   b  decrease stepwise in a direction from the reference axis S toward the outside of the dielectric member  105 . 
     More specifically, the first member  44   a  and the second member  44   b  are connected to each other, for example, at a portion at which the distance r from the reference axis S is 0 mm to a predetermined value rx 2 . 
     In the portion, a notch  52   a  is formed in an end portion of the first member  44   a  opposite the second member  44   b . In the portion, a notch  52   b  is formed in an end portion of the second member  44   b  opposite the first member  44   a . Consequently, the equivalent relative dielectric constant ε re  of the portion of the dielectric member  105  is about 2. 
     In the case where the first member  44   a  and the second member  44   b  are composed of the material that has a relative dielectric constant ε rM  of about 3, and the equivalent relative dielectric constant ε re  of the portion of the dielectric member  105  through which the reference axis S passes is about 2, it is necessary to dispose a substance that has a relative dielectric constant ε rM  of less than 2 in the portion. 
     In the dielectric member  105  according to the fourth modification to the first embodiment of the present disclosure, thereforeupon, the notch  52   a  is formed in the end portion of the first member  44   a  opposite the second member  44   b , and the notch  52   b  is formed in the end portion of the second member  44   b  opposite the first member  44   a.    
     With this structure, the equivalent relative dielectric constant ε re  of the portion of the dielectric member  105  through which the reference axis S passes can be set to about 2, and the strength can be increased by connecting the first member  44   a  and the second member  44   b  to each other. 
     The first member  44   a  and the second member  44   b  may be connected by using a coupling member such as a screw. In this case, the depths of the notches  52   a  and  52   b  are designed depending on, for example, the size of the coupling member. 
     The coupling member such as a screw is preferably composed of resin so as not to affect radio waves. In this case, the sizes and depths of the notches  52   a  and  52   b  are designed such that the equivalent relative dielectric constant ε re  of the dielectric member  105  is the desired value in consideration of the relative dielectric constant ε rM  of the screw. 
     The coupling member may be composed of material containing metal or metal to ensure sufficient strength. In this case, the coupling member is preferably thin to decrease an influence on radio waves. 
     [Fifth Modification] 
       FIG.  14    illustrates the structure of a body portion of a dielectric member according to a fifth modification to the first embodiment of the present disclosure. 
     Referring to  FIG.  14   , a body portion  45  of a dielectric member  106  according to the fifth modification includes a first member  45   a  and a second member  45   b . The first member  45   a  and the second member  45   b  are plane-symmetrical to each other with the plane P centered. 
     The thicknesses h r  of the first member  45   a  and the second member  45   b  at a position the distance r away from the reference axis S in the horizontal plane continuously decrease in a direction from the reference axis S toward the outside of the dielectric member  106 , for example, such that the relationship of expression (4) described above is satisfied, when the relative dielectric constant ε rM1  of the air that is the substance M is 1. 
     In the expression described above, a is the distance between an upper member and a lower member, not illustrated, in the dielectric member  106 , R is the radius of the dielectric member  106 , and ε rM  is the relative dielectric constant of material of which the body portion  45  is composed. 
     The equivalent relative dielectric constant ε re  of the lens  201  according to the fifth modification to the first embodiment of the present disclosure continuously decreases in the direction from the reference axis S toward the outside of the dielectric member  106 . 
     With this structure, the radio wave radiation direction can be more flexibly set. 
     The first member  45   a  and the second member  45   b  can be manufactured, for example, by grinding a single member consist of resin and having a columnar shape by using a lathe. For this reason, manufacturing is easier than in the case of a dielectric lens disclosed in PTL 1. 
     Some features according to the first modification to the fifth modification described above can be combined. 
     Other embodiments of the present disclosure will now be described with reference to drawings. In the drawings, portions like or corresponding to each other are designated by like reference signs, and a description thereof is not repeated. 
     Second Embodiment 
     According to the first embodiment described above, the dielectric member  101  has a plane-symmetrical structure in the perpendicular direction. According to a second embodiment of the present disclosure, however, a dielectric member  111  of an antenna  302  has an asymmetrical structure in the perpendicular direction. 
       FIG.  15    illustrates the structure of the antenna according to the second embodiment of the present disclosure. 
     Referring to  FIG.  15   , the antenna  302  includes a lens  202  and one or more radio wave radiators  221  that are disposed around the lens  202 . 
     The lens  202  includes the dielectric member  111 . The dielectric member  111  is, for example, a columnar member and has an upper surface  13  that is defined by an upper member  25  and a lower surface  14  that is defined by a lower member  26 . 
     The upper surface  13  and the lower surface  14  of the dielectric member  111  have, for example, a circular shape that has a radius R of 30 mm. 
     The dielectric member  111  has an equivalent relative dielectric constant ε re  that decreases in a direction from the reference axis S that passes through the upper surface  13  and the lower surface  14  toward the outside of the dielectric member  111 . The reference axis S passes through, for example, the center of the upper surface  13  and the center of the lower surface  14  and extends in the perpendicular direction. 
       FIG.  16    is a side view of the structure of the dielectric member according to the second embodiment of the present disclosure. 
     Referring to  FIG.  16   , the dielectric member  111  includes a body portion  24  and the substance M that has a relative dielectric constant ε rM  of less than 2. The body portion  24  and the substance M are provided between the upper member  25  and the lower member  26 . Here, the substance M is air. The body portion  21  according to the first embodiment has the upper surface  11  and the lower surface  12 . The body portion  24  according to the second embodiment has the lower surface  14  but does not have the upper surface  13 . The upper surface  13  is defined by the upper member  25  adjacent to the substance M that is the air as described above. 
     The upper member  25  and the lower member  26  are composed of, for example, material containing metal or metal. A distance a between the upper member  25  and the lower member  26  is, for example, 7.112 mm. 
     The body portion  24  is composed of material that has a relative dielectric constant ε rM  of 2 or more, for example, resin that has a relative dielectric constant ε rM  of 3. 
     More specifically, the thickness hx of the body portion  24  decreases in a direction from the reference axis S toward the outside of the dielectric member  111 , and the volume of the air between the upper member  25  and the lower member  26  consequently increases in the direction from the reference axis S toward the outside of the dielectric member  111 . 
     Consequently, for example, the equivalent relative dielectric constant ε re  of a portion of the dielectric member  111  through which the reference axis S passes is about 2, and the equivalent relative dielectric constant ε re  of an outer edge portion thereof is about 1. The equivalent relative dielectric constant ε re  of the dielectric member  111  changes stepwise from 2 to 1 in the direction from the reference axis S toward the outside of the dielectric member  111 . 
     Specifically, as for a relationship among the relative dielectric constant ε rM  of the material of which the body portion  24  is composed, the radius R of the dielectric member  111 , the distance a between the upper member  25  and the lower member  26 , and the thickness hxr of the body portion  24  at a position the distance r away from the reference axis S in the horizontal plane, the thickness hxr is designed so as to approximately satisfy an expression:
 
 hxr=a ×(2−( r/R ) 2 −1)/(ε rM2 −1),  (5)
 
where the relative dielectric constant ε rM1  of the air that is the substance M is 1, for example, as in the relationships in the graph G 1  and the graph G 2  illustrated in  FIG.  6   .
 
     Here, the thickness hx of the body portion  24  changes stepwise in the direction from the reference axis S toward the outside of the dielectric member  111 . 
     More specifically, the body portion  24  includes components  32  that are stacked along the reference axis S. Examples of the components  32  include a disk-shaped member, and the reference axis S passes through the center of a main surface. Here, the body portion  24  includes the eight components  32 , that is, components  32   a ,  32   b ,  32   c ,  32   d ,  32   e ,  32   f ,  32   g , and  32   h.    
     The components  32   a  to  32   h  have the same relative dielectric constant ε rM  and are stacked in a direction from the lower member  26  toward the upper member  25  in order of the components  32   h ,  32   g ,  32   f ,  32   e ,  32   d ,  32   c ,  32   b , and  32   a.    
     The body portion  24  has a conical shape or a truncated cone shape as a whole, and a section that passes through the reference axis S and that is along a YZ plane has a trapezoidal shape or a triangle shape as a whole. The trapezoidal shape includes a shape of a trapezoid that has a stair-like leg portion. Specifically, the radii of the components  32   a  to  32   h  are referred to as radii r 11  to r 18 , and the radii r 11  to r 18  satisfy a relationship in magnitude: r 11 &lt;r 12 &lt;r 13 &lt;r 14 &lt;r 15 &lt;r 16 &lt;r 17 &lt;r 18 . 
     In  FIG.  16   , the body portion  24  has a trapezoid shape in which a short side is near the upper member  25  and a long side is near the lower member  26  in a section that passes through the reference axis S and that is along the YZ plane, but the short side may be near the lower member  26 , and the long side may be near the upper member  25 . 
     The body portion  24  is not limited to being disposed near the lower member  26  and may be disposed near the upper member  25 . 
     The lens  202  may not include the upper member  25 , or the lower member  26 , or both. In this case, the thickness of the dielectric member  111  is preferably set to a thickness equal to or more than a predetermined value. This predetermined value is a value that enables radio waves that propagate in the radial direction in the dielectric member  111  to pass through the inside of the dielectric member  111  before the radio waves leak out through the upper surface  13  or the lower surface  14  of the dielectric member  111 , or both. 
     In the antenna  302  according to the second embodiment of the present disclosure, the body portion  24  is thus a member that has a conical shape or a conical trapezoidal shape. 
     With this structure, the radio wave radiation direction of radio waves that are transmitted and received along a plane perpendicular to the reference axis S can be changed. 
     In addition, the body portion  24  can be manufactured by stacking the components  32 , and manufacturing is easier than in the case where both of the first member  21   a  and the second member  21   b  are manufactured as in the body portion  21  illustrated in  FIG.  5   . 
     In addition, the thickness of the outer edge portion of the body portion  24  can be more than that of the body portion  21  illustrated in  FIG.  5   , and the strength can be increased. 
     [First Modification] 
       FIG.  17    illustrates the structure of a body portion of a dielectric member according to a first modification to the second embodiment of the present disclosure. 
     Referring to  FIG.  17   , a body portion  61  of a dielectric member  112  according to the first modification includes a first member  61   a  and a second member  61   b.    
     The thicknesses of the first member  61   a  and the second member  61   b  decrease stepwise in a direction from the reference axis S toward the outside of the dielectric member  112 . The first member  61   a  and the second member  61   b  are asymmetrical to each other with the plane P centered. 
     More specifically, the thickness of a portion of the first member  61   a  at which the distance r from the reference axis S satisfies r=ra (0 mm≤ra≤R) is less than the thickness of a portion of the second member  61   b  at which the distance r satisfies r=ra. 
     Also, with this structure, the thickness of the body portion  61  decreases stepwise in the direction from the reference axis S toward the outside of the dielectric member  112 . For this reason, the equivalent relative dielectric constant ε re  of the dielectric member  112  decreases stepwise in the direction from the reference axis S toward the outside of the dielectric member  112 . 
     [Second Modification] 
       FIG.  18    illustrates the structure of a body portion of a dielectric member according to a second modification to the second embodiment of the present disclosure. 
     Referring to  FIG.  18   , a body portion  62  of a dielectric member  113  according to the second modification includes a first member  62   a  and a second member  62   b.    
     The thicknesses of the first member  62   a  and the second member  62   b  decrease stepwise in a direction from the reference axis S toward the outside of the dielectric member  113 . The first member  62   a  and the second member  62   b  are asymmetrical to each other with the plane P centered. 
     More specifically, the thickness of the first member  62   b  decreases stepwise at positions at which the distance r from the reference axis S ranges from 0 mm to R. The thickness of the second member  62   a  decreases stepwise at positions at which the distance r from the reference axis S ranges from 0 mm to rx 3  (rx 3 &lt;R) and is 0 mm at positions at which the distance r ranges from rx 3  to R. 
     Also, with this structure, the thickness of the body portion  62  decreases stepwise in the direction from the reference axis S toward the outside of the dielectric member  113 . For this reason, the equivalent relative dielectric constant ε re  of the dielectric member  113  decreases stepwise in the direction from the reference axis S toward the outside of the dielectric member  113 . 
     [Third Modification] 
       FIG.  19    illustrates the structure of a body portion of a dielectric member according to a third modification to the second embodiment of the present disclosure. 
     Referring to  FIG.  19   , a body portion  63  of a dielectric member  114  according to the third modification includes a first member  63   a  and a second member  63   b.    
     The thicknesses of the first member  63   a  and the second member  63   b  decrease stepwise in a direction from the reference axis S toward the outside of the dielectric member  114 . The first member  63   a  and the second member  63   b  have the same shape. 
     The first member  63   a  and the second member  63   b  have different relative dielectric constants ε rM . For example, the relative dielectric constant ε rM  of the first member  63   a  is about 2, and the relative dielectric constant ε rM  of the second member  63   b  is about 3. 
     Also, with this structure, the thickness of the body portion  63  decreases stepwise in the direction from the reference axis S toward the outside of the dielectric member  114 . For this reason, the equivalent relative dielectric constant ε re  of the dielectric member  114  decreases stepwise in the direction from the reference axis S toward the outside of the dielectric member  114 . 
     The other structures are the same as those of the antenna  301  according to the first embodiment of the present disclosure described above, and a detailed description is not repeated herein. 
     Third Embodiment 
     The dielectric member  101  according to the first embodiment of the present disclosure described above includes the components  31  that have the same relative dielectric constant ε rM  and that are stacked along the reference axis S. In a dielectric member  115  according to a third embodiment of the present disclosure, in contrast, components that have different relative dielectric constants ε rM  are stacked into layers in a direction from the reference axis S toward the outside of the dielectric member  115 . 
     [Structure] 
       FIG.  20    is a perspective view of the structure of an antenna according to the third embodiment of the present disclosure. 
     Referring to  FIG.  20   , an antenna  303  according to the third embodiment of the present disclosure includes a lens  203  and one or more radio wave radiators  221  that are disposed around the lens  203 . 
     The lens  203  includes the dielectric member  115 . The dielectric member  115  is, for example, a columnar member and has an upper surface  15  and a lower surface  16 . 
     The thickness of the dielectric member  115  is equal to or more than a predetermined value. This predetermined value is a value that enables radio waves that propagate in the radial direction in the dielectric member  115  to pass through the inside of the dielectric member  115  before the radio waves leak out through the upper surface  15 , or the lower surface  16  of the dielectric member  115 , or both. That is, since the thickness of the dielectric member  115  is equal to or more than the predetermined value, it is not necessary to dispose, for example, members composed of metal near the upper surface  15  and the lower surface  16  of the dielectric member  115 , and the radio waves are prevented from leaking in the vertical direction of the dielectric member  115 . 
     The lens  203  may include an upper member that is disposed near the upper surface  15  of the dielectric member  115 , or a lower member that is disposed near the lower surface  16  of the dielectric member  115 , or both. 
     The upper surface  15  and the lower surface  16  of the dielectric member  115  have, for example, a circular shape that has a radius R of 30 mm. 
     The dielectric member  115  has an equivalent relative dielectric constant ε re  that decreases in a direction from the reference axis S that passes through the upper surface  15  and the lower surface  16  toward the outside of the dielectric member  115 . The reference axis S passes through, for example, the center of the upper surface  15  and the center of the lower surface  16  and extends in the perpendicular direction. 
     In the dielectric member  115 , there is a single kind of substance at the position the distance r away from the reference axis S. For this reason, the equivalent relative dielectric constant ε re  at the position of the distance r is equal to the relative dielectric constant ε rM  of the substance at the position of the distance r. 
     More specifically, the dielectric member  115  includes the components that have different relative dielectric constants ε rM  and that are stacked in the direction from the reference axis S toward the outside of the dielectric member  115 . Specifically, the dielectric member  115  includes a columnar member  71  and annular members  72  as the components. The columnar member  71  is disposed in a portion through which the reference axis S passes. 
     The number of the annular members  72  is seven. The seven annular members  72  are referred to as annular members  72   a ,  72   b ,  72   c ,  72   d ,  72   e ,  72   f , and  72   g , and the annular members  72   a  to  72   g  have a hollow shape and has an annular-shaped section perpendicular to the reference axis S. 
     The annular member  72   a  surrounds the outer circumference of the columnar member  71 , the annular member  72   b  surrounds the outer circumference of the annular member  72   a , the annular member  72   c  surrounds the outer circumference of the annular member  72   b , the annular member  72   d  surrounds the outer circumference of the annular member  72   c , the annular member  72   e  surrounds the outer circumference of the annular member  72   d , the annular member  72   f  surrounds the outer circumference of the annular member  72   e , and the annular member  72   g  surrounds the outer circumference of the annular member  72   f.    
     The waveguides  151  are connected to, for example, the annular member  72   g.    
     The descending order of the magnitudes of the relative dielectric constants ε rM  is the order of those of the columnar member  71 , the annular member  72   a , the annular member  72   b , the annular member  72   c , the annular member  72   d , the annular member  72   e , the annular member  72   f , and the annular member  72   g . Specifically, the relative dielectric constant ε rM  of the columnar member  71  is about 2, and the relative dielectric constant ε rM  of the annular member  72   g  that forms an outer edge portion of the dielectric member  115  is about 1. 
     Consequently, the equivalent relative dielectric constant ε re  of the dielectric member  115  decreases stepwise from 2 to 1 in a direction form the reference axis S toward the outside of the dielectric member  115 . 
     [Manufacturing Method] 
       FIG.  21    illustrates a flowchart in which procedures for a method of manufacturing the antenna according to the third embodiment of the present disclosure are defined. 
     Referring to  FIG.  21   , an operator first prepares the components of the dielectric member  115 , that is, a member that includes the columnar member  71 , the annular members  72   a  to  72   g , and the waveguides  151 , and the radio wave radiators  221  (step S 21 ). 
     Subsequently, the operator stacks the columnar member  71  and the annular members  72   a  to  72   g  into layers in the direction from the reference axis S toward the outside of the dielectric member  115  (step S 22 ). 
     The operator disposes the radio wave radiators  221  at positions at which the radio wave radiators  221  face the respective waveguides  151  around the lens  203  in which the columnar member  71  and the annular members  72   a  to  72   g  are stacked (step S 23 ). 
     In the antenna  303  according to the third embodiment of the present disclosure, the dielectric member  115  thus includes the components that have different relative dielectric constants ε rM , that is, the columnar member  71  and the annular members  72   a  to  72   g . The columnar member  71  and the annular members  72   a  to  72   g  are stacked into the layers in the direction from the reference axis S toward the outside of the dielectric member  115 . 
     The dielectric member  115  can be readily manufactured such that the equivalent relative dielectric constant ε re  changes by a simple element of stacking the columnar member  71  and the annular members  72   a  to  72   g  into the layers. 
     In the antenna  303  according to the third embodiment of the present disclosure, the thickness of the dielectric member  115  is designed such that the radio waves that propagate in the dielectric member  115  are inhibited from leaking out through the upper surface  15  and the lower surface  16 . 
     With this structure, it is not necessary to dispose, for example, members composed of metal near the upper surface  15  and the lower surface  16  of the dielectric member  115 , and the radio waves are prevented from leaking in the vertical direction of the dielectric member  115 . 
     In the method of manufacturing the antenna  303  according to the third embodiment of the present disclosure, the operator first prepares the columnar member  71  and the annular members  72   a  to  72   g  that have different relative dielectric constants ε rM . The operator manufactures the dielectric member  115  by stacking the columnar member  71  and the annular members  72   a  to  72   g  into the layers in the direction from the reference axis S toward the outside described above such that the equivalent relative dielectric constant ε re  decreases in the direction from the reference axis S that passes through the upper surface  15  and the lower surface  16  of the dielectric member  115  toward the outside of the dielectric member  115 . 
     Since the equivalent relative dielectric constant ε re  of the lens  203  thus decreases in the direction from the reference axis S toward the outside of the dielectric member  115 , the radio wave radiation direction can be readily changed. Since the dielectric member  115  has the upper surface  15  and the lower surface  16 , a specific mold, for example, is not needed, and the lens  203  can be readily manufactured unlike the case where a spherical lens is manufactured. 
     Moreover, the dielectric member  115  can be readily manufactured such that the equivalent relative dielectric constant ε re  changes by a simple method of stacking the columnar member  71  and the annular members  72   a  to  72   g  into the layers. 
     Accordingly, the method of manufacturing the antenna  303  according to the third embodiment of the present disclosure enables the lens  203  that can change the radio wave radiation direction to be more readily manufactured. 
     The other structures are the same as those of the antenna  301  according to the first embodiment of the present disclosure described above, and a detailed description is not repeated herein. 
     The features of the antenna  301  according to the first embodiment of the present disclosure and the first modification to the fifth modification to the first embodiment, the antenna  302  according to the second embodiment and the first modification to the third modification to the second embodiment, and the antenna  303  according to the third embodiment can be appropriately combined. 
     It should be thought that the embodiments are described above by way of example in all aspects and are not restrictive. The scope of the present invention is not shown by the above description but is shown by the scope of claims and includes all modifications having the same meaning and range as the scope of the claims. 
     [Additional Remarks] 
     (A-1) A lens including a dielectric member that has an upper surface and a lower surface and having an equivalent relative dielectric constant that decreases in a direction from a reference axis that passes through the upper surface and the lower surface toward the outside of the dielectric member. 
     Since the equivalent relative dielectric constant of the lens decreases in the direction from the reference axis toward the outside of the dielectric member as in (A-1) described above, the radio wave radiation direction can be readily changed. Since the dielectric member has the upper surface and the lower surface, a specific mold, for example, is not needed, and the lens can be readily manufactured unlike the case where a spherical lens is manufactured. Accordingly, the lens that can change the radio wave radiation direction can be more readily manufactured. 
     (A-2) The lens described in (A-1), in which the dielectric member is a columnar member. 
     With the structure in (A-2) described above, the degree of freedom of a position at which a radio wave radiator can be disposed so as to face a side surface of the dielectric member increases, and the radio wave radiation direction can be consequently changed within an increased range. 
     (A-3) The lens described in (A-1) or (A-2), in which the dielectric member includes multiple components that have different relative dielectric constants, and the multiple components are stacked into layers in the direction from the reference axis toward the outside of the dielectric member. 
     The dielectric member can be readily manufactured such that the equivalent relative dielectric constant changes by a simple element of stacking the components into the layers as in (A-3) described above. 
     (A-4) The lens described in any one of (A-1) to (A-3), in which the thickness of the dielectric member is set such that radio waves that propagate in the dielectric member are inhibited from leaking out through the upper surface and the lower surface. 
     With the structure in (A-4) described above, it is not necessary to dispose, for example, members composed of metal near the upper surface and the lower surface of the dielectric member, and the radio waves can be prevented from leaking in the vertical direction of the dielectric member. 
     (A-5) The lens described in (A-1) or (A-2), in which the dielectric member includes a body portion that has a relative dielectric constant of 2 or more, and the thickness of the body portion decreases in the direction from the reference axis toward the outside of the dielectric member. 
     The dielectric member can be readily manufactured such that the equivalent relative dielectric constant changes because of a simple structure in which the thickness of the body portion decreases in the direction from the reference axis toward the outside of the dielectric member as in (A-5) described above. Since the relative dielectric constant of the body portion is 2 or more, the equivalent relative dielectric constant of a portion of the dielectric member through which the reference axis passes can be set to 2 or more. 
     (A-6) The lens described in (A-5), in which the body portion is a member that has a conical shape or a conical trapezoidal shape. 
     With the structure in (A-6) described above, the radio wave radiation direction of radio waves that are transmitted and received along a plane perpendicular to the reference axis can be changed. 
     (A-7) The lens described in (A-5) or (A-6), in which a portion of the dielectric member other than the body portion contains a substance that has a relative dielectric constant of less than 2. 
     With the structure in (A-7) described above, the equivalent relative dielectric constant of the dielectric member can be readily changed by changing a volume ratio between the body portion that has a relative dielectric constant of 2 or more and the substance that has a relative dielectric constant of less than 2. 
     (A-8) The lens described in (A-7), in which the dielectric member includes, as the substance, a member that has a relative dielectric constant of more than 1. 
     With the structure in (A-8) described above, for example, the body portion is supported by using the member described above, and the strength of the dielectric member can be increased. 
     (A-9) The lens described in any one of (A-5) to (A-8), in which the lens has an equivalent relative dielectric constant that continuously decreases in the direction from the reference axis toward the outside of the dielectric member. 
     With the structure in (A-9) described above, the radio wave radiation direction can be more flexibly set. 
     (A-10) The lens described in any one of (A-5) to (A-8), in which the body portion includes multiple components that are stacked along the reference axis and that have the same relative dielectric constant. 
     The body portion can be readily manufactured such that the thickness changes by a simple element of stacking the components that have the same relative dielectric constant along the reference axis as in (A-10) described above. 
     (A-11) The lens described in any one of (A-5) to (A-10), in which the body portion is formed by a cutting process. 
     With the structure in (A-11) described above, a work such as stacking and sticking members is not needed, the body portion of the dielectric member can be manufactured from an integral component, a manufacturing work can be consequently simplified, and manufacturing costs can be consequently reduced. 
     (A-12) The lens described in any one of (A-5) to (A-11), in which the body portion includes a first member and a second member that is connected to the first member at the portion through which the reference axis passes. 
     With the structure in (A-12) described above, the strength of the dielectric member can be increased in the case where the body portion includes the members. 
     (A-13) The lens described in any one of (A-1) to (A-12), in which the lens further includes an upper member that is disposed near the upper surface of the dielectric member and a lower member that is disposed near the lower surface of the dielectric member. 
     With the structure in (A-13) described above, radio waves can be prevented from leaking in the vertical direction of the dielectric member. 
     (A-14) An antenna including a lens that includes a dielectric member and a radio wave radiator that is disposed around the lens, in which the dielectric member has an upper surface and a lower surface and has an equivalent relative dielectric constant that decreases in a direction from a reference axis that passes through the upper surface and the lower surface toward the outside of the dielectric member. 
     Since the equivalent relative dielectric constant of the lens decreases in the direction from the reference axis toward the outside of the dielectric member as in (A-14) described above, the radio wave radiation direction can be readily changed. Since the dielectric member has the upper surface and the lower surface, a specific mold, for example, is not needed, and the lens can be readily manufactured unlike the case of a spherical lens. Accordingly, the antenna that includes the lens that can change the radio wave radiation direction can be more readily manufactured. 
     (A-15) The antenna described in (A-14), in which the height of an opening portion of the radio wave radiator is equal to or less than the thickness of the dielectric member. 
     With the structure in (A-15) described above, a radio wave is inhibited from leaking near the boundary between the radio wave radiator and the dielectric member when the radio wave is radiated from the antenna. 
     (A-16) The antenna described in (A-14), in which the height of the opening portion of the radio wave radiator is equal to or more than the thickness of the dielectric member. 
     With the structure in (A-16) described above, a radio wave can be inhibited from leaking near the boundary between the radio wave radiator and the dielectric member when the radio wave is received by the antenna. 
     (A-17) The antenna described in (A-14), in which the height of the opening portion of the radio wave radiator is equal to the thickness of the dielectric member. 
     With the structure in (A-17) described above, radio waves can be inhibited from leaking near the boundary between the radio wave radiator and the dielectric member when the radio wave is radiated from the antenna and when the radio wave is received by the antenna. 
     (A-18) The antenna described in any one of (A-15) to (A-17), in which the opening portion and the dielectric member are coupled with each other with a member that is composed of material that contains metal or metal interposed therebetween. 
     With the structure in (A-18) described above, a radio wave can be prevented from leaking near the boundary between the radio wave radiator and the dielectric member with more certainty. 
     (A-19) A device for vehicle including an antenna, in which the antenna includes a lens that includes a dielectric member and a radio wave radiator that is disposed around the lens, and the dielectric member has an upper surface and a lower surface and has an equivalent relative dielectric constant that decreases in a direction from a reference axis that passes through the upper surface and the lower surface toward the outside of the dielectric member. 
     Since the equivalent relative dielectric constant of the lens decreases in the direction from the reference axis toward the outside of the dielectric member as in (A-19) described above, the radio wave radiation direction can be readily changed. Since the dielectric member has the upper surface and the lower surface, a specific mold, for example, is not needed, and the lens can be readily manufactured unlike the case of a spherical lens. Accordingly, the antenna that includes the lens that can change the radio wave radiation direction can be more readily manufactured. 
     (A-20) A method of manufacturing a lens that includes a dielectric member, including a step of preparing multiple components that have different relative dielectric constants, and a step of manufacturing the dielectric member by stacking the multiple components into layers in a direction from a reference axis toward the outside such that an equivalent relative dielectric constant decreases in the direction from the reference axis that passes through an upper surface and a lower surface of the dielectric member toward the outside of the dielectric member. 
     Since the equivalent relative dielectric constant of the lens decreases in the direction from the reference axis toward the outside of the dielectric member as in (A-20) described above, the radio wave radiation direction can be readily changed. Since the dielectric member has the upper surface and the lower surface, a specific mold, for example, is not needed, and the lens can be readily manufactured unlike the case of a spherical lens. 
     In addition, the dielectric member can be readily manufactured such that the relative dielectric constant changes by a simple method of stacking the components into the layers. 
     Accordingly, the lens that can change the radio wave radiation direction can be more readily manufactured. 
     (A-21) A method of manufacturing a lens that includes a dielectric member, including a step of preparing multiple components that have the same relative dielectric constant, and a step of manufacturing the dielectric member by stacking the multiple components along a reference axis such that an equivalent relative dielectric constant decreases in a direction from the reference axis that passes through an upper surface and a lower surface of the dielectric member toward the outside of the dielectric member. 
     Since the equivalent relative dielectric constant of the lens decreases in the direction from the reference axis toward the outside of the dielectric member as in (A-21) described above, the radio wave radiation direction can be readily changed. Since the dielectric member has the upper surface and the lower surface, a specific mold, for example, is not needed, and the lens can be readily manufactured unlike the case of a spherical lens. 
     In addition, the dielectric member can be readily manufactured such that the dielectric constant changes by a simple method of stacking the components that have the same relative dielectric constant along the reference axis. 
     Accordingly, the lens that can change the radio wave radiation direction can be more readily manufactured. 
     (A-22) A method of manufacturing a lens that includes a dielectric member, including a step of preparing a component and a step of cutting the component such that an equivalent relative dielectric constant decreases in a direction from a reference axis that passes through an upper surface and a lower surface of the dielectric member toward the outside of the dielectric member. 
     Since the equivalent relative dielectric constant of the lens decreases in the direction from the reference axis toward the outside of the dielectric member as in (A-22) described above, the radio wave radiation direction can be readily changed. Since the dielectric member has the upper surface and the lower surface, a specific mold, for example, is not needed, and the lens can be readily manufactured unlike the case of a spherical lens. 
     In addition, a work such as stacking and sticking members is not needed, the dielectric member can be manufactured from an integral component, a manufacturing work can be consequently simplified, and manufacturing costs can be consequently reduced. 
     (B-1) A lens including a dielectric member that has an upper surface and a lower surface, in which 
     an equivalent relative dielectric constant decreases in a direction from a reference axis that passes through the upper surface and the lower surface toward the outside of the dielectric member, 
     the reference axis passes through the center of the upper surface and the center of the lower surface and extends in the perpendicular direction, 
     the dielectric member includes multiple components that are stacked along the reference axis and that have the same relative dielectric constant, 
     the components are disk-shaped members, and 
     the reference axis passes through the center of a main surface of each component. 
     (B-2) An antenna including a lens that includes a dielectric member, and 
     a radio wave radiator that is disposed around the lens, in which 
     the dielectric member has an upper surface and a lower surface, and an equivalent relative dielectric constant decreases in a direction from a reference axis that passes through the upper surface and the lower surface toward the outside of the dielectric member, 
     the lens further includes a waveguide, and 
     the radio wave radiator is a horn antenna and is disposed at a position at which the radio wave radiator faces the waveguide. 
     REFERENCE SIGNS LIST 
     
         
         
           
               11 ,  13 ,  15  first surface (upper surface) 
               12 ,  14 ,  16  second surface (lower surface) 
               18  outer circumference 
               21 ,  24 ,  41 ,  42 ,  43 ,  44 ,  45 ,  61 ,  62 ,  63  first substance (body portion) 
               21   a ,  41   a ,  42   a ,  43   a ,  44   a ,  45   a ,  61   a ,  62   a ,  63   a  first member 
               21   b ,  41   b ,  42   b ,  43   b ,  44   b ,  45   b ,  61   b ,  62   b ,  63   b  second member 
               22 ,  25  upper member 
               23 ,  26  lower member 
               31 ,  31   a  to  31   h ,  32 ,  32   a  to  32   h  component 
               51  low relative dielectric constant member 
               52   a ,  52   b  notch 
               71  columnar member 
               72 ,  72   a  to  72   g  annular member 
               101  to  106 ,  111  to  115  dielectric member 
               151 ,  151   a  to  151   g  waveguide 
               171   a  to  171   g  focal point 
               161  wireless base station device 
               201 ,  202 ,  203  lens 
               221  radio wave radiator 
               301 ,  302 ,  303  antenna 
               401  device for vehicle 
             B radio wave 
             M second substance 
             P two-dimensional plane 
             S reference axis 
             Z direction of the reference axis