Abstract:
A coaxial connector includes an outer conductive member including a board mounting portion and a main body portion; an insulation member disposed in the outer conductive member; a central conductive member supported with the insulation member; and a metal member disposed in the outer conductive member below the insulation member. The metal member includes a through hole for retaining the central conductive member therein. The central conductive member is situated in the through hole away from an inner surface of the through hole by a first distance at an upper portion of the through hole. The central conductive member is situated in the through hole away from the inner surface of the through hole by a second distance at a lower portion of the through hole. The first distance is greater than the second distance.

Description:
BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT 
     The present invention relates to a coaxial connector. In particular, the present invention relates to a coaxial connector with improved impedance characteristics. 
     Patent Reference has disclosed a conventional coaxial connector.  FIG. 13  shows a configuration of the conventional coaxial connector. 
     Patent Reference: Japanese Patent Application Publication No. 2003-178844 
     As shown in  FIG. 13 , the conventional coaxial connector includes a first outer conductive member  101 , an insulating body  102 , a second outer conductive member  103 , and a central conductive member pin  105 . The conventional coaxial connector is to be secured on a printed circuit board with screws  116  through screw holes  114  provided on a bottom surface of the first outer conductive member  101 . The central conductive member pin  105  can elastically contact with a circuit pattern  112  in a stable state via a rotatable spherical structure  107 . 
     In these days, there is a demand for a coaxial connector for inspection, which can be secured on a circuit board surface at a high density with screws and can exhibit satisfactory impedance characteristics relative to high-frequency signals. In the conventional coaxial connector disclosed in Patent Reference, however, it is necessary to have a complicated structure, in which the central conductive member pin  105  is provided with elasticity in a vertical direction, and a rotatable spherical structure is disposed at an end of the central conductive member pin  105 . In addition, in case of standard products such as typical BNC and SMB, when the conventional coaxial connector is used to process high-frequency signals higher than 3 GHz, impedance mismatch easily tends to occur, thereby deteriorating the impedance characteristics. 
     In view of the problems described above, an object of the present invention is to provide a coaxial connector with improved impedance characteristics. 
     Further objects and advantages of the present invention will be apparent from the following description of the present invention. 
     SUMMARY OF THE PRESENT INVENTION 
     In order to achieve the above object, based on knowledge that impedance characteristics are determined by a ratio between an outer diameter of the central conductive member and an inner diameter of the outer conductive member, it is assumed that it could be possible to improve the impedance characteristics by adjusting the ratio. Here, the impedance characteristics have close relationship with insertion loss and a voltage standing wave ratio (VSWR), so that those characteristics are also taken into consideration. After various simulations, it is confirmed that the impedance characteristics can be improved with the following configurations. 
     According to a first aspect of the present invention, a coaxial connector includes an outer conductive member; an insulation member; a central conductive member; and an annular metal member. The outer conductive member includes a board mounting portion and a cylindrical main body portion, which is vertically provided along an axial direction from the board mounting portion. The outer conductive member has a through hole, with which the board mounting portion and the main body portion are connected to each other. The insulation member is to be accommodated inside of the through hole of the outer conductive member. 
     According to the first aspect of the present invention, the central conductive member is supported by the insulation member, and is to be disposed inside of the through hole of the outer conductive member along the axial direction. The annular metal member has a facing surface that faces at least a part of the insulation member in a surface that extends in a radial direction perpendicular to the axial direction, and is to be accommodated in the through hole of the outer conductive member on a side of the board mounting portion relative to the insulation member. The central conductive member supported by the insulation member penetrates a through hole of the annular metal member along the axial direction, and has an air layer that expands in the radial direction between an outer surface of the central conductive member and an inner circumferential surface of the through hole. The diameter of the air layer of the annular metal member near the facing surface is configured to be larger than a diameter of the air layer of the annular metal member at a position that is away from the facing surface. 
     According to a second aspect of the invention, the coaxial connector may include an annular step portion on an inner circumferential surface of the through hole of the annular metal member. The annular step portion extends from the facing surface towards a surface opposite to the facing surface. As a result, the diameter of the air layer of the annular metal member near the facing surface can be set larger than the diameter of the air layer of the annular metal member at a position that is away from the facing surface. 
     According to a third aspect of the invention, the coaxial connector may include a plurality of step portions on the inner circumferential surface of the through hole of the annular metal member. The step portions extend from the facing surface towards the surface opposite the facing surface. As a result, the diameter of the air layer of the annular metal member near the facing surface is set larger than the diameter of the air layer of the annular metal member at a position that is away from the facing surface. 
     According to a fourth aspect of the invention, the coaxial connector may include a tapered portion on the inner circumferential surface of the through hole of the annular metal member. The tapered portion extends from the facing surface towards a surface opposite the facing surface. As a result, the diameter of the air layer of the annular metal member near the facing surface can be set larger than the diameter of the air layer of the annular metal member at a position that is away from the facing surface. In addition, the diameter of the air layer at the tapered portion can be set so as to become larger from a side that is away from the facing side to a side that is close to the facing surface. 
     According to a fifth aspect of the invention, the coaxial connector may include a plurality of groove portions formed in the facing surface. The groove portions extend from a center of the through hole of the annular metal member towards outside of the annular metal member. As a result, at least at the groove portions, the diameter of the air layer of the annular metal member near the facing surface can be set larger than the diameter of the air layer of the annular metal member at a position that is away from the facing surface. 
     According to a sixth aspect of the invention, in the coaxial connector, the groove portions may be provided at a plurality of positions radially around the center of the through hole of the annular metal member. In addition, a plurality of generally fan-shaped portions may be formed on the facing surface of the annular metal member. 
     According to a seventh aspect of the invention, in the coaxial connector, the central conductive member is preferably provided to protrude more than the annular metal member in a direction from the main body portion to the board mounting portion. The annular metal member is preferably provided to protrude more than the board mounting portion in a direction from the main body portion towards the board mounting portion. 
     According to an eighth aspect of the invention, in the coaxial connector, the annular metal member is preferably plated with a material of higher conductivity than the material of the annular metal member. 
     According to a ninth aspect of the invention, in the coaxial connector, the through hole of the outer conductive member may include a engaging portion, which engages with the insulation member accommodated in the through hole of the outer conductive member, and another engaging portion, which engages with the annular metal member accommodated in the through hole of the outer conductive member. 
     As described above, according to the present invention, it is achievable to provide a coaxial connector with improved impedance characteristics. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing an outer appearance of a coaxial connector used in simulations; 
         FIG. 2  is an exploded perspective view showing the coaxial connector used in the simulations; 
         FIG. 3  is a vertical sectional view showing the coaxial connector used in the simulations; 
         FIG. 4  is a view showing an arrangement of the coaxial connector used in the simulations; 
         FIG. 5  is a diagram showing simulation results regarding insertion loss; 
         FIG. 6  is a diagram showing simulation results regarding voltage standing wave ratio (VSWR); 
         FIG. 7  is a diagram showing simulation results regarding impedance; 
         FIGS. 8( a ) and 8( b )  are views showing a configuration of a coaxial connector according to Example 1; 
         FIGS. 9( a ) and 9( b )  are views showing a configuration of a coaxial connector according to Example 2; 
         FIGS. 10( a ) and 10( b )  are views showing a configuration of a coaxial connector according to Example 3; 
         FIGS. 11( a ) and 11( b )  are views showing a configuration of a coaxial connector according to Example 4; 
         FIGS. 12( a ) and 12( b )  are views showing a configuration of a coaxial connector according to Comparative Example; and 
         FIG. 13  is a view showing a configuration of a conventional coaxial connector. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereunder, an embodiment of the present invention will be described with reference to the accompanying drawings. 
     [Configuration of a Coaxial Connector] 
     A simulation was conducted to study influences of a configuration of a coaxial connector at and around an interface between an annular metal member and an insulation member, which face each other on impedance characteristics of the coaxial connector. In the simulations, a coaxial connector  1  having a configuration shown in  FIGS. 1  through  3  was used.  FIG. 1  is a perspective view of outer appearance of the coaxial connector  1 .  FIG. 2  is an exploded perspective view of the coaxial connector  1 .  FIG. 3  is a vertical sectional view of the coaxial connector  1 . Here, the coaxial connector  1  shown in those figures is not a conventional one, but the one invented by the present inventors upon conducting the simulations. 
     The coaxial connector  1  can be used, for example, as a coaxial connector for inspection, which is to be vertically screwed on an evaluation board surface for high-speed transmission. With advancement in achieving higher speed transmission of higher frequency signals, the coaxial connector requires a connector for an evaluation board, to which it is possible to send coaxial signals of high frequency at high density. The coaxial connector  1  is suitable as such a connector for an evaluation board. Here, the use of the coaxial connector  1  is not limited to the one for an evaluation board, and of course. The coaxial connector  1  can be also used for general connections. 
     The coaxial connector  1  mainly includes an outer conductive member  10 , a central conductive member  20 , an insulation member  30 , and an annual metal member  40 . 
     The outer conductive member  101  may be produced, for example, by cutting metal such as stainless steel and brass. The outer conductive member  10  mainly includes a board mounting portion  12  and a main body portion  11 . The board mounting portion  12  is a portion to be mounted on a board (not illustrated) and is formed as a flat body having a certain thickness. 
     On left and right sides of the board mounting portion  12 , there are provided screw holes  16  that are through holes provided to secure the outer conductive member  10  on a board. With those screw holes  16 , it is possible to vertically secure the annular metal member  40 , which is for securing the insulation member  30  on a surface of the mounting board, on a surface of the mounting board as a secure grounding surface of the outer conductive member  10 . The main body portion  11  is a portion that is vertically provided in an axial direction from the board mounting portion  12 , and has a cylindrical shape as a whole. On an insertion side of a mating terminal of the main body portion  11 , there are provided screw portions  17  that enable connection to mating terminals (not illustrated) by screws. 
     On the board mounting portion  12  and the main body portion  11 , there is provided a through hole  15  that is continuous therebetween. The through hole  15  includes a large-diameter portion  15 - 1 , to which a part of a mating terminal is inserted, a small diameter portion  15 - 2 , which is formed by cutting so as to have the diameter thereof gradually small from the board mounting surface and is for disposing the central conductive member  20  and the insulation member  30  therein, and a medium-sized diameter portion  15 - 3  to dispose the annular metal member  40  therein. 
     The insulation member  30  may be produced, for example, from resin. The insulation member  30  can include, for example, three coaxial ring portions, i.e., a small-diameter ring  37 , a medium-diameter ring  38 , and a large diameter ring  39 . Alternatively, the insulation member  30  can have a shape of those rings piled up in the order. Each ring includes a holding hole  35  having a certain diameter, so that all those rings form a through hole. The insulation member  30  is inserted to be accommodated in the small diameter hole  15 - 2  of the through hole  15 , which is continuous between the board mounting portion  12  and the main body portion  11 , via the medium diameter portion  15 - 3  from the board mounting side of the outer conductive member  10 . 
     In order to position the insulation member  30  to a specified position of the through hole  15 , it is possible to use a flange  31  formed between the medium diameter ring  38  and the large diameter ring  39  of the insulation member  30 . It is also possible to restrict excess press-in of the insulation member  30  in the through hole  15  by having the flange  31  abut against the engaging step portion (engaging portion)  18  provided in the through hole  15 . 
     The central conductive member  20  can be also produced, from example, by fabricating sheet metal of phosphor bronze. On a side for insertion of a mating terminal, there is formed a plurality of elastic arms  28  to elastically contact with the mating terminal. The elastic arms  28  are formed by having slotted end portions  21 . The central conductive member  20  is supported by a holding hole  35  of the insulation member  30 . 
     In order to position the central conductive member  20  at a specified position of the holding hole  35 , there are provided an annular flange  27  and a slanted protrusion  22  near a middle part of the central conductive member  20 . When the central conductive member  20  is inserted in the holding hole  35  of the insulation member  30  from a side near the small diameter ring  37 , the annular flange  27  abuts an upper surface of the small diameter ring  37 , and the slanted protrusion  22  is secured in a dent  32  of the insulation member  30 . With the configuration, the central conductive member  20  is supported by the insulation member  30 , and at the same time, is disposed inside of the through hole  15  of the outer conductive member  10  along the axial direction via the insulation member  30 . 
     The annular metal member  40  has a ring shape, in which a through hole  45  having a certain diameter is provided at a center thereof. The annular metal member  40  can be produced, for example, by cutting metal such as stainless steel and bronze. In order to increase conductivity, it is preferable to apply plating on a side of a mounting surface of the annular metal member  40  or the whole thereof with a highly conductive material such as metal plating and silver plating, also in view of electrical performance and cost reduction. The annular metal member  40  is inserted to be accommodated in the medium diameter portion  15 - 3  on a board-mounting side of the outer conductive member  10  on a board-mounting side relative to the insulation member  30 . 
     In order to position the annular metal member  40  to a specified position of the through hole  15 , there is provided an engaging step portion (engaging portion)  19  on the through hole  15 . On to the engaging portion  19 , one surface  44  of the annular metal member  40 , which is present within a surface perpendicular to the axial direction, is disposed so as to face at least a part (the facing surface  34 ) of the insulation member  30 . It is also possible to restrict excess press-in of the annular metal member  40  to the through hole  15  by abutting the annular metal member  40  to the engaging step portion (engaging portion)  19 . 
     In order to make smooth insertion of the annular metal member  40  into the outer conductive member  10 , it is also possible to provide a small annular chamfered portion  42  on an edge of an outer surface  47  of the annular metal member  40  on a side for inserting a mating terminal. With the chamfered portion  42 , an outer diameter of the one surface  44  of the annular metal member  40  is slightly smaller than an outer diameter of the facing surface  34  of the annular metal member  40 , but it is still larger than the outer diameter of the insulation member  30 . 
     Moreover, the diameter of the through hole  45  of the annular metal member  40  is larger than the outer diameter of the central conductive member  20 , but it is smaller than the outer diameter of the insulation member  30 . With the dimensional relation, it is achievable to prevent the insulation member  30  from coming off by disposing the annular metal member  40  so as to face at least a part (facing surface  34 ) of the insulation member  30 . 
     The coaxial connector  1  is assembled by inserting the insulation member  30  in the small diameter portion  15 - 2  of the through hole  15  of the outer conductive member  10 , then inserting the annular metal member  40  in the medium diameter portion  15 - 3 , and lastly inserting the central conductive member  20  into the holding hole  35  of the insulation member  30  accommodated in the through hole  15 . 
     After assembling, the central conductive member  20 , which is supported by the holding hole  35  of the insulation member  30 , is disposed, having the through hole  45  be penetrated along the axial direction (a direction along the arrow “A” in  FIG. 3 ). With the central conductive member  20  being disposed in the through hole  45 , there is formed an air layer  33 , which extends in a radial direction being perpendicular to the axial direction, between the outer surface  24  of the central conductive member  20  and the inner circumferential surface  48  of the through hole  45 . 
     Here, the central conductive member  20  is preferably in a state of protruding for the same amount as or slightly less than the annular metal member  40  in the direction from the main body portion  11  to the board mounting portion  12  along the axial direction (direction indicated as the arrow “A” in  FIG. 3 ). In addition, the annular metal member  40  is preferably provided in a state of protruding slightly more than the board mounting portion  12  in the direction from the main body portion  11  to the board mounting direction (the direction “A”). 
     When the central conductive member  20  and the annular metal member  40  are in those states, even if the board mounting portion  12  is secured without using solder, e.g. by screwing onto a board surface, the mounting surface  23  of the central conductive member  20  surely contacts with the board. Similarly, the mounting surface of the annular metal member  40  also surely contacts with the board. 
     [Simulation Software] 
     Simulation software used in the invention was ANSYS HFSS Ver. 15, which is common software and can be easily obtained. 
     [Simulation Method] 
     Two coaxial connectors shown in  FIGS. 1 through 3  were used. As shown in  FIG. 4 , the two coaxial connectors  1  and  1 ′ were disposed so as to have the mounting portions  12  and  12 ′ of the outer conductive members  10  and  10 ′ face each other. Here, as described above, the central conductive member  20  is provided in a state of protruding more than the annular metal member  40  in a direction from the main body portion  11  to the board mounting portion  12  (the direction “A” in  FIG. 3 ). 
     At the same time, the annular metal member  40  is provided in a state of protruding more than the board mounting portion  12  in the direction from the main body portion  11  to the board mounting portion  12  (the direction “A”). Therefore, the portions that face each other when the coaxial connectors  1  and  1 ′ are mechanically secured with screws (not illustrated) are electrically connected to each other. More specifically, the mounting surface  43  of the annular metal member  40  of one coaxial connector  1  is connected to the mounting surface  43 ′ of the annular metal member  40 ′ of the other coaxial connector  1 ′. The mounting surface  23  of the central conductive member  20  of the one coaxial connector  1  is connected to the mounting surface  23 ′ of the central conductive member  20 ′ of the other coaxial connector  1 ′. 
     Here, the outer conductive member  10  and the annular metal member  40  of the one coaxial connector  1  are electrically connected. Moreover, the outer conductive member  10 ′ and the annular metal member  40 ′ of the other coaxial connector  1 ′ are electrically connected. Therefore, the outer conductive member  10  and the outer conductive member  10 ′ are electrically connected, similarly to the connection between the annular metal member  40  and  40 ′. 
     To the coaxial connectors  1  and  1 ′ disposed as described above, connected are coaxial cables (not illustrated), which are respectively connected to input and an output of a network analyzer. Then, to the one coaxial connector  1 , electric signals of up to 50 GHz were input. Upon this input, based on that impedance characteristics are determined by a ratio between the outer diameter  29  of the central conductive member  20  and the inner diameter  49  of the annular metal member  40 , it is anticipated that it may be possible to improve the impedance characteristics by changing a configuration at and around an interface between facing annular metal member  40  and insulation member  30 . Accordingly, we conducted simulations of impedance and insertion loss and voltage standing wave ratio (VSWR) that influences the impedance for annular metal members of various shapes (annular metal members  40   a - 40   e  shown in  FIGS. 8( a )-8( b ) to 12( a )-12( b ) , which will be described later). 
     [Simulation Results] 
     Detailed simulation results are shown in  FIGS. 5 to 7  only for shapes that gave relatively good results. 
       FIG. 5  shows simulation results of the insertion loss in all Examples and Comparative Example in one sheet, which were seen in output signals obtained by the other coaxial connector  1 ′ when electrical signals of up to 50 GHZ were input in the one coaxial connector  1 . Frequencies (GHz) of up to 50 GHz is taken at the abscissa and the insertion loss (dB) is taken at the ordinate. Obviously, as the insertion loss (dB) is closer to “0”, loss is less, so that the value is close to an ideal one as it is closer to “0”. 
       FIG. 6  shows simulation results regarding voltage standing wave ratio (VSWR) in all Examples and Comparative Example in one sheet, and shows signals reflected to the one coaxial connector  1  when electric signals of up to 50 GHZ were input in the one coaxial connector  1 . In the diagram, the frequency of up to 50 GHz is taken at the abscissa and a value of standing wave ratio is taken at the ordinate. Obviously, as the voltage standing wave ratio is closer to “1”, the reflection is less, so that the value is ideal if the value is closer to “1”. 
       FIG. 7  shows simulation results of impedance in all Examples and Comparative Example in one sheet, which were calculated from output signals obtained in the other coaxial connector  1 ′ when electrical signals of up to 50 GHz were input to the one coaxial connector  1 . The abscissa represents time (ns) and the ordinate represents resistance (Ω), respectively. Since a coaxial line of 50Ω is assumed, when the value of impedance in  FIG. 7  is closer to 50Ω, the impedance match is more satisfactory. In addition, when the insertion loss is even smaller and the voltage standing wave ratio is close to “1”, the impedance matching collapses. Therefore, needless to say, in this case, the impedance characteristics are definitely good. 
     Here, there is no obvious relation with parts of the coaxial connectors  1  and  1 ′ for waveform that indicates the insertion loss in  FIG. 5  and the waveform showing the voltage standing wave ratio in  FIG. 6 . On the other hand, between the waveform showing the impedance and parts of the coaxial connectors  1  and  1 ′, there is some correlation recognized, although it is impossible to strictly compare since there is a difference between electrical length and a physical length. For this reason, for  FIG. 7 , we showed parts of the coaxial connectors  1  and  1 ′ that correspond to the waveforms along with the waveforms showing impedance for convenience, and we revealed the correlation therebetween. 
       FIGS. 8( a )-8( b ) to 11( a )-11( b )  show portions including the annual metal members  40   a  to  40   d  used in respective Examples, including portions therearound. Furthermore,  FIGS. 12( a ) and 12( b )  show a portion including the annular metal member  40  used in Comparative Example and a portion therearound. 
     As a result of the simulations, from comparison between Examples shown in  FIGS. 8( a )-8( b ) to 11( a )-11( b )  and Comparative Example shown in  FIGS. 12( a ) and 12( b ) , it was found that satisfactory simulation results were obtained, when the diameter  49 ′ of the air layer  33  of the annular metal member  40  near the facing surface  44  is set larger than the diameter  49  of the air layer  33  of the annular metal member  40  at a position that is away from the facing surface  44 . Here, for convenience, in  FIGS. 8( a )-8( b ) to 11( a )-11( b )  (and  FIGS. 12( a ) and 12( b ) ), similar reference numerals to those in  FIGS. 1 to 3  are used for members that correspond to members in  FIGS. 1 to 3 . 
     Example 1 
     Single-Step Configuration 
     Using the annular metal member  40   a  shown in  FIG. 8( a ) , simulation was conducted.  FIG. 8( b )  is a partial sectional view that corresponds to  FIG. 3 , when the annular metal member  40   a  was used. Being different from those shown in  FIGS. 1 to 3 , in case of the annular metal member  40   a , there is provided an annular step portion  51  on an inner circumferential surface  48  of the through hole  45 , which extends from the facing surface  44  to the mounting surface  43  that is provided opposite the facing surface  44 . As a result, the diameter  49 ′ of the air layer  33  of the annular metal member  40   a  near the facing surface  44  is larger than the diameter  49  of the air layer  33  of the annular metal member  40   a  at a position that is away from the facing surface  44 . 
     As shown in  FIGS. 5 to 7 , in this case, quite better results were obtained for any of the insertion loss, voltage standing wave ration, and impedance, in comparison with the configuration shown in  FIGS. 1 to 3 , in which the annular step portion is not provided. 
     Example 2 
     Double-Step Configuration 
     Using the annular metal member  40   b  shown in  FIG. 9( a ) , simulation was conducted.  FIG. 9( b )  is a partial sectional view equivalent to  FIG. 3  when the annular metal member  40   b  was used. Being different from those shown in  FIGS. 1 to 3 , the annular metal member  40   b  includes a plurality of the annular step portions (double-step portion in this Example)  51 ′ and  51 ″ on the inner circumferential surface  48  of the through hole  45 , which extends from the facing surface  44  to the mounting surface  43  that is provided opposite the facing surface  44 . A difference from Example 1 shown in  FIGS. 8( a ) and 8( b )  is that a plurality of (two in this Example) annular step portions is provided. Here, in the annular step portion  51 ′ and  51 ″, the diameter  49 ′ of the air layer  33  in the annular step portion  51 ′ that is closer to the facing surface  44  is set larger than the diameter  49 ″ of the air layer  33  in the annular step portion  51 ″ that is away from the facing surface  44 . 
     As shown in  FIGS. 5 to 7 , in this case, similar to the configuration of Example 1, in which the annular step portion  51  has a single-step configuration, results of the insertion loss, voltage standing wave ratio, and impedance are much better than those for the configurations shown in  FIGS. 1 to 3 . Moreover, in comparison with Example 1, the results were slightly better. 
     Example 3 
     Tapered Configuration 
     Using an annular metal member  40   c  shown in  FIG. 10( a ) , simulation was conducted.  FIG. 10( b )  is a partial sectional view equivalent to  FIG. 3  when the annular metal member  40   c  was used. Being different from those shown in  FIGS. 1-3 , in case of the annular metal member  40   c , there is provided a tapered portion  52  on the inner circumferential surface  48  of the through hole  45 , which extends from the facing surface  44  to the mounting surface  43  that is opposite the facing surface  44 . Here, an inner diameter of the tapered portion  52  is set large from a side that is away from the facing surface  44  to a side close to the facing surface  44 . As a result, the diameter  49 ′ of the air layer  33  of the annular metal member  40   c  near the facing surface  44  is larger than the diameter  49  of the air layer  33  of the annular metal member  40   c  at a position that is away from the facing surface  44 . 
     As shown in  FIGS. 5 to 7 , in this case, similarly to Examples 1 and 2, results were slightly better in any of the insertion loss, the voltage standing wave ratio, and the impedance, in comparison with those in  FIGS. 1 to 3 , in which the annular step portion is not provided. In addition, in this case, slightly better results were obtained in comparison with Example 1, but values were comparable to those in Example 2. 
     Example 4 
     Slit Configuration 
     Using an annular metal member  40   d  shown in  FIG. 11( a ) , simulation was conducted.  FIG. 11( b )  is a partial sectional view equivalent to  FIG. 3  when the annular metal member  40   d  was used. Being different from those shown in  FIGS. 1-3 , the annular metal member  40   d  includes a plurality of groove portions (slits)  60  on the facing surface  44  from the center of the through hole  45  of the annular metal member  40   d  towards outside of the annular metal member  40   d . Furthermore, the groove portions  60  are provided in plurality radially around the center of the through hole  45  of the annular metal member  40   d  at equal intervals. As a result, the diameter  49 ′ at a certain part of the air layer  33  of the annular metal member  40   d  near the facing surface  44  is larger than the diameter  49  of the air layer  33  of the annular metal member  40   d  at a position away from the facing surface  44 . 
     As shown in  FIGS. 5 to 7 , in this case, similarly to Examples 1 and 2, results are better than those of the configurations shown in  FIGS. 1 to 3  in any of the insertion loss, the voltage standing wave ratio, and the impedance. However, in comparison with Examples 1 to 3, the results are slightly poor. 
     Comparative Example 
     Without Step-Like Structure 
     For a comparative example, using the annular metal member  40  shown in  FIGS. 1 to 3 , simulation was conducted. As in the Examples, the inner diameter of the annular metal member was not fabricated to have a special structure. Therefore, the diameter  49  of the air layer  33  of the annular metal member  40  near the facing surface  44  is equal to the diameter  49  of the air layer  33  of the annular metal member  40  at a position that is away from the facing surface  44 . 
     As shown in  FIGS. 5 to 7 , in this case, only poor results than those in Examples 1 to 4 were obtained for any of the insertion loss, the voltage standing wave ration, and the impedance. 
     CONCLUSION AND DISCUSSION 
     As is obvious from  FIGS. 5 to 7 , in Examples 1 to 4, in which certain fabrication was applied to the annular metal members, better results were obtained in any of the insertion loss, the voltage standing wave ratio, and the impedance, in comparison with Comparative Example. Therefore, it is revealed that it is generally possible to obtain satisfactory results when the diameter  49  of the air layer  33  of the annular metal member  40   a  to  40   d  near the facing surface  44  is set larger than the diameter  49  of the air layer  33  of the annular metal member  40   a  to  40   d  at a position that is away from the facing surface  44 . 
     High-frequency characteristics depend on an outer diameter of a central conductive member, an inner diameter of an outer conductive member, and permittivity of an insulating material provided between the central conductive member and the outer conductive member. Therefore, points where the inner diameter of the outer conductive member changes are considered as changing points of permittivity. Providing step-like structure or the like so as to enable forming an air layer, the permittivity of which is stable, the amount of change can be mild and mismatching of impedance was restrained. 
     Here, although details are not provided herein, similarly good results were obtained when embodiments of Examples 1 to 4 were employed in combination. Therefore, the scope of the present invention also includes all aspects in those variations, alterations, and modifications. 
     The disclosure of Japanese Patent Applications No. 2014-021311, filed on Feb. 6, 2014, is incorporated in the application by reference. 
     While the present invention has been explained with reference to the specific embodiments of the present invention, the explanation is illustrative and the present invention is limited only by the appended claims.