Abstract:
A coaxial connector that includes: a first inner conductor and a second inner conductor; a capacitor that is electrically coupled to the first inner conductor and the second inner conductor; an outer conductor that surrounds the first and second inner conductors, and the capacitor; a first support member that fixes the first inner conductor to the outer conductor; a second support member that fixes the second inner conductor to the outer conductor; a first dielectric material that is provided between the outer conductor and the first inner conductor and between the outer conductor and the second inner conductor, and a second dielectric material that is provided between the outer conductor and the capacitor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of and claims priority benefit to U.S. patent application Ser. No. 12/929,826, filed Feb. 17, 2011 now U.S. Pat. No. 8,026,774, allowed, which is a continuation of U.S. Ser. No. 12/285,802, filed Oct. 14, 2008, now U.S. Pat. No. 7,952,449, which application in turn is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2008-0181883, filed Jul. 11, 2008, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Field 
     The embodiment discussed herein is directed to a coaxial connector for transmitting an electric signal. 
     2. Description of the Related Art 
     Generally, a coaxial connector is used to connect signal lines for transmitting a high-speed (radio frequency) electric signal. An inner conductor serving as a signal line is provided in a central part of a coaxial connector, and an outer conductor serving as a grounding line is provided to surround the inner conductor. A dielectric material is filled between the inner conductor and the outer conductor. An outer diameter of the inner conductor and an inner diameter of the outer conductor are set to predetermined diameters so as to match a specific impedance (for example, 50Ω). 
     In the above-mentioned coaxial connector, there is a cutoff frequency fc at which a signal having a frequency higher than a fixed frequency cannot be transmitted. The cutoff frequency fc is determined by the outer diameter of the inner conductor, the inner diameter of the outer conductor, and a specific dielectric constant of the dielectric material filled between the inner conductor and the outer conductor. The cutoff frequency fc becomes higher as the diameters become smaller and the specific dielectric constant becomes lower. Accordingly, in order to transmit a radio frequency signal, it is necessary to make the diameter of the coaxial connector small and make the specific dielectric constant of the filled dielectric material low. Generally, in order to obtain a radio frequency transmission band of about more than 60 GHz, the outer diameter of the inner conductor is reduced to about 1 mm and an air (∈r=1.0) is used as a dielectric material. 
     In recent years, miniaturization and speeding up have progressed in measuring instruments and optical transmission and reception devices that handle a high-speed (radio frequency) electric signal. With such a progress, there is a demand for miniaturizing coaxial connectors used for those devices are required. Although connectors having a screw-type connecting part, which are represented by a 2.92 mm connector or a 1.85 mm connector, were in popular use, connectors having a push-on type connecting part, such as an SMP connector or an SMPM connector, have become popular with the demand for miniaturization (for example, refer to Non-Patent Document 1). 
     In many cases, a coaxial connector used for connection between measuring instrument or devices is provided with functions such as a DC block or a frequency filter. The DC block is provided for interrupting a direct current component and to transmit only an alternating current (AC) signal. The frequency filter is provided for attenuating a specific frequency component of a signal. 
     Specifically, the DC block and the frequency filter are formed by inserting a capacitor in the middle of the inner conductor. For example, it is suggested to divide the inner conductor into a first inner conductor and a second inner conductor and connecting the first and second inner conductors with two flat-plate capacitors located therebetween in series (for example, refer to Patent Document 1). Additionally, it is suggested to divide the inner conductor into a first inner conductor and a second inner conductor while forming surfaces parallel to the axis and connecting the first and second inner conductors with a dielectric material located therebetween (for example, refer to Patent Document 2). 
     According to the structures of the DC blocks, a strength of a connecting part (a part where the DC block is formed) between the first inner conductor and the second inner conductor is small, and the connecting part may be damaged due to a thermal stress of the inner conductor or the like. Thus, it is suggested to provide a stress relaxation mechanism for absorbing and relaxing a stress in the axial direction (for example, refer to Patent Document 3)
     Patent Document 1: U.S. Pat. No. 6,496,353   Patent Document 2: U.S. Pat. No. 7,180,392   Patent Document 3: U.S. Pat. No. 5,576,675   Non-Patent Document 1: U.S. military standard MIL_STD — 348A   

     If a capacitor is interposed in the middle of the inner conductor as mentioned above, it is difficult to equalize an impedance between the capacitor and the outer conductor and an impedance between the inner conductor and the outer conductor. That is, a distance between the inner conductor and the outer conductor, which is set to maintain a predetermined impedance, is changed at the portion of the capacitor, which results in a change in the impedance. Accordingly, an impedance mismatch occurs at the portion where the capacitor is provided, which causes degradation of a radio frequency signal transmission characteristic. 
     Accordingly, it is desirous to develop a small coaxial connector having a structure in which, even if a capacitor is inserted in a middle of an inner conductor, an impedance mismatch at a portion where the capacitor is provided is suppressed. 
     SUMMARY 
     There is provided a coaxial connector comprising: a first inner conductor and a second inner conductor; a capacitor connecting between the first inner conductor and the second inner conductor; an outer conductor extending along and surrounding the first inner conductor, the second inner conductor, and the capacitor; a first dielectric material filled in a gap between the outer conductor and the first and second inner conductors; a support member supporting the first and second inner conductors with respect to the outer conductor; and a second dielectric material for impedance matching provided between the capacitor and the outer conductor. 
     There is provided a radio frequency signal transmission method for transmitting a radio frequency signal through an inner conductor serving as a signal line, the radio frequency signal transmission method comprising: causing the radio frequency signal to be input to and propagate through the inner conductor, an impedance between the inner conductor and an outer conductor serving as a grounding line being adjusted to a predetermined impedance; and causing a component of the radio frequency signal to propagate through a capacitor provided in a middle of the inner conductor, an impedance at a portion of the capacitor installed being adjusted to match said predetermined impedance by a dielectric material provided around said capacitor. 
     Additional objects and advantages of the embodiment will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The object and advantages of the embodiment will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary explanatory only and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a coaxial connector having a basic structure; 
         FIG. 2  is a circuit diagram of an equivalent circuit of a transmission path of the coaxial connector illustrated in  FIG. 1 ; 
         FIG. 3  is a cross-sectional view of a coaxial connector according to a first embodiment; 
         FIG. 4  is a cross-sectional view of a first variation of the coaxial connector illustrated in  FIG. 3 ; 
         FIG. 5  is a cross-sectional view of a second variation of the coaxial connector illustrated in  FIG. 3 ; 
         FIG. 6  is a cross-sectional view of a third variation of the coaxial connector illustrated in  FIG. 3 ; 
         FIG. 7  is an illustration indicating a manufacturing method of the coaxial connector illustrated in  FIG. 6 ; 
         FIG. 8  is a graph indicating the impedance of a coaxial connector acquired by an electromagnetic field simulation; 
         FIG. 9  is a graph indicating a reflection characteristic and a transmission characteristic of a coaxial connector acquired by an electromagnetic field simulation; 
         FIG. 10  is a graph indicating actual measurement values of a reflection characteristic and a transmission characteristic of a coaxial connector; 
         FIG. 11  is a cross-sectional view of a coaxial connector according to a second embodiment; 
         FIG. 12  is a cross-sectional view of a coaxial connector according to a third embodiment; 
         FIG. 13  is a cross-sectional view of a coaxial connector according to a fourth embodiment; 
         FIG. 14A  is a cross-sectional view of a connector when the structure of the coaxial connector illustrated in  FIG. 6  is applied to a connector having a fitting part (connecting part) of a push-type in a state before the connector is connected to another connector; and 
         FIG. 14B  is a cross-sectional view of the connector shown in  FIG. 14A  in a state after the connector is connected to another connector. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Preferred embodiments of the present invention will be explained with reference to the accompanying drawings. 
     A description will now be given, with reference to  FIG. 1 , of a basic structure of a coaxial connector. The coaxial connector illustrated in  FIG. 1  includes an inner conductor  2 , an outer conductor  4  surrounding the inner conductor  2  and a dielectric material  3  as a first dielectric material filled in a gap between the inner conductor  2  and the outer conductor  4 . The inner conductor  2  and the outer conductor  4  are formed of an electrically conductive metal such as a copper alloy. A predetermined gap is formed between the inner conductor  2  and the outer conductor  4 . It is preferable that a material having a small specific dielectric constant ∈r is filled in the gap. In many cases, a fluorocarbon resin is used as the material having a small specific dielectric constant ∈r to be filled in the gap. However, the gap may be an air gap. In such a case, an air in the gap corresponds to the material filled in the gap. Here, it is assumed that an air is filled in the gap between the inner conductor  2  and the outer conductor  4 , and an air is filled in the gap as the dielectric material  3 . 
     The inner conductor  2  is divided into two portions, i.e., an inner conductor  2 A and an inner conductor  2 B, and a capacitor  6  is inserted between the inner conductor  2 A and the inner conductor  2 B. The capacitor  6  is connected and fixed to the inner conductor  2 A and the inner conductor  2 B by a joining material such as a solder  8 . Although a laminated ceramic chip capacitor, which is formed as a mount part to be mounted to a generally used substrate, is used as the capacitor  6 , the capacitor  6  is not limited to such a chip capacitor. It should be noted that, in the example illustrated in  FIG. 1 , the inner conductors  2 A and  2 B are mechanically connected to each other by joining and fixing them with the capacitor  6  interposed therebetween. Thus, a connecting strength of the inner conductors  2 A and  2 B is equal to a connecting strength of the solder  8 . 
     The inner conductor  2  into which the capacitor  6  is incorporated is fixed to the outer conductor  4  via a support member  10 . It is preferable to use a resin as a material to form the support member  10 . Since a specific dielectric constant ∈r of a resin is generally 2 to 4 (∈r=2 to 4), the specific dielectric constant ∈r of the portion where the support member  10  is provided is larger than that of portions (air gap) other than the portion where the support member  10  is provided. Thus, impedance matching is achieved by enlarging the gap by providing grooves to the inner conductor  2  and the outer conductor  4  where the support member  10  is provided. It should be noted that the grooves serve as engaging portions for attaching the support member  10  to the inner conductor  2  and the outer conductor  4 . 
     A circuit illustrated in  FIG. 2  is an equivalent circuit of a signal transmission path in the structure of the coaxial connector illustrated in  FIG. 1 . Because outer diameters of internal electrodes of the capacitor  6  are smaller than an outer diameter of the inner conductor  2 , a width of the gap between the capacitor  6  and the outer conductor  4  is larger than a width of a gap in other portions. Thus, a parasitic capacitance (a capacitor Cp of  FIG. 2 ) generated by the capacitor  6  being provided is smaller than an electrostatic capacitance (a capacitor Cn of  FIG. 2 ) generated between the inner conductor  2  and the outer conductor  4 . 
     Here, on the assumption that the equivalent circuit illustrated in  FIG. 2  is a single distribution constant circuit, an impedance Z thereof is represented by Z=(L/C) 1/2  where L is an inductance per unit length and C is a capacitance per unit length. According to the equation, the inductance dependency is large, that is, it is regarded that an inductance Lp is increased as a capacitance Cp is decreased, and, thus, the impedance Z is increased. That is, the impedance in the portion where the capacitor  6  is provided is larger than impedances of other portions, which causes generation of an impedance mismatch. 
     If an impedance mismatch occurs as mentioned above, a reflection of a radio frequency signal occurs in that portion, which results in a degradation of a radio frequency signal transmission characteristic. Thus, the impedance of the portion where the capacitor  6  is provided is matched by adjusting the parasitic capacitance Cp of the capacitor  6  so as to improve the radio frequency signal transmission characteristic. 
       FIG. 3  is a cross-sectional view of a coaxial connector according to a first embodiment. A basic structure of the coaxial connector  20  illustrated in  FIG. 3  is the same as that of the coaxial connector illustrated in  FIG. 1 , and parts that are the same as the parts illustrated in  FIG. 1  are given the same reference numerals and descriptions thereof will be omitted. 
     In  FIG. 3 , a dielectric material ring  22  as a second dielectric material is attached to an outer circumference of the capacitor  6 . The dielectric material ring  22  serves as a material for matching the parasitic capacitance Cp of the capacitor  6 . The dielectric material ring  22  can be formed of any material having an insulation property and a specific dielectric constant larger than the specific dielectric constant of the dielectric material  3  (in this case, larger than the specific dielectric material ∈r=1 of air). For example, the dielectric material ring  22  may be formed of the same fluorocarbon resin as the support member  10  or a rubber such as a fluorocarbon rubber. Although the dielectric material ring  22  is described as a ring, the same effect can be obtained if it is a semi-circular shape or a shape to be applied partially around the capacitor  6 . 
     By arranging the dielectric material ring  22  around the capacitor  6 , the parasitism capacitance Cp generated between the capacitor  6  and the outer conductor  4  can be increased. Therefore, the impedance matching can be achieved in the portion where the capacitor  6  is provided. That is, the impedance can be constant (for example, a specific impedance of 50Ω) also in the portion where the capacitor  6  is provided by arranging the dielectric material ring  22  having a large specific dielectric constant ∈r around the capacitor  6 , thereby suppressing reflection of a radio frequency signal. As a result, even if the capacitor  6  is provided in the middle of the inner conductor  2 , reflection of a radio frequency due to an impedance change can be reduced, and the radio frequency signal transmission characteristic of the coaxial connector  20  can be maintained well. 
     It should be noted that, like a coaxial connector  20 A illustrated in  FIG. 4 , concave portions of a size almost equal to the outer configuration of the capacitor  6  may be formed in the end surfaces of the inner conductors  2 A and  2 B so that the capacitor  6  is joined to the inner conductors  2 A and  2 B by a solder or the like after fitting the capacitor  6  in the concave portions. Thereby, strength of the connecting part by the capacitor  6  can be increased. The concave portions may be recesses or notches of a channel shape, or may be formed by members connected to the inner conductors  2 A and  2 B. 
     Here, if the outer diameter of the capacitor  6  is close to or larger than the outer diameter of the inner conductors  2 A and  2 B and the end surfaces of the inner conductors  2 A and  2 B do not have a sufficient size to form the concave portions, the outer diameter of the inner conductors  2 A and  2 B may be increased so as to form the large diameter portions like a coaxial connector illustrated in  FIG. 5 . In such a case, it is necessary to form concave portion  4   a  on the inner surface of the outer conductor  4  at a position facing the large diameter portions having a large diameter near the end surfaces of the inner conductors  2 A and  2 B. That is, it is necessary to set the impedance to a desirable value by a distance between the outer conductor  4  and each of the inner conductors  2 A and  2 B even in the portions having the large diameter near the end surfaces of the inner conductors  2 A and  2 B. 
     Further, like a coaxial connector  20 C illustrated in  FIG. 6 , grooves formed in the inner surface of the outer conductor  4  into which the support members  10  are fit and the above-mentioned concave portion  4   a  for impedance matching may be formed as a single groove or concave portion by shifting the support members  10  toward the connecting part of the capacitor  6 . Thereby, the portion where the capacitor  6  is provided can be made small, which permits the entire coaxial connector  20 C to be made small. Additionally, since the configuration of the inner surface of the outer conductor  4  can be simplified, cutting work of the outer conductor  4  can be performed easily. 
     A description will now be given, with reference to  FIG. 7 , of an example of an assembling method of the coaxial connector  20 C illustrated in  FIG. 6 . According to the assembling method indicated in  FIG. 7 , the outer conductor  4  is divided into two pieces, outer conductors  4 A and  4 B, so that the outer conductors  4 A and  4 B are fit to each other to be a single piece forming the outer conductor  4 . Although a description will be given of a fitting method of the outer conductors  4 A and  4 B using press-fitting here, the assembling method is not limited to the press-fitting and may include fitting by screw and electrical or physical connection. 
     First, as illustrated in FIG.  7 -( a ), the capacitor  6  on which the dielectric material ring  22  is fit is inserted into the concave portions of the end surfaces of the inner conductors  2 A and  2 B, and fixed by solder or the like so as to form an inner conductor assembly  2 C. Then, the support members  10  are attached to the inner conductors  2 A and  2 B of the inner conductor assembly  2 C, respectively. Thereafter, as illustrated in FIG.  7 -( b ), the inner conductor assembly  2 C is assembled to the outer conductor  4 B so that the support member  10  fits in the concave portion  4   a  of the outer conductor  4 B. Then, as illustrated in FIG.  7 -( c ), the outer conductor  4 A is press-fitted into the outer conductor  4 B. Thereby, as illustrated in FIG.  7 -( d ), the outer conductor  4  is formed and the inner conductor assembly  2 C is fixed inside the outer conductor  4  in a state where the support members  10  are fixed to the concave portion  4   a  in the inner surface of the outer conductor  4 . 
     As mentioned above, the small-size coaxial connector  20 C can be assembled very easily by press-fitting the outer conductor  4 A into the outer conductor  4 B after inserting the inner conductor assembly  2 C into the outer conductor  4 B. The assembling method by press-fitting the two-divided outer conductors can be applied to other coaxial connectors mentioned above, and is also applicable to coaxial connectors explained below. 
       FIG. 8  is a graph indicating the impedance acquired by an electromagnetic field simulation using the coaxial connector  20 C of the structure illustrated in  FIG. 6  as a model. In the graph of  FIG. 8 , a solid line indicates the impedance of the coaxial connector  20 C provided with the dielectric material ring  22 , and a dashed line indicates the impedance of a coaxial connector, which is not provided with the dielectric material ring  22 . 
     As apparent from the graph of  FIG. 8 , an impedance change at the portion where the capacitor  6  is provided is suppressed by providing the dielectric material ring  22 . That is, by providing the dielectric material ring  22 , impedance matching can be achieved and an impedance mismatch can be suppressed. 
       FIG. 9  is a graph indicating a reflection characteristic S 11  and a transmission characteristic S 21  acquired by an electromagnetic filed simulation using the coaxial connector  2 C of the structure illustrated in  FIG. 6  as a model. In the graph of  FIG. 9 , solid lines indicate the reflection characteristic S 11  and the transmission characteristic S 21  of the coaxial connector  20 C provided with the dielectric material ring  22 , and dashed lines indicate the reflection characteristic S 11  and the transmission characteristic S 21  of a coaxial connector, which is not provided with the dielectric material ring  22 . The two curves (solid line and dashed line) indicated in a lower part of the graph indicate the reflection characteristic S 11 , and the generally flat two curves (solid line and dashed line) indicated in an upper part of the graph indicate the transmission characteristic S 21 . 
     The transmission characteristic S 21  of the coaxial connector, which is not provided with the dielectric material ring  22  is indicated by the dashed line, which indicates that the transmission characteristic S 21  decreases as the frequency increases. On the other hand, the transmission characteristic S 21  of the coaxial connector  20 C provided with the dielectric material ring  22  is almost zero over the entire band, which indicates that there is almost no transmission loss. Thus, it can be appreciated that the transmission characteristic S 21  in the radio frequency band is improved by providing the dielectric material ring  22 . 
     The reflection characteristic S 11  of the coaxial connector, which is not provided with the dielectric material ring  22 , indicates that it is below −20 dB in the portion where the frequency is low but reflection increases higher than −20 dB at a frequency exceeding 20 GHz. On the other hand, the reflection characteristic S 11  of the coaxial connector  20 C provided with the dielectric material ring  22  is below −20 dB in a radio frequency band from a low frequency to about 55 GHz. Thus, it can be appreciated that the reflection characteristic S 11  in the radio frequency band is greatly improved by providing the dielectric material ring  22 . 
     The coaxial connector  20 C of the structure illustrated in  FIG. 6  was fabricated and the reflection characteristic S 11  and the transmission characteristic S 21  were measured, and a result indicated in the graph of  FIG. 10  was obtained. It can be appreciated from the graph that the reflection characteristic S 11  was below −20 dB in a radio frequency band from a low frequency to about 55 GHz, which indicates that the reflection characteristic S 11  was greatly improved. On the other hand, since the transmission characteristic S 21  was maintained at a value of almost zero to the frequency of about 60 GHz, it was confirmed that a good transmission characteristic was maintained also in a radio frequency band. 
     A description will now be given, with reference to  FIG. 11  of a coaxial connector according to a second embodiment. In  FIG. 11 , parts that are the same as the parts illustrated in  FIG. 6  and  FIG. 7  are given the same reference numerals, and descriptions thereof will be omitted. 
     Although the coaxial connector  20 D according to the second embodiment has the same structure as the above-mentioned coaxial connector  20 C, it differs in that the dielectric material ring  22  is replaced by a modified dielectric material ring  24 . The modified dielectric material ring  24  does not have a shape to be attached to an outer circumference of the capacitor  6 , but is made in a shape to cover circumferences of the inner conductors  2 A and  2 B. The length of the modified dielectric material ring  24  is equal to a distance between the support members  10 , and opposite ends of the modified dielectric material ring  24  are brought into contact with the respective support members  10 . 
     The thickness of the modified dielectric material ring  24  is set so that impedances between sections B, C and D are equal to the impedance of a section A. Specifically, the thickness of the modified dielectric material ring  24  in the section C is small and the thickness of the modified dielectric material ring  24  in the section D is large so that the portion of the modified dielectric ring  24  in the section D forms a protruding part. Although the protruding part of the modified dielectric material ring  24  protrudes outwardly, it may protrude inwardly so as to maintain a desired thickness. Also the cross-section of the modified dielectric material ring  24  is not always required to be a square shape as illustrated in  FIG. 11 . The modified dielectric material ring  24  can be various shapes in order to achieve impedance matching. 
     According to the present embodiment, the modified dielectric material ring  24  is interposed between the support members  10 , and the joint part between the inner conductors  2 A and  2 B can be strengthened by the modified dielectric material ring  24 . That is, if a force to compress the capacitor  6  is applied to the inner conductors  2 A and  2 B when connecting and disconnecting the coaxial connector, a portion of the force can be absorbed by the modified dielectric material ring  24 , which can reduce a force applied to the capacitor  6  and the joint part. 
     A description will be given below, with reference to  FIG. 12 , of a coaxial connector according to a third embodiment. In  FIG. 12 , parts that are the same as the parts illustrated in  FIG. 6  and  FIG. 7  are given the same reference numerals, and descriptions thereof will be omitted. 
     Although the coaxial connector  20 E according to the third embodiment has the same structure as the above-mentioned coaxial connector  20 C, it differs in that an adhesive  26  is provided to an outer circumference of the capacitor  6  instead of the dielectric material ring  22 . By using a resin such as, for example, an epoxy resin as for the adhesive  26 , an electrostatic capacitance can be adjusted to achieve the impedance matching as the same as the dielectric material ring  22 . 
     The adhesive  26  may be provided by applying onto the outer circumference of the capacitor  6  and cured, or may be provided on the circumference of the capacitor  6  over an entire area between the inner conductors  2 A and  2 B. If the adhesive  26  is provided to only the outer circumference of the capacitor  6 , the capacitor  6  can be strengthened by the adhesive  26 . If the adhesive  26  is provided to cover the outer circumference of the capacitor  6  and the joint part of the capacitor  6 , the capacitor  6  is strengthened and also the joint part is strengthened. 
     A description will be given below, with reference to  FIG. 13 , of a coaxial connector according to a fourth embodiment. In  FIG. 13 , parts that are the same as the parts illustrated in  FIG. 11  and  FIG. 12  are given the same reference numerals, and descriptions thereof will be omitted. 
     The coaxial connector  20 F according to the fourth embodiment is a combination of the modified dielectric material ring  24  illustrated in  FIG. 11  and the adhesive  26  illustrated in  FIG. 12 . The adhesive  26  is filled in a space between the modified dielectric material ring  24  and the outer circumference of the capacitor  6 , and the joint part of the capacitor  6  is strengthened strongly by the modified dielectric material ring  24  and the adhesive  26 . 
     The structures of the above-mentioned coaxial connectors  20  to  20 F can be used for a connector having a fitting part (joint part) of a push-on type such as SMP or SMPM. The specifications of SMP and SMPM are provided in U.S. military standard MIL_STD — 348A.  FIGS. 14A and 14B  are cross-sectional views of a coaxial connector when the structure of the coaxial connector  20 C illustrated in  FIG. 6  as an example is applied to a connector  30  having a fitting part (joint part)  30   a  of a push-on type.  FIG. 14A  illustrates a state before the connector  30  is connected to another connector  32 .  FIG. 14B  illustrates a state after the connector  30  is connected to the connector  32 . 
     In  FIGS. 14A and 14B , a fitting part (joint part)  30   a  is formed on each of opposite ends of the connector  30  having the structure of the coaxial connector  20 C. The fitting part (joint part)  30   a  is configured to be fitted to a fitting part (joint part)  32   a  of the connector  32 . The connector  30  can be connected to the connector  32  quickly and easily by placing the fitting part  30   a  of the connector  30  to opposite to the fitting part  32   a  of the connector  32  and pushing the fitting part  30   a  into the fitting part  32   a.    
     It should be noted that, by using the above-mentioned coaxial connectors  20  to  20 F, a radio frequency signal transmission method to transmit a radio frequency signal while suppressing a signal degradation can be achieved. That is, when transmitting a radio frequency signal through a signal transmission path in which the outer conductor  4  as a grounding line is provided around the inner conductor  2  as a signal line, a method of transmitting a radio frequency signal while maintaining excellent reflection characteristic and transmission characteristic to suppress a signal degradation can be achieved. 
     In the radio frequency transmission method, first, a radio frequency signal is input to and caused to propagate through the inner conductor  2  as a signal line provided with a predetermined impedance. Then, the radio frequency signal is caused to propagate further through the capacitor  6  inserted in the middle of the inner conductor  2 . While the radio frequency signal propagates through the capacitor  6 , a component of the radio frequency signal is limited by the capacitor  6 . That is, a DC component of the radio frequency signal is removed by the capacitor  6 , or only a frequency component of a certain band is removed by the capacitor  6 . Because a dielectric material (the dielectric material ring  22 , the modified dielectric material ring  24 , the adhesive  26 ) is provided on the outer circumference of the capacitor  6  and the impedance of the portion where the capacitor  6  is provided is matched, a reflection of the radio frequency signal hardly occurs and the radio frequency signal is transmitted without attenuating in the portion where the capacitor  6  is provided. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor to furthering the art, and are to be construed a being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relates to a showing of the superiority and inferiority of the invention. Although the embodiment(s) of the present invention (s) has(have) been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.