Patent Publication Number: US-11394100-B2

Title: High-frequency connection structure for connecting a coaxial line to a planar line using adhesion layers

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a national phase entry of PCT Application No. PCT/JP2019/015301, filed on Apr. 8, 2019, which claims priority to Japanese Application No. 2018-079624, filed on Apr. 18, 2018, which applications are hereby incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a high-frequency line connection structure, and more particularly, to a technique of connecting a coaxial line and a planar line. 
     BACKGROUND 
     In recent years, in the field of optoelectronics, a high-frequency interface constituting an optoelectronic component is required to have low reflection characteristics and a low insertion loss over a wide frequency range. The structure of such a high-frequency interface adopts a mode of using a lead pin and a flexible printed circuit, but may, in some cases, use a coaxial interface. 
     Particularly, electronic components and optical module components having a 1 mm interface with band characteristics at 100 GHz or higher are expected to be used as key components for next-generation optical communication at 1 Tbps or more, and are being developed in and outside of Japan. 
     Various components are disposed on a plane inside an electronic component or an optical module component as described above, and a high-frequency line that electrically connects the various components is generally fabricated on an insulating dielectric substrate. For its part, the 1 mm interface has a coaxial line structure including an inner conductor and a cylindrical ground, which is clearly different from the structure of the high-frequency line that is fabricated on the dielectric substrate described above. 
     Because of such a difference in the structures, a new connection mechanism for a high-frequency line is desired to be implemented, the new connection mechanism having a low insertion loss with respect to high-frequency characteristics and low return loss characteristics at a connection part at which a high-frequency line fabricated on a dielectric substrate and a coaxial line are mechanically and electrically connected. 
     Accordingly, Patent Literature 1 discloses a high-frequency line connection structure  500 A as shown in  FIG. 5A , where an inner conductor  514  constituting a coaxial line  510  is structured to protrude from a line end, the inner conductor  514  is electrically connected to a signal line  522  at a line end of a grounded coplanar line  520 , and a dielectric layer  513  and a radio wave absorption layer  500  are disposed on a connection part. 
     More specifically, as shown in  FIG. 5A , with the high-frequency line connection structure  500 A, the coaxial line  510  and the grounded coplanar line  520  are connected. 
     The coaxial line  510  includes a cylindrical earth ground  511  covered by the radio wave absorption layer  500 , an insulator  512  filling the inside of the earth ground  511 , and the inner conductor  514  covered by the insulator  512 . A part at a line end of the coaxial line  510  where the inner conductor  514  protrudes is covered by the dielectric layer  513 . 
     The grounded coplanar line  520  includes a pair of grounds  521  formed on a surface of a dielectric substrate  523 , the signal line  522  formed sandwiched between the pair of grounds  521  while being separated by predetermined distances, and an earth ground  524  formed on a back surface of the dielectric substrate  523 . Furthermore, the grounded coplanar line  520  is formed on metal bases  530 ,  540 . 
     With the high-frequency line connection structure  500 A, a fundamental mode of electromagnetic waves to be propagated is different between the coaxial line  510  and the grounded coplanar line  520 . Accordingly, the dielectric layer  513  is introduced for the purpose of facilitating conversion of the fundamental mode at a connection section  550  (see  FIGS. 5D and 5E ), and the radio wave absorption layer  500  is introduced for the purpose of absorbing unwanted radiation occurring at the connection section  550 . 
     An increase in the insertion loss or a return loss is thereby suppressed at the high-frequency line connection structure  500 A. Therefore, according to frequency characteristics of the insertion loss and frequency characteristics of the return loss at the high-frequency line connection structure  500 A, ripple and dip are removed, and desirable transmission characteristics may be obtained over a wide band. 
     However, the dielectric layer  513  causes a high-frequency loss. Furthermore, energy that is a source of unwanted radiation that is absorbed by the radio wave absorption layer  500  is based on a high-frequency signal that is propagated through a line. Accordingly, the high-frequency line connection structure  500 A is a connection mechanism which assumes occurrence of energy loss at the connection section  550 . Generally, with respect to a high-frequency signal at a high frequency such as 100 GHz, an output amplitude at an IC or the like that generates the high-frequency signal is small in the first place. Moreover, it is commonly known that unwanted radiation is more notably generated, as the frequency increases. 
     Accordingly, in a case where a high-frequency signal at a high frequency such as 100 GHz is propagated by the high-frequency line connection structure  500 A, the return loss is effectively reduced by the radio wave absorption layer  500 , but there is still an occurrence of energy loss, and a total equivalent loss is reduced. 
       FIGS. 5B and 5C  are perspective views showing main structures of the high-frequency line connection structure  500 A shown in  FIG. 5A , excluding the dielectric layer  513  and the radio wave absorption layer  500 .  FIGS. 5D and 5E  are side views of the high-frequency line connection structure  500 A shown in  FIGS. 5B and 5C . 
     An arrow drawn in the side view shown in  FIG. 5D  indicates a high-frequency signal path P 1 . Furthermore, an arrow drawn in the side view shown in  FIG. 5E  indicates a return current path P 2  corresponding to the high-frequency signal in  FIG. 5D . As shown in  FIGS. 5D and 5E , the arrows have different lengths, and there is concern that apparent reflection will appear at a frequency corresponding to λ/4 the difference in the lengths. 
       FIG. 6  shows calculation results of a return loss and an insertion loss of the high-frequency line connection structure  500 A. As shown in  FIG. 6 , a dip appears in the return loss at a specific frequency, and the insertion loss is deteriorated at the frequency. In this manner, with the high-frequency line connection structure  500 A, because different line structures are connected, deterioration in the return loss is caused due to a bypass of a return current path at the connection part. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: Japanese Patent No. 3144576, published Mar. 12, 2001. 
       
    
     SUMMARY OF THE INVENTION 
     Technical Problem 
     As described above, and referring to  FIGS. 5A, 5B, 5C, 5D, and 5E , with the high-frequency line connection structure  500 A described in Patent Literature 1 including the dielectric layer  513  and the radio wave absorption layer  500  (see  FIGS. 5A-5E ), it is difficult to achieve a connection structure having low-loss characteristics and a superior return loss. 
     Embodiments of the present invention have been made to solve the problems described above, and has as its object to provide a high-frequency line connection structure having a low return loss, and having low insertion loss characteristics over a wide band. 
     Means for Solving the Problem 
     To solve the problems described above, a high-frequency line connection structure according to embodiments of the present invention is a high-frequency line connection structure for connecting a coaxial line and a planar line, where the coaxial line includes an inner conductor extending in an axial direction, the inner conductor having a cross-section formed in a circular shape around an axis, the cross-section being perpendicular to the axial direction, an outer conductor including a penetrating hole for housing the inner conductor, the penetrating hole having a columnar shape, and an insulation layer for insulating between the inner conductor and the outer conductor, the insulation layer being provided in the penetrating hole between the inner conductor and the outer conductor the inner conductor includes a leading end portion extending in the axial direction from an end surface of the outer conductor, the planar line includes a substrate that is formed of dielectric, a signal line that is formed on a surface of the substrate, the signal line having a strip-shape, a pair of first conductive thin films that are formed in regions, on the surface of the substrate, that are adjacent to the coaxial line, the pair of first conductive thin films being formed on respective sides of the signal line across a predetermined distance, and a second conductive thin film that covers a back surface of the substrate, the second conductive thin film being electrically connected to the pair of first conductive thin films, the high-frequency line connection structure includes a first adhesion layer that is conductive, and that is formed to cover the leading end portion of the inner conductor and an end of the signal line included in the planar line, and a second adhesion layer that is conductive, and that is formed on a side of the coaxial line along edges of the pair of first conductive thin films included in the planar line to connect the pair of first conductive thin films and the outer conductor of the coaxial line, and when seen along the axial direction, end portions of the pair of first conductive thin films that are close to the signal line coincide with a position of an inner wall of the penetrating hole formed in the outer conductor and having the columnar shape. 
     Furthermore, with the high-frequency line connection structure according to embodiments of the present invention, when viewed along the axial direction, an end portion of the second conductive thin film that is adjacent to the coaxial line may coincide with the position of the inner wall of the penetrating hole formed in the outer conductor and having the columnar shape. 
     Furthermore, with the high-frequency line connection structure according to the embodiments of present invention, a length of the substrate of the planar line in a direction perpendicular to a lengthwise direction of the signal line may be smaller than a radius of a concentric circle of the coaxial line, a cutaway part may be formed in the second conductive thin film of the planar line, the cutaway part may be formed by selectively removing a region including a connection section as viewed from top, the connection section being formed by connecting the leading end portion of the inner conductor of the coaxial line and a part of a surface of the planar line by the first adhesion layer, and the coaxial line of the second conductive thin film and an end portion of the second conductive thin film that is adjacent to the cutaway part may coincide with the position of the inner wall of the penetrating hole formed in the outer conductor and having the columnar shape. 
     Furthermore, with the high-frequency line connection structure according to embodiments of the present invention, the planar line may further include a plurality of through holes for providing electrical continuity between the pair of first conductive thin films and the second conductive thin film, the through holes penetrating the substrate. 
     Furthermore, with the high-frequency line connection structure according to the embodiments of present invention, the planar line may further include a plurality of half through holes for providing electrical continuity between the pair of first conductive thin films and the second conductive thin film, the half through holes being formed in an end surface of the substrate that is adjacent to the coaxial line in a manner penetrating the substrate, and the second adhesion layer may fill the plurality of half through holes. 
     Effects of Embodiments of the Invention 
     According to embodiments of the present invention, end portions of an opposing pair of first conductive thin films included in a planar line that are adjacent to a coaxial line, and an end portion of a second conductive thin film that is adjacent to the coaxial line are disposed to coincide with a position of an inner wall of a columnar penetrating hole formed in an outer conductor included in the coaxial line, and a second adhesion layer is formed along edges of the pair of first conductive thin films that are adjacent to the coaxial line, and thus, a high-frequency line connection structure having a low return loss, and having low insertion loss characteristics over a wide band may be achieved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is an exploded view of a high-frequency line connection structure according to a first embodiment of the present invention. 
         FIG. 1B  is a perspective view of the high-frequency line connection structure according to the first embodiment of the present invention. 
         FIG. 1C  is a side view of the high-frequency line connection structure according to the first embodiment of the present invention. 
         FIG. 1D  is a diagram for describing a signal current path and a return current path of the high-frequency line connection structure according to the first embodiment of the present invention. 
         FIG. 2  is a diagram for describing an effect of the first embodiment of the present invention. 
         FIG. 3A  is an exploded view of a high-frequency line connection structure according to a second embodiment of the present invention. 
         FIG. 3B  is a perspective view of the high-frequency line connection structure according to the second embodiment of the present invention. 
         FIG. 3C  is a side view of the high-frequency line connection structure according to the second embodiment of the present invention. 
         FIG. 3D  is a diagram for describing a signal current path and a return current path of the high-frequency line connection structure according to the second embodiment of the present invention. 
         FIG. 4A  is an exploded view of a high-frequency line connection structure according to a third embodiment of the present invention. 
         FIG. 4B  is a perspective view of the high-frequency line connection structure according to the third embodiment of the present invention. 
         FIG. 4C  is a front view of the high-frequency line connection structure according to the third embodiment of the present invention. 
         FIG. 4D  is a side view of the high-frequency line connection structure according to the third embodiment of the present invention. 
         FIG. 4E  is a diagram for describing a signal current path and a return current path of the high-frequency line connection structure according to the third embodiment of the present invention. 
         FIG. 5A  is a front view of a conventional high-frequency line connection structure. 
         FIG. 5B  is an exploded view of the conventional high-frequency line connection structure. 
         FIG. 5C  is a perspective view of the conventional high-frequency line connection structure. 
         FIG. 5D  is a diagram for describing a signal current path of the conventional high-frequency line connection structure. 
         FIG. 5E  is a diagram for describing a return current path of the conventional high-frequency line connection structure. 
         FIG. 6  is a diagram for describing a return loss and an insertion loss of the conventional high-frequency line connection structure. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     Hereinafter, preferred embodiments of the present invention will be described in detail with reference to  FIGS. 1A, 1B, 1C, 1D, 2, 3A, 3B, 3C, 3D, 4A, 4B, 4C, 4D, and 4E . Structural elements common among the drawings are denoted by same reference signs. 
     First Embodiment 
       FIG. 1A  is an exploded view of a high-frequency line connection structure  1  according to a first embodiment.  FIG. 1B  is a perspective view of the high-frequency line connection structure  1 . Furthermore,  FIG. 1C  is a side view of the high-frequency line connection structure  1 . 
     As shown in  FIGS. 1A to 1C , a coaxial line  10  and a planar line  20  are disposed on a cuboid metal base  50 , and are connected to each other. Furthermore, an outer conductor  11  of the coaxial line  10  is disposed on one surface of the metal base  50 , and the planar line  20  is disposed on the same surface of the metal base  50  across a metal base  40 . 
     The high-frequency line connection structure  1  according to the present embodiment includes the coaxial line  10 , the planar line  20 , a first adhesion layer  30  (see  FIGS. 1B and 1C ), the metal base  40 , the metal base  50 , and a second adhesion layer  60  (see  FIG. 1B ). 
     The coaxial line  10  includes the outer conductor  11 , an inner wall  12  of the outer conductor  11 , an inner conductor  13 , and an insulation layer  14 . The outer conductor  11 , the inner wall  12  of the outer conductor  11 , and the inner conductor  13  are formed to have a coaxial structure. 
     The outer conductor  11  is formed to have a block shape, and includes, on the inside, a columnar penetrating hole that extends in an axial direction. The outer conductor  11  houses the inner conductor  13  in the columnar penetrating hole. The outer conductor  11  is formed from a metal material. As shown in  FIGS. 1A and 1B , the columnar penetrating hole formed in the outer conductor  11  is formed coaxially with the inner conductor  13 . 
     The inner wall  12  is an inner peripheral surface at the columnar penetrating hole formed in the outer conductor  11 , and is formed into a cylindrical shape. Furthermore, predetermined end portions that are of a pair of first conductive thin films  23  (see  FIGS. 1A and 1B ) and a second conductive thin film  22  (see  FIGS. 1A and 1B ) of the planar line  20  described later and that are adjacent to the coaxial line  10  are aligned and positioned to coincide with the position of the inner wall  12  when seen along the axial direction. 
     A cross-section of the inner conductor  13  that is perpendicular to the axial direction is formed to have a circular shape around the axis. The inner conductor  13  is a signal core wire of the coaxial line  10  formed by including the inner wall  12  of the outer conductor  11  and the insulation layer  14 . 
     As shown in  FIGS. 1A and 1B , the inner conductor  13  includes a leading end portion  13   a  extending in the axial direction from an end surface of the block-shaped outer conductor  11 . The leading end portion  13   a  of the inner conductor  13  is electrically connected to a signal line  25  provided on a surface of the planar line  20  by the first adhesion layer  30  (see  FIGS. 1B and 1C ). The inner conductor  13  is formed from a metal material. 
     The insulation layer  14  is provided in the penetrating hole between the inner conductor  13  and the outer conductor  11 , and insulates between the inner conductor  13  and the outer conductor  11 . 
     Next, a description will be given of the planar line  20  to which the coaxial line  10  is connected. 
     The planar line  20  is on an extension of the coaxial line  10  that is formed from the outer conductor  11 , the inner wall  12 , the inner conductor  13 , and the insulation layer  14 . 
     The planar line  20  includes a substrate  21 , the second conductive thin film  22 , the pair of first conductive thin films  23 , through holes  24 , and the signal line  25 . 
     The planar line  20  is provided on a surface of the metal base  40 . The planar line  20  forms a well-known grounded coplanar line at a connection section  70  where the leading end portion  13   a  of the inner conductor  13  of the coaxial line  10  is connected. 
     The substrate  21  is a planar substrate formed of dielectric. For example, the substrate  21  may be formed of low-loss ceramics such as alumina. The signal line  25  and the pair of first conductive thin films  23  are formed on a surface of the substrate  21 , the pair of first conductive thin films  23  being formed on respective sides of the signal line  25  across a predetermined distance. Moreover, the second conductive thin film  22  is disposed on a back surface of the substrate  21 . 
     The second conductive thin film  22  is formed covering the entire back surface of the substrate  21 . The second conductive thin film  22  is disposed on a surface of the metal base  40 . The second conductive thin film  22  serves as a ground of the planar line  20  of a grounded coplanar line type. 
     An end portion  22   a  (see  FIG. 1B ) of the second conductive thin film  22  that is adjacent to the coaxial line  10  is positioned to coincide with the position of the inner wall  12  of the outer conductor  11  of the coaxial line  10 , and is electrically connected to the inner wall  12  by solder, conductive adhesive or the like (not shown). 
     The pair of first conductive thin films  23  are formed in regions, on the surface of the substrate  21 , that are adjacent to the coaxial line  10 , on respective sides of the signal line  25  across a predetermined distance. The predetermined distance of the pair of first conductive thin films  23  from the signal line  25  may be set such that characteristic impedance of the planar line  20  takes a predetermined value. 
     End portions  23   a ,  23 ′ a  (see  FIG. 1B ) of the pair of first conductive thin films  23  that are close to the signal line  25  are disposed to coincide with the position of the inner wall  12  of the columnar penetrating hole formed in the outer conductor  11  of the coaxial line  10 , and are electrically connected to the inner wall  12  by solder, conductive adhesive or the like (not shown). 
     A plurality of through holes  24  are formed penetrating the substrate  21  from the surface to the back surface. More specifically, a conductive material is vapor-deposited or filled on inner wall surfaces of the through holes  24 , and the through holes  24  electrically connect and provide electrical continuity between the pair of first conductive thin films  23  formed on the surface of the substrate  21  and the second conductive thin film  22  formed on the back surface. Because the plurality of through holes  24  are formed, the pair of first conductive thin films  23  become more stable equipotential surfaces. The plurality of through holes  24  are formed along a direction perpendicular to a lengthwise direction of the signal line  25 , in regions where the pair of first conductive thin films  23  are formed and with predetermined spaces therebetween. An appropriate space may be selected as the space between the plurality of through holes  24  taking into account the characteristics of transmission lines of the high-frequency line connection structure  1 . 
     The signal line  25  is formed into a strip shape on the surface of the substrate  21 , and propagates high-frequency signals. The signal line  25  is formed from a metal material. One end of the signal line  25  that is adjacent to the coaxial line  10  is electrically connected to the leading end portion  13   a  of the inner conductor  13  of the coaxial line  10 . 
     As shown in  FIG. 1B , the first adhesion layer  30  is formed covering the leading end portion  13   a  of the inner conductor  13  of the coaxial line  10  and a part of a surface of the signal line  25  of the planar line  20 . The first adhesion layer  30  is conductive, and mechanically and electrically connects the coaxial line  10  and the planar line  20 . Solder, conductive adhesive or the like may be used as the first adhesion layer  30 . The leading end portion  13   a  of the inner conductor  13  of the coaxial line  10  and the part of the surface of the signal line  25  of the planar line  20  that are connected by the first adhesion layer  30  form the connection section  70 . 
     The metal base  50  is provided on a back surface of the metal base  40 , and supports the entire coaxial line  10  and the planar line  20 . The high-frequency line connection structure  1  is integrally formed by the metal base  50 . A surface of the metal base  50  is electrically connected to the metal base  40  and the outer conductor  11  of the coaxial line  10  by solder, conductive adhesive or the like (not shown). 
     Exactly the same potential, or in other words, a ground potential, is thereby achieved with respect to the outer conductor  11  of the coaxial line  10  and the second conductive thin film  22  of the planar line  20 . 
     A height of the metal base  40  (a length in a direction perpendicular to a propagation direction of high-frequency signals) is adjusted in such a way that the end portion  22   a  (see  FIG. 1B ) of the second conductive thin film  22  of the planar line  20  is adjacent to the coaxial line  10 , and is at the position of the inner wall  12  of the columnar penetrating hole formed in the outer conductor  11  of the coaxial line  10 . The surface of the metal base  40  and the second conductive thin film  22  of the planar line  20  are electrically connected by solder, conductive adhesive or the like (not shown). Furthermore, an end surface of the metal base  40  that is adjacent to the coaxial line  10  is electrically connected to an end surface of the outer conductor  11  by solder, conductive adhesive or the like (not shown). 
     The entire second conductive thin film  22  of the planar line  20  thereby has a stable ground potential. 
     As shown in  FIG. 1B , the second adhesion layer  60  is formed along edges that are of the pair of first conductive thin films  23  of the planar line  20  and that are adjacent to the coaxial line  10 , and electrically and mechanically connects the pair of first conductive thin films  23  and the outer conductor  11  of the coaxial line  10 . Solder, conductive adhesive or the like may be used as the second adhesion layer  60 . 
     The planar line  20  and the coaxial line  10  configured in the above manner are electrically connected, and the planar line  20  thus forms a grounded coplanar line. 
     Furthermore, the planar line  20  in a region where the connection section  70  is not formed has a microstrip line structure in a direction away from the coaxial line  10 . 
     The high-frequency line connection structure  1  thus minimizes a difference between a fundamental mode of an electromagnetic field formed by lines of electric force that are radially generated from an outer peripheral surface of the inner conductor  13  of the coaxial line  10  toward the inner wall  12  of the outer conductor  11 , and a fundamental mode of an electromagnetic field formed by lines of electric force from the signal line  25  of the grounded coplanar line (planar line  20 ) to the pair of first conductive thin films  23  and the second conductive thin film  22 . Generation of radiation due to non-coincidence between the fundamental modes is thereby suppressed. 
     Next, a description will be given of a signal current path P 1  and a return current path P 2  of the high-frequency line connection structure  1 .  FIG. 1D  is a diagram showing the signal current path P 1  and the return current path P 2  of the high-frequency line connection structure  1  as viewed from a side. 
     As can be seen in  FIG. 1D , the return current path P 2  does not make a bypass at the connection section  70  between the coaxial line  10  and the planar line  20 , and a route having a same length as the signal current path P 1  is formed. Resulting effects of characteristics of the high-frequency line connection structure  1  are shown in  FIG. 2 . Solid curved lines shown in  FIG. 2  indicate a return loss (in dB) versus frequency (in GHz) and an insertion loss (in dB) versus frequency (in GHz) of the high-frequency line connection structure  1  according to the present embodiment. Furthermore, dotted curved lines indicate a return loss and an insertion loss of a high-frequency line connection structure  500 A ( FIGS. 5A, 5B, 5C, 5D, 5D, and 6 ) of a conventional example. 
     As can be seen in  FIG. 1D , characteristics of the high-frequency line connection structure  1  according to the present embodiment are more clearly improved with respect to the return loss, compared with characteristics of the high-frequency line connection structure  500 A of the conventional example. Furthermore, also with respect to the insertion loss, characteristics of the high-frequency line connection structure  1  according to the present embodiment are improved. 
     As described above, the high-frequency line connection structure  1  according to the first embodiment includes the conductive second adhesion layer  6   o  (see  FIG. 1B ) that is formed along the edges of the pair of first conductive thin films  23  of the planar line  20 . Furthermore, the end portions  23   a ,  23 ′ a  (see  FIG. 1B ) of the pair of first conductive thin films  23  and the end portion  22   a  (see  FIG. 1B ) of the second conductive thin film  22  that is adjacent to the coaxial line  10  are disposed to coincide with the position of the inner wall  12  of the columnar penetrating hole formed in the outer conductor  11 . Accordingly, the high-frequency line connection structure  1  may have a low return loss, and have low insertion loss characteristics over a wide band. 
     As a result, the high-frequency line connection structure  1  enables provision of electronic components and optical module components having next-generation broadband characteristics of 1 Tbps or more. 
     Second Embodiment 
     Next, a description will be given of a second embodiment of the present invention. Additionally, in the following description, structures the same as those in the first embodiment described above will be denoted by same reference signs, and description thereof will be omitted. 
     In the first embodiment, a case is described where a plurality of through holes  24  are provided, the through holes  24  electrically connecting the pair of first conductive thin films  23  and the second conductive thin film  22  formed at the planar line  20 , on the surface and the back surface of the substrate  21 , respectively. In contrast, in the second embodiment, a plurality of half through holes  24 A are used instead of the plurality of through holes  24 . 
       FIG. 3A  is an exploded view of a high-frequency line connection structure  1 A according to the present embodiment.  FIG. 3B  is a perspective view of the high-frequency line connection structure  1 A.  FIG. 3C  is a side view of the high-frequency line connection structure  1 A. In the following, structures different from those in the first embodiment will be mainly described. 
     The half through holes  24 A (see  FIGS. 3A and 3B ) electrically connect a pair of first conductive thin films  23 A (see  FIGS. 3A and 3B ) formed on the surface of the substrate  21  of the planar line  20 A and the second conductive thin film  22  formed on the back surface of the substrate  21 . The half through holes  24 A (see  FIGS. 3A and 3B ) are semi-cylindrical through holes. The plurality of half through holes  24 A (see  FIGS. 3A and 3B ) are formed with predetermined spaces therebetween, along an end surface of the substrate  21  that is adjacent to the coaxial line  10 . 
     As shown in  FIG. 3B , an end surface of the planar line  20 A where the plurality of half through holes  24 A are formed and an end surface of the coaxial line  10 , on the side of the leading end portion  13   a  of the inner conductor  13 , are positioned and connected in the manner as described in the first embodiment. 
     More specifically, a second adhesion layer  60 A is formed on the side of the coaxial line  10  along edges of the pair of first conductive thin films  23 A (see  FIGS. 3A and 3B ) and the pair of first conductive thin films  23 A (see  FIGS. 3A and 3B ) and the outer conductor  11  are electrically connected. At this time, the second adhesion layer  60 A also fills semi-cylindrical gaps formed between the half through holes  24 A (see  FIGS. 3A and 3B ) and the outer conductor  11  of the coaxial line  10 . For example, the second adhesion layer  60 A permeates into the gaps of the half through holes  24 A (see  FIGS. 3A and 3B ) by capillary action. Due to the second adhesion layer  60 A also filling the half through holes  24 A (see  FIGS. 3A and 3B ), the coaxial line  10  and a planar line  20 A are mechanically adhered and fixed, in addition to being electrically connected. Solder, conductive adhesive or the like may be used as the second adhesion layer  60 A. 
       FIG. 3D  is a diagram for describing the signal current path P 1  and the return current path P 2  of the high-frequency line connection structure  1 A as viewed from a side. 
     As shown in  FIG. 3D , the return current path P 2  does not make a bypass at a connection section  70 A of the high-frequency line connection structure  1 A between the coaxial line  10  and the planar line  20 A, and a route having a same length as the signal current path P 1  is formed. Accordingly, characteristics of the high-frequency line connection structure  1 A according to the present embodiment are improved in the same manner as in the first embodiment ( FIG. 2 ). That is, compared with high-frequency characteristics of the high-frequency line connection structure  500 A of the conventional example, characteristics of the high-frequency line connection structure  1 A according to the present embodiment are more clearly improved with respect to the return loss, and characteristics are also improved with respect to the insertion loss. 
     As described above, with the high-frequency line connection structure IA according to the second embodiment, a plurality of half through holes  24 A are formed in the planar line  20 A, and the second adhesion layer  60 A fills the half through holes  24 A. Accordingly, the high-frequency line connection structure IA may increase strength of mechanical connection between the coaxial line  10  and the planar line  20 A, and may have low return loss and low insertion loss characteristics over a wide band. 
     Third Embodiment 
     Next, a description will be given of a third embodiment of the present invention. Additionally, in the following description, structures the same as those in the first and second embodiments described above will be denoted by same reference signs, and description thereof will be omitted. 
     The first and second embodiments each describe a case where the end portion  22   a  (see  FIG. 1B ) that is of the second conductive thin film  22  of the planar line  20 ,  20 A and that is adjacent to the coaxial line  10  is positioned to coincide with the position of the inner wall  12  of the columnar penetrating hole formed in the outer conductor  11 . In contrast, in the third embodiment, a substrate  21 B is formed to have a thickness (a length in a direction perpendicular to a lengthwise direction of the signal line  25 ) smaller than a thickness of the substrate  21  of the planar line  20 ,  20 A described in the first and second embodiments. 
       FIG. 4A  is an exploded view of a high-frequency line connection structure  1 B according to the third embodiment.  FIG. 4B  is a perspective view of the high-frequency line connection structure  1 B.  FIG. 4C  is a front view of the high-frequency line connection structure  1 B. Furthermore,  FIG. 4D  is a side view of the high-frequency line connection structure  1 B. In the following, structures different from those in the first and second embodiments will be mainly described. 
     As shown in the front view in  FIG. 4C , a thickness a 1  of the substrate  21 B of a planar line  20 B, or in other words, the length in the direction perpendicular to the lengthwise direction of the signal line  25 , is sufficiently smaller than a radius r of a concentric circle of the coaxial line  10 . More specifically, the thickness a 1  of the substrate  21 B is smaller than a length a 2  from a point on a circumference of the inner conductor  13  along the radius r to the inner wall  12  of the outer conductor  11 . 
     As shown in  FIGS. 4A and 4B , a cutaway part A is formed in a second conductive thin film  22 B provided on a back surface of the substrate  21 B of the planar line  20 B. More specifically, the cutaway part A is formed by selectively removing a region including a connection section  70 B, such as a region immediately below the connection section  70 B, for example. The substrate  21 B is exposed at the region where the second conductive thin film  22 B is removed. 
     The cutaway part A has a rectangular shape in plan view, and may be formed, for example, such that a length a 3  (see  FIG. 4D ) of one side along the lengthwise direction of the signal line  25  is substantially the same as a length of the leading end portion  13   a  of the inner conductor  13  of the coaxial line  10  in an extension direction. 
     Furthermore, as shown in  FIG. 4C , a length a 4  of another side of the cutaway part A, along a widthwise direction of the signal line  25 , is a length by which end portions  22   b ,  22 ′ b  of the second conductive thin film  22 B coincide with the position of the inner wall  12  of the columnar penetrating hole of the outer conductor  11 . The end portions  22   b ,  22 ′ b  of the second conductive thin film  22 B that are adjacent to the cutaway part A thus coincide with the position of the inner wall  12  of the outer conductor  11 . 
     A height of the metal base  40 B (a length in a direction perpendicular to a propagation direction of high-frequency signals) is adjusted according to a thickness of the planar line  20 B. A cutaway part A′ corresponding to a shape of the cutaway part A formed in the second conductive thin film  22 B is formed in the metal base  40 B. More specifically, the cutaway part A′ is oriented in a direction away from an end surface of the metal base  40 B that is adjacent to the coaxial line  10 , and is formed penetrating the metal base  40 B from a surface to a back surface. An opening is formed in the end surface of the metal base  40 B that is adjacent to the coaxial line  10  due to the cutaway part A′ being formed. 
     For example, when the planar line  20 B is viewed from top, the cutaway part A′ has a rectangular cross-section that has lengths a 3 , a 4  (see  FIGS. 4D and 4C , respectively) that are substantially the same as those of the cutaway part A formed in the second conductive thin film  22 B. Additionally, the cutaway part A′ is not limited to have a rectangular cross-section, but may be formed according to the shape of the cutaway A formed in the second conductive thin film  22 B. 
     As described above, in the present embodiment, the substrate  21 B having a smaller thickness than those in the first and second embodiments is used. Generally, characteristic impedance is proportional to the square root of a reciprocal of electrical capacitance. An increase in the electrical capacitance causes reduction in the characteristic impedance. 
     In the present embodiment, the region A and the cutaway part A′ are formed immediately below the connection section  70 B, and a region where the second conductive thin film  22 B and the metal base  40 B are selectively removed is provided. Reduction in the characteristic impedance caused by an increase in the electrical capacitance may thereby be suppressed. 
       FIG. 4E  is a diagram for describing the signal current path P 1  and the return current path P 2  of the high-frequency line connection structure  1 B as viewed from a side. 
     As shown in  FIG. 4E , a bypass of the return current path P 2  is almost non-existent at the connection section  70 B between the coaxial line  10  and the planar line  20 B. Accordingly, high-frequency characteristics of the high-frequency line connection structure  1 B according to the present embodiment are also improved in substantially the same manner as in the first and second embodiments ( FIG. 2 ). 
     Accordingly, compared with the high-frequency line connection structure  500 A of the conventional example, the high-frequency line connection structure  1 B according to the present embodiment is more clearly improved with respect to the return loss, and furthermore, with respect to the insertion loss. 
     As described above, with the high-frequency line connection structure  1 B according to the third embodiment, the thickness a 1  of the substrate  21 B is sufficiently smaller than the radius r of the concentric circle of the coaxial line  10 . Furthermore, the end portions  23   a ,  23 ′ a  (see  FIG. 4C ) that are of the pair of first conductive thin films  23  of the planar line  20 B and that are close to the signal line  25  are disposed to coincide with the position of the inner wall  12  of the outer conductor  11 , and also, the end portions  22   b ,  22 ′ b  of the second conductive thin film  22 B that are adjacent to the cutaway part A are disposed to coincide with the position of the inner wall  12  of the columnar penetrating hole formed in the outer conductor  11 . 
     The high-frequency line connection structure  1 B may thus achieve a low return loss, and low insertion loss characteristics over a wide band. Furthermore, mechanical strength of the high-frequency line connection structure  1 B is increased because the coaxial line  10  and the planar line  20 B are mechanically connected by the first adhesion layer  30  (see  FIG. 4B ) and the second adhesion layer  60 , in addition to being electrically connected. 
     Heretofore, embodiments of the high-frequency line connection structure of the present invention have been described, but the present invention is not limited to the embodiments described, and may be modified in various ways conceivable to those skilled in the art within the scope of the invention described in the claims. 
     Additionally, in the embodiments described above, the substrate  21  forming the grounded coplanar line (planar line  20 ,  20 A,  20 B) is low-loss ceramics such as alumina, but liquid crystal polymer, polyimide, quartz glass or the like may also be used as the substrate  21 . 
     Furthermore, in the embodiments described above, at the time of electrically connecting the coaxial line  10  and the grounded coplanar line (planar line  20 ,  20 A,  20 B) by the first adhesion layer  30  and the second adhesion layer  60 ,  60 A, such as solders, gold plating is generally applied to the connection section  70 ,  70 A,  70 B at the lines to improve wettability of solders. However, gold plating is not an essential feature of the present invention, and description thereof is omitted. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1 ,  1 A,  1 B high-frequency line connection structure 
               10  coaxial line 
               11  outer conductor 
               12  inner wall 
               13  inner conductor 
               13   a  leading end portion 
               14  insulation layer 
               20  planar line 
               21  substrate 
               22  second conductive thin film 
               23  first conductive thin film 
               24  through hole 
               25  signal line 
               30  first adhesion layer 
               60  second adhesion layer 
               40 ,  50  metal base 
               70  connection section.