Patent Publication Number: US-7586386-B2

Title: Transmission line transition from a coplanar strip line to a conductor pair using a semi-loop shape conductor

Description:
TECHNICAL FIELD OF THE INVENTION 
   The present invention relates to a transmission line transition, which is suited for a communication system using a microwave or millimeter wave band, and which is capable of making conversion from a microstrip transmission line to a coplanar strip transmission line or from a conductor for a coplanar strip transmission line to a microstrip transmission line. 
   BACKGROUND OF THE INVENTION 
   Coplanar strip transmission lines have been generally utilized as transmission lines, which feed a signal to a planar antenna or transmit a signal received by a planar antenna when the planar antenna is utilized for communication using a microwave or millimeter-wave band. 
   A transmission line transition, which has been utilized to make conversion from a microstrip transmission line to a slot transmission line possible and conversion from the slot transmission line to a coplanar strip transmission line possible, is shown in  FIG. 6 . In the example shown in  FIG. 6 , a first dielectric substrate  21  has an electromagnetically coupling conductor  24  for a coplanar strip transmission line disposed in a substantially dew-shaped form thereon, and the first dielectric substrate  21 , a dielectric layer  27 , a grounding conductor  26  and a second dielectric substrate  22  are laminated in this order. The second dielectric substrate  22  has the grounding conductor  26  disposed on a surface thereof close to the dielectric layer  27 , and the grounding conductor  26  has a linear slot  25  formed therein. The second dielectric substrate  22  has an electromagnetically coupling conductor  20  for a microstrip line disposed on a surface thereof remote from the dielectric layer  27 . All parts in the example shown in  FIG. 6  except for the second dielectric substrate  22  and the electromagnetically coupling conductor  20  for a microstrip line are disclosed in “Microstrip Lines and Slotlines”, Second Edition, p. 440-441, 7.7.5 CPS-to-Slotline Transitions, coauthored by K. C. Gupta, Ramesh Garg, Inder Bahl, Prakash Bhartia. However, there has been a problem that a transmission line transition, which partly utilizes the prior art, is not suitable for miniaturization. 
   Additionally, the above-mentioned prior art reference is silent on specific dimensions of the electromagnetically coupling conductor  24  for a coplanar strip transmission line and the like. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to solve the above-mentioned problems involved in the prior art and to provide a novel transmission line transition. 
   The present invention provides a transmission line transition comprising: 
   a first dielectric substrate; 
   a second dielectric substrate spaced from the first dielectric substrate; 
   a dielectric layer interposed between the first dielectric substrate and the second dielectric substrate; 
   the first dielectric substrate having a pair of conductors for a coplanar strip transmission line and an electromagnetically coupling conductor for the coplanar strip transmission line disposed on a surface close to the dielectric layer; 
   the electromagnetically coupling conductor for the coplanar strip transmission line being formed in a semi-loop shape with a discontinuity partly formed therein; 
   respective portions of the electromagnetically coupling conductor for the coplanar strip transmission line, which are located at both ends of the discontinuity or in the vicinity of both ends of the discontinuity, being connected to respective ends of the paired conductors for the coplanar strip transmission line or portions of the paired conductors in the vicinity of the respective ends of the paired conductors, the paired conductors extending toward a direction to be apart from the electromagnetically coupling conductor; 
   the semi-loop shape being a rectangular shape or a substantially rectangular shape; 
   the second dielectric substrate having a grounding conductor disposed on a surface close to the dielectric layer, the grounding conductor having a first slot and a second slot formed therein so as to be parallel or substantially parallel to each other; 
   the grounding conductor further having a connecting slot formed therein so as to connect between the first slot and the second slot so that the first slot, the second slot and the connecting slot form an electromagnetic coupling slot in an H-character shape or substantially H-character shape; 
   the electromagnetic coupling slot being disposed so that the connecting slot intersects a longitudinal direction of the rectangular shape or the substantially rectangular shape of the semi-loop shape as viewed in a plan view; and 
   the second dielectric substrate having an electromagnetically coupling conductor for a microstrip transmission line disposed on a surface remote from the dielectric layer so as to pass over or under the connecting slot. 
   The present invention also provides a transmission line transition comprising: 
   a first dielectric substrate; 
   a second dielectric substrate spaced from the first dielectric substrate; 
   a dielectric layer interposed between the first dielectric substrate and the second dielectric substrate; 
   the first dielectric substrate having a pair of conductors for a coplanar strip transmission line and an electromagnetically coupling conductor for the coplanar strip transmission line disposed on a surface close to the dielectric layer; 
   the electromagnetically coupling conductor for the coplanar strip transmission line being formed in a semi-loop shape with a discontinuity partly formed therein; 
   respective portions of the electromagnetically coupling conductor for the coplanar strip transmission line, which are located at both ends of the discontinuity or in the vicinity of both ends of the discontinuity, being connected to respective ends of the paired conductors for the coplanar strip transmission line or portions of the paired conductors in the vicinity of the respective ends of the paired conductors, the paired conductors extending toward a direction to be apart from the electromagnetically coupling conductor; 
   the semi-loop shape being a square shape or a substantially square shape; 
   the second dielectric substrate having a grounding conductor disposed on a surface close to the dielectric layer, the grounding conductor having a first slot and a second slot formed therein so as to be parallel or substantially parallel to each other; 
   the grounding conductor further having a connecting slot formed therein so as to connect between the first slot and the second slot so that the first slot, the second slot and the connecting slot form an electromagnetic coupling slot in an H-character shape or substantially H-character shape; 
   the electromagnetic coupling slot being disposed so that a portion of the connecting slot extending in a longitudinal direction passes over or under a side of the electromagnetically coupling conductor for the coplanar strip transmission line remote from the discontinuity; and 
   the second dielectric substrate having an electromagnetically coupling conductor for a microstrip transmission line disposed on a surface remote from the dielectric layer so as to pass over or under the connecting slot. 
   In accordance with the present invention, the electromagnetically coupling conductor for a coplanar strip transmission line is formed in a semi-loop shape with a discontinuity formed therein, and the respective portions of the electromagnetically coupling conductor for the coplanar strip transmission line, which are located at both ends of the discontinuity or in the vicinity of both ends of the discontinuity, are connected to the respective ends of the paired conductors for the coplanar strip transmission line or portions of the paired conductors for the coplanar strip transmission line in the vicinity of the respective ends of the paired conductors. When the semi-loop shape is a rectangular shape or a substantially rectangular shape, the transmission line transition can be made compact by 8.5 to 61% in comparison with the prior art. 
   When the semi-loop shape is a square shape or a substantially square shape, the transmission line transition can be made compact by 20 to 30% in comparison with the prior art. 
   The present invention can utilize the above-mentioned structure to realize transmission line conversion and impedance matching between the microstrip transmission line and the coplanar strip transmission line. Additionally, the present invention has an advantage of being capable of fabricating a transmission line transition at a low cost by a simple structure. 
   When a transmission line transition according to the present invention is utilized as a planar antenna transmission line, which is disposed at the front windshield or the rear windshield of an automobile, it is possible to effectively produce a high frequency antenna. In particular, it is possible to fabricate a high frequency antenna, which is suited for SDARS (Satellite Digital Audio Radio Service for about 2.6 GHz), GPS (Global Positioning System), VICS (Vehicle Information and Communication System), ETC (Electronic Toll Collection System), DSRC (Dedicated Short Range Communication) system and the like. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings, wherein like reference numbers represent like parts, and which may not be described in detail for all drawing figures: 
       FIG. 1  is a schematic view showing the transmission line transition according to an embodiment of the present invention; 
       FIG. 2  is a plan view showing an electromagnetic coupling slot and an electromagnetic coupling conductor for a microstrip line in the embodiment; 
       FIG. 3  is a plan view showing the electromagnetic coupling conductor for the microstrip line and an electromagnetic coupling conductor for a coplanar strip transmission line in the embodiment; 
       FIG. 4  is a plan view showing the electromagnetic coupling conductor for the coplanar strip transmission line in the embodiment; 
       FIG. 5  is a frequency characteristic graph in an Example; 
       FIG. 6  is a schematic view showing a transmission line transition utilizing a conventional electromagnetically coupling conductor for a coplanar strip transmission line; 
       FIG. 7  is a plan view showing an electromagnetic coupling conductor for a coplanar strip transmission line, according to another embodiment, which is different from the embodiment shown in  FIGS. 1 and 4 ; 
       FIG. 8  is a plan view for explanation of L offx1 ; 
       FIG. 9  is a plan view for explanation of L offx2 ; 
       FIG. 10  is a plan view for explanation of L offy  wherein the value of L offy  is positive; 
       FIG. 11  is a plan view for explanation of L offy  wherein the value of L offy  is negative; 
       FIG. 12  is a characteristic graph for L offx1  or L offx2  to insertion loss in Example 2; 
       FIG. 13  is a characteristic graph for L offy  to insertion loss in Example 3; 
       FIG. 14  is a characteristic graph for length L 5  to insertion loss in Example 4; and 
       FIG. 15  is a plan view showing an electromagnetic coupling conductor for a microstrip line and an electromagnetic coupling conductor for a coplanar strip transmission line according to another embodiment, which is different from the embodiments shown in  FIGS. 8 ,  9 ,  10  and  11 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Now, a transmission line transition according to the present invention will be described based on preferred embodiments shown in the accompanying drawings.  FIG. 1  is a schematic view showing the transmission line transition according to one embodiment of the present invention,  FIG. 2  is a plan view showing an electromagnetic coupling slot  5  and an electromagnetic coupling conductor  10  for a microstrip line in the embodiment shown in  FIG. 1 , and  FIG. 3  is a plan view showing the electromagnetic coupling conductor  10  for the microstrip line and an electromagnetic coupling conductor  4  for a coplanar strip transmission line in the embodiment shown in  FIG. 1 . 
   In FIGS.  1 , 2  and  3 , reference numeral  1  ( FIG. 1 ) designates a first dielectric substrate, reference numeral  2  ( FIGS. 1 ,  2 ) designates a second dielectric substrate, reference numeral  3  ( FIG. 1 ) designates a pair of conductors for the coplanar strip transmission line, reference numeral  3   a  ( FIG. 1 ) designates a first conductor for the coplanar strip transmission line, reference numeral  3   b  ( FIG. 1 ) designates a gap for the coplanar strip transmission line, which is disposed between the paired conductors for the coplanar strip transmission line, reference numeral  3   c  ( FIG. 1 ) designates a second conductor for the coplanar strip transmission line, reference numeral  4  ( FIGS. 1 ,  3 ) designates the electromagnetically coupling conductor for the coplanar strip transmission line, reference numerals  4   b  ( FIG. 3) and 4   c  ( FIG. 3 ) designate portions of the electromagnetically coupling conductor for the coplanar strip transmission line, which are located at both ends of a discontinuity  4   a  or in the vicinity of both ends of the discontinuity, reference numeral  4   d  ( FIG. 3 ) designates a side of the electromagnetically coupling conductor for the coplanar strip transmission line, which is remote from the discontinuity, reference numeral  5  ( FIG. 1 ) designates the electromagnetically coupling slot, which is formed in an H-character shape or a substantially H-character shape, reference numeral  5   a  ( FIG. 2 ) designates a first slot, reference numeral  5   b  ( FIG. 2 ) designates a second slot, reference numeral  5   c  ( FIG. 2 ) designates a connecting slot, reference numeral  6  ( FIG. 3 ) designates arrows showing the longitudinal direction of a semi-loop shape of the electromagnetically coupling conductor  4  for the coplanar strip transmission line, reference numeral  7  ( FIG. 1 ) designates a dielectric layer, reference numeral  10  ( FIG. 1 ) designates the electromagnetic coupling conductor for the microstrip line, reference numeral  12  ( FIG. 1 ) designates a grounding conductor, reference L 1  ( FIG. 2 ) designates the distance between the center of the connecting slot  5   c  and an open end of the electromagnetic coupling conductor  10  for the microstrip line, reference L 2  ( FIG. 2 ) designates the length of the connecting slot  5   c , reference L 3  ( FIG. 2 ) designates the length of the first slot, reference W 1  ( FIG. 2 ) designates the conductor width of the electromagnetic coupling conductor  10  for the microstrip line, reference W 2  ( FIG. 2 ) designates the width of the first slot  5   a , and reference W 2 ′, ( FIG. 2 ) designates the width of the second slot  5   b.    
   In the embodiment shown in  FIG. 1 , all parts shown in  FIG. 1  are laminated so as to be put one after the other in the direction of arrows. 
     FIG. 4  is a plan view of the electromagnetically coupling conductor  4  for the coplanar strip transmission line in the embodiment shown in  FIG. 1 . In  FIG. 4 , reference G 1  designates the distance between the first conductor  3   a  for the coplanar strip transmission line and the second conductor  3   c  for the coplanar strip transmission line, reference L 4  designates the length of a short side of the electromagnetically coupling conductor  4  for the coplanar strip transmission line, reference L 5  designates the length of a long side of the electromagnetically coupling conductor  4  for the coplanar strip transmission line, W 3  designates the conductor width of the first conductor  3   a  for the coplanar strip transmission line, reference W 3 ′ designates the conductor width of the second conductor  3   c  for the coplanar strip transmission line, and reference W 4  designate the conductor width of the electromagnetically coupling conductor  4  for the coplanar strip transmission line.  FIG. 4  shows a side  4   e  and a side  4   f  of the electromagnetically coupling conductor  4 . 
   The transmission line transition according to the present invention as best shown in  FIG. 1  comprises the first dielectric substrate  1 , the second dielectric substrate  2  disposed so as to be spaced from the first dielectric substrate  1 , and the dielectric layer  7  disposed between the first dielectric substrate  1  and the second dielectric substrate  2 . The first dielectric substrate  1  has the paired conductors  3  for the coplanar strip transmission line and the electromagnetically coupling conductor  4  for the coplanar strip transmission line disposed on a surface thereof close to the dielectric layer  7 . The electromagnetically coupling conductor  4  for the coplanar strip transmission line is formed in a semi-loop shape with the discontinuity  4   a  ( FIG. 4 ) formed therein. In the present invention, the semi-loop shape means an incomplete loop shape wherein the loop has a discontinuity partly formed therein. 
   As best shown in  FIG. 4 . the portions  4   b  and  4   c  of the electromagnetically coupling conductor  4  for the coplanar strip transmission line, which are located at both ends of the discontinuity  4   a  or in the vicinity of both ends of the discontinuity, are connected to respective ends of the paired conductors  3  of the coplanar strip transmission line or portions of the paired conductors  3  in the vicinity of the respective ends of the paired conductors  3 . The paired conductors for the coplanar strip transmission line extend in a direction to be apart from the electromagnetically coupling conductor  4  for the coplanar strip transmission line. 
   In the present invention, as best shown in  FIG. 4 , it is preferred from the viewpoint of improving transmission efficiency that the conductor width W 3  of the first conductor  3   a  for the coplanar strip transmission line be the same or substantially the same as the conductor width W 3 ′ of the second conductor  3   c  for the coplanar strip transmission line. It is also preferred that the conductor width W 3  of the first conductor  3   a  for the coplanar strip transmission line and the conductor width W 3 ′ of the second conductor  3   c  for the coplanar strip transmission line be narrower than the conductor width W 4  of the electromagnetically coupling conductor  4  for the coplanar strip transmission line. Further, it is more preferred that the following conditions be satisfied.
 
Conductor width W 3 ≦0.6×conductor width W 4 , and
 
Conductor width W 3 ′0.6×conductor width W 4  
 
   In the embodiment shown in  FIGS. 1 and 3 , the semi-loop shape of the electromagnetically coupling conductor  4  for the coplanar strip transmission line is a rectangular shape or a substantially rectangular shape, and the longitudinal direction of the rectangular shape or the substantially rectangular shape intersects the extension direction of portions of the paired conductors  3  for the coplanar strip transmission line, which are located in the vicinity of the discontinuity  4   a . It is preferred from the viewpoint decreasing insertion loss and improving transmission efficiency that the semi-loop shape be a rectangular shape or a substantially rectangular shape. However, the semi-loop shape is not limited to have a such a shape. Even if the semi-loop shape is a square shape or a substantially square shape, the present invention is operable. It should be noted that the longitudinal direction of the semi-loop shape of the electromagnetically coupling conductor  4  for the coplanar strip transmission line accords (i.e., is in accordance) with the longitudinal direction of the side  4   d  of the electromagnetically coupling conductor  4  for the coplanar strip transmission line, which is remote from the discontinuity. 
   The second dielectric substrate  2  has the grounding conductor  12  disposed on a surface thereof close to the dielectric layer  7 , and the grounding conductor  12  has the first slot  5   a  and the second slot  5   b  formed therein so as to be parallel or substantially parallel to each other. The grounding conductor  12  additionally has the connecting slot  5   c  formed therein to connect the first slot  5   a  and the second slot  5   b , and the first slot  5   a , the second slot  5   b  and the connecting slot  5   c  form the electromagnetically coupling slot  5  in an H-character shape or a substantially H-character shape. 
   In the embodiments shown in  FIGS. 8 ,  9  and  10  stated later, the electromagnetically coupling slot is disposed in such a direction that a portion of the electromagnetically coupling slot overlaps with the electromagnetically coupling conductor  4  for the coplanar strip transmission line as viewed in a plan view and that the connecting slot  5   c  passes over or under a portion of the rectangular or substantially rectangular semi-loop shape extending in the longitudinal direction. The connecting slot  5   c  orthogonally or substantially orthogonally passes over or under the side  4   d  of the electromagnetically coupling conductor  4  for the coplanar strip transmission line, which is remote from the discontinuity. 
   The second dielectric substrate  2  has the electromagnetically coupling conductor  10  for the microstrip line disposed on a surface remote from the dielectric layer  7  so as to pass over or under the connecting slot  5   c . In the embodiment shown in  FIG. 1 , the angle, at which the connecting slot  5   c  and the electromagnetically coupling conductor  10  for the microstrip line intersect each other as viewed a plan view, is a right angle or a substantially right angle. This arrangement is preferred to improve transmission efficiency. However, the present invention is not limited to have this arrangement. The present invention is operable even if the angle formed by the connecting slot  5   c  and the electromagnetically coupling conductor  10  for the microstrip line is not a right angle or a substantially right angle. 
   When it is assumed that an imaginary straight line extends in a direction perpendicular to the first dielectric substrate  1  and passes through the center of the connecting slot  5   c , and when the center of the connecting slot  5   c  is viewed from the imaginary straight line, it is preferred from the viewpoint of improving transmission efficiency that the point where the connecting slot  5   c  and the electromagnetically coupling conductor  10  for the microstrip line intersect each other overlap or substantially overlap with the center of the connecting slot  5   c.    
   When it is assumed that the imaginary straight line extends in the direction perpendicular to the first dielectric substrate  1  and passes through the center of the connecting slot  5   c , and when the center of the connecting slot  5   c  is viewed from the imaginary straight line, it is preferred from the viewpoint of improving transmission efficiency that the center of the connecting slot  5   c  overlap or substantially overlap with the side  4   d  of the electromagnetically coupling conductor  4  for the coplanar strip transmission line. 
   When it is assumed that the imaginary straight line extends in the direction perpendicular to the first dielectric substrate  1  and passes through the center of the connecting slot  5   c , and when the center of the connecting slot  5   c  is viewed from the imaginary straight line, it is preferred from the viewpoint of improving transmission efficiency that the center of the connecting slot  5   c  overlap or substantially overlap with the center of the side  4   d  of the electromagnetically coupling conductor  4  for the coplanar strip transmission line. 
   As stated above, it is preferred from the viewpoint of improving transmission efficiency that a portion of the electromagnetically coupling slot  5  overlap with the electromagnetically coupling conductor  4  for the coplanar strip transmission line as viewed in such a plan view. However, the present invention is not limited to have this arrangement. The present invention is operable even if all portions of the electromagnetically coupling slot  5  are disposed inside an inner peripheral edge of the electromagnetically coupling conductor  4  for the coplanar strip transmission line ( FIG. 15 ).  FIG. 15  shows an outer edge  40  of a side of the electromagnetically coupling conductor  4  close to the first conductor  3   a  and the second conductor  3   c.    
   In the embodiment shown in  FIGS. 1 and 4 , in a case wherein the electromagnetically coupling conductor  4  for the coplanar strip transmission line has a certain conductor width, when it is assumed that the electromagnetically coupling conductor  4  for the coplanar strip transmission line has no discontinuity  4   a  formed therein, and that the electromagnetically coupling conductor  4  for the coplanar strip transmission line is disposed so as to be continuous at the portion with the shown discontinuity  4   a ; the electromagnetically coupling conductor  4  for the coplanar strip transmission line thus assumed has an outer peripheral edge and an inner peripheral edge formed in a square or substantially square shape, respectively. 
   The electromagnetically coupling conductor  4  for the coplanar strip transmission line according another embodiment, which is different from the embodiment shown in  FIGS. 1 and 4 , is shown in  FIG. 7 . In the embodiment shown in  FIG. 7 , in a case wherein the respective four apexes of the four corners of the square shape or the substantially square shape defined by the outer peripheral edge of the electromagnetically coupling conductor  4  for the coplanar strip transmission line are called outer peripheral apexes, wherein the respective four apexes of the four corners of the square shape or the substantially square shape defined by the inner peripheral edge of the electromagnetically coupling conductor for the coplanar strip transmission line are called inner peripheral apexes, and wherein explanation is made about an outer peripheral apex  15  as an example, which is located at an upper left position in  FIG. 7 ; when it is assumed that a first imaginary straight line connects between the outer peripheral apex  15  and an inner peripheral apex  14  closest to the outer peripheral apex  15 , when the first imaginary straight line is called a first imaginary line  11 , when it is assumed that a second imaginary straight line extends orthogonally or substantially orthogonally to the first imaginary line  11  and passes through the center or a position in the vicinity of the center between the outer peripheral apex  15  and the inner peripheral apex  14 , and when the second imaginary straight line is called a second imaginary line  8 , a portion of the electromagnetically coupling conductor  4  for the coplanar strip transmission line, which is located outside the second imaginary line, is cut out, forming a cut-out portion. 
   It is preferred that each of all four outer peripheral apexes have a cut-out portion formed therein as in the embodiment shown in  FIG. 7 . However, the present invention is not limited to have this arrangement. The present invention is operable as long as at least one of the four outer peripheral apexes has a cut-out portion. 
   In the embodiment shown in  FIG. 7 , when it is assumed that an outer peripheral edge of the portion  4   b  of the electromagnetically coupling conductor  4  for the coplanar strip transmission line, which is close to the first conductor  3   a  for the coplanar strip transmission line and forms one of both end portions of the electromagnetically coupling conductor  4  for the coplanar strip transmission line at both ends of the discontinuity  4   a , is linearly extended toward a central portion of the discontinuity, when a point where the extended outer peripheral edge intersects an inner peripheral edge of the first conductor  3   a  for the coplanar strip transmission line is called a first intersection  16 , when it is assumed that an outer peripheral edge of the first conductor  3   a  for the coplanar strip transmission line is linearly extended toward the central portion of the electromagnetically coupling conductor  4  for the coplanar strip transmission line, and that a point where the extended outer peripheral edge of the first conductor intersects the inner peripheral edge of the electromagnetically coupling conductor  4  for the coplanar strip transmission line is called a second intersection  17 , when it is assumed that a third imaginary straight line connects between the first intersection  16  and the second intersection  17 , and when the imaginary straight line is called a third imaginary line  13 ; a portion of the electromagnetically coupling conductor  4  for the coplanar strip transmission line or the first conductor  3   a  for the coplanar strip transmission line, which is closer to the central portion of the electromagnetically coupling conductor  4  for the coplanar strip transmission line than the third imaginary straight line, is cut out, forming a cut-out portion (a first inner cut-out portion). 
   The outer peripheral edge of the first conductor  3   a  for the coplanar strip transmission line means a peripheral edge of the first conductor  3   a  for the coplanar strip transmission line, which is remote from the gap  3   b  for the coplanar strip transmission line. The inner peripheral edge of the first conductor  3   a  for the coplanar strip transmission line means a peripheral edge of the first conductor  3   a  for the coplanar strip transmission line, which is close to the gap  3   b  for the coplanar strip transmission line. 
   Additionally, in the embodiment shown in  FIG. 7 , when it is assumed that an outer peripheral edge of the portion  4   c  of the electromagnetically coupling conductor for the coplanar strip transmission line, which is close to the second conductor  3   c  for the coplanar strip transmission line and forms the other one of both end portions of the electromagnetically coupling conductor for the coplanar strip transmission line at both ends of the discontinuity  4   a , is linearly extended toward the central portion of the discontinuity, when the point where the extended outer peripheral edge intersects an inner peripheral edge of the second conductor  3   c  for the coplanar strip transmission line is called a third intersection, when it is assumed that an outer peripheral edge of the second conductor  3   b  for the coplanar strip transmission line is linearly extended toward the central portion of the electromagnetically coupling conductor  4  for the coplanar strip transmission line, and that the point where the extended outer peripheral edge of the second conductor intersects the inner peripheral edge of the electromagnetically coupling conductor  4  for the coplanar strip transmission line is called a fourth intersection, when it is assumed that a fourth imaginary straight line connects between the third intersection and the fourth intersection, and when the fourth imaginary straight line is called a fourth imaginary line; a portion of the electromagnetically coupling conductor  4  for the coplanar strip transmission line or the second conductor for the coplanar strip transmission line, which is closer to the central portion of the electromagnetically coupling conductor  4  for the coplanar strip transmission line than the fourth imaginary straight line, is cut out, forming a cut-out portion (a second inner cut-out portion). 
   It is preferred that the electromagnetically coupling conductor  4  for the coplanar strip transmission line have both the first inner cut-out portion and the second inner cut-out portion formed therein as shown in  FIG. 7 . However, the present invention is not limited to have this arrangement. The present invention is operable even if the electromagnetically coupling conductor  4  for the coplanar strip transmission line has only one of the first inner cut-out portion and the second inner cut-out portion formed therein. 
   When the electromagnetically coupling conductor  4  for the coplanar strip transmission line has a short side length of L 4 , and when the electromagnetically coupling conductor for the coplanar strip transmission line has a long side length L 5 , it is preferred from the viewpoint of improving transmission efficiency that the formula of 0.11≦(L 4 /L 5 )&lt;1.0, in particular 0.11≦(L 4 /L 5 )&lt;0.65, be satisfied. 
   It is preferred from the viewpoint of improving transmission efficiency that the length L 3  of the first slot and the length of the second slot be the same or substantially the same as each other. However, the present invention is not limited to have this arrangement. The present invention is operable even if the length L 3  of the first slot and the length of the second slot are different from each other. It is preferred from the viewpoint of improving transmission efficiency that the length L 3  of the first slot of the length of the second slot be normally shorter than the length L 5 . 
   It is preferred from the viewpoint of improving transmission efficiency that the width W 2  of the first slot  5   a  and the width W 2 ′ of the second slot  5   b  be from 0.1 to 1.0 mm, in particular from 0.2 to 0.6 mm. It is preferred from the viewpoint of improving transmission efficiency that the conductor width W 1  of the electromagnetically coupling conductor  10  of the microstrip line be from 1.0 to 2.0 mm, in particular from 1.3 to 1.6 mm. It is preferred from the viewpoint of improving transmission efficiency that the distance L 1  be from 3.0 to 15.0 mm, in particular from 5.0 to 10.0 mm. 
   In the present invention, when the operating frequency is from 1.95 to 2.93 GHz, it is preferred that the length L 5  of the side of the electromagnetically coupling conductor  4  for the coplanar strip transmission line, which is remote from the discontinuity  4   a , be from 5.0 to 46.1 mm. The reason why the operating frequency is set at a value from 1.95 to 2.93 GHz is that the formula of (2.34/1.2)−(2.34/0.8) GHz≈1.95−2.93 GHz is established, providing a tolerance range of 20% from 2.34 GHz that is the frequency of SDARS in the United States. The permissible range is preferably from 2.13 to 2.6 GHz with a tolerance range of 10%, more preferably from 2.23 to 2.46 GHz with a tolerance range of 5%. 
   The length L 5  preferably ranges from 8.0 to 40.8 mm, more preferably ranges from 12.0 to 37.2 mm. 
   Under the condition of the above-mentioned operating frequency range, it is preferred that the length L 4  of two sides of the electromagnetically coupling conductor  4  for the coplanar strip transmission line, which are adjacent the side  4   d  opposite the discontinuity  4   a , be from 5.0 to 46.1 mm. The length L 4  more preferably ranges from 8.0 to 40.8 mm, most preferably ranges from 12.0 to 37.2 mm. 
     FIGS. 8 and 9  are plan views for explanation of L offx1  and L offx2  described later. Explanation of the following conditions will be made when it is assumed that an imaginary straight line passes through the center of the gap  3   b  for the coplanar strip transmission line and extends toward the center of the electromagnetically coupling conductor  4  for the coplanar strip transmission line under the condition of the above-mentioned operating frequency range, and wherein the electromagnetically coupling conductor  4  for the coplanar strip transmission line is viewed, being divided into a portion close to the first conductor  3   a  for the coplanar strip transmission line and a portion close to the second conductor  3   c  for the coplanar strip transmission line with this imaginary straight line used as the boundary. 
   The inner edge of a side  4   e  of the electromagnetically coupling conductor  4  for the coplanar strip transmission line, which forms one of the two sides adjacent the side  4   d  opposite the discontinuity  4   a  and is close to the first conductor  3   a  for the coplanar strip transmission line, is called a first inner edge. The inner edge of a side  4   f  of the electromagnetically coupling conductor  4  for the coplanar strip transmission line, which forms the other one of the two sides adjacent the side  4   d  opposite the discontinuity  4   a  and is close to the second conductor  3   c  for the coplanar strip transmission line, is called a second inner edge. 
   When the leading edge of the first slot  5   a , which is close to the first inner edge, is called a first leading edge  5   a   1  ( FIG. 8 ) of the first slot; when the leading edge of the first slot  5   a , which is close to the second inner edge, is called a second leading edge  5   a   2  ( FIG. 8 ) of the first slot; when the leading edge of the second slot  5   b , which is close to the first inner edge, is called a first leading edge  5   b   1  of the second slot; when the leading edge of the second slot  5   b , which is close to the second inner edge, is called a second leading edge  5   b   2  of the second slot; when a shorter one of the shortest distance between the first leading edge  5   a   1  of the first slot and the first inner edge, and the shortest distance between the first leading edge  5   b   1  of the second slot and the first inner edge is called L offx1  ( FIG. 8 ); and when a shorter one of the shortest distance between the second leading edge  5   a   2  of the first slot and the second inner edge, and the shortest distance between the second leading edge  5   b   2  of the second slot and the second inner edge is called L offx2  ( FIG. 9 ); it is preferred that the formulas of L offx1 ≧−2 mm and L offx2 ≧−2 mm are satisfied, where L offx1  is negative when the closer of the first leading edge  5   a   1  and the first leading edge  5   b   1  to the first inner edge is disposed beyond the first inner edge relative to the side  4   f , and L offx2  is negative when the closer of the second leading edge  5   a   2  and the second leading edge  5   b   2  to the second inner edge is disposed beyond the second inner edge relative to the side  4   e.    
   It is determined that the value of L offx1  is positive when the first leading edge  5   a   1  of the first slot approaches toward the center of the electromagnetically coupling conductor  4  for the coplanar strip transmission line in a direction (indicated by an arrow  41  in  FIG. 8 ), which is parallel to the longitudinal direction of the side  4   d  of the electromagnetically coupling conductor  4  for the coplanar strip transmission line remote from the discontinuity  4   a , and that the value of L offx1  is negative when the first leading edge  5   a   1  of the first slot recedes from the center in such a direction. 
   It is determined that the value of L offx2  is positive when the second leading edge  5   a   2  of the first slot approaches toward the center of the electromagnetically coupling conductor  4  for the coplanar strip transmission line in a direction (indicated by an arrow  42  in  FIG. 9 ), which is parallel to the longitudinal direction of the side  4   d  of the electromagnetically coupling conductor  4  for the coplanar strip transmission line remote from the discontinuity  4   a , and that the value of L offx2  is negative when the second leading edge  5   a   2  of the first slot recedes from the center in such a direction. 
   It is preferred that the values of L offx1  and L offx2  satisfy the formulas of L offx1 ≧0 mm and L offx2 ≧0 mm. It is particularly preferred that the values of L offx1  and L offx2  satisfy the formulas of L offx1 ≧1 mm and L offx2 ≧1 mm. 
     FIGS. 10 and 11  are plan views for explanation of L offy  described later. In  FIGS. 10 and 11 , when the distance between an outer edge  40  ( FIG. 10 ) of a side of the electromagnetically coupling conductor  4  for the coplanar strip transmission line close to the paired conductors  3   a ,  3   c  for the coplanar strip transmission line and an edge  5   a   3  ( FIG. 10 ) of the first slot close to the connecting slot  5   c  is called L offy , it is preferred that the formula of −4.3 mm≦L offy ≦8.0 mm is satisfied, where L offy  is negative when edge  5   a   3  is disposed beyond the outer edge  40  relative to side  4   d.    
   It is determined that the value of L offy  is positive when the edge  5   a   3  of the first slot close to the connecting slot  4   c  is disposed so as to be close to the side  4   d  of the electromagnetically coupling conductor  4  for the coplanar strip transmission line remote from the discontinuity  4   a  with the outer edge  40  of the side of the electromagnetically coupling conductor  4  for the coplanar strip transmission line close to the paired conductors ( 3   a ,  3   c ) for the coplanar strip transmission line used as the boundary ( FIG. 10 ), and that the value of L offy  is negative when the edge  5   a   3  of the first slot is disposed beyond the outer edge  40  relative to side  4   d  and so as to be close to the paired conductors ( 3   a ,  3   c ) for the coplanar strip transmission line ( FIG. 11 ). 
   The value of L offy  preferably satisfies the formula of −3.5 mm≦L offy ≦7.3 mm, particularly preferably satisfies the formula of 1.0 mm≦L offy ≦6.5 mm. 
   There is no particular limitation to the thickness of the first dielectric substrate  1  since the thickness of the first dielectric substrate is not directly related to electromagnetic coupling. For example, when the first dielectric substrate comprises an automobile windshield, it is preferred to use a glass sheet having a thickness of from 2.0 to 6.0 mm and a dielectric constant of (ε 1 ) of from 5.0 to 9.0 as in a normal automobile windshield. 
   When the first dielectric substrate  1  comprises an automobile windshield, it is preferred that the grounding conductor  12  have a peripheral edge spaced from the opening edge of an automobile body by a length of 1 mm or more. However, the present invention is not limited to have this arrangement. The present invention is operable even if the peripheral edge of the grounding conductor  12  is connected to the opening edge of an automobile body. In this case, the opening edge means a peripheral edge of an opening of an automobile body, into which a windshield is fitted, which serves as ground connection through the automobile body, and which is made of a conductive material, such as metal. 
   It is preferred that the second dielectric substrate  2  have dimensions (an area) of from 2.6×26.0 mm (67.6 mm 2 ) to 15.0×31.0 mm (465 mm 2 ). It is preferred from the viewpoint of improving transmission efficiency that the second dielectric substrate have a dielectric constant (ε 2 ) of from 1.0 to 8.0. The second dielectric substrate  2  may be normally a circuit board comprising a synthetic resin, ceramics or the like. It is preferred that the second dielectric substrate  2  have a thickness of from 0.1 to 6.0 mm. This is because it is easy to fabricate a substrate in such a thickness range in terms of production technique. 
   It is preferred that the dielectric layer  7  be interposed between the first dielectric substrate  1  and the second dielectric substrate  2  and have an insulating property. The dielectric layer  7  may normally comprise a dielectric composition containing, e.g., a synthetic resin, such as an adhesive or a filler, having an insulating property, or ceramics. The dielectric layer may comprise a gas layer. However, the present invention is not limited to have such arrangements. Any dielectric substance is applicable as the dielectric layer, and a dielectric substrate is also applicable as the dielectric layer. 
   An example of the adhesive having an insulating property is an adhesive containing an epoxy resin or the like. It is preferred to use an adhesive having a dielectric constant ranging from 1.0 to 4.0 since such an adhesive is easily available at a low cost. An example of the filler is a filler containing silicone having an insulating property. 
   When the dielectric layer  7  comprises a gas layer, an air layer is normally used because of being inexpensive. The present invention is not limited to use such an air layer. The gas layer may comprise an inert gas, such as nitrogen or argon. It is preferred that such a gas layer be sufficiently dried so as to prevent the moisture contained in the gas from being condensed according to temperatures. 
   It is preferred that the dimensions or area of the dielectric layer  7  be the same as the dimensions or area of the second dielectric substrate  2 . It is preferred from the viewpoints of improving transmission efficiency that the dielectric layer  7  have a thickness of from 0.1 to 1.6 mm. It is preferred from the viewpoint of improving transmission efficiency that the dielectric layer  7  have a dielectric constant (ε 3 ) of from 1.0 to 3.0. It is preferred that the present invention be applied to a frequency range of from 1 to 30 GHz, in particular a frequency range from 2 to 6 GHz. 
   EXAMPLE 
   Now, the present invention will be described referring to examples. The present invention is not limited to these examples. It should be noted that various improvement and modifications may be made without departing from the spirit and the scope of the present invention. Now, the examples will be described in detail, referring to the accompanying drawings. 
   Example 1 
   On the assumption that a transmission line transition was fabricated just as shown in  FIGS. 1 ,  2 ,  3  and  4 , transmission characteristics from a pair of conductors  3  for a coplanar strip transmission line to an electromagnetically coupling conductor  10  for a microstrip line were calculated by the FDTD method (Finite Difference Time Domain method). The operating frequency were set at 2.34 GHz. The dimensions of respective parts are shown below, and the frequency characteristics of this example is shown in  FIG. 5 . In  FIG. 5 , reference numeral  19  designates reflection loss, and reference numeral  18  designates insertion loss. 
   
     
       
         
             
             
             
           
             
                 
             
           
          
             
               Dimensions or area of second dielectric 
               12.25 × 32.0 
               mm (392.0 mm 2 ) 
             
             
               substrate 2: 
             
             
               Thickness of second dielectric 
               0.8 
               mm 
             
             
               substrate 2: 
             
             
               Thickness of dielectric layer 7: 
               0.42 
               mm 
             
             
               ε 1   
               7.0 
             
             
               ε 2   
               4.0 
             
             
               ε 3   
               2.0 
             
             
               L 1   
               8.0 
               mm 
             
             
               L 2   
               4.6 
               mm 
             
             
               L 3   
               21.0 
               mm 
             
             
               L 4   
               7.0 
               mm 
             
             
               L 5   
               28.0 
               mm 
             
             
               W 1   
               1.45 
               mm 
             
             
               W 2 , W 2 ′ 
               0.4 
               mm 
             
             
               W 3 , W 3 ′ 
               0.5 
               mm 
             
             
               W 4   
               1.0 
               mm 
             
             
               G 1   
               0.5 
               mm 
             
             
                 
             
          
         
       
     
   
   Example 2 
   On the assumption that a transmission line transition was fabricated so as to be the same as the one in Example 1 except that L offx1  and L offx2  were modified, transmission characteristics from a pair of conductors  3  for a coplanar strip transmission line to an electromagnetically coupling conductor  10  for a microstrip line were calculated according to the FDTD method. The operating frequency was set at 2.34 GHz. Characteristics of L offx1  to insertion loss, which were obtained when the values of L offx1  and L offx2  were modified, are shown in  FIG. 12 . 
   Example 3 
   On the assumption that a transmission line transition was fabricated so as to be the same as the one in Example 1 except that L offy  was modified, transmission characteristics from a pair of conductors  3  for a coplanar strip transmission line to an electromagnetically coupling conductor  10  for a microstrip line were calculated according to the FDTD method. The operating frequency was set at 2.34 GHz. Characteristics of L offy  to insertion loss, which were obtained when the value of L offy  was modified, are shown in  FIG. 13 . 
   Example 4 
   On the assumption that a transmission line transition was fabricated so as to be the same as the one in Example 1 except that the long side width L 5  of an electromagnetically coupling conductor  4  for a coplanar strip transmission line was modified, transmission characteristics from a pair of conductors  3  for the coplanar strip transmission line to an electromagnetically coupling conductor  10  for a microstrip line were calculated according to the FDTD method. The operating frequency was set at 2.34 GHz. Characteristics of length L 5  to insertion loss, which were obtained when the value of the with L 5  was modified, are shown in  FIG. 14 . 
   The present invention is applicable to a transmission line transition for a high frequency antenna, which is suitable for use in SDARS, GPS, satellite digital broadcasting, VICS, ETC and DSRC system. 
   The entire disclosure of Japanese Patent Application No. 2005-73190 filed on Mar. 15, 2005 including specification, claims, drawings and summary is incorporated herein by reference in its entirety.