Electrical Coupler

Abstract

An electrical coupler is provided for connecting a coaxial transmission line to a microstrip transmission line which provides a close impedance match over a wide band of microwave frequencies. The coupler comprises an inner and an outer member which have a cross section at one end that is identical to that of the coaxial transmission line and a cross section at their other ends which includes a pair of spaced straight lines for connection to the spaced conductive strips of a microstrip transmission line. The ends of the coupler are connected by means which provide for a uniform transition from the configuration at one end to the configuration at the other end while maintaining a substantially constant impedance.

Description

BACKGROUND OF THE INVENTION

This invention relates to electrical couplers, and particularly to electrical couplers for connecting coaxial transmission lines to microstrip transmission lines.

Due to advances in thin and thick film technology in recent years, microstrip has found increasing applications in microwave technology. Complex microwave circuits containing a number of active and passive components can be produced using microstrip transmission lines. The problem remains, however, to interconnect several microstrip circuits, or to connect them to circuits or components of other configurations, since not all microwave components and circuits can be realized in microstrip. Such interconnections generally have been effected with coaxial couplers and coaxial transmission lines.

An interconnection between a coaxial transmission line and a microstrip transmission line obviously requires a transition between the configuration of the coaxial transmission line and that of the microstrip transmission line. To avoid impedance mismatches, and subsequent loss or distortion of signals, etc., the transition must be effected while closely matching the characteristic impedance of both transmission lines. In this regard, the coaxial conductor and microstrip transmission line to be connected generally have identical impedances, i.e. the characteristic impedance referred to hereinbefore. While it has proven to be relatively easy to obtain a good impedance match over narrow bands of microwave frequencies, electrical couplers having a close impedance match over wide bands of microwave frequencies have heretofore not been available.

It is the general object of this invention to provide an electrical coupler for coupling coaxial transmission lines to microstrip transmission lines which provides a close impedance mach over wide bands of microwave frequencies.

SUMMARY OF THE INVENTION

According to the present invention, there is provided an electrical coupler for connecting a coaxial transmission to a microstrip transmission line which can be matingly connected to the coaxial transmission line at one end and generally matingly connected to a microstrip transmission line at its other end. The aforesaid electrical coupler includes a transition region which uniformly and smoothly effects a change in the configuration of the coupler at one end to the configuration of the coupler at its other end while maintaining a substantially constant impedance.

In accordance with a preferred embodiment of the invention, the aforesaid uniform and smooth transition is achieved by the cooperation of a pair of conductive members. One of said pair of members comprises a ramplike conductive member which is disposed in a conductive conduit and terminates at its smaller end within the conduit and at its larger end at the end of the coupler to be connected to a microstrip transmission line thereby providing a line contact with one of the strips of conductive material of the microstrip transmission line. The other of said pair of members has one end shaped to matingly engage the inner conductor of a coaxial conductor, a tapered central portion which tapers in a direction away from the connection to the coaxial conductor, and a planar conductive portion adapted to provide line contact with the other conductive strip of the microstrip transmission line.

In accordance with a further feature of the invention, the tapered portion of the inner member is in the shape of a truncated cone and has a flat strip of conductive material disposed in a generally central slot of the truncated cone and of a substantially uniform width approximately equal to the diameter of the inner conductor of said coaxial conductor. The flat strip begins adjacent the larger end of the truncated cone and extends beyond the smaller end of the truncated cone for engagement with the said other conductor strip of said microstrip transmission line.

In accordance with a still further feature of the invention, the said flat strip of planar member extends beyond the microwave transmission linelike end of the coupler and includes a springlike portion which is continually urged into engagement with the said other conductive strip of the microwave transmission line.

In accordance with yet another feature of the invention, the length of the truncated cone portion is 10 times the difference between its initial and final diameters.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective partial cross-sectional view of a standard coaxial conductor;

FIG. 2 is a perspective view of a standard microstrip transmission line;

FIG. 3 is a perspective partial cross-sectional view illustrating an electrical coupler according to the present invention connecting a coaxial conductor to a microstrip transmission line;

FIG. 4 is a plan partial cross-sectional view of the assembly of FIG. 3;

FIG. 5 is an end view of the assembly of FIG. 3; and

FIG. 6 is a perspective view of a component of the electrical coupler of FIG. 3.

DETAILED DESCRIPTION OF THE DRAWINGS

A standard coaxial conductor 10 is shown in FIG. 1 which comprises an outer conductor member 12, an inner conductor member 14, and spaced dielectric supports 16 (only one being shown) maintaining the inner conductor 14 and outer conductor 12 in the desired coaxial relationship. FIG. 2 is a perspective view of a standard microstrip transmission line 18 which comprises a thin flat conductive strip 19 disposed in parallel with and spaced from a second wider flat conductive strip 20 by a dielectric sheet 22.

A preferred embodiment of an electrical coupler 24 in accordance with the present invention will now be described with reference to FIGS. 3, 4, 5 and 6. The coupler 24 comprises a conductive conduit member 26, an inner conductive member 28, and a ramplike conductive member 30. It will be appreciated that the ramplike conductive member 30 could be formed integral with the conductive conduit member 26.

Before going into detail with respect to the specifics of the various components of the coupler 24, it is believed that a general discussion of the concepts which have led to the development of the present electrical coupler will be helpful to a better understanding of the invention. The electrical coupler 24 was developed while bearing in mind that the following features were desired:

1. An initial acceptable impedance match with the coaxial conductor;

2. An initial acceptable impedance match with the microstrip transmission line; and

3. A transition from the coaxial conductor connection to the microstrip transmission line connection which provides a minimum of impedance discontinuities. The foregoing is accomplished in accordance with the present invention by using an arrangement of mechanically simple cross sections Generally, the impedance of a coaxial conductor to be connected to a microstrip transmission line is identical to the impedance of the microstrip transmission line. Therefore, the transition between the respective configurations is accomplished by the presently preferred embodiment while maintaining a substantially constant impedance.

The ramplike conductive member 30 is disposed within conduit member 26 with the smaller end 32 of the ramplike member terminating short of a coaxial conductorlike end 34 of the coupler 24. The larger end of the ramplike member 30 terminates at the transmission linelike end 36 of the coupler 24 where it provides a line contact 38 with conductive strip 20. It will be appreciated, particularly from FIGS. 3 and 6, that ramplike member 30 has a curved surface 40 which matingly engages a portion of the inner wall 42 of conduit member 26, a flat end 42 which terminates in the plane defined by the microstrip transmission linelike end 36 of coupler 24, and a flat surface 44 disposed at an angle to a center line passing through the axis of conduit member 26.

Inner conductive member 28 comprises an end portion 46 adapted to matingly engage inner conductor member 14 of the coaxial conductor 10. Inner member 28 includes a central tapered portion 48 which begins adjacent end portion 46 with a diameter equal to that of inner conductor member 14 and tapers to a smaller diameter at its other end 50. In a preferred embodiment of the invention, the diameter of the tapered portion at its said other end 50 is a multiple of the width of the conductive strip 19. In accordance with a specific embodiment of the invention, the diameter of the said other end 50 of the tapered portion 48 is twice the width of the conductive strip 19, the radius of the end thus being equal to the width of the conductive strip 19.

It has been found that the length of the tapered portion 48 in relation to the difference between the diameter of the tapered portion 48 adjacent end portion 46 (which is equal to the diameter of inner conductor 14) and the diameter of the tapered portion 48 at its said other end 50 is most important. In particular, it has been found that when the length of the tapered portion 48 is less than five times the difference between its initial and final diameter, a close impedance match is obtained only over a rather small band of microwave frequencies. When the length of the tapered portion 48 approaches 10 times the difference between its initial and final diameters, an acceptable impedance match is obtained over a wide band of microwave frequencies, such as from zero to 12 HGz. Even wider bands of frequencies could, it is believed, be accommodated by couplers in which the length of the tapered portion 48 relative to the difference between its initial and final diameters exceeds 10.

The inner conductive member 28 further includes a generally planar member 52, which could be made integral with the tapered portion 48. In the illustrated embodiment, the planar member 52 is inserted in a slot 53 of the tapered portion 48. It can be seen in FIG. 4 that the width of planar member 50 is substantially constant and is equal to the diameter of end portion 46. Though this is preferred, the width of planar member 52 could be equal to or less than the diameter of end portion 46.

Planar member 52 terminates in a flat strip portion 54 having a width equal to the width of conductive strip 19. The portion 54 is connected to the remainder of planar member 52 by rounded portions 56, 58 which smooth the transition from the initial wide width to the final narrow width.

A springlike portion 60 is connected to the flat end portion 54 and engages conductive strip 19. Portion 50 could be provided by a separate small contact spring attached to planar member 52 if desired. The spring effect can be used to insure conductive engagement between the springlike portion 60 and conductive strip 19. An alternative would be to solder portion 60 to conductive strip 19. Many other possibilities of course exist. In this regard, the relationship of inner conductive member 28 to conduit member 26 is effected in accordance with the illustrated embodiment by their connections to the coaxial conductor 10 and microstrip transmission line 18. Other possibilities of course exist for the assembly of coupler 24 such as dielectric spacers or the like. If planar member 52 is not employed, a direct contact between the said other end 50 of tapered portion 48 and conductive strip 19 would be effected. In such case, the connection would be a line contact connection directly above line contact 38, the geometry of the parts being so selected that this can be accomplished.

Referring again to the fact that it is desirable that the length of the tapered portion 48 of inner member 28 be approximately equal to or greater than 10 times the difference between its initial and final diameter, it has been found that a length of exactly 10 times the said difference is preferred since this length represents an increasing portion of a wave length as frequency is increased causing smaller discontinuities where they are most critical. The taper alone would, of course, result in an increase in characteristic impedance of the line. To avoid, this, the ramplike member 30 is disposed with its flat surface 44 facing inner member 28. The ramplike member 30 serves a dual purpose in that is lowers the impedance in the transition region and gradually changes the geometry of the coupler from a coaxial cross section to one where conduit 26 has a flat surface which provides the line contact 38. In addition, the gradual increase in impedance due to the taper is nullified by a corresponding gradual decrease in impedance due to the ramplike member.

It will be noted that surface 44 and planar member 52 are in generally opposed relationship and terminate at the microstrip transmission like end 36 of coupler 26 in approximately the same relationship as the conductive strips 19 and 20 of the microstrip transmission line 18. As a result, the electromagnetic field is more and more concentrated between the flat surface 44 and the flat surface 62 on the bottom (as viewed in FIG. 3) of planar member 52. Though this effect would be generally achieved utilizing tapered portion 48 and conductive member 30 alone, it is enhanced by the use of planar member 52 so that essentially all of the electromagnetic energy is concentrated between the flat surface 44 and the opposed flat surface 62 of planar member 52. The aforesaid concentration is gradual thereby maintaining the impedance at a substantially constant value.

While the principles of the invention have been described in connection with a specific embodiment thereof, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of the invention since various modifications, additions and changes are within such scope.

Claims

What is claimed is:

1. An electrical coupler for electrically conductively connecting a coaxial transmission line comprising inner and outer coaxial conductor members to a microstrip transmission line comprising a pair of spaced flat conductive strips, said coupler comprising a conductive conduit member and an inner conductive member adapted at one of their common ends to mate with the respective outer and inner coaxial conductor members of said coaxial transmission line, said inner conductive member being adapted at its other end to provide a straight line portion adapted to approximately mate with one of the said pair of conductive strips of said microstrip transmission line, and a ramplike conductive member disposed within said conductive conduit member and providing a straight line portion adapted to approximately mate with the other of said pair of said conductive strips of said microstrip transmission line, said ramplike conductive member comprising a substantially planar surface displaced from said inner conductive member and disposed at an angle thereto to provide a substantially constant impedance between said inner conductive member and said conductive conduit member.

2. An electrical coupler according to claim 1, wherein said inner conductive member comprises an end portion adapted to mate with the inner coaxial conductor member of said coaxial transmission line, and a truncated conelike portion having an initial diameter adjacent said end portion equal to the diameter of said end portion and a final diameter smaller than the diameter of said end portion.

3. An electrical coupler according to claim 2, wherein the length of said truncated conelike portion of said inner conductive member is greater than five times the difference between its initial and final diameters.

4. An electrical coupler according to claim 2, wherein the length of said truncated conelike portion is equal to or greater than 10 times the difference between its initial and final diameters.

5. An electrical coupler according to claim 2, wherein said inner conductive member comprises a flat planar member of substantially uniform width disposed generally centrally of said truncated conelike portion and extending from the smaller end thereof to provide one of said straight line portions adapted to mate with one of said pair of conductive strips of said microstrip transmission line.

6. An electrical coupler according to claim 5, wherein said substantially uniform width of said planar member is equal to the diameter of said inner coaxial conductor member of said coaxial transmission line.

7. An electrical coupler according to claim 5, wherein said planar member terminates at its end beyond said truncated conelike member in a smaller width than its said generally uniform width and said smaller width portion is connected to the generally uniform width portion by a gradually tapered portion.

8. An electrical coupler according to claim 7, wherein said generally tapered portion of said planar member comprises rounded shoulder portions connecting said substantially uniform width portion to said smaller width portion.

9. An electrical coupler according to claim 5, wherein said planar member terminates in a springlike portion at its end extending beyond said truncated conelike portion which springlike portion is adapted to be continuously urged into conductive engagement with said one of said pair of conductive strips of said microstrip transmission line.

10. An electrical coupler according to claim 5, wherein said ramplike member terminates at its larger end in a plane defined by the said other end of said conductive conduit member to provide at its upper edge the other of said straight line portions adapted to mate with the other of said pair of conductive strips of said microstrip transmission line.

11. An electrical coupler according to claim 10, wherein said one of said straight line portions has a width that is a multiple of the width of said one of said pair of conductive strips of said microstrip transmission line.

12. An electrical coupler according to claim 5, wherein said one of said straight line portions has a width that is a multiple of the width of said one of said pair of conductive strips of said microstrip transmission line.

13. An electrical coupler according to claim 12, wherein said multiple is two.

14. An electrical coupler according to claim 2, wherein said inner conductive member comprises a substantially planar member disposed approximately centrally of said truncated conelike portion, said planar member beginning at the larger end of said truncated conelike portion and extending beyond the smaller end thereof to provide coplanar flangelike extending portions extending outwardly from the surface of said truncated conelike portion with a substantially uniform distance between the edges of said flangelike extending portions, and said portions being disposed in opposed relation to said planar surface of said ramplike member.

15. An electrical coupler according to claim 14, wherein said substantially uniform distance between the edges of said flangelike extending portions is approximately equal to the diameter of said inner coaxial conductor member of said coaxial transmission line.

16. An electrical coupler according to claim 14, wherein said ramplike member terminates at its larger end in a plane defined by the said other end of said conductive conduit member to provide at its upper edge the other of said straight line portions adapted to mate with the other of said pair of conductive strips of said microstrip transmission line.

17. An electrical coupler according to claim 16, wherein said one of said straight line portions has a width that is a multiple of the width of said one of said pair of conductive strips of said microstrip transmission line.

18. An electrical coupler according to claim 17, wherein said multiple is two.

19. An electrical coupler according to claim 14, wherein said one of said straight line portions has a width that is a multiple of the width of said one of said pair of conductive strips of said microstrip transmission line.