Millimeter wave microstrip to coaxial line side-launch transition
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to electronic circuitry operable at millimeter wave frequencies, and more particularly to a transition for efficiently joining a coaxial transmission line to a microstrip transmission line. 
     II. Discussion of the Prior Art 
     In applications requiring a high degree of microminiaturization, operation at millimeter wavelength frequencies allows tiny active and passive components to be employed. For example, only a small length of printed wiring may exhibit a capacitive reactance comparable to that provided by a ceramic capacitor operating at lower frequencies. 
     One mode of transmitting these RF signals is the so-called microstrip transmission line. It comprises a conductive strip of a relatively narrow width on one major surface of a dielectric planar substrate and a relatively wide conductive ground plane disposed on the opposite major surface of the dielectric substrate. High frequency signals in the millimeter wavelength range propagate along the surface of the microstrip line. Such microstrip transmission lines may be exposed, but oftentimes will be contained within a conductive box-like enclosure or housing to prohibit stray radiation emanating from the transmission line to deleteriously affect other components within the electronic system embodying the microstrip transmission line. Where a closed system is utilized, it is necessary to employ a feed-through to bring the transmitted signal in and out of the enclosure. Such a feed-through may typically be a length of coaxial transmission line. Even with open or uncased microstrip transmission lines, it is still necessary to provide a means for coupling the signal output from that medium to a load such as, for example, an antenna. Again, this necessitates a transition device for mating the microstrip media to the coaxial line media. 
     With uncased microstrip transmission lines, practice in the past has been to utilize an edge transition in which the outer conductive shield and the center insulation of the coaxial line is stripped back to expose the center conductor and then that center conductor is laid upon the microstrip conductor and a solder bond is used to join the two. Similarly, the outer solid tubular conductive shield of the coaxial line is conductively joined to the ground plane of the microstrip media, again using solder. This type of edge-launch transition is not altogether satisfactory in many applications, primarily because of packaging limitations, weakness of the joint and space considerations. For example, the physical geometry of the particular electronics package may preclude the use of an edge-launch transition between the microstrip and the coax line. 
     When the microstrip transmission line is contained within a conductive housing, the ground plane of the microstrip transmission line assembly can be soldered to the base or floor of the housing and, likewise, the tubular conductive shield of the coaxial line may also be soldered to the housing. This results in a stronger, more rugged construction but, as pointed out below, physical constraints of the electronic package itself may preclude this type of edge-launch transition. 
     For the foregoing reasons, a need exists for a more efficient means of joining a microstrip transmission line to a coaxial transmission line in a side-launch configuration. The present invention fulfills such a need. In particular, the present invention provides a means for coupling microstrip devices or systems to other transmission media, coaxial line in this case. In accordance with the present invention, a side-launch transition has been devised. To the best of our knowledge, we are the first to devise a side-launched coax to microstrip transition operable at millimeter wave frequencies. It is our belief that no one has earlier attempted a side-launch transition at millimeter wave frequencies because of the difficulty in simultaneously converting from coaxial mode to microstrip mode and in compensating for the large parasitic reactances present at millimeter wave frequencies. 
     OBJECTS 
     It is accordingly a principal object of the present invention to provide an improved method and apparatus for efficiently coupling a coaxial transmission line to a microstrip transmission line in a side-launch configuration. 
     Another object of the present invention is to provide a side-launch transition for applications where an edge-launch transition will not physically fit or where mechanical constraints makes it more convenient. 
     Still another object of the invention is to provide a side-launched transition between a coaxial transmission line and a microstrip transmission line in which attention is paid to compensating parasitic reactances. 
     Yet still another object of the invention is to provide a side-launched transition between a microstrip transmission line and a coaxial cable in which two approximately half wavelength matching transmission line sections are used to bisect the side-launch coaxial cable. 
     A yet further object of the invention is to provide a side-launched transition between a coaxial transmission line and a microstrip transmission line which is small in size and, thus, compatible with mechanical system requirements in many microminiaturized radio frequency systems. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, the microstrip on the first side of the dielectric substrate is provided with an integrally formed, generally rectangular conductive pad area where the length of the pad is approximately equal to the wavelength of the signal being transmitted. Formed through the conductive ground plane on the other side of the substrate from the conductive pad area is a circular aperture of a predetermined diameter which is equal to or slightly less than the diameter of the coax transmission line to be joined to the microstrip transmission line. A small hole is drilled through the center of this aperture and extending through both the dielectric substrate layer and the conductive pad. This hole is dimensioned to receive the center conductor of the coaxial line. Because this geometry in a plan view resembles an eye, the aperture is referred to herein as an &#34;iris&#34;. In that the iris formed through the ground plane is centrally aligned within the rectangular pad area, two conductive segments, each approximately one-half wavelength long, extend outwardly on either side of the coax line&#39;s center conductor when that conductor is inserted through the dielectric layer and through the microstrip conductive pad area. Once so inserted, the coax transmission line&#39;s center conductor is conductively bonded to the conductive pad by soldering. Following that, the outer shield of the coaxial transmission line is joined to the ground plane by soldering the two together around the perimeter of the aperture. 
     When the microstrip transmission line is enclosed in a conductive, box-like housing having a floor surface abutting the ground plane of the microstrip transmission line, a reliable joint between the outer tubular conductor of the coaxial line and the ground plane can be achieved by boring an appropriate sized hole through the floor of the housing and then fitting the coaxial transmission line into that bore with the center conductor again extending through a small hole drilled through the microstrip assembly at the site of the iris and then soldering the exterior surface of the coaxial cable to the wall of the bore drilled through the floor of the housing. Alternatively, a coaxial connector (male or female) can be bolted to the exterior of the housing floor with the connector&#39;s dielectric extending through a predrilled hole in the housing floor so as to butt up against the substrate ground plane with its pre-etched iris. The connector&#39;s center pin then extends through a hole drilled both through the substrate and through the rectangular conductive pad area on the opposed side of the substrate. The coaxial cable can then be joined to the connector in a conventional fashion (e.g., using a mating connector). 
     As will be explained further hereinbelow, both of these transitions effectively match the characteristic impedance of the coaxial line to that of the microstrip line which is, of course, a condition resulting in a maximum power transfer. Furthermore, by paying particular attention to the thickness of the dielectric substrate, the diameter of the coaxial cable and the size of the iris or opening through the ground plane, it is possible to compensate the parasitic reactances present in the region where the electromagnetic fields make a 90° &#34;bend&#34;, further minimizing power losses and reflections at the site of the transition. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     These and other objects and advantages of the invention will become apparent to those skilled in the art from the following detailed description of a preferred embodiment in which: 
     FIG. 1 is a perspective view of a prior art edge launched coaxial to microstrip transmission line transition; 
     FIG. 2 is a perspective view of the side-launched transition in accordance with a first embodiment of the present invention; 
     FIG. 3 is a side cross-sectional view taken along the line 3--3 in FIG. 2; 
     FIG. 4 is a side cross-sectional view of the side-launched transition in accordance with an alternative embodiment of the invention; 
     FIG. 5 are curves illustrating the performance of the present invention when compared to the prior art arrangement of FIG. 1; and 
     FIG. 6 shows a side-launched transition joining two microstrip transmission lines. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Before going into the detailed description of the preferred embodiments, it is deemed helpful to an overall understanding of the invention to first consider the well-known prior art edge-launched transition whereby coaxial and microstrip transmission lines are joined. With reference to FIG. 1, the coaxial transmission line is identified by the numeral 10 and is entirely conventional in including an outer tubular conductive shield 12 which, typically, will be formed from a solid outer conductor and a center conductor 14 coaxially disposed within the tubular shield and spaced therefrom by a dielectric medium, such as any one of a number of available plastics. This insulating layer is identified by numeral 16. 
     The microstrip transmission line is indicated generally by numeral 18 and it includes a microstrip conductor 20 which is disposed on a first major surface 22 of a dielectric substrate 24. The strip 20 has a relatively narrow width dimension and of a length tailored to the particular application. On the opposite major surface 26 of the dielectric substrate 24 is a relatively wide ground plane 28. That is to say, the width of the ground plane is several times the width of the conductive microstrip 20. The conductors 20 and 28 may be defined using conventional printed circuit techniques. 
     As can be seen from FIG. 1 in forming the edge-launched transition, the end portion of the coaxial cable 10 is made to abut the edge surface 30 of the microstrip assembly 18 and is generally longitudinally aligned with the microstrip 20. An end portion of the outer shield 12 and the underlying dielectric 16 is stripped off to expose a predetermined length of the center conductor 14 and this exposed length is made to overlay the conductive microstrip 20. A connection is then made between the two using solder, as at 32, or other known conductive joining process. An exposed end portion of the outer conductive shield 12 is likewise joined to the ground plane 28 of the microstrip assembly as at 34 to complete the transition. 
     Another commercially-available edge-launched coaxial-to-microstrip transition is available through Wiltron Corporation of Mountain View, Calif. Rather than directly soldering the center conductor of the coaxial cable to the microstrip and its conductive tubular shield to the ground plane, in the Wiltron transition, the microstrip transition line is mounted within a housing having a small diameter bore and a larger diameter threaded counterbore. Fitted into the small diameter bore is a center conductor member which is supported within a glass bead. The center conductor is attached at one end to the microstrip by a conductive bond and the other end of the center conductor extends into the threaded counterbore. The counterbore is configured to receive a threaded double female plug, one end mating with the center conductor and the other end mating with a male connector to which the coaxial cable is attached. 
     Referring now to FIGS. 2 and 3, it may be observed that the side-launch transition of the present invention again comprises a coaxial cable 40 having a center conductor 42, a surrounding cylindrical body of dielectric material 44 and an outer concentric conductive tubular sleeve 46. 
     The microstrip line is disposed within a conductive housing, only the base or floor of which is shown in the drawings and is identified by numeral 49. A bore 47 is formed through the floor 49 and is of a diameter slightly greater than the O.D. of the tubular shield conductor 46. The microstrip transmission line comprises an insulating substrate 48 of a predetermined thickness dimension having a microstrip conductor 50 formed on one major surface thereof and a conductive ground plane 51 formed on the opposed surface which is conductively joined to the floor 49. At the site of the transition, the microstrip line 50 widens out to form a generally rectangular conductive pad area 52. The length dimension of this conductive pad area is designed to approximately equal one wavelength of the signal with which the transmission system will be used. 
     With continued reference to FIGS. 2 and 3, disposed directly beneath the conductive pad area 52 and centrally disposed relative thereto is an etched circular aperture 56 which extends only through the thickness dimension of the ground plane 51. The circular aperture 56 is concentric with the bore 47 in the housing floor. The diameter of the opening 56 is generally equal to or a predetermined slight degree less than the outside diameter of the dielectric layer 44 of the coaxial transmission line. Centrally located within the aperture 56 and extending through the insulating dielectric substrate layer 48 and the thickness dimension of the conductive pad area 52 is a small drilled hole 58 whose diameter is slightly greater than the diameter of the central conductor 42 of the coaxial line 40. The coaxial line is inserted into bore 47 and an exposed end portion of the central conductor 42 of the coaxial cable is fitted upward through the hole 58 so as to project slightly above the top level of the conductive pad 52. A bead of solder 60 is then used to conductively join the center conductor of the coaxial cable to the pad area of the microstrip transmission line. Likewise, the portion of the coaxial outer conductor inserted into the bore 47 in the housing base 49 is soldered to the inside of the bore 47 to make a connection to the ground plane. The coaxial outer conductor is also soldered to the housing base 49, as at 61, for added strength. 
     Taking into account the geometry of the circular aperture 56 and the centrally disposed hole 58 which extends through the insulation layer and the conductive pad area, the overlaying circular patterns may conveniently be referred to as an iris. The center of this iris lies on the perpendicular bisector of the length dimension of the pad area 52 and, as such, the portions of the pad area on either side of the center of the iris are approximately each of a length λ/2. This approximately half-wave length microstrip structure on either side of the center conductor 42 acts as a pair of open circuit resonant devices which function to transform the transverse electromagnetic energy carried by the coax line to a quasi transverse electromagnetic signal carried by the microstrip transmission line and vice versa. Furthermore, the circular iris 56 etched into the ground plane 51 is used to compensate the parasitic reactances induced by the 90° change in direction of propagation and by the change from a coaxial mode to a microstrip mode. Empirical testing has revealed that the reactance of the circular iris 56 is a function of its diameter and is determined as a function of the diameter of the coaxial cable employed as well as the thickness of the microstrip&#39;s insulating substrate 48. 
     FIG. 4 illustrates an exploded side view of a side-launch microstrip to coaxial cable transition which utilizes a coaxial connector. In this arrangement, the housing floor 49 has holes as at 70 drilled into the thickness dimension thereof from the bottom and the holes are tapped to receive fasteners 72 which pass upward through a flange 74 extending laterally from the coaxial connector body 76. In this way, the coaxial cable connector body is firmly secured to the microstrip transmission line housing. The exposed dielectric cylinder 44 of the coaxial connector is trimmed to a length equal to the thickness dimension of the housing floor 49. The exposed dielectric cylinder 44 fits through the bore 82 formed through the thickness dimension of the housing floor 49 and is aligned with the iris 56 formed in the ground plane 51 when the center conductor 42 of the coaxial cable is inserted through the aperture 58. Once so assembled, the center conductor 42 may be soldered or otherwise conductively bonded to the conductive pad area 52 of the microstrip conductor as at 60 in FIGS 2 and 3. 
     An external thread is provided on the connector body 76 as illustrated and within the body, the central conductor has a longitudinal opening formed therein for receiving a mating pin on a cable connector (not shown) having an internally threaded ring for mating with the threads on the connector body 76 which thus serves to removably couple the two connector halves together. 
     The arrangement of this alternative embodiment offers the advantage of greater structural strength and ease of assembly when compared to the embodiment of FIG. 1. Thus, it is a preferred version in those applications which provide sufficient room to allow for the presence of a coaxial cable connector. To better appreciate the performance characteristics of the side-launched microstrip to coaxial cable transition, there is shown in FIG. 5 a plot of return loss and insertion loss vs. frequency for an embodiment of the type shown in FIG. 4. In the device on which the measurements shown in FIG. 5 were taken, the length of the pad 52 was 0.270 inches and its width was 0.070 inches. The width of microstrip transmission line 50 was 0.031 inches and was fabricated on 0.010 inch thick Duroid® 5880 material manufactured by Rogers Corporation of Chandler, Ariz., using one-half ounce rolled copper. The diameter of the iris was 0.080 inches. 
     The curves reflect a return loss greater than 15 dB corresponding to a VSWR of 1.43:1 over a 3 GHZ bandwidth. The insertion loss is less than 0.75 dB over this same bandwidth, which is deemed to be more than adequate performance for such a transition. 
     Referring next to FIG. 6, there is illustrated the manner in which a side-launch transition of the present invention may be 
     from one microstrip transmission line used to feed an RF signal from one microstrip transmission line substrate to a second substrate disposed beneath it. Again, each of the microstrip transmission line assemblies includes a dielectric substrate 48 carrying a ground plane 51 on one surface thereof and a microstrip transmission line 50 having a pad area 52 formed on the opposed surface. The pad area 52 is again dimensioned to function as a resonator and includes a central opening 58 formed through the thickness dimension thereof. Etched or cut through the ground plane layer 51 and surrounding the aperture 58 is an iris 56. 
     With continued reference to FIG. 6, sandwiched between the two microstrip transmission line assemblies is a metal housing member 49 having a dielectric cylinder 80 inserted into a circular bore 82 formed through the thickness dimension of the housing member 49. This dielectric cylinder 80 has a dimension which is greater than that of the iris 56. The center conductor 42 passes through the drilled hole 58 formed in the microstrip transmission line assemblies and a bead of solder as at 60 is used to electrically join the center conductor 42 to the resonator pad 52. In this arrangement, then, a short length of coaxial cable is formed and used to connect the two side-launch transitions between adjacent microstrip transmission line assemblies. Thus, an input signal may be applied to the microstrip transmission line 50 on one substrate with the output being taken from the microstrip transmission line 50 on the other substrate. 
     This invention has been described herein in considerable detail in order to comply with the Patent Statutes and to provide those skilled in the art with the information needed to apply the novel principles and to construct and use such specialized components as are required. However, it is to be understood that the invention can be carried out by specifically different equipment and devices, and that various modifications, both as to equipment details and assembly procedures, can be accomplished without departing from the scope of the invention itself.