US 3771075 A
A coupling arrangement for microstrip systems, each system characterized by a dielectric with a ground plane contacting one side thereof and a conductive strip fastened to the other side. A small coupling hole in the ground plane immediately underlies the strip conductor at a current maximum for signal along the line. Two such microstrip systems are placed in back-to-back relation with their ground planes in physical contact and their coupling holes in registry to transfer signal being propagated in a transverse electromagnetic mode via one of the strip conductors to the other of the strip conductors and ultimately to a desired point or points of utilization. In a specific application of the invention, the coupled microstrip systems are used to transfer signal energy from a feed network to a phased array of antenna elements.
Description (OCR text may contain errors)
[ Nov. 6, 1973 1 MICROSTRIP TO MICROSTRIP TRANSITION  Inventor: Harry R. Phelan, lndialantic, Fla.
 Assignee: Harris-Intertype Corporation,
Cleveland, Ohio  Filed: May 25, 1971  Appl. No.: 146,736
 US. Cl 333/84 M, 333/24 R  Int. Cl. 1101p 5/02, HOlp 3/08  Field of Search 333/84 M, 84 R, 10', 333/73 W, 24 R, 6, 9; 317/101 CM, 101 CE; 340/174 GP  References Cited UNITED STATES PATENTS 3,150,336 9/1964 Gonda 333/84 M 2,654,842 10/1953 Engelmann.. 333/84 M 2,760,169 8/1956 Engelmann 333/84 M 2,794,185 5/1957 Sichak 333/84 M 2,962,716 11/1960 Engelmann.. 333/84 M 2,976,499 3/1961 Sferrazza 333/84 M 3,012,210 12/1961 Nigg 333/84 M X 3,176,275 3/1965 Gribble et al.... 333/24 X 3,368,169 2/1968 Carter et a1. 333/73 S 3,587,110 6/1971 Woodward 333/9 X FOREIGN PATENTS OR APPLICATIONS 828,241 2/1960 Great Britain 333/10 159,017 2/1953 Australia 333/9 1,191,414 10/1959 France 333/10 OTHER PUBLICATIONS Cohn, S. B. Slot Line on a Dielectric Substrate, M'IT-17 l01969, pp. 768-778.
Dukes, V. M. C. Broad-Band Slot-Coupled Micr0- strip Directional Couplers, IEE Vol. 105B, 1958, pp. 147-154.
Cohn, S, B. Microwave Coupling by Large Apertures Proc. IRE 6-1952, pp. 696-698.
Primary Examiner-Rudolph V. Rolinec Assistant Examiner-Wm. H. Punter AttarneyDonald R. Greene  ABSTRACT A coupling arrangement for microstrip systems, each system characterized by a dielectric with a ground plane contacting one side thereof and a conductive strip fastened to the other side. A small coupling hole in the ground plane immediately underlies the strip conductor at a current maximum for signal along the line. Two such microstrip systems are placed in back- 3 Claims, 7 Drawing Figures PATENTEU NOV 6 i975 SHEET 1 CF 3 INVENTOR H. R. PHELAN 1 MICROSTRIP TO MICROSTRIP TRANSITION BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to transmission lines, and more particularly to microstrip circuits and transmission lines and to techniques of coupling such lines.
2. Discussion of Prior Art As is well known, a strip transmission line employs conductors in the form of fiat strips or plates. In the typical microstrip transmission line the conductive strip is separated from a ground plane by a dielectric layer, and in general the circuit is fabricated in the form. ofa dielectric sheet clad on each sidewith copper foil of predetermined width. The foil on one side constitutes the ground plane, and the foil on the other side is the signal transmission conductor strip. Microstrip lines find wideuse in signal transmission in the transverse electromagnetic (TEM) mode at microwave frequencies. A common application of such lines is in the field of antenna systems, in which the microstrip line may be employed as part of a feed for a phased array, for example.
In the past, in order to couple a pair of microstrip circuits it has been necessary to utilize radio frequency (RF) connectors. Implementation of a quick disconnect connector, as would be desirable, for example, for removal and replacement of faulty modules in a phased array antenna system is expensive and unreliable. Moreover, coaxial and pin connectors add considerable expense to the cost of the system in which theyare used, particularly where a large number of such connectors are required, as in the typical phased array application.
SUMMARY OF THE INVENTION Briefly, according to the present invention, the ground plane of the microstrip circuit is provided with a narrow slot extending generally transverse to the direction of propagation of signal along the overlying conductive strip, and beyond the edge of the strip generally symmetrically to either side thereof. In the case of an open circuit line transition, the center of the slot is disposed approximately a quarter wavelength (X/4) back from the terminus of the conductive strip constituting the end of the transmission line. Alternatively, as a transition for a short circuit line, the microstrip may be short circuited to the edge of a metallic carrier for the microstrip circuit, immediately after crossing the slot (i.e., at the opposite side of the slot).
Two such microstrip circuits or transmission lines are readily coupled without need for the usual RF connectors by placing the circuits in back-to-back relation with the ground plane of each in contact with the ground plane of the other and with the respective coupling slots in registry. The end of the conductive strip of each transmission line may extend to the same or to the opposite side of the registered slots relative to the end of the conductive strip of the other transmission line. The conductive strip may undergo a turn, relative to its orientation across the slot, after it has traversed a distance from the slot sufficient to preclude disruption of the field distribution at the transition.
Accordingly, it is a principal object of the present invention to provide a coupling arrangement, or transition, for microstrip transmission lines, by which good signal coupling is achieved between the microstrip lines without need for conventional connectors.
It is another object of the invention to provide a microstrip to microstriptransition which achieves relatively localized coupling between lines at a point along the lines displaying a current maximum in the signal wave, and yet with virtually negligible discontinuity in the ground plane of each line.
BRIEF DESCRIPTION OF THE DRAWING In describing certain preferred embodiments of the invention, reference will be made to the accompanying FIGURES of drawing, in which:
FIG. 1 is a fragmentary perspective view of an em bodiment of a transition for an open circuit line;
FIG. 2 is a section view taken along the line 22 of FIG. 1;
FIG. 3 is a detailed fragmentary section view of microstrip transmission line illustrating the orientation of electric field lines for a signal transmitted along the line in the transverse electromagnetic mode;
FIG. 4 is a section view taken along line 44 looking in the direction of the arrows in FIG. 5 of a microstrip to microstrip transition in which the circuits are maintained in place by a carrier medium;
FIG. 5 is a plan view taken along line 5-5 looking in the direction of the arrows in FIG. 4 of the arrangement of FIG. 4, showing several embodiments of the transition according to the present invention;
FIG. 6 is a partial section view of a phased array antenna and feed network using microstrip to microstrip transitions;
FIG. 7 is a plan view of the configuration of FIG. 6.
DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGS. 1 and 2, each of a pair of microstrip lines l0, 11, which are shown as coupled in accordance with the teachings of the present invention, comprises a dielectric sheet or substrate 12 clad or bonded on each side thereof with a layer of suitable signal conductive metal such as a copper foil. The substrate 12 may be aceramic, such as alumina (aluminum oxide). The metal layer on one side of substrate 12 constitutes aground plane 13, while the metal layer on the other side of the substrate is a relatively narrow strip 15, following the desired path of the transmission line or sigrial-carrying circuit.
According to the invention, a narrow slot 14 is provided in each ground plane 13 at a position immediately underlying the conductive strip 15 and at an angle thereto, where a transition from one microstrip line to another is desired. Although in FIGS. 1 and 2 the slot 14 is illustrated as being oriented at a right angle to the longitudinal path of the strip 15, that orientation is not essential to the invention and it is to be understood that the slot may be canted or folded relative to thestrip. It is desired that a current maximum for signal propagated along the line exist in the vicinity of the slot in order for the slot to act as a coupling hole for the line. Such a current maximum occurs over the coupling hole, as shown in FIGS. 1 and 2, along an open circuit line which extends a quarter wavelength ()t/4 or an odd multiple thereof at approximately the center of the range of frequencies to be accommodated by the microstrip line) beyond the slot. The longer dimension of the slot 14 is preferably about M2, but the narrower dimension is not critical and may typically be about A; the size of the longer dimension, i.e., approximately M16.
Of importance to the present invention is the fact that signal is propagated along the microstrip transmission line in a transverse electromagnetic (TEM) mode, in which the TEM E-field configuration is illustrated by the dotted lines and arrows 17 in FIG. 3. When two microstrip transmission lines constructed as indicated above are placed in back-to-back configuration, as in FIGS. 1 and 2, with the ground plane 13 of each microstrip circuit in contact with the ground plane of the other and with the slots 14 in the respective ground planes in registry, the strip conductors 15 of the two microstrip circuits are electromagnetically coupled through the current maximum existing over the coupling slot. Thus, a localized transition occurs at the otherwise negligible discontinuity (i.e., the coupling slot) in the ground plane. In each instance, the slot may be vacant (i.e., open) or it may be filled with a dielectric insert. While the coupled lines are shown in FIGS. 1 and 2 with the ends of the strip conductors 15 extending in opposite directions beyond slot 14, it is to be understood that such an arrangement is not essential to the invention and that the strips may terminate at the same side of the slot according to the desired path of signal into and away from the transition.
With reference to FIGS. 4 and 5, two such back-toback microstrip circuits 10, 11 are conveniently supported by sandwiching/or otherwise electrically connecting the ground planes of the two circuits between the walls or blocks 19, 20 of a metal carrier. As is more clearly shown in FIG. 5, the metal carrier in this particular example has a rectangular cross-section with an open center, and the microstrip circuits are held captive in the carrier by a plurality of hold-down fasteners 22. The arrangement shown in FIGS. 4 and constitutes a module which is readily disassembled to permit rapid replacement of faulty microstrip circuit portions.
Referring specifically to FIG. 5, several embodiments of transitions in accordance with the present invention are shown by way of example to illustrate the versatility of the invention. The transition designated A is substantially identical to that shown and described with reference to FIGS. 1 and 2. The transition denoted B illustrates the variation of the path of strip conductor 15 which is possible, if desired, at either side of coupling hole, but it is to be understood that a bend at any angle away from the slot may be utilized, rather than a 90 angle, if desired. However, it is essential that the strip conductor not undergo a change in orientation within two strip widths from the side of the coupling slot at which the turn or bend is desired. This is because a smaller distance between the point of the bend and the side of the slot may disrupt the field distribution which is necessary at the point of transition, thereby producing an impedance mismatch. Typically, the width of the strip is approximately 0.050 inch, but it is to be understood that other widths may be encountered in practice, and that the invention is not to be considered as limited to use of a strip of specific width.
A different embodiment of transition according to the invention is illustrated by the configuration designated C in FIG. 5. Here, the required current maximum over coupling hole 14 occurs as a consequence of the direct short circuit of the line to a point of reference potential, i.e., ground, at the edge of the microstrip circuit substrate. In particular, the strip conductor 15, or
an extension electrically connected thereto, may project across coupling hole 14 to contact the conductive wall 20 of the circuit carrier. Reliable contact between the strip conductor and the metallic wall is assured by use of a screw fastener 24, as shown, which is threaded tightly down upon the projection strip conductor. Of course, other short circuiting techniques will readily suggest themselves to those skilled in the art, and may be employed as alternatives in the short circuit line configuration which has been described. The advantage of the short circuited line is its capacity for operation over much wider bandwidths than the quarter wavelength open circuit stub configurations A and B. In theory, the short circuit configuration is frequency independent. However, the open circuit transition embodiment has the advantage of simplicity of implementation anywhere within the boundaries of the microstrip circuit substrate, since it does not require a direct mechanical shortcircuit to a point of ground potential.
Microstrip transition embodiment D (FIG. 5) is a version of the short circuit line similar to that designated B for the open circuit technique, in that the strip conductor undergoes a bend at a point exceeding two strip widths from the side of the coupling slot.
One example of an application of the microstrip-tomicrostrip transition described above is as a modular feed network for a phased array antenna. Such an arrangement, shown in FIGS. 6 and 7, eliminates the need for RF connectors between the array modules and the feed network, as would be required with conventional techniques. The feed network 30 comprises a microstrip circuit 31 including a substrate 32 having a feed network strip 33 bonded thereto and which is itself attached to the wall 34 of a metallic enclosure 35 serving as the ground plane for the circuit. As is more clearly shown in FIG. 7 the path of microstrip feed network line 33 (dotted line) is defined according to the configuration of the phased array of radiating elements.
The array modules 40 comprise separate RF microstrip circuits of the type which has generally been described with reference to FIGS. 4 and 5, and which comprise a dielectric substrate 41 fastened to a wall 42 of a metallic carrier serving as a ground plane. A strip conductor 44 fastened to the opposite side of the substrate is directly connected to a radiating element 48 constituting one of the elements of the phased array. The ground plane of each array module 40 is provided with a coupling hole 45 which, when the module is fastened in appropriate position to the feed network enclosure 35, registers with an associated coupling hole 37 in the ground plane formed by the feed network enclosure. A set of two or more quick disconnect fasteners 50 is employed with each module to permit the rapid and convenient removal and replacement of any faulty module of the array, while requiring but a single input-connector S1 for the entire feed network.
While the use of readily separable ground planes has been described, it should be understood that the present invention is also applicable to the coupling of microstrip circuits in which the coupled circuits share a common ground plane. While such an arrangement does not have a quick disconnect capability, it does possess the other advantages such as overall simplicity of the transition and the absence of any requirement of a hole extending through the microstrip circuit substrate.
Accordingly, while certain preferred embodiments of the present invention have been disclosed herein, those skilled in the art to which the invention pertains will recognize that variations of the specific details of construction which have been illustrated and described may be resorted to without departing from the spirit and scope of the invention, as defined by the appended claims.
1. A direct coupling configuration for coupling the sole output of one microstrip line to the sole input of another microstrip line comprising:
first and second microstrip lines, each of said lines having only one input port for power and only one output port for power and each including a strip conductor having a longitudinal centerline and separated from a ground plane by a dielectric layer, for propagation of signal within a predetermined frequency range in a TEM mode via said strip conductor, and
a coupling hole in said ground plane at a point immediately opposite said strip conductor;
said first and second microstrip lines being disposed in back-to-back relation with the ground plane of each in direct physical and electrical contact with the other and the coupling holes in registry, and with said centerlines substantially parallel to each other at the place nearest said hole, to provide direct electromagnetic coupling between the two strip conductors,
first means for terminating a first one of said microstrip lines in a low-loss impedance different from its characteristic impedance, with said coupling hole intennediate said input port of said first line and said means for terminating, to produce a standing wave thereon upon excitation and to block any significant power transfer therebeyond, and
second means for terminating the second one of said microstrip lines in a low-loss impedance different from its characteristic impedance, with said coupling hole intermediate said output port of said second line and said second means for terminating, to produce a standing wave thereon upon excitation and to block any significant power transfer therebeyond, whereby said coupling hole is the sole power output port for said first microstrip line and is the sole power input port for said second microstrip line,
said coupling hole being a slot extending transverse to said strip conductors approximately one-half wavelength long in said transverse direction at the wavelength of signal to be accommodated by said circuits and being of a width substantially less than said length and said coupling hole of each microstrip circuit being located at a point of current maximum for said standing wave on said strip conductor so that essentially all of the input power applied to said first microstrip circuit is directly transferred to said second microstrip circuit through said coupling holes with no intermediate transmission medium.
2. The coupling configuration of claim 1, wherein at least one of the strip conductors terminates approximately one-quarter wavelength beyond one side of the coupling hole, at the wavelength of signal to be accommodated by said microstrip lines.
3. The coupling configuration of claim 1, wherein at least one of the strip conductors terminates at one side of the coupling hole in a short circuit.