Suspended Substrate Elliptic Rat-Race Coupler

The present invention relates generally to the field of microwave devices and, more particularly, to suspended substrate circuits. Still more particularly, the present invention relates to rat-race couplers.

The hybrid ring circuit, also known as the rat-race coupler, has been used for many years and is still an essential part of many complex microwave circuits such as mixers, phase shifters and power dividers. The rat-race coupler has three of the four transmission lines between ports equal to one-quarter wavelength and one line between ports equal to three-quarter wavelengths at the midband operating frequency of the device.

Referring to FIG. 1 a prior art rat-race coupler is schematically illustrated. As can be seen in FIG. 1, the rat-race coupler 12 is comprised of a circular ring 14 and four conductive arms or ports 16, 18, 20 and 22. The arms 16 and 18 are separated by an electrical length of .lambda./4 where .lambda. is the wavelength at the midband operating frequency of the device.

Similarly, ports 18, 20 and 22 are separated, respectively, by an electrical length of .lambda./4. Ports 16 and 22 are separated by an electrical length of 3.lambda./4. It can readily be appreciated that an input on port 18 will result in an equal, in-phase division of the input signal on port 18 between ports 16 and 20. The port 22 is isolated from port 18 by reason of cancellation of the signals that would propagate around ring 14 in opposite directions. Particularly, any signal entering ring 14 via port 18 and propagating along the direction indicated by arrow 24 would propagate along an electrical length of .lambda. and any signal entering ring 14 via port 18 propagating in the direction indicated by arrow 26 along the ring would propagate through an electrical length of .lambda./2 to port 22. The propagation distance between port 18 and 22 via the direction of arrow 24 is therefore 180.degree. out of phase with the signal propagating along ring 14 via the direction of arrow 26 and, therefore, cancellation of these two signals occurs (assuming equal loads on ports 16 and 20). Thus, by inputting a signal on input& port 18, and using appropriate loads on ports 16 and 20, the rat-race 12 acts as a splitter, with all the input power: absorbed by the two loads. The rat-race can also be used as a mixer if, for example, a local oscillator (L.0.) signal is injected on port 22. With an L.0. signal injected on port 22, equal, 180.degree. phase shifted signals will appear on ports 16 and 20 and port 18 will be isolated. This is because the electrical propagation distance between ports 20 and 22 is .lambda./4 and the electrical propagation distance between ports 16 and 22 is 3.lambda./4. Diodes at ports 16 and 20 appear as loads to both the L.O. and input radio frequency (R.F.) signals. Often the resulting intermediate frequency (I.F.) is removed at the symmetric position 27.

Cutoff frequency is defined as the lowest frequency which can propagate through a guide in a waveguide mode. A below cutoff frequency waveguide or below cutoff waveguide is a guide having dimensions which will not allow waveguide propagation modes at frequencies below the cutoff frequency. Below cutoff waveguide housings are always used with suspended substrate circuits and often with millimeter wave microstrip circuits. In these applications, rat-race couplers cannot be used without widening the enclosing channel and providing mode suppresion pins or other mode suppression devices. The provision of mode suppression devices usually requires drilling holes in the circuit substrates.

One method of coupling both R. F. signals and L. O. power into two diodes is through the use of a rat-race mixer as described above with respect to FIG. 1. The use of two diodes in a balanced arrangement helps to cancel noise due to L. 0. sidebands. The rat-race mixer involves the use of a circular ring of about one-half wavelength in diameter. The width of the ring, e.g. the width of ring 14 in FIG. 1, and the addition of the four conductor arms, e.g. 16, 18, 20 and 22 in FIG. 1, to the hybrid ring does not allow its use in a suspended substrate housing. This is because the suspended substrate housing must have a channel width less than one-half the wavelength of the highest propagating frequency in order to prevent propagation in the waveguide TE.sub.10 mode.

FIG. 2 is a schematic illustration of a stripline channel 24 within a metallic housing and having a suspended substrate conductor 26 formed on a suspended substrate card (not shown) positioned within the stripline channel 24. In this prior art configuration TEM mode electromagnetic energy can propagate within the below cutoff waveguide 24. FIG. 3 is, likewise, a schematic illustration of a below cutoff waveguide channel 28 having a suspended substrate conductor 30 extending therethrough and also having a below cutoff waveguide channel 32 extending orthogonally from the primary channel 28. This configuration also permits the propagation of TEM mode energy within the below cutoff channels 28 and 32. FIG. 4 illustrates schematically two crossed below cutoff waveguides or channels 34 and 36 and having a suspended substrate rat-race coupler 38 positioned within the intersection of the crossed channels 34 and 36. It has been discovered that a suspended substrate transmission line within a housing having two crossed channels as shown in FIG. 4 will suffer extreme circuit loses. It is believed that the energy losses are due to higher order modes which occur when there are no sidewalls in each of the guides 34 and 36. The lack of one sidewall, as would be the case for the suspended substrate configuration of FIG. 3, does not cause any appreciable signal loss, however. The failure of the device constructed as shown in FIG. 4 is also believed to be attributable to the fact the sole sidewall current flow is necessary in order to support TEM propagation.

The present invention is a solution to the problem of forming a suspended substrate or microstrip circuit within a below cutoff waveguide housing and provides a simple means of allowing the use of a rat-race coupler within below cutoff channels. In accordance with the present invention no mode suppression pins are used and, therefore, the substrate need not be drilled. The suspended substrate rat-race coupler of the present invention is believed to be equal to or better in performance than microstrip circular rat-races tested in the same millimeter frequency range.

The present invention solves the suspended substrate rat-race problem by offsetting portions of one of the two crossed below cutoff waveguide channels to form what may be considered to be a primary below cutoff waveguide channel with two offset perpendicular channels. An elliptic suspended substrate rat-race coupler is positioned within the area formed by the juncture of the primary below cutoff waveguide channel and the offset below cutoff waveguide channels. A ring formed of a conductor having an appropriate width can thus be fit reasonably well into the juncture and extensions of the rat-race conductive arms or ports can be made into the below cutoff waveguide channels.

Accordingly, it is the primary object of the present invention to disclose a novel suspended substrate rat-race coupler.

It is also the object of the present invention to disclose the first suspended substrate rat-race coupler.

A still further object of the present invention is to disclose a rat-race coupler that has the capability of enabling its four ports to be connected within below cutoff waveguide channels without disrupting the TEM propagation mode.

These and other objects of the invention will become more readily apparent from the ensuing specification and claims when taken in conjunction with the attached drawings.

FIG. 1 is a schematic illustration of a prior art rat-race coupler.

FIG. 2 is a schematic illustration of a prior art suspended substrate line segment.

FIG. 3 is a schematic illustration of a suspended substrate line within a channel having a T-channel connection.

FIG. 4 is a schematic illustration of a conventional configuration rat-race coupler and illustrating how such a coupler could be introduced into two crossed below cutoff waveguide channels, albeit ineffectively.

FIG. 5 is a perspective view of the elliptic rat-race coupler on a suspended substrate card in accordance with the present invention.

FIG. 6 is a schematic illustration of how the elliptic rat-race coupler of the present invention can be positioned within offset below cutoff waveguide channels.

FIG. 6A is a top view of the offset below cutoff waveguide channels of the present invention illustrating the offset feature.

FIG. 7 is an exploded view of the suspended substrate elliptic rat-race coupler of the present invention.

FIG. 8 is an isometric view of the top portion of the device illustrated in FIG. 7 rotated from right to left 180.degree..

FIG. 9 is a graph of the frequency response characteristics of the device of the present invention illustrated in FIG. 7.

FIG. 10 is a schematic illustration of an alternate version of the present invention wherein the below cutoff waveguide channels extending from the primary channel are extended on the same side rather than the opposite side of the below cutoff waveguide channel.

FIG. 11 is a partial cross half-section taken through lines 11--11 of FIG. 7 and showing, by way of example, suitable dimensions for the present invention.

Referring to FIG. 5 the suspended substrate card portion of the elliptical rat-race coupler of the present invention is illustrated. Specifically, suspended substrate card 40 is the supporting structure for the suspended substrate circuit 42. Card 40 may be, for example, a dielectric material such as "Duroid" which is fiberglass impregnated "Teflon" or it may be, for further example, a flexible ceramic dielectric made from powdered ceramic material a is well known. The suspended substrate circuit 42 is preferably made of copper which is formed on the dielectric 40 by well known techniques. The circuit 42 is comprised of an elliptically shaped conductive portion 44 that constitutes the rat-race ring. Four ports or conductive arms 46, 48, 50 and 52 are electrically and physically connected to the elliptical ring 44 and are generally spaced as illustrated in FIG. 5. Specifically, the ports or arms 46 and 48 are spaced from each other along ring 44 by an electrical length of .lambda./4, where .lambda. is the wavelength at the midband operating frequency of the device. Likewise, the ports 48 and 50, and 50 and 52 are separated, respectively, by electrical lengths of .lambda./4. Ports 46 and 52 are separated along ring 44 by an electrical length of 3.lambda./4. Because of the electrical length separation between the input/output ports 46, 48, 50 and 52, the device functions as a rat-race coupler as described above with respect to FIG. 1.

Referring now to FIG. 6 there is a schematic, top view illustration of how the rat-race circuit configuration 42 can be placed within below cutoff metallic housing channels to operate as a suspended substrate rat-race coupler. The circuit configuration 42 on card 40 is placed within a primary below cutoff waveguide channel 54. Displaced or offset below cutoff channels 56 and 58 extend or&orthogonally and in opposite directions as T's from the primary channel 54 as illustrated in FIG. 6. As can be seen in FIG. 6 the major axis of the elliptic ring 44 is positioned such that its distal ends 60 and 62 lie generally within the intersections, respectively, of the channel 54 with the channel 56 and of the channel 54 with the channel 58. By so orienting the major axis 59 of the elliptical ring 44, the conductive arms or ports of the rat-race circuit configuration can be positioned to be extended into the adjacent, respective below cutoff channels. Thus, port 46 can be extended via conductor 64 into the left hand portion of below cutoff channel 54, port 48 can be extended via conductor 66 into below cutoff channel 56, port 50 can be extended via conductor 68 into the right hand side of below cutoff channel 54 and, finally, port 52 can be extended via conductor 70 into below cutoff channel 58. Thus, two of the four ports of the rat-race configuration 42 can be extended outside of the main channel 54 within the other below cutoff waveguide channels 56 and 58. Applicant has discovered that the channel 58 should be offset with respect to the channel 56 such that no part of the volume within channel 58, if extended, would overlap any of the volume within channel 56 and such that the left sidewall 58a of channel 58 would not be collinear with the right sidewall 56a of the channel 56. There is, thus, at least a small separation "d" between the imaginary extension of left sidewall 58a and the right sidewall 56a of channels 58 and 56, respectively. This is illustrated in detail in FIG. 6a. By way of example the offset "d" may be 0.010".

In an alternate embodiment of the present invention depicted in FIG. 10, an elliptical rat-race suspended substrate coupler 74 can be formed on a suspended substrate card (not shown) and positioned within a primary below cutoff waveguide channel 76 with the primary axis of the ellipse of elliptic conductor 78 being collinear with the longitudinal axis of the below waveguide channel 76. Further, a first below cutoff waveguide channel 80 extends from primary waveguide channel 76 and a second below waveguide channel 82 extends from primary below waveguide channel 76, both below waveguide channels 80 and 82 extending from primary waveguide 76 on the same side and being separated by metallic wall 84. The four ports or conducting arms 85, 86, 88 and 90 of the rat-race 74 are thus extended into, respectively, the left side, as illustrated in FIG. 10, of primary below cutoff waveguide channel 76, below cutoff waveguide channel 80, below cutoff waveguide channel 82 and the right side, as illustrated in FIG. 10, of primary below cutoff waveguide channel 76.

FIG. 7 illustrates a three dimensional exploded isometric view of the suspended substrate rat-race coupler of the present invention illustrated in FIG. 6. In FIG. 7, the suspended substrate card 40 is shown as being positioned on a small ledge 92 formed within the inner surface of the lower half 94 of the metallic waveguide housing within which the below cutoff waveguide channels are formed. The top half 96 of the housing includes a first input/output port 98 and a second input/output port 100. The inner surface of top half 96 of the metallic housing also includes a first back short cavity 102 and a second back short cavity 104 as is illustrated in FIG. 8. Similarly, bottom half 94 of the metallic housing includes a first input/output port 106 and a second input/output port 108 which extends completely through the housing portion 94. Bottom half 94 of the metallic housing also includes a first back short cavity 110 and a second back short cavity 12. When the suspended substrate card 40 is in position, sandwiched between the top and bottom halves 94 and 96, respectively, of the metallic housing, the ends 114, 116, 118 and 120 of the circuit configuration 42 extend into and lie over the back short cavities 102, 110, 104 and 112, respectively.

The below cutoff waveguides channels 54, 56 and 58 illustrated schematically in FIG. 6 are thus formed by the channel halves 54a, 56a, 58a in metallic housing top half portion 96 and channel halves 54b, 56b and 58b formed metallic housing bottom half portion 94. It can thus be appreciated that R.F. energy can be input into or output from the rat-race suspended substrate coupler of the present invention via any of the input/output ports 98, 100, 106 or 108.

The half section depicted in FIG. 11 illustrates, by way of example, suitable dimensions for the present invention for a suspended substrate rat-race coupler for operation in the 26.5 to 40 GHz frequency band for a suspended substrate coupler formed on a flexible ceramic substrate. The dimensions t, p, w.sub.1, h.sub.t, w.sub.2, w.sub.3, H, h.sub.1, h.sub.2, and h.sub.3 illustrated in FIG. 11 may be, by way of example, as follows:

Waveguide Band (GHz)--26-40

t--(Substrate Thickness)--0.005"

p--(Probe Length)--0.080"

w.sub.1 --(Line Width)--0.028"

h.sub.t --(Channel Height)--0.057"

w.sub.2 --(Channel Width)--0.116"

w.sub.3 --(Substrate Width)--0.124"

H--(Waveguide Height)--0.124"

h.sub.1 & h.sub.2 --(Substrate to Upper & Lower Walls)--0.026"

h.sub.3 --(Backshort Distance)--0.100"

Finally, FIG. 9 illustrates, by way of example, the measured frequency response of the suspended substrate rat-race coupler of the present invention for a device operating in the frequency band between 26.5 and 40 GHz. In the graph of FIG. 9 curve 1 indicates the return loss of the input signal, curves 2 and 3 indicate the signals propagating on equally divided coupled ports of the rat-race coupler of the present invention and curve 4 illustrates the signal attenuation on the isolated port. Transition losses are indicated.

Obviously, many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.