6-Port directional orthogonal mode transducer having corrugated waveguide coupling for transmit/receive isolation

Abstract

A directional orthogonal mode transducer has an inner, circular waveguide for propagating 6 GHz transmit signals and an outer, circular, coaxial waveguide for propagating 4 GHz receive signals. The terminal end of the outer waveguide is joined to an enlarged, cylindrical coupling section provided with a plurality of spaced, inwardly projecting corrugations in the form of washer-like annular rings. The corrugations, when properly dimensioned, establish surface reactance conditions that result in an inner, circular field distribution at the transmit frequency and a surrounding, annular field distribution at the receive frequency. This effectively decouples or isolates the transmit and receive signals whereby they are separately propagated in their respective waveguides.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a directional orthogonal mode transducer having a corrugated waveguide coupling section to implement the separation of the transmit and receive signals between two separate, coaxial waveguide systems.

2. Description of the Prior Art

In certain satellite communications systems it is desirable to reuse frequencies on orthogonal polarizations. Only two previous approaches to satisfy this requirement are known. The first is based on a wide-band 4-port orthogonal mode transducer design where transmit and receive signals appear at all ports. This approach has the disadvantage that transmit and receive signals must be separated by reactive diplexing. Furthermore, independent control of transmit and receive polarizations can be achieved only with the addition of external variable coupling circuits, and the addition of such variable coupling circuits will generally degrade polarization orthogonality. The second approach is based on designing a directional junction, but the directional property of the junction is realized with two balanced pairs of multihole directional couplers. This main disadvantage with this design is its inherent mechanical complexity, particularly in view of the very severe requirements on balancing or symmetry.

A dielectric rod configuration has also been proposed to implement waveguide coupling, but is difficult to meet the severe requirements of mechanical symmetry when using dielectric materials necessary to suppress higher order mode generation. The elemination of higher order modes is mandatory to obtain a high degree of polarization isolation.

SUMMARY OF THE INVENTION

The 6-port directional orthogonal mode transducer of the present invention functions in the dual-polarized transmit and dual-polarized receive modes, and provides inherent transmit-to-receive isolation by virtue of the reactive nature of its combining junction. It may be used in both earth stations and spacecraft or satellite antenna systems. In linear polarization it allows for independent control of the two orthogonal transmit polarizations with respect to the two orthogonal receive polarizations. It employs separate, coaxial waveguide systems which allow optimum narrow band polarizers to be used, greatly improving the polarization isolation in the dual circularly-polarized mode of operation.

Structurally, the instant transducer comprises an inner, circular waveguide for propagating a transmit signal, and an outer, circular, coaxial waveguide for propagating a lower frequency receive signal between its inner surface and the outer surface of the inner waveguide. Adjacent the terminal end of the inner waveguide an enlarged, cylindrical coupling section is secured to the outer waveguide. Within this coupling section are mounted a plurality of spaced, inwardly projecting, annular rings, generically referred to in the art as corrugations. The depth of the corrugations is less than .lambda./4 of the receive signal and greater than .lambda./4 of the transmit signal, and the spacing between them is less than .lambda./2 of the transmit signal.

The effect of these corrugations is to establish a surface reactance condition that changes from inductive to capacitive causing a 180.degree. phase shift between the lowest order symmetric modes in the transmit and receive signals. Stated another way, the reactive effect of the corrugations renders the TE.sub.11 and TM.sub.11 modes out of phase at the lower receive frequency and the electric field pattern that results from their vector additon is an annular ring of energy, with a void in the middle, that couples to the outer, coaxial waveguide. Conversely, the TE.sub.11 and TM.sub.11 modes are in phase at the higher transmit frequency, and their resultant field pattern is a central circle of energy, surrounded by a void annular ring, that couples to the inner, circular waveguide.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 shows a perspective view, partly in cutaway, of a directional orthogonal mode transducer constructed in accordance with the teachings of the present invention,

FIG. 2 shows a sectional view of the transducer of FIG. 1 taken along lines 2--2,

FIG. 3 shows a sectional view of a corrugated waveguide structure which will be used in analyzing the operation of the present invention,

FIG. 4 shows an end view of the waveguide structure of FIG. 3,

FIG. 5 shows simplified electric field distribution diagrams illustrating the vectorial addition of the out of phase TE.sub.11 and TM.sub.11 modes in the waveguide structure of FIG. 3, and

FIG. 6 shows diagrams similar to those of FIG. 5 but illustrating the addition of the in phase TE.sub.11 and TM.sub.11 modes.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, a corrugated structure as shown in FIGS. 3 and 4 is constructed by a sequential placement of annular ring or washer-like irises 10 in a circular waveguide 12. Such a corrugated structure may be analyzed by a number of methods varying from mathematically involved treatments such as hybridmode analysis to simple circuit representations of the shunt and series reactances of the sequential irises. For purposes of a simplified analysis herein, the two lowest order symmetric modes in a circular waveguide, the TE.sub.11 and TM.sub.11 modes, will be used to approximate the field distributions that exist in the corrugated waveguide of FIGS. 3 and 4. For example, if the corrugation depth D in FIG. 4 is less than 1/4 of the wavelength of the propagated signal, the resultant electric field may be synthesized as an out-of-phase addition of the TE.sub.11 and TM.sub.11 modes. This is shown in FIG. 5. This is similar to the electric field distribution one would expect from a dielectric cylinder mounted just inside the waveguide wall.

As may be clearly seen, the central portions of the TE.sub.11 field are opposed and cancelled by the central portions of the TM.sub.11 field, leaving a central void as shown in the resultant field diagram to the right of the equal sign. At the same time, the peripheral portions of the respective fields are similarly oriented in a directional sense and reinforce one another, resulting in an annular ring of energy surrounding the central void.

Conversely, when the corrugation depth D exceeds 1/4 of the wavelength of the propagated signal, the resultant field may be synthesized as an in-phase addition of the TE.sub.11 and TM.sub.11 modes, as shown in FIG. 6. Essentially, the central portions of the TE.sub.11 and TM.sub.11 fields are now similarly oriented and reinforce each other to produce a central core of energy as seen in the resultant field diagram on the right side of FIG. 6. At the same time, the peripheral portions of the respective fields are opposed to and cancel each other, leaving a void annular ring surrounding the central core of energy.

This simple modal synthesis was applied to the design of the 6-port directional orthogonal mode transducer of the present invention shown in FIGS. 1 and 2, which will now be described in greater detail. The transducer comprises an inner, circular waveguide 14 surrounded by an outer, coaxial waveguide 16. The two waveguides are coupled by a rotational joint 18 to enable Faraday effect compensation. The inner waveguide 14 is used to propagate a transmit signal of approximately 6 GHz, and is provided with direct and shunt coupled input ports 20, 22, respectively. The outer waveguide 16 is used to propagate a receive signal of approximately 4 GHz, and is provided with orthogonal, shunt coupled output ports 24, 26. The size of the inner waveguide 14 is selected to be below cutoff in the 4 GHz receive band.

A coupling section 28 of larger diameter than the outer waveguide 16 is secured to a terminal end of the latter, and is provided with a plurality of corrugations in the form of annular, washer-like rings 30. The depth of the corrugations is chosen to be less than .lambda./4 of the 4 GHz receive signal and greater than .lambda./4 of the 6 GHz transmit signal, and the spacing between the corrugations is less than .lambda./2 of the transmit signal. The transducer is completed by a circular, terminal waveguide section 32 secured to the other end of the coupling section 28. The section 32 is of equal diameter to the outer waveguide 16, and defines the overall input and output ports of the transducer.

As developed above, the surface reactance effect of the corrugations 30, given the specified depth and spacing parameters, produces the resultant electric field distribution patterns for the 4 GHz receive and 6 GHz transmit signals shown in FIGS. 5 and 6, respectively. These field distributions are synthesized from the TM.sub.11 mode being out-of-phase with the TE.sub.11 mode at the receive frequency and in-phase with the TE.sub.11 mode at the transmit frequency, as previously discussed. It can be intuitively seen that the annular ring pattern of FIG. 5 will efficiently couple to the outer, coaxial waveguide 16 of FIG. 1, while the central core pattern of FIG. 6 will similarly couple to the circular, inner waveguide 14. The net result is a very effective and complete degree of isolation between the transmit and receive signals, both propagated through the same overall, structurally simple transducer.

While in the foregoing description the transducer is structurally configured in circular cross-section waveguides, it could equally well be implemented in square cross-section waveguides. Further, the corrugations could be defined by patterns of inwardly projecting rods, screws or teeth, as is known in the art. From a fabrication standpoint, however, the circular geometry and the annular ring corrugation form disclosed are to be preferred.

Claims

What is claimed is:

1. In a directional orthogonal mode transducer including an inner, circular waveguide for propagating a first signal and a surrounding, outer, coaxial waveguide for propagating a second signal having a lower frequency than the first signal, an improved signal coupling section comprising:

a. a cylindrical waveguide axially aligned with and attached to one end of the outer waveguide and having a diameter greater than that of the outer waveguide, and

b. a plurality of inwardly projecting corrugations spaced around the inner periphery of the cylindrical waveguide, said corrugations being dimensioned and configured to produce, by reason of their surface reactance effect, resultant electric field distribution patterns in the form of a central core of energy with a surrounding, void annular ring for the first signal, and an annular ring of energy with a central void for the second signal, whereby the first and second signals are substantially isolated from each other with the first signal coupling to the inner waveguide and the second signal coupling to the outer waveguide.

2. A transducer as defined in claim 1 wherein the corrugations are annular, washer-like rings.

3. A transducer as defined in claim 2 wherein the depth of each ring is less than 1/4 the wavelength of the second signal and greater than 1/4 the wavelength of the first signal, and the spacing between the rings is less than 1/2 the wavelength of the first signal.