Overmoded Waveguide Elbow and Fabrication Process

It is well-known that standard or fundamental waveguide is severely restricted in maximum power capacity and in minimum loss because of its required cross sectional dimensions. It is also well-known that overmoded waveguide has the advantages that it can be designed to have arbitrarily high power capacity and arbitrarily low attenuation by appropriately increasing the waveguide cross section. In overmoded waveguide, the required suppression of unwanted modes is achieved using dielectric and metallic structures to restrict unwanted allowable modes (e.g. see "Trunk Waveguide Communication" by A. E. Karbowiak, Chapmen and Hall, Ltd., London, 1965). Overmoded waveguide have been utilized as telecommunications trunk transmission lines and to connect transmitters to communications or radar antennas.

The most common type of overmoded waveguide supports the circular TE.sub.01 mode which has the unique property of decreasing transmission loss with increasing frequency for a given diameter. Circular overmoded waveguide can take the form of a plain metallic waveguide, metallic waveguide with a dielectric liner, or a sheathed-helix waveguide consisting of a closely wound insulated wire surrounded by a dielectric layer encapsulated by a good conductor. Various processes have been proposed for fabricating helical waveguide structures; examples are disclosed in U.S. Pat. Nos. 3,605,046, 4,043,029, 4,066,987, 4,071,834, and 4,090,280. However, one significant problem associated with the practical application of circular overmoded waveguide is the need for an elbow structure which is efficient and practical for overmoded circular waveguide applications, and which can be fabricated in a feasible manner in practical sizes and configurations.

The present invention relates generally to a waveguide elbow structure and its novel method of fabrication, and particularly to an elbow useful for practical applications of circular overmoded waveguide. In accordance with the preferred embodiment of this invention, the elbow is fabricated as a sheathed-helix waveguide by a process which has been successfully used in practice to construct overmoded waveguide elbows suitable for use at X-band (approximately 2.5 inches inside diameter) and at S-band (approximately 6 inches inside diameter). The overall design goal was to provide for 6-10 MW peak power handling capability at S-band with continuous operating temperatures of 150.degree. C. and no cooling water for the component materials. A close tolerance was maintained on the circularity and positioning of the internal helical winding, as well as the roundness and uniform thickness of the adjacent dielectric.

One object of the present invention is to provide a method for fabricating an overmoded waveguide elbow structure.

Another object of the invention is to provide a method for fabricating an overmoded waveguide elbow structure as a sheathed-helix waveguide consisting of an internal, closely wound insulated wire surrounded by a dielectric layer encapsulated by an outer conductor.

Other objects, purposes and characteristic features of the present invention will be pointed out as the description of the invention progresses and/or be obvious from the accompanying drawings wherein:

FIG. 1 illustrates a completed waveguide elbow fabricated in accordance with the present invention;

FIG. 2 is a simplified cross sectional view of the waveguide elbow showing the basic components thereof;

FIG. 3 is a partial side view taken along line 3--3 in FIG. 2;

FIG. 4 is a block diagram illustrating the preferred embodiment of the fabrication process proposed in accordance with the present invention; and

FIG. 5 is a diagrammatic illustration of the various fabrication steps comprising the preferred embodiment of the invention.

As discussed above, the present invention relates to circular overmoded waveguide and, in particular, to the fabrication of a sheathed-helix waveguide elbow designed for overmoded operation. FIG. 1 illustrates the completed elbow structure 10; whereas, FIGS. 2 and 3 show the basic components of the elbow as comprising an internal helical wound insulated wire 11, a dielectric sheath or layer 12, and an external encapsulating conductor 13.

The process by which the sheathed-helix waveguide elbow of FIG. 1 is fabricated, in accordance with the presently preferred embodiment of the invention, is illustrated in FIGS. 4 and 5 of the drawings. Referring simultaneously to FIGS. 4 and 5, at (a), the proposed process begins with a suitable rigid core 14. In practical application of the process, to fabricate an X-band elbow having an inside diameter of approximately 2 and 1/2 inches, the rigid core 14 comprised a flexible metal bellows; whereas, for fabricating an S-band elbow having an inside diameter of about 6 inches, the core 14 was constructed of short pieces of hollow pipe bolted end-to-end for the desired length of elbow. The core 14 is made hollow so that hot water can be passed through the core as will be discussed later. In step two of the process, as shown at (b), a coating of low melting temperature alloy 15 such as Woods Metal (158.degree. F.) is molded onto the outer surface of the core 14. This might be accomplished in a suitable mold 15a of cornu bend configuration, having a continuously variable radius of bend.

To prevent adhesion of the alloy 15 to the insulated helical wound wire (reference 11 in FIG. 2 and 3), the alloy 15 is first coated with a suitable rubber-base paint to form a placenta-like skin 16 of suitable thickness (reference (c) in FIGS. 4 and 5). The next step (d) in the process involves helically winding the insulated wire onto the form. This step preferably is performed such that each turn of wire is perpendicular to the centerline of the waveguide structure. To accomplish this, a novel constant tension wire winding device was invented by one of the present inventors and is disclosed in detail in copending and commonly assigned U.S. patent application Ser. No. 115,291 filed Nov. 2, 1987.

Following the wire winding step, it was found desirable to first coat the outer surface of the helical wire with a highly adhesive dielectric material such as grey RTV to assure a good bond between the winding and the subsequently applied RTV dielectric sheath. This is represented at step (e) in FIG. 5 where the highly adhesive dielectric, designated at 12a, is applied as a thin film to fill any spaces between the winding and then screed off flush with the outer surface of the helical wire 11. After the dielectric layer 12a has cured, flanges 17 are attached to the ends of the bend structure and the structure is placed in a second mold 17a, where a selected liquid dielectric material is molded, at step (f), onto the helical wire. In FIGS. 2 and 3, the dielectric is referenced generally at 12. In the practical application referred above, the dielectric layer or sheath 12 is formed of two part liquid RTV which is injected under pressure into the mold 17a surrounding the insulated helical wire winding. To assure circularity and uniform thickness of the dielectric sheath 12 and angular coverage of the elbow, the helical wire wound structure is mounted concentrically in the mold 17a with the flanges 17; e.g. by suitable chaplets formed of solid RTV disposed at selected locations along the length of the wound structure to support it centered in the mold. Preferably, the RTV was deaerated prior to injection into the mold 17a, to assure a uniform density.

The layer 12 is then cured, to form a solid dielectric layer surrounding the helical wire.

At this stage in the proposed process, an appropriate metallic conductor skin 13 is placed on the outer surface of the structure along the entire length of the bend, from flange to flange. In the practical application referred to above, this outer conductor 13 (step (g)) was formed by wrapping aluminum foil around the outside of the dielectric layer. The outer metallic skin 13 need not be very thick so long as good electrical conductivity is achieved along the length of the bend's outside conductor from flange to flange. It was found that wrapping a sticky-back aluminum tape overlapped approximately 50% was adequate. As a finishing, two fiberglass and resin layers (step (h)) FIG. 4 were applied over the aluminum foil skin.

As illustrated in FIGS. 4 and 5, the core 14 is removed by first melting and removing the low melt temperature alloy, at step (i). This was accomplished by simply running hot water through the center of the hollow core and then pouring out the molten alloy. The core 14 is thereby freed for removal as depicted at step (j) in FIG. 5. Finally, the placenta 16 is removed at step (k) and, following any necessary trimming of the ends (step (1)) FIG. 4, the illustrated process is complete.

Obviously, various modifications and alterations to the above-described process are possible in light of the foregoing discussion, and therefore, within the scope of the appended claims, the invention may be practiced otherwise than as specifically shown and described hereinabove.