Abstract:
A waveguide expansion joint for connecting two circular waveguides operating in the TE 01  mode while allowing for waveguide expansion and contraction due to temperature variations. The expansion joint includes first and second rings on the outside surface at one end thereof. When the expansion joint is inserted into a waveguide the rings are proximate to the inside diameter of the waveguide for absorbing undesired propagating modes, for pressurization of the expansion joint, and for coaxially aligning the waveguide and the expansion joint.

Description:
FIELD OF THE INVENTION 
     This invention relates to an expansion joint for connecting sections of a waveguide that permits expansion and contraction of the waveguide sections without distortion of the electromagnetic energy propagating therein. 
     BACKGROUND OF THE INVENTION 
     Electromagnetic transmission systems often employ waveguide sections that are rigidly supported within a surrounding conduit. The waveguide requires the use of expansion joints to accommodate for temperature-induced expansion and contraction of the connected waveguide sections. For example, in long runs of low-loss circular waveguide operating in the TE 01  mode, the variation in overall length of the waveguide with temperature may be on the order of inches. A slip joint or expansion joint is necessary to allow for this length variation in such a run where the ends are fixed. The waveguide may also be pressurized to form a barrier against the leakage of moisture into the waveguide. Thus the slip joint must be sealed to prevent loss of pressurization. 
     Various tapered joints have been employed in expansion joints for TE 01  mode transmission waveguides. One such tapered expansion section is exemplified by U.S. Pat. No. 4,369,413 issued to Devan et al. The Devan expansion joint provides a mechanical expansion capability and taper transition between different operating waveguide diameters, while minimizing the generation of unwanted modes, especially the TE 02  mode. Devan accomplishes this with a two-piece expansion joint; the first piece increases in diameter linearly from the diameter of the waveguide to which it is to be connected, to a diameter that is sufficiently small to prevent the generation of spurious modes (especially the TE 02  mode) and equal to the inside diameter of the second piece. The second waveguide piece is concentrically and precisely fitted in slideable engagement with the first waveguide piece. The first and second waveguide pieces are arranged to permit sliding with respect to each other to provide the expansion and contraction capability. The second waveguide piece has at one end thereof an internal diameter that is equal to the enlarged diameter of the first waveguide piece. At the other end of the second waveguide piece there is a cosine or other specially shaped section to enlarge the joint to the diameter of the other waveguide to which it is to be connected. 
     Although this prior art expansion joint provides some expansion and contraction capability, it is limited by the frictional forces created by the precise fit where the outside surface of the first piece and the inside surface of the second piece are in contact. In the prior art designs this frictional force is necessary to provide alignment of the waveguide and the expansion joint and to prevent the emission of the propogating energy from the waveguide into free space. These disadvantages are overcome by the present invention. 
     SUMMARY OF THE INVENTION 
     Although the prior art expansion/contraction waveguide joints provide satisfactory operation and attenuation of undesired modes, it is desirable to provide an expansion joint having a simplified geometry, lower frictional forces to provide easier sliding between waveguide sections, substantial attenuation of undesired modes, and ease of manufacture and installation. 
     A waveguide expansion joint is adapted for joining a first circular waveguide with a second circular waveguide for propagating electromagnetic energy in a preselected mode, typically the TE 01  mode. The first end of the waveguide expansion joint has an outside diameter approximately equal to the inside diameter of the first circular waveguide to which it is to be connected. The first end of the expansion joint and the first circular waveguide are connected by slideable concentric engagement. The second end of the expansion joint has an outside diameter slightly less than the inside diameter of the second circular waveguide. The second end of the expansion joint and the second circular waveguide are connected by slideable engagement of the expansion joint into the waveguide, with the volume between the outside diameter of the waveguide and the inside diameter of the expansion joint (at the second end thereof) filled by a ring member concentric with the expansion joint. In one embodiment, the ring member serves three purposes: (1) absorbs electromagnetic energy propagating in undesirable modes in the gap between the inside diameter of the waveguide and the outside diameter of the expansion joint, thus preventing the radiation thereof into free space; (2) seals the gap to maintain waveguide pressurization; and (3) coaxially aligns the expansion joint and the waveguide. In other embodiments, these three functions are provided by one or more rings of proper size and composition. For example, in one embodiment a ferrite ring provides the absorption characteristic, a first &#34;O&#34; ring provides the sealing, and a second &#34;O&#34; ring (in conjunction with the first) provides the required alignment. The electromagnetic energy propagates through the expansion joint from the first end to the second end thereof. 
     The expansion joint of the present invention also offers ease of installation at the site. All the waveguide is first put in place, except the final section and the expansion joint contained within it. After measuring the gap to determine the length of the final section, the final section is cut a few inches shorter than this gap. The expansion joint is adjusted as needed within the final section to complete the waveguide run. This adjustable feature of the inventive expansion joint offers a significant advantage when the waveguide run is fabricated by removing the necessity for close tolerances for the waveguide sections. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention may be better understood, and the further advantages and uses thereof more readily apparent, when considered in view of the following detailed description of exemplary embodiments, taken with the accompanying drawings in which: 
     FIG. 1 is a cross-sectional illustration of a preferred embodiment of an expansion joint constructed according to the teachings of the present invention; 
     FIG. 2 is a cross-sectional illustration of a preferred embodiment of a coupling joint constructed according to the teachings of the present invention; 
     FIG. 3 is a cross-sectional illustration of a second embodiment of an expansion joint constructed according to the teachings of the present invention; and 
     FIGS. 4 through 8 are cross-section illustrations of additional embodiments of an expansion joint constructed according to the teachings of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     It is known that the TE 0N  propagating modes have only circumferential wall currents, while all other modes have some longitudinal wall currents. Therefore, any circumferential cut in the wall of a waveguide is virtually invisible to the TE 0N  modes but allows all other modes to radiate through the cut. High power waveguide runs are usually sized to allow only the TE 01  mode to propagate, with the TE 02  mode and the remaining TE 0N  modes cut off. There are, therefore, eight modes that can propagate in the waveguide, i.e., the desired TE 01  mode and seven undesirable modes. Any circular asymmetry in the waveguide causes mode conversion from the TE 01  mode to one of the undesired modes. 
     The expansion joint of the present invention is shown in cross-sectional form in FIG. 1 and designated generally by the reference character 10. The expansion joint 10 includes a waveguide member 11 having a step transition therein and slideably engaged in coaxial orientation with a waveguide 12 and a waveguide 14. In use, the waveguide 12 would typically be a long waveguide run, only the end of which is shown in FIG. 1; the other end being fixed. Assuming the direction of propagation indicated, the waveguide 14 would typically be connected to a receiving device. The expansion joint 10 is, however, reciprocal allowing electromagnetic energy to propogate in either direction through it. The waveguide 14 would therefore be a relatively short length and also rigidly fixed. To allow for expansion and contraction of the waveguide 12, the expansion joint 10 is, as discussed, slideably engaged in the waveguide 12. At one end of the waveguide member 11 the outside diameter thereof equals the inside diameter of the waveguide 14 into which the waveguide member 11 fits, forming a rigid junction and preventing relative movement thereat. The other end of the waveguide member 11 has an outside diameter slightly less than the inside diameter of the waveguide 12 into which the waveguide member 11 fits. Thus, a gap 17 is formed at this junction so as to allow expansion or contraction movement of the waveguide 12 within the expansion joint 10. The expansion joint 10 also includes an absorber and pressure seal 16 oriented coaxially with the waveguide member 11 at the end thereof that is to fit into the waveguide 12. The absorber and pressure seal 16 is necessary to absorb undesired modes that might radiate from the gap 17 between the inside diameter of the waveguide 12 and the outside diameter of the waveguide member 11. In the preferred embodiment, the absorber and pressure seal 16 is constructed of a carbon-loaded silicone rubber that provides both mode absorption and pressure sealing of the joint between the waveguide member 11 and the waveguide 12. This pressure seal is necessary to seal the joint and maintain waveguide pressure. The absorber and pressure seal 16 also holds the waveguide member 11 and the waveguide 12 in the proper orientation as shown in FIG. 1. An alignment ring 18 is also placed in coaxial orientation with the waveguide member 11 as shown in FIG. 1. As shown, the absorber and pressure seal 16 and the alignment ring 18 provide proper coaxial alignment of the expansion joint 10 and the waveguide 12. The alignment ring 18 can be fabricated of any stable, low-friction, dielectric (usually) material. In lieu of using the alignment ring 18, in another embodiment of the present invention the final alignment may be accomplished with a series of pins, studs, or a split ring as is well known in the art. Alternatively, the absorber and pressure seal 16 can provide only the mode-absorbing function, with two or more rings for alignment (including the alignment ring 18) and one or more rings for pressure sealing. FIG. 5 shows an embodiment of the expansion joint 10 wherein the absorber ring 16 provides only the mode-absorbing function, and a pressure seal 40 provides pressure sealing of the gap 17. 
     The expansion joint 10 also includes one section at each end thereof having a slightly larger inside diameter than the main diameter of the expansion joint 10. These sections are shown generally with reference character 20 in FIG. 1. The sections 20 are quarter-wavelength transformers for providing impedance matching between the expansion joint and the waveguides 12 and 14. 
     An exemplary embodiment of an expansion joint 10 constructed according to the teachings of the present invention was designed in 3.027 inch inside-diameter waveguide for the 7.9 to 8.4 GHz frequency band. The return loss in this frequency band was approximately 35 dB or less than 1.04:1 VSWR. The higher order mode leakage in this exemplary embodiment was more than 70 dB down. This exemplary embodiment of expansion joint 10 also allowed for more than six inches of length variation due to temperature changes. 
     FIG. 2 shows a coupling joint 22 constructed according to the teachings of the present invention. The coupling joint 22 couples two waveguides 12 and 14 without the use of flanges, as required by the well-known prior art designs. The ends of the waveguides 12 and 14 may be rough cut and the coupling joint 22 inserted between them. In this embodiment there are two gaps 17 to allow expansion and contraction movement of both waveguides 12 and 14, two absorber and pressure seals 16, and two alignment rings 18 for aligning the waveguides 12 and 14. As discussed in conjunction with the expansion joint 10 illustrated in FIG. 1, the absorber and pressure seal 16 absorbs undesired modes propagating in the waveguide 12 or the waveguide 14, maintains a pressure seal, and provides the frictional forces to hold the coupling joint 22 inside the waveguides 12 and 14. In conjunction with the alignment rings 18, the absorber and pressure seals 16 ensure coaxial alignment of the waveguides 12 and 14 and the coupling joint 22. Electromagnetic energy can propagate in either direction in the FIG. 2 embodiment (and in the embodiments of FIGS. 1 and 3). The embodiment of FIG. 2 is especially suitable for use as feed-through for a metal electromagnetic pulse (EMP) or electromagnetic interference (EMI) barrier that requires rigid and tight attachment of the waveguide to the wall. A hole is cut through the EMP/EMI wall (identified by reference character 23) and a flange 24 is attached (by suitable means such as silver solder) to the expansion joint 22. The flange 24 is welded to the EMP/EMI wall 23 as shown generally by reference character 25. FIG. 6 illustrates another embodiment of the coupling joint 22 including two alignment rings 18 and 18&#39; at each end of the coupling joint 22. 
     The expansion joint 10 and the coupling joint 22 are used when the frequency band of operation is near but slightly below the TE 02  cutoff frequency. If the operation band is significantly below the TE 02  cutoff frequency, but very near the TE 01  cutoff, an inverse type expansion joint, shown in FIG. 3, can be used. Operation of the inverse expansion joint 26 is similar to that of the expansion joint 10 and the coupling joint 22 except the waveguide member 11 includes a section 11&#39; located between the gap 17 and the waveguide 12. The inside diameter of the waveguide member 11 should be beyond cutoff for the TE 02  mode in the band of operation. As indicated, electromagnetic energy can propagate in either direction through the expansion joint 26. 
     FIG. 4 is a slight modification of FIG. 1, wherein two alignment rings I8 and 18&#39; are used to provide proper alignment between the waveguide 12 and the waveguide member 11. 
     In FIG. 7 there is another embodiment having a single alignment ring 18, a pressure seal 44, and an absorber 16. In this embodiment the absorber 16 provides only the electromagnetic energy absorption function as discussed above, while the pressure seal 44 pressure seals the gap 17. 
     Although several embodiments in accordance with the present invention have been shown and described, it is understood that the same is not limited thereto but is susceptible of numerous changes and modifications as known to a person skilled in the art, and I therefore do not wish to be limited to the details shown and described herein, but intend to cover all such changes and modifications as are obvious to one of ordinary skill in the art.