Abstract:
A connecting joint for a radio frequency waveguide line comprises a tubular metallic braid wire shield connecting two beam line pipe ends thereby making the inside surface appear as a continuous beam tube. The metallic braid wire is secured within a bellows assembly that allows the connecting joint to expand or contract as is appropriate for changes in ambient conditions or conditions within the waveguide line. The connecting joint is capable of supporting either positive or negative pressure within the joint while the exterior of the joint may vary with ambient conditions.

Description:
FIELD OF THE INVENTION 
     This invention relates to shielding radio frequency (RF) waves within a waveguide, and in particular, to a braid shielded RF bellows assembly. A metallic tubular braided-wire shield connects two beam line pipe ends thereby making the inside surface appear as a continuous beam tube. A bellows surrounds this braided-wire shielding. The tube may be fastened by welding, brazing, or other suitable method. 
     BACKGROUND OF THE INVENTION 
     Waveguides that transmit RF energy commonly require flexible joints to connect fixed portions of the waveguide. These flexible joints must be capable of expanding or contracting with temperature changes to compensate for the expansion or contraction of the fixed portions of the waveguides that they join together. The flexible joints must be capable of not attenuating or degrading the RF signals that are transmitted within them. In addition, the flexible joints must be capable of maintaining the positive or extreme negative pressure that is commonly held within the waveguide. 
     In the prior art, flexible joints for waveguides have been proposed which include a bellows assembly within a telescoping outer shield. Having the bellows in the inner part of the wave guide, immediately surrounding the RF energy, can lead to degradation of the signals within the bellows as a result of the irregular shape of the bellows. This design, having a telescoping outer shield and an inner bellows assembly is of complex construction. 
     As described by these several limitations, flexible waveguide joints of the present art have not proven fully satisfactory for transmitting RF energy simply and without signal degradation. 
     SUMMARY OF THE INVENTION 
     This invention provides an improved flexible joint for wave guides which includes a bellows assembly on the outer periphery of the flexible joint to allow for expansion and contraction as caused by temperature changes. The invention furthermore provides a tubular wire braid having a smooth and even inner diameter that acts as the wave guide within the flexible joint to hold the RF energy with minimum degradation. The tubular wire braid is free to expand and contract with the bellows and yet allow a non-degraded RF signal to pass therethrough. 
     The improved flexible joint of this invention provides a joint that is of simple construction, easy to fabricate, maintains a proper hermetic seal between the inner waveguide pressure and the pressure outside the joint, and holds RF signal degradation to a minimum. 
     OBJECTS AND ADVANTAGES 
     A principal object of the present invention is to provide a flexible wave guide joint that will not degrade the signal within the wave guide. 
     A second object of the present invention is to provide a flexible wave guide joint that is simple and easy to construct. 
     A third object of the present invention is to provide a flexible wave guide joint that will hold a hermetic seal between the positive or negative pressure within the wave guide and the ambient pressure outside of the wave guide. 
     Other objects and advantages of the preferred embodiment will become apparent when reading the attached description of the invention and referring to the associated drawings. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of the preferred embodiment of the braid shielded RF bellows. 
     FIG. 2 is a detail view of the section of FIG. 1 showing one end of the tubular wire braid pressed by the jam ring into the cuff. 
    
    
     DESCRIPTION OF THE INVENTION 
     The invention is a braid shielded RF bellows for providing a flexible connecting joint for joining two RF wave guide sections. A cross-sectional view is provided of the braid shielded RF bellows 30 in FIG. 1. The braid shielded RF bellows 30 includes the bellows sub assembly 14 consisting of a bellows 12, a first cuff 16a and a second cuff 16b, a piece of tubular braid wire 10, a first jam ring 18a and a second jam ring 18b, two conflat adapter rings 20, and two conflat flanges 22. The first cuff 16a and second cuff 16b each have a flange portion 24 and a tubular portion 26 as shown in FIG. 1. All these separate pieces comprise the braid shielded RF bellows 30 as shown in FIG. 1 and provide a longitudinal passageway or channel 32 along central axis 34 through which RF waves may be transmitted with little degradation in strength. The bellows sub assembly 14 also provides a hermetic seal through which either positive or negative pressure within the channel 32 is isolated from ambient pressure outside of the bellows sub assembly 14. 
     FIG. 2 is a detail view of a section of FIG. 1 showing a first end 10a of the tubular braid wire 10 pressed by the first jam ring 18a into the first cuff 16a. As shown in FIG. 2, the first cuff 16a includes a flange portion 24 that is secured to the bellows sub assembly 14 and a tubular portion 26 through which the tubular braid wire 10 is threaded. A recess 28 is formed on the inner periphery of the tubular portion 26 of the first cuff 16a enabling the first jam ring 18a to be pressed into the channel 32 until the first jam ring 18a is flush with the end 36 of the tubular portion 26 of the first cuff 16a. The excess wire braid that extends beyond the end 36 of the tubular portion 26 of the first cuff 16a may then be trimmed off with a clean sharp tin snips. The opposite end of the braid shielded RF bellows 30 has parts on its opposite end analogous to those on the first end, including a second cuff 16b having a flange portion 24 and a tubular portion 26 also with a recess in the inner periphery of the tubular portion 26. The second jam ring 18b is pressed into the second cuff 16b in an analogous manner as was done with the first end of the braid shielded RF bellows 30, thereby pressing and holding the second end of the tubular braid wire 10b between the second jam ring 18b and the second cuff 16b. 
     The braid shielded RF bellows 30 is constructed, referring to FIG. 1, by first providing a piece of braid wire. A typical specification of the braid wire as used in this invention may be obtained from New England Wire Corporation, Lisbon, N.H. The wire braid used was New England Wire Corporation&#39;s type 304 stainless steel wire braid part number NES480130SS. 
     The braid wire is cut to the desired length, typically 18 inches. The cut piece of braid wire is then verified as being free of any bends or any crossed wires. The cut piece of braid wire is then squeezed into a circular cross section from the oval as provided, typically 1.375 inches in diameter. The resulting tubular braid wire 10 is then cleaned ultrasonically for at least 10 minutes in a minimum of 2 positions and at two different states of length. 
     The bellows sub assembly 14 in FIG. 1 is constructed by first cutting a bellows 12 to the desired length, typically 4.13 inches. A metallic cuff is then formed to fit each end of the bellows 12, in FIG. 1 a first cuff 16a and a second cuff 16b are shown. The bellows sub assembly 14 is then formed by securing the bellows 12 to the cuffs 16a, 16b by welding, brazing, or some other appropriate means. The bellows sub assembly 14 is then cleaned appropriately for use in ultra high vacuum. The bellows sub assembly 14 is then installed in a clean fixture to hold it to an appropriate length, typically 4.13 inches, and to keep the cuffs 16a, 16b parallel. 
     With the bellows sub assembly 14 held in the clean fixture, the tubular braid wire 10 is slid through its interior channel 32. A first jam ring 18a is then pressed into the inner periphery of the first end 10a of the tubular braid wire thereby jamming the tubular braid wire 10 between the first jam ring 18a and the first cuff 16a. A recess 28 is formed on the inner periphery of the tubular portion 26 of the first cuff 16a allowing a space for the first end 10a of the tubular braid wire to be held within. The first jam ring 18a is then welded in several places to the first cuff 16a in several places to hold it fast. Any excess braid wire extending beyond the end 36 of the tubular portion 26 of the first cuff 16a is then cut off the end of the bellows sub assembly 14 using clean sharp tin snips. A fusion weld is then completed around the perimeter of the first cuff 16a. 
     With the tubular braid wire 10 jammed flush, welded, and thereby secured at its first end 10a, the inside diameter of the tubular braid wire 10 within the bellows sub assembly 14 is manually smoothed to the desired diameter of approximately 1.375 inches. The second jam ring 18b is then pressed into the inner periphery of the second end 10b of the tubular braid wire 10 thereby forcing the second end 10b into the recess 28 in the inner periphery of the tubular portion 26 of the second cuff 16b. The second end 10b of the tubular braid wire 10 is then held fast between the second jam ring 18b and the second cuff 16b, with the end 36 of the tubular portion 26 of the second cuff 16b flush with the second jam ring 18b. The tubular braid wire 10 is then verified that it remains at the desired inside diameter of 1.375 inches. The second jam ring 18b is then welded to the second cuff 16b in several places and the excess braid wire is cut off of the second end of the bellows sub assembly 14 using clean sharp tin snips. A fusion weld is then made around the perimeter of the second cuff 16b. 
     The tubular braid wire 10 is now smoothed to the correct inside diameter and secured to each end of the bellows sub assembly 14. With this assembly still held within a clean fixture, conflat adapter rings 20 are aligned with the cuffs 16a, 16b at each end of the bellows sub assembly 14. The conflat adapter rings 20 are then welded with a full penetration weld to each of said cuffs, 16a and 16b. 
     The bellows sub assembly 14 with the secured tubular braid wire 10, jam rings 18a and 18b, and conflat adapter rings 20 is then baked in a vacuum furnace at 600° C. for one hour and allowed to cool under vacuum. Conflat flanges 22, as shown in FIG. 1, are then aligned with each of the conflat adapter rings 20 on their respective ends of the bellows sub assembly. The conflat flanges 22 are then welded to the conflat adapter rings 20 to complete the braid shielded RF bellows 30. The completed braid shielded RF bellows 30 is then double bagged with first a nylon bag and then a polyethylene bag until needed for joining two sections of the RF wave guide. 
     As can be seen in FIG. 2, the inside diameter of the assembly is constant at typically 1.375 inches. To keep this constant diameter the jam ring outside diameter and shape and the inside diameter and shape of recess 28 complement each other including the thickness of the braid 10 therebetween. Typically the inside dimensions of the recess 28 are 1.422 inches diameter at the inner end with the recess starting at the outer end with a diameter of 1.485 inches. This diameter is constant for 0.100 inches from the outer end then starts to taper inward for a distance of 0.179 inches until the 1.422 inches diameter is reached. The entire depth of the cuff is typically 0.500 inches. The jam ring 18a has a depth that is typically 0.270 inches, a uniform inside diameter of 1.375 inches and an outside diameter that complements the recess 28 including taking in consideration the thickness of the braid. The outside diameter on the inner end is 1.375 inches tapering for 0.170 inches to an outside diameter of 1.435 inches which continues as the uniform outside diameter for an additional 0.100 inches at the outer end.