COAXIAL IMPEDANCE STANDARDS 



697 



stant and the conductance losses. For this reason all insulating cyUnders 

 have been pierced longitudinally so that low pressure dry air may be cir- 

 culated through the assembled coaxials when they are in use. 



The cylinders were cut with a high-speed fly cutter, and the center hole 

 drilled in the same operation. It was determined by sensitive electrical tests 

 that centering precision in the assembled coaxials was equivalent to that 

 obtained in coaxials with lathe turned diics of solid materials. 



Cylinder spacing of 3'' on centers was determined as about the maximum 

 permissible to prevent detectable sag in the center wire between points of 

 support. There is no specific strength requirement on the cylinders except 

 that they must support the weight of the straight drawn wire. However, a 

 single polyethylene disc is used at the test end to resist radial thrusts which 

 may occur in making test connections. 





-^— — *^M 



Fig. 3 — Partially assembled coaxial standard. 



Assembly Features 



It is necessary to have mechanical protection for the copper tubes of the 

 standards, and this is provided for each by an aluminum tube slipped over 

 the copper tube, with insulation consisting of a helical wrap of 0.0015 x |" 

 paper ribbon first applied to the copper tube. The aluminum tube is locked 

 on the copper tube by short lengths of fibre tubing wedged between at each 

 end. The wedging action occurs when the end fittings are screwed to the 

 aluminum tube. Figure 3 is a photograph of a partially assembled standard 

 to illustrate further the construction. The presence of the aluminum tube 

 may be disregarded in so far as its electrical effects are concerned because 

 of the self-shielding qualities of the copper tube. 



End Effects 



Experimental data indicate that if a coaxial tube is longer than its center 

 conductor and the wire is then lengthened incrementally until it is as long 

 as the tube, the capacitance of the coaxial remains directly proportional to 



