COAXIAL IMPEDANCE STANDARDS 695 



intermediate annealing. The laboratory operation consisted of drawing the 

 0.110" stock straight out through a 0.1004" I. D. diamond die pre-selected 

 as to circularity and finish of the bore. 



The copper which was used is known in the trade as "electrolytic tough 

 pitch" chemically composed of 99.95 per cent copper, 0.02 per cent oxygen 

 and 0.03 per cent divided between six other minor contaminants. 



There has always been some concern as to the possibility that a wire 

 drawn from annealed stock might well have a thin full hard shell over a 

 relatively annealed core. The d-c. resistivity measurements would then de- 

 termine a weighted average resistivity, instead of the surface resistivity 

 needed for calculations. To settle this point, tests were made on full-hard 

 wire, semi-hard wire as used in commercial coaxials, fully annealed wire, 

 and on annealed wire plated with a 0.2 mil layer of silver. The tests con- 

 sisted of (a) comparing the a-c. resistance of the wires by precise methods 

 at 1 mc where transmission is at a skin depth of 2 mils, and (b) microscopic 

 study of grain structure of polished-etched cross sections of the samples at 

 magnifications to 2000 diameters. The conclusion from both studies was 

 that no thin skin exists. The wire is therefore treated as homogeneous 

 throughout its cross section in computation of d-c. resistivity and in com- 

 putation of a-c. resistance. 



Copper Tubing 



The effects on the resistance of a coaxial traceable to the physical con- 

 stants of the outer conductor are scaled down about 5 : 1 so that the require- 

 ments on the tubing are not so severe as on the wire. However, stock cop- 

 per tubing from a distributor's warehouse cannot normally be used. Such 

 tubing may have unacceptable inside surface roughness, ellipticity of bore, 

 and a high and variable resistivity. Roughness and ellipticity are the result 

 of worn plug dies frequently used in the drawing of commercially acceptable 

 tubing, or omission of the plug die in drawing the tubing to final diameter. 

 Most stock tubing contains phosphorous and, even though the percentage of 

 phosphorous may be very small, the effect in increasing the resistivity is 

 marked. The full-hard tubing used in the coaxial standards here described 

 was procured directly from a mill, and was largely drawn in consecutive 

 lengths from a single casting of oxygen-free electrolytic copper using selected 

 dies. 



Insulation 



Expanded polystyrene is the dielectric material used in the standards 

 and its applicable properties are shown in Table III. In solid form it has a 

 dielectric constant, e', about 3% greater than that of air and, when used 



