692 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1951 



It is with the new set of impedance standards that this paper is chiefly 

 concerned. 



The new standards have been constructed in lengths varying from six 

 inches to thirteen feet in increments of length so that, at a given frequency 

 and far-end condition, eighteen standard impedances are available. 



The completed standards have been, provided with a permanent storage 

 cabinet, Fig. 1, located in an air-conditioned cable development laboratory 

 adjacent to measurement facilities. The special tools developed for con- 

 struction, assembly, and use of the standards are also stored in the cabinet 

 together with spare materials for maintenance. 



Each standard consists of a seamless hard drawn copper tube f I. D. as 

 outer conductor and a straight hard drawn copper wire, nominally No. 10 

 A.W.G., as center conductor. The insulation is expanded polystyrene in the 

 form of spaced cyhnders. An aluminum tube is used over the copper tube 

 for mechanical protection but is insulated from it. Stainless steel fittings 

 are provided at each end to exclude dust, to facilitate connection for cir- 

 culation of dry air, and to provide the short-circuit necessary for Zsh meas- 

 urements. The properties, selection, and preparation of the three compo- 

 nents — v/ire, tubing, and insulation and the provision of a repeatable method 

 for short circuiting the coaxials are the basic problems in construction, and 

 are discussed in the following paragraphs. 



Physical Constants 



The measured physical constants of the copper wire and tubing which 

 are of interest in this appHcation are given in Table II, and those of the 

 expanded polystyrene insulation in Table III. Wherever practicable the 

 absolute accuracies of the measuring instruments were checked against 

 secondary standards of weights and measures, periodically referred to the 

 U. S. Bureau of Standards laboratory for calibration. 



Dimensions 



The I. D. dimension quoted in Table II is the average of a number of 

 tests on end samples of tubing and was obtained from dimensional and 

 weight relationships as expressed by the equation: 



-/■>•-:-?• 



(5) 



where V is the volume of copper in the sample as measured by the displace- 

 ment technique. Do is the measured O. D. of the sample and C is its length. 

 The I. D. was also determined by direct measurements of O. D. and wall 

 thickness, and agreement obtained with the figure quoted for the above 



