Coaxial Impedance Standards 



By R. A. KEMPF 



{Manuscript Received Mar. 7, 1951) 



The calibrations of bridge networks used in developmental tests on coaxial 

 cable are obtained by comparison of the networks with calculable standards 

 of impedance consisting of a group of short-length precision copper coaxial lines. 

 The standards are calculable by reason of the availability of precise formulae 

 relating the distributed primary constants to the measurable physical constants 

 and dimensions of the coaxials. This paper outUnes the constructional problems 

 and design features of a group of such standards of impedance which provide a 

 range of values over a broad band of frequencies. 



Introduction 



The "mile of standard cable" was for a long time the basis for rating the 

 transmission qualities of telephonic apparatus and networks.^* ^ The title 

 of this paper suggests that a return to the old standard has been accom- 

 pUshed. This is true in a restricted sense, but with important differences. 

 The standards here described consist of varying lengths of a rigid coaxial 

 transmission line structure. Their sole function is to supply primary refer- 

 ences of resistance, inductance, capacitance and conductance which are 

 numerically comparable to typical unknowns encountered in laboratory 

 cable measurements. Unlike the mile of standard cable, the rigid coaxial is 

 simple structurally, its physical constants and dimensions may be deter- 

 mined accurately, and precise formulae are available for translating these 

 properties into electrical constants at any frequency. It is thus an excellent 

 means for the objective — calculable radio-frequency laboratory standards 

 of R, L, G, and C of the restricted numerical range needed to calibrate the 

 bridge networks used in measurements on the short lengths of cable avail- 

 able to the cable development engineer. 



Because developmental cables are not usually available in the longer 

 lengths on which the secondary constants of attenuation, phase, and char- 

 acteristic impedance may be measured directly, laboratory measurements 

 on a cable sample are usually confined to determination of the four dis- 

 tributed primary parameters or constants. From these the secondary con- 

 stants may then be calculated. 



Measurement of the distributed primary constants of a given line struc- 

 ture is an indirect process, except under limited or restricted circumstances. 



1 R. V. L. Hartley, "The Transmission Unit," Electrical Communication, Vol. /, No. 

 1, July, 1924. 



2 W. L. Everitt, Communication Engineering, pp. 101-2. 



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