690 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1951 



This is because (1) an impedance bridge can do no more than measure the 

 unknown impedance which may be placed across its terminals; and (2) the 

 line structure can be measured only by making bridge readings at its input 

 or output terminals, from which points the true distributed series properties 

 of the line appear to be altered by the shunt properties, and vice-versa. 

 Statement (2) applies to all observations at the end of a section of trans- 

 mission line, except when the line is very short electrically. 



Assuming that accurate bridge measurements of the impedance at the 

 terminals of a line are available, standard transmission formulae may be 

 used to calculate rigorously the distributed primary constants. Uncertainty 

 as to the accuracy of impedance bridge measurements led to the develop- 



Table I 



Dimensions and Physical Properties which Determine The Primary Electrical 



Constants of Any Coaxial Transmission Line 



R, L, G, and C are distributed resistance, inductance, conductance and capacitance, 

 respectively, at any frequency, F. 



p is the dc volume resistivity of the copper conductors which have diameters ID and d, 

 and wall thickness t. 



c and Fp are the composite dielectric constant and power factor respectively, assumed 

 independent of frequency. 



ment of coaxial impedance standards as a means of checking the accuracy 

 of test apparatus. As this work progressed, and the merit of the standards 

 was more fully appreciated, it came about that the bridges were not merely 

 checked against the coaxial standards, but instead the bridge calibrations 

 were derived from the coaxial standards. 



Input Impedance of a Coaxial 



The distributed primary constants of any coaxial with a uniform structure 

 may be precisely computed in terms of dimensions and physical constants 

 using formulae which have been developed by SchelkunofP and others. 

 Table I indicates the physically measurable quantities used to compute the 

 respective distributed electrical constants, R, L, G, and C. 



' S. A. SchelkunoflF, "Electromagnetic Theory of Coaxial Transmission Lines and 

 Cylindrical Shields," B.S.TJ., Oct. 1934. 



