PROPORTIONING OF CIRCUITS FOR ATTENUATION 259 



Stranded Conductors 



With conductors having; soHd walls, or composed of non-insulated 

 strips or filaments, the currents at high frequencies are largely crowded 

 toward the inner surface of the outer conductor and the outer surface 

 of the inner conductor, due to skin effect. Since the losses in the 

 conductors themselves ordinarily comprise the major portion of the 

 attenuation in a coaxial circuit, interest attaches to the possibility of 

 counteracting the increase in conductor resistance due to skin effect by 

 using a conductor composed of a number of individually insulated 

 strands so twisted or interwoven as to distribute the current more 

 nearly uniformly over the cross-section.^' Chief attention naturally 

 focuses upon the inner conductor, which is by far the greater contribu- 

 tor to the resistance, and this discussion will be largely limited to the 

 case where only the inner coaxial conductor is stranded.* 



Types of stranded conductors suitable for use as the inner conductor 

 of a coaxial circuit include both those in which the conductor cross- 

 section is completely filled with insulated strands and those in which 

 the insulated strands form an annular cross -section, surrounding a 

 core of non-conducting or conducting material. Of various possible 

 methods of stranding, one simple and effective process is similar to that 

 used in the construction of rope. A few strands are twisted together to 

 form a group, several such groups are twisted into a larger group, and 

 so on until the desired conductor cross-section is obtained. 



The high-frequency resistance of a stranded conductor may be 

 determined either by measurement or computation. For a completely 

 stranded inner conductor of any diameter, size, number of strands, and 

 thickness of insulation, the high-frequency resistance is given by S. 

 Butterworth '^ and in unpublished material by J. R. Carson. The 

 resistance values obtained in measurements of stranded conductors 

 approximate very closely the theoretical results. 



In evaluating the results obtained with stranding, it is convenient to 

 compare the resistance of a stranded conductor with that of a non- 

 stranded conductor of the same overall size. For the case of a stranded 

 inner conductor, the ratio of the resistance of the stranded conductor 

 at any given frequency to the resistance at the same frequency of a 

 solid conductor having the same outer diameter and composed of the 

 same material used in the strands may be designated as ni. 



The values of the resistance ratio m which may be realized in practice 

 depend upon the frequency and the design of stranded conductor. 

 Some idea of these values for two specific conductors may be obtained 



* "Stranded" is used to mean "composed of insulated strands." 



