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THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1953 



these dimensions the cross sectional area of the outer conductor is nearly 

 twice that of the inner conductor, so that the optimum arises from other 

 causes than matching the conductivity of the two conductors. 



The problem of determining the relative optimum values of Clogston 

 I, in genera), is complicated, but numerical studies indicate that for 

 values of the fill factor other than one-half, the values for T and U are 

 not greatly different. 



Another factor in Clogston cables which can be optimized is the ratio 

 of the layer thicknesses of the conducting and the insulating layers. 

 Clogston* has shown that in the frequency range where the attenuation 

 is substantially fiat with frequency, this optimum ratio is equal to: 



w/t = 2. 



(6) 



For this condition, the dc resistance of the laminated conductor is 

 increased by {w + t)/w or 3/2 over a solid conductor. The dielectric 

 constant of the main insulation, according to (2) above must equal 

 €i = 3c, w^hich reduces the speed of propagation by \/3, assuming 

 Mi = M- 



At frequencies where the attenuation begins to increase, other op- 

 timimi values of w/t can be obtained, and the ratio will depend upon 

 what top frequency is considered. 



In a practical case a fill factor of unity will probably not be used. 

 A little space in the center will be made available for a solid conductor for 

 energizing or other purposes. 



Fig. 3 — Conventional coaxial cable. 



* Loo. cit. 



