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BELL SYSTEM TECHNICAL JOURNAL 



RATIO OF DISTANCE BETWEEN CONDUCTOR AXES 

 TO INNER RADIUS OF OUTER CONDUCTOR 



Fig. 8 — Increase in attenuation of coaxial circuit due to eccentricity. 



attenuation ratios plotted as a function of eccentricity, assuming a 

 fixed ratio of conductor diameters and substantially air insulation. 



Temperature Coefficient 

 With a coaxial circuit, as with other types of circuits, the tempera- 

 ture coefficient of resistance decreases as the frequency is increased, 

 due to the action of skin effect, and approaches a value of one-half the 

 d.-c. temperature coefficient.^^ Thus, for conductors of copper the 

 a.-c. coefficient at high frequencies is approximately .002 per degree 

 Centigrade. When the dielectric losses are small, the temperature 

 coefficient of attenuation at high frequencies is the same as the tempera- 

 ture coefficient of resistance. 



Diameter Ratio 

 An interesting condition exists with regard to the relative sizes of 

 the two conductors. For a given size of outer conductor there is a 

 unique ratio of inner diameter of outer conductor to outer diameter of 

 inner conductor which gives a minimum attenuation. At high fre- 

 quencies, this optimum ratio of diameters (or radii) is practically inde- 

 pendent of frequency. When the conductivity is the same for both 

 conductors, and either the dielectric losses are small or the insulation 

 is distributed so that the dielectric flux follows radial lines, the value 

 of the optimum diameter ratio is approximately 3.6. When the outer 

 and inner conductors do not have the same conductivity, the optimum 

 diameter ratio differs from this value. For a lead outer conductor and 

 copper inner conductor, for example, the ratio should be about 5.3. 



