696 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1951 



to insulate the coaxial standards in the form of spaced cylinders, the com- 

 posite dielectric constant is increased only about 0.4% over theoretical for 

 air. Errors in determining the constant of the expanded material are thus 

 scaled down by a factor of nearly 10: 1 in their effect on the composite con- 

 stant. The figure quoted for dielectric constant in Table III was obtained 

 by adding incremental amounts of dielectric to a 12" length of standard 

 coaxial and plotting the measured capacitance as increments above the 

 computed capacitance for air dielectric, as in Fig. 2. 



The distributed conductance, G, is derived from the power factor of the 

 dielectric which, in the case of any reasonably good material, is so small 





 (AIR) 



0.2 0.3 0.4 



INSULATION IN GRAMS 



0.5 



0.6 



Fig. 2 — Data for determination of dielectric constant of expanded polystyrene. 



that experimental determination is subject to large error. The apparent 

 conductance G' from (4) is the value at the input terminals of the coaxial. 

 An important feature of the standards is that G' becomes independent of 

 G at high frequencies and, therefore, it is desirable to reduce G (representa- 

 tive of the loss in the dielectric) to as small a value as possible in order that 

 G' may become independent at the lowest possible frequency. This is ac- 

 complished by the use of expanded polystyrene as the dielectric of the stand- 

 ards. 



Polystyrene is of such molecular structure that it is not hygroscopic. 

 However, under certain conditions, water vapor may condense in cells of 

 the expanded material in sufficient amount to increase the dielectric con- 



