PROPORTIONING OF CIRCUITS FOR ATTENUATION 249 



It is well at this juncture to understand the function of shielding 

 in a high-frequency transmission circuit. Such shielding serves one or 

 both of these purposes: {a) keeping interference due to external sources 

 from entering the circuit, and {b) preventing the circuit from causing 

 interference in external circuits. The shielding may either supplement 

 or completely replace the use of electrical balance to reduce inter- 

 ference. The design of shield, that is, its construction, material, 

 thickness, etc., is determined by the degree of shielding required and by 

 considerations of mechanical performance and cost. The degree of 

 shielding needed depends in turn upon such factors as the type and 

 length of circuit, the nature and frequency of the signals to be trans- 

 mitted, and the magnitudes of external interference. These interesting 

 aspects of shield design, some of which have been dealt with else- 

 where,'' -• ^ will not be discussed here. 



Attention will rather be directed to an intriguing property of any 

 individually shielded circuit, namely, that, for given conditions of 

 design, there always exists an optimum proportioning or configuration 

 which makes the transmission efficiency of the circuit a maximum, or, 

 in other words, makes the attenuation a minimum. One such condi- 

 tion of design which may be imposed is that the cross-sectional area 

 enclosed within the shield is to be a constant. In what follows, 

 determination will be made of such optimum proportioning for a wide 

 variety of types of individually shielded circuits. Since the attenua- 

 tion is generally of outstanding importance in a high-frequency trans- 

 mission line, the results should be not only of theoretical interest but 

 also of practical value. Moreover, the different methods which are 

 used in solving these problems should find further application, both 

 in the many other known problems which must perforce be omitted 

 for lack of space, and in those problems which may be conceived in 

 the future. 



The principal types of individually shielded circuits to be discussed 

 are: 



(1) Coaxial or concentric circuits, in which an outer conductor, which 



serves also as a shield, completely surrounds a centrally disposed 

 inner conductor. 



(2) Shielded pairs, consisting of a pair of conductors which form the 



transmission circuit, these being surrounded by an individual 

 conducting shield. 



The coaxial circuit is unbalanced, and relies solely upon shielding for 

 protection against interference from or into its exterior. In contrast 

 to this is the balanced type of circuit, in which the go and return 



