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Crosstalk Between Coaxial Transmission Lines 



By S. A. SCHELKUNOFF and T. M. ODARENKO 



The general theory of coaxial pairs was dealt with in an article on 

 "The Electromagnetic Theory of Coaxial Transmission Lines and 

 Cylindrical Shields" by S. A. Schelkunoff {B. S. T. J., Oct., 1934). 

 The present paper considers a specific aspect of the general theory, 

 namely, crosstalk. 



Formulae for the crosstalk are developed in terms of the dis- 

 tributed mutual impedance, the constants of the transmission lines 

 and the terminal impedances. Some limiting cases are given 

 special consideration. The theory is then applied to a few special 

 types of coaxial structures studied experimentally and a close 

 agreement is shown between the results of calculations and of 

 laboratory measurements. 



If the outer members of coaxial pairs are complicated structures 

 rather than solid cylindrical shells, the crosstalk formulae still apply 

 but the mutual impedances and the transmission constants which 

 are involved in these formulae must be determined experimentally 

 since these quantities cannot always be calculated with sufficient 

 accuracy. 



The crosstalk between coaxial pairs with solid outer conductors 

 rapidly decreases with increasing frequency while the crosstalk 

 between unshielded balanced pairs increases. In the low frequency 

 range there is less crosstalk between such balanced pairs than 

 between coaxial pairs but at high frequencies the reverse is true. 

 The diminution of crosstalk between coaxial pairs with increasing 

 frequency is caused by an ever increasing shielding action furnished 

 by the outer conductors of the pairs. 



Finally, crosstalk in long lines using coaxial conductors is 

 discussed and the conclusion is reached that, unlike the case of the 

 balanced structure, the far-end crosstalk imposes a more severe 

 condition than the near-end crosstalk in two-way systems which 

 involve more than two coaxial conductors. 



A COAXIAL line consists of an outer conducting tube which 

 envelops a centrally disposed inner conductor. The circuit is 

 formed between the inner surface of the outer conductor and the outer 

 surface of the inner conductor. Since any kind of high-frequency 

 external interference tends to concentrate on the outer surface of the 

 outer conductor and the transmitted current on the inner surface of 

 the outer circuit, the outer conductor serves also as a shield, the 

 shielding effect being more effective the higher the frequency. 



Due to this very substantial shielding at high frequencies, this type 

 of circuit has been a matter of increased interest for use as a connector 



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