Crosstalk Between Coaxial Conductors in Cable 



By R. p. BOOTH and T. M. ODARENKO 



The available literature on crosstalk between coaxial conductors 

 in contact makes it clear that the presence of any other conducting 

 material in continuous or frequent contact with the coaxial outer 

 conductors simply reduces the coupling per unit length without 

 altering the law of crosstalk summation with length. 



When the conducting material is insulated from the coaxials, 

 as in the case of quads and sheath in coaxial cables, the situation is 

 more complicated. Instead of simply reducing the coupling per 

 unit length the quads and sheath, with the outer conductors for a 

 return, provide a tertiary circuit in which interaction crosstalk can 

 take place between elementary line sections. The summation with 

 length for this type of crosstalk is quite different from that between 

 two coaxials in contact and therefore the combined summation is 

 obviously more involved. 



Tests on sections of a five-mile length of coaxial cable were made 

 at Princeton, New Jersey, in the latter part of 1937 and early in 

 1938 in order to obtain experimental verification of the manner in 

 which the quads and sheath affect crosstalk summation with length. 

 It is shown that the crosstalk component due to the presence of the 

 sheath and quads opposes the component which is present between 

 two coaxials in free space so that the resultant crosstalk is con- 

 siderably lower than would be computed ignoring the tertiary 

 effects. 



Introduction 



In spite of the geometrical and electrical symmetry of the coaxial 

 circuit and the excellent shielding properties of the outer conductor, 

 a part of the electromagnetic energy escapes from the circuit through 

 the outer conductor and sets up an electromagnetic field in the space 

 around it. Any circuit, be it even another coaxial placed in this field 

 will absorb a part of the energy stored in the field and deliver it to the 

 terminals of the circuit in the form of an unwanted or interfering 

 current — the crosstalk current. The magnitude of this crosstalk 

 current depends on a variety of factors, such as the physical character- 

 istics of the conductors and of the intervening space, the frequency 

 and the length of the circuit. 



Expressions for two important cases of crosstalk between two coaxial 

 circuits in free space, namely, the so-called "direct" crosstalk with the 

 outer conductors in continuous contact and the "indirect" crosstalk 

 with the outer conductors insulated from each other, were determined 



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