CROSSTALK BETWEEN COAXIAL TRANSMISSION LINES 151 



very good. Only equation (19) must not be pushed to its absurd 

 implication, that for long enough transmission lines most energy will 

 eventually travel via the disturbed line. The true limiting condition is 

 that the energy will ultimately be divided equally between the two 

 lines. 



Crosstalk Via an Intermediate Circuit 



The simplest case of the coaxial conductor system where the only 

 crosstalk present is of the direct crosstalk type, as considered in the 

 previous section, is the triple coaxial conductor. The mutual coupling 

 in this case is due only to the transfer impedance between two circuits, 

 as there are no other physical circuits involved. The case of two single 

 coaxial conductors, the outer shells of which are in continuous electrical 

 contact or strapped at frequent intervals, approximates the condition 

 for the direct crosstalk if the system is sufficiently removed from any 

 conducting matter. When two single parallel conductors in free space 

 do not touch, an extra transmission line, an "intermediate circuit," is 

 present consisting of the two outer shells of the coaxial conductors. 

 Even two conductors, the shells of which are electrically connected, 

 will form an intermediate circuit consisting of the outer shells and the 

 other parallel conductors. 



The voltage impressed on the disturbing coaxial circuit induces 

 currents' and voltages in the intermediate circuit, which now acts as a 

 disturbing qi^rcuit for the second coaxial circuit, thus causing crosstalk. 

 We shall rrow consider the near-end and far-end components of this 

 indirect type of crosstalk. 



Indirect Near-End Crosstalk 

 Let us consider a system shown in Fig. 3. The circuit (3) is the 



Z\ 



Z3 



T 



k I 



P-(z,3dx) 



■ X »-| dx 



(1) Ee" 



C3J 



p q 



^^Usgdyf 



(2) 



-J dyk- 



Fig. 3 — Indirect crosstalk between two coaxial pairs. 



Yf 



Z| 



•Z3 



