SYSTEM DESIGN — NEWFOUNDLAND-NOVA SCOTIA LINK 225 



dropped by 4 kc to 20-260 kc, so that the loAvest frequency is the same 

 as on the Atlantic cables. This enables common frequency-generating 

 equipment to be used at Clarenville for the two links and minimizes 

 crosstalk problems. The main transmission bands and the allocation of 

 the five 12-circuit groups are shown in Fig. 5, together with the ancillary 

 channels; the facilities pro^dded are discussed later. 



Submerged Repeaters 



The submerged repeaters emploj^ed are fully described elsewhere/" 

 and it will suffice to note here that they are rigid units, approximately 

 cylindrical in shape, 9 feet long and 10^ inch maximum diameter. The}' 

 are capable of withstanding the full laying pressure in deep water, al- 

 though this of little importance in the present application. 



They are arranged for both-way transmission through a common am- 

 plifier which has two forward paths in parallel, with a single feedback 

 path. The two halves of the amplifier are so arranged that practically 

 any component can fail in one, without affecting the other. 



Power-Feeding Arraiigements 



The submerged repeaters are energized by constant-current dc sup- 

 plies between the center conductor and ground, the power units at the two 

 ends being in series aiding and the repeater power circuits being in 

 series with the center conductor, i.e., without earth connections, as in 

 Fig. 6. This is the only arrangement by which it is possible to control 

 the supply accurately at every repeater, the insulation resistance of cable 

 and repeaters being sufficiently great that the current in the center 

 conductor is virtually the same at all points. The constant-current fea- 

 ture of the supply ensures that repeaters cannot be overrun in the event 

 of an earth fault on the system. 



On the Oban-Clarenville link the anode voltage is derived from the 

 drop across the electron tube heaters. This results in the heaters 

 being at a positive potential with respect to the cathodes, a condition 

 which tends to break down the heater-cathode insulation." In the Ameri- 

 can electron tubes this insulation is very robust and the risk is considered 

 to be negligible, but in the current British electron tubes, which have 

 a much higher performance, the arrangement is undesirable. In view of 

 the much smaller number of repeaters it was possible to derive the 

 heater and anode supplies as in Fig. 6 and thus to reverse the sense of the 

 heater-cathode ^•oltage and also to proA'ide an anode voltage of 90, 

 against 55 in the longer link. 



