TWELVE-CHANNEL OPEN-WIRE CARRIER SYSTEM 



379 



Noise 



Observed external sources of noise in J systems are atmospheric 

 static, dust storms, radio stations, power line carrier and power supply 

 systems. 



Of these possible sources the more important will usually be atmos- 

 pheric static which will be greatest during the summer months. In 

 regions where dust storms occur, their effects are expected to exceed 

 that of atmospheric static but will be more likely to occur during the 

 winter and early spring. 



The following table shows values of noise at 140 kilocycles, caused 

 by atmospheric static, found at the open-wire line terminals of one 

 repeater section; the values are those which it is expected will be 

 exceeded during one per cent of the summer season extending from 

 May to September. If the repeater spacings shown were used, the 

 total static noise in the top channel at the end of a circuit with 20 

 repeaters would be 20 db above reference noise at the — 9 db level. 

 However, other factors such as ice may require the use of shorter 

 spacings. 



* Above reference noise, 10 ^ watt at 1000 cycles. 



Line Impedance 



As mentioned previously in the discussion of crosstalk, it is important 

 that the line impedances be matched closely and large irregularities be 

 avoided. Because of the different wire sizes and pair spacings, a wide 

 range of open-wire line impedances may be encountered. Novel 

 construction arrangements and the development of new lead-in circuits 

 have made it possible to secure a reflection coefficient of about five 

 per cent at the junction between the open-wire pair and the toll 

 entrance and office equipment at the highest transmitted frequency. 



The transposition arrangement and wire spacing of a pair affect the 

 smoothness of its impedance because they affect the reactions between 

 circuits which cause absorption eff'ects. The marked improvement 

 which can be obtained by proper design is illustrated by comparison of 

 Curves A and B of Fig. 11. Curve A shows the impedance of a 



