128 BELL SYSTEM TECHNICAL JOURNAL 



expert ear and that which is required for a pubUc service communica- 

 tion system which must provide sufificient operating margin to enable 

 the average person to converse with ease and certainty under all 

 ordinary conditions. Under favorable static conditions, the trans- 

 mission level can be permitted to fall to extraordinarily low values. 

 When this condition is accompanied by a substantial reduction in the 

 effective attenuation, which sometimes occurs at short wave lengths 

 especially at night, apparently due to the effective absence of either 

 absorption, then it becomes possible to "get thru" over relatively 

 long distances with powers diminutive as compared with those re- 

 quired for giving a regular service. With these exceptional trans- 

 mission conditions we are, of course, familiar. They are exemplified 

 by the long distances reached at night by the amateurs, as across the 

 Atlantic, and by the hearing of the normally 30-mile (48 km.) Catalina 

 Island system in Australian waters. The transmission curves of 

 Fig. 3 account for these unusual long distance transmissions if we 

 assume that the attenuation due to absorption is eliminated on these 

 occasions by some natural cause. Thus, at 3,000 miles (4,800 km.) 

 the curves for 1,000 kilocycles (300 meters), for example, show that 

 were the absorption eliminated, the transmission equivalent would be 

 improved by the difference between about 10.8 and 5.2 for logio 



h . 



", or 5.6, an improvement equivalent to a little over 100 miles of 



standard cable. The remaining or purely spreading-out loss of about 

 5 units, or 100 miles of standard cable, is then taken care of by the 

 sending and receiving amplification. 



Interference may occur in either or both of two ways — by the 

 interference level rising to a point comparable with the normal trans- 

 mission level at the receiving end of the ether circuit, or by the 

 transmission level of the waves themselves dropping so low, due to 

 excessive atmospheric absorption, as to fall below that of the atmos- 

 pheric disturbances. For reliable transmission it is necessary, there- 

 fore, to deliver normally at the receiving end, a wave intensity suffi- 

 cient to allow for the fluctuations which occur in atmospheric absorp- 

 tion and in the intensity level of the atmospherics. The importance 

 of working to transmission level standards which give an adequate 

 operating margin against interference, for the types of service re- 

 quired, will be appreciated from the foregoing. The following values 

 of minimum transmission levels will be of value to know: 



(a) For carrier wire telephone transmission at frequencies in the 

 tens of thousandths of cycles, the limiting interference may be 



