124 BELL SYSTEM TECHNICAL JOURNAL 



which can be employed on wires without Hkewise running into large 

 amplification requirements. For example, the loss at 100,000 cycles 

 for a distance of 200 miles (320 km.) is about as great over wires as 

 the minimum loss which it is theoretically possible to obtain over 

 radio. 



Referring now to the attenuations for longer distances, as given in 

 Fig. 3, it is of interest to note that for distances of the order of 2,000 

 to 3,000 miles (3,200 to 4,800 km.) the lower radio frequency curves 

 cross the 1,000 cycle wire curve, meaning that for these distances 

 it is possible for radio transmission to be as efficient as straight tele- 

 phone transmission. The wires present to carrier frequencies for 

 these long distances losses which are generally greater than prevail 

 for radio. 



These attenuation relations cannot be directly converted into an 

 economic comparison, however, for the economies depend not upon 

 the attenuation itself but upon, among other factors, the cost in- 

 volved in overcoming the losses by means of amplification; and this 

 cost in turn depends largely upon the extent to which the amplifica- 

 tion can be applied at weak powers, as by the frequent application 

 of telephone repeaters. By applying repeaters every few hundred 

 miles in the wire case, the attenuation is prevented from piling up 

 and the amplification is handled at relatively weak and therefore 

 economical power levels. This brings us to the point of requiring 

 that the attenuation values given above, be considered in reference 

 to the amplification and power required to overcome them and yield 

 the necessary volume of transmission at the receiving end over and 

 above interference. 



Interference and Its Effect Upon the Transmitting Power 



Required 



In both the radio and wire cases there is always present in the 

 transmission medium a certain amount of stray wave energy which 

 tends to interfere with the proper reception of the message-carrying 

 waves. It is necessary that the communication waves arrive at the 

 receiving end of the system with such power as to be large compared 

 with the interfering waves — by a factor determined by the type and 

 grade of communication involved. Inasmuch as the stray energy 

 always has some finite value, this requirement of freedom from inter- 

 ference will determine in the radio case as well as in some types of 

 wire transmission the minimum wave power required at the receiving 

 end of the transmission system. 



