APPLICATION OF WIRE TRANSMISSION TO RADIO 127 



where this level leaves off and the original zero level, measures the 

 over-all transmission equivalent of the circuit, shown in this case as 



about logio y^ = 0.57 or about 12 miles of standard cable. This cor- 



■I2 

 responds to a current ratio of about 4, a value ample for good " talk." 

 Of course, in a one-way circuit the terminal amplification can be 

 raised to any value desired. In a two-way circuit, however, a limit 

 in the terminal amplification is imposed by interference between the 

 two transmissions, as will be understood subsequently. 



We may make i.he following useful observations from this curve: 



1. The net transmission equivalent represents the difference be- 

 tween the over-all loss and the over-all gain. 



2. The over-all gain is divided betw^een the transmitting and re- 

 ceiving ends. We should like to throw as much of this amplifica- 

 tion as possible to the receiving end because of the economy 

 with which amplification can be provided at low powers. 



3. The extent to which we can do this, however, is distinctly limited 

 by the fact that the transmission level obtaining at the receiving 

 end in the transmission medium must be held above a certain 

 amount in order to overcome interference. 



4. It is, then, the absolute intensity of the interference which 

 determines the receiving power level required, and in turn this 

 together with the attenuation back to the transmitting station 

 which determines the transmitting power required. 



Thus the two transmission features most fundamentally important 

 in a radio communication system are (1) the interference level and 

 (2) the transmission loss thru the medium. These once given, the 

 other engineering considerations follow naturally. There are an- 

 alogously fundamental factors in wire communication systems. In 

 the latter case, however, the art has advanced to a point where means 

 of controlling the interference level are available, so that the ratio of 

 interference to transmitted power may be made small by decreasing 

 the former rather than increasing the latter. 



Minimum Transmission Levels Obtaining in Practice 



The working value w^hich should be assumed for the ratio between 

 the transmission level of the received signals and the interference, 

 depend upon the type of communication involved, whether it is 

 telephone or telegraph, for example, and upon the grade of service 

 to be given. There is a wide difference between the transmission 

 level which will enable telephonic signals to be barely discerned by an 



