126 BELL SYSTEM TECHNICAL JOURNAL 



the interference relation but merely the absolute amount of terminal 

 amplification required. On the other hand the transmission loss 

 occasioned by getting into the "ether" at the sending end aflfects 

 the interference relation vitally, as we shall see. 



Transmission Levels 



This necessity of having to keep the power of the received waves 

 above the interference level may be visualized by reference to Fig. 4. 

 Here we have what in wire practice is called a "transmission level" 

 diagram. Such a diagram is useful in showing what goes on in the 

 system from the power and interference standpoints. The vertical 

 scale is plotted in terms of the transmission level expressed as the 

 logarithm of the current or field intensity ratios, and the horizontal 

 scale represents progression along the system. For illustration 

 purposes, the presence of interference is indicated at the bottom of 

 the transmission-level scale by the shading. 



Tracing thru the diagram we proceed as follows: 



The point of "zero "level is taken roughly as that corresponding to 

 the power delivered into a telephone circuit by a certain telephone 

 transmitter when spoken into by the average talker, and is here taken 

 to be 0.01 watt. As the voice currents are amplified to power pro- 

 portions in the transmitting station, at the left, the transmission level 

 is greatly increased, as illustrated by the vertical jump in the curve. 

 The amplified voice currents are assumed to be converted by modula- 

 tion into high frequency currents at this high power level and put into 

 the antenna. The high frequency loss in the antenna system is in- 

 dicated by the perpendicular jog in the curve. The drooping-off curve 

 then commences, starting with a point which represents the power 

 usefully applied to the ether in accordance with the expression PR, 

 where / is the antenna current and R is the radiation resistance. 

 The level curve falls off in accordance with the transmission loss 

 curves previously discussed, as it extends across the transmitting 

 medium to the receiving station. It will not do to permit the trans- 

 mission level to fall as low as that of the interference, so I have shown 

 that the transmission reaches the receiving station before dropping 

 down very far into the interference level. At the receiving point a 

 further transmission loss occurs in getting into the receiving antenna 

 circuit, shown by the drop in the curve, but this loss obtains for the 

 interference as well as the desired signals and does not affect the 

 interference ratio. The terminal amplification brings the level up 

 to that required for suitable audition and the difference between 



