APPLICATION OF WIRE TRANSMISSION TO RADIO 



119 



which simply expresses the fact that, as the wave proceeds along 

 the wire, the losses in the resistance of the conductor and in the insu- 

 lation, extract for each mile a certain definite proportion of the volt- 

 age and current which arrives at that point. After traveling (/) 

 miles the original current /i is attenuated down to a value /i«~"' 

 which represents the received current h- This is the same general 

 law of damping as applies to the dying down of the voltage and cur- 

 rent in an oscillation circuit, except that here the damping is with 

 respect to distance along the line rather than time. We are assuming, 

 of course, that the circuit is so terminated as to avoid reflection 

 effects at the terminals — a condition readily met, by making the 

 terminal impedance equal to the characteristic line impedance. This 

 is indicated in the figure by the designations, Z (internal) equals Z 

 (line). A similar relation is taken for the radio case. The " line " 

 impedance is here the antenna radiation resistance while the " termi- 



WIRE AND RADIO TRANSMISSION SYSTEMS 



nal " impedance is the resistance internal to the antenna and the 

 apparatus, assuming resonance; thus R (internal) equals R (radiation). 



We know that in radio there are tw^o distinct causes of the trans- 

 mission loss: (1) that, due to the spreading out of the waves, which 

 is characteristic of non-guided wave transmission; and (2) that due 

 to absorption in the air and earth's surface, which extracts a definite 

 percentage loss for each mile of the radio circuit and which conforms, 

 therefore, to an exponential law similar in its general nature to that 

 of wire attenuation. 



This transmission law, as expressed by the familiar Austin-Cohen 



