IMPEDANCE CONCEPT AND APPLICATION 



39 



tion loss. Inasmuch as the radial impedance in air is proportional 

 to the first power of the frequency and in metal it is proportional only 

 to the square root of the frequency, a point is reached beyond which 

 the radial impedance in air always exceeds the radial impedance in 

 metals. Thus the air-to-magnetic metal impedance ratio is less than 

 unity near / = and greater than unity for sufficiently high frequen- 

 cies. Consequently, the absolute value of this impedance ratio is 

 equal to unity at some intermediate frequency at which the reflection 



1.0 



0.1 



01 



u 0.001 



0.0001 



0.00001 



10' 



10^ 



10' 



10- 



10^ 



10' 



10= 



FREQUENCY IN CYCLES PER SECOND 



Fig. 4 — The radial impedances in air, copper and iron at a distance of 2 centi- 

 meters from the axis for cylindrical waves generated by line doublets comprised ot 

 infinitely long electric current filiments. The conductivity of copper = 5.8005 X 

 10^ mhos per meter, the conductivity of iron = 10^ mhos per meter, the permeability 

 of air and copper = 1.257 X 10^^ henries per meter, the permeability of iron = 

 1.257 X 10"'' henries per meter. 



loss will be quite small. ^^ Some typical curves of radial impedances 

 are shown in Fig. 4. The radial impedances in non-magnetic metals 

 are always less than the impedance in air. 



At high frequencies the reflection loss between metals is substan- 

 tially independent of the frequency. At copper-iron boundaries this 

 loss is always high and at copper-air boundaries it increases steadily 

 with the frequency and becomes quite substantial at frequencies as 



" A small reflection loss exists because the impedances have different phases. 



