652 



ELECTRICAL METHODS 



[Chap. 10 



than the wave length. Because of the differences in the variations with 

 distance, the radiation in the vicinity of the antenna consists almost en- 

 tirely of space waves, while at greater distances it approaches more and 

 more the character of a surface wave. In addition to geometric spreading, 

 the amplitude decreases because of absorption. The change from space 

 to surface wave occurs more rapidly the shorter the wave and the lower 

 the conductivitv and dielectric constant of the surface and near-surface 



>' F 

 I 



^z. 



I I 



I 1 1 I 



(o) 



^ 



c 



^ 



^^ Cathode Qatf 



I — I Oscillograph Phfes 



(c) 



Fig. 10-17. Measurement of phase shift in soil specimen by cathode-ray method 



(after Ratcliffe and White). 



beds. Thus, the variation of the electric and magnetic fields of the 

 radiation may be expressed by the relations: 



_ ^ ^„ ireJi /o (amp) -Sr / , , -u 

 Er = 120 — — • — -. — ~ ' e ^ (volts cm ); 

 A r(cm) 



H, = 4^^^".^. e-^'C.G.S., 



X r 



(10-17) 



where E is the electric and H the magnetic field, a is the form factor and 

 h the height of the antenna, r the distance from the source, g an absorp- 

 tion coefficient, 7o the current in the antinode of the antenna, and X the 

 wave length. 



If the earth were a perfect conductor, the electrical field would be at 

 right angles and the magnetic field parallel with the earth's surface, and 

 the electrical and magnetic fields would be in phase. For finite conduc- 

 tivity at the surface or at shallow depth, a forward inclination of the 

 electrical wave front is produced, while the magnetic field remains sub- 

 stantially parallel with the surface. In other words, the electrical field 

 now consists of a vertical component Z and a small horizontal component 

 which are out of phase with respect to each other. The ratio of the peak 



