2 . GROUND WAVE 



The ground wave travels over the transmission path near the earth's 

 surface . Three types of ground waves may be propagated, as illustrated in 

 Figure III -5. 



TRANSMITTER 



FIGURE III -5 



GROUND -WAVE PROPAGATION 



Because of the curvature of the earth's surface, only the direct and 

 the ground -reflected waves enter into consideration at frequencies above approxi- 

 mately 30 mc. At these frequencies the useful range is essentially line of sight. 

 For the frequencies of interest here, surface waves are the most important of 

 the ground waves . 



Surface waves are transmitted horizontally, and the earth's atmos - 

 phere and ground conductivity cause the propagated wave to remain near the 

 earth's surfaces and follow its curvature . The rotation of the vertical plane of 

 polarization of the wave front, due to the finite conductivity of the earth, gives 

 rise to phase errors . This type of error is particularly important in long-range 

 navigation problems where very low frequencies (VLF) are used . 



During propagation, the ground wave becomes more attenuated than in 

 free space . Surface wave attenuation increases as the frequency is increased, 

 until at a frequency of about 30 mc, the surface wave is almost totally absorbed. 

 At frequencies above 30 mc, only sky-wave transmission remains for long-range 

 navigation and communication purposes . 



19 



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