3 . SKY WAVES 



The index of refraction does not vary linearly with height. In addition, 

 ionized layers are always present in the upper atmosphere . As a result of these 

 effects, rf energy when propagated at a sufficient angle to the earth's surface is 

 refracted in the atmospheric layers and bent earthward. If conditions are favor- 

 able the rf wave will eventually return to earth at some distance from the point 

 of origin . Upon reaching the earth the wave front may be reflected again toward 

 the ionosphere and the process repeated until all the energy is finally attenuated. 

 (See Figure III -6.) 



Both the altitude and density of the three most important ionized layers 

 vary as a function of the time of day, season, and year . Further variations occur 

 each solar cycle of about 1 1 years . 



RELATIVE IONIZATION DENSITY- 



FIGURE III- 6 SKY-WAVE PROPAGATION WITH REFLECTION 



FROM F2 LAYER 



a . Propagation at Frequencies of 30 - 300 kc 



Frequencies in the range of 30 - 300 kc are known as low -frequency 

 (LF) waves and are only slightly attenuated when propagated as a surface wave. 

 With adequate amounts of power the surface wave can be propagated over dis - 

 tances of 500 nautical miles or more . 



20 



