Because the frequency is low, the sky wave is heavily refracted by 

 the ionosphere and is bent back toward earth after penetrating only a very short 

 distance within the ionized layers; thus only a very small attenuation is incurred. 

 The wave may be reflected at the earth's surface and redirected back toward the 

 ionosphere . Because of the limited altitude of the layers and the curvature of the 

 earth's surface, which limits the maximum skip distance, the sky waves propa- 

 gated in this frequency range may travel over distances of 2000 miles or more . 



A small energy loss occurs at each reflection. The path covered de- 

 pends on the distribution of ionization. The field strength at the receiver shows 

 both diurnal and seasonal variations . In general, propagation conditions are 

 more favorable in winter than in summer . 



b. Propagation at Frequencies of 300 kc - 3 mc 



In the medium frequency (MF) range of 300 kc - 3 mc surface -wave 

 propagation is less satisfactory; the wave becomes more attenuated than in the 

 case of long-wave propagation. The range of effective coverage by powerful 

 transmitters is generally less than 500 miles . 



At the medium frequencies the sky wave is refracted less than at the 

 lower frequencies, and a greater penetration into the ionosphere takes place. 

 During daytime hours when the E -layer is low -lying, a large amount of absorp- 

 tion occurs because of the high density of this ionized layer . As a result the 

 sky wave becomes highly attenuated and seldom reaches the ground to complete 

 the first skip . During daylight hours, therefore, sky-wave transmission is not 

 considered reliable, and surface -wave propagation must be used. At night, how- 

 ever, there generally is considerable sky-wave return in this frequency range 

 because of the disappearance of ionization at the lower altitude . At sunset the 

 absorption of the sky wave rapidly decreases, because the density of the low- 

 altitude ionized layer decreases . A few hours after sunset ionospheric propaga- 

 tion as well as surface propagation takes place and continues until dawn . 



During the evening hours this frequency range is characterized by 

 three areas of reception . The surface wave predominates at the relatively short 

 ranges, while at longer ranges the sky wave predominates . At the intermediate 

 ranges both waves have nearly equal strength, and severe interference may re- 

 sult. The interference is in the form of phase distortion, because the sky wave 

 travels varying distances as the height and strength of the ionospheric layers 

 shift throughout the evening hours . Field strength also varies, and signal fading 

 is common. Sky-wave reception is required beyond the intermediate area; re- 

 ception is good but not as reliable as the surface -wave reception at the short 

 ranges . 



21 



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