220 



TROPOSPHERIC PROPAGATION AND RADIO METEOROLOGY 



exceeding 2 db. It varies in intensity according to 

 the state of turbulence in the air along the propaga- 

 tion path. In perfectly calm air the fluctuation is 

 practically nonexistent but becomes quite noticeable 

 in turbulent air. This sort of fading is analogous to 

 the scintillation of the fixed stars or the unsteadiness 

 of the telescopic picture of distant objects occurring 

 especially on warm summer days. The physical 

 explanation for the scintillations is found in the 

 fact that the turbulent motion of the air produces 

 irregular variations in refractive index. The conse- 

 quent irregular bending of rays passing through such 

 a medium produces a patchy distribution of intensity 

 over the wave front. In the case of stellar scintilla- 

 tions the main change in refractive index is caused 

 by fluctuations in air density, and the significant 

 level of turbulence is at an elevation of several 

 thousand feet. For radio waves fluctuations of water 

 vapor density are the chief cause of the scintillations, 

 and the active region is consequently close to the 

 ground. For typical radio scintillations see Figure 

 35A. 



LOW SIGNAL 

 h,, = 125 FT h z = 50 FT 



Figure 35. Signal strengths for X = 10 cm over sea. 



Duct Fades 



A duct is normally accompanied by fades in the 

 signal strength of large amplitude (up to 30 db) and 

 of moderate periods (of the order of 15 min). A 

 detailed theory of this type of fluctuation in signal 

 strength is not available. When the duct is fully 

 developed, there is a large-scale deviation from 

 standard conditions with regard to mean field 

 strength. If, in particular, both transmitter and 

 receiver are situated inside the duct, there is a great 

 increase in received field strength. Suppose, however, 

 that for some reason the duct does not function 

 according to the simple theory. The field strength at 

 the receiver may then drop to the value correspond- 

 ing to standard conditions. The observed fades 

 exhibit just this characteristic in that they consist 

 in sharp drops of signal strength down from a mean 

 upper level. The conditions are illustrated in Figure 

 35, which shows three records obtained for a 22-mile 

 path over sea. Figure 35A shows the normal record 

 on a calm day when the only disturbances are due 

 to scintillations. The record shown in Figure 35B, 

 on the other hand, was obtained for a condition of 

 simple surface trapping, with transmitter and receiver 

 inside the duct. It will be noted that the signal 

 strength is considerably above the 95-db average as 

 given in Figure 35 A. 



Duct-type fades have been observed over land as 

 well as over sea and appear to form a characteristic 

 feature from which the presence of superrefraction 

 may be inferred. 



Blackout 



Figure 35C shows a fade in which the signal level 

 is far below average and which for this reason is 

 called "blackout." This type is liable to occur when 

 warm, moist air is cooled from below (see the sub- 

 standard M curve lb in Figure 20) and is often 

 correlated with fog. The main irregularities in signal 

 strength are again on a time scale of the order of 34 

 hour; the amplitude of variation is smaller than in 

 the preceding case and rarely exceeds 10 db. 



Fronts and Thunderstorms 



On several occasions marked variations in signal 

 strength have been observed when fronts pass 

 between the transmitter and receiver. The passage 

 of the front itself is marked by very rapid and deep 

 fluctuations, followed by less violent changes on a 



