TROPOSPHERIC: PROPAGATION AND RADIO METEOROLOGY 161 
conditions, however, are due to changing meteoro- 
logical conditions. A characteristic type is an irregular 
fluctuation in signal strength on a time scale of the 
order of a minute and with an amplitude rarely 
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. Figure 35A shows that the signal 
strength is considerably above the 95-db average . 
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. 
OB BELOW 1 WATT 
A STEADY SIGNAL AVERAGE BENE 
4, 7 125 FT h,=50 F 
NWN NN 
| \SIMPLE SURFACE TRAPPING \\ ss \ 
Eze) 
a ae || | a, 
| SRE ITTY (| BSE | 
(e/a / = 9/ TIMED 
j2ommnnn/ S20 87 
B HIGH AVERAGE SIGNAL WITH DEEP FADING 
h,= 125 FT haz 25 FT 
DOB BELOW 1 WATT 
DB BELOW 1 WATT 
Cc LOW SIGNAL 
hy = 125 FT hg= 50 FT 
Figure 35. Signal strengths for 1 = 10 cm over sea. 
Duct FaprEs 
A duct is normally accompanied by fades in the 
signal strength of large amplitude (up to 30 db) and 
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. 
BLackoutT 
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 Ib 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 14 
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 
longer time scale (see Figure 36). It appears that 
similar effects are likely to occur during thunder- 
storms. 
INPUT 
DB ABOVE 1pV RECEIVER 
TIME GMT 
Ficure 36. Effect of a front on signal strength (Hasle- 
mere-Wembley Link, England). 
