162 TECHNICAL SURVEY 
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Some peculiar effects were observed by transmission 
through fog over an experimental overland radio link 
in England. The effect of a shallow layer of radiation 
fog in the early autumn (September-October) was 
to produce a nearly complete fade-out of signal 
strength which lasted for hours and rose to normal 
as the fog cleared. The explanation of this effect is 
probably the same as in the case of the ‘“‘blackout”’ 
tvpe fades discussed above, indicating that radiation 
fog produces a substandard M curve. Later in the 
autumn (November-December) or winter (January) 
it was found that the effect of fog was quite different. 
In this season the signal strength was increased and 
deep fades appeared which are reminiscent of the 
duct-type fades described earlier. 
FapDING ON DIFFERENT WAVELENGTHS 
Several experiments have been performed in which 
transmitters working on different wavelengths oper- 
ate simultaneously over the same path and the 
received field inténsities are recorded on the same 
chart. Figure 37 shows one such record for the 42.5- 
mile (optical) path from the Empire State Building, 
New York City, to Hauppauge, Long Island, for 
May 14 and 15, 1943, at frequencies of 474 mc and 
2,800 me. It will be noticed that on May 14 up to 
about 5:45 p.m. the two records show a close 
agreement. At 6:00 p.m. violent fading sets in on 
both frequencies, but with great’ diversity in detail. 
Not infrequently the signal on one frequency increases 
while on the other frequency it decreases. About 1:00 
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a.m. on May 16 the disturbance dies down, and the 
initial harmony in the two records is restored. 
Experiments over the longer (nonoptical) path 
from the Empire State Building, New York City, to 
Riverhead, Long Island (range 70.1 miles), showed 
much greater diversity in the fading patterns for the 
different frequencies. On the other hand, observa- 
tions over the British radio link from Guernsey to 
Chalden on 60 me and 37.5 me (range 85 miles) 
showed that if there were marked variations on one 
frequency similar results were likely to be found on 
the other frequency. 
RELIABILITY OF CIRCUITS 
The reader must be warned that the amount of 
the fading in the signal strength is not a measure of 
the performance of radar and communication circuits. 
These will operate successfully so long as the periods 
of low signal are relatively short. Neither the scin- 
tillations of Figure 35A nor the larger dips of Figure 
35B would seriously affect operation, but a prolonged 
signal such as in Figure 35C would certainly interfere 
seriously with communication and radar performance. 
Some quantitative data are available from the 
transmission path referred to in the previous para- 
graph. On the optical path, New York to Hauppauge, 
the range of signal fluctuations increased rapidly with 
inereasing frequency. On 45 mc the “undisturbed” 
level (the observational equivalent of standard) was 
21 db below free space with an amplitude of fluctua- 
tions that very rarely exceeded +4 db. On the 474- 
me circuit the undisturbed level was 3.5 db below 
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May 14,'43 
Figure 37. Simultaneous variations of signal strength with frequency. (Empire State Bldg. to Hauppauge, L. I., N.Y.) 
