RADIO MKTEOKOI.OGY 



221 



longer time scale (see Figure 36). It appears that 

 similar effects are likely to occur (luring thunder- 

 storms. 



o 



50 

 40 

 30 



20 

 10 



19 

 TIME GMT 



20 



Figure 36. Effect of a front on signal strength (Hasle- 

 mere-Wembley Link, England). 



Fog 



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" 

 type 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. 



Fading 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 intensities 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 mc. 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 

 a.m. on May 16 the disturbance dies down, and the 

 initial harmony in the two records is restored. 



Figure 37. Simultaneous variations of signal strength with frequency. (Empire State Bldg. to Hauppauge, L. I., N. Y.) 



