ijh ins ii i:x pick i mi: NTs 



54 



5. Low level ocelli's commonly, hut nut always in 

 conditions of fog or low visibility. 



6. Low signal level is usually observed at the 

 passage of warm fronts and high level at the passage 

 of cold fronts. 



7. Generally speaking, high signal level tends to 

 occur in periods of anticyclonic weather. 



A typical record of signal strength for 9-cm waves, 

 representing hourly mean values for a month, is 

 shown in Figure 1. These records are from two 



Match and the General Electric Laboratories at 

 Wembley. The wavelength is in the 10-cm band, 

 and transmission, monitoring, frequency control, and 

 recording are fully automatic. The path is optical 

 except for some houses and trees near the receiver 

 which introduce a diffraction loss estimated at 30 

 dl). As is generally the case with paths that. are 

 optical or nearly so, the fluctuations of received 

 intensity are far less than in the case of long non- 

 optical paths. Figure 2 shows a record for one month 



I 2 3 



5 8 7 



9 10 II 12 .3 14 15 |£ 17 is 19 20 21 22 23 24 23 26 27 28 29 30 



I 2 3 13 5 7 



Figure 1. Signal strength in decibels above 1 fiv receiver input. S band hourly means, June 1944. (Irish Sea experiment.) 

 (C = cold front, W = warm front, = occluded front.) 



links of the short path, both nonoptical. Important 

 meteorological phenomena, especially passage of 

 fronts, are shown at the top of the diagram. W indi- 

 cates warm, C cold, occluded. Note in particular 

 the standard and the free space level indicated on 

 the lower record and the free space level on the 

 upper. The standard level for the latter would be 

 about 33 db below the zero line. This record, which 

 is by no means exceptional, gives a fair idea of how 

 vastly the signal exceeds the magnitude calculated 

 for standard conditions. At the same time it shows 

 the highly irregular character of these phenomena 

 and the difficulty of correlating them in a simple 

 way with the weather or other conditions. 



Overland Path 



An experimental overland path 38 miles has long 

 been operated in the neighborhood of London between 

 the Admiralty Signal Establishment at Whitwell 



Figure 2. Whitwell Hatch-Wembley path, March 1944. 

 S band hourly mean intensities in decibels above 1 /uv 

 receiver input. 



in 1944. The large diurnal fluctuations in amplitude 

 with maxima above normal in the early morning 

 hours occur in the beginning of the month and at 

 several occasions later, especially from the 21st to 

 the 26th. These are related to weather conditions 



