50 



TRANSMISSION EXPERIMENTS IN ENGLAND 



ticularly IVir tlir .jT-niile paths Al) and BU.- Similar i. W'liile puriods ui' liigli lc\cl are sumt'liiiies char- 

 study of tlie S band and 3y2-meter data will follow, aftorizcd by large gradients of water vapor (soiiiid- 

 Figure 3 shows a plot of hourly mean signal level ings usually made for the first 300 ft, at oue point near 

 for the S-band signal over the links AD and BD for the center of the path), no satisfactory correlation 

 June 1944, with a record of some meteorological fac- has been found between the character of the M curve 

 tors — fronts, precipitation, and fog — with which com- and tlie major \ai-iations in signal level IVir the peri- 

 parison has lieen made. ods which have licen studied. In gcnci'id. as is com- 



I 2 3 4 J • T • • 10 II 12 ,3 l« IS le 17 IS l» 20 21 22 23 24 29 26 27 28 2« 30 



RAIN k-i > 

 -irOC AND - 



rupNTsj 



WCNCAR AREA) 



W CC WC 



POOR VISISII.' 



k 



t 



I llfli-l 



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TtsOM 



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►rRtt SPACt 



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J. 4 ■ J « ■ 7 » « ' 10 ■ II 12 ' 13 ■ 14 ■ 15 H 17 18 19 20 21 22 23 24 25 28 27 28 29 30 



F'iGDRE 2. Signal strength in decibels above 1 fiv receiver injiiit, .June 1944, drawn from hourly mean values. 



It should be emphasized that analysis of the data 

 obtained during this period has not yet been com- 

 pleted, but the following general conclusions may be 

 drawn : 



1. There is general agreement between signal vari- 

 ations for the two paths, though the short-period 

 variations often differ. 



3. Signals are obtained over the 300-mile path 

 only when signals over 57-mile path BD exceed about 

 30 db above 1 fj.\. But if the latter condition is ful- 

 filled the former does not always follow. 



3. There is a marked diurnal variation, when the 

 general level is low or moderate, with high signal in 

 the late afternoon or evening and low level in the 

 early morning. 



4. There is evidence for an appreciable seasonal 

 variation with high level for a greater fraction of the 

 time in summer than in winter or spring. 



5. Low level occurs commonly in conditions of 

 fog or low visibility (e.g., low level on 174 occasions 

 out of 333 on which fog was recorded between Feb- 

 ruary and June 1944). 



6. Low level is usually observed at the passage of 

 fronts (e.g., on 78 occasions out of 106 on which 

 fronts were recorded). 



mon experience for similar paths, high levels tend to 

 occur in anticyclonic periods. 



The general character of the S-ljand signal varia- 

 tions for the four 57-mile paths is illustrated by the 

 plots of hourly mean levels for 5 days in August 1944 

 showji in Figure 3. The range of variation in level 

 increases with the excess of path length over optical 

 range, and there is an obvious similarity between the 

 three nonoptical paths as regards the larger changes 

 in level. This similarity does not extend to the optical 

 [lath AC. which shows signs of an inverse correlation 

 with the nuijor variations of the nonoptical paths. 

 For a standard atmosphere (% earth radius) the 

 rccei\er on the AC path is near the record maximum 

 of the interference field. For fairly small departures 

 from standard (curvature corresponding to, say, 1.0 

 to 1.7 times earth radius) the range of variation 

 caused by interference is quite small, aljout -(-4 to 

 — 7 db relative to free space field: the smallness of 

 the variation is due to the appreciable etfect of diverg- 

 ence of the reflected ray in this case. 



AVhile other factors beside interference with the 

 reflected ray are almost certainly operative in produc- 

 ing variations over the optical path, slow fading with 

 a range of the order of 10 db is quite common; and 



