BRITISH TRANSMISSION EXPERIMENTS 



53 



iiiululatinii', Imilt-u]) eoiuitn'. For stamlavd conditions 

 tlie path is cluar except for tlic last mile where trees 

 and houses i'orni a barrier elevated about ^2 degree 

 above the ray path. This introduces a local diit'raction 

 loss at the receiver which has been estimated roughly 

 at about 30 db. This estimate is necessarily an \\n- 

 eertain one, both because of the complexity of the 

 real barrier (which is approximated as one or more 

 opaque straight edges) and because of possible sea- 

 sonal variations. 



Seasonal variations in general signal level have been 

 observed with a maximum in late summer of the order 

 of 10 to 1.5 db higher than the single winter minimimi 

 recorded so far. An attempt to explain this variation 

 in terms of changes in the horizontal plane diffraction 

 pattern of part of the barrier with varying opacity of 

 the tree background does not appear to be supported 

 by the residts of the past few months (Summer 1944). 

 The mean level for the whole period is, however, close 

 to 30 db below free space (52 db above 1 ;u.v receiver 

 input) and is thus at least of the same order of mag- 

 nitude as the estimated standard level. The unfor- 

 tunate effect of this uncertainty regarding standard 

 level is mitigated to a considerable extent by the fact 

 that a land path of this kind gives an easily definable 

 "general"' level, which is in fact that obtained under 

 "well-mixed" meteorological conditions. 



Further details of the path and a discussion of the 

 results in relation to general meteorological conditions 

 over the path have been given in two National Physical 

 Ijaboratory reports,^"-^' which cover the first year's 

 operation. A further report is in preparation. The aim 

 here is limited to a general description of the type 

 of results obtained, with examples of some character- 

 istic signal records. 



Figure 5 gives a plot of hourly mean level for March 

 1944 which clearly shows the two main characteristics 

 of the signal : the reasonably constant general level 

 and the regular diurnal cycle which occurs with radia- 

 tion nights. The period March 31 to 26 is typical of 

 an undisturbed run of clear nights: note the period 

 of marked substandard signal in the early morning of 

 March 27, indicating that condensation near the 

 ground has reduced the water vapor content there suffi- 

 ciently to make the lapse rate negative. Intermittent 

 rain in bad weather periods usually gives a more vari- 

 able level than cloudy weather with no precipitation; 

 a small rise in level is often observed with continuous 

 rain (direct effects of rain on the equipment have been 

 carefully guarded against and may be assumed negli- 



a I 9 I 10 I II 



J^'i/^A} ' 



^^Ay^ - \M^h 



■^ 



18 , 19 , 20 , 



' 7 ' a ' 9 ' 10 ' II ' 12 ' 13 

 , 22 I 23 , 24 , 25 . 26 , 27 , 28 



l/u^ 



v,f-\f< 



' 17 ' 18 ' 19 '50 ' 21 ' 22 ' 23 '24 • 25 ■ 26 ■ 27 ' 28 ' 29 ' 30 



WHlTWELL MATCH WEMBLEr PATH, MARCH IM4, S-BANO 

 HOURUr MEAN INTENSITIES IN DB ABOVE I/.V RECEIVER INPUT 



Figure 5. WhitwcU Hatch to Wembley path, March 

 1944. S-band liourly mean intensities in decibels above 

 1 idV receiver in]jut. 



gible) and a more marked rise with clear skies in 

 daylight following rain. These effects are readily ex- 

 plicable in terms of changes in water vapor distribu- 

 tion. 



The work of the past 6 months (Summer 1944) has 

 shown a definite correlation of high level at night with 

 temperature in\ersion whether with clear or with 

 variable skies; on the other hand, clear or variable 

 skies with no temperature inversion (e.g., wdtli incom- 

 ing cold air) show no night peak of signal. In general 

 the increase of level on an initially clear night is 

 arrested by the development of low cloud or of fog. 

 Double maxima are often observed in the night peaks 

 (e.g., March 15 in Figure 5). 



The magnitude of the peaks on radiation nights is 

 usually 5 to 10 db; it can occasionally reach 15 to 

 20 db particularly in summer. It seems very probable 

 from the geometry of the path that earth-reflected rays 

 play little part, at least for moderate degrees of bend- 

 ing. It is therefore reasonable to seek to explain the 

 larger variations as resulting from increasing ray 

 curvature. In terms of the rough estimate mentioned 

 above, a change from standard to "flat earth" condi- 

 tions would give an increase in level of the order of 

 10 db, which is a typical figure for the observed rise 

 on an undisturbed radiation night. It is of interest to 

 note that free space level is never reached on this 

 path : the highest instantaneous level reached is 10 to 

 12 db below free space. In other words complete, or 

 nearly complete, reflection regions do not exist at 

 heights of the order of 1,000 ft or more (required to 

 "clear" the barrier) over this path. This is in line with 

 the observed lack of any effect of high inversion on the 

 signal level. 



