BRITISH TRANSMISSION EXPERIMENTS 



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Figure 3. Signal strength in cleciljels above 1 ii\ receiver input. FS; Free space signal. STD: Standard signal. 

 SL: Sensitivity limit. 



a slightly substancltu-d imlex gi'iidient, which would 

 produce a marked decrease in level for the noiioptical 

 paths, would leave the AC path in the neighborhood 

 of the first interference maximum. 



The free space levels (4-8 db al)ove 1 \x\ from A, 

 44 db from B; difference due to different radiated 

 powers at this period; \\'-i db estimated atmos])heric 

 absorption allowed for) are marked on the plots of 

 Figure 3. For all four paths the highest levels reached 

 are in the neighborhood of the free space value ; levels 

 several decibels above free space are occasionally 

 reached during good periods, Ijut they are only rarely 

 maintained for as much as a few hours (e.g., path 

 BD, May 12, 13 and August 5, 6). The long-time 

 average le\'el for path AC should be close to free space 

 level, probably alsout 2 db above. It is actually about 

 5 db below for the first half of August 1944 and 

 about 3 db below over a period of several months. 



While some of this discrepancy may be due to residual 

 experimental error, the fact that it appears to be 

 least during periods of poor transmission between the 

 low stations (e.g., August 10 in Figure 3) may be 

 significant. 



For the nonoptical paths BC, AD, and BD the 

 standard level is 20, 37, and 79 db below free sjiace 

 le\el, respectively, for S band. The corresjionding 

 figures for X band are 28, 53, and 113 db. The stand- 

 ard level is shown in Figure 2, and in Figure 3 for 

 paths BC and AD ; for path BD it is over 30 db below 

 the receiver threshold. While it is rare, during the 

 sunnner mouths, for the level to i-emain near stand- 

 ard for a large fraction of the time (in February, 

 liowever, the AD level was within 2 db of standard 

 for about 25 per cent of the time), the minima of 

 the major signal variations usually lie within about 

 ±5 db of the standard level except (1) during runs 



