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TROPOSPHERIC PROPAGATION AND RADIO METEOROLOGY 



Experiments over the longer (nonoptical) path 

 from the Empire State Building, New York City, to 

 Biverhead, Long Island (range 70.1 miles), showed 

 much greater diversity in the fading patterns for the 

 different frequencies. On the other hand, observa- 

 tions over the British radio link from Guernsey to 

 Chaldon on 60 mc and 37.5 mc (range 85 miles) 

 showed that if there were marked variations on one 

 frequency similar results were likely to be found on 

 the other frequency. 



Beliability of Circuits 



The reader must be warned that the amount of 

 the fading in the signal strength is not a measure of 

 the performance of radar and communication circuits. 

 These will operate successfully so long as the periods 

 of low signal are relatively short. Neither the scin- 

 tillations of Figure 35A nor the larger dips of Figure 

 35B would seriously affect operation, but a prolonged 

 signal such as in Figure 35C would certainly interfere 

 seriously with communication and radar performance. 



Some quantitative data are available from the 

 transmission path referred to in the previous para- 

 graph. On the optical path, New York to Hauppauge, 

 the range of signal fluctuations increased rapidly with 

 increasing frequency. On 45 mc the "undisturbed" 

 level (the observational equivalent of standard) was 

 21 db below free space with an amplitude of fluctua- 

 tions that very rarely exceeded ±4 db. On the 474- 

 mc circuit the undisturbed level was 3.5 db below 

 free space while the fluctuations varied between 10.5 

 db above to more than 30 db below free space. The 

 level was 5 db or more below the undisturbed value 

 during 0.01 per cent of the time in January and 

 during 0.4 per cent of the time in July. On the 2,800- 

 mc circuit the undisturbed level was — 2 db below 

 free space; the maximum was 12 db above and the 

 minimum more than 25 db below free space. During 

 0.15 per cent of the time the signal was 5 db or more 

 below the undisturbed level in January; the corre- 

 sponding figure for July was 3.6 per cent. The conclu- 

 sion may be drawn from this and similar experiments 

 that over optical paths transmission becomes gradu- 

 ally less reliable as the frequency is raised. 



Over the nonoptical path, New York to Biverhead, 

 the margin of fluctuations was much larger. On the 

 45-mc circuit the undisturbed value was 35 db below 

 free space, the maximum 18 db below, and the 

 minimum more than 50 db below free space. During 

 1.6 per cent of the time the signal was 5 db or more 



below the undisturbed level. On the 474-mc circuit 

 the undisturbed signal was 30 to 35 db below free 

 space, the maximum 10 db above, and the minimum 

 44 db below free space. During 0.47 per cent of the 

 time the signal was 5 db or more below the undis- 

 turbed value. At 2,800 mc the undisturbed signal 

 was 50 to 60 db below free space near the limit of 

 sensitivity; the observed maximum was 13 db above 

 free space, and the minimum could not be observed. 

 In this case the effects of superrefraction were quite 

 pronounced. In January the signal was less than 40 

 db below free space during 6.5 per cent of the time; 

 the corresponding figure for July is as high as 33 

 per cent. 



The reliability of these transmission circuits is 

 shown in Figure 38. Here, both for the optical and 

 nonoptical paths, the percentage of time during 

 which the signal strength was below specified values 

 is plotted for the various frequencies used. The 

 specified values of signal strength, for each frequency 

 and path, are measured relative to the corresponding 

 undisturbed value. The results, which give averages 

 of the performance during July 1943 and January 

 1944, indicate that the reliability increases appreci- 

 ably with decreasing frequency. 



It must be said that the New York area where 

 these experiments were made is not particularly 

 affected by blackout situations, and the results are 

 probably not typical for locations where blackouts 

 are a frequent occurrence. The general nature of 

 these data is confirmed by results of extensive 

 experiments in England and in Massachusetts Bay. 



17.3.10 Scattering and Absorption 



by Water Drops 



As microwave sets have come into general use in 

 recent years the "rain echoes" frequently seen on 

 the scope have attracted attention. The possibility 

 of using microwave radar as an aid to meteorological 

 forecasting and for aerial navigation was early recog- 

 nized and is now being put to operational use. 



At first sight, ground clutter resulting from trap- 

 ping of radiation in a ground-based duct and rain 

 reflections look somewhat alike on the scope of a 

 radar set. At closer inspection differences appear; 

 the cloud pictures are usually more fuzzy and less 

 sharply defined than the echoes received from ground 

 targets. An experienced operator usually has little 

 difficulty in distinguishing rain echoes from echoes 

 of targets or objects at the ground, but occasional 



