EXPERIMENTS AT THE EASTERN COAST OK THE U. S. 



63 



rather closely similar to eaeh other, and the graphs 

 presented here may be taken as being characteristic 

 of all of them. 



Figure 3 shows the signal types observed at the 

 microwave frequencies, S band and X band. The 

 first type is well above the standard level with high 

 signal on the average. It has roller fades with periods 

 of from 2 min to an hour or so which may go down 

 to the minimum detectable level. These periods are 

 generally shorter at any time on the X than on the 

 S band. When this type of signal was present on the 

 S band, it was almost invariably present on the X 

 band and on both the short and long paths. It always 

 occurred simultaneously on the high and low receivers 

 at any frequency. 



The second type is high and steady at anywhere 

 from 5 to 30 db above the standard, generally higher 

 on the X than on the S band. Most of the time this 

 type occurred simultaneously on both bands, but 

 there were some occasions when the S-band signal 

 was of the high and steady type while the X one 

 was of the first type, high with roller fades. 



The third type of signal is about standard and 

 fairly steady which may be a limiting case of the 

 high and steady variety. It does not necessarily occur 

 on both frequencies and on both high and low 

 receivers at the same time. 



The fourth type is standard on the average, with 

 scintillations of more than 10 db. The reason for the 

 difference between this and the preceding type has 

 not yet been established. The scintillations may 

 occur on either the S or X band while at the same 

 time the other signal is steady. 



The fifth type, known as "blackout," is far below 

 standard and shows strong scintillations. In general 

 it occurs simultaneously on both frequencies, both 

 paths, and on both high and low receivers. 



Figure 4 shows a similar set of signal types as 

 observed with 256-cm waves. These are distinct 

 from those observed at the microwave frequencies 

 not only in appearance but also in times of occur- 

 rence. In general no relation has been found to exist 

 between the signal type at this frequency and that 

 observed simultaneously on S or X band, although 

 'on rare occasions such a relation is indicated; the 

 type may remain constant on one frequency and 

 change on the other. Steady signal is most frequent 

 at 256 cm, but the other types shown also occur 

 fairly often. Variations of 30 to 40 db overall take 

 place, and the variations may be fast or slow. 



A statistical study of the frequency of occurrence 



of various signals reveals some rather interesting 

 features. Table 1 shows the frequency of occurrence 

 of above standard, standard, and below standard 

 types on the S and X bands during three typical 

 weeks in the summer of 1944. In these statistics the 

 range of the standard signal was taken as +5 db 

 for the S band and ±10 db for the X band. The 

 behavior of the K-band signal is quite similar to 

 that of the other two. 



Table 1. S and X bands, July and August. 



Date 



Per cent of 



time above 



standard 



Per cent of 



time below 



standard 



Per cent of 



time 



standard 



July 10-16 

 Aug. 21-27 

 Aug. 28-Sept. 3 



63 



97 

 80 



36 

 3 



15 



As the season progressed into the fall, standard 

 signal became more common and substandard signal 

 less frequent especially in the S band. This is shown 

 in Table 2. 



Table 2. S and X bands, September and October. 



These statistical results are characteristic of the 

 over-water path near a coast used in the experiments 

 of the Radiation Laboratory; and, while the signal 

 types shown in Figures 3 and 4 are about the same 

 in overland paths, the relative frequency of incidence 

 for the various types is quite different. This frequency 

 depends not only on the location of the path but, 

 also as shown above, on the season. A more detailed 

 analysis shows that it also depends on the particular 

 weather situation, which may prevail for periods of 

 several days or longer. 



It has been mentioned before that the signal 

 patterns on the S and X bands and those on the 

 high and low receivers are closely parallel. Figures 5 

 and 6 show these correlations graphically; the first is 

 between the S and X bands and the second is between 

 the high and the low S-band receivers. In contradis- 

 tinction there is practically no correlation between 



