RADIO AND RADAR TRANSMISSION MEASUREMENTS 



frequencies although on rare occasions sucli a relation 

 is indicated ; indeed the type may remain constant on 

 one and eliange on either of the others. Steady signal 

 is most frequent at 256 cm, but the otlier types shown 

 also occur fairly often. Variations of 30 to 40 db 

 overall take place, and the variations may be fast or 

 slow. 



Statistics 



A fairly detailed statistical study has been made of 

 the S and X signals at the top level. These were 

 chosen because they were available for the longest 

 periods, and because they gave the most reliable re- 

 sults (because of the receiver characteristics the re- 

 lation of the standard to the minimum detectable 

 level was most suitable). The other microwave records 

 gave similar results. As for the 256-cm transmission, 

 the most imj^ortant result was that the signal level 

 was above the mininumi detectable very nearly 100 

 per cent of the time, although fades to this level were 

 fairly frequent. If a choice had to be made of the 

 most reliable frequency for transmission over the 

 circuit, there would be no question in the choice of the 

 longer wavelength. 



The statistics available on K band are very similar 

 to those on X band as far as can be determined. 

 Signal levels less than about 20 db above standard 

 cannot be detected on the K band. 



The study was made of the average signal level on 

 a weekly basis; it showed marked differences from 

 week to week, depending upon the speciilc weather 

 situation. For purposes of the statistics a range of 

 values around the standard was included in the 

 standard signal (allowance for scintillation, tides, 

 etc.). This range was taken as ±5 db on S band and 

 ±10 db on X band, values determined by inspection 

 of the entire record and thought to give comparable 

 results. 



The most interesting result of this analysis was 

 the discovery that standard signal occurs extremely 

 rarely over this path. High signal is most frequent; 

 depending upon the wavelength and the season of the 

 year, substandard and standard signal occur less fre- 

 quently. In the summer no significant frequency de- 

 pendence was observed in the statistics. Some typical 

 weeks gave the figures shown in Table 1. 



As the season progressed to the fall, however, sev- 

 eral related trends became apparent: (a) the increas- 

 ing incidence of standard signal, especially on S 

 band; (b) the increasing incidence of high, steady 



signal, especially on X band, with the level higher 

 above the standard on X than on S; (c) the fre- 

 quency effect on the incidence of above-standard 

 signal indicated in (b) ; and (d) the decreasing oc- 

 currence of substandard signal. These trends are illus- 

 trated in Table 2. 



Table 2. 

 the fall. 



Statistics of S- and X-band transmission in 



Date 



Per cent of Per cent of Per cent 

 time above time below of time 



standard standard standard* 



Sept. 25-Oct. 1 

 Oct. 16-22 



S 

 X 



58 

 80 



15 

 10 



S 76 

 X 92 



•11 

 10 



22 



8 



*By this term is to be understood tlie percentage time in which the 

 signal is ± 2.5 db of standard on S band and =fc 5 db on X band. 



No diurnal effect was found in the signal except 

 under some very sjiecial circumstances. Not only was 

 no such trend apparent upon visual inspection, but 

 also an analysis of the material by 6-hour intervals 

 confirmed appearances. 



Correlations 



In addition to the statistical study, another type 

 of analysis has been made to look for correlations 

 between the variations of signal strength with fre- 

 quency at a given location or with height at a given 

 frequency. Figures 4 to 6 show some typical graphs 

 of such correlations, each point representing average 

 hourly values, for 1 week. Figure 4 shows the varia- 

 tion of the high S- and X-band signal strengths. It 

 is clear that in most cases the two wavelengths change 

 together. This was the predominant behavior through- 

 out the simimer. The notable exceptions are those 

 points where X is high and S nearly standard; this 

 is the frequency effect remarked in the discussion 

 of the high and steady signal which became common 

 in the fall. As will be seen later, this occurs with 

 very low modified index inversions, less than 20 ft 

 high. 



