TRANSMISSION CHARACTERISTICS OF AN OVER-WATER PATH 



15 



rates" for higher ducts. The 117-mc signal, however, 

 is affected only by ducts considerably more than 100 

 ft high. Similarly, only a thin substandard layer is 



90 



100 



< 110 



120 



130 



140 



•• • 





• M • • • 



A •••• 



• • 



• •• 

 • • • 



• • « 



• • •» 



•• • 



• ••• •• • 



• • —» • 

 t«« ••••• 



' •• •• ••• 



• ••••• 



m» ••• 

 • • ••• • • 



• • •■•• 



• • • 

 • •• 



ALL PERIODS INCLUDED 





100 



80 60 40 



DB BELOW 1 WATT 

 HIGH S BAND 



20 



Figure 10. Field strengths: 117 mc and S band, July 

 31 to August 17, 1944. 



signal level was above standard 49 per cent of the 

 time, standard 38 per cent of the time, and substand- 

 ard 13 per cent of the time. Of the superstandard 

 period 46 per cent has been correlated with elevated 

 superstandard M layers, 36 per cent with thick sur- 

 face ducts, and 4 per cent with situations in which 

 elevated layers and thick surface ducts coexisted. Only 

 14 per cent of the time remains in doubt, and this in- 

 cludes many jDeriods of exceedingly complex meteoro- 

 logical situations for which the analysis was incon- 

 clusive. In addition to correlation of field strengths 

 with M curves, comparisons have been made between 

 measured and theoretical values of field strengths. The 

 theoretical values were calculated on the assumption of 

 bilinear modified index curves, that is, curves made 

 up of two straight-line segments. The M curve is taken 

 to be standard above the joint, and two parameters are 

 used : the height of the joint, or duct thickness, g, and 

 the ratio s of the slope. The straight lines are drawn 

 not in terms of M deficits but to give the best possible 

 fit to the actual M curve. For the range of values of 

 these parameters for which the contribution of the first 

 mode only is of importance the curves of field strength 

 shown in the following two figures are representative. 

 Figure 11 shows the effect of changing duct height, 

 to 500 ft, on the 117-mc field strength for various 

 values of the slope of the lower segment. The field 



100 200 300 400 



DUCT HEIGHT IN FEET 



500 



Figure 11. Theoretical field strength versus duct 

 height, bilinear index, first mode, 117 mc. 



required to affect the S-band signal, but not until the 



layer is rather thick is the low frequency affected by it. 



In the period so far studied (960 hours total) the 



strength is measured relative to free space value and 

 — 33 db is standard, (s = — 3 corresponds to a value 

 of cIM/dh about —100/100 ft ; s = — 2 is —30 per 1 00 



