ONIvWAY TRANSMISSION EXPERIMENTS OVER THE SEA 



23 



tiiiK.'. This coudilion euinpliciitos any llieureiiwil treat- 

 ment of the problem. 



The analysis thus far indicates that the variation in 



tn 6 



u 



a 



is' 



o <•♦ 



lU I- » 

 m ^ 



100 MC 





-30 -20 -10 10 20 -30 -20 -10 



OB ABOVE FREE SPACE 



Figure 6. Xumber of modes trapped. 



actual index of refraction through the layer has to 

 be used to explain the magnitude of the fields observed 

 on the one-way link. When the layer is thin the longer 



400 



D. STRATUM THICKNESS 

 A' WAVELENGTH 



Figure 7. Reflection ratio. 



700 



wave radiation might be expected to leak more readily 

 through the stratum and thus show less trapping at 

 the greater distances. Aetuallj^, the vertical sections 

 of field strength taken in the plane (Figures 8 and 9) 



■show rather large fields above the layer at the lunger 

 ranges. This might be interpreted iu favor of the 

 modified index over the measured index of refraction. 

 However, on the other hand it could be diffraction due 

 to the low elevation of the layer, or a storage field when 

 the actual index of refraction is used. A study of the 

 attenuation along the path should clear up this last 

 point. 



2 1.2 Vertical Field Strength Sections 



Two typical sets of field strength data are shown 

 in Figure 8 and in Figure 9. Figure 8 illustrates a 

 case for ■\\liieh there was definite trapping predicted 

 by the modified index criterion. It will be noted that 

 the 63-mc radiation shows little variation in field 

 strength with altitude. In most cases it shows e^'en 

 less variation with time at a given altitude. The higher 

 frequencies show more variation of signal with alti- 

 tude, and the field strength distribution varies more 

 with time. This variation with time is iu complete 

 agreement with the data taken on the San Pedro to 

 San Diego one-way link. The minimum field above the 

 minimum point of the M curve, as predicted by ray 

 theory, is certainly missing at the lower frequencies 

 and rather uncertain at the higher frequencies. 



Figure 9 in the following paper shows field strength 

 sections for a day when the reflecting layer was at an 

 elevation of around 3,000 ft. Here the solid line rep- 

 resents the first run and the dotted line the repeat sec- 

 tion. The time interval between sections was from an 

 liour to an hour and a half. The sections at about 75 

 miles from the laboratory show results compatible with 

 the one-way link data. At low elevations the lower 

 frequencies show stronger fields than the higher fre- 

 quencies. This again is in agreement with reflection 

 theory. 



SUMMAEY 



The modified index of refraction, in conjunction 

 with ray theory, is a poor criterion for trapping. 

 Strong fields are observed well below the horizon Avhen 

 the observed modified index would indicate that no 

 trapping would be taking place. The vertical distribu- 

 tion of field strength for the lower frequencies appears 

 to have little in common with the fields predicted by 

 ray tracing methods where the energy is assumed to 

 follow the rays. 



There is no apparent correlation between the experi- 

 mental data and the siniifle wave guide analysis. 



