TRANSMISSION CHARACIERISIICS OF AN OVERWATKR PAl H 



17 



rcacli liie high Nahic uiie might expect oii the basis of 

 au atteimation proportional to l/Il but I'atlier remains 

 near tlie ordinary free space value. This results from 

 the fact that coincident with the reduction in attenua- 

 tion which occurs with trapping, there is also an ap- 

 preciable reduction in the height-gain function within 

 the duct, as shown in Figure 14. The balance of the 

 two couutereffeets prevents extreme increases in field 

 strengths at all ranges of practical interest for micro- 

 waves. 



To sum up. the 117-mc transmission is noticeably 

 affected both by thick surface ducts or substandard 

 layers and by elevated superstandard layers up to 1,500 

 ft altitude, which need not neces-sarily overhang. The 

 ^\'ave theory for elevated layers is not yet sufficiently 

 advanced to permit drawing definite conclusions. As 

 for surface phenomena, an excellent qualitative agree- 

 ment has been obtained between theoretical and ob- 

 served results. There has been no indication of a need 

 to revise the formula used for computing the modified 

 index of refraction. 



Following presentation of this paper the following 

 data ^^'ere presented on a similar experiment^ made on 

 an over-water path between San Pedro and San Diego, 

 California. Transmitting and receiving antennas were 

 at 100-ft elevation, with continuous wave transmission 

 conducted from the San Pedro end of the link simul- 

 taneously on 53, 100, and 550 mc. The typical non- 

 standard condition in this area is produced by dry air 

 aloft subsiding over moist air near the sea surface. 



B Index Modified Relative To 4a/ 3 



Figure 15. Modified inde.\ B. 



This gives rise to a sharp discontinuity in the index 

 of refraction distribution with altitude at some eleva- 

 tion above the earth. 



In analyzing the data from this experiment, the 



index of refraction modi lied for -ia/o, instead of the 

 modified index M, was used. The new modified refrac- 

 tive index, B, thus obtained is shown in Figure 15. 

 The pertinent factors for reflection considerations are 

 as follows : /( , the height of the layer above the earth ; 

 AB, the total change in index through the layer; and 

 D, the thickness of the layer. For moderately high 

 layers, D is much less than h. 



IMaximum field strength measured during the hour 

 in which a meteorological sounding was taken is 

 plotted against height of the layer above the ocean. 

 The data are segregated into groups for different 

 ranges of change in index of refraction through the 

 layer. Figure 16 shows the data for changes in AB 

 between 30 aiid 40 by means of crosses ; for AB of 40 

 to 50 with dots; and for AB of 50 to 60 with circles. 



If reflections are assumed to take place midway 

 between the transmitters and receivers, the field 

 strength may vary roughly as shown in Figure 16. 

 The height-gain function holds the lower frequency 

 fields down when the layer is low, whereas the added 

 adx'antage in the reflection coefficient produces rela- 

 tiAely stronger fields for the lower frequencies Avhen 

 the layer is high. A complete report will be made soon 

 on the experimental data and its relationship with 

 this consideration. 



It was further pointed out that maximum observed 

 field strength need not always coincide with complete 

 trapping. The experimental evidence that for a given 

 frequency the signal strength over a low fixed path 

 first increases as the height of the base of the M in- 

 \ersion increases and then decreases does not neces- 

 sarily contradict the wave guide theory. When the base 

 is low, transmission is by means of well-excited modes 

 with low attenuation. As the base height increases, the 

 attenuation of some of the modes decreases and the 

 field strength therefore increases. Further increase 

 in base height results in well-locked modes which are 

 more and more difficult to excite. It is then that the 

 most effective mode is one which leaks sufficiently to 

 be excited by a transmitter outside the duct and yet 

 does not leak sufficiently to be strongly attenuated 

 before reaching the receiver. As the height continues 

 to increase, modes which can be excited are all strongly 

 attenuated, and the ones which are only slightly at- 

 tenuated cannot be excited. Thus signal strength ul- 

 timately decreases with increasing height. 



