22 



TRANSMISSION EXPERIMENTS NEAR SAN DIEGO 



straliun tliickiu'SS U) \va\cleiiglli, D/X. At iionual 

 incidence the reflection coefficient is small, even for 

 D/k-^0; however, such reflections have been oIj- 

 served exi5erimentally.^ At obliqne incidence, for the 



Figure 4. Signal types on one-way link for low in- 

 version (trapping). 



cases where the index of refi'action varies nionotoni- 

 cally through the layer, the reflectiou ratio increases 

 as Z)/A~0. For the modified index type the reflection 

 ratio increases as D/X decreases, passing through a 

 maximum after which it again decreases/ 



Figure 7 shows the reflection ratio as a function of 

 D/X. for various angles of incidence, where here the 

 index of I'efraction is a monotonically decreasing func- 

 tion of height through the layer. The total change in 

 n through the layer is taken to be 60 X 10"" which is 

 the order of magnitude of the changes noted in this 

 area during the summer season. For a given stratum 

 thickness and height above the earth such that the 

 radiation will be incident upon the layer at angles 

 slightly less than the critical angle, the lower fre- 



quencies will be rellected more strongly from the 

 layer. In addition, any deviation of the layer from 

 the horizontal plane will affect the higher frequency 

 I'adiation more than the lower frequencies. This is 

 manifested by the greater fading range of the 547- 

 mc signal as shown in Figure 3. 



Consider the case where the layer is 330 ft thick. 

 Since the angle at which the radiation will be inci- 

 dent upon the layer will depend upon its elevation, 

 it is possible to compare the experimental data with 

 theoretically calculated reflection ratios. In Figure 2, 

 the curves indicate the theoretically predicted varia- 

 tion of field strength as the layer rises. The absolute 

 decibel scale does not apply to the theoretical curves; 

 only the slope is significant. The actual layer thick- 

 ness and the effective chan2;e of the index of refraction 



Figure 5. Signal types for high inversion. Xote the 

 low level of the 547-mc signal. 



through the layer varied around the values used, and 

 so an exact correspondence between theory and experi- 

 ment should not be expected. However, the agreement 

 is fair. In addition, at any given time the reflecting 

 stratum is a warped surface which changes shape with 



