UL TRA-SHOR T \VA\ 'E PROP A GA TION 



147 



Even with vertical polarization, the reflection coefficient is also 

 approximately — 1 for transmission over smooth land with near- 

 grazing incidence. The same inverse square tendency is therefore to 

 be expected with vertical polarization under these conditions. 



Non-Optical Paths 



We shall now discuss one type of non-optical path which is of interest 

 both because it occurs frequently and because on the basis of the 

 assumptions made it is amenable to approximate calculation. It is 

 represented in simplified form in Fig. 15. 



T and R are located on opposite sides of a hill, M, and the distances 

 TM and TR are great compared with the altitudes involved. The 

 low land on both sides of the hill is comparatively flat, though not 



Fig. 15 



necessarily coplanar. As previously discussed, the magnitude of the 



coefficient of reflection to be expected will be close to unity ^^ for many 



conditions likely to be met and the phase change will be not much 



different from 180°. In other words, the wave reflected from the 



ground between T and M will appear to have come from a negative 



virtual image, T' . The disturbance above the mountain, M, will be 



made up of two components corresponding to the antenna and its 



negative image. In passing from the region above M to the receiver, 



R, each of these components is broken down into two new components 



due to reflection between M and R. One of these proceeds directly 



to the receiving antenna. The other proceeds indirectly, being 



reflected by the intervening ground; it may be thought of as traveling 



-' An exception of this occurs in the case of vertical polarization over surfaces 

 having appreciable conductivity, such as sea-water. Recent experimental work not 

 described in this paper indicates that the assumption is incorrect over land at fre- 

 quencies considerably higher than those of the present experiment. In such cases 

 the theory is still tenable if appropriate constants are used. 



