RADIO PROPAGATION FUNDAMENTALS 



599 



10 20 40 60 100 200 400 600 



FREQUENCY IN MEGACYCLES PER SECOND 



1000 



Fig. 3 — Minimum effective antenna height. 



grazing angles over smooth surfaces, its magnitude ma,y be less than 

 unity when the terrain is rough. The classical Rayleigh criterion of 

 roughness indicates that specular reflection occurs when the phase devia- 

 tions are less than about ±(7r/2) and that the reflection coefficient will 

 be substantially less than unity when the phase deviations are greater 

 t han ± (7r/2) . In most cases this theoretical boundary between specular 

 and diffuse reflection occurs when the variations in terrain exceed | to j 

 of the first Fresnel zone clearance. Experimental results with microwave 

 transmission have shown that most practical paths are "rough" and 

 ordinarily have a reflection coefficient in the range of 0.2-0.4. In addi- 

 tion, experience has shown that the reflection coefficient is a statistical 

 problem and cannot be predicted accurately from the path profile.^ 



