Chapter 1. The Radio Refractive 

 Index of Air 



1.1. Introduction 



The last few decades have seen a remarkable increase in the practical 

 utilization of the radio spectrum above 30 Mc/s. This, in turn, has 

 focussed attention upon the mechanisms by which these radio waves are 

 propagated. Since radio energy at these frequencies is not normally 

 reflected by the ionosphere, variability in the characteristics of the re- 

 ceived fields is attributed to variations in the lower atmosphere and, in 

 particular, to the radio refractive index. 



The radio refractive index is central to all theories of radio propagation 

 through the lower atmosphere. The atmosphere causes a downward 

 curvature of horizontally launched radio waves which is normally about 

 one quarter of that of the earth. Under unusual meteorological con- 

 ditions, however, the radio energy may be confined to thin layers near 

 the earth's surface with resultant abnormally high field strengths being 

 observed beyond the normal radio horizon. At other times a transition 

 layer between differing air masses will give rise to the reflection of radio 

 energy. In addition to these gross profile effects, the atmosphere is 

 always more or less turbulent, with the result that radio energy is scattered 

 out of the normal antenna pattern. 



It is not the purpose of the present discussion to emphasize the inter- 

 play of various propagation mechanisms, as has been done, for example, 

 in a classic paper by Saxton [1]', but rather to emphasize that the refrac- 

 tive index of the troposphere is of central concern in the propagation of 

 radio waves at frequencies above 30 Mc/s. In what follows, then, the 

 classical equation for the radio refractive index will be considered, a sum- 

 mary of the recent determinations of the constants in this equation given, 

 the errors in the practical use of the equation will be analyzed, and, finally, 

 the normal gross features of atmospheric refractive index structure will 

 be described. 



1.2. Dielectric Constant of Moist Air 



Debye [2] has considered the effect of an impressed electric field upon 

 the dielectric constant of both non-polar and polar molecules, a polar 



1 Figures in brackets indicate the literature references on p. 21. 



