INTRODUCTION 247 



6.2. An Analysis of VHF Field Strength Variations 

 and Refractive Index Profiles 



6.2.1. Introduction 



It is evident from the discussion in section 6.1 that the further develop- 

 ment of radio-meteorological parameters would be assisted by a better 

 understanding of the propagation mechanism on non-optical paths. In 

 particular, the influence of thermal stability on signal level, fading rate 

 and wavelength dependence is an important topic requiring further study. 



The effect of varying meteorological conditions on signal characteristics 

 is especially marked in the case of paths of "intermediate" length, with 

 terminals just beyond the radio horizon. As noted in section 6.1 on such 

 a path the residual scattered field in the absence of stable layers or surface 

 ducts will often be comparable with the weak diffracted field. Further- 

 more, at frequencies up to, say 300 Mc/s (X > Im) relatively strong 

 fields will frequently be observed in conditions favorable to the production 

 of temperature inversions in the first 2 km or so above the earth's surface. 

 It is the purpose of this section to discuss some aspects of radiometeor- 

 ology relevant to this situation, especially the field, strength distribution 

 observed on a 200 km path at frequencies between 72 and 180 Mc/s. 

 The signal characteristics are analyzed in terms of a classification of 

 refractive index profiles, with the objective of clarifying the relative im- 

 portance of different propagation mechanisms and their influence on the 

 measured field strength distribution. 



Table 6.3 lists characteristic profile types, the assumed mechanism 

 associated with each type, and typical meteorological conditions. Se- 

 lected references are given for each category, and special mention may be 

 made here of recent work [28] in France which, to a large extent, unifies 

 and extends earlier analyses based on the separate concepts of "reflection" 

 and "scattering." 



6.2.2. Radio and Meteorological Data 



The analysis to follow is limited to paths between Chicago and Urbana, 

 111. (fig. 6.12), since several years of radio data were available for four 

 separate wavelengths between 1.67 and 4.18 m. Moreover, two radio- 

 sonde stations are located on or near the path, a unique situation in radio- 

 meteorological investigations. Details of the radio paths are given in 

 table 6.4 (in which 6 is the total angle between the horizon rays from trans- 

 mitter and receiver on a 4/3-earth profile). 



The meteorological data were obtained from the simultaneous radio- 

 sonde observations made by the Weather Bureau at Joliet and at 

 Chanute Air Force Base, near Rantoul, 111. The results used were those 

 from the significant levels reported whenever the temperature or humidity 

 departed by ±10 percent from predetermined values. 



