Chapter 17 

 TROPOSPHERIC PROPAGATION AND RADIO METEOROLOGY" 



17 ' FUNDAMENTALS OF PROPAGATION 



111 Significance of Propagation Problems 



hie central problkm of short and microwave 



t; 



propagation (at frequencies greater than 40 to 

 (50 inc) is the determination of accurate coverage 

 patterns for a given transmitter. These patterns are 

 usually calculated from electromagnetic theory and 

 then may be checked by experiment. For communi- 

 cation work the check is simple, namely, the estab- 

 lishment of satisfactory communication. In the case 

 of radar it is necessary to calibrate by time-consum- 

 ing airplane flights. 



Experience has shown that actual coverage is not 

 constant in time but suffers large variations which 

 are caused by the changeable refraction of the at- 

 mosphere. The variations in weather conditions that 

 influence the refraction often are irregular and very 

 rapid, and it is technically impossible to test all these 

 conditions. Coverage diagrams, therefore, must be 

 based on the physical principles of wave propaga- 

 tion, assuming that the characteristics of the atmos- 

 phere remain constant for reasonable periods. These 

 principles are outlined here. 



At the present stage of technical development it is 

 not always permissible to ascribe an observed varia- 

 tion in coverage to changing atmospheric conditions. 

 Variations in transmitter output or receiver sensi- 

 tivity are always likely to be present to a degree 

 sufficient to influence results considerably. In prac- 

 tice it is often extremely difficult to tell these causes 

 apart. In fact, investigations carried out with opera- 

 tional radar equipment make it probable that an in- 

 crease in surface coverage due to favorable conditions 

 of refraction frequently passes unnoticed because of 

 poor set performance. The coverage appears normal, 

 while the set in reality is operating considerably be- 

 low peak efficiency. 



A knowledge and understanding of the effects of 

 weather upon propagation therefore will also be of 

 help in checking set performance in the absence of 

 suitable electrical equipment for measuring output 

 and sensitivity. In dealing with coverage problems 

 this double aspect of propagation phenomena should 

 always be kept in mind. By a suitable analysis of the 



various factors determining coverage, and by an in- 

 telligent understanding of their interplay, the re- 

 sponsible officer may achieve a better control of the 

 operational performance of his equipment. 



In tactical operations and in planning, a knowl- 

 edge of the nonvariable factors affecting propaga- 

 tion, such as dielectric constant and conductivity of 

 the ground or sea, contours of the terrain, vegetation, 

 etc., is equally important. Many problems concern- 

 ing these factors cannot be considered in this manual 



Factors Influencing Propagation 



This volume is confined to the propagation of 

 waves within the troposphere and hence is not con- 

 cerned with ionospheric propagation, which is re- 

 sponsible for the long distance transmission of short 

 waves (high frequency band). The higher the fre- 

 quency above 30 mc, the less frequently radio waves 

 are returned to the earth by the ionosphere. Conse- 

 quently very short radio waves are confined to the 

 troposphere, and the treatment given here does not 

 need to be supplemented by a study of the ionos- 

 phere. Propagation in the lower atmosphere is called 

 "tropospheric propagation" (see Figure 1). 



i Mo«o|ph| re - 



a By Columbia University Wave Propagation Group. 



TRANSMITTER*' 



Figure 1. Tropospheric versus ionospheric propagation. 



The main factors influencing the shape of a cover- 

 age diagram under these circumstances are: (1) re- 

 flection by the ground, (2) diffraction by the ground 

 contour, (3) refraction by the atmosphere, and (4) 

 guided propagation by superrefraction in the lower 

 atmosphere. The present chapter deals mainly with 

 refraction phenomena, but reflection and diffraction 

 will be briefly considered. 



Refraction is influenced by the physical state of the 



189 



