PART III 
CONFERENCE REPORTS ON NONSTANDARD PROPAGATION 
Chapter 13 
TROPOSPHERIC PROPAGATION AND RADIO METEOROLOGY* 
FUNDAMENTALS OF PROPAGATION 
Significance of Propagation Problems 
HE CENTRAL PROBLEM of short and microwave 
propagation (at frequencies greater than 40 to 
60 mc) 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 aegree 
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 
®By Columbia University Wave Propagation Group. 
134 
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 me, 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). 
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 
