Chapter 1 
PROPAGATION OF RADIO WAVES: 
INTRODUCTION AND OBJECTIVES . 
FACTORS INFLUENCING PROPAGATION 
HE PROPAGATION of radio waves through the 
Gh atmosphere and around the curve of the earth, 
at frequencies above 30 mc, is influenced by so many 
factors that it is desirable to give first an overall 
survey of the problem. This chapter presents the 
problem of propagation in broad perspective in 
contrast with many of the later chapters which are 
devoted to detailed consideration of special phases 
and methods of calculation. 
Our purpose has been to provide information de- 
signed for men with college training in radio, physics, 
or electrical engineering, which: 
1. States the basic laws of propagation, -that is, 
shows how the characteristics of the earth and the 
atmosphere control the propagation of radio waves; 
2. Gives the fundamental properties of the basic 
types of antenna systems, particularly their direc- 
tivity and gain; 
3. Gives the reflecting properties of targets such 
as airplanes for use in detection by radar; 
4. Teaches the reader how to calculate field 
strength or obtain the coverage diagrams, given a 
particular set, power, and site; 
5. Gives the fundamental information required in 
the above calculations for application to the radar 
and communication sets used in operational theaters; 
6. Provides illustrative material and sample cal- 
culations which. show how the laws of propagation 
may advantageously be used in the location and 
operation of radar systems, communication sets, and 
countermeasure equipment designed to deceive the 
enemy and to prevent jamming of equipment by 
enemy action or by mutual interaction of our own 
sets. 
FUNDAMENTAL PROBLEMS 
AND LIMITATIONS 
Meaning of Propagation 
By propagation is meant the movement of radio 
waves through the atmosphere, and the transfer, by a 
wave mechanism, of radiant energy from a transmit- 
ting antenna to a receiving antenna. The problem 
of propagation requires an understanding of the 
manner in which the wave energy is emitted and 
received as well as of the manner in which it flows 
through the atmosphere. The radio engineer must 
understand this general mechanism, be able to eval- 
327 
uate the factors which play contributory roles, and, 
for a given amount of power emitted from a given 
transmitter, be able to compute the strength of the 
radiation field at any point in space or to locate all 
the points in space where a given field strength 
occurs. / 
The problem divides naturally into two parts, (1) 
the one-way transmission or communication problem, 
and (2) the two-way transmission or radar problem. 
In the former the prime requisite is to calculate the 
amount by which the wave and its field strength are 
attenuated in passing from the transmitter to a 
receiver and yet permit a field at the receiver suffi- 
cient at least to produce the minimum detectable 
signal. In the latter problem the attenuation must 
be calculable for the two-way journey from trans- 
mitter to the target and back to the receiver, which 
frequently uses the same antenna as the transmitter. 
In this type of problem, due. account must also be 
taken of the reflecting and reradiating properties of 
the target. 
Knowledge of these factors is indispensable for the 
design, installation, and successful operation of both 
communication and radar systems. 
Atmospheric Layers 
The atmosphere from one point of view is com- 
posed of two layers, the troposphere and the strato- 
sphere. The former is a layer adjacent to the earth 
which extends upward approximately 10 km, in 
which the temperature decreases about 6.5 C per 
kilometer with increasing altitude to a value, at the 
upper boundary, of about — 50 C. Above this is 
the stratosphere in which the temperature remains 
approximately constant at — 50 C. 
The ionosphere, as its name implies, is a layer (or 
series of layers) composed of ions and free electrons 
lying at an elevation of approximately 100 km. See 
Figure 1. These layers play an important role in the 
transmission of waves at frequencies below 30 me 
and are responsible for transmission over very long 
distances. At the higher frequencies, which are the 
concern of this volume, the portion of the waves 
which penetrate the ionosphere is not useful for 
transmission. 
From this it follows that propagation at the higher 
frequencies (above 30 mc), to be useful, must occur 
entirely in the troposphere. This volume therefore 
is concerned only with tropospheric propagation. 
