PROPAGATION OF RADIO WAVES 331 
the transmitter is 
_ 8V5 VP; 
— 
P, is the power radiated by the doublet transmitting 
antenna (see pages 336, 337). 
Ey (12) 
SURVEY OF PROPAGATION 
Outline 
We will first consider those factors which are 
instrumental in modifying the transmission or the 
attenuation that arises from the presence of the 
earth, then give typical curves of both vertical and 
horizontal variation of field strength, and lastly, 
consider the problem of coverage. 
Factors Modifying Transmission 
The important factors which affect the distribu- 
tion of field strength are the following: 
1. Antenna characteristics. For many applications 
the most important feature is the gain which is a 
measure of the directivity of the antenna. From a 
value of 1.09 for the half-wave dipole, the gain may 
increase to several thousand for the highly directive 
parabolic antennas used in the microwave range. 
Antennas with high gains which concentrate the 
radiated energy into beams of small angles require 
less power to produce a detectable signal. This is 
particularly important in radar where the attenua- 
tion of the two-way path is pronounced. 
Qualitative radiation patterns for the doublet 
antenna and an antenna with high directivity are 
illustrated in Figure 4. The radial distance to the 
pattern gives the relative amount of power per unit 
area radiated in that direction. 
2. Polarization. The wave is said to be polarized 
horizontally or vertically according to whether the 
electric vector E is parallel to the earth’s surface or 
is in a plane perpendicular thereto. A horizontal 
electric doublet (axis parallel to the earth’s surface) 
radiates horizontally polarized waves, whereas a 
vertical doublet radiates vertically polarized waves. 
Too many factors are involved to make it possible 
Transmitter 
¥ Reflection 
Point 
CE COMED, Tae 
Doublet 
Antenna 
Antenna ntenna With High 
Directivity 
Reflector 
Fiaure 4. Antenna radiation patterns, 
to state in general which type of polarization should 
be used in a particular case. 
3. Refraction. As explained in text on page 329, 
refraction in the standard atmosphere can be taken 
into account by using an equivalent earth with a 
radius equal to 4/; that of the actual earth and a 
homogeneous atmosphere in which the rays traverse 
straight line paths. 
4. Reflection. Well above the line of sight (see 
Figure 5) the field at the receiver R is,the vector sum 
of the fields radiated along the paths of the direct 
and reflected rays. The contribution from the 
reflected ray path depends primarily on the manner 
in which the earth (or sea) acts as a reflecting body. 
Upon reflection, the angle of incidence (90° — y) 
is equal to the angle of reflection, irrespective of the 
polarization of the wave, but the strength of the 
field in the reflected ray relative to that in the inci- 
dent ray depends upon (a) the grazing angle y, 
(b) the type of polarization, (c) the reflecting proper- 
ties of the earth or sea, and (d) the divergence factor. 
The incident beam or bundle of rays, in general, is 
partially absorbed by the earth, while the reflected 
portion is reduced in strength and suffers a phase 
shift relative to the incident beam. (In the case of 
sea water with horizontal polarization, the earth 
acts substantially as a perfect reflector, for which 
the reflection is 100 per cent complete and the phase 
shift is 180 degrees for all grazing angles. This is 
true for vertical polarization only at zero grazing 
angle.) 
The divergence factor is introduced to account for 
the fact that an incident bundle of rays striking a 
spherical surface diverges upon reflection and pro- 
duces a further decrease in strength of the reflected 
beam. 
Reflection from hills, trees, and other obstacles 
must frequently be taken into account, particularly 
Recelver 
Interference 
Region 
Ficure 5. Geometrical relations for rays. 
