PROPAGATION OF RADIO WAVES 



and the free-space field strength at distance d from 

 the transmitter is 



Eo = 



3V5 VPi 



(12) 



Pj is the power radiated by the doublet transmitting 

 antenna (see Section 2.1.1). 



1.3 



1.3.1 



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. 



1.3.2 Paetors Modifying Transmission 



The important factors which affect the distribu- 

 tion of field strength are the following: 



1. A^itejina characteristics. For many applications 

 the most important feature is the gain which is a 

 measure of the directivity of the antenna. From a 



pattern gives the relative amount of power per unit 

 area radiated in that direction. 



00 



Doublet 

 Antenna 



^:^^ 



Reflector 



Anienno with High 

 Directivity 



Figure 4. Antenna radiation patterns. 



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 

 to state in general which type of polarization should 

 be used in a particular case. 



3. Refraction. As explained in Section 1.2.4, 

 refraction in the standard atmosphere can be taken 

 into account by using an ecjuivalent earth with a 

 radius equal to ^/s that of the actual earth and a 

 homogeneous atmosphere in which the rays traverse 

 straight line paths. 



Receiver 

 R 



Interference 

 Region 



•'-'^x 



!&-on 



-"'■//> 



^'00 



Figure 5. Geometrical relations for rays. 



value of 1.09 for the half-w-ave 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 w^here 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 



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° — i^) 

 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 ^, 



