SURVEY OF PROPAGATION 



(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 wth 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 

 in the siting of very high frequency [VHF] com- 

 munication sets. 



5. Diffraction. The mechanism by which radio 

 waves curve around edges and penetrate into the 

 shadow region behind an opaque obstacle is called 

 diffraction. The explanation usually given is based 

 on Huyghens' principle. This, in effect, states that 

 every elementary area on a wa\'efront (see P in 

 Figure 6) is a center which radiates in all directions 



Relotive 

 Field 



Figure 6. Diffraction around an obstacle. 



on the forward side of the wavefront; the intensity 

 of radiation is a maximum in the direction per- 

 pendicular to the wavefront and depends on angle 

 6 according to the function (1 + cos 6). The field at 

 any point, either inside or outside the shadow zone, is 

 obtained by summing the contributions from all the 

 elementary areas comprising the wavefront. 



As a result of these calculations, the field along the 



line AA' in Figure 6 varies approximately as indi- 

 cated in the curve. Unity represents the field value 

 if the obstacle were removed. It is seen that the 

 field strength rises from a minimum at point A to 0.5 

 at the edge of the shadow zone and thereafter oscil- 

 lates about unity. The field outside the shadow zone, 

 therefore, at certain points is stronger and at other 

 points is weaker than it would be if there were no 

 obstacle. The curve, of course, varies with the 

 position of the line AA', the size and shape of the 

 obstacle, the wavelength of the radiation, and the 

 t3'pe of polarization. The diffraction of radiant 

 energy into the shadow zone increases with increas- 

 ing wavelength. 



Of prime importance for propagation of radio 

 waves is the diffraction of these waves into the 

 diffraction region below the line of sight (see Figure 

 5). But it should be noted that the influence of 

 diffraction is not confined to this region but extends 

 well above the line of sight. [In general, the influ- 

 ence extends upward far enough to affect the shape 

 of the lower part of the first lobe in a coverage Q 

 diagram (see Figures 25 and 26 of Chapter 5). In 

 this region the diffraction contribution must be 

 added to the contributions of the direct and reflected 

 rays to give the correct value of the field strength 

 at R in Figure 5.] 



Of importance in communication problems is 

 diffraction of waves around obstacles such as hills, 

 trees, houses, etc. This is illustrated in Figure 6. 

 Again diffraction is important in problems involving 

 propagation above two different earth conditions. 

 An especially important case is that of a radar set 

 well inland and searching far out over the sea. Here 

 the shore line is treated as a diffracting edge for the 

 radiation from the image antenna. 



6. Absorption and scattering. No account is taken 

 in this book of the absorption and scattering of radio 

 waves by the various constituents of the atmos- 

 phere. Oxjrgen, water vapor, water droplets, and 

 rain all contribute to absorption. Their influence, 

 however, is important only in the microwave range 

 and in general tends to increase mth frequency. 



1.3.J 



General Nature 

 of the Radiation Field 



In Section 1.3.2, reference has been made to the 

 role of reflection by the earth. The resultant of the 

 direct and indirect rays at points in the region above 

 the line of sight gives rise to the lobes of an inter- 



