PROPAGATION OF RADIO WAVES 



ference pattern (see Figures 9 to 12). The maximum 

 number of lobes is the largest integral number of 

 times that the quarter \va\'elength is contained in the 

 transmitter height. 



In the case of horizontal polarization over a 

 smooth surface, e.g., a calm sea, the reflected and 

 direct rays are comparable in strength, so that at 

 certain points (on lines for which the points corre- 

 spond to a path difference of a half wavelength) 

 where the reinforcement is a maximum, the field 

 ma}^ be as much as twice the free-space field. More 

 exactly, the free-space field is multiplied at points of 

 maxima by (1 + FD), where D is the value of the 

 di\'ergence factor for the point and F gi\'es the rela- 

 tive strength of the reflected and direct rays attribut- 

 able to the antenna beam pattern. At points of 

 minima (the nulls) the field is (1 — FD) times the 

 free-space field. 



In general the magnitude of the reflected wave is 

 reduced both by the increased divergence resulting 

 from reflection from the convex surface of the earth 

 but also because the electrical properties of the 

 earth are such that only part of the incident energy 

 is reflected. The magnitude of the reflection coeffi- 

 cient is then pD instead of the D used in the preced- 

 ing paragraph, where p is the magnitude of the 

 reflection coefficient for plane waves impinging on 

 a plane surface. The field strength, then, lies be- 

 tween (1 + pFD) and (1 — pFD). As a result of 

 the smaller value of pFD for \'ertical polarization 

 the maxima of the interference pattern are reduced 

 and the nulls strengthened. 



At low heights (see Figure 3 of Chapter 5) the 

 effect of diff'raction is important, so that when refer- 

 ence is made to the optical interference region, it 

 should be understood that the portion of the optical 

 region near the earth is not included. It must be 

 considered instead as part of the diffraction region. 



The diffraction region, accordingly, designates a 

 layer in the optical region as weW as the region behjw 

 the line of sight (.see Figure 3 in Chapter 5). Below 

 the line of sight the field falls off exponentially. 

 Within the diffraction region, fields are strengthened 

 lay raising the receiver or transmitter antennas. 



1.3.4 



Typical Radio Gain Curves 



Three types of graphical representation of radio 

 gain in a vertical plane through the transmitter 

 antenna are possible, namely, (1) at a specified 

 distance, radio gain against height; (2) at a specified 



height, radio gain against distance; and (3) a .set of 

 contour lines representing constant radio gain. 



10.000 



y 1000 -I 



-240 -220 



-180 -160 -140 

 20 LOS A IN DB 



-80 



Figure 7. Radio gain vs receiver height for horizontal 

 polarization. 



In Figure 7, curves of type (1) are exhibited for 

 various frequencies. The transmission is over sea 

 water with horizontally polarized \\'aves. It may be 

 observed that the higher the frequency the lower the 

 first maximum and the narrower the lobe. Figure 8 

 gives similar information for vertically polarized 



IZiooo 



2 



-180 -160 -140 

 20 LOG A IN OB 



Figure 8. Radio gain v.s receiver lieight for vertical 

 polarization. 



waves. Note that the minima are not so deep with 

 vertical polarization. Curves of type (2) exhibit 

 similar characteristics (see Figure 6 of Chapter 5). 

 In Figures 9 to 12'', ^'ertical coverage diagrams of 



^ Figures 7 to 12 have been adapted from Radiation Lab- 

 oratory Report C-6. 



