()02 THE BELL SYSTEM TECHNICAL JOIKXAL, iMAV 1!J57 



Larger antennas (more narrow beamwidths) will decrease the fast multi- 

 path fading and widen the frecjuency separation between uncorrelated 

 fading but at the risk of increasing the long term fading associated with 

 the ^'ariations in the angle of arrival. 



Optimum space diversity, when ground reflections are controlling, 

 requires that the separation between antennas be sufficient to place one 

 antenna on a field intensity maximum while the other is in a field in- 

 tensity minimum. In practice, the best spacing is usually not known be- 

 cause the principal fading is caused by multipath variations in the 

 atmosphere. However, adequate diversity can usually be achie^'ed with 

 a vertical separation of 100-200 wa\'elengths. 



At frec^uencies above 5,000-10,000 mc, the presence of rain, snow, or 

 fog introduces an absorption in the atmosphere which depends on the 

 amount of moisture and on the frequency. During a rain of cloud burst 

 proportions the attenuation at 10,000 mc may reach 5 db per mile and 

 at 25,000 mc it may be in excess of 25 db per mile.^ In addition to the 

 effect of rainfall some selective absorption may result from the oxygen 

 and water vapor in the atmosphere. The first absorption peak due to 

 water vapor occurs at about 24,000 mc and the first absorption peak for 

 oxygen occurs at about 60,000 mc. 



III. TROPOSPHERIC TRANSMISSION BEYOND LINE OF SIGHT 



A basic characteristic of electromagnetic waves is that the energy is 

 propagated in a direction perpendicular to the surface of uniform phase. 

 Radio waves travel in a straight line only as long as the phase front is 

 plane and is infinite in extent. 



Energy can be transmitted beyond the horizon by three principal 

 methods: reflection, refraction and diff"raction. Reflection and refrac- 

 tion are associated with either sudden or gradual changes in the direc- 

 tion of the phase front, while diffraction is an edge effect that occurs 

 because the phase surface is not infinite. When the resulting phase front 

 at the receiving antenna is irregular in either amplitude or position, the 

 distinctions between reflection, refraction, and diffraction tend to break 

 down. In this case the energy is said to l)e scattered. Scattering is fre- 

 quently pictured as a result of irregular reflections although irregular 

 refraction plus diffraction may be equally important. 



The following paragraphs describe first the theories of refraction and 

 of diffraction over a smooth sphere and a knife edge. This is followed by 

 empirical data derived from experimental results on the transmission to 

 points far beyond the horizon, on the eftects of hills and trees, and on 

 fading phenomena. 



