RADIO PROPAGATION FUNDAMENTALS 



609 



and for diffraction over a knife edge obstruction. Various theories have 

 been advanced to explain these effects but none has been reduced to a 

 simple form for every day use.^^ The explanation most commonly ac- 

 cepted is that energy is reflected or scattered from turbulent air masses 

 in the volume of air that is enclosed by the intersection of the beamwidths 

 of the transmitting and receiving antennas.'^ 



The variation in the long term median signals with distance has 

 been derived from experimental results and is shown in Fig. 9 for two 

 frequencies.^^ The ordinate is in db below the signal that would have 

 been expected at the same distance in free space with the same power 

 and the same antennas. The strongest signals are obtained by pointing 

 the antennas at the horizon along the great circle route. The values 

 shown on Fig. 9 are essentially annual averages taken from a large num- 

 ber of paths, and substantial variations are to be expected with terrain, 

 climate, and season as well as from day to day fading. 



Antenna sites with sufficient clearance so that the horizon is several 

 miles away will, on the average, provide a higher median signal (less 

 loss) than shown on Fig. 9. Conversely, sites for which the antenna must 

 be pointed upward to clear the horizon will ordinarily result in ap- 

 preciably more loss than shown on Fig. 9. In many cases the effects of 

 path length and angles to the horizon can be combined by plotting the 

 experimental results as a function of the angle between the lines drawn 

 tangent to the horizon from the transmitting and receiving sites. ^^ 



20 



u 

 < 



CL 

 If) 



UJ 

 UJ 



40 



5 



o 



_i 



HI 

 CD 



HI 



m 

 u 



HI 



a 



60 



80 



100 



120 



40 50 60 80 100 200 300 400 600 800 1000 



DISTANCE IN MILES 



Fig. 9 — Beyond-horizon transmission — median signal level versus distance. 



