ULTRA-SHORT WAVE PROPAGATION 



149 



is obtained by assuming a reflection coefficient of — 0.8. The resulting 

 curve is shown by the broken line in Fig. 16. In the case of the 

 optical path of Fig. 13 reflection coefficients of — 1 and — 0.8 agree 

 equally well with the experimental point. (In Fig. 16 a correction 

 has been applied to the experimental data eliminating the effect of 

 local reflections at the receiver.) 



This curve brings out the important fact that even for non-optical 

 paths, one may expect to find optimum frequencies. On this particular 



LEBANON 



z ^ 



Sq 



100 200 500 1000 2000 

 FREQUENCY— MEGACYCLES 



5000 10000 



Fig. 16 — Above: Profile of "non-optical" path between Deal and Lebanon. 

 Below: Frequency characteristics for this path calculated on various assumptions. 



Curve I takes only the shadow effect (diffraction) into account. Note that the 

 experimental points fall far below it. 



Curve II is calculated on the basis of diffraction and reflection (coefficient — 1.0). 

 Note that this gives a better check with experiment, but values are too low. 



Curve III adds to II a correction for refraction. 



Curve IV assumes diffraction, refraction and a reflection coefficient of — 0.8. 

 It checks the experimental points to within experimental error. 



The original experimental data have been corrected to eliminate the effect of 

 ground reflection near the receiver. The transmitter, being above level ground, 

 needed no such correction. 



path the simple assumptions give 1200 mc. (25 cm.) for the lowest of 

 these. On other paths which have been calculated, optimum fre- 

 quencies would be expected in the range between 1 and 10 meters. 



It is fully realized that the details of these curves will probably not 

 be found experimentally. We do not as yet have sufficient experience 

 to pick the simple picture that will in effect represent a complicated 

 topography and transmission mechanism, and it is obvious that this 

 may never be possible. It is encouraging, however, that the limited 

 number of measurements which have already been made experi- 

 mentally, agree reasonably well with the theory proposed. 



