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quencies in the short wave range he found that the simple optical 

 picture cannot always explain the results obtained with vertical 

 polarization. With horizontal polarization, however, satisfactory 

 agreement was obtained. The propriety of the simple optical picture 

 is therefore much clearer for horizontal than for vertical polarization. 

 Reasons have been given in an earlier section for expecting regularity 

 of reflection even in the case of rugged land, if the incidence is near 

 enough to grazing. It was also shown that there probably exists an 

 effective coefficient of reflection which is actually near to — 1 for both 

 polarizations. At the receiver the phase relation between the direct 

 and reflected waves, and hence the field, thus depend only on the path 





— K ___ 





30 



20 



100 200 500 OOO 2000 5000 10000 

 FREQUENCY - MEGACYCLES 



Fig. 13 — Above: Profile of "optical" path between Beer's Hill and Lebanon. 

 Below: Calculated frequency characteristics for this path. 



Curve I, reflection only (coefficient, — 1). 



Curve 11, refraction and reflection (coefficient, — 1). 



Curve III, refraction and reflection (coefficient — 0.8). 



difference measured in wave-lengths. A set of interference fringes 

 will therefore be set up, and the received signal at any given point 

 will be a function of the frequency. 



Making these assumptions as to reflection and taking refraction into 

 account, it is interesting to calculate the frequency characteristic of a 

 typical path. For this purpose, we may choose the path from Beer's 

 Hill to Lebanon, which is discussed by Englund, Crawford and 

 Mumford. The characteristic which would be obtained from the fore- 

 going considerations of reflection and refraction is shown in Fig. 13. 

 The light curve shows the frequency characteristic that results by 

 neglecting refraction. It can be seen that below about 500 mc. the 

 expected gain due to refraction is about five db., a gain which is by 



