ULTRA-SHORT WAVE PROPAGATION 145 



no means inconsiderable. For 70 mc. the field strength indicated by 

 the curve is in fair agreement with measurements made over this path 

 by Englund, Crawford and Mumford. 



The effect of reducing the reflection coefficient to — 0.8 is to raise 

 the low frequency end of the curve, to reduce the maxima to 5.1 db. 

 and to raise the minima to — 14 db. This is shown by the dashed 

 curve of Fig. 13. 



Another point in connection with the solid curve in Fig. 13 is of 

 interest. At 715 mc. (42 cm.) the path difference is half of a wave- 

 length and the two components now add in phase. This is the 

 optimum phase relation since it gives the largest possible resultant. 

 Hence 715 mc. is an optimum frequency for this particular path on 

 these assumptions and a field 6 db above the inverse distance value 

 would be expected. Even at one third this frequency, 240 mc. 

 (126 cm.), fields equal to the inverse distance value might be expected. 

 For higher frequencies many maxima and minima are indicated. 



Since the lowest optimum frequency depends on the difference 

 between the path lengths of the direct and reflected components, it 

 should be possible to obtain much lower optimum frequencies by 

 picking paths in which the terminals are located very much higher 

 than the valley between them. Thus, optical paths more than one 

 hundred miles long may be found in California for which the lowest 

 optimum frequencies may be considerably less than 30 mc. (10 m.). 



Error in the assumption of a phase shift of 180° would change the 

 frequency at which maximum and minimum fields occur, and failure 

 to obtain a reflection coefficient of unity might materially reduce the 

 difference between the received field and the free space value. 



The profile shown in Fig. 14 is used to illustrate the effects of change 

 in polarization and ground constants as indicated by calculations based 

 on simple optical theory. In the computations indicated by the 

 various frequency characteristics of this figure, the same profile has 

 always been used, but two different sets of ground constants, and both 

 horizontal and vertical polarizations, have been employed. The 

 curves are self-explanatory. It is especially to be noted that for 

 horizontal polarization the field decreases with decrease in frequency 

 and is nearly the same for land as for sea-water, i.e., it is nearly 

 independent of conductivity and dielectric constant. For vertical 

 polarization this trend is reversed for frequencies such that the con- 

 duction currents are large compared with the displacement currents. 

 In this example, this occurs in the neighborhood of 60 mc. in the case 

 of sea-water and 5 mc. in the case of "average" land. Thus for 

 vertical polarization there exists a "poorest" frequency separating the 



