ULTRA-SHORT WAVE PROPAGATION 135 



earth as well as any of similar length that exists in this part of the 



country. 



Stability of Signals 



Speaking generally, the signals received in ultra-short wave trans- 

 mission vary little, if at all. In this respect they are in marked con- 

 trast with signals of lower frequencies in the transmission of which the 

 Kennelly-Heaviside layer is involved. In this work, definite indica- 

 tions of fading have been found only in the case of paths in which the 

 attenuation in excess of that represented by the inverse distance 

 formula has been in the order of 30 to 40 db. The variations were in 

 the order of one or two decibels, and the period was a few seconds. 

 This may have been due to variable atmospheric refraction. On the 

 other hand, it is not inconceivable that it may have been due to 

 reflection from clouds. It is, of course, easy to show that there is so 

 little moisture in clouds that reflections must be extremely weak. 

 But w^e have to explain coeflicients of reflection in the order of only 

 0.01. This is plausible since we are concerned with reflection from 

 the cloud at near-grazing incidence for which the coeflicient tends to 

 be unity regardless of the difference in dielectric constant. Further 

 investigation is needed along these lines. 



Part II — Theory 



Before entering into a quantitative explanation of some of the 

 results which have been presented, it may be well to direct attention 

 to certain ways in which the present problem is related to the familiar 

 concepts of optical reflection, diffraction and refraction. 



Reflection 



Reflection constants are readily calculated in the case of smooth 

 surfaces such as still water. Having obtained these, the resultant 

 amplitude at the receiver can be calculated for different ground 

 constants. (See Appendix I.) 



Even if the surface is rough, It Is to be expected that an ultra- 

 short radio wave may be reflected regularly from a body of water. 

 The existence of regular reflection is less obvious when transmission 

 occurs over rolling land. In the first case we have the most simple 

 conditions since the surface waves on the water are irregularities of a 

 single general type and range of dimensions. They are merely 

 deviations from a plane, or rather from a sphere. But in the second 

 case, the irregularities of the land are of all forms and dimensions and 

 the existence of regular reflection cannot be granted without con- 

 sideration. 



