STUDIES IN RADIO BROADCAST TRANSMISSION 161 



An endeavor to form an explanation of the cause of this selective 

 action in fading must be largely in the nature of speculation. Fur- 

 thermore, since our data consist in the results of things which have 

 happened rather than in any first hand information on the processes 

 of the happening, the building of an explanation is a synthetic pro- 

 cess. In general for any given set of facts it is possible to synthesize 

 a number of explanations. Bearing this philosophy in mind we have 

 considered various theories in connection with our observations and 

 have concluded that simple wave interference as a major cause of 

 the signal variations is at present the most likely explanation. While 

 wave interference may be called a major cause it should perhaps also 

 be called a secondary cause since the assumption of wave interference 

 presupposes for its origin, primary causation by some physical state or 

 configuration of the transmission medium. Speculation as to the 

 nature of this primary cause is one stage further removed from the 

 data contained in our oscillographic records than is the assumption 

 of wave interference. 



Since it is desirable in the remainder of this discussion to point 

 out the evidences of wave interference, let us consider briefly the 

 nature of this phenomenon. 



To avoid any possible confusion of terms let it be said that by 

 "wave interference" we mean a particular physical phenomenon in 

 wave transmission and have no reference whatever to static, signals 

 from other stations, or any other of the forms of radio noise which are 

 commonly designated by the word "interference" when they hinder 

 the reception of desired signals. 



When two single frequency plane polarized wave trains start out 

 at the same time from a common source and travel by different 

 routes to meet again at a distant point the nature of disturbance at 

 that point is determined by the relative space phases of the planes of 

 polarization and time phases of the amplitude of the two arriving 

 waves. 



If we let E represent the vertical resultant of the electric field, 

 which would be the only part affecting a simple vertical antenna, 

 such as we have used in most of our tests, then 



£=ei sin 27r(F/+^i)+g2 sin 2Tr{Ft+d^) (1) 



where F is the frequency and dx and ^2 are the distances along the 

 respective paths measured in wave lengths and ex and e^ are the 

 vertical components of the two waves. These two sine terms may 

 be thought of as two vectors differing in phase. 



