498 BELL SYSTEM TECHNICAL JOURNAL 



Such a curve can also serve to check on the theoretical explanation of 

 the cause of fading in certain cases. Thus if the fading is due to the 

 combination of two radiation components in assigned random ampli- 

 tude relation and arbitrary or random phase relation, a curve can be 

 calculated from probability considerations and compared with the 

 experimental curve. ^ Such a simple mechanism was inadequate for 

 our fading most of the time. Moreover, the fading changed enor- 

 mously from day to day. It is hoped that the samples given in the 

 figures will give an adequate idea of this phenomenon. 



Only rarely was fading practically absent for periods of an hour or 

 two. Such a period is illustrated in Curve a, Fig. 2. Two days later 

 the extreme fading of Curve b was recorded. It is significant, as will 

 later appear, that the non-fading situation was the one of higher signal. 

 The amplitude range of curve b is nearly normal; the fading rate is 

 much greater than normal, for vertical polarization. In Fig. 3 the 

 fading rate is normal but the amplitude range is excessive. In Fig. 4 

 a characteristic type of fading, which we have termed "scintillation," 

 is recorded. In this case the fading, initially erratic and of a fairly 

 wide amplitude range, subsides in a characteristic manner to a steady, 

 fast rate oscillation, or scintillation, of moderate amplitude. 



In Fig. 5 a 24-hour run is recorded. The rambling erratic character 

 of the fading is well shown here. Characteristic deep short-period 

 minima occur at intervals, occasionally they are twinned, some of them 

 have a fine structure at the bottom. There are several "dropouts" 

 where the signal practically disappeared. 



No sunrise-sunset variations in fading were noticed, though looked 

 for. Diurnal variations could not be established since automatic 

 recording was not available. A seasonal falling olif in average signal 

 was noticed in the winter; the 1.6 meter wave, because of its normally 

 low level, dropped below the noise level in the winter of '34-35. No 

 effect of ocean waves, clouds, or other visible weather phenomena 

 could be established. It is true, however, that to be certain of the 

 non-effect of such phenomena as clouds, a clbud observer at the mid- 

 way point should have been present. In so far as cloud layers make 

 air mass boundaries visible they may well affect the transmission. 

 Cloud bottoms which represent merely the adiabatic dew point level 

 should apparently not cause much signal reflection at these wave- 

 lengths. 



Effect of Polarization on Fading 



After some preliminary experimenting it was found that comparisons 

 of two transmissions were worthless unless made on simultaneous 

 recordings. The recorders were therefore fitted with telechron motors 



