98 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1952 



Fig. 7 — Received pulses — disturbed transmission. 



at all surprising that pulse shapes and amplitudes change very rapidly 

 at times. 



Looking again at the photograph, Fig. 7, we see that at 7:54-05 there 

 was a complete fade as far as our system is concerned. To produce this 

 degree of cancellation the path difference must have been very small 

 though still sufficient to give a relative phase angle of 180° at the radio 

 frequencies involved. At 8:00-15 and 8:08-00 pulse distortion is the most 

 noticeable effect of the "fading," the pulses being considerably shorter 

 than their normal value. Pictures, not shown here, taken between 7:54 

 and 8:08 show definite evidence of two-path transmission with a path 

 difference of 2 to 3 feet; therefore the pulses of 8:00-15 and 8:08-00 

 are probably also the result of two-path effects. 



The first two pictiu'es of Fig. 8 show another form of pulse distortion 

 observed on a number of occasions. Here the pulse is flattened out on 

 top probably due to energy coming in over a second path differing in 

 length by only one or two feet from the main path. Each time this type 

 of pulse was observed a check was made to be sure that the flattening 

 Avas not dtie to overload in our equipment. The pictures presented up to 

 now have all shown comparatively slow changes of conditions. Very 

 rapid changes were, however, quite common. In many cases pulse shape 

 or amplitude changed considerably during the 5 to 15 second exposure 

 time ordinarily used. The picture taken at 2:20-45 A.M. on August 27 

 is one example of such a rapid change, there being two definite sets of 

 conditions shown on the one photograph. The remaining picture on Fig. 

 8 shows the pulses used for obtaining time calibration of the system. 

 These pulses were spaced 9 millimicroseconds apart and by adjust- 



