STUDIES IN RADIO BROADCAST TRANSMISSION 179 



The record in Fig. 25 represents a slow picture of the changes shown 

 in (b) and (c) of ¥\g. 24. If these wobbled frequency waves are 

 studied carefully it will be noted that where a single peak stands at 

 one moment there gradually comes in view another as if it were sliding 

 from behind the first. The cycle length being about 1/10-second 

 we may get some idea from this series of the rate at which the changes 



Fig. 25 — Sample slow record showing distortion produced3>y intentional I'reqiiency 

 modulation. Night record 



take place. The presence of so many peaks in these records is at- 

 tributed in part to the fact that the rotating condenser used gave a 

 frequency change which was far from a simple sinusoidal relation. 



Let us now return to the stepped-frequency method of obtaining 

 the band fading pictures and ascertain why it has certain advantages. 

 In (b) of Fig. 22 is shown the "equivalent" characteristic for the 

 stepped condenser. During 1/2000 of a second (for the conditions so 

 far assumed) in each step distortion may occur due to transient 

 conditions, but during the remainder of the quarter second assigned 

 to each step (for the records so far taken) a steady state is reached. 

 Thus, theoretically, distortion occurs only during about 1/500 of the 

 step interval. In (b) of Fig. 22 the lag is greatly exaggerated for pur- 

 poses of illustration. This means simply that we have maintained 

 constant frequency for a sufficient length of time to establish, before 

 taking our picture, a fixed interference condition over the region 

 including transmitter and receiver at least. 



Daytime Field Strength Distribution 



Thus far we have been dealing with the unstable phenomena of 

 night-time transmission. Our interest has been directed almost 



