166 



BELL SYSTEM TECHNICAL JOURNAL 



During the day signals substantially uniform in amplitude are 

 received. An example of the type of transmission obtained in the 

 daytime is given in Fig. 15, which is a record of the carrier and side- 

 band signals received with substantially the same terminal condi- 

 tions with the exception of the time as that existed when the records 

 shown in Fig. 12 were made. 



The abrupt change in the amplitude of the side-band signals was 

 due to an intentional change at the transmitter in the input level 

 of the tone modulating the carrier, and accordingly the amplitude 

 of the carrier did not change. The timing interval is 5 seconds. 



Band Fading Records 



The familiar fading record is limited to two axes, amplitude and 

 time. So far we have extended this cramped perspective somewhat 

 by observing as many as three separate fading records spaced at 

 audio-frequency intervals along the frequency axis. Even these 

 three narrow lookouts upon the wide range of ether transmission 

 have indicated amplitude relations along the frequency axis which 



TRANSMITTER 



W- 



Amplifier 



(Stepped 

 Frequency ) 



I A Monitoring 



|-y Speaker 



] (F2-F,l) 



TZ3 Osciliigrapl- 

 — ' ^ Element 



Beating 

 Oscillatoi 



Fig. 16 — -Diagram of system used to obtain records of selective fading or "band 



fading" records 



promise to open a new line of attack upon the problem of night-time 

 fading. But the desirability of knowing what takes place in the 

 interval unrevealed by these cracks in the fence becomes obvious. 

 We should like to know the relative amplitude of frequencies over a 

 wide band, and the change in this relation with time. 



Since it is not a simple matter to record simultaneously the ampli- 

 tude of a large number of waves of frequencies separated by say one 

 hundred cycles in the radio-frequency range a single frequency in 

 combination with a frequency stepping device at the transmitter 

 has been adopted. The circuit arrangement is shown diagrammatic- 

 ally in Fig. 16. The rotary contactor bringing into the circuit sue- 



