CARRIER AND SIDE-BANDS IN RADIO TRANSMISSION 101 



means of increasing the intensity of the reproduced signal, since this 

 is proportional to the carrier component at the receiver as well as to 

 the side-band. Also, by making the carrier large, k is made very 

 small and the distorting currents due to interaction of the side-bands 

 become negligible. The use of a large local carrier in homodyne 

 radio telephony assists in frequency selection in the same way as does 

 the hetrodyne wave in radio telegraph reception. Suppose an inter- 

 fering message is separated from the desired one by only a few thou- 

 sand cycles and so is not entirely suppressed by the receiving selective 

 circuits. Currents of voice frequency can be reproduced from its 

 side-bands only by interaction with its own carrier, and hence they 

 will be small compared with those of the desired message, which are 

 proportional to the local carrier. On the other hand, the large cur- 

 rents due to the interfering message and local carrier will all have 

 frequencies above the voice range, and so can be suppressed by selec- 

 tive circuits in the output of the detector. 



The same general reasoning applies also to static interference. 

 Appreciable interfering currents of signal frequency can result only 

 from those components of the static wave which lie in the frequency 

 range of the side-bands. Moreover, they will bear the same ratio 

 to the signal currents as do the static components to the side-band 

 components. We may conclude, then, that when means are pro- 

 vided for eliminating all of the static except that which is inherently 

 inseparable from the signal, the disturbing effect of the residue is 

 determined solely by the relative magnitude of the side-band com- 

 ponents and the static components which lie in the same frequency 

 range. As the object of high power stations is to make the signals 

 large compared with the static, the importance of concentrating the 

 power in the side-bands rather than in the carrier is obvious. 



Effect of Radio Distortion 



Let us pass now from the intentional modifications of a modulated 

 wave and consider the effects of unintentional distortions. Limiting 

 our attention first to systems in which the carrier is transmitted, we 

 have to consider the effect of distortion such as might be introduced 

 by the sending and receiving circuits and the transmitting medium. 

 Assuming the characteristics of these to be known in terms of their 

 transmission of sinusoidal components of various radio frequencies, 

 we wish to determine their effect on the amplitudes and phases of 

 the components of the reproduced wave. We shall assume the 

 current-voltage relations in the transmission system to be linear, so 



