RADIO BROADCASTING TRANSMITTERS 123 



are susceptible of serious amplitude distortion unless the conditions 

 under which the tubes operate (direct plate and grid voltages and 

 impedance of the connected load) are carefully predetermined. For 

 this purpose, the harmonic analyzer has proved to be invaluable. 

 Through its use, commercial transmitters are now available in which, 

 at the working upper limit of modulation, the harmonics generated 

 are not greater than 5 per cent. 



The attainment of such high standards for fidelity leaves little 

 opportunity for progress, and it is improbable that significant ad- 

 vances in this direction will be made in the near future. Accordingly, 

 in continuing their efforts toward further improvements in broad- 

 casting service, transmitter engineers have been led to divert their 

 attention to the problem of rendering less conspicuous and objection- 

 able the background of noise and interference which, in the past, has 

 so seriously impaired the artistic effect of programs except in the 

 immediate vicinity of transmitting stations. This is the principal 

 justification for the present movement toward higher power outputs 

 for broadcasting stations. It has also resulted in increased emphasis 

 on the maintenance of a high average degree of modulation, a develop- 

 ment which is rapidly bringing about a very perceptible improvement 

 in general broadcasting conditions. 



The degree of modulation of the carrier in a radio telephone trans- 

 mitter is a somewhat intangible factor which necessarily varies rapidly 

 through wide limits during the rendition of a program. With every 

 transmitter, however, there is a definite modulation limit which is a 

 characteristic of the design and which cannot be exceeded without 

 bringing about serious distortion. This limit is an important per- 

 formance index which, for lack of a better name, has been called 

 "modulation capability." The modulation capability of a trans- 

 mitter may be defined as the maximum degree of modulation (expressed 

 as a percentage) that is possible without appreciable distortion, em- 

 ploying a single-frequency sine-wave input and using a straight-line 

 rectifier coupled to the antenna in conjunction with an oscillograph 

 or harmonic analyzer to indicate the character of the output. 



For a number of reasons, some technical and some economic, many 

 of the broadcasting transmitters in use have been so constructed that 

 overloading of the audio power stage with consequent distortion 

 occurs whenever the degree of modulation exceeds approximately 

 50 per cent. The usual practice in placing broadcasting transmitters 

 into service consists of determining, by means of a suitable vacuum- 

 tube voltmeter or other "volume indicator," the audio level at the 

 input of the set for which distortion becomes evident. The average 



