Radio Broadcasting Transmitters and Related Trans- 

 mission Phenomena ^ 



By EDWARD L. NELSON 



This paper is a brief discussion of recent developments in American 

 practice concerning radio broadcasting transmitters. Descriptive material 

 and photographs pertaining to several new commercial transmitting equip- 

 ments are included. Reference is also made to the more important aspects 

 of the related transmission problem. On account of the scope of the 

 subject, the treatment is necessarily superficial, but it may serve to indi- 

 cate the present status of the transmitter art and its relative position with 

 respect to the industry as a whole. A short bibliography containing some 

 of the more important recent contributions to the subject is attached as 

 an appendix, to which reference may be had for more detailed information. 



Radio Transmitters 



THE radio transmitter is essentially a focal point in the present- 

 day broadcasting system, since upon it the program circuits 

 converge and from it the radio distribution network emanates. For 

 this reason, the requirements which have been imposed on trans- 

 mitting apparatus are extremely rigorous, and all phases of trans- 

 mitter performance have been subjected to the most careful scrutiny. 

 Under these stimulating influences, the last few years have brought 

 about some very noteworthy advances in this portion of the broad- 

 casting field. 



As long as music and entertainment continue to hold a prominent 

 place on broadcasting programs, fidelity of transmission will probably 

 remain the most sought-for characteristic, not only for the radio 

 transmitter itself, but for all of the apparatus units in the system. 

 A very high standard of performance has now been attained in this 

 respect. Fig. 1, below, shows the overall frequency-response charac- 

 teristic of a new type 50-kw. equipment, the first of which has gone 

 into service at one of the leading American broadcasting stations 

 within the past few months. It will be noted that this characteristic 

 is substantially flat between 30 and 10,000 cycles. The greatest 

 departure from the horizontal line which is the ideal characteristic 

 is less than 1 db. The frequency discrimination which this represents 

 is of such a low order that it probably could not be detected in ordi- 

 nary listening tests, even by a skilled musician. 



Another recognized prerequisite to a high degree of fidelity is exact 

 proportionality between audio input and sideband output. Increased 



\Read before the World Engineering Congress, Tokio, Japan, October, 1929; 

 Proceedings of Institute of Radio Engineers, November, 1929. 



121 



