Some Developments in Common Frequency 

 Broadcasting * 



By G. D. GILLETT 



This paper describes the results of the simultaneous operation of radio 

 stations WHO and WOC broadcasting the same program on a common 

 frequency using independent crystal controlled oscillators. These stations 

 had previously been compelled to share time on 1000 kc and each is now able 

 to render full time service. 



The exceptional stability of the crystal controlled oscillators used at each 

 station is described. Since even these oscillators require occasional read- 

 justment to maintain them in isochronism, a monitoring receiver was estab- 

 lished midway between the stations and the resultant program is sent back 

 by wire line to WOC to provide an indication for readjusting its frequency to 

 exact isochronism with WHO. An audio oscillator used to modulate the 

 carriers in the monitoring receiver provides a tone independent of the 

 program for the guidance of the operator. Curves are presented showing 

 the quality impairment caused by different degrees of isochronism and 

 signal strength ratios. 



The improvement in distant reception with simultaneous operation is 

 reported and an explanation given. The impaired reception in the area 

 midway between the stations and outside their normal service range is shown 

 to be a function of the degree of modulation of each transmitter, of the field 

 strength ratio and of the audio phase angle and independent of the carrier 

 phase at the transmitters. It is pointed out that reception equal to that 

 from either station alone may still be obtained in this area by the use of a 

 simple directive antenna. 



The marked increase in the service rendered by these stations through 

 simultaneous operation is indicative of the improved service that can be 

 rendered to urban areas by common frequency broadcasting. Although it 

 is probable that the high powered station on a cleared channel will remain 

 the best means of affording a high-grade service to a metropolitan area while 

 also rendering an acceptable service to large rural areas, common frequency 

 broadcasting now appears to offer definite means by which to provide an 

 improved coverage to a number of noncontiguous communities. 



THE development of chain broadcasting and the congestion in 

 the broadcast frequency range has naturally led to a considera- 

 tion of the possibilities of operating a group of stations on a single 

 frequency.12 xhe possible usefulness of such a system has resulted 

 in a number of attempts to secure the additional coverage offered by 

 the simultaneous operation of two or more stations broadcasting the 

 same program on a common frequency. This problem has been at- 

 tacked in two different ways. 



♦Presented at Sixth Annual Convention of I. R. E., June 4-6, 1931. Published in 

 Proc. I. R. E., 19, 1347-1369; August, 1931. 



1 DeLoss K. Martin, Glenn D. Gillett, and Isabel S. Bemis, "Some Possibilities 

 and Limitations in Common Frequency Broadcasting," Proc. I. R. E., 15, 213-223; 

 March, 1927. , . , ^.^ 



= Charles B. Aiken, "The Detection of Two Modulated Waves which Differ 

 Slightly in Carrier Frequency." Proc. 1. R. E., January, 1931; B. S. T. J., January. 

 1931. 



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