DETECTION OF TWO MODULATED WAVES 17 



5 and 6, while the second value corresponds to curves 8. We observe 

 that there is a tremendous difference between the areas which may be 

 considered to be free from displaced side band interference and those 

 which will be free from carrier l)eat interference, in case the beat 

 frequency is allowed to wander into the audible range. The comparison 

 between the two areas is given by curves 1 and 5 for the 40 db interval 

 and by curves 5 and 8 for the 20 db interval. 



The speech from the interfering station will now be the same as the 

 desired speech and can have effect only in so far as it adds to or subtracts 

 from the desired speech. It will be noted from F'igs. 1 and 5 that for 

 carrier ratios of less than 0.1 this component is always down more than 

 40 db and may be safely neglected. 



The foregoing discussion serves to illustrate the types of interference 

 which may be expected when two stations are operated on approxi- 

 mately the same frequency. The data discussed have involved low 

 values of attenuation. This is of particular interest when the distance 

 between stations is large since with high values of attenuation either 

 station will have very little effect on the service area of the other. Of 

 course at night time we may have signal strengths which will be of the 

 order of magnitude of that given by the simple inverse distance kiw 

 invoU'ing zero attenuation. This possibility probably presents a 

 serious limitation on night time common frequency broadcasting but 

 should be of little consequence during the daylight hours. Conditions 

 will be somewhat different for stations that are placed nearer together 

 and specific results can be readily computed for any given spacing. 

 The equations which have been discussed can be applied to any such 

 case and the areas corresponding to those in Figs. 5 and 6 determined. 



One point which is emphasized by the results which have been 

 obtained is, that with a carrier frequency difference of several cycles 

 satisfactory reception cannot be expected in the regions which lie 

 midway between two transmitters. The field strength of one station 

 must be at all times predominately higher than that of the other and 

 consequently the use of pseudocommon frequency broadcasting should 

 be restricted to stations of wide geographic separation. It should then 

 be possible to furnish high grade service to relatively small densely 

 populated areas in the immediate vicinity of either transmitter, 

 reception at a considerable distance from both stations being ad- 

 mittedly unsatisfactory. However, if the carriers are strictly isoch- 

 ronous much larger service areas should be feasible. 



