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BELL SYSTEM TECHNICAL JOURNAL 



invoked. The FDM-FM system of "minimum band width" listed in Ta- 

 ble IV is intended to possess this 44 db tolerance. To illustrate the require- 

 ment of new frequencies let us take the case of several FDM-FM routes 



130 140 150 160 170 180 170 160 150 140 130 



Fig. 21 — Assumed directional selectivity of 10 ft. x 10 ft. antenna at 4000 mc. 



5 6 



Fig. 22 — Simplified route pattern for studj' of selectivity required in congested localities. 



converging in the manner suggested by any of the diagrams of Fig. 22. A 

 critical situation occurs in regard to the frequencies used for receiving at 

 the point toward which the systems converge. If the same receiving 



