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required. The range of frequencies transmitted may then extend 

 from 58.9 to 61.1 kilocycles with the suppressed carrier at 58.5 kilo- 

 cycles, and the radio receiver must be designed to accept this band of 

 frequencies. The transmission-frequency characteristic of the overall 

 radio receiving set should not vary more than ± 2 TU within the 

 band specified above to give a good telephone communication circuit. 

 6. Selectivity Requirements. The selectivity required in the receiving 

 set is such that when the desired signal is at the assumed minimum 

 value no deleterious effects will be caused by undesired signals. 



10 20 30 40 50 60 70 80 90 100 110 120 130 140 ISO 160 170 180 190200 

 FREQUENCY IN KILOCYCLES 



Fig. 16 — Selectivity requirements for the long wave transatlantic radio-telephone 



receiving system. 



In Fig. 16 there are shown measured daylight field strengths of 

 various existing radio-telegraph stations as observed at Riverhead, 

 New York, and at Cupar, Scotland. Since these measurements could 

 not be indefinitely extended in frequency nor could they take into 

 account all stations which might exist in this range, they may be con- 

 sidered only as a guide in obtaining a curve of the maximum telegraph 

 interference to be expected. These data, in Fig. 16, have been 

 expressed as ratios (in TU) to the minimum desired signal to be 

 received. It is important to note that the directional selectivity of 

 the receiving antenna system used materially decreases the relative 

 magnitude of many of these interfering signals, particularly at the 

 higher frequencies. The American receiving station is now located in 



