342 BELL SYSTEM TECHNICAL JOURNAL 



It is believed that the directional diagram, shown in Fig. 15, repre- 

 sents about the ultimate that can be done economically in a general 

 reduction of back-end area and narrowing of the diagram by means of 

 wave-antennas. Future extensions or redesign of the array at Houlton 

 must be based on the reduction of the relative receptivity in distinct 

 directions determined either by statistical study of the noise received 

 by the antenna system or actual measurements of the direction of 

 arrival of the noise which limits the operation of the transatlantic 

 radio-telephone circuit. 



The Radio Receiver 



A description of the design and performance of the radio-telephone 

 receiving set will constitute another paper. The radio receiving 

 equipment employed in connection with the antenna systems was 

 developed and constructed by Bell Telephone Laboratories. 



The major transmission requirements upon which design must be 

 based are as follows: 



1. The limiting values of the signal field to be received; 



2. The output power of the receiving antenna for a given signal 



strength ; 



3. Power output required from the radio receiver; 



4. The type of telephone transmission to be received; 



5. The frequency band to be received; 



6. The nature and strength of interference from other radio stations 



and from noise; The selectivity required to reduce un- 

 desired modulation 



a. In amplifiers, 



b. In demodulators; 



7. Stability of frequency, gain, and transmission-frequency character- 



istic. 



1. Limiting Values of the Signal Field to be Received. The range of 

 daily averages of signal field at 60 kilocycles for all daylight path hours 

 is shown in Fig. 4. The fields, as previously published data indi- 

 cate,'*'®'^ vary diurnally between much wider limits. At night the 

 field frequently approaches, as a maximum, the value calculated on 

 the basis of the inverse distance law. During sunrise and sunset dip 

 periods the field frequently goes to a value less than one microvolt 

 per meter with even 50 kilowatts radiated from a transmitter 5,000 

 kilometers away. Suppose we take as being approximately correct 

 values, field strengths of 0.4 microvolts per meter as the lower limit 

 and 400 microvolts per meter as the upper limit. We then have deter- 

 mined that the receiving set should have a variation of gain of 60 TU. 



