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BELL SYSTEM TECHNICAL JOURNAL 



directions of arrival of static and other disturbances including unwanted 

 radio signals are random, it is obvious that sharp directive discrimina- 

 tion aids very materially in excluding them from the receiver. On 

 the other hand, the antennas are not sharply resonant systems and they 

 do not distinguish between waves from substantially the same direction 

 and closely adjacent in frequency. This duty is left to the circuits 

 of the radio receiver. 



BEATING 

 OSCILLATOR 



DIRECTIONAL 

 ANTENNA 



TRANSMISSION 

 ^ LINE 



LINE 



TERMINATION 



AND REPEATER 



FIRST 

 DETECTOR 



SECOND 

 DETECTOR 



TWO 



STAGES 



RADIO 



AMPLIFIER 



INTERMEDIATE 



FREQUENCY AMPLIFIER 



AND FILTER 



TELEPHONE 

 LINE 



AUDIO AMPLIFIER 

 AND MONITORING 

 APPARATUS 



Fig. 7- — Block schematic of receiving system. 



Having collected the signal with a directional antenna the energy is 

 conveyed to the receiving set by means of concentric pipe transmission 

 lines of small diameter. The use of concentric conductors simplifies 

 the prevention of direct signal pick-up by the lines, it reduces losses 

 and prevents external objects from influencing the transmission proper- 

 ties, thus allowing the line to be buried in the ground or placed a few 

 inches above the surface where it will have no appreciable adverse 

 effect on the antenna performance. 



Referring now to Fig. 7, the radio currents arriving over the trans- 

 mission line are first amplified by two stages of radio amplification 

 involving tuned circuits which discriminate further in favor of the 

 wanted signal. The signal delivered by the radio amplifier is at 

 a suitable level for efficient demodulation and is applied to the grid of 

 the first detector. By means of a beating oscillator whose frequency is 

 suitably adjusted, the first detector steps the signal carrier frequency 

 down to a fixed value of 400 kilocycles from one in the range 9000 to 

 21,000 kilocycles which depends, of course, on the distant transmitting 

 station assignment. The intermediate frequency signal at 400 kilo- 



