TRANSATLANTIC RADIO TELEPHONY 313 



antenna, but at this frequency reasonably satisfactory results are 

 obtained. 



3. Receiving Antenna Efficiency. The use of directional receiving 

 antennas is essential to satisfactory and economic results over such 

 distances as the transatlantic radio path (see later in this paper). 

 The directivity of an antenna system of a given kind, size, and cost in 

 general increases with frequency, since the directivity is a direct func- 

 tion of the ratio of the dimensions of the antenna system to the 

 wave-length employed. 



4. Available Space in the Frequency Spectrum. Each of the above 

 factors operates to make the frequency of 60 kilocycles about the 

 best which could be used in the present state of the art for this trans- 

 mission path. Fortunately this frequency was so located in the radio 

 spectrum that a band of the desired width free from interference could 

 be obtained. 



It has been noted that the radio noise as shown in Fig. 2 varies 

 very little with frequency above 40 kilocycles. There is some doubt 

 as to whether or not this accurately represents the actual state of 

 affairs, since the measurement sets used for measuring the noise would 

 not satisfactorily measure much below one microvolt per meter on 

 account of tube noise. At frequencies of 40 kilocycles and above, 

 especially in the winter, there are many days during which the radio 

 noise is practically absent. On these days the measurements tended 

 to approach the minimum determined by the set noise. The fact that 

 many such readings were incorporated in the average probably tends 

 to mask the true variations of radio noise with frequency in this 

 range. On the other hand, however, they indicate a very real limita- 

 tion which tends to operate against the use of frequencies higher than 

 about 60 kilocycles unless fields were increased by increase in trans- 

 mitting power. This would be particularly true during the sunset 

 and sunrise dips and during periods of abnormally poor transmission 

 when the fields fall much below the average. If the set noise limitation 

 could be removed it is quite possible that frequencies above 60 kilo- 

 cycles would become more useful. Higher frequencies for radio 

 telephone use would be particularly advantageous because of the 

 greater band width which could be obtained from the transmitting 

 antenna and because of the greater directivity which could be obtained 

 in the receiving system at the same cost. 



Selection of a Satisfactory Receiving Location 



The selection of a suitable receiving location is based upon three 

 major considerations; namely, maximum received signal-to-noise ratio, 



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