EXTENSION OE THE TELEPHONE SYSTEM TO SHIPS 165 



th.it with the circuit conditions .is illustrated, the over-all equivalent 

 between, say, a Chicago subscriber and a ship, including a 10-unit 

 toll circuit loss, is approximately 25 units, which should give a good 

 talk. 



Power Levels and Interference 



It is necessary that the magnitude of stray currents be so kept 

 down in comparison to the transmission currents thruout the system 

 as to obviate noise interference with telephone conversation. This 

 requirement is particularly difficult of realization in the radio link 

 because of static and, especially in the vicinity of New York, inter- 

 ference from spark telegraph stations. It is, of course, this inter- 

 ference, caused by the presence in the ether, on the wave length band 

 being used, of extraneous wave components, which sets the actual 

 range limit of the radio link. Actually it was found that in trans- 

 mitting on about 400 meters in the vicinity of New York the receiving 

 field strength could not be permitted to go on the average below 

 about 200 micro-volts per meter, and even then the spark situation 

 is so bad in the present art as to give periods of prohibitive inter- 

 ference. In less congested zones along the coast to the north, prob- 

 ably lower field strengths could be permitted. 



Transmission Variations in Radio Circuit 



One of the outstanding transmission characteristics of a ship-to- 

 shore radio telephone system is the variation which the attenuation of 

 the radio link undergoes as a result of the movement of the vessel. 

 In order to determine the magnitude of these variations, a series of 

 measurements were made of the telephone transmission over the 

 radio circuit as the vessel proceeded on her course. 



The method of making these measurements is shown schematically 

 in Fig. 13. Take for example the case of measuring a one-way circuit 

 as distinguished from a circuit looped back. A 1,000-cycle current 

 of predetermined power of the order of one miliwatt is impressed 

 upon the input circuit of the radio transmitter. This tone is received 

 in the output of the distant radio receiver. There it is passed to the 

 measuring apparatus where it is amplified, rectified, and made to 

 operate an indicating instrument. The receiving end measuring 

 apparatus is then switched to a local source of 1,000-cycle current 

 giving the same power as was applied to the transmitter at the sending 

 end. The proportion of this power which enters the measuring ap- 

 paratus is then varied by a variable network calibrated in power ratio 



