410 BELL SYSTEM TECHNICAL JOURNAL 



path, the rise of the field to high values as the ship passed out of the 

 harbor, and the gradual diminution as the vessel continued on her 

 course. It will he oljserved that transmission on this frequency was 

 effective at night all the way to Bermuda, but that during the daytime 

 the transmission failed for distances greater than a few hundred miles. 

 Corresponding measurements showed that daylight transmission could 

 be secured by means of a higher frequency, such as 9 megacycles (v33 

 meters). Measurements of this kind, supplemented by data obtained 

 for a wide range of distances over land, and for transatlantic distances, 

 have built up a fairly complete set of quantitative data on short-wave 

 transmission over different distances and for various times of the day 

 and year. 



Along with this study of transmission conditions, there was carried 

 on the development of short-wave apparatus technicjue for telephony. 

 The first application was in the field of point-to-point transatlantic 

 operation and the considerable art built up there, including the design 

 of transmitters, receivers, directive antennas, and the working out of 

 two-way operating methods, served as a very useful basis from which to 

 develop the coastal and ship stations for the maritime system. 



With this background of development, preparations were made to 

 set up a two-way, short-wave radio telephone system for commercial 

 service. This service was centered upon New York because of the 

 large concentration of ocean-going trafftc at that port. 



The Technical Problem 



One of the most important problems to be solved in the design of a 

 short-wave system is that of determining the frequencies necessary for 

 giving the service involved. The frequencies which are best suited to 

 the different distances, time of day, and season of the year for trans- 

 mission over the North Atlantic are indicated in the curves of Fig. 2. 

 The curves for the greater distances refer to the transmission which 

 appears to take place in the upper regions of the earth's atmosphere 

 and is usually referred to as sky-wave transmission. Each of the sky- 

 wave curves traces the optimum frequency-distance relation for the 

 time of day and season of the year indicated. The curves merely give 

 a general picture of the frequency relation and do not take account of 

 other effects, such as fading, magnetic storms, etc. 



The figure brings out very clearly the necessity for using a variety of 

 wavelengths if the ship lanes are to be adequately covered. Fortu- 

 nately, there is a considerable band on each side of the curves shown, in 

 which good transmission can be obtained, and this enables one to choose 

 a small number of frequencies in the short-wave range which are ade- 



