RADIO TELEPHONE SERVICE TO SHIPS AT SEA 



411 



quate to cover the conditions. Actually, it is found that a set of about 

 four frequencies will suffice to cover the North Atlantic. For distances 

 greater than a few hundred miles this characteristic obtains irrespective 

 of whether the transmission is over water or over land, by reason of the 

 fact that the transmission appears to take place in the upper regions of 

 the earth's atmosphere. 



Closer in to the transmitting station, however, there is the so-called 

 surface component, the attenuation of which is much less over sea water 

 than over land. It will be seen that the surface wave may be relied 

 upon for distances of the order of 200-300 miles, for frequencies of 

 about 4 megacycles. The transmission of this component is much 

 more stable and reliable than is the transmission of the sky wave. It 



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DISTANCE. - STATUTE UILE.S 



Fig. 2. — Distance-frequency characteristic. 



seemed important, therefore, to utilize the surface wave to the maxi- 

 mum extent possible. 



W'ith this in mind, a series of transmission measurements was made 

 over a stretch of water between New Jersey, Long Island, and Nan- 

 tucket for the purpose of more accurately evaluating the effectiveness 

 of the surface wave component, particularly in so far as it bears upon 

 the question of how close to the water front the coastal station need be 

 placed. Transportable transmitting and receiving stations were used 

 in these tests. It was found that as the transmitting or the receiving 

 station was moved away from the water front, the attenuation in- 

 creased materially. For example, moving either terminal a mile back 

 from the coast line increases the attenuation some 8 decibels at 4.5 

 megacycles. On the other hand, a narrow stretch of land, such as a 

 sand bar, out a few miles from the coast, introduces relatively little loss. 



