VLF Transmission 



Undersea reception of very low 

 frequency signals suffers from large 

 amplitude attenuation and severe phase 

 perturbations. The latter result from 

 waves which cause variations in the 

 distance from the sea surface to the 

 point of reception. Experiments utilizing 

 a frequency of 17. 2 kc/s show that the 

 change in phase amounts to 8. 2 degrees 

 per foot of sea water path. This is of 

 major concern in a phase- coherent sig- 

 naling system. Signal attenuation, on 

 the order of 1. 3 dB per foot of immersion, 

 is also observed. 



Studies at the tower are aimed at 

 establishing adequate control for a phase- 

 compensating system. Pressure mea- 

 surements are made which are known to 

 correlate with the length of the subsurface 

 transmission path. Other factors such as 

 temperature, salinity, antenna orientation, 

 antenna motion, and the relative orienta- 

 tion of water wave to electromagnetic 

 wave are also related to the overall 

 problem of vlf propagation. 



An underwater-loop antenna and pressure transducer 

 are extended 12 feet out from the tower rail before 

 being lowered to varying depths for studies of 

 transmission. With the subsurface wooden extensions, 

 the tower had negligible effect on vlf attenuation 

 measurements down to a depth of 35 feet (over half- 

 way to the bottom). 



PRESSURE 

 TRANSDUCER 



66 



