electrodes. Any material whose conductivity is greater than that of the sea water 

 will distort the electric field pattern and consequently alter the sensitivity of the 

 antenna. Exposed metal could also react with the electrodes and result in unwanted 

 corrosion currents. These DC corrosion currents could be modulated by mechanical 

 motion of the line and cause unwanted noise. 



The second feature concerned specific gravity. The specific gravity of the line 

 must be close to the specific gravity of sea water in order to maintain a known dis- 

 tance between electrodes and hence measure the horizontal component accurately. 

 The only material that would fulfill the above requirements was polypropylene. It 

 is non-metallic, and its specific gravity is 0.9. 



In addition to having a neutral buoyancy and being non- metallic, the cable had to 

 have sufficient strength to withstand the tension developed from towing. The cable 

 elongation, while towing, had to be kept to a minimum to prevent the electrical 

 connecting wires from being permanently deformed and weakened. As fabricated, 

 the trailing cable, in addition to the electrical leads, contained an inner core, an 

 intermediate braid, and a cover braid (Fig. 4). Most of the strength was in the 

 core and intermediate braid, while the cover braid protected the line from abrasion. 

 With this construction the line would only elongate 5 percent at the top speed of an 

 AGOR. 



The line was manufactured on the cordage company's normal production machines 

 with the three twisted pairs of electrical leads constrained in the center by the two 

 outer layers of multi-filament polypropylene braid. The nominal diameter of the 

 line was 1. 25 inches and was essentially neutrally buoyant throughout its entire 

 length. The stainless-steel electrodes, 60 inches long by 2 inches wide, were 

 spiraled on top of the intermediate braid and covered with the outer braid. 



Two of the major electrical problems in the trailing electrode antenna were the 

 lead resistance and susceptibility to electromagnetic interference. The wire size 

 had to be large enough not to add unduly to the thermal noise of the overall antenna 

 impedance. The No. 14 wire used in connecting the electrode pair to the instrumen- 

 tation adds 6. 5 ohms to the nominal 1. 5 ohms of the electrode pair. The wire was 

 covered with extruded teflon and was tested in manufacture with a high voltage to 

 insure that there were no ruptures in the jacket. A tightly twisted pair was selected 

 over a coaxial type cable because it is easier to manufacture, tolerates physical 

 elongation, and provides lower susceptibility to surrounding magnetic fields. 



For example, in a one-gauss field the voltage induced in a single twist of the 

 twisted pair of 100 cycles is 1 x 10~ volt. The coefficient of coupling between two 

 of the twisted pairs was 5 x 10~ . With this low susceptibility and coefficient of 

 coupling, the transmission line would be unaffected by anabient electromagnetic 

 noise fields on or near the ship. 



250 



