To keep the noise of the amplifying device below or equal to the thermal noise of 

 the antenna, a geophysical- type transformer working into an RCA Nuvistor was 

 used in the input circuit of the preamplifier. 



There are two basic sensors that can be used for detecting electromagnetic 

 energy in the sea. They are a loop antenna, which responds to the magnetic field 

 component, and a pair of electrodes, which respond to the electric component. 

 When measurements are being made at sea or in the sea from a moving platform, 

 the sensor is subjected to motion with respect to the earth's magnetic field. Since 

 such motion induces large currents in loop antennas, an electrode antenna was 

 chosen as the sensor. The sensor is towed astern of the ship. At low speeds this 

 trailing electrode antenna is essentially insensitive to vibration. 



The overall length of the trailing electrode antenna line installed on the GILLISS 

 was 360 meters (Fig. 3). Two stainless-steel electrodes were located at the trailed 

 end of the line and were spaced 75 meters apart. A miniature, bellows-type pres- 

 sure transducer was installed near the end of the line to monitor the depth of the 

 antenna. A dummy antenna was located in the line near the electrode closest to the 

 ship to monitor the transmission line for cable contamination. The line wfis depressed 

 to approximately 30 meters by a 12-inch by 12-inch depressor designed by Taylor 

 Model Basin. 



The voltage that is induced in a pair of electrodes in a medium of uniform 

 conductivity is directly proportional to electrode spacing. Thus, the electrodes 

 were spaced 75 meters apart to increase the system sensitivity. Sources of noise, 

 such as velocity-induced noise, thermal noise, and polarization noise are independent 

 of spacing. 



The major fixed noise source affecting sensitivity is the phenomenon of polariza- 

 tion. This is the gradual build-up of corrosion by-products at the electrode-sea 

 water interface. This coating, in the case of most stainless steels, builds up a 

 lossy capacitive film that further inhibits corrosion. However, the coating is not 

 stable; it is constantly breaking down, changing the corrosion process, and conse- 

 quently changing the electrical equilibrium. This changing corrosion process 

 produces the electrode self-noise. Continuing experiments with various materials 

 have found carbon to be unaffected by polarization, and a new trailing electrode 

 antenna that utilizes carbon as the electrode material is being designed. 



The trailing cable was manufactured by Samson Cordage Works, Shirley, Mass. , 

 from a design developed by the Underwater Sound Laboratory and Samson. The 

 physical characteristics of the trailing electrode antenna line are iiiieresting. 

 Several special features had to be incorporated into the manufactur of this line. 

 The first feature was that the supporting line be non-metallic. Because of the 

 nature of the electric dipoles, it is essential that no metal be in the vicinity of the 



248 



