74 



USRL TEST STATIONS 



Figure 8. 15 c to 150 kc recording system. Bay on left 

 shows receiving amplifier and detector. To the right, 

 shown in order, are signal and noise generators, recorder 

 and thyratron control panel, and power amplifiers. 



While the lead-covered coaxial lines provide excel- 

 lent transmission, equally good performance for com- 

 paratively short runs can be obtained from a twisted 

 pair of flexible, single coaxial cables (Figures 11B 

 and 1 1C). The rubber-covered cable is used for lines 

 that are exposed to the weather; the cotton-braid 

 covered one for inside connections. 



Projector Coupling Equipment 



The projector coupling apparatus is housed near 

 the test area in the transmitting booth. The primary 

 function of this equipment is to provide suitable im- 

 pedances for matching the various test projectors as 

 they are connected to the 135-ohm transmitting line. 

 Repeating coils provide various sending impedances. 

 H-type resistance pads, designed to be used between 

 each sending impedance and 135 ohms, permit the 

 measurement of available power at any sending im- 

 pedance. Two repeating coils are available for han- 

 dling power outputs up to 100 watts. One of these has 

 seven secondary windings terminating in a multi- 

 contact receptacle. The sending impedance is varied 

 by inserting in this receptacle one of seven plugs, 

 with contacts strapped together in various patterns 

 so that impedances of 4, 9, 16, 25, 36, 49, or 64 ohms 

 may be provided. The second coil provides imped- 

 ances of 135, 600, or 2,400 ohms. A third coil handles 

 power outputs up to 1,000 watts at 50, 100, or 500 

 ohms, and resistance pads are available for measuring 

 the available power at these high outputs. A watt- 

 meter circuit for measuring actual power delivered 

 to the projector is also available. 



Hydrophone Coupling Equipment 



The hydrophone coupling apparatus is housed on 

 the piers in the receiving booths. Its primary function 

 is to provide suitable coupling between hydrophones 

 and the 135-ohm receiving lines to the laboratory. 



A battery-operated preamplifier of novel design 

 provides for either balanced or unbalanced opera- 

 tion. A switch in one position sets the input circuit 

 for balanced operation. In this case impedance may 

 be represented by a shunt resistance of 100 megohms 

 and a shunt capacitance of about 5^/xf, with a ground 

 at the electric center. In the other position the input 

 impedance is set for unbalanced operation and may 

 be represented as a shunt resistance of 50 megohms 

 and a shunt capacitance of about lO^if, with one 

 terminal at ground. The amplifier output has been 

 designed to feed into the 1 35-ohm receiving line. The 



