L 



20 kc + 3DB 



BANDPASS FOR 

 + V 



10 DB 



435 REGION OF -V 455 kc CARRIER 475+ 

 Fig. 3- Receiver bandpass characteristics. 



found to optimize the signal to noise ratio and 

 minimize the effects of head turbulence. 



Adequate frequency stability was obtained by 

 use of 0.002$ tolerance quartz crystals in both 

 the transmitter and receiver. A drift of the 

 receiver's local oscillator causes a shift in 

 the received signals with respect to the IF band- 

 pass but does not affect the instrument accuracy. 

 Transmitter frequency drift effect is discussed 

 in the section on theory. 



THE EXPERIMENT 



Several experiments during a one year period 

 were conducted at the Institute of Marine Science 

 laboratory dock in 8 feet of sea water. The head 

 assembly was tripod-mounted k feet off the bottom 

 and aligned parallel with the current flow. The 

 heads faced toward the Bear Cut bridge 1,000 feet 

 away and were h feet to one side of the dock 

 pilings. Various head angles and crossover points 

 were tried as well as positions parallel and normal 

 to the current flow. 



A series of tests was run in the towing tank 

 at Stevens Institute of Technology and the output 

 was tape recorded. The tank was 80 feet square 

 with damping beaches and was filled to a depth of 

 h feet with still fresh water in an air condi- 

 tioned room. 



Further tests were conducted off the west shore 

 of Bimini, Bahamas, in 20 feet of water during 

 slack tide when wind and current were nil. The 

 transducer head assembly was tripod mounted 

 k feet from the bottom and heads were successively 

 positioned in all k quadrants, starting normal to 

 the shoreline facing seaward. Various head 

 spacings and angles were tried. The meter output 

 was monitored and tape recorded. 



The receiver is a 7-transistor circuit 

 employing an RF stage, mixer, oscillator (crystal- 

 controlled), 2-stage IF strip, diode detector 

 and 2 audio stages. AVtJ was not incorporated 

 in an effort to maintain system linearity. The 

 IF amplifier was tuned as shown in Fig. 3 so that 

 the carrier and upper IF sideband only were 

 passed. This limited the instrument response 

 to read current flow toward the transducer faces 

 and reduced response to turbulent flow in one- 

 half of the possible directions . A receiver 

 sensitivity of 1 microvolt at 0.15 VPP output for 

 a 12 db (S+N)/N ratio was achieved by the use of 

 70 Mcps cut-off PADT transistors (post alloy 

 diffused. Amperex, Inc.) in the critical circuits. 

 The receiver was powered by a 12 volt mercury 

 cell. 



The transmitter utilized a conventional crystal 

 controlled vacuum tube oscillator. A variable 

 voltage regulated power supply provided control 

 of transmitter power input from 15 watts to less 

 than 10 milliwatts. In practice, a power input 

 of k$ volts at 6 milliamps to plate and screen 

 of the oscillator tube was found to be optimum. 

 The low impedance of the barium titanate trans- 

 ducer (-J = 2 ohms, R<2 ohms) made it necessary 

 to place it in series with the final tank circuit 

 in order to transfer sufficient driving power. 



No environmental housings were fabricated and 

 the heads were fed remotely by means of coaxial 

 cable. Under average sea reflectivity condi- 

 tions, audio outputs of 0.1 to 0.35 VPP were 

 obtained with a signal to noise ratio of 10:1. 

 Higher signal to noise ratios can be obtained by 

 closer transducer frequency matching and/or 

 higher sensitivity crystals. 



Tests in the bay at Bimini off the Lerner 

 Marine Laboratory dock were conducted with the 

 transducers 8 inches from the surface in 2 feet 

 of water. The heads faced into the current well 

 forward of the dock pilings but 2 feet from an 

 anchored barge. The water was clear and the sur- 

 face was smooth. A small drifting wood chip was 

 carefully timed over the length of the barge to 

 get an approximate flow velocity. Tapes were 

 made of the meter output . The head spacing was 

 8 inches with a crossover of 10 inches . 



ANALYSIS 



Three principal types of equipment were used 

 for readout . An Airpax magmeter read frequency 

 of the Doppler shift. Integrating time constants 

 were varied from 0.25 second to k seconds in 

 0.25 second steps. A 4-second integration gave 

 the most stable readings. A Panoramic sonic 

 analyzer, Model LP1A, was used for spectrum 

 analysis using one second duration and to 20 

 Kcps log frequency sweeps at linear amplitude 

 settings. Relative amplitude relationships were 

 studied. A Tektronix oscilloscope, Model 535; 

 was used to display output wave shape and fre- 

 quency . 



Tests were recorded on magnetic tape and 

 analyzed. A ^-second tape loop was made of a 

 representative portion of each test. Succes- 

 sively 1, 20 and k-0 sweeps were recorded on the 

 analyzer. Single sweeps at rates of land 2 cm per 

 second were recorded on the scope and magmeter 

 readings at k seconds integration time were noted. 

 In addition, 15 and 60 consecutive one second 

 sweeps were taken of continuous segments of the 



129 



