on approximately 69 megacycles. The tube also operates as a frequency 

 doubler and amplifier with an output at 138 megacycles. The lever arm of 

 the accelerometer forms one plate of a variable capacitor which is in the 

 oscillator frequency-determining circuit. When the accelerometer arm 

 moves, it changes the value of this capacitance, which results in a fre- 

 quency change of the oscillator. The change in frequency due to the 

 movement of the accelerometer arm is very nearly proportional to the 

 acceleration that the accelerometer senses. The capacitance change is 

 adjusted so that a 1/2-g acceleration results in a frequency change of 

 approximately 50 kc . The output of the transmitter is fed through a 

 coaxial cable to the ground plane antenna. It should be noted that the 

 transmitter was designed with low cost in mind, and because of its simpli- 

 city, the frequency of its output drifts with temperature and other changes. 

 This effect will be noticeable in operation and will require the user to 

 occasionally retune the receiver during operation. 



BATTERY PACK 



The battery pack used to power the transmitter is composed of six 

 45-volt dry batteries that furnish plate and screen voltage for the tube 

 and one 3-volt battery for the tube filament. The batteries are wedged 

 into the bottom of the instrument box and are held in place by wooden 

 braces to prevent them from shifting in a rough sea. The batteries are 

 of sufficient capacity to operate the transmitter for a period of more 

 than 8 hours. 



RECEIVING ANTENNA 



The receiving antenna is of the stacked coaxial type with a ground 

 plane and has a gain of 6 db over a simple dipole. This antenna was used 

 because it provides uniform reception from all directions and has a low 

 angle of radiation. Also, temporary installation aboard ship is quite 

 simple. It should always be installed as high as practicable above the 

 water, and clear of obstructions in all directions, to provide the great- 

 est line of sight path from the transmitting antenna. The antenna is 



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