Page 611 radio acoustic ranging 6544 



in 6545. The keying threshold may also be indicated by the aid of a radio receiver. 

 After replacing the hydrophone, the sono-radio buoy is ready to be put on station. 



6544. Other Methods of Gain Adjustment 



A method used to a limited extent for gain adjustment is called the loud-speaker 

 method. The hydrophone is placed at a fixed distance in front of a loud speaker from 

 which a tone of fixed amplitude and frequency can be emitted. The gain of the sono- 

 radio buoy is then adjusted to give some amplifier output voltage, or to a value which 

 will just cause the radio trajiismitter of the unit to operate. The intensity of the sound 

 from the loud speaker and its distance from the hydrophone are fixed by the value of 

 gain needed by the sono-radio buoy for optimum operation as determined by 654(1). 

 This method has the advantage of including all the equipment that is subject to 

 change, but the measurements cannot always be accurately repeated and the method 

 cannot be considered as accurate as that described in 6543. 



To make these adjustments, a loud speaker or headphones are needed near enough 

 to the buo}^ so that a radio signal can be heard and, since the adjustments are often 

 made at some distance from the radio room, an extension must be provided from the 

 radio receiver — or a portable radio receiver may be used. 



A most inaccurate method, which unfortunately is frequently used, is to use a 

 sound made orally or by clapping one's hands in front of the hydrophone. The gain 

 is then adjusted until the radio transmitter just starts to operate. The loudness of 

 noise and the distance from the hydrophone for optimum gain adjustment are deter- 

 mined by test. A variation of this method is used with the sono-radio buoy suspended 

 at the ship's rail with the hydrophone in the water. The sliip's macliinery then becomes 

 the source of sound. The gain of the sono-radio buoy is then adjusted as above until 

 this noise causes the radio transmitter to operate. The success of this method depends 

 on the judgment and experience of the teclmician. Its only advantages are that little 

 time is consumed, no extra apparatus is needed, and the test involves all parts of the 

 buoy. 



6545, Adjustment of the Radio Transmitter 



Just before a sono-radio buoy is put on station its radio-frequency circuits must 

 be tuned. Most of the sono-radio buoys used by the Coast and Geodetic Survey are 

 arranged so that the radio transmitter can be tuned by adjusting the circuits in the 

 antenna coupler. (The other radio-frequency circuits need be tuned only occasionally, 

 as they should remain in adjustment for long periods of time.) Ideally, the final tuning 

 should be done after the sono-radio buoy has been put in the water, as then the antenna 

 characteristics are not influenced by the metal of the ship. However, this can be done 

 only from a small boat and it is time-consuming; moreover, it is difficult to make such 

 adjustments from a boat, unless the sea is extremely smooth. For practical reasons then 

 the final tuning must be done wliile the sono-radio buoy is still on board ship. 



The antenna should be vertical during the tuning, so as to be less affected by the 

 metal of the vessel. A convenient method is to tune the sono-radio buoy while it is 

 held over the side of the vessel, with its antenna in a vertical position, ready to be 

 anchored. Another satisfactory method is to use a dummy antenna circuit wliich is 

 equivalent to the antenna when the buoy is in the water. The advantages of this 

 latter method are that the metal of the ship has no effect on the dummy antenna circuit 

 and the adjustment will be nearer what is required when the sono-radio buoy is in the 



