Page 631 radio acoustic ranging 6723 



6723. Ship^s Antenna for R. A. R. 



Because of the low radio power radiated by sono-radio buoys, the receiving system 

 on the survey vessel must be very efficient. In this respect the ship's antenna used 

 for receiving the radio signals from the sono-radio buoy is very important. An ideal 

 system would be a vertical antenna one-quarter to one-half wave length long, with no 

 marked directivity characteristics in the horizontal plane. However, few survey 

 vessels have masts tall enough to support such an antenna, and the influence of metal 

 of the vessel in the field of the antenna usually makes it somewhat directive. Much 

 can be done, however, to make the antenna approach the desired characteristics. Wliere 

 space permits, the length of the antenna should be one-quarter to one-half of the wave 

 length used by the sono-radio buoy, even though all or part of the antenna is sloped. 

 The antenna should be kept as far as practicable from stays, rigging, smokestack, and 

 other antennas. 



The antenna wire should be strong and flexible, and suitable for shipboard use. 

 A stranded 7 by 22 phosphor-bronze wire is a preferred type. Insulators of a high 

 quality, such as pyrex 7K-inch, should be used to support the antenna. Two or more 

 such insulators should be used at each point where any are needed, to increase the 

 leakage path. The antenna should be kept taut at all times to prevent its swinging. 



The following test may be made to determine whether an antenna has the desired 

 directivity characteristics: Tune in a distant radio signal of about the same frequency 

 as that of the sono-radio buoy. The ship should then be turned slowly through 360° 

 while the strength of the radio signal is noted on some metering device incorporated in 

 or attached to the receiver. The antenna can be considered satisfactory if the minimum 

 signal strength observed is not less than one-half the maximum observed. 



If the test shows that the antenna is unsatisfactory, the entire system should be 

 carefully overhauled, and reinstalled if necessary, bearing in mind the points empha- 

 sized here. 



6724. Chronograph Amplifier 



Numerous types of audio amplifiers are used by the Coast and Geodetic Survey 

 for the operation of the chronograph. Their performance is essentially the same; they 

 differ only in design. Only one type of amplifier which has given good results will be 

 described. A circuit diagram of this chronograph amplifier is included in figure 140. 



The sensitivity of this amplifier is about medium and its frequency characteristics are those of 

 any fairly good audio amplifier. The last two tubes of the amplifier operate on receipt of either the 

 bomb or radio signals, whereas the first tube amplifies only the bomb signal. Located between these 

 two sections of the amplifier is a switch, known as the hydro-radio switch, which is intended for switch- 

 ing the last two stages that operate the signal stylus of the chronograph either to the output of the 

 hydrophone amplifier tube or to the output of the radio receiver. The final tube in the chronograph 

 amplifier is a type 6L6 beam power tube, triode connected. The plate circuit of this tube is connected 

 to the signal stylus magnet. This tube is biased to cutoff so that only the positive peaks of the 

 signals cause the plate current to flow. A filter in the plate circuit removes the audio-frequency 

 component leaving only the envelope of the audio-frequency signal. A miUiammeter in the plate 

 circuit of the type 6L6 tube gives the integrated value of its plate current caused by the bomb 

 or radio signal. This meter should be located where it can be observed by the radio technician 

 or chronograph attendant or both; it gives an indication of signal intensity. Preceding the type 

 6L6 tube is a type 6C5 triode tube which is arranged to operate as a class A amplifier. A line-to- 

 grid transformer with 125- and 500-ohm primary taps operates into the grid of the type 6C5 tube. 

 The 125-ohm primary tap is connected through a switch to the 125-ohm secondary of a plate-to-line 

 transformer, which is connected in the plate circuit of the hydrophone amplifier tube. The 500-ohm 



