Page 599 radio acoustic ranging 644 



obtained from a quartz crystal with a low temporatiirc coefficient, but temperature 

 control is not necessary. The frequencies of all the sono-radio buoys used in a scheme 

 of control need not be exactly the same — in fact, small differences of a few hundred 

 cycles in frequency aid in distinguishing one sono-radio buoy from another. 



Under normal operating conditions, at a wave length of about 72 meters (4160 kc), 

 3 watts of radio-frequency power will furnish a signal of adequate strength to cover the 

 distance range desired of a sono-radio buoy — the maximum range required in most 

 surveys being about 100 nautical miles (see also 6443). 



The transmitter may be of conventional design — either a crystal-controlled oscilla- 

 tor driving a class C amplifier or a single-tube crystal-controlled oscillator has been 

 found satisfactory. Both of these types are used by the Coast and Geodetic Survey to 

 good advantage (see 6513 and 6523). 



For convenience, the transmitter should be designed so that once it is properly 

 tuned it will not require retuning for a long time. This applies principally to the 

 equipment in the drum. Any slight adjustment to the tuning of the output circuit, 

 or to the antenna-coupling circuit, can be readily made. 



Wliere the length of the antenna is only a small fraction of the radio wave length, 

 the base of the antenna is at a high impedance point and must be elevated above the 

 water surface to prevent it from being effectively shorted during rough weather. This 

 makes it necessary to couple the transmitter to the antenna through a low-impedance 

 waterproof transmission line. This may be a two-conductor rubber-covered cable. 

 The transmission line is terminated in a watertight container mounted on the buoy 

 structure about 3 feet above the water, which contains the antenna-coupling circuit. 



It is desirable to make the vertical antenna as long as practicable, consistent with 

 convenient handling on the ship. The ideal length of antenna would be about a quarter 

 wave length, but of course this is impracticable for a frequency of 4 megacycles. Sono- 

 radio buoys, used by the Coast and Geodetic Survey, operate on a wave length of 72 

 meters, but their antennas are generally not more than 18 feet long. An 18-foot 

 antenna represents about one-thirteenth of the wave length. 



644. Radio Frequencies and Power 

 6441. Radio Frequencies 



The most suitable frequency for radio transmission of R.A.R. stations requires 

 some consideration. The ideal frequency would be one at which reliable radio signals 

 could be received without interference 24 hours a day in all seasons at all necessary 

 operating distances. However, since available power is limited and due to the varying 

 distance to which small amounts of radio energy will produce satisfactory signals, the 

 ideal frequency is generally unattainable. 



Except for some slight directivity of the ordinary antennas used in R.A.R. , the 

 radio waves are transmitted in all directions of radii of a hemisphere drawn about the 

 antenna as a center. Radio waves starting from the antenna at zero angle with the 

 earth's surface are called ground waves and all others are called sky waves. The 

 ground wave follows the curvature of the earth and the distance to which it is propa- 

 gated is a function of the conductivity of the earth's surface. Fortunately for hydro- 

 graphic surveying, the conductivity of sea water is much greater than that of the land 

 and consequently the ground wave travels much farther over the sea than over land. 

 Except for slight refraction and diffraction a sky wave travels in a straight line until 



